Nitrocarburized crankshaft member and steel for nitrocarburized crankshafts

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≦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 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.