Steel part having long rolling contact fatigue life and method for producing the same

Abstract

A steel part having a long rolling contact fatigue life and capable of further increasing the life of a bearing under severer using condition than usual conditions. The steel part includes steel having a composition containing 0.7% by mass to 1.1% by mass of C, 0.5% by mass to 2.0% by mass of Si, 0.4% by mass to 2.5% by mass of Mn, 1.6% by mass to 5.0% by mass of Cr, 0.1% by mass to less than 0.5% by mass of Mo, 0.010% by mass to 0.050% by mass of Al, less than 0.0015% by mass of Sb as an impurity, and the balance composed of Fe and inevitable impurities, the steel being hardened and tempered. In the steel structure of a portion from the surface to a depth of 5 mm, residual cementite has a grain diameter of 0.05 to 1.5 μm, prior austenite has a grain diameter of 30 μm or less, and the ratio by volume of the residual austenite is less than 25%.

1 . A steel part having a long rolling contact fatigue life, comprising steel having a composition containing:
 C: about 0.7% by mass to about 1.1% by mass;   Si: about 0.5% by mass to about 2.0% by mass;   Mn: about 0.4% by mass to about 2.5% by mass;   Cr: about 1.6% by mass to about 5.0% by mass;   Mo: about 0.1% by mass to less than about 0.5% by mass;   Al: about 0.010% by mass to about 0.050% by mass;   less than about 0.0015% by mass of Sb as an impurity, and the balance composed of Fe and inevitable impurities, the steel being hardened and tempered, wherein a portion from the surface to a depth of about 5 mm has a steel structure in which residual cementite has a grain diameter of about 0.05 to about 1.5 μm, prior austenite has a grain diameter of about 30 μm or less, and the ratio by volume of the residual austenite is less than about 25%.   
   
   
       2 . The steel part according to  claim 1 , wherein the steel further comprises at least one selected from the following:
 Ni: about 0.5% by mass to about 2.0% by mass;   V: about 0.05% by mass to about 1.00% by mass; and   Nb: about 0.005% by mass to about 0.50% by mass.   
   
   
       3 . A method for producing a steel part having a long rolling contact fatigue life, comprising:
 hot-working steel,   spheroidizing annealing the steel by maintaining the steel at about 800° C. to about 850° C. for about 5 hours or more,   cooling the steel to about 700° C. or less at a rate of about 0.01° C./s or less, and   hardening and tempering the steel, wherein the steel has a composition comprising:
 C: about 0.7% by mass to about 1.1% by mass; 
 Si: about 0.5% by mass to about 2.0% by mass; 
 Mn: about 0.4% by mass to about 2.5% by mass; 
 Cr: about 1.6% by mass to about 5.0% by mass; 
 Mo: about 0.1% by mass to less than about 0.5% by mass; 
 Al: about 0.010% by mass to 0.050% by mass; 
    less than about 0.0015% by mass of Sb as an impurity, and the balance composed of Fe and inevitable impurities.   
   
   
       4 . A method for producing a steel part having a long rolling contact fatigue life, comprising:
 hot-working steel,   cooling the steel to about 200° C. at a cooling rate of about 0.5° C./s or less,   spheroidizing annealing the steel by maintaining at about 750° C. to about 850° C.,   cooling the steel to about 700° C. or less at a rate of about 0.015° C./s or less, and   hardening and tempering the steel, wherein the steel has a composition comprising:
 C: about 0.7% by mass to about 1.1% by mass; 
 Si: about 0.5% by mass to about 2.0% by mass; 
 Mn: about 0.4% by mass to about 2.5% by mass; 
 Cr: about 1.6% by mass to about 5.0% by mass; 
 Mo: about 0.1% by mass to less than about 0.5% by mass; 
 Al: about 0.010% by mass to about 0.050% by mass; 
    less than about 0.0015% by mass of Sb as an impurity, and the balance composed of Fe and inevitable impurities.   
   
   
       5 . The method according to  claim 3 , wherein the steel further comprises at least one selected from the following:
 Ni: about 0.5% by mass to about 2.0% by mass;   V: about 0.05% by mass to about 1.00% by mass; and   Nb: about 0.005% by mass to about 0.50% by mass.   
   
   
       6 . The method according to  claim 4 , wherein the steel further comprises at least one selected from the following:
 Ni: about 0.5% by mass to about 2.0% by mass;   V: about 0.05% by mass to about 1.00% by mass; and   Nb: about 0.005% by mass to about 0.50% by mass.