Study On The Strain Induced Precipitation Behavior Of MnS In Fe-3%Si Alloy

As one of the most important soft magnetic materials, grain-oriented silicon steel is widely used as core material in transformers. Its excellent magnetic property comes from the sharp Goss texture developed by secondary recrystallization process. The morphology and distribution of inhibitor plays a key role for complete secondary recrystallization. MnS, one of the most popular inhibitors used in grain-oriented silicon steel, normally precipitates in hot deformation process. The hot deformation microstructure affects the distribution of MnS and further the property of grain-oriented silicon steel by changing the strain induced precipitation thermodynamics and kinetics of MnS. So far, there is no unified view on microstructure evolution mechanism of ferrite during hot deformation and on the nucleation site of strain induced precipitation. Therefore, it has a theoretical value and engineering sigificane to stuy the microstructure evolution and its effect on the strain induced precipitatioin of MnS.A Fe-3%Si alloy with low carbon (<0.015wt.%) was used in present paper as the model material of grain-oriented silicon steel to maintain the ferrite state in the experimental temperature range. Moreover, the relatively lower content of Mn (0.062wt.%) and S (0.012wt.%) compared with industrial practice was chosen to decrease the solution temperature. The isothermal deformation at constant strain rates was carried out by Gleeble machine, and the microstructure, subgrain boundary characteristic and dislocation configuration were analyzed by OM, EBSD and TEM. The main results obtained are as follows:The Fe-3%Si was isothermally deformed to60%at1073-1473K and1-5s-1to study the flow behavior. The relationship of yield stress, steady state stress and effective stress with deformation temperature and strain rate can be well described by power law equation. The apparent activation energy and stress exponent are289kJ/mol and4.4for steady state stress, while377kJ/mol and5.5for yield stress. The high apparent activation energy for steady state stress can be attributed to the effect of high yield stress. By introducing yield stress into Bergstrom model, accompanied with the constitutive equations for steady state stress and yield stress as well as the empirical formula between yield strain and stress, the whole stress-strain curves can be satisfactorily predicted. The microstructure after deformation was analyzed by OM, EBSD and TEM, and it was found that the microstructure was affected by the Zener-Hollomon parameter (Z) and can be devided into dynamic recovery as Z>2×10-and partial dynamic recrystallization as Z<2×1011. The subgrain size and subgrain boundary misorientation in the range of1-10" can be expressed as a function of Z. For studying the mechanism of dynamic recrystallization, Fe-3%Si was hot deformed to20%,40%and70%at1173K and0.01s-1. It was found that dynamic recrystallization of Fe-3%Si occurred by lattice rotation mechanism at lower strain and geometric dynamic recrystallization mechanism at higher strain.Fe-3%Si was hot deformed to5%-20%at1073-1373K and0.01-5s-1and then relaxed to study the static recovery. It was found that the static recovery rate decreased with deformation temperature increase due to the strain energy decreasing with deformation temperature increase, while the strain or strain rate had little effect on the static recovery rate. The substructure after dynamic reovery can be further developed in the static recovery process, while the dislocation density inside the substructure remains relatively constant. Complete static recrystallization of Fe-3%Si deformed to40%-60%at higher temperature of1223-1423K and higher strain rate of5-80s-1took place and the static recrystallization kinetics was discribed by Avrami equation.For studying the effect of temperature on strain induced precipitation behavior of MnS, Fe-3%Si was hot deformed to60%at Is-1and1073-1373K, and held for Is at deformation temperature. It was found that the distribution of MnS precipited after deformation was affected by deformation temperature, precipiation mainly on sub-grain boundary in the higher temperature range (T>1273K) and homogeneous precipitation in the lower temperature range (T<1173K). The dependence of the distribution of MnS on deformation temperature can be attributed to the effect of deformation temperature on the critical nucleation energy and the number of effective nucleation site. Due to the well-developed subgrains in the higer temperature range, the interface energy of subgrain boundary contributes to the critical nucleation energy of MnS more significantly than the distortion energy of dislocation inside subgrain, therefore, MnS has thermodynamic advantage to nucleate on subgrain boundary. In the lower temperature range, the lower misorientation of subgrain boundary leads to the weakened contribution of interface energy of subgrain boundary, so that MnS has thermodynamic advantage to nucleate on the dislocations inside subgrain. The density and size of MnS precipitated after deformation are affected by dislocation density in the case of MnS homogeneous precipitation. The higher density and smaller size of MnS ocuured with the strain increase in transient stage, while the desity and size of MnS keep relatively constant in the steady state stage. Moreover, the homogeneity of MnS precipitated after deformation was affected by the solution depleted area around the MnS precipitated before deformation, which can be weakened by increasing strain. In contrast, in the temperature range where subgrain boundary acts as the preferential nucleation site, the distribution of MnS is dominantly affected by subgrain size.Partial static recrystallization before MnS precipitation has an evident effct on the distribution of MnS. In the steady state stage, the effect of strain on static recrystallization kinetics is larger than on strain induced precipitation kinetics. Therefore, the sequence of precipitation and recrystallization can be controlled by modifying strain to avoid the detrimental effect of static recrystallization on precipitation of MnS. Moreover, the occurrence of precipitation and recrystallization can also be controlled in terms of deformation temperature and the cooling rate.It should to be noted that the Fe-3%Si alloy used in the experiment and deformation mode are somewhat different from the grain-oriented silicon steel containing a small anount of austenite and the continuously decreasing temperature in industrial hot deformation process. Consequently, it is necessary to consider the effects of temperature and strain gradients, the austenite fraction on microstructure evolution and strain induced precipitation when the above results are to be put into industrial application.