Modelling strain induced precipitation of niobium carbonitride during hot rolling of microalloyed steel

Because of its industrial importance the behaviour of microalloyed steels during thermomechanical processing has been extensively studied in the laboratory. Semi-empirical models have been developed and successfully applied. Nevertheless there is still not a full understanding, which can be used as a physical basis for a complete model of industrial rolling involving multiple passes in roughing and in finishing. Such a model should define when and where carbonitride precipitates form, and possibly redissolve, their composition and how much micro alloy element is precipitated in the austenite. This paper considers for a simple niobium microalloyed steel (i) the effect of particle size distribution during reheating (ii) deformation in the roughing temperature range leading to significant precipitation, even though recrystallisation is not retarded, and to acceleration of the kinetics of precipitation during finishing, (iii) repeated nucleation and possible reversion of strain induced precipitation between passes in finishing. It poses a number of questions which have important implications for modelling. Finally some critical experimental observations required to answer the questions are discussed.