Surface layer grain refining hot-shearing method and workpiece obtained by surface layer grain refining hot-shearing

Abstract

Provided is a surface layer grain refining hot-shearing method including: heating and keeping a steel sheet in a temperature range of from Ac3 to 1400° C. to austenitize the steel sheet; subsequently shearing the steel sheet in a state in which the steel sheet is placed on a die; and quenching by rapidly cooling the sheared steel sheet, wherein a start temperature of the shearing is set to be a temperature (° C.) obtained by adding a temperature of from 30° C. to 140° C. to a previously measured Ar3 of the steel sheet.

1 . A surface layer grain refining hot-shearing method comprising:
 heating and keeping a steel sheet having a carbon content of 0.15% or more by mass in a temperature range of from Ac3 to 1400° C. to austenitize the steel sheet;   subsequently shearing the steel sheet in a state in which the steel sheet is placed on a die; and   quenching by rapidly cooling the sheared steel sheet,   wherein a start temperature of the shearing is set to be a temperature (° C.) obtained by adding a temperature of from 30° C. to 140° C. to a previously measured Ar3 of the steel sheet.   
     
     
         2 . A surface layer grain refining hot-shearing method comprising:
 heating and keeping a steel sheet having a carbon content of 0.15% or more by mass in a temperature range of from Ac3 to 1400° C. to austenitize the steel sheet;   subsequently shearing the steel sheet in a state in which the steel sheet is placed on a die; and   quenching by rapidly cooling the sheared steel sheet,   wherein a start temperature of the shearing is set to be a temperature (° C.) obtained by adding a value, which is calculated by multiplying an amount of equivalent plastic strain of a surface layer in a sheared portion by a coefficient from 40 to 60, to a previously measured Ar3 of the steel sheet.   
     
     
         3 . The surface layer grain refining hot-shearing method according to  claim 2 , wherein the amount of equivalent plastic strain of the surface layer in the sheared portion is calculated as an average value of an amount of equivalent plastic strain of a region in a range of from 5% to 20% of a thickness of the steel sheet from a shear plane of the sheared portion to an inside of the steel sheet in a normal direction of the shear plane and in a range of from 20% to 50% of the thickness of the steel sheet in a thickness direction of the steel sheet from a bottom on a burr side of the sheared portion. 
     
     
         4 . The surface layer grain refining hot-shearing method according to  claim 2 , wherein the amount of equivalent plastic strain of the surface layer in the sheared portion is calculated by a numerical simulation that is performed based on a stress-strain diagram at a steel sheet temperature of from 500° C. to 800° C. 
     
     
         5 . The surface layer grain refilling hot-shearing method according to  claim 2 , wherein the amount of equivalent plastic strain of the surface layer in the sheared portion is calculated based on a Mises yield function represented by the following Formula (1) 
       
         
           
             
               
                 
                   
                     
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         6 . The surface layer grain refining hot-shearing method according to  claim 1 , wherein the shearing of the steel sheet starts within three seconds after the steel sheet comes in contact with the die. 
     
     
         7 . The surface layer grain refining hot-shearing method according to  claim 1 , wherein the rapid cooling is performed when the steel sheet comes in contact with the die. 
     
     
         8 . The surface layer grain refining hot-shearing method according to  claim 1 , wherein the rapid cooling is performed when water jetting from a puncture formed in a contacting portion of the steel sheet with the die passes through a groove provided in the contacting portion of the steel sheet. 
     
     
         9 . The surface layer grain refining hot-shearing method according to  claim 1 , wherein press forming not accompanying fracture of the steel sheet is performed between the heating and the shearing of the steel sheet. 
     
     
         10 . A workpiece obtained by surface layer grain refining hot-shearing, comprising:
 a steel sheet having a carbon content of 0.15% or more by mass, a surface layer of a sheared portion of a steel sheet having a carbon content of 0.15% or more by mass including a ferrite phase and a remainder, the surface layer being defined as a range up to 100 μm inside of the steel sheet in a normal direction of a shear plane from a fracture plane of the sheared portion,   wherein the remainder includes at least one phase of a bainite phase, a martensite phase, or a residual austenite phase which has a crystal grain diameter of 3 μm or less, and includes cementite and inevitably generated inclusions,   wherein the ferrite phase has an average grain size of 3 μm or less,   wherein the surface layer contains 5% or more grains by number having an aspect ratio of 3 or more, and   wherein a region out of the range of 100 μm includes: martensite and inevitably generated inclusions; or bainite, martensite, and inevitably generated inclusions.   
     
     
         11 . The workpiece obtained by surface layer grain refining hot-shearing according to  claim 10 , wherein, in the surface layer, the cementite has a number density of 0.8 pieces/μm 3  or less and the cementite has a maximum length of 3 μm or less. 
     
     
         12 . The workpiece obtained by surface layer grain refining hot-shearing according to  claim 10 , wherein a total area ratio of the bainite phase, the martensite phase, and the residual austenite phase, which are measured by an electron-beam backscattering diffraction (EBSD) method, is from 10% to 50% in the surface layer. 
     
     
         13 . A workpiece obtained by surface layer grain refining hot-shearing, the workpiece produced by: heating and keeping a steel sheet having a carbon content of 0.15% or more by mass in a temperature range of from Ac3 to 1400° C. to austenitize the steel sheet; subsequently shearing the steel sheet in a state in which the steel sheet is placed on a die; and quenching by rapidly cooling the sheared steel sheet,
 wherein a start temperature of the shearing is set to be a temperature (° C.) obtained by adding a temperature of from 30° C. to 140° C. to a previously measured Ar3 of the steel sheet. 
 
     
     
         14 . A workpiece obtained by surface layer grain refining hot-shearing, the workpiece produced by heating and keeping a steel sheet having a carbon content of 0.15% or more by mass in a temperature range of from Ac3 to 1400° C. to austenitize the steel sheet; subsequently shearing the steel sheet in a state in which the steel sheet is placed on a die; and quenching by rapidly cooling the sheared steel sheet, and
 a start temperature of the shearing is set to be a temperature (° C.) obtained by adding a value, which is calculated by multiplying an amount of equivalent plastic strain of a surface layer in a sheared portion by a coefficient from 40 to 60, to a previously measured Ar3 of the steel sheet. 
 
     
     
         15 . The surface layer grain refining hot-shearing method according to  claim 2 , wherein the shearing of the steel sheet starts within three seconds after the steel sheet comes in contact with the die. 
     
     
         16 . The surface layer grain refining hot-shearing method according to  claim 2 , wherein the rapid cooling is performed when the steel sheet comes in contact with the die. 
     
     
         17 . The surface layer grain refining hot-shearing method according to  claim 2 , wherein the rapid cooling is performed when water jetting from a puncture formed in a contacting portion of the steel sheet with the die passes through a groove provided in the contacting portion of the steel sheet. 
     
     
         18 . The surface layer grain refining hot-shearing method according to  claim 2 , wherein press forming not accompanying fracture of the steel sheet is performed between the heating and the shearing of the steel sheet.