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A novel concept of microstructure design involving warm rolling and low-temperature annealing has been proposed to greatly improve the strength-ductility combination of medium Mn steel. The unique transformation induced plasticity has been triggered by firstly microbands and secondly deformation twins, enhancing the ductility significantly and the reasonable dislocation density in non-recrystallized austenite grains increases the yield strength greatly. The individual contribution of solid solution strengthening, grain boundary strengthening, V-precipitates strengthening and dislocation strengthening to yield strength is discussed by constitutive models, in which the tangled dislocation plays a dominant role. Best strength-ductility combination with yield strength (1239MPa), ultimate tensile strength (1341MPa), total elongation (41.8%) and product of strength and elongation (56.1GPa·%) has been obtained, which are comparable and superior to the reported medium-Mn steels. [Display omitted] [ABSTRACT FROM AUTHOR]

The relationship between crystallographic orientation, microstructure characteristic and mechanical properties in 3.5Mn transformation induced plasticity (TRIP) steel that subjected to different cold rolling reduction and annealing time was elucidated. Before the hot-rolled plates were cold rolled to different thicknesses, large-fractioned Mn-enriched cementite particles dispersed in ferrite matrix were deliberately prepared to enhance Mn-partitioning and improve austenite stability. The evolution of microstructure was performed by electron probe micro-analyzer (EPMA), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and X-ray diffraction (XRD), and the mechanical properties of the intercritical annealed samples were evaluated by uniaxial tensile test at room temperature. The results show that the cementite dissolution, ferrite recrystallization and austenite formation are strongly coupled and interdependent, resulting in a complex ultra-fine microstructure composed of retained austenite, ferrite and cementite in the intercritical annealed samples. Moreover, the Schmid factor (SF) distribution within the range of 0.25–0.5 was studied to understand its correlation with the crystallographic orientation and mechanical property of the test steel. It is concluded that the mechanical properties of 3.5Mn TRIP steel can be improved when the annealing period, orientation and austenite stability are optimized. [ABSTRACT FROM AUTHOR]

A novel ultra-high strength transformation induced plasticity (TRIP)-assisted steel with a better combination of ductility and stretch flangeability was fabricated by introducing a banded structure into the resulting microstructure. The corresponding relationships between microstructures and mechanical properties were studied in detail by the tensile test and the hole expansion test. These tests reveal that some pancake-like ferrite grains obtained in cold rolling are retained during intercritical annealing and finally form a banded structure together with martensite/austenite (M-A) islands along the rolling direction. The steel can be further strengthened in the subsequent punching process, while uniformly-distributed bainite matrix with fine M-A islands dominates the overall microstructure after annealing at higher annealing temperatures. The banded structure of the soft ferrite phase will promote crack propagation along the rolling direction rather than in the thickness direction, thereby favoring improvement of the hole expansion ratio (HER). Instead, the mixed bainite-M-A islands microstructure led to increased micro-voids density at the edge of the shear affected zone (SAZ) after the hole punching process and a great number of cracks rapidly propagated through the thickness direction in the matrix, which is responsible for a decrease in HER. Excellent mechanical properties were obtained in the low-temperature annealed sample with an HER of 29–35%, elongation of 21–26%, tensile strength of 941–1016 MPa, and yield strength of 426–566 MPa. [ABSTRACT FROM AUTHOR]