| 摘要 |
PROBLEM TO BE SOLVED: To provide a steel material capable of obtaining TS (tensile strength) of 900 MPa or higher in the state of as-rolled even without adding V at all, and further excellent in toughness.SOLUTION: There is provided a steel material including a prescribed chemical composition, and having a gradient structure where, provided that the distance from the steel center to the steel surface is defined as r, 95% (volume ratio) or more of the region in the periphery from the steel center to the radius 10 mm is made of a structure of ferrite and pearlite, and the structure in the region from the steel center to the r/2 position or more is subsequently changed from bainite (upper bainite and/or lower bainite) and further tempered martensite from the central side to the surface side.SELECTED DRAWING: Figure 3 |
