Blog

Yellow discoloration on silicon steel sheets after heating is mainly caused by surface oxidation reactions and improper temperature regulation. At high temperatures, iron reacts with oxygen to produce iron oxides, and temperature fluctuations accelerate this process. This article analyzes the root causes and provides targeted countermeasures.

I. Mechanism of Yellow Discoloration from Oxidation

When silicon steel sheets are exposed to high-temperature environments, iron on the surface undergoes oxidation with oxygen to form ferric oxide (Fe₂O₃), a compound that appears yellow or tan. The degree of oxidation is governed by three core factors:

• Ambient oxygen concentration: Open heating environments lead to far more severe oxidation than sealed annealing furnaces.
• Heating duration: Prolonged high-temperature exposure thickens the oxide layer.
• Silicon content: High-silicon steel sheets deliver superior oxidation resistance compared to low-silicon grades.

💡 Core Verdict: Yellow discoloration is a visual indicator of accumulated iron oxide, proving chemical alteration of the material surface.

Above 650 °C, the lattice structure of silicon steel begins to reconstruct. If temperature fluctuation exceeds ±10 °C at this stage, local overheating occurs. Uneven heating creates drastic differences in oxidation reaction rates, resulting in patchy yellow oxide layers.

Heat treatment equipment equipped with temperature feedback systems effectively stabilizes furnace temperature, which is especially critical for cold-rolled silicon steel thinner than 0.3 mm.

⚠️ Risk Warning: Repeated thermal cycling causes peeling of the oxide layer, exposing fresh metal substrate and accelerating further oxidation.

II. Application of Anti-Oxidation Coatings

Coating silicon steel surfaces with anti-oxidation paint is the most straightforward protective measure. Such coatings generally contain silicate or phosphate components that form a dense protective film between 300 °C and 800 °C:

• Water-based coatings suit continuous roller coating processes.
• Solvent-based coatings are used for single-piece spray applications.
• High-temperature cured coatings withstand temperatures up to 1000 °C.

✅ Reliable Operation: Clean surface oil contaminants with dedicated silicon steel cleaners prior to coating to guarantee coating adhesion.

Optimizing heat treatment processes requires coordinated control of three parameters:

• Limit heating rate to 50–100 °C/min to prevent thermal shock.
• Introduce shielding gas such as nitrogen during the soaking stage.
• Deploy zone-controlled annealing furnaces to eliminate temperature gradients.

For grain-oriented silicon steel, dwell time within the critical temperature range of 700–850 °C must be strictly regulated.

III. Material Selection Strategy

Non-oriented silicon steel generally resists high-temperature oxidation better than grain-oriented types thanks to uniform silicon distribution. For applications requiring repeated heating such as motor cores, high-silicon steel with silicon content above 3% is preferred.

🔍 Key Distinction: Although grain-oriented silicon steel boasts high magnetic permeability, its aligned grain structure accelerates oxidation rates.

Selection of supporting processing equipment also impacts final surface quality:

• Silicon steel cutting machines shall be fitted with cooling systems to avoid oxidation at cut edges.
• On-line detectors monitor surface color variation in real time.
• Sharp stamping dies minimize tensile deformation of the material.

These measures preserve the original surface condition of silicon steel to the maximum extent.

IV. Long-Term Maintenance Solutions

Regular maintenance extends the service life of silicon steel products:

• Inspect coating integrity every three months.
• Apply silicon steel anti-rust oil for short-term storage protection.
• Chemically clean oxidized areas with weak acid before recoating.

📌 Note: Professional rework treatment is required once the oxide layer thickness exceeds 5 μm.

Storage environment relative humidity shall be kept below 60%, with insulating paper interleaved between stacked sheets. Sealed silicon steel packaging materials are recommended during transportation to prevent erosion by rainwater and salt spray.

The yellow discoloration issue of silicon steel sheets demands systematic solutions covering three dimensions: material selection, process control and post-service maintenance, among which temperature precision and surface protection represent the most critical breakthrough points.