Study On The Preparation Of {100} Textured Electrical Steels By Phase Transformation

With the coming of the high efficiency of electrical equipment, magnetic properties of the traditional non-oriented electrical steel are difficult to meet the needs of the people, especially the high core loss and low magnetic induction of high grade sheets. The strategies for a better magnetic property in non-oriented electrical steels are generally to purify the steels eliminating precipitates and to obtain a uniform grain size distribution. In contrast, the texture is not well controlled. It is well-known that the ideal texture for non-oriented electrical steel is{001}<uvO>, where each grain in this orientation has two<100> directions in the plane of the sheet to give excellent magnetic performances and nearly isotropic properties. Therefore, the preparation of {100} textured columnar grain structure in a non-oriented electrical steel is the key of developing non-oriented electrical steel.There are many methods to prepare{100} texture, such as by making use of surface energy, strain energy control, the two stages of decarburization annealing process, two stage cold rolling and annealing process, cross rolling process, the strip casting process and so on. Because of a long processing time, complicated process, and high requirement on equipment, it is difficult to produce{100} textured non-oriented electrical steel in the industrialized production.Low carbon intermediate Si-content and ultra-low carbon non-oriented electrical steels are used in this study. It is difficult to obtain the{100} texture by decarburizing annealing under wet hydrogen atmosphere in the low carbon intermediate Si-containing non-oriented electrical steel. A strong{111} texture is obtained and a notable decrease in magnetic properties is observed (P15/50=4.49W·kg-1, B50=1.69T). The optimized texture and excellent magnetic properties are obtained in intermediate Si-content non-oriented electrical steel by process adjustments, such as using pure hydrogen atmosphere during final annealing and coarsening initial microstructure by hot-band annealing. A number of near{100} oriented columnar grains are obtained, and a notable increase in magnetic properties is observed, with magnetic induction at5000/Am (B50) above1.76T and core loss at1.5T by50Hz (P15/50) below2.6W/kg. Meanwhile, the {100} texture with columnar grains is obtained by y to a phase transformation under pure hydrogen atmosphere in Fe-0.5%Mn non-oriented electrical steel. The magnetic performance of laboratory-prepared samples is significantly superior (P,5/50=4.30W·kg1, B50=1.80T).Further discussions on texture optimization for intermediate Si-contents non-oriented electrical steel are required. First, pure hydrogen atmosphere is used to weaken the surface oxidation layer and to enhance the effect of anisotropic strain energy. Driven by anisotropic strain energy during phase transformation, a number of near{100} oriented columnar grains are obtained. Second, high amount of non-gamma fiber-oriented recrystallized grains, which strongly influence the final transformation texture, are observed during the rapid heating process in the final annealing because of the coarse initial microstructure. Non-gamma fiber-oriented grains grow from the surface into the center layer along the normal direction and form poor columnar grains by carbon diffusion during slow y/a transformation in final annealing under pure hydrogen atmosphere. In contrast, high temperature gradient in sheet surface region in pure hydrogen atmosphere induces a significant strain energy difference across grain boundaries during y to a phase transformation, leading to the formation of {100} texture with columnar grains in Fe-0.5%Mn non-oriented electrical steel. In addition, different textures and microstructures are developed under various atmosphere conditions. The γâ†'α transformation interface in microstructure moves from the surface of sheets towards the inner part along ND under a high temperature gradient in pure hydrogen atmosphere, calling the process as "directional" phase transformation. Driven by the anisotropic strain energy, the strong{100} textured columnar grains are obtained during the "directional" phase transformation in pure hydrogen atmosphere with a high flow rate. However, driven by the anisotropies in both strain energy and surface energy, the fine{100} and{110} textured columnar grains are developed in pure hydrogen atmosphere with a relatively low flow rate. By contrast, the transformation process is "global" when specimens are annealed in pure nitrogen atmosphere. As a consequence, a{111} texture with equiaxed grains is resulted.Given its simplicity in processing and its ability to obtain good texture-related magnetic properties, the proposed approach is helpful to the development of new types of non-oriented electrical steels.