Analysis of the causes of corner cracks and edge cracks in continuous casting slabs and steel leakage

This article discusses the hazards of steel breakouts in slab continuous casting, analyzes the causes of corner cracks and edge cracks in slabs, and proposes preventive measures and issues that should be paid attention to.

Keywords: slab; steel leakage; crack; continuous casting

Introduction

Steel breakage during continuous casting is a destructive production accident, and it is also an accident that cannot be completely avoided. Slab casting machines have large cross-sections, many equipments, and complex structures, so the losses caused by steel breakouts are much greater than those of square billet casting machines. During the slab production process, steel breakouts are inevitable, but based on experience and strengthened inspections, breakout accidents and losses can be reduced. Reducing the number of continuous casting breakouts is a common concern for technicians and operators; it is an important way to reduce losses and costs, and can roughly reflect the management level and employee quality of a full continuous casting production plant. Therefore, it is necessary to investigate and analyze the causes of steel leakage.

Current situation survey

There are two types of slab continuous casting molds currently used in production: curved and straight molds. Since there is two-dimensional heat transfer at the corners, the molten steel entering the mold first begins to crust at the corners. Because the arc-shaped mold has a certain curvature, the billet shell at the corners is subject to bending stress during movement. The friction force experienced by the billet shell when moving in the mold is relatively large, so there is generally a 3-5 mm thick angled copper strip at the corner of the arc-shaped mold to smooth the surface between the copper plates on the side of the mold and the inner and outer arc copper plates.  When using a straight mold, because the shell is not subject to bending stress when moving in the mold, there is no need for a smooth transition between the side and the inner and outer arc copper plates. Generally, four flat copper plates are combined together without a transition copper strip.

A steelmaking plant currently has an ultra-low-head slab continuous casting machine with a basic arc radius of 5700 mm, a slab cross-section size of 1050 mm × 180 mm, and a mold length of 900 mm. It is a combined adjustable mold. The upper opening size is 187.7 mm × 1 078 mm, the lower opening size is 187.0 mm × 1070 mm, there is an inverse taper of 8 mm in the width direction, and a reverse taper of 0. 7 mm in the thickness direction. A foot roller is installed at the bottom, and the side and internal and external arc copper plates There is a 3~5mm thick copper strip in between. In 1999, there were 13 steel leaks, 10 of which were edge cracks and corner cracks. In the first eight months of 2000, there were 9 steel leaks, of which 7 were edge cracks. Each steel leakage will cause a loss of 100,000 to 200,000 yuan, so reducing the number of slab edge cracks and corner cracks and steel leakage is an important direction to reduce slab production accidents.

Reason Analysis

Side cracks caused by bending of wide-surface foot rollers

During the use of the crystallizer, due to the long-term thermal stress and external force, the wide-faced foot rollers are deformed and bent. After the billet shell comes out of the crystallizer, it cannot get the support of the wide-faced foot rollers. At this time, the billet shell is affected by the molten steel. If the strength of the billet shell cannot withstand the gravity of the molten steel, it will break at the weakest point of the billet shell, causing a steel leakage accident. After the molten steel enters the crystallizer, due to the two-dimensional heat transfer effect at the corners, the molten steel in the corners solidifies first. In the lower part of the crystallizer, due to the solidification shrinkage of the molten steel in the corners and leaves the copper plate, the cooling intensity weakens, the solidification speed decreases, and the liquid steel exits. When molding, the billet shell at the corners is the thinnest, so steel breakouts will form at the corners of the billet. From the analysis of the shell thickness data after multiple measurements of steel breakouts, the thickness of the shell at the lower mouth of the mold is 2~3 mm thinner at the corners than at other parts.

Edge cracks and steel leakage caused by the narrow surface foot roller and the side copper plate not being on the same plane

During the adjustment process before using the crystallizer, if the narrow-faced foot rollers and side copper plates are not adjusted to the same plane, the billet shell will not be effectively supported after coming out of the mold, or the foot rollers will cause severe compression of the billet shell. , will cause the billet shell to be subjected to unbalanced external forces, resulting in deformation or crack defects. When the billet shell at the defective part cannot withstand the static pressure of the molten steel, a serious steel leakage accident will occur. The cause and wide range of such steel leakage accidents will occur. The steel breakouts caused by the bending of the face and foot rollers are basically the same. They are all caused by the lack of effective support after the shell comes out of the crystallizer. However, the former type of steel breakouts is less affected by human factors, while the latter type can be avoided (see Figure 1) .

Figure 1 Cracks on the side of the slab

Figure 2 Corner cracks in billet

Corner cracks and steel leakage caused by thick copper strip or wear

Due to its characteristics, the arc-shaped crystallizer needs to be equipped with copper strips to buffer the bending stress during the movement of the billet shell; there is cooling water on the copper plates on the four sides of the crystallizer to take away the heat transferred to the copper plates in time. Although the copper strips are not cooled water, but when its thickness is 3 ~ 5 mm, the accumulated heat can be transferred to the copper plate in time. If the thickness of the copper strip is greater than 5 mm, the accumulated heat will increase, and the temperature gradient between the shell and the copper strip will decreases, the thermal resistance increases, and the solidification speed at the corners decreases. When exiting the mold, the billet shell at the chamfer of the billet becomes thinner and is torn under the action of the shrinkage stress of the inner and outer arcs and side billet shells, forming corner cracks and steel leakage. (See Figure 2). If the mold is used for a long time and the copper strip is worn, it will continue to be used. Because the corners of the mold cannot be smoothly transitioned, the corners of the billet shell will be irregular and not smooth, which will easily lead to steel leaks or steel shell bending. Corner cracks and steel leakage are formed under stress.

Corner cracks and steel leakage caused by molten steel turning and entraining slag at the corners of the mold

Due to the design problem of the bottom shape of the immersed nozzle, after the molten steel flows out of the side hole, the flow field of the molten steel in the mold is unevenly distributed, causing serious disturbance of the molten steel level, causing part of the mold slag to be rolled into the billet shell without time to melt. Without melting, it cannot be well lubricated. Unmelted mold slag is rolled into the shell and forms inclusions. Both cases will cause the thermal resistance of the shell to increase, the solidification speed of the shell to slow down, and the strength of the shell to be weaker than the surrounding shells. Due to the effects of thermal stress and solidification shrinkage stress, stress concentration occurs at the weakest point of the shell and cracks occur, resulting in corner cracks and steel leakage.

Side cracks and steel leakage caused by small inverse taper on the side of the crystallizer or low cooling intensity

This kind of steel breakout accident is the most common one. Due to the small inverse taper of the mold or the low cooling intensity of the side, the cast slab is severely bulged and deformed. After the molten steel solidifies into a billet shell in the mold, the molten steel creeps forward. At this time, the inverse taper of the mold is appropriate, the billet shell is close to the copper plate, the thermal resistance is small, and the heat can be transferred away in time. The billet shell gradually thickens. After the billet shell comes out of the crystallizer, it The strength is enough to withstand the static pressure of molten steel, and the cast slab will not have edge cracks or dents; if the reverse taper of the mold is small or the cooling intensity is low, the billet shell cannot be close to the copper plate, the thermal resistance is large, and the heat cannot be transferred away in time, and the edge The billet shell is relatively thin. Due to the concentration of thermal stress and shrinkage stress, the billet shell will crack at the weakest point due to the static pressure of the molten steel and loss of support after leaving the mold, causing edge cracks and steel leakage.

 Corner cracks and steel breakouts caused by large reverse taper on the side of the mold

When the reverse taper on the side of the mold is large, although edge cracks and bulging of the billet can be avoided, the resistance of the billet shell when moving in the mold increases, which can easily cause transverse cracks at the corners (as shown in Figure 3), and in severe cases can lead to leakage.

Figure 3 Transverse cracks at the corners of the slab

 Preventive measures

(1) The edge cracks and steel leakage caused by the bending of the wide-surface foot roller are irregular. To ensure smooth production, it is necessary to ensure that the equipment is in good condition. Timely inspection is an effective way to discover hidden dangers and avoid accidents. Therefore, it is stipulated that inspections must be carried out after shutdown, and hidden dangers must be rectified immediately.

(2) It can be avoided that the narrow surface foot roller and the side copper plate are not on the same plane. This problem is greatly affected by human factors, mainly the adjustment process of the crystallizer before use. If the adjustment quality is good, problems will generally not occur during use. Therefore, before using the crystallizer, the side feet must be rolled and crystallized. The side panels of the mold must be adjusted to a flat surface so that the cast billet can be effectively supported after it comes out of the mold, thereby avoiding bulge deformation or corner and edge cracks in the billet shell. Carry out inspection after the casting machine is shut down, and deal with any problems immediately if any problems are found.

(3) The thicker copper strip causes more corner cracks and edge cracks and steel leakage. In view of the current situation of high cost pressure and high consumption of copper plates in our factory, in order to reduce the consumption of copper plates per ton of steel and extend the use times of copper plates, the thickness of the copper strips is sometimes greater than 5 mm or even 8 mm; in addition, in order to reduce the number of maintenance times , extending the primary service life of the crystallizer, causing serious wear of the copper plates and copper strips in the later stages of use of the crystallizer, and hindering heat transfer. In order to reduce steel leakage accidents and stabilize production, it is stipulated that the thickness of the copper strip equipped with the mold must be less than 5 mm. If it does not meet the requirements, it will be rejected. When the steel poured into the mold reaches about 10,000 tons, it must be replaced. It must be inspected at any time during production and use. If the wear and tear of the crystallizer copper plates and copper strips cannot meet production requirements, they must be replaced immediately.

(4) The mold liquid level turning over and steel slag entrainment are very harmful. Firstly, it causes slag inclusions and corner cracks and steel leakage; secondly, it increases the inclusions in the steel and affects the quality of the cast slab. In order to solve the problem of mold liquid level turning, various types of nozzles were designed for testing. Based on the test results, a nozzle shape that meets the slab cross section and production needs of our factory was summarized. The sum of the side hole area of the immersed nozzle and the nozzle were The inner hole area ratio is set at 3:2. In addition, when the casting machine is started and the nozzle and tundish are replaced, and the molten steel level in the mold is not higher than the side hole of the immersed nozzle, no protective slag is added to prevent the protective slag from entering the molten steel and causing corner cracks or inclusion leakage. steel.

(5) The reverse taper of the mold is an important parameter of the slab continuous casting machine. Because molten steel undergoes a volume reduction process from liquid to solid, the crystallizer must have an appropriate inverse taper to meet the volume shrinkage requirements of the billet shell. If the inverse taper is too large or too small, it will affect the quality of the cast slab. It must be based on the cross section of the cast slab. Size, characteristics of molten steel, actual shrinkage ratio, and appropriate inverse taper of the slab mold. Slab casting machine mold size: upper opening 187.7 mm Production. In addition, pay attention to the surface quality of the slab during production, and immediately stop the machine for processing if any abnormalities are found.

Effects

Through the implementation of the above measures, slab production has been relatively stable since September 2000. A total of one vicious steel breakout accident occurred, and it was an operational error. Slab corner cracks and side cracks and steel breakouts rarely occurred. In order to ensure stability, Production, cost reduction and waste reduction have made positive contributions.

Conclusion

From the perspective of equipment and operations, try to avoid steel breakouts as much as possible. The specific measures are:

(1) Strict crystallizer acceptance system, and those that do not meet the process requirements will be rejected for use.

(2) The casting machine inspection system must be strictly observed to prevent malfunctioning of the casting machine.

(3) Observe the quality of the slab frequently during production, and stop the machine immediately for processing if any abnormalities are found.

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