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Research and analysis of stripping ccm billet defects

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In view of the problem of stripping of continuous cast billets, the causes of stripping defects were analyzed, and measures such as improving the crystallizer structure and strengthening the uniform cooling of secondary cooling water were proposed to eliminate the stripping defects of billets.

Anyang Steel’s Second Steelmaking Plant currently has three four-machine four-strand continuous casters, two with an arc radius of 5250mm and one with an arc radius of 6000mm. The pouring section is mainly 120mm×120mm billet. The types of cast steel are mainly carbon structural steel and low alloy steel. During the production process, sometimes quality defects such as slab stripping may occur. As a unique shape defect of billet continuous casting, billet peeling is a quality problem in billet continuous casting.

The mechanism and hazards of taking off prescriptions

The square billet (also known as rhombus deformation) refers to the tangential deformation of the square billet (or rectangular billet) (one angle is less than 90°, and the other opposite angle is greater than 90°). The difference between the two diagonal lines of the cast slab section is often used to indicate the size of the deformation.

The square cast slab has adjacent acute and obtuse angles on the cross section, and the diagonal lengths are not equal. Depressions appear at obtuse angles, and cracks appear vertically on the surface of the cast slab and along the diagonal lines under the skin. The length of the cracks increases linearly with the degree of off-squareness. Depressions often appear between the obtuse angles and cracks of the desquared slab, and bulges appear between the acute angles and cracks. When the slab is deeply desquared, there are varying degrees of bulging on all sides.

he mechanism of slab stripping is when high-temperature molten steel is injected into the crystallizer for forced cooling. During the solidification process of molten steel, there is two-dimensional heat transfer in the corner area of the mold, where the billet shell solidifies the fastest, shrinks earlier and forms an air gap earlier. The air gap blocks the heat transfer of the billet shell. When the billet comes out of the mold, the corners are the thinnest and most likely to induce deformation. This is a prerequisite for the billet to easily come off. ​​​​

When the green shell of the meniscus of the mold is cooled unevenly on all sides in the crystallizer, the shell will grow unevenly and will fall off after leaving the mold due to different solidification shrinkage and the static pressure of molten steel. If the billet enters the secondary cooling zone and is cooled unevenly, it will aggravate the desquatting of the billet; in other cases, even if the primary billet shell is cooled evenly on all sides in the crystallizer. If the billet enters the secondary cooling zone and is cooled unevenly, the billet will also easily become de-squared. In short, the cause of stripping of continuous cast slabs is mainly caused by uneven heat transfer around the crystallizer or uneven heat transfer in the secondary cooling zone.

The hazards of cast slab stripping include the following aspects:

(1) Desquareness induces internal cracks in the cast slab, especially diagonal cracks;

(2) Stripping and accompanied by a certain degree of bulging will cause steel leakage accidents and affect the normal production of the casting machine;

(3) The removal of the slab will cause difficulty in lifting the slab, and loose clamping during the lifting process can easily damage equipment and cause safety accidents;

(4) The stripping will cause the steel to be pushed backwards in the heating furnace, making it difficult for the billet to bite into the pass of the rolling mill, causing certain difficulties in rolling, and it is easy to twist and cause folding defects.

Factors affecting billet stripping

1. Copper mould tube

(1) Copper mould tube taper: During the steel drawing process, the upper shell 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, and the heat transfer effect between the shell and the crystallizer is better. The billet shell grows rapidly; in the lower part of the mold, when the thickness of the billet shell continues to increase to resist the static pressure of molten steel, due to the shrinkage of the billet shell, the mold wall and the billet shell are out of contact, resulting in an air gap. ​​​​

As the billet drawing movement proceeds, 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 restrictive link in the cooling heat transfer, which greatly affects the heat transfer from the solidification shell to the crystallizer. The mold copper tube is made with an appropriate inverse taper to adapt to the shrinkage of the billet shell, which is beneficial to delaying the formation of air gaps and reducing the width of the air gaps, thereby increasing heat flow and improving heat transfer. However, as the number of times the crystallizer is used continues to increase, the size of the inner cavity of the crystallizer copper tube 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 desquamation.

The inverse taper curve of the copper pipe shown in the figure is drawn based on the actual measured data after the 802 furnace (5614 tons of steel production) was planned to be off the machine. The taper of the copper tube in the figure basically maintains a parabolic curve, which is in line with the shrinkage law of the cast slab. The cast slab of this mold is not off-square.

Research and analysis of stripping ccm billet defects

The inverse taper curve of the copper pipe shown in the figure is drawn based on actual measured data after using furnace 423 (steel production capacity of 2961 tons) after being removed from the machine due to stripping. It can be seen that within the range of nearly 350mm below 500mm from the upper mouth of the copper pipe, the taper of the copper pipe is very small, almost forming a smooth straight line. With such a small inverse taper in the lower part of the copper tube, the shell of the casting billet shrinks and breaks away from the crystallizer wall, resulting in a large air gap, which weakens the heat transfer of the copper wall. The shell becomes thin and uneven. Loss of prescription is aggravated by cold.

Research and analysis of stripping ccm billet defects

(2) Crystallizer water gap:

The cooling of the 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 the cooling capacity. Uneven water gaps will cause uneven cooling of the crystallizer copper tubes.

The inner water jacket is stamped and formed from stainless steel, and the copper pipe is installed in the inner water jacket. The size of the water gap can be adjusted by the inner water jacket adjustment screw. If the water jacket is deformed or the copper pipe size is inconsistent, the water seam will be uneven. The water flow rate on the narrow side is lower than that on other sides, which takes away less heat and has poor cooling effect. The slab cannot form a uniform shell in the crystallizer, causing the slab to fall off.                      

At the same time, the two-dimensional heat transfer at the corners of the crystallizer has strict requirements on the uniformity of cooling. Rounded corner water jacket, this kind of water jacket has small water seams at the corners. Since the narrow water seam has relatively strict accuracy requirements, it is difficult to guarantee the accuracy of the corners during the actual installation process. The cooling of the corners can easily cause unevenness, and the cast slab can become desquared very seriously. The area of the water seam at the corner of the right-angle water jacket is more than double that of the corner area of the rounded water jacket. Under the same water pressure, the flow rate increases in proportion to the area, which increases the water flow rate at the corners, which relatively offsets the impact of uneven narrow water gaps at the corners.

(3) Crystallizer water quality:

Although the cooling water flow rate in the water gap of the crystallizer is high, the flow rate of the water film along the outer wall of the copper tube is low due to the boundary effect during the flow process. If the water hardness is high, Ca and Mg plasma in the water will easily deposit to form scale, which will affect the heat transfer efficiency of the copper tube. At the same time, the temperature of the cold and hot surfaces will be uneven, which may even lead to excessive local accumulation of heat, causing permanent deformation of the copper tube. Severe local boiling will also lead to permanent deformation of the crystallizer water jacket.

2. Cooling in the secondary cooling zone

From the molten steel to the cast slab, 80% of the molten steel solidifies in the secondary cooling zone, which is the main area that aggravates the deformation of the cast slab. Therefore, the cooling uniformity of the secondary cooling section is equally important for shedding defects. In actual production, due to the difficulty of arc alignment of the billet and the wear of the roller table (support roller, idler roller, tension and straightening roller) and other reasons, the billet often deviates. In addition to the harsh working environment of the secondary cooling section and the impact of production accidents such as steel leakage, the secondary cooling spray water pipes are deformed, and the nozzles are clogged and fallen off. This affects the uniformity of cooling in the secondary cooling zone to a great extent. At this time, if the cast slab is deformed when it comes out of the mold, it will rapidly deteriorate in the secondary cooling zone and even cause scrap.

3. Other process factors:

(1) Temperature and drawing speed: The higher the superheat of the molten steel, the greater the chance of stripping. Highly superheated molten steel has a large amount of heat transfer, which can easily cause uneven heat flow on the cross section of the slab, leading to the formation of square defects. Increasing the drawing speed will increase the heat flow density. High heat flow can easily lead to the uneven conduction of heat in the cross-section direction of the slab, thereby causing uneven growth of the billet shell in different directions on the cross-section, inducing desquamation. The mismatch between the pulling speed and the superheat will aggravate the desquatting of the slab and be accompanied by the bulging of the slab. This has been confirmed in actual production.

(2) Nozzle centering: When molten steel is injected into the crystallizer through the nozzle of the tundish tank. Whether the impact point is centered and the level of the liquid level in the mold will directly affect the heat flow distribution in the mold. Misalignment of the nozzle will cause the shell to grow unevenly in the mold and cause the slab to fall off.                      

(3) Influence of steel type: Due to differences in chemical composition, the shrinkage rate of steel is inconsistent among steel types. When the carbon content of steel types is different, the shrinkage of the billet shell on the meniscus varies greatly. The steel type Q235B that is easy to peel off has a carbon content of 0.12-0.17% and is a peritectic steel. The shell line shrinks greatly, and the gap formed from the meniscus to the inner wall of the copper tube is large and irregular. As a result, the primary green shell solidifies unevenly and forms shedding. The shrinkage of low carbon steel wire is greater than that of medium and high carbon steel, so the shedding 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%, desulfurization is more likely to occur, and the desulfurization exceedance rate increases significantly.

3. Other process factors:

(1) Temperature and drawing speed: The higher the superheat of the molten steel, the greater the chance of stripping. Highly superheated molten steel has a large amount of heat transfer, which can easily cause uneven heat flow on the cross section of the slab, leading to the formation of square defects. Increasing the drawing speed will increase the heat flow density. High heat flow can easily lead to the uneven conduction of heat in the cross-section direction of the slab, thereby causing uneven growth of the billet shell in different directions on the cross-section, inducing desquamation. The mismatch between the pulling speed and the superheat will aggravate the desquatting of the slab and be accompanied by the bulging of the slab. This has been confirmed in actual production.

(2) Nozzle centering: When molten steel is injected into the crystallizer through the nozzle of the tundish tank. Whether the impact point is centered and the level of the liquid level in the mold will directly affect the heat flow distribution in the mold. Misalignment of the nozzle will cause the shell to grow unevenly in the mold and cause the slab to fall off.                      

(3) Influence of steel type: Due to differences in chemical composition, the shrinkage rate of steel is inconsistent among steel types. When the carbon content of steel types is different, the shrinkage of the billet shell on the meniscus varies greatly. The steel type Q235B that is easy to peel off has a carbon content of 0.12-0.17% and is a peritectic steel. The shell line shrinks greatly, and the gap formed from the meniscus to the inner wall of the copper tube is large and irregular. As a result, the primary green shell solidifies unevenly and forms shedding. The shrinkage of low carbon steel wire is greater than that of medium and high carbon steel, so the shedding 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%, desulfurization is more likely to occur, and the desulfurization exceedance rate increases significantly.

Research and analysis of stripping ccm billet defects

4. Improvement measures

(1) Selection of crystallizer

The mold copper tube plays an important role in whether the cast slab will be peeled off. According to the actual production conditions, a mold with appropriate taper is selected to conform to the solidification shrinkage law of the slab, thereby reducing the occurrence of air gaps and ensuring the uniformity of the slab shell. Organize relevant professional and technical personnel to measure the newly entered copper pipes one by one, and grasp the relevant process parameters of each copper pipe in a timely manner. Before installation and use, the water gap around the crystallizer must be measured and the width of the water gap adjusted. If the copper pipe positioning pin is loose or falls off, it should be dealt with in time. Make sure the uneven error is less than 0.1mm. At the same time, the copper pipes and water jackets should be strictly inspected and there should be no deformation or scaling. At the same time, the crystallizer inspection and management system will be improved. The replaced crystallizer ensures that the inverse taper of each copper tube must be measured, recorded in a timely manner, and managed in files. If the inverse taper of the copper pipe is inappropriate, replace it with a new copper pipe. If the copper pipe is severely deformed and exceeds the specification, it must be replaced with a new copper pipe no matter how long it is used. At the same time, the structure of the crystallizer copper tube should be improved, such as using a right-angle water jacket. In addition, attention must be paid to improving the quality of the cooling water, testing and treating the water quality regularly to ensure that the hardness of the soft water used for crystallization reaches the standard.

(2) Optimize the distribution of secondary cold water

Secondary cooling is the root cause of secondary desquatting after the slab comes out of the crystallizer. Therefore, it is necessary to ensure that the secondary cooling water is evenly distributed. ​​​​

Timely replace seriously worn support rollers, guide rollers, and tension and straightening rollers to ensure that the arc of the casting machine is normal. ​​

Appropriately adjust the flow rate of the secondary cold water and increase the pressure, thereby increasing the water spray force, increasing the atomization effect, and increasing 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 must be checked and replaced in time to ensure the arc-like performance of the spray pipe. The nozzles should be inspected and cleaned frequently. Especially when a steel leakage accident occurs, cleaning and replacement must be actively arranged; the assessment of steel leakage accidents of casting operators should be strengthened to reduce the occurrence of steel leakage accidents. ​

When the slab is de-squared, replacing the small nozzle, appropriately reducing the drawing speed, weakening the secondary cooling intensity, etc. can reduce the degree of de-square of the slab.

(3)Constant pulling speed pouring

Constant-speed pouring will be fully implemented starting from raw material quality, production organization, equipment operation, smelting, production preparation, steel pouring process and other processes. Increasing the constant drawing speed ratio ensures smooth production and also helps reduce slab stripping.

5. Improve other process parameters

(1) Reduce the superheat of the molten steel and control the temperature of the molten steel in the middle package to 10-20°C above the freezing point.

(2) Select the appropriate pulling speed based on specific factors such as superheat.                      

(3) When aligning the tundish nozzle and the crystallizer, it is necessary to reduce the center deviation of the crystallizer. If the deviation is too large, the shell will grow unevenly. The center deviation of the tundish nozzle and the crystallizer is required to be less than 3mm.                      

(4) Control the composition of molten steel, especially the carbon content.

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As professional one-stop solution provider, LIAONING MINERAL & METALLURGY GROUP CO., LTD(LMM GROUP) Established in 2007, and focus on engineering research & design, production & delivery, technology transfer, installation & commissioning, construction & building, operation & management for iron, steel & metallurgical industries globally. 

Our product  have been supplied to world’s top steel manufacturer Arcelormittal, TATA Steel, EZZ steel etc. We do OEM for Concast and Danieli for a long time.

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As professional one-stop solution provider, LIAONING MINERAL & METALLURGY GROUP CO., LTD(LMM GROUP) Established in 2007, and focus on engineering research & design, production & delivery, technology transfer, installation & commissioning, construction & building, operation & management for iron, steel & metallurgical industries globally. 

Our product  have been supplied to world’s top steel manufacturer Arcelormittal, TATA Steel, EZZ steel etc. We do OEM for Concast and Danieli for a long time.

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