| 摘要 |
PROBLEM TO BE SOLVED: To provide a steel member in which the part extremely deteriorated in toughness caused by strain aging is not generated in the member even in the case cold bending working in which the bend radius is small is performed, and also, treatment for improving the properties of the surface is not required after the working and further to provide an earthquake resistant steel member high in safety. SOLUTION: A steel sheet which is formed of a layered structure different in crystal grain sizes in the sheet thickness direction, in which the surface and back layer parts 2 and 4 are provided with fine-grained structures in which the crystal grain size is <=3 micron by the average diameter of the equivalent circle and in which the one side thickness of the surface and back layer parts 2 and 4 lies in the range of 2 to 40% of the sheet thickness is welded or cold- worked to produce the steel member.
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