Electromagnetic stirring control method

Abstract

To provide an electromagnetic stirring control method in which molten steel is stirred by electromagnetic force in a continuous casting process, wherein the method being capable of producing a more homogeneous cast slab and, in particular, suppressing the formation of white bands.SOLUTION: A method includes a current value determination step of determining a current value for operation by setting a parameter region corresponding to a central property of a cast slab, which is obtained using a casting speed at a stirring position and a current value of an induced current that generates electromagnetic force as parameters. The current value determination step includes a boundary setting step of setting a boundary function for defining a boundary line of the parameter region, wherein the boundary function is defined as a linear function having the casting speed and the current value as variables, based on an equiaxed crystal ratio of molten steel.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an electromagnetic stirring control method for stirring molten steel by electromagnetic force in a continuous casting process. [Background technology]

[0002] After continuous casting, voids tend to form in the center of the slab, which is the final solidification location. This is thought to be caused by a localized shortage of molten steel due to bridging at the end of solidification. One effective countermeasure is to suppress bridging using electromagnetic stirring. Electromagnetic stirring not only eliminates voids in the center but also improves segregation.

[0003] For example, Patent Document 1 discloses a continuous casting method for slab slabs using unsolidified reduction and electromagnetic stirring. It describes that, although negative segregation is formed in most of the thickness center of the slab, positive segregation tends to remain in a portion of the thickness center in the width direction of the slab. Furthermore, it describes that an electromagnetic stirring device that electromagnetically stirs unsolidified molten steel upstream of the reduction rolls stably forms negative segregation in the thickness center of the slab slab across the width direction, thereby eliminating the segregation. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-87249 Summary of the Invention [Problem to be solved by the invention]

[0005] As described in Patent Document 1, negative segregation bands called white bands (WB) are observed during electromagnetic stirring in the continuous casting process, and are generally formed due to strong stirring. These white bands have undesirable effects such as a decrease in the uniformity of physical properties within the cross section of the steel material and deterioration of the product appearance. Therefore, there has been a demand for an electromagnetic stirring control method that can suppress this.

[0006] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an electromagnetic stirring control method for stirring molten steel by electromagnetic force in a continuous casting process, which is capable of producing a more homogeneous cast slab, in particular, capable of suppressing the formation of white bands. [Means for solving the problem]

[0007] The present invention provides an electromagnetic stirring control method for stirring molten steel by electromagnetic force in a continuous casting process, comprising a current value determination step of setting a parameter region corresponding to the central portion properties of a cast slab obtained using as parameters the casting speed at a stirring position and the current value of an induced current that generates the electromagnetic force, and determining the current value in operation, wherein the current value determination step includes a boundary setting step of setting a boundary function that demarcates the boundary line of the parameter region, and the boundary function is defined as a linear function with the casting speed and the current value as variables based on the equiaxed crystal fraction of the molten steel.

[0008] According to this feature, a boundary function is provided in the equation for applying a predetermined stirring force to molten steel, and the current value is determined based on this boundary function, thereby making it possible to provide a more homogeneous cast slab.

[0009] In the above-described invention, the central portion properties may include the formation of white bands. According to this feature, it is possible to suppress the formation of white bands and obtain a more homogeneous cast slab.

[0010] In the above-described invention, the boundary setting step may include a step of determining the boundary function by obtaining the equiaxed crystal fraction corresponding to the casting speed from a structural observation of the slab at each casting speed. Also, the boundary function may include a step of determining the equiaxed crystal fraction corresponding to the casting speed from a relational expression weighted for each elemental component of the molten steel, including the casting speed. According to this feature, it is possible to easily provide a more homogeneous slab for steel types with a wide range of component compositions.

[0011] In the above-described invention, the stirring may be performed by alternately reversing the direction of rotation around the axis of the flow path of the molten steel. With this feature, electromagnetic stirring can be reliably performed, and a more homogeneous cast slab can be obtained. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a scatter diagram showing the presence or absence of white band formation, with current value and casting speed as parameters. [Figure 2] 1 is a graph showing the relationship between the equiaxed crystal ratio E and the constant B of a linear function. DETAILED DESCRIPTION OF THE INVENTION

[0013] An electromagnetic stirring control method will be described as one embodiment of the present invention.

[0014] As mentioned above, continuous casting tends to produce voids in the center of the slab, which can lead to problems such as a decrease in the center filling ratio. Electromagnetic stirring, which uses electromagnetic force to stir the molten steel in the final solidification zone, is an effective solution, and it is known that the stronger the stirring, the higher the center filling ratio. However, strong electromagnetic stirring can sometimes result in the formation of white bands. Therefore, electromagnetic stirring can be performed by determining the current value of the induced current that generates an electromagnetic force that produces a stirring force that does not form white bands at a high center filling ratio. However, the results vary depending on the casting speed and the chemical composition of the molten steel, making it difficult to determine the appropriate current value.

[0015] Therefore, the inventors collected data on the central properties of slabs produced for a plurality of steel types under various conditions, particularly on the presence or absence of white bands, using as parameters the casting speed at the stirring position where electromagnetic stirring is performed and the current value of the induced current that generates the electromagnetic force of electromagnetic stirring.

[0016] As shown in Figure 1, the casting speed and current were organized, and a boundary function indicating the boundary between the occurrence of white bands (WB) and the non-occurrence of WB was determined as a linear function for steel grade (1) ([C]: 0.21%), steel grade (2) ([C]: 0.35%), and steel grade (3) ([C]: 0.48%). It is known that increasing the equiaxed crystal fraction reduces the stirring power of the electromagnetic stirring. Furthermore, the equiaxed crystal fraction varies depending on the steel grade's composition. Therefore, the difference in the boundary function for each steel grade was attributed to the equiaxed crystal fraction. Microstructural observations revealed that the average equiaxed crystal fraction was 28.2% for steel grade (1), 33.4% for steel grade (2), and 22.8% for steel grade (3). Furthermore, it was found that the position of the boundary function in this figure shifted toward higher current values ​​for steel grades with a higher equiaxed crystal fraction.

[0017] In this embodiment, the position in continuous casting where electromagnetic stirring is performed is the final solidification zone. In the final solidification zone, the molten steel is in a solid-liquid mixed phase where a solid phase and a liquid phase coexist. The stirring force in this solid-liquid mixed phase is thought to be affected by the shape of particles crystallized in the solid molten steel, and the shape of the particles is thought to be reflected in the equiaxed crystal fraction after solidification.

[0018] Based on these, the electromagnetic stirring control method can be as follows.

[0019] First, the properties of the central portion are investigated by observing the structure of a slab obtained by continuous casting using the above-mentioned casting speed and current value as parameters, and a parameter range corresponding to this central portion property is set. Here, the central portion property particularly includes the formation of white bands. Then, a boundary function is obtained, which represents the boundary line between the region where white bands are formed and the region where they are not formed, as a function of parameters. Here, as described above, the boundary function defining the parameter range is a linear function. In this manner, the boundary function can be set in the boundary setting step.

[0020] In the figure, the parameter region where the white band is formed is the upper right side of the boundary function. Therefore, if the current value is determined according to the casting speed so that it is outside the parameter region, the formation of the white band can be suppressed.

[0021] By performing continuous casting while performing electromagnetic stirring at a determined current value through the current value determination process as described above, a more homogeneous cast slab can be produced. As described above, the greater the stirring force, i.e., the greater the current value, the higher the center filling rate. Therefore, in the current value determination process, it is preferable to determine the current value so that it is outside the parameter region but close to the boundary function.

[0022] The boundary function is a linear function, so it can be expressed as, for example, I = A × Vc + B. I is the current value, Vc is the casting speed, A is a coefficient, and B is a constant. In other words, the boundary function is a linear function with the casting speed and current value as variables. In particular, coefficient A is the same regardless of the steel type. On the other hand, constant B varies depending on the steel type and is determined based on the equiaxed crystal fraction. In other words, the boundary function determined by constant B is determined based on the equiaxed crystal fraction. The equiaxed crystal fraction can be determined by observing the structure of the cast slab with respect to the casting speed. Here, the determined equiaxed crystal fraction was averaged for each steel type. Note that constant B corresponds to the value of I when Vc = 0, i.e., the I-intercept of the boundary function.

[0023] For example, as shown in Figure 2, the relationship between the equiaxed crystal fraction and the constant B is plotted on a graph, and an approximate curve is drawn between the plotted points. This approximate curve then allows the unique relationship between the equiaxed crystal fraction and the constant B to be determined.

[0024] The coefficient A and the constant B are values ​​specific to the equipment, including the position where electromagnetic induction stirring is performed. Therefore, the above method can be applied to steel types for which sufficient data is not available, as long as the equiaxed crystal ratio can be calculated by collecting a certain amount of data after determining the equipment and stirring position.

[0025] After a certain amount of data has been accumulated, the equiaxed crystal ratio corresponding to the casting speed can be calculated from a relational expression in which the elemental components of the molten steel and the casting speed are weighted, respectively. For example, the contents of the elements contained in the molten steel and the casting speed are used as explanatory variables (x1, x2, x3, ..., and Vc), and the coefficients (β1, β2, β3, ..., and β) that weight the explanatory variables are used. v ) and add them together to obtain the equiaxed crystal ratio E, which is the objective variable. In other words, E=β1x1+β2x2+β3x3…+β v A multiple regression equation is created to obtain Vc. Then, each coefficient is determined by multiple regression analysis using the least squares method, and a multiple regression equation is determined to obtain the equiaxed crystal ratio E.

[0026] In this way, by creating a multiple regression equation for calculating the equiaxed crystal fraction, it is possible to calculate the equiaxed crystal fraction corresponding to the casting speed even for steel types that have no operational history, and to determine the current value described above. In other words, it is possible to easily obtain more homogeneous cast slabs for steel types with a wide range of component compositions.

[0027] In addition, in electromagnetic stirring, devices for several stirring methods, such as unidirectional flow stirring, have been proposed, and any of them may be used, but stirring by alternately reversing the direction of rotation around the axis of the flow path of the molten steel is preferable because it can reliably provide electromagnetic stirring.

[0028] While typical embodiments of the present invention and modifications based thereon have been described above, the present invention is not necessarily limited to these, and a person skilled in the art will be able to find various alternative embodiments and modifications without departing from the spirit of the present invention or the scope of the appended claims.

Claims

1. An electromagnetic stirring control method for stirring molten steel by electromagnetic force in a continuous casting process, comprising: a current value determination step of setting a parameter range corresponding to the central portion properties of the resulting slab using the casting speed at the stirring position and the current value of the induced current that generates the electromagnetic force as parameters, and determining the current value in operation; the current value determination step includes a boundary setting step of determining a boundary function that defines a boundary line of the parameter region; The electromagnetic stirring control method is characterized in that the boundary function is determined as a linear function with the casting speed and the current value as variables based on the equiaxed crystal ratio of the molten steel.

2. 2. The electromagnetic stirring control method according to claim 1, wherein the central portion properties include the formation of a white band.

3. 2. The electromagnetic stirring control method according to claim 1, wherein the boundary setting step includes a step of determining the equiaxed crystal ratio corresponding to the casting speed from a structural observation of the slab for the casting speed, and determining the boundary function.

4. 4. The electromagnetic stirring control method according to claim 3, further comprising a step of determining the boundary function by calculating the equiaxed crystal ratio corresponding to the casting speed from a relational expression weighted for each elemental component of the molten steel including the casting speed.

5. 5. The electromagnetic stirring control method according to claim 1, wherein the stirring is performed by alternately reversing the direction of rotation around the axis of the flow path of the molten steel.

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