What Is Silicon Steel?
Silicon steel is a soft magnetic silicon-iron alloy with very low carbon content, typically having 1.0%–4.5% silicon alloying components. Silicon is a useful deoxidizer for steel because it reacts with oxygen to generate stable SiO2 that is not reduced for carbon, thereby avoiding iron lattice distortion caused by oxygen atom doping. Silicon solidifies in Fe, increasing resistivity while also assisting in the separation of the harmful impurity carbon. As a result, the inclusion of silicon enhances iron's resistivity and maximum permeability, decreases coercivity, core loss (iron loss), and magnetic aging, and necessitates a certain degree of flexibility to satisfy the needs of punching and shearing processing when making appliances. It is necessary to have a lower quantity of detrimental contaminants, as well as a flat plate shape and acceptable surface quality, in order to improve magnetic susceptibility and reduce hysteresis loss. However, increasing the silicon percentage makes the material hard and brittle decreases thermal conductivity and toughness reduces heat dissipation, and makes mechanical processing difficult; thus, the general silicon content of steel sheets should not exceed 4.5%.
Classifications of Silicon Steel
- According to the manufacturing technique, silicon steel can be classified as hot-rolled or cold-rolled; hot-rolled silicon steel has a limited availability and a significant energy loss, and it is gradually being phased out. According to the internal grain orientation, cold-rolled silicon steel is classified as oriented silicon steel or non-oriented silicon steel.
- Electrical steel is classified into two types based on their silicon content: low-silicon sheets (low grade) and high-silicon sheets (high grade). Low-silicon steel sheet has a certain mechanical strength and is commonly referred to as "dynamo silicon steel sheet." High-silicon steel sheet is magnetically good but more brittle and is commonly used in the construction of transformer cores and is referred to as "transformer silicon steel sheet."
| Type | Silicon Content (%) | Nominal Thickness (mm) | Applications | ||
| Hot-Rolled Silicon Steel (Non-Oriented) | Hot-Rolled Low Silicon Steel (Dynamo Steel) | 1.0-2.5 | 0.5 | Household motors and micro-motors | |
| Hot-Rolled High Silicon Steel (Transformer Steel) | 3.0-4.5 | 0.35, 0.50 | Transformers | ||
| Cold-Rolled Electrical Steel | Cold-Rolled Non-Oriented Electrical Steel (Dynamo Steel) | Low Carbon Electrical Steel | ≤0.5 | 0.50, 0.65 | Household motors, micro-motors, small transformers, and ballasts |
| Silicon Steel | >0.5-3.5 | 0.35, 0.50 | Large and medium-sized motors, generators, and transformers | ||
| Cold-Rolled Grain-Oriented Silicon Steel (Transformer Steel) | General Grain-Oriented Silicon Steel (CGO) | 2.9-3.3 | 0.18, 0.23, 0.27, 0.30, 0.35 | Small, medium, and large transformers and ballasts | |
| High Magnetic Induction Grain-Oriented Silicon Steel (HiB) | |||||
| Special-Purpose Silicon Steel | Cold-Rolled Grain-Oriented Thin Silicon Steel Strip | 2.9-3.3 | 0.03, 0.05, 0.10 | Pulse transformers, magnetic amplifiers, high-frequency transformers, and welding machines | |
| Cold-Rolled Non-Oriented Thin Silicon Steel Strip | 3.0 | 0.15, 0.20 | High-frequency motors and generators | ||
| Cold-Rolled Non-Oriented Silicon Steel for Magnetic Switches | 3.0 | 0.70 | Relays and magnetic switches | ||
| Cold-Rolled High Silicon Steel | 6.5 | 0.1-0.5 | High-frequency motors, transformers and magnetic shielding | ||
Difference between Grain-Oriented and Non-Oriented Silicon Steel
The grain distribution of grain-oriented silicon steel is disordered, has a low silicon content, and is primarily used in motor manufacturing.
The grain distribution of non-oriented silicon steel is chaotic with low silicon content and is primarily used in transformer manufacture. The typical limit for silicon concentration is 3%. Cold-rolled silicon steel is classified into two types: non-oriented silicon steel and grain-oriented silicon steel:
- Manufacturing cold-rolled non-oriented silicon steel is quite simple, with silicon mass fractions ranging from 0.5% to 3.0%. The thickness of the completed product after cold rolling is typically between 0.35 and 0.5 mm. It has a greater BS than oriented silicon steel, as well as uniform thickness, excellent dimensional precision, a smooth and flat surface, and better filling factor and magnetic characteristics.
- The silicon mass percentage in cold-rolled grain-oriented silicon steel is 3.0% or higher, and the carbon mass fraction is 0.03%–0.05%. Because the oxide inclusions in the steel are minor, inhibitors are required. Grain-oriented silicon steel offers lower steel losses, strong directional magnetism, and superior high permeability and low loss characteristics in its rolling direction as compared to cold-rolled non-oriented silicon steel. Cold-rolled grain-oriented silicon steel, also known as "cold-rolled transformer silicon steel," is primarily used in the production of transformers.
Cold-rolled oriented silicon steel is further divided into two types: common oriented silicon steel (CGO) and high magnetic induction oriented silicon steel (Hi-B). High magnetic induction cold-rolled oriented silicon steel is a single-oriented steel strip that is primarily utilized in the production of electronic instruments such as chokes, transformers, and other electromagnetic components.
Performance Requirements of Silicon Steel Sheet
The main quality characteristics of silicon steel sheets are iron loss value, magnetic flux density, hardness, flatness, thickness uniformity, coating type, and sheet punch ability.
- Low Iron Loss
Low iron loss is an important indicator of silicon steel sheet quality; the iron loss of silicon steel sheet is determined by the various classes of silicon steel sheet; the higher the grade, the lower the iron loss value.
- High Magnetic Flux Density
Another essential electromagnetic feature of silicon steel sheet is flux density, which reflects the ease with which it can be magnetized. The magnetic flux density is the quantity of magnetic flux per unit area at a specific frequency of magnetic field strength. The flux density of a silicon steel sheet is typically measured in Tesla at a frequency of 50 or 60 Hz with an applied magnetic field of 5000 A/m, known as B50.
The magnetic flux density is affected by the overall structure of the silicon steel sheet, impurities, internal tensions, and other factors. The energy efficiency of motors, transformers, and other motor equipment is directly affected by magnetic flux density. The higher the flux density, the bigger the flux per unit area and therefore the better the energy efficiency; thus, the higher the flux density of silicon steel sheet, the better; nevertheless, most specifications simply demand the minimum value of flux density.
- High Hardness
One of the qualities of silicon steel sheets is its hardness. When current automatic punching machines are used for sheet punching, the hardness requirements are even more rigorous, as too low a hardness is not favorable to the automatic punching machine's feeding operation and tends to produce excessively long burrs, making assembly more difficult. The hardness of the silicon steel sheet must be greater than a particular hardness value in order to achieve these standards. The hardness of 50AI300 silicon steel sheet, for example, is usually not lower than the HR30T hardness rating of 47. As the quality of silicon steel sheeting grows, so does its hardness. The higher the quality of the silicon steel sheet, the more silicon content is usually added, and the harder the steel is due to the solid solution strengthening effect of the alloy.
- Smooth, Flat, and Uniform Surface Thickness
Flatness and thickness homogeneity are significant quality criteria for silicon steel sheets. Excellent flatness makes sheet punching and assembly easier. The rolling and annealing technologies are intimately related to flatness. The advancement of rolling and annealing technology and processes benefits flatness; for example, the use of continuous annealing is superior to batch annealing. It is also critical that the thickness of the silicon steel sheet be consistent. The thickness of the core after assembly will be affected if the thickness difference between the center and the edge of the steel sheet is too large or if the fluctuation in the thickness of the steel sheet is too large along its length. Because different thicknesses of the core cause huge variations in magnetic conductivity, which directly affect the characteristics of the motor and transformer, the lower the thickness variation of the silicon steel sheet, the better. Steel sheet thickness uniformity is strongly related to hot and cold rolling technology and processes, so sheet thickness fluctuation can be reduced by improving the rolling technology.
- Coating
The coating is a critical quality component for silicon steel sheets. The silicon steel sheet's surface is chemically coated, and a thin film is applied to offer insulation, rust protection, and lubrication. Insulation decreases eddy current loss between the cores of the silicon steel sheet; rust prevention keeps the sheet from rusting during production and storage; and lubrication improves punchability and dies life.
- Good punchability
One of the most essential qualities of silicon steel sheets is punchability. Punchability extends die life and reduces burrs in the punch. The punchability of silicon steel sheet is closely connected to the type of coating and hardness. Newer coating types primarily enhance the punchability of silicon steel sheets. Furthermore, the hardness of the steel sheet is too low, causing major burrs and making punching difficult; nevertheless, the hardness is too high, reducing die life; hence, the hardness of the silicon steel sheet must be managed in an optimum range.
Production Process of Silicon Steel Sheet
- Pickling
To prevent problems on the surface of completed cold-rolled items, remove oxides from hot-rolled steel strips using descaling equipment with a hydrochloric acid tank.
- Stamping with a cold stamp
The reduction ratio is chosen at 40%–90% to ensure the thickness and material for varied applications, and advanced control equipment is provided for the realization of automatic thickness control and automatic shape control.
- Annealing
It is a method used in cold stamping to soften tough steel strip material. We make deep-process steel and high-tension steel by heating and rapidly cooling the metal, and we employ box (cover) annealing and continuous annealing processes.
- Coating for insulation
Using continuous coating equipment, silicon steel plates are processed into cores with an insulating coating solution sprayed underneath the plates to improve their processing qualities and eliminate eddy current losses corresponding to the thickness of the plates themselves.
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