U.S. patent application number 15/720168 was filed with the patent office on 2018-01-25 for steel sheet for warm press forming, warm-pressed member, and manufacturing methods thereof.
The applicant listed for this patent is POSCO. Invention is credited to Yeol-Rae CHO, Eul-Yong CHOI, Ki-Soo KIM, Kyoo-Young LEE, Jin-Keun OH.
Application Number | 20180023171 15/720168 |
Document ID | / |
Family ID | 48290247 |
Filed Date | 2018-01-25 |
United States Patent
Application |
20180023171 |
Kind Code |
A1 |
OH; Jin-Keun ; et
al. |
January 25, 2018 |
STEEL SHEET FOR WARM PRESS FORMING, WARM-PRESSED MEMBER, AND
MANUFACTURING METHODS THEREOF
Abstract
Provided is a warm-pressed member comprising, by weight %, C:
0.01% to 0.5%, Si: 3.0% or less (excluding 0%), Mn: 3% to 15%, P:
0.0001% to 0.1%, S: 0.0001% to 0.03%, Al: 3.0% or less (excluding
0%), N: 0.03% or less (excluding 0%), and the balance of Fe and
inevitable impurities. After a warm press forming process and a
cooling process, the warm-pressed member has a microstructure
comprising: 5 volume % to 50 volume % of retained austenite; and at
least one of ferrite, martensite, tempered martensite, and bainite
as a remainder.
Inventors: |
OH; Jin-Keun; (Gwangyang-si,
KR) ; LEE; Kyoo-Young; (Gwangyang-si, KR) ;
CHO; Yeol-Rae; (Gwangyang-si, KR) ; CHOI;
Eul-Yong; (Gwangyang-si, KR) ; KIM; Ki-Soo;
(Gwangyang-si, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
POSCO |
Pohang-si |
|
KR |
|
|
Family ID: |
48290247 |
Appl. No.: |
15/720168 |
Filed: |
September 29, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14356300 |
May 5, 2014 |
|
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PCT/KR2012/009244 |
Nov 5, 2012 |
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15720168 |
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Current U.S.
Class: |
420/73 ; 148/620;
148/621; 148/648; 420/103; 420/104; 420/120; 420/72; 420/76;
72/206; 72/47 |
Current CPC
Class: |
C21D 8/0463 20130101;
C21D 8/0478 20130101; C22C 38/04 20130101; C22C 38/002 20130101;
C22C 38/008 20130101; C22C 38/02 20130101; C22C 38/12 20130101;
C22C 38/14 20130101; C22C 38/08 20130101; C21D 8/0226 20130101;
C21D 2211/008 20130101; C21D 8/02 20130101; B21B 15/00 20130101;
C21D 2211/002 20130101; C21D 2211/001 20130101; C23C 2/12 20130101;
C22C 38/38 20130101; C23C 2/02 20130101; B21B 2015/0057 20130101;
C21D 6/005 20130101; C22C 38/60 20130101; C21D 8/0473 20130101;
C22C 38/18 20130101; C22C 38/001 20130101; C21D 7/13 20130101; C22C
38/16 20130101; C21D 2211/005 20130101; C22C 38/06 20130101; C23C
2/06 20130101; C21D 9/46 20130101; B21B 1/04 20130101; C22C 38/58
20130101 |
International
Class: |
C22C 38/38 20060101
C22C038/38; B21B 1/04 20060101 B21B001/04; C23C 2/02 20060101
C23C002/02; C22C 38/60 20060101 C22C038/60; C22C 38/58 20060101
C22C038/58; C22C 38/18 20060101 C22C038/18; C22C 38/16 20060101
C22C038/16; C22C 38/14 20060101 C22C038/14; C22C 38/12 20060101
C22C038/12; 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; C23C 2/06 20060101
C23C002/06; B21B 15/00 20060101 B21B015/00; C21D 6/00 20060101
C21D006/00; C21D 8/02 20060101 C21D008/02; C21D 8/04 20060101
C21D008/04; C21D 9/46 20060101 C21D009/46; C23C 2/12 20060101
C23C002/12 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 7, 2011 |
KR |
10-2011-0115331 |
Claims
1. A warm-pressed member comprising, by weight %, C: 0.01% to 0.5%,
Si: 3.0% or less (excluding 0%), Mn: 3% to 15%, P: 0.0001% to 0.1%,
S: 0.0001% to 0.03%, Al: 3.0% or less (excluding 0%), N: 0.03% or
less (excluding 0%), and the balance of Fe and inevitable
impurities, wherein after a warm press forming process and a
cooling process, the warm-pressed member has a microstructure
comprising: 5 volume % to 50 volume % of retained austenite; and at
least one of ferrite, martensite, tempered martensite, and bainite
as a remainder.
2. The warm-pressed member of claim 1, wherein the warm-pressed
member has a tensile strength of 1000 MPa or greater and an
elongation of 10% or greater.
3. The warm-pressed member of claim 1, further comprising 0.001% to
0.4% of at least one selected from the group consisting of Ti, Nb,
Zr, and V.
4. The warm-pressed member of claim 1, further comprising 0.005% to
2.0% of at least one of Cu and Ni.
5. The warm-pressed member of claim 1, further comprising 0.0001%
to 1.0% of at least one of Sb and Sn.
6. The warm-pressed member of claim 1, further comprising 0.0001%
to 0.01% of B.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to a steel sheet for
automobile structural members or reinforcement members, and more
particularly, to a steel sheet that may be increased in strength,
elongation, shock-absorbing ability, and plating corrosion
resistance after a warm press forming process. In addition, the
present disclosure relates to a warm-pressed member formed of the
steel sheet, and methods of manufacturing the steel sheet and the
warm-pressed member.
BACKGROUND ART
[0002] Automobiles are increasingly required to have high fuel
efficiency and crashworthiness in order to protect both the
environment and automobile passengers. Thus, a great deal of
research has been conducted to develop lightweight and crashworthy
automobiles using high-strength chassis.
[0003] For example, hot pressing methods have been proposed to
produce high-strength steel sheets improved in terms of formability
and shape controllability. Such methods are disclosed in Patent
Documents 1 and 2. In such methods, a steel sheet having a single
phase of austenite that is low in strength but high in formability
is subjected to a heat treatment process and a pressing process,
and is then rapidly cooled by dies. Therefore, ultra-high-strength
final products having martensite as a main microstructure phase are
manufactured.
[0004] However, since a steel sheet having a single phase of
austenite is heated at high temperature in the methods, oxide scale
may have to be removed from the surfaces of the steel sheet after
the heat treatment if the steel sheet is not a plated steel sheet,
and high costs may be incurred in heating the steel sheet to a high
temperature.
[0005] If Zn-plated or Al-plated steel sheets are processed by the
methods, plating materials may be evaporated or fused to cause a
decrease in productivity. Since the melting point of zinc (Zn) is
500.degree. C. or less and the melting point of aluminum (Al) is
lower than 700.degree. C., if a steel sheet plated with zinc (Zn)
or aluminum (Al) is heat-treated at high temperature as described
above, the zinc (Zn) or aluminum (Al) may be partially melted and
thus may not properly function as a plating material. In addition,
the zinc (Zn) or aluminum (Al) may be fused to dies or forming
machines to deteriorate the formability of the steel sheet.
[0006] Furthermore, although the strength of a steel sheet is
increased through such a high-temperature forming process, the
elongation of the steel sheet is reduced to lower than 10% because
90% or more of the microstructure of the steel sheet is formed by
martensite, and thus the steel sheet may not have sufficient
crashworthiness. Therefore, the steel sheet may only be used to
manufacture limited kinds of automotive components.
[0007] (Patent Document 1) Korean Patent Application Laid-open No.
2007-0057689
[0008] (Patent Document 2) U.S. Pat. No. 6,296,805
DISCLOSURE
Technical Problem
[0009] An aspect of the present disclosure may provide a steel
sheet for warm press forming having high strength, good elongation,
and thus improved crashworthiness after being warm pressed, and a
member formed by warm-pressing the steel sheet.
[0010] An aspect of the present disclosure may also provide a
plated steel sheet for warm press forming that can have good
corrosion resistance even after a heat treatment such as a heat
treatment of a warm press forming process, and a warm-pressed
member.
Technical Solution
[0011] According to an aspect of the present disclosure, a steel
sheet for warm press forming may include, by weight %, C: 0.01% to
0.5%, Si: 3.0% or less (excluding 0%), Mn: 3% to 15%, P: 0.0001% to
0.1%, S: 0.0001% to 0.03%, Al: 3.0% or less (excluding 0%), N:
0.03% or less (excluding 0%), and the balance of Fe and inevitable
impurities.
[0012] According to another aspect of the present disclosure, a
method of manufacturing a steel sheet for warm press forming may
include: heating a steel slab to a temperature within a temperature
range of 1000.degree. C. to 1400.degree. C., the steel slab
including the above-mentioned composition of the steel sheet;
forming a hot-rolled steel sheet by performing a hot rolling
process on the steel slab and then a finish-rolling process on the
steel slab at a temperature within a temperature range of Ar3 to
1000.degree. C.; and coiling the hot-rolled steel sheet at a
temperature higher than Ms but equal to or lower than 800.degree.
C.
[0013] According to another aspect of the present disclosure, a
warm-pressed member may include the above-mentioned composition of
the steel sheet, wherein after a warm press forming process and a
cooling process, the warm-pressed member may have a microstructure
formed by: 3 volume % to 50 volume % of retained austenite; and at
least one of ferrite, martensite, tempered martensite, and bainite
as a remainder.
[0014] According to another aspect of the present disclosure, a
method of manufacturing a member by warm press forming may include:
performing a warm press forming process on a steel sheet including
the above-mentioned composition of the steel sheet; and cooling the
steel sheet, wherein the warm press forming process may include a
heat treatment process including: heating the steel sheet to a
temperature within a temperature range of Ac1 to Ac3 at a heating
rate of 1.degree. C./sec to 1000.degree. C./sec; and maintaining
the steel sheet at the temperature within the temperature range for
1 second to 10000 seconds.
Advantageous Effects
[0015] The present disclosure relates a method of manufacturing an
ultra-high-strength steel sheet that can be used for manufacturing
structural members, reinforcement members, and shock-absorbing
members of automobiles, and a member formed by warm-pressing the
steel sheet. According to the method of the present disclosure, a
steel sheet having a ultra-high tensile strength of 1000 MPa or
greater and good elongation after a heat treatment of a warm press
forming process can be manufactured, and a heat-treatment member
formed of the steel sheet can be provided. That is, according to
the present disclosure, the application of a heat treatment type
ultra-high-strength steel sheet can be extended to impact
members.
DESCRIPTION OF DRAWINGS
[0016] FIG. 1 is a graph illustrating a thermal history of a hot
press forming process of the related art.
[0017] FIG. 2 is a graph illustrating a thermal history of a warm
press forming process of the present disclosure.
BEST MODE
[0018] In the present disclosure, the term "warm press forming"
refers to forming a steel sheet to have a certain shape after
heat-treating the steel sheet at a temperature equal to or lower
than the austenite single phase region. That is, the term "warm
press forming" is contrasted with the term "hot press forming"
referring to forming a steel sheet into a certain shape after
heat-treating the steel sheet at a temperature higher than the
austenite single phase region.
[0019] In the present disclosure, the warm press forming includes a
heat treatment process and a forming process and may be performed
in the order of a heat treatment process and a forming process or
the order of a forming process and a heat treatment process.
[0020] The inventors have found that when a member (component) is
manufactured through a warm press forming process, the elongation
of the member can be improved by properly adjusting the
composition, microstructure, and heat treatment temperature of the
member, and have invented the present invention based on the
knowledge.
[0021] In a hot press forming process of the related art, a steel
sheet is heated to a temperature higher than the austenite single
phase region so as to form martensite as a main microstructure
phase of the steel sheet while suppressing the formation of
ferrite, and then the steel sheet is formed to have a desired shape
and rapidly cooled to a temperature lower than a Mf (martensite
finishing point), so as to form a high-strength member having
martensite as a main microstructure phase.
[0022] However, in a warm press forming method of the present
disclosure, a steel sheet is heat-treated at a temperature lower
than the austenite single phase region and is subjected to a
forming process and a cooling process. The warm press forming
method of the present disclosure is proposed based on the knowledge
that if a steel sheet is heated and maintained at a temperature
lower than the austenite single phase region, elements such as C
and Mn are concentrated in austenite formed in grains or grain
boundaries, and thus the austenite can be stable at room
temperature after the forming process and the cooling process.
[0023] Hereinafter, a steel sheet for warm press forming will be
described in detail according to an embodiment of the present
disclosure.
[0024] (Steel Sheet for Warm Press Forming)
[0025] First, the composition of the steel sheet for warm press
forming will be described in detail (hereinafter, concentrations
are expressed in weight % unless otherwise specified).
[0026] Carbon (C): 0.01% to 0.5%
[0027] Carbon (C) is an element for increasing the strength of the
steel sheet, and the concentration of carbon (C) is properly
adjusted to ensure the formation of retained austenite in the steel
sheet. If the concentration of carbon (C) is less than 0.01%, the
strength of the steel sheet may not be sufficient, and it may be
difficult to maintain 3 volume % or more of retained austenite in
the steel sheet during a warm press forming process. Therefore,
0.01% or more (preferably, 0.05% or more) of carbon (C) is included
in the steel sheet. If the concentration of carbon (C) is greater
than 0.5%, it may be difficult to cold-roll the steel sheet after
the steel sheet is hot-rolled, and due to excessively high strength
of the steel sheet, it may be difficult to obtain a desired
elongation of the steel sheet. In addition, the weldability of the
steel sheet may be lowered. Therefore, 0.5% or less (preferably,
0.4% or less, and more preferably, 0.3% or less) of carbon (C) is
included in the steel sheet.
[0028] Silicon (Si): 3.0% or Less (Excluding 0%)
[0029] Silicon (Si) functions as a deoxidizer during a steel making
process and suppresses the formation of carbides during a heat
treatment process. If the concentration of silicon (Si) is greater
than 3%, it may be difficult to plate the steel sheet. Thus, the
concentration of silicon (Si) in the steel sheet may be 3% or less
(preferably, 2.5% or less, and more preferably, 2% or less).
[0030] Aluminum (Al): 3.0% or Less (Excluding 0%)
[0031] Aluminum (Al) removes oxides during a steel making process,
and thus a clean steel sheet may be obtained. In addition, like
silicon (Si), aluminum (Al) suppresses the formation of carbides
during a heat treatment process. If the concentration of aluminum
(Al) is high, a two-phase region is extended, and thus the
temperature range of the annealing process is widened. However, if
the concentration of aluminum (Al) is greater than 3%, it may be
difficult to plate the steel sheet, and the manufacturing cost of
the steel sheet may be increased. Therefore, the concentration of
aluminum (Al) in the steel sheet is set to be 3% or less
(preferably, 2.5% or less, and more preferably, 2.0% or less.
[0032] Manganese (Mn): 3% to 15%
[0033] Manganese (Mn) has an important function in the embodiment
of the present disclosure. Manganese (Mn) functions as a solid
solution strengthening element and lowers the Ms (martensite start
point) temperature for improving the stability of austenite at room
temperature. In addition, since manganese (Mn) lowers the Ac1 and
Ac3 temperatures, manganese (Mn) has an important function in a
warm press forming process of the embodiment of the present
disclosure. Furthermore, manganese (Mn) diffuses into austenite
during a heat treatment performed at a temperature within the range
of Ac1 to Ac3 in a warm press forming process, and thus the
stability of the austenite may be further improved at room
temperature. If the concentration of manganese (Mn) in the steel
sheet is less than 3%, these effects may not be sufficiently
obtained. Thus, the concentration of manganese (Mn) in the steel
sheet may be 3% or greater (preferably, 4% or greater, and more
preferably, 5% or greater). However, if the concentration of
manganese (Mn) is greater than 15%, the manufacturing cost of the
steel sheet is increased, and the amount of retained austenite may
be too large. In this case, although the elongation of the steel
sheet is increased, the strength of the steel sheet may be
insufficient. Therefore, the concentration of manganese (Mn) in the
steel sheet may be 15% or less (preferably, 13% or less, and more
preferably 11% or less).
[0034] Phosphorus (P): 0.0001% to 0.1%
[0035] Like silicon (Si), phosphorus (P) suppresses the formation
of carbides when martensite is heat-treated. However, in the case
that the amount of phosphorus (P) is excessive, the weldability and
grain boundary characteristics of the steel sheet may be
deteriorated. Therefore, the upper limit of the concentration of
phosphorus (P) may be set to be 0.1%. In addition, since
manufacturing costs increase to maintain the concentration of
phosphorus (P) at a level lower than 0.0001%, the lower limit of
the concentration of phosphorus (P) may be set to be 0.0001%.
[0036] Sulfur (S): 0.0001% to 0.03%
[0037] Sulfur (S) exists in the steel sheet as an impurity lowering
the ductility and weldability of the steel sheet. Such effects are
not large if the concentration of sulfur (S) is 0.03% or less, the
upper limit of the concentration of sulfur (S) is set to be 0.03%.
Since manufacturing costs increase to maintain the concentration of
sulfur (S) at a level lower than 0.0001%, the lower limit of the
concentration of sulfur (S) is set to be 0.0001%.
[0038] Nitrogen (N): 0.03% or Less (Excluding 0%)
[0039] Nitrogen (N) exists in the steel sheet as an impurity. In
the steel sheet, nitrogen (N) forms nitrides which improve
resistance to delayed fractures caused by hydrogen. If the
concentration of nitrogen (N) is greater than 0.03%, a steel slab
may become sensitive to cracks during a continuous casting process,
and pores may be easily formed in the steel slab. Therefore, the
upper limit of the concentration of nitrogen (N) is set to be 0.03%
(preferably 0.02%, and more preferably, 0.01%).
[0040] In addition to the above-mentioned elements, the steel sheet
of the embodiment of the present disclosure may further include: at
least one of chromium (Cr), molybdenum (Mo), and tungsten (W) as an
element improving hardenability; at least one of titanium (Ti),
niobium (Nb), zirconium (Zr), and vanadium (V) as a precipitation
strengthening element; at least one of copper (Cu) and nickel (Ni)
as an element improving strength; boron (B) as an element improving
grain boundary strengthening and hardenability; and at least one of
antimony (Sb) and tin (Sn) as an element improving plating
characteristics.
[0041] Combination of at Least One of Chromium (Cr), Molybdenum
(Mo), and Tungsten (W): 0.001% to 2.0%
[0042] Chromium (Cr), molybdenum (Mo), and tungsten (W) improve
hardenability and precipitation strengthening, and thus increase
the strength of the steel sheet. If the concentration of chromium
(Cr), molybdenum (Mo), or tungsten (W) is lower than 0.001%,
sufficient hardenability and precipitation strengthening may not be
obtained, and if the concentration of chromium (Cr), molybdenum
(Mo), or tungsten (W) is greater than 2.0%, such effects may not be
further obtained although manufacturing costs increase. Therefore,
the upper limit of the concentration of chromium (Cr), molybdenum
(Mo), or tungsten (W) is set to be 2.0%.
[0043] Combination of at Least One of Titanium (Ti), Niobium (Nb),
and Vanadium (V): 0.001% to 0.4%
[0044] Titanium (Ti), niobium (Nb), and vanadium (V) are effective
in improving the strength, grain refinement, and heat-treatment
characteristics of the steel sheet. If the concentration of
titanium (Ti), niobium (Nb), or vanadium (V) is lower than 0.001%,
such effects may not be obtained, and if the concentration of
titanium (Ti), niobium (Nb), or vanadium (V) is greater than 0.4%,
manufacturing costs increase. Therefore, the concentration of
titanium (Ti), niobium (Nb), or vanadium (V) may be set to be
within 0.001% to 0.4%.
[0045] Combination of at Least One of Copper (Cu) and Nickel (Ni):
0.005% to 2.0%
[0046] Copper (Cu) forms a fine Cu precipitate to improve the
strength of the steel sheet. If the concentration of copper (Cu) is
lower than 0.005%, the strength of the steel sheet may not be
sufficiently increased, and if the concentration of copper (Cu) is
greater than 2.0%, the processability of the steel sheet may be
deteriorated. Therefore, it may be preferable that the
concentration of copper (Cu) be set to be within 0.005% to 2.0%.
Nickel (Ni) improves the strength and heat-treatment
characteristics of the steel sheet. However, if the concentration
of nickel (Ni) is less than 0.005%, such effects may not be
obtained, and if the concentration of nickel (Ni) is greater than
2.0%, manufacturing costs increase. Therefore, the concentration of
nickel (Ni) may be set to be within 0.005% to 2.0%.
[0047] Boron (B): 0.0001% to 0.01%
[0048] Boron (B) improves the hardenability of the steel sheet, and
although a small amount of boron (B) is added to the steel sheet,
the strength of the steel sheet may be markedly increased through a
heat treatment. In addition, boron (B) enhances grain boundaries
and thus suppresses grain boundary embrittlement of the steel sheet
having a large amount of manganese (Mn). However, if the
concentration of boron (B) in the steel sheet is less than 0.0001%,
such effects may not be obtained. In addition, if the concentration
of boron (B) is greater than 0.01%, such effects may not be further
obtained, and the high-temperature processability of the steel
sheet may be deteriorated. Therefore, the upper limit of the
concentration of boron (B) may be set to be 0.01%.
[0049] Combination of at Least One of Antimony (Sb) and Tin (Sn):
0.0001% to 1.0%
[0050] Antimony (Sb) and tin (Sn) may be concentrated on the
surface and grain boundaries of the steel sheet. Thus, antimony
(Sb) and tin (Sn) may prevent the manganese (Mn) included in the
steel sheet in a high concentration from concentrating on the
surface of the steel sheet and generating oxides during an
annealing process of the steel sheet. Therefore, the steel sheet
may be easily plated in a plating process. However, if the
concentration of antimony (Sb) or tin (Sn) in the steel sheet is
less than 0.0001%, such effects may not be obtained. In addition,
if the concentration of antimony (Sb) or tin (Sn) is greater than
1.0%, the high-temperature processability of the steel sheet may be
deteriorated. Therefore, the upper limit of the concentration of
the antimony (Sb) or tin (Sn) may be set to be 1.0%.
[0051] The steel sheet may include iron (Fe) and inevitable
impurities as the remainder of constituents. However, the steel
sheet may further include other elements as well as the
above-mentioned elements.
[0052] In the embodiment of the present disclosure, the steel sheet
for warm press forming may be one of a hot-rolled steel sheet, a
cold-rolled steel sheet, and a plated steel sheet. However, the
steel sheet of the present disclosure is not limited but may be any
kind of steel sheet. The plated steel sheet may be a Zn-based
plated steel sheet or an Al-based plated steel sheet.
[0053] The steel sheet for warm press forming may have a main
microstructure formed by 30 volume % or more of martensite,
bainite, or a combination thereof. If the steel sheet has a main
microstructure formed by less than 30 volume % of martensite,
bainite, or a combination thereof, austenite may not be
sufficiently formed in the steel sheet during a heat treatment of a
warm press forming process, and the strength of the steel sheet may
not be sufficiently high.
[0054] Hereinafter, a method of manufacturing a steel sheet for
warm press forming will be described in detail according to an
embodiment of the present disclosure.
[0055] (Method of Manufacturing Steel Sheet for Warm Press
Forming)
[0056] A steel slab including the above-described composition is
heated to 1000.degree. C. to 1400.degree. C., and is hot-rolled. If
the heating temperature of the steel slab is lower than
1000.degree. C., the microstructure of the steel slab formed after
a continuous casting process may not be sufficiently homogenized,
and if the heating temperature of the steel slab is higher than
1400.degree. C., manufacturing costs may be increased.
[0057] Thereafter, the steel slab is subjected to a finish hot
rolling process at a temperature within a temperature range of Ar3
to 1000.degree. C. to form a hot-rolled steel sheet. If the process
temperature of the finish hot rolling process is lower than Ar3,
two-phase rolling may occur to cause a mixed grain size
distribution and lower processability. On the contrary, if the
process temperature of the finish hot rolling process is greater
than 1000.degree. C., the grains of the steel slab may be
coarsened, and a large amount of oxide scale may be generated.
[0058] Thereafter, the hot-rolled steel sheet is coiled at a
temperature higher than Ms but equal to or lower than 800.degree.
C. If the hot-rolled steel sheet is coiled at a temperature equal
to or lower than Ms, a large load may be applied to a hot-rolling
coiler, and if the hot-rolled steel sheet is coiled at a
temperature higher than 800.degree. C., the thickness of an oxide
layer of the hot-rolled steel sheet may be increased.
[0059] The hot-rolled steel sheet manufactured as described above
may be used in a warm press forming process or may be additionally
treated through a pickling process. Furthermore, after the
hot-rolled steel sheet is pickled, the steel sheet may be plated
with a Zn-based material or an Al-based material, and then the
plated steel sheet may be used in a warm press forming process.
[0060] In addition, the hot-rolled steel sheet may be subjected to
a pickling process and a cold rolling process to produce a
cold-rolled steel sheet. The pickling process may be performed
according to a general method, and the reduction ratio of the cold
rolling process is not limited. For example, the reduction ratio of
the cold rolling process may be selected from general values used
in the related art.
[0061] For example, before the hot-rolled steel sheet is
cold-rolled, the hot-rolled steel sheet may be batch-annealed.
Since the hot-rolled steel sheet manufactured as described above
has a high degree of strength, the hot-rolled steel sheet may be
batch-annealed to reduce the strength thereof and thus to reduce
the load of the cold rolling process. That is, the cold rolling
processability of the hot-rolled steel sheet may be improved. It
may be preferable that the batch annealing be performed within the
temperature range of Ac1 to Ac3. If the process temperature of the
batch annealing is lower than Ac1, the strength of the hot-rolled
steel sheet may not be sufficiently lowered. On the contrary, if
the process temperature of the batch annealing is higher than Ac3,
manufacturing costs may be increased, and a large amount of
martensite may be formed in the hot-rolled steel sheet when the
hot-rolled steel sheet is slowly cooled after the batch annealing.
In this case, the strength of the hot-rolled steel sheet may not be
sufficiently lowered. After the batch annealing, the hot-rolled
steel sheet may be cold-rolled to produce a cold-rolled steel
sheet.
[0062] The cold-rolled steel sheet may be treated through a
continuous annealing process to produce an annealed steel sheet.
Process conditions of the continuous annealing process are not
limited. For example, preferably, the continuous annealing process
may be performed at a temperature within the temperature range of
700.degree. C. to 900.degree. C. If the process temperature of the
continuous annealing process is lower than 700.degree. C., the
steel sheet may not be sufficiently recrystallized. If the process
temperature of the continuous annealing process is greater than
900.degree. C., manufacturing costs may be increased, and
processability may be lowered. The annealed steel sheet may be
plated through a Zn--Ni electroplating process to produce a Zn--Ni
electroplated steel sheet.
[0063] Alternatively, the cold-rolled steel sheet may be plated
with a Zn-based material or an Al-based material so as to improve
the corrosion resistance and thermal resistance of the cold-rolled
steel sheet. Heat-treatment and Zn-plating conditions for the
cold-rolled steel sheet are not limited. For example, the
cold-rolled steel sheet may be hot-dip galvanized to produce a
product known as a GI (galvanized iron) sheet or may be hot-dip
galvannealed to produce a product known as a GA (galvannealed)
steel sheet. In addition, heat-treatment and Al-plating conditions
for the cold-rolled steel sheet are not limited. For example,
conditions generally used in the related art may be used.
[0064] Hereinafter, a warm-pressed member manufactured through a
warm press forming process using the above-described steel sheet
will be described according to an embodiment of the present
disclosure.
[0065] (Warm-Pressed Member)
[0066] In the embodiment of the present disclosure, the
warm-pressed member includes the above-described composition of the
steel sheet for warm press forming. The microstructure of the
warm-pressed member may include: 3 volume % to 50 volume % of
retained austenite; and at least one of ferrite, martensite,
tempered martensite, and bainite as a remainder.
[0067] If the volume fraction of retained austenite is lower than
3%, the warm-pressed member may not have an ultra high degree of
strength and a high degree of elongation desired in the embodiment
of the present disclosure. On the contrary, if the volume fraction
of retained austenite is higher than 50%, it may be difficult to
produce the warm-pressed member because large amounts of C and Mn
have to be included in the warm-pressed member. In addition to the
retained austenite, the microstructure of the warm press forming
may include at least of ferrite, martensite, tempered martensite,
and bainite.
[0068] Ferrite may be formed in the warm-pressed member during a
heat treatment of a warm press forming process (described later) or
may be partially formed before the heat treatment. Preferably, the
fraction of ferrite in the warm-pressed member may be 30% or less.
If the fraction of ferrite is greater than 30%, the warm-pressed
member may not have sufficient strength.
[0069] Martensite may be formed in the warm-pressed member during a
heat treatment of a warm press forming process or may be partially
formed before the heat treatment. At this time, carbides may be
partially formed in the martensite. The fraction of martensite in
the warm-pressed member may be within the range of 50% to 95%. If
the fraction of martensite is lower than 50%, the warm-pressed
member may not have sufficient strength, and if the fraction of
martensite is greater than 95%, retained austenite may not be
sufficient included in the warm-pressed member.
[0070] Hereinafter, a method of manufacturing a warm-pressed member
will be described in detail according to an embodiment of the
present disclosure.
[0071] (Method of Manufacturing Warm-Pressed Member)
[0072] In the embodiment of the present disclosure, a warm press
forming method is used to form a member having a high degree of
elongation. The inventors have researched into a method of
manufacturing a member having desired properties through a warm
press forming process based on the knowledge that a desired degree
of thermal resistance of a plating layer can be guaranteed if a
heat treatment is performed at a temperature lower than Ac3. As a
result, it is found that if a steel sheet having the
above-mentioned composition is heat-treated at a temperature lower
than Ac3, the steel sheet can have retained austenite.
[0073] That is, it is found that if a steel sheet including
manganese (Mn) is properly subjected to a hot rolling process,
and/or a cold rolling process, and an annealing process, the steel
sheet can have a microstructure of 5 .mu.m or less before a heat
treatment. In addition, it is found that if a steel sheet includes
sufficient amounts of martensite and/or bainite before a heat
treatment, nano-sized lath grains of the martensite and/or bainite
are converted into austenite or manganese (Mn) and carbon (C)
stabilize the austenite during a heat treatment of a warm press
forming process to form a stable austenite structure even at room
temperature. As described above, it may be preferable that the main
microstructure of a steel sheet for warm press forming be formed by
30% or more of martensite, bainite, or a combination thereof. If
the fraction of martensite, bainite, or a combination thereof in a
steel sheet is low, a sufficient amount of austenite may not be
formed in the steel sheet during a heat treatment of a warm press
forming process, and the steel sheet may not have a desired degree
of strength.
[0074] A member manufactured based on the above-mentioned knowledge
has 3 volume % or more of retained austenite and thus good
elongation.
[0075] In the method of manufacturing a warm-pressed member, a
steel sheet manufactured as described above is subjected to a warm
press forming process. In the warm press forming process, a forming
process may be performed after or before a heat treatment.
[0076] The heat treatment of the warm press forming process may be
performed by heating the steel sheet to a temperature within a
temperature range of Ac1 to Ac3 with a heating rate of 1.degree.
C./sec to 1000.degree. C./sec. Then, the steel sheet is maintained
at the temperature within the temperature range for 1 second to
10000 seconds.
[0077] If the heating rate is lower than 1.degree. C./sec,
manufacturing costs may be increased, and productivity may be
lowered. Therefore, the lower limit of the heating rate may be set
to be 1.degree. C./sec. Although the heating rate is greater than
1000.degree. C./sec, the effect of the heat treatment is not
increased but an excessive amount of heating equipment may be
required. Therefore, the upper limit of the heating rate may be set
to be 1000.degree. C./sec.
[0078] The temperature range of Ac1 to Ac3 is important to
guarantee the formation of retained austenite. If the heat
treatment is performed at a temperature lower than Ac1, austenite
may not be formed in grains or grain boundaries of martensite or
bainite, and thus retained austenite may not be obtained.
Therefore, the heat treatment may be performed at a temperature
equal to or greater than Ac1 (preferably, Ac1+10.degree. C. and
more preferably, Ac1+20.degree. C.). If the heat treatment is
performed at a temperature greater than Ac3, carbon (C) and
manganese (Mn) may not be sufficiently concentrated on austenite,
and thus the stability of retained austenite may be low. That is, a
sufficient amount of retained austenite may not be obtained, and
thus the elongation of the steel sheet may not be sufficient even
though the strength of the steel sheet may be increased. Therefore,
the upper limit of the temperature range of the heat treatment may
be set to be Ac3 (preferably, Ac3-10.degree. C., and more
preferably, Ac3-20.degree. C.)
[0079] If the steel sheet is maintained within the heat-treatment
temperature range for a period of time longer than 10000 seconds,
productivity may be decreased, and martensite may disappear to
lower the strength of the steel sheet. Therefore, the upper limit
of the period of time may be set to 10000 seconds.
[0080] Thereafter, the steel sheet is warm-pressed and cooled. At
this time, the cooling rate is not limited. For example, it may be
preferable that the cooling rate range from 1.degree. C./sec to
1000.degree. C./sec. If the cooling rate is lower than 1.degree.
C./sec, productivity may be lowered, and additional equipment may
be used to control the cooling rate. Therefore, manufacturing costs
may be increased. If the cooling rate is greater than 1000.degree.
C./sec, additional equipment may be used to rapidly cool the steel
sheet, and the microstructure of a warm-pressed member formed of
the steel sheet may not be appropriate.
MODE FOR INVENTION
[0081] Hereinafter, examples of the present disclosure will be
described in detail. The following examples are for illustrative
purposes and are not intended to limit the scope of the present
disclosure.
Examples
[0082] Steel slabs having compositions as shown in Table 1 were
produced by a vacuum melting process, and the steel slabs were
reheated in a heating furnace at 1200.degree. C. for 1 hour and
were hot-rolled. The hot rolling of the steel slabs were finished
at 900.degree. C., and the hot-rolled steel slabs (hot-rolled steel
sheets) were cooled at 680.degree. C. in a furnace. A warm press
forming process was performed on the hot-rolled steel sheets under
simulated conditions.
[0083] Meanwhile, the hot-rolled steel sheets were pickled and then
a cold rolling process was performed on the pickled hot-rolled
steel sheets with a cold rolling reduction ratio of 50% so as to
produce cold-rolled steel sheets. Particularly, steel sheets M and
N were treated through a batch annealing process after the cold
rolling process. In the batch annealing process, the steel sheets M
and N were heated at a heating rate of 30.degree. C./h and
maintained at 600.degree. C. for 10 hours. Thereafter, the steel
sheets M and N were cooled at a cooling rate of 30.degree. C./h. A
continuous annealing process was performed on the other steel
sheets instead of the batch annealing process. The continuous
annealing process was performed at 780.degree. C.
[0084] In addition, the picked hot-rolled steel sheets and the
cold-rolled steel sheets were plated through a zinc (Zn) or
aluminum (Al) plating process so as to produce plated steel sheets.
Specifically, in the zinc (Zn) or aluminum (Al) plating process,
the steel sheets were annealed at 780.degree. C. and then were
dipped in a zinc (Zn) or Aluminum (Al) plating bath.
[0085] The pickled hot-rolled steel sheets, the cold-rolled steel
sheets, and the plated steel sheets were treated under simulated
heat treatment conditions of the warm press forming process. The
heat treatment conditions are shown in Table 2 below. The heating
rate of the heat treatment was 3.degree. C./sec.
[0086] Tension test specimens of the steel sheets processed through
the warm press forming process under simulated conditions were
prepared according to JIS Z 2201 #5, and mechanical properties of
the tension test specimens were measured. In addition, the fraction
of retained austenite in each of the steel sheets was measured by
an X-ray diffraction test. In detail, the fraction of retained
austenite were calculated by a 5 peak method expressed in Equation
1 using the areas of austenite (200), (220), and (311) peaks and
the areas of ferrite (200) and (211) peaks obtained in the X-ray
diffraction test. In Equation 1, V.sub..gamma. refers to an
austenite fraction, I.sub..alpha. refers to a ferrite peak area,
and I.sub..gamma. refers to an austenite peak area.
V .gamma. XRD = [ 1 / 2.19 ( I .alpha. 200 / I .gamma. 200 ) + 1 ]
+ [ 1 / 1.35 ( I .gamma. 220 / I .alpha. 220 ) + 1 ] + [ 1 / 1.5 (
I .alpha. 200 / I .gamma. 311 ) + 1 ] + [ 1 / 1.12 ( I .alpha. 211
/ I .gamma. 200 ) + 1 ] + [ 1 / 0.7 ( I .alpha. 211 / I .gamma. 220
) + 1 ] + [ 1 / 0.78 ( I .alpha. 211 / I .gamma. 311 ) + 1 ] 6 [
Equation 1 ] ##EQU00001##
[0087] Mechanical properties and retained austenite fractions of
the steel sheets measured as described above are shown in Table 2
below.
TABLE-US-00001 TABLE 1 Steel sheets C Si Mn P S Al N Others Notes A
0.08 0.1 5.1 0.014 0.003 0.04 0.004 -- *IS B 0.07 0.1 7.0 0.012
0.004 0.03 0.003 -- IS C 0.07 0.1 10.0 0.014 0.003 0.02 0.004 -- IS
D 0.15 1.56 6.1 0.010 0.005 2.29 0.004 -- IS E 0.16 0.1 5.0 0.014
0.003 0.04 0.004 B: 0.0026 IS F 0.31 0.1 5.0 0.014 0.003 0.03 0.004
Ti: 0.02 IS G 0.32 1.6 5.0 0.014 0.003 0.04 0.004 Nb: 0.03 IS H
0.16 0.1 6.9 0.013 0.003 0.03 0.003 Zr: 0.05 IS I 0.30 0.1 6.9
0.013 0.003 0.03 0.003 W: 0.04 IS J 0.30 0.7 6.9 0.013 0.003 0.03
0.003 Cr: 0.3 IS K 0.29 0.6 7.1 0.015 0.004 0.05 0.005 Mo: 0.05 IS
L 0.03 0.1 9.1 0.013 0.003 0.02 0.004 Cu: 0.05 IS M 0.04 0.1 9.5
0.015 0.003 0.05 0.004 Ni: 0.11 IS N 0.15 0.1 9.9 0.014 0.002 0.01
0.004 V: 0.05 IS O 0.14 0.1 9.8 0.015 0.005 0.11 0.005 Sb: 0.05 IS
P 0.02 0.1 14.2 0.014 0.003 0.04 0.004 Sn: 0.04 IS Q 0.23 0.2 1.3
0.011 0.003 0.03 0.005 Cr: 0.17, **CS Ti: 0.03, B: 0.0026 R 0.28
1.5 1.5 0.010 0.003 0.02 0.007 Nb: 0.05, CS B: 0.003 *IS: Inventive
steel, **CS: Comparative steel
TABLE-US-00002 TABLE 2 Heat treatment conditions Mechanical
Retained Cooling properties austenite Steel Product Temp. Time rate
TS El fraction sheets types (.degree. C.) (sec) (.degree. C./sec)
(MPa) (%) (%) Notes A CR 700 300 45 1054 17 7.3 *IE Zn 700 300 5
1031 18 7.8 IE Zn 850 300 45 1201 6 2.1 CE B CR 650 300 45 1124 20
9.0 IE C HR 500 300 45 1356 15 13.4 IE Al 600 300 45 1330 19 20.6
IE D CR 740 300 45 1042 31 16.9 IE E CR 700 300 45 1127 14 11.6 IE
F CR 700 300 45 1297 13 9.6 IE G Zn 700 300 45 1102 27 10.9 IE H CR
600 300 45 1121 20 16.7 IE Zn 650 300 5 1249 26 16.8 IE I CR 650
300 45 1206 28 18.8 IE J CR 650 300 45 1189 23 28.1 IE K CR 650 300
45 1236 21 25.6 IE L Zn 500 300 45 1052 16 6.9 IE M CR 500 300 45
1063 18 8.1 IE N CR 500 300 45 1491 18 18.3 IE CR 600 300 45 1428
17 22.8 IE O CR 600 300 45 1436 17 21.5 IE P Zn 550 300 45 1015 26
31.4 IE Q Al 600 300 45 541 22 0.5 **CE Al 900 300 45 1629 6 0.3 CE
R CR 750 300 45 786 21 1.7 CE CR 850 300 45 1899 7 0.7 CE *IE:
Inventive Example, **CE: Comparative Example
[0088] Products produced using steel sheets A to P having
compositions according to the present disclosure have retained
austenite fractions of 3% or greater and good elongation. However,
products produced using comparative steel sheets Q and R have
retained austenite fractions of less than 3% regardless of heat
treatment conditions and have poor elongation.
[0089] When the steel sheet A was heat-treated at 850.degree. C.
higher than Ac3 in the warm press forming process, the strength of
the steel sheet A was sufficiently high but the elongation thereof
was decreased because of insufficient amount of retained
austenite.
* * * * *