U.S. patent application number 14/917845 was filed with the patent office on 2016-08-04 for method for manufacturing press-molded article, and 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 | 20160222483 14/917845 |
Document ID | / |
Family ID | 52665201 |
Filed Date | 2016-08-04 |
United States Patent
Application |
20160222483 |
Kind Code |
A1 |
MURAKAMI; Toshio ; et
al. |
August 4, 2016 |
METHOD FOR MANUFACTURING PRESS-MOLDED ARTICLE, AND PRESS-MOLDED
ARTICLE
Abstract
A method for manufacturing a press-formed article includes:
heating a specific steel sheet for hot-pressing at 900.degree. C.
or more and 1,100.degree. C. or less; 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 and equal to or more than a martensite transformation starting
temperature Ms, while ensuring an average cooling rate of
20.degree. C./sec or more in a mold during forming and after the
completion of forming; and further cooling the steel sheet, which
has been cooled, to 200.degree. C. or less at an average cooling
rate of less than 20.degree. C./sec.
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: |
52665201 |
Appl. No.: |
14/917845 |
Filed: |
September 10, 2013 |
PCT Filed: |
September 10, 2013 |
PCT NO: |
PCT/JP13/74425 |
371 Date: |
March 9, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 1/20 20130101; C21D
8/005 20130101; C21D 2211/002 20130101; C22C 38/26 20130101; C22C
38/28 20130101; C22C 38/14 20130101; C22C 38/20 20130101; C22C
38/002 20130101; C21D 1/673 20130101; C22C 38/001 20130101; C22C
38/50 20130101; C22C 38/54 20130101; C22C 38/06 20130101; C22C
38/04 20130101; C22C 38/02 20130101; C22C 38/38 20130101; C21D 9/46
20130101; C21D 9/0068 20130101; C22C 38/22 20130101; C22C 38/32
20130101; C22C 38/005 20130101 |
International
Class: |
C21D 9/00 20060101
C21D009/00; C22C 38/50 20060101 C22C038/50; C22C 38/38 20060101
C22C038/38; C22C 38/32 20060101 C22C038/32; C22C 38/28 20060101
C22C038/28; C22C 38/26 20060101 C22C038/26; C22C 38/22 20060101
C22C038/22; C22C 38/20 20060101 C22C038/20; 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 1/20 20060101
C21D001/20; C22C 38/54 20060101 C22C038/54 |
Claims
1. A method for manufacturing a press-formed article, the method
comprising: heating a steel sheet for hot-pressing at 900.degree.
C. or more and 1,100.degree. C. or less, to obtain a heated steel
sheet; beginning press forming of the heated steel sheet; cooling
the steel sheet to a temperature equal to or less than a
temperature 100.degree. C. below a bainite transformation starting
temperature (Bs) and equal to or more than a martensite
transformation starting temperature (Ms), while ensuring an average
cooling rate of 20.degree. C./sec or more in a mold during the
press forming and after completion of the press forming to obtain a
cooled steel sheet; and further cooling the cooled steel sheet to
200.degree. C. or less at the average cooling rate of less than
20.degree. C./sec, wherein the steel sheet comprises C: from 0.15
to 0.5% by mass, Si: from 0.2 to 3% by mass, Mn: from 0.5 to 3% by
mass, P: 0.05% by mass or less (exclusive of 0%), S: 0.05% by mass
or less (exclusive of 0%), Al: from 0.01 to 1% by mass, B: from
0.0002 to 0.01% by mass. Ti: equal to or more than 3.4[N]+0.01% by
mass and equal to or less than 3.4[N]+0.1% by mass, wherein [N]
indicates a content (mass %) of N, N: from 0.001 to 0.01%, iron,
and unavoidable impurities; 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 6 nm or less; and a precipitated Ti amount and a
total Ti amount in a steel of the steel sheet satisfies formula
(1): Precipitated Ti amount (mass %)-3.4[N]<0.5.times.[total Ti
amount (mass %)-3.4[N]] (1)
2. The method according to claim 1, wherein the steel sheet for
hot-pressing further comprises, as the other element(s), at least
one of the following (a) to (c): (a) at least one selected from the
group consisting of V, Nb and Zr, in an amount of 0.1% or less
(exclusive of 0%) in total; (b) at least one 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) at least one 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 press-formed article of a steel sheet comprising: C: from 0.15
to 0.5% by mass; Si: from 0.2 to 3% by mass; Mn: from 0.5 to 3% by
mass; P: 0.05% by mass or less (exclusive of 0%); S: 0.05% by mass
or less (exclusive of 0%); Al: from 0.01 to 1% by mass; B: from
0.0002 to 0.01% by mass; Ti: equal to or more than 3.4[N]+0.01% by
mass and equal to or less than 3.4[N]+0.1% by mass, wherein [N]
indicates a content (mass %) of N; from 0.001 to 0.01%; iron; and
unavoidable impurities, wherein: a metal microstructure of the
press-formed article includes bainitic ferrite: from 60 to 97 area
%, martensite: 37 area % or less, retained austenite: from 3 to 20
area %, and remainder microstructure: 5 area % or less; an average
equivalent-circle diameter of Ti-containing precipitate having an
equivalent-circle diameter of 30 nm or less among Ti-containing
precipitates contained in the press-formed article is 10 nm or
less; and a precipitated Ti amount and a total Ti amount in a steel
of the steel sheet satisfies formula (1): Precipitated Ti amount
(mass %)-3.4[N]<0.5.times.[total Ti amount (mass %)-3.4[N]]
(1)
4. A press-formed article of a steel sheet comprising: C: from 0.15
to 0.5% by mass; Si: from 0.2 to 3% by mass; Mn: from 0.5 to 3% by
mass; P: 0.05% by mass or less (exclusive of 0%); S: 0.05% by mass
or less (exclusive of 0%); Al: from 0.01 to 1% by mass; B: from
0.0002 to 0.01% by mass; Ti: equal to or more than 3.4[N]+0.01% by
mass and equal to or less than 3.4[N]+0.1% by mass, wherein [N]
indicates a content (mass %) of N; N: from 0.001 to 0.01%; iron;
and unavoidable impurities, wherein: the steel sheet for
hot-pressing further comprises, as the other element(s), at least
one of the following (a) to (c): (a) at least one selected from the
group consisting of V, Nb and Zr, in an amount of 0.1% or less
(exclusive of 0%) in total; (b) at least one 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) at least one selected from the
group consisting of Mg, Ca and REM, in an amount of 0.01% or less
(exclusive of 0%) in total; a metal microstructure of the
press-formed article includes bainitic ferrite: from 60 to 97 area
%, martensite: 37 area % or less, retained austenite: from 3 to 20
area %, and remainder microstructure: 5 area % or less; an average
equivalent-circle diameter of Ti-containing precipitate having an
equivalent-circle diameter of 30 nm or less among Ti-containing
precipitates contained in the press-formed article is 10 nm or
less; and a precipitated Ti amount and a total Ti amount in a steel
of the steel sheet satisfies formula (1): Precipitated Ti amount
(mass %)-34[N]<0.5.times.[total Ti amount (mass %)-3.4[N]] (1).
Description
TECHNICAL FIELD
[0001] The present invention relates to a press-formed article to
be used when manufacturing an automotive structural component, and
a method for manufacturing such a press-formed article. More
specifically, the present invention relates to a press-formed
article manufactured by applying, when forming a previously heated
steel sheet (blank) into a predetermined shape, a press forming
method of imparting a shape together with applying a heat treatment
to obtain a predetermined strength, 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 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 members 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 and the die) by
cooling the punch and the die in parallel with forming, and
quenching of the material 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 1,270 MPa or
more is about 12.7% at the maximum, and further improvement is
demanded.
[0008] On the other hand, an automotive component needs to be
joined mainly by spot welding, but in a hot-stamped formed article
having, a microstructure mainly including martensite, it is known
that strength in the weld heat affected zone (HAZ) is reduced
significantly and the welded joint is subject to a strength
reduction (softening) (for example, Non-Patent Document 1).
RELATED ART
Patent Document
[0009] Patent Document 1: JP-A-2010-65292
[0010] Patent Document 2: JP-A-2010-65293
[0011] Patent Document 3: JP-A-2010-65294
[0012] Patent Document 4: JP-A-2010-65295
Non-Patent Document
[0013] Non-Patent Document 1: Hirosue et al. "Nippon Steel
Technical Report", No. 378, pp. 15-20 (2003)
SUMMARY OF THE INVENTION
Problems that the Invention is to Solve
[0014] The present invention has been made under these
circumstances, and an object thereof is to provide: a method useful
for manufacturing a press-formed article which is capable of
achieving a high-level balance between high strength and elongation
and has good anti-softening property in HAZ; and a press-formed
article which exerts the above properties.
Means for Solving the Problems
[0015] In the method for manufacturing a press-formed article in
the present invention, which can attain the object above, a steel
sheet for hot-pressing is heated at 900.degree. C. or more and
1,100.degree. C. or less, the steel sheet for hot-pressing
including:
[0016] C: from 0.15 to 0.5% (mass %; hereinafter, the same applies
to the chemical component composition),
[0017] Si: from 0.2 to 3%,
[0018] Mn: from 0.5 to 3%,
[0019] P: 0.05% or less (exclusive of 0%),
[0020] S: 0.05% or less (exclusive of 0%),
[0021] Al: from 0.01 to 1%,
[0022] B: from 0.0002 to 0.01%,
[0023] 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
[0024] N: from 0.001 to 0.01%, with the remainder being iron and
unavoidable impurities, in which 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 6 nm or less, and a precipitated Ti
amount and a total Ti amount in a steel satisfy the following
formula (1),
[0025] and thereafter press forming 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 and equal to or more than a martensite transformation starting
temperature Ms, while ensuring an average cooling rate of
20.degree. C./sec or more in a mold during forming and after the
completion of forming, and thereafter the steel sheet is cooled to
200.degree. C. or less at an average cooling rate of less than
20.degree. C./sec. 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]<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).
[0026] In the steel sheet for hot-pressing to be used in the
manufacturing method 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.
[0027] (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
[0028] (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
[0029] (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
[0030] In the press-formed article obtained by this manufacturing
method, the metal microstructure of the press-formed article
includes bainitic ferrite: from 60 to 97 area %, martensite: 37
area % or less, retained austenite: from 3 to 20 area %, and
remainder microstructure: 5 area % or less, the average
equivalent-circle diameter of Ti-containing precipitate having an
equivalent-circle diameter of 30 nm or less among Ti-containing,
precipitates contained in the press-formed article is 10 nm or
less, and a relationship of the formula (1) is satisfied, and thus,
a high-level balance between high strength and elongation can be
achieved as uniform properties in the formed article.
Advantage of the Invention
[0031] According to the present invention, a steel sheet where the
chemical component composition is strictly specified, the size of
the Ti-containing precipitate is controlled and the precipitation
rate of Ti not forming TiN is controlled is used, so that by
hot-pressing the steel sheet under predetermined conditions, the
strength-elongation balance of the formed article can be made to be
a high-level balance and the anti-softening property in HAZ is
improved.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] [FIG. 1] A schematic explanatory view showing the mold
configuration for carrying out hot-press forming.
MODE FOR CARRYING OUT THE INVENTION
[0033] The present inventors have made studies from various aspects
to realize a press-formed article 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.
[0034] 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 the
steel sheet is hot-press formed under predetermined conditions, a
predetermined amount of retained austenite is ensured after press
forming and a press-formed article having increased intrinsic
ductility (residual ductility) and good anti-softening property in
HAX is obtained. The present invention has been accomplished based
on this finding.
[0035] In the steel sheet for hot-pressing to be used 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%)
[0036] C is an important element in lowering the bainite
transformation starting temperature Bs to refine bainitic ferrite
produced in the cooling process, and increasing the dislocation
density in bainitic ferrite to enhance the strength. In addition,
the amount of fine retained austenite formed between bainitic
ferrite laths is increased, and a high-level balance between high
strength and elongation can be ensured. If the C content is less
than 0.15%, the bainite transformation starting temperature Bs
elevates to bring about coarsening of bainitic ferrite and
reduction in the dislocation density, and the strength of a hot
press-formed article cannot be ensured. If the C content is too
large and exceeds 0.5%, the strength is excessively high, and good
ductility is not obtained. The lower limit of the C content is
preferably 0.18% 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%)
[0037] Si exerts an effect of suppressing cementite formation due
to decomposition of retained austenite formed between bainitic
ferrite laths during cooling of mold quenching, and forming
retained austenite thereby. 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%, ferrite is readily formed, making it difficult to
produce a single phase of austenite during heating, and the
fraction of a microstructure other than bainitic ferrite and
retained austenite in the steel sheet for hot-pressing exceeds 5
area %. 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%)
[0038] Mn is an element effective in enhancing the quenchability
and suppressing the formation of a soft microstructure such as
ferrite and pearlite during cooling of mold quenching. In addition,
this is an important element in lowering the bainite transformation
starting temperature Bs to refine bainitic ferrite produced in the
cooling process and increasing the dislocation density in bainitic
ferrite to enhance the strength. Furthermore, this 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 content 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%))
[0039] 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 Willis of manufacture to reduce the content to 0%. For
this reason, the content thereof 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%))
[0040] 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 content thereof 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%)
[0041] Al is useful as a deoxidizing element and allows the solute
N present in the steel to be fixed as AlN, 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%)
[0042] B is an element having an action of suppressing ferrite
transformation and pearlite transformation, 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)
[0043] Ti exerts an effect of improving the quenchability 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. In addition, when Ti added
excessively relative to N is caused to be present in a solid
solution state in a hot-stamp formed article and the precipitated
compound is finely dispersed, the strength reduction in HAZ can be
suppressed by virtue of precipitation strengthening due to
formation, as TiC, of Ti dissolved in solid during welding of the
hot-stamp formed article or by virtue of an effect such as delaying
increase of the dislocation density due to the dislocation
movement-preventing effect of TiC. However, if the Ti content is
too large and exceeds 3.4[N]+0.1%, the Ti-containing precipitate
(e.g., TiN) formed is coarsened to deteriorate the ductility of the
steel sheet. The lower limit of the Ti content is more preferably
3.4[N]+0.02% or more (further preferably 3.4[N]+0.05% or more), and
the upper limit is more preferably 3.4[N]+0.09% or less (further
preferably 3.4[N]+0.08% or less).
(N: from 0.001 to 0.01%)
[0044] N decrease the improvement effect of the hardenability
during quenching by fixing B as BN, and thus, the content thereof
is preferably reduced as much as possible, but the reduction in an
actual process is limited and therefore, the lower limit is set to
0.001%. If the N content is too large, the Ti-containing
precipitate (e.g., TiN) formed is coarsened, and this precipitate
works as a fracture origin to deteriorate the ductility of the
steel sheet. 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).
[0045] The basic chemical components in the steel sheet for
hot-pressing to be used 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 to
be used in the present invention, it is also useful to further
contain, as the other element(s), at least one of the following (a)
to (c), if desired. The properties of 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.
[0046] (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
[0047] (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
[0048] (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)
[0049] 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)
[0050] Cu Ni, Cr and Mo suppress ferrite transformation and
pearlite transformation, and therefore, effectively act to prevent
the formation of ferrite and perlite 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, in an Amount of 0.01% or Less (Exclusive of 0%) in Total)
[0051] 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.
[0052] In the steel sheet for hot-pressing to be used 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 6 nm or less, and (B) the relationship of
"precipitated Ti amount (mass %)-3.4[N]<0.5.times.[total Ti
amount (mass %)-3.4[N]]" (the relationship of the formula (1)) is
satisfied, are also important requirements.
[0053] The Ti-containing precipitate and formula (1) is controlled
for preventing softening of HAZ and such a control is originally a
control required of a formed article, but these values are little
changed between before and after hot-press forming. Therefore, the
control needs to be already done at the stage before forming (the
steel sheet for hot-pressing). When excessive Ti relative to N in
the steel sheet before forming is cause to be present in a solid
solution state or refined state, the Ti-containing precipitate can
be maintained in a solid solution state or refined state during
heating of hot pressing. As a result, the amount of Ti precipitated
in the press-formed article can be controlled to not more than a
predetermined amount, and softening in HAZ can be prevented,
whereby the joint properties can be improved.
[0054] 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 6 nm or less (requirement of
(A) above). The size (average equivalent-circle diameter) of the
Ti-containing precipitate is preferably 5 nm or less, more
preferably 3 nm or less. Examples of the Ti-containing precipitate
targeted in the present invention include TiC, TiN and other
Ti-containing precipitates such as TiVC, TiNbC, TiVCN and
TiNbCN.
[0055] As described later, the average equivalent-circle diameter
of Ti-containing precipitates in the press-formed article is
specified to be 10 nm or less, whereas that before forming (steel
sheet for hot-pressing) is specified to be 6 nm or less. The reason
why the size of the precipitate is specified to be larger in the
formed article than in the steel sheet is that Ti is present as a
fine precipitate or in a solid solution state in the steel sheet
and when heated at near 800.degree. C. for 15 minutes or more, the
Ti-containing precipitate is slightly coarsened. In order to ensure
the properties as a formed article, the average equivalent-circle
diameter of Ti-containing precipitates must be 10 nm or less, and
for realizing this precipitation state in a hot-stamp formed
article, it is necessary that in the state of the steel sheet for
hot-stamping, the average equivalent-circle diameter of fine
precipitates of 30 nm or less is adjusted to 6 nm or less and many
of Ti is caused to be present in a solid solution state.
[0056] 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 solid solution state or
refined 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 smaller than 0.5 times the remainder after
deduction of Ti that forms TiN from total Ti (i.e.,
0.5.times.[(total Ti amount)-3.4[N]]) (requirement of (B) above).
The "precipitated Ti amount-3.4[N]" is preferably 0.4.times.[(total
Ti amount)-3.4[N]] or less, more preferably 0.3.times.[(total Ti
amount)-3.4[N]] or less.
[0057] For manufacturing the above steel sheet (steel sheet for
hot-pressing), 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 850.degree. C. or more (preferably 900.degree. C. or
more) and 1,000.degree. C. or less (preferably 950.degree. C. or
less), and immediately after that, it may be cooled (rapid cooling)
at an average cooling rate of 20.degree. C./sec or more(preferably
30.degree. C./sec or more) until 500.degree. C. or less (preferably
450.degree. C. or less) and after that, it may be wound at a
temperature of 350.degree. C. or more (preferably 380.degree. C. or
more) and 450.degree. C. or less (preferably 430.degree. C. or
less).
[0058] In the method above, (1) rolling is terminated in a
temperature region where a dislocation introduced into austenite by
hot rolling remains, (2) rapid cooling is performed immediately
thereafter to allow a Ti-containing precipitate such as TiC to be
finely formed on the dislocation, and (3) rapid cooling is further
performed, followed by winding, whereby bainite transformation or
martensite transformation is controlled to occur.
[0059] The steel sheet for hot-pressing which has the
above-described chemical component composition 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 10 to
80% (preferably from 20 to 70%) after pickling and then used for
the manufacture by hot pressing. The steel sheet for hot-pressing
or a cold rolled material thereof may be subjected to a heat
treatment including heating at 830.degree. C. or more (preferably
850.degree. C. or more and 900.degree. C. or less), then rapid
cooling at a cooling rate of 20.degree. C./sec or more (preferably
30.degree. C./sec or more) until 500.degree. C. or less (preferably
450.degree. C. or less), and then holding at 500.degree. C. or less
for 10 seconds or more and 1,000 seconds or less, or tempering at a
temperature of 500.degree. C. or less. In addition, 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.
[0060] Using the above-described steel sheet for hot-pressing, the
steel sheet is heated at a temperature of 900.degree. C. or more
and 1,100.degree. C. or less, and after press forming is started,
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.) and equal to or more
than the martensite transformation starting temperature Ms, 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, and
then cooled to 200.degree. C. or less at an average cooling rate of
less than 20.degree. C./sec, whereby an optimal microstructure as a
formed article with predetermined strength and high ductility
(microstructure mainly including bainitic ferrite) can be produced
in a press-formed article having a single property. The reason for
specifying each requirement in this forming method is as
follows.
[0061] If the heating temperature of the steel sheet is less than
900.degree. C., a sufficient amount of austenite cannot be obtained
during heating, and the martensite fraction is too large in the
final microstructure (microstructure of a formed article). If the
heating temperature of the steel sheet exceeds 1,100.degree. C.,
the austenite grain size grows during heating, the martensite
transformation starting temperature Ms and martensite
transformation finishing temperature Mf are elevated, retained
austenite cannot be ensured during quenching, and good formability
is not achieved. The heating temperature is preferably 950 or more
and 1,050.degree. C. or less. At this time, if the heating time is
too long, the Ti-containing precipitate in the steel sheet can be
hardly refined, and a Ti-containing, precipitate even in a small
amount is formed during heating and coarsened to reduce the effect
of improving weldability. For this reason, the heating time is
preferably shorter. The heating time is preferably 3,600 seconds or
less, and more preferably 20 seconds or less.
[0062] For allowing austenite formed in the heating step above to
be a desired microstructure (microstructure mainly including
bainitic ferrite) 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 and equal
to or more than martensite transformation starting temperature Ms.
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 while impeding production of a
microstructure such as ferrite or pearlite, whereby fine austenite
is caused to remain between bainitic ferrite laths and a
predetermined amount of retained austenite is assured while
ensuring the amount of bainitic ferrite.
[0063] 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. When the cooling is performed to a temperature less
than the martensite transformation starting temperature Ms, the
production amount of martensite is increased and the elongation
(ductility) of the formed article is deteriorated.
[0064] After reaching a temperature equal to or less than a
temperature 100.degree. C. below the bainite transformation
starting temperature Bs and equal to or more than the martensite
transformation starting temperature Ms, rapid cooling is stopped,
and cooling to 200.degree. C. or less is performed at an average
cooling rate of less than 20.degree. C./sec. By adding such a
cooling step, bainitic ferrite transformation is promoted. If the
average cooling rate here is 20.degree. C./sec or more, martensite
is formed and although the strength may be increased, good
elongation is not obtained. The average cooling rate is preferably
15.degree. C./sec or less, more preferably 10.degree. C./sec or
less. The reason why the steel sheet is cooled to 200.degree. C. or
less in this cooling is that the amount of retained austenite
remaining at room temperature is increased by distributing carbon
from bainitic ferrite to untransformed austenite.
[0065] After performing the above-described two-stage cooling,
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, from 1.degree. C./sec or more and
100.degree. C./sec or less. The control of the average cooling rate
during press 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.
[0066] In the press-formed article obtained by this manufacturing
method, the metal microstructure includes bainitic ferrite: from 60
to 97 area %, martensite: 37 area % or less, retained austenite:
from 3 to 20 area %, and remainder microstructure: 5 area % or
less, and the amount of carbon in the retained austenite is 0.50%
or more, so that 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 (the
amount of carbon in basic microstructure and retained austenite) in
this hot press-formed article is as follows.
[0067] When the main microstructure of a press-formed article is
high-strength bainitic ferrite rich in ductility, both of high
strength and high ductility of a press-formed article can be
satisfied. From such a standpoint, the area fraction of bainitic
ferrite must be 60 area % or more. However, if this fraction
exceeds 97 area %, the retained austenite fraction is insufficient,
and the ductility (residual ductility) is reduced. The lower limit
of the bainitic ferrite fraction is preferably 65 area % or more
(more preferably 70 area % or more), and the upper limit is
preferably 95 area % or less (more preferably 90 area % or
less).
[0068] The strength of a hot press-formed article can be increased
by partially incorporating high-strength martensite, but if the
amount thereof is large, the ductility (residual ductility) is
reduced. From such a standpoint, the area fraction of martensite
must be 37 area % or less. The lower limit of the martensite
fraction is preferably 5 area % or more (more preferably 10 area %
or more), and the upper limit is preferably 30 area % or less (more
preferably 25 area % or less).
[0069] 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 he 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).
[0070] As for the microstructure other than those described above,
ferrite, pearlite, and the like 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 %). However,
an area fraction up to 5 area % is acceptable. The area fraction of
the remainder microstructure is preferably 4 area % or less, more
preferably 3 area % or less.
[0071] In the press-formed article above, 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 (i.e., in the
steel sheet constituting the press-formed article) is 10 nm or
less. When this requirement is satisfied, a press-formed article
capable of achieving a high-level balance between high strength and
elongation can be obtained. The average equivalent-circle diameter
of the Ti-containing precipitate is preferably 8 nm or less, more
preferably 6 nm or less.
[0072] In addition, in the press-formed article, the amount of Ti
present as a precipitate other than TiN (i.e., precipitated Ti
amount-3.4[N]) is smaller than 0.5 times the remainder Ti after
deduction of Ti that forms TiN from total Ti (i.e., smaller than
0.5.times.[total Ti amount (%)-3.4[N]]). When this requirement is
satisfied, Ti dissolved in solid during welding is finely
precipitated in HAZ or the existing fine Ti-containing precipitate
suppresses recovery, etc, of the dislocation, and as a result,
softening in HAZ is prevented, and the weldability is improved. The
"precipitated Ti amount-3.4[N]" is preferably 0.4.times.[total Ti
amount)-3.4[N]] or less, more preferably 0.3.times.[total Ti
amount)-3.4[N]] or less.
[0073] According to the method in the present invention, the
properties such as strength and elongation of a 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 hot press-formed
article can be hardly applied. This is very useful in expanding the
application range of a hot press-formed article.
[0074] 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
[0075] Steel materials (Steel Nos. 1 to 31) having the chemical
component composition shown in Table 1 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: 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 Table 2 below. Here, in Table 1, Ac.sub.3 transformation
point, Ms point, and Bs point were determined using the following
formulae (2) to (4) (see, for example, The Physical Metallurgy of
Steels, Leslie, Maruzen, (1985)). In addition, treatments (1) and
(2) shown in Remarks of Table 2 mean that each treatment (rolling,
cooling and alloying) described below was performed.
Ac.sub.3 transformation 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] (2)
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] (3)
Bs point (.degree.
C.)=830-270.times.[C]-90.times.[Mn]-37.times.[Ni]-70.times.[Cr]-8:3.times-
.[Mo] (4)
wherein [C], [Si], [Mn], [P], [Al], [Ti], [V], [Cl], [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 (4) is not contained, the
calculation is done assuming that the term is not present.
[0076] Treatment (1): The hot-rolled steel sheet was cold-rolled
(sheet thickness: 1.6 nun), 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.
[0077] 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 -- -- -- 4 0.220 0.25 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 0.044 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 -- -- -- 16 0.350 1.20 1.20 0.0050
0.0020 0.030 0.0020 0.044 0.0040 -- -- -- 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 0.040 -- -- -- Ac.sub.3
Steel Chemical Component Composition* (mass %) transformation
Bs-100.degree. C. Ms Point No. Ni Zr Ca Mg REM Cr Mo point
(.degree. C.) (.degree. C.) (.degree. C.) 1 -- -- -- -- -- -- --
845 563 425 2 -- -- -- -- -- 0.20 -- 880 568 446 3 -- -- -- -- --
0.20 -- 812 549 421 4 -- -- -- -- -- 0.20 -- 821 549 421 5 -- -- --
-- -- 0.20 -- 853 549 421 6 -- -- -- -- -- 0.20 -- 863 549 421 7 --
-- -- -- -- 0.20 -- 863 549 421 8 -- -- -- -- -- 0.20 -- 863 549
421 9 -- -- -- -- -- 0.20 -- 863 549 421 10 -- -- -- -- -- 0.20 --
863 549 421 11 -- -- -- -- -- 0.20 -- 863 549 421 12 -- -- -- -- --
0.20 -- 863 549 421 13 -- -- -- -- -- 0.20 -- 863 549 421 14 -- --
-- -- -- 0.20 -- 863 549 421 15 -- -- -- -- -- 0.20 -- 899 549 421
16 -- -- -- -- -- 0.20 -- 838 514 374 17 -- -- -- -- -- 0.20 -- 786
414 240 18 -- -- -- -- -- 0.20 -- 875 585 436 19 -- -- -- -- --
0.20 -- 827 441 374 20 -- -- -- -- -- 0.20 -- 886 549 421 21 -- --
-- -- -- 0.20 -- 926 549 421 22 -- -- -- -- -- 0.20 -- 980 549 432
23 -- -- -- -- -- 0.20 -- 866 549 421 24 -- -- -- -- -- 0.20 -- 863
549 421 25 -- -- -- -- -- 0.20 -- 859 549 419 26 0.20 -- -- -- --
0.20 -- 860 541 417 27 -- -- -- -- -- 0.20 0.20 869 532 420 28 --
0.015 -- -- -- 0.20 -- 863 549 421 29 -- -- 0.002 -- -- 0.20 -- 863
549 421 30 -- -- -- 0.002 -- 0.20 -- 863 549 421 31 -- -- -- --
0.002 0.20 -- 863 549 421 *Remainder: Iron and unavoidable
impurities except for P, S and N.
TABLE-US-00002 TABLE 2 Steel Sheet Manufacturing Conditions Average
Cooling Rate from Heating Finish Rolling Tem- Finish Temperature to
Tempera- pera- Rolling Winding ture Steel ture Temperature
Temperature Winding No. (.degree. C.) (.degree. C.) (.degree.
C./sec) (.degree. C.) Remarks 1 1200 950 20 500 -- 2 1200 950 20
500 -- 3 1200 950 20 500 -- 4 1200 950 20 500 -- 5 1200 950 20 500
-- 6 1200 950 20 500 -- 7 1200 800 20 500 -- 8 1200 950 20 500
treatment (1) 9 1200 950 20 500 treatment (2) 10 1200 950 20 500 --
11 1200 950 20 500 -- 12 1200 950 20 500 -- 13 1200 950 20 500 --
14 1200 950 20 500 -- 15 1200 950 20 500 -- 16 1200 950 20 500 --
17 1200 950 20 500 -- 18 1200 950 20 500 -- 19 1200 950 20 500 --
20 1200 950 20 500 -- 21 1200 950 20 500 -- 22 1200 980 20 500 --
23 1200 950 20 500 -- 24 1200 950 20 500 -- 25 1200 950 20 500 --
26 1200 950 20 500 -- 27 1200 950 20 500 -- 28 1200 950 20 500 --
29 1200 950 20 500 -- 30 1200 950 20 500 -- 31 1200 950 20 500
--
[0078] With respect to the steel sheets (steel sheets for
press-forming) obtained, analysis of the Ti precipitation state
("precipitated Ti amount-3.4[M]" and average equivalent-circle
diameter of Ti-containing precipitates) was performed in the
following manner. The results obtained are shown in Table 3 below
together with the calculated value of 0.5.times.[total Ti
amount-3.4[N]].
(Analysis of Ti Precipitation State of Steel Sheet)
[0079] 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-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-3.4[N]" was
determined. In the case where the Ti-containing precipitate
partially contained V or Nb, the contents of these precipitates
were also measured.
TABLE-US-00003 TABLE 3 Steel Sheet for Press-Forming Average
Equivalent-Circle Precipitated Ti Diameter of Steel Amount - 3.4[N]
0.5 .times. [Total Ti Amount - Ti-Containing No. (mass %) 3.4[N]
(mass %) Precipitates (nm) 1 0.009 0.015 4.0 2 0.008 0.015 2.5 3
0.008 0.015 2.3 4 0.006 0.015 3.3 5 0.001 0.003 3.0 6 0.008 0.015
3.2 7 0.018 0.015 9.2 8 0.008 0.015 3.4 9 0.010 0.015 3.2 10 0.008
0.015 2.8 11 0.008 0.015 2.8 12 0.008 0.015 2.8 13 0.008 0.015 2.8
14 0.008 0.015 2.8 15 0.009 0.015 3.9 16 0.009 0.015 3.7 17 0.008
0.015 3.2 18 0.008 0.015 2.7 19 0.006 0.015 2.1 20 0.025 0.043 3.1
21 0.128 0.093 10.8 22 0.008 0.015 2.7 23 0.009 0.015 3.1 24 0.006
0.015 3.6 25 0.007 0.015 3.6 26 0.008 0.015 3.6 27 0.008 0.015 2.7
28 0.006 0.015 3.0 29 0.007 0.015 3.3 30 0.006 0.015 3.2 31 0.006
0.015 3.0
[0080] 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, heating time average
cooling rate, and rapid cooling finishing temperature during press
forming) are shown in Table 4 below.
TABLE-US-00004 TABLE 4 Press-Forming Conditions Cooling Rate After
Rapid Cooling Finishing Heating Heating Average Finishing Rapid
Steel Temperature time Cooling Rate Temperature Cooling No.
(.degree. C.) (sec) (.degree. C./sec) (.degree. C.) (.degree.
C./sec) 1 900 600 40 450 3 2 900 15 40 450 3 3 900 15 40 450 3 4
900 15 40 450 3 5 900 15 40 450 3 6 900 15 40 450 3 7 900 15 40 450
3 8 900 15 40 450 3 9 900 15 40 450 3 10 900 15 40 450 3 11 900 15
40 450 25 12 800 15 40 300 3 13 900 15 5 450 3 14 900 15 40 600 3
15 900 15 40 450 3 16 900 15 40 450 3 17 900 15 40 400 3 18 900 15
40 450 3 19 900 15 40 400 3 20 900 15 40 450 3 21 900 15 40 450 3
22 900 15 40 450 3 23 900 15 40 450 3 24 900 15 40 450 3 25 900 15
40 450 3 26 900 15 40 450 3 27 900 15 40 450 3 28 900 15 40 450 3
29 900 15 40 450 3 30 900 15 40 450 3 31 900 15 40 450 3
[0081] 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 hardness reduction amount after heat treatment
were measured by the following methods, and the Ti precipitation
state was analyzed by the method described above.
(Measurement of Tensile Strength (TS) and Elongation (Total
Elongation EL))
[0082] 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 1,180 MPa or more and an elongation
(EL) of 12.0% or more were satisfied and the strength-elongation
balance (TS.times.EL) was 16,000 (MPa. %) or more.
(Observation of Metal Microstructure (Fraction of Each
Microstructure))
[0083] (1) With respect to the microstructures of bainitic ferrite,
martensite and ferrite in the formed article, the steel sheet was
corroded with nital and after distinguishing, bainitic ferrite,
martensite and ferrite from each other by SEM observation
(magnifications: 1,000 times or 2,000 times), the fraction (area
ratio) of each microstructure was determined.
[0084] (2) The retained austenite fraction in the formed article
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).
(Hardness Reduction Amount After Heat Treatment)
[0085] As the heat history based on spot welding, the hardness
reduction amount (.DELTA.Hv) relative to the original hardness
(Vickers hardness) was measured after heating to 700.degree. C. at
an average heating rate of 50.degree. C./sec in a heat treatment
simulator and then cooling at an average cooling rate of 50.degree.
C./sec. The anti-softening property in HAZ was judged as good when
the hardness reduction amount (.DELTA.Hv) was 50 Hv or less.
[0086] The observation results (fraction of each microstructure, Ti
precipitation state, and precipitated Ti amount-34[N]) of the metal
microstructure are shown in Table 5 below. In addition, the
mechanical properties (tensile strength TS, elongation EL,
TS.times.EL, and hardness reduction amount .DELTA.Hv) of the
press-formed article are shown in Table 6 below. Here, the value of
"precipitated Ti amount-34[N]" in the press-formed article is
slightly different from the value of "precipitated Ti
amount-3.4[N]" in the steel sheet for press-forming, but this is a
measurement error.
TABLE-US-00005 TABLE 5 Metal Microstructure of Press-Formed Article
Bainitic Ferrite Martensite Average Equivalent-Circle Precipitated
Ti Steel Fraction Fraction Ferrite Fraction Retained Austenite
Diameter of Ti-Containing Amount-3.4[N] No. (area %) (area %) (area
%) Fraction (area %) Precipitates (nm) (mass %) 1 87 8 0 5 2.7
0.010 2 81 8 5 6 2.5 0.010 3 94 6 0 0 3.0 0.010 4 89 7 0 4 2.5
0.009 5 86 6 0 8 3.5 0.000 6 86 7 0 7 3.3 0.011 7 85 8 0 7 8.0
0.023 8 86 7 0 7 2.9 0.010 9 87 6 0 7 2.1 0.011 10 88 5 0 7 3.0
0.011 11 6 88 0 6 3.4 0.011 12 0 92 0 8 3.9 0.011 13 26 5 48 6 3.2
0.011 14 55 6 33 6 3.4 0.012 15 91 0 0 9 2.4 0.013 16 86 7 0 7 2.3
0.012 17 72 8 8 12 2.8 0.013 18 88 5 0 7 2.4 0.008 19 87 6 0 7 3.0
0.012 20 88 6 0 6 3.8 0.021 21 87 6 0 7 13.8 0.180 22 85 7 0 8 2.1
0.012 23 88 6 0 6 3.1 0.011 24 86 7 0 7 3.6 0.009 25 85 7 0 8 3.0
0.012 26 89 5 0 6 2.0 0.013 27 87 7 0 6 2.0 0.011 28 85 8 0 7 3.6
0.008 39 84 9 0 7 3.0 0.010 30 86 7 0 7 3.3 0.009 31 85 7 0 8 3.0
0.011
TABLE-US-00006 TABLE 6 Mechanical Properties of Press-Formed
Article Tensile Steel Strength TS Elongation EL TS .times. EL
Hardness Reduction No. (MPa) (%) (MPa %) Amount .DELTA.Hv (Hv) 1
1217 13.3 16151 42 2 1189 15.5 16188 38 3 1239 10.1 12551 38 4 1210
13.3 16115 35 5 1254 13.0 16296 35 6 1263 12.8 16220 39 7 1241 11.9
14786 68 8 1213 13.6 16488 44 9 1218 13.4 16263 36 10 1206 13.3
16057 42 11 1495 10.6 15847 44 12 1532 10.3 15735 40 13 890 18.1
16096 40 14 955 15.2 14516 43 15 1260 13.0 16329 42 16 1320 12.3
16187 38 17 1992 3.5 6972 40 18 1217 13.4 16311 41 19 1245 13.2
16481 38 20 1215 13.3 16160 44 21 1253 12.9 16174 98 22 1266 12.9
16275 37 23 1221 13.3 16256 40 24 1220 13.2 16063 40 25 1224 13.2
16136 42 26 1244 13.1 16271 41 27 1232 13.2 16236 40 28 1233 13.5
16646 39 29 1250 13.2 16479 42 30 1244 13.1 16271 40 31 1229 13.2
16196 41
[0087] These results allow for the following consideration. It is
found that in the case of Steel Nos. 1, 2, 4 to 6, 8 to 10, 15, 16,
18 to 20, and 22 to 31, which are Examples satisfying the
requirements specified in the present invention, a formed article
having a good strength-ductility balance and a good anti-softening
property is obtained.
[0088] On the other hand, in the case of Steel Nos. 3, 7, 11 to 14,
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 having a small Si content is used, the
retained austenite fraction is not ensured in the press-formed
article and since only low elongation EL is obtained, the
strength-elongation balance (TS.times.EL) is deteriorated. In the
case of Steel No. 7 where the finish rolling temperature in the
manufacture of a steel sheet is low, the relationship of the
formula (1) is not satisfied, and not only the Ti-containing
precipitate is coarsened to reduce the strength-elongation balance
(TS.times.EL) but also the anti-softening property is
deteriorated.
[0089] In the case of Steel No. 11 where the cooling rate after
rapid cooling during press forming is high, martensite is
excessively produced and not only the strength is too high,
resulting in obtaining only low EL, but also the
strength-elongation balance (TS.times.EL) is deteriorated. In the
case of Steel No. 12 where the rapid cooling finishing temperature
during press forming is low, martensite is excessively produced and
not only the strength is too high, resulting in obtaining only low
EL, but also the strength-elongation balance (TS.times.EL) is
deteriorated.
[0090] In the case of Steel No. 13 where the average cooling rate
during press forming is low, the area ratio of bainitic ferrite
cannot be ensured and not only the strength is too low but also the
strength-elongation balance (TS.times.EL) is deteriorated. In the
case of Steel No. 14 where the rapid cooling finishing temperature
during press forming is high, the area ratio of bainitic ferrite
cannot be ensured due to production of ferrite and not only the
strength is too low but also the strength-elongation balance
(TS.times.EL) is deteriorated.
[0091] In the case of Steel No. 17 where a steel sheet having an
excessive C content is used, the strength of a formed article is
high, but only low elongation EL is obtained. In the case of Steel
No. 21 where a steel sheet having an excessive Ti content is used,
a press-formed article does not satisfy the relationship of the
formula (1), and not only the Ti-containing precipitate in the
press-formed article is coarsened but also the anti-softening,
property is deteriorated.
INDUSTRIAL APPLICABILITY
[0092] In the present invention, a steel sheet for hot-pressing
which has a predetermined chemical component composition, where the
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 6 nm or less and the
precipitated Ti amount and the total Ti amount in the steel satisfy
a predetermined relationship, is heated at a temperature of
900.degree. C. or more and 1,100.degree. C. or less, and after
press forming is started, 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 and equal
to or more than the martensite transformation starting temperature
Ms, 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, and then cooled to 200.degree. C. or less at an average
cooling rate of less than 20.degree. C./sec, whereby a press-formed
article capable achieving a high-level balance between high
strength and elongation can be obtained and moreover, a
press-formed article having good anti-softening property in HAZ can
be realized.
DESCRIPTION OF REFERENCE NUMERALS AND SIGNS
[0093] 1: Punch
[0094] 2: Die
[0095] 3: Blank holder
[0096] 4: Steel sheet (blank)
* * * * *