Steel for machine structure use for surface hardening and steel part for machine structure use

Abstract

Steel for machine structure use for surface hardening containing, by mass %, C: 0.3 to 0.6%, Si: 0.02 to 2.0%, Mn: 1.5% to 3.0%, W: 0.0025 to 0.5%, Al: 0.001 to 0.5%, N: 0.003 to 0.02%, S: 0.0001 to 0.025%, P: 0.0001 to 0.03%, and O: 0.0001 to 0.0050%, having an Mn/S of 70 to 30000, and having a balance of substantially Fe and unavoidable impurities.

1 . Steel for machine structure use for surface hardening containing, by mass %,
 C: 0.3 to 0.6%,   Si: 0.02 to 2.0%,   Mn: 1.5% to 3.0%,   W: 0.0025 to 0.5%,   Al: 0.001 to 0.5%,   N: 0.003 to 0.02%,   S: 0.0001 to 0.025%,   P: 0.0001 to 0.03%, and   O: 0.0001 to 0.005%,   having an Mn/S of 70 to 30000, and   having a balance of substantially Fe and unavoidable impurities.   
     
     
         2 . Steel for machine structure use for surface hardening as set forth in  claim 1 , characterized by further containing, by mass %, one or more of
 Cr: 0.01 to 2.0%,   Mo: 0.01 to 1.0%, and   V: 0.01 to 1.0%.   
     
     
         3 . Steel for machine structure use for surface hardening as set forth in  claim 1 , characterized by further containing, by mass %,
 B: 0.0005 to 0.005%.   
     
     
         4 . Steel for machine structure use for surface hardening as set forth in  claim 1 , characterized by further containing, by mass %, one or more of
 Nb: 0.005 to 0.3%,   Ti: 0.005 to 0.2%,   Ni: 0.05 to 2.0%, and   Cu: 0.01 to 2.0%.   
     
     
         5 . Steel for machine structure use for surface hardening as set forth in  claim 1 , characterized by further containing, by mass %, one or more of
 Ca: 0.0005 to 0.01%,   Mg: 0.0005 to 0.01%,   Zr: 0.0005 to 0.05%, and   Te: 0.0005 to 0.1%.   
     
     
         6 . Steel for machine structure use for surface hardening as set forth in  claim 1 , characterized in that said steel for machine structure use for surface hardening is steel which is nitrided, then induction hardened. 
     
     
         7 . Steel for machine structure use for surface hardening as set forth in  claim 6  characterized in that said nitriding is soft nitriding. 
     
     
         8 . A steel part for machine structure use obtained by machining steel for machine structure use for surface hardening as as set forth in  claim 1 , nitriding it, then induction hardening it, said steel part for machine structure use characterized in that the surface layer from the surface down to a depth of 0.4 mm or more is a nitrided layer and the hardness of the nitrided layer from the surface down to a depth of 0.2 mm is a Vicker's hardness at the time of tempering at 300° C. of 650 or more. 
     
     
         9 . A steel part for machine structure use as set forth in  claim 8 , characterized in that said nitriding is soft nitriding. 
     
     
         10 . A steel part for machine structure use as set forth in  claim 8 , characterized in that said nitrided layer from the surface down to a depth of 5 μm or more includes pores of a circle equivalent diameter of 0.1 to 1 μm in an amount of 10000/mm 2  or more.