HIGH-STRENGTH STEEL PIPE WITH EXCELLENT LOW-TEMPERATURE TOUGHNESS FOR LINE PIPE, HIGH-STRENGTH STEEL PLATE FOR LINE PIPE, AND PROCESSES FOR PRODUCING THESE

摘要

The present invention provides high strength steel pipe for line pipe superior in low temperature toughness suppressed in drop of toughness of the HAZ and a method of production of the same, more particularly high strength steel plate for line pipe used as a material for high strength steel pipe for line pipe and a method of production of the same, in particular high strength steel pipe for line pipe superior in low temperature toughness characterized in that the chemical compositions of the base metal is, by mass%, C: 0.020 to 0.080%, Si: 0.01 to 0.50%, Mo: 0.01 to 0.15%, Al: 0.0005 to 0.030%, and Nb: 0.0001 to 0.030% contained in a range of C+0.25Si+0.1Mo+Al+Nb: 0.100% or less and the mixture of austenite and martensite present along prior austenite grain boundaries of the reheated part of the heat affected zone has a width of 10 µm or less and a length of 50 µm or less.