Steel sheet and method of production of same

Abstract

A steel sheet improved in hardenability and material formability having a predetermined chemical composition, characterized in that, in the metal structure of the steel sheet, an average grain size of carbides is 0.4 μm to 2.0 μm, an area ratio of pearlite is 6% or less, when a number of carbides in ferrite grains is A and a number of carbides at ferrite grain boundaries is B, B/A>l, and when an X-ray diffraction intensity at {211}<011>at a plane of a part of ½ sheet thickness of the steel sheet is denoted by “I1” and an X-ray diffraction intensity at {100}<011>is denoted by “I0”, I1/I0<1 is satisfied, and the steel sheet has a Vickers hardness of 100 HV to 150 HV.

1 . A steel sheet consisting of, by mass %,
 C: 0.10 to 0.70%,   Si: 0.01 to 0.30%,   Mn: 0.30 to 3.00%,   Al: 0.001 to 0.10%,   Cr: 0.010 to 0.50%,   Mo: 0.0010 to 0.50%,   B: 0.0004 to 0.01%,   Ti: 0.001 to 0.10%,   P: 0.02% or less,   S: 0.01% or less,   N: 0.0200% or less,   O: 0.0200% or less,   Sn: 0.05% or less,   Sb: 0.05% or less,   As: 0.05% or less,   Nb: 0.10% or less,   V: 0.10% or less,   Cu: 0.10% or less,   W: 0.10% or less,   Ta: 0.10% or less,   Ni: 0.10% or less,   Mg: 0.05% or less,   Ca: 0.05% or less,   Y: 0.05% or less,   Zr: 0.05% or less,   La: 0.05% or less, and   Ce: 0.05% or less and   a balance of Fe and unavoidable impurities,
 wherein a metal structure of the steel sheet includes 
 carbide having an average grain size of 0.4 μm to 2.0 μm, 
 perlite having an area ratio of 6% or less and 
 ferrite wherein a ratio of a number of the carbides at ferrite grain boundaries to a number of the carbides in ferrite grains of over 1; and 
 I1/I0<1 is satisfied when an X-ray diffraction intensity at {211}<011>at a plane of a part of ½ sheet thickness of the steel sheet is denoted by “I1” and an X-ray diffraction intensity at {100}<011>is denoted by “I0”, 
 the steel sheet having a Vickers hardness of 100 HV to 150 HV. 
   
     
     
         2 . A method of production for producing steel sheet according to  claim 1  comprising
 hot rolling a steel slab of a chemical composition according to  claim 1  with finish rolling temperature between 820° C. and 950° C., to obtain hot rolled steel sheet; 
 coiling the hot rolled steel sheet at 400° C. to 550° C.; 
 pickling the coiled hot rolled steel sheet; 
 heating the pickled hot rolled steel sheet to an annealing temperature of 650° C. to 720° C. by a heating rate of 30° C/hour to 150° C/hour and holding the steel sheet for 3 hours to 60 hours as a first stage of annealing; next, 
 heating the hot rolled steel sheet to an annealing temperature of 725° C. to 790° C. by a heating rate of 1° C/hour to 80° C/hour and holding the steel sheet for 3 hours to less than 10 hours as a second stage of annealing; and, next, 
 cooling the annealed hot rolled steel sheet to 650° C. by a cooling rate of 1° C/hour to 100° C/hour.