A brief analysis of the causes and countermeasures of stripping of billets in continuous casting

This article analyzes the causes of billet peeling from the aspects of continuous casting equipment and technology and proposes preventive measures.

Keywords: continuous casting; billet; stripping; crystallizer; cooling intensity; measures

Introduction

A steelmaking plant currently has two R5.25 m four-machine four-stream billet continuous casters with a design capacity of 400,000 tons per year. The billet continuous casting machine is the Demark R5.25 m model introduced in 1982. The current main production varieties include: Q195, Q215, Q235, HRB335, HRB400, TGLZ, as well as welding wire, welding rod and other steel billets. Stripping is one of the main quality problems that often occurs in billet production. The squareness of the billet refers to the unequal length of the two diagonals on the cross section of the billet, which directly affects the “bite” of the rolling mill in the next rolling process, causing rolling difficulties. In serious cases, it will cause direct scrap of steel rolling or lead to corner steel leakage accidents in the continuous casting machine due to longitudinal cracks at the corners and longitudinal cracks on the face. This article analyzes the causes and preventive measures of billet peeling from the perspective of process and equipment factors.

Solidification characteristics of molten steel in the crystallizer

During continuous casting, the solidification of molten steel begins in the crystallizer. The molten steel contacts the copper wall in the crystallizer to form a meniscus with a small radius. At the root of the meniscus, due to the fast cooling rate (100°C/s), it solidifies into a primary green shell. The forming primary shell shrinks due to the δ→γ phase transformation, causing the shell to separate from the copper wall, forming an air gap, and the static pressure of the molten steel causes the shell to expand outward. As the billet shell moves downward, the surface of the billet shell begins to reheat, the temperature of the billet shell increases, the strength becomes lower, and the static pressure of the molten steel deforms the billet shell. In the corner area of the crystallizer, due to two-dimensional heat transfer, the shell solidifies the fastest and shrinks earliest. The air gap is formed first, and then the heat transfer slows down and the solidification speed decreases. As the shell moves downward, the air gap expands from the corners to the face. At this time, the air gap in the center of the billet face is smaller than that in the corners. The heat flow of the billet shell in the corners is minimal and the billet shell is thin. Under the action of the static pressure of the molten steel, it is easy to deform.

Basic principles of formation of desquamation

According to the literature, there are two basic mechanisms for the formation of flakes. One is that the flakes are related to the uneven solidification of the four sides of the billet shell, and the other is that the flakes are related to the uneven solidification of the four corners of the billet shell. What these two theories have in common is that desquamation is formed in the place where the heat flow is greatest in the crystallizer due to uneven solidification of the billet shell.

The desquatting is caused by uneven cooling of the four corners of the billet in the crystallizer. Constrained by the four walls of the crystallizer, the cast billet is square. Due to the uneven cooling of the crystallizer, the thickness of the billet shell is also uneven. The corners with strong cooling form acute angles, and the corners with weak cooling form obtuse angles. The shell is thicker near acute angles and thinner near obtuse angles. When water is sprayed in the secondary cooling zone for cooling, even if the four sides are cooled evenly, the shell temperature caused by uneven thickness of the shell is also inconsistent, resulting in uneven shrinkage of the shell and further development of shedding.

Desquareness is caused by uneven cooling of the four sides of the billet in the crystallizer. The asynchronous intermittent boiling on the four sides of the crystallizer causes uneven cooling of the four sides of the shell, resulting in uneven thickness of the shell, and the directionality of the peeling is determined by the weakness of the shell at the corners. For example, in the meniscus area of the crystallizer at a certain moment, the cooling water on two adjacent surfaces is in a boiling state, while the cooling water on the other two adjacent surfaces is in a non-boiling state. Due to the different cooling intensities and different shell thicknesses, the two adjacent surfaces with strong cooling form an acute angle, and the two adjacent surfaces with weak cooling form an obtuse angle. At the next moment, the two adjacent faces of the crystallizer undergo a boiling transition, causing the stripping direction to change accordingly.

The influence of process factors on the removal of recipes

Effect of chemical components on detoxification

The shrinkage of steel depends on the carbon content and temperature (phase change) of the steel. The linear shrinkage of the high-temperature billet shell has a direct impact on the shedding. When [C]=0.1%, the linear shrinkage of the billet shell in the crystallizer is the largest, and it is also most likely to cause peeling. The reason why high carbon steel is more prone to peeling than low carbon steel is that at 20~50mm below the meniscus, high carbon steel has smaller linear shrinkage than low carbon steel and greater heat flow, which can easily cause intermittent boiling on the outer wall of the copper tube. This results in uneven shell thickness and thinning of the effective shell thickness.

When P in the steel is high, the effective thickness of the billet shell in the crystallizer can be reduced, which increases the heat flow at 20 to 50 mm below the meniscus, making it easy to produce intermittent boiling, and it is easy to detach. P is more likely to desquare high carbon steel than low carbon steel. Because the effective shell thickness of high carbon steel is thinner than that of low carbon steel, the proportion of γ-Fe in the solidification interval of molten steel is large, and the equilibrium distribution coefficient and diffusion coefficient of P in γ-Fe are smaller than those in α-Fe. When [S]<0.025% and Mn/S>30 in steel, it is helpful to slow down stripping.

The influence of the deflection of the tundish nozzle on the stripping

When the injection flow is deflected, it is easy to make the solidified billet shell thinner in the area close to the injection flow, while the solidified billet shell formed in the opposite area is correspondingly thicker, which intensifies the unevenness of the billet shell.

Effect of pouring temperature and pouring speed on stripping

When the molten steel is superheated and the drawing speed is high, the billet is prone to falling off. Because these two factors promote the intermittent boiling of water in the water gap of the crystallizer and the deformation of the shell, and make the thickness of the shell thinner, the strength and stiffness of the shell are reduced. It is easy to deform the green shell under the action of unbalanced force.

Influence of equipment factors on recipe removal

The crystallizer is the throat of the continuous casting machine. If the thickness of the shell of molten steel in the crystallizer is uneven, it will easily break off when it comes out of the crystallizer. If there is an obvious temperature difference between the two corners of the inner arc of the billet at the bottom of the mold, one corner will be bright and the opposite corner will be dark. The corners with bright colors have low cooling intensity and form obtuse angles, while the corners with dark colors have high cooling intensity and form acute angles. At this time, the cast slab has already formed a square shape, which gradually intensifies in the secondary cooling zone. The degree of uneven cooling in the crystallizer and in the secondary cooling zone is variable and may inhibit or aggravate shedding. The cast slab that does not come out of the crystallizer is subject to strong uneven cooling in the secondary cooling zone, which may also cause it to become de-squared. The main equipment influencing factors are:

1. The smaller the inverse taper or the positive taper of the crystallizer copper tube, the more serious the degree of off-squareness.

2. Wear, deformation, uneven inner surface of the mold copper tube, and peeling of the chrome plating layer can all be caused during the billet drawing process. Due to large changes in the air gap and uneven cooling, the thickness of the green billet shell in the crystallizer is uneven, which easily causes and intensifies the desquareness of the billet. The desquareness of the billet is proportional to the number of times the copper tube is used.

3. The impact of the water jacket on the removal of the prescription. If the thickness of the water seam around the copper tube of the crystallizer is uneven and the cooling water velocity is uneven, the heat flux density on the four walls of the copper tube will be different, which will cause uneven cooling of the green shell and lead to desquamation. Before the renovation of our factory in 1996, the crystallizer water jacket was welded with plain carbon steel plates, which was susceptible to corrosion and serious deformation, affecting the size of the water seam. At the same time, if the corners of the water jacket are not rounded, the water flow gap at the corners will be larger than that on the face, which will partially reduce the resistance of the water flow and make the water flow to the corners preferentially, thereby reducing the water flow speed on the face. Generally speaking, the cooling endured by the corners is sufficient, while the face requires high water flow velocity to reduce copper pipe deformation. In addition, the water flow gap opened at the corner has the function of separating the water flow on two adjacent surfaces. Therefore, the velocity at the water inlet is uneven and cannot be adjusted. To ensure a uniform water seam, the rounded corners of the water jacket must be concentric with the rounded corners of the copper pipe.

4. Influence of crystallizer cooling water quality. The quality of the crystallizer cooling water seriously affects the temperature of the copper wall and the deformation of the copper tube. The scale deposited from the cooling water on the outer wall of the copper tube will cause the local thermal resistance of the copper wall to increase, aggravate intermittent boiling and deformation of the copper tube, thereby causing the slab to fall off.

5. The impact of vibration conditions on the removal of prescriptions. Unstable vibration of the crystallizer will cause uneven stress on the slab within the mold, increase the unevenness of the air gap between the slab and the copper wall of the mold, and lead to uneven heat transfer. This aggravates the uneven thickness of the shell and intensifies the peeling.

6. Wear and deformation of the guide section can cause the casting billet to deviate from the arc, resulting in uneven stress between the casting billet and the copper tube, and uneven air gaps between the billet shell and the copper tube. Affects the uneven local heat transfer and aggravates the problem.

7. The desquared slab develops rapidly during the initial period of entering the secondary cooling zone. If the centering of the secondary cooling section is not good, the arc alignment is inaccurate, and the nozzle is clogged or fallen off, it will aggravate the degree of shedding.

8. If the pressure of the tension and leveling machine is too high, it will cause the cast slab to be non-square (rectangular); if the tension and leveling rollers are not parallel, it will also cause the cast slab to be off-square.

Countermeasures to prevent prescription loss

It is necessary to ensure the normal connection of continuous casting molten steel, ensure the argon blowing treatment of the molten steel to achieve uniform molten steel temperature and composition, and ensure the relative stability of the molten steel temperature in the tundish.

Stabilize the height of the tundish liquid level and the crystallizer liquid level, stabilize the pulling speed, and ensure the centering of the nozzle.

The crystallizer cooling water must be softened to meet the crystallizer water quality requirements.

Strengthen the production, installation, use and maintenance of the crystallizer, especially to ensure uniform water seam thickness and regular measurement of the inverse taper of the crystallizer.

Strengthen the inspection of vibration systems to ensure the quality of maintenance and installation.

Carefully carry out the backflush and sewage discharge work of the secondary cooling water filter and the regular slag discharge of the secondary cooling standpipe. Strengthen the management of the secondary cooling section to ensure that the secondary cooling standpipe is accurately aligned and the nozzles are not blocked or falling off.

Check the guide section frequently to ensure the normal curvature of the casting machine.

Strengthen inspections of hydraulic stations and car pulling machines, and promptly replace seriously worn tensioning and straightening rollers.

Conclusion

Billet stripping is a common shape defect in billet continuous casting, which is the result of a combination of process and equipment factors. The root cause of desquamation is the uneven cooling of the slab in the crystallizer, which is also related to the uneven cooling in the secondary cooling zone. The crystallizer is the internal cause of deformation, and the process factors are the external factors that affect depreciation. By strengthening the management of crystallizers and equipment maintenance, optimizing process parameters, and stricting process disciplines, the occurrence of crystallization can be effectively prevented.

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