U.S. patent application number 14/232784 was filed with the patent office on 2014-06-05 for hot press forming steel plate, formed member using same, and method for manufacturing the plate and member.
The applicant listed for this patent is Tae-Kyo Han, Jong-Sang Kim, Kyoo-Young Lee, Jin-Keun Oh. Invention is credited to Tae-Kyo Han, Jong-Sang Kim, Kyoo-Young Lee, Jin-Keun Oh.
Application Number | 20140150930 14/232784 |
Document ID | / |
Family ID | 47558269 |
Filed Date | 2014-06-05 |
United States Patent
Application |
20140150930 |
Kind Code |
A1 |
Lee; Kyoo-Young ; et
al. |
June 5, 2014 |
HOT PRESS FORMING STEEL PLATE, FORMED MEMBER USING SAME, AND METHOD
FOR MANUFACTURING THE PLATE AND MEMBER
Abstract
The present invention relates to a hot press forming steel plate
made of a composition comprising: 0.3-1.0 wt % of C; 0.0-4.0 wt %
of Mn; 1.0-2.0 wt % of Si; 0.01-2.0 wt % of Al; 0.015 wt % or less
of S; 0.01 wt % or less of N; and the remainder being Fe and
unavoidable impurities. Further, the present. invention relates to
a method for manufacturing the hot press forming steel plate,
characterized by comprising the steps of: heating, to between 1100
and 1300.degree. C., a steel slab having the composition;
performing hot rolling finishing between. an Ar3 transformation
point and 950.degree. C.; and performing winding between MS and
720.degree. C. Further, the present invention. relates to a hot
press formed member characterized by having the composition, and
having a dual phase microstructure made of bainite and residual
austenite. In addition, the present invention relates to a method
for manufacturing the hot press formed member, characterized by
comprising the steps of: heating the steel plate having the
composition to a temperature of an Ac3 point or higher; hot press
molding the heated steel plate; performing cooling at a cooling
speed of 20.degree. C./sec or higher to a temperature between MS
and 350.degree. C.; and performing a heat treatment in a heating
furnace between MS and 550.degree. C.
Inventors: |
Lee; Kyoo-Young;
(Gwangyang-si, KR) ; Oh; Jin-Keun; (Gwangyang-si,
KR) ; Kim; Jong-Sang; (Gwangyang-si, KR) ;
Han; Tae-Kyo; (Gwangyang-si, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Lee; Kyoo-Young
Oh; Jin-Keun
Kim; Jong-Sang
Han; Tae-Kyo |
Gwangyang-si
Gwangyang-si
Gwangyang-si
Gwangyang-si |
|
KR
KR
KR
KR |
|
|
Family ID: |
47558269 |
Appl. No.: |
14/232784 |
Filed: |
July 15, 2011 |
PCT Filed: |
July 15, 2011 |
PCT NO: |
PCT/KR2011/005242 |
371 Date: |
January 14, 2014 |
Current U.S.
Class: |
148/518 ;
148/320; 148/328; 148/330; 148/334; 148/336; 148/337; 148/529;
148/531; 148/533; 148/603; 148/653; 420/100; 420/103; 420/104;
420/117; 420/118; 420/119; 420/120; 420/121; 420/123; 420/127;
420/128; 420/99; 428/653; 428/659; 72/66 |
Current CPC
Class: |
C21D 2211/002 20130101;
C22C 38/06 20130101; B21B 1/026 20130101; C23C 2/02 20130101; C21D
8/0226 20130101; C21D 2211/001 20130101; C22C 38/12 20130101; C22C
38/04 20130101; C23C 2/12 20130101; C22C 38/22 20130101; C23C 2/06
20130101; Y10T 428/12757 20150115; C22C 38/32 20130101; C21D 8/0273
20130101; C21D 8/0263 20130101; C22C 38/08 20130101; C22C 38/18
20130101; C22C 38/34 20130101; C22C 38/14 20130101; C22C 38/002
20130101; C21D 8/0236 20130101; C22C 38/28 20130101; C22C 38/38
20130101; C22C 38/02 20130101; C22C 38/001 20130101; B21C 47/26
20130101; C21D 9/46 20130101; Y10T 428/12799 20150115 |
Class at
Publication: |
148/518 ; 72/66;
148/529; 148/531; 148/533; 148/603; 148/653; 148/320; 148/328;
148/330; 148/334; 148/336; 148/337; 420/99; 420/100; 420/103;
420/104; 420/117; 420/118; 420/119; 420/120; 420/121; 420/123;
420/127; 420/128; 428/653; 428/659 |
International
Class: |
C22C 38/38 20060101
C22C038/38; C21D 8/02 20060101 C21D008/02; C22C 38/08 20060101
C22C038/08; C22C 38/00 20060101 C22C038/00; C22C 38/34 20060101
C22C038/34; C22C 38/02 20060101 C22C038/02; C22C 38/28 20060101
C22C038/28; C22C 38/22 20060101 C22C038/22; C22C 38/14 20060101
C22C038/14; C22C 38/12 20060101 C22C038/12; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; B21B 1/02 20060101
B21B001/02; C22C 38/32 20060101 C22C038/32 |
Claims
1. A steel sheet for hot press forming, comprising, by wt %, C:
0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01% to
2.0%, S: 0.015% or less, N: 0.01% or less, and the balance of Fe
and inevitable impurities.
2. The steel sheet for hot press forming of claim 1, further
comprising at least one selected from the group consisting of Mo:
0.5% or less (excluding 0%), Cr: 1.5% or less (excluding 0%), Ni:
0.5% or less (excluding 0%), Nb: 0.005% to 0.1%, and V: 0.005% to
0.1%.
3. The steel sheet for hot press forming of claim 1, further
comprising B: 0.005% or less (excluding 0%) and Ti: 0.06% or less
(excluding 0%).
4. The steel sheet for hot press forming of claim 1, wherein the
steel sheet is one of a hot-rolled steel sheet, a cold-rolled steel
sheet, and a plated cold-rolled steel sheet coated with a plating
layer.
5. A method for manufacturing a steel sheet for hot press forming,
the method comprising: heating a steel slab to a temperature range
of 1100.degree. C. to 1300.degree. C., the steel slab comprising,
by wt %, C: 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al:
0.01% to 2.0%, S: 0.015% or less, N: 0.01% or less, and the balance
of Fe and inevitable impurities; performing a finish hot-rolling
process at a temperature within a range of Ar.sub.3 transformation
point to 950.degree. C. to form a steel sheet; and coiling the
steel sheet at a temperature within a range of M.sub.s to
720.degree. C.
6. The method of claim 5, wherein the steel slab further comprises
at least one selected from the group consisting of Mo: 0.5% or less
(excluding 0%), Cr: 1.5% or less (excluding 0%), Ni: 0.5% or less
(excluding 0%), Nb: 0.005% to 0.1%, and V: 0.005% to 0.1%.
7. The method of claim 5, wherein the steel slab further comprises
B: 0.005% or less (excluding 0%) and Ti: 0.06% or less (excluding
0%).
8. A method for manufacturing a steel sheet for hot press forming,
the method comprising: heating a steel slab to a temperature range
of 1100.degree. C. to 1300.degree. C., the steel slab comprising,
by wt %, C: 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al:
0.01% to 2.0%, S: 0.015% or less, N: 0.01% or less, and the balance
of Fe and inevitable impurities; performing a finish hot-rolling
process at a temperature within a range of Ar.sub.3 transformation
point to 950.degree. C. to form a steel sheet; coiling the steel
sheet at a temperature within a range of M.sub.s to 720.degree. C.;
pickling the steel sheet; cold-rolling the steel sheet;
continuously annealing the steel sheet at a temperature within a
range of 750.degree. C. to 900.degree. C.; and overaging the steel
sheet at a temperature within a range of M.sub.s to 550.degree.
C.
9. The method of claim 8, wherein the steel slab further comprises
at least one selected from the group consisting of Mo: 0.5% or less
(excluding 0%), Cr: 1.5% or less (excluding 0%), Ni: 0.5% or less
(excluding 0%), Nb: 0.005% to 0.1%, and V: 0.005% to 0.1%.
10. The method of claim 8, wherein the steel slab further comprises
B: 0.005% or less (excluding 0%) and Ti: 0.06% or less (excluding
0%).
11. A method for manufacturing a steel sheet for hot press forming,
the method comprising: heating a steel slab to a temperature range
of 1100.degree. C. to 1300.degree. C., the steel slab comprising,
by wt %, C: 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al:
0.01% to 2.0%, S: 0.015% or less, N: 0.01% or less, and the balance
of Fe and inevitable impurities; performing a finish hot-rolling
process at a temperature within a range of Ar.sub.3 transformation
point to 950.degree. C. to form a steel sheet; coiling the steel
sheet at a temperature within a range of M.sub.s to 720.degree. C.;
pickling the steel sheet; cold-rolling the steel sheet;
continuously annealing the steel sheet at a temperature within a
range of 750.degree. C. to 900.degree. C.; overaging the steel
sheet at a temperature within a range of M.sub.s to 550.degree. C.;
and plating the overaged steel sheet by any one of hot-dip
galvanizing, galvannealing, electro galvanizing, and hot-dip
aluminizing.
12. The method of claim 11, wherein the steel slab further
comprises at least one selected from the group consisting of Mo:
0.5% or less (excluding 0%), Cr: 1.5% or less (excluding 0%), Ni:
0.5% or less (excluding 0%), Nb: 0.005% to 0.1%, and V: 0.005% to
0.1%.
13. The method of claim 11, wherein the steel slab further
comprises B: 0.005% or less (excluding 0%) and Ti: 0.06% or less
(excluding 0%).
14. A hot-press formed member comprising, by wt %, C: 0.3% to 1.0%,
Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01% to 2.0%, S: 0.015%
or less, N: 0.01% or less, and the balance of Fe and inevitable
impurities, wherein the hot-press formed member has a dual phase
microstructure formed by bainite and retained austenite.
15. The hot-press formed member of claim 14, further comprising at
least one selected from the group consisting of Mo: 0.5% or less
(excluding 0%), Cr: 1.5% or less (excluding 0%), Ni: 0.5% or less
(excluding 0%), Nb: 0.005% to 0.1%, and V: 0.005% to 0.1%.
16. The hot-press formed member of claim 14, further comprising B:
0.005% or less (excluding 0%) and Ti: 0.06% or less (excluding
0%).
17. The hot-press formed member of claim 14, wherein the hot-press
formed member has a TS(MPa)*EI(%) value of 25,000 MPa % or
greater.
18. The hot-press formed member of claim 16, wherein the hot-press
formed member has a TS(MPa)*EI(%) value of 25,000 MPa % or
greater.
19. A method for manufacturing a hot-press formed member, the
method comprising: heating a steel sheet to a temperature equal to
or higher than Ac.sub.3, the steel sheet comprising, by wt %, C:
0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01% to
2.0%, S: 0.015% or less, N: 0.01% or less, and the balance of Fe
and inevitable impurities; hot-press forming the heated steel
sheet; cooling the hot-press formed steel sheet to a temperature
range of M.sub.s to 550.degree. C. at a rate of 20.degree. C./sec
or higher; and heat-treating the cooled steel sheet at a
temperature within a range of M.sub.s to 550.degree. C. in a
heating furnace.
20. The method of claim 19, wherein the steel sheet further
comprises at least one selected from the group consisting of Mo:
0.5% or less (excluding 0%), Cr: 1.5% or less (excluding 0%), Ni:
0.5% or less (excluding 0%), Nb: 0.005% to 0.1%, and V: 0.005% to
0.1%.
21. The method of claim 19, wherein the steel sheet further
comprises B: 0.005% or less (excluding 0%) and Ti: 0.06% or less
(excluding 0%).
22. The method of claim 19, wherein the steel sheet is one of a
hot-rolled steel sheet, a cold-rolled steel sheet, and a plated
cold-rolled steel sheet coated with a plating layer.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to a steel sheet for hot
press forming, a member formed using the steel sheet, and methods
for manufacturing the steel sheet and the member, and more
particularly, to a steel sheet for manufacturing high-strength and
high-ductility products suitable for impact members and crashworthy
members of automobiles through a hot press forming process, a
member formed using the steel sheet, and methods of manufacturing
the steel sheet and the member.
BACKGROUND ART
[0002] Recently, safety regulations for protecting automobile
occupants and fuel efficiency regulations for protecting the
environment have been greatly tightened, and social requirements
for vehicle weight reductions have markedly increased. The use of
high-strength steel sheets is necessary to reduce the weight of
automotive parts while maintaining the rigidity thereof and the
crash safety of automobiles.
[0003] However, if steel sheets for automobiles are improved in
strength, the yield strength thereof is inevitably increased, and
the elongation thereof is reduced. These factors significantly
lower the formability of such steel sheets. In addition, due to
excessive spring-back in high-strength steel sheets, the dimensions
of components formed of high-strength steel sheets may be varied
after a forming process. That is, the shape fixability of
components may be lowered.
[0004] To address these limitations, advanced high strength steel
(AHSS), such as dual phase (DP) steel in which martensite is
included in a ferrite matrix to lower the yield ratio thereof and
transformation induced plasticity (TRIP) steel in which bainite and
retained austenite are included in a ferrite matrix to markedly
increase the strength-elongation balance thereof, have been
developed and commercialized.
[0005] However, such steel sheets have a tensile strength of about
500 MPa to 1,000 MPa which may be insufficient to satisfy current
rigidity and crash safety requirement while allowing for the
lightening of automobiles
[0006] Therefore, a steel forming method known as hot press forming
has been commercialized to overcome such limitations and realize
ultra high-strength automotive components. In the hot press forming
method, after blanking, a steel sheet is subjected to heating to an
Ac.sub.3 temperature or higher for transformation into austenite,
extracting, press forming, and die quenching, so as to form a
martensite or mixed martensite-bainite microstructure. Ultra
high-strength members having a tensile strength of 1 GPa or greater
and high dimensional precision owning to high-temperature forming
can be obtained using the hot press forming method.
[0007] Although such a hot press forming method of the related art
is suitable for satisfying rigidity and crash. safety requirements
while lightening automotive components, final products have an
elongation of 10% or less. That is, final products have a very low
level of ductility, In other words, components manufactured by a
hot press forming method may be used as impact members in
automobiles, but may not be suitable for use as crashworthy members
that absorb crash energy to protect vehicle occupants in a
crash.
[0008] Therefore, to use hot-press formed members as crashworthy
members of automobiles, research into members having a high degree
of ductility after being hot-press formed and steel sheets for
forming such members through a hot press forming process is
required.
DISCLOSURE
Technical Problem
[0009] Aspects of the present disclosure may provide a steel sheet
for manufacturing a. hot-press formed member having high strength
and high ductility, a. member formed using the steel sheet, and
methods for manufacturing the steel sheet and the member.
Technical Solution
[0010] According to an aspect of the present disclosure, a steel
sheet for hot press forming may include, by wt %, C: 0.3% to 1.0%,
Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01% to 2.0%, S: 0.015%
or less, N: 0.01% or less, and the balance of Fe and inevitable
impurities.
[0011] According to another aspect of the present disclosure, a
method for manufacturing a steel sheet for hot press forming may
include: heating a steel slab to a temperature within a range of
1100.degree. C. to 1300.degree. C., the steel slab including, by wt
%, C: 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01%
to 2.0%, 0.015% or less, N: 0.01% or less, and the balance of Sc
and inevitable impurities; performing a finish hot-rolling process
at a temperature within a range of an Ar.sub.3 transformation point
to 950.degree. C. to form a steel, sheet; and coiling the steel
sheet at a temperature within a range of M.sub.s to 720.degree.
C.
[0012] According to another aspect of the present disclosure, a
hot-press formed member may include, by wt %, C: 0.3% to 1.0%, Mn:
0.01% to 4.0%, Si: 1.0% to 2.0, Al: 0.01% to 2.0%, S: 0.015% or
less, N: 0.01% or less, and the balance of Fe and inevitable
impurities, wherein the hot-press formed member has a dual phase
microstructure formed by bainite and retained austenite.
[0013] According to another aspect of the present disclosure, a
method for manufacturing a hot-press formed member may include:
heating a steel sheet to a temperature equal to or higher than Ac:,
the steel sheet including, by wt %, C: 0.3% to 1.0%, Mn: 0.01% to
4.0%, Si: 1.0% to 2.0%, Al: 0.01% to 2.0%, S: 0.015% or less, N;
0.01% or less, and the balance of Fe and inevitable impurities;
hot-press forming the heated steel sheet; cooling the hot-press
formed steel sheet to a temperature within a range of M.sub.s to
550.degree. C. at a rate of 20.degree. C./sec or higher; and
heat-treating the cooled steel sheet at a temperature within a
range of M.sub.s to 550.degree. C. in a heating furnace.
Advantageous Effects
[0014] The present disclosure provides a high-strength,
high-ductility steel sheet for hot press forming. The present
disclosure also provides a member formed using the steel. sheet and
having dual phase microstructure constituted by bainite and
retained austenite and a TS(MPa)*E1(%)value of 25,000 MPa % or
greater. Since the member has high ductility as well as high
strength, the member may be usefully used as a crashworthy member
of an automobile.
DESCRIPTION OF DRAWINGS
[0015] FIG. is a temperature-time graph illustrating manufacturing
processes of a hot-press formed member according to an embodiment
of the present disclosure.
[0016] FIGS. 2A to 2C are images showing microstructures of
hot-press formed members according to cooling rates after a forming
process in a method for manufacturing a hot-press formed member, in
which FIG. 2A is the case of a cooling rate of 30.degree. C./sec,
FIG. 2B is the case of a cooling rate of 5.degree. C./sec, and FIG.
2C is an enlarged image of FIG. 2B.
BEST MODE
[0017] Embodiments of the present disclosure provide a method for
manufacturing a formed member having a high degree of ductility as
well as high strength for use as a crashworthy member of an
automobile, and a steel sheet having a high. degree of ductility
for use in manufacturing such a formed member. In detail, the
present disclosure provides four categories: a steel sheet for hot
press forming having a high degree of ductility, a method for
manufacturing the steel sheet, a hot-press formed member, and a
method for manufacturing the hot-press formed member.
[0018] (Steel sheet for hot press forming)
[0019] Hereinafter, a steel sheet for hot press forming will be
described in detail according to an embodiment of the present
disclosure.
[0020] The steel sheet for hot press forming has a high degree of
ductility as well as a high degree of strength so that a member
formed of the steel sheet through a hot press forming process may
have a high degree of ductility and a high degree of strength. The
steel sheet for hot press forming includes, by wt %, C: 0.3% to
1.0%, Mn: 0.01% to 40%, Si; 1.0% to 2.0%, Al: 0.01% to 2.0%, S:
0.015% or less, N: 0.01% or less, and the balance of Fe and
inevitable impurities.
[0021] Carbon (C) is an element included in the steel sheet to
enhance the strength thereof. Furthermore, in the embodiment of the
present disclosure, carbon (C) is diffused into retained austenite
by elements such as silicon (Si) to stabilize the retained
austenite and thus to prevent transformation to martensite. The
steel sheet for hot press forming may include 0.3 wt % to 1.0 wt %
of carbon (C). If the carbon. content is less than 0.3%, the amount
of retained austenite is low after forming, and thus it may be
difficult to guarantee both strength and ductility. If the carbon
content is greater than 1.0%, bainite transformation is markedly
slowed, and the formation of pearlite is facilitated, thereby
deteriorating properties of the steel sheet.
[0022] Manganese (Mn) is included. in the steel sheet to prevent
red shortness caused by FeS formed by sulfur (S) inevitably
included in the steel sheet during a manufacturing process. The
content of manganese (M) may be within the range of 0.01% to 4.0%.
If the content of manganese (M) is less than 0.01%, red shortness
may be caused by FeS. If the content of manganese (M) is greater
than 4.0%, bainite transformation may be slowed. to increase the
time required for a heat treatment in hot press forming process. As
a result, the productivity of the hot press forming process may be
lowered, and the manufacturing cost of the steel sheet may be
increased.
[0023] Silicon (Si) is an element included in the steel sheet to
guarantee the ductility of a final product. Silicon (Si)
facilitates ferrite transformation and diffuses carbon (C) into
retained austenite to stabilize the retained austenite by an
increased amount of carbon (C) in the retained austenite, thereby
preventing transformation to martensite. The content of silicon
(Si) may be within the range of 1.0 wt % to 2.0 wt %. If the
content of silicon (Si) is less than 1.0%, the effect of
stabilizing retained austenite may be poor. If the content of
silicon (Si) greater than 2.0%, the roiling characteristics of the
steel sheet may be deteriorated. For example, the steel sheet may
be cracked during a rolling process. Therefore, the upper limit, of
the content of silicon (Si) is set as 2.0%.
[0024] Aluminum (Al) removes oxygen from the steel sheet to prevent
the inclusion of nonmetallic substances therein during
solidification. In addition, like silicon (Si), aluminum (Al)
facilitates the diffusion of carbon (C) into retained austenite to
stabilize the retained austenite. The content of aluminum (Al) may
be within the range of 0.01% to 2.0%. If the content of aluminum
(Al) is less than 0.01%, oxygen may be insufficiently removed, and
thus it may be difficult to prevent the inclusion of nonmetallic
substances. If the content of aluminum (Al) is greater than 2.0%,
the unit cost of steel making may be increased.
[0025] Sulfur (S) is an element inevitably included in the steel
sheet during a manufacturing process thereof. Sulfur (S) combines
with iron (Fe) to form FeS causing red shortness. Therefore, it may
be necessary to keep the content of sulfur (S) as low as possible.
For example, the content of sulfur (S) may be limited to 0.015% or
less.
[0026] Nitrogen (N) is an element inevitably included in the steel
sheet during a manufacturing process. The content of nitrogen (N)
may be kept as low as possible. For example, the content of
nitrogen (N) may be limited to 0.01% or less.
[0027] In addition to the above-mentioned elements, the steel sheet
for hot press forming may further include at least one element
selected from the group consisting of Mo: 0.5% or less (excluding
0%), Cr: 1.5% or less (excluding 0%), Ni: 0.5% or less (excluding
0%), Nb: 0.005% to 0.1%, and V: 0.005% to 0.1%.
[0028] Molybdenum (Mo) may be added to the steel sheet to suppress
the formation of pearlite. Since molybdenum (Mo) is relatively
expensive and may increase the manufacturing cost of the steel
sheet, 0.5 wt % or less of molybdenum (Mo) may be added.
[0029] Chromium (Cr) may be added to the steel sheet to suppress
the formation of ferrite and expand bainite transformation if the
content of chromium (Cr) is greater than 1.5 wt %, chromium carbide
may he formed to lower the amount of dissolved. carbon (C).
Therefore, 1.5 wt % or less of chromium (Cr) may be added.
[0030] Nickel (Ni) may be added to increase the faction of
austenite and the hardenability of the steel sheet. Since nickel
(Ni) is expensive and increases the manufacturing cost of the steel
sheet, 0.5 wt % or less of nickel (Ni) may be added.
[0031] Niobium (Nb) may be added to improve the strength, grain
refining characteristics, and ductility of the steel sheet. During
reheating, niobium (Nb) suppresses grain. growth, and during
cooling, niobium (Nb) delays transformation of austenite into
ferrite, 0.005 wt % to 0.1 wt % of niobium (Nb) may be added, if
the content of niobium (Nb) is less than 0.005%, it is difficult to
assure the effect of grain refinement, and if the content of
niobium (Nb) is greater than 0.1%, carbonitrides may excessively
precipitate to cause delayed fractures in the steel sheet or
decrease the workability of the steel sheet.
[0032] Vanadium (V) may be added to improve the strength, drain
refining characteristics, and hardenability of the steel sheet.
0.005 wt % to 0.1 wt % of vanadium (V) may be added. If the content
of vanadium (V) is less than 0.005%, such effects may not be
obtained, and if the content of vanadium (V) is greater than 0.1%,
carbonitrides may excessively precipitate to cause delayed
fractures in the steel sheet or decrease the workability of the
steel sheet.
[0033] In addition, the steel sheet for hot press forming may
further include B: 0.005% or less (excluding 0%) and Ti: 0.06% or
less (excluding 0%).
[0034] Boron (B) may be added to suppress the formation of ferrite.
If the content of boron (B) is greater than 0.005 wt %, boron (B)
may combine with iron (Fe) or carbon (C) to form a compound
facilitating the formation of ferrite. Therefore, 0.005 wt % of
less of boron (B) may be added.
[0035] Titanium (Ti) may be added to maximize the effect of boron
(B), Titanium (Ti) combines with nitrogen (N) existing as an
impurity in the steel sheet to form TiN, and thus boron (B) may not
combine with nitrogen (N). Therefore, the formation of ferrite may
be suppressed by boron (B). This effect may be assured by adding
0.06 wt % or less of titanium (Ti).
[0036] The steel sheet may be a hot-rolled or cold-rolled steel
sheet. For example, the steel sheet may be a cold-rolled steel
sheet coated with a plating layer for improving corrosion
resistance and suppressing the formation of a surface oxide
layer.
[0037] According to the embodiment of the present disclosure, since
the steel sheet for hot press forming has high strength and high
ductility owing to the above-described composition, the steel sheet
may be usefully used to manufacture hot-press formed members
(described later) having high strength and ductility.
[0038] (Method for manufacturing steel sheet for hot press
forming)
[0039] Hereafter, a method for manufacturing a steel sheet for hot
press forming will be described in detail according to an
embodiment of the present disclosure. This embodiment is an
exemplary example for manufacturing a steel sheet suitable for
manufacturing a hot-press formed member having improved
ductility.
[0040] The method for manufacturing a steel sheet for hot press
forming includes: heating a steel slab to a temperature within a
range of 1100.degree. C. to 1300.degree. C., the steel slab
including, by wt %, 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to
2.0%, Al: 0.01% to 2.0%, 0.015% or less, N: 0.01% or less, and the
balance of Fe and inevitable impurities; performing a finish
hot-rolling process at a temperature within a range of Ar.sub.3
transformation point to 950.degree. C. to form a steel sheet; and
coiling the steel sheet at a temperature within a range of M.sub.s
to 720.degree. C.,
[0041] If the steel slab is heated to a temperature lower than
1100.degree. C., the continuous-casting structure of the steel slab
may be insufficiently uniformized, and it may be difficult to
assure a finish rolling temperature. If the steel slab is heated to
a temperature greater than 1300.degree. C., the size of crystal
grains and the possibility of surface oxidation may increase to
deteriorate the strength and surface properties of the steel slab.
Therefore, the steel slab may be heated to a temperature within a
range of 1100.degree. C. to 1300.degree. C. If the finish
hot-rolling temperature is lower than Ar.sub.3 transformation
point, dual phase rolling may occur to result in hot-rolling mixed
grain sizes, and if the finish hot-rolling temperature is higher
than 950.degree. C., crystal grains may be coarsened and surface
oxidation may occur during the finish hot-rolling process.
Therefore, the finish hot-rolling temperature may be within the
range of the Ar.sub.3 transformation point to 950.degree. C. In
addition, if the coiling temperature is lower than M.sub.s,
austenite mar transform to martensite to decrease the ductility of
the steel sheet and thus to make it difficult to perform a hot
coiling process on the steel sheet. If the coiling temperature is
higher than 720.degree. C., a thick surface oxide layer may be
formed on the steel sheet together with internal oxidation in the
steel sheet. Therefore, the coiling temperature may be within the
range of M.sub.s to 720.degree. C.
[0042] The method for manufacturing a steel sheet for hot press
forming may include: heating a steel slab a temperature within a
range of 1100.degree. C. to 1300.degree. C., the steel slab
including, by wt %, 0: 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to
2.0%, Al: 0.01% to 2.0%, S: 0.015% or less, N: 0.01% or less, and
the balance of Fe and inevitable impurities; performing a finish
hot-rolling process at a temperature within a range of Ar.sub.3
transformation point to 950.degree. C. to form a steel sheet;
coiling the steel sheet at a temperature within a range of M.sub.s
to 720.degree. C.; pickling the steel sheet; cold-rolling the steel
sheet; continuously annealing the steel sheet at a. temperature
within a range of 750.degree. C. to 900.degree. C.; and overaging
the steel sheet at temperature within a range of M.sub.s to
550.degree. C.
[0043] The pickling of the steel sheet is performed to remove
surface oxides formed during the heating and finish hot-rolling
processes. Thereafter, the cold-rolling
[0044] process is performed. If the continuous annealing
temperature for the cold-rolled steel sheet is lower than
750.degree. C., recrystallization may occur insufficiently, and
thus a desired degree of workability of the steel sheet may not be
obtained. If the continuous annealing temperature is higher than
900.degree. C., it may difficult to heat the steel sheet to the
continuous annealing temperature due to the limitation of heating
equipment. In addition, if the overaging temperature is lower than
M.sub.s, martensite may be formed to excessively increase the
strength of the steel sheet and negatively affect the ductility of
the steel sheet. Therefore, before a hot press forming process,
blanking may not be easily performed. If the overaging temperature
is higher than 550.degree. C., the processability of the steel
sheet may be lowered due to roll surface deterioration in an
annealing furnace, and intended carbide precipitation and bainite
transformation may not occur in the overaging process.
[0045] The method for manufacturing a steel sheet for hot press
forming may include: heating a steel slab to a temperature range of
1100.degree. C. to 1300.degree. C. the steel slab including, by wt
%, C: 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01%
to 2.0%, 0.015% or less, N: 0.01% or less, and the balance of Fe
and inevitable impurities; performing a finish hot-rolling process
at a temperature within a range of Ar.sub.3 transformation point to
950.degree. C. to form a steel sheet; coiling the steel sheet at a
temperature within a range of M.sub.s to 20.degree. C.; pickling
the steel sheet; cold-rolling the steel sheet; continuously
annealing the steel sheet at a temperature within a range of
750.degree. C. to 900.degree. C.; overaging the steel sheet at a
temperature within a range of M.sub.s to 550.degree. C.; and
plating the overaged steel sheet by any one of hot-dip galvanizing,
galvannealing, electro galvanizing, and hot-dip aluminizing.
[0046] A hot-dip galvanized steel, sheet may be manufactured by
dipping a cold-rolling steel sheet in a galvanizing bath. A
galvannealed steel sheet may be manufactured by dipping a
cold-rolled steel sheet in a plating bath and performing an
alloying heat-treatment process on the steel sheet. An
electro-galvanized steel sheet may be manufactured by performing an
electro galvanizing process or a Zn--Fe electroplating process on a
cold-rolled steel sheet in a continuous electroplating line. A
hot-dip aluminized steel sheet may be manufactured by heating a
cold-rolled steel sheet, dipping the steel sheet in an aluminum
plating bath, and cooling the steel sheet at room temperature at a
cooling rate of 5.degree. C./sec to 15.degree. C./sec.
[0047] The steel slab may further include at least one selected
from the group consisting of Mo: 0.5% or less (excluding 0%), Cr:
1.5% or less (excluding 0%), Ni: 0.5% or less (excluding 0%), Nb:
0.005% to 0.1%, and V: 0.005% to 0.1%. In addition, the steel slab
may further include B: 0.005% or less (excluding 0%) and Ti: 0.06%
or less (excluding 0%).
[0048] (Hot-press formed member)
[0049] Hereinafter, a hot-press formed member will be described in
detail according to an embodiment of the present disclosure.
[0050] The hot-press formed member has high ductility and high
strength. For this, the hot-press formed member includes, by wt %,
C: 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01% to
2.0%, S: 0.015% or less, N: 0.01% or less, and the balance of Fe
and inevitable impurities. The hot-press formed member may have a
microstructure formed of bainite and retained austenite without
martensite.
[0051] The hot-press formed member may further include at least one
selected from the group consisting of Mo: 0.5% or less (excluding
0%), Cr: 1.5% or less (excluding 0%), Ni: 0.5% or less (excluding
0), Nb: 0.005% to 0.1%, and V: 0.005% to 0.1%. In addition, the
hot-press formed. member may further include B: 0.005% or less
(excluding 0%) and Ti: 0.06% or less (excluding 0%)
[0052] Hot-press formed members of the related art are manufactured
to have ultra high strength, and thus martensite requisitely formed
therein. However, martensite lowers the ductility of such hot-press
formed. members and thus makes such hot-press formed members
unsuitable to be used as crashworthy members of automobiles.
Therefore, in the embodiment of the present disclosure, the
formation of martensite in the hot-press formed member is
suppressed, and the amount of retained austenite is increased.
Thus, the hot-press formed member has dual phases: bainite and
retained austenite.
[0053] The hot-press formed member having the above-mentioned
composition and microstructure has good strength-ductility balance.
For example, TS*E1 of the hot-press formed member may be 25,000 or
greater so as to be used as a crashworthy member of an automobile
as well as being used as an impact member, where TS denotes tensile
strength [MPa] as and E1 denotes elongation [%].
[0054] (Method for manufacturing hot-press formed member)
[0055] Hereinafter, a method for manufacturing a hot-press formed
member will be described in detail according to an embodiment of
the present disclosure.
[0056] The method is for performing a hot press forming process on
the above-described steel sheet to provide an ultra high-strength
automotive component having high ductility. For this, the method
includes: heating a steel sheet to a temperature equal to or higher
than Ac.sub.3, the steel sheet including, by wt %, C: 0.3% to 1.0%,
Mn: 0.0l% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01% to 2.0%, S: 0.015%
or less, N: 0.01% or less, and the balance of Fe and inevitable
impurities; hot-press forming the heated steel sheet; cooling the
hot-press formed steel sheet to temperature range of M.sub.s to
550.degree. C. at a rate of 20.degree. C./sec or higher; and
heat-treating the cooled steel sheet in a heating furnace heated at
a temperature within a range of M.sub.s to 550.degree. C.
[0057] The steel sheet may further include at least one selected
from the group consisting of Mo: 0.5% or less (excluding 0%), Cr:
1.5% or less (excluding 0%), 0.5% or less (excluding 0%), Nb:
0.005% to 0.1%, and V: 0.005% to 0.1%. In addition, the steel sheet
may further include B: 0.005% or less (excluding 0%) and Ti: 0.06%
or less (excluding 0%). The steel sheet may be one of a hot-rolled
steel sheet, a cold-rolled steel sheet, and a plated cold-rolled
steel sheet coated with a plating layer.
[0058] In the method for manufacturing a hot-press formed member
according to the embodiment of the present. disclosure, the
heat-treating after the hot-press forming is controlled differently
as compared with the case of the related art, so as to manufacture
a hot-press formed member having a different microstructure for
improving ductility as compared with a hot-press formed member of
the related art. That is, in the related art, heat-treatment
conditions are adjusted to form martensite as a main microstructure
to finally obtain an ultra high-strength member. However, since
such a technique of the related art is not suitable to manufacture
a highly ductile member usable as a crashworthy member of an
automobile, the inventors have suggested heat treatment conditions
for forming a microstructure constituted by bainite and retained
austenite without martensite.
[0059] First, the steel sheet is heated to a temperature equal to
Ac.sub.3 or higher for transformation to austenite, and is then
hot-press formed.
[0060] The heat-treatment conditions after the hot-press forming
have a major effect on determining the microstructure of a product
the related art, generally a hot-press formed steel sheet is
directly die-quenched to a temperature equal to or lower than
M.sub.s so as to form martensite as a main microstructure in a
final product and thus to enhance the strength of the final
product.
[0061] However, in the embodiment of the present disclosure,
martensite is excluded from the microstructure of a final product
so as to improve the ductility of the final product while
maintaining the strength of the final product at a level suitable
for weight reduction. To this end, instead of cooling the hot-press
formed steel sheet directly to room temperature equal to or lower
than M.sub.s, the hot-press formed steel sheet is cooled to a
temperature range of M.sub.s to 550.degree. C., and heat-treated in
a heating furnace at a temperature within a range of M.sub.s to
550.degree. C. so as to cause the hot-press formed steel sheet to
undergo transformation to bainite. If the steel sheet is cooled to
a temperature equal to or lower than M.sub.s, martensite may be
formed to lower the ductility of the steel sheet, and if the steel
sheet is cooled to a temperature higher than 550.degree. C.,
pearlite may be formed to deteriorate properties of the steel
sheet. Therefore, the cooling rate is adjusted to be within the
range of M.sub.s to 550.degree. C. to form a dual phase
microstructure constituted by bainite and retained austenite.
[0062] In the bainite formed as described above, Fe.sub.3C carbide
may not be formed because elements such as silicon are sufficiently
included in the steel sheet to diffuse carbon (C) into the retained
austenite. That is, carbon (C) does not form carbides but is
dissolved in the retained austenite to stabilize the retained
austenite and thus to lower M.sub.s. Therefore, in the next cooling
process, transformation to martensite is suppressed. Therefore, in
a final product, the retained austenite remains instead of
undergoing transformation to martensite, thereby improving
ductility.
[0063] The cooling rate may be 20.degree. C./sec or higher. If the
cooling rate is lower than 20.degree. C./sec, transformation to
pearlite may easily occur to lower properties of a final product.
Referring to FIG. 2A, bainite was formed at a cooling rate of
30.degree. C./sec. However, referring to FIGS. 2B and 2C, a
pearlite structure in which ferrite and Fe.sub.3C were layered was
formed at a cooling rate of 5.degree. C./sec.
[0064] For example, the above-described processes for manufacturing
a hot-press formed member according to the embodiment of the
present disclosure may be summarized as follows. First, a steel
sheet is inserted in a heating furnace to heat the steel sheet to
Ac.sub.3 or higher for forming austenite, and then the heated steel
sheet is hot-press formed. After the hot press forming, the steel
sheet is cooled to a temperature range of M.sub.s to 550.degree. C.
at a cooling rate of 20.degree. C. sec or higher so as not to form
pearlite, and is then heat-treated in a heating furnace at a
temperature within a range of M.sub.s to 550.degree. C.. These
processes are for transformation to bainite, and during the
processes, carbon (C) diffuses into austenite to lower M.sub.s.
Although a hot-press formed member manufactured through the
above-described processes is cooled to room temperature without any
controlling, transformation to martensite does not occur. That is,
a dual phase microstructure constituted by bainite and retained
austenite may be obtained.
[0065] Hereinafter, the embodiments of the present disclosure will
be described more specifically according to examples. The following
examples are merely provided to allow for a clear understanding of
the present disclosure, rather than to limit the scope thereof.
Mode for Invention
Examples
[0066] Steel ingots 90 mm in length and 175 mm. in width having
compositions shown in Table 1 were manufactured. by vacuum melting,
and were then re-heated. at 1200.degree. C. for 1 hour, Thereafter,
the steel ingots were hot-rolled to obtain steel sheets having a
thickness of 3 mm, At that time, a finish hot-rolling temperature
was Ar.sub.3 or higher. Then, after cooling the steel sheets, the
steel sheets were inserted into a heating furnace previously heated
to 600.degree. C. and left in the heating furnace for 1 hour.
Thereafter, the steel sheets were cooled in the heating furnace to
simulate hot coiling. Next, the steel sheets were cold-rolled at a
reduction ratio of 60% to a thickness of 1.2 mm and were annealed
at 900.degree. C. Then, the steel sheets were allowed to undergo
bainite transformation at 400.degree. C., In Table 1, the contents
of elements are given in wt % except for the contents of sulfur (S)
and nitrogen (N) given in ppm.
TABLE-US-00001 TABLE 1 Steels C Si Mn Al Mo Cr Ni Ti B Nb V S N IS*
1 0.40 1.51 3.01 0.04 30 20 IS 2 0.63 1.49 0.72 0.60 20 20 IS 3
0.61 1.52 0.63 0.51 0.30 30 20 IS 4 0.61 1.50 0.65 0.50 0.015 30 20
IS 5 0.62 1.49 1.61 1.53 0.02 30 20 IS 6 0.60 1.50 2.91 0.04 0.25
1.20 20 20 IS 7 0.71 1.47 0.70 0.52 0.010 0.002 20 20 IS 8 0.68
1.48 0.71 0.04 0.24 30 20 IS 9 0.70 1.15 0.72 0.51 0.24 30 20 IS 10
0.71 1.15 0.71 0.04 0.24 0.010 0.002 30 20 IS 11 0.69 1.55 0.18
0.04 0.24 0.50 0.010 0.002 30 20 IS 12 0.82 1.49 0.51 0.54 30 20 IS
13 0.82 1.51 1.01 0.53 30 20 CS** 1 0.23 1.5 1.5 0.04 30 20 CS 2
0.20 0.5 1.5 0.03 30 20 CS 3 0.22 1.5 2 0.03 0.20 20 20 CS 4 0.68
0.42 0.70 0.52 20 20 *IS: Inventive Steel, **CS: Comparative
Steel
[0067] To simulate a heat treatment in a heating furnace during a
hot press forming process, the 1.2 mm thickness steel sheets
manufactured as described above were heated to a temperature of
900.degree.C. and maintained at the temperature for 30 seconds,
Then, the steel sheets were cooled to cooling temperatures at a
rate of 30.degree.C./sec. Next, the steel sheets were inserted into
a heating furnace and heat-treated in the heating furnace at the
same temperatures as the cooling temperatures for 400 seconds to
10,800 seconds. Thereafter, the steel sheets were air-cooled. In
this way, hot-press formed. members were obtained. The process
conditions and mechanical properties of the hot-press formed
members are shown in Table 2 below.
TABLE-US-00002 TABLE 2 Cooling Rate Cooling Time YS YS E1 TS * E1
M.sub.s Steels (.degree. C./sec) Temperature (.degree. C.) (sec)
(MPa) (MPa) (%) (MPa %) (.degree. C.) Is* 1 30 400 3600 732 1265 28
35420 295 IS 2 30 400 3600 899 1187 39 46244 298 IS 3 30 400 3600
869 1196 37 44252 302 IS 4 30 400 3600 915 1289 35 45115 307 IS 5
30 400 3600 883 1185 36 42660 272 IS 6 30 400 10800 856 1420 26
36920 199 30 300 10800 985 1610 22 35420 199 IS 7 30 400 3600 900
1185 40 46923 273 5 400 3600 719 1128 11 12408 273 IS 8 30 400 600
816 1310 21 27510 280 IS 9 30 400 600 915 1240 29 35960 277 IS 10
30 400 600 845 1318 25 32950 270 IS 11 30 400 600 940 1288 26 33488
280 IS 12 30 400 3600 881 1229 35 43556 245 IS 13 30 400 3600 725
1306 39 50934 228 CS** 1 30 400 3600 640 1125 15 16875 399 30 250
3600 1295 1511 6 9066 399 CS 2 30 250 3600 1220 1450 7 10150 420 CS
3 30 250 3600 1280 1490 6 8940 384 CS 4 30 400 3600 870 1201 16
19216 295 *IS: Inventive Steel, **CS: Comparative Steel
[0068] Since TS*E1 of Comparative Steel 1 cooled. at a cooling rate
of 400.degree. C. is 16,735 MPa %, Comparative Steel 1 is not
suitable as a crashworthy member of an automobile. The reason for
this may be that the insufficient content of carbon (C) led to
failure in stabilizing retained austenite. in the case that the
cooling rate was 250.degree. C., Comparative Steel 1 was cooled to
a temperature lower than M.sub.s to result in a large amount of
transformation to martensite, and thus Comparative Steel 1 had high
strength but low ductility. In this case, TS*El of Comparative
Steel 1 is 9,066 MPa %, and Comparative Steel 1 is not suitable to
form a crashworthy member of an automobile.
[0069] The carbon (C) content and silicon (Si) content of
Comparative Steel 2 are also not sufficient to stabilize retained
austenite, and the cooling temperature of Comparative Steel 2 is
equal to or lower than M.sub.s to result in transformation to
martensite. Therefore, Comparative Steel 2 has low ductility, and
TS*E1 thereof is low at 10,150 MPa %. Comparative Steel 3 also has
an insufficient content of carbon (C), and the cooling temperature
of Comparative Steel 3 is equal to or lower than M.sub.s.
Therefore, TS*E1 of Comparative Steel 3 is low at 8,940 MPa %, and
Comparative Steel 3 is not suitable to form a. crashworthy member
of an automobile.
[0070] Although Comparative Steel 4 has a sufficient content of
carbon (C), the silicon (Si) content of Comparative Steel 4 is not
sufficient to fully diffuse carbon (C) into retained austenite.
Therefore, although TS*E1 of Comparative Steel 4 is relatively high
at 19,216 MPa % as compared with other comparative steels, TS*E1 of
Comparative Steel 4 is not greater than 25,000 MPa %. That is,
Comparative Steel 4 is not suitable for forming a crashworthy
member of an automobile.
[0071] Samples of inventive Steel 7 having a composition within the
range of the present disclosure were cooled at a cooling rate of
30.degree. C./sec and at a cooling rate of 5.degree. C./sec,
respectively. In the case that the cooling rate was 30.degree.
C./sec, TS*E1 of Inventive Steel 7 was high at 46,923 MPa % and
suitable for a crashworthy member of an automobile. However, in the
case that the cooling rate was 5.degree. C./sec, TS*E1 of Inventive
Steel 7 was low at 12,480 MPa % and not suitable for a crashworthy
member of an automobile. The reason for this may be that the low
cooling rate led. to the formation of pearlite as shown in FIGS. 2A
to 2C and deterioration of properties thereof.
* * * * *