Tailoring Low-Alloyed Cryogenic Steel Strong and Tough Introduces Fine-Grained Equiaxed Ferrite by Intercritical Subquenching

Herein, a novel approach is developed to enhance the strength, ductility, and low-temperature toughness of high-strength low-alloy steels, overcoming the traditional trade-off. By incorporating an intercritical subquenching step during the conventional quenching and tempering (QT) process, a microstructure featuring refined lath bainite and a small amount of fine equiaxed ferrite is achieved. The resulting steel exhibits a strength similar to that of QT steel with significantly improved ductility and low-temperature impact toughness. Compared to traditional QT steel, the new process steel maintains a similar strength. In addition, the elongation increases from similar to 22.2% to similar to 24.9%, and the impact energy significantly increases at -50 C-degrees from similar to 233 to similar to 347 J. The fine grain size and dense lath structure contribute to the minimal reduction in strength. The presence of equiaxed ferrite and bainite improves the ductility, while plastic deformation and special grain boundaries play a crucial role in enhancing the low-temperature toughness. This innovative heat treatment method offers an economical solution for producing high-strength low-alloy cryogenic vessel steels without compromising strength while greatly improving ductility and low-temperature toughness.