CC-DR Roll Failure: Thermal Crack Mechanism and Remanufacturing

CC-DR Process Background and Roll Failure Challenges

In the evolution of the contemporary steel industry, Continuous Casting and Direct Rolling (CC-DR) technology has become a core pathway for improving energy efficiency and shortening production cycles. This process links the solidification of liquid steel directly with the plastic deformation of the finished product, maximizing thermal energy utilization. However, this integrated production mode presents unprecedented technical challenges for rolls.

In CC-DR lines, rolls are subjected not only to massive mechanical compressive and shear stresses but also to extreme alternating thermal loads. The generation and propagation of CC-DR roll thermal cracks (also known as fire cracks or thermal fatigue cracks) have become a major bottleneck limiting roll life, affecting strip surface quality, and increasing unplanned downtime.

Thermal crack formation is a complex process. During rolling, the roll surface undergoes violent temperature spikes and drops. When the roll contacts high-temperature slabs (1000 to 1100 degrees Celsius), the surface layer temperature surges to over 600 degrees Celsius; subsequently, in the cooling water spray zone, the temperature drops rapidly to around 100 degrees Celsius. This periodic thermal cycling leads to massive alternating thermal stresses. When these stresses exceed the material’s fatigue limit, microscopic dislocation accumulation and grain boundary debonding evolve into visible network-like thermal cracks.

Physical Triggers and Formation Mechanism of Roll Thermal Cracks

In-depth analysis of the formation mechanism is the prerequisite for scientific prevention. Thermal cracking is essentially low-cycle thermal fatigue damage.

Cyclic Thermal Stress and Plastic Strain Accumulation

Under CC-DR conditions, the roll surface is in a non-steady-state thermal load state. According to thermoelastic theory, the generated Thermal Stress is proportional to the Temperature Difference, the Thermal Expansion Coefficient, and the Elastic Modulus.

To quantitatively evaluate this damage, researchers introduced the Surface Damage Coefficient (referred to as kappa). Studies show that this coefficient is highly correlated with calculated plastic strain. When operational parameters cause the Surface Damage Coefficient to exceed 375, the roll failure mode shifts from wear to significant thermal fatigue cracking; when it is below this threshold, other damage mechanisms (such as oxidative wear) dominate.

Influence of Microstructure on Crack Resistance

The microstructural characteristics of the roll determine its ability to “capture” cracks. Traditional cast iron or high-chromium steel rolls typically have a matrix containing large amounts of M7C3 carbides. During thermal cycling, the asynchronous expansion between carbides and the matrix is the primary site for crack initiation. Especially in high-chromium iron rolls, these carbides are prone to brittle fracture under thermal stress, and the resulting microcracks rapidly spread along the carbide network .

Coupling of Oxidation and Thermal Fatigue

In high-temperature environments (500 to 700 degrees Celsius), oxidation significantly accelerates crack propagation. Oxygen penetrates the roll matrix along initial microcracks, forming brittle oxides at the crack tips. In subsequent thermal cycles, these oxides are crushed under pressure, further weakening grain boundary strength and driving cracks deeper into the material .

Roll Material Selection and Optimization of Thermal Crack Resistance of HSS Rolls

To address thermal cracking, roll material development has evolved from alloy cast iron to high-performance High-Speed Steel (HSS). Currently, optimization of thermal crack resistance of HSS rolls is the primary focus for early finishing stands.

Metallurgical Advantages of High-Speed Steel (HSS) Rolls

HSS rolls contain high levels of vanadium, molybdenum, tungsten, and chromium. Their core advantage lies in their unique carbide types:

• Carbide Hardness and Toughness: HSS contains large amounts of MC-type carbides (such as vanadium carbide) with hardness reaching 2800-3000 HV, significantly higher than the M7C3 carbides (1300-1800 HV) in high-chromium materials . These hard points are uniformly dispersed, breaking the continuous carbide network.
• Fracture Toughness and Blunting: Experiments prove that crack propagation inside HSS is extremely difficult. When a crack encounters MC-type carbides, it often blunts or deflects at the interface, making it hard to form deep cracks .
• High-Temperature Red Hardness: HSS maintains a hardness of approximately 500 HV even at 600 degrees Celsius, ensuring the surface does not undergo violent plastic deformation during service .

Material Performance Comparison

Design of Cooling Systems for CC-DR Rolls and Precision Thermal Management

Effective cooling is about reducing localized thermal stress concentrations through uniform heat exchange. Scientific design of cooling systems for CC-DR rolls is critical for prevention .

Nozzle Configuration and Flow Field Optimization

1. Water Pressure: Using a cooling water pressure of 1.0 MPa is more effective than conventional pressures at penetrating the steam film (Leidenfrost Effect) on the roll surface.
2. Spray Angle: Experimental validation shows that spray angles between 45 and 60 degrees achieve the best heat transfer coefficients.
3. Segmented Cooling: Advanced lines use segmented cooling to ensure a more uniform roll surface temperature across the length of the roll, directly reducing localized thermal stress .

Advancement of Air-Mist Cooling Technology

Modern CC-DR mills often utilize air-mist cooling. Air-mist nozzles use compressed air to atomize water into fine particles (50-100 micrometers) .

• High Flexibility: Air-mist systems have a high water adjustment ratio (up to 3.7 to 1), allowing real-time adjustment.
• Reduced Thermal Shock: Fine mist droplets evaporate instantaneously, making the heat exchange process smoother and lowering the peak instantaneous tensile stress .

Laser Cladding Remanufacturing Technology for Roll Surfaces

When roll surfaces suffer severe damage, laser cladding remanufacturing technology for roll surfaces offers a high-performance solution for restoration .

Materials Science of Laser Cladding

Laser cladding has a low dilution rate (typically less than 5%), enabling a strong metallurgical bond. For thermal crack applications, Nickel-based alloy powders mixed with 10% to 60% Tungsten Carbide (WC) are commonly used .

• Nickel-based Matrix: Offers excellent heat and corrosion resistance.
• Tungsten Carbide (WC) Reinforcement: Particles (100-300 mesh) significantly increase hardness and provide high thermal creep resistance under high-temperature cycling.

Key Process Parameter Specifications

1. Preheating: Rolls must be preheated to 150-450 degrees Celsius before cladding to reduce the thermal gradient.
2. Laser Parameters: Power is typically controlled between 2000 and 5500 Watts with scanning speeds of 0.1 to 6 meters per minute.
3. Post-Treatment: After cladding, a tempering treatment at 500-600 degrees Celsius is mandatory to prevent delayed cracks.

Online Detection and Maintenance Standards

The continuity of CC-DR requires roll management to shift to “predictive maintenance”.

Quantitative Evaluation via Eddy Current Testing (ECT)

Integrated ECT devices monitor microscopic cracks and thermal soft spots during grinding.

• Abnormality Thresholds: Industry standards for identifying defects are a Crack Value of 0.2 or a Soft Point Value of 0.4 .
• Monitoring: If testing shows values are stable or decreasing, it indicates the thermal crack is weakening rather than propagating .

Scientific Basis for “Operating with Damage”

According to fracture mechanics, a crack will not expand unstably as long as the Stress Intensity Factor (KI) is less than the material’s Fracture Toughness (KIc) .

The standard assessment for safe operation is:

KI = 1.12 * Tensile Stress * Square Root of (pi * Crack Depth) <= KIc

Theoretically, for HSS rolls in early finishing stands, thermal cracks with a depth of less than 10.59 mm typically do not lead to catastrophic failure under a normal rolling force of 34 MN .

Conclusion and Industry Outlook

Solving roll thermal cracking in CC-DR requires a closed-loop management system. HSS rolls provide intrinsic resistance, while air-mist cooling and precise lubrication provide the ideal external environment. Future directions will focus on “Digital Twin Rolls” and AI-driven predictive maintenance, which can reduce roll consumption by over 50% while ensuring the highest surface quality for finished products