Impact of Mo/Ni alloying on microstructural modulation and low-temperature toughness of high-strength low-alloy steel

The high-strength low-alloy steel plates with varying Ni/Mo contents were manufactured using the thermos-mechanical control process. The investigation was conducted to explore the effect of Ni/Mo microalloying on microstructure evolution and mechanical properties of the steel. The results revealed that the increase in Ni content from 1 to 2 wt.% reduced the transition temperature of ferrite and the growth range of ferritic grain was narrowed, which promoted grain refinement. The optimized combination of grain size, high-angle grain boundaries (HAGBs), and martensite-austenite (M–A) islands parameter contributed to the excellent impact toughness of S1 steel at –100 °C (impact absorbed energy of 218.2 J at –100 °C). As the Mo increases from 0 to 2 wt.%, the matrix structure changes from multiphase structure to granular bainite, which increases the average effective grain size to ~ 4.62 μm and reduces HAGBs proportion to ~ 36.22%. With these changes, the low-temperature impact toughness of S3 steel is weakened. In addition, based on the analysis of the characteristics of crack propagation path, it was found that M–A islands with low content (~ 2.21%) and small size (~ 1.76 μm) significantly retarded crack propagation, and the fracture model of M–A islands with different morphologies was further proposed. Furthermore, correlation between behaviour of delamination and toughness was further analysed by observing delamination size and impact energy parameters.