Superplastic medium manganese steel and method of produing the same

Abstract

A superplastic medium manganese steel according to the present invention preferably has a composition containing 4 to 8 wt. % of manganese (Mn) and 3 wt. % or less (excluding 0 wt. %) of aluminum (Al), with the remainder being iron (Fe) and inevitable impurities. In another embodiment, a superplastic medium manganese steel according to the present invention preferably has a composition containing 4 to 8 wt. % of manganese (Mn) and 3 wt. % or less (excluding 0 wt. %) of silicon (Si), with the remainder being iron (Fe) and inevitable impurities.

1 . A superplastic medium manganese steel having a composition containing 4 to 8 wt. % of manganese (Mn) and 3 wt. % or less (excluding 0 wt. %) of aluminum (Al), with the remainder being iron (Fe) and inevitable impurities. 
     
     
         2 . A superplastic medium manganese steel having a composition containing 4 to 8 wt. % of manganese (Mn) and 3 wt. % or less (excluding 0 wt. %) of silicon (Si), with the remainder being iron (Fe) and inevitable impurities. 
     
     
         3 . The superplastic medium manganese steel of  claim 1 , wherein the composition further contains 0.2 wt. % or less (excluding 0 wt. %) of niobium (Nb). 
     
     
         4 . The superplastic medium manganese steel of  claim 1 , wherein the composition further contains 0.03 wt. % or less (excluding 0 wt. %) of boron (B). 
     
     
         5 . The superplastic medium manganese steel of  claim 1 , wherein the composition further contains 0.2 wt. % or less (excluding 0 wt. %) of carbon (C). 
     
     
         6 . The superplastic medium manganese steel of  claim 3 , wherein the composition further contains 0.2 wt. % or less (excluding 0 wt. %) of carbon (C). 
     
     
         7 . The superplastic medium manganese steel of  claim 4 , wherein the composition further contains 0.2 wt. % or less (excluding 0 wt. %) of carbon (C). 
     
     
         8 . The superplastic medium manganese steel of  claim 1 , wherein the medium manganese steel is annealed in a temperature range of a ferrite-austenite dual-phase region to form ferrite and austenite. 
     
     
         9 . The superplastic medium manganese steel of  claim 8 , wherein the temperature range of the ferrite-austenite dual-phase region ranges from 600 to 900° C. 
     
     
         10 . The superplastic medium manganese steel of  claim 8 , wherein grains of the ferrite and austenite formed in the temperature range of the ferrite-austenite dual-phase region have an average grain size of 2 μm or less. 
     
     
         11 . A method of producing a superplastic medium manganese steel, the method comprising the steps of:
 (S1) melting either a medium manganese steel having a composition containing 4 to 8 wt. % of manganese (Mn) and 3 wt. % or less (excluding 0 wt. %) of aluminum (Al), with the remainder being iron (Fe) and inevitable impurities, or a medium manganese steel having a composition containing 4 to 8 wt. % of manganese (Mn) and 3 wt. % or less (excluding 0 wt. %) of silicon (Si), with the remainder being iron (Fe) and inevitable impurities, and then homogenizing the medium manganese steel;   (S2) hot-rolling the homogenized medium manganese steel;   (S3) cooling the hot-rolled steel;   (S4) cold-rolling the cooled steel; and   (S5) annealing the cold-rolled steel at a predetermined elevated temperature.   
     
     
         12 . The method of  claim 11 , wherein a temperature for the homogenizing in step (S1) is 1200° C., and the melting in step (S1) is performed at a temperature equal to or higher than the temperature for the homogenizing. 
     
     
         13 . The method of  claim 11 , wherein the hot rolling in step (S2) is performed at a temperature in a range of 1000 to 1200° C. 
     
     
         14 . The method of  claim 11 , wherein step (S3) is performed by at least one cooling method selected from among water quenching, oil quenching and air cooling. 
     
     
         15 . The method of  claim 11 , wherein the cold rolling in step (S4) is performed at a reduction ratio of 90% or less (excluding 0%). 
     
     
         16 . The method of  claim 15 , wherein the reduction ratio ranges from 60 to 80%. 
     
     
         17 . The method of  claim 11 , wherein the cold rolling in step (S4) is performed at room temperature. 
     
     
         18 . The method of  claim 11 , wherein a dual phase formed in step (S5) is ferrite and austenite. 
     
     
         19 . The method of  claim 18 , wherein a temperature for the annealing in step (S5) is in a temperature range of a ferrite-austenite dual-phase region. 
     
     
         20 . The method of  claim 19 , wherein the temperature range of the ferrite-austenite dual-phase region ranges from 600 to 900° C. 
     
     
         21 . The method of  claim 19 , wherein each of the ferrite and the austenite has an average grain size of 2 μm. 
     
     
         22 . The superplastic medium manganese steel of  claim 2 , wherein the composition further contains 0.2 wt. % or less (excluding 0 wt. %) of niobium (Nb). 
     
     
         23 . The superplastic medium manganese steel of  claim 2 , wherein the composition further contains 0.03 wt. % or less (excluding 0 wt. %) of boron (B). 
     
     
         24 . The superplastic medium manganese steel of  claim 2 , wherein the composition further contains 0.2 wt. % or less (excluding 0 wt. %) of carbon (C). 
     
     
         25 . The superplastic medium manganese steel of  claim 22 , wherein the composition further contains 0.2 wt. % or less (excluding 0 wt. %) of carbon (C). 
     
     
         26 . The superplastic medium manganese steel of  claim 23 , wherein the composition further contains 0.2 wt. % or less (excluding 0 wt. %) of carbon (C). 
     
     
         27 . The superplastic medium manganese steel of  claim 2 , wherein the medium manganese steel is annealed in a temperature range of a ferrite-austenite dual-phase region to form ferrite and austenite. 
     
     
         28 . The superplastic medium manganese steel of  claim 27 , wherein the temperature range of the ferrite-austenite dual-phase region ranges from 600 to 900° C. 
     
     
         29 . The superplastic medium manganese steel of  claim 27 , wherein grains of the ferrite and austenite formed in the temperature range of the ferrite-austenite dual-phase region have an average grain size of 2 μm or less.