Method of quenching sheared portion, steel product including sheared portion that is quenched and method of manufacturing steel product including sheared portion that is quenched

Abstract

A method of quenching a sheared portion includes a heating process heating a steel product to a temperature equal to or greater than an austenitizing temperature, a shearing process shearing the steel product which is heated in the heating process to form a sheared portion at the steel product while the steel product is maintained at the temperature equal to or greater than the austenitizing temperature, and a cooling process cooling the steel product at which the sheared portion is formed in the shearing process to a temperature equal to or smaller than a martensitic transformation start point.

1 . A method of quenching a sheared portion, comprising:
 a heating process heating a steel product to a temperature equal to or greater than an austenitizing temperature;   a shearing process shearing the steel product which is heated in the heating process to form a sheared portion at the steel product while the steel product is maintained at the temperature equal to or greater than the austenitizing temperature; and   a cooling process cooling the steel product at which the sheared portion is formed in the shearing process to a temperature equal to or smaller than a martensitic transformation start point.   
     
     
         2 . The method according to  claim 1 , further comprising a heat reserving process maintaining the steel product at the temperature equal to or greater than the austenitizing temperature after the sheared portion is formed at the steel product in the shearing process, wherein the cooling process is performed after the heat reserving process is performed. 
     
     
         3 . The method according to  claim 2 , wherein in the heat reserving process, the steel product is maintained at the temperature equal to or greater than the austenitizing temperature until a crystal grain size at the sheared portion becomes equal to or greater than 4 μm. 
     
     
         4 . The method according to  claim 1 , wherein the steel product is a plate-shaped member including a first surface and a second surface which is disposed at an opposite side from the first surface,
 the shearing process is performed with a die device including a die onto which the steel product is placed and which makes contact with the first surface of the steel product, a stripper which makes contact with the second surface of the steel product placed onto the die, and a punch for shearing the steel product,   at least the die and the stripper are made of a material including a lower heat conductivity than a metal.   
     
     
         5 . The method according to  claim 4 , wherein the die and the stripper are made of ceramics. 
     
     
         6 . The method according to  claim 5 , wherein the ceramics are zirconia ceramics. 
     
     
         7 . A steel product comprising:
 a sheared portion formed by a shearing;   a portion including the sheared portion and being quenched;   a crystal grain size at the sheared portion being equal to or greater than 4 μm; and   a DI value less than 1.8 inches.   
     
     
         8 . A method of manufacturing a steel product including a sheared portion which is quenched, comprising:
 a heating process heating a steel product to a temperature equal to or greater than an austenitizing temperature, the steel product including a DI value less than 1.8 inches;   a shearing process shearing the steel product which is heated in the heating process to form a sheared portion at the steel product while the steel product is maintained at the temperature equal to or greater than the austenitizing temperature; and   a cooling process cooling the steel product at which the sheared portion is formed in the shearing process to a temperature equal to or smaller than a martensitic transformation start point.