Steel for low-temperature service having excellent surface processing quality and method for manufacturing same

Abstract

Provided is a steel sheet for low-temperature service, which can be used at a wide temperature range from low temperature to room temperature in liquefied gas storage tanks and transport facilities. The steel sheet for low-temperature service has an excellent surface processing quality even after a processing processes is performed, such as a tension process. The a steel sheet contains manganese (Mn, 15-35 wt %), carbon (C, satisfying 23.6C+Mn≧28 and 33.5C—Mn≦23), copper (Cu, 5 wt % or less (excluding 0 wt %)), chrome (Cr, satisfying 28.5C+4.4Cr≦57 (excluding 0 wt %)), titanium (Ti, 0.01-0.5 wt %), nitrogen (N, 0.003-0.2 wt %), the balance iron (Fe), and other inevitable impurities. Ti and N satisfy relational expression 1 below. [Relational expression 1] 1.0≦Ti/N≦4.5 (Mn, C, Cr, Ti, and N in the respective expressions mean wt % of respective ingredient contents).

1 . Steel for low temperature environments having excellent surface processing qualities, comprising:
 15 wt % to 35 wt % of manganese (Mn), carbon (C) satisfying 23.6C+Mn≧28 and 33.5C—Mn≦23, 5 wt % or lower of copper (Cu) (excluding 0 wt %), chrome (Cr) satisfying 28.5C+4.4Cr≦57 (excluding 0 wt %), 0.01 wt % to 0.5 wt % of titanium (Ti), 0.003 wt % to 0.2 wt % of nitrogen (N), iron (Fe) as a residual component, and inevitable impurities,   wherein Ti and N satisfy Relational Formula 1 below,
   1.0≦Ti/N≦4.5,  [Relational Formula 1]
 
   where Mn, C, Cr, Ti, and N in each expression refer to wt % of a content of each component.   
     
     
         2 . The steel for low temperature environments having excellent surface processing qualities of  claim 1 , wherein the steel comprises a TiN precipitate having a size of 0.01 μm to 0.3 μm. 
     
     
         3 . The steel for low temperature environments having excellent surface processing qualities of  claim 1 , wherein the steel comprises a TiN precipitate in an amount of 1.0×10 7  to 1.0×10 10  per 1= 2 . 
     
     
         4 . The steel for low temperature environments having excellent surface processing qualities of  claim 1 , wherein a number of austenite grains having a size of 200 μm or greater is 5 or less per 1 cm 2  in a microstructure of the steel. 
     
     
         5 . The steel for low temperature environments having excellent surface processing qualities of  claim 1 , wherein a microstructure of the steel comprises austenite in an area fraction of 95% or greater. 
     
     
         6 . The steel for low temperature environments having excellent surface processing qualities of  claim 5 , wherein a carbide present in a grain boundary of austenite is lower than or equal to 5% in an area fraction. 
     
     
         7 . The steel for low temperature environments having excellent surface processing qualities of  claim 1 , wherein impact toughness of the steel is higher than or equal to 41 J at a temperature of −196° C. 
     
     
         8 . A method of manufacturing steel for low temperature environments having excellent surface processing qualities, comprising:
 providing a slab including 15 wt % to 35 wt % of Mn, C satisfying 23.6C+Mn≧28 and 33.5C—Mn≦23, 5 wt % or lower of Cu (excluding 0 wt %), Cr satisfying 28.5C+4.4Cr≦57 (excluding 0 wt %), 0.01 wt % to 0.5 wt % of Ti, 0.003 wt % to 0.2 wt % of N, Fe as a residual component, and inevitable impurities, Ti and N satisfying Relational Formula 1 below;   heating the slab at a temperature of 1050° C. to 1250° C.; and   manufacturing heat-rolled steel by heat rolling the slab that has been heated,
   1.0≦Ti/N≦4.5,  [Relational Formula 1]
 
   where Mn, C, Cr, Ti, and N in each expression refer to wt % of a content of each component.   
     
     
         9 . The method of  claim 8 , wherein the steel comprises a TiN precipitate having a size of 0.01 μm to 0.3 μm. 
     
     
         10 . The method of  claim 8 , wherein the steel comprises a TiN precipitate in an amount of 1.0×10 7  to 1.0×10 10  per 1 mm 2 . 
     
     
         11 . The method of  claim 8 , wherein a number of austenite grains having a size of 200 μm or greater is 5 or less per 1 cm 2  in a microstructure of the steel.