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

Abstract

A hot-rolled steel sheet having a sheet thickness of 2-10 mm and containing specific amounts of C, Mn, Al, and N with iron and unavoidable impurities. With regard to all grains existing at a position of t/4 in depth, t being sheet thickness, an area ratio of grains having a sheet-plane orientation within 10° from (123) plane is 20% or more, a total area ratio of grains having a crystal direction within 10° from <001> direction and grains having a crystal direction within 10° from <110> direction, in a rolling direction, is 25% or less, and an average grain size of all grains is 3-50 μm.

1 . A hot-rolled steel sheet excellent in drawability and surface hardness after a carburizing heat treatment, wherein:
 the hot-rolled steel sheet has a sheet thickness of from 2 to 10 mm; and   the hot-rolled steel sheet comprises 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%, 
 N: from 0.003 to 0.30%, 
 iron, and 
 unavoidable impurities; 
   a microstructure of the hot-rolled steel sheet mainly comprises ferrite and pearlite; and,   with respect to all grains including the ferrite and the pearlite, existing at a position of t/4 in depth (t: sheet thickness),
 an area ratio of grains having a sheet-plane orientation within 10° from (123) plane is 20% or more, 
 a total area ratio of grains having a crystal direction within 10° from <001> direction and grains having a crystal direction within 10° from <110> direction, in a rolling direction, is 25% or less, 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 hot-rolled steel sheet further comprises at least one member 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   (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 hot-rolled steel sheet further comprises at least one member 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   (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%).