Steel for nitrocarburizing use, steel product for nitrocarburizing use and crankshaft

Abstract

A steel for nitrocarburizing use, which comprises by mass percent, C: more than 0.45% to not more than 0.60%, Si<0.50%, Mn: more than 1.30% to not more than 1.70%, P≦0.05%, S: 0.02 to 0.10%, Cr≦0.30% and N: more than 0.007% to not more than 0.030%, and which further contains one or two elements selected from Al: more than 0.010% to not more than 0.10% and Ti: more than 0.005% to not more than 0.035%, with Al+Ti being 0.015% or more, with the balance being Fe and impurities, wherein V among the impurities is not more than 0.010%, and further satisfies the following 2 formulas, has high fatigue strength and excellent straightenability after the nitrocarburizing treatment, without performing the expensive heat treatment of quenching and tempering. Consequently, they are suitable as raw materials for nitrocarburized components: fn 1=1.25C+Mn−0.1Cr, fn 2=N−0.45Al−( 1/22)Ti.

1 . A steel for nitrocarburizing use, which comprises by mass percent, C: more than 0.45% to not more than 0.60%, Si: less than 0.50%, Mn: more than 1.30% to not more than 1.70%, P: not more than 0.05%, S: 0.02 to 0.10%, Cr: not more than 0.30% and N: more than 0.007% to not more than 0.030%, and which further contains one or two elements selected from Al: more than 0.010% to not more than 0.10% and Ti: more than 0.005% to not more than 0.035%, with Al+Ti being 0.015% or more, with the balance being Fe and impurities, wherein V among the impurities is not more than 0.010%, “fn1” expressed by the following formula (1) is not less than 1.90, and “fn2” expressed by the following formula (2) is more than 0:
     fn 1=1.25C+Mn−0.1Cr  (1),       fn 2=N−0.45Al−( 1/22)Ti  (2);   
     wherein each element symbol in the formulas (1) and (2) represents the content by mass percent of the element concerned. 
   
   
       2 . The steel for nitrocarburizing use according to  claim 1 , which further contains by mass percent, Ca: not more than 0.005%. 
   
   
       3 . The steel for nitrocarburizing use according to  claim 1 , which further contains by mass percent, one or two elements selected from Cu: not more than 0.3% and Ni: not more than 0.2%. 
   
   
       4 . A steel product for nitrocarburizing use, which has a chemical composition by mass percent, C: more than 0.45% to not more than 0.60%, Si: less than 0.50%, Mn: more than 1.30% to not more than 1.70%, P: not more than 0.05%, S: 0.02 to 0.10%, Cr: not more than 0.30% and N: more than 0.007% to not more than 0.030%, and which further contains one or two elements selected from Al: more than 0.010% to not more than 0.10% and Ti: more than 0.005% to not more than 0.035%, with Al+Ti being 0.015% or more, with the balance being Fe and impurities, wherein V among the impurities is not more than 0.010%, “fn1” expressed by the following formula (1) is not less than 1.90, and “fn2” expressed by the following formula (2) is more than 0; and has a microstructure which comprise ferrite-pearlite phase with a ratio of the ferrite of 10% or less:
     fn 1=1.25C+Mn−0.1Cr  (1),       fn 2=N−0.45Al−( 1/22)Ti  (2);   
     wherein each element symbol in the formulas (1) and (2) represents the content by mass percent of the element concerned. 
   
   
       5 . The steel product for nitrocarburizing use according to  claim 4 , whose chemical composition further contains by mass percent, Ca: not more than 0.005%. 
   
   
       6 . The steel product for nitrocarburizing use according to  claim 4 , whose chemical composition further contains by mass percent, one or two elements selected from Cu: not more than 0.3% and Ni: not more than 0.2%. 
   
   
       7 . A crankshaft manufactured by using the steel product for nitrocarburizing use according to  claim 4 . 
   
   
       8 . A crankshaft manufactured by using the steel product for nitrocarburizing use according to  claim 5 . 
   
   
       9 . A crankshaft manufactured by using the steel product for nitrocarburizing use according to  claim 6 . 
   
   
       10 . The steel for nitrocarburizing use according to  claim 2 , which further contains by mass percent, one or two elements selected from Cu: not more than 0.3% and Ni: not more than 0.2%. 
   
   
       11 . The steel product for nitrocarburizing use according to  claim 5 , whose chemical composition further contains by mass percent, one or two elements selected from Cu: not more than 0.3% and Ni: not more than 0.2%.