Hot deformation behavior of HSLA steel Q690 and phase transformation during compression

A hot compression experiment was conducted on the High Strength Low Alloy (HSLA) steel Q690E, using Gleeble-3500 thermal/mechanical simulator, with a temperature of 950 °C and maximum compression amount of 80%. The deformation induced ferrite (DIF) was obtained under different compression amounts and cooling conditions. The stress-strain diagram during compression was mapped and the micro-hardness of the thermal simulated specimens measured. Results show that a 40% compression effectively accelerates the formation of DIF and refines the grain size. The grain sizes after 80% deformation are in a concentrating distribution ranges from 1 μm to 3 μm, and the average grain size is 2.34 μm. The formation of DIF leads to an obvious drop on the stress-strain curve and makes it shows a hump-like distribution characteristic, and also leads to a decrease of microhardness. The mechanism of DIFT is analyzed. A mathematical model is established between the effective grain boundary area and the compression amount. And its calculations indicate that the increasing of the effective grain boundaries area caused by the compressive deformation is one of the essential reasons for DIFT. The ferrite phase transformation driving force can be increased by the dislocation energy, and the critical dislocation energy to drive DIFT in this experiment is calculated to be 17.56 J/mol.