Research On Wrapped Non-metallic Inclusions In High Speed Wheel Steel

In order to reduce the adverse effects of brittle Al₂O₃inclusions on the fatigue properties of high-speed wheel steels,plastic inclusions are usually used to wrap the brittle inclusions.In order to evaluate the wrapping state of wrapped inclusions in high-speed wheel steel,this paper uses scanning electron microscope and ASPEX automatic scanning electron microscope to analyze and study the wrapped non-metallic inclusions of core-shell structure in two high-speed wheel steels,including core-shell structure wrapping.The morphology and composition of non-metallic inclusions,as well as the composition of the core and shell,inclusion-like inclusions and the matrix interface were analyzed.And statistical analysis of the size of the core and shell.The wrapped inclusions were characterized by nanoindentation,and the mechanical properties of non-metallic inclusions were studied.The precipitation behavior of non-metallic inclusions during the solidification of two wheel steels was studied by thermodynamic calculation.The details are as follows:Non-metallic inclusions in the steel core shell structure of high-speed wheels are generally elliptical or circular,and the length is mainly distributed between 1 and 10μm,and the size of the core is between 0.3 and 2μm.Reasonably control the size of Al₂O₃in this range.Contribute to the formation of wrapped inclusions.There is a composition gradient from the core to the outer shell of the shell-core structure wrapping non-metallic inclusions.It is inferred that the outer layer of the wrapping inclusions is chemically bonded between Mn S and Al₂O₃.Comparing the two wheel steels,it is found that the wrapped state of the wrapped non-metallic inclusions in the bainitic wheel steel is more uniform than that in the pearlitic wheel steel,and the wrapping effect is better.By comparing the residual indentation depth of the pearlite steel matrix,pure Mn S and Mn S-composite inclusions,it can be seen that the plastic deformation of pure Mn S is the largest,followed by the matrix,and the Al₂O₃-composite inclusion has the worst plasticity.Compared with pure Mn S,the yield strength of Mn S wrapped in the outer layer is increased and the plasticity index is decreased.As the thickness of the Mn S coating increases,the plastic deformation of the Mn S-composite inclusions becomes larger.The thicker the wrapping layer,the better it is to improve its hardness and brittleness.The calculation results of thermodynamic software show that the increase of S and Mn,especially S content,can increase the precipitation temperature of Mn S.The S content of pearlitic wheel steel is higher than 70ppm and the Mn content is higher than 0.6%;the S content of bainitic wheel steel is higher than 40ppm and the Mn content is higher than 2.2%when the Mn S precipitation temperature is higher than the solidus temperature.The zone can promote the diffusion of Mn and S,help Mn S nucleate on Al₂O₃to form Mn S-Al₂O₃core-shell non-metallic inclusions,and improve the hard and brittleness of Al₂O₃inclusions.