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 improve ment in ductility at the low temperature end of the hot ductility trou gh in coarse grained steels tested at low strain rates, always corresp onds to the presence of a significant volume fraction of ferrite; ferr ite having excellent ductility. For good ductility 30-40% ferrite is r equired and this can be achieved by deforming 20-30 K below the Ar-3 u ndeformed temperature. The higher the Ar-3, the easier it is to deform and the narrower is the trough. Higher Ar-3 temperatures can be obtai ned by lowering the C and Mn levels, refining the grain size, or slowi ng down the cooling rate to the test temperature. These methods are di scussed 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 tem peratures only gives rise to small improvements in ductility (and thes e have not always been found adequate to eliminate transverse and edge cracking), straightening at lower temperatures when a significant amo unt of ferrite is present can give excellent ductility, more than enou gh to prevent cracking. The presence of ferrite will also ensure that the load requirements for bending are not excessive. (C) 1996 The Inst itute of Materials.