Heavy gauge structural steel plates are often (at least partially) rolled in the two-phase region due to the large temperature gradients existing both over thickness and width. In some cases, the last passes are deliberately applied in the intercritical region in order to increase plate strength. Unfortunately, industrial practice shows that intercritical rolling may be detrimental for Charpy V-notch (CVN) impact toughness.
The exact effect of each process parameter (initial ferrite fraction, reduction applied and cooling strategy) on the strength/toughness balance was investigated during an extensive rolling campaign carried-out at pilot scale. The CVN impact results confirmed that plates rolled in austenitic region have lower ductile brittle transition temperatures (DBTT) and higher upper shelf energy (USE) than the intercritically rolled plates. The impact toughness properties resulting from intercritical rolling are linked to microstructures and textures obtained by an in-depth EBSD characterisation. The texture evolution during ferrite rolling alone does not explain the considerable decrease in toughness. On the contrary, a good correlation is found between DBTT and grain size delimited by high-angle boundaries (HAGBs), whose misorientation is above 15° (effective grain size). The lack of ferrite recrystallization explains in turn the lower USE values measured.
Finally, the results are summarized in strength/toughness plots and process window maps. These tools are aimed at helping steel producers to define robust hot rolling practice and ensure homogeneous plate properties.