| 摘要 |
PROBLEM TO BE SOLVED: To allow a steel sheet to have superfine particles and to impart an excellent notch fatigue resistance and fatigue characteristics thereto by allowing it to have a structure in which the average particle size of ferrite is controlled to be below a specified value, the average particle size of 2nd phase particles is controlled to be a value equal to or below a specified value, the aspect ratio is controlled to a value equal to or below a specified value, and the ratio in which the interval between the most adjacent 2nd phase particles is made to be a value of the 2nd phase particle size or above is controlled to the ratio equal to or above the specified one and allowing the notch sensitivity coefficient and tensile strength to satisfy a specified relation. SOLUTION: This steel sheet is the one formed of ferrite as the main phase and a 2nd phase, the average particle size of ferrite is controlled to <4μm, the average particle size of the 2nd phase particles is controlled to <=8μm, the aspect ratio is controlled to <=2.0, and the ratio in which the interval between the most adjacent 2nd phase particles is made the one equal to or above the particle size of the 2nd phase particles is controlled to >=80%. The relation between the notch sensitivity coefficient (q) in the formula (α: the stress concentration coefficient in the notched part with a pierced hole shape, andβ: the ratio of deterioration in fatigue strength caused by a notch = (the smooth fatigue strength of the steel sheet)/(the fatigue strength to a notch with a pierced hole shape in the steel sheet) and static tensile strength TS (MPa) lies in the relation in the inequality II (where TS>500 MPa).
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