Integrated Computational Modeling of Run-Out Table Phase Transformations
Creators: Karl, Ry
Publisher: name: University of Alberta Library
Year: 2023

<p>A thermal-microstructural model was developed to predict the effect of laminar cooling during TMCP on the through thickness microstructure of an X70 steel. Model development was conducted in three sections, beginning with microstructure modelling of dilatometer samples with cooling rates ranging from 1 - 120 ℃/s. This was followed by thermal modelling using a finite element method. The thermal and microstructure models were then unified into one using a modified form of the Scheil additive principle. Using this unified model, microstructure predictions for an industrially produced 15 mm X70 sample were made, and shown to be in good agreement with measured values. </p>

<p>A microstructure model was developed for the simultaneous transformation of two phases during continuous cooling. This model was used on the data acquired from dilatometry testing of samples cooled at 1-, 5-, 15-, 22-, 30-, 50-, 80-,<br>
and 120 °C/s. As a result, transformation curves for high and low temperature phases were produced for each dilatometer sample. A combination of optical and scanning electron microscopy were then used to characterize the phases in each sample. From this, it was found that below 50 °C/s, the primary phases were ferrite and acicular ferrite, while above this they were acicular ferrite and bainite. Model validation was done using EBSD band contrasting, where the fraction of each phase was determined based on the pattern quality of each pixel. </p>

<p>Thermal modelling was carried out in ABAQUS, and used to capture the effect of different laminar cooling configurations on the through thickness temperature profile. Infrared thermography and pyrometer temperature measurements were gathered during plant trials and used as model inputs, while a modified spray boiling curve was used as a fitting parameter. Assessment of infrared data lead to the observation of abnormal temperature fluctuations across the width of the skelp. A method was developed to characterize these anomalous temperatures, and it was determined that they were the result of oxides on the skelp surface. A program in Python was developed to track the location and size of these oxides prior to coiling, and additional thermal models that included the presence of oxides were developed. </p>

<p>The thermal and microstructure models were then combined into one by using a modified form of the Scheil additive principle. In this approach, austenite decomposition curves were generated for predicted temperature profiles from thermal modelling, and then decomposed using the microstructure model. Validation of this combined model was done by conducting EBSD band contrasting on an industrially produced X70 sample. Once validated, the model was used to make predictions for alternate laminar cooling configurations: presence of oxides, lower coiling temperature, early cooling, and late cooling. </p>

<p>Based on the results from the thermal microstructure model, it was observed that changing the cooling configuration had little influence on the end microstructure. What this illustrates is the inability to produce significant changes to the microstructure through cooling. From dilatormety testing, it was shown that for this grade of steel, at cooling rates achievable on the ROT, a combination of ferrite (~ 80%) and acicular ferrite is always produced. From an industrial standpoint, these results emphasize the importance of chemistry and their pivotal role in determining the end microstructure of the steel. Further work should focus on determining how to manipulate steel chemistry to achieve desired types and fractions of phases.  </p>