Solid-siliconizing Treatment Of Cold-rolled Non-grain Oriented Silicon Steel—Microstructural Evolution And Properties

Silicon steel usually exhibites excellent magnetic property,such as the lowest iron loss under high frequency,the largest magnetic permeability and the smallest magnetostriction coefficient,when its silicon reaches 6.5wt.%(refer as high silicon steel),therefore it is the ideal(high frequency,high efficiency,low noise)core material for the electronic equioment.However,with the increment of silicon content,its brittleness increases and elongation decrease sharply,then it is difficult to be produced by rolling,that limits its application.Though NKK Corporation successfully applied the CVD method to the small-scale production of 6.5wt.%silicon steel sheets in Japan,there are still problems to be solved,such as high deposition temperature,high energy consumption,corrosion of equipment and plates by the SiCl4 decomposion,and subsequent warm rolling and flattening operations.These technological obstacles restrict the production scale,and the 6.5wt.%silicon steel sheet produced through the CVD mat ho d can not meet the market demand.Solid siliconizing mathod can synthesize ideal compound layer(i.e.densed and well-bonded with the matrix)on the sample surface,when both the siliconizing temperature and the corrosive halide content are much lower than that of conventional CVD method.So,it is valuable for the solid siliconizing method to be researched and applied.Based on the previous research,the work in this paper was focused on the investigation of solid siliconizion of cold-rolled 3wt.%non-grain oriented silicon steel sheet,in order to explore the formation mechanism of the compound layer and the influence of the related factors.Diffusion annealing was carried for the selected siliconized sample,to study the influence of annealing temperature on micro structure,phase and silicon distribution along the depth.Magnetic properties and hardness wrer measured for the typical samples,to determine the relationship between technological parameter,microstructure and properties.The main conclusions are as follows:(1)Densed compound layer can be obtained on the surface of cold-rolled non-oriented silicon steel after the solid siliconizing in the range of 800?850?.The thickness of compound layer increases with the increment of the siliconizing temperature and the duration,and the influence of the temperature is greater.(2)The compound layer is composed of the surface FeSi phase layer and the adjacent Fe3Si phase layer.During the growth of Fe3Si phase layer toward the matrix,the thickness of FeSi phase remains nearly unchanged.When the Fe3Si phase layer grows to the center of the sheet,the thickness of FeSi phase begins to increase.(3)After diffusion annealing at 850?1050? for 60 min,the interface between the compound layer and the matrix of siliconized sample disappears,and the silicon content tends to be uniform distribution along the depth.Increasing the annealing temperature is helpful for this process.(4)The FeSi phase on the surface of the siliconized samples is transformed into the Fe3Si phase after diffusion annealing at 850? and 950?,and further transformed into?-Fe phase with the increment of the temperature into 1050?.(5)The sample iron loss increases at low frequency and decreases significantly at high frequency after siliconizing and diffusion annealing.At 0.005 T,the iron loss of the sample of 0.2 mm thick can be reduced by 40.5?80.0%.Meanwhile,the intensity of magnetic induction is slightly reduced.(6)The hardness of the cold-rolled samples after solid siliconizingis remains constant in the compound layer,and drop supptly at the interfacebe tween the compound layer and matrix;The hardness of the FeSi and Fe3Si phases is similar.