Morphology Analysis And Control Of MnS Inclusion In Heavy Rail Steels

High speed heavy rail steel is usually a fine grain pearlite steel with special requirements of low amount of non-metallic inclusions, good fatigue resistance, excellent wear resistance and weldability, high purity, high strength and toughness. The non-metallic inclusions had a detrimental effect on the performance of steel significantly. The rating of class A inclusions (mainly MnS) is high and even more than 3.0 in domestic steel plant. In the current study, the bloom and rail of U75V were investigated. The morphological and distributive characterization of MnS inclusions and solidification structure were obtained. The mechanism of MnS formation was analyzed and calculated by the thermodynamics and kinetics and the strategies of control MnS inclusions were proposed. The optimal parameters of control were given through the laboratory experiment in order to improve the mass of heavy rail steel. The following conclusions were obtained:The solidification structure and combination two-dimensional and three-dimensional morphology of MnS inclusions in the bloom and rail were investigated. It is found that the morphology and distribution of MnS in bloom as follows:The morphology of MnS changed from elliptical and spherical near the surface of bloom to strip-like, petal-like, polyhedral and irregular in the center of bloom gradually. The size of both pure MnS particles and complex inclusions which consisted of a MnS outer layer and an oxide core increased from the edge to the center of bloom. The content of MnS in the complex inclusions decreased from 80 mass% near the edge of bloom to 2 mass% in the center of bloom, where MnS was precipitated dispersedly on the surface of oxides. In the rail, MnS was elongated along the rolling direction of rail during rolling process, and then it was rolled into circular, elliptical and fold shape in direction of the cross-section. The morphology of MnS inclusions was reclassified on the basis of the integrated 3D morphology under the following categories:elliptical, spherical, rod-like, plate-like, irregular or polyhedron, dendrite and patch MnS precipitated on the surface of oxides.It is shown that the MnS inclusions began to precipitate at 1619 K. which is the solidus temperature of the heavy rail steels through the thermodynamic calculation during solidification. This result was in good agreement with the 1645 K calculated by Thermo-Calc and 1631 K calculated by FactSage 6.4, respectively. The MnS inclusions started to precipitate in the solid which fraction was 0.94 using micro-segregation model. The degree of S segregation was greater than that of Mn, the content of Mn and S in the residual liquid were 1.427% and 0.120% which were 1.64 and 9.23 times of the initial mass fraction when the solidification fraction is 0.9, respectively. In addition, the size of MnS inclusions decreased obviously with increasing cooling rate through the dynamic calculation. The results of dynamics calculation indicated that MnS particles were dominated by homogeneous nucleation and grain boundary nucleation during solidification. The effective temperature of MnS nucleation is 1634 K. Precipitation of MnS inclusions was put off and nucleation rate of MnS was decreased by decreasing content of S less than 50 ppm. The critical radius of MnS precipitation was also decreased by increasing the cooling rate and gave rise to reducing the size of MnS inclusions.The morphological evolution of MnS inclusions in the bloom and rail were studied during reheating and isothermal holding. It was found that the variation of MnS morphology and characteristic was often slow by the holding time at 1200 ?. Next, it was helps for splitting of MnS particles under the condition of 1300 ? when the holding time extended more than 30 min. However, at 1400?, the MnS particles were dissolved into steel matrix. Then, a large number of MnS generated between of grain during solidification again. The size of individual MnS was small, but it leads to the large-size of MnS inclusions when all of them were together. During process of reheating and isothermal holding, it was found that the restrictive factors of evolution of MnS morphology and characteristic were determined in the high sulfur samples by prolonging the holding time over 30 min at 1300? because of the transformation from diffusion to rate of solid solution of Mn and S elements. Corresponding to the rail, it was found that the transitional nodes of restrictive factors for the evolution of the long striped MnS were proved when the holding time from 30 min to 60 min at 1300?.The precipitation behavior of MnS inclusions under the conditions of remelting?cooling?isothermal holding was studied through changing the cooling rate, holding temperature and holding time. It was found that the morphology of MnS changed from the nearly spherical and spindle into rod and strip by decreasing the cooling rate. What's more, the transitional nodes of restrictive factors for MnS evolution were proved when the holding time from 30 min to 60 min at 1300?. Comparison with the high and low sulfur samples, the number density of MnS was reduced in the high sulfur samples, but increased in low sulfur samples at 1200?. However, it was the opposite in the low sulfur samples at 1400?. It shows that the high heating temperature is to help MnS control in low sulfur samples. The conclusion is that the effect of heating temperature and holding time on control MnS inclusions are more reasonable and effective by decreasing the content of S less than 81 ppm.The optimal content of sulfur is less 40 ppm on the basis of the actual situation of the steel plant. Another suggestion was proposed as follow:Increasing the area of columnar crystal in the bloom with corresponding to the location of detection rating in rail head. Further, the grade of class A inclusion were reduced and the quality of heavy rail steel were improved into one of the most advanced level in China and even in the world.