U.S. patent application number 17/639414 was filed with the patent office on 2022-09-22 for steel sheet for hot forming, hot-formed member, and method for manufacturing same.
This patent application is currently assigned to POSCO. The applicant listed for this patent is POSCO. Invention is credited to Dongchul Chae, Hyosik Chun, Gyujin Jo, Jae-hwa Lee, Hyunsung Son.
Application Number | 20220298595 17/639414 |
Document ID | / |
Family ID | 1000006433905 |
Filed Date | 2022-09-22 |
United States Patent
Application |
20220298595 |
Kind Code |
A1 |
Lee; Jae-hwa ; et
al. |
September 22, 2022 |
STEEL SHEET FOR HOT FORMING, HOT-FORMED MEMBER, AND METHOD FOR
MANUFACTURING SAME
Abstract
Disclosed are a high-strength and non-plated steel sheet which
is for hot forming and may be suitable for use in automotive
structural members that require collision resistance
characteristics, a hot-formed member, and a method for
manufacturing same. A steel sheet for hot forming and a hot-formed
member according to an embodiment of the present disclosure
contain, in percent by weight (wt %), 0.05 to 0.3% of carbon (C),
0.5 to 3.0% of silicon (Si), 0.1 to 2.0% of manganese (Mn), 3.0 to
9.0% of chromium (Cr), more than 0% and less than 0.2% of nitrogen
(N), and 0.03 to 1.0% of niobium (Nb), with the remainder
comprising iron (Fe) and inevitable impurities.
Inventors: |
Lee; Jae-hwa; (Pohang-si,
Gyeongsangbuk-do, KR) ; Jo; Gyujin; (Pohang-si,
Gyeongsangbuk-do, KR) ; Chun; Hyosik; (Gwangyang-si,
Jeollanam-do, KR) ; Chae; Dongchul; (Pohang-si,
Gyeongsangbuk-do, KR) ; Son; Hyunsung; (Incheon,
KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
POSCO |
Pohang-si, Gyeongsangbuk-do |
|
KR |
|
|
Assignee: |
POSCO
Pohang-si, Gyeongsangbuk-do
KR
|
Family ID: |
1000006433905 |
Appl. No.: |
17/639414 |
Filed: |
September 1, 2020 |
PCT Filed: |
September 1, 2020 |
PCT NO: |
PCT/KR2020/011684 |
371 Date: |
March 1, 2022 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 9/46 20130101; C22C
38/54 20130101; C22C 38/04 20130101; C21D 6/004 20130101; C22C
38/002 20130101; C22C 38/02 20130101; C21D 8/0226 20130101; C21D
8/0263 20130101; C21D 6/005 20130101; C22C 38/48 20130101; C22C
38/001 20130101; C21D 2211/004 20130101; C21D 6/008 20130101; C22C
38/60 20130101; C21D 8/0236 20130101; C21D 2211/005 20130101; C22C
38/06 20130101; C21D 8/0205 20130101 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C21D 8/02 20060101 C21D008/02; C21D 6/00 20060101
C21D006/00; C22C 38/60 20060101 C22C038/60; C22C 38/54 20060101
C22C038/54; C22C 38/48 20060101 C22C038/48; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 3, 2019 |
KR |
10-2019-0108827 |
Claims
1. A steel sheet for hot forming comprising, in percent by weight
(wt %), 0.05 to 0.3% of carbon (C), 0.5 to 3.0% of silicon (Si),
0.1 to 2.0% of manganese (Mn), 3.0 to 9.0% of chromium (Cr), more
than 0% and less than 0.2% of nitrogen (N), 0.03 to 1.0% of niobium
(Nb), and the remainder of iron (Fe) and inevitable impurities,
wherein a microstructure comprises a ferrite phase and 20 vol % or
less of a carbonitride.
2. The steel sheet according to claim 1, wherein the ferrite phase
has an average grain size of 100 .mu.m or less.
3. The steel sheet according to claim 1, wherein the steel sheet
satisfies Expression (1) below:
0.80*Si+0.57*Cr-3.53*C-1.45*Mn-1.9>0 (1) (wherein Si, Cr, C, and
Mn denote contents (wt %) of the elements, respectively).
4. The steel sheet according to claim 1, wherein a content of Cr is
from 3.5 to 5.5%.
5. The steel sheet according to claim 1, further comprising less
than 3.0% of nickel (Ni).
6. The steel sheet according to claim 1, further comprising less
than 0.1% of phosphorus (P) and less than 0.01% of sulfur (S).
7. A hot-formed member comprising, in percent by weight (wt %),
0.05 to 0.3% of carbon (C), 0.5 to 3.0% of silicon (Si), 0.1 to
2.0% of manganese (Mn), 3.0 to 9.0% of chromium (Cr), more than 0%
and less than 0.2% of nitrogen (N), 0.03 to 1.0% of niobium (Nb),
and the remainder of iron (Fe) and inevitable impurities.
8. The hot-formed member according to claim 7, wherein the
hot-formed member satisfies Expression (1) below:
0.80*Si+0.57*Cr-3.53*C-1.45*Mn-1.9>0 (1) (wherein Si, Cr, C, and
Mn denote contents (wt %) of the elements, respectively).
9. The hot-formed member according to claim 7, wherein an average
oxygen content is 20 wt % or less at a point of 0.1 .mu.m depth
from the surface.
10. The hot-formed member according to claim 7, wherein the
hot-formed member has a yield strength of 1,100 MPa or more and a
tensile strength of 1,500 MPa or more.
11. The hot-formed member according to claim 7, wherein a content
of Cr is from 3.5 to 5.5%.
12. The hot-formed member according to claim 7, further comprising
less than 3.0% of nickel (Ni).
13. The hot-formed member according to claim 7, further comprising
less than 0.1% of phosphorus (P) and less than 0.01% of sulfur
(S).
14. A method for manufacturing a hot-formed member, the method
comprising: preparing a steel sheet for hot forming comprising, in
percent by weight (wt %), 0.05 to 0.3% of carbon (C), 0.5 to 3.0%
of silicon (Si), 0.1 to 2.0% of manganese (Mn), 3.0 to 9.0% of
chromium (Cr), more than 0% and less than 0.2% of nitrogen (N),
0.03 to 1.0% of niobium (Nb), and the remainder of iron (Fe) and
inevitable impurities; heating the steel sheet at a rate of 1 to
1,000.degree. C./sec to a temperature range of Ac3+50.degree. C. to
Ac3+200.degree. C. and maintaining for 1 to 1,000 seconds; and
hot-forming the heated and maintained steel sheet and cooling the
steel sheet at a rate of 1 to 1000.degree. C./sec to a temperature
below Mf.
15. The method according to claim 14, wherein the steel sheet for
hot forming satisfies Expression (1) below.
0.80*Si+0.57*Cr-3.53*C-1.45*Mn-1.9>0 (1) (wherein Si, Cr, C, and
Mn denote contents (wt %) of the elements, respectively).
16. The method according to claim 14, wherein the steel sheet for
hot forming comprises a microstructure comprising a ferrite phase
and 20 vol % or less of a carbonitride, wherein an average grain
size of the ferrite phase is 100 .mu.m or less.
17. The method according to claim 14, wherein a content of Cr in
the steel sheet for hot forming is from 3.5 to 5.5%.
18. The method according to claim 14, wherein the steel sheet for
hot forming further comprises less than 3.0% of nickel (Ni).
19. The method according to claim 14, wherein the steel sheet for
hot forming further comprises less than 0.1% of phosphorus (P) and
less than 0.01% of sulfur (S).
20. The method according to claim 14, wherein the preparing of the
steel sheet for hot forming comprises: reheating a slab in a
temperature range of 1,000 to 1,300.degree. C.; preparing a
hot-rolled steel sheet by finish-rolling the reheated slab in a
temperature range higher than Ar3 and equal to or lower than
1,000.degree. C.; coiling the hot-rolled steel sheet in a
temperature range higher than Ms and equal to or lower than
850.degree. C.; and acid-pickling the coiled, hot-rolled steel
sheet.
21. The method according to claim 20, further comprising: preparing
a cold-rolled steel sheet by rolling the acid pickled, hot-rolled
steel sheet with a reduction ratio of 30 to 80%; and continuously
annealing the cold-rolled steel sheet in a temperature range of 700
to 900.degree. C.
22. The method according to claim 20, further comprising
batch-annealing the coiled, hot-rolled or acid-pickled steel sheet
in a temperature range of 500 to 850.degree. C. for 1 to 100 hours.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to a steel sheet for hot
forming, a hot-formed member using the same, and a method for
manufacturing the same, and more particularly, to a high-strength
and non-plated steel sheet which is for hot forming and may be
suitable for use in automotive structural members that require
collision resistance characteristics, a hot-formed member, and a
method for manufacturing the same.
BACKGROUND ART
[0002] As various safety regulations for protecting passengers of
vehicles have been strengthened and interest in environmental
issues has grown recently, regulations on fuel efficiency and
CO.sub.2 emission have also been strengthened.
[0003] Accordingly, thickness of materials used for the vehicles
may be reduced to increase fuel efficiency, but a decrease in
thickness may cause a stability problem in the vehicles and thus
enhancement of strength of the material should be accompanied
therewith.
[0004] A process of increasing strength of a material causes a
decrease in elongation together with an increase in yield strength,
resulting in deterioration of formability in most cases. Therefore,
advanced high strength steels (AHSS) such as dual phase (DP) steels
and TRIP steels have been developed based on studies on various
materials and have been actually applied to automobile parts, and
such steel sheets exhibit excellent formability compared to
conventional high-strength steels for vehicles.
[0005] However, a higher forming force is required to form
automobile parts with an increased strength of a material as
described above, and thus capacity and load of a press need to be
increased. Also, molding of the high-strength material may cause a
decrease in mold life and a decrease in productivity.
[0006] Although a martensitic steel having an ultra-high strength
of 1,000 MPa or more may be effective on reducing a weight of the
body of a vehicle when used in the vehicle, commercialization of
the martensitic structure is difficult due to poor formability.
[0007] As methods for commercialization using a martensitic steel,
a method of preparing a high-strength martensitic structure by cold
forming an initial ferritic structure having excellent formability,
forming an austenite by heat treatment at a high temperature, and
quenching the resultant has been used. However, a problem of poor
shape fixability may occur according to the above-described forming
method due to phase transformation in a non-constrained sate.
Particularly, a volume change is accompanied by a change in the
crystal structure from FCC to BCT in phase transformation from
austenite to martensite occurring during a cooling process, and
accordingly dimensional precision deteriorates. Thus, an additional
process of performing dimensional correction is required.
[0008] To solve these problems, a forming method called hot press
forming (HPF) or hot forming has recently been proposed. Hot press
forming is a forming method to increase a strength of a final
product by preparing an austenite single phase by heating a steel
sheet at a high temperature higher than Ac1 at which processing is
easily performed, hot forming the steel sheet by press forming, and
forming a low-temperature structure such as martensite by
quenching. The hot forming is advantageous in that a problem in
formability caused during preparation of a high-strength material
may be minimized.
[0009] However, because the steel sheet is heated to a high
temperature in the case of using the hot press forming method, the
surface of the steel sheet is oxidized, and thus a process of
removing oxides from the surface of the steel sheet needs to be
added after the press forming.
[0010] To solve these problems, a method disclosed in Patent
Document 1 has been proposed. In Patent Document 1, although a
steel sheet coated with Al--Si is heated at a temperature of
850.degree. C. or higher and then hot-pressed to form a martensite
structure, the steel sheet is not oxidized during heating due to an
Al--coating layer formed on the surface of the steel sheet. When
hot press forming is performed using the Al-coated steel sheet, not
only a product having an ultra-high strength of 1,000 MPa may be
easily obtained but also a product having high dimensional
precision may be obtained, and thus the hot press forming has drawn
attention and interest as a very effective method for forming
automobile parts on a decrease in weight and an increase in
rigidity of vehicles.
[0011] However, several problems have recently be raised in the hot
press forming method using an Al-coated steel sheet during a
forming process and a subsequent bonding/welding process between
other members.
[0012] Among them, according to Patent Document 2, because a
plating layer includes aluminum as a main phase, aluminum may be
liquified at a temperature higher than a melting point of the
plating layer to be fused to a roll in a heating furnace when a
blank is heated in a heating furnace or partial exfoliation may
occur due to stress.
[0013] Also, according to Patent Document 3, a hot-pressed, formed
member may be prepared by bonding two or more members using an
adhesive. In this case, a sufficient adhesive strength needs to be
maintained to verify adhesive strength. A method of testing whether
the bonded portion is easily maintained at a high strength by
applying a tensile stress in a direction perpendicular to the
bonded surface is often used. In this case, a plating layer is
often detached from the inside of the plating layer or an interface
between the plating layer and a steel sheet. In this case, a
problem of separation of the two members may occur even under a low
stress.
[0014] In addition, according to Patent Document 4, a tailored
welded blank (TWB), which is made by pre-bonding different steel
sheets having different thicknesses for decreasing a weight of a
vehicle, has been used as a major material in hot press forming.
The TWB is mainly prepared by laser bonding and it is known that
combination of the surface condition of a material and strength of
the raw material considerably affects properties. However, in the
case of a hot dip Al plated steel sheet, breakage of a welded part
was observed when deformed by press forming after heat treatment.
This is because Al on the plating layer of the surface penetrates
into the welded part during laser welding of a TWB material and
thus a ferrite phase remains in the welded part after heat
treatment to embrittle the welded part. To overcome this, an
additional process of removing a surface film is suggested before
laser welding of the hot dip Al plated steel sheet.
[0015] As described above, aluminum plating is essential in order
to prevent oxidation during heating for hot press forming of a
martensitic steel, but there is a need to develop technologies for
solving various problems occurring thereby.
[0016] [Patent Document 1] US Patent Publication No. 6,296,805
(Oct. 2, 2001)
[0017] [Patent Document 2] Korean Patent Publication No. 10-1696121
(Jan. 6, 2017)
[0018] [Patent Document 3] Korean Patent Application Publication
No. 10-2018-0131943 (Dec. 11, 2018)
[0019] [Patent Document 4] Korean Patent Application Publication
No. 10-2015-0075277 (Jul. 3, 2015)
DISCLOSURE
Technical Problem
[0020] Embodiments of the present disclosure have been proposed to
solve problems described above and provided are a steel sheet for
hot forming having ultra-high strength while preventing surface
oxidation during hot press forming without using a plating layer, a
hot-formed member, and a method for manufacturing the same.
Technical Solution
[0021] In accordance with an aspect of the present disclosure, a
steel sheet for hot forming includes, in percent by weight (wt %),
0.05 to 0.3% of carbon (C), 0.5 to 3.0% of silicon (Si), 0.1 to
2.0% of manganese (Mn), 3.0 to 9.0% of chromium (Cr), more than 0%
and less than 0.2% of nitrogen (N), 0.03 to 1.0% of niobium (Nb),
and the remainder of iron (Fe) and inevitable impurities, wherein a
microstructure comprises a ferrite phase and 20 vol % or less of a
carbonitride.
[0022] Also, according to an embodiment of the present disclosure,
tThe ferrite phase may have an average grain size of 100 .mu.m or
less.
[0023] Also, according to an embodiment of the present disclosure,
the steel sheet may satisfy Expression (1) below:
0.80*Si+0.57*Cr-3.53*C-1.45*Mn-1.9>0
[0024] Also, according to an embodiment of the present disclosure,
a content of Cr may be from 3.5 to 5.5%.
[0025] Also, according to an embodiment of the present disclosure,
the steel sheet may further include less than 3.0% of nickel
(Ni).
[0026] Also, according to an embodiment of the present disclosure,
the steel sheet may further include less than 0.1% of phosphorus
(P) and less than 0.01% of sulfur (S).
[0027] In accordance with another aspect of the present disclosure,
a hot-formed member includes, in percent by weight (wt %), 0.05 to
0.3% of carbon (C), 0.5 to 3.0% of silicon (Si), 0.1 to 2.0% of
manganese (Mn), 3.0 to 9.0% of chromium (Cr), more than 0% and less
than 0.2% of nitrogen (N), 0.03 to 1.0% of niobium (Nb), and the
remainder of iron (Fe) and inevitable impurities.
[0028] Also, according to an embodiment of the present disclosure,
the hot-formed member may satisfy Expression (1) below:
0.80*Si+0.57*Cr-3.53*C-1.45*Mn-1.9>0
[0029] Also, according to an embodiment of the present disclosure,
an average oxygen content may be 20 wt % or less at a point of 0.1
.mu.m depth from the surface.
[0030] Also, according to an embodiment of the present disclosure,
the hot-formed member may have a yield strength of 1,100 MPa or
more and a tensile strength of 1,500 MPa or more.
[0031] Also, according to an embodiment of the present disclosure,
a content of Cr may be from 3.5 to 5.5%.
[0032] Also, according to an embodiment of the present disclosure,
the hot-formed member may further include less than 3.0% of nickel
(Ni).
[0033] Also, according to an embodiment of the present disclosure,
the hot-formed member may further include less than 0.1% of
phosphorus (P) and less than 0.01% of sulfur (S).
[0034] In accordance with another aspect of the present disclosure,
a method for manufacturing a hot-formed member includes: preparing
a steel sheet for hot forming comprising, in percent by weight (wt
%), 0.05 to 0.3% of carbon (C), 0.5 to 3.0% of silicon (Si), 0.1 to
2.0% of manganese (Mn), 3.0 to 9.0% of chromium (Cr), more than 0%
and less than 0.2% of nitrogen (N), 0.03 to 1.0% of niobium (Nb),
and the remainder of iron (Fe) and inevitable impurities; heating
the steel sheet at a rate of 1 to 1,000 .degree. C./sec to a
temperature range of Ac3+50.degree. C. to Ac3+200.degree. C. and
maintaining for 1 to 1,000 seconds; and hot-forming the heated and
maintained steel sheet and cooling the steel sheet at a rate of 1
to 1000.degree. C./sec to a temperature below Mf
[0035] Also, according to an embodiment of the present disclosure,
the steel sheet for hot forming may satisfy Expression (1)
below.
0.80*Si+0.57*Cr-3.53*C-1.45*Mn-1.9>0 (1)
[0036] Also, according to an embodiment of the present disclosure,
the steel sheet for hot forming may include a microstructure
comprising a ferrite phase and 20 vol % or less of a carbonitride,
wherein an average grain size of the ferrite phase is 100 .mu.m or
less.
[0037] Also, according to an embodiment of the present disclosure,
a content of Cr in the steel sheet for hot forming may be from 3.5
to 5.5%.
[0038] Also, according to an embodiment of the present disclosure,
the steel sheet for hot forming may further include less than 3.0%
of nickel (Ni).
[0039] Also, according to an embodiment of the present disclosure,
the steel sheet for hot forming may further include less than 0.1%
of phosphorus (P) and less than 0.01% of sulfur (S).
[0040] Also, according to an embodiment of the present disclosure,
the preparing of the steel sheet for hot forming may include:
reheating a slab in a temperature range of 1,000 to 1,300.degree.
C.; preparing a hot-rolled steel sheet by finish-rolling the
reheated slab in a temperature range higher than Ar3 and equal to
or lower than 1,000.degree. C.; coiling the hot-rolled steel sheet
in a temperature range higher than Ms and equal to or lower than
850.degree. C.; and acid-pickling the coiled, hot-rolled steel
sheet.
[0041] Also, according to an embodiment of the present disclosure,
the method may further include: preparing a cold-rolled steel sheet
by rolling the acid pickled, hot-rolled steel sheet with a
reduction ratio of 30 to 80%; and continuously annealing the
cold-rolled steel sheet in a temperature range of 700 to
900.degree. C.
[0042] Also, according to an embodiment of the present disclosure,
the method may further include batch-annealing the coiled,
hot-rolled or acid-pickled steel sheet in a temperature range of
500 to 850.degree. C. for 1 to 100 hours.
Advantageous Effects
[0043] In the steel sheet for hot forming and the hot-formed member
according to an embodiment of the present disclosure, surface
oxidation is prevented during hot press forming by improving
oxidation resistance by controlling alloying elements, and thus
conventional aluminum plating may be omitted.
[0044] In addition, problems, which may occur during a hot press
forming process and a bonding/welding process performed between
different members when an Al-coated steel sheet is used, may be
solved.
[0045] In addition, high strength at an equivalent level to that of
conventional Al-plated steel materials may be obtained.
DESCRIPTION OF DRAWINGS
[0046] FIG. 1 is an electron microscope image showing a
microstructure of a steel sheet for hot forming according to an
embodiment of the present disclosure.
[0047] FIG. 2 is a photograph exemplarily illustrating good
formability (a) and poor formability (b) obtained when hot forming
is performed using a mini-bumper mold.
[0048] FIG. 3 is a graph illustrating tensile test results of
samples of examples and comparative examples which are hot-formed
using a plate-shaped mold.
[0049] FIGS. 4 and 5 are electron microscope images of
microstructures of steel sheets for hot forming according to an
example and a comparative example prior to formation,
respectively.
[0050] FIGS. 6 and 7 are graphs illustrating GDS analysis results
of hot-formed members obtained using a mini-bumper mold according
to an example exhibiting good oxidation resistance and a
comparative example exhibiting inferior oxidation resistance with
respect to depth from the surface.
BEST MODE
[0051] A steel sheet for hot forming according to an embodiment of
the present disclosure may include, in percent by weight (wt %),
0.05 to 0.3% of carbon (C), 0.5 to 3.0% of silicon (Si), 0.1 to
2.0% of manganese (Mn), 3.0 to 9.0% of chromium (Cr), more than 0%
and less than 0.2% of nitrogen (N), 0.03 to 1.0% of niobium (Nb),
and the remainder of iron (Fe) and inevitable impurities, wherein a
microstructure includes a ferrite phase and 20 vol % or less of a
carbonitride.
MODES OF THE INVENTION
[0052] Hereinafter, embodiments of the present disclosure will be
described in detail with reference to the accompanying drawings.
The following embodiments are provided to fully convey the spirit
of the present disclosure to a person having ordinary skill in the
art to which the present disclosure belongs. The present disclosure
is not limited to the embodiments shown herein but may be embodied
in other forms. In the drawings, parts unrelated to the
descriptions are omitted for clear description of the disclosure
and sizes of elements may be exaggerated for clarity.
[0053] All of the above-described problems occurring during the hot
forming process and the bonding/welding process are caused by
presence of a plating layer. The present inventors have designed
optimum alloying elements such as Cr, Si, and Mn to obtain high
strength at an equivalent level to that of conventional
plated-steel sheet, to inhibit surface oxidation without using a
plated layer, and to have excellent formability suitable for
preparation of a formed member.
[0054] A steel sheet for hot forming and a hot-formed member
according to an embodiment of the present disclosure may include,
in percent by weight (wt %), 0.05 to 0.3% of carbon (C), 0.5 to
3.0% of silicon (Si), 0.1 to 2.0% of manganese (Mn), 3.0 to 9.0% of
chromium (Cr), more than 0% and less than 0.2% of nitrogen (N),
0.03 to 1.0% of niobium (Nb), and the remainder of iron (Fe) and
inevitable impurities.
[0055] Hereinafter, reasons for numerical limitations on the
contents of alloying elements in the embodiment of the present
disclosure will be described. Hereinafter, the unit is wt % unless
otherwise stated.
[0056] The content of C is from 0.05 to 0.3%.
[0057] C is an element not only effective on stabilization of an
austenite phase but also effective on obtaining high strength by
solid solution strengthening effects. However, an excess of C may
not only deteriorate processibilty due to an increase in a carbide
in a microstructure but also deteriorate physical and mechanical
properties (e.g., ductility, toughness, and corrosion resistance)
of a welded part and a heat-affected portion. Therefore, an upper
limit thereof is set to 0.3%. In addition, as described above, C
needs to be added in an amount of 0.05% or more to obtain stability
of the austenite stability and target mechanical properties.
Preferably, C may be added in an amount of 0.15% or more to obtain
high strength. However, because high strength may be complemented
by adding N and formation of a Cr carbide deteriorates oxidation
resistance, the C content is not necessarily 0.15% or more.
[0058] The content of Si is from 0.5 to 3.0%.
[0059] Si, serving as a deoxidizer during a steelmaking process, is
effective on enhancing corrosion resistance and oxidation
resistance, and these properties are effective when the Si content
is 0.5% or more. However, because Si is an element effective on
stabilizing a ferrite phase, an excess of Si may promote formation
of delta (.delta.) ferrite in a cast slab, thereby not only
deteriorating hot processibility but also deteriorating ductility
and toughness of a steel material due to solid solution
strengthening effects. Therefore, an upper limit thereof is set to
0.7%. Preferably, Si may be added in an amount of 1.0 to 2.0%.
[0060] The content of Mn is from 0.1 to 2.0%.
[0061] Mn, as an element effective on stabilizing an austenite
phase, is essential to obtain the austenite phase at a high
temperature during heat treatment and is added in an amount of 0.1%
or more. However, an excess of Mn not only causes an increases in
S-based inclusions (MnS) leading to deterioration of ductility,
toughness, and corrosion resistance of a steel material but also
deteriorates oxidation resistance due to an increase in MnO on the
surface of the steel material during heat treatment at a high
temperature in an oxidizing atmosphere for forming an austenite
structure. Therefore, an upper limit thereof is set to 2.0%.
[0062] The content of Cr is from 3.0 to 9.0%.
[0063] Cr, as a ferrite-stabilizing element, is effective on
improving corrosion resistance and oxidation resistance, and these
properties are effective when the Cr content is 3.0% or more.
However, an excess of Cr may cause an increase in Ac1 due to
enhancement of stability of ferrite making it difficult to obtain
an austenite phase during heat treatment of a steel material.
Therefore, an upper limit thereof is set to 9.0%. In consideration
of hot formability and economic efficiency, the Cr content may be
from 3.5 to 7.0%, preferably, from 3.5 to 5.5%.
[0064] The content of N is more than 0% and less than 0.2%.
[0065] N, as not only an austenite phase-stabilizing element but
also an element effective on obtaining high strength by solid
solution strengthening effects, may decrease the amounts of Ni and
Mn, thereby preventing an increase in costs of materials. However,
an excess of N may cause formation of a large amount of a nitride
in a microstructure, thereby deteriorating processibility. Also,
when more than a certain level of N is added, delta (.delta.)
ferrite formed during a cooling process after casting may cause
local formation of nitrogen pin holes, thereby deteriorating
quality. Therefore, an upper limit thereof is set to 0.2%.
[0066] The content of Nb is from 0.03 to 1.0%.
[0067] Nb, forming a carbonitride of Nb(C,N) at a high temperature,
is effective on preventing coarsening of grains during heat
treatment and this property is effective when the Nb content is
0.03% or more. Such grain refinement is effective not only on
improving processibilty of a steel material at a high temperature
but also on enhancing impact resistance. However, an excess of Nb
may cause formation of a large amount of the Nb(C,N) carbonitride,
thereby decreasing amounts of solute C and N, making it difficult
to obtain target mechanical properties. Therefore, an upper limit
thereof is set to 1.0%, preferably 0.3%.
[0068] The content of Ni is less than 3.0%.
[0069] Although used as a strong austenite phase-stabilizing
element, Ni is not an essential element in the present disclosure
because manufacturing costs are increased thereby. However, when Ni
is added within an upper limit of 3.0%, an austenite phase may be
easily formed at a high temperature. However, when the Ni content
is 3.0% or more, residual austenite is excessively formed in a
cooled structure after heat treatment and thus strength may
deteriorate. Therefore, an upper limit thereof is set to 3.0%.
[0070] The content of P is less than 0.1%.
[0071] Because P deteriorates corrosion resistance or hot
processibilty, an upper limit thereof is set to 0.1%.
[0072] The content of S is less than 0.01%.
[0073] Because S deteriorates corrosion resistance or hot
processibilty, an upper limit thereof is set to 0.01%.
[0074] The remaining component of the composition of the present
disclosure is iron (Fe). However, the composition may include
unintended impurities inevitably incorporated from raw materials or
surrounding environments, and thus addition of other alloy
components is not excluded. The impurities are not specifically
mentioned in the present disclosure, as they are known to any
person skilled in the art of manufacturing.
[0075] The steel sheet for hot forming of the present disclosure
has a microstructure including a ferrite phase and 20 vol % or less
of a carbonitride. Because good hot formability is required to
prevent cracks or bursts on the surface during hot forming, e.g.,
hot press forming (HPF), grain refinement is required in a ferrite
phase.
[0076] The steel sheet for hot forming according to an embodiment
of the present disclosure may include a ferrite phase having an
average grain size of 100 .mu.m or less. In the present disclosure,
the average grain size of the ferrite phase is controlled by the
chemical composition of alloying elements. By adding Nb as
described above, a carbonitride is formed to reduce in size of
grains and coarsening of grains may be prevented at a high
temperature, and thus addition of Nb is essential. The ranges of
contents of C and N which form the carbonitride with Nb are also
important to control the average grain size. When the content of Cr
is too low, e.g., less than 3.0%, grains are coarsened, thereby
deteriorating formability. As will be descried below, the steel
sheet for hot forming may be a hot-rolled steel sheet obtained by
batch annealing, a cold-rolled steel sheet obtained by continuous
annealing, or a hot-rolled steel sheet obtained by acid pickling
without performing annealing. Although the grain size of steel
sheets provided to hot forming may generally be controlled by
annealing, excellent formability may be obtained during hot forming
regardless of performing annealing when the range of the chemical
composition of alloying elements of the present disclosure is
satisfied.
[0077] In addition, according to an embodiment of the present
disclosure, the steel sheet for hot forming may satisfy Expression
(1) below.
0.80*Si+0.57*Cr-3.53*C-1.45*Mn-1.9>0 (1)
[0078] According to the present disclosure, excellent oxidation
resistance may be obtained by adjusting the contents of Si, Cr, C,
and Mn to satisfy Expression (1) although a plating layer is not
formed. Although the contents of oxidation-inhibiting elements such
as Cr and Si have the greatest influence on oxidation resistance of
a hot-formed member, the oxidation resistance is also sensitive to
the contents of C and Mn that promote formation of precipitates and
oxides as well thereby deriving Expression (1) above. When the
contents of Cr and Si are low, dense formation of Cr and Si oxides
is inhibited and a thick Fe oxide is formed on the surface layer.
In addition, when a large amount of C is added, formation of a Cr
carbide increases to reduce the Cr content in a matrix, thereby
causing formation of an Fe oxide. In addition, when a large amount
of Mn is locally added, a Mn oxide is formed thereby deteriorating
oxidation resistance on the surface.
[0079] Oxidation behavior of the surface layer sensitively changes
during hot forming due to influence of various alloying elements as
described above, It is important to define the quality of oxidation
resistance of the surface layer, and the hot-formed member
according to an embodiment of the present disclosure may have an
average oxygen content of 20 wt % or less at a point of 0.1 .mu.m
depth from the surface.
[0080] Then, a method of preparing a steel sheet for hot forming
and a hot-formed member will be described.
[0081] First, a steel sheet for hot forming may be manufactured
according to a well-known manufacturing process as a cold-rolled
steel sheet or an acid-pickled, hot-rolled steel sheet, but
manufacturing conditions are not particularly limited. An example
of the method for manufacturing the steel sheet for hot forming is
as follows.
[0082] An ingot or slab having the above-described chemical
composition of alloying elements is heated in a temperature range
of 1,000 to 1,300.degree. C. and hot-rolled. At a heating
temperature below 1,000.degree. C., it is difficult to homogenize
the slab structure, and at a heating temperature exceeding
1,300.degree. C., an oxide layer may be excessively formed and
manufacturing costs may increase.
[0083] Subsequently, hot finish rolling is performed in a
temperature range higher than Ar3 and equal to or lower than
1,000.degree. C. At a finish rolling temperature of Ar3 or less,
recrystallization rolling may be easily induced making it difficult
to control formation of a surface mixed structure and a steel
sheet. When the finish rolling temperature exceeds 1,000.degree.
C., hot-rolled grains may be easily coarsened.
[0084] The hot-rolled steel sheet may be coiled in a temperature
range higher than Ms and equal to or lower than 850.degree. C. When
a coiling temperature is Ms or below, it is difficult to perform a
subsequent cold rolling due to too high strength of the hot-rolled
steel. When the coiling temperature is higher than 850.degree. C.,
a thickness of an oxide layer excessively increases making it
difficult to perform acid pickling on the surface.
[0085] The hot-rolled steel sheet may be hot-formed immediately
after acid pickling. Meanwhile, the acid pickling and cold rolling
may be performed to control the thickness of the steel sheet more
precisely. Although a cold rolling reduction ratio after acid
pickling is not particularly limited, the cold rolling may be
performed with a reduction ratio of 30 to 80% to obtain a target
thickness. In this regard, to reduce a rolling load of the cold
rolling, if required, the hot-rolled steel sheet or the previously
acid-pickled, hot-rolled steel sheet may be batch-annealed. In this
regard, although batch annealing conditions are not particularly
limited, the batch annealing may be performed at a temperature of
500 to 850.degree. C. for 1 to 100 hours to reduce strength of the
hot-rolled steel sheet.
[0086] The cold-annealed, cold-rolled steel sheet may be
continuously annealed. Although a continuous annealing heat
treatment process is not particularly limited, the heat treatment
may be performed in a temperature range of 700 to 900.degree.
C.
[0087] Subsequently, the hot-rolled steel sheet or cold-rolled,
annealed steel sheet prepared as described above may be hot-formed
to prepare a hot-formed member.
[0088] The prepared steel sheet for hot forming is heated to a
temperature range of Ac3+50.degree. C. to Ac3+200.degree. C. at a
heating rate of 1 to 1,000.degree. C./sec. At a heating rate below
1.degree. C./sec, it is difficult to obtain sufficient
productivity. Also, a too long heating time not only excessively
increases a grain size to deteriorate impact toughness but also
excessively forms oxides on the surface of the formed member to
deteriorate spot weldability. To increase the heating rate to
exceed 1,000.degree. C./sec, expensive equipment is required.
[0089] Subsequently, the heat treatment may be maintained in the
temperature range of Ac3+50.degree. C. to Ac3+200.degree. C. for 1
to 1,000 seconds. At a heating temperature below Ac3+50.degree. C.,
there is a high possibility that ferrite is formed while a blank is
transferred from a heating furnace to a mold, thereby failing to
obtain a target strength. When the heating temperature exceeds
Ac3+200.degree. C., an excess of oxides on the surface of the
formed member makes it difficult to obtain spot weldability and
coating property during a subsequent process.
[0090] The hot-formed member is cooled to a temperature below Mf
simultaneously with the hot forming and a cooling rate may be
controlled in a range of 1 to 1000.degree. C./sec. At a cooling
rate below 1.degree. C./sec, undesirable ferrite is formed making
it difficult to obtain a tensile strength 1,500 MPa or more. On the
contrary, to obtain a cooling rate exceeding 1,000.degree. C./sec,
expensive, specified equipment is required.
[0091] Hereinafter, the present disclosure will be described in
more detain with reference to the following examples.
EXAMPLES
[0092] Ingot materials having chemical compositions of alloying
elements shown in Table 1 were below melted, heated in a furnace at
a temperature of 1,180.degree. C. for 2 hours, and hot-rolled to
obtain hot-rolled steel sheets having a final thickness of 3 mm.
Subsequently, the hot-rolled steel sheets were acid-pickled for
cold rolling, cold-rolled with a reduction ratio of 60%, and
annealed at 760.degree. C. to obtain steel sheets for hot
forming.
TABLE-US-00001 TABLE 1 Steel type (wt %) C Si Mn P S Cr Ni N Nb
Others Comparative Example 1 0.219 1.47 0.5 0.012 0.002 5.0 0.2
0.019 0 Comparative Example 2 0.222 1.51 0.5 0.014 0.004 3.97 0.196
0.016 0 Comparative Example 3 0.22 1.51 1.48 0.018 0.002 4.0 0.198
0.02 0 Comparative Example 4 0.215 1.99 1.5 0.016 0.003 4.0 0.201
0.016 0 Comparative Example 5 0.217 2.45 1.48 0.012 0.002 3.98
0.197 0.016 0 Comparative Example 6 0.223 1.55 1.51 0.012 0.003
3.93 0.203 0.018 0 Al: 0.51 Comparative Example 7 0.225 1.49 1.48
0.014 0.004 3.93 201 0.02 0 Al: 1.02 Comparative Example 8 0.223
1.49 0.5 0.016 0.002 7.05 0.198 0.021 0 Comparative Example 9 0.225
1.48 1.47 0.017 0.002 6.93 0.2 0.027 0 Comparative Example 10 0.14
0.4 0.48 0.016 0.003 11.3 0.39 0.05 0.16 B: 0.0038 Comparative
Example 11 0.179 1.5 0.52 0.013 0.002 3.98 0.2 0.027 0 Comparative
Example 12 0.182 1.5 0.5 0.014 0.004 4.0 0.2 0.028 0 B: 0.0054
Comparative Example 13 0.135 1.47 0.49 0.012 0.003 3.87 0.2 0.027 0
Comparative Example 14 0.14 1.5 0.49 0.018 0.003 4.04 0.2 0.03 0 B:
0.0038 Comparative Example 15 0.139 1.51 0.51 0.014 0.002 4.0 0.2
0.029 0 B: 0.0083 Comparative Example 16 0.265 1.49 0.498 0.016
0.002 3.97 0.203 0.031 0 Comparative Example 17 0.295 1.49 0.492
0.018 0.002 4.05 0.206 0.033 0 Comparative Example 18 0.216 1.5
0.512 0.014 0.002 4.0 0.2 0.031 0.096 Sb: 0.043 Comparative Example
19 0.202 1.49 0.495 0.013 0.002 3.88 0.196 0.026 0.103 Sb: 0.046
Comparative Example 20 0.25 1.53 0.512 0.014 0.002 4.99 0.201 0.026
0.101 Sb: 0.055 Comparative Example 21 0.225 1.5 0.506 0.016 0.004
2.97 0.212 0.028 0.098 Sb: 0.048 Comparative Example 22 0.216 1.51
0.495 0.012 0.004 1.92 0.204 0.028 0.101 Sb: 0.05 Comparative
Example 23 0.258 1.5 0.496 0.012 0.003 2.94 0.204 0.03 0.047 Sb:
045 Example 1 0.215 1.49 0.495 0.013 0.002 3.96 0.203 0.032 0.095
Example 2 0.217 1.49 0.496 0.016 0.004 4.99 0.196 0.031 0.1 Example
3 0.215 1.49 0.493 0.012 0.002 4.49 0.197 0.035 0.07 Example 4
0.238 1.5 0.505 0.016 0.004 5.0 0.2 0.028 0.102 Example 5 0.242
1.52 0.497 0.011 0.003 5.02 0.201 0.027 0.105 Sn: 0.052 Example 6
0.234 1.64 0.61 0.016 0.004 4.61 0.28 0.021 0.096 Al: 1.12 Example
7 0.217 1.5 0.496 0.012 0.003 4.0 0.206 0.031 0.052
[0093] FIG. 1 is an electron microscope image illustrating a
microstructure of a steel sheet for hot forming according to an
embodiment of the present disclosure. Referring to FIG. 1, it may
be confirmed that a microstructure of a cold-rolled, annealed steel
sheet for hot forming includes 20 vol % of a carbonitride in a
ferrite matrix structure.
[0094] The steel sheets for hot forming prepared as described above
were hot-formed and heat treatment conditions therefor are shown in
Table 2 below. The steel sheets were put into a furnace pre-heated
to 950.degree. C., maintained for 5.5 minutes, air-cooled for 12
seconds, hot-formed in a mold, and quenched to room temperature at
a cooling rate of 30.degree. C./sec or more.
[0095] Two types of molds were used to form the hot-formed member.
A first mold was a plate-shaped mold for forming the hot-formed
member and performing a tensile test to evaluate physical
properties after hot forming, and a second mold was prepared as a
mini-bumper mold to evaluate formability and oxidation
resistance.
[0096] Samples of the formed members obtained using the
plate-shaped mold were evaluated by the tensile test according to
the JIS 13 B standards and the results are shown in Table 2. In
addition, formability and oxidation resistance of the formed
members obtained by using the mini-bumper mold were evaluated by
applying the same hot forming heat treatment conditions and the
results are shown in Table 2.
[0097] FIG. 2 is a photograph exemplarily illustrating good
formability (a) and poor formability (b) obtained when hot forming
is performed using a mini-bumper mold after hot forming. As shown
in (b) of FIG. 2, cracks or bursts occurred on the surfaces during
hot forming in some of the comparative examples and they were
indicated as "poor" in Table 2. On the contrary, good formability
as shown in (a) of FIG. 2 was indicated as "good".
[0098] Oxidation resistance of the hot-formed members obtained
using the mini-bumper mold was evaluated based on whether excessive
oxide scales were locally formed on the surface. A case in which
surface oxidation was inhibited was indicated as "good" and a case
in which excessive oxide scales were locally formed was indicated
as "inferior".
TABLE-US-00002 TABLE 2 Heat treatment conditions Tensile test
properties Properties of hot-formed for hot forming Yield Tensile
member Temperature Time strength strength Elongation Expression
Oxidation Example Atmosphere (.degree. C.) (min) (MPa) (MPa) (%)
Formability (1) resistance Comparative Example 1 air 950 5.5 1,075
1,564 7.7 poor 0.628 good Comparative Example 2 air 950 5.5 1,029
1,517 8.2 poor 0.062 good Comparative Example 3 air 950 5.5 1,107
1,643 7.6 poor -1.335 inferior Comparative Example 4 air 950 5.5
1,176 1,744 7.1 poor -0.962 inferior Comparative Example 5 air 950
5.5 1,202 1,814 7.5 poor -0.583 inferior Comparative Example 6 air
950 5.5 1,108 1,605 6.8 poor -1.397 good Comparative Example 7 air
950 5.5 969 1,491 8.9 poor -1.408 good Comparative Example 8 air
950 5.5 1,141 1,644 6.8 poor 1.798 good Comparative Example 9 air
950 5.5 1,180 1,731 7.2 poor 0.308 good Comparative Example 10 air
950 5.5 1,086 1,411 8.5 poor 3.671 good Comparative Example 11 air
950 5.5 995 1,411 8.1 poor 0.183 good Comparative Example 12 air
950 5.5 979 1,405 9.2 poor 0.213 good Comparative Example 13 air
950 5.5 905 1,304 9.9 poor 0.295 good Comparative Example 14 air
950 5.5 920 1,303 9 poor 0.398 good Comparative Example 15 air 950
5.5 897 1,286 8.8 poor 0.358 good Comparative Example 16 air 950
5.5 1,206 1,723 7.2 good -0.103 inferior Comparative Example 17 air
950 5.5 1,256 1,804 7.3 good -0.154 inferior Comparative Example 18
air 950 5.5 1,180 1,657 8.2 good 0.075 inferior Comparative Example
19 air 950 5.5 1,411 1,796 10.2 good 0.073 inferior Comparative
Example 20 air 950 5.5 1,189 1,704 7.4 good 0.543 inferior
Comparative Example 21 air 950 5.5 1,150 1,645 9.1 poor -0.535
inferior Comparative Example 22 air 950 5.5 1,089 1,599 9.3 poor
-1.078 inferior Comparative Example 23 air 950 5.5 1,187 1,701 8.6
poor -0.654 inferior Example 1 air 950 5.5 1,140 1,596 8.8 good
0.073 good Example 2 air 950 5.5 1,127 1,597 7.6 good 0.651 good
Example 3 air 950 5.5 1,135 1,597 8.2 good 0.378 good Example 4 air
950 5.5 1,165 1,662 7.6 good 0.578 good Example 5 air 950 5.5 1,174
1,679 7.6 good 0.602 good Example 6 air 950 5.5 1,203 1,735 7.8
good 0.329 good Example 7 air 950 5.5 1,110 1,553 8.5 good 0.095
good
[0099] FIG. 3 is a graph illustrating tensile test results of the
hot-formed samples of examples and comparative examples using a
plate-shaped mold, and the tensile test was performed according to
JIS 13 B standards. Upon comparison among all of the tensile test
curves of the examples and comparative examples, it was confirmed
that fracture did not occur before exhibiting a maximum strength
but occurred after the maximum tensile strength was obtained as
shown in FIG. 3.
[0100] With regard the results, to evaluate hydrogen delayed
fracture resistance of an Al-plated hot-formed member, a method of
measuring the H content in a steel sheet has been known. According
to Patent Document 2 (Korean Patent Publication No. 10-1696121),
occurrence of a fracture was observed before a maximum strength was
obtained in a tensile curve, and a normal fracture was not observed
in the tensile test due to the high H content in the steel sheet.
That is, this indicates that hydrogen delayed fracture resistance
may be judged based on the results of the tensile curve obtained
from the tensile test. In the case of the hot-formed member
prepared using the chemical composition of the alloying elements
according to the present disclosure, a tensile behavior, in which
fracture occurred after a tensile strength reached a maximum level,
was observed and thus excellent hydrogen delayed fracture
resistance was confirmed.
[0101] Upon evaluation of formability of the hot-formed members
shown in Table 2, grain size of the steel sheets for hot forming
was confirmed as a factor the most significantly affecting the
formability. That is, in most cases in which formability of steel
types indicated as "poor" in Table 2, the C content was low or the
grain refining element such as Nb was not added, and this result
was more clearly identified by observing a microstructure thereof.
FIGS. 4 and 5 are electron microscope images of microstructures of
steel sheets for hot forming according to an example and a
comparative example prior to formation, respectively. FIG. 4 is a
photograph of the microstructure of Example 2 before hot forming,
and FIG. 5 is a photograph of the microstructure of Comparative
Example 1 before hot forming. It was confirmed that the steel types
having formability indicated by "poor" had a coarse ferrite grain
size of 100 .mu.m or more before hot forming as shown in FIG. 5.
Based on these results, it was confirmed that the average grain
size of ferrite in the microstructure needs to be controlled to 100
.mu.m or less to obtain good formability in the final hot-formed
member.
[0102] Meanwhile, it was confirmed that excellent oxidation
resistance of the hot-formed member was obtained when the contents
of Cr and Si which are oxidation-suppressing elements and the
contents of C and Mn which are elements forming precipitates and
oxides satisfy Expression (1) as described above based on Table
2.
[0103] Oxidation resistance quality of surface layers during hot
forming were classified into good and inferior by visual
observation based on glow discharge spectrometer (GDS) analysis
results, and representative results are shown in FIGS. 6 and 7.
FIGS. 6 and 7 are graphs illustrating GDS analysis results of
hot-formed members using a mini-bumper mold according to an example
exhibiting good oxidation resistance and a comparative example
exhibiting inferior oxidation resistance with respect to depth from
the surface. As a result of analyzing contents of the alloying
elements with respect to depth in the thickness direction from the
surface by the GDS, a difference of oxygen contents between the
hot-formed member having good oxidation resistance and that having
inferior oxidation resistance was clearly observed. While the
average oxygen content exceeds 20 wt % at a point of 0.1 .mu.m
depth from the surface in the comparative example exhibiting
inferior oxidation resistance of FIG. 7, it was confirmed that the
average oxygen content was about 2 to 3 wt % at a point of 0.1
.mu.m depth from the surface in the example exhibiting good
oxidation resistance of FIG. 6. Based on these results, it was
confirmed that the average oxygen content needs to be controlled to
20 wt % or less at a point of 0.1 .mu.m depth from the surface to
obtain good oxidation resistance of a final hot-formed member.
[0104] The comparative examples and examples of Table 2 will be
described in more detail below.
[0105] In Comparative Examples 1 to 9 to which Nb was not added,
grain refinement did not occur before hot forming, and thus poor
formability was obtained. Among them, inferior oxidation resistance
was observed in Comparative Examples 3 to 5 due to negative values
of Expression (1). However, in the cases of Comparative Examples 6
and 7, good oxidation resistance was obtained despite negative
values of Expression (1) because Al, effective on oxidation
resistance, was added in an amount of 0.5% or more.
[0106] In Comparative Example 10, poor formability was obtained
despite addition of Nb due to the high Cr content and good
oxidation resistance was obtained despite the low Si content
because Expression (1) was satisfied by the high Cr content.
[0107] In Comparative Examples 10 to 15 where the C content was
slightly low even within the range proposed by the present
disclosure, and thus it was confirmed that the yield strength and
the tensile strength did not reach 1,100 MPa and 1,500 Mpa,
respectively. However, in Comparative Example 10 where the N
content was high as 0.05%, a result close to the target strength
was obtained and thus it was confirmed that high strength property
may be complemented by adding N.
[0108] Good formability was obtained in Comparative Examples 16 and
17 although Nb was not added and this was because oxidation
resistance more deteriorated by formation of a large amount of a
carbide due to a slightly high C content but formability was
improved due to oxide scales.
[0109] Sb was further added to the steel types of Comparative
Examples 18 to 23. Sb was oxidized at a hot forming temperature of
950.degree. C. to be present as scales in the form of ash resulting
in inferior oxidation resistance although Expression (1) was
satisfied in Comparative Examples 18 to 20.
[0110] In Comparative Examples 21 to 23, poor formability was
obtained despite addition of Nb, and it was confirmed that this is
because the grains coarsened due to the low Cr content to
deteriorate formability.
[0111] While the present disclosure has been particularly described
with reference to exemplary embodiments, it should be understood by
those of skilled in the art that various changes in form and
details may be made without departing from the spirit and scope of
the present disclosure.
INDUSTRIAL APPLICABILITY
[0112] The steel sheet for hot forming according to the present
disclosure may be applied to automotive structural members because
ultra-high strength may be obtained simultaneously inhibiting
surface oxidation during hot press forming without using a plating
layer.
* * * * *