Effect Of Nb On Inhibitors Precipitation,microstructure And Texture Evolution Of Grain Oriented Silicon Steel Manufactured By Low Temperature Slab Reheating Technology

Grain oriented silicon steels are wildly used as the core materials for transformers due to the high magnetic permeability and low core loss,which are charactered by the sharp Goss texture({110}<001>)obtained by secondary recrystallization annealing.Currently,the commonly used inhibitors in grain oriented silicon steel are Al N and Mn S.In order to acquire fine and dispersed inhibitors during hot rolling and normalization process,the ingots are usually reheated to above 1350 ^oC or even higher to ensure the coarse particles fully dissolve into the iron matrix before hot rolling.In this case,it wastes a great deal of energy.Based on the environmental protection and sustainable development,producing high quality grain oriented silicon steel by low temperature slab reheating technique has become an important development direction.Nb C,Nb N and Nb(C,N)have low solution temperature,and possess the general properties as the inhibitor of grain oriented silicon steel.Therefore,it is expected to reduce the slab reheating temperature by adding niobium as inhibitor forming element into grain oriented silicon steel.In this study,the evolution of precipitates,microstructure and texture along the process of Nb-bearing grain oriented silicon steel were studied systematically,and the role of Nb in grain oriented silicon steel was discussed.The specific research contents and results are as follows:(1)The effect of niobium on the microstructure evolution and the behaviors of second phase particles of the grain oriented silicon steel(0-0.022 wt%Nb)ingots during reheating process was studied.The results show that the homogeneity of microstructure of reheated specimens can be improved by Nb addition.During the cooling process,the precipitation of second phase particles can be divided into two stages,Mn S and Nb(C,N)precipitates at the first stage,while Al N precipitates at the second stage.Meanwhile,the second phase particles precipitated at first stage in Nb-bearing steel are finer and mor dispersed than those in steel without Nb,Nb addition makes little effect on the behaviors of precipitation of Al N.(2)The kinds,number density and size of inhibitors in Nb-bearing grain oriented silicon steel hot-rolled bands and normalized bands(High-Nb Low-Al,0.028-0.052 wt%Nb)were investigated.The results show that the main inhibitors are Mn S,composite precipitates with Mn S as the core and Nb(C,N).The precipitates in steel with 0.028 wt%Nb are finer and more dispersed than those in steel with 0.052 wt%Nb.In hot-rolled bands with 0.028 wt%Nb,the average size and number density of inhibitors are 40 nm and 13.8×10⁵/mm²,respectively.While those in hot-rolled bands with 0.052 wt%Nb are66 nm and 9.4×10⁵/mm²,respectively.After normalization annealing,the number of inhibitors with small size in steels increased significantly,which makes the average size and number density of inhibitors in normalized bands decrease and increase,respectively.(3)The microstructure,texture evolution and magnetic properties of Nb-bearing grain oriented silicon steel(High-Nb Low-Al,0.028-0.052 wt%Nb)were investigated.The results show that the content and intensity of Goss texture in steel with 0.028 wt%Nb are higher than those in steel with 0.052 wt%Nb.Meanwhile,the steel with 0.028wt%Nb performs stronger pinning force during the whole heat treatment process,resulting in smaller grain size in normalized bands and primary annealed bands of low Nb steel.The magnetic properties of Nb-bearing grain oriented silicon steel can be improved by normalization annealing,the secondary annealed bands without normalization annealing show poor magnetic properties,B₈₀₀ is lower than 1.50 T,P_1.7/50is higher than 2.41 W/kg,because the secondary recrystallization was not completed during final annealing.While large macrostructure is obtained in steel with normalization annealing,and the grain size reaches the centimeter scale with the maximum size of about 1 cm.The best magnetic properties were obtained in steel with0.028 wt%Nb,and B₈₀₀=1.70 T,P_1.7/50=1.77 W/kg.(4)The effect of nitriding and decarburization annealing on the carbon content,precipitates,microstructure and texture of Nb-bearing grain oriented silicon steel(0.052wt%Nb)were studied.The carbon content in steel decreased to below 0.003 wt%after decarburization annealing 150 s,which is satisfied with the requirement of decarburization annealing of grain oriented silicon steel.Nitriding treatment before decarburization annealing can improve the homogeneity of primary recrystallization structure,but it makes little effect on the texture evolution.For the specimens without nitriding,after decarburization annealing 150 s,the primary recrystallization microstructure is obviously inhomogeneous along the thickness direction,fine grains were observed in the center region,while larger grains were observed at the edge,the average grain diameter is 18.4?m,but the standard deviation reached to 10.3?m.For the specimens with nitriding,the primary recrystallization microstructure is homogeneous and fine,and the average grain diameter and standard deviation were only10.2?m and 4.6?m,respectively.After final annealing,secondary recrystallization took place in the specimens with nitriding,which shows better magnetic properties.(5)The inclusions and mechanical properties of Nb-bearing grain oriented silicon steel(Low-Nb High-Al,0-0.025 wt%Nb)were investigated.The results show that Nb addition can reduce the number of inclusions.With the increase of Nb content from 0 to0.025 wt%,the number density of inclusions decreases from 578/mm² to 157/mm²,especially,the number density of inclusions with large size(>2?m)decreases significantly.In addition,the yield strength and tensile strength of ingots increases from361 MPa and 385 MPa to 450 MPa and 566 MPa,respectively.It contributes to the smooth construction of hot rolling and cold rolling during the production process of grain oriented silicon steel.High strength grain oriented silicon was obtained by Nb addition to meet the special requirements.(6)The evolution of inhibitors,microstructure,texture and magnetic properties of Nb-bearing grain oriented silicon steel(Low-Nb High-Al,0-0.025 wt%Nb)were investigated.The number of Nb(C,N)decreases after decarburization annealing due to the decrease of C content,resulting in significantly decrease of number density of inhibitors.After decarburization annealing,the distribution of inhibitors,microstructure and texture in primary recrystallized bands with 0.009 wt%Nb shows advantages in promoting Goss grains grow up abnormally during final annealing.After final annealing,perfect secondary recrystallization took place,and excellent magnetic properties were obtained in steel with 0.009 wt%Nb,and B₈₀₀=1.872 T,P_1.7/50=1.25 W/kg.