| 摘要 |
PURPOSE:To prevent the generation of red rust and sacrifical corrosion at a low cost, by subjecting a steel pipe consisting of a mild steel material formed with Ni layers of a prescribed thickness on the inside or/and outside circumferential surfaces to a heat treatment in an atmosphere of reducing gas or the like thereby forming the Ni diffused layer on the surface of the base material of the steel pipe. CONSTITUTION:Ni layers 2 of 0.5-10mm. are formed by an electroplating method or the like on both surfaces of a steel strip 1 consisting of a mild steel material in the case of adapting this surface treatment method to, for example, an electric welded steel pipe. The strip 1 formed thereon with the layers 2 is welded by a high-frequency induction welding method or the like through a forming stage to form an electric welded steel pipe 3. The dotted line A in the figure is a welded section. The steel pipe 3 is then subjected to a heat treatment under the conditions of 800-1,100 deg.C and >=5min in a reducing or inert gaseous atmosphere to form an Ni diffused layer 5 on the surface of the base material 1 of the steel pipe. The dotted line B in the figure denotes the boundary between the base material 1 and the layer 2. In other words, Ni forms a solution solution of Fe on the surface of said material 1, and the diffused layer 5 joined to one body with the base material contg. increasingly rich Ni toward the surface side is formed. |
