Q&P process induced pure-lath structure with dual-phase to optimize the strength-toughness of low-yield ratio HSLA steels

In order to ensure security in the buildings, bridges and submarine pipelines, a good combination of high yield strength with a low yield ratio and excellent toughness should be achieved in the high-strength low-alloy (HSLA) steels. This study innovatively introduces salt-quenching combined with direct partitioning at 400 °C (Q&P-400) or 250 °C (Q&P-250) to produce a dual-phase structure of “hard” tempered martensite and “soft” bainite in a 0.16C-1.5Mn-1.5Ni-0.9Cu steel. Compared to the Q&P-400 sample, the Q&P-250 sample exhibited lower yield strength of ∼908 MPa but higher tensile strength of ∼1350 MPa, leading to a significantly lower yield ratio of 0.67 and improved low-temperature toughness of 65.5 J at −40 °C. The phase transformation during Q&P was analyzed by the OM, XRD, SEM, EBSD, TEM with EDS and thermal dilation. The superior mechanical performance is attributed to the pure-lath structure with large crystalline misorientation, which can effectively alleviate the interface sensitivity and hinder the crack propagation. The lath-structured bainite is transformed from retained austenite during partitioning process under the combined effects of elemental diffusion and coordinate deformation between recovery martensite. This new salt-bath Q&P treatment is notably more efficient and economical compared to the traditional off-line quenching and tempering (QT) method used for medium-thick plate production, which means the plates need to be transferred from the hot rolling line to the quenching line for the following heat treatment. This study presents a HSLA steel with low alloying contents and an on-line Q&P method with short process and low energy consumption for medium-thick plates production.