Oriented Electrical Steel

Grain-oriented silicon steel, also referred to as oriented electrical steel, is a premium soft magnetic alloy tailored for static electrical equipment cores. Its most distinctive feature is the highly aligned grain structure—the crystal grains are arranged in a (110)[001] orientation parallel to the rolling direction of the steel sheet. This structure endows the material with ultra-low iron loss and high magnetic permeability along the rolling direction, making it an irreplaceable core material for high-efficiency power transformers.

Core Composition and Manufacturing Principles

Chemical Composition Design

Silicon (Si): The core alloying element, with a typical content of 2.8%–3.5%. A moderate silicon content enhances electrical resistivity, suppresses eddy current loss, and optimizes magnetic properties. Excessively high silicon content will drastically reduce the steel’s ductility, making it difficult to process.

Iron (Fe): The base matrix, providing the fundamental magnetic properties of the alloy.

Impurity control: Strictly limits the content of carbon (C), sulfur (S), phosphorus (P) and other harmful elements. Carbon increases hysteresis loss; sulfur and phosphorus cause embrittlement and degrade magnetic uniformity.

Auxiliary elements: Trace amounts of manganese (Mn) and aluminum (Al) are added to form fine precipitates, which inhibit grain growth during annealing and refine the grain structure.

Key Manufacturing ProcessesThe production of grain-oriented silicon steel is a highly precise and complex process, with two core steps determining its magnetic properties:

Secondary recrystallization annealing: The steel sheet undergoes cold rolling and intermediate annealing multiple times, then is subjected to high-temperature annealing (above 1,100°C). This process promotes the selective growth of grains with (110)[001] orientation, forming a fully aligned grain structure along the rolling direction.

MgO coating annealing: A magnesium oxide coating is applied to the steel sheet surface before high-temperature annealing. It reacts with the steel surface to form a glassy forsterite insulation layer, which not only prevents eddy current loss between core laminations but also controls grain growth orientation during annealing.

Key Performance Indicators

The performance of grain-oriented silicon steel is directionally dependent—excellent magnetic properties are only exhibited along the rolling direction, while properties in the transverse direction are significantly weaker. Core indicators include:

Iron Loss (P₁₇/₅₀, P₁₇/₆₀)

The most critical index for evaluating energy efficiency, representing the energy loss per unit mass of steel sheet under a sinusoidal magnetic field with a magnetic induction intensity of 1.7 T and a frequency of 50 Hz/60 Hz. It is composed of hysteresis loss, eddy current loss and abnormal loss. Ultra-low iron loss can significantly reduce the no-load loss of transformers.

Magnetic Induction Intensity (B₈, B₂₅)

Refers to the magnetic flux density when the magnetic field strength is 8 A/cm and 25 A/cm, respectively. Higher magnetic induction intensity allows the transformer core to achieve higher magnetic flux with smaller size, reducing the overall volume and weight of the equipment.

Magnetic Permeability

It has extremely high magnetic permeability along the rolling direction, which means the steel sheet can be magnetized to the rated magnetic induction intensity with a very small excitation current, further reducing the no-load current of the transformer.

Insulation Coating Adhesion

The surface insulation layer must have good adhesion and insulation performance to ensure the reliability of the core lamination structure and prevent short-circuit eddy current loss.

Classification and Typical Applications

Grain-oriented silicon steel is mainly classified by iron loss level and magnetic induction intensity, and its applications are highly concentrated in static power equipment:

Development Trends

Ultra-low loss and high magnetic induction: Developing Hi-B grades with lower iron loss (e.g., P₁₇/₅₀ < 0.8 W/kg) to meet the high-efficiency and energy-saving requirements of UHV power grids.

Thin-gauge and narrow-width customization: Producing thinner steel sheets (e.g., 0.23 mm, 0.20 mm) to reduce eddy current loss, and customizing narrow-width specifications for special transformers.

Green production process: Optimizing the high-temperature annealing process, reducing energy consumption and carbon emissions, and developing recyclable oriented silicon steel products.

Specialization for new energy systems: Developing oriented silicon steel suitable for new energy grid-connected transformers, which can adapt to the characteristics of fluctuating loads and high-frequency operation.