Method for producing steel component having locally softened part

Abstract

A method for producing a steel includes preparing a steel sheet having a chemical composition in % by mass of: C: 0.05 to 0.40%, Si: 0 to 2.0%, Mn: 1.0 to 3.0%, Al: 0.010 to 1.0%, P: more than 0% and 0.100% or less, S: more than 0% and 0.010% or less, N: more than 0% and 0.010% or less, B: 0.0005 to 0.010%, and iron; heating the steel sheet to Ac1 point (° C.) or higher and lower than Ac3 point (° C.)+10° C.; processing the steel sheet by applying a strain of 0.5% or more thereto at 675° C. or higher and lower than Ac3 point+10° C.; holding or gradually cooling the steel sheet at an average cooling rate of 0 to 15° C./sec for 1 second or more and 120 seconds or less; and cooling the steel sheet to Ms point (° C.)−50° C.

1 . A method for producing a steel component, the method comprising:
 preparing a steel sheet having a chemical composition comprising:   C: 0.05% to 0.40% by mass,   Si: 0% to 2.0% by mass,   Mn: 1.0% to 3.0% by mass,   Al: 0.010% to 1.0% by mass,   P: more than 0% by mass and 0.100% by mass or less,   S: more than 0% by mass and 0.010% by mass or less,   N: more than 0% by mass and 0.010% by mass or less,   B: 0.0005% to 0.010% by mass, and   heating the steel sheet to a temperature of Ac1 point (° C.) or higher and lower than Ac3 point (° C.)+10° C.;   after the heating, processing the steel sheet by applying a strain of 0.5% or more thereto at a processing temperature of 675° C. or higher and lower than Ac3 point (° C.)+10° C.;   after the processing, holding the steel sheet at the processing temperature for 1 second or more and 120 seconds or less, or gradually cooling the steel sheet at an average cooling rate of more than 0° C./sec and 15° C./sec or less for 1 second or more and 120 seconds or less; and   after the holding or gradually cooling, cooling the steel sheet to a temperature of Ms point (° C.)−50° C.,   wherein an average cooling rate from the temperature of the heating to the Ms point (° C.)−50° C. is controlled to be 10° C./sec or more.   
     
     
         2 . A method for producing a steel component, the method comprising:
 preparing a steel sheet having a chemical composition comprising:   C: 0.05% to 0.40% by mass,   Si: 0% to 2.0% by mass,   Mn: 1.0% to 3.0% by mass,   Al: 0.010% to 1.0% by mass,   P: more than 0% by mass and 0.100% by mass or less,   S: more than 0% by mass and 0.010% by mass or less,   N: more than 0% by mass and 0.010% by mass or less,   B: 0.0005% to 0.010% by mass, and   heating the steel sheet to a temperature of Ac3 point (° C.)+10° C. or higher and 1,100° C. or lower;   after the heating, processing the steel sheet by applying a strain of 10% or more thereto at a processing temperature of Ms point (° C.)+50° C. or higher and lower than Ac3 point (° C.)+10° C.;   after the processing, holding the steel sheet at the processing temperature for 1 second or more and 120 seconds or less, or gradually cooling the steel sheet at an average cooling rate of more than 0° C./sec and 15° C./sec or less for 1 second or more and 120 seconds or less; and   after the holding or gradually cooling, cooling the steel sheet to a temperature of Ms point (° C.)−50° C.,   wherein an average cooling rate from the temperature in the heating to the Ms point (° C.)−50° C. is controlled to be 10° C./sec or more.   
     
     
         3 . The method according to  claim 1 , wherein the steel sheet further comprises at least one selected from the group consisting of (a) and (b), wherein:
 (a) is at least one selected from the group consisting of Cu: more than 0% by mass and 0.50% by mass or less, and Ni: more than 0% by mass and 0.50% by mass or less, and   (b) is at least one selected from the group consisting of Ti: more than 0% by mass and 0.10% by mass or less, Cr: more than 0% by mass and 3.0% by mass or less, and Nb: more than 0% by mass and 0.10% by mass or less.   
     
     
         4 . The method according to  claim 2 , wherein the steel sheet further comprises at least one selected from the group consisting of (a) and (b), wherein:
 (a) is at least one selected from the group consisting of Cu: more than 0% by mass and 0.50% by mass or less, and Ni: more than 0% by mass and 0.50% by mass or less, and   (b) is at least one selected from the group consisting of Ti: more than 0% by mass and 0.10% by mass or less, Cr: more than 0% by mass and 3.0% by mass or less, and Nb: more than 0% by mass and 0.10% by mass or less.   
     
     
         5 . The method according to  claim 1 , further comprising applying the strain by stretch forming. 
     
     
         6 . The method according to  claim 1 , further comprising applying the strain by forging. 
     
     
         7 . The method according to  claim 1 , further comprising applying the strain by return bending during draw forming. 
     
     
         8 . The method according to  claim 1 , further comprising applying the strain by shearing. 
     
     
         9 . The method according to  claim 1 , further comprising applying the strain by a plurality of times of processing. 
     
     
         10 . The method according to  claim 9 , wherein the plurality of times of processing comprises processing for applying deformation and processing for restoring the deformation. 
     
     
         11 . The method according to  claim 2 , further comprising applying the strain by stretch forming. 
     
     
         12 . The method according to  claim 2 , further comprising applying the strain by forging. 
     
     
         13 . The method according to  claim 2 , further comprising applying the strain by return bending during draw forming. 
     
     
         14 . The method according to  claim 2 , further comprising applying the strain by shearing. 
     
     
         15 . The method according to  claim 2 , further comprising applying the strain by a plurality of times of processing. 
     
     
         16 . The method according to  claim 15 , wherein the plurality of times of processing comprises processing for applying deformation and processing for restoring the deformation. 
     
     
         17 . The method according to  claim 3 , further comprising applying the strain by stretch forming. 
     
     
         18 . The method according to  claim 3 , further comprising applying the strain by a plurality of times of processing. 
     
     
         19 . The method according to  claim 4 , further comprising applying the strain by stretch forming. 
     
     
         20 . The method according to  claim 4 , further comprising applying the strain by a plurality of times of processing.