Investigation On The 3D Morphology And Precipitation Of MnS Inclusions In U75V Heavy Rail Steel

As the demand for railway transport speed and carrying capacity becomes higher and higher,the quality requirements of rail steel also increase.At present,heavy rail steel widely used has high strength and toughness,high wear resistance,high resistance to crushing and brittleness,but non-metallic inclusions have a serious impact on the performance of heavy rail steel.In this paper,we choose the U75V heavy rail steel continuous casting billet as the experimental materials,and get the formation of solidified microstructure by experiment.We can obtain MnS inclusions morphology and distribution by electron probe,metallographic microscope.The MnS inclusions were ionized out of the steel by the method of anhydrous electrolysis and analyzed.The 3D morphology and accurate distribution of MnS inclusions were obtained.Based on the thermodynamic and kinetic calculations,the precipitation regularity of MnS was obtained.According to the isothermal treatment experiment to explore the effective measures to control MnS to provide a theoretical basis for further improving the purity of heavy rail steel slab.The main conclusions are as follows:(1)In this paper,the non-water-to-water non-aqueous electrolysis method is proposed for the first time,which can effectively display the three-dimensional morphology of inclusions and avoid the inherent defects that the shape of the traditional inclusion two-dimensional morphology detection can not be directly detected and measured.Combined with scientific characterization method,the shape and size distribution of MnS inclusions at different positions of the heavy rail steel continuous casting slab have been systematically examined and studied,which is not random and contingent.(2)The results of thermodynamic and kinetic calculations show that MnS inclusions are precipitated at the end of solidification.The content of MnS inclusions is closely related to the content of solute in molten steel,which is mainly affected by S content and followed by Mn,C and P contents.After calculation,the spheroids MnS which aspect ratio is about 1:1 always can be seen in the edge of the slab,while in the middle of the slab,MnS inclusions are mostly long strip with an aspect ratio of about 4.Due to the strong plasticity of MnS,it is very easy to deform and form the crack source during the later rolling.The larger aspect ratio inclusions are more easily elongated in the rolling.Therefore,it is necessary to control the aspect ratio and reduce incidental deformation of the rail which caused adverse consequences.(3)Dendrite distribution from the surface to the center of the slab was obtained by low-temperature hot-acid corrosion test and dendritic corrosion test on the slab.The dendrite distributions were as follows:surface fine grain region,columnar grain region,mixed grain region,central equiaxial crystal zone and secondary dendrite arm spacing gradually increase.The formation of different crystal regions is related to the local cooling rate of the slab by the Procast software and the thermodynamic calculation.The larger the local cooling rate is,the smaller the spacing of the secondary dendrite arms is.The dendrites in the crystalline zone are arranged more densely;the smaller the local cooling rate,the larger the spacing of the secondary dendrite arms is,and it is easy to form an equiaxed zone.(4)Isothermal homogenization treatment will change the size and morphology of MnS inclusions in steel.During the heat preservation of MnS,the dissolution and growth are mainly controlled by the diffusion and solution reaction of Mn or S elements.When the diffusion effect is greater than the solution effect,MnS inclusions are coarsen;when the solid solution effect is larger than the diffusion effect,MnS inclusions will crack or shrink.Center equiaxed part of the inclusions morphology will change from the original sheet into a block or polyhedral and then into a rod with the holding time.1300℃ heat treatment effect can achieve the purpose of reducing the size of inclusions,and the effect is more obvious.The thermal insulation effect of 1200℃ slightly less effect.The factory should choose the rail in the rolling process before the insulation temperature and time according to the actual production,in order to achieve the minimum economic consumption of inclusions to control the size and shape of the purpose.