U.S. patent application number 14/382158 was filed with the patent office on 2015-01-29 for steel sheet for hot pressing use, press-formed product, and method for manufacturing press-formed product.
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 | 20150027602 14/382158 |
Document ID | / |
Family ID | 49116646 |
Filed Date | 2015-01-29 |
United States Patent
Application |
20150027602 |
Kind Code |
A1 |
Murakami; Toshio ; et
al. |
January 29, 2015 |
STEEL SHEET FOR HOT PRESSING USE, PRESS-FORMED PRODUCT, AND METHOD
FOR MANUFACTURING PRESS-FORMED PRODUCT
Abstract
A steel sheet for hot pressing use according to the present
invention has a specified chemical component composition, wherein
some of Ti-containing precipitates contained in the steel sheet,
each of which having an equivalent circle diameter of 30 nm or
less, have an average equivalent circle diameter of 3 nm or more,
the precipitated Ti amount and the total Ti amount in the steel
fulfill the relationship represented by formula (1) shown below,
and the sum total of the fraction of bainite and the fraction of
martensite in the metal microstructure is 80 area % or more.
Precipitated Ti amount (mass %)-3.4[N]>0.5.times.[total Ti
amount (mass %)-3.4[N]] (1) (In the formula (1), [N] represents the
content (mass %) of N in the steel.)
Inventors: |
Murakami; Toshio; (Kobe-shi,
JP) ; Naitou; Junya; (Kobe-shi, JP) ; Okita;
Keisuke; (Kobe-shi, JP) ; Ikeda; Shushi;
(Nagoya, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KABUSHIKI KAISHA KOBE SEIKO SHO (Kobe Steel, Ltd.) |
Kobe-shi, Hyogo |
|
JP |
|
|
Assignee: |
KABUSHIKI KAISHA KOBE SEIKO SHO
(Kobe Steel, Ltd.)
Kobe-shi, Hyogo
JP
|
Family ID: |
49116646 |
Appl. No.: |
14/382158 |
Filed: |
March 1, 2013 |
PCT Filed: |
March 1, 2013 |
PCT NO: |
PCT/JP13/55680 |
371 Date: |
August 29, 2014 |
Current U.S.
Class: |
148/653 ;
148/330; 420/103; 420/104; 420/106; 420/112; 420/121; 420/90 |
Current CPC
Class: |
C21D 2211/008 20130101;
C22C 38/50 20130101; C22C 38/38 20130101; C22C 38/001 20130101;
C22C 38/04 20130101; C22C 38/24 20130101; C21D 2211/004 20130101;
C21D 1/18 20130101; C22C 38/14 20130101; C22C 38/02 20130101; C21D
8/005 20130101; C22C 38/06 20130101; C22C 38/20 20130101; C22C
38/60 20130101; C21D 1/673 20130101; B21D 37/16 20130101; B21D
22/208 20130101; C22C 38/002 20130101; C22C 38/32 20130101; C22C
38/54 20130101; C22C 38/26 20130101; C21D 2211/002 20130101; C21D
2221/00 20130101; C22C 38/22 20130101; C21D 2221/10 20130101; C21D
9/48 20130101; C22C 38/28 20130101; C22C 38/34 20130101; C21D
2211/001 20130101 |
Class at
Publication: |
148/653 ;
148/330; 420/90; 420/103; 420/106; 420/112; 420/104; 420/121 |
International
Class: |
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 38/14 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 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 9, 2012 |
JP |
2012-053844 |
Claims
1. A steel sheet, comprising, in mass %, with respect to a chemical
component composition: C: 0.15-0.5%; Si: 0.2-3%; Mn: 0.5-3%; P:
0.05% or less, exclusive of 0%; S: 0.05% or less, exclusive of 0%;
Al: 0.01-1%; B: 0.0002-0.01%; Ti: 3.4[N]+0.01% or more and
3.4[N]+0.1% or less, wherein [N] represents N content in mass %;
and N: 0.001-0.01% respectively, with a remainder comprising iron
and inevitable impurities, wherein some of Ti-containing
precipitates contained in the steel sheet, each of which having an
equivalent circle diameter of 30 nm or less, have an average
equivalent circle diameter of 3 nm or more, a precipitated Ti
amount and a total Ti amount in the steel fulfill a relationship of
formula (1), and a sum total of a fraction of bainite and a
fraction of martensite in a metal microstructure is 80 area % or
more: precipitated Ti amount (mass %)-3.4[N]>0.5.times.[(total
Ti amount (mass %))-3.4[N]] (1) wherein [N] represents the content
(mass %) of N in the steel.
2. The steel sheet according to claim 1, further comprising at
least one of (a)-(c) below as other elements: (a) at least one
element selected from the group consisting of V, Nb and Zr by 0.1%
or less, exclusive of 0%, in total; (b) at least one element
selected from the group consisting of Cu, Ni, Cr and Mo by 1% or
less, exclusive of 0%, in total; and (c) at least one element
selected from the group consisting of Mg, Ca and REM by 0.01% or
less, exclusive of 0%, in total.
3. A method for manufacturing a press-formed product, comprising:
hot pressing with a steel sheet according to claim 1; heating the
steel sheet to a temperature of Ac.sub.1 transformation
point+20.degree. C. or above and Ac.sub.3 transformation
point-20.degree. C. or below; thereafter starting press forming;
and executing cooling to a temperature or below, the temperature
being lower than a bainite transformation starting temperature Bs
by 100.degree. C., while securing an average cooling rate of
20.degree. C./s or more within a tool during forming and after
completion of forming.
4. A press-formed product obtained by the method according to claim
3, wherein the metal microstructure comprises retained austenite:
3-20 area %, annealed martensite and/or annealed bainite: 30-87
area %, and martensite as quenched: 10-67 area %, and an amount of
carbon in the retained austenite is 0.60% or more.
5. A method for manufacturing a press-formed product, comprising:
hot pressing with the steel sheet according to claim 1; dividing a
heating region of the steel sheet into two regions; heating one
region thereof to a temperature of Ac.sub.3 transformation point or
above and 950.degree. C. or below; heating the other region to a
temperature of Ac.sub.1 transformation point+20.degree. C. or above
and Ac.sub.3 transformation point-20.degree. C. or below;
thereafter starting press forming; and executing cooling to a
temperature of martensite transformation starting temperature Ms or
below while securing an average cooling rate of 20.degree. C./s or
more within a tool during forming and after completion of
forming.
6. A press-formed product obtained by the method according to claim
5, comprising a first region whose metal microstructure comprises
retained austenite: 3-20 area % and martensite: 80 area % or more,
and a second region whose metal microstructure comprises retained
austenite: 3-20 area %, annealed martensite and/or annealed
bainite: 30-87 area %, and martensite as quenched: 10-67 area %
with an amount of carbon in the retained austenite being 0.60% or
more.
Description
TECHNICAL FIELD
[0001] The present invention relates to a steel sheet for hot
pressing use used in manufacturing structural components of an
automobile and suitable for hot press forming, a press-formed
product obtained from such a steel sheet for hot pressing use, and
a method for manufacturing the press-formed product, and relates
more specifically to a steel sheet for hot pressing use that is
useful in being applied to a hot press forming method securing a
predetermined strength by being subjected to heat treatment
simultaneously with impartation of the shape in forming a
pre-heated steel sheet (blank) into a predetermined shape, a
press-formed product, and a useful method for manufacturing such a
press-formed product.
BACKGROUND ART
[0002] As one of the fuel economy improvement measures of an
automobile triggered by global environment problems, weight
reduction of the vehicle body is advancing, and it is necessary to
high-strengthen a steel sheet used for an automobile as much as
possible. On the other hand, when a steel sheet is
high-strengthened, shape accuracy in press forming comes to
deteriorate.
[0003] On this account, a hot press forming method has been
employed for manufacturing components in which a steel sheet is
heated to a predetermined temperature (for example, a temperature
at which a state of an austenitic phase is achieved), the strength
is lowered, the steel sheet is thereafter formed using a tool of a
temperature (room temperature for example) lower than the steel
sheet, thereby impartation of a shape and rapid heat treatment
(quenching) utilizing the temperature difference of the both are
executed simultaneously, and the strength after forming is secured.
Also, such a hot-press forming method is referred to by various
names such as a hot forming method, hot stamping method, hot stamp
method, die quench method, and the like in addition to the hot
press method.
[0004] FIG. 1 is a schematic explanatory drawing showing a tool
configuration for executing hot press forming described above, 1 in
the drawing 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 punch
shoulder radius, rd is die shoulder radius, and CL is punch/die
clearance respectively. Also, out of these components, in the punch
1 and the die 2, passages 1a, 2a through which a cooling medium
(water for example) can pass are formed inside of each, and it is
configured that these members are cooled by making the cooling
medium pass through these passages.
[0005] In hot press forming (hot deep drawing for example) using
such a tool, forming is started in a state the steel sheet (blank)
4 is heated to a two-phase zone temperature (between Ac.sub.1
transformation point and Ac.sub.3 transformation point) or a
single-phase zone temperature of Ac.sub.3 transformation point or
above and is softened. That is, in a state the steel sheet 4 in a
high temperature state is sandwiched between the die 2 and the
blank holder 3, the steel sheet 4 is pressed in to the inside of a
hole of the die 2 by the punch 1, and is formed into a shape
corresponding to the shape of the outer shape of the punch 1 while
reducing the outside diameter of the steel sheet 4. Also, by
cooling the punch 1 and the die 2 in parallel with forming, heat
removal from the steel sheet 4 to the tools (the punch 1 and the
die 2) is executed, holding and cooling are further executed at a
forming bottom dead point (the temporal point the tip of the punch
is positioned at the deepest point: the state shown in FIG. 1), and
thereby quenching of the raw material is executed. By executing
such a forming method, a formed product of 1,500 MPa class with
excellent dimensional accuracy can be obtained, the forming load
can be reduced compared with a case a component of a same strength
class is cold-formed, and therefore less capacity of the press
machine is needed.
[0006] As a steel sheet for hot pressing use widely used at
present, one using 22Mn--B5 steel as a raw material is known. The
steel sheet has the tensile strength of approximately 1,500 MPa and
the elongation of approximately 6-8%, and is applied to a shock
resistant member (a member not causing deformation as much as
possible and not causing breakage in collision). However,
application to a component requiring deformation such as an energy
absorption member is difficult because elongation (ductility) is
low.
[0007] As a steel sheet for hot pressing use exerting excellent
elongation, technologies such as the patent literatures 1-4 for
example have also been proposed. According to these technologies,
the basic strength class of each steel sheet is adjusted by setting
the carbon content in the steel sheet to various ranges, and
elongation is improved by introducing ferrite with high
deformability and reducing the average grain size of ferrite and
martensite. Although these technologies are effective in improving
elongation, they are still insufficient from the viewpoint of
improving elongation matching the strength of the steel sheet. For
example, those having 1,470 MPa or more of the tensile strength TS
have the elongation EL of approximately 10.2% at the maximum, and
further improvement is required.
[0008] On the other hand, even the formed product having a lower
strength class compared to the hot stamp formed product, 980 MPa
class or 1,180 MPa class of the tensile strength TS for example,
having been studied until now has a problem in forming accuracy of
cold press forming, and there are needs for low-strength hot press
forming as an improvement measure therefor. At that time, it is
necessary to largely improve energy absorption properties in the
formed product.
[0009] Particularly, in recent years, development of the technology
for differentiating the strength within a single component is
proceeding. As such a technology, a technology has been proposed in
which the portion that must be prevented from deforming has high
strength (high strength side: shock resistant portion side), and
the portion that needs energy absorption has low strength and high
ductility (low strength side: energy absorption portion side). For
example, in a passenger car of the middle class or above, there is
a case that portions having both functions of shock resistant
property and energy absorption property are provided within a
component of a B-pillar and rear side member considering
compatibility in a side collision and a rear collision (a function
for protecting the counterpart side also when a small-sized car
collides with). For the purpose of manufacturing such members, (a)
a method of joining a steel sheet becoming of low strength even in
being heated to a same temperature and tool-quenched to a normal
steel sheet for hot pressing use (tailored weld blank: TWB), (b) a
method for differentiating the strength for each region of a steel
sheet by differentiating the cooling rate in the tool, (c) a method
for differentiating the strength by differentiating the heating
temperature for each region of a steel sheet, and the like have
been proposed.
[0010] Although the tensile strength: 1,500 MPa class is achieved
on the high strength side (shock resistant portion side) according
to these technologies, the maximum tensile strength is 700 MPa and
the elongation EL is approximately 17% on the low strength side
(energy absorption portion side), and achievement of higher
strength and higher ductility are required in order to further
improve the energy absorption properties.
CITATION LIST
Patent Literature
[0011] [Patent Literature 1] JP-A 2010-065292 [0012] [Patent
Literature 2] JP-A 2010-065293 [0013] [Patent Literature 3] JP-A
2010-065294 [0014] [Patent Literature 4] JP-A 2010-065295
SUMMARY OF INVENTION
Technical Problems
[0015] The present invention has been developed in view of such
circumstances as described above, and its object is to provide a
steel sheet for hot pressing use capable of obtaining a hot
press-formed product that can achieve the balance of high strength
and elongation with a high level when uniform property is required
within a formed product and is useful in obtaining a press-formed
product that can achieve the balance of high strength and
elongation with a high level according to each region when regions
corresponding to a shock resistant portion and an energy absorption
portion are required within a single formed product, a press-formed
product exerting the properties described above, and a useful
method for manufacturing such a hot press-formed product.
Solution to Problems
[0016] The steel sheet for hot pressing use of the present
invention which could achieve the object described above
contains:
[0017] C: 0.15-0.5% (means mass %, hereinafter the same with
respect to the chemical component composition);
[0018] Si: 0.2-3%;
[0019] Mn: 0.5-3%;
[0020] P: 0.05% or less (exclusive of 0%);
[0021] S: 0.05% or less (exclusive of 0%);
[0022] Al: 0.01-1%;
[0023] B: 0.0002-0.01%;
[0024] Ti: 3.4[N]+0.002% or more and 3.4[N]+0.1% or less ([N]
expresses N content (mass %)), and
[0025] N: 0.001-0.01% respectively, with the remainder consisting
of iron and inevitable impurities, in which
[0026] some of Ti-containing precipitates contained in the steel
sheet, each of which having an equivalent circle diameter of 30 nm
or less, have an average equivalent circle diameter of 3 nm or
less, the precipitated Ti amount and the total Ti amount in the
steel fulfill the relationship represented by formula (1) shown
below, and the sum total of the fraction of bainite and the
fraction of martensite in the metal microstructure is 80 area % or
more. Also, "equivalent circle diameter" is the diameter of an
imaginary circle having an area same to the size (area) of Ti
containing precipitates (TiC for example) ("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] represents the content (mass %) of N in
the steel.)
[0027] In the steel sheet for hot pressing use of the present
invention, according to the necessity, it is also useful to
contain, as other elements, (a) at least one element selected from
the group consisting of V, Nb and Zr by 0.1% or less (exclusive of
0%) in total, (b) at least one element selected from the group
consisting of Cu, Ni, Cr and Mo by 1% or less (exclusive of 0%) in
total, (c) at least one element selected from the group consisting
of Mg, Ca and REM by 0.01% or less (exclusive of 0%) in total, and
the like, and the properties of the press-formed product is
improved further according to the kind of the elements
contained.
[0028] The method for manufacturing a press-formed product of the
present invention which could achieve the object described above
includes the steps of using such a steel sheet for hot pressing use
of the present invention as described above, heating the steel
sheet to a temperature of Ac.sub.1 transformation point+20.degree.
C. or above and Ac.sub.3 transformation point-20.degree. C. or
below, thereafter starting press forming, and executing cooling to
a temperature or below, the temperature being lower than the
bainite transformation starting temperature Bs by 100.degree. C.,
while securing the average cooling rate of 20.degree. C./s or more
within a tool during forming and after completion of forming.
[0029] In the press-formed product obtained by the method for
manufacturing, the metal microstructure includes retained
austenite: 3-20 area %, annealed martensite and/or annealed
bainite: 30-87 area %, and martensite as quenched: 10-67 area %,
the amount of carbon in the retained austenite is 0.60% or more,
and the balance of high strength and elongation can be achieved
with a high level and as a uniform property within the formed
product. Also, the area ratio of annealed martensite and/or
annealed bainite means the total area ratio of both microstructures
when both microstructures are included, and means, when either one
microstructure is included, the area ratio of the
microstructure.
[0030] Also, another method for manufacturing a press-formed
product of the present invention which could achieve the object
described above includes the steps of using such a steel sheet for
hot pressing use of the present invention as described above,
dividing a heating region of the steel sheet into two regions,
heating one region thereof to a temperature of Ac.sub.3
transformation point or above and 950.degree. C. or below, heating
the other region to a temperature of Ac.sub.1 transformation
point+20.degree. C. or above and Ac.sub.3 transformation
point-20.degree. C. or below, thereafter starting press forming,
and executing cooling to a temperature of martensite transformation
starting temperature Ms or below while securing the average cooling
rate of 20.degree. C./s or more within a tool during forming and
after completion of forming.
[0031] In the press-formed product obtained by the method for
manufacturing, a first region whose metal microstructure includes
retained austenite: 3-20 area % and martensite: 80 area % or more
and a second region whose metal microstructure includes retained
austenite: 3-20 area %, annealed martensite and/or annealed
bainite: 30-87 area %, and martensite as quenched: 10-67 area %
with the amount of carbon in the retained austenite being 0.60% or
more are included, the balance of high strength and elongation can
be achieved with a high level according to each region, and regions
corresponding to a shock resistant portion and an energy absorption
portion are present within a single formed product.
Advantageous Effects of Invention
[0032] According to the present invention, because a steel sheet is
used in which the chemical component composition is strictly
stipulated, the size of Ti-containing precipitates is controlled,
the precipitation rate is controlled for Ti that does not form TiN,
and the ratio of tempered hard phase (martensitic phase, bainitic
phase and the like), hard phase (as-quenched martensite phase) and
retained austenite phase is adjusted with respect to the metal
microstructure, by hot-pressing the steel sheet under a
predetermined condition, high strength-elongation balance of the
press-formed product can be made a high level. Also, when
hot-pressing is executed under different conditions in plural
regions, the shock resistant portion and the energy absorption
portion can be formed within a single formed product, the balance
of high strength and elongation can be achieved with a high level
for each portion.
BRIEF DESCRIPTION OF DRAWINGS
[0033] FIG. 1 is a schematic explanatory drawing showing a tool
configuration for executing hot press forming.
DESCRIPTION OF EMBODIMENTS
[0034] The present inventors carried out studies from various
aspects in order to achieve such a steel sheet for hot pressing use
that can obtain a press-formed product exhibiting excellent
ductility (elongation) also while securing high strength after
press-forming in manufacturing the press-formed product by heating
a steel sheet to a predetermined temperature and thereafter
executing hot press forming.
[0035] As a result of the studies, it was found out that, when the
chemical component composition of the steel sheet for hot pressing
use was strictly stipulated, the size of Ti-containing precipitates
and precipitated Ti amount were controlled and the metal
microstructure was made an appropriate one, by hot press forming of
the steel sheet under a predetermined condition, a press-formed
product in which retained austenite of a predetermined amount was
secured after press forming and intrinsic ductility (residual
ductility) was enhanced could be obtained, and the present
invention was completed.
[0036] In the steel sheet for hot pressing use of the present
invention, it is necessary to strictly stipulate the chemical
component composition, and the reasons for limiting the range of
each chemical component are as follows.
[C: 0.15-0.5%]
[0037] C is an important element in achieving the balance of high
strength and elongation of a case uniform properties are required
within a formed product with a high level or in securing retained
austenite particularly in the low strength/high ductility portion
of a case the regions corresponding to a shock resistant portion
and an energy absorption portion are required within a single
formed product. Also, by concentration of C to austenite in heating
of hot press forming, retained austenite can be formed after
quenching. Also, C contributes to increase of the amount of
martensite, and increases the strength. In order to exert such
effects, C content should be 0.15% or more.
[0038] However, when C content becomes excessive and exceeds 0.5%,
two phase zone heating range becomes narrow, and the balance of
high strength and elongation of a case uniform properties are
required within a formed product is not achieved with a high level,
or it becomes hard to adjust the metal microstructure to that
targeted particularly in the low strength/high ductility portion (a
microstructure in which a predetermined amount of annealed
martensite and/or annealed bainite is secured) of a case the
regions corresponding to a shock resistant portion and an energy
absorption portion are required within a single formed product.
Preferable lower limit of C content is 0.17% or more (more
preferably 0.20% or more), and more preferable upper limit is 0.45%
or less (further more preferably 0.40% or less).
[Si: 0.2-3%]
[0039] Si exerts an effect of forming retained austenite by
suppressing that martensite is tempered during cooling of
tool-quenching and cementite is formed, or that untransformed
austenite is disintegrated. In order to exert such an effect, Si
content should be 0.2% or more. Also, when Si content becomes
excessive and exceeds 3%, ferrite is liable to be formed, formation
of single-phase microstructure becomes hard in heating, and
required fractions of bainite and martensite cannot be secured in a
steel sheet for hot pressing use. Preferable lower limit of Si
content is 0.5% or more (more preferably 1.0% or more), and
preferable upper limit is 2.5% or less (more preferably 2.0% or
less).
[Mn: 0.5-3%]
[0040] Mn is an element effective in enhancing quenchability and
suppressing formation of a microstructure (ferrite, pearlite,
bainite and the like) other than martensite and retained austenite
during cooling of tool-quenching. Also, Mn is an element
stabilizing austenite, and is an element contributing to increase
of retained austenite amount. In order to exert such effects, Mn
should be contained by 0.5% or more. Although Mn content is
preferable to be as much as possible when only properties are
considered, because the cost of adding alloy increases, Mn content
is made 3% or less. Preferable lower limit of Mn content is 0.7% or
more (more preferably 1.0% or more), and preferable upper limit is
2.5% or less (more preferably 2.0% or less).
[P: 0.05% or Less (Exclusive of 0%)]
[0041] Although P is an element inevitably included in steel,
because P deteriorates ductility, P is preferable to be reduced as
much as possible. However, because extreme reduction causes
increase of the steel making cost and to make it 0% is difficult in
manufacturing, P content is made 0.05% or less (exclusive of 0%).
Preferable upper limit of P content is 0.045% or less (more
preferably 0.040% or less).
[S: 0.05% or Less (Exclusive of 0%)]
[0042] Similar to P, S is also an element inevitably included in
steel, S deteriorates ductility, and therefore S is preferable to
be reduced as much as possible. However, because extreme reduction
causes increase of the steel making cost and to make it 0% is
difficult in manufacturing, S content is made 0.05% or less
(exclusive of 0%). Preferable upper limit of S content is 0.045% or
less (more preferably 0.040% or less).
[Al: 0.01-1%]
[0043] Al is useful as a deoxidizing element, fixes solid-solution
N present in steel as AlN, and is useful in improving ductility. In
order to effectively exert such an effect, Al content should be
0.01% or more. However, when Al content becomes excessive and
exceeds 1%, Al.sub.2O.sub.3 is formed excessively, and ductility is
deteriorated. Also, preferable lower limit of Al content is 0.02%
or more (more preferably 0.03% or more), and preferable upper limit
is 0.8% or less (more preferably 0.6% or less).
[B: 0.0002-0.01%]
[0044] B is an element contributing to prevention of formation of
ferrite, pearlite and bainite during cooling after heating to a
two-phase zone temperature of (Ac.sub.1 transformation
point-Ac.sub.3 transformation point) because B has an action of
suppressing ferrite transformation, pearlite transformation and
bainite transformation on the high strength portion side, and to
secure retained austenite. In order to exert such effects, B should
be contained by 0.0002% or more, however, even when B is contained
excessively exceeding 0.01%, the effects saturate. Preferable lower
limit of B content is 0.0003% or more (more preferably 0.0005% or
more), and preferable upper limit is 0.008% or less (more
preferably 0.005% or less).
[Ti: 3.4[N]+0.01% or More and 3.4[N]+0.1% or Less: [N] Expresses N
Content (Mass %)]
[0045] Ti develops improvement effect of quenchability by fixing N
and holding B in a solid solution state. In order to exert such an
effect, it is important to contain Ti more than the stoichiometric
ratio of Ti and N (3.4 times of N content) by 0.01% or more.
However, when Ti content becomes excessive to be more than
3.4[N]+0.1%, Ti-containing precipitates formed are finely dispersed
and impede the growth of martensite during cooling after heating to
the two phase zone temperature, a lath (lath-like martensite) with
a small aspect ratio is formed, discharging of carbon (C) to
retained austenite between the laths becomes slow, and the carbon
amount in the retained austenite reduces. Preferable lower limit of
Ti content is 3.4[N]+0.02% or more (more preferably 3.4[N]+0.05% or
more), and preferable upper limit is 3.4[N]+0.09% or less (more
preferably 3.4[N]+0.08% or less).
[N: 0.001-0.01%]
[0046] N is an element inevitably mixed in and is preferable to be
reduced, however, because there is a limit in reducing N in an
actual process, 0.001% is made the lower limit. Also, when N
content becomes excessive, the ductility deteriorates because of
time aging, N precipitates as BN, the quenchability improvement
effect by solid-dissolved B is deteriorated, and therefore the
upper limit is made 0.01%. Preferable upper limit of N content is
0.008% or less (more preferably 0.006% or less).
[0047] The basic chemical composition in the steel sheet for hot
pressing use of the present invention is as described above, and
the remainder is iron and inevitable impurities other than P, S (0,
H and the like for example). Further, in the steel sheet for hot
pressing use of the present invention, according to the necessity,
it is also useful to further contain (a) at least one element
selected from the group consisting of V, Nb and Zr by 0.1% or less
(exclusive of 0%) in total, (b) at least one element selected from
the group consisting of Cu, Ni, Cr and Mo by 1% or less (exclusive
of 0%) in total, (c) at least one element selected from the group
consisting of Mg, Ca and REM (rare earth elements) by 0.01% or less
(exclusive of 0%) in total, and the like, and the properties of the
steel sheet for hot pressing use are improved further according to
the kind of the element contained. Preferable range when these
elements are contained and reasons for limiting the range are as
follows.
[0048] [At Least One Element Selected from the Group Consisting of
V, Nb and Zr by 0.1% or Less (Exclusive of 0%) in Total]
[0049] V, Nb and Zr have effects of forming fine carbide and
miniaturizing the microstructure by a pinning effect. In order to
exert such effects, it is preferable to contain them by 0.001% or
more in total. However, when the content of these elements becomes
excessive, coarse carbide is formed and becomes a start point of
breakage, and ductility is deteriorated adversely. Therefore, it is
preferable to contain these elements by 0.1% or less in total. More
preferable lower limit of the content of these elements in total is
0.005% or more (further more preferably 0.008% or more), and more
preferable upper limit in total is 0.08% or less (further more
preferably 0.06% or less).
[0050] [At Least One Element Selected from the Group Consisting of
Cu, Ni, Cr and Mo: 1% or Less (Exclusive of 0%) in Total]
[0051] Cu, Ni, Cr and Mo suppress ferrite transformation, pearlite
transformation and bainite transformation, therefore prevent
formation of ferrite, pearlite and bainite during cooling after
heating, and act effectively in securing retained austenite. In
order to exert such effects, it is preferable to contain them by
0.01% or more in total. Although the content is preferable to be as
much as possible when only the properties are considered, because
the cost for adding alloys increases, 1% or less in total is
preferable. Also, because there is an action of largely increasing
the strength of austenite, the load of hot rolling increases,
manufacturing of the steel sheet becomes difficult, and therefore
1% or less is also preferable from the viewpoint of
manufacturability. More preferable lower limit of these elements in
total is 0.05% or more (further more preferably 0.06% or more), and
more preferable upper limit in total is 0.5% or less (further more
preferably 0.3% or less).
[0052] [At Least One Element Selected from the Group Consisting of
Mg, Ca and REM by 0.01% or Less (Exclusive of 0%) in Total]
[0053] Because these elements miniaturize inclusions, they act
effectively in improving ductility. In order to exert such effects,
it is preferable to contain them by 0.0001% or more in total.
Although the content is preferable to be as much as possible when
only the properties are considered, because the effects saturate,
0.01% or less in total is preferable. More preferable lower limit
of these elements in total is 0.0002% or more (further more
preferably 0.0005% or more), and more preferable upper limit in
total is 0.005% or less (further more preferably 0.003% or
less).
[0054] In the steel sheet for hot pressing use of the present
invention, (A) some of Ti-containing precipitates contained in the
steel sheet, each of which having an equivalent circle diameter of
30 nm or less, have an average equivalent circle diameter of 3 nm
or less, (B) 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) described above) is fulfilled, and
(C) the metal microstructure contains at least either one of
bainite and martensite, and the sum total of the fraction of
bainite and the fraction of martensite is 80 area % or more, are
also important requirements.
[0055] When Ti that is excessive with respect to N is dispersed
finely or majority thereof is present in a solid solution state in
the steel sheet before hot press forming, much amount of Ti comes
to be present while it is fine in heating of hot press forming.
Thus, in martensite transformation that occurs during rapid cooling
within the tool after heating, the growth of martensite lath in the
longitudinal direction is impeded, the growth in the width
direction is promoted, and the aspect ratio reduces. As a result,
discharge of carbon from the martensite lath to surrounding
retained austenite delays, the carbon amount in retained austenite
reduces, the stability of retained austenite deteriorates, and
therefore the improvement effect of the elongation cannot be
obtained sufficiently.
[0056] From such a viewpoint, Ti-containing precipitates should be
dispersed coarsely, and, for that purpose, it is necessary that
some of Ti-containing precipitates contained in the steel sheet,
each of which having an equivalent circle diameter of 30 nm or
less, have an average equivalent circle diameter of 3 nm or more
(the requirement of (A) described above). Also, the reason the
equivalent circle diameter of the Ti-containing precipitates of the
object is stipulated to be 30 nm or less is that it is necessary to
control the Ti-containing precipitates and excluding TiN formed
coarsely in the melting stage that does not affect microstructure
change and properties thereafter. The size of the Ti-containing
precipitates (the average equivalent circle diameter of the
Ti-containing precipitates whose equivalent circle diameter is 30
nm or less) is preferably 5 nm or more, more preferably 10 nm or
more. Further, the Ti-containing precipitates of the object of the
present invention also include precipitates containing Ti such as
TiVC, TiNbC, TiVCN, TiNbCN and the like in addition to TiC and
TiN.
[0057] Also, in the steel sheet for hot pressing use, it is
necessary that, out of Ti, majority of Ti other than that used for
precipitating and fixing N is present in the precipitated state.
For that purpose, it is necessary that the Ti amount present as the
precipitates other than TiN (that is, precipitated Ti amount (mass
%)-3.4[N]) is more than 0.5 times of the balance obtained by
deducting Ti that forms TiN from total Ti (that is, more than
0.5.times.[total Ti amount (mass %)-3.4[N]]) (the requirement of
(B) described above). 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.
[0058] Although control of the metal microstructure is
intrinsically necessary for achieving desired strength-elongation
balance in the formed product, the metal microstructure cannot be
controlled only by the hot pressing condition, and it is necessary
to control the microstructure of the raw material steel thereof
(the steel sheet for hot pressing use) beforehand. In order to
secure the proper amount of annealed martensite and annealed
bainite which are fine and largely contributing to ductility in the
press forming steel sheet, it is necessary to make the sum total of
the fraction of bainite and the fraction of martensite in the steel
sheet 80 area % or more. When the sum total of the fraction of
bainite and the fraction of martensite is less than 80 area %, the
fraction of annealed martensite and/or annealed bainite targeted is
hardly secured, and the amount of other microstructure (ferrite for
example) increases to deteriorate the strength-elongation balance.
The sum total of the fraction of bainite and the fraction of
martensite is preferably 90 area % or more, more preferably 95 area
% or more.
[0059] Further, in the steel sheet for hot pressing use of the
present invention, although the remainder of the metal
microstructure is not particularly limited, at least any of
ferrite, pearlite or retained austenite can be cited for
example.
[0060] The steel sheet (the steel sheet for hot pressing use) of
the present invention as described above can be manufactured by
that a billet obtained by melting steel having the chemical
component composition as described above is subjected to hot
rolling with the heating temperature: 1,100.degree. C. or above
(preferably 1,150.degree. C. or above) and 1,300.degree. C. or
below (preferably 1,250.degree. C. or below) and the finish rolling
temperature of 750.degree. C. or above (preferably 780.degree. C.
or above) and 850.degree. C. or below (preferably 830.degree. C. or
below), cooling thereafter (slow cooling: intermediate cooling) so
as to stay for 10 s or more (preferably 50 s or more) between
700-750.degree. C. (preferably 720-740.degree. C.), cooling (rapid
cooling) thereafter to 450.degree. C. or below (preferably
350.degree. C. or below) at 20.degree. C./s or more (preferably
30.degree. C./s or more), and winding at 100.degree. C. or above
(preferably 150.degree. C. or above) and 450.degree. C. or below
(preferably 400.degree. C. or below).
[0061] The method described above is for executing control so that
(1) rolling is finished at a temperature range where dislocation
introduced by hot rolling remains within austenite, (2)
Ti-containing precipitates such as TiC and the like are formed
finely on the dislocation by rapid cooling immediately thereafter,
and (3) bainite transformation or martensite transformation is
caused by rapid cooling and winding thereafter.
[0062] The steel sheet for hot pressing use having the chemical
component composition, metal microstructure and Ti-precipitation
state as described above may be used as it is for manufacturing by
hot press forming, and may be subjected to cold rolling with the
draft: 10-80% (preferably 20-70%) after pickling. Further, the
steel sheet for hot pressing use or the material obtained by cold
rolling thereof may be subjected to such heat treatment of heating
to such a temperature range where TiC is not dissolved by 100%
(1,000.degree. C. or below: for example 870-900.degree. C.),
rapidly cooling thereafter to 450.degree. C. or below (preferably
400.degree. C. or below) at a cooling rate of 20.degree. C./s or
more (preferably 30.degree. C./s or more), and holding thereafter
at 450.degree. C. or below for 10 s or more and 1,000 s or less or
tempering at a temperature of 450.degree. C. or below. Also, the
steel sheet for hot pressing use of the present invention may be
subjected to plating containing at least one element out of Al, Zn,
Mg and Si on the surface thereof (the surface of the base steel
sheet).
[0063] By using the steel sheet for hot pressing use as described
above, executing heating to a temperature of Ac.sub.1
transformation point+20.degree. C. or above and Ac.sub.3
transformation point-20.degree. C. or below, thereafter starting
press-forming, and executing cooling to a temperature or below, the
temperature being lower than the bainite transformation starting
temperature Bs by 100.degree. C., while securing the average
cooling rate of 20.degree. C./s or more within the tool during
forming and after completion of forming, the press formed product
having a single property (may be hereinafter referred to as "single
region formed product") can have an optimum microstructure of low
strength and high ductility. The reasons for stipulating each
requirement in this forming method are as described below.
[0064] In order to form austenite between laths of martensite and
bainite within the steel sheet and to form annealed martensite and
annealed bainite excellent in ductility by annealing martensite and
bainite, the heating temperature should be controlled to a
predetermined range. When the heating temperature of the steel
sheet is below Ac.sub.1 transformation point+20.degree. C.,
sufficient amount of austenite cannot be secured in heating, and a
predetermined amount of retained austenite cannot be secured in the
final microstructure (the microstructure of the formed product).
Also, when the heating temperature of the steel sheet exceeds
Ac.sub.3 transformation point-20.degree. C., the transformation
amount to austenite increases excessively in heating, and a
predetermined amount of annealed martensite and annealed bainite
cannot be secured in the final microstructure (the microstructure
of the formed product).
[0065] In order to make austenite formed in the heating step
described above a desired microstructure while preventing formation
of the microstructure such as ferrite or pearlite, it is necessary
to properly control the average cooling rate and the cooling
finishing temperature during forming and after forming. From such a
viewpoint, it is necessary to make the average cooling rate during
forming 20.degree. C./s or more and to make the cooling finishing
temperature a temperature or below, the temperature being lower
than the bainite transformation starting temperature Bs by
100.degree. C. The average cooling rate during forming is
preferably 30.degree. C./s or more (more preferably 40.degree. C./s
or more). By transforming austenite having been present in heating
to bainite and martensite while preventing formation of the
microstructure such as ferrite or martensite by making the cooling
finishing temperature a temperature equal to or below the bainite
transformation starting temperature Bs, fine austenite is made
remain between the laths of bainite and martensite, and a
predetermined amount of retained austenite is secured while
securing bainite and martensite.
[0066] When the cooling finishing temperature becomes higher than
the temperature that is lower than the bainite transformation
starting temperature Bs by 100.degree. C. and the average cooling
rate is less than 20.degree. C./s, the microstructure such as
ferrite, pearlite and the like is formed, a predetermined amount of
retained austenite cannot be secured, and elongation (ductility) in
the formed product deteriorates.
[0067] Although control of the average cooling rate basically
becomes unnecessary at the stage the temperature becomes equal to
or below the temperature lower than the bainite transformation
starting temperature Bs by 100.degree. C., cooling may be executed
to the room temperature with the average cooling rate of 1.degree.
C./s or more and 100.degree. C./s or less for example. Also,
control of the average cooling rate during forming and after
completion of forming can be achieved by means such as (a) to
control the temperature of the forming tool (the cooling medium
shown in FIG. 1 above), and (b) to control the thermal conductivity
of the tool.
[0068] In the press-formed product manufactured by hot press
forming as described above, the metal microstructure is formed of
retained austenite: 3-20 area %, annealed martensite and/or
annealed bainite: 30-87 area %, and martensite as quenched: 10-67
area %, the carbon amount in the retained austenite is 0.60% or
more, and the balance of high strength and elongation can be
achieved with a high level and as a uniform property within the
formed product. The reasons for setting the range of each
requirement (the basic microstructure and the carbon amount in the
retained austenite) in such a hot press-formed product are as
described below.
[0069] Retained austenite has an effect of increasing the work
hardening ratio (transformation induced plasticity) and improving
ductility of the press-formed product by being transformed to
martensite during plastic deformation. In order to exert such an
effect, the fraction of retained austenite should be made 3 area %
or more. Ductility becomes more excellent as the fraction of
retained austenite is higher. In the composition used for a steel
sheet for an automobile, retained austenite that can be secured is
limited, and approximately 20 area % becomes the upper limit.
Preferable lower limit of retained austenite is 5 area % or more
(more preferably 7 area % or more).
[0070] By making the main microstructure annealed martensite and/or
annealed bainite which is fine and has low dislocation density,
ductility (elongation) of the press-formed product can be enhanced
while securing a predetermined strength. From such a viewpoint, the
fraction of annealed martensite and/or annealed bainite is made 30
area % or more. However, when this fraction exceeds 87 area %, the
fraction of retained austenite becomes insufficient, and ductility
(residual ductility) deteriorates. Preferable lower limit of
annealed martensite and/or annealed bainite is 40 area % or more
(more preferably 50 area % or more), and preferable upper limit is
less than 80 area % (more preferably less than 70 area %).
[0071] Because martensite as quenched is a microstructure inferior
in ductility, when much amount thereof is present, elongation is
deteriorated, however, in order to achieve high strength of over
100 kg/mm.sup.2 class in a microstructure with low matrix strength
such as annealed martensite, it is necessary to secure a
predetermined amount of martensite as quenched. From such a
viewpoint, the fraction of martensite as quenched is made 10 area %
or more. However, when the fraction of martensite as quenched
increases excessively, strength increases excessively and
elongation becomes insufficient, and therefore the fraction thereof
should be 67 area % or less. Preferable lower limit of the fraction
of martensite as quenched is 20 area % or more (more preferably 30
area % or more), and preferable upper limit is 60 area % or less
(more preferably 50 area % or less).
[0072] With respect to the microstructure other the above, ferrite,
pearlite, bainite and the like may be included as the remainder
microstructure, however, these microstructures are inferior in
contribution to strength and contribution to ductility compared to
other microstructures, and it is basically preferable not to be
contained (it may also be 0 area %). However, up to 20 area % is
allowable. The remainder microstructure is preferably 10 area % or
less, more preferably 5 area % or less.
[0073] The carbon amount in retained austenite affects the timing
of work induced transformation of retained austenite to martensite
at the time of deformation such as the tensile test and the like,
and enhances the transformation induced plasticity (TRIP) effect by
causing the work induced transformation at a higher strain zone as
the carbon amount is higher. In the case of the process of the
present invention, carbon is discharged during cooling from the
martensite lath formed to surrounding austenite. At that time, if
Ti-carbide or carbonitride dispersed in steel is dispersed
coarsely, growth of the martensite lath in the longitudinal
direction proceeds without being impeded, and therefore the
martensite lath narrow in the width, long, and having a large
aspect ratio is obtained. As a result, carbon is easily discharged
from the martensite lath to the width direction, the carbon amount
in retained austenite increases, and the ductility improves. From
such a viewpoint, in the press-formed product of the present
invention, the carbon amount in retained austenite in steel was
stipulated to be 0.60% or more. Further, although the carbon amount
in retained austenite can be concentrated to approximately 0.70%,
approximately 1.0% is the limit.
[0074] When the steel sheet for hot pressing use of the present
invention is used, by properly adjusting the press forming
condition (heating temperature and cooling rate), the properties
such as strength, elongation and the like of the press-formed
product can be controlled, the press-formed product with high
ductility (residual ductility) is obtained, and therefore
application to a portion (energy absorption member for example) to
which it has been difficult to apply conventional press-formed
products becomes also possible which is very useful in expanding
the application range of the press-formed product. Also, not only
the single region formed product described above, a press-formed
product exerting strength-ductility balance according to each
region (may be hereinafter referred to as "plural region formed
product") is obtained when the heating temperature and the
condition of each region in forming are properly controlled and the
microstructure of each region is adjusted in manufacturing the
press-formed product by press forming of a steel sheet using a
press-forming tool.
[0075] The plural region formed product can be manufactured as
described above using the steel sheet for hot pressing use of the
present invention by dividing a heating region of the steel sheet
into at least two regions, heating one region thereof (hereinafter
referred to as the first region) to a temperature of Ac.sub.3
transformation point or above and 950.degree. C. or below, heating
another region (hereinafter referred to as the second region) to a
temperature of Ac.sub.1 transformation point+20.degree. C. or above
and Ac.sub.3 transformation point-20.degree. C. or below,
thereafter starting press forming of both of the first and second
regions, and executing cooling to a temperature of martensite
transformation starting temperature Ms or below while securing the
average cooling rate of 20.degree. C./s or more within a tool in
both of the first and second regions during forming and after
forming.
[0076] According to the method described above, by dividing the
heating region of the steel sheet into at least two regions (high
strength side region and low strength side region) and controlling
the manufacturing condition according to each region, such a
press-formed product that strength-ductility balance according to
each region is exerted is obtained. The second region out of two
regions corresponds to the low strength side region, and the
manufacturing condition, microstructure and properties in this
region is basically same to those of the single region formed
product described above. Below, the manufacturing condition for
forming the other first region (corresponding to the high strength
side region) will be described. Also, in executing this
manufacturing method, it is required to form regions with different
heating temperature by a single steel sheet, however, by using an
existing heating furnace (for example, far infrared furnace,
electric furnace+shield), controlling while making the boundary
section of the temperature 50 mm or less is possible.
(Manufacturing Condition of the First Region/High Strength Side
Region)
[0077] In order to properly adjust the microstructure of the
press-formed product, it is necessary to control the heating
temperature to a predetermined range. By properly controlling this
heating temperature, transformation to a microstructure mainly of
martensite is caused while securing a predetermined amount of
retained austenite in the cooling step after heating, and a desired
microstructure can be achieved within the range of the final hot
press-formed product. When the steel sheet heating temperature in
this region is below Ac.sub.3 transformation point, a sufficient
amount of austenite cannot be obtained in heating, and a
predetermined amount of retained austenite cannot be secured in the
final microstructure (the microstructure of the formed product).
Also, when the heating temperature of the steel sheet exceeds
950.degree. C., the grain size of austenite becomes large in
heating, martensite transformation starting temperature (Ms point)
and martensite transformation finishing temperature (Mf point)
rise, retained austenite cannot be secured in quenching, and
excellent formability is not achieved. The heating temperature of
the steel sheet is preferably Ac.sub.3 transformation
point+50.degree. C. or above and 900.degree. C. or below.
[0078] In order to make austenite formed in the heating step
described above a desired microstructure while preventing formation
of the microstructure such as ferrite or pearlite, it is necessary
to properly control the average cooling rate and the cooling
finishing temperature during forming and after forming. From such a
viewpoint, the average cooling rate during forming should be
20.degree. C./s or more and the cooling finishing temperature
should be martensite transformation starting temperature (Ms point)
or below. The average cooling rate during forming is preferably
30.degree. C./s or more (more preferably 40.degree. C./s or more).
By transforming austenite having been present in heating to
martensite while preventing formation of the microstructure such as
ferrite or pearlite by making the cooling finishing temperature the
martensite transformation starting temperature (Ms point) or below,
martensite is secured. Specifically, the cooling finishing
temperature is 400.degree. C. or below, preferably 300.degree. C.
or below.
[0079] In the press-formed product obtained by such a method, the
metal microstructure, precipitates and the like are different
between the first region and the second region. In the first
region, the metal microstructure is of retained austenite: 3-20
area % (the action and effect of retained austenite are same to the
above), and martensite: 80 area % or more. In the second region,
the metal microstructure same to that of the single region formed
product described above and 0.60% or more of the carbon amount in
retained austenite are fulfilled.
[0080] By making the main microstructure of the first region
martensite with high strength containing a predetermined amount of
retained austenite, ductility and high strength in a specific
region in the hot press-formed product can be secured. From such a
viewpoint, the area fraction of martensite should be 80 area % or
more. The fraction of martensite is preferably 85 area % or more
(more preferably 90 area % or more). Also, as the microstructure in
the first region, ferrite, pearlite, bainite and the like may be
included in a part thereof.
[0081] Although the effect of the present invention will be shown
below more specifically by examples, the examples described below
do not limit the present invention, and any of the design
alterations judging from the purposes described above and below is
to be included in the technical range of the present invention.
EXAMPLES
Example 1
[0082] Steel (steel Nos. 1-32) having the chemical component
composition shown in Table 1 below was molten in vacuum, was made a
slab for experiment, was thereafter made a steel sheet by hot
rolling, was thereafter cooled, and was subjected to a treatment
that simulates winding (sheet thickness: 3.0 mm). The winding
simulated treatment method included cooling to the winding
temperature, putting the sample thereafter into a furnace heated to
the winding temperature, holding for 30 min, and cooling in the
furnace. The manufacturing condition for the steel sheet at that
time is shown in Table 2 below. Also, Ac.sub.1 transformation
point, Ac.sub.3 transformation point, Ms point, and Bs point in
Table 1 were obtained using the formula (2)-formula (5) below
(refer to "The physical Metallurgy of Steels", Leslie, Maruzen
Company, Limited (1985) for example). Also, the treatments (1)-(3)
shown in the remarks column in Table 2 express that each treatment
(rolling, cooling, alloying) shown below was executed.
Ac.sub.1 transformation point (.degree.
C.)=723+29.1.times.[Si]-10.7.times.[Mn]+16.9.times.[Cr]-16.9.times.[Ni]
(2)
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] (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)
[0083] wherein [C], [Si], [Mn], [P], [Al], [Ti], [V], [Cr], [Mo],
[Cu] and [Ni] represent the content (mass %) of C, Si, Mn, P, Al,
Ti, V, Cr, Mo, Cu and Ni respectively. Also, when the element shown
in each term of the formulae (2)-(5) above is not contained,
calculation is done assuming that the term is null.
[0084] Treatment (1): After finish rolling, cooling was executed to
650.degree. C. with the average cooling rate of 50.degree. C./s,
cooling was thereafter executed for 10 s from 650.degree. C. with
the average cooling rate of 5.degree. C./s, and cooling was
thereafter executed to the winding temperature with the average
cooling rate of 50.degree. C./s. The front and back surfaces were
thereafter polished and the thickness was reduced to 1.6 mm so as
to match the thickness to that of the treatments (2) and (3).
[0085] Treatment (2): The hot-rolled steel sheet was cold-rolled,
was heated thereafter to 860.degree. C. simulating continuous
annealing, was cooled thereafter to 400.degree. C. with the average
cooling rate of 30.degree. C./s, and was held.
[0086] Treatment (3): The hot-rolled steel sheet was cold-rolled,
was heated thereafter to 860.degree. C. for simulating continuous
hot dip galvanizing line, was cooled thereafter to 400.degree. C.
with the average cooling rate of 30.degree. C./s, was held, was
thereafter heated further by (500.degree. C..times.10 s), and was
cooled thereafter.
TABLE-US-00001 TABLE 1 Steel Chemical component composition* (mass
%) No. C Si Mn P S Al B Ti N V Nb 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 0.50 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 0.450 1.20 1.20 0.0050 0.0020
0.030 0.0020 0.044 0.0040 -- -- 18 0.720 1.20 1.20 0.0050 0.0020
0.030 0.0020 0.044 0.0040 -- -- 19 0.220 1.20 0.80 0.0050 0.0020
0.030 0.0020 0.044 0.0040 -- -- 20 0.220 1.20 2.40 0.0050 0.0020
0.030 0.0020 0.044 0.0040 -- -- 21 0.220 1.20 1.20 0.0050 0.0020
0.030 0.0020 0.100 0.0040 -- -- 22 0.220 1.20 1.20 0.0050 0.0020
0.030 0.0020 0.200 0.0040 -- -- 23 0.220 0.50 1.20 0.0050 0.0020
0.40 0.0020 0.044 0.0040 -- -- 24 0.220 1.20 1.20 0.0050 0.0020
0.030 0.0020 0.044 0.0040 0.030 -- 25 0.220 1.20 1.20 0.0050 0.0020
0.030 0.0020 0.044 0.0040 -- 0.020 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 -- -- Chemical component Steel
composition* (mass %) Ac.sub.3 - Ac.sub.1 + 20.degree. C. Bs - Ms
point No. Cu Ni Cr Mo 20.degree. C. (.degree. C.) (.degree. C.)
100.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 4 -- -- 0.20 -- 812 748 549 421 5 -- -- 0.20 -- 843 768 549 421
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 16 --
-- 0.20 -- 818 768 514 374 17 -- -- 0.20 -- 802 768 487 338 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
-- 0.20 -- 839 768 549 419 27 -- 0.20 0.20 -- 840 765 541 417 28 --
-- 0.20 0.20 849 768 532 420 *The remainder: iron and inevitable
impurities other than P, S, N.
TABLE-US-00002 TABLE 2 Steel sheet manufacturing condition Steel
Heating Finish rolling Cooling time Average cooling rate Winding
No. temperature (.degree. C.) temperature (.degree. C.) of
750-700.degree. C. (s) of 700.degree. C.-450.degree. C. (.degree.
C./s) temperature (.degree. C.) Remarks 1 1200 800 15 50 200 -- 2
1200 800 15 50 200 -- 3 1200 800 15 50 200 -- 4 1200 800 15 50 200
-- 5 1200 800 15 50 200 -- 6 1200 800 15 50 200 -- 7 1200 800 15 50
200 -- 8 1200 800 15 50 200 -- 9 1200 800 1 17 200 Treatment (1) 10
1200 900 1 17 450 Treatment (1) 11 1200 800 15 50 400 Cold rolling:
20% 12 1200 800 15 50 400 Treatment (2) 13 1200 800 15 50 400
Treatment (3) 14 1200 800 15 50 580 -- 15 1200 800 15 50 200 -- 16
1200 800 15 50 200 -- 17 1200 800 15 50 200 -- 18 1200 800 15 50
200 -- 19 1200 800 15 50 200 -- 20 1200 800 15 50 200 -- 21 1200
800 15 50 200 -- 22 1200 800 15 50 200 -- 23 1200 800 15 50 200 --
24 1200 800 15 50 200 -- 25 1200 800 15 50 200 -- 26 1200 800 15 50
200 -- 27 1200 800 15 50 200 -- 28 1200 800 15 50 200 --
[0087] With respect to the steel sheet obtained, analysis of the
precipitation state of Ti and observation of the metal
microstructure (the fraction of each microstructure) were executed
by the procedure described below. The result is shown in Table 3
below along with the calculated value of 0.5.times.[total Ti
amount(mass %)-3.4[N]] (shown as 0.5.times.(total Ti
amount-3.4[N])).
[Analysis of Precipitation State of Ti of Steel Sheet]
[0088] 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, by
composition analysis of the precipitates using an energy dispersion
type X-ray spectrometer (EDX), Ti-containing precipitates were
identified. The area of the Ti-containing precipitates of at least
100 pieces was measured by image analysis, those having the
equivalent circle diameter of 30 nm or less were extracted, and the
average value thereof was made the size of the precipitates. Also,
in the table, the size is shown as "average equivalent circle
diameter of Ti-containing precipitates". Further, with respect to
precipitated Ti amount (mass %)-3.4[N] (the Ti amount present as
the precipitates), extraction residue analysis (in extraction
treatment, the precipitates coagulate, and fine precipitates also
can be measured) was executed using a mesh with mesh diameter: 0.1
.mu.m, and precipitated Ti amount (mass %)-3.4[N] (expressed as
"precipitated Ti amount-3.4[N]" in Table 3) was obtained. Also,
when the Ti-containing precipitates partly contained V and Nb, the
contents of these precipitates were also measured.
[Observation of Metal Microstructure (Fraction of Each
Microstructure)]
[0089] (1) With respect to the microstructure of martensite and
bainite in the steel sheet, the steel sheet was corroded by nital,
martensite and bainite were distinguished from each other by SEM
observation (magnifications: 1,000 times or 2,000 times), and each
fraction (area ratio) was obtained.
[0090] (2) The retained austenite fraction in the steel sheet was
measured by X-ray diffraction method after the steel sheet was
ground up to 1/4 thickness thereof and was thereafter subjected to
chemical polishing (for example, ISJJ Int. Vol. 33. (1933), No. 7,
P. 776).
TABLE-US-00003 TABLE 3 Steel sheet for press forming use Average
equivalent Precipitated 0.5 .times. (total circle diameter of Steel
Ti amount - 3.4[N] Ti amount - 3.4[N]) Ti-containing Fraction of
Fraction of No. (mass %) (mass %) precipitates (nm) martensite
(area %) bainite (area %) Others 1 0.024 0.015 10.0 100 0 -- 2
0.024 0.015 10.0 100 0 -- 3 0.025 0.015 10.0 100 0 -- 4 0.027 0.015
10.0 100 0 -- 5 0.029 0.015 10.0 100 0 -- 6 0.026 0.015 10.0 100 0
-- 7 0.023 0.015 10.0 100 0 -- 8 0.026 0.015 10.0 100 0 -- 9 0.006
0.015 5.0 100 0 -- 10 0.012 0.015 2.0 0 100 -- 11 0.026 0.015 10.0
0 100 -- 12 0.030 0.015 10.0 0 100 -- 13 0.027 0.015 10.0 0 100 --
14 0.025 0.015 10.0 0 0 Ferrite + Pearlite: 100% 15 0.028 0.015
10.0 100 0 -- 16 0.023 0.015 10.0 90 0 Retained austenite 10% 17
0.026 0.015 10.0 80 0 Retained austenite 20% 18 0.030 0.015 10.0 60
0 Retained austenite 40% 19 0.023 0.015 10.0 100 0 -- 20 0.023
0.015 10.0 100 0 -- 21 0.084 0.043 10.0 100 0 -- 22 0.144 0.093
18.0 100 0 -- 23 0.029 0.015 10.0 100 0 -- 24 0.024 0.015 10.0 100
0 -- 25 0.024 0.015 10.0 100 0 -- 26 0.026 0.015 10.0 100 0 -- 27
0.028 0.015 10.0 100 0 -- 28 0.024 0.015 10.0 100 0 --
[0091] Each steel sheet described above (1.6 mm.sup.t.times.150
mm.times.200 mm) (with respect to those other than the treatments
of (1)-(3) described above, the thickness was adjusted to 1.6 mm by
hot rolling) was heated to a predetermined temperature in a heating
furnace, and was thereafter subjected to press forming and cooling
treatment using the tool (FIG. 1 above) of a hat shape to obtain
the press-formed product. The press forming conditions (heating
temperature, average cooling rate, and rapid cooling finishing
temperature in press forming) are shown in Table 4 below.
TABLE-US-00004 TABLE 4 Press forming condition Steel Heating
Average cooling Rapid cooling finishing No. temperature (.degree.
C.) rate (.degree. C./s) temperature (.degree. C.) 1 790 40 300 2
800 40 300 3 760 40 300 4 770 40 300 5 810 40 300 6 950 40 300 7
820 5 300 8 800 40 600 9 800 40 300 10 810 40 300 11 800 40 300 12
820 40 300 13 810 40 300 14 800 40 300 15 830 40 300 16 790 40 300
17 790 40 300 18 770 40 300 19 800 40 300 20 780 40 300 21 810 40
300 22 820 40 300 23 830 40 300 24 800 40 300 25 810 40 300 26 800
40 300 27 810 40 300 28 820 40 300
[0092] With respect to the formed product obtained, tensile
strength (TS), elongation (total elongation EL), and observation of
the metal microstructure (the fraction of each microstructure) were
measured by methods described below.
[Measurement of Tensile Strength (TS) and Elongation (Total
Elongation EL)]
[0093] The tensile test was executed using JIS No. 5 test specimen,
and the tensile strength (TS) and the elongation (EL) were
measured. At this time, the strain rate of the tensile test was
made 10 mm/s. In the present invention, the case 980-1,179 MPa of
the tensile strength (TS) and 20% or more of the elongation (EL)
were satisfied and the strength-elongation balance (TS.times.EL)
was 24,000 (MPa%) or more was evaluated to have passed.
[0094] (Observation of Metal Microstructure (Fraction of Each
Microstructure))
[0095] (1) With respect to the microstructure of annealed
martensite, bainite and annealed bainite in the steel sheet, the
steel sheet was corroded by nital, annealed martensite, bainite and
annealed bainite were distinguished from each other by SEM
observation (magnifications: 1,000 times or 2,000 times), and each
fraction (area ratio) was obtained.
[0096] (2) The retained austenite fraction in the steel sheet was
measured by X-ray diffraction method after the steel sheet was
ground up to 1/4 thickness thereof and was thereafter subjected to
chemical polishing (for example, ISJJ Int. Vol. 33. (1933), No. 7,
P. 776). At this time, the carbon amount in retained austenite was
also measured.
[0097] (3) With respect to the fraction of martensite as quenched,
the steel sheet was LePera-corroded, the area ratio of the white
contrast was measured as the mixture microstructure of martensite
as quenched and retained austenite, the retained austenite fraction
obtained by X-ray diffraction was deducted therefrom, and the
fraction of martensite as quenched was calculated.
[0098] The observation results (fraction of each microstructure) of
the metal microstructure are shown in Table 5 below. Also, the
mechanical properties (tensile strength TS, elongation EL, and
TS.times.EL) of the formed product are shown in Table 6 below.
TABLE-US-00005 TABLE 5 Metal microstructure of formed product
Fraction of annealed martensite Fraction of martensite Fraction of
retained Carbon amount in retained Steel No. and/or annealed
bainite (area %) as quenched (area %) austenite (area %) austenite
(mass %) Others 1 70 23 7 0.65 -- 2 67 23 7 0.65 Ferrite 3% 3 70 28
2 0.45 -- 4 70 25 5 0.65 -- 5 70 23 7 0.65 -- 6 70 23 7 0.65 -- 7
70 2 0 0.65 Pearlite 18%, Ferrite 10% 8 70 6 3 0.65 Pearlite 12%,
Ferrite 9% 9 70 23 7 0.65 -- 10 70 23 7 0.65 -- 11 70 23 7 0.65 --
12 70 23 7 0.65 -- 13 70 23 7 0.65 -- 14 -- 33 7 0.52 Ferrite 60%
15 70 23 9 0.65 -- 16 60 29 11 0.65 -- 17 55 32 13 0.65 -- 18 16 62
22 0.65 -- 19 70 25 5 0.68 -- 20 70 21 9 0.61 -- 21 70 23 7 0.65 --
22 70 23 7 0.65 -- 23 70 23 7 0.65 -- 24 70 23 7 0.65 -- 25 70 23 7
0.65 -- 26 70 23 7 0.65 -- 27 70 23 7 0.65 -- 28 70 23 7 0.65
--
TABLE-US-00006 TABLE 6 Mechanical properties of formed product
Steel Tensile strength Elongation EL TS .times. EL No. TS (MPa) (%)
(MPa %) 1 1074 23.5 25239 2 1022 26.1 26674 3 1014 12.0 26364 4 983
25.5 25067 5 1016 26.2 26619 6 1524 11.3 17221 7 781 19.7 15386 8
856 19.0 16264 9 1026 22.6 23188 10 1044 22.0 22968 11 1051 25.9
27221 12 1018 25.3 25755 13 1039 24.1 25040 14 1063 19.9 21154 15
1038 25.6 26573 16 1192 23.5 28012 17 1260 28.0 35280 18 1321 10.2
13474 19 1031 24.3 25053 20 1021 29.0 29609 21 1045 25.9 27066 22
1045 18.1 18915 23 1013 26.1 26439 24 1049 24.0 25176 25 1033 26.0
26858 26 1007 26.1 26283 27 1059 25.2 26687 28 1045 24.2 25289
[0099] From these results, following consideration can be made.
Those of the steel Nos. 1, 2, 4, 5, 11-13, 15-17, 19-21, 23-32 are
examples fulfilling the requirements stipulated in the present
invention, and it is known that components excellent in
strength-elongation balance have been obtained.
[0100] On the other hand, those of the steel Nos. 3, 6-10, 14, 18,
22 are the comparative examples not fulfilling any of the
requirements stipulated in the present invention, and any of the
properties is deteriorated. That is, in that of the steel No. 3, a
steel sheet with low Si content is used, the fraction of retained
austenite in the formed product is not secured, the carbon amount
in the retained austenite drops, and the elongation is not enough.
In that of the steel No. 6, the heating temperature in forming is
high, only low elongation EL is obtained, and the
strength-elongation balance (TS.times.EL) also deteriorates.
[0101] In that of the steel No. 7, the average cooling rate in
press forming is slow, pearlite and ferrite are formed, the
fraction of martensite as quenched cannot be secured, and the
strength-elongation balance (TS.times.EL) is deteriorated. In that
of the steel No. 8, the rapid cooling finishing temperature is
high, pearlite and ferrite are formed, the fraction of martensite
as quenched cannot be secured, only low elongation is obtained, and
the strength-elongation balance (TS.times.EL) is also
deteriorated.
[0102] In those of the steel Nos. 9, 10, the condition in
manufacturing the steel is not appropriate, the amount of
precipitated Ti is insufficient (steel Nos. 9, 10), Ti-containing
precipitates are small (steel No. 10), and, when press forming is
executed using such a steel sheet, the strength-elongation balance
(TS.times.EL) is deteriorated even if the forming condition is
appropriate.
[0103] In that of the steel No. 14, a steel sheet whose metal
microstructure is of ferrite+pearlite of 100 area % which is caused
by the winding temperature is used, the fraction of annealed
martensite and/or annealed bainite during forming cannot be
secured, and the strength-elongation balance (TS.times.EL) is
deteriorated. In that of the steel No. 18, the steel sheet with
excessive C content is used, the strength becomes high, and only
low elongation EL is obtained. In that of the steel No. 22, the
steel sheet with excessive Ti content is used, and the
strength-elongation balance (TS.times.EL) is deteriorated.
Example 2
[0104] Steel (steel Nos. 33-37) having the chemical component
composition shown in Table 7 below was molten in vacuum, was made a
slab for experiment, was thereafter hot-rolled, and was thereafter
cooled and wound (sheet thickness: 3.0 mm). The manufacturing
condition for the steel sheet at that time is shown in Table 8
below.
TABLE-US-00007 TABLE 7 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.) 29 0.220
1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 -- -- -- -- 0.20
-- 843 768 549 421 30 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020
0.044 0.0040 -- -- -- -- 0.20 -- 843 768 549 421 31 0.350 1.20 1.20
0.0050 0.0020 0.030 0.0020 0.044 0.0040 -- -- -- -- 0.20 -- 818 768
514 374 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 *The remainder: iron and
inevitable impurities other than P, S, N.
TABLE-US-00008 TABLE 8 Steel sheet manufacturing condition Heating
Steel temperature Finish rolling Cooling time Average cooling rate
Winding No. (.degree. C.) temperature (.degree. C.) of
750-700.degree. C. (s) of 700.degree. C.-450.degree. C. (.degree.
C./s) temperature (.degree. C.) Remarks 29 1200 800 15 50 200 -- 30
1200 800 15 50 200 -- 31 1200 800 15 50 200 -- 32 1200 800 1 17 200
Treatment (1)
[0105] With respect to the steel sheet obtained, analysis of the
precipitation state of Ti-containing precipitates and observation
of the metal microstructure (the fraction of each microstructure)
were executed similarly to Example 1. The result is shown in Table
9 below.
TABLE-US-00009 TABLE 9 Steel sheet for press forming use
Precipitated 0.5 .times. (total Average equivalent circle Fraction
of Steel Ti amount - 3.4[N] Ti amount - 3.4[N]) diameter of
Ti-containing martensite Fraction of No. (mass %) (mass %)
precipitates (nm) (area %) bainite (area %) Others 29 0.030 0.015
10.0 100 0 -- 30 0.023 0.015 10.0 100 0 -- 31 0.028 0.015 10.0 90 0
Retained austenite 10% 32 0.006 0.015 2.3 100 0 --
[0106] Each steel sheet described above (3.0 mm.sup.t.times.150
mm.times.200 mm) was heated to a predetermined temperature in a
heating furnace, and was subjected thereafter to press forming and
cooling treatment using the tool (FIG. 1 above) of a hat shape to
obtain the formed product. At this time, the steel sheet was put in
an infrared furnace and the portion intended to be
high-strengthened (the steel sheet portion corresponding to the
first region) was configured so that infrared rays directly hit so
as to allow high temperature heating, whereas the portion intended
to be low-strengthened (the steel sheet portion corresponding to
the second region) was shielded with a cover so that a part of the
infrared rays was blocked so as to allow low temperature heating,
and thereby the heating temperature was differentiated. Therefore,
the formed product has the regions with different strength within a
single component. The press forming conditions (heating
temperature, average cooling rate, and rapid cooling finishing
temperature of each region in press forming) are shown in Table 10
below.
TABLE-US-00010 TABLE 10 Press forming condition Heating Average
Rapid cooling Steel temperature cooling finishing No. Region
(.degree. C.) rate (.degree. C./s) temperature (.degree. C.) 29 Low
strength side 810 40 300 High strength side 920 40 300 30 Low
strength side 800 40 300 High strength side 850 40 300 31 Low
strength side 790 40 300 High strength side 920 40 300 32 Low
strength side 810 40 300 High strength side 920 40 300
[0107] With respect to the formed product obtained, tensile
strength (TS), elongation (total elongation EL), observation of the
metal microstructure (the fraction of each microstructure), and the
carbon amount in retained austenite in each region were obtained
similarly to Example 1.
[0108] The observation results (fraction of each microstructure) of
the metal microstructure are shown in Table 11 below. Also, the
mechanical properties (tensile strength TS, elongation EL, and
TS.times.EL) of the formed product are shown in Table 12 below.
Further, the case 1,470 MPa or more of the tensile strength (TS)
and 8% or more of the elongation (EL) were fulfilled and the
strength-elongation balance (TS.times.EL) was 14,000 (MPa%) or more
on the high strength side was evaluated to have passed (the
evaluation criteria of the low strength side are same to those of
Example 1).
TABLE-US-00011 TABLE 11 Metal microstructure of formed product
Fraction Carbon amount Steel Fraction of annealed martensite
Fraction of martensite of retained in retained No. Region and/or
annealed bainite (area %) as quenched (area %) austenite (area %)
austenite (mass %) Others 29 Low strength side 70 23 7 0.65 -- High
strength side 0 94 6 0.55 -- 30 Low strength side 70 23 7 0.65 --
High strength side 0 61 6 0.52 Ferrite 33% 31 Low strength side 60
29 11 0.65 -- High strength side -- 95 5 0.53 -- 32 Low strength
side 70 23 7 0.65 -- High strength side 0 94 6 0.54 --
TABLE-US-00012 TABLE 12 Mechanical properties of formed product
Steel Tensile strength Elongation EL TS .times. EL No. Region TS
(MPa) (%) (MPa %) 29 Low strength side 1058 24.1 25498 High
strength side 1511 11.0 16621 30 Low strength side 1063 23.9 25406
High strength side 1278 13.0 16614 31 Low strength side 1192 25.0
29800 High strength side 1820 10.1 18382 32 Low strength side 1049
21.5 22554 High strength side 1499 11.0 16489
[0109] From this result, following consideration can be made. Those
of the steel Nos. 33, 35, 37 are examples fulfilling the
requirements stipulated in the present invention, and it is known
that components excellent in the strength-elongation balance in
each region have been obtained.
[0110] On the other hand, those of the steel Nos. 34, 36 are the
comparative examples not fulfilling any of the requirements
stipulated in the present invention, and any of the properties is
deteriorated. That is, in that of the steel No. 34, the heating
temperature in press forming is low, and the strength on the high
strength side drops. In that of the steel No. 36, a steel sheet
with small size of Ti-containing precipitates is used, only low
strength is obtained on the high strength side, and the
strength-elongation balance (TS.times.EL) is deteriorated on the
low strength side.
[0111] Although the present invention has been described in detail
and referring to specific embodiments, it is obvious for a person
with an ordinary skill in the art that various alterations and
amendments can be effected without departing from the spirit and
the range of the present invention.
[0112] The present application is based on Japanese Patent
Application (JP-A-No. 2012-053844) applied on Mar. 9, 2012, and the
contents thereof are hereby incorporated by reference.
INDUSTRIAL APPLICABILITY
[0113] The present invention is suitable to a steel sheet for hot
pressing use that is used in manufacturing structural components of
an automobile.
REFERENCE SIGNS LIST
[0114] 1 . . . punch [0115] 2 . . . die [0116] 3 . . . blank holder
[0117] 4 . . . steel sheet (blank)
* * * * *