This article introduces various problems that affect the smooth production of the fan-shaped segments of Chenggang slab continuous casting machine in the early stage of production: such as hydraulic system, secondary cooling and continuous casting machine opening, etc. It also introduces the equipment transformation in detail and the results achieved after the transformation and improvement.
In July 2008, the Chenggang 4-machine 4-stream large slab continuous casting machine was put into production. The continuous casting machine was generally contracted by Xi’an Heavy Machinery Research Institute. The clamping frame is equipped with four hydraulic cylinders to ensure the pre-adjustment of the roll gap, and the pressing device is equipped with two hydraulic cylinders to ensure the transportation of the ingot and the billet. Each stream has eleven fan-shaped segments, 1-6 segments are arc segments, and 7-8 segments are straightening segments. 9-11 are horizontal straightening segments. The motor power is 7.5KW.’
During a period of time after the production was put into operation, due to the imperfections of the continuous casting process and equipment maintenance and adjustment, defects such as centre segregation and centre looseness appeared inside the ingot during production. In severe cases, internal cracks occurred, affecting the internal quality and processing of the steel; the fan-shaped rollers were cracked due to the lack of external spray water, and the service life was very short, which did not meet the use requirements, causing a large number of rollers to be offline; the unreasonable design of the fan-shaped roller bearing seat caused scratches on the ingot, seriously affecting the smooth progress of the next process.
How to solve the problem has become an urgent mission for the continuous casting machine. After exploring the continuous casting process parameters and the continuous casting equipment conditions, the process and equipment factors that affect the quality of the slab were obtained and improved.
1. Improve equipment accuracy and process measures to eliminate defects in ingot quality
1) Curvature and opening degree
From the beginning of the solidification of molten steel in the copper mould tube to the end of solidification, the shell of the billet always has a tendency to bulge due to the static pressure of the molten steel. As the molten steel gradually solidifies and the thickness of the billet shell increases, the size of the billet should be gradually reduced. However, in actual production, the billet running channel formed by the clamping roller cannot meet the above requirements, and a large bulge and extrusion force is generated on the billet shell. When the tensile stress exceeds the strength of the steel at the solidification front or the tensile strain exceeds a certain critical value, the solidification front will crack along the columnar intergranular space to form an internal crack. According to the metallurgical and mechanical properties of the continuous casting slab. The formation of the centre crack of the billet is the result of the combined effect of metallurgical characteristics and mechanical factors during the continuous casting process. According to the equipment mechanism of continuous casting, the higher the accuracy of the CCM segment roll gap, the more effective it is in suppressing centre segregation and internal cracks.
In actual production, due to various accidents, the frame needs to be loosened, there are sundries mixed in the pad, or the number of pads is relatively large, and the roll gap accuracy is often about 1-5mm, which seriously affects the accuracy of the equipment. By improving the material of the gasket, limiting the number of gaskets, ensuring the gasket is clean, ensuring that the gasket is pressed tightly when measuring the opening, making a fine adjustment after the initial adjustment, and adjusting the gasket thickness, the opening must be re-measured after adjustment. Optimization measures such as ensuring that the casting machine opening is within the corresponding allowable range, and paying attention to controlling the opening inlet and outlet of each section to make the opening precise and accurate.
In addition, high-precision and long-life bearings are used to ensure that the water channel of the water-cooled bearing seat of the segmented roller is unobstructed, strengthen bearing lubrication management, limit the bending and eccentricity of the clamping rod, strengthen the inspection of off-line spare parts, and only use them online after they are qualified, and replace the CCM segments regularly. If the steel leakage causes the billet to stagnate in the segment, the online segment is arranged to gradually shorten the service life and be removed from the machine in advance, so that the arc opening of the casting machine can be guaranteed. When the casting machine is shut down for inspection, the arc should not only be by the arc plate but also by the contact and wear between the clamping roller and the billet shell.
2) Hydraulic system
The pressure of the roller cylinder is one of the basic guarantees for the opening of the casting machine. Since the pulling roller directly contacts the surface of the billet, the hydraulic pressure caused by the loss of pressure control of the pulling roller cylinder and the failure of the valve table, cylinder, and pipeline network will lead to abnormal hydraulic pressure and improper pressure of the cylinder, which will have a great impact on the bulging and compression of the billet shell and will cause the liquid phase flow in the billet shell, which will have a great impact on the centre segregation. The cylinder pressure release easily causes the billet to bulge, so the working pressure of each pulling roller cylinder should be adjusted accordingly according to the different sections and the end position of solidification, and the stable operation of the hydraulic system should be ensured.
3) Secondary cooling process
During the solidification process of the billet, due to the arrangement and injection of the nozzles, the cooling intensity and heat dissipated to the outside of each part of the ingot are different, resulting in inconsistency in the width direction of the ingot. Improper distribution of cooling water in the longitudinal direction of the billet leads to a decrease in the billet temperature or a too-fast recovery speed. Because the specific heat capacity of steel changes with the temperature, the shrinkage of steel is different at different billet temperatures, which will lead to uneven thickness of the billet shell.
If the billet is too cold, the columnar crystals will be developed and the interdendritic separation will be strengthened. If the billet is not cooled enough, the billet shell will be too thin, causing production accidents. Therefore, it is necessary to ensure the normal use and regular cleaning of the backwash filter and the ordinary filter and to drain the compressed air regularly to prevent the blockage of granular objects and sludge in the water and gas pipelines. The spare parts of the pipelines on the casting machine are put on the machine after the water test is qualified outside the line. The water and gas system is carefully checked and handled during each overhaul, which changes the phenomenon of a large number of nozzle blockages and unblocked positions during the overhaul cycle. The nozzles with deformed outlets are resolutely eliminated to ensure the stability of the secondary cooling effect.
2. Reconstruct unreasonable equipment structure to eliminate surface defects of billet
In the early stage of production, scratches appeared on the billet, which was located on the bearing seat of the segmented roller and appeared frequently, bringing a heavy burden to the finishing work of the billet and also brought quality problems to the production of the hot-rolled coil plant. After a careful study of the segmented roller drawings, it was found that the upper cover of the bearing seat was originally made of Q235-C material with a thickness of 5MM, and the roller skin was 5MM from the upper edge of the bearing seat. During the production process, due to the high temperature and humid environment, the upper cover of the middle bearing seat of the segmented roller always leaked. The bearing seat at the leaking place was deformed due to insufficient cooling. As time went on, slag accumulated in the deformed place. When the slag was higher than the roller skin, scratches appeared on the ingot.
After careful research, it was decided to change the cover plate on the upper part of the bearing seat to 1Cr18Ni9Ti and change the original distance of the roller skin from the upper edge of the bearing seat from 5MM to 7MM. This not only avoids water leakage from the bearing seat, but also effectively avoids slag accumulation on the bearing seat, and completely eliminates the scratches on the billet. Since the free optimization of the CCM segment, the surface quality has increased by 4.96%, the heat delivery rate has increased by 18.41%, the finishing operation rate has been reduced to 5%, and the power consumption per ton of steel has been reduced by 2 yuan.
3. Install a roller cooling system to extend the service life of the equipment.
According to the solidification mechanism of the billet, the amount of secondary cooling water is gradually reduced when the billet enters the straightening and horizontal sections. However, the service life of the CCM segment roller drops sharply under the high-temperature condition of lack of external spray water, which is manifested as the roller bearing locking and the cladding layer cracking due to high temperature. The roller is not flexible in rotation and cannot be drawn normally. The roller cracking is caused by the thinning of the roller diameter, which leads to errors in the precision of the CCM segment opening, and the billet will have a series of quality problems such as internal cracks. Therefore, the fan-shaped segment must be replaced.
The replacement cycle of the CCM segment is relatively long, which increases the labour intensity of the workers and the operation rate of the casting machine drops sharply. On the one hand, the problem is that the segment is frequently offline, resulting in the inability to properly circulate the segment’s spare parts. (The maintenance cycle of the segment is relatively long, generally about 7 days) On the other hand, the problem is that the roller spare parts cannot be normally circulated, and the spare parts cost has risen sharply. The cost of the roller spare parts alone has increased by 4 yuan/ton. How to solve the problem of short roller life has become a problem that must be solved. The technicians carefully explored the slab continuous casting technology and equipment, and through on-site practice, decided to add external spray water to the CCM segment roller.
There are two options: If spray water is added to the roller on the basis of the original secondary cooling water, it should be a feasible option, but in the actual operation on site, there is not enough space. And the cost of transformation is very huge. The technicians decided to use another option to add spray water on the side of the CCM segment. As shown in Figure 2
Figure 1: Schematic diagram of the transformation of the fan-shaped segment bearing seat
Figure 2: Spray rack after the transformation of the fan-shaped segment
The materials required for a fan-shaped segment are as follows: 2 straight seam welded pipes of 2000mmDN15, 12 DN15 wire heads, and 12 nozzles of the model 1/2PZ6800810ZY.
Nozzle introduction: Material CU, the spray angle is 50, and the required pressure is 0.3MPa. The nozzle has a novel structure and a spray shape of double-hole semicircular arc flat. It is mainly used for cooling the rollers of the continuous casting machine and can also be used on some special occasions.
In combination with the status of the CCM segment of the slab continuous casting machine, this paper obtains the process and equipment factors that affect the quality of the slab, proposes corresponding improvement strategies, and formulates and implements a production test plan based on technical principles and equipment parameters of the continuous casting machine.
During implementation, continuous improvement is continuously made to meet the needs of high-quality slab production of the slab continuous casting machine. The qualified rate of Q345B C510L SPHC steel varieties such as SS400 and SS490 has reached 99.7%. The production quality of ordinary carbon steel slabs has been continuously improved, and various indicators have been significantly optimized. Among them, the qualified rate of internal quality of the billet is above 99.5%, the qualified rate of subcutaneous inclusions and vibration mark depth has been increased to above 99%, the qualified rate of surface quality is also above 98%, the hot delivery rate of the ingot has been increased to above 95%, and the consumption of molten steel has been reduced to about 1016kg/t. steel.
The improved CCM segment has greatly ensured the stable operation of the equipment, saving energy and reducing costs. After several months of production operation, the improved host system has shown its stability. The operation rate of the casting machine has been steadily improved, the quality of the billet has been significantly improved, the production capacity has been brought into play, and various economic and technical indicators have been greatly improved. At the same time, after the transformation of the casting machine, the consumption of spare parts has been greatly reduced, and the maintenance workload has been reduced, the labour intensity has been reduced, and huge economic benefits have been achieved. It also laid the foundation for the smooth production of the entire production line.
One Response
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