Microstructure-property relationship of a high strength-toughness Cr-Mo-V steel

The demand for oil casing steel with ultra-high strength and excellent impact toughness for safe application in ultra-deep wells is pressing. Aiming at improving the combination of strength, ductility and impact toughness, the designed Cr-Mo-V micro-alloyed oil casing steel was quenched at 800°C, 900°C, 1000°C, followed by tempering at 600°C, 680°C, 760°C respectively to obtain distinct microstructures. The results showed that the microstructure of the samples quenched at 800°C followed by tempering consisted of untransformed ferrite and large undissolved carbides, which deteriorated both tensile strength and impact toughness significantly. For other conditions, the nucleated carbides and the boundaries are key factors that balance the tensile strength from 1226 MPa to 971 MPa and impact toughness from 65 J to 236 J. From the perspective of carbide, optimal precipitation strengthening is achieved with a small carbide size obtained by low tempering temperature of 600℃, while larger-sized carbides would significantly soften the matrix to improve the toughness but deteriorate the tensile strength. Additionally, an increase in prior austenite grain size with the corresponding enlarged sub-boundaries obtained by high quenching temperature substantially diminishes grain refinement strengthening, dislocation strengthening and the energy absorbed in crack propagation process, which is unfavorable to strength and toughness.