Nitrocarburized crankshaft member and steel for nitrocarburized crankshafts

摘要

A nitrocarburized crankshaft member made of a steel that includes C in an amount by weight of 0.25 to 0.32% as a required element and an optional element that may be included, and Fe and inevitable impurities in a remaining portion. The steel-made crankshaft member mainly includes ferrite and perlite, wherein at least a portion of the steel surface thereof having a ferrite surface area of 50% or greater is imparted with a nitrocarburized hard layer. The nitrocarburized hard layer includes a surface compound layer suppressed to a thickness of 10 to 35 μm, and a nitrogen diffusion zone below the surface compound layer having a diffusion depth of 700 μm or greater. The steel includes C, Si, Mn, Cu, Ni, and Cr as the required elements and Mo, N, s-Al, and Ti as the optional elements.

主权项

1. A nitrocarburized crankshaft member made of a steel comprising ferrite and perlite and at least a portion of a steel surface thereof having a ferrite surface area of 50% or greater is imparted with a nitrocarburized hard layer,
wherein the steel consists of C, Si, Mn, Cu, Ni, and Cr as required elements, Mo, N, s-Al, and Ti 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 the range of 0.01 to 0.15%, Mn within the range of 0.55 to 0.90%, Cu within the range of 0.10 to 0.60%, Ni within the range of 0.05 to 0.30%, Cr within the range of 0.10 to 0.20%, Mo within the range of 0.05% or less, N within the range of 0.020% or less, s-Al within the range of 0.020% or less, Ti within the range of 0.020% or less, and Fe and the inevitable impurities in a remaining portion, and satisfies the conditions that 0.430≦Ceq≦0.530, where Ceq=C+0.07×Si+0.16×Mn+0.19×Cu+0.17×Ni+0.2×Cr+0.4×Mo, and 0.380≦Creq≦0.470, where Creq=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 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 formed during establishment of a diffusion depth of a nitrogen diffusion zone below said surface compound layer of 700 μm or greater.