A method for controlling inclusions in a casting process

Abstract

The application discloses a method for controlling inclusions in a casting process. In order to further avoid the inclusions in molten steel exceeding the standard and reduce the inclusions in the non-steady casting process, the tundish is improved in design to improve the flow field and ensure a proper high superheat of the molten steel, so that the inclusions in the cast blank are obviously reduced, the flaw detection qualified rate of the steel plate is increased from 97.5% to more than 99%, the product unqualified rate is obviously reduced, and the economic benefit and social benefit of the steel plant are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of continuous casting in the metallurgical industry, and in particular to a method for controlling inclusions in a casting process. BACKGROUND

[0002] The control of inclusions in molten steel has always been an important direction of pure steel smelting. As is known, non-metallic inclusions in steel are an important factor affecting product quality, and the control of inclusions in steel involves the number, size, size distribution, chemical composition, morphology, and homogenization control of inclusions in steel. In converter steelmaking, especially in the smelting of high-performance steel, preventing molten steel pollution and reducing the content of non-metallic inclusions in steel are important prerequisites for improving the performance of steel grades. By optimizing the smelting process, i.e., optimizing the deoxidizing slagging system, alloy addition method, argon blowing system, calcium treatment process, and protective casting process, large inclusions in steel are fully floated to avoid secondary oxidation of molten steel, and inclusions in steel are effectively controlled.

[0003] Although the above measures are taken in the smelting process to effectively control the inclusions in the steel, the interaction of molten steel with refractory materials, the interaction of molten steel with air, and the non-steady casting of the casting machine during the start, stop, and change of the package process during the casting process can all lead to excessive inclusions in the molten steel. All major steel plants have adopted protective casting to prevent secondary oxidation of molten steel, but there are still a large number of steel plates that fail to meet the requirements due to excessive inclusions, resulting in detection failure and various performance failures, which seriously affect the economic benefits of enterprises.

[0004] The detection failure of steel plates in a certain steel plant is only about 97.5%, and the main reason for the failure is the excessive inclusions in the molten steel, which leads to the failure of the steel plate at the 1 / 4 position in the thickness direction, and the performance failure of high-strength steel due to excessive inclusions. These steel plates can only be used at a lower grade, significantly increasing the manufacturing cost of the steel plant.

[0005] A method for controlling large inclusions in ultra-low carbon steel and its application are disclosed in Chinese Patent No. 202210392994.0, which addresses the technical problem of reducing the amount of slag entering the immersion tube during the RH process. However, the method described in this document further reduces the inclusions in the molten steel during the smelting process and cannot solve the inclusions caused by the casting machine during the casting process. SUMMARY

[0006] To solve the above technical problems, the purpose of the present application is to provide a method for controlling inclusions in a casting process. On the basis of normal protective casting, by improving the operation during non-steady casting, improving the structure of the tundish, and ensuring the superheat of the molten steel during casting, the content of inclusions in the molten steel is maximally controlled to improve the quality of the casting blank and reduce the manufacturing cost.

[0007] To solve the above technical problems, the application adopts the following technical solutions:

[0008] The application is a method for controlling inclusions in a casting process, comprising:

[0009] 1) Improve the operation of non-steady casting

[0010] The non-steady casting is the casting of the casting machine under the conditions of starting casting, stopping casting, changing ladle, etc.

[0011] (1) Downgrade the starting casting blank to a lower grade steel or arrange the rolling thickness to be less than 20mm;

[0012] (2) Improve the stop casting curve and reduce the low liquid level casting time

[0013] When tailing out, the tailing out curve acceleration is changed from -0.2m / min 2 to -0.4m / min 2 , so that the casting speed at the time of tailing out is reduced from 0.8m / min to 0.4m / min, and the time is reduced from 120 seconds to 60 seconds. After the casting speed is reduced to 0.4m / min, the fast tailing out program is executed, and the time to close the stopper is reduced from 150 seconds to 30 seconds. Thus, the low liquid level and low casting speed time at the time of tailing out is reduced from the previous 270 seconds to the improved 90 seconds, and the quality of the tailing out blank is greatly improved;

[0014] (3) Improve the method of changing ladle between heats

[0015] Try to pull the next ladle of molten steel before the "vortex" is generated in the tundish, that is, perform "full ladle" operation: the working liquid level of the tundish of the wide and thick plate caster is 38 tons, and the liquid level of the tundish is ensured to be not less than 30 tons when the next ladle is pulled out. If it is not possible to ensure more than 30 tons due to mixed casting of different steel grades or tight molten steel rhythm, etc., control the flow to 30 tons after the next ladle is pulled out, and slowly control the flow to the working liquid level after 30 tons, so as to minimize the inclusions entering the molten steel;

[0016] 2) Reform the internal structure of the tundish to promote the floating of inclusions in the molten steel

[0017] (1) Appropriately increase the distance between the retaining wall and the dam

[0018] Increase the distance between the retaining wall and the dam of the tundish from 65mm to 130±10mm. By increasing the distance, the molten steel obtains a greater flow rate, reaches the dam and makes the molten steel fully rise, achieving the purpose of making the inclusions in the molten steel fully float;

[0019] (2) Intermittently arranged with refractory bricks between the retaining wall and the dam

[0020] The area through which the molten steel flows between the original retaining wall and retaining dam is made porous, the flow speed of the molten steel through the area is increased without changing the molten steel flow amount, and the purpose of increasing the flow field uplift is achieved.

[0021] 3) Ensure the pouring molten steel superheat is 20-30 DEG C

[0022] The superheat is controlled between 20-30 DEG C, and the electromagnetic stirring in the secondary cooling zone is used, which can ensure that the inclusions in the molten steel are removed sufficiently, and can also reduce the center segregation of the casting blank.

[0023] Further, the distance between the retaining wall and retaining dam of the tundish is increased from 65mm to 130mm.

[0024] Further, the inclusions of the prepared casting blank are obviously reduced, the steel plate flaw detection qualified rate is increased from 97.5% to more than 99%, and the product unqualified rate is significantly reduced.

[0025] Compared with the prior art, the beneficial technical effects of the present application are:

[0026] The inclusions control method in the pouring process of the present application can further avoid the inclusions exceeding the standard, reduce the inclusions in the non-steady state pouring process, improve the tundish design to improve the flow field, ensure the appropriate high molten steel superheat, obviously reduce the inclusions of the prepared casting blank, increase the steel plate flaw detection qualified rate from 97.5% to more than 99%, significantly reduce the product unqualified rate, improve the economic benefit and social benefit of the steel plant. BRIEF DESCRIPTION OF DRAWINGS

[0027] The present application will be further described below in combination with the description of the drawings.

[0028] Figure 1 It is the tundish liquid level control curve when the tundish is replaced in the present application.

[0029] Figure 2 It is the flaw detection picture of the casting blank rolled into a steel plate prepared by the embodiment of the present application. DETAILED DESCRIPTION

[0030] An inclusions control method in a pouring process, comprising:

[0031] 1. Improve the operation during non-steady state pouring

[0032] The non-steady state pouring is the pouring of the caster under the conditions of starting pouring, stopping pouring, replacing the tundish, etc.

[0033] (1) The starting pouring blank is degraded into a low-grade steel grade or arranged to have a rolling thickness less than 20mm

[0034] The molten steel in the tundish has no covering agent when the ladle is just opened, at this time the molten steel is severely oxidized to form oxide inclusions, and the tundish just roasted has fallen refractory and other impurities, these inclusions into the molten steel, seriously affect the purity of the molten steel, at this time the billet cast contains a large amount of inclusions, easy to cause steel plate flaw detection and other performance do not agree. So the corresponding billet at the beginning of pouring is downgraded to low-grade steel or arranged to roll 20mm below specification, can avoid the quality defects of the first billet of the pouring time to cause the quality defects of the steel plate.

[0035] (2) Improve the stop pouring curve and reduce the low liquid level pouring time

[0036] When the stop pouring program is executed, as the tundish liquid level decreases, due to the "vortex" effect, a large amount of inclusions enters the crystallizer, and the corresponding casting billet contains a large amount of inclusions, which affects the quality of the casting billet.

[0037] Because of the low liquid level, only the low liquid level and low speed time can be shortened. When tailing out, the tailing out curve acceleration is changed from-0.2m / min 2 To-0.4m / min 2, , so that the speed from the tailing out is reduced from 0.8m / min to 0.4m / min, from the previous 120 seconds to 60 seconds, and the speed is reduced to 0.4m / min. After executing the fast tailing out program, the time to close the stopper is reduced from the previous 150 seconds to 30 seconds. In this way, the low liquid level and low speed time when tailing out is reduced from the previous 270 seconds to the improved 90 seconds, and the quality of the tail billet of the pouring time is greatly improved.

[0038] (3) Improve the tundish changing operation method between heats

[0039] When the tundish is changed between heats, the tundish liquid level continues to decrease, and due to the "vortex" effect, a large amount of inclusions enters the crystallizer, and the corresponding casting billet contains a large amount of inclusions, which affects the quality of the casting billet.

[0040] The improved method is to try to pull the next ladle of molten steel before the "vortex" of the tundish liquid level is generated, that is, to carry out "full ladle" operation: the working liquid level of the tundish of the plate and heavy plate caster is 38 tons, and the next ladle is pulled to ensure that the tundish liquid level is not less than 30 tons. If mixed pouring of different steel grades or due to tight molten steel rhythm, it is not possible to ensure more than 30 tons, after the next ladle is pulled, the flow is controlled to 30 tons (13 tons per minute), and after 30 tons, the flow is controlled to the working liquid level at a slow speed (7 tons per minute), so as to minimize the inclusions entering the molten steel. Figure 1 .

[0041] The purpose of controlling the flow to 30 tons when the liquid level of the tundish is below 30 tons is to reduce the casting time of low liquid level and the inclusion entering caused by the vortex phenomenon; the purpose of slowly controlling the flow to the working liquid level when the liquid level of the tundish is above 30 tons is to reduce the disturbance of the steel flow to the molten steel in the tundish and further reduce the inclusion entering the molten steel.

[0042] 2. Transforming the internal structure of the tundish to promote the floating of the inclusions in the molten steel.

[0043] (1) Appropriately increasing the distance between the retaining wall and the retaining dam.

[0044] The retaining wall and the retaining dam play the role of blocking the steel slag and improving the flow in the tundish; the distance between the retaining wall and the retaining dam of the tundish of the wide plate caster in our factory is 65 mm, and we increase the distance to 130 mm, so that the molten steel obtains a greater flow rate, reaches the retaining dam and fully rises, thereby achieving the purpose of making the inclusions in the molten steel fully float.

[0045] (2) Separately arranging the refractory bricks between the retaining wall and the retaining dam.

[0046] The area through which the molten steel flows between the original retaining wall and the retaining dam is changed into a plurality of holes, and the flow rate of the molten steel flowing through the area is increased under the condition that the amount of the molten steel passing through the area is unchanged, thereby achieving the purpose of improving the rising of the flow field.

[0047] Through the transformation of the internal structure of the tundish, the rising force of the molten steel and the flow field are more active, and the inclusions in the steel are effectively promoted to float and be fully absorbed by the covering agent.

[0048] 3. Guaranteeing the superheat of the cast molten steel to be 20-30 ℃.

[0049] From the perspective of improving the center segregation of the cast slab, the low superheat casting should be guaranteed, especially when the temperature of the molten steel is close to the liquidus temperature, which is the best. However, the practice has proved that although the low superheat casting can reduce the center segregation of the cast slab, the inclusions cannot be removed in time, which easily causes the non-conformity of the steel plate detection. The practice has proved that the superheat is controlled to be 20-30 ℃, and the electromagnetic stirring in the secondary cooling zone is used, so that the inclusions in the molten steel can be fully removed, and the center segregation of the cast slab can be reduced.

[0050] Implementation case:

[0051] The implementation case uses the control method of the application to control the casting of the wind power steel.

[0052] Transforming the internal structure of the tundish before casting: increasing the distance between the retaining wall and the retaining dam of the tundish from 65 mm to 130 mm, separately arranging the refractory bricks between the retaining wall and the retaining dam, and changing the area through which the molten steel flows between the original retaining wall and the retaining dam into a plurality of holes;

[0053] The casting blank is degraded into a low-grade steel or arranged to be rolled to a thickness less than 20 mm;

[0054] The low liquid level and low pulling speed time at the time of stopping casting and tailing out is reduced from the previous 270 seconds to the improved 90 seconds;

[0055] Full ladle operation is performed during ladle changing;

[0056] The steel liquid superheat degree is controlled at 20-30°C during casting.

[0057] The cast blank is rolled into a steel plate without flaw detection disqualification phenomenon, and the flaw detection picture is as shown in Figure 2 .

[0058] It can be seen that, in the steel casting process, by taking measures in the non-steady state processes such as starting casting, stopping casting and ladle changing, and improving the tundish design, the inclusions are fully floated, and the appropriate superheat degree is ensured during the casting process, so that the inclusions can be obviously reduced, and the steel plate flaw detection qualified rate is significantly improved.

[0059] The above-described embodiments are only used to describe the preferred modes of the present application, and do not limit the scope of the present application, and various modifications and improvements to the technical solutions of the present application made by those skilled in the art without departing from the design spirit of the present application shall fall within the protection scope of the present application defined by the claims.

Claims

1. A method for controlling inclusions during casting, characterized in that: include: 1) Improve the operation during unsteady-state casting The unsteady-state casting refers to casting under conditions of casting machine start-up, stop-up, and ladle change; (1) Downgrade the billet to a lower grade of steel or arrange for rolling a thickness of less than 20 mm; (2) Improve the casting stop curve and reduce the casting time at low liquid level. At tail exit, the tail exit curve acceleration is changed from -0.2 m / min 2 Change to -0.4m / min 2 This reduces the casting speed from 0.8 m / min at the tail exit to 0.4 m / min, and the time from 120 seconds to 60 seconds. After the casting speed drops to 0.4 m / min, a rapid tail exit procedure is executed, and the time until the stopper is closed is reduced from 150 seconds to 30 seconds. This reduces the time of low liquid level and low casting speed at the tail exit from 270 seconds to an improved 90 seconds, greatly improving the quality of the tail billet. (3) Improve the inter-furnace ladle changing operation method Try to open the next ladle of molten steel before a "vortex" forms in the tundish level, i.e., perform a "full ladle" operation: The working liquid level in the tundish of the Baotou Steel wide and thick plate casting machine is 38 tons. Ensure that the liquid level in the tundish is not lower than 30 tons when the next ladle is opened. If different steel grades are mixed or the liquid level cannot be guaranteed to be above 30 tons due to the tight molten steel flow, control the flow to 30 tons after the next ladle is opened. After 30 tons, slowly control the flow to the working liquid level to minimize the amount of inclusions entering the molten steel. 2) Modify the internal structure of the tundish to promote the floating of inclusions in the molten steel. (1) Appropriately increase the distance between the retaining wall and the dam. The distance between the tundish retaining wall and the dam is increased from 65mm to 130±10mm. By increasing the distance, the molten steel can achieve a greater flow velocity and reach the dam, allowing the molten steel to rise fully and achieving the purpose of making the inclusions in the molten steel float to the surface. (2) Refractory bricks are used to intersperse between retaining walls and dams. The area through which molten steel flows between the original retaining wall and dam is made porous. While keeping the amount of molten steel constant, the flow velocity of molten steel through this area is increased, thereby achieving the goal of increasing the upward flow of the flow field. 3) Ensure the superheat of the cast steel is between 20 and 30°C. The superheat is controlled between 20 and 30°C, and electromagnetic stirring in the secondary cooling zone is used to ensure that inclusions in the molten steel are fully removed and to reduce segregation in the center of the billet.

2. The method for controlling inclusions during the casting process according to claim 1, characterized in that: The distance between the intermediate liner retaining wall and the dam was increased from 65mm to 130mm.

3. The method for controlling inclusions during the casting process according to claim 1, characterized in that: The resulting billet had significantly fewer inclusions, and the steel plate flaw detection pass rate increased from 97.5% to over 99%, significantly reducing the product defect rate.

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