Crack Appearance in Hot Rolled Billets
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 a
- PDF / 3,047,991 Bytes
- 16 Pages / 439.37 x 666.142 pts Page_size
- 80 Downloads / 167 Views
Advanced Materials Analysis, Enschede, The Netherlands Budapest University of Technology and Economics, Budapest, Hungary
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 macromechanisms 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.
372
S.T. Mandziej, G. Krallics
Introduction During hot rolling after continuous casting of steel various cracks may occur resulting from the size and shape of the product as well as from temperature, strain and strain rate of the applied processing. Thus during rolling of 410 mm diameter continuously cast ingots into square 200 mm cross section billets, periodic large internal cavities (bursts) were observed. They often appeared in a medium-carbon Cr-Mo construction steel, however never in any plain carbon steel of similar carbon content, although the casting and rolling conditions were identical.
Fig. 1. Drawing stress (Vxf) as a function of reduction (r%), with indicated burstsensitive zone [1]
373
Relative Reduction, ∈
Crack Appearance in Hot Rolled Billets
Relative Thickness, hc /Rc
Fig. 2. Relation between cross-reduction and size of rolls as a factor affecting the central bursting in bar rolling [2]
Fig. 3. Schematic of rolling with marked area where the pulling force is created due to friction [3]
The central bursts
Data Loading...
