Effect Of Laves Phase Precipitates In Mn12Ni2MoTi(Al) Steel

Transformation induced plasticity?TRIP?assisted steel has excellent mechanical properties of high strength and high plasticity.Precipitation strengthening is an effective method to improve the yield strength.In this work,Mn12Ni2MoTi?Al?steel was selected as experimental marterials.The microstructures of Mn12Ni2MoTi?Al?steels containing Laves phases were investigated by X-ray diffraction,scanning electron microscopy,transmission electron microscopy,hardness testing and tensile tests.The morphology,evolution of Laves phase precipitats in Mn12Ni2MoTi?Al?steel and its effect on the microstructure and properties of the steel were studied.The main conclusions are as follows:?1?When supersaturated solid solution Mn12Ni2MoTi?Al?steel was annealed at745°C,the Laves phases with rich Mo,Ti,and Si must precipitate quickly.TEM and XRD analyses consistently show that the crystal structure of the Laves phase is?Fe,Mn?₂?Mo,Ti?with a hexagonal structure.The morphology of the Laves phase precipitats in the cold-rolled specimen is spherical,while in the solution-quenched specimen,in addition to the spherical particles,there are elongated particles that precipitate along the martensite interface of the lath.Under the same annealing condition,the larger the cold rolling deformation,the larger the size of the Laves phase precipitates,the average size of the Laves phase precipitates in the solution quenched sample is the smallest.Whether it is the solution quenched or cold state,a large number of fine dispersed dispersed Laves phase particles are formed at the annealing time of 5min,and the size of Laves phase particles increases with the annealing time.The volume fraction of Laves phase reaches a maximum of 6.8%when the cold-rolled sample is annealed at 8 h,and the Laves phase particles have coarsened obviously;On the other hand,the volume fraction of the Laves phase reached 7.7%at the annealing of the solution quenched sample for 24 h,and then the coarsening began.The coarsening of the Laves phase in the cold-rolled sample mainly depends on its grain boundary diffusion mechanism,so its coarsening rate is h igh.The diffusion rate of the coarsening rate of the Laves phase in the solid-solution samples is much lower than that of the Laves phase in the cold-rolled samples.?2?The Laves phase preferentially precipitated at grain boundaries are pinned at grain boundaries,which hindering the migration of recrystallized grain boundaries,effectively suppresses grain coarsening.The pinning pressure of Laves for the grain boundary increases first and then decreases with the extension of annealing time and reaches the maximum value at of the annealing time 4 h.Although the annealing time exceeds 8 h,the Laves phase coarsens and the pinning pressure on grain boundary decreases,but the migration of grain boundary can still be hindered.Therefore,the grain size in the sample remains at the submicron level when the annealing time is extended to 24 h.?3?Laves phase improves the stability of the annealed structure.After annealing at 745°C for 24 h,cold-rolled Mn12Ni2MoTi?Al?steels can form submicron ultrafine grained duplex phase microstructures composed of austenite grains and ferrite grains.The submicron austenite is very stable so that does nor transform during water cooling.For 65%and 80%cold-rolled samples,the relative volume fraction of austenite reached61.2%and 50.7%,respectively after annealing at 745°C for 1 h.The relative volume fraction of austenite in the as-quenched samples reached a maximum of 7.6 at the annealing time of 5 min.?4?The hardness of the cold-rolled sample is very high,and the greater the amount of cold-rolled,the higher the hardness.For 65%CR and 80%CR samples annealed at745°C for 5 min,their yield strength and elongation were 792 MPa,873 MPa and 23.4%,16.44%,respectively,which are higher than the yield strength and total elongation of quenched martensite samples of the same material.