Microstructural transformation and precipitation of an ultra-high strength steel under continuous cooling

Abstract

We investigated phase transition and precipitation of ultra-high strength steel (UHSS) in a new “short process” with controlled rolling and controlled cooling. Thermal expansion test combined with metallographic observation was used to research the continuous cooling transformation (CCT) curve. Moreover, the microstructural transformation and precipitation law was revealed by morphological observation and alloying elements by electron probe micro-analyzer (EPMA). Transmission electron microscopy (TEM) was utilized to analyze the composition and grain orientation of microstructure. The study showed that the measured critical transformation temperatures of Ac1 and Ac3 were 746 and 868 °C, respectively. The CCT curve indicated that the undercooled austenite was transformed into proeutectoid ferrite and bainite with HV 520 in a broad range of cooling rate of 1 °C•s−1. When subjected to a cooling rate of 1 °C•s−1, the undercooled austenite was divided into small-sized blocks by formed martensite. With further increase of cooling rate, micro-hardness increased dramatically, the microstructure of specimen was mainly lathe bainite(LB), granular bainite(GB), lath martensite (LM) and residual austenite. By diffraction test analysis, it was identified that there was K-S orientation relationship between martensite and austenite for {110}α//{111}γ, {111}α//{101}γ. EPMA clearly showed that carbon diffused adequately due to staying for a long time at high temperature with a lower cooling rate of 2 °C•s−1. Phase transition drive force was lower and the residual austenite existed in the block form of Martensite austenite island (M-A). With the increase of cooling rate to 10 °C•s−1, the block residual austenite reduced, the carbon content of residual austenite increased and α phase around the residual austenite formed into a low carbon bainite form.