Modelling austenite decomposition: bainite, acicular ferrite, heat of transformation and effect of deformation
Important progress has been made in the development of metallurgical models that describe the behaviour of steel both during steel processing and during its application. However, the successful application of through process models in new steel product development still faces a number of scientific challenges. In modern strip steel grades, the steel production has a significant influence on its final properties. Furthermore, the trend towards complex multi-phase microstructures requires very sophisticated models to describe the mechanical properties of these steel grades.
Background of research
In the 3D microstructure evolution model framework, currently known as Cellular Automata Sharp Interface Phase Transformation (CASIPT), the austenite to polygonal ferrite and pearlite transformations are in most situations described with acceptable accuracy. However, the transformation of austenite to bainite or to acicular ferrite is not described or not accurate enough. Furthermore, the influence of transforming from austenite in a deformed state (as opposed to from recrystallized austenite) is also not (accurately) included in either the online or the CASIPT framework. These omissions limit the use of the CASIPT model for product development and limit the accuracy of the online control models. For controlling the temperatures in the hot strip mill run-out-table and in the continuous annealing lines it is also required that a model becomes available that describes the heat of transformation of bainite.
Goal of the project
The main goal of this research project was to develop and implement a model capable to reproduce the bainite reaction in the CASIPT framework. The bainite product is extremely relevant for the development of dual-phase steels: on one side it contributes to the mechanical properties of the steel, but on the other side, it would in principle be desirable to avoid the formation of bainite in such steel grades. A physically-based model that is able to predict the conditions in which bainite forms and at which rate, is valuable for industrial applications. It leads to better process control and allows for the design of new alloys. Furthermore, other steel products, such as complex phase and TRIP steels, rely on the presence of the bainite microconstituent for achieving certain properties. While the currently implemented submodels for growth of pro-eutectoid ferrite from austenite rely on the isotropic nature of the growth of the ferrite phase, the bainite growth is anisotropic, leading to the development of lath or plate-like morphologies.
In the bainite submodel developed in the present work, lengthening of the bainite microconstituent is considered most significant during growth, and thickening and widening areas are neglected. The shape of the bainitic product is controlled by the initial shape of the nucleus (see figure).
The martensite transformation creates microstructures very similar to bainitic microstructures. Additionally, due to the importance of martensite transformation, a martensite submodel was developed in the CASIPT framework.
The project was performed by postdoctoral researcher Dr. Arthur Nishikawa under supervision of Prof. Maria Santofimia Navarro at TU Delft in collaboration with Kees Bos from Tata Steel.
The project T17019h is part of DENS program and got PPS funding in 2018-2021.
Dr. Kees Bos (Tata Steel)
The bainite and martensite submodels developed by Arthur have already been integrated into our Through Process Model (TPM). This leads to more accurate description of the microstructural evolution during thermal cycles, which can be used for the optimization of process parameters. Also, microstructures generated by CASIPT simulations have been used as input for crystal plasticity simulations using the DAMASK model. This creates the possibility of using the CASIPT simulations for designing new steel products aiming at specific mechanical properties.
Dr. Arthur Nishikawa (TU Delft)
The bainite transformation is one of the most complex – and still not fully understood – phenomena in the metallurgy of iron alloys. I like to think that the skills I have acquired during my education in Brazil prepared me well for this challenge. Working in such a challenging field is definitely a motivational factor. On the other hand, moments of frustration are inevitable and the support by my supervisors and colleagues has been fundamental for coping with that. It has been a privilege to work among so many competent researchers in the DENS programme. From both professional and personal sides, it is a lifetime opportunity.