High-efficiency transformation and practice of billet continuous caster crystallizer

 The design defects of the crystallizer of a 1″ billet continuous caster in a certain plant were analyzed. After being transformed to make it more efficient, good results were achieved in practical applications.

Keywords: billet; crystallizer; efficiency improvement; transformation

For a long time, a steelmaking plant has been using a three-furnace-to-four-machine production organization model due to the low casting machine speed and small production capacity. In order to improve the casting machine production capacity and achieve efficient furnace-machine matching, the billet continuous caster was subsequently modified. The system’s efficient production technology is optimized, and the core technology to achieve efficient production in continuous casting is high casting speed. One of the key technologies to achieve high casting speed is the efficient design of the crystallizer. This article only analyzes the original crystallizer of the 1* billet of this factory. Design defects are systematically analyzed, and measures for comprehensive and efficient technical optimization are introduced in detail. Through the efficient optimization of the crystallizer, the casting machine’s casting speed has been greatly improved and the casting machine’s production potential has been released. The maximum design casting speed of the casting machine has been increased from 3.2m/min to 4.0m/min, and the maximum working speed has reached 4.5 m/min, the annual production capacity of a single stream reaches 200,000 tons.

1*Main process parameters of casting machine

(1) Radius of curvature of casting machine: 5250 mm;

(2) Cast slab section: 120 mm×120mm;

(3) Design pulling speed: 2.8~3.2 m/min;

(4) Number of casting machine streams: four machines and four streams.

Main parameters of the original 1″ billet continuous caster crystallizer

The main parameters of the original 1* billet continuous caster crystallizer are shown in Table 1.

Crystallizer assembly and designBefore optimizationOptimized
Steel pipe length (mm)812850
Copper tube wall thickness (mm)1012
Copper pipe materialPhosphorus deoxidized coppersilver copper alloy
Copper tube tapersingle tapercontinuous taper
Inlet water pressure (MPa)0.6~0.70.8~1.0
Diversion inner water jacket formFour-plate right-angle butt weldingOverall extension fillet
water jacketOrdinary carbon steelStainless steel
Water seam width (mm)54.0
Steel pipe fixation methodUpper card plate fixedPress the flange on the flange
Crystallizer outlet coolingNo zero segmentappend zero segment
Average amount of steel passed through copper pipes (tons/piece)32156715

Analysis of original crystallizer process structure and parameters

Copper pipe support and fixation methods

The original crystallizer was generally called a pallet structure. Due to its earlier design, as the requirements for continuous casting in steelmaking became higher and higher, it gradually showed many shortcomings that it was not suitable for high-efficiency continuous casting: as the casting speed increased, The copper tube of the crystallizer will withstand greater heat, especially the thermal stress and the static pressure of molten steel at the top. However, the clamped structure has no measures to limit its thermal stress deformation; the copper wall at the slot is thinner, and the thermal stress it withstands is very high. , the friction force is greater, and the strength here is extremely weak; at the same time, the clamping plate restricts the cooling water from continuing to cool upward, so that there is no cooling water about 30 mm above the clamping plate. Under the action of heat transfer, the copper wall in the upper waterless area The deformation intensifies, and in severe cases, the original inverse taper trend of the inner cavity of the mold copper tube is destroyed, affecting the quality of the casting billet and the service life of the mold copper tube.

Water jacket form

The inner water jacket has a right-angle four-plate butt welding structure. For 1* billet, as the billet casting speed increases (3.8~4.0 m/min), the heat exchange also accelerates, which requires the The water speed and water volume can meet the exchange balance of superheat, latent heat, sensible heat and water of the cast slab in the crystallizer. At the same time, it requires slow cooling of the meniscus to reduce the temperature gradient of the upper cast slab and make the solidification stress uniform. Under high drawing speed, the four-plate butt welding structure has poor precision, the corners are at right angles, there is welding stress, and it is easy to deform, causing the meniscus to be unable to be cooled slowly, which cannot meet the requirements of efficient continuous casting.

Copper tube wall thickness

The wall thickness of the copper tube of the crystallizer is 10 mm. After the inner cavity of the copper tube was inspected after the assembly of the crystallizer, it was found that the inverse taper had changed, especially the taper of the lower port increased by 0.01~0.12 mm (lower port 0~200 mm), which destroyed the original design. The inverted taper increases the drawing resistance and the liquid level cannot be raised (200 to 250 mm from the upper opening); at the same time, the thin copper wall increases the sensitivity of heat transfer at high drawing speeds, causing greater stress on the primary billet shell, and It has poor resistance to deformation at high temperatures, and the cold surface temperature is high, which is prone to intermittent boiling, affecting the uniformity and effect of heat transfer.

Copper tube length

The current mold copper tube is 812 mm. As the drawing speed increases, the casting billet stays in the copper tube for a shorter time, the mold shell becomes thinner, and the probability of steel leakage increases. The highest monthly leakage rate reaches 0.68%.

Copper pipe material

The common material currently used is phosphorus deoxidized copper. The recrystallization temperature of ordinary phosphorus deoxidized copper is generally only 270 to 280°C. When the drawing speed increases, the temperature difference between the inside and outside along the thickness of the copper tube can reach more than 200°C, below the meniscus. At 150 mm, the hot surface temperature of the copper tube is as high as over 250°C. When the temperature of the hot surface reaches or exceeds the recrystallization temperature, the copper tube will undergo thermal recrystallization. The crystal grains in the structure will grow and the hardness will decrease, causing the copper tube to plastically deform, expand and bulge upward and outward, and form a local excessive taper or negative taper. , increasing the drawing resistance and the uneven solidification of the billet shell.

 Crystallizer taper

The reverse taper of the original crystallizer is a single taper. Due to the deformation of the crystallizer at the meniscus, a negative taper is often formed. The negative taper interacts with the primary green shell during negative sliding, which increases the depth and unevenness of the vibration marks, causing four sides of the mold. Uneven heat transfer results in uneven billet shells, which increases the drawing resistance and the risk of steel breakouts.

Crystallizer cooling water parameters

The current water slit width and water speed cannot meet the production process requirements. Under the same conditions, the water slit is wide (>4 mm), the flow rate is slow (<12 m/sec), and the temperature difference between the inlet and outlet water is large (>7℃), it is possible If the cold surface temperature of the copper tube exceeds 100°C, nucleate boiling of water will occur, causing permanent deformation of the copper tube.

The current cooling water pressure of the crystallizer is low and the water volume is too small, which cannot meet the minimum requirements of water speed >12 m/sec and avoid intermittent boiling.

Crystallizer assembly

The current crystallizer assembly is complex, which on the one hand causes a large cumulative error during the processing and affects the overall assembly accuracy; on the other hand, it increases maintenance costs and increases employee labor intensity.

Optimization of high-efficiency crystallizer technology

Modification of the fixed support method of the crystallizer

The crystallizer is a high-load heat exchanger, which enables good heat transfer, which is beneficial to the quality of the slab and its own life. Therefore, the upper flange is pressed and the periphery is sealed and fixed, so that the crystallizer copper tube has almost no cooling dead zone, improving heat transfer and reducing thermal deformation; because it has fewer assembly parts, the cumulative assembly error is small, and the overall precision is high. .

Modification of water jacket form

The domestically advanced stainless steel integrally extended fillet inner water jacket is used, and the integral extended pressure processing is performed. In order to prevent the deformation of the inner water jacket, the wall thickness is designed to be increased to 8 mm. At the same time, the integrity of the cylinder also restrains deformation; the inner cavity adopts an inverted taper. , the water gap changes from the lower mouth to the upper mouth at 3.5~4 mm, and the water speed is 13.39~11.58 m/s, making the upper water speed lower than the lower water speed, which has the effect of slow cooling of the meniscus; at high casting speed , the corners are rounded so that the water seams at the corners are consistent with the water seams on the face, so that the slab is cooled evenly; because the curvature is consistent with the copper tube, the positioning of the inner cavity is more convenient, thus ensuring that the slab is cooled evenly in the crystallizer , so that the slab has sufficient and uniform thickness when it comes out of the mold, which is crucial to increasing the drawing speed.

Increase the wall thickness of copper pipes

Increasing the wall thickness of the original mold copper tube to 12 mm can improve its strength, which is about 8% higher than the original 10 mm wall thickness, and enhance its ability to withstand the static pressure of molten steel; in addition, the wall thickness of the crystallizer copper tube is increased Finally, its thermal resistance is increased by about 2.5% compared to 10mm, which can reduce the sensitivity of the billet to cooling intensity and ease the meniscus cooling speed. The relationship between crystallizer heat transfer and various process factors is shown in Figure 1.

Figure 1 Relationship between heat transfer in the crystallizer and various process factors

The material of the crystallizer copper tube is TAgO (see Table 2)

Table 2 Crystallizer copper tube materials

MaterialChemical ingredientsMechanical behaviorRecrystallization Temp
TP2Cu≥99.9P:0.015~0.04σb:245 MPsσ3:196 MPs280~320℃
TAg0.1Cu≥99.9Ag:0.08~0.12σb:265~343  MP₈σs:195~275 MPs≥350℃

As can be seen from Table 2, the strength and recrystallization temperature of silver-copper alloy are higher than those of phosphorus deoxidized copper, and the ability to resist the static pressure and thermal stress of molten steel is stronger.

Increase the length of the crystallizer copper tube

Increasing the length of the mold copper tube to 850 mm will extend the solidification time of the billet shell and increase the thickness of the billet by 3% at the same pulling speed.

Inverse taper optimization

The inverted taper design of the crystallizer copper tube adopts a new taper design concept. It is designed from a single taper to a continuous parabola, which is closer to the solidification shrinkage curve of the billet shell, greatly improving the cooling effect of the crystallizer, effectively reducing the chance of steel leakage, and extending the length of the crystallizer. The life of copper pipes.

Improve cooling water parameters

The water gap is designed to be 3.5~4 mm, the water speed is > 12m/sec, the inlet water pressure is 0.8~1.0 MPa, and the inlet and outlet water temperature difference is 4~7°C, keeping the cold surface temperature below 100°C, increasing the temperature gradient. It can prevent intermittent boiling, help reduce the temperature of the hot surface and reduce the deformation of the copper tube.

Simplifying crystallizer assembly

The copper pipe is assembled by flanging the upper flange and pressing it tightly to prevent the upper mouth of the crystallizer from deforming and leaking. After the upper flange is disassembled, the copper pipe of the crystallizer can be replaced; a simple zero-section spray structure is adopted. Realize fast online replacement, reduce labor intensity, and ensure the overall cooling effect of the crystallizer.

Production practice effects

(1) The casting machine casting speed is increased. After the crystallizer has been optimized with high-efficiency technology, the designed casting speed has been increased from the original 2.8 to 3.2 m/min to 3.8 to 4.0 m/min, and the maximum working speed has reached 4.5 min.

(2) The steel flow rate of the crystallizer copper tube is increased. After the crystallizer was efficiently optimized by the system, the average steel flow rate of a single mold copper tube reached 6715 t (the maximum was 9876 t), which was 3480 t higher than the original mold copper pipe flow rate.

(3) Casting machine leakage accidents have been significantly reduced, the casting machine spillage rate has dropped from 0.48% before the transformation to the current level of about 0.10%, and the quality of cast slabs has steadily improved.

(4) The increase in casting speed and production operation rate has greatly released the production capacity of the casting machine, achieving efficient matching production of the converter and the casting machine.

Conclusion

(1) The high-efficiency optimization technology of the crystallizer is a key technology to increase the casting machine’s casting speed and release the casting machine’s production capacity. With a small investment at this stage, efficient transformation of conventional crystallizers can fully meet production needs.

(2) The high-efficiency crystallizer technology is a system technology and must be comprehensively optimized from the process structure and process parameters of the crystallizer to achieve efficient production.

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