| 摘要 |
<p>PROBLEM TO BE SOLVED: To provide a high strength steel sheet having high fatigue strength and excellent in press formability. SOLUTION: When a hole of 10 mm diameter is punched at 10% clearance, the residual stressσR (MPa), in a peripheral direction, of a hole 2 in the surface S1 of a steel sheet on the punch admission side and the tensile strength TS of the steel sheet satisfy the relation ofσR /TS<=0.05. Moreover, hardness Hve, in the position P at a distance of 0.2 mm from the lower end D of the shear drop occurring at the surface S1 of the steel sheet on the punch admission side and also from the internal surface S2 of the hole, and hardness Hvo, in the steel-sheet surface free from the influence of punching, satisfy the relation of Hve-Hvo>=35. This steel sheet has a dual-phase structure consisting of ferrite and a low-temp. transformation product composed essentially of martensite, and further, it is preferable to regulate the ratio between the hardness Hvs of the low-temp. transformation product and the hardness Hvf of the ferrite. Hvs/Hvf, to >=3.5.</p> |
