U.S. patent application number 14/382995 was filed with the patent office on 2015-03-05 for warm press forming method and automobile frame component.
The applicant listed for this patent is JFE Steel Corporation. Invention is credited to Takeshi Fujita, Toru Minote, Yoshikiyo Tamai, Yuichi Tokita.
Application Number | 20150064052 14/382995 |
Document ID | / |
Family ID | 49116317 |
Filed Date | 2015-03-05 |
United States Patent
Application |
20150064052 |
Kind Code |
A1 |
Tokita; Yuichi ; et
al. |
March 5, 2015 |
WARM PRESS FORMING METHOD AND AUTOMOBILE FRAME COMPONENT
Abstract
A method of forming a steel sheet having a tensile strength of
440 MPa or more into a press-formed part including a flange portion
and other portions by press forming includes: heating the steel
sheet to a temperature of 400.degree. C. to 700.degree. C.; and
press-forming the heated steel sheet using draw forming to obtain a
press-formed part, with the steel sheet being held at a press
bottom dead point in the die for one second to five seconds.
Geometric changes such as springback that occur in a panel can thus
be suppressed, the dimensional accuracy of the panel can be
enhanced, and the desired mechanical properties can easily be
obtained in the press-formed part.
Inventors: |
Tokita; Yuichi; (Tokyo,
JP) ; Tamai; Yoshikiyo; (Tokyo, JP) ; Minote;
Toru; (Tokyo, JP) ; Fujita; Takeshi; (Tokyo,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE Steel Corporation |
Tokyo |
|
JP |
|
|
Family ID: |
49116317 |
Appl. No.: |
14/382995 |
Filed: |
March 4, 2013 |
PCT Filed: |
March 4, 2013 |
PCT NO: |
PCT/JP2013/001316 |
371 Date: |
September 4, 2014 |
Current U.S.
Class: |
420/82 ; 420/109;
420/121; 420/122; 420/126; 420/83; 420/84; 72/364; 72/46 |
Current CPC
Class: |
C22C 38/001 20130101;
C22C 38/50 20130101; C22C 38/04 20130101; B21K 7/12 20130101; B21D
22/208 20130101; C22C 38/08 20130101; C22C 38/105 20130101; C22C
38/06 20130101; C23C 2/06 20130101; C22C 38/14 20130101; C23C 2/28
20130101; C21D 2211/005 20130101; C22C 38/005 20130101; C22C 38/44
20130101; C22C 38/12 20130101; C22C 38/28 20130101; C21D 9/46
20130101; C22C 38/20 20130101; C22C 38/00 20130101; B21D 53/88
20130101; C21D 1/673 20130101; C21D 8/0226 20130101; C22C 38/002
20130101; B21J 1/06 20130101; C22C 38/007 20130101; C22C 38/24
20130101; C21D 2211/004 20130101; C22C 38/02 20130101; C22C 38/60
20130101 |
Class at
Publication: |
420/82 ; 420/83;
420/84; 420/109; 420/121; 420/122; 420/126; 72/364; 72/46 |
International
Class: |
B21K 7/12 20060101
B21K007/12; C22C 38/50 20060101 C22C038/50; C22C 38/44 20060101
C22C038/44; C22C 38/28 20060101 C22C038/28; C22C 38/24 20060101
C22C038/24; C22C 38/20 20060101 C22C038/20; C22C 38/14 20060101
C22C038/14; C22C 38/12 20060101 C22C038/12; C22C 38/10 20060101
C22C038/10; C22C 38/08 20060101 C22C038/08; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00; B21J 1/06 20060101
B21J001/06; C22C 38/60 20060101 C22C038/60 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 6, 2012 |
JP |
2012-048725 |
Claims
1-12. (canceled)
13. A warm press forming method of forming a steel sheet having a
tensile strength of 440 MPa or more into a press-formed part
including flange portions and other portions by press forming, the
method comprising: heating the steel sheet to a temperature of
400.degree. C. to 700.degree. C.; and press-forming the heated
steel sheet using draw forming to obtain a press-formed part, with
the steel sheet being held at a press bottom dead point in a die
for one second to five seconds.
14. The method according to claim 13, wherein a difference in
average temperature among flange portions and other portions of the
press-formed part immediately after draw forming is kept within
150.degree. C.
15. The method according to claim 13, wherein the press-formed part
has a tensile strength of 80% to 110% of a tensile strength of the
steel sheet.
16. The method according to claim 13, wherein the steel sheet has a
chemical composition containing, by mass %, C: 0.015% to 0.16%, Si:
0.2% or less, Mn: 1.8% or less, P: 0.035% or less, S: 0.01% or
less, Al: 0.1% or less, N: 0.01% or less, and Ti: 0.13% to 0.25%,
provided that a relation defined by Expression (1) is satisfied,
and the balance including Fe and incidental impurities, and wherein
the steel sheet has a microstructure containing a ferrite phase by
95% or more on an area ratio basis with respect to the entire
microstructure, ferrite crystal grains constituting the ferrite
phase have an average grain size of 1 .mu.m or more, and carbides
having an average particle size of 10 nm or less are dispersed and
precipitated in the ferrite crystal grains 2.00.gtoreq.([%
C]/12)/([% Ti]/48).gtoreq.1.05 (1) where [% M] indicates the
content by mass % of element M.
17. The method according to claim 14, wherein the press-formed part
has a tensile strength of 80% to 110% of a tensile strength of the
steel sheet.
18. The method according to claim 14, wherein the steel sheet has a
chemical composition containing, by mass %, C: 0.015% to 0.16%, Si:
0.2% or less, Mn: 1.8% or less, P: 0.035% or less, S: 0.01% or
less, Al: 0.1% or less, N: 0.01% or less, and Ti: 0.13% to 0.25%,
provided that a relation defined by Expression (1) is satisfied,
and the balance including Fe and incidental impurities, and wherein
the steel sheet has a microstructure containing a ferrite phase by
95% or more on an area ratio basis with respect to the entire
microstructure, ferrite crystal grains constituting the ferrite
phase have an average grain size of 1 .mu.m or more, and carbides
having an average particle size of 10 nm or less are dispersed and
precipitated in the ferrite crystal grains 2.00.gtoreq.([%
C]/12)/([% Ti]/48).gtoreq.1.05 (1) where [% M] indicates the
content by mass % of element M.
19. The method according to claim 17, wherein the steel sheet has a
chemical composition containing, by mass %, C: 0.015% to 0.16%, Si:
0.2% or less, Mn: 1.8% or less, P: 0.035% or less, S: 0.01% or
less, Al: 0.1% or less, N: 0.01% or less, and Ti: 0.13% to 0.25%,
provided that a relation defined by Expression (1) is satisfied,
and the balance including Fe and incidental impurities, and wherein
the steel sheet has a microstructure containing a ferrite phase by
95% or more on an area ratio basis with respect to the entire
microstructure, ferrite crystal grains constituting the ferrite
phase have an average grain size of 1 .mu.m or more, and carbides
having an average particle size of 10 nm or less are dispersed and
precipitated in the ferrite crystal grains 2.00.gtoreq.([%
C]/12)/([% Ti]/48).gtoreq.1.05 (1) where [% M] indicates the
content by mass % of element M.
20. The method according to claim 19, wherein the steel sheet
comprises a coating or plating layer on a surface thereof.
21. The method according to claim 20, wherein the chemical
composition further contains at least one group selected from (A)
to (F), wherein (A) by mass %, at least one selected from V: 1.0%
or less, Mo: 0.5% or less, W: 1.0% or less, Nb: 0.1% or less, Zr:
0.1% or less, and Hf: 0.1% or less, provided that a relation
defined by Expression (1)' is satisfied: 2.00.gtoreq.([% C]/12)/([%
Ti]/48+[% V]/51+[% W]/184+[% Mo]/96+[% Nb]/93+[% Zr]/91+[%
Hf]/179).gtoreq.1.05 (1)' where [% M] indicates the content by mass
% of element M, (B) by mass %, B: 0.003% or less, (C) by mass %, at
least one selected from Mg: 0.2% or less, Ca: 0.2% or less, Y: 0.2%
or less, and REM: 0.2% or less, (D) by mass %, at least one
selected from Sb: 0.1% or less, Cu: 0.5% or less, and Sn: 0.1% or
less, (E) by mass %, at least one selected from Ni: 0.5% or less
and Cr: 0.5% or less, (F) by mass %, at least one selected from O,
Se, Te, Po, As, Bi, Ge, Pb, Ga, In, Tl, Zn, Cd, Hg, Ag, Au, Pd, Pt,
Co, Rh, Ir, Ru, Os, Tc, Re, Ta, Be and Sr, in a total amount of
2.0% or less.
22. An automobile frame component produced by the method according
to claim 21.
23. The method according to claim 15, wherein the steel sheet has a
chemical composition containing, by mass %, C: 0.015% to 0.16%, Si:
0.2% or less, Mn: 1.8% or less, P: 0.035% or less, S: 0.01% or
less, Al: 0.1% or less, N: 0.01% or less, and Ti: 0.13% to 0.25%,
provided that a relation defined by Expression (1) is satisfied,
and the balance including Fe and incidental impurities, and wherein
the steel sheet has a microstructure containing a ferrite phase by
95% or more on an area ratio basis with respect to the entire
microstructure, ferrite crystal grains constituting the ferrite
phase have an average grain size of 1 .mu.m or more, and carbides
having an average particle size of 10 nm or less are dispersed and
precipitated in the ferrite crystal grains 2.00.gtoreq.([%
C]/12)/([% Ti]/48).gtoreq.1.05 (1) where [% M] indicates the
content by mass % of element M.
24. The method according to claim 23, wherein the chemical
composition further contains at least one group selected from (A)
to (F), wherein (A) by mass %, at least one selected from V: 1.0%
or less, Mo: 0.5% or less, W: 1.0% or less, Nb: 0.1% or less, Zr:
0.1% or less, and Hf: 0.1% or less, provided that a relation
defined by Expression (1)' is satisfied: 2.00.gtoreq.([% C]/12)/([%
Ti]/48+[% V]/51+[% W]/184+[% Mo]/96+[% Nb]/93+[% Zr]/91+[%
Hf]/179).gtoreq.1.05 (1)' where [% M] indicates the content by mass
% of element M, (B) by mass %, B: 0.003% or less, (C) by mass %, at
least one selected from Mg: 0.2% or less, Ca: 0.2% or less, Y: 0.2%
or less, and REM: 0.2% or less, (D) by mass %, at least one
selected from Sb: 0.1% or less, Cu: 0.5% or less, and Sn: 0.1% or
less, (E) by mass %, at least one selected from Ni: 0.5% or less
and Cr: 0.5% or less, (F) by mass %, at least one selected from O,
Se, Te, Po, As, Bi, Ge, Pb, Ga, In, Tl, Zn, Cd, Hg, Ag, Au, Pd, Pt,
Co, Rh, Ir, Ru, Os, Tc, Re, Ta, Be and Sr, in a total amount of
2.0% or less.
25. The method according to claim 23, wherein the steel sheet
comprises a coating or plating layer on a surface thereof.
26. An automobile frame component produced by the method according
to claim 23.
27. The method according to claim 24, wherein the steel sheet
comprises a coating or plating layer on a surface thereof.
28. An automobile frame component produced by the method according
to claim 24.
29. An automobile frame component produced by the method according
to claim 27.
30. The method according to claim 16, wherein the steel sheet
comprises a coating or plating layer on a surface thereof.
31. The warm press forming method according to claim 30, wherein
the chemical composition further contains at least one group
selected from (A) to (F), wherein (A) by mass %, at least one
selected from V: 1.0% or less, Mo: 0.5% or less, W: 1.0% or less,
Nb: 0.1% or less, Zr: 0.1% or less, and Hf: 0.1% or less, provided
that a relation defined by Expression (1)' is satisfied:
2.00.gtoreq.([% C]/12)/([% Ti]/48+[% V]/51+[% W]/184+[% Mo]/96+[%
Nb]/93+[% Zr]/91+[% Hf]/179).gtoreq.1.05 (1)' where [% M] indicates
the content by mass % of element M, (B) by mass %, B: 0.003% or
less, (C) by mass %, at least one selected from Mg: 0.2% or less,
Ca: 0.2% or less, Y: 0.2% or less, and REM: 0.2% or less, (D) by
mass %, at least one selected from Sb: 0.1% or less, Cu: 0.5% or
less, and Sn: 0.1% or less, (E) by mass %, at least one selected
from Ni: 0.5% or less and Cr: 0.5% or less, (F) by mass %, at least
one selected from O, Se, Te, Po, As, Bi, Ge, Pb, Ga, In, Tl, Zn,
Cd, Hg, Ag, Au, Pd, Pt, Co, Rh, Ir, Ru, Os, Tc, Re, Ta, Be and Sr,
in a total amount of 2.0% or less.
32. An automobile frame component produced by the method according
to claim 31.
Description
TECHNICAL FIELD
[0001] This disclosure relates to a warm press forming method that
can suppress defects in dimensional accuracy due to geometric
changes such as springback that occur in a high strength steel
sheet being press-formed.
[0002] The disclosure also relates to an automobile frame component
produced by the warm press forming method.
BACKGROUND
[0003] To achieve a reduction in the weight of automobile bodies to
improve fuel efficiency and crash safety of automobiles to protect
occupants, high strength steel sheets have been increasingly
applied to automotive parts. It is generally known, however, that
high strength steel sheets exhibit poor press formability, undergo
considerable geometric changes (springback) caused by elastic
recovery after being removed from the die, and are prone to defects
in dimensional accuracy. Thus, there are currently a limited number
of parts that can be obtained by applying press forming to high
strength steel sheets.
[0004] Therefore, to improve press formability and shape fixability
(to reduce springback), JP 2005-205416 A discloses an example of
hot press forming applied to a high strength steel sheet in which a
steel sheet is press-formed after being heated to a predetermined
temperature.
[0005] The aforementioned hot press forming involves forming of a
steel sheet at temperatures higher than those at which cold press
forming is performed to reduce the deformation resistance of the
steel sheet for press forming, in other words, to increase the
deformation capacity thereof, aiming to improve the shape
fixability and at the same time prevent the occurrence of press
cracking.
[0006] With the hot press forming disclosed in JP 2005-205416 A,
press forming is based on draw forming. In draw forming, edges of
the heated steel sheet (which will be also called a "blank") are
compressed between a die and a blank holder during the formation
process and, accordingly, the edges of the blank and other portions
thereof contact with, e.g., the die for different times. In
addition, a drop in the temperature of the contact zone of the
blank during the press forming process leads to a non-uniform
temperature distribution in the press-formed part immediately after
the formation (hereinafter also called "panel") due to the
difference in the contact time with the aforementioned die, and so
on.
[0007] This results in a problem that panels, in particular,
automobile frame components to which high strength steel sheets are
applied, undergo geometric changes during the air cooling process
after the hot press forming, which prevents the provision of panels
with sufficiently satisfactory dimensional accuracy.
[0008] In addition, general hot press forming involves heating of a
steel sheet to the austenite region as well as cooling of the steel
sheet accompanying quenching and phase transformation and,
consequently, the microstructure of the steel sheet tends to change
after formation, causing the problem of large variations in the
tensile properties such as strength and ductility, of the
press-formed part.
[0009] It could therefore be helpful to provide a warm press
forming method that can suppress geometric changes such as
springback that occur in panels, thereby improving the dimensional
accuracy of the panels and obtaining the desired mechanical
properties in the resulting press-formed parts.
[0010] It could also be helpful to provide an automobile frame
component produced by the warm press forming method.
SUMMARY
[0011] When a high strength steel sheet is applied, we tried to
limit the heating temperature of the high strength steel sheet,
which would otherwise need to be heated to the austenite region
with conventional hot press forming, below the austenite
transformation temperature.
[0012] We also studied forming methods and forming conditions to
determine the conditions under which geometric changes caused by
springback can be suppressed.
[0013] As a result, we discovered that when forming a high strength
steel sheet into a press-formed part including flange portions and
other portions by press forming, the intended results can be
achieved advantageously by
[0014] (1) heating a steel sheet to a so-called warm-forming
temperature range; and
[0015] (2) then press-forming the heated steel sheet using draw
forming to obtain a press-formed part, with the steel sheet being
held at a press bottom dead point over a certain period of
time.
[0016] We thus provide:
[0017] [1] A warm press forming method for forming a steel sheet
having a tensile strength of 440 MPa or more into a press-formed
part including flange portions and other portions by press forming,
the method comprising:
[0018] heating the steel sheet to a temperature range of
400.degree. C. to 700.degree. C.; and
[0019] then press-forming the heated steel sheet using draw forming
to obtain a press-formed part, with the steel sheet being held at a
press bottom dead point in the die for one second to five
seconds.
[0020] [2] The warm press forming method according to the aspect
[1], wherein a difference in average temperature among flange
portions and other portions of the press-formed part immediately
after the draw forming is kept within 150.degree. C.
[0021] [3] The warm press forming method according to the aspect
[1] or [2], wherein the press-formed part has a tensile strength of
80% to 110% of a tensile strength of the steel sheet.
[0022] [4] The warm press forming method according to any one of
the aspects [1] to [3], wherein the steel sheet has a chemical
composition containing, by mass %,
[0023] C: 0.015% to 0.16%,
[0024] Si: 0.2% or less,
[0025] Mn: 1.8% or less,
[0026] P: 0.035% or less,
[0027] S: 0.01% or less,
[0028] Al: 0.1% or less,
[0029] N: 0.01% or less, and
[0030] Ti: 0.13% to 0.25%,
provided that a relation defined by Expression (1) below is
satisfied, and
[0031] the balance including Fe and incidental impurities, and
[0032] wherein the steel sheet has a microstructure containing a
ferrite phase by 95% or more on an area ratio basis with respect to
the entire microstructure, ferrite crystal grains constituting the
ferrite phase have an average grain size of 1 .mu.M or more, and
carbides having an average particle size of 10 nm or less are
dispersed and precipitated in the ferrite crystal grains
2.00.gtoreq.([% C]/12)/([% Ti]/48).gtoreq.1.05 (1)
where [% M] indicates the content by mass % of element M.
[0033] [5] The warm press forming method according to the aspect
[4], wherein the chemical composition further contains, by mass %,
at least one selected from
[0034] V: 1.0% or less,
[0035] Mo: 0.5% or less,
[0036] W: 1.0% or less,
[0037] Nb: 0.1% or less,
[0038] Zr: 0.1% or less, and
[0039] Hf: 0.1% or less,
provided that a relation defined by Expression (1)' is
satisfied:
2.00.gtoreq.([% C]/12)/([% Ti]/48+[% V]/51+[% W]/184+[% Mo]/96+[%
Nb]/93+[% Zr]/91+[% Hf]/179).gtoreq.1.05 (1)'
where [% M] indicates the content by mass % of element M.
[0040] [6] The warm press forming method according to the aspect
[4] or [5], wherein the chemical composition further contains, by
mass %, B: 0.003% or less.
[0041] [7] The warm press forming method according to any one of
the aspects [4] to [6], wherein the chemical composition further
contains, by mass %, at least one selected from Mg: 0.2% or less,
Ca: 0.2% or less, Y: 0.2% or less, and REM: 0.2% or less.
[0042] [8] The warm press forming method according to any one of
the aspects [4] to [7], wherein the chemical composition further
contains, by mass %, at least one selected from Sb: 0.1% or less,
Cu: 0.5% or less, and Sn: 0.1% or less.
[0043] [9] The warm press forming method according to any one of
the aspects [4] to [8], wherein the chemical composition further
contains, by mass %, at least one selected from Ni: 0.5% or less
and Cr: 0.5% or less.
[0044] [10] The warm press forming method according to any one of
the aspects [4] to [9], wherein the chemical composition further
contains, by mass %, at least one selected from O, Se, Te, Po, As,
Bi, Ge, Pb, Ga, In, Tl, Zn, Cd, Hg, Ag, Au, Pd, Pt, Co, Rh, Ir, Ru,
Os, Tc, Re, Ta, Be and Sr, in a total amount of 2.0% or less.
[0045] [11] The warm press forming method according to any one of
the aspects [1] to [10], wherein the steel sheet comprises a
coating or plating layer on a surface thereof.
[0046] [12] An automobile frame component produced by the warm
press forming method according to any one of the aspects [1] to
[11].
[0047] It is possible to suppress geometric changes made to a panel
being air-cooled after the press forming process, allowing
manufacture of automobile frame components having good dimensional
accuracy. Consequently, high strength steel sheets, which could not
conventionally be applied to automobile frame components due to
defects in dimensional accuracy, can be applied thereto to allow a
reduction in weight of automotive body, which may greatly
contribute to solving environmental issues.
[0048] In addition, the warm press forming does not involve
quenching and/or phase transformation before and after the forming
process and can directly make use of the mechanical properties of
steel sheets as blank material, thereby allowing for stable
production of press-formed parts with desired properties.
BRIEF DESCRIPTION OF THE DRAWINGS
[0049] Our methods and components will be further described below
with reference to the accompanying drawings, wherein:
[0050] FIGS. 1a-1c illustrate a press forming process using draw
forming, where (a) shows a state when the forming process starts,
(b) shows a state during the forming process, and (c) shows a state
at the press bottom dead point (a state when the forming process
ends).
[0051] FIG. 2(a) illustrates an exemplary automobile frame
component produced from a panel obtained by press forming.
[0052] FIG. 2(b) illustrates flange portions of a panel obtained by
press forming using draw forming.
[0053] FIG. 3(a) is a graph showing the relationship between the
difference in average temperature among flange portions and other
portions of a panel obtained by warm press forming using draw
forming and the amount of geometric changes made to the panel from
the time immediately after press forming (the time when the panel
was removed from the die) until the end of air cooling.
[0054] FIG. 3(b) is a diagram explaining the amount of geometric
changes made to the panel from the time immediately after press
forming (the time when the panel was removed from the die) until
the end of air cooling.
[0055] FIG. 4(a) is a graph showing the relationship between the
difference in average temperature among flange portions and other
portions of panels, each being obtained by warm press forming using
draw forming, and the holding time at press bottom dead point.
[0056] FIG. 4(b) is a graph showing the relationship between the
amount of geometric changes made to the panels from the time
immediately after warm press forming using draw forming (the time
when the panels were removed from the die) until the end of air
cooling and the holding time at press bottom dead point.
[0057] FIG. 5(a) schematically illustrates a center pillar upper
press panel.
[0058] FIG. 5(b) is a diagram explaining the amount of geometric
changes made to the panel from the time immediately after press
forming (the time when the panel was removed from the die) until
the end of air cooling.
REFERENCE SIGNS LIST
[0059] 1 Die [0060] 2 Punch [0061] 3 Blank holder [0062] 4 Heated
steel sheet (blank) [0063] 5 Press-formed part (panel) [0064] 6
Flange portion [0065] 7 Sidewall portion [0066] 8 Reference panel
(panel at the time of being removed from the die immediately after
press forming) [0067] 9 Air-cooled panel [0068] 10 Panel at press
bottom dead point [0069] 11 Center pillar upper press panel
DETAILED DESCRIPTION
[0070] Our methods and components will be described in detail
below.
[0071] First, the reasons for heating a steel sheet to temperatures
of 400.degree. C. to 700.degree. C. prior to press forming will be
described below.
Heating Temperature of Steel Sheet: 400.degree. C. to 700.degree.
C.
[0072] When a steel sheet is heated to temperatures of 400.degree.
C. or higher, the strength is reduced and the ductility increases.
This may facilitate deformation of the steel sheet in conformity
with the die during press forming, thereby preventing the
occurrence of press cracking and suppressing the formation of
wrinkles. If the heating temperature of the steel sheet exceeds
700.degree. C., however, the material strength is reduced so much
as to incur the risk of cracking, fracture, and the like.
Therefore, the heating temperature of the steel sheet is
400.degree. C. to 700.degree. C. In particular, when the heating
temperature of the steel sheet is 400.degree. C. or higher and
lower than 650.degree. C., it is possible to suppress oxidation of
surfaces of the steel sheet and/or formation of cracks and,
furthermore, to prevent an excessive increase in press load, which
is still more advantageous.
[0073] Next, the reasons for holding a steel sheet at a press
bottom dead point in the die for one second to five seconds prior
to a press forming process using draw forming will be described
below.
[0074] For a panel requiring high sidewall portions, press forming
is usually performed using draw forming. In performing the draw
forming, even a warm (or hot) press forming process is generally
carried out by a blank holder arranged as shown in FIG. 1 to
suppress wrinkles that would occur during the forming process,
while applying tension to sidewall portions with edges of the blank
being compressed between the blank holder and the upper die.
[0075] In FIG. 1, a die is labeled 1, a punch is labeled 2, a blank
holder is labeled 3, a heated steel sheet (blank) is labeled 4, a
press-formed part (panel) after the formation is labeled 5, flange
portions are labeled 6, and sidewall portions are labeled 7.
[0076] As shown in FIG. 2(a), for example, an automobile frame
component is often worked to form a closed cross section by joining
members having a substantially hat-shaped cross section by spot
welding and the like. In this case, the edges of the blank
compressed as shown in FIG. 2(b) provide flange portions of the
panel after the formation. The flange portions are required to be
flat since they are points at which panels are joined together by
spot welding and the like. This is the reason why the formation is
performed while applying blank holding force to edges of the blank
as mentioned above.
[0077] In the case of the aforementioned draw forming, the edges of
the blank are continuously compressed between the blank holder and
the upper die from the early stage of the forming process until the
completion of the process. Consequently, the heated steel sheet
(blank) is subject to a heat transfer from edges of the blank to
the die during the press forming process, with the result that the
edges of the blank are susceptible to a temperature drop, leading
to a large difference in temperature among flange portions and
other portions of the panel immediately after the formation.
[0078] Such a difference in temperature in the panel results in
different rates of thermal contraction at different points in the
panel in the course of cooling to room temperature and,
consequently, causes residual stress in the panel, which in turn is
subject to geometric changes to release the stress. We have
identified this mechanism as the major cause of geometric changes
that would occur during the cooling process.
[0079] Then, we focused on and investigated the relationship, in
the case of a press forming process using draw forming, between the
difference in average temperature among flange portions and other
portions of a panel and the amount of geometric changes made to the
panel from the time immediately after press forming until the end
of air cooling.
[0080] As used herein, the term "difference in average temperature"
means a difference in average temperature immediately after press
forming, unless otherwise specified. As used herein, the phrase
"immediately after press forming" refers to a point in time that
represents the end of a holding process at a press bottom dead
point and the start of air cooling of a panel after being removed
from the die. In addition, the term "the amount of geometric
changes" means a difference (variation) between the geometry of a
panel after removal from the die immediately after warm press
forming and the geometry of the panel after air cooling.
[0081] Further, FIG. 3(a) is a graph showing the relationship
between the difference in average temperature among flange portions
and other portions of a panel obtained by warm press forming using
draw forming and the amount of geometric changes made to the panel
from the time immediately after press forming (the time when the
panel was removed from the die) until the end of air cooling. In
this case, a steel sheet of 980 MPa grade was used and the heating
temperature thereof was set to be 600.degree. C. In addition, the
aforementioned amount of geometric changes was determined by an
opening amount a, which was measured at the edges of the flanges in
relation to a reference panel (a panel removed from the die
immediately after press forming), as shown in FIG. 3(b). In the
figure, a reference panel is labeled 8 (dashed line), an air-cooled
panel is labeled 9 (thick solid line), and a panel at the press
bottom dead point is labeled 10 (thin solid line).
[0082] It can be seen from FIG. 3(a) that the larger the
aforementioned difference in average temperature in a panel, the
larger the amount of geometric changes made to the panel from the
time it is removed from the die immediately after press forming
until the end of air cooling. In particular, the amount of
geometric changes becomes greater than 1.0 mm where the difference
in average temperature exceeds 150.degree. C., it is important that
the difference in average temperature be kept within 150.degree.
C., preferably within 100.degree. C., to reduce the amount of
geometric changes caused by the temperature difference in the
panel.
[0083] From the results of the aforementioned investigation, we
found that the difference in average temperature among flange
portions and other portions of a panel closely correlates to the
amount of geometric changes made to the panel from the time it is
removed from the die immediately after press forming until the end
of air cooling. Based on this finding, we studied how to suppress
the aforementioned difference in average temperature during draw
forming. As a result, we held a steel sheet at the press bottom
dead point as shown in FIG. 1(c) over a certain period of time.
[0084] The mechanism by which the aforementioned difference in
average temperature can be suppressed by holding a steel sheet at
the press bottom dead point will be described below.
[0085] That is, when a panel formed from a blank is held at the
press bottom dead point, not only the flange portions constrained
by the die and the blank holder, but also other portions than the
flange portions such as sidewall portions, are cooled by contact
with the die and the punch die. This facilitates soaking in the
panel and, therefore, suppresses the difference in average
temperature among the flange portions and the other portions.
[0086] FIG. 4(a) shows the relationship between the difference in
average temperature among flange portions and other portions of
those panels having a substantially hat-shaped cross section that
were obtained warm press forming using draw forming and the holding
time at the press bottom dead point; and FIG. 4(b) shows the
relationship between the amount of geometric changes made to the
panels from the time they were removed from the die immediately
after press forming until the end of air cooling and the holding
time at the press bottom dead point. In this case, steel sheets of
980 MPa grade were used and the heating temperatures thereof were
600.degree. C., 650.degree. C., and 700.degree. C.,
respectively.
[0087] It can be seen from FIGS. 4(a) and 4(b) that for the heating
temperature of 600.degree. C., the difference in average
temperature among flange portions and other portions of the panel
may be kept within 150.degree. C. and the amount of geometric
changes made to the panel may be suppressed to 1.0 mm or less, by
setting the holding time at press bottom dead point to one second
or more.
[0088] It can also be understood that even for the heating
temperatures of 650.degree. C. and 700.degree. C., the difference
in average temperature among flange portions and other portions of
each panel may be kept within 150.degree. C. and the amount of
geometric changes made to each panel may be suppressed to 1.0 mm or
less, by setting the holding time at press bottom dead point to
three seconds or more.
[0089] However, a holding time at press bottom dead point exceeding
five seconds is disadvantageous in terms of production efficiency,
although the amount of geometric changes is kept substantially
constant for any of the heating temperatures.
[0090] In view of the above, a steel sheet is held at the press
bottom dead point for one second to five seconds, and preferably
three to five seconds, at the time of press forming using draw
forming.
[0091] As described above, to suppress the difference in average
temperature in steel sheets of any tensile strength grade within
150.degree. C., it suffices to set the heating temperature of the
steel sheet to 400.degree. C. to 700.degree. C. and the holding
time at press bottom dead point to three seconds or more. In this
case, while no particular limitation is placed on the draw forming
conditions, the pressing speed is preferably 10 spm to 15 spm
(strokes per minute, which represents the number of parts that can
be formed in one minute plus any additional time, if applicable,
taken to hold parts at the press bottom dead point).
[0092] In addition, with draw forming, flange portions are
continuously compressed during formation, which provides the
benefit of making the flange portions less prone to wrinkle
formation. Further, we hold the steel sheet at the press bottom
dead point as described above, which makes it possible to suppress
wrinkle formation in flange portions in a more effective
manner.
[0093] It is assumed that heating the steel sheet has the same
effect irrespective of the heating method used such as heating in
an electric furnace, electrical heating, and rapid heating using
far infrared heating.
[0094] In addition, as mentioned earlier, the warm press forming
method is applied to a steel sheet having a tensile strength of 440
MPa or more. Further, the warm press forming method may preferably
be applied to a steel sheet having a tensile strength of 780 MPa or
more, and even 980 MPa or more.
[0095] Additionally, as mentioned earlier, the warm press forming
method makes it possible to directly make use of the mechanical
properties of steel sheets as blanks, thereby allowing a
press-formed part obtained by press forming of a steel sheet to
have a tensile strength which is not greatly different from, or 80%
to 110% of, that of the steel sheet before press forming.
[0096] Furthermore, it is possible to obtain a press-formed part
that retains, even after the press forming process, a tensile
strength which is almost as high as that of the steel sheet before
press forming (or, that has a tensile strength of 95% to 100% of
the tensile strength of the steel sheet prior to the press forming
process), depending on the forming conditions and the properties of
the steel sheet.
[0097] Therefore, depending on the properties required for
press-formed parts, the use of steel sheets having the
corresponding properties as blanks allows for stable production of
press-formed parts with desired properties.
[0098] The chemical composition range of a steel sheet that can
preferably be used as a blank will be described below. Note that
the unit "%" of each component is "mass %" unless otherwise
specified.
C: 0.015% to 0.16%
[0099] Carbon (C) is an important element in that it forms carbides
with other elements such as Ti, V, Mo, W, Nb, Zr, and Hf, which
exhibit fine particle distribution in the matrix to thereby
increase the strength of a steel sheet. In this case, to achieve a
tensile strength as high as 440 MPa or more, the content of C in
steel is preferably 0.015 or more. However, if the content of C
exceeds 0.16%, the ductility and toughness are significantly
reduced, which makes it impossible to ensure good impact absorption
ability (such as expressed by "tensile strength TS.times.total
elongation El"). Therefore, the content of C is preferably 0.015%
to 0.16%, more preferably 0.03% to 0.16%, and still more preferably
0.04% to 0.14%.
Si: 0.2% or Less
[0100] Silicon (Si) is a solid-solution-strengthening element that
suppresses the reduction of strength in a high temperature range
and, consequently, adversely affects formability in a warm-forming
temperature range (warm formability). Therefore, the content of Si
in steel is preferably kept as low as possible in the present
invention, but a Si content of up to 0.2% is tolerable. In view of
this, the content of Si is preferably 0.2% or less, more preferably
0.1% or less, and still more preferably 0.06% or less. Note that
the content of Si may be reduced to impurity level.
Mn: 1.8% or Less
[0101] Manganese (Mn) is also a solid-solution-strengthening
element, like Si, that suppresses the reduction of strength in a
high temperature range and, consequently, adversely affects the
formability in a warm-forming temperature range (warm formability).
Therefore, the content of Mn in steel is preferably kept as low as
possible in the present invention, but a Mn content of up to 1.8%
is tolerable. In view of this, the content of Mn is preferably 1.8%
or less, more preferably 1.3% or less, and still more preferably
1.1% or less. Note that if the content of Mn is too low, the
austenite (.gamma.) to ferrite (.alpha.) transformation temperature
may rise excessively, which could lead to coarsening of carbides.
Therefore, the content of Mn is preferably 0.5% or more.
P: 0.035% or Less
[0102] Phosphorus (P) is an element that has a very high,
solid-solution-strengthening ability, suppresses the reduction of
strength in a high temperature range, and consequently adversely
affects formability in a warm-forming temperature range (warm
formability). Additionally, P exists in a segregated manner at
grain boundaries, thereby lowering the ductility during and after
warm forming. In view of this, the content of P in steel is
preferably kept as low as possible, but a P content of up to 0.035%
is tolerable. Accordingly, the content of P is preferably 0.035% or
less, more preferably 0.03% or less, and still more preferably
0.02% or less.
S: 0.01% or Less
[0103] Sulfur (S) is an element that exists as inclusion in steel.
S reduces the strength of the steel sheet when bonded to Ti, while
forming sulfides when bonded to Mn, leading to a reduction of the
ductility of the steel sheet at room temperature, under warm
condition, and the like. Therefore, the content of S is preferably
kept as low as possible, but a S content of up to 0.01% is
tolerable. Accordingly, the content of S is preferably 0.01% or
less, more preferably 0.005% or less, and still more preferably
0.004% or less.
Al: 0.1% or Less
[0104] Aluminum (Al) is an element that acts as a deoxidizer. To
obtain this effect, it is desirable that Al is contained in steel
by 0.02% or more. However, if the content of Al exceeds 0.1%, more
oxide-based inclusions form, significantly reducing the ductility
under warm condition. Therefore, the content of Al is preferably
0.1% or less, and more preferably 0.07% or less.
N: 0.01% or Less
[0105] Nitrogen (N) is an element that forms coarse nitrides when
bonded to Ti, Nb, and the like at the steelmaking stage.
Accordingly, the strength of the steel sheet significantly
decreases if it contains a large amount of N. In view of this, the
content of N is preferably kept as low as possible, but a N content
of up to 0.01% is tolerable. Therefore, the content of N is
preferably 0.01% or less, and more preferably 0.007% or less.
Ti: 0.13% to 0.25%
[0106] Titanium (Ti) is an element that forms carbides when bonded
to C and thereby contributes to increased strength of the steel
sheet. To ensure that the steel sheet has a tensile strength as
high as 440 MPa or more at room temperature, as targeted by the
present invention, the content of Ti is preferably 0.13% or more.
On the other hand, if the content of Ti exceeds 0.25%, coarse TiC
particles remain and micro voids form during heating of the steel
material. Therefore, the content of Ti is preferably 0.25% or less,
more preferably 0.14% to 0.22%, and still more preferably 0.15% to
0.22%.
[0107] In the foregoing, the preferred composition ranges of the
components have been described. However, it does not suffice for
the components to only satisfy the aforementioned ranges, and it is
also important for C and Ti, in particular, to satisfy Expression
(1):
2.00.gtoreq.([% C]/12)/([% Ti]/48).gtoreq.1.05 (1)
where [% M] indicates the content by mass % of element M.
[0108] That is, Expression (1) is a requirement to enable the
strengthening by precipitation with carbides, which will be
described later, and to ensure a high strength as desired after
warm forming. When the contents of C and Ti satisfy Expression (1),
it is possible to allow precipitation of a desired amount of
carbides, thereby ensuring a high strength as desired.
[0109] In addition, if the result of ([% C]/12)/([% Ti]/48) is less
than 1.05, not only does the grain boundary strength decrease, but
also the carbides exhibit lower thermal stability upon heating.
Accordingly, the carbides are more prone to coarsening, which makes
it impossible to achieve a high strength as desired. On the other
hand, if the result of ([% C]/12)/([% Ti]/48) exceeds 2.00,
cementite precipitates excessively. This results in formation of
micro voids, and consequently cause cracks during warm forming.
Note that the result of ([% C]/12)/([% Ti]/48) is more preferably
1.05 to 1.85.
[0110] In addition to the aforementioned basic components, the
steel sheet that can preferably be used in the warm press forming
method may optionally contain the following elements as
appropriate.
At Least One Selected from V: 1.0% or Less, Mo: 0.5% or Less, W:
1.0% or Less, Nb: 0.1% or Less, Zr: 0.1% or Less, and Hf: 0.1% or
Less
[0111] Vanadium (V), molybdenum (Mo), tungsten (W), niobium (Nb),
zirconium (Zr), and hafnium (Hf) are elements, like Ti, that form
carbides to contribute to increasing the strength of the steel
sheet. Therefore, the steel sheet may contain at least one element
in addition to Ti, selected from V, Mo, W, Nb, Zr, and Hf, if a
further enhancement of its strength is required. To obtain this
effect, it is preferred that the content of V is 0.01% or more, the
content of Mo is 0.01% or more, the content of W is 0.01% or more,
the content of Nb is 0.01% or more, the content of Zr is 0.01% or
more, and the content of Hf is 0.01% or more.
[0112] On the other hand, if the content of V exceeds 1.0%,
carbides are more prone to coarsening; in particular, coarsening of
carbides in a warm-forming temperature range makes it difficult to
control the average particle size of the carbides after being
cooled to room temperature to be 10 nm or less. Accordingly, the
content of V is preferably 1.0% or less, more preferably 0.5% or
less, and still more preferably 0.2% or less.
[0113] In addition, if the contents of Mo and W are more than 0.5%
and 1.0%, respectively, the .gamma.-to-.alpha. transformation is
exceedingly delayed. As a result, bainite phase and martensite
phase exist in a mixed manner in the microstructure of the steel
sheet, which makes it difficult to obtain ferrite single phase,
which will be described later. In view of this, the contents of Mo
and W are preferably 0.5% or less and 1.0% or less,
respectively.
[0114] Additionally, if Nb, Zr, and Hf are contained in steel by
more than 0.1%, respectively, coarse carbides are not completely
dissolved and remain in slab being reheated. Consequently, micro
voids form more easily during warm forming. In view of this, the
contents of Nb, Zr, and Hf are preferably 0.1% or less,
respectively.
[0115] Note that if the above elements are also contained in steel,
the following Expression (1)', instead of Expression (1), needs to
be satisfied. The reason for this requirement is the same as stated
in conjunction with Expression (1).
2.00.gtoreq.([% C]/12)/([% Ti]/48+[% V]/51+[% W]/184+[% Mo]/96+[%
Nb]/93+[% Zr]/91+[% Hf]/179).gtoreq.1.05 (1)'
where [% M] indicates the content by mass % of element M.
[0116] Furthermore, the steel sheet that can preferably be used in
the warm press forming method may optionally contain the following
elements as appropriate. B: 0.003% or less
[0117] Boron (B) is an element that acts to inhibit nucleation of
the .gamma.-to-.alpha. transformation to lower the
.gamma.-to-.alpha. transformation point, thereby contributing to
the refinement of carbides. To obtain this effect, it is desirable
that the content of B is 0.0002% or more. However, containing over
0.003% of B does not increase this effect, but is rather
economically disadvantageous. Therefore, the content of B is
preferably 0.003% or less, and more preferably 0.002% or less.
At Least One Selected from Mg: 0.2% or Less, Ca: 0.2% or Less, Y:
0.2% or Less, and REM: 0.2% or Less
[0118] Magnesium (Mg), calcium (Ca), yttrium (Y), and REM all act
as refining inclusions, which action provides an effect of
suppressing stress concentration in the vicinity of inclusions and
the base material during warm forming, and thereby improving the
ductility. Therefore, these elements may optionally be contained in
steel. Note that the REM, which is an abbreviation for Rare Earth
Metal, represents lanthanoid elements.
[0119] However, if Mg, Ca, Y, and REM are contained in steel in an
excessive amount over 0.2%, respectively, these elements compromise
castability (which is the ability of a molten steel to flow through
a mold before solidification; higher castability represents better
flowability of a molten steel), rather leading to lower ductility.
It is thus preferred that the content of Mg is 0.2% or less, the
content of Ca is 0.2% or less, the content of Y is 0.2% or less,
and the content of REM is 0.2% or less. More preferably, the
content of Mg is 0.001% to 0.1%, the content of Ca is 0.001% to
0.1%, the content of Y is 0.001% to 0.1%, and the content of REM is
0.001% to 0.1%.
[0120] It is also desirable that the total amount of these elements
is adjusted to be 0.2% or less, and more preferably 0.1% or
less.
At Least One Selected from Sb: 0.1% or Less, Cu: 0.5% or Less, and
Sn: 0.1% or Less
[0121] Antimony (Sb), copper (Cu), and tin (Sn) are elements that
concentrate near surfaces of a steel sheet and has an effect of
suppressing softening of the steel sheet that would be caused by
nitriding of the surfaces of the steel sheet during warm forming.
Therefore, at least one of these elements may optionally be
contained in steel. Note that Cu is also effective to improve
anti-corrosion property. To obtain this effect, it is desirable
that Sb, Cu, and Sn are contained in steel by 0.005% or more,
respectively. However, if Sb, Cu, and Sn are contained in steel in
excessive amounts over 0.1%, 0.5%, and 0.1%, respectively, the
resulting steel sheet has a poor surface texture. Therefore, it is
preferred that the content of Sb is 0.1% or less, the content of Cu
is 0.5% or less, and the content of Sn is 0.1% or less.
At Least One Selected from Ni: 0.5% or Less and Cr: 0.5% or
Less
[0122] Both Ni and Cr are elements that contribute to increased
strength of steel. At least one of these elements may optionally be
contained in steel. Ni is an austenite-stabilizing element that
suppresses formation of ferrite at high temperature and contributes
to increased strength of the steel sheet. In addition, Cr is a
quench-hardenability-improving element that suppresses, as is the
case with Ni, formation of ferrite at high temperature and
contributes to increased strength of the steel sheet.
[0123] To obtain this effect, it is preferred that Ni and Cr are
contained in steel by 0.01% or more. However, if Ni and Cr are
contained in steel in an excessive amount over 0.5%, respectively,
formation of a low temperature transformation phase, such as
martensite phase and bainite phase, is induced. A low temperature
transformation phase, such as martensite phase and bainite phase,
shows recovery during heating, thereby causing a reduction in the
strength after warm forming. To obtain this effect, it is preferred
that Ni and Cr are contained in steel by 0.5% or less, and more
preferably by 0.3% or less, respectively.
At Least One Selected from O, Se, Te, Po, as, Bi, Ge, Pb, Ga, in,
TI, Zn, Cd, Hg, Ag, Au, Pd, Pt, Co, Rh, Ir, Ru, Os, Tc, Re, Ta, be
and Sr in a Total Amount of 2.0% or Less
[0124] A total amount of 2.0% or less of the above elements is
tolerable since it does not affect the strength or warm formability
of the steel sheet. The total amount is more preferably 1.0% or
less.
[0125] The balance other than the aforementioned components
includes Fe and incidental impurities.
[0126] Next, a preferred microstructure of the aforementioned steel
sheet will be described.
Area Ratio of Ferrite Phase with Respect to the Entire
Microstructure: 95% or More
[0127] The steel sheet has a metal structure of ferrite single
phase. As used herein, the term "ferrite single phase" is not only
intended to represent a situation where the area ratio of ferrite
phase is 100%, but also to encompass a substantially ferrite single
phase where the area ratio of ferrite phase is 95% or more.
[0128] For the steel sheet having a ferrite single phase as its
metal structure, it is possible to retain excellent ductility and
even suppress changes to the material properties caused by heating.
The coexistence of hard phases such as bainite phase and martensite
phase in the microstructure causes recovery of dislocations
introduced to the hard phases by heating and, consequently, the
hard phases soften, which makes it impossible to maintain the
strength of the steel sheet even after warm forming. Accordingly,
the absence of pearlite, bainite phase, and martensite phase
delivers better results, although the coexistence of such hard
phases and even a retained austenite phase is tolerable as long as
the area ratio of these phases with respect to the entire
microstructure is 5% or less.
[0129] In this case, if a steel sheet has a metal structure of
substantially ferrite single phase, the metal structure remains as
substantially ferrite single phase even when the steel sheet is
heated to a temperature range of 400.degree. C. to 700.degree. C.
(warm-forming temperature range). Additionally, the aforementioned
steel sheet may show an increase in ductility as it is heated,
achieving good total elongation in the warm-forming temperature
range.
[0130] Moreover, in the case where the steel sheet is subjected to
a forming process in the warm-forming temperature range, the
forming process is conducted in connection with recovery of
dislocation and, consequently, with little reduction in ductility
during warm forming. Furthermore, since the steel sheet does not
show any microstructural changes even when cooled to room
temperature after warm forming, it maintains the metal structure of
substantially ferrite single phase and exhibits excellent
ductility.
Average Grain Size of Ferrite: 1 .mu.m or More
[0131] For ferrite having an average grain size of less than 1
.mu.M, crystal grains tend to grow during warm forming, with the
result that the material properties of a press-formed part after
warm forming considerably differ from those observed before warm
forming, reducing the stability of the steel sheet as a material.
Therefore, ferrite preferably has an average grain size of 1 .mu.m
or more.
[0132] On the other hand, if ferrite has an excessively large,
average grain size over 15 .mu.m, it is not possible to achieve
strengthening through grain refinement of the microstructure, which
makes it difficult to ensure a desired strength of the steel sheet.
Therefore, ferrite preferably has an average grain size of 15 .mu.m
or less, and more preferably 12 .mu.m or less.
[0133] To obtain a microstructure with ferrite having an average
grain size of 1 .mu.M or more, it is effective to prevent
nucleation sites for ferrite from excessively increasing in number.
The number of nucleation sites is closely related to the amount of
strain energy to be stored in the steel sheet during the rolling
process. Consequently, to prevent refinement of ferrite grains, it
is necessary to prevent excessive storage of strain energy. To this
end, the finisher delivery temperature is preferably at 840.degree.
C. or higher.
Average Particle Size of Carbides in the Ferrite Crystal Grains: 10
Nm or Less
[0134] With the aforementioned ferrite single phase structure, it
is difficult to obtain a steel sheet having a sufficiently high
tensile strength and/or yield ratio. In this regard, the strength
of the steel sheet may be increased by allowing fine carbides
having an average particle size of 10 nm or less to be precipitated
in the ferrite crystal grains. In this case, if the average
particle size of the carbides is more than 10 nm, it is difficult
to obtain the aforementioned high tensile strength and/or yield
ratio. Note that the average particle size of the carbides is more
preferably 7 nm or less.
[0135] Examples of the fine carbides include Ti carbides, and
furthermore, V carbides, Mo carbides, W carbides, Nb carbides, Zr
carbides, and Hf carbides. These carbides do not undergo coarsening
and the average particle size thereof remains 10 nm or less, as
long as the heating temperature of the steel sheet is held at
700.degree. C. or lower. The coarsening of the carbides is thus
suppressed even when the steel sheet is heated to a warm-forming
temperature of 400.degree. C. to 700.degree. C. for warm forming,
with the result that the steel sheet will not show a considerable
reduction in its strength after cooled to room temperature
following the warm forming process. Thus, by providing a steel
sheet with a microstructure that contains the aforementioned
carbides having an average particle size of 10 nm or less in a
matrix of substantially ferrite single phase, it is possible to
effectively suppress the reduction of yield strength of a
press-formed part obtained by warm forming of the steel sheet while
heating it to the warm-forming temperature of 400.degree. C. to
700.degree. C.
[0136] Note that the aforementioned steel sheet may comprise a
coating or plating layer such as a hot dip galvanized layer.
Examples of such a coating or plating layer include an
electroplated layer, an electroless-plated layer, a hot-dipped
layer, and so on. Further, a galvannealed layer may also be
used.
[0137] Next, a method of manufacturing a steel sheet that can
preferably be used in the warm press forming method will be
described.
[0138] The steel sheet that can preferably used in the warm press
forming method is obtained by heating a steel material, then
subjecting the steel material to hot rolling including rough
rolling and finish rolling and, subsequently, coiling the steel
material to obtain a hot rolled steel sheet.
[0139] In this case, the method of manufacturing a steel raw
material preferably includes, without any particular limitation:
preparing a molten steel having the aforementioned composition by a
well-known steelmaking method such as a converter and an electric
furnace; subjecting the molten steel to optional secondary refining
in a vacuum degassing furnace; and casting the molten steel to
obtain a steel raw material, such as slab, by a well-known casting
method such as a continuous casting. Note that the continuous
casting is preferred in terms of productivity and quality.
[0140] Preferred manufacturing conditions will now be
described.
Heating Temperature of Steel Raw Material: 1100.degree. C. to
1350.degree. C.
[0141] Coarse carbides fail to be dissolved if the heating
temperature of the steel raw material is below 1100.degree. C. and,
consequently, fewer fine carbides are dispersed and precipitated in
the resulting steel sheet, which makes it difficult to ensure a
high strength as desired. On the other hand, if the heating
temperature of the steel raw material is above 1350.degree. C.,
oxidation progresses so much as to form oxide scales during hot
rolling and to deteriorate the surface texture of the steel sheet,
thereby lowering the warm formability of the steel sheet.
Therefore, the heating temperature of the steel raw material is
preferably 1100.degree. C. to 1350.degree. C. A more preferable
range is 1150.degree. C. to 1300.degree. C.
Finisher Delivery Temperature: 840.degree. C. or Higher
[0142] If the finisher delivery temperature is below 840.degree.
C., the microstructure contains extended ferrite grains and ends up
with a mixed-grain-size microstructure in which individual ferrite
grains are greatly different in grain size, with the result that
the strength of the steel sheet significantly decreases. In
addition, a finisher delivery temperature below 840.degree. C.
results in excessive strain energy being stored in the steel sheet
during the rolling process, which makes it difficult to obtain a
microstructure containing ferrite grains having an average grain
size of 1 .mu.m or more. Therefore, the finisher delivery
temperature is preferably 840.degree. C. or higher, and more
preferably 860.degree. C. or higher.
Time to Initiate Forced Cooling after Completion of Hot Rolling:
Within Three Seconds
[0143] After completion of the aforementioned hot rolling, the
resulting hot rolled steel sheet is subjected to forced cooling. If
more than three seconds elapse before the forced cooling is
initiated after completion of the hot rolling, a large amount of
carbides are subject to strain-induced precipitation, which makes
it difficult to ensure desired precipitation of fine carbides.
Therefore, the forced cooling is preferably initiated within three
seconds after completion of the hot rolling, and more preferably
within two seconds.
Average Cooling Rate from the Start to the End of Cooling:
30.degree. C./s or Higher
[0144] If the average cooling rate from the start to the end of
cooling is lower than 30.degree. C./s, the steel sheet is
maintained at a high temperature for a longer period of time, which
accelerates coarsening of carbides caused by strain-induced
precipitation. Therefore, the aforementioned forced cooling after
the hot rolling is preferably performed at an average cooling rate
of 30.degree. C./s or higher to rapidly cool the steel sheet to a
predetermined temperature. The average cooling rate is more
preferably 50.degree. C./s or higher.
[0145] Note that a cooling stop temperature is set such that a
coiling temperature eventually falls within a target temperature
range, taking into account the temperature drop that would occur in
the steel sheet during a period from the end of cooling to the
start of coiling. That is, since the steel sheet experiences a drop
in temperature as it is air cooled after the end of cooling, the
cooling stop temperature is normally set to be approximately equal
to the temperature of coiling temperature+5.degree. C. to
+10.degree. C.
Coiling Temperature: 500.degree. C. to 700.degree. C.
[0146] A coiling temperature below 500.degree. C. results in an
insufficient amount of carbides being precipitated in the steel
sheet for providing the steel sheet with as high strength as
desired. On the other hand, a coiling temperature above 700.degree.
C. induces coarsening of precipitated carbides, which also makes it
difficult to provide the steel sheet with as high strength as
desired. Therefore, the coiling temperature is preferably
500.degree. C. to 700.degree. C., and more preferably 550.degree.
C. to 680.degree. C.
[0147] In addition, the resulting hot rolled steel sheet may be
subjected to a coating or plating process using a well-known method
to form a coating or plating layer on its surface. The coating or
plating layer is preferably a hot-dip galvanized layer, a
galvannealed layer, an electroplated layer, or the like.
[0148] Next, the mechanical properties of the steel sheet that may
be obtained by the aforementioned manufacturing method and
preferably be used in the warm press forming method will be
described.
[0149] Specifically, the preferred steel sheet has the following
mechanical properties:
[0150] (a) tensile strength at room temperature: 780 MPa or more,
and yield ratio at room temperature: 0.85 or more;
[0151] (b) yield strength YS.sub.2 in a warm-forming temperature
range of 400.degree. C. to 700.degree. C.: 80% or less of yield
strength YS.sub.1 at room temperature; and (c) total elongation
El.sub.1 in a warm-forming temperature range of 400.degree. C. to
700.degree. C.: 1.1 times or more total elongation El.sub.1 at room
temperature
[0152] The aforementioned properties will be described below.
Tensile Strength at Room Temperature: 780 MPa or More, and Yield
Ratio at Room Temperature: 0.85 or More
[0153] While the warm press forming method is applied to a steel
sheet having a tensile strength at room temperature of 440 MPa or
more, the aforementioned manufacturing method may be used to obtain
a steel sheet having TS.sub.1 of 780 MPa or more and a yield ratio
at room temperature of 0.85 or more.
As used herein, "TS.sub.1" represents a tensile strength at room
temperature and "room temperature" refers to a temperature of
(22.+-.5).degree. C.
Yield Strength YS.sub.2 in a Warm-Forming Temperature Range of
400.degree. C. to 700.degree. C.: 80% or Less of Yield Strength
YS.sub.1 at Room Temperature
[0154] For a steel sheet having a yield strength YS.sub.2 in a
warm-forming temperature range of 400.degree. C. to 700.degree. C.
which is more than 80% of a yield strength YS.sub.1 at room
temperature, the deformation resistance of the steel sheet is not
sufficiently reduced at the time of warm forming and accordingly
increased load (press load) is required for warm forming, leading
to a shortened die life. Additionally, the body size of the
processing machine (press machine) must be necessarily increased
for applying a large load (press load). As the body size of the
processing machine (press machine) increases, it takes a longer
time to transfer a steel sheet heated to a warm forming temperature
to a processing machine, which causes a temperature drop in the
blank and accordingly makes it difficult to perform warm forming at
a desired temperature range. Moreover, shape fixability is not
improved sufficiently and, consequently, the effect to be obtained
by warm forming is reduced.
[0155] Therefore, the yield strength YS.sub.2 in the warm-forming
temperature of 400.degree. C. to 700.degree. C. is preferably 80%
or less, and more preferably 70% or less of the yield strength
YS.sub.1 at room temperature.
Total Elongation El.sub.2 in a Warm-Forming Temperature Range of
400.degree. C. to 700.degree. C.: 1.1 Times or More Total
Elongation El.sub.1 at Room Temperature
[0156] For a steel sheet having a total elongation El.sub.2 at the
warm-forming temperature of 400.degree. C. to 700.degree. C. which
is 1.1 times or more the total elongation El.sub.1 at room
temperature, formability for warm forming is improved sufficiently
to allow the steel sheet to be formed more easily into a member
having a complicated shape, without causing any defects such as
cracking.
[0157] Therefore, the total elongation El.sub.2 in the warm-forming
temperature of 400.degree. C. to 700.degree. C. is preferably 1.1
times or more, and more preferably 1.2 times or more the total
elongation El.sub.1 at room temperature.
[0158] Further, a steel sheet, which exhibits the following
mechanical properties in addition to the above after being formed
into a press-formed part, may more preferably be used in the warm
press forming method.
Yield Strength YS.sub.3 at Room Temperature and Total Elongation
El.sub.3 at Room Temperature of a Press-Formed Part: 80% or More of
the Yield Strength YS.sub.1 at Room Temperature and the Total
Elongation El.sub.1 at Room Temperature of the Material Steel Sheet
Prior to Press Forming
[0159] For a press-formed part having a yield strength YS.sub.3 at
room temperature and a total elongation El.sub.3 at room
temperature that are less than 80% of the yield strength YS.sub.1
at room temperature and the total elongation El.sub.1 at room
temperature of the material steel sheet prior to press forming,
respectively, the strength and total elongation of the resulting
member after warm forming are insufficient. If such a steel sheet
is subjected to warm press forming to produce an automobile
component of desired shape, the resulting component offers
insufficient crash worthiness upon crash of the automobile,
resulting in reduced reliability as an automobile component.
[0160] In view of this, it is preferred that a press-formed part
has a yield strength YS.sub.3 at room temperature and a total
elongation El.sub.3 at room temperature that are 80% or more, and
more preferably 90% or more of the yield strength YS.sub.1 at room
temperature and the total elongation El.sub.1 at room temperature
of the material steel sheet prior to press forming.
EXAMPLES
Example 1
[0161] Steel sheets, each having a sheet thickness of 1.6 mm and a
tensile strength of 440 MPa grade to 1180 MPa grade, were heated
under the conditions shown in Table 1 and subjected to draw forming
to obtain center pillar upper press panels as shown in FIG. 5(a),
respectively, which are one of automobile frame components.
[0162] In this case, the steel sheets were heated in an electric
furnace. The in-furnace time was set to be 300 seconds so that each
blank can be heated in the furnace, resulting in a uniform
temperature distribution throughout the blank. The heated blanks
were then removed from the furnace and fed into a press machine
after a transfer time of 10 seconds, respectively, where the blanks
were subjected to forming processes with different holding times at
the press bottom dead point as shown in Table 1.
[0163] Immediately thereafter, the temperature difference between
flange portions and other portions of each of the formed panels was
measured. That is, the temperature was measured in each panel at
six points (indicated by "X" in FIG. 5(a)) in flange portions and
five points in other portions (indicated by "Y" in FIG. 5(a)) using
a contactless thermometer, and the difference between the average
temperature of the X points and the average temperature of the Y
points was defined as the difference in average temperature among
the flange portions and the other portions.
[0164] In addition, a servo press was used as a press machine,
where the pressing speed was set to be 15 spm (strokes per minute,
which represents the number of parts that can be formed in one
minute plus any additional time, if applicable, taken to hold the
parts at the press bottom dead point).
[0165] The formed panels were air cooled for a sufficiently long
period of time, after which, regarding the cross sectional shape of
each center pillar upper press panel as shown in FIG. 5(b),
measurements were made with a laser displacement sensor of the
amount of geometric changes a made to the edges of each panel until
the end of air cooling, in relation to the reference panel shape
(which is the shape the panel took when it was removed from the die
immediately after the press forming process). The measurement
results are also shown in Table 1.
TABLE-US-00001 TABLE 1 Difference in Average Nominal Holding Time
Temperature among Amount of Tensile Heating at Press Flange
Portions Geometric Strength of Temperature Bottom Dead and Other
Portions Changes Steel Sheet of Steel Sheet Point of Press-formed
Part .alpha. No. (MPa) (.degree. C.) (sec) (.degree. C.) (mm)
Remarks 1 980 700 3 148 0.8 Inventive Example 2 980 700 5 95 0.4
Inventive Example 3 980 700 10 46 0.4 Reference Example 4 980 700
15 28 0.4 Reference Example 5 980 650 3 122 0.6 Inventive Example 6
980 650 5 75 0.3 Inventive Example 7 980 600 1 143 0.9 Inventive
Example 8 980 600 3 92 0.4 Inventive Example 9 980 600 5 58 0.2
Inventive Example 10 780 700 -- 258 2.5 Comparative Example 11 980
700 -- 263 2.6 Comparative Example 12 1180 700 -- 260 2.5
Comparative Example 13 980 400 -- 168 1.2 Comparative Example 14
980 500 -- 183 1.3 Comparative Example 15 980 600 -- 203 1.4
Comparative Example 16 980 650 -- 231 1.8 Comparative Example
[0166] As Table 1 shows, each of steel Nos. 1, 2, 5 to 9 of our
examples, in which steel sheets were held at the press bottom dead
point for one second or more, yielded good dimensional accuracy
such that the difference in average temperature among flange
portions and other portions of each press-formed part was kept
within 150.degree. C. and the amount of geometric changes a was 1.0
mm or less.
[0167] In contrast, none of steel Nos. 10 to 16 of the comparative
examples, in which steel sheets were held at the press bottom dead
point for less than one second, yielded sufficient dimensional
accuracy, because the difference in average temperature among
flange portions and other portions of each press-formed part was
greater than 150.degree. C. and the amount of geometric changes a
was 1.2 mm to 2.6 mm.
[0168] It is clearly understood from the above results that the
warm press forming method may suppress the difference in average
temperature among flange portions and other portions of a panel,
and thereby reduce the amount of geometric changes made to the
panel from the time immediately after press forming until the end
of air cooling, thereby providing the press-formed part with
significantly improved dimensional accuracy.
Example 2
[0169] Molten steels having the chemical compositions shown in
Table 2 were prepared by steelmaking in a converter, and subjected
to continuous casting to obtain slabs (steel raw materials). The
slabs (steel raw materials) were heated to the heating temperatures
shown in Table 3, then subjected to soaking, rough rolling, finish
rolling under the hot rolling conditions shown in Table 3, cooling,
and subsequent coiling to obtain hot rolled steel sheets (sheet
thickness: 1.6 mm). Note that each of the steel sheets a, i, k, m
was heated to 700.degree. C. in a continuous galvanizing line and
immersed in a hot-dip galvanizing bath at a liquid temperature of
460.degree. C. to form a hot-dip galvanized layer on the surfaces
of the steel sheet, and the hot-dip galvanized layer thus obtained
was subjected to alloying treatment at 530.degree. C. to form a
galvannealed layer. The coating weight was set to be 45 g/m.sup.2
for each steel sheet.
[0170] Then, test pieces were collected from the hot rolled steel
sheets thus obtained and analyzed by microstructure observation,
precipitation observation, and tensile tests. The analysis was
carried out as follows.
[0171] (1) Microstructure Observation
[0172] Test pieces were collected from the obtained hot rolled
steel sheets for microstructure observation. Each test piece was
polished and etched (etching solution: 5% nital solution) at its
cross section parallel to the rolling direction (L-section), and
then its center part in the sheet thickness direction was observed
and imaged in ten fields of view under a scanning electron
microscope (at magnification of .times.400). The micrographs thus
obtained were analyzed using an image processing technique to
identify the microstructure and to measure the microstructure
proportion and the average grain size of each phase.
[0173] That is, the obtained micrographs were used to distinguish
ferrite phase from other phases so as to measure the area of the
ferrite phase, thereby determining an area ratio of the ferrite
phase to the entire fields of view being observed. While the
ferrite phase is observed with smoothly curved grain boundaries
with no corrosion marks appeared in the grains, any grain
boundaries appeared in linear form were construed as part of the
ferrite phase. The obtained micrographs were also used to determine
the average grain size of ferrite by a cutting method in conformity
with ASTM E 112-10.
[0174] (2) Precipitate Observation
[0175] In addition, test pieces were collected from the center
portions in the sheet thickness direction of the obtained hot
rolled steel sheets, and subjected to mechanical and chemical
polish to obtain thin films for observation under a transmission
electron microscope (TEM). The thin films thus obtained were
observed under a TEM (at magnification of .times.120,000) for
precipitates (carbides). Measurements were made of the particle
size of 100 or more carbides to determine an arithmetic mean value
thereof, which was defined as the average particle size of carbides
in each steel sheet. Note that coarse cementite and nitride
particles greater than 1 .mu.m in diameter were excluded from the
measurements.
[0176] (3) Tensile Test
[0177] JIS No. 13B tensile test pieces were collected from the
obtained hot rolled steel sheets with a direction orthogonal to the
rolling direction being the tensile direction, in accordance with
JIS Z 2201 (1998). The collected test pieces were subjected to
tensile tests in accordance with JIS G 0567 (1998) to measure
mechanical properties (yield strength YS.sub.1, tensile strength
TS.sub.1, total elongation El.sub.1) at room temperature
(22.+-.5.degree. C.) and high-temperature mechanical properties
(yield strength YS.sub.2, tensile strength TS.sub.2, total
elongation El.sub.2) at temperatures shown in Table 4. Note that
all of the tensile tests were conducted with a cross-head speed of
10 mm/min. In addition, in the case of measuring high-temperature
mechanical properties, tensile tests were carried out in such a way
that test pieces were heated in an electric furnace and retained
for 15 minutes after they had reached a condition where they were
stably maintained at temperatures of .+-.3.degree. C. of the test
temperature.
[0178] Tables 3 and 4 list the test results (1) to (3).
TABLE-US-00002 TABLE 2 Chemical Composition (mass %) ([% C]/ V, Mo,
Sb, Cu, 12)/ Steel W, Nb, Mg, Ca, Sn, Ni, ([% Ti]/ ID C Si Mn P S
Al N Ti B Zr, Hf Y, REM Cr Others 48)* A 0.048 0.01 0.95 0.01
0.0018 0.041 0.0038 0.158 -- -- -- -- -- 1.22 B 0.075 0.02 1.05
0.02 0.0025 0.040 0.0029 0.165 -- -- -- -- -- 1.82 C 0.063 0.01
1.01 0.02 0.0022 0.041 0.0039 0.221 0.0014 -- -- -- -- 1.14 D 0.082
0.02 0.75 0.01 0.0009 0.039 0.0026 0.165 -- V: 0.12 -- -- -- 1.18 E
0.062 0.02 0.65 0.01 0.0031 0.035 0.0048 0.151 -- W: 0.13, -- -- --
1.08 Mo: 0.09 F 0.132 0.01 0.85 0.02 0.0013 0.045 0.0039 0.141 --
V: 0.36 Mg: 0.002 -- O: 0.0008, 1.10 As: 0.0007, Ag: 0.0001, Tc:
0.0007, Be: 0.0004, Ta: 0.0001, Sr: 0.0001, Pt: 0.0001, Rh: 0.0001,
Ru: 0.0001 G 0.121 0.03 0.53 0.02 0.0038 0.041 0.0028 0.151 -- Mo:
0.27, -- Sb: 0.06 Te: 0.0001, 1.54 Nb: 0.02, Bi: 0.0002, Zr: 0.02,
Ge: 0.0003, Hf: 0.03 Zn: 0.001, Re: 0.0001 H 0.091 0.02 0.58 0.01
0.0029 0.039 0.0033 0.190 -- -- Mg: 0.002, Sn: 0.05, Cd: 0.0001,
1.92 Ca: 0.002 Ni: 0.3 Au: 0.000 1, Co: 0.002, Ir: 0.0001, Os:
0.0001 I 0.085 0.02 0.53 0.01 0.0029 0.039 0.0029 0.166 -- V: 0.10
REM: 0.001, Cu: 0.2, Se: 0.0001, 1.31 Y: 0.001 Cr: 0.l Po: 0.0001,
Pb: 0.0001, Ga: 0.0002, In: 0.0001, T1: 0.0002, J 0.029 0.02 0.65
0.02 0.0023 0.044 0.0034 0.169 -- -- -- -- -- 0.69 K 0.191 0.01
0.75 0.02 0.0019 0.046 0.0036 0.166 -- -- -- -- -- 4.60 L 0.115
0.03 0.85 0.01 0.0015 0.041 0.0023 0.153 -- -- -- -- -- 3.01 M
0.085 0.03 0.25 0.02 0.0025 0.043 0.0035 0.165 0.0015 V: 0.15 -- --
-- 1.11 N 0.091 0.02 0.65 0.01 0.0031 0.045 0.0041 0.153 -- Mo:
0.31 -- Cr: 0.04, -- 1.18 Ni: 0.03 O 0.050 0.02 0.65 0.01 0.0031
0.047 0.0045 0.090 -- -- -- -- -- 2.22 *[% M] is the content of
element M (mass %). However, if V, W, Mo, Nb, Zr, Hf are contained,
the following expression needs to be satisfied instead of ([%
C]/12)/([% Ti]/48): ([% C]/12)/([% Ti]/48 + [% V]/51 + [% W]/184 +
[% Mo]/96 + [% Nb]/93 + [% Zr]/91 + [% Hf]/179).
TABLE-US-00003 TABLE 3 Hot Rolling Conditions, etc. Time to
Initiate Steel Sheet Microstructure Forced Area Average Average
Finisher Cooling Ratio Grain Particle Heating Delivery after
Average Coiling of Size Size of Temper- Temper- Completion Cooling
Temper- Ferrite of Precip- Steel Steel ature ature of Rolling Rate
ature Phase Ferrite itates Sheet ID (.degree. C.) (.degree. C.)
(sec) (.degree. C./sec) (.degree. C.) Type* (%) (.mu.m) (nm) a A
1220 900 1.1 75 600 F + .theta. 99 5 3 b A 1050 890 1.3 80 620 F
100 5 18 c A 1230 800 1.2 80 600 F + 92 9 6 Deformed F d A 1230 870
4.6 75 650 F 100 7 11 e A 1220 880 1.2 20 600 F 100 7 14 f A 1230
890 1.8 85 730 F 100 6 14 g A 1220 890 1.2 80 480 F + B 85 4 3 h B
1250 950 1.6 75 680 F 100 4 4 i C 1260 910 1.5 55 640 F 100 4 2 j D
1250 970 1.8 60 620 F 100 5 5 k E 1250 920 1.3 90 590 F 100 3 3 l F
1320 960 1.5 85 620 F 100 4 5 m G 1330 960 1.4 95 630 F 100 4 3 n H
1330 900 1.3 65 620 F + .theta. 98 4 4 o I 1250 980 1.7 70 640 F +
.theta. 99 4 4 p J 1250 920 1.6 75 650 F 100 7 11 q K 1250 930 1.4
70 650 F + P 92 4 3 r L 1260 920 1.3 80 640 F + P 93 4 4 s M 1250
910 1.1 65 610 F 100 4 3 t N 1250 920 1.2 70 640 F 100 3 3 u O 1230
910 1.1 65 610 F + .theta. 94 4 3 *F: ferrite phase, Deformed F:
deformed ferrite phase, .theta.: cementite, P: pearlite, B: bainite
phase
TABLE-US-00004 TABLE 4 Mechanical Properties of Steel Sheet at Room
Temperature Mechanical Properties of Steel Sheet Total at High
Temperature Yield Tensile Elon- Yield Tensile Total YS.sub.2/
Strength Strength gation Yield Temper- Strength Strength Elon-
YS.sub.1 .times. Steel Steel YS.sub.1 TS.sub.1 El.sub.1 Ratio ature
YS.sub.2 TS.sub.2 gation 100 Sheet ID (MPa) (MPa) (%) YR (.degree.
C.) (MPa) (MPa) El.sub.2(%) (%) El.sub.2/El.sub.1 a A 738 820 20
0.9 400 539 607 23 73 1.16 500 413 476 29 56 1.46 600 273 328 36 37
1.78 700 148 189 53 20 2.65 800 125 164 58 17 2.92 b A 567 689 22
0.82 600 221 290 38 39 1.73 c A 677 768 14 0.88 600 365 439 21 54
1.50 d A 634 767 24 0.83 600 234 306 41 37 1.71 e A 622 745 24 0.83
600 228 399 39 37 1.63 f A 590 726 23 0.81 600 215 288 38 36 1.65 g
A 621 757 17 0.82 600 373 445 18 57 1.06 h B 771 845 20 0.91 600
278 329 36 36 1.80 i C 860 945 19 0.91 600 298 354 35 35 1.84 j D
912 997 18 0.91 600 340 401 31 37 1.72 k E 852 932 21 0.91 600 321
377 37 38 1.76 l F 1141 1201 15 0.95 600 374 452 28 33 1.87 m G
1123 1195 18 0.94 600 330 395 32 29 1.78 n H 884 951 20 0.93 600
296 350 35 33 1.75 o I 893 971 21 0.91 600 310 372 39 35 1.86 p J
607 731 23 0.83 600 193 310 43 28 1.87 q K 745 834 19 0.89 400 574
649 18 77 0.95 r L 736 822 19 0.9 400 563 635 18 76 0.95 s M 954
1015 18 0.94 600 345 406 31 36 1.72 t N 945 1027 18 0.92 600 312
363 35 33 1.94 u O 671 721 23 0.93 600 251 305 25 37 1.09
[0179] Then, the steel sheets thus obtained were heated under the
conditions shown in Table 5, and then subjected to warm draw
forming to obtain center pillar upper press panels as shown in FIG.
5(a), respectively, which are one of automobile frame components.
Note that the conditions for heating and draw forming other than
those shown in Table 5 are the same as described in Example 1.
[0180] Additionally, under the same conditions as those in Example
1, measurements were made of the temperature difference between
flange portions and other portions of each panel immediately after
the formation, and of the amount of geometric changes a made to the
edges of each panel until the end of the air cooling process, in
relation to the reference panel shape (which is the shape the panel
took when it was removed from the die immediately after press
forming).
[0181] Moreover, JIS No. 13B tensile test pieces were collected
from the formed panels and subjected to tensile tests at room
temperature under the same conditions as described above, to
measure their mechanical properties (yield stress (YS.sub.3),
tensile strength (TS.sub.3), and total elongation (El.sub.3)).
[0182] The obtained results are shown in Table 5.
TABLE-US-00005 TABLE 5 Difference in Holding Average Temper-
Heating Time ature among Temper- at Flange Portions Amount ature
Press and Other of Mechanical Properties of Press-formed Part
(Panel) of Bottom Portions of Geometric Yield Tensile Total
YS.sub.3/ TS.sub.3/ El.sub.3/ Steel Dead Press-formed Changes
Strength Strength Elon- YS.sub.1 .times. TS.sub.1 .times. El.sub.1
.times. Steel Steel Sheet Point Part .alpha. YS.sub.3 TS.sub.3
gation 100 100 100 No. Sheet ID (.degree. C.) (sec) (.degree. C.)
(mm) (MPa) (MPa) E1.sub.3(%) (%) (%) (%) Remarks 17 a A 400 1 98
0.32 741 823 21 100 100 105 Inventive Example 18 500 1 140 0.80 735
818 22 100 100 110 Inventive Example 19 600 1 148 0.95 740 822 22
100 100 110 Inventive Example 20 700 5 99 0.50 730 812 24 99 99 120
Inventive Example 21 500 3 80 0.30 738 819 22 100 100 110 Inventive
Example 22 500 5 70 0.25 761 852 15 103 104 75 Inventive Example 23
b A 600 3 92 0.30 566 690 22 100 100 100 Inventive Example 24 c A
600 3 95 0.31 694 777 16 103 101 114 Inventive Example 25 d A 600 3
90 0.31 641 771 24 101 101 100 Inventive Example 26 e A 600 3 98
0.32 619 740 24 100 99 100 Inventive Example 27 f A 600 3 97 0.32
582 723 24 99 100 104 Inventive Example 28 g A 600 3 96 0.33 671
812 12 108 107 71 Inventive Example 29 h B 600 3 90 0.33 768 842 23
100 100 115 Inventive Example 30 i C 600 3 95 0.35 863 946 21 100
100 111 Inventive Example 31 j D 600 3 92 0.38 916 1002 19 100 101
106 Inventive Example 32 k E 600 3 93 0.36 855 936 22 100 100 105
Inventive Example 33 l F 600 3 90 0.40 1139 1185 16 100 99 107
Inventive Example 34 m G 600 3 94 0.35 1125 3194 21 100 100 117
Inventive Example 35 n H 600 3 92 0.35 875 948 19 99 100 95
Inventive Example 36 o I 600 3 96 0.36 905 986 19 101 102 90
Inventive Example 37 p J 600 3 95 0.15 405 476 28 67 65 122
Inventive Example 38 q K 400 1 95 0.32 739 830 14 99 100 74
Inventive Example 39 r L 400 1 98 0.31 737 819 14 100 100 74
Inventive Example 40 s M 600 3 92 0.36 948 1007 19 99 99 106
Inventive Example 41 t N 600 3 93 0.35 940 1021 19 99 99 106
Inventive Example 42 u O 600 3 94 0.30 681 732 18 101 102 78
Inventive Example
[0183] As Table 5 shows, each of steel Nos. 17 to 42 of our
examples yielded good dimensional accuracy such that the difference
in average temperature between flange portions and other portions
was kept within 150.degree. C. and the amount of geometric changes
a was 1.0 mm or less.
[0184] In particular, steel Nos. 17 to 22, 29 to 36, 40, and 41 of
our examples using steel sheets having preferred chemical
compositions and microstructures yielded good dimensional accuracy
in the press-formed parts after the formation, despite the use of
high strength steel sheets having a tensile strength of 780 MPa or
more, and furthermore, the press-formed parts exhibited extremely
good mechanical properties such that, for example, the tensile
strength TS.sub.3 of these press-formed parts was 99% to 104% of
the tensile strength TS.sub.1 of the respective material steel
sheets before press forming.
* * * * *