Rolling elements

Abstract

The pitting resistance of a gear is increased by hardening its tooth flanks through application of carburizing/quenching, bright hardening and induction hardening to a steel material capable of providing significantly improved softening resistance in tempering at a low temperature of 300 to 350° C. To this end, the steel material prepared so as to satisfy the relationship described by: 5≦4.3×Si (wt %)+7.3×Al (wt %)+3.1×V (wt %)+1.5×Mo (wt %)+1.2×Cr (wt %)×(0.45÷C (wt %)) is carburized such that the carbon concentration of its carburized surface layer is adjusted to 0.6 to 0.9 wt %; and the steel material is subjected to quenching and tempering at 300° C. or less subsequently to the carburization process, or alternatively the steel material is once cooled after the carburization process and then subjected to treatments of re-heating hardening and tempering at 300° C. or less so that a hardness of HRC 58 or more is ensured by the tempering process at 300° C.

What is claimed is:  
     
         1 . A rolling element which is made from a steel material containing at least 0.15 to 0.35 wt % C; further containing either 1.0 to 3.0 wt % Si or 0.35 to 1.5 wt % Al or alternatively, 0.5 to 3.0 wt % (Si+Al); and further containing one or more alloy elements selected from the group consisting of Mn, Ni, Cr, Mo, V, Cu, W, Ti, Nb, B, Zr, Ta, Hf, and Ca, unavoidable impurities such as P, S, N and O, and balance essentially consisting of Fe; said steel material being prepared so as to satisfy the relationship described by:  
       5≦4.3×Si (wt %)+7.3×Al (wt %)+3.1×V (wt %)+1.5×Mo (wt %)+1.2×Cr (wt %)×(0.45÷C (wt %)),  
       and 
 which is formed by carburizing said steel material such that the carbon concentration of a carburized surface layer of the steel material is adjusted to 0.6 to 0.9 wt %; quenching the steel material subsequently to the carburization process and then tempering the steel material at 300° C. or less, or alternatively cooling the steel material once after the carburization process and then applying treatments of re-heating hardening and tempering at 300° C. or less to the steel material so that a hardness of HRC 58 or more is ensured by the tempering process at 300° C.  
 
     
     
         2 . The rolling element according to  claim 1 , wherein, in said steel material, the amount of Cr is limited to no more than 1.4 times the amount of Si, and one or more elements selected from the group consisting of 0.35 wt % or less Mo, 0.4 wt % or less V, 1.0 to 2.5 wt % (Mn+Ni) are added in an amount which satisfies the relationship described by:  
       −0.146×Si (wt %)+0.03×Mn (wt %)−0.024×Ni (wt %)+0.075×Cr (wt %)+0.043×Mo (wt %)+0.133×V (wt %)≦0.  
     
     
         3 . The rolling element according to  claim 1  or  2 , wherein said steel material contains 1.5 to 2.5 wt % Si or (Si+Al) and less than 2.0 wt % Cr to prevent precipitation of cementite during the carburization process.  
     
     
         4 . The rolling element according to  claim 1  or  2 , wherein said steel material having an Al content of 0.3 wt % or more contains 0.3 to 1.5 wt % Ni.  
     
     
         5 . A rolling element which is made from a steel material containing at least 0.15 to 0.35 wt % C; further containing either 1.0 to 3.0 wt % Si or 0.35 to 1.5 wt % Al or alternatively, 0.5 to 3.0 wt % (Si+Al); and further containing one or more alloy elements selected from the group consisting of Mn, Ni, Cr, Mo, V, Cu, W, Ti, Nb, B, Zr, Ta, Hf., and Ca, unavoidable impurities such as P, S, N and O, and balance essentially consisting of Fe; said steel material being prepared so as to satisfy the relationship described by:  
       5≦4.3×Si (wt %)+7.3×Al (wt %)+3.1×V (wt %)+1.5×Mo (wt %)+1.2×Cr (wt %)×(0.45÷C (wt %)),  
       and 
 which is formed by carburizing said steel material such that the carbon concentration of a carburized surface layer of the steel material is adjusted to 0.9 to 1.5 wt %; applying treatments of reheating hardening and tempering at 300° C. or less to the steel material after cooling to a temperature equal to or lower than the Al temperature from a state where no cementite precipitates in the surface layer during the carburization process, so that fine cementite particles having a size of 1 μm or less are dispersed within the tempered martensitic phase of the carburized surface layer and a hardness of HRC 62 or more is ensured by the tempering process at 300° C.  
 
     
     
         6 . The rolling element according to  claim 5 , wherein, in said steel material, the amount of Cr is limited to no more than 1.4 times the amount of Si, and one or more elements selected from the group consisting of 0.35 wt % or less Mo, 0.4 wt % or less V, 1.0 to 2.5 wt % (Mn+Ni) are added in an amount which satisfies the relationship described by:  
       −0.146×Si (wt %)+0.03×Mn (wt %)−0.024×Ni (wt %)+0.075×Cr (wt %)+0.043×Mo (wt %)+0.133×V (wt %)≦0.  
     
     
         7 . The rolling element according to  claim 5  or  6 , wherein said steel material contains 1.5 to 2.5 wt % Si or (Si+Al) and less than 2.0 wt % Cr to prevent precipitation of cementite during the carburization process.  
     
     
         8 . The rolling element according to  claim 5  or  6 , wherein said steel material having an Al content of 0.3 wt % or more contains 0.3 to 1.5 wt % Ni.  
     
     
         9 . A rolling element which is made from a steel material containing at least 0.35 to 0.60 wt % C; further containing either 1.0 to 3.0 wt % Si or 0.35 to 1.5 wt % Al or alternatively, 0.5 to 3.0 wt % (Si+Al); and further containing one or more alloy elements selected from the group consisting of Mn, Ni, Cr, Mo, V, Cu, W, Ti, Nb, B, Zr, Ta, Hf, and Ca, unavoidable impurities such as P, S, N and O, and balance essentially consisting of Fe; said steel material being prepared so as to satisfy the relationship described by:  
       5≦4.3×Si (wt %)+7.3×Al (wt %)+3.1×V (wt %)+1.5×Mo (wt %)+1.2×Cr (wt %)×(0.45÷C (wt %)),  
       and 
 which is formed by tempering said steel material at 300° C. or less after quenching treatment such as induction hardening so that a hardness of HRC 55 or more is ensured for a hardened surface layer of the steel material by the tempering process at 300° C.  
 
     
     
         10 . The rolling element according to  claim 9 , wherein said steel material contains one or more elements selected from the group consisting of 0.3 to 1.5 wt % Mn, 0.5 wt % or less Cr, 0.35 wt % or less Mo, 0.4 wt % or less V and 0.0005 to 0.005 wt % B.  
     
     
         11 . The rolling element according to  claim 9  or  10 , wherein said steel material having an Al content of 0.3 wt % or more contains 0.3 to 1.5 wt % Ni.  
     
     
         12 . A rolling element which is made from a steel material containing at least 0.35 to 0.60 wt % C; further containing either 0.8 to 3.0 wt % Si or 0.5 to 1.5 wt % Al or alternatively, 0.8 to 3.0 wt % (Si+Al); and further containing one or more alloy elements selected from the group consisting of Mn, Ni, Cr, Mo, V, Cu, W, Ti, Nb, B, Zr, Ta, Hf, and Ca, unavoidable impurities such as P, S, N and O, and balance essentially consisting of Fe; said steel material being prepared so as to satisfy the relationship described by:  
       5≦4.3×Si (wt %)+7.3×Al (wt %)+3.1×V (wt %)+1.5×Mo (wt %)+1.2×Cr (wt %)×(0.45÷C (wt %)),  
       and 
 which is formed by carburizing said steel material such that the carbon concentration of a carburized surface layer of the steel material is adjusted to 0.9 to 1.5 wt %; applying treatments of reheating hardening and tempering at 300° C. or less to the steel material after cooling to a temperature equal to or lower than the A1 transformation temperature subsequently to the carburization process, such that fine cementite particles having a size of 1 μm or less are dispersed within the martensitic phase of the carburized surface layer and a hardness of HRC 62 or more is ensured by the tempering process at 300° C.  
 
     
     
         13 . The rolling element according to  claim 12 , wherein, in said steel material, the amount of Cr is limited to no more than 1.4 times the amount of Si, and one or more elements selected from the group consisting of 0.35 wt % or less Mo, 0.4 wt % or less V, 1.0 to 2.5 wt % (Mn+Ni) are added in an amount which satisfies the relationship described by:  
       −0.146×Si (wt %)+0.03×Mn (wt %)−0.024×Ni (wt %)+0.075×Cr (wt %)+0.043×Mo (wt %)+0.133×V (wt %)≦0.  
     
     
         14 . The rolling element according to  claim 12  or  13 , wherein the steel material contains one or more elements selected from the group consisting of 0.3 to 1.5 wt % Mn, 0.5 wt % or less Cr, 0.35 wt % or less Mo, 0.4 wt % or less V and 0.0005 to 0.005 wt % B.  
     
     
         15 . The rolling element according to  claim 12  or  13 , wherein said steel material having an Al content of 0.3 wt % or more contains 0.3 to 1.5 wt % Ni.  
     
     
         16 . A rolling element which is made from a steel material containing at least 0.60 to 1.50 wt % C; further containing either 1.0 to 3.0 wt % Si or 0.35 to 1.5 wt % Al or alternatively, 0.5 to 3.0 wt % (Si+Al); and further containing one or more alloy elements selected from the group consisting of Mn, Ni, Cr, Mo, V, Cu, W, Ti, Nb, B, Zr, Ta, Hf, and Ca, unavoidable impurities such as P, S, N and O, and balance essentially consisting of Fe; said steel material being prepared so as to satisfy the relationship described by:  
       5≦4.3×Si (wt %)+7.3×Al (wt %)+3.1×V (wt %)+1.5×Mo (wt %)+1.2×Cr (wt %)×(0.45÷C (wt %)),  
       and 
 which is formed by tempering said steel material at 300° C. or less after quenching treatment such as induction hardening so that a hardness of HRC 58 or more is ensured for a hardened surface layer of the steel material by the tempering process at 300° C.  
 
     
     
         17 . The rolling element according to  claim 16 , wherein said steel material contains one or more elements selected from the group consisting of 0.3 to 1.5 wt % Mn, 0.5 wt % or less Cr, 0.35 wt % or less Mo, 0.4 wt % or less V and 0.0005 to 0.005 wt % B.  
     
     
         18 . The rolling element according to  claim 16  or  17 , wherein said steel material having an Al content of 0.3 wt % or more contains 0.3 to 1.5 wt % Ni.