Basic knowledge of silicon steel (electrical steel)

Silicon steel, also known as electrical steel, is a special low-carbon steel with a silicon content ranging from 0.5% to 4.5%. Due to its excellent magnetic properties, it has become a core material for electromagnetic energy conversion in power and electronic equipment, from transformers in power stations to motors in household refrigerators, all relying on its application.

1. Core characteristics: why silicon steel is the "magnetic choice"

The core value of silicon steel lies in its unique magnetic properties, which are determined by both composition design and production process. These properties are primarily manifested in three key dimensions:

High magnetic permeability: Magnetic permeability measures the ease with which a material can be magnetized by a magnetic field. The high magnetic permeability of silicon steel means that it can be more easily magnetized under the same external magnetic field, generating a stronger internal magnetic field and thus reducing energy loss. This is crucial for devices that require efficient magnetic field conversion, such as transformers and motors, allowing them to achieve high magnetic flux output with low energy consumption.

Low iron loss: Iron loss refers to the energy loss (usually dissipated in the form of heat) incurred by a material in an alternating magnetic field (such as an AC environment) due to hysteresis and eddy current effects. Silicon steel, through precise control of silicon content (silicon increases the resistivity of steel, suppressing eddy currents) and rolling processes, reduces iron loss to an extremely low level. For example, in the long-term continuous operation of power transformers, low iron loss can significantly reduce their annual energy consumption, serving as the core material support for "energy-saving transformers".

Excellent magnetic aging stability: Magnetic aging refers to the phenomenon where the magnetic properties of a material degrade over time during long-term use or storage. Silicon steel, through processes such as decarburization annealing and insulating coating, can effectively inhibit the slow changes in its internal structure, ensuring that the magnetic properties remain stable throughout the decades-long service life of the equipment, thus avoiding efficiency reductions or malfunctions caused by declining magnetic properties.

II. Classification logic: divided by purpose and performance

The classification of silicon steel primarily revolves around "application scenario requirements", with the core falling into two major categories. Significant differences exist between these categories in terms of production processes and performance priorities:

1. Cold-rolled silicon steel: the "mainstay of magnetic cores" in high-end equipment

Cold-rolled silicon steel, produced using the cold-rolling process, boasts a thinner thickness (typically ranging from 0.15mm to 0.5mm) and superior magnetic properties, making it the preferred choice for medium and high-frequency, high-precision equipment. It can be further subdivided into:

Cold-rolled non-oriented silicon steel: Its magnetic properties are essentially uniform in all directions of the steel (with no significant directionality), and it is primarily used in rotating equipment such as the stator and rotor of electric motors. When these devices operate, the direction of the magnetic field continuously changes as the rotor rotates. The non-oriented magnetic properties ensure stable power output from the motor at different rotational angles, commonly seen in household air conditioning compressors, electric vehicle drive motors, and the like.

Cold-rolled oriented silicon steel: Through special rolling and annealing processes, the magnetic grains of the steel are aligned in a specific direction (usually the rolling direction), resulting in extremely high magnetic permeability and low iron loss in that direction, but poor performance in the perpendicular direction. It is mainly used for the core of transformers - when a transformer operates, the magnetic field direction is relatively fixed (along the core magnetic circuit). Oriented silicon steel can minimize energy loss and is the core material for high-voltage and ultra-high-voltage transformers.

2. Hot-rolled silicon steel: the "economic choice" for medium and low voltage scenarios

Hot-rolled silicon steel is produced using hot rolling technology, with a relatively thick thickness (typically 0.5mm - 1.0mm). Its magnetic properties are inferior to those of cold-rolled silicon steel, but its production cost is low and the process is relatively simple. It is mainly used in medium and low voltage equipment where magnetic performance is not highly required, such as small distribution transformers, electric fan motors, washing machine motors, and other civilian or low-end industrial applications. It is still widely used in cost-sensitive fields.

III. Production process: the "key link" determining performance

The production of silicon steel is a field with extremely high technical thresholds in steel manufacturing, where each step of the process directly affects the final magnetic properties. The core processes include:

Smelting and casting: It is necessary to strictly control the content of impurities such as carbon, sulfur, and nitrogen (impurities can hinder magnetic domain movement and reduce magnetic properties), while precisely adding silicon to ensure uniform silicon content. Subsequently, steel billets are produced through continuous casting process. Some high-end silicon steels also adopt "thin strip continuous casting" technology to shorten the subsequent rolling process and improve microstructural homogeneity.

Rolling: It is the core step that determines the thickness and texture (grain orientation) of silicon steel. Cold-rolled silicon steel undergoes multiple passes of cold rolling to gradually reduce the steel billet to the target thickness (such as 0.3mm, 0.23mm). For oriented silicon steel, a "secondary cold rolling" is also required at a specific temperature to guide the grains to align along the rolling direction. Hot-rolled silicon steel is directly rolled into thick strips by a hot rolling mill, which is a relatively simplified process but with lower precision.

Annealing treatment: It is divided into "decarburization annealing" and "finished product annealing". Decarburization annealing is mainly used for cold-rolled silicon steel, where excess carbon in the steel is removed at high temperatures (the carbon content needs to be controlled below 0.005%, otherwise it will increase iron loss), while forming a uniform grain structure. Finished product annealing involves heating the rolled steel at high temperatures to further optimize grain size and texture, and enhance magnetic permeability. The finished product annealing of oriented silicon steel needs to be carried out in a specific magnetic field environment ("magnetic field annealing") to ensure more precise grain orientation.

Insulating Coating: The final step involves applying a thin insulating coating (typically an inorganic coating such as magnesium oxide or magnesium phosphate) onto the surface of silicon steel. The primary functions of this coating are: firstly, insulation, to prevent eddy currents from occurring when silicon steel sheets are stacked (if the sheets are electrically connected, large eddy currents will form, increasing losses); secondly, rust prevention, to protect the silicon steel from oxidation during storage and use; and thirdly, to improve lamination, allowing the silicon steel sheets to be stacked tightly and evenly to form an iron core.

IV. Application Fields: "Full Chain Coverage" from Power Generation to Household Use

The application of silicon steel spans the entire industry chain from "power generation - power transmission - power consumption", serving as the "basic framework" of power systems and electronic equipment:

Power generation end: The stator core of large generators is made of cold-rolled non-oriented silicon steel, which converts mechanical energy (such as the power of steam turbines and water turbines) into electrical energy through electromagnetic induction. The high magnetic permeability ensures high power generation efficiency;

Transmission end: The iron core of high-voltage and ultra-high-voltage transformers is made of cold-rolled oriented silicon steel, which steps down (or steps up) the high voltage output from the power station to achieve long-distance low-loss transmission. Low iron loss is the key to reducing transmission energy consumption;

Electrical equipment: Almost all equipment containing motors and transformers requires silicon steel - cold-rolled non-oriented silicon steel for air conditioning, refrigerator, and washing machine motors in the household sector; cold-rolled or hot-rolled non-oriented silicon steel for water pump and fan motors in the industrial sector; hot-rolled silicon steel for small cores in household low-power transformers and chargers; and high-end cold-rolled non-oriented silicon steel (requiring higher magnetic properties and temperature resistance) for electric vehicle drive motors and charging pile transformers in the new energy sector.