Hot-rolled steel sheet exhibiting excellent cold formability and excellent surface hardness after forming

Abstract

Disclosed is a hot-rolled steel sheet that has a thickness of 3 to 20 mm and includes, in a chemical composition in mass percent, C of 0.3% or less (excluding 0%), Si of 0.5% or less (excluding 0%), Mn of 0.2% to 1%, P of 0.05% or less (excluding 0%), S of 0.05% or less (excluding 0%), Al of 0.01% to 0.1%, and N of 0.008% to 0.025%, with the remainder consisting of iron and inevitable impurities. A solute nitrogen content is 0.007% or more, and the carbon and nitrogen contents meet a condition as specified by 10C+N≦3.0. The microstructure of the steel sheet includes pearlite of less than 20% in area percentage, with the remainder approximately consisting of ferrite. The average grain size of the ferrite is 3 to 35 μm. The steel sheet has good cold formability during forming and still has predetermined surface hardness after forming.

1 . A hot-rolled steel sheet having excellent cold formability and satisfactory surface hardness after forming,
 the hot-rolled steel sheet having a thickness of 3 to 20 mm,   the hot-rolled steel sheet comprising, in chemical composition   in percent by mass (hereinafter the same for chemical composition):   C in a content of 0.3% or less (excluding 0%);   Si in a content of 0.5% or less (excluding 0%);   Mn in a content of 0.2% to 1%;   Pin a content of 0.05% or less (excluding 0%);   S in a content of 0.05% or less (excluding 0%);   Al in a content of 0.01% to 0.1%;   N in a content of 0.008% to 0.025%;   with the remainder consisting of iron and inevitable impurities,   a content of solute nitrogen being 0.007% or more, and   the contents of carbon (C) and nitrogen (N) meeting a condition as specified by expression: 10C+N≦3.0,   a microstructure of the hot-rolled steel sheet comprising:   pearlite in a content of less than 20% in area percentage based on the total microstructure,   with the remainder approximately consisting of ferrite,   the ferrite having an average grain size of 3 to 35 μm.   
     
     
         2 . The hot-rolled steel sheet according to  claim 1 ,
 further comprising, in the chemical composition, at least one element selected from the group consisting of:   Cr in a content of 2% or less (excluding 0%);   Mo in a content of 2% or less (excluding 0%);   Ti in a content of 0.2% or less (excluding 0%);   Nb in a content of 0.2% or less (excluding 0%)   V in a content of 0.2% or less (excluding 0%)   B in a content of 0.005% or less (excluding 0%);   Cu in a content of 5% or less (excluding 0%);   Ni in a content of 5% or less (excluding 0%);   Co in a content of 5% or less (excluding 0%);   Ca in a content of 0.05% or less (excluding 0%);   at least one rare-earth element (REM) in a total content of 0.05% or less (excluding 0%);   Mg in a content of 0.02% or less (excluding 0%);   Li in a content of 0.02% or less (excluding 0%);   Pb in a content of 0.5% or less (excluding 0%); and   Bi in a content of 0.5% or less (excluding 0%).