Non-oriented electrical steel is a ferrosilicon soft magnetic alloy with very low carbon content. It is an indispensable and important material in the power, electronics and military industries. Non-oriented silicon steel is the core material of motor and generator rotors that work in rotating magnetic fields, requiring good magnetic properties and process performance. In recent years, with the development of high-speed and miniaturized motors, higher performance requirements have been put forward for non-oriented silicon steel, such as low iron loss and high magnetic induction at high frequencies.
Due to the complex manufacturing process and equipment of electrical steel and strict composition control, companies regard the manufacturing technology of electrical steel as their life and protect it in the form of patents. The manufacturing technology and product quality of electrical steel have become one of the important indicators to measure the development level of a country’s steel technology.
Generally speaking, the electricity consumption of motors accounts for about 65% to 70% of the total power generation. If the loss of motors can be reduced by 10%, based on 2006 statistics, motors nationwide can save 20 billion kW·h/ a or above, which is equivalent to saving 7.7 million tons of standard coal, reducing sulfur dioxide emissions by more than 120,000 tons, reducing nitrogen dioxide emissions by more than 70,000 tons, and reducing carbon dioxide emissions by more than 20 million tons. Therefore, improving the performance of non-oriented electrical steel is of great significance for energy conservation and emission reduction.
The main technical key to improving the performance of non-oriented electrical steel is to further achieve strict control of steel composition, because trace elements have a great impact on the magnetic properties of non-oriented silicon steel.
The main harmful elements that are detrimental to its magnetic properties are:
(1) Carbon: It will worsen iron loss, cause aging, and form fine carbides.
(2) Nitrogen, sulfur and oxygen: Sulfides such as MnS, nitrides such as AlN and TiN, and various oxides and other fine particles are precipitated to prevent domain wall movement.
(3) Titanium: TiC and TiN fine particles are precipitated, which increases the recrystallization temperature, delays recrystallization and grain growth, and promotes the development of unfavorable orientation.
(4) Vanadium, Zirconium and Niobium: generates the precipitation of fine particles of VC, VN, ZrC, ZrN, NbC and NbN, hindering recrystallization and grain growth.
(5) Arsenic: Promotes the precipitation of sulfides such as MnS.
(6) Copper: CuS particles are generated, which hinders the movement of magnetic domain walls and the growth of crystal grains.
(7) Molybdenum: generates related oxide, sulfide, and nitride particles, which affects performance.
Elements that have a beneficial effect or dual impact on the magnetic properties of non-oriented silicon steel are:
(1) Aluminum: The role of aluminum is similar to that of silicon. Aluminum can increase resistance, shrink the austenite phase area, and promote grain growth, so it has certain beneficial effects. However, the role of aluminum is affected by the nitrogen content in silicon steel. Aluminum and nitrogen easily form an AlN precipitate phase, which reduces the magnetic properties of the silicon steel sheet. When the size of the precipitated AlN particles is less than 0.5 μm, they pin the grain boundaries and hinder the growth of the grains, thereby increasing iron loss. However, when the size of the precipitated AlN particles is larger than 1 μm, their pinning effect on the grain boundaries is very light, and therefore has little impact on the magnetic properties of the sample.
(2) Phosphorus: Phosphorus can improve the magnetic properties of iron-silicon alloy. Phosphorus can form iron phosphide at the grain boundaries, which can improve the punching properties of silicon steel. The segregation of phosphorus at the grain boundaries can hinder the nucleation and growth of unfavorably oriented recrystallized grains and increase the magnetic induction intensity. At the same time, phosphorus will increase the resistance of silicon steel and reduce iron loss.
(3) Manganese: Manganese can increase the resistance of silicon steel and reduce iron loss. But the role of manganese has a lot to do with sulfur content. When the hot rolling heating temperature is below the MnS solid solution temperature, the generated MnS can be coarsened; if it exceeds the MnS solid solution temperature, MnS will dissolve and disperse and precipitate during the subsequent cooling process, thereby reducing the magnetic properties.
(4) Tin: A trace amount of tin under certain limits will promote the formation of favorable texture, improve magnetic induction and reduce iron loss.