| 摘要 |
PROBLEM TO BE SOLVED: To provide a forged steel product formed of carbon steel having improved hydrogen crack resistance through control of the form and number density of sulfide-based inclusions and oxide-based inclusions, without means for addition of expensive alloy elements.SOLUTION: The forged steel product satisfying a predetermined component composition includes sound parts formed of a ferrite structure or a mixed structure of ferrite and pearlite and the balance (hereinafter described as "macrosegregation parts") in observation of a steel cross section at a depth of D/4 (D: circle equivalent diameter of the cross section of a forged steel product) in a measurement area of 5 cm by 5 cm. The proportion of the sound parts is 90% by area or more in the steel cross section. The sulfide-based inclusions having a major axis of 1 μm or more in the macrosegregation parts have an average circle equivalent diameter of 200 μm or less, an average aspect ratio of 50 or less, and a number density of 300 pieces/cmor less. The oxide-based inclusions having a major axis of 1 μm or more in the macrosegregation parts have an average circle equivalent diameter of 100 μm or less, an average aspect ratio of 20 or less, and a number density of 300 pieces/cmor less. |
