Medium Mn steels are promising candidates for the third-generation advanced high strength steels. However, it shows a distinct limit of mechanical property under conventional hot-rolling and intercritical annealing procedures. Aiming at improving the combination of strength, ductility and toughness, the present study proposed a multi-alloyed design principle together with a direct intercritical rolling process. The results show that the microstructure of the hot rolled steel consisted of wide delta ferrite, lath martensite, twinned-martensite and coarse retained austenite with a low stability, corresponding to a product of strength and elongation (PSE) of 24.0 GPa% and an impact toughness of 13.8 J/cm 2 at −40 °C. In contrast, the intercritical rolling produced a laminated microstructure containing thin delta ferrite, submicron recrystallized ferrite and retained austenite. The PSE and impact toughness at −40 °C simultaneously increased to 46.1 GPa% and 46.7 J/cm 2 , respectively, which was also much higher than the hot rolled steel subjected to low temperature and high temperature annealing. The significant enhancement of mechanical property was mainly attributed to the highest content and optimal stability of retained austenite caused by the submicron grain size, high density of dislocations, granular morphology, and elemental enrichment. The optimized austenite stability provided persistent transformation-induced plasticity (TRIP) effect for work hardening and damage resistance. Moreover, the nano-sized VC precipitates in the intercritical rolled steel played a significant role in reaching an ultrahigh yield strength of ∼1 GPa. This study not only provides a very simple pathway to improve the mechanical property of medium Mn steel, but also sheds light on the design and development of high-performance materials. 

Tailoring retained austenite and mechanical property improvement in Al–Si–V containing medium Mn steel via direct intercritical rolling

2.3 GPa cryogenic strength through thermal-induced and deformation-induced body-centered cubic martensite in a novel ferrous medium entropy alloy

Making low-alloyed steel strong and tough by designing a dual-phase layered structure

Maraging steel with ultra-high strength and good ductility is usually achieved via semi-coherent nanometer-sized precipitates and a dual-phase structure. In this work, we studied the effect of solution treatment parameters on the microstructural evolution and the mechanical properties of cobalt-free maraging steel, with high strength (1852 MPa) and satisfactory tensile elongation (11.5%) at room temperature was obtained. The nano-scaled precipitates in the martensite matrix were analyzed, in which the η-Ni3(Ti, Mo) precipitates were distributed in width of 7.1 nm and length of 19 nm. It was found that the needle-like Ni3Ti phase plays a predominant role in enhancing the strength of the maraging steel, while the ductility can be attributed to the nano laminated austenite structure, which could effectively prevent the propagation of microcracks in the martensite matrix. A heterogeneous microstructure of precipitates was formed in the martensite matrix. Besides, the size and number density of precipitates could be regulated by tuning the solution treatment conditions, in which a finer size and a larger number of precipitates could be gained by annealing at a high temperature for a short time. 

Yuxuan Liu

Papers

Ultrafine-grained dual-phase maraging steel with high strength and excellent cryogenic toughness