THICK HIGH-STRENGTH STEEL PLATE AND PROCESS FOR PRODUCING THE SAME

摘要

A thick high-strength steel plate with the following characteristics. The steel plate consists of a Ni-containing steel, having a bainite main structure and exhibiting a pearlite fraction of 5% or below. In front and backside surface layer regions of 5% plate thickness, the fraction of coarse ferrite of over 25 µm equivalent circle diameter is 10% or below, and the average equivalent circle diameter of cementite is 0.5 µm or less. When the interior region excluding the above surface layer regions in a cross section perpendicular to the rolling direction of the plate is divided into isotropic areas and when measurement lines according to a sectioning method are drawn in a T-direction parallel to the plate thickness and the isotropic area wherein on the measurement lines, excluding isotropic areas of less than 8 µm equivalent circle diameter, the angle made by axes closest to the T-direction among the axes of multiple isotropic areas continuously adjacent to each other is less than 20° is regarded as one equal crack propagation resistance area, the average equivalent circle diameter (d) of the equal crack propagation resistance area is defined by the formula d=(7.11 x [Ni]+11) x (1.2-t/300) (µm).