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

Abstract

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The influence of composition and cooling rate on the hot ductility of steels has been reviewed. Models to predict hot ductility behaviour have been discussed and the parts of the trough which can be used to predict the likelihood of cracking occurring are highlighted. On tensile testing both deformation induced ferrite in sufficient quantity to improve ductility and dynamic recrystallisation occur but not when straightening during continuous casting; the strain being too low. This limits the use of the hot ductility curve in predicting cracking behaviour. The temperature range in which straightening of the continuously cast strand should be carried out is either 30°C below the Ar ₃ when there is a large amount of ferrite (∼40%) present before deformation or above the T _d, the temperature at which dynamic recrystallisation starts to take place in a tensile test; this being when the ferrite film no longer forms and precipitates are sufficiently coarse and few in number to influence the ductility. The influence of C,Mn,N, Nb,V,B,Ti,Al and the residuals Cu and Sn on hot ductility are examined and recommendations made with regard to the amounts and cooling conditions required to give freedom from transverse and edge cracks. The hot ductility of the new transformation induced plasticity (TRIP) and twin induced plasticity (TWIP) steels has also been reviewed. P has been shown to have little influence on the hot ductility, but for Al based TRIP and TWIP steels, Al levels need to be closely controlled and high Al levels, (1·5 to 2%Al) are favoured.