Analysis on the Causes of Wear of Copper Plate in Continuous Casting Mold

The copper mould tube is the core equipment of the entire continuous casting process, which directly affects the productivity, product quality and production cost of continuous casting. During the production process, the copper mold tube has to constantly withstand the impact of high temperature, high pressure and strong friction, and the working environment is extremely harsh. Understanding the causes of copper mold tube wear is of great significance to the design and use of copper mold tubes. Here is a brief introduction to the copper mold tube wear mechanism.

Cracks in the copper plate of the copper mold tube

Copper plate cracks often appear in the meniscus area of the copper mold tube. The main reason is the extremely high heat flow caused by the increase in the surface temperature of the copper plate. At this high temperature, the copper plate tends to expand relative to the steel support facility. This is especially true for thin slab continuous casting molds. Due to the fast drawing speed, the surface temperature of the copper plate rises rapidly, and the temperature the copper plate withstands exceeds its recovery and recrystallization temperature, thereby greatly reducing its strength and hardness, causing the funnel-shaped transition zone to experience an obvious three-dimensional expansion movement. This combination of thermal and mechanical strain combined with a decrease in hardness (up to 50%) leads to the appearance of cracks on the surface of the copper plate, and the cracks tend to propagate further in an intergranular manner.

Excessive heat levels during the casting process and the resulting macroscopic plastic strain/deformation in the surface and subsurface areas of the mold copper plate are the main causes of cracks in the mold meniscus. This damage mechanism is intensified by extremely high temperature regions. For example, the transition region from the funnel-shaped region of the copper mould tube to the parallel region of the copper mould tube. At this time, the local temperature gradient between the hot and cold sides of the copper mould tube can reach hundreds of degrees Celsius. The working side surface (hot side) of the copper plate of the copper mould tube is more or less affected by diffusion. Zn, S, Cd in the molten steel and F in the mold powder all diffuse to the surface and subsurface of the copper plate. These diffusive elements can cause thermal brittleness of the mold copper plate, leading to the formation and spread of cracks. The key factors for crack damage are the maximum temperature experienced by the copper mould tube and the service time of the copper mould tube.

When high-speed continuous casting occurs, another effect of the ultra-high temperature on the mold material is the plastic strain of the copper plate of the mold, resulting in deformation of the copper plate under the meniscus. And because Zn in the molten steel diffuses into the copper plate to form brass, the so-called “brassing” phenomenon occurs. The latter problem is particularly problematic for electric furnace steel mills or mini-rolling mills, because electric furnace steel mills or mini-rolling mills usually make steel from scrap steel. The formation of brass causes the mold copper plate to become thermally brittle, and the stress caused by the high operating temperature causes cracks in the mold copper plate.

The bulge of the mold copper plate

The bulge of the mold copper plate is caused by the gap between the wide and narrow sides of the mold copper plate. The reason is that the temperature of the inner surface (hot surface) of the mold copper plate is too high, which causes the thermal expansion of the mold copper plate in the lower area of the meniscus. This thermal expansion is limited on the cold surface of the outer surface of the copper plate of the copper mould tube. As a result, the copper mould tube produces macroscopic elastic/plastic deformation. The degree of deformation mainly depends on the creep characteristics of the material and the strength of the material. When the copper mould tube that has undergone macroelastic/plastic deformation is cooled, the copper plate of the copper mould tube cannot completely return to its original state, resulting in grooves or dents on the wide surface of the copper mould tube. As the groove or dent continues to expand, a gap is formed between the wide groove or dent of the mold copper plate and the narrow surface of the mold copper plate. This void often determines when the mold copper plate must be removed and reprocessed, making it a decisive factor in the mold’s service life.

In addition to causing the gap between the wide and narrow surfaces of the copper mould tube, the bulge will also cause the copper plate material of the copper mould tube to move. That is, when the width of the copper mould tube is adjusted online, material accumulation is formed on the front side of the narrow sliding edge of the copper plate of the copper mould tube. Reducing the clamping force of the narrow surface can effectively eliminate this effect, but of course there is a limit to the reduction in clamping force. This is due to the static pressure of the molten steel in the copper mould tube. When the clamping force is reduced to a certain level, the copper mould tube will open. Therefore, a high-hardness copper mould tube material must be used to reduce this effect.

Uneven heat flow in the copper mould tube

The transverse and longitudinal heat flow of the copper mold tube wall is not uniform in local areas. However, most mold copper plates are designed only for uniform heat transfer rates, resulting in local temperature differences on the hot surface of the mold, especially along the meniscus.

The design of mold powder generally has a service temperature range. Due to the different melting and penetration behaviors of the mold slag, there will also be differences in temperature in local areas of the mold copper plate, especially in the middle of the mold copper plate and the parallel plane areas of the mold copper plate. This difference in melting of the mold flux results in differences in the thickness of the adiabatic mold flux film being formed, resulting in inconsistent heat transfer rates. This result, due to uneven thermal conductivity of the mold walls, is often referred to as “casting creases” or thin slab longitudinal cracks.

The molten steel injected into the mold forms a standing wave at the meniscus, which aggravates the above problem. As a result, the temperature of the mold wall is the highest in the standing wave formation area (the standing wave is the highest point of the steel water surface in the mold). This is because the molten mold slag flows into the lowest point of the meniscus molten steel, which is located in the middle of the mold. The mold flux insulation film formed at this position is the thickest, while only a thinner mold flux insulation film is formed between the casting slab and the mold copper plate. The final result is that the mold copper plate has the lowest temperature in the area with the thickest mold slag insulation film, and the highest temperature in the area with the thinnest mold slag insulation film. In order to eliminate this effect, many thin slab continuous casting machines use electromagnetic braking to adjust and control the mold liquid level and reduce the standing wave of the mold liquid level.

Wear at the outlet of the mold copper plate billet

In addition to the above reasons, another factor that determines the life of the mold and reprocessing is the mechanical wear caused by the billet shell under the mold. The way to solve the wear of the lower part of the copper mould tube is to plate nickel, nickel alloy or other alloy materials on the surface of the copper mould tube, which can also prevent the casting shell from absorbing copper from the wall of the copper mould tube. The absorption of copper by the cast slab may produce star-shaped cracks on the surface of the cast slab, eventually leaving star-shaped crack defects on the surface of the finished product.

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