On Influence Of Grain Size On Strength,Toughness And Strain Hardening Behavior Of A High Manganese Austenitic Cryogenic Steel

The high-manganese austenitic cryogenic steel has obvious technical and economic advantages over the conventional nickel-based cryogenic steel and is expected to replace the nickel-based cryogenic steel to be widely used in the LNG tank system in future.High-manganese cryogenic steel has austenitic structure with relatively low yield strength,and it cannot be strengthened through phase transformation,so grain refinement strengthening is one of the most important strengthening methods.In this paper,different grain sizes were obtained by rolling and subsequent solution treatment with different processes.The effect of grain size on mechanical properties,microstructural evolution,and work hardening behavior of high manganese low-temperature steels was systematically studied in order to provide the theoretical basis for the production and application of cryogenic steel.Studies have shown that as the austenite grain size decreases,both the yield strength and tensile strength of the experimental steel increase,and the total elongation decreases.The quantitative relationship between theyield strength and grain size at room temperature was determined as:?_ys=?₀+6.98?±0.37?D^-0.5,where the strengthening coefficient,K_y value(6.98MPa·mm^0.5)was significantly lower than the values reported in literatures.It is considered that the lower carbon content of the experimental steel leads to the lower critical stress?*and the shear modulus G of the yield phenomenon in the material,resulting in a lower K_y value.The yield strength and tensile strength of the experimental steel at-196°C increase significantly in comparison with that at room temperature,and the plasticity was so greatly reduced that premature fracture occurs at the uniform deformation stage.The tensile fracture surface morphology changes from dimples at room temperature to a mixture of quasi-cleavage and dimplesw at cryogenic temperature.The grain refinement strengthening coefficient K_y of experimental steel at low temperature is much higher than that at room temperature,which indicates that the experimental steel has stronger grain refinement strengthening effect at cryogenic temperature.The effect of grain size on the work hardening behavior of experimental steel was investigated.The results show that the twinning strengthening effect becomes not obvious gradually with the increase of grain size.At room temperature,the work hardening index n of the experimental steel increases continuously with increasing grain size,while at-196°C,the work hardening index n of different grain sizes does not differ greatly.The impact toughness of the experimental steel at room temperature increases as the grain size increases.At cryogenic cryogenic,the impact toughness of the experimental steel was slightly lower than room temperature,but the impact toughness of the specimens with different grain sizes remained basically stable.