METHOD OF PRODUCING LOW YIELD RATIO THICK STEEL PLATE FOR BUILDING STRUCTURE HAVING EXCELLENT TOUGHNESS IN HEAT-AFFECTED ZONE BY ULTRAHIGH HEAT INPUT WELDING

摘要

PROBLEM TO BE SOLVED: To provide a method of producing a low yield ratio, thick steel plate for a building structure which has excellent toughness in a weld heat-affected zone by ultrahigh heat input welding in a heat gain of 400 kJ/cm.;SOLUTION: Si and/or Mn is added to molten steel, and the content of dissolved oxygen is controlled to 0.0030 to 0.0120 mass%. After that, rare earth metals are added thereto to control the dissolved oxygen content to 0.0010 to 0.0050 mass%, and, the contents of C, Si, Mn and P are controlled to the proper ranges. Further, the molten steel is made into the one having a composition containing 0.0005 to 0.0060% S, 0.0030 to 0.0200% rare earth metals and 0.0040 to 0.0070% N, and containing one or more kinds of metals selected from Nb, Cu and Ni as well, and in which the contents of Al and Ti are limited to ≤0.004%, respectively. The molten steel is next cast to form into a steel stock. The steel stock is thereafter heated to ≥1,000°C, and is subsequently subjected to hot rolling at a rolling finishing temperature of an Ar₃ transformation point or higher. The steel plate is then subjected to accelerated cooling at a mean cooling rate of ≥1°C/s and at a cooling stopping temperature of 600 to 250°C, and is next air-cooled.;COPYRIGHT: (C)2004,JPO