Optimal design of crystallizer for billet continuous caster

The problems existing in the existing continuous casting machine are pointed out, and the crystallizer solidification heat transfer mechanism and its optimization principles are described, focusing on five aspects of the continuous casting machine: mold length, cooling water supply, taper, structure and vibration parameters. , analyzed the optimal design plan of the crystallizer in detail, and determined the optimal design technical parameters. The design results have been well confirmed in practice.

Keywords: billet; casting machine; optimized design

The main function of the crystallizer is to provide uniform and rapid cooling of the molten steel, promote rapid solidification of the molten steel, and uniform growth of the shell, ensuring that the shell after the copper tube of the crystallizer can withstand the static pressure of the unsolidified molten steel inside. The copper tube length of the existing crystallizer is 850mm, which is relatively short, and the design taper is not reasonable. It must be improved at the same time to better meet production requirements.

The crystallizer vibration device is mainly used to support the crystallizer and cause the crystallizer to vibrate up and down along the arc direction to prevent the molten steel from bonding with the inner wall of the crystallized copper tube during solidification, and to close the cracked shell through negative slipping motion. . The crystallizer vibration device used by most casting machines in my country is a short-arm four-bar linkage mechanism, which has shortcomings such as large deflection, high transmission failure rate, and heavy daily maintenance workload. Therefore, it is particularly urgent to improve the technical parameters of the crystallizer and improve the quality of special steel billets.

Crystallizer heat transfer mechanism and its optimization principles

The molten steel in the mold solidifies. When the stiffness and strength of the billet shell are sufficient to withstand the static pressure of the molten steel, the billet shell begins to separate from the inner wall of the mold, forming an air gap. The air gap increases the thermal resistance and reduces the cooling capacity. From the analysis of the heat transfer effect of the crystallizer, the proportion of various thermal resistances in the crystallizer to the total thermal resistance is: 25% of the billet shell, 71% of the air gap, 1% of the crystallizer wall, and 2% of the crystallizer wall and cooling water interface %. Among various thermal resistances, the air gap thermal resistance controls the cooling effect of the crystallizer. In order to further improve the heat transfer efficiency of the crystallizer, the shell formed in it has sufficient thickness and uniform peripheral thickness when it exits the crystallizer. , the key to improving the solidification conditions of the crystallizer is to change the influence of the air gap, which needs to be considered from two aspects: crystallizer design and improving crystallizer lubrication.

According to the energy balance:

where, t is the time the molten steel stays in the mold (min); v is the pulling speed (m/s); s is the cross-section perimeter of the slab (m); q(t) is the heat flow of the mold Density (W/m²). Through the analysis of micro heat transfer, we get:

Taking special steel as an example, substituting the physical property parameters of steel into:

Taking special steel as an example, substituting the physical parameters of the steel is: K=0.0236m/√min, then K=23.6mm/min. The solidification coefficient K reflects the parabolic law of the solidified shell, but does not consider the influence of the air gap.

Another important factor affecting the cooling capacity of the crystallizer is the water velocity in the water gap of the crystallizer. The design ensures that the water velocity is greater than 10~12m/s. In fact, the impact of the water flow rate in the water gap of the crystallizer on the cooling capacity of the crystallizer has attracted enough attention. Due to the uneven water flow rate in the water gap, uneven cooling is caused, which in turn leads to uneven shell thickness. Important factors affecting the quality of cast slabs.

The design of the crystallizer must ensure that the thickness of the outlet shell meets the strength requirements. The design principles are:

(1) Ensure high-efficiency heat conduction function, that is, high cooling intensity and high cooling efficiency, so that the cast slab can reach a sufficient thickness in the crystallizer;

(2) The heat flow intensity of the crystallizer is uniform, and the uniform heat flow intensity makes the billet shell uniform;

(3) The drawing resistance is small and the drawing speed is uniform;

(4) Ensure long life of the crystallizer, especially the use of copper tube crystallizers.

Crystallizer optimization design

Determination of crystallizer length

Increase the length of the mold to extend the time the primary solidified shell of the slab stays in the mold, and increase the thickness of the shell at the outlet of the mold so that the uniform shell at the outlet of the mold supports the slab itself and reduce steel breakouts.

According to the solidification theory, the crystallizer length is calculated according to the formula:

In the formula, [ ] is the thickness of the mold safety blank shell (mm, take 11mm for small billet, 14mm for bloom); K is the solidification coefficient of the mold (take 22mm/min’); V is the casting speed (mm/min ).

The casting speed reaches 2000mm/min, the thickness of the safety billet shell is calculated as 14mm, and the length of the mold copper tube is 910mm. As the length of the copper tube of the crystallizer increases, the drawing resistance increases, while the cooling effect of the too long crystallizer is not obvious. It is comprehensively determined that the design length of the crystallizer is 900mm. Using a 900mm copper pipe, it can be calculated that the maximum drawing speed of billet is 3.20m/min and the maximum drawing speed of bloom is 1.97m/min.

Determination of crystallizer cooling water flow rate

The cooling water volume is determined based on the design basis of ensuring the water gap flow rate is 10-12m/s. Water volume calculation:

Q=((a+28+2D)(b+28+2D)-(a+28)(b+26))×V×3600/ 1000

In the formula, Q is the cooling water flow rate (m³h); a is the length of the wide side (m); b is the length of the narrow side (m); δ is the thickness of the copper pipe (m); D is the width of the water gap (m); V is Flow velocity (m/s, take 10~12m/s). The designed flow rate of each section is: 150mm×150mm, 105-125 m³/h; 200mm×200mm, 138-165m³/h; 200mm×240mm, 152-182m³h.

Calculation analysis shows that by improving the cooling intensity of the crystallizer and increasing the pulling speed, the way to ensure that the shell at the outlet of the crystallizer has sufficient strength and uniform thickness is: (1) Design a reasonable crystallizer taper to reduce the influence of air gap thermal resistance; (2) Appropriately extend the length of the crystallizer and increase the solidification time of the shell; (3) Improve the lubrication conditions and reduce the resistance to drawing; (4) Improve the accuracy of the water seam in the crystallizer, improve the uniformity of the water flow, and reduce the damage caused by uneven water flow rates. The cooling is uneven.

Determination of crystallizer taper

In order to eliminate or reduce the influence of the air gap, the inner cavity of the crystallizer should be designed with a certain inverse taper according to the different shrinkage properties of the steel types and the degree of thermal deformation of the crystallized copper plate. Research shows that using a crystallizer with a taper (single taper) can increase heat transfer efficiency by 20%. The inner cavity of the continuously tapered crystallizer can better reduce the thickness of the air gap, better meet the process requirements of high-speed continuous casting, and can increase the heat transfer efficiency of the crystallizer by 43%.

For the selection of the initial taper of the crystallizer, factors such as the high-temperature mechanical properties of the billet shell, steel type, drawing speed and lubrication form need to be comprehensively considered. When the pulling speed is high, the molten steel stays in the mold for less time, and the shrinkage is reduced, requiring the taper of the mold to be reduced accordingly; when pouring high carbon steel, the taper of the mold is less affected by changes in the pulling speed, while for low carbon steel is more sensitive. That is to say, when the pulling speed of high carbon steel changes greatly, the adaptability of the mold is strong. When pouring low carbon steel, if the pulling speed changes greatly, the original design taper of the mold is no longer suitable. . The parabolic crystallizer makes the blank shell in contact with the inner wall of the crystallizer without any gaps or extrusion. It obeys the law of maximum thermal deformation of the crystallizer and phase change of the solidified shell. The taper here is designed to be the largest, generally 2.0 %~4.0%, while the lower part is mainly the solidification shrinkage of the solidification shell, so the taper in the lower half of the crystallizer is smaller, generally 0.5%~0.8%. If the upper taper is too small, in addition to causing a large air gap, a negative taper will be generated during the negative slippage of the mold, severely squeezing the green shell, causing heat transfer to reach the limit, causing surface ripples, micro transverse cracks and other defects. Therefore, when pouring low carbon steel, the designed casting speed should be strictly followed. Otherwise, excessive contact between the mold and the shell will cause large friction or a large air gap. High carbon steel requires a larger upper taper, while mild steel requires a larger lower taper. P In summary, it is determined to use continuous taper, with a total taper of 0.9%~1.1%/m.

Adopt exquisite copper water jacket technology

The traditional water gap parameter between the crystallizer copper tube and the water jacket is 6-8. This parameter cannot meet the requirements of high pulling speed. In order to adapt to the increase of heat flow in the crystallizer at high pulling speed, it is necessary to improve the cooling water of the crystallizer. Make corresponding improvements, specific measures: adopt narrow water gap design to create conditions for obtaining higher cooling water flow rate, and use exquisite copper water jacket to ensure uniform cooling water flow rate. The optimized design parameters of the crystallizer are as follows: the copper tube material is deoxidized phosphor copper; the crystallizer taper is parabolic taper; the cadmium tube wall thickness (minimum) is 13mm; the copper tube internal fillet; the meniscus position is 100~150mm; the water seam The width is 4.0mm; the cooling water flow rate is greater than 10~11m/s; the copper pipe fixation method is the upper fixed type; the vibration negative slip time is 0.12~0.15s; the crystallizer advance amount is 3~5mm.

Crystallizer vibration device and vibration parameters

The main function of crystallizer vibration is to remove the blank. The vibration device of the continuous casting machine is designed to be in the form of a half leaf spring, short arm and four links. Its structure is shown in Figure 1:

Figure 1 Structure of crystallizer vibration device

(1 vibration table; 2 connecting rods; 3 reduction gearbox and eccentric shaft; 4 coupling; 5 motor; 6 vibration base; 7 steel plate connecting rod; 8 vibration rod; 9 balance spring)

The vibration device is mainly composed of a fixed frame, connecting rod, vibration frame, transmission device and overload protection device. Each hinge point uses a spherical bearing. It adopts the semi-leaf spring short arm four-link form, which ensures smooth and reliable operation and reduces maintenance. AC frequency conversion control is adopted to ensure that the vibration frequency is automatically adjusted with the pulling speed and saves oil and gas lubrication. From the perspective of mold vibration, to improve the quality of the slab and reduce the depth of vibration marks, it is necessary to reduce the negative slip time (tw). In addition, increasing the positive slip time (tp) is beneficial to reducing crystallization friction resistance. Therefore, in order to better control the bonded steel breakout, under the same vibration and pulling speed, using non-sinusoidal vibration can obtain lower negative slippage time and higher positive slippage time than sinusoidal vibration. The selection principles for determining vibration parameters: First, the negative slip time tw>0, and the change is smooth; second, under the same pulling speed conditions, the lower the tw, the better, and the higher the better.

In order to ensure negative slippage vibration, there is always a minimum frequency requirement under certain pulling speed conditions. At the same pulling speed, the minimum vibration frequency required for non-sinusoidal vibration is smaller, that is, the maximum allowable pulling speed for non-sinusoidal vibration under certain amplitude and frequency conditions is greater, which is why non-sinusoidal vibration can further improve Pull speed. Non-sinusoidal vibration can better meet the process requirements of casting speed. It is the best way to choose crystallizer vibration and is also a key technology for efficient continuous casting.

Conclusion

The optimized crystallizer parameter design results are: the length of the crystallizer is 900mm; a crystallizer with an internal parabolic taper is selected, with a total taper of 0.9%~1.1%m; a refined copper water jacket made of deoxidized phosphorus copper is used, with a water gap width of 3.5mm, copper The tube fixation method is upper fixed type; a vibration device in the form of a half-leaf spring, short arm and four links is used, and reasonable vibration parameters are selected; the negative slippage time of sinusoidal vibration is 0.12~0.15s. The design results have achieved good results in practical applications.

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