Method for manufacturing ingot steel for bearings and bearing steel

Abstract

An ingot steel for bearings has a composition containing C: 0.56% by mass or more and 0.70% by mass or less, Si: 0.15% by mass or more and less than 0.50% by mass, Mn: 0.60% by mass or more and 1.50% by mass or less, Cr: 0.50% by mass or more and 1.10% by mass or less, P: 0.025% by mass or less, S: 0.025% by mass or less, Al: 0.005% by mass or more and 0.500% by mass or less, O: 0.0015% by mass or less, N: 0.0030% by mass or more and 0.015% by mass or less, and a remainder of Fe and incidental impurities, wherein eutectic carbide formation index Ec satisfies 0<Ec≦0.25.

1 . An ingot steel for bearings, the ingot steel having a composition comprising:
 C: 0.56% by mass or more and 0.70% by mass or less,   Si: 0.15% by mass or more and less than 0.50% by mass,   Mn: 0.60% by mass or more and 1.50% by mass or less,   Cr: 0.50% by mass or more and 1.10% by mass or less,   P: 0.025% by mass or less,   S: 0.025% by mass or less,   Al: 0.005% by mass or more and 0.500% by mass or less,   O: 0.0015% by mass or less,   N: 0.0030% by mass or more and 0.015% by mass or less, and   a remainder of Fe and incidental impurities,   wherein eutectic carbide formation index Ec defined by the formula (I) satisfies
   0 <Ec≦ 0.25, 
   wherein 
     Ec =(−0.07×[% Si]−0.03×[% Mn]+0.04×[% Cr]−0.36×[% Al]+0.79)−[% C]  (1)
 
   wherein [ ] indicates the amount of component described in parentheses (% by mass).   
     
     
         2 . The ingot steel according to  claim 1 , wherein the composition further comprises one or more selected from the group consisting of:
 Cu: 0.005% by mass or more and 0.5% by mass or less,   Ni: 0.005% by mass or more and 1.00% by mass or less, and   Mo: 0.01% by mass or more and 0.5% by mass or less.   
     
     
         3 . The ingot steel according to  claim 1 , wherein the composition further comprises one or more selected from the group consisting of:
 W: 0.001% by mass or more and 0.5% by mass or less,   Nb: 0.001% by mass or more and 0.1% by mass or less,   Ti: 0.001% by mass or more and 0.1% by mass or less,   Zr: 0.001% by mass or more and 0.1% by mass or less, and   V: 0.002% by mass or more and 0.5% by mass or less.   
     
     
         4 . The ingot steel according to  claim 1 , wherein the composition further comprises B: 0.0002% by mass or more and 0.005% by mass or less. 
     
     
         5 . A method for manufacturing a bearing steel, comprising:
 casting a steel ingot into an ingot steel by ingot casting;   heating the ingot steel to a temperature of 1000° C. to 1350° C.; and   hot forging the ingot steel at a forging ratio of 2.0 or more to produce a steel bar having a square section with a short side length of 400 mm or more or a circular section having a diameter of 400 mm or more,   wherein the steel ingot has a composition containing:   C: 0.56% by mass or more and 0.70% by mass or less,   Si: 0.15% by mass or more and less than 0.50% by mass,   Mn: 0.60% by mass or more and 1.50% by mass or less,   Cr: 0.50% by mass or more and 1.10% by mass or less,   P: 0.025% by mass or less,   S: 0.025% by mass or less,   Al: 0.005% by mass or more and 0.500% by mass or less,   O: 0.0015% by mass or less,   N: 0.0030% by mass or more and 0.015% by mass or less, and   a remainder of Fe and incidental impurities,   wherein eutectic carbide formation index Ec defined by the formula (I) satisfies
   0 <Ec≦ 0.25, 
   wherein 
     Ec =(−0.07×[% Si]−0.03×[% Mn]+0.04×[% Cr]−0.36×[% Al]+0.79)−[% C]  (1)
 
   wherein [ ] indicates the amount of component described in parentheses (% by mass).   
     
     
         6 . The method according to  claim 5 , wherein the composition further comprises one or more selected from the group consisting of:
 Cu: 0.005% by mass or more and 0.5% by mass or less,   Ni: 0.005% by mass or more and 1.00% by mass or less, and   Mo: 0.01% by mass or more and 0.5% by mass or less.   
     
     
         7 . The method according to  claim 5 , wherein the composition further comprises one or more selected from the group consisting of:
 W: 0.001% by mass or more and 0.5% by mass or less,   Nb: 0.001% by mass or more and 0.1% by mass or less,   Ti: 0.001% by mass or more and 0.1% by mass or less,   Zr: 0.001% by mass or more and 0.1% by mass or less, and   V: 0.002% by mass or more and 0.5% by mass or less.   
     
     
         8 . The method for manufacturing a bearing steel according to  claim 5 , wherein the composition further comprises B: 0.0002% by mass or more and 0.005% by mass or less. 
     
     
         9 . The ingot steel according to  claim 2 , wherein the composition further comprises one or more selected from the group consisting of:
 W: 0.001% by mass or more and 0.5% by mass or less,   Nb: 0.001% by mass or more and 0.1% by mass or less,   Ti: 0.001% by mass or more and 0.1% by mass or less,   Zr: 0.001% by mass or more and 0.1% by mass or less, and   V: 0.002% by mass or more and 0.5% by mass or less.   
     
     
         10 . The ingot steel according to  claim 2 , wherein the composition further comprises B: 0.0002% by mass or more and 0.005% by mass or less. 
     
     
         11 . The ingot steel according to  claim 3 , wherein the composition further comprises B: 0.0002% by mass or more and 0.005% by mass or less. 
     
     
         12 . The method according to  claim 6 , wherein the composition further comprises one or more selected from the group consisting of:
 W: 0.001% by mass or more and 0.5% by mass or less,   Nb: 0.001% by mass or more and 0.1% by mass or less,   Ti: 0.001% by mass or more and 0.1% by mass or less,   Zr: 0.001% by mass or more and 0.1% by mass or less, and   V: 0.002% by mass or more and 0.5% by mass or less.   
     
     
         13 . The method for manufacturing a bearing steel according to  claim 6 , wherein the composition further comprises B: 0.0002% by mass or more and 0.005% by mass or less. 
     
     
         14 . The method for manufacturing a bearing steel according to  claim 7 , wherein the composition further comprises B: 0.0002% by mass or more and 0.005% by mass or less.