Through a Controlled Quenching to Achieve a Good Combination of Mechanical Properties in Low‐Yield Ratio 900 MPa High‐Strength Low‐Alloy Steels

Two distinct heat treatments, that is, intercritical quenching combined with low‐temperature tempering (QT) to control the initial quenching temperature, and salt‐bath quenching combined with partitioning (Q&P) to regulate the final quenching temperature, were employed on high‐strength low‐alloy (HSLA) steels to achieve multiphase microstructures characterized by high strength, low yield ratio, and good impact toughness. Comprehensive experiments involving tension test, low‐temperature impact, microstructural observation, and in situ tension have been conducted to compare the microstructures and mechanical properties. It is found that both kinds of specimens can achieve a good match between high strength‐toughness and low yield ratio. The microstructures are composed of lath martensite with ferrite for QT specimen and tempered martensite with bainite for the Q&P specimen, a combination of “soft” and “hard” phases thus resulting in a yield ratio lower than 0.85 for 900 MPa low‐alloy steels. Notably, the Q&P specimen exhibited a markedly superior uniform elongation of 6.1% compared to the QT counterpart of 3.2%, a phenomenon attributed to the work hardening rate during deformation. A combination of strong and weak lath structures in tempered martensite and bainite can induce dislocation propagation and the carbides can act as obstacles to dislocation motion, jointly enhancing work hardening.