Steel sheet for nitriding and production method therefor

Abstract

A steel sheet for nitriding has excellent formability and punchability. The steel sheet has a composition including, in percent by mass, 0.02% to 0.08% of C, 0.1% or less of Si, 0.2% to 1.8% of Mn, 0.05% or less of P, 0.02% or less of S, 0.01% to 0.06% of Al, 0.5% to 1.5% of Cr, 0.01% or less of N, and the balance being Fe and incidental impurities; and has a microstructure including ferrite as a main phase and pearlite and/or bainite as a secondary phase. The ferrite has a fraction of 70% or more in the entire microstructure and an average grain diameter of 5 to 25 gm. An average length of the major axis of cementite present in the secondary phase in a cross section in the rolling direction of the steel sheet is 3.0 μm or less.

1 .- 4 . (canceled) 
     
     
         5 . A steel sheet for nitriding having a composition and a microstructure:
 the composition comprising in percent by mass,   0.02% to 0.08% of C, 0.1% or less of Si;   0.2% to 1.8% of Mn, 0.05% or less of P;   0.02% or less of S, 0.01% to 0.06% of Al;   0.5% to 1.5% of Cr, 0.01% or less of N; and   the balance being Fe and incidental impurities; and   a microstructure including ferrite as a main phase and pearlite and/or bainite as a secondary phase,   wherein the ferrite has a fraction of 70% or more in the entire microstructure and an average grain diameter of 5 to 25 μm, and cementite present in the secondary phase has an average length of a major axis of 3.0 μm or less in a cross section in a rolling direction of the steel sheet.   
     
     
         6 . The steel sheet for nitriding according to  claim 5 , wherein the composition further comprises, in percent by mass, at least one selected from the group consisting of 0.005% to 0.075% of V, 0.005% to 0.025% of Nb, and 0.005% to 0.025% of Ti. 
     
     
         7 . A method of producing a steel sheet for nitriding, comprising:
 heating a steel slab having a composition including, in percent by mass, 0.02% to 0.08% of C, 0.1% or less of Si, 0.2% to 1.8% of Mn, 0.05% or less of P, 0.02% or less of S, 0.01% to 0.06% of Al, 0.5% to 1.5% of Cr, 0.01% or less of N, and the balance being Fe and incidental impurities, to 1,050° C. to 1,250° C.;   hot rolling the heated steel slab at a finishing temperature ranging from the Ar 3  transformation temperature to (the Ar 3  transformation temperature+100° C.);   cooling the hot rolled steel sheet at a cooling rate of 40° C./s to 80° C./s in the temperature range from the finishing temperature to 750° C. at a cooling rate of 15° C./s to 35° C./s in the temperature range from 750° C. to a cooling stop temperature of 500° C. to 650° C.; and   coiling the cooled steel sheet at a coiling temperature of 500° C. to 650° C.   
     
     
         8 . The method according to  claim 7 , wherein the composition of the steel slab further includes, in percent by mass, at least one selected from the group consisting of 0.005% to 0.075% of V, 0.005% to 0.025% of Nb, and 0.005% to 0.025% of Ti. 
     
     
         9 . The method according to  claim 7 , wherein the steel sheet has a microstructure including ferrite as a main phase and pearlite and/or bainite as a secondary phase, wherein the ferrite has a fraction of 70% or more in the entire microstructure and an average grain diameter of 5 to 25 μm, and cementite present in the secondary phase has an average length of a major axis of 3.0 μm or less in a cross section in a rolling direction of the steel sheet.