Copper Mould Tube Induction

A copper mold tube is a crucial component in continuous casting machines, specifically designed to solidify molten steel into a desired shape. It acts as a water-cooled mold that facilitates rapid heat transfer from the molten steel to cooling water, causing the steel to solidify quickly. Copper’s excellent thermal conductivity makes it ideal for this process, while its resistance to thermal erosion and deformation ensures the mold’s longevity and consistent performance.

Function

Copper mold tubes are at the heart of continuous casting machines, where they play a vital role in the solidification process of molten steel.

Molds play an integral role in the process of continuous casting liquid steel. Molds are the most critical component of the continuous casting process. Use of a high quality copper mold tube can increase productivity and steel quality by minimizing defects in the required bloom, billet, or slab.

Copper mold tubes play a critical role in increasing the efficiency of the steel manufacturing process and improving steel quality. But before we get into the details of how copper mold tubes help increase steel mills’ production, let’s take a closer look at understanding the continuous casting process.

Copper is chosen for its excellent thermal conductivity, which allows for efficient heat transfer and rapid solidification of the molten metal.

Material

These tubes are typically designed with specific dimensions and shapes to produce billets, blooms, or slabs of steel.
In JISteel, we provide three kinds of Copper Mould Tube of different material—- CuAg, TP2, and CuCrZr.
CuAg: The silver-copper alloy has 0.1% silver added to pure copper. It has extremely high thermal conductivity, close to that of pure copper and greater than 380 W/m·K, which is beneficial for rapid cooling to obtain a thin slab shell. Silver can also significantly increase the recrystallization temperature of copper, which is about 70 – 100°C higher than that of pure copper. Moreover, it has good ductility and toughness.

TP2: When smelting pure copper, 0.015% – 0.040% phosphorus is added as a deoxidizer, making the oxygen content usually lower than 0.005%. It has good ductility and processing properties, low cost, and good thermal conductivity, and is a commonly used material in modern smelting.

CuCrZr: The Chromium-Zirconium-Copper alloy has copper as the matrix, with the addition of 0.5-1.2 wt% chromium and 0.03-0.3 wt% zirconium. Sometimes, trace amounts of magnesium, silicon, or rare earth elements are also added. Through precipitation hardening, it has high strength and high hardness. Its thermal conductivity coefficient is usually above 300 – 350 W/m·K. It has good resistance to softening, the re-crystallization temperature can reach above 500°C, and it has excellent thermal fatigue resistance.

Coating

JISteel provide two coating—- the traditional hard chromium layer and the Ni – Co alloy + hard chromium composite coating.

Hard chromium layer: Features:

– High hardness (about 800–1000 HV) and relatively good wear resistance.

– Mature technology, low cost, and easy to operate.

Ni – Co Alloy plus hard Chromium composite coating.

Structural Design

Electroplate a 0.4–0.5 mm nickel-cobalt (Ni-Co) alloy layer on the copper substrate, and then deposit a hard chromium layer to form a composite structure of “transition layer and wear-resistant layer”.

Advantages

-**Strong bonding force**: The thermal expansion coefficient of the Ni-Co alloy is close to that of the copper substrate, reducing interfacial stress.

– **Anti-corrosion isolation**: Block the diffusion of harmful elements (such as Zn, S) in the molten steel to the copper substrate.

– **Double wear resistance**: After the hard chromium layer is worn, the Ni-Co layer (with a hardness of 300–600 HV) continues to provide protection, extending the service life.

– **Significant lifespan improvement**: Compared with a single chromium coating, the service life is increased by about 30%.

Production and Delivery

There are general five procedures for making copper mould tubes.

1. JI-Steel produce Copper Mould Tube in a vacuum environment. Vacuum melting (such as Vacuum Induction Melting, VIM) or melting under a protective atmosphere (such as argon protection) is adopted to to strictly control the alloy composition, especially the content of harmful gases such as oxygen and hydrogen, reduce inclusions, ensure the purity and uniformity of the material. After melting, an alloy ingot is formed.

The melted copper alloy liquid is poured into a specific mold through vertical semi-continuous casting or horizontal continuous casting processes. The goal is to obtain an ingot with a dense internal structure, uniform composition, and free from defects such as macro-segregation, pores, and shrinkage porosity. The diameter of the ingot is usually larger than the outer diameter of the final copper tube, preparing for subsequent hot working.

• The role of hot extrusion

Put the ingot heated to a specific temperature (usually in the range of 700°C – 900°C, depending on the alloy) into the extrusion cylinder.

Under great pressure (several thousand tons or even tens of thousands of tons), the copper ingot softened at high temperature is extruded through a die with an annular gap to form a thick-walled hollow tube blank.

The hot extrusion process can break the casting structure, refine the grains, improve the density and mechanical properties of the material, and initially form a tubular structure. 

• Solution treatment and cold working

Heat the extruded tube blank to the solution temperature (for chromium-zirconium copper, usually between 950°C – 1000°C) to allow alloying elements (such as Cr, Zr) to dissolve fully into the copper matrix.

After holding for a certain period of time, perform rapid cooling (usually water quenching). The aim is to obtain a supersaturated solid solution, preparing for subsequent age hardening, and at the same time softening the material for cold working.

Cold working (cold rolling/cold drawing):

* At room temperature, carry out multiple passes of cold rolling or cold drawing on the solution-treated tube blank.

* The purposes are: to accurately reduce the wall thickness and diameter of the tube blank to dimensions close to those of the finished product (usually leaving a small amount of finishing allowance). Greatly improve the strength, hardness, and dimensional accuracy of the material through cold deformation. Obtain good surface finish.

• Machining (finishing)

Taper machining:

The inner cavity of the mold copper tube is not a simple straight cylinder but is designed with a specific taper (linear taper, multi-stage taper, or a more complex parabolic taper) to adapt to the solidification shrinkage of molten steel and reduce the air gap, ensuring uniform growth of the billet shell and smooth demoulding. This requires precise internal boring on a precision lathe (such as a CNC lathe) or a special boring machine.

Outer shape machining: Machine the outer circle, both end faces, flanges, water channels (cooling water channels), positioning grooves/holes, etc. of the copper tube to ensure precise fitting, sealing with the mold back plate (water tank), and uniform distribution of the cooling water flow. The machining accuracy of the water channel is crucial for the cooling efficiency.

Chamfering and polishing: Chamfer the key parts (such as the upper and lower openings) and perform high-precision polishing on the inner surface (especially in the meniscus area) to achieve an extremely low surface roughness, reducing the frictional resistance with the solidified billet shell and preventing scratching of the casting billet and adhesive breakout.

5. Inner surface plating treatment (key step)

To improve the wear resistance, thermal crack resistance, copper chip adhesion resistance of the inner wall (working surface) of the copper tube and extend its service life, the inner surfaces of the vast majority of mold copper tubes need to be plated. The most commonly used are traditional hard chromium plating (Cr), Ni-Co alloy + hard chromium composite plating. The thickness usually ranges from 0.1 mm to several millimeters (commonly 1 – 3 mm), depending on the usage requirements.

Process: Electroplating (which requires precise control of current density, temperature, plating solution composition, etc.) or electroforming (for manufacturing thicker coatings) is mainly used. Before plating, the inner wall of the copper tube must be strictly pre-treated, such as cleaning and activation. The coating must be dense, uniform, and firmly bonded to the copper matrix without defects.

Post-treatment: After the plating is completed, grinding or polishing is usually required to achieve the final required dimensional accuracy and surface finish.

• Quality inspection standards

Dimensional accuracy: Use precision measuring tools (such as Coordinate Measuring Machine – CMM) to comprehensively measure the key geometric dimensions of the copper tube (inner diameter, taper, wall thickness, length, flange size, water channel size, etc.).

Surface quality: Visually inspect, use an endoscope and a surface roughness tester to check for scratches, pits, cracks, coating defects, etc. on the inner and outer surfaces.

Non-destructive testing: Commonly use ultrasonic testing to detect defects (such as cracks, inclusions, pores, poor bonding) inside the copper tube matrix and coatings; use penetrant testing or eddy current testing to detect surface and near-surface defects.

Pressure test: Conduct a hydraulic pressure test on the cooling water channel to ensure there are no leaks.

Hardness test: Check whether the hardness of the copper tube matrix and coatings meets the requirements.

Metallographic analysis: Take samples for inspection of the microstructure (grain size, distribution of precipitated phases, etc.) when necessary. Assembly and packaging measures

Pre-assemble or accurately match the qualified copper tube with other components of the mold, and then carry out anti-rust and anti-knock packaging for the copper tube, such as applying anti-rust oil, covering with a protective film, and using a special packaging box to protect the copper tube from damage during transportation.