21 questions on basic knowledge of continuous casting and measures to improve the quality of continuous casting slabs

1. What are the conditions for molten steel to transform from liquid to solid?

We put a cup of water (for example, 20℃) in a -20℃ cold storage. When the temperature of the water drops to 0℃, crystals will appear in the cup. At this time, water and water crystals coexist, and the temperature is still 0℃. , only when the water is completely frozen, the entire temperature of the cup drops to the same temperature as the cold storage temperature. Therefore, the temperature at which water begins to freeze is called the freezing temperature. The solidification and copper mould tube process of molten steel is the same as that of water. When the temperature drops to the solidification temperature (1535°C), crystals appear. It can be seen that in order to realize the process of transforming a liquid into a solid, two conditions must be met, namely a certain degree of supercooling and a crystallization core. The so-called subcooling degree is the degree by which the actual temperature is lower than the solidification temperature. For example, in pure iron, only when the supercooling degree reaches 295°C, many small-sized, short-range and orderly arranged atomic groups in the liquid metal can form embryonic crystal nuclei and gradually grow as the crystallization core. However, in actual production, when molten steel is poured into the mold, the degree of supercooling required for copper mould tube is only a few degrees. This is because: (1) The temperature of the mold is low, the temperature of the molten steel is high, and the mold wall provides cooling power. (2) The uneven surface of the model provides “support” and facilitates the formation of crystal nuclei. (3) The particles suspended in the molten steel can also serve as copper mould tube cores.

2.What does the shrinkage of molten steel during the solidification process include?

Molten steel changes from liquid to solid. As the temperature drops, the shrinkage can be divided into: (1) Liquid shrinkage: the shrinkage from the pouring temperature to the liquidus temperature. For low carbon steel, it is generally 1%; (2) Solidification shrinkage: the volume shrinkage when the liquid completely turns into a solid. For steel, it is generally 3 to 4%. Volume shrinkage will leave shrinkage cavities in the steel ingot. (3) Solid state shrinkage: shrinkage when cooling from solidus temperature to room temperature. Generally it is 7~8%. Solid-state shrinkage manifests itself as linear shrinkage of the entire steel ingot, which is related to the phase transformation of the steel during cooling. It has an important influence on the occurrence of cracks in steel ingots. The density of liquid steel is 7.0g/cm3 and the density of solid steel is 7.8g/cm3. The shrinkage of liquid into solid is: ((7.8-7.0)/7.0)×100%=11.4%, of which the liquid shrinkage is about 1% , the solidification shrinkage is 3 to 6%, and the solid state shrinkage is 7 to 8%. The 3-4% volume shrinkage during solidification will leave shrinkage cavities in the steel ingot. Use a protective cap to concentrate the shrinkage cavities at the head of the steel ingot. During continuous casting, the molten steel is continuously replenished to the liquid phase, so there are no concentrated shrinkage cavities in the continuous casting billet. The linear shrinkage of the cast slab with liquid core as it continues to solidify has an important impact on the quality of the cast slab and production safety. Therefore, the copper mould tube should maintain a certain inverse taper, and the roll gap of the support roller in the secondary cooling zone should conform to the law of slab line shrinkage from top to bottom. That is to say, the cast slab with liquid core runs in the inner and outer arc envelope space formed by many pairs of rollers. The opening between the rollers should decrease with the linear shrinkage of the slab during cooling. For example, a vertical bending slab continuous caster with an upright section has 99 pairs of rollers arranged in the secondary cooling zone. It is difficult in terms of mechanical structure to make the opening of the rollers continuously decrease from top to bottom. Therefore, the opening degree of the roller gap is set to shrink in a stepped manner. For example, if the thickness of the slab is 250mm, the thickness of the narrow surface of the upper mouth of the mold is 258mm, and the thickness of the lower mouth is 257mm. After coming out of the crystallizer, it is divided into 10 steps and reduced to the 99th pair of rollers with a spacing of 253mm.

3. What are the characteristics of the solidification process of continuous casting billet?

Compared with die casting, the characteristics of the solidification process of continuous casting are: (1) The solidification of continuous casting billet is a heat transfer process. The molten steel is poured into the copper mould tube while transferring heat, solidifying, and running, forming a continuous casting slab with a relatively long liquid cavity (the slab is more than 20 meters long). In order to accelerate solidification, three cooling zones are arranged in the continuous casting machine: ① Primary cooling zone: The molten steel forms a sufficiently thick and uniform shell in the crystallizer to ensure no leakage when exiting the copper mould tube. ②Secondary cooling zone: water spray cooling to accelerate the transfer of internal heat to completely solidify the slab. ③Third cooling zone: uniformize the temperature of the slab. (2) Solidification of continuous casting billet is the process of transforming liquid into solid along the liquid phase in the solidification temperature range. The continuous casting billet can be regarded as a steel ingot with a long liquid phase, which moves along an arc track in the continuous casting machine at a fixed speed. The slab solidifies in motion. In essence, it is the process of latent heat release and transfer along the liquid-solid-liquid interface. The crystal strength at the solidification interface is very small (only 1~3N/mm2), and the strain from deformation to fracture is 0.2~0.4%. Therefore, when the external force (such as bulging force, straightening force, thermal stress, etc.) on the cast slab exceeds the above critical value, cracks will occur at the solid-liquid interface and expand along the columnar crystals until the solidified shell can resist the external force. This is the reason why the cast slab has internal cracks. (3) The solidification of continuous casting billet is a staged solidification process. Solidification growth goes through three stages: ① Molten steel forms a primary green shell in the crystallizer. ② The cast slab with liquid core grows stably in the secondary cooling zone. ③The growth of the liquid phase near the end of solidification is accelerated. During the solidification process, the flow and mixing caused by the mold injection flow in the liquid phase have an important impact on the solidification of the slab. Research points out that the upper part of the liquid phase is a forced convection zone, and the height of the convection zone is determined by the flow injection method, the type of immersed nozzle and the cross section of the cast slab. The liquid flow in the lower part of the liquid phase is mainly natural convection caused by the shrinkage of the billet shell and the sinking of the crystal, or the flow caused by the bulging of the billet. Flow has an important influence on the structure of the cast slab, the floating of inclusions and the segregation of solute elements. (4) The cooling of the solidified shell in the continuous casting machine can be regarded as undergoing deformation heat treatment. The solidified shell is affected by force on the one hand and cooled by water spray on the other hand. As the temperature decreases, phase changes occur and the structure also changes. Sulfide and nitride material points may precipitate at the grain boundaries, which increases high-temperature brittleness and is the source of surface cracks in the cast slab. Therefore, it is necessary to have a deep understanding of the interconnectedness and mutual restriction of the above four aspects in order to formulate correct countermeasures in terms of equipment and technology, so that the continuous casting machine can achieve high production efficiency and good quality of cast slabs.

4. What parts of the heat released by the solidification of molten steel include?

The heat released when the molten steel is cooled from the pouring temperature to room temperature includes three parts: (1) Overheating of the molten steel: the heat released when the molten steel is cooled from the pouring temperature to the solidification temperature. (2) Latent heat of solidification: the heat s released when the molten steel is cooled from the liquidus temperature (TL) to the solidus temperature (Ts) (3) Physical sensible heat: the heat released when the steel is cooled from the solidus temperature to room temperature. The latent heat of solidification mainly depends on the steel composition. For pure iron it is 273kJ/kg and for low carbon steel it is 310kJ/kg. Only when latent heat is released can molten steel solidify. To increase the solidification speed, we must accelerate the release of latent heat. Therefore, the release rate of latent heat is directly related to the productivity of continuous casting.

5.What is solidification segregation?

After refining outside the furnace and blowing and stirring, the composition of the molten steel at any position in the ladle is uniform. After solidification, the chemical composition of the steel ingot or continuous casting billet is different from the surface to the center, and some are very different. This uneven composition is called segregation. Segregation can be divided into two types: one is called microsegregation, which is the difference in composition between the dendrite trunk and the dendrites. Generally, the distance is very small, ranging from a few microns to segregation. The other type, called macrosegregation, is compositional differences over long distances (measured in centimeters or meters). Take a longitudinal or transverse section sample from the slab and conduct a sulfur print or acid leaching inspection. You can observe the segregation status with the naked eye, also called low-magnification segregation. The reasons for segregation are: (1) The difference in solubility of elements in liquid and solid states. Define the distribution coefficient K to characterize the degree of segregation: K=Cι (concentration of elements in the liquid phase)/CS (concentration of elements in the solid phase). If K=1, then Cι=CS means there is no segregation in the solidified product. K<1, indicating that the solidified product is segregated. The K values measured for different elements are: C, 0.13; S, 0.02, O, 0.02, P, 0.13, Si, 0.66, N, 0.28, Mn, 0.84, Cr, 0.95. It can be seen that S, P, O, and C are strongly segregating elements. (2) Cooling rate. The faster the cooling rate, the smaller the degree of segregation. (3) The diffusion speed of elements in the solid phase. Elements diffuse quickly in high-temperature solids, which can reduce segregation. For example, the K value of carbon is 0.13, which is also a strongly segregating element. However, during high-temperature annealing, carbon atoms have strong diffusion ability, which is conducive to homogenization. (4) The stronger the flow in the liquid phase at the solidification front, the more serious the macrosegregation will be. For example, the bulging belly of the continuous casting billet is an important reason for severe center segregation of the casting billet.

6. What is the meaning of continuous casting billet quality?

The quality of the final product depends on the quality of the supplied slab. Broadly speaking, the so-called continuous casting billet quality refers to the severity of billet defects allowed to obtain qualified products. Its meaning is: ① Purity of cast slab (number, shape, distribution, gas, etc. of inclusions). ②Surface defects of the cast slab (cracks, slag inclusions, pores, etc.). ③ Internal defects of the cast slab (cracks, segregation, inclusions, etc.). The purity of the cast billet mainly depends on the treatment process before the molten steel enters the copper mould tube. In other words, to make the molten steel “cleaner”, efforts must be made in each process before the molten steel enters the copper mould tube, such as selecting appropriate refining outside the furnace, tundish metallurgy, protective pouring, etc. The surface defects of the cast slab are mainly determined by the solidification process of the molten steel in the mold. It is related to factors such as the formation of the mold shell, the fluctuation of the crystallizer liquid level, the design of the immersed nozzle, and the performance of the protective slag. All parameters affecting surface quality must be controlled within target values to produce defect-free slabs, which is the prerequisite for hot feeding and direct rolling. The internal defects of the slab are mainly determined by the cooling process of the slab in the secondary cooling zone and the slab support system. Reasonable secondary cooling water distribution, centering of the support rollers, and preventing billet bulging are the prerequisites for improving the internal quality of the billet. Therefore, in order to obtain good slab quality, different process technologies can be used in different stages of continuous casting, such as ladle, tundish, crystallizer and secondary cooling zone, according to different requirements of steel types and products. Effectively control the quality of cast slabs.

7. What measures can be taken to improve the purity of continuous cast steel?

Purity refers to the quantity, shape and distribution of non-metallic inclusions in steel. To reduce the inclusions in the steel to the required level according to the steel type and product quality, the following five aspects should be taken: ① Reduce the [O] content in the steel as much as possible. ②Prevent the interaction between molten steel and air. ③Reduce the interaction between molten steel and refractory materials. ④ Reduce slag being involved in the molten steel. ⑤ Improve the flow and promote the floating of inclusions in the molten steel. In terms of process operation, the following measures should be taken: (1) Slag-free tapping: the converter uses slag-stopping balls, and the electric furnace uses eccentric furnace bottom tapping to prevent tapping slag from falling into the ladle in large quantities. (2) Ladle refining: Choose an appropriate refining method according to the steel type to uniform temperature, fine-tune ingredients, reduce oxygen content, remove gas inclusions, etc. (3) Oxidation-free pouring: After the molten steel is processed by the ladle, the total oxygen content in the steel can be reduced from 130ppm to less than 20ppm. If the ladle → tundish injection flow is not protected or poorly protected, the total oxygen content in the tundish molten steel will rise to the range of 60 to 100 ppm, returning to the level before outside-furnace refining, and the effect of outside-furnace refining will be wasted. (4) Tundish metallurgy: The tundish has a large capacity, and adding retaining walls and dams are effective measures to promote the floating of inclusions. For example, for a 6t tundish, the slab inclusion waste rate is 12%, and the inclusions are 0.82/m2; for a 12t tundish + retaining wall, the slab inclusions are 0, and the inclusions are 0.04/m2. (5) Immersed nozzle + protective powder: The protective powder should be able to fully absorb inclusions. The material of the immersed nozzle, the shape of the nozzle and the insertion depth should be conducive to the floating and separation of inclusions.

8. What measures can be taken to improve the surface quality of continuous casting slabs?

Surface defects of cast slabs mainly include slag inclusions, cracks, etc. If the surface defects are serious. Finishing must be carried out before hot processing, otherwise the metal yield and cost will be affected. Producing cast slabs with no surface defects is a prerequisite for hot delivery and hot charging. The surface defects of the cast slab have different shapes and the causes are complex. Generally speaking, the surface defects of the cast slab are mainly controlled by the solidification process of the molten steel in the mold. In order to ensure the surface quality, the following points must be paid attention to during operation: (1) Stability of the mold liquid level: Fluctuations in the steel liquid level will cause uneven growth of the billet shell, and slag will also be involved in the billet shell. The test pointed out that the relationship between liquid level fluctuation and the depth of subcutaneous slag inclusion in the slab is as follows:. Liquid level fluctuation range, mm depth of subcutaneous slag inclusion, mm ±20<2±40<4 >40<7 When the depth of subcutaneous slag inclusion is <2mm, the billet can be eliminated when heated. When the slag inclusion depth is 2 to 5 mm, the surface of the slab must be cleaned. The steel liquid level fluctuates within ±10mm, which can eliminate subcutaneous slag inclusions. Therefore, it is very important to choose a sensitive and reliable liquid level control system to ensure that liquid level fluctuations are within the allowable range. (2) copper mould tube vibration: The weak point on the surface of the cast slab is the “vibration trace” formed by the meniscus shell. The harm caused by vibration marks to surface quality is: ① The trough of vibration marks is the origin of transverse cracks, ② The trough of vibration marks is the accumulation area of bubbles and slag particles. For this reason, a copper mould tube vibration mechanism with high frequency and small amplitude can be used to reduce the depth of vibration marks. (3) Uniformity of the primary green shell: Uneven primary green shells on the meniscus of the mold will cause longitudinal cracks and depressions in the cast slab, resulting in leakage. The uniformity of shell growth depends on the steel composition, mold cooling, steel liquid level stability and slag lubrication properties. (4) Molten steel flow in the mold: The forced flow of the mold caused by injection flow should not involve the slag on the liquid surface inside. If the insertion depth of the immersed nozzle is less than 50mm, the slag powder on the liquid surface will be involved in the solidified shell, forming subcutaneous slag inclusions; if the insertion depth of the immersed nozzle is >170mm, subcutaneous slag inclusions will also increase. Therefore, the insertion depth of the immersed water inlet and the outlet inclination are very important parameters. (5) Mold slag performance: It should have good inclusion absorption ability and slag film lubrication ability.

9. What measures should be taken to improve the internal quality of continuous casting billets?

The internal quality of the cast slab refers to the low magnification structure, component segregation, center porosity, center segregation and cracks, etc. After the billet is thermally processed, some defects may disappear, some may be deformed, and some may remain intact, causing varying degrees of harm to product performance. The generation of internal defects in the slab involves the effects of solidification heat transfer, mass transfer and stress. The generation mechanism is extremely complex. But in general, the internal defects of the slab are controlled by the solidification process of the slab in the secondary cooling zone. Measures to improve the internal quality of the slab include: (1) Control the structure of the slab: The first thing is to expand the equiaxed crystal area in the center of the slab and inhibit the growth of columnar crystals. This reduces center segregation and center loosening. For this reason, the use of low superheat pouring of molten steel, electromagnetic stirring and other technologies are effective ways to expand the equiaxed crystal area. (2) Reasonable secondary cooling system: the surface temperature of the slab should be uniformly distributed in the secondary cooling zone, and the surface temperature at the straightening point should be greater than 900°C. Straightening should be carried out without liquid core as much as possible. For this purpose, computers are used to control secondary cooling water distribution, air-water spray cooling, etc. (3) Control the stress and deformation of the cast slab in the secondary cooling zone: The stress and deformation of the solidified shell in the secondary cooling zone are the root causes of cracks. For this purpose, multi-point bending and straightening, accurate arc alignment, roll gap centering, compression casting technology, etc. are adopted. (4) Control the flow of molten steel in the liquid phase cavity to promote the floating of inclusions and improve their distribution. For example, the copper mould tube adopts electromagnetic stirring technology and improves the immersed nozzle design.

10.What are the types of defects in continuous casting billets?

Surface defects of continuous cast slabs are important defects that affect the output of continuous casting machines and the quality of cast slabs. According to statistics, cracks account for 50% of all types of defects. If there are cracks in the cast slab, in serious cases it will lead to leakage or scrap, in light cases it will need to be finished. This not only affects the productivity of the casting machine, but also affects the quality of the product, thus increasing the cost. Internal defects in the cast billet affect the mechanical properties, performance and service life of the product. (1) Surface defects: including surface longitudinal cracks, transverse cracks, reticular cracks, subcutaneous slag inclusions, subcutaneous pores, surface depressions, etc. (2) Internal defects: including middle cracks, subcutaneous cracks, compression cracks, inclusions, central cracks and segregation, etc. (3) Shape defects: The billet is deformed (off square) and the slab is bulging.

11. What are the causes of longitudinal cracks on the surface of continuous casting slabs and what are the methods to prevent them?

Longitudinal cracks on the surface of continuous casting billets will affect the quality of rolled products. For example, a longitudinal crack with a length of 300mm and a depth of 2.5mm leaves a delamination defect of 1125mm on the rolled plate. Severe longitudinal cracks will cause leakage and scrap. Research points out that longitudinal cracks originate from the uneven thickness of the primary green shell on the meniscus of the mold. The tensile stress acting on the billet shell exceeds the allowable strength of the steel, causing stress concentration at the weak points of the billet shell, leading to fracture, which expands in the secondary cooling zone after exiting the mold. The reasons for longitudinal cracking can be summarized as follows: (1) The misalignment of the nozzle and the copper mould tube causes bias flow to wash away the solidified shell. (2) The melting performance of the mold slag is poor and the liquid slag layer is too thick or too thin, resulting in uneven thickness of the slag film and making the local solidification shell too thin. The liquid slag layer is <10mm, and longitudinal cracks are significantly increased. (3) The copper mould tube liquid level fluctuates. If the liquid level fluctuates >10mm, the probability of longitudinal cracking is 30%. (4) S+P content in steel. When S>0.02% and P>0.017% in steel, the high-temperature strength and plasticity of the steel are significantly reduced, and the tendency of longitudinal cracking increases. (5) When C in steel is 0.12 to 0.17%, the tendency of longitudinal cracking increases. Measures to prevent longitudinal cracking are: (1) The nozzle and the crystallizer must be aligned. (2) The crystallizer liquid level fluctuation is stable at ±10mm. (3) Appropriate insertion depth of the immersed nozzle. (4) Suitable copper mould tube taper. (5) The arc alignment between the copper mould tube and the upper part of the secondary cooling zone must be accurate. (6) Suitable protective powder performance. (7) A hot-top copper mould tube is used, that is, materials with poor thermal conductivity such as stainless steel are embedded in a 75mm copper plate in the meniscus area. The heat flow in the meniscus area is reduced by 50-70%, which delays the shrinkage of the shell and reduces the dents, thereby also reducing the probability of longitudinal cracking.

12. What are the causes of transverse cracks on the surface of continuous casting billets and what are the methods to prevent them?

Transverse cracks are located at the troughs of the vibration marks on the inner arc surface of the slab, and are usually hidden and invisible. After pickling inspection, it was pointed out that the crack depth can reach 7mm and the width is 0.2mm. The cracks are located in the ferrite network area, and the network area happens to be the primary austenite grain boundary. And there are precipitations of fine particles (such as AlN) on the grain boundaries. Especially C-Mn-Nb(V) steel is more sensitive to cracks. Causes of transverse cracks: (1) Vibration marks that are too deep are the origin of transverse cracks. (2) The content of A1 and Nb in steel increases, which promotes the precipitation of particles (A1N) at the grain boundaries and induces transverse cracks. (3) The cast slab is straightened at the brittleness temperature of 900 to 700°C. (4) The secondary cooling is too strong. Measures to prevent the occurrence of transverse cracks: (1) Using high frequency (200-400 times/min) and small vibration amplitude (2-4mm) in the copper mould tube is an effective way to reduce the depth of vibration marks. (2) The secondary cooling zone uses steady weak cooling to make the surface temperature of the slab greater than 900°C during straightening. (3) The copper mould tube liquid level is stable, and a mold powder with good lubrication properties and low viscosity is used. (4) Use flame to clean surface cracks.

13. What are the causes of network cracks on the surface of continuous casting slabs and what are the methods to prevent them?

This kind of crack can only be discovered after pickling the surface of the cast slab, and the depth can reach 5mm. Causes: (1) The surface of the high-temperature cast slab absorbs the copper from the mold, and the copper becomes liquid and then penetrates along the austenite grain boundary. (2) The selective oxidation of iron on the surface of the cast slab causes residual elements in the steel (such as Cu, Sn, etc.) to remain on the surface and penetrate along the grain boundaries to form cracks. Studies have shown that elements such as Cu, Sn, and Sb are enriched in the crack area. If the Cu content in the steel is greater than 0.1%, the cracks will be aggravated; if the Al content in the steel increases, the network cracks will be aggravated. Prevention methods: (1) Plate Cr or Ni on the surface of the copper mould tube to increase the hardness. (2) Appropriate secondary cooling water volume. (3) Control residual elements such as Cu in steel to <0.2%. (4) Control Mn/S>40. 

14. What are the causes and preventive measures for longitudinal cracks at the corners of continuous casting billets?

Longitudinal cracks at the corners may be located near the edge where the wide surface and the narrow surface meet. Some are 10 to 15 mm away from the edge, and some are just on the edge. In severe cases, steel leakage may occur. Reasons for formation: For square shapes, it may be that the thickness of the water seam along the height of the copper mould tube is uneven, resulting in poor cooling at the corners of the copper mould tuber; the taper of the copper mould tube is too small, and the radius of the corner of the copper mould tube is too small. For slabs, it may be due to: (1) Improper support of the narrow surface causing the narrow surface to bulge. The narrow surface has a bulge of 6 to 12 mm, accompanied by longitudinal cracks at the corners, resulting in steel leakage. (2) The taper is inappropriate. (3) Insufficient cooling water for narrow surfaces. Improvement method: For billets: (1) Control the geometry of the copper mould tube to prevent deformation. (2) Appropriate fillet radius. (3) When assembling the copper mould tube, keep the thickness of the cooling water seam consistent to ensure uniform cooling. For slabs: 1) Adjust the gap between narrow-surface foot rollers to limit bulge inward by 1 to 2 mm. 2) Appropriate taper (1.0%/m). 3) Appropriate amount of cooling water. 4) Align the nozzle and the copper mould tube so that the flow does not deviate.

15. What are the causes and preventive measures of transverse cracks at the corners of continuous casting billets?

This is a small transverse crack located at the corner of the slab. The possible reasons are: (1) The taper of the copper mould tube is too large. (2) The surface of the copper mould tube is scratched. (3) The arc between the copper mould tube outlet and the zero section is not accurate. Improvement method: adjust the taper of the mold, strictly align the arc, and adjust the secondary cooling so that the corner temperature of the billet cannot be less than 800°C during straightening.

16. How are subcutaneous bubbles formed in continuous casting slabs?

Below the surface of the cast slab, there are large bubbles growing in the direction of columnar crystals with diameters and lengths of more than 1 mm and 10 mm respectively. These bubbles are called surface bubbles if they are exposed, subcutaneous bubbles if they are not exposed, and subcutaneous pinholes that are smaller and denser than the bubbles. In the heating furnace, the surface bubbles of the cast slab or the inner surface of the subcutaneous bubbles are oxidized to form a decarburization layer, which cannot be welded after rolling and form surface defects. Shallowly buried bubbles can be removed with grinding wheels, air shovels, and flame cleaning. Deeply buried bubbles are difficult to find and can cause cracks in the product. Insufficient deoxidation of molten steel is the main cause of bubbles. If enhanced deoxidation is used to reduce the oxygen content in the steel, the aluminum content in the molten steel will reach 0.01~0.015%, thereby eliminating the bubbles. In addition, the gas content in molten steel (especially hydrogen) is also an important reason for the generation of bubbles. Therefore, all materials added to the molten steel (such as ferroalloy, slag powder, etc.) should be dried, the ladle and tundish should be baked, the amount of lubricating oil should be appropriate, and the injection flow should be protected by pouring, which has an obvious effect on reducing bubbles.

17.What are surface folding defects of continuous casting billet?

There are transverse folding marks on the surface of the cast slab, which may be accompanied by transverse cracks in severe cases. Reasons for formation: (1) The suspension in the copper mould tube causes the solidification shell to tear. Due to the strong cooling of the copper mould tube, the molten steel leaking out of the tear immediately solidifies on the surface to form folding marks. (2) Improper adjustment of copper mould tube vibration parameters. (3) The arc alignment between the copper mould tube outlet and the secondary cooling section is poor. (4) The copper mould tube is poorly lubricated and the billet shell is bonded to the copper wall.

18. What is the reason for the “cold mole” on the surface of the cast slab?

The metal lumps or slag embedded under the surface of the billet are called “cold moles”. The reasons are: (1) The splash of the steel flow during open pouring sticks to the cold steel on the surface of the copper mould tube and is embedded in the solidification shell; (2) The liquid level of the copper mould tube fluctuates too much, and the insoluble matter in the slag is rolled into the solidification shell.

19.What is the double skin defect on the surface of continuous casting billet?

There are transverse discontinuities on the surface of the cast slab, and there are obvious traces of incomplete welding called double skin. Reasons: (1) The injection flow of the copper mould tube suddenly stops pouring, or the billet drawing stops instantaneously. If the pouring time is too long, obvious reconnections will be formed on the surface of the slab; (2) The molten steel is too sticky, the temperature is too low, the nozzle is blocked, the injection flow deviates, etc. may cause heavy skinning.

20.Why is the surface of continuous casting billet sometimes concave?

This defect is common on the narrow sides of billets or slabs. Reasons for formation: (1) The copper mould tube taper is too large; (2) Uneven cooling in the secondary cooling zone. This can be prevented by using a suitable copper mould tube taper and uniform secondary cooling.

21.Why does the surface of continuous casting billet sometimes appear convex?

This defect is common in rectangular blanks and narrow sides of slabs. The reason is that the solidification shell bulges due to poor arc alignment and weak cooling of the foot roller or support roller out of the copper mould tube.

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