Crystallographic Understanding of the Effect of Ni Content on the Hardenability of High-Strength Low-Alloy Steel

A matrix structure with high strength, such as lath martensite/bainite is created via quenching to achieve conventional high-strength low-alloy ultra-heavy plates. Subsequently, this structure is tempered to improve its toughness. However, it is usually impossible to avoid the low cooling rate in the center of the ultra-heavy plates during cooling, causing inhomogeneous microstructure and mechanical properties along the normal direction. Therefore, it is necessary to enhance the hardenability of the alloy. At lower cooling rates, granular bainite/ferrites are formed in the center of the plates with low hardenability. While this leads to the incompletely transformed martensite/austenite islands (M/A islands), which often cause cracks, fewer high angle grain boundaries (HAGBs) are also formed, which can effectively impede crack propagation. Therefore, improving the strength, toughness, and hardenability is crucial for the development of high-strength low-alloy steel. The addition of nickel can improve the hardenability as well as the toughness of the heavy plates. In this study, two high-strength low-alloy steels with different nickel contents are designed. In addition, the effect of nickel content on hardenability and phase transition temperature is tested using end quenching and thermal mechanical simulation testing. The effects of nickel content on the microstructure and crystallographic characteristics of coherent phase-transformed products are characterized using SEM and EBSD. The results reveal that the increased nickel content greatly improves the hardenability and significantly reduces the phase transition temperature. At a low cooling rate of 0.5 degrees C/s, the microstructure of 2.94Ni steel is lath bainite, and the M/A islands are dispersed on a thin film, forming a phase transformation mode with higher HAGB density, block boundary density and V1/V2 variant pair content, and high hardness. This mode is dominated by the close-packed plane group. While the microstructure of 0.92Ni steel is granular bainite and the M/A islands are distributed in coarse blocks, forming a phase transformation mode with lower HAGB density, block boundary density and V1/V2 variant pair content, and significantly low hardness. Moreover, this mode is dominated by the Bain group. Additionally, the results demonstrate that at the cooling rate of 0.5 degrees C/s, as nickel content increases, the driving force of phase transformation is greatly improved to obtain a higher transformation rate than the steel with low nickel content. The maximum carbon content of untransformed austenite is higher, which promotes the complete transformation of bainite and produces fewer M/A islands. Therefore, this research possesses great potential for the composition design and process control of high-strength low-alloy steel.