Patent Publication Number: US-10329650-B2

Title: High manganese steel

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to Korean Patent Application No. 10-2016-0131805, filed Oct. 12, 2016, the entire contents of which is incorporated herein for all purposes by this reference. 
     TECHNICAL FIELD 
     The present invention relates to a high manganese steel. 
     BACKGROUND 
     The conventional high manganese steels have excellent strength and elongation by controlling contents of manganese (Mn) and aluminum (Al) to control stacking fault energy (SFE). However, the conventional high manganese steels still have a high density, and thus, it is not possible to expect improvement of fuel efficiency by lightweight when it is applied to bodywork components such as a center pillar, a front side member, a side sill, a front pillar, a floor cross member, etc. 
     Korean Patent Laid-Open Publication No. KR 10-2016-0078840 aims to produce high manganese having high yield strength and high elongation by increasing the content of manganese (Mn), but has a limitation in that the content of aluminum (Al) is only 2.5 to 5.0 wt %, such that a density is high. 
     The contents described as the related art have been provided only for assisting in the understanding for the background of the present invention and should not be considered as corresponding to the related art known to those skilled in the art. 
     SUMMARY 
     Embodiments of the present invention provide a high manganese steel capable of having high strength and high elongation by controlling contents of manganese (Mn), aluminum (Al), etc., and capable of being lightened by lowering a density. 
     According to an exemplary embodiment of the present invention, a high manganese steel includes: 0.5 to 1.2 wt % of carbon (C), 0.1 to 2.3 wt % of silicon (Si), 15 to 30 wt % of manganese (Mn), 7.0 to 13.0 wt % of aluminum (Al), 0.01 to 3.0 wt % of nickel (Ni), 0.01 to 0.5 wt % of chromium (Cr), 0.01 to 0.4 wt % of molybdenum (Mo), 0.01 to 0.5 wt % of vanadium (V), 0.005 to 0.3 wt % of niobium (Nb), 0.005 to 0.3 wt % of titanium (Ti), and remainder iron (Fe) and other inevitable impurities. 
     A density may be 7.1 (g/cm 3 ) or less. 
     Yield strength may be 705 MPa or more, and tensile strength may be 1120 MPa or more. 
     An elongation may be 41.6% or more, and a work hardening exponent (n) may be 0.208 or more. 
     Stacking fault energy (SFE) may be 35.3 to 44.1 (mJ/m 2 ). 
     A fraction of carbide present in an organization may be 1.34% or more. 
     A fraction of inclusion present in an organization may be 0.062% or less. 
     A β-Mn phase may be formed in an organization by containing 25 to 30 wt % of manganese (Mn). 
     According to another embodiment, a high manganese steel consists essentially of 0.5 to 1.2 wt % of carbon (C), 0.1 to 2.3 wt % of silicon (Si), 15 to 30 wt % of manganese (Mn), 7.0 to 13.0 wt % of aluminum (Al), 0.01 to 3.0 wt % of nickel (Ni), 0.01 to 0.5 wt % of chromium (Cr), 0.01 to 0.4 wt % of molybdenum (Mo), 0.01 to 0.5 wt % of vanadium (V), 0.005 to 0.3 wt % of niobium (Nb), 0.005 to 0.3 wt % of titanium (Ti), and iron (Fe). The high manganese steel has a density of 7.1 (g/cm 3 ) or less, a yield strength of 705 MPa or more, a tensile strength of 1120 MPa or more, an elongation of 41.6% or more, and a work hardening exponent (n) of 0.208 or more. 
     According to another embodiment, a high manganese steel consists essentially of 0.5 to 1.2 wt % of carbon (C), 0.1 to 2.3 wt % of silicon (Si), 15 to 30 wt % of manganese (Mn), 7.0 to 13.0 wt % of aluminum (Al), 0.01 to 3.0 wt % of nickel (Ni), 0.01 to 0.5 wt % of chromium (Cr), 0.01 to 0.4 wt % of molybdenum (Mo), 0.01 to 0.5 wt % of vanadium (V), 0.005 to 0.3 wt % of niobium (Nb), 0.005 to 0.3 wt % of titanium (Ti), and iron (Fe). The high manganese steel has a stacking fault energy (SFE) of 35.3 to 44.1 (mJ/m 2 ). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graph showing properties of a high manganese steel according to the present invention. 
         FIG. 2  schematically shows a structure of β-Mn phase according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Hereinafter, preferable exemplary embodiments of the present invention will be described with reference to the accompanying drawings. 
     A high manganese steel according to the present invention includes: 0.5 to 1.2 wt % of carbon (C), 0.1 to 2.3 wt % of silicon (Si), 15 to 30 wt % of manganese (Mn), 7.0 to 13.0 wt % of aluminum (Al), 0.01 to 3.0 wt % of nickel (Ni), 0.01 to 0.5 wt % of chromium (Cr), 0.01 to 0.4 wt % of molybdenum (Mo), 0.01 to 0.5 wt % of vanadium (V), 0.005 to 0.3 wt % of niobium (Nb), 0.005 to 0.3 wt % of titanium (Ti), and remainder iron (Fe) and other inevitable impurities. 
     Hereinafter, a reason for limiting condition of steel components in the high manganese steel of the present invention is described in detail. 
     Carbon (C): 0.5 to 1.2% 
     Carbon (C) is an austenite stabilizing element and acts to increase strength and stacking fault energy. (Fe, Mn)3AlC type κ-carbide, VC, (V,Nb)C, etc., are formed. It is possible to deduce optimum strength and elongation by controlling contents under condition of high contents of manganese (Mn) and aluminum (Al). 
     When the content of carbon (C) is less than 0.5%, machining crack may occur due to formation of α-martensite. Production of carbides may be reduced and strength and ductility may be reduced. On the other hand, when the content of carbon (C) is more than 1.2%, high-strength brittleness may occur. An elongation may be reduced by precipitation of cementite. Further, weldability may be lowered and workability may be lowered due to excessive slip deformation. The stacking fault energy may be excessively increased. Accordingly, the content of carbon (C) is limited to 0.5 to 1.2%. 
     Silicon (Si): 0.1 to 2.3% 
     Silicon (Si) may act as a deoxidizer and may act to strength solidification. Yield strength may be increased. When high content manganese is added, formation of a manganese oxide layer may be suppressed. Corrosion may be prevented and surface quality may be improved. 
     When the content of silicon (Si) is less than 0.1%, strength may be lowered and deoxidation effect may not be large. On the other hand, when the content of silicon (Si) is more than 2.3%, toughness, quenching ability, and weldability may be lowered. At the time of hot rolling, acidity may be deteriorated and plating ability may be deteriorated by the formation of the oxide layer. Accordingly, the content of silicon (S) is limited to 0.1 to 2.3%. 
     Manganese (Mn): 15 to 30% 
     Manganese (Mn) is an austenite stabilizing element and may contribute to stabilization of stacking fault energy. A β-manganese (Mn) phase may be formed, and thus, mechanical properties may be largely changed. 
     When the content of manganese (Mn) is 15% or less, ferrite/martensite may be generated in a cooling process due to a reduction in stability of the austenite. Accordingly, the ductility may be reduced. On the other hand, when the content of manganese (Mn) is more than 30%, mechanical properties may be lowered. At the time of hot rolling, crack may occur. Accordingly, the content of manganese (Mn) is limited to 15 to 30%. 
     Aluminum (Al): 7.0 to 13.0% 
     Aluminum (Al) is a deoxidizer and may improve the ductility. It is possible to achieve lightweight and to increase the stacking fault energy though a low density. Due to suppression of formation of ε-martensite phase, the ductility may be improved, and corrosion resistance, oxidation resistance, and high temperature toughness may be increased. Moldability may be improved. A strain softening effect may be enhanced by controlling production of κ-carbide. A density of a slip band may be lowered and strain hardening may be reduced. 
     When the content of aluminum (Al) is less than 7.0%, lightweight may be insignificant and the ductility may be lowered. In addition, the production of the κ-carbide may be lowered, and moldability may be lowered. Corrosion resistance and oxidation resistance may be lowered. On the other hand, when the content of aluminum (Al) is more than 13.0%, castability may be lowered, and at the time of hot rolling, surface quality may be deteriorated due to surface oxidation. The elongation may be lowered, and cold rolling property may be lowered. Accordingly, the content of aluminum (Al) is limited to 7.0 to 13.0%. 
     Nickel (Ni): 0.01 to 3.0% 
     By adding nickel (Ni), (Fe,Ni)Al which is a B2 phase, may be precipitated, and may be utilized as a reinforcing phase. The B2 phase of 1 μm or less in an austenite base may be precipitated up to 40 vol. %. 
     When the content of nickel (Ni) is less than 0.01%, the toughness may be lowered, and impact resistance may be lowered. On the other hand, when the content of nickel (Ni) is more than 3.0%, the strength may be increased, but the toughness may be reduced rapidly. Accordingly, the content of nickel (Ni) is limited to 0.01 to 3.0%. 
     Chromium (Cr): 0.01 to 0.5% 
     Chromium (Cr) is an element that forms carbide. The chromium may act to appropriately delay the production of κ-carbide. Stability at high temperature may be increased and the quenching ability may be improved. Further, hardenability may be provided, and an organization may be refined. 
     When the content of chromium (Cr) is less than 0.01%, the strength may be lowered and a precipitation amount of the carbide may be reduced. On the other hand, when the content of chromium (Cr) is more than 0.5%, the strength may be increased, but the toughness may be reduced rapidly. Accordingly, the content of chromium (Cr) is limited to 0.01 to 0.5%. 
     Molybdenum (Mo): 0.01 to 0.4% 
     Molybdenum (Mo) is an element that forms carbide. Brittleness, corrosion resistance and heat resistance may be improved. In addition, cutting ability may be increased. 
     When the content of molybdenum (Mo) is less than 0.01%, the strength may be lowered and a precipitation amount of the carbide may be reduced. Brittleness resistance may be lowered. On the other hand, when the content of molybdenum (Mo) is more than 0.4%, a bainite fraction may be reduced and the elongation may be lowered. Accordingly, the content of molybdenum (Mo) is limited to 0.01 to 0.4%. 
     Vanadium (V): 0.01 to 0.5% 
     Vanadium (V) is an element that forms carbide. The vanadium may reduce the density, may preserve the strength, and may provide excellent balance of strength and elongation. Fine precipitates may be formed. (V,Nb)C may be formed by adding niobium (Nb). 
     When the content of vanadium (V) is less than 0.01%, the strength may be lowered and a precipitation amount of the carbide may be reduced. Brittleness resistance may be lowered. On the other hand, when the content of vanadium (V) is more than 0.5%, formation of the carbide may be saturated and the elongation may be lowered. Accordingly, the content of vanadium (V) is limited to 0.01 to 0.5%. 
     Niobium (Nb): 0.005 to 0.3% 
     Niobium (Nb) is an element that forms carbide. A crystal grain may be refined, and the density may be lowered. The strength may be preserved, and balance of strength and elongation may be excellent. Fine precipitates may be formed. (V,Nb)C may be formed by adding vanadium (V). 
     When the content of niobium (Nb) is less than 0.005%, the carbide formation may be insignificant. The organization may be coarsened and the strength may be lowered. On the other hand, when the content of niobium (Nb) is more than 0.3%, the formation of the carbide may be saturated, a crystal grain boundary segregation may be formed, and precipitation phase may be coarsened. Accordingly, the content of niobium (Nb) is limited to 0.005 to 0.3%. 
     Titanium (Ti): 0.005 to 0.3% 
     Titanium (Ti) is an element that forms carbide. The crystal grain may be refined, and the density may be lowered. The titanium may preserve the strength, and may provide excellent balance of strength and elongation. 
     When the content of titanium (Ti) is less than 0.005%, an effect that the strength is improved and the density is lowered may be insignificant. On the other hand, when the content of titanium (Ti) is more than 0.3%, the formation of the carbide may be saturated, the crystal grain boundary segregation may be formed, and the precipitation phase may be coarsened. At the time of cold rolling, crack may occur, and the weldability may be lowered. Accordingly, the content of titanium (Ti) is limited to 0.005 to 0.3%. 
     Examples and Comparative Examples on the basis of specimens produced with different composition components and contents are described in Tables 1 and 2 below. The samples were subjected to reheating at 1100 to 1300° C., hot rolling at about 800 to 1000° C., coiling at about 500° C., cold rolling at ambient temperature, and cold-rolling annealing at 700 to 900° C. to be used. 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Carbon 
                 Silicon 
                 Manganese 
                 Aluminum 
                 Nickel 
                 Chromium 
                 Molybdenum 
                 Vanadium 
                 Niobium 
                 Titanium 
               
               
                 wt % 
                 (C) 
                 (Si) 
                 (Mn) 
                 (Al) 
                 (Ni) 
                 (Cr) 
                 (Mo) 
                 (V) 
                 (Nb) 
                 (Ti) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Example 1 
                 0.52 
                 0.15 
                 15.3 
                 7.2 
                 0.09 
                 0.12 
                 0.05 
                 0.04 
                 0.009 
                 0.007 
               
               
                 Example 2 
                 0.84 
                 2.25 
                 24.3 
                 10.5 
                 1.81 
                 0.39 
                 0.12 
                 0.23 
                 0.21 
                 0.16 
               
               
                 Example 3 
                 1.19 
                 1.38 
                 29.8 
                 12.7 
                 2.76 
                 0.48 
                 0.37 
                 0.46 
                 0.28 
                 0.27 
               
               
                 Comparative 
                 0.48 
                 1.66 
                 15.2 
                 7.6 
                 0.08 
                 0.16 
                 0.03 
                 0.06 
                 0.006 
                 0.006 
               
               
                 Example 1 
               
               
                 Comparative 
                 1.23 
                 0.12 
                 22.7 
                 11.5 
                 1.86 
                 0.34 
                 0.29 
                 0.33 
                 0.27 
                 0.19 
               
               
                 Example 2 
               
               
                 Comparative 
                 0.62 
                 0.08 
                 23.2 
                 12.3 
                 2.71 
                 0.42 
                 0.36 
                 0.46 
                 0.26 
                 0.09 
               
               
                 Example 3 
               
               
                 Comparative 
                 0.76 
                 2.35 
                 28.2 
                 8.3 
                 0.05 
                 0.24 
                 0.12 
                 0.07 
                 0.019 
                 0.02 
               
               
                 Example 4 
               
               
                 Comparative 
                 1.07 
                 0.19 
                 14.7 
                 11.2 
                 1.48 
                 0.36 
                 0.16 
                 0.45 
                 0.25 
                 0.006 
               
               
                 Example 5 
               
               
                 Comparative 
                 0.53 
                 2.14 
                 30.3 
                 12.3 
                 2.15 
                 0.41 
                 0.47 
                 0.46 
                 0.22 
                 0.21 
               
               
                 Example 6 
               
               
                 Comparative 
                 0.57 
                 1.84 
                 25.5 
                 6.9 
                 2.56 
                 0.03 
                 0.35 
                 0.46 
                 0.25 
                 0.005 
               
               
                 Example 7 
               
               
                 Comparative 
                 0.74 
                 1.52 
                 17.6 
                 13.2 
                 0.29 
                 0.09 
                 0.15 
                 0.43 
                 0.009 
                 0.13 
               
               
                 Example 8 
               
               
                 Comparative 
                 1.14 
                 0.11 
                 21.2 
                 10.9 
                 0.007 
                 0.29 
                 0.32 
                 0.23 
                 0.19 
                 0.21 
               
               
                 Example 9 
               
               
                 Comparative 
                 0.59 
                 2.11 
                 29.4 
                 12.1 
                 3.05 
                 0.25 
                 0.35 
                 0.46 
                 0.28 
                 0.014 
               
               
                 Example 
               
               
                 10 
               
               
                 Comparative 
                 0.77 
                 1.42 
                 19.3 
                 8.9 
                 0.69 
                 0.008 
                 0.19 
                 0.03 
                 0.006 
                 0.007 
               
               
                 Example 
               
               
                 11 
               
               
                 Comparative 
                 1.19 
                 0.33 
                 27.1 
                 9.4 
                 1.01 
                 0.52 
                 0.23 
                 0.41 
                 0.11 
                 0.15 
               
               
                 Example 
               
               
                 12 
               
               
                 Comparative 
                 0.64 
                 2.06 
                 26.5 
                 12.7 
                 2.72 
                 0.44 
                 0.006 
                 0.46 
                 0.18 
                 0.008 
               
               
                 Example 
               
               
                 13 
               
               
                 Comparative 
                 0.92 
                 1.72 
                 16.2 
                 9.3 
                 0.03 
                 0.32 
                 0.41 
                 0.14 
                 0.10 
                 0.23 
               
               
                 Example 
               
               
                 14 
               
               
                 Comparative 
                 0.65 
                 1.92 
                 19.9 
                 7.1 
                 0.29 
                 0.11 
                 0.015 
                 0.008 
                 0.007 
                 0.20 
               
               
                 Example 
               
               
                 15 
               
               
                 Comparative 
                 1.06 
                 0.42 
                 24.1 
                 9.5 
                 1.61 
                 0.22 
                 0.26 
                 0.53 
                 0.29 
                 0.21 
               
               
                 Example 
               
               
                 16 
               
               
                 Comparative 
                 0.98 
                 0.35 
                 28.1 
                 9.2 
                 1.48 
                 0.14 
                 0.18 
                 0.53 
                 0.004 
                 0.19 
               
               
                 Example 
               
               
                 17 
               
               
                 Comparative 
                 0.71 
                 1.63 
                 21.3 
                 12.4 
                 2.73 
                 0.38 
                 0.36 
                 0.46 
                 0.31 
                 0.12 
               
               
                 Example 
               
               
                 18 
               
               
                 Comparative 
                 0.94 
                 1.44 
                 16.3 
                 8.2 
                 0.69 
                 0.17 
                 0.22 
                 0.16 
                 0.007 
                 0.003 
               
               
                 Example 
               
               
                 19 
               
               
                 Comparative 
                 1.15 
                 0.74 
                 20.9 
                 9.5 
                 1.77 
                 0.48 
                 0.16 
                 0.27 
                 0.16 
                 0.32 
               
               
                 Example 
               
               
                 20 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                   
                   
                   
                 Work 
                 Stacking 
                   
                   
               
               
                   
                   
                 Yield 
                 Tensile 
                   
                 hardening 
                 Fault 
                 Carbide 
                 Inclusion 
               
               
                   
                 Density 
                 Strength 
                 Strength 
                 Elongation 
                 exponent 
                 Energy 
                 Fraction 
                 Fraction 
               
               
                   
                 (g/cm 3 ) 
                 (MPa) 
                 (MPa) 
                 (%) 
                 (n) 
                 (mJ/m 2 ) 
                 (%) 
                 (%) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Example1 
                 7.1 
                 705 
                 1120 
                 41.6 
                 0.208 
                 35.3 
                 1.34 
                 0.062 
               
               
                 Example2 
                 6.8 
                 746 
                 1147 
                 45.2 
                 0.226 
                 38.2 
                 1.55 
                 0.041 
               
               
                 Example3 
                 6.5 
                 820 
                 1198 
                 50.3 
                 0.252 
                 44.1 
                 1.92 
                 0.045 
               
               
                 Comparative 
                 6.9 
                 665 
                 1010 
                 40.5 
                 0.203 
                 26.2 
                 0.46 
                 0.051 
               
               
                 Example 1 
               
               
                 Comparative 
                 7.3 
                 725 
                 1199 
                 32.6 
                 0.163 
                 50.6 
                 1.96 
                 0.057 
               
               
                 Example 2 
               
               
                 Comparative 
                 6.7 
                 685 
                 1006 
                 42.5 
                 0.213 
                 35.8 
                 1.32 
                 0.053 
               
               
                 Example 3 
               
               
                 Comparative 
                 6.8 
                 822 
                 1169 
                 34.2 
                 0.171 
                 32.1 
                 1.45 
                 0.052 
               
               
                 Example 4 
               
               
                 Comparative 
                 6.8 
                 748 
                 1149 
                 31.5 
                 0.158 
                 28.8 
                 1.22 
                 0.042 
               
               
                 Example 5 
               
               
                 Comparative 
                 6.7 
                 695 
                 1105 
                 41.2 
                 0.206 
                 50.1 
                 1.49 
                 0.053 
               
               
                 Example 6 
               
               
                 Comparative 
                 7.7 
                 715 
                 1132 
                 41.3 
                 0.207 
                 19.2 
                 0.69 
                 0.038 
               
               
                 Example 7 
               
               
                 Comparative 
                 6.2 
                 736 
                 1136 
                 34.2 
                 0.171 
                 39.2 
                 1.03 
                 0.134 
               
               
                 Example 8 
               
               
                 Comparative 
                 7.0 
                 735 
                 1136 
                 31.2 
                 0.156 
                 34.3 
                 1.24 
                 0.035 
               
               
                 Example 9 
               
               
                 Comparative 
                 6.8 
                 774 
                 1146 
                 31.2 
                 0.156 
                 36.6 
                 1.01 
                 0.062 
               
               
                 Example 
               
               
                 10 
               
               
                 Comparative 
                 7.1 
                 695 
                 1095 
                 40.2 
                 0.201 
                 33.2 
                 0.62 
                 0.052 
               
               
                 Example 
               
               
                 11 
               
               
                 Comparative 
                 7.1 
                 782 
                 1163 
                 34.6 
                 0.173 
                 36.2 
                 1.42 
                 0.064 
               
               
                 Example 
               
               
                 12 
               
               
                 Comparative 
                 7.1 
                 666 
                 998 
                 41.2 
                 0.206 
                 35.6 
                 1.01 
                 0.058 
               
               
                 Example 
               
               
                 13 
               
               
                 Comparative 
                 6.9 
                 813 
                 1173 
                 33.6 
                 0.168 
                 37.4 
                 1.26 
                 0.055 
               
               
                 Example 
               
               
                 14 
               
               
                 Comparative 
                 6.6 
                 653 
                 995 
                 44.7 
                 0.224 
                 32.6 
                 0.36 
                 0.044 
               
               
                 Example 
               
               
                 15 
               
               
                 Comparative 
                 6.9 
                 770 
                 1148 
                 36.6 
                 0.183 
                 34.5 
                 1.53 
                 0.038 
               
               
                 Example 
               
               
                 16 
               
               
                 Comparative 
                 7.1 
                 668 
                 1002 
                 40.4 
                 0.202 
                 36.2 
                 0.95 
                 0.051 
               
               
                 Example 
               
               
                 17 
               
               
                 Comparative 
                 6.6 
                 744 
                 1144 
                 33.2 
                 0.166 
                 35.1 
                 1.06 
                 0.062 
               
               
                 Example 
               
               
                 18 
               
               
                 Comparative 
                 7.3 
                 669 
                 1003 
                 43.2 
                 0.216 
                 36.4 
                 0.84 
                 0.126 
               
               
                 Example 
               
               
                 19 
               
               
                 Comparative 
                 6.9 
                 720 
                 1139 
                 36.9 
                 0.185 
                 34.6 
                 1.02 
                 0.042 
               
               
                 Example 
               
               
                 20 
               
               
                   
               
            
           
         
       
     
     Table 1 shows composition components and contents of Examples and Comparative Examples. In addition, Table 2 shows the density, yield strength, tensile strength, elongation, work hardening exponent, stacking fault energy, carbide fraction, and inclusion fraction of Examples and Comparative Examples. 
     The density was measured using a density meter such as a underwater substitution type hydrometer, etc., and the yield strength, tensile strength and elongation were measured according to KS B 0802, and the work hardening exponent was calculated using an average value for a strain rate ranging from 5 to 15%. The stacking fault energy was estimated by using a transmission electron microscopy (TEM), etc. 
     It could be confirmed that the high manganese steel according to the present invention had excellent strength and high elongation as shown in Table 2 and  FIG. 1 . 
     In Comparative Example 1 and Comparative Example 2, only the content of carbon (C) was controlled to be under or over the limit range of the high manganese steel according to the present invention while the contents of other components were controlled to be the same range as those of Examples within the limit range of the high manganese steel according to the present invention. 
     As shown in Table 2, it could be confirmed that when the content of carbon (C) was under the limit range, the yield strength, the tensile strength, and the carbide fraction were lower than those of Examples, and when the content of carbon (C) was over the limit range, the elongation, and the work hardening exponent were lower than those of Examples. 
     In Comparative Example 3 and Comparative Example 4, only the content of silicon (Si) was controlled to be under or over the limit range of the high manganese steel according to the present invention while the contents of other components were controlled to be the same range as those of Examples within the limit range of the high manganese steel according to the present invention. 
     As shown in Table 2, it could be confirmed that when the content of silicon (Si) was under the limit range, the yield strength and the tensile strength were lower than those of Examples, and when the content of silicon (Si) was over the limit range, the elongation and the work hardening exponent were lower than those of Examples. 
     In Comparative Example 5 and Comparative Example 6, only the content of manganese (Mn) was controlled to be under or over the limit range of the high manganese steel according to the present invention while the contents of other components were controlled to be the same range as those of Examples within the limit range of the high manganese steel according to the present invention. 
     As shown in Table 2, it could be confirmed that when the content of manganese (Mn) was under the limit range, the elongation and the work hardening exponent were lower than those of Examples, and when the content of manganese (Mn) was over the limit range, the yield strength and the tensile strength were lower than those of Examples. 
     In Comparative Example 7 and Comparative Example 8, only the content of aluminum (Al) was controlled to be under or over the limit range of the high manganese steel according to the present invention while the contents of other components were controlled to be the same range as those of Examples within the limit range of the high manganese steel according to the present invention. 
     As shown in Table 2, it could be confirmed that when the content of aluminum (Al) was under the limit range, the density was higher than that of Examples, and when the content of aluminum (Al) was over the limit range, the elongation and the work hardening exponent were lower than those of Examples. 
     In Comparative Example 9 and Comparative Example 10, only the content of nickel (Ni) was controlled to be under or over the limit range of the high manganese steel according to the present invention while the contents of other components were controlled to be the same range as those of Examples within the limit range of the high manganese steel according to the present invention. 
     As shown in Table 2, it could be confirmed that when the content of nickel (Ni) was under or over the limit range, the elongation and the work hardening exponent of were lower than those of Examples. 
     In Comparative Example 11 and Comparative Example 12, only the content of chromium (Cr) was controlled to be under or over the limit range of the high manganese steel according to the present invention while the contents of other components were controlled to be the same range as those of Examples within the limit range of the high manganese steel according to the present invention. 
     As shown in Table 2, it could be confirmed that when the content of chromium (Cr) was under the limit range, the yield strength and the tensile strength were lower than those of Examples, and when the content of chromium (Cr) was over the limit range, the elongation and the work hardening exponent were lower than those of Examples. 
     In Comparative Example 13 and Comparative Example 14, only the content of molybdenum (Mo) was controlled to be under or over the limit range of the high manganese steel according to the present invention while the contents of other components were controlled to be the same range as those of Examples within the limit range of the high manganese steel according to the present invention. 
     As shown in Table 2, it could be confirmed that when the content of molybdenum (Mo) was under the limit range, the yield strength and the tensile strength were lower than those of Examples, and when the content of molybdenum (Mo) was over the limit range, the elongation and the work hardening exponent were lower than those of Examples. 
     In Comparative Example 15 and Comparative Example 16, only the content of vanadium (V) was controlled to be under or over the limit range of the high manganese steel according to the present invention while the contents of other components were controlled to be the same range as those of Examples within the limit range of the high manganese steel according to the present invention. 
     As shown in Table 2, it could be confirmed that when the content of vanadium (V) was under the limit range, the yield strength and the tensile strength were lower than those of Examples, and when the content of vanadium (V) was over the limit range, the elongation and the work hardening exponent were lower than those of Examples. 
     In Comparative Example 17 and Comparative Example 18, only the content of niobium (Nb) was controlled to be under or over the limit range of the high manganese steel according to the present invention while the contents of other components were controlled to be the same range as those of Examples within the limit range of the high manganese steel according to the present invention. 
     As shown in Table 2, it could be confirmed that when the content of niobium (Nb) was under the limit range, the yield strength and the tensile strength were lower than those of Examples, and when the content of niobium (Nb) was over the limit range, the elongation and the work hardening exponent were lower than those of Examples. 
     In Comparative Example 19 and Comparative Example 20, only the content of titanium (Ti) was controlled to be under or over the limit range of the high manganese steel according to the present invention while the contents of other components were controlled to be the same range as those of Examples within the limit range of the high manganese steel according to the present invention. 
     As shown in Table 2, it could be confirmed that when the content of titanium (Ti) was under the limit range, the density was higher than that of Examples, and the yield strength and the tensile strength were lower than those of Examples, and when the content of titanium (Ti) was over the limit range, the elongation and the work hardening exponent were lower than those of Examples. 
     Due to the addition of aluminum (Al), overall density of the steel may be lowered to achieve lightweight. Preferably, the density of the high manganese steel according to the present invention may be 7.1 (g/cm 3 ) or less. The aluminum (Al) may replace iron (Fe) as a substitutional lightweight element. An atomic weight of iron (Fe) is two times higher than that of aluminum (Al). On the contrary, an atomic radius of iron (Fe) is smaller than that of aluminum (Al). Thus, when aluminum (Al) replaces iron (Fe), the density of the steel is lowered by expanding a lattice. 
     On the other hand, even if aluminum (Al) replaces iron (Fe), it is possible to increase specific strength while maintaining the same level of strength. In addition, the formation of ε-martensite phase having deformation twinning defect and brittleness may be delayed to increase resistance to hydrogen embrittlement. 
     Preferably, the high manganese steel according to the present invention may have yield strength of 705 MPa or more, and tensile strength of 1120 MPa or more. In order to achieve lightweight and thinness, it is required to satisfy that the yield strength is 700 MPa or more and the tensile strength is 1100 MPa or more. As confirmed from Table 2, Example 1 having the lowest yield strength and tensile strength had yield strength of 705 MPa and tensile strength of 1120 MPa. 
     This is related to the formation of fine carbides due to the addition of chromium (Cr), molybdenum (Mo), vanadium (V), niobium (Nb), titanium (Ti), etc., and preferably, the fraction of carbides such as (Fe, Mn)3AlC, VC, (V,Nb)C, etc., is present at 1% or more, such that the strength and the toughness of the steel may be increased. As confirmed from Table 2, Example 1 having the lowest carbide fraction had a carbide fraction of 1.34%. 
     Meanwhile, since the inclusion may cause deterioration of the strength and fatigue durability, it is preferable that the inclusion fraction is present at 0.07% or less. As confirmed from Table 2, Example 1 having the highest inclusion fraction had an inclusion fraction of 0.062%. 
     It is preferable that the elongation is 40% or more. This is a numerical value for securing the moldability and workability. The elongation results from the balance of strength and elongation according to the control of contents of vanadium (V), niobium (Nb), and titanium (Ti). As confirmed from Table 2, Example 1 having the lowest elongation had an elongation of 41.6%. 
     The work hardening exponent indicates a hardening degree at the time of machining, which means a strain rate at the moment when stress begins to decrease. Therefore, the higher the work hardening exponent, the higher the moldability. As a numerical value for securing such moldability, the n value is preferably 0.2 or more. As confirmed from Table 2, Example 1 having the lowest work hardening exponent had a work hardening exponent of 0.208. 
     In general, for steels having a high content of manganese (Mn) related with stacking fault energy, a deformation behavior may depend on the stacking fault energy. The lower the stacking fault energy, the lower the deformation twinning defect and the organization recovery, and the lower the moldability. On the contrary, as the stacking fault energy becomes higher, a limited strain level is exceeded, and thus, the workability is deteriorated. Accordingly, the stacking fault energy preferably has a range of 30 to 50 (mJ/m 2 ). As shown in Table 2, it could be appreciated that Examples 1 to 3 had the stacking fault energy within the above-described range. 
     The β-Mn phase is formed in a microstructure depending on the composition of carbon (C), manganese (Mn) and aluminum (Al). Due to the formation of the β-Mn phase, mechanical properties such as yield strength, tensile strength and elongation may be changed. 
     The β-Mn phase has a cubic structure as shown in  FIG. 2 . The β-Mn phase may be produced when the content of manganese (Mn) is 25 wt % or more at the time of Fe—Al—Mn—C phase transformation. The β-Mn phase may be formed mainly at an interface of an austenite crystal grain boundary or an austenite and ferrite phase. 
     Specifically, when the aluminum has a low content of 10 wt % or less, the β-Mn phase and the ferrite grow while forming a colony in which a lamellar form is mixed. When aluminum (Al) has a high content of 10 wt % or more, the β-Mn phase rapidly grows along the austenite grain boundary, and exhibits a growth behavior while having a Widmanstatten structure inside the grain. 
     The high manganese steel according to the present invention may control the contents of elements such as manganese (Mn) and aluminum (Al), etc., as described above, thereby having excellent strength and elongation, and simultaneously, lowering the density to achieve lightweight. Therefore, the high manganese steel may have high strength and excellent workability and moldability, may achieve thinness and integration of components, and may be applied to bodywork components such as a center pillar, a front side member, a side sill, a front pillar, and a floor cross member, etc. 
     According to the high manganese steel of the present invention as described above, the carbide may be formed by controlling the contents of manganese (Mn), aluminum (Al), etc., such that the yield strength and the tensile strength may be high, and the elongation and the work hardening exponent may be high. 
     Further, it is possible to achieve the lightweight by lowering the density. 
     Hereinabove, although the present invention has been described with reference to exemplary embodiments and the accompanying drawings, the present invention is not limited thereto, but may be variously modified and altered by those skilled in the art to which the present invention pertains without departing from the spirit and scope of the present invention claimed in the following claims.