The term “failure” (or “fission/rupture”) regarding copper mold tubes typically refers to issues such as cracking, rupturing, or structural deformation (e.g., bulging) that occur during continuous casting. These problems directly lead to breakouts (molten steel leakage), reduced cast strand quality, and a shortened equipment service life. Based on actual continuous casting operations, the causes of such failures can be categorized into four main areas: thermal stress, mechanical stress, medium corrosion, and improper process control.
Thermal Stress and Thermal Fatigue (Most Common Causes)
Localized High Temperature and Thermal Shock: The heat flux density in the meniscus region of the mold is extremely high, resulting in localized high temperatures. This causes uneven thermal expansion in the copper tube, generating immense thermal stress. When the temperature exceeds the recrystallization temperature of the copper material, its hardness and strength decrease, making it highly susceptible to thermal cracking (cracks often propagate along grain boundaries).
Excessive Temperature Gradients: The localized temperature difference between the hot face (in contact with molten steel) and the cold face (in contact with cooling water) of the mold can reach hundreds of degrees Celsius. Drastic temperature fluctuations lead to thermal fatigue, accelerating crack initiation and propagation.
Mechanical Stress and Deformation
Casting Resistance and Friction: Excessive resistance during the casting pull (e.g., due to poor alignment accuracy of the withdrawal and straightening unit or abnormalities in the oscillation table) or improper design/adjustment of the mold taper can subject the copper tube to excessive localized tensile or frictional stress, resulting in transverse or longitudinal cracks.
Water Pressure and Deformation Stress: Excessive water pressure during post-installation testing or during the primary cooling stage of production can cause localized deformation of the copper tube (such as buckling of the narrow face or deformation at the bottom outlet). This generates additional internal stress within the tube, leading to cracking or tearing of the coating. Static Pressure of Molten Steel: During high-speed casting or the production of large-section billets, the static pressure of the molten steel is significant. If the copper tube lacks sufficient rigidity, localized bulging can occur, leading to stress concentration and cracking.
Medium Corrosion and Microstructural Damage
Element Diffusion and Hot Embrittlement: Elements such as sulfur (S) and zinc (Zn) from the molten steel, as well as components from the mold flux, can diffuse into the surface and subsurface of the copper tube. This leads to the formation of sulfides or “dezincification” (brass formation), increasing the tube’s surface hot brittleness and making it highly susceptible to cracking under the combined action of thermal and mechanical stresses.
Environmental Medium Erosion: Environmental media (such as sulfur) can penetrate micro-cracks in the copper tube, accelerating crack propagation under tensile stress.
Process and Operational Factors
Uneven Cooling: Irregular adjustment of the mold water gap, scaling due to water quality, or abnormalities in cooling water flow rate/pressure can cause uneven cooling. This results in localized thermal stress concentration, triggering cracks (such as folding marks or longitudinal cracks in the continuous casting strand).
Influence of Mold Flux: If the liquid slag layer of the mold flux is too thin or melts unevenly, it causes variations in local heat transfer rates. This leads to uneven solidification of the strand shell and increased localized thermal stress, subsequently inducing cracks.
Summary
Cracking in copper mold tubes results from the coupled effects of thermal, mechanical, environmental, and operational factors. In actual production, the formation of cracks can be effectively prevented or minimized by optimizing mold taper, controlling cooling intensity, ensuring equipment assembly precision, and optimizing process parameters.