Steel Material and Process for Producing the Same

Abstract

A steel material of 85 to 95 H RB exhibiting a ratio of area occupied by ferrite of 30% or higher, which steel material comprises given amounts of C, Si, Mn, S, Ti, Cr, Al and B and the balance of iron and unavoidable impurities, is subjected to induction hardening. After the hardening, the surface of the steel material exhibits an H v value of 640 to 730. When t refers to the effective hardening depth, namely, the distance from the surface to a region exhibiting an H v value of 392 and r refers to the radius of the steel material, the hardened layer ratio, t/r, is 0.4 or higher.

1 . A steel member comprising, in percent by mass, 0.47% to 0.52% of C, 0.03% to 0.15% of Si, 0.6% to 0.7% of Mn, 0.005% to 0.03% of S, 0.025% to 0.04% of Ti, 0.05% to 0.3% of Cr, 0.04% to 0.09% of Mo, 0.02% to 0.04% of Al, and 0.0005% to 0.004% of B, the balance being iron and inevitable impurities,
 wherein said steel member has a surface with a Vickers hardness of 640 to 730, and   when said steel member has a radius r and an effective hardening depth t that is a distance between said surface and a region with a Vickers hardness of 392, a hardened layer ratio t/r of said steel member is 0.4 or more.   
   
   
       2 . A steel member according to  claim 1 , wherein a nitrogen content of said steel member is 0.01% by mass or less. 
   
   
       3 . A steel member according to  claim 1 , wherein said steel member is a drive shaft. 
   
   
       4 . A method for producing a steel member, wherein said steel member has a surface with a Vickers hardness of 640 to 730, and when said steel member has a radius r and an effective hardening depth t that is a distance between said surface and a region with a Vickers hardness of 392, a ratio t/r of said steel member is 0.4 or more,
 said method comprising subjecting a steel material to induction hardening,   wherein said steel material comprises, in percent by mass, 0.47% to 0.52% of C, 0.03% to 0.15% of Si, 0.6% to 0.7% of Mn, 0.005% to 0.03% of S, 0.025% to 0.04% of Ti, 0.05% to 0.3% of Cr, 0.04% to 0.09% of Mo, 0.02% to 0.04% of Al, and 0.0005% to 0.004% of B, the balance being iron and inevitable impurities, and has an occupied area ratio by ferrite of 30% or more and a B-scale Rockwell hardness of 85 to 95.   
   
   
       5 . A method according to  claim 4 , wherein said steel material is formed into a drive shaft and then subjected to said induction hardening.