Effects of vanadium on the microstructure and mechanical properties of a high strength low alloy martensite steel

• Precipitation behavior of (Ti, V)C precipitates was elucidated.
• (Ti, V)C precipitation can induce the grain refinement after reheating-quenching process.
• Ultra-fine austenite grains with the average size smaller than 5 μm were obtained.
• The microstructure transformed from ultra-fine austenite was analyzed.
• Excellent mechanical properties were achieved in ultra-fine grained steel.

The precipitation behavior of vanadium-rich carbides formed during the reheating processes and its influence on the microstructure and mechanical properties have been systematically investigated in a high strength low alloy martensite steel. It was found that the nano-sized (Ti, V)C particles mainly precipitated during reheating rather than soaking at the austenitization process. For their pinning effect on grain growth, ultra-fine austenite grain with the average size of 3.5 μm formed during holding at 880 °C for 1 h. As the austenitization temperature increased, the prior nano-sized (Ti, V)C coarsened and its volume fraction decreased, which led to increment of austenite grain size. Excellent combined mechanical properties such as tensile strength 1670 MPa, yield strength 1460 MPa, elongation 10% at room temperature and impact energy (Akv) 57 J at −40 °C were obtained in the ultra-fine grained steels. In particular, the low-temperature toughness was improved by grain refinement. The strengthening mechanisms were also investigated by comparing the experimental results with theoretical calculation. The variation of yield strength with the condition of heat treatment was discussed in detail.