Cr-Mn CONTAINING DUPLEX STEELS WITH EXCELLENT CRYOGENIC TOUGHNESS AND MANUFACTURING METHOD THEREOF

Abstract

The present invention provides an alloy composition having a two-phase structure with excellent cryogenic toughness, which can store various liquefied gases such as hydrogen, LNG and nitrogen more safely because it exhibits excellent cryogenic impact toughness compared to conventional cryogenic high-manganese steel, and significantly increase resistance to container damage even in accidents where external impact is applied, chromium-manganese steel and a manufacturing method thereof.

1 . A chromium-manganese steel with excellent cryogenic toughness, comprising:
 15 to 25 wt. % of manganese (Mn), 5 to 15 wt. % of chromium (Cr), more than 0 wt. % to 4 wt. % or less of aluminum (Al), more than 0 wt. % to 0.05 wt. % or less of carbon (C) and a remainder on an Fe base,   wherein the remainder comprises Fe and inevitable impurities,   wherein the chromium-manganese steel has a two-phase microstructure consisting of austenite and δ-ferrite phases,   wherein the impact absorption energy obtained through an impact test on a Charpy V-notch standard specimen at liquid nitrogen temperature (approximately −196° C.) is 200 J or more, and   wherein the chromium-manganese steel satisfies all of Relationship Formulas (1) and (2) below:   (1) Yield strength of 200 MPa or more   (2) Ultimate tensile strength of 550 MPa or more.   
     
     
         2 . The chromium-manganese steel of  claim 1 , wherein silicon (Si) is comprised in addition to the aluminum (Al) such that the wt. % of aluminum (Al) and silicon is 6 wt. % or less. 
     
     
         3 . The chromium-manganese steel according to  claim 1 ,
 wherein the chromium-manganese steel exhibits a low stacking fault energy of 30 mJ/m 2  or less such that under externally applied stress and impact conditions, phase transformation from austenite to ε-martensite occurs, and phase transformation from ε-martensite to α′-martensite occurs.   
     
     
         4 . The chromium-manganese steel of  claim 3 , wherein the ratio (b/a) of a length (b) in the shortest direction to a length (a) in the longest direction of δ-ferrite grains is 0.5 or less. 
     
     
         5 . (canceled) 
     
     
         6 . (canceled) 
     
     
         7 . The chromium-manganese steel of  claim 3 , wherein the chromium-manganese steel satisfies Relationship Formula (3) below:
 (3) Total elongation of 60% or more.   
     
     
         8 . A method for manufacturing chromium-manganese steel according to  claim 1 , comprising:
 step 1 of melting an alloy composition of chromium-manganese steel with excellent cryogenic toughness having a two-phase structure of austenite and δ-ferrite;   step 2 of heat treating for homogenization by maintaining the molten alloy composition in an ingot in a temperature range of 900° C. to 1,200° C. for 1 hour to 10 hours, followed by air cooling or water cooling;   step 3 of hot working (rolling or forging) to 50% to 90% reduction in thickness in a temperature range of 900° C. to 1,200° C. after the heat treating for homogenization; and   step 4 of heat treating for recrystallization in a temperature range of 600° C. to 900° C. for 0.5 to 10 hours after the hot working, followed by air cooling or water cooling.   
     
     
         9 . The method of  claim 8 , wherein through step 2, a δ-ferrite phase having an area fraction of 5% or more to 30% or less is homogeneously generated. 
     
     
         10 . The method of  claim 8 , further comprising the step of:
 cold working to a sheet of the desired thickness and then performing heat treatment again, after performing step 4.   
     
     
         11 . The chromium-manganese steel according to  claim 2 ,
 wherein the chromium-manganese steel exhibits a low stacking fault energy of 30 mJ/m 2  or less such that under externally applied stress and impact conditions, phase transformation from austenite to ε-martensite occurs, and phase transformation from ε-martensite to α′-martensite occurs.   
     
     
         12 . The chromium-manganese steel of  claim 11 , wherein the ratio (b/a) of a length (b) in the shortest direction to a length (a) in the longest direction of δ-ferrite grains is 0.5 or less. 
     
     
         13 . The chromium-manganese steel of  claim 11 , wherein the chromium-manganese steel satisfies Relationship Formula (3) below:
 (3) Total elongation of 60% or more.   
     
     
         14 . A method for manufacturing chromium-manganese steel according to  claim 2 , comprising:
 step 1 of melting an alloy composition of chromium-manganese steel with excellent cryogenic toughness having a two-phase structure of austenite and δ-ferrite;   step 2 of heat treating for homogenization by maintaining the molten alloy composition in an ingot in a temperature range of 900° C. to 1,200° C. for 1 hour to 10 hours, followed by air cooling or water cooling;   step 3 of hot working (rolling or forging) to 50% to 90% reduction in thickness in a temperature range of 900° C. to 1,200° C. after the heat treating for homogenization; and   step 4 of heat treating for recrystallization in a temperature range of 600° C. to 900° C. for 0.5 to 10 hours after the hot working, followed by air cooling or water cooling.   
     
     
         15 . The method of  claim 14 , wherein through step 2, a δ-ferrite phase having an area fraction of 5% or more to 30% or less is homogeneously generated. 
     
     
         16 . The method of  claim 14 , further comprising the step of:
 cold working to a sheet of the desired thickness and then performing heat treatment again, after performing step 4.