Microstructure Evolution as an Effect of Strain Reversal During the Hot Deformation of Microalloyed Steels

Susceptibility of the steel for further cold processing strongly depends on the whole prior history of deformation in the hot working regime.In the industrial hot working processes,e.g.at the roughing stage of hot rolling,or forging,material undergoes complex deformation modes that leads to gradients of both microstructure and properties across the deformed cross-section.Torsion and multiaxial compression tests with different amounts of strain reversals were conducted at elevated temperatures to study the effect of applied strain path on the phenomena occurring during microstructure evolution of Nb-microalloyed steels.Gathered results will provide data for the modification of existing constitutive equations that mostly do not account for the strain path changes.Better through-process modelling will then enable to achieve better properties and quality of the products for further cold processing.

Effect of Strain Path Reversal on Austenite Grain Subdivision and Deformation Induced Ferrite Transformation Studied by Hot Torsion

The effects of large strain and strain path reversal on the deformation microstructure evolution in austenite below the recrystallisation temperature were studied by hot torsion using a non-transforming Fe-30wt%Ni model austenitic alloy.Results show that the high angle boundaries (HABs) can be generated by both microstructure mechanism through dislocation accumulation and texture mechanism via subgrain rotation.However,multiple strain path reversals lead to less well-developed HABs in the original grains compared to single reversal deformed to the same amount of total accumulative strain.This effect is attributed to the subgrain rotation mechanism being less effective at small strains.In comparison,the same hot torsion tests were conducted using a microalloyed steel at a temperature between Ae 3 and Ar 3.After single strain path reversal,substantial deformation-induced austenite-to-ferrite phase transformation was observed.Meanwhile,a test with multiple strain path reversals but with the same total strain produces much lower levels of strain-induced ferrite formation.This difference is correlated to the observations made in the Fe-30wt%Ni model alloy.It is believed that the different amount of strain-induced ferrite originated from the different levels of strain-induced HABs within the austenite which act as ferrite nucleation sites.