【DESCRIPTION】
【Invention Title】
STEEL SHEET FOR HOT PRESS FORMED PRODUCT HAVING
SUPERIOR BENDABILITY AND ULTRA-HIGH STRENGTH, HOT PRESS
FORMED PRODUCT USING SAME, AND METHOD FOR MANUFACTURING
SAME
【Technical Field】
The present disclosure relates to a steel sheet for
manufacturing a product such as a pillar reinforcing member,
a cross member, a side member, or front or rear bumper
through a hot press forming process, a hot press formed
product manufactured using the steel sheet, and methods for
manufacturing the steel sheet and the hot press formed
product. More particularly, the present disclosure relates
to a steel sheet for manufacturing a hot press formed
product having high bendability and ultra-high strength, a
hot press formed product manufactured using the steel sheet,
and methods for manufacturing the steel sheet and the hot
press formed product.
【Background Art】
Safety regulations for protecting vehicle passengers
as well as fuel efficiency regulations for protecting the
environment have recently been tightened, and thus there is
increasing interest in techniques for improving the
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stiffness of automobile components and reducing the weight
of automobiles. For example, along with attempts to reduce
the weight of parts such as pillar reinforcing members or
cross members forming passenger safety cage zones in
automobiles as well as side members or front/rear bumpers
forming crash zones in automobiles, the use of highstrength
parts has been increased to guarantee stiffness
and crashworthiness.
In automotive steel sheets, the increase of strength
may inevitably result in the increase of yield strength,
decrease in elongation, and significantly decreased
formability. Thus, as a forming method for solving problems
related to the formability of high-strength steel and
providing high-strength automotive parts having a tensile
strength grade of 1470 MPa or greater, a hot press forming
method or a hot forming method has been developed and
widely used.
Hot press forming guarantees various degrees of
strength. For example, in the early 2000s, hot press formed
products having a tensile strength grade of 1500 MPa could
be manufactured using 22MnB5 steel, as stated in DIN. In
general, before hot press forming process, a steel sheet
blank having a tensile strength of 500 MPa to 800 MPa is
heated to a temperature within an austenite temperature
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range of an Ac3 transformation temperature or higher and is
transferred to the press equipped with a cooling device to
form the blank and quench the press formed blank (product)
in the dies. Therefore, a press formed product ultimately
contains martensite or a mixture of martensite and bainite,
and thus the press formed product may have ultra-high
strength, on the level of 1500 MPa or greater. In addition,
since a press formed product is rapidly cooled within dies,
the press formed product may have precise dimensions.
The basic concept of the hot press forming method and
the use of boron bearing steel in the hot press forming
method were first proposed in Patent Document 1 (UK Patent
No. 1490535) and have subsequently been widely used. In
addition, an aluminum or aluminum alloy coated steel sheet
has been proposed in Patent Document 2 (US Patent No.
6296805) to suppress the formation of surface oxide layer
during heating in the hot press forming process. In
addition, Zn-coated galvanized or galvannealed steel sheets
have been proposed for applications which required
sacrificial protection such as wet area of automotive body.
In addition, so as to improve the fuel efficiency of
automobiles, automobile manufacturers have been
increasingly interested in the higher tensile strength
grade of steel sheets for hot press forming. In this regard,
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a steel sheet for manufacturing a hot press formed product
having a tensile strength grade of 1800 MPa has been
proposed. Compared to steel sheets for manufacturing hot
press formed products having a tensile strength grade of
1500 MPa, the proposed steel sheet has a relatively high
carbon content, and niobium (Nb) effective in refinement of
initial austenite grains is added to the proposed steel
sheet to improve the toughness of hot press formed products.
However, the above-described methods of the related
art for improving the strength of hot press formed products
result in the formation of cracks, an increase in
susceptibility to crack propagation, and accordingly, poor
bendability.
【Disclosure】
【Technical Problem】
Aspects of the present disclosure may provide a steel
sheet for manufacturing a hot press formed product having
high bendability and ultra-high strength, and a method for
manufacturing the steel sheet.
Aspects of the present disclosure may also provide a
hot press formed product having high bendability and ultrahigh
strength, and a method for manufacturing the hot press
formed product.
【Technical Solution】
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According to an aspect of the present disclosure, a
steel sheet for a formed product having high bendability
and ultra-high strength may include C: 0.28 wt% to 0.40 wt%,
Si: 0.5 wt% to 1.5 wt%, Mn: 0.8 wt% to 1.2 wt%, Al: 0.01
wt% to 0.1 wt%, Ti: 0.01 wt% to 0.1 wt%, Cr: 0.05 wt% to
0.5 wt%, P: 0.01 wt% or less, S: 0.005 wt% or less, N: 0.01
wt% or less, B: 0.0005 wt% to 0.005 wt%, and at least one
selected from the group consisting of Mo: 0.05 wt% to 0.5
wt%, Cu: 0.05 wt% to 0.5 wt%, and Ni: 0.05 wt% to 0.5 wt%,
wherein Mn and Si may satisfy 0.05 ≤ Mn/Si ≤ 2, and the
steel sheet may include a balance of Fe and other
inevitable impurities.
According to another aspect of the present disclosure,
a formed product having high bendability and ultra-high
strength may be manufactured by performing a hot press
forming process on a steel sheet, the steel sheet including
C: 0.28 wt% to 0.40 wt%, Si: 0.5 wt% to 1.5 wt%, Mn: 0.8
wt% to 1.2 wt%, Al: 0.01 wt% to 0.1 wt%, Ti: 0.01 wt% to
0.1 wt%, Cr: 0.05 wt% to 0.5 wt%, P: 0.01 wt% or less, S:
0.005 wt% or less, N: 0.01 wt% or less, B: 0.0005 wt% to
0.005 wt%, and at least one selected from the group
consisting of Mo: 0.05 wt% to 0.5 wt%, Cu: 0.05 wt% to 0.5
wt%, and Ni: 0.05 wt% to 0.5 wt%, wherein Mn and Si may
satisfy 0.05 ≤ Mn/Si ≤ 2, and the steel sheet may include a
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balance of Fe and other inevitable impurities.
According to another aspect of the present disclosure,
a method for manufacturing a steel sheet for a formed
product having high bendability and ultra-high strength may
include: preparing a slab, the slab including C: 0.28 wt%
to 0.40 wt%, Si: 0.5 wt% to 1.5 wt%, Mn: 0.8 wt% to 1.2 wt%,
Al: 0.01 wt% to 0.1 wt%, Ti: 0.01 wt% to 0.1 wt%, Cr: 0.05
wt% to 0.5 wt%, P: 0.01 wt% or less, S: 0.005 wt% or less,
N: 0.01 wt% or less, B: 0.0005 wt% to 0.005 wt%, and at
least one selected from the group consisting of Mo: 0.05
wt% to 0.5 wt%, Cu: 0.05 wt% to 0.5 wt%, and Ni: 0.05 wt%
to 0.5 wt%, wherein Mn and Si may satisfy 0.05 ≤ Mn/Si ≤ 2,
and the slab may include a balance of Fe and other
inevitable impurities; reheating the slab to a temperature
within a range of 1150°C to 1250°C; hot rolling the
reheated slab at a temperature within a finish rolling
temperature range of an Ar3 transformation temperature to
950°C so as to form a hot-rolled steel sheet; and coiling
the hot-rolled steel sheet at a temperature within a range
of 500°C to 730°C.
According to another aspect of the present disclosure,
a method for manufacturing a formed product having high
bendability and ultra-high strength may include: preparing
a blank of a steel sheet, the steel sheet including C: 0.28
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wt% to 0.40 wt%, Si: 0.5 wt% to 1.5 wt%, Mn: 0.8 wt% to 1.2
wt%, Al: 0.01 wt% to 0.1 wt%, Ti: 0.01 wt% to 0.1 wt%, Cr:
0.05 wt% to 0.5 wt%, P: 0.01 wt% or less, S: 0.005 wt% or
less, N: 0.01 wt% or less, B: 0.0005 wt% to 0.005 wt%, and
at least one selected from the group consisting of Mo: 0.05
wt% to 0.5 wt%, Cu: 0.05 wt% to 0.5 wt%, and Ni: 0.05 wt%
to 0.5 wt%, wherein Mn and Si may satisfy 0.05 ≤ Mn/Si ≤ 2,
and the steel sheet may include a balance of Fe and other
inevitable impurities; heating the blank to a temperature
within a range of 850°C to 950°C; and manufacturing a
formed product by performing a hot press forming process on
the blank to form a formed product and cooling the formed
product in dies to a temperature of 200°C or lower.
【Advantageous Effects】
Embodiments of the present disclosure provide a steel
sheet for manufacturing a hot press formed product having
ultra-high strength and high bendability, and a hot press
formed product manufactured using the steel sheet. The
steel sheet and the hot press formed product may be applied
to automobile bodies or parts for weight reduction and
crashworthiness improvements.
【Best Mode】
Embodiments of the present disclosure relate to a
steel sheet for manufacturing a hot press formed product
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having high bendability and ultra-high strength, a hot
press formed product formed of the steel sheet, and methods
for manufacturing the steel sheet and the hot press formed
product.
In general, steel sheets for manufacturing 1500 MPa
grade hot press formed products are formed of steel having
a chemical composition corresponding to that of 22MnB5
steel, and the content of carbon (C) in such steel sheets
may be increased to obtain a higher strength by heat
treatment. For example, boron bearing steels such as 30MnB5
steel or 34MnB5 steel may have a degree of strength
corresponding to the strength grade of 1800 MPa or 2000 MPa,
respectively.
However, the content of manganese (Mn) in such steels
is fixed to a range of 1.2 wt% to 1.4 wt%. If the strength
of steel sheets for manufacturing hot press formed products
or the strength of hot press formed products is increased
by adjusting the carbon contents thereof while fixing the
content of manganese (Mn) within this range, the formation
of cracks and an increase in susceptibility to crack
propagation are observed in a bending test. That is, in
this case, the bendability of steel sheets for hot press
formed products or the bendability of hot press formed
products are decreased.
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To address these problems, the inventors have
reviewed metallographic factors improving the bendability
of steel and found that if the formation of a banded
structure caused by micro-segregation is decreased before a
hot press forming process and a secondary phase is
uniformly distributed, bendability can be increased after a
hot press forming process, and if a painting baking
treatment is performed after a hot press forming process,
bendability can be improved as a whole. These improvements
are markedly affected by the addition of particular
elements.
Thus, so as to solve problems such as a low
bendability of a hot press formed product having a high
strength, the inventors have invented a new steel sheet for
manufacturing a hot press formed product. The
metallographic non-uniformity of the steel sheet is reduced
by adjusting the composition of the steel sheet and a
thermal history that the steel sheet experiences during
manufacturing processes, and the steel sheet includes
elements increasing the amount of austenite retained in
martensite during a painting baking treatment process after
a hot press forming process. Thus, the steel sheet has a
markedly improved degree of bendability compared to steel
sheets of the related art for manufacturing hot press
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formed products.
Herein, the term "steel sheet for a hot press formed
product" or "steel sheet for manufacturing a hot press
formed product" may refer to a hot-rolled steel sheet, a
cold-rolled steel sheet, or a plated steel sheet for
manufacturing a hot press formed product.
Hereinafter, a steel sheet for a hot press formed
product having high bendability and ultra-high strength
will be described in detail.
According to an exemplary embodiment of the present
disclosure, a steel sheet for a hot press formed product
having high bendability and ultra-high strength includes C:
0.28 wt% to 0.40 wt%, Si: 0.5 wt% to 1.5 wt%, Mn: 0.8 wt%
to 1.2 wt%, Al: 0.01 wt% to 0.1 wt%, Ti: 0.01 wt% to 0.1
wt%, Cr: 0.05 wt% to 0.5 wt%, P: 0.01 wt% or less, S: 0.005
wt% or less, N: 0.01 wt% or less, B: 0.0005 wt% to 0.005
wt%, and at least one selected from the group consisting of
Mo: 0.05 wt% to 0.5 wt%, Cu: 0.05 wt% to 0.5 wt%, and Ni:
0.05 wt% to 0.5 wt%, wherein Mn and Si satisfies 0.05 ≤
Mn/Si ≤ 2, and the steel sheet includes the balance of Fe
and other inevitable impurities.
Hereinafter, reasons for setting the contents of
alloying elements of the steel sheet to be within the
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above-stated ranges will be described.
Carbon (C): 0.28 wt% to 0.40 wt%
Carbon (C) increases the hardenability of the steel
sheet, and after the steel sheet is cooled in dies or
quenched, the strength of the steel sheet is markedly
affected by the content of carbon (C). If the content of
carbon (C) in the steel sheet is less than 0.28 wt%, it may
be difficult to obtain a strength of 1800 MPa or greater.
Conversely, if the content of carbon (C) in the steel sheet
is greater than 0.4 wt%, although a high degree of strength
is obtained, the possibility of cracking increases due to
the concentration of stress around a weld nugget in a spot
welding process after a product forming process. In
addition, stress may concentrate around weld zone
connecting steel coil-to-coil in the manufacturing process,
and thus strip breakage is likely to occur. Therefore, the
content of carbon (C) is adjusted to be less than 0.4 wt%.
Silicon (Si): 0.5 wt% to 1.5 wt%
Silicon (Si) markedly helps the steel sheet to have a
uniform microstructure and stable strength rather than
improving the hardenability of the steel sheet. Like
manganese (Mn), silicon (Si) markedly affects the
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bendability of the steel sheet. As the content of silicon
(Si) increases, the formation of a banded structure rich in
manganese (Mn) and carbon (C) is reduced, and secondary
phases including pearlite are uniformly distributed in the
microstructure of the steel sheet before a hot press
forming process. In addition, silicon (Si) markedly
improves the bendability of the steel sheet by painting
baking treatment process after a hot press forming process.
If the content of silicon (Si) is less than 0.5 wt%, the
microstructure of the steel sheet may not be uniform before
a hot press forming process, and thus the bendability of
the steel sheet may not be improved after a hot press
forming process. Conversely, if the content of silicon (Si)
is greater than 1.5 wt%, red scale may be easily formed on
a hot-rolled steel sheet, and thus the surface quality of a
final product may be negatively affected. In addition, the
A3 transformation point of the steel sheet may rise, and
thus the heating temperature (solution treatment
temperature) of a hot press forming process may be
inevitably increased. Therefore, the upper limit of the
content of silicon (Si) may be set to be 1.5 wt%.
Manganese (Mn): 0.8 wt% to 1.2 wt%
Like carbon (C), manganese (Mn) improves the
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hardenability of the steel sheet, and manganese (Mn) has
the most decisive effect next to carbon (C) on the strength
of the steel sheet after the steel sheet is cooled in dies
or quenched. However, as the content of manganese (Mn)
increases, the microstructure of the steel sheet becomes
less uniform before hot press forming process because
banded structure having large amounts of carbon (C) and
manganese (Mn) is easily formed. As a result, the
bendability of the steel sheet may be poor after the steel
sheet is cooled in dies or quenched. If the content of
manganese (Mn) is less than 0.8 wt%, although the
uniformity of the microstructure of the steel sheet is
improved, the steel sheet may not have an intended degree
of tensile strength after a hot press forming process.
Conversely, if the content of manganese (Mn) is greater
than 1.2 wt%, although the strength of the steel sheet is
improved, the bendability of the steel sheet is decreased.
Therefore, the upper limit of the content of manganese (Mn)
may be set to be 1.2 wt%.
Aluminum (Al): 0.01 wt% to 0.1 wt%
Aluminum (Al) is a representative deoxidizer, and
this effect may be sufficiently obtained if the content of
aluminum (Al) is 0.02 wt% or greater. If the content of
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aluminum (Al) is less than 0.01 wt%, deoxidation may not
sufficiently occur. However, if the content of aluminum
(Al) is excessively high, aluminum (Al) induces the
precipitation of nitrogen (N) during a continuous casting
process, thereby leading to surface defects. Therefore, the
upper limit of the content of aluminum (Al) may be set to
be 0.1 wt%.
Phosphorus (P): 0.01 wt% or less
Phosphorus (P) is an inevitably added impurity and
has substantially no effect on the strength of the steel
sheet after a hot press forming process. Moreover, in
austenitizing treatment process followed by a hot press
forming process, phosphorus (P) may segregate along grain
boundaries of austenite and may thus worsen the bendability
or fatigue characteristics of the steel sheet. Therefore,
in the exemplary embodiment of the present disclosure, the
content of phosphorus (P) is limited to 0.01 wt% or less.
Sulfur (S): 0.005 wt% or less
Sulfur (S) is an impurity, and if sulfur (S) combines
with manganese (Mn) and exists in the form of elongated
sulfide inclusion, the ductility of the steel sheet may
decrease after the steel sheet is cooled in dies or
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quenched. Therefore, the content of sulfur (S) is adjusted
to be 0.005 wt% or less.
Titanium (Ti): 0.01 wt% to 0.1 wt%
During heating in a hot press forming process, TiN,
TiC, or TiMoC precipitate suppresses the growth of
austenite grains. In addition, if the precipitation of TiN
occurs sufficiently, the effective amount of boron (B)
improving the hardenability of austenite is increased, and
thus the strength of the steel sheet may stably be improved
after the steel sheet is cooled in dies or quenched. If the
content of titanium (Ti) is less than 0.01 wt%,
microstructure refinement or strength improvements may
occur insufficient. Conversely, if the content of titanium
(Ti) is greater than 0.1 wt%, the strength of the steel
sheet may not be improved as much as the added amount of
titanium (Ti). Therefore, the upper limit of the content of
titanium (Ti) may be set to be 0.1 wt%.
Chromium (Cr): 0.05 wt% to 0.5 wt%
Like manganese (Mn) and carbon (C), chromium (Cr)
improves the hardenability of the steel sheet and increases
the strength of the steel sheet after the steel sheet is
cooled in dies or quenched. In a process of adjusting
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martensite, chromium (Cr) has an effect on a critical
cooling rate, and thus martensite may be easily formed by
the addition of chromium (Cr). Furthermore, in a hot press
forming process, chromium (Cr) lowers the A3 transformation
point of the steel sheet. These effects may be obtained if
the content of chromium (Cr) is 0.05 wt% or greater.
However, if the content of chromium (Cr) is greater than
0.5 wt%, the surface quality of a coated steel sheet may be
decreased, and the spot weldability of the steel sheet may
be worsened when hot press formed products are welded
together. Therefore, the content of chromium (Cr) may be
adjusted to be 0.5 wt% or less.
Boron (B): 0.0005 wt% to 0.005 wt%
Boron (B) is highly effective in improving the
hardenability of the steel sheet. Even a very small amount
of boron (B) may lead to an increase in the strength of the
steel sheet after the steel sheet is cooled in dies or
quenched. However, as the content of boron (B) increases,
the effect of improving the quenching characteristics of
the steel sheet is not increased in proportion to the
content of boron (B), and corner defects of slab may be
formed during continuous casting process. Conversely, if
the content of boron (B) is less than 0.0005 wt%, the
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quenching characteristics or strength of the steel sheet
may not be improved as intended in the exemplary embodiment.
Therefore, the upper and lower limits of the content of
boron (B) may be set to be 0.005 wt% and 0.0005 wt%,
respectively.
Nitrogen (N): 0.01 wt% or less
Nitrogen (N) is an inevitably added impurity leading
to the precipitation of AlN during continuous casting
process and cracks in corners of continuous cast slab. In
addition, precipitates such as TiN are known as absorbing
sites of diffusional hydrogen. Thus, if the precipitation
of nitrogen (N) is properly controlled, resistance to
hydrogen delayed fracture may be improved. Thus, the upper
limit of the content of nitrogen (N) may be set to be 0.01
wt%.
In addition to the above-described alloying elements,
the steel sheet may further include at least one selected
from the group consisting of molybdenum (Mo), copper (Cu),
and nickel (Ni).
Molybdenum (Mo): 0.05 wt% to 0.5 wt%
Like chromium (Cr), molybdenum (Mo) improves the
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hardenability of the steel sheet and stabilizes the
strength of the steel sheet after quenching. In addition,
molybdenum (Mo) added to steel widens an austenite
temperature range toward a lower temperature and thus
broadens a process window when the steel is annealed in hot
rolling process and cold rolling process and the steel is
heated during hot press forming process. If the content of
molybdenum (Mo) is less than 0.05 wt%, the effect of
improving hardenability or widening an austenite
temperature range may not be obtained. Conversely, if the
content of molybdenum (Mo) is greater than 0.5 wt%, even
though strength is increased, the strength increasing
effect is not high compared to the amount of molybdenum
(Mo). That is, it is not economical. Thus, the upper limit
of the content of molybdenum (Mo) may be set to be 0.3 wt%.
Copper (Cu): 0.05 wt% to 0.5 wt%
Copper (Cu) improves the corrosion resistance of the
steel sheet. In addition, when a tempering process is
performed to improve ductility after a hot press forming
process, supersaturated copper (Cu) may lead to the
precipitation of ε-carbide and thus age-hardening. If the
content of copper (Cu) is less than 0.05 wt%, these effects
may not be obtained. Thus, the lower limit of the content
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of copper (Cu) may be set to be 0.05 wt%. Conversely, if
copper (Cu) is excessively added, surface defects may be
formed during steel sheet manufacturing process, and the
corrosion resistance of the steel sheet may not be highly
increased as compared to the amount of copper (Cu). That is,
it may be uneconomical. Thus, the upper limit of the
content of copper (Cu) may be set to be 0.5 wt%.
Nickel (Ni): 0.05 wt% to 0.5 wt%
Nickel (Ni) is effective in improving the strength,
ductility, quenching characteristics of the steel sheet.
If copper (Cu) is only added to the steel sheet, the steel
sheet may become susceptible to hot shortening. However,
nickel (Ni) decreases the susceptibility of the steel sheet
to hot shortening. In addition, nickel (Ni) added to steel
widens an austenite temperature range toward a lower
temperature and thus broadens a process window when the
steel is annealed in a hot rolling process and a cold
rolling process and the steel is heated in a hot press
forming process. If the content of nickel (Ni) is less than
0.05 wt%, the above-mentioned effects may not be obtained.
Conversely, if the content of nickel (Ni) is greater than
0.5 wt%, even though the quenching characteristics and
strength of the steel sheet are improved, the effect of
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improving quenching characteristics is not high compared to
the amount of nickel (Ni). That is, it is not economical.
Thus, the upper limit of the content of nickel (Ni) may be
set to be 0.5 wt%.
The contents of manganese (Mn) and silicon (Si) may
satisfy 0.05 ≤ Mn/Si ≤ 2.
In terms of the ratio of Mn and Si contents (Mn/Si),
as the content of manganese (Mn) increases, a banded
structure is easily formed in the microstructure of the
steel sheet before a hot press forming process, and thus
the bendability of the steel sheet may be worsened after
the steel sheet is cooled in dies or quenched. In addition,
as the content of silicon (Si) increases, the formation of
a banded structure rich in manganese (Mn) and carbon (C) is
reduced, and a secondary phase structure including pearlite
is uniformly distributed in the microstructure of the steel
sheet before a hot press forming process. In addition,
silicon (Si) markedly improves the bendability of the steel
sheet in a painting baking treatment process after a hot
press forming process. These effects are determined by the
ratio of Mn/Si. If silicon (Si) is excessively added and
thus the ratio of Mn/Si is equal to or less than 0.05,
coating quality is worsened. Conversely, if manganese (Mn)
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is excessively added and thus the ratio of Mn/Si is greater
than 2, a banded structure may be formed, and thus the
bendability of the steel sheet may be decreased. Therefore,
the upper and lower limits of the ratio of Mn/Si are set to
be 2.0 and 0.05, respectively.
In the exemplary embodiment of the present disclosure,
the other component of the steel sheet is iron (Fe).
However, impurities of raw materials or manufacturing
environments may be inevitably included in the steel sheet,
and such impurities may not be able to be removed from the
steel sheet. Such impurities are well-known to those of
ordinary skill in the art to which the present disclosure
relates, and thus descriptions thereof will not be given in
the present disclosure.
The steel sheet may be one selected from the group
consisting of a hot-rolled steel sheet, a cold-rolled steel
sheet, and a coated steel sheet.
The steel sheet of the exemplary embodiment having
the above-described chemical composition may be used in the
form of a hot-rolled steel sheet, a pickled and oiled steel
sheet, or a cold-rolled steel sheet, or coated steel sheet.
In this coated steel case, surface oxidation of the steel
sheet may be prevented, and the corrosion resistance of the
steel sheet may be improved.
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The coated steel sheet may be an aluminum alloy
coated steel sheet obtained by forming an aluminum alloy
coating layer on a hot-rolled steel sheet, a pickled and
oiled steel sheet, or a cold-rolled steel sheet. The
aluminum alloy coating steel sheet may include an alloy
coating layer containing at least one selected from the
group consisting of silicon (Si): 8 wt% to 10 wt% and
magnesium (Mg): 4 wt% to 10 wt%, and the balance of
aluminum (Al), iron (Fe), and other impurities. An
inhibition layer may be disposed between the alloy coating
layer and the steel sheet (base steel sheet).
The steel sheet may have a microstructure including
ferrite and pearlite or a microstructure including ferrite,
pearlite, and bainite. Preferably, the microstructure of
the steel sheet may include ferrite and less than 40% of
pearlite, or the microstructure of the steel sheet may
include ferrite and less than 40% of pearlite and bainite.
Preferably, the strength of the steel sheet may be
within the range of 800 MPa or less in tensile strength.
The reason for this is as follows. Before a hot press
forming process is performed on the steel sheet prepared as
a hot-rolled pickled steel sheet, a cold-rolled steel sheet,
or a coated steel sheet as described above, blanks of the
steel sheet corresponding to the shapes of products to be
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manufactured are prepared. At this time, if the strength of
the steel sheet is excessively high, blanking dies may
easily wear and break, and the noise of a blanking process
may increase in proportion to the strength of the steel
sheet.
Therefore, preferably, the steel sheet may have a
tensile strength within the range of 800 MPa or less, and
may include ferrite and less than 40% of secondary phases
such as pearlite and bainite.
Hereinafter, a hot press formed product will be
described in detail according to an exemplary embodiment of
the present disclosure.
The hot press formed product of the exemplary
embodiment is manufactured by performing a hot press
forming process on the above-described steel sheet. The hot
press formed product may have high bendability and ultrahigh
strength. The steel sheet may be one selected from the
group consisting of a hot-rolled steel sheet, a cold-rolled
steel sheet, and a coated steel sheet. The coated steel
sheet may be an aluminum alloy coated steel sheet obtained
by forming an aluminum alloy coated layer on a hot-rolled
steel sheet, a pickled steel sheet, or a cold-rolled steel
sheet.
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The hot press formed product may be manufactured by
performing a hot press forming process on the aluminum
alloy coated steel sheet. The hot press formed product may
include an Fe-Al film layer containing at least one
selected from the group consisting of silicon (Si): 4 wt%
to 10 wt% and magnesium (Mg): 2 wt% to 10 wt%, and other
impurities. The Fe-Al film layer may be formed as the
coating layer of the aluminum alloy coated steel sheet
undergoes alloying in the hot press forming process. The
Fe-Al film layer may include an Fe3Al+FeAl layer (inter
diffusion layer), an Fe2Al5 layer, and an Fe-Al layer that
are sequentially formed on a base steel sheet (that is, on
an iron surface of the aluminum alloy coated steel sheet).
In addition, since alloying occurs between the alloying
layer and the base steel sheet during the hot press forming
process, the Fe-Al film layer may have a relatively high
iron content and thus a relatively low silicon content
and/or a relatively low manganese content when compared to
the plating layer before the hot press forming process.
The microstructure of the hot press formed product
may include martensite in an amount of 90 area% or greater
and the balance of at least one of bainite and ferrite.
Preferably, the hot press formed product may have a
tensile strength of 1700 MPa or greater.
Page 26
If the hot press formed product is manufactured using
a hot-rolled steel sheet or a cold-rolled steel sheet, the
hot press formed product may preferably have a tensile
strength of 1800 MPa or greater and a tensile strength x
bendability balance of 115,000 MPa•° or greater.
If the hot press formed product is manufactured using
an aluminum alloy coated steel sheet, the hot press formed
product may preferably have a tensile strength of 1800 MPa
or greater and a tensile strength x bendability balance of
100,000 MPa•° or greater.
If the hot press formed product is manufactured using
a hot-rolled steel sheet or a cold-rolled steel sheet, the
hot press formed product may preferably have a tensile
strength of 2000 MPa or greater and a tensile strength x
bendability balance of 95,000 MPa•° or greater.
If the hot press formed product is manufactured using
an aluminum alloy plated steel sheet, the hot press formed
product may preferably have a tensile strength of 2000 MPa
or greater and a tensile strength x bendability balance of
85,000 MPa•° or greater.
Hereinafter, a method of manufacturing a steel sheet
for a hot press formed product will be described in detail
according to an exemplary embodiment of the present
Page 27
disclosure.
According to the exemplary embodiment of the present
disclosure, a steel sheet having high bendability and
ultra-high strength and suitable for a hot press forming
process is manufactured. The method includes: preparing a
slab having the composition of the steel sheet of the
previous embodiment; reheating the slab to a temperature
within a range of 1150°C to 1250°C; hot rolling the
reheated slab at a temperature within a finish rolling
temperature range of an Ar3 transformation temperature to
950°C so as to form a hot-rolled steel sheet; and coiling
the hot-rolled steel sheet at a temperature within a range
of 500°C to 730°C.
Since the slab is reheated to a temperature within a
range of 1150°C to 1250°C, the microstructure of the slab
may become uniform, and carbonitride precipitates such as
titanium (Ti) precipitates may be sufficiently re-dissolved,
thereby preventing grains of the slab from growing
excessively.
The hot rolling process is performed at a finish
rolling temperature of an Ar3 transformation temperature to
950°C. If the finish rolling temperature is lower than an
Ar3 transformation temperature, austenite may be partially
transformed into ferrite, and a two phase region (in which
Page 28
ferrite and austenite exist together) may be formed. In
this state, if a hot rolling process is performed,
deformation resistance may not be uniform, and thus the
mass flow of the strip may be negatively affected. In
addition, stress may concentrate on ferrite phases, and
fracture may occur. Conversely, if the finish rolling
temperature is higher than 950°C, surface detects such as
sand-like scale may be formed. Therefore, the finish
rolling temperature may be set to be within the range of an
Ar3 transformation temperature to 950°C.
Next, when the hot-rolled steel sheet is cooled and
coiled, the coiling temperature may be properly adjusted so
as to reduce widthwise mechanical property deviation of the
hot-rolled steel sheet and prevent the formation of a lowtemperature
phase such as martensite having a negative
influence on the mass flow of the steel sheet in a
subsequent cold rolling process. That is, preferably, the
coiling temperature may be set to be within the range of
500°C to 730°C.
If the coiling temperature is lower than 500°C, a
low-temperature microstructure such as martensite may be
formed, and thus the strength of the hot-rolled steel sheet
may be excessively increased. Particularly, if the hotrolled
steel sheet is overcooled in the egdes of coil,
Page 29
material properties of the coiled steel sheet may be varied
in the width direction, and the mass flow of the steel
sheet may be negatively affected in a subsequent cold
rolling process, thereby making it difficult to control the
thickness of the steel sheet.
Conversely, if the coiling temperature is higher than
730°C, oxides may be formed on the surface region of the
steel sheet, and cracks may be formed on the surface region
of the steel sheet after such internal oxides are removed
through a pickling process. In this state, if the steel
sheet is coated, the interface between the steel sheet
(base steel sheet) and a coating layer may be uneven. This
may worsen the bendability of the steel sheet together with
the internal oxides in a subsequent hot press forming
process. Therefore, the upper limit of the coiling
temperature may be set to be 730°C.
According to the exemplary embodiment, the hot-rolled
steel sheet may be pickled and cold rolled. Then, a
continuous annealing process may be performed on the steel
sheet at a temperature within a range of 750° to 850°C, and
an overaging heat treatment process may be performed on the
steel sheet at a temperature within a range of 400°C to
600°C. In this manner, a cold-rolled steel sheet may be
manufactured.
Page 30
The pickling and cold rolling are not limited to
particular methods. For example, the pickling and cold
rolling may be performed by generally-used methods. A
reduction ratio of the cold rolling is not limited. For
example, it may be preferable that the reduction ratio be
within the range of 40% to 70%.
The continuous annealing process may be performed at
a temperature within a range of 750°C to 850°C. If the
continuous annealing temperature is lower than 750°C,
recrystallization may not sufficiently occur. If the
continuous annealing temperature is higher than 850°C,
coarse grains may be formed, and much heating cost may be
required.
Next, the overaging heat treatment process may be
performed at a temperature within a range of 400°C to 600°C
so as to obtain a final microstructure in which pearlite or
bainite is partially included in a ferrite matrix. In this
case, the cold-rolled steel sheet may have strength range
within 800 MPa or less like the hot-rolled steel sheet.
According to the exemplary embodiment of the present
disclosure, after the hot-rolled steel sheet is pickled and
cold rolled, the steel sheet may be annealed at a
temperature within a range of 700°C to an Ac3
transformation temperature and may be coated with an
Page 31
aluminum alloy coating layer to manufacture an aluminum
alloy coated steel sheet.
Preferably, the annealing process may be performed at
a temperature within a range of 700°C to an Ac3
transformation temperature. The annealing temperature may
be determined by taking the final softening of the steel
sheet and the temperature at which the steel sheet is
dipped into a coating path in a subsequent coating process
into consideration. If the annealing temperature is too low,
recrystallization may occur insufficiently, and the
temperature of the steel sheet may be low when being dipped
into a coating bath, thereby leading to unstable adhesion
of a coating layer and poor coating quality. Therefore, the
lower limit of the annealing temperature may be set to be
700°C. If the annealing temperature is too high, coarse
grains may be formed, and the strength of a coated steel
sheet may be excessively increased by the formation of a
low temperature transformation phase from austenite during
annealing, coating, and cooling processes. Therefore, the
upper limit of the annealing temperature may be set to be
an Ac3 transformation temperature.
An alloy coating bath used in the process of forming
the aluminum alloy coated steel sheet may include at least
one selected from the group consisting of silicon (Si): 8
Page 32
wt% to 10 wt% and magnesium (Mg): 4 wt% to 10 wt%, and the
balance of aluminum (Al) and other impurities.
The amount of the coated layer may preferably be 120
g/m2 to 180 g/m2 based on both sides.
The coating layer may be formed by a hot dipping
method.
In the hot dipping method, when the steel sheet is
cooled after coating the steel sheet by dipping the steel
sheet in the coating bath, the rate of cooling and the
speed of a cooling line are not limited.
This is allowed because of the annealing temperature
lower than an Ac3 transformation temperature and one of the
characteristics of the manufacturing method of the
exemplary embodiment. That is, if the steel sheet is heated
to Ac3 transformation temperature or higher in the
annealing process and dipped into the coating bath, and
then the coated steel sheet is cooled at a critical cooling
rate or faster, the strength of the coated steel sheet may
be excessively increased because of the formation of
martensite. However, according to the exemplary embodiment,
since the annealing process is performed at an Ar3
transformation temperature or below, factors leading to
phase-transformation-induced material property variations
may be markedly decreased, and thus the above-mentioned
Page 33
problems may not occur.
Thus, the cooling rate and cooling line speed may be
determined by taking the productivity of a coating line and
economical aspects into account. In view of the
microstructure of the steel sheet dependent on the cooling
rate, the cooling rate may be adjusted to enable the
formation of a ferrite-pearlite microstructure or a
microstructure in which spheroidized cementite exists in a
ferrite matrix.
Hereinafter, a method of manufacturing a hot press
formed product will be described in detail according to an
exemplary embodiment of the present disclosure.
The method of the exemplary embodiment may include:
preparing a blank of the above-described steel sheet;
heating the blank to a temperature within a range of 850°C
to 950°C; and performing a hot press forming process on the
heated blank to manufacture a hot press formed product.
The blank is heated to a temperature within a range
of 850°C to 950°C. If the heating temperature is lower than
850°C, ferrite transformation may occur from the surface of
the blank because the blank is cooled during transfer of
the blank from furnace to die. In this case, even after a
subsequent heat treatment, martensite may not be
sufficiently formed throughout the thickness of the blank,
Page 34
and an intended degree of strength may not be obtained.
Conversely, if the heating temperature is higher than 950°C,
austenite grains may become coarse, and more heating power
may be consumed, thereby increasing manufacturing costs. In
addition, if the steel sheet from which the blank is
prepared is a cold-rolled steel sheet, decarbonization may
be facilitated, and thus after a final heat treatment
process, the strength of hot press formed products may be
low. Thus, the upper limit of the heating temperature may
be set to be 950°C.
After heating the blank to the temperature within a
range of 850°C to 950°C, the blank may be maintained within
the temperature range for 60 seconds to 600 seconds. The
temperature range is basically set for heating the blank to
an austenite region. According to another aspect, if the
temperature range is lower than 850°C, ferrite may not be
completely dissolved, and if the temperature range is
higher than 950°C, surface oxidation may occur along
austenite grain boundaries, thereby decreasing interfacial
strength and worsening bendability. Therefore, the upper
limit of the temperature range may be set to be 950°C. If
the heated blank is maintained within the temperature range
for a period of time shorter than 60 seconds, ferrite is
likely to remain unintendedly. If the heated blank is
Page 35
maintained with the temperature range for a period of time
longer than 600 seconds, a thick aluminum-containing oxide
layer may be formed on the surface, thereby leading to poor
spot weldability. Therefore, the heated blank may be
maintained within the temperature range of 850°C to 950°C
for 60 seconds to 600 seconds.
The blank heated as described above may be hot-formed
and simultaneously cooled in dies within 12 seconds after
the blank is removed from the heating furnace. As described
above, the blank having the chemical composition proposed
in the exemplary embodiment of the present disclosure is
cooled at a critical cooling rate or faster so as to obtain
a microstructure having a martensite matrix. Although the
cooling rate of the blank is increased to be higher than
critical cooling rate to obtain martensite matrix at which
transformation to martensite occurs, the strength of the
blank is not highly increased compared to the increased
cooling rate, but additional pieces of cooling equipment
may be required. That is, it is not economical. Therefore,
the cooling rate of the blank may be set to be 300°C/s or
less.
After the blank is hot-formed (hot press forming),
the hot press formed product may be cooled in the dies to a
temperature lower than 200°C to finish transformation to
Page 36
martensite.
In addition, a trimming process may be performed on
the hot press formed product, and other parts may be
coupled to the hot press formed product to form an assembly.
Then, a painting baking treatment process may be performed
on the assembly preferably at a temperature within a range
of 150°C to 200°C for 10 minutes to 30 minutes. The
temperature range and process time of the painting baking
treatment process are set as described above in
consideration of optimal drying conditions after painting.
That is, if the temperature range is lower than 150°C, a
drying time may be excessively long, and if the temperature
range is higher than 200°C, strength may decrease. In
addition, if the process time (maintaining period of time)
is shorter than 10 minutes, bake hardening may occur
insufficiently, and if the process time is excessively long,
bake hardening may occur excessively and strength may
decrease.
For example, the hot press formed product may be
manufactured using an aluminum alloy coated steel sheet
through the above-described method. In this case, the hot
press formed product manufactured using an aluminum alloy
coated steel sheet may include an Fe-Al film layer
containing at least one selected from the group consisting
Page 37
of silicon (Si): 4 wt% to 10 wt% and magnesium (Mg): 2 wt%
to 10 wt%, and other impurities.
Preferably, the hot press formed product may have a
microstructure including martensite in an amount of 90
area% or greater, retained austenite in an amount of less
than 5 area%, and the balance of at least one selected from
retained bainite and ferrite.
Preferably, the hot press formed product may have a
tensile strength of 1700 MPa or greater.
If the hot press formed product is manufactured using
a hot-rolled steel sheet or a cold-rolled steel sheet, the
hot press formed product may preferably have a tensile
strength of 1800 MPa or greater and a tensile strength x
bendability balance of 115,000 MPa•° or greater.
If the hot press formed product is manufactured using
an aluminum alloy plated steel sheet, the hot press formed
product may preferably have a tensile strength of 1800 MPa
or greater and a tensile strength x bendability balance of
100,000 MPa•° or greater.
If the hot press formed product is manufactured using
a hot-rolled steel sheet or a cold-rolled steel sheet, the
hot press formed product may preferably have a tensile
strength of 2000 MPa or greater and a tensile strength x
bendability balance of 95,000 MPa•° or greater.
Page 38
If the hot press formed product is manufactured using
an aluminum alloy plated steel sheet, the hot press formed
product may preferably have a tensile strength of 2000 MPa
or greater and a tensile strength x bendability balance of
85,000 MPa•° or greater.
In the above, "°" denotes a angle complementary to a
bend angle at a maximum load in a three-point bending test,
and the bendability is high, as the bend angle
(complementary angle) is large in a bending test.
【Mode for Invention】
Hereinafter, the present disclosure will be described
more specifically according to examples. However, the
following examples should be considered in a descriptive
sense only and not for purposes of limitation. The scope of
the present invention is defined by the appended claims,
and modifications and variations may reasonably made
therefrom.
[Example 1]
Hot press formed products having a strength of 1700
MPa or greater after a hot press forming process,
specifically, 1800 Mpa grade hot press formed products,
were manufactured as follows. First, slabs having
compositions as illustrated in Table 1 were heated to
1200°C to homogenize the microstructure of the slabs.
Page 39
Thereafter, the slabs are rough rolled, finish rolled, and
then coiled at 650°C so as to manufacture hot-rolled steel
sheets having a thickness of 3.0 mm. Then, the hot-rolled
steel sheets were pickled and cold rolled at a reduction
ratio of 50% so as to manufacture cold rolled full hard
steel sheets having a thickness of 1.5 mm. Thereafter, some
of the cold rolled full hard steel sheets were annealed at
800°C, and an overaging process was performed while
maintaining an entrance temperature to be 500°C and an exit
temperature to be 450°C, so as to manufacture cold-rolled
steel sheets. The other of the cold rolled full hard steel
sheets were annealed at 780°C and were dipped into a
coating bath including 90%Al-9%Si and a balance of iron
(Fe) and other impurities, so as to manufacture aluminum
coated (Al-Si coated) steel sheets having a coating weight
of 150 g/m2 to 160 g/m2 based on both sides.
Referring to Table 1, since inventive steels included
silicon (Si) in an amount of 0.5 wt% or greater, the
inventive steels were clearly distinguishable from steels
of the related art for hot press forming in terms of the
ratio of Mn/Si. Inventive Steels 1 to 9 had an Mn/Si ratio
within the range of 0.5 to 2, and steels to which silicon
(Si) and manganese (Mn) were added according to the related
art had an Mn/Si ratio within the range of 3.6 to 5.0. The
Page 40
steels of the related art were denoted as Comparative
Steels 1 to 8 in Table 1. Inventive Steel 5 had an
excessive amount of silicon (Si) even though the Mn/Si
ratio of Inventive Steel 5 was within the range proposed in
the embodiments of the present disclosure. Thus, Inventive
Steel 5 had aluminum coating failure and poor coating
quality. In Table 1 below, if the content of an element is
in ppm, * is attached to the symbol of the element.
[Table 1]
No. Composition (wt%) Mn/
Si
C Si Mn P* S* s-Al Ti Cr B* Mo Cu Ni N*
CS 1 0.29 0.26 1.25 110 24 0.029 0.029 0.16 26 - - - 40 4.8
CS 2 0.28 0.25 0.92 58 12 0.030 0.030 0.40 28 0.10 - - 40 3.7
IS 1 0.27 0.7 0.9 55 15 0.031 0.029 0.40 26 0.11 - - 40 1.3
IS 2 0.27 1.2 0.91 67 11 0.029 0.032 0.38 25 0.09 - - 40 0.8
IS 3 0.33 1.1 0.50 55 14 0.031 0.029 0.40 25 0.10 - - 40 0.5
CS 3 0.32 0.25 0.91 79 3 0.034 0.030 0.21 26 0.10 - - 27 3.6
CS 4 0.32 0.26 0.89 65 8 0.040 0.028 0.21 20 0.08 - - 46 3.4
CS 5 0.32 0.25 0.89 120 25 0.034 0.034 0.15 17 0.17 - - 35 3.6
CS 6 0.32 0.26 0.88 120 24 0.027 0.029 0.15 17 - - - 38 3.4
IS 4 0.32 0.6 0.90 82 0.025 0.023 0.17 24 0.15 - - 45 1.5
IS 5 0.30 1.5 0.90 77 16 0.030 0.027 0.20 27 - - - 40 0.6
CS 7 0.32 0.26 0.89 65 8 0.040 0.028 0.21 20 0.08 - - 46 3.4
IS 6 0.32 0.6 0.95 73 0.033 0.030 0.15 33 0.15 - - 27 1.6
IS 7 0.32 0.7 1.10 55 0.031 0.025 0.15 26 0.15 0.1 - 40 1.6
IS 8 0.32 0.6 0.94 68 0.023 0.027 0.20 23 0.15 - 0.15 35 1.6
IS 9 0.31 0.8 0.90 47 0.025 0.025 0.15 27 0.20 0.33 0.20 55 1.1
CS 8 0.32 0.26 1.25 109 0.030 0.029 0.20 30 - - - 52 5.0
CS: Comparative Steel, IS: Inventive Steel
The cold-rolled steel sheets and the aluminum coated
steel sheets manufactured as described above were heated to
930°C for 5 minutes to 7 minutes and were transferred from
a heating furnace to a press machine equipped with flat
Page 41
dies in which the steel sheets were cooled. At that time, a
period of time from time at which the steel sheets were
removed from the heating furnace to time at which the flat
dies were closed was 8 seconds to 12 seconds, and the steel
sheets were cooled in the flat dies at a cooling rate of
50°C/s to 100°C/s. Then, for painting baking treatment
process, the steel sheets were maintained at a temperature
of 170°C to 180°C for 20 minutes and were air cooled, and
the tensile characteristics and bendability of the steel
sheets were evaluated. Oxide scale formed on the surfaces
of the cold-rolled steel sheets during the above-described
processes was removed through a shot blasting process after
heat treatment process.
Tensile specimens were taken from the steel sheets in
the direction parallel to the rolling direction of the
steel sheets according to ASTM370A. A bending test was
performed by bending each of 60 mm x 20 mm specimens using
a 1R punch in the direction perpendicular to the rolling
direction (a bend line was parallel with the rolling
direction), and measuring a bend angle at the maximum load.
Table 2 below illustrates results of evaluation of
tensile characteristics and bendability of Inventive Steels
1 to 9 and Comparative Steels 1 to 8 after a hot press
forming process and a painting baking treatment process. In
Page 42
Table 2, YS, TS, and El refer to yield strength, tensile
strength, and elongation, respectively. In Table 2,
Inventive Steels 1 to 4 and Comparative Steels 1 to 6 are
those used to form the cold-rolled steel sheets, and
Inventive Steels 5 to 9 and Comparative Steels 7 and 8 are
those used to form the aluminum coated steel sheets.
[Table 2]
No. Mn/
Si
Properties after hot press forming
(HPF) heat treatment
Properties after HPF heat
treatment and painting
baking treatment
YS TS El Bend
angle
TS x
Bend
angle
Reference YS TS El Bend
angle
TS x
Bend
angle
CS 1 4.8 1264 1827 6.8 57.2 104,453 >110,000 1361 1701 6.3 60.1 102,230
CS 2 3.7 1194 1728 7.6 57.5 99,374 >110,000 1372 1694 7.3 64.4 109,085
IS 1 1.3 1234 1760 7.5 65.5 115,311 >110,000 1315 1650 6.2 75.2 124,009
IS 2 0.8 1156 1730 7.8 74.8 129,380 >110,000 1281 1632 7.3 79.3 129,453
IS 3 0.5 1069 1629 8.7 78.2 127,352 >110,000 1316 1611 7.6 88.3 142,165
CS 3 3.6 1270 1890 7.3 57.4 108,486 >110,000 1804 63.4 114,374
CS 4 3.4 1281 1880 6.5 56.7 106,596 >110,000 1451 1799 6.5 63.6 114,416
CS 5 3.6 1252 1810 6.4 52.0 94,120 >110,000 1299 1720 6.0 57.0 98,040
CS 6 3.4 1264 1844 6.2 48.2 88,881 >110,000 1286 1740 5.9 49.1 85,434
IS 4 1.5 1264 1832 6.8 67.1 122,744 >110,000 1399 1736 6.5 73.2 127,075
IS 5 0.6 - - - - - >100,000 - - - - -
CS 7 3.4 1324 1934 5.8 47.0 90,898 >100,000 1460 1825 6.3 53.0 96,725
IS 6 1.6 1254 1844 6.5 55.2 101,420 >100,000 1407 1754 6.3 64.3 112,782
IS 7 1.6 1246 1860 6.7 56.2 104,160 >100,000 1414 1768 6.2 61.4 108,555
IS 8 1.6 1295 1850 6.5 56.3 103,600 >100,000 1432 1768 6.3 62.2 109,970
IS 9 1.1 1328 1870 6.3 55.1 102,850 >100,000 1430 1785 6.1 64 114,240
CS 8 5.0 1377 1940 5.8 43.4 84,196 >100,000 1425 1800 6.0 53 95,400
CS: Comparative Steel, IS: Inventive Steel
First, material properties after a hot press forming
(HPF) heat treatment process were compared to evaluate the
test results on the bendability of the cold-rolled steel
sheets (Inventive Steels 1 to 4 and Comparative Steels 1 to
Page 43
6).
As illustrated in Table 2, when values of strength x
bend angle of Comparative Steels 1 to 6 having a relatively
high Mn/Si ratio were compared with values of strength x
bend angle of Inventive Steels 1 to 4 having an Mn/Si ratio
within the range proposed in the embodiments of the present
disclosure, although Inventive Steels 1 to 4 had a
relatively low Mn/Si ratio, the values of strength x bend
angle of Inventive Steels 1 to 4 were relatively high. That
is, before hot press forming process, non-uniform
microstructures such as a banded structure were reduced
owing to reduced Mn content and increased Si content, and
thus the bendability of the inventive steels were markedly
improved after the hot press forming process. In general,
when painting baking treatment process is performed on
steel sheets after the steel sheets are cooled in dies,
yield strength and bendability increase, and tensile
strength decreases slightly. After a painting baking
treatment process, the bendability of the inventive steels
having an Mn/Si ratio within the range of 2 or less was
improved much more than the comparative steels as shown in
tensile strength x bendability balance values.
The aluminum coated steel sheets (Inventive Steels 5
to 9 and Comparative Steels 7 and 8) had similar properties.
Page 44
However, when cold-rolled steel sheets and aluminum coated
steel sheets having the same composition were compared, the
bendability of the aluminum coated steel sheets was lower
than the bendability of the cold-rolled steel sheets by
about 5° to 10°. Reasons for this were the suppression of
surface decarbonization by coated layers and the
concentration of stress caused by cracks in the coated
layers. Therefore, due to this characteristics, a reference
range for the tensile strength x bendability balance of
cold-rolled steel sheets was set to be 110,00 MPa•° or
greater, and a reference range for the tensile strength x
bendability balance of aluminum coated steel sheets was set
to be 100,000 MPa•° or greater. The cold-rolled steel
sheets formed of the inventive steels had tensile strength
x bendability balance values within the range of 115,000
MPa•° to 129,000 MPa•°, and the aluminum coated steel
sheets of the inventive steels had tensile strength x
bendability balance values within the range of 101,000
MPa•° to 104,000 MPa•°. That is, both the cold-rolled steel
sheets and the aluminum coated steel sheets satisfied the
reference ranges.
[Example 2]
Hot press formed products having a strength of 1900
Page 45
MPa or greater after a hot press forming process,
specifically, 2000 MPa grade hot press formed products,
were manufactured as follows. First, slabs having
compositions as illustrated in Table 3 were heated to
1200°C to homogenize the microstructure of the slabs.
Thereafter, the slabs are rough rolled, finish rolled, and
then coiled at 650°C so as to manufacture hot-rolled steel
sheets having a thickness of 3.0 mm. Then, the hot-rolled
steel sheets were pickled and cold rolled at a reduction
ratio of 50% so as to manufacture cold rolled full hard
steel sheets having a thickness of 1.5 mm. Thereafter, some
of the cold rolled full hard steel sheets were annealed at
780°C, and an overaging process was performed while
maintaining an entrance temperature to be 500°C and an exit
temperature to be 450°C, so as to manufacture cold-rolled
steel sheets. The other of the cold rolled full hard steel
sheets were annealed at 760°C and were dipped into a
coating bath including 90%Al-9%Si and a balance of iron
(Fe) and other impurities, so as to manufacture aluminum
coated (AlSi coated) steel sheets having a coating weight
of 150 g/m2 to 160 g/m2 based on both sides.
Referring to Table 3, since inventive steels included
silicon (Si) in an amount of 0.5 wt% or greater, the
inventive steels were clearly distinguishable from steels
Page 46
of the related art for hot press forming in terms of the
ratio of Mn/Si. The inventive Steels had an Mn/Si ratio
within the range of 0.5 to 2, and steels to which silicon
(Si) and manganese (Mn) were added according to the related
art had an Mn/Si ratio within the range of 3.6 to 4.5. The
steels of the related art were mentioned as comparative
steels. Although Inventive Steel 5 had an Mn/Si ratio
within the range proposed in the embodiments of the present
disclosure, the content of silicon (Si) in Inventive Steel
5 was excessive, and thus red scale was markedly formed on
the surface of hot-rolled steel sheet of Inventive Steel 5.
The red scale remained in the shape of bands having
different surface roughness after the cold rolling process,
and thus an intended degree of surface quality could not be
obtained.
[Table 3]
No. Composition (wt%) Mn/
Si
C Si Mn P* S* s-Al Ti Cr B* Mo Cu Ni N*
CS 1 0.36 0.26 1.1 110 27 0.033 0.030 0.195 18 0.08 - - 44 4.2
CS 2 0.36 0.25 1.1 110 27 0.027 0.029 0.196 18 - - - 43 4.4
CS 3 0.35 0.28 1.1 57 6 0.042 0.031 0.20 20 0.08 - - 40 3.9
IS 1 0.37 0.55 0.89 73 16 0.032 0.025 0.20 30 0.11 - - 53 1.6
IS 2 0.36 0.7 0.90 67 26 0.026 0.031 0.20 26 0.12 - - 45 1.3
IS 3 0.37 1.07 0.89 57 14 0.03 0.024 0.48 27 0.09 - - 49 0.8
IS 4 0.36 1.00 1.30 80 18 0.022 0.025 0.48 32 0.09 - - 51 1.3
IS 5
(red
scale)
0.35 1.60 0.90 82 22 0.025 0.03 0.20 25 0.12 - - 33 0.6
CS 4 0.35 0.25 0.90 54 11 0.030 0.030 0.20 25 - - - 40 3.6
CS 5 0.35 0.28 1.1 57 6 0.042 0.031 0.20 20 0.08 - - 40 3.9
IS 6 0.35 0.6 1.10 67 8 0.025 0.031 0.20 22 0.10 - - 33 1.8
IS 7 0.35 0.65 0.90 72 18 0.029 0.025 0.20 26 0.11 - - 25 1.4
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IS 8 0.35 0.70 0.90 57 8 0.024 0.028 0.20 30 0.15 0.10 - 22 1.3
IS 9 0.34 0.60 1.00 45 12 0.03 0.032 0.20 19 0.10 - 0.20 28 1.7
IS 10 0.34 0.55 1.00 87 18 0.025 0.03 0.20 22 0.07 0.30 0.16 30 1.8
CS 6 0.35 0.20 0.90 112 20 0.036 0.035 0.20 25 0.10 - - 23 4.5
CS: Comparative Steel, IS: Inventive Steel
The cold-rolled steel sheets and the aluminum coated
steel sheets manufactured as described above were heated to
930°C for 5 minutes to 7 minutes and were transferred from
a heating furnace to a press machine equipped with flat
dies in which the steel sheets were cooled. At that time, a
period of time from time at which the steel sheets were
removed from the heating furnace to time at which the flat
dies were closed was 8 seconds to 12 seconds, and the steel
sheets were cooled in the flat dies at a cooling rate of
50°C/s to 100°C/s. Then, for painting baking treatment
process, the steel sheets were maintained at a temperature
of 170°C to 180°C for 20 minutes and were air cooled, and
the tensile characteristics and bendability of the steel
sheets were evaluated. Oxide scale formed on the surfaces
of the cold-rolled steel sheets during the above-described
processes was removed through a shot blasting process after
a heat treatment process.
Tensile specimens were taken from the steel sheets in
the direction parallel to the rolling direction of the
steel sheets according to ASTM370A. A bending test was
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performed by bending each of 60 mm x 20 mm specimens using
a 1R punch in the direction perpendicular to the rolling
direction (a bend line was parallel with the rolling
direction), and measuring a bend angle at the maximum load.
[Table 4]
No. Mn/
Si
Properties after HPF heat treatment Properties after HPF heat
treatment and painting
baking treatment
YS TS El Bend
angle
TS x
Bend
angle
Reference YS TS El Bend
angle
TS x
Bend
angle
CS 1 4.2 1439 2094 5.9 43.1 90,251 >100,000 1590 1966 5.9 47.0 92,402
CS 2 4.4 1361 2059 4.9 44.6 91,831 >100,000 1555 1920 6.3 49.0 94,080
CS 3 3.9 1345 2023 5.6 45.3 91,642 >100,000 1502 1914 6.1 53.1 101,633
IS 1 1.6 1320 2040 6.3 49.5 100,980 >100,000 1525 1925 6.0 50.6 97,405
IS 2 1.3 1377 2034 5.7 53 107,802 >100,000 1544 1920 6 55 105,600
IS 3 0.8 1375 2125 6.0 49.6 105,400 >100,000 1560 2015 5.9 60.1 121,102
IS 4 1.3 1420 2170 5.6 44.4 96,348 >100,000 1566 2035 5.8 54.4 110,704
IS 5
(red
scale)
0.6 1344 2001 6.2 54 108,054 >100,000 1480 1890 6.5 61 115,290
CS 4 3.6 1306 1977 6.5 51.7 102,186 >100,000 1506 1877 5.5 55.9 105,033
CS 5 3.9 1395 2047 5.2 35.5 72,669 >90,000 1514 1924 6 43.4 83,502
IS 6 1.8 1356 2040 5.8 45.6 93,024 >90,000 1535 1933 6 50.1 96,843
IS 7 1.4 1355 2033 6 46.2 93,925 >90,000 1539 1920 5.5 49.3 94,656
IS 8 1.3 1366 2030 5.4 45 91,350 >90,000 1544 1924 5.4 53.1 102,164
IS 9 1.7 1320 2015 6.1 46 92,690 >90,000 1512 1905 5.6 50.2 95,631
IS 10 1.8 1333 2032 6.2 45.5 92,456 >90,000 1533 1932 5.6 51.2 98,918
CS 6 4.5 1356 2043 5.8 40 81,720 >90,000 1557 1945 5.3 44.4 86,358
CS: Comparative Steel, IS: Inventive Steel
Table 4 above illustrates results of evaluation on
tensile characteristics and bendability of Inventive Steels
1 to 10 and Comparative Steels 1 to 6 after a hot press
forming process and a painting baking treatment process. In
Table 4, YS, TS, and El refer to yield strength, tensile
strength, and elongation, respectively. In Table 4,
Page 49
Inventive Steels 1 to 5 and Comparative Steels 1 to 4 are
those used to form the cold-rolled steel sheets, and
Inventive Steels 6 to 10 and Comparative Steels 5 and 6 are
those used to form the aluminum coated steel sheets.
First, material properties after hot press forming
(HPF) heat treatment process were compared to evaluate the
test results on the bendability of the cold-rolled steel
sheets (Inventive Steels 1 to 5 and Comparative Steels 1 to
4). When values of strength x bendability of Comparative
Steels 1 to 4 having a relatively high Mn/Si ratio were
compared with values of strength x bendability of Inventive
Steels 1 to 5 having an Mn/Si ratio within the range
proposed in the embodiments of the present disclosure,
although Inventive Steels 1 to 5 had a relatively low Mn/Si
ratio, the values of strength x bendability of Inventive
Steels 1 to 5 were relatively high. That is, before a hot
press forming process, non-uniform microstructures such as
a banded structure were reduced owing to reduced Mn content
and increased Si content, and thus the bendability of the
inventive steels was markedly improved after the hot press
forming process. In general, when a painting baking
treatment process is performed on steel sheets after the
steel sheets are cooled in dies, yield strength and
bendability increase, and tensile strength decreases
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slightly. After painting baking treatment process, the
bendability of the inventive steels having an Mn/Si ratio
within the range of 2 or less was improved much more than
the comparative steels as shown in tensile strength x
bendability balance values.
The aluminum coated steel sheets (Inventive Steels 6
to 10 and Comparative Steels 5 to 6) had similar properties.
However, when cold-rolled steel sheets and aluminum coated
steel sheets having the same composition were compared, the
bendability of the aluminum coated steel sheets was lower
than the bendability of the cold-rolled steel sheets by
about 5° to 10°. Reasons for this were the suppression of
surface decarbonization by coating layers and the
concentration of stress caused by cracks in the coating
layers. Therefore, due to this characteristics, a reference
range for the tensile strength x bendability balance of
cold-rolled steel sheets was set to be 95,000 MPa•° or
greater, and a reference range for the tensile strength x
bendability balance of aluminum coated steel sheets was set
to be 85,000 MPa•° or greater. The cold-rolled steel sheets
formed of the inventive steels had tensile strength x
bendability balance values within the range of 96,000 MPa•°
to 108,000 MPa•°, and the aluminum coated steel sheets
formed of the inventive steels had tensile strength x
Page 51
bendability balance values within the range of 91,000 MPa•°
to 93,000 MPa•°. That is, both the cold-rolled steel sheets
and the aluminum coated steel sheets satisfied the
reference ranges.
While exemplary embodiments have been shown and
described above, it will be apparent to those skilled in
the art that modifications and other embodiments could be
made therefrom. That is, such modifications and other
embodiments could be made without departing from the scope
of the present invention as defined by the appended claims.
Page 52
【WE CLAIM:】
【Claim 1】
A steel sheet for a formed product having high
bendability and ultra-high strength, the steel sheet
comprising C: 0.28 wt% to 0.40 wt%, Si: 0.5 wt% to 1.5 wt%,
Mn: 0.8 wt% to 1.2 wt%, Al: 0.01 wt% to 0.1 wt%, Ti: 0.01
wt% to 0.1 wt%, Cr: 0.05 wt% to 0.5 wt%, P: 0.01 wt% or
less, S: 0.005 wt% or less, N: 0.01 wt% or less, B: 0.0005
wt% to 0.005 wt%, and at least one selected from the group
consisting of Mo: 0.05 wt% to 0.5 wt%, Cu: 0.05 wt% to 0.5
wt%, and Ni: 0.05 wt% to 0.5 wt%, wherein Mn and Si
satisfies 0.05 ≤ Mn/Si ≤ 2, and the steel sheet comprises a
balance of Fe and other inevitable impurities.
【Claim 2】
The steel sheet of claim 1, wherein the steel sheet
is at least one selected from the group consisting of a
hot-rolled steel sheet, a pickled steel sheet, a coldrolled
steel sheet, and a coated steel sheet.
【Claim 3】
The steel sheet of claim 2, wherein the coated steel
sheet is an aluminum alloy coated steel sheet manufactured
by forming an aluminum alloy coating layer on a hot-rolled
steel sheet, a pickled steel sheet, or a cold-rolled steel
sheet.
Page 53
【Claim 4】
The steel sheet of claim 3, wherein the aluminum
alloy coated steel sheet comprises an alloy coating layer
comprising at least one selected from the group consisting
of Si: 8 wt% to 10 wt% and Mg: 4 wt% to 10 wt%, and a
balance of Al, Fe, and other impurities.
【Claim 5】
The steel sheet of claim 1, wherein the steel sheet
has a microstructure comprising ferrite and pearlite or a
microstructure comprising ferrite, pearlite, and bainite.
【Claim 6】
A formed product having high bendability and ultrahigh
strength and manufactured by performing a hot press
forming process on a steel sheet, the steel sheet
comprising C: 0.28 wt% to 0.40 wt%, Si: 0.5 wt% to 1.5 wt%,
Mn: 0.8 wt% to 1.2 wt%, Al: 0.01 wt% to 0.1 wt%, Ti: 0.01
wt% to 0.1 wt%, Cr: 0.05 wt% to 0.5 wt%, P: 0.01 wt% or
less, S: 0.005 wt% or less, N: 0.01 wt% or less, B: 0.0005
wt% to 0.005 wt%, and at least one selected from the group
consisting of Mo: 0.05 wt% to 0.5 wt%, Cu: 0.05 wt% to 0.5
wt%, and Ni: 0.05 wt% to 0.5 wt%, wherein Mn and Si
satisfies 0.05 ≤ Mn/Si ≤ 2, and the steel sheet comprises a
balance of Fe and other inevitable impurities.
【Claim 7】
Page 54
The formed product of claim 6, wherein the steel
sheet is an aluminum alloy coated steel sheet, and the
formed product comprises an Fe-Al film layer, wherein the
Fe-Al film layer comprises at least one selected from the
group consisting of Si: 4 wt% to 10 wt% and Mg: 2 wt% to 10
wt%, and other impurities.
【Claim 8】
The formed product of claim 6, wherein the formed
product comprises a microstructure comprising martensite in
an amount of 90 area% or greater, retained austenite in an
amount of less than 5 area%, and a balance of at least one
selected from retained bainite and ferrite.
【Claim 9】
The formed product of claim 6, wherein the formed
product has a tensile strength of 1700 MPa or greater.
【Claim 10】
A method for manufacturing a steel sheet for a formed
product having high bendability and ultra-high strength,
the method comprising:
preparing a slab, the slab comprising C: 0.28 wt% to
0.40 wt%, Si: 0.5 wt% to 1.5 wt%, Mn: 0.8 wt% to 1.2 wt%,
Al: 0.01 wt% to 0.1 wt%, Ti: 0.01 wt% to 0.1 wt%, Cr: 0.05
wt% to 0.5 wt%, P: 0.01 wt% or less, S: 0.005 wt% or less,
N: 0.01 wt% or less, B: 0.0005 wt% to 0.005 wt%, and at
Page 55
least one selected from the group consisting of Mo: 0.05
wt% to 0.5 wt%, Cu: 0.05 wt% to 0.5 wt%, and Ni: 0.05 wt%
to 0.5 wt%, wherein Mn and Si satisfies 0.05 ≤ Mn/Si ≤ 2,
and the slab comprises a balance of Fe and other inevitable
impurities;
reheating the slab to a temperature within a range of
1150°C to 1250°C;
hot rolling the reheated slab at a temperature within
a finish rolling temperature range of an Ar3 transformation
temperature to 950°C so as to form a hot-rolled steel
sheet; and
coiling the hot-rolled steel sheet at a temperature
within a range of 500°C to 730°C.
【Claim 11】
The method of claim 10, further comprising forming a
cold-rolled steel sheet by pickling and cold rolling the
hot-rolled steel sheet, continuously annealing the pickled
and cold-rolled steel sheet at a temperature within a range
of 750°C to 850°C, and overaging the annealed steel sheet
at a temperature within a range of 400°C to 600°C.
【Claim 12】
The method of claim 10, further comprising forming an
aluminum alloy coated steel sheet by pickling and cold
rolling the hot-rolled steel sheet, annealing the pickled
Page 56
and cold-rolled steel sheet at a temperature within a range
of 700°C to an Ac3 transformation temperature, and forming
an aluminum alloy coating layer on the annealed steel sheet.
【Claim 13】
The method of claim 12, wherein the forming of the
aluminum alloy coated steel sheet is performed using an
alloy coating bath, and the alloy coating layer comprises
at least one selected from the group consisting of Si: 8
wt% to 10 wt% and Mg: 4 wt% to 10 wt%, and a balance of Al,
Fe, and other impurities.
【Claim 14】
The method of claim 12, wherein aluminum alloy
coating layer has a coating weight of 120 g/m2 to 180 g/m2.
【Claim 15】
The method of claim 14, wherein the aluminum alloy
coating layer is formed by a hot dipping method.
【Claim 16】
A method for manufacturing a formed product having
high bendability and ultra-high strength, the method
comprising:
preparing a blank of a steel sheet, the steel sheet
comprising C: 0.28 wt% to 0.40 wt%, Si: 0.5 wt% to 1.5 wt%,
Mn: 0.8 wt% to 1.2 wt%, Al: 0.01 wt% to 0.1 wt%, Ti: 0.01
wt% to 0.1 wt%, Cr: 0.05 wt% to 0.5 wt%, P: 0.01 wt% or
Page 57
less, S: 0.005 wt% or less, N: 0.01 wt% or less, B: 0.0005
wt% to 0.005 wt%, and at least one selected from the group
consisting of Mo: 0.05 wt% to 0.5 wt%, Cu: 0.05 wt% to 0.5
wt%, and Ni: 0.05 wt% to 0.5 wt%, wherein Mn and Si
satisfies 0.05 ≤ Mn/Si ≤ 2, and the steel sheet comprises a
balance of Fe and other inevitable impurities;
heating the blank to a temperature within a range of
850°C to 950°C; and
manufacturing a formed product by performing a hot
press forming process on the blank to form a formed product
and cooling the formed product in dies to a temperature of
200°C or lower.
【Claim 17】
The method of claim 16, further comprising performing
a painting baking treatment process on the formed product
at a temperature within a range of 150°C to 200°C for 10
minutes to 30 minutes after the formed product is cooled in
the dies.
【Claim 18】
The method of claim 16, wherein the steel sheet is an
aluminum alloy coated steel sheet, and the formed product
comprises an Fe-Al film layer, wherein the Fe-Al film layer
comprises at least one selected from the group consisting
of Si: 4 wt% to 10 wt% and Mg: 2 wt% to 10 wt%, and other
Page 58
impurities.
【Claim 19】
The method of claim 16, wherein the heating of the
blank comprises maintaining the blank at the temperature
within the range of 850°C to 950°C for 60 seconds to 600
seconds.
【Claim 20】
The method of claim 16, wherein the cooling of the
formed product in the dies is performed by cooling the
formed product to 200°C or lower at a cooling rate ranging
from a critical cooling rate to 300°C/s.