A Two‐Step Thermomechanical Process for Improving High‐Angle Grain Boundary Proportion and Achieving Delamination Toughening in Hot‐Rolled Steel with a Limited Slab‐to‐Plate Ratio

Abstract

A two-step thermomechanical process is utilized to increase high-angle grain boundary (HAGB) proportion and achieve delamination cracks in hot-rolled steel produced by a limited slab-to-plate ratio. Three plates are austenitized at 900 °C and hot rolled with 65%, 57%, and 50% reduction at 880-800 °C after rolling at 1100-1050 °C followed by water cooling. Water quenching tests are carried out to prove the advantages of the two-step thermomechanical process. The steels having 69.9-71.8% HAGB proportion and delamination cracks are fabricated with a final thickness of 48 mm and a total rolling reduction ratio of 3.33. The high HAGB proportion is attributed to the abnormally refined prior austenite grain of 18.5 AE 10.5 μm caused by the heating temperature of 900 °C. Thus, 93-129% more effective interfacial area (193-229 mm À1 ) can be provided than the conventional process (100 mm À1 ) during rolling below the nonrecrystallization temperature of austenite. Abundant ferrite grains that form and impinge from the different prior austenite grains obtain 33.6-34.5% frequency distribution of the grain boundary misorientation angles, which fall into 20°-47°. The premises of delamination toughening are that the delamination cracks appear at the opposite side of the V-notch with a reduced number.