Silicon steel is a type of soft magnetic material, mainly used in manufacturing motor and transformer cores, etc. The performance requirements for silicon steel mainly include iron loss, magnetic induction intensity, magnetic anisotropy, magnetic aging, brittleness, etc.; the smelting process of silicon steel is more complex than other steel types, requiring precise process technology in various links such as refining, converter, RH vacuum, and continuous casting.
Silicon steel is a kind of soft magnetic material, with a silicon content in the range of 1.0% to 4.5%, mainly used in manufacturing motor and transformer cores, ballasts in fluorescent lamps, magnetic switches and relays, magnetic shielding, and magnets in high-energy accelerators. The following requirements are imposed on the performance of silicon steel:
(1) Low iron loss. Because high iron loss will increase power loss, the main function of adding silicon to steel is to reduce iron loss; reducing sulfur content helps reduce iron loss; appropriately increasing the phosphorus content in steel is beneficial to reducing iron loss.
(2) High magnetic induction intensity. High magnetic induction intensity can reduce the exciting current (no-load current) of the iron core, reduce copper loss and iron loss caused by wire resistance, and save electricity.
(3) Requirements for magnetic anisotropy. Motors operate in a rotating state, requiring silicon steel to be magnetically isotropic, manufactured using non-oriented silicon steel; transformers operate in a static state, manufactured using cold-rolled oriented silicon steel.
(4) Small magnetic aging. The phenomenon that the magnetic properties of the iron core change with the time of use is called magnetic aging. Magnetic aging is mainly caused by the precipitation of fine carbides and nitrides of supersaturated carbon and nitrogen in the steel. Therefore, the carbon content in high-quality non-oriented silicon steel should be less than 0.0035%, and the nitrogen content should be less than 0.005%.
(5) Low brittleness. Silicon steel sheets must be stamped into shape when making iron cores, and the stamping performance must be good; if the steel is brittle, it will reduce the yield and affect the life of the die. Sulfur is not only harmful to magnetism but also causes the steel to be hot brittle, so it should be reduced as much as possible.
In addition, the surface of the silicon steel sheet should be smooth and flat, the thickness should be uniform with small deviations, and the insulating film should be good, etc.
The smelting process must meet the requirements of different grades of silicon steel, and requires ultra-low carbon, low sulfur, and low nitrogen content for non-oriented silicon steel. In general, the key points of silicon steel smelting are as follows:
A. Raw Material Refining
(1) Using low-manganese molten iron for smelting, requiring wMn < 0.35%, using low-manganese scrap steel, requiring wMn < 0.35%, high manganese content in steel will deteriorate the magnetic properties of silicon steel sheets;
(2) Desulfurization pretreatment of molten iron, w[s] < 0.005% of molten iron entering the furnace;
(3) Using high-grade ferrosilicon alloy (low carbon and manganese content);
(4) Stable composition and few impurities in auxiliary materials.
B. Converter Blowing
Top and bottom combined blowing process, decarbonization, demanganization, good temperature control, and the final carbon content is controlled at 0.04%.
C. Ladle Alloying and Bottom Argon Blowing
Alloys should be added evenly, and all additions should be completed before 70% of the steel is tapped.
D. RH Vacuum Treatment
Fine-tune the molten steel composition to meet the specified requirements and control the final temperature.
E. Continuous Casting Process
(1) Due to the high silicon content and poor thermal conductivity of the molten steel, slow pouring is adopted, and the secondary cooling uses a weak cooling system.
(2) Full-process protection pouring, and the crystallizer uses a special protective slag.
(3) Electromagnetic stirring is used in the secondary cooling zone to improve the proportion of equiaxed crystals and eliminate the corrugated defects on the surface of the silicon steel sheet.
(4) Hot delivery of continuously cast billets, using insulated bogies to prevent cracking.
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