Crack Appearance in Hot Rolled Billets

Abstract

Hot cracks may appear in metal alloys on heating or on cooling when the tensile strains and related stresses, caused by thermal expansion or shrinkage and usually enhanced by various restraints, cannot be compensated by a local plastic deformation of an alloy. In general, these metal alloys, which have large thermal expansion coefficients, are susceptible to hot cracking during casting, welding or e.g. re-heating for hot rolling. The metallurgical quality of an alloy affects its susceptibility to hot cracking, in particular the chemical and microstructural inhomogeneities influence this susceptibility. Additional plastic strains, applied in hot forming, usually extend the cracks leading to damage. In this work an attempt is made to describe the micro- and macro-mechanisms of the cracking as well as microstructural and mechanical factors assisting the damage, based on observations of continuously cast ingots in which internal hot cracks and cavities were formed during initial steps of hot rolling. Characteristic of these ingots was chemical segregation resulting in differences of hot ductility in different parts of the ingots. Thus during plastic deformation, due to interaction between the microstructurally and mechanically different regions, cracks and cavities did nucleate and grow. Discussed here are physical data necessary to adequately describe the behaviour of material during deformation and cracking, as well as physical simulation methods to gain these data. The gained data and identification of the damage processes are then used for computer modelling with an aim to determine critical conditions of controlling the application process in order to avoid the cracking and to assure the manufacture of sound products. The micro-scale conditions characteristic of e.g. welds and the macro-scale situations typical for rolling of steel billets are discussed.