Study On The Second Phase Precipitation Behavior Of Microalloyed Medium Manganese Steel By Thermomechanical Treatment

At present,the combination of microalloy technology,controlled rolling technology and new heat treatment technology is the most wildly used process in modern industrial.The microstructure and mechanical properties of the steel are further optimized by the miniaturization and homogenization of second phase particles rationally and scientifically.In this study,precipitation strengthening microalloyed medium manganese steel is researched which is the third generation advanced high strength steel.Through thermomechanical treatments(controlled rolling + austenite reverse phase transformation annealing),the strength and toughness of the medium Mn steel are improved by utilizing methods such as refinement strengthening,precipitation hardening,and dislocation strengthening.The effects of thermomechanical treatment scheme,carbide precipitation behavior and mechanical properties of experimental steel are investigated by scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD)and room temperature tensile tests.The correspondence between deformation parameters,microstructure morphology and the product of strength and elongation is established.The hot deformation behavior of the experimental steel have been studied by means of Gleeble-3800 thermal-mechanical simulation machine.The critical temperature of dynamic recrystallization was between 950°C and 1000°C which was tested by the austenite dynamic recrystallization experiment.The isothermal kinetics curve(PTT)of carbide precipitation in austenite region and the fastest “nose” temperature(about 925°C)for precipitate were obtained by stress relaxation experiments.The austenite transformation temperatures and the martensite transformation temperatures of the experimental steel were obtained by a combination of thermal expansion experiments and metallographic analysis.These results have provided a theoretical basis for the thermomechanical processing parameters.The effects of thermal deformation on the precipitation phase and microstructure and properties of the annealed medium Mn steel in the austenite recrystallized temperature(1050°C),unrecrystallized temperature(950°C and 850°C)and intercritical region temperature(750°C)were studied to obtain the optimum deformation temperature.The thermomechanical treatment at a deformation temperature of 950°C(near the nose temperature)promotes the precipitation of the finely dispersed second phase.MC carbide has high stability and can effectively refines the grains.The interaction of MC carbide with dislocation makes the optimal product of strength and elongation(32.5GPa·%).When the steel is deformed at 750°C which is two phase region,the martensite lath width is small(0.26μm)while the number of precipitated phases is small and inhomogeneously distributed,resulting in a relatively low product of strength and elongation.The microstructure,properties and carbide precipitation behavior of medium Mn steel after thermomechanical treatment were studied for different deformations(respectively 0%,30%,55% and 70%)in the unrecrystallized temperature.After thermomechanical treatment,the structure consists of modified lath martensite,a small amount of reverse austenite and other phases,as well as dispersed nano-sized needle-like M3 C and granular MC carbide.With the increase of deformation,the carbide particle size and the width of martensitic laths are refined and the strength and elongation are both increased.The smallest average size of second phase particles(20nm)and the best product of strength and elongation(32.5GPa·%)can be obtained with the deformation amount of 70% at 950°C.It is proved that increasing the deformation of the non-recrystallization zone can optimize the comprehensive mechanical properties of the experimental steel.