STEEL INGOT FOR FORGING AND INTEGRAL CRANKSHAFT

摘要

A steel ingot for forging that excels in fatigue performance and hydrogen cracking resistance; and an integral crankshaft therefrom. The steel ingot for forging is produced with use of a casting mold, wherein the density (DBOT) of inclusions of 5 to 10μm major diameter observed on a steel section in an inferior portion of the steel ingot is 10 to 80 inclusions/cm2; the density (DTOP) of inclusions of 5 to 10μm major diameter observed on a steel section in a superior portion of the steel ingot is 20 to 90 inclusions/cm2; the density of inclusions of 40μm or greater major diameter observed on a steel section in both inferior and superior portions of the steel ingot is 5 inclusions/cm2 or less; and the densities satisfy the relationship (DTOP)/(DBOT)>=[S(ppm)]/18. The integral crankshaft is produced by hot forging of this steel ingot for forging.