Patent ID: 8696834
Filing Date: 2014-04-15
Classification: C21D,C22C

Abstract:
1. A method for manufacturing seamless steel pipes in which a billet consisting of, by mass percent, C: 0.15 to 0.35%, Si: 0.05 to 0.5%, Mn: 0.1 to 1.5%, Cr: 0.2 to 1.5%, Mo: 0.1 to 1.5%, Ti: 0.005 to 0.50%, and Al: 0.001 to 0.50%, the balance being Fe and impurities, the impurities having a composition of 0.1% or less of Ni, 0.04% or less of P, 0.01% or less of S, 0.01% or less of N, and 0.01% or less of 0, is hot pierced and hot rolled, and further heat treatment is performed, wherein (1) a hot rolled steel pipe is direct quenched from a temperature of not lower than the Ar the steel pipe is controlled to HRC 42 or lower in hardness by means of a heat treatment in a heat treatment equipment connected to a quenching apparatus for performing the direct quenching or the inline quenching, wherein heat treatment temperature T (° C.) and heat treatment time period t (hr) satisfy that a PL value defined by the formula (1) below is in the range of 14,000 to 18,600 in condition of 450° C.<T≦the Ac wherein T is heat treatment temperature (° C.), t is heat treatment time period (hr), and log is common logarithm; and further the steel pipe is reheated, quenched from a temperature of not lower than the Ac tempered at a temperature of not higher than the Ac