Strengthening Mechanism Of Mo-containing High Manganese Austenitic Steel

High manganese TWIP(Twinning Induced Plasticity)steel has attracted wide attention from the automotive industry because of its high tensile strength and excellent plastic deformation ability,but its applicability is limited because of its low yield strength.In this paper,Fe-25Mn-0.5C high manganese steel is used as a control group.Different mass fractions of Mo elements were added.By adjusting the rolling and heat treatment processes,the effect of different strengthening methods on the yield strength of the Mo-containing high manganese steel was investigated,and the effect of the Mo in the system was clarified.Firstly,the strengthening effect of precipitation strengthening in the Mo-containing high manganese steel was investigated.By depositing the hot-rolled sheet to an aging treatment at a temperature of 800℃ for 45 minutes,dispersedly distributed nano-scale Mo carbides were precipitated in the coarse austenite grains.The micro-hardness curve shows that the precipitation of Mo carbides is closely related to the Mo content,and the obvious precipitation strengthening effect occurs when the Mo content reaches 5%.Room temperature tensile test and XRD phase analysis show that the precipitation of the Mo-rich second phase increases the yield strength of the 5Mo specimen by 100 MPa and improves its work hardening ability significantly.However,the plastic deformation mechanism is still dominated by the Twinning Induced Plasticity(TWIP)effect.Experiments have shown that the precipitation strengthening of the second nanophase is still an effective strengthening method for austenitic high manganese steel.Then,the strengthening effect of deformation strengthening on Mo-containing high manganese steel was investigated.The effect of cold plastic deformation on the mechanical properties of high manganese steel was studied.In this paper,it is found that Mo can inhibit the recrystallization behavior of high manganese steel obviously and increase its recrystallization temperature significantly.To obtain a mixed structure of a cold-rolled structure and recrystallized grains,the recrystallization process is controlled by changing the annealing temperature.The tensile test results show that samples which have mixed structure show better strength-plasticity matching.Finally,through the rolling at room temperature,high density nano twins and εmartensite were introduced into the high manganese steel.The higher the Mo content,thehigher the nano twin content,and the lower the ε martensite content.At the same time,the addition of Mo element can significantly improve the thermal stability of the matrix.After the annealing treatment at 600℃ for 15 minutes,the TEM results show that as the Mo content increases,the nano twins can still stably exist in the matrix and contribute strength;The XRD results show that the ε martensite in the steel has been thermally reversed to austenite.This part of the inversely transformed austenite and the recovery effect make the matrix have good plastic deformation ability.Therefore,the Fe-25Mn-0.5C-5Mo sample can still have an elongation of 17% while maintaining a yield strength close to 1.6 GPa.Experiments have proved that the introduction of nano-twin microstructures in high-manganese steels and their stable existence during high-temperature annealing through Mo alloying is an effective way to significantly improve the yield strength of austenitic high-manganese steels.In addition,combining the research contents in Chapters 3 and 5 reveals the precipitation behavior of Mo carbides under different matrix structures.Precipitation characteristics in nano-twin regions,nano-recrystallized grains and coarse-grained grains are significantly different.The size of Mo carbides precipitated in the nano-twin region is the smallest,only a few nanometers to a dozen nanometers.It is generally nucleated at the interface of the primary twin and the secondary twin or at the twin step.The precipitated Mo carbides in the nano-recrystallized region have complex morphology,including spherical shapes with a diameter of several nanometers and round rods with a diameter of several tens of nanometers,and the precipitated phase inherits a linear arrangement characteristic of twin directions;Mo carbides in the coarse grains are obviously round rods or flakes,with sizes ranging from tens to hundreds of nanometers,which are dispersed on the matrix.