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

Abstract

This hot-rolled steel sheet 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. The contents of solid-solved N, C and N are within specific ranges, and bainitic ferrite having a specific average crystal grain size and pearlite have specific area occupancies in the structure, with the balance occupied by polygonal ferrite. This hot-rolled steel sheet has a specific hardness distribution in the thickness direction.

1 . A hot-rolled steel sheet excellent in heavy cold workability and surface hardness after working, wherein:
 a sheet thickness is from 3 to 20 mm;   a component composition comprises, in mass %,   C: more than 0% and 0.3% or less,   Si: more than 0% and 0.5% or less,   Mn: from 0.2 to 1%,   P: more than 0% and 0.05% or less,   S: more than 0% and 0.05% or less,   Al: from 0.01 to 0.1%, and   N: from 0.008 to 0.025%,   with the remainder being iron and unavoidable impurities, wherein   solute N: 0.007% or more and   the contents of C and N satisfy the relationship 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;   an average grain size of the bainitic ferrite is in a range of from 3 to 50 μm; and   (Hv max −Hv min )/Hv min ≦0.3, wherein, in a hardness distribution in a thickness direction, Hv max  and Hv min ≦0.3, are respectively the maximal value and the minimal value of Vickers hardness values of three portions that are a surface portion, a t/4 portion where t is the sheet thickness, and a central portion.   
     
     
         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 member selected from the group consisting of Cr: more than 0% and 2% or less and Mo: more than 0% and 2% or less, in mass %;   (b) at least one member selected from the group consisting of Ti: more than 0% and 0.2% or less, Nb: more than 0% and 0.2% or less, and V: more than 0% and 0.2% or less, in mass %;   (c) B: more than 0% and 0.005% or less in mass %;   (d) at least one member selected from the group consisting of Cu: more than 0% and 5% or less, Ni: more than 0% and 5% or less, and Co: more than 0% and 5% or less, in mass %; and   (e) at least one member selected from the group consisting of Ca: more than 0% and 0.05% or less, REM: more than 0% and 0.05% or less, Mg: more than 0% and 0.02% or less, Li: more than 0% and 0.02% or less, Pb: more than 0% and 0.5% or less, and Bi: more than 0% and 0.5% or less.