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Spatial distribution in size and frequency of microalloy precipitates have been characterized in two continuous-cast high-strength, low-alloy steel slabs, one containing Nb, Ti, and V and the other containing only Ti. Microsegregation during casting resulted in an inhomogeneous distribution of Nb and Ti precipitates in as-cast slabs. A model has been proposed in this study based on the detailed characterization of cast microalloy precipitates for predicting the spatial distribution in size and volume fraction of precipitates. The present model considers different models, which have been proposed earlier. Microsegregation during solidification has been predicted from the model proposed by Clyne and Kurz. Homogenization of alloying elements during cooling of the cast slab has been predicted following the approach suggested by Kurz and Fisher. Thermo-Calc software predicted the thermodynamic stability and volume fraction of microalloy precipitates at interdendritic and dendritic regions. Finally, classical nucleation and growth theory of precipitation have been used to predict the size distribution of microalloy precipitates at the aforementioned regions. The accurate prediction and control over the precipitate size and fractions may help in avoiding the hot-cracking problem during casting and selecting the processing parameters for reheating and rolling of the slabs.

Development of austenite grain structures have been compared in two different microalloyed steels (Nb–Ti and Nb–V steels) and one Al killed C–Mn steel, after soaking at 950–1250°C for 1 h. Minimum austenite grain size in Nb–V steel at the lower soaking temperature (<1075°C) can be attributed to the pinning effect from AlN, Nb(C,N) and V(C,N) precipitates. At the intermediate soaking temperatures (1150–1200°C) dissolution of Nb precipitates led to an abnormal austenite grain growth and the formation of bimodal grain size distributions in microalloyed steels. Grain size bimodality was more severe in Nb–V steel as compared to Nb–Ti steel. Complete absence of precipitates allowed the austenite grains to grow freely at higher soaking temperature (>1200°C) in all the steels. Higher stability of TiN precipitate restricted the grain growth in Nb–Ti steel at higher soaking temperature. An effort has been made to predict the austenite grain size considering both solute drag and Zener drag.

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