U.S. patent application number 12/865527 was filed with the patent office on 2011-02-10 for high-strength steel sheet and method for manufacturing the same.
This patent application is currently assigned to JFE STEEL CORPORATION. Invention is credited to Yoshimasa Funakawa, Hiroshi Matsuda, Saiji Matsuoka, Reiko Mizuno, Tatsuya Nakagaito, Yasushi Tanaka.
Application Number | 20110030854 12/865527 |
Document ID | / |
Family ID | 40912934 |
Filed Date | 2011-02-10 |
United States Patent
Application |
20110030854 |
Kind Code |
A1 |
Matsuda; Hiroshi ; et
al. |
February 10, 2011 |
HIGH-STRENGTH STEEL SHEET AND METHOD FOR MANUFACTURING THE SAME
Abstract
A high strength steel sheet has a tensile strength of 900 MPa or
higher that can achieve both high strength and good formability and
a composition including, on a mass basis, C: 0.1% or more and 0.3%
or less; Si: 2.0% or less; Mn: 0.5% or more and 3.0% or less; P:
0.1% or less; S: 0.07% or less; Al: 1.0% or less; and N: 0.008% or
less, with the balance Fe and incidental impurities. The steel
sheet microstructure includes, on an area ratio basis, 5% or more
and 80% or less of ferrite, 15% or more of autotempered martensite,
10% or less of bainite, 5% or less of retained austenite, and 40%
or less of as-quenched martensite; the mean hardness of the
autotempered martensite is HV.ltoreq.700; and the mean number of
precipitated iron-based carbide grains each having a size of 5 nm
or more and 0.5 .mu.m or less and included in the autotempered
martensite is 5.times.10.sup.4 or more per 1 mm.sup.2.
Inventors: |
Matsuda; Hiroshi; (Tokyo,
JP) ; Mizuno; Reiko; (Tokyo, JP) ; Funakawa;
Yoshimasa; (Tokyo, JP) ; Tanaka; Yasushi;
(Tokyo, JP) ; Nakagaito; Tatsuya; (Tokyo, JP)
; Matsuoka; Saiji; (Tokyo, JP) |
Correspondence
Address: |
IP GROUP OF DLA PIPER LLP (US)
ONE LIBERTY PLACE, 1650 MARKET ST, SUITE 4900
PHILADELPHIA
PA
19103
US
|
Assignee: |
JFE STEEL CORPORATION
Tokyo
JP
|
Family ID: |
40912934 |
Appl. No.: |
12/865527 |
Filed: |
January 29, 2009 |
PCT Filed: |
January 29, 2009 |
PCT NO: |
PCT/JP2009/051915 |
371 Date: |
October 22, 2010 |
Current U.S.
Class: |
148/504 ;
148/328; 148/624 |
Current CPC
Class: |
C21D 2211/002 20130101;
C21D 1/25 20130101; C21D 8/0426 20130101; C21D 8/0436 20130101;
C22C 38/04 20130101; C23C 2/06 20130101; C23C 2/02 20130101; C21D
6/005 20130101; C21D 2211/005 20130101; C21D 8/0447 20130101; C23C
2/28 20130101; C21D 2211/004 20130101; C21D 8/0468 20130101; C21D
2211/008 20130101 |
Class at
Publication: |
148/504 ;
148/328; 148/624 |
International
Class: |
C21D 11/00 20060101
C21D011/00; C22C 38/00 20060101 C22C038/00; C21D 8/02 20060101
C21D008/02; C22C 38/04 20060101 C22C038/04; C22C 38/18 20060101
C22C038/18; C22C 38/12 20060101 C22C038/12; C22C 38/02 20060101
C22C038/02; C22C 37/06 20060101 C22C037/06; C22C 38/60 20060101
C22C038/60 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 31, 2008 |
JP |
2008-021403 |
Claims
1. A high strength steel sheet having a tensile strength of 900 MPa
or higher, comprising a composition including, on a mass basis: C:
0.1% or more and 0.3% or less; Si: 2.0% or less; Mn: 0.5% or more
and 3.0% or less; P: 0.1% or less; S: 0.07% or less; Al: 1.0% or
less; and N: 0.008% or less, with the balance Fe and incidental
impurities, with a steel microstructure including, on an area ratio
basis, 5% or more and 80% or less of ferrite, 15% or more of
autotempered martensite, 10% or less of bainite, 5% or less of
retained austenite, and 40% or less of as-quenched martensite; a
mean hardness of the autotempered martensite is HV.ltoreq.700; and
the mean number of precipitated iron-based carbide grains each
having a size of 5 nm or more and 0.5 .mu.m or less and included in
the autotempered martensite is 5.times.10.sup.4 or more per 1
mm.sup.2.
2. The high strength steel sheet according to claim 1, further
comprising, on a mass basis, at least one element selected from:
Cr: 0.05% or more and 5.0% or less; V: 0.005% or more and 1.0% or
less; and Mo: 0.005% or more and 0.5% or less.
3. The high strength steel sheet according to claim 1, further
comprising, on a mass basis, at least one element selected from:
Ti: 0.01% or more and 0.1% or less; Nb: 0.01% or more and 0.1% or
less; B: 0.0003% or more and 0.0050% or less; Ni: 0.05% or more and
2.0% or less; and Cu: 0.05% or more and 2.0% or less.
4. The high strength steel sheet according to claim 1, further
comprising, on a mass basis, at least one element selected from:
Ca: 0.001% or more and 0.005% or less; and REM: 0.001% or more and
0.005% or less.
5. The high strength steel sheet according to claim 1, wherein the
area ratio of autotempered martensite in which the number of
precipitated iron-based carbide grains each having a size of 0.1
.mu.m or more and 0.5 .mu.m or less is 5.times.10.sup.2 or less per
1 mm.sup.2 to the entire autotempered martensite is 3% or more.
6. The high strength steel sheet according to claim 1, wherein a
galvanized layer is disposed on a surface of the steel sheet.
7. The high strength steel sheet according to claim 1, wherein a
galvannealed layer is disposed on a surface of the steel sheet.
8. A method for manufacturing a high strength steel sheet,
comprising: hot-rolling and then cold-rolling a slab to be formed
into a steel sheet having the composition according to claim 1 to
form a cold-rolled steel sheet; annealing the cold-rolled steel
sheet in a first temperature range of 700.degree. C. or higher and
950.degree. C. or lower for 15 seconds or longer and 600 seconds or
shorter; in a second temperature range, which is a temperature
range from the first temperature range to 420.degree. C., cooling
the steel sheet from the first temperature range to 550.degree. C.
at an average cooling rate of 3.degree. C./s or higher and cooling
the steel sheet from 550.degree. C. to 420.degree. C. within 600
seconds; and cooling the steel sheet at a cooling rate of
50.degree. C./s or lower in a third temperature range of
250.degree. C. or higher and 420.degree. C. or lower to perform, in
the third temperature range, autotempering treatment in which
martensite transformation is caused while at the same time the
transformed martensite is tempered.
9. The method according to claim 8, wherein, when the steel sheet
is cooled at a cooling rate of 50.degree. C./s or lower in the
third temperature range of 250.degree. C. or higher and 420.degree.
C. or lower, the steel sheet is cooled at a cooling rate of
1.0.degree. C./s or higher and 50.degree. C./s or lower in a
temperature range of at least (Ms temperature-50).degree. C. or
lower to perform, in the third temperature range, autotempering
treatment in which martensite transformation is caused while at the
same time the transformed martensite is tempered.
10. The method according to claim 8, wherein martensite start
temperature Ms of the slab is approximated by M represented by
Formula (1) below, and the M is 300.degree. C. or higher:
M(.degree. C.)=540-361.times.{[C %]/(1-[.alpha.%]/100)}-6.times.[Si
%]-40.times.[Mn %]+30.times.[Al %]-20.times.[Cr %]-35.times.[V
%]-10.times.[Mo %]-17.times.[Ni %]-10.times.[Cu %] (1) where [X %]
is mass % of a constituent element X of the slab and [.alpha.%] is
an area ratio (%) of polygonal ferrite.
11. The method according to claim 9, wherein martensite start
temperature Ms of the slab is approximated by M represented by
Formula (1) below, and the M is 300.degree. C. or higher:
M(.degree. C.)=540-361.times.{[C %]/(1-[.alpha.%]/100)}-6.times.[Si
%]-40.times.[Mn %]+30.times.[Al %]-20.times.[Cr %]-35.times.[V
%]-10.times.[Mo %]-17.times.[Ni %]-10.times.[Cu %] (1) where [X %]
is mass % of a constituent element X of the slab and [.alpha.%] is
an area ratio (%) of polygonal ferrite.
12. The high strength steel sheet according to claim 2, further
comprising, on a mass basis, at least one element selected from:
Ti: 0.01% or more and 0.1% or less; Nb: 0.01% or more and 0.1% or
less; B: 0.0003% or more and 0.0050% or less; Ni: 0.05% or more and
2.0% or less; and Cu: 0.05% or more and 2.0% or less.
13. The high strength steel sheet according to claim 2, further
comprising, on a mass basis, at least one element selected from:
Ca: 0.001% or more and 0.005% or less; and REM: 0.001% or more and
0.005% or less.
14. The high strength steel sheet according to claim 3, further
comprising, on a mass basis, at least one element selected from:
Ca: 0.001% or more and 0.005% or less; and REM: 0.001% or more and
0.005% or less.
15. The high strength steel sheet according to claim 2, wherein the
area ratio of autotempered martensite in which the number of
precipitated iron-based carbide grains each having a size of 0.1
.mu.m or more and 0.5 .mu.m or less is 5.times.10.sup.2 or less per
1 mm.sup.2 to the entire autotempered martensite is 3% or more.
16. The high strength steel sheet according to claim 3, wherein the
area ratio of autotempered martensite in which the number of
precipitated iron-based carbide grains each having a size of 0.1
.mu.m or more and 0.5 .mu.m or less is 5.times.10.sup.2 or less per
1 mm.sup.2 to the entire autotempered martensite is 3% or more.
17. The high strength steel sheet according to claim 4, wherein the
area ratio of autotempered martensite in which the number of
precipitated iron-based carbide grains each having a size of 0.1
.mu.m or more and 0.5 .mu.m or less is 5.times.10.sup.2 or less per
1 mm.sup.2 to the entire autotempered martensite is 3% or more.
18. The high strength steel sheet according to claim 2, wherein a
galvanized layer is disposed on a surface of the steel sheet.
19. The high strength steel sheet according to claim 3, wherein a
galvanized layer is disposed on a surface of the steel sheet.
20. The high strength steel sheet according to claim 4, wherein a
galvanized layer is disposed on a surface of the steel sheet.
Description
RELATED APPLICATIONS
[0001] This is a .sctn.371 of International Application No.
PCT/JP2009/051915, with an international filing date of Jan. 29,
2009 (WO 2009/096596 A1, published Aug. 6, 2009), which is based on
Japanese Patent Application Nos. 2008-021403, filed Jan. 31, 2008,
and 2009-015840, filed Jan. 27, 2009, the subject matter of which
is incorporated by reference.
TECHNICAL FIELD
[0002] This disclosure relates to a high strength steel sheet that
is used in industrial fields such as the automobile and electrical
industries, has good formability, and has a tensile strength of 900
MPa or higher and a method for manufacturing the same. The high
strength steel sheet includes steel sheets whose surface is
galvanized or galvannealed.
BACKGROUND
[0003] In recent years, the improvement in the fuel efficiency of
automobiles has been an important subject from the viewpoint of
global environmental conservation. Therefore, by employing a high
strength automobile material, there has been an active move to
reduce the thickness of components and thus to lighten the
automobile body itself. However, since an increase in the strength
of steel sheets reduces workability, the development of materials
having both high strength and good workability has been demanded.
To satisfy such a demand, various multiple-phase steel sheets such
as a ferrite-martensite dual-phase steel (DP steel) and a TRIP
steel that uses transformation-induced plasticity of retained
austenite have been developed.
[0004] For example, the following disclose DP steels. JP 1853389
discloses a high strength steel sheet with a low yield ratio that
is excellent in surface quality and bendability and has a tensile
strength of 588 to 882 MPa and a method for manufacturing the steel
sheet, by specifying the composition and the hot-rolling and
annealing conditions. JP 3610883 discloses a high strength
cold-rolled steel sheet with excellent bendability and a method for
manufacturing the steel sheet, by specifying the hot-rolling,
cold-rolling, and annealing conditions of steel having a certain
composition. JP 11-61327 discloses a steel sheet that is excellent
in collision safety and formability and a method for manufacturing
the steel sheet, by specifying the volume fraction and grain
diameter of martensite and the mechanical properties. JP
2003-213369 discloses a high strength steel sheet, a high strength
galvanized steel sheet, and a high strength galvannealed steel
sheet that are excellent in stretch-flangeability and
crashworthiness and a method for manufacturing the steel sheets, by
specifying the composition and the volume fraction and grain
diameter of martensite. JP 2003-213370 discloses a high strength
steel sheet, a high strength galvanized steel sheet, and a high
strength galvannealed steel sheet that are excellent in
stretch-flangeability, shape fixability, and crashworthiness and a
method for manufacturing the steel sheets, by specifying the
composition, the grain diameter and microstructure of ferrite, and
the volume fraction of martensite. JP 2003-505604 discloses a high
strength steel sheet having excellent mechanical properties and a
method for manufacturing the steel sheet, by specifying the
composition, the amount of martensite, and the manufacturing
method. JP 6-093340 and JP 6-108152 each disclose a high strength
galvanized steel sheet that is excellent in stretch-flangeability
and bendability and a method and facility for manufacturing the
steel sheet, by specifying the composition and the manufacturing
conditions in a galvanizing line.
[0005] The following disclose steel sheets having a microstructure
including a phase other than martensite as a hard second phase. JP
7-011383 discloses a steel sheet that is excellent in fatigue
properties, by employing martensite and/or bainite as a hard second
phase and specifying the composition, the grain diameter, the
hardness ratio, and the like. JP 10-060593 discloses a steel sheet
that is excellent in stretch-flangeability, by mainly employing
bainite or pearlite as a second phase and specifying the
composition and the hardness ratio. JP 2005-281854 discloses a
high-strength and ductility galvanized steel sheet that is
excellent in hole expandability and a method for manufacturing the
steel sheet, by employing bainite and martensite as a hard second
phase. JP 3231204 discloses a multiple-phase steel sheet that is
excellent in fatigue properties by employing bainite and martensite
as a hard second phase and specifying the fraction of constituent
phases, the grain diameter, the hardness, and the mean free path of
the entire hard phase. JP 2001-207234 discloses a high strength
steel sheet that is excellent in ductility and hole expandability,
by specifying the composition and the amount of retained austenite.
JP 7-207413 discloses a high strength multiple-phase cold-rolled
steel sheet that is excellent in workability, by employing a steel
sheet including bainite and retained austenite and/or martensite
and specifying the composition and the fraction of phases. JP
2005-264328 discloses a high strength steel sheet that is excellent
in workability and a method for manufacturing the steel sheet, by
specifying the distribution state of the grains of a hard second
phase in ferrite and the ratio of the grains of tempered martensite
and bainite to the grains of ferrite. JP 2616350 discloses an
ultra-high strength cold-rolled steel sheet that is excellent in
delayed fracture resistance and has a tensile strength of 1180 MPa
or higher and a method for manufacturing the steel sheet, by
specifying the composition and the manufacturing process. JP
2621744 discloses an ultra-high strength cold-rolled steel sheet
that is excellent in bendability and has a tensile strength of 980
MPa or higher and a method for manufacturing the steel sheet, by
specifying the composition and the manufacturing method. JP 2826058
discloses an ultra-high strength thin steel sheet that has a
tensile strength of 980 MPa or higher and whose hydrogen
embrittlement is prevented by limiting the number of iron-based
carbide grains in tempered martensite to a certain number and a
method for manufacturing the steel sheet.
[0006] However, the above-described disclosures pose the problems
below. JP 1853389, JP 3610883, JP 11-061327, JP 2003-213369, JP
2003-213370, JP 2003-505604, JP 6-093340, JP 7-011383, JP
10-060593, JP 3231204, JP 2001-207234 and JP 7-207413 disclose
steel sheets having a tensile strength of lower than 900 MPa, and
the workability often cannot be maintained if the strength is
further increased. JP 1853389 describes that annealing is performed
in a single phase region and the subsequent cooling is performed to
400.degree. C. at a cooling rate of 6 to 20.degree. C./s. However,
in the case of a galvanized steel sheet, the adhesion of a coating
needs to be taken into account and heating needs to be performed
before coating because 400.degree. C. is lower than the temperature
of a coating bath. Thus, the galvanized steel sheet cannot be
manufactured in a continuous galvanizing and galvannealing line
having no heating equipment before the coating bath. In JP 6-093340
and JP 6-108152, since tempered martensite needs to be formed
during the heat treatment in a galvanizing line, there is required
equipment for reheating the steel sheet after the cooling to Ms
temperature or lower. In JP 2005-281854, bainite and martensite are
employed as a hard second phase and the fraction is specified.
However, the characteristics significantly vary in the specified
range, and the operating conditions need to be precisely controlled
to suppress the variation. In JP 2005-264328, since cooling is
performed to Ms temperature or lower to form martensite before
bainite transformation, equipment for reheating the steel sheet is
required. Furthermore, the operating conditions need to be
precisely controlled to achieve stable characteristics.
Consequently, the costs for equipment and operation are increased.
In JP 2616350 and JP 2621744, the steel sheet needs to be
maintained in a bainite-formation temperature range after annealing
to obtain a microstructure mainly composed of bainite, which makes
it difficult to achieve ductility. In the case of a galvanized
steel sheet, the steel sheet needs to be reheated to a temperature
higher than the temperature of a coating bath. JP 2826058 only
describes the improvement in hydrogen embrittlement of a steel
sheet, and there is little consideration for workability although
bendability is considered to some extent.
[0007] In general, the ratio of a hard second phase to the entire
microstructure needs to be increased to increase the strength of a
steel sheet. However, when the ratio of a hard second phase is
increased, the workability of a steel sheet is strongly affected by
that of the hard second phase. The reason is as follows. When the
ratio of the hard second phase is low, minimal workability is
achieved by the deformation of ferrite itself that is a parent
phase even if the workability of the hard second phase is
insufficient. However, when the ratio of the hard second phase is
high, the formability of a steel sheet is directly affected by the
deformability of the hard second phase, not the deformation of
ferrite. If the workability is insufficient, the formability is
considerably degraded.
[0008] Therefore, in the case of a cold-rolled steel sheet, for
example, martensite is formed through water quenching by adjusting
the fraction of ferrite and a hard second phase using a continuous
annealing furnace that can perform water quenching. Subsequently,
the temperature is increased and held to temper martensite, whereby
the workability of the hard second phase is improved.
[0009] However, in the case where equipment has no ability to
temper the thus-formed martensite by increasing temperature and
holding high temperature, the strength can be ensured, but it is
difficult to ensure the workability of the hard second phase such
as martensite.
[0010] To achieve stretch-flangeability using a hard phase other
than martensite, the workability of a hard second phase is ensured
by employing ferrite as a parent phase and bainite or pearlite
containing carbides as a hard second phase. Unfortunately, in this
case, sufficient ductility cannot be achieved.
[0011] When bainite is used, there is a problem in that the
characteristics significantly vary due to the variation in a
bainite-formation temperature range and the holding time. When
martensite or retained austenite (including bainite containing
retained austenite) is employed as a second phase, for example, a
mixed microstructure of martensite and bainite is considered to be
used as a second phase microstructure to ensure both ductility and
stretch-flangeability.
[0012] However, to employ a mixed microstructure composed of
various phases as a second phase and precisely control the fraction
or the like, the heat treatment conditions need to be precisely
controlled, which often poses a problem of manufacturing
stability.
[0013] It would therefore be helpful to provide a high strength
steel sheet having a tensile strength of 900 MPa or higher that can
minimize the formation of bainite, which easily causes a variation
in characteristics such as strength and formability, and can have
both high strength and good formability and to provide an
advantageous method for manufacturing the high strength steel
sheet.
SUMMARY
[0014] The formability is evaluated using TS.times.T. El and a
.lamda. value that represents stretch-flangeability. TS.times.T.
El.gtoreq.14500 MPa% and .lamda..gtoreq.15% are target
characteristics.
[0015] We studied the formation process of martensite, in
particular, the effect of the cooling conditions of a steel sheet
on martensite. We subsequently found that a high strength steel
sheet having both good formability and high strength with a tensile
strength of 900 MPa or higher that can be obtained by the following
method. By suitably controlling the heat treatment conditions after
cold-rolling, martensite transformation is caused while at the same
time the transformed martensite is tempered. The ratio of the
thus-formed autotempered martensite is controlled to a certain
ratio and also the distribution state of iron-based carbide grains
included in the autotempered martensite is suitably controlled,
whereby such a high strength steel sheet can be obtained.
[0016] We thus provide: [0017] 1. A high strength steel sheet
having a tensile strength of 900 MPa or higher, includes a
composition including, on a mass basis: [0018] C: 0.1% or more and
0.3% or less; [0019] Si: 2.0% or less; [0020] Mn: 0.5% or more and
3.0% or less; [0021] P: 0.1% or less; [0022] S: 0.07% or less;
[0023] Al: 1.0% or less; and [0024] N: 0.008% or less, with the
balance Fe and incidental impurities, wherein a steel
microstructure includes, on an area ratio basis, 5% or more and 80%
or less of ferrite, 15% or more of autotempered martensite, 10% or
less of bainite, 5% or less of retained austenite, and 40% or less
of as-quenched martensite; a mean hardness of the autotempered
martensite is HV.ltoreq.700; and the mean number of precipitated
iron-based carbide grains each having a size of 5 nm or more and
0.5 .mu.m or less and included in the autotempered martensite is
5.times.10.sup.4 or more per 1 mm.sup.2. [0025] 2. The high
strength steel sheet according to the above-described 1, further
includes, on a mass basis, at least one element selected from:
[0026] Cr: 0.05% or more and 5.0% or less; [0027] V: 0.005% or more
and 1.0% or less; and [0028] Mo: 0.005% or more and 0.5% or less.
[0029] 3. The high strength steel sheet according to the
above-described 1 or 2, further includes, on a mass basis, at least
one element selected from: [0030] Ti: 0.01% or more and 0.1% or
less; [0031] Nb: 0.01% or more and 0.1% or less; [0032] B: 0.0003%
or more and 0.0050% or less; [0033] Ni: 0.05% or more and 2.0% or
less; and [0034] Cu: 0.05% or more and 2.0% or less. [0035] 4. The
high strength steel sheet according to any one of the
above-described 1 to 3, further includes, on a mass basis, at least
one element selected from: [0036] Ca: 0.001% or more and 0.005% or
less; and [0037] REM: 0.001% or more and 0.005% or less. [0038] 5.
The high strength steel sheet according to any one of the
above-described 1 to 4, wherein the area ratio of autotempered
martensite in which the number of precipitated iron-based carbide
grains each having a size of 0.1 .mu.m or more and 0.5 .mu.m or
less is 5.times.10.sup.2 or less per 1 mm.sup.2 to the entire
autotempered martensite is 3% or more. [0039] 6. The high strength
steel sheet according to any one of the above-described 1 to 5,
wherein a galvanized layer is disposed on a surface of the steel
sheet. [0040] 7. The high strength steel sheet according to any one
of the above-described 1 to 5, wherein a galvannealed layer is
disposed on a surface of the steel sheet. [0041] 8. A method for
manufacturing a high strength steel sheet, includes the steps of
hot-rolling and then cold-rolling a slab to be formed into a steel
sheet having the composition according to any one of the
above-described 1 to 4 to form a cold-rolled steel sheet; annealing
the cold-rolled steel sheet in a first temperature range of
700.degree. C. or higher and 950.degree. C. or lower for 15 seconds
or longer and 600 seconds or shorter; in a second temperature
range, which is a temperature range from the first temperature
range to 420.degree. C., cooling the steel sheet from the first
temperature range to 550.degree. C. at an average cooling rate of
3.degree. C./s or higher and cooling the steel sheet from
550.degree. C. to 420.degree. C. within 600 seconds; and cooling
the steel sheet at a cooling rate of 50.degree. C./s or lower in a
third temperature range of 250.degree. C. or higher and 420.degree.
C. or lower to perform, in the third temperature range,
autotempering treatment in which martensite transformation is
caused while at the same time the transformed martensite is
tempered. [0042] 9. The method for manufacturing a high strength
steel sheet according to the above-described 8, wherein when the
steel sheet is cooled at a cooling rate of 50.degree. C./s or lower
in the third temperature range of 250.degree. C. or higher and
420.degree. C. or lower, the steel sheet is cooled at a cooling
rate of 1.0.degree. C./s or higher and 50.degree. C./s or lower in
a temperature range of at least (Ms temperature-50).degree. C. or
lower to perform, in the third temperature range, autotempering
treatment in which martensite transformation is caused while at the
same time the transformed martensite is tempered. [0043] 10. The
method for manufacturing a high strength steel sheet according to
the above-described 8 or 9, wherein martensite start temperature Ms
of the slab is approximated by M represented by Formula (1) below,
and the M is 300.degree. C. or higher:
[0043] M(.degree. C.)=540-361.times.{[C
%]/(1-[.alpha.%]/100)}-6.times.[Si %]-40.times.[Mn %]+30.times.[Al
%]-20.times.[Cr %]-35.times.[V %]-10.times.[Mo %]-17.times.[Ni
%]-10.times.[Cu %] (1) [0044] where [X %] is mass % of a
constituent element X of the slab and [.alpha.%] is an area ratio
(%) of polygonal ferrite.
[0045] A high strength steel sheet having a tensile strength of 900
MPa or higher that can achieve high strength, good workability, and
good ductility can be obtained by forming an appropriate amount of
autotempered martensite in a steel sheet and suitably controlling
the distribution state of carbide grains included in the
autotempered martensite. Therefore, our steel sheets significantly
contribute to the weight reduction of automobile bodies.
[0046] In the method for manufacturing a high strength steel sheet,
since reheating the steel sheet after quenching is not needed,
special manufacturing equipment is not required and the method can
be easily applied to a galvanizing or galvannealing process.
Therefore, our methods contributes to decreases in the number of
steps and in the cost.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] FIG. 1 is a schematic view showing quenching and tempering
steps performed to obtain typical tempered martensite.
[0048] FIG. 2 is a schematic view showing an autotempering
treatment step performed to obtain autotempered martensite.
DETAILED DESCRIPTION
[0049] The reason for the above-described limitation of the
microstructure of a steel sheet according to the present invention
will be described below.
Area Ratio of Ferrite: 5% or More and 80% or Less
[0050] To achieve both workability and a tensile strength of 900
MPa or higher, the ratio between ferrite and a hard phase described
below is important and thus the area ratio of ferrite needs to be
5% or more and 80% or less. If the area ratio of ferrite is less
than 5%, ductility is not ensured. If the area ratio of ferrite is
more than 80%, the area ratio of the hard phase becomes
insufficient and thus the strength becomes insufficient. The area
ratio of ferrite is preferably set in the range of 10% or more and
65% or less.
Area Ratio of Autotempered Martensite: 15% or More
[0051] Autotempered martensite is a microstructure obtained by
simultaneously causing martensite transformation and tempering of
the martensite through autotempering treatment, and not so-called
"tempered martensite" obtained through quenching and tempering
treatments as in the related art. The microstructure is not a
uniformly tempered microstructure formed by completing martensite
transformation through quenching and then performing tempering
through a temperature increase as in typical quenching and
tempering treatments, but is a microstructure including martensites
in different tempered states obtained by performing martensite
transformation and tempering of the martensite in stages through
the control of a cooling process in a temperature range of Ms
temperature or lower.
[0052] This autotempered martensite is a hard phase for increasing
strength. If the area ratio of autotempered martensite is less than
15%, sufficient strength cannot be achieved and work hardening of
ferrite cannot be facilitated. Thus, the area ratio of autotempered
martensite needs to be 15% or more and is preferably 30% or
more.
[0053] The microstructure of a steel sheet is preferably composed
of ferrite and autotempered martensite within the above-described
range. When such phases are formed, other phases such as bainite,
retained austenite, and as-quenched martensite are sometimes
formed. These phases may be formed as long as some parameters are
within the tolerable ranges described below. The tolerable ranges
will now be described.
Area Ratio of Bainite: 10% or Less (Including 0%)
[0054] Bainite is a hard phase that contributes to an increase in
strength, but the characteristics significantly vary in accordance
with the formation temperature range and the variation in the
quality of material is sometimes increased. Therefore, the area
ratio of bainite in a steel microstructure is desirably as low as
possible, but up to 10% of bainite is tolerable. The area ratio of
bainite is preferably 5% or less.
Area Ratio of Retained Austenite: 5% or Less (Including 0%)
[0055] Retained austenite is transformed into hard martensite when
processed, which decreases stretch-flangeability. Thus, the area
ratio of retained austenite in a steel microstructure is desirably
as low as possible, but up to 5% of retained austenite is
tolerable. The area ratio of retained austenite is preferably 3% or
less.
Area Ratio of as-Quenched Martensite: 40% or Less (Including
0%)
[0056] Since as-quenched martensite has considerably poor
workability, the area ratio of as-quenched martensite in a steel
microstructure is desirably as low as possible, but up to 40% of
as-quenched martensite is tolerable. The area ratio of as-quenched
martensite is preferably 30% or less. Herein, as-quenched
martensite can be differentiated from autotempered martensite in
that carbides of as-quenched martensite are not observed with a
scanning electron microscope (SEM) or a transmission electron
microscope (TEM).
Mean Hardness of Autotempered Martensite: HV.ltoreq.700
[0057] If the mean hardness of autotempered martensite is
700<HV, stretch-flangeability is considerably degraded. Thus,
HV.ltoreq.700 needs to be satisfied and HV.ltoreq.630 is preferably
satisfied.
Iron-Based Carbide in Autotempered Martensite
[0058] Size: 5 nm or More and 0.5 .mu.m or Less, Mean Number of
Precipitated Carbide Grains: 5.times.10.sup.4 or More per 1
mm.sup.2
[0059] Autotempered martensite is martensite subjected to the heat
treatment (autotempering treatment) performed by the method.
However, even if the mean hardness of autotempered martensite is
HV.ltoreq.700, the workability is decreased when the autotempering
treatment is improperly performed. The degree of autotempering
treatment can be confirmed through the formation state
(distribution state) of iron-based carbide grains in autotempered
martensite. When the mean number of precipitated iron-based carbide
grains each having a size of 5 nm or more and 0.5 .mu.m or less is
5.times.10.sup.4 or more per 1 mm.sup.2, it can be judged that
desired autotempering treatment has been performed. Iron-based
carbide grains each having a size of less than 5 nm are removed
from the target of judgment because such carbide grains do not
affect the workability of autotempered martensite. On the other
hand, iron-based carbide grains each having a size of more than 0.5
.mu.m are also removed from the target of judgment because such
carbide grains may decrease the strength of autotempered martensite
but hardly affect the workability. If the number of iron-based
carbide grains is less than 5.times.10.sup.4 per 1 mm.sup.2, it is
judged that the autotempering treatment has been improperly
performed because workability, particularly stretch-flangeability,
is not improved. The number of iron-based carbide grains is
preferably 1.times.10.sup.5 or more and 1.times.10.sup.6 or less
per 1 mm.sup.2, more preferably 4.times.10.sup.5 or more and
1.times.10.sup.6 or less per 1 mm.sup.2. Herein, an iron-based
carbide is mainly Fe.sub.3C, and .epsilon. carbides and the like
may be further contained.
[0060] To confirm the formation state of carbide grains, it is
effective to observe a mirror-polished sample using a SEM (scanning
electron microscope) or a TEM (transmission electron microscope).
Carbide grains can be identified by, for example, performing
SEM-EDS (energy dispersive X-ray spectrometry), EPMA (electron
probe microanalyzer), or FE-AES (field emission-Auger electron
spectrometry) on samples whose section is polished.
[0061] In the steel sheet, the amount of autotempered martensite
narrowed down by further limiting the size and number of iron-based
carbide grains precipitated in the above-described autotempered
martensite can be suitably set as follows.
Autotempered Martensite in which the Number of Precipitated
Iron-Based Carbide Grains Each Having a Size of 0.1 .mu.m or More
and 0.5 .mu.m or Less is 5.times.10.sup.2 or Less per 1 mm.sup.2:
the Area Ratio of the Autotempered Martensite to the Entire
Autotempered Martensite is 3% or More
[0062] By increasing the ratio of autotempered martensite in which
the number of precipitated iron-based carbide grains each having a
size of 0.1 .mu.m or more and 0.5 .mu.m or less is 5.times.10.sup.2
or less per 1 mm.sup.2, ductility is further improved. To produce
such an effect, the area ratio of autotempered martensite in which
the number of precipitated iron-based carbide grains each having a
size of 0.1 .mu.m or more and 0.5 .mu.m or less is 5.times.10.sup.2
or less per 1 mm.sup.2 to the entire autotempered martensite is
preferably 3% or more. If a large amount of autotempered martensite
in which the number of precipitated iron-based carbide grains each
having a size of 0.1 .mu.m or more and 0.5 .mu.m or less is
5.times.10.sup.2 or less per 1 mm.sup.2 is contained in a steel
sheet, workability is considerably degraded. Thus, the area ratio
of such autotempered martensite to the entire autotempered
martensite is preferably 40% or less, more preferably 30% or
less.
[0063] When the area ratio of autotempered martensite in which the
number of precipitated iron-based carbide grains each having a size
of 0.1 .mu.m or more and 0.5 .mu.m or less is 5.times.10.sup.2 or
less per 1 mm.sup.2 to the entire autotempered martensite is 3% or
more, the number of fine iron-based carbide grains is increased in
autotempered martensite. Therefore, the mean number of precipitated
iron-based carbide grains in the entire autotempered martensite is
increased. Thus, the mean number of precipitated iron-based carbide
grains each having a size of 5 nm or more and 0.5 .mu.m or less in
autotempered martensite is preferably 1.times.10.sup.5 or more and
5.times.10.sup.6 or less per 1 mm.sup.2, more preferably
4.times.10.sup.5 or more and 5.times.10.sup.6 or less per 1
mm.sup.2.
[0064] The specific reason why ductility is further improved as
described above is not clear, but it is believed to be as follows.
When the area ratio of autotempered martensite in which the number
of precipitated iron-based carbide grains each having a relatively
large size of 0.1 .mu.m or more and 0.5 .mu.m or less is
5.times.10.sup.2 or less per 1 mm.sup.2 to the entire autotempered
martensite is 3% or more, the autotempered martensite
microstructure includes a portion that contains a large number of
iron-based carbide grains having a relatively large size and a
portion that contains a small number of iron-based carbide grains
having a relatively large size in a mixed manner. The portion that
contains a small number of iron-based carbide grains having a
relatively large size is hard autotempered martensite because a
large number of fine iron-based carbide grains are contained. On
the other hand, the portion that contains a large number of
iron-based carbide grains having a relatively large size is soft
autotempered martensite. By providing the hard autotempered
martensite such that the hard autotempered martensite is surrounded
by the soft autotempered martensite, the degradation of
stretch-flangeability caused by the hardness difference in
autotempered martensite can be suppressed. Furthermore, by
dispersing the hard martensite in the soft autotempered martensite,
work hardenability is improved and thus ductility is improved.
[0065] The reason why the composition is set in the above-described
range in the steel sheet will be described below. The symbol "%"
below used for each component means "% by mass."
C: 0.1% or More and 0.3% or Less
[0066] C is an essential element for increasing the strength of a
steel sheet. A C content of less than 0.1% causes difficulty in
achieving both strength and workability such as ductility or
stretch-flangeability of the steel sheet. On the other hand, a C
content of more than 0.3% causes a significant hardening of welds
and heat-affected zones, thereby reducing weldability. Thus, the C
content is set in the range of 0.1% or more and 0.3% or less,
preferably 0.12% or more and 0.23% or less.
Si: 2.0% or Less
[0067] Si is a useful element for solution hardening of ferrite,
and the Si content is preferably 0.1% or more to ensure the
ductility and the hardness of ferrite. However, the excessive
addition of Si causes the degradation of surface quality due to the
occurrence of red scale and the like and the degradation of the
adhesion of a coating. Thus, the Si content is set to 2.0% or less,
preferably 1.6% or less.
Mn: 0.5% or More and 3.0% or Less
[0068] Mn is an element that is effective in strengthening steel,
stabilizes austenite, and is necessary for ensuring the area ratio
of a hard phase. To achieve this, a Mn content of 0.5% or more is
required. On the other hand, an excessive Mn content of more than
3.0% causes the degradation of castability or the like. Thus, the
Mn content is set in the range of 0.5% or more and 3.0% or less,
preferably 1.5% or more and 2.5% or less.
P: 0.1% or Less
[0069] P causes embrittlement due to grain boundary segregation and
degrades shock resistance, but a P content of up to 0.1% is
tolerable. Furthermore, in the case where a steel sheet is
galvannealed, a P content of more than 0.1% significantly reduces
the rate of alloying. Thus, the P content is set to 0.1% or less,
preferably 0.05% or less.
S: 0.07% or Less
[0070] S is formed into MnS as an inclusion that causes the
degradation of shock resistance and causes cracks along a flow of a
metal in a weld zone. Thus, the S content is preferably minimized.
However, a S content of up to 0.07% is tolerable in terms of
manufacturing costs. The S content is preferably 0.04% or less.
Al: 1.0% or Less
[0071] Al is an element that contributes to ferrite formation and a
useful element for controlling the amount of the ferrite formation
during manufacturing. However, an excessive Al content degrades the
quality of a slab during steelmaking. Thus, the Al content is set
to 1.0% or less, preferably 0.5% or less. Since an excessively low
Al content sometimes makes it difficult to perform deoxidization,
the Al content is preferably 0.01% or more.
N: 0.008% or Less
[0072] N is an element that most degrades the anti-aging property
of steel. Therefore, the N content is preferably minimized. A N
content of more than 0.008% causes significant degradation of an
anti-aging property. Thus, the N content is set to 0.008% or less,
preferably 0.006% or less.
[0073] If necessary, the steel sheet can suitably contain the
components described below in addition to the basic components
described above.
At Least One Element Selected from Cr: 0.05% or More and 5.0% or
Less, V: 0.005% or More and 1.0% or Less, and Mo: 0.005% or More
and 0.5% or Less
[0074] Cr, V, and Mo have an effect of suppressing the formation of
pearlite when a steel sheet is cooled from the annealing
temperature and thus can be optionally added. The effect is
produced at a Cr content of 0.05% or more, a V content of 0.005% or
more, or a Mo content of 0.005% or more. On the other hand, an
excessive Cr content of more than 5.0%, an excessive V content of
more than 1.0%, or an excessive Mo content of more than 0.5%
excessively increases the area ratio of a hard phase, thereby
unnecessarily increasing the strength. Thus, when these elements
are incorporated, the Cr content is preferably set in the range of
0.05% or more and 5.0% or less, the V content is preferably set in
the range of 0.005% or more and 1.0% or less, and the Mo content is
preferably set in the range of 0.005% or more and 0.5% or less.
[0075] Furthermore, at least one element selected from Ti, Nb, B,
Ni, and Cu can be incorporated. The reason for the limitation of
the content ranges is as follows.
Ti: 0.01% or More and 0.1% or Less and Nb: 0.01% or More and 0.1%
or Less
[0076] Ti and Nb are useful for precipitation strengthening of
steel and the effect is produced at a Ti content of 0.01% or more
or a Nb content of 0.01% or more. On the other hand, a Ti content
of more than 0.1% or a Nb content of more than 0.1% degrades the
workability and shape flexibility. Thus, the Ti content and the Nb
content are each preferably set in the range of 0.01% or more and
0.1% or less.
B: 0.0003% or More and 0.0050% or Less
[0077] B has an effect of suppressing the formation and growth of
ferrite from austenite grain boundaries and thus can be optionally
added. The effect is produced at a B content of 0.0003% or more. On
the other hand, a B content of more than 0.0050% decreases
workability. Thus, when B is incorporated, the B content is
preferably set in the range of 0.0003% or more and 0.0050% or less.
Herein, when B is incorporated, the formation of BN is preferably
suppressed to produce the above-described effect. Thus, B is
preferably added together with Ti.
Ni: 0.05% or More and 2.0% or Less and Cu: 0.05% or More and 2.0%
or Less
[0078] In the case where a steel sheet is galvanized, Ni and Cu
promote internal oxidation, thereby improving the adhesion of a
coating. The effect is produced at a Ni content of 0.05% or more or
a Cu content of 0.05% or more. On the other hand, a Ni content of
more than 2.0% or a Cu content of more than 2.0% degrades the
workability of a steel sheet. Ni and Cu are useful elements for
strengthening steel. Thus, the Ni content and the Cu content are
each preferably set in the range of 0.05% or more and 2.0% or
less.
At Least One Element Selected from Ca: 0.001% or More and 0.005% or
Less and REM: 0.001% or More and 0.005% or Less
[0079] Ca and REM are useful elements for spheroidizing the shape
of a sulfide and improving an adverse effect of the sulfide on
stretch-flangeability. The effect is produced at a Ca content of
0.001% or more or an REM content of 0.001% or more. On the other
hand, a Ca content of more than 0.005% or an REM content of more
than 0.005% increases the number of inclusions or the like and
causes, for example, surface defects and internal defects. Thus,
when Ca and REM are incorporated, the Ca content and the REM
content are each preferably set in the range of 0.001% or more and
0.005% or less.
[0080] Components other than the components described above are Fe
and incidental impurities. However, a component other than the
components described above may be contained to the extent that the
advantages are not impaired.
[0081] As described below, the composition of the steel sheet
according to the present invention preferably satisfies
M.gtoreq.300.degree. C. that represents a relation between the
composition and the area ratio of polygonal ferrite to perform
stable production, that is, to suppress the variation in
characteristics due to the variation in manufacturing
conditions.
[0082] A galvanized layer or a galvannealed layer may be disposed
on a surface of a steel sheet.
[0083] A preferred method for manufacturing a steel sheet and the
reason for the limitation of the conditions will now be
described.
[0084] A slab prepared to have the above-described preferred
composition is produced, hot-rolled, and then cold-rolled to obtain
a cold-rolled steel sheet. These processes are not particularly
limited, and can be performed by typical methods.
[0085] The preferred manufacturing conditions will now be described
below. A slab is heated to 1100.degree. C. or higher and
1300.degree. C. or lower and subjected to finish hot-rolling at a
temperature of 870.degree. C. or higher and 950.degree. C. or
lower, which means that the hot-rolling end temperature is set to
870.degree. C. or higher and 950.degree. C. or lower. The
thus-obtained hot-rolled steel sheet is wound at a temperature of
350.degree. C. or higher and 720.degree. C. or lower. Subsequently,
the hot-rolled steel sheet is pickled and cold-rolled at a
reduction ratio of 40% or higher and 90% or lower to obtain a
cold-rolled steel sheet.
[0086] It is assumed that the hot-rolled steel sheet is produced
through the typical steps of steel making, casting, and
hot-rolling, but the hot-rolled steel sheet may be produced by thin
slab casting without performing part or all of the hot-rolling
steps.
[0087] The resultant cold-rolled steel sheet is annealed for 15
seconds or longer and 600 seconds or shorter in a first temperature
range of 700.degree. C. or higher and 950.degree. C. or lower,
specifically, in an austenite single-phase region or a dual-phase
region of an austenite phase and a ferrite phase. If the annealing
temperature is lower than 700.degree. C. or the annealing time is
shorter than 15 seconds, a carbide in the steel sheet is sometimes
not sufficiently dissolved, or the recrystallization of ferrite is
not completed and thus desired ductility and stretch-flangeability
are sometimes not achieved. On the other hand, if the annealing
temperature exceeds 950.degree. C., austenite grains are
significantly grown and the constituent phases produced by cooling
performed later are coarsened, which may degrade ductility and
stretch-flangeability. If the annealing time exceeds 600 seconds, a
vast amount of energy is consumed and thus the cost is increased.
Therefore, the annealing temperature is set in the range of
700.degree. C. or higher and 950.degree. C. or lower, preferably
760.degree. C. or higher and 920.degree. C. or lower. The annealing
time is set in the range of 15 seconds or longer and 600 seconds or
shorter, preferably 30 seconds or longer and 400 seconds or
shorter.
[0088] In a second temperature range, which is a temperature range
from the first temperature range to 420.degree. C., the annealed
cold-rolled steel sheet is cooled to 550.degree. C. from the first
temperature range at a cooling rate of 3.degree. C./s or higher,
and is then cooled from 550 to 420.degree. C. within 600 seconds.
Subsequently, the steel sheet is cooled at a cooling rate of
50.degree. C./s or lower in a third temperature range of
250.degree. C. or higher and 420.degree. C. or lower.
[0089] The cooling conditions in a second temperature range from
the first temperature range to 420.degree. C. are essential to
suppress the precipitation of phases other than intended ferrite
and autotempered martensite phases. In the temperature range from
the first temperature range to 550.degree. C., pearlite
transformation easily occurs. If the average cooling rate is lower
than 3.degree. C./s in the range from 700.degree. C., which is the
lower limit temperature of the first temperature range, to
550.degree. C., pearlite or the like is precipitated and a desired
microstructure is sometimes not obtained. Therefore, the cooling
rate needs to be 3.degree. C./s or higher, and is preferably
5.degree. C./s or higher. The upper limit of the cooling rate is
not particularly specified, but special cooling equipment is
required to achieve a cooling rate of 200.degree. C./s or higher.
Thus, the cooling rate is preferably 200.degree. C./s or lower.
[0090] When the steel sheet is held for a long time in a
temperature range of 550.degree. C. to 420.degree. C., bainite
transformation is caused. If the time required for cooling from
550.degree. C. to 420.degree. C. exceeds 600 seconds, bainite
transformation is caused and thus a desired microstructure is
sometimes not obtained. Therefore, the time required for cooling
from 550.degree. C. to 420.degree. C. is 600 seconds or shorter,
preferably 400 seconds or shorter.
[0091] After the process in the second temperature range, the steel
sheet is processed in the third temperature range. The most
important feature is that, in the third temperature range,
autotempering treatment in which martensite transformation is
caused while at the same time the transformed martensite is
tempered is performed to obtain autotempered martensite in which
the precipitation state of carbide grains is suitably
controlled.
[0092] Typical martensite is obtained by performing annealing and
then performing quenching with water cooling or the like. The
martensite is a hard phase, and contributes to an increase in the
strength of a steel sheet but degrades workability. To change the
martensite into tempered martensite having satisfactory
workability, a quenched steel sheet is normally heated again to
perform tempering. FIG. 1 schematically shows the steps described
above. In such normal quenching and tempering treatments, after
martensite transformation is completed by quenching, the
temperature is increased to perform tempering. Consequently, a
uniformly tempered microstructure is obtained.
[0093] In contrast, autotempering treatment is a treatment in which
a steel sheet is cooled in a certain cooling-rate range in the
third temperature range as shown in FIG. 2. In the autotempering
treatment, quenching and tempering through reheating are not
performed, which is a method with high productivity. The steel
sheet including autotempered martensite obtained through this
autotempering treatment has strength and workability equal to or
higher than those of the steel sheet obtained by performing
quenching and tempering through reheating shown in FIG. 1. In the
autotempering treatment, martensite transformation and the
tempering can be made to occur continuously or stepwise by
performing continuous cooling (including stepwise cooling and
holding) in the third temperature range. Consequently, a
microstructure including martensites in different tempered states
can be obtained. Although the martensites in different tempered
states have different characteristics in terms of strength and
workability, desired characteristics as the entire steel sheet can
be achieved by suitably controlling the amounts of martensites in
different tempered states through autotempering treatment.
Furthermore, since the autotempering treatment is performed without
rapidly cooling a steel sheet to a low temperature range in which
the martensite transformation is fully completed, the residual
stress in the steel sheet is low and a steel sheet having a good
plate shape is obtained, which is advantageous.
[0094] The third temperature range is 250.degree. C. or higher and
420.degree. C. or lower. If the temperature exceeds 420.degree. C.,
bainite transformation is easily caused as described above. If the
temperature is lower than 250.degree. C., autotempering treatment
requires a long time and thus proceeds insufficiently in a
continuous annealing line or a continuous galvanizing and
galvannealing line. In the third temperature range, the cooling
rate of a steel sheet needs to be 50.degree. C./s or lower to cause
martensite transformation while at the same time tempering the
transformed martensite and thus to obtain autotempered martensite.
If the cooling rate exceeds 50.degree. C./s, the autotempering
treatment insufficiently proceeds and the workability of martensite
is sometimes not ensured. If the cooling rate is less than
0.1.degree. C./s, bainite transformation occurs or autotempering
treatment excessively proceeds, whereby strength sometimes cannot
be ensured. Thus, the cooling rate is preferably 0.1.degree. C./s
or higher.
[0095] In the method for manufacturing a steel sheet, the following
configuration can be suitably added if necessary.
[0096] When a steel sheet is cooled at a cooling rate of 50.degree.
C./s or lower in a third temperature range of 250.degree. C. or
higher and 420.degree. C. or lower, the steel sheet is preferably
cooled at a cooling rate of 1.0.degree. C./s or higher and
50.degree. C./s or lower in a temperature range of at least (Ms
temperature-50).degree. C. or lower. This is because, by further
appropriately controlling the precipitation state of carbide grains
included in autotempered martensite, the area ratio of autotempered
martensite in which the number of precipitated iron-based carbide
grains each having a size of 0.1 .mu.m or more and 0.5 .mu.m or
less is 5.times.10.sup.2 or less per 1 mm.sup.2 to the entire
autotempered martensite can be set to 3% or more. If the cooling
rate exceeds 50.degree. C./s, autotempering treatment
insufficiently proceeds and desired autotempered martensite is not
obtained. Consequently, the workability of martensite is sometimes
not ensured. If the cooling rate is less than 1.0.degree. C./s, 3%
or more of the area ratio of autotempered martensite in which the
number of precipitated iron-based carbide grains each having a size
of 0.1 .mu.m or more and 0.5 .mu.m or less is 5.times.10.sup.2 or
less per 1 mm.sup.2 to the entire autotempered martensite cannot be
achieved, and desired ductility and strength are not ensured. Thus,
the cooling rate is set to 1.0.degree. C./s or higher. Herein, Ms
temperature can be obtained in a typical manner through the
measurement of thermal expansion or electrical resistance during
cooling. Alternatively, M obtained from an approximate expression
(1) of Ms temperature described below may be used.
[0097] In the method for manufacturing a steel sheet, autotempering
treatment can be stably performed when M represented by the
approximate expression (1) below is 300.degree. C. or higher:
M(.degree. C.)=540-361.times.{[C %]/(1-[.alpha.%]/100)}-6.times.[Si
%]-40.times.[Mn %]+30.times.[Al %]-20.times.[Cr %]-35.times.[V
%]-10.times.[Mo %]-17.times.[Ni %]-10.times.[Cu %] (1)
where [X %] is mass % of an alloy element X and [.alpha.%] is the
area ratio (%) of polygonal ferrite.
[0098] M represented by the above-described expression (1) is an
empirical approximate expression of Ms temperature from which
martensite transformation starts. It is believed that M is highly
related to the precipitation behavior of iron-based carbide grains
from martensite. Thus, M can be used as an indicator that indicates
whether autotempered martensite in which the number of iron-based
carbide grains each having a size of 5 nm or more and 0.5 .mu.m or
less is 5.times.10.sup.4 or more per 1 mm.sup.2 can be stably
obtained. Even if M is less than 300.degree. C., autotempered
martensite is obtained. However, since the temperature is low,
martensite transformation and autotempering treatment tend to
slowly proceed. Compared with the case of M.gtoreq.300.degree. C.,
a steel sheet needs to be cooled slowly or held at a low
temperature for a long time to obtain desired autotempered
martensite, which may considerably lower manufacturing efficiency.
Thus, M is preferably 300.degree. C. or higher.
[0099] The area ratio of polygonal ferrite is measured, for
example, through the image processing and analysis of a SEM
micrograph taken at 1000 to 3000 power. Polygonal ferrite is
observed in the steel sheet that has been annealed and cooled under
the above-described conditions. To ensure that M is 300.degree. C.
or higher, after a cold-rolled steel sheet having a desired
composition is produced, the area ratio of polygonal ferrite is
measured and thus M is obtained from the expression (1) using the
contents of alloy elements that can be calculated from the
composition of the steel sheet. In the case where M is less than
300.degree. C., the heat treatment conditions are suitably adjusted
such that the area ratio of polygonal ferrite becomes lower, to
obtain desired M. For example, the annealing temperature in the
first temperature range is further increased and the average
cooling rate from the first temperature range to 550.degree. C. is
further increased. Alternatively, the contents of the components in
the expression (1) may be adjusted.
[0100] The steel sheet can be galvanized and galvannealed. The
galvanizing and galvannealing treatments are preferably performed
in a continuous galvanizing and galvannealing line while the
above-described annealing and cooling conditions are satisfied. The
galvanizing and galvannealing treatments are preferably performed
in a temperature range of 420.degree. C. or higher and 550.degree.
C. or lower. In this case, the time required for cooling a steel
sheet from 550.degree. C. to 420.degree. C., that is, the holding
time in the temperature range of 420.degree. C. or higher and
550.degree. C. or lower needs to be 600 seconds or shorter, the
time including galvanizing treatment time and/or galvannealing
treatment time.
[0101] A method of galvanizing and galvannealing treatments is as
follows. First, a steel sheet is immersed in a coating bath and the
coating weight is adjusted using gas wiping or the like. In the
case where the steel sheet is galvanized, the amount of dissolved
Al in the coating bath is in the range of 0.12% or more and 0.22%
or less. In the case where the steel sheet is galvannealed, the
amount of dissolved Al is in the range of 0.08% or more and 0.18%
or less.
[0102] In the case where the steel sheet is galvanized, the
temperature of the coating bath is desirably 450.degree. C. or
higher and 500.degree. C. or lower. In the case where the steel
sheet is galvannealed by further performing alloying treatment, the
temperature during alloying is preferably 450.degree. C. or higher
and 550.degree. C. or lower. If the alloying temperature exceeds
550.degree. C., an excessive amount of carbide grains are
precipitated from untransformed austenite or the transformation
into pearlite is caused, whereby desired strength and ductility are
sometimes not achieved. Powdering is also degraded. If the alloying
temperature is less than 450.degree. C., the alloying does not
proceed.
[0103] The coating weight is preferably in the range of 20 to 150
g/m.sup.2 per surface. If the coating weight is less than 20
g/m.sup.2, corrosion resistance is degraded. Meanwhile, even if the
coating weight exceeds 150 g/m.sup.2, the corrosion resistance is
saturated, which merely increases the cost. The degree of alloying
is preferably in the range of 7 to 15% by mass on a Fe content
basis in the coating layer. If the degree of alloying is less than
7% by mass, uneven alloying is caused and the surface appearance
quality is degraded. Furthermore, a so-called ".zeta. phase" is
formed in the coating layer and thus the slidability is degraded.
If the degree of alloying exceeds 15% by mass, a large amount of
hard brittle F phase is formed and the adhesion of the coating is
degraded.
[0104] The holding temperature in the first temperature range, in
the second temperature range, or the like is not necessarily
constant. Even if the holding temperature is varied, the purpose of
this step is not impaired as long as the holding temperature is
within a predetermined temperature range. The same is true for the
cooling rate. Furthermore, a steel sheet may be subjected to
annealing and autotempering treatments with any equipment as long
as heat history is just satisfied. Moreover, it is also included in
the scope of this disclosure that, after autotempering treatment,
temper rolling is performed on the steel sheet for shape
correction.
EXAMPLES
Example 1
[0105] Our steel sheets and methods will now be further described
with respect to selected Examples. The disclosure is thus not
limited to the Examples. It will be understood that modifications
may be made without departing from the scope of this
disclosure.
[0106] A slab to be formed into a steel sheet having the
composition shown in Table 1 was heated to 1250.degree. C. and
subjected to finish hot-rolling at 880.degree. C. The hot-rolled
steel sheet was wound at 600.degree. C., pickled, and cold-rolled
at a reduction ratio of 65% to obtain a cold-rolled steel sheet
having a thickness of 1.2 mm. The resultant cold-rolled steel sheet
was subjected to heat treatment under the conditions shown in Table
2. Quenching was not performed on any sample shown in Table 2.
Herein, the holding time in Table 2 was a time held at the holding
temperature shown in Table 2. The annealing time in a first
temperature range of 700.degree. C. or higher and 950.degree. C. or
lower was 600 seconds or shorter under any of the conditions shown
in Table 2.
[0107] In the galvanizing treatment, both surfaces were subjected
to plating in a coating bath having a temperature of 463.degree. C.
at a coating weight of 50 g/m.sup.2 per surface. In the
galvannealing treatment, the alloying treatment was performed such
that Fe % (iron content) in the coating layer was adjusted to 9% by
mass. The resultant steel sheet was subjected to temper rolling at
a reduction ratio (elongation ratio) of 0.3% regardless of the
presence or absence of a coating.
TABLE-US-00001 TABLE 1 Steel (mass %) type C Si Mn Al P S N Cr V Mo
Ti Nb B Ni Cu Ca REM Remarks A 0.16 1.59 2.2 0.040 0.011 0.005
0.0039 0.5 -- -- -- -- -- -- -- -- -- Suitable steel B 0.15 1.51
2.3 0.036 0.012 0.004 0.0023 0.9 -- -- -- -- -- -- -- -- --
Suitable steel C 0.15 1.40 2.3 0.041 0.012 0.004 0.0029 -- -- -- --
0.04 -- -- -- -- -- Suitable steel D 0.15 1.00 2.2 0.039 0.009
0.004 0.0037 1.0 -- -- 0.021 -- 0.0010 -- -- -- -- Suitable steel E
0.14 1.48 2.2 0.040 0.025 0.002 0.0038 -- -- -- -- -- -- -- -- --
-- Suitable steel F 0.21 1.42 2.3 0.041 0.010 0.004 0.0037 -- -- --
-- -- -- -- -- -- -- Suitable steel G 0.29 1.50 2.1 0.040 0.010
0.003 0.0041 -- -- -- -- -- -- -- -- -- -- Suitable steel H 0.16
0.51 2.2 0.039 0.013 0.004 0.0032 1.5 -- -- 0.020 -- 0.0008 -- --
-- -- Suitable steel I 0.15 1.49 2.8 0.037 0.012 0.003 0.0033 -- --
-- 0.019 -- 0.0005 -- -- -- -- Suitable steel J 0.12 1.52 2.3 0.037
0.029 0.003 0.0041 1.0 -- -- 0.020 -- 0.0009 -- -- -- -- Suitable
steel K 0.21 0.49 1.6 0.037 0.029 0.003 0.0041 -- -- -- 0.022 --
0.0012 -- -- -- -- Suitable steel L 0.15 1.50 2.3 0.043 0.013 0.002
0.0043 1.0 -- -- 0.050 -- 0.0010 -- -- -- -- Suitable steel M 0.11
1.48 2.0 0.039 0.013 0.003 0.0037 0.9 -- 0.03 0.021 -- 0.0008 -- --
-- -- Suitable steel N 0.16 1.50 2.3 0.038 0.012 0.003 0.0041 0.8
0.10 -- -- -- -- -- -- -- -- Suitable steel O 0.12 0.99 1.2 0.040
0.013 0.003 0.0041 1.0 -- -- -- -- -- 1.00 -- -- -- Suitable steel
P 0.15 1.53 2.1 0.041 0.011 0.004 0.0029 0.5 -- -- -- -- -- -- 0.3
-- -- Suitable steel Q 0.15 1.48 2.3 0.044 0.009 0.004 0.0031 1.0
-- -- 0.019 -- 0.0008 -- -- -- 0.002 Suitable steel R 0.05 1.05 2.1
0.037 0.008 0.004 0.0030 -- -- -- -- -- -- -- -- -- -- Comparative
steel S 0.33 1.25 2.7 0.035 0.010 0.004 0.0042 -- -- -- -- -- -- --
-- -- -- Comparative steel T 0.23 1.51 3.5 0.040 0.008 0.004 0.0039
-- -- -- -- -- -- -- -- -- -- Comparative steel Note) Underline
means the value is outside the suitable range.
TABLE-US-00002 TABLE 2 Second temperature Third range temperature
Average range cooling Time Average rate required cooling First from
for rate temperature first cooling from range temperature from
420.degree. C. Holding Holding range 550.degree. C. to Sample Steel
Temperature time to 550.degree. C. to 420.degree. C. 250.degree. C.
No. type (.degree. C.) (second) (.degree. C./s) (second) (.degree.
C./s) Plating*.sup.2 Remarks 1 A 820 180 10 90 15 CR Invention
Example 2 B 830 100 10 60 10 GI Invention Example 3 C 820 180 10 80
55 CR Comparative Example 4 C 850 180 45 60 10 CR Invention Example
5 D 830 250 40 60 10 GI Invention Example 6 E 820 180 10 60 100 CR
Comparative Example 7 F 810 180 8 70 60 CR Comparative Example 8 G
820 180 5 60 55 GA Comparative Example 9 H 820 180 10 120 15 GA
Invention Example 10 I 870 180 15 60 10 CR Invention Example 11 J
830 200 30 60 10 CR Invention Example 12 J 830 300 7 60 5 CR
Invention Example 13 K 860 40 10 45 10 GI Invention Example 14 L
860 90 10 60 10 CR Invention Example 15 M 850 180 10 60 9 CR
Invention Example 16 N 800 450 10 80 10 CR Invention Example 17 O
820 180 10 60 8 GI Invention Example 18 P 860 400 10 75 9 GI
Invention Example 19 Q 800 180 10 60 10 CR Invention Example 20 R
800 180 10 80 10 CR Comparative Example 21 S 800 180 10 80 10 CR
Comparative Example 22 T 800 180 10 80 10 CR Comparative Example 23
A 880 200 50 20 70 CR Comparative Example 24 E 810 200 10 90 10 CR
Invention Example 25 G 850 350 5 650 5 CR Comparative Example 26 G
870 150 15 120 3 CR Invention Example *.sup.1Underline means the
value is outside the suitable range. *.sup.2CR: no plating
(cold-rolled steel sheet), GI: galvanizing, and GA:
galvannealing
[0108] The characteristics of the resultant steel sheets were
evaluated by the following methods.
[0109] To examine the microstructure of the steel sheets, two test
pieces were cut from each of the steel sheets. One of the test
pieces was polished without performing any treatment. The other of
the test pieces was polished after heat treatment was performed at
200.degree. C. for 2 hours. The polished surface was a section in
the sheet thickness direction, the section being parallel to the
rolling direction. By observing a steel microstructure of the
polished surface with a scanning electron microscope (SEM) at a
magnification of 3000.times., the area ratio of each phase was
measured to identify the phase structure of each crystal grain. The
observation was performed for 10 fields and the area ratio was an
average value of the 10 fields. The area ratios of autotempered
martensite, polygonal ferrite, and bainite were obtained using the
test pieces polished without performing any treatment. The area
ratios of as-quenched martensite (untempered martensite) and
retained austenite were obtained using the test pieces polished
after heat treatment was performed at 200.degree. C. for 2 hours.
The test pieces polished after heat treatment was performed at
200.degree. C. for 2 hours were prepared to differentiate
untempered martensite from retained austenite in the SEM
observation. In the SEM observation, it is difficult to
differentiate untempered martensite from retained austenite. When
martensite is tempered, an iron-based carbide is formed in the
martensite. The iron-based carbide makes it possible to
differentiate martensite from retained austenite. The heat
treatment at 200.degree. C. for 2 hours does not affect the phases
other than martensite, that is, martensite can be tempered without
changing the area ratio of each phase. As a result, martensite can
be differentiated from retained austenite due to the formed
iron-based carbide. By comparing the test pieces polished without
performing any treatment to the test pieces polished after heat
treatment was performed at 200.degree. C. for 2 hours through SEM
observation, it was confirmed that phases other than martensite
were not changed.
[0110] The size and number of iron-based carbide grains included in
autotempered martensite were measured through SEM observation. The
test pieces were the same as those used in the microstructure
observation. Obviously, the test pieces polished without performing
any treatment were observed. The test pieces were observed at a
magnification of 10000.times. to 30000.times. in accordance with
the precipitation state and size of the iron-based carbide grains.
The size of the iron-based carbide grains was evaluated using an
average value of the major axis and minor axis of individual
precipitates. The number of iron-based carbide grains each having a
size of 5 nm or more and 0.5 .mu.m or less was counted and thus the
number of iron-based carbide grains per 1 mm.sup.2 of autotempered
martensite was calculated. The observation was performed for 5 to
20 fields. The mean number was calculated from the total number of
all the fields of each sample, and the mean number was employed as
the number (per 1 mm.sup.2 of autotempered martensite) of
iron-based carbide grains of each sample.
[0111] The hardness HV of autotempered martensite was measured
using an ultramicro-Vickers hardness meter at a load of 0.02 N.
After the microstructure of autotempered martensite in which
iron-based carbide grains were precipitated was confirmed by
observing an indentation with a SEM, the average value of ten or
more measurement values was employed as the hardness HV.
[0112] A tensile test was performed in accordance with JIS Z2241
using a JIS No. 5 test piece taken from the steel sheet in the
rolling direction of the steel sheet. Tensile strength (TS), yield
strength (YS), and total elongation (T. El) were measured. The
product of the tensile strength and the total elongation
(TS.times.T. El) was calculated to evaluate the balance between the
strength and the elongation. When TS.times.T. El.gtoreq.14500
(MPa%), the balance was determined to be satisfactory.
[0113] Stretch-flangeability was evaluated in compliance with The
Japan Iron and Steel Federation Standard JFST 1001. The resulting
steel sheet was cut into pieces each having a size of 100
mm.times.100 mm. A hole having a diameter of 10 mm was made in the
piece by punching at a clearance of 12% of the thickness. A cone
punch with a 60.degree. apex was forced into the hole while the
piece was fixed with a die having an inner diameter of 75 mm at a
blank-holding pressure of 88.2 kN. The diameter of the hole was
measured when a crack was initiated. The maximum hole-expanding
ratio (%) was determined with Formula (2) to evaluate
stretch-flangeability using the maximum hole-expanding ratio:
Maximum hole-expanding ratio
.lamda.(%)={(D.sub.f-D.sub.0)/D.sub.0}.times.100 (2)
where D.sub.f represents the hole diameter (mm) when a crack was
initiated, and D.sub.0 represents an initial hole diameter (mm).
.lamda..gtoreq.15% was determined to be satisfactory.
[0114] Table 3 shows the evaluation results.
TABLE-US-00003 TABLE 3 Mean Number of hardness of iron-based Area
ratio (%) auto- carbide Auto- As- tempered grains Sample Steel
tempered Retained quenched martensite per No. type martensite
Ferrite austenite Bainite martensite (HV) 1 mm.sup.2 *1 1 A 57 43 0
0 0 602 1 .times. 10.sup.5 2 B 72 28 0 0 0 550 1 .times. 10.sup.5 3
C 35 63 0 0 2 661 1 .times. 10.sup.4 4 C 37 52 4 7 0 601 1 .times.
10.sup.5 5 D 83 17 0 0 0 526 5 .times. 10.sup.5 6 E 0 72 0 0 28 857
None 7 F 22 60 1 2 15 771 1 .times. 10.sup.3 8 G 58 37 0 0 5 691 5
.times. 10.sup.3 9 H 91 9 0 0 0 492 1 .times. 10.sup.6 10 I 84 12 1
3 0 503 1 .times. 10.sup.6 11 J 84 16 0 0 0 470 3 .times. 10.sup.6
12 J 30 67 0 0 3 667 7 .times. 10.sup.4 13 K 90 10 0 0 0 523 5
.times. 10.sup.5 14 L 90 10 0 0 0 505 5 .times. 10.sup.5 15 M 84 15
1 0 0 480 1 .times. 10.sup.6 16 N 93 7 0 0 0 495 1 .times. 10.sup.6
17 O 59 39 2 0 0 513 8 .times. 10.sup.5 18 P 88 12 0 0 0 503 1
.times. 10.sup.6 19 Q 86 14 0 0 0 502 1 .times. 10.sup.6 20 R 18 82
0 0 0 470 4 .times. 10.sup.6 21 S 70 20 0 0 10 802 1 .times.
10.sup.3 22 T 82 12 0 0 6 790 1 .times. 10.sup.3 23 A 49 8 1 0 42
563 5 .times. 10.sup.5 24 E 52 48 0 0 0 589 1 .times. 10.sup.5 25 G
10 27 14 46 0 603 1 .times. 10.sup.6 26 G 93 7 0 0 0 592 1 .times.
10.sup.6 Sample M YS TS T El TS .times. T El .lamda. No. (.degree.
C.) (MPa) (Mpa) (%) (MPa %) (%) Remarks 1 332 771 1255 14.8 18574
16 Invention Example 2 346 924 1341 12.0 16092 22 Invention Example
3 293 687 1238 13.4 16589 5 Comparative Example 4 330 660 1220 14.0
17080 21 Invention Example 5 361 849 1393 11.1 15462 45 Invention
Example 6 261 576 1066 18.8 20041 13 Comparative Example 7 250 667
1226 14.2 17409 5 Comparative Example 8 281 817 1521 7.5 11408 1
Comparative Example 9 355 946 1385 10.9 15097 36 Invention Example
10 358 908 1392 11.0 15239 42 Invention Example 11 367 772 1270
13.9 17653 35 Invention Example 12 288 601 1021 17.5 17868 22
Invention Example 13 389 903 1449 10.9 15794 32 Invention Example
14 359 916 1418 11.8 16732 34 Invention Example 15 386 883 1305
12.5 16313 49 Invention Example 16 357 838 1420 12.4 17608 20
Invention Example 17 378 617 1125 15.2 17100 25 Invention Example
18 372 900 1389 10.5 14585 36 Invention Example 19 356 837 1371
12.2 16726 24 Invention Example 20 420 593 781 20.9 16323 29
Comparative Example 21 276 1081 1520 9.2 13984 1 Comparative
Example 22 297 931 1481 9.6 14218 2 Comparative Example 23 370 1123
1481 9.7 14365 10 Comparative Example 24 346 763 1197 16.1 19272 18
Invention Example 25 304 1008 1423 14.1 20064 4 Comparative Example
26 334 1118 1635 11.2 18312 19 Invention Example *1 The size of
iron-based carbide grains is 5 nm or more and 0.5 .mu.m or less.
*2) Underline means the value is outside the suitable range.
[0115] As is clear from Table 3, our steel sheets have a tensile
strength of 900 MPa or higher, a value of TS.times.T.
El.gtoreq.14500 (MPa%), and a value of .lamda..gtoreq.15% that
represents stretch-flangeability and thus has both high strength
and good workability. In Invention Examples, the steel sheets
having an M of 300.degree. C. or higher are excellent in
stretch-flangeability, particularly stretch-flangeability that is
not degraded even if strength is increased.
[0116] In contrast, in sample Nos. 6 and 7, the hardness of
martensite is 700<HV and the number of iron-based carbide grains
included in martensite is less than 5.times.10.sup.4 per 1 mm.sup.2
or martensite does not include iron-based carbide grains.
Therefore, a tensile strength of 900 MPa is satisfied, but a value
of .lamda., is less than 15%, which provides poor workability. This
is because, in sample Nos. 6 and 7, the cooling rate in the third
temperature range is high, which does not satisfy 50.degree. C./s.
In sample Nos. 3 and 8, the hardness of martensite is
satisfactorily HV.ltoreq.700, but the number of iron-based carbide
grains included in martensite is less than 5.times.10.sup.4 per 1
mm.sup.2. Therefore, a tensile strength of 900 MPa or higher is
satisfied, but a value of .lamda., is less than 15%, which provides
poor workability. This is because, in sample Nos. 3 and 8, the
cooling rate in the third temperature range is 55.degree. C./s,
which does not satisfy 50.degree. C./s or lower. In particular,
since sample No. 8 has a relatively high C content, TS.times.. El
is 14500 MPa% or less.
[0117] It can be confirmed from the above description that steel
sheets that include autotempered martensite sufficiently subjected
to autotempering treatment such that the hardness of martensite is
HV.ltoreq.700 and the number of iron-based carbide grains in
martensite is 5.times.10.sup.4 or more per 1 mm.sup.2 have both
high strength and good workability.
Example 2
[0118] To confirm the effect of further improvement in ductility
achieved by suitably controlling the distribution state of
iron-based carbide grains included in autotempered martensite,
samples were manufactured in the same manner as the samples shown
in Table 2, except that the cooling rate in a temperature range of
250.degree. C. or higher and (Ms temperature-50).degree. C. or
lower of the third temperature range was changed as shown in Table.
4. In Table 4, sample Nos. 9, 11, 13, 14, and 26 are the same as
those shown in Table 2 and listed in Table 4 to clarify the
temperature range of 250.degree. C. or higher and (Ms
temperature-50).degree. C. or lower. Note that M (.degree. C.) was
used as the Ms temperature.
TABLE-US-00004 TABLE 4 Second Third temperature temperature range
range First Average Time Average Average cooling temperature
cooling required cooling rate from range rate from first for
cooling rate from (Ms temperature Holding Holding temperature from
550.degree. C. 420.degree. C. -50) Steel temperature time range to
to 420.degree. C. to 250.degree. C. .degree. C. to 250.degree. C.
Sample No. type (.degree. C.) (second) 550.degree. C. (.degree.
C./s) (second) (.degree. C./s) (.degree. C./s) Plating *.sup.1
Remarks 9 H 820 180 10 120 15 0.8 GA Invention Example 11 J 830 200
30 60 10 20 CR Invention Example 13 K 860 40 10 45 10 0.5 GI
Invention Example 14 L 860 90 10 60 10 0.8 CR Invention Example 26
G 870 150 15 120 3 10 CR Invention Example 27 H 820 180 10 120 15
30 GA Invention Example 28 J 830 200 30 60 10 0.5 CR Invention
Example 29 K 860 40 10 45 10 25 GI Invention Example 30 L 860 90 10
60 10 20 CR Invention Example 31 G 870 150 15 120 3 0.4 CR
Invention Example *.sup.1CR: no plating (cold-rolled steel sheet),
GI: galvanizing, and GA: galvannealing
[0119] The characteristics of the thus-obtained steel sheets were
evaluated in the same manner as in Example 1. Herein, the amount of
autotempered martensite in which the number of precipitated
iron-based carbide grains each having a size of 0.1 .mu.m or more
and 0.5 .mu.m or less is 5.times.10.sup.2 or less per 1 mm.sup.2 in
the entire autotempered martensite was obtained as follows.
[0120] As described above, the test pieces polished without
performing any treatment were observed at a magnification of
10000.times. to 30000.times. using a SEM. The size of the
iron-based carbide grains was evaluated using an average value of
the major axis and minor axis of individual precipitates. The area
ratio of autotempered martensite in which the iron-based carbide
grains have a size of 0.1 .mu.m or more and 0.5 .mu.m or less was
measured. The observation was performed for 5 to 20 fields.
[0121] Table 5 shows the results.
[0122] As is apparent from Table 5, in sample Nos. 11, 26, 27, 29,
and 30 with a cooling rate of 1.0.degree. C./s or higher and
50.degree. C./s or lower in the temperature range of 250.degree. C.
or higher and (Ms temperature-50).degree. C. or lower, the
distribution state of iron-based carbide grains included in
autotempered martensite is suitably controlled and thus TS.times.T.
El.gtoreq.17000 MPa% is exhibited, that is, ductility is
improved.
TABLE-US-00005 TABLE 5 Number of Mean iron-based hardness carbide
Area ratio (%) of auto- grains Auto- As- tempered (5 nm to Sample
Steel tempered Retained quenched martensite 0.5 .mu.m) per No. type
martensite Ferrite austenite Bainite martensite (HV) 1 mm2 9 H 91 9
0 0 0 492 1 .times. 106 11 J 84 16 0 0 0 470 3 .times. 106 13 K 90
10 0 0 0 523 5 .times. 105 14 L 90 10 0 0 0 505 5 .times. 105 26 G
93 7 0 0 0 592 1 .times. 106 27 H 91 9 0 0 0 564 1 .times. 106 28 J
84 16 0 0 0 460 3 .times. 106 29 K 90 10 0 0 0 553 5 .times. 105 30
L 90 10 0 0 0 526 5 .times. 105 31 G 93 7 0 0 0 541 1 .times. 106
Area ratio of auto- tempered martensite in which the number of
precipitated iron-based carbide grains (5 nm to 0.5 .mu.m) is 5
.times. 102 or less per 1 mm2 to the entire Sample autotempered M
YS TS T El TS .times. T El .lamda. No. martensite (%) (.degree. C.)
(MPa) (MPa) (%) (MPa %) (%) Remarks 9 2 355 946 1385 10.9 15097 36
Invention Example 11 14 367 772 1270 13.9 17653 35 Invention
Example 13 1 389 903 1449 10.9 15794 32 Invention Example 14 2 359
916 1418 11.8 16732 34 Invention Example 26 16 334 1118 1635 11.2
18312 19 Invention Example 27 24 355 952 1538 11.1 17072 32
Invention Example 28 2 367 797 1207 13.8 16657 36 Invention Example
29 16 389 907 1524 11.2 17069 29 Invention Example 30 14 359 918
1457 12.1 17630 30 Invention Example 31 0 334 1094 1497 11.3 16916
18 Invention Example
* * * * *