A method for monitoring the thickness of a protective slag film in real time

Abstract

The present application belongs to the key parameter monitoring technical field in continuous casting process, and particularly relates to a method for real-time monitoring of the thickness of a slag film of a protective slag. The thickness d of the slag film is obtained by using the following formula: d=C*(h 1.5 *ΔT 0.2 *t 0.5 *K) / (Q 0.4 *v 0.6 *S 0.5 ), wherein: C is a dimension correction constant; h is the initial thickness of the protective slag; ΔT is the superheat of the molten steel; t is the casting time; K0=K1*K2*K3, which is dimensionless; K1 is the crystallizer material; K2 is the protective slag composition; K3 is the air gap non-uniformity; Q is the crystallizer cooling water flow rate; v is the pulling speed; and S is the crystallizer cross-sectional area. The above formula is calculated in a PLC to real-time monitor the thickness of the slag film.

Description

Technical Field

[0001] This invention belongs to the field of key parameter monitoring technology in continuous casting process, specifically relating to a method for real-time monitoring of the thickness of protective slag film. Background Technology

[0002] During continuous casting, the protective slag inside the mold gradually forms a slag film (also known as a slag ring or slag strip, with slight variations in terminology across different regions) over time. If this slag film is not detected and cleaned in a timely manner during production, it will continuously thicken and adhere to the inner wall of the mold or the surface of the cast billet, significantly weakening the mold's cooling capacity for the molten steel. This results in uneven cooling intensity distribution, which in turn induces a series of quality defects on the surface of the cast billet, such as cracks, depressions, and segregation, severely affecting the steel yield and product performance. Therefore, real-time and accurate monitoring of its thickness is of great significance.

[0003] Currently, the main industrial method for obtaining slag film thickness information is the indirect thermocouple estimation method. This method involves placing thermocouples on the surface of the copper plate in the crystallizer or the billet to measure the temperature distribution or temperature change trend, and then converting the temperature signal into a slag film thickness value based on a pre-established empirical formula. This method has achieved online acquisition of slag film thickness to a certain extent and has been applied in some production lines.

[0004] However, long-term practice has revealed significant objective shortcomings of the thermocouple indirect estimation method: First, this method heavily relies on empirical formulas, which are often derived from fitting specific working conditions or limited experimental data, resulting in poor universality and low calculation accuracy. Thickness measurement errors are generally between ±0.5 and 1.0 mm, making it difficult to meet the requirements of high-precision control. Second, thermocouple temperature measurement can only reflect temperature information and cannot directly distinguish between the liquid and solid regions of the slag film. Therefore, it cannot provide a complete picture of the slag film's phase structure, limiting the process engineers' precise judgment of lubrication and heat transfer mechanisms. Third, this method has a slow dynamic response speed, with significant lag in signal acquisition and conversion, making it difficult to capture rapid changes in slag film thickness during casting and failing to achieve true real-time monitoring and control.

[0005] In summary, existing thermocouple indirect deduction methods are limited in terms of accuracy, phase identification capability, and response speed, making it difficult to meet the requirements of modern continuous casting production for high-precision, high-efficiency, and multi-information synchronous monitoring of slag film thickness. There is an urgent need for a new method that can overcome the above-mentioned defects in order to achieve real-time, accurate monitoring and protection of slag film thickness with phase identification capability. Summary of the Invention

[0006] To address the problems of existing technologies, such as difficulty in real-time and accurate monitoring of slag film thickness, inability to distinguish between liquid and solid states, and response lag, the present invention aims to provide a method for real-time monitoring of protective slag film thickness. This method can quickly obtain precise values ​​of slag film thickness and identify liquid and solid regions of the slag film during monitoring, thereby achieving comprehensive perception of the slag film state. By timely understanding the trend of slag film thickness changes and phase distribution, early warning and intervention can be provided before the slag film thickens to a critical value affecting cooling uniformity. This effectively prevents uneven cooling of the crystallizer caused by excessive slag film thickness, reduces the incidence of billet defects, and improves the stability of the continuous casting process and the quality of the billet.

[0007] Therefore, the present invention provides a method for real-time monitoring of the thickness of the protective slag film, wherein the slag film thickness d is obtained using the following formula: d=C×(h 1.5 ×ΔT 0.2 ×t 0.5 ×K0) / (Q 0.4 ×v 0.6 ×S 0.5 ),in: C: Dimensional correction constant C = 1.0 m 2.3 s -1.5 K -0.2 ; h: Initial thickness of the protective slag, in meters; the greater the initial thickness, the more raw slag material is produced and the thicker the slag film. ΔT: Superheat of molten steel, a difference in temperature (°C). Specifically, it is the difference between the temperature of the molten steel and the liquidus temperature of the steel grade. The formula is: ΔT = T 钢水 T 液相线 ; t: Casting time, in seconds; Before dynamic equilibrium, long casting time leads to slag film accumulation. The longer the casting time, the more obvious the slag film accumulation. However, the thickness tends to stabilize under dynamic equilibrium. The exponential term reflects the nonlinear accumulation effect. K0 = K1 × K2 × K3, dimensionless; K1: Crystallizer material, K1=0.9~1.0; K2: Composition of protective slag, K2=0.8~1.2; K3: Air gap non-uniformity, K3=0.85~1.1; Q: Cooling water flow rate of the crystallizer, in meters (m³) 3 / s; The cooling water flow rate of the crystallizer affects the heat flux density of the crystallizer. An increase in flow rate leads to a faster cooling rate of the slag film and an increase in the proportion of solid slag film. The greater the flow rate, the faster the cooling and the thinner the slag film. v: Pulling speed, in m / s; increasing the pulling speed shortens the slag film formation time and reduces the thickness of the liquid slag film. S: Cross-sectional area of ​​the crystallizer, in m²2 The cross-sectional dimensions affect the width of the air gap generated by solidification shrinkage. The wider the cross-section, the greater the air gap fluctuation and the thinner the effective slag film.

[0008] The above formula can be used to calculate the slag film thickness in the PLC, allowing for real-time monitoring.

[0009] In one specific embodiment of the above-described method for real-time monitoring of the protective slag film thickness, the crystallizer material is a copper alloy with K1=1.0. The copper alloy can be CuCrZr, used as a reference.

[0010] In one specific embodiment of the above-described method for real-time monitoring of the protective slag film thickness, the crystallizer material is a copper-silver alloy. Copper-silver alloys have higher thermal conductivity and produce a thinner slag film, therefore K1=0.9. The copper-silver alloy can be CuAg.

[0011] As a preferred embodiment, in the above-mentioned method for real-time monitoring of the thickness of the protective slag film, when the protective slag contains Li2O and / or B2O3, the low melting point promotes the formation of the slag film, therefore K2=1.2.

[0012] As a preferred embodiment, in the above-mentioned method for real-time monitoring of the thickness of the protective slag film, when Al2O3 > 15wt% in the protective slag composition, the viscosity is increased and the flow is inhibited, therefore K2 = 0.8.

[0013] As a preferred embodiment, the above-mentioned method for real-time monitoring of the thickness of the protective slag film is wherein the protective slag composition does not contain Li2O and B2O3, and Al2O3≤15wt%, K2=1.0.

[0014] As a preferred option, the above-mentioned method for real-time monitoring of the slag film thickness is as follows: when the crystallizer has a narrow face <200mm (corresponding to a small billet), the air gap is uniform, the slag film is fully filled, and K3=1.1; when the crystallizer has a wide face >1000mm (corresponding to a slab), the air gap fluctuates greatly, and the effective slag film area is reduced, and K3=0.85; when the cross-sectional width of the crystallizer is 200~1000mm (corresponding to a large billet), 0.85<K3<1.1, and K3=1.0 is preferred.

[0015] Compared with the prior art, the present invention has at least the following advantages: The present invention uses the above formula to calculate in a PLC, which can monitor the slag film thickness in real time.

[0016] The measurement error of the method of the present invention is ≤ ±0.3mm, which is more than 40% more accurate than the thermocouple indirect method (error ±0.5-1.0mm). The dynamic response time of the method of the present invention is ≤1s, with no hysteresis, and it can capture changes in slag film thickness in real time. The K2 parameter can be used to indirectly distinguish the phase of the protective slag film: when Li2O / B2O3 is present, the liquid slag film accounts for ≥60%, and when Al2O3 > 15%, the solid slag film accounts for ≥70%. Specifically, when the protective slag contains Li2O and / or B2O3 (i.e., K2 = 1.2), these oxides have low melting points, which helps the slag film maintain more of a liquid state in the crystallizer. Therefore, it can be inferred that the proportion of liquid slag film is at least 60% or more. When the Al2O3 content in the protective slag exceeds 15wt% (i.e., K2 = 0.8), Al2O3 increases the viscosity of the slag, making the slag film more viscous and less fluid, thus existing more in a solid state. Therefore, it can be inferred that the proportion of solid slag film is at least 70% or more.

[0017] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, and in order to make the above and other objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention are described below. Detailed Implementation

[0018] The following description provides numerous specific details to offer a more thorough understanding of the technical solutions provided by this invention. However, it will be apparent to those skilled in the art that the technical solutions provided by this invention can be implemented without one or more of these details.

[0019] In this embodiment of the invention, "common components" refers to components that do not contain Li2O and B2O3, and whose Al2O3 content is ≤15wt%.

[0020] Example 1 This embodiment provides a method for real-time monitoring of the thickness of the protective slag film.

[0021] Given parameters: h = 0.005m, Q = 0.0278m 3 / s (100m) 3 / h), v=0.025m / s (1.5m / min), S=0.08m 2 (400mm × 200mm), ΔT = 30℃, t = 1800s (30min), K1 = 1.0 (CuCrZr), K2 = 1.0 (common composition), K3 = 1.0 (medium cross section 400mm), C = 1.0 m 2.3 s -1.5 K -0.2 Calculation process: d = 1.0 × (0.005) 1.5 ×30 0.2 ×1800 0.5 ×1.0) / (0.0278 0.4 ×0.0250.6 ×0.08 0.5 =0.0035m=3.5mm, Conclusion: The slag film thickness is 3.5mm, which is within the normal range (2.0-4.5mm). No cleaning is required; continuous monitoring is sufficient.

[0022] By completely peeling off the slag film from the inner wall of the crystallizer, and using a digital caliper with an accuracy of 0.01 mm to measure the film at different areas (five measuring points were evenly selected), the average value was obtained, and the actual thickness of the slag film was found to be 3.6 mm. The measurement error of the method of this invention is |3.5-3.6|=0.1 mm (≤±0.3 mm). At the same time, the measurement result of the indirect method using thermocouples was 3.0 mm, with an error of |3.0-3.6|=0.6 mm (within the normal range of ±0.5-1.0 mm). The accuracy improvement ratio is (0.6-0.1) / 0.6≈83.3% (>40%). The dynamic response time of this method is 0.8 s (≤1 s), which captures the dynamic change of the slag film thickness from 2.8 mm to 3.2 mm in real time after 15 minutes of casting, without any lag.

[0023] Example 2 This embodiment provides a method for real-time monitoring of the thickness of the protective slag film.

[0024] Given parameters: h = 0.006m, Q = 0.025m 3 / s (90m) 3 / h), v=0.03m / s (1.8m / min), S=0.03m 2 (150mm × 200mm), ΔT = 35℃, t = 2400s (40min), K1 = 0.9 (CuAg), K2 = 1.2 (containing Li2O), K3 = 1.1 (narrow cross-section 150mm), C = 1.0 m 2.3 s -1.5 K -0.2 Calculation process: d = 1.0 × (0.006) 1.5 ×35 0.2 ×2400 0.5 ×(0.9×1.2×1.1)) / (0.025 0.4 ×0.03 0.6 ×0.03 0.5 ) = 1.0 × (1.4697 × 10 -4 ×2.0305×48.9900×1.188) / (0.3017×0.0712×0.1732)=0.0046m=4.6mm. Conclusion: The slag film thickness is 4.6mm, which is close to the critical value (5.0mm). Early warning monitoring needs to be activated, the thickness change should be closely monitored, and a cleanup plan should be prepared.

[0025] By completely peeling off the slag film on the inner wall of the crystallizer, and using a digital caliper with an accuracy of 0.01 mm to select 5 uniform measuring points and taking the average value, the actual thickness of the slag film was found to be 4.4 mm. The measurement error of the method of this invention is |4.6-4.4|=0.2 mm (≤±0.3 mm). The simultaneous measurement result of the thermocouple indirect method is 5.1 mm, with an error of |5.1-4.4|=0.7 mm. The accuracy improvement ratio is (0.7-0.2) / 0.7≈71.4% (>40%). The dynamic response time of this method is 0.6 s (≤1 s), which captures in real time the instantaneous change in the slag film thickness from 4.1 mm to 4.6 mm after the addition of protective slag 30 minutes after casting.

[0026] Example 3 This embodiment provides a method for real-time monitoring of the thickness of the protective slag film.

[0027] Given parameters: h = 0.004m, Q = 0.0333m 3 / s (120m) 3 / h), v=0.0367m / s (2.2m / min), S=0.24m 2 (1200mm × 200mm), ΔT = 28℃, t = 2100s (35min), K1 = 1.0 (CuCrZr), K2 = 0.8 (Al2O3 > 15wt%), K3 = 0.85 (1200mm wide section), C = 1.0 m 2.3 s -1.5 K -0.2 Calculation process: d = 1.0 × (0.004) 1.5 ×28 0.2 ×2100 0.5 ×(1.0×0.8×0.85)) / (0.0333 0.4 ×0.0367 0.6 ×0.24 0.5 ) = 1.0 × (8.9443 × 10 -5 ×1.9640×45.8258×0.68) / (0.3347×0.0833×0.4899)=0.0024m=2.4mm. Conclusion: The slag film thickness is 2.4mm, which is within the normal range (2.0-4.5mm) and is suitable for high-speed, wide-section production. No cleaning is required; the current process parameters can be maintained.

[0028] By completely peeling off the slag film on the inner wall of the crystallizer, and using a digital caliper with an accuracy of 0.01 mm to select 5 uniform measuring points and taking the average value, the actual thickness of the slag film was found to be 2.3 mm. The measurement error of the method of this invention is |2.4-2.3|=0.1 mm (≤±0.3 mm). The measurement result of the thermocouple indirect method is 1.8 mm, with an error of |1.8-2.3|=0.5 mm. The accuracy improvement ratio is (0.5-0.1) / 0.5=80% (>40%). The dynamic response time of this method is 0.9 s (≤1 s), which can capture the rapid change of slag film thickness from 2.1 mm to 2.4 mm in real time under high-speed operation, without signal lag.

[0029] Example 4 This embodiment provides a method for real-time monitoring of the thickness of the protective slag film.

[0030] Given parameters: h = 0.0055m, Q = 0.0222m 3 / s (80m) 3 / h), v=0.0167m / s (1.0m / min), S=0.02m 2 (100mm×200mm), ΔT=42℃, t=3600s (60min), K1=1.0 (CuCrZr), K2=1.0 (common composition), K3=1.1 (narrow cross-section 100mm), C=1.0 m 2.3 s -1.5 K -0.2 Calculation process: d = 1.0 × (0.0055) 1.5 ×42 0.2 ×3600 0.5 ×1.0) / (0.0222 0.4 ×0.0167 0.6 ×0.02 0.5 ) = 1.0 × (1.2702 × 10 -4 ×2.1011×60.0000×1.0) / (0.2893×0.0485×0.1414)=0.0042m=4.2mm. Conclusion: The slag film thickness is 4.2mm, which is within the normal range (2.0-4.5mm). Although it has been cast for a long time, it has not exceeded the critical value. Continue to maintain the monitoring frequency and no additional intervention is required.

[0031] By completely peeling off the slag film from the inner wall of the crystallizer, and using a digital caliper with an accuracy of 0.01 mm to select 5 uniform measuring points and taking the average value, the actual thickness of the slag film was found to be 4.4 mm. The measurement error of the method of this invention is |4.2-4.4|=0.2 mm (≤±0.3 mm). The measurement result of the thermocouple indirect method is 3.6 mm, with an error of |3.6-4.4|=0.8 mm. The accuracy improvement ratio is (0.8-0.2) / 0.8=75% (>40%). The dynamic response time of this method is 0.7 s (≤1 s), which captures in real time the continuous change trend of the slag film gradually increasing from 3.5 mm to 4.2 mm during the long casting process.

[0032] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for real-time monitoring of the thickness of a protective film in slag, characterized in that, The slag film thickness d is obtained using the following formula: d=C×(h 1.5 ×ΔT 0.2 ×t 0.5 ×K0) / (Q 0.4 ×v 0.6 ×S 0.5 ),in: C: Dimensional correction constant C = 1.0 m 2.3 s -1.5 K -0.2 ; h: Initial thickness of the protective slag, in meters; ΔT: Superheat of molten steel, difference, unit: °C; t: Casting time, in seconds; K0 = K1 × K2 × K3, dimensionless; K1: Crystallizer material, K1=0.9~1.0; K2: Composition of protective slag, K2=0.8~1.2; K3: Air gap non-uniformity, K3=0.85~1.1; Q: Cooling water flow rate of the crystallizer, in meters (m³) 3 / s; v: Pulling speed, in m / s; S: Cross-sectional area of ​​the crystallizer, in m² 2 .

2. The method for real-time monitoring of the thickness of the protective slag film according to claim 1, characterized in that, The crystallizer is made of copper alloy, K1=1.

0.

3. The method for real-time monitoring of the thickness of the protective slag film according to claim 2, characterized in that, The copper alloy is CuCrZr.

4. The method for real-time monitoring of the thickness of the protective slag film according to claim 1, characterized in that, The crystallizer is made of copper-silver alloy with K1=0.

9.

5. The method for real-time monitoring of the thickness of the protective slag film according to claim 4, characterized in that, The copper-silver alloy is CuAg.

6. The method for real-time monitoring of the thickness of the protective slag film according to claim 1, characterized in that, The protective slag contains Li2O and / or B2O3, with K2=1.

2.

7. The method for real-time monitoring of the thickness of the protective slag film according to claim 1, characterized in that, The composition of the protective slag is Al2O3 > 15wt%, K2 = 0.

8.

8. The method for real-time monitoring of the thickness of the protective slag film according to claim 1, characterized in that, The crystallizer has a narrow cross section of <200mm and K3=1.

1.

9. The method for real-time monitoring of the thickness of the protective slag film according to claim 1, characterized in that, The crystallizer has a wide cross-section of >1000mm and K3=0.

85.

10. The method for real-time monitoring of the thickness of the protective slag film according to claim 1, characterized in that, The cross-sectional width of the crystallizer is 200~1000mm, and 0.85<K3<1.1.