Steel Sheet Excellent In Fine Blanking Performance and Manufacturing Method of the Same

Abstract

A steel sheet excellent in FB performance-and also excellent in fabrication performance after FB working and a manufacturing method of the same are provided. The steel sheet is a steel sheet having a composition containing from 0.1 to 0.5% of C, not more than 0.5% of Si and from 0.2 to 1.5% of Mn in terms of % by mass, with P and S being adjusted at proper ranges, and having a structure in which a ferrite has an average grain size of from 1 to 10 μm, cementite has a spheroidization ratio of 80% or more, and of the cementite, an amount S gb of ferrite intergranular cementite which is defined by the following expression (1): S gb (%)={S on /(S on +S in )}×100 (wherein S on represents a total occupied area of cementite present on the ferrite grain boundary of the cementite present per unit area; and Sn in represents a total occupied area of cementite present in a ferrite grain of the cementite present per unit area) is 40% or more. In this way, the steel sheet becomes a steel sheet excellent in FB performance, mold life and performance after FB working.

1 - 8 . (canceled) 
   
   
       9 . A steel sheet excellent in fine blanking performance comprising:
 a composition containing from 0.1 to 0.5% of C, not more than 0.5% of Si, from 0.2 to 1.5% of Mn, not more than 0.03% of P and not more than 0.02% of S in terms of % by mass, with the remainder being Fe and unavoidable impurities and having a structure mainly composed of ferrite and cementite, wherein said ferrite has an average grain size of from 1 to 10 μm, said cementite has a spheroidization ratio of 80% or more, and of said cementite, an amount S gb  of ferrite intergranular cementite which is an amount of a cementite present on a crystal grain boundary of ferrite and which is defined by the following expression (1) is 40% or more:
     S   gb (%)={ S   on /( S   on   +S   in )}×100  (1) 
   
     wherein S on  represents a total occupied area of cementite present on the ferrite grain boundary of the cementite present per unit area; and S in  represents a total occupied area of cementite present in a ferrite grain of the cementite present per unit area. 
   
   
       10 . The steel sheet according to  claim 9 , wherein the cementite present on the crystal grain boundary of said ferrite has an average particle size of not more than 5 μm. 
   
   
       11 . The steel sheet according to  claim 9 , wherein the composition further contains not more than 0.1% of Al in terms of % by mass. 
   
   
       12 . The steel sheet according to  claim 9 , wherein the composition further comprises one or more members selected from the group consisting of not more than 3.5% of Cr, not more than 0.7% of Mo, not more than 3.5% of Ni, from 0.01 to 0.1% of Ti and from 0.0005 to 0.005% of B in terms of % by mass. 
   
   
       13 . A method of manufacturing a steel sheet excellent in fine blanking performance including successively applying hot rolling by heating and rolling a raw steel material to form a hot rolled sheet and hot rolled sheet annealing by applying annealing to the subject hot rolled sheet, wherein the raw steel material is a composition comprising from 0.1 to 0.5% of C, not more than 0.5% of Si, from 0.2 to 1.5% of Mn, not more than 0.03% of P and not more than 0.02% of S in terms of % by mass, with the remainder being Fe and unavoidable impurities; and said hot rolling is a treatment in which a termination temperature of finish rolling is from 800 to 950° C., after completion of the subject finish rolling, cooling is carried out at an average cooling rate of 50° C./s or more, the subject cooling is stopped at a temperature in the range of from 500 to 700° C., and coiling is carried out at from 450 to 600° C. 
   
   
       14 . The method according to  claim 13 , wherein the composition further comprises not more than 0.1% of Al in terms of % by mass. 
   
   
       15 . The method according to  claim 13 , wherein the composition further comprises one or more members selected from the group consisting of not more than 3.5% of Cr, not more than 0.7% of Mo, not more than 3.5% of Ni, from 0.01 to 0.1% of Ti and from 0.0005 to 0.005% of B in terms of % by mass. 
   
   
       16 . The method according to  claim 14 , wherein the composition further comprises one or more members selected from the group consisting of not more than 3.5% of Cr, not more than 0.7% of Mo, not more than 3.5% of Ni, from 0.01 to 0.1% of Ti and from 0.0005 to 0.005% of B in terms of % by mass. 
   
   
       17 . The method according to  claim 13 , wherein the hot rolled sheet annealing is carried out at an annealing temperature of from 600 to 750° C. 
   
   
       18 . The method according to  claim 14 , wherein the hot rolled sheet annealing is carried out at an annealing temperature of from 600 to 750° C. 
   
   
       19 . The method according to  claim 15 , wherein the hot rolled sheet annealing is carried out at an annealing temperature of from 600 to 750° C. 
   
   
       20 . The method according to  claim 16 , wherein the hot rolled sheet annealing is carried out at an annealing temperature of from 600 to 750° C.