Hot-rolled steel sheet having excellent surface hardness after carburizing heat treatment and excellent cold workability

Abstract

A hot-rolled steel sheet that: has a sheet thickness of 2-10 mm, contains specific amounts of C, Mn, Al, and N, the remainder being iron and unavoidable impurities, and contains ferrite and pearlite in a specified area ratio, the remainder being bainite. In the hot-rolled steel sheet, a number of grains having an aspect ratio of major axis to minor axis of 3 or less is 60% or more of a number of all grains and an average grain size of the all grains is from 3 to 50 μm.

1 . A hot-rolled steel sheet excellent in cold formability and surface hardness after a carburizing heat treatment, having:
 a sheet thickness of from 2 to 10 mm;   a component composition comprising, in mass % (hereinafter, the same applies to chemical components),   C: from 0.05 to 0.30%,   Mn: from 0.3 to 3.0%,   Al: from 0.015 to 0.1%, and   N: from 0.003 to 0.30%,   with the remainder being iron and unavoidable impurities;   
       and
 a microstructure comprising, in area ratio, 
 ferrite: from 10 to 50%, 
 pearlite: from 15 to 50% and 
 remainder: bainite, 
 
       wherein
 with respect to grains of all phases including the ferrite and the pearlite (hereinafter, referred to as “all grains”), 
 a number of grains having an aspect ratio (major axis/minor axis) of 3 or less is 60% or more of a number of the all grains, and an average grain size of the all grains is from 3 to 50 μm. 
 
     
     
         2 . The hot-rolled steel sheet according to  claim 1 , wherein, in the unavoidable impurities, Si is 0.5% or less, P is 0.030% or less and S is 0.035% or less. 
     
     
         3 . The hot-rolled steel sheet according to  claim 1 , wherein the component composition further comprises at least any one of the following (a) to (f):
 (a) at least one member selected from the group consisting of Cr: 3.0% or less (exclusive of 0%), Mo: 1.0% or less (exclusive of 0%) and Ni: 3.0% or less (exclusive of 0%);   (b) at least one member selected from the group consisting of Cu: 2.0% or less (exclusive of 0%) and Co: 5% or less (exclusive of 0%);   (c) at least one member selected from the group consisting of V: 0.5% or less (exclusive of 0%), Ti: 0.1% or less (exclusive of 0%) and Nb: 0.1% or less (exclusive of 0%);   (d) at least one member selected from the group consisting of Ca: 0.08% or less (exclusive of 0%) and Zr: 0.08% or less (exclusive of 0%);   (e) Sb: 0.02% or less (exclusive of 0%); and/or   (f) at least one member selected from the group consisting of REM: 0.05% or less (exclusive of 0%), Mg: 0.02% or less (exclusive of 0%), Li: 0.02% or less (exclusive of 0%), Pb: 0.5% or less (exclusive of 0%), and Bi: 0.5% or less (exclusive of 0%).   
     
     
         4 . The hot-rolled steel sheet according to  claim 2 , wherein the component composition further comprises at least any one of the following (a) to (f):
 (a) at least one member selected from the group consisting of Cr: 3.0% or less (exclusive of 0%), Mo: 1.0% or less (exclusive of 0%) and Ni: 3.0% or less (exclusive of 0%);   (b) at least one member selected from the group consisting of Cu: 2.0% or less (exclusive of 0%) and Co: 5% or less (exclusive of 0%);   (c) at least one member selected from the group consisting of V: 0.5% or less (exclusive of 0%), Ti: 0.1% or less (exclusive of 0%) and Nb: 0.1% or less (exclusive of 0%);   (d) at least one member selected from the group consisting of Ca: 0.08% or less (exclusive of 0%) and Zr: 0.08% or less (exclusive of 0%);   (e) Sb: 0.02% or less (exclusive of 0%); and/or   (f) at least one member selected from the group consisting of REM: 0.05% or less (exclusive of 0%), Mg: 0.02% or less (exclusive of 0%), Li: 0.02% or less (exclusive of 0%), Pb: 0.5% or less (exclusive of 0%), and Bi: 0.5% or less (exclusive of 0%).