U.S. patent application number 15/554875 was filed with the patent office on 2018-02-15 for high-strength cold-rolled steel sheet having excellent formability and collision characteristics and having tensile strength of 980 mpa or more, and method 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, Tadao MURATA.
Application Number | 20180044752 15/554875 |
Document ID | / |
Family ID | 57322703 |
Filed Date | 2018-02-15 |
United States Patent
Application |
20180044752 |
Kind Code |
A1 |
MURATA; Tadao ; et
al. |
February 15, 2018 |
HIGH-STRENGTH COLD-ROLLED STEEL SHEET HAVING EXCELLENT FORMABILITY
AND COLLISION CHARACTERISTICS AND HAVING TENSILE STRENGTH OF 980
MPa OR MORE, AND METHOD FOR PRODUCING SAME
Abstract
Disclosed herein is a high-strength cold-rolled steel sheet, in
which the metal structure at a position of 1/4 of the sheet
thickness satisfies (1) to (4): (1) an area ratio of ferrite is
more than 10% to 65% or less, with a balance being a hard phase
including quenched martensite and retained austenite and including
at least one selected from the group consisting of bainitic
ferrite, bainite, and tempered martensite; (2) a volume ratio
V.sub..gamma. of retained austenite is 5% to 30%; (3) an area ratio
V.sub.MA of an MA structure in which quenched martensite and
retained austenite are combined is 3% to 25%, and an average
circle-equivalent diameter of the MA structure is 2.0 .mu.m or
less; and (4) a ratio V.sub.MA/V.sub..gamma. of the area ratio
V.sub.MA of the MA structure to the volume ratio V.sub..gamma. of
the retained austenite is 0.50 to 1.50.
Inventors: |
MURATA; Tadao;
(Kakogawa-shi, JP) ; FUTAMURA; Yuichi;
(Kakogawa-shi, JP) ; KASUYA; Koji; (Kakogawa-shi,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) |
Kobe-shi |
|
JP |
|
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel, Ltd.)
Kobe-shi
JP
|
Family ID: |
57322703 |
Appl. No.: |
15/554875 |
Filed: |
March 1, 2016 |
PCT Filed: |
March 1, 2016 |
PCT NO: |
PCT/JP2016/056169 |
371 Date: |
August 31, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 6/005 20130101;
C21D 6/001 20130101; C21D 2211/001 20130101; C22C 38/08 20130101;
C22C 38/06 20130101; C23C 2/40 20130101; C21D 9/46 20130101; C21D
8/0226 20130101; C22C 38/12 20130101; C22C 38/002 20130101; C21D
8/02 20130101; C21D 2211/008 20130101; C23C 2/28 20130101; C25D
3/22 20130101; C21D 6/008 20130101; C22C 38/60 20130101; C21D
8/0205 20130101; C22C 38/005 20130101; C22C 38/14 20130101; C22C
38/16 20130101; C22C 38/38 20130101; C21D 6/002 20130101; C21D
8/0247 20130101; C21D 2211/005 20130101; C22C 38/02 20130101; C22C
38/001 20130101 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C22C 38/16 20060101 C22C038/16; C22C 38/14 20060101
C22C038/14; C22C 38/12 20060101 C22C038/12; C22C 38/08 20060101
C22C038/08; C25D 3/22 20060101 C25D003/22; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00; C21D 8/02 20060101
C21D008/02; C21D 6/00 20060101 C21D006/00; C23C 2/28 20060101
C23C002/28; C23C 2/40 20060101 C23C002/40; C22C 38/38 20060101
C22C038/38; C22C 38/06 20060101 C22C038/06 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 31, 2015 |
JP |
2015-071438 |
Nov 18, 2015 |
JP |
2015-225507 |
Claims
1: A high-strength cold-rolled steel sheet having a tensile
strength of 980 MPa or more and being excellent in formability and
crashworthiness, the high-strength cold-rolled steel sheet
comprising, in mass %: C: 0.10% or more to 0.5% or less, Si: 1.0%
or more to 3% or less, Mn: 1.5% or more to 7% or less, P: more than
0% to 0.1% or less, S: more than 0% to 0.05% o or less, Al: 0.005%
or more to 1% or less, N: more than 0% to 0.01% or less, and O:
more than 0% to 0.01% or less, wherein a metal structure at a
position of 1/4 of a sheet thickness satisfies (1) to (4) below:
(1) when the metal structure is observed with a scanning electron
microscope, an area ratio of ferrite relative to a whole of the
metal structure is more than 10% to 65% or less, with a balance
being a hard phase including quenched martensite and retained
austenite and including at least one selected from the group
consisting of bainitic ferrite, bainite, and tempered martensite,
(2) when the metal structure is measured by X-ray diffractometry, a
volume ratio V.sub..gamma. of retained austenite relative to the
whole of the metal structure is 5% or more to 30% or less, (3) when
the metal structure is observed with an optical microscope, an area
ratio V.sub.MA of an MA structure, in which quenched martensite and
retained austenite are combined, relative to the whole of the metal
structure is 3% or more to 25% or less, and an average
circle-equivalent diameter of the MA structure is 2.0 .mu.m or
less, and (4) a ratio V.sub.MA/V.sub..gamma. of the area ratio
V.sub.MA of the MA structure to the volume ratio V.sub..gamma. of
the retained austenite satisfies a formula (i) below:
0.50.ltoreq.V.sub.MA/V.sub..gamma..ltoreq.1.50 (i).
2: The high-strength cold-rolled steel sheet according to claim 1,
further comprising, as other elements, one or more of any of (a) to
(e) below, in mass %: (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.
3: A high-strength electrogalvanized steel sheet having an
electrogalvanized layer on a surface of the high-strength
cold-rolled steel sheet according to claim 1.
4: A high-strength hot-dip galvanized steel sheet having a hot-dip
galvanized layer on a surface of the high-strength cold-rolled
steel sheet according to claim 1.
5: A high-strength hot-dip galvannealed steel sheet having a
hot-dip galvannealed layer on a surface of the high-strength
cold-rolled steel sheet according to claim 1.
6: A method for producing a high-strength cold-rolled steel sheet
having a tensile strength of 980 MPa or more and being excellent in
formability and crashworthiness, the method comprising: hot rolling
a steel with a rolling rate at a final stand of finish rolling
being 5 to 25% and with a finish rolling end temperature being an
Ar.sub.3 point or higher to 900.degree. C. or lower, coiling the
steel with a coiling temperature being 600.degree. C. or lower, and
cooling the steel to room temperature; cold rolling the steel;
heating the steel, at an average heating rate of 10.degree.
C./second or more, to a temperature region of 800.degree. C. or
higher and lower than an Ac.sub.3 point, and soaking the steel
while holding the steel in the temperature region for 50 seconds or
more; cooling the steel at an average cooling rate of 10.degree.
C./second or more, to an arbitrary cooling stop temperature
T.degree. C. that lies in a temperature range of 50.degree. C. or
higher and an Ms point or lower; and heating and holding the steel
in a temperature region of higher than the cooling stop temperature
T.degree. C. and 550.degree. C. or lower for 50 seconds or more,
and thereafter cooling the steel to room temperature, wherein the
steel comprises, in mass %: C: 0.10% or more to 0.5% or less, Si:
1.0% or more to 3% or less, Mn: 1.5% or more to 7% or less, P: more
than 0% to 0.1% or less, S: more than 0% to 0.05% or less, Al:
0.005% or more to 1% or less, N: more than 0% to 0.01% or less, and
O: more than 0% to 0.01% or less.
7: A method for producing a high-strength hot-dip galvanized steel
sheet having a tensile strength of 980 MPa or more and being
excellent in formability and crashworthiness, the method
comprising: hot rolling a steel with a rolling rate at a final
stand of finish rolling being 5 to 25% and with a finish rolling
end temperature being an Ar.sub.3 point or higher to 900.degree. C.
or lower, coiling the steel with a coiling temperature being
600.degree. C. or lower, and cooling the steel to room temperature;
cold rolling the steel; heating the steel, at an average heating
rate of 10.degree. C./second or more, to a temperature region of
800.degree. C. or higher and lower than an Ac.sub.3 point, and
soaking the steel while holding the steel in the temperature region
for 50 seconds or more; cooling the steel at an average cooling
rate of 10.degree. C./second or more, to an arbitrary cooling stop
temperature T.degree. C. that lies in a temperature range of
50.degree. C. or higher and an Ms point or lower; and heating and
holding the steel in a temperature region of higher than the
cooling stop temperature T.degree. C. and 550.degree. C. or lower
for 50 seconds or more, and after performing hot-dip galvanizing
within a holding time, cooling the steel to room temperature,
wherein the steel comprises, in mass %: C: 0.10% or more to 0.5% or
less, Si: 1.0% or more to 3% or less, Mn: 1.5% or more to 7% or
less, P: more than 0% to 0.1% or less, S: more than 0% to 0.05% or
less, Al: 0.005% o or more to 1% or less, N: more than 0% to 0.01%
or less, and O: more than 0% to 0.01% or less.
8: A method for producing a high-strength hot-dip galvannealed
steel sheet having a tensile strength of 980 MPa or more and being
excellent in formability and crashworthiness, the method
comprising: hot rolling the steel with a rolling rate at a final
stand of finish rolling being 5 to 25% and with a finish rolling
end temperature being an Ar.sub.3 point or higher to 900.degree. C.
or lower, coiling the steel with a coiling temperature being
600.degree. C. or lower, and cooling the steel to room temperature;
cold rolling the steel; heating the steel, at an average heating
rate of 10.degree. C./second or more, to a temperature region of
800.degree. C. or higher and lower than an Ac.sub.3 point, and
soaking the steel while holding the steel in the temperature region
for 50 seconds or more; cooling the steel at an average cooling
rate of 10.degree. C./second or more, to an arbitrary cooling stop
temperature T.degree. C. that lies in a temperature range of
50.degree. C. or higher and an Ms point or lower; and heating and
holding the steel in a temperature region of higher than the
cooling stop temperature T.degree. C. and 550.degree. C. or lower
for 50 seconds or more, and after performing hot-dip galvanizing
within a holding time, further performing an alloying treatment and
thereafter cooling the steel to room temperature, wherein the steel
comprises, in mass %: C: 0.10% or more to 0.5% or less, Si: 1.0% or
more to 3% or less, Mn: 1.5% or more to 7% or less, P: more than 0%
to 0.1% or less, S: more than 0% to 0.05% or less, Al: 0.005% or
more to 1% or less, N: more than 0% to 0.01% or less, and O: more
than 0% to 0.01% or less.
Description
TECHNICAL FIELD
[0001] The present invention relates to a high-strength cold-rolled
steel sheet having a tensile strength of 980 MPa or more and being
excellent in formability and crashworthiness and to a method for
producing the same. In further detail, the present invention
relates to the high-strength cold-rolled steel sheet described
above, a high-strength electrogalvanized steel sheet having an
electrogalvanized layer formed on a surface of the high-strength
cold-rolled steel sheet, a high-strength hot-dip galvanized steel
sheet having a hot-dip galvanized layer formed on a surface of the
high-strength cold-rolled steel sheet, and a high-strength hot-dip
galvannealed steel sheet having a hot-dip galvannealed layer formed
on a surface of the high-strength cold-rolled steel sheet, and to a
method for producing the same.
BACKGROUND ART
[0002] In order to achieve fuel cost reduction of automobiles,
transport aircrafts and the like, it is desired to reduce the
weight of the automobiles, transport aircrafts and the like. In
order to achieve weight reduction, it is effective, for example, to
reduce the sheet thickness by using a high-strength steel sheet.
However, when the steel sheet is made to have a higher strength,
the steel sheet comes to have poorer ductility and
stretch-flangeability, thereby degrading the formability into a
product shape.
[0003] Also, in steel parts for automobiles, a steel sheet whose
surface has been subjected to galvanization such as
electrogalvanization (which may hereafter be denoted as EG),
hot-dip galvanizing (which may hereafter be denoted as GI), or
hot-dip galvannealing (which may hereafter be denoted as GA), which
may hereafter be comprehensively referred to as galvanized steel
sheet, is often used from the viewpoint of corrosion resistance. In
these galvanized steel sheets as well, increase in strength and
formability is demanded in the same manner as in the above
high-strength steel sheet.
[0004] For example, Patent Literature 1 discloses a hot-dip
galvannealed steel sheet having a metal structure in which
martensite and retained austenite are mixedly present in ferrite
and having a tensile strength TS of 490 to 880 MPa by reinforcement
of the complex structure thereof, thus having a good press
formability.
[0005] Also, Patent Literature 2 discloses a high-strength
cold-rolled steel sheet having a TS (Tensile Strength) of 590 MPa
or more and being excellent in formability, specifically, with
TS.times.EL (EL: Elongation, elongation) being 23000 MPa % or more,
and being excellent in corrosion resistance after application even
in a severe environment such as a hot salt water test, a salt water
spraying test, or a combined cyclic corrosion test. The metal
structure of this steel sheet is a structure including ferrite,
retained austenite, bainite, and/or martensite. It is described
that the retained austenite has a function of enhancing ductility
of the steel sheet, the function being known as a TRIP effect.
[0006] In the meantime, it is demanded that the steel parts for
automobiles are excellent in crashworthiness which is an ability to
efficiently absorb an impact generated when the automobiles come
into collision. There is known, for example, Patent Literature 3 as
a technique for improving the crashworthiness. Patent Literature 3
discloses a high-strength galvanized steel sheet having a maximum
tensile strength of 900 MPa or more and being excellent in
collision absorption energy in which a dynamic/static ratio as
large as that of a steel sheet of 590 MPa class and a maximum
tensile strength of 900 MPa or more are compatible with each other,
as well as a method for producing the same. This production method
is characterized in that, after performing galvanization, cooling
is performed, and rolling is performed with use of a roll having a
roughness (Ra) of 3.0 or less.
CITATION LIST
Patent Literature
[0007] Patent Literature 1: Japanese Patent No. 3527092
[0008] Patent Literature 2: Japanese Patent No. 5076434
[0009] Patent Literature 3: Japanese Patent No. 5487916
SUMMARY OF INVENTION
Problems to be Solved by the Invention
[0010] According to the techniques disclosed in Patent Literatures
1 and 2, the formability of a steel sheet can be improved. However,
no consideration is made on the crashworthiness. In contrast,
according to the technique disclosed in Patent Literature 3, the
crashworthiness of the steel sheet can be improved. However, no
consideration is made on the formability as evaluated by ductility
and stretch-flangeability.
[0011] The present invention has been made in view of the
aforementioned circumstances, and an object thereof is to provide a
high-strength cold-rolled steel sheet having a tensile strength of
980 MPa or more, having good formability as evaluated by ductility
and stretch-flangeability, and having excellent crashworthiness.
Another object of the present invention is to provide a
high-strength electrogalvanized steel sheet having an
electrogalvanized layer on a surface of the high-strength
cold-rolled steel sheet, a high-strength hot-dip galvanized steel
sheet having a hot-dip galvanized layer on a surface of the
high-strength cold-rolled steel sheet, and a high-strength hot-dip
galvannealed steel sheet having a hot-dip galvannealed layer on a
surface of the high-strength cold-rolled steel sheet. Still another
object of the present invention is to provide a method for
producing a high-strength cold-rolled steel sheet, a high-strength
hot-dip galvanized steel sheet, and a high-strength hot-dip
galvannealed steel sheet having the above properties in
combination.
Means for Solving the Problems
[0012] A high-strength cold-rolled steel sheet having a tensile
strength of 980 MPa or more according to the present invention that
has solved the aforementioned problems is a steel sheet containing,
in mass %, C: 0.10% or more to 0.5% or less, Si: 1.0% or more to 3%
or less, Mn: 1.5% or more to 7% or less, P: more than 0% to 0.1% or
less, S: more than 0% to 0.05% or less, Al: 0.005% or more to 1% or
less, N: more than 0% to 0.01% or less, and O: more than 0% to
0.01% or less, with a balance being iron and inevitable impurities.
Further, the gist lies in that a metal structure at a position of
1/4 of a sheet thickness satisfies (1) to (4) below. The term "MA"
is an abbreviation for Martensite-Austenite Constituent.
(1) When the metal structure is observed with a scanning electron
microscope, an area ratio of ferrite relative to a whole of the
metal structure is more than 10% to 65% or less, with a balance
being a hard phase including quenched martensite and retained
austenite and including at least one selected from the group
consisting of bainitic ferrite, bainite, and tempered martensite.
(2) When the metal structure is measured by X-ray diffractometry, a
volume ratio V.sub..gamma. of retained austenite relative to the
whole of the metal structure is 5% or more to 30% or less. (3) When
the metal structure is observed with an optical microscope, an area
ratio V.sub.MA of an MA structure, in which quenched martensite and
retained austenite are combined, relative to the whole of the metal
structure is 3% or more to 25% or less, and an average
circle-equivalent diameter of the MA structure is 2.0 .mu.m or
less. (4) A ratio V.sub.MA/V.sub..gamma. of the area ratio V.sub.MA
of the MA structure to the volume ratio V.sub..gamma. of the
retained austenite satisfies a formula (i) below:
0.50.ltoreq.V.sub.MA/V.sub..gamma..ltoreq.1.50 (i).
[0013] The steel sheet may further contain, as other elements, in
mass %:
(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, (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, and the like.
[0014] A high-strength electrogalvanized steel sheet having an
electrogalvanized layer on a surface of the high-strength
cold-rolled steel sheet, a high-strength hot-dip galvanized steel
sheet having a hot-dip galvanized layer on a surface of the
high-strength cold-rolled steel sheet, and a high-strength hot-dip
galvannealed steel sheet having a hot-dip galvannealed layer on a
surface of the high-strength cold-rolled steel sheet are also
comprised within the scope of the present invention.
[0015] The high-strength cold-rolled steel sheet having a tensile
strength of 980 MPa or more and being excellent in formability and
crashworthiness according to the present invention can be produced
by subjecting a steel satisfying a component composition described
above to hot rolling with a rolling rate at a final stand of finish
rolling being 5 to 25% and with a finish rolling end temperature
being the Ar.sub.3 point or higher to 900.degree. C. or lower,
coiling with a coiling temperature being 600.degree. C. or lower,
and cooling to room temperature; cold rolling; heating, at an
average heating rate of 10.degree. C./second or more, to a
temperature region of 800.degree. C. or higher and lower than the
Ac.sub.3 point, and soaking by holding in the temperature region
for 50 seconds or more; cooling at an average cooling rate of
10.degree. C./second or more, to an arbitrary cooling stop
temperature T.degree. C. that lies in a temperature range of
50.degree. C. or higher and the Ms point or lower; and heating and
holding in a temperature region of higher than the cooling stop
temperature T.degree. C. and 550.degree. C. or lower for 50 seconds
or more, and thereafter cooling to room temperature.
[0016] A high-strength hot-dip galvanized steel sheet having a
tensile strength of 980 MPa or more and being excellent in
formability and crashworthiness according to the present invention
can be produced by subjecting a steel satisfying a component
composition described above to hot rolling with a rolling rate at a
final stand of finish rolling being 5 to 25% and with a finish
rolling end temperature being the Ar.sub.3 point or higher to
900.degree. C. or lower, coiling with a coiling temperature being
600.degree. C. or lower, and cooling to room temperature; cold
rolling; heating, at an average heating rate of 10.degree.
C./second or more, to a temperature region of 800.degree. C. or
higher and lower than the Ac.sub.3 point, and soaking by holding in
the temperature region for 50 seconds or more; cooling at an
average cooling rate of 10.degree. C./second or more, to an
arbitrary cooling stop temperature T.degree. C. that lies in a
temperature range of 50.degree. C. or higher and the Ms point or
lower; and heating and holding in a temperature region of higher
than the cooling stop temperature T.degree. C. and 550.degree. C.
or lower for 50 seconds or more, and after performing hot-dip
galvanizing within a holding time, cooling to room temperature.
[0017] A high-strength hot-dip galvannealed steel sheet having a
tensile strength of 980 MPa or more and being excellent in
formability and crashworthiness according to the present invention
can be produced by subjecting a steel satisfying a component
composition described above to hot rolling with a rolling rate at a
final stand of finish rolling being 5 to 25% and with a finish
rolling end temperature being the Ar.sub.3 point or higher to
900.degree. C. or lower, coiling with a coiling temperature being
600.degree. C. or lower, and cooling to room temperature; cold
rolling; heating, at an average heating rate of 10.degree.
C./second or more, to a temperature region of 800.degree. C. or
higher and lower than the Ac.sub.3 point, and soaking by holding in
the temperature region for 50 seconds or more; cooling at an
average cooling rate of 10.degree. C./second or more, to an
arbitrary cooling stop temperature T.degree. C. that lies in a
temperature range of 50.degree. C. or higher and the Ms point or
lower; and heating and holding in a temperature region of higher
than the cooling stop temperature T.degree. C. and 550.degree. C.
or lower for 50 seconds or more, and after performing hot-dip
galvanizing within a holding time, further performing an alloying
treatment and thereafter cooling to room temperature.
Effects of the Invention
[0018] According to the present invention, the component
composition and the metal structure are suitably controlled, so
that a high-strength cold-rolled steel sheet, a high-strength
electrogalvanized steel sheet, a high-strength hot-dip galvanized
steel sheet, and a high-strength hot-dip galvannealed steel sheet
having a tensile strength of 980 MPa or more and being excellent
both in formability as evaluated by ductility and
stretch-flangeability and in crashworthiness can be provided. The
high-strength cold-rolled steel sheet, the high-strength
electrogalvanized steel sheet, the high-strength hot-dip galvanized
steel sheet, and the high-strength hot-dip galvannealed steel sheet
according to the present invention is particularly excellent in
ductility among the formability properties. The present invention
can also provide a method for producing the high-strength
cold-rolled steel sheet, the high-strength electrogalvanized steel
sheet, the high-strength hot-dip galvanized steel sheet, and the
high-strength hot-dip galvannealed steel sheet described above. The
high-strength cold-rolled steel sheet, the high-strength
electrogalvanized steel sheet, the high-strength hot-dip galvanized
steel sheet, and the high-strength hot-dip galvannealed steel sheet
according to the present invention are extremely useful in the
fields of industry such as automobiles.
BRIEF DESCRIPTION OF DRAWINGS
[0019] FIG. 1 is a schematic descriptive view showing one example
of a heat treatment pattern performed in the Examples.
DESCRIPTION OF EMBODIMENTS
[0020] The present inventors have repeatedly made eager studies in
order to improve all of ductility, stretch-flangeability, and
crashworthiness in a high-strength cold-rolled steel sheet having a
tensile strength of 980 MPa or more. As a result, the present
inventors have found out that, in order to improve the ductility
while ensuring the tensile strength by setting a ferrite fraction
in the metal structure to be within a predetermined range and
setting the balance structure to be a hard phase, it is effective
to generate a predetermined amount of ferrite and to appropriately
control a ratio V.sub.MA/V.sub..gamma. of an area ratio V.sub.MA of
an MA structure, in which quenched martensite and retained
austenite are combined, to a volume ratio V.sub..gamma. of retained
austenite relative to the whole of the metal structure and that, in
order to improve the stretch-flangeability, it is effective to make
the MA structure finer and, in order to improve the
crashworthiness, it is effective to make the MA structure finer and
to appropriately control the above ratio V.sub.MA/V.sub..gamma.,
thereby completing the present invention.
[0021] First, the metal structure characterizing the present
invention will be described.
[0022] The high-strength cold-rolled steel sheet according to the
present invention is characterized in that the metal structure at a
position of 1/4 of the sheet thickness satisfies (1) to (4)
below.
(1) When the metal structure is observed with a scanning electron
microscope, the area ratio of ferrite relative to the whole of the
metal structure is more than 10% and 65% or less, with the balance
being a hard phase including quenched martensite and retained
austenite and including at least one selected from the group
consisting of bainitic ferrite, bainite, and tempered martensite.
(2) When the metal structure is measured by X-ray diffractometry,
the volume ratio V.sub..gamma. of retained austenite relative to
the whole of the metal structure is 5% or more and 30% or less. (3)
When the metal structure is observed with an optical microscope,
the area ratio V.sub.MA of an MA structure, in which quenched
martensite and retained austenite are combined, relative to the
whole of the metal structure is 3% or more and 25% or less, and an
average circle-equivalent diameter of the MA structure is 2.0 .mu.m
or less. (4) The volume ratio V.sub..gamma. of the retained
austenite and the area ratio V.sub.MA of the MA structure satisfy a
formula (i) below:
0.50.ltoreq.V.sub.MA/V.sub..gamma..ltoreq.1.50 (i).
[0023] The observation of the above metal structure is carried out
all at the position of 1/4 of the sheet thickness, as representing
the steel sheet.
[0024] Methods of measuring the fractions in the metal structure as
defined in the above (1) to (3) may differ from, each other, so
that a sum of the fractions may exceed 100%. In other words, in the
above (1), the metal structure is observed with a scanning electron
microscope, so that the measured area ratio is a ratio obtained
when the whole of the metal structure is assumed to be 100%. The
area ratio measured with use of a scanning electron microscope
includes that of quenched martensite and retained austenite as an
area ratio of the hard phase. On the other hand, in the above (2),
the retained austenite fraction in the metal structure is
calculated by X-ray diffractometry, while in the above (3), the
area ratio of the MA structure in which quenched martensite and
retained austenite are combined is observed with an optical
microscope. For this reason, the fraction of retained austenite and
quenched martensite is measured in a duplicated manner by a
plurality of methods. Also, hereafter, the retained austenite may
be denoted as retained .gamma.. Accordingly, a sum of the fractions
in the metal structure as defined in the above (1) to (3) may
exceed 100%. Also, the structure in which quenched martensite and
retained .gamma. are combined may be denoted as MA structure.
[0025] (1) In the present invention, the area ratio of ferrite
relative to the whole of the metal structure is set to be more than
10% to 65% or less when the metal structure is observed with a
scanning electron microscope. Ferrite is a structure that
particularly enhances ductility among the formability properties of
the steel sheet. In order that such an effect may be exhibited, the
area ratio of ferrite is set to be more than 10% in the present
invention. The area ratio of ferrite is preferably 15% or more,
more preferably 20% or more. However, when ferrite is excessive in
amount, the strength of the steel sheet decreases, so that a
tensile strength of 980 MPa or more cannot be ensured. Accordingly,
in the present invention, the area ratio of ferrite is set to be
65% or less. The area ratio of ferrite is preferably 60% or less,
more preferably 50% or less.
[0026] The balance of the above metal structure is a hard phase
including quenched martensite and retained .gamma. as an essential
structure and including at least one selected from the group
consisting of bainitic ferrite, bainite, and tempered martensite.
These hard phases constitute a structure that is harder than
ferrite, so that, by generating a predetermined amount of ferrite
and making the balance structure be a hard phase, the strength of
the steel sheet can be enhanced to be 980 MPa or more. The reason
why quenched martensite and retained .gamma. are contained as an
essential structure is, as described later, for the purpose of
generating a predetermined amount of an MA structure in which
quenched martensite and retained .gamma. are combined.
[0027] In addition to ferrite and the hard phase, the above metal
structure may contain at least one selected from the group
consisting of pearlite and cementite. A sum area ratio of pearlite
and cementite is not particularly limited as long as the effect of
the present invention is not deteriorated; however, the sum area
ratio is preferably, for example, 20% or less. The sum area ratio
is more preferably 15% or less, still more preferably 10% or
less.
[0028] The area ratio of the above metal structure may be
calculated by performing observation with a scanning electron
microscope after the position of 1/4 of the sheet thickness is
corroded with nital, and the observation magnification may be set
to be, for example, 1000 times.
[0029] (2) In the present invention, when the metal structure is
measured by X-ray diffractometry, the volume ratio V.sub..gamma. of
retained .gamma. relative to the whole of the metal structure is
set to be 5% or more to 30% or less. The retained .gamma. produces
an effect of suppressing concentration of strain by receiving the
strain so as to be deformed and transformed into martensite when
the steel sheet is processed, thereby promoting hardening of the
deformed portion during the processing. For this reason, the
strength-elongation balance of the steel sheet is enhanced, and the
ductility can be improved. In order that such an effect may be
exhibited, it is necessary that the volume ratio of retained
.gamma. is set to be 5% or more. The volume ratio of retained
.gamma. is preferably 6% or more, more preferably 7% or more.
However, when the volume ratio of retained .gamma. increases
excessively, the stretch-flangeability becomes deteriorated.
Accordingly, the volume ratio of retained .gamma. is set to be 30%
or less in the present invention. The volume ratio of retained
.gamma. is preferably 25% or less, more preferably 20% or less.
[0030] The above volume ratio of retained .gamma. may be determined
by measuring the position of 1/4 of the sheet thickness by X-ray
diffractometry. The retained .gamma. exists between the laths of
bainitic ferrite or by being included in the MA structure. The
above effect by the retained .gamma. is exhibited irrespective of
the existence form, so that, in the present invention, the volume
ratio was determined by calculating a sum of the amounts of all the
retained .gamma. measured by X-ray diffractometry irrespective of
the existence form.
[0031] (3) In the present invention, when the metal structure is
observed with an optical microscope, the area ratio V.sub.MA of the
MA structure relative to the whole of the metal structure is set to
be 3% or more to 25% or less. The above MA structure is a structure
that enhances the strength-elongation balance of the steel sheet
and can improve the ductility. In order that such an effect may be
exhibited, it is necessary that the area ratio of the MA structure
is set to be 3% or more. The area ratio of the MA structure is
preferably 4% or more, more preferably 5% or more. However, when
the area ratio of the MA structure increases excessively, the
crashworthiness becomes deteriorated. Accordingly, the area ratio
of the MA structure is set to be 25% or less in the present
invention. The area ratio of the MA structure is preferably 23% or
less, more preferably 20% or less.
[0032] Also, in the present invention, the average
circle-equivalent diameter of the MA structure is set to be 2.0
.mu.m or less. By making the MA structure be finer, the
stretch-flangeability and the crashworthiness can be enhanced. In
order that such an effect may be exhibited, it is necessary that
the average circle-equivalent diameter of the MA structure is set
to be 2.0 m or less. The average circle-equivalent diameter of the
MA structure is preferably 1.8 .mu.m or less, more preferably 1.5
.mu.m or less. According as the MA structure becomes finer, the
stretch-flangeability and the crashworthiness will be better, so
that a lower limit of the average circle-equivalent diameter of the
MA structure is not particularly limited; however, from an
industrial point of view, the lower limit is about 0.1 .mu.m.
[0033] The above MA structure is a structure in which quenched
martensite and retained .gamma. are combined. The quenched
martensite means a structure in a state in which untransformed
austenite is transformed into martensite during the process in
which the steel sheet is cooled from the heating temperature down
to room temperature. By observation with an optical microscope, the
quenched martensite can be distinguished from the tempered
martensite that has been tempered by a heating treatment. In other
words, when the metal structure is observed with an optical
microscope after being subjected to LePera corrosion, the quenched
martensite is observed to be white whereas the tempered martensite
is observed to be gray.
[0034] The quenched martensite and the retained .gamma. are hardly
distinguished from each other by observation with an optical
microscope, so that the structure in which quenched martensite and
retained .gamma. are combined is measured as the MA structure in
the present invention.
[0035] The area ratio of the above MA structure is a value as
measured at the position of 1/4 of the sheet thickness of the steel
sheet.
[0036] The average circle-equivalent diameter of the MA structure
is a value determined by calculating a circle-equivalent diameter
based on the area of each MA structure for all the MA structures
that are recognized in the field of observation and calculating an
average of the obtained circle-equivalent diameters.
[0037] (4) In the present invention, it is important that the ratio
V.sub.MA/V.sub..gamma. of the area ratio V.sub.MA of the MA
structure to the volume ratio V.sub..gamma. of the retained .gamma.
satisfies the following formula (i):
0.50.ltoreq.V.sub.MA/V.sub..gamma..ltoreq.1.50 (i).
[0038] The ductility and the crashworthiness are rendered
compatible with each other when the value of the above ratio
V.sub.MA/V.sub..gamma. is controlled to satisfy the above formula
(i). In other words, as described above, the retained .gamma. is
positively generated in the present invention in order to enhance
the strength-elongation balance that constitutes an index of
ductility. As a result of this, the MA structure is inevitably
formed in the steel sheet. Further, upon further studies on the
strength-elongation balance, it has been found out that, when a
predetermined amount of retained .gamma. is generated, it is good
to control the area ratio V.sub.MA of the MA structure so that the
value of the above ratio V.sub.MA/V.sub..gamma. may become 0.50 or
more. The value of the above ratio V.sub.MA/V.sub..gamma. is
preferably 0.55 or more, more preferably 0.60 or more. However,
when the value of the above ratio V.sub.MAN/V.sub..gamma. becomes
excessively large, the MA structure is excessively generated. The
quenched martensite that exists in the MA structure is a very hard
structure, so that, when the MA structure is excessively generated,
cracks are liable to be generated at the interface to other
structures at the time of collision, and accordingly, the
crashworthiness is rather deteriorated. Therefore, in the present
invention, the value of the above ratio V.sub.MA/V.sub..gamma. is
set to be 1.50 or less in order to reduce the area ratio of
quenched martensite in the MA structure to ensure the
crashworthiness. The value of the above ratio
V.sub.MA/V.sub..gamma. is preferably 1.40 or less, more preferably
1.30 or less.
[0039] As shown above, the metal structure of the high-strength
cold-rolled steel sheet that characterizes the present invention
has been described.
[0040] Next, the component composition of the high-strength
cold-rolled steel sheet according to the present invention will be
described. Hereafter, "%" with regard to the component composition
of a steel sheet means "mass %".
[0041] [C: 0.10% or More to 0.5% or Less]
[0042] C is an element that is necessary for ensuring the tensile
strength of 980 MPa or more and for enhancing the stability of
retained .gamma. to ensure a predetermined amount of the retained
.gamma.. In the present invention, the C amount is set to be 0.10%
or more. The C amount is preferably 0.12% or more, more preferably
0.15% or more. However, when the C amount is excessively large, the
strength after hot rolling increases, so that cracks may be
generated during the cold rolling, or the weldability of a final
product may decrease. Accordingly, the C amount is set to be 0.5%
or less. The C amount is preferably 0.40% or less, more preferably
0.30% or less, and still more preferably 0.25% or less.
[0043] [Si: 1.0% or More to 3% or Less]
[0044] Si is an element that acts as a solute-strengthening element
and contributes to a higher strength of the steel. Also, Si
suppresses generation of carbide and effectively acts for
generation of ferrite and retained .gamma., so that Si is an
element that is necessary for ensuring an excellent
strength-elongation balance. In the present invention, the Si
amount is set to be 1.0% or more. The Si amount is preferably 1.2%
or more, more preferably 1.35% or more, and still more preferably
1.5% or more. However, when the Si amount is excessively large, a
considerable scale is formed during the hot rolling to generate
scale marks on the surface of the steel sheet, thereby degrading
the surface property. Also, the pickling property is degraded as
well. Accordingly, the Si amount is set to be 3% or less. The Si
amount is preferably 2.8% or less, more preferably 2.6% or
less.
[0045] [Mn: 1.5% or More to 7% or Less]
[0046] Mn is an element that contributes to a higher strength of
the steel sheet by enhancing the hardenability. Further, Mn is an
element that is necessary for stabilizing .gamma. to generate
retained .gamma.. In the present invention, the Mn amount is set to
be 1.5% or more. The Mn amount is preferably 1.6% or more, more
preferably 1.7% or more, still more preferably 1.8% or more, and
furthermore preferably 2.0% or more. However, when the Mn amount is
excessively large, the strength after hot rolling increases, so
that cracks may be generated during the cold rolling, or the
weldability of the final product may decrease. Also, when Mn is
added in an excessively large amount, Mn is segregated to
deteriorate the ductility and the stretch-flangeability.
Accordingly, the Mn amount is set to be 7% or less. The Mn amount
is preferably 5.0% or less, more preferably 4.0% or less, and still
more preferably 3.0% or less.
[0047] [P: More than 0% to 0.1% or Less]
[0048] P is an impurity element that is inevitably contained and,
when contained in an excessively large amount, deteriorates the
weldability of the final product. Accordingly, the P amount is set
to be 0.1% or less in the present invention. The P amount is
preferably 0.08% or less, more preferably 0.05% or less. The
smaller the P amount is, the better it is. However, it is
industrially difficult to set the P amount to be 0%. A lower limit
of the P amount is 0.0005% from the industrial point of view.
[0049] [S: More than 0% to 0.05% or Less]
[0050] As with P, S is an impurity element that is inevitably
contained and, when contained in an excessively large amount,
deteriorates the weldability of the final product. Also, S forms
sulfide-based inclusions in the steel sheet, thereby causing
deterioration of the ductility and the stretch-flangeability of the
steel sheet. Accordingly, the S amount is set to be 0.05% or less
in the present invention. The S amount is preferably 0.01% or less,
more preferably 0.005% or less. The smaller the S amount is, the
better it is. However, it is industrially difficult to set the S
amount to be 0%. A lower limit of the S amount is 0.0001% from the
industrial point of view.
[0051] [Al: 0.005% or More to 1% or Less]
[0052] Al is an element that acts as a deoxidizer. In order that
such an action may be exhibited, the Al amount is set to be 0.005%
or more in the present invention. The Al amount is more preferably
0.01% or more. However, when the Al amount is excessively large,
the weldability of the final product is considerably deteriorated.
Accordingly, the Al amount is set to be 1% or less in the present
invention. The Al amount is preferably 0.8% or less, more
preferably 0.6% or less.
[0053] [N: More than 0% to 0.01% or Less]
[0054] N is an impurity element that is inevitably contained and,
when N is contained in an excessively large amount, nitride is
deposited in a large amount to deteriorate the ductility,
stretch-flangeability, and crashworthiness. Accordingly, the N
amount is set to be 0.01% or less in the present invention. The N
amount is preferably 0.008% or less, more preferably 0.005% or
less. Since nitride in a small amount contributes to a higher
strength of the steel sheet, the N amount may be 0.001% or
more.
[0055] [O: More than 0% to 0.01% or Less]
[0056] O is an impurity element that is inevitably contained and,
when contained in an excessively large amount, deteriorates the
ductility and the crashworthiness. Accordingly, the O amount is set
to be 0.01% or less in the present invention. The O amount is
preferably 0.005% or less, more preferably 0.003% or less. The
smaller the O amount is, the better it is. However, it is
industrially difficult to set the O amount to be 0%. A lower limit
of the O amount is 0.0001% from the industrial point of view.
[0057] The cold-rolled steel sheet according to the present
invention satisfies the aforementioned component composition, and
the balance is made of iron and inevitable impurities. The
inevitable impurities may include the above-mentioned elements such
as P, S, N, and O, which may be brought into the steel depending on
the circumstances of raw materials, facility materials, production
equipment, and the like, and may also include tramp elements such
as Pb, Bi, Sb, and Sn.
[0058] The cold-rolled steel sheet of the present invention may
further contain, as other elements,
(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, (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, and the like.
[0059] These elements of (a) to (e) may be contained either alone
or in an arbitrary combination. The reason why such ranges have
been set is as follows.
[0060] [(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]
[0061] Cr and Mo are each an element that acts to improve the
strength of the steel sheet by enhancing hardenability. In order
that such an action may be effectively exhibited, the amount of
each of Cr and Mo is preferably set to be 0.1% or more, more
preferably 0.3% or more. However, when these elements are contained
in an excessively large amount, the ductility and the
stretch-flangeability decrease. Also excessive addition leads to
higher costs. Accordingly, when Cr or Mo is contained alone, the
amount is preferably 1% or less, more preferably 0.8% or less,
still more preferably 0.5% or less. Cr and Mo may be used either
alone or in combination. When Cr and Mo are used in combination, it
is preferable that each amount is within the above range of the
content when used alone, and a sum of the contents of Cr and Mo is
1.5% or less.
[0062] [(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]
[0063] Ti, Nb, and V are each an element that acts to improve the
strength of the steel sheet by forming carbide and nitride in the
steel sheet and to make prior .gamma. grains finer. In order that
such an action may be effectively exhibited, the amount of each of
Ti, Nb, and V is preferably set to be 0.005% or more, more
preferably 0.010% or more. However, when these elements are
contained in an excessively large amount, carbide is deposited at
the grain boundary, so that the stretch-flangeability and the
crashworthiness of the steel sheet are deteriorated. Accordingly,
in the present invention, the amount of each of Ti, Nb, and V is
preferably set to be 0.15% or less, more preferably 0.12% or less,
and still more preferably 0.10% or less. These elements may be used
either alone or in combination of two or more that are arbitrarily
selected.
[0064] [(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]
[0065] Cu and Ni are each an element that acts effectively for
generation and stabilization of retained .gamma.. Also, Cu and Ni
act to improve the corrosion resistance of the steel sheet. In
order that such an action may be effectively exhibited, the amount
of each of Cu and Ni is preferably set to be 0.05% or more, more
preferably 0.10% or more. However, when Cu is contained in an
excessively large amount, the hot formability is deteriorated.
Accordingly, when Cu is added alone, the amount of Cu is preferably
set to be 1% or less, more preferably 0.8% or less, and still more
preferably 0.5% or less. On the other hand, when Ni is contained in
an excessively large amount, a higher cost is invited, so that the
amount of Ni is preferably set to be 1% or less, more preferably
0.8% or less, and still more preferably 0.5% or less. Cu and Ni may
be used either alone or in combination. When Cu and Ni are used in
combination, the above action is more likely to be exhibited, and
also, by incorporation of Ni, the deterioration of hot formability
caused by addition of Cu is more likely to be suppressed. When Cu
and Ni are used in combination, a sum of the amounts of Cu and Ni
is preferably set to be 1.5% or less, more preferably 1.0% or
less.
[0066] [(d) B: More than 0% to 0.005% or Less]
[0067] B is an element that improves hardenability and is an
element that acts to allow austenite to exist stably down to room
temperature. In order that such an action may be effectively
exhibited, the amount of B is preferably set to be 0.0005% or more,
more preferably 0.0010% or more, and still more preferably 0.0015%
or more. However, when B is contained in an excessively large
amount, boride may be generated to deteriorate the ductility.
Accordingly, the amount of B is preferably set to be 0.005% or
less. The amount of B is more preferably 0.004% or less, still more
preferably 0.0035% or less.
[0068] [(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]
[0069] Ca, Mg, and REM are elements that act to finely disperse the
inclusions in the steel sheet. In order that such an action may be
effectively exhibited, the amount of each of Ca, Mg, and REM is
preferably set to be 0.0005% or more, more preferably 0.0010% or
more. However, when these elements are added in an excessively
large amount, the castability, hot formability, and the like may be
deteriorated. Accordingly, the amount of each of Ca, Mg, and REM is
preferably set to be 0.01% or less, more preferably 0.008% or less,
and still more preferably 0.007% or less. These elements may be
used either alone or in combination of two or more that are
arbitrarily selected. In the present invention, REM is an
abbreviation for Rare earth metal (rare earth element), and is
meant to include lanthanoid elements which are fifteen elements
from La to Lu, and Sc and Y.
[0070] As shown above, the high-strength cold-rolled steel sheet
according to the present invention has been described.
[0071] An electrogalvanized layer, a hot-dip galvanized layer, or a
hot-dip galvannealed layer may be formed on a surface of the
high-strength cold-rolled steel sheet. In other words, the scope of
the present invention includes a high-strength electrogalvanized
steel sheet (which may hereafter be referred to as EG steel sheet)
having an electrogalvanized layer formed on a surface of the
high-strength cold-rolled steel sheet, a high-strength hot-dip
galvanized steel sheet (which may hereafter be referred to as GI
steel sheet) having a hot-dip galvanized layer formed on a surface
of the high-strength cold-rolled steel sheet, and a high-strength
hot-dip galvannealed steel sheet (which may hereafter be referred
to as GA steel sheet) having a hot-dip galvannealed layer formed on
a surface of the high-strength cold-rolled steel sheet.
[0072] Next, a method for producing the high-strength cold-rolled
steel sheet according to the present invention will be
described.
[0073] The high-strength cold-rolled steel can be produced by
subjecting a steel satisfying a component composition described
above to hot rolling with a rolling rate at a final stand of finish
rolling being 5 to 25% and with a finish rolling end temperature
being the Ar.sub.3 point or higher and 900.degree. C. or lower,
coiling with a coiling temperature being 600.degree. C. or lower,
and cooling to room temperature; cold rolling; heating, at an
average heating rate of 10.degree. C./second or more, to a
temperature region of 800.degree. C. or higher and lower than the
Ac.sub.3 point, and soaking by holding in the temperature region
for 50 seconds or more; cooling at an average cooling rate of
10.degree. C./second or more, to an arbitrary cooling stop
temperature T.degree. C. that lies in a temperature range of
50.degree. C. or higher and the Ms point or lower; and heating and
holding in a temperature region of higher than the cooling stop
temperature ToC and 550.degree. C. or lower for 50 seconds or more,
and thereafter cooling to room temperature.
[0074] Hereafter, the steps will be sequentially described.
[0075] [Rolling Rate at a Final Stand of Finish Rolling: 5 to
25%]
[0076] First, a steel satisfying the aforementioned component
composition is heated in accordance with a conventional method. A
heating temperature is not particularly limited; however, the
heating temperature is preferably set to be, for example, 1000 to
1300.degree. C. When the heating temperature is lower than
1000.degree. C., solid solution of carbide is insufficiently
formed, and a sufficient strength is hardly obtained. On the other
hand, when the heating temperature is higher than 1300.degree. C.,
the structure of the hot-rolled steel sheet becomes coarse, and
also the MA structure of the cold-rolled steel sheet is liable to
become coarse. As a result, the crashworthiness tends to
decrease.
[0077] After the heating, hot rolling is carried out. In the
present invention, it is important that the rolling rate at a final
stand of finish rolling is set to be 5 to 25%. When the rolling
rate is less than 5%, the austenite grain size after hot rolling
becomes coarse, and the average circle-equivalent diameter of the
MA structure in the cold-rolled steel sheet after annealing becomes
large. As a result, the stretch-flangeability decreases.
Accordingly, in the present invention, it is necessary that the
rolling rate is set to be 5% or more. The rolling rate is
preferably 6% or more, more preferably 7% or more, and still more
preferably 8% or more. However, when the rolling rate exceeds 25%,
the average circle-equivalent diameter of the MA structure also
becomes large, leading to deterioration of the
stretch-flangeability and crashworthiness. The mechanism therefor
is not clear; however, this seems to be because the structure after
hot rolling is made non-homogeneous. In the present invention, it
is necessary that the rolling rate is set to be 25% or less. The
rolling rate is preferably 23% or less, more preferably 20% or
less.
[0078] [Finish Rolling End Temperature: Ar.sub.3 Point or Higher
and 900.degree. C. or Lower]
[0079] When the finish rolling end temperature is lower than the
temperature of the Ar.sub.3 point, the steel sheet structure after
hot rolling becomes non-homogeneous, and the stretch-flangeability
decreases. On the other hand, when the finish rolling end
temperature exceeds 900.degree. C., recrystallization of austenite
occurs to make the crystal grains become coarse, and the average
circle-equivalent diameter of the MA structure in the cold-rolled
steel sheet becomes large. As a result, the stretch-flangeability
decreases. Accordingly, in the present invention, it is necessary
that the finish rolling end temperature is set to be 900.degree. C.
or lower. The finish rolling end temperature is preferably
890.degree. C. or lower, more preferably 880.degree. C. or
lower.
[0080] The temperature of the Ar.sub.3 point was calculated on the
basis of the following formula (ii). In the formula, brackets [ ]
indicate the content of each element (mass %), and calculation may
be made by assuming that the content of an element that is not
contained in the steel sheet is 0 mass %.
Ar.sub.3 point(.degree.
C.)=910-310.times.[C]-80.times.[Mn]-20.times.[Cu]-15.times.[Cr]-55.times.-
[Ni]-80.times.[Mo] (ii)
[0081] [Coiling Temperature: 600.degree. C. or Lower]
[0082] When the coiling temperature exceeds 600.degree. C., the
crystal grains become coarse, and the average circle-equivalent
diameter of the MA structure in the cold-rolled steel sheet becomes
large. As a result, the stretch-flangeability decreases.
Accordingly, in the present invention, the coiling temperature is
set to be 600.degree. C. or lower. The coiling temperature is
preferably 580.degree. C. or lower, more preferably 570.degree. C.
or lower, and still more preferably 550.degree. C. or lower.
[0083] [Cold Rolling]
[0084] After the hot rolling, the steel sheet may be coiled, cooled
to room temperature, pickled by a conventional method in accordance
with the needs, and subsequently cold-rolled by a conventional
method. The cold rolling rate in the cold rolling may be set to be,
for example, 30 to 80%.
[0085] [Annealing]
[0086] After the cold rolling, annealing is carried out by heating,
at an average heating rate of 10.degree. C./sec or more, to a
temperature region of 800.degree. C. or higher and lower than the
Ac.sub.3 point, and soaking by holding in the temperature region
for 50 seconds or more. When the average heating rate of heating to
the above temperature region after the cold rolling is lower than
10.degree. C./sec, the austenite grains grow and become coarse
during the heating, so that the average circle-equivalent diameter
of the MA structure in the cold-rolled steel sheet becomes large,
and the stretch-flangeability decreases. Accordingly, in the
present invention, the average heating rate is set to be 10.degree.
C./sec or more. The average heating rate is preferably 12.degree.
C./sec or more, more preferably 15.degree. C./sec or more. An upper
limit of the above average heating rate is not particularly
limited; however, the average heating rate is typically about
100.degree. C./sec at the maximum.
[0087] By setting the soaking temperature to be 800.degree. C. or
higher and lower than the Ac.sub.3 point, a desired ferrite amount
can be ensured. When the soaking temperature is lower than
800.degree. C., reverse transformation to austenite becomes
insufficient, and the processed structure remains in the
cold-rolled steel sheet, so that the formability decreases.
Accordingly, in the present invention, the soaking temperature is
set to be 800.degree. C. or higher. The soaking temperature is
preferably 805.degree. C. or higher, more preferably 810.degree. C.
or higher. However, when the soaking temperature is higher than the
temperature of the Ac.sub.3 point, a desired ferrite amount cannot
be ensured, and the ductility is deteriorated. Accordingly, in the
present invention, the soaking temperature is set to be lower than
the temperature of the Ac.sub.3 point. The soaking temperature is
preferably (Ac.sub.3 point-10.degree. C.) or lower, more preferably
(Ac.sub.3 point-20.degree. C.) or lower.
[0088] When the soaking time is less than 50 seconds, the processed
structure remains in the cold-rolled steel sheet, and the ductility
is deteriorated. Accordingly, in the present invention, the soaking
time is set to be 50 seconds or more. The soaking time is
preferably 60 seconds or more. An upper limit of the soaking time
is not particularly limited; however, when the soaking time is too
long, concentration of Mn into the austenite phase proceeds, and
the Ms point may decrease, leading to increase or coarsening of the
MA structure. Accordingly, the soaking time is preferably set to be
3600 seconds or less, more preferably 3000 seconds or less.
[0089] Regarding the soaking holding in the above temperature
region, the steel sheet need not be thermostatically held at the
same temperature, so that the steel sheet may be heated and cooled
in a fluctuating manner within the above temperature region.
[0090] The temperature of the aforementioned Ac.sub.3 point can be
calculated on the basis of the following formula (iii) disclosed in
"The Physical Metallurgy of Steels" (William C. Leslie, published
by Maruzen Co., Ltd. on May 31, 1985, page 273). In the formula,
brackets [ ] indicate the content of each element (mass %), and
calculation may be made by assuming that the content of an element
that is not contained in the steel sheet is 0 mass %.
Ac.sub.3(.degree.
C.)=910-203.times.[C].sup.1/2-15.2.times.[Ni]+44.7.times.[Si]+104.times.[-
V]+31.5.times.[Mo]+13.1.times.[W]-(30.times.[Mn]+11.times.[Cr]+20.times.[C-
u]-700.times.[P]-400.times.[Al]-120.times.[As]-400.times.[Ti])
(iii)
[0091] [Cooling]
[0092] After the above soaking holding, the steel sheet is cooled
to an arbitrary cooling stop temperature T.degree. C. that lies in
a temperature range of 50.degree. C. or higher and the Ms point or
lower. By cooling down to this temperature range, untransformed
austenite can be transformed to martensite and hard bainite phase,
and the MA structure also can be made finer. During this period,
martensite exists as quenched martensite immediately after the
transformation; however, the martensite is tempered while being
reheated and held in a later step and remains as tempered
martensite. This tempered martensite does not give adverse effects
on any of the ductility, stretch-flangeability, and crashworthiness
of the steel sheet. However, when the above cooling stop
temperature T exceeds the Ms point, martensite is not generated,
and the MA structure generated in the reheating holding step at a
high temperature becomes coarse, so that the local deformation
capability decreases, and the stretch-flangeability cannot be
improved. Accordingly, in the present invention, the cooling stop
temperature T is set to be equal to or lower than the temperature
of the Ms point. The cooling stop temperature T is preferably (Ms
point-20.degree. C.) or lower, more preferably (Ms point-50.degree.
C.) or lower. On the other hand, when the cooling stop temperature
T is lower than 50.degree. C., retained .gamma. and the MA
structure are generated only in a slight amount, so that the
ductility cannot be improved. Accordingly, in the present
invention, a lower limit of the cooling stop temperature T is set
to be 50.degree. C. or higher. The cooling stop temperature T is
preferably 60.degree. C. or higher, more preferably 70.degree. C.
or higher.
[0093] The temperature of the aforementioned Ms point can be
calculated on the basis of the following formula (iv). In the
formula, brackets [ ] indicate the content of each element (mass
%), and calculation may be made by assuming that the content of an
element that is not contained in the steel sheet is 0 mass %. Also,
Vf in the formula indicates the area ratio of ferrite relative to
the whole of the metal structure.
Ms point(.degree.
C.)=561-474.times.[C]/(1-Vf/100)-33.times.[Mn]-17.times.[Ni]-17.times.[Cr-
]-21.times.[Mo] (iv)
[0094] After performing the above soaking and holding, it is
important that an average cooling rate down to the cooling stop
temperature T that lies in the above temperature range is set to be
10.degree. C./sec or more. Excessive generation of ferrite can be
suppressed by appropriately controlling the cooling rate down to
the cooling stop temperature T after soaking and holding. In other
words, when the average cooling rate is lower than 10.degree.
C./sec, ferrite is excessively generated during the cooling, and
the tensile strength decreases. Accordingly, in the present
invention, the average cooling rate is set to be 10.degree. C./sec
or more. The average cooling rate is preferably 15.degree. C./sec
or more, more preferably 20.degree. C./sec or more. An upper limit
of the above average cooling rate is not particularly limited, and
the steel sheet may be cooled by cooling with water or cooling with
oil.
[0095] [Reheating Step]
[0096] After the steel sheet is cooled down to an arbitrary cooling
stop temperature T.degree. C. in the temperature range of
50.degree. C. or higher and the Ms point or lower, it is important
that the steel sheet is reheated to a temperature region of higher
than the cooling stop temperature T.degree. C. and 550.degree. C.
or lower, and the steel sheet is held in this temperature region
for 50 seconds or more. By reheating to the temperature region of
higher than the cooling stop temperature T.degree. C. and
550.degree. C. or lower, the hard phase such as martensite can be
tempered, and untransformed austenite can be transformed to
bainitic ferrite or bainite. When the reheating is not carried out,
the balance between the amounts of generation of retained .gamma.
and the MA structure becomes degraded, and the ratio
V.sub.MA/V.sub..gamma. of the area ratio V.sub.MA of the MA
structure to the volume ratio V.sub..gamma. of the retained .gamma.
cannot be controlled to be within an appropriate range. As a
result, the crashworthiness cannot be improved. Further, the hard
phase cannot be tempered, and dislocation at a high density is
generated. Accordingly, in the present invention, the steel sheet
is reheated to a temperature exceeding the cooling stop temperature
T after the steel sheet is cooled to the cooling stop temperature
T. The reheating temperature is preferably (T+20.degree. C.) or
higher, more preferably (T+30.degree. C.) or higher, and still more
preferably (T+50.degree. C.) or higher. However, when the reheating
temperature exceeds 550.degree. C., retained .gamma. and the MA
structure are generated only in a slight amount, so that the
tensile strength decreases, and the value of TS.times..lamda.
becomes smaller, making it impossible to improve the
stretch-flangeability. Accordingly, in the present invention, the
reheating temperature is set to be 550.degree. C. or lower. The
reheating temperature is preferably 520.degree. C. or lower, more
preferably 500.degree. C. or lower, and still more preferably
450.degree. C. or lower.
[0097] Here, in the present invention, "reheating" means, as it is
stated, heating, that is, raising the temperature from the above
cooling stop temperature T. Accordingly, the reheating temperature
is a temperature higher than the above cooling stop temperature T.
Therefore, even if the reheating temperature is, for example,
within a temperature region of 50.degree. C. or higher and
550.degree. C. or lower, this does not fall under the category of
the reheating of the present invention if the cooling stop
temperature T and the reheating temperature are the same as each
other or if the reheating temperature is lower than the cooling
stop temperature T.
[0098] After the steel sheet is reheated to the temperature region
of higher than the cooling stop temperature T.degree. C. and
550.degree. C. or lower, the steel sheet is held in the temperature
region for 50 seconds or more. When the reheating holding time is
less than 50 seconds, the MA structure is excessively generated,
and the ductility cannot be improved. Further, the MA structure
becomes coarse, and the average circle-equivalent diameter cannot
be appropriately controlled, so that the stretch-flangeability
cannot be improved either. Also, the ratio V.sub.MA/V.sub..gamma.
of the area ratio V.sub.MA of the MA structure to the volume ratio
V.sub..gamma. of the retained .gamma. cannot be appropriately
controlled, so that the crashworthiness cannot be improved either.
Furthermore, the hard phase cannot be sufficiently tempered, and
also the transformation of untransformed austenite to bainitic
ferrite or bainite does not proceed sufficiently. Accordingly, in
the present invention, the reheating holding time is set to be 50
seconds or more. The reheating holding time is preferably 80
seconds or more, more preferably 100 seconds or more, and still
more preferably 200 seconds or more. An upper limit of the
reheating holding time is not particularly limited. However, when
the holding time is long, the productivity decreases, and the
tensile strength tends to decrease. From such viewpoints, the
reheating holding time is preferably set to be 1500 seconds or
less, more preferably 1000 seconds or less.
[0099] After the steel sheet is reheated and held, the steel sheet
is cooled to room temperature. An average cooling rate during the
cooling is not particularly limited; however, the average cooling
rate is preferably, for example, 0.1.degree. C./sec or more, more
preferably 0.4.degree. C./sec or more. Further, the average cooling
rate is preferably, for example, 200.degree. C./sec or less, more
preferably 150.degree. C./sec or less.
[0100] [Plating Treatment]
[0101] After the reheating holding, the high-strength cold-rolled
steel sheet according to the present invention obtained by cooling
to room temperature may be subjected to electrogalvanization,
hot-dip galvanizing, or hot-dip galvannealing in accordance with a
conventional method.
[0102] The electrogalvanization may be carried out, for example, by
subjecting the above high-strength cold-rolled steel sheet to
energization while immersing the steel sheet into a zinc solution
of 50 to 60.degree. C. (particularly 55.degree. C.) so as to
perform an electrogalvanization treatment. The plating adhesion
amount is not particularly limited and may be, for example, about
10 to 100 g/m.sup.2 per one surface.
[0103] The hot-dip galvanizing may be carried out, for example, by
immersing the above high-strength cold-rolled steel sheet into a
hot-dip galvanizing bath of 300.degree. C. or higher and
550.degree. C. or lower, so as to perform a hot-dip galvanizing
treatment. The plating time may be suitably adjusted so that a
desired plating adhesion amount can be ensured. The plating time is
preferably set to be, for example, 1 to 10 seconds.
[0104] The hot-dip galvannealing may be carried out by performing
an alloying treatment after the above hot-dip galvanizing. The
alloying treatment temperature is not particularly limited;
however, when the alloying treatment temperature is too low, the
alloying does not proceed sufficiently, so that the alloying
treatment temperature is preferably 450.degree. C. or higher, more
preferably 460.degree. C. or higher, and still more preferably
480.degree. C. or higher. However, when the alloying treatment
temperature is too high, the alloying proceeds too much, and the Fe
concentration in the plating layer becomes high, thereby
deteriorating the plating adhesion property. From such a viewpoint,
the alloying treatment temperature is preferably 550.degree. C. or
lower, more preferably 540.degree. C. or lower, and still more
preferably 530.degree. C. or lower. The alloying treatment time is
not particularly limited and may be adjusted so that the hot-dip
galvanized layer may be alloyed. The alloying treatment time is
preferably, for example, 10 to 60 seconds.
[0105] A high-strength hot-dip galvanized steel sheet having a
tensile strength of 980 MPa or more and being excellent in
formability and crashworthiness according to the present invention
can also be produced by subjecting a steel satisfying a component
composition described above to hot rolling with a rolling rate at a
final stand of finish rolling being 5 to 25% and with a finish
rolling end temperature being the Ar.sub.3 point or higher and
900.degree. C. or lower, coiling with a coiling temperature being
600.degree. C. or lower, and cooling to room temperature; cold
rolling; heating, at an average heating rate of 10.degree.
C./second or more, to a temperature region of 800.degree. C. or
higher and lower than the Ac.sub.3 point, and soaking by holding in
the temperature region for 50 seconds or more; cooling at an
average cooling rate of 10.degree. C./second or more, to an
arbitrary cooling stop temperature T.degree. C. that lies in a
temperature range of 50.degree. C. or higher and the Ms point or
lower; and heating and holding in a temperature region of higher
than the cooling stop temperature T.degree. C. and 550.degree. C.
or lower for 50 seconds or more, and after performing hot-dip
galvanizing within a holding time, cooling to room temperature. In
other words, the steps until heating to the temperature region of
higher than the cooling stop temperature T.degree. C. and
550.degree. C. or lower are the same as those of the
above-described method for producing a high-strength cold-rolled
steel sheet according to the present invention, so that the hot-dip
galvanizing and the holding for 50 seconds or more that is carried
out in the above temperature region of higher than the cooling stop
temperature T.degree. C. and 550.degree. C. or lower may be
simultaneously carried out in the same step.
[0106] The hot-dip galvanizing may be carried out within the
holding time in the reheating temperature region, that is, in the
temperature region of higher than the cooling stop temperature
T.degree. C. and 550.degree. C. or lower, and a conventional method
can be adopted as a specific plating method. For example, the steel
sheet heated to the temperature region of higher than the cooling
stop temperature T.degree. C. and 550.degree. C. or lower may be
immersed into a plating bath adjusted to have a temperature within
the range of higher than the cooling stop temperature TOC and
550.degree. C. or lower, so as to perform a hot-dip galvanizing
treatment. The plating time may be suitably adjusted so that a
desired plating amount can be ensured within the time of the
reheating holding. The plating time is preferably set to be, for
example, 1 to 10 seconds.
[0107] There are the following three patterns of (I) to (III) as a
combination of the hot-dip galvanizing treatment; and only the
heating and without performing the plating treatment, in the
reheating.
[0108] (I) Only the heating is carried out, and thereafter, the
hot-dip galvanizing treatment is carried out.
[0109] (II) The hot-dip galvanizing treatment is carried out, and
thereafter, only the heating is carried out.
[0110] (III) Only the heating is carried out, and thereafter, the
hot-dip galvanizing treatment is carried out, and further, only the
heating is carried out, in this order.
[0111] The reheating temperature at which only the heating is
carried out and the temperature of the plating bath used for
performing the hot-dip galvanizing may be different from each
other. In the present invention, heating or cooling may be carried
out from one temperature to the other temperature. Furnace heating,
induction heating, or the like may be adopted as a method for the
heating.
[0112] A high-strength hot-dip galvannealed steel sheet having a
tensile strength of 980 MPa or more and being excellent in
formability and crashworthiness according to the present invention
can also be produced by subjecting a steel satisfying a component
composition described above to hot rolling with a rolling rate at a
final stand of finish rolling being 5 to 25% and with a finish
rolling end temperature being the Ar.sub.3 point or higher and
900.degree. C. or lower, coiling with a coiling temperature being
600.degree. C. or lower, and cooling to room temperature; cold
rolling; heating, at an average heating rate of 10.degree.
C./second or more, to a temperature region of 800.degree. C. or
higher and lower than the Ac.sub.3 point, and soaking by holding in
the temperature region for 50 seconds or more; cooling at an
average cooling rate of 10.degree. C./second or more, to an
arbitrary cooling stop temperature T.degree. C. that lies in a
temperature range of 50.degree. C. or higher and the Ms point or
lower; and heating and holding in a temperature region of higher
than the cooling stop temperature T.degree. C. and 550.degree. C.
or lower for 50 seconds or more, and after performing hot-dip
galvanizing within a holding time, further performing an alloying
treatment and thereafter cooling to room temperature. In other
words, the steps until heating to the temperature region of higher
than the cooling stop temperature TOC and 550.degree. C. or lower
are the same as those of the above-described method for producing a
high-strength cold-rolled steel sheet according to the present
invention, so that the hot-dip galvanizing and the holding for 50
seconds or more that is carried out in the above temperature region
of higher than the cooling stop temperature T.degree. C. and
550.degree. C. or lower may be simultaneously carried out in the
same step, and thereafter the hot-dip galvanized layer may be
alloyed, followed by cooling down to room temperature.
[0113] The alloying treatment temperature is not particularly
limited; however, when the alloying treatment temperature is too
low, the alloying does not proceed sufficiently, so that the
alloying treatment temperature is preferably 450.degree. C. or
higher, more preferably 460.degree. C. or higher, and still more
preferably 480.degree. C. or higher. However, when the alloying
treatment temperature is too high, the alloying proceeds too much,
and the Fe concentration in the plating layer becomes high, thereby
deteriorating the plating adhesion property. From such a viewpoint,
the alloying treatment temperature is preferably 550.degree. C. or
lower, more preferably 540.degree. C. or lower, and still more
preferably 530.degree. C. or lower.
[0114] The alloying treatment time is not particularly limited and
may be adjusted so that the hot-dip galvanized layer may be
alloyed. The alloying treatment time is preferably, for example, 10
to 60 seconds. The alloying treatment is carried out after
performing the hot-dip galvanizing treatment for a predetermined
period of time within the temperature region of higher than the
cooling stop temperature T.degree. C. and 550.degree. C. or lower,
so that the time needed for the alloying treatment is not included
in the holding time within the temperature region of higher than
the cooling stop temperature T.degree. C. and 550.degree. C. or
lower.
[0115] After performing the hot-dip galvanizing within the holding
time in the temperature region of higher than the cooling stop
temperature T.degree. C. and 550.degree. C. or lower and performing
the alloying treatment in accordance with the needs, the steel
sheet may be cooled down to room temperature. The average cooling
rate during the cooling is not particularly limited; however, the
average cooling rate is preferably, for example, 0.1.degree. C./sec
or more, more preferably 0.4.degree. C./sec or more. Further, the
average cooling rate is preferably, for example, 200.degree. C./sec
or less, more preferably 150.degree. C./sec or less.
[0116] The high-strength cold-rolled steel sheet according to the
present invention has a tensile strength of 980 MPa or more. The
tensile strength is preferably 1000 MPa or more, more preferably
1010 MPa or more. Further, the above high-strength cold-rolled
steel sheet is excellent in formability as evaluated by ductility
and stretch-flangeability, and also is excellent in
crashworthiness.
[0117] The ductility can be evaluated by strength--elongation
balance. In the present invention, those in which a product of the
tensile strength TS (MPa) and the elongation EL (%) is 17000 MPa%
or more are rated as acceptable. The value of TS.times.EL is
preferably 17100 MPa% or more, more preferably 17200 MPa% or
more.
[0118] The stretch-flangeability can be evaluated by strength-hole
expansion ratio balance. In the present invention, those in which a
product of the tensile strength TS (MPa) and the hole expansion
ratio .lamda. (%) is 20000 MPa% or more are rated as acceptable.
The value of TS.times..lamda. is preferably 21000 MPa% or more,
more preferably 22000 MPa% or more.
[0119] The crashworthiness can be evaluated by strength-VDA bending
angle balance. In the present invention, those in which a product
of the tensile strength TS (MPa) and the VDA bending angle
(.degree.) is 90000 MPa.degree. or more are rated as acceptable.
The value of TS.times.VDA bending angle is preferably 90500
MPa.degree. or more, more preferably 91000 MPa.degree. or more.
[0120] The thickness of the high-strength cold-rolled steel sheet
according to the present invention is not particularly limited;
however, the steel sheet is preferably a thin steel sheet having a
thickness of, for example, 6 mm or less.
[0121] The present application claims the rights of priority based
on Japanese Patent Application No. 2015-071438 filed on Mar. 31,
2015 and Japanese Patent Application No. 2015-225507 filed on Nov.
18, 2015. The entire contents of the specifications of Japanese
Patent Application No. 2015-071438 and Japanese Patent Application
No. 2015-225507 are incorporated in the present application by
reference.
EXAMPLES
[0122] Hereafter, the present invention will be described more
specifically by way of Examples; however, the invention is not
limited by the following Examples and can be carried out while
including additional modifications within a scope conforming to the
gist disclosed heretofore and hereinafter, all such modifications
being encompassed within the technical scope of the invention.
[0123] A steel containing the components given in the following
Table 1 with the balance being iron and inevitable impurities was
prepared by ingot-making and subjected to hot rolling, cold
rolling, and continuous annealing to produce a cold-rolled steel
sheet. In the following Table 1, "-" means that the corresponding
element is not contained. The following Table 1 also show the
temperature of the Ar.sub.3 point calculated on the basis of the
above formula (ii) and the temperature of the Ac.sub.3 point
calculated on the basis of the above formula (iii). Further, FIG. 1
shows one example of a heat treatment pattern that was carried out
in the continuous annealing. In FIG. 1, the reference sign 1
denotes a heating step, 2 a soaking step, 3 a cooling step, 4 a
reheating holding step, and 5 a cooling stop temperature.
[0124] [Hot Rolling]
[0125] A slab obtained by ingot-making was heated to 1250.degree.
C., and hot rolling was carried out to a sheet thickness of 2.3 mm
with the rolling reduction in the final stand of finish rolling
being set to be a rolling reduction shown in the following Table
2-1 or 2-2 and with the finish rolling end temperature being set to
be a temperature shown in the following Table 2-1 or 2-2. After the
hot rolling, the steel sheet was cooled down to a coiling
temperature shown in the following Table 2-1 or 2-2 at an average
cooling rate of 30.degree. C./sec, followed by coiling. After the
coiling, the steel sheet was cooled in air to room temperature, so
as to produce a hot-rolled steel sheet.
[0126] [Cold Rolling]
[0127] After the obtained hot-rolled steel sheet was pickled to
remove surface scale, cold rolling was carried out to produce a
cold-rolled steel sheet having a thickness of 1.2 mm.
[0128] [Continuous Annealing]
[0129] The obtained cold-rolled steel sheet was subjected to
continuous annealing based on the heat treatment pattern shown in
FIG. 1. That is, the obtained cold-rolled steel sheet was heated as
a heating step at an average heating rate shown in the following
Table 2-1 or 2-2 up to the soaking temperature shown in the
following Table 2-1 or 2-2, and was held at the soaking temperature
as a soaking step. The following Table 2-1 or 2-2 shows the soaking
time. Further, the following Table 2-1 or 2-2 shows a value
calculated by subtracting the soaking temperature from the
temperature of the Ac.sub.3 point.
[0130] After the soaking, the steel sheet was cooled as a cooling
step at an average cooling rate shown in the following Table 2-1 or
2-2 down to the cooling stop temperature T.degree. C. shown in the
following Table 2-1 or 2-2.
[0131] After the cooling, the steel sheet was heated to the
reheating temperature shown in the following Table 2-1 or 2-2 and
was held at the reheating temperature as a reheating holding step,
followed by cooling down to room temperature to produce a test
sample material. The following Table 2-1 or 2-2 shows the reheating
holding time. Also, the following Table 2-1 or 2-2 shows a value
calculated by subtracting the cooling stop temperature T from the
reheating temperature.
[0132] Nos. 8 and 11 shown in the following Table 2-1 are samples
in which the reheating holding step was not carried out after the
cooling was stopped at the cooling stop temperature T shown in the
following Table 2-1. That is, in No. 8, the steel sheet was cooled
with the cooling stop temperature T set to be 480.degree. C., and
thereafter cooled to 350.degree. C., which was lower than that
temperature, and held at 350.degree. C. for 300 seconds. For the
sake of convenience, the following Table 2-1 gives 350.degree. C.
in the section of the reheating temperature and gives 300 seconds
in the section of the reheating holding time. In No. 11, the steel
sheet was cooled with the cooling stop temperature T set to be
330.degree. .degree. C., and thereafter cooled to 300.degree. C.,
which was lower than that temperature, and held at 300.degree. C.
for 300 seconds. For the sake of convenience, the following Table
2-1 gives 300.degree. C. in the section of the reheating
temperature and gives 300 seconds in the section of the reheating
holding time.
[0133] [Electrogalvanization]
[0134] No. 2 shown in the following Table 2-1 is a sample in which
the above test sample material was immersed into a galvanizing bath
of 55.degree. C. to perform an electrogalvanization treatment and
thereafter washed with water and dried to produce an
electrogalvanized steel sheet. The electrogalvanization treatment
was carried out with an electric current density set to be 40
A/dm.sup.2. The galvanizing adhesion amount was 40 g/m.sup.2 per
one surface. In the electrogalvanization treatment, washing
treatments such as degreasing with alkaline aqueous solution
immersion, washing with water, and pickling or the like were
carried out as appropriate, so as to produce a test sample material
having an electrogalvanized layer on the surface of the cold-rolled
steel sheet. In the following Table 2-1, the section of
classification for No. 2 gives "EG".
[0135] [Hot-Dip Galvanizing]
[0136] No. 36 shown in the following Table 2-2 is a sample in which
the above test sample material was immersed into a hot-dip
galvanizing bath of 460.degree. C. to perform a hot-dip galvanizing
treatment, thereby to produce a hot-dip galvanized steel sheet. The
hot-dip galvanizing adhesion amount was 30 g/m.sup.2 per one
surface. In the following Table 2-2, the section of classification
for No. 36 gives "GI".
[0137] [Hot-Dip Galvannealing]
[0138] No. 18 shown in the following Table 2-1 is a sample in which
the above test sample material was immersed into a hot-dip
galvanizing bath of 460.degree. C. to perform a hot-dip galvanizing
treatment, followed by heating to 500.degree. C. to perform an
alloying treatment, thereby to produce a hot-dip galvannealed steel
sheet. The hot-dip galvannealing adhesion amount was 30 g/m.sup.2
per one surface. In the following Table 2-1, the section of
classification for No. 18 gives "GA".
[0139] Test sample materials in which none of the
electrogalvanization treatment, hot-dip galvanizing treatment, and
hot-dip galvannealing treatment was carried out are denoted as
"cold-rolled" in the section of classification in the following
Tables 2-1 and 2-2.
[0140] With respect to the obtained test sample materials, a metal
structure was observed by the following procedure.
[0141] [Observation of Metal Structure]
[0142] (Area Ratio of Ferrite and Hard Phase)
[0143] After the cross-section of the obtained test sample material
was polished, the test sample material was subjected to nital
corrosion, followed by performing observation at the position of
1/4 of the sheet thickness in three fields of view at a
magnification of 1000 times with a scanning electron microscope, so
as to capture a photomicrograph image. The observation field of
view was such that one field of view had a size of 100
.mu.m.times.100 .mu.m. With the lattice interval set to be 5 .mu.m,
the area ratio of ferrite was measured by the point counting method
with the number of lattice points being 20.times.20, and an average
value Vf of the three fields of view was calculated. The
calculation results are shown in the following Tables 3-1 and 3-2.
The area ratio of ferrite was calculated by excluding the area
ratio of the hard phase that existed in the ferrite phase.
[0144] Further, the Ms point was calculated in accordance with the
above formula (iv) based on the component composition shown in the
following Table 1 and the average area ratio Vf of ferrite shown in
the following Tables 3-1 and 3-2. The results are shown in the
following Tables 2-1 and 2-2. The following Tables 2-1 and 2-2 also
show a value obtained by subtracting the temperature of the Ms
point from the cooling stop temperature T.
[0145] In a similar manner, a sum area ratio of pearlite and
cementite was measured by the point counting method, and an average
value of the three fields of view was calculated. The calculation
results are shown in the following Tables 3-1 and 3-2. The sum area
ratio of pearlite and cementite is denoted as "other structures" in
the following Tables 3-1 and 3-2.
[0146] In the present Examples, the structure other than ferrite,
pearlite, and cementite calculated by the point counting method was
assumed to be a hard phase. In other words, a value obtained by
subtracting the area ratio of ferrite and the sum area ratio of
pearlite and cementite from 100% was calculated as an area ratio of
the hard phase. The results are shown in the following Tables 3-1
and 3-2.
[0147] As a result of observation of a specific structure
constituting the hard phase, it was found out that the hard phase
included quenched martensite and retained .gamma. and included at
least one selected from the group consisting of bainitic ferrite,
bainite, and tempered martensite.
[0148] (Volume Ratio V.sub..gamma. of Retained .gamma.)
[0149] The obtained test sample material was polished down to the
position of 1/4 of the sheet thickness with use of a sandpaper of
#1000 to #1500, and further the surface was subjected to
electrolytic polishing down to the depth of 10 to 20 .mu.m,
followed by measuring the volume ratio V.sub..gamma. of retained
.gamma. with use of an X-ray diffractometer. Specifically, "RINT
1500" manufactured by Rigaku Corporation was used as the X-ray
diffractometer and, with use of a Co target, a power of 40 kV-200
mA was output to measure the range of 40.degree. to 130.degree. in
terms of 2.theta.. The volume ratio V.sub..gamma. of retained
.gamma. was quantitated on the basis of the obtained bcc (.alpha.)
diffraction peaks (110), (200), and (211) and fcc (.gamma.)
diffraction peaks (111), (200), (220), and (311). The results are
shown in the following Tables 3-1 and 3-2.
[0150] (Area Ratio V.sub.MA and Average Circle-Equivalent Diameter
of MA Structure)
[0151] After the cross-section of the obtained test sample material
was polished, the test sample material was subjected to LePera
corrosion, followed by performing observation at the position of
1/4 of the sheet thickness in three fields of view at a
magnification of 1000 times with an optical microscope, so as to
capture a photomicrograph image. The observation field of view was
such that one field of view had a size of 100 .mu.m.times.100
.mu.m. The portion whitened by LePera corrosion was regarded as the
MA structure. With the lattice interval set to be 5 .mu.m, the area
ratio of the MA structure was measured by the point counting method
with the number of lattice points being 20.times.20, and an average
value of the three fields of view was calculated. The calculation
results are shown in the following Tables 3-1 and 3-2.
[0152] Upon subjecting the photomicrograph image captured with the
optical microscope to image analysis, the average circle-equivalent
diameter d of each MA structure was calculated, and an average
value was determined. The results are shown in the following Tables
3-1 and 3-2.
[0153] (Ratio of Area Ratio V.sub.MA of MA Structure to Volume
Ratio V.sub..gamma. of Retained .gamma.)
[0154] The ratio V.sub.MA/V.sub..gamma. of the area ratio V.sub.MA
of the MA structure to the volume ratio V.sub..gamma. of the
retained .gamma. was calculated on the basis of the volume ratio
V.sub..gamma. of the retained .gamma. and the area ratio V.sub.MA
of the MA structure calculated by the above-described procedure.
The calculation results are shown in the following Tables 3-1 and
3-2.
[0155] Next, with respect to the obtained test sample material, the
mechanical properties, ductility, stretch-flangeability, and
crashworthiness were evaluated by the following procedure.
[0156] [Evaluation of Mechanical Properties and Ductility]
[0157] A No. 5 test piece defined in JIS Z2201 was cut out so that
the direction perpendicular to the rolling direction of the
obtained test sample material would be a longitudinal direction.
With use of this test piece, a tensile test was carried out so as
to measure the tensile strength TS and the elongation EL. The
measurement results are shown in the following Tables 3-1 and
3-2.
[0158] In the present Examples, the samples in which the tensile
strength was 980 MPa or more were evaluated as having a high
strength and being acceptable, whereas the samples in which the
tensile strength was less than 980 MPa were evaluated as having an
insufficient strength and being a reject.
[0159] Also, the value of tensile strength TS.times.elongation EL
was calculated on the basis of the measured values of tensile
strength TS and elongation EL. The calculation results are shown in
the following Tables 3-1 and 3-2. The value of TS.times.EL
indicates a strength-elongation balance and serves as an index for
evaluating the ductility.
[0160] In the present Examples, the samples in which the value of
TS.times.EL was 17000 MPa% or more were evaluated as having an
excellent ductility and being acceptable, whereas the samples in
which the value of TS.times.EL was less than 17000 MPa% were
evaluated as having a poor ductility and being a reject.
[0161] [Evaluation of Stretch-Flangeability]
[0162] In order to evaluate the stretch-flangeability of the test
sample material, a hole expansion test was carried out according to
the Japan Iron and Steel Federation Standard JFS T 1001, so as to
measure the hole expansion ratio .lamda.. The measurement results
are shown in the following Tables 3-1 and 3-2.
[0163] Also, the value of tensile strength TS.times.hole expansion
ratio .lamda. was calculated on the basis of the measured values of
tensile strength TS and hole expansion ratio .lamda.. The
calculation results are shown in the following Tables 3-1 and 3-2.
The value of TS.times..lamda. indicates a strength-hole expansion
ratio balance and serves as an index for evaluating the
stretch-flangeability.
[0164] In the present Examples, the samples in which the value of
TS.times..lamda. was 20000 MPa% or more were evaluated as having an
excellent stretch-flangeability and being acceptable, whereas the
samples in which the value of TS.times..lamda. was less than 20000
MPa% were evaluated as having a poor stretch-flangeability and
being a reject.
[0165] [Evaluation of Crashworthiness]
[0166] It is disclosed in the following literature that the
crashworthiness is correlated to a bending angle. [0167]
Literature: P. Larour, H. Pauli, T. Kurz, T. Hebesberger:
"Influence of post uniform tensile and bending properties on the
crash behaviour of AHSS and press-hardening steel grades",
IDDRG2010
[0168] Accordingly, a bending test was carried out under the
following conditions on the basis of the VDA standard (VDA238-100)
defined by the German Association of the Automotive Industry. The
displacement at the maximum load measured by the bending test was
converted into an angle according to the VDA standard, so as to
determine the bending angle. The conversion results are shown in
the following Tables 3-1 and 3-2.
[0169] (Measurement Conditions)
[0170] Test method: support with rolls, pressing-in of punch
[0171] Roll diameter: .phi.30 mm
[0172] Punch shape: tip end R=0.4 mm
[0173] Distance between rolls: 2.9 mm
[0174] Punch pressing-in speed: 20 mm/min
[0175] Test piece dimension: 60 mm.times.60 mm
[0176] Bending direction: direction perpendicular to the rolling
direction
[0177] Testing machine: SIMAZU AUTOGRAPH 20 kN
[0178] Also, the value of tensile strength TS.times.VDA bending
angle.degree. was calculated on the basis of the values of the
tensile strength TS measured in the tensile test and the VDA
bending angle. The calculation results are shown in the following
Tables 3-1 and 3-2.
[0179] In the present Examples, the samples in which the value of
TS.times.VDA was 90000 MPa.degree. or more were evaluated as having
an excellent crashworthiness and being acceptable, whereas the
samples in which the value of TS.times.VDA was less than 90000
MPa.degree. were evaluated as having a poor crashworthiness and
being a reject.
[0180] On the basis of the above results, samples satisfying all of
the requirements: the value of TS being 980 MPa or more, the value
of TS.times.EL being 17000 MPa% or more, the value of
TS.times..lamda. being 20000 MPa% or more, and the value of
TS.times.VDA being 90000 MPa.degree. or more were regarded as the
present invention examples and listed as being acceptable in the
totaltotal evaluation section of the following Tables 3-1 and 3-2.
On the other hand, samples in which one or more of the value of TS,
the value of TS.times.EL, the value of TS.times..lamda., and the
value of TS.times.VDA failed to satisfy the above acceptance
standard were regarded as the comparative examples and listed as
being a reject in the total evaluation section of the following
Tables 3-1 and 3-2.
TABLE-US-00001 TABLE 1 Steel Component (mass %) type C Si Mn P S Al
Cr Mo Ti Nb V A 0.16 1.98 2.05 0.03 0.001 0.01 -- -- -- -- -- B
0.22 1.86 2.06 0.02 0.001 0.03 -- -- -- -- -- C 0.19 1.55 2.34 0.03
0.001 0.01 -- -- -- -- -- D 0.16 2.55 2.05 0.03 0.003 0.01 -- -- --
-- -- E 0.17 2.20 1.84 0.04 0.003 0.04 -- -- -- -- -- F 0.20 1.96
2.53 0.01 0.001 0.02 -- -- -- -- -- G 0.12 2.17 2.53 0.03 0.002
0.02 0.2 -- -- -- -- H 0.28 1.98 2.09 0.02 0.002 0.04 -- 0.5 -- --
-- I 0.42 1.73 2.35 0.04 0.001 0.04 -- -- 0.10 -- -- J 0.22 1.08
2.02 0.02 0.002 0.04 -- -- -- 0.08 -- K 0.23 1.37 2.08 0.04 0.002
0.03 -- -- -- -- 0.13 L 0.23 2.32 1.66 0.03 0.003 0.03 -- -- -- --
-- M 0.20 2.04 1.87 0.01 0.003 0.02 -- -- -- -- -- N 0.20 2.03 2.86
0.01 0.002 0.03 -- -- -- -- -- O 0.17 2.16 4.13 0.02 0.001 0.10 --
-- -- -- -- P 0.14 2.42 6.27 0.03 0.003 0.15 -- -- -- -- -- Q 0.07
1.84 2.17 0.03 0.003 0.03 -- -- -- -- -- R 0.18 0.55 2.06 0.02
0.001 0.05 -- -- -- -- -- S 0.16 1.65 1.10 0.03 0.001 0.03 -- -- --
-- -- Ar.sub.3 Ac.sub.3 Steel Component (mass %) point point type
Cu Ni B Ca Mg REM N O (.degree. C.) (.degree. C.) A -- -- -- -- --
-- 0.004 0.002 697 881 B -- -- -- -- -- -- 0.003 0.001 676 861 C --
-- -- -- -- -- 0.003 0.001 663 845 D -- -- -- -- -- -- 0.004 0.001
696 906 E -- -- -- -- -- -- 0.004 0.001 710 914 F -- -- -- -- -- --
0.005 0.001 647 847 G -- -- -- -- -- -- 0.005 0.001 668 888 H -- --
-- -- -- -- 0.004 0.001 616 874 I -- -- -- -- -- -- 0.004 0.001 593
870 J -- -- -- -- -- -- 0.002 0.001 680 832 K -- -- -- -- -- --
0.003 0.001 673 865 L 0.20 0.20 -- -- -- -- 0.002 0.001 692 893 M
-- -- 0.0015 -- -- -- 0.005 0.001 697 869 N -- -- -- 0.0022 -- --
0.003 0.001 620 844 O -- -- -- -- 0.0030 0.003 0.001 526 852 P --
-- -- -- -- 0.0025 0.003 0.001 364 835 Q -- -- -- -- -- -- 0.004
0.002 716 908 R -- -- -- -- -- -- 0.003 0.001 691 822 S -- -- -- --
-- -- 0.003 0.001 771 902
TABLE-US-00002 TABLE 2-1 Annealing step Reheating holding Re-
Cooling heating Cool- temper Hot rolling step Aver- Cool- ing
ature- Finish Heating Soaking age ing stop cooling rolling Final
Average Soak- Ac.sub.3- cool- stop temper- Re- stop Re- end stand
Coiling heating ing soaking Soak- ing temper- ature heating temper-
heating temper- rolling temper- rate temper- temper- ing rate ature
Ms T-Ms temper- ature holding Steel ature rate ature (.degree. C./
ature ature time (.degree. C./ T point point ature T time Classifi-
No. type (.degree. C.) (%) (.degree. C.) sec) (.degree. C.)
(.degree. C.) (sec) sec) (.degree. C.) (.degree. C.) (.degree. C.)
(.degree. C.) (.degree. C.) (sec) cation 1 A 880 15 550 15 820 61
300 15 250 349 -99 450 200 600 Cold-rolled 2 A 880 15 550 15 840 41
300 15 200 383 -183 380 180 600 EG 3 A 1000 15 550 15 820 61 300 15
300 323 -23 420 120 600 Cold-rolled 4 A 880 40 550 15 820 61 300 15
300 352 -52 450 150 600 Cold-rolled 5 A 880 3 550 15 820 61 300 15
250 344 -94 420 170 400 Cold-rolled 6 B 880 20 500 15 820 41 300 15
190 310 -120 420 230 600 Cold-rolled 7 B 880 20 550 15 900 -39 300
20 90 386 -296 380 290 600 Cold-rolled 8 B 880 20 500 15 850 11 300
15 480 372 108 350 -130 300 Cold-rolled 9 C 880 15 500 15 840 5 300
15 300 364 -64 420 120 600 Cold-rolled 10 C 880 15 500 15 840 5 300
15 25 369 -344 480 455 600 Cold-rolled 11 C 880 30 500 15 820 25
300 15 330 350 -20 300 -30 300 Cold-rolled 12 D 880 15 500 15 850
56 300 20 150 387 -237 380 230 600 Cold-rolled 13 D 880 7 500 15
850 56 300 20 200 384 -184 440 240 600 Cold-rolled 14 D 880 15 500
15 880 26 300 20 250 402 -152 450 200 5 Cold-rolled 15 E 880 10 550
20 850 64 300 20 250 387 -137 400 150 600 Cold-rolled 16 E 880 10
550 3 850 64 300 20 250 382 -132 450 200 600 Cold-rolled 17 F 880
15 550 15 830 17 300 15 170 353 -183 360 190 600 Cold-rolled 18 F
880 15 550 15 810 37 300 15 170 337 -167 440 270 300 GA 19 F 880 15
550 15 810 37 300 15 400 345 55 450 50 600 Cold-rolled 20 G 880 10
550 15 820 68 300 15 300 392 -92 450 150 600 Cold-rolled
TABLE-US-00003 TABLE 2-2 Annealing step Reheating holding Re-
Cooling heating Cool- temper- Hot rolling step Aver- Cool- ing
ature Finish Heating Soaking age ing stop cooling rolling Final
Average Soak- Ac.sub.3- cool- stop temper- Re- stop Re- end stand
Coiling heating ing soaking Soak- ing temper- ature heating temper-
heating temper- rolling temper- rate temper- temper- ing rate ature
Ms T-Ms temper- ature holding Steel ature rate ature (.degree. C./
ature ature time (.degree. C./ T point point ature T time Classifi-
No. type (.degree. C.) (%) (.degree. C.) sec) (.degree. C.)
(.degree. C.) (sec) sec) (.degree. C.) (.degree. C.) (.degree. C.)
(.degree. C.) (.degree. C.) (sec) cation 21 H 880 15 550 15 820 54
300 15 170 283 -113 450 280 600 Cold-rolled 22 I 880 20 550 15 850
20 300 20 100 257 -157 500 400 600 Cold-rolled 23 J 880 15 600 15
810 22 300 10 300 362 -62 450 150 600 Cold-rolled 24 K 880 15 550
15 820 45 300 10 200 350 -150 420 220 600 Cold-rolled 25 K 880 15
550 15 820 45 300 10 300 346 -46 600 300 1000 Cold-rolled 26 L 880
15 550 15 830 63 300 20 250 297 -47 420 170 600 Cold-rolled 27 L
880 15 550 15 830 63 300 1 300 106 194 420 120 600 Cold-rolled 28 M
880 15 550 15 840 29 300 20 250 333 -83 440 190 600 Cold-rolled 29
M 880 15 700 15 840 29 300 20 250 321 -71 460 210 600 Cold-rolled
30 N 880 15 600 15 810 34 300 10 200 345 -145 400 200 600
Cold-rolled 31 O 880 15 600 15 810 42 300 10 190 325 -135 480 290
600 Cold-rolled 32 P 880 15 600 15 805 30 300 10 150 278 -128 500
350 600 Cold-rolled 33 Q 880 15 500 15 840 68 300 20 200 443 -243
420 220 600 Cold-rolled 34 R 880 15 500 15 810 12 300 20 200 403
-203 420 220 600 Cold-rolled 35 S 880 15 500 15 830 72 300 10 200
247 -47 480 280 600 Cold-rolled 36 E 880 10 550 20 880 34 400 20
200 401 -201 440 240 200 GI 37 F 880 15 550 7 830 17 300 15 320 344
-24 460 140 600 Cold-rolled 38 E 880 10 550 20 820 94 300 7 300 204
96 400 100 300 Cold-rolled 39 C 880 15 500 15 830 15 300 15 210 356
-146 370 160 100 Cold-rolled 40 B 880 15 500 10 840 21 300 30 220
335 -115 380 160 600 Cold-rolled 41 A 880 15 550 20 830 51 100 20
220 380 -160 420 200 300 Cold-rolled
TABLE-US-00004 TABLE 3-1 Average circle- Structure fraction
equivalent MA diameter Ferrite Hard Other Retained .gamma.
structure of MA Material properties Vf phase structures
V.sub..gamma. V.sub.MA structure TS EL .lamda. VDA TS .times. EL TS
.times. .lamda. TS .times. VDA Total No. (area %) (area %) (area %)
(vol %) (area %) (.mu.m) V.sub.MA/V.sub..gamma. (MPa) (%) (%)
(.degree.) (MPa %) (MPa %) (MPa .degree.) evaluation 1 48 50 2 12
15 1.5 1.25 1022 22 24 92 22484 24528 94024 Acceptable 2 32 65 3 8
8 1.2 1.00 1216 17 31 76 20672 37696 92416 Acceptable 3 56 41 3 14
18 2.5 1.29 1051 20 17 86 21020 17867 90386 Reject 4 47 48 5 16 22
2.8 1.38 1073 22 15 79 23606 16095 84767 Reject 5 50 47 3 12 15 2.2
1.25 1031 20 18 89 20620 18558 91759 Reject 6 42 57 1 17 20 1.1
1.18 1055 26 32 98 27430 33760 103390 Acceptable 7 1 97 2 5 6 0.7
1.20 1281 10 42 82 12810 53802 105042 Reject 8 12 84 4 13 24 1.9
1.85 1057 19 21 75 20083 22197 79275 Reject 9 24 73 3 13 18 1.2
1.38 1036 19 22 94 19684 22792 97384 Acceptable 10 21 76 3 3 1 0.7
0.33 992 15 42 91 14880 41664 90272 Reject 11 32 65 3 7 12 2.3 1.71
1354 13 18 61 17602 24372 82594 Reject 12 28 68 4 10 13 1.1 1.30
1246 17 26 75 21182 32396 93450 Acceptable 13 30 68 2 12 15 1.6
1.25 1053 20 21 88 21060 22113 92664 Acceptable 14 16 83 1 14 35
3.4 2.50 1521 7 8 55 10647 12168 83655 Reject 15 29 66 5 13 16 1.4
1.23 1041 21 26 92 21861 27066 95772 Acceptable 16 32 66 2 12 17
2.4 1.42 992 21 17 91 20832 16864 90272 Reject 17 26 71 3 9 7 1.0
0.78 1222 17 31 78 20774 37882 95316 Acceptable 18 34 64 2 13 14
1.2 1.08 1062 22 28 86 23364 29736 91332 Acceptable 19 30 66 4 13
16 2.3 1.23 1034 18 15 88 18612 15510 90992 Reject 20 31 67 2 10 11
1.0 1.10 1029 17 31 91 17493 31899 93639 Acceptable
TABLE-US-00005 TABLE 3-2 Average circle- Structure fraction
equivalent MA diameter Ferrite Hard Other Retained .gamma.
structure of MA Material properties Vf phase structures
V.sub..gamma. V.sub.MA structure TS EL .lamda. VDA TS .times. EL TS
.times. .lamda. TS .times. VDA Total No. (area %) (area %) (area %)
(vol %) (area %) (.mu.m) V.sub.MA/V.sub..gamma. (MPa) (%) (%)
(.degree.) (MPa %) (MPa %) (MPa .degree.) evaluation 21 33 65 2 18
21 1.2 1.17 1238 18 22 76 22284 27236 94088 Acceptable 22 13 86 1
22 24 1.2 1.09 1421 14 21 65 19894 29841 92365 Acceptable 23 21 77
2 11 15 1.6 1.36 1053 17 23 87 17901 24219 91611 Acceptable 24 24
74 2 12 15 0.9 1.25 1017 20 24 100 20340 24408 101700 Acceptable 25
26 60 14 2 2 0.9 1.00 819 21 16 82 17199 13104 67158 Reject 26 48
49 3 16 19 1.0 1.19 1004 25 28 95 25100 28112 95380 Acceptable 27
73 22 5 11 16 1.5 1.45 921 29 16 101 26709 14736 93021 Reject 28 42
56 2 13 16 1.6 1.23 997 25 26 92 24925 25922 91724 Acceptable 29 46
50 4 12 17 2.3 1.42 1011 21 17 91 21231 17187 92001 Reject 30 23 75
2 9 10 0.9 1.11 1194 16 32 77 19104 38208 91938 Acceptable 31 18 82
0 13 17 1.5 1.31 1432 14 23 65 20048 32936 93080 Acceptable 32 12
88 0 12 17 1.7 1.42 1501 13 25 61 19513 37525 91561 Acceptable 33
32 66 2 3 5 1.0 1.67 1023 15 34 97 15345 34782 99231 Reject 34 8 89
3 5 6 1.1 1.20 1038 14 56 102 14532 58128 105876 Reject 35 72 20 8
11 14 1.7 1.27 938 25 15 97 23450 14070 90986 Reject 36 19 77 4 11
14 1.6 1.27 1005 20 31 94 20100 31155 94470 Acceptable 37 31 66 3
10 11 2.4 1.10 1027 17 17 93 17459 17459 95511 Reject 38 73 22 5 13
16 1.8 1.23 932 22 19 102 20504 17708 95064 Reject 39 29 69 2 10 9
1.0 0.90 1248 15 34 76 18720 42432 94848 Acceptable 40 33 64 3 13
16 1.5 1.23 1214 16 27 76 19424 32778 92264 Acceptable 41 34 63 3
10 12 1.3 1.20 1064 18 27 87 19152 28728 92568 Acceptable
[0181] From Tables 1, 2-1, 2-2, 3-1, and 3-2, the following
considerations can be made.
[0182] In Tables 3-1 and 3-2, all of the samples rated as
"acceptable" in the total evaluation section are steel sheets
satisfying the requirements defined in the present invention, and
all of the value of TS.times.EL, the value of TS.times..lamda., and
the value of TS.times.VDA determined in accordance with the tensile
strength TS satisfy the acceptance standard values. It will be
understood that these steel sheets have good formability as
evaluated by ductility and stretch-flangeability, and are excellent
in ductility in particular, and also in crashworthiness.
[0183] In contrast, the samples rated as "reject" in the total
evaluation section are steel sheets that do not satisfy one or more
of the requirements defined in the present invention, and at least
one of ductility, stretch-flangeability, and crashworthiness could
not be improved. The details are as follows.
[0184] No. 3 is a sample in which the MA structure was coarsened
because the finish rolling end temperature was too high. As a
result, the value of TS.times..lamda. was small, so that the
stretch-flangeability could not be improved.
[0185] No. 4 is a sample in which the MA structure was coarsened
because the rolling reduction at the final stand during the finish
rolling was too high and exceeded the range defined in the present
invention. As a result, the value of TS.times..lamda. was small, so
that the stretch-flangeability could not be improved. Also, the
value of TS.times.VDA was small, so that the crashworthiness could
not be improved.
[0186] No. 5 is a sample in which the MA structure was coarsened
because the rolling reduction at the final stand during the finish
rolling was too low and was below the range defined in the present
invention. As a result, the value of TS.times..lamda. was small, so
that the stretch-flangeability could not be improved.
[0187] No. 7 is a sample in which a ferrite amount within the range
defined in the present invention could not be ensured because the
soaking was carried out at a high temperature exceeding the
temperature region of 800.degree. C. or higher and lower than the
Ac.sub.3 point. As a result, the value of TS.times.EL was small, so
that the ductility could not be improved.
[0188] No. 8 is a sample in which the value of
V.sub.MA/V.sub..gamma. was too large because the cooling stop
temperature T after the soaking was too high and exceeded the
temperature region of 50.degree. C. or higher and the Ms point or
lower and because the reheating holding was not carried out after
the cooling. As a result, the value of TS.times.VDA was small, so
that the crashworthiness could not be improved.
[0189] No. 10 is a sample in which a predetermined amount of
retained .gamma. and the MA structure could not be ensured because
the cooling stop temperature T after the soaking was below
50.degree. C., so that the value of V.sub.MA/V.sub..gamma. was
small and below the defined range. As a result, the value of
TS.times.EL was small, so that the ductility could not be
improved.
[0190] No. 11 is a sample in which the MA structure was coarsened
and the value of V.sub.MA/V.sub..gamma. was too large because the
rolling reduction at the final stand during the finish rolling was
too high and exceeded the range defined in the present invention
and because the reheating holding was not carried out after the
cooling. As a result, the value of TS.times.VDA was small, so that
the crashworthiness could not be improved.
[0191] No. 14 is a sample in which the MA structure was coarsened
because the reheating holding time was too short. As a result, the
value of TS.times..lamda. was small, so that the
stretch-flangeability could not be improved. Further, the MA
structure was generated excessively. As a result, the value of
TS.times.EL was small, so that the ductility could not be improved.
Also, the value of V.sub.MA/V.sub..gamma. was too large. As a
result, the value of TS.times.VDA was small, so that the
crashworthiness was deteriorated.
[0192] Nos. 16 and 37 are samples in which the MA structure was
coarsened because the average heating rate after the coiling was
too small. As a result, the value of TS.times..lamda. was small, so
that the stretch-flangeability could not be improved.
[0193] No. 19 is a sample in which the MA structure was coarsened
because the cooling stop temperature T after the soaking was too
high and exceeded the temperature region of 50.degree. C. or higher
and the Ms point or lower. As a result, the value of
TS.times..lamda. was small, so that the stretch-flangeability could
not be improved.
[0194] No. 25 is a sample in which decomposition of austenite
occurred and a predetermined amount of retained .gamma. and the MA
structure could not be ensured because the reheating temperature
carried out after the cooling was too high. As a result, TS was
small. Also, the value of TS.times..kappa. was small, so that the
stretch-flangeability could not be improved. Further, the value of
TS.times.VDA was small, so that the crashworthiness could not be
improved.
[0195] Nos. 27 and 38 are samples in which ferrite was excessively
generated because the average cooling rate after the soaking was
too small. As a result, TS was small. Also, the value of
TS.times..lamda. was small, so that the stretch-flangeability could
not be improved.
[0196] No. 29 is a sample in which the MA structure was coarsened
because the coiling temperature was too high. As a result, the
value of TS.times..lamda. was small, so that the
stretch-flangeability could not be improved.
[0197] No. 33 is a sample in which the C amount was too small, so
that a retained .gamma. amount within the range defined in the
present invention could not be ensured, and the value of
V.sub.MA/V.sub..gamma. was so large as to exceed the range defined
in the present invention. As a result, the value of TS.times.EL was
small, so that the ductility was deteriorated.
[0198] No. 34 is a sample in which the Si amount was too small, so
that a ferrite amount within the range defined in the present
invention could not be ensured. As a result, the value of
TS.times.EL was small, so that the ductility was deteriorated.
[0199] No. 35 is a sample in which the Mn amount was too small, so
that the hardenability was insufficient, and ferrite was
excessively generated. As a result, TS was low. Further, the value
of TS.times..lamda. was small, so that the stretch-flangeability
was deteriorated.
REFERENCE SIGNS
[0200] 1 Heating step [0201] 2 Soaking step [0202] 3 Cooling step
[0203] 4 Reheating holding step [0204] 5 Cooling stop
temperature
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