The Influence Of Niobium And Chromium On Behaviors Of Precipitates And Hot Rolled Microstructure Of Grain-oriented Silicon Steel

At present,high quality low slab reheating temperature grain-oriented silicon steel becomes the major developing product of industry for its advantages of high quality and low cost.The key to produce it is to add suitable grain boundary segregation elements or inhibitors forming elements into the steel.And the inhibitors with low solution temperature take effect on inhibiting from normal grain growth.Niobium and chromium chosen as auxiliary inhibitors are added into grain-oriented silicon steel to aim at providing reference data for the application of niobium and chromium in low temperature grain-oriented silicon steel in this paper.In order to explore the influence of niobium and chromium on the behaviors of precipitates and hot rolled microstructure of grain-oriented silicon steel,the characteristics of precipitates in grain-oriented silicon steel with different content of niobium and chromium were investigated in this paper.The optimized design content of niobium and chromium was proposed by thermodynamic calculation,and the formation mechanisms of inclusions and precipitates were analyzed.Then the microstructure evolution of as-cast slabs was observed by confocal scanning laser microscope.The influence mechanisms of the characteristics of precipitates on the microstructure,texture of hot rolled plates with different content of niobium and chromium were analyzed.?1?The composition of grain-oriented silicon steel with different content of niobium and chromium was optimized,and thermodynamics conditions of inhibitors for precipitating were calculated.The thermodynamics calculation showed that during hot rolling process the sequence of precipitation of the second phase particles is MnS,NbN and NbC in specimens bearing more niobium,NbN in specimens bearing less niobium,Al N.The second phase particles such as Cr₇C₃,CrN and NbC in specimens bearing less niobium precipitate until coiling process in the steel.?2?The in-situ observation at high temperature shows that the second phase particles in specimen bearing more niobium and less chromium dissolve best at 1350?.The precipitation of heterogeneous microstructures in this specimen is divided into three stages.Every stage lasts very short and the integrity of phase change is better.While the microstructure evolution of other as-cast slabs is only divided into two stages.?3?The heterogeneous microstructures are more homogeneous in specimens bearing more niobium and are mainly banding-like or net-like,which are aggregated with Nb-Al-S and Fe3 C.The heterogeneous microstructures in specimens bearing less niobium have various morphologies and similar composition with specimens bearing more niobium.There is a large amount of Mn S in the specimens and the small-sized MnS are dispersed distributing around the heterogeneous microstructures and the large-sized MnS is the core of the heterogeneous microstructures.Chromium content has little influence on the distribution and composition of heterogeneous microstructures.?4?The precipitates in specimens bearing more niobium are mainly Nb N and NbC which dissolve at low temperature when the as-cast slabs are quickly quenching after soaking.The area density of the precipitates is larger and the average particle size is smaller.So,adding suitable niobium into grain-oriented silicon steel can reduce the soaking temperature of reheating process.The influence of chromium content on precipitates only reflects in the composition.The influence degree is very small.?5?Compared with the specimens bearing less niobium,there are more NbN,NbC and less AlN in specimens bearing more niobium.The average particle size of precipitates in specimens bearing more niobium is smaller and the volume density is larger.Under high niobium conditions,it can be known that the average particle size of precipitates in specimen bearing less chromium is smaller and the volume density is larger.The inhibition effect on recrystallization is stronger.?6?The microstructure gradient along the longitudinal section is better in specimens bearing more niobium.Under high niobium conditions,it can be known that the recrystallization fraction is fewer,there are more restored and deformed microstructure,and Goss texture component has higher proportion and stronger intensity in specimen bearing less chromium while compared with the specimen bearing more chromium.Therefore,0.08% niobium is recommended to add into grain oriented silicon steels to form more fine low temperature inhibitors and obtain higher proportion and stronger intensity Goss texture in the designed composition of grain oriented silicon steel.But the effect of chromium is not obvious and further study need to be done.