【Technical Field】
The present disclosure relates to an ultrahighstrength
steel sheet and a method for manufacturing the
ultrahigh-strength steel sheet.
【Background Art】
Recently, automobile manufacturers have increasingly
used lightweight, high-strength materials as materials for
automobiles to prevent environmental pollution and improve
the fuel efficiency and safety of automobiles, and such
lightweight and high-strength materials have also been used
as materials for automotive structural members.
In the related art, high-strength steel sheets formed
of low carbon steel having a ferrite matrix have been used
as steel sheets for automobiles. Although low-carbon, highstrength
steel sheets are used to manufacture automobiles,
it has been difficult to obtain commercially-viable lowcarbon,
high-strength steel sheets having a maximum
elongation of 30% or greater if the low-carbon, highstrength
steel sheets have a tensile strength of about 800
MPa or greater. Therefore, it is difficult to use high3
strength steel sheets having a strength of about 800 MPa or
greater for manufacturing complex components. That is, the
use of such high-strength steel sheets only allows for the
manufacturing of simple components and makes it difficult
to manufacture freely designed components.
In addition, when current steel sheet manufacturing
techniques are considered, it is difficult to manufacture
steel sheets having a high degree of strength on the level
of 1300 Mpa or greater and processable through a cold
pressing process or a roll forming process.
Patent Documents 1 and 2 have proposed methods for
solving the above-mentioned problems. Patent Documents 1
and 2 disclose high-manganese austenitic steels having high
degrees of ductility and strength.
In Patent Document 1, a large amount of manganese
(Mn) is added to steel to obtain a steel sheet having a
high degree of ductility. However, work hardening occurs
severely in deformed portions of the steel sheet, and thus
the steel sheet is easily fractured after being worked. In
addition, although Patent Document 2 provides a steel sheet
having an intended degree of ductility, the characteristics
of the steel sheet for electroplating and hot dip plating
are poor because of the addition of a large amount of
silicon (Si). Furthermore, although Patent Documents 1 and
4
2 provide steel sheets having high degrees of workability,
the yield strength of the steel sheets is low, and thus the
crashworthiness of the steel sheets is poor. Moreover,
since the steel sheet disclosed in Patent Document 2 has
poor weldability in three-sheet lap welding, poor delayed
fracture resistance, and a degree of strength on the level
of 1200 MPa or less, the marketability of the steel sheet
was low, and the steel sheet was not successfully
commercialized.
In addition, automobile manufactures have recently
increased the use of twining-induced plasticity (TWIP)
steel because the formation of twins in high-manganese
steel during plastic deformation increases the work
hardening of high-manganese steel and thus the formability
of high-manganese steel.
However, there is a limit to increasing the tensile
strength of TWIP steel containing austenite, and thus it is
difficult to manufacture a ultrahigh-strength steel sheet
using TWIP steel.
(Patent Document 1) Japanese Patent Application Laidopen
Publication No.: 1992-259325
(Patent Document 2) International Patent Publication
No.: WO02/101109
【Disclosure】
5
【Technical Problem】
An aspect of the present disclosure may provide a
technique for manufacturing an ultrahigh-strength steel
sheet having an ultrahigh degree of strength, a high degree
of ductility, a high degree of crashworthiness, and a high
degree of three-sheet spot weldability by controlling the
contents of austenite stabilizing elements and
manufacturing conditions so that the ultrahigh-strength
steel sheet may be used for manufacturing automotive
structural members of vehicle bodies and complex internal
plates owing to high workability such as bendability.
【Technical Solution】
According to an aspect of the present disclosure, an
ultrahigh-strength steel sheet may include, by wt%, carbon
(C): 0.4% to 0.7%, manganese (Mn): 12% to 24%, aluminum
(Al): 0.01% to 3.0%, silicon (Si): 0.3% or less, phosphorus
(P): 0.03% or less, sulfur (S): 0.03% or less, nitrogen
(N): 0.04% or less, and a balance of iron (Fe) and
inevitable impurities, wherein the ultrahigh-strength steel
sheet may include single phase austenite as a
microstructure.
According to another aspect of the present disclosure,
a method for manufacturing an ultrahigh-strength steel
sheet may include: heating a steel ingot or a continuously
6
cast slab having the above-described composition to 1050°C
to 1300°C for homogenization; hot rolling the homogenized
steel ingot or continuously cast slab at a finish hot
rolling temperature of 850°C to 1000°C so as to form a hotrolled
steel sheet; coiling the hot-rolled steel sheet
within a temperature range of 200°C to 700°C; cold rolling
the coiled steel sheet at a reduction ratio of 30% to 80%
to form a cold-rolled steel sheet; continuously annealing
the cold-rolled steel sheet within a temperature range of
400°C to 900°C; and re-rolling the continuously annealed
steel sheet.
【Advantageous Effects】
According to the present disclosure, an ultrahighstrength
steel sheet having high degrees of strength and
ductility may be provided by controlling types of alloying
elements and contents of the elements, and performing a rerolling
process after a cold rolling process or a plating
process so as to induce work hardening and thus to impart
tensile strength on the level of 1300 MPa or greater and
yield strength on the level of 1000 MPa to the steel sheet.
The ultrahigh-strength steel sheet may be used for
manufacturing front side members of vehicles as well as
automotive structural members of vehicle bodies or complex
internal plates.
7
【Description of Drawings】
FIG. 1 is a view illustrating the aspect ratio of
grains of a microstructure of inventive steel 5 of Table 1
in a rolling direction before and after a re-rolling
process according to an exemplary embodiment of the
pressure difference.
FIG. 2 is a schematic view illustrating the
definition of an aspect ratio of grains of a microstructure
in a rolling direction.
FIG. 3 is a view illustrating grains of a
microstructure of inventive steel 5 of Table 3 after a rerolling
process according to an exemplary embodiment of the
pressure difference.
FIG. 4 is a view illustrating the average grain size
of a microstructure of inventive steel 7 of Table 5 before
and after a re-rolling process according to an exemplary
embodiment of the pressure difference.
FIG. 5 is a graph illustrating the tensile strength
and yield strength of inventive samples and comparative
samples of Table 7.
【Best Mode】
The inventors have conducted research to improve high
manganese steel having a high degree of strength owing to
containing a large amount of manganese (Mn) but a low
8
degree of ductility and thus a low degree of formability.
As a result, the inventors have found that an ultrahighstrength
steel sheet having high degrees of strength,
ductility, and workability for manufacturing automotive
components could be manufactured by controlling alloying
elements and inducing work hardening through a re-rolling
process.
In addition, the inventors have found that if types
and contents of alloying elements in steel are optimally
adjusted, a steel sheet having high degrees of
crashworthiness, platability, and three-sheet weldability
could be manufactured. Based on this knowledge, the
inventors have invented the present invention.
The present disclosure relates to an ultrahighstrength
steel sheet. The contents of alloying elements,
that is, the contents of austenite stabilizing elements
such as manganese (Mn), carbon (C), and aluminum (Al) in
the ultrahigh-strength steel sheet are adjusted so as to
guarantee the formation of intact austenite at room
temperature and to optimize the formation of deformation
twins during a plastic deformation. In addition, the
ultrahigh-strength steel sheet is processed through a rerolling
process so as to improve the strength of the steel
sheet and control the microstructure of the steel sheet for
9
improving the workability, crashworthiness, platability,
and weldability of the steel sheet.
Embodiments of the present disclosure will now be
described in detail.
First, reasons for regulating the contents of
alloying elements of the ultrahigh-strength steel sheet
will be described according to an exemplary embodiment of
the present disclosure. In the following description, the
content of each element is in wt% unless otherwise
specified.
Carbon (C): 0.4% to 0.7%
Since carbon (C) is an element stabilizing austenite,
as the content of carbon (C) increases, the formation of
austenite is facilitated. However, if the content of carbon
(C) in steel is less than 0.4%, when the steel is deformed,
α’-martensite is formed, causing cracks in a working
process and decreases the ductility of the steel.
Conversely, if the content of carbon (C) in steel is
greater than 0.7%, the electrical resistance of the steel
may increase, and thus the weldability of the steel may
decrease when a spot welding process using electrical
resistance is performed on three sheets of the steel.
Therefore, according to the exemplary embodiment of the
present disclosure, it may be preferable that the content
10
of carbon (C) be within the range of 0.4% to 0.7%.
Manganese (Mn): 12% to 24%
Like carbon (C), manganese (Mn) is an element
stabilizing austenite. However, if the content of manganese
(Mn) in steel is less than 12%, α’-martensite, decreasing
the formability of the steel, is formed, and thus even
though the strength of the steel is increased, the
ductility of the steel is markedly decreased. In addition,
the work hardening of the steel is decreased. Conversely,
if the content of manganese (Mn) is greater than 24%, the
strength of the steel is increased because the formation of
twins is suppressed. However, the ductility of the steel is
decreased, and the electrical resistance of the steel is
increased to result in poor weldability. Moreover, as the
content of manganese (Mn) in steel increases, cracks may be
easily formed during a hot rolling process, and in terms of
economics, the manufacturing costs of steel are increased.
Therefore, according to the exemplary embodiment of the
present disclosure, it may be preferable that the content
of manganese (Mn) be within the range of 12% to 24%.
Aluminum (Al): 0.01% to 3.0%
In general, aluminum (Al) is added to steel as a
11
deoxidizer. In the exemplary embodiment of the present
disclosure, however, aluminum (Al) is added to the steel
sheet to improve ductility and delayed fracture resistance.
That is, although aluminum (Al) stabilizes ferrite,
aluminum (Al) increases stacking fault energy on a slip
plane, thereby suppressing the formation of ε-martensite
and improving the ductility and delayed fracture resistance
of steel. In addition, although the content of manganese
(Mn) is low, aluminum (Al) suppresses the formation of ε-
martensite, and thus the addition of aluminum (Al) has an
effect of improving the workability of steel while
minimizing the addition of manganese (Mn). Therefore, if
the content of aluminum (Al) in steel is less than 0.01%,
although the strength of the steel is increased owing to
the formation of ε-martensite, the ductility of the steel
is markedly decreased. Conversely, if the content of
aluminum (Al) in steel is greater than 3.0%, the formation
of twins is suppressed, and thus the ductility of the steel
is decreased. In addition, the castability of the steel is
lowered in a continuous casting process, and when the steel
is hot-rolled to form a steel sheet, the surface of the
steel sheet is easily oxidized, thereby decreasing the
surface qualities of the steel sheet. Therefore, according
to the exemplary embodiment of the present disclosure, it
12
may be preferable that the content of aluminum (Al) be
within the range of 0.01% to 3.0%.
Silicon (Si): 0.3% or less
Silicon (Si) is an element promoting solid-solution
strengthening. When dissolved in steel, silicon (Si)
decreases the grain size of the steel and thus increases
the yield strength of the steel. It is known that if the
content of silicon (Si) in steel is excessive, the hot-dip
platability of the steel deteriorates because a silicon
oxide layer is formed on the surface of the steel.
However, if a proper amount of silicon (Si) is added
to steel containing a large amount of manganese (Mn), the
oxidation of manganese (Mn) is suppressed owing to
containing a thin silicon oxide layer formed on the surface
of the steel. Therefore, the formation of a thick manganese
oxide layer on a cold-rolled steel sheet may be prevented
after a rolling process, and the corrosion of the coldrolled
steel sheet may be prevented after an annealing
process, thereby improving the surface qualities of the
cold-rolled steel sheet and maintaining the surface
qualities of the cold-rolled steel sheet in an
electroplating process. However, if the content of silicon
(Si) in steel is increased by too much, large amounts of
13
silicon oxides may be formed on the surface of a steel
sheet in a hot rolling process, and thus the steel sheet
may not be easily pickled and may have poor surface
qualities. In addition, when a steel sheet is annealed at a
high temperature in a continuous annealing process or a
continuous hot-dip plating process, silicon (Si) may be
concentrated on the surface of the steel sheet. Thus, when
the steel sheet is processed through a hot-dip plating
process, the steel sheet may not be easily wetted with
molten zinc, and thus the platability of the steel sheet
may be lowered. Moreover, if a large amount of silicon (Si)
is added to steel, the weldability of the steel is
decreased. Therefore, to avoid the above-mentioned problems,
it may be preferable that the content of silicon (Si) be
0.3% or less.
Phosphorus (P) and sulfur (S): each 0.03% or less
In general, phosphorus (P) and sulfur (S) are
inevitably added to steel during manufacturing processes,
and thus the contents of phosphorus (P) and sulfur (S) are
limited to 0.03% or less, respectively. Particularly,
phosphorus (P) inducing segregation decreases the
workability of steel, and sulfur (S) forming coarse
manganese sulfide (MnS) causes defects such as flange
14
cracks and decreases the hole extension ratio (HER) of
steel. Therefore, the contents of phosphorus (P) and sulfur
(S) are maintained to be as low as possible.
Nitrogen (N): 0.04% or less
During solidification, nitrogen (N) contained in
austenite grains reacts with aluminum (Al) and precipitates
as nitrides, thereby facilitating the formation of twins.
That is, nitrogen (N) increases the strength and ductility
of a steel sheet during a forming process. However, if the
content of nitrogen (N) in steel is greater than 0.04%,
nitrides may be excessively precipitated, and thus the hotrolling
characteristics and elongation of the steel are
worsened. Therefore, it may be preferable that the content
of nitrogen (N) be 0.04% or less.
According to the exemplary embodiment of the present
disclosure, in addition to the above-mentioned elements,
nickel (Ni), chromium (Cr), and tin (Sn) may be further
included in the ultrahigh-strength steel sheet so as to
further improve characteristics such as crashworthiness and
platability.
Ni: 0.05% to 1.0%
15
Nickel (Ni) an effective element for stabilizing
austenite and increasing the strength of steel sheets.
However, if the content of nickel (Ni) is less than 0.05%,
it may be difficult to obtain the above-mentioned effects,
and if the content of nickel (Ni) is greater than 1.0%, it
is uneconomical because manufacturing costs increase.
Therefore, according to the exemplary embodiment of the
present disclosure, it may be preferable that the content
of nickel (Ni) be within the range of 0.05% to 1.0%.
Chromium (Cr): 0.05% to 1.0%
Chromium (Cr) is an effective element for improving
the platability and strength of steel sheets. However, if
the content of chromium (Cr) is less than 0.05%, it may be
difficult to obtain the above-mentioned effects, and if the
content of chromium (Cr) is greater than 1.0%, it is
uneconomical because manufacturing costs increase.
Therefore, according to the exemplary embodiment of the
present disclosure, it may be preferable that the content
of chromium (Cr) be within the range of 0.05% to 1.0%.
Tin (Sn): 0.01% to 0.1%
Like chromium (Cr), tin (Sn) is an effective element
for improving the platability and strength of steel sheets.
16
However, if the content of tin (Sn) is less than 0.01%, it
may be difficult to obtain the above-mentioned effects, and
if the content of tin (Sn) is greater than 0.1%, it is
uneconomical because manufacturing costs increase.
Therefore, according to the exemplary embodiment of the
present disclosure, it may be preferable that the content
of tin (Sn) be within the range of 0.01% to 0.1%.
Furthermore, according to the exemplary embodiment of
the present disclosure, titanium (Ti) and boron (B) may be
further included in the ultrahigh-strength steel sheet so
as to further improve weldability and workability. In this
case, one or both of nickel (Ni) and chromium (Cr) may be
added to the ultrahigh-strength steel sheet together with
titanium (Ti) and boron (B). If one or both of nickel (Ni)
and chromium (Cr) are added, the contents thereof may be
within the above-mentioned ranges.
Titanium (Ti): 0.005% to 0.10%
Titanium (Ti) is a strong carbide forming element,
and since titanium carbide suppresses the growth of grains,
titanium (Ti) is effective in grain refinement. If titanium
(Ti) is added to steel together with boron (B), hightemperature
compounds are formed along columnar crystal
17
boundaries, and thus grain boundary cracks may be prevented.
However, if the content of titanium (Ti) is less than
0.005%, it may be difficult to obtain the above-mentioned
effects, and if the content of titanium (Ti) is greater
than 0.10%, excessive titanium (Ti) may segregate along
grain boundaries to cause grain boundary embrittlement or
may form excessively coarse precipitates to hinder the
growth of grains. Therefore, according to the exemplary
embodiment of the present disclosure, it may be preferable
that the content of titanium (Ti) be within the range of
0.005% to 0.10%.
Boron (B): 0.0005% to 0.0050%
If boron (B) is added to steel together with titanium
(Ti), high-temperature compounds are formed along grain
boundaries, and thus the formation of grain boundary cracks
is prevented. However, if the content of boron (B) is less
than 0.0005%, it may be difficult to obtain the abovementioned
effect, and if the content of boron (B) is
greater than 0.0050%, boron compounds may be formed to
worsen the platability of steel. Therefore, according to
the exemplary embodiment of the present disclosure, it may
be preferable that the content of boron (B) be within the
range of 0.0005% to 0.0050%.
18
The ultrahigh-strength steel sheet having the abovementioned
composition may include single phase austenite as
a microstructure, and preferably, the microstructure of the
ultrahigh-strength steel sheet may include grains in an
amount of 70% or greater that have an aspect ratio of 2 or
greater in a rolling direction by the effect of work
hardening.
If the aspect ratio of the grains of the
microstructure is less than 2 in the rolling direction, the
ultrahigh-strength steel sheet may not have intended
degrees of strength and ductility. That is, since grains
deformed by work hardening to have an aspect ratio of 2 or
greater are included in the ultrahigh-strength steel sheet
in an amount of 70% or greater, the ultrahigh-strength
steel sheet may have high degrees of strength and ductility
and thus a high degree of crashworthiness.
In addition, the microstructure of the ultrahighstrength
steel sheet of the exemplary embodiment of the
present disclosure may preferably have an average grain
size of 2 μm to 10 μm. If the average grain size is greater
than 10 μm, the ultrahigh-strength steel sheet may not have
intended degree of strength and ductility. Although the
ultrahigh-strength steel sheet has a higher degree of
19
strength as the average grain size decreases, the lower
limit of the average grain size is preferably set to 2 μm
because of limitations in processing. More preferably, if
the average grain size is within the range of 2 μm to 5 μm,
the strength and ductility of the ultrahigh-strength steel
sheet may be further improved.
If the composition of the ultrahigh-strength steel
sheet is controlled as described above according to the
exemplary embodiment of the present disclosure, the range
of current in a welding process for the ultrahigh-strength
steel sheet may be within the range of 1.0 kA to 1.5 kA.
Among welding techniques, spot welding is a technique
of fusing a base metal using heat generated by electrical
resistance. If a base metal containing excessive amounts of
alloying elements is spot-welded, the electrical resistance
of the base metal may unexpectedly increase or vary due to
substances such as oxides formed on a contact surface, and
thus spot welding conditions may be restricted. In addition,
even though welding is performed, welding defects may
remain. That is, the weldability of the base metal may be
poor. That is, steel containing large amounts of carbon (C)
and manganese (Mn) has a low degree of spot weldability
because the electrical resistance of the steel is markedly
increased by carbon (C) and manganese (Mn). However,
20
according to the exemplary embodiment of the present
disclosure, the contents of carbon (C) and manganese (Mn)
in the ultrahigh-strength steel sheet are properly adjusted,
and thus the range of current in a spot welding process for
the ultrahigh-strength steel sheet may be within the range
of 1.0 kA to 1.5 kA.
The inventors have invented a method for
manufacturing the ultrahigh-strength steel sheet having the
above-described composition, and the method will now be
described in detail according to an exemplary embodiment of
the present disclosure.
According to the exemplary embodiment of the present
disclosure, a steel ingot or a continuously cast slab
having the above-described elements and element contents
within the above-described ranges may be heated for
homogenization. Thereafter, the steel ingot or continuously
cast slab may be subjected to a hot rolling process and a
hot strip coiling process to form a hot-rolled steel sheet.
In addition, the hot-rolled steel sheet may be subjected to
a cold rolling process and an annealing process to form a
cold-rolled steel sheet. In addition, the cold-rolled steel
sheet may be subjected to an electrogalvanizing process or
a hot-dip galvanizing process. In the present disclosure,
the steel ingot or continuously cast slab may be simply
21
referred to as a slab.
Hereinafter, process conditions for manufacturing the
steel sheet will be described in detail.
Heating Process (Homogenization): 1050°C to 1300°C
In the exemplary embodiment of the present disclosure,
when a slab of high manganese steel is heated for
homogenization, it may be preferable that the heating
temperature be within the range of 1050°C to 1300°C.
When the slab is heated for homogenization, as the
heating temperature increases, the size of grains may
increase, and surface oxidation may occur to cause a
decrease in strength or a deterioration surface qualities.
In addition, a liquid phase layer may be formed along
columnar boundaries of the slab, and thus when the slab is
hot rolled, cracks may be formed. Therefore, it may be
preferable that the upper limit of the heating temperature
be 1300°C. Conversely, if the heating temperature is lower
than 1050°C, it may be difficult to maintain the slab at a
certain temperature in a finish rolling process, and thus
the rolling load may increase because of a temperature
decrease. That is, the slab may not be sufficiently rolled
to an intended thickness. Therefore, it may be preferable
that the lower limit of the heating temperature be 1050°C.
22
Rolling Process: finish hot rolling temperature 850°C
to 1000°C
The slab homogenized through the heating process may
be subjected to a hot rolling process to form a hot-rolled
steel sheet. In this case, preferably, the temperature of
finish hot rolling may be set to be within the range of
850°C to 1000°C.
If the finish hot rolling temperature is lower than
850°C, the rolling load may increase. Thus, a rolling mill
may be damaged, and the interior quality of the steel sheet
may be worsened. Conversely, if the finish hot rolling
temperature is higher than 1000°C, surface oxidation may
occur during a rolling process. Therefore, preferably, the
finish hot rolling temperature may be set to be within the
range of 850°C to 1000°C, and more preferably within the
range of 900°C to 1000°C.
Coiling Process: 200°C to 700°C
The hot-rolled steel sheet may be subjected to a hot
strip coiling process. In this case, the coiling
temperature of the hot strip coiling process may preferably
be 700°C or lower.
If the coiling temperature of the hot strip coiling
23
process is higher than 700°C, a thick oxide layer may be
formed on the surface of the hot-rolled steel sheet, and
oxidation may occur inside the hot-rolled steel sheet. In
this case, the oxide layer may not be easily removed in a
pickling process. Thus, the coiling temperature may
preferably be 700°C or lower. However, to adjust the
coiling temperature to be lower than 200°C, it may be
necessary to spray a large amount of cooling water on the
hot-rolled steel after the hot rolling process. In this
case, coiling may not smoothly proceed, and workability may
decrease. Therefore, it may be preferable that the lower
limit of the coiling temperature be 200°C.
Cold Rolling Process: reduction ratio 30% to 80%
After performing the hot rolling process under the
above-mentioned conditions, a cold rolling process may be
performed under general conditions so as to form a coldrolled
steel sheet having an intended shape and thickness.
In this case, the reduction ratio of the cold rolling
process may be set according to customer requirements. For
example, preferably, the reduction ratio may be set to be
within the range of 30% to 80% so as to adjust the strength
and elongation of the steel sheet.
24
Continuous Annealing Process: 400°C to 900°C
The cold-rolled steel sheet may be subjected to a
continuous annealing process. In this case, the temperature
of the continuous annealing process may preferably be
within the range of 400°C to 900°C, and then the
platability and strength of the cold-rolled steel sheet may
be improved.
In detail, if the temperature of the continuous
annealing process is too low, the workability of the coldrolled
steel sheet may not be sufficiently improved, and
transformation into austenite may not sufficiently occur
such that austenite may not be maintained at a low
temperature. Therefore, preferably, the temperature of the
continuous annealing process may be 400°C or higher.
However, if the temperature of the continuous annealing
process is too high, recrystallization may excessively
occur, or the strength of the steel sheet may be decreased
to 1000 MPa or less because of the growth of grains.
Particularly, large amounts of surface oxides may be formed
on the steel sheet in a hot-dip plating process, and thus
the platability of the steel sheet may deteriorate.
Therefore, the upper limit of the temperature of the
continuous annealing process may be set to be 900°C.
In the exemplary embodiment of the present disclosure,
25
since the high manganese steel is austenitic steel not
undergoing phase transformation, if the high manganese
steel is heated to its recrystallization temperature or
higher, the workability of the high manganese steel may be
sufficiently improved. Therefore, general annealing
conditions may be used.
A hot-dip plated steel sheet, an electroplated steel
sheet, or an hot-dip alloy plated steel sheet may be
manufactured by immersing the cold-rolled steel sheet
manufactured under the above-described conditions into a
plating bath, or performing an electroplating process or a
hot-dip alloy plating process on the cold-rolled steel
sheet.
The electroplated steel sheet may be manufactured
using a general electroplating method and conditions. In
addition, the hot-dip alloy plated steel sheet may be
manufactured by performing a general hot-dip alloy plating
process on the cold-rolled steel sheet after the continuous
annealing process.
Generally, in an electroplating process or a hot-dip
alloy plating process, heat treatment conditions have an
effect on steel undergoing phase transformations, and thus
proper heat treatment conditions may be required. According
to the exemplary embodiment of the present disclosure,
26
however, the high manganese steel has single phase
austenite and does not undergo phase transformation, and
thus the mechanical characteristics of the high manganese
steel may be markedly independent on heat treatment
Therefore, the steel sheet may be plated under general
conditions.
The steel sheet manufactured as described above, such
as the cold-rolled steel sheet, the hot-dip plated steel
sheet, the hot-dip alloy plated steel sheet, or the
electroplated steel sheet, may be re-rolled through one of
a skin pass milling process, a double reduction rolling
process, a hot rolling finishing process, and a continuous
rolling process so as to increase the strength of the steel
sheet by work hardening.
At this time, the reduction ratio of the re-rolling
process may preferably be 30% or greater so as to
efficiently improve the tensile strength of the steel sheet
while not markedly increasing the rolling load. More
preferably, the reduction ratio of the re-rolling process
may be within the range of 30% to 50%.
Referring to FIG. 1, the microstructure of the steel
sheet varied by the re-rolling process was observed by
Electron Backscattered Diffraction (EBSD). Before the rerolling
process, the aspect ratio of grains of the steel
27
sheet in the rolling direction was less than about 1.
However, after the re-rolling process, the aspect ratio of
grains of the steel sheet in the rolling direction was 2 or
greater, and the amount of such grains was 70% or more. In
addition, the faction of twins was also increased.
Therefore, according to the exemplary embodiment of the
present disclosure, the high manganese steel could have an
ultrahigh degree of strength and a high degree of
crashworthiness through the re-rolling process. In other
words, it may be preferable that grains having an aspect
ratio of 2 or greater in the rolling direction after the
re-rolling process be included in the steel sheet in an
amount of 70% or greater.
Herein, the term “aspect ratio” refers to a ratio of
the height (b) to the width (a) of grains as shown in FIG.
2.
In addition, FIG. 4 illustrates the grain size of the
steel sheet before and after the re-rolling process. Before
the re-rolling process, the steel sheet had an average
grain size of about 10 μm, and after the re-rolling process,
the steel sheet had an average grain size of about 5 μm and
an increase twin fraction.
In general, if steel is deformed by cold rolling or
tension, grains of the steel are stretched in the
28
deformation direction of the steel. However, if high
manganese twinning-induced plasticity (TWIP) steel is
deformed, twins are formed in the steel as well as grains
of the steel being stretched. In the grains of the steel,
the twins form a new grain orientation and induce grain
refinement. That is, the re-rolling process induces grain
refinement and thus guarantees ultrahigh strength.
According to the exemplary embodiment of the present
disclosure, after the re-rolling process, the
microstructure of the steel sheet may preferably have an
average grain size of 2 μm to 10 μm and thus have ultrahigh
strength.
Unlike corrosion resistance of a plating layer,
crashworthiness relates to the mechanical characteristics
of an internal primary phase of a metal, and heat treatment
conditions for plating high manganese steel having single
phase austenite do not have an effect on the mechanical
characteristics of the high manganese steel. Therefore, the
steel sheet of the exemplary embodiment of the present
disclosure may have crashworthiness after being plated.
As described above, the steel sheet having elements
and contents of the elements and conditions for
manufacturing as described in the exemplary embodiment of
the present disclosure may have an ultrahigh degree of
29
strength within the range of 1300 MPa or greater and a high
degree of yield strength within the range of 1000 MPa or
greater.
That is, according to the exemplary embodiment of the
present disclosure, the steel sheet may have a high degree
of ductility as well as a high degree of strength, and thus
the workability of the steel sheet may be satisfactory in a
forming process.
Hereinafter, the present disclosure will be described
more specifically according to examples. However, the
examples are provided for clearly explaining the
embodiments of the present disclosure and are not intended
to limit the scope of the present invention.
【Mode for Invention】
(Example 1)
Steel ingots having compositions as illustrated in
Table 1 were maintained in a heating furnace at 1200°C for
one hour and were subjected to a hot rolling process to
form hot-rolled steel sheets. At that time, the temperature
of finish hot rolling was set to be 900°C, and after the
hot rolling process, the hot-rolled steel sheets were
coiled at 650°C. Thereafter, the hot-rolled steel sheets
were pickled and were cold rolled at a reduction ratio of
50%. Next, samples of the cold-rolled steel sheets were
30
heat treated at an annealing temperature of 800°C and an
overaging temperature of 400°C to simulate a continuous
annealing process, and were then re-rolled with reduction
ratios as illustrated in Table 2 below.
After the cold-rolled steel sheets were re-rolled, a
tension test was performed to measure mechanical
characteristics of the re-rolled steel sheets such as
strength and elongation according to reduction ratios, and
results of the tension test are illustrated in Table 2. The
tension test was performed on samples prepared from the rerolled
steel sheets according to JIS 5 by using a universal
testing machine.
[Table 1]
Samples C Al Mn P S Si N Note
1 0.35 1.48 11.50 0.01 0.01 0.01 0.0080 Comparative
Steel
2 0.59 0.00 14.92 0.02 0.01 0.01 0.0080 Comparative
Steel
3 0.55 1.55 15.27 0.01 0.01 0.01 0.0071 Inventive Steel
4 0.58 1.81 15.13 0.01 0.01 0.01 0.0082 Inventive Steel
5 0.59 2.02 15.18 0.01 0.00 0.01 0.0077 Inventive Steel
6 0.60 0.05 25.00 0.01 0.01 0.06 0.0068 Comparative
Steel
[Table 2]
Steels Reduction (%)
in re-rolling
YS(MPa) TS(MPa) T-El(%) Note
1-1 20.1 654.9 1078.6 40.1 Comparative Sample
31
1-2 29.9 802.1 1249.5 31.2 Comparative Sample
1-3 39.7 949.3 1420.3 22.3 Comparative Sample
2-1 15.1 614.0 980.0 42.2 Comparative Sample
2-2 30.9 824.0 1130.0 6.3 Comparative Sample
3-1 37.3 1250.0 1596.0 11.2 Inventive Sample
4-1 37.6 1261.0 1587.0 11.6 Inventive Sample
5-1 36.4 1260.0 1604.0 10.9 Inventive Sample
5-2 36.4 1226.0 1546.0 8.7 Inventive Sample
5-3 40.8 1271.0 1615.0 10.4 Inventive Sample
5-4 43.4 1287.0 1633.0 10.3 Inventive Sample
6-1 19.9 651.9 1111.9 27.2 Comparative Sample
6-2 27.8 800.6 1281.0 18.4 Comparative Sample
6-3 39.9 952.3 1453.6 5.4 Comparative Sample
Table 2 illustrates results of an evaluation of the
strength of the steel sheets which were prepared from the
steel ingots having the compositions shown in Table 1
through the hot rolling process, the cold rolling process,
and the re-rolling process inducing work hardening. In
Table 2, steel sheets having high degrees of tensile
strength, yield strength, and elongation according to the
reduction ratios in the re-rolling process are inventive
samples.
As illustrated in Table 2, the contents of carbon (C)
and manganese (Mn) in steels 1-1 to 1-3 prepared using
sample 1 of Table 1 were lower than the ranges proposed in
the present disclosure, and thus the yield strength and
tensile strength of steels 1-1 and 1-3 were low.
32
Particularly, steels 1-1 and 1-2 re-rolled at a reduction
ratio of less than 30% had lower yield strength and tensile
strength than steel 1-3 re-rolled at a reduction ratio of
30% or greater.
In addition, steels 2-1 and 2-2 prepared using sample
2 of Table 1 not including aluminum (Al) had low degrees of
yield strength and tensile strength. Similarly, steel 2-1
re-rolled at a reduction ratio of less than 30% had yield
strength and tensile strength lower than those of steel 2-2
re-rolled at a reduction ratio of 30% or greater.
The contents of manganese (Mn) and silicon (Si) in
steels 6-1 to 6-3 prepared using sample 6 of Table 1 were
outside the ranges proposed in the present disclosure, and
thus the yield strength of steels 6-1 to 6-3 was low. In
addition, steels 6-1 and 6-2 re-rolled at a reduction ratio
of less than 30% had yield strength and tensile strength
lower than those of steel 6-3 re-rolled at a reduction
ratio of 30% or greater.
Therefore, it can be understood that when a rerolling
process is performed at a reduction ratio of 30% or
greater, high degrees of yield strength and tensile
strength are guaranteed.
However, samples (steels 3-1 to 5-4) having
compositions as proposed in the present disclosure had high
33
degrees of yield strength and tensile strength.
Along with this, so as to evaluate the effect of the
re-rolling process on the microstructure of steel and the
yield strength and tensile strength of the steel, the
microstructure of inventive steel 5 was observed by
electron backscattered diffraction (EBSD) before and after
the re-rolling process, as illustrated in FIG. 1.
As shown in FIG. 1, before the re-rolling process,
the aspect ratio of grains of inventive steel 5 in the
rolling direction was about 1. However, after the rerolling
process, the aspect ratio of grains of inventive
steel 5 in the rolling direction was 2 or greater, and the
amount of such grains was 70% or more. In addition, the
twin faction of inventive steel 5 was also increased owing
to the re-rolling process. As described above, it may be
understood that since a re-rolling process increases the
aspect ratio of grains of steel in the rolling direction
and the formation of twins in the steel, the yield strength
and tensile strength of the steel were increased. Thus, the
yield strength and tensile strength of other inventive
samples were also increased after the re-rolling process,
and thus had a high degree of crashworthiness.
Therefore, the high manganese steel of the present
disclosure may have an ultrahigh degree of strength and a
34
high degree of crashworthiness through the re-rolling
process.
(Example 2)
Steel ingots having compositions as illustrated in
Table 3 were maintained in a heating furnace at 1200°C for
one hour and were subjected to a hot rolling process to
form hot-rolled steel sheets. At that time, the temperature
of finish hot rolling was set to be 900°C, and after the
hot rolling process, the hot-rolled steel sheets were
coiled at 650°C. Thereafter, the hot-rolled steel sheets
were pickled and were cold rolled at a reduction ratio of
50%. Next, samples of the cold-rolled steel sheets were
heat treated (continuously annealed) at an annealing
temperature of 800°C and an overaging temperature of 400°C
to simulate a continuous annealing process. In addition,
after the cold-rolled steel sheets were heat treated as
described above, a test for simulating a hot-dip
galvanizing process was performed on the steel sheets using
a hot-dip galvanizing bath adjusted to a temperature of
460°C. In addition, as described in the above example, the
continuously annealed steel sheets were re-rolled with
different reduction ratios as illustrated in Table 4 below.
The platability of the hot-dip galvanized steel
35
sheets was measured as illustrated in Table 4. In detail,
the steel sheets were hot-dip galvanized by setting the
temperature of the hot-dip galvanizing bath to be 460°C and
immersing the steel sheets into the hot-dip galvanizing
bath. Thereafter, the platability of the hot-dip galvanized
steel sheets was evaluated by observing the appearance of
the hot-dip galvanized steel sheets with the naked eye. A
steel sheet with a uniform plating layer was evaluated as
being “good”, and a steel sheet with a non-uniform plating
layer was evaluated as being “poor” as illustrated in Table
4.
In addition, after the cold-rolled steel sheets were
re-rolled, a tension test were performed to measure
mechanical characteristics of the cold-rolled steel sheets
such as strength and elongation according to reduction
ratios, and results of the tension test were illustrated in
Table 4. The tension test was performed on samples prepared
from the re-rolled steel sheets according to JIS 5 by using
a universal testing machine.
[Table 3]
Samples C Al Mn P S Si Ni Cr Sn N Note
1 0.35 1.48 12.00 0.01 0.01 0.01 0.255 0.31 0.03 0.0080 Comparative
Steel
2 0.59 0.00 14.92 0.02 0.01 0.01 0.004 0.30 0.00 0.0080 Comparative
Steel
3 0.75 1.01 15.24 0.02 0.01 0.01 0.004 0.31 0.00 0.0088 Comparative
36
Steel
4 0.59 2.02 15.18 0.01 0.00 0.01 0.009 0.31 0.00 0.0077 Comparative
Steel
5 0.51 1.31 15.42 0.02 0.01 0.01 0.255 0.31 0.03 0.0078 Inventive
Steel
6 0.50 1.79 15.23 0.01 0.00 0.01 0.253 0.31 0.03 0.0083 Inventive
Steel
7 0.62 1.60 18.20 0.01 0.01 0.01 0.210 0.20 0.03 0.0078 Inventive
Steel
8 0.60 0.05 24.00 0.01 0.01 0.06 - - - 0.0068 Comparative
Steel
[Table 4]
Steels Platability Reduction YS TS T-El Note
1-1 Good 20.1 654.9 1078.6 40.1 Comparative
Sample
1-2 Good 29.9 802.1 1249.5 31.2 Comparative
Sample
1-3 Good 39.7 949.3 1420.3 22.3 Comparative
Sample
2-1 Poor 20.1 1154.0 1480.0 16.2 Comparative
Sample
2-2 Poor 30.9 1324.0 1730.0 6.3 Comparative
Sample
3-1 Poor 34.5 1300.0 1655.0 12.4 Comparative
Sample
4-1 Poor 36.4 1260.0 1604.0 10.9 Comparative
Sample
4-2 Poor 36.4 1226.0 1546.0 8.7 Comparative
Sample
4-3 Poor 40.8 1271.0 1615.0 10.4 Comparative
Sample
4-4 Poor 43.4 1287.0 1633.0 10.3 Comparative
Sample
37
5-1 Good 32.4 1178.0 1498.0 11.8 Inventive
Sample
5-2 Good 36.9 1233.0 1563.0 10.3 Inventive
Sample
5-3 Good 38.2 1262.0 1594.0 10.0 Inventive
Sample
5-4 Good 41.9 1325.0 1666.0 9.3 Inventive
Sample
6-1 Good 18.0 918.0 1240.0 20.2 Comparative
Sample
6-2 Good 30.5 1088.0 1390.0 12.2 Inventive
Sample
6-3 Good 36.7 1188.0 1499.0 10.7 Inventive
Sample
6-4 Good 39.6 1231.0 1541.0 10.4 Inventive
Sample
6-5 Good 44.7 1294.0 1613.0 8.0 Inventive
Sample
7-1 Good 20.1 858.9 1286.3 41.5 Comparative
Sample
7-2 Good 31.2 1004.6 1452.0 32.8 Inventive
Sample
7-3 Good 39.7 1153.3 1621.2 24.0 Inventive
Sample
8-1 Poor 19.9 651.9 1111.9 27.2 Comparative
Sample
8-2 Poor 29.8 800.6 1281.0 18.4 Comparative
Sample
8-3 Poor 39.9 952.3 1453.6 5.4 Comparative
Sample
The platability evaluation results illustrated in
Table 4 were obtained from the cold-rolled steel sheets
formed from the steels illustrated in Table 3 before the
38
cold rolled steel sheets were re-rolled after the hot-dip
galvanizing simulation test. In addition, after the steel
sheets were formed of the steel ingots having compositions
as illustrated in Table 3 through the hot rolling process,
the cold rolling process, and the re-rolling process for
inducting work hardening, the strength of the steel sheets
were measured as illustrated in Table 4.
As illustrated in Table 4, the contents of elements
having an effect on platability such as nickel (Ni),
chromium (Cr), or tin (Sn) in steels 1-1 to 1-3 formed of
samples 1 of table 3 were within the ranges proposed in the
present disclosure, and thus platability of steels 1-1 to
1-3 were good. However, the content of carbon (C) having an
effect on strength was lower than the range proposed in the
present disclosure, and thus the tensile strength and yield
strength of steels 1-1 to 1-3 were not guaranteed after
work hardening. Particularly, when the reduction ratio of
the re-rolling process was less than 30%, strength was low
compared to the case in which the reduction ratio of the
re-rolling process was 30% or greater.
In addition, steels 2-1, 2-2, 3-1, and 4-1 to 4-4
formed of samples 2 to 4 of Table 3 not including tin (Sn)
having an effect on platability had a low degree of
platability.
39
Steels 8-1 to 8-3 formed of sample 8 of Table 3 not
including any one of nickel (Ni), chromium (Cr), and tin
(Sn) having an effect on platability were observed as
having very poor platability.
However, steels 5-1 to 5-4, 6-2 to 6-5, 7-2, and 7-3
formed of samples 5 – 7 having compositions as proposed in
the present disclosure had high degrees of yield strength
and tensile strength as well as having a high degree of
platability. However, steels 6-1 and 7-1 re-rolled at a
reduction ratio of less than 30% had not satisfied the
degrees of tensile strength and yield strength of the
present disclosure. That is, when the reduction ratio of
the re-rolling process was increased, for example, to 30%
or greater, yield strength and tensile strength were
further increased. Therefore, it could be understood that
when a re-rolling process is performed at a reduction ratio
of 30% or greater, high degrees of yield strength and
tensile strength are guaranteed.
Along with this, so as to evaluate the effect of the
re-rolling process on the microstructure of steel and the
yield strength and tensile strength of the steel, the
microstructure of inventive steel 5 was observed by
electron backscattered diffraction (EBSD) after the rerolling
process, as illustrated in FIG. 3.
40
As shown in FIG. 3, after the re-rolling process, the
aspect ratio of grains in the rolling direction was 2 or
greater, and the amount of such grains was 70% or greater.
In addition, many twins were formed.
As described above, it may be understood that since a
re-rolling process increases the aspect ratio of grains of
steel in the rolling direction and the formation of twins
in the steel, the yield strength and tensile strength of
the steel are increased. Thus, the yield strength and
tensile strength of other inventive samples were also
increased after the re-rolling process, and thus had a high
degree of crashworthiness.
Therefore, the high manganese steel of the present
disclosure may have an ultrahigh degree of strength and a
high degree of crashworthiness through the re-rolling
process.
(Example 3)
Steel ingots having compositions as illustrated in
Table 5 were maintained in a heating furnace at 1200°C for
one hour and were subjected to a hot rolling process to
form hot-rolled steel sheets. At that time, the temperature
of finish hot rolling was set to be 900°C, and after the
hot rolling process, the hot-rolled steel sheet was coiled
41
at 650°C. Thereafter, the hot-rolled steel sheets were
pickled and were cold rolled at a reduction ratio of 50%.
Next, samples of the cold-rolled steel sheets were heat
treated at an annealing temperature of 800°C and an
overaging temperature of 400°C to simulate a continuous
annealing process. In addition, after the cold-rolled steel
sheets were continuously annealed at 800°C as described
above, a test for simulating a hot-dip galvanizing process
was performed on the steel sheets using a hot-dip
galvanizing bath adjusted to a temperature of 460°C.
Thereafter, tension test samples were prepared from
the cold-rolled steel sheets by JIS 5, and a tension test
was performed using a universal testing machine. Results of
the tension test are illustrated in Table 6.
In addition, a current range for welding three sheets
was measured using the cold-rolled steel sheets processed
through the heat treatment simulating a continuous
annealing process, and the plated steel sheets. In detail,
three sheets of each of the steel (twining-induced
plasticity (TWIP) steel) of the present disclosure, mild
steel, and dual phase (DP) steel were welded together
within a set current range according to a standard spot
welding test method by ISO. Results of the test are
illustrated in Table 6.
42
In addition, standard cup samples were formed of the
cold-rolled steel sheets, and the formation of cracks
caused by delayed fracture were checked under salt spray
test (SST) conditions. In detail, standard cup samples were
prepared through a drawing process with a drawing ratio of
1.8, and time periods until cracks were formed in the cup
samples under SST conditions were measured. Cup samples in
which cracks were not formed for a reference time period
(240 hours) were determined as being “good.” Results of the
test are shown in Table 6.
In addition, after the cold-rolled steel sheets were
re-rolled, a tension test were performed to measure
mechanical characteristics of the steel sheets such as
strength and elongation according to the compositions and
manufacturing conditions of the steel sheets, and results
of the tension test were illustrated in Table 7 and FIG. 5.
[Table 5]
Samples C Al Mn P S Si Ni Cr Ti B N Note
1 0.35 1.48 11.50 0.01 0.01 0.01 - - - - 0.0080 *CS
2 0.59 0.00 14.92 0.02 0.01 0.01 0.140 0.30 0.044 0.0015 0.0080 CS
3 0.75 1.01 15.24 0.02 0.01 0.01 0.140 0.31 0.068 0.0017 0.0088 CS
4 0.59 1.29 15.31 0.01 0.01 0.01 0.140 0.31 0.065 0.0016 0.0080 **IS
5 0.55 1.55 15.27 0.01 0.01 0.01 0.140 0.31 0.065 0.0017 0.0071 IS
6 0.58 1.81 15.13 0.01 0.01 0.01 0.140 0.31 0.064 0.0016 0.0082 IS
7 0.59 2.02 15.18 0.01 0.00 0.01 0.190 0.31 0.063 0.0016 0.0077 IS
8 0.51 1.31 15.42 0.02 0.01 0.01 0.255 0.31 0.064 0.0016 0.0078 IS
9 0.50 1.56 15.04 0.02 0.00 0.01 0.256 0.31 0.064 0.0016 0.0074 IS
43
10 0.50 1.79 15.23 0.01 0.00 0.01 0.253 0.31 0.063 0.0017 0.0083 IS
11 0.72 1.60 18.20 0.01 0.01 0.01 0.210 0.20 0.076 0.0015 0.0078 CS
12 0.60 0.05 25.00 0.01 0.01 0.06 - - - - 0.0068 CS
*CS: Comparative Steel, IS: Inventive Steel
[Table 6]
Steels YS
(MPa)
TS
(MPa)
T-El
(%)
Current in threesheet
welding
Cracking by
delayed
fracture
Note
1 353.0 737.0 58.0 1kA or greater Did not occur Comparative Sample
2 500.0 1007.0 28.6 1kA or greater Occurred Comparative Sample
3 570.0 1004.0 41.3 Less than 1kA Did not occur Comparative Sample
4 568.0 995.0 59.1 1kA or greater Did not occur Inventive Sample
5 575.0 958.0 45.4 1kA or greater Did not occur Inventive Sample
6 578.0 940.0 48.5 1kA or greater Did not occur Inventive Sample
7 602.0 929.0 49.2 1kA or greater Did not occur Inventive Sample
8 530.0 936.0 48.9 1kA or greater Did not occur Inventive Sample
9 537.0 909.0 52.2 1kA or greater Did not occur Inventive Sample
10 542.0 885.0 55.8 1kA or greater Did not occur Inventive Sample
11 557.0 973.0 59.4 Less than 1kA Did not occur Comparative Sample
12 353.0 772.0 45.0 1kA or greater Occurred Comparative Sample
As shown in Table 6, steel sheets having satisfactory
welding current ranges and delayed fracture resistance are
inventive samples.
Referring to Table 6, steel 1 formed of sample 1 of
Table 5 having a carbon content and a manganese content
lower than the ranges proposed in the present disclosure
had low degrees of strength, ductility, and delayed
fracture resistance. Steel 2 formed of sample 2 of Table 5
44
not including aluminum (Al) had a low degree of delayed
fracture resistance, and cracks were formed in steel 2.
Furthermore, in the case of steels 3 and 11 formed of
samples 3 and 11 of Table 5 each having a carbon content
high than the range proposed in the present disclosure, a
current range in which three-sheet spot welding was
possible was less than 1 kA. In addition, steel 12 formed
of sample 12 having a manganese content and a silicon
content outside the ranges proposed in the present
disclosure had insufficient degrees of strength, ductility,
and delayed fracture resistance.
However, steels 3 to 10 formed of inventive steels of
Table 5 having optimized contents of carbon (C), manganese
(Mn), and aluminum (Al) had a current range of 1 kA or
higher for three-sheet spot welding and a satisfactory
degree of delayed fracture resistance.
[Table 7]
Steels Reduction
(%)
YS
(MPa)
TS
(MPa)
T-El
(%)
Note
1-1 20.1 654.9 1078.6 40.1 Comparative Sample
1-2 29.9 802.1 1249.5 31.2 Comparative Sample
1-3 39.7 949.3 1420.3 22.3 Comparative Sample
2-1 20.1 820.0 1180.0 16.2 Comparative Sample
2-2 30.9 941.0 1248.0 6.3 Comparative Sample
3 34.5 980.0 1299.5 12.4 Comparative Sample
4 35.0 1233.0 1593.0 12.3 Inventive Sample
5 37.3 1250.0 1596.0 11.2 Inventive Sample
45
6 37.6 1261.0 1587.0 11.6 Inventive Sample
7-1 36.4 1260.0 1604.0 10.9 Inventive Sample
7-2 36.4 1226.0 1546.0 8.7 Inventive Sample
7-3 40.8 1271.0 1615.0 10.4 Inventive Sample
7-4 43.4 1287.0 1633.0 10.3 Inventive Sample
8-1 32.4 1178.0 1498.0 11.8 Inventive Sample
8-2 36.9 1233.0 1563.0 10.3 Inventive Sample
8-3 38.2 1262.0 1594.0 10.0 Inventive Sample
8-4 41.9 1325.0 1666.0 9.3 Inventive Sample
9-1 32.4 1152.0 1451.0 11.6 Inventive Sample
9-2 35.3 1209.0 1525.0 10.4 Inventive Sample
9-3 39.9 1259.0 1576.0 9.8 Inventive Sample
9-4 40.8 1283.0 1612.0 9.5 Inventive Sample
10-1 18.0 918.0 1240.0 20.2 Comparative Sample
10-2 31.0 1088.0 1390.0 12.2 Inventive Sample
10-3 36.7 1188.0 1499.0 10.7 Inventive Sample
10-4 39.6 1231.0 1541.0 10.4 Inventive Sample
10-5 44.7 1294.0 1613.0 8.0 Inventive Sample
11-1 20.1 858.9 1286.3 41.5 Comparative Sample
11-2 30.5 934.3 1150.0 32.2 Comparative Sample
11-3 39.7 980.0 1276.0 24.0 Comparative Sample
12-1 19.9 651.9 1111.9 27.2 Comparative Sample
12-2 29.8 800.6 1281.0 18.4 Comparative Sample
12-3 39.9 952.3 1453.6 5.4 Comparative Sample
Table 7 illustrates results of evaluation of the
strength of the steel sheets which were prepared from the
steel ingots having the compositions shown in Table 5
through the hot rolling process, the cold rolling process,
and the re-rolling process inducing work hardening.
Referring to Table 7, steel sheets having high
46
degrees of tensile strength, yield strength, and elongation
according to the reduction ratios in the re-rolling process
are Inventive Samples.
As shown in Table 7, steels formed of sample 1 of
Table 5 having contents of carbon (C) and manganese (Mn)
lower than the ranges proposed in the present disclosure
had low degrees of yield strength. Particularly, when the
reduction ratio of the re-rolling process was less than 30%,
yield strength was relatively low compared to the case in
which the reduction ratio of the re-rolling process was 30%
or greater. In addition, steel sheets formed of samples 3
or 11 having a carbon content higher than the range
proposed in the present disclosure had a low degree of
yield strength or tensile strength even though the
reduction ratio of the re-rolling process was greater than
30%. Particularly, when the reduction ratio of the rerolling
process was less than 30%, strength was further
decreased. In addition, the contents of manganese (Mn) and
silicon (Si) in steels prepared using sample 12 of Table 5
were outside the ranges proposed in the present disclosure,
and thus the yield strength of the steels was low. In
addition, when the reduction ratio of the re-rolling
process was less than 30%, yield strength was lower than
the case in which the reduction ratio of the re-rolling
47
process was 30% or higher. Therefore, it could be
understood that when a re-rolling process is performed at a
reduction ratio of 30% or greater, high degrees of yield
strength and tensile strength are guaranteed.
Along with this, so as to evaluate the effect of the
re-rolling process on the microstructure of steel and the
yield strength and tensile strength of the steel, the
microstructure of inventive steel 7 was observed by
electron backscattered diffraction (EBSD) before and after
the re-rolling process, as illustrated in FIG. 4.
As illustrated in FIG. 4, the average size of grains
was about 10 μm before the re-rolling process. However,
after the re-rolling process, the average size of grains
was about 5 μm owing to grain refinement. In addition, the
twin faction of inventive steel 7 was also increased owing
to the re-rolling process. As described above, it may be
understood that since a re-rolling process induces grain
refinement and the formation of twins, the yield strength
and tensile strength of steel are increased.
FIG. 5 is a graph illustrating the tensile strength
and yield strength of comparative examples and inventive
examples of Table 7. That is, the ranges of the tensile
strength and yield strength of comparative examples and
inventive examples are illustrated in FIG. 5. As
48
illustrated in FIG. 5, according to the reduction ratio of
a re-rolling process, a yield strength range of 1000 MPa or
greater and a tensile strength range of 1300 MPa or greater
that are required for automotive crashworthy members may be
obtained according to the present disclosure.
49

WE CLAIM:
【Claim 1】
An ultrahigh-strength steel sheet comprising, by wt%,
carbon (C): 0.4% to 0.7%, manganese (Mn): 12% to 24%,
aluminum (Al): 0.01% to 3.0%, silicon (Si): 0.3% or less,
phosphorus (P): 0.03% or less, sulfur (S): 0.03% or less,
nitrogen (N): 0.04% or less, and a balance of iron (Fe) and
inevitable impurities, wherein the ultrahigh-strength steel
sheet comprises single phase austenite as a microstructure.
【Claim 2】
The ultrahigh-strength steel sheet of claim 1,
wherein the microstructure of the ultrahigh-strength steel
sheet comprises grains in an amount of 70% or greater that
have an aspect ratio of 2 or greater in a rolling direction
by an effect of work hardening.
【Claim 3】
The ultrahigh-strength steel sheet of claim 1,
further comprising nickel (Ni): 0.05% to 1.0%, chromium
(Cr): 0.05% to 1.0%, and tin (Sn): 0.01% to 0.10%.
【Claim 4】
The ultrahigh-strength steel sheet of claim 1,
50
further comprising titanium (Ti): 0.005% to 0.10%, boron
(B): 0.0005% to 0.0050%, and at least one of nickel (Ni):
0.05% to 1.0% and chromium (Cr): 0.05% to 1.0%.
【Claim 5】
The ultrahigh-strength steel sheet of claim 4,
wherein the microstructure of the ultrahigh-strength steel
sheet has an average grain size within a range of 2 μm to
10 μm by an effect of work hardening.
【Claim 6】
The ultrahigh-strength steel sheet of claim 4,
wherein the ultrahigh-strength steel sheet has a current
range of 1.0 kA to 1.5 kA during a welding process.
【Claim 7】
The ultrahigh-strength steel sheet of claim 1,
wherein the steel sheet has a tensile strength of 1300 MPa
or greater and a yield strength of 1000 MPa or greater.
【Claim 8】
The ultrahigh-strength steel sheet of claim 1,
wherein the ultrahigh-strength steel sheet is one of a
cold-rolled steel sheet, a hot-dip plated steel sheet, a
51
hot-dip alloy plated steel sheet, and an electroplated
steel sheet.
【Claim 9】
A method for manufacturing an ultrahigh-strength
steel sheet, the method comprising:
heating a steel ingot or a continuously cast slab to
1050°C to 1300°C for homogenization, the steel ingot or
continuously cast slab comprising, by wt%, carbon (C): 0.4%
to 0.7%, manganese (Mn): 12% to 24%, aluminum (Al): 0.01%
to 3.0%, silicon (Si): 0.3% or less, phosphorus (P): 0.03%
or less, sulfur (S): 0.03% or less, nitrogen (N): 0.04% or
less, and a balance of iron (Fe) and inevitable impurities;
hot rolling the homogenized steel ingot or
continuously cast slab at a finish hot rolling temperature
of 850°C to 1000°C so as to form a hot-rolled steel sheet;
coiling the hot-rolled steel sheet within a
temperature range of 200°C to 700°C;
cold rolling the coiled steel sheet at a reduction
ratio of 30% to 80% to form a cold-rolled steel sheet;
continuously annealing the cold-rolled steel sheet
within a temperature range of 400°C to 900°C; and
re-rolling the continuously annealed steel sheet.
52
【Claim 10】
The method of claim 9, wherein the steel ingot or
continuously cast slab further comprises nickel (Ni): 0.05%
to 1.0%, chromium (Cr): 0.05% to 1.0%, and tin (Sn): 0.01%
to 0.10%.
【Claim 11】
The method of claim 9, wherein the steel ingot or
continuously cast slab further comprises titanium (Ti):
0.005% to 0.10%, boron (B): 0.0005% to 0.0050%, and at
least one of nickel (Ni): 0.05% to 1.0% and chromium (Cr):
0.05% to 1.0%.
【Claim 12】
The method of claim 9, wherein the re-rolling is
performed through one of a skin pass milling process, a
double reduction rolling process, a hot rolling finishing
process, and a continuous rolling process.
【Claim 13】
The method of claim 9, wherein the re-rolling is
performed at a reduction ratio of 30% to 50%.
【Claim 14】
53
The method of claim 9, wherein after the continuous
annealing, the method further comprises electroplating or
hot-dip plating the continuously annealed steel sheet.