to which he has now returned.The hot ductility behaviour of steels in the temperature range 700-11 oooe 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.

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In constant strain rate tests, the occurrence of dynamic recrystallization (DRX) is traditionally identified from the presence of stress peaks in flow curves. However, not all materials display well-defined peaks when tested under these conditions. Using plain carbon, Nb-bearing and 321 austenitic stainless steels, it is shown that the onset of DRX can also be detected from inflections in plots of the strain hardening rate q against stress s or, equivalently, from inflections in ln q-ln s and ln q-e plots regardless the presence of stress peaks in the flow curves. These observations are verified by means of metallography. A unified description of the flow curve is introduced based on normalization of the stress and strain by the respective peak or steady state values. This approach reveals that, in a given material, the ratio of DRX critical stress to the peak or steady state stress is constant, as is that of the critical strain to the corresponding strain values. Furthermore, it is shown that the present technique can be used to establish the occurrence of DRX when this cannot be determined unambiguously from the shape of the flow curve.KEY WORDS: hot deformation; austenite; dynamic recrystallization. Fig. 1. Constant strain rate stress-strain curve typical of DRX.

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The use of heavy gauge steel sheets for structural applications often requires a combination of high yield strength and adequate toughness. The most cost effective way to achieve high yield strength and high ductility in low alloyed steels is through grain refinement. In industrial practice, such refinement is commonly obtained by thermomechanical controlled processing (TMCP). This approach comprises slab reheating to well defined temperatures, a large amount of hot deformation below the non-recrystallisation temperature T-nr and accelerated cooling. In practice, the T-nr is generally raised by the addition of microalloying elements such as Nb and Ti. As these elements contribute substantially to the alloying costs, optimisation of their use allows for a decrease in production cost. Better understanding of the T-nr assists in tuning the rolling process so that optimum mechanical properties can be produced. One area of importance is to recognise that the concept of the T-nr was originally developed for reversing mills and the production of plate steels. Methods of defining and determining it must be modified if it is to be applied to strip mills and their associated short interpass times. The main goal of this review is to provide a concise and complete overview of the current understanding of the fundamental mechanisms that control the T-nr and to address the different methods that can be used to determine it