A novel phase transition behavior during dynamic partitioning and analysis of retained austenite in quenched and partitioned steels

Abstract

We have studied here the carbon partitioning during continuous cooling at different cooling rates using dilatometer. An interesting phase transition was observed involving multiple stages of transformation during dynamic partitioning and a mechanism was proposed to explain the observations. Two morphologies of nano-sized retained austenite were observed, film-like and block-type. It was proposed that the amount of retained austenite was stable in a certain range of cooling rate. When the cooling rate was between 0.05°C/s and 1°C/s, the carbon partitioning was adequate and ~ 11 to 14% retained austenite was obtained. With increase in cooling rate from 5 and 10°C/s, carbon partitioning was not adequate, which resulted in decrease in film-like austenite. The austenite adjacent to ferrite had two different transformation products, M/A (martensite-austenite island) and twin martensite, based on the degree of carbon transfer from ferrite. However, macro-hardness test showed that cooling rate had little effect on hardness and the hardness was between 398 HV and 407 HV at cooling rate of ~ 0.05 to 10℃/s. Additionally, samples cooled at low cooling rate indicated continuous TRIP effect and excellent combination of high strength 1030MPa and high elongation of ~ 25% was obtained in the plate cooled at 0.1℃/s. On the basis of results, a strategy was proposed to obtain high performance hot rolled Q&P steels. The study confirmed the viability of implementing quenching and partitioning process in the hot rolling production line.