U.S. patent application number 16/082519 was filed with the patent office on 2019-03-07 for high-strength steel sheet and method for manufacturing same.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). The applicant listed for this patent is Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). Invention is credited to Yuichi FUTAMURA, Elijah KAKIUCHI, Toshio MURAKAMI, Tadao MURATA, Shigeo OTANI.
Application Number | 20190071757 16/082519 |
Document ID | / |
Family ID | 60044002 |
Filed Date | 2019-03-07 |
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United States Patent
Application |
20190071757 |
Kind Code |
A1 |
MURATA; Tadao ; et
al. |
March 7, 2019 |
HIGH-STRENGTH STEEL SHEET AND METHOD FOR MANUFACTURING SAME
Abstract
A high-strength steel sheet comprising, as component
composition, by mass: C: 0.15 to 0.35%; total of Si and Al: 0.5 to
2.5%; Mn: 1.0 to 4.0%; P: more than 0% and 0.05% or less; and S:
more than 0% and 0.01% or less, with the balance being Fe and
inevitable impurities, wherein a steel structure satisfies, in
ratio with respect to the whole structure: ferrite: more than 5
area % and 50 area % or less; total of tempered martensite and
bainite: 30 area % or more; and retained austenite: 10 volume % or
more, the steel structure further includes MA, and the steel
structure satisfies: an average circle equivalent diameter of the
MA: 1.0 .mu.m or less; an average circle equivalent diameter of the
retained austenite: 1.0 .mu.m or less; and a volume ratio of
retained austenite with a circle equivalent diameter of 1.5 .mu.m
or more to the whole retained austenite: 5% or more.
Inventors: |
MURATA; Tadao;
(Kakogawa-shi, JP) ; MURAKAMI; Toshio; (Kobe-shi,
JP) ; KAKIUCHI; Elijah; (Kobe-shi, JP) ;
OTANI; Shigeo; (Kobe-shi, JP) ; FUTAMURA; Yuichi;
(Nagoya-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) |
Kobe-shi |
|
JP |
|
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel, Ltd.)
Kobe-shi
JP
|
Family ID: |
60044002 |
Appl. No.: |
16/082519 |
Filed: |
February 22, 2017 |
PCT Filed: |
February 22, 2017 |
PCT NO: |
PCT/JP2017/006608 |
371 Date: |
September 5, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 6/008 20130101;
C22C 38/002 20130101; C21D 8/0226 20130101; C21D 8/0236 20130101;
C21D 8/0263 20130101; C21D 2211/002 20130101; C21D 6/004 20130101;
C22C 38/02 20130101; C21D 9/46 20130101; C22C 38/48 20130101; C21D
2211/008 20130101; C22C 38/32 20130101; C22C 38/44 20130101; C22C
38/54 20130101; C21D 2211/001 20130101; C22C 38/20 20130101; C21D
6/005 20130101; C22C 38/42 20130101; C22C 38/26 20130101; C22C
38/005 20130101; C22C 38/06 20130101; C22C 38/04 20130101; C21D
2211/005 20130101; C22C 38/24 20130101; C22C 38/22 20130101; C21D
11/005 20130101 |
International
Class: |
C22C 38/04 20060101
C22C038/04; C21D 6/00 20060101 C21D006/00; C21D 9/46 20060101
C21D009/46; C21D 11/00 20060101 C21D011/00; C22C 38/00 20060101
C22C038/00; C22C 38/02 20060101 C22C038/02; C22C 38/06 20060101
C22C038/06; C22C 38/20 20060101 C22C038/20; C22C 38/22 20060101
C22C038/22; C22C 38/24 20060101 C22C038/24; C22C 38/26 20060101
C22C038/26; C22C 38/32 20060101 C22C038/32; C22C 38/42 20060101
C22C038/42; C22C 38/44 20060101 C22C038/44; C22C 38/48 20060101
C22C038/48; C22C 38/54 20060101 C22C038/54 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 31, 2016 |
JP |
2016-072740 |
Dec 27, 2016 |
JP |
2016-253902 |
Claims
1. A high-strength steel sheet comprising a component composition
comprising Fe and, by mass: C: 0.15 to 0.35%; a total of Si and Al:
0.5 to 2.5%; Mn: 1.0 to 4.0%; P: more than 0% and 0.05% or less;
and S: more than 0% and 0.01% or less, wherein the component
composition forms a steel structure comprising: ferrite: more than
5 area % and 50 area % or less; a total of tempered martensite and
bainite: 30 area % or more; and a total retained austenite: 10
volume % or more, wherein the steel structure further comprises a
martensite-austenite composite, and wherein the steel structure
satisfies: an average circle equivalent diameter of the
martensite-austenite composite: 1.0 .mu.m or less; an average
circle equivalent diameter of the retained austenite: 1.0 .mu.m or
less; and a volume ratio of a retained austenite with a circle
equivalent diameter of 1.5 .mu.m or more to the total retained
austenite: 5% or more.
2. The high-strength steel sheet of claim 1, wherein the amount of
C in the component composition is 0.30% or less by mass.
3. The high-strength steel sheet of claim 1, wherein the amount of
Al in the component composition is less than 0.10% by mass.
4. The high-strength steel sheet of claim 1, further comprising at
least one of, by mass: (a) at least one selected from the group
consisting of Cu, Ni, Mo, Cr and B, in a total amount of more than
0% and 1.0% or less; (b) at least one selected from the group
consisting of V, Nb, Ti, Zr and Hf, in a total amount of more than
0% and 0.2% or less; and (c) at least one selected from the group
consisting of Ca, Mg and REM, in a total amount of more than 0% and
0.01% or less.
5. A method for manufacturing the high-strength steel sheet of
claim 1, the method comprising (i)-(v) in order: (i) heating an
original sheet comprising the component composition of claim 1,
wherein the original sheet has an Ac.sub.1 point and an Ac.sub.3
point, to a temperature in a range of T1 or higher to lower than
the Ac.sub.3 point, wherein T1 satisfies: T1=the Ac.sub.1
point.times.0.8+the Ac.sub.3 point.times.0.2; (ii) rapidly cooling
from a temperature T2 of 650.degree. C. or more to a temperature
T3a of 300 to 500.degree. C. at an average cooling rate of
30.degree. C./s or more and less than 200.degree. C./s; (iii)
gradually cooling from T3a to a temperature T3b not less than
300.degree. C. at an average cooling rate of 0.degree. C./s or more
and 10.degree. C./s or less for 10 seconds or more and less than
300 seconds; (iv) cooling from a temperature T3b to a temperature
T4 of 100 to 300.degree. C. at an average cooling rate of more than
10.degree. C./s; and (v) reheating to a temperature T5 of 300 to
500.degree. C.
6. The method of claim 5, wherein (iii) further comprises retaining
a constant temperature of 300 to 500.degree. C.
7. A method for manufacturing the high-strength steel sheet of
claim 2, the method comprising (i)-(v) in order: (i) heating an
original sheet comprising the component composition of claim 2,
wherein the original sheet has an Ac.sub.1 point and an Ac.sub.3
point, to a temperature in a range of T1 or higher to lower than
the Ac.sub.3 point, wherein T1 satisfies: T1=the Ac.sub.1
point.times.0.8+the Ac.sub.3 point.times.0.2; (ii) rapidly cooling
from a temperature T2 of 650.degree. C. or more to a temperature
T3a \of 300 to 500.degree. C. at an average cooling rate of
30.degree. C./s or more and less than 200.degree. C./s; (iii)
gradually cooling from T3a to a temperature T3b not less than
300.degree. C. at an average cooling rate of 0.degree. C./s or more
and 10.degree. C./s or less for 10 seconds or more and less than
300 seconds; (iv) cooling from a temperature T3b to a temperature
T4 of 100 to 300.degree. C. at an average cooling rate of more than
10.degree. C./s; and (v) reheating to a temperature T5 of 300 to
500.degree. C.
8. A method for manufacturing the high-strength steel sheet of
claim 3, the method comprising (i)-(v) in order: (i) heating an
original sheet comprising the component composition of claim 3,
wherein the original sheet has an Ac.sub.1 point and an Ac.sub.3
point, to a temperature in a range of T1 or higher to lower than
the Ac.sub.3 point, wherein T1 satisfies: T1=the Ac.sub.1
point.times.0.8+the Ac.sub.3 point.times.0.2; (ii) rapidly cooling
from a temperature T2 of 650.degree. C. or more to a temperature
T3a of 300 to 500.degree. C. at an average cooling rate of
30.degree. C./s or more and less than 200.degree. C./s; (iii)
gradually cooling from T3a to a temperature T3b not less than
300.degree. C. at an average cooling rate of 0.degree. C./s or more
and 10.degree. C./s or less for 10 seconds or more and less than
300 seconds; (iv) cooling from a temperature T3b to a temperature
T4 of 100 to 300.degree. C. at an average cooling rate of more than
10.degree. C./s; and (v) reheating to a temperature T5 of 300 to
500.degree. C.
9. A method for manufacturing the high-strength steel sheet of
claim 4, the method comprising (i)-(v) in order: (i) heating an
original sheet comprising the component composition of claim 4,
wherein the original sheet has an Ac.sub.1 point and an Ac.sub.3
point, to a temperature in a range of T1 or higher to lower than
the Ac.sub.3 point, wherein T1 satisfies: T1=the Ac.sub.1
point.times.0.8+the Ac.sub.3 point.times.0.2; (ii) rapidly cooling
from a temperature T2 of 650.degree. C. or more to a temperature
T3a of 300 to 500.degree. C. at an average cooling rate of
30.degree. C./s or more and less than 200.degree. C./s; (iii)
gradually cooling from T3a to a temperature T3b not less than
300.degree. C. at an average cooling rate of 0.degree. C./s or more
and 10.degree. C./s or less for 10 seconds or more and less than
300 seconds; (iv) cooling from a temperature T3b to a temperature
T4 of 100 to 300.degree. C. at an average cooling rate of more than
10.degree. C./s; and (v) reheating to a temperature T5 of 300 to
500.degree. C.
10. The method of claim 7, wherein (iii) further comprises
retaining a constant temperature of 300 to 500.degree. C.
11. The method of claim 8, wherein (iii) further comprises
retaining a constant temperature of 300 to 500.degree. C.
12. The method of claim 9, wherein (iii) further comprises
retaining a constant temperature of 300 to 500.degree. C.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to a high-strength steel
sheet usable for various applications including an automotive
component, and a method for manufacturing the high-strength steel
sheet.
BACKGROUND ART
[0002] A steel sheet provided for manufacturing steel components
such as a component for vehicles is required to be reduced in
thickness to realize improvement of fuel efficiency, and high
strengthening of the steel sheet is required to achieve the
reduction in thickness and securement of strength of the
components. Patent Document 1 discloses as the high-strength steel
sheet, a steel sheet that has a tensile strength of 980 to 1180 MPa
and exhibits good deep drawability.
PRIOR ART DOCUMENT
Patent Document
[0003] Patent Document 1: JP 2009-203548 A
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0004] The steel sheet, however, is required to have not only high
tensile strength and excellent deep drawability but also further
excellent strength-ductility balance, high yield ratio and
excellent hole expandability, i.e., a high hole expansion ratio
when it is used for steel components for various application
including a component for vehicles.
[0005] Specifically, each of the tensile strength, the
strength-ductility balance, the yield ratio, the deep drawability,
and the hole expansion ratio is required to attain a following
level. First, the tensile strength is required to be 780 MPa or
more. Further, the steel sheet needs to have high yield strength
(YS) in addition to high tensile strength (TS) in order to increase
stress that can be applied to a component during usage. Further,
from a viewpoint of securing collision safety and the like, it is
necessary to increase the yield strength of the steel sheet. Thus,
a yield ratio (YR=YS/TS) of 0.60 or more is required
specifically.
[0006] As to the strength-ductility balance, a product of TS and
total elongation EL, TS.times.EL is required to be 22000 MPa % or
more. Furthermore, for securement of formability during forming a
component, an LDR (Limiting Drawing Ratio) is required to be 2.05
or more, which is an index that indicates the deep drawability and
is derived by a method in Examples described later, and the hole
expansion ratio 2 indicating the hole expandability is also
required to be 20% or more.
[0007] The high-strength steel sheet disclosed in Patent Document
1, however, has difficulty satisfying all these requirements, and a
high-strength steel sheet has been demanded that is capable of
satisfying all these requirements.
[0008] Embodiments of the present invention have been made to meet
these requirements, and an object of the embodiments is to provide
a high-strength steel sheet that attains a high level in any of the
tensile strength TS, the yield ratio YR, the product of TS and
total elongation EL, TS.times.EL, the LDR indicating the deep
drawability, and the hole expansion ratio .lamda. indicating the
hole expandability, and a method for manufacturing the
high-strength steel sheet.
Means for Solving the Problems
[0009] A high-strength steel sheet according to the embodiment of
the present invention, which has been capable of solving the above
problems, contains, as component compositions, by mass:
C: 0.15 to 0.35%;
[0010] total of Si and Al: 0.5 to 2.5%;
Mn: 1.0 to 4.0%;
[0011] P: more than 0% and 0.05% or less; and S: more than 0% and
0.01% or less, with the balance being Fe and inevitable impurities,
wherein a steel structure satisfies, in ratio with respect to the
whole structure: ferrite: more than 5 area % and 50 area % or less;
total of tempered martensite and bainite: 30 area % or more; and
retained austenite: 10 volume % or more, the steel structure
further includes MA (martensite-austenite constituent), and the
steel structure satisfies: an average circle equivalent diameter of
the MA: 1.0 .mu.m or less, an average circle equivalent diameter of
the retained austenite: 1.0 .mu.m or less, and a volume ratio of
retained austenite with a circle equivalent diameter of 1.5 .mu.m
or more to the whole retained austenite: 5% or more.
[0012] An amount of C in the component composition is preferably
0.30% or less. An amount of Al in the component composition may be
less than 0.10%.
[0013] The high-strength steel sheet may further contain by
mass:
(a) at least one selected from the group consisting of Cu, Ni, Mo,
Cr and B in a total amount of more than 0% and 1.0% or less; (b) at
least one selected from the group consisting of V, Nb, Ti, Zr and
Hf in a total amount of more than 0% and 0.2% or less; and (c) at
least one selected from the group consisting of Ca, Mg and REM
(rare earth metal) in a total amount of more than 0% and 0.01% or
less.
[0014] A method for manufacturing a high-strength steel sheet
according to the embodiment of the present invention, which has
been capable of solving the above problems, is a method for
manufacturing the high-strength steel sheet and includes steps A to
E in this order:
[0015] the step A of heating to a temperature T1 of (Ac.sub.1
point.times.0.8+Ac.sub.3 point.times.0.2) or higher and lower than
Ac.sub.3 point, using an original sheet that satisfies the
foregoing component composition;
[0016] the step B of rapidly cooling after the heating, from a
rapid cooling start temperature T2 of 650.degree. C. or higher to a
cooling stop temperature T3a of 300 to 500.degree. C. at an average
cooling rate of 30.degree. C./s or more and less than 200.degree.
C./s;
[0017] the step C of performing gradual cooling after the rapid
cooling, in a temperature range from 300 to 500.degree. C. at an
average cooling rate of 0.degree. C./s or more and 10.degree. C./s
or less for 10 seconds or more and less than 300 seconds;
[0018] the step D of cooling after the gradual cooling, from a
gradual cooling finish temperature T3b of 300.degree. C. or higher
to a cooling stop temperature T4 of 100 to 300.degree. C. at an
average cooling rate of more than 10.degree. C./s; and the step E
of reheating to a reheating temperature T5 of 300 to 500.degree.
C.
[0019] The step C may include retaining at constant temperature in
the temperature range from 300 to 500.degree. C.
Effects of Invention
[0020] According to the embodiment of the present invention, it is
possible to provide a high-strength steel sheet that attains a high
level in any of the tensile strength TS, the yield ratio YR, the
product of TS and total elongation EL, TS.times.EL, the hole
expandability, and the deep drawability, and a method for
manufacturing the high-strength steel sheet.
BRIEF DESCRIPTION OF THE DRAWING
[0021] FIG. 1 shows a diagram schematically illustrating a heat
treatment of a manufacturing method according to an embodiment of
the present invention.
MODE FOR CARRYING OUT THE INVENTION
[0022] As a result of earnest studies, the prevent inventors have
found that it is possible to obtain a high-strength steel sheet
that attains a high level in any of the tensile strength TS, the
yield ratio YR, the product of TS and total elongation EL,
TS.times.EL, the hole expandability, and the deep drawability when
the steel sheet has predetermined components and a steel structure.
In particular, the present inventors have found that it is possible
to achieve especially excellent deep drawability by suppressing an
average particle diameter of retained austenite to a certain size
or smaller while securing a certain ratio or more of relatively
large retained austenite in the steel structure, and further that
especially a multi-step austempering treatment is to be performed
in a manufacturing step in order to obtain this steel structure.
Thus, the embodiments of the present invention have been completed.
First, the steel structure is described below.
[0023] 1. Steel Structure
[0024] Hereinafter, the high-strength steel sheet according to the
embodiments of the present invention is described in detail in
terms of steel structure. In the description of the steel structure
below, mechanisms are sometimes described due to which such
structure is capable of improving various characteristics. These
mechanisms have been devised on the basis of findings having been
obtained by the present inventors until now, and it is to be noted
that these mechanisms are not to limit a technical scope of the
present invention.
[0025] [Ferrite: More than 5 Area % and 50 Area % or Less]
[0026] Ferrite is generally excellent in ductility but has a
problem of being low in strength. In the embodiments of the present
invention, an amount of ferrite is set to more than 5 area % in
order to improve the ductility and obtain excellent
strength-ductility balance. The amount of ferrite is preferably 7
area % or more, more preferably 10 area % or more. On the other
hand, an excess amount of ferrite decreases the hole expandability
and/or the yield ratio. Therefore, the amount of ferrite is set to
50 area % or less in order to secure an excellent hole expansion
ratio .lamda. and a high yield ratio. The amount of ferrite is
preferably 45 area % or less, more preferably 40 area % or
less.
[0027] [Total of Tempered Martensite and Bainite: 30 Area % or
More]
[0028] Total of tempered martensite and bainite is set to 30 area %
or more in the whole structure to be capable of attaining both high
strength and high hole expandability. Bainite in the embodiments of
the present invention refers to tempered bainite, untempered
bainite and bainitic ferrite. The total of tempered martensite and
bainite is preferably 35 area % or more. An upper limit of the
total of tempered martensite and bainite is about 85 area %, in
view of fractions of ferrite and retained austenite as other
essential structures.
[0029] [Retained Austenite: 10 Volume % or More]
[0030] Retained austenite causes, during processing such as press
working, TRIP phenomenon where retained austenite transforms into
martensite due to strain-induced transformation and is a structure
capable of giving large elongation. Formed martensite has high
hardness. Therefore, it is possible to obtain excellent
strength-ductility balance. Further, it is also possible to
increase the deep drawability by controlling a particle diameter
distribution of retained austenite as described later. An amount of
retained austenite is set to 10 volume % or more in order to obtain
excellent strength-ductility balance especially with a TS.times.EL
of 22000 MPa % or more and sufficiently produce an effect of
improving the deep drawability by controlling a particle diameter
distribution of retained austenite described later. The amount of
retained austenite is preferably 12 volume % or more, more
preferably 13 volume % or more, further preferably 14 volume % or
more, still further preferably 15 volume % or more. An upper limit
of the amount of retained austenite is around 40 volume %, in view
of the component composition and the manufacturing conditions that
are defined in the embodiments of the present invention, and the
like.
[0031] In the high-strength steel sheet according to the
embodiments of the present invention, much of retained austenite
exists in a form of an MA. The MA is an abbreviation of a
martensite-austenite constituent and a composite structure of
martensite and austenite.
[0032] [Average Circle Equivalent Diameter of MA: 1.0 .mu.m or
Less]
[0033] The MA is a hard phase, and a vicinity of an interface
between a parent phase and the hard phase serves as a void
formation site during deformation. The MA having a coarser size
more causes concentration of strain in the interface between the
parent phase and the hard phase to easily generate fracture from
the void, as a base point, formed in the vicinity of the interface
between the parent phase and the hard phase. This results in a
decrease of the hole expandability. Therefore, an average circle
equivalent diameter of the MA is suppressed to 1.0 .mu.m or less.
The average circle equivalent diameter of the MA is preferably 0.8
.mu.m or less, more preferably 0.7 .mu.m or less. The average
circle equivalent diameter of the MA is preferably smaller to
increase the hole expandability. In view of the component
composition and the manufacturing conditions that are defined in
the embodiments of the present invention, however, a lower limit of
the average circle equivalent diameter of the MA is around 0.2
.mu.m.
[0034] In the embodiments of the present invention, an area
proportion of the MA including retained austenite is not especially
limited, and an area proportion of retained austenite may be set to
within the above range as described above. As described above, the
amount of retained austenite is 10 volume % or more in the
embodiments of the present invention, so that the MA where much of
retained austenite exists may also exist by 10 area % or more. In
the embodiments of the present invention, it is necessary to secure
desired area proportions of ferrite, and tempered martensite and
bainite, so that an upper limit of the area proportion of the MA is
less than 65 area %.
[0035] [Average Circle Equivalent Diameter of Retained Austenite:
1.0 .mu.m or Less]
[0036] [Ratio of Retained Austenite with Circle Equivalent Diameter
of 1.5 .mu.m or More to Whole Retained Austenite: 5% or More]
[0037] In the embodiments of the present invention, as described
above, the present inventors have found that it is possible to
achieve especially excellent deep drawability by suppressing an
average particle diameter of retained austenite to a certain size
or smaller while securing a certain ratio or more of relatively
large retained austenite. Hereinafter, reasons for this definition
are described in detail.
[0038] When flow stress in a flange portion of a steel component
formed during deep drawing is smaller than tensile stress in a
longitudinal wall of the steel component, drawing easily proceeds
to give good deep drawability. The flange portion is, in its
deformation behavior, deformed while subjected to compressive
stress due to strong compressive stress along a circumference of a
board surface. On the other hand, martensite transformation is
accompanied by volume expansion, so that the martensite
transformation is less likely to occur under compressive stress. As
described above, strain-induced martensite transformation of
retained austenite is suppressed in the flange portion, so that
work hardening is small, as a result, the deep drawability is
improved. Existence of coarse retained austenite increases an
amount of volume expansion during martensite transformation. In
other words, martensite transformation is more suppressed under
compressive stress, so that the deep drawability is considered to
be more improved.
[0039] On the other hand, it is necessary to maintain a high work
hardening rate during deformation in order to increase the tensile
stress in the longitudinal wall portion for obtaining good deep
drawability. The present inventors have found that in order to
maintain a high work hardening rate during deformation as described
above, it is effective to cause strain-induced transformation over
a wide stress range. Thus, the present inventors have focused on
mixing unstable retained austenite that easily undergoes
strain-induced transformation under relatively low stress, with
stable retained austenite that undergoes strain-induced
transformation only under high stress.
[0040] From these viewpoints, the present inventors have studied to
obtain steel structures including at predetermined amounts:
relatively coarse retained austenite as the unstable retained
austenite that contributes to suppressing martensite transformation
in the flange portion and easily undergoes strain-induced
transformation under relatively low stress in the longitudinal wall
portion; and fine retained austenite as the stable retained
austenite that undergoes strain-induced transformation only under
high stress in the longitudinal wall portion.
[0041] Then, the present inventors have found that it is possible
to maintain a high work hardening rate during deformation and thus
exhibit excellent deep drawability by setting an average circle
equivalent diameter of retained austenite to 1.0 .mu.m or less and
setting a volume ratio of retained austenite with a circle
equivalent diameter of 1.5 .mu.m or more to the whole retained
austenite to 5% or more. In the embodiments of the present
invention, "retained austenite with a circle equivalent diameter of
1.5 .mu.m or more" is sometimes referred to as "relatively coarse
retained austenite."
[0042] The average circle equivalent diameter of retained austenite
is preferably 0.95 .mu.m or less, more preferably 0.90 .mu.m or
less. A lower limit of the average circle equivalent diameter of
retained austenite is around 0.40 .mu.m, in view of the component
composition and the manufacturing conditions that are defined in
the embodiments of the present invention.
[0043] The volume ratio of retained austenite with a circle
equivalent diameter of 1.5 .mu.m or more to the whole retained
austenite is preferably 10% or more, more preferably 15% or more.
When the ratio of relatively coarse retained austenite is
excessively large, the size of the MA is easily coarsened to easily
decrease the hole expandability. From this viewpoint, the volume
ratio of retained austenite with a circle equivalent diameter of
1.5 .mu.m or more is preferably 50% or less, more preferably 40% or
less.
[0044] As described above, it is possible to obtain large
elongation when retained austenite undergoes strain-induced
transformation to cause the TRIP phenomenon. On the other hand,
martensite structure formed by the strain-induced transformation is
hard and actions as an origin of fracture. A larger martensite
structure is more likely to be the origin of fracture. By setting
the average circle equivalent diameter of retained austenite to 1.0
.mu.m or less as described above, however, it is possible to reduce
the size of martensite formed by the strain-induced transformation
and thus suppress fracture.
[0045] The average circle equivalent diameter of retained
austenite, and the volume ratio of retained austenite with a circle
equivalent diameter of 1.5 .mu.m or more to the whole retained
austenite can be derived, as indicated in Examples described later,
by preparing a phase map, using an EBSD (Electron Back Scatter
Diffraction patterns) method, as a crystal analysis method with use
of an SEM (Scanning Electron Microscope).
[0046] The metallographic structure of the steel sheet according to
the embodiments of the present invention include, as described
above, ferrite, tempered martensite, bainite, retained austenite,
and the MA. The metallographic structure may be composed of only
these structures but may also include a balance structure such as
perlite within a scope that does not impair the effects of the
present invention.
[0047] 2. Component Composition
[0048] Hereinafter, the high-strength steel sheet according to the
embodiments of the present invention is described in terms of
composition. All the unit expression % in component composition
means mass %.
[0049] [C: 0.15.about.0.35%]
[0050] C is an essential element for obtaining a desired structure
such as retained austenite to secure high characteristics such as
TS.times.EL. In order to effectively produce such an action, an
amount of C is set to 0.15% or more. The amount of C is preferably
0.17% or more, more preferably 0.20% or more. On the other hand,
when the amount of C is more than 0.35%, the MA and retained
austenite become coarse, as a result, both the deep drawability and
the hole expandability decrease. Further, when the amount of C is
excessively large, the steel sheet is also inferior in weldability.
Therefore, the amount of C is set to 0.35% or less. The amount of C
is preferably 0.33% or less, more preferably 0.30% or less.
[0051] [Total of Si and Al: 0.5 to 2.5%]
[0052] Si and Al respectively serve to suppress precipitation of
cementite and promote formation of retained austenite. In order to
effectively produce such an action, Si and Al are contained in an
amount of 0.5% or more in total. The total of Si and Al is
preferably 0.7% or more, more preferably 1.0% or more. On the other
hand, when the total of Si and Al is more than 2.5%, tempered
martensite and bainite cannot be secured and the MA and retained
austenite become coarse. Therefore, the total of Si and Al is set
to 2.5% or less, preferably 2.0% or less.
[0053] Among Si and Al, Al may be contained in an addition amount
that is the degree to which Al functions as a deoxidation element,
that is, less than 0.10%. Alternatively, for a purpose of, for
example, suppressing formation of cementite and increasing the
amount of retained austenite, Al may be contained in a much amount,
for example, in an amount of 0.7% or more.
[0054] [Mn: 1.0.about.4.0%]
[0055] Mn is a necessary element to suppress formation of ferrite
and secure tempered martensite/bainite and the like other than
ferrite. In order to effectively produce such an action, an amount
of Mn is set to 1.0% or more. The amount of Mn is preferably 1.5%
or more, more preferably 2.0% or more. When the amount of Mn is
more than 4.0%, however, bainite transformation is suppressed in a
manufacturing process and the relatively coarse retained austenite
described above cannot be obtained sufficiently, as a result,
excellent deep drawability cannot be secured. Therefore, the amount
of Mn is set to 4.0% or less, preferably 3.7% or less, more
preferably 3.5% or less.
[0056] [P: More than 0% and 0.05% or Less]
[0057] P inevitably exists as an impurity element. When an amount
of P is more than 0.05%, the total elongation EL and the hole
expandability are degraded. Therefore, the amount of P is set to
0.05% or less, preferably 0.03% or less.
[0058] [S: More than 0% and 0.01% or Less]
[0059] S inevitably exists as an impurity element. When an amount
of S is more than 0.01%, a sulfide-based inclusion such as MnS is
formed that functions as an origin of breaking to decrease the hole
expandability. Therefore, the amount of S is 0.01% or less,
preferably 0.005% or less.
[0060] In one preferable embodiment, the balance of the component
composition of the high-strength steel sheet is Fe and inevitable
impurities. As the inevitable impurities, trace elements such as,
As, Sb and Sn are allowed to be incorporated, the trace elements
being brought in depending on a situation such as a raw material, a
material or manufacturing facilities. As in cases of P and S, for
example, there is an element whose content is usually preferably
smaller and thus that is an inevitable impurity but whose
composition range is separately defined as described above.
Therefore, in the present specification, the "inevitable
impurities" that constitute the balance are a concept other than an
element whose composition range is separately defined.
[0061] The high-strength steel sheet according to the embodiments
of the present invention may further contain any other element as
long as the steel sheet can retain the characteristics. Exemplified
below are other elements that are experimentally considered to be
selectively contained without impairing the effects of the present
invention.
[0062] [(a) at Least One Selected from Group Consisting of Cu, Ni,
Mo, Cr and B: More than 0% and 1.0% or Less in Total]
[0063] Cu, Ni, Mo, Cr and B serve as elements that increase the
tensile strength of the steel sheet, and are elements that
stabilize retained austenite and effectively action to secure a
predetermined amount of retained austenite. In order to effectively
produce such effects, the elements are contained in a total amount
of preferably 0.001% or more, more preferably 0.01% or more. When
the elements are excessively contained, however, the effects of the
elements are saturated to cause economical waste, so that the total
amount of the elements is preferably 1.0% or less, more preferably
0.5% or less. The elements can be contained alone or in combination
of any two or more elements selected.
[0064] [(b) at Least One Selected from Group Consisting of V, Nb,
Ti, Zr and Hf: More than 0% and 0.2% or Less in Total]
[0065] V, Nb, Ti, Zr and Hf are elements for manufacturing effects
of precipitation strengthening and refining a structure and
usefully action for high strengthening of the steel sheet. In order
to effectively produce such effects, the elements are contained in
a total amount of preferably 0.01% or more, more preferably 0.02%
or more. When the elements are excessively contained, however, the
effects of the elements are saturated to cause economical waste, so
that the total amount of the elements is preferably 0.2% or less,
more preferably 0.1% or less. The elements can be contained alone
or in combination of any two or more elements selected.
[0066] [(c) at Least One Selected from Group Consisting of Ca, Mg
and REM: More than 0% and 0.01% or Less in Total]
[0067] Ca, Mg and REM are elements that control a form of a sulfide
in steel and effectively action to improve processability. In order
to effectively produce such effects, the elements are contained in
a total amount of preferably 0.001% or more, more preferably 0.002%
or more. When the elements are excessively contained, however, the
effects of the elements are saturated to cause economical waste, so
that the total amount of the elements is preferably 0.01% or less
and is further preferably set to 0.005% or less. The elements can
be contained alone or in combination of any two or more elements
selected. Examples of the REM include Sc, Y, lanthanoids and the
like.
[0068] 3. Characteristics
[0069] As described above, the high-strength steel sheet according
to the embodiments of the present invention attains a high level in
any of the TS, the YR, the TS.times.EL, the and the LDR.
Hereinafter, the high-strength steel sheet according to the
embodiments of the present invention is described in detail in
terms of these characteristics.
[0070] (1) Tensile Strength TS
[0071] The high-strength steel sheet according to the embodiments
of the present invention has a tensile strength of 780 MPa or more.
The high-strength steel sheet has a tensile strength of preferably
880 MPa or more, more preferably 980 MPa or more. The strength is
preferably higher, but an upper limit of the tensile strength is
around 1300 MPa, in view of the component composition, the
manufacturing conditions and the like of the steel sheet according
to the embodiments of the present invention.
[0072] (2) Yield Ratio YR
[0073] The high-strength steel sheet according to the embodiments
of the present invention has a yield ratio of 0.60 or more. Such a
yield ratio together with the high tensile strength described above
can realize high yield strength. As a result, a final product
obtained by processing such as deep drawing is usable without
deformation even if a high load is applied to it. The high-strength
steel sheet preferably has a yield ratio of 0.65 or more. The yield
ratio is preferably higher, but an upper limit of the yield ratio
is around 0.75, in view of the component composition, the
manufacturing conditions and the like of the steel sheet according
to the embodiments of the present invention.
Product of tensile strength TS and total elongation EL, TS.times.EL
(3)
[0074] The high-strength steel sheet according to the embodiments
of the present invention has a TS.times.EL of 22000 MPa % or more.
The high-strength steel sheet having a TS.times.EL of 22000 MPa %
or more is capable of simultaneously having high strength and high
ductility, attaining a high level of strength-ductility balance.
The TS.times.EL is preferably 23000 MPa or more. The TS.times.EL is
preferably higher, but an upper limit of the TS.times.EL is around
35000 MPa %, in view of the component composition, the
manufacturing conditions and the like of the steel sheet according
to the embodiments of the present invention.
[0075] (4) Hole Expansion Ratio .lamda.
[0076] The high-strength steel sheet according to the embodiments
of the present invention has a hole expansion ratio .lamda. of 20%
or more. This can give excellent processability such as press
formability. The high-strength steel sheet preferably has a hole
expansion ratio .lamda. of 30% or more. The hole expansion ratio
.lamda. is preferably higher, but an upper limit of the hole
expansion ratio is around 60%, in view of the component
composition, the manufacturing conditions and the like of the steel
sheet according to the embodiments of the present invention.
[0077] (5) Deep Drawability
[0078] In the embodiments of the present invention, the deep
drawability of the steel sheet is evaluated by the LDR as indicated
in Examples described later. The high-strength steel sheet
according to the embodiments of the present invention has an LDR of
2.05 or more, preferably 2.10 or more and thus has excellent deep
drawability. The LDR is preferably higher, but an upper limit of
the LDR is around 2.25, in view of the component composition, the
manufacturing conditions and the like of the steel sheet according
to the embodiments of the present invention.
[0079] The high-strength steel sheet according to the embodiments
of the present invention is excellent in any of the TS, the YR, the
TS.times.EL, the and the LDR, so that the high-strength steel sheet
is capable of being suitably used for steel-formed components for
various application including a component for vehicles.
[0080] 4. Manufacturing Method
[0081] Next, a method for manufacturing the high-strength steel
sheet according to the embodiments of the present invention is
described. The present inventors have found that the high-strength
steel sheet is obtained that has the desired steel structure
described above and thus has the desired characteristics described
above, by performing, on an original sheet having the predetermined
composition, a heating treatment including steps A to E described
in detail below, especially, a multi-step austempering treatment.
Hereinafter, details are described.
[0082] As the original sheet on which the heat treatment is
performed, there can be exemplified a hot-rolled steel sheet that
has undergone hot rolling and a cold-rolled steel sheet that has
further undergone cold rolling. Conditions of the hot rolling and
the cold rolling are not especially limited.
[0083] The heat treatment performed on the original sheet is
described with reference to FIG. 1 schematically illustrating the
heat treatment.
[0084] [Step A: Step of Heating to Temperature T1 of (Ac.sub.1
Point.times.0.8+Ac.sub.3 Point.times.0.2) or Higher and Lower than
Ac.sub.3 Point]
[0085] The step A corresponds to [2] in FIG. 1. As illustrated by
[2] in FIG. 1, heating is performed to a temperature T1 of
(Ac.sub.1 point.times.0.8+Ac.sub.3 point.times.0.2) or higher and
lower than Ac.sub.3 point. When the heating temperature T1 is lower
than (Ac.sub.1 point.times.0.8+Ac.sub.3 point.times.0.2), reverse
transformation into austenite is insufficient to allow a processed
structure to remain, so that it is impossible to secure a certain
amount of ferrite and thus the strength-ductility balance is
decreased. Therefore, the heating temperature T1 is set to
(Ac.sub.1 point.times.0.8+Ac.sub.3 point.times.0.2) or higher,
preferably (Ac.sub.1 point.times.0.8+Ac.sub.3
point.times.0.2)+5.degree. C. or higher, more preferably (Ac.sub.1
point.times.0.8+Ac.sub.3 point.times.0.2)+10.degree. C. or higher.
When the heating temperature T1 is Ac.sub.3 point or higher,
however, it is impossible to secure a desired amount of ferrite and
thus the ductility is degraded. Therefore, the heating temperature
T1 is set to lower than Ac.sub.3 point. The heating temperature T1
is preferably Ac.sub.3 point--10.degree. C. or lower, more
preferably Ac.sub.3 point--20.degree. C. or lower.
[0086] A retention time t1 in the heating temperature T1 is
preferably 1 to 1800 seconds, in view of securement of the effects
by the heating, productivity and the like.
[0087] The Ac.sub.1 and Ac.sub.3 points are derived from following
formulae (1) and (2) respectively that are described in "The
Physical Metallurgy of Steels" by Leslie (published from Maruzen
Co., Ltd., May 31, 1985, p. 273). In either of the formulae, each
of the elements in the formula represents a content by mass % of
the element in a steel sheet, and calculation is performed with an
element that is not contained set as zero.
Ac.sub.1 point=723-10.7Mn+29.1Si+16.9Cr (1)
Ac.sub.3
point=910-203C.sup.0.5+44.7Si+104V+31.5Mo-30Mn-11Cr+700P+400Al+-
400Ti (2)
[0088] In a temperature rising stage illustrated by [1] in FIG. 1,
the temperature may be raised at arbitrary heating rate. For
example, the temperature is raised from room temperature to the
heating temperature T1 at an average heating rate HR1 of 1.degree.
C./s or more and 20.degree. C./s or less.
[0089] [Step B: step of cooling after heating, from rapid cooling
start temperature T2 of 650.degree. C. or higher to cooling stop
temperature T3a of 300 to 500.degree. C. at average cooling rate of
30.degree. C./s or more and less than 200.degree. C./s]
[0090] The step B corresponds to [4] in FIG. 1. As illustrated by
[4] in FIG. 1, rapid cooling is performed between a rapid cooling
start temperature T2 of 650.degree. C. or higher and a cooling stop
temperature T3a of 300 to 500.degree. C. at an average cooling rate
CR2 of 30.degree. C./s or more, thereby formation of excessive
ferrite can be supressed. The average cooling rate CR2 is
preferably 35.degree. C./s or more, more preferably 40.degree. C./s
or more. When the average cooling rate CR2 is too high, however,
excessive thermal strain is likely to be generated by sharp
cooling, and thus the average cooling rate CR2 is set to less than
200.degree. C./s, preferably 100.degree. C./s or less.
[0091] The rapid cooling at an average cooling rate CR2 of
30.degree. C./s or more is started from the rapid cooling start
temperature T2 of 650.degree. C. or higher. When the rapid cooling
start temperature T2 is lower than 650.degree. C., excessive growth
of ferrite occurs before the start of the rapid cooling to cause an
inconvenience such as being unable to secure a desired amount of
ferrite. The rapid cooling start temperature T2 is preferably
680.degree. C. or higher, more preferably 700.degree. C. or higher.
An upper limit of the rapid cooling start temperature T2 is not
especially limited and may be the heating temperature T1 or
lower.
[0092] The cooling stop temperature T3a of 300 to 500.degree. C. is
rapid cooling finish temperature at which the rapid cooling is
finished and is also start temperature of gradual cooling in the
step C described below. When the temperature T3a is higher than
500.degree. C., that is, if the rapid cooling is stopped in a high
temperature range, a carbon-condensed region excessively increases
to coarsen not only retained austenite but also the MA, and thus
the hole expansion ratio is decreased. The temperature T3a is
preferably 480.degree. C. or lower, more preferably 460.degree. C.
or lower. On the other hand, when the temperature T3a is lower than
300.degree. C., that is, if the rapid cooling is performed to a low
temperature range, the carbon-condensed region decreases to result
in being unable to secure relatively coarse retained austenite with
a circle equivalent diameter of 1.5 .mu.m or more, and thus the
deep drawability is decreased. The temperature T3a is preferably
320.degree. C. or higher, more preferably 340.degree. C. or
higher.
[0093] In the embodiments of the present invention, an average
cooling rate CR1 is not especially limited, which is employed in a
temperature decrease from the heating temperature T1 to 650.degree.
C. illustrated by [3] in FIG. 1. As the average cooling rate CR1,
there can be exemplified cooling at a relatively low average
cooling rate of 0.1.degree. C./s or more and 10.degree. C./s or
less.
[0094] [Step C: step of performing gradual cooling after rapid
cooling, in temperature range from 300 to 500.degree. C. at average
cooling rate of 0.degree. C./s or more and 10.degree. C./s or less
for 10 seconds or more and less than 300 seconds after rapid
cooling]
[0095] The step C corresponds to [5] in FIG. 1. As illustrated by
[5] in FIG. 1, a temperature range from a gradual cooling start
temperature, i.e., the rapid cooling finish temperature T3a to a
gradual cooling finish temperature T3b illustrated in FIG. 1 is 300
to 500.degree. C. An average cooling rate from the T3a to the T3b
is set to 0.degree. C./s or more and 10.degree. C./s or less, and a
time from the T3a to the T3b, that is, a gradual cooling time t3 is
set to 10 seconds or more and less than 300 seconds. These
conditions allow partial formation of bainite. Bainite has a lower
solid solubility limit of carbon than a solid solubility limit of
austenite, so that carbon exceeding the solid solubility limit is
discharged. This results in forming a region of austenite where
carbon is condensed around bainite. This region undergoes cooling
in the step D and reheating in the step E that are described later
to become relatively coarse retained austenite. Existence of this
relatively coarse retained austenite enables an increase of the
deep drawability as described above.
[0096] Described below are reasons for setting the temperature
range of the gradual cooling to 300 to 500.degree. C. That is, when
the gradual cooling start temperature T3a is higher than
500.degree. C., as described above, the carbon-condensed region
becomes excessively large to coarsen not only retained austenite
but also the MA, and thus the hole expansion ratio is decreased.
The temperature T3a is preferably 480.degree. C. or lower, more
preferably 460.degree. C. or lower. On the other hand, when the
gradual cooling finish temperature T3b is lower than 300.degree.
C., the carbon-condensed region is refined due to progress of
bainite transformation at low temperature to be unable to
sufficiently secure relatively coarse retained austenite, and thus
the deep drawability is decreased. The temperature T3b is
preferably 320.degree. C. or higher, more preferably 340.degree. C.
or higher.
[0097] When the gradual cooling time t3 is less than 10 seconds,
the area of the carbon-condensed region decreases and the amount of
retained austenite with a circle equivalent diameter of 1.5 .mu.m
or more becomes insufficient, and thus the deep drawability is
decreased. Therefore, the gradual cooling time t3 is set to 10
seconds or more, preferably 30 seconds or more, more preferably 50
seconds or more. On the other hand, when the gradual cooling time
t3 is 300 seconds or more, the carbon-condensed region becomes
excessively large to coarsen not only retained austenite but also
the MA, and thus the hole expansion ratio is decreased. Therefore,
the gradual cooling time t3 is set to less than 300 seconds,
preferably 200 seconds or less, more preferably 100 seconds or
less, further preferably 80 seconds or less, still further
preferably 60 seconds or less.
[0098] When the average cooling rate of the gradual cooling is more
than 10.degree. C./s, sufficient bainite transformation does not
occur, as a result, sufficient carbon-condensed region does not
form and thus the amount of relatively coarse retained austenite
becomes insufficient. The average cooling rate is preferably
8.degree. C./s or less, more preferably 3.degree. C./s or less.
Retaining at an average cooling rate of 0.degree. C./s, that is, at
constant temperature may be performed. Alternatively, the cooling
rate may be changed within the above range in the above temperature
range, or the gradual cooling may be combined with the retaining at
constant temperature.
[0099] As preferable conditions of the gradual cooling, there can
be exemplified gradual cooling in a temperature range from 320 to
480.degree. C. at an average cooling rate of 0.degree. C./s or more
and 8.degree. C./s or less for 30 to 80 seconds. More preferably,
the gradual cooling is performed in a temperature range from 340 to
460.degree. C. at an average cooling rate of 0.degree. C./s or more
and 3.degree. C./s or less for 50 to 60 seconds.
[0100] In Patent Document 1 described above, it is considered that
the gradual cooling is not performed but reheating after excessive
cooling is carried out. As a result, in Patent Document 1, it is
considered that the amount of relatively coarse retained austenite,
that is, the ratio of retained austenite with a circle equivalent
diameter of 1.5 .mu.m or more is low and thus deep drawability is
inferior to that of the steel sheet of the present invention.
[0101] [Step D: step of cooling after gradual heating, from gradual
cooling finish temperature T3b of 300.degree. C. or higher to
cooling stop temperature T4 of 100 to 300.degree. C. at average
cooling rate of more than 10.degree. C./s]
[0102] The step D corresponds to [6] in FIG. 1. As illustrated by
[6] in FIG. 1, cooling is performed from the gradual cooling finish
temperature T3b of 300.degree. C. or higher to a cooling stop
temperature T4 of 100 to 300.degree. C. at an average cooling rate
CR3 of more than 10.degree. C./s to allow a part of the
carbon-condensed region to undergo martensite transformation. This
enables refining of the remaining carbon-condensed region that has
not undergone martensite transformation, and thus fine MA is
obtained. When the average cooling rate CR3 is 10.degree. C./s or
less, the carbon-condensed region outspreads more than necessary
during the cooling to coarsen the MA, and thus the hole expansion
ratio is decreased. The average cooling rate CR3 is preferably
15.degree. C./s or more, more preferably 20.degree. C./s or more.
The gradual cooling finish temperature T3b is 500.degree. C. or
lower as described in the step C.
[0103] The stop temperature T4 of the cooling at the average
cooling rate CR3 is controlled in a temperature range from 100 to
300.degree. C. to adjust an amount of remaining austenite that does
not undergo martensite transformation and thus control an amount of
a final amount of retained austenite. When the cooling stop
temperature T4 is lower than 100.degree. C., the amount of retained
austenite becomes short. This results in an increase of the TS but
a decrease of the EL to make the TS.times.EL balance insufficient.
The cooling stop temperature T4 is preferably 120.degree. C. or
higher, more preferably 140.degree. C. or higher. On the other
hand, when the cooling stop temperature T4 is higher than
300.degree. C., coarse untransformed austenite increases and
remains even after following cooling, and eventually the size of
the MA is coarsened and thus the hole expansion ratio .lamda. is
decreased. Therefore, the cooling stop temperature T4 is set to
300.degree. C. or lower. The cooling stop temperature T4 is
preferably 280.degree. C. or lower, more preferably 260.degree. C.
or lower. In one preferable embodiment, the gradual cooling finish
temperature T3b is set to a cooling start temperature as
illustrated by [6] in FIG. 1.
[0104] After the cooling at the average cooling rate CR3, retaining
at the cooling stop temperature T4 may be performed as illustrated
by [7] in FIG. 1 or reheating treatment described later may be
performed without the retaining. When the retaining is performed at
the cooling stop temperature T4, a retention time t4 is preferably
set to 1 to 600 seconds. This is because the retention time t4
hardly affects the characteristics even when being long, but
decreases productivity when being more than 600 seconds.
[0105] [Step E: Step of Reheating to Reheating Temperature T5 of
300 to 500.degree. C.]
[0106] The step E corresponds to [9] in FIG. 1. As illustrated by
[9] in FIG. 1, reheating to a reheating temperature T5 of 300 to
500.degree. C. is performed and retaining is performed at the
reheating temperature T5 to discharge carbon in martensite and thus
promote condensation of carbon in austenite around martensite, and
austenite can be stabilized. This enables an increase in the amount
of retained austenite eventually obtained.
[0107] When the reheating temperature T5 is lower than 300.degree.
C., diffusion of carbon becomes insufficient to give no sufficient
amount of retained austenite and thus the TS.times.EL is decreased,
and to give no effect of improving the deep drawability by control
of a particle diameter distribution of retained austenite.
Therefore, the reheating temperature T5 is set to 300.degree. C. or
higher, preferably 320.degree. C. or higher, more preferably
340.degree. C. or higher. On the other hand, when the reheating
temperature T5 is higher than 500.degree. C., carbon is
precipitated as cementite to give no sufficient amount of retained
austenite. As described above, such shortage of retained austenite
decreases the TS.times.EL and gives no effect of improving the deep
drawability by control of a particle diameter distribution of
retained austenite. Therefore, the reheating temperature T5 is set
to 500.degree. C. or lower, preferably 480.degree. C. or lower,
more preferably 460.degree. C. or lower.
[0108] A reheating time t5 during retaining at the reheating
temperature T5 is not especially limited and is, for example, 50
seconds or more, further 70 seconds or more, further 90 seconds or
more. An upper limit of the reheating time t5 is, for example, 1200
seconds or less, further 900 seconds or less, further 600 seconds
or less.
[0109] An average heating rate HR2 is not especially limited in a
temperature rising step illustrated by [8] in FIG. 1 from the
cooling stop temperature T4 to the reheating temperature T5.
Cooling illustrated by [10] in FIG. 1 after the reheating may be
preferably performed to 200.degree. C. or lower, for example, room
temperature at an average cooling rate CR4 of 2 to 20.degree.
C./s.
EXAMPLES
[0110] Hereinafter, the embodiments of the present invention will
be more specifically described with reference to examples. The
present invention is not limited by the following examples, and can
be carried out by adding a modification thereto within a range that
is suitable for the gist described above and below, and any
modifications are included in the technical range of the present
invention.
[0111] 1. Preparation of Sample
[0112] A casting material having component composition (with the
balance being iron and inevitable impurities) described in Table 1
was produced by vacuum melting and then subjected to hot forging to
form a sheet with a sheet thickness of 30 mm, followed by hot
rolling. Table 1 also shows values of the Ac.sub.1 and Ac.sub.3
points and "0.8.times.Ac.sub.1 point+0.2.times.Ac.sub.3 point" that
were calculated from the component composition. In the hot rolling,
the sheet that had been heated to 1200.degree. C. was subjected to
multistage rolling to have a sheet thickness of 2.5 mm. At this
time, hot rolling finish temperature was set to 880.degree. C.
Then, the sheet was cooled to 600.degree. C. at 30.degree. C./s,
inserted into a furnace heated to 600.degree. C. after the cooling
was stopped, retained for 30 minutes, and then furnace cooled to
give a hot-rolled steel sheet. The hot-rolled steel sheet was
subjected to acid pickling to remove scale on a surface of the
steel sheet and subjected to cold rolling to give a 1.4 mm
cold-rolled steel sheet as an original sheet. This original pate
was subjected to a heat treatment under conditions indicated in
Tables 2-1 and 2-2 to give a sample. In the present examples except
No. 17, the average heating rate H2 was set to 30.degree. C./s in
the temperature rising step from the cooling stop temperature T4 to
the reheating temperature T5.
[0113] The samples obtained were evaluated for a steel structure
and characteristics as described below.
TABLE-US-00001 TABLE 1 Component composition (mass %), balance
being iron and 0.8Ac.sub.1 + Steel inevitable impurities Ac.sub.1
Ac.sub.3 0.2Ac.sub.3 code C Si Mn P S Al Si + Al (.degree. C.)
(.degree. C.) (.degree. C.) a 0.21 1.63 2.18 0.008 0.001 0.03 1.66
747 842 766 b 0.24 1.32 2.50 0.010 0.002 0.04 1.36 735 818 751 c
0.17 1.87 1.62 0.006 0.003 0.04 1.91 760 881 784 d 0.33 1.08 1.98
0.007 0.002 0.03 1.11 733 799 746 e 0.21 1.75 2.66 0.014 0.003 0.03
1.78 745 837 764 f 0.23 1.35 1.95 0.014 0.003 0.03 1.38 741 836 760
g 0.22 1.15 2.24 0.010 0.001 0.03 1.18 732 818 750 h 0.21 0.81 2.31
0.010 0.002 0.03 0.84 722 803 738 i 0.23 2.15 2.32 0.007 0.002 0.02
2.17 761 852 779 j 0.22 1.72 3.53 0.005 0.003 0.04 1.76 735 805 749
k 0.22 1.70 2.70 0.007 0.001 0.43 2.13 744 987 792 l 0.12 1.95 1.17
0.009 0.002 0.02 1.97 767 906 795 m 0.48 1.07 2.02 0.009 0.002 0.02
1.09 733 771 740 n 0.34 0.12 2.57 0.012 0.002 0.04 0.16 699 744 708
o 0.27 3.49 1.44 0.013 0.002 0.04 3.53 809 942 836 p 0.18 1.84 0.77
0.007 0.003 0.03 1.87 768 900 795 q 0.23 1.29 4.33 0.014 0.003 0.04
1.33 714 766 725 r 0.19 1.99 2.62 0.014 0.001 0.02 2.01 753 850 772
s 0.29 1.58 2.07 0.008 0.003 0.04 1.62 747 831 764 t 0.17 2.34 2.02
0.012 0.002 0.04 2.38 769 895 795
TABLE-US-00002 TABLE 2-1 [3] [4] [5] [1] [2] Average Rapid Average
Rapid Gradual Heating Heating Retention cooling cooling start
cooling cooling stop cooling rate temperature time rate temperature
rate temperature time Steel HR1 T1 t1 CR1 T2 CR2 T3a t3 No. code
(.degree. C./s) (.degree. C.) (s) (.degree. C./s) (.degree. C.)
(.degree. C./s) (.degree. C.) (s) 1 a 10 800 120 10 700 40 400 50 2
a 10 800 120 5 700 40 440 80 3 a 10 850 120 10 700 40 400 50 4 a 10
800 120 10 700 40 550 50 5 a 10 800 120 10 700 40 400 50 6 a 10 800
120 10 700 40 400 50 7 b 10 780 120 10 700 40 400 50 8 b 10 780 120
10 700 40 400 50 9 b 10 800 120 10 700 40 250 50 10 b 10 800 120 10
700 40 400 50 11 c 10 830 120 10 700 40 400 50 12 d 10 780 120 10
700 40 400 50 13 d 10 760 200 10 700 40 360 50 14 e 10 800 120 10
700 40 400 50 15 f 10 800 120 10 700 40 400 50 16 f 10 800 120 10
700 40 420 70 17 f 10 800 120 10 700 40 400 300 18 f 10 800 120 10
700 40 -- -- 19 f 10 800 120 10 700 40 400 400 20 g 10 800 120 10
700 40 400 50 [5] Gradual [6] [10] cooling Average [7] [9] Average
finish cooling Cooling stop Retention Reheating Reheating cooling
temperature rate temperature time temperature time rate T3b CR3 T4
t4 T5 t5 CR4 No. (.degree. C.) (.degree. C./s) (.degree. C.) (s)
(.degree. C.) (s) (.degree. C./s) 1 350 30 200 50 400 300 10 2 420
30 170 50 400 300 10 3 400 30 200 50 400 300 10 4 500 30 200 50 400
300 10 5 400 30 200 50 250 300 10 6 400 30 50 50 400 300 10 7 400
30 200 50 400 300 10 8 400 30 200 50 550 300 10 9 250 30 200 50 400
300 10 10 350 1 200 50 400 300 10 11 400 30 200 -- 400 300 10 12
400 30 200 50 400 300 10 13 360 30 170 50 430 300 10 14 400 30 200
50 400 300 10 15 400 30 200 50 400 300 10 16 400 30 230 50 360 300
10 17 -- -- -- -- -- -- 10 18 -- -- 200 50 400 300 10 19 400 30 200
50 400 300 10 20 350 30 200 50 400 300 10
TABLE-US-00003 TABLE 2-2 [3] [4] [5] [1] [2] Average Rapid Average
Rapid Gradual Heating Heating Retention cooling cooling start
cooling cooling stop cooling rate temperature time rate temperature
rate temperature time Steel HR1 T1 t1 CR1 T2 CR2 T3a t3 No. code
(.degree. C./s) (.degree. C.) (s) (.degree. C./s) (.degree. C.)
(.degree. C./s) (.degree. C.) (s) 21 g 10 800 120 10 700 40 400 5
22 g 10 800 120 10 700 40 400 150 23 h 10 780 120 10 700 40 400 50
24 i 10 830 120 10 700 40 400 50 25 i 10 820 120 10 700 40 370 50
26 j 10 780 120 10 700 40 400 50 27 k 10 900 120 10 700 40 400 50
28 l 10 840 120 10 700 40 400 50 29 m 10 750 120 10 700 40 400 50
30 n 10 720 120 10 700 40 400 50 31 o 10 850 120 10 700 40 400 50
32 p 10 800 120 10 700 40 400 50 33 q 10 730 120 10 700 40 400 50
34 r 10 820 120 10 700 40 480 30 35 r 10 800 120 10 700 40 400 50
36 s 10 800 120 10 700 40 420 50 37 s 10 780 120 10 700 40 330 50
38 t 10 830 120 10 700 40 400 50 39 t 10 840 120 10 700 40 430 50
40 t 10 815 120 20 700 40 360 100 [5] Gradual [6] [10] cooling
Average [7] [9] Average finish cooling Cooling stop Retention
Reheating Reheating cooling temperature rate temperature time
temperature time rate T3b CR3 T4 t4 T5 t5 CR4 No. (.degree. C.)
(.degree. C./s) (.degree. C.) (s) (.degree. C.) (s) (.degree.C/s)
21 400 30 200 50 400 300 10 22 320 30 200 50 400 200 10 23 400 30
200 50 400 300 10 24 400 30 200 50 400 300 10 25 340 30 250 50 420
300 10 26 350 30 200 50 400 300 10 27 350 30 200 50 400 300 10 28
400 30 200 50 400 300 10 29 400 30 200 50 400 300 10 30 400 30 200
50 400 300 10 31 400 30 200 50 400 300 10 32 400 30 200 50 400 300
10 33 400 30 200 50 400 300 10 34 440 30 270 50 420 300 10 35 400
15 230 50 460 300 10 36 400 30 120 50 400 300 10 37 330 30 200 30
420 300 10 38 380 20 200 50 400 300 10 39 430 30 220 20 380 300 10
40 350 30 140 50 420 300 10
[0114] 2. Evaluation of Steel Structures
[0115] The samples obtained were measured as described below for
the steel structure at a position of the sheet thickness t/4.
[0116] [Area Proportion of Ferrite]
[0117] The sample was electrolytically polished, then subjected to
LePera etching, observed by 3 fields (100 .mu.m.times.100 .mu.m
size/field) with an optical microscope (1000 times), and measured
for an area proportion of ferrite by a point arithmetic method with
20.times.20 grating points at a grating space of 5 .mu.m, and an
average value of area proportions of ferrite was calculated. The
value derived in the area ratio can be directly used as a value of
a volume proportion (volume %).
[0118] [Area Proportion of Total of Tempered Martensite and
Bainite]
[0119] An area proportion of the total of tempered martensite and
bainite was derived by deducting the fraction of ferrite described
above and the fraction of the MA derived by a following method,
that is, the total of retained austenite and martensite that has
been only quenched, from the whole structure.
[0120] [Area Proportion and Average Circle Equivalent Diameter of
MA]
[0121] The sample was electrolytically polished, then subjected to
nital etching, observed by 3 fields (20 .mu.m.times.20 .mu.m
size/field) with an SEM (5000 times), and measured for an average
circle equivalent diameter of the MA by drawing 20 straight lines
with a length of 10 .mu.m at arbitrary positions in a photograph,
measuring length of segments of the MA that intersect with the
straight lines, and calculating an average value of the length of
the segments. The area proportion of the MA is calculated on the
basis of the average circle equivalent diameter of the MA and the
number of MAs observed in a field, and an average of the three
fields was derived.
[0122] [Volume Ratio of Retained Austenite]
[0123] The amount of retained austenite can be obtained by deriving
through X-ray diffraction a diffraction intensity ratio between
ferrite, bainite, tempered martensite and martensite as
body-centered cubic lattices or body-centered tetragonal lattices
and austenite as a face-centered cubic lattice and by calculation
based on the diffraction intensity ratio. In detail, measurement
was as follows. That is, the sample was polished to a position of
1/4 the sheet thickness using #1000 to #1500 sand paper, and then
the surface was further electrolytically polished to a depth of
about 10 to 20 .mu.m and measured using a two dimensional micro
area X-ray diffractometer (RINT-RAPIDII) manufactured by Rigaku
Corporation, as a X-ray diffractometer. Specifically, the
measurement was performed using a Co target, using a Co-K.alpha.
ray as an X-ray source, at an output of about 40 kV-200 mA, in a 20
range from 40.degree. to 130.degree.. Then, retained .gamma. was
quantitatively measured from obtained diffraction peaks (110),
(200) and (211) of the bcc (.alpha.) and diffraction peaks (111),
(200), (220) and (311) of the fcc (.gamma.).
[0124] [Average Circle Equivalent Diameter of Retained Austenite,
and Ratio of Retained Austenite with Circle Equivalent Diameter of
1.5 .mu.m or More]
[0125] The average circle equivalent diameter of retained
austenite, and the ratio of the amount of retained austenite with a
circle equivalent diameter of 1.5 .mu.m or more to the amount of
the whole retained austenite are derived as described above by
preparing a phase map according to an EBSD method which is a
crystal analysis with use of an SEM. In detail, a surface of the
sample was electrolytically polished, and EBSD measurement (OIM
system manufactured by TexSEM Laboratories, Inc.) was performed at
a position of 1/4 the sheet thickness by total three regions (100
.mu.m.times.100 .mu.m size per region) at 180,000 points with one
step of 0.25 .mu.m. From the phase map obtained by this
measurement, the area of each austenite phase (retained austenite)
was derived, the circle equivalent diameter of each austenite phase
was derived from the area, and an average value of circle
equivalent diameters was defined as the average circle equivalent
diameter of retained austenite.
[0126] Integrated area was derived for retained austenite with a
circle equivalent diameter of 1.5 .mu.m or more, and the ratio of
the integrated area to the total area of retained austenite phases
was derived to obtain the ratio of retained austenite with a circle
equivalent diameter of 1.5 .mu.m or more to the whole retained
austenite. The thus-derived ratio of retained austenite with a
circle equivalent diameter of 1.5 .mu.m or more to the whole
retained austenite is an area ratio but is equivalent to a volume
ratio.
[0127] Table 3 shows results of measuring these steel structures.
Table 3 indicates ferrite as "F," tempered martensite and bainite
as "tempered MB," retained austenite with a circle equivalent
diameter of 1.5 .mu.m or more as "retained .gamma. with 1.5 .mu.m
or more."
TABLE-US-00004 TABLE 3 MA Retained .gamma. Ratio of average average
Amount of amount of circle circle retained .gamma. retained .gamma.
Tempered equivalent equivalent with 1.5 .mu.m with 1.5 .mu.m F M/B
MA diameter Retained .gamma. diameter.gamma. or more or more No.
(area %) (area %) (area %) (.mu.m) (volume %) (.mu.m) (volume %)
(%) 1 22 52 26 0.58 14.1 0.83 3.3 23.1 2 29 45 26 0.55 14.0 0.80
4.0 28.2 3 0 73 27 0.60 13.6 0.85 3.6 26.3 4 23 49 28 1.54 14.4
0.79 3.8 26.3 5 29 43 28 0.55 7.0 0.85 0.9 13.0 6 30 57 13 0.30 5.1
0.59 1.0 18.6 7 19 52 29 0.43 16.2 0.80 3.0 18.5 8 14 57 29 0.65
7.0 0.84 0.6 7.9 9 13 58 29 0.60 14.4 0.84 0.6 4.2 10 16 55 29 1.20
14.4 0.80 4.5 31.5 11 33 40 27 0.45 12.0 0.82 4.7 39.1 12 14 51 35
0.55 20.5 0.77 2.9 14.3 13 26 43 31 0.50 21.1 0.79 4.4 20.9 14 24
50 26 0.50 14.3 0.86 3.8 26.9 15 24 48 28 0.50 15.3 0.86 4.0 25.9
16 23 46 31 0.48 14.8 0.87 4.5 30.7 17 22 52 26 1.18 13.9 1.65 4.6
32.8 18 20 53 27 0.55 15.8 0.80 0.7 4.2 19 29 43 29 1.50 14.5 0.77
4.3 29.3 20 18 53 29 0.63 13.6 0.83 3.9 28.9 21 14 59 27 0.50 14.2
0.86 0.5 3.5 22 14 59 27 0.63 14.8 0.82 4.6 30.9 23 17 57 26 0.53
13.7 0.84 4.7 33.9 24 17 55 28 0.65 14.4 0.81 3.6 24.8 25 20 49 31
0.55 14.7 0.86 4.0 27.4 26 18 53 29 0.58 15.2 0.79 4.3 28.0 27 21
52 27 0.50 13.8 0.84 2.9 21.2 28 42 34 24 0.63 6.2 0.81 0.8 12.9 29
12 49 39 1.26 29.8 1.18 16.4 55.0 30 6 62 32 0.53 6.2 0.82 0.9 15.0
31 42 28 30 1.12 18.1 1.32 3.5 19.1 32 55 19 26 0.48 11.3 0.77 5.3
47.0 33 16 55 29 0.45 15.1 0.86 0.2 1.3 34 18 51 31 0.55 13.3 0.88
3.6 27.1 35 33 39 28 0.48 12.9 0.90 4.7 36.1 36 17 55 28 0.58 18.2
0.66 3.9 21.2 37 33 38 29 0.45 18.2 0.80 4.3 23.6 38 38 36 26 0.60
11.6 0.82 3.5 30.1 39 31 43 26 0.68 11.6 0.87 3.5 30.5 40 45 34 21
0.60 11.7 0.69 3.5 29.7
[0128] 3. Evaluation of Characteristics
[0129] The sample obtained was subjected to a tensile test as
indicated below to measure the yield strength YS, the tensile
strength TS, and the total elongation EL, and the yield ratio YR
and the TS.times.EL were calculated. Further, the hole expansion
ratio .lamda. was measured by a method indicated below and the deep
drawability was evaluated.
[0130] (Tensile Strength)
[0131] No. 5 test piece specified in JIS Z2201 was taken from the
sample such that a direction perpendicular to the rolling direction
is a longitudinal direction. Then, the test piece was subjected to
a tensile test with a tensile tester under conditions of JIS Z2241
to derive the YS, the TS, the YR, the EL and the TS.times.EL. Then,
a test piece having the TS of 780 MPa or more, the YR of 0.60 or
more, and the TS.times.EL of 22000 MPa % or more was evaluated as
having high strength, a high yield ratio, and excellent
strength-ductility balance.
[0132] (Measurement of Hole Expansion Ratio)
[0133] The hole expansion ratio .lamda. was derived according to
The Japan Iron and Steel Federation Standard JFS T1001. In detail,
a test piece was punched to form a punched hole with a diameter
d.sub.0 (d.sub.0=10 mm), a punch with a tip angle of 60.degree. was
pushed into the punched hole, a diameter d of the punched hole was
measured when a generated crack penetrated through the sheet
thickness of the test piece, and the hole expansion ratio was
derived from a following formula. Then, a test piece having a hole
expansion ratio .lamda. of 20% or more was evaluated as having
excellent processability such as press formability.
.lamda.(%)={(d-d.sub.0)/d.sub.0}.times.100
[0134] (Evaluation of Deep Drawability)
[0135] The deep drawability was evaluated by the LDR. The LDR is
derived as follows. That is, d is defined as a diameter of a
cylinder obtained by cylinder drawing, and D is defined as a
maximum diameter of a disk-shaped steel sheet (blank) capable of
giving a cylinder without fracture in one time of deep drawing.
Then, the LDR is derived from d/D. In the present examples,
disk-shaped specimens with a sheet thickness of 1.4 mm and various
diameters were subjected to cylinder deep drawing with a die having
a punch diameter of 50 mm, a punch angle radius of 6 mm, a die
diameter of 55.2 mm, and a die angle radius of 8 mm, the diameter
(maximum diameter D) of a specimen that had been drawn without
fracture was measured, and the d/D was derived and defined as the
LDR. A specimen having an LDR of 2.05 or more was evaluated as
having excellent deep drawability.
[0136] Table 4 shows these results.
TABLE-US-00005 TABLE 4 Deep YS TS EL TS .times. EL .lamda.
drawability No. (MPa) (MPa) YR (%) (MPa %) (%) LDR 1 682 1063 0.64
21.9 23280 42 2.09 2 615 996 0.62 24.2 24103 36 2.11 3 963 1167
0.83 17.6 20539 30 2.09 4 700 1074 0.65 22.1 23735 15 2.11 5 832
1344 0.62 14.2 19085 26 2.02 6 518 843 0.61 21.0 17703 44 1.94 7
735 1121 0.66 21.5 24102 31 2.09 8 591 865 0.68 20.2 17473 22 2.01
9 809 1173 0.69 19.1 22404 26 1.90 10 761 1129 0.67 20.4 23032 18
2.14 11 604 990 0.61 22.9 22671 24 2.13 12 829 1206 0.69 22.2 26773
25 2.07 13 655 1031 0.64 27.8 28662 22 2.13 14 698 1072 0.65 21.8
23370 35 2.12 15 690 1065 0.65 22.9 24389 32 2.13 16 771 1211 0.64
19.8 23978 27 2.15 17 1185 1820 0.65 12.6 22932 16 2.15 18 719 1096
0.66 22.2 24331 38 1.86 19 652 1062 0.61 22.6 24001 16 2.10 20 728
1095 0.66 20.6 22557 28 2.10 21 777 1130 0.69 19.9 22487 41 1.80 22
782 1139 0.69 20.1 22894 22 2.11 23 751 1105 0.68 20.2 22321 28
2.14 24 717 1076 0.67 21.4 23026 30 2.08 25 737 1123 0.66 20.6
23134 42 2.11 26 764 1141 0.67 20.6 23505 38 2.10 27 727 1114 0.65
20.5 22837 31 2.08 28 507 929 0.55 21.0 19509 43 2.07 29 883 1274
0.69 26.0 33124 18 1.99 30 885 1228 0.72 13.1 16087 30 2.06 31 582
1034 0.56 26.9 27815 19 2.16 32 378 732 0.52 33.2 24302 24 2.08 33
766 1141 0.67 20.5 23391 38 1.83 34 746 1115 0.67 20.0 22300 32
2.11 35 557 895 0.62 26.0 23270 36 2.13 36 740 1097 0.67 23.3 25560
27 2.11 37 607 991 0.61 27.4 27153 32 2.13 38 625 992 0.63 23.3
23114 28 2.11 39 653 1076 0.61 20.7 22273 35 2.11 40 508 841 0.60
28.0 23548 26 2.09
[0137] From Tables 1 to 4, following things are understood. Any of
Nos. 1, 2, 7, 11 to 16, 20, 22 to 27 and 34 to 40 have the
component composition defined in the embodiments of the present
invention and was produced under the defined conditions to obtain
the desired steel structure, so that these exhibit not only high
tensile strength and excellent deep drawability but also excellent
strength-ductility balance, a high yield ratio, and excellent hole
expandability, i.e., a high hole expansion ratio. The examples
other than described above, however, did not satisfy either the
defined component composition or the defined manufacturing
conditions to obtain no desired steel structures, as a result, at
least any one of the characteristics was inferior. Details are as
follows.
[0138] No. 3 was manufactured by the condition of an excessively
high heating temperature T1 to be unable to secure ferrite, and
thus TS.times.EL was decreased.
[0139] No. 4 was manufactured by the condition of an excessively
high rapid cooling stop temperature, i.e., an excessively high
gradual cooling start temperature T3a to give a coarse MA. This
resulted in a decrease of the hole expandability.
[0140] No. 5 was manufactured by the condition of an excessively
low reheating temperature T5 to be unable to secure a certain
amount or more of retained austenite and thus the TS.times.EL and
the deep drawability were decreased.
[0141] No. 6 was manufactured by the condition of an excessively
low cooling stop temperature T4 after the cooling at the average
cooling rate CR3 to be unable to secure a certain amount or more of
retained austenite, and thus the TS.times.EL and the deep
drawability were decreased.
[0142] No. 8 was manufactured by the condition of an excessively
high reheating temperature T5 to be unable to secure a certain
amount or more of retained austenite, and thus the TS.times.EL and
the deep drawability were decreased.
[0143] No. 9 was manufactured by the condition of excessively low
temperatures T3a and T3b for the gradual cooling, to be unable to
secure retained austenite with a circle equivalent diameter of 1.5
.mu.m or more, and thus the deep drawability was decreased.
[0144] No. 10 was manufactured by the condition of an excessively
low average cooling rate CR3 after the gradual cooling to the
cooling stop temperature T4 of 100 to 300.degree. C. to coarsen the
MA, and thus the hole expandability was decreased.
[0145] No. 17 is an example of retaining at 400.degree. C. for 300
seconds after the rapid cooling illustrated by [4] in FIG. 1 and
cooling to room temperature, without performing [6] to [9] in FIG.
1. In the case of this example, the cooling at the average cooling
rate CR3 and the reheating were not performed to cause no
martensite transformation, and the MA and retained austenite were
coarsened. This resulted in a decrease of the hole expandability.
No. 17, however, had a remarkably different structure from the
structures defined in the embodiments of the present invention, so
that retained austenite had a coarse average circle equivalent
diameter but the deep drawability was secured.
[0146] No. 18 is an example of rapidly cooling the original sheet
from a rapid cooling start temperature of 700.degree. C. to a stop
temperature of 200.degree. C. at an average cooling rate of
40.degree. C. without a holding step illustrated by [5] in FIG. 1.
In this case, it was impossible to secure retained austenite with a
circle equivalent diameter of 1.5 .mu.m or more, so that the deep
drawability was decreased.
[0147] No. 19 was manufactured by the condition of a long gradual
cooling time t3 to coarsen the MA, and thus the hole expandability
was decreased.
[0148] No. 21 was manufactured by the condition of a short gradual
cooling time t3 to be short of the amount of retained austenite
with a circle equivalent diameter of 1.5 .mu.m or more, and thus
the deep drawability was decreased.
[0149] No. 28 had an excessively small amount of C to be short of
the amount of retained austenite, and thus the TS.times.EL was
especially decreased.
[0150] No. 29 had an excessive amount of C to coarsen the MA and
retained austenite, and thus both the deep drawability and the hole
expandability were decreased.
[0151] No. 30 was short of the amount of Si+Al to be short of the
amount of retained austenite, and thus the TS.times.EL was
decreased.
[0152] No. 31 had an excessive amount of Si+Al to be short of
tempered martensite/bainite, and the MA and retained austenite were
coarsened. This resulted in a low yield ratio and a decrease of the
hole expandability. No. 31, however, had a remarkably different
structure from the structures defined in the embodiments of the
present invention, so that retained austenite had a coarse average
circle equivalent diameter but the deep drawability was
secured.
[0153] No. 32 is an example of obtaining no sufficient tempered
martensite and bainite because the amount of Mn was short to
excessively increase ferrite. This resulted in a decrease of the TS
and the YR.
[0154] No. 33 had an excessive amount of the Mn to be short of the
amount of retained austenite with a circle equivalent diameter of
1.5 .mu.m or more, and thus the deep drawability was decreased.
[0155] The disclosure of the present specification includes the
following aspects.
Aspect 1:
[0156] A high-strength steel sheet containing, as component
composition, by mass: [0157] C: 0.15 to 0.35%; [0158] total of Si
and Al: 0.5 to 2.5%; [0159] Mn: 1.0 to 4.0%; [0160] P: more than 0%
and 0.05% or less; and [0161] S: more than 0% and 0.01% or less,
with the balance being Fe and inevitable impurities, wherein [0162]
a steel structure satisfies, in ratio with respect to the whole
structure: [0163] ferrite: more than 5 area % and 50 area % or
less; [0164] total of tempered martensite and bainite: 30 area % or
more; and [0165] retained austenite: 10 volume % or more, [0166]
the steel structure further includes MA, and [0167] the steel
structure satisfies: [0168] an average circle equivalent diameter
of the MA: 1.0 .mu.m or less; [0169] an average circle equivalent
diameter of the retained austenite: 1.0 .mu.m or less; and [0170] a
volume ratio of retained austenite with a circle equivalent
diameter of 1.5 .mu.m or more to the whole retained austenite: 5%
or more.
Aspect 2:
[0171] The high-strength steel sheet according to aspect 1, wherein
the amount of C in the component composition is 0.30% or less.
Aspect 3:
[0172] The high-strength steel sheet according to aspect 1 or 2,
wherein the amount of Al in the component composition is less than
0.10%.
Aspect 4:
[0173] The high-strength steel sheet according to any one of
aspects 1 to 3, further containing, by mass: at least one selected
from the group consisting of Cu, Ni, Mo, Cr and B in a total amount
of more than 0% and 1.0% or less.
Aspect 5:
[0174] The high-strength steel sheet according to any one of
aspects 1 to 4, further containing, by mass: at least one selected
from the group consisting of V, Nb, Ti, Zr and Hf in a total of
more than 0% and 0.2% or less.
Aspect 6:
[0175] The high-strength steel sheet according to any one of
aspects 1 to 5, further containing, by mass: at least one selected
from the group consisting of Ca, Mg and REM in a total more than 0%
and 0.01% or less.
[0176] Aspect 7:
[0177] A method for manufacturing the high-strength steel sheet
according to any one of aspects 1 to 6, the method includes steps A
to E in this order: [0178] the step A of heating to a temperature
T1 of (Ac.sub.1 point.times.0.8+Ac.sub.3 point.times.0.2) or higher
and lower than Ac.sub.3 point, using an original sheet that
satisfies the component composition according to any one of aspects
1 to 6; [0179] the step B of rapidly cooling after the heating,
from a rapid cooling start temperature T2 of 650.degree. C. or
higher to a cooling stop temperature T3a of 300 to 500.degree. C.
at an average cooling rate of 30.degree. C./s or more and less than
200.degree. C./s; [0180] the step C of performing gradual cooling
after the rapid cooling, in a temperature range from 300 to
500.degree. C. at an average cooling rate of 0.degree. C./s or more
and 10.degree. C./s or less for 10 seconds or more and less than
300 seconds; [0181] the step D of cooling after the gradual
cooling, from a gradual cooling finish temperature T3b of
300.degree. C. or higher to a cooling stop temperature T4 of 100 to
300.degree. C. at an average cooling rate of more than 10.degree.
C./s; and [0182] the step E of reheating to a reheating temperature
T5 of 300 to 500.degree. C.
Aspect 8:
[0183] The manufacturing method according to aspect 7, wherein the
step C includes retaining at constant temperature in the
temperature range from 300 to 500.degree. C.
[0184] The present application claims priority to Japanese Patent
Application No. 2016-072740 filed on Mar. 31, 2016 and Japanese
Patent Application No. 2016-253902 filed on Dec. 27, 2016. Japanese
Patent Application Nos. 2016-072740 and 2016-253902 are
incorporated herein by reference.
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