Effect Of Nano-scale Sulfides On The Microstructure Of Non-oriented Silicon Steel And Their Control Technique

Non-oriented silicon steel is mainly used in the laminated cores of motor and generator rotor.Therefore,both of excellent mechanical properties and magnetic properties are required.How to reduce iron loss and improve the magnetic induction of silicon steel has been a key research direction.Impurity content,inclusion quantity,thickness of steel plate and surface state have effects on the iron loss and magnetic induction of silicon steel.Above all,the impurity elements have a great influence on the performance of the silicon steel.With the increase of sulfur content in steel,the iron loss of silicon steel is deteriorated sharply.In fact,it is the nano-scale sulfides formed in the case of high sulfur content affect magnetic properties of silicon steel.Hence,further study on the effect of nano-scale sulfides on magnetic propertites and microstrcure of non-oriented silicon steel,and the precipitation mechanism of nanoscale sulfide should be put forward,which has an important theoretical significance and application value.The nanoscale sulfide in the low-grade non-oriented silicon.steel was investigated.The samples were cut from a sheet of non-oriented silicon steel in the processes of hot-rolling,continuous annealing and final stress relief annealing.Then,the samples were labeled as HR,CR and CA,respectively.The precipitation characteristic and evolution law of nanoscale sulfide are analyzed.Precipitates in the HR,CR and CA samples were detected using scanning electron microscopy(SEM,Apollo 300)equipped with energy dispersive spectrometry,the results show that the precipitates in the size range of 0.2-1 μn is mainly manganese sulfide(MnS)and a small number of manganese-copper sulfide((Mn,Cu)S)and cupper sulfide(Cu2S),the two-dimensional and three-dimensional morphology of these sulfides is irregular circular and ellipsoid shape,repectivily;Considering the small number of Cu2S,the effect on the grain size could be ignored,the effect of MnS on the grain growth was emphasized.A small number of MnS were detected in HR and CA samples,which distributed with in the grain.A large amount of MnS with size range of 0.3-0.7μm were precipitated in SA samples,and mainly distributed in the grain boundary.The precipitation kinetics of MnS was studied,it was found that the segregation of sulfur on grain boundary occurred in the non-equilibrium manner during the final annealing process,and the maximum segregation of sulfur can be achieved through 14min.The maximum segregation of sulfur at the grain boundary is 5.7 times of that in the intragranular.It is revealed that the critical nucleation size of MnS at grain boundaries is 0.3-0.8nm.It is revealed that the grain boundary nucleation rate of MnS is much larger than that of homogeneous nucleation.Hence,grain boundary nucleation is the main nucleation mode of MnS.The effect of MnS on the grain growth was characterized by SEM instrument equipped with an electron back-scattered diffraction detector,and analysed quantitatively.The calculation of pinning force provided by MnS(0.3-0.7μm)and driving force provided by grain boundary energy proved that MnS(0.3-0.7μm)could pin the grain growth effectively.Combined with the experimental results and theoretical calculations,the critical parameters of the sulfur content in the steel with innocuous MnS were predicted.The critical sulfur content in the Zener model,the RBM and the FBM model is 11ppm,10ppm and 2ppm,respectively.The relation between the microstructure and the magnetic properties of non-oriented silicon steel plate were confirmed by experimental results,which show that the iron loss decreased with the increased of grain size,while the magnetic induction intensity increased with it.The experimental scheme for controlling more sulphides precipitate on oxides during the solidification process and reducing the precipitation of nanoscale MnS is proposed.Through the analysis of the morphology and composition of inclusions in the continuous casting billet and the calculation of the mismatch degree of MnS and oxide,the type and distribution of oxide in the slab are studied,and it is revealed that the mismatch between the compound Al2O3-SiO2 oxide and the MnS is smaller,the mismatch between Al2O3·SiO2 inclusion and the MnS is the least,which is the most suitable heterogeneous nucleation site for the precipitation of MnS.The detection and analysis of compound oxide and sulfide in the billet revealed that MnS was precipitated on the submicron Al2O3·SiO2 oxide with fully wrapped type.The MnS embedded type is precipitated at micron scale containing MnO inclusions,mainly through the diffusion of Mn and S into the MnS embryo.MnS precipitates at the micron scale outer layer of MnO-free oxide,mainly because there is no MnS embryo,and the oxide is only act as nucleation site.Through the statistical analysis of the compound oxide sulfide,it is found that the precipitation ratio of MnS on the oxide increases with the gradual enrichment of silicon in the Al2O3-SiO2 compound oxide,and the MnS is more easily precipitated on the small size compound oxide.The experiment result that MnS precipitates on the Al2O3-depleted area of Al2O3-SiO2 and the silicon-rich Al2O3·SiO2 inclusion is consistent with that the mismatch between the silicon-rich oxide and the MnS is the least.The formation of silicon-rich oxide by changing the deoxidization system in high aluminum silicon steel is studied.It is proved that the composition of the composite Al2O3-SiO2 inclusions is affected by the deoxidization mode(Si/Al or Al/Si).The effect of different deoxygenation methods(Si/Al or Al/Si)on the composition of Al2O3-SiO2 in non-oriented silicon steel have been carried out,the result show that small size spheroidizing Al2O3-SiO2-(MnO)composite inclusions were formed in both deoxygenation methods,while the amount of Al2O3-SiO2-(MnO)is more,and the composite is more closer to the low melting area,and silicon-rich region in the Si/Al deoxygenation.In this study,the size range of harmfull MnS in non-oriented silicon steel is determined,and the critical sulfur content in steel is predicted according to the experimental results,which provides guidance.for the steel plant to reduce the harm of sulfide.It is of great theoretical and practical significance for steel enterprise to reduce production cost and shorten production cycle by using oxides to control sulfides more concentrated in the process of solidification.