Mechanism and preventive measures of defects in continuous casting billets

Continuous casting billets produce many defects during the production process, resulting in quality defects in subsequent products of the billets. This article studies the generation mechanism of defects in billets, and combines production experience to propose methods to prevent defects in order to improve product quality, save energy, and increase productivity.

Keywords: billet; defect generation mechanism; preventive measures

Types and generation mechanisms of billet defects

The defects of billets are divided into internal defects and external defects. The internal defects of billets include internal cracks, shrinkage cavities, loose centers, center inclusions, (or center slag inclusions) and subcutaneous bubbles. The external defects of billets include transverse cracks, longitudinal cracks, corner cracks, joints, missing squares, distortions, etc. The external defects of the billet are all reflected on the surface of the billet.

After the molten steel enters the crystallizer, it first contacts the crystallizer wall and is quenched to produce a quenching layer. Due to the strong cooling intensity of the quenching layer, fine equiaxed grains with basically no fractional crystallization are formed. The characteristics of fine equiaxed grains It has uniform ingredients, fine grains and superior mechanical properties. After the quench layer is formed, due to volume shrinkage, a gap will be formed between the green shell and the inner wall of the crystallizer. Due to the creation of the gap, gas and mold slag will enter and be filled.

Due to the increase in the heat transfer process of the gaps and the mold slag layer in the heat transfer, the entire crystallization cooling process is slowed down. The best cooling is perpendicular to the direction of the crystallizer. Due to the slowed down cooling (heat transfer), the selective crystallization becomes It is possible and becomes the main form of crystallization. As time goes by, the most advantageous crystallization is crystallization in the direction perpendicular to the surface of the crystallizer, forming the backbone of columnar crystals. Secondary columnar crystals are formed in the direction perpendicular to the main trunk, and tertiary columnar crystals are produced in the vertical direction of the branches, until the secondary and tertiary columnar crystals fully grow to fill the entire gap, so that at the front of each columnar crystal, due to As a result of fractional crystallization, many inclusions (SiO2, Al2O3, MnO, S, P, [N], [O], [H]) are accumulated. During the entire crystallization process, they are accompanied by heat loss and temperature drop, and they accumulate. A large number of inclusions meet the conditions for simultaneous crystallization. At this time, the center of the billet will solidify in an instant, forming a thick and fine grain belt. Because the thick and fine grains support each other, the cast billet The center is relatively loose. If the secondary cold water is not used properly at this time, the liquid molten steel cannot be fully filled, resulting in loose center and shrinkage holes. Since there are also a large number of inclusions that are enriched in crystals and retained between the columnar crystals, the bonding force or strength between the columnar crystals is insufficient. When the secondary cold water is cooled unevenly, cracks will occur between the columnar crystals and cracks (internal cracks) will occur.

Factors affecting billet quality

Crystallizer soft water quality

The continuous casting crystallizer is called the “heart” of the continuous casting equipment, so the soft water in the crystallizer is called the blood of the continuous casting machine. It can be seen that the quality of the soft water in the crystallizer is important to continuous casting, and the quality of the soft water in the crystallizer is important to continuous casting. The impact is manifested in four aspects: First, the quality of the crystallizer soft water. Substandard crystallizer soft water will inevitably lead to scaling of the soft water pipes and crystallizer water seams, making the cooling intensity of the originally designed crystallizer fail to meet the requirements. The heat taken away is reduced, and the entire crystallization process of the slab changes. The billet shell is too thin, surface and internal defects increase, the quality of the cast billet deteriorates, and the cast slab appears to be off-square, internal cracked, and the center is loose. Second, due to scaling, the cooling effect of the copper tube is poor, causing the temperature of the copper tube to rise, the wear resistance to deteriorate, and the life of the copper tube to decrease. Third, due to the thin shell of the scaled billet, longitudinal cracks will appear as soon as the continuous casting billet comes out of the crystallizer, and in severe cases, steel leakage accidents will occur.

Second cold water

Generally speaking, in continuous casting, the secondary cooling water is required to be cooled evenly, and the amount of secondary cooling water is adjusted according to different steel types and different casting speeds. If the secondary cooling water is not adjusted properly, the cooling of the slab in the secondary cooling chamber may be too strong or too weak. , too strong secondary cooling water will accelerate the temperature drop of the billet in the secondary cooling chamber, accelerate the solidification of the liquid core of the billet, and cause defects such as loose shrinkage holes in the center. In addition, too strong secondary cooling water will reduce the temperature of the billet. , increase the tension and straightening force of the tension and leveling machine, and in severe cases, frozen billet accidents may occur. Uneven secondary cooling water will also cause the cast billet to bend or twist, exacerbating quality defects. Too weak secondary cooling water will increase the surface temperature of the cast slab, the liquid core will exist for a long time, the selected fraction will be fully crystallized, and the internal defects will be aggravated.

Vibration

When the vibration of the continuous casting machine fails, firstly, the liquid level inside the crystallizer fluctuates greatly, which can easily cause slag entrapment and steel leakage; secondly, it will cause defects such as transverse cracks and corner cracks in the billet. In severe cases, the billet will fall out of the mold. The mouth was broken. The way to judge the crystallizer vibration failure is that the side with quality defects is not the crystallizer vibration failure point, but the failure point is exactly on the side without defects.

Mold powder

The main functions of the mold powder are to improve and control the heat transfer between the shell and the crystallizer; to lubricate the mold and the shell; to adsorb inclusions floating from the molten steel; to insulate and maintain heat; to isolate air to prevent oxidation. The performance of the mold flux has an important influence on the surface cracks of the continuous casting billet. The viscosity of the mold flux is an important parameter that reflects the flow performance of the mold flux after it forms liquid slag. If the viscosity is too small, more slag will flow in, and the slag film formed will be thicker. It is easy to cause the slag film between the billet shell and the copper plate to be uneven. If the viscosity is too high, it will affect the spreadability and meltability of the mold slag, causing the vibration marks at the corners to deepen and transverse cracks at the corners. If the viscosity is too high, the slag film between the crystallizer and the shell will be too thin, resulting in poor lubrication.

Defect analysis and control measures

Folding

The main reason for folding is that the rolled piece before the finished product has ears. Secondly, the flash and severe scratches produced in each pass during the rolling process, as well as damage to the roll ring and severe wear of the roll groove, etc., can all cause the folding. Folding will occur on the surface of the finished product; in addition, if the blank has serious defects and improper cleaning may also cause folding. Therefore, the main measures to reduce or avoid folding include: rationally designing the pass pattern, accurately estimating the width, accurately adjusting the position of the roller groove and guides, reducing or eliminating defects such as ears, flashes, scratches and other defects before the finished product is rolled, and carefully Clean up serious defects on the surface of the blank.

Scarring

The occurrence of bar scarring defects is closely related to the original surface quality of the billet, rolling operation, head and tail shearing after rolling, and equipment status. Although scarring defects have different shapes, compared with hidden defects such as cracks and folds, they are relatively intuitive defects and are easy to analyze and locate their occurrence.

The small scab defect was discovered after eddy current inspection. A review of the production conditions at the time found that the descaling water for this batch of steel was too alkaline, causing the descaling nozzle to be blocked, the descaling effect was poor, and there was a lot of oxide scale remaining on the surface. Some iron oxide scale adhered to the surface of the billet will appear as discontinuous small scars on the steel surface after rolling; in addition, it will also cause a large number of pit defects. Therefore, ensuring the quality of the descaling water and regularly cleaning and maintaining the nozzles can reduce such defects.

Scarring defects mostly occur at the head and tail of the billet. They are the cutting residues produced by flame cutting of the continuous casting billet. They adhere to the head and tail of the steel during the rolling process and are not completely cut during the head and tail shearing. Therefore, when cutting continuous casting billet to length, it is necessary to ensure cutting quality and minimize residues. Appropriately increasing the cutting head and tail length after rolling is an effective measure to control such defects.

Scratches

Scratches on the steel surface are completely a processing defect. If the scratches are not detected, they may cause surface cracking during the user’s use. This is especially the case when the user uses cold heading, which is more harmful and often results in batch waste. It mainly occurs on the conveyor roller table and channel side wall after steel rolling. Because the roller bearings are locked, iron sheets and other foreign objects are stuck, the rollers stop rotating, the vertical rollers on the side walls of the channel are improperly adjusted or the joints are protruding. These immovable hard points will cause scratches when they come into contact with the moving red-hot steel. This kind of scratches are often shallow; there is also a more serious scratch, which occurs at the entrance guide splint and outlet duct of the rolling mill. Due to improper installation or alignment of the guide duct, scratches occur when the steel passes through. To prevent scratches, we need to start with regular inspection and maintenance of equipment, use of alarm devices, and regular inspections of rolling mill guides by rolling workers.

 Roller damage

Roller damage occurs at defective rotating parts, which may be rolling mill rolls, rolling guide rollers, channel ground rollers, channel side vertical rollers, straightening rollers, etc. The main characteristic of roll defects is that they are regularly distributed in the length direction of the bar, and defects appear in a fixed length period.

After a steel piling accident occurs, the surfaces of the rolls and guide rolls should be confirmed to see if iron sheets are adhered; for high-alloy varieties with greater deformation resistance, the heads should be cut off to avoid hard points that may damage the rolls due to rapid cooling of the heads. Afterwards, check whether there are hard damage pits on the surface of the roll; take small samples for rolling logistics, and the length of the small-size material samples should be appropriately lengthened. Pickling logistics inspection can detect roll injuries in time, and find out the location of the occurrence in time according to the distribution pattern of roll injuries. To reduce the amount of roller damage waste.

 Conclusion

Based on the research on the mechanism of billet defects, and through long-term production experience accumulation and summary, the influencing factors of defects produced in billets during the production process were analyzed, and it was concluded that the defects can be effectively reduced in practical production operations. Methods for Billet Defects. The big bales are opened at one time, the middle bales are poured when the whole bales are full, and the black slag of the crystallizer is operated. These measures can smooth the process flow, effectively reduce the occurrence of defects, and improve the quality of billets.

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