Hot-rolled steel sheet having excellent cold workability and excellent surface properties and hardness after working

Abstract

A hot-rolled steel sheet which has a thickness of 3-20 mm and contains specific amounts of C, Si, Mn, P, S, Al and N, with the balance made up of iron and unavoidable impurities. This hot-rolled steel sheet contains a specific amount of solid-solved N, and the contents of C and N satisfy the relation 10C+N≦3.0. This hot-rolled steel sheet contains bainitic ferrite and pearlite, respectively in an area ratio of 5% or more and in an area ratio of less than 20% relative to the whole structure, with the balance made up of polygonal ferrite. The average crystal grain size of the bainitic ferrite is 3-50 μm.

1 . A hot-rolled steel sheet excellent in cold workability as well as in surface property and hardness after working, wherein:
 a sheet thickness is from 3 to 20 mm;   a component composition comprises, in mass % (hereinafter, the same applies to chemical components),   C: 0.3% or less (exclusive of 0%),   Si: 0.5% or less (exclusive of 0%),   Mn: from 0.2 to 1%,   P: 0.05% or less (exclusive of 0%),   S: 0.05% or less (exclusive of 0%),   Al: from 0.01 to 0.1%, and   N: from 0.008 to 0.025%,   with a remainder being iron and unavoidable impurities, wherein solute N: 0.007% or more and   the contents of C and N satisfy the relationship of 10C+N≦3.0;   a microstructure comprises, in terms of area ratio relative to an entire microstructure,   bainitic ferrite: 5% or more,   pearlite: less than 20%, and   remainder: polygonal ferrite; and   an average grain size of the bainitic ferrite is in a range of from 3 to 50 μm.   
     
     
         2 . The hot-rolled steel sheet according to  claim 1 , wherein the component composition further comprises at least one member of the following (a) to (e):
 (a) at least one of Cr: 2% or less (exclusive of 0%) and Mo: 2% or less (exclusive of 0%),   (b) at least one member selected from the group consisting of Ti: 0.2% or less (exclusive of 0%), Nb: 0.2% or less (exclusive of 0%) and V: 0.2% or less (exclusive of 0%),   (c) B: 0.005% or less (exclusive of 0%),   (d) at least one member selected from the group consisting of Cu: 5% or less (exclusive of 0%), Ni: 5% or less (exclusive of 0%) and Co: 5% or less (exclusive of 0%), and   (e) at least one member selected from the group consisting of Ca: 0.05% or less (exclusive of 0%), 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%).