| 摘要 |
PURPOSE:To provide the method for manufacturing welded steel tube low in yield ratio required for a transportation line pipe of petroleum and gas and for a building structural steel tube. CONSTITUTION:In a multi-layered type welded steel tube composed of two kinds of components, the inner layer part is made of a high carbon component and the outer layer part is made of a low carbon component and by heat treating the whole of a tube, the strength of the inner layer part is increased and that of the outer layer part is made lower than that of the inner layer part, as a result, the yield ratio of the whole of a steel tube is lowered. When a tension test is carried out to the multi-layered steel tube having the strengh different between the inner layer and outer layer, the yield strength depends on the low strength layer, on the other hand, the tensile strength depends on the high strength layer, consequently, the multi-layered steel tube has low yield ratio. |
