Analysis of causes of damage to continuous casting rollers and measures to improve service life

Abstract: Periodically changing thermal stress and mechanical stress are the main causes of thermal fatigue cracks, cracks and flesh loss in continuous casting rollers. Improvements have been made in terms of design, cooling methods, and materials. The life of the continuous casting rolls is mainly improved through surfacing. The surfacing material is an ultra-low carbon material with nitrogen as a solid solution strengthening element. It is also pointed out that the fatigue resistance of the surfacing material should be further improved.

Keywords: continuous casting roller; cause of damage; service life; thermal stress; surfacing welding

Introduction

Continuous casting rollers are one of the key spare parts in continuous casting production. Generally, there are about 200 rollers with different roller diameters and structures on a continuous casting line. Their working quality directly restricts the output and quality of continuous casting production. With the development of continuous casting technology in my country, the manufacturing technology of continuous casting rollers has also become increasingly perfect. In recent years, the manufacturing materials and variety specifications of continuous casting rollers have been gradually improved, and the manufacturing and surfacing repair processes have been continuously improved. After the roller surface has been surfacing, the online service life of the continuous casting roller can be increased by more than 1 to 3 times, which meets the needs of steelmaking in my country. The needs of continuous development of continuous casting production.

Analysis of causes of damage to continuous casting rollers

The temperature of the continuous casting billet when it comes out of the copper mould tube is generally 1200°C, and it is still around 800°C when it reaches the horizontal section. From the copper mould tube foot roller to the horizontal section continuous casting roller, the roller surface temperature changes in a hot and cold cycle. Xu Baobi and Dong Shaohua’s research on the temperature field and thermal stress of the rollers of the continuous casting machine showed that the maximum temperature at the contact point between the continuous casting roller and the slab can reach nearly 500°C. Therefore, the periodically changing temperature field will inevitably cause periodically changing thermal stress. The continuous casting roller also bears mechanical stresses such as the cast slab’s self-sagging gravitational force, bulging force, straightening force, billet drawing force, and roller transmission resistance. After analysis and testing, the main cause of damage to the continuous casting roll is the combined effect of thermal stress and mechanical stress. The mechanical stress is only about 1/10 of the thermal stress. It is mainly the periodic changes in thermal stress that cause thermal fatigue cracks, cracks and flesh loss in the continuous casting roller.

Measures to improve the service life of continuous casting rollers

Improve design

The design of the VAI continuous casting machine changes the single continuous casting roller (maximum deflection 8 mm) into two sections (maximum deflection 3mm) or three sections (maximum deflection 1 mm) to reduce the deflection of the roller when it is loaded. Its design optimizes the relationship between the load of the continuous casting roller and the distance between the rollers, reduces the bending moment of the stress, reduces the strain, and also reduces the sensitivity of the continuous casting roller to crack expansion.

A U-shaped spiral cooling water tank is opened on the surface of the copper mould tube foot roller to enhance the surface cooling of the foot roller. In recent years, the design has been changed to a double-spiral U-shaped cooling groove and air mist cooling to further reduce the roller surface temperature, reduce thermal stress and thermal fatigue, and improve the service life of the continuous casting roller.

Improve cooling methods

The original spray cooling system only uses cooling water with a pressure of 0.4 MPa. In order to reduce the temperature of the roller surface, compressed air with a pressure of 0.04MPa is added to the horizontal nozzle of the water pipe to atomize the water droplets. After atomization, most of the water droplets have a diameter of <100 μm. This is beneficial to improving cooling efficiency, reducing the roller surface temperature by about 200°C, and reducing thermal stress and thermal fatigue. The service life of the roller only depends on the mechanical wear caused by the friction between the roller itself and the cast slab.

Improve materials

The continuous casting roller is in a high-temperature working environment, and its material should naturally be given priority to heat-resistant steel. Therefore, the first generation of continuous casting rollers used 15CrMo and 15CrMoV heat-resistant steel, which were forged, processed, tempered, and surface quenched for direct use. Due to the small thickness of the hardened layer and other reasons, the service life of the continuous casting roller is short, and the steel passing capacity is generally only 120,000 to 200,000 tons.

The second-generation continuous casting rollers used 1Cr13 and 2Cr13 stainless steel. In normal production, cyclic thermal stress generally does not exceed its high-temperature strength limit, and it is corrosion-resistant and has good plasticity. 1Cr13 has a cross-section shrinkage of 50% and an elongation of 18%, making it an ideal material for slowing down the development of cracks. However, if the entire continuous casting roll is made of stainless steel, the cost will be too high. Therefore, composite continuous casting rolls have been further developed.

The third generation continuous casting roller adopts surfacing strengthening technology. The base material is 15CrMo or 30CrMo or 42CrMo, and the roller surface is strengthened by surfacing. The surfacing material is 1Cr13Ni4 or 1Cr13NiMo, etc. For example, Baosteel initially adopted H102F patented by Nippon Iron & Steel Co., Ltd. of Japan, Anshan Iron and Steel Co., Ltd. adopted the fifth-generation surfacing materials KRC14 and KRC32 developed by Kobe Steel, and Lincore423Cr from Lincoln Company in the United States. Most of them are Cr-Ni series surfacing materials. The service life of the continuous casting roller is improved, and the steel passing capacity can reach 300,000 to 600,000 tons.

The fourth generation continuous casting roller further improves the cladding material. The surfacing materials used in the third generation of continuous casting rolls use carbon as a solid solution strengthening element to increase the hardness of the surfacing layer. However, carbon can easily form intermetallic compounds such as Cr23C6 with Cr at high temperatures (400~900°C), resulting in the depletion of chromium in the surfacing layer, especially in the overlap transition zone. The continuous casting roller has a large amount of cooling water under thermal fatigue for a long time, which can easily cause cracking and damage of intergranular chromium-poor surfacing layer. At this time, the bearings, bearing seats, mandrels, rotating joints, lubrication and other system parts in the continuous casting roll system are still intact. To this end, research has been carried out to further improve the cladding materials. The British Welding Alloy Company has developed Crome core 414N-0 (WA414N-O), an ultra-low C surfacing material using nitrogen as a solid solution strengthening element. The alloy composition is OCr13Ni4MoVN to avoid the formation of Cr23C6. Its strengthening mechanism is to form uniformly dispersed nitrides to avoid intergranular cracking caused by the depletion of chromium. Its steel passing capacity can reach more than 1 million tons. However, it uses open arc cladding and the price of welding wire is high (it once reached 60,000 to 70,000 yuan/t). After long-term testing, domestic enterprises have now generally adopted the domestic welding wire (1Cr13Ni4Mo) submerged arc surfacing process, with the cost greatly reduced and the effect basically the same. The comparison of the metallographic structure of the surfacing layer using imported welding wire and domestic welding wire is shown in Figure 1.

Figure 1 Metallographic structure of the weld layer cladded with different welding wires and cladding methods

Conclusion

The service life of a casting roll depends on a variety of factors, including roll design, cooling methods, and maintenance during production. For example, steel leakage and billet stagnation (cast billet stalling) occur; poor equipment maintenance and poor cooling water quality cause the nozzle to be clogged and cannot be replaced in time, causing the temperature of the roller surface to rise; poor lubrication causes the bearing to seize and the roller to stop rotating; Roller surface wear and tear will cause the roller surface temperature to rise and cause cracks. Therefore, the service life of continuous casting rollers is a systematic project. In addition, there are many units for manufacturing, surfacing and repairing continuous casting rollers, and the quality of manufacturing and repairing of continuous casting rollers varies. In addition to its own reasons, insufficient understanding of the operating characteristics of the continuous casting machine and lack of pertinence are also one of the reasons for the low service life of the continuous casting roller.

When manufacturing and repairing continuous casting rollers, surfacing materials and surfacing methods should be selected based on their design characteristics, cooling methods, and application environments. On the other hand, improving the fatigue resistance of cladding materials can also increase the service life of continuous casting rollers.

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