Wear-resistant steel sheet having excellent toughness and production method

Abstract

A steel sheet has a composition containing, in mass %, from 0.60 to 1.25% of C, 0.50% or less of Si, from 0.30 to 1.20% of Mn, 0.030% or less of P, 0.030% or less of S, from 0.30 to 1.50% of Cr, from 0.10 to 0.50% of Nb, from 0 to 0.50% of Ti, from 0 to 0.50% of Mo, from 0 to 0.50% of V, from 0 to 2.00% of Ni, the balance being Fe with unavoidable impurities. A ferrite phase as a metal matrix has dispersed therein cementite particles and Ni—Ti based carbide particles. A number density (cross section parallel to rolling and thickness directions) of the carbide particles having a circle equivalent diameter of 0.5 μm or more is from 3,000 to 9,000 per mm 2 and a number density of voids having a circle equivalent diameter of 1.0 μm or more is 1,250 per mm 2 or less.

1 . A steel sheet: having a chemical composition containing, in terms of percentage by mass, from 0.60 to 1.25% of C, 0.50% or less of Si, from 0.30 to 1.20% of Mn, 0.030% or less of P, 0.030% or less of S, from 0.30 to 1.50% of Cr, from 0.10 to 0.50% of Nb, from 0 to 0.50% of Ti, from 0 to 0.50% of Mo, from 0 to 0.50% of V, from 0 to 2.00% of Ni, and the balance of Fe, with unavoidable impurities; having a metal structure containing a ferrite phase as a metal matrix having dispersed therein cementite particles and particles of a carbide containing one or more kind of Nb and Ti (which is hereinafter referred to as a “Nb.Ti based carbide”); and having, on a cross sectional surface in parallel to a rolling direction and a thickness direction (i.e., an L cross section), a number density of the Nb.Ti based carbide particles having a circle equivalent diameter of 0.5 μm or more of from 3,000 to 9,000 per mm 2  and a number density of voids having a circle equivalent diameter of 1.0 μm or more of 1,250 per mm 2  or less. 
     
     
         2 . A method for producing the steel sheet according to  claim 1 , comprising, in this order:
 a step of producing a cast piece while controlling a cooling rate in cooling a molten steel from a liquidus line temperature to a solidus line temperature to from 5 to 20° C./min (casting step);   a step of heating and retaining the cast piece to from 1,200 to 1,350° C. for from 0.5 to 4 hours (cast piece heating step);   a step of performing hot rolling (hot rolling step);   a step of performing once or more a procedure including cold rolling at a rolling reduction ratio of 35% or less, and then retaining at a temperature of 500° C. or more and less than an Ac 1  point for from 10 to 50 hours and then cooling (intermediate cold rolling annealing step); and   a step of performing cold rolling at a rolling reduction ratio of 60% or less (finish cold rolling step).   
     
     
         3 . The method for producing the steel sheet according to  claim 2 , further comprising, between the hot rolling step and the intermediate cold rolling and annealing step:
 a step of performing annealing by retaining a hot rolled steel sheet obtained in the hot rolling step at a temperature of 500° C. or more and less than an Ac 1  point for from 10 to 50 hours, and then cooling (hot rolled steel sheet annealing step).   
     
     
         4 . The method for producing the steel sheet according to  claim 2 , further comprising, after the finish cold rolling step:
 a step of performing annealing by retaining at from 300 to 500° C. for from 1 to 5 hours (stress relief annealing step).