Microstructural characteristics with various cooling paths and the mechanism of embrittlement and toughening in low-carbon high performance bridge steel

Based on ultra fast cooling (UFC), the microstructural characteristics and mechanical properties with various cooling paths and the mechanism of enbrittlement and toughening for different microstructural characteristics in low-carbon high performance bridge steel were investigated in details using optical microscope (OM), scanning electron microscope (SEM), electron back-scattered diffraction (EBSD), electron probe micro-analyzer (EPMA) and transmission electron microscope (TEM). The results show that using UFC can effectively refine the size of M/A constituent, promote the formation of lath bainite with high misorientation between laths, suppress the re-partition of carbon, and enhance the relative frequency of high-angle grain boundaries during bainite transformation. However, at the higher UFC cooling finish temperature of 560 °C, the bainite transformation mainly takes place during air cooling. The larger block-form M/A constituent is almost twin martensite with zone axis of B=[113] and twin plan of (pqr)=(21−1) due to sufficient re-partition of carbon and carbon concentration of approx. 0.22 wt% not making residual austenite so stable that they become twin martensite below the martensite transformation start temperature (Ms). The balance of high strength with yield strength of 876 MPa and better toughness with ductile–brittle transition temperature (DBTT) of lower than −60 °C was realized using the cooling path of UFC→400 °C→air cooling. ition, based on observation and analysis of cracks initiation and cracks propagation, the mechanism of embrittlement and toughening for the cooling paths of UFC→560 °C→air cooling and UFC→400 °C→air cooling, respectively, was discussed in details. For the cooling path of UFC→560 °C→air cooling, the microcracks can easily nucleate at larger block-form brittle twin martensite or twin martensite–matrix interface and easily propagate through twin martensite or along twin-martensite–matrix interface; furthermore, low-angle grain boundaries, even high-angle grain boundaries, cannot effectively arrest cracks propagation, resulting in the higher DBTT. However, for the cooling path of UFC→400 °C→air cooling, the microvoids can hardly nucleate at fine M/A constituent or carbides and their growth is not along lath boundaries, but through bainite laths with high misorientation between laths. Moreover, the larger plastic deformation is observed at turning sites or coalescence sites, resulting in the lower DBTT.