Effect of hot deformation and cooling rate on phase transformations in low carbon HN5MVNb bainitic steel

摘要

Deformations at temperatures 900 ℃, 860 ℃, 810 ℃ and 780 ℃ in the consecutive amounts 24%, 20%, 19% and 18.5% were applied to low carbon HN5MVNb bainitic steel using hot compression testing in dilatometer Bahr 805 followed by continuous cooling. The results show clearly that the kinetics of the austenite decomposition were depended on local equilibrium conditions between recovery, recrystallization and phase transformation processes for a given cooling rate. Bainite transformation was accelerated when sample was cooled after deformation at cooling rate 60 ℃/s. At lower cooling rates than 5 ℃/s down to 0.5 ℃/s, bainite transformation was postpone when comparing its kinetics with those for non deformed steel. The bainitic transformation cannot be fitted to a single transformation mechanism owing to the formation of carbides. Different behavior was observed for austenite to ferrite transformation. Usually it was accelerated with consecutive deformations of the steel for all cooling rates used in experiments.