Analysis of De-squareness Defects in Continuous Casting Billet

Aiming at the problem of de-squareness of continuous casting small square billet, the causes of de-squareness defect were analyzed, and measures such as improving the structure of crystallizer and strengthening the uniform cooling of secondary cooling water were proposed to achieve the purpose of eliminating the de-squareness defect of the casting billet.

A steel plant currently has three four-machine four-stream continuous casting machines, two of which have an arc radius of 5250mm and one has an arc radius of 6000mm. The casting section is mainly 120mm×120mm square billets. The casting steel types are mainly carbon structural steel and low alloy steel.
During the production process, the quality defect of billet de-squareness sometimes occurs. Billet de-squareness is a shape defect unique to billet continuous casting and is a quality problem for small billet continuous casting.

The mechanism and harm of detoxification

De-squareness (also known as rhombus deformation) refers to the tangential deformation of a square billet (or rectangular billet) (one angle is less than 90°, and the other angle is greater than 90°). The difference between the two diagonals of the billet section is usually used to indicate the magnitude of the de-squareness deformation.

De-squared billets have adjacent acute and obtuse angles on the cross section, the length of the diagonals are not equal, depressions appear at the obtuse angles, and cracks perpendicular to the billet surface and along the diagonals appear under the skin. The length of the cracks increases linearly with the degree of de-squaredness. De-squared billets often have depressions between the obtuse angles and cracks, and bulges between the acute angles and cracks. When the de-squaredness is deep, there are bulges on all four sides to varying degrees.

The mechanism of billet de-squareness is that when high-temperature molten steel is injected into the crystallizer for forced cooling, during the solidification of the molten steel, the corner area of ​​the crystallizer is a two-dimensional heat transfer, where the billet shell solidifies fastest, shrinks earlier and forms air gaps earlier, which hinders the heat transfer of the billet shell. When the billet shell leaves the crystallizer, the corner is the thinnest and most likely to induce deformation, which is the prerequisite for the small square billet to de-square easily.

When the primary shell of the meniscus of the crystallizer is cooled unevenly on all sides in the crystallizer, the shell grows unevenly, and after leaving the crystallizer, it will be out of square due to different solidification shrinkage and the static pressure of molten steel. If the billet enters the secondary cooling zone and is cooled unevenly, the billet will be more square; in another case, even if the primary shell of the billet is cooled evenly on all sides in the crystallizer, if the billet enters the secondary cooling zone and is cooled unevenly, the billet is also prone to square. In short, the main cause of the continuous casting billet out of square is the uneven heat transfer around the crystallizer or the uneven heat transfer in the secondary cooling zone.

The hazards of billet de-squareness are as follows:

• Square off induces internal cracks in the ingot, especially diagonal cracks;
• Square off is accompanied by a certain degree of bulging, which will cause steel leakage accidents and affect the normal production of the casting machine;
• Square off will make it difficult to lift the ingot, and loose clamps during lifting are likely to damage equipment and cause safety accidents;
• Square off will cause the heating furnace to push and reverse the steel, making it difficult for the ingot to bite into the mill hole, which brings certain difficulties to rolling, and is easy to twist and cause folding defects.

Factors Affecting the De-squareness of Billets

• Copper mould tube

During the steel drawing process, the shell of the upper part of the crystallizer is thinner. Under the action of the static pressure of the molten steel, the shell sticks to the wall of the crystallizer, the heat transfer effect between the shell and the crystallizer is better, and the shell grows faster; in the lower part of the crystallizer, when the shell thickness continues to increase to be able to withstand the static pressure of the molten steel, the shell shrinks and the crystallizer wall and the shell are out of contact, resulting in an air gap.

As the billet drawing movement progresses, the air gap between the billet shell and the crystallizer wall gradually increases.
At this time, in the heat transfer between the molten steel and the crystallizer, the air gap becomes a limiting link in the cooling heat transfer, which greatly affects the heat transfer from the solidified shell to the crystallizer.
The copper tube of the crystallizer is made with an appropriate inverted taper to adapt to the contraction of the billet shell, which is conducive to delaying the formation of the air gap and reducing the width of the air gap, thereby increasing the heat flow and improving the heat transfer.
However, as the number of times the crystallizer is used increases, the inner cavity size of the copper tube of the crystallizer is also constantly changing, resulting in changes in the taper and even shape of the crystallizer, affecting the heat transfer of the crystallizer and causing de-squareness.

As shown in the figure, the copper tube inverted taper curve is drawn based on the measured data after the planned unloading of the 802 furnace (5614 tons of steel). The copper tube taper basically maintains a parabolic curve, which conforms to the contraction law of the casting billet. The casting billet of the crystallizer has no de-squareness.

As shown in the figure, the copper tube back taper curve is drawn based on the measured data after the 423 furnace (2961 tons of steel) was removed from the machine due to de-squareness. It can be seen that within the range of 500mm and nearly 350mm below the upper end of the copper tube, the copper tube taper is very small, almost forming a smooth straight line. With such a small back taper at the bottom of the copper tube, the ingot shell shrinks and breaks away from the crystallizer wall, resulting in a large air gap and weakening the heat transfer of the copper wall. The shell becomes thinner and uneven, and the de-squareness is aggravated under the action of secondary cooling after leaving the crystallizer.

• Crystallizer water gap:

The cooling of molten steel in the crystallizer is carried out by heat exchange with the cooling water in the crystallizer. The flow density of water in the water gap strongly affects the heat transfer coefficient and is an important factor in ensuring cooling capacity. Uneven water gaps will lead to uneven cooling of the crystallizer copper tube. As shown in the figure.

The inner water jacket is made of stainless steel by stamping, and the copper tube is installed in the inner water jacket. The size of the water gap is adjusted by the inner water jacket adjusting screws. If the water jacket is deformed or the size of the copper tube is inconsistent, resulting in uneven water gaps, the water flow on the narrow side is lower than that on other sides, taking away less heat, and having a poor cooling effect. The ingot cannot form a uniform shell in the crystallizer, causing the ingot to de-square.

At the same time, the two-dimensional heat transfer at the corner of the crystallizer has strict requirements on the uniformity of cooling. As shown in the figure, the water jacket with rounded corners has small water gaps at the corners. Since the narrow water gaps have relatively strict precision requirements, and the precision of the corners is difficult to ensure during the actual installation process, the corner cooling is very likely to cause unevenness, and the casting is seriously out of square. The water gap area of ​​the right-angle water jacket is more than doubled than that of the rounded corner water jacket. Under the same water pressure, the flow rate increases in proportion to the area, increasing the water flow at the corners, which relatively offsets the impact of the uneven narrow water gaps at the corners.

• Crystallizer water quality:

Although the cooling water flow rate in the water seam of the crystallizer is very high, the water film flow rate along the outer wall of the copper tube is relatively low due to the boundary effect during the flow process. If the water hardness is high, Ca, Mg and other ions in the water are easily deposited to form scale, affecting the heat transfer efficiency of the copper tube. At the same time, the temperature of the cold and hot surfaces will be uneven, and even excessive heat accumulation will occur locally, causing permanent deformation of the copper tube. Severe local boiling will also cause permanent deformation of the crystallizer water jacket.

Second cooling zone cooling

From molten steel to billet, 80% of the molten steel solidifies in the secondary cooling zone, which is the main area that aggravates the deformation of the billet. Therefore, for the de-square defect, the cooling uniformity of the secondary cooling section is equally important. In actual production, the billet often deviates due to the difficulty of arcing of small square billets and the wear of rollers (support rollers, idlers, and straightening rollers). In addition, the working environment of the secondary cooling section is relatively bad and the influence of production accidents such as steel leakage, the deformation of the secondary cooling spray pipe, the blockage and shedding of the nozzle, etc., greatly affect the cooling uniformity of the secondary cooling zone. At this time, if the billet is de-squared when it comes out of the crystallizer, it will rapidly deteriorate in the secondary cooling zone and even cause waste.

Other process factors

• Temperature and drawing speed:

The higher the superheat of the molten steel, the greater the chance of de-squareness. The heat conduction of the molten steel with high superheat is large, which easily causes uneven heat flow on the cross section of the ingot, thus leading to the formation of de-squareness defects. The increase in drawing speed will increase the heat flux density. High heat flow is very likely to cause uneven heat conduction in the cross section direction of the ingot, thereby causing uneven growth of the ingot shell in different directions on the cross section, inducing de-squareness. The mismatch between the drawing speed and the superheat will aggravate the de-squareness of the ingot, accompanied by bulging of the ingot. This has been confirmed in actual production.

• Nozzle alignment:

When molten steel is injected into the crystallizer through the tundish nozzle, whether the position of the impact point is aligned and the height of the liquid level in the crystallizer will directly affect the heat flow distribution in the crystallizer. Misalignment of the nozzle will cause uneven growth of the billet shell in the crystallizer and de-squareness of the billet.

• Influence of steel type:

ifferent steel types have different shrinkage rates due to differences in chemical composition. When the carbon content of steel types is different, the shrinkage of the shell on the meniscus is very different. The carbon content of Q235B, a steel type that is easy to de-square, is 0.12-0.17%. It is a peritectic steel. The linear shrinkage of the shell is large. The gap formed between the meniscus and the inner wall of the copper tube is large and irregular, which leads to uneven solidification of the primary shell and de-square. The linear shrinkage of low-carbon steel is greater than that of medium and high carbon steel, so the de-square rate of low-carbon steel is greater than that of medium and high carbon steel. The high sulfur content in carbon steel leads to better heat transfer. When the sulfur content in the steel is greater than 0.03%, de-square is more likely to occur, and the de-square exceeding standard rate is significantly increased.

Improvement measures

Selection of copper mould tube

The copper tube of the crystallizer plays an important role in whether the billet is out of square. According to the actual production situation, a crystallizer with appropriate taper is selected to conform to the solidification shrinkage law of the billet, thereby reducing the occurrence of air gap sections and ensuring the uniformity of the billet shell.

Organize relevant professional and technical personnel to measure the newly-added copper tubes one by one and keep track of the relevant process parameters of each copper tube in time. Before installation and use, the water seams around the crystallizer must be measured and the water seam width must be adjusted. If the copper tube positioning pins are loose or fall off, they should be handled in time to ensure that the uneven error is less than 0.1mm;
At the same time, the copper tube and water jacket are strictly inspected and there should be no deformation or scaling. At the same time, the inspection and management system of the crystallizer is improved. The replaced crystallizer ensures that the inverted taper of each copper tube must be measured, and timely records are made for file management. Replace the copper tube with an inappropriate inverted taper.

If the copper tube is severely deformed, once it exceeds the standard, it must be replaced with a new copper tube regardless of how long it has been used. At the same time, improve the structure of the copper tube of the crystallizer, such as using a right-angle water jacket.
In addition, attention should be paid to improving the quality of cooling water, and water quality should be tested and treated regularly to ensure that the hardness of soft water used for crystallization meets the standard.

Optimize secondary cooling water distribution

econdary cooling is the root cause of secondary de-squareness after the billet leaves the crystallizer. Therefore, it is necessary to ensure that the secondary cooling water is evenly distributed.

• Replace the severely worn support rollers, guide rollers, and straightening rollers in time to ensure the normal curvature of the casting machine.
• Properly adjust the secondary cooling water flow rate and increase the pressure, thereby increasing the water spray force, increasing the atomization effect, and improving the cooling intensity.
• Strengthen the management, inspection, and maintenance of the secondary cooling section, purchase spray pipes with good rigidity, and ensure accurate arc alignment. When the spray pipe is deformed, it should be calibrated and replaced in time to ensure the arc-matching property of the spray pipe; the nozzle should be checked and cleaned frequently, especially when a steel leakage accident occurs, and cleaning and replacement should be actively arranged; strengthen the assessment of steel leakage accidents of casting operators to reduce the occurrence of steel leakage accidents.
• When the billet is de-squared, replace the small water nozzle, appropriately reduce the pulling speed, weaken the secondary cooling intensity, etc., which can reduce the degree of de-squareness of the billet.

Constant pulling speed casting

Taking raw material quality, production organization, equipment operation, smelting, production preparation, steel pouring and other processes as the starting point, constant speed casting is fully implemented.

Improve other process parameters

• Reduce the superheat of the molten steel. The temperature of the molten steel in the tundish should be controlled at 10-20℃ above the solidification point.
• Select the appropriate pulling speed according to specific factors such as superheat.
• When aligning the tundish water inlet with the crystallizer, reduce the center deviation of the crystallizer. If the deviation is too large, it will also cause uneven growth of the billet shell. The center deviation between the tundish water inlet and the crystallizer is required to be less than 3mm.
• Control the composition of the molten steel, especially the carbon content.

Conclusion

• Most of the square removal is caused by the non-uniform cooling of the molten steel in the crystallizer. It is necessary to comprehensively consider and optimize the various factors that affect the uniform cooling of the crystallizer. Use copper tubes with reasonable parameters such as water seam size and taper to improve the quality of cooling water, control the thickness of the billet shell, and effectively control the shape of the billet.
• Uneven secondary cooling water will lead to secondary square removal after the billet leaves the crystallizer; strong uneven distribution of secondary cooling water directly leads to square removal of the billet.
• The square removal of the billet is caused by many factors. Strengthening the control of molten steel composition, superheat, pulling speed, etc. has a certain effect on improving the square removal of the billet.