Investigation of panel crack formation in steel ingots: Part II. Off-corner panel cracks

Abstract

Model predictions of the thermal and stress evolution in steel ingots, combined with a metallurgical study, have confirmed the mechanism of formation of off-corner panel cracks. The cracks are generated initially in the subsurface of the ingot during the early stages of reheating in the soaking pit. The volume change associated with γ-α transformation plays a major role in the generation of the tensile stresses as in the case of the mid-face panel cracks. Early in the reheating, a subsurface twophase zone of α and γ contracts within a surrounding field of austenite and is placed in tension. Crack formation is enhanced by the precipitation of AlN which embrittles the steel. The subsurface cracks subsequently may penetrate to the surface of the ingot immediately upon its withdrawal from the soaking pit owing to the generation of surface tensile stresses caused by rapid cooling of the ingot surface. Measures to minimize formation of off-corner panel cracks, therefore, include reducing air cooling time (prior to charging ingots to the soaking pit) to prevent γ-α transformation, reheating the ingots slowly in an initially cold soaking pit to reduce the tensile stresses, increasing the air cooling time substantially to force the cracks farther from the surface, and minimizing soaking times and temperatures.