Medium-Mn steels are characterized by ultrafine-grained (UFG) duplex microstructure consisting of ferrite and a large amount of retained austenite. Intercritical annealing is of great importance to achieving the UFG duplex microstructure and adjusting amount as well as stability of retained austenite. In the present work, the influence of intercritical annealing temperature on the microstructure and mechanical properties was investigated in a cold-rolled medium-Mn steel Fe-12Mn-3Al-0.05C. Particularly, the dependence of microstructural morphology on intercritical annealing temperature was emphasized to reveal the genesis of the microstructural morphology in medium-Mn steels. The ferrite-austenite duplex microstructure manifested an elongated morphology in the specimen annealed at 555 • C, which inherited the lath structure of the cold-rolled state. The medium-Mn steel exhibited a continuous yielding behavior and a relatively low strain-hardening rate. With an increase in intercritical annealing temperature up to 650 • C, the amount of retained austenite increased and microstructure was partially recrystallized, showing a mixture of elongated and equiaxed grain morphologies. When the intercritical annealing was applied at 700 • C, the medium-Mn steel mainly exhibited recrystallized microstructure with equiaxed morphology. The optimal balance between the amount and the stability of retained austenite led to an enhancement of strain hardening and ductility. With a further increase in the intercritical annealing temperature to 750 • C, the medium-Mn steel possessed pronounced strain-hardening behavior at the beginning of the tensile deformation with deteriorated ductility.

No abstract

The low permeability of shale, in accretionary complexes and passive continental margins, influences pore pressure generation, and thus induces deformation and fault slip behavior. It is also key in hindering the ability to evaluate shale gas production. Gas is commonly used in measuring the permeability of shale and generally yields high values than that of liquids, due to slippage effect. Separation of the slippage effect from gas permeability is a critical task in the study of low‐permeability media. This study measured the gas permeability (Kg) of the Longmaxi shale parallel to bedding (L1P) and perpendicular to bedding (L1V) under different confining pressures (Cp) and pore pressures (Pp), while simultaneously measuring porosity for different values of Cp. Additionally, a new approach is proposed to define the effective stress coefficient and boundary condition for the Klinkenberg correction. The results show that the Kg of L1P is almost unaffected by the slippage effect, while, for L1V, the Klinkenberg correction is required. The Klinkenberg plot of L1V follows the quadratic model from the study by Ashrafi Moghadam and Chalaturnyk (2014, https://doi.org.10/1016/j.coal.2013.10.008), rather than the classic linear Klinkenberg equation. The results also show that, as the Cp increases and/or the Pp decreases, the contribution of the slippage effect to the L1V Kg increased from 20% to 86.5%. Finally, based on the Knudsen number (Kn), the flow regime of L1P changes from Darcy flow (Kn < 0.01) to slip flow (0.01 < Kn < 0.1), and that of L1V is slip flow.