Research On Strengthening-Ductility Mechanism Of High Manganese Steels Based On Nano-Precipitation

High manganese steels with high strength,high ductility and excellent processing properties have been widely used in important areas,such as automobiles,high-speed train tracks,cryogenic vessels.However,strength-ductility has always been one of the key issues facing the development and application of high manganese steels.In this paper,nanoprecipitation strengthening phases were introduced into high-manganese steels,and the strength of high-manganese steel was improved by nano-precipitation strengthening while maintaining ductility.The effect of aging treatment on the strength-ductility of high-manganese steel during precipitation was studied.At the same time,cryogenic treatment was also used to stabilize austinites and the influencing mechanism on the strength-ductility was discussed,which showed theoretical and application foundations for the R&D of high manganese steels.In this paper,two high-manganese steels with different compositions were designed to compare the strengthening effects of different nano-precipitates using tensile mechanical testing,hardness testing,X-ray diffraction(XRD),scanning electron microscopy(SEM),and electron probe micro analysis(EPMA)and transmission electron microscopy(TEM).The effects of the addition of Ni/Al elements,aging treatment and cryogenic treatment on the stacking fault energy,microstructure,mechanical properties and work hardening behavior of coppercontaining high-manganese steel were studied to reveal the strength-ductility regulation mechanism of nano-precipitated high manganese steels.(1)The effect of Ni and Al elements on copper-containing high-manganese steels was studied.It showed that after the Ni and Al were added,the stacking fault energies were increased from 15 mJ/m2 to 30 mJ/m2,indicating that a single 1Cu high-manganese steel was induced by transformation plasticity(TRIP),and 1Cu2Ni1Al high manganese steel was induced by twinning induced plasticity(TWIP).The addition of Ni and Al elements improved the stability of austenite,and the strengthening mechanism was changed from TRIP to TWIP during the deformation process,thus improving the strength-ductility of the copper-containing high manganese steels.(2)The effects of aging heat treatments on the microstructural evolution and mechanical properties of Cu high manganese steel and 1 Cu2NilAl high manganese steel were studied.It showed that the grains of the high manganese steels after aging treatments were obviously refined,the matrix phase constitutions were changed,the annealing twins were denser,and a large number of nano-precipitated phases(Cu-rich phases and NiAl precipitates)were precipitated at the interface between the matrix and the annealing twins,which ultimately caused significant increases in the strength and ductility of high manganese steels.(3)The effect of cryogenic treatment and aging heat treatment on the strength-ductility and microstructure of 1 Cu high manganese steel was studied.It showed that after cryogenic treatment,the C element in the martensite of 1 Cu high manganese steel was partitioned to the adjacent austenites.The stability of austenites was increased,the joint strengthening of the TRIP and TWIP effects was achieved,and the strength-ductility of the 1 Cu high manganese steel was improved.At the same time,a large number of nano-size Cu-rich precipitates were precipitated out of the austenite matrix and annealing twins interface of 1 Cu high manganese steel after cryogenic tretment and aging treatment,which further enhanced the strength-ductility of the alloys.