Simulation of hot strip rolling by compression testing: Effect of the variation of strain-rate on the constitutive behaviour of a niobium-vanadium microalloyed steel

The major objective of this investigation was to test experimentally the relative validity of simulating hot strip rolling by compression testing at constant true strain rates under axisymmetric conditions.;Isothermal, single- and multi-stage compression tests were carried out on a microprocessor-controlled servohydraulic testing system by using a niobium-vanadium microalloyed steel. Cylindrical specimens were subjected to deformation at rates of strain either constant or varying as during a rolling pass/passes. True stress-true strain flow curves were obtained by correcting the measured stress-strain data for the effect of friction at the specimen/die interface. The constant friction shear factor was determined quantitatively by hot ring tests. Optical microscopy was used to observe the prior austenite microstructures.;Based on the results of single-stage, constant true strain-rate tests, the dependence of the flow strength as a function of strain, rate of strain and temperature was modelled by using an Arrhenius-type equation.;The comparison of the responses of the material to the two types of compression revealed no significant differences between flow curves except at the very end of the tests. This observation agreed with the predictions of the model, which was developed based on the constant strain-rate flow curves, indicating that the material follows a mechanical equation of state in hot rolling processes.;For the purpose of the simulation, the material's flow through the roll gap was theoretically modelled.;The flow curves and the average flow strengths were compared between the constant and varying strain-rate, four-stage tests. Differences were also found negligible. The variation of the average flow strength during multistage testing was dependent of the length of the interpass time and the average strain rate in a significant manner.;It was found that the microstructures developed from single- as well as multi-stage compression tests were independent of the variation of the strain rate during deformation.;This study showed that the constant true strain-rate compression testing is a very useful technique in simulating hot flat strip rolling processes.