Speaker
Description
Austenite grain size is an important parameter for controlling the mechanical properties of microalloyed steels during hot strip rolling. With the installation of a laser-ultrasonic (LUS) measurement gauge at SSAB’s hot strip mill in Borlänge, a sensitive, non-contact inline method is available that allows the grain structure to be measured immediately after the final finishing stand.
This paper focuses on the development and use of the Pass Schedule Calculation (PSC®) model to describe and influence austenite grain size by optimized pass schedules. The process model PSC® represents, amongst other things, the thermo-mechanical conditions during rolling including temperature evolution, deformation distribution, recrystallization, precipitation and grain growth/refinement. In recent years, the model performance has been steadily improved by means of comparison with a large set of LUS measurements.
With the model thus improved, targeted optimizations of the pass schedule are now possible to specifically influence the mechanical properties of the final product. It was found that, for example, the adjustment of the transfer bar thickness has a significant impact. By increasing the transfer bar thickness, the accumulated deformation in the finishing mill increases, which tends to promote a more uniform recrystallization and a more homogeneous austenite grain size distribution along the strip length. The comparison between PSC®-predicted austenite grain size and LUS measurements shows that the model can reproduce the measured trends reasonably well.
The more uniform grain structure also influences downstream processes, especially cold rolling, where yield strength scatter is reduced. Overall, the results suggest that LUS measurements can be used as feedback information for PSC®-based pass schedule optimization.
In the future, combining PSC® and LUS measurements could further support microstructure-based pass schedule control in hot strip mills – for example to support grain refinement and reduce property variations.