Importance of Ar 3 temperature in controlling ductility and width of hot ductility trough in steels,and its relationship to transverse cracking

A review of the literature and current work indicates that the improvement in ductility at the low temperature end of the hot ductility trough in coarse grained steels tested at low strain rates, always corresponds to the presence of a significant volume fraction of ferrite;ferrite having excellent ductility. For good ductility 30–40%ferrite is required and this can be. achieved by deforming 20–30 K below the Ar₃ undeformed temperature. The higher the Ar₃, the easier it is to deform and the narrower is the trough. Higher Ar₃ temperatures can be obtained by lowering the C and Mn levels, refining the grain size, or slowing down the cooling rate to the test temperature. These methods are discussed and the advantages of straightening during continuous casting, at lower temperatures rather than the currently preferred route using higher temperatures are outlined. Whereas straightening at higher temperatures only gives rise to small improvements in ductility (and these have not always been found adequate to eliminate transverse and edge cracking), straightening at lower temperatures when a significant amount of ferrite is present can give excellent ductility, more than enough to prevent cracking. The presence of ferrite will also ensure that the load requirements for bending are not excessive.

MST/3257