Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
HIGH STRENGTH STEEL SHEET AND METHOD FOR MANUFACTURING THE
SAME
[Technical Field]
[0001]
The present invention relates to a high strength steel
sheet, which is used in industrial fields of automobile,
electric apparatus, and the like and which has excellent
workability, especially elongation and stretch-flangeability,
and a tensile strength (TS) of 980 MPa or more, and a method
for manufacturing the same.
[Background Art]
[0002]
In recent years, enhancement of fuel economy of the
automobile has become an important issue from the viewpoint
of global environmental conservation. Consequently, there
is an active movement afoot to reduce the thicknesses of car
components through increases in strength of car body
materials, so as to reduce the weight of a car body itself.
[0003]
In general, in order to increase the strength of a
. steel sheet, it is necessary to increase the proportion of a
hard phase, e.g., martensite or bainite, relative to a whole
microstructure of the steel sheet. However, the increase in
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strength of the steel sheet through the increase in
proportion of the hard phase causes a reduction in
workability. Therefore, development of a steel sheet having
high strength and excellent workability in combination has
been desired. Heretofore, various complex microstructure
steel sheets, e.g., a ferrite-martensite double phase steel
(DP steel) and a TRIP steel taking the advantage of the
transformation induced plasticity of retained austenite,
have been developed.
[0004]
In the case where the proportion of the hard phase in
the complex microstructure steel sheet increases, the
workability of the steel sheet is affected by the
workability of the hard phase significantly. This is
because in the case where the proportion of the hard phase
is small and that of soft polygonal ferrite is large, the
deformability of polygonal ferrite is predominant over the
workability of the steel sheet, and even in the case where
the workability of the hard phase is inadequate, the
workability, e.g., the elongation, is ensured, whereas in
the case where the proportion of the hard phase is large,
the deformability of the hard phase in itself rather than
deformation of polygonal ferrite exerts an influence
directly on the formability of the steel sheet and,
therefore, if the workability of the hard phase in itself is
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inadequate, deterioration of the workability of the steel
sheet becomes significant.
[0005]
Consequently, as for a cold rolled steel sheet, after
conducting a heat treatment to adjust the amount of
polygonal ferrite generated during annealing and cooling
thereafter, the steel sheet is water-quenched so as to
generate martensite, the temperature is raised again, and
the steel sheet is kept at high temperatures, so that
martensite is tempered, carbides are generated in martensite,
which is a hard phase, and thereby, the workability of
martensite is improved. However, such quenching¨tempering
of martensite needs a specific production facility, for
example, a continuous annealing facility having a water
quenching function. Therefore, in the case where a common
facility is used, in which after the steel sheet is water-
quenched, it is not possible to raise the temperature again
and keep at high temperatures, the strength of the steel
sheet can be increased but the workability of martensite,
which is a hard phase, cannot be improved.
[0006]
Furthermore, as for a steel sheet, in which the hard
phase is other than martensite, there is a steel sheet, in
which a primary phase is specified to be polygonal ferrite,
a hard phase is specified to be bainite and pearlite, and
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carbides are generated in these bainite and pearlite serving
as the hard phase. This steel sheet is a steel sheet, in
which the workability is improved not only by polygonal
ferrite, but also by generating carbides in the hard phase
so as to improve the workability of the hard phase in itself,
and in particular, an improvement of the stretch-
flangeability is intended. However, since the primary phase
is specified to be polygonal ferrite, it is difficult to
allow an increase in strength to 980 MPa or more in terms of
tensile strength (TS) and the workability to become mutually
compatible. In this connection, even when the workability
of the hard phase in itself is improved by generating
carbides in the hard phase, the level of workability is
inferior to that of polygonal ferrite. Therefore, if the
amount of polygonal ferrite is reduced in order to increase
the strength to 980 MPa or more in terms of tensile strength
(TS), adequate workability cannot be obtained.
[0007]
Patent Document 1 proposes a high strength steel sheet
having excellent bendability and impact characteristic,
wherein alloy components are specified and the steel
microstructure is specified to be fine uniform bainite
including retained austenite.
[0008]
Patent Document 2 proposes a complex microstructure
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steel sheet having excellent bake hardenability, wherein
predetermined alloy components are specified, the steel
microstructure is specified to be bainite including retained
austenite, and the amount of retained austenite in the
bainite is specified.
[0009]
Patent Document 3 proposes a complex microstructure
steel sheet having excellent impact resistance, wherein
predetermined alloy components are specified, the steel
microstructure is specified in such a way that bainite
including retained austenite is 90% or more in terms of area
percentage and the amount of austenite in the bainite is 1%
or more, and 15% or less, and the hardness (HV) of the
bainite is specified.
[Prior Art Documents]
[Patent Documents]
[0010]
[Patent Document 1] Japanese Unexamined Patent
Application Publication No. 4-235253
[Patent Document 2] Japanese Unexamined Patent
Application Publication No. 2004-76114
[Patent Document 3] Japanese Unexamined Patent
Application Publication No. 11-256273
[Disclosure of Invention]
[Problems to be Solved by the Invention]
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[0011]
However, the above-described steel sheets have problems
as described below.
Regarding the component composition described in Patent
Document 1, it is difficult to ensure the amount of stable
retained austenite, which exerts a TRIP effect in a high
strain region in the case where a strain is applied to a
steel sheet. Therefore, although the bendability is
obtained, the elongation is low when the plasticity becomes
unstable, and the punch stretchability is poor.
[0012]
Regarding the steel sheet described in Patent Document
2, the bake hardenability is obtained. However, in the case
where an increase in strength is intended in such a way that
the tensile strength (TS) becomes 980 MPa or more, or
furthermore, 1,050 MPa or more, it is difficult to ensure
the strength or ensure the workability, e.g., the elongation
and the stretch-flangeability, when the strength increases
because the microstructure primarily contains bainite and,
furthermore, ferrite while martensite is minimized.
[0013]
The steel sheet described in Patent Document 3 is for
the purpose of improving the impact resistance, and the
microstructure contains bainite having a hardness of HV 250
or less as a primary phase, specifically at a content
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exceeding 90%. Therefore, it is difficult to make the
tensile strength (TS) 980 MPa or more.
[0014]
The present invention solves the above-described
problems advantageously. Accordingly, it is an object to
provide a high strength steel sheet having excellent
workability, especially the elongation and the stretch-
flangeability, and a tensile strength (TS) of 980 MPa or
more, as well as an advantageous method for manufacturing
the same.
The high strength steel sheets according to the present
invention include a steel sheet, in which galvanizing or
galvannealing is applied to a surface of the steel sheet.
Incidentally, in the present invention, excellent
workability refers to that the value of TS x T.EL satisfies
20,000 MPa.% or more and the value of TS x A, satisfies
25,000 MPa.% or more. In this regard, TS represents a
tensile strength (MPa), T.EL represents total elongation (%),
and k represents a hole-expansion limit (%).
[Means for Solving the Problems]
[0015]
In order to solve the above-described problems, the
present inventors conducted intensive research on the
component composition and the microstructure of a steel
sheet. As a result, it was found that the strength was
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increased through the use of a lower bainite microstructure
and/or a martensite microstructure, stable retained austenite,
which was advantageous to obtain a TRIP effect, was able to be
ensured through the use of upper bainite transformation while the
C content was increased in such a way that the amount of C in the
steel sheet became 0.17% or more, a part of martensite was
converted to tempered martensite and, thereby, a high strength
steel sheet having excellent workability, especially a balance
between the strength and the elongation and a balance between the
strength and the stretch-flangeability in combination, and a
tensile strength of 980 MPa or more was obtained.
[0016]
The present invention is based on the above-described
findings, and the gist and the configuration thereof are as
described below.
1. A high strength steel sheet having a tensile strength of
980 MPa or more characterized by having a composition
comprising, on a percent by mass basis,
C: 0.17% or more, and 0.73% or less;
Si: 3.0% or less;
Mn: 0.5% or more, and 3.0% or less;
P: 0.1% or less;
S: 0.07% or less;
Al: 3.0% or less; and
N: 0.010% or less,
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while it is satisfied that Si + Al is 0.7% or more, and the
remainder includes Fe and incidental impurities,
wherein regarding the steel sheet microstructure, it is
satisfied that the area percentage of a total amount of lower
bainite and whole martensite is 10% or more, and 90% or less
relative to the whole steel sheet microstructure, the amount
of retained austenite is 5% or more, and 50% or less, the area
percentage of bainitic ferrite in upper bainite is 5% or more
relative to the whole steel sheet microstructure, as-quenched
martensite is 75% or less of the total amount of lower bainite
and whole martensite, and the area percentage of polygonal
ferrite is 10% or less, including 0%, relative to the whole
steel sheet microstructure, and the average amount of C in the
retained austenite is 0.70% or more.
[0017]
2. The high strength steel sheet having a tensile strength of
980 MPa or more according to item 1, characterized in that
the steel sheet further comprises at least one type of
element selected from, on a percent by mass basis,
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.
[0018]
3. The high strength steel sheet having a tensile strength of
980 MPa or more according to item 1 or item 2, characterized
in that
the steel sheet further comprises at least one type of
element selected from, on a percent by mass basis,
Ti: 0.01% or more, and 0.1% or less; and
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Nb: 0.01% or more, and 0.1% or less.
[0019]
4. The high strength steel sheet having a tensile strength of
980 MPa or more according to any one of items 1 to 3,
characterized in that
the steel sheet further comprises, on a percent by mass
basis,
B: 0.0003% or more, and 0.0050% or less.
[0020]
5. The high strength steel sheet having a tensile strength of
980 MPa or more according to any one of items 1 to 4,
characterized in that
the steel sheet further comprises at least one type of
element selected from, on a percent by mass basis,
Ni: 0.05% or more, and 2.0% or less; and
Cu: 0.05% or more, and 2.0% or less.
[0021]
6. The high strength steel sheet having a tensile strength of
980 MPa or more according to any one of items 1 to 5,
characterized in that
the steel sheet further comprises at least one type of
element selected from, on a percent by mass basis,
Ca: 0.001% or more, and 0.005% or less; and
REM: 0.001% or more, and 0.005% or less.
[0022]
7. A high strength steel sheet having a tensile strength of
980 MPa or more characterized by comprising a galvanized layer
or a galvannealed layer on a surface of the steel sheet
according to any one of items 1 to 6.
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[0023]
8. A method for manufacturing a high strength steel sheet
having a tensile strength of 980 MPa or more, characterized by
comprising the steps of hot-rolling a billet having a
component composition according to any one of items 1 to 6,
conducting cold-rolling so as to produce a cold-rolled steel
sheet, annealing the resulting cold-rolled steel sheet for 15
seconds or more, and 600 seconds or less in an austenite
single phase region and, thereafter, conducting cooling to a
cooling termination temperature: T C determined in a first
temperature range of 350 C or higher, and 490 C or lower,
wherein cooling to at least 550 C is conducted while the
average cooling rate is controlled at 5 C/s or more,
subsequently, the steel sheet is held in the first temperature
range for 15 seconds or more, and 1,000 seconds or less and,
then, the steel sheet is held in a second temperature range of
200 C or higher, and 350 C or lower for 15 seconds or more,
and 1,000 seconds or less.
[0024]
9. The method for manufacturing a high strength steel sheet
having a tensile strength of 980 MPa or more according to item
8, characterized in that a galvanizing treatment or a
galvannealing treatment is applied during cooling to the
cooling termination temperature: T C or in the first
temperature range.
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[Advantages]
[0025]
According to the present invention, a high strength steel
sheet having excellent workability, especially the elongation and
the stretch-flangeability, and a tensile strength (TS) of 980 MPa
or more, as well as an advantageous method for manufacturing the
same can be provided. Therefore, the utility value in industrial
fields of automobile, electric, and the like is very large, and
in particular, the usefulness in weight reduction of an
automobile body is significant.
[Brief Description of Drawing]
[0026]
[Fig. 1] Fig. 1 is a diagram showing a temperature pattern of a
heat treatment in a manufacturing method according to
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the present invention.
[Best Modes for Carrying Out the Invention]
[0027]
The present invention will be specifically described
below.
Initially, the reason for limitation of the steel sheet
microstructure in such a way that described above in the
present invention will be described. Hereafter the area
percentage refers to an area percentage relative to the
whole steel sheet microstructure.
[0028]
Area percentage of total amount of lower bainite and whole
martensite: 10% or more, and 90% or less
The lower bainite and the martensite are
microstructures necessary for increasing the strength of the
steel sheet. If the area percentage of a total amount of
lower bainite and whole martensite is less than 10%, the
steel sheet does not satisfy the tensile strength (TS) of
980 MPa or more. On the other hand, if the total amount of
lower bainite and whole martensite exceeds 90%, the upper
bainite is reduced and, as a result, stable retained
austenite, in which C is concentrated, cannot be ensured.
Consequently, a problem occurs in that the workability, e.g.,
elongation, deteriorates. Therefore, the area percentage of
the total amount of lower bainite and whole martensite is
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specified to be 10% or more, and 90% or less. A preferable
range is 20% or more, and 80% or less. A more preferable
range is 30% or more, and 70% or less.
[0029]
Proportion of as-quenched martensite in total amount of
lower bainite and whole martensite: 75% or less
If the proportion of as-quenched martensite in the
martensite exceeds 75% of the total amount of lower bainite
and whole martensite present in the steel sheet, the tensile
strength becomes 980 MPa or more, but the stretch-
flangeability is poor. The as-quenched martensite is very
hard, and the deformability of the as-quenched martensite in
itself is very low. Therefore, the workability, especially
stretch-flangeability, of the steel sheet deteriorates
significantly. Furthermore, since the difference in
hardness between the as-quenched martensite and the upper
bainite is significantly large, if the amount of as-quenched
martensite is large, the interface between the as-quenched
martensite and the upper bainite increases. Consequently,
fine voids are generated at the interface between the as-
quenched martensite and the upper bainite during punching or
the like, and in stretch-flange forming conducted after the
punching, voids are coupled to each other, so that cracking
develops easily and, thereby, stretch-flangeability
deteriorates. Therefore, the proportion of as-quenched
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martensite in the martensite is specified to be 75% or less
relative to the total amount of lower bainite and whole
martensite present in the steel sheet. Preferably, the
proportion is 50% or less. In this regard, the as-quenched
martensite is a microstructure, in which no carbide is
detected in the martensite, and can be observed with SEM.
[0030]
Amount of retained austenite: 5% or more, and 50% or less
The retained austenite undergoes martensitic
transformation through a TRIP effect during working and,
thereby, strain dispersive power is enhanced so as to
improve the elongation.
Regarding the steel sheet according to the present
invention, in particular, retained austenite, in which the
amount of concentrated C is increased, is formed in the
upper bainite through the use of upper bainite
transformation. As a result, retained austenite capable of
making the TRIP effect apparent even in a high strain region
during working can be obtained. In the case where such
retained austenite and martensite are present in combination
and used, good workability is obtained even in a high
strength region, in which the tensile strength (TS) is 980
MPa or more. Specifically, the value of TS x T.E1 can be
made 20,000 MPa-% or more, and a steel sheet having an
excellent balance between the strength and the elongation
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can be obtained.
Here, since the retained austenite in the upper bainite
is formed between laths of bainitic ferrite in the upper
bainite and distributes finely, large amounts of measurement
at high magnification is necessary for determination of the
amount (area percentage) thereof through microstructure
observation, and it is difficult to quantify accurately.
However, the amount of retained austenite formed between
laths of the bainitic ferrite is an amount corresponding to
the amount of formed bainitic ferrite to some extent. Then,
the present inventors conducted research. As a result, it
was found that an adequate TRIP effect was able to be
obtained and the tensile strength (TS) of 980 MPa or more
and TS x T.E1 of 20,000 MPa.% or more were able to be
achieved if the area percentage of bainitic ferrite in the
upper bainite was 5% or more, and the amount of retained
austenite determined by an intensity measurement with X-ray
diffraction (XRD), which was a previously employed technique -
to measure the amount of retained austenite, specifically,
an X-ray diffraction intensity ratio of ferrite to austenite,
was 5% or more. In this regard, it has been ascertained
that the amount of retained austenite determined by the
previously employed technique to measure the amount of
retained austenite is equivalent to the area percentage of
retained austenite relative to the whole steel sheet
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microstructure.
In the case where the amount of retained austenite is
less than 5%, an adequate TRIP effect is not obtained. On
the other hand, if the amount exceeds 50%, hard martensite
generated after the TRIP effect is made apparent becomes
excessive, deterioration of tenacity and the like become
problems. Therefore, the amount of retained austenite is
specified to be within the range of 5% or more, and 50% or
less. The range is preferably more than 5%, and more
preferably within the range of 10% or more, and 45% or less.
The range is further preferably within the range of 15% or
more, and 40% or less.
[0031]
Average amount of C in retained austenite: 0.70% or more
Regarding a high strength steel sheet having a tensile
strength (TS) of 980 MPa to 2.5 GPa class, in order to
obtain excellent workability through the use of the TRIP
effect, the amount of C in the retained austenite is
important. In the steel sheet according to the present
invention, C is concentrated into the retained austenite
formed between laths of bainitic ferrite in the upper
bainite. It is difficult to accurately evaluate the amount
of C concentrated into the retained austenite between the
laths. However, as a result of research of the present
inventors, it was found that excellent workability was
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obtained in the present invention when the average amount of
C in the retained austenite determined from the amount of
shift of a diffraction peak in the X-ray diffraction (XRD),
which was a previously employed method for measuring the
average amount of C in the retained austenite (an average of
the amount of C in the retained austenite), was 0.70% or
more.
In the case where the average amount of C in the
retained austenite is less than 0.70%, martensitic
transformation occurs in a low strain region during working,
so that the TRIP effect in a high strain region to improve
the workability is not obtained. Therefore, the average
amount of C in the retained austenite is specified to be
0.70% or more. The amount is preferably 0.90% or more. On
the other hand, if the average amount of C in the retained
austenite exceeds 2.00%, the retained austenite becomes
excessively stable, martensitic transformation does not
occur during working, and the TRIP effect is not made
apparent, so that the elongation deteriorates. Therefore,
it is preferable that the average amount of C in the
retained austenite is specified to be 2.00% or less. more
preferably, the average amount is 1.50% or less.
[0032]
Area percentage of bainitic ferrite in upper bainite: 5% or
more
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Generation of bainitic ferrite due to upper bainite
transformation is necessary for concentrating C in
untransformed austenite so as to obtain retained austenite,
which makes the TRIP effect apparent in a high strain region
during working and which enhances strain resolution. The
transformation from austenite to bainite occurs over a wide
temperature range of about 150 C to 550 C, and bainite
generated in this temperature range include various types.
In many cases in the previous technology, such various types
of bainite has been specified as bainite simply. However,
in order to obtain the workability desired in the present
invention, it is necessary that the bainite microstructure
is specified clearly. Therefore, the upper bainite and the
lower bainite are defined as described below.
The upper bainite is characterized in that lath-shaped
bainitic ferrite and retained austenite and/or carbides
present between bainitic ferrite are included and fine
carbides regularly arranged in the lath-shaped bainitic
ferrite are not present. On the other hand, the lower
bainite is characterized in that lath-shaped bainitic
ferrite and retained austenite and/or carbides present
between bainitic ferrite are included, as is common to the
upper bainite, and in the lower bainite, fine carbides
regularly arranged in the lath-shaped bainitic ferrite are
present.
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That is, the upper bainite and the lower bainite are
distinguished on the basis of presence or absence of fine
carbides regularly arranged in the bainitic ferrite. The
above-described difference in the generation state of
carbides in the bainitic ferrite exerts a significant
influence on concentration of C into the retained austenite.
That is, in the case where the area percentage of bainitic
ferrite in the upper bainite is less than 5%, even when
bainite transformation proceeds, the amount of C formed into
carbides in the bainitic ferrite increases. As a result,
the amount of concentration of C into the retained austenite
present between laths decreases, and a problem occurs in
that the amount of retained austenite, which exerts the TRIP
effect in a high strain region during working, decreases.
Therefore, it is necessary that the area percentage of
bainitic ferrite in the upper bainite is 5% or more in terms
of area percentage relative to the whole steel sheet
microstructure. On the other hand, if the area percentage
of bainitic ferrite in the upper bainite exceeds 85%
relative to the whole steel sheet microstructure, it may
become difficult to ensure the strength. Consequently, it
is preferable that the area percentage is specified to be
85% or less.
[0033]
Area percentage of polygonal ferrite: 10% or less (including
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0%)
If the area percentage of polygonal ferrite exceeds 10%,
it becomes difficult to satisfy the tensile strength (TS):
980 MPa or more and, at the same time, strain is
concentrated on soft polygonal ferrite present together in
the hard microstructure during working, so that cracking
occurs easily during working. As a result, a desired
workability is not obtained. Here, if the area percentage
of the polygonal ferrite is 10% or less, even when the
polygonal ferrite is present, a state, in which a small
amount of polygonal ferrite is discretely dispersed in a
hard phase, is brought about, concentration of strain can be
suppressed, and deterioration of the workability can be
avoided. Therefore, the area percentage of the polygonal
ferrite is specified to be 10% or less. The area percentage
is preferably 5% or less, further preferably 3% or less, and
may be 0%.
[0034]
Incidentally, regarding the steel sheet according to
the present invention, the hardness of the hardest
microstructure in the steel sheet microstructure is HV < 800.
That is, in the case where as-quenched martensite is not
present in the steel sheet according to the present
invention, any one of the tempered martensite, the lower
bainite, and the upper bainite becomes the hardest phase.
'
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All of these microstructures are phases which become HV 800.
Alternatively, in the case where as-quenched martensite is
present, the as-quenched martensite becomes the hardest
microstructure. Regarding the as-quenched martensite in the steel
sheet according to the present invention, the hardness becomes HV
800, a significantly hard martensite exhibiting HV > 800 is not
present, and good stretch-flangeability can be ensured.
[0035]
The steel sheet according to the present invention may
include pearlite, and Widmanstaetten ferrite as the remainder
microstructure. In that case, it is preferable that the allowable
content of the remainder microstructure is specified to be 20% or
less in terms of area percentage. More preferably, the allowable
content is 10% or less.
[0036]
The basic configuration of the steel sheet microstructure of
the high strength steel sheet according to the present invention
is as described above, and the following configuration may be
added as necessary.
[0037]
Next, the reason for limitation of component composition of
the steel sheet in such a way that described above in the present
invention will be described. In this
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connection, % hereafter representing the following component
composition refers to percent by mass.
C: 0.17% or more, and 0.73% or less
The element C is an indispensable element to increase
the strength of the steel sheet and ensure the amount of
stable retained austenite, and an element necessary to
ensure the amount of martensite and retain austenite at room
temperature. If the amount of C is less than 0.17%, it is
difficult to ensure the strength and the workability of the
steel sheet. On the other hand, if the amount of C exceeds
0.73%, hardening of a welded zone and a heat-affected zone
is significant, so that the weldability deteriorates.
Therefore, the amount of C is specified to be within the
range of 0.17% or more, and 0.73% or less. The range is
preferably within the range of more than 0.20%, and 0.48% or
less, and further preferably 0.25% or more.
[0038]
Si: 3.0% or less (including 0%)
The element Si is a useful element, which contributes
to an improvement in the strength of steel by strengthening
through solid solution. However, if the amount of Si
exceeds 3.0%, an increase in the amount of solid solution
into the polygonal ferrite and the bainitic ferrite causes
deterioration of the workability and the tenacity, and
causes deterioration of surface characteristics due to
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occurrence of red scale and the like and deterioration of
the wettability and the adhesion of the coating in the case
where hot dipping is applied. Therefore, the amount of Si
is specified to be 3.0% or less. The amount is preferably
2.6% or less. The amount is further preferably 2.2% or less.
Moreover, Si is an element useful for suppressing
generation of carbides and facilitating generation of
retained austenite. Therefore, it is preferable that the
amount of Si is specified to be 0.5% or more. However, in
the case where generation of carbides is suppressed by
merely Al, Si is not necessarily added, and amount of Si may
be 0%.
[0039]
Mn: 0.5% or more, and 3.0% or less
The element Mn is an element useful for strengthening
steel. If the amount of Mn is less than 0.5%, carbides are
deposited in a temperature range higher than the temperature,
at which bainite and martensite are generated, during
cooling after annealing. Consequently, it is not possible
to ensure the amount of hard phase, which contributes to
strengthening of steel. On the other hand, the amount of Mn
exceeding 3.0% causes deterioration of castability and the
like. Therefore, the amount of Mn is specified to be within
the range of 0.5% or more, and 3.0% or less. The range is
preferably 1.5% or more, and 2.5% or less.
CA 02734978 2011-02-21
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[0040]
P: 0.1% or less
The element P is an element useful for strengthening
steel. If the amount of P exceeds 0.1%, the impact
resistance deteriorates due to embrittlement based on grain
boundary segregation, and in the case where galvannealing is
applied to a steel sheet, the alloying rate is reduced
significantly. Therefore, the amount of P is specified to
be 0.1% or less. The amount is preferably 0.05% or less. In
this connection, it is preferable that,the amount of P is
reduced. However, reduction to less than 0.005% causes a
significant increase in cost. Therefore, it is preferable
that the lower limit thereof is specified to be about 0.005%.
[0041]
S: 0.07% or less
The element S generates MnS so as to become an
inclusion and causes deterioration of the impact resistance
and cracking along a metal flow of a welded zone. Therefore,
it is preferable that the amount of S is minimized. However,
since excessive reduction in the amount of S causes an
increase in a production cost, the amount of S is specified
to be 0.07% or less. Preferably, the amount is 0.05% or
less, and more preferably 0.01% or less. In this connection,
reduction of S to less than 0.0005% is attended with a
significant increase in production cost. Therefore, the
CA 02734978 2011-02-21
- 26 -
lower limit thereof is about 0.0005% from the viewpoint of
the production cost.
[0042]
Al: 3.0% or less
The element Al is an element useful for strengthening
steel and, in addition, is a useful element, which is added
as a deoxidizing agent in a steel making process. If the
amount of Al exceeds 3.0%, inclusion in a steel sheet
increases and the elongation deteriorates. Therefore, the
amount of Al is specified to be 3.0% or less. The amount is
preferably 2.0% or less.
Moreover, Al is an element useful for suppressing
generation of carbides and facilitating generation of
retained austenite. Furthermore, it is preferable that the
amount of Al is specified to be 0.001% or more in order to
obtain a deoxidation effect, and more preferably 0.005% or
more. In this regard, the amount of Al in the present
invention is the amount of Al contained in the steel sheet
after deoxidation.
[0043]
N: 0.010% or less
The element N is an element, which causes maximum
deterioration of the aging resistance of steel, and is
preferably minimized. If the amount of N exceeds 0.010%,
deterioration of the aging resistance becomes significant
CA 02734978 2011-02-21
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and, therefore, the amount of N is specified to be 0.010% or
less. In this connection, reduction of N to less than
0.001% causes a significant increase in production cost, so
=
that the lower limit thereof is about 0.001% from the
viewpoint of the production cost.
[0044]
Up to this point, the basic components have been
described. However, in the present invention, only
satisfaction of the above-described component ranges is not
adequate, and it is necessary that the following formula is
satisfied.
Si + Al 0.7%
As described above, both Si and Al are elements useful
for suppressing generation of carbides and facilitating
generation of retained austenite. Regarding suppression of
generation of carbides, an effect is exerted by containing
Si or Al alone, but it is necessary to satisfy that a total
of the amount of Si and the amount of Al is 0.7% or more.
In this connection, the amount of Al in the above-described
formula is the amount of Al contained in the steel sheet
after deoxidation.
[0045]
In addition, in the present invention, the components
described below can be contained appropriately besides the
above-described basic components.
CA 02734978 2011-02-21
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,
At least one type 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
The elements Cr, V, and Mo are elements having a
function of suppressing generation of pearlite during
cooling from an annealing temperature. The effect thereof
is obtained at Cr: 0.05% or more, V: 0.005% or more, and Mo:
0.005% or more. On the other hand, if Cr: 5.0%, V: 1.0%,
and Mo: 0.5% are exceeded, the amount of hard martensite
becomes too large, and the strength becomes high more than
necessary. Therefore, in the case where Cr, V, and Mo are
contained, the ranges are specified to be 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.
[0046]
At least one type selected from Ti: 0.01% or more, and 0.1%
or less and Nb: 0.01% or more, and 0.1% or less
The elements Ti and Nb are elements useful for
strengthening steel through deposition, and the effect
thereof is obtained when the individual contents are 0.01%
or more. On the other hand, if the individual contents
exceed 0.1%, the workability and the shape fixability
deteriorate. Therefore, in the case where Ti and Nb are
contained, the ranges are specified to be Ti: 0.01% or more,
and 0.1% or less and Nb: 0.01% or more, and 0.1% or less.
CA 02734978 2011-02-21
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[0047]
B: 0.0003% or more, and 0.0050% or less
The element B is an element useful for suppressing
generation.growth of ferrite from austenite grain boundaries.
The effect thereof is obtained when the content is 0.0003%
or more. On the other hand, if the content exceeds 0.0050%,
the workability deteriorates. Therefore, in the case where
B is contained, the range is specified to be B: th0003% or
more, and 0.0050% or less.
[0048]
At least one type selected from Ni: 0.05% or more, and 2.0%
or less and Cu: 0.05% or more, and 2.0% or less
The elements Ni and Cu are elements useful for
strengthening steel. Furthermore, in the case where
galvanizing or galvannealing is applied to a steel sheet,
internal oxidation of a steel sheet surface layer portion is
facilitated and, thereby, adhesion of the coating is
improved. These effects are obtained when individual
contents are 0.05% or more. On the other hand, if the
individual contents exceed 2.0%, the workability of the
steel sheet deteriorates. Therefore, in the case where Ni
and Cu are contained, the ranges are specified to be Ni:
0.05% or more, and 2.0% or less and Cu: 0.05% or more, and
2.0% or less.
[0049]
CA 02734978 2011-02-21
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At least one type selected from Ca: 0.001% or more, and
0.005% or less and REM: 0.001% or more, and 0.005% or less
The elements Ca and REM are useful for spheroidizing
the shape of sulfides and improve the adverse effect of
sulfides on the stretch-flangeability. The effects thereof
are obtained when individual contents are 0.001% or more.
On the other hand, if the individual contents exceed 0.005%,
increases of inclusion and the like are invited so as to
cause surface defects, internal defects, and the like.
Therefore, in the case where Ca and REM are contained, the
ranges are specified to be Ca: 0.001% or more, and 0.005% or
less and REM: 0.001% or more, and 0.005% or less.
[0050]
In the steel sheet according to the present invention,
the components other than those described above are Fe and
incidental impurities. However, components other than those
described above may be contained within the bounds of not
impairing the effects of the present invention.
[0051]
Next, a method for manufacturing a high strength steel
sheet according to the present invention will be described.
After a billet adjusted to have the above-described
favorable component composition is produced, hot-rolling is
conducted and, then, cold-rolling is conducted so as to
produce a cold-rolled steel sheet. In the present invention,
CA 02734978 2011-02-21
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these treatments are not specifically limited and may be
conducted following usual methods.
Favorable production conditions are as described below.
After the billet is heated to a temperature within the range
of 1,000 C or higher, and 1,300 C or lower, the hot rolling
is terminated in a temperature range of 870 C or higher, and
950 C or lower. The resulting hot-rolled steel sheet is
taken up in a temperature range of 350 C or higher, and
720 C or lower. Subsequently, the hot-rolled steel sheet is
pickled and, thereafter, cold-rolling is conducted at a
reduction ratio within the range of 40% or more, and 90% or
less, so as to produce a cold-rolled steel sheet.
In this connection, in the present invention, it is
assumed that the steel sheet is produced through usual
individual steps of steel making, casting, hot rolling,
pickling, and cold rolling. However, for example,
production may be conducted through thin slab casting or
strip casting while a part of or an entire hot rolling step
is omitted.
[0052]
A heat treatment shown in Fig. 1 is applied to the
resulting cold-rolled steel sheet. The explanation will be
conducted below with reference to Fig. 1.
Annealing is conducted for 15 seconds or more, and 600
seconds or less in an austenite single phase region. The
CA 02734978 2011-02-21
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steel sheet according to the present invention contains
upper bainite, lower bainite, and martensite, which are
transformed from untransformed austenite in a relatively low
temperature range of 350 C or higher, and 490 C or lower, as
primary phases. Therefore, it is preferable that polygonal
ferrite is minimized, and annealing in an austenite single
phase region is required. The annealing temperature is not
specifically limited insofar as it is in the austenite
single phase region. If the annealing temperature exceeds
1,000 C, growth of austenite grains is significant, coarser
configuration phases are generated by downstream cooling,
and the tenacity and the like deteriorate. On the other
hand, in the case where the annealing temperature is lower
than A3 point (austenite transformation point), polygonal
ferrite has already been generated in an annealing stage,
and it becomes necessary that a temperature range of 500 C
or more is cooled very rapidly in order to suppress growth
of polygonal ferrite during cooling. Therefore, it is
necessary that the annealing temperature is specified to be
the A3 point or higher, and preferably, 1,000 C or lower.
Furthermore, if the annealing time is less than 15
seconds, in some cases, reverse transformation to austenite
does not proceed adequately or carbides in the steel sheet
are not dissolved adequately. On the other hand, if the
annealing time exceeds 600 seconds, an increase in cost is
CA 02734978 2011-02-21
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invited along with high energy consumption. Therefore, the
annealing time is specified to be within the range of 15
seconds or more, and 600 seconds or less. Preferably, the
annealing time is within the range of 60 seconds or more,
and 500 seconds or less. Here, the A3 point can be
calculated on the basis of
A3 point ( C) = 910 - 203 x [C%]1/2 + 44.7 x [Si%] - 30 x
[Mn%] + 700 x [P%] + 130 x [Al%] - 15.2 x [Ni%] - 11 x [Cr%]
- 20 x [Cu%] + 31.5 x [Mo%] + 104 x [V%] + 400 x [Ti%]
In this connection, [X%] represents percent by mass of
component element X of the steel sheet.
[0053]
The cold-rolled steel sheet after annealing is cooled
to a cooling termination temperature: T C determined in a
first temperature range of 350 C or higher, and 490 C or
lower, wherein cooling to at least 550 C is conducted while
the average cooling rate is controlled at 5 C/s or more. In
the case where the average cooling rate is less than 5 C/s,
excessive generation and growth of polygonal ferrite,
deposition of pearlite, and the like occur, so that a
desired steel sheet microstructure is not obtained.
Therefore, the average cooling rate from the annealing
temperature to the first temperature range is specified to
be 5 C/s or more. Preferably, the average cooling rate is
C/s or more. The upper limit of the average cooling rate
CA 02734978 2011-02-21
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is not specifically limited insofar as variations do not
occur in the cooling termination temperature. Regarding
general facility, if the average cooling rate exceeds
100 C/s, variations in microstructure in a longitudinal
direction and a sheet width direction of a steel sheet
becomes large significantly. Therefore, 100 C/s or less is
preferable.
r0054]
The steel sheet cooled to 550 C is cooled succeedingly
to the cooling termination temperature: T C. The rate of
cooling of the steel sheet in the temperature range of T C
or higher, and 550 C or lower is not specifically limited
except that a keeping time in the first keeping temperature
range is specified to be 15 seconds or more, and 1,000
seconds or less. However, in the case where the steel sheet
is cooled at a too low rate, carbides are generated from
untransformed austenite and, thereby, there is a high
probability that a desired microstructure is not obtained.
Therefore, it is preferable that the steel sheet is cooled
at an average rate of 1 C/s or more in a temperature range
of T C or higher, and 550 C or lower.
[0055]
The steel sheet cooled to the cooling termination
temperature: T C is kept in the first temperature range of
350 C or higher, and 490 C or lower for a period of 15
CA 02734978 2011-02-21
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seconds or more, and 1,000 seconds or less. If the upper
limit of the first temperature range exceeds 490 C, carbides
are deposited from the untransformed austenite and, thereby,
a desired microstructure is not obtained. On the other hand,
in the case where the lower limit of the first temperature
range is lower than 350 C, a problem occurs in that lower
bainite is generated rather than upper bainite and the
amount of C concentrated into austenite is reduced.
Therefore, the first temperature range is specified to be
within the range of 350 C or higher, and 490 C or lower.
Preferably, the range is 370 C or higher, and 460 C or lower.
Moreover, in the case where the keeping time in the
first temperature range is less than 15 seconds, a problem
occurs in that the amount of upper bainite transformation is
reduced and the amount of C concentrated into untransformed
austenite is reduced. On the other hand, in the case where
the keeping time in the first temperature range exceeds
1,000 seconds, carbides are deposited from untransformed
austenite which serves as retained austenite in the final
microstructure of the steel sheet, stable retained austenite,
into which C has been concentrated, is not obtained and, as
a result, a desired workability is not obtained. Therefore,
the keeping time is specified to be 15 seconds or more, and
1,000 seconds or less. preferably, the range is 30 seconds
or more, and 600 seconds or less.
CA 02734978 2011-02-21
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[0056]
After keeping in the first temperature range is
completed, the resulting steel sheet is cooled to a second
temperature range of 200 C or higher, and 350 C or lower at
any rate and is kept in the second temperature range for a
period of 15 seconds or more, and 1,000 seconds or less. If
the upper limit of the second temperature range exceeds
350 C, a problem occurs in that lower bainite transformation
does not proceed and, as a result, the amount of as-quenched
martensite increases. On the other hand, in the case where
the lower limit of the second temperature range is lower
than 200 C as well, a problem occurs in that lower bainite
transformation does not proceed and the amount of as-
quenched martensite increases. Therefore, the second
= temperature range is specified to be within the range of
200 C or higher, and 350 C or lower. Preferably, the range
is 250 C or higher, and 340 C or lower.
Moreover, in the case where the keeping time is less
than 15 seconds, an adequate amount of lower bainite is not
obtained, and desired workability is not obtained. On the
other hand, in the case where the keeping time exceeds 1,000
seconds, carbides are deposited from the stable retained
austenite in the upper bainite generated in the first
temperature range and, as a result, desired workability is
not obtained. Therefore, the keeping time is specified to
CA 02734978 2011-02-21
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be 15 seconds or more, and 1,000 seconds or less.
preferably, the range is 30 seconds or more, and 600 seconds
or less.
[0057]
In this regard, in a series of heat treatments
according to the present invention, the keeping temperature
is not necessarily a constant insofar as the keeping
temperature is within the above-described predetermined
temperature range, and fluctuation within a predetermined
temperature range does not impair the gist of the present
invention. The same goes for the cooling rate. Furthermore,
The steel sheet may be heat-treated with any facility
insofar as only the thermal history is satisfied. In
addition, the scope of the present invention includes that
temper rolling is applied to the surface of the steel sheet
or a surface treatment, e.g., electroplating, is applied
after the heat treatment in order to correct the shape.
[0058]
The method for manufacturing a high strength steel
sheet according to the present invention can further include
a galvanizing treatment or a galvannealing treatment, in
which an alloying treatment is further added to the
galvanizing treatment. The galvanizing treatment or,
furthermore, the galvannealing treatment may be conducted
during the above-described cooling to the first temperature
CA 02734978 2011-02-21
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range or in the first temperature range. In this case, the
keeping time in the first temperature range is specified to
be 15 seconds or more, and 1,000 seconds or less, in which a
treatment time of the galvanizing treatment or the
galvannealing treatment in the first temperature range is
included. In this connection, it is preferable that the
galvanizing treatment or the galvannealing treatment is
conducted with a continuous galvanizing and galvannealing
line.
[0059]
Furthermore, the method for manufacturing a high
strength steel sheet according to the present invention can
include that the high strength steel sheet is produced
following the above-described manufacturing method according
to the present invention, where steps up to the heat
treatment have been completed, and thereafter, the
galvanizing treatment or, furthermore, the galvannealing
treatment is conducted.
Alternatively, after the keeping in the second
temperature range following the manufacturing method
according to the present invention, the galvanizing
treatment or the galvannealing treatment can be conducted
succeedingly.
[0060]
A method for applying a galvanizing treatment or a
CA 02734978 2011-02-21
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galvannealing treatment to a steel sheet is as described
below.
The steel sheet is immersed into a plating bath, and
the amount of adhesion is adjusted through gas wiping or the
like. It is preferable that the amount of Al dissolved in
the plating bath is specified to be within the range of
0.12% or more, and 0.22% or less in the case of the
galvanizing treatment and within the range of 0.08% or more,
and 0.18% or less in the case of the galvannealing treatment.
Regarding the treatment temperature, as for the
galvanizing treatment, the temperature of the plating bath
may be within the range of usual 450 C or higher, and 500 C
or lower, and furthermore, in the case where the
galvannealing treatment is applied, it is preferable that
the temperature during alloying is specified to be 550 C or
lower. In the case where the alloying temperature exceeds
550 C, carbides are deposited from untransformed austenite
and in some cases, pearlite is generated. Consequently, the
strength or the workability, or the two are not obtained.
In addition, the powdering property of the coating layer
deteriorates. On the other hand, if the temperature during
alloying is lower than 450 C, in some cases, alloying does
not proceed. Therefore, it is preferable that the alloying
temperature is specified to be 450 C or higher.
It is preferable that the coating mass is specified to
CA 02734978 2011-02-21
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be within the range of 20 g/m2 or more, and 150 g/m2 or less
per surface. If the coating mass is less than 20 g/m2, the
corrosion resistance becomes inadequate. On the other hand,
even when 150 g/m2 is exceeded, the corrosion-resisting
effect is saturated and merely an increase in the cost is
invited.
It is preferable that the degree of alloying of the
coating layer (Fe percent by mass (Fe content)) is within
the range of 7 percent by mass or more, and 15 percent by
mass or less. If the degree of alloying of the coating
layer is less than 7 percent by mass, alloying variations
occur, so that the quality of outward appearance
deteriorates, or a so-called a phase is generated in the
coating layer, so that the sliding property of the steel
sheet deteriorates. On the other hand, if the degree of
alloying of the coating layer exceeds 15 percent by mass,
large amounts of hard brittle F phase is formed, so that the
adhesion of the coating deteriorates.
[EXAMPLES]
[0061]
The present invention will be described below in
further detail with reference to the examples. However, the
following examples do not limit the present invention. In
this connection, modification of the configuration within
the range of the gist configuration of the present invention
CA 02734978 2011-02-21
- 41 -
is included in the scope of the present invention.
[0062]
An ingot obtained by melting a steel having a component
composition shown in Table 1 was heated to 1,200 C and was
subjected to finish hot rolling at 870 C. The resulting
hot-rolled steel sheet was taken up at 650 C and,
subsequently, the hot-rolled steel sheet was pickled.
Thereafter, cold rolling was conducted at a reduction ratio
of 65% so as to produce a cold-rolled steel sheet having a
sheet thickness: 1.2 mm. The resulting cold-rolled steel
sheet was subjected to a heat treatment under the condition
shown in Table 2. In this connection, the cooling
termination temperature: T in Table 2 refers to a
temperature at which cooling of a steel sheet is terminated
in cooling of the steel sheet from the annealing temperature.
Furthermore, a part of cold-rolled steel sheets were
subjected to a galvanizing treatment or a galvannealing
treatment. Here, as for the galvanizing treatment, plating
was conducted on both surfaces at a plating bath
temperature: 463 C in such a way that a mass per unit area
(per surface): 50 g/m2 was ensured. Moreover, as for the
galvannealing treatment, plating was conducted on both
surfaces while the alloying condition was adjusted in such a
way that a mass per unit area (per surface): 50 g/m2 was
ensured and the degree of alloying (Fe percent by mass (Fe
CA 02734978 2011-02-21
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content)) became 9 percent by mass. In this connection, the
galvanizing treatment and the galvannealing treatment were
conducted after cooling was once conducted to T C shown in
Table 2.
[0063]
The resulting steel sheet was subjected to temper
rolling at a reduction ratio (elongation percentage): 0.3
after a heat treatment in the case where a plating treatment
is not conducted, or after a galvanizing treatment or a
galvannealing treatment in the case where these treatments
were conducted.
[0064]
[Table 1]
Table 1
(percent by mass)
Steel
A3point
C Si Mn Al P N Cr V Mo Ti Nb B
Ni Cu Ca REM Si+Al Remarks
type
( C)
A 0.311 1.96 1.54 0.041 0.009 0.0024 0.0025 - - - - - -
- - - - 2.00 850 Invented steel
B 0.299 1.98 1.99 0.042 0.013
0.0019 0.0034 - - - - - - - - - - 2.02
842 Invented steel
C 0.305 2.52 2.03 0.043 0.010 0.0037 0.0042 - - - - - -
- - - - 2.56 862 Invented steel
D 0.413 2.03 1.51 0.038 0.012
0.0017 0.0025 - - - - - - - - - - 2.07
838 Invented steel
E 0.417 1.99 2.02 0.044 0.010 0.0020 0.0029 - - - - - -
- - - - 2.03 820 Invented steel
F 0.330 1.45 2.82 0.040 0.012 0.0031 0.0043 - - - - - -
- - - - 1.49 787 Invented steel
G 0.185 1.52 2.32 0.048 0.020
0.0050 0.0044 - - - - - - - - - - 1.57
841 Invented steel
H 0.522 1.85 1.48 0.040 0.011
0.0028 0.0043 - - - - - - - - - - 1.89
815 Invented steel
I 0.320 0.99 2.25 0.041 0.014 0.0018 0.0042 - - - - - -
- - - - 1.03 787 Invented steel
1.)
J 0.263 1.50 2.29 0.039 0.011
0.0010 0.0036 0.9 - - - - - - - - - 1.54 807
Invented steel
K 0.270 1.35 2.27 0.043 0.004 0.0020 0.0035 - 0.21 - - -
- - - - - 1.39 827 Invented steel
CO
L 0.221 1.22 1.99 0.040 0.040
0.0030 0.0043 - - 0.19 - - - - - -
- 1.26 849 Invented steel 1.)
M 0.202 1.75 2.52 0.045 0.044 0.0020 0.0044 - - - 0.035 -
- - - - - 1.80 872 Invented steel
oI
N 0.175 1.51 2.18 0.042 0.022
0.0020 0.0044 - - - - 0.07 - - -
- - 1.55 848 Invented steel 1.)
1.)
0 0.212 1.51 2.37 0.043 0.030 0.0010 0.0029 - - - 0.020 -
0.0011 - - - - 1.55 848 Invented steel
P 0.480 1.52 1.33 0.044 0.015
0.0020 0.0038 - - - - - - 0.52 - - - 1.56
806 Invented steel
Q 0.310 1.42 2.02 0.043 0.015
0.0030 0.0023 - - - - - - - 0.55 - - 1.46
805 Invented steel
R 0.335 2.01 2.22 0.043 0.004 0.0028 0.0041 - - - - - -
- - 0.003 - 2.05 824 Invented steel
S 0.329 1.88 1.65 0.040 0.021
0.0020 0.0031 - - - - - - - - 0.002 1.92
848 Invented steel
T 0.330 0.01 2.33 1.010 0.025 0.0020 0.0033 - - - - - -
- - - - 1.02 873 Invented steel
U 0.291 - 2.75 0.042 0.012 0.0040
0.0024 - - - - - - - - - - ao4 732
Comparative steel
/ 0.290 0.48 2.22 0.130 0.006
0.0020 0.0035 - - - - - - - - 0.61 777
Comparative steel
W 0.145 0.50 1.42 0.320 0.007
0.0018 0.0041 - - - - - - - - - - 0.82
859 Comparative steel
X 0.190 1.00 0.41 0.036 0.013 0.0020 0.0038 - - - - - -
- - - - 1.04 868 Comparative steel
Note) Underline indicates that the value is out of the appropriate range.
Table 2
Annealing Average cooling Cooling rate
Cooling Second temperature range
Coating
Keeping time in first
Sample No. Steel type temperature Annealing rate
to 550 C 550 C to T C termination Keeping I Remarks
*2 time ta) temperature range (s)
Keeping times (s)
( C) ( C/s) ( C/s)
temperature ( C) temperature ( C)
1 A CR 880 180 4 15 430 100 300
100 Comparative example
2 A CR 900 180 20 20 400 5 320
90 Comparative example
3 A , CR 900 200 50 50 _ 420 100 330
180 Invention example
_
4 A CR 900 200 50 50 400 100 330
300 Invention example
B CR 800 200 20 20 400 120 300
100 Comparative example
6 B CR 880 _ 200 20 20 520 200 330
300 Comparative example
7 B CR 880 350_ 35 35 400 100 330
350 Invention example
8 C CR 890 150 25 25 400 80 110
120 Comparative example
9 C CR 900 200 20 20 380 120 310
300 Invention example
D CR 900 200 20 20 400 100 330
300 Invention example
11 D CR 900 200 _ 50 50 400 300 250
10 Comparative example
_
12 E CR 880 250 15 15 400 200 340
550 Invention example
_
13 F CR 870 300 20 20 450 100 330
250 Invention example
n
14 F GI 870 300 12 12 450 100 330
200 Invention example
G CR 890 200 20 20 400 , 90 240
420 Invention example o
iv
16 H CR 880 200 25 25 370 _ 400 200
500 Invention example ---1
L...)
17 I CR 900 250 30 30 400 150 250
300 Invention example
kok
..
---1
18 I GA 900 250 20 20 450 _ 100 280
100 Invention example
-
19 J CR 900 200 20 20 , 370 , 90 300
300 Invention example
¨
o
K CR 900 200 40 40 _ 420 _ 90 300
300 Invention example 1 H
.
H
21 L CR 900 200 30 30 420 , 200 300
300 Invention example
o1
22 M CR 900 200 20 20 420 , 180 300
300 Invention example "
1
23 N CR 900 200 20 20 420 _ 100 300
300 Invention example "
H
24 0 CR 900 . 200 20 20 420 100. 300
300 Invention example
P CR 900 200 20 20 420 . 300 300
300 Invention example
26 Q CR 900 200 30 30 420 120_ 300
300 Invention example
27 R CR 900 200 30_ 30 420 , 100 300
300 Invention example
28 S CR 900 200 30 30 420 100_ 300
300 Invention example
29 T CR 900 200 30 30 420 120 300
300 Invention example
_ _
U CR 900 200 13 13 420 100 300
300 Comparative example
_
31 V CR 900 200 20 20 420 - _
100
300 300 Comparative example
_
32 W_ CR 900 200 40 40 420 60 _
300 300 Comparative example
-
33 X CR 900 200 15 15 420 60 300
300 Comparative example
*1 Underline indicates that the value is out of the appropriate range.
*2 CR:No coating (cold¨rolled steel sheet) GI:Galvanized steel sheet GA:
Galvannealed steel sheet
=
CA 02734978 2011-02-21
- 45 -
[0066]
Various characteristics of the thus obtained steel
sheet were evaluated by the following methods.
A sample was cut from each steel sheet and was polished.
Microstructures of ten fields of view of a surface parallel
to the rolling direction were observed with a scanning
electron microscope (SEM) at 3,000-fold magnification, the
area percentage of each phase was measured, and a phase
structure of each crystal grain was identified.
[0067]
The steel sheet was ground.polished up to one-quarter
of a sheet thickness in the sheet thickness direction and
the amount of retained austenite was determined by X-ray
diffractometry. As for an incident X-ray, Co-Ka was used
and the amount of retained austenite were calculated from
the average value of the intensity ratio of each of (200),
(220), and (311) faces of austenite to the diffraction
intensity of each of (200), (211), and (220) faces of
ferrite.
[0068]
As for the average amount of C in the retained
austenite, a lattice constant was determined from the
intensity peak of each of (200), (220), and (311) faces of
austenite based on the X-ray diffractometry, and the average
amount of C (percent by mass) in the retained austenite was
CA 02734978 2011-02-21
- 46 -
determined from the following calculation formula.
a0 = 0.3580 + 0.0033 x [C%] + 0.00095 x [Mn%] + 0.0056 x
[A196] + 0.022 x [N%]
where, a0 represents a lattice constant (nm) and [X%]
represents percent by mass of an element X. In this
connection, the percent by mass of an element other than C
was percent by mass relative to whole steel sheet.
[0069]
The tensile test was conducted based on JIS Z2241 by
using a test piece of JIS No. 5 size taken in a direction
perpendicular to the rolling direction of the steel sheet.
The TS (tensile strength) and the T.E1 (total elongation)
were measured, a product of the strength and the total
elongation (TS x T.E1) was calculated and, thereby, the
balance between the strength and the workability
(elongation) was evaluated. In this connection, in the
present invention, the case where TS x T.E1 20,000 MPa.%
was evaluated as good.
[0070]
The stretch-flangeability was evaluated on the basis of
the Japan Iron and Steel Federation Standard JFST 1001.
Each of the resulting steel sheets was cut into 100 mm x 100
mm, a hole having a diameter: 10 mm was punched with a
clearance of 12% of sheet thickness. Thereafter, a dice
having an inside diameter: 75 mm was used, a 60 circular
CA 02734978 2011-02-21
- 47 -
cone punch was pushed into the hole while holding was
conducted with a holddown force: 88.2 kN, a hole diameter at
crack occurrence limit was measured, and a hole-expansion
limit X (%) was determined from the formula (1). .
hole-expansion limit X (%) = {(Df - DO)/D0} x 100 === (1)
where Df represents a hole diameter (mm) at occurrence
of crack and DO represents an initial hole diameter (mm).
The thus measured k was used, the product of the
strength and the hole-expansion limit (TS x k) was
calculated and, thereby, the balance between the strength
and the stretch-flangeability was evaluated.
In this connection, in the present invention, the
stretch-flangeability was evaluated as good in the case
where TS x k 25,000 MPa-96.
[0071]
Furthermore, the hardness of the hardest microstructure
in the steel sheet microstructure was determined by a method
described below. That is, as a result of microstructure
observation, in the case where as-quenched martensite was
observed, 10 points of the as-quenched martensite were
measured with an ultramicro-Vickers at a load: 0.02 N, and
an average value thereof was assumed to be the hardness of
the hardest microstructure in the steel sheet microstructure.
In this connection, in the case where as-quenched martensite
is not observed, as described above, the microstructure. of
CA 02734978 2011-02-21
- 48 -
any one of the tempered martensite, the upper bainite, and
the lower bainite becomes the hardest phase in the steel
sheet according to the present invention. In the case of
steel sheet according to the present invention, the hardest
phase was a phase showing HV 800.
=
[0072]
The above-described evaluation results are shown in
Table 3.
[0073]
[Table 3]
Table 3
Area percentage relative to whole steel sheet microstructure (%) (As-quenched
Average
Sample Steel As- M) amount of C in
TS T.EL A. TS x TEL TS x A
No. type ab* 2 LB*2+M* 2 quenched a*2 r *2 *5 Remainder Oi b+LB
AM-FLB) retained r
(MPa) (%) (%) (MPa=%) (MPa=%) Remarks
+NA+ r
M (%) (%)
-
1 A 3 _ 6 058 1 32 10 0 - 841
21 38 17661 31958 Comparative example
_
_
2 A 4 89 , 10 3 4 0 97 11
0.91 ' 1492 12 20 17904 29840 Comparative example
3 A 54 31 10 2 13 0 98 32 1.14
1166 19 34 , 22154 39644 Invention example
_
4 A 56 30 7 2 12 0 98 23 1.23
1156 21 34 24276 39304 Invention example
B 21 49 10 21 6 3 76 20_ 0.68 1296 13
20 16848 25920 Comparative example
6 B 37 49 10 3 8 3 94 20 0.57
1467 11 22 16137 32274 Comparative example
7 B 50 38 10 0 12 o 100 26 1.22
1302 18 23 23436 29946 Invention example
_
8 C 50 35 28 0 15 0 100 80 0.94
1482 20 5 29640 7410 Comparative example
9 C 52 34 11 0 14 0 100 32 1.16
1371 19 20 26049 27420 Invention example
D 45 39 9 0 16 0 100 23 1.36 1335 24
21 32040 28035 Invention example
_
11 D 58 21 18 0 21 0 100 86 1.20
1203 29 8 34887 9624 Comparative example n
12 E 25 63 30 0 12 0 100 48 1.15 ,
1695 15 18 25425 30510 Invention example
o
13 F 15 78 30 0 _ 7 o 100 , 38 0.81
1710 14 19 23940 32490 Invention example iv
---1
14 F 14 76 _ 30 2 8 o 98 39
0.75 1632 13 19 21216 31008 Invention example 1
(A
11.
G 70 14 2 7 9 0 93 14 0.74 1098 21
45 23058 49410 Invention example ko
---1
16 H 16 78 17 0 6_ 0 100 22 1.09
1820 14 18 25480 32760 Invention example 'ICD co
17 I 37 52 20 0 11 o 100 38 0.85
1395 16 21 22320 29295 Invention example iv
o
i
H
18 I 36 , 55 38 0 9 o 100 69 0.82
1314 17 20 22338 26280 Invention example H
oI
19 J 16 75 , 14 1 9 0 100 19
0.81 1783 12 17 21396 30311 Invention example
iv
K 22 69 , 21 0 9 0 100 , 30 0.83
1612 13 19 20956 , 30628 Invention example
1
iv
21 L 20 69 , 22 0 11 0 100 32
0.73 1870 11 14 20570 26180 Invention example
H
22 M 36 56 13 . 0 8 0 100 23 . 0.82
1285 21 20 26985 25700 Invention example
23 N 33 58 35_ 0 9 0 100 60 0.79
1045 25 38 26125 39710 Invention example
24 0 35 55 15 o 10 o 100 27 _ 0.86
1230 19 28 23370 34440 Invention example
P 30 57 25 0 13 0 100 44 0.92 1771 14
19 24794 33649 Invention example
26 Q 40 44 , 18 0 16 0 100 41
0.95 1596 13 20 20748 31920 Invention example
27 R 22 68 , 17 0 10 0 100 25
0.96 1482 14 37 20748 54834 Invention example
28 S 60 29 12 0 11 o 100 41 1.05
, 1465 , 17 28 24905 41020 Invention example
29 T 42 47 25 0 _ 11 0 100 53 1.07
1355 19 33 25745 44715 Invention example
_
U 40 41 20 9 2 , 8 83 , 49 - 1183 13
23 15379 27209 Comparative example
_
_
31 V 39 54 24 4 3 0 96 44 -
1288 12 23 15456 29624 Comparative example
_
32 W 78 8 3 0 3 11 89 38 . - 901
14 32 12614 28832 Comparative example
_
33 X 8 1 1 70 0 21 9 100 - 735
14 30 10290 22050 _Comparative example
_
*1 Underline indicates that the value is out of the appropriate range.
*2 a b: Bainitic ferrite in upper bainite LB: Lower bainite M: Martensite a
: Polygonal ferrite y : Retained austenite
*3 Amount of retained austenite determined by X¨ray diffractometry was assumed
to be area percentage relative to whole steel sheet microstructure.
CA 02734978 2011-02-21
- 50 -
[0074]
As is clear from Table 3, every steel sheet according
to the present invention satisfies that the tensile strength
is 980 MPa or more, the value of TS x T.E1 is 20,000 MPa.%
or more, and TS x X 25,000 MPa.%. Therefore, it was able
to be ascertained that high strength and excellent
workability, especially excellent stretch-flangeability,
were provided in combination.
[0075]
On the other hand, regarding Sample No. 1, the average
cooling rate to 550 C was out of the appropriate range.
Therefore, a desired steel sheet microstructure was not
obtained. Although TS 'x X 25,000
MPa.96 was satisfied, the
tensile strength (TS) 980 MPa and TS x T.E1 20,000
MPa.96
were not satisfied. Regarding Sample No. 2, the keeping
time in the first temperature range was out of the
appropriate range. Regarding Sample No. 5, the annealing
temperature was lower than A3 point. Regarding Sample No. 6,
the cooling termination temperature: T was out of the first
temperature range. Regarding Sample No. 8, the keeping
temperature in the second temperature range was out of the
appropriate range. Regarding Sample No. 11, the keeping
time in the second temperature range was out of the
appropriate range. Therefore, a desired steel sheet
microstructure was not obtained. Although the tensile
CA 02734978 2011-02-21
- 51 -
strength (TS) 980 MPa
was satisfied, any one of TS x T.E1
20,000 MPa.9,5 and TS x X 25,000
MPa.% was not satisfied.
Regarding Sample Nos. 30 to 34, the component compositions
were out of the appropriate range. Therefore, a desired
steel sheet microstructure was not obtained, and at least
one of the tensile strength (TS) 980 MPa, TS x T.E1
20,000 MPa.96, and TS x X 25,000 MPa.% was not satisfied.
