U.S. patent application number 15/126936 was filed with the patent office on 2017-04-06 for high strength cold rolled steel sheet and high strength galvanized steel sheet having excellent ductility and bendability, and methods for producing same.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). The applicant listed for this patent is Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). Invention is credited to Yuichi FUTAMURA, Koji KASUYA, Sae MIZUTA.
Application Number | 20170096723 15/126936 |
Document ID | / |
Family ID | 54144092 |
Filed Date | 2017-04-06 |
United States Patent
Application |
20170096723 |
Kind Code |
A1 |
KASUYA; Koji ; et
al. |
April 6, 2017 |
HIGH STRENGTH COLD ROLLED STEEL SHEET AND HIGH STRENGTH GALVANIZED
STEEL SHEET HAVING EXCELLENT DUCTILITY AND BENDABILITY, AND METHODS
FOR PRODUCING SAME
Abstract
Provided is a high strength cold rolled steel sheet having a
tensile strength of 980 MPa or higher and excellent ductility and
bendability. The high strength cold rolled steel sheet has a
specified component composition. The structures at a position of
1/4 sheet thickness in the steel sheet has certain area ratio of
ferrite relative to the entire structure, certain area ratio of a
mixed structure of fresh martensite and retained austenite relative
to the entire structure, certain region in which the concentration
of Mn is enriched, certain standard deviation of the fraction of a
region in which the concentration of Mn is enriched, and certain
concentration of Mn in a ferrite phase.
Inventors: |
KASUYA; Koji; (Kakogawa-shi,
JP) ; MIZUTA; Sae; (Kakogawa-shi, JP) ;
FUTAMURA; Yuichi; (Kakogawa-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) |
Kobe-shi, Hyogo |
|
JP |
|
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel, Ltd.)
Kobe-shi, Hyogo
JP
|
Family ID: |
54144092 |
Appl. No.: |
15/126936 |
Filed: |
December 25, 2014 |
PCT Filed: |
December 25, 2014 |
PCT NO: |
PCT/JP2014/084315 |
371 Date: |
September 16, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/02 20130101;
C22C 38/06 20130101; C22C 38/12 20130101; C22C 38/60 20130101; C22C
38/001 20130101; C22C 38/16 20130101; C23C 2/06 20130101; C22C
38/04 20130101; C21D 8/0236 20130101; C22C 38/28 20130101; C21D
2211/008 20130101; C22C 38/005 20130101; C21D 2211/005 20130101;
C21D 9/46 20130101; C21D 8/0278 20130101; C21D 8/0226 20130101;
C21D 8/0247 20130101; C22C 38/14 20130101; C22C 38/002 20130101;
C22C 38/22 20130101; C22C 38/08 20130101; C23C 2/28 20130101 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C22C 38/00 20060101
C22C038/00; C23C 2/06 20060101 C23C002/06; C22C 38/14 20060101
C22C038/14; C22C 38/28 20060101 C22C038/28; C22C 38/22 20060101
C22C038/22; C22C 38/08 20060101 C22C038/08; C23C 2/28 20060101
C23C002/28; C21D 8/02 20060101 C21D008/02; C22C 38/12 20060101
C22C038/12 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 17, 2014 |
JP |
2014-053399 |
Mar 17, 2014 |
JP |
2014-053400 |
Sep 22, 2014 |
JP |
2014-192757 |
Claims
1. A high strength cold rolled steel sheet having a tensile
strength of 980 MPa or higher and excellent ductility and
bendability, the steel sheet satisfying a component composition
comprising in percent by mass: C: 0.10% to 0,30%, Si: 1.2% to 3%;
Mn: 0.5% to 3.0%; P: more than 0% to 0.1% or less; S: more than 0%
to 0.05% or less; Al: 0.005% to 0.2%; N: more than 0% to 0.01% or
less; and O: more than 0% to 0.01% or less, the balance being iron
and inevitable impurities, and structures at a position of 1/4
sheet thickness in the steel sheet satisfying all of (1) to (5)
below: (1) the area ratio of ferrite relative to the entire
structure is from 5% or more to less than 50%, and the balance is a
hard phase, when observed under a scanning electron microscope; (2)
the area ratio of a mixed structure of fresh martensite and
retained austenite relative to the entire structure is more than 0%
to 30% or less, when observed under an optical microscope after
LePera etching; (3) a region in which the concentration of Mn is
enriched to 1.2 times or more the concentration of Mn in the steel
sheet is present at 5 area% or more, when analyzed with an electron
probe microanalyzer; (4) when the fraction of a region in which the
concentration of Mn is enriched to 1.2 times or more the
concentration of Mn in the steel sheet is measured in 2-.mu.m
square sections, a standard deviation in measurements for 100
sections is 4.0% or more; and (5) the concentration of Mn in a
ferrite phase is 0.90 times or less the concentration of Mn in the
steel sheet, when analyzed with an electron probe
microanalyzer.
2. The high strength cold rolled steel sheet according to claim 1,
wherein the volume ratio of retained austenite relative to the
entire structure is 5% or more, when measured by an X-ray
diffraction method.
3. The high strength cold rolled steel sheet according to claim 1,
wherein the hard phase comprises a mixed structure of fresh
martensite and retained austenite, and at least one structure
selected from the group consisting of bainitic ferrite, bainite,
and tempered martensite.
4. The high strength cold rolled steel sheet according to claim 1,
wherein the component composition further comprises one or more of
(A) to (E) below, in percent by mass, as another element: (A) at
least one selected from the group consisting of Cr: more than 0% to
1% or less and Mo: more than 0% to 1% or less; (B) at least one
selected from the group consisting of Ti: more than 0% to 0,15% or
less, Nb: more than 0% to 0.15% or less, and V: more than 0% to
0.15% or less; (C) at least one selected from the group consisting
of Cu: more than 0% to 1% or less and Ni: more than 0% to 1% or
less; (D) B: more than 0% to 0.005% or less; and (E) at least one
selected from the group consisting of Ca: more than 0% to 0.01% or
less, Mg: more than 0% to 0.01% or less, and REM: more than 0% to
0,01% or less.
5. A high strength electrogalvanized steel sheet in which an
electrogalvanized layer is formed on the surface of the high
strength cold rolled steel sheet according to claim 1.
6. A high strength galvanized steel sheet in which a galvanized
layer is formed on the surface of the high strength cold rolled
steel sheet according to claim 1.
7. A high strength galvannealed steel sheet in which a galvannealed
layer is formed on the surface of the high strength cold rolled
steel sheet according to claim 1.
8. A method for producing a high strength cold rolled steel sheet
having a tensile strength of 980 MPa or higher and excellent
ductility and bendability, the steel sheet being the high strength
cold rolled steel sheet according to claim 1, comprising: in hot
rolling a steel sheet having the component composition, coiling at
a coiling temperature of 500.degree. C. to 800.degree. C., then
holding for 3 hours or more at 500.degree. C. to 800.degree. C.,
then cooling to room temperature, soaking and holding in a
temperature range from (Ac.sub.1 point+20.degree. C.) to less than
Ac.sub.3 point after cold rolling, then cooling to a temperature
range of 500.degree. C. or less at an average cooling rate of
10.degree. C./s or more to 500.degree. C. and at an average cooling
rate of 10.degree. C./s or more at 500.degree. C. or less, then
reheating to a temperature range of 250.degree. C. to 500.degree.
C., holding for 30 seconds or more, and cooling to room
temperature.
9. A method for producing a high strength electrogalvanized steel
sheet, wherein the high strength cold rolled steel sheet obtained
by the producing method according to claim 8 is fluffier subjected
to electrogalvanizing.
10. A method for producing a high strength galvanized steel sheet
having a tensile strength of 980 MPa or higher and excellent
ductility and bendability, the steel sheet being the high strength
galvanized steel sheet according to claim 6. comprising: in hot
rolling a steel sheet having the component composition, coiling at
a coiling temperature of 500.degree. C. to 800.degree. C., then
holding for 3 hours or more at 500.degree. C. to 800.degree. C.,
then cooling to room temperature, soaking and holding in a
temperature range from (Ac.sub.1 point+20.degree. C.) to less than
Ac.sub.3 point after cold rolling, then cooling to a temperature
range of 500.degree. C. or less at an average cooling rate of
10.degree. C./s or more to 500.degree. C. and at an average cooling
rate of 10.degree. C./s or more at 500.degree. C. or less, then
reheating to a temperature range of 250.degree. C. to 500.degree.
C., holding for 30 seconds or more, galvanizing within the holding
period, and then cooling to room temperature.
11. The method for producing a high strength galvanized steel sheet
according to claim 10, wherein alloying in a temperature range from
450.degree. C. to 550.degree. C. is performed after the
galvanization.
12. A steel sheet comprising percent by mass: C: 0.10% to 0.30%,
Si: 1.2% to 3%; Mn: 0.5% to 3.0%; P: more than 0% to 0.1% or less;
S: more than 0% to 0.05% or less; Al: 0.005% to 0.2%; N: more than
0% to 0.01% or less; and O: more than 0% to 0.01% or less, the
balance being iron and inevitable impurities; wherein structures at
a position of 1/4 sheet thickness in the steel sheet satisfy (1) to
(5) below: (1) an area ratio of ferrite relative to the entire
structure is from 5% or more to less than 50%, and the balance is a
hard phase, when observed under a scanning electron microscope; (2)
an area ratio of a mixed structure of fresh martensite and retained
austenite relative to the entire structure is more than 0% to 30%
or less, when observed under an optical microscope after LePera
etching; (3) a region in which the concentration of Mn is enriched
to 1.2 times or more the concentration of Mn in the steel sheet is
present at 5 area% or more, when analyzed with an electron probe
microanalyzer; (4) when the fraction of a region in which the
concentration of Mn is enriched to 1.2 times or more the
concentration of Mn in the steel sheet is measured in 2-m square
sections, a standard deviation in measurements for 100 sections is
4.0% or more; and (5) a concentration of Mn in a ferrite phase is
0.90 times or less the concentration of Mn in the steel sheet, when
analyzed with an electron probe microanalyzer.
Description
TECHNICAL FIELD
[0001] The present invention relates to a high strength cold rolled
steel sheet and a high strength galvanized steel sheet having
excellent ductility and bendability and a method for producing
same. More specifically, the present invention relates to a high
strength cold rolled steel sheet, a high strength electrogalvanized
steel sheet, a high strength galvanized steel sheet, and a high
strength galvannealed steel sheet having excellent ductility and
bendability in a tensile strength range of 980 MPa or higher, and
to a method for producing same suitable for efficient production of
such steel sheets.
BACKGROUND ART
[0002] In order to improve fuel efficiency of automobiles and
transportation machines and the like, the weight of the automobiles
and transportation machines needs to be reduced. For example, the
weight can be effectively reduced by using high-strength steel
sheets and reducing the sheet thickness. However, where the
strength of steel sheets is increased, the ductility is decreased,
and therefore processability is degraded. Accordingly, excellent
bendability and ductility necessary for press molding are required
for high-strength steel sheets. Further, to ensure etching
resistance, steel sheets plated with zinc by electrogalvanizing
(EG), galvanizing (GI, Galvanized Iron), or gall/annealing (GA) are
often used for steel automotive parts. The electrogalvanized steel
sheet, galvanized steel sheet, and galvannealed steel sheet will be
sometimes referred to hereinbelow as "galvanized steel sheets".
Properties required for the galvanized steel sheets are the same as
those required for high-strength steel sheets.
[0003] Examples of ultrahigh-strength steel sheets having excellent
bendability which are suggested in Patent Literature 1 to 4
illustrate techniques for improving the processability of
high-strength steel sheets.
CITATION LIST
Patent Literature
[0004] Patent Literature 1: Japanese Unexamined Patent Publication
No. 2011-179030
[0005] Patent Literature 2: Japanese Patent No. 5299591
[0006] Patent Literature 3: Japanese Unexamined Patent Publication
No. 2011-225976
[0007] Patent Literature 4: WO 2012/036269
SUMMARY OF INVENTION
[0008] However, a steel sheet having excellent ductility and
bendability in a tensile strength range of 980 MPa or higher has
not yet been suggested.
[0009] The present invention has been created with the foregoing in
view,and it is an objective thereof to provide a high-strength
steel sheet and a high strength galvanized steel sheet having a
tensile strength of 980 MPa or higher and excellent ductility and
bendability and also a method suitable for production same with
good productivity, and more specifically to provide a high strength
cold rolled steel sheet, a high strength electrogalvanized steel
sheet, a high strength galvanized steel sheet, and a high strength
galvannealed steel sheet having excellent ductility and bendability
in a tensile strength range of 980 MPa or higher and a method for
manufacturing same suitable for efficient production of such steel
sheets.
[0010] The gist of the present invention which can resolve the
problems is that the component composition of a steel sheet
comprises, in percent by mass: C: 0.10% to 0.30%; Si: 1.2% to 3%;
Mn: 0,5% to 3.0%; P: more than 0% to 0.1% or less; S: more than 0%
to 0.05% or less; Al: 0.005% to 0.2%; N: more than 0% to 0.01% or
less; and O: more than 0% to 0.01% or less, the balance being iron
and inevitable impurities, and structures at a position of 1/4
sheet thickness in the steel sheet satisfy all of (1) to (5) below.
Hereinbelow, "%" in relation to the component composition of the
steel sheets means "% by mass".
[0011] (1) The area ratio of ferrite relative to the entire
structure is from 5% or more to less than 50% and the balance is a
hard phase, when observed under a scanning electron microscope.
[0012] (2) The area ratio of a mixed structure of fresh martensite
and retained austenite relative to the entire structure is more
than 0% to 30% or less, when observed under an optical microscope
after LePera etching.
[0013] (3) A region in which the concentration of Mn is enriched to
1.2 times or more the concentration of Mn in the steel sheet is
present at 5 area % or more, when analyzed with an electron probe
microanalyzer; and
[0014] (4) When the fraction of a region in which the concentration
of Mn is enriched to 1.2 times or more the concentration of Mn in
the steel sheet is measured in 2-.mu.m square sections, a standard
deviation in measurements for 100 sections is 4.0.degree. or
more
[0015] (5) The concentration of Mn in a ferrite phase is 0.90 times
or less the concentration of Mn in the steel sheet, when analyzed
with an electron probe microanalyzer.
[0016] In the preferred embodiment of the present invention, the
volume ratio of retained austenite relative to the entire structure
is 5.sup.0 or more, when measured with an X-ray diffraction
method.
[0017] Further, it is preferred that the hard phase be composed of:
the mixed structure of fresh martensite and retained austenite; and
at least one structure selected from the group consisting of
bainitic ferrite, bainite, and tempered martensi.
[0018] When the present invention is implemented, in the preferred
embodiment, at least any one of the following other elements is
further comprised: (A) at least one selected from the group
consisting of Cr: more than 0% to 1% or less and Mo: more than 0%
to 1% or less; (B) at least one selected from the group consisting
of Ti: more than 0% to 0.15% or less, Nb: more than 0% to 0.15% or
less; and V: more than 0% to 0.15% or less; (C) at least one
selected from the group consisting of Cu: more than 0% to 1% or
less and Ni: more than 0% to 1% or less; (D) B: more than 0% to
0.005% or less; and (E) at least one selected from the group
consisting of Ca: more than 0% to 0.01% or less, Mg: more than 0%
to 0.01% or less, and REM (Rare Earth Metal): more than 0% to 0.01%
or less.
[0019] The present invention is also inclusive of a high strength
electrogalvanized steel sheet in which an electrogalvanized layer
is formed on the surface of the high strength cold rolled steel
sheet; a high strength gair lized steel sheet in which a galvanized
layer is formed on the surface of the high strength cold rolled
steel sheet; and a high strength gal vannealed steel sheet in which
a galvannealed layer is formed on the s face of the high strength
cold rolled steel sheet.
[0020] The method for producing the high strength cold rolled steel
sheet in accordance with the present invention which can resolve
the problems includes: in a step of hot rolling a steel sheet
having the component composition, coiling at a coiling temperature
of 500.degree. C. to 800.degree. C., then holding for 3 hours or
more at 500.degree. C. to 800.degree. C., then cooling to room
temperature, soak ng d holding in a temperature range from
(Ac.sub.1 point+20.degree. C.) to less than Ac.sub.3 point after
cold rolling, then cooling to a temperature range of 500.degree. C.
or less at an average cooling rate of 10.degree. C./s or more to
500.degree. C. and at an average cooling rate of 10.degree. C./s or
more at 500.degree. C. or less, then reheating to a temperature
range of 250.degree. C. to 500.degree. C., holding for 30 seconds
or more, and cooling to room temperature.
[0021] In accordance with the present invention, the steel sheet
obtained by the producing method is preferably further subjected to
electrogalvanizing.
[0022] Further, a method for producing the high strength cold
rolled steel sheet in accordance with the present invention, which
can resolve the problems, includes: in a step of hot rolling a
steel sheet having the component composition, coiling at a coiling
temperature of 500.degree. C. to 800.degree. C., then holding for 3
hours or more at 500.degree. C. to 800.degree. C., then cooling to
room temperature, soaking and holding in a temperature range from
(Ac.sub.1 point+20.degree. C.) to less than Ac.sub.3 point after
cold rolling, then cooling to a temperature range of 500.degree. C.
or less at an average cooling rate of 10.degree. C./s or more to
500.degree. C. and at an average cooling rate of 10.degree. C./s or
more at 500.degree. C. or less, then reheating to a temperature
range of 250.degree. C. to 500.degree. C., holding for 30 seconds
or more, galvanizing within the holding period, and then cooling to
room temperature.
[0023] In accordance with the present invention, alloying in a
temperature range from 450.degree. C. to 550.degree. C. is
preferably performed after the galvanization.
[0024] The present invention can provide a high strength cold
rolled steel sheet and a high strength galvanized steel sheet
having excellent ductility and bendability even at 980 MPa or more,
and more particularly a high strength cold rolled steel sheet, a
high strength electrogalvanized steel sheet, a high strength
galvanized steel sheet, and a high strength galvannealed steel
sheet that excel in the properties. With the method producing same
in accordance with the present Tention, these steel sheets can be
produced with good efficiency. Therefore, the high strength cold
rolled steel sheet, etc., in accordance with the present invention
is particularly very useful in industrial fields, such as
automotive field.
BRIEF DESCRIPTION OF DRAWINGS
[0025] FIG. 1 is a conceptual diagram illustrating an example of a
heat treatment pattern in the producing method of the present
invention.
DESCRIPTION OF EMBODIMENTS
[0026] The inventors have conducted a comprehensive study to
improve ductility and bendability of high strength cold rolled
steel sheets and high strength galvanized steel sheets, in
particular, having a tensile strength of 980 MPa or higher.
[0027] The results have demonstrated that ductility and bendability
can be improved, while ensuring a high strength of 980 MPa or
higher by optimizing a ferrite phase and a hard phase in the metal
structure of the steel sheet and adequately controlling the
segregation of Mn, provided that the component composition is
adequately controlled.
[0028] The reason for specifying the metal structures in the
present invention is described below. A fraction measured by
microscopic observations means a fraction in 100% of the entire
structure of the steel sheet. As for the metal structure in the
present invention, the measurement method differs depending on the
metal structure. As a consequence, when the metal structures
specified by the present invention are all added up, 100% is
sometimes exceeded. This is because the retained y constituting the
mixed structure of fresh martensite and retained austenite is
measured not only by the optical microscope observations, but also
by X-ray diffraction. The retained austenite can be referred to
hereinbelow as "retained .gamma.", and the mixed structure of fresh
martensite and retained austenite can be referred to as "MA
(Martensite-Austenite Constituent) structure".
[0029] Area ratio of ferrite: 5% or more to less than 50%
[0030] Ferrite is a structure that increases ductility and
bendability of the steel sheet. In the present invention the
ductility and bendability in a high-strength region with a tensile
strength of 980 MPa or higher can be increased by increasing the
area fraction of ferrite. For such as effect to be demonstrated,
the area ratio of ferrite is 5% or more, preferably 7% or more,
more preferably 10% or more. However, where the amount of ferrite
is excessively high, the strength of the steel sheet decreases and
a high strength of 980 MPa or higher is difficult to ensure.
Therefore, the area ratio of ferrite is set to less than 50%,
preferably 45% or less, more preferably 40% or less. The area ratio
of ferrite is measured by scanning electron microscope (SEM)
observations at a position of 1/4 sheet thickness in the steel
sheet.
[0031] Hard Phase
[0032] The hard phase is a structure necessary for increasing the
tensile strength. In the present invention,a high strength of 980
MPa or higher can be achieved, while ensuring the presence of soft
ferrite within the area ratio range, by increasing the area
fraction of the hard phase. For this effect to be demonstrated, the
remaining metal structure other than ferrite needs to be a hard
phase. The hard phase, as referred to in the present invention, is
a phase harder than the ferrite and is at least one selected from
the group consisting of bainitic ferrite, bainite, tempered
martensite, and MA structure. In the present invention, as
indicated hereinbelow, at least the MA structure is included. Among
the hard phases, bainitic ferrite, bainite, and tempered martensite
are represented by values measured by SEM observations at a
position of 1/4 sheet thickness in the steel sheet. The retained
.gamma. is present between the laths of bainitic ferrite or
included in the MA structure.
[0033] Area ratio of MA structure relative to the entire structure:
more than 0% to 30% or less
[0034] Where the MA structure is present, the strength and
ductility can be increased. Therefore, from the standpoint of
increasing the strength-ductility balance, it is preferred that the
area ratio of the MA structure be 3% or more, more preferably 4% or
more. Meanwhile, where the area ratio of the MA structure becomes
too large, benda.bility is degraded. Accordingly, in the present
invention, the area ratio of the MA structure is set to 30% or
less, preferably 20% or less, more preferably 15% or less.
[0035] The fresh martensite constituting the MA structure, as
referred to herein, is a product of martensitic transformation of
non-transformed austenite in the process of cooling the steel sheet
form the heating temperature to room temperature and is
distinguished from the tempered martensite after the heating
treatment. In the present invention, a location which appears white
in optical microscope observations after LePera etching is taken as
the MA structure. Since the fresh martensite and retained -y are
difficult to distinguish from one another in optical microscope
observations, the composite structure of fresh martensite and
retained y is measured as the MA structure. The MA structure is
represented by a value measured by optical microscope observations
at a position of 1/4 sheet thickness in the steel sheet.
[0036] A region in which the concentration of Mn is enriched to 1.2
times or more the concentration of Mn in the steel sheet: 5 area %
or more, and the standard deviation of the area fractions of the
regions in which the concentration of Mn is enriched to 1.2 times
or more the concentration of Mn in the steel sheet in 2-.mu.m
squares: 4.0% or more
[0037] In the present invention, a region in which the
concentration of Mn is enriched is specified by using the
concentration of Mn obtained by analysis using an electron probe
microanalyzer (EPMA) with a beam diameter of 1 .mu.m or less in a
20 .mu.m.times.20 .mu.m range in the cross section of the steel
sheet. The "concentration of Mn in the steel sheet", as referred to
herein, is the concentration of Mn obtained by chemical analysis of
the base steel sheet by an inductively coupled plasma emission
spectroscopy. Therefore, the region in which the concentration of
Mn is enriched to 1.2 times or more the concentration of Mn in the
st sheet, as referred to herein, is a region in which the measured
value of the concentration of Mn obtained by the EPMA analysis is
1.2 times or more higher than the concentration of Mn in the base
steel sheet, the measurements being performed in a 20
.mu.m.times.20 .mu.m range.
[0038] The "2-.mu.m square", as referred to herein, is a
quadrangular section with a 2-.mu.m side. In the present invention,
the 2 .mu.m.times.20 .mu.m EPMA measurement region is divided into
100 sections, one section being a quadrangle with a 2-.mu.m side,
which are obtained by drawing vertical and horizontal lines at
2-.mu.m intervals, the area fraction of a region in which the
concentration of Mn is 1.2 times or more higher is measured in each
section, and the standard. deviation is determined, statistically
in 100 sections.
[0039] In the present invention, it has been found that bendability
is greatly increased when the region in which the concentration of
Mn is enriched to 1.2 times or more the concentration of Mn in the
steel sheet is present at 5 area% or more, and the standard
deviation obtained when the fraction of the region in which the
concentration of Mn is enriched to 1.2 times or more in 2-.mu.m
squares is 4.0% or more. The region in which the concentration of
Mn in the steel sheet is enriched to 1.2 times or more can be
referred to hereinbelow as the "region with the concentration of Mn
of 1.2 times or more", and the standard deviation obtained when
measuring the fraction of the region in which the concentration of
Mn is enriched to 1.2 times or more in 2-.mu.m squares can be
referred to hereinbelow as the "standard deviation of the regions
in which the concentration of Mn is enriched to 1.2 times or more"
or simply the "standard deviation".
[0040] Thus, the region with the concentration of Mn of 1.2 times
or more is mainly a hard phase. As the area ratio of the region
with the concentration of Mn of 1.2 times or more increases, the
concentration of Mn in the ferrite phase becomes relatively
smaller, the hardness of the ferrite phase can be decreased, and
the bendability can be increased. The larger is the segregation of
Mn, the greater is the standard deviation, but the concentration of
Mn in the ferrite phase, which contributes to the increase in
bendability, decreases and the hardness of the ferrite phase can be
reduced.
[0041] In order to obtain such an effect, the area ratio of the
region with the concentration of Mn of 1.2 times or more is set to
5.0 area % or more, preferably 5.2 area % or more, more preferably
5.5 area % or more. Meanwhile, where the ratio occupied by the
region with the concentration of Mn of 1.2 times or more is too
large, the Ms point of austenite decreases and the amount of the MA
structure can increase. Therefore, the area ratio of the region
with the concentration of Mn of 1.2 times or more is set preferably
to 20 area % or less, more preferably 15 area % or less.
[0042] Further, the standard deviation of the regions in which the
concentration of Mn is enriched to 1.2 times or more is set to 4.0%
or more, preferably 4.5% or more, more preferably 5.0% or more.
Where the standard deviation is less than 4.0%, the distribution of
Mn becomes insufficiently uniform and the decrease in hardness of
the ferrite phase which contributes to the increase in bendability
is insufficient. Meanwhile, no specific upper limit is set for the
standard deviation, and the standard deviation is preferably 10% or
less.
[0043] Where the region with the concentration of Mn of 1.2 times
or more is 5 area% or more and the standard deviation is 4.0% or
more, a spheroidized hard phase can be dispersed in the ferrite
phase, and the effect of increasing the strength of the steel
material can be combined with the effect of increasing the
bendability by the ferrite. The spheroidized hard phase is referred
to hereinbelow as "globular hard phase". Here, the globular hard
phase is part of the hard phase and is configured, similarly to the
hard phase, of bainitic ferrite, bainite, tempered martensite, MA
structure, or the like. It has been conventionally assumed that
where a difference in hardness between the hard phase and ferrite
phase is large, interface cracking occurs during bending and
bendability is degraded, but it has been found that the interface
cracking can be suppressed by the globular hard phase in the
ferrite phase. In order to obtain such an effect, a smaller amount
of the globular hard phase in the ferrite phase is preferred. The
aspect ratio of the globular hard phase is preferably set to 3 or
less, more preferably 2.5 or less, even more preferably 2 or less,
and the equivalent circle diameter of the globular phase is 2 .mu.m
or less, preferably 1.8 .mu.m or less, even more preferably 1.5
.mu.m or less. For the effect to be demonstrated, it is preferred
that the globular hard phase take 0.70 vol % or more, more
preferably 0.75 vol % or more, even more preferably 0.80 vol % or
more relative to the hard phase.
[0044] The concentration of Mn in ferrite is 0.90 times or less the
concentration of Mn in the steel sheet.
[0045] Where the concentration of Mn in the ferrite phase is too
high, the hardness of the ferrite phase is not sufficiently reduced
and the bendability is degraded. Therefore, the concentration of Mn
in the ferrite needs to he made lower than the concentration of Mn
in the steel sheet. As a consequence, the concentration of Mn in
the ferrite phase is set to 0.90 times or less, preferably 0.85
times or less, more preferably 0.80 times or less the concentration
of Mn in the steel sheet. Meanwhile, where the concentration of Mn
in the ferrite is made too low, the ferrite hardness decreases and
strength can be insufficient. For this reason, the concentration of
Mn in the ferrite phase is preferably set to 0.3 times or more,
more preferably to 0.4 times or more the concentration of Mn in the
steel sheet. The concentration of Mn in the ferrite phase can be
measured by EPMA.
[0046] Volume ratio of retained .gamma. relative to entire
structure: 5% or more
[0047] When the steel sheet is processed, the retained y receives
strains, deforms, and undergoes martensitic transformation, thereby
making it possible to ensure good ductility and demonstrating an
effect of enhancing hardening in the deformation zone during
processing and suppressing the concentration of strains. Therefore,
the retained austenite is a structure necessary for improving the
strength-ductility balance of the steel sheet. For such effects to
be advantageously demonstrated, the volume ratio of the retained
.gamma. is set preferably to 5% or more, more preferably 6% or
more, even more preferably 7% or more. No specific upper limit is
set for the volume ratio of the retained .gamma., but within the
ranges of the component composition and producing conditions of the
present invention, the volume ratio is, at the largest, 20% or
less. The ratio of the retained y is a measured value determined by
the X-ray diffraction at a position of 1/4 plate thickness in the
steel sheet.
[0048] The retained .gamma. is present between the laths of
bainitic ferrite and included in the MA structure. Since the effect
of the retained .gamma. is demonstrated regardless of the form of
presence thereof, in the present invention, the retained .gamma.
that can be verified during the measurements is taken as the
retained y regardless of the form of presence.
[0049] The component composition of the high-strength steel sheet
in accordance with the present invention will be explained
hereinbelow.
[0050] C: 0.10% to 0.15%
[0051] C is an element required to ensure the strength of the steel
sheet and increase the stability of the retained .gamma.. In order
to ensure the tensile strength of 980 MPa or higher, the amount of
C is set to 0.10% or more, preferably 0.12% or more, more
preferably 0.15% or more. However, where the amount of C is too
high, the strength after hot rolling rises, cracks occur during
cold rolling, and weldability of the final product decreases, For
this reason, the amount of C is set to 0.30% or less, preferably
0.26% or less, more preferably 0.23% or less.
[0052] Si: 1.2% to 3%
[0053] Si is a solid solution strengthening element and contributes
to the strengthening of steel. Silicon also effectively suppresses
the generation of carbides, effectively induces the generation of
the retained .gamma., and ensures excellent TS.times.EL balance.
For those effects to be advantageously demonstrated, the amount of
Si is set to 1.2% or more, preferably 1.35% or more, more
preferably 1,5% or more. However, where the amount of Si is too
high, scale is intensively formed during the hot rolling, scale
traces are transferred to the steel sheet surface, and the surface
state is degraded. Pickling ability is also degraded. For this
reason, the amount of Si is set to 3% or less, preferably 2.8% or
less, more preferably 2.6% or less.
[0054] Mn: 0.5% to 3.0%
[0055] Mn is an element that improves hardenability and contributes
to strengthening of steel sheets. Manganese is also effective in
stabilizing .gamma. and inducing the generation of retained
.gamma.. For these effects to be advantageously demonstrated, the
amount of Mn is set to 0.5% or more, preferably 0.6% or more, more
preferably 1.0% or more, even more preferably 1.5% or more, and
still more preferably to 2.0% or more. However, where the amount of
Mn is too high, the strength after hot rolling increases, cracks
appear during the cold rolling, and weldability of the final
product is degraded. Further, where Mn is added in excess, Mn
segregates and processability is degraded. For this reason, the
amount of Mn is set to 3.0% or less, preferably 2.8% or less, more
preferably 2.6% or less.
[0056] P: more than 0% to 0.1% or less
[0057] P is an element that is contained unavoidably and degrades
the weldability of steel sheets. For this reason, the amount of P
is set to 0.1% or less, preferably 0.08% or less, more preferably
0.05% or less. Since it is desirable that the amount of P be made
as small as possible, the lower limit thereof is not particularly
specified, but the industrially attainable lower limit is
0.0005%.
[0058] S: more than 0% to 0.05% or less
[0059] Similarly to P. S is also an element that is contained
unavoidably and degrades the weldability of steel sheets. S also
generates sulfur-containing inclusions in steel sheets and degrades
the processability of steel sheet. For this reason, the amount of S
is set to 0.05% or less, preferably 0.01% or less, more preferably
0.005% or less. Since it is desirable that the amount of S be made
as small as possible, the lower limit thereof is not particularly
specified, but the industrially attainable lower limit is
0.0001%.
[0060] Al: 0.005% to 0.2%
[0061] Al is an element acting as a deoxidizing agent. For the
effect thereof to be advantageously demonstrated, the amount of Al
is set to 0.005% or more, preferably 0.01% or more. However, where
the amount of Al is too high, the weld.ability of steel sheets is
greatly degraded. For this reason, the amount of Al is set to 0.2%
or less, preferably 0.15% or less, more preferably 0.10% or
less.
[0062] N: more than 0% to 0.01% or less
[0063] N is an element that is contained unavoidably, causes the
precipitation of nitrides in steel sheets, and contributes to the
strengthening of steel sheets. From this standpoint, it is
preferred that the amount of N be 0.001% or more. However, where
the amount of N is too high, a large amount of nitrides
precipitate, thereby causing the degradation of elongation,
stretch-flangeability .lamda., and bendability. Therefore, the
amount of N is set to 0.01% or less, preferably 0.008% or less,
more preferably 0.005% or less.
[0064] O: more than 0% to 0.01% or less
[0065] O is an element that is contained unavoidably, and where the
amount thereof is too high, ductility and bendability during the
processing are deQraded. For this reason, the amount of O is set to
0.01% or less, preferably 0.005% or less, more preferably 0.003% or
less. Since it is desirable that the amount of O be made as small
as possible, the lower limit thereof is not particularly specified,
but the industrially attainable lower limit is 0.0001%.
[0066] Other components
[0067] The steel sheet in accordance with the present invention has
the component composition, the balance being iron and inevitable
impurities. The inevitable impurities include the P, S, N, and O
which can be included in steel for the reasons associated with
starting materials, resources, production facilities, and the like,
and also tramp elements such as Pb, Bi, Sb, and Sn. The following
elements can he also actively includes as other elements within
ranges in which the effects of the present invention are not
adversely affected.
[0068] The steel sheet in accordance with the present invention may
further comprise the following other elements:
[0069] (A) at least one selected from the group consisting of Cr:
more than 0% to 1% or less and Mo: more than 0% to 1% or less;
[0070] (B) at least one selected from the group consisting of Ti:
more than 0% to 0.15% or less, Nb: more than 0% to 0.15% or less,
and V: more than 0% to 0,15% or less
[0071] (C) at least one selected from the group consisting of Cu:
more than 0% to 1% or less and Ni: more than 0% to 1% or less;
[0072] (D) B: more than 0% to 0.005% or less; and
[0073] (E) at least one selected from the group consisting of Ca:
more than 0% to 0.01% or less, Mg: more than 0% to 0.01% or less,
and REM: more than 0% to 0.01% or less. These elements (A) to (E)
can be included individually or in appropriate combinations
thereof. The reasons for setting such ranges are described
below
[0074] (A) At least one selected from the group consisting of Cr:
more than 0% to 1% or less and Mo: more than 0% to 1% or less
[0075] Cr and Mo are elements that effectively increase quenching
ability and increase the strength of steel sheets. These elements
can be used individually or together. For such effects to be
advantageously demonstrated,the contents of Cr and Mo are set
preferably to 0.1% or more, more preferably 0.3% or more,
respectivelyr. However, where the account of these elements is too
high, the processability is degraded and cost is increased. For
this reason, when Cr and Mo are used individually, the amount
thereof is set preferably to 1% or less, more preferably 0.8% or
less, even more preferably 0.5% or less. When Cr and Mo are used
together, each is included within a range with the upper limit, and
the total amount thereof is preferably 1.5% or less.
[0076] (B) At least one selected from the group consisting of Ti:
more than 0% to 0.15% or less, Nb: more than 0% to 0,15% or less,
and V: more than 0% to 0.15% or less
[0077] Ti, Nb, and V are elements that effectively form carbide or
nitride precipitates in steel sheets, increase the strength of
steel sheets, and refine old y grains. These elements can be used
individually or together. For these effects to be advantageously
demonstrated, the amount of each of Ti, Nb, and V is set preferably
to 0.005% or more, more preferably 0.010% or more. However where
the amount of the elements is too high, carbides precipitate on
grain boundaries and stretch-flangeability and bendability of steel
sheets are degraded. Therefore, the amount of each of Ti, Nb, and V
is set preferably to 0.15% or less, more preferably 0.12% or less,
even more preferably 0.10% or less.
[0078] (C) At least one selected from the group consisting of Cu:
more than 0% to 1% or less and Ni: more than 0% to 1% or less
[0079] Cu and Ni are elements effective in generating and
stabilizing retained austenite and also effective in increasing the
etching resistance. These elements can be used individually or
together. For these effects to be advantageously demonstrated, the
amount of each of Cu and Ni is set preferably to 0.05% or more,
more preferably 0.10% or more. However, where Cu is comprised in
excess, hot processability is degraded. For this reason, where Cu
is added individually, the amount thereof is set preferably to 1%
or less, more preferably 0.8% or less, more preferably 0.5% or
less, Where Ni is comprised in excess, cost is increased. For this
reason, the amount of Ni is set preferably to 1% or less, more
preferably 0.8% or less, even more preferably 0.5% or less. Where
Cu and Ni are comprised together, the effects are easily
demonstrated, Furthermore, the addition of Ni suppresses the
degradation of hot processability caused by the addition of Cu.
Therefore, when Cu and Ni are used together, the combined amount
thereof is set preferably to 1.5% or less, more preferably 1.0% or
less.
[0080] (D) B: more than 0% to 0,005% or less
[0081] B is an element increasing quenching ability and effective
in causing austenite to be stably present at room temperature. For
these effects to be advantageously demonstrated, the amount of B is
set preferably to 0.0005% or more, more preferably 0.0010% or more,
even more preferably 0.0015% or more. However, where boron is
comprised in excess, borides are generated and ductility is
degraded. For this reason, the amount of B is set preferably to
0.005% or less, more preferably 0.004% or less, even more
preferably 0.0035% or less.
[0082] (E) At least one selected from the group consistina of Ca:
more than 0% to 0.01% or less, Mg: more than 0% to 0.01% or less,
and REM: more than 0% to 0.01% or less
[0083] Ca, Mg, and REM are elements effectively refinina and
dispersing inclusions in steel sheets. These elements may be used
individually or in appropriate combinations of two or more thereof.
For the effects thereof to be advantageously demonstrated, the
amount of each of Ca, Mg and REM is set preferably to 0.0005% or
more preferably to 0.0010% or more. However, where these elements
are comprised in excess, castability, hot processability, or the
like are degraded. For this reason, the amount of eaCh of Ca, Mg
and REM is set preferably to 0.01% or less, more preferably 0.008%
or less, even more preferably 0.007% or less.
[0084] In the present invention, REM is an abbreviation of rare
earth metals and is intended to include lanthanoids, that is 15
elements from La to Lu, and also scandium and yttrium.
[0085] Methods for producing the high strength cold rolled steel
sheet, high strength electrogalvanized stud sheet, high strength
galvanized stud sheet, and high strength gal vannealed steel sheet
in accordance with the present invention, in particular methods for
producing the high strength cold rolled steel sheet and high
strength galvanized steel sheet will be explained hereinbelow. In
the methods for producing the high strength cold rolled steel sheet
and high strength galvanized steel sheet, "in a step of hot rolling
a steel sheet having the component composition, the steps of
coiling at a coiling temperature of 500.degree. C. to 800.degree.
C., then holding for 3 h or more at 500.degree. C. to 800.degree.
C., then cooling to room temperature, soak ng uud holding in a
temperature range from (Ac.sub.1 point+20.degree. C.) to less than
Ac.sub.3 point after the cooling, then cooling to a temperature
range of 500.degree. C. or less at an average cooling rate of
10.degree. C./s or more to 500.degree. C. and at an average cooling
rate of 10.degree. C./s or more at 500.degree. C. or less" are the
same Therefore, the explanation of these steps refers to both
methods. The step of reheating after the "cooling to a temperature
range of 500.degree. C. or less" differs among the two methods.
Therefore, this step is explained separately for the high strength
cold rolled steel sheet and high strength galvanized steel
sheet.
[0086] In the methods for producing the high strength cold rolled
steel sheet and high strength galvanized steel sheet in accordance
with the present invention, the below-described annealing is
performed with respect to steel sheets obtained by hot rolling and
cold rolling of the steel having the component composition. In the
method for producing the high strength cold rolled steel sheet,
reheating is performed after the annealing. If necessary, a high
strength electrogalvanized steel sheet can be obtained by
appropriately combining electrogalvanization. In the method for
producing the high strength galvanized steel sheet, galvanization
is performed together with reheating after the annealing. If
necessary, a high-strength galvannealed steel sheet can be obtained
by appropriately combining annealing processing. In accordance with
the present invention, a high strength cold rolled steel sheet or a
high strength galvanized steel sheet and the like having the
desired structure can be obtained by appropriately controlling the
producing conditions.
[0087] For example, hot rolling is performed, as depicted in FIG.
1, by the usual method by using the steel having the component
composition. In the hot rolling, for example, the steel is hot
rolled such that the finish rolling temperature is Ac.sub.3 point
or higher, and then coiling is performed at a coiling temperature
of 500.degree. C. to 800.degree. C., Then, the steel is held for 3
h or more at 500.degree. C. to 800.degree. C., then cooled to room
temperature and cold rolled. The operational upper limit for
cooling after the finish rolling is about 500.degree. C./s.
[0088] After the cold rolling, the steel is annealed by soaking and
holding in a two-phase temperature range from (Ac.sub.1
point+20.degree. C.) to less than Ac.sub.3 point and then cooling
to a temperature range of 500.degree. C. or less at an average
cooling rate of 10.degree. C./s or more to 500.degree. C. and at an
average cooling rate of 10.degree. C./s or more at 500.degree. C.
or less. The method for producing a high strength cold rolled steel
sheet further includes a step of reheating to a temperature range
of 250.degree. C. to 500.degree. C., holding for 30 s or more, and
then cooling to room temperature. The method for producing a high
strength galvanized steel sheet further includes a step of
reheating to a temperature range of 250.degree. C. to 500.degree.
C. after cooling to the temperature range of 500.degree. C. or
less, holding for 30 s or more, galvanizing within the holding
period, and then cooling to room temperature.
[0089] The reasons for specifying the conditions are described in
details hereinbelow.
[0090] Coiling at a coiling temperature of 500.degree. C. to
800.degree. C., then holding for 3 h or more at 500.degree. C. to
800.degree. C., then cooling to room temperature
[0091] As a result of coiling at a coiling temperature of
500.degree. C. to 800.degree. C. after the hot rolling and then
holding for 3 h or more at 500.degree. C. to 800.degree. C., the
predetermined Mn concentration distribution is generated,
Mn-containing carbides are precipitated, and a hard phase
spheroidized in the ferrite phase by annealing after the cold
rolling is obtained. To obtain such effects, the coiling
temperature is set to 500.degree. C. or more, preferably
550.degree. C. or more, more preferably 600.degree. C. or more.
However, where the coiling temperature is too high, a large amount
of scale or grain boundary oxidation appear in the steel sheet and
pickling ability is degraded. For this reason, the coiling
temperature is set to 800.degree. C. or less, preferably
750.degree. C. or less, more preferably 700.degree. C. or less.
Further, the temperature range of holding after the coiling is set
to 500.degree. C. or more, preferably 510.degree. C. or more, more
preferably 520.degree. C. or more, even more preferably 550.degree.
C. or more, and still more preferably 580.degree. C. or more.
Meanwhile, where the holding temperature is too high, a large
amount of scale, grain boundary oxidation, or the like, can appear
in the steel sheet and pickling ability can be degraded in the same
manner as when the coiling temperature is too high. For this
reason, the holding temperature is set to 800.degree. C. or less,
preferably 780.degree. C. or less, even more preferably 750.degree.
C. or less, still more preferably 700.degree. C. or less. The
holding time in this temperature range is 3 h or more, preferably 4
h or more, more preferably 5 h or more, even more preferably 7 h or
more, and still more preferably 10 h or more Meanwhile, where the
holding time is too long, a large amount of scale, grain boundary
oxidation, or the like, can appear in the steel sheet and pickling
ability can be degraded in the same manner as when the coiling
temperature is too high. For this reason, the holding time is set
preferably to 72 h or less, more preferably 60 h or less.
[0092] In the present invention, holding at a predetermined
temperature it the concept such that the holding is not necessarily
continued at the same temperature and the temperature may change,
provided it is within a predetermined temperature range. For
example, holding may be isothermal holding within a range of the
holding temperature and is inclusive of variations within this
range, that is, a decrease in temperature, increase in temperature
caused by heating, and increase in temperature caused by
recuperation associated with transformation.
[0093] In the present invention, cooling to room temperature is
performed after holding for the preset time in the temperature
range, but the cooling rate in this step is not particularly
limited. For example, air cooling may be performed.
[0094] Pickling, cold rolling
[0095] After the hot rolling, optional pickling is performed and
then cold rolling is performed at a cold rolling draft of about 30%
to 80%.
[0096] Annealing
[0097] In the annealing step after the cold rolling, soaking and
holding are performed in a two-phase region in a temperature range
from (Ac.sub.1 point+20.degree. C.) to less than Ac.sub.3 point,
and then cooling is performed to a temperature range of 500.degree.
C. or less by cooling at an average cooling rate of 10.degree. C./s
or more in a temperature range to 500.degree. C. and then cooling
at an average cooling rate of 10.degree. C./s or more in a
temperature range from 500.degree. C. or less.
[0098] As a result of soaking and holding in a two-phase region in
a temperature range from (Ac.sub.1 point+20.degree. C.) to less
than Ac.sub.3 point, it is possible to ensure the desirable amount
of ferrite, while maintaining, the Mn concentration distribution in
accordance with the present invention. Where the holding and
soaking temperature is lower than Ac.sub.1 point+20.degree. C., the
amount of ferrite in the metal structure of the finally obtained
steel sheet is too large and sufficient strength cannot be ensured.
For this reason, the holding and soaking temperature is set equal
to or higher than Ac.sub.1 point+20.degree. C., preferably equal to
or higher than Ac.sub.1 point+25.degree. C., more preferably equal
to or higher than Ac.sub.1 point+50.degree. C., and even more
preferably equal to or higher than Ac.sub.1 point+80.degree. C.
Meanwhile where the Ac.sub.3 point is reached or exceeded, ferrite
cannot be sufficiently generated and grown during soaking and
holding, the ductility decreases, the Mn concentration distribution
becomes even, and the amount of the globular hard phase generated
in the ferrite phase decreases. For this reason the holding and
soaking temperature is set less than Ac.sub.3 point, preferably
equal to or less than Ac.sub.3 point-5.degree. C., more preferably
equal to or less than Ac.sub.3 point-10.degree. C., and even more
preferably equal to or less than Ac.sub.3 point-20.degree. C.
[0099] The average temperature increase rate when the temperature
rises in the soaking and holding temperature range is not
particularly limited and can be selected as appropriate. For
example, the average temperature increase rate may be about
0.5.degree. C./s to 50.degree. C./s.
[0100] In the present invention, the holding time in the soaking
and holding temperature range is not particularly limited. However,
where the holding time is too short, the processed structure
remains and the ductility of steel can decrease. For this reason,
the holding time is set preferably to 40 s or more, more preferably
to 60 s or more. Meanwhile, where the holding time is too long, the
enrichment of the austenite phase with Mn is advanced, the Ms point
decreases, and the amount of MA structure can be increased. For
this reason, the holding time is set preferably to 3600 s or less,
more preferably 3000 s or less.
[0101] As mentioned hereinabove, in the present invention, holding
at a predetermined temperature it the concept such that the holding
is not necessarily continued at the same temperature and the
temperature may change, provided it is within a predetermined
temperature range. For example, when holding is performed at the
soaking and holding temperature, it may be isothermal holding
within a range from a temperature equal to or higher than
Ac.sub.1+20.degree. C. to less than Ac.sub.3, and the holding
temperature may vary within this range.
[0102] The Ac.sub.1 point and Ac.sub.3 points can be calculated
from formulas (a) and (b) below which are described in in "The
Physical Metallurgy of Steels", published by Maruzen on May 31,
1985, p. 273. In the formulas, the square parentheses represent the
amount of each element in mass %. The amount of elements which are
not contained in the steel sheet may be calculated as 0 mass %.
Ac.sub.1 (.degree.
C.)=723-10.7.times.[Mn]-16.9.times.[Ni]+29.1.times.[Si]+16.9.times.[Cr]+2-
90.times.[As]+6.38.times.[W] (a)
Ac.sub.3 (.degree. C.)=910-203.times.
[C]-15.2.times.[Ni]+44.7.times.[Si]+10.sup.4.times.[V]+31.5.times.[Mo]13.-
1.times.[W](30.times.[Mn]-11.times.[Cr]-20.times.[Cu]+700.times.[P]+400.ti-
mes.[Al]+120.times.[As]+400.times.[Ti]) (b)
[0103] After the soaking and holding, cooling is performed in a
temperature range to 500.degree. C. at an average cooling rate of
10.degree. C./s or more. By controlling the cooling rate from the
soaking and cooling temperature, it is possible to suppress the
generation of ferrite with a high concentration of Mn and reduce
the amount of generated ferrite. Where the average cooling rate is
low, ferrite with a high concentration of Mn is generated during
the cooling, the bendability can be degraded, and the strength can
be reduced. For this reason, the average cooling rate is set to
10.degree. C./s or more, preferably 15.degree. C./s or more, more
preferably 20.degree. C./s or more. No upper limit is set for the
average cooling rate, and cooling may be performed with water or
oil.
[0104] After the cooling at the average cooling rate in a
temperature range to 500.degree. C., cooling is performed at
500.degree. C. or less at an average cooling rate of 10.degree.
C./s or more. By setting the average cooling rate at 500.degree. C.
or less to 10.degree. C./s or more, it is possible to suppress the
generation of soft high-temperature bainite, suppress the
auto-tempering martensite, and increase the strength. For such
effects to be obtained, the average cooling rate at 500.degree. C.
or less is set to 10.degree. C./s or more, preferably 15.degree.
C./s or more, more preferably 20.degree. C./s or more. No upper
limit is set for the average cooling rate, and cooling may be
performed with water or oil.
[0105] The average cooling rate to 500.degree. C. and the average
cooling rate at 500.degree. C. or less may be the same or different
and may be adjusted, as appropriate, within the ranges.
[0106] The cooling stop temperature when the cooling is performed
at an average cooling rate of 10.degree. C./s or more at
500.degree. C. or less is in a temperature range of 500.degree. C.
or less. Where the cooling stop temperature is higher than
500.degree. C., the amount of the hard phase decreases, the
strength cannot be ensured, and the amount of the MA structure
increases, thereby degrading the bendability. For this reason, the
cooling stop temperature is set to 500.degree. C. or less,
preferably 400.degree. C. or less, more preferably 350.degree. C.
or less, even more preferably 300.degree. C. or less. No lower
limit is set for the cooling stop temperature, and it can be as low
as room temperature to simplify the operations.
[0107] The reheating step will be explained hereinbelow separately
for the method for producing a cold-rolled steel sheet and the
method for producing a galvanized steel sheet.
[0108] Reheating in the method for producing a cold-rolled steel
sheet
[0109] After the cooling in a temperature range of 500.degree. C.
or less has been stopped, reheating is performed to a temperature
range of 250.degree. C. to 500.degree. C., followed by holding for
30 s or more, and cooling to room temperature.
[0110] By reheating to a temperature range of 250.degree. C. to
500.degree. C. and holding for 30 s or more after the cooling stop,
it is possible to temper the hard phase such as martensite and
cause the transformation of the non-transformed austenite. When the
reheating is not performed or the holding temperature is too low,
the tempering of the hard phase does not advance, high-density
dislocations are generated, a large amount of the MA structure
remains, and the bendability is degraded. For this reason, the
reheating temperature is set to 250.degree. C. or more, preferably
300.degree. C. or more, more preferably 350.degree. C. or more.
Meanwhile, where the holding temperature is too high, the strength
decreases. For this reason, the reheating temperature is set to
500.degree. C. or less, preferably 470.degree. C. or less, more
preferably 450.degree. C. or less. In the present invention, the
"reheating" is literary the heating, that is, increase in
temperature, from the cooling stop temperature to 500.degree. C. or
less. Therefore, the reheating temperature is a temperature higher
than the cooling stop temperature, and even in a temperature range
of 250.degree. C. to 500.degree. C., the isothermal holding in
which the cooling stop temperature is equal to the reheating
temperature, or the process of cooling from the cooling stop
temperature to an even lower temperature is not included in the
reheating.
[0111] Further, where the holding time in the reheating temperature
range is too short, the hard phase cannot be sufficiently tempered,
and the non-transformed austenite cannot he transformed. For this
reason, the holding time is set to 30 s or more, preferably 50 s or
more, more preferably 100 s or more, even more preferably 200 s or
more. Meanwhile, no upper limit is set for the holding time, but
where the holding time is too long, the productivity decreases. In
addition, the strength also decreases. For this reason, the holding
time is set preferably to 1500 s or less, more preferably 1000 s or
less.
[0112] After holding for the predetermined time in the reheating
temperature range, cooling is performed to room temperature. The
average cooling rate in this case is not particularly limited, and
the cooling may he performed at an average cooling rate of
preferably 0.1.degree. C./s or more, more preferably 0.4.degree.
C./s or more, and preferably 200.degree. C./s or less, more
preferably 150.degree. C./s or less.
[0113] In accordance with the present invention, an
electrogalvanized layer may be formed on the surface of the
obtained steel sheet.
[0114] A method for forming the electrogalvanized layer is not
particularly limited, and the usual electrogalvanizing method can
be used. For example, when an electrogalvanized steel sheet is
produced, the electrogalvanizing treatment can be performed by
immersing in a zinc solution at 55.degree. C. and allowing an
electric current to flow in the solution. The amount plated on one
side is not particularly limited and can be about 10 g/m.sup.2 to
100 g/m.sup.2 when an electrogalvanized steel sheet is
produced.
[0115] Reheating in the method for producing a galvanized steel
sheet
[0116] After the cooling in a temperature range of 500.degree. C.
or less has been stopped, reheating is performed to a temperature
range of 250.degree. C. to 500.degree. C., followed by holding for
30 s or more, galvanizing in the holding time, and then cooling to
room temperature.
[0117] By reheating to a temperature range of 250.degree. C. to
500.degree. C. and after the cooling stop and holding for 30 s or
more, it is possible to temper the hard phase such as martensite
and cause the transformation of the non-transformed austenite. When
the reheating is not performed or the holding temperature is too
low, the tempering of the hard phase does not advance, high-density
dislocations are generated, a large amount of the MA structure
remains, and the bendability is degraded. For this reason, the
reheating temperature is set to 250.degree. C. or more, preferably
300.degree. C. or more, more preferably 350.degree. C. or more.
Meanwhile, where the holding temperature is too high, the strength
decreases. For this reason, the reheating temperature is set to
500.degree. C. or less, preferably 470.degree. C. or less, more
preferably 450.degree. C. or less. In the present invention, the
"reheating" is literary the heating, that is, increase in
temperature, from the cooling stop temperature to 500.degree. C. or
less. Therefore, the reheating temperature is a temperature higher
than the cooling stop temperature, and even in a temperature range
of range of 250.degree. C. to 500.degree. C., the isothermal
holding in which the cooling stop temperature is equal to the
reheating temperature, or the process of cooling from the cooling
stop temperature to an even lower temperature is not included in
the reheating.
[0118] Further, where the holding time in the reheating temperature
range is too short, the hard phase cannot be sufficiently tempered,
and the non-transformed austenite cannot be transformed. For this
reason, the holding time is set to 30 s or more, preferably 50 s or
more, more preferably 100 s or more, even more preferably 200 s or
more. Meanwhile, no upper limit is set for the holding time, but
where the holding time is too long, the productivity decreases. In
addition, the strength also decreases. For this reason, the holding
time is set preferably to I50( )s or less, more preferably 1000 s
or less.
[0119] In the present invention a galvanized layer is formed on the
steel sheet surface by galvanization in the holding time of 30 s or
more in the reheating temperature range. In the present invention,
the galvanization and holding in the reheating temperature range
can be performed together. Thus, in order to control as
appropriate, for example, the metal structure or strength by
reheating, it is necessary that galvanization be performed within
the holding time period in the reheating time range. A method for
forming the galvanized layer is not particularly limited and the
usual galvanization method can be used. For example, the
galvanization may be performed by dipping the steel sheet in a
plating bath which has been adjusted to the reheating temperature
range. The plating time may be the holding time and may be
adjusted, as appropriate, such as to ensure the desired amount of
plated metal. For example, the plating time is preferably 1 s to 10
s.
[0120] The galvanization and heating without the plating process
can be combined in the reheating process according to the following
patterns:
[0121] (i) only heating is performed and then galvanization is
performed;
[0122] (ii) galvanization is performed and then heating is
performed; and
[0123] (iii) the treatment is performed in the order of: only
heating, galvanization, only heating.
[0124] Where the reheating temperature in the case in which only
heating is performed is different from the galvanization
temperature, that is, the plating bath temperature, heating or
cooling from one temperature to the other temperature can be
included. Examples of the heating method include furnace heating
and induction heating.
[0125] When the galvannealed layer is formed on the steel sheet
surface, alloying may be performed after the galvanization. The
alloying temperature is not particularly limited, but where the
alloying temperature is too low, the alloying does not sufficiently
advance. For this reason, the alloying temperature is preferably
450.degree. C. or more, more preferably 460.degree. C. or more, and
even more preferably 480.degree. C. or more. Meanwhile, where the
alloying temperature is too high, the alloying advances too much,
the concentration of Fe in the plated layer increases, and the
adhesion of the plated layer is degraded. For this reason, the
alloying temperature is preferably 550.degree. C. or less, more
preferably 540.degree. C. or less, and even more preferably
530.degree. C. or less. The time of the alloying treatment is not
particularly limited and may be adjusted to obtain the desired
alloying. The alloying treatment time is preferably from 10 s to 60
s. Since the alloying treatment is performed after holding for the
predetermined time in the reheating temperature range, the alloying
treatment time is not included in the time of holding in the
reheating temperature range.
[0126] After holding for the predetermined time in the reheating
temperature range, cooling is performed to room temperature. The
average cooling rate in this case is not particularly limited, and
the cooling may be performed at an average cooling rate of
preferably 0.1.degree. C./s or more, more preferably 0,4.degree.
C./s or more, and preferably 200.degree. C./s or less, more
preferably 150.degree. C./s or less.
[0127] The technique of the present invention can be particularly
advantageously applied to thin steel sheets with a thickness of 6
mm or less.
[0128] The objects of the high strength cold rolled steel sheet and
high strength galvanized steel sheet in accordance with the present
invention are steel sheets with a tensile strength of 980 MPa or
more, preferably 1000 MPa or more, and more preferably 1010 MPa or
more. The ductility is represented by the balance of strength and
ductility, that is, by "(tensile strength in MPA
units).times.(ductility in % units)", and is set preferably to
15,000 MPa-% or more, more preferably to 15,100 MPa-% or more, even
more preferably to 15,200 MPa-% or more. The bendability is
represented by the balance of strength and VDA. (V rband der
Automobilindustrie) bending angle determined by the method
disclosed in the below-described examples, that is, by "(tensile
strength in MPA units).times.(VDA bending angle in degree units)",
and is set preferably to 100,000 MPa-.degree. or more, more
preferably to 100,500 MPa-.degree. or more, even more preferably to
101,000 MPa-.degree. or more.
[0129] This application claims the benefit of priority to Japanese
Patent Application No. 2014-053399 filed on Mar. 17, 2014, Japanese
Patent Application No. 2014-053400 filed on Mar. 17, 2014, and
Japanese Patent Application No. 2014-192757 filed on Sep. 22, 2014.
The entire contents of Japanese Patent Application No. 2014-053399
filed on Mar. 17, 2014, the contents of Japanese Patent Application
No. 2014-053400 filed on Mar. 17, 2014, and the entire contents of
Japanese Patent Application No. 2014-192757 filed on Sep. 22, 2014
are incorporated herein by reference.
EXAMPLES
[0130] The present invention will be explained hereinbelow in
greater detail by the examples thereof, but the present invention
is not limited to the examples and can be practiced with
modifications adaptable to the purposes described above and below,
and all those modification are also included in the technical scope
of the present invention.
Example 1
[0131] The steels with the component compositions presented in
Table 1 were melted and cold-rolled steel sheets were produced by
hot rolling.fwdarw.cold rolling.fwdarw.continuous annealing undo
the below-described conditions. In the steels with the component
compositions presented in Table 1, the balance is iron and
inevitable impurities. The empty column means that the element was
not added,
[0132] Hot rolling
[0133] A slab was heated to 1250.degree. C. and hot rolled to a
sheet thickness of 2.3 mm at a draft ratio of 90% and a finish
rolling temperature of 920.degree. C. Then, the steel sheet was
cooled to the "Coiling temperature (.degree. C.)" shown in Table 2
or Table 3 at an average cooling rate of 30.degree. C./s from this
temperature, coiled, and then held under the conditions of the
"Holding temperature 1 (.degree. C.)" and "Holding time (h)" shown
in Table 2, or the "Holding start temperature (.degree. C.)",
"Holding end temperature (.degree. C.)", and "Holding time (h)"
shown in Table 3, A hot-rolled steel sheet was then produced by air
cooling to room temperature.
[0134] Cold Rolling
[0135] The obtained hot-rolled steel sheet was pickled to remove
the surface scale, and then cold rolling was performed to produce a
cold-rolled steel sheet with a thickness of1:2 mm.
[0136] Annealing of Cold-Rolled Steel Sheet
[0137] A sample steel was produced by
soaking/holding.fwdarw.cooling.fwdarw.reheating the obtained
cold-rolled steel sheet under the conditions shown in Table 2 and
Table 3. No. 32 in Table 2 is a comparative example in which no
reheating was performed. Instead of reheating the steel sheet was
cooled from the cooling stop temperature of 480.degree. C. to
350.degree. C. and then held for 300 s at this temperature. This
process is indicated in the reheating column.
[0138] In the tables, the soaking and holding temperature is
represented by "Soaking temperature (.degree. C.)", the average
cooling rate to 500.degree. C. after the soaking is represented by
"Average cooling rate 1 (.degree. C./s)", the cooling rate at
500.degree. C. or less is represented by "Average cooling rate 2
(.degree. C./s), the cooling stop temperature is represented by
"Cooling stop temperature (.degree. C.)", the holding temperature
during reheating after the cooling stop is represented by
"Reheating holding temperature (.degree. C.)", and the holding time
at the holding temperature is represented by "Reheating holding
time (s)". In the present example, the holding time at the soaking
and holding temperature was set to 100 s to 600 s. After the
"Reheating holding time (s)" at the "Reheating holding temperature
(.degree. C.)", the steel sheet was allowed to cool to room
temperature obtain a sample steel. The cold-rolled steel sheets
which were not subjected to the below-described
electrogalvanization were assigned with a "CR" symbol in the
"Product type" column in the tables.
[0139] Production of Electrogalvanized Steel Sheet
[0140] Electrogalvanized steel sheets were obtained from some of
the sample steels by immersing in a zinc plating bath at 55.degree.
C., electrogalvanizing at a current density of 30 A/dm.sup.2 to 50
A/dm.sup.2, and then washing with water and drying. The amount of
zinc plated on one side was 10 g/m.sup.2 to 100 g/m.sup.2. In the
plating treatment, sample steels having an electrogalvanized layer
on the surface were obtained by performing washing treatment such
as degreasing by immersion in an appropriate aqueous alkali
solution, washing with water, and pickling. The electrogalvanized
steel sheets were assigned with an "EG" symbol in the "Product
type" column in the tables.
[0141] The metal structures, concentration of Mn, and mechanical
properties were evaluated, as described hereinbelow, for each
sample steel. The results are shown in Table 4 and Table 5.
[0142] Measurement of Metal Structures
[0143] The area ratio of ferrite, the area ratio of the hard phase,
the area ratio of the MA structure, the volume ratio of the
retained .gamma., and the ratio of the globular hard phase were
measured in the following manner. Thus, the cross section of the
sample steel was polished and etched as described hereinabove, and
the position at 1/4 of the sheet thickness was then observed under
an optical microscope or a scanning electron microscope. The ratio
of each structure was measured by image analysis of the metal
structure image captured with the optical microscope or SEM. The
details are described below.
[0144] Surface Area of Ferrite
[0145] The polished sample was subjected to nital etching, a total
of 3 fields of view, each field of view having a size of 100
.mu.m.times.100 .mu.m, were observed under the SEM with a
magnification of 1000, the area ratio of ferrite was measured with
a point algorithm method with 20.times.20 grating points at a
grating spacing of 5 .mu.m, and the average value for the 3 fields
of view was calculated. The results are shown in "Ferrite (area %)"
in the tables. The hard phase in the ferrite phase is excluded from
the a ratio of ferrite.
[0146] Area Ratio of Hard Phase
[0147] The structures other than the ferrite are a hard phase, and
a value obtained by subtracting the ferrite area ratio from 100
area % of the observation fields of view was taken as the area
ratio of the hard phase. The results are shown in "Hard phase (area
%)" in the tables. The structures of the hard phase were also
observed, and it was confirmed that the hard phase is at least one
selected from the group including bainitic ferrite, bainite,
tempered martensite, retained .gamma., and MA structure.
[0148] Area Ratio of MA Structure
[0149] The polished sample was subjected to LePera etching, a total
of 3 fields of view, each field of view having a size of 100
.mu.m.times.100 .mu.m, were observed under the optical microscope
with a magnification of 1000, the area ratio of the MA structure
was measured with a point algorithm method with 20.times.20 grating
points at a grating spacing of 5 .mu.m, and the average value for
the 3 fields of view was calculated. The results are shown in "MA
(area %)" in the tables. A zone which was colored white in the
LePera etching was observed as the MA structure.
[0150] Volume ratio of retained .gamma.
[0151] The sample was polished to a position of 1/4 sheet thickness
with sandpapers #1000 to #1500, and the surface was then
electrolytically polished to a depth of 10 .mu.m to 20 .mu.m,
followed by measurements using an X-ray diffraction device RINT1500
produced by Rigaku Corporation. More specifically, a Co target was
used, the measurements were performed in a 2.theta. range of
40.degree. to 130.degree. at an output of 40 kV-200 mA, and the
retained .gamma. was quantitatively measured from the obtained
diffraction peaks (110), (200), (211) of the bcc (.alpha.) and
diffraction peaks (111), (200), (220), and (311) of fcc (.gamma.).
The results are presented in "Retained .gamma. (vol %)" in the
tables.
[0152] Globular Hard Phase in Ferrite Phase
[0153] After the polishing, the sample was subjected to nital
etching, a globular hard phase with an aspect ratio of 1 to 3 and
an equivalent circle diameter of 2 .mu.m or less which was present
in the ferrite phase was observed in a total of 3 fields of view,
each field of view having a size of 100 .mu.m.times.100 under a SEM
with a magnification of 1000, and the ratio of the globular hard
phase in the hard phase was determined by image analysis. The
results are presented in "Globular hard phase (area %)" in the
tables,
[0154] Ratio of the Region in which the Concentration of Mn is
Enriched to 1.2 Times or more the Concentration of Mn in the Steel
Sheet
[0155] The sample steel was cut along the transverse cross section,
embedded in a resin, and polished. The concentration of Mn was then
measured in a 20 .mu.m.times.20 .mu.m range by using EPMA under the
condition of a beam diameter being 1 .mu.m of less. The obtained
concentration of Mn was divided by the concentration of Mn in the
steel sheet determined by chemical analysis performed by
inductively coupled plasma emission spectroscopy, and the ratio of
the region in which the concentration of Mn was enriched to 1.2
times or more the concentration of Mn in the steel sheet was
determined. Then, the region with the concentration of Mn of 1.2
times or more and the region with the concentration of Mn of less
than 1.2 times were represented by different colors, and the area
percentage of the region with the concentration of Mn of 1.2 times
or more was determined. The results are presented in "Area ratio of
region with concentration of Mn of 1.2 times (%)" in the
tables.
[0156] Standard Deviation of the Regions in which the Concentration
of Mn is Enriched to 2 Times or more the Concentration of Mn in the
Steel Sheet
[0157] An image colored according to the concentration of Mn in the
steel sheet was divided into 100 sections, each section being a
2-.mu.m square, the fraction of the region in which the
concentration of Mn was enriched to 1.2 times or more the
concentration of Mn in the steel sheet was measured in each
section, and the standard deviation of 100 sections was determined.
The results are presented in "Standard deviation of regions with
concentration of Mn of 1.2 times (area %)" in the tables.
[0158] Ratio of the Concentration of Mn in the Ferrite Phase in the
Concentration of Mn in the Steel Sheet
[0159] The concentration of Mn was observed under a SEM in the same
20 .mu.m.times.20 .mu.m fields of view measured by the EPMA
analysis. The ferrite grains and distribution of Mn concentration
therein were identified by comparing the ERMA analysis results with
the SEM image. The intersection point of the long and short axes of
a ferrite grain was taken as the center position of the ferrite
grain, and the concentration of Mn in the center position was taken
as the concentration of Mn of this ferrite grain. The concentration
of Mn at the ferrite grain center position in 20 .mu.m.times.20
.mu.m ranges was identified by the method, and the ratio of the
concentration of Mn in the ferrite phase in the concentration of Mn
in the steel sheet was determined by dividing the concentration of
Mn of the ferrite grain the highest concentration of Mn among the
20 .mu.m.times.20 .mu.m ranges by the concentration of Mn in the
steel sheet. In the present invention, a 20 .mu.m.times.20 .mu.m
field was taken as one field of view, and the average value for 3
fields of view was taken as the ratio of the concentration of Mn in
the ferrite phase in the concentration of Mn in the steel sheet.
The results are presented in "Ratio of concentration of Mn in
ferrite phase" in the tables.
[0160] Evaluation of Mechanical Properties
[0161] A tensile test was performed using a test piece No. 5
stipulated by JIS Z2201, and the tensile strength and ductility
were measured. The test piece was cut out from the sample steel
such that the longitudinal direction was orthogonal to the rolling
direction. A strength-ductility balance was calculated from the
obtained tensile strength and ductility. In the tables, the tensile
strength, ductility, and strength-ductility balance are presented
by "TS (MPa)", "EL (%)", and "TS.times.EL (MPa. %)".
[0162] In the present invention, when the tensile strength was 980
MPa or higher, the strength was high and evaluated as acceptable,
and when the tensile strength was less than 980 MPa, the strength
was insufficient and evaluated as unacceptable.
[0163] Further, the ductility was evaluated by the
strength-ductility balance, and where TS.times.EL was 15,000 MPa-%
or higher, the ductility was evaluated as excellent and acceptable.
Where TS.times.EL was less than 15,000 MPa-%, the ductility was
evaluated as poor and unacceptable.
[0164] Evaluation of Bendability
[0165] The bendability was evaluated under the following
measurement conditions on the basis of the VDA standard
"VDA238-100" of Verband der Deutschen Automobilindustrie (the
German Association of the Automotive Industry). In the present
invention, the bending angle was determined by converting the
displacement under the maxim load, which was obtained by the
bending test, into the angle stipulated by the VDA standard. The
results are presented in "VDA bending angle (.degree.)" in the
table. The bendability was evaluated from the tensile strength and
bending angle. The results are presented in "TS.times.VDA
(MPa-.degree.)" in the tables, Where TS.times.VDA was 100,000
MPa-.degree. or more, the bendability was evaluated as excellent
and acceptable, Where TS.times.VDA was less than 100,000
MPa-.degree., the bendability was evaluated as insufficient and and
acceptable.
Measurement Conditions
[0166] Test method: roll support, punch pushing
[0167] Roll diameter: .phi.30 mm
[0168] Punch shape: tip R=0.4 mm
[0169] Roll spacing: 2.9 mm
[0170] Pushing speed: 20 mm/mm
[0171] Test piece dimensions: 60 mm.times.60 mm
[0172] Bending direction: rolling right angle direction
Testing machine: SHIMADZU AUTOGRAPH 20kN
TABLE-US-00001 TABLE 1 Steel Component composition (mass %)
Ac.sub.1 Ac.sub.3 grade C Si Mn P S Al N O Cr Mo Ti Nb V Cu Ni B Ca
Mg REM (.degree. C.) (.degree. C.) A 0.23 1.5 2.4 0.01 0.001 0.04
0.001 0.001 0.015 741 837 B 0.19 2.0 2.6 0.01 0.001 0.04 0.003
0.001 0.015 753 862 C 0.23 1.8 2.2 0.01 0.002 0.04 0.003 0.001
0.015 752 856 D 0.23 1.5 2.0 0.01 0.001 0.04 0.004 0.001 0.015 745
849 E 0.19 1.5 2.6 0.01 0.001 0.04 0.003 0.001 739 834 F 0.23 2.2
2.0 0.01 0.001 0.03 0.004 0.001 0.3 771 867 G 0.17 1.35 2.2 0.01
0.001 0.03 0.004 0.001 0.3 739 849 H 0.17 1.35 2.0 0.02 0.002 0.03
0.005 0.001 0.015 741 853 I 0.15 1.35 2.2 0.01 0.002 0.04 0.003
0.001 0.015 739 850 J 0.17 1.85 2.3 0.01 0.001 0.04 0.004 0.001 0.1
752 861 K 0.19 1.85 2.2 0.02 0.001 0.05 0.003 0.001 0.3 748 868 L
0.21 1.85 2.1 0.01 0.002 0.04 0.005 0.001 0.1 0.3 749 853 M 0.22
1.35 2.1 0.01 0.001 0.05 0.004 0.001 0.015 0.0025 740 845 N 0.17
1.35 2.2 0.02 0.001 0.04 0.004 0.001 0.001 739 851 O 0.22 1.5 2.4
0.02 0.001 0.06 0.004 0.001 0.015 0.001 741 854 P 0.22 1.5 2.4 0.01
0.001 0.07 0.004 0.001 0.3 0.015 0.001 746 848 Q 0.19 2.8 2.0 0.01
0.001 0.01 0.003 0.001 0.015 0.0025 783 904 R 0.26 1.2 0.8 0.01
0.002 0.04 0.003 0.001 1.0 0.3 0.1 766 856 S 0.15 2.6 2.6 0.01
0.002 0.06 0.003 0.001 0.3 0.015 0.0025 0.001 771 916 T 0.12 1.8
2.8 0.01 0.001 0.07 0.004 0.001 0.9 0.9 0.1 0.0015 0.001 761 888 U
0.35 1.2 2.6 0.02 0.001 0.05 0.005 0.001 730 800 V 0.26 1.5 3.5
0.01 0.001 0.04 0.002 0.001 729 792 W 0.12 2 2.2 0.02 0.003 0.05
0.006 0.001 758 897 X 0.19 1.9 2.2 0.01 0.001 0.05 0.004 0.001 755
867 Y 0.21 1.8 2.1 0.02 0.001 0.05 0.003 0.001 753 868 Z 0.23 1.3
2.1 0.02 0.002 0.04 0.004 0.001 738 838 AA 0.28 1.2 2.0 0.02 0.002
0.05 0.002 0.001 737 830 AB 0.23 1.8 2.4 0.02 0.002 0.05 0.004
0.001 750 855 AC 0.18 1.5 2.8 0.01 0.001 0.03 0.001 0.001 737 826 *
REM in steel grade P is Y: 0.0005, Ce: 0.0005.
TABLE-US-00002 TABLE 2 Average Average Cooling Reheating Reheating
Coiling Holding Holding Ac.sub.1 + Soaking cooling cooling stop
holding holding Test Steel temp. temp. 1 time Ac.sub.1 20 Ac.sub.3
temp. rate 1 rate 2 temp. temp. time Product No. grade (.degree.
C.) (.degree. C.) (h) (.degree. C.) (.degree. C.) (.degree. C.)
(.degree. C.) (.degree. C./s) (.degree. C./s) (.degree. C.)
(.degree. C.) (s) type 1 A 600 600 10 741 761 837 810 50 50 180 350
500 CR 2 B 600 600 10 753 773 862 810 60 60 180 350 300 CR 3 C 620
600 9 752 772 856 830 20 20 300 380 600 CR 4 D 630 620 8 745 765
849 830 30 30 240 380 500 CR 5 E 580 580 24 739 759 834 820 50 50
240 350 700 CR 6 F 550 550 30 771 791 867 810 60 60 200 300 800 CR
7 G 500 500 48 739 759 849 820 60 60 180 300 500 CR 8 H 500 500 40
741 761 853 800 50 50 150 400 1000 CR 9 I 700 700 5 739 759 850 800
60 60 150 400 1000 CR 10 J 660 600 5 752 772 861 810 50 50 100 420
1200 CR 11 K 650 650 3 748 768 868 820 30 30 20 420 800 CR 12 L 700
700 3 749 769 853 820 20 20 20 350 700 CR 13 M 600 600 8 740 760
845 830 50 50 150 300 700 CR 14 N 650 650 10 739 759 851 830 60 60
180 390 700 CR 15 O 550 550 20 741 761 854 840 60 60 20 400 800 CR
16 P 580 580 12 746 766 848 830 50 50 20 420 300 CR 17 Q 700 680 10
783 803 904 860 30 30 200 400 700 CR 18 R 640 620 10 766 786 856
820 20 20 200 430 500 CR 19 S 600 580 3 771 791 916 870 30 30 300
50 500 CR 20 T 600 580 7 761 781 888 850 30 30 180 450 500 CR 21 B
600 -- -- 753 773 862 820 60 60 180 380 600 CR 22 K 300 300 10 748
768 868 830 30 30 180 380 600 CR 23 A 500 500 1 741 761 837 820 20
20 180 350 300 CR 24 C 600 600 10 752 772 856 830 30 30 200 370 500
EG 25 M 600 600 10 740 760 845 840 30 30 200 400 400 EG 26 H 600
600 10 741 761 853 840 30 30 180 400 500 CR 27 B 550 550 10 753 773
862 910 60 60 300 380 300 CR 28 K 600 600 10 748 768 868 830 20 20
200 420 45 EG 29 I 600 600 10 739 759 850 810 30 30 200 410 600 EG
30 G 600 600 10 739 759 849 810 30 -- 600 410 500 CR 31 B 600 600
10 753 773 862 830 5 30 200 410 500 CR 32 B 600 600 10 753 773 862
820 20 30 480 350 300 CR
TABLE-US-00003 TABLE 3 Holding Holding Average Average Cooling
Reheating Reheating Coiling start end Holding Ac.sub.1 + Soaking
cooling cooling stop holding holding Test Steel temp. temp. temp.
time Ac.sub.1 20 Ac.sub.3 temp. rate 1 rate 2 temp. temp. time
Product No. grade (.degree. C.) (.degree. C.) (.degree. C.) (h)
(.degree. C.) (.degree. C.) (.degree. C.) (.degree. C.) (.degree.
C./s) (.degree. C./s) (.degree. C.) (.degree. C.) (s) type 33 W 600
600 600 5 758 778 897 860 40 30 180 350 60 CR 34 W 620 600 550 20
758 778 897 840 50 30 190 320 300 CR 35 X 620 600 580 10 755 775
867 830 30 20 170 420 600 CR 36 X 600 620 620 15 755 775 867 850 50
15 150 380 500 CR 37 Y 580 580 550 24 753 773 868 830 30 20 180 420
120 CR 38 Y 600 600 600 30 753 773 868 840 50 25 190 450 900 CR 39
Z 600 600 580 10 738 758 838 820 50 20 380 400 500 CR 40 AA 600 600
610 3 737 757 830 820 60 30 120 400 1000 CR 41 AB 650 650 650 5 750
770 855 840 30 20 180 255 1000 CR 42 AC 700 680 680 10 737 757 826
810 20 20 200 420 1200 CR 43 AC 650 650 600 10 737 757 826 800 40
20 180 350 1000 CR
TABLE-US-00004 TABLE 4 Area ratio of Standard deviation Globular
region with of regions with Hard Retained hard concentration of
concentration of Test Steel Ferrite phase MA .gamma. phase Mn of
1.2 times Mn of 1.2 times No. grade (area %) (area %) (area %)
(volume %) (area %) (%) (area %) 1 A 30 70 7 10 0.84 7.1 5.5 2 B 32
68 6 11 0.86 5.7 5.4 3 C 24 76 14 15 0.79 6.2 5.4 4 D 21 79 9 10
0.75 6.3 5.1 5 E 15 85 8 8 0.95 5.5 6.2 6 F 24 76 5 9 0.94 6.4 7.1
7 G 21 79 4 9 1.01 7.5 7 8 H 37 63 2 10 1.32 6.8 7.5 9 I 34 66 1 11
0.76 5.7 5 10 J 35 65 1 11 0.71 7.1 5.1 11 K 32 68 1 7 0.73 6.0 4.7
12 L 30 70 1 10 0.68 6.2 4.2 13 M 15 85 3 10 0.80 6.3 5.2 14 N 15
85 4 9 0.85 6.8 5.8 15 O 8 92 1 3 1.00 5.4 6 16 P 24 76 1 5 0.89
5.8 5.1 17 Q 24 76 5 10 0.86 5.8 5.4 18 R 20 80 8 10 0.84 5.9 5.6
19 S 18 82 45 10 0.74 6.3 4.2 20 T 5 95 6 10 0.75 6.8 4.8 21 B 30
70 8 10 0.45 5.2 3.2 22 K 20 80 5 10 0.62 3.1 1.2 23 A 30 70 8 10
0.56 5.4 3.9 24 C 25 75 10 9 0.86 7.0 5.8 25 M 24 76 8 11 0.84 6.8
5.5 26 H 21 79 9 12 0.87 5.7 5.4 27 B 0 100 4 9 0.00 1.0 0.8 28 K
20 80 8 11 0.85 6.1 5.2 29 I 20 80 7 11 0.86 6.3 5.8 30 G 60 40 33
10 0.87 6.1 5.5 31 B 41 59 12 13 0.87 5.1 4.2 32 B 28 72 35 16 0.81
5.6 5.2 Ratio of VDA bend- Test concentration of TS EL TS .times.
EL ing angle TS .times. VDA No. Mn in ferrite phase (MPa) (%)
(MPa-%) (.degree.) (MPa-.degree.) 1 0.76 1243 16 19888 84 104412 2
0.57 1235 17 20995 85 104975 3 0.76 1240 18 22320 82 101680 4 0.80
1220 14 17080 83 101260 5 0.83 1254 13 16302 86 107844 6 0.80 1187
15 17805 87 103269 7 0.78 1012 16 16192 110 111320 8 0.58 1042 16
16672 106 110452 9 0.64 1021 15 15315 98 100058 10 0.62 987 21
20727 106 104622 11 0.63 1028 21 21588 104 106912 12 0.74 1064 24
25536 100 106400 13 0.79 1026 21 21546 98 100548 14 0.77 1048 18
18864 99 103752 15 0.82 1034 15 15510 97 100298 16 0.81 1028 15
15420 98 100744 17 0.64 1181 15 17715 91 107471 18 0.76 1025 17
17425 104 106600 19 0.71 1199 13 15587 65 77935 20 0.84 1341 15
20115 78 104598 21 0.64 1231 16 19696 78 96018 22 0.65 1175 14
16450 75 88125 23 0.80 1140 15 17100 84 95760 24 0.74 1120 14 15680
95 106400 25 0.88 1040 19 19760 101 105040 26 0.83 1025 22 22550 98
100450 27 -- 1342 9 12078 81 108702 28 0.68 1186 16 18976 88 104368
29 0.68 1042 18 18756 101 105242 30 0.74 810 16 12960 89 72090 31
0.92 1025 21 21525 89 91225 32 0.65 1210 14 16940 72 87120
TABLE-US-00005 TABLE 5 Area ratio Standard deviation Globular of
region with of regions with Hard Retained hard concentration of
concentration of Test Steel Ferrite phase MA .gamma. phase Mn of
1.2 times Mn of 1.2 times No. grade (area %) (area %) (area %)
(volume %) (area %) (%) (area %) 33 W 27 73 5 11 0.92 15 6.2 34 W
41 59 5 13 1.45 14 5.7 35 X 44 56 4 15 1.24 15 6.7 36 X 15 85 7 12
1.13 13 5.7 37 Y 46 54 4 15 1.48 15 5.6 38 Y 25 75 5 12 1.39 13 5.1
39 Z 22 78 10 15 1.20 10 5.5 40 AA 15 85 7 12 1.08 9 6.5 41 AB 14
86 4 8 1.09 10 5.4 42 AC 18 82 7 11 1.33 12 5.6 43 AC 29 71 5 10
1.21 11 5.9 Ratio of VDA bend- Test concentration of TS EL TS
.times. EL ing angle TS .times. VDA No. Mn in ferrite phase (MPa)
(%) (MPa-%) (.degree.) (MPa-.degree.) 33 0.77 1035 22 22770 105
108675 34 0.79 1022 21 21462 99 101178 35 0.77 1032 25 25800 101
104232 36 0.80 1203 15 18045 88 105864 37 0.77 1051 25 26275 106
111406 38 0.80 1103 21 23163 92 101476 39 0.84 1061 25 26525 103
109283 40 0.85 1308 12 15696 83 108564 41 0.84 1522 10 15220 79
120238 42 0.81 1055 21 22155 102 107610 43 0.83 1204 14 16856 87
104748
[0173] Tables 1 to 5 lead to the following conclusions. The steel
sheets of Tests No. 1 to 18, 20, 24 to 26, 28, 29, and 33 to 43
which were produced using the steel grades A to T and W to AC
having the component composition in accordance with the present
invention and under the annealing conditions stipulated by the
present invention had excellent ductility and bendability in a
tensile strength range of 980 MPa and higher.
[0174] By contrast, other steel sheets did not have the component
compositions or were not produced under the conditions stipulated
by the present invention, as described hereinbelow, and the desired
properties could not be obtained.
[0175] The amount of C in steel grade U and the amount of Mn in
steel grade V in Table 1 exceeded the respective upper limits of
the present invention and fracture occurred during cold rolling. As
a result, sample steels could not be produced.
[0176] Test No. 19 is an example in which no reheating was
performed after cooling to 500.degree. C. or less. The amount of MA
structure increased and the bendability was degraded.
[0177] Test No. 21 is an example in which no holding at the
predetermined temperature was performed after coiling. The standard
deviation of the regions with the concentration of Mn of 1.2 times
was low and the bendability was degraded.
[0178] Test No. 22 is an example in which the coiling temperature
and holding temperature were low. The area ratio of the region with
the concentration of Mn of 1.2 times and the standard deviation of
the regions with the concentration of Mn of 1.2 times were low and
the bendability was degraded.
[0179] Test No. 23 is an example in which the time of holding at
the predetermined temperature after coiling was short. The standard
deviation of the regions with the concentration of Mn of 1.2 times
was low and the bendability was degraded.
[0180] Test No. 27 is an example in which the soaking temperature
was high. No ferrite was generated and the area ratio of the region
with the concentration of Mn of 1.2 times and. the standard
deviation of the regions with the concentration of Mn of 1.2 times
were low. As a result, the ductility was degraded.
[0181] Test No. 30 is an example in which the cooling stop
temperature was high. The amount of ferrite and MA structure
increased, the strength was low, and the ductility and bendability
were degraded.
[0182] Test No. 31 is an example in which the cooling rate to
500.degree. C. was low. The concentration of Mn in the ferrite
phase became too high. As a result, the bendability was
degraded.
[0183] Test No. 32 is an example in which no reheating was
performed after cooling to 500.degree. C. or less. The amount of
the MA structure increased and the bendability was degraded,
Example 2
[0184] The steels with the component compositions presented in
Table 6 were melted and cold-rolled steel sheets were produced by
hot rolling.fwdarw.cold rolling.fwdarw.continuous annealing undo
the below-described conditions. In the steels with the component
compositions presented in Table 6, the balance is iron and
inevitable impurities. The empty column means that the element was
not added.
[0185] Hot rolling
[0186] A slab was heated to 1250.degree. C. and hot rolled to a
sheet thickness of 2,3 mm at a draft ratio of 90% and a finish
temperature of 920.degree. C. Then, the steel sheet was cooled to
the "Coiling temperature (.degree. C.)" shown in Table 7 or Table 8
at an average cooling rate of 30.degree. C./s from this
temperature, coiled, and then held under the conditions of the
"Holding temperature 1 (.degree. C.)" and "Holding time (h)" shown
in Table 7, or the "Holding start temperature (.degree. C.)",
"Holding end temperature (.degree. C.)", and "Holding time (h)"
shown in Table 8, A hot-rolled steel sheet was then produced by air
cooling to room temperature.
[0187] Cold Rolling
[0188] The obtained hot-rolled steel sheet was pickled to remove
the surface scale, and then cold rolling was performed to produce a
cold-rolled steel sheet with a thickness of 1.2 mm.
[0189] Annealing of Cold-Rolled Steel Sheet and Production of
Galvanized Steel Sheet or Galvannealed Steel Sheet
[0190] A sample steel was produced by
soaking/holding.rarw.cooling.rarw.reheating.rarw.plating the
obtained cold-rolled steel sheet under the conditions shown in
Table 9 and Table 10. No. 29 in Table 7 is a comparative example in
which no reheating was performed. Instead of reheating the steel
sheet was cooled from the cooling stop temperature of 470.degree.
C. to 400.degree. C. and then held for 45 s at this temperature.
This process is indicated in the reheating column.
[0191] In the tables, the soaking and holding temperature is
represented by "Soaking temperature (.degree. C.), the average
cooling rate to 500.degree. C. after the soaking is represented by
"Average cooling rate 1 (.degree. C./s)", the cooling rate at
500.degree. C. or less is represented by "Average cooling rate 2
(.degree. C.)", the cooling stop temperature is represented by
"Cooling stop temperature (.degree. C.)", the holding temperature
during reheating after the cooling stop is represented by
"Reheating holding temperature (.degree. C.)", the holding time at
the holding temperature is represented by "Reheating holding time
(s)", the temperature of the plating bath is represented by
"Plating bath temperature (.degree. C.)", and the plating treatment
time is represented by "Galvanization treatment time (s)". The
"Reheating holding time (s)" is a total time including, the
"galvanization treatment time (s)".
[0192] After holding for about ("Reheating holding time
(s)"--"Galvanization treatment time (s)") at the "Reheating holding
temperature (.degree. C.)", the steel sheet is dipped into a
galvanization bath and the galvanized layer is formed for the
"Galvanization treatment time (s)", in Tests No. 31 and 33, the
steel was immersed into the galvanization bath after heating from
the "Reheating holding temperature (.degree. C.)" to "Plating bath
temperature (.degree. C.)", which are shown in Table 8, immediately
prior to dipping into the galvanization bath. Some steel sheets
were alloyed after the galvanization treatment. In the tables, the
alloying temperature is represented by "Alloying temperature
(.degree. C.)" and the holding time at the alloying temperature is
represented by "Alloying treatment time (s)". After holding for the
predetermined time, the steel sheet was allowed to cool to room
temperature to obtain a sample steel. The steel sheets which were
subjected only to galvanization were assigned with a "GI" symbol
and the steel sheets that were also alloyed were assigned with a
"GA" symbol in the "Product type" column in the tables.
[0193] The metal structures, concentration of Mn, and mechanical
properties were evaluated, as described hereinbelow, for each
sample steel in the same manner as in Example 1. The results are
shown in Table 9 and Table 10.
TABLE-US-00006 TABLE 6 Steel Component composition (mass %)
Ac.sub.1 Ac.sub.3 grade C Si Mn P S Al N O Cr Mo Ti Nb V Cu Ni B Ca
Mg REM (.degree. C.) (.degree. C.) A 0.21 1.85 2.1 0.01 0.001 0.04
0.004 0.001 754 860 B 0.17 2 2.3 0.01 0.001 0.03 0.005 0.001 757
866 C 0.21 1.2 2 0.01 0.002 0.04 0.005 0.001 737 834 D 0.22 1.5 1.9
0.01 0.001 0.04 0.004 0.001 0.4 753 843 E 0.22 1.5 1.9 0.01 0.001
0.04 0.004 0.001 0.3 746 857 F 0.22 1.8 1.8 0.01 0.001 0.04 0.003
0.001 0.02 756 872 G 0.19 1.5 2.1 0.01 0.002 0.03 0.004 0.001 0.02
744 845 H 0.15 2.6 2.4 0.01 0.001 0.05 0.004 0.001 0.02 773 905 I
0.23 1.35 2 0.01 0.001 0.03 0.003 0.001 0.2 0.2 738 825 J 0.19 2 2
0.02 0.001 0.04 0.003 0.001 0.003 760 881 K 0.26 1.35 2.2 0.01
0.002 0.04 0.004 0.001 0.002 739 824 L 0.12 1.8 2.8 0.01 0.001 0.04
0.003 0.001 0.003 745 859 M 0.26 1.8 1.6 0.01 0.001 0.05 0.003
0.001 0.002 758 866 N 0.19 1.85 2.2 0.02 0.001 0.04 0.002 0.001
0.03 0.2 753 876 O 0.35 1.2 2.6 0.01 0.001 0.04 0.004 0.001 730 789
P 0.26 1.5 3.5 0.01 0.001 0.04 0.004 0.001 729 792 Q 0.21 2 1.8
0.01 0.001 0.01 0.006 0.001 762 863 R 0.22 1.9 2.4 0.02 0.001 0.04
0.004 0.001 753 858 S 0.19 2.4 1.8 0.02 0.001 0.03 0.004 0.001 774
901 T 0.16 1.6 1.6 0.01 0.002 0.03 0.003 0.001 743 854 U 0.19 1.95
2.6 0.01 0.001 0.04 0.003 0.001 752 854 * REM in steel grade M is
Y: 0.001, Ce: 0.001.
TABLE-US-00007 TABLE 7 Uniform Uniform Coiling Holding Holding
Ac.sub.1 + Soaking cooling cooling Test Steel temp. temp. 1 time
Ac.sub.1 20 Ac.sub.3 temp. rate 1 rate 2 No. grade (.degree. C.)
(.degree. C.) (h) (.degree. C.) (.degree. C.) (.degree. C.)
(.degree. C.) (.degree. C./s) (.degree. C./s) 1 A 600 580 10 754
774 860 830 40 40 2 A 600 580 10 754 774 860 830 40 40 3 A 600 580
10 754 774 860 830 30 40 4 A 580 580 10 754 774 860 820 40 40 5 B
550 550 24 757 777 866 830 20 20 6 B 650 -- -- 757 777 866 830 20
20 7 B 300 300 10 757 777 866 830 20 20 8 B 650 650 1 757 777 866
830 20 20 9 B 550 530 15 757 777 866 790 20 20 10 C 600 580 8 737
757 834 800 20 20 11 D 550 550 10 753 773 854 810 40 40 12 D 550
550 10 753 773 854 900 40 40 13 E 550 550 10 746 766 864 820 40 40
14 F 500 500 48 756 776 872 830 40 40 15 F 500 500 48 756 776 872
820 40 40 16 F 500 500 48 756 776 872 820 40 40 17 F 500 500 48 756
776 872 830 40 40 18 G 600 600 8 744 764 852 810 20 20 19 G 550 550
10 744 764 852 810 30 30 20 H 550 580 10 773 793 912 870 40 40 21 I
600 600 3 738 758 825 800 20 20 22 I 550 550 12 738 758 825 790 30
30 23 J 700 700 3 760 780 881 840 20 20 24 J 550 550 8 760 780 881
860 5 30 25 K 550 550 10 739 759 824 800 20 20 26 L 550 550 10 745
765 859 820 40 40 27 M 600 580 8 758 778 866 830 20 20 28 N 600 600
10 753 773 876 810 20 20 29 N 600 600 24 753 773 876 820 20 20
Cooling Reheating Reheating Plating Galvaniza- Alloying stop
holding holding bath tion treat- Alloying treat- Test temp. temp.
time temp. ment time temp. ment time Product No. (.degree. C.)
(.degree. C.) (s) (.degree. C.) (s) (.degree. C.) (s) type 1 100
450 50 460 3 550 20 GA 2 100 400 50 450 3 500 20 GA 3 300 450 10
460 3 450 25 GA 4 150 430 80 460 3 -- 25 GI 5 200 430 300 460 4 500
25 GA 6 200 430 300 460 4 500 25 GA 7 200 430 300 460 4 500 25 GA 8
200 430 100 460 4 450 25 GA 9 100 150 40 420 4 -- 25 GI 10 150 430
50 460 3 500 25 GA 11 150 430 50 460 3 500 25 GA 12 150 430 50 460
3 500 25 GA 13 150 430 50 460 3 500 25 GA 14 100 400 50 460 3 450
25 GA 15 100 450 300 460 3 550 50 GA 16 100 450 200 460 3 500 30 GA
17 600 450 500 460 3 500 45 GA 18 150 370 50 440 5 500 25 GA 19 150
450 70 460 5 -- 25 GI 20 250 400 50 450 3 450 25 GA 21 150 430 50
460 3 480 25 GA 22 200 430 70 460 3 -- 25 GI 23 150 430 50 460 3
500 25 GA 24 150 400 80 460 3 500 25 GA 25 150 470 200 470 3 500 25
GA 26 350 400 500 450 3 450 25 GA 27 50 430 30 460 3 500 25 GA 28
200 420 45 460 3 -- 25 GI 29 470 400 45 450 3 500 25 GA
TABLE-US-00008 TABLE 8 Holding Holding Uniform Uniform Coiling
start end Holding Ac.sub.1 + Soaking cooling cooling Test Steel
temp. temp. temp. time Ac.sub.1 20 Ac.sub.3 temp. rate 1 rate 2 No.
grade (.degree. C.) (.degree. C.) (.degree. C.) (h) (.degree. C.)
(.degree. C.) (.degree. C.) (.degree. C.) (.degree. C./s) (.degree.
C./s) 30 Q 580 660 640 4 762 782 863 810 50 50 31 Q 600 640 600 24
762 782 863 840 50 50 32 R 650 580 500 4 753 773 858 800 50 50 33 S
580 600 550 12 774 794 901 820 40 40 34 T 600 640 620 3 743 763 854
800 70 60 35 U 650 660 640 3 752 772 854 800 20 20 Cooling
Reheating Reheating Plating Galvaniza- Alloying stop holding
holding bath tion treat- Alloying treat- Test temp. temp. time
temp. ment time temp. ment time Product No. (.degree. C.) (.degree.
C.) (s) (.degree. C.) (s) (.degree. C.) (s) type 30 100 430 50 450
3 470 30 GA 31 150 250 600 440 3 450 20 GA 32 100 400 50 460 3 470
25 GA 33 100 300 50 450 3 500 25 GA 34 420 450 700 460 3 -- -- GI
35 190 380 500 450 3 450 25 GA
TABLE-US-00009 TABLE 9 Area ratio Standard deviation Globular of
region with of region with Hard Retained hard concentration of
concentration of Test Steel Ferrite phase MA .gamma. phase Mn of
1.2 times Mn of 1.2 times No. grade (area %) (area %) (area %)
(volume %) (area %) (%) (area %) 1 A 34 66 2 9 0.84 6.3 5.3 2 A 28
72 11 9 0.79 5.5 5.2 3 A 39 61 45 6 0.79 6.1 5.1 4 A 29 71 5 11
0.88 5.9 5.0 5 B 36 64 6 13 1.20 7.1 6.0 6 B 31 69 7 11 0.31 5.4
3.1 7 B 36 64 8 11 0.54 3.8 1.9 8 B 34 66 8 13 0.43 5.2 3.4 9 B 48
52 41 6 0.88 6.0 6.0 10 C 35 65 8 14 0.65 6.2 5.4 11 D 31 69 14 13
0.71 5.8 5.2 12 D 4 96 2 6 0.69 5.1 4.4 13 E 28 72 8 13 0.81 5.9
5.2 14 F 38 62 11 14 1.18 6.2 7.2 15 F 40 60 6 15 1.10 7.4 7.0 16 F
36 64 8 13 0.90 6.2 7.3 17 F 54 46 33 9 1.01 6.4 7.2 18 G 43 57 13
14 0.80 6.8 5.0 19 G 38 62 8 12 0.84 6.5 5.0 20 H 16 84 10 14 0.88
6.0 5.1 21 I 27 73 9 14 0.72 6.0 4.6 22 I 32 68 11 10 0.85 6.3 5.1
23 J 39 61 12 15 0.77 6.8 4.8 24 J 48 52 14 9 0.82 6.1 4.7 25 K 19
81 2 17 0.92 5.7 5.2 26 L 8 92 1 11 0.85 7.1 5.4 27 M 45 55 19 16
0.79 6.3 5.4 28 N 25 75 8 11 0.85 6.1 5.2 29 N 23 77 32 9 1.01 7.0
5.8 Ratio of VDA bend- Test concentration of TS EL TS .times. EL
ing angle TS .times. VDA No. Mn in ferrite phase (MPa) (%) (MPa-%)
(.degree.) (MPa-.degree.) 1 0.72 1090 14 15260 96 104640 2 0.74
1175 15 17625 89 104575 3 0.72 1251 12 15012 61 76311 4 0.67 1178
17 20026 90 106020 5 0.69 1155 14 16170 92 106260 6 0.71 1165 13
15145 75 87375 7 0.72 1138 14 15932 85 96730 8 0.70 1150 14 16100
84 96600 9 0.62 1022 17 17374 68 69496 10 0.70 997 20 19940 103
102691 11 0.64 1010 20 20200 102 103020 12 0.93 1055 13 13715 98
103390 13 0.65 1021 19 19399 101 103121 14 0.68 982 26 25532 106
104092 15 0.63 989 19 18791 105 103845 16 0.65 987 19 18753 109
107583 17 0.70 978 18 17604 89 87042 18 0.68 1006 20 20120 103
103618 19 0.71 1100 18 19800 97 106700 20 0.68 1209 14 16926 85
102765 21 0.76 1046 21 21966 98 102508 22 0.74 1009 20 20180 106
106954 23 0.69 1037 18 18666 98 101626 24 0.91 1028 19 19532 88
90464 25 0.78 1206 16 19296 88 106128 26 0.70 1189 13 15457 88
104632 27 0.68 994 24 23856 106 105364 28 0.69 1186 16 18976 88
104368 29 0.68 1199 14 16786 80 95920
TABLE-US-00010 TABLE 10 Area ratio Standard deviation Globular of
region with of region with Hard Retained hard concentration of
concentration of Test Steel Ferrite phase MA .gamma. phase Mn of
1.2 times Mn of 1.2 times No. grade (area %) (area %) (area %)
(volume %) (area %) (%) (area %) 30 Q 33 67 8 13 0.80 6.0 4.7 31 Q
16 84 17 7 1.10 7.3 6.4 32 R 23 77 7 12 0.72 6.3 4.5 33 S 44 56 14
12 0.71 5.8 4.4 34 T 12 88 18 9 0.80 5.5 4.5 35 U 25 67 9 10 0.81
6.4 4.5 Ratio of VDA bend- Test concentration of TS EL TS .times.
EL ing angle TS .times. VDA No. Mn in ferrite phase (MPa) (%)
(MPa-%) (.degree.) (MPa-.degree.) 30 0.77 996 24 23904 105 104580
31 0.69 1015 25 25375 103 104545 32 0.63 1295 13 16835 83 107485 33
0.70 1051 22 23122 97 101947 34 0.72 1040 20 20800 99 102960 35
0.64 1365 11 15015 80 109200
[0194] Tables 6 to 10 lead to the following conclusions. The steel
sheets of Tests No. 1, 2, 4, 5, 10, 11, 13 to 16, 18 to 23, 25 to
28, and 30 to 35 which were produced using the steel grades A to N
and Q to U having the component composition in accordance with the
present invention and under the annealing conditions stipulated by
the present invention had excellent ductility and bendability in a
tensile strength range of 980 MPa and higher.
[0195] By contrast, other steel sheets did not have the component
compositions or were not produced under the conditions stipulated
by the present invention, as described hereinbelow, and the desired
properties could not be obtained.
[0196] The amount of C in steel grade O and the amount of Mn in
steel grade P in Table 6 exceeded the respective upper limits of
the present invention and fracture occurred during cold rolling. As
a result, sample steels could not be produced.
[0197] Test No. 3 is an example in which the time of holding at the
reheating holding temperature after cooling to 500.degree. C. or
less was short. The amount of MA structure increased and the
bendability was degraded.
[0198] Test No. 6 is an example in which no holding at the
predetermined temperature was performed after coiling. The standard
deviation of the regions with the concentration of Mn of 1.2 times
was low and the bendability was degraded.
[0199] Test No. 7 is an example in which the coiling temperature
and holding temperature were low. The area ratio of the region with
the concentration of Mn of 1.2 times and the standard deviation of
the regions with the concentration of Mn of 1.2 times were low and
the bendability was degraded.
[0200] Test No. 8 is an example in which the time of holding at the
predetermined temperature after coiling was short. The standard
deviation of the regions with the concentration of Mn of 1.2 times
was low and the bendability was degraded.
[0201] Test No, 9 is an example in which the reheating holding
temperature after cooling to 500.degree. C. or less was low. The
amount of MA structure increased and the bendability was
degraded.
[0202] Test No. 12 is an example in which the soaking temperature
was high. Ferrite was generated insufficiently and the
concentration of Mn in the ferrite phase was too high. As a result,
the ductility was degraded.
[0203] Test No. 17 is an example in which the cooling stop
temperature was high. The amount of ferrite and MA structure
increased, the strength was low, and the bendability degraded.
[0204] Test No, 24 is an example in which the cooling rate to
500.degree. C. was low. The concentration of Mn in the ferrite
phase became too high. As a result, the bendability was
degraded.
[0205] Test No. 29 is an example in which no reheating was
performed after cooling to 500.degree. C. or less. The amount of
the MA structure increased and the bendability was degraded.
* * * * *