Nitrocarburized crankshaft member and steel for nitrocarburized crankshafts

Abstract

A nitrocarburized crankshaft member made of a steel having essentially ferrite and perlite, and at least a portion of a steel surface thereof having a ferrite surface area of 50% or greater that is imparted with a nitrocarburized hard layer. The steel consists of C, Si, Mn, Cu, Ni, and Cr as required elements and Mo, N, s-Al, Ti, Pb, Bi, and Ca as optional elements that may be included, and Fe and inevitable impurities. C is within a range of 0.25 to 0.32%. The nitrocarburized crankshaft member includes a thickness of a surface compound layer of the nitrocarburized hard layer of 10 to 35 μm that is formed during establishment of a diffusion depth of a nitrogen diffusion zone below the surface compound layer of 700 μm or greater.

1 . A nitrocarburized crankshaft member made of a steel having essentially ferrite and perlite, and at least a portion of a steel surface thereof having a ferrite surface area of 50% or greater that is imparted with a nitrocarburized hard layer,
 wherein the steel consists of C, Si, Mn, Cu, Ni, and Cr as required elements and Mo, N, s-Al, Ti, Pb, Bi, and Ca as optional elements that may be included, and Fe and inevitable impurities,   wherein the required elements and the optional elements are in amounts by weight of:
 C within a range of 0.25 to 0.32%, 
 Si within a range of 0.01 to 0.15%, 
 Mn within a range of 0.55 to 0.90%, 
 Cu within a range of 0.10 to 0.60%, 
 Ni within a range of 0.05 to 0.30%, 
 Cr within a range of 0.10 to 0.20%, 
 Mo within a range of 0.05% or less, 
 N within a range of 0.020% or less, 
 s-Al within a range of 0.020% or less, and 
 Ti within a range of 0.020% or less, 
 at least one of the optional elements Pb, Bi, and Ca to improve machinability, and 
 Fe and the inevitable impurities in a remaining portion, and, 
   satisfying conditions as follows:
 0.430≦C eq ≦0.530, where C eq =C+0.07×Si+0.16×Mn+0.19×Cu+0.17×Ni+0.2×Cr+0.4×Mo, and 
 0.380≦C req ≦0.470, where C req =0.55×C+0.06×Si+0.18×Mn+0.09×Cu+0.47×Mo+1.0×Cr; 
 a Ni to Cu weight ratio is 0.50≦Ni/Cu≦1.00; and 
   amounts by weight of P and S, which are the inevitable impurities, are suppressed to 0.03% or less and 0.15% or less, respectively,   wherein the nitrocarburized crankshaft member includes a thickness of a surface compound layer of the nitrocarburized hard layer of 10 to 35 μm that is formed during establishment of a diffusion depth of a nitrogen diffusion zone below the surface compound layer of 700 μm or greater.   
     
     
         2 . The nitrocarburized crankshaft member according to  claim 1 , wherein an HV ratio of a maximum hardness of the nitrogen diffusion zone, at a depth of at least 50 μm or greater from said surface compound layer, to a core hardness is 1.65 or greater. 
     
     
         3 . The nitrocarburized crankshaft member according to  claim 2 , wherein the maximum hardness of the nitrogen diffusion zone is within the range of HV 280 to 330. 
     
     
         4 . The nitrocarburized crankshaft member according to  claim 2 , wherein the core hardness is within the range of HV 160 to 190. 
     
     
         5 . The nitrocarburized crankshaft member according to  claim 4 , wherein the maximum hardness of the nitrogen diffusion zone is within the range of HV 280 to 330.