Control process for reducing shrinkage cavities of slab continuous casting tail blank

Abstract

The invention provides a control process for reducing shrinkage cavities of a slab continuous casting tail blank. Comprising the steps that S1, a compensatory soft reduction mode is adopted after final casting, and tail blank shrinkage caused by cooling shrinkage of a casting blank after final casting is compensated; according to the method, an SMART fan-shaped section is utilized, a casting blank is pressed down in an unsolidified area of the fan-shaped section after final casting, and tail blank shrinkage caused by solidification shrinkage is compensated through slight deformation of the casting blank; a compensatory soft reduction mode is associated with a final casting instruction, and after final casting of a casting machine, the opening degree of each fan-shaped section is adjusted according to a set position, so that the feeding function of a casting blank through slight reduction after final casting is realized. S2, the secondary cooling water amount is reduced after final pouring, so that the cooling strength of the tail blank is reduced; and S3, top sealing and cooling are conducted after final pouring. By means of the compensatory soft reduction mode, secondary cooling control optimization and top sealing cooling after final pouring, the tail blank shrinkage cavity length is effectively reduced, the casting blank yield and quality are improved, meanwhile, the production cost is reduced, the production efficiency is improved, and the remarkable effect on improvement of the continuous casting process is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of slab continuous casting, and in particular to a control process for reducing shrinkage cavities in tail slabs during slab continuous casting. Background Art

[0002] Shrinkage cavities at the tail end of the slab are a common technical challenge during the slab continuous casting process. The working principle of a slab continuous casting machine is to gradually solidify the slab in a sector through continuous casting, forming a long liquid phase cavity. As the density of molten steel increases and its volume shrinks during solidification, additional molten steel is required to replenish the volume required for solidification. This process proceeds smoothly under normal steel pouring conditions because the molten steel in the upper crystallizer of the slab can be continuously replenished from the tundish, thus avoiding the formation of shrinkage cavities in the center of the slab. However, at the end of the pour, the situation changes. After the final pouring of the tundish, the molten steel in the crystallizer loses the replenishment of the molten steel from the tundish. At this time, the shrinkage cavities required for the solidification of the molten steel in the liquid phase cavity can only be replenished by the molten steel at the tail end of the slab. This inevitably leads to a section with shrinkage cavities in the middle of the tail end of the casting. During the subsequent soaking process before hot rolling, the surface of the ingot with shrinkage cavities is easily oxidized. During the rolling process, the shrinkage cavities cannot be welded, which will cause problems such as stratification of the ingot, making it unusable as a qualified product. Therefore, the tail ingot affected by the shrinkage cavities can only be cut and scrapped, which undoubtedly increases production costs and reduces production efficiency.

[0003] To reduce the length of shrinkage cavities affecting the tail billet and improve the yield of the ingot, existing technologies employ a variety of methods. In terms of pouring operations, operators will control the timing of the final pouring of the tundish as precisely as possible to ensure that at the end of the pour, a certain amount of liquid steel remains at the tail of the billet for shrinkage compensation. At the same time, in terms of casting speed control, the casting speed will be appropriately reduced based on the actual conditions at the end of the pour, extending the residence time of the molten steel in the liquidus cavity, allowing more molten steel to fully solidify and compensate for shrinkage. Furthermore, in terms of equipment, some continuous casting machines are equipped with specialized hold-down devices that apply a certain amount of pressure to the tail billet to promote the flow of molten steel and compensate for shrinkage, thereby reducing the length of the shrinkage cavity.

[0004] However, although the existing technologies have made certain efforts to reduce the shrinkage holes in the tail billet, there are still many shortcomings. On the one hand, the existing casting operation and casting speed control process are difficult to be accurate and stable. They are affected by various factors such as the operator's experience, equipment accuracy and production environment, and are prone to deviations, resulting in the length of the tail billet shrinkage hole cannot be effectively controlled. On the other hand, the existing pressure-down shrinkage compensation measures have limited shrinkage compensation effects in actual applications, and may have an adverse effect on the surface quality and internal structure of the billet. More importantly, these existing technologies are often optimized in isolation for a certain link, lacking systematicity and synergy, and it is difficult to significantly reduce the affected length of the tail billet shrinkage hole as a whole, and thus cannot effectively improve the billet yield of the continuous casting machine to meet the demand for efficient and high-quality billets in modern steel production. Summary of the Invention

[0005] Based on the above technical problems proposed in the prior art in reducing the length of the tail billet affected by shrinkage cavities, such as inaccurate operation, limited shrinkage compensation effect, and lack of systematic coordination, a control process for reducing the shrinkage cavities of the tail billet in continuous casting of slabs is provided. The present invention mainly utilizes the existing process facilities and final pouring operations to coordinate the tail billet capping, casting speed control, and pressure compensation in order to reduce the length of the tail billet affected by shrinkage cavities, thereby reducing the length of the tail billet affected by shrinkage cavities and improving the yield rate of the continuous casting machine. The technical means adopted by the present invention are as follows: A control process for reducing shrinkage holes in the tail of a slab continuous casting process comprises the following steps: S1. After the final pouring, a compensatory soft reduction mode is used to compensate for the shrinkage cavity of the tail billet caused by the cooling and shrinkage of the billet after the final pouring; S2. Reduce the amount of secondary cooling water after the final pouring to reduce the cooling intensity of the tail billet; S3. After the final pouring, cap and cool the roof.

[0006] Furthermore, S1 specifically includes: using SMART segments to press down the slab in the unsolidified area of ​​the segments after the final pour, using slight deformation of the slab to compensate for shrinkage holes in the tail slab caused by solidification shrinkage; setting additional reduction according to the thickness of the slab and the liquidus cavity ratio of the segments, and designing a three-stage reduction to avoid cracks in the slab caused by concentrated reduction; The compensatory light pressure mode is associated with the final pouring instruction. After the final pouring of the casting machine, the opening degree of each sector is adjusted according to the set position to realize the shrinkage compensation function of the billet through slight pressure after the final pouring.

[0007] Furthermore, S3 specifically includes: after the slag is removed from the continuous casting final casting crystallizer, nail chips are added into the crystallizer according to the casting section and stirred to cool down.

[0008] Furthermore, the weight range of the added nail chips is 4-8 kg.

[0009] Compared with the prior art, the present invention has the following advantages: The present invention develops a compensatory light reduction mode by utilizing the existing SMART sector segments, and provides an effective compensation means for the fundamental cause of the tail billet shrinkage cavity - the solidification shrinkage of the billet during the solidification process. After the final pour, the billet is pressed down in the unsolidified area of ​​the sector segment, and the slight deformation of the billet is used to compensate for the tail billet shrinkage cavity caused by solidification shrinkage. According to the thickness of the billet and the liquid phase hole ratio of the sector segment, a reasonable additional reduction amount (5.8mm) is set, and in order to avoid cracks in the billet due to concentrated reduction, the reduction is designed to be carried out in three sections, and the reduction is achieved by setting the opening degree of the entrance roller and exit roller of each section. This segmented reduction method can not only effectively reduce the length of the tail billet shrinkage cavity, but also avoid damage to the billet, thereby improving the quality stability of the billet. At the same time, the compensatory light pressure mode and the final pouring instruction are linked together. After the final pouring of the casting machine, the opening degree of each fan-shaped segment is automatically adjusted according to the set position, which can achieve precise control of the shrinkage cavity of the tail billet, thereby significantly reducing the length of the tail billet affected by the shrinkage cavity, reducing the amount of waste cutting and processing, and improving the yield rate of the billet.

[0010] The present invention optimizes the secondary cooling control after the final pouring, and develops a secondary cooling final pouring model to replace the original normal secondary cooling water model by appropriately reducing the secondary cooling water volume. According to the characteristics of the tail of the billet, the water volume of the first 8 circuits after the final pouring is set at 80% of the original design water volume, which is associated with the final pouring instruction. Since the static pressure of the molten steel of the tail billet after the final pouring of the casting machine no longer increases with the billet drawing, appropriately reducing the cooling intensity can increase the shrinkage feeding molten pool at the tail of the billet, thereby further improving the shrinkage feeding ability of the tail billet and helping to reduce the length of the shrinkage cavity. This optimized secondary cooling control method not only improves the internal quality of the billet, but also reduces the internal stress of the billet caused by excessive cooling, thereby improving the comprehensive performance of the billet.

[0011] The present invention proposes an innovative solution to the problem of capping after the final pour of continuous casting. After the slag is removed from the crystallizer of the final pour of continuous casting, 4-8 kg of nail chips are added to the crystallizer according to the casting section and stirred to cool down. The specific amount added can be adjusted according to the actual situation. This approach can ensure that the tail billet is quickly capped after the final pour, avoiding problems such as tail billet hanging due to shrinkage pressure and reduced cooling. Rapid capping can not only shorten the production cycle and improve production efficiency, but also ensure the surface quality of the billet, reduce the scrapping of the billet due to defects such as hanging steel, and further improve the qualified rate and yield rate of the billet. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0013] Figure 1 It is a process flow chart of the present invention. DETAILED DESCRIPTION

[0014] It should be noted that, in the absence of conflict, the embodiments and features of the embodiments of the present invention can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0015] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is in no way intended to limit the present invention and its application or use. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0016] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0017] Unless otherwise specified, the relative arrangement of the parts and steps, numerical expressions and numerical values ​​set forth in these embodiments do not limit the scope of the present invention. At the same time, it should be clear that, for ease of description, the sizes of the various parts shown in the accompanying drawings are not drawn according to actual proportional relationships. The technology, methods and equipment known to those of ordinary skill in the relevant art may not be discussed in detail, but in appropriate cases, the technology, methods and equipment should be considered as a part of the specification. In all examples shown and discussed here, any specific value should be interpreted as being merely exemplary, rather than as a limitation. Therefore, other examples of the exemplary embodiments can have different values. It should be noted that similar numbers and letters represent similar items in the following drawings, and therefore, once an item is defined in an accompanying drawing, it does not need to be further discussed in subsequent drawings.

[0018] In the description of the present invention, it should be understood that the directions or positional relationships indicated by directional words such as "front, back, up, down, left, right", "horizontal, vertical, vertical, horizontal" and "top, bottom" are usually based on the directions or positional relationships shown in the accompanying drawings. They are only for the convenience of describing the present invention and simplifying the description. Unless otherwise specified, these directional words do not indicate or imply that the device or element referred to must have a specific direction or be constructed and operated in a specific direction. Therefore, they cannot be understood as limiting the scope of protection of the present invention: the directional words "inside and outside" refer to the inside and outside relative to the outline of each component itself.

[0019] For ease of description, spatially relative terms such as "above," "above," "on the upper surface of," and "above" may be used herein to describe the spatial positional relationship of a device or feature to other devices or features as shown in the figures. It should be understood that spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is inverted, a device described as "above" or "on top of" another device or structure would then be positioned as "below" or "below" the other device or structure. Thus, the exemplary term "above" may include both the orientations of "above" and "below." The device may also be positioned in other different ways (rotated 90 degrees or in other orientations), and the spatially relative descriptions used herein should be interpreted accordingly.

[0020] Currently, the shrinkage of molten steel during solidification is approximately 4%, while the length of the liquid phase cavity during normal slab casting is approximately 20-25m. Based on the liquid phase cavity state of the second-cold solidification model of the slab in the fan-shaped section, the liquid phase cavity of the slab accounts for 60% of the total slab. Based on the above data, it can be calculated that after the final pour, the maximum impact length of the normal tail slab is 0.6m. If factors such as soft pressure are taken into account, the length will be even shorter. However, in actual production, the average tail slab cutting loss length is 0.9m, and sometimes it can reach more than 1.5m. The main reasons include the following: The solidification of the liquid phase hole of the billet in the secondary cooling section of the tail billet casting process is a process from top to bottom, and the billet shell at the feeding molten steel position is thickening at the same time.

[0021] After the final pour of the continuous casting, in order to avoid steel hanging on the tail billet, "capping" and secondary cooling are required, which will increase the thickness of the billet shell and reduce the molten pool at the tail of the tail billet for shrinkage compensation.

[0022] The factor that has the greatest impact on the shrinkage of the tail billet is the cutting loss caused by the holes in the tail billet.

[0023] The present invention is used on a large slab continuous casting machine of 230*1100-2300. On the basis of existing process equipment, through the design of relevant process parameters, a control method is formed to reduce the cutting loss of the slab tail and improve the slab yield, achieving good results.

[0024] like Figure 1 As shown, the present invention provides a control process for reducing shrinkage holes in the tail of a slab continuous casting, comprising the following steps: S1. After the final pouring, a compensatory soft reduction mode is used to compensate for the shrinkage cavity of the tail billet caused by the cooling and shrinkage of the billet after the final pouring; The existing SMART segments are used to develop a compensatory soft reduction mode to compensate for the shrinkage cavity in the tail billet caused by the cooling and shrinkage of the billet after the final pouring.

[0025] Since the shrinkage cavity of the tail billet is caused by the formation of the billet shell inside and outside the sector segment, and the liquid phase cavity inside the billet shell is caused by the 4% solidification shrinkage during the solidification process, the present invention utilizes the existing SMART sector segment, and presses the billet in the unsolidified area of ​​the sector segment after the final pouring, and uses the slight deformation of the billet to compensate for the shrinkage cavity of the tail billet caused by solidification shrinkage.

[0026] Based on the thickness of the slab and the liquidus-hole ratio of the segments, the additional reduction was set at 5.8 mm. To avoid cracking caused by concentrated reduction, the reduction was designed to be carried out in three stages. The casting machine segments and the distance between each segment and the liquidus level of the mold are shown in Table 1.

[0027] Table 1 Segment distribution and position of each segment of the casting machine

[0028] According to the calculation of the secondary cooling model, sections 3, 4, and 5 where the liquid core exists are selected for reduction setting. The reduction is achieved by setting the opening of the inlet and outlet rollers of each section. The specific opening design of each section in the compensatory light reduction mode is shown in Table 2.

[0029] Table 2 Opening of each section in the compensatory soft reduction mode

[0030] The compensatory light pressure mode and the final pouring instruction are associated in the system. After the final pouring of the casting machine, the opening degree of each sector is adjusted according to the set position, thereby realizing the shrinkage compensation function of the billet through slight pressure after the final pouring.

[0031] S2. Reduce the amount of secondary cooling water after the final pouring to reduce the cooling intensity of the tail billet; The secondary cooling water in continuous casting is used to cool the strands in the fan-shaped sections, ensuring the strand shell grows thicker and preventing bulging caused by the static ferrostatic pressure within the strand. A thicker tail strand shell reduces the shrinkage-feeding capacity of the liquid phase cavity within the tail strand, increasing its length. Since the static ferrostatic pressure of the tail strand after the final casting of the caster does not increase with the strand drawing, the cooling intensity of the tail strand can be appropriately reduced.

[0032] This patent sets the water volume of the first eight circuits after the final pour at 80% of the original design, and develops a secondary cooling final pour model to replace the original normal secondary cooling water model. This model is only used for water volume control after the final pour and is associated with the final pour instruction. The specific water volume is shown in Table 3. It is used to increase the shrinkage molten pool at the tail of the cast billet.

[0033] Table 3 Cooling water volume of each circuit in the secondary cooling final pouring model

[0034] S3. Cap and cool after final pouring.

[0035] In order to ensure that the tail billet can be quickly capped after the final pouring and avoid the tail billet hanging on the steel due to the shrinkage compensation pressure and reduced cooling, after the slag is removed from the continuous casting final pouring crystallizer, 4-8 kg of nail chips are added to the crystallizer according to the casting section and stirred to cool it down. The specific amount added is shown in Table 4.

[0036] Table 4 Amount of nail chips added for different casting sections

[0037] The present invention was applied to the production of various types of steel on a 230*1100-2300 large slab continuous casting machine. The process was normal and no production problems such as tail billet hanging on the steel occurred. Table 5 shows the shrinkage cavity conditions of the tail billet in the five castings before implementation; Table 6 shows the shrinkage cavity conditions of the tail billet in the five castings after implementation. From the actual situation, after adopting this method, the length of the tail billet shrinkage cavity was reduced by an average of 29.94% compared with the original.

[0038] Table 5 Length of shrinkage cavity in tail billet during the first five pourings

[0039] Table 6 Length of shrinkage cavity in tail billet after 5 pouring times

[0040] Example 1 Steel grade: X42, superheat of the last shot of the tundish: 23°C; production section: 230×1610mm, production process: 1. The remaining molten steel in the tundish is 8.1 tons, and the final casting is carried out at a casting speed of 0.4m / min; 2. After pressing the final pouring button, the second cold water will automatically select the final pouring model; 3. The sector segment automatically selects the compensatory light pressing mode when pressing down; 4. Carry out routine slag removal after final pouring; 5. Add 6 kg of nail chips to the crystallizer and stir; 6. After the tail billet leaves the crystallizer, slowly increase the speed to 1.3m / min and pull it out. The actual tail blank was cut twice, with a cutting length of 0.67m.

[0041] Example 2 Steel grade: CCSB, superheat of the last shot of the tundish: 25°C; production section: 230×2030mm, production process: 1. The remaining molten steel in the tundish is 8.51 tons, and the final casting is carried out at a casting speed of 0.4m / min; 2. After pressing the final pouring button, the second cold water will automatically select the final pouring model; 3. The sector segment automatically selects the compensatory light pressing mode when pressing down; 4. Carry out routine slag removal after final pouring; 5. Add 8 kg of nail chips to the crystallizer and stir; 6. After the tail billet leaves the crystallizer, slowly increase the speed to 1.5m / min and pull it out. The actual tail blank was cut in one shot, and the cutting length was 0.57m.

[0042] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A control process for reducing shrinkage cavity of tail slab in continuous casting, characterized in that: The steps include: S1. After the final pouring, a compensatory soft reduction mode is used to compensate for the shrinkage cavity of the tail billet caused by the cooling and shrinkage of the billet after the final pouring; S2. Reduce the amount of secondary cooling water after the final pouring to reduce the cooling intensity of the tail billet; S3. Cap and cool after final pouring.

2. The control process for reducing shrinkage cavity of the tail billet of slab continuous casting according to claim 1 is characterized in that: S1 specifically includes: using SMART segments to press down the slab in the unsolidified area of ​​the segments after the final pour, using slight deformation of the slab to compensate for shrinkage holes in the tail slab caused by solidification shrinkage; setting additional reduction based on the slab thickness and the liquidus cavity ratio of the segments, and designing a three-stage reduction to avoid cracks in the slab caused by concentrated reduction; The compensatory light pressure mode is associated with the final pouring instruction. After the final pouring of the casting machine, the opening degree of each sector is adjusted according to the set position to realize the shrinkage compensation function of the billet through slight pressure after the final pouring.

3. The control process for reducing shrinkage cavity of tail slab in continuous casting of slab according to claim 1, characterized in that: S3 specifically includes: after the slag is removed from the continuous casting final casting mold, nail chips are added into the mold according to the casting section to stir and cool it down.

4. The control process for reducing shrinkage cavity of tail slab in continuous casting of slab according to claim 3, characterized in that: The weight range of the added nail chips is 4-8 kg.

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