A method for calibrating a continuous casting solidification heat transfer model by means of a slab macrograph

Abstract

This invention relates to a method for calibrating a continuous casting solidification heat transfer model using a low-magnification model of a cast billet. Without requiring precise data on the model's physical properties, and without relying on nail guns or temperature measurement, the model is calibrated by reading the low-magnification value of the cast billet and determining the shell thickness during the process. The cast billet is obtained using a light-pressure continuous casting process. The shell thickness H corresponding to at least two rolls in the light-pressure region of the continuous casting is read from the low-magnification value of the cast billet. The shell thickness corresponding to the N# roll is recorded as Hn, and the shell thickness of the N+1# roll is Hn+1. The solidification position of the rolls corresponding to the shell thickness equal to H is found from the model's calculation results. The spacing between the aforementioned two or more roll positions is calculated. The calculated roll spacing is compared with the actual roll spacing. The equivalent thermal conductivity m is adjusted until the calculated roll spacing is closest to the actual roll spacing. Then, the equivalent thermal conductivity m is solidified, at which point the model is considered calibrated.

Description

Technical Field

[0001] This invention pertains to the segregation control technology of steel continuous casting billets, and specifically relates to a method for calibrating the heat transfer model of continuous casting solidification by reading the low-magnification billet shell thickness without relying on nail guns, temperature measurement, or accurate physical property parameters. Background Technology

[0002] Segregation and other internal quality characteristics are important indicators for evaluating the quality of steel materials, especially special steels. Poor internal quality can seriously harm the performance of processed steel products. To improve internal quality, a continuous casting solidification heat transfer model is typically used for calculation, followed by selection of end-stage electromagnetic stirring, light reduction equipment, and adjustment of relevant process parameters. However, model adjustment usually takes a long time, and sometimes even years pass without finding a suitable process, resulting in unsatisfactory quality improvements. The difficulties in adjustment and calibration are mainly due to two factors: firstly, current calibration methods have some problems; and secondly, it is difficult to obtain accurate physical properties such as latent heat of solidification, specific heat capacity, and thermal conductivity required in the model.

[0003] The solidification heat transfer model for continuous casting is a commonly used tool for studying light reduction and segregation improvement. The modeling process always requires some assumptions, so it must be calibrated before being applied to actual production. The accuracy of the calculation results is affected not only by the precision of the modeling strategy but also by physical property parameters. These parameters include liquidus temperature, solidus temperature, latent heat of solidification, specific heat capacity, and thermal conductivity. While there are relatively mature regression formulas for liquidus temperature and solidus temperature within the industry, readily available data for latent heat of solidification, specific heat capacity, and thermal conductivity are difficult to find for different steel grades, and they are also difficult to measure accurately. Methods used in the industry include nail guns (such as CN103940351A) and infrared thermography (such as CN114905020A) for model calibration, but both methods have limitations and difficulties in application.

[0004] Nail shooting is dangerous, involving high temperatures in the operating area and the risk of accidental discharge. The preparation process is complex, typically only measuring the thickness of the blank at one location at a time; if multiple locations are to be tested simultaneously, the preparation workload increases exponentially. Furthermore, firearms are prone to malfunction, and bullets may deviate due to installation inaccuracies or operational errors. The sample preparation process is complex, and sample processing becomes even more difficult when the nail is misaligned. Therefore, nail shooting cannot be performed frequently, and obtaining effective low-magnification data through nail shooting is not easy. Moreover, different people may interpret the measurement results for the same sample differently. In short, obtaining effective data using the nail shooting method is not easy.

[0005] Using infrared thermometry data to calibrate the solidification heat transfer model between different rollers in a continuous casting machine is a relatively simple method. However, the surface temperature of the continuously cast billet is very high during production, which poses certain safety risks to the temperature measurement work. The iron oxide scale on the surface of the billet is relatively large and its peeling is uneven due to roller crushing and thermal expansion and contraction, which brings significant inaccuracies to the temperature measurement results. Handheld infrared thermometry is usually used, and the possibility of deviation in the temperature measurement location may affect the measurement results. Sometimes, the temperature measurement results of the same location at the same time by different temperature guns also show significant differences, affecting the accuracy of infrared thermometry.

[0006] The physical property parameters such as latent heat of solidification, specific heat capacity, and thermal conductivity, which must be used in the mathematical model of the solidification heat transfer model, are difficult to obtain accurate data from the literature. Self-testing requires special equipment, and the accuracy, reproducibility, and repeatability of the test data require a lot of time to verify. Summary of the Invention

[0007] The purpose of this invention is to provide a method for calibrating a continuous casting solidification heat transfer model by low magnification of the cast billet. This method calibrates the model without requiring precise data on the model's physical properties, and without relying on nail guns or temperature measurement. It involves reading the low magnification of the cast billet to determine the thickness of the billet shell during the process.

[0008] The method of the present invention is suitable for the verification of solidification heat transfer models for continuous casting of square or rectangular billets.

[0009] The solidification heat transfer model of continuous casting (hereinafter also referred to as the mathematical model) has many assumptions. The physical properties vary depending on the elemental composition of the billet, the production process and the production environment. Therefore, the mathematical model needs to be calibrated after development before it can be used for actual production research.

[0010] The mathematical model must use physical property parameters such as latent heat of solidification, specific heat capacity, and thermal conductivity, as well as the equivalent thermal conductivity m in the mathematical model. m refers to the equivalent thermal conductivity of liquid steel relative to solid steel in the continuous casting process, which is generally taken as 4 to 7. It can be replaced by data of other steel grades with known parameters for the initial calculation.

[0011] The specific technical solution adopted in this application is as follows: a method for calibrating a solidification heat transfer model by low magnification of a billet. First, the liquidus temperature and solidus temperature of the current steel grade are obtained using general formulas. The continuous casting production process parameters of the current steel grade are then input into the model. The latent heat of solidification, specific heat capacity, thermal conductivity, and equivalent thermal conductivity m in the model are first replaced with data of other steel grades with known parameters for initial calculation.

[0012] The billet is obtained by light-pressure continuous casting process. The shell thickness H corresponding to at least two rolls in the light-pressure continuous casting area is read from the low magnification of the billet. The shell thickness corresponding to the N# roll is recorded as Hn, and the shell thickness of the N+1# roll is Hn+1. The solidification position of the roll corresponding to the shell thickness equal to H is found from the model calculation results. The spacing between the aforementioned two or more roll positions is calculated. The roll spacing calculated by the model is compared with the actual roll spacing. If the calculated roll spacing is less than the actual roll spacing, it means that the cooling rate between these two roll intervals is too large and the m value needs to be reduced. Conversely, if the calculated roll gap is greater than the actual roll gap, it means that the cooling rate between these two roll intervals is too low, and the value of m needs to be increased. The increment or decrement is determined according to the size of the gap difference, with each increment or decrement being 0.1 or 0.2. The equivalent thermal conductivity m is adjusted until the calculated roll gap is closest to the actual roll gap, and then the equivalent thermal conductivity m is solidified. At this point, the model is considered to be calibrated. When the model is calibrated and used for calculation results, the center solid fraction at other roll rows is calculated based on the relative position value with respect to roll N#.

[0013] Preferably, the value of the equivalent heat conduction system m of the current steel grade is in the range of 4 to 7.

[0014] Preferably, based on the billet shell thickness read from the low magnification of the billet, the corresponding distance from the meniscus is found in the model calculation results, and the distance between adjacent rolls is calculated.

[0015] Preferably, when the calculated roll spacing is closest to the actual roll spacing, the billet shell thickness, billet shell temperature, billet center temperature and center solid fraction at all rolls in the light pressing area of ​​the continuous casting machine are determined according to the on-site roll spacing of the continuous casting machine, that is, the relative position of each roll in the model based on the roll spacing identification model. At this time, the difference between the distance from the meniscus in the model calculation and the actual distance is ignored. At this time, the temperature and solid fraction calculated by the model can be directly used for the process design and analysis of the current steel grade.

[0016] This invention provides a method for calibrating a model by determining the shell thickness of a cast billet through low-magnification readings, without requiring precise data on the latent heat of solidification, specific heat capacity, thermal conductivity, or other physical properties of the current steel grade, and without the need for nail guns or temperature measurement. Compared to existing technologies, the advantages of this invention are:

[0017] (1) This method has no safety risks. The high-temperature working environment of nail guns is harsh, and there is also the risk of accidental discharge or ricochet injury from the gun; temperature measurement is generally carried out by hand-held infrared thermometers, which requires the personnel to be in the gaps of the continuous casting equipment and in a scorching environment, which also has certain safety risks.

[0018] (2) This method is convenient and efficient. The low-magnification of the cast billet completes the normal production sample test, so the processing time is negligible. The work of determining the thickness of the two roll shells from the low-magnification of the cast billet can usually be completed in half an hour. Model calculations of 3-5 times can basically complete the verification of the roll gap, i.e., the solidification model, with each calculation taking about half an hour. In total, it takes about 3 hours to complete the model calibration work for a new steel grade. Nail-shooting requires finding a research institute or purchasing the necessary firearms. Nail-shooting usually needs to be performed multiple times at different locations on the continuous casting machine, and the preparation and judgment of the sample after each nail-shooting takes several weeks or even months. Relying solely on model calculations requires very accurate physical property parameters, which are difficult to find readily available data for different steel grades. Testing requires specialized equipment and has a long cycle, usually taking several months to obtain a single data point. The temperature measurement method requires an infrared thermometer. Handheld temperature measurement requires personnel, while fixed temperature measurement requires finding suitable locations to ensure that the distance and angle between each temperature measurement point and the cast billet meet the requirements, and multiple temperature measurement guns are needed.

[0019] (3) This method has good accuracy. The nail-shot method, when interpreting the billet shell thickness from the sample, may yield different results from different individuals, and the reproducibility error is large for different samples at the same location. The temperature measurement method suffers from significant differences in measurement results due to the large amount of iron oxide scale on the surface of the continuously cast billet, which is subject to varying degrees of peeling caused by roller rolling and thermal expansion and contraction. The distance and angle between the temperature measurement point and the billet, as well as the deviation of the temperature measurement point from its location on the billet, can all affect the accuracy of the measurement results. Attached Figure Description

[0020] Figure 1 This is a flowchart of a low-magnification calibration model for the solidification heat transfer of a cast billet in an embodiment of the present invention.

[0021] Figure 2 This is a schematic diagram of the billet shell thickness read out at low magnification in an embodiment of the present invention (roller #3: 64mm, roll #4: 74mm). Detailed Implementation

[0022] The present invention will be further described in detail below with reference to the accompanying drawings. The embodiments described are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0023] This embodiment demonstrates the solidification heat transfer model for X steel used in a continuous casting machine.

[0024] Given: continuous casting machine parameters such as the spacing between each roll column, low-magnification sample, the light reduction process at the time, and a preliminary solidification model. The preliminary solidification model refers to a model with complete physical properties of steel A and has been calibrated for analysis of steel A.

[0025] Solution: The solidification model is calibrated to meet the process analysis conditions for X steel.

[0026] Step 1: Use general formulas to obtain the liquidus temperature, solidus temperature, and continuous casting process parameters of X steel. Input these parameters into the model. The unknown physical properties of X steel, such as latent heat of solidification, specific heat capacity, and thermal conductivity, as well as the equivalent thermal conductivity m, are taken from those of A steel. Run the model initially to obtain the first results.

[0027] Step Two: Using a low magnification method, the shell thickness of the X-steel billet produced by the previous light reduction process is read. In this embodiment, the actual light reduction process used five rollers, from roller #2 to roller #6. Analyzing the low magnification of the billet, the shell thicknesses of rollers #3 and #4 can be obtained. The reduction amount of roller #2 is small, and the cracks are not obvious. The cracks from rollers #5 and #6 are close to the geometric center of the billet and intersect with the loose segregation zone at the center of the billet, making them difficult to distinguish and therefore not requiring strict attention. In this embodiment, the thickness of roller #3 is 64mm, and that of roller #4 is 74mm. See the table below for details. Figure 2 .

[0028]

[0029] Step 3: Based on the billet shell thickness read from the low magnification of the cast billet, find the corresponding distance from the meniscus in the model calculation results and calculate the adjacent spacing. By adjusting the m value, compare the calculated spacing with the actual roll spacing.

[0030] In this embodiment, the initial calculation m in step two uses 5.5. Comparison revealed that the calculated spacing of 1.25 meters is less than the actual roller spacing of 1.31 meters. After adjusting m to 5.4, the calculated roller spacing and the actual roller spacing are calculated and compared again. This process is repeated several times until the calculated spacing is close to the actual spacing, at which point the model calibration is considered complete. At this point, based on the actual roller spacing of the continuous casting machine, the billet shell thickness, billet shell temperature, billet center temperature, and center solids content at all rollers in the light pressing area of ​​the casting machine can be determined. The difference between the distance from the meniscus calculated in the model and the actual distance is ignored. The temperatures and solids contents calculated by the model can then be used for process design and analysis.

[0031] The calculation results of this embodiment are shown in the table below.

[0032]

[0033] Note: In the table above, the column below the value of m represents the distance (in meters) from the meniscus in the model when the blank thickness calculated using the model is equal to the value in step two (64mm for roll #3 and 74mm for roll #4). The bottom row shows the roll spacing between roll #3 and roll #4 after calculation for different values ​​of m.

[0034] In this embodiment, when m is ultimately selected as 5.0, the model-calculated roll spacing is close to the actual roll spacing. At this point, the model calibration for X steel is considered complete. Based on the equipment roll spacing of 1.31 meters, the position of each roll relative to the lunar surface in the model can be calculated by moving it upwards and downwards. Data such as the central solid fraction can be read from the model calculation results based on these positions for process research. In this embodiment, the difference between the roll position displayed by the model and the actual position is approximately 0.3 meters.

Claims

1. A method for calibrating a solidification heat transfer model using a low-magnification cast billet, characterized in that: First, use general formulas to obtain the liquidus temperature and solidus temperature of the current steel grade. Then, input the continuous casting production process parameters of the current steel grade into the model. The latent heat of solidification, specific heat capacity, thermal conductivity, and equivalent thermal conductivity m in the model are replaced with data of other steel grades with known parameters for the initial calculation. Read the billet shell thickness H corresponding to at least two rolls in the continuous casting light reduction area from the low magnification of the billet. Record the billet shell thickness corresponding to the N# roll as Hn, and the billet shell thickness corresponding to the N+1# roll as Hn+1. Find the roll solidification position corresponding to the billet shell thickness equal to H from the model calculation results, and calculate the spacing between the aforementioned two or more roll positions. Compare the roll spacing calculated by the model with the actual roll spacing. If the calculated roll spacing is less than the actual roll spacing, it means that the cooling rate between these two roll intervals is too large, and the value of m needs to be reduced. Conversely, if the calculated roll spacing is greater than the actual roll spacing, it means that the cooling rate between these two roll intervals is too small, and the value of m needs to be increased. The increment or decrement is determined according to the size of the spacing difference. Each increment or decrement is 0.1 or 0.

2. Adjust the equivalent thermal conductivity m until the calculated roll spacing is closest to the actual roll spacing. Then solidify the equivalent thermal conductivity m. At this point, the model is considered to be calibrated. When the model is used for calculation results after calibration, calculate the central solid fraction at other roll rows based on the relative position value with the N# roll.

2. The method for calibrating a solidification heat transfer model by low-magnification of a cast billet according to claim 1, characterized in that: The equivalent thermal conductivity m is taken as 4 to 7.

3. The method for calibrating a solidification heat transfer model by low-magnification of a cast billet according to claim 1, characterized in that: Based on the billet shell thickness read from the low magnification of the billet, the corresponding distance from the meniscus is found in the model calculation results, and the distance between adjacent rolls is calculated.

4. The method for calibrating a solidification heat transfer model by low-magnification of a cast billet according to claim 1, characterized in that: When the calculated roll spacing is closest to the actual roll spacing, the billet shell thickness, billet shell temperature, billet center temperature and center solid fraction at all rolls in the light pressing area of ​​the continuous casting machine are determined according to the roll spacing on site, that is, the relative position of each roll in the model based on the roll spacing identification model. At this time, the difference between the distance from the meniscus in the model calculation and the actual distance is ignored. At this time, the temperature and solid fraction calculated by the model can be directly used for the process design and analysis of the current steel grade.

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