STEEL INGOT FOR FORGING AND INTEGRAL CRANKSHAFT

摘要

The present invention provides a forging steel ingot and a solid type crankshaft excellent in both fatigue properties and hydrogen cracking resistance. A forging steel ingot cast with a mold is produced wherein: the number density (D BOT ) of inclusions 5 to 10 µm in longest diameter observed on a steel cross-section is 10 to 80 pieces/cm 2 at the steel ingot lower portion; the number density (D TOP ) of inclusions 5 to 10. µm in longest diameter observed on a steel cross-section is 20 to 90 pieces/cm 2 at the steel ingot upper portion; the number density of inclusions 40 µm or more in longesf diameter observed on a steel cross-section is 5 pieces/cm 2 or less at both the steel ingot lower portion and the steel ingot upper portion; and the expression (D TOP )/(D BOT ) ‰¥ [S (ppm)]/18 is satisfied. A solid type crankshaft is produced by hot-forging the forging steel ingot.