Steel sheet

Abstract

In a steel sheet, a cleanliness of a metal structure is 0.08% or less, a which is an Mn segregation degree is 1.6 or less, and a difference ΔHv between a low strain formed portion that undergoes a plastic strain of 5% or less and a high strain formed portion that undergoes a plastic strain of 20% or higher in a hot forming in average hardness after the hot forming is 40 or less.

1 . A steel sheet comprising as a chemical composition, by mass %,
 C: 0.18% to 0.275%;   Si: 0.02% to 0.15%;   Mn: 1.85% to 2.75%;   sol.Al:  0 . 0002 % to 0.5%;   Cr: 0.05% to 1.00%;   B: 0.0005% to 0.01%;   P: 0.1% or less;   S: 0.0035% or less;   N: 0.01% or less;   Ni: 0% to 0.15%;   Cu: 0% to 0.05%;   Ti: 0% to 0.1%;   Nb: 0% to 0.2%; and   a remainder including Fe and impurities,   wherein a cleanliness of a metal structure is 0.08% or less,   α which is an Mn segregation degree expressed by the following expression 1 is 1.6 or less, and   in a hot forming, a difference ΔHv in an average hardness after a hot forming between a low strain formed portion that undergoes a plastic strain of 5% or less and a high strain formed portion that undergoes a plastic strain of 20% or higher is 40 or less.   α=(a maximum Mn concentration, by mass %, in a thickness center portion of the steel sheet)/(an average Mn concentration, by mass %, at a position at a depth of ¼ of a sheet thickness from a surface of the steel sheet) . . . expression 1.   
     
     
         2 . The steel sheet according to  claim 1 , wherein the chemical composition further includes, instead of a portion of Fe, by mass %, one or two selected from the group consisting of Ni: 0.02% to 0.15%, and Cu: 0.003% to 0.05%. 
     
     
         3 . The steel sheet according to  claim 1  or  2 , wherein the chemical composition further includes, instead of a portion of Fe, by mass %, one or two selected from the group consisting of Ti: 0.005% to 0.1%, and Nb: 0.005% to 0.2%. 
     
     
         4 . The steel sheet according to  claim 1  or  2 , wherein the surface of the steel sheet further includes a coated layer. 
     
     
         5 . The steel sheet according to  claim 3 , wherein the surface of the steel sheet further includes a coated layer.