Patent ID: 8808469

Claim:
A nitrocarburized raw material for crankshafts that has a composition composed of from 0.35 to 0.55% of C, from 0.05 to 0.5% of Si, from 0.6 to 1.2% of Mn, from 0.01 to 0.5% of Cu, from 0.01 to 0.5% of Ni, from 0.05 to 0.6% of Cr, from 0.15 to 0.40% of V, from 0.04 to 0.1% of S, from 0.001 to 0.01% of s-Al, from 0.0005 to 0.02% of Ca and from 0.001 to 0.04% of N in terms of % by mass, with the rest being Fe and inevitable impurities, wherein the respective components are regulated in such a manner that carbon equivalent defined as C [eq.] represented by the following expression: C [eq.]=C+0.07×Si+0.16×Mn+0.19×Cu+0.17×Ni+0.2×Cr+[V] satisfies from 0.58 to 0.89%, [V] representing the content of V capable of being solid-soluted in austenite at the time of normalizing heating under a normalizing treatment, and wherein the nitrocarburized raw material has a structure of ferrite+pearlite after being subjected to hot forging and the normalizing treatment, and comprises a resultant material obtained by smelting a steel having the foregoing composition, performing the hot forging, applying the normalizing treatment to the hot forged material at a treatment temperature in the range of from 780° C. to 850° C. which enables V to be solid-soluted in austenite by an amount of [V] which is capable of keeping C[eq.] in the range from 0.58 to 0.89%, preventing the size of the pearlite block from being enlarged, further increasing hardness and further enhancing fatigue strength without lowering straightening ability, applying a nitrocarburizing treatment under a condition at a treatment temperature of from 500 to 650° C. for a treatment time of from 1 to 5 hours, and then applying a straightening process for correcting bending caused by the nitrocarburizing treatment to provide straightening ability of not less than 25 μm.