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[Original Article] Research and Analysis on Defects of Continuous Casting Billets

[vc_row][vc_column][vc_column_text]There are three four-machine four-strand continuous casters in the second steelmaking plant of Anyang Iron and Steel Co., Ltd., two arc-shaped radii of 5250mm, and one arc-shaped

In the production process, the quality defects of the cast slab fall off sometimes appear. As a unique shape of billet continuous casting shape defect is a quality problem of billet continuous casting[/vc_column_text][/vc_column][/vc_row][vc_row][vc_column][vc_column_text]

  • The mechanism and harm of leaving party

Cast billet de-square (also known as rhombohedral deformation) refers to the tangential deformation of the billet (or rectangular billet) (one pair of angles is less than 90°, and the other is large at 90°). The difference between the two diagonals of the cast slab section is often used to indicate the size of the out-of-square deformation. On the cross section of the cast slab.

Research and Analysis on Defects of Continuous Casting Billets

 

The cast slab has adjacent acute and obtuse angles on the cross section, and the diagonal lengths are not equal, and they appear at the obtuse angle. It is dented, and there are cracks perpendicular to the surface of the cast slab and along the diagonal line under the skin. The length of the crack and the degree of de-squareness increase linearly relationship. There is often a depression between the obtuse angle and the crack of the cast slab, and the bulge appears between the acute angle and the crack. There are different degrees of bulging.

 

The mechanism of casting billet splitting is when high-temperature molten steel is injected into the copper mould tube for forced cooling.during the solidification of molten steel,the corner area of the copper mould tube is two dimensional heat transfer,where the billet shell solidifies fastest,shrink earlier and forms air gap earlier,the air gap hinders the heat transfer of the blank shell.when the shell of the billet comes out of the copper mould tube,the corners are the thinnest and the deformation is most likely to be induced,this is the prerequisite for the billet to easily fall off.

 

When the primary blank shell of the meniscus of the mold is cooled unevenly on all sides of the mold, the blank shell grows unevenly. After exiting the mold, the blank shell is separated due to the different solidification shrinkage and the hydrostatic pressure of the steel. If the slab enters the secondary cooling zone and cools unevenly, it will aggravate the cast squaring; in addition, even if the primary shell is cooled evenly on all sides of the mold, if the slab enters the secondary cooling zone and cools unevenly, the cast slab will also be cooled. It is easy to get out of the way. In short, the cause of continuous casting billet squaring is mainly caused by uneven heat transfer around the mold or uneven heat transfer in the secondary cooling zone.

 

The hazards of cast slab falling off have the following aspects:

(1) De-square induced internal cracks in the cast slab, especially diagonal cracks;

(2) Leaving the square and accompanied by a certain degree of bulging will cause a steel breakout accident and affect the normal production of the casting machine;

(3) Leaving the side will cause difficulty in lifting the casting billet. During the lifting process, the loose clamp will easily damage the equipment and cause safety accidents;

(4) De-square will cause the phenomenon of reheating furnace pushing steel and falling steel. The casting billet is difficult to bite into the rolling mill pass, which brings certain difficulties to rolling, and it is easy to twist and cause folding defects.

 

  • Factors affecting the de-square billet

1.  copper mould tube

(1) Copper mould tube taper: During the drawing process, the upper shell of the mold is relatively thin. Under the action of the hydrostatic pressure of the steel, The shell is attached to the mold wall, the heat transfer effect between the shell and the mold is better, and the shell grows faster; in the lower part of the mold, when the thickness of the blank shell continues to increase to resist the static pressure of the molten steel,due to the shrinkage of the billet shell,the mold wall and the billet shell are out of contact and an air gap is generated.

 

The air gap between the shell of the blank and the wall of the mold gradually increases as the drawing movement progresses. At this time, in the heat transfer between the molten steel and the mold, the air gap becomes a restrictive part of cooling and heat transfer, which greatly affects the heat transfer from the condensate to the mold. The mold copper tube is made with an appropriate inverted taper to adapt it to the shrinkage of the blank shell to delay the formation of the air gap and reduce the width of the air gap, thereby increasing heat flow and improving heat transfer. However, with the continuous increase in the use of the copper mould tube, the size of the cavity of the copper tube of the copper mould tube is also constantly changing, which leads to changes in the taper and even shape of the copper mould tube which affects the heat transfer of the copper mould tube and causes a square off.

 

The inverted taper curve of the copper pipe shown in Figure  is drawn based on the measured data after the planned shutdown of the 802 furnace (with a steel volume of 5614 tons). Figure  The taper of the copper tube basically maintains the parabolic curve, which conforms to the shrinkage law of the slab, and the slab of the mold does not fall off.

Research and Analysis on Defects of Continuous Casting Billets

Research and Analysis on Defects of Continuous Casting Billets

 

The inverted taper curve of the copper pipe shown in Fig.  is drawn based on the measured data after the 423 furnace (with a steel volume of 2961 tons) was off the machine due to the square off. It can be seen that the taper of the copper pipe is very small in the range of 350mm below 500mm from the copper pipe mouth, almost forming a smooth straight line. With such a small inverted taper in the lower part of the copper tube, the billet shell shrinks and disengages from the mold wall, resulting in a large air gap, which weakens the heat transfer of the copper wall, and the billet shell becomes thin and uneven. After exiting the copper mould tube,the squaring is intensified under the action of the secondary cooling.

 

(2) Mold water gap: The cooling of molten steel in the mold is carried out by heat exchange with the cooling water in the mold. The flow density of water in the water gap strongly affects the heat transfer coefficient, which is an important factor to ensure the cooling capacity. The uneven water gap will cause uneven cooling of the mold copper tube. As shown in Figure

Research and Analysis on Defects of Continuous Casting Billets

 

The inner water jacket is formed by stamping and forming of stainless steel. The copper pipe is installed in the inner water jacket. The water gap is adjusted by the inner water jacket adjusting screw. If the water jacket is deformed or the copper pipe size is the same, the water gap will be uneven and the water flow on the narrow side will be uneven. Lower than other surfaces, the heat taken away is less, the cooling effect is poor, and the cast billet cannot form a uniform shell in the mold, causing the cast billet to fall off.

 

At the same time, the two-dimensional heat transfer at the corners of the mold has strict requirements on the uniformity of cooling. Figure 5shows a water jacket with rounded corners. This kind of water jacket has small water gaps at the corners. Because the narrow water gaps have relatively strict accuracy requirements, the accuracy of the corners is difficult to guarantee during the actual installation process, and the corner cooling is easy to cause unevenness , The cast slab de-square is very serious. The right-angle water jacket in Figure  has more than doubled the area of the corner water gap than the corner area of the fillet water jacket. Under the same water pressure, the flow rate increases in proportion to the area, which increases the water at the corner. The flow, relatively speaking, offsets the influence of the unevenness of the narrow water gap at the corner.

 

Research and Analysis on Defects of Continuous Casting Billets

 

(3) Copper mould tube  water quality: Although the cooling water flow rate in the water gap of the copper mould tube is high, the flow rate of the water film along the outer wall of the copper pipe is low due to the boundary effect during the flow process. If the water hardness is high, the Ca and Mg plasma in the water will easily deposit and form scale, which will affect the heat transfer efficiency of the copper pipe. At the same time, the temperature of the cold and hot surfaces will be uneven, and even lead to excessive accumulation of heat locally, resulting in permanent deformation of the copper tube, and serious local boiling will cause permanent deformation of the mold water jacket.

 

2. Cooling in the second cold zone

From molten steel to slab, 80% of the molten steel is solidified in the secondary cold zone, which is the main area that aggravates the deformation of the slab. Therefore, the cooling uniformity of the secondary cooling section is equally important for the defect of square off. In actual production, the billet often deviates due to the difficulty of arc alignment and the wear of the roller table (support roller, idler roller, pull-straightening roller), etc., and the working environment of the second cold section is relatively harsh and the production of steel leaks. The impact of the accident, the deformation of the second cold spray water pipe, nozzle blockage, and falling off, to a large extent affect the uniformity of cooling in the second cold zone. At this time, if the casting slab exits the mold when there is de-square deformation, It will deteriorate rapidly in the second cold zone and even cause waste.

 

3. Other process factors:

(1) Temperature and drawing speed: The higher the degree of superheat of molten steel, the greater the probability of falling square. The high superheated molten steel has a large heat output, which is easy to cause uneven heat flow on the section of the cast slab, which leads to the formation of detachment defects. The increase of the drawing speed will increase the heat flux density, and the high heat flux is very easy to cause the uneven heat output in the section direction of the cast slab, which will cause the uneven growth of the slab shell in different directions on the section, and induce the generation of detachment. The mismatch between the drawing speed and the degree of overheating will aggravate the cast blank and bulge. This has been confirmed in actual production.

 

(2)Nozzle alignment: When molten steel is injected into the mold through the nozzle of the intermediate tank, whether the position of 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, and the misalignment of the nozzle will cause the billet shell The growth in the crystallizer is uneven, and the cast billet falls off.

 

(3) The influence of steel grade: The shrinkage rate of steel is inconsistent due to the difference in chemical composition of various steel grades. When the carbon content of steel grade is different, the shrinkage of the slab shell in the meniscus varies greatly. The easy-to-square steel Q235B has a carbon content of 0.12-0.17%. It belongs to peritectic steel. The shell line shrinks greatly. The gap between the meniscus and the inner wall of the copper tube is large and irregular, which causes the primary shell to not solidify. The de-square formation is uniform, and the shrinkage of low-carbon steel wire 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 steel is greater than 0.03%, it is more prone to squaring, and the rate of squaring exceeding the standard is significantly increased.

 

  • improvement measures

1.Selection of copper mould tube

The mold copper tube plays an important role in whether the cast slab is out of square. According to the actual production situation, choose a mold with a suitable taper to make it conform to the solidification and shrinkage law of the cast slab, thereby reducing the occurrence of air gaps and ensuring the casting The uniformity of the blank shell. Organize relevant professional and technical personnel to measure the new copper tubes one by one, and grasp the relevant process parameters of each copper tube in time. Before installation and use, the water gap around the mold must be measured, and the width of the water gap must be adjusted. If the copper pipe positioning pin is loose or falling off, it should be treated in time to ensure that the uneven error is less than 0.1mm; at the same time, the copper pipe and water jacket should be strictly inspected , There should be no deformation and no scaling. At the same time, improve the mold inspection management system. The replaced mold ensures that the inverted taper of each copper tube must be measured, and records are made in time and file management is carried out. If the inverted taper of the copper pipe is not suitable, replace the new copper pipe. If the copper pipe is severely deformed, once it exceeds the standard, it will be replaced no matter how long it is used. At the same time, the structure of the mold copper tube is improved, such as the use of a right-angle water jacket. In addition, pay attention to improving the cooling water. The water quality is tested and treated regularly to ensure that the hardness of the soft water used for crystallization reaches the standard. Figure

Research and Analysis on Defects of Continuous Casting Billets

 

2. Optimize the distribution of secondary cooling water

The secondary cooling is the root cause of the secondary de-square after the cast slab exits the mold. Therefore, the uniform distribution of the secondary cooling water must be ensured.

 

(1) Replace the severely worn support rollers, guide rollers, and pull-straightening rollers in time to ensure the normal curvature of the casting machine.

(2) Properly adjust the flow of the secondary cooling water and increase the pressure, thereby increasing the water spraying force, increasing the atomization effect and increasing the cooling intensity.

(3) Strengthen the management, inspection and maintenance of the second cold section, choose a spray pipe with good rigidity and ensure accurate arc alignment. When the spray pipe is deformed, it should be checked and replaced in time to ensure the arc-like performance of the spray pipe; the nozzle should be checked and cleaned frequently, especially in the event of a steel breakout, actively arrange for cleaning and replacement; strengthen the casting operator Accident assessment to reduce the occurrence of steel leakage accidents

(4) When the cast slab is de-squared, replace the small nozzle to appropriately reduce the drawing speed, weakening = secondary cooling strength, etc., which can reduce the degree of cast slab de-square.

 

3.Constant drawing speed pouring

Our factory focuses on the quality of raw materials, production organization, equipment operation, smelting, production preparation, steel casting and other workers, and fully implements constant speed casting. (The following table shows the percentage of constant pulling speed in each quarter in 2014) Figure

Research and Analysis on Defects of Continuous Casting Billets

 

It can be seen from the above table that the constant drawing speed ratio has increased quarter by season, and the constant drawing speed ratio has increased to ensure that production goes forward, and it is also beneficial to reduce cast blanks.

 

4. Improve other process parameters

(1) Reduce the superheat of molten steel, and control the temperature of tundish molten steel at 10-20°C above the freezing point.

(2) Choose an appropriate pulling speed according to specific factors such as overheating.

(3) When the tundish nozzle is aligned with the mold, the deviation of the mold center should be reduced. If the deviation is too large, the green shell will grow.

The uneven length requires that the centering deviation between the tundish nozzle and the copper mould tube is less than 3mm.

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

 

  • Conclusion

  1. Most of the deviations are due to the non-uniform cooling of molten steel in the mold, and various factors affecting the uniform cooling of the mold must be comprehensively considered and optimized. Using copper pipes with reasonable parameters such as water gap size and taper to improve the cooling water quality, control the thickness of the billet shell, and effectively control the shape of the cast billet.
  2. The unevenness of the secondary cooling water will lead to secondary de-squares after the cast slab exits the mold; the strong uneven distribution of the secondary cold water will directly lead to the de-squares of the cast slab.
  3. The slab cast-off is caused by many factors. Strengthening the control of the molten steel composition, superheat, drawing speed, etc., has a certain effect on improving the cast slab cast-off.

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