Determination of Thermal Effects of Diffusionless Phase Transformations in Low-Carbon Low-Alloy Steels at High Cooling Rates

摘要

We have designed an original laboratory bench allowing one to model technological rates of cooling of hot-rolled sheets in the process of controlled thermomechanical processing. The bench consists of a heating furnace with a mechanism for sample extraction and fixation, a temperature measurement unit comprising three pyrometers, an air-cooling unit with a pneumatic system, a unit for management and information recording. The sample cooling rate is controlled through sample thickness and pressure in the air cooling system. We use samples cut from an industrial sheet of the 06G2MB-type low-carbon low-alloy pipe steel, which is intended for production of large pipes of the X80 strength class. We have obtained time dependencies of sample temperatures at cooling rates ranging from 50 to 700 ℃/s. The heat effect of bainitic transformation is equal to 120 ± 5 kJ/kg, this being twice the thermal effect of martensitic transformation for low-carbon steels.