Modeling of Austenite Grain Size Distribution in Nb Microalloyed Steels Processed by Thin Slab Casting and Direct Rolling (TSDR) Route

Abstract

A mathematical model has been developed to predict the austenite microstructure evolution of Nb microalloyed steels during “Thin slab casting” and “Hot direct rolling” (TSDR) processing. The model is based on empirical equations specifically derived for the microstructural and processing features typical in these new technologies. Its main novelty is that it works with austenite grain size distributions instead of the typical mean values as used in conventional models to represent the microstructure. This fact is particularly important in working with as-cast austenite due to the wide range of grain sizes present in this microstructure. In the model the different softening and hardening mechanisms that can operate during hot working in austenite are considered: static, dynamic and metadynamic recrystallization, grain growth after recrystallization and Nb(C, N) strain induced precipitation. The model uses the initial austenite grain size distribution as input and provides the size distribution of recrystallized and unrecrystallized grains at the entry of any rolling pass. A validation of the model has been carried out in the laboratory by multipass torsion tests. The model is capable of predicting any heterogeneities that may appear in the final microstructure after this kind of processing and that are not well predicted by using conventional models based on mean values. Additionally, it can calculate the deformation history, in terms of the strain accumulated in the austenite, and stress behavior, in terms of the mean flow stress (MFS) corresponding to each deformation pass.