Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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DESCRIPTION
HIGH STRENGTH STEEL SHEET AND METHOD FOR MANUFACTURING THE
SAME
Technical Field
The present 'invention relates to a high strength steel
sheet that is used in industrial fields such as an
automobile industry and an electrical industry, has good
formability, and has a tensile strength of 900 MPa or higher
and a method for manufacturing the same. The high strength
steel sheet of the present invention includes steel sheets
whose surface is galvanized or galvannealed.
Background Art
In recent years, the improvement in the fuel efficiency
of automobiles has been an important subject from the
viewpoint of global environment conservation. Therefore, by
employing a high strength automobile material, there has
been an active move to reduce the thickness of components
and thus to lighten the automobile body itself. However,
since an increase in the strength of steel sheets reduces
workability, the development of materials having both high
strength and good workability has been demanded. To satisfy
such a demand, various multiple-phase steel sheets such as a
ferrite-martensite dual-phase steel (DP steel) and a TRIP
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steel that uses transformation-induced plasticity of
retained austenite have been developed.
For example, the following Patent Documents disclose DP
steels. Patent Document 1 discloses a high strength steel
sheet with a low yield ratio that is excellent in surface
quality and bendability and has a tensile strength of 588 to
882 MPa and a method for manufacturing the steel sheet, by
specifying the composition and the hot-rolling and annealing
conditions. Patent Document 2 discloses a high strength
cold-rolled steel sheet with excellent bendability and a
method for manufacturing the steel sheet, by specifying the
hot-rolling, cold-rolling, and annealing conditions of steel
having a certain composition. Patent Document 3 discloses a
steel sheet that is excellent in collision safety and
formability and a method for manufacturing the steel sheet,
by specifying the volume fraction and grain diameter of
martensite and the mechanical properties. Patent Document 4
discloses a high strength steel sheet, a high strength
galvanized steel sheet, and a high strength galvannealed
steel sheet that are excellent in stretch-flangeability and
crashworthiness and a method for manufacturing the steel
sheets, by specifying the composition and the volume
fraction and grain diameter of martensite. Patent Document
discloses a high strength steel sheet, a high strength
galvanized steel sheet, and a high strength galvannealed
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steel sheet that are excellent in stretch-flangeability,
shape fixability, and crashworthiness and a method for
manufacturing the steel sheets, by specifying the
composition, the grain diameter and microstructure of
ferrite, and the volume fraction of martensite. Patent
Document 6 discloses a high strength steel sheet having
excellent mechanical properties and a method for
manufacturing the steel sheet, by specifying the composition,
the amount of martensite, and the manufacturing method.
Patent Documents 7 and 8 each disclose a high strength
galvanized steel sheet that is excellent in stretch-
flangeability and bendability and a method and facility for
manufacturing the steel sheet, by specifying the composition
and the manufacturing conditions in a galvanizing line.
The following Patent Documents disclose steel sheets
having a microstructure including a phase other than
martensite as a hard second phase. Patent Document 9
discloses a steel sheet that is excellent in fatigue
properties, by employing martensite and/or bainite as a hard
second phase and specifying the composition, the grain
diameter, the hardness ratio, and the like. Patent Document
discloses a steel sheet that is excellent in stretch-
flangeability, by mainly employing bainite or pearlite as a
second phase and specifying the composition and the hardness
ratio. Patent Document 11 discloses a high-strength and
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ductility galvanized steel sheet that is excellent in hole
expandability and a method for manufacturing the steel sheet,
by employing bainite and martensite as a hard second phase.
Patent Document 12 discloses a multiple-phase steel sheet
that is excellent in fatigue properties by employing bainite
and martensite as a hard second phase and specifying the
fraction of constituent phases, the grain diameter, the
hardness, and the mean free path of the entire hard phase.
Patent Document 13 discloses a high strength steel sheet
that is excellent in ductility and hole expandability, by
specifying the composition and the amount of retained
austenite. Patent Document 14 discloses a high strength
multiple-phase cold-rolled steel sheet that is excellent in
workability, by employing a steel sheet including bainite
and retained austenite and/or martensite and specifying the
composition and the fraction of phases. Patent Document 15
discloses a high strength steel sheet that is excellent in
workability and a method for manufacturing the steel sheet,
by specifying the distribution state of the grains of a hard
second phase in ferrite and the ratio of the grains of
tempered martensite and bainite to the grains of ferrite.
Patent Document 16 discloses an ultra-high strength cold-
rolled steel sheet that is excellent in delayed fracture
resistance and has a tensile strength of 1180 MPa or higher
and a method for manufacturing the steel sheet, by
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specifying the composition and the manufacturing process.
Patent Document 17 discloses an ultra-high strength cold-
rolled steel sheet that is excellent in bendability and has
a tensile strength of 980 MPa or higher and a method for
manufacturing the steel sheet, by specifying the composition
and the manufacturing method. Patent Document 18 discloses
an ultra-high strength thin steel sheet that has a tensile
strength of 980 MPa or higher and whose hydrogen
embrittlement is prevented by limiting the number of iron-
based carbide grains in tempered martensite to a certain
number and a method for manufacturing the steel sheet.
However, the above-described inventions pose the
problems below. Patent Documents 1 to 7, 9 to 10, and 12 to
14 disclose the inventions regarding steel sheets having a
tensile strength of lower than 900 MPa, and the workability
often cannot be maintained if the strength is further
increased. Patent Document 1 describes that annealing is
performed in a single phase region and the subsequent
cooling is performed to 400 C at a cooling rate of 6 to
20 C/s. However, in the case of a galvanized steel sheet,
the adhesion of a coating needs to be taken into account and
heating needs to be performed before coating because 400 C
is lower than the temperature of a coating bath. Thus, the
galvanized steel sheet cannot be manufactured in a
continuous galvanizing and galvannealing line having no
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heating equipment before the coating bath. In Patent
Documents 7 and 8, since tempered martensite needs to be
formed during the heat treatment in a galvanizing line,
there is required equipment for reheating the steel sheet
after the cooling to Ms temperature or lower. In Patent
Document 11, bainite and martensite are employed as a hard
second phase and the fraction is specified. However, the
characteristics significantly vary in the specified range,
and the operating conditions need to be precisely controlled
to suppress the variation. In Patent Document 15, since
cooling is performed to Ms temperature or lower to form
martensite before bainite transformation, equipment for
reheating the steel sheet is required. Furthermore, the
operating conditions need to be precisely controlled to
achieve stable characteristics. Consequently, the costs for
equipment and operation are increased. In Patent Documents
16 and 17, the steel sheet needs to be maintained in a
bainite-formation temperature range after annealing to
obtain a microstructure mainly composed of bainite, which
makes it difficult to achieve ductility. In the case of a
galvanized steel sheet, the steel sheet needs to be reheated
to a temperature higher than the temperature of a coating
bath. Patent Document 18 only describes the improvement in
hydrogen embrittlement of a steel sheet, and there is little
consideration for workability although bendability is
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considered to some extent.
In general, the ratio of a hard second phase to the
entire microstructure needs to be increased to increase the
strength of a steel sheet. However, when the ratio of a
hard second phase is increased, the workability of a steel
sheet is strongly affected by that of the hard second phase.
The reason is as follows. When the ratio of the hard second
phase is low, minimal workability is achieved by the
deformation of ferrite itself that is a parent phase even if
the workability of the hard second phase is insufficient.
However, when the ratio of the hard second phase is high,
the formability of a steel sheet is directly affected by the
deformability of the hard second phase, not the deformation
of ferrite. If the workability is insufficient, the
formability is considerably degraded.
Therefore, in the case of a cold-rolled steel sheet,
for example, martensite is formed through water quenching by
adjusting the fraction of ferrite and a hard second phase
using a continuous annealing furnace that can perform water
quenching. Subsequently, the temperature is increased and
held to temper martensite, whereby the workability of the
hard second phase is improved.
However, in the case where equipment has no ability to
temper the thus-formed martensite by increasing temperature
and holding high temperature, the strength can be ensured,
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but it is difficult to ensure the workability of the hard
second phase such as martensite.
To achieve stretch-flangeability using a hard phase
other than martensite, the workability of a hard second
phase is ensured by employing ferrite as a parent phase and
bainite or pearlite containing carbides as a hard second
phase. Unfortunately, in this case, sufficient ductility
cannot be achieved.
When bainite is used, there is a problem in that the
characteristics significantly vary due to the variation in a
bainite-formation temperature range and the holding time.
When martensite or retained austenite (including bainite
containing retained austenite) is employed as a second phase,
for example, a mixed microstructure of martensite and
bainite is considered to be used as a second phase
microstructure to ensure both ductility and stretch-
flangeability.
However, to employ a mixed microstructure composed of
various phases as a second phase and precisely control the
fraction or the like, the heat treatment conditions need to
be precisely controlled, which often poses a problem of
manufacturing stability.
Patent Document 1: Japanese Patent No. 1853389
Patent Document 2: Japanese Patent No. 3610883
Patent Document 3: Japanese Unexamined Patent
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Application Publication No. 11-61327
Patent Document 4: Japanese Unexamined Patent
Application Publication No. 2003-213369
Patent Document 5: Japanese Unexamined Patent
Application Publication No. 2003-213370
Patent Document 6: Japanese Unexamined Patent
Application Publication (Translation of PCT Application) No.
2003-505604
Patent Document 7: Japanese Unexamined Patent
Application Publication No. 6-93340
Patent Document 8: Japanese Unexamined Patent
Application Publication No. 6-108152
Patent Document 9: Japanese Unexamined Patent
Application Publication No. 7-11383
Patent Document 10: Japanese Unexamined Patent
Application Publication No. 10-60593
Patent Document 11: Japanese Unexamined Patent
Application Publication No. 2005-281854
Patent Document 12: Japanese Patent No. 3231204
Patent Document 13: Japanese Unexamined Patent
Application Publication No. 2001-207234
Patent Document 14: Japanese Unexamined Patent
Application Publication No. 7-207413
Patent Document 15: Japanese Unexamined Patent
Application Publication No. 2005-264328
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Patent Document 16: Japanese Patent No. 2616350
Patent Document 17: Japanese Patent No. 2621744
Patent Document 18: Japanese Patent No. 2826058
Disclosure of Invention
The present invention advantageously solves the
problems described above. An object of the present
invention is to provide a high strength steel sheet having a
tensile strength of 900 MPa or higher that can minimize the
formation of bainite, which easily causes a variation in
characteristics such as strength and formability, and can
have both high strength and good formability and to provide
an advantageous method for manufacturing the high strength
steel sheet.
The formability is evaluated using TS ?< T. El and a X
value that represents stretch-flangeability. In the present
invention, TS x T. El 14500 MPa.% and X 15% are target
characteristics.
To solve the problems described above, the inventors of
the present invention have studied about the formation
process of martensite, in particular, the effect of the
cooling conditions of a steel sheet on martensite.
Consequently, the inventors have found that a high
strength steel sheet having both good formability and high
strength with a tensile strength of 900 MPa or higher that
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are targeted in the present invention can be obtained by the
following method. By suitably controlling the heat
treatment conditions after cold-rolling, martensite
transformation is caused while at the same time the
transformed martensite is tempered. The ratio of the thus-
formed autotempered martensite is controlled to a certain
ratio and also the distribution state of iron-based carbide
grains included in the autotempered martensite is suitably
controlled, whereby such a high strength steel sheet can be
obtained.
The present invention has been completed through
further investigation on the basis of the above-described
findings. The gist of the invention is described below.
1. A high strength steel sheet having a tensile strength of
900 MPa or higher, includes a composition including, on a
mass basis:
C: 0.1% or more and 0.3% or less;
Si: 2.0% or less;
Mn: 0.5% or more and 3.0% or less;
P: 0.1% or less;
S: 0.07% or less; ,
Al: 1.0% or less; and
N: 0.008% or less, with the balance Fe and incidental
impurities, wherein a steel microstructure includes, on an
area ratio basis, 5% or more and 80% or less of ferrite, 15%
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or more of autotempered martensite, 10% or less of bainite,
5% or less of retained austenite, and 40% or less of as-
quenched martensite; a mean hardness of the autotempered
martensite is HV 700; and the mean number of precipitated
iron-based carbide grains each having a size of 5 nm or more
and 0.5 pm or less and included in the autotempered
martensite is 5 x 104 or more per 1 mm2.
2. The high strength steel sheet according to the above-
described 1, further includes, on a mass basis, at least one
element selected from:
Cr: 0.05% or more and 5.0% or less;
V: 0.005% or more and 1.0% or less; and
No: 0.005% or more and 0.5% or less.
3. The high strength steel sheet according to the above-
described 1 or 2, further includes, on a mass basis, at
least one element selected from:
Ti: 0.01% or more and 0.1% or less;
Nb: 0.01% or more and 0.1% or less;
B: 0.0003% or more and 0.0050% or less;
Ni: 0.05% or more and 2.0% or less; and
Cu: 0.05% or more and 2.0% or less.
4. The high strength steel sheet according to any one of
the above-described 1 to 3, further includes, on a mass
basis, at least one element selected from:
Ca: 0.001% or more and 0.005% or less; and
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REM: 0.001% or more and 0.005% or less.
5. The high strength steel sheet according to any one of the
above-described 1 to 4, wherein the area ratio of autotempered
martensite in which the number of precipitated iron-based
carbide grains each having a size of 0.1 m or more and 0.5 m or
less is 5 x 102 or less per 1 mm2 to the entire autotempered
martensite is 3% or more.
6. The high strength steel sheet according to any one of the
above-described 1 to 5, wherein a galvanized layer is disposed
on a surface of the steel sheet.
7. The high strength steel sheet according to any one of the
above-described 1 to 5, wherein a galvannealed layer is disposed
on a surface of the steel sheet.
8. A method for manufacturing a high strength steel sheet,
includes the steps of hot-rolling a slab having the composition
according to any one of the above-described 1 to 4 to form a
hot-rolled steel sheet; cold-rolling the hot rolled steel sheet
to form a cold-rolled steel sheet; annealing the cold-rolled
steel sheet in a first temperature range of 700 C or higher and
950 C or lower for 15 seconds or longer and 600 seconds or
shorter; in a second temperature range, which is a temperature
range from the first temperature range to 420 C, cooling the
steel sheet from the first temperature range to 550 C at an
average cooling rate of 3 C/s or higher and cooling the steel
sheet from 550 C to 420 C within 600
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seconds; and cooling the steel sheet at a cooling rate of
50 C/s or lower in a third temperature range of 250 C or
higher and 420 C or lower to perform, in the third
temperature range, autotempering treatment in which
martensite transformation is caused while at the same time
the transformed martensite is tempered.
9. The method for manufacturing a high strength steel sheet
according to the above-described 8, wherein when the steel
sheet is cooled at a cooling rate of 50 C/s or lower in the
third temperature range of 250 C or higher and 420 C or
lower, the steel sheet is cooled at a cooling rate of
1.0 C/s or higher and 50 C/s or lower in a temperature
range of at least (Ms temperature - 50) C or lower to
perform, in the third temperature range, autotempering
treatment in which martensite transformation is caused while
at the same time the transformed martensite is tempered.
10. The method for manufacturing a high strength steel
sheet according to the above-described 8 or 9, wherein
martensite start temperature Ms of the slab is approximated
by M represented by Formula (1) below, and the M is 300 C or
higher:
M ( C) = 540 - 361 x {[C%]/(1 - [a%]/100)1 - 6 x [Si%] -
40 x [Mn%] + 30 x [AIM - 20 x [Cr%] - 35 x [V%] - 10 x [Mo%]
- 17 x [Ni%] - 10 x [Cu%] === (1)
where [X%] is mass% of a constituent element X of the slab
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and [a%] is an area ratio (%) of polygonal ferrite.
According to the present invention, a high strength
steel sheet having a tensile strength of 900 MPa or higher
that can achieve high strength, good workability, and good
ductility can be obtained by forming an appropriate amount
of autotempered martensite in a steel sheet and suitably
controlling the distribution state of carbide grains
included in the autotempered martensite. Therefore, the
present invention significantly contributes to the weight
reduction of automobile bodies.
In the method for manufacturing a high strength steel
sheet according to the present invention, since the
reheating of a steel sheet after quenching is not needed,
special manufacturing equipment is not required and the
method can be easily applied to a galvanizing or
galvannealing process. Therefore, the present invention
contributes to decreases in the number of steps and in the
cost.
Brief Description of Drawings
[Fig. 1] Fig. 1 is a schematic view showing quenching
and tempering steps performed to obtain typical tempered
martensite.
[Fig. 2] Fig. 2 is a schematic view showing an
autotempering treatment step performed to obtain
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autotempered martensite in accordance with the present
invention.
Best Mode for Carrying Out the Invention
The present invention will now be specifically
described.
The reason for the above-described limitation of the
microstructure of a steel sheet according to the present
invention will be described below.
Area ratio of ferrite : 5% or more and 80% or less
To achieve both workability and a tensile strength of
900 MPa or higher, the ratio between ferrite and a hard
phase described below is important and thus the area ratio
of ferrite needs to be 5% or more and 80% or less. If the
area ratio of ferrite is less than 5%, ductility is not
ensured. If the area ratio of ferrite is more than 80%, the
= area ratio of the hard phase becomes insufficient and thus
the strength becomes insufficient. The area ratio of
ferrite is preferably set in the range of 10% or more and
65% or less.
Area ratio of autotempered martensite: 15% or more
In the present invention, autotempered martensite is a
microstructure obtained by simultaneously causing martensite
transformation and the tempering of the martensite through
autotempering treatment, and not so-called tempered
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martensite obtained through quenching and tempering
treatments as in the related art. The microstructure is not
a uniformly tempered microstructure formed by completing
martensite transformation through quenching and then
performing tempering through a temperature increase as in
typical quenching and tempering treatments, but is a
microstructure including martensites in different tempered
states obtained by performing martensite transformation and
the tempering of the martensite in stages through the
control of a cooling process in a temperature range of Ms
temperature or lower.
This autotempered martensite is a hard phase for
increasing strength. If the area ratio of autotempered
martensite is less than 15%, sufficient strength cannot be
achieved and work hardening of ferrite cannot be facilitated.
Thus, the area ratio of autotempered martensite needs to be
15% or more and is preferably 30% or more.
In the present invention, the microstructure of a steel
sheet is preferably composed of ferrite and autotempered
martensite within the above-described range. When such
phases are formed, other phases such as bainite, retained
austenite, and as-quenched martensite are sometimes formed.
These phases may be formed as long as some parameters are
within the tolerable ranges described below. The tolerable
ranges will now be described.
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Area ratio of bainite: 10% or less (including 0%)
Bainite is a hard phase that contributes to an increase
in strength, but the characteristics significantly vary in
accordance with the formation temperature range and the
variation in the quality of material is sometimes increased.
Therefore, the area ratio of bainite in a steel
microstructure is desirably as low as possible, but up to
10% of bainite is tolerable. The area ratio of bainite is
preferably 5% or less.
Area ratio of retained austenite: 5% or less (including 0%)
Retained austenite is transformed into hard martensite
when processed, which decreases stretch-flangeability. Thus,
the area ratio of retained austenite in a steel
microstructure is desirably as low as possible, but up to 5%
of retained austenite is tolerable. The area ratio of
retained austenite is preferably 3% or less.
Area ratio of as-quenched martensite: 40% or less (including
0%)
Since as-quenched martensite has considerably poor
workability, the area ratio of as-quenched martensite in a
steel microstructure is desirably as low as possible, but up
to 40% of as-quenched martensite is tolerable. The area
ratio of as-quenched martensite is preferably 30% or less.
Herein, as-quenched martensite can be differentiated from
autotempered martensite in that carbides of as-quenched
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=
martensite are not observed with a scanning electron
microscope (SEM) or a transmission electron microscope (TEM).
Mean hardness of autotempered martensite: HV 700
If the mean hardness of autotempered martensite is 700
< HV, stretch-flangeability is considerably degraded. Thus,
HV 700 needs to be satisfied and HV 630 is preferably
satisfied.
Iron-based carbide in autotempered martensite
Size: 5 nm or more and 0.5 m or less, Mean number of
precipitated carbide grains: 5 x 104 or more per 1 mm2
Autotempered martensite is martensite subjected to the
heat treatment (autotempering treatment) performed by the
method of the present invention. However, even if the mean
hardness of autotempered martensite is HV 700, the
workability is decreased when the autotempering treatment is
improperly performed. The degree of autotempering treatment
can be confirmed through the formation state (distribution
state) of iron-based carbide grains in autotempered
martensite. When the mean number of precipitated iron-based
carbide grains each having a size of 5 nm or more and 0.5 pm
or less is 5 x 104 or more per 1 mm2, it can be judged that
desired autotempering treatment has been performed. Iron-
based carbide grains each having a size of less than 5 nm
are removed from the target of judgment because such carbide
grains do not affect the workability of autotempered
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martensite. On the other hand, iron-based carbide grains
each having a size of more than 0.5 m are also removed from
the target of judgment because such carbide grains may
decrease the strength of autotempered martensite but hardly
affect the workability. If the number of iron-based carbide
grains is less than 5 x 104 per 1 mm2, it is judged that the
autotempering treatment has been improperly performed
because workability, particularly stretch-flangeability, is
not improved. The number of iron-based carbide grains is
preferably 1 x 105 or more and 1 x 106 or less per 1 mm2,
more preferably 4 x 105 or more and 1 x 106 or less per 1 mm2.
Herein, an iron-based carbide is mainly Fe3C, and s carbides
and the like may be further contained.
To confirm the formation state of carbide grains, it is
effective to observe a mirror-polished sample using a SEM
(scanning electron microscope) or a TEN (transmission
electron microscope). Carbide grains can be identified by,
for example, performing SEM-EDS (energy dispersive X-ray
spectrometry), EPMA (electron probe microanalyzer), or FE-
AES (field emission-Auger electron spectrometry) on samples
whose section is polished.
In the steel sheet of the present invention, the amount
of autotempered martensite narrowed down by further limiting
the size and number of iron-based carbide grains
precipitated in the above-described autotempered martensite
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can be suitably set as follows.
Autotempered martensite in which the number of precipitated
iron-based carbide grains each having a size of 0.1 m or
more and 0.5 m or less is 5 x 102 or less per 1 mm2: the
area ratio of the autotempered martensite to the entire
autotempered martensite is 3% or more
By increasing the ratio of autotempered martensite in
which the number of precipitated iron-based carbide grains
each having a size of 0.1 m or more and 0.5 m or less is 5
x 102 or less per 1 mm2, ductility is further improved. To
produce such an effect, the area ratio of autotempered
martensite in which the number of precipitated iron-based
carbide grains each having a size of 0.1 m or more and 0.5
m or less is 5 x 102 or less per 1 mm2 to the entire
autotempered martensite is preferably 3% or more. If a
large amount of autotempered martensite in which the number
of precipitated iron-based carbide grains each having a size
of 0.1 m or more and 0.5 m or less is 5 x 102 or less per 1
2 i
MM s contained in a steel sheet, workability is
considerably degraded. Thus, the area ratio of such
autotempered martensite to the entire autotempered
martensite is preferably 40% or less, more preferably 30% or
less.
When the area ratio of autotempered martensite in which
the number of precipitated iron-based carbide grains each
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having a size of 0.1 m or more and 0.5 m or less is 5 x 102
or less per 1 mm2 to the entire autotempered martensite is
3% or more, the number of fine iron-based carbide grains is
increased in autotempered martensite. Therefore, the mean
number of precipitated iron-based carbide grains in the
entire autotempered martensite is increased. Thus, the mean
number of precipitated iron-based carbide grains each having
a size of 5 nm or more and 0.5 m or less in autotempered
martensite is preferably 1 x 105 or more and 5 x 106 or less
per 1 mm2, more preferably 4 x 105 or more and 5 x 106 or
less per 1 mm2.
The specific reason why ductility is further improved
as described above is not clear, but it is believed to be as
follows. When the area ratio of autotempered martensite in
which the number of precipitated iron-based carbide grains
each having a relatively large size of 0.1 m or more and
0.5 m or less is 5 x 102 or less per 1 mm2 to the entire
autotempered martensite is 3% or more, the autotempered
martensite microstructure includes a portion that contains a
large number of iron-based carbide grains having a
relatively large size and a portion that contains a small
number of iron-based carbide grains having a relatively
large size in a mixed manner. The portion that contains a
small number of iron-based carbide grains having a
relatively large size is hard autotempered martensite
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because a large number of fine iron-based carbide grains are
contained. On the other hand, the portion that contains a
large number of iron-based carbide grains having a
relatively large size is soft autotempered martensite. By
providing the hard autotempered martensite such that the
hard autotempered martensite is surrounded by the soft
autotempered martensite, the degradation of stretch-
flangeability caused by the hardness difference in
autotempered martensite can be suppressed. Furthermore, by
dispersing the hard martensite in the soft autotempered
martensite, work hardenability is improved and thus
ductility is improved.
The reason why the composition is set in the above-
described range in the steel sheet according to the present
invention will be described below. The symbol "%" below
used for each component means "% by mass".
C: 0.1% or more and 0.3% or less
C is an essential element for increasing the strength
of a steel sheet. A C content of less than 0.1% causes
difficulty in achieving both strength and workability such
as ductility or stretch-flangeability of the steel sheet.
On the other hand, a C content of more than 0.3% causes a
significant hardening of welds and heat-affected zones,
thereby reducing weldability. Thus, in the present
invention, the C content is set in the range of 0.1% or more
CA 02713181 2010-07-26
- 24 -
and 0.3% or less, preferably 0.12% or more and 0.23% or less.
Si: 2.0% or less
Si is a useful element for solution hardening of
ferrite, and the Si content is preferably 0.1% or more to
ensure the ductility and the hardness of ferrite. However,
the excessive addition of Si causes the degradation of
surface quality due to the occurrence of red scale and the
like and the degradation of the adhesion of a coating. Thus,
the Si content is set to 2.0% or less, preferably 1.6% or
less.
Mn: 0.5% or more and 3.0% or less
Mn is an element that is effective in strengthening
steel, stabilizes austenite, and is necessary for ensuring
the area ratio of a hard phase. To achieve this, a Mn
content of 0.5% or more is required. On the other hand, an
excessive Mn content of more than 3.0% causes the
degradation of castability or the like. Thus, the Mn
content is set in the range of 0.5% or more and 3.0% or less,
preferably 1.5% or more and 2.5% or less.
P: 0.1% or less
P causes embrittlement due to grain boundary
segregation and degrades shock resistance, but a P content
of up to 0.1% is tolerable. Furthermore, in the case where
a steel sheet is galvannealed, a P content of more than 0.1%
significantly reduces the rate of alloying. Thus, the P
CA 02713181 2010-07-26
- 25 -
content is set to 0.1% or less, preferably 0.05% or less.
S: 0.07% or less
S is formed into MnS as an inclusion that causes the
degradation of shock resistance and causes cracks along a
flow of a metal in a weld zone. Thus, the S content is
preferably minimized. However, a S content of up to 0.07%
is tolerable in terms of manufacturing costs. The S content
is preferably 0.04% or less.
Al: 1.0% or less
Al is an element that contributes to ferrite formation
and a useful element for controlling the amount of the
ferrite formation during manufacturing. However, an
excessive Al content degrades the quality of a slab during
steelmaking. Thus, the Al content is set to 1.0% or less,
preferably 0.5% or less. Since an excessively low Al
content sometimes makes it difficult to perform
deoxidization, the Al content is preferably 0.01% or more.
N: 0.008% or less
N is an element that most degrades the anti-aging
property of steel. Therefore, the N content is preferably
minimized. A N content of more than 0.008% causes
significant degradation of an anti-aging property. Thus,
the N content is set to 0.008% or less, preferably 0.006% or
less.
If necessary, the steel sheet of the present invention
CA 02713181 2010-07-26
- 26 -
can suitably contain the components described below in
addition to the basic components described above.
At least one element selected from Cr: 0.05% or more and
5.0% or less, V: 0.005% or more and 1.0% or less, and Mo:
0.005% or more and 0.5% or less
Cr, V, and Mo have an effect of suppressing the
formation of pearlite when a steel sheet is cooled from the
annealing temperature and thus can be optionally added. The
effect is produced at a Cr content of 0.05% or more, a V
content of 0.005% or more, or a Mo content of 0.005% or more.
On the other hand, an excessive Cr content of more than 5.0%,
an excessive V content of more than 1.0%, or an excessive Mo
content of more than 0.5% excessively increases the area
ratio of a hard phase, thereby unnecessarily increasing the
strength. Thus, when these elements are incorporated, the
Cr content is preferably set in the range of .05% or more
and 5.0% or less, the V content is preferably set in the
range of 0.005% or more and 1.0% or less, and the Mo content
is preferably set in the range of 0.005% or more and 0.5% or
less.
Furthermore, at least one element selected from Ti, Nb,
B, Ni, and Cu can be incorporated. The reason for the
limitation of the content ranges is as follows.
Ti: 0.01% or more and 0.1% or less and Nb: 0.01% or more and
0.1% or less
CA 02713181 2010-07-26
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Ti and Nb are useful for precipitation strengthening of
steel and the effect is produced at a Ti content of 0.01% or
more or a Nb content of 0.01% or more. On the other hand, a
Ti content of more than 0.1% or a Nb content of more than
0.1% degrades the workability and shape flexibility. Thus,
the Ti content and the Nb content are each preferably set in
the range of 0.01% or more and 0.1% or less.
B: 0.0003% or more and 0.0050% or less
B has an effect of suppressing the formation and growth
of ferrite from austenite grain boundaries and thus can be
optionally added. The effect is produced at a B content of
0.0003% or.more. On the other hand, a B content of more
than 0.0050% decreases workability. Thus, when B is
incorporated, the B content is preferably set in the range
of 0.0003% or more and 0.0050% or less. Herein, when B is
incorporated, the formation of EN is preferably suppressed
to produce the above-described effect. Thus, B is
preferably added together with Ti.
Ni: 0.05% or more and 2.0% or less and Cu: 0.05% or more and
2.0% or less
In the case where a steel sheet is galvanized, Ni and
Cu promote internal oxidation, thereby improving the
adhesion of a coating. The effect is produced at a Ni
content of 0.05% or more or a Cu content of 0.05% or more.
On the other hand, a Ni content of more than 2.0% or a Cu
CA 02713181 2010-07-26
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content of more than 2.0% degrades the workability of a
steel sheet. Ni and Cu are useful elements for
strengthening steel. Thus, the Ni content and the Cu
content are each preferably set in the range of 0.05% or
more and 2.0% or less.
At least one element selected from Ca: 0.001% or more and
0.005% or less and REM: 0.001% or more and 0.005% or less
Ca and REM are useful elements for spheroidizing the
shape of a sulfide and improving an adverse effect of the
sulfide on stretch-flangeability. The effect is produced at
a Ca content of 0.001% or more or an REM content of 0.001%
or more. On the other hand, a Ca content of more than
0.005% or an REM content of more than 0.005% increases the
number of inclusions or the like and causes, for example,
surface defects and internal defects. Thus, when Ca and REM
are incorporated, the Ca content and the REM content are
each preferably set in the range of 0.001% or more and
0.005% or less.
In the steel sheet of the present invention, components
other than the components described above are Fe and
incidental impurities. However, a component other than the
components described above may be contained to the extent
that the advantages of the present invention are not
impaired.
As described below, the composition of the steel sheet
CA 02713181 2010-07-26
- 29 -
according to the present invention preferably satisfies M
300 C that represents a relation between the composition and
the area ratio of polygonal ferrite to perform stable
production, that is, to suppress the variation in
characteristics due to the variation in manufacturing
conditions.
In the present invention, a galvanized layer or a
galvannealed layer may be disposed on a surface of a steel
sheet.
A preferred method for manufacturing a steel sheet
according to the present invention and the reason for the
limitation of the conditions will now be described.
A slab prepared to have the above-described preferred
composition is produced, hot-rolled, and then cold-rolled to
obtain a cold-rolled steel sheet. In the present invention,
these processes are not particularly limited, ahd can be
performed by typical methods.
The preferred manufacturing conditions will now be
described below. A slab is heated to 1100 C or higher and
1300 C or lower and subjected to finish hot-rolling at a
temperature of 870 C or higher and 950 C or lower, which
means that the hot-rolling end temperature is set to 870 C
or higher and 950 C or lower. The thus-obtained hot-rolled
steel sheet is wound at a temperature of 350 C or higher and
720 C or lower. Subsequently, the hot-rolled steel sheet is
CA 02713181 2010-07-26
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pickled and cold-rolled at a reduction ratio of 40% or
higher and 90% or lower to obtain a cold-rolled steel sheet.
It is assumed that the hot-rolled steel sheet is
produced through the typical steps of steel making, casting,
and hot-rolling, but the hot-rolled steel sheet may be
produced by thin slab casting without performing part or all
of the hot-rolling steps.
The resultant cold-rolled steel sheet is annealed for
15 seconds or longer and 600 seconds or shorter in a first
temperature range of 700 C or higher and 950 C or lower,
specifically, in an austenite single-phase region or a dual-
phase region of an austenite phase and a ferrite phase. If
the annealing temperature is lower than 700 C or the
annealing time is shorter than 15 seconds, a carbide in the
steel sheet is sometimes not sufficiently dissolved, or the
recrystallization of ferrite is not completed and thus
desired ductility and stretch-flangeability are sometimes
not achieved. On the other hand, if the annealing
temperature exceeds 950 C, austenite grains are
significantly grown and the constituent phases produced by
cooling performed later are coarsened, which may degrade
ductility and stretch-flangeability. If the annealing time
,exceeds 600 seconds, a vast amount of energy is consumed and
thus the cost is increased. Therefore, the annealing
temperature is set in the range of 700 C or higher and 950 C
CA 02713181 2010-07-26
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or lower, preferably 760 C or higher and 920 C or lower.
The annealing time is set in the range of 15 seconds or
longer and 600 seconds or shorter, preferably 30 seconds or
longer and 400 seconds or shorter.
In a second temperature range, which is a temperature
range from the first temperature range to 420 C, the
annealed cold-rolled steel sheet is cooled to 550 C from the
first temperature range at a cooling rate of 3 C/s or
higher, and is then cooled from 550 to 420 C within 600
seconds. Subsequently, the steel sheet is cooled at a
cooling rate of 50 00/5 or lower in a third temperature
range of 250 C or higher and 420 C or lower.
The cooling conditions in a second temperature range
from the first temperature range to 420 C are essential to
suppress the precipitation of phases other than intended
ferrite and autotempered martensite phases. In the
temperature range from the first temperature range to 550 C,
pearlite transformation easily occurs. If the average
cooling rate is lower than 3 C/s in the range from 700 C,
which is the lower limit temperature of the first
temperature range, to 550 C, pearlite or the like is
precipitated and a desired microstructure is sometimes not
obtained. Therefore, the cooling rate needs to be 3 C/s or
higher, and is preferably 5 C/s or higher. The upper limit
of the cooling rate is not particularly specified, but
CA 02713181 2010-07-26
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special cooling equipment is required to achieve a cooling
rate of 200 C/s or higher. Thus, the cooling rate is
preferably 200 C/s or lower.
When the steel sheet is held for a long time in a
temperature range of 550 C to 420 C, bainite transformation
is caused. If the time required for cooling from 550 C to
420 C exceeds 600 seconds, bainite transformation is caused
and thus a desired microstructure is sometimes not obtained.
Therefore, the time required for cooling from 550 C to 420 C
is 600 seconds or shorter, preferably 400 seconds or shorter.
After the process in the second temperature range, the
steel sheet is processed in the third temperature range.
The most important feature of the present invention is that,
in the third temperature range, autotempering treatment in
which martensite transformation is caused while at the same
time the transformed martensite is tempered is performed to
obtain autotempered martensite in which the precipitation
state of carbide grains is suitably controlled.
Typical martensite is obtained by performing annealing
and then performing quenching with water cooling or the like.
The martensite is a hard phase, and contributes to an
increase in the strength of a steel sheet but degrades
workability. To change the martensite into tempered
martensite having satisfactory workability, a quenched steel
sheet is normally heated again to perform tempering. Fig. 1
CA 02713181 2010-07-26
- 33 -
schematically shows the steps described above. In such
normal quenching and tempering treatments, after martensite
transformation is completed by quenching, the temperature is
increased to perform tempering. Consequently, a uniformly
tempered microstructure is obtained.
In contrast, autotempering treatment is a treatment in
which a steel sheet is cooled in a certain cooling-rate
range in the third temperature range as shown in Fig. 2. In
the autotempering treatment, quenching and tempering through
reheating are not performed, which is a method with high
productivity. The steel sheet including autotempered
martensite obtained through this autotempering treatment has
strength and workability equal to or higher than those of
the steel sheet obtained by performing quenching and
tempering through reheating shown in Fig. 1. In the
autotempering treatment, martensite transformation and the
tempering can be made to occur continuously or stepwise by
performing continuous cooling (including stepwise cooling
and holding) in the third temperature range. Consequently,
a microstructure including martensites in different tempered
states can be obtained. Although the martensites in
different tempered states have different characteristics in
terms of strength and workability, desired characteristics
as the entire steel sheet can be achieved by suitably
controlling the amounts of martensites in different tempered
CA 02713181 2010-07-26
- 34 -
states through autotempering treatment. Furthermore, since
the autotempering treatment is performed without rapidly
cooling a steel sheet to a low temperature range in which
the martensite transformation is fully completed, the
residual stress in the steel sheet is low and a steel sheet
having a good plate shape is obtained, which is advantageous.
In the present invention, the third temperature range
is 250 C or higher and 420 C or lower. If the temperature
exceeds 420 C, bainite transformation is easily caused as
described above. If the temperature is lower than 250 C,
autotempering treatment requires a long time and thus
proceeds insufficiently in a continuous annealing line or a
continuous galvanizing and galvannealing line. In the third
temperature range, the cooling rate of a steel sheet needs
to be 50 C/s or lower in order to cause martensite
transformation while at the same time tempering the
transformed martensite and thus to obtain autotempered
martensite. If the cooling rate exceeds 50 C/s, the
autotempering treatment insufficiently proceeds and the
workability of martensite is sometimes not ensured. If the
cooling rate is less than 0.1 C/s, bainite transformation
occurs or autotempering treatment excessively proceeds,
whereby strength sometimes cannot be ensured. Thus, the
cooling rate is preferably 0.1 C/s or higher.
In the method for manufacturing a steel sheet according
CA 02713181 2010-07-26
- 35 -
to the present invention, the following configuration can be
suitably added if necessary.
When a steel sheet is cooled at a cooling rate of
50 C/s or lower in a third temperature range of 250 C or
higher and 420 C or lower, the steel sheet is preferably
cooled at a cooling rate of 1.0 C/s or higher and 50 C/s
or lower in a temperature range of at least (Ms temperature
- 50) C or lower. This is because, by further appropriately
controlling the precipitation state of carbide grains
included in autotempered martensite, the area ratio of
autotempered martensite in which the number of precipitated
iron-based carbide grains each having a size of 0.1 m or
more and 0.5 m or less is 5 x 102 or less per 1 mm2 to the
entire autotempered martensite can be set to 3% or more. If
the cooling rate exceeds 50 C/s, autotempering treatment
insufficiently proceeds and desired autotempered martensite
is not obtained. Consequently, the workability of
martensite is sometimes not ensured. If the cooling rate is
less than 1.0 C/s, 3% or more of the area ratio of
autotempered martensite in which the number of precipitated
iron-based carbide grains each having a size of 0.1 m or
more and 0.5 m or less is 5 x 102 or less per 1 mm2 to the
entire autotempered martensite cannot be achieved, and
desired ductility and strength are not ensured. Thus, the
cooling rate is set to 1.0 C/s or higher. Herein, Ms
CA 02713181 2010-07-26
- 36 -
temperature can be obtained in a typical manner through the
measurement of thermal expansion or electrical resistance
during cooling. Alternatively, M obtained from an
approximate expression (1) of Ms temperature described below
may be used.
In the method for manufacturing a steel sheet according
to the present invention, autotempering treatment can be
stably performed when M represented by the approximate
expression (1) below is 300 C or higher:
M ( C) = 540 - 361 x ([0%]/(1 - [a%]/100)1 - 6 x [Si%] -
40 x [Mn%] + 30 x [Al%] - 20 x [Cr%] - 35 x [V%] - 10 x [Mo%]
- 17 x [Ni%] - 10 x [Cu%] === (1)
where [X%] is mass% of an alloy element X and [a%] is the
area ratio (%) of polygonal ferrite.
M represented by the above-described expression (1) is
an empirical approximate expression of Ms temperature from
which martensite transformation starts. It is believed that
M is highly related to the precipitation behavior of iron-
based carbide grains from martensite. Thus, M can be used
as an indicator that indicates whether autotempered
martensite in which the number of iron-based carbide grains
each having a size of 5 nm or more and 0.5 m or less is 5 x
104 or more per 1 mm2 can be stably obtained. Even if M is
less than 300 C, autotempered martensite is obtained.
However, since the temperature is low, martensite
CA 02713181 2010-07-26
- 37 -
transformation and autotempering treatment tend to slowly
proceed. Compared with the case of M 300 C,
a steel sheet
needs to be cooled slowly or held at a low temperature for a
long time to obtain desired autotempered martensite, which
may considerably lower manufacturing efficiency. Thus, M is
preferably 300 C or higher.
The area ratio of polygonal ferrite is measured, for
example, through the image processing and analysis of a SEM
micrograph taken at 1000 to 3000 power. Polygonal ferrite
is observed in the steel sheet that has been annealed and
cooled under the above-described conditions. To ensure that
M is 300 C or higher, after a cold-rolled steel sheet having
a desired composition is produced, the area ratio of
polygonal ferrite is measured and thus M is obtained from
the expression (1) using the contents of alloy elements that
can be calculated from the composition of the steel sheet.
In the case where M is less than 300 C, the heat treatment
conditions are suitably adjusted such that the area ratio of
polygonal ferrite becomes lower, to obtain desired M. For
example, the annealing temperature in the first temperature
range is further increased and the average cooling rate from
the first temperature range to 550 C is further increased.
Alternatively, the contents of the components in the
expression (1) may be adjusted.
The steel sheet of the present invention can be
CA 02713181 2010-07-26
- 38 -
galvanized and galvannealed. The galvanizing and
galvannealing treatments are preferably performed in a
continuous galvanizing and galvannealing line while the
above-described annealing and cooling conditions are
satisfied. The galvanizing and galvannealing treatments are
preferably performed in a temperature range of 420 C or
higher and 550 C or lower. In this case, the time required
for cooling a steel sheet from 550 C to 420 C, that is, the
holding time in the temperature range of 420 C or higher and
550 C or lower needs to be 600 seconds or shorter, the time
including galvanizing treatment time and/or galvannealing
treatment time.
A method of galvanizing and galvannealing treatments is
as follows. First, a steel sheet is immersed in a coating
bath and the coating weight is adjusted using gas wiping or
the like. In the case where the steel sheet is galvanized,
the amount of dissolved Al in the coating bath is in the
range of 0.12% or more and 0.22% or less. In the case where
the steel sheet is galvannealed, the amount of dissolved Al
is in the range of 0.08% or more and 0.18% or less.
In the case where the steel sheet is galvanized, the
temperature of the coating bath is desirably 450 C or higher
and 500 C or lower. In the case where the steel sheet is
galvannealed by further performing alloying treatment, the
temperature during alloying is preferably 450 C or higher
CA 02713181 2010-07-26
- 39 -
and 550 C or lower. If the alloying temperature exceeds
550 C, an excessive amount of carbide grains are
precipitated from untransformed austenite or the
transformation into pearlite is caused, whereby desired
strength and ductility are sometimes not achieved.
Powdering is also degraded. If the alloying temperature is
less than 450 C, the alloying does not proceed.
The coating weight is preferably in the range of 20 to
150 g/m2 per surface. If the coating weight is less than 20
g/m2, corrosion resistance is degraded. Meanwhile, even if
the coating weight exceeds 150 g/m2, the corrosion
resistance is saturated, which merely increases the cost.
The degree of alloying is preferably in the range of 7 to
15% by mass on a Fe content basis in the coating layer. If
the degree of alloying is less than 7% by mass, uneven
alloying is caused and the surface appearance quality is
degraded. Furthermore, a so-called phase is formed in the
coating layer and thus the slidability is degraded. If the
degree of alloying exceeds 15% by mass, a large amount of
hard brittle F phase is formed and the adhesion of the
coating is degraded.
In the present invention, the holding temperature in
the first temperature range, in the second temperature range,
or the like is not necessarily constant. Even if the
holding temperature is varied, the purport of the present
CA 02713181 2010-07-26
- 40 -
invention is not impaired as long as the holding temperature
is within a predetermined temperature range. The same is
true for the cooling rate. Furthermore, a steel sheet may
be subjected to annealing and autotempering treatments with
any equipment as long as heat history is just satisfied.
Moreover, it is also included in the scope of the present
invention that, after autotempering treatment, temper
rolling is performed on the steel sheet of the present
invention for shape correction.
Examples
Example 1
The present invention will now be further described
with Examples. The present invention is not limited to
Examples. It will be understood that modifications may be
made without departing from the scope of the invention.
A slab to be formed into a steel sheet having the
composition shown in Table I was heated to 1250 C and
subjected to finish hot-rolling at 880 C. The hot-rolled
steel sheet was wound at 600 C, pickled, and cold-rolled at
a reduction ratio of 65% to obtain a cold-rolled steel sheet
having a thickness of 1.2 mm. The resultant cold-rolled
steel sheet was subjected to heat treatment under the
conditions shown in Table 2. Quenching was not performed on
any sample shown in Table 2. Herein, the holding time in
CA 02713181 2010-07-26
- 41 -
Table 2 was a time held at the holding temperature shown in
Table 2. The annealing time in a first temperature range of
700 C or higher and 950 C or lower was 600 seconds or
shorter under any of the conditions shown in Table 2.
In the galvanizing treatment, both surfaces were
subjected to plating in a coating bath having a temperature
of 463 C at a coating weight of 50 g/m2 per surface. In the
galvannealing treatment, the alloying treatment was
performed such that Fe% (iron content) in the coating layer
was adjusted to 9% by mass. The resultant steel sheet was
subjected to temper rolling at a reduction ratio (elongation
ratio) of 0.3% regardless of the presence or absence of a
coating.
Table I
(mass%)
Steel type C Si Mn Al P s N Cr V Mo Ti Nb
B Ni Cu Ca REM Remarks
A 0.16 1.59 2.2 0.040 0.011 0.005 0.0039 0.5 - -
- - - - - . _ Suitable steel
B 0.15 1.51 2.3 0.036 0.012 0.004 0.0023 0.9 - - -
- - - - - - Suitable steel
C 0.15 1.40 2.3 , 0.041 0.012 0.004 0.0029 -- -
- 0.04
- - - - - Suitable steel
,
D 0.15 1.00 2.2 0.039 0.009 0.004
0.0037 1.0 - - 0.021 - 0.0010 - - - -
Suitable steel
E 0.14 1.48 2.2 0.040 0.025 0.002 0.0038 - - - -
- - - - - - Suitable steel
-
n
F 0.21 1.42 2.3 0.041 0.010 0.004 0.0037 - - -
- - - - - - Suitable steel 0
-
1.)
G 0.29 1.50 2.1 0.040 0.010 0.003
0.0041 - - - - - - - - - Suitable steel
H
CA
H
H 0.16 0.51 2.2 , 0.039 0.013 0.004 0.0032 1.5 - -
0.020 - 0.0008 - - - - Suitable steel
I CO
H
I 0.15 ,_. 1.49 2.8 0.037 0.012 0.003 0.0033 - - -
0.019 - 0.0005 - - - - Suitable steel
0
-
H
0
J 0.12 1.52 2.3 0.037 0.029 0.003 0.0041 1.0
- - 0.020 - 0.0009 - - - - Suitable
steel 1 1
= 0
-.3
I
K 0.21 0.49 1.6 , 0.037 0.029 0.003 0.0041 - - -
0.022 - 0.0012 - - - - Suitable steel
K)
c7,
L 0.15 1.50 2.3 0.043 0.013 0.002
0.0043 1.0 - - 0.050 - 0.0010 - - - -
Suitable steel
M 0.11 1.48 2.0 0.039 0.013 0.003 0.0037 0.9 - 0.03
0.021 - 0.0008 - - - - Suitable steel
N 0.16 1.50 2.3 _., 0.038 0.012 0.003 0.0041 0.8 0.10
- - - - - - - - Suitable steel
0 0.12 0.99 1.2 0.040 0.013 0.003 0.0041 1.0 - - -
- - 1.00 - - - Suitable steel
P 0.15 1.53 2.1 0.041 0.011 0.004 0.0029 0.5 -
- - - - - 0.3 - - Suitable steel
Q 0.15 1.48 2.3 0.044 0.009 0.004 0.0031 1.0 - -
0.019 - 0.0008 - - - 0.002 Suitable steel
R 0.05 1.05 2.1 0.037 0.008 0.004 0.0030 - - - - -
- - - - - Comparative steel
_
S 0.33 1.25 2.7 0.035 0.010 0.004 0.0042 - - - -
- - - - - - Comparative steel
_
T 0.23 1.51 3.5 0.040 0.008 0.004 0.0039 - - - - -
- - - - - Comparative steel
Note) Underline means the value is outside the suitable range.
CA 02713181 2010-07-26
- 4 3
Table 2 ¨
First temperature range Second temperature range Third
temperature
range
Sample
Steel type Holding Average cooling rate
Time required for Average cooling rate Plating.' Remarks
No. Holding time from first
Temperature cooling from 550 C from 420 C to 250
(CC) (second) temperature range to
to 420 C (second)
550 C (T/s)
1 A 820 180 10 90 15 CR Invention Example
2 B 830 100 10 60 10 GI Invention Example
3 C 820 180 10 80 55 CR Comparative Example
,
4 C 850 180 45 60 10 CR Invention Example
D 830 250 40 60 10 GI Invention Example
6 E 820 180 10 60 100 CR Comparative Example
7 F 810 180 8 70 60 CR Comparative Example
8 G 820 180 5 60 55 GA Comparative Example ,
9 H 820 180 10 120 15 GA Invention Example
I 870 180 15 60 10 CR Invention Example
_
11 J 830 200 30 60 10 CR Invention
Example
12 J 830 300 7 60 5 CR Invention
Example
13 K 860 40 10 45 10 GI Invention
Example
14 L 860 90 10 60 10 CR Invention
Example
M 850 180 10 60 9 CR Invention Example
16 N 800 450 10 80 10 CR Invention
Example
17 0 820 180 10 60 8 GI Invention
Example
18 P 860 400 10 75 9 GI Invention
Example
19 Q 800 180 10 60 10 CR Invention
Example
R 800 180 10 80 10 CR Comparative Example
21 S 800 180 10 80 10 CR Comparative Example
22 T 800 180 10 80 10 CR Comparative Example
23 A 880 200 50 20 70 CR Comparative
Example
24 E 810 200 10 90 10 CR Invention
Example
G 850 350 5 650 5 CR Comparative Example
26 G 870 150 15 120 3 CR Invention
Example
*1) Underline means the value is outside the suitable range.
*2) CR: no plating (cold-rolled steel sheet), GI: galvanizing, and GA:
galvannealing
CA 02713181 2010-07-26
- 44 -
The characteristics of the resultant steel sheets were
evaluated by the following methods.
To examine the microstructure of the steel sheets, two
test pieces were cut from each of the steel sheets. One of
the test pieces was polished without performing any
treatment. The other of the test pieces was polished after
heat treatment was performed at 200 C for 2 hours. The
polished surface was a section in the sheet thickness
direction, the section being parallel to the rolling
direction. By observing a steel microstructure of the
polished surface with a scanning electron microscope (SEM)
at a magnification of 3000x, the area ratio of each phase
was measured to identify the phase structure of each crystal
grain. The observation was performed for 10 fields and the
area ratio was an average value of the 10 fields. The area
ratios of autotempered martensite, polygonal ferrite, and
bainite were obtained using the test pieces polished without
performing any treatment. The area ratios of as-quenched
martensite (untempered martensite) and retained austenite
were obtained using the test pieces polished after heat
treatment was performed at 200 C for 2 hours. The test
pieces polished after heat treatment was performed at 200 C
for 2 hours were prepared in order to differentiate
untempered martensite from retained austenite in the SEM
observation. In the SEM observation, it is difficult to
CA 02713181 2010-07-26
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differentiate untempered martensite from retained austenite.
When martensite is tempered, an iron-based carbide is formed
in the martensite. The iron-based carbide makes it possible
to differentiate martensite from retained austenite. The
heat treatment at 200 C for 2 hours does not affect the
phases other than martensite, that is, martensite can be
tempered without changing the area ratio of each phase. As
a result, martensite can be differentiated from retained
austenite due to the formed iron-based carbide. By
comparing the test pieces polished without performing any
treatment to the test pieces polished after heat treatment
was performed at 200 C for 2 hours through SEM observation,
it was confirmed that phases other than martensite were not
changed.
The size and number of iron-based carbide grains
included in autotempered martensite were measured through
SEM observation. The test pieces were the same as those
used in the microstructure observation. Obviously, the test
pieces polished without performing any treatment were
observed. The test pieces were observed at a magnification
of 10000x to 30000x in accordance with the precipitation
state and size of the iron-based carbide grains. The size
of the iron-based carbide grains was evaluated using an
average value of the major axis and minor axis of individual
precipitates. The number of iron-based carbide grains each
CA 02713181 2010-07-26
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having a size of 5 nm or more and 0.5 m or less was counted
and thus the number of iron-based carbide grains per 1 mm2
of autotempered martensite was calculated. The observation
was performed for 5 to 20 fields. The mean number was
calculated from the total number of all the fields of each
sample, and the mean number was employed as the number (per
1 mm2 of autotempered martensite) of iron-based carbide
grains of each sample.
The hardness HV of autotempered martensite was measured
using an ultramicro-Vickers hardness meter at a load of 0.02
N. After the microstructure of autotempered martensite in
which iron-based carbide grains were precipitated was
confirmed by observing an indentation with a SEM, the
average value of ten or more measurement values was employed
as the hardness HV.
A tensile test was performed in accordance with JIS
Z2241 using a JIS No. 5 test piece taken from the steel
sheet in the rolling direction of the steel sheet. Tensile
strength (TS), yield strength (YS), and total elongation (T.
El) were measured. The product of the tensile strength and
the total elongation (TS x T. El) was calculated to evaluate
the balance between the strength and the elongation. In the
present invention, when TS x T. El 14500 (MPa.%), the
balance was determined to be satisfactory.
Stretch-flangeability was evaluated in compliance with
CA 02713181 2010-07-26
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The Japan Iron and Steel Federation Standard JFST 1001. The
resulting steel sheet was cut into pieces each having a size
of 100 mm x 100 mm. A hole having a diameter of 10 mm was
made in the piece by punching at a clearance of 12% of the
thickness. A cone punch with a 600 apex was forced into the
hole while the piece was fixed with a die having an inner
diameter of 75 mm at a blank-holding pressure of 88.2 kN.
The diameter of the hole was measured when a crack was
initiated. The maximum hole-expanding ratio (%) was
determined with Formula (2) to evaluate stretch-
flangeability using the maximum hole-expanding ratio:
Maximum hole-expanding ratio X (%) = {(Df - D0) /D0} x 100
=== (2)
where pf represents the hole diameter (mm) when a crack was
initiated, and Do represents an initial hole diameter (mm).
In the present invention, X ?_ 15% was determined to be
satisfactory.
Table 3 shows the evaluation results.
Table 3
Area ratio (%)
Mean hardness Number of iron-
Sample Steel M YS TS
T. El TS x T. El X.
No. type Autotempered
Ferrite Retained As-quenched of autote
basedcarbide grains Remarks
Bainite mpered ase 2 (CC) (mpa)
(mpa) (%) (mpa.%) (%)
martensite austenite martensite martensite (HV)
per 1 mm *1
1 A 57 43 0 0 0 602 1 x 105 332 771
1255 14.8 18574 16 Invention Example _
2 B 72 28 0 0 0 550 1 x 105 346 924 1341
12.0 16092 22 Invention Example _
-
3 C 35 63 0 0 2 661 1 x 104 293 687 1238
13.4 16589 5 Comparative Example
4 C 37 52 4 7 0 601 lx 105 330 660
1220 14.0 17080 21 _ Invention Example _
_ D 83 17 0 0 0 526 5 x 105 361 849 1393
11.1 15462 45 _ Invention Example
6 E 0 72 0 0 28 857 None 261 576 1066 18.8
20041 13 Comparative Example
_
7 F 22 60 1 2 15 771 1 x 103 250 667 1226
14.2 17409 5 Comparative Example_
8 G 58 37 0 0 5 691 5 x 103 281 817 1521
7.5 11408 1 _ Comparative Example n
, 9 H 91 9 0 0 0 492 1 x 106 355 946 1385
10.9 15097 36 Invention Example
I 84 12 1 3 0 503 1 x 106 358 908 1392
11.0 15239 42 Invention Example 0
N)
-.3
11 J 84 16 0 0 0 470 3 x 106 367 772 1270
13.9 17653 35 Invention Example H
u.)
12 J 30 67 0 0 3 667 7 x 104 288 601
1021 17.5 17868 22 Invention Example 1 H
CO
H
13 K 90 10 0 0 0 523 5 x 105 389 903 1449
10.9 15794 32 _ Invention Example
14 L 90 10 0 0 , 0 505 5 x 105 359 916 1418
11.8 16732 34 Invention Example m 0
H
0
M 84 15 , 1 0 0 480 1 x 106 386 883 1305
12.5 16313 49 Invention Example I '
- _
0
16 N 93 7 0 0 0 495 lx 106 357 838 1420 12.4
17608 20 Invention Example
1
17 0 59 39 2 0 0 513 8 x 105 378 617
1125 15.2 17100 25 Invention Example I\)
c7,
18 P 88 12 0 0 0 503 1 x 106 372 900 1389
10.5 14585 36 Invention Example
19 Q 86 14 0 0 _ 0 502 1 x 106
356 837 1371 12.2 16726 24 Invention Example
_ 20 R 18 82 0 0 0 470 4 x 106
420 593 781 20.9 16323 29 Comparative Example
_ 21 S 70 20 , 0 0 10 802 1 x 103
276 1081 1520 9.2 13984 1 Comparative Example
22 T 82 12 0 0 6 790 1 x 103 297 931 1481
9.6 14218 2 Comparative Example_
23 A 49 , 8 1 0 42 563 5 x 105 370 1123 1481
9.7 14365 10 _ Comparative Example
_
24 E 52 48 0 0 0 589 1 x 105 346 763
1197 16.1 19272 18 Invention Example
G 10 27 14 46 0 603 lx 106 304 1008 1423 14.1
20064 4 Comparative Example
26 G 93 7 0 0 0 592 lx 106
334 1118 1635 11.2 18312 19 Invention Example
*1) The size of iron-based carbide grains is 5 rim or more and 0.5 um or less.
*2) Underline means the value is outside the suitable range.
CA 02713181 2010-07-26
- 49 -
As is clear from Table 3, any steel sheet of the
present invention has a tensile strength of 900 MPa or
higher, a value of TS x T. El 14500 (MPa.%), and a value
of X 15% that represents stretch-flangeability and thus
has both high strength and good workability. In Invention
Examples, the steel sheets having an M of 300 C or higher
are excellent in stretch-flangeability, particularly
stretch-flangeability that is not degraded even if strength
is increased.
In contrast, in sample Nos. 6 and 7, the hardness of
martensite is 700 < HV and the number of iron-based carbide
grains included in martensite is less than 5 x 104 per 1 mm2
or martensite does not include iron-based carbide grains.
Therefore, a tensile strength of 900 MPa is satisfied, but a
value of X is less than 15%, which provides poor workability.
This is because, in sample Nos. 6 and 7, the cooling rate in
the third temperature range is high, which does not satisfy
50 C/s. In sample Nos. 3 and 8, the hardness of martensite
is satisfactorily MV 700, but the number of iron-based
carbide grains included in martensite is less than 5 x 104
per 1 mm2. Therefore, a tensile strength of 900 MPa or
higher is satisfied, but a value of X is less than 15%,
which provides poor workability. This is because, in sample
Nos. 3 and 8, the cooling rate in the third temperature
range is 55 C/s, which does not satisfy 50 C/s or lower.
CA 02713181 2010-07-26
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In particular, since sample No. 8 has a relatively high C
content, TS x T. El is 14500 MPa.% or less.
It can be confirmed from the above description that the
steel sheet of the present invention that includes
autotempered martensite sufficiently subjected to
autotempering treatment such that the hardness of martensite
is HV 700 and the number of iron-based carbide grains in
martensite is 5 x 104 or more per 1 mm2 has both high
strength and good workability.
Example 2
To confirm the effect of further improvement in
ductility achieved by suitably controlling the distribution
state of iron-based carbide grains included in autotempered
martensite, samples were manufactured in the same manner as
the samples shown in Table 2, except that the cooling rate
in a temperature range of 250 C or higher and (Ms
temperature - 50) C or lower of the third temperature range
was changed as shown in Table. 4. In Table 4, sample Nos. 9,
11, 13, 14, and 26 are the same as those shown in Table 2
and listed in Table 4 to clarify the temperature range of
250 C or higher and (Ms temperature - 50) C or lower. Note
that M ( C) was used as the Ms temperature.
Table 4
First temperature range Second temperature range Third
temperature range
Sample
Average cooling
Average cooling
9
Steel type Holding Time required for Average
cooling rate from (Ms Plating Remarks
No. Holding time rate from first
temperature cooling from 550 C rate
from 420 C temperature -
(second) temperature range to
( C) to 420 C (second) to 250 C ( C/s) 50) C to 250 C
550 C ( C/s)
( C/s)
9 H 820 180 10 120 15
0.8 GA Invention Example _
0
11 J 830 200 30 60 10
20 CR Invention Example
0
13 K 860 40 10 45 10
0.5 GI Invention Example iv
-.3
- H
u.)
14 L 860 90 10 60 10
0.8 CR Invention Example 1 H
CO
H
26 G 870 150 15 120 3
10 CR Invention Example (i_ ,-,, "
-,0
H
27 H 820 180 10 120 15
30 GA Invention Example I 0
1
- 0
28 J 830 200 30 60 10
0.5 CR Invention Example .-.1
I
"
c7,
29 K 860 40 10 45 10
25 GI Invention Example
30 L 860 90 10 60 10
20 CR Invention Example
, 31 G 870 150 15 120 3
0.4 CR Invention Example
*1) CR: no plating (cold-rolled steel sheet), GI: galvanizing, and GA:
galvannealing
CA 02713181 2010-07-26
- 52 -
The characteristics of the thus-obtained steel sheets
were evaluated in the same manner as in Example 1. Herein,
the amount of autotempered martensite in which the number of
precipitated iron-based carbide grains each having a size of
0.1 m or more and 0.5 m or less is 5 x 102 or less per 1
2 i
mm n the entire autotempered martensite was obtained as
follows.
As described above, the test pieces polished without
performing any treatment were observed at a magnification of
10000x to 30000x using a SEN. The size of the iron-based
carbide grains was evaluated using an average value of the
major axis and minor axis of individual precipitates. The
area ratio of autotempered martensite in which the iron-
based carbide grains have a size of 0.1 m or more and 0.5
m or less was measured. The observation was performed for
to 20 fields.
Table 5 shows the results.
As is apparent from Table 5, in sample Nos. 11, 26, 27,
29, and 30 with a cooling rate of 1.0 C/s or higher and
50 C/s or lower in the temperature range of 250 C or higher
and (Ms temperature - 50) C or lower, the distribution state
of iron-based carbide grains included in autotempered
martensite is suitably controlled and thus TS x T. El >
17000 MPa.% is exhibited, that is, ductility is improved.
Table 5
Area ratio (%) Area ratio of
autotempered
martensite in which
the number of
Number of iron-
Mean hardness of based carbide precipitated iron-
Sample Steel based carbide
M YS TS T. El TS x T. El X.
autotempered grains (5 nm to
Remarks
No. type Autotempered Ferrite Retained
Bainite As-quenched grains (5 nm to
0.5 ( C) (MPa) (MPa) (%) (MPa.%) (%)
martensite austenite martensite martensite (HV)
0.5 1.1m) per 1
pm) is 5 x 102 or
mm2
less per 1 nun2 to
the entire
autotempered
martensite (%)
0
9 H 91 9 0 0 0 492 1 x 106 2
355 946 1385 10.9 15097 36 Invention Example
_
0
11 J 84 16 0 0 0 470 3 x 106 14 367
772 1270 13.9 17653 35 Invention Example
1..)
- -,1
13 K 90 10 0 0 0 523 5x 105 I 389
903 1449 10.9 15794 32 Invention Example H
L...)
-
' I H
14 L 90 10 0 0 0 505 5 x 105 2 359
916 1418 11.8 16732 34 Invention Example op
H
-
oi
26 G 93 7 0 0 0 592 lx 106 16 334
1118 1635 11.2 18312 19 Invention Example w
1..)
-
0
27 H 91 9 0 0 0 564 lx 106 24 355
952 1538 11.1 17072 32 Invention Example I H
0
o1
28 J 84 16 0 0 0 460 3 x 106 2 367
797 1207 13.8 16657 36 Invention Example
I
29 K 90 10 0 0 0 553 5x 105 16 389
907 1524 11.2 17069 29 Invention Example
1..)
0-,
30 , L 90 10 0 0 0 526 5 x 105 14 - 359
918 1457 12.1 17630 30 Invention Example
31 G 93 7 0 0 0 541 lx 106 0 334
1094 1497 11.3 16916 18 Invention Example
