As core continuous casting components, copper mould tubes endure high-temperature erosion, billet friction and thermal stress. Surface coatings determine their service life and billet quality. Mainstream types include chromium-based, nickel-based alloy and functional composite coatings, whose characteristics, application scenarios and core advantages are analyzed below.

1. Chromium-Based Coatings: Traditional Mainstream Wear-Resistant Option

With high hardness, excellent chemical stability and good thermal conductivity, chromium-based coatings are widely used in medium-low speed continuous casting, mainly including hard chromium and microcrack chromium coatings.

Hard chromium coatings (conventional electroplating, 0.15–0.30 mm, HV 800–1000) resist molten steel erosion and billet friction, suitable for low-speed casting (≤2.0 m/min) of plain carbon and low-alloy steel. Drawbacks: thermal expansion mismatch with copper substrate causes high-temperature microcracking and spalling; poor lubricity increases high-speed sticking and breakout risks.

Microcrack chromium coatings (optimized from hard chromium) form 100–300 cracks/cm via process adjustment. Retaining hard chromium's key properties, they disperse thermal stress, store mold flux for better lubrication, and suit medium-speed casting (2.0–3.0 m/min) as the mainstream chromium-based type.

2. Nickel-Based Alloy Coatings: Core for Medium-High Speed Continuous Casting

For high-speed, high-quality continuous casting, nickel-based alloy coatings are preferred due to good copper substrate compatibility, strong bonding strength and superior lubrication. Nickel-iron and nickel-cobalt alloys are industrially mature.

Nickel-iron (Ni-Fe) coatings (90%–95% Ni, 5%–10% Fe; electroplated, 0.10–0.25 mm, HV 400–500) offer high cost-effectiveness, good thermal conductivity, toughness and thermal fatigue resistance. Suitable for mass high-speed casting of plain carbon/low-carbon steel. Limitations: slightly lower chemical stability than chromium-based coatings; reduced toughness with higher hardness.

Nickel-cobalt (Ni-Co) coatings (85%–90% Ni, 10%–15% Co; electroplated) use cobalt to enhance high-temperature hardness (HV 500–600) and stability (50–60 W/(m·K)), with good bonding and lubrication to prevent high-speed sticking. Suitable for high-speed casting (3.0–4.5 m/min) of medium-high carbon/low-alloy steel. Limitations: high cobalt cost, large internal stress at high hardness, poor thermal cycling resistance.

3. Composite Coatings: Supplementary for Severe Working Conditions

For high-hardness steel, narrow-face copper tubes and complex conditions, composite coatings ("matrix + functional particles" structure) enhance wear resistance and lubrication. Nickel-cobalt + chromium and nickel-cobalt-iron coatings are widely used.

Nickel-cobalt + chromium composite coatings adopt a "transition layer + surface layer" structure. The nickel-cobalt transition layer improves bonding with the copper substrate, while the chromium surface layer enhances wear resistance, thereby reducing the risk of spalling. These coatings are suitable for high-carbon steel casting and narrow-face copper tubes subjected to concentrated stress.

By adjusting the proportion of constituent metals, nickel-cobalt-iron composite coatings achieve a balance between hardness, toughness, and thermal cycling resistance. They are applicable to a wider range of steel grades and demonstrate stable performance under high-temperature and high-frequency thermal shock conditions.

Conclusion

The selection of copper mould tube coatings depends on the steel grade, casting speed, and working conditions: chromium-based coatings are preferred for medium and low-speed casting, nickel-based alloy coatings for medium and high-speed casting, and composite coatings for severe working conditions. These proven coatings balance performance, cost, and adaptability, ensuring the efficient operation of continuous casting processes.