PROCESS FOR PRODUCING SEAMLESS STEEL PIPE

摘要

A process for producing a seamless steel pipe, in which the occurrence of inner surface flaw in the pipe can be reduced through lowering of the friction coefficient at draw rolling by means of a mandrel mill. At the draw rolling of hollow stock pipe, a lubricant composed mainly of at least one of graphite and mica is applied to the surface of mandrel bar, and further a lubricant composed mainly of a borate of monovalent alkali metal is fed onto the inner surface of hollow stock pipe, satisfying the following requirements (a) to (d). (a) The temperature of inner surface of stock pipe immediately after the completion of piercing is set for >= 1150˚C, and the time from the completion of piercing to feeding of the lubricant composed mainly of a borate of monovalent alkali metal, or the time from the completion of descaling to feeding of the lubricant is set for 5 to 60 sec. (b) The temperature of inner surface of stock pipe during the feeding of the lubricant composed mainly of a borate of monovalent alkali metal is set for >= 1100˚C. (c) The time from the feeding of the lubricant composed mainly of a borate of monovalent alkali metal to the initiation of draw rolling is set for >= 10 sec. (d) The temperature of inner surface of stock pipe immediately before the draw rolling is set so as to range from 1000˚ to 1170˚C.