By comprehensively analyzing the different conditions of multiple continuous casting machine molds used on site, we summarized the main factors that affect the increase in steel throughput of the mold. Through follow-up research and adjustments to the mold foot roller modification, foot roller opening, vibration table deflection arc accuracy, and guide section arc, the wear amount of the copper tube was effectively reduced and the steel flow rate of the copper tube was increased.
Keyword: Copper mould tube
The crystallizer is a key component of the continuous casting machine and the most critical link that restricts the efficient production of the casting machine. The requirement for the mold for high-efficiency billet continuous casting is that under high pulling speed conditions, the molten steel can uniformly form a shell in the mold and reach a certain shell thickness at the exit of the mold. Specifically, it requires uniform cooling around the crystallizer and at the corners, and sufficient cooling efficiency.
At present, most of the same industry uses research on the inner cavity of the mold, taper, and Cr plating technology, and the steel throughput is around 8000t. After on-site tracking analysis, the modification of the crystallizer foot rollers, the adjustment of the foot roller openings, the deflection of the vibration table, etc. were tracked and studied and adjusted. It can effectively reduce the wear of the copper tube, ensure uniform cooling of the cast slab in the mold, improve the surface quality of the cast slab and reduce the probability of steel leakage. The copper tube steel throughput can reach more than 13,000t.
Basic information introduction
1.1 Overview of casting machine
3 continuous casting machine models: two R9m and one arc continuous casting R8.
Casting blank section: 155*155mm.
Working pulling speed: 0~5m/min.
Vibration mechanism: non-sinusoidal vibration mechanism of servo motor electric cylinder.
Copper tube material: silver copper.
Copper tube length: 1000mm.
Copper tube inner cavity: surface hard chrome plated, thickness 0.1mm.
1.2 Current status of crystallizer operation
As the billet drawing speed continues to increase, the accuracy requirements for each equipment are also getting higher and higher. The original designed maximum casting speed of the continuous casting machine was 3.5m/min. Through a series of on-site improvements and innovations, the current normal pulling speed is around 4.8m/min. The maximum casting speed in the test flow reached 5.73m/min, and the quality of the slab fluctuated significantly after the casting speed was increased. As the pulling speed increases, problems with the crystallizer also expose many potential production hazards.
At present, it is mainly the steel flow rate of the mold and the wear of the lower mouth of the mold that limit the increase in production capacity and the service life of the copper tube. Due to the high wear of the mold, there are more breakouts and interruptions. At the same time, the wear is large, which causes the casting machine to slow down the speed of steel drawing and stripping. The average steel volume of copper pipes is about 7,500t, and the consumption cost is more than 2.8 yuan/t of steel. This has seriously hampered the cost control and stable production of steelmaking plants.
Analysis of factors affecting the steel flow rate of copper pipes
(1) The cast slab has insufficient support at the lower mouth of the mold, and the copper tube is subject to concentrated stress on one side and the wear is aggravated.
As the pulling speed continues to increase, since the strength of the crystallizer foot roller frame and the number of roller rows have not changed, the accuracy of the foot roller opening is seriously lost during the production process. The green green shell is eccentric in the mold, and the integration of mold slag cannot meet its lubrication, filling air gap and heat transfer requirements, and the demoulding effect becomes worse. The cast slab cannot be effectively supported and guided after coming out of the mold, resulting in cracks and leakage due to uneven force and internal stress, and abnormal internal wear of the steel pipe.
(2) The vibration operation accuracy exceeds the standard, causing the slab to move irregularly in the mold.
The crystallizer vibration device is one of the important equipment of the continuous casting machine. Its main function is to prevent the bonding between the molten steel and the inner wall of the copper tube. When the vibration table vibrates, the crystallizer and the green shell in the copper tube move relative to each other to achieve demolding. When bonding occurs, vibration is used to force demoulding to eliminate the bonding. In this case, if the arc simulation accuracy and deflection of the vibration device exceed the standard, it will cause the slab to move irregularly in the copper tube of the mold.
(3) The inner cavity size of the copper tube deviates due to thermal expansion, contraction and water pressure.
The thermal expansion and contraction coefficient of copper is 17.7×10-6m/m·℃. By establishing a three-dimensional slotted copper tube finite element model, the thermal-structural coupling calculation is performed. The picture below shows the deformation diagram of the meniscus position interface. The dotted line is the contour line before deformation. The appearance after expansion and deformation is shown in Figure 2. Although the middle of the copper tube is concave as a whole, compared with the original position, the outward increase is > 0.14mm, resulting in changes in the taper of the copper tube, deformation of the inner cavity, and loss of coating.
(4) Crystallizer assembly and repair: the roller opening and water gap adjustment do not meet the requirements.
Multi-taper copper tubes are used. During the installation process of the crystallizer foot rollers, the inner cavity surface arc plate of the copper tube is used for “zero-to-zero” adjustment. After adjusting the opening of the foot roller, it was found that the copper pipe was severely worn on one side during use. After the steel flow reached 3000t, the length of the worn surface was 270mm and the wear amount was 0.8mm, which seriously affected the service life of the copper pipe.
According to actual on-site tracking analysis and research, the main research direction is to improve the steel throughput of the mold. Do a good job of supporting the casting billet in the copper tube, ensuring the concentricity of the casting billet and the copper tube, ensuring that the air gaps on the four sides of the primary billet shell and the inner cavity of the copper tube are uniform, and eliminating irregular movements between the vibration device and the copper tube.
Research on technology to improve copper pipe steel throughput
Research on the application of converting single-row foot rollers into multi-row foot rollers in the crystallizer
The single row of foot rollers cannot meet the supporting and guiding functions of the cast slab. It is prone to cracks and steel pipe leakage due to uneven force and internal stress after the slab leaves the mold, and abnormal internal wear of the steel pipe. Change the single row of foot rollers into multiple rows of foot rollers. When producing different steel types, the opening of the foot roller can be adjusted step by step according to the different solidification shrinkage rates of molten steel and the inverse taper of the copper pipe. The support effect of multiple rows of foot rollers can effectively improve the concentration of stress on a single point of the slab and the eccentricity in the copper tube.
Optimizing the adjustment size of the opening of the crystallizer foot roller
The crystallizer is the heart of the continuous casting machine. Its function is to mold the molten steel injected into the mold and form a uniform shell thickness. After coming out of the crystallizer, it will not be bulged or deformed by the static pressure of molten steel, so the heat transfer of the crystallizer plays a decisive role in the stability of production. The main factor affecting the use of the crystallizer is the excessive wear of the lower mouth of the crystallizer.
The current standard set by our crystallizer is that if the lower mouth wear exceeds 1mm, it will be scrapped. The main reason for the wear of the crystallizer: due to the large opening of the foot roller, it cannot effectively support the cast slab. Affected by the accuracy of the secondary cooling chamber, the cast slab is biased to one side, causing wear to the copper wall of the crystallizer. After the lower mouth is worn, it seriously affects the cooling of the slab at the end of the mold, causing cracks on the surface and corners of the slab, which expands under the influence of the static pressure of the molten steel in the mold and causes steel leakage.
Research and application of high-precision diversion water jacket
Now the maximum deviation of the matching gap between the crystallizer water jacket and the copper tube reaches 1mm. According to experiments, it is proved that the water gap difference is 1mm, and the cooling water flow velocity changes by up to 20%. The main function of high-speed drawing copper tubes is to increase the cooling coefficient and improve cooling uniformity through the research, development and application of high-precision diversion water jackets. The water flow in the grooves on the four sides of the copper tube is evenly distributed to increase the water flow velocity, thereby achieving uniform cooling of the cast slab in the mold and improving the cooling coefficient. Eliminate the matching gap between the water jacket and the copper pipe, and play a positioning and full-coverage support role for the copper pipe. It can effectively reduce the deformation effect of the inner cavity size of the copper pipe due to thermal expansion, reduce the accident rate, inhibit the desquamation of the cast slab, and eliminate abnormal wear caused by the deformation of the inner cavity of the copper pipe.
Research on the relationship between the deflection amount, accuracy and steel flow rate of the crystallizer vibration device
The greater the deflection of the vibrating table, the lower the arc imitation accuracy. During operation, the concentricity between the casting billet and the copper tube in the mold cannot be guaranteed, resulting in concentrated stress on one side of the copper tube during operation. The negative slip rate of the slab becomes worse, which means that the steel flow rate of the mold cannot be increased, and the mold is scrapped due to steel breakout accidents.
It is a very common phenomenon that the steel flow rate of the mold is relatively low. As long as the mold repair process is standardized in real time according to the actual use of the equipment. Formulating precision maintenance standards for vibration devices and casting machine sectors, and doing a good job of supporting and guiding the cast slabs in the mold copper tubes can effectively improve the copper and steel beams of the mold copper tubes and improve the overall working efficiency of the continuous casting machine equipment.