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

In addition, 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.