| 摘要 |
PURPOSE:To obtain the pipe material having a high ability for absorbing energy by providing the softening range at the weld heat influencing part, specifying the ratio of min. hardness of this softening range to the average hardness of base material to the specific relation, and producing the steel pipe so that the ratio of the yield strength of pipe to the tensile strength is made over than the specific value. CONSTITUTION:The high strength thin steel sheet which is strengthened with the transformation structure by continuous annealing, etc., after hot rolling or cold rolling is executed to the pipe with the usual electric resistance welding. There is a softening range at the welding heat influencing part, the ratio (Hv1/Hv2) of the lowest hardness Hv1 of this softening range to the average hardness Hv2 satisfies the equation I, and the ratio (sigmay/sigmaB) of the yield strength sigmay to the tensile strength sigmaB is made over than 0.7. The high strength thin steel sheet is made to the pipe with the usual electric resistance welding, the hardness distribution of the welded part and the yield ratio of pipe are made to the prescribed value, so the high absorbing energy is obtained. Therefore, without breaking at the welded part at the time of forcibly breaking down, the high strength pipe material having high absorbing energy can be presented.
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