Balancing strength and toughness by QLT process in a low-Ni heavy steel plate with GPa grade

Given the developing high-performance and cost-effective heavy steel plate, the quenching-lamellarizing-tempering (QLT) treatment was conducted on a designed low-Ni steel. Results revealed that the introduction of the “L” process can achieve impact absorbed energy of up to ∼130 J in the transverse direction, while ensuring GPa-grade high strength. Microstructural analysis showed that high-temperature lamellarizing promoted matrix recovery and recrystallization, facilitated the formation of reversed austenite, and resulted in a composite structure predominantly featuring tempered martensite (TM), intercritical ferrite (IF), and a considerable volume of retained austenite (RA). However, the DICTRA simulations indicated that with increasing lamellarizing temperatures, the C content within the reversed austenite decreased, compromising its thermal stability and leading to the transformation of some unstable reversed austenite into fresh martensite (FM). Consequently, samples treated at 700 °C exhibited a comparable RA content to those lamellarized at 675 °C, while the dislocation density increased appreciably due to FM formation, culminating in elevated ultimate tensile strength (UTS) and reduced total elongation (TEL). After tempering, the intensive precipitation of Cu-rich particles in FM compensated softening associated with high-temperature lamellarizing through precipitation strengthening and increased the yield strength (YS) by 87 MPa. Notably, tempering did not alter the RA content in the experimental steel; instead, it enhanced the enrichment of C elements in the RA, thus the sustained transformation-induced plasticity (TRIP) effect can be obtained by stabilizing RA. In addition, the excellent cryogenic toughness of QLT samples was attributed to the pronounced plastic deformation of the soft TM/IF matrix and the inhibition of crack propagation by RA. Despite the minimal impact of tempering on low-temperature toughness, the elastic strain energy was noticeably improved due to the precipitation of nano Cu-rich particles.