MANUFACTURE OF NONTEMPERED HIGH-TENSILE STEEL PLATE OF 6070KG/MM2 CLASS AND HAVING EXCELLENT LOW-TEMPERATURE TOUGHNESS

摘要

PURPOSE:To manufacture of a nontempered high-tensile steel plate of 60-70kg/ mm.2 class, having excellent low-temperature toughness and excellent weldability, by a method in which a low-carbon equivalent steel containing Mb and Mo is heated to a high temperature, rolled orderly in a high temperature recrystallization gamma region, unrecrystallization gamma region, two-phase region, and three-phase region, and then air-cooled. CONSTITUTION:A steel, containing 0.02-0.12% C, 0.10-0.50% Si, 1.50-2.10% Mn, 0.008-0.060% total Al, 0.005% S, 0.03-0.10% Mb, 0.05-0.33% Mo, and Fe and inevitable impurities, is heated to 1,100-1,250 deg.C to uniformly mix Mb and then rolled at an accumulated draft of 50% or more in a high-temperature (950 deg.C or higher) recrystallization gamma region. Then, the steel is rolled at an accumulated draft of 50% or more in an unrecrystallization gamma region at a high temperature of 950 deg.C or more and in a two-phase region of unrecrystallization gamma + fine ferrite, and then rolled at an accumulated draft of 10-60% in three- phase region of unrecrystallization gamma + fine ferrite + fine bentonite in a temperature range of (Ar3 -50 deg.C)-(Ar3 -50 deg.C). Lastly, the steel is air-cooled.