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

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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.