CN115026250A - Control method for continuous casting large round billet tail end near liquidus electromagnetic stirring process - Google Patents

Abstract

The invention belongs to the technical field of continuous casting production, and in particular relates to a method for controlling a near liquidus electromagnetic stirring process at the end of a continuous casting large round billet. Aiming at the technical problems that the position of the solidification end is difficult to predict and the stirring parameters are difficult to determine during the continuous casting of the large round billet, the present invention proposes a predictable and adjustable method for controlling the near liquidus electromagnetic stirring process of the end of the continuous casting billet. The volume average method predicts the distribution of columnar crystals and equiaxed crystals during the solidification process of the continuous casting slab, determines the position of the solidification end (near liquidus), and adjusts the electromagnetic stirring according to the position of the solidification end. Electromagnetic stirring was applied at the position of the line). The invention can accurately predict the position of the solidification end of the continuous casting large round billet during the solidification process, so that the position of the agitator can be accurately controlled according to the technical requirements of the electromagnetic stirring at the solidification end, and further, the molten steel flow state at the solidification end can be effectively improved, and the large circle can be improved. The quality of the core of the billet.

Description

A kind of process control method of continuous casting billet end near liquidus electromagnetic stirring

technical field

The invention belongs to the technical field of continuous casting production, and in particular relates to a method for controlling a near liquidus electromagnetic stirring process at the end of a continuous casting large round billet.

Background technique

Large round billets are an important type of casting products, which are mainly used for the production of large-sized and super-sized high-pressure boiler tubes, oil well pipes, bearing sleeves, gears, high-speed train wheels and other high value-added rings, and have broad application prospects. At present, the method of die casting is mainly used, but the efficiency of die casting is low and the yield is low. Replacing die casting with continuous casting can achieve high efficiency and high yield of die steel production. However, with the expansion of the section of the continuous casting slab, the heat capacity per unit length of the slab increases, the heat dissipation area decreases, and the solidification mode changes from rapid solidification to slow solidification, resulting in intensified thermal convection of molten steel in the core and redistribution of solute elements. Some macrosegregation of solute elements and loose shrinkage caused by slow solidification are also more and more prominent.

According to different types and specifications of continuous casting billets, metallurgical workers at home and abroad have adopted technologies such as improving the quality of molten steel, pouring with low superheat, and lightly pressing the end to improve the internal quality of continuous casting billets. However, the improvement of molten steel quality is limited. Excessive requirements will increase the difficulty of smelting and increase the cost of smelting. In actual production, reducing the superheat to near the liquidus line will cause nozzle blockage and affect inclusions in the steel. It is difficult to determine the process parameters of the end of the light pressure, and the requirements for equipment and control technology are high.

Electromagnetic stirring is the most effective technical means to improve the quality of continuous casting billets and inhibit the segregation of elements and loose shrinkage cavities. Among them, the electromagnetic stirring technology at the end of solidification can generate electromagnetic force through the rotating magnetic field applied by the electromagnetic stirrer, and form forced convection of molten steel to break or fuse the columnar crystal to form an equiaxed crystal nucleus; It can promote the function of inclusions to float, so as to control the development of center segregation within a certain range.

The electromagnetic stirring process at the end of solidification can optimize the solidification process of continuous casting billets, but since the solidification mode of continuous casting billets changes from rapid solidification to slow solidification, the position of the solidification end is also difficult to predict, and the stirring parameters are also different according to different Sections and different steel grades are experimentally determined, while the trial and error method is costly and has a long cycle.

SUMMARY OF THE INVENTION

Aiming at the technical problems that the position of the solidification end is difficult to predict and the stirring parameters are difficult to determine in the process of continuous casting of the large round billet, the present invention proposes a predictable and adjustable method for controlling the near liquidus electromagnetic stirring process of the end of the continuous casting billet. The volume average method predicts the distribution of columnar crystals and equiaxed crystals during the solidification process of the continuous casting slab, determines the position of the solidification end (near liquidus), and adjusts the electromagnetic stirring according to the position of the solidification end. Electromagnetic stirring was applied at the position of the line).

Specifically, the method of the present invention comprises the following steps:

S1. Create a geometric model: According to the size of the continuous casting large round billet in the actual production process, create a model of the large round billet.

S2. Meshing: Using finite element simulation software, the model is divided into structured meshes, and the edge meshes are refined.

S3. Formulate boundary conditions and physical parameters: According to the operating parameters of the actual production conditions, determine each calculation boundary condition, and determine the physical parameters of the casting alloy according to the steel type.

The physical parameters of casting alloys include density, specific heat capacity, thermal conductivity, liquid-phase dynamic viscosity, liquid-phase solute diffusivity, solid-phase solute diffusivity, latent heat of phase transition, thermal expansion coefficient, and liquidus slope.

Boundary conditions can be set according to actual production conditions and operating parameters. For example, convection heat transfer can be used as the boundary condition, and segmental cooling can be used for the boundary of the large round billet.

S4. Perform simulation calculation: use the three-phase volume average method to simulate and calculate the temperature inside the large round billet and the distribution of each phase during the continuous casting process. The interior of the large round billet is divided into three continuous phases: liquid phase l, equiaxed crystal phase e and columnar crystal phase c and modeled to determine the source terms of mass, momentum and component transport between the phases.

The corresponding phase volume fractions are f _l , f _e and f _c , respectively. The liquid phase and the equiaxed crystal phase are movable phases, and the corresponding NS equations can be used to solve their motion; the columnar crystal phase is the immovable phase that adheres to the cooling wall and grows toward the liquid phase. For each local node of the large round billet and the whole of the large round billet, the sum of the three-phase volume fractions is 1, that is, f _l +f _e +f _c =1.

By determining the source terms of mass, momentum and composition transport between the phases and solving the governing equations, the distribution of liquid phase, columnar crystal phase and equiaxed crystal phase during the solidification of the continuous casting slab is predicted. Governing equations include continuity equations, momentum equations, species transport equations, energy equations, and other custom scalar equations for simulating transport or diffusion, depending on the situation. The governing equations can be solved based on the control volume of the finite difference method, using more than 60 iterations per time step, and the convergence criterion for the normalized residuals of the continuity equations, momentum equations, species transport equations and custom scalar equations is 10 ^-4 or less, the convergence criterion of the normalized residual of the energy equation is 10 ^-7 or less.

S5. Determining the position of the solidification end: the area between the area boundary contour line of f _l =1 to the area boundary contour line of f _l =0 is the solidification end of the large round billet.

Among them, the regional boundary contour line of f _l = 1 is also the isotherm of the end point (T _l ) of the liquidus temperature of the large round billet alloy, and the regional boundary contour line of f _l = 0 is also the end point of the solidus temperature of the large round billet alloy (T _s ). ) of the isotherm.

S6. Adjust the electromagnetic stirring according to the position of the coagulation end, and apply the required magnetic field at the coagulation end. Preferably, the position of the electromagnetic stirrer for applying the magnetic field is within an interval of 3 meters above the end point of the area boundary contour of f _l =1, and can be adjusted up and down within the interval.

Further, step S6 can be realized by the following scheme: according to the different requirements of the large round billets of different cross-sectional sizes for internal flow and heat transfer, with the help of low-frequency electromagnetic field simulation software such as Maxwell, the finite element method is used to calculate the effect of the continuous casting large round billet electromagnetic stirrer, Determine the electromagnetic stirring parameters (current and frequency) and predict the distribution of the magnetic field, and apply a suitable region of the magnetic field (such as a region with relatively high magnetic induction intensity) to the previously determined coagulation end position. Specifically, it can be achieved through the following steps:

S6.1 Create a geometric model: According to the type and size of the electromagnetic stirrer used at the solidification end of the continuous casting billet in the actual production process, create a model and delineate the computational domain.

S6.2 Meshing: The computational domain is divided into unstructured meshes based on electromagnetic field simulation software such as Maxwell, and the meshes at the edges of the geometry are refined.

S6.3 Set the solution type and excitation: set to solve the transient magnetic field, activate the eddy current analysis inside the slab; generate the cross section of the winding coil and input the number of turns, and set the current and phase of each winding coil.

S6.4 Assign its material properties to each component in the computational domain, and formulate boundary conditions; the material properties include copper wire, yoke, billet and air domain, and the air domain is the outer edge of the computational domain; due to the constraint of the magnetic yoke on the magnetic field, the Neiman Boundary condition, i.e. no flux passing through.

S6.5 Carry out simulation calculation: According to the different requirements of large round billets with different cross-sectional sizes for internal flow and heat transfer, determine the electromagnetic stirring parameters (mainly current and frequency), and predict the magnetic induction intensity distribution and induced current inside the continuous casting billet accordingly. and electromagnetic force distribution.

S6.6 According to the simulation results of magnetic induction intensity distribution, induced current and electromagnetic force distribution, and the position of the solidification end determined in step S5, adjust the position of the electromagnetic stirrer so that the appropriate area of the magnetic field acts on the solidification end.

The method applies electromagnetic stirring at the end of solidification (near the liquidus). The electromagnetic stirrer can move up and down according to the electromagnetic stirring parameters (current and frequency), adjust the position to apply electromagnetic stirring at the end of solidification, passivate the edge of the dendrite, form a near-sphere, promote the stabilization of the spherulite and the transformation of the dendrite to the spheroid, and increase its nucleation rate.

Compared with the prior art, the present invention has the following advantages:

1. The present invention implements electromagnetic stirring near the liquidus line at the solidification end of the continuous casting billet, which can realize the passivation of the dendrite edge, make it form a near sphere, promote nucleation, and also improve the uniformity of the billet and promote inclusions. Object floating function.

2. The present invention adopts the numerical simulation method to determine the position of the solidification end and the electromagnetic stirring parameters (current and frequency), which can reduce the cost, shorten the cycle and improve the efficiency compared with the method of experimental trial and error.

3. The method for predicting the solidification end of the continuous casting billet provided by the present invention can accurately predict the position of the solidification end of the continuous casting billet during the solidification process, so that the position of the agitator can be accurately controlled according to the technological requirements of the electromagnetic stirring at the solidification end. Further, the molten steel flow state at the solidification end can be effectively improved, and the core quality of the large round billet can be improved.

4. The predictable and adjustable method for controlling the near-liquidus electromagnetic stirring process at the end of continuous casting large round billets provided by the present invention has wide applicability and is suitable for large round billets of different sizes and materials in actual production, and is set for on-site electromagnetic stirring. Provide guidance.

In conclusion, the application of the technical solution of the present invention can optimize the existing continuous casting large round billet electromagnetic stirring process, and can be widely promoted in the fields of iron and steel and non-ferrous metal metallurgy casting.

Description of drawings

Fig. 1 is a schematic diagram of the electromagnetic stirring zone near the liquidus solidification end in the present invention; 1-f _l =1 area boundary contour, 2-f _l =0 area boundary contour, 3-f _l =1 area boundary contour Line endpoints, 4- _fl =0 area boundary contour endpoints, 5-Electromagnetic stirrer.

Fig. 2 Three-phase distribution results predicted by the volume average method in the present invention (a) liquid phase (b) equiaxed crystal (c) columnar crystal.

Fig. 3 embodiment of the present invention with the aid of low-frequency electromagnetic field simulation software Maxwell, adopts finite element method to carry out the model schematic diagram and calculation result of continuous casting large round billet electromagnetic stirrer effect calculation: (a) geometric model (b) magnetic induction intensity distribution (c) electromagnetic force Vector distribution. Among them, 6-cast billet, 7-iron core, 8-coil W1, 9-coil W2, 10-coil W3.

Detailed ways

The method of the present invention will be described in detail below with reference to specific embodiments.

Firstly, the distribution of liquid phase, columnar crystal phase and equiaxed crystal phase in the casting process of the continuous casting billet with a diameter of 600mm is simulated by the volume average method, and the position of the solidification end (near liquidus) is determined. The specific steps are as follows:

S1. Create a geometric model: Create a model according to the size of the continuous casting billet in the actual production process.

S2. Grid division: perform structured grid division and refine the edge grid.

S3. Formulate boundary conditions and physical parameters: According to the operating parameters of the actual production conditions, determine each calculation boundary condition. In this embodiment, the boundary conditions adopt convection heat transfer, and the boundary of the large round billet adopts segment cooling conditions, ignoring the crystallizer. Vibration, ignoring the heat transfer, inclusion and reaction between mold flux and molten steel.

At the same time, the physical parameters of the cast alloy are determined according to the steel type, including density, specific heat capacity, thermal conductivity, liquid dynamic viscosity, liquid solute diffusivity, solid solute diffusivity, latent heat of phase transformation, thermal expansion coefficient and liquidus slope.

S4. Perform simulation calculation: use the three-phase volume average method to simulate the temperature and phase distribution inside the large round billet, and divide the inside of the large round billet into three continuous phases: liquid phase l, equiaxed crystal phase e and columnar crystal phase c. Modeling was performed to determine the source terms for mass, momentum, and component transport between the phases, with the corresponding phase volume fractions f _l , f _e , and f _c , respectively. The liquid phase and the equiaxed crystal phase are movable phases, and the corresponding NS equations can be used to solve their motion; the columnar crystal phase is the immovable phase that adheres to the cooling wall and grows toward the liquid phase.

For each local node of the large round billet and the whole of the large round billet, the sum of the three-phase volume fractions is 1, that is, f _l +f _e +f _c =1.

By determining the source terms of mass, momentum and composition transport between the phases and solving the governing equations, the distribution of liquid phase, columnar crystal phase and equiaxed crystal phase during the solidification of the continuous casting slab is predicted; the governing equations include the continuity equation, Momentum equations, species transport equations, energy equations, and other custom scalar equations for simulating transport or diffusion, depending on the situation. The governing equations are solved based on the control volume of the finite difference method, and each time step uses more than 60 iterations. The convergence criteria of the normalized residuals of the continuity equation, momentum equation, species transport equation and custom scalar equation are 10 ^-4 or less, the convergence criterion of the normalized residual of the energy equation is 10 ^-7 or less.

The three-phase distribution obtained by simulation in this embodiment is shown in FIG. 2 .

S5. Determine the position of the solidification end: as shown in Figure 1, 1 is the boundary contour line of the f _l = 1 region, that is, the boundary line of the pure liquid phase region in the large round billet, which can also be regarded as the end point of the liquidus temperature of the large round billet alloy ( T _l ) isotherm, the area above the boundary contour line of f _l = 1 is the complete liquid phase area of f _l = ₁ ; The boundary line of the phase region can also be regarded as the isotherm of the solidus temperature end point (T _s ) of the large round billet alloy, and the region below the boundary isoline 2 of the f _l = 0 region is the complete solid phase region of f _l = 0; The area between 1 and 2 is the location of the coagulation end.

S6. Adjust the electromagnetic stirring according to the position of the coagulation end, and apply the required magnetic field at the coagulation end. Specifically, the electromagnetic stirrer 5 can be set in the interval of 3 meters above the end point 3 of the boundary contour 1 of the f _l =1 region (which is also the starting point of the complete liquid phase region in the center of the slab), and can be adjusted up and down.

In this example, according to the requirements for internal flow and heat transfer of the large round billet of this size, with the help of the low-frequency electromagnetic field simulation software Maxwell, the finite element method is used to calculate the effect of the electromagnetic stirrer for the continuous casting large round billet, and the electromagnetic stirring parameters (current and frequency) are determined. Predict the distribution of the magnetic field, and apply a suitable region of the magnetic field (such as a region with relatively high magnetic induction) to the previously determined coagulation end position. Specifically include the following steps:

S6.1 Create a geometric model: According to the type and size of the electromagnetic stirrer used at the solidification end of the continuous casting billet in the actual production process, create a model and delineate the computational domain. The created model is shown in Fig. 3(a) and includes a casting billet 6 (a large round billet to be cast), an iron core 7 (as a yoke), three coils 8-10 wound with copper wire, and these parts surrounding airspace.

S6.2 Meshing: The computational domain is divided into unstructured meshes based on electromagnetic field simulation software such as Maxwell, and the meshes at the edges of the geometry are refined.

S6.3 Set the solution type and excitation: set to solve the transient magnetic field, activate the eddy current analysis inside the slab; generate the cross section of the winding coil and input the number of turns, and set the current and phase of each winding coil.

S6.4 Assign its material properties to each component in the computational domain, and formulate boundary conditions; the material properties include copper wire, yoke, billet and air domain, and the air domain is the outer edge of the computational domain; due to the constraint of the magnetic yoke on the magnetic field, the Neiman Boundary condition, i.e. no flux passing through.

S6.5 Carry out simulation calculation: According to the different requirements of large round billets with different cross-sectional sizes for internal flow and heat transfer, determine the electromagnetic stirring parameters (mainly current and frequency), and predict the magnetic induction intensity distribution and induced current inside the continuous casting billet accordingly. and electromagnetic force distribution, as shown in Figures 3(b) and 3(c).

S6.6 According to the simulation results of the distribution of magnetic induction intensity, induced current and electromagnetic force, and the position of the solidification end determined in step S5, adjust the position of the electromagnetic stirrer, so that the appropriate area of the magnetic field acts on the solidification end, and apply the required amount to the solidification end. magnetic field.

Claims (8)

1. A near-liquidus electromagnetic stirring process control method for the tail end of a large continuous casting round billet is characterized in that a volume average method is used for predicting the distribution of columnar crystals and equiaxed crystals in the continuous casting billet solidification process, the solidification tail end position is determined, and electromagnetic stirring is adjusted according to the solidification tail end position.

2. The near-liquidus electromagnetic stirring process control method for the tail end of the continuous casting round billet according to claim 1, characterized by comprising the following steps:

s1, creating a geometric model: establishing a model according to the size of a continuous casting large round billet in the actual production process;

s2, grid division: adopting finite element simulation software to perform structured grid division on the model and encrypt edge grids;

s3, formulating boundary conditions and physical property parameters: determining each calculation boundary condition according to the operation parameters of the actual production working condition, and determining the physical parameters of the cast alloy according to the steel grade;

s4, performing simulation calculation: utilizing a three-phase volume average method to carry out analog calculation on the temperature and phase distribution in the large round billet, dividing the interior of the large round billet into three continuous phases of a liquid phase l, an equiaxed crystal phase e and a columnar crystal phase c for modeling, wherein the corresponding phase volume fractions are respectively f _l 、f _e And f _c Wherein the liquid phase and the equiaxed crystal phase are movable phases, and the columnar crystal phase is an immovable phase which is adhered on the cooling wall surface and grows towards the liquid phase direction;

predicting the distribution of liquid phase, columnar crystal phase and equiaxed crystal phase in the solidification process of the continuous casting billet by determining the mass, momentum and component transmission source items among all phases and solving a control equation;

s5, determining the position of a solidification tail end: from f _l Region boundary contour to f of 1 _l The region between the region boundary contour lines which are 0 is the solidification tail end of the round billet;

s6, adjusting electromagnetic stirring according to the position of the solidification tail end, and applying a required magnetic field at the solidification tail end.

3. The method for controlling the near-liquidus electromagnetic stirring process of the tail end of the continuous casting round billet according to claim 2, wherein in the step S3, the boundary conditions are as follows: convection heat transfer is adopted, and the boundary of the large round billet is cooled in a sectional manner.

4. The method for controlling a near-liquidus electromagnetic stirring process at the end of a continuous casting round billet according to claim 2, wherein in the step S4, the control equation is solved based on the control volume of the finite difference method, more than 60 iterations are adopted in each time step, and the convergence criterion of the normalized residuals of the continuity equation, the momentum equation, the species transport equation and the custom scalar equation is 10 ^-4 The convergence criterion for the normalized residual of the energy equation is 10 below ^-7 The following.

5. The method for controlling a near-liquidus electromagnetic stirring process of a terminal of a large round billet for continuous casting according to claim 2, wherein the position of the electromagnetic stirrer is f in step S6 _l The region boundary contour is 1 in a section 3 meters above the end point of the contour and can be adjusted up and down within the section.

6. The method for controlling the near-liquidus electromagnetic stirring process of the tail end of the continuous casting round billet according to any one of claims 1 to 5, wherein the step S6 specifically comprises the following steps:

s6.1, creating a geometric model: according to the type and the size of an electromagnetic stirrer adopted at the solidification end of a continuous casting round billet in the actual production process, a model is created, and a calculation domain is defined;

s6.2, grid division: dividing a computational domain into unstructured grids based on electromagnetic field simulation software, and encrypting geometric body edge position grids;

s6.3, setting solving type and excitation: setting to solve a transient magnetic field and activating eddy current analysis inside the casting blank; generating the section of a winding coil, inputting the number of turns, and setting the current and the phase of each winding coil;

s6.4, distributing material attributes of each part in the calculation domain, and formulating boundary conditions;

s6.5, performing simulation calculation: determining electromagnetic stirring parameters according to different requirements of large round billets with different section sizes on internal flow and heat transfer, and predicting magnetic induction intensity distribution, induced current and electromagnetic force distribution in the continuous casting billets;

s6.6 adjusts the position of the electromagnetic stirrer so that an appropriate region of the magnetic field acts on the solidification end, based on the simulation results of the magnetic induction intensity distribution, the induced current, and the electromagnetic force distribution, and the solidification end position determined in step S5.

7. The method for controlling a near-liquidus electromagnetic stirring process of the tail end of a continuous casting round billet as claimed in claim 2, wherein the physical properties of the cast alloy in the step S3 include: density, specific heat capacity, thermal conductivity, liquid dynamic viscosity, liquid solute diffusion coefficient, solid solute diffusion coefficient, latent heat of phase change, coefficient of thermal expansion, and liquidus slope.

8. The method for controlling the near-liquidus electromagnetic stirring process of the tail end of the continuous casting round billet according to the claim 6, characterized in that in the step S6.4: the material properties comprise a copper wire, a magnet yoke, a casting blank and an air domain, wherein the air domain is the outer edge of the calculation domain; the niemann boundary condition without magnetic flux crossing is adopted.

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