STEEL PLATE WITH EXCELLENT HYDROGEN-INDUCED CRACKING RESISTANCE AND TOUGHNESS OF THE WELD HEAT AFFECTED ZONE, AND STEEL TUBE FOR USE AS LINE PIPE

摘要

This steel plate has excellent hydrogen-induced cracking resistance and toughness of the weld heat affected zone (HAZ toughness), and this steel tube is obtained using said steel plate. This steel plate is characterized by comprising prescribed elements with the remainder being iron and unavoidable impurities, wherein the ratio (Ca/S) of Ca content to S content is 2.0 or greater, and the Ca content, S content and O content satisfy (Ca-1.25S)/O ≦̸ 1.8, and further, in the region from the surface to 5mm deep in the thickness direction, there are 2.0 per mm2 or fewer Ca-based inclusions that have a 50μm or longer major axis or long side, and 5×102 per μm2 or more TiN's that have a 300nm or shorter major axis or long side.