Longitudinal slab cracks are a common and potentially detrimental surface defect in continuous casting. In-depth research into the impacts and causes of longitudinal slab cracks, along with the development of effective control measures, is crucial for improving steel product quality, increasing production efficiency, and reducing production costs.
Impacts of Longitudinal Slab Cracks
Impact of Longitudinal Slab Cracks on Product Quality
(1) Deterioration of Surface Quality: Longitudinal cracks on the slab surface may further expand during the subsequent hot rolling process, resulting in defects such as edge warping and delamination. These defects seriously affect the surface flatness and finish of the steel, reducing the product’s appearance. For example, in the production of weathering steel for containers, longitudinal cracks on the surface of thin slabs can form dark lines on the coil after hot rolling, making the product unable to meet the quality requirements for container panels and columns.
(2) Impairment of Internal Properties: The presence of longitudinal cracks disrupts the continuity of the steel’s internal structure, potentially causing stress concentration under load, reducing mechanical properties such as strength and toughness. Cracks also easily become corrosion initiation sites, reducing the steel’s corrosion resistance and shortening the product’s service life.
(3) Product Downgrade and Scrap: For products with high requirements for surface quality and internal performance, such as automotive steel and household appliance steel, even small longitudinal cracks can lead to product degradation. When longitudinal cracks are severe, the product may even be scrapped, resulting in significant waste of resources and economic losses.
Impact of Longitudinal Slab Cracks on Production Efficiency
(1) Off-line Cleaning of Slabs: After longitudinal cracks are discovered in slabs, they must be cleaned off-line to ensure the quality of subsequent products. This not only increases labor and time costs but also reduces the availability of the casting machine, impacting the smoothness of production.
(2) Reduced Hot Transfer and Hot Charging Rates: Hot transfer and hot charging are important methods for improving steel production efficiency and reducing energy consumption. However, slabs with longitudinal cracks often cannot be directly hot transferred and hot charged, requiring additional handling or waiting. This reduces the hot transfer and hot charging rate, increasing energy consumption and production costs.
(3) Increased Risk of Production Interruptions: In severe cases, longitudinal cracks can lead to steel breakouts. Once a breakout occurs, the casting machine must be shut down for cleaning and repair, resulting in a long production interruption and significant economic losses for the company. It also causes significant damage to the equipment and increases maintenance costs.
Causes of Longitudinal Cracks in Slabs
Molten Steel Composition
⑴. Effect of Carbon Content: When the carbon content in molten steel is within the peritectic range (generally 0.10%-0.16%), a δ→γ transformation occurs during solidification, accompanied by significant volumetric contraction. This causes the slab shell to separate from the copper plate in the mold wall, forming an air gap. This minimizes heat flow, reduces the shell thickness, and creates a surface depression. The depression cools and solidifies more slowly than other areas, resulting in a coarsened microstructure and increased crack sensitivity. Under the influence of thermal stress and ferrostatic pressure, stress concentration in the depression can easily lead to longitudinal cracks. For example, in Q235 steel, the incidence of longitudinal cracking increases significantly when the carbon content is within the peritectic range.
⑵. Effect of Sulfur Content: Sulfur exists in steel as FeS, which has a relatively low melting point. The melting point of the eutectic composed of Fe and FeS is only 985°C. When the sulfur content in steel exceeds 0.020%, the steel is susceptible to hot brittleness under pressure. During the continuous casting process, this hot brittleness can cause cracks in the ingot due to thermal stress. For example, when producing Q235 using the BOF-CC process, if the converter end-of-converter S is ≥ 0.020% and the manganese content in the molten steel is reduced, resulting in an inability to maintain the Mn/S ratio, this can exacerbate the occurrence of longitudinal cracks in the ingot.
⑶. Effect of Oxygen Content: High oxygen content in steel produces the low-melting-point eutectic compound FeO-FeS (melting point 940°C), further exacerbating the steel’s hot brittleness. When using the BOF-LF-CC process, the free oxygen and sulfur contents in the molten steel are both low, resulting in a lower incidence of longitudinal cracking. However, when using the BOF-CC process, if the lower limit for Als in the molten steel is eliminated, the free oxygen content in the molten steel will be high. When (O) in the molten steel reaches ≥40 ppm, the number of longitudinal cracks in the slabs per heat increases dramatically.
Mold-Related Factors
⑴. Uneven Cooling: Uneven cooling of the primary slab shell in the mold is a major cause of longitudinal cracking. Improper water flow rates on the wide and narrow sides of the mold can lead to uneven heat flux distribution, resulting in inconsistent shell thickness. On the mold thermogram, a low-temperature “cold tooth” phenomenon will appear. Initially located at the bottom of the thermogram, these “cold teeth” grow toward the meniscus over a short period of time, leading to severe cracks. If not addressed promptly, these cracks can easily cause steel breakouts. For some high-phosphorus and alloy-rich steel grades, cooling shrinkage is significant, and using a forced cooling mode can exacerbate the development of primary cracks.
⑵. Poor Mold Flux Performance: The performance of the mold flux significantly influences longitudinal cracking on the slab surface. The viscosity of mold slag is a key performance indicator. If the viscosity is too low, excessive slag flow will occur, resulting in a thicker slag film and uneven heat transfer, which can easily cause longitudinal cracks in the cast strand. Excessive viscosity can hinder slag flow, thinning the slag film, impairing slag flowability, and poor lubrication, similarly leading to longitudinal cracks. Changes in the composition of the molten steel can also alter the mold slag, affecting its fluidity and heat transfer at the meniscus, which in turn affects the uniformity of heat transfer in the strand shell and increases the likelihood of surface cracks.
⑶. Copper Plate Condition: The smoothness of the slag line on the meniscus of the mold copper plate can affect the flow of mold slag. A rough copper plate surface can lead to uneven mold slag flow, resulting in uneven heat transfer in the strand shell and cracks in weak areas of the primary strand shell. Long-term use of the mold copper plate may lead to wear and deformation, which can impair heat transfer and lubrication between it and the strand shell, increasing the risk of longitudinal cracks.
Other Process Factors
⑴ Unstable Casting Speed: Variations in casting speed have a significant impact on strand quality. At low casting speeds, the upward flow of molten steel from the submerged nozzle is weakened, resulting in insufficient heat supply to the mold meniscus, a low temperature, and a thick and uneven primary shell. Furthermore, the mold slag melts poorly due to its low temperature, making cracks more likely to form. At high casting speeds, within a certain superheat level, mold solidification is delayed, resulting in a thinner shell and an increase in the average shell surface temperature, exacerbating the tendency for surface cracking in the slab. Frequent casting speed fluctuations can also lead to insufficient slag supply and the influx of solid slag particles, disrupting the thickness and uniformity of the slag film, impairing heat transfer, and increasing the incidence of longitudinal cracking.
(2) Improper nozzle alignment: Inappropriate nozzle immersion depth can affect the flow distribution of the molten steel within the mold. Too shallow a nozzle immersion depth can easily cause slag curl, and the return flow pushes the slag in the corners, affecting the uniformity of heat transfer in the slab. Too deep a nozzle insertion depth can shift the heat center downward, making it difficult for the return flow to reach the top of the molten steel surface. This results in a low topside molten steel temperature, poor mold slag melting, and a tendency for longitudinal surface cracking. Inaccurate nozzle alignment can also lead to uneven distribution of molten steel within the mold, causing uneven solidification of the billet shell and increasing the risk of longitudinal cracking.
⑶ Molten steel superheat: Molten steel superheat also affects billet quality. When the superheat is between 25°C and 35°C, the incidence of longitudinal cracking in weathering steel is minimal; when the superheat is <25°C or >35°C, longitudinal cracking is more pronounced. When the superheat is <25°C, mold slag melts poorly, resulting in reduced slag consumption per ton of steel. This indicates poor lubrication between the billet shell and the copper plate, making the billet highly susceptible to longitudinal cracking. Excessive superheat prolongs the solidification time of the molten steel, leading to uneven billet shell thickness and an increased likelihood of longitudinal cracking.
