Research On Formation Mechanism And Control Process Of MnS In Non-Quenched And Tempered Steel For Heavy Crankshaft

The control process of MnS inclusions in non-quenched and tempered steel for heavy crankshaft has been a common concern to steel manufacturers and downstream users.MnS inclusions with proper size and uniform distribution are not only beneficial to the improvement of cutting performance,but also can refine the microstructures by pinning austenite grain boundary at high temperature and inducing intragranular ferrite at low temperature.However,large or heavily clustered MnS inclusions are inoperative and likely to cause serious anisotropy in mechanical properties and magnetic trace defects on crankshaft surface.On the background of the production of non-quenched and tempered steel for heavy truck crankshafts by Xining Special Steel Co.,Ltd.,the influences of MnS inclusions on the transverse mechanical properties of rolled non-quenched and tempered steel and the magnetic trace defects on the crankshaft surface were studied.Besides,the formation mechanism of MnS inclusions in different morphologies of non-quenched and tempered crankshaft steel and the influence of different deoxidation methods on the morphology of MnS inclusions were studied in the laboratory.Combined with the experimental results of the factory,the control mechanism of using fine magnesium-aluminum spinel oxides to improve the morphology of MnS inclusions has been proposed.After systematic sampling of non-quenched and tempered crankshaft steel produced by Xining Special Steel Co.,Ltd.,the evolution behaviors of various inclusions in the smelting process were analyzed,then the related smelting processes were optimized,and finally satisfactory results were obtained.The research contents and conclusions of the full text are divided into the following aspects:(1)By studying the relationship between the surface quality of the finished crankshaft and the characteristics of MnS inclusions,it is found that large-sized,aggregated MnS inclusions can cause magnetic trace defects on the surface of the crankshaft,mainly concentrated on the inner side of some connecting rod journals.The reason for its occurrence is that some large-sized MnS inclusions at center of the original rolled material have been squeezed and exposed to the journal surface of the connecting rods during the forging process.(2)By studying the relationship between the mechanical properties of rolled non-quenched and tempered steel and the characteristics of MnS inclusions,it is found that long striped and aggregated MnS inclusions can significantly reduce the transverse plasticity of non-quenched and tempered steel.The main reason is that during the stretching process,a large number of micro-cracks are formed simultaneously at the sites of MnS aggregation,resulting in stress concentration,leading to early fracture of steel and woody fracture morphology.In contrast,uniformly distributed MnS inclusions with low aspect ratio have less detriment of the transverse plasticity of the rolled bars.(3)The effects of different deoxidation methods such as Si-Mn,Al,Al-Zr,Zr,Ti and rare earth Ce deoxidation on the morphology of MnS in steel were studied in the laboratory,and pilot plant test of Zr deoxidation was also conducted.It is found that the magnesium-aluminum spinel oxides formed by Al deoxidation are small in size,large in number.And they have a low planar disregistry with MnS inclusion,which means they are good at controlling MnS morphology in steel.However,the effects of Si-Mn,Al-Zr,Zr,Ti and rare earth Ce deoxidation on MnS nucleation are poor and the MnS aggregates in the as-cast ingots.After analyzing characteristics of oxides and MnS inclusions in Al-Zr and Zr killed steels,it is found that the oxide composition and quantity are most critical to the morphology change of MnS in non-quenched and tempered steel for crankshaft.(4)By comparing the MnS morphology and oxide characteristics of two commercial Al-killed non-quenched and tempered steels,it is found that the high-efficiency oxide core in the steel is fine magnesium-aluminum spinel oxides.The main reason is that magnesium-aluminum spinel oxides and MnS have a low planar disregistry,which means they have high nucleation ability.Besides,they are small in size and large in number.(5)Through the systematic sampling of the actual smelting of non-quenched and tempered steel,it was found that the inclusion evolution behavior in Al-deoxidized steel was:Al2O3 after EAF?Al2O3-MgO during LF-CaO-MgO-Al2O3 during VD?CaO-MgO-Al2O3 or Al2O3-MgO before casting.Combined with thermodynamic software analysis,it is found that Ca element has a great influence on the inclusion components in smelting process.A small amount of Ca would convert Al2O3 or Al2O3-MgO oxides in the molten steel into CaO-MgO-Al2O3 or CaO-Al2O3 oxides.(6)The Al content after tapping of the electric arc furnace is controlled to be 0.015 to 0.020%,the alkalinity of slag in smelting process is controlled between 2.5 and 3.5,and the Ca content before casting is controlled below 3ppm.After these modification,the ratio of fine Al2O3-MgO oxides in the rolled materials increased from 41.9%to 65.2%,the average ability of oxides for MnS nucleation increased from 3.1 to 5.6,the MnS morphology has been improved,and the first qualified rate of cross sectional rate of reduction increased from 58.8% to 84.5%.