U.S. patent application number 15/021193 was filed with the patent office on 2016-08-04 for hot-pressing steel plate, press-molded article, and method for manufacturing press-molded article.
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 Shushi IKEDA, Toshio MURAKAMI, Junya NAITOU, Keisuke OKITA.
Application Number | 20160222485 15/021193 |
Document ID | / |
Family ID | 52665203 |
Filed Date | 2016-08-04 |
United States Patent
Application |
20160222485 |
Kind Code |
A1 |
MURAKAMI; Toshio ; et
al. |
August 4, 2016 |
HOT-PRESSING STEEL PLATE, PRESS-MOLDED ARTICLE, AND METHOD FOR
MANUFACTURING PRESS-MOLDED ARTICLE
Abstract
A steel sheet for hot-pressing includes a specific chemical
component composition. In the steel sheet, an average
equivalent-circle diameter of a Ti-containing precipitate having an
equivalent-circle diameter of 30 nm or less among Ti-containing
precipitates contained in the steel sheet is 3 nm or more. In the
steel, a precipitated Ti amount and a total Ti amount in a steel
satisfy the relationship of: Precipitated Ti amount (mass
%)-3.4[N].gtoreq.0.5.times.[(total Ti amount (mass %))-3.4[N]] in
which [N] indicates the content (mass %) of N in the steel. In the
steel sheet, a ferrite fraction in a metal microstructure is 30
area % or more.
Inventors: |
MURAKAMI; Toshio; (Hyogo,
JP) ; NAITOU; Junya; (Hyogo, JP) ; OKITA;
Keisuke; (Hyogo, JP) ; IKEDA; Shushi; (Aichi,
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: |
52665203 |
Appl. No.: |
15/021193 |
Filed: |
September 10, 2013 |
PCT Filed: |
September 10, 2013 |
PCT NO: |
PCT/JP2013/074427 |
371 Date: |
March 10, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/18 20130101;
C22C 38/24 20130101; C22C 38/28 20130101; C21D 6/008 20130101; C22C
38/20 20130101; C22C 38/14 20130101; C21D 6/004 20130101; C21D
9/0068 20130101; B21D 22/022 20130101; C22C 38/12 20130101; C22C
38/04 20130101; C22C 38/26 20130101; C21D 9/46 20130101; B32B
15/013 20130101; C22C 38/50 20130101; C22C 38/32 20130101; C22C
38/02 20130101; C22C 38/38 20130101; C21D 1/18 20130101; B21D
22/208 20130101; C21D 7/13 20130101; C22C 38/00 20130101; C22C
38/22 20130101; C22C 38/005 20130101; C21D 2221/00 20130101; C21D
2211/004 20130101; C22C 38/06 20130101; C21D 1/20 20130101; C22C
38/002 20130101; C22C 38/34 20130101; C21D 2211/008 20130101; C21D
2211/005 20130101; C22C 38/001 20130101; C21D 2211/001 20130101;
C21D 1/673 20130101; C21D 6/005 20130101; C21D 8/005 20130101; C22C
38/54 20130101 |
International
Class: |
C21D 9/00 20060101
C21D009/00; C22C 38/54 20060101 C22C038/54; C22C 38/50 20060101
C22C038/50; C22C 38/38 20060101 C22C038/38; C22C 38/34 20060101
C22C038/34; C22C 38/32 20060101 C22C038/32; C22C 38/28 20060101
C22C038/28; C22C 38/26 20060101 C22C038/26; C22C 38/24 20060101
C22C038/24; C22C 38/22 20060101 C22C038/22; C22C 38/20 20060101
C22C038/20; C22C 38/18 20060101 C22C038/18; C22C 38/14 20060101
C22C038/14; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C22C 38/00 20060101
C22C038/00; C21D 8/00 20060101 C21D008/00; C21D 6/00 20060101
C21D006/00; C21D 1/20 20060101 C21D001/20; B21D 22/02 20060101
B21D022/02 |
Claims
1. A steel sheet for hot-pressing, comprising: C: from 0.15 to 0.5%
(mass %; hereinafter, the same applies to the chemical component
composition), Si: from 0.2 to 3%, Mn: from 0.5 to 3%, P: 0.05% or
less (exclusive of 0%), S: 0.05% or less (exclusive of 0%), Al:
from 0.01 to 1%, B: from 0.0002 to 0.01%, Ti: equal to or more than
3.4[N]+0.01% and equal to or less than 3.4[N]+0.1% (wherein [N]
indicates a content (mass %) of N), and N: from 0.001 to 0.01%,
with the remainder being iron and unavoidable impurities, wherein
an average equivalent-circle diameter of a Ti-containing
precipitate having an equivalent-circle diameter of 30 nm or less
among Ti-containing precipitates contained in the steel sheet is 3
nm or more, a precipitated Ti amount and a total Ti amount in a
steel satisfy a relationship of the following formula (1), and a
ferrite fraction in a metal microstructure is 30 area % or more:
Precipitated Ti amount (mass %)-3.4[N].gtoreq.0.5.times.[(total Ti
amount (mass %))-3.4[N]] (1) (in the formula (1), [N] indicates the
content (mass %) of N in the steel).
2. The steel sheet for hot-pressing according to claim 1,
comprising, as the other element(s), at least one of the following
(a) to (c): (a) one or more kinds selected from the group
consisting of V, Nb and Zr, in an amount of 0.1% or less (exclusive
of 0%) in total; (b) one or more kinds selected from the group
consisting of Cu, Ni, Cr and Mo, in an amount of 1% or less
(exclusive of 0%) in total; and (c) one or more kinds selected from
the group consisting of Mg, Ca and REM, in an amount of 0.01% or
less (exclusive of 0%) in total.
3. A method for manufacturing a press-formed article, the method
comprising: heating the steel sheet for hot-pressing as defined in
claim 1 at a temperature equal to or more than Ac.sub.1
transformation point+20.degree. C. and equal to or less than
Ac.sub.3 transformation point-20.degree. C.; starting press forming
of the steel sheet which has been heated; and then cooling the
steel sheet to a temperature equal to or less than a temperature
100.degree. C. below a bainite transformation starting temperature
Bs while ensuring an average cooling rate of 20.degree. C./sec or
more in a mold during forming and after a completion of
forming.
4. A press-formed article of a steel sheet having a chemical
component composition as defined in claim 1, wherein a metal
microstructure of the press-formed article includes retained
austenite: from 3 to 20 area %, ferrite: from 30 to 80 area %,
bainitic ferrite: less than 30 area % (exclusive of 0 area %), and
martensite: 31 area % or less (exclusive of 0 area %), and an
average equivalent-circle diameter of a Ti-containing precipitate
having an equivalent-circle diameter of 30 nm or less among
Ti-containing precipitates contained in the press-formed article is
3 nm or more, and a carbon amount in the retained austenite is
0.50% or more.
5. A method for manufacturing a press-formed article, the method
comprising: wherein using the steel sheet for hot-pressing as
defined in claim 1 dividing a heating region of the steel sheet
into at least two regions; heating one region of the divided
regions at a temperature of Ac.sub.3 transformation point or more
and 950.degree. C. or less; heating another region of the divided
regions at a temperature equal to or more than Ac.sub.1
transformation point+20.degree. C. and equal to or less than
Ac.sub.3 transformation point-20.degree. C.; and then, starting
press forming of both regions, and then cooling the steel sheet to
a temperature equal to or less than a martensite transformation
starting temperature Ms while ensuring an average cooling rate of
20.degree. C./sec or more in a mold in both of the regions during
forming and after a completion of forming.
6. A press-formed article of a steel sheet having a chemical
component composition as defined in claim 1, which has a first
region having a metal microstructure including retained austenite:
from 3 to 20 area % and martensite: 80 area % or more and a second
region having a metal microstructure including retained austenite:
from 3 to 20 area %, ferrite: from 30 to 80 area %, bainitic
ferrite: less than 30 area % (exclusive of 0 area %), and
martensite: 31 area % or less (exclusive of 0 area %), and a carbon
amount in the retained austenite is 0.50% or more.
7. A method for manufacturing a press-formed article, the method
comprising: heating the steel sheet for hot-pressing as defined in
claim 2 at a temperature equal to or more than Ac.sub.1
transformation point+20.degree. C. and equal to or less than
Ac.sub.3 transformation point-20.degree. C.; starting press forming
of the steel sheet which has been heated; and then, cooling the
steel sheet to a temperature equal to or less than a temperature
100.degree. C. below a bainite transformation starting temperature
Bs while ensuring an average cooling rate of 20.degree. C./sec or
more in a mold during forming and after a completion of
forming.
8. A press-formed article of a steel sheet having a chemical
component composition as defined in claim 2, wherein a metal
microstructure of the press-formed article includes retained
austenite: from 3 to 20 area %, ferrite: from 30 to 80 area %,
bainitic ferrite: less than 30 area % (exclusive of 0 area %), and
martensite: 31 area % or less (exclusive of 0 area %), and an
average equivalent-circle diameter of a Ti-containing precipitate
having an equivalent-circle diameter of 30 nm or less among
Ti-containing precipitates contained in the press-formed article is
3 nm or more, and a carbon amount in the retained austenite is
0.50% or more.
9. A method for manufacturing a press-formed article, the method
comprising: using the steel sheet for hot-pressing as defined in
claim 2; dividing a heating region of the steel sheet into at least
two regions; heating one region of the divided regions at a
temperature of Ac.sub.3 transformation point or more and
950.degree. C. or less; heating another region of the divided
regions at a temperature equal to or more than Ac.sub.1
transformation point+20.degree. C. and equal to or less than
Ac.sub.3 transformation point-20.degree. C.; and then, starting
press forming of both regions; and then, cooling the steel sheet to
a temperature equal to or less than a martensite transformation
starting temperature Ms while ensuring an average cooling rate of
20.degree. C./sec or more in a mold in both of the regions during
forming and after a completion of forming.
10. A press-formed article of a steel sheet having a chemical
component composition as defined in claim 2, which has a first
region having a metal microstructure including retained austenite:
from 3 to 20 area % and martensite: 80 area % or more and a second
region having a metal microstructure including retained austenite:
from 3 to 20 area %, ferrite: from 30 to 80 area %, bainitic
ferrite: less than 30 area % (exclusive of 0 area %), and
martensite: 31 area % or less (exclusive of 0 area %), and a carbon
amount in the retained austenite is 0.50% or more.
Description
TECHNICAL FIELD
[0001] The present invention relates to a steel sheet for
hot-pressing to be used for an automotive structural component and
suitable for hot-press forming, a press-formed article obtained
from the steel sheet for hot-pressing, and a method for
manufacturing a press-formed article. More specifically, the
present invention relates to a steel sheet for hot-pressing which
is useful, when forming a previously heated steel sheet (blank)
into a predetermined shape, for the application to a hot-press
forming method of imparting a shape, and applying a heat treatment
to obtain a predetermined strength, a press-formed article, and a
method useful for the manufacture of such a press-formed
article.
BACKGROUND ART
[0002] As one of the measures for automotive fuel economy
improvement triggered by global environmental problems, weight
saving of a vehicle body is proceeding, and in turn, the strength
of a steel sheet used for automobiles must be increased as much as
possible. On the other hand, when the strength of a steel sheet is
increased, the shape accuracy during press forming decreases.
[0003] For this reason, a component (press-formed article) is
manufactured by employing a hot-press forming method where a steel
sheet is heated to a given temperature (e.g., a temperature for
forming an austenite phase) to lower the strength and then formed
with a mold at a temperature (e.g., room temperature) lower than
that of the steel sheet to impart a shape and, perform
rapid-cooling heat treatment (quenching) by making use of a
temperature difference therebetween so as to ensure the strength
after forming. Such a hot-press forming method is referred to by
various names such as hot forming method, hot stamping method, hot
stamp method and die quenching method, in addition to hot-pressing
method.
[0004] FIG. 1 is a schematic explanatory view showing the mold
configuration for carrying out the above-described hot-press
forming. In FIG. 1, 1 is a punch, 2 is a die, 3 is a blank holder,
4 is a steel sheet (blank), BHF is a blank holding force, rp is a
punch shoulder radius, rd is a die shoulder radius, and CL is a
punch-to-die clearance. Of these parts, the punch 1 and the die 2
are configured such that passages 1a and 2a allowing for passing of
a cooling medium (e.g., water) are formed in respective insides and
the parts are cooled by passing a cooling medium through the
passage.
[0005] When hot-press forming (for example, hot deep drawing) is
performed using such a mold, the forming is started in a state
where the steel sheet (blank) 4 is softened by heating at a
two-phase zone temperature of (Ac.sub.1 transformation point to
Ac.sub.3 transformation point) or a single-phase zone temperature
equal to or more than Ac.sub.3 transformation point. More
specifically, in the state of the steel sheet 4 at a high
temperature being sandwiched between the die 2 and the blank holder
3, the steel sheet 4 is pushed into a hole of the die 2 (between 2
and 2 in FIG. 1) by the punch 1 and formed into a shape
corresponding to the outer shape of the punch 1 while reducing the
outer diameter of the steel sheet 4. In addition, heat is removed
from the steel sheet 4 to the mold (the punch 1 and the die 2) by
cooling the punch and the die in parallel with forming, and
quenching of the material (steel sheet) is carried out by further
holding and cooling the steel sheet at the forming bottom dead
center (the point when the punch head is positioned at the deepest
part: the state shown in FIG. 1). By carrying out such a forming
method, a formed article of 1500 MPa class can be obtained with
high dimensional accuracy and moreover, the forming load can be
reduced as compared with a case of forming a component of the same
strength class by cold working, so that the volume required of the
pressing machine can be small.
[0006] As the steel sheet for hot-pressing which is widely used at
present, a steel sheet using 22MnB5 steel as the material is known.
This steel sheet has a tensile strength of 1,500 MPa and an
elongation of approximately from 6 to 8% and is applied to an
impact-resistant member (a member that undergoes as little a
deformation as possible at the time of collision and is not
fractured). However, its application to a component requiring a
deformation, such as energy-absorbing member, is difficult because
of low elongation (ductility).
[0007] As the steel sheet for hot-pressing which exerts good
elongation, the techniques of, for example, Patent Documents 1 to 4
have also been proposed. In these techniques, the carbon content in
the steel sheet is set in various ranges to adjust the fundamental
strength class of respective steel sheets, and the elongation is
enhanced by introducing a ferrite having high deformability and
reducing the average particle diameters of ferrite and martensite.
The techniques above are effective in enhancing the elongation but
in view of elongation enhancement according to the strength of the
steel sheet, it is still insufficient. For example, the elongation
EL of a steel sheet having a tensile strength TS of 1,470 MPa or
more is about 10.2% at the maximum, and further improvement is
demanded.
[0008] On the other hand, a formed article of a low strength class
as compared with hot-stamp formed articles which have been
heretofore studied, for example, a formed article having a tensile
strength TS of 980 MPa class or 1,180 MPa class, also has a problem
with the forming accuracy in the cold pressing, and as an
improvement measure thereof, there is a need for low-strength hot
pressing. In this case, the energy absorption properties in a
formed article must be greatly improved.
[0009] Particularly, in recent years, a technique for
differentiating the strength within a single component is being
developed. As such a technique, a technique of imparting high
strength to a site that must be prevented from deforming (high
strength side: impact resistant site-side) and imparting low
strength and high ductility to a site that must absorb energy (low
strength side: energy absorption site-side) has been proposed. For
example, in a passenger car of middle or higher class, both
functional sites of impact resistance and energy absorption are
sometimes provided in a component of B-pillar or rear side member
by taking into account the compatibility at the time of side
collision and rear collision (a function of protecting also the
counterpart side when involved in a collision with a small car).
For manufacturing such a member, there have been proposed, for
example, (a) a method where a steel sheet having low strength even
when heated/mold quenched at the same temperature is joined to a
normal steel sheet for hot-pressing (tailored weld blank: TWB), (b)
a method where the cooling rate in the mold is differentiated to
create a difference in the strength among respective regions of a
steel sheet, (c) a method where a difference in the heating
temperature is created among respective regions of a steel sheet to
differentiate the strength.
[0010] In these techniques, a tensile strength of 1,500 MPa class
is achieved on the high strength side (impact resistant site-side),
but the low strength side (energy absorption site-side) stays at a
maximum tensile strength of 700 MPa and an elongation EL of about
17% and in order to further improve the energy absorption
properties, it is required to realize higher strength and higher
ductility.
[0011] In addition, in order to realize a complicated shape by hot
stamping, applicability to an approach of performing press forming
at room temperature to create a shape to a certain degree and then
performing hot stamping is required, or since a steel sheet for use
in press forming of hot stamping is cut out, the strength of a
steel sheet for hot-stamping is also required not to be excessively
high.
RELATED ART
Patent Document
[0012] Patent Document 1: JP-A-2010-65292 [0013] Patent Document 2:
JP-A-2010-65293 [0014] Patent Document 3: JP-A-2010-65294 [0015]
Patent Document 4: JP-A-2010-65295
SUMMARY OF THE INVENTION
Problems that the Invention is to Solve
[0016] The present invention has been made under these
circumstances, and an object thereof is to provide a steel sheet
for hot-pressing which makes it possible to easily conduct forming
or working before hot pressing, obtain a press-formed article
capable of achieving a high-level balance between high strength and
elongation when uniform properties are required in a formed
article, achieve a high-level balance between high strength and
elongation according to respective regions when regions
corresponding to an impact resistant site and an energy absorption
site are required in a single formed article; a press-formed
article exerting the above-described properties; and a method
useful for manufacturing such a press-formed article.
Means for Solving the Problems
[0017] The steel sheet for hot-pressing in the present invention,
which can attain the object above, contains:
[0018] C: from 0.15 to 0.5% (mass %; hereinafter, the same applies
to the chemical component composition),
[0019] Si: from 0.2 to 3%,
[0020] Mn: from 0.5 to 3%,
[0021] P: 0.05% or less (exclusive of 0%),
[0022] S: 0.05% or less (exclusive of 0%),
[0023] Al: from 0.01 to 1%,
[0024] B: from 0.0002 to 0.01%,
[0025] Ti: equal to or more than 3.4[N]+0.01% and equal to or less
than 3.4[N]+0.1% (wherein [N] indicates a content (mass %) of N),
and
[0026] N: from 0.001 to 0.01%, with the remainder being iron and
unavoidable impurities, in which
[0027] an average equivalent-circle diameter of a Ti-containing
precipitate having an equivalent-circle diameter of 30 nm or less
among Ti-containing precipitates contained in the steel sheet is 3
nm or more, a precipitated Ti amount and a total Ti amount in a
steel satisfy a relationship of the following formula (1), and a
ferrite fraction in a metal microstructure is 30 area % or more.
Here, the "equivalent-circle diameter" is the diameter of a circle
having the same area as the size (area) of a Ti-containing
precipitate (e.g., TiC) when the precipitate is converted to a
circle ("the average equivalent-circle diameter" is the average
value thereof).
Precipitated Ti amount(mass %)-3.4[N].gtoreq.0.5.times.[(total Ti
amount(mass %))-3.4[N]] (1)
(in the formula (1), [N] indicates the content (mass %) of N in the
steel).
[0028] In the steel sheet for hot-pressing in the present
invention, it is also useful to contain, as the other element(s),
at least one of the following (a) to (c), if desired. The
properties of the press-formed article are further improved
according to the kind of the element that is contained according to
need.
[0029] (a) One or more kinds selected from the group consisting of
V, Nb and Zr, in an amount of 0.1% or less (exclusive of 0%) in
total
[0030] (b) One or more kinds selected from the group consisting of
Cu, Ni, Cr and Mo, in an amount of 1% or less (exclusive of 0%) in
total
[0031] (c) One or more kinds selected from the group consisting of
Mg, Ca and REM, in an amount of 0.01% or less (exclusive of 0%) in
total
[0032] In the method for manufacturing a press-formed article in
the present invention, which can attain the object above, the steel
sheet for hot-pressing in the present invention is heated at a
temperature equal to or more than Ac.sub.1 transformation
point+20.degree. C. and equal to or less than Ac.sub.3
transformation point-20.degree. C., then press forming of the steel
sheet is started, and the steel sheet is cooled to a temperature
equal to or less than a temperature 100.degree. C. below a bainite
transformation starting temperature Bs while ensuring an average
cooling rate of 20.degree. C./sec or more in a mold during forming
and after a completion of forming.
[0033] In the press-formed article in the present invention, the
metal microstructure in the press-formed article includes retained
austenite: from 3 to 20 area %, ferrite: from 30 to 80 area %,
bainitic ferrite: less than 30 area % (exclusive of 0 area %), and
martensite: 31 area % or less (exclusive of 0 area %), and an
average equivalent-circle diameter of a Ti-containing precipitate
having an equivalent-circle diameter of 30 nm or less among
Ti-containing precipitates contained in the press-formed article is
3 nm or more, a carbon amount in the retained austenite is 0.50% or
more, and a high-level balance between high strength and elongation
can be achieved as uniform properties in the press-formed
article.
[0034] On the other hand, in another method for manufacturing a
press-formed article in the present invention, which can attain the
object above, the above steel sheet for hot-pressing is used, a
heating region of the steel sheet is divided into at least two
regions, one region of them is heated at a temperature of Ac.sub.3
transformation point or more and 950.degree. C. or less, another
region of them is heated at a temperature equal to or more than
Ac.sub.1 transformation point+20.degree. C. and equal to or less
than Ac.sub.3 transformation point-20.degree. C., then press
forming of both regions is started, and the steel sheet is cooled
to a temperature equal to or less than a martensite transformation
starting temperature Ms while ensuring an average cooling rate of
20.degree. C./sec or more in a mold in both of the regions during
forming and after a completion of forming.
[0035] Another press-formed article in the present invention is a
press-formed article of a steel sheet having the chemical component
composition above, and the press-formed article has a first region
having a metal microstructure including retained austenite: from 3
to 20 area % and martensite: 80 area % or more and a second region
having a metal microstructure including retained austenite: from 3
to 20 area %, ferrite: from 30 to 80 area %, bainitic ferrite: less
than 30 area % (exclusive of 0 area %), and martensite: 31 area %
or less (exclusive of 0 area %), and the carbon amount in the
retained austenaite in the second region is 0.50% or more. In this
press-formed article, a high-level balance between high strength
and elongation can be achieved depending on respective regions, and
regions corresponding to an impact resistant site and an energy
absorption site are present in a single formed article.
Advantage of the Invention
[0036] According to the present invention, a steel sheet where the
chemical component composition is strictly specified and the size
of the Ti-containing precipitate is controlled, and where the
precipitation rate of Ti not forming TiN is controlled, and as to
the metal microstructure, the ratio of ferrite is adjusted, is
used, so that by hot-pressing the steel sheet under predetermined
conditions, the strength-elongation balance of the press-formed
article can be made to be a high-level balance. In addition, when
hot-pressing is performed under different conditions among a
plurality of regions, an impact resistant site and an energy
absorption site can be formed in a single formed article, and a
high-level balance between high strength and elongation can be
achieved in respective sites.
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] FIG. 1 A schematic explanatory view showing the mold
configuration for carrying out hot-press forming.
MODE FOR CARRYING OUT THE INVENTION
[0038] The present inventors have made studies from various aspects
to realize a steel sheet for hot-pressing which ensures that, in
the manufacture of a press-formed article by heating a steel sheet
at a predetermined temperature and then hot-press forming the steel
sheet, a press-formed article exhibiting good ductility
(elongation) is obtained while assuring high strength after press
forming.
[0039] As a result, it has been found that when the chemical
component composition of the steel sheet for hot-pressing is
strictly specified and the size of the Ti-containing precipitate as
well as the precipitated Ti amount are controlled and when a proper
metal microstructure is created and the steel sheet is hot-press
formed under predetermined conditions, a predetermined amount of
retained austenite is ensured after forming and a press-formed
article having increased intrinsic ductility (residual ductility)
is obtained. The present invention has been accomplished based on
this finding.
[0040] In the steel sheet for hot-pressing in the present
invention, the chemical component composition needs to be strictly
specified, and the reason for limiting the range of each chemical
component is as follows.
(C: from 0.15 to 0.5%)
[0041] C is an important element in achieving a high-level balance
between high strength and elongation when uniform properties are
required in a press-formed article, or in ensuring retained
austenite particularly in the low strength/high ductility site when
regions corresponding to an impact resistant site and an energy
absorption site are required in a single formed article. In
addition, C is enriched into austenite during heating in the hot
press forming, so that retained austenite can be formed after
quenching. Furthermore, this element contributes to increasing the
amount of martensite and increases the strength. In order to exert
such effects, the C content must be 0.15% or more.
[0042] However, if the C content is too large and exceeds 0.5%, the
two-phase zone heating region becomes narrow, and when uniform
properties are required in a formed article, the balance between
high strength and elongation is not achieved at a high level, or
when regions corresponding to an impact resistant site and an
energy absorption site are required in a single formed article,
adjustment to a metal microstructure (microstructure where
predetermined amounts of ferrite, bainitic ferrite and martensite
are ensured) targeted particularly in the low strength/high
ductility site is difficult. The lower limit of the C content is
preferably 0.17% or more (more preferably 0.20% or more), and the
upper limit is preferably 0.45% or less (more preferably 0.40% or
less).
(Si: from 0.2 to 3%)
[0043] Si exerts an effect of forming retained austenite by
preventing martensite from being tempered during cooling of mold
quenching to form cementite or by suppressing decomposition of
untransformed austenite. In order to exert such an effect, the Si
content must be 0.2% or more. If the Si content is too large and
exceeds 3%, solid-solution hardening amount is excessively large,
and the ductility is greatly reduced. The lower limit of the Si
content is preferably 0.5% or more (more preferably 1.0% or more),
and the upper limit is preferably 2.5% or less (more preferably
2.0% or less).
(Mn: from 0.5 to 3%)
[0044] Mn is an element effective in enhancing the hardenability
during quenching and suppressing the formation of a microstructure
(e.g., ferrite, pearlite, bainite) other than martensite and
retained austenite during cooling of mold quenching. In addition,
Mn is an element capable of stabilizing austenite and is an element
contributing to an increase in the retained austenite amount. In
order to exert such effects, Mn must be contained in an amount of
0.5% or more. In the case of considering only the properties, the
Mn content is preferably larger, but since the cost of alloying
addition rises, the upper limit is set to 3% or less. The lower
limit of the Mn content is preferably 0.7% or more (more preferably
1.0% or more), and the upper limit is preferably 2.5% or less (more
preferably 2.0% or less).
(P: 0.05% or less (exclusive of 0%))
[0045] P is an element unavoidably contained in the steel but
deteriorates the ductility and therefore, the P content is
preferably reduced as much as possible. However, an extreme
reduction causes an increase in the steelmaking cost, and it is
difficult in terms of manufacture to reduce the content to 0%. For
this reason, the upper limit is set to 0.05% or less (exclusive of
0%). The upper limit of the P content is preferably 0.045% or less
(more preferably 0.040% or less).
(S: 0.05% or less (exclusive of 0%))
[0046] S is an element unavoidably contained in the steel, as with
P, and deteriorates the ductility and therefore, the S content is
preferably reduced as much as possible. However, an extreme
reduction causes an increase in the steelmaking cost, and it is
difficult in terms of manufacture to reduce the content to 0%. For
this reason, the upper limit is set to 0.05% or less (exclusive of
0%). The upper limit of the S content is preferably 0.045% or less
(more preferably 0.040% or less).
(Al: from 0.01 to 1%)
[0047] Al is useful as a deoxidizing element and allows the solute
N present in the steel to be fixed as AIN, which is useful in
enhancing the ductility. In order to effectively exert such an
effect, the Al content must be 0.01% or more. However, if the Al
content is too large and exceeds 1%, Al.sub.2O.sub.3 is excessively
produced to deteriorate the ductility. The lower limit of the Al
content is preferably 0.02% or more (more preferably 0.03% or
more), and the upper limit is preferably 0.8% or less (more
preferably 0.6% or less).
(B: from 0.0002 to 0.01%)
[0048] B is an element having an action of suppressing ferrite
transformation, pearlite transformation and bainite transformation
on the high strength site-side and therefore, contributes to
preventing the formation of ferrite, pearlite and bainite during
cooling after heating at a two-phase zone temperature of (Ac.sub.1
transformation point to Ac.sub.3 transformation point), and
ensuring retained austenite. In order to exert such effects, B must
be contained in an amount of 0.0002% or more, but even when this
element is contained excessively over 0.01%, the effects are
saturated. The lower limit of the B content is preferably 0.0003%
or more (more preferably 0.0005% or more), and the upper limit is
preferably 0.008% or less (more preferably 0.005% or less).
(Ti: equal to or more than 3.4[N]+0.01% and equal to or less than
3.4[N]+0.1%: [N] is the content (mass %) of N)
[0049] Ti exerts an effect of improving the hardenability during
quenching by fixing N and maintaining B in a solid solution state.
In order to exert such an effect, it is important to contain this
element in an amount larger than the stoichiometric ratio of Ti and
N (3.4 times the N content) by 0.01% or more. However, if the Ti
content is too large and exceeds 3.4[N]+0.1%, the Ti-containing
precipitate (for example, TiN) to be formed is finely dispersed and
impedes growth in the longitudinal direction of martensite formed
into a lath shape during cooling after heating to the austenite
region, resulting in a lath microstructure having a small aspect
ratio. Conversely, when the precipitate is sufficiently large, a
martensite microstructure having a large aspect ratio is produced,
and stable retained austenite is obtained even with the same C
amount in retained austenite, and as a result, the property
(elongation) is enhanced. The lower limit of the Ti content is
preferably 3.4[N]+0.02% or more (more preferably 3.4[N]+0.05% or
more), and the upper limit is preferably 3.4[N]+0.09% or less (more
preferably 3.4[N]+0.08% or less).
(N: from 0.001 to 0.01%)
[0050] N decreases the effect of improving the hardenability during
quenching by fixing B as BN and therefore, the content thereof is
preferably reduced as much as possible, but since the reduction in
an actual process is limited, the lower limit is set to 0.001%. If
the N content is too large, the ductility deteriorates due to
strain aging, and this elements precipitates as BN, leading to
reduction of effect of improving the hardenability during quenching
by solute B. For this reason, the upper limit is set to 0.01%. The
upper limit of the N content is preferably 0.008% or less (more
preferably 0.006% or less).
[0051] The basic chemical components in the steel sheet for
hot-pressing in the present invention are as described above, and
the remainder is iron and unavoidable impurities (e.g., O, H) other
than P, S and N. In the steel sheet for hot-pressing in the present
invention, it is also useful to further contain at least one of the
following (a) to (c), if desired. The properties of the steel sheet
for hot-pressing (i.e., press-formed article) are further improved
according to the kind of the element that is contained according to
need. In the case of containing such an element, the preferable
range and the reason for limitation on the range are as
follows.
[0052] (a) One or more kinds selected from the group consisting of
V, Nb and Zr, in an amount of 0.1% or less (exclusive of 0%) in
total
[0053] (b) One or more kinds selected from the group consisting of
Cu, Ni, Cr and Mo, in an amount of 1% or less (exclusive of 0%) in
total
[0054] (c) One or more kinds selected from the group consisting of
Mg, Ca and REM, in an amount of 0.01% or less (exclusive of 0%) in
total
(One or more kinds selected from the group consisting of V, Nb and
Zr, in an amount of 0.1% or less (exclusive of 0%) in total)
[0055] V, Nb and Zr have an effect of forming fine carbide and
refining the microstructure by a pinning effect. In order to exert
such an effect, these elements are preferably contained in an
amount of 0.001% or more in total. However, if the content of these
elements is too large, coarse carbide is formed and works out to a
fracture origin to conversely deteriorate the ductility. For this
reason, the content of these elements is preferably 0.1% or less in
total. The lower limit of the content of these elements is more
preferably 0.005% or more (still more preferably 0.008% or more) in
total, and the upper limit is more preferably 0.08% or less (still
more preferably 0.06% or less) in total.
(One or more kinds selected from the group consisting of Cu, Ni, Cr
and Mo: 1% or less (exclusive of 0%) in total)
[0056] Cu, Ni, Cr and Mo suppress ferrite transformation, pearlite
transformation and bainite transformation and therefore,
effectively act to prevent the formation of ferrite, perlite and
bainite during cooling after heating and ensure retained austenite.
In order to exert such an effect, these are preferably contained in
an amount of 0.01% or more in total. In the case of considering
only the properties, the content is preferably larger, but since
the cost of alloying addition rises, the content is preferably 1%
or less in total. In addition, these elements have an action of
greatly increasing the strength of austenite and put a large load
on hot rolling, making it difficult to manufacture a steel sheet.
Therefore, also from the standpoint of manufacturability, the
content is preferably 1% or less. The lower limit of the content of
these elements is more preferably 0.05% or more (still more
preferably 0.06% or more) in total, and the upper limit is more
preferably 0.5% or less (still more preferably 0.3% or less) in
total.
(One or more kinds selected from the group consisting of Mg, Ca and
REM (rare earth element), in an amount of 0.01% or less (exclusive
of 0%) in total)
[0057] These elements refine the inclusion and therefore,
effectively act to enhance the ductility. In order to exert such an
effect, these elements are preferably contained in an amount of
0.0001% or more in total. In the case of considering only the
properties, the content is preferably larger, but since the effect
is saturated, the content is preferably 0.01% or less in total. The
lower limit of the content of these elements is more preferably
0.0002% or more (still more preferably 0.0005% or more) in total,
and the upper limit is more preferably 0.005% or less (still more
preferably 0.003% or less) in total.
[0058] In the steel sheet for hot-pressing in the present
invention, (A) the average equivalent-circle diameter of
Ti-containing precipitates having an equivalent-circle diameter of
30 nm or less among Ti-containing precipitates contained in the
steel sheet is 3 nm or more, (B) the relationship of "precipitated
Ti amount (mass %)-3.4[N].gtoreq.0.5.times.[total Ti amount (mass
%)-3.4[N]]" (the relationship of the formula (1)) is satisfied, and
(C) the ferrite fraction in the metal microstructure is 30 area %
or more, are also important requirements.
[0059] The existence state of Ti-containing precipitate in a formed
article and the condition itself of the formula (1) little affect
the strength or elongation of the steel sheet but affect the
microstructure produced when the steel sheet is hot-pressed,
thereby enhancing the elongation in a final formed article.
Therefore, it must be already controlled at a stage before forming
(steel sheet for hot-pressing). When excess Ti relative to N in the
steel sheet before forming is finely dispersed or mostly present in
a solid solution state in the steel sheet before hot pressing, this
is, while remaining fine, present in a large amount during heating
in hot pressing. Then, in martensite transformation occurring
during rapid cooling in a mold after heating, growth in the
longitudinal direction of a martensite lath is impeded, and growth
in the width direction is promoted, leading to a small aspect
ratio. As a result, delivery of carbon to the surrounding retained
austenite from the martensite lath is delayed and since the carbon
amount in retained austenite decreases and the stability of
retained austenite is reduced, the effect of elongation enhancement
is not sufficiently obtained.
[0060] From such a standpoint, Ti-containing precipitates needs to
be finely dispersed and to this end, the average equivalent-circle
diameter of Ti-containing precipitates having an equivalent-circle
diameter of 30 nm or less among Ti-containing precipitates
contained in the steel sheet must be 3 nm or more (requirement of
(A) above). Here, the equivalent-circle diameter of the target
Ti-containing precipitate is specified to be 30 nm or less, because
it is necessary to control Ti-containing precipitates excluding TiN
that is formed coarsely in the melting stage and thereafter does
not affect the microstructural change or properties. The size
(average equivalent-circle diameter) of the Ti-containing
precipitate is preferably 5 nm or more, more preferably 10 nm or
more. Examples of the Ti-containing precipitate targeted in the
present invention include TiC and other Ti-containing precipitates
such as TiVC and TiNbC.
[0061] In addition, in the steel sheet for hot-pressing, the
majority of Ti except for Ti to be used for precipitating and
fixing N must be caused to be present in a precipitated state. To
this end, the amount of Ti present as a precipitate other than TiN
(i.e., precipitated Ti amount-3.4[N]) needs to be an amount equal
to or more than 0.5 times the remainder after deduction of Ti that
forms TiN from total Ti (i.e., 0.5.times.[(total Ti amount (mass
%))-3.4[N]]) (requirement of (B) above). The "precipitated Ti
amount (mass %)-3.4[N]" is preferably 0.6.times.[(total Ti amount
(mass %))-3.4[N]] or more, more preferably 0.7.times.[(total Ti
amount (mass %))-3.4[N]] or more.
[0062] The steel material must be necessarily processed before hot
stamping and is sometimes subjected to press forming, and in such a
case, a predetermined amount of ferrite as soft microstructure
needs to be ensured. From such a standpoint, the ferrite fraction
in the steel sheet for hot-pressing must be 30 area % or more
(requirement of (C) above). The ferrite fraction is preferably 50
area % or more, more preferably 70 area % or more.
[0063] In the steel sheet for hot-pressing, the remainder of the
metal microstructure is not particularly limited but includes, for
example, at least any one of pearlite, bainite, martensite and
retained austenite.
[0064] For manufacturing the steel sheet (steel sheet for
hot-pressing) in the present invention, a slab prepared by melting
a steel material having the above-described chemical component
composition may be hot-rolled at a heating temperature:
1,100.degree. C. or more (preferably 1,150.degree. C. or more) and
1,300.degree. C. or less (preferably 1,250.degree. C. or less) and
a finish rolling temperature of 750.degree. C. or more (preferably
780.degree. C. or more) and 850.degree. C. or less (preferably
830.degree. C. or less), and after that, it may be hold for 10
seconds or more in a temperature region of 700 to 650.degree. C.,
and thereafter, it may be wound at a temperature of 450.degree. C.
or more (preferably 480.degree. C. or more) and 650.degree. C. or
less (preferably 630.degree. C. or less).
[0065] In the method above, the Ti-containing precipitate such as
TiC formed during ferrite transformation is coarsened by allowing
ferrite transformation to sufficiently proceed at a high
temperature. In addition, the Ti-containing precipitate such as TiC
formed is grown and coarsened by setting the winding temperature to
a high temperature.
[0066] The steel sheet for hot-pressing which has the
above-described chemical component composition, metal
microstructure and Ti-precipitation state may be directly used for
the manufacture by hot pressing or may be subjected to cold rolling
at a rolling reduction of 60% or less (preferably 40% or less)
after pickling and then used for the manufacture by hot pressing.
In addition, the steel sheet for hot-pressing or a cold rolled
material thereof may be subjected to a heat treatment in a
temperature range where the whole amount of Ti-containing
precipitate is not dissolved in solid (for example, 1,000.degree.
C. or less). Furthermore, the surface of the steel sheet for
hot-pressing (the surface of the base steel sheet) in the present
invention may be subjected to plating containing one or more kinds
of Al, Zn, Mg and Si.
[0067] Using the above-described steel sheet for hot-pressing, the
steel sheet is heated at a temperature equal to or more than
Ac.sub.1 transformation point+20.degree. C. (Ac.sub.1+20.degree.
C.) and equal to or less than Ac.sub.3 transformation
point-20.degree. C. (Ac.sub.3-20.degree. C.) and after starting
press forming, the steel sheet is cooled to a temperature equal to
or less than a temperature 100.degree. C. below the bainite
transformation starting temperature Bs (Bs-100.degree. C.) while
ensuring an average cooling rate of 20.degree. C./sec or more in a
mold during forming as well as after the completion of forming,
whereby an optimal microstructure as a formed article with low
strength and high ductility can be produced in a press-formed
article having a single property (hereinafter, sometimes referred
to as "single-region formed article"). The reason for specifying
each requirement in this forming method is as follows.
[0068] In a steel sheet containing a predetermined amount of
ferrite, in order to cause a partial transformation to austenite
while allowing part of the ferrite to remain, the heating
temperature must be controlled to a predetermined range. If the
heating temperature of the steel sheet is less than Ac.sub.1
transformation point+20.degree. C., a sufficient amount of
austenite cannot be obtained during heating, and a predetermined
amount of retained austenite cannot be ensured in the final
microstructure (microstructure of a formed article). If the heating
temperature of the steel sheet exceeds Ac.sub.3 transformation
point-20.degree. C., the transformation amount to austenite is
excessively increased during heating, and a predetermined amount of
ferrite cannot be ensured in the final microstructure
(microstructure of a formed article).
[0069] For allowing austenite formed in the heating step above to
be a desired microstructure while impeding production of a
microstructure such as ferrite or pearlite, the average cooling
rate during forming as well as after forming and the cooling
finishing temperature must be appropriately controlled. From such a
standpoint, it is necessary that the average cooling rate during
forming is 20.degree. C./sec or more and the cooling finishing
temperature is equal to or less than a temperature 100.degree. C.
below the bainite transformation starting temperature Bs. The
average cooling rate during forming is preferably 30.degree. C./sec
or more (more preferably 40.degree. C./sec or more). When the
cooling finishing temperature is equal to or less than a
temperature 100.degree. C. below the bainite transformation
starting temperature Bs, austenite present during heating is
transformed to bainite or martensite while impeding production of a
microstructure such as ferrite or pearlite, whereby fine austenite
is caused to remain between bainite or martensite laths and a
predetermined amount of retained austenite is assured while
ensuring bainite and martensite.
[0070] If the cooling finishing temperature exceeds the temperature
100.degree. C. below the bainite transformation starting
temperature Bs or the average cooling rate is less than 20.degree.
C./sec, a microstructure such as ferrite and pearlite is formed,
and a predetermined amount of retained austenite cannot be ensured,
resulting in deterioration of the elongation (ductility) in a
formed article. The cooling finishing temperature is not
particularly limited as long as it is equal to or less than a
temperature 100.degree. C. below Bs, and the cooling finishing
temperature may be, for example, equal to or less than the
martensite transformation starting temperature Ms.
[0071] After reaching a temperature equal to or less than the
temperature 100.degree. C. below the bainite transformation
starting temperature Bs, fundamentally, the average cooling rate
need not be controlled, but the steel sheet may be cooled to room
temperature at an average cooling rate of, for example, 1.degree.
C./sec or more and 100.degree. C./sec or less. Control of the
average cooling rate during forming as well as after the completion
of forming can be achieved by a technique of, for example, (a)
controlling the temperature of the forming mold (the cooling medium
shown in FIG. 1), or (b) controlling the thermal conductivity of
the mold.
[0072] As to the press-formed article (single-region formed
article) manufactured by the above-described press forming, the
metal microstructure in the formed article (i.e., in the steel
sheet constituting the formed article) includes retained austenite:
from 3 to 20 area %, ferrite: from 30 to 80 area %: bainitic
ferrite: less than 30 area % (exclusive of 0 area %), and
martensite: 31 area % or less (exclusive of 0 area %), the average
equivalent-circle diameter of Ti-containing precipitates having an
equivalent-circle diameter of 30 nm or less among Ti-containing
precipitates contained in the press-formed article is 3 nm or more
(the form of Ti-containing precipitate is the same as in the steel
sheet), the carbon amount in retained austenite is 0.50% or more,
and a high-level balance between high strength and elongation can
be achieved as a uniform property in a formed article. The reason
for setting the range of each requirement (basic microstructure) in
this hot press-formed article is as follows.
[0073] Retained austenite has an effect of increasing the work
hardening ratio (transformation induced plasticity) and enhancing
the ductility of the press-formed article by undergoing
transformation to martensite during plastic deformation. In order
to exert such an effect, the retained austenite fraction must be 3
area % or more. The ductility is more improved as the retained
austenite fraction is higher. In the composition to be used for an
automotive steel sheet, the assurable retained austenite is
limited, and the upper limit is about 20 area %. The lower limit of
the retained austenite is preferably 5 area % or more (more
preferably 7 area % or more).
[0074] When the main microstructure is fine ferrite having high
ductility, the ductility (elongation) of a press-formed article can
be enhanced. From such a standpoint, the ferrite fraction is 30
area % or more. However, if this fraction exceeds 80 area %, the
strength of a formed article cannot be ensured. The lower limit of
the ferrite fraction is preferably 35 area % or more (more
preferably 40 area % or more), and the upper limit is preferably 75
area % or less (more preferably 70 area % or less).
[0075] The bainitic ferrite is a microstructure effective in
enhancing the strength of a formed article but is a structure
slightly lacking in ductility and therefore when present in a large
amount, it deteriorates the elongation. From such a standpoint, the
bainitic ferrite fraction is less than 30 area %. The upper limit
of the bainitic ferrite fraction is preferably 25 area % or less
(more preferably 20 area % or less).
[0076] The martensite (as-quenched martensite) is a microstructure
effective in enhancing the strength of a formed article but is a
structure lacking in ductility and therefore when present in a
large amount, it deteriorates the elongation. From such a
standpoint, the martensite fraction is 31 area % or less. The upper
limit of the martensite fraction is preferably 25 area % or less
(more preferably 20 area % or less).
[0077] The microstructure other than those described above is not
particularly limited, and pearlite, etc. may be contained as a
remainder microstructure, but such a microstructure is inferior to
other microstructures in terms of contribution to strength or
contribution to ductility, and it is fundamentally preferable not
to contain such a microstructure (may be even 0 area %).
[0078] The carbon amount in retained austenite affects the timing
of work induced transformation of retained austenite to martensite
during the deformation such as tensile test, and as the carbon
amount is larger, work induced transformation occurs in a higher
strain region, leading to the increase of the transformation
induced plasticity (TRIP) effect. In the case of the process in the
present invention, carbon is delivered to the surrounding austenite
from the formed martensite lath during cooling. At this time, when
the Ti carbide or carbonitride dispersed in the steel is coarsely
dispersed, the growth of martensite lath in the longitudinal
direction proceeds without impeding the growth, and a martensite
lath having large aspect ratio with a narrow long width is
produced. As a result, carbon is easily delivered in the width
direction from the martensite lath, and not only the carbon amount
in retained austenite is increased but also the ductility is
enhanced. From such a standpoint, in the press-formed article in
the present invention, the carbon amount in retained austenite in
the steel is specified to be 0.50% or more (preferably 0.60% or
more). The carbon amount in retained austenite can be enriched to
about 0.70%, but about 1.0% is the limit.
[0079] When the steel sheet for hot-pressing in the present
invention is used, the properties such as strength and elongation
of a press-formed article can be controlled by appropriately
adjusting the press-forming conditions (heating temperature and
cooling rate) and moreover, a press-formed article having high
ductility (residual ductility) is obtained, making its application
possible to a site (e.g., energy absorption member) to which the
conventional press-formed article can be hardly applied. This is
very useful in expanding the application range of a press-formed
article. In addition to the above-described single-region formed
article, in the manufacture of a press-formed article by
press-forming a steel sheet by use of a press-forming mold, when
the heating temperature and the conditions in each region during
press-forming are appropriately controlled and the microstructure
of each region is thereby adjusted, a press-formed article exerting
a strength-ductility balance depending on respective regions
(hereinafter, sometimes referred to as "multiple-region formed
article") is obtained.
[0080] When manufacturing a multiple-region formed article as
described above by using the steel sheet for hot-pressing in the
present invention, the manufacture may be performed by diving a
heating region of the steel sheet into at least two regions,
heating one region (hereinafter, referred to as first region) at a
temperature of Ac.sub.3 transformation point or more and
950.degree. C. or less, heating another region (hereinafter,
referred to as second region) at a temperature equal to or more
than Ac.sub.1 transformation point+20.degree. C. and equal to or
less than Ac.sub.3 transformation point-20.degree. C., then
starting press forming of both the first and second regions, and
cooling the steel sheet to a temperature equal to or less than the
martensite transformation starting temperature Ms while ensuring an
average cooling rate of 20.degree. C./sec or more in a mold in both
of the first and second regions during forming as well as after the
completion of forming.
[0081] In the method above, a heating region of the steel sheet is
divided into at least two regions (high strength-side region and
low strength-side region), and the manufacturing conditions are
controlled according to respective regions, whereby a press-formed
article exerting a strength-ductility balance depending on
respective regions is obtained. Out of two regions, the second
region corresponds to the low strength-side region, and the
manufacturing conditions, microstructure and properties in this
region are basically the same as those of the above-described
single-region formed article. In the following, the manufacturing
conditions for forming the first region (corresponding to the high
strength-side region) are described. Here, when conducting this
manufacturing method, regions different in the heating temperature
need to be formed in a single steel sheet, but the temperature can
be controlled while keeping a temperature boundary portion of 50 mm
or less, by using an existing heating furnace (e.g., far infrared
furnace, electric furnace+shield).
(Manufacturing Conditions of First Region/High Strength-Side
Region)
[0082] In order to appropriately adjust the microstructure of the
hot press-formed article, the heating temperature must be
controlled to a predetermined range. By appropriately controlling
this heating temperature, in the subsequent cooling process,
transformation to a microstructure mainly including martensite can
be caused to occur while ensuring a predetermined amount of
retained austenite, and a desired microstructure can be produced in
the region of a final hot press-formed article. If the steel sheet
heating temperature in this region is less than the Ac.sub.3
transformation point, a sufficient amount of austenite cannot be
obtained during heating, and a predetermined amount of retained
austenite cannot be ensured in the final microstructure (the
microstructure of a formed article). If the heating temperature of
the steel sheet exceeds 950.degree. C., the austenite grain size
grows during heating, the martensite transformation starting
temperature (Ms point) and the martensite transformation finishing
temperature (Mf point) are elevated, retained austenite cannot be
ensured during quenching, and good formability is not achieved. The
heating temperature of the steel sheet is preferably Ac.sub.3
transformation point+50.degree. C. or more and 930.degree. C. or
less.
[0083] In order to allow austenite formed in the heating step above
to be a desired microstructure while impeding production of a
microstructure such as ferrite or pearlite, the average cooling
rate during forming as well as after forming and the cooling
finishing temperature must be appropriately controlled. From such a
standpoint, the average cooling rate during forming needs to be
20.degree. C./sec or more, and the cooling finishing temperature
needs to be equal to or less than the martensite transformation
starting temperature (Ms point). The average cooling rate during
forming is preferably 30.degree. C./sec or more (more preferably
40.degree. C./sec or more). When the cooling finishing temperature
is equal to or less than the martensite transformation starting
temperature (Ms point), austenite present during heating is
transformed to martensite while impeding production of a
microstructure such as ferrite or pearlite, whereby martensite is
ensured. Specifically, the cooling finishing temperature is
400.degree. C. or less, preferably 300.degree. C. or less.
[0084] In the press-formed article obtained by such a method, the
metal microstructure, precipitate, etc. are different between the
first region and the second region. In the first region, the metal
microstructure includes retained austenite: from 3 to 20 area %
(the action and effect of retained austenite are the same as
above), and martensite: 80 area % or more. The second region
satisfies the metal microstructure and the carbon amount in the
retained austenite which are the same as in the above-described
single-region formed article.
[0085] When the main microstructure of the first region is
high-strength martensite containing a predetermined amount of
retained austenite, a press-formed article can be assured of
ductility in a specific region and high strength. From such a
standpoint, the area fraction of martensite needs to be 80 area %
or more. The martensite fraction is preferably 85 area % or more
(more preferably 90 area % or more). The first region may partially
contain ferrite, pearlite, bainite, etc. as a remainder
microstructure.
[0086] The effects in the present invention are described more
specifically below by referring to Examples, but the present
invention is not limited to the following Examples, and all design
changes made in light of the gist described above or later are
included in the technical range in the present invention.
EXAMPLES
Example 1
[0087] Steel materials (Steel Nos. 1 to 3, 5 to 15 and 17 to 31)
having the chemical component composition shown in Tables 1 and 2
below were melted in vacuum to make an experimental slab, then
hot-rolled to prepare a steel sheet, followed by cooling and
subjecting to a treatment simulating the winding (sheet thickness:
1.6 mm or 3.0 mm). As to the method for treatment simulating the
winding, the sample was cooled to a winding temperature, and put in
a furnace heated at the winding temperature, followed by holding
for 30 minutes and then cooling in the furnace. The manufacturing
conditions of the steel sheets are shown in Tables 3 and 4 below.
Here, in Tables 1 and 2, the Ac.sub.1 transformation point,
Ac.sub.3 transformation point, Ms point, and Bs point were
determined using the following formulae (2) to (5) (see, for
example, The Physical Metallurgy of Steels, Leslie, Maruzen,
(1985)). In addition, treatments (1) and (2) shown in Remarks of
Table 3 mean that each treatment (rolling, cooling and alloying)
described below was performed.
Ac.sub.1 transformation point (.degree.
C.)=723+29.1.times.[Si]-10.7.times.[Mn]+16.9.times.[Cr]-16.9[Ni]
(2)
Ac.sub.3transformation point (.degree.
C.)=910-203.times.[C].sup.1/2+44.7.times.[Si]-30.times.[Mn]+700.times.[P]-
+400.times.[Al]+400.times.[Ti]+104.times.[V]-11.times.[Cr]+31.5.times.[Mo]-
-20.times.[Cu]-15.2.times.[Ni] (3)
Ms point (.degree.
C.)=550-361.times.[C]-39.times.[Mn]-10.times.[Cu]-17.times.[Ni]-20.times.-
[Cr]-5.times.[Mo]+30.times.[Al] (4)
Bs point (.degree.
C.)=830-270.times.[C]-90.times.[Mn]-37.times.[Ni]-70.times.[Cr]-83.times.-
[Mo] (5)
wherein [C], [Si], [Mn], [P], [Al], [Ti], [V], [Cr], [Mo], [Cu] and
[Ni] represent the contents (mass %) of C, Si, Mn, P, Al, Ti, V,
Cr, Mo, Cu and Ni, respectively. In the case where the element
shown in each term of formulae (2) to (5) is not contained, the
calculation is done assuming that the term is not present.
[0088] Treatment (1): The hot-rolled steel sheet was cold-rolled
(sheet thickness: 1.6 mm), then heated at 800.degree. C. for
simulating continuous annealing in a heat treatment simulator, held
for 90 seconds, cooled to 500.degree. C. at an average cooling rate
of 20.degree. C./sec, and held for 300 seconds.
[0089] Treatment (2): The hot-rolled steel sheet was cold-rolled
(sheet thickness: 1.6 mm), then heated at 860.degree. C. for
simulating a continuous hot-dip galvanizing line in a heat
treatment simulator, cooled to 400.degree. C. at an average cooling
rate of 30.degree. C./sec, held, further held under the conditions
of 500.degree. C..times.10 seconds for simulating immersion in a
plating bath and alloying treatment, and thereafter cooled to room
temperature at an average cooling rate of 20.degree. C./sec.
TABLE-US-00001 TABLE 1 Steel Chemical Component Composition* (mass
%) No. C Si Mn P S Al B Ti N V Nb Cu 1 0.220 1.20 1.20 0.0050
0.0020 0.030 0.0020 0.044 0.0040 -- -- -- 2 0.150 1.20 1.20 0.0050
0.0020 0.030 0.0020 0.044 0.0040 -- -- -- 3 0.220 0.05 1.20 0.0050
0.0020 0.030 0.0020 0.044 0.0040 -- -- -- 5 0.220 1.20 1.20 0.0050
0.0020 0.030 0.0020 0.024 0.0040 -- -- -- 6 0.220 1.20 1.20 0.0050
0.0020 0.030 0.0020 0.044 0.0040 -- -- -- 7 0.220 1.20 1.20 0.0050
0.0020 0.030 0.0020 0.044 0.0040 -- -- -- 8 0.220 1.20 1.20 0.0050
0.0020 0.030 0.0020 0.044 0.0040 -- -- -- 9 0.220 1.20 1.20 0.0050
0.0020 0.030 0.0020 0.044 0.0040 -- -- -- 10 0.220 1.20 1.20 0.0050
0.0020 0.030 0.0020 0.044 0.0040 -- -- -- 11 0.220 1.20 1.20 0.0050
0.0020 0.030 0.0020 0.044 0.0040 -- -- -- 12 0.220 1.20 1.20 0.0050
0.0020 0.030 0.0020 0.044 0.0040 -- -- -- 13 0.220 1.20 1.20 0.0050
0.0020 0.030 0.0020 0.044 0.0040 -- -- -- 14 0.220 1.20 1.20 0.0050
0.0020 0.030 0.0020 0.044 0.0040 -- -- -- 15 0.220 2.00 1.20 0.0050
0.0020 0.030 0.0020 0.044 0.0040 -- -- -- Chemical Component Steel
Composition* (mass %) Ac.sub.3 - 20.degree. C. Ac.sub.1 +
20.degree. C. Bs - 100.degree. C. Ms Point No. Ni Zr Mg Ca REM Cr
Mo (.degree. C.) (.degree. C.) (.degree. C.) (.degree. C.) 1 -- --
-- -- -- -- -- 845 765 563 425 2 -- -- -- -- -- 0.20 -- 860 768 568
446 3 -- -- -- -- -- 0.20 -- 792 735 549 421 5 -- -- -- -- -- 0.20
-- 833 768 549 421 6 -- -- -- -- -- 0.20 -- 843 768 549 421 7 -- --
-- -- -- 0.20 -- 843 768 549 421 8 -- -- -- -- -- 0.20 -- 843 768
549 421 9 -- -- -- -- -- 0.20 -- 843 768 549 421 10 -- -- -- -- --
0.20 -- 843 768 549 421 11 -- -- -- -- -- 0.20 -- 843 768 549 421
12 -- -- -- -- -- 0.20 -- 843 768 549 421 13 -- -- -- -- -- 0.20 --
843 768 549 421 14 -- -- -- -- -- 0.20 -- 843 768 549 421 15 -- --
-- -- -- 0.20 -- 879 792 549 421 *Remainder: Iron and unavoidable
impurities except for P, S and N.
TABLE-US-00002 TABLE 2 Steel Chemical Component Composition* (mass
%) No. C Si Mn P S Al B Ti N V Nb Cu 17 0.720 1.20 1.20 0.0050
0.0020 0.030 0.0020 0.044 0.0040 -- -- -- 18 0.220 1.20 0.80 0.0050
0.0020 0.030 0.0020 0.044 0.0040 -- -- -- 19 0.220 1.20 2.40 0.0050
0.0020 0.030 0.0020 0.044 0.0040 -- -- -- 20 0.220 1.20 1.20 0.0050
0.0020 0.030 0.0020 0.100 0.0040 -- -- -- 21 0.220 1.20 1.20 0.0050
0.0020 0.030 0.0020 0.200 0.0040 -- -- -- 22 0.220 0.50 1.20 0.0050
0.0020 0.40 0.0020 0.044 0.0040 -- -- -- 23 0.220 1.20 1.20 0.0050
0.0020 0.030 0.0020 0.044 0.0040 0.030 -- -- 24 0.220 1.20 1.20
0.0050 0.0020 0.030 0.0020 0.044 0.0040 -- 0.020 -- 25 0.220 1.20
1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 -- -- 0.20 26 0.220
1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 -- -- -- 27 0.220
1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 -- -- -- 28 0.220
1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 -- -- -- 29 0.220
1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 -- -- -- 30 0.220
1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 -- -- -- 31 0.220
1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 -- -- -- Chemical
Component Steel Composition* (mass %) Ac.sub.3 - 20.degree. C.
Ac.sub.1 + 20.degree. C. Bs - 100.degree. C. Ms Point No. Ni Zr ME
Ca REM Cr Mo (.degree. C.) (.degree. C.) (.degree. C.) (.degree.
C.) 18 -- -- -- -- -- 0.20 -- 766 768 414 240 19 -- -- -- -- --
0.20 -- 855 773 585 436 20 -- -- -- -- -- 0.20 -- 807 756 441 374
21 -- -- -- -- -- 0.20 -- 866 768 549 421 22 -- -- -- -- -- 0.20 --
906 768 549 421 23 -- -- -- -- -- 0.20 -- 960 748 549 432 24 -- --
-- -- -- 0.20 -- 846 768 549 421 25 -- -- -- -- -- 0.20 -- 843 768
549 421 26 -- -- -- -- -- 0.20 -- 839 768 549 419 27 0.20 -- -- --
-- 0.20 -- 840 765 541 417 28 -- -- -- -- -- 0.20 0.20 849 768 532
420 29 -- 0.015 -- -- -- 0.20 -- 843 768 549 421 30 -- -- 0.002 --
-- 0.20 -- 843 768 549 421 31 -- -- -- 0.002 -- 0.20 -- 843 768 549
421 32 -- -- -- -- 0.002 0.20 -- 843 768 549 421 *Remainder: Iron
and unavoidable impurities except for P, S and N.
TABLE-US-00003 TABLE 3 Steel Sheet Manufacturing Conditions Steel
Heating Finish Rolling Cooling time from Winding No. Temperature
(.degree. C.) Temperature (.degree. C.) 700 to 650.degree. C. (sec)
Temperature (.degree. C.) Remarks 1 1200 800 12 500 -- 2 1200 800
12 500 -- 3 1200 800 12 500 -- 5 1200 800 12 500 -- 6 1200 800 1
500 -- 7 1200 900 12 500 -- 8 1200 800 12 500 treatment (1) 9 1200
800 12 500 treatment (2) 10 1200 800 12 500 -- 11 1200 800 12 500
-- 12 1200 800 12 500 -- 13 1200 800 12 500 -- 14 1200 800 12 500
-- 15 1200 800 12 500 --
TABLE-US-00004 TABLE 4 Steel Sheet Manufacturing Conditions Steel
Heating Finish Rolling Cooling time from Winding No. Temperature
(.degree. C.) Temperature (.degree. C.) 700 to 650.degree. C. (sec)
Temperature (.degree. C.) Remarks 17 1200 800 12 500 -- 18 1200 800
12 500 -- 19 1200 800 12 500 -- 20 1200 800 12 500 -- 21 1200 800
12 500 -- 22 1200 800 12 500 -- 23 1200 800 12 500 -- 24 1200 800
12 500 -- 25 1200 800 12 500 -- 26 1200 800 12 500 -- 27 1200 800
12 500 -- 28 1200 800 12 500 -- 29 1200 800 12 500 -- 30 1200 800
12 500 -- 31 1200 800 12 500 --
[0090] With respect to the steel sheets (steel sheets for
press-forming) obtained, analysis of the Ti precipitation state and
observation of the metal microstructure (the fraction of each
microstructure) were performed in the following manner. In
addition, the tensile strength (TS) of each steel sheet was
measured by the later-described method. The results obtained are
shown in Tables 5 and 6 below together with the calculated value of
0.5.times.[total Ti amount (mass %)-3.4[N]] [indicated as
0.5.times.[total Ti amount-3.4[N]].
(Analysis of Ti Precipitation State of Steel Sheet)
[0091] An extraction replica sample was prepared, and a
transmission electron microscope image (magnifications: 100,000
times) of Ti-containing precipitates was photographed using a
transmission electron microscope (TEM). At this time, the
Ti-containing precipitate (those having an equivalent-circle
diameter of 30 nm or less) was identified by the composition
analysis of precipitates by means of an energy dispersive X-ray
spectrometer (EDX). At least 100 pieces of Ti-containing
precipitates were measured for the area by image analysis, the
equivalent-circle diameter was determined therefrom, and the
average value thereof was defined as the precipitate size (average
equivalent-circle diameter of Ti-containing precipitates). As for
the "precipitated Ti amount (mass %)-3.4[N]" (the amount of Ti
present as a precipitate), extraction residue analysis was
performed using a mesh having a mesh size of 0.1 .mu.m (during
extraction treatment, a fine precipitate resulting from aggregation
of precipitates could also be measured), and the "precipitated Ti
amount (mass %)-3.4[N]" (in Tables 5 and 6, indicated as
"Precipitated Ti Amount-3.4[N]") was determined. In the case where
the Ti-containing precipitate partially contained V or Nb, the
contents of these were also measured.
(Observation of Metal Microstructure (Fraction of Each
Microstructure))
[0092] (1) As to the microstructures of ferrite (and bainitic
ferrite and pearlite) in the steel sheet, the steel sheet was
corroded with nital and after distinguishing each of
microstructures from each other by SEM observation (magnifications:
1,000 times or 2,000 times), the ferrite fraction (area ratio) was
determined.
[0093] (2) The retained austenite fraction in the steel sheet was
measured by X-ray diffraction method after the steel sheet was
ground to 1/4 thickness and then subjected to chemical polishing
(for example, ISM Int. Vol. 33. (1933), No. 7, P. 776). The carbon
amount of the retained austenite was also measured.
TABLE-US-00005 TABLE 5 Steel Sheet for Press-Forming Average
Equivalent- Circle 0.5 .times. [Total Diameter of Precipitated Ti
Ti- Ti Amount- Amount- Containing Ferrite Tensile Steel 3.4[N]
3.4[N] Precipitates Fraction Remainder Strength No. (mass %) (mass
%) (nm) (area %) Microstructure* (MPa) 1 0.028 0.015 12.3 51 B 745
2 0.028 0.015 11.2 65 B 675 3 0.024 0.015 11.8 56 P + B 719 5 0.005
0.005 12.0 58 B 708 6 0.025 0.015 11.7 14 B 1021 7 0.003 0.015 2.6
41 B 951 8 0.026 0.015 11.7 55 B 726 9 0.029 0.015 11.5 60 B 701 10
0.029 0.015 10.7 58 B 711 11 0.029 0.015 10.7 58 B 711 12 0.029
0.015 10.7 58 B 711 13 0.029 0.015 10.7 58 B 711 14 0.029 0.015
10.7 58 B 711 15 0.026 0.015 12.6 60 B 701 *B: Bainitic ferrite, P:
pearlite.
TABLE-US-00006 TABLE 6 Steel Sheet for Press-Forming Average
Equivalent- Circle 0.5 .times. [Total Diameter of Precipitated Ti
Ti- Ti Amount- Amount- Containing Ferrite Tensile Steel 3.4[N]
3.4[N] Precipitates Fraction Remainder Strength No. (mass %) (mass
%) (nm) (area %) Microstructure* (MPa) 17 0.027 0.015 12.1 5 B + M
1180 18 0.026 0.015 10.6 45 P + B 775 19 0.028 0.015 13.0 53 B + M
735 20 0.076 0.043 13.7 58 B 710 21 0.168 0.093 17.5 41 B 797 22
0.023 0.015 12.9 51 B 746 23 0.023 0.015 12.3 46 B 768 24 0.027
0.015 10.4 53 B 735 25 0.027 0.015 12.9 56 B 720 26 0.024 0.015
10.9 57 B 716 27 0.030 0.015 12.5 52 B 740 28 0.026 0.015 12.3 58 B
745 29 0.027 0.015 12.0 60 B 731 30 0.025 0.015 11.8 55 B 722 31
0.024 0.015 12.2 63 B 747 *B: Bainitic ferrite, P: pearlite, M:
Martensite.
[0094] Each of the steel sheets above (1.6 mm.sup.t.times.150
mm.times.200 mm) (the thickness t of those other than the treatment
(1) and (2) was adjusted to 1.6 mm by hot rolling) was heated at a
predetermined temperature in a heating furnace, followed by
subjecting to press forming and cooling treatment using a
hat-shaped mold (FIG. 1) to obtain a formed article. The press
forming conditions (heating temperature, average cooling rate, and
rapid cooling finishing temperature during press forming) are shown
in Table 7 below.
TABLE-US-00007 TABLE 7 Press-Forming Conditions Steel Heating
Average Cooling Rapid Cooling Finishing No. Temperature (.degree.
C.) Rate (.degree. C./sec) Temperature (.degree. C.) 1 810 40 300 2
810 40 300 3 760 40 300 5 800 40 300 6 810 40 300 7 810 40 300 8
810 40 300 9 810 40 300 10 810 40 300 11 900 40 300 12 810 5 300 13
810 40 600 14 810 40 100 15 840 40 300 17 770 40 300 18 810 40 300
19 780 40 300 20 820 40 300 21 840 40 300 22 850 40 300 23 810 40
300 24 810 40 300 25 800 40 300 26 800 40 300 27 810 40 300 28 810
40 300 29 810 40 300 30 810 40 300 31 810 40 300
[0095] With respect to the press-formed articles obtained, the
tensile strength (TS), elongation (total elongation EL),
observation of metal microstructure (fraction of each
microstructure), and the carbon amount in the retained austenite
was measured by the method described above.
(Measurement of Tensile Strength (TS) and Elongation (Total
Elongation EL))
[0096] A tensile test was performed using a JIS No. 5 test piece,
and the tensile strength (TS) and elongation (EL) were measured. At
this time, the strain rate in the tensile test was set to 10
mm/sec. In the present invention, the test piece was rated "passed"
when a tensile strength (TS) of 980 MPa or more and an elongation
(EL) of 18% or more were satisfied and the strength-elongation
balance (TS.times.EL) was 20,000 (MPa-%) or more.
(Observation of Metal Microstructure (Fraction of Each
Microstructure))
[0097] (1) With respect to the microstructures of ferrite and
bainitic ferrite in the steel sheet, the steel sheet was corroded
with nital and after distinguishing ferrite amd bainitic ferrite
from each other (including distinguishing from tempered martensite)
by SEM observation (magnifications: 1,000 times or 2,000 times),
the fraction (area ratio) of each microstructure was
determined.
[0098] (2) The retained austenite fraction in the steel sheet was
measured by X-ray diffraction method after the steel sheet was
ground to 1/4 thickness and then subjected to chemical polishing
(for example, ISJJ Int. Vol. 33. (1933), No. 7, P. 776).
[0099] (3) As to the fraction of martensite (as-quenched
martensite), after LePera corrosion of the steel sheet, the area
ratio of a white contrast regarded as a mixed microstructure of
as-quenched martensite and retained austenite was measured, and the
retained austenite fraction determined by X-ray diffraction was
subtracted therefrom, whereby the martensite fraction was
calculated.
[0100] The observation results (fraction of each microstructure,
and carbon amount in the retained austenite) of the metal
microstructure are shown in Tables 8 and 9 below. In addition, the
mechanical properties (tensile strength TS, elongation EL and
TS.times.EL) of the press-formed article are shown in Table 10
below.
TABLE-US-00008 TABLE 8 Metal Microstructure of Press-Formed Article
Ferrite Bainitic Ferrite Martensite Retained Carbon Amount in Steel
Fraction Fraction Fraction Austenite Retained Austenite No. (area
%) (area %) (area %) Fraction (area %) (mass %) Others 1 49 19 25 7
0.64 -- 2 64 16 12 8 0.64 -- 3 51 19 30 0 -- -- 5 53 18 22 7 0.62
-- 6 7 18 14 6 0.64 tempered martensite: 55% 7 55 19 19 7 0.63 -- 8
48 16 28 8 0.63 -- 9 48 18 27 7 0.64 -- 10 48 15 30 7 0.64 -- 11 0
0 95 5 0.52 -- 12 85 0 8 7 0.52 -- 13 72 0 7 1 0.48 pearlite: 20%
14 51 15 27 7 0.63 -- 15 49 16 26 9 0.63 --
TABLE-US-00009 TABLE 9 Metal Microstructure of Press-Formed Article
Ferrite Bainitic Ferrite Martensite Retained Carbon Amount in Steel
Fraction Fraction Fraction Austenite Retained Austenite No. (area
%) (area %) (area %) Fraction (area %) (mass %) Others 17 8 0 80 12
0.75 -- 18 62 15 17 6 0.68 -- 19 52 18 23 7 0.61 -- 20 45 19 29 7
0.63 -- 21 46 18 30 6 0.45 -- 22 51 19 22 8 0.64 -- 23 48 19 26 7
0.65 -- 24 46 16 31 7 0.63 -- 25 51 15 27 7 0.63 -- 26 46 16 31 7
0.62 -- 27 46 17 29 8 0.62 -- 28 49 19 25 7 0.63 -- 29 51 19 22 8
0.64 -- 30 52 19 23 6 0.63 -- 31 51 18 23 8 0.62
TABLE-US-00010 TABLE 10 Mechanical Properties of Press-Formed
Article Steel Tensile Strength TS Elongation EL No. (MPa) (%) TS
.times. EL (MPa-% ) 1 1063 19.2 20410 2 1024 21.1 21606 3 981 11.5
11282 5 1072 21.0 22512 6 1034 24.1 24919 7 1052 18.0 18936 8 1004
22.0 22088 9 1048 21.0 22008 10 1044 20.9 21820 11 1511 10.2 15412
12 889 19.6 17424 13 811 15.2 12327 14 1017 22.2 22577 15 1068 21.7
23176 17 1682 6.5 10933 18 1056 21.7 22915 19 1075 20.5 22038 20
1023 20.3 20767 21 1013 16.0 16208 22 1046 22.1 23117 23 1021 22.4
22870 24 1010 21.8 22018 25 1026 21.6 22162 26 1004 22.5 22590 27
1061 21.4 22705 28 1063 22.0 23386 29 1024 22.3 22835 30 1023 21.9
22404 31 1031 22.0 22682
[0101] These results allow for the following consideration. It is
found that in the case of Steel Nos. 1, 2, 5, 8 to 10, 14, 15, 18
to 20, and 22 to 31, which are Examples satisfying the requirements
specified in the present invention, a press-formed article having a
good strength-ductility balance is obtained.
[0102] On the other hand, in the case of Steel Nos. 3, 6, 7, 11 to
13, 17, and 21, which are Comparative Examples failing in
satisfying any of the requirements specified in the present
invention, any of the properties is deteriorated. More
specifically, in the case of Steel No. 3 where a steel sheet for
press-forming which has a small Si content is used, the retained
austenite fraction is not ensured in the press-formed article and
due to low elongation, the strength-elongation balance is
deteriorated. In the case of Steel No. 6 where the cooling time in
the range from 700.degree. C. to 650.degree. C. in the manufacture
of a steel sheet is insufficient, ferrite transformation does not
sufficiently proceeds, failing in ensuring the ferrite fraction in
a steel sheet, and it is expected that the strength is increased to
make the forming or working before press forming difficult.
[0103] In the case of Steel No. 7 where the finish rolling
temperature in the manufacture of a steel sheet is high, the steel
sheet for press-forming does not satisfy the relationship of the
formula (1), and the strength-elongation balance of the
press-formed article is deteriorated. In the case of Steel No. 11
where the heating temperature during press forming is high,
martensite is produced in a large amount, and ferrite is not
produced, and as a result, the strength is increased, and only low
elongation EL is obtained (the strength-elongation balance
(TS.times.EL) is also deteriorated).
[0104] In the case of Steel No. 12 where the average cooling rate
during press forming is low, ferrite is produced in a large amount
at the stage of the press-formed article, and strength-elongation
balance (TS.times.EL) is deteriorated. In the case of Steel No. 13
where the rapid cooling finishing temperature during press forming
is high, pearlite is produced in a large amount at the stage of the
press-formed article, failing in ensuring the retained austenite
fraction, and the carbon amount in retained austenite is
insufficient, and as a result, not only the strength and elongation
are reduced but also the strength-elongation balance (TS.times.EL)
is deteriorated.
[0105] In the case of Steel No. 17 where a steel sheet for
press-forming which has an excessive C content is used, the ferrite
fraction of the steel sheet is decreased, failing in ensuring the
ferrite fraction in the press-formed article, and the martensite
fraction is increased, and as a result, the strength is high, and
only low elongation EL is obtained (the strength-elongation balance
(TS.times.EL) is also deteriorated). In the case of Steel No. 21
where a steel sheet for press-forming which has an excessive Ti
content is used (the carbon amount in retained austenite is
insufficient), the elongation and strength-elongation balance
(TS.times.EL) is deteriorated.
Example 2
[0106] Steel materials (Steel Nos. 32 to 36) having the chemical
component composition shown in Table 11 below were melted in vacuum
to make an experimental slab, and then it was hot-rolled, followed
by cooling and winding (sheet thickness: 3.0 mm). The steel sheet
manufacturing conditions here are shown in Table 12 below.
TABLE-US-00011 TABLE 11 Ac.sub.3 - Ac.sub.1 + Bs - Ms Steel
Chemical Component Composition* (mass %) 20.degree. C. 20.degree.
C. 100.degree. C. Point No. C Si Mn P S Al B Ti N V Nb Cu Ni Cr Mo
(.degree. C.) (.degree. C.) (.degree. C.) (.degree. C.) 32 0.220
1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 -- -- -- -- 0.20
-- 843 768 549 421 33 0.350 1.20 1.20 0.0050 0.0020 0.030 0.0020
0.044 0.0040 -- -- -- -- 0.20 -- 818 768 514 374 34 0.220 1.20 1.20
0.0050 0.0020 0.030 0.0020 0.044 0.0040 -- -- -- -- 0.20 -- 843 768
549 421 35 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040
-- -- -- -- -- -- 845 765 563 425 36 0.220 1.20 1.20 0.0050 0.0020
0.030 0.0020 0.044 0.0040 -- -- -- -- 0.20 -- 843 768 549 421
*Remainder: Iron and unavoidable impurities except for P, S and
N.
TABLE-US-00012 TABLE 12 Steel Sheet Manufacturing Conditions Steel
Heating Finish Rolling Cooling Time from 700 Winding No.
Temperature (.degree. C.) Temperature (.degree. C.) to 620.degree.
C. (sec) Temperature (.degree. C.) Remarks 32 1200 800 12 500 -- 33
1200 800 12 500 -- 34 1200 800 12 500 treatment (1) 35 1200 800 12
500 -- 36 1200 800 12 500 --
[0107] With respect to the steel sheets (steel sheets for
press-forming) obtained, analysis of the precipitation state of Ti
precipitates, observation of the metal microstructure (the fraction
of each microstructure), and measurement of the tensile strength
were performed in the same manner as in Example 1. The results are
shown in Table 13 below.
TABLE-US-00013 TABLE 13 Steel Sheet for Press-Forming Precipitated
Ti 0.5 .times. [Total Ti Average Equivalent-Circle Tensile Steel
Amount-3.4 [N] Amount-3.4 [N] Diameter of Ti-Containing Ferrite
Fraction Remainder Strength No. (mass %) (mass %) Precipitates (nm)
(area %) Microstructure* (MPa) 33 0.028 0.015 10.9 52 B 735 34
0.025 0.015 10.5 56 B 830 35 0.025 0.015 10.8 60 B 702 36 0.026
0.015 10.9 52 B 735 37 0.023 0.015 11.0 52 B 740 *B: Bainitic
ferrite.
[0108] Each of the steel sheets above (3.0 mm.sup.t.times.150
mm.times.200 mm) was heated at a predetermined temperature in a
heating furnace and then subjected to press forming and cooling
treatment in a hat-shaped mold (FIG. 1) to obtain a formed article.
At this time, the steel sheet was placed in an infrared furnace,
and a temperature difference was created by applying an infrared
ray directly to a portion intended to have high strength (the steel
sheet portion corresponding to the first region) so that the
portion could be heated at a high temperature, and by putting a
cover on a portion intended to have low strength (the steel sheet
portion corresponding to the second region) so that the portion
could be heated at a low temperature by blocking part of the
infrared ray. Accordingly, the press-formed article has regions
differing in the strength in a single component. The press forming
conditions (heating temperature, average cooling rate, and rapid
cooling finishing temperature of each region during press forming)
are shown in Table 14 below.
TABLE-US-00014 TABLE 14 Press Forming Conditions Heating Average
Rapid Cooling Steel Temperature Cooling Rate Finishing No. Region
(.degree. C.) (.degree. C./sec) Temperature (.degree. C.) 32 low
strength side 790 40 300 high strength side 920 40 300 33 low
strength side 800 40 300 high strength side 920 40 300 34 low
strength side 810 40 300 high strength side 920 40 300 35 low
strength side 800 40 300 high strength side 920 40 300 36 low
strength side 800 40 300 high strength side 850 40 300
[0109] With respect to the press-formed articles obtained, the
tensile strength (TS), elongation (total elongation EL),
observation of metal microstructure (fraction of each
microstructure), and measurement of carbon amount in the retained
austenite, in each region, were determined in the same manner as in
Example 1.
[0110] The observation results (fraction of each microstructure) of
the metal microstructure and the carbon amount in the retained
austenite are shown in Table 15 below. In addition, the mechanical
properties (tensile strength TS, elongation EL and TS.times.EL) of
the press-formed article are shown in Table 16 below. Here, the
test piece was rated "passed" when a tensile strength (TS) of 1,470
MPa or more and an elongation (EL) of 8% or more were satisfied on
the high strength side and the strength-elongation balance
(TS.times.EL) was 14,000 (MPa%) or more (the evaluation criteria of
the low strength side are the same as in Example 1).
TABLE-US-00015 TABLE 15 Metal Microstructure of Press-Formed
Article Ferrite Bainitic Ferrite Martensite Retained Carbon Amount
in Steel Fraction Fraction Fraction Austenite Retained Austenite
No. Region (area %) (area %) (area %) Fraction (area %) (mass %)
Others 32 low strength side 54 18 21 7 0.63 -- high strength side 0
0 94 6 0.52 -- 33 low strength side 50 19 25 6 0.63 -- high
strength side 0 0 95 5 0.53 -- 34 low strength side 54 18 23 8 0.64
-- high strength side 0 0 94 6 0.55 -- 35 low strength side 50 18
25 7 0.63 -- high strength side 0 0 94 6 0.53 -- 36 low strength
side 50 18 39 7 0.63 -- high strength side 25 0 69 6 0.55 --
TABLE-US-00016 TABLE 16 Mechanical Properties of Press-Formed
Article Steel Tensile Strength Elongation EL TS .times. EL No.
Region TS (MPa) (%) (MPa-% ) 32 low strength side 1038 21.6 22421
high strength side 1511 12.3 18585 33 low strength side 1192 18.7
22290 high strength side 1820 11.3 20566 34 low strength side 1057
21.3 22514 high strength side 1499 12.0 17988 35 low strength side
1035 21.0 21735 high strength side 1520 11.5 17480 36 low strength
side 1052 20.5 21566 high strength side 1288 12.3 15842
[0111] These results allow for the following consideration. It is
found that in the case of Steel Nos. 32 to 35, which are Examples
satisfying the requirements specified in the present invention, a
component having a good strength-ductility balance in each region
is obtained. On the other hand, in the case of Steel No. 36 where
the heating temperature during press forming is low, the ferrite
fraction on the high strength side is low, and the martensite
fraction on the high strength side is high (the difference in the
strength from the low strength side is less than 300 MPa).
INDUSTRIAL APPLICABILITY
[0112] In the present invention, the steel sheet has a
predetermined chemical component composition, the average
equivalent-circle diameter of Ti-containing precipitates having an
equivalent-circle diameter of 30 nm or less among Ti-containing
precipitates contained in the steel sheet is 3 nm or more, the
precipitated Ti amount and the total Ti amount in the steel satisfy
a predetermined relationship, and the ferrite fraction in the metal
microstructure is 30 area % or more, whereby there can be realized
a steel sheet for hot-pressing which is useful to obtain a
press-formed article ensuring that forming or working before hot
pressing is facilitated, a press-formed article capable of
achieving a high-level balance between high strength and elongation
when uniform properties are required in the formed article can be
obtained, and the press-formed article can achieve a high-level
balance between high strength and elongation according to
respective regions when regions corresponding to an impact
resistant site and an energy absorption site are required in a
single formed article.
DESCRIPTION OF REFERENCE NUMERALS AND SIGNS
[0113] 1: Punch [0114] 2: Die [0115] 3: Blank holder [0116] 4:
Steel sheet (blank)
* * * * *