U.S. patent application number 16/080571 was filed with the patent office on 2019-03-21 for high strength steel sheet and manufacturing method therefor.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). The applicant listed for this patent is Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). Invention is credited to Yuichi FUTAMURA, Muneaki IKEDA, Koji KASUYA, Toshio MURAKAMI, Tadao MURATA, Kenji SAITO.
Application Number | 20190085426 16/080571 |
Document ID | / |
Family ID | 59808247 |
Filed Date | 2019-03-21 |
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United States Patent
Application |
20190085426 |
Kind Code |
A1 |
IKEDA; Muneaki ; et
al. |
March 21, 2019 |
HIGH STRENGTH STEEL SHEET AND MANUFACTURING METHOD THEREFOR
Abstract
One aspect of the present invention is a high strength steel
sheet having a specific component composition, wherein a metal
structure of the steel sheet comprises polygonal ferrite, bainite,
tempered martensite, and retained austenite; when the metal
structure is observed with a scanning electron microscope, the
metal structure satisfies polygonal ferrite: 10 to 50 area %,
bainite: 10 to 50 area %, and tempered martensite: 10 to 80 area %
with respect to the metal structure overall; and when the metal
structure is measured by X-ray diffractometry, the metal structure
satisfies retained austenite: 5.0 volume % or more, retained
austenite with a carbon concentration of 1.0 mass % or less: 3.5
volume % or more, and retained austenite with a carbon
concentration of 0.8 mass % or less: 2.4 volume % or less with
respect to the metal structure overall.
Inventors: |
IKEDA; Muneaki;
(Kakogawa-shi, JP) ; KASUYA; Koji; (Shinagawa-ku,
JP) ; MURATA; Tadao; (Kakogawa-shi, JP) ;
SAITO; Kenji; (Kakogawa-shi, JP) ; MURAKAMI;
Toshio; (Kobe-shi, JP) ; FUTAMURA; Yuichi;
(Nagoya-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) |
Kobe-shi |
|
JP |
|
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel, Ltd.)
Kobe-shi
JP
|
Family ID: |
59808247 |
Appl. No.: |
16/080571 |
Filed: |
February 8, 2017 |
PCT Filed: |
February 8, 2017 |
PCT NO: |
PCT/JP2017/004594 |
371 Date: |
August 28, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 8/0447 20130101;
C22C 38/28 20130101; C22C 38/001 20130101; C22C 38/02 20130101;
C22C 38/06 20130101; C22C 38/14 20130101; C21D 6/005 20130101; C22C
38/002 20130101; C22C 38/38 20130101; C21D 2211/001 20130101; C22C
38/34 20130101; C22C 38/08 20130101; C23C 2/02 20130101; C23C 2/06
20130101; C21D 1/785 20130101; C21D 1/185 20130101; C22C 38/16
20130101; C21D 2211/008 20130101; C23C 2/40 20130101; C21D 1/19
20130101; C21D 8/0247 20130101; C22C 38/005 20130101; C21D 2211/005
20130101; C22C 38/12 20130101; C22C 38/60 20130101; C21D 2211/002
20130101; C22C 38/32 20130101; C21D 9/46 20130101; C21D 6/008
20130101; C22C 38/04 20130101 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C22C 38/00 20060101 C22C038/00; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C22C 38/06 20060101
C22C038/06; C23C 2/02 20060101 C23C002/02; C23C 2/06 20060101
C23C002/06; C23C 2/40 20060101 C23C002/40 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 29, 2016 |
JP |
2016-038304 |
Sep 20, 2016 |
JP |
2016-182966 |
Claims
1. A high strength steel sheet satisfying comprising, in mass %: C:
0.10% to 0.5%, Si: 1.0% to 3%, Mn: 1.5% to 3%, P: more than 0% and
0.1% or less, S: more than 0% and 0.05% or less, Al: 0.005% to 1%,
N: more than 0% and 0.01% or less, and iron wherein (1) a metal
structure of the steel sheet comprises polygonal ferrite, bainite,
tempered martensite, and retained austenite; (2) when the metal
structure is observed with a scanning electron microscope, the
metal structure satisfies: polygonal ferrite: 10 to 50 area %,
bainite: 10 to 50 area %, and tempered martensite: 10 to 80 area %,
with respect to the metal structure overall; and (3) when the metal
structure is measured by X-ray diffractometry, the metal structure
satisfies: retained austenite: 5.0 volume % or more, retained
austenite with a carbon concentration of 1.0 mass % or less: 3.5
volume % or more, and retained austenite with a carbon
concentration of 0.8 mass % or less: 2.4 volume % or less, with
respect to the metal structure overall.
2. The high strength steel sheet according to claim 1, further
comprising one or more selected from the group consisting of: Cr:
more than 0% and 1% or less, Mo: more than 0% and 1% or less, Ti:
more than 0% and 0.15% or less, Nb: more than 0% and 0.15% or less,
V: more than 0% and 0.15% or less, Cu: more than 0% and 1% or less,
Ni: more than 0% and 1% or less, B: more than 0% and 0.005% or
less, Ca: more than 0% and 0.01% or less, Mg: more than 0% and
0.01% or less, and a rare earth element: more than 0% and 0.01% or
less, in mass %.
3. The high strength steel sheet according to claim 1, having an
electrogalvanized layer, a hot-dip galvanized layer, or a hot-dip
galvannealed layer on a surface of the steel sheet.
4. A method for manufacturing the high strength steel sheet
according to claim 1, the method comprising: a steel sheet
satisfying the component composition of claim 1 to a T1 temperature
region, which is 800.degree. C. or higher and an Ac.sub.3 point or
lower, and holding the steel sheet in the T1 temperature region for
40 seconds or more for soaking; a first cooling after the soaking,
wherein in cooling the steel sheet down to an arbitrary cooling
stop temperature T2 satisfying 350.degree. C. or lower and
100.degree. C. or higher when an Ms point represented by the
following formula (I) is 350.degree. C. or higher, or else, in
cooling the steel sheet down to an arbitrary cooling stop
temperature T2 satisfying the Ms point or lower and 100.degree. C.
or higher when the Ms point represented by the following formula
(1) is lower than 350.degree. C., the steel sheet is cooled at an
average cooling rate of 5.degree. C./sec or more from 700.degree.
C. down to a temperature which is the higher one of 300.degree. C.
and the cooling stop temperature T2; reheating the steel sheet to a
T3 temperature region exceeding 350.degree. C. and being
540.degree. C. or lower and holding the steel sheet for 50 seconds
or more in the T3 temperature region; and a second cooling after
the holding, wherein the steel sheet is cooled at an average
cooling rate of 10.degree. C./sec or more from the T3 temperature
region down to 300.degree. C., and further the steel sheet is
cooled at an average cooling rate exceeding 0.degree. C./sec and
being less than 10.degree. C./sec from 300.degree. C. down to
150.degree. C., the Ms point satisfying: Ms point (.degree.
C.)=561-474.times.[C]/(1-Vf/100)-33.times.[Mn]-17.times.[Ni]-17.times.[Cr-
]-21.times.[Mo] (I) where, in the formula (I), Vf represents a
polygonal ferrite fraction (area %) in a sample that is separately
obtained by performing the soaking under the same conditions as in
the manufacturing conditions for the high strength steel sheet and
thereafter cooling down to room temperature at the same average
cooling rate as in the manufacturing conditions for the high
strength steel sheet in the first cooling, and in the formula (I),
brackets [ ] represents a content (mass %) of each element, where
calculation is made assuming that the content of elements not
contained in the steel sheet is 0 mass %.
5. The method for manufacturing a high strength steel sheet
according to claim 4, further comprising electrogalvanization is
carried out after the second cooling.
6. The method for manufacturing a high strength steel sheet
according to claim 4, further comprising hot-dip galvanization or
hot-dip galvannealing is carried out in the reheating.
Description
TECHNICAL FIELD
[0001] The present invention relates to a high strength steel sheet
and a manufacturing method therefor. In further detail, the present
invention relates to a high strength steel sheet being excellent in
formability at room temperature and having a tensile strength of
980 MPa or more in which a molding load for forming at a hot
temperature of 100 to 350.degree. C. is outstandingly reduced as
compared with the molding load for forming at room temperature, as
well as to a manufacturing method therefor.
BACKGROUND ART
[0002] In a steel sheet used for structural components in an
automobile or the like, a high strength of 980 MPa or more is
demanded for realizing collision safety for passengers and
improvement in fuel cost performance. Meanwhile, the steel sheet is
typically molded into a component shape at room temperature, and
press-forming is carried out for this molding. Accordingly, it is
demanded that the steel sheet has a good press-formability (which
may hereafter be simply referred to as formability).
[0003] As a steel sheet having both of strength and formability, a
TRIP (Transformation Induced Plasticity; transformation induced
plasticity) steel sheet is known in the art (See, for example,
Patent Literature 1). The TRIP steel sheet is a steel sheet
containing metastable austenite (which may hereafter be referred to
as retained austenite and may be denoted as retained .gamma.) and,
when the steel sheet is deformed by receiving stress, the steel
sheet produces an effect of prompting hardening of the deformed
part to prevent concentration of stain by being transformed into
martensite, whereby uniform deformability is improved to exhibit a
good elongation. Though the elongation of the TRIP steel sheet is
good in this manner, the strength of the steel sheet itself is
high, so that the molding load during the press-forming is large,
thereby imposing an excessively large burden on a pressing machine.
Accordingly, depending on the component shape, there may be cases
in which the TRIP steel sheet cannot be applied. Thus, it is
desired to reduce the burden on the pressing machine and reduce the
molding load during the press-forming. In other words, it is
recommended that the steel sheet has a low strength during the
press-forming and has a high strength during use after the
press-forming.
[0004] As a method for reducing the molding load during the
press-forming, there can be considered, for example, a method of
press-molding the steel sheet after heating the steel sheet to a
temperature of about 100.degree. C. to an A.sub.1 point. Generally,
by heating the steel sheet, the deformation resistance decreases,
so that the molding load during the press-forming can be
reduced.
[0005] As a technique for reducing the molding load during the
press-forming by molding the steel sheet after heating, techniques
disclosed in Patent Literatures 2 and 3 are known in the art.
[0006] Patent Literature 2 discloses a high tension thin steel
sheet having a tensile strength of 450 MPa or more and being
excellent in hot temperature moldability and shape fixing property
that is suitable for the forming method that performs press-molding
by heating the steel sheet to a temperature region of from
350.degree. C. to the A.sub.1 point. This high tension thin steel
sheet satisfies a predetermined component composition; the ratio of
the tensile strength at 450.degree. C. relative to the tensile
strength at room temperature is 0.7 or less; and the crystal
structure of the steel is such that the volume ratio of a
martensite phase is 10% or more and 80% or less; an average
diameter of each of the dispersed martensite phases is 8 .mu.m or
less; and the volume ratio of the ferrite phase is the largest in
the structure other than martensite. In the high tension thin steel
sheet disclosed in Patent Literature 2, degree of decrease in the
tensile strength at 150.degree. C. is small and, in order to obtain
a sufficient load reducing effect during the molding, there is
eventually a need to heat the steel sheet to the temperature region
of from 350.degree. C. to the A.sub.1 point for press-molding.
However, when the steel sheet is heated to such a high temperature,
the surface state of the steel sheet is deteriorated by oxidation,
and also the energy for heating the steel sheet increases.
[0007] Patent Literature 3 discloses a high strength steel sheet
providing sufficient decrease in strength during the hot
temperature molding at 150 to 250.degree. C. and being capable of
ensuring a high strength of 980 MPa or more during use at room
temperature after the molding. This high strength steel sheet
contains retained austenite at 5 to 20% in terms of area ratio
relative to the overall structure, where the C concentration of the
retained austenite (C.gamma..sub.R) is controlled to be 0.5 to 1.0
mass %.
[0008] Meanwhile, depending on the shape of the structural
components and the like, there are cases in which not only the hot
temperature pressing but also the press-molding at room temperature
is further carried out in addition to the press-molding at a hot
temperature. Accordingly, it is demanded that the steel sheet used
for the aforementioned components and the like is excellent in
formability at room temperature as well.
[0009] The heating temperature of the high strength steel sheet
disclosed in Patent Literature 3 is 150 to 250.degree. C., so that
the problems such as generated in the high tension thin steel sheet
disclosed in Patent Literature 2 do not arise; however, in Patent
Literature 3, no consideration is made on the formability at room
temperature.
CITATION LIST
Patent Literature
[0010] Patent Literature 1: Japanese Patent No. 5728115
[0011] Patent Literature 2: Japanese Unexamined Patent Publication
No. 2003-113442
[0012] Patent Literature 3: Japanese Unexamined Patent Publication
No. 2013-181184
SUMMARY OF INVENTION
[0013] The present invention has been made by paying attention to
the circumstances such as mentioned above, and an object thereof is
to provide a high strength steel sheet having a tensile strength of
980 MPa or more and being excellent in formability at room
temperature, particularly in elongation and hole expansion
formability, in which the molding load for forming at a hot
temperature of 100 to 350.degree. C. is reduced, as well as a
manufacturing method therefor.
[0014] One aspect of the present invention is a high strength steel
sheet satisfying, in mass %, C: 0.10% to 0.5%, Si: 1.0% to 3%, Mn:
1.5% to 3%, P: more than 0% and 0.1% or less, S: more than 0% and
0.05% or less, Al: 0.005% to 1%, and N: more than 0% and 0.01% or
less, with a balance being iron and inevitable impurities, in which
(1) a metal structure of the steel sheet contains polygonal
ferrite, bainite, tempered martensite, and retained austenite; (2)
when the metal structure is observed with a scanning electron
microscope, the metal structure satisfies polygonal ferrite: 10 to
50 area %, bainite: 10 to 50 area %, and tempered martensite: 10 to
80 area %, with respect to the metal structure overall; and (3)
when the metal structure is measured by X-ray diffractometry, the
metal structure satisfies retained austenite: 5.0 volume % or more,
retained austenite with a carbon concentration of 1.0 mass % or
less: 3.5 volume % or more, and retained austenite with a carbon
concentration of 0.8 mass % or less: 2.4 volume % or less, with
respect to the metal structure overall.
[0015] The foregoing and other objects, features, and advantages of
the present invention will be apparent from the following detailed
description and the attached drawings.
BRIEF DESCRIPTION OF DRAWINGS
[0016] FIG. 1 is a model view showing a diffraction peak of
retained .gamma. as measured by X-ray diffractometry.
[0017] FIG. 2 is a model view for describing a mode in which at
least one selected from the group consisting of retained austenite
and carbide is arrayed in series.
[0018] FIG. 3A is a model view for describing a distribution state
of bainite and tempered martensite.
[0019] FIG. 3B is a model view for describing a distribution state
of bainite and tempered martensite.
[0020] FIG. 4 is a model view showing an annealing pattern in
manufacturing a high strength steel sheet according to the present
invention.
DESCRIPTION OF EMBODIMENTS
[0021] Hereafter, embodiments according to the present invention
will be described; however, the present invention is not limited to
these alone.
[0022] First, a metal structure of a high strength steel sheet
according to an embodiment of the present invention will be
described.
[0023] The metal structure of the high strength steel sheet is a
mixed structure containing polygonal ferrite, bainite, tempered
martensite, and retained austenite.
[0024] Further, in the metal structure of the high strength steel
sheet, it is important that
[0025] (A) when the metal structure is observed with a scanning
electron microscope, the metal structure satisfies:
[0026] polygonal ferrite: 10 to 50 area %,
[0027] bainite: 10 to 50 area %, and
[0028] tempered martensite: 10 to 80 area %, with respect to the
metal structure overall; and
[0029] (B) when the metal structure is measured by X-ray
diffractometry, the metal structure satisfies:
[0030] retained austenite: 5.0 volume % or more,
[0031] retained austenite with a carbon concentration of 1.0 mass %
or less: 3.5 volume % or more, and
[0032] retained austenite with a carbon concentration of 0.8 mass %
or less: 2.4 volume % or less, with respect to the metal structure
overall.
[0033] First, the requirements of the above (A) will be described
after the requirements of the above (B) characterizing the present
invention are described.
[0034] [Retained .gamma.]
[0035] The retained .gamma. is a structure needed for improving the
uniform deformability by the TRIP effect to ensure a good
elongation. Also, the retained .gamma. is a structure needed for
ensuring the strength.
[0036] In the high strength steel sheet, in order that such an
effect may be exhibited, the volume ratio of retained .gamma.
(which may hereafter be denoted as V.gamma..sub.R) is set to be
5.0% or more, preferably 8% or more, and more preferably 10% or
more, with respect to the metal structure overall. However, when
the amount of generation of retained .gamma. is excessive, the hole
expansion ratio .lamda., decreases, and formability at room
temperature cannot be improved. Accordingly, in the high strength
steel sheet, the volume ratio of retained .gamma. (V.gamma..sub.R)
is set to be preferably 30% or less, more preferably 25% or less,
with respect to the metal structure overall.
[0037] The retained .gamma. may be generated between laths, or may
exist in an agglomerate form as a part of an MA mixture phase in
which fresh martensite and retained .gamma. are combined, in a
collective body of a lath-shaped structure, for example, a block, a
packet, or the like, or on a prior .gamma. grain boundary. MA is an
abbreviation for Martensite-Austenite Constituent.
[0038] The volume ratio of retained .gamma. (V.gamma..sub.R) is a
value measured by X-ray diffractometry.
[0039] In the high strength steel sheet, it is particularly
important that, when the metal structure is measured by X-ray
diffractometry, the volume ratio of retained austenite with a
carbon concentration of 1.0 mass % or less [which may hereafter be
denoted as V.gamma..sub.R(C.ltoreq.1.0%)] is 3.5 volume % or more,
and the volume ratio of retained austenite with a carbon
concentration of 0.8 mass % or less [which may hereafter be denoted
as V.gamma..sub.R(C.ltoreq.0.8%)] is 2.4 volume % or less, with
respect to the metal structure overall. In other words, it is
important that retained .gamma. having a carbon concentration
exceeding 0.8 mass % and being 1.0 mass % or less is generated
appropriately, as will be described below.
[0040] First, description will be given on a fact that the load at
a hot temperature can be sufficiently reduced by setting the volume
ratio of retained .gamma. with a carbon concentration of 1.0 mass %
or less to be a predetermined value or more.
[0041] As an index of load reduction during the hot temperature
molding, it is possible to use .DELTA.TS, that is, the value
obtained by subtracting the tensile strength at a hot temperature
(which may hereafter be denoted as hot temperature TS) from the
tensile strength at room temperature (which may hereafter be
denoted as room temperature TS) (room temperature TS--hot
temperature TS), as described in the above Patent Literature 3. It
can be stated that the larger the .DELTA.TS is, the more
sufficiently the load at a hot temperature is reduced.
[0042] In order to increase the value of .DELTA.TS, there can be
considered a method of using:
[0043] (1) decrease in the deformation resistance by increase in
the forming temperature; and
[0044] (2) the fact that retained .gamma. improves the tensile
strength TS because of being unstable at room temperature, but does
not improve the tensile strength TS because of being stable at a
hot temperature.
[0045] Further, the present inventors have found out that, since
the above (1) is not dependent on the material, it is effective to
allow the retained .gamma. shown in the above (2) to exist in order
to obtain a steel sheet having a larger .DELTA.TS, and that, for
that purpose, it is effective to positively generate retained
.gamma. having a low carbon concentration, specifically, the volume
ratio V.gamma..sub.R(C.ltoreq.1.0%) of retained .gamma. with a
carbon concentration of 1.0 mass % or less, at 3.5 mass % or more
with respect to the metal structure overall. The
V.gamma..sub.R(C.ltoreq.1.0%) is preferably 4.0 volume % or more,
more preferably 4.5 volume % or more. An upper limit of the
V.gamma..sub.R(C.ltoreq.1.0%) is not particularly limited, and the
maximum value of V.gamma..sub.R(C.ltoreq.1.0%) is equal to the
volume ratio of retained .gamma. contained in the steel sheet. The
V.gamma..sub.R(C 1.0%) is preferably 10 volume % or less, more
preferably 8 volume % or less.
[0046] As described above, it has been found out that the molding
load at a hot temperature can be reduced by positively generating
retained .gamma. with a carbon concentration of 1.0 mass % or less.
However, when retained .gamma. with a carbon concentration being
too low is generated in a large amount, the retained .gamma. is
transformed into hard fresh martensite (FM) at an initial stage
when hole expansion forming is carried out at room temperature, to
generate a site of strain concentration, thereby considerably
decreasing the hole expansion ratio .lamda., and deteriorating the
formability at room temperature. Accordingly, generation of
retained .gamma. with a carbon concentration being too low must be
suppressed in order to increase the hole expansion ratio .lamda.,
and to improve the formability at room temperature.
[0047] Based on such a viewpoint, studies were made on a
relationship between the carbon concentration of retained .gamma.
and the formability at room temperature, whereupon it has been
found out that it is effective to set the volume ratio
V.gamma..sub.R(C.ltoreq.0.8%) of retained .gamma. with a carbon
concentration of 0.8 mass % or less to be 2.4 volume % or less with
respect to the metal structure overall. The
V.gamma..sub.R(C.ltoreq.0.8%) is preferably 2.3 volume % or less,
more preferably 2.2 volume % or less, and still more preferably 2.1
volume % or less. The smaller the V.gamma..sub.R(C.ltoreq.0.8%) is,
the more preferable it is. The V.gamma..sub.R(C.ltoreq.0.8%) is
most preferably 0 volume %.
[0048] As described above, by setting the
V.gamma..sub.R(C.ltoreq.1.0%) to be 3.5 volume % or more, the
.DELTA.TS can be increased, and the molding load at a hot
temperature can be reduced as compared with the molding load at
room temperature; and, by suppressing the Vy.sub.R(C.ltoreq.0.8%)
to be 2.4 volume % or less, the hole expansion ratio .lamda., when
hole expansion forming is carried out at room temperature can be
increased, and the formability at room temperature can be
improved.
[0049] Here, the present inventors have confirmed that, since
retained .gamma. with a carbon concentration exceeding 1.0 mass %
is stable both at room temperature and at a hot temperature, little
influence is given to .DELTA.TS and the hole expansion ratio
.lamda., at room temperature.
[0050] Here, a relationship between the prior art technique and the
present invention will be described.
[0051] An idea of controlling the stability of retained .gamma. by
means of the carbon concentration of retained .gamma. is
conventionally well known. Further, for example, as disclosed in
the above Patent Literature 3, a technique is already known in
which the strength during the molding at 150 to 250.degree. C. is
lowered by controlling the average value of the carbon
concentration of retained .gamma. to be within a predetermined
range.
[0052] In contrast, the present invention is largely different from
the prior art technique in that attention is paid not to the
average carbon concentration of retained .gamma. but to the carbon
concentration of individual retained .gamma., and that retained
.gamma. with a carbon concentration exceeding 0.8 mass % and being
1.0 mass % or less is positively generated. In other words, the
present inventors have found out a knowledge that, even when the
amount of generation of retained .gamma. is the same and the
average value of the carbon concentration of retained .gamma. is
the same, the obtained characteristics greatly change when the
amount of generation of retained .gamma. with a carbon
concentration of 0.8 mass % or less is different from the amount of
generation of retained .gamma. with a carbon concentration of 1.0
mass % or less.
[0053] A further comment on the above Patent Literature 3 is that,
since the average value of carbon concentration in retained .gamma.
contained in the steel sheet disclosed in Patent Literature 3 is
extremely low, it is conjectured that the amount of generation of
retained .gamma. with a carbon concentration of 0.8 mass % or less
is considerably large and that the configuration structure is
different.
[0054] Next, a method for measuring each of the volume ratio
(V.gamma..sub.R) of retained .gamma., the average carbon
concentration (which may hereafter be denoted as % (C.sub.avg) of
retained .gamma., and the carbon concentration distribution of
retained .gamma. with respect to the metal structure overall will
be described.
[0055] The volume ratio of retained .gamma. and the average carbon
concentration of retained .gamma. are measured by X-ray
diffractometry after grinding down to the thickness of 1/4 of the
steel sheet and then chemically polishing the steel sheet. For the
principle of measurement, reference can be made to ISIJ Int. Vol.
33, year of 1933, No. 7, p. 776. Here, in this X-ray
diffractometry, an X-ray diffractometer (RINT-1500) manufactured by
Rigaku Corporation was used as an X-ray diffraction apparatus, and
a Co-K.alpha. beam was used as the X-ray.
[0056] The carbon concentration distribution of retained .gamma.
was determined in the following manner by using three diffraction
peaks of (200).sub..gamma., (220).sub..gamma., and
(311).sub..gamma. measured with use of the above X-ray diffraction
apparatus.
[0057] First, as shown in the model view of FIG. 1, for each of the
three diffraction peaks of (200).sub..gamma., (220).sub..gamma.,
and (311).sub..gamma., 2.theta. (2.theta..sub.avg(hkl)) at which
the diffraction intensity attains the maximum and the half value
width .DELTA.2.theta.(hkl) thereof were determined. Here, (hkl)
represents (200), (220), or (311) (The same applies hereafter).
Here, FIG. 1 is a model view showing a diffraction peak of retained
.gamma. as measured by X-ray diffractometry.
[0058] Subsequently, from the above 2.theta..sub.avg(hkl), d(hkl)
was determined from the following formula (1) using the Bragg
conditions: .lamda..sub.B=2d sin .theta. (d: lattice surface
interval, .lamda..sub.B: wavelength of Co-K 0 beam).
d(hkl)=.lamda..sub.B/{2 sin(2.theta..sub.avg(hkl)/(2)} (1)
[0059] Further, by the following formula (2), the lattice constants
a.sub.0(hkl) were determined, and the lattice constant a.sub.0 was
determined by calculating an arithmetic mean of those three lattice
constants a.sub.0(hkl).
a.sub.0(hkl)=d(hkl) (h.sup.2+k.sup.2+l.sup.2) (2)
[0060] Further, the carbon concentration % C.sub.avg (unit: mass %)
was determined by using the following formula (3).
% C.sub.avg=(1/0.033).times.(a.sub.0-3.572) (3)
[0061] Next, the half value width .DELTA.% C of the carbon
concentration distribution of retained .gamma. was determined by
the following procedure.
[0062] First, the diffraction angles at the upper limit and lower
limit of the half value width .DELTA.2.theta.(hkl) of the
diffraction angle 2.theta.(hkl) of each peak were determined by the
following formulas (4) and (5) (See FIG. 1).
2.theta..sub.L(hkl)=2.theta..sub.avg(hkl)-.DELTA.2.theta.(hkl)/2
(4)
2.theta..sub.H(hkl)=2.theta..sub.avg(hkl)+.DELTA.2.theta.(hkl)/2
(5)
[0063] Then, the upper and lower limit values % C.sub.L and %
C.sub.H of the half value width of the carbon concentration
distribution were determined by using the Bragg conditions and the
above formulas (1) to (3) according to the same procedure as
described above by using the above 2.theta..sub.L(hkl) and
2.theta..sub.H(hkl), respectively. Further, the half value width
.DELTA.% C of the carbon concentration distribution was determined
by the following formula (6).
.DELTA.% C=% C.sub.H-% C.sub.L (6)
[0064] Further, assuming that the carbon concentration distribution
is the normal distribution, the standard deviation .sigma.% C was
calculated in the following manner from the above half value width
.DELTA.% C. That is to say, the probability density function f(x)
of the normal distribution is represented by the following formula
(7) from the average value u and the standard deviation
.sigma..
f(x)=}1/
(2.pi..sigma..sup.2)}.times.exp}-(x-u).sup.2/(2.sigma..sup.2)}
(7)
[0065] The probability f(u) at the average value is determined
using the following formula (8) by substituting x=u in the above
formula (7).
f(u)=1/ (2.pi..sigma..sup.2) (8)
[0066] Further, since the probability density f(%
C.sub.avg.+-..DELTA.% C/2) at the values (% C.sub.avg.+-..DELTA.%
C/2) obtained by moving upward and downward from the average value
u=% C.sub.avg by 1/2 of the half value width .DELTA.% C is 1/2 of
the probability density f(u)=f(% C.sub.avg) at the average value
u=% C.sub.avg, the relationship of (9) is obtained from the above
formulas (7) and (8).
}1/ (2.pi..sigma.% C.sup.2)}.times.exp{-(.DELTA.%
C/2).sup.2/(2.sigma.% C.sup.2)}=1/{2 (2.pi..sigma.% C.sup.2)}
(9)
[0067] By modification of the above formula (9), the following
formula (10) is deduced as a formula for determining the standard
deviation .sigma.% C from the half value width .DELTA.% C, so that
the standard deviation .sigma.% C was calculated by substituting
the half value width .DELTA.% C in this formula (10).
.sigma.% C= {.DELTA.% C/2).sup.2/(2ln2) (10)
[0068] Further, the following formula (12) was deduced as a formula
for determining the volume ratio V.gamma..sub.R(C.ltoreq.1.0%) of
retained .gamma. with a carbon concentration of 1.0 mass % or less
with respect to the metal structure overall by the cumulative
distribution function g(x) shown in the following formula (11)
using the average value % C.sub.avg and the standard deviation
.sigma.% C of the carbon concentration distribution in the retained
.gamma. determined in the above-described manner, and
V.gamma..sub.R(C.ltoreq.1.0%) was calculated using this formula
(12).
g(x)=(1/2).times.[1+erf}(x-u)/ (2.sigma..sup.2)}] (11)
V.gamma..sub.R(C.ltoreq.1.0%)=V.gamma..sub.R.times.g(1.0)=V.gamma..sub.R-
.times.(1/2).times.[1+erf{(1.0-% C.sub.avg)/ (2.sigma.% C.sup.2)}]
(12)
[0069] Also, the following formula (13) was deduced as a formula
for determining the volume ratio V.gamma..sub.R(C.ltoreq.0.8%) of
retained .gamma. with a carbon concentration of 0.8 mass % or less
with respect to the metal structure overall by the cumulative
distribution function g(x) shown in the following formula (11)
using the average value % C.sub.avg and the standard deviation
.sigma.% C of the carbon concentration distribution in the retained
.gamma. determined in the above-described manner, and
V.gamma..sub.R(C.ltoreq.0.8%) was calculated using this formula
(13).
V.gamma..sub.R(C.ltoreq.0.8%)=V.gamma..sub.R.times.g(0.8)=V.gamma..sub.R-
.times.(1/2).times.[1+erf{0.8-% C.sub.avg)/ (2.sigma.% C.sup.2)}]
(13)
[0070] In the above formulas (12) and (13), V.gamma..sub.R is a sum
volume ratio of retained .gamma. with respect to the metal
structure overall.
[0071] Next, the requirements of the above (A) will be
described.
[0072] [Polygonal Ferrite]
[0073] Polygonal ferrite is soft as compared with bainite and is a
structure that functions to improve the formability at room
temperature by enhancing the elongation of the steel sheet. In
order that such a function may be exhibited, the area ratio of
polygonal ferrite is set to be 10% or more, preferably 20% or more,
and more preferably 25% or more, with respect to the metal
structure overall. However, when the amount of generation of
polygonal ferrite is excessive, the strength decreases, so that the
area ratio of polygonal ferrite is set to be 50% or less,
preferably 45% or less, and more preferably 40% or less, with
respect to the metal structure overall.
[0074] The area ratio of polygonal ferrite can be measured with use
of a scanning electron microscope.
[0075] [Bainite]
[0076] Bainite generated by bainite transformation is a structure
that concentrates C into austenite and functions effectively in
obtaining retained .gamma.. Also, since bainite has a strength that
lies between the strength of polygonal ferrite and the strength of
tempered martensite, bainite is a structure that enhances both of
strength and elongation with a good balance. In order that such a
function may be exhibited, the area ratio of bainite is set to be
10% or more, preferably 15% or more, and more preferably 20% or
more, with respect to the metal structure overall. However, when
the amount of generation of bainite is excessive, the strength
decreases, so that the area ratio of bainite is set to be 50% or
less, preferably 40% or less, and more preferably 30% or less, with
respect to the metal structure overall.
[0077] The above bainite is a structure in which an average of an
interval of at least one kind selected from the group consisting of
retained .gamma. and carbide with each other is 1 .mu.m or more
when the cross-section of the steel sheet is subjected to nital
corrosion and then observed with a scanning electron microscope,
and the scope of bainite means to include one in which carbide is
partially deposited besides the bainitic ferrite in which no
carbide is deposited.
[0078] [Tempered Martensite]
[0079] Tempered martensite is a structure that functions to enhance
both of strength and hole expansion ratio .lamda., with a good
balance. In order that such a function may be exhibited, the area
ratio of tempered martensite is set to be 10% or more, preferably
15% or more, and more preferably 20% or more, with respect to the
metal structure overall. However, when the amount of generation of
tempered martensite is excessive, decrease in the amount of
generation of retained .gamma. is conspicuous, and the elongation
decreases, so that the area ratio of tempered martensite is set to
be 80% or less, preferably 70% or less, and more preferably 60% or
less, with respect to the metal structure overall.
[0080] The above tempered martensite is a structure in which an
average of an interval of at least one kind selected from the group
consisting of retained .gamma. and carbide with each other is less
than 1 .mu.m when the cross-section of the steel sheet is subjected
to nital corrosion and then observed with a scanning electron
microscope.
[0081] Here, the "average of an interval of at least one kind
selected from the group consisting of retained .gamma. and carbide
with each other" will be described. The average is a value obtained
by calculating an average of the results obtained by measuring the
distance between the center positions of adjacent retained .gamma.s
with each other, the distance between the center positions of
adjacent carbides with each other, or the distance between the
center positions of a certain retained .gamma. and a carbide
adjacent to the retained .gamma. when the cross-section of the
steel sheet is subjected to nital corrosion and then observed with
a scanning electron microscope. The distance between the center
positions means a distance that is obtained by determining the
center position of each retained .gamma. or each carbide and
measuring the distance between the center positions of the most
adjacent retained .gamma.s with each other, carbides with each
other, or retained .gamma. and carbide. The center position is
obtained by determining the major axis and the minor axis of
retained .gamma. or carbide, and the center position is defined to
be a position at which the major axis and the minor axis intersect
with each other.
[0082] However, when the retained .gamma. or carbide is deposited
on a boundary of a lath, a plurality of retained .gamma. and
carbide are arrayed in series, and the mode assumes a needle-like
shape or a plate-like shape. In this case, the distance between the
center positions is not the distance between at least one kind
selected from the group consisting of retained .gamma. and carbide
with each other but is defined to be a distance between a line 11
and a line 11 formed by at least one kind selected from the group
consisting of retained .gamma. and carbide arrayed in series in the
major axis direction as shown in FIG. 2, that is, the lath-to-lath
distance is defined to be the distance 12 between the center
positions. Here, FIG. 2 is a model view for describing a mode in
which at least one selected from the group consisting of retained
austenite and carbide is arrayed in series.
[0083] The distribution state of bainite and tempered martensite is
not particularly limited, so that both of bainite and tempered
martensite may be generated in a prior austenite grain, or bainite
and tempered martensite may be respectively generated in the prior
austenite grains.
[0084] The distribution state of bainite and tempered martensite is
schematically shown in FIGS. 3A and 3B. FIG. 3A shows a mode in
which both of a bainite 21 and a tempered martensite 22 are mixedly
generated in a prior austenite grain 23, and FIG. 3B shows a mode
in which the bainite 21 and the tempered martensite 22 are
respectively generated in the prior austenite grains 23. A black
solid circle 24 shown in each of the figures represents an MA
mixture phase. Here, FIGS. 3A and 3B are model views for describing
a distribution state of bainite and tempered martensite.
[0085] [Others]
[0086] The metal structure of the high strength steel sheet may be
made of polygonal ferrite, bainite, tempered martensite, and
retained .gamma.; however, the metal structure may include balance
structures such as an MA mixture phase, pearlite, and fresh
martensite as other structures within a range that does not
deteriorate the function of the present invention. Any of these
balance structures constitutes a point of start of cracks and
deteriorates the formability at room temperature, so that the
amount of the balance structures is preferably as small as
possible. The amount of the balance structures is preferably 25
area % or less in total when the cross-section of the steel sheet
is subjected to nital corrosion and then observed with a scanning
electron microscope.
[0087] Here, the area ratios of polygonal ferrite, bainite, and
tempered martensite are measured with use of a scanning electron
microscope, whereas the volume ratio of retained .gamma. is
measured by X-ray diffractometry, so that the measurement methods
are different. For this reason, a sum of the area ratios and the
volume ratios of these structures may exceed 100%.
[0088] Next, a component composition of the high strength steel
sheet according to the present embodiment will be described.
Hereafter, % in the component composition means mass %.
[0089] The high strength steel sheet satisfies C: 0.10% to 0.5%,
Si: 1.0% to 3%, Mn: 1.5% to 3%, P: more than 0% and 0.1% or less,
S: more than 0% and 0.05% or less, Al: 0.005% to 1%, and N: more
than 0% and 0.01% or less.
[0090] C is an element that enhances the strength of the steel
sheet and is also an element that is necessary for ensuring
retained .gamma. by stabilizing austenite. In order that such an
effect may be exhibited, the C amount is set to be 0.10% or more.
The C amount is preferably 0.13% or more, more preferably 0.15% or
more. However, when C is contained in an excessively large amount,
the weldability is degraded, so that the C amount is set to be 0.5%
or less. The C amount is preferably 0.30% or less, more preferably
0.25% or less.
[0091] Si is a solute-strengthening element and is an element that
contributes to achievement of a higher strength of the steel sheet.
Also, Si is an element that is important in condensing and
stabilizing C in austenite by suppressing deposition of carbide and
in ensuring retained .gamma.. In order that such an effect may be
exhibited, the Si amount is set to be 1.0% or more. The Si amount
is preferably 1.2% or more, more preferably 1.3% or more. However,
when Si is contained in an excessively large amount, reverse
transformation of polygonal ferrite into austenite does not occur
during the heating and soaking in annealing, so that polygonal
ferrite remains in an excessively large amount, thereby causing
insufficient strength. Also, a considerable scale is formed during
the hot rolling to generate scale marks on the surface of the steel
sheet, thereby degrading the surface property. Accordingly, the Si
amount is set to be 3% or less. The Si amount is preferably 2.5% or
less, more preferably 2.0% or less.
[0092] Mn functions as a hardenability improving element and is an
element that enhances the strength of the steel sheet by
suppressing generation of polygonal ferrite in an excessively large
amount during the cooling. Also, Mn contributes to stabilization of
retained .gamma.. In order that such an effect may be exhibited,
the Mn amount is set to be 1.5% or more. The Mn amount is
preferably 1.8% or more, more preferably 2.0% or more. However,
when Mn is contained in an excessively large amount, generation of
bainite is considerably suppressed, so that a desired amount of
bainite cannot be ensured, giving a poor balance between strength
and elongation. Also, adverse effects such as generation of ingot
cracks are given. From these reasons, the Mn amount is set to be 3%
or less. The Mn amount is preferably 2.8% or less, more preferably
2.7% or less.
[0093] P is an inevitable impurity and, when contained in an
excessively large amount, promotes grain boundary embrittlement by
grain boundary segregation to degrade the formability at room
temperature, so that the P amount is set to be 0.1% or less. The P
amount is preferably 0.08% or less, more preferably 0.05% or less.
The P amount is preferably as little as possible; however, P is
typically contained at about 0.001%.
[0094] S is an inevitable impurity and, when contained in an
excessively large amount, forms a sulfide-based inclusions such as
MnS to generate a point of start of cracks to degrade the
formability at room temperature, so that the S amount is set to be
0.05% or less. The S amount is preferably 0.01% or less, more
preferably 0.005% or less. The S amount is preferably as little as
possible; however, S is typically contained at about 0.0001%.
[0095] As with Si, Al is an element that is important in ensuring
retained .gamma. by suppressing deposition of carbide. Also, Al is
an element that functions as a deoxidizing material. In order that
such an effect may be exhibited, the Al amount is set to be 0.005%
or more. The Al amount is preferably 0.010% or more, more
preferably 0.03% or more. However, when Al is contained in an
excessively large amount, inclusions are deposited in a large
amount in the steel sheet, and the formability at room temperature
is degraded, so that the Al amount is set to be 1% or less. The Al
amount is preferably 0.8% or less, more preferably 0.5% or
less.
[0096] N is an inevitable impurity and, when N is contained in an
excessively large amount, nitride is deposited in a large amount to
generate a point of start of cracks to degrade the formability at
room temperature, so that the N amount is set to be 0.01% or less.
The N amount is preferably 0.008% or less, more preferably 0.005%
or less. The N amount is preferably as little as possible; however,
N is typically contained at about 0.001%.
[0097] The basic components of the high strength steel sheet are as
described above, and the balance is made of iron and inevitable
impurities. As the inevitable impurities, mingling of elements that
are brought into the steel depending on the circumstances of raw
materials, facility materials, production equipment, and the like
is permitted within a range that does not deteriorate the effects
of the present invention.
[0098] The high strength steel sheet may further contain, as other
elements, at least one kind belonging to the following (a) to (e).
Also, the elements belonging to the following (a) to (e) may be
contained either alone or as a combination of a plurality of
elements belonging to the following (a) to (e):
[0099] (a) at least one selected from the group consisting of Cr:
more than 0% and 1% or less and Mo: more than 0% and 1% or
less,
[0100] (b) at least one selected from the group consisting of Ti:
more than 0% and 0.15% or less, Nb: more than 0% and 0.15% or less,
and V: more than 0% and 0.15% or less,
[0101] (c) at least one selected from the group consisting of Cu:
more than 0% and 1% or less and Ni: more than 0% and 1% or
less,
[0102] (d) B: more than 0% and 0.005% or less, and
[0103] (e) at least one selected from the group consisting of Ca:
more than 0% and 0.01% or less, Mg: more than 0% and 0.01% or less,
and a rare earth element: more than 0% and 0.01% or less.
[0104] (a) Cr and Mo are elements that prevent decrease in strength
by suppressing generation of polygonal ferrite in an excessively
large amount during the cooling. In order that such an effect may
be effectively exhibited, the Cr amount is preferably 0.02% or
more, more preferably 0.1% or more, and still more preferably 0.2%
or more. The Mo amount is preferably 0.02% or more, more preferably
0.1% or more, and still more preferably 0.2% or more. However, when
Cr and Mo are contained in an excessively large amount, generation
of bainite is considerably suppressed in the same manner as in the
case of Mn, so that a desired amount of bainite cannot be ensured,
and a poor balance between strength and elongation may be given.
Accordingly, the Cr amount is preferably set to be 1% or less, more
preferably 0.8% or less, and still more preferably 0.5% or less.
The Mo amount is preferably set to be 1% or less, more preferably
0.8% or less, and still more preferably 0.5% or less. Either one or
both of Cr and Mo may be contained.
[0105] (b) Ti, Nb, and V are each an element that functions to
improve the strength and toughness of the steel sheet by making the
metal structure finer. In order that such an effect may be
effectively exhibited, the Ti amount is preferably 0.01% or more,
more preferably 0.015% or more, and still more preferably 0.020% or
more. The Nb amount is preferably 0.01% or more, more preferably
0.015% or more, and still more preferably 0.020% or more. The V
amount is preferably 0.01% or more, more preferably 0.015% or more,
and still more preferably 0.020% or more. However, when Ti, Nb, and
V are contained in an excessively large amount, the effect is
saturated. Also, a carbide may be deposited at the grain boundary,
and the formability at room temperature may be degraded. Due to
these reasons, the Ti amount is preferably set to be 0.15% or less,
more preferably 0.12% or less, and still more preferably 0.10% or
less. The Nb amount is preferably set to be 0.15% or less, more
preferably 0.12% or less, and still more preferably 0.10% or less.
The V amount is preferably set to be 0.15% or less, more preferably
0.12% or less, and still more preferably 0.10% or less. Any one
kind selected from Ti, Nb, and V may be contained, or else,
arbitrarily selected two or more kinds may be contained.
[0106] (c) Cu and Ni are elements that function to improve the
corrosion resistance of the steel sheet. In order that such an
effect may be effectively exhibited, the Cu amount is preferably
0.01% or more, more preferably 0.05% or more, and still more
preferably 0.10% or more. The Ni amount is preferably 0.01% or
more, more preferably 0.05% or more, and still more preferably
0.10% or more. However, when Cu and Ni are contained in an
excessively large amount, the effect is saturated. Also, the hot
temperature formability may be degraded. Due to these reasons, the
Cu amount is preferably set to be 1% or less, more preferably 0.8%
or less, and still more preferably 0.5% or less. The Ni amount is
preferably set to be 1% or less, more preferably 0.8% or less, and
still more preferably 0.5% or less. Either one or both of Cu and Ni
may be contained.
[0107] (d) B is an element that prevents decrease in strength by
suppressing generation of polygonal ferrite in an excessively large
amount during the cooling in the same manner as in the cases of Cr
and Mn. In order that such an effect may be effectively exhibited,
the B amount is preferably 0.0001% or more, more preferably 0.0005%
or more, and still more preferably 0.0010% or more. However, when B
is contained in an excessively large amount, generation of bainite
is considerably suppressed in the same manner as in the cases of Cr
and Mn, so that a desired amount of bainite cannot be ensured, and
a poor balance between strength and elongation may be given. Due to
these reasons, the B amount is preferably set to be 0.005% or less,
more preferably 0.004% or less, and still more preferably 0.003% or
less.
[0108] (e) Ca, Mg, and a rare earth element (Rare Earth Metal; REM)
are each an element that functions to finely disperse the
inclusions in the steel sheet. In order that such an effect may be
effectively exhibited, the Ca amount is preferably 0.0001% or more,
more preferably 0.0005% or more, and still more preferably 0.0010%
or more. The Mg amount is preferably 0.0001% or more, more
preferably 0.0005% or more, and still more preferably 0.0010% or
more. The rare earth element amount is preferably 0.0001% or more,
more preferably 0.0005% or more, and still more preferably 0.0010%
or more. However, when Ca, Mg, and a rare earth element are
contained in an excessively large amount, the forgeability and the
hot temperature formability may become poor. Due to this reason,
the Ca amount is preferably set to be 0.01% or less, more
preferably 0.005% or less, and still more preferably 0.003% or
less. The Mg amount is preferably set to be 0.01% or less, more
preferably 0.005% or less, and still more preferably 0.003% or
less. The rare earth element amount is preferably set to be 0.01%
or less, more preferably 0.005% or less, and still more preferably
0.003% or less. Any one kind selected from Ca, Mg, and a rare earth
element may be contained, or else, arbitrarily selected two or more
kinds may be contained.
[0109] Here, in the high strength steel sheet, the rare earth
elements mean to include lanthanoid elements (15 elements from La
to Lu), Sc (scandium), and Y (yttrium).
[0110] An electrogalvanized (EG: Electro-Galvanized) layer, a
hot-dip galvanized (GI: Hot Dip Galvanized) layer, or a hot-dip
galvannealed (GA: Hot Dip Galvannealed) layer may be provided on a
surface of the high strength steel sheet. In other words, the scope
of the present invention encompasses a high strength
electrogalvanized steel sheet, a high strength hot-dip galvanized
steel sheet, and a high strength hot-dip galvannealed steel
sheet.
[0111] Next, a method for manufacturing a high strength steel sheet
according to the present embodiment will be described with
reference to FIG. 4. FIG. 4 is a model view showing an annealing
pattern in manufacturing the high strength steel sheet, where the
lateral axis represents time (second), and the longitudinal axis
represents temperature (.degree. C.).
[0112] [Soaking Step]
[0113] First, a steel sheet satisfying the above component
composition is heated to a T1 temperature region, which is
800.degree. C. or higher and an Ac.sub.3 point or lower, and is
held for 40 seconds or more in the T1 temperature region for
soaking (soaking step). The steel sheet may be a hot-rolled steel
sheet or a cold-rolled steel sheet. Here, the soaking temperature
in the T1 temperature region may hereafter be denoted as "T1", and
the soaking time in the T1 temperature region may hereafter be
denoted as "t1". Also, the holding means to include a mode in which
the temperature fluctuates within the T1 temperature region besides
the constant-temperature holding.
[0114] A predetermined amount of polygonal ferrite can be generated
by soaking in a two-phase temperature region of polygonal ferrite
and austenite. When the soaking temperature T1 in the T1
temperature region is too low, the polygonal ferrite is generated
in an excessively large amount, and the strength decreases. Also,
when the soaking temperature T1 is too low, the expansion structure
generated during the cold rolling remains, and the elongation
decreases, so that the formability at room temperature cannot be
improved. Accordingly, the soaking temperature T1 is set to be
800.degree. C. or higher. The soaking temperature T1 is preferably
810.degree. C. or higher, more preferably 820.degree. C. or higher.
However, when the soaking temperature T1 is too high, there will be
an austenite single-phase region, and the amount of generation of
polygonal ferrite becomes insufficient, so that the elongation
decreases, and the formability at room temperature cannot be
improved. Accordingly, the soaking temperature T1 is set to be the
Ac.sub.3 point or lower. The soaking temperature T1 is preferably
(Ac.sub.3 point--10.degree. C.) or lower, more preferably (Ac.sub.3
point--20.degree. C.) or lower.
[0115] When the soaking time t1 in the T1 temperature region is too
short, the steel sheet cannot be uniformly heated, so that carbide
remains without being dissolved as a solid solute, and the
generation of retained .gamma. is suppressed. As a result, the
elongation decreases, and the formability at room temperature
cannot be improved. Accordingly, the soaking time tl is set to be
40 seconds or more. The soaking time t1 is preferably 50 seconds or
more, more preferably 80 seconds or more. An upper limit of the
soaking time t1 is not particularly limited; however, when the
soaking time t1 is too long, the productivity is aggravated.
Accordingly, the soaking time t1 is preferably set to be 500
seconds or less, more preferably 450 seconds or less.
[0116] The temperature of the Ac.sub.3 point of the steel sheet can
be calculated on the basis of the following formula (II) disclosed
in "The Physical Metallurgy of Steels" (William C. Leslie,
published by Maruzen Co., Ltd. on May 31, 1985, p. 273). In the
formula (II), brackets [ ] indicate the content (mass %) of each
element, and calculation may be made by assuming that the content
of an element that is not contained in the steel sheet is 0 mass
%.
Ac.sub.3 point (.degree.
C.)=910-203.times.[C].sup.1/2+44.7.times.[Si]-30.times.[Mn]-11.times.[Cr]-
+31.5.times.[Mo]-20.times.[Cu]-15.2.times.[Ni]+400.times.[Ti]+104.times.[V-
]+700.times.[P]+400.times.[Al] (II)
[0117] [First Cooling Step]
[0118] After the soaking, the steel sheet is cooled down to an
arbitrary cooling stop temperature T2 satisfying 350.degree. C. or
lower and 100.degree. C. or higher when an Ms point represented by
the following formula (I) is 350.degree. C. or higher, or else, the
steel sheet is cooled down to an arbitrary cooling stop temperature
T2 satisfying the Ms point or lower and 100.degree. C. or higher
when the Ms point represented by the following formula (I) is lower
than 350.degree. C. (first cooling step). Further, in the first
cooling step, the steel sheet is cooled at an average cooling rate
of 5.degree. C./sec or more from 700.degree. C. down to a
temperature which is the higher one of 300.degree. C. and the
cooling stop temperature T2.
[0119] By controlling the average cooling rate (which may hereafter
be denoted as CR1) in an interval from 700.degree. C. to the
temperature which is the higher one of 300.degree. C. and the
cooling stop temperature T2 after soaking, a predetermined amount
of soft polygonal ferrite can be generated. In other words, when
the average cooling rate CR1 in the aforementioned interval is
smaller than 5.degree. C./sec, the polygonal ferrite is generated
in an excessively large amount, and the strength decreases.
Accordingly, the average cooling rate CR1 in the interval must be
controlled to be 5.degree. C./sec or larger, and is preferably
10.degree. C./sec or larger, more preferably 15.degree. C./sec or
larger. An upper limit of the average cooling rate CR1 in the
interval is not particularly limited; however, when the average
cooling rate CR1 is too large, it will be difficult to perform
temperature control. Accordingly, the average cooling rate CR1 in
the interval is preferably 80.degree. C./sec or smaller, more
preferably 60.degree. C./sec or smaller.
[0120] The cooling stop temperature T2 is set to be 100 to
350.degree. C. However, when an Ms point calculated by the
following formula (I) is lower than 350.degree. C., the cooling
stop temperature T2 is set to be 100.degree. C. to the Ms
point.
[0121] When the cooling stop temperature T2 is too low, tempered
martensite is generated in an excessively large amount, and the
retained .gamma. amount becomes small, so that the elongation
decreases, and the formability at room temperature cannot be
improved. Also, when the cooling stop temperature T2 is too low,
retained .gamma. with a carbon concentration exceeding 1.0 mass %
is generated in a large amount, and the amount of retained .gamma.
with a carbon concentration of 1.0 mass % or less becomes
comparatively small, so that .DELTA.TS decreases, and the molding
load at a hot temperature cannot be sufficiently reduced as
compared with the molding load at room temperature. The reason why
the retained .gamma. with a carbon concentration exceeding 1.0 mass
% is generated in a large amount seems to be that retained .gamma.
having a film form remains between the laths within the tempered
martensite, and the carbon concentration of this retained .gamma.
is high. Accordingly, the cooling stop temperature T2 is set to be
100.degree. C. or higher. The cooling stop temperature T2 is
preferably 110.degree. C. or higher, more preferably 120.degree. C.
or higher. However, when the cooling stop temperature T2 is too
high, the amount of generation of tempered martensite becomes
small, so that the bainite transformation occurring thereafter
hardly proceeds, and the C concentration into austenite hardly
proceeds. As a result, there will be a large amount of retained
.gamma. with a carbon concentration of 0.8 mass % or less, so that
the hole expansion ratio .lamda., decreases, and the formability at
room temperature cannot be improved. Accordingly, when the Ms point
is 350.degree. C. or higher, the cooling stop temperature T2 is set
to be 350.degree. C. or lower. The cooling stop temperature T2 is
preferably 330.degree. C. or lower, more preferably 300.degree. C.
or lower. On the other hand, when the Ms point is lower than
350.degree. C., the cooling stop temperature T2 is set to be the Ms
point or lower. The cooling stop temperature T2 is preferably (Ms
point--20.degree. C.) or lower, more preferably (Ms
point--50.degree. C.) or lower.
[0122] The temperature of the Ms point can be calculated on the
basis of the following formula (I) obtained by taking the polygonal
ferrite fraction (Vf) into consideration in the formula disclosed
in the above "The Physical Metallurgy of Steels" (p. 231). In the
following formula (I), brackets [ ] indicate the content of each
element (mass %), and calculation may be made by assuming that the
content of an element that is not contained in the steel sheet is 0
mass %. The value Vf represents the polygonal ferrite fraction
(area %); however, since it is difficult to directly measure the
polygonal ferrite fraction during the production, Vf may be defined
as the polygonal ferrite fraction in a sample separately obtained
by performing the soaking step under the same conditions as the
production conditions of the high strength steel sheet and
thereafter cooling to room temperature at the same average cooling
rate as in the production conditions of the high strength steel
sheet in the first cooling step.
Ms point (.degree.
C.)=561-474.times.[C]/(1-Vf/100)-33.times.[Mn]-17.times.[Ni]-17.times.[Cr-
]-21.times.[Mo] (I)
[0123] [Reheating Step]
[0124] After the steel sheet is cooled to the cooling stop
temperature T2, the steel sheet is reheated to a T3 temperature
region exceeding 350.degree. C. and being 540.degree. C. or lower,
and the steel sheet is held for 50 seconds or more in the T3
temperature region (reheating step). Here, the reheating
temperature in the T3 temperature region may hereafter be denoted
as "T3", and the holding time in the T3 temperature region may
hereafter be denoted as "t3". Also, the holding means to include a
mode in which the temperature fluctuates within the T3 temperature
region besides the constant-temperature holding.
[0125] By holding the steel sheet in the T3 temperature region for
50 seconds or more, retained .gamma. with a carbon concentration
exceeding 0.8 mass % and being 1.0 mass % or less can be generated,
so that it is possible to realize a high strength steel sheet in
which the molding load at a hot temperature is reduced while the
formability at room temperature is maintained to be good.
[0126] When the reheating temperature T3 is too low, the amount of
generation of retained .gamma. with a carbon concentration of 1.0
mass % or less becomes small, and .DELTA.TS decreases, so that the
molding load at a hot temperature cannot be reduced. Accordingly,
the reheating temperature T3 is set to be higher than 350.degree.
C. The reheating temperature T3 is preferably 360.degree. C. or
higher, more preferably 370.degree. C. or higher. However, when the
reheating temperature T3 is too high, the bainite transformation
does not proceed sufficiently, so that the retained .gamma. amount
decreases, and the elongation EL decreases. Also, within the
retained .gamma., the amount of retained .gamma. with a carbon
concentration of 0.8 mass % or less becomes large, and the hole
expansion ratio .lamda., decreases. As a result, the formability at
room temperature cannot be improved. Accordingly, the reheating
temperature T3 is set to be 540.degree. C. or lower. The reheating
temperature T3 is preferably 520.degree. C. or lower, more
preferably 500.degree. C. or lower.
[0127] Also, when the holding time t3 is too short, the bainite
transformation does not proceed sufficiently, so that C
concentration into austenite does not proceed sufficiently, and the
amount of generation of retained .gamma. decreases. As a result,
the elongation EL decreases. Also, there will be variation in the
degree of concentration of C into each austenite, so that the
amount of retained .gamma. with a carbon concentration of 0.8 mass
% or less becomes large, and the hole expansion ratio .lamda.,
decreases. As a result, the formability at room temperature is
degraded. Accordingly, the holding time t3 is set to be 50 seconds
or more. The holding time t3 is preferably 80 seconds or more, more
preferably 100 seconds or more. An upper limit of the holding time
t3 is not particularly limited;
[0128] however, in consideration of the productivity, the holding
time t3 is preferably, for example, 20 minutes or less.
[0129] [Second Cooling Step]
[0130] After the steel sheet is held in the reheating step, the
steel sheet is cooled at an average cooling rate of 10.degree.
C./sec or more from the T3 temperature region down to 300.degree.
C., and further the steel sheet is cooled at an average cooling
rate exceeding 0.degree. C./sec and being less than 10.degree.
C./sec from 300.degree. C. down to 150.degree. C. (second cooling
step). In cooling the steel sheet from the above T3 temperature
region to 150.degree. C. after the holding, it is important to
carry out two-stage cooling with 300.degree. C. being a boundary.
On the high-temperature side till 300.degree. C., .DELTA.TS can be
increased by quick cooling, so that the molding load at a hot
temperature can be reduced. On the low-temperature side from
300.degree. C., the hole expansion ratio .lamda., can be increased
by slow cooling, so that the formability at room temperature can be
improved.
[0131] In other words, when the average cooling rate down to
300.degree. C. (which may hereafter be denoted as CR2) after
reheating is too small, the bainite transformation and
concentration of C into untransformed austenite proceed during the
cooling, so that the amount of retained .gamma. with a carbon
concentration exceeding 1.0 mass % increases, while the amount of
retained .gamma. with a carbon concentration of 1.0 mass % or less
decreases. As a result, .DELTA.TS decreases, and the molding load
at a hot temperature cannot be reduced. Accordingly, the average
cooling rate CR2 must be controlled to be 10.degree. C./sec or more
and is preferably 15.degree. C./sec or more, more preferably
20.degree. C./sec or more. An upper limit of the average cooling
rate CR2 is not particularly limited; however, when the average
cooling rate CR2 is too large, it will be difficult to perform
temperature control. Accordingly, the average cooling rate CR2 is
preferably 80.degree. C./sec or smaller, more preferably 60.degree.
C./sec or smaller.
[0132] On the other hand, though bainite transformation does not
proceed during the cooling from 300.degree. C. to 150.degree. C.,
diffusion of C proceeds. Accordingly, by setting the average
cooling rate in this temperature region (which may hereafter be
denoted as CR3) to be small, the amount of generation of retained
.gamma. with a carbon concentration of 0.8 mass % or less can be
reduced. The reason for this is unknown; however, it can be
considered that, among the variations of the C concentration
generated in each retained .gamma., C is diffused into the retained
.gamma. having a C concentration lower than the original C
concentration during the cooling, so that the amount of generation
of retained .gamma. having a low carbon concentration can be
reduced. A supply source of C seems to be the tempered martensite
or the like. Also, in the fresh martensite (FM) transformed
inevitably from y, self-tempering proceeds during the cooling. As a
result, the hole expansion ratio .lamda. increases, and the
formability at room temperature is improved. Accordingly the
average cooling rate CR3 in the interval must be controlled to be
smaller than 10.degree. C./sec, and is preferably 5.degree. C./sec
or smaller, more preferably 2.degree. C./sec or smaller.
[0133] After the steel sheet is cooled to 150.degree. C., the steel
sheet may be cooled to room temperature in accordance with a
conventional method.
[0134] [Plating]
[0135] An electrogalvanized layer, a hot-dip galvanized layer, or a
hot-dip galvannealed layer may be formed on a surface of the high
strength steel sheet.
[0136] The conditions for forming the electrogalvanized layer, the
hot-dip galvanized layer, or the hot-dip galvannealed layer are not
particularly limited, and an electrogalvanization (EG) treatment, a
hot-dip galvanization (GI) treatment, or a hot-dip galvannealing
(GA) treatment of a conventional method can be adopted. By this, an
electrogalvanized steel sheet (which may hereafter be referred to
as "EG steel sheet"), a hot-dip galvanized steel sheet (which may
hereafter be referred to as "GI steel sheet"), and a hot-dip
galvannealed steel sheet (which may hereafter be referred to as "GA
steel sheet") can be obtained.
[0137] As a method for manufacturing the EG steel sheet, there can
be considered, for example, a method in which, after the steel
sheet is subjected to the second cooling step, the steel sheet may
be energized while being immersed in a zinc solution of 55.degree.
C., so as to perform an electrogalvanization treatment.
[0138] As a method for manufacturing the GI steel sheet, a hot-dip
galvanization treatment may be carried out simultaneously with the
reheating step. That is, after the steel sheet is reheated to the
T3 temperature region, the steel sheet may be immersed into a
plating bath adjusted to have a temperature within the T3
temperature region to perform hot-dip galvanization, so that the
hot-dip galvanization and the holding in the T3 temperature region
may be simultaneously carried out. At this time, it is sufficient
that the staying time in the T3 temperature region satisfies the
requirements of the holding time t3.
[0139] As a method for manufacturing the GA steel sheet, hot-dip
galvanization may be carried out at a temperature within the above
T3 temperature region, and thereafter an alloying treatment may be
carried out successively in the T3 temperature region. At this
time, it is sufficient that the staying time in the T3 temperature
region satisfies the requirements of the holding time t3.
[0140] The galvanizing adhesion amount is not particularly limited
and may be set to be, for example, about 10 to 100 g/m.sup.2 per
one surface.
[0141] The sheet thickness of the high strength steel sheet is not
particularly limited; however, the steel sheet may be, for example,
a thin steel sheet having a sheet thickness of 3 mm or less.
[0142] In the high strength steel sheet, the tensile strength (TS)
is 980 MPa or more, preferably 1100 MPa or more.
[0143] The high strength steel sheet is excellent in formability at
room temperature (TS.times.EL, .lamda.). Specifically, in the high
strength steel sheet, TS.times.elongation (EL) is preferably 16000
MPa% or more, more preferably 18000 MPa% or more. Further, in the
high strength steel sheet, the hole expansion ratio .lamda. is
preferably 20% or more, more preferably 25% or more.
[0144] As described above, the high strength steel sheet is
excellent in formability at room temperature (TS.times.EL,
.lamda.), and moreover the molding load at a hot temperature is
sufficiently reduced. Specifically, in the high strength steel
sheet, .DELTA.TS is preferably 150 MPa or more, more preferably 180
MPa or more. This hot-temperature forming means that molding is
carried out at a temperature of about 100 to 350.degree. C.
[0145] The high strength steel sheet can be suitably used as a
material of a structural component of an automobile. Examples of
the structural component of an automobile include crash parts such
as front and rear side-members and crash boxes, reinforcements such
as pillars (for example, bears, center pillar reinforcements and
the like), car body components such as roof rail reinforcements,
side sills, floor members, and kick sections, shock resistant
absorbing components such as bumper reinforcements and door impact
beams, and sheet components.
[0146] While the present specification discloses various modes of
techniques as described above, principal techniques among these
will be summarized as follows.
[0147] One aspect of the present invention is a high strength steel
sheet satisfying, in mass %, C: 0.10% to 0.5%, Si: 1.0% to 3%, Mn:
1.5% to 3%, P: more than 0% and 0.1% or less, S: more than 0% and
0.05% or less, Al: 0.005% to 1%, and N: more than 0% and 0.01% or
less, with a balance being iron and inevitable impurities, in which
(1) a metal structure of the steel sheet contains polygonal
ferrite, bainite, tempered martensite, and retained austenite; (2)
when the metal structure is observed with a scanning electron
microscope, the metal structure satisfies polygonal ferrite: 10 to
50 area %, bainite: 10 to 50 area %, and tempered martensite: 10 to
80 area %, with respect to the metal structure overall; and (3)
when the metal structure is measured by X-ray diffractometry, the
metal structure satisfies retained austenite: 5.0 volume % or more,
retained austenite with a carbon concentration of 1.0 mass % or
less: 3.5 volume % or more, and retained austenite with a carbon
concentration of 0.8 mass % or less: 2.4 volume % or less, with
respect to the metal structure overall.
[0148] According to such a configuration, a high strength steel
sheet having a good elongation and hole expansion formability at
room temperature and having a tensile strength of 980 MPa or more
in which the molding load for forming at a hot temperature of 100
to 350.degree. C. is outstandingly reduced as compared with the
molding load for forming at room temperature, can be provided.
[0149] The high strength steel sheet may further contain, as other
elements, one or more selected from the group consisting of Cr:
more than 0% and 1% or less and Mo: more than 0% and 1% or less, in
mass %.
[0150] The high strength steel sheet may further contain, as other
elements, one or more selected from the group consisting of Ti:
more than 0% and 0.15% or less, Nb: more than 0% and 0.15% or less,
and V: more than 0% and 0.15% or less, in mass %.
[0151] The high strength steel sheet may further contain, as other
elements, one or more selected from the group consisting of Cu:
more than 0% and 1% or less and Ni: more than 0% and 1% or less, in
mass %.
[0152] The high strength steel sheet may further contain, as
another element, B: more than 0% and 0.005% or less in mass %.
[0153] The high strength steel sheet may further contain, as other
elements, one or more selected from the group consisting of Ca:
more than 0% and 0.01% or less, Mg: more than 0% and 0.01% or less,
and a rare earth element: more than 0% and 0.01% or less, in mass
%.
[0154] The scope of the high strength steel sheet described above
encompasses a high strength steel sheet having an electrogalvanized
layer, a hot-dip galvanized layer, or a hot-dip galvannealed layer
on a surface of the steel sheet.
[0155] Also, another aspect of the present invention is a method
for manufacturing a high strength steel sheet, the method including
a soaking step of heating the steel sheet satisfying the aforesaid
component composition to a T1 temperature region, which is
800.degree. C. or higher and an Ac.sub.3 point or lower, and
holding the steel sheet in the Ti temperature region for 40 seconds
or more for soaking; a first cooling step which is carried out
after the soaking and in which, in cooling the steel sheet down to
an arbitrary cooling stop temperature T2 satisfying 350.degree. C.
or lower and 100.degree. C. or higher when an Ms point represented
by the following formula (I) is 350.degree. C. or higher, or else,
in cooling the steel sheet down to an arbitrary cooling stop
temperature T2 satisfying the Ms point or lower and 100.degree. C.
or higher when the Ms point represented by the following formula
(I) is lower than 350.degree. C., the steel sheet is cooled at an
average cooling rate of 5.degree. C./sec or more from 700.degree.
C. down to a temperature which is the higher one of 300.degree. C.
and the cooling stop temperature T2; a reheating step of reheating
the steel sheet to a T3 temperature region exceeding 350.degree. C.
and being 540.degree. C. or lower and holding the steel sheet for
50 seconds or more in the T3 temperature region; and a second
cooling step which is carried out after the holding and in which
the steel sheet is cooled at an average cooling rate of 10.degree.
C./sec or more from the T3 temperature region down to 300.degree.
C., and further the steel sheet is cooled at an average cooling
rate exceeding 0.degree. C./sec and being less than 10.degree.
C./sec from 300.degree. C. down to 150.degree. C.
Ms point (.degree. C.)=561-474.times.[C9
/(1-Vf/100)-33.times.[Mn]-17.times.[Ni]-17.times.[Cr]-21.times.[Mo]
(I)
[0156] In the formula (I), Vf represents a polygonal ferrite
fraction (area %) in a sample that is separately obtained by
performing the aforesaid soaking step under the same conditions as
in the manufacturing conditions for the aforesaid high strength
steel sheet and thereafter cooling down to room temperature at the
same average cooling rate as in the manufacturing conditions for
the aforesaid high strength steel sheet in the aforementioned first
cooling step. Also, in the formula (I), brackets [ ] represents a
content (mass %) of each element, where calculation is made
assuming that the content of elements not contained in the steel
sheet is 0 mass %.
[0157] Also, in the method for manufacturing a high strength steel
sheet, electrogalvanization may be carried out after the second
cooling step.
[0158] Also, in the method for manufacturing a high strength steel
sheet, hot-dip galvanization or hot-dip galvannealing may be
carried out in the reheating step.
[0159] According to the present invention, the component
composition and metal structure of the steel sheet are
appropriately controlled, and in particular, the carbon
concentration of retained .gamma. is strictly controlled, so that a
high strength steel sheet having a good elongation and hole
expansion formability at room temperature and having a tensile
strength of 980 MPa or more in which the molding load for forming
at a hot temperature of 100 to 350.degree. C. is outstandingly
reduced as compared with the molding load for forming at room
temperature, as well as a manufacturing method therefor, can be
provided.
[0160] Hereafter, the present invention will be explained more
specifically by way of examples; however, the present invention is
not limited by the following examples, and it goes without saying
that the invention can be carried out while including additional
modifications within a scope conforming to the gist disclosed
heretofore and hereinafter, all such modifications being
encompassed within the technical scope of the invention.
EXAMPLES
[0161] A steel material was produced by melting a steel containing
the components shown in the following Table 1 with a balance being
iron and inevitable impurities. The obtained steel material was
heated and held at 1250.degree. C. for 30 minutes and thereafter
hot-rolled with the rolling reduction being set to be about 90% and
with the finish rolling end temperature being set to be 920.degree.
C. Thereafter, the steel sheet was cooled down from this
temperature to a coiling temperature of 600.degree. C. at an
average cooling rate of 30.degree. C./sec, followed by coiling.
After the coiling, the steel sheet was cooled to room temperature,
so as to produce a hot-rolled steel sheet having a sheet thickness
of 2.6 mm.
[0162] After the obtained hot-rolled steel sheet was pickled to
remove surface scale, cold rolling was carried out at a cold
rolling rate of 46% to produce a cold-rolled steel sheet having a
sheet thickness of 1.4 mm.
[0163] The obtained cold-rolled steel sheet was subjected to
continuous annealing to produce a test sample material. That is,
the obtained cold-rolled steel sheet was heated to a soaking
temperature T1 (.degree. C.) shown in the following Table 2-1 or
2-2 and held for a soaking time tl (sec) shown in the following
Table 2-1 or 2-2 for soaking. Thereafter, the resultant was cooled
down to a cooling stop temperature T2 (.degree. C.) shown in the
following Table 2-1 or 2-2. The average cooling rate CR1 (.degree.
C./sec) for cooling from 700.degree. C. down to the higher one of
300.degree. C. and the cooling stop temperature T2 is shown in the
following Table 2-1 or 2-2.
[0164] Also, the following Table 2-1 and 2-2 show together the Ms
point (.degree. C.) calculated on the basis of the component
composition shown in the following Table 1 and the formula (I)
shown above.
[0165] Subsequently, the steel sheet was heated from the cooling
stop temperature T2 (.degree. C.) up to a reheating temperature T3
(.degree. C.) shown in the following Table 2-1 or 2-2 and held at
this temperature for a holding time t3 (sec) shown in the following
Table 2-1 or 2-2.
[0166] After the holding, the steel sheet was cooled to room
temperature. During this, the steel sheet was cooled at an average
cooling rate CR2 (.degree. C./sec) shown in the following Table 2-1
or 2-2 from the reheating temperature T3 (.degree. C.) down to
300.degree. C., and further the steel sheet was cooled at an
average cooling rate CR3 (.degree. C./sec) shown in the following
Table 2-1 or 2-2 from 300.degree. C. down to 150.degree. C.
[0167] Some of the test sample materials obtained by continuous
annealing were subjected to the following plating treatment to
produce an EG steel sheet, a GI steel sheet, and a GA steel
sheet.
[0168] [Electrogalvanization (EG) Treatment]
[0169] After the continuous annealing was carried out, the test
sample material was cooled to room temperature. Subsequently, the
test sample material was immersed into a galvanizing bath of
55.degree. C. to perform an electrogalvanization treatment at an
electric current density of 30 to 50 A/dm.sup.2, followed by
washing with water and drying to produce an EG steel sheet. The
adhesion amount of electrogalvanization was set to be 10 to 100
g/m.sup.2 per one surface.
[0170] [Hot-dip Galvanization (GI) Treatment]
[0171] After the test sample material was heated from the cooling
stop temperature T2 (.degree. C.) to the reheating temperature T3
(.degree. C.) shown in the following Table 2-1 or 2-2, the test
sample material was immersed into a hot-dip galvanization bath
having a temperature of 460.degree. C. to perform a plating
treatment and thereafter cooled to room temperature to produce a GI
steel sheet. The staying time in the T3 temperature region is shown
in the section of the holding time t3 (sec) shown in the following
Table 2-1 or 2-2. The adhesion amount of hot-dip galvanization was
set to be 10 to 100 g/m.sup.2 per one surface.
[0172] [Hot-Dip Galvanealing (GA) Treatment]
[0173] After immersion into the above hot-dip galvanization bath,
an alloying treatment was further carried out at a temperature
shown in the following Table 2-1 or 2-2, followed by cooling down
to room temperature to produce a GA steel sheet. The staying time
in the T3 temperature region is shown in the section of the holding
time t3 (sec) shown in the following Table 2-1 or 2-2. The adhesion
amount of hot-dip galvannealing was set to be 10 to 100 g/m.sup.2
per one surface.
[0174] The classification of the obtained test sample materials is
shown in the following Table 2-1 or 2-2. In the Tables, the
notations of cold-rolled, EG, GI, and GA represent a cold-rolled
steel sheet, an EG steel sheet, a GI steel sheet, and a GA steel
sheet, respectively.
[0175] With respect to the obtained test sample materials (which
mean to include a cold-rolled steel sheet, an EG steel sheet, a GI
steel sheet, and a GA steel sheet. The same applies hereinafter.),
observation of the metal structure and evaluation of the mechanical
properties were carried out by the following procedure.
[0176] <<Observation of Metal Structure>>
[0177] In the metal structure, the area ratio of each of polygonal
ferrite, bainite, and tempered martensite was calculated on the
basis of the result of observation by a scanning electron
microscope. The volume ratio of retained .gamma. was calculated by
X-ray diffractometry.
[0178] [Polygonal Ferrite, Bainite, and Tempered Martensite]
[0179] After the cross-section of the test sample material parallel
to the rolling direction was polished, the test sample material was
subjected to nital corrosion, followed by performing observation at
the position of 1/4 of the sheet thickness in five fields of view
at a magnification of 3000 times with a scanning electron
microscope. The observation field of view was set to be about 40
.mu.m.times.about 30 .mu.m. The area ratio of polygonal ferrite can
be measured by the observation with this scanning electron
microscope. Also, the area ratios of bainite and tempered
martensite were measured as follows. An average of an interval
(average interval) of at least one kind selected from the group
consisting of retained .gamma. and carbide, which are observed as
being white or faint gray in the observation field of view, with
each other was measured by the method described above, and
classification into bainite and tempered martensite was made by the
measured average interval, so as to measure the area ratios by the
point counting method.
[0180] Results of the measurement are shown in the following Table
3-1 or 3-2. In the Table 3-1 or 3-2, F represents the area ratio of
polygonal ferrite; B represents the area ratio of bainite; and TM
represents the area ratio of tempered martensite. The balance is
retained .sub.7, an MA mixture phase in which fresh martensite and
retained .gamma. are combined, pearlite, and fresh martensite.
[0181] [Retained .gamma.]
[0182] After the test sample material was ground down to a position
of 1/4 of the sheet thickness, the ground surface was chemically
polished, and thereafter the volume ratio of retained .gamma.
relative to the metal structure overall was measured by X-ray
diffractometry (ISIJ Int. Vol. 33, year of 1933, No. 7, p.
776).
[0183] Also, the carbon concentration in the retained .gamma. was
measured by the procedure described above, so as to calculate the
volume ratio of retained .gamma. with a carbon concentration of 1.0
mass % or less [V.gamma..sub.R(C.ltoreq.1.0%)] and the volume ratio
of retained .gamma. with a carbon concentration of 0.8 mass % or
less [V.gamma..sub.R(C.ltoreq.0.8%)] with respect to the metal
structure overall.
[0184] Here, as reference data, the following Table 3-1 or 3-2
shows together the average value % C.sub.avg and the standard
deviation .sigma.% C of the carbon concentration distribution in
the retained .gamma..
[0185] <<Evaluation of Mechanical Properties>>
[0186] [Tensile Strength (TS) and Elongation (EL) at Room
Temperature]
[0187] The tensile strength (TS) and the elongation (EL) at room
temperature (25.degree. C.) were measured by performing a tensile
test in accordance with JIS Z2241. The test piece that was put to
use was one obtained by cutting out a No. 5 test piece defined in
JIS Z2201 from the test sample material so that the direction
perpendicular to the rolling direction of the test sample material
would be a longitudinal direction. The tensile speed of the tensile
test was set to be 10 mm/min.
[0188] The results of TS and EL measured at room temperature are
shown in the following Table 3-1 or 3-2.
[0189] Also, a product of TS and EL (TS.times.EL) measured at room
temperature was calculated, and the results are shown in the
following Table 3-1 or 3-2.
[0190] [.DELTA.TS]
[0191] In order to evaluate the degree of load reduction during the
hot temperature molding, a value obtained by subtracting the
tensile strength at a hot temperature (200.degree. C.) (hot
temperature TS) from the tensile strength at room temperature
(25.degree. C.) (room temperature TS) (room temperature TS--hot
temperature TS=.DELTA.TS) was calculated. The tensile test was
carried out under the same conditions as those described above
except that the tensile speed was set to be 1000 mm/min in order to
simulate the speed during the press-forming, and the tensile test
was carried out at two levels of room temperature and 200.degree.
C. The calculated .DELTA.TS is shown in the following Table 3-1 or
3-2.
[0192] [Hole Expansion Formability]
[0193] The hole expansion formability was evaluated by the hole
expansion ratio (.lamda.) measured by performing a hole expansion
test in accordance with JIS Z2256. The measurement results are
shown in the ".lamda.(%)" section of the following Table 3-1 or
3-2.
[0194] In the present invention, the cases in which TS was 980 MPa
or more were evaluated as having a high strength. Also, the cases
in which TS.times.EL was 16000 MPa% or more and .lamda. was 20% or
more were evaluated as having an excellent formability at room
temperature. Also, the cases in which .DELTA.TS was 150 MPa or more
were evaluated as having a reduced molding load at a hot
temperature. Further, the cases in which all of TS, TS.times.EL,
.lamda., and .DELTA.TS satisfied the standard values were evaluated
as being acceptable. On the other hand, the cases in which one or
more of TS, TS.times.EL, .lamda., and .DELTA.TS did not satisfy the
standard values were evaluated as being a reject.
[0195] The following observations arise from Tables 1, 2-1, 2-2,
3-1, and 3-2 given below.
[0196] Nos. 1, 7, 8, 10, 13, 15, 17, 20, 22, 23, 26 to 31, 33, 37,
38, 40, and 42 are examples satisfying the requirements defined in
the present invention, where TS measured at room temperature was
980 MPa or higher, thereby providing a high strength. Also,
TS.times.EL and .lamda. satisfied the acceptance standards of the
present invention, thereby providing a good formability at room
temperature. Further, since .DELTA.TS satisfied the acceptance
standard of the present invention, the molding load at a hot
temperature could be reduced.
[0197] In contrast, Nos. 2 to 6, 9, 11, 12, 14, 16, 18, 19, 21, 24,
25, 32, 34 to 36, 39, and 41 are examples that did not satisfy one
or more of the requirements defined in the present invention, and
at least one property among strength, formability at room
temperature, and molding load reduction at a hot temperature was
degraded.
[0198] Hereafter, description will be given in detail.
[0199] Nos. 2, 6, 12, and 41 are examples in which the average
cooling rate CR3 from 300.degree. C. to 150.degree. C. after
holding in the reheating step was too large, and retained .gamma.
having a carbon concentration of 0.8 mass % or less was generated
in a large amount, so that .lamda. was small, and the formability
at room temperature could not be improved.
[0200] Nos. 3, 5, and 39 are examples in which the average cooling
rate CR2 from the reheating temperature (holding temperature in the
reheating step) to 300.degree. C. was too small, and the amount of
retained .gamma. having a carbon concentration of 1.0 mass % or
less could not be ensured, so that .DELTA.TS was small, and the
molding load at a hot temperature could not be reduced. Here, No. 3
is an example simulating the above Patent Literature 1 and, as
described in the paragraph [0128] of the above Patent Literature 1,
the average cooling rate down to room temperature after holding was
set to be 5.degree. C./sec.
[0201] Nos. 4, 19, and 32 are examples in which the cooling stop
temperature T2 after soaking was too high. In Nos. 4, 19, and 32,
tempered martensite was not generated, or the generated amount was
small, so that TS at room temperature was low, and the strength
could not be ensured. Also, in Nos. 4, 19, and 32, retained .gamma.
having a carbon concentration of 0.8 mass % or less was generated
in a large amount, so that .lamda. was small, and the formability
at room temperature could not be improved. Here, it seems that, in
No. 19, the holding time t3 was comparatively long, and the amount
of generated polygonal ferrite was comparatively small, so that
bainite was generated in an excessive amount. In other words, the
smaller the amount of generated polygonal ferrite is, the more
difficult it is for C to be concentrated in the surrounding
austenite, so that bainite transformation seems to occur
rapidly.
[0202] No. 9 is an example in which the average cooling rate CR1 in
the interval of from 700.degree. C. to 300.degree. C. after soaking
was too small, so that polygonal ferrite was generated in an
excessive amount. As a result, No. 9 could not ensure a desired
TS.
[0203] No. 11 is an example in which the soaking temperature T1 was
too high, so that little amount of polygonal ferrite was generated.
As a result, in No. 11, TS.times.EL was low, and the formability at
room temperature could not be improved.
[0204] No. 14 is an example in which the cooling stop temperature
T2 after soaking was too low. In No. 14, bainite was not generated,
and tempered martensite was generated in an excessive amount, so
that the amount of retained .gamma. could not be ensured. As a
result, in No. 14, TS.times.EL was low, and the formability at room
temperature could not be improved. Also, in No. 14, the amount of
retained .gamma. having a carbon concentration of 1.0 mass % or
less could not be ensured, so that .DELTA.TS was small, and the
molding load at a hot temperature could not be reduced.
[0205] No. 16 is an example in which the soaking temperature T1 was
too low, and polygonal ferrite was generated in an excessive
amount. As a result, a desired TS could not be ensured. Also, in
No. 16, TS.times.EL lowered, and the formability at room
temperature could not be improved. This seems to be because the
formed structure introduced during the cold rolling remained.
[0206] No. 18 is an example in which the reheating temperature T3
was too high. In No. 18, little amount of bainite was generated,
and the amount of generation of retained .gamma. could not be
ensured, so that TS.times.EL lowered, and the formability at room
temperature could not be improved. Also, in No. 18, retained
.gamma. having a carbon concentration of 0.8 mass % or less was
generated in an excessive amount, so that X, was small, and the
formability at room temperature could not be improved.
[0207] No. 21 is an example in which the reheating temperature T3
was too low, and the amount of retained .gamma. having a carbon
concentration of 1.0 mass % or less could not be ensured. As a
result, in No. 21, .DELTA.TS was small, and the molding load at a
hot temperature could not be reduced.
[0208] No. 24 is an example in which the soaking time t1 was too
short, and a desired amount of retained .gamma. could not be
ensured. As a result, in No. 24, TS.times.EL lowered, and the
formability at room temperature could not be improved.
[0209] No. 25 is an example in which the holding time t3 was too
short. In No. 25, the amount of generation of bainite and retained
.gamma. could not be ensured, so that TS.times.EL lowered. Also, in
No. 25, retained .gamma. having a carbon concentration of 0.8 mass
% or less was generated in an excessive amount, so that .lamda. was
small, and the formability at room temperature could not be
improved.
[0210] Nos. 34 to 36 are examples in which the component
composition did not satisfy the requirements defined in the present
invention.
[0211] In No. 34, the Si amount was too small, so that TS lowered,
and the strength could not be ensured. Also, in No. 34, the amount
of generation of retained .gamma. could not be ensured, so that
TS.times.EL lowered, and the formability at room temperature could
not be improved. Also, No. 34 is an example in which the amount of
retained .gamma. having a carbon concentration of 1.0 mass % or
less could not be ensured, so that .DELTA.TS was small, and the
molding load at a hot temperature could not be reduced.
[0212] In No. 35, the C amount was too small, so that TS lowered,
and the strength could not be ensured. Also, in No. 35, the amount
of generation of retained .gamma. could not be ensured, so that
TS.times.EL lowered, and the formability at room temperature could
not be improved. Also, No. 35 is an example in which the amount of
retained .gamma. having a carbon concentration of 1.0 mass % or
less could not be ensured, so that .DELTA.TS was small, and the
molding load at a hot temperature could not be reduced.
[0213] In No. 36, the Mn amount was too small, so that polygonal
ferrite was generated in an excessive amount; TS lowered; and the
strength could not be ensured. Also, in No. 36, the amount of
generation of retained .gamma. could not be ensured, so that
TS.times.EL lowered, and the formability at room temperature could
not be improved. Also, No. 36 is an example in which the amount of
retained .gamma. having a carbon concentration of 1.0 mass % or
less could not be ensured, so that .DELTA.TS was small, and the
molding load at a hot temperature could not be reduced.
TABLE-US-00001 TABLE 1 Steel Components (mass %) Ac.sub.3 type C Si
Mn P S Al Cr Mo Ti Nb V Cu Ni B Ca Mg REM N (.degree. C.) A 0.19
1.85 2.20 0.02 0.001 0.04 -- -- -- -- -- -- -- -- -- -- -- 0.004
868 B 0.21 1.85 2.10 0.01 0.002 0.03 -- -- -- -- -- -- -- -- -- --
-- 0.004 856 C 0.17 1.75 2.40 0.01 0.002 0.04 -- -- -- -- -- -- --
-- -- -- -- 0.005 856 D 0.22 1.35 2.10 0.01 0.004 0.05 -- -- 0.04
-- -- -- -- -- -- -- -- 0.004 855 E 0.17 1.35 2.20 0.01 0.002 0.03
-- 0.2 -- -- -- -- -- -- -- -- -- 0.004 846 F 0.22 1.85 1.73 0.01
0.002 0.04 0.2 -- 0.09 -- -- -- -- -- -- -- -- 0.004 902 G 0.22
1.85 2.20 0.02 0.001 0.04 0.2 -- 0.02 -- -- -- -- 0.0025 -- -- --
0.005 867 H 0.18 2.00 2.65 0.02 0.003 0.04 -- -- -- 0.02 -- -- --
-- -- -- -- 0.004 864 I 0.42 1.53 1.76 0.03 0.001 0.05 -- -- -- --
0.09 -- -- -- -- -- -- 0.005 844 J 0.12 1.80 2.80 0.01 0.002 0.02
-- -- -- -- -- 0.3 0.2 -- -- -- -- 0.002 842 K 0.22 1.80 1.80 0.01
0.002 0.04 -- -- -- 0.10 -- -- -- -- -- -- -- 0.003 864 L 0.18 1.50
2.42 0.01 0.001 0.04 -- -- -- -- -- -- -- -- 0.0020 -- -- 0.004 841
M 0.18 2.50 2.03 0.01 0.002 0.03 -- -- -- -- -- -- -- -- -- 0.0026
-- 0.004 894 N 0.28 1.11 1.60 0.01 0.002 0.03 -- -- -- -- -- -- --
-- -- -- 0.0021 0.004 823 O 0.16 1.90 2.87 0.01 0.003 0.03 -- -- --
-- -- -- -- -- -- -- -- 0.003 847 P 0.18 0.52 2.05 0.02 0.002 0.03
-- -- -- -- -- -- -- -- -- -- -- 0.004 812 Q 0.09 1.61 2.45 0.01
0.001 0.03 -- -- -- -- -- -- -- -- -- -- -- 0.004 867 R 0.18 1.60
1.30 0.01 0.001 0.03 -- -- -- -- -- -- -- -- -- -- -- 0.003 875 S
0.22 1.85 2.00 0.01 0.001 0.04 -- -- -- -- -- -- -- -- -- -- --
0.004 860 T 0.22 1.85 1.90 0.01 0.001 0.04 -- -- 0.06 -- -- -- --
-- -- -- -- 0.004 887
TABLE-US-00002 TABLE 2-1 Average Average Average cooling cooling
cooling Soaking Soaking rate Cooling stop Reheating Holding rate
rate Alloying Steel temperature time CR1 Ms temperature temperature
time CR2 CR3 temperature No. type T1 (.degree. C.) t1 (sec)
(.degree. C./sec) (.degree. C.) T2 (.degree. C.) T3 (.degree. C.)
t3 (sec) (.degree. C./sec) (.degree. C./sec) (.degree. C.)
Classification 1 A 830 80 20 322 170 400 350 30 5 -- Cold-rolled 2
A 830 80 20 325 170 400 350 20 20 -- Cold-rolled 3 A 830 80 20 336
170 400 350 5 5 -- Cold-rolled 4 A 810 150 15 315 380 380 580 20 8
530 GA 5 A 850 80 8 352 200 420 150 8 3 450 GA 6 A 850 80 8 343 200
420 150 25 13 450 GA 7 B 810 240 20 323 180 420 720 20 8 --
Cold-rolled 8 B 850 100 15 361 280 350 300 30 5 -- Cold-rolled 9 B
810 100 3 249 150 420 720 20 8 -- Cold-rolled 10 C 830 200 5 338
200 420 300 10 2 -- Cold-rolled 11 C 900 100 40 400 330 410 80 15 5
-- Cold-rolled 12 C 800 150 45 348 200 400 300 15 15 -- Cold-rolled
13 D 840 100 10 369 120 400 130 12 2 540 GA 14 D 840 100 10 366 80
400 130 12 2 450 GA 15 E 810 60 40 354 120 370 300 15 5 --
Cold-rolled 16 E 780 150 10 240 150 450 300 15 5 -- Cold-rolled 17
F 840 80 15 300 100 450 65 17 3 480 GA 18 F 860 80 40 343 170 560
65 17 3 -- GI 19 F 890 80 40 369 400 400 900 15 5 450 GA 20 G 830
80 30 317 110 430 70 15 5 470 GA
TABLE-US-00003 TABLE 2-2 Average Average Average cooling cooling
cooling Soaking Soaking rate Cooling stop Reheating Holding rate
rate Alloying Steel temperature time CR1 Ms temperature temperature
time CR2 CR3 temperature No. type T1 (.degree. C.) t1 (sec)
(.degree. C./sec) (.degree. C.) T2 (.degree. C.) T3 (.degree. C.)
t3 (sec) (.degree. C./sec) (.degree. C./sec) (.degree. C.)
Classification 21 G 830 80 30 308 110 300 70 15 5 470 GA 22 G 830
520 30 311 110 430 70 15 5 470 GA 23 H 830 150 20 348 150 430 300
25 1 -- GI 24 H 830 30 20 344 150 430 250 25 1 -- GI 25 H 830 150
20 355 150 430 40 25 1 -- Cold-rolled 26 I 810 360 8 223 150 370
500 12 2 440 GA 27 J 825 200 30 370 220 370 50 10 5 -- Cold-rolled
28 K 830 50 40 308 100 400 70 17 3 450 GA 29 L 810 80 30 352 180
450 80 15 5 470 GA 30 M 860 80 30 333 180 450 80 15 5 470 GA 31 N
815 60 8 316 300 420 300 15 5 -- EG 32 N 800 45 5 253 350 450 1000
15 5 500 GA 33 O 810 60 30 344 250 420 300 15 5 -- EG 34 P 800 150
30 384 200 440 150 15 5 -- Cold-rolled 35 Q 840 150 30 401 200 440
150 15 5 -- Cold-rolled 36 R 860 150 30 281 150 440 150 15 5 --
Cold-rolled 37 S 820 60 20 305 120 450 65 20 2 500 GA 38 S 820 60
20 305 120 450 65 12 2 500 GA 39 S 820 60 20 305 120 450 65 6 2 500
GA 40 S 820 60 20 305 120 450 65 24 8 500 GA 41 S 820 60 20 305 120
450 65 24 15 500 GA 42 T 830 60 20 327 120 450 65 25 4 500 GA
TABLE-US-00004 TABLE 3-1 Material characteristics Metal structure
(area %) Metal structure (volume %) % C.sub.avg .sigma.% C TS EL TS
.times. EL .lamda. .DELTA.TS No. F B TM Balance V.gamma..sub.R
V.gamma..sub.R (C .ltoreq.1.0%) V.gamma..sub.R (C .ltoreq.0.8%)
(mass %) (mass %) (MPa) (%) (MPa %) (%) (MPa) 1 46 20 22 12 14.0
6.1 2.1 1.04 0.23 1078 19 20482 32 202 2 45 17 26 12 13.8 5.9 2.6
1.05 0.29 1075 19 20425 17 213 3 41 16 32 11 12.1 3.2 1.7 1.28 0.45
1076 18 19368 34 128 4 48 34 0 18 11.6 4.9 2.5 1.07 0.34 924 20
18480 18 192 5 34 27 25 14 15.2 3.4 2.0 1.42 0.56 1062 19 20178 27
141 6 38 20 31 11 13.0 4.0 2.5 1.27 0.54 1058 20 21160 15 162 7 41
26 23 10 14.7 6.3 1.9 1.04 0.21 1081 22 23782 30 194 8 24 46 20 10
13.2 4.1 1.8 1.16 0.33 1092 17 18564 38 158 9 59 18 12 11 13.2 4.5
2.0 1.13 0.32 952 21 19992 23 198 10 44 22 21 13 11.8 5.6 2.3 1.02
0.25 1042 18 18756 26 166 11 2 47 28 23 8.4 6.2 2.3 0.90 0.16 1198
11 13178 60 250 12 40 26 18 16 15.4 6.4 2.9 1.06 0.30 1187 19 22553
16 202 13 15 10 72 3 8.0 5.0 1.5 0.95 0.17 1190 14 16660 38 180 14
17 0 82 1 3.6 1.4 0.6 1.08 0.29 1233 8 9864 49 98 15 38 10 42 10
12.5 5.8 2.3 1.02 0.25 1052 18 18936 22 184 16 67 10 10 13 6.8 3.5
2.0 0.99 0.35 881 17 14977 25 168 17 48 12 31 9 16.8 5.7 2.2 1.12
0.28 1038 24 24912 27 203 18 34 9 34 23 4.8 3.6 2.7 0.74 0.39 1067
11 11737 12 151 19 21 54 8 17 12.0 6.2 2.8 0.99 0.26 970 18 17460
16 226 20 38 13 41 8 16.8 6.4 2.1 1.07 0.24 1146 21 24066 35
208
TABLE-US-00005 TABLE 3-2 Material characteristics Metal structure
(area %) Metal structure (volume %) % C.sub.avg .sigma.% C TS EL TS
.times. EL .lamda. .DELTA.TS No. F B TM Balance V.gamma..sub.R
V.gamma..sub.R (C .ltoreq.1.0%) V.gamma..sub.R (C .ltoreq.0.8%)
(mass %) (mass %) (MPa) (%) (MPa %) (%) (MPa) 21 41 10 39 10 13.8
2.8 1.0 1.27 0.32 1184 17 20128 39 98 22 40 12 40 8 15.5 5.8 2.1
1.08 0.26 1138 17 19346 22 200 23 32 15 42 11 7.8 6.2 2.3 0.88 0.15
1224 14 17136 30 211 24 34 11 46 9 4.7 3.8 2.3 0.81 0.22 1220 11
13420 28 154 25 28 7 42 23 4.5 4.0 3.1 0.66 0.27 1251 8 10008 11
161 26 29 29 21 21 19.0 3.9 1.4 1.26 0.32 1399 15 20985 23 168 27
40 34 21 5 5.8 3.7 2.2 0.89 0.30 984 17 16728 42 195 28 46 11 32 11
17.6 6.0 2.1 1.11 0.26 1000 24 24000 25 213 29 34 25 28 13 12.2 6.8
2.3 0.97 0.19 1083 18 19494 38 197 30 47 19 30 4 17.0 3.6 0.9 1.20
0.24 1052 24 25248 47 175 31 31 38 16 15 12.5 5.1 2.3 1.07 0.30
1123 17 19091 20 226 32 48 30 6 16 16.0 5.5 2.7 1.14 0.36 931 24
22344 18 202 33 38 34 24 4 9.7 3.8 1.2 1.06 0.23 1181 18 21258 59
189 34 22 31 43 4 4.0 2.8 1.7 0.85 0.28 975 13 12675 42 113 35 46
16 35 3 3.2 2.1 0.8 0.93 0.19 921 16 14736 39 108 36 64 11 15 10
3.8 2.6 1.1 0.91 0.19 784 18 14112 52 111 37 45 10 30 15 17.1 6.8
1.6 1.05 0.19 1034 25 25850 32 210 38 45 15 30 10 14.5 4.5 0.8 1.09
0.18 1025 24 24600 33 176 39 45 18 30 7 13.6 3.3 0.4 1.12 0.17 1023
23 23529 33 145 40 45 10 28 17 15.1 7.0 2.2 1.02 0.21 1042 24 25008
25 227 41 45 10 30 15 14.2 7.6 2.9 0.98 0.22 1045 23 24035 17 232
42 39 10 34 17 17.5 5.7 1.3 1.09 0.20 1023 24 24552 33 198
[0214] This application is based on Japanese Patent Application No.
2016-038304 filed on Feb. 29, 2016 and Japanese Patent Application
No. 2016-182966 filed on Sep. 20, 2016, the entire contents of
which are incorporated in the present application.
[0215] While the present invention has been fully and appropriately
described in the above with reference to the drawings by way of
embodiments in order to express the present invention, it is to be
recognized that those skilled in the art can readily change and/or
modify the embodiments described above. Therefore, it is to be
interpreted that the changes or modifications made by those skilled
in the art are encompassed within the scope of the claims unless
those changes or modifications are at a level that departs from the
scope of the claims described in the claims section of the present
application.
INDUSTRIAL APPLICABILITY
[0216] According to the present invention, the component
composition and metal structure of the steel sheet are
appropriately controlled, and in particular, the carbon
concentration of retained .gamma. is strictly controlled, so that a
high strength steel sheet having a good elongation and hole
expansion formability at room temperature and having a tensile
strength of 980 MPa or more in which the molding load for forming
at a hot temperature of 100 to 350.degree. C. is outstandingly
reduced as compared with the molding load for forming at room
temperature, as well as a manufacturing method therefor, are
provided.
* * * * *