U.S. patent application number 15/550180 was filed with the patent office on 2019-01-31 for ultra-high-strength steel sheet having excellent yield ratio and workability.
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 Toshio MURAKAMI, Toshiya NAKATA, Takahiro OZAWA, Kenji SAITO, Kosuke SHIBATA, Atsuhiro SHIRAKI, Yukihiro UTSUMI, Fumio YUSE.
Application Number | 20190032166 15/550180 |
Document ID | / |
Family ID | 56615325 |
Filed Date | 2019-01-31 |
United States Patent
Application |
20190032166 |
Kind Code |
A1 |
SHIBATA; Kosuke ; et
al. |
January 31, 2019 |
ULTRA-HIGH-STRENGTH STEEL SHEET HAVING EXCELLENT YIELD RATIO AND
WORKABILITY
Abstract
An ultra-high-strength steel sheet having a component
composition that includes specific amounts of each of C, Si, Mn,
and Al and a remainder of iron and unavoidable impurities, and in
which the amounts of each of P, S, and N among the unavoidable
impurities are limited to a specific amount. The
ultra-high-strength steel sheet includes 1 area % or more of a
region in which martensite constitutes 90 area % or more, residual
austentite constitutes 0.5 area % or more, and the local Mn
concentration is at least 1.2 times that of the Mn content of the
entire steel sheet. The ultra-high-strength steel sheet has a
tensile strength of 1470 MPa or more, a yield ratio of 0.75 or
more, and a total elongation of 10% or more.
Inventors: |
SHIBATA; Kosuke; (Hyogo,
JP) ; NAKATA; Toshiya; (Hyogo, JP) ; MURAKAMI;
Toshio; (Hyogo, JP) ; OZAWA; Takahiro; (Hyogo,
JP) ; YUSE; Fumio; (Hyogo, JP) ; SHIRAKI;
Atsuhiro; (Hyogo, JP) ; SAITO; Kenji; (Hyogo,
JP) ; UTSUMI; Yukihiro; (Hyogo, 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: |
56615325 |
Appl. No.: |
15/550180 |
Filed: |
February 8, 2016 |
PCT Filed: |
February 8, 2016 |
PCT NO: |
PCT/JP2016/053640 |
371 Date: |
August 10, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/04 20130101;
C21D 9/46 20130101; C22C 38/54 20130101; C21D 8/0205 20130101; C22C
38/00 20130101; C21D 6/001 20130101; C21D 2211/002 20130101; C22C
38/002 20130101; C21D 6/002 20130101; C21D 2211/009 20130101; C22C
38/12 20130101; C21D 8/0226 20130101; C21D 6/008 20130101; C21D
6/005 20130101; C22C 38/06 20130101; C22C 38/08 20130101; C22C
38/18 20130101; C21D 2211/008 20130101; C22C 38/02 20130101; C22C
38/14 20130101 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C22C 38/14 20060101 C22C038/14; C22C 38/12 20060101
C22C038/12; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C22C 38/00 20060101
C22C038/00; C22C 38/08 20060101 C22C038/08; C22C 38/18 20060101
C22C038/18; C21D 8/02 20060101 C21D008/02; C21D 6/00 20060101
C21D006/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 13, 2015 |
JP |
2015-026736 |
Claims
1. An ultra-high-strength steel sheet excellent in yield ratio and
workability, having a composition comprising, by mass %, C: 0.15%
to 0.35%, Si: 0.5% to 3.0% Mn: 0.5% to 1.5%, Al: 0.001% to 0.10%
and the balance being iron and inevitable impurities, wherein each
of P, S and N of the inevitable impurities is limited to P: 0.1% or
less S: 0.01% or less and N: 0.01% or less, the ultra-high-strength
steel sheet having a structure comprising, by area ratio based on a
whole structure, martensite: 90% or more and residual austenite:
0.5% or more, the ultra-high-strength steel sheet having 1% or more
by area ratio of a region where a local Mn concentration is at
least 1.2 times a Mn content in a whole steel sheet, and the
ultra-high-strength steel sheet having a tensile strength of 1470
MPa or more, a yield ratio of 0.75 or more and an elongation of 10%
or more.
2. The ultra-high-strength steel sheet excellent in yield ratio and
workability according to claim 1, wherein the composition further
comprises, by mass %, at least one of the following (a) to (c): (a)
one or two or more of Cu: 0.05% to 1.0%, Ni: 0.05% to 1.0% and B:
0.0002% to 0.0050%, (b) one or two or more of Mo: 0.01% to 1.0%,
Cr: 0.01% to 1.0%, Nb: 0.01% to 0.3%, Ti: 0.01% to 0.3% and V:
0.01% to 0.3%, and (c) one or two of Ca: 0.0005% to 0.01% and Mg:
0.0005% to 0.01%.
Description
[0001] TECHNICAL FIELD
[0002] The present invention relates to an ultra-high-strength
steel sheet excellent in yield ratio and workability. The steel
sheet type of the ultra-high-strength steel sheet in accordance
with the present invention shall be considered to include not only
cold-rolled steel sheets, but also various plated steel sheets such
as hot-dip galvanized steel sheets and hot-dip galvanized and
alloyed steel sheets.
BACKGROUND ART
[0003] For the purpose of improvement of fuel consumption by weight
reduction of vehicle bodies, steel sheets used for skeleton
components of automobiles have recently been required to be
increased in strength, and in order to ensure collision safety, a
high yield ratio is also required. On the other hand, in order to
form parts with complicated shapes, excellent workability is also
required.
[0004] It has therefore been eagerly desired to provide an
ultra-high-strength steel sheet increased in elongation (EL) while
having a high yield ratio. More specifically, a steel sheet having
a tensile strength of 1470 MPa or more, a yield ratio of 0.75 or
more, and an elongation of 10% or more has been required.
[0005] In addition, although steel sheets for automobiles are
subjected to welding during assembly of vehicle bodies or during
mounting of parts, weldability heavily depends on compositions of
the steel sheets. In particular, when C and Mn are added in large
amounts, it is known that the weldability is degraded. It has
therefore been required for the steel sheets for automobiles to
fulfill the above-mentioned mechanical properties, while having a
composition satisfying 0.35 mass % or less of C and 1.5 mass % or
less of Mn.
[0006] Conventionally herein, in order to increase the elongation
of the high-strength steel sheet, mainly the following two means
have been used.
[0007] (1) The amount of residual austenite is increased to utilize
a TRIP action thereof.
[0008] (2) The amount of soft ferrite (including bainitic ferrite)
is increased.
[0009] However, in order to allow a large amount of austenite to
remain, the means of the above (1) requires the increase of the
added amount of C or Mn, resulting in a failure to satisfy
C.ltoreq.0.35 mass % and Mn.ltoreq.1.5 mass %. There has been
therefore a problem that sufficient weldability cannot be
ensured.
[0010] On the other hand, in order to ensure the elongation, the
means of the above (2) requires a predetermined amount of a soft
phase, resulting in a failure to satisfy a yield ratio of 0.75 or
more. There has been therefore a problem that sufficient collision
safety cannot be ensured.
[0011] For example, Patent Literature 1 proposes a steel sheet that
is increased in resistance to hydrogen embrittlement and is also
excellent in resistance to delayed fracture at a punching hole
processing part, in an ultra-high-strength region having a tensile
strength of 1180 MPa or more, by allowing a large amount of
austenite to remain by increasing the Mn content in the steel
sheet.
[0012] However, with respect to the above-mentioned steel sheet,
the Mn content in the steel sheet is more than 1.5 mass % for all
the invention steels as shown in the examples thereof, and there
has been room for improvement in terms of the weldability.
[0013] In addition, Patent Literature 2 proposes a steel sheet that
can realize a tensile strength of 1470 MPa or more and an
elongation of 10% or more, in a composition satisfying 0.35 mass %
or less of C and 1.5 mass % or less of Mn, by increasing the
fraction of a soft ferrite phase.
[0014] However, the above-mentioned steel sheet cannot realize a
yield ratio of 0.75 or more as shown in the examples thereof, and
there is a problem that sufficient collision safety cannot be
ensured.
CITATION LIST
Patent Literatures
[0015] Patent Literature 1: JP-A-2008-81788
[0016] Patent Literature 2: JP-A-2010-90432
SUMMARY OF INVENTION
Technical Problems
[0017] Therefore, an object of the present invention is to provide
an ultra-high-strength steel sheet excellent in yield ratio and
workability, which can satisfy a tensile strength of 1470 MPa or
more, a yield ratio of 0.75 or more and an elongation of 10% or
more.
Solution to Problems
[0018] In a first invention of the present invention which is an
ultra-high-strength steel sheet excellent in yield ratio and
workability, the ultra-high-strength steel sheet has a composition
comprising, by mass %,
[0019] C: 0.15% to 0.35%,
[0020] Si: 0.5% to 3.0%
[0021] Mn: 0.5% to 1.5%,
[0022] Al: 0.001% to 0.10% and
[0023] the balance being iron and inevitable impurities,
[0024] wherein each of P, S and N of the inevitable impurities is
limited to
[0025] P: 0.1% or less
[0026] S: 0.01% or less and
[0027] N: 0.01% or less,
[0028] the ultra-high-strength steel sheet has a structure
comprising, by area ratio based on a whole structure,
[0029] martensite: 90% or more and
[0030] residual austenite: 0.5% or more,
[0031] the ultra-high-strength steel sheet has 1% or more by area
ratio of a region where a local Mn concentration is at least 1.2
times a Mn content in a whole steel sheet, and
[0032] the ultra-high-strength steel sheet has a tensile strength
of 1470 MPa or more, a yield ratio of 0.75 or more and an
elongation of 10% or more.
[0033] In a second invention of the present invention which is the
ultra-high-strength steel sheet excellent in yield ratio and
workability according to the first invention, the composition
further comprises, by mass %, one or two or more of
[0034] Cu: 0.05% to 1.0%,
[0035] Ni: 0.05% to 1.0% and
[0036] B: 0.0002% to 0.0050%.
[0037] In a third invention of the present invention which is the
ultra-high-strength steel sheet excellent in yield ratio and
workability according to the first or second invention, the
composition further comprises, by mass %, one or two or more of
[0038] Mo: 0.01% to 1.0%,
[0039] Cr: 0.01% to 1.0%,
[0040] Nb: 0.01% to 0.3%,
[0041] Ti: 0.01% to 0.3% and
[0042] V: 0.01% to 0.3%.
[0043] In a fourth invention of the present invention which is the
ultra-high-strength steel sheet excellent in yield ratio and
workability according to any one of the first to third inventions,
the composition further comprises, by mass %, one or two of
[0044] Ca: 0.0005% to 0.01% and
[0045] Mg: 0.0005% to 0.01%.
Advantageous Effects of Invention
[0046] In accordance with the present invention, martensite is used
as a main structure of steel, and Mn is concentrated in residual
austenite, without increasing the average concentration of C and Mn
in the whole steel sheet, whereby it has become possible to provide
an ultra-high-strength steel sheet that has a high strength and a
high yield ratio and is excellent in workability, while ensuring
weldability.
DESCRIPTION OF EMBODIMENTS
[0047] The present invention will be explained below in greater
detail.
[0048] First, a structure characterizing an ultra-high-strength
steel sheet excellent in yield ratio and workability in accordance
with the present invention (hereinafter also referred to as "the
steel sheet in the present invention") will be explained.
[Structure of the Steel Sheet in the Present Invention]
[0049] As described above, in the steel sheet in the present
invention, martensite is used as a matrix, and moreover residual
austenite in which Mn is concentrated is contained in a
predetermined amount (hereinafter, austenite is sometimes
represented by .gamma.).
<Martensite: 90% or More>
[0050] In order to realize the steel sheet having a tensile
strength of 1470 MPa or more and achieve a high yield ratio of 0.75
or more, martensite is required to be, by area ratio, 90% or more,
preferably 92% or more, and more preferably 94% or more. In the
present description, martensite is used to mean including both
fresh martensite not subjected to tempering and tempered martensite
subjected to tempering.
[0051] Since all except for residual austenite may be martensite,
the upper limit of the martensite area ratio is 99.5%, and it is
preferably 99% or less, in consideration of the lower limit (0.5%)
of residual austenite.
<Residual Austenite: 0.5% or More>
[0052] In order to use its TRIP action to thereby improve the
elongation, the residual austenite is required to be, by area
ratio, 0.5% or more, preferably 0.6% or more, and more preferably
0.7% or more.
[0053] Since all except for martensite may be residual austenite,
the upper limit of the residual austenite area ratio is 10%, and it
is preferably 5% or less, more preferably 3% or less, and
particularly preferably 2% or less, in consideration of the lower
limit (90%) of martensite.
[0054] As described above, although the steel sheet in the present
invention may be composed of only two phases of martensite and
residual austenite (the total area ratio of the two phases is
100%), it is possible to inevitably generate other phases (such as
ferrite, bainite and pearlite). The presence of such other phases
is allowed as long as the total area ratio thereof is 9.5% or less.
The total area ratio of the other phases is preferably 7.5% or
less, and more preferably 5.5% or less.
[0055] <Region Where the Local Mn Concentration is at Least 1.2
Times the Mn Content in the Whole Steel Sheet: 1% or More by Area
Ratio>
[0056] Residual austenite is allowed to remain even in a high
strain region by concentrating Mn in residual austenite to increase
stability of the residual austenite, thereby further improving the
elongation to ensure an elongation of 10% or more. On the other
hand, from the viewpoint of ensuring weldability, the average Mn
concentration in the steel sheet is required to fulfill 1.5 mass %
or less. In the steel sheet in the present invention, therefore, a
Mn-concentrated region is formed. That is, residual austenite
formed in the Mn-concentrated region is stabilized while keeping
low the Mn concentration in the matrix. This results in that a part
of a region where the local Mn concentration is at least 1.2 times
the Mn content in the whole steel sheet is present as residual
austenite to contribute to further improvement of the
elongation.
[0057] Then, the composition constituting the steel sheet in the
present invention will be explained. All the units of chemical
components are hereinafter by mass %.
[Composition of Steel Sheet in the Present Invention]
C: 0.15% to 0.35%
[0058] C is an important element having a large influence on the
strength of the steel sheet. In order to ensure the strength of the
steel sheet, C is contained in an amount of 0.15% or more,
preferably 0.16% or more and more preferably 0.17% or more.
However, when C is excessively contained, the weldability is
degraded. Therefore, C is contained in an amount of 0.35% or less,
preferably 0.3% or less, and more preferably 0.25% or less.
Si: 0.5% to 3.0%
[0059] Si is a useful element for suppressing the formation of
carbides and promoting the formation of the residual austenite. In
order to effectively exhibit such an action, Si is contained in an
amount of 0.5% or more, and is preferably 0.8% or more, and is more
preferably 1.1% or more. However, when Si is excessively contained,
the weldability is remarkably degraded. Therefore, Si is contained
in an amount of 3.0% or less, preferably 2.5% or less, and more
preferably 2.0% or less.
Mn: 0.5% to 1.5%
[0060] Mn is a useful element contributing to an increase in the
strength of the steel sheet as a solid solution hardening element.
It has also an effect of suppressing ferrite transformation during
cooling by increasing hardenability during quenching. In addition,
since it has also an effect of stabilizing austenite, residual
austenite having high stability can be formed. In order to
effectively exhibit such actions, Mn is contained in an amount of
0.5% or more, preferably 0.7% or more, and more preferably 0.9% or
more. However, the Mn amount is preferably lower from the
standpoint of ensuring the weldability, and Mn is contained in an
amount of 1.5% or less, preferably 1.3% or less, and more
preferably 1.15% or less.
Al: 0.001% to 0.10%
[0061] Al is a useful element added as a deoxidizing agent, and in
order to obtain such an action, it is contained in an amount of
0.001% or more, preferably 0.01% or more, and more preferably 0.03%
or more. However, when Al is excessively contained, cleanliness of
the steel is degraded. Therefore, Al is contained in an amount of
0.10% or less, preferably 0.08% or less, and more preferably 0.06%
or less.
[0062] The steel sheet in the present invention contains the
above-mentioned elements as essential elements, the balance being
iron and inevitable impurities (such as P, S, N and O). Of the
inevitable impurities, P, S and N can be contained up to respective
allowable ranges as described below.
P: 0.1% or less
[0063] P is inevitably present as an impurity element, and
contributes to an increase in the strength by solid solution
hardening. However, the segregation thereof to prior austenite
grain boundary embrittles the grain boundary, thereby degrading
workability. Therefore, the P amount is limited to 0.1% or less,
preferably 0.05% or less, and more preferably 0.03% or less.
S: 0.01% or less
[0064] S is also inevitably present as an impurity element, and
forms MnS inclusions, which may be starting points of cracks during
deformation, thereby decreasing the workability. Therefore, the S
amount is limited to 0.01% or less, preferably 0.005% or less, and
more preferably 0.003% or less.
N: 0.01% or less
[0065] N is also inevitably present as an impurity element, and
decreases the workability of the steel sheet by strain aging.
Therefore, the N amount is limited to 0.01% or less, preferably
0.005% or less, and more preferably 0.003% or less.
[0066] In addition to these, the following allowable components may
be contained within the ranges not impairing the actions of the
present invention.
One or two or more of
Cu: 0.05% to 1.0%,
Ni: 0.05% to 1.0% and
B: 0.0002% to 0.0050%
[0067] These elements are useful elements having an effect of
increasing hardenability during quenching and suppressing
transformation from austenite. In order to obtain such an action,
the respective elements are preferably contained in an amount equal
to or more than the above-mentioned lower limits, respectively. The
above-mentioned elements may be contained either alone or as a
combination of two or more thereof. However, even when these
elements are excessively contained, the effect becomes saturated,
resulting in an economic waste. Therefore, the respective elements
are contained in an amount equal to or less than the
above-mentioned upper limits, respectively.
One or two or more of
Mo: 0.01% to 1.0%,
Cr: 0.01% to 1.0%,
Nb: 0.01% to 0.3%,
Ti: 0.01% to 0.3% and
V: 0.01% to 0.3%
[0068] These elements are useful for improving the strength without
degrading the workability. In order to obtain such an action, the
respective elements are preferably contained in an amount equal to
or more than the above-mentioned lower limits, respectively. The
above-mentioned elements may be contained either alone or as a
combination of two or more thereof. However, when these elements
are excessively contained, coarse carbides are formed to degrade
the workability. Therefore, the respective elements are contained
in an amount equal to or less than the above-mentioned upper
limits, respectively.
One or two of
Ca: 0.0005% to 0.01% and
Mg: 0.0005% to 0.01%
[0069] These elements are useful for improving the workability by
decreasing starting points of fracture by refining inclusions. In
order to obtain such an action, the elements are each preferably
contained in an amount of 0.0005% or more. The above-mentioned
elements may be contained either alone or as a combination of two
of them. However, when excessively contained, the inclusions are
coarsened on the contrary to degrade the workability. Therefore,
the elements are each contained in an amount of 0.01% or less.
[0070] Then, preferred production conditions for obtaining the
above-mentioned steel sheet in the present invention will be
explained below.
[Preferred Production Method of Steel Sheet in the Present
Invention]
[0071] First, the steel having the above-mentioned composition is
melted, and a slab (steel material) is obtained by ingot making or
continuous casting. Thereafter, hot rolling is performed under
conditions of a soaking temperature of 1200.degree. C. or lower
(more preferably 1150.degree. C. or lower) and a finishing
temperature of 900.degree. C. or lower (more preferably 880.degree.
C. or lower), followed by cooling from the finishing temperature to
the Ac1 point or lower, thereby forming a bainite or pearlite
single-phase structure or a two-phase structure as containing
ferrite.
[0072] After the above-mentioned hot rolling, annealing treatment
is performed under conditions of holding at 680.degree. C. to the
Ac1 point (more preferably 690.degree. C. to [Ac1-10.degree. C.])
for 0.8 hours or longer (more preferably 1 hour or longer). By this
annealing treatment, carbides are spheroidized and coarsened, and
Mn is concentrated in the carbides to at least 1.2 times the amount
of Mn added to the steel sheet. This annealing treatment may be
performed by holding as such in the above-mentioned temperature
region after cooling to the Ac1 point or lower, may be performed by
gradual cooling in this temperature region, or may be performed
after once cooled to lower than 680.degree. C. after the hot
rolling.
[0073] The Ac1 point can be determined from chemical components of
the steel sheet using the following formula (1) described in
Leslie, "The Physical Metallurgy of Steels", translated by
Shigeyasu Kouda, Maruzen, 1985, p. 273.
Ac1 (.degree.
C.)=723-10.7.times.Mn-16.9.times.Ni+29.1.times.Si+16.9.times.Cr
(1)
[0074] Here, each element symbol in the above-mentioned formula
represents the content (mass %) of each element.
[0075] After the above-mentioned annealed sheet is cold rolled, the
cold-rolled sheet is subjected to heat treatment (y-transformation
heat treatment) under conditions of holding it at an austenite
single-phase region temperature (the Ac3 point or higher) for 52 s
or longer, thereby austenitizing the carbides. Since Mn has been
concentrated in the carbides by the annealing treatment in the
prior stage, austenite having a high Mn concentration is formed. By
rapid cooling from the austenite single-phase region temperature to
room temperature at a cooling rate of 100.degree. C./s or more,
residual austenite where Mn has been concentrated to at least 1.2
times the amount of Mn added to the steel sheet can be formed in
martensite that is the matrix.
[0076] The Ac3 point can be determined from chemical components of
the steel sheet using the following formula (2) described in
Leslie, "The Physical Metallurgy of Steels", translated by
Shigeyasu Kouda, Maruzen, 1985, p. 273.
Ac3 (.degree. C.)=910-203.times.
C-30.times.Mn+44.7.times.Si+700.times.P+400.times.Al-15.2.times.Ni-11.tim-
es.Cr-20.times.Cu+400.times.Ti+31.5.times.Mo+104.times.V (2)
[0077] Here, each element symbol in the above-mentioned formula
represents the content (mass %) of each element.
[0078] Then, tempered martensite is formed by tempering the
above-mentioned heat-treated sheet under conditions of holding it
at 150 to 300.degree. C. for 30 to 1200 s, and strength-elongation
balance can be improved to obtain the steel sheet in the present
invention (the ultra-high-strength steel sheet excellent in the
yield ratio and workability).
[0079] The present invention will be explained below in greater
detail with reference to Examples, but it goes without saying that
the present invention is not limited to the Examples described
below and can be implemented with appropriate modifications without
departing from the spirit described above and later, and all such
modification are included in the technical scope of the present
invention.
EXAMPLES
[Test Method]
[0080] Steels having respective compositions of A to K shown in
Table 1 described below were melted, and ingots having a thickness
of 120 mm were prepared. Using these ingots, hot rolling was
performed to a thickness of 2.8 mm, and thereafter, annealing was
performed under the annealing conditions shown in Table 2 described
below. After the annealed sheets were pickled, they were cold
rolled to a thickness of 1.0 mm to obtain cold-rolled sheets. Then,
the cold-rolled sheets were subjected to y-transformation heat
treatment and tempering under the respective conditions shown in
Table 2 described below.
TABLE-US-00001 TABLE 1 Transformation Steel Chemical composition*
(mass %) temperature (.degree. C.) type C Si Mn Al P S N Others
Ac.sub.1 Ac.sub.3 A 0.20 1.78 0.99 0.045 0.015 0.0015 0.0041 B:
0.002, Ti: 0.015 743 898 B 0.20 1.84 1.28 0.041 0.011 0.0015 0.0042
-- 740 887 C 0.19 1.75 1.35 0.045 0.012 0.0012 0.0037 Ca: 0.004,
Mg: 0.005 739 886 D 0.25 1.20 1.08 0.046 0.008 0.0016 0.0041 Ti:
0.05 742 854 E 0.10 1.45 1.02 0.045 0.011 0.0017 0.0038 -- 743 906
F 0.22 1.44 0.49 0.045 0.009 0.0011 0.0035 -- 748 889 G 0.21 1.53
0.95 0.046 0.013 0.0008 0.0041 Cr: 0.50 752 900 H 0.22 1.64 1.25
0.045 0.010 0.0016 0.0042 Cu: 0.10 740 874 I 0.21 1.46 1.11 0.045
0.009 0.0012 0.0041 Ni: 0.10 740 871 J 0.22 1.39 1.06 0.045 0.016
0.0008 0.0037 Nb: 0.05 742 874 K 0.20 1.52 1.08 0.043 0.011 0.0011
0.0041 Mo: 0.10 742 920 L 0.19 1.44 1.03 0.045 0.009 0.0012 0.0037
V: 0.05 743 884 (Underlined: outside the range of the present
invention, *: balance: iron and inevitable impurities, --: not
added)
TABLE-US-00002 TABLE 2 Production Steel Annealing after hot rolling
.gamma.-transformation heat treatment Tempering No. type
Temperature (.degree. C.) Time (h) Temperature (.degree. C.) Time
(s) Cooling rate (.degree. C./s) Temperature (.degree. C.) Time (s)
1 A 500 1 930 90 >150 200 360 2 A 700 0.5 930 90 >150 200 360
3 A 700 1 930 90 >150 200 360 4 A 700 1 850 90 >150 200 360 5
A 800 1 930 90 >150 200 360 6 B 500 1 930 90 >150 200 360 7 B
700 0.5 930 90 >150 200 360 8 B 700 1 930 90 >150 200 360 9 B
700 1 850 90 >150 200 360 10 B 800 1 930 90 >150 200 360 11 C
700 1 930 90 >150 200 360 12 D 700 1 930 90 >150 200 360 13 E
700 1 930 90 >150 200 360 14 F 700 1 930 90 >150 200 360 15 G
700 1 930 90 >150 200 360 16 H 700 1 930 90 >150 200 360 17 I
700 1 930 90 >150 200 360 18 J 700 1 930 90 >150 200 360 19 K
700 1 930 90 >150 200 360 20 L 700 1 930 90 >150 200 360
(Underlined: outside the range of the present invention, Hatched:
outside the recommended conditions of the present invention)
[Measurement Methods]
[0081] Using each steel sheet obtained, the area ratio of
martensite and residual austenite and the local Mn concentration
were measured. In order to evaluate mechanical properties of the
steel sheet, the yield strength (YS), the tensile strength (TS) and
the elongation (EL) were also measured. These measurement methods
are shown below.
(Area Ratio of Martensite)
[0082] The area ratio of martensite was measured as follows. Each
steel sheet was mirror polished, and a surface thereof was corroded
with a 3% Nital liquid to expose a metal structure. Thereafter,
using an SEM (scanning electron microscope), a structure of a
portion of 1/4 the sheet thickness was observed under a
magnification of 2000 for 5 fields of view of an approximately 40
.mu.m.times.30 .mu.m region, and a region looking grey was defined
as martensite. The area ratios determined for the respective fields
of view were arithmetically averaged as the area ratio of
martensite.
(Area Ratio of Residual Austenite)
[0083] The area ratio of residual austenite was determined by
grinding and polishing each steel sheet to 1/4 the sheet thickness
in a sheet thickness direction and measuring X-ray diffraction
intensity.
(Local Mn Concentration)
[0084] The local Mn concentration was determined by quantitatively
analyzing 3 fields of view of an approximately 20 .mu.m.times.20 mm
region using a field emission electron probe microanalyzer
(FE-EPMA), dividing a measurement region to small regions of 1
.mu.m.times.1 mm in each field of view, and averaging the Mn
concentrations in the respective small regions. The ratio of small
regions where the average Mn concentration is at least 1.2 times
the Mn content in the steel sheet was defined as the area ratio of
the Mn-concentrated region in each field of view, and calculated.
Evaluation was performed by arithmetically averaging the area
ratios of the Mn-concentrated regions in the 3 fields of view.
(Yield Strength, Tensile Strength and Elongation)
[0085] Using each steel sheet to be evaluated, a No. 5 testpiece
described in JIS Z 2201 was prepared while taking a major axis to a
direction perpendicular to a rolling direction, and measurement was
performed in accordance with JIS Z 2241 to determine the yield
strength (YS), tensile strength (TS) and elongation (EL), and then,
yield ratio (YR) was determined from YS/TS.
[Measurement Results]
[0086] The measurement results are shown in Table 3 described
below. In these examples, the sheet having a tensile strength (TS)
of 1470 MPa or more, a yield ratio (YR) of 0.75 or more and an
elongation (EL) of 10% or more was represented by "A" and evaluated
as passed, and determined as an ultra-high-strength steel sheet
that excelled in the yield ratio and the workability. On the other
hand, the sheet having a tensile strength (TS) of less than 1470
MPa, a yield ratio (YR) of less than 0.75 or an elongation (EL) of
less than 10% was represented by "B" and determined as failed.
TABLE-US-00003 TABLE 3 Area ratio in structure (%) Mechanical
properties Steel Steel Production Mn-concentrated YS TS YR EL No.
type No. Martensite Residual .gamma. region (MPa) (MPa) (-) (%)
Evaluation 1 A 1 95 1.1 0.0 1176 1512 0.78 8.2 B 2 A 2 95 1.1 0.6
1174 1513 0.78 9.2 B 3 A 3 95 0.7 1.3 1177 1506 0.78 10.5 A 4 A 4
84 1.0 1.5 845 1355 0.62 11.4 B 5 A 5 94 0.9 0.0 1168 1510 0.77 7.9
B 6 B 6 98 1.3 0.0 1194 1535 0.78 7.8 B 7 B 7 97 1.1 0.7 1187 1533
0.77 8.8 B 8 B 8 98 0.8 1.4 1224 1545 0.79 10.1 A 9 B 9 85 1.1 1.7
897 1398 0.64 11.2 B 10 B 10 96 1.0 0.0 1187 1520 0.78 7.5 B 11 C
11 99 0.8 1.5 1235 1556 0.79 10.2 A 12 D 12 99 0.6 1.1 1156 1487
0.78 10.0 A 13 E 13 31 0.0 0.2 412 845 0.49 19.1 B 14 F 14 78 0.0
1.8 752 1233 0.61 10.6 B 15 G 15 98 0.7 1.0 1203 1534 0.78 10.5 A
16 H 16 99 0.5 1.1 1208 1522 0.79 10.3 A 17 I 17 98 0.9 1.3 1194
1535 0.78 10.2 A 18 J 18 99 1.0 1.0 1223 1555 0.79 10.1 A 19 K 19
98 1.1 1.4 1234 1565 0.79 10.1 A 20 L 20 99 1.0 1.2 1242 1574 0.79
10.3 A (Underlined: outside the range of the present invention,
Hatched: outside the recommended conditions of the present
invention)
[0087] As shown in Table 3, all the invention steels (steel Nos. 3,
8, 11, 12 and 15 to 20) fulfilling the requirements of the present
invention (the above-mentioned component requirements and the
above-mentioned structure requirements) satisfy a tensile strength
TS of 1470 MPa or more, a yield ratio YR of 0.75 or more and an
elongation EL of 10% or more, and the ultra-high-strength steel
sheets excellent in the yield ratio and the workability have been
obtained.
[0088] By contrast, the comparative steels (steel Nos. 1, 2, 4 to
7, 9, 10, 13 and 14) not satisfying at least one of the
requirements of the present invention (the above-mentioned
component requirements and the above-mentioned structure
requirements) are degraded in at least any one property of the
tensile strength TS, the yield ratio YR and the elongation EL.
[0089] For example, in steel Nos. 1 and 6, the annealing
temperature after hot rolling is too low and is outside the
recommended range as shown in production Nos. 1 and 6 of Table 2,
respectively. Thus, Mn is not sufficiently concentrated in residual
austenite to degrade the elongation EL, as shown in Table 3.
[0090] On the other hand, in steel Nos. 5 and 10, the annealing
temperature after hot rolling is too high and is outside the
recommended range as shown in production Nos. 5 and 10 of Table 2,
respectively. Thus, Mn is homogenized by diffusion, and Mn is not
sufficiently concentrated in residual austenite to degrade the
elongation EL, as shown in Table 3.
[0091] Further, in steel Nos. 2 and 7, the annealing holding time
after hot rolling is too short and is outside the recommended range
as shown in production Nos. 2 and 7 of Table 2, respectively. Thus,
Mn is not sufficiently concentrated in residual austenite to
degrade the elongation EL, as shown in Table 3.
[0092] In addition, in steel Nos. 4 and 9, the y-transformation
heat treatment temperature is too low and is outside the
recommended range as shown in production Nos. 4 and 9 of Table 2,
respectively. Thus, austenitization is not sufficiently achieved,
and martensite is insufficient, resulting in poor tensile strength
TS and yield ratio YR, as shown in Table 3.
[0093] Furthermore, in steel No. 13, the C content is too low as
shown in steel type E of Table 1. Thus, both martensite and
residual austenite are insufficient, and Mn is not sufficiently
concentrated in residual austenite, resulting in poor tensile
strength TS and yield ratio YR, as shown in Table 3.
[0094] Further, in steel No. 14, the Mn content is too low as shown
in steel type F of Table 1. Thus, both martensite and residual
austenite are insufficient, resulting in poor tensile strength TS
and yield ratio YR, as shown in Table 3.
[0095] As described above, it has been confirmed that the
ultra-high-strength steel sheets excellent in the yield ratio and
the workability are obtained by satisfying the requirements of the
present invention.
[0096] While the present invention has been described in detail and
with reference to specific embodiments thereof, it will be apparent
to one skilled in the art that various changes and modifications
can be made therein without departing from the spirit and scope
thereof.
[0097] This application is based on Japanese Patent Application No.
2015-026736 filed on Feb. 13, 2015, the entire subject matter of
which is incorporated herein by reference.
INDUSTRIAL APPLICABILITY
[0098] The ultra-high-strength steel sheet of the present invention
is excellent in yield ratio and workability, and is useful for
vehicle bodies as cold-rolled steel sheets and various plated steel
sheets.
* * * * *