Impact of second phase precipitation and solute drag effects on austenite grain growth

In metallurgical downstream processes, the high-temperature austenitic microstructure plays a critical role in determining material performance. For example, a coarse microstructure can lead to issues such as slab surface cracks in continuous casting (CC), embrittlement, hot tears, and even fracture. To mitigate these risks and enhance both surface quality and mechanical properties, refining the microstructure is essential. Austenite grain growth (AGG) is governed by multiple mechanisms that correlate with chemical composition and thermal processing conditions.

The focus of the dissertation was to conduct experiments and incorporate those into a model for predicting austenite grain growth for steelmaking and metallurgical processes.

Alloys were examined in situ for austenite grain growth in isothermal trials using high-temperature laser scanning confocal microscopy (HT-LSCM) at temperatures of 950 °C to 1350 °C. Microstructural quantification was performed using the linear intercept method, following ASTM E112 standards. The data obtained were incorporated into the grain growth model that accounts for the growth inhibition effects of impurity-induced solute drag and pinning forces exerted by second-phase particle precipitations. Key parameters in this model include grain boundary mobility (GBM), binding energy (E_0) of impurities segregating to grain boundaries (GB) and pinning force (P_Z) of precipitates.

This dissertation comprises several publications covering various aspects of austenite grain growth and its inhibition mechanisms. As a starting point, high-purity Fe (99.98 %) was examined to determine the intrinsic GBM of pure austenite. Results demonstrated that increasing the annealing temperature leads to higher GBM, following an Arrhenius-type correlation.
Next, Fe-P alloys were systematically investigated and the experiments revealed that P induces a solute drag effect, reducing GBM and slowing down grain growth rates. In the modeling framework, this effect was quantified using the parameter E_0.
Further investigations into Fe-C alloys revealed that even traces of S induce a strong solute drag effect. While low additions of C did not significantly affect GBM, exceeding a certain threshold of C led to an effectively displacing of S from grain boundaries. This resulted in a sharp increase in GBM, approximating to the intrinsic mobility of pure iron, as C itself does not induce a solute drag effect. These findings were supported by density functional theory (DFT) calculations, which too revealed the competition of S and C at GBs. The results from DFT experiments were successfully reproduced in a mean-field model, demonstrating that competitive segregation between C and S promotes enhanced grain growth at higher C levels.
In Fe-C-P alloys, GB interactions between C and P were systematically investigated. The results demonstrated that the segregation tendency of P can be mitigated by the presence of C, and thereby enhances GBM. Compared to Fe-P alloys with similar P content, Fe-C-P alloys exhibited significantly higher GBM. The addition of 0.20 wt.-% C was shown to suppress P-induced solute drag at elevated temperatures, promoting accelerated austenite grain growth. DFT simulations corroborated the experimental findings, revealing that C and P competitively segregate to GBs. This atomistic insight supported the hypothesis that C weakens the drag effect of P by reducing its grain boundary coverage. The close agreement between in situ experiments and ab initio simulations reinforces the proposed mechanism and aligns well with existing literature on competitive C - P segregation behavior in austenite.
Finally, the role of Nb(C,N) precipitations on AGG was investigated in Fe-C-Nb-N alloys. Results demonstrated that even small amounts of Nb(C,N) effectively pin grain boundaries, thereby inhibiting effectively grain growth. However, as the annealing temperature increases, precipitates tend to coarsen and cluster, reducing their pinning forces. At even higher temperatures, they dissolve, which enables higher grain growth rates.
The developed methodology utilized in situ HT-LSCM experiments to observe austenite grain growth. The results were used to parameterize solute drag and pinning effects in a mean-field model for austenite grain growth. Additionally, DFT simulations were employed to calculate grain boundary segregation tendencies of impurities, which were then used as solute drag parameters in a separate mean-field model. A key achievement of this work is the strong alignment between the predictions of both mean-field models, confirming the validity of the developed methodology. This approach serves as a robust parameterization method for grain growth models, particularly for industrial applications. The model can thus aid in optimizing microstructures, for example, in continuous casting, contributing to the production of higher-quality slabs with reduced surface defects and cracks.