Technical transformation of continuous casting mold to increase steel throughput of copper tubes

Since the R12M five-machine five-strand bloom continuous casting machine of a company was put into operation, the steel flow rate of the mold copper tube has been low, which has seriously affected the operation rate of the casting machine and the cost has remained high. By thickening the upper flange of the crystallizer, installing an exhaust device, and changing the sealing method of the upper flange, the steel flow rate of the copper pipe has been significantly increased.

Key words: continuous casting; crystallizer; steel flow rate

The crystallizer is the heart of the continuous casting machine. Since the 5-machine and 5-strand bloom continuous casting machine of a company was put into operation, the use effect of the copper tubes of the crystallizer has been poor. The average steel throughput of the copper tubes is 13,000t. The reason for the offline is not bulging deformation. The chrome plating on the upper lip has fallen off, and the sealing ring has burned out. By thickening the upper flange of the crystallizer, installing a crystallizer water gap exhaust device, and changing the sealing method of the upper flange, after more than a year of actual use, the amount of steel flowing through the copper pipe has been significantly increased.

Performance that a good crystallizer should have

1) Good thermal conductivity enables rapid solidification of molten steel. Due to the limited length of the crystallizer, our company is 800mm. To be able to take away a large amount of heat within such a short distance, it must have good thermal conductivity. If the thermal conductivity is poor, the shell of the billet coming out of the crystallizer will become thinner and prone to steel leakage, which will also have a great impact on the service life of the copper tube.

2) The structural rigidity should be good. The inner wall of the crystallizer is in contact with high-temperature molten steel, and cooling water flows through the outer wall, and its wall thickness is very thin (only 18 to 30 mm). Therefore, the temperature gradient in its thickness direction is extremely large, and the thermal stress is considerable. Its structure must have greater stiffness to adapt to the large thermal stress.

3) Easy to assemble, disassemble and adjust. In order to quickly replace the section or repair the crystallizer, it can improve the production capacity and operating rate of the continuous casting machine.

4) Long working life. The high temperature of the mold is accompanied by sliding friction between the cast slab and the inner wall of the mold. Therefore, the inner wall should have good wear resistance and high recrystallization temperature.

Design requirements for crystallizer cooling water system

1) In order to remove impurities mixed into the cooling water, a self-cleaning filter is installed on the continuous casting machine;

2) The cooling water loop of the crystallizer is collected into the main pipeline and returns to the water treatment system;

3) Ensure that the cooling water can fill the cooling water tank and each pipeline, and install an air exhaust pipe and a bleed valve at the highest point to exhaust the air in the pipeline;

4) Cooling water quality should have strict requirements, pH value 7-8, hardness

Main technical parameters and structural description of 5-stream bloom crystallizer

The main technical parameters are shown in Table 1:

Pouring section230*230mm230*350mm320*410mm
structure typeTubularCopper pipe materialCuAg alloy
Copper tube length800mmWater seam4mm
Water gap flow rate>10m/swater pressure>0.8MPa

Crystallizer structure

The company uses tubular crystallizers. The molten steel flowing into the crystallizer is rapidly cooled to form a cast slab with a certain shell thickness. The main structure (Figure 1) consists of chrome-plated copper pipes on the inner wall, water jacket, stainless steel structural shell, positioning key plate, upper and lower sealing flanges, various sealing rings, foot rollers, foot roller section spray devices, etc. The copper pipe is made of copper-silver alloy and the inner wall is chrome plated. The inner water jacket is a stainless steel integrally formed water jacket to ensure uniform water seams. The sealing ring material is silicone rubber. The upper end of the copper pipe is fixed with a key plate, and the lower end is free to adapt to the longitudinal expansion and contraction caused by the rapid changes in heat and cold of the copper pipe. The copper tube is fixed circumferentially with bumps. The copper pipe of this structure is firmly fixed, easy to disassemble, accurate in arc alignment, and can be quickly replaced. The water inlet and outlet are sealed with O-rings, automatically connected, easy to install, and reliable in positioning.

Crystallizer cooling

Equipment functions and control requirements

The clean water flowing through the inner cavity of the crystallizer is used to rapidly cool the molten steel in contact with the inner wall of the crystallizer and cause it to crystallize rapidly. The amount of cooling water in the crystallizer is adjustable.

Electrical equipment and control methods

1) Each flow of crystallizer cooling water uses an electric stop valve for on-off control. This control can be performed manually or automatically. When the automatic operation state is selected, the stop valve automatically opens when the flow working state transitions to the casting state;

2) Use an electromagnetic flowmeter to detect the ultra-low limit alarm of the cooling water flow rate of each crystallizer;

3) The cooling water flow rate is manually adjusted according to the process requirements;

4) Thermal resistors are used to detect the temperature difference between the inlet and outlet water of each cooling cavity, and an alarm will occur if the upper limit exceeds 9℃;

5) When the cooling water flow rate of the crystallizer falls below the lower limit, pouring will be stopped immediately and an alarm will be issued;

6) The crystallizer cooling system can be operated locally and display flow rate and remote data display.

Analysis of production status and problems before crystallizer transformation

According to statistics, since the continuous casting machine was put into operation, taking the 230*230 cross-section as an example, the average steel throughput of the copper tubes in the mold has been about 13,000t. Although it meets the process design requirements, judging from the copper pipe scrap standards, the upper flange leaks due to the off-line reasons, the chromium plating layer on the upper mouth falls off, and the upper mouth of the copper pipe is severely deformed. From the removal of the copper pipe, it was found that the side sealing rubber ring was burned. After analysis, the reason is that the molten steel level in the mold is too close to the upper opening, the side sealing rubber ring is close to the copper tube wall, and the cooling water in the inner cavity of the mold cannot fully cool the side sealing area and the area 50mm above. If this problem is solved, there is still great potential for the amount of steel passing through copper pipes. First, employees were required to operate at a low liquid level during process operations and pass through a 50mm waterless cooling zone. The results were somewhat alleviated, but the difficulty for the operators increased. Due to the limited length of the copper pipe, the risk of steel leakage increased. This brings difficulties to production, and the mold is often forced to stop casting due to water leakage, interrupting continuous production and seriously affecting the productivity of the casting machine. For this reason, it is imperative to carry out technical transformation of the crystallizer.

Crystallizer technical transformation measures

Modification of the key board

After our repeated offline comparison tests, the final solution was to remove the side sealing rubber ring. Open a row of evenly arranged water inlet holes (Φ3.5mm) around the outer wall of the original key board close to the copper tube. Let the cooling water from the water gap of the crystallizer enter the 50mm water-free zone, and open four (Φ30mm) drainage holes on the four sides of the outer ring of the key plate to communicate with the inner cavity of the crystallizer. In this way, the 50mm water-free cooling area becomes a water cooling area, which greatly improves the cooling conditions in this area.

Modification of the upper flange

In order to prevent the upper flange from being deformed and leaking during the pouring process due to insufficient strength of the upper flange due to the elimination of the side sealing rubber ring. Thicken the upper flange from 35mm to 50mm. And open an annular groove on the lower surface of the upper flange, place a sealing rubber ring, and press it tightly on the lower flange to prevent the cooling water above the key plate from escaping. Through offline maintenance of 1.2MPa for 8 hours, the upper flange does not deform and is well sealed.

Install exhaust return device

Since the inner wall of the copper tube of the mold is in contact with high-temperature molten steel during the pouring process, the cooling water in the water gap is easily vaporized instantly, resulting in a large number of bubbles, which are finally collected in the upper part of the mold. Due to the buoyancy of the gas, the bubbles will not be discharged with the return pipe. This has a great impact on the cooling effect of the crystallizer and is also the main factor in the short service life of the copper tube. The original design had a vent hole on the upper flange, which was sealed with a nut to pass water through the cold test and eliminate the gas in the pipe. It was closed during production and could not play a very good role. Now remove the nut, connect a high-pressure resistant metal hose, and connect the other end to the drain pipe of the crystallizer to form a closed loop. During production, the bubbles generated will flow back into the return water pipeline along with this pipeline at any time. This does not affect the entire net water pressure, but also solves the problem of air bubbles in the crystallizer that cannot be eliminated. see picture 1.

Effects of crystallizer technical transformation

Through various technical transformations of the crystallizer, problems such as deformation of the ends of the copper tubes, peeling off of the chromium plating layer, and bulging of the bellies have been better solved. The amount of steel flowing through copper pipes has increased significantly. The steel throughput has increased from the original average of 13,000t/piece to 26,000t/piece, and the use of individual copper pipes has exceeded 30,000t. The economic benefits of cost reduction are obvious, and the copper consumption per ton of steel dropped from 1.113 yuan/ton to 0.649 yuan/ton. The operating conditions of employees have been improved, the requirements for controlling the molten steel level in the mold have been relaxed, and the occurrence of steel leakage accidents has been reduced. After the technical transformation, there was no unplanned stoppage of pouring due to crystallizer problems during production, which improved the operation rate of the continuous casting machine. It provides a good reference for similar crystallizer transformation.

Conclusion

(1) By changing the cooling water sealing method of the crystallizer, the cooling system of the crystallizer is optimized, making the crystallizer cooling water circulation flow field more reasonable and improving the cooling effect;

(2) The improvement of the cooling effect is directly reflected in the substantial increase in the steel flow rate of the copper tube, which reduces the production cost, has obvious economic benefits, and improves the operation rate of the continuous casting machine;

(3) Reduce the operating difficulty for operators and reduce the occurrence of steel breakout accidents.

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