The key to the properties of steel grades lies within their microstructure. So, metallurgists are very keen to know how the microstructure looks like in order to improve the properties of the steel.
However, during the processing of hot-rolled grades, the final microstructure – and, consequently, the steel’s properties – depends strongly on the successive deformations taking place during hot rolling, especially before the decomposition of the high temperature phase called austenite.
The main issue to be resolved, of course, is that the austenite itself has disappeared after phase transformation. To reveal the “original” microstructure from the transformed structure, our team studied various methods – such as chemical etching of the phosphorus segregation where the austenite grain boundaries were located, or oxide etching of the austenite grain boundaries at high temperature – but these methods can only be used if certain conditions are met. Now, thanks to diffraction tools, a new and more reliable method can be used based on crystallographic calculations.
Recovering the grain shape and orientation of the pre-transformed phase is crucial for understanding microstructural evolution during any hot deformation process (hot rolling, welding, etc.). In turn, this enables better control over the material properties to be obtained.
In 2012, OCAS developed a model, based on crystallographic considerations and using experimental electron backscatter diffraction (EBSD) mappings as input. This original method succeeds in overcoming the limitations of traditional techniques and has already received international recognition.
Making toughness predictable
In 2013, our project proposal “AUSTOUGH: Austenite reconstruction tool for low-temperature toughness control in heavy gauge steels” was accepted for funding by the Research Fund for Coal and Steel (RFCS). Together with the other project partners, fundamental knowledge on the link between austenite microstructure and final toughness will be gathered so that the model can be refined. The objective is to optimise the rolling parameters to improve the toughness of steel even further.
For OCAS, the outcome of the AUSTOUGH project will be valuable in further improving our lab-scale rolling simulations of our customers’ industrial processing.
“We have already made huge progress in the past two years with regard to austenite reconstruction. But we continue to strive for perfection!”