U.S. patent application number 16/754135 was filed with the patent office on 2020-10-22 for hot-pressed steel sheet member and method for producing same.
This patent application is currently assigned to JFE STEEL CORPORATION. The applicant listed for this patent is JFE STEEL CORPORATION. Invention is credited to Yoshimasa FUNAKAWA, Takayuki FUTATSUKA, Yoshihiko ONO, Kentaro SATO, Katsutoshi TAKASHIMA, Shimpei YOSHIOKA.
Application Number | 20200332382 16/754135 |
Document ID | / |
Family ID | 1000004972491 |
Filed Date | 2020-10-22 |
United States Patent
Application |
20200332382 |
Kind Code |
A1 |
TAKASHIMA; Katsutoshi ; et
al. |
October 22, 2020 |
HOT-PRESSED STEEL SHEET MEMBER AND METHOD FOR PRODUCING SAME
Abstract
Disclosed is a hot-pressed steel sheet member having a tensile
strength of 1780 MPa or more and excellent bending collapsibility.
The hot-pressed steel sheet member has: a specific chemical
composition where a ratio of a C content in mass % to a Nb content
in mass %, C/Nb, is from 22 to 100; a microstructure in which an
average grain size of prior austenite grains is 8 .mu.m or less, a
volume fraction of martensite is 90% or more, and a solute C
content is 25% or less of a total C content; and a tensile strength
of 1780 MPa or more.
Inventors: |
TAKASHIMA; Katsutoshi;
(Chiyoda-ku, Tokyo, JP) ; FUTATSUKA; Takayuki;
(Chiyoda-ku, Tokyo, JP) ; SATO; Kentaro;
(Chiyoda-ku, Tokyo, JP) ; YOSHIOKA; Shimpei;
(Chiyoda-ku, Tokyo, JP) ; ONO; Yoshihiko;
(Chiyoda-ku, Tokyo, JP) ; FUNAKAWA; Yoshimasa;
(Chiyoda-ku, Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE STEEL CORPORATION |
Chiyoda-ku Tokyo |
|
JP |
|
|
Assignee: |
JFE STEEL CORPORATION
Chiyoda-ku Tokyo
JP
|
Family ID: |
1000004972491 |
Appl. No.: |
16/754135 |
Filed: |
November 7, 2018 |
PCT Filed: |
November 7, 2018 |
PCT NO: |
PCT/JP2018/041343 |
371 Date: |
April 7, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/16 20130101;
C22C 38/04 20130101; C22C 38/12 20130101; C22C 38/105 20130101;
C21D 2211/008 20130101; C22C 38/008 20130101; C21D 2211/001
20130101; C23C 2/12 20130101; C22C 38/32 20130101; C22C 38/14
20130101; C22C 38/001 20130101; C21D 8/0236 20130101; C22C 38/06
20130101; C23C 2/06 20130101; C21D 9/46 20130101; C22C 38/02
20130101 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C21D 8/02 20060101 C21D008/02; C22C 38/00 20060101
C22C038/00; C22C 38/02 20060101 C22C038/02; C22C 38/04 20060101
C22C038/04; C22C 38/06 20060101 C22C038/06; C22C 38/10 20060101
C22C038/10; C22C 38/12 20060101 C22C038/12; C22C 38/14 20060101
C22C038/14; C22C 38/16 20060101 C22C038/16; C22C 38/32 20060101
C22C038/32; C23C 2/06 20060101 C23C002/06; C23C 2/12 20060101
C23C002/12 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 13, 2017 |
JP |
2017-218301 |
Claims
1. A hot-pressed steel sheet member, comprising: a chemical
composition containing, in mass %, C: 0.30% or more and less than
0.50%, Si: 0.01% or more and 2.0% or less, Mn: 0.5% or more and
3.5% or less, Nb: 0.001% or more and 0.10% or less, P: 0.05% or
less, S: 0.01% or less, Al: 0.01% or more and 1.00% or less, and N:
0.01% or less, with the balance being Fe and inevitable impurities,
where a ratio of a C content in mass % to a Nb content in mass %,
C/Nb, is from 22 to 100; a microstructure in which an average grain
size of prior austenite grains is 8 .mu.m or less, a volume
fraction of martensite is 90% or more, and a solute C content is
25% or less of a total C content; and a tensile strength of 1780
MPa or more.
2. The hot-pressed steel sheet member according to claim 1, wherein
the chemical composition further contains, in mass %, at least one
selected from the group consisting of Mo: 0.35% or less, Cr: 0.35%
or less, Ti: 0.15% or less, B: 0.0050% or less, Ca: 0.005% or less,
V: 0.05% or less, Cu: 0.50% or less, Ni: 0.50% or less, Sn: 0.50%
or less, Zn: 0.10% or less, Co: 0.10% or less, Zr: 0.10% or less,
Ta: 0.10% or less, and W: 0.10% or less.
3. The hot-pressed steel sheet member according to claim 1, further
comprising, on a surface thereof, an Al or Al alloy coated or
plated layer or a Zn or Zn alloy coated or plated layer.
4. A method for producing a hot-pressed steel sheet member,
comprising: heating a cold-rolled steel sheet to a heating
temperature of Ac3 transformation temperature or higher and 1000
.degree. C. or lower, the cold-rolled steel sheet comprising a
chemical composition containing, in mass%, C: 0.30% or more and
less than 0.50%, Si: 0.01% or more and 2.0% or less, Mn: 0.5% or
more and 3.5% or less, Nb: 0.001% or more and 0.10% or less, P:
0.05% or less, S: 0.01% or less, Al: 0.01% or more and 1.00% or
less, and N: 0.01% or less, with the balance being Fe and
inevitable impurities, where a ratio of a C content in mass % to a
Nb content in mass %, C/Nb, is from 22 to 100; hot pressing the
heated cold-rolled steel sheet to obtain a hot-pressed steel sheet;
cooling the hot-pressed steel sheet to Mf point or lower; and
subjecting the cooled hot-pressed steel sheet to heat treatment
under a set of conditions including a heating temperature of
50.degree. C. to 300.degree. C. and a holding time of 5 seconds to
3600 seconds.
5. The method for producing a hot-pressed steel sheet member
according to claim 4, wherein the chemical composition further
contains, in mass %, at least one selected from the group
consisting of Mo: 0.35% or less, Cr: 0.35% or less, Ti: 0.15% or
less, B: 0.0050% or less, Ca: 0.005% or less, V: 0.05% or less, Cu:
0.50% or less, Ni: 0.50% or less, Sn: 0.50% or less, Zn: 0.10% or
less, Co: 0.10% or less, Zr: 0.10% or less, Ta: 0.10% or less, and
W: 0.10% or less.
6. The hot-pressed steel sheet member according to claim 2, further
comprising, on a surface thereof, an Al or Al alloy coated or
plated layer or a Zn or Zn alloy coated or plated layer.
Description
TECHNICAL FIELD
[0001] This disclosure relates to a hot-pressed steel sheet member,
and in particular to a hot-pressed steel sheet member having both a
tensile strength of 1780 MPa or more and excellent bending
collapsibility. This disclosure also relates to a method for
producing the hot-pressed steel sheet member.
BACKGROUND
[0002] In recent years, along with growing concern over
environmental problems, CO.sub.2 emission regulations are being
further tightened, and in the automobile field, reduction of
automobile body weight for fuel efficiency improvement has become
an issue. Therefore, sheet metal thinning of automotive parts by
using high-strength steel sheets has been advanced, specifically,
application of steel sheets having a tensile strength (TS) of 1780
MPa or more is being considered.
[0003] However, high-strength steel sheets used in structural parts
and reinforcing members of automobiles are required to have
excellent formability and high dimensional accuracy after forming.
Since steel sheets having a tensile strength of 1780 MPa or more
are low in ductility, there is a problem that cracks are likely to
occur during cold press forming. Further, since steel sheets having
a tensile strength of 1780 MPa or more are high in yield stress,
the steel sheets are susceptible to large springback after
subjection to cold press forming. Accordingly, in the way of cold
pressing a steel sheet having a tensile strength of 1780 MPa or
more, high dimensional accuracy can not be obtained after
forming.
[0004] Therefore, in recent years, as a technique for achieving
both high strength and high dimensional accuracy, press forming in
hot pressing (also called hot stamping, die quenching, press
quenching, or the like) has attracted attention. Hot pressing is a
molding process in which a steel sheet is heated to a temperature
range of the austenite single phase, and molded at a high
temperature while being rapidly cooled (quenched) by being brought
into contact with the mold. Since molding is performed in a state
where the steel sheet is softened by being heated and the strength
of the steel sheet is increased by quenching, hot pressing makes it
possible to obtain a member having both high strength and high
dimensional accuracy. From such features, hot pressing is utilized
in the manufacture of members such as automotive members for which
strength and accuracy are required.
[0005] For example, JP2013-040390A (PTL1) proposes a method for
producing a hot-pressed member used as an automotive member. In the
method of PTL 1, quenching treatment and heat treatment are
performed after hot pressing to improve the toughness of the
hot-pressed member.
CITATION LIST
Patent Literature
[0006] PTL 1: JP2013-040390A
SUMMARY
Technical Problem
[0007] Automotive members, in particular framework parts, are
required to have excellent bending collapsibility in addition to
excellent strength. The bending collapsibility refers to the
property of collapsing in plastic deformation without causing
cracks in the member upon bending deformation. In order to ensure
the crashworthiness of automobiles, automotive members are required
to be excellent in bending collapsibility.
[0008] However, a problem was found that when the member is
quenched during hot pressing as described above so as to have a
tensile strength of 1780 MPa or more after being pressed,
martensite remains as-quenched, making the steel sheet prone to
cracking without plastic deformation upon bending deformation.
[0009] Further, according to the method of PTL 1, a certain
improvement in toughness is achieved by performing heat treatment.
It was found, however, that even in the hot-pressed member
described in PTL 1, sufficient bending collapsibility can not be
obtained when the member has a tensile strength of 1780 MPa or more
after being pressed.
[0010] For assurance of weight reduction and crashworthiness
required for automotive framework parts, the member after hot
pressing is required to have both a high tensile strength of 1780
MPa or more and excellent bending collapsibility. However, it is
difficult to improve the bending collapsibility of a hot-pressed
steel sheet member having TS of 1780 MPa or more, methods for
obtaining a hot-pressed steel sheet member having both of these
properties have not been developed in the art.
[0011] It would thus be helpful to provide a hot-pressed steel
sheet member having both a tensile strength of 1780 MPa or more and
excellent bending collapsibility.
Solution to Problem
[0012] The present inventors reached the following findings as a
result of study in order to solve the above problems.
[0013] (1) To improve the bending collapsibility of the hot-pressed
member, it is important to control the solute C content of the
hot-pressed member. In the case where the components of the steel
sheet are adjusted to ensure a tensile strength of 1780 MPa or
more, the solute C content in the martensite as-quenched increases.
As a result, the hardness of the member is increased, but the
member becomes brittle. Therefore, by performing heat treatment to
cause precipitation of solute C, it is possible to improve the
bending collapsibility while ensuring a certain degree of hardness.
Adjustment of the solute C content can be made by performing heat
treatment under a predetermined condition after the steel sheet
microstructure is transformed into martensite by hot pressing.
[0014] (2) To improve the bending collapsibility of the hot-pressed
member, in addition to the control of the solute C content, it is
also important to control the ratio of C content to Nb content,
C/Nb, in a predetermined range. Since Nb precipitates as NbC, and
NbC has the effect of refining austenite grains, it is effective
for improving the bending collapsibility. Therefore, from the
viewpoint of generating NbC, it is considered desirable to add a
sufficient amount of Nb. However, since C, which is necessary for
strength improvement, is consumed by the formation of NbC, the
required strength can not be obtained when the C content is too
small as compared with the Nb content. On the other hand, if the C
content is too large as compared with the Nb content, the bending
collapsibility decreases due to an increase in the solute C content
or a decrease in the NbC content. Therefore, in order to achieve
both strength and bending collapsibility, it is insufficient to
simply control the C content and Nb content individually, it is
necessary to control the ratio of C content to Nb content, C/Nb, in
a predetermined range.
[0015] The present disclosure was completed based on the above
findings, and the primary features thereof are as follows.
[0016] 1. A hot-pressed steel sheet member, comprising: a chemical
composition containing (consisting of), in mass%, C: 0.30% or more
and less than 0.50%, Si: 0.01% or more and 2.0% or less, Mn: 0.5%
or more and 3.5% or less, Nb: 0.001% or more and 0.10% or less, P:
0.05% or less, S: 0.01% or less, Al: 0.01% or more and 1.00% or
less, and N: 0.01% or less, with the balance being Fe and
inevitable impurities, where a ratio of a C content in mass % to a
Nb content in mass %, C/Nb, is from 22 to 100; a microstructure in
which an average grain size of prior austenite grains is 8 .mu.m or
less, a volume fraction of martensite is 90% or more, and a solute
C content is 25% or less of a total C content; and a tensile
strength of 1780 MPa or more.
[0017] 2. The hot-pressed steel sheet member according to 1.,
wherein the chemical composition further contains, in mass %, at
least one selected from the group consisting of Mo: 0.35% or less,
Cr: 0.35% or less, Ti: 0.15% or less, B: 0.0050% or less, Ca:
0.005% or less, V: 0.05% or less, Cu: 0.50% or less, Ni: 0.50% or
less, and Sn: 0.50% or less.
[0018] 3. The hot-pressed steel sheet member according to 1. or 2.,
further comprising, on a surface thereof, an Al or Al alloy coated
or plated layer or a Zn or Zn alloy coated or plated layer.
[0019] 4. A method for producing a hot-pressed steel sheet member,
comprising: heating a cold-rolled steel sheet to a heating
temperature of Ac3 transformation temperature or higher and
1000.degree. C. or lower, the cold-rolled steel sheet comprising a
chemical composition containing (consisting of), in mass %, C:
0.30% or more and less than 0.50%, Si: 0.01% or more and 2.0% or
less, Mn: 0.5% or more and 3.5% or less, Nb: 0.001% or more and
0.10% or less, P: 0.05% or less, S: 0.01% or less, Al: 0.01% or
more and 1.00% or less, and N: 0.01% or less, with the balance
being Fe and inevitable impurities, where a ratio of C content in
mass % to Nb content in mass %, C/Nb, is from 22 to 100; hot
pressing the heated cold-rolled steel sheet to obtain a hot-pressed
steel sheet; cooling the hot-pressed steel sheet to Mf point or
lower; and subjecting the cooled hot-pressed steel sheet to heat
treatment under a set of conditions including a heating temperature
of 50.degree. C. to 300.degree. C. and a holding time of 5 seconds
to 3600 seconds.
[0020] 5. The method for producing a hot-pressed steel sheet member
according to 4., wherein the chemical composition further contains,
in mass %, at least one selected from the group consisting of Mo:
0.35% or less, Cr: 0.35% or less, Ti: 0.15% or less, B: 0.0050% or
less, Ca: 0.005% or less, V: 0.05% or less, Cu: 0.50% or less, Ni:
0.50% or less, and Sn: 0.50% or less.
Advantageous Effect
[0021] According to the present disclosure, it is possible to
provide a hot-pressed steel sheet member having a tensile strength
of 1780 MPa or more and excellent bending collapsibility.
DETAILED DESCRIPTION
[0022] Hereinafter, a method for carrying out the present
disclosure will be specifically described.
Chemical Composition
[0023] The hot-pressed steel sheet member according to the present
disclosure comprises a steel sheet portion having the above
chemical composition. Hereinafter, the reasons for the limitations
will be described. The "%" representations below relating to the
chemical composition are in "mass %".
[0024] C: 0.30% or more and less than 0.50%
[0025] C is an element effective for increasing the strength of a
steel sheet, and is important for enhancing the strength of the
steel by strengthening martensite after hot pressing. However, when
the C content is less than 0.30%, the hardness of martensite after
hot pressing is insufficient, and the expected tensile strength can
not be obtained. Therefore, the C content is 0.30% or more. On the
other hand, when the C content is 0.50% or more, it becomes
difficult to sufficiently reduce the solute C content in the heat
treatment after cooling, and the bending collapsibility is lowered.
Therefore, the C content is less than 0.50%, preferably less than
0.45%, and more preferably less than 0.40%.
[0026] Si: 0.01% or more and 2.0% or less
[0027] Si has the action of solid solution strengthening the
ferrite, and is an effective element for increasing the strength.
However, since the excessive addition of Si lowers the chemical
convertibility, the Si content is 2.0% or less, and preferably 1.3%
or less. On the other hand, providing an ultra-low Si content
increases the cost, and thus the Si content is 0.01% or more.
[0028] Mn: 0.5% or more and 3.5% or less
[0029] Mn is an element having the effect of increasing the quench
hardenability, and contributes to the formation of martensite,
i.e., the increase in strength, during cooling after hot pressing.
To obtain this effect, the Mn content is set to 0.5% or more, and
preferably 1.0% or more. On the other hand, when the Mn content is
more than 3.5%, Mn bands are excessively generated, and it is
impossible to sufficiently reduce the solute C content through heat
treatment, resulting in lower bending collapsibility. Therefore,
the Mn content is 3.5% or less, and preferably 2.5% or less.
[0030] Nb: 0.001% or more and 0.10% or less
[0031] Nb is an element that contributes to increasing the strength
by forming fine carbonitrides. In addition, Nb is an element that
contributes to the improvement of bending collapsibility since it
achieves the refinement of the austenite grain size during hot
pressing. To obtain this effect, the Nb content is 0.001% or more,
and preferably 0.003% or more. However, adding Nb in large
quantities does not increase the above effect, rather resulting in
increased cost. Therefore, the Nb content is 0.10% or less,
preferably 0.07% or less, more preferably 0.03% or less, and even
more preferably 0.02% or less.
[0032] P: 0.05% or less
[0033] P is an element that contributes to increasing the strength
by solid solution strengthening. However, when P is added
excessively, segregation at grain boundaries becomes significant
and causes embrittlement of the grain boundaries, resulting in
lower bending collapsibility. Therefore, the P content is 0.05% or
less, and preferably 0.04% or less. On the other hand, the lower
limit of the P content is not particularly limited, yet providing
an ultra-low P content leads to an increase in steel manufacturing
cost. Therefore, the P content is preferably 0.0005% or more.
[0034] S: 0.01% or less
[0035] When the S content is excessively high, sulfide inclusions
such as MnS are generated in large quantities, and cracking occurs
originating from the inclusions, resulting in lower bending
collapsibility. Therefore, the S content is 0.01% or less, and
preferably 0.005% or less. On the other hand, the lower limit of
the Si content is not particularly limited, yet providing an
ultra-low S content causes an increase in steel manufacturing cost.
Therefore, the S content is preferably 0.0002% or more.
[0036] Al: 0.01% or more and 1.00% or less
[0037] Al is an element required for deoxidation. To obtain this
effect, the Al content is set to 0.01% or more. However, the effect
is saturated when the Al content exceeds 1.00%. Therefore, the Al
content is 1.00% or less, and preferably 0.50% or less.
[0038] N: 0.01% or less
[0039] N forms a coarse nitride and lowers the bending
collapsibility. When the N content is more than 0.01%, the effect
on the bending collapsibility becomes significant. Therefore, the N
content is 0.01% or less, and preferably 0.008% or less.
[0040] The steel sheet according to an embodiment of the present
disclosure may have a chemical composition containing the above
components, with the balance being Fe and inevitable
impurities.
[0041] According to another embodiment of the present disclosure,
the chemical composition may optionally contain at least one of the
following elements.
[0042] Mo: 0.35% or less
[0043] Mo is an element having the effect of increasing the quench
hardenability, and contributes to the formation of martensite,
i.e., the increase in strength, during cooling after hot pressing.
However, excessively adding Mo does not increase the effect, rather
resulting in increased cost. Further, excessive Mo addition lowers
the chemical convertibility. Therefore, when Mo is added, the Mo
content is 0.35% or less. On the other hand, the lower limit of the
Mo content is not particularly limited, yet from the viewpoint of
increasing the addition effect of Mo, the Mo content is preferably
0.005% or more, and more preferably 0.01% or more.
[0044] Cr: 0.35% or less
[0045] Cr, like Mo, is also an element having the effect of
increasing the quench hardenability, and contributes to the
formation of martensite, i.e., the increase in strength, during
cooling after hot pressing. However, excessively adding Cr does not
increase the effect, but rather ends up increasing the cost.
Further, Cr forms a surface oxide, lowering the coatability.
Therefore, when Cr is added, the Cr content is 0.35% or less. On
the other hand, the lower limit of the Cr content is not
particularly limited, yet from the viewpoint of increasing the
addition effect of Cr, it is preferable that the Cr content is
0.005% or more, and more preferably 0.01% or more.
[0046] Ti: 0.10% or less
[0047] Ti is an element that contributes to increasing the strength
by forming a fine carbonitride. Ti also contributes to improving
the bending collapsibility by refining the austenite grain size
during hot pressing. However, when a large amount of Ti is added,
elongation after hot pressing is significantly reduced. Therefore,
when Ti is added, the Ti content is 0.10% or less, and preferably
0.08% or less. On the other hand, the lower limit of the Ti content
is not particularly limited, yet from the viewpoint of increasing
the addition effect of Ti, the Ti content is preferably 0.005% or
more.
[0048] B: 0.0050% or less
[0049] B is an element having the effect of increasing the quench
hardenability, and contributes to the formation of martensite,
i.e., the increase in strength during cooling after hot pressing.
In addition, since B segregates at the grain boundaries and
improves the grain boundary strength, B is effective in improving
the bending collapsibility. However, excessive addition of B leads
to formation of coarse precipitates with C, lowering the bending
collapsibility. Therefore, when B is added, the B content is
0.0050% or less, and preferably 0.0035% or less. On the other hand,
the lower limit of the B content is not particularly limited, yet
from the viewpoint of increasing the addition effect of B, it is
preferable that the B content is 0.0002% or more.
[0050] Ca: 0.005% or less
[0051] Ca is an element that controls the shapes of sulfides and
oxides, that has the effect of suppressing the formation of coarse
MnS, and that improves the bending collapsibility. However,
excessive addition of Ca deteriorates the workability. Therefore,
when Ca is added, the Ca content is 0.005% or less. On the other
hand, the lower limit of the Ca content is not particularly
limited, yet from the viewpoint of increasing the addition effect
of Ca, the Ca content is preferably 0.0005% or more.
[0052] V: 0.05% or less
[0053] V is an element that contributes to increasing the strength
by forming a fine carbonitride. However, excessive addition of V
deteriorates the bending collapsibility. Therefore, when V is
added, the V content is 0.05% or less. On the other hand, the lower
limit of the V content is not particularly limited, yet from the
viewpoint of increasing the addition effect of V, the V content is
preferably 0.01% or more.
[0054] Cu: 0.50% or less
[0055] Cu is an element that contributes to increasing the strength
by solid solution strengthening. Cu also contributes to improving
the delayed fracture resistance by increasing the corrosion
resistance. However, excessive addition does not increase the
effect, but rather surface defects caused by Cu are more likely to
occur. Therefore, when Cu is added, the Cu content is 0.50% or
less. On the other hand, the lower limit of the Cu content is not
particularly limited, yet from the viewpoint of increasing the
addition effect of Cu, the Cu content is preferably 0.05% or
more.
[0056] Ni: 0.50% or less
[0057] Ni, like Cu, is an element that contributes to improving the
delayed fracture resistance by increasing the corrosion resistance.
When added with Cu, Ni also has the effect of suppressing surface
defects caused by Cu. Therefore, the addition of Ni is particularly
effective when adding Cu. However, a large amount of Ni addition
decreases the bending collapsibility, resulting in lower tensile
shear stress. Therefore, when Ni is added, the Ni content is 0.50%
or less. On the other hand, the lower limit of the Ni content is
not particularly limited, yet from the viewpoint of increasing the
addition effect of Ni, the Ni content is preferably 0.05% or
more.
[0058] Sn: 0.50% or less
[0059] Sn, like Cu, is an element that contributes to increasing
the delayed fracture resistance by improving the corrosion
resistance. However, a large amount of Sn addition decreases the
bending collapsibility. Therefore, when Sn is added, the Sn content
is 0.50% or less. On the other hand, the lower limit of the Sn
content is not particularly limited, yet from the viewpoint of
increasing the addition effect of Sn, the Sn content is preferably
0.05% or more.
[0060] Zn: 0.10% or less
[0061] Zn is an element that increases the quench hardenability
during hot pressing, and contributes to the formation of
martensite, i.e., the increase in strength, after hot pressing.
However, since a large amount of Zn addition decreases the bending
collapsibility, when Zn is added, the Zn content is 0.10% or less.
On the other hand, the lower limit of the Zn content is not
particularly limited, yet from the viewpoint of increasing the
addition effect of Zn, the Zn content is preferably 0.005% or
more.
[0062] Co: 0.10% or less
[0063] Co, like Cu and Ni, is also an element having the effect of
improving the corrosion resistance by improving the hydrogen
overpotential. Thus, the delayed fracture resistance can be
improved by Co addition. However, since a large amount of Co
addition decreases the bending collapsibility, when Co is added,
the Co content is 0.10% or less. On the other hand, the lower limit
of the Co content is not particularly limited, yet from the
viewpoint of increasing the addition effect of Co, the Co content
is preferably 0.005% or more.
[0064] Zr: 0.10% or less
[0065] Zr, like Cu and Ni, is an element that contributes to
improving the delayed fracture resistance by increasing the
corrosion resistance. However, a large amount of Zr addition
decreases the bending collapsibility. Therefore, when Zr is added,
the Zr content is 0.10% or less. On the other hand, the lower limit
of the Zr content is not particularly limited, yet from the
viewpoint of increasing the addition effect of Zr, the Zr content
is preferably 0.005% or more.
[0066] Ta: 0.10% or less
[0067] Ta, like Ti, is an element that contributes to increasing
the strength by forming carbides or nitrides. However, excessively
adding Ta does not increase the addition effect, but rather ends up
increasing the alloy cost. Therefore, when Ta is added, the Ta
content is 0.10% or less. On the other hand, the lower limit of the
Ta content is not particularly limited, yet from the viewpoint of
increasing the addition effect of Ta, the Ta content is preferably
0.005% or more.
[0068] W: 0.10% or less
[0069] W, like Cu and Ni, is an element that contributes to
improving the delayed fracture resistance by improving the
corrosion resistance. However, a large amount of W addition
decreases the bending collapsibility. Therefore, when W is added,
the W content is 0.10% or less. On the other hand, the lower limit
of the W content is not particularly limited, yet from the
viewpoint of increasing the addition effect of W, the W content is
preferably 0.005% or more.
[0070] Further, the hot-pressed steel sheet member in another
embodiment of the present disclosure may comprise a chemical
composition containing
[0071] C: 0.30% or more and less than 0.50%,
[0072] Si: 0.01% or more and 2.0% or less,
[0073] Mn: 0.5% or more and 3.5% or less,
[0074] Nb: 0.001% or more and 0.10% or less,
[0075] P: 0.05% or less,
[0076] S: 0.01% or less,
[0077] Al: 0.01% or more and 1.00% or less, and
[0078] N: 0.01% or less, and
optionally, at least one selected from the group consisting of
[0079] Mo: 0.35% or less,
[0080] Cr: 0.35% or less,
[0081] Ti: 0.15% or less,
[0082] B: 0.0050% or less,
[0083] Ca: 0.005% or less,
[0084] V: 0.05% or less,
[0085] Cu: 0.50% or less,
[0086] Ni: 0.50% or less,
[0087] Sn: 0.50% or less,
[0088] Zn: 0.10% or less,
[0089] Co: 0.10% or less,
[0090] Zr: 0.10% or less,
[0091] Ta: 0.10% or less, and
[0092] W: 0.10% or less,
with the balance being Fe and inevitable impurities.
[0093] C/Nb: from 22 to 100
[0094] In the present disclosure, it is important that the ratio of
the C content in mass % to the Nb content in mass %, C/Nb, in the
steel sheet be from 22 to 100. When the C/Nb exceeds 100, the
amount of formation of Nb-based carbides is reduced and the solute
C content increases. Since Nb-based carbides have the effect of
suppressing the growth of austenite grains by the pinning effect,
the reduction of Nb-based carbides leads to the coarsening of
grains after hot pressing, resulting in lower bending
collapsibility. In addition, when the solute C content is
increased, the toughness decreases, resulting in lower bending
collapsibility. Therefore, the C/Nb is set to 100 or less,
preferably 80 or less, and more preferably 70 or less. On the other
hand, when the C/Nb is less than 22, Nb-based carbides serving as
the origins of cracking are generated in large quantities,
resulting in lower bending collapsibility. Although C is an element
having the effect of increasing the strength of the steel, when
C/Nb is below 22, the proportion of C consumed by the formation of
Nb-based carbides increases, and as a result, the tensile strength
is lowered. Therefore, C/Nb is set to 22 or more, preferably 25 or
more, and more preferably 30 or more.
Microstructure
[0095] Furthermore, it is important for the hot-pressed steel sheet
member according to the present disclosure to have a steel sheet
portion having a microstructure that satisfies the following
conditions.
[0096] Average grain size of prior austenite grains: 8 .mu.m or
less When the average grain size of prior austenite grains exceeds
8 .mu.m, the toughness at the time of bending collapse is lowered,
deteriorating the bending collapsibility. Therefore, the average
grain size of prior austenite grains is set to 8 .mu.m or less, and
preferably 7 .mu.m or less. On the other hand, the lower limit is
not particularly limited, yet it is preferably 2 .mu.m or more,
more preferably 3 .mu.m or more, and even more preferably 5 .mu.m
or more.
[0097] Martensite volume fraction: 90% or more
[0098] When the volume fraction of martensite is less than 90%, it
is difficult to obtain a tensile strength of 1780 MPa or more.
Therefore, the volume fraction of martensite is 90% or more, and
preferably 95% or more. On the other hand, the upper limit of the
volume fraction of martensite is not particularly limited, yet it
may be 100%. The microstructures other than martensite are not
particularly limited, and any microstructures may be contained. For
example, the residual microstructures other than martensite may be
one or more selected from the group consisting of ferrite, bainite,
and pearlite.
[0099] Solute C content: 25% or less of the total C content
[0100] When the solute C content exceeds 25% of the total C
content, the toughness at the time of bending collapse decreases.
Therefore, the solute C content is 25% or less of the total C
content, preferably 20% or less, and more preferably 15% or less.
On the other hand, the lower limit of the solute C content is not
particularly limited, yet it is preferably 5% or more, more
preferably 6% or more, of the total C content.
Tensile Strength
[0101] TS: 1780 MPa or more
[0102] The hot-pressed steel sheet member according to the present
disclosure has a tensile strength (TS) of 1780 MPa or more. TS is
preferably 1800 MPa or more, more preferably 1850 MPa or more, and
even more preferably 1900 MPa or more. On the other hand, the upper
limit of the TS is not particularly limited, yet it may usually be
2500 MPa or less or 2450 MPa or less.
Yield Ratio
[0103] In the case of the hot-pressed steel sheet member having a
high yield ratio (YR), it is possible to further improve the
collision safety when using the hot-pressed steel sheet member as
an automotive member. Therefore, the yield ratio is preferably 65%
or more, and more preferably 70% or more. Note that the yield ratio
(YR) is a value defined as the ratio of yield strength YS to
tensile strength TS, and specifically, can be calculated as
YR=YS/TS.times.100 (%).
Coated or Plated Layer
[0104] The hot-pressed steel sheet member according to the present
disclosure may have no coated or plated layer. In that case, the
hot-pressed steel sheet member is formed from a steel material
having the chemical composition, microstructure, and tensile
strength as described above. However, in order to prevent oxidation
during hot pressing or to improve the corrosion resistance of the
hot-pressed steel sheet member, it is preferable that the
hot-pressed steel sheet member further comprises a coated or plated
layer on the surface of the steel sheet. If the hot-pressed steel
sheet member comprises a coated or plated layer on a surface
thereof, the steel sheet portion excluding the coated or plated
layer (i.e., the base steel sheet) has the chemical composition and
microstructure described above.
[0105] As the coated or plated layer, an Al or Al alloy coated or
plated layer or a Zn or Zn alloy coated or plated layer is
preferred. By applying such a coated or plated layer to a surface
of the steel sheet, it is possible to prevent the oxidation of the
steel sheet surface caused by hot pressing, and to improve the
corrosion resistance of the hot-pressed steel sheet member.
[0106] As used herein, the term "Zn or Zn alloy coated or plated
layer" is intended to refer to a coated or plated layer containing
Zn in an amount of 50 mass % or more. The Zn or Zn alloy coated or
plated layer may be any of a Zn coated or plated layer and a Zn
alloy coated or plated layer. The Zn or Zn alloy coated or plated
layer may be a coated or plated layer made of an alloy containing
Zn as a main component and at least one selected from the group
consisting of Si, Mg, Ni, Fe, Co, Mn, Sn, Pb, Be, B, P, S, Ti, V,
W, Mo, Sb, Cd, Nb, Cr, and Sr. Examples of the Zn or Zn alloy
coated or plated layer that can be suitably used include a Zn--Ni
alloy coated or plated layer.
[0107] As used herein, the Al or Al alloy coated or plated layer is
intended to refer to a coated or plated layer containing Al in an
amount of 50 mass % or more. The Al or Al alloy coated or plated
layer may be any of an Al coated or plated layer and an Al alloy
coated or plated layer. The Al or Al alloy coated or plated layer
may be, for example, a coated or plated layer made of an alloy
containing Al as a main component as well as at least one selected
from the group consisting of Si, Mg, Ni, Fe, Co, Mn, Sn, Pb, Be, B,
P, S, Ti, V, W, Mo, Sb, Cd, Nb, Cr, and Sr. One example of the Al
or Al alloy coated or plated layer that can be suitably used is an
Al--Si coated or plated layer.
[0108] The forming method of the coated or plated layer is not
particularly limited, and the coated or plated layer may be formed
in any way. For example, a hot dip coated layer which is a coated
layer formed by hot dip coating, an electroplating layer which is a
plated layer formed by electroplating, a vapor deposition coated
layer which is a coated layer formed by vapor deposition coating,
and the like are all applicable. In addition, the coated or plated
layer may be a galvannealed layer which is a coated layer formed by
applying an alloying treatment after the coating or plating
step.
[0109] Examples of the Al or Al alloy coated or plated layer that
can be suitably used include a hot-dip Al--Si coated or plated
layer formed by hot dip coating. Further, examples of the Zn or Zn
alloy coated or plated layer that can be suitably used include a
hot-dip galvanized layer formed by hot dip coating, a galvannealed
layer formed by alloying of a hot-dip galvanized layer, and a Zn
electroplated layer or Zn--Ni alloy electroplated layer formed by
electroplating. In particular, from the viewpoint of further
improving the corrosion resistance of the hot pressed member and
preventing the liquid metal brittle cracking caused by molten Zn
during hot press forming, it is preferable to use a Zn--Ni alloy
coated or plated layer as the Zn or Zn alloy coated or plated
layer.
[0110] Note that when the steel sheet to which a coated or plated
layer has been applied is subjected to hot pressing, some or all of
the elements contained in the coated or plated layer are diffused
into the base steel sheet, and may produce a solid solution phase
or an intermetallic compound. Similarly, Fe, which is one of the
components of the base steel sheet, is also diffused into the
coated or plated layer, and may produce a solid solution phase or
an intermetallic compound. In addition, an oxide layer may be
formed on a surface of the coated or plated layer.
[0111] For example, when an Al--Si coated or plated layer is
heated, the coated or plated layer changes to a coated or plated
layer that is mainly composed of an Fe--Al intermetallic compound
containing Si. Further, when a hot-dip galvanized layer, a
galvannealed layer, an electroplated Zn layer, or the like is
heated, an FeZn solid solution phase in which Zn is solid-dissolved
in Fe, a ZnFe intermetallic compound, a ZnO layer on a surface
layer, or the like is formed. Furthermore, when a Zn--Ni alloy
electroplated layer is heated, a solid solution layer containing Ni
in which plated layer components are solid-dissolved in Fe, an
intermetallic compound mainly composed of ZnNi, a ZnO layer on the
surface layer, or the like is formed.
[0112] As used herein, an Al-containing coated or plated layer that
is formed by heating a cold-rolled steel sheet for hot pressing to
which an Al or Al alloy coated or plated layer has been applied is
referred to as an Al or Al alloy coated or plated layer, and a
Zn-containing coated or plated layer that is formed by heating a
cold-rolled steel sheet for hot pressing to which a Zn or Zn alloy
coated or plated layer has been applied is referred to as a Zn or
Zn alloy coated or plated layer.
[0113] The coating weight of the coated or plated layer is not
particularly limited, and may be arbitrarily selected. However, a
coating weight per one side of less than 5 g/m.sup.2 may make it
difficult to secure proper corrosion resistance. Therefore, the
coating weight per one side is preferably 5 g/m.sup.2 or more. On
the other hand, a coating weight per one side of more than 150
g/m.sup.2 may deteriorate the resistance to coating exfoliation.
Therefore, the coating weight per one side is preferably 150
g/m.sup.2 or less.
Production Method
[0114] Next, a method for producing a hot-pressed steel sheet
member according to the present disclosure will be described. The
method for producing a hot-pressed steel sheet member according to
the present disclosure is not particularly limited, yet in one
embodiment, the following steps (1) to (4) may be applicable. The
steps are specifically described below.
(1) Heating of a cold-rolled steel sheet (2) Hot pressing (3)
Cooling (quenching) (4) Heat treatment
Cold-Rolled Steel Sheet
[0115] The cold-rolled steel sheets used as the material have the
above-described chemical composition. That is, the chemical
composition of the steel sheet portion of each resulting
hot-pressed steel sheet member is basically the same as that of the
cold-rolled steel sheet used as the material.
[0116] The method for producing the cold-rolled steel sheet is not
particularly limited, and any conventional method is applicable.
For example, the cold-rolled steel sheet may be produced by hot
rolling and then cold rolling a steel material (steel slab) having
the above-described chemical composition. The cold-rolled steel
sheet may be further subjected to temper rolling. When performing
temper rolling, a preferred elongation ratio is 0.05% to 2.0%.
[0117] For example, a steel material (slab) having the
above-described chemical composition is hot rolled under the
condition of a finisher delivery temperature of 860.degree. C. to
950.degree. C. to obtain a hot-rolled steel sheet. Then, the
hot-rolled steel sheet is coiled at a coiling temperature of
650.degree. C. or lower. At this point, after completion of the hot
rolling, the hot-rolled steel sheet is cooled to the coiling
temperature at a cooling rate of 5.degree. C./s or higher. Then,
the coiled hot-rolled steel sheet is taken out and pickled, and
further cold rolled. After completion of the cold rolling, the
steel sheet is subjected to a heat treatment whereby it is heated
to a temperature range of 650.degree. C. to 950.degree. C. at an
average heating rate of 2.degree. C./s or higher, and subjected to
5 seconds or more of soaking in the temperature range. Then, the
steel sheet is subjected to cooling whereby it is cooled to a
cooling stop temperature of 600.degree. C. or lower at an average
cooling rate of 2.degree. C./s or higher to obtain a cold-rolled
steel sheet.
[0118] Note that the above production conditions are given by way
of example, and the present disclosure is not limited thereto. This
is because, in the method according to the present disclosure, it
is possible to control the microstructure of the steel sheet by
heating before hot pressing and cooling after the hot pressing.
Coating or Plating Treatment
[0119] Although the above cold-rolled steel sheet may be subjected
to the subsequent heating directly (i.e., without being subjected
to coating or plating treatment), it may optionally be subjected to
coating or plating treatment prior to the heating. The way of
coating or plating treatment is not particularly limited, and any
method such as hot-dip coating, electroplating, or vapor deposition
plating can be used. After the coating or plating treatment, the
steel sheet may also be subjected to alloying treatment.
Heating
[0120] Then, the cold-rolled steel sheet is heated to a heating
temperature of an Ac3 transformation temperature or higher and
1000.degree. C. or lower. When the heating temperature is lower
than the Ac3 point, the austenite fraction in the heated steel
sheet is lowered such that the volume ratio of martensite is less
than 90% after hot pressing, making it impossible to ensure the
intended tensile strength. Further, when the heating temperature is
higher than 1000.degree. C., the grain size becomes excessively
coarse, resulting in lower bending collapsibility. Here, the Ac3
transformation temperature can be determined by the following
Expression (1):
[0121] Ac3 transformation temperature (.degree. C.)=
881-206C+53Si -15Mn-20Ni-1Cr-27Cu+41Mo (1),
where each element symbol represents the content in mass % of the
corresponding element. The content of any element not contained is
calculated as 0.
[0122] The heating may be performed in any way without limitation,
yet may generally be carried out using a heating furnace. The
heating furnace may be, for example, an electric furnace, a gas
furnace, an electrical resistance heating furnace, or a far
infrared heating furnace.
[0123] Although the cold-rolled steel sheet may be subjected to hot
pressing immediately after being heated to the heating temperature,
it may be preferably held at the heating temperature for 0 seconds
to 600 seconds. A holding time exceeding 600 seconds makes the
grain size excessively coarse, resulting in lower bending
collapsibility. Therefore, when performing holding, the holding
time is 600 seconds or less.
Hot Pressing
[0124] Then, the heated cold-rolled steel sheet is conveyed to a
press machine and subjected to hot pressing. The hot pressing is
not limited to a particular method, and may be performed in any
manner. The pressing temperature is not particularly limited, yet
the hot pressing is preferably performed in a range of 550.degree.
C. to 800.degree. C.
Cooling
[0125] The hot-pressed steel sheet is cooled to the Mf point or
lower. By this cooling, the steel sheet heated to the Ac3
transformation temperature or higher by the above heating is cooled
to the Mf point or lower. With this setup, the volume fraction of
the austenite may be 90% or more. Note that the Mf point is the
temperature at which the martensite transformation ends, and can be
determined based on the continuous cooling transformation curve
(CCT curve).
[0126] The cooling is not limited to a particular method, and may
be performed in any manner. Usually, as is performed in a common
hot pressing, it is suffice to perform cooling by contact with the
mold. Note that the cooling may be started simultaneously with the
hot pressing.
[0127] The rate of the cooling is not particularly limited, yet
from the viewpoint of microstructural control, it is preferable
that the average cooling rate within a temperature range of the
cooling start temperature to 150.degree. C. is 10.degree. C./s or
higher. For example, after the steel sheet is cooled by contact
with the mold to a temperature of 150.degree. C. or lower at an
average cooling rate of 10.degree. C./s or higher, the mold is
released. Thereafter, the steel sheet may also be optionally
allowed to naturally cool to room temperature.
Heat Treatment
[0128] Then, the cooled hot-pressed steel sheet is subjected to
heat treatment under a set of conditions including a heating
temperature of 50.degree. C. to 300.degree. C. and a holding time
of 5 seconds to 3600 seconds. Through this heat treatment, the
solute C content of the steel sheet can be adjusted to 25% or less
of the total C content. When the heating temperature is lower than
50.degree. C., the solute C content is increased, resulting in
lower bending collapsibility. On the other hand, when the heating
temperature exceeds 300.degree. C., precipitated carbides are
coarsened, resulting in lower tensile strength. Further, when the
holding time is shorter than 5 seconds, the solute C content is
increased, resulting in lower bending collapsibility. On the other
hand, when the holding time exceeds 3600 seconds, precipitated
carbides are coarsened, resulting in lower tensile strength.
EXAMPLES
[0129] Examples of the present disclosure will now be described
below. It should be noted that the present disclosure is not
limited by the embodiments described below, but may be implemented
with appropriate modifications to the extent that may conform to
the spirit of the present disclosure, all of which modifications
are included in the technical scope of the present disclosure.
[0130] First, cold-rolled steel sheets used as the material for
manufacture of hot-pressed steel sheet members were produced as
follows.
[0131] Steel samples having the chemical compositions listed in
Table 1 were prepared by steelmaking and cast into steel slabs.
Each steel slab was hot rolled under a set of conditions including
a hot rolling heating temperature of 1250.degree. C. and a finisher
delivery temperature (FDT) of 900.degree. C. to obtain a hot-rolled
steel sheet. The resulting hot-rolled steel sheet was coiled at a
coiling temperature of 600.degree. C.
[0132] The resulting hot-rolled steel sheet was taken out and
pickled, and then cold rolled to obtain a cold-rolled steel sheet
having a sheet thickness of 1.4 mm. The obtained cold-rolled steel
sheet was subjected to annealing treatment in a continuous
annealing line (CAL) or a continuous galvanizing line (CGL) to
obtain a cold-rolled steel sheet (CR) or a hot-dip galvanized steel
sheet (GI) as the final material.
[0133] Note that some of the steel sheets were subjected to
alloying treatment after subjection to the hot-dip galvanizing
treatment to obtain galvannealed steel sheets (GA). In addition,
others were subjected to molten aluminum coating treatment to
obtain hot-dip aluminum-coated steel sheets (GAS). Still others
were subjected to plating in an electrogalvanization line (EGL)
after subjection to the annealing at CAL, to obtain
electrogalvanized steel sheets (EG) or zinc-nickel electroplated
steel sheets (EZN).
[0134] Then, each of the resulting cold-rolled steel sheets was
heated to the heating temperature listed in Table 2 and held for 60
seconds at the heating temperature. The heating was carried out in
ambient air using an infrared heating furnace or an atmosphere
heating furnace. The heating rate at the time of heating was
5.degree. C./s.
[0135] Then, each of the heated steel sheets was conveyed to the
press machine and hot pressed into a hat-shaped, hot-pressed steel
sheet member. The steel sheet temperature during the hot pressing
was set to 700.degree. C. The mold used for the hot pressing had a
punch width of 120 mm, a punch shoulder radius of 6 mm, and a die
shoulder radius of 6 mm, and the forming depth was set to 40
mm.
[0136] Cooling was accomplished by combining contact cooling by
squeezing each hot-pressed steel sheet member between the punch and
the die and air cooling on the die after being released from the
squeezing. The average cooling rate was adjusted to 100.degree.
C./s within a temperature range of the press start temperature to
150.degree. C. The average cooling rate was adjusted by changing
the time to hold the punch at the bottom dead center in the range
of 1 second to 60 seconds.
[0137] After the air cooling to room temperature, each hot-pressed
steel sheet member was subjected to heat treatment at the
corresponding heating temperature and holding time listed in Table
2. Specifically, in an atmospheric furnace, each hot-pressed steel
sheet member was heated to the above-described heating temperature,
held for the above-described holding time, and subjected to air
cooling.
[0138] Then, for each of the obtained hot-pressed steel sheet
members, the microstructure, tensile property, and bending
collapsibility of the steel sheet portion were evaluated as
described below.
Microstructure
[0139] Volume Fraction of Martensite
[0140] The volume fraction of martensite in the steel sheet portion
of each hot-pressed steel sheet member was evaluated as follows.
First, a cross section of each steel sheet taken along the sheet
thickness direction to be parallel to the the rolling direction was
polished and etched with a 3 vol % nital, and observed under a
scanning electron microscope (SEM) at .times.2000 and .times.5000
magnification. Then, the area ratio of martensite was measured by a
point counting method (in conformity with ASTM E562-83 (1988)), and
the result was used as the volume fraction.
[0141] Average Grain Size of Prior Austenite Grains
[0142] The average grain size of prior austenite grains was
determined by image interpretation of the micrographs obtained by
SEM observations in measuring the volume fraction of martensite
described above. Specifically, the equivalent circular diameters
were calculated by identifying the prior austenite grains in the
micrographs, and the result of averaging the equivalent circular
diameters was used as the average grain size of prior austenite
grains. In the image interpretation, Image-Pro available from Media
Cybernetics was used.
[0143] Solute C Content
[0144] The solute C content was determined by subtracting the C
content precipitated as carbides (C.sub.p) from the total C content
in steel (C.sub.total).
[0145] C.sub.total
[0146] As the total C content in steel (C.sub.total), the C content
of each steel sheet listed in Table 1 (mass %) was used.
[0147] C.sub.p
[0148] The carbides at first include cementite (M.sub.3C). In
addition, when Nb, Ti, and/or V are contained, their carbides (NbC,
TiC, and/or VC) are precipitated. Therefore, the amount of C
precipitated as carbides (C.sub.p) can be obtained as the sum of
the amount of C precipitated as cementite (C.sub.p1) and the amount
of C precipitated as NbC, TiC, and/or VC (C.sub.p2).
[0149] C.sub.p1
[0150] The amount of C precipitated as cementite was determined by
the combined use of analysis by transmission electron
microscope-energy dispersive X-ray spectroscopy (TEM-EDX) and
ICP-emission analysis of the extracted residue obtained by
electrolytic extraction. First, from each of the obtained
hot-pressed steel sheet members, a TEM observation sample to be
measured was prepared, and the concentrations of the metallic
elements constituting the cementite were measured using EDX
analysis. Here, the metallic elements constituting the cementite
are Fe, Cr, and Mn. As the concentration, the result of averaging
the measurements at 10 locations was used. From the obtained
concentrations, the atomic ratios of Fe, Mn, and Cr represented as
F.sub.Fe, F.sub.Cr, and F.sub.Mn, respectively, were determined,
where F.sub.Fe+F.sub.Cr+F.sub.Mn=1.
[0151] Then, an extracted residue was obtained from each of the
hot-pressed steel sheet members using electrolytic extraction. As
an electrolyte solution, 10% acetylacetone-based electrolyte
solution was used. The resulting extracted residue was analyzed by
radio frequency inductively coupled plasma (ICP) optical emission
spectrometry to determine the amount of Fe precipitated as
cementite in the steel, C.sub.Fe (mass %).
[0152] Using the values obtained in the above measurements, the
amount of C precipitated as cementite (C.sub.p1) was calculated
by:
C.sub.p1(in mass
%)=12/(M.times.3).times.C.sub.Fe.times.1/(F.sub.Fe),
where
M=(56.times.F.sub.Fe+52.times.F.sub.Cr+54.times.F.sub.Mn).
[0153] C.sub.p2
[0154] The amount of C precipitated as NbC, TiC, and/or VC,
(C.sub.p2), was determined as follows. First, an extracted residue
was obtained from each of the hot-pressed steel sheet members using
electrolytic extraction. As the electrolyte solution, a 10%
acetylacetone-based electrolyte solution was used. The resulting
extraction residue was analyzed by radio frequency inductively
coupled plasma (ICP) optical emission spectrometry, and the content
of Nb, Ti, and/or V were measured. The measured amounts of Nb, Ti,
and/or V are the amounts of the metal elements precipitated as NbC,
TiC, and/or VC. Therefore, from the measured values, the amounts of
C precipitated as NbC, TiC, and/or VC, (C.sub.p2), were
calculated.
[0155] From C.sub.p1 and C.sub.p1 obtained as described above, the
solute C content was calculated by:
[solute C content (in mass %)]=C.sub.total-(C.sub.p1+C.sub.p2).
Tensile Property
[0156] A JIS No. 5 tensile test piece was taken from a position at
the hat bottom of each of the obtained hot-pressed steel sheet
members, and subjected to tensile test in accordance with JIS Z
2241 to measure the yield strength (YS) and the tensile strength
(TS).
Bending Collapsibility
[0157] Each of the obtained hot-pressed steel sheet members was
subjected to a three-point bending deformation to measure the
strokes and loads. In the three-point bending deformation, a mold
with 280 mm span and punch 100R was used and the test speed was set
to 0.1 m/s. The bending collapsibility was judged "good" when the
bottom dead center was reached without cracking even when the
maximum load was exceeded, or "poor" when cracking occurred.
[0158] The measured steel sheet microstructure, tensile property,
and bending collapsibility are listed in Table 3. Note that the
solute C content was represented as a ratio (percentage) with
respect to the total C content (C.sub.total). As can be seen from
this result, each hot-pressed steel sheet member satisfying the
conditions of the present disclosure had a tensile strength of 1780
MPa or more and excellent bending collapsibility.
TABLE-US-00001 TABLE 1 Steel Chemical composition (mass %) * ID C
Si Mn Nb P S Al N Other components C/Nb Remarks A 0.34 0.34 2.13
0.013 0.01 0.001 0.03 0.002 -- 26 Conforming steel B 0.35 0.22 1.55
0.010 0.01 0.001 0.03 0.002 Ti: 0.02, B: 0.0021, Cr: 0.20 35
Conforming steel C 0.46 1.87 0.62 0.005 0.01 0.001 0.02 0.003 Mo:
0.24, Cu: 0.15, Sn: 0.08, Zr: 0.03 92 Conforming steel D 0.31 0.05
3.12 0.008 0.02 0.001 0.02 0.002 V: 0.02, Co: 0.05, Zn: 0.03, W:
0.02 39 Conforming steel E 0.44 0.22 2.40 0.019 0.01 0.002 0.88
0.002 Ni: 0.21, Ta: 0.03, Ca: 0.001 23 Conforming steel F 0.31 0.33
1.73 0.006 0.04 0.008 0.03 0.002 Ca: 0.001 52 Conforming steel G
0.21 0.23 1.33 0.004 0.01 0.001 0.03 0.003 -- 53 Comparative steel
H 0.55 0.34 1.88 0.015 0.01 0.001 0.03 0.002 -- 37 Comparative
steel I 0.34 0.15 1.49 0.001 0.01 0.001 0.03 0.003 -- 340
Comparative steel J 0.31 1.02 1.88 0.082 0.01 0.001 0.03 0.002 -- 4
Comparative steel * The balance is Fe and inevitable
impurities.
TABLE-US-00002 TABLE 2 Heating before hot Heat treatment pressing
Heating Holding Steel Coating or Heating temp. time No. sample ID
plating * temp. (.degree. C.) (.degree. C.) (s) Remarks 1 A EZN 900
150 1200 Example 2 B EZN 900 170 1200 Example 3 C EZN 900 160 1000
Example 4 D EZN 900 60 1200 Example 5 E EZN 900 170 3000 Example 6
F EZN 900 160 1200 Example 7 A CR 900 170 1800 Example 8 B GI 900
250 20 Example 9 C GA 900 170 1200 Example 10 D GAS 900 120 2000
Example 11 E EG 900 150 1400 Example 12 F GA 900 150 1400 Example
13 A CR 900 200 1000 Example 14 B CR 900 100 2000 Example 15 B EZN
700 150 1400 Comparative Example 16 A GA 1100 150 1400 Comparative
Example 17 B EZN 900 -- -- Comparative Example 18 B GA 900 40 1000
Comparative Example 19 B GI 900 350 1000 Comparative Example 20 B
GAS 900 170 1 Comparative Example 21 B CR 900 170 6000 Comparative
Example 22 G GAS 900 150 1400 Comparative Example 23 H GAS 900 150
1400 Comparative Example 24 I EG 900 150 1400 Comparative Example
25 J GAS 900 150 1400 Comparative Example * CR: cold-rolled steel
sheet, GI: hot-dip galvanized steel sheet, GA: galvannealed steel
sheet GAS: hot-dip aluminum coated steel sheet, EG:
electrogalvanized steel sheet, EZN: zinc-nickel electroplated steel
sheet
TABLE-US-00003 TABLE 3 Microstructure of hot-pressed steel sheet
member Volume Average grain Solute C fraction of size of prior
content/total C Tensile properties martensite austenite grains
content YS TS Bending No. (%) (.mu.m) (%) (MPa) (MPa)
collapsibility Remarks 1 100 6 12 1430 2015 good Example 2 100 5 6
1420 2033 good Example 3 99 6 13 1511 2122 good Example 4 99 6 22
1288 1991 good Example 5 99 6 8 1621 2381 good Example 6 99 5 13
1392 1845 good Example 7 98 6 8 1420 1994 good Example 8 100 7 11
1443 2041 good Example 9 100 7 8 1489 2124 good Example 10 99 6 7
1384 1899 good Example 11 99 6 8 1699 2388 good Example 12 99 7 10
1472 1931 good Example 13 99 6 8 1488 1881 good Example 14 100 7 9
1473 2013 good Example 15 62 7 10 1002 1413 good Comparative
Example 16 100 10 10 1443 1992 poor Comparative Example 17 99 7 32
1182 1913 poor Comparative Example 18 100 6 31 1177 1932 poor
Comparative Example 19 100 7 3 1221 1721 good Comparative Example
20 100 7 28 1181 1923 poor Comparative Example 21 99 6 4 1231 1688
good Comparative Example 22 99 7 9 1188 1531 good Comparative
Example 23 100 7 9 1721 2513 poor Comparative Example 24 98 10 10
1301 1833 poor Comparative Example 25 99 7 8 1288 1910 poor
Comparative Example
* * * * *