LOW-YIELD-RATIO HIGH-STRENGTH HOT-ROLLED STEEL PLATE WITH EXCELLENT LOW-TEMPERATURE TOUGHNESS AND PROCESS FOR PRODUCING SAME

摘要

A low-yield-ratio high-strength hot-rolled steel plate having excellent low-temperature toughness that has a composition which contains, in terms of mass%, 0.03-0.10% C, 0.01-0.50% Si, 1.4-2.2% Mn, 0.005-0.10% Al, 0.02-0.10% Nb, 0.001-0.030% Ti, 0.05-0.50% Mo, 0.05-0.50% Cr, and 0.01-0.50% Ni and which preferably has a value of Moeq in the range of 1.4-2.2%, and that has a structure which comprises bainitic ferrite having an average grain diameter of 10 µm or less as the main phase and massive martensite having an aspect ratio less than 5.0 as a second phase, the content of the massive martensite being 1.4-15% in terms of areal proportion. It is preferred that the massive martensite should have a maximum size of 5.0 µm or smaller and an average size of 0.5-3.0 µm. This steel plate has a YS, measured in a direction making an angle of 30º with the rolling direction, of 480 MPa or higher, a tensile strength in the plate-width direction of 600 MPa or higher, a vTrs of -80ºC or lower, and a yield ratio of 85% or less.