The prediction and avoidance of cracking in long product hot rolling. Phase II (Pacrolp-II) : final report

Pacrolp-II project is aimed at minimising ductility break-ups on 'apparently sound' as-cast semis (blooms/billets), which are prone to surface cracking during reheating/rolling. The main singularity of this project has been a multiscale analysis combined with a through process evaluation (casting/ reheating/rolling) of damage by experiments and simulation. The study of as-cast structures has led to identify the microstructural features that are relevant in subsequent process steps (grain distributions, nature, distribution size and location of MnS inclusions, incipient solidification damage ...). The reheating is an important step that can eliminate, enhance or provoke surface defects, depending on the steel grade and furnace conditions. A wide range of mechanical tests, combined with fine microstructural evaluation and FEM models for analysis of damage levels, have allowed: the definition of thresholds of triaxiality and strain for nucleation and cracking, the evolution of plasticity of inclusions with applied strain, a better understanding on the interaction between austenite grain boundaries and MnS inclusions in the early stages of damage nucleation and the relevance of inclusions spatial distribution in all the steps of damage evolution. Multiscale modelling has been developed to study the effect of macro processing and as-cast conditions on the MnS inclusions at the scale of interest. These models, in combination with laboratory tests and analysis, have allow the identification and quantification of a high number of factors (micro-macro) intervening in the damage process. The relevant role of stress triaxiality/strain path suggests the convenience of proper design of entry bite geometry and grove radius in roughing passes.