[Technical Field]
[0001] The present invention relates to high manganese-nitrogen containing steel sheets
having high strength and high ductility, and more particularly to high manganese-nitrogen
containing steel sheets and a method of manufacturing the same, which may be used
as steel sheets for automobiles requiring high workability and impact absorbing materials
such as bumper reinforcing materials for automobiles.
[Background Art]
[0002] Generally, steel sheets for an automobile body require high workability. To satisfy
such requirements, ultra-low carbon steel having a low tensile strength of about 200
to 300 MPa and good workability has generally been used for automobiles steel sheets.
Recently, various attempts have been made to improve fuel efficiency of automobiles
in order to solve environmental problems such as air pollution. Particularly, as weight
reduction of automobiles becomes essential for improvement of fuel efficiency, it
is necessary for automobile steel sheets to have not only high workability but also
high strength.
[0003] Further, since automobile components such as bumper reinforcing materials for automobiles
or impact absorbing materials in a car door are directly related to passenger safety,
there is an urgent need for commercialization of ultra high-strength steel which generally
has a tensile strength of 780 MPa or more and high elongation.
[0004] Examples of such high strength steel for automobiles include dual phase (DP) steel,
transformation induced plasticity (TRIP) steel, twin induced plasticity (TWIP) steel,
and the like.
[0005] First, DP steel has a dual phase of ferrite and martensite transformed from austenite
at room temperature and is manufactured by setting a cooling finish temperature below
a martensite start temperature (Ms) to transform part of the austenite into martensite
when cooling a hot rolled steel sheet to room temperature. Such DP steel may have
various mechanical properties through regulation of mole fraction of martensite and
ferrite.
[0006] TRIP steel exhibits good workability and is obtained by partially forming retained
austenite, followed by transformation of the austenite into martensite during component
machining. TRIP steel has high strength resulting from significant work hardening
based on martensite transformation, but has a drawback of excessively low elongation.
[0007] In other words, both DP steel and TRIP steel have a work hardening mechanism mainly
based on a martensite structure, which is a hard phase and exhibits a highly increasing
rate in the degree of work hardening during plastic deformation, thereby enabling
manufacture of high strength hot-rolled steel sheets. In this case, however, the steel
sheets have significantly low ductility, thereby making it difficult to guarantee
an elongation of 30% or more.
[0008] On the other hand, TWIP steel contains a large amount of manganese and has a single
austenite phase, which is stable at room temperature and allows formation of mechanical
twins in the austenite structure during component machining, thereby increasing the
degree of work hardening. Namely, TWIP steel has an austenite structure instead of
a ferrite structure as a matrix structure and has improved elongation through additional
work hardening by continuously generating mechanical twins in austenite grains to
obstruct movement of dislocations during plastic deformation. Further, TWIP steel
may have high elongation and high tensile strength due to the mechanical twins causing
a high degree of work hardening. In particular, TWIP steel has elongation 50% higher
than that of conventional DP steel or TRIP steel and is thus preferably applied to
steel sheets for automobiles.
[0009] However, current TWIP steel has a high manganese content in the range of about 18
to 30% in order to guarantee austenite stability and adjust stacking fault energy,
and requires addition of large amounts of aluminum or silicon together with manganese,
causing a significant increase in material and manufacturing costs. Moreover, there
is a need for development of TWIP steel which has a low Mn content in order to avoid
additional increase in manufacturing costs caused by volatilization of Mn or temperature
decrease during a steel manufacturing process or continuous casting process. Further,
in terms of mechanical properties, since currently developed TWIP steel has a low
yield strength of about 300 MPa and a tensile strength of 1 GPa or less, there is
a need for steel sheets which have higher strength without deteriorating elongation.
[Disclosure]
[Technical Problem]
[0010] The present invention provides a steel sheet capable of solving problems of DP steel,
TRIP steel and TWIP steel in the related art.
[0011] Specifically, the present invention provides a steel sheet which has both high strength
and high ductility with reduced amounts of manganese.
[0012] In addition, the present invention provides a steel sheet which contains inexpensive
elements instead of manganese while guaranteeing higher strength and ductility and
easier working than steel sheets having a high manganese content.
[0013] Further, the present invention provides a method of manufacturing a high manganese-nitrogen
containing steel sheet, which allows an increase in nitrogen content of the steel
sheet.
[Technical Solution]
[0014] In accordance with an aspect of the present invention, a high manganese-nitrogen
containing steel sheet includes: 0.5 to 1.0 wt% of carbon, 10 to 20 wt% of manganese,
0.02 to 0.2 wt% of nitrogen, and the balance of Fe and unavoidable impurities.
[0015] In accordance with another aspect of the present invention, a high manganese-nitrogen
containing steel sheet includes: 0.5 to 1.0 wt% of carbon, 10 to 20 wt% of manganese,
4.0 wt% or less of chrome, 0.02 to 0.3 wt% of nitrogen, and the balance of Fe and
unavoidable impurities.
[0016] In accordance with a further aspect of the present invention, a high manganese-nitrogen
containing steel sheet includes: 0.5 to 1.0 wt% of carbon, 10 to 20 wt% of manganese,
4.0 wt% or less of chrome, 0.02 to 0.3 wt% of nitrogen, at least one of less than
4 wt% of silicon, less than 3 wt% of aluminum, less than 0.30 wt% of niobium, less
than 0.30 wt% of titanium and less than 0.30 wt% of vanadium, and the balance of Fe
and unavoidable impurities.
[0017] In this case, at least part of the nitrogen may be contained in the steel sheet through
arc-melting.
[0018] The steel sheet may have a tensile strength and total elongation (TS× El) of 50,000
MPa% or more.
[0019] Manganese may be present in an amount of 15 to 18 wt%.
[0020] Nitrogen may be present in an amount of 0.10 to 0.3 wt%.
[0021] The steel sheet may be a hot-rolled steel sheet.
[0022] The steel sheet may be a cold-rolled annealed steel sheet.
[0023] In accordance with yet another aspect of the present invention, a method of manufacturing
a high manganese-nitrogen containing steel sheet includes: placing electrolytic iron,
electrolytic manganese and carbon powder in a chamber; filling the chamber with an
argon-nitrogen atmosphere; and arc-melting the electrolytic iron, electrolytic manganese
and carbon powder.
[0024] The arc-melting may be repeated plural times.
[0025] The nitrogen-argon atmosphere may have a nitrogen fraction of 0.2 to 0.8.
[0026] The method may further include: hot rolling the high nitrogen-containing steel sheet
at 900°C or more; and air cooling or forced air cooling the hot rolled steel sheet.
[0027] The method may further include: cold rolling the cooled steel sheet at a reduction
rate of 50% or more at room temperature; annealing the cold rolled steel sheet at
800°C or more; and air cooling or forced air cooling the annealed steel sheet.
[0028] The manufactured steel sheet may include: 0.5 to 1.0 wt% of carbon, 10 to 20 wt%
of manganese, 0.02 to 0.2 wt% of nitrogen, and the balance of Fe and unavoidable impurities.
[0029] In the method, a raw material for chrome may be further placed in the chamber.
[0030] In this case, the manufactured steel sheet may include 0.5 to 1.0 wt% of carbon,
10 to 20 wt% of manganese, 4.0 wt% or less of chrome, 0.02 to 0.2 wt% of nitrogen,
and the balance of Fe and unavoidable impurities.
[0031] Further, raw materials for chrome and at least one of silicon, aluminum, niobium,
titanium and vanadium may be placed in the chamber.
[0032] In this case, the manufactured steel sheet may include: 0.5 to 1.0 wt% of carbon,
10 to 20 wt% of manganese, 4.0 wt% or less of chrome, 0.02 to 0.3 wt% of nitrogen,
at least one of less than 4 wt% of silicon, less than 3 wt% of aluminum, less than
0.30 wt% of niobium, less than 0.30 wt% of titanium and less than 0.30 wt% of vanadium,
and the balance of Fe and unavoidable impurities.
[Advantageous Effects]
[0033] According to exemplary embodiments of the invention, high manganese-nitrogen containing
steel sheets have an austenite structure formed at room temperature and allow effective
regulation of stacking fault energy through addition of chrome and nitrogen. Thus,
the steel sheets allow a high degree of work hardening, and have high tensile strength
and excellent workability by mechanical twins formed during plastic deformation of
the steel sheets. Namely, the high manganese-nitrogen containing steel sheets according
to the exemplary embodiments have a very high product of tensile strength to total
elongation (TS×E1) of 50,000 MPa% or more, which is much higher than that of conventional
TWIP steel, thereby guaranteeing a significantly high product of tensile strength
to total elongation while reducing manufacturing costs.
[0034] Further, the high manganese-nitrogen containing steel sheets according to the exemplary
embodiments may be used in various ways such as hot rolled steel sheets, cold-rolled
annealed steel sheets, and the like.
[Description of Drawing]
[0035]
Fig. 1 is an electron micrograph of a high manganese-nitrogen containing steel sheet
according to one example of the present invention; and
Fig. 2 is a tensile strength curve of steel according to Example 9.
[Mode for Invention]
[0036] Exemplary embodiments of the present invention will now be described in detail with
reference to the accompanying drawings.
[0037] In high manganese-nitrogen containing steel sheets according to exemplary embodiments,
carbon and nitrogen are added while lowering the manganese content to be in the range
of 10∼20 wt% to have a single austenite phase structure at room temperature, as compared
with conventional twin induced plasticity (TWIP) steel containing 20 wt% of manganese.
Particularly, nitrogen induces not only solid solution strengthening effects, but
also mechanical twins by affecting the stacking fault energy.
[0038] Thus, the high manganese-nitrogen containing steel sheets according to the exemplary
embodiments include the aforementioned alloy elements, thereby achieving an elongation
of 50% or more and higher yield strength and tensile strength than conventional TWIP
steel while reducing the amounts of expensive alloy elements such as manganese or
aluminum below those of the conventional TWIP steel.
[0039] First, according to a first exemplary embodiment, a high manganese-nitrogen containing
steel sheet includes 0.5 to 1.0 wt% of carbon, 10 to 20 wt% of manganese, 0.02 to
0.3 wt% of nitrogen, and the balance of Fe and unavoidable impurities.
[0040] Specifically, the high manganese-nitrogen containing steel sheet according to the
first exemplary embodiment includes 10 to 20 wt% of manganese. Namely, since TWIP
steel has mechanical twins formed in an austenite matrix at room temperature during
plastic deformation, it is important to expand an austenite region of high temperature
to an austenite region at room temperature on a Fe-carbon phase diagram. In this embodiment,
manganese is used as an austenite stabilizing element.
[0041] More preferably, manganese is present in an amount of 15 to 18 wt% in the steel sheet.
If the Mn content reaches 15 wt%, austenite stability can be secured and stacking
fault energy can be effectively lowered to promote formation of mechanical twins during
plastic deformation, thereby providing a very high product of tensile strength to
elongation.
[0042] If the Mn content is less than 10 wt%, austenite stability is significantly deteriorated,
causing formation of ferrite or martensite in the austenite region during cooling
after hot rolling. Further, if the Mn content is less than 10 wt%, stacking fault
energy of the austenite phase excessively increases, thereby making it difficult to
form mechanical twins.
[0043] If the Mn content exceeds 20 wt%, the stacking fault energy excessively increases,
so that the twins are not formed and plastic deformation of austenite occurs, thereby
causing deterioration of mechanical properties.
[0044] Further, the high manganese-nitrogen containing steel sheet according to this embodiment
includes 0.5 to 1.0 wt% of carbon. Namely, Fe-Mn binary alloys containing 20 wt% or
less of Mn have ε-martensite or α-martensite partially formed therein instead of a
single austenite phase microstructure at room temperature. Thus, according to this
embodiment, in order to form a single austenite phase microstructure at room temperature,
carbon is added as an austenite stabilizing element which is inexpensive and highly
effective.
[0045] If the carbon content is less than 0.5 wt%, it is difficult to obtain a single austenite
phase during cooling after hot rolling due to insufficient austenite stability, or,
even in the case where the single austenite phase is obtained at room temperature,
phase transformation occurs from austenite to martensite during plastic deformation
to form TRIP steel, and thus desired TWIP steel cannot be obtained.
[0046] If the carbon content exceeds 1.0 wt%, stable austenite can be obtained at room temperature,
but cementite precipitation occurs, causing deterioration in elongation or weldability.
Further, if the carbon content exceeds 1.0 wt%, the stacking fault energy excessively
increases thereby making it difficult to form the mechanical twins during deformation.
[0047] Further, the high manganese-nitrogen containing steel sheet according to the first
exemplary embodiment contains 0.02 to 0.30 wt% of nitrogen. Specifically, nitrogen
acts as an interstitial element which stabilizes the austenite structure, and as in
carbon, austenite stability increases and strength resulting from solid solution strengthening
increases with increasing amount of nitrogen. Further, although the nitrogen content
increases, the stacking fault energy does not increase, thereby facilitating formation
of the mechanical twins.
[0048] According to this embodiment, when the nitrogen content is 0.10 wt% or more, the
degree of solid solution hardening increases, thereby providing advantageous effects
of significantly increased yield strength of the steel sheet.
[0049] The nitrogen content less than 0.02 wt% is an amount of nitrogen added as an impurity
in manufacture of typical steel sheets and makes it difficult to obtain austenite
stability. Thus, ferrite or martensite is not formed at room temperature after hot
rolling, and it is difficult to obtain a function of regulating the stacking fault
energy. On the other hand, although it is very difficult to increase the nitrogen
content without adding elements such as chrome, the present invention enables an increase
of the nitrogen content to 0.1 wt% or more, more preferably 0.2 wt% or more, through
an arc-melting process described below.
[0050] Next, according to a second exemplary embodiment, a high manganese-nitrogen containing
steel sheet includes 0.5 to 1.0 wt% of carbon, 10 to 20 wt% of manganese, 4.0 wt%
or less of chrome, 0.02 to 0.3 wt% of nitrogen, 4 wt% or less of chrome, and the balance
of Fe and unavoidable impurities.
[0051] First, chrome improves not only corrosion resistance but also nitrogen solubility
of steel. Further, chrome reduces the stacking fault energy, which increases due to
addition of carbon, thereby promoting formation of the mechanical twins. However,
since chrome is a ferrite stabilizing element, the chrome content exceeding 4.0 wt%
can cause partial formation of ferrite during hot rolling. Further, since chrome is
expensive, use of large amounts of chrome increases manufacturing costs. Thus, the
content of chrome may be set to 4 wt% or less.
[0052] Further, if the nitrogen content exceeds 0.30 wt%, it is necessary to increase the
Cr content added in order to allow nitrogen to be dissolved in a large amount in the
steel sheet, which results in an undesirable increase of manufacturing costs.
[0053] Further, the amounts of other elements added to the steel sheet according to the
second embodiment are the same as those of the steel sheet according to the first
embodiment, and a detailed description thereof will thus be omitted herein.
[0054] Next, a high manganese-nitrogen containing steel sheet according to a third exemplary
embodiment of the invention includes 0.5 to 1.0 wt% of carbon, 10 to 20 wt% of manganese,
4.0 wt% or less of chrome, 0.02 to 0.3 wt% of nitrogen, at least one of less than
4 wt% of silicon, less than 3 wt% of aluminum, less than 0.30 wt% of niobium, less
than 0.30 wt% of titanium and less than 0.30 wt% of vanadium, and the balance of Fe
and unavoidable impurities.
[0055] Specifically, if the silicon content is 4 wt% or less, solid solution hardening obtained
by silicon results in reduction of grain size, thereby improving strength through
increase of yield strength. Further, addition of silicon reduces the stacking fault
energy of steel, thereby facilitating formation of mechanical twins during plastic
deformation.
[0056] However, if the added amount of silicon exceeds 4 wt%, a silicon oxide layer is formed
on the steel sheet, thereby deteriorating wettability. Further, the stacking fault
energy of the steel is excessively lowered to decrease austenite stability, thereby
promoting formation of ε-martensite. Thus, the silicon content of silicon may be set
to 4 wt% or less.
[0057] Further, if the aluminum content is 3 wt% or less, deoxidation effects cannot be
obtained. Further, aluminum suppresses formation of ε-martensite through increase
in stacking fault energy at a slip plane, thereby improving ductility. In addition,
aluminum may suppress formation of ε-martensite even with a low amount of manganese,
thereby enabling minimization of manganese content in manufacture of steel while improving
workability.
[0058] However, if the aluminum content exceeds 3 wt%, formation of twins is suppressed
due to excessive increase in stacking fault energy, thereby deteriorating ductility
and casting properties upon continuous casting. Moreover, surface oxidation severely
occurs upon hot rolling, thereby deteriorating surface quality of finished products.
[0059] Further, niobium, titanium and vanadium are strong carbide forming elements coupling
with carbon to form carbide, which effectively prevents grain growth to form fine
grains while providing precipitation hardening effects by formation of precipitate
phases. However, if the amount of niobium, titanium or vanadium exceeds 0.30 wt%,
segregation of niobium, titanium or vanadium can occur in grain boundaries causing
grain boundary brittlement, or the precipitate phases can become excessively coarse,
thereby deteriorating grain growth effects. Thus, niobium, titanium or vanadium may
be added in an amount of 0.30 wt% or less.
[0060] Further, the amounts of other elements added to the steel sheet according to the
third embodiment are the same as those of the steel sheets according to the first
and second embodiments, and a detailed description thereof will thus be omitted herein.
[0061] Next, a method of manufacturing a high manganese-nitrogen containing steel sheet
according to an exemplary embodiment of the invention will be described.
[0062] The method of manufacturing a high manganese-nitrogen containing steel sheet according
to the exemplary embodiment is as follows. First, electrolytic iron, electrolytic
manganese, and carbon powder are placed in a chamber. Here, the composition of a final
steel sheet may be controlled by controlling the amounts of such raw materials supplied
to the chamber. Then, the chamber is evacuated and is filled with argon and nitrogen
to create an argon-nitrogen atmosphere therein. Here, the argon-nitrogen atmosphere
has a total pressure of 1 atm., and nitrogen may have a partial pressure in the range
of 0.2 to 0.8 atm. If the ratio of nitrogen is less than 20 wt%, the amount of nitrogen
added becomes too low in the steel during arc-melting, thereby deteriorating arc-melting
efficiency. If the ratio of nitrogen exceeds 80 wt%, the pressure of inert gas, that
is, the pressure of argon, is excessively reduced, causing severe generation of manganese
fumes, by which the interior of the chamber is severely contaminated. Further, if
the ratio of nitrogen is too high, scattering of the raw materials severely occurs
due to melting of a tungsten electrode rod, causing a very rough surface of the steel
sheet after arc-melting. Next, the raw materials are subjected to arc-melting using
an electrode rod within the chamber, followed by cooling for an appropriate period
of time, thereby providing desired steel. Here, although arc-melting and cooling may
be performed once, arc-melting and cooling are desirably repeated plural times. In
addition, the nitrogen content increases with increasing the number of times of repeating
the processes of arc-melting and cooling.
[0063] Particularly, since the nitrogen content is limited to 0.02 to 0.1 wt% in a typical
method of manufacturing TWIP steel, it is very difficult to form a high manganese-nitrogen
containing steel sheet having the composition according to the first exemplary embodiment
without adding an element for promoting dissolution of nitrogen in the steel sheet,
such as chrome. However, when the steel is formed by arc-melting in the argon-nitrogen
atmosphere as described above, it is possible to add a larger amount of nitrogen without
adding an expensive element such as chrome than in the case of forming the steel sheet
using a typical method, and, particularly, the high manganese-nitrogen containing
steel sheet having the composition according to the first embodiment may be obtained.
However, the method of manufacturing a steel sheet using arc-melting may be applied
not only to the steel sheet having the composition according to the first embodiment
but also to steel sheets having various compositions.
[0064] After manufacturing the high manganese-nitrogen containing steel sheet using the
arc-melting, the steel sheet is subjected to hot rolling at 900°C or more, followed
by air cooling or forced air cooling.
[0065] Further, more preferably, the hot rolled and cooled steel sheet is subjected to cold
rolling at a reduction rate of 50% at room temperature, followed by annealing at 800°C
or more and air cooling or forced air cooling.
[0066] Alternatively, the high manganese-nitrogen containing steel sheets according to the
embodiments of the invention may be manufactured by a typical method.
[0067] Specifically, the typical method of manufacturing the steel sheet includes heating
a steel sheet having a desired composition to 1100°C or more, hot rolling the heated
steel sheet at 900°C or more to provide a steel sheet, and air cooling or forced air
cooling the hot rolled steel sheet. Then, the method may further include cold rolling
the cooled steel sheet at a reduction rate of 50% or more, annealing the cold rolled
steel sheet at 800°C or more, and air cooling or forced air cooling the annealed steel
sheet.
[Mode for Invention]
[0068] Samples of Examples 1 to 6 and Comparative Examples 1 to 6 were each produced by
heating steels having compositions listed in Table 1 to 1100°C or more, hot rolling
at 900°C or more to provide steel sheets with a thickness of 3mm, and air cooling
the hot rolled steel sheet. In particular, the sample of Example 4 is a cold rolled
steel sheet sample which was produced by cold rolling the hot rolled steel sheet sample
of Example 3 from a thickness of 3mm to a thickness of 1.5 mm, followed by annealing
at 800°C for 10 minutes.
Table 1
Sample No. |
Composition (wt%) |
Note |
C |
Mn |
Cr |
N |
Al |
Si |
Example 1 |
0.594 |
14.96 |
1.83 |
0.068 |
|
- |
hot-rolled steel sheet |
Example 2 |
0.618 |
15.03 |
1.82 |
0.086 |
- |
- |
hot-rolled steel sheet |
Example 3 |
0.560 |
14.90 |
2.51 |
0.210 |
- |
- |
hot-rolled steel sheet |
Example 4 |
0.560 |
14.90 |
2.51 |
0.210 |
- |
|
cold-rolled annealed steel sheet |
Example 5 |
0.580 |
17.05 |
0.209 |
0.023 |
0.005 |
1.59 |
hot-rolled steel sheet |
Example 6 |
0.610 |
19.01 |
0.302 |
0.020 |
0.96 |
- |
hot-rolled steel sheet |
Comparati ve Example 1 |
0.607 |
9.00 |
1.73 |
0.060 |
- |
- |
hot-rolled steel sheet |
Comparati ve Example 2 |
0.0006 |
23.8 |
- |
- |
2.70 |
3.0 |
hot-rolled steel sheet |
Comparati ve Example 3 |
0.580 |
17.49 |
- |
- |
1.50 |
- |
hot-rolled steel sheet |
Comparati ve Example 4 |
0.933 |
12.76 |
- |
- |
- |
0.010 |
hot-rolled steel sheet |
Comparati ve Example 5 |
1.16 |
9.87 |
- |
- |
- |
0.066 |
hot-rolled steel sheet |
Comparati ve Example 6 |
1.19 |
8.08 |
- |
- |
- |
0.067 |
hot-rolled steel sheet |
[0069] Next, strength and elongation of the samples were measured, and results are shown
in the following table 2.
Table 2
Sample |
Yield strength No. (YS) (MPa) |
Tensile strength (TS) (MPa) |
Total elongation El(%) |
TS×E1 (MPa%) |
Note |
Example 1 |
361.4 |
900.9 |
60.0 |
54054 |
hot-rolled steel sheet |
Example 2 |
366.3 |
880.1 |
62.4 |
54918 |
hot-rolled steel sheet |
Example 3 |
653.1 |
1050.6 |
59.6 |
62616 |
hot-rolled steel sheet |
Example 4 |
607.7 |
1155.3 |
61.3 |
70820 |
cold-rolled annealed steel sheet |
Example 5 |
343.2 |
803.2 |
68.2 |
53413 |
hot-rolled steel sheet |
Example 6 |
358.4 |
818.3 |
66.5 |
54417 |
hot-rolled steel sheet |
Comparative Example 1 |
650.1 |
928.7 |
15.5 |
14395 |
hot-rolled steel sheet |
Comparative Example 2 |
339.0 |
666.0 |
67.0 |
44622 |
hot-rolled steel sheet |
Comparative Example 3 |
313.3 |
711.4 |
61.4 |
43680 |
hot-rolled steel sheet |
Comparative Example 4 |
387.0 |
1021.2 |
33.9 |
34619 |
hot-rolled steel sheet |
Comparative Example |
461.2 |
908.8 5 |
7.61 |
6916 |
hot-rolled steel sheet |
Comparative Example 6 |
470.5 |
937.3 |
5.04 |
4724 |
hot-rolled steel sheet |
[0070] As seen from the Table, each of the steel sheets according to Examples 1 to 4 has
a yield strength (YS) exceeding 300 MPa and a tensile strength (TS) exceeding 880Mpa.
Further, each of the steel sheets according to Examples 1 to 4 has a total elongation
(EL) of about 60%, and a very high product of tensile strength to elongation (TS×EL)
of 50,000 MPa%. In other words, it can be seen that each of the steel sheets according
to the examples has higher yield strength and higher tensile strength than conventional
TWIP steel (Comparative Examples 2 and 3), and similar elongation to the conventional
TWIP steel. Particularly, as can be seen from Example 3, when the nitrogen content
exceeds 0.2 wt%, the steel sheet sample have very high yield strength and tensile
strength provided by solid solution hardening effects of nitrogen. Namely, the steel
sheet according to Example 3 has a tensile strength exceeding 1 GPa and an elongation
approaching 60%, thereby providing a product of tensile strength and elongation (TS×EL)
exceeding 60,000 MPa%. Further, Example 4 is a steel sheet produced by cold rolling
and annealing the hot rolled steel sheet of Example 3, and it was confirmed that Example
4 had improved tensile strength and elongation.
[0071] Formation of mechanical twins can be confirmed from Fig. 1. Specifically, Fig. 1
is an electron micrograph of a high manganese-nitrogen containing steel sheet according
to one example of the present invention, and as seen from Fig.1, the steel sheet according
to Example 3 has mechanical twins.
[0072] Further, as compared with the steel sheets according to Examples 1 to 4, the steel
sheet of Example 5 further including aluminum and silicon and the steel sheet of Example
6 further including aluminum have large amounts of nitrogen despite a significant
decrease in Cr content, thereby exhibiting excellent yield strength and tensile strength.
[0073] On the other hand, the steel sheets of the comparative examples were produced by
a conventional method and had lower tensile strength or elongation than those of the
examples. First, the steel sheet according to Comparative Example 1 was a TRIP steel
sheet and had a high tensile strength of 928.7 MPa and a low total elongation of 15.5%.
Other TWIP steel sheets (Comparative Examples 2 and 3), which did not contain nitrogen
and were produced by a conventional method, have high total elongation of 60% or more,
but have a relatively low tensile strength of about 700 MPa, thereby providing a product
of tensile strength to elongation (TS×EL) of about 40,000 MPa%. Further, it could
be seen that the steel sheets according to Comparative Examples 4 to 6 rapidly reduced
in elongation with increasing carbon content.
[0074] Next, the steel sheets according to Examples 7 to 9 were produced using arc-melting.
Specifically, electrolytic iron, electrolytic manganese, and carbon powder were placed
in a predetermined ratio in a chamber, which in turn was evacuated and filled with
argon and nitrogen to create an argon-nitrogen atmosphere in the chamber. Here, advantageously,
the argon-nitrogen atmosphere had a total pressure of 1 atm., and nitrogen had a partial
pressure in the range of 0.2 to 0.8 atm. Then, the raw materials were subjected to
arc-melting using an arc electrode rod at an electric current of 400A for 30 minutes
while advancing the arc electrode rod in a state of being separated a distance of
2 to 5 cm from each of the samples, followed by cooling for 30 minutes. The process
of arc-melting and cooling was repeated three times.
[0075] Next, for the steel sheet according to Example 10, a raw material for chrome was
further placed together with electrolytic iron, electrolytic manganese and carbon
powder in the chamber, followed by arc-melting. Other conditions for producing the
steel sheet according to Example 10 were the same as those for the steel sheets according
to Examples 1 to 3.
[0076] Next, the steel sheet according to Comparative Example 7 was produced by melting
the raw materials in a nitrogen atmosphere without arc-melting.
[0077] Detailed compositions of the high Mn steel sheets according to Examples 7 to 10 and
Comparative Example 7 are as follows.
Table 3
Sample No. |
Composition (wt%) |
Note |
C |
Mn |
Cr |
N |
Example 7 |
0.003 |
11.95 |
- |
0.093 |
Arc-melting |
Example 8 |
0.760 |
14.27 |
- |
0.109 |
Arc-melting |
Example 9 |
0.570 |
16.47 |
- |
0.090 |
Arc-melting |
Example 10 |
0.004 |
14.42 |
1.99 |
0.141 |
Arc-melting |
Comparative Example 7 |
0.618 |
15.03 |
1.82 |
0.086 |
Melting in nitrogen atmosphere |
[0078] As in Examples 7 to 10, it was possible to produce a high-Mn steel sheet by arc-melting
Cr-free steel in an argon-nitrogen atmosphere. Further, as in Example 4, it was possible
to form higher-nitrogen steel by arc-melting Cr-containing steel in an argon-nitrogen
atmosphere.
[0079] On the other hand, the steel according to the comparative example included chrome
and was produced by a typical steel manufacturing method in a nitrogen atmosphere.
It could be confirmed that, even in the case where Cr-containing steel was subj ected
to melting in a nitrogen atmosphere as in the comparative example, the nitrogen content
of the steel was less than that of the steel sheets according to the examples. Namely,
as in the comparative example, when steel containing 1.73 wt% of Cr is produced by
melting, the steel contained 0.086 wt% of nitrogen, which was much lower than the
nitrogen content of the steel sheet according to Example 10, which contained a similar
amount of Cr to that of the comparative example and 0.141 wt% of nitrogen.
[0080] Fig. 2 is a tensile strength curve of steel according to Example 9. As shown in Fig.
2, advantageously, the steel according to Example 9 has excellent strength and elongation,
that is, 985 MPa and 56%, and has a product of tensile strength to elongation of about
55,000 MPa%, which is much higher than high Mn steel containing 20 wt% or less of
Mn without containing Cr, and which is similar to high Mn steel containing more than
20% Mn and an expensive metal element such as Cr or the like.
[0081] Although some embodiments have been described herein, it should be understood by
those skilled in the art that these embodiments are given by way of illustration only,
and that various modifications, variations, and alterations can be made without departing
from the spirit and scope of the invention. Therefore, the scope of the invention
should be limited only by the following claims and equivalents thereof.
1. A high manganese-nitrogen containing steel sheet comprising: 0.5 to 1.0 wt% of carbon;
10 to 20 wt% of manganese; 0.02 to 0.2 wt% of nitrogen; and the balance of Fe and
unavoidable impurities.
2. A high manganese-nitrogen containing steel sheet comprising: 0.5 to 1.0 wt% of carbon;
10 to 20 wt% of manganese; 4.0 wt% or less of chrome; 0.02 to 0.3 wt% of nitrogen;
and the balance of Fe and unavoidable impurities.
3. A high manganese-nitrogen containing steel sheet comprising: 0.5 to 1.0 wt% of carbon;
10 to 20 wt% of manganese; 4.0 wt% or less of chrome; 0.02 to 0.3 wt% of nitrogen;
at least one of less than 4 wt% of silicon, less than 3 wt% of aluminum, less than
0.30 wt% of niobium, less than 0.30 wt% of titanium and less than 0.30 wt% of vanadium;
and the balance of Fe and unavoidable impurities.
4. The high manganese-nitrogen containing steel sheet of any one of claims 1 to 3, wherein
at least part of the nitrogen is contained in the steel sheet through arc-melting.
5. The high manganese-nitrogen containing steel sheet of any one of claims 1 to 3, wherein
the steel sheet has a product of tensile strength to total elongation (TS× El) of
50,000 MPa% or more.
6. The high manganese-nitrogen containing steel sheet of any one of claims 1 to 3, wherein
manganese is present in an amount of 15 to 18 wt%.
7. The high manganese-nitrogen containing steel sheet of any one of claims 1 to 3, wherein
nitrogen is present in an amount of 0.10 to 0.3 wt%.
8. The high manganese-nitrogen containing steel sheet of any one of claims 1 to 3, wherein
the steel sheet is a hot rolled steel sheet.
9. The high manganese-nitrogen containing steel sheet of any one of claims 1 to 3, wherein
the steel sheet is a cold rolled annealed steel sheet.
10. A method of manufacturing a high manganese-nitrogen containing steel sheet, comprising:
placing electrolytic iron, electrolytic manganese and carbon powder in a chamber;
filling the chamber with an argon-nitrogen atmosphere; and
arc-melting the electrolytic iron, electrolytic manganese and carbon powder.
11. The method of claim 10, wherein the arc-melting is repeated plural times.
12. The method of claim 10, wherein the nitrogen-argon atmosphere has a nitrogen fraction
of 0.2 to 0.8.
13. The method of any one of claims 10 to 12, further comprising:
hot rolling the high nitrogen-containing steel sheet at 900°C or more; and
air cooling or forced air cooling the hot rolled steel sheet.
14. The method of claim 13, further comprising:
cold rolling the cooled steel sheet at a reduction rate of 50% or more at room temperature;
annealing the cold rolled steel sheet at 800°C or more; and
air cooling or forced air cooling the annealed steel sheet.
15. The method of any one of claims 10 to 12, wherein the manufactured steel sheet comprises
0.5 to 1.0 wt% of carbon, 10 to 20 wt% of manganese, 0.02 to 0.2 wt% of nitrogen,
and the balance of Fe and unavoidable impurities.
16. The method of any one of claims 10 to 12, wherein a raw material for chrome is further
placed in the chamber.
17. The method of claim 16, wherein the manufactured steel sheet comprises 0.5 to 1.0
wt% of carbon, 10 to 20 wt% of manganese, 4.0 wt% or less of chrome, 0.02 to 0.2 wt%
of nitrogen, and the balance of Fe and unavoidable impurities.
18. The method of any one of claims 10 to 12, wherein raw materials for chrome and at
least one of silicon, aluminum, niobium, titanium and vanadium are further placed
in the chamber.
19. The method of claim 18, wherein the manufactured steel sheet comprises 0.5 to 1.0
wt% of carbon, 10 to 20 wt% of manganese, 4.0 wt% or less of chrome, 0.02 to 0.3 wt%
of nitrogen, at least one of less than 4 wt% of silicon, less than 3 wt% of aluminum,
less than 0.30 wt% of niobium, less than 0.30 wt% of titanium and less than 0.30 wt%
of vanadium, and the balance of Fe and unavoidable impurities.