HIGH STRENGTH STEEL PLATE WITH EXCELLENT HYDROGEN INDUCED CRACKING RESISTANCE, ITS MANUFACTURING METHOD, AND STEEL PIPE FOR LINE PIPE

摘要

PROBLEM TO BE SOLVED: To stably manufacture a high strength steel plate having a high strength of API X52 grade or higher and capable of exhibiting excellent HIC (hydrogen-induced cracking) resistance even under severe NACE conditions. SOLUTION: The high strength steel plate has a composition which contains components, such as Mn, Nb, Ti, Al, O (oxygen), N and Ca, and satisfies Ca/S≥2 and in which, among impurities, the total of respective contents of elements combining with S, O (oxygen) and N to form an inclusion is made to≤0.01%. Further, neither the above inclusion formed by the combination with S, O (oxygen) and N nor an Nb-Ti-C-N inclusion exceeds 5μm in size. The method for manufacturing the high strength steel plate comprises steps of: subjecting a steel material with the above composition to heating in such a way that temperature in the central part of the steel material reaches≥1,125°C; carrying out finish rolling while regulating the draft at each rolling pass in hot rolling to≥10%; starting cooling from≥(Ar3point+20)°C; regulating cooling rate to (5 to 50)°C/s; and stopping water cooling at≤550°C. COPYRIGHT: (C)2006,JPO&NCIPI