CA 02531616 2005-12-23
HIGH STRENGTH THIN STEEL SHEET HAVING HIGH HYDROGEN
EMBRITTLEMENT RESISTING PROPERTY AND HIGH WORKABILITY
BACKGROUND OF THE INVENTION
Field of the Invention
[0001]
The present invention relates to a high strength thin
steel sheet that has high hydrogen embrittlement resisting
property (particularly the hydrogen embrittlement resisting
property after being subjected to forming process) and high
workability, especially to a high strength thin steel sheet
that has high resistance against fractures due to hydrogen
embrittlement such as season crack and delayed fracture that
pose serious problems for steel sheets having tensile strength
of 1180 MPa or higher, and has high workability.
Description of the Related Art
[0002]
There are increasing demands for the steel sheet, that
is pressed or bent into a form of a high-strength component of
automobile or industrial machine, to have both high strength
and high ductility at the same time. In recent years, there
are increasing needs for high strength steel sheets having
strength of 1180 MPa or higher, as the automobiles are being
designed with less weight. A type of steel sheet that is
regarded as promising to satisfy these needs is TRIP
(transformation induced plasticity) steel sheet.
[0003]
The TRIP steel sheet includes residual austenite
structure and, when processed to deform, undergoes
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considerable elongation due to induced transformation of the
residual austenite (residual y) into martensite by the action
of stress. Known examples of the TRIP steel include TRIP type
composite-structure steel (TPF steel) that consists of
polygonal ferrite as the matrix phase and residual austenite;
TRIP type tempered martensite steel (TAM steel) that consists
of tempered martensite as the matrix phase and residual
austenite; and TRIP type bainitic steel (TBF steel) that
consists of bainitic ferrite as the matrix phase and residual
austenite. Among these, the TBF steel has long been known
(described, for example, in NISSIN STEEL TECHNICAL REPORT, No.
43, Dec. 1980, ppl-10), and has such advantages as the
capability to readily provide high strength due to the hard
bainitic ferrite structure, and the capability to show
outstanding elongation because fine residual austenite grains
can be easily formed in the boundary of lath-shaped bainitic
ferrite in the bainitic ferrite structure. The TBF steel also
has such an advantage related to manufacturing, that it can be
easily manufactured by a single heat treatment process
(continuous annealing process or plating process).
[0004]
In the realm of high strength of 1180 MPa upward,
however, the TRIP steel sheet is known to suffer a newly
emerging problem of delayed fracture caused by hydrogen
embrittlement, similarly to the conventional high strength
steel. Delayed fracture refers to the failure of high-strength
steel under stress, that occurs as hydrogen originating in
corrosive environment or the atmosphere infiltrates and
diffuses in microstructural defects such as dislocation, void
and grain boundary, and makes the steel brittle. This results
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in decreases in ductility and toughness of the metallic
material.
[0005]
It has been well known that the high strength steel that
is widely used in the manufacture of PC steel wire and line
pipe experiences hydrogen embrittlement (pickling
embrittlement, plating embrittlement, delayed fracture, etc.)
caused by the infiltration of hydrogen into the steel when
tensile strength of the steel becomes 980 MPa or higher.
Accordingly, most of technologies of improving hydrogen
embrittlement resisting property have been developed aiming at
steel members such as bolt. "New Development in Elucidation of
Delayed Fracture" (published by The Iron and Steel Institute
of Japan in January, 1997), for example, describes that it is
effective in improving the resistance against delayed fracture
to add element such as Cr, Mo or V that demonstrates
resistance against temper softening to the metal structure
that is based on tempered martensite as the major phase. This
technology is intended to cause the delayed fracture to take
place within grains instead of in the grain boundaries,
thereby to constrain the fracture from occurring, by
precipitating alloy carbide and making use thereof as the site
for trapping hydrogen.
[0006]
Thin steel sheets having strength higher than 780 MPa
have rarely been used for the reason of workability and
weldability. Also hydrogen embrittlement has rarely been
regarded as a problem for thin steel sheets where hydrogen
that has infiltrated therein is immediately released due to
the small thickness. For these reasons, much efforts have not
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been dedicated to counter the hydrogen embrittlement. In
recent years, however, higher strength is required of the
reinforcement members such as bumper, impact beam and seat
rail, etc., in order to meet the requirement of weight
reduction of the automobile and to improve the collision
safety. Automobile components that are shaped by pressing or
bending process such as pillar are also required to have
higher strength. As a result, there have been increasing
demands for high strength steel sheet having strength of 980
MPa or higher for the manufacture of these parts. This makes
it necessary to improve hydrogen embrittlement resisting
property of the high strength steel sheet.
[0007]
Use of the technology addressed to the bolt steel
described above may be considered for improving the hydrogen
embrittlement resisting property of the high strength steel
sheet. However, in the case of "New Development in Elucidation
of Delayed Fracture" (published by The Iron and Steel
Institute of Japan in January, 1997), for example, 0.40 or
higher of C content and much alloy elements are contained, and
therefore application of this technology to a thin steel sheet
compromises the workability required of the thin steel sheet.
The technology also has a drawback related to the
manufacturing process, since it takes several hours or longer
period of heat treatment to cause the alloy carbide to
precipitate. Therefore, improvement of the hydrogen
embrittlement resisting property of a thin steel sheet
requires it to develop a novel technology.
[0008]
It is relatively easy to achieve a high strength with
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quench-hardened (tempered) martensite steel that has been
commonly used as a high-strength steel. However, improvement
of the workability without variability essentially requires it
to provide a tempering process which makes it necessary to
strictly control the temperature and duration of the process.
This also sometimes increases the possibility of tempering
embrittlement and makes it difficult to reliably improve
workability. Although there is a steel of composite structure
of martensite and ferrite or the like developed to improve
ductility, such a steel has a high notch sensitivity due to
mixed presence of hard phase and soft phase, thus making it
difficult to achieve sufficient improvement of hydrogen
embrittlement resisting property.
[0009]
Hydrogen-induced delayed fracture is believed to occur
in such a steel that contains martensite, because hydrogen is
concentrated in grain boundaries of prior austenite thereby to
form voids or other defects that become the starting points of
the fracture. Common practice that has been employed to
decrease the sensitivity to delayed fracture is to diffuse
fine grains of carbide or the like uniformly as the site for
trapping hydrogen, thereby to decrease the concentration of
diffusive hydrogen. However, even when a large number of
carbide grains or the like are diffused as the trap site for
hydrogen, there is a limitation to the hydrogen trapping
capability and delayed fracture attributable to hydrogen
cannot be fully suppressed.
[0010]
Japanese Unexamined Patent Publication (Kokai) No. 11-
293383 describes a technology to improve the hydrogen
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embrittlement resisting property of steel sheet, where
hydrogen-induced defects can be suppressed by having oxides
that include Ti and Mg exist as the main components in the
structure. However, this technology is intended for thick
steel sheets and, although consideration is given to delayed
fracture after welding with a large input heat, no
consideration is given to the environment (for example,
corrosive environment, etc.) in which automobile parts
manufactured by using thin steel sheets are used.
[0011]
Japanese Unexamined Patent Publication (Kokai) No. 2003-
166035 describes that it is made possible to improve the
ductility and delayed fracture resistance after being
subjected to forming process, by controlling the mutual
relationships between 1) the form (standard deviation and mean
grain size) in which oxide, sulfide, composite crystallization
product or composite precipitate of Mg is dispersed, 2)
volumetric proportion of residual austenite and 3) strength of
the steel sheet. However, it is difficult to improve the
hydrogen embrittlement resisting property in such an
environment as hydrogen is generated through corrosion of the
steel sheet simply through the trapping effect achieved by
controlling the form of precipitate.
[0012]
It has been a common practice in the past to reduce the
residual austenite that was believed to have an adverse effect
on the hydrogen embrittlement resisting property. In recent
years, however, the effect of residual austenite on the
improvement of hydrogen embrittlement resisting property has
been recognized and accordingly much attention has been paid
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to the TRIP steel that contains residual austenite.
[0013]
Tomohiko HOJO et. al "Hydrogen Embrittlement of High
Strength Low Alloy TRIP Steel (Part 1: Hydrogen Absorbing
Characteristic and Ductility", The Society of Materials
Science, Japan, proceedings of 51St academic lecture meeting,
2002, vol. 8, ppl7-18 and Tomohiko HOJO et. al "Influence of
Austempering Temperature on Hydrogen Embrittlement of High,
for example, describe investigations into the hydrogen
embrittlement resisting property of the TRIP steel. It is
pointed out that, among the TRIP steels, TBF steel has
particularly high hydrogen absorbing capacity, and observation
of a fracture surface of the TBF steel shows the restriction
of quasi cleavage fracture due to storage of hydrogen. However,
the TBF steels reported in the documents described above show
delayed fracture characteristic of about 1000 seconds at the
most in terms of the time before crack occurrence measured in
cathode charging test, indicating that these steels are not
meant to endure the harsh operating environment such as that
of automobile parts over a long period of time. Moreover,
since the heat treatment conditions reported in the documents
described above involve heating temperature being set higher,
there are such problems as low efficiency of practical
manufacturing process. Thus it is strongly required to develop
a new species of TBF steel that provides high production
efficiency as well. Also there has been such a problem that
press forming operation leads to lower hydrogen embrittlement
resisting property.
[0014]
As described above, there have been virtually no TRIP
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steels containing residual austenite that have been developed
so as to demonstrate high workability when processed to form
parts, by taking measures to counter hydrogen embrittlement
after the forming process in consideration of the harsh
operating environment such as that of automobile parts over a
long period of time.
SUMMARY OF THE INVENTION
[0015]
The present invention has been made with the background
described above, and has an object of providing a high
strength thin steel sheet that shows high hydrogen
embrittlement resisting property in a harsh operating
environment over a long period of time after the process of
forming the steel sheet into a part, and has improved
workability and tensile strength of 1180 MPa or higher.
[0016]
In order to achieve the object described above, the
present inventors conducted a research on a steel sheet that
shows high hydrogen embrittlement resisting property after the
forming process, and demonstrates improved workability which
is the characteristic property of the TRIP steel sheet during
the forming process. Through the research, it was found that
it is very important to control the metallurgical structure
after the forming process in order to achieve high hydrogen
embrittlement resisting property after the forming process.
Specifically, it was found that it is important that the metal
structure after the stretch forming process is constituted
from:
1% or more of residual austenite;
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80~ or more in total of bainitic ferrite and martensite; and
90 or less (may be 0%) in total of ferrite and pearlite in
terms of the proportion of area to the entire structure,
wherein the mean axis ratio (major axis/minor axis) of the
residual austenite grains is 5 or higher.
A first high strength thin steel sheet having high
hydrogen embrittlement resisting property according to the
present invention comprises higher than 0.25 and up to 0.600
of C (contents of components given in terms of percentage in
this patent application all refer to percentage by weight),
1.0 to 3.0~ of Si, 1.0 to 3.50 of Mn, 0.150 or less P, 0.020
or less S and 1.5% or less (higher than 0%) of A1, while iron
and inevitable impurities making up the rest, wherein the
metallurgical structure comprises:
to or more residual austenite;
80% or more in total of bainitic ferrite and martensite; and
90 or less (may be 0%) in total of ferrite and pearlite in the
proportion of area to the entire structure, and wherein the
mean axis ratio (major axis/minor axis) of the residual
austenite grains is 5 or higher, and the steel has tensile
strength of 1180 MPa or higher.
[0017]
The present inventors also conducted another research
from a point of view that was different from that of the
former research, and found that high hydrogen embrittlement
resisting property after the forming process can be achieved
by controlling the metal structure after the forming process
as follows. It is important that the metal structure after the
forming process comprises:
to or more residual austenite;
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the mean axis ratio (major axis/minor axis) of the residual
austenite grains is 5 or higher.
mean length of minor axes of the residual austenite grains is
1 um or less; and
minimum distance between the residual austenite grains is 1 um
or less.
When the metal structure is controlled as described above,
hydrogen embrittlement resisting property of the high strength
thin steel sheet can be sufficiently improved without adding
much alloy elements. The phrase "after the forming process"
means the state of the steel sheet after being stretched with
an elongation ratio of 30. Specifically, the steel sheet is
subjected to uniaxial stretching of 3°s at the room temperature
(the stretching process of 3°s elongation may hereinafter be
referred to simply as "processing").
[0018]
A second high strength thin steel sheet having high
hydrogen embrittlement resisting property according to the
present invention comprises higher than 0.25 and up to 0.60a
of C, 1.0 to 3.Oa of Si, 1.0 to 3.50 of Mn, 0.150 or less P,
0.020 or less S, 0.50 or less (higher than 0~) Al, while iron
and inevitable impurities making up the rest, wherein the
metal structure after the stretch forming process of 30
elongation comprises:
to or more residual austenite;
the mean axis ratio (major axis/minor axis) of the residual
austenite grains is 5 or higher;
mean length of minor axes of the residual austenite grains is
1 ~m or less;
minimum distance between the residual austenite grains is 1 um
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or less; and
tensile strength is 1180 MPa or higher.
According to the present invention, it is made possible
to manufacture, with a high level of productivity, a high
strength thin steel sheet having tensile strength of 1180 MPa
or higher that neutralizes hydrogen that infiltrates from the
outside after the steel sheet has been formed into a part
thereby to maintain satisfactory hydrogen embrittlement
resisting property, and demonstrates high workability during
the forming process. Use of the high strength thin steel sheet
makes it possible to manufacture high strength parts that
hardly experience delayed fracture, such as bumper, impact
beam and other reinforcement members and other automobile
parts such as seat rail, pillar, etc.
$RIEF DESCRIPTION OF THE DRAWINGS
[0019]
Fig. 1 is a schematic perspective view of a part used in
pressure collapse test in Example 1.
Fig. 2 is a side view schematically showing the setup of
pressure collapse test in Example 1.
Fig. 3 is a schematic perspective view of a part used in
impact resistance test in Example 1.
Fig. 4 is a sectional view along A-A in Fig. 3.
Fig. 5 is a side view schematically showing the setup of
impact resistance test in Example 1.
Fig. 6 is a photograph of TEM observation (magnification
factor 15000) of No.101 (inventive steel) of Example 1.
Fig. 7 is a photograph of TEM observation (magnification
factor 15000) of No.120 (comparative steel) of Example 1.
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Fig. 8 is a photograph of TEM observation (magnification
factor 15000) of No.201 (inventive steel) of Example 2.
Fig. 9 is a photograph of TEM observation (magnification
factor 15000) of No.220 (comparative steel) of Example 2.
Fig. 10 is a graph showing the relationship between the
mean axis ratio of the residual austenite grains and hydrogen
embrittlement risk index.
Fig. 11 is a diagram schematically showing the minimum
distance between residual austenite grains.
Fig. 12 is a photograph of TEM observation
(magnification factor 15000) of No.301 (inventive steel) of
Example 3.
Fig. 13 is a photograph of TEM observation
(magnification factor 60000) of No.301 (inventive steel) of
Example 3.
Fig. 14 is a photograph of TEM observation
(magnification factor 15000) of No.313 (comparative steel) of
Example 3.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0020]
(First Embodiment)
The first high strength thin steel sheet according to
the present invention is constituted from higher than 0.25 and
up to 0.600 of C (contents of components given in terms of
percentage in this patent application all refer to percentage
by weight), 1.0 to 3.0% of Si, 1.0 to 3.50 of Mn, 0.150 or
less P, 0.020 or less S, 1.50 or less (higher than 0~) of Al,
1.0% or less (higher than 0%) of Mo and 0.1% or less (higher
than 0%) of Nb, while iron and inevitable impurities making up
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the rest, and is characterized in that:
(i) the metal structure after the forming process contains:
1% or more residual austenite;
800 or more in total of bainitic ferrite and martensite; and
90 or less (may be Oo) in total of ferrite and pearlite
in terms of the proportion of area to the entire structure,
and
the mean axis ratio (major axis/minor axis) of the residual
austenite grains is 5 or higher; and
(ii) the steel contains a specified amount of Mo and/or Nb.
The requirements described above have reasons as follows.
[0021]
(Metal structure after stretch forming by 3% elongation)
Metal structure after stretch forming process by 30
elongation was specified because, in various experiments
conducted for the actual processing conditions in the
manufacture of a part, best correlation between the result of
laboratory test and the actual occurrence of cracks in the
part was observed when the part was processed by stretch
forming with an elongation ratio of 30.
The phrase "after the forming process" means the state
of the steel sheet after being stretch formed with elongation
of 30. Specifically, the steel sheet is subjected to
elongation of 3s by uniaxial stretching at the room
temperature (the stretch forming process of 3o elongation may
hereinafter be referred to simply as "process").
[0022]
(lo or more residual austenite in the area proportion to the
entire structure)
It is necessary that the metal contains to or more
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residual austenite in the area proportion to the entire
structure after the process of forming the part, in order to
achieve high hydrogen embrittlement resisting property in
harsh operating environment over an extended period of time
after forming the part. Content of the residual austenite is
preferably 2% or higher, and more preferably 3% or higher.
Since the desired level of high strength cannot be obtained
when an excessive amount of residual austenite is contained
after processing, it is recommended to set an upper limit of
20% (more preferably 15%) to the residual austenite content.
[0023]
(Mean axis ratio (major axis/minor axis) of the residual
austenite grains: 5 or higher)
Lath-shaped grains of residual austenite after the
process have far higher capacity of trapping hydrogen than
carbide. When the mean axis ratio (major axis/minor axis) of
the residual austenite grains is 5 or higher, in particular,
it was found that hydrogen that infiltrates from the outside
through atmospheric corrosion can be substantially neutralized
thereby to achieve remarkable achievement in hydrogen
embrittlement resisting property. The mean axis ratio of the
residual austenite grains is preferably 10 or higher, and more
preferably 15 or higher.
[0024]
The residual austenite refers to a region that is
observed as FCC (face centered cubic lattice) by the FE-
SEM/EBSP method which will be described later. Measurement by
the EBSP may be done, for example, by measuring a measurement
area (about 50 by 50 um) at an arbitrarily chosen position in
a surface parallel to the rolled surface at a position of one
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quarter of the thickness at measuring intervals of 0.1 um. The
measuring surface is prepared by electrolytic polishing in
order to prevent the residual austenite from transforming.
Then the test piece is set in the lens barrel of an FE-SEM
equipped with an EBSP detector (of which details will be
described later) and is irradiated with electron beam. An EBSP
image projected onto a screen is captured by a high
sensitivity camera (VE-1000-SIT manufactured by Dage-MTI Inc.)
and is sent to a computer. The computer carries out image
analysis and generates color mapping of the FCC phase through
comparison with a structural pattern simulated with a known
crystal system (FCC (face centered cubic lattice) phase in the
case of residual austenite). Area proportion of the region
that is mapped as described above is taken as the area
proportion of the residual austenite. This analysis was
carried out by means of hardware and software of OIM
(Orientation Imaging MicroscopyTM) system of TexSEM
Laboratories Inc.
[0025]
The mean axis ratio was determined by measuring the
major axis and minor axis of residual austenite crystal grains
existing in each of three arbitrarily chosen fields of view in
the observation by means of TEM (transmission electron
microscope) with magnification factor of 15000, and averaging
the ratios of major axis to minor axis.
[0026]
(800 or more in total of bainitic ferrite and martensite)
In order to decrease the number of intergranular
fracture initiating points in the steel thereby to surely
decrease the concentration of diffusive hydrogen to a harmless
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level and achieve a high strength, it is desirable to form the
matrix phase of the steel structure after processing from a
binary phase structure of bainitic ferrite and martensite with
the bainitic ferrite acting as the main phase, instead of the
single phase structure of martensite that is generally used
for high strength steels.
[0027]
In the single phase structure of martensite, a carbide
(for example, film-like cementite) is likely to precipitate in
the grain boundaries, thus making intergranular fracture
likely to occur. In the case of the binary structure of
bainitic ferrite and martensite with the bainitic ferrite
acting as the main phase, in contrast, the bainitic ferrite is
a hard phase and therefore it is easy to increase the strength
of the entire structure as in the case of the single phase of
martensite. The hydrogen embrittlement resisting property can
also be improved as much hydrogen is trapped in the
dislocations. It also has such an advantage that coexistence
of the bainitic ferrite and the residual austenite which will
be described later prevents the generation of carbide that
acts as the intergranular fracture initiating points, and it
becomes easier to create the lath-shaped residual austenite in
the boundaries of lath-shaped bainitic ferrite.
[0028]
Accordingly, it is required in the present invention
that the binary structure of bainitic ferrite and martensite
occupy 800 or more, preferably 85% or more and more preferably
900 or more of the entire structure after the stretch forming
processing to elongate by 30. Upper limit of the proportion
may be determined by the balance with other structure
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(residual austenite), and is set to 99% when the other
structures (ferrite, etc.) than the residual austenite is not
contained.
[0029]
The bainitic ferrite referred to in the present
invention is plate-shaped ferrite having a lower structure of
high density of dislocations. It is clearly distinguished from
polygonal ferrite that has lower structure including no or
very low density of dislocations, by SEM observation as
follows.
[0030]
Area proportion of bainitic ferrite structure is
determined as follows. A test piece is etched with Nital
etchant. A measurement area (about 50 by 50 um) at an
arbitrarily chosen position in a surface parallel to the
rolled surface at a position of one quarter of the thickness
is observed with SEM (scanning electron microscope)
(magnification factor of 1500) thereby to determine the area
proportion.
[0031]
Bainitic ferrite is shown with dark gray color in SEM
photograph (bainitic ferrite, residual austenite and
martensite may not be distinguishable in the case of SEM
observation), while polygonal ferrite is shown black in SEM
photograph and has polygonal shape that does not include
residual austenite and martensite inside thereof.
[0032]
The SEM used in the present invention is a high-
resolution FE-SEM (Field Emission type Scanning Electron
Microscope XL30S-FEG manufactured by Philips Inc.) equipped
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with an EBSP (Electron Back Scattering Pattern) detector, that
has a merit of being capable of analyzing the area observed by
the SEM at the same time by means of the EBSP detector. EBSP
detection is carried out as follows. When the sample surface
is irradiated with electron beam, the EBSP detector analyzes
the Kikuchi pattern obtained from the reflected electrons,
thereby to determine the crystal orientation at the point
where the electron beam has hit upon. Distribution of
orientations over the sample surface can be measured by
scanning the electron beam two-dimensionally over the sample
surface while measuring the crystal orientation at
predetermined intervals. The EBSP detection method has such an
advantage that different structures that are regarded as the
same structure in the ordinary microscopic observation but
have different crystal orientations can be distinguished by
the difference in color tone.
[0033]
(9% or less (may be 0%) in total of ferrite and pearlite)
The steel sheet after the processing may be constituted
either from only the structures described above (namely, a
mixed structure of bainitic ferrite + martensite and residual
austenite), or may include other structure such as ferrite
(the term ferrite used herein refers to polygonal ferrite,
that is a ferrite structure that includes no or very few
dislocations) or pearlite to such an extent that the effect of
the present invention is not compromised. Such additional
components are structures that can inevitably remain in the
manufacturing process of the present invention, of which
concentration is preferably as low as possible, within 9%,
preferably less than 5% and more preferably less than 3%
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according to the present invention.
[0034]
In order to maintain high hydrogen embrittlement
resisting property after the forming process, for example,
large content of residual austenite of 5% or more may be
contained in the steel sheet prior to the forming process, or
large amount of fine residual austenite grains may be
dispersed in the structure. Alternatively, forming process
conditions may be controlled so as to make the residual
austenite less likely to transform (for example, form the part
by bending operation or control the forming temperature and/or
stretching speed). The most desirable means of improving the
workability and hydrogen embrittlement resisting property at
the same time while maintaining the content of residual
austenite before and after the processing substantially
constant within an appropriate range and maintaining other
properties (high strength, etc.) is to satisfy the following
requirements (A) and (B).
[0035]
(A) Increase C content in the composition and increase
the concentration of C in the residual austenite.
[0036]
Although residual austenite transforms into martensite
when the steel sheet is deformed (processed), high content of
C in the residual austenite stabilizes it so that further
transformation becomes unlikely to occur. Thus residual
austenite can be retained after the forming process, thereby
maintaining the high hydrogen embrittlement resisting property.
[0037]
According to the present invention, higher than 0.250 of
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C is contained in order to achieve the effects described above.
C is also an element required to achieve a high strength of
1180 MPa or higher, and 0.27% or more, preferably 0.30% or
more C is contained. However, in order to ensure corrosion
resistance, concentration of C is limited within 0.60,
preferably 0.55% or lower and more preferably 0.50 or lower
in the present invention.
[0038]
It is recommended to increase the C content in the steel
sheet as described above, thereby to maintain the
concentration of C in the residual austenite (CYR) of 0.8% or
higher. Controlling the value of CyR to 0.8% or higher enables
it to effectively improve the elongation property, which is
preferably 1.00 or higher and more preferably 1.2% or higher.
While it is preferable that CyR is as high as possible, it is
considered that in practice there is an upper limit of around
1.60.
[0039]
(B) Form the residual austenite in fine lath-shaped
grains.
[0040]
Residual austenite formed in fine lath-shaped grains
does not undergo excessive transformation during the forming
process, thus enabling it to maintain the residual austenite.
[0041]
Some of the TRIP steels of the prior art have
unsatisfactory hydrogen embrittlement resisting property
despite sufficient content of residual austenite. The reason
may be that, since residual austenite existing in the TRIP
steel of the prior art generally has block shape of size on
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micrometer order, it can easily transform into martensite when
being stressed and may act as the starting point of mechanical
destruction. Through a research conducted by the present
inventors, it was found that residual austenite formed in lath
shape is more stable and less likely to transform into
martensite than the residual austenite of the prior art that
has block shape, given the same amount of deformation. This
difference may be caused by the difference in the way in which
the stress is applied and in the difference in spatial
restriction, although not fully elucidated. Stabilization of
residual austenite during processing has no influence on the
lowering of workability of TRIP steel sheet due to induced
transformation. According to the present invention, induced
transformation proceeds efficiently and high workability can
be achieved without hardly reducing the residual austenite,
when the residual austenite is formed into fine lath shape as
described above.
[0042]
Lath-shaped grains of residual austenite having mean
axis ratio (major axis/minor axis) of 5 or higher (preferably
10 or higher, and more preferably 15 or higher) minimizes the
decrease of residual austenite during processing and makes it
possible to easily achieve mean axis ratio (major axis/minor
axis) of 5 or higher after processing, put the hydrogen
absorbing capability of the residual austenite into full play
and greatly improve hydrogen embrittlement resisting property.
While no upper limit of the mean axis ratio is specified for
the consideration of improvement in hydrogen embrittlement
resisting property, the residual austenite grains are required
to have certain level of thickness in order to achieve the
- 21 -
CA 02531616 2005-12-23
TRIP effect during processing. Thus it is preferable to set an
upper limit to 30, more preferably to 20 or less.
[0043]
According to a preferred embodiment of the present
invention, Mo and Nb are added for the purpose of reducing the
size of the residual austenite grains. Mo has the effects of
strengthening the grain boundary so as to suppress hydrogen
embrittlement from occurring, in addition to reducing the size
of the residual austenite grains. Mo also has the effect of
improving the hardenability of the steel sheet. It is
recommended to add 0.005°s or more of Mo in order to achieve
these effects. More preferably O.lo or more of Mo is added.
However, since the effects described above reach saturation
when the Mo content exceeds 1.0%, resulting in economical
disadvantage, Mo content is limited to 0.8% or less and more
preferably to 0.50 or less.
[0044]
Nb, in cooperation with Mo, acts very effectively to
decrease the grain size of the structure. Nb also has the
effect of increasing the strength of the steel sheet. It is
recommended to add 0.005% or more of Nb in order to achieve
these effects. More preferably 0.010 or more of Nb is added.
However, since the effects described above reach saturation
when an excessive Nb content is included, resulting in
economical disadvantage, Nb content is limited to O.lo or less
and more preferably to 0.085 or less.
[0045]
In order to readily obtain the structure described above
after processing, it is recommended to make the steel sheet
constituted from 80s or more (preferably 850 or more, and more
- 22 -
CA 02531616 2005-12-23
preferably 90% or more) in total of bainitic ferrite and
martensite, and 9% or less (preferably less than 5%, and more
preferably less than 3% containing Oo) in total of ferrite and
pearlite making up the rest of the residual austenite before
processing. This is because it is preferable that the steel
sheet has high hydrogen embrittlement resisting property prior
to the processing as well as after the processing, and this
constitution makes it easier to achieve the specified strength.
[0046]
While this embodiment is characterized in that metal
structure is controlled after processing, it is necessary to
control the other components as described below, in order to
form the metal structure and efficiently improve hydrogen
embrittlement resisting property and strength thereby to
ensure ductility required for the thin steel sheet.
[0047]
<Si: 1.0 to 3.0%>
Si is an important element that effectively suppresses
the residual austenite from decomposing and carbide from being
generated, and is also effective in enhancing substitution
solid solution for hardening the material. In order to make
full use of these effects, it is necessary to include Si in a
concentration of 1.0% or higher, preferably 1.2% or higher and
more preferably 1.5% or higher. However, excessively high
content of Si leads to conspicuous formation of scales due to
hot rolling and makes it necessary to remove flaws, thus
adding up to the manufacturing cost and resulting in
economical disadvantage. Therefore Si content is controlled
within 3.Oo, preferably within 2.5% and more preferably within
2.Oo.
- 23 -
CA 02531616 2005-12-23
[0048]
<Mn: 1.0 to 3.5%>
Mn is an element required to stabilize austenite and
obtain desired residual austenite. In order to make full use
of this effect, it is necessary to add Mn in concentration of
1.0% or higher, preferably 1.2% or higher, and more preferably
1.5% or higher. However, adding an excessive amount Mn leads
to conspicuous segregation and poor workability. Therefore
upper limit to the concentration of Mn is set to 3.5% and more
preferably to 3.0% or less.
[0049]
<P: 0.15% or lower (higher than 0%)>
P intensifies intergranular fracture due to
intergranular segregation, and the content thereof is
therefore preferably as low as possible. Upper limit to the
concentration of P is set to 0.15%, preferably 0.1% or less
and more preferably to 0.05% or less.
[0050]
<S: 0.02% or lower (higher than 0%)>
S intensifies the absorption of hydrogen into the steel
sheet in corrosive environment, and the content thereof is
therefore preferably as low as possible. Upper limit to the
concentration of S is set to 0.02%.
[0051]
<A1: 1.5% or less (higher than 0%)> (In the case of inventive
steel 1)
<A1: 0.5% or less (higher than 0%)> (In the case of inventive
steel 2)
O.Olo or higher content of A1 may be included for the
purpose of deoxidation. In addition to deoxidation, A1 also
- 24 -
CA 02531616 2005-12-23
has the effects of improving the corrosion resistance and
improving hydrogen embrittlement resisting property.
[0052]
The mechanism of improving the corrosion resistance is
supposedly based on the improvement of corrosion resistance of
the matrix phase per se and the effect of formation rust
generated by atmospheric corrosion, while the effect of the
formation rust presumably has greater contribution. This is
supposedly because the formation rust is denser and better in
protective capability than ordinary iron rust, and therefore
depresses the progress of atmospheric corrosion so as to
decrease the amount of hydrogen generated by the atmospheric
corrosion, thereby to effectively suppress the occurrence of
hydrogen embrittlement, and hence the delayed fracture.
[0053]
While details of the mechanism of improvement of the
hydrogen embrittlement resistance by Al is not known, it is
supposed that condensing of Al on the surface of the steel
makes it difficult for hydrogen to infiltrate into the steel,
and the decreasing diffusion rate of hydrogen in the steel
makes it difficult for hydrogen to migrate so that hydrogen
embrittlement becomes less likely to occur. In addition,
stability of lath-shaped residual austenite improved by the
addition of A1 is believed to contribute to the improvement of
hydrogen embrittlement resisting property.
[0054]
In order to effectively achieve the effects of A1 in
improving the corrosion resistance and improving the hydrogen
embrittlement resisting property, A1 content is controlled to
0.20 or higher, preferably 0.50 or higher.
- 25 -
CA 02531616 2005-12-23
[0055]
However, A1 content must be controlled within 1.5% in
order to prevent inclusions such as alumina from increasing in
number and size so as to ensure satisfactory workability,
ensure the generation of fine residual austenite grains,
suppress corrosion from proceeding from the inclusion
containing A1 as the starting point, and prevent the
manufacturing cost from increasing. In view of the
manufacturing process, it is preferable to control so that A3
point is not higher than 1000°C.
[0056]
As the Al content increases, inclusions such as alumina
increase and workability becomes poorer. In order to suppress
the generation of the inclusions such as alumina and make a
steel sheet having higher workability, A1 content is
restricted within 0.5o, preferably within 0.3o and more
preferably within O.lo.
[0057]
While constituent elements (C, Si, Mn, P, S, A1, Mo, Nb)
of the steel of this embodiment is as described above with the
rest substantially being Fe, it may include inevitable
impurities introduced into the steel depending on the stock
material, production material, manufacturing facility and
other circumstances, containing 0.0010 or less of N (nitrogen).
In addition, other elements as described below may be
intentionally added to such an extent that does not adversely
affect the effects of the present invention.
[0058]
<B: 0.0002 to O.Olo>
B is effective in increasing the strength of the steel
- 26 -
CA 02531616 2005-12-23
sheet, and it is preferable that 0.0002% or more (more
preferably 0.0005% or more) B is contained. However, an
excessive content of B leads to poor hot processing property.
Therefore, it is preferable to control the concentration of B
to within 0.01% (more preferably within 0.005%).
[0059]
<At least one selected from among Ca: 0.0005% to 0.005%, Mg:
0.0005% to 0.01% and REM: 0.0005% to 0.01%)
Ca, Mg and REM (rare earth element) are effective in
suppressing an increase in hydrogen ion concentration, that is,
a decrease in pH in the atmosphere of the interface due to
corrosion of the steel sheet surface, thereby to improve the
corrosion resistance of the steel sheet. These elements are
also effective in controlling the form of sulfide contained in
the steel and improve the workability of the steel. In order
to achieve the effects described above, it is recommended to
add each of Ca, Mg and REM in concentration of 0.0005% or
higher. However, since excessive contents of these elements
leads to poor workability, it is preferable to keep the
concentration of Ca to 0.005% or less and concentration of Mg
and REM each within 0.01%.
[0060]
(Second Embodiment)
The second high strength thin steel sheet according to
the present invention is constituted from higher than 0.25%
and up to 0.60% of C (contents of components given in terms of
percentage in this patent application all refer to percentage
by weight), 1.0 to 3.0% of Si, 1.0 to 3.5% of Mn, 0.15% or
less of P, 0.02% or less of S, 1.5% or less (higher than 0%)
of Al while iron and inevitable impurities constitute the rest,
- 27 -
CA 02531616 2005-12-23
wherein:(i) the structure after the forming process comprises:
1% or more residual austenite;
mean axis ratio (major axis/minor axis) of the residual
austenite grains is 5 or higher;
800 or more in total of bainitic ferrite and martensite; and
9% or less (may be Oo) in total of ferrite and pearlite in the
proportion of area to the entire structure, and
(ii) the steel contains specified amount of Cu and/or Ni.
[0061]
The requirements (i) have the reasons as described above.
[0062]
The requirement(ii) described above has the reason as
follows.
[0063]
Specific measures were studied to retain residual
austenite after processing, control the shape of the residual
austenite grains, improve the hydrogen trapping capability and
reliably reduce the concentration of diffusive hydrogen in the
steel sheet to a harmless level by:(a) sufficiently
suppressing the generation of hydrogen from the steel sheet in
corrosive environment; and (b) suppressing hydrogen that has
been generated from infiltrating the steel sheet.
[0064]
It was found that it is very effective to include 0.003
to 0.50 of Cu and/or 0.003 to l.Oo of Ni in achieving the
objectives of (a) and (b), and that the effect of improving
hydrogen embrittlement resisting property through control of
the structure can be achieved further by containing these
elements.
[0065]
- 28 -
CA 02531616 2005-12-23
Specifically, presence of Cu and Ni improves the
corrosion resistance of the steel, and effectively suppresses
the generation of hydrogen due to corrosion of the steel sheet.
These elements also have the effect of promoting the
generation of iron oxide, a-Fe00H, that is believed to be
particularly stable thermodynamically and have protective
property among various forms of rust generated in the
atmosphere. By assisting the generation of this rust, it is
made possible to suppress hydrogen that has been generated
from infiltrating into the spring steel thereby to
sufficiently improve the hydrogen embrittlement resisting
property to endure in harsh corrosive environment. This effect
can be achieved particularly satisfactorily when Cu and Ni are
contained at the same time.
[0066]
In order to achieve the effects described above,
concentration of Cu, if added, should be 0.0030 or higher,
preferably 0.050 or higher and more preferably O.lo or higher.
Concentration of Ni, if added, should be 0.0030 or higher,
preferably 0.050 or higher and more preferably O.lo or higher.
[0067]
Since excessively high concentration of either Cu or Ni
is detrimental to workability, it is preferable to limit the
Cu content to 0.5% or lower and limit the Ni content to 1.0%
or lower.
[0068]
In order to achieve high hydrogen embrittlement
resisting property after the forming process by retaining the
predetermined amount of residual austenite after the forming
process as in (i) described above, for example, 5% or more
- 29 -
CA 02531616 2005-12-23
residual austenite may be contained in the steel sheet prior
to the forming process, or large amount of fine residual
austenite grains may be dispersed in the structure.
Alternatively, forming process conditions may be controlled so
as to make the residual austenite less likely to transform
(for example, form the part by bending operation or control
the forming temperature and/or stretching speed). The most
desirable means of improving the workability and hydrogen
embrittlement resisting property at the same time while
maintaining the content of residual austenite before and after
the processing substantially constant within an appropriate
range and maintaining other properties (high strength, etc.)
is to satisfy the requirements (A) and (B) described
previously.
[0069]
While this embodiment is characterized in that metal
structure is controlled after processing and predetermined
amount of Cu and/or Ni are added, it is necessary to control
the other components as described below, in order to readily
form the metal structure and efficiently improve hydrogen
embrittlement resisting property and strength thereby to
ensure ductility required for the thin steel sheet.
[0070]
While constituent elements (C, Si, Mn, P, S, Al, Cu
and/or Ni) of the steel of this embodiment are as described
above with the rest substantially being Fe, it may include
inevitable impurities introduced into the steel depending on
the stock material, production material, manufacturing
facility and other circumstances, containing O.OOlo or less of
N (nitrogen). In addition, other elements as described below
- 30 -
CA 02531616 2005-12-23
may be intentionally added to such an extent that does not
adversely affect the effects of the present invention.
[0071]
<Ti and/or V: 0.003 to l.Oo in total>
Ti has the effect of assisting in the generation of
protective rust, similarly to Cu and Ni. The protective rust
has a very valuable effect of suppressing the generation of [i-
Fe00H that appears in chloride environment and has adverse
effect on the corrosion resistance (and hence on the hydrogen
embrittlement resisting property). Formation of such a
protective rust is promoted particularly by adding Ti and V
(or Zr). Ti renders the steel high corrosion resistance, and
also has the effect of cleaning the steel.
[0072]
V is effective in increasing the strength of the steel
sheet and decreasing the size of crystal grains, in addition
to having the effect of improving hydrogen embrittlement
resistance through cooperation with Ti, as described
previously.
[0073]
In order to fully achieve the effect of Ti and/or V
described above, it is preferable to add Ti and/or V in total
concentration of 0.003% or higher (more preferably 0.01% or
higher). For the purpose of improving hydrogen embrittlement
resisting property, in particular, it is preferable to add
more than 0.030 of Ti, more preferably 0.050 or more of Ti.
However, the effects described above reach saturation when an
excessive amount of Ti is added, resulting in economical
disadvantage. Excessive V content also increases the
precipitation of much carbonitride and leads to poor
- 31 -
CA 02531616 2005-12-23
workability and lower hydrogen embrittlement resisting
property. Therefore, it is preferable to control the total
concentration of Ti and/or V to within 1.0%, more preferably
within 0.5a.
[0074]
<Zr: 0.003 to 1.0%>
Zr is effective in increasing the strength of the steel
sheet and decreasing the crystal grain size, and also has the
effect of improving hydrogen embrittlement resisting property
through cooperation with Ti. In order to sufficiently achieve
these effects, it is preferable that 0.003% or more of Zr is
contained. However, excessive Zr content increases the
precipitation of carbonitride and leads to poor workability
and lower hydrogen embrittlement resisting property. Therefore,
it is preferable to control the concentration of Zr to within
1.0%.
[0075]
<Mo: 1.0% or less (higher than 0%)>
Mo has the effects of stabilizing austenite so as to
retain the residual austenite, and suppress the infiltration
of hydrogen thereby to improve hydrogen embrittlement
resisting property. Mo also has the effect of improving the
hardenability of the steel sheet. In addition, Mo strengthens
the grain boundary so as to suppress hydrogen embrittlement
from occurring. It is recommended to add 0.005% or more Mo in
order to achieve these effects. More preferably 0.1% or more
Mo is added. However, since the effects described above reach
saturation when the Mo content exceeds 1.0%, resulting in
economical disadvantage, Mo content is limited to 0.8% or less
and more preferably to 0.5% or less.
- 32 -
CA 02531616 2005-12-23
[0076]
<Nb: 0.1% or less (higher than 0%)>
Nb is very effective in increasing the strength of the
steel sheet and decreasing the grain size of the structure. Nb
achieves these effects particularly effectively in cooperation
with Mo. In order to achieve these effects, it is recommended
to include 0.005% or more of Nb. More preferably 0.01% or more
of Nb is added. However, since the effects described above
reach saturation when an excessive Nb content is included,
resulting in economical disadvantage, Nb content is limited to
0.1% or less and more preferably to 0.08% or less.
[0077]
<B: 0.0002 to 0.01%>
B is effective in increasing the strength of the steel
sheet, and it is preferable that 0.0002% or more (more
preferably 0.0005% or more) B is contained in order to achieve
these effects. However, an excessive content of B leads to
poor hot processing property. Therefore, it is preferable to
control the concentration of B within 0.01% (more preferably
within 0.005%).
[0078]
<At least one kind selected from among a group consisting of
Ca: 0.0005% to 0.005%, Mg: 0.0005% to 0.01% and REM: 0.0005%
to 0.01%)
Ca, Mg and REM (rare earth element) are effective in
suppressing an increase in hydrogen ion concentration, that is,
a decrease in pH in the atmosphere of the interface due to
corrosion of the steel sheet surface, thereby to improve the
corrosion resistance of the steel sheet. It is also effective
in controlling the form of sulfide in the steel and improving
- 33 -
CA 02531616 2005-12-23
the workability of the steel. In order to achieve the effects
described above, it is recommended to add each of Ca, Mg and
REM in concentration of 0.00050 or higher. However, since
excessive contents of these elements leads to poor workability,
it is preferable to keep the concentrations of Ca within
0.0050, Mg and REM each within 0.01%.
[0079]
(Third Embodiment)
A third high strength thin steel sheet according to the
present invention is constituted from higher than 0.25 and up
to 0.600 of C (contents of components given in terms of
percentage in this patent application all refer to percentage
by weight), 1.0 to 3.0% of Si, 1.0 to 3.50 of Mn; 0.15% or
less of P, 0.020 or less of S, 1.5% or less (higher than Oo)
of Al, while iron and inevitable impurities making up the rest,
wherein:(iii) the structure satisfies the following
requirements after forming:
to or more residual austenite;
the mean axis ratio (major axis/minor axis) of the residual
austenite grains is 5 or higher;
mean length of minor axes of the residual austenite grains is
1 um or less; and
minimum distance between residual austenite grains is 1 um or
less.
TiVhen the metal structure is controlled as described
above, hydrogen embrittlement resisting property of the high
strength thin steel sheet can be sufficiently improved without
adding much alloy elements.
The phrase "after the forming process" means the state
of the steel sheet after being stretch formed with elongation
- 34 -
CA 02531616 2005-12-23
of 3%. Specifically, the steel sheet is subjected to
elongation of 3% by uniaxial stretching at the room
temperature (the stretch forming process of 3% elongation may
hereinafter be referred to simply as ~~process").
[0080]
The requirements for the residual austenite of the
present invention will now be described in detail below.
[0081]
<1% or more residual austenite>
<Mean axis ratio (major axis/minor axis) of the residual
austenite grains is 5 or higher >
It is necessary that the metal structure contains 1% or
more residual austenite in terms of area proportion to the
entire structure after processing, in order to achieve high
hydrogen embrittlement resisting property in harsh operating
environment over an extended period of time after forming the
part. Residual austenite contributes not only to the
improvement of hydrogen embrittlement resisting property as
described above, but also to the improvement of total
elongation as has been known in the prior art. Content of the
residual austenite is preferably 20 or higher, and more
preferably 3% or higher. Since the desired level of high
strength cannot be obtained when an excessive amount of
residual austenite is contained, it is recommended to set an
upper limit of 15% (more preferably loo) to the residual
austenite content.
[0082]
Lath-shaped grains of residual austenite after
processing have far higher capacity of trapping hydrogen than
carbide. Fig. 1 is a graph showing the relationship between
- 35 -
CA 02531616 2005-12-23
the mean axis ratio of the residual austenite grains measured
by a method to be described later and hydrogen embrittlement
risk index (measured by a method to be described later in an
example, lower value of this index means better hydrogen
embrittlement resisting property). From Fig. 1, it can be seen
that hydrogen embrittlement risk index sharply decreases when
the mean axis ratio (major axis/minor axis) of the residual
austenite grains increases beyond 5. This is supposedly
because, when the mean axis ratio of the residual austenite
grains becomes 5 or higher, intrinsic capability of the
residual austenite to absorb hydrogen is put into full play,
so that the residual austenite attains far higher capacity of
trapping hydrogen than carbide and substantially neutralizes
the hydrogen that infiltrates from the outside through
atmospheric corrosion thereby to achieve remarkable
achievement in hydrogen embrittlement resisting property. The
mean axis ratio of the residual austenite grains is preferably
10 or higher, and more preferably 15 or higher.
[0083]
<Mean length of minor axes of the residual austenite grains is
1 um or less>
According to the present invention, it has been found
that hydrogen embrittlement resisting property can be
effectively improved by dispersing fine grains of residual
austenite of lath shape. Specifically, hydrogen embrittlement
resisting property can be surely improved by dispersing the
lath-shape grains of residual austenite having sizes of 1 um
or less (submicrometer order). This is supposedly because
surface area of the residual austenite grains (interface)
increases resulting in larger hydrogen trapping capability,
- 36 -
CA 02531616 2005-12-23
when larger number of fine lath-shape grains of residual
austenite having smaller mean length of minor axis are
dispersed. Mean length of minor axes of the residual austenite
grains is preferably 0.5 um or less, more preferably 0.25 um
or less.
[0084]
According to the present invention, hydrogen trapping
capability of the fine lath-shape grains of residual austenite
can be made far greater than that in the case of dispersing
carbide, and thereby to substantially neutralize hydrogen that
infiltrates from the outside through atmospheric corrosion,
even when the same proportion by volume of residual austenite
is contained, by controlling the mean axis ratio and mean
length of minor axes of the residual austenite grains as
described above.
[0085]
<Minimum distance between residual austenite grains is 1 um or
less>
According to the present invention, it has been found
that hydrogen embrittlement resisting property can be improved
further by controlling the minimum distance between adjacent
residual austenite grains, in addition to the above.
Specifically, hydrogen embrittlement resistance can be surely
improved when the minimum distance between residual austenite
grains is 1 um or less. This is supposedly because propagation
of cracks is suppressed so that the structure demonstrates
higher resistance against fracture, when a large number of
fine lath-shape grains of residual austenite are dispersed in
proximity to each other. Minimum distance between adjacent
residual austenite grains is preferably 0.8 um or less, and
- 37 -
CA 02531616 2005-12-23
more preferably 0.5 um or less.
[0086]
The residual austenite refers to a region that is
observed as FCC (face centered cubic lattice) by the FE-
SEM/EBSP method which will be described later. Measurement by
the EBSP may be done, for example, by measuring a measurement
area (about 50 by 50 um) at an arbitrarily chosen position in
a surface parallel to the rolled surface at a position of one
quarter of the thickness at measuring intervals of 0.1 um. The
measuring surface is prepared by electrolytic polishing in
order to prevent the residual austenite from transforming.
Then the test piece is set in the lens barrel of an FE-SEM
equipped with the EBSP detector (of which details will be
described later) and is irradiated with electron beam. An EBSP
image projected onto a screen is captured by a high
sensitivity camera (VE-1000-SIT manufactured by Dage-MTI Inc.)
and is sent to a computer. The computer carries out image
analysis and generates color mapping of the FCC phase through
comparison with a structural pattern simulated with a known
crystal system (FCC (face centered cubic lattice) phase in the
case of residual austenite). Area proportion of the region
that is mapped as described above is taken as the area
proportion of the residual austenite. This analysis was
carried out by means of hardware and software of OIM
(Orientation Imaging MicroscopyTM) system of TexSEM
Laboratories Inc.
[0087]
The mean axis ratio, mean length of minor axes and
minimum distance between residual austenite grains were
determined as follows. The mean axis ratio of the residual
- 38 -
CA 02531616 2005-12-23
austenite grains was determined by measuring the major axis
and minor axis of residual austenite crystal grain existing in
each of three arbitrarily chosen fields of view in the
observation by means of TEM (transmission electron microscope)
with magnification factor of 15000, and averaging the ratios
of major axis to minor axis. The mean length of minor axes of
the residual austenite grains was determined by averaging the
lengths of minor axes measured as described above. The minimum
distance between adjacent residual austenite grains was
determined by measuring the distance between adjacent residual
austenite grains that were aligned in the direction of major
axis as shown as (a) in Fig. 11 (distance (b) in Fig. 11 is
not regarded as the minimum distance between the grains) by
observing with TEM (magnification factor of 15000) in each of
three arbitrarily chosen fields of view and averaging the
distances measured in the three fields of view.
[0088]
In order to decrease the number of intergranular
fracture initiating points in the steel thereby to surely
decrease the concentration of diffusive hydrogen to a harmless
level and achieve a high strength, it is desirable to form the
matrix phase of the steel sheet after processing from a binary
phase structure of bainitic ferrite and martensite with the
bainitic ferrite acting as the main phase, instead of the
single phase structure of martensite that is generally used
for high strength steels.
[0089]
In the case of single phase structure of martensite, a
carbide (for example, film-like cementite) is likely to
precipitate in the grain boundaries, thus making intergranular
- 39 -
CA 02531616 2005-12-23
fracture likely to occur. In the case of the binary phase
structure of bainitic ferrite and martensite with the bainitic
ferrite acting as the main phase, in contrast, the bainitic
ferrite is a hard phase and therefore it is easy to increase
the strength of the entire structure as in the case of the
single phase of martensite. The hydrogen embrittlement
resisting property can also be improved as much hydrogen is
trapped in the dislocations. It also has such an advantage
that coexistence of the bainitic ferrite and residual
austenite which will be described later prevents the
generation of carbide that acts as the intergranular fracture
initiating points, and it becomes easier to create the lath-
shaped residual austenite in the boundaries of lath-shaped
bainitic ferrite.
[0090]
Accordingly, it is required in the present invention
that the binary phase structure of bainitic ferrite and
martensite occupy 800 or more, preferably 850 or more and more
preferably 90% or more of the entire structure after the
stretch forming processing to elongate by 30. Upper limit of
the proportion may be determined by the balance with other
structure (residual austenite), and is set to 99o when other
structure (ferrite, etc.) than the residual austenite is not
contained.
[0091]
The bainitic ferrite referred to in the present
invention is plate-shaped ferrite having a lower structure of
high density of dislocations. It is clearly distinguished from
polygonal ferrite that has lower structure including no or
very low density of dislocations, by SEM observation as
- 40 -
CA 02531616 2005-12-23
fO110WS.
[0092]
Area proportion of bainitic ferrite structure is
determined as follows. A test piece is etched with Nital
etchant. A measurement area (about 50 by 50 um) at an
arbitrarily chosen position in a surface parallel to the
rolled surface at a position of one quarter of the thickness
is observed with SEM (scanning electron microscope)
(magnification factor of 1500) thereby to determine the area
proportion.
[0093]
Bainitic ferrite is shown with dark gray color in SEM
photograph (bainitic ferrite, residual austenite and
martensite may not be distinguishable in the case of SEM
observation), while polygonal ferrite is shown black in SEM
photograph and has polygonal shape that does not contain
residual austenite and martensite inside thereof.
[0094]
The SEM used in the present invention is a high-
resolution FE-SEM (Field Emission type Scanning Electron
Microscope XL30S-FEG manufactured by Philips Inc.) equipped
with an EBSP (Electron Back Scatter diffraction Pattern)
detector, that has a merit of being capable of analyzing the
area observed by the SEM at the same time by means of the EBSP
detector. EBSP detection is carried out as follows. When the
sample surface is irradiated with electron beam, the EBSP
detector analyzes the Kikuchi pattern obtained from the
reflected electrons, thereby to determine the crystal
orientation at the point where the electron beam has hit upon.
Distribution of orientations over the sample surface can be
- 41 -
CA 02531616 2005-12-23
measured by scanning the electron beam two-dimensionally over
the sample surface while measuring the crystal orientation at
predetermined intervals. The EBSP detection method has such an
advantage that different structures that are regarded as the
same structure in the ordinary microscopic observation but
have different crystal orientations can be distinguished by
the difference in color tone.
[0095]
The metal structure after the processing may be
constituted either from only the structures described above
(namely, a mixed structure of bainitic ferrite + martensite
and residual austenite), or may include other structure such
as ferrite (the term ferrite used herein refers to polygonal
ferrite, that is a ferrite structure that includes no or very
few dislocations) or pearlite to such an extent that the
effect of the present invention is not compromised. Such
additional components are structures that can inevitably
remain in the manufacturing process of the present invention,
of which concentration is preferably as low as possible,
within 9%, preferably less than 5o and more preferably less
than 3o according to the present invention.
[0096]
In order to maintain high hydrogen embrittlement
resisting property after the forming process, for example,
high proportion of residual austenite, 50 or more, may be
contained in the steel sheet prior to the forming process, or
a large amount of fine residual austenite grains may be
dispersed in the structure. Alternatively, forming process
conditions may be controlled so as to make the residual
austenite less likely to transform (for example, form the part
- 42 -
CA 02531616 2005-12-23
by bending operation or control the forming temperature and/or
stretching speed). The most desirable means of improving the
workability and hydrogen embrittlement resisting property at
the same time while maintaining the content of residual
austenite before and after the processing substantially
constant within an appropriate range and maintaining other
properties (high strength, etc.) is to satisfy the
requirements (A) and (B) described previously.
[0097]
While this embodiment is characterized in that the metal
structure is controlled after processing, it is necessary to
control the other components as described previously, in order
to form the metal structure and efficiently improve hydrogen
embrittlement resisting property and strength thereby to
ensure the level of ductility required for the thin steel
sheet.
[0098]
While the present invention does not specify the
manufacturing conditions, it is recommended to apply heat
treatment in the following procedure after hot rolling or cold
rolling conducted thereafter, in order to form the structure
described above that can be easily worked and has high
strength and high hydrogen embrittlement resistance after the
processing, by using the steel material of the composition
described above. The recommended procedure is to keep the
steel the composition described above at a temperature (T1) in
a range from A3 point to (A3 point + 50°C) for a period of 10
to 1800 seconds (tl), cool down the steel at a mean cooling
rate of 3°C/s or higher to a temperature (T2) in a range from
Ms point to Bs point and keep the material at this temperature
- 43 -
CA 02531616 2005-12-23
for a period of 60 to 3600 seconds (t2).
[0099]
It is not desirable that the temperature T1 becomes
higher than (A3 point + 50°C) or the period tl is longer than
1800 seconds, in which case austenite grains grow resulting in
poor workability (elongation flanging property). When the
temperature T1 is lower than A3 point, on the other hand,
desirable bainitic ferrite structure cannot be obtained. When
the period tl is shorter than 10 seconds, austenitization does
not proceed sufficiently and therefore cementite and other
alloy carbides remain. The period tl is preferably in a range
from 30 to 600 seconds, more preferably from 60 to 400 seconds.
[0100]
Then the steel sheet is cooled down. The steel is cooled
at a mean cooling rate of 3°C/s or higher, for the purpose of
preventing pearlite structure from being generated while
avoiding the pearlite transformation region. The mean cooling
rate should be as high as possible, and is preferably 5°C/s or
higher, and more preferably 10°C/s or higher.
[0101]
After quenching to the temperature between Ms point and
Bs point at the rate described above, the steel is subjected
to isothermal transformation so as to transform the matrix
phase into binary phase structure of bainitic ferrite and
martensite. When the heat retaining temperature T2 is higher
than Bs, much pearlite that is not desirable for the present
invention is formed, thus hampering the formation of the
predetermined bainitic ferrite structure. When T2 is below Ms,
on the other hand, the amount of residual austenite decreases.
[0102]
- 44 -
CA 02531616 2005-12-23
When the temperature holding period t2 is longer than
1800 seconds, density of dislocations in bainitic ferrite
becomes low, the amount of trapped hydrogen decreases and the
desired residual austenite cannot be obtained. When t2 is less
than 60 second, on the other hand, desired bainitic ferrite
structure cannot be obtained. The length of t2 is preferably
from 90 to 1200 seconds, and more preferably from 120 to 600
seconds. There is no restriction on the method of cooling
after maintaining the heating temperature, and air cooling,
quenching or air-assisted water cooling may be employed.
[0103]
In the practical manufacturing process, the annealing
process described above can be carried out easily by employing
a continuous annealing facility or a batch annealing facility.
In case that a cold rolled sheet is plated with zinc by hot
dipping, the heat treatment process may be replaced by the
plating process by setting the plating conditions so as to
satisfy the heat treatment conditions. The plating may also be
alloyed.
[0104]
There is no restriction on the hot rolling process (or
cold rolling process as required) that precedes the continuous
annealing process described above, and commonly employed
process conditions may be used. Specifically, the hot rolling
process may be carried out in such a procedure as, after hot
rolling at a temperature above Ar3 point, the steel sheet is
cooled at a mean cooling rate of about 30°C/s and is wound up
at a temperature from about 500 to 600°C. In case that the hot
rolled steel sheet has unsatisfactory appearance, cold rolling
may be applied in order to rectify the appearance. It is
- 45 -
CA 02531616 2005-12-23
recommended to set the cold rolling ratio in a range from 1 to
700. Cold rolling beyond 700 leads to excessive rolling load
that makes it difficult to carry out the cold rolling.
[0105]
While the present invention is addressed to thin steel
sheet, there is no limitation to the form of product, and may
be applied, in addition to steel sheet made by hot rolling or
steel sheet made by cold rolling, to those subjected to
annealing after hot rolling or cold rolling, followed by
chemical conversion treatment, hot-dip coating, electroplating,
vapor deposition, painting, priming for painting, organic
coating treatment or the like.
[0106]
The plating process may be either galvanizing or
aluminum plating. The method of plating may be either hot-dip
coating or electroplating, and the plating process may also be
followed by alloying heat treatment or multi-layer plating. A
steel sheet, that is plated or not plated, may also be
laminated with a film.
[0107]
When the coating operation described above is carried
out, chemical conversion treatment such as phosphating or
electrodepositing coating may be applied in accordance to the
application. The coating material may be a known resin that
can be used in combination with a known hardening agent such
as epoxy resin, fluorocarbon resin, silicone acrylic resin,
polyurethane resin, acrylic resin, polyester resin, phenol
resin, alkyd resin, or melamine resin. Among these, epoxy
resin, fluorocarbon resin or silicone acrylic resin is
preferably used in consideration of corrosion resistance.
- 46 -
CA 02531616 2005-12-23
Known additives that are added to coating materials such as
coloring agent, coupling agent, leveling agent, sensitization
agent, antioxidant agent, anti-UV protection agent, flame
retarding agent or the like may be used.
[0108]
There is also no restriction on the coating and solvent-
based coating, powder coating, water-based coating, water-
dispersed coating, electrodeposition coating or like may be
employed. Desired coating layer of the coating material
described above can be formed on the steel by a known
technique such as dipping, roll coater, spraying, or curtain
flow coater. The coating layer may have any proper thickness.
[0109]
The high strength thin steel sheet of the present
invention may be applied to high-strength automotive
components such as bumper, door impact beam, pillar and other
reinforcement members and interior parts such as seat rail,
etc. Automobile components that are manufactured by forming
process also have sufficient properties (strength) and high
hydrogen embrittlement resisting property.
[0110]
The present invention will now be described below by way
of examples, but the present invention is not limited to the
examples. Various modifications may be conceived without
departing from the technical scope of the present invention.
jExample 1]
[0111]
Sample steels A-1 through Y-1 having the compositions
described in Table 1 were melt-refined in vacuum to make test
slabs. The slabs were processed in the following procedure
- 47 -
CA 02531616 2005-12-23
(hot rolling --~ cold rolling -continuous annealing) thereby to
obtain hot-rolled steel plates measuring 3.2 mm in thickness.
The steel plates were pickled to remove scales from the
surface and then cold rolled so as to reduce the thickness to
1.2 mm.
[0112]
<Hot rolling> Starting temperature (SRT): Held at a
temperature between 1150 and 1250°C for 30 minutes.
Finishing temperature (FDT): 850°C
Cooling rate: 40°C/s
Winding-up temperature: 550°C
<Cold rolling> Rolling ratio: 500
<Continuous annealing> Each steel specimen was kept at a
temperature of A3 point + 30°C for 120 seconds, then cooled in
air at a mean cooling rate of 20°C/s to temperature TO shown
in Table 2, and was kept at TO for 240 seconds, followed by
air-assisted water cooling to the room temperature.
[0113]
No. 116 shown in Table 2 was made by heating a cold-
rolled steel sheet to 830°C, keeping at this temperature for 5
minutes followed by quenching in water and tempering at 300°C
for 10 minutes, thereby to form a martensite steel as a
comparative example of the high-strength steel of the prior
art. No. 120 was made by heating a cold-rolled steel sheet to
800°C, keeping at this temperature for 120 seconds, cooling
down at a mean cooling rate of 20°C/s to 350°C and keeping at
this temperature for 240 seconds.
[0114]
JIS No. 5 test pieces were prepared from the steel
sheets obtained as described above, and were subjected to
- 48 -
CA 02531616 2005-12-23
stretch forming process with elongation of 3o mimicking the
actual manufacturing process. Metal structures of the test
pieces were observed before and after the processing, tensile
strength (TS) and elongation (total elongation E1) before the
processing and hydrogen embrittlement resisting property after
the processing were measured by the following procedures.
[0115]
Observation of metal structure
Metal structures of the test pieces were observed before
and after the processing as follows. A measurement area (about
50 by 50 um) at an arbitrarily chosen position in a surface
parallel to the rolled surface at a position of one quarter of
the thickness was photographed at measuring intervals of 0.1
um, and area proportions of bainitic ferrite (BF), martensite
(M) and residual austenite (residual Y) were measured by the
method described previously. Then similar measurements were
made in two fields of view that were arbitrarily selected, and
the measured values were averaged. Area proportions of other
structures (ferrite, pearlite, etc.) were subtracted from the
entire structure.
[0116]
Mean axis ratio of the residual austenite grains of the
steel sheet before and after the processing were measured by
the method described previously. Test pieces having mean axis
ratio of 5 or higher were regarded to satisfy the requirements
of the present invention (o), and those having mean axis ratio
of lower than 5 were regarded to fail to satisfy the
requirements of the present invention (x).
[oll~]
Measurement of tensile strength (TS) and elongation (E1)
- 49 -
CA 02531616 2005-12-23
Tensile test was conducted on the JIS No. 5 test piece
before processing, so ws to measure the tensile strength (TS)
and elongation (E1). Stretching speed of the tensile test was
set to 1 mm/sec. Among the steel sheets having tensile
strength of 1180 MPa as measured by the method described
previously, those which showed elongation of 100 or more were
evaluated as high in elongation property.
[0118]
Evaluation of hydrogen embrittlement resisting property
In order to evaluate hydrogen embrittlement resisting
property, the JIS No. 5 test piece was stretched so as to
elongate by 30. Then after bending with a radius of curvature
of 15 mm, load of 1000 MPa was applied and the test piece was
immersed in 5o solution of hydrochloric acid, and the time
before crack occurred was measured.
[0119]
Hydrogen-charged 4-point bending test was also conducted
for some steel species. Specifically, a rectangular test piece
measuring 65 mm by 10 mm made of each steel sheet elongated by
3% was immersed in a solution of 0.5 mol of HZS04 and 0.01 mol
of KSCN and was subjected to cathode hydrogen charging.
Maximum stress endured without breaking for 3 hours was
determined as the critical fracture stress (DFL).
[0120]
Results of these tests are shown in Table 2.
- 50 -
CA 02531616 2005-12-23
_ alO M rla1M00~'l~l~d''V'C' lfl61O l~'-1l0Ml0M M I~M
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MM N N M NN N N N NN M ~'MM ~-iN M MM M M M
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- 51 -
CA 02531616 2005-12-23
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- 52 -
CA 02531616 2005-12-23
[0123]
The results shown in Tables 1 and 2 can be interpreted
as follows (numbers in the following description are test Nos.
in Table 2).
[0124]
Test pieces Nos. 101 through 113 (inventive steel sheets
2) and test pieces Nos. 121 through 125 (inventive steel
sheets 1) that satisfy the requirements of the present
invention have high strength of 1180 MPa or higher, and high
hydrogen embrittlement resisting property in harsh environment
after the forming process. They also have high elongation
property required of the TRIP steel sheet, thus providing
steel sheets best suited for reinforcement parts of
automobiles that are exposed to corrosive atmosphere. Test
pieces Nos. 121 through 125, in particular, show even better
hydrogen embrittlement resisting property.
[0125]
Test pieces Nos. 114 through 120 and 126 that do not
satisfy the requirements of the present invention, in contrast,
have the following drawbacks.
[0126]
No. 114 made of steel species N-1 that includes
insufficient C content does not have good workability.
[0127]
No. 115 made of steel species 0-1 that includes
insufficient Mn content does not retain sufficient residual
austenite and is inferior in hydrogen embrittlement resisting
property after the processing.
[0128]
No. 116, martensite steel that is a conventional high
- 53 -
CA 02531616 2005-12-23
strength steel made of steel species P-1 that includes
insufficient Si content, hardly contains residual austenite
and is inferior in hydrogen embrittlement resisting property.
It also does not show the elongation property required of a
thin steel sheet.
[0129]
No. 117 made of steel species Q-1 that includes
excessive C content has precipitation of carbide and is
inferior in both forming workability and hydrogen
embrittlement resisting property after processing.
[0130]
No. 118 made of steel species R-1 that includes
excessive Mo content and No. 119 made of steel species S-1
that includes excessive Nb content are inferior in forming
workability. Nos. 118 and 119 could not undergo the processing,
making it impossible to investigate the property after the
processing.
[0131]
No. 120, that was made of a steel that has the
composition specified in the present invention but was not
manufactured under the recommended conditions, resulted in the
conventional TRIP steel. As a result, the residual austenite
does not have the mean axis ratio specified in the present
invention, while the matrix phase is not formed in binary
phase structure of bainitic ferrite and martensite, and
therefore sufficient level of hydrogen embrittlement resisting
property is not achieved.
[0132]
No. 126 includes A1 content higher than that specified
for the inventive steel sheet 1. As a result, although the
- 54 -
CA 02531616 2005-12-23
predetermined amount of residual austenite is retained, the
residual austenite does not have the mean axis ratio specified
in the present invention, the desired matrix phase is not
obtained and inclusions such as AlN are generated thus
resulting in poor hydrogen embrittlement resisting property.
[0133]
Then parts were made by using steel species A-1, J-1
shown in Table 1 and comparative steel sheet (590 MPa class
high strength steel sheet of the prior art). Performance
(pressure collapse resistance and impact resistance) of the
formed test piece were studied by conducting pressure collapse
test and impact resistance test as follows.
[0134]
Pressure collapse test
The part 1 (hat channel as test piece) shown in Fig. 1
was made by using steel species A-1, J-1 shown in Table 1 and
the comparative steel sheet, and was subjected to pressure
collapse test. The part was spot welded at the positions 2 of
the part shown in Fig. 1 at 35 mm intervals as shown in Fig. 1
by supplying electric current of a magnitude less than the
expulsion generating current by 0.5 kA from an electrode
measuring 6 mm in diameter at the distal end. Then a die 3 was
pressed against the part 1 from above the mid portion thereof
in the longitudinal direction as shown in Fig. 2, and the
maximum tolerable load was determined. Absorbed energy was
determined from the area under the load-deformation curve. The
results are shown in Table 3.
- 55 -
CA 02531616 2005-12-23
[0135]
Table 3
Steel sheet used Evaluation
of test
piece
Residual Maximum Energy
Steel species TS E~, load absorbed
...............................................................................
...............................................................................
...............................................................................
.............................................................................
(MPa)(s) (Area ~) (kN) (kJ)
Symbol A-1 1510 15 9 14.1 0.72
Symbol J-1 1491 12 11 14 0.68
Comparative steel
613 22 0 5.7 0.33
sheet
[0136]
From Table 3, it can be seen that the part (test piece)
made from the steel sheet of the present invention has higher
load bearing capability and absorbs greater energy than a part
made of the conventional steel sheet having lower strength,
thus showing high pressure collapse resistance.
[0137]
Impact resistance test
The parts 4 (hat channel as test piece) shown in Fig. 3
were made by using steel species A-1, J-1 shown in Table 1 and
the comparative steel sheet, and were subjected to impact
resistance test. Fig. 4 is a sectional view along A-A of the
part 4 shown in Fig. 3. In the impact resistance test, after
the part was spot welded at the positions 5 of the part 4
similarly to the pressure collapse test, the part 4 was placed
on a base 7 as schematically shown in Fig. 5. A weight 6
(weighing lOkg) was dropped onto the part 4 from a height of
11 meters, and the energy absorbed before the part 4 underwent
deformation of 40 mm in the direction of height. The results
are shown in Table 4.
- 56 -
CA 02531616 2005-12-23
[0138]
Table 4
Steel sheet Evaluation of test piece
used
Steel species TS EL Residual Energy absorbed
...............................................................................
....Y
...............................................................................
...............................................................................
.................
(MPa) (g)
...............................................................................
....(kJ)
(Area $)
Symbol A-1 1510 15 9 6.94
Symbol J-1 1491 12 11 6.65
Comparative 613 22 0 3.56
steel sheet
[0139]
From Table 4, it can be seen that the part (test piece)
made from the steel sheet of the present invention absorbs
greater energy than a part made of the conventional steel
sheet that has lower strength, thus showing higher impact
resistance.
[0140]
TEM photograph of the test piece made in this example is
shown as reference. Fig. 6 is a photograph of TEM observation
of No. 101 of the present invention. From Fig. 6, it can be
seen that the high strength thin steel sheet of the present
invention contains lath-shaped residual austenite (black
portion of bar shape in Fig. 6) specified in the present
invention dispersed therein. Fig. 7 is a photograph of TEM
observation of No. 120 of a comparative example. From Fig. 7,
it can be seen that the high strength thin steel sheet of No.
120 contains residual austenite (black portion of somewhat
round shape in Fig. 7), although the residual austenite has a
block shape that does not satisfy the requirements of the
present invention.
[Example 2]
- 57 -
CA 02531616 2005-12-23
[0141]
Sample steels A-2 through Y-2 having the compositions
described in Table 5 were melt-refined in vacuum to make test
slabs. The slabs were processed in the following procedure
(hot rolling-cold rolling-continuous annealing) thereby to
obtain hot-rolled steel plates measuring 3.2 mm in thickness.
The steel plates were pickled to remove scales from the
surface and then cold rolled so as to reduce the thickness to
1.2 mm.
[0142]
<Hot rolling>
Starting temperature (SRT): Held at a temperature between 1150
and 1250°C for 30 minutes.
Finishing temperature (FDT): 850°C
Cooling rate: 40°C/s
Winding-up temperature: 550°C
<Cold rolling>
Rolling ratio: 50%
<Continuous annealing>
Each steel specimen was kept at a temperature of A3
point + 30°C for 120 seconds, then rapidly cooled (air
cooling) at a mean cooling rate of 20°C/s to temperature TO
shown in Table 6, and was kept at TO for 240 seconds, followed
by air-assisted water cooling to the room temperature.
[0143]
No. 217 in Table 6 was made by heating a cold-rolled
steel sheet to 830°C, keeping at this temperature for 5
minutes followed by quenching in water and tempering at 300°C
for 10 minutes, thereby to form a martensite steel as a
comparative example of the high-strength steel of the prior
- 58 -
CA 02531616 2005-12-23
art. No. 220 was made by heating a cold-rolled steel sheet to
800°C, keeping at this temperature for 120 seconds, cooling
down at a mean cooling rate of 20°C/s to 350°C and keeping at
this temperature for 240 seconds.
[0144]
JIS No. 5 test pieces were prepared from the steel
sheets obtained as described above, and were subjected to
stretch forming process with elongation of 3o mimicking the
actual manufacturing process. Metal structures of the test
pieces were observed before and after the processing, tensile
strength (TS) and elongation (total elongation El) before the
processing and hydrogen embrittlement resisting property after
the processing were measured by the following procedures.
[0145]
Observation of metal structure
Metal structures of the test pieces were observed before
and after the processing as follows. A measurement area (about
50 by 50 um) at an arbitrarily chosen position in a surface
parallel to the rolled surface at a position of one quarter of
the thickness was photographed at measuring intervals of 0.1
um, and area proportions of bainitic ferrite (BF), martensite
(M) and residual austenite (residual Y) were measured by the
method described previously. Then similar measurements were
made in two fields of view that were arbitrarily selected, and
the measured values were averaged. Area proportions of other
structures (ferrite, pearlite, etc.) were subtracted from the
entire structure.
[0146]
Mean axis ratio of the residual austenite grains of the
steel sheet before and after the processing were measured by
- 59 -
CA 02531616 2005-12-23
the method described previously. Test pieces having mean axis
ratio of 5 or higher were regarded to satisfy the requirements
of the present invention (o), and those having mean axis ratio
of lower than 5 were regarded to fail to satisfy the
requirements of the present invention (x).
[0147]
Measurement of tensile strength (TS) and elongation (E1)
Tensile test was conducted on the JIS No. 5 test piece
before processing, so as to measure the tensile strength (TS)
and elongation (E1). Stretching speed of the tensile test was
set to 1 mm/sec. Among the steel sheets having tensile
strength of 1180 MPa as measured by the method described
previously, those which showed elongation of 10% or more were
evaluated as high in elongation property.
[0148]
Evaluation of hydrogen embrittlement resisting property
In order to evaluate the hydrogen embrittlement
resisting property, the JIS No. 5 test piece was stretched so
as to elongate by 3%. Then after bending with a radius of
curvature of 15 mm, load of 1000 MPa was applied and the test
piece was immersed in 5% solution of hydrochloric acid, to
measure the time before crack occurred.
[0149]
The bent test pieces prepared as described above were
subjected to accelerated exposure test in which 3o solution of
NaCl was sprayed once every day for 30 days simulating the
actual operating environment, and the number of days before
crack occurred was determined.
[0150]
Hydrogen-charged 4-point bending test was also conducted
- 60 -
CA 02531616 2005-12-23
for some steel species. Specifically, a rectangular test piece
measuring 65 mm by 10 mm made of each steel sheet elongated by
3% was immersed in a solution of 0.5 mol of HZS04 and 0.01 mol
of KSCN and was subjected to cathode hydrogen charging.
Maximum stress endured without breaking for 3 hours was
determined as the critical fracture stress (DFL). Then the
ratio (DFL ratio) of this value to the value of DFL of test No.
203 (steel species C-2) shown in Table 6 was determined.
Results of these tests are shown in Table 6.
- 61 -
CA 02531616 2005-12-23
.-irl~f7CO10rlN N ~ NO M ON ~'f ~ MC~rlr-I~--I~ f
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- 62 -
CA 02531616 2005-12-23
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- 63 -
CA 02531616 2005-12-23
[0153]
The results shown in Tables 5 and 6 can be interpreted
as follows (numbers in the following description are test Nos.
in Table 6).
[0154]
Test pieces Nos. 201 through 214 (inventive steel sheets
2) and test pieces Nos. 221 through 225 (inventive steel
sheets 1) that satisfy the requirements of the present
invention have high strength of 1180 MPa or higher, and high
hydrogen embrittlement resisting property in harsh environment
after the forming process. They also have high elongation
property required of the TRIP steel sheet, thus providing
steel sheets best suited for reinforcement parts of
automobiles that are exposed to corrosive atmosphere. Test
pieces Nos. 221 through 225, in particular, show even better
hydrogen embrittlement resisting property.
[0155]
Test pieces Nos. 215 through 220 and 226 that do not
satisfy the requirements of the present invention, in contrast,
have the following drawbacks.
[0156]
No. 215 made of steel species 0-2 that includes
insufficient C content has the amount of residual austenite
significantly decreased after the processing, and fails to
show the required level of hydrogen embrittlement resisting
property of the present invention.
[0157]
No. 216 made of steel species P-2 that includes
insufficient Mn content does not retain sufficient residual
austenite and is inferior in hydrogen embrittlement resisting
- 64 -
CA 02531616 2005-12-23
property after the processing.
[0158]
No. 217, martensite steel that is a conventional high
strength steel made of steel species Q-2 that includes
insufficient Si content, hardly contains residual austenite
and is inferior in hydrogen embrittlement resisting property.
It also does not show the elongation property required of a
thin steel sheet.
[0159]
No. 218 made of steel species R-2 that includes
excessive C content has precipitation of carbide and is
inferior in both the forming workability and the hydrogen
embrittlement resisting property after processing.
[ 0160 ]
No. 219 made of steel species S-2 that does not include
Cu and/or Ni shows insufficient corrosion resistance and fails
to show the required level of hydrogen embrittlement resisting
property of the present invention.
[0161]
No. 220, that was made of a steel that has the
composition specified in the present invention but was not
manufactured under the recommended conditions, resulted in the
conventional TRIP steel. As a result, the residual austenite
does not have the mean axis ratio specified in the present
invention, while the matrix phase is not formed in binary
phase structure of bainitic ferrite and martensite, and
therefore sufficient level of hydrogen embrittlement resisting
property is not achieved.
[0162]
No. 226 includes A1 content higher than that specified
- 65 -
CA 02531616 2005-12-23
for the inventive steel sheet 1. As a result, although the
predetermined amount of residual austenite is retained, the
residual austenite does not have the mean axis ratio specified
in the present invention, the desired matrix phase is not
obtained and inclusions such as A1N are generated thus
resulting in poor hydrogen embrittlement resisting property.
[0163]
Then parts were made by using steel species A-2, K-2
shown in Table 5 and comparative steel sheet (590 MPa class
high strength steel sheet of the prior art). Performance
(pressure collapse resistance and impact resistance) of the
formed test piece were studied by conducting pressure collapse
test and impact resistance test as follows.
[0164]
Pressure collapse test
Maximum tolerable load was determined similarly to
Example 1 by using steel species A-2, K-2 shown in Table 5 and
the comparative steel sheet. Absorbed energy was determined
from the area lying under the load-deformation curve. The
results are shown in Table 7.
[0165]
Table 7
Steel sheet used Evaluation
of test
piece
Residual Maximum Energy
Steel species TS EL Y load absorbed
-............_.__ ~.._.._.. __.................._.
... ..._. ..
..
-
(MPa~ ( ~ ~ ( ( k J)
~ (Area kNj
j
Symbol A-2 1512 14 13 14.1 0.7
Symbol K-2 1485 14 13 13.9 0.68
Comparative steel
613 22 0 5.7 0.33
sheet
- 66 -
CA 02531616 2005-12-23
[ 0166]
From Table 7, it can be seen that the part (test piece)
made from the steel sheet of the present invention has higher
load bearing capability and absorbs greater energy than a part
made of the conventional steel sheet that has lower strength,
thus showing higher pressure collapse resistance.
[0167]
Impact resistance test
The impact resistance test was conducted similarly to
Example 1 on the steel sheets made of steel species A-2, K-2
shown in Table 5 and the comparative steel sheet. The results
are shown in Table 8.
[ 0168 ]
[Table 8]
Evaluation of test
Steel sheet used
piece
TS EL Res Energy absorbed
idual
Y
Steel species _ . _._." J
(MPa) (o) _._ ....,..-
(Area %)~ (kJ)
Symbol A-2 1512 14 13 7.06
Symbol K-2 1485 14 13 6.92
Comparative steel
613 22 0 3.56
sheet
[0169]
From Table 8, it can be seen that the part (test piece)
made from the steel sheet of the present invention absorbs
greater energy than a part made of the conventional steel
sheet having lower strength, thus showing higher impact
resistance.
[0170]
TEM photograph of the test piece made in this exampJ.e is
shown as reference. Fig. 8 is a photograph of TEM observation
- 67 -
CA 02531616 2005-12-23
of No. 201 of the present invention. From Fig. 8, it can be
seen that the high strength thin steel sheet of the present
invention contains lath-shaped residual austenite (black
portion of bar shape in Fig. 8) specified in the present
invention dispersed therein. Fig. 9 is a photograph of TEM
observation of No. 220 of a comparative example. From Fig. 9,
it can be seen that the high strength thin steel sheet of No.
220 contains residual austenite (black portion of somewhat
round shape in Fig. 9), although the residual austenite has a
block shape that does not satisfy the requirements of the
present invention.
[Example 3]
[0171]
Sample steels A-3 through Q-3 having the compositions
shown in Table 9 were melt-refined in vacuum to make test
slabs. The slabs were processed in the following procedure
(hot rolling-cold rolling-.continuous annealing) thereby to
obtain hot-rolled steel plates measuring 3.2 mm in thickness.
The steel plates were pickled to remove scales from the
surface and then cold rolled so as to reduce the thickness to
1.2 mm.
[0172]
<Hot rolling> Starting temperature (SRT): Held at a
temperature between 1150 and 1250°C for 30 minutes.
Finishing temperature (FDT): 850°C
Cooling rate: 40°C/s
Winding-up temperature: 550°C
<Cold rolling> Rolling ratio: 50%
<Continuous annealing> Each steel specimen was kept at a
temperature of A3 point + 30°C for 120 seconds, then rapidly
- 68 -
CA 02531616 2005-12-23
cooled (air cooling) at a mean cooling rate of 20°C/s to
temperature TO shown in Table 10, and was kept at TO for 240
seconds, followed by air-assisted water cooling to the room
temperature.
[0173]
No. 311 shown in Table 10 was made by heating a cold-
rolled steel sheet to 830°C, keeping at this temperature for 5
minutes followed by quenching in water and tempering at 300°C
for 10 minutes, thereby to form a martensite steel as a
comparative example of the high-strength steel of the prior
art. No. 312 was made by heating a cold-rolled steel sheet to
800°C, keeping at this temperature for 120 seconds, cooling at
a mean cooling rate of 20°C/s down to 350°C and keeping at
this temperature for 240 seconds.
[0174]
JIS No. 5 test pieces were prepared from the steel
sheets obtained as described above, and were subjected to
stretch forming process with elongation of 3o mimicking the
actual manufacturing process. Metal structures of the test
pieces were observed before and after the processing, tensile
strength (TS) and elongation (total elongation El) before the
processing and hydrogen embrittlement resisting property after
the processing were measured by the following procedures.
[0175]
Observation of metal structure
Metal structures of the test pieces were observed before
and after the processing as follows. A measurement area (about
50 by 50 um) at an arbitrarily chosen position in a surface
parallel to the rolled surface at a position of one quarter of
the thickness was photographed at measuring intervals of 0.1
- 69 -
CA 02531616 2005-12-23
um, and area proportions of bainitic ferrite (BF), martensite
(M) and residual austenite (residual Y) were measured by the
method described previously. Then similar measurements were
made in two fields of view that were arbitrarily selected, and
the measured values were averaged. Area proportions of other
structures (ferrite, pearlite, etc.) were subtracted from the
entire structure.
[0176]
Mean axis ratio, mean length of minor axes and minimum
distance between the residual austenite grains of the steel
sheet before and after the processing were measured by the
method described previously. Test pieces having mean axis
ratio of 5 or higher were regarded to satisfy the requirements
of the present invention (o), and those having mean axis ratio
of lower than 5 were regarded to fail to satisfy the
requirements of the present invention (x).
[0177]
Measurement of tensile strength (TS) and elongation (E1)
Tensile test was conducted on the JIS No. 5 test piece
before processing, so as to measure the tensile strength (TS)
and elongation (E1). Stretching speed of the tensile test was
set to 1 mm/sec. Among the steel sheets having tensile
strength of 1180 MPa as measured by the method described
previously, those which showed elongation of 10% or more were
evaluated as high in elongation.
[0178]
Evaluation of hydrogen embrittlement resisting property
In order to evaluate the hydrogen embrittlement
resisting property, flat test piece 1.2 mm in thickness was
subjected to slow stretching rate test (SSRT) with a
- 70 -
CA 02531616 2005-12-23
stretching speed of 1X10-4/sec, to determine hydrogen
embrittlement risk index (%) defined by the equation shown
below.
Hydrogen embrittlement risk index (o) - 100 X (1-E1/EO)
[0179]
EO represents the elongation before rupture of a steel
test piece that does not substantially contain hydrogen, E1
represents the elongation before rupture of a steel test piece
that has been charged with hydrogen electrochemically in
sulfuric acid. Hydrogen charging was carried out by immersing
the steel test piece in a mixed solution of H2S09 (0.5 mol/L)
and KSCN (0.01 mol/L) and supplying constant current (100A/m2)
at room temperature.
[0180]
A steel sheet having hydrogen embrittlement risk index
higher than 50°s is likely to undergo hydrogen embrittlement
during use. In the present invention, steel sheets having
hydrogen embrittlement risk index not higher than 50o were
evaluated to have high hydrogen embrittlement resisting
property.
[0181]
Results of the test are shown in Table 10.
_ 71 _
CA 02531616 2005-12-23
_ O f~O Ml0f~M ~ ~ O M f 61O~O O l~
~ rl~ MO N M O 61a1alM tn~ 61O M
L~O I~610061O ~' ~ I~N l~lfl~ U7l0 l~
M M N NN N M N N -IM M MM M M M
_ f~I~~ ~~ O ~ ~--I~ N 61~ l9~'v--IM 00
cn ~ M ~ m~ t~N ~o aoW o uWo00~
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- 72 -
CA 02531616 2005-12-23
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- 73 -
CA 02531616 2005-12-23
[0184]
The results shown in Tables 9 and 10 can be interpreted
as follows (numbers in the following description are test Nos.
in Table 10).
[0185]
Test pieces Nos. 301 through 309 (inventive steel sheets
2) and test pieces Nos. 313 through 317 (inventive steel
sheets 1) that satisfy the requirements of the present
invention have high strength of 1180 MPa or higher, and show
high hydrogen embrittlement resisting property in harsh
environment after the forming process. They also have high
elongation property required of the TRIP steel sheet, thus
providing steel sheets best suited for reinforcement parts of
automobiles that are exposed to corrosive atmosphere.
[0186]
Test pieces Nos. 310 through 312 and 318 that do not
satisfy the requirements of the present invention, in contrast,
have the following drawbacks.
[0187]
No. 310 made of steel species J-3 that includes
excessive C content has carbide precipitated and residual
austenite of longer mean length of minor axis, thus resulting
poor performance in both workability and hydrogen
embrittlement resisting property after processing.
[0188]
No. 311, martensite steel that is a conventional high
strength steel made of steel species K-3 that includes
insufficient Si content, hardly contains residual austenite
and is inferior in hydrogen embrittlement resisting property.
It also does not show the elongation property required of a
- 74 -
CA 02531616 2005-12-23
thin steel sheet.
[0189]
No. 312, that was made of a steel that has the
composition specified in the present invention but was not
manufactured under the recommended conditions, resulted in the
conventional TRIP steel. As a result, the residual austenite
does not have the mean axis ratio and the mean length of minor
axis specified in the present invention, while the matrix
phase is not formed in binary phase structure of bainitic
ferrite and martensite, thus resulting in low strength and
poor hydrogen embrittlement resisting property.
[0190]
No. 318 includes A1 content higher than that specified
for the inventive steel sheet 1. As a result, although the
predetermined amount of residual austenite is retained, the
residual austenite does not have the mean axis ratio specified
in the present invention, the desired matrix phase is not
obtained and inclusions such as A1N are generated thus
resulting in poor hydrogen embrittlement resisting property.
[0191]
Then parts were made by using steel species A-3, G-3
shown in Table 9 and comparative steel sheet (590 MPa class
high strength steel sheet of the prior art). Performance
(pressure collapse resistance and impact resistance) of the
formed test piece were studied by conducting pressure collapse
test and impact resistance test as follows.
[0192]
Pressure collapse test
Maximum tolerable load was determined similarly to
Example 1 by using steel species A-3, G-3 shown in Table 9 and
- 75 -
CA 02531616 2005-12-23
the comparative steel sheet. Absorbed energy was determined
from the area under the load-deformation curve. The results
are shown in Table 11.
[0193]
Table 11
Steel sheet used Evaluation
of test
piece
Residual Maximum Energy
TS EL Y load absorbed
Steel s eCies
...............................................................................
...............................................................................
...............................................................................
...............................................................................
............
p
(MPa) (%) (Area (kN) (kJ)
%)
Symbol A-3 1221 14 8.5 11.3 0.58
Symbol G-3 1480 14 10 13.8 0.69
Comparative steel613 22 0 5.7 0.33
sheet
[0194]
From Table 11, it can be seen that the part (test piece)
made from the steel sheet of the present invention has higher
load bearing capability and absorbs greater energy than a part
made of the conventional steel sheet having lower strength,
thus showing high pressure collapse resistance.
[0195]
Impact resistance test
The impact resistance test was conducted similarly to
Example 1 on the steel sheets made of steel species A-3, G-3
shown in Table 9 and the comparative steel sheet. The results
are shown in Table 12.
- 76 -
CA 02531616 2005-12-23
[0196]
Table 12
Steel sheet used Evaluation of test
piece
TS EL Residual Energy absorbed
y
Steel s ecies
p
(MPa ( o ) (Area o ) ( kJ)
)
Symbol A-3 1221 14 8.5 5.72
Symbol G-3 1480 14 10 6.88
Comparative steel
613 22 0 3
56
sheet .
[0197]
From Table 12, it can be seen that the part (test piece)
made from the steel sheet of the present invention absorbs
greater energy than a part made of the conventional steel
sheet that has lower strength, thus showing high impact
resistance.
[0198]
TEM photographs of the test pieces made in this example
are shown as reference. Fig. 12 is a photograph of TEM
observation (magnification factor 15000) of No. 301 of the
present invention. Fig. 13 is a photograph of TEM observation
(magnification factor 60,000) of a portion shown in the
photograph of Fig. 12. From Figs. 12, 13, it can be seen that
the high strength thin steel sheet of the present invention
contains fine residual austenite grains (black portion of bar
shape in Figs. 12, 13) specified in the present invention
dispersed therein, and that the residual austenite has the
lath shape specified in the present invention. Fig. 14 is a
photograph of TEM observation of No. 313 of a comparative
example. From Fig. 14, it can be seen that the high strength
_ 77 _
CA 02531616 2005-12-23
thin steel sheet of No. 313 contains residual austenite (black
portion of somewhat round shape in Fig. 14), although the
residual austenite has a block shape that does not satisfy the
requirements of the present invention.
_ 78 _
