Patent Publication Number: US-2012045358-A1

Title: High manganese nitrogen-containing steel sheet having high strength and high ductility, and method for manufacturing the same

Description:
TECHNICAL FIELD 
     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 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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 
     The present invention provides a steel sheet capable of solving problems of DP steel, TRIP steel and TWIP steel in the related art. 
     Specifically, the present invention provides a steel sheet which has both high strength and high ductility with reduced amounts of manganese. 
     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. 
     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 
     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. 
     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. 
     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. 
     In this case, at least part of the nitrogen may be contained in the steel sheet through arc-melting. 
     The steel sheet may have a tensile strength and total elongation (TS×El) of 50,000 MPa % or more. 
     Manganese may be present in an amount of 15 to 18 wt %. 
     Nitrogen may be present in an amount of 0.10 to 0.3 wt %. 
     The steel sheet may be a hot-rolled steel sheet. 
     The steel sheet may be a cold-rolled annealed steel sheet. 
     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. 
     The arc-melting may be repeated plural times. 
     The nitrogen-argon atmosphere may have a nitrogen fraction of 0.2 to 0.8. 
     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. 
     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. 
     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. 
     In the method, a raw material for chrome may be further placed in the chamber. 
     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. 
     Further, raw materials for chrome and at least one of silicon, aluminum, niobium, titanium and vanadium may be placed in the chamber. 
     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 
     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×El) 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. 
     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 
         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 
     Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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 c-martensite or a-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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     Further, if the aluminum content is 3 wt % or less, deoxidation effects cannot be obtained. Further, aluminum suppresses formation of c-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. 
     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. 
     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. 
     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. 
     Next, a method of manufacturing a high manganese-nitrogen containing steel sheet according to an exemplary embodiment of the invention will be described. 
     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. 
     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. 
     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. 
     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. 
     Alternatively, the high manganese-nitrogen containing steel sheets according to the embodiments of the invention may be manufactured by a typical method. 
     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 
     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 3 mm, 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 3 mm to a thickness of 1.5 mm, followed by annealing at 800° C. for 10 minutes. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Composition (wt %) 
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Sample No. 
                 C 
                 Mn 
                 Cr 
                 N 
                 Al 
                 Si 
                 Note 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 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 
               
               
                 Comparative 
                 0.607 
                 9.00 
                 1.73 
                 0.060 
                 — 
                 — 
                 hot-rolled 
               
               
                 Example 1 
                   
                   
                   
                   
                   
                   
                 steel sheet 
               
               
                 Comparative 
                 0.0006 
                 23.8 
                 — 
                 — 
                 2.70 
                 3.0 
                 hot-rolled 
               
               
                 Example 2 
                   
                   
                   
                   
                   
                   
                 steel sheet 
               
               
                 Comparative 
                 0.580 
                 17.49 
                 — 
                 — 
                 1.50 
                 — 
                 hot-rolled 
               
               
                 Example 3 
                   
                   
                   
                   
                   
                   
                 steel sheet 
               
               
                 Comparative 
                 0.933 
                 12.76 
                 — 
                 — 
                 — 
                 0.010 
                 hot-rolled 
               
               
                 Example 4 
                   
                   
                   
                   
                   
                   
                 steel sheet 
               
               
                 Comparative 
                 1.16 
                 9.87 
                 — 
                 — 
                 — 
                 0.066 
                 hot-rolled 
               
               
                 Example 5 
                   
                   
                   
                   
                   
                   
                 steel sheet 
               
               
                 Comparative 
                 1.19 
                 8.08 
                 — 
                 — 
                 — 
                 0.067 
                 hot-rolled 
               
               
                 Example 6 
                   
                   
                   
                   
                   
                   
                 steel sheet 
               
               
                   
               
            
           
         
       
     
     Next, strength and elongation of the samples were measured, and results are shown in the following table 2. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Yield 
                 Tensile 
                   
                   
                   
               
               
                   
                 strength 
                 strength 
                 Total 
               
               
                   
                 (YS) 
                 (TS) 
                 elongation 
                 TS × El 
               
               
                 Sample No. 
                 (MPa) 
                 (MPa) 
                 El (%) 
                 (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 
                 650.1 
                 928.7 
                 15.5 
                 14395 
                 hot-rolled steel 
               
               
                 Example 1 
                   
                   
                   
                   
                 sheet 
               
               
                 Comparative 
                 339.0 
                 666.0 
                 67.0 
                 44622 
                 hot-rolled steel 
               
               
                 Example 2 
                   
                   
                   
                   
                 sheet 
               
               
                 Comparative 
                 313.3 
                 711.4 
                 61.4 
                 43680 
                 hot-rolled steel 
               
               
                 Example 3 
                   
                   
                   
                   
                 sheet 
               
               
                 Comparative 
                 387.0 
                 1021.2 
                 33.9 
                 34619 
                 hot-rolled steel 
               
               
                 Example 4 
                   
                   
                   
                   
                 sheet 
               
               
                 Comparative 
                 461.2 
                 908.8 
                 7.61 
                 6916 
                 hot-rolled steel 
               
               
                 Example 5 
                   
                   
                   
                   
                 sheet 
               
               
                 Comparative 
                 470.5 
                 937.3 
                 5.04 
                 4724 
                 hot-rolled steel 
               
               
                 Example 6 
                   
                   
                   
                   
                 sheet 
               
               
                   
               
            
           
         
       
     
     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 880 Mpa. 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. 
     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. 
     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. 
     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. 
     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 400 A 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. 
     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. 
     Next, the steel sheet according to Comparative Example 7 was produced by melting the raw materials in a nitrogen atmosphere without arc-melting. 
     Detailed compositions of the high Mn steel sheets according to Examples 7 to 10 and Comparative Example 7 are as follows. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
             
            
               
                   
                   
               
               
                   
                 Composition (wt %) 
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Sample No. 
                 C 
                 Mn 
                 Cr 
                 N 
                 Note 
               
               
                   
               
               
                 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 
                 0.618 
                 15.03 
                 1.82 
                 0.086 
                 Melting in 
               
               
                 Example 7 
                   
                   
                   
                   
                 nitrogen 
               
               
                   
                   
                   
                   
                   
                 atmosphere 
               
               
                   
               
            
           
         
       
     
     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. 
     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 subjected 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. 
       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. 
     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.