Steel parts for machine structure, material therefor, and method for manufacture thereof

Abstract

The present invention provides a machine structural part composed of 0.3% to 0.7% of C, 0.30% or less of Si, 0.2% to 2.0% of Mn, 0.005% to 0.25% of Al, 0.005% to 0.1% of Ti, 0.05% to 0.6% of Mo, 0.0003% to 0.006% of B, 0.06% or less of S, 0.020% or less of P, 0.0030% or less of O on a mass basis, and the balance being Fe and unavoidable impurities. In addition, the machine structural part has a hard layer on at least a part of a surface thereof formed by induction quenching, an average prior austenite grain diameter of the hard layer is 7 μm or less, and the maximum diameter of a non-metal inclusion composed of oxides is 15 μm or less. Since having a high fatigue strength, this machine structural part is suitable for an automobile drive shaft, constant velocity joint, and the like.

1 . A machine structural part comprising 0.3% to 0.7% of C, 0.30% or less of Si, 0.2% to 2.0% of Mn, 0.005% to 0.25% of Al, 0.005% to 0.1% of Ti, 0.05% to 0.6% of Mo, 0.0003% to 0.006% of B, 0.06% or less of S, 0.020% or less of P, 0.0030% or less of O on a mass basis, and the balance being Fe and unavoidable impurities, wherein the machine structural part has a hard layer on at least a part of a surface thereof formed by induction quenching, an average prior austenite grain diameter of the hard layer is 7 μm or less, and the maximum diameter of a non-metal inclusion composed of oxides is 15 μm or less.  
   
   
       2 . The machine structural part according to  claim 1 , further comprising at least one element selected from the group consisting of 2.5% or less of Cr, 1.0% or less of Cu, 3.5% or less of Ni, 1.0% or less of Co, 0.1% or less of Nb, 0.5% or less of V, 0.5% or less of Ta, 0.5% or less of Hf, and 0.015% or less of Sb on a mass basis.  
   
   
       3 . The machine structural part according to  claim 1 , further comprising at least one element selected from the group consisting of 1.0% or less of W, 0.005% or less of Ca, 0.005% or less of Mg, 0.005% or less of Te, 0.1% or less of Se, 0.5% or less of Bi, 0.5% or less of Pb, 0.01% or less of Zr, and 0.1% or less of REM on a mass basis.  
   
   
       4 . The machine structural part according to  claim 2 , further comprising at least one element selected from the group consisting of 1.0% or less of W, 0.005% or less of Ca, 0.005% or less of Mg, 0.005% or less of Te, 0.1% or less of Se, 0.5% or less of Bi, 0.5% or less of Pb, 0.01% or less of Zr, and 0.1% or less of REM on a mass basis.  
   
   
       5 . The machine structural part according to  claim 1 , wherein the number of precipitates containing Mo is 500 or more per 1 μm 3 , and the average grain diameter thereof is 20 nm or less.  
   
   
       6 . The machine structural part according to  claim 2 , wherein the number of precipitates containing Mo is 500 or more per 1 μm 3 , and the average grain diameter thereof is 20 nm or less.  
   
   
       7 . The machine structural part according to  claim 3 , wherein the number of precipitates containing Mo is 500 or more per 1 μm 3 , and the average grain diameter thereof is 20 nm or less.  
   
   
       8 . The machine structural part according to  claim 4 , wherein the number of precipitates containing Mo is 500 or more per 1 μm 3 , and the average grain diameter thereof is 20 nm or less.  
   
   
       9 . A raw material for a machine structural part, comprising 0.3% to 0.7% of C, 0.30% or less of Si, 0.2% to 2.0% of Mn, 0.005% to 0.25% of Al, 0.005% to 0.1% of Ti, 0.05% to 0.6% of Mo, 0.0003% to 0.006% of B, 0.06% or less of S, 0.020% or less of P, 0.0030% or less of O on a mass basis, and the balance being Fe and unavoidable impurities, wherein the raw material has a bainite microstructure and a martensite microstructure, the total volume fraction of the bainite microstructure and the martensite microstructure is 10% or more, the maximum diameter of a non-metal inclusion composed of oxides is 15 μm or less, and an average prior austenite grain diameter of a hard layer formed by induction quenching is 7 μm or less.  
   
   
       10 . The raw material for a machine structural part according to  claim 9 , further comprising at least one element selected from the group consisting of 2.5% or less of Cr, 1.0% or less of Cu, 3.5% or less of Ni, 1.0% or less of Co, 0.1% or less of Nb, 0.5% or less of V, 0.5% or less of Ta, 0.5% or less of Hf, and 0.015% or less of Sb on a mass basis.  
   
   
       11 . The raw material for a machine structural part according to  claim 9 , further comprising at least one element selected from the group consisting of 1.0% or less of W, 0.005% or less of Ca, 0.005% or less of Mg, 0.005% or less of Te, 0.1% or less of Se, 0.5% or less of Bi, 0.5% or less of Pb, 0.01% or less of Zr, and 0.1% or less of REM on a mass basis.  
   
   
       12 . The raw material for a machine structural part according to  claim 10 , further comprising at least one element selected from the group consisting of 1.0% or less of W, 0.005% or less of Ca, 0.005% or less of Mg, 0.005% or less of Te, 0.1% or less of Se, 0.5% or less of Bi, 0.5% or less of Pb, 0.01% or less of Zr, and 0.1% or less of REM on a mass basis.  
   
   
       13 . The raw material for a machine structural part according to  claim 9 , wherein the number of precipitates containing Mo is 500 or more per 1 μm 3 , and the average grain diameter thereof is 20 nm or less.  
   
   
       14 . The raw material for a machine structural part according to  claim 10 , wherein the number of precipitates containing Mo is 500 or more per 1 μm 3 , and the average grain diameter thereof is 20 nm or less.  
   
   
       15 . The raw material for a machine structural part according to  claim 11 , wherein the number of precipitates containing Mo is 500 or more per 1 μm 3 , and the average grain diameter thereof is 20 nm or less.  
   
   
       16 . The raw material for a machine structural part according to  claim 12 , wherein the number of precipitates containing Mo is 500 or more per 1 μm 3 , and the average grain diameter thereof is 20 nm or less.  
   
   
       17 . The raw material for a machine structural part according to  claim 9 , wherein the raw material is obtained through a process having a total reduction rate of 80% or more performed at a temperature of 800° C. to 1,000° C., followed by cooling at a cooling rate of 0.2° C./sec or more in a temperature region of 500° C. to 700° C.  
   
   
       18 . The raw material for a machine structural part according to  claim 17 , wherein the raw material is obtained through second working having a rate of 20% or more performed in cooling in a region of 700° C. to less than 800° C., after cooling, or at the Ar1 transformation point or less after cooling.  
   
   
       19 . A method for manufacturing a machine structural part, comprising a step of heating the raw material for a machine structural part, according to  claim 9 , to 800° C. to 1,000° C. at least one time by induction quenching at a heating rate of 300° C./sec or more in a temperature region of 600° C. to 800° C.  
   
   
       20 . The method for manufacturing a machine structural part, according to  claim 19 , wherein the heating is performed at a heating rate of 700° C./sec or more.  
   
   
       21 . The method for manufacturing a machine structural part, according to  claim 19 , wherein a residence time of the raw material in a temperature region of 800° C. or more is set to 5 seconds or less.  
   
   
       22 . The method for manufacturing a machine structural part, according to  claim 20 , wherein a residence time of the raw material in a temperature region of 800° C. or more is set to 5 seconds or less.