Abstract
In low-carbon steel, obtaining ultra-high-strength inevitably deteriorates ductility and cryogenic toughness. Therefore, in this study, we investigated the role of “lamellarization” on the fraction and distribution of retained austenite on a low-carbon multi-alloyed steel subjected to a novel quench-lamellarization-intercritical tempering (QLIT) treatment and elucidated its effects on mechanical properties and fracture mechanisms. The performance of the proposed method was compared with that of quench-intercritical tempering (QIT) treatment. The results showed that compared with QIT (800 °C + 620 °C) treatment, adding an L-step in QL680IT and QL710IT significantly increased the fraction of retained austenite and stimulated VC precipitates inside tempered martensite. Lamellarization temperature modified the alloying element enrichment and the first martensitic formation, which further influenced the nucleation and kinetics of reverse austenitic transformation during the subsequent IT process. Ni-, Mn-, and Cr-enriched retained austenite had high thermal stability after holding at −196 °C. Nearly all the retained austenite transformed to martensite under constant tensile and fast impact loads. The transformation-induced plasticity of the high fraction retained austenite significantly improved the strain hardening rate and delayed necking, increasing the elongation, and maintaining tensile strength of 1 GPa. Moreover, the transformation-induced toughness of retained austenite with homogeneous distribution increased the impact energy from 27 to 84 J at −196 °C. The filmy retained austenite at the tempered martensite packet and block boundaries delayed the crack initiation and arrested its propagation via crack deflections, crack-tip passivation, and crack closure, which improved the crack initiation and propagation energies and changed the impact fracture from cleavage facets to large dimples. In addition, the dispersion of nanoscale vanadium carbides in multi-step heat treatment effectively improved the strength without reducing the low-temperature toughness.
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