Bainite type non-refined steel for nitriding, method for production thereof and nitrided product

Abstract

An object of the present invention is to provide an untempered free-machining steel to be nitrided, not containing Pb, capable of securing a predetermined strength and hardness and having machinability higher than steel containing Pb; and nitrided products. To be more specific, the present invention relates to a bainite-type untempered free-machining steel to be nitrided containing C: 0.05 to 0.8 wt %, Si: 0.01 to 2.5 wt %, Mn: 0.1 to 3.5 wt %, P: 0.001 to 0.2 wt %, S: 0.01 to 0.2 wt %, Cr: 1.0 to 3.5 wt %, V: 0.1 to 0.5 wt %, Al: 0.001 to 0.020 wt %, Ca: 0.0005 to 0.02 wt %, and O: 0.0005 to 0.01 wt % and the residue with an alloy composition of unavoidable impurities and Fe as its basic alloy composition, provided that a content of each of the Mn, the S and the Cr satisfy the formula: ([Mn]− 55 ×[S]/ 32 +[Cr])>2.0, in which an occupation area of a sulfide inclusion present in contact with an oxide inclusion containing 8 to 62 wt % of CaO and containing not less than 1.0 wt % of Ca is not less than 2.0×10 −4 mm 2 per visual field area of 3.5 mm 2 . The present invention also relates to a method of manufacturing the above bainite-type untempered free-machining steel to be nitrided and nitrided products manufactured by the said manufacturing method.

1 . A bainite-type untempered steel to be nitrided comprising alloy compositions C: 0.05 to 0.8 wt %, Si: 0.01 to 2.5 wt %, Mn: 0.1 to 3.5 wt %, P: 0.001 to 0.2 wt %, S: 0.01 to 0.2 wt %, Cr: 1.0 to 3.5 wt %, V: 0.1 to 0.5 wt %, Al: 0.001 to 0.020 wt %, Ca: 0.0005 to 0.02 wt % and O: 0.0005 to 0.01 wt %, and the residue composed of unavoidable impurities and Fe wherein an occupation area of a sulfide inclusion present in contact with an oxide inclusion containing 8 to 62 wt % of CaO and containing not less than 1.0 wt % of Ca is not less than 2.0×04 mm 2  per visual field area of 3.5 mm 2 , provided that each content of said Mn, said S, and said Cr satisfies the following formula:  
         ([Mn]−55×[S]/32+[Cr])>2.0  
       (where [indicates wt % (wt/wt) of each element).  
     
     
         2 . A bainite-type untempered steel to be nitrided claimed in  claim 1 , further comprising, in addition to the alloy components specified in  claim 1 , one or more element(s) selected from Mo: 2.0% or less, Cu: 2.0% or less, Ni: 4.0% or less and B: 0.0005 to 0.01%, wherein the formula:  
         ([Mn]−55×[S]/32+[Cr]+[Mo])>2.0  
       (where [ ] indicates wt % (wt/wt) of each element) is established when Mo is comprised.  
     
     
         3 . A bainite-type untempered steel to be nitrided claimed in  claim 1  or  2 , further comprising one or two element(s) selected from Nb: 0.2% or less and Ti: 0.2% or less.  
     
     
         4 . A bainite-type untempered steel to be nitrided claimed in  claim 1  or  2 , further comprising one or more element(s) selected from Ta: 0.5% or less, Zr: 0.5% or less, and Mg: 0.02% or less.  
     
     
         5 . A bainite-type untempered steel to be nitrided claimed in  claim 1  or  2 , further comprising one or more element(s) selected from Pb: 0.4% or less, Bi: 0.4% or less, Se: 0.4% or less, and Te: 0.2% or less.  
     
     
         6 . A method of manufacturing untempered steel claimed in  claim 1  or  2 , which comprises melting an alloy having alloy components specified in any one of claims  1  through  5  and the residue composed of unavoidable impurities and Fe, provided that each content of Mn, S, Cr and Mo satisfies the formula: ([Mn]−55×[S]/32+[Cr]+[Mo])>2.0 (where [ ] indicates wt % (wt/wt) of each element), wherein an operation satisfying the following conditions 1) through 3) is performed: 
 1) [ H S]/[ H O]: 8 to 80    where [ H S]=[S]× 10   logFs , and [ H O]=[O]×10 logFs        logFs=0.113[C]+0.065[Si]−0.025[Mn]+0.043[P]−0.028[S]−0.013[Cu]−0.011 [Cr]+0.0027[Mo]−0.27[O]+0.024[N]+0.054[Al] logFo=−0.44[C]−0.131 [Si]−0.02[Mn]−0.147[P]+0.133[S]−0.0095[Cu]+0.006[Ni]−0.041 [Cr]+0.0035[Mo]−1.00[O]−0.1834[N]−0.20 μl]+ 0.11[V]     2) [sol-Al]: 0.02 to 0.20% and    3) [Ca]×[S]: 1×10 −5  to 1×10 −3 .    
     
     
         7 . A nitrided product formed by shaping the bainite-type untempered steel claimed in  claim 1  or  2  into a configuration of a mechanical component part, followed by nitriding said shaped bainite-type untempered steel.  
     
     
         8 . A steel for a rocker arm, wherein a hard oxide in contact with a sulfide composed of MnS and CaS is dispersed in a matrix; said hard oxide contains Al 2 O 3  and CaO; a ratio of the content of CaO to the said hard oxide is 8.0 to 62% (wt/wt); a ratio of the content of Ca to the sulfide is 1.0 to 45% (wt/wt), and an occupation area of the sulfide in the steel is 2.0×10 −4  to 1.0×10 −1  mm 2  per area of 3.5 mm 2 .  
     
     
         9 . A steel for a rocker arm claimed in  claim 8 , comprising: 
 C: 0.1 to 0.5% (wt/wt), Si: 0.01 to 2.5% (wt/wt), Mn: 0.1 to 3.5% (wt/wt), P: 0.001 to 0.2% (wt/wt), S: 0.01 to 0.2% (wt/wt), Cr: 1.0 to 3.5% (wt/wt), V: 0.1 to 0.5% (wt/wt), Al: 0.001 to 0.02% (wt/wt), Ca: 0.0005 to 0.02% (wt/wt) and O: 0.0005 to 0.01% (wt/wt), and the residue composed of Fe and unavoidable impurities,    wherein each content of said Cr and said V satisfies the formula:      [Cr]+1.97×[V])≧2.15    (where [ ] indicates wt % of each element).    
     
     
         10 . A steel for a rocker arm claimed in  claim 9 , having an inner hardness of 20 to 35 HRC and having a structure of ferrite+bainite, wherein carbon equivalent Ceq indicated by ([C]+0.27×([Mn]−55×[S]/32)+0.31×[Cr]+0.3[V]) (where [ ] indicates wt % of each element) is set in a range of 0.8 to 1.1% (wt/wt).  
     
     
         11 . A steel for a rocker arm claimed in  claim 9  or  10 , further comprising one or more element(s) selected from Mo: ≦2.0% (wt/wt), Cu: ≦2.0% (wt/wt), Ni: ≦4.0% (wt/wt), and B: 0.0005 to 0.01% (wt/wt).  
     
     
         12 . A steel for a rocker arm claimed in  claim 9  or  10 , wherein each content of elements Mn, S, and Cr satisfies the formula:  
         ([Mn]−55×[S]/32+[Cr])>2.0 wt %  
       (where [ ] indicates wt % of each element); 
 or each content of said elements Mn, S, Cr and Mo satisfies the formula:  
   ([Mn]−55×[S]/32+[Cr]+[Mo])>2.0 wt %  
 (where [ ] indicates wt % of each element).  
 
     
     
         13 . A steel for a rocker arm claimed in  claim 9  or  10 , further comprising one or two element(s) selected from Nb: ≦0.2% (wt/wt) and Ti: ≦0.2% (wt/wt).  
     
     
         14 . A steel for a rocker arm claimed in  claim 9  or  10 , further comprising one or more element(s) selected from Ta: ≦0.5% (wt/wt), Zr: ≦0.5% (wt/wt), and Mg: ≦0.02% (wt/wt).  
     
     
         15 . A method of manufacturing a steel for a rocker arm described in any one of claims  8  through  10 , which comprises melting an alloy having alloy components specified in any one of claims  8  through  10  and the residue composed of unavoidable impurities and Fe, provided that each content of Mn, S, Cr, and Mo satisfies the formula: ([Mn]−55×[S]/32+[Cr]+[Mo])>2.0 wt % (where [ ] indicates wt % (wt/wt) of each element), and conducting the above operation in such a manner that the said alloy in a molten states satisfies the following conditions 1) through 3): 
 1) [ H S]/[ H O]: 8 to 80    where [ H S]=[S]×10 logFs , and [ H O]=[O]×10 logFo        logFs=0.113[C]+0.065[Si]−0.025[Mn]+0.043[P]−0.028[S]−0.013[Cu]−0.011 [Cr]+0.0027[Mo]−0.27[O]+0.024[N]+0.054[Al] logFo=−0.44[C]−0.131 [Si]−0.02[Mn]−0.147[P]+0.133[S]−0.0095[Cu]+0.006[Ni]−0.041 [Cr]+0.0035[Mo]−1.00[O]−0.1834[N]−0.20[Al]+0.11[V]   2) [sol-Al]: 0.02 to 0.20% and    3) [Ca] x [S]: 1×10 h−5  to 1×10 −3      (where [ ] indicates wt % (wt/wt) of each element).    
     
     
         16 . A rocker arm manufactured from the steel for a rocker arm claimed in any one of claims  8  through  10 .