Hot ductility of steels and its relationship to the problem of transverse cracking during continuous casting

Abstract

The hot ductility behaviour of steels in the temperature range 700–1100°C when tested in tension at low strain rates is examined. Three distinct ranges of behaviour are displayed: (a) a high ductility, low temperature range which results from the presence of a large volume fraction of the more ductile ferrite phase; (b) a high temperature ductile region covering the range within which grain boundary particles are dissolved and boundaries are able to migrate so that any initiated cracks are prevented from linking up and enlarging; and (c) a trough between these two ranges in which low ductility intergranular failures often occur. These intergranular failures arise as a result of stress and strain concentrations at the austenite boundaries caused either by the presence of thin films of the softer deformation induced ferrite enveloping the austenite grains, or by grain boundary sliding in the austenite. Both failure mechanisms are encouraged by the presence of grain boundary precipitates and inclusions (the finer, the more detrimental), coarser grain sizes, and lower strain rates. The factors which control intergranular failure are analysed, leading to estimates of both the width and depth of the trough. The relation between the hot ductility behaviour in tensile testing and the occurrence of transverse cracking in straightening operations is discussed. It is shown that information from hot ductility tests can be used to predict the likelihood of transverse cracking because the variables that influence the depth of the ductility trough are also responsible for transverse cracking. The steps that can be taken to reduce the incidence of transverse cracking are considered in detail: these include adjustments to the chemical composition, grain size reductions, and control of the volume fraction and size distribution of the inclusions and precipitates. In terms of continuous casting process variables, control of the secondary cooling flow is probably most effective in reducing transverse cracking.