Development of Cladding Materials and Processes for Long-Life Continuous Casting Rolls

Overview and Background

Continuous casting is the primary method for producing steel slabs, billets, and blooms in modern steel mills. The casting rolls, which support and shape the solidifying steel shell, are subjected to extreme thermal and mechanical conditions. The surface temperature of the casting roll can reach 800°C to 1000°C, and the roll is exposed to the corrosive action of molten steel, slag, and flux. Additionally, the roll undergoes cyclic thermal loading due to the alternating contact with the hot steel shell and the water-cooled interior. These conditions lead to severe wear, thermal fatigue cracking, and surface degradation, which limit the service life of the casting roll. Weld overlay cladding is a proven technology for extending the life of casting rolls by providing a wear-resistant and thermally stable surface layer. This study note examines the development of cladding materials and processes for long-life continuous casting rolls.

Operating Conditions and Failure Modes of Casting Rolls

The operating environment of a continuous casting roll is one of the most demanding in the metallurgical industry. The following table summarizes the key operating conditions:

The primary failure modes of casting rolls include:

1. Thermal fatigue cracking: Caused by cyclic thermal loading, leading to surface and subsurface cracks.
2. Abrasive wear: Caused by the sliding contact with the solidifying steel shell and entrained oxide particles.
3. Adhesive wear: Caused by the transfer of material from the roll surface to the steel shell.
4. Corrosive wear: Caused by the chemical interaction with molten steel, slag, and flux.
5. Surface scaling: Caused by oxidation at high temperatures, leading to a loss of surface integrity.
6. Indentation: Caused by the pressure of the steel shell on the roll surface.

The service life of an unclad casting roll is typically 20 to 50 days, depending on the steel grade being cast and the operating conditions. With proper cladding, the service life can be extended to 150 to 300 days, representing a 3 to 6 times improvement.

Cladding Material Selection

The selection of cladding material for casting rolls is a critical decision that directly affects the service life and performance of the roll. The ideal cladding material must possess:

• High hardness at elevated temperatures (hot hardness)
• Excellent thermal fatigue resistance
• Good wear resistance against steel and slag
• Low thermal expansion coefficient to minimize thermal stress
• Good metallurgical bond with the roll base material
• Resistance to oxidation and scaling at high temperatures

The following table compares common cladding materials for casting rolls:

For slab casting rolls, high chromium cast iron (HCRI) is the most commonly used cladding material. HCRI contains 12% to 25% chromium, which forms a stable chromium carbide (Cr₇C₃) network that provides excellent wear resistance. The chromium content also provides good oxidation resistance at elevated temperatures. The hardness of HCRI is typically 500 to 700 HV, which is sufficient to resist abrasive wear from the steel shell.

For billet casting rolls, high manganese steel (such as Hadfield steel, containing 12% to 14% manganese) is often used. The high manganese content promotes the formation of an austenitic microstructure that work-hardens rapidly under impact loading, providing excellent wear resistance. However, the thermal fatigue resistance of high manganese steel is lower than that of HCRI, making it less suitable for slab casting applications.

Weld Overlay Processes for Casting Roll Cladding

Several weld overlay processes are used for cladding casting rolls. The selection of the process depends on the roll geometry, the required cladding thickness, and the production volume.

For slab casting rolls, submerged arc welding (SAW) is the most commonly used process. SAW provides a high deposition rate, good penetration, and a stable arc that is suitable for automated welding on cylindrical surfaces. The cladding is typically applied in multiple passes, with each pass being 2 to 3 mm thick. The total cladding thickness is typically 8 to 12 mm.

The SAW cladding process for casting rolls involves the following steps:

1. Surface preparation: The roll surface is cleaned and ground to remove any existing coating, oxide scale, or contamination.
2. Preheating: The roll is preheated to 200°C to 300°C to reduce the thermal gradient and minimize the risk of cracking.
3. First pass: A transition layer is deposited using a filler metal that is compatible with both the base material and the final cladding material. For example, if the base material is a low-carbon steel and the cladding material is HCRI, a 309L stainless steel transition layer may be used.
4. Subsequent passes: The HCRI cladding material is deposited in multiple passes, with each pass being ground flush before the next pass is applied.
5. Post-weld heat treatment: The roll is heated to 600°C to 700°C and held for 1 to 2 hours to relieve residual stresses and improve the microstructure of the cladding.
6. Machining: The cladded surface is machined to the required profile and surface finish.

Microstructural Analysis and Performance Evaluation

The microstructure of the cladding layer is critical to its performance. In HCRI cladding, the microstructure typically consists of a matrix of pearlite and ferrite with a network of chromium carbides (Cr₇C₃). The carbide network provides the primary wear resistance, while the matrix provides toughness. The size and distribution of the carbides are influenced by the cooling rate and the composition of the cladding material.

A typical microstructural analysis of HCRI cladding reveals:

• Carbide network: Cr₇C₃ carbides forming a continuous network along the grain boundaries. The carbide size is typically 2 to 5 μm.
• Matrix: A mixture of pearlite and ferrite, with the pearlite fraction increasing with chromium content.
• Dilution zone: A transition zone at the interface between the cladding and the base material, where the composition is a mixture of the cladding and base materials.

The wear resistance of the cladding is evaluated using standardized tests such as the ASTM G65 pin-on-disk test or the ASTM G99 high-speed disk test. For casting roll applications, a more relevant test is the thermal fatigue test, which subjects the cladding to cyclic thermal loading similar to the operating conditions.

A typical thermal fatigue test involves:

1. Heating the cladding surface to 900°C using an induction heater.
2. Quenching with water to cool the surface to 100°C.
3. Repeating the cycle for a specified number of cycles.
4. Inspecting the surface for cracks after each cycle.

The number of cycles to initiate a crack is a measure of the thermal fatigue resistance. For HCRI cladding, the thermal fatigue life is typically 2000 to 5000 cycles, compared to 200 to 500 cycles for the unclad base material.

Engineering Practice and Performance Data

In a steel mill in China, a series of slab casting rolls were clad with HCRI using the SAW process. The rolls were operated in a continuous casting line producing carbon steel slabs at a casting speed of 1.2 m/min. The following performance data was collected:

The results demonstrate that the HCRI cladding extended the roll life by a factor of 7 to 9 compared to the unclad condition. The ESW cladding provided a slightly longer service life than the SAW cladding, likely due to the thicker cladding layer and lower residual stress.

A key finding from this case study was that the quality of the cladding surface finish had a significant impact on the roll performance. Rolls with a surface roughness of Ra ≤ 1.6 μm showed significantly better performance than rolls with Ra > 3.2 μm. This is because a smoother surface reduces the contact pressure between the roll and the steel shell, which reduces both abrasive wear and thermal fatigue cracking.

Summary and Conclusions

The development of cladding materials and processes for long-life continuous casting rolls is a critical area of research and engineering practice. The selection of the cladding material must be based on a thorough understanding of the operating conditions and failure modes of the casting roll. High chromium cast iron is the most widely used cladding material for slab casting rolls, offering an excellent balance of wear resistance, thermal fatigue resistance, and cost-effectiveness. The submerged arc welding process is the most commonly used method for applying the cladding, providing high deposition rates and good quality. The performance of the cladding is evaluated through microstructural analysis, hardness testing, and thermal fatigue testing. Engineering practice has demonstrated that proper cladding can extend the service life of casting rolls by a factor of 5 to 10, resulting in significant cost savings and reduced downtime. The key to success lies in the careful selection of materials, precise control of the welding process, and rigorous quality assurance.