Mechanisms of strength-plasticity enhancement and stress-induced phase transition in a medium-carbon low-alloy steel

A medium-carbon low-alloy steel with designed chemical composition was investigated.The steel exhibits an excellent product of strength and elongation value of 31,832 MPa%through quenching and partitioning treatment,with a tensile strength of 1413 MPa and elongation of 22%.X-ray diffraction analysis and transmission electron microscopy characterizations confirm that the retained austenite in the specimens undergoes stress-induced phase transformation to the martensite and hexagonal phases,namely the transformation-induced plasticity(TRIP)effect is triggered.This TRIP effect,triggered by the stress-induced phase transition of retained austenite,is responsible for the excellent mechanical properties obtained in the steel.For further investigating the stress-induced phase transition mechanism,thermodynamic methods are applied.Gibbs free energy of face-centered cubic-Fe,ε-Fe,ω-Fe and body-centered cubic-Fe associated with the stress-induced phase transition was calculated using molecular dynamics simulations,and a calculation method of strain energy in thermodynamic units for the stress-induced martensitic transformation is presented.The final results reveal the process and thermodynamic mechanism of stress-induced martensitic transformation in medium-carbon steels,in which the hexagonal phase can participate in the process as an intermediate product.

Carbide precipitation behavior and mechanical properties of micro-alloyed medium Mn steel

The carbide precipitation behavior and mechanical properties of advanced high strength steel deformed at different temperatures are investigated by X-ray diffractometer(XRD),scanning electron microscope(SEM),transmission electron microscope(TEM) equipped with an energy dispersing spectroscopy(EDS),and tensile tests.The medium Mn steel was subjected to controlled deformation up to 70% at 750℃,850℃,950℃,and 1050℃,and then quenched with water to room temperature,followed by intercritical annealing at 630℃ for 10 min.In comparison with the undeformed and quenched specimen,it can be concluded that acicular cementite precipitates during the quenching and cooling process,while granular NbC is the deformation induced precipitate and grows during the following annealing process.As the deformation temperature increases from 750℃ to 1050℃,the product of strength and elongation increases at first and then decreases.The smallest average size of second phase particles(20 nm) and the best mechanical properties(32.5 GPa%) can be obtained at the deformation temperature of 950℃.

Effects of annealing temperature on microstructure and properties of tailor-rolled blank of medium-manganese steel

The microstructure and mechanical properties of tailor-rolled blank (TRB) of medium-manganese steel during annealing were investigated. The annealing process of austenite-reverted transformation of the experimental steel was formulated. The effects of different morphologies on properties with different annealing temperatures in different thickness zones of TRB were analyzed. In the thin zone, the morphology is blocky, and in the thick zone, the morphology is lamellar. At the same annealing temperature, the tensile strength of the thick zone is lower than that of the thin zone, and the elongation and product of strength and elongation (PSE) of the thick zone are higher than those of the thin zone. With the increase in annealing temperature, the tensile strength increases, while the yield strength, the elongation and the PSE all decrease in the same thickness zone. Because the stability of lamellar austenite is higher than that of blocky austenite, the comprehensive mechanical properties in the thick zone are good. At the annealing temperature of 640 ℃, the experimental steel has the best comprehensive mechanical properties, and the maximum PSE is more than 40.0 GPa% in different thickness zones. Particularly, PSE is up to 45.6 GPa% in the thickness zone of 2.0 mm.