This article describes the causes of mold wear in the continuous casting process, which are roughly divided into cracks, bulging, uneven heat flow, and wear at the exit.
Key words: continuous casting; mold wear; cause analysis
Mold copper tube is the core equipment of the whole continuous casting process, which directly affects the productivity, product quality and production cost of continuous casting. During the production process, the crystallizer has to continuously withstand the impact of high temperature, high pressure and strong friction, and the working environment is extremely harsh. Understanding the cause of mold wear is of great significance to the design and use of the mold. Here is a brief introduction to the wear mechanism of the copper tube of the mold.
Mold copper tube crack
Cracks in the copper plates often occur in the meniscus region of the mold, mainly due to the extremely high heat flux caused by the increased surface temperature of the copper plates, at which temperatures the copper plates tend to expand relative to the steel support. This is especially true for thin slab continuous casting molds. Due to the rapid casting speed, the surface temperature of the copper plate rises rapidly, and the temperature of the copper plate exceeds its recovery recrystallization temperature, thereby greatly reducing its strength and hardness. The funnel-shaped transition zone experienced an obvious 3-dimensional expansion motion. This combination of thermal and mechanical strain, together 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 casting and the resulting macroscopic plastic strain/deformation in the surface and subsurface regions of the mold copper plate are the main causes of cracks at the mold meniscus. This damage mechanism is intensified by extremely high temperature regions. For example, the transition zone from the funnel zone of the crystallizer to the parallel zone of the crystallizer. At this time, the local temperature gradient between the hot and cold sides of the crystallizer can reach hundreds of degrees Celsius.
The working side surface (hot surface) of the mold copper plate is more or less affected by diffusion, and the Zn, S, Cd in the molten steel and the F in the mold slag all diffuse to the surface and subsurface of the copper plate. These diffusing elements can lead to thermal embrittlement of the copper plate of the mold, leading to the formation and propagation of cracks. The key factors for crack damage are the maximum temperature and service time of the mold.
During high-speed continuous casting, another effect of the crystallizer material being subjected to ultra-high temperature is the plastic strain of the copper plate of the mold, which leads to the deformation of the copper plate under the meniscus, and the formation of brass due to the diffusion of Zn in the molten steel into the copper plate, that is, the so-called “Brassification” phenomenon. The latter problem is particularly acute for EAF or mini-rolling mills, which typically make steel from scrap. The formation of brass causes thermal embrittlement of the copper plate of the crystallizer, and the stress caused by the high working temperature causes cracks in the copper plate of the crystallizer.
Belly of crystallizer copper tube
The bulging 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 copper plate of the crystallizer is too high, which causes the thermal expansion of the copper plate of the mold in the lower part of the meniscus; Elastic/plastic deformation, the degree of deformation mainly depends on the creep characteristics of the material and the strength of the material. When the mold with macroscopic elastic/plastic deformation is cooled, the copper plate of the mold cannot be completely restored to its original state, resulting in grooves or dents on the wide surface of the mold. As the grooves or dents continue to expand, gaps are formed between the wide surface grooves or dents of the mold copper plate and the narrow surfaces of the mold copper plate. This gap often determines when the mold copper plate must be removed and reworked, making it a decisive factor affecting the life of the mold.
In addition to causing the above-mentioned gap between the wide and narrow sides of the mold, the bulging can also cause the material of the mold copper plate to nest, that is, when the mold is adjusted in-line, it will be on the front side of the sliding edge of the narrow side of the mold copper plate. Build up of material. Reducing the clamping force of the narrow surface can effectively eliminate this effect. Of course, the reduction of the clamping force is limited. This is because, due to the effect of the hydrostatic pressure in the mold, when the clamping force is reduced to a certain extent , the crystallizer opens. Therefore, mold materials with high hardness must be used to reduce this effect.
Uneven heat flow in the crystallizer
The lateral and longitudinal heat flow of the crystallizer wall is not uniform in local areas. However, most mold copper plates are only designed for a uniform heat transfer rate, resulting in localized temperature differences on the mold hot surfaces, especially along the meniscus.
Mold powder design generally has an operating temperature range. Due to the different melting and penetration behaviors of the mold powder, there will also be differences in the temperature of the mold copper plate, especially in the middle of the mold copper plate and the parallel surface of the mold copper plate. area. This difference in melting of the flux can lead to differences in the thickness of the insulating flux film being formed, resulting in inconsistent heat transfer rates. This result, due to the non-uniform heat transfer rate of the mold wall, is commonly referred to as “caster folds” or thin slab longitudinal cracks.
The molten steel injected into the mold forms a standing wave at the meniscus, which aggravates the above problems. As a result, the temperature of the mold wall is the highest in the area where the standing wave is formed (the standing wave is the highest point of the molten steel surface of the mold). This is because, since the molten mold slag flows into the lowest point of the meniscus molten steel, this point is located in the middle of the mold, and the mold slag heat insulation film formed at this position is the thickest. Thinner mold flux insulation film. The final result is that the temperature of the mold copper plate is the lowest in the region where the mold slag insulation film is the thickest, and the temperature is the highest in the region where the mold slag insulation film is the thinnest. In order to eliminate this effect, many thin slab continuous casting machines use electromagnetic brakes to adjust and control the mold liquid level and reduce the standing wave of the mold liquid level.
Abrasion at the Copper Slab Exit of the Mold
In addition to the above reasons, another factor that determines the mold life and reprocessing is the mechanical wear caused by the slab shell under the copper tube of the mold. The way to solve the wear of the lower part of the crystallizer is to plate nickel, nickel alloy or other alloy materials on the surface of the crystallizer, and at the same time, it can also prevent the slab shell from absorbing copper from the mold wall. The absorption of copper by the slab may produce star-shaped cracks on the surface of the slab, and finally leave star-shaped crack defects on the surface of the finished product.
