Thermomechanical processing of eutectoid steels: strategies to improve the microstructure of the hot rolled strips

Eutectoid steel strips are designed for the production of parts for intensive use such as clutches, seat slides, and springs as they exhibitexcellent strength levels and wear resistance. These properties arise from the unique morphology of lamellar pearlite which can be consideredas a self-laminated nanoscale composite. However, a spheroidization annealing step is nowadays necessary to improve the cold forming properties before further cold rolling steps.This thesis is aimed at improving the tensile ductility of the hot rolled products of eutectoid composition in order to eliminate the intermediateannealing step. Two strategies are proposed.The first is to transpose the concept of controlled rolling developed for HSLA toeutectoid steels. Through a strict adjustment of the austenite processing and of the cooling strategy, it is possible to improve the ductilityof the final lamellar microstructure. The way the processing parameters influence the hot deformation of austenite, the eutectoid transformation and of the subsequent spheroidization annealing is deeplyinvestigated. It is found that refinement and pancaking of austeniteis beneficial as it reduces the pearlite block size improving the totaltensile elongation. Accelerated cooling is of paramount importance toachieve fine Interlamellar spacing (ILS), which lead to high strengthlevels and accelerate spheroidization during subsequent annealing.The second approach involves intercritical or warm deformation. Warm processing of eutectoid steels is first explored by torsion testingand then up-scaled to a pilot rolling-line. The interactions between thermomechanical parameters, rolling forces generated and microstructuralevolution are carefully scrutinized. During concurrent hot deformation, spheroidization of cementite takes place almost instantaneouslyin both torsion and rolling. The restoration processes occurring in the ferrite matrix depends on the strain path and the strain rates. Low strain rates (0,1 s−1) and simple shear promotes the formation of a recrystallized-like HABs network of about 3μm in size.Plane strain compression and high strain rates (10 s−1) leads to the formation of a typical recovered dislocation substructure (LABs) of 1μm in size. During annealing, no recrystallization occurs and the LABs substructure remains stable. This substructure influences drammatically the mechanical properties: the strength is very high and the work-hardening behavior is poor due to high recovery rate in the region close to the LABs. However, due to the presence of spheroidizedcementite particles the ductility of warm rolled eutectoid steels is higher than that of ultra fine grained low carbon steels.Doctorat en Sciences de l'ingénieur