A method for determining the taper of the inner cavity of the crystallizer for ultra-high casting speed billet continuous casting

Abstract

This patent discloses a method for determining the inner cavity taper of an ultra-high-speed billet continuous casting crystallizer. 1) The contraction of the solidified shell in the crystallizer area and the high-temperature deformation of the copper tube are obtained by comprehensive numerical simulation to obtain the three-dimensional change law of the taper of the copper tube of the billet crystallizer in the longitudinal and transverse directions under ultra-high drawing speed; 2) This taper is obtained based on ideal working conditions and is adapted to the length and corner fillet radius of the ultra-high-speed crystallizer used; 3) For ease of manufacturing, several characteristic points are selected in the transverse direction of the solidified billet, and numerical regression is used to obtain the change curve of the crystallizer taper in the vertical direction. The change of the taper between different characteristic points should gradually transition naturally during processing; 4) Based on the difference in solidification shrinkage characteristics of peritectic steel and non-peritectic steel, the tapers of two types of crystallizer copper tubes are designed to meet production needs. According to the present invention, an inner cavity taper curve that ensures efficient heat transfer in the crystallizer can be obtained, laying a foundation for realizing ultra-high-speed continuous casting of billets and ensuring billet quality and smooth continuous casting.

Description

Technical Field

[0001] The present invention provides a method for determining the taper of the inner cavity of a crystallizer in the process of ultra-high-speed billet continuous casting, by which a taper curve of the inner cavity of the crystallizer can be obtained to ensure efficient heat transfer in the crystallizer, thereby realizing ultra-high-speed continuous casting of billets and ensuring the quality of the billet and smooth continuous casting. This method is mainly used in the design of the inner cavity taper of the crystallizer in the continuous casting process of steel and in the field of crystallizer manufacturing and use. Background Art

[0002] As one of the main development trends of continuous casting technology today, ultra-high speed billet continuous casting can not only significantly improve productivity, reduce production costs and save energy consumption, but also has great significance for the realization of billet continuous casting and rolling process. Compared with conventional billet continuous casting, ultra-high speed billet continuous casting has greatly increased the drawing speed and significantly accelerated the solidification speed. Ensuring efficient heat transfer in the crystallizer is the key to the crystallizer design. Among the many factors that affect the heat transfer of the continuous casting crystallizer, such as cooling water, copper tube cooling structure, operating conditions and inner cavity taper, the inner cavity taper of the crystallizer is the most significant.

[0003] During the continuous casting process, the molten steel injected into the inner cavity of the crystallizer through the immersion nozzle is cooled by the forced water-cooled copper tube of the crystallizer, and gradually solidifies from the meniscus until a solidified shell with a certain thickness is formed at the outlet of the crystallizer. During the solidification of the molten steel, the shrinkage and deformation of the solidified ingot will occur simultaneously, which will cause the solidified shell surface and the hot surface of the crystallized copper tube to separate and produce an air gap, especially in the middle and lower parts and corners of the crystallizer. The thickness of the air gap will continue to increase along the direction of the crystallizer billet drawing; in addition, the expansion and deformation of the crystallizer copper tube after heating will further increase the thickness of the air gap. Since the thermal conductivity of the air gap itself is very weak, the presence of an air gap in the crystallizer will significantly reduce its heat transfer capacity, causing the temperature distribution and thickness of the solidified ingot to become uneven, which will be reflected in the final product as surface and sub-surface cracks, longitudinal cracks and other quality defects of the ingot, and even steel leakage accidents in severe cases. In order to eliminate the air gap in the crystallizer and improve its heat transfer capacity, the most effective means is to design the inner cavity of the crystallizer into an inverted taper to compensate for the air gap that occurs after the solidification of the ingot shrinkage and the crystallizer deformation.

[0004] Therefore, in order to ensure efficient heat transfer in the crystallizer and achieve ultra-high casting speed, it is urgent to refine the taper of the inner cavity of the ultra-high-speed continuous casting crystallizer of the square billet. Regarding the method for determining the taper of the inner cavity of the continuous casting crystallizer, the results of the literature and patent search are as follows.

[0005] 1) Each steel mill first determines a taper based on the accumulated production experience according to the billet type, steel type, casting speed and other process parameters, and then corrects the previously proposed taper of the crystallizer cavity by analyzing the quality of the produced continuous casting billets and observing whether the continuous casting is smooth. The main problems of this method are strong dependence on experience, long design cycle and high cost.

[0006] 2) Formula derivation and calculation: Under the premise of known solidification coefficient, the thickness of the billet along the billet drawing direction is first calculated according to the root mean square formula of liquid steel solidification, and then the total shrinkage of the billet cross section is calculated by the relationship between carbon content and solidification shrinkage, phase change shrinkage, liquid steel shrinkage during cooling, and solid state shrinkage during cooling, and then the connection taper curve along the billet drawing direction is obtained. However, this taper is fixed along the circumference of the crystallizer, and it is impossible to fully fit the billet and the inner wall, especially at the corners of the crystallizer; in addition, this method does not consider the influence of copper plate deformation on the determination of taper.

[0007] 3) Establish a two-dimensional high-temperature stress-strain model of the solidified ingot, apply empirical heat flow as the heat transfer boundary condition and the corresponding mechanical boundary, and use relevant commercial finite element software to perform thermal-mechanical coupling simulation to obtain the solidification shrinkage strain information of the ingot; then use the same method to simulate the copper plate of the crystallizer to obtain its deformation information; finally, superimpose the two deformations of the ingot and the copper plate to obtain the taper curve of the crystallizer cavity. The main problem with this method is that after the two-dimensional modeling of the ingot, the influence of the flow behavior of the molten steel on the solidification heat transfer cannot be considered. Especially for ultra-high pulling speeds, not considering the significantly increased forced convection heat transfer in the crystallizer will reduce the reliability of the final simulation results.

[0008] 4) In addition, some scholars proposed to first establish a three-dimensional model of the flow and solidification heat transfer of molten steel in the crystallizer, and use the inverse algorithm to obtain the heat flow boundary, and simulate the three-dimensional temperature distribution of the solidified ingot; based on this, the relevant formula is used to calculate the solidification shrinkage of the solidified ingot. A three-dimensional stress-strain model is established for the crystallizer to perform a full thermal-mechanical coupling simulation to obtain the deformation information of the crystallizer. Finally, the two are combined to obtain the multi-linear inverted taper of the inner cavity of the square billet crystallizer. This method uses a formula to calculate the solidification shrinkage of the ingot, and the processing method is simple. No explanation is given for the selection of relevant parameters during the calculation; in addition, when the final crystallizer taper is determined, the simulated crystallizer cavity taper is manually changed significantly.

[0009] 5) The Chinese invention patent "Method for Determining the Taper of the Inner Cavity of a Continuous Casting Crystallizer" (application number: 201310383942.8) provides a method for first simulating the three-dimensional flow and heat transfer solidification model that couples the parameters of the molten metal, the crystallizer, and the nozzle to obtain the three-dimensional temperature distribution of the solidified ingot and the crystallizer; then the temperature is used as a heat load to import the established two-dimensional high-temperature stress-strain model of the ingot and the three-dimensional finite element model of the crystallizer to perform thermal-mechanical coupling calculations to obtain the deformation information of both; finally, the two strains are superimposed to obtain the taper curve of the inner cavity of the crystallizer. Although this patent considers the influence of the flow behavior of molten steel on solidification heat transfer, it is aimed at the design of the inner cavity taper of the crystallizer at conventional drawing speeds, and does not consider the design of the crystallizer taper at ultra-high drawing speeds, and does not explain the machining problem of the transverse inner cavity taper curve of the crystallizer; different types of crystallizer inner cavity tapers are not designed based on the differences in the shrinkage characteristics of steel grades.

[0010] 6) The Chinese utility model patent "A Plum Blossom-shaped Crystallizer Copper Tube" (application number: 201520168790.4) proposes that the cross-section of the upper curved surface of the crystallizer copper tube is convex, the middle curved surface cross-section is rectangular, and the lower curved surface cross-section is a rectangle with raised corners to fit the curve formed by the bulging of the billet, while ensuring the lateral temperature uniformity of the billet, thereby achieving a high drawing speed. The crystallizer cavity structure designed by this patent has a maximum drawing speed of more than 4.0m / min for a 150mm×150mm square billet, and does not involve ultra-high drawing speed; the patent does not propose to design the inner cavity taper of two types of crystallizers according to the type of steel.

[0011] 7) The Chinese utility model patent "Hyperbolic Crystallizer Copper Tube" (application number: 201110324570.5) designs the taper curve of the crystallizer cavity as a hyperbola, and has a larger taper near the meniscus, so as to better adapt to the contraction of the billet shell in the longitudinal direction of the crystallizer, improve the heat transfer effect of the crystallizer, and meet the requirements of high drawing speed. This patent targets low drawing speed and does not involve ultra-high drawing speed; it only considers the taper curve of the crystallizer cavity in the longitudinal direction of the crystallizer, and does not elaborate on the design and machining issues of the taper curve of the crystallizer cavity in the transverse direction; it does not design different types of crystallizer cavity tapers based on the differences in the shrinkage characteristics of steel grades.

[0012] 8) China Utility Model Patent "Double W-type High-speed Crystallizer Copper Tube" (Application No.: 200520008088.8), the inner wall longitudinal section curve of the crystallizer copper tube is composed of an upper straight line segment and a lower pseudo-parabola segment, the two segments are tangent, and the inner wall longitudinal section curve equation of the crystallizer copper tube is given. The inner cavity of the entire crystallizer copper tube has a continuous inverted taper. This patent also does not involve the ultra-high drawing speed of square billet continuous casting, does not explain the design and machining of the transverse inner cavity taper curve of the crystallizer; does not design different types of crystallizer inner cavity tapers based on the differences in steel shrinkage characteristics.

[0013] In summary, the current method for determining the inner cavity taper of the continuous casting crystallizer and the existing design of the inner cavity taper structure of the billet continuous casting crystallizer have the following shortcomings: they are basically designed for the inner cavity taper of the crystallizer of slabs, billets and round billets at conventional drawing speeds, and there is no report on the method for determining the inner cavity taper of the billet ultra-high-speed continuous casting crystallizer that needs to ensure efficient cooling of the crystallizer; the design and machining issues of the transverse inner cavity taper curve of the crystallizer are not described; and different types of crystallizer inner cavity tapers designed according to the differences in the shrinkage characteristics of steel grades are not proposed. Based on this, the present invention proposes a method for determining the inner cavity taper of the ultra-high drawing speed billet continuous casting crystallizer, which is original. Summary of the invention

[0014] Compared with conventional billet continuous casting, the drawing speed of ultra-high-speed billet continuous casting is greatly increased, and the solidification speed is significantly accelerated. Ensuring efficient heat transfer in the crystallizer is the key to the design of the crystallizer. In order to ensure efficient heat transfer in the crystallizer and the realization of ultra-high drawing speed, it is necessary to design an ultra-high-speed crystallizer structure and taper that are different from the conventional drawing speed. Ultra-high-speed crystallizers are generally longer in length and have a larger radius of the arc structure at the corners to ensure that the uniformity of the lateral heat transfer of the crystallizer and the billet quality meet the requirements and achieve ultra-high drawing speed. The taper of the crystallizer for ultra-high-speed billet continuous casting cannot be determined by the conventional continuous casting crystallizer taper due to the fast solidification speed, different crystallizer structures, and changes in the distribution law of the solidified shell thickness in the height direction of the crystallizer. Instead, it must be determined by understanding the contraction of the solidified shell and the deformation law of the copper crystallizer and by mathematical simulation methods.

[0015] In summary, the taper design of the ultra-high-speed billet continuous casting mold is very important. Its design should ensure that no air gap is generated to the greatest extent during the continuous casting process (no air gap is generated under ideal conditions, and the degree of air gap generated under other conditions is very low).

[0016] The solidification shrinkage of the billet varies during the continuous casting of different steel grades, and the solidification shrinkage of peritectic steel is larger. In order to achieve ultra-high speed continuous casting of multi-steel billet, two types of tapers, peritectic steel and non-peritectic steel, should be designed and implemented for ultra-high speed billet continuous casting.

[0017] The main contents of the present invention are:

[0018] 1) By establishing a mathematical model of flow, heat transfer, and high-temperature stress-strain that integrates the steel solidification shell, protective slag film, and copper tube water cooling structure in the ultra-high-speed lower billet continuous casting crystallizer without considering the air gap, and performing simulation calculations, the accurate temperature distribution in the billet solidification shell and the copper tube of the crystallizer, the solidification shrinkage of the billet solidification shell in the height direction of the crystallizer and the horizontal direction of the billet, and the high-temperature deformation law of the copper tube can be obtained. The three-dimensional change law of the copper tube taper in the crystallizer area in the longitudinal and lateral directions can be obtained by synthesizing (adding) the solidification shrinkage of the billet solidification shell and the high-temperature deformation law of the copper tube. Since the process parameters such as the pulling speed and the superheat of the steel liquid may change during continuous casting, the change law of the mold taper obtained by calculation is also different. The final taper design of the ultra-high-speed billet continuous casting crystallizer is based on the ideal operating conditions (the most commonly used pulling speed and target superheat, etc.), and refers to the taper calculation data that deviates from the ideal operating conditions; the final design of the taper should also be adapted to the length and corner fillet radius (crystallizer inner cavity structure) of the ultra-high-speed continuous casting crystallizer used.

[0019] 2) The taper of the copper tube of the ultra-high-speed billet continuous casting mold obtained by the study changes continuously in three dimensions in the longitudinal and transverse directions of the casting billet. The structure of the inner cavity of the copper tube is a three-dimensional curved surface, which is difficult to machine. Therefore, in order to facilitate machining, based on the final determined law of the taper change of the ultra-high-speed billet continuous casting mold, several characteristic points are selected in the transverse (circumferential) direction of the solidified casting billet, see Figure 1 , data processing regression is used to obtain the change curve of the mold taper in the vertical direction. During processing, the change of taper between different feature points should be gradually and naturally transitioned, so that smooth processing and manufacturing can be achieved while ensuring that the mold taper accuracy meets the requirements.

[0020] 3) Due to the large solidification shrinkage of peritectic steel, the inverted taper of the crystallizer should be larger in theory. In order to achieve ultra-high speed of billet continuous casting and ensure the smoothness of continuous casting, the simulation calculation and data processing methods of the above contents 1) and 2) can be used for ultra-high speed billet continuous casting, and the taper change curves of the crystallizer copper tube for peritectic steel and non-peritectic steel can be designed, and the crystallizers with these two tapers can be used in production. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The characteristic points selected along the circumferential surface of the solidified ingot are: 1—surface center point, 2—1 / 4L point, 3—1 / 2L point, 4—3 / 4L point, 5—arc starting point, 6—arc center point, 7—arc end point. DETAILED DESCRIPTION

[0022] The present invention provides a method for determining the taper of the inner cavity of an ultra-high-speed billet continuous casting crystallizer, and the specific implementation method thereof is as follows:

[0023] 1) For the small square billet continuous casting crystallizer with a cross section less than or equal to 200mm×200mm and a pulling speed of 6.0m / min or above at an ultra-high pulling speed, the solidification shrinkage of the solidified shell of the casting in the crystallizer area and the high-temperature deformation law of the copper tube are obtained by comprehensive (additive) numerical simulation. The three-dimensional change law of the taper of the copper tube in the longitudinal and transverse directions of the billet continuous casting crystallizer under ultra-high pulling speed can be obtained.

[0024] 2) The variation of the calculated mold taper is affected by process parameters such as the casting speed and superheat of molten steel during continuous casting. The final taper design of the ultra-high-speed billet continuous casting mold is based on ideal working conditions (most commonly used casting speed and target superheat, etc.), and refers to the taper calculation data that deviates from the ideal working conditions.

[0025] 3) The final design of the taper should also be compatible with the length of the ultra-high-speed continuous casting crystallizer used (total length is 1000-1500 mm, effective length is 900-1400 mm) and the corner fillet radius (crystallizer inner cavity structure).

[0026] 4) The taper of the copper tube of the ultra-high-speed billet continuous casting crystallizer obtained above changes continuously in three dimensions in the longitudinal and transverse directions of the billet. The structure of the inner cavity of the copper tube is a three-dimensional curved surface, which is difficult to machine. In order to facilitate machining, several characteristic points are selected in the transverse (circumferential) direction of the solidified billet, see Figure 1 , the curve of the change of the mold taper in the vertical direction is obtained by data processing regression. The change of the taper between different feature points during processing should be gradually and naturally transitioned, so that the processing and manufacturing can be smoothly carried out under the premise of ensuring that the mold taper accuracy meets the requirements. It should be noted that the feature points are selected at the typical positions on the circumferential surface of the solidified billet, and their number can be appropriately increased according to the cross-sectional size of the small square billet, and is not limited to the schematic diagram. Figure 1 The number of feature points shown.

[0027] 5) In the actual continuous casting process, almost all steel grades are produced. In order to achieve ultra-high speed continuous casting of square billet with multiple steel grades and ensure smooth continuous casting, based on the difference in solidification shrinkage characteristics between peritectic steel and non-peritectic steel, the simulation calculation and data processing methods from 1) to 4) are used to obtain the taper change curves of the ultra-high speed square billet continuous casting mold for the two types of steel grades, and the two types of taper molds are used in production.

Claims

1. A method for determining the taper of the inner cavity of an ultra-high-speed billet continuous casting crystallizer, characterized in that: The following steps are included: Step 1: Establish a mathematical model of flow, heat transfer, and high-temperature stress and strain that integrates the solidified shell of molten steel, protective slag film, and copper tube water cooling structure in the ultra-high-speed lower billet continuous casting crystallizer without considering the air gap, and obtain the solidification shrinkage of the solidified shell of the billet and the high-temperature deformation law of the copper tube in the height direction of the crystallizer and the transverse direction of the billet by simulation calculation; Step 2: The solidification shrinkage of the solidified shell of the ingot obtained in step 1 and the high-temperature deformation of the copper tube are added to obtain the three-dimensional variation law of the taper of the copper tube in the ultra-high-speed crystallizer area of ​​the square billet in the longitudinal and transverse directions; Step 3: The structure of the inner cavity of the copper tube of the ultra-high-speed square billet continuous casting crystallizer obtained in step 3 is a three-dimensional curved surface, which is difficult to machine. In order to ensure that the taper accuracy of the crystallizer meets the requirements and can be smoothly processed and manufactured, several characteristic points are selected in the horizontal direction of the solidified billet, and the change curve of the taper of the crystallizer in the vertical direction is obtained by data processing and regression. The change of the taper between different characteristic points during processing should be gradually and naturally transitioned; Step 4: In view of the difference in solidification shrinkage characteristics between peritectic steel and non-peritectic steel, the simulation calculation and data processing methods of steps 1-3 are used for ultra-high-speed square billet continuous casting to design the taper change curves of the copper tube of the crystallizer for the two types of steel, and crystallizers with these two tapers are used in production.

2. The method for determining the taper of the inner cavity of the ultra-high speed billet continuous casting crystallizer according to claim 1 is characterized in that The cross-sectional size of the billet in step 1-4 is less than or equal to 200 mm×200 mm, and the pulling speed is above 6.0 m / min.

3. The method for determining the taper of the inner cavity of the ultra-high speed billet continuous casting crystallizer according to claim 1 is characterized in that The taper design of the ultra-high speed billet continuous casting crystallizer in step 2 is obtained based on ideal working conditions and refers to the taper calculation data deviating from the ideal working conditions, wherein the ideal working conditions refer to the most commonly used casting speed and target superheat.

4. The method for determining the taper of the inner cavity of the ultra-high speed billet continuous casting crystallizer according to claim 1 is characterized in that The taper of the ultra-high-speed billet continuous casting crystallizer in step 2 is adapted to the length and corner fillet radius of the ultra-high-speed continuous casting crystallizer used, wherein the total length of the crystallizer is 1000-1500mm, the effective length is 900-1400mm, and the corner fillet radius R of the inner cavity of the crystallizer is 6-30mm.

5. The method for determining the taper of the inner cavity of the ultra-high speed billet continuous casting crystallizer according to claim 1, characterized in that As described in step 3, an appropriate number of feature points are selected in the transverse direction of the solidified ingot to ensure smooth processing and manufacturing under the premise that the taper accuracy of the crystallizer meets the requirements. The number of feature points can be appropriately increased or decreased according to the change in the cross-sectional size of the small square billet. It is recommended to select about 6-9 feature points.

6. The method for determining the taper of the inner cavity of the ultra-high speed billet continuous casting mold according to claim 1 is characterized in that There are differences in the solidification shrinkage characteristics and heat transfer characteristics of the two types of steel, peritectic steel and non-peritectic steel, described in step 4. This difference specifically refers to the inconsistent volume changes caused by solidification cooling and solid-state phase transformation, as well as different high-temperature physical performance parameters. In order to achieve ultra-high speed continuous casting of square billets of multiple steel grades and ensure smooth continuous casting, it is proposed to design a taper change curve of ultra-high-speed square billet continuous casting crystallizers for the two types of steel grades, and use these two types of taper crystallizers in production.

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