Method and facility for manufacturing seamless steel pipe with excellent toughness

Abstract

It is difficult in the related art to realize not only a decrease in material variability due to non-uniform microstructure distribution in the wall thickness direction of a pipe body but also the maintenance of satisfactory productivity of the whole heat treatment line at the same time. A method includes determining in advance whether or not the pipe body is made of a steel grade having an Ms point lower than 200° C.; leaving the pipe body of a steel grade having an Ms point lower than 200° C. additionally at room temperature (it is preferable to be transported to a holding bed 6 and left) until the temperature difference between the portion having the highest temperature and the portion having the lowest temperature in a cross section in a direction at a right angle to the pipe axis becomes less than 2.0° C. after the quenching treatment has been performed, and then performing the tempering treatment; and, on the other hand, performing a tempering treatment on the pipe body of a steel grade not having an Ms point lower than 200° C. without leaving the pipe body at room temperature after a quenching treatment has been performed.

1 . A method for manufacturing a seamless steel pipe including a process of performing quenching and tempering on a pipe body which is an intermediate product of a seamless steel pipe, the method comprising determining in advance whether or not the pipe body is made of a steel grade having an Ms point lower than 200° C.; additionally leaving the pipe body of a steel grade having an Ms point lower than 200° C. at room temperature until the temperature difference between the portion having the highest temperature and the portion having the lowest temperature in a cross section in a direction at a right angle to the pipe axis becomes less than 2.0° C. after the quenching treatment has been performed, and then performing the tempering treatment; and, on the other hand, performing a tempering treatment on the pipe body of a steel grade not having an Ms point lower than 200° C. without leaving the pipe body at room temperature after a quenching treatment has been performed. 
     
     
         2 . A facility for manufacturing a seamless steel pipe including a facility where a pipe body which is an intermediate product of a seamless steel pipe is subjected to quenching and tempering, the facility comprising a means for determining in advance whether or not the pipe body is made of a steel grade having an Ms point lower than 200° C. and a holding bed for leaving the pipe body additionally at room temperature until the temperature difference between the portion having the highest temperature and the portion having the lowest temperature in a cross section in a direction at a right angle to the pipe axis becomes less than 2.0° C. before performing the tempering treatment.