Process modeling of microalloyed steel for near net shape casting

Strip casting is an emerging technology for flat products; it is based on the concept of near-net shape casting; it holds potential for substantial energy saving and reduction in environmentally undesirable emission. In conventional thermo-mechanical processing, the pinning pressure due to the effects of Nb and NbC on boundary mobility is used to advantage to retard static recrystallisation and thus aid strain accumulation during “pancaking of austenite”. The timetemperature-deformation schedule characteristic of thin slabs (30-50 mm) and strips are such that it is not possible to take full advantage of the conventional approach to microalloying technology. Quantitative analysis of thin slab direct rolling (TSDR) shows that it is possible to obtain ultrafine austenite grains (< 3 micron diameter) via dynamic recrystallization; this requires that a critical accumulated strain be exceeded. Recent research at McMaster University has shown that strain-induced precipitation at dislocation nodes is effective in retarding static softening by recovery, especially at the short interpass times characteristic of thin slab processing. The inhibition of static softening then permits the accumulation of the large driving forces required for lower-temperature dynamic recrystallization. The available information on the effects of niobium on the critical strain for dynamic recrystallization generated by Lutz Meyer and coworkers relates to the total niobium content of the steel; this data-base has been used in a series of simulations of austenite grain size development in multi-pass rolling schedules. Several of these have been validated by experimental studies using the thermo-mechanical simulator WUMSI. Even in the case of thicker strip (10-15 mm), the time-temperature-deformation schedule is too rapid to take advantage of the strain-induced precipitation of NbC. There is however an opportunity to utilize upstream processing: Previous research has shown that TiN can serve as a precursor for the epitaxial precipitation of NbC. To take advantage of this mechanism, it is essential to obtain a fine dispersion of TiN particles. Quantitative modelling shows that this can be done through the design of base chemistry, the control of solidification variables, and the control of post-solidification cooling schedules. Each of these variables is used to increase the thermodynamic potential for the precipitation of TiN from austenite, thereby promoting a good dispersion of TiN via strain-induced precipitation.