Investigation of panel crack formation in steel ingots: Part I. Mathematical analysis and mid face panel cracks

An investigation of panel crack formation in steel ingots has been undertaken to improve understanding of the mechanisms by which the cracks develop and to evaluate possible solutions to this problem that has plagued the steel industry intermittently for decades. The investigation features the application of two-dimensional, finite-element, heat-flow, and stress models, which have been described in earlier publications^1,2 for steel ingot processing. The model predictions have clarified the role of stress generation in panel crack formation and demonstrate the importance of the γ→ α phase transformation. It has been revealed that two distinct types of panel cracks, both of which are partly caused by intermediate-temperature embrittlement of steel involving aluminum nitride precipitation, operate under different mechanisms. Mid-face panel cracks, which are analyzed in Part I of this paper, apparently form during air cooling when the mid-face surface is between the Ar₁ and 500 °C. The cracks can be prevented by ensuring the ingot surface does not cool below the Ar₁, and preferably the Ar₃ temperature. In case of a 335 mm square ingot, this would require reheating and rolling within the first hour after being stripped from the mold. Alternatively charging the ingots to a holding furnace to slow the surface cooling rate through the critical Ar₁ - 500 °C temperature zone should be beneficial. Off-corner panel cracks are discussed in Part II.