Effect Of Trace Ce On Microstructures And Mechanical Properties Of Low Carbon High Manganese Steels

Low carbon high manganese steels with high strength, good low-temperature toughness, and excellent weldability combination are widely used for shipbuilding, engineering components, bridge and offshore platform applications. With the development of microalloying and control rolling and cooling(TMCP) techniques, the production of low carbon high manganese steel adopts commonly microalloying and TMCP techniques abroad and at home.However, TMCP can not unlimited control the rolled reduction and cooling velocity, so the effect of grain refinement will be limited to a certain range.Microalloying elements addition can improve low carbon high manganese steel’s mechanical property through grain refinement and precipitation strength,but it will increase production cost. Excessive addition of microalloying elements will increase carbon equivalent, deteriorate welding property and impact toughness of steel.Therefore, this paper takes low carbon high manganese steel as its object of study, four steel samples with different Ce content were melted in medium frequency vacuum induction furnace, and the effect of trace Ce on microstructures and mechanical properties of low carbon high manganese steels were studied. The methods and contents of this research are as follows:(1) Four steel samples with different Ce content were melted in medium frequency vacuum induction furnace, chemical composition of steel samples were analyzed and the mechanism of purifying molten steel by trace Ce addition was analyzed and discussed combined with modern metallurgical theory.(2)Collecting experimental data by means of optical microscope(OM), scanning electron microscope(SEM), energy dispersive spectrometer(EDS) and other means, describing the evolution mechanism of Ce contained submicron inclusions in the refining process of molten steel through thermodynamic calculation and dynamic analysis.(3) To explain the mechanism of microstructure refinement by Ce contained submicron inclusions, the lattice misfit between different inclusions and primary δ-Fe phase of liquid steel were calculated using two-dimensional lattice misfit formula.(4) The mechanism of trace Ce improving mechanical properties of low carbon high manganese steels was analysed and discussed using experimental data collected by mechanical property experiment. This study not only can provide basic experiment data for the application of rare earth elements in low carbon high manganese steel, but is important for developing manufacture technology of low carbon highmanganese steel with high strength and toughness at low cost.This paper draws the following conclusions through experiment and research:(1) Additions of 0-0.0025% Ce to steels resulted in 0.037-0.014% retained sulphur while oxygen was reduced from 0.014% to 0.0064%, which demonstrate Ce has strong desulphurization and deoxidation ability.(2) The microstructures of low carbon high manganese steels were remarkably refined by trace Ce, with Ce content increasing from 0% to0.0025%, average ferrite grain size was reduced from 15.45μmto 7.18μm, and pearlite lamella spacing was reduced from 0.25μm to 0.14μm.(3) After adding Ce into molten steel, angular Al2O3 particles with large size and elongated MnS particles were modified into small-size and spherical Ce contained inclusions, and these Ce contained submicron inclusions could effectively act as heterogeneous nuclei of initial δ-Fe, therefore, the microstructures of low carbon high manganese steels were remarkably refined.(4) The strength, ductility and impact toughness of low carbon high manganese steels improved obviously with increasing Ce content, which was mainly attributed to the microstructure refinement and inclusions modification.