Continuous Casting Copper Tube Life Extension: Billet Mold Production Practices

Industry professionals commonly recognize the mold as the “heart” of a continuous casting machine, since its condition directly determines billet quality and production costs. In actual high-efficiency continuous casting operations, severe wear frequently occurs at the lower end of the mold’s copper tube, resulting in a short service life, frequent replacements, and persistently high costs. Based on production practices at Yangchun New Steel’s small billet continuous casting machine, this paper systematically analyzes the primary causes of wear at the lower end of the copper tube. It proposes a series of improvement measures from multiple dimensions, including equipment structure, process control, and operating procedures. Through comprehensive measures—optimizing support roller layout, adjusting opening width, enhancing structural rigidity, and standardizing procedures—we have signifiantly extended copper tube service life, achieving an average steel throughput exceeding 10,000 t per tube. This provides a valuable reference for similar enterprises to reduce consumption and stabilize production.

The mold is a critical component in the continuous casting process, often referred to as the “heart” of the continuous casting machine. Its quality and operating conditions play a vital role in determining the machine’s production capacity and the quality of the cast billets. The level of steel throughput in the mold copper tubes is a key indicator of whether the overall mold parameters and continuous casting process parameters are appropriate. Low steel throughput not only significantly increases spare part costs but also extends offline maintenance time, disrupts normal production rhythms, and affects the effective turnover of the mold. It is evident that the service life of the copper tube directly affects the operating rate of the continuous casting machine, billet quality, and yield, which in turn impacts the production costs of the continuous casting process. The causes of wear at the lower end of the copper tube include, from a process perspective, molten steel temperature and quality; from an equipment perspective, the support effectiveness of the mold foot rollers and the internal profile of the copper tube; and from an operational perspective, the adjustment method of the foot rollers and the operation of the billet feed and pull rods. Currently, domestic steel mills primarily producing mild carbon steel grades have a mold steel throughput of only 3,000 t/mold. Severe wear at the lower end of the copper tube compromises uniform heat transfer between the ingot and the copper tube, leading to high production accident rates and elevated copper tube costs. At CYKC Group, Casting Machines No. 1 and No. 3 have a mold arc length of 9 m, with an average mold steel throughput lifespan of 8,000 t/mold; Casting Machine No. 2 has a curvature of 8 m, with an average service life of 6,000 t per mold. To further reduce spare parts costs and stabilize production, this paper addresses extending the service life of copper tubes by improving mold design, optimizing maintenance processes, promoting technological upgrades, and refining process technologies.

1.1 Application of Multi-Row Full-Roll Crystallizers

Molten steel solidifies in the crystallizer, forming a 10–12 mm shell. Copper tubes prioritize thermal conductivity over wear resistance. Shell growth and casting deviation cause up to 150 mm wear at tube’s lower end. To address misalignment, we added support rollers at the outlet and reconfigured nozzles to a “2-1-1” three-row layout for better cooling. Since single-row rollers wore quickly, we adopted a three-row, twelve-roller design (see Figure 2). This distributes mechanical stress, maintains centerline alignment, extends mold function, offsets static pressure, prevents shrinkage deformation, and reduces lower-end copper tube wear.

1.2 Improved Method for Adjusting the Opening of the Mold Foot Rollers

Integral forged copper tubes ensure high inner-cavity precision, while multi-component foot rollers suffer poor assembly accuracy, requiring manual adjustment. Traditional adjustment using curved plates fails to address lower-end tube wear, as ingot shrinkage after secondary cooling reduces section size below the roller opening. To fix this, we changed the roller configuration from “0-to-0” to “0-to-negative” based on shrinkage rate—matching the second roller to the arc plate while setting the third smaller.This stabilizes ingot support, prevents one-sided contact, reduces localized copper tube wear, and extends service life.

1.3 Improvements to the Mold Foot Roller Support

To address foot roller bracket deformation, analysis revealed insufficient weld strength under high temperature and billet stress. We added stiffening plates at the welds and introduced additional fixed supports to enhance rigidity (see Figure 4). These modifications significantly reduced deformation, extending foot roller service life from 5 to 15 casting cycles. Installation accuracy improved; deviations decreased, ensuring stable caster operation and minimizing copper tube wear.

1.4 Optimization of the Copper Tube Inner Lumen

We employ a high-precision reverse taper gauge. We collect data on changes in the copper tube’s inner wall using high-precision displacement sensors mounted on a measurement cart and transmit it to a computer via serial communication. Specialized software then plots the inner wall contour curve based on multi-angle sensor measurements, ultimately calculating the taper value with high precision (see Figure 5). This measurement method enables more precise control of the mold taper, improves measurement accuracy, and provides a precise basis for mold maintenance and the establishment of copper tube discharge standards. It helps increase the mold’s steel throughput and reduces production accidents.
Based on cooling intensity requirements, we adopted a small-range, high-taper design for the crescent section. In actual operation, we found that molten steel static pressure kept the ingot shell dimensions unchanged despite increased cooling intensity, matching the upper opening size. This exacerbated friction and wear at the lower opening, while we observed increased drawing resistance below normal speeds, indicating that a steep taper design is unsuitable for the crescent surface. Through testing, we optimized the taper from the original fixed design (1.98% at crescent, 1.93% at lower end) to a gradient taper (maximum 1.98% at crescent, gradually decreasing to 1.79% toward the lower end). This ensures uniform cooling of the ingot shell within the copper tube while effectively reducing wear on the copper tube.

1.5 Standardizing Operations for Stopping Pouring and Advancing the Ingot Rod

Prior to production, the casting machine equipment and programs must undergo a test run to ensure smooth startup. During test runs, we must advance the ingot head into the mold to check for misalignment issues (see Figure 6). Since the steel ingot head is significantly harder than the copper tube, its insertion causes wear and scratches at the lower end. To address this, first, we install an ingot cap when inserting the ingot rod to protect the copper tube from scraping. Second, we require a pre-production test run without inserting the ingot rod, reducing contact frequency and minimizing wear. Finally, we standardize pulling speed requirements during casting stoppage, controlling normal speed below 2.6 m/min to effectively prevent excessive speed during abnormal conditions from accelerating lower-end copper tube wear.

1.6 Effects of Process Conditions

Excessive superheat in the continuous casting melt can easily cause deformation of the copper tube’s inner cavity, resulting in a sudden change in the tube’s taper angle so that it no longer follows a continuous parabolic curve. This is the primary cause of steel leakage due to adhesion and cracking in continuous casting machines. Once a steel leakage incident occurs, the lower end of the copper tube will sustain some damage when the billet is removed or when the pulling rod is reinserted. Strictly controlling the superheat (liquidus temperature + control temperature) of each steel grade within the range of 10–20 °C (see Figure 7) can effectively mitigate deformation of the copper tube’s inner cavity and reduce the occurrence of steel leakage incidents, thereby minimizing wear on the lower end of the copper tube.
When the molten steel temperature is too low or its fluidity is poor, nodules are likely to form at the ladle nozzle. Cooled molten steel (referred to as “cold steel”) may adhere to the copper wall or the upper end of the copper tube. When removing this cold steel using an oxygen lance or cutting torch (see Figure 7), the copper tube is prone to burning. During the production of ordinary carbon steel, due to poor deoxidation of the molten steel, such situations are more common, and the risk of the copper tube being burned is higher.

1.7 Improving Equipment Conditions and Stabilizing Equipment Operation

We use casting downtime to align the ingot pulling rods and promptly replace severely deformed rods that cannot be realigned. We strictly adjust the casting machine’s arc alignment. During alignment, we insert the alignment template 100 mm into the mold, then pass it sequentially downward through each piece of equipment, ending at the lower roll of the first drawing and straightening machine. The optimal condition occurs when the arc-alignment template fully contacts the inner wall of the copper tube, the roller surfaces of the guide rollers in each section, and the lower roller surface of the first drawing and straightening machine. We consider a gap exceeding 2 mm abnormal. Based on the arc-alignment results, inspect and adjust issues such as roller misalignment, tilt, and suspension one by one, and make adjustments promptly.

During maintenance, inspect the arc adjustment rollers and roller bases in the secondary cooling chamber, and promptly replace any loose bolts. At the same time, inspect the arc adjustment rollers in each section for any noticeable movement, as well as for wear on the roller surfaces and edges. If the circumference is <600 mm, the roller edge height is <30 mm, or the roller surface is uneven with wear depth exceeding 5 mm, the roller must be replaced. After replacing the rollers, verify that their curvature is normal to ensure the secondary cooling section is aligned and in contact with the arc plate, conforming to the billet’s running trajectory to prevent wear on the lower end of the copper tube caused by billet deviation. Refer to Figure 8 for the alignment of the arc template with the copper tube and roller surface.

1.8 Improve Mold Maintenance Standards

Strict maintenance standards were implemented: bearing housing deviation <1 mm, coaxiality <0.2 mm, opening deviation <0.6 mm, and bearings soaked in oil. Arc plates ensure tight roller fit. Standardized procedures eliminated arbitrary operations. Results: uniform wear <50 mm, throughput >10,000 t per tube, stable high-speed production.

Conclusion

(1) Optimizing the design of the copper tube’s inner cavity and adopting a gradient taper to reduce friction between the primary billet shell and the copper tube has reduced the wear length at the lower end of the copper tube from 150 mm to less than 50 mm.
(2) We increased the number of mold foot rollers to extend the curvature of the copper tube, maintaining a stable billet travel path and preventing unilateral wear on the lower end of the copper tube caused by arc deviation in the secondary cooling chamber after the billet exits the mold.

(3) We enhanced the strength of the mold foot roller supports to ensure their precision under high-temperature and high-pulling-speed conditions, increasing the support service life from 5 to over 15 uses.

(4) Based on the ingot’s solidification shrinkage characteristics, we optimized the adjustment precision of the foot roller opening to ensure that the mold foot rollers remain in constant contact with the copper tube.

Contact： cykc@cykcgroup.com.cn