This article introduces the current status of the slab heat transfer rate of the 4# slab continuous caster in a steelmaking plant. It is analyzed that the main reason for the low heat transfer rate is unstable equipment accuracy and poor process execution leading to unstable slab quality. We start with equipment and technology respectively to stabilize the quality of the slab and improve the heat transfer rate.
Keywords: heat transfer rate; equipment accuracy; technology
As far as the 4# continuous casting machine is concerned, the hot feed rate in 2017 was only 61.22%. The heat on the surface of the cast slab is lost during the entire production process: first, the batch of cast slabs is taken off the line; second, the grinding process. The huge waste of energy caused by this has delayed the delivery date of the product contract.
Analysis of the reasons
For the front and back processes, continuous casting, which plays a connecting role in steelmaking, is not only affected by the accuracy of its own equipment, but also affected by two aspects: one is refining; the other is front-end steelmaking. Therefore, it is a particularly important link in steelmaking. Specifically, the main factors affecting the hot delivery rate of machine 4 are as follows.
Vertical bending section equipment accuracy
(1) The arc connection in these two aspects is particularly unstable: one is the 1# sector section; the other is the vertical bending section. The phenomenon of arc running occurred 2-5 days after the shutdown and adjustment. The arc connection fluctuated between 0.7mm and 3.8mm. The incidence rate of corner cracks in the billet was 12%. In terms of these two aspects, on-site inspection found that the contact locations were severely worn: one was the trunnion; the other was the zero-section U-shaped base. The arc connection during the production process is unstable when there is a gap. Its typical manifestation is the frequent occurrence of arc running phenomenon.
(2) After going online, the hydraulic plate of section 0 leaked particularly frequently in 2017. The consequence is that, in this case, the quality of the slab surface is affected by minor occurrences such as scratches on the surface of the slab due to roller jamming: First, the surface of the bearing seat and roller is easy to accumulate slag; second, the water atomization effect is not good; third, the zero-stage cold water pressure is unstable. For minor cases, with the help of the adjustment of the bottom U-shaped seat gasket, the two aspects are consistent: one is the base; the other is the zero-section hydraulic plate. In this way, the accuracy of arc connection of the casting machine is maintained. In severe cases, the arc joining process in two aspects and zero segments will exceed the process standards: one is the crystallizer; the other is the 1# sector segment. The replacement of the zero segment is inevitable.
For the previous and later processes, a particularly important link like continuous casting plays a connecting role. Therefore, these three aspects all have a certain impact on it: one is refining; the other is front-end steelmaking; and the third is the accuracy of its own equipment. The main factors affecting the hot delivery rate of machine 4 are as follows.
Vertical bending section equipment accuracy
As far as 2-5 days after shutdown adjustment is concerned, arc running will occur when the arc connection is particularly unstable in two aspects: one is the 1# sector section; the other is the vertical bending section. The arc fluctuates between 0.7mm and 3.8mm, and the incidence rate of corner cracks in the billet is 12%. On-site inspection found that the contact positions of these two parts were seriously worn and there was a gap: The first one is the trunnion; the second one is the zero-section U-shaped base and the frequent occurrence of thin arcs, which is caused by unstable arc connection during the technical production process.
The pressure of the secondary cooling water is unstable and the atomization effect is poor.
The number of spray guns controlled by each circuit is not the same for the secondary cooling zone divided into 9 compressed air circuits. The maximum and minimum circuits control up to 108 and only 56 guns respectively. Whether it is the actual atomization effect of each spray gun or the clogging of each spray square tube, they are all different. The surface of the cast slab is prone to cracks due to uneven temperature and accumulation of water.
Unstable production rhythm
In terms of the amount of cast slabs rolled off the production line, machine #4 accounted for 4.8% of the total output in 2017, despite the abnormal composition. On the one hand, the rhythm of the converter is tight and the phosphorus of the steel discharge is high; on the other hand, the slag during the steel discharge is caused by the failure of the slag blocking truck; and on the other hand, the bottom blowing of the steel discharge is not effective when the oxidation of the molten steel is high. . All these make the carbon content unstable under the tense steel-laying situation, resulting in abnormal composition.
Modifications to the flaw detection plate process
The number of billets that require flaw detection reaches about 40% in the 4# continuous caster, which smelts many types of steel. The current technological level requires that the rolling thickness of the flaw detection plate ≥40mm must be pitted, and in principle, the thickness of 20~40mm must be pitted. If the pit is full, it must be piled at the designated pile position for cooling. If the RH process does not meet the requirements, the steel cutting stock must be replaced and accumulated in the steelmaking site, which greatly affects the hot delivery of the billet.
Relevant measures and their effects
The scratches on the cast slab should be reduced on the premise of stable arc connection.
(1) First, replace the trunnions of all zero sections. All U-shaped bases and gaskets will be replaced during downtime for maintenance. While using 8 screws and double nuts to tighten the two gasketed parts of the U-shaped base, the gaps near the edge of the inner arc were welded: one is the bottom; the other is the side. In this way, for the U-shaped base, the phenomenon of displacement caused by two forces in its production disappears: one is the static pressure of the molten steel; the other is the tensile force of the casting billet.
(2) As for the zero-section hydraulic plate, when the elastic hydraulic plate base is used, it will automatically fit due to the gravity pressure of the zero-section. In this way, the number of water leaks from the pressure plate will be reduced. After the elastic hydraulic pressure plate was replaced in July 2018, the number of leaks in the zero-section pressure plate was reduced to zero.
The accuracy of the driving roller can be improved with the optimization of the dressing process
The workload can be reduced by reducing the number of times on and off the internal arc drive roller arch. The measures taken are: first, rough adjustment of the inner arc drive roller; second, adding gaskets to the arch base of the inner arc drive roller. In order to ensure that the concentricity of the roller is less than 0.2mm, it is necessary to measure 4 points on each roller at the same time and rotate each measuring point 90°.
Improve the atomization effect under the premise of stabilizing the secondary cooling pressure
The actual pouring conditions were simulated under the conditions of standard secondary cold water total pressure of 11.5 Bar and compressed air total pressure of 5.0 Bar. For the secondary cold water circuit at each pulling speed, set the water volume. At the same time, as far as the on-site circuit is concerned, when 2-3 people check the atomization effect of each spray gun, it is finally ensured that the atomization effect of each spray gun is similar.
First, the cooling temperature of the slab is relatively uniform; second, there is no obvious water accumulation on the surface of the slab. In addition to purging the accumulated water on the surface of the cast slab, not only the wide-surface secondary cold water in all circuits is closed during production, but the compressed air is also turned on. In this way, the phenomenon of casting slabs being taken offline in batches due to corner cracks can be avoided.
Stabilize the rhythm while shortening the smelting cycle
(1) Shorten the sample production time. With the help of the purchase of a new spectrum analyzer, the main operation room on the second floor in front of the converter obtained such equipment in 2018. The accuracy of the analysis results can be ensured when regular calibration of the spectrometer and good daily maintenance are in place. In this way, the time for waiting for samples and sending samples is reduced. The converter smelting cycle will be shortened by one to three minutes.
(2) Reduce the phosphorus return of the slag. With the help of the introduction of advanced technology from foreign factories and technical exchanges, the problem of phosphorization caused by the low hit rate of slag blocking and the difficulty in maintenance of converter slag blocking trucks can be solved. In the early stage of the transformation project regarding the use of sliding plates to retain slag in the converter, relevant parties went to the Nanjing Iron and Steel Plant for inspection. The original use of slag blocking plugs has been abandoned when the actual situation is combined. The effect of steel discharge and slag blocking in the converter has been improved, resulting in a reduction in the number of times of slag discharge and phosphorization.
(3) Reduce tapping time. By adopting large-diameter tapping holes (the original tapping hole has an upper opening of 140mm and a lower opening of 130mm, and the new tapping hole has an upper opening of 160mm and a lower opening of 140mm), the steel-laying cooling time and steel-laying time can be improved and reduced respectively. This provides sufficient conditions for shortening the converter smelting cycle.
(4) Shorten the refining time. When 60-80kg of fluorite is added first, 200-300kg must be added during the specific process of placing steel. In this way we can implement the slag washing operation. The alloy is fully melted when the surface slag is uniform. The amount of refining fluorspar and the refining de-S pressure will decrease and decrease respectively. The smelting cycle is constantly improving as the number of times of molten steel such as continuous casting and refining power consumption are reduced respectively. This creates favorable conditions for a reasonable sequence of production rhythms.
Optimize and adjust the flaw detection plate process
(1) In order to ensure the supply of steam, the dispatching room must coordinate steam 1-2 hours in advance according to the RH plan.
(2) In terms of daily inspection of steam pipelines, the refining workshop must do a good job. Effective measures are very necessary to detect air leaks in pipelines.
(3) The vacuum degree cannot meet the process requirements when the elbow sealing ring of the moving vacuum tank is often displaced. As for the problem of sealing ring displacement, when the vacuum processing time is long, it can be completely solved on the premise that the dovetail groove sealing strip is used.
(4) The vacuum degree is not high when the vacuum tank trolley is used for a long time.
(5) Treatment of RH furnace: the time required for vacuum treatment is ≥15min, the vacuum degree is ≤133Pa, and the ultimate vacuum holding time is ≥10min. The RH vacuum furnace determines H once every two days (originally stipulated 2 times/month). When producing various types of steel, priority must be given to determining hydrogen for various types of steel, and hydrogen determination should be made no less than 15 times per month.
(6) The dispatching room must closely monitor the residual elements of molten iron. In this way, the use of high-quality hot metal in the RH furnace for high-grade steel is ensured.
(7) In response to the problem of RH process inconsistency, it is necessary to speed up the implementation of steel modification. In terms of the outbound hydrogen volume of the RH furnace in 2018, due to the various measures mentioned above, it was reduced by 0.09ppm compared with the previous year. Hot feeding of all flaw detection plates can be realized, increasing the hot feeding rate by >20%.
As far as the secondary cooling water vapor atomization effect is concerned, it is constantly improving when the accuracy of the driving roller is improved and the arc connection of the vertical bending section is stabilized. With a stable production rhythm and all hot-feeding measures for flaw detection plates, the hot-feeding rate of slabs in 2018 reached 82.38%, an increase of 21.16% from last year.