Steel for machine structural purposes for surface hardening use and steel parts for machine structural purposes and method of production of same

Abstract

A steel for surface hardening for machine structural use which is characterized by containing, in terms of mass %, 0.30-0.60% C, 0.02-2.0% Si, 0.35-1.5% Mn, 0.001-0.5% Al, 0.05-2.0% Cr, 0.001-1.0% Sn, 0.0001-0.021% S, 0.0030-0.0055% N, 0.01-2.0% Ni, 0.01-2.0% Cu, up to 0.030% P, and up to 0.005% O, with the remainder comprising Fe and incidental impurities, the contents of Sn, Cu, Ni, Mn, and S satisfying relationships (1) and (2). −0.19≦0.12×Sn+Cu−0.1×Ni≦0.15  (1) 60≦Mn/S≦300  (2)

1 - 9 . (canceled) 
     
     
         10 . Steel for machine structural purposes for surface hardening use characterized by containing, by mass %,
 C: 0.30 to 0.60%,   Si: 0.02 to 2.0%,   Mn: 0.35 to 1.5%,   Al: 0.001 to 0.5%,   Cr: 0.05 to 2.0%,   Sn: 0.001 to 1.0%,   S: 0.0001 to 0.021%,   N: 0.0030 to 0.0055%,   Ni: 0.01 to 2.0%,   Cu: 0.01 to 2.0%,   P: 0.030% or less, and   O: 0.005% or less,   having a balance of Fe and unavoidable impurities,   having contents of Sn, Cu, Ni, Mn, and S satisfying the following formulas (1) and (2), and   having contents of Sn and Cr satisfying the following formula (3);
   −0.19≦0.12×Sn+Cu−0.1×Ni≦0.15  (1)
 
   60≦Mn/S≦300  (2)
 
   Sn≧0.2×Cr  (3).
 
   
     
     
         11 . Steel for machine structural purposes for surface hardening use as set forth in  claim 10  characterized in that said steel further contains, by mass %, one or more of
 B: 0.0003 to 0.005%, 
 W: 0.0025 to 0.5%, 
 Mo: 0.05 to 1.0%, 
 V: 0.05 to 1.0%, 
 Nb: 0.005 to 0.3%, 
 Ti: 0.005 to 0.2%, 
 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%. 
 
     
     
         12 . A steel part for machine structural purposes which is produced by nitriding or ferritic nitrocarborizing then induction hardening steel for machine structural purposes for surface hardening use as set forth in  claim 10 , said steel part characterized in that
 (a) a surface layer of a depth of 0.4 mm or more from the surface is a nitrided layer and   (b) a Vicker's hardness is 650 or more at the surface layer of a depth of 0.2 mm from the surface when treating the steel by tempering at 300° C.   
     
     
         13 . A steel part for machine structural purposes as set forth in  claim 12   
       characterized in that at a layer inside said nitrided layer at a depth of 5 μm or more from the surface, there are 5000/mm 2  or more of pores of a circle equivalent diameter of 0.1 to 1 μm. 
     
     
         14 . A steel part for machine structural purposes as set forth in  claim 12  characterized in that a layer which corresponds to the compound layer which is formed by nitriding or ferritic nitrocarborizing at the surface side in said nitrided layer is removed by machining. 
     
     
         15 . A method of production of a steel part for machine structural purposes characterized by forming steel for machine structural purposes for surface hardening use as set forth in  claim 10  into a predetermined shape by forging or/and cutting, then treating the shaped steel member by nitriding or ferritic nitrocarborizing at 550 to 650° C. for 0.5 to 5 hours, then treating it by induction heating at 900 to 1100° C. for 0.05 to 5 seconds for quenching, and (a) forming a nitrided layer at a surface layer of a depth of 0.2 mm or more from the surface and (b) making a Vicker's hardness 650 or more at the surface layer of a depth of 0.2 mm from the surface when tempering at 300° C. 
     
     
         16 . A method of production of a steel part for machine structural purposes as set forth in  claim 15  characterized by performing said induction hardening, then grinding away the surface layer by an amount of grinding (mm) satisfying the following formula (4):
   0.05×{1−0.006×(600−nitriding or ferritic nitrocarborizing temperature(° C.))}×{(nitriding or ferritic nitrocarborizing time(hr)+1)/3≦}amount of grinding≦0.2 mm  (4).
 
 
     
     
         17 . A steel part for machine structural purposes which is produced by nitriding or ferritic nitrocarborizing then induction hardening steel for machine structural purposes for surface hardening use as set forth in  claim 11 , said steel part characterized in that
 (a) a surface layer of a depth of 0.4 mm or more from the surface is a nitrided layer and   (b) a Vicker's hardness is 650 or more at the surface layer of a depth of 0.2 mm from the surface when treating the steel by tempering at 300° C.   
     
     
         18 . A steel part for machine structural purposes as set forth in  claim 17   
       characterized in that at a layer inside said nitrided layer at a depth of 5 μm or more from the surface, there are 5000/mm 2  or more of pores of a circle equivalent diameter of 0.1 to 1 μm. 
     
     
         19 . A steel part for machine structural purposes as set forth in  claim 13  characterized in that a layer which corresponds to the compound layer which is formed by nitriding or ferritic nitrocarborizing at the surface side in said nitrided layer is removed by machining. 
     
     
         20 . A steel part for machine structural purposes as set forth in  claim 17  characterized in that a layer which corresponds to the compound layer which is formed by nitriding or ferritic nitrocarborizing at the surface side in said nitrided layer is removed by machining. 
     
     
         21 . A steel part for machine structural purposes as set forth in  claim 18  characterized in that a layer which corresponds to the compound layer which is formed by nitriding or ferritic nitrocarborizing at the surface side in said nitrided layer is removed by machining. 
     
     
         22 . A method of production of a steel part for machine structural purposes characterized by forming steel for machine structural purposes for surface hardening use as set forth in  claim 11  into a predetermined shape by forging or/and cutting, then treating the shaped steel member by nitriding or ferritic nitrocarborizing at 550 to 650° C. for 0.5 to 5 hours, then treating it by induction heating at 900 to 1100° C. for 0.05 to 5 seconds for quenching, and (a) forming a nitrided layer at a surface layer of a depth of 0.2 mm or more from the surface and (b) making a Vicker's hardness 650 or more at the surface layer of a depth of 0.2 mm from the surface when tempering at 300° C. 
     
     
         23 . A method of production of a steel part for machine structural purposes as set forth in  claim 22  characterized by performing said induction hardening, then grinding away the surface layer by an amount of grinding (mm) satisfying the following formula (4):
   0.05×{1−0.006×(600−nitriding or ferritic nitrocarborizing temperature(° C.))}×{(nitriding or ferritic nitrocarborizing time(hr)+1)/3≦}amount of grinding≦0.2 mm  (4).