Surface Decarburization-Restrained Steel and Manufacturing Method Thereof

Abstract

Decarburization-restrained steel and manufacturing method thereof are disclosed. Steel includes a boron (B)-concentrated layer formed on its surface to prevent carbon of the steel from being in contact with oxygen in atmosphere to thus restrain decarburization of the steel. The steel includes a boron-concentrated layer with a thickness of 3 mm or larger formed on the surface of the steel. The method of manufacturing decarburization-restrained steel includes cooling steel containing 0.001 wt % to 0.02 wt % of boron (B) at a cooling speed of 0.5° C./s to 25° C./s at an austenite+ferrite two-phase region.

1 . Surface decarburization-restrained steel comprising a boron-concentrated layer with a thickness of 3 mm or larger formed on the surface of the steel. 
     
     
         2 . The steel of  claim 1 , wherein the boron-concentrated layer is a region where the content of boron is 10 times or larger the average composition of steel. 
     
     
         3 . The steel of  claim 1 , wherein the steel contains 0.001 wt % to 0.02 wt % of boron. 
     
     
         4 . The steel of  claim 3 , wherein the steel further contains 0.02 wt % or less of nitrogen (N) and 0.005 wt % to 0.5 wt % of titanium (Ti). 
     
     
         5 . The steel of  claim 4 , wherein the steel further contains 0.2 wt % to 1.0 wt % of carbon (C), 0.1 wt % to 3.5 wt % of silicon (Si), 0.3 wt % to 1.0 wt % of manganese (Mn), 0.1 wt % or less of aluminum (Al), 0.02 wt % or less of phosphorus (P), and 0.02 wt % or less of sulfur (S). 
     
     
         6 . The steel of  claim 5 , wherein the steel further contains 0.1 wt % to 1.5 wt % of chromium (Cr), 0.01 wt % to 1.0 wt % of nickel (Ni), 0.01 wt % to 1.0 wt % of copper (Cu), 0.0020 wt % or less of oxygen (O), 0.005 wt % to 0.5 wt % of vanadium (V), and 0.005 wt % to 0.5 wt % of niobium (Nb). 
     
     
         7 . A method of manufacturing surface decarburization-restrained steel, the method comprising:
 cooling steel containing 0.001 wt % to 0.02 wt % of boron (B) at a cooling speed of 0.5° C./s to 25° C./s at an austenite+ferrite two-phase region.   
     
     
         8 . The method of  claim 7 , wherein the steel further contains 0.02 wt % or less of nitrogen (N) and 0.005 wt % to 0.5 wt % of titanium (Ti). 
     
     
         9 . The method of  claim 8 , wherein the steel further contains 0.2 wt % to 1.0 wt % of carbon (C), 0.1 wt % to 3.5 wt % of silicon (Si), 0.3 wt % to 1.0 wt % of manganese (Mn), 0.1 wt % or less of aluminum (Al), 0.02 wt % or less of phosphorus (P), and 0.02 wt % or less of sulfur (S). 
     
     
         10 . The method of  claim 9 , wherein the steel further contains 0.1 wt % to 1.5 wt % of chromium (Cr), 0.01 wt % to 1.0 wt % of nickel (Ni), 0.01 wt % to 1.0 wt % of copper (Cu), 0.0020 wt % or less of oxygen (O), 0.005 wt % to 0.5 wt % of vanadium (V), and 0.005 wt % to 0.5 wt % of niobium (Nb). 
     
     
         11 . The steel of  claim 2 , wherein the steel contains 0.001 wt % to 0.02 wt % of boron. 
     
     
         12 . The steel of  claim 11 , wherein the steel further contains 0.02 wt % or less of nitrogen (N) and 0.005 wt % to 0.5 wt % of titanium (Ti). 
     
     
         13 . The steel of  claim 12 , wherein the steel further contains 0.2 wt % to 1.0 wt % of carbon (C), 0.1 wt % to 3.5 wt % of silicon (Si), 0.3 wt % to 1.0 wt % of manganese (Mn), 0.1 wt % or less of aluminum (Al), 0.02 wt % or less of phosphorus (P), and 0.02 wt % or less of sulfur (S). 
     
     
         14 . The steel of  claim 13 , wherein the steel further contains 0.1 wt % to 1.5 wt % of chromium (Cr), 0.01 wt % to 1.0 wt % of nickel (Ni), 0.01 wt % to 1.0 wt % of copper (Cu), 0.0020 wt % or less of oxygen (O), 0.005 wt % to 0.5 wt % of vanadium (V), and 0.005 wt % to 0.5 wt % of niobium (Nb).