Method for predicting risk of longitudinal cracking of cast slab before delivery and continuous casting method based on prediction before delivery

Abstract

This invention discloses a method for pre-production prediction of longitudinal crack risk in cast billets and a continuous casting method based on pre-production prediction. The method assesses the condition of casting equipment using a rating system with four levels: A, B, C, and D. For crystallizer vibration: A level is defined as longitudinal runout < ±0.2mm and lateral runout < ±0.05mm; B level is defined as longitudinal runout ≤ ±0.2mm and ±0.05mm ≤ lateral runout < ±0.2mm; C level is defined as ±0.2mm ≤ longitudinal runout < ±0.25mm and lateral runout ≤ ±0.2mm; and D level is defined as ±0.25mm ≤ longitudinal runout ≤ ±0.3mm and lateral runout ≤ ±0.2mm. This method, by establishing equipment rating rules for pre-production prediction, can effectively avoid the generation of longitudinal crack defects on the surface of cast billets and the risk of these defects flowing into subsequent processes. It improves the surface quality of cast billets without increasing production costs, achieving high efficiency and high quality.

Description

Technical Field

[0001] This invention belongs to the field of continuous casting technology, and in particular to a method for predicting the risk of longitudinal cracks in billets before production and a continuous casting method based on pre-production prediction. Background Technology

[0002] Longitudinal cracks in continuously cast slabs are a common surface defect in the production process, occurring on both the surface and subsurface of the slab. Extensive research has been conducted by scholars both domestically and internationally on this defect, finding its root cause to originate within the crystallizer and worsen through expansion in the secondary cooling zone. Surface longitudinal cracks can be detected through hot or cold inspection and can be eliminated through manual finishing, without affecting subsequent rolling processes. However, this results in increased steel material loss and labor costs, and also impacts production scheduling. Subsurface cracks, also known as subcutaneous cracks, are difficult to detect during slab inspection and easily propagate to subsequent processes, affecting hot and cold rolling, causing linear quality defects on the surface of steel coils, severely impacting product yield and the fulfillment of contracts with key customers.

[0003] Longitudinal cracks in cast billets are a typical defect that occurs within the mold. They can range in length from a few millimeters to 3-4 meters, and in severe cases, even penetrate the entire surface of the billet. The root cause of longitudinal cracks lies in the meniscus region of the mold. During the solidification process of molten steel to form a billet shell, volume shrinkage occurs, creating air gaps between the billet shell and the mold wall. This increases thermal resistance and reduces heat transfer efficiency. Because these air gaps are non-uniform within the mold, they affect the uniformity of heat transfer from the billet shell to the mold, further causing uneven distribution of the billet shell thickness and internal stress. Under the static pressure of the molten steel, tiny micro-cracks form at the weakest points of the billet shell. If the casting machine is in poor condition, the cooling intensity of the billet entering the secondary cooling zone will be uneven, and the micro-cracks formed at the exit of the mold will further expand and extend, eventually forming longitudinal cracks on the billet surface. Therefore, in addition to the close relationship between the properties of the protective slag, the heat flow of the crystallizer, the superheat of the molten steel, the constant casting speed, the insertion depth of the submerged nozzle, and whether the surface of the copper plate of the crystallizer is worn, the precision of the casting equipment and the cooling intensity of the secondary cooling system should also be given special attention in preventing longitudinal cracks in the billet.

[0004] Current domestic and international research mainly focuses on analyzing and studying the causes of longitudinal crack defects on the surface of cast billets and rapid identification through online detection. However, there are few reports on classifying different steel grades for pre-production prediction based on casting machine arc and spray status to avoid the risk of longitudinal cracks.

[0005] Chinese patent application CN114346193A discloses a process method for preventing longitudinal cracks and sticking / stainless steel leakage during the start-up process of a slab continuous casting machine. This method reduces longitudinal cracks and leakage by controlling the casting speed after the billet exits the tundish. The method initially sets the casting speed to 0.28 m / min automatically; after maintaining this speed for 2 minutes, it increases by 0.03 m / min every 30 seconds. Once the speed reaches 0.70 m / min, it is manually controlled to increase by 0.03 m / min every 40 seconds to reach the normal casting speed. This method does not address other process parameters that affect longitudinal cracking.

[0006] Chinese patent application CN101985166A discloses a method for predicting longitudinal crack leakage in continuously cast slabs. This method analyzes and judges based on temperature data detected by thermocouples, comprehensively considers the transverse temperature difference of thermocouples, the longitudinal temperature change law and amplitude, and combines the characteristics of steel grades to determine the occurrence of longitudinal cracks, overcoming the limitations of relying solely on transverse temperature difference or the temperature change rate of a single longitudinal thermocouple. However, it does not study the related influencing factors such as the performance of protective slag, the taper of the crystallizer, and the steady-state control of the liquid level.

[0007] Chinese patent application CN113584254A discloses a method for reducing the incidence of longitudinal cracks in ultra-high carbon steel billets. This method reduces the incidence of longitudinal cracks in ultra-high carbon steel continuous casting billets by controlling the superheat and H content of the molten steel after LF refining, as well as controlling the basicity, viscosity and melting temperature of the protective slag. However, it does not involve the optimization of parameters such as crystallizer vibration, crystallizer taper, and the state of the fan-shaped section.

[0008] Chinese patent application CN115586215 A discloses a method for logically judging longitudinal cracks on the surface of continuously cast billets based on temperature characteristics. This method involves establishing a sample database of longitudinal cracks on the surface of continuously cast billets; calculating the temperature characteristic values ​​of thermocouples in the continuous casting mold; obtaining the temperature characteristic value range of longitudinal cracks on the surface of continuously cast billets; and performing rapid and accurate identification of longitudinal cracks on the surface of continuously cast billets. However, this method only provides an effective method for online detection of longitudinal cracks on the surface of continuously cast billets, and does not conduct corresponding research on the control, evaluation, and prediction of longitudinal cracks.

[0009] Chinese patent application CN109136782A discloses a method for controlling longitudinal cracks in 1000MPa grade cold-rolled strip steel billets. This method controls cracks with a depth ≤1mm generated inside the mold by means of continuous casting mold mold flux, mold taper, vibration mode, heat flux density, etc., but does not analyze the control after exiting the mold.

[0010] "Control of Longitudinal Cracks on the Surface of Slabs in No. 4 Slab Continuous Casting Machine" (Proceedings of the 2007 Pan-Pearl River Delta Eleven Provinces (Regions) Steelmaking and Continuous Casting Annual Conference, Issue F09, 2007, pp. 264-267) This paper comprehensively analyzes and studies the parameters affecting longitudinal cracks from aspects such as steel composition, protective slag performance, steel temperature, crystallizer vibration, crystallizer liquid level fluctuation control, and the state of the sector section. Summary of the Invention

[0011] The technical problem to be solved by the present invention is to provide a method for predicting the risk of longitudinal cracks in cast billets before production; the present invention also provides a continuous casting method based on pre-production prediction to improve the surface quality of cast billets.

[0012] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: to evaluate the condition of casting equipment, and to evaluate it according to the evaluation rules from four levels: A, B, C and D.

[0013] Crystallizer vibration: Longitudinal runout < ±0.2mm and lateral runout < ±0.05mm is Grade A; longitudinal runout < ±0.2mm and ±0.05mm ≤ lateral runout < ±0.2mm is Grade B; ±0.2mm ≤ longitudinal runout < ±0.25mm and lateral runout ≤ ±0.2mm is Grade C; ±0.25mm ≤ longitudinal runout ≤ ±0.3mm and lateral runout ≤ ±0.2mm is Grade D.

[0014] Deviation of the outer arc of the sector segment: Grade A: all points < ±0.5mm; Grade B: ±0.5mm ≤ deviation of 1-3 points on the outer arc < ±1.0mm, and all other deviations ≤ ±0.5mm; Grade C: ±0.5mm ≤ deviation of 4-5 points on the outer arc < ±1.0mm, and all other deviations ≤ ±0.5mm; Grade D: ±0.5mm ≤ deviation of 6-8 points on the outer arc < ±1.0mm, and no more than 2 points with deviations ≥ ±1.0mm, and all other deviations ≤ ±0.5mm.

[0015] Spraying status: During continuous casting, the blockage rate before the straightening section is ≤3%, which is Grade A; 3% < blockage rate ≤5%, which is Grade B; 5% < blockage rate ≤8%, which is Grade C; 8% < blockage rate, which is Grade D;

[0016] The equipment rating prediction is based on the weakest link system, using the worst-case conditions of crystallizer vibration, deviation of the outer arc of the fan-shaped section, and spray condition as the standard to predict the risk of longitudinal cracks on the surface of the billet: the comprehensive evaluation grade A is the lowest risk of longitudinal cracks, followed by grades B and C, and the comprehensive evaluation grade D is the highest risk of longitudinal cracks.

[0017] The continuous casting method of this invention uses the above-mentioned method for pre-production prediction: the casting machine equipment is comprehensively evaluated as Grade A for producing sensitive steel grades with carbon equivalent of 0.3 ≤ carbon equivalent < 0.6; Grade B for producing pickled steel and structural steel with carbon equivalent of 0.1 ≤ carbon equivalent < 0.3; and Grades C and D for producing ordinary ultra-low carbon and low carbon steel.

[0018] Furthermore, the casting speed control for the start of continuous casting is as follows: after initial casting, the casting speed passes through the lower opening of the crystallizer at 0.4 m / min, and then at 0.3–0.4 m / min. 2 The acceleration rate increases to 0.8 m / min, remains constant for 2-3 minutes, and then decreases to 0.2 m / min. 2 The growth benchmark was raised to the target acceleration.

[0019] Furthermore, during the continuous casting process, the slag layer thickness is controlled to be 8-15 mm, and the consumption of protective slag is calculated according to formula (I);

[0020] Q=0.8(1 / A 0.3 )(60 / f)(nV 2 ) -0.5 +0.13 (Ⅰ)

[0021] In the formula, Q: consumption of protective slag, kg / m³ 2 A: Amplitude of crystallizer vibration, mm; f: Frequency of crystallizer vibration, times / min; n: Viscosity of protective slag, Pa·s; V: Target pulling speed, m / min.

[0022] Furthermore, during the continuous casting process, the heat flow ratio between the wide and narrow sides of the crystallizer is controlled at 1.1 to 1.25, and the superheat of the molten steel is controlled at 15 to 30°C.

[0023] Furthermore, during the continuous casting process, the crystallizer vibration adopts a smooth sawtooth wave negative slip mode with a skew rate of 60%, a smoothness of 25%, a negative slip time controlled at 0.12 to 0.15 s, and a negative slip time rate controlled at 39 to 43%.

[0024] The beneficial effects of adopting the above technical solution are as follows: By establishing equipment rating rules for pre-production prediction, the present invention can quickly and effectively determine the risk of longitudinal cracks on the surface of the billet, providing a basis for further production.

[0025] This invention's continuous casting method, through the establishment of equipment rating rules for pre-production prediction, can effectively avoid the generation of longitudinal cracks on the billet surface and the risk of these defects flowing into the next process. Without increasing production costs, it improves the surface quality of the billet, achieving high efficiency and high quality. In particular, by comprehensively controlling steady-state factors such as the carbon equivalent of molten steel, crystallizer vibration, spraying status, fan-shaped section accuracy, temperature difference between the inlet and outlet of the crystallizer cooling water, rapid expansion speed during initial casting, and superheat of the molten steel, the risk of longitudinal cracks on the billet surface can be further avoided, thus improving the surface quality of the billet. Attached Figure Description

[0026] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0027] Figure 1 This is a photograph of the longitudinal crack defect on the surface of the cast billet obtained by conventional methods.

[0028] Figure 2 This is a photograph of the longitudinal crack defects on the surface of the strip obtained by conventional methods.

[0029] Figure 3 This is a real image of the surface of the cast billet obtained in Example 2 of the method of the present invention;

[0030] Figure 4 This is a real image of the surface of the strip steel obtained in Embodiment 2 of the method of the present invention. Detailed Implementation

[0031] Longitudinal cracks in cast billets originate within the crystallizer and will worsen after exiting the crystallizer if the casting machine is in poor condition. Considering the different alloy compositions and carbon equivalents of different steel grades, the varying thermal expansion and contraction stresses generated during billet shell solidification within the crystallizer lead to differences in crack sensitivity. Furthermore, abnormal equipment conditions after exiting the crystallizer will further amplify crack defects. Based on this understanding, this pre-production method for predicting the risk of longitudinal cracks in cast billets establishes a set of equipment condition rating rules. These rules evaluate equipment in four levels: A, B, C, and D, where A represents the best equipment condition, B represents good equipment condition, C represents average equipment condition, and D represents poor equipment condition. The evaluation process is as follows:

[0032] (1) Crystallizer vibration: Longitudinal sway < ±0.2mm and lateral sway < ±0.05mm is Grade A; longitudinal sway < ±0.2mm and ±0.05mm ≤ lateral sway < ±0.2mm is Grade B; ±0.2mm ≤ longitudinal sway < ±0.25mm and lateral sway ≤ ±0.2mm is Grade C; ±0.25mm ≤ longitudinal sway ≤ ±0.3mm and lateral sway ≤ ±0.2mm is Grade D. The longitudinal sway ≤ ±0.2mm should be understood as the distance of sway in both longitudinal directions being 0.2mm or less, with the longitudinal position point of the crystallizer when it is not vibrating as the reference center; the lateral sway ≤ ±0.05mm should be understood as the distance of sway in both lateral directions being 0.05mm or less, with the lateral position point of the crystallizer when it is not vibrating as the reference center; other distance ranges are understood in the same way.

[0033] (2) Deviation of the outer arc of the sector segment: The casting machine has a total of 17 sector segments, each with 4 points; all points of the outer arc < ±0.5mm is grade A; ±0.5mm ≤ deviation of 1 to 3 points of the outer arc < ±1.0mm, and other deviations ≤ ±0.5mm is grade B; ±0.5mm ≤ deviation of 4 to 5 points of the outer arc < ±1.0mm, and other deviations ≤ ±0.5mm is grade C; ±0.5mm ≤ deviation of 6 to 8 points of the outer arc < ±1.0mm, and ≤ 2 points with deviations ≥ ±1.0mm, and other deviations ≤ ±0.5mm is grade D. The deviation of 1 to 3 points of the outer arc < ±1.0mm should be understood as 1 to 3 points offset in both directions by a distance exceeding or equal to 0.5mm but not exceeding 1.0mm, with the original position of the outer arc of the sector segment as the reference center; other deviation ranges are understood in the same way.

[0034] (3) Spraying status: The normal spraying status is a mist. The presence of blockage is an undesirable status. During the continuous casting process, the area before the straightening section, i.e., sections 1 to 8, is the sensitive area of ​​the billet shell. The blockage rate before the straightening section is ≤3% for Grade A; 3% < blockage rate before the straightening section ≤5% for Grade B; 5% < blockage rate before the straightening section ≤8% for Grade C; and blockage rate before the straightening section >8% for Grade D.

[0035] (4) The equipment rating prediction is based on the short board system. The worst conditions of crystallizer vibration, fan-shaped section outer arc deviation and spraying status are used as the standard to predict the risk of longitudinal cracks on the surface of the billet: the comprehensive evaluation is that the risk of longitudinal cracks is the lowest for grade A, followed by grade B, then grade C, and the comprehensive evaluation is that the risk of longitudinal cracks is the highest for grade D.

[0036] This continuous casting method based on pre-production prediction adopts the above method for pre-production prediction, including the following process control: (1) According to the pre-production prediction results, if the casting machine is rated A, then the casting machine is used to produce sensitive steel grades with carbon equivalent of 0.3≤0.6, including high-strength steel of grade 780MP and above and pipeline steel of grade X65 and above; if the comprehensive evaluation is B, then the casting machine is used to produce pickled steel and structural steel with carbon equivalent of 0.1≤0.3 and containing trace alloys; if the comprehensive evaluation is C and D, ordinary ultra-low carbon and low carbon steel and other steel grades that are not prone to longitudinal cracking are produced. The carbon equivalent calculation formula is: Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15.

[0037] (2) Rapid expansion speed control is implemented in the initial stage of continuous casting, that is, after starting, the speed is 0.4 m / min through the lower end of the crystallizer; then the speed is 0.3 to 0.4 m / min. 2 The acceleration rate increased to 0.8 m / min; after maintaining this rate for 2–3 minutes, it decreased to 0.2 m / min. 2The speed increase benchmark is raised to the target casting speed to shorten the unsteady control time of the speed increase; finally, the casting control is maintained at the target casting speed, preferably a constant casting speed; the target casting speed is preferably 0.9 to 1.6 m / min. This control process shortens the unsteady control time of the speed increase by rapidly increasing the casting speed at the beginning of continuous casting, and reduces the occurrence of uneven shell thickness and air gaps during the unsteady control time of the speed increase.

[0038] (2) The protective slag for the crystallizer should be added in small, frequent, and uniform amounts, i.e., once every 5 to 8 minutes, with each addition being 4 to 7 kg, and evenly dispersed on the surface of the molten steel in the crystallizer to ensure "black surface operation," i.e., no exposed molten steel surface, no local redness, and no molten steel sparks. During the casting process, slag strips should not be picked up, and the molten steel surface in the crystallizer should be kept stable during casting to ensure a slag layer thickness of 8 to 15 mm. The consumption of protective slag should be calculated according to formula (I), and it is best to control it at 0.35 to 0.6 kg / t;

[0039] Q=0.8(1 / A 0.3 )(60 / f)(nV 2 ) -0.5 +0.13 (Ⅰ)

[0040] In formula (Ⅰ), Q: consumption of protective slag, kg / m³ 2 A: Amplitude of crystallizer vibration, mm; f: Frequency of crystallizer vibration, times / min; n: Viscosity of protective slag, Pa·s; V: Target pulling speed, m / min.

[0041] This control process ensures that the molten slag lubricates the meniscus and fills the air gap between the copper plate and the billet shell by controlling the appropriate slag layer thickness and consumption of the molded mold protective slag.

[0042] (3) Ensure steady-state casting with heat flow and control heat flow deviation ≤ 0.2MW / m 2 The heat flow ratio between the wide and narrow sides of the crystallizer is controlled at 1.1 to 1.25; the temperature difference between the inlet and outlet of the cooling water in the crystallizer is controlled at 3 to 8℃, preferably 3 to 6℃ in summer and 5 to 8℃ in winter; narrow superheat temperature control is adopted to ensure that the superheat of molten steel is controlled at 15 to 30℃.

[0043] (4) The crystallizer vibration adopts a smooth sawtooth wave negative slip vibration mode with a skewness of 60% and a smoothness of 25%. When the pulling speed is 0.9m / min to 1.6m / min, the frequency is 130 to 180 (1 / min), the negative slip time is controlled at 0.12 to 0.15s, the negative slip displacement is controlled at 2.5 to 3.5mm, and the negative slip time rate is controlled at 39 to 43%.

[0044] The skewness is calculated as: (total time of the upper stroke / total time of one vibration cycle) * 100.

[0045] The negative slip time rate is the ratio of the negative slip time within one vibration cycle to half a vibration cycle, Ts = 2S / T; where, negative slip time rate: Ts; negative slip time: S; vibration cycle: T.

[0046] In this control process, the crystallizer vibration adopts a smooth sawtooth wave negative slip mode with a skew rate of 60% and a smoothness of 25%. Its characteristics include a longer rise time than fall time and a gentler speed. The advantages are increased consumption of protective slag, reduced friction of molten steel near the meniscus of the crystallizer, reduced tensile stress on the initial billet shell, and lower risk of leakage. Negative slip vibration occurs when the crystallizer vibrates downwards at a speed greater than the billet pulling speed, forming negative slip. During this period, the billet shell experiences greater compressive than tensile forces, and the solidified shell "welds" the cracked sections caused by compression, promoting shell healing. However, excessively long negative slip times result in deeper vibration marks and more severe slag inclusions in the billet shell at these marks. Therefore, the negative slip time is controlled between 0.12 and 0.15 seconds, and the negative slip time rate is controlled between 39% and 43%.

[0047] (5) Implementation results: Figure 1 and Figure 2 These are images showing the actual appearance of longitudinal cracks on the surface of cast billets and strips obtained using conventional methods. Figure 1 The depth of the longitudinal crack is 0.25 mm. Figure 2 There are long, dark, linear longitudinal cracks in the middle; Figure 3 and Figure 4 The figures show the surface appearance of the slabs and strips obtained by this method. As can be seen from the figures, this continuous casting method effectively improves the surface quality of the slabs and strips. After adopting this continuous casting method, the incidence of longitudinal cracks on the slab surface decreased from 16%–21% in the conventional method to 1.5%–1.8%, and the incidence of longitudinal cracks on the strip surface decreased from 8%–13% in the conventional method to 0.9%–1.2%.

[0048] Example 1: The method for predicting the risk of longitudinal cracks in the billet before production and the continuous casting method based on the pre-production prediction are described in detail below.

[0049] (1) Pre-production assessment: The condition of the casting machine is rated. The longitudinal runout of the crystallizer is ≤ ±0.1mm and the lateral runout is ±0.08~±0.17mm, which is grade B;

[0050] The deviation of two points on the outer arc of the sector segment is ≥0.5mm (one point is 0.5mm and the other point is 0.7mm), and the deviation of all other points is ≤0.5mm, which is Grade B;

[0051] The overall spray pattern is a diffused mist. There are a total of 140 spray nozzles in sections 1 to 8 (70 on the inner arc and 70 on the outer arc). There are 3 nozzles blocked (2 on the inner arc and 1 on the outer arc), with a blockage rate of 2.1%, which meets the requirement of a blockage rate of ≤3%, and is classified as Grade A.

[0052] The overall equipment rating is predicted to be B.

[0053] (2) Continuous casting process: used to produce pickled product QStE420TM, with the following chemical composition and mass percentage: C 0.08%, Si 0.06%, Mn 1.21%, P 0.011%, S 0.006%, Al 0.03%, Nb 0.022%, Ti 0.011%, and the balance being Fe and unavoidable impurities. Based on the carbon equivalent formula, the carbon equivalent of the pickled product QStE420TM is 0.28, which meets the requirement of 0.1 ≤ carbon equivalent < 0.3 for equipment Class B.

[0054] The QStE420TM billet has a cross-section of 1500mm and a thickness of 240mm. Initially, the casting speed is rapidly increased, starting at 0.4m / min through the bottom of the crystallizer, and then decreasing to 0.4m / min. 2 The acceleration rate increased to 0.8 m / min, remained at that level for 2.5 minutes, and then decreased to 0.2 m / min. 2 The growth rate benchmark was raised to the target casting speed of 1.3 m / min, and the casting speed was kept constant.

[0055] The mold protective slag is added every 6-8 minutes, with each addition being 5-7 kg. It is evenly dispersed on the surface of the molten steel in the mold, with no exposed molten steel, no local redness, and no molten steel sparks. The slag strips are not picked up during the casting process, the slag layer thickness is 10 mm, and the slag consumption is 0.45 kg / t.

[0056] The heat flux of the crystallizer is controlled at ≤0.15MW / m. 2 Within this range, the heat flow ratio between the wide and narrow sides is controlled within the range of 1.1 to 1.2, the temperature difference between the inlet and outlet of the cooling water in the crystallizer is 5℃ (in winter), and the superheat of the narrow side is controlled within the range of 18 to 30℃;

[0057] The crystallizer vibration control adopts a smooth sawtooth wave negative slip vibration mode with a skew rate of 60%, smoothness of 25%, a pulling speed of 1.3 m / min, a vibration frequency of 165 m / min, a negative slip time of 0.12 s, a negative slip displacement of 2.8 mm, and a negative slip time rate of 40%.

[0058] (3) Product inspection: The obtained billet was cold inspected and no longitudinal crack defects were found on the surface of the billet; the subsequent rolling was followed up and the surface condition of the cold coil was good, with no longitudinal crack defects.

[0059] Example 2: The method for predicting the risk of longitudinal cracks in the billet before production and the continuous casting method based on the pre-production prediction are described in detail below.

[0060] (1) Pre-production assessment: The condition of the casting machine is rated. The longitudinal runout of the crystallizer is ≤ ±0.18mm and the lateral runout is ≤ ±0.03mm, which is Grade A;

[0061] The deviation of the outer arc of the sector segment is ≤0.5mm, which is Grade A;

[0062] The overall spray pattern is a diffused mist. There are a total of 140 spray nozzles in sections 1 to 8 (70 on the inner arc and 70 on the outer arc). There are 4 nozzles blocked (2 on the inner arc and 2 on the outer arc), with a blockage rate of 2.8%, which meets the requirement of a blockage rate of ≤3%, and is classified as Grade A.

[0063] The overall equipment rating is predicted to be Grade A.

[0064] (2) Continuous casting process: used to produce high-strength steel product DP780, with the following chemical composition and mass percentage: C 0.13%, Si 0.3%, Mn 1.75%, P 0.015%, S 0.01%, Al 0.045%, Cr 0.3%, with the balance being Fe and unavoidable impurities. Based on the carbon equivalent formula, the carbon equivalent of high-strength steel product DP780 is 0.47, which meets the requirements for sensitive steel grades (0.3 ≤ carbon equivalent < 0.6) for equipment Class A.

[0065] The DP780 casting has a cross-section of 1300mm and a thickness of 240mm. Initially, the casting speed is rapidly increased, starting at 0.4m / min through the bottom of the crystallizer, then decreasing to 0.3m / min. 2 The acceleration rate increased to 0.8 m / min, remained constant for 2 minutes, and then decreased to 0.2 m / min. 2 The growth rate benchmark was raised to the target casting speed of 1.4 m / min, and the casting speed was kept constant.

[0066] The mold protective slag is added every 5-7 minutes, with each addition being 4-6 kg. It is evenly dispersed on the surface of the molten steel in the mold, with no exposed molten steel, no local redness, and no molten steel sparks. The slag strips are not picked up during the casting process, the slag layer thickness is 15 mm, and the slag consumption is 0.6 kg / t.

[0067] The heat flux of the crystallizer is controlled at ≤0.2MW / m. 2 Within the specified range, the heat flow ratio between the wide and narrow sides is controlled within the range of 1.15 to 1.2, the temperature difference between the inlet and outlet of the cooling water in the crystallizer is 6℃ (in winter), and the superheat of the narrow side is controlled within the range of 15 to 23℃;

[0068] The crystallizer vibration control adopts a smooth sawtooth wave negative slip vibration mode with a skew rate of 60%, smoothness of 25%, a pulling speed of 1.4 m / min, a vibration frequency of 175 m / min, a negative slip time of 0.13 s, a negative slip displacement of 2.6 mm, and a negative slip time rate of 43%.

[0069] (3) Product inspection: Cold inspection of the obtained casting billet, such as... Figure 3 , Figure 4As shown, no longitudinal cracks were found on the surface of the billet; subsequent rolling showed that the surface condition of the cold coil was good, with no longitudinal cracks.

[0070] Example 3: The method for predicting the risk of longitudinal cracks in the billet before production and the continuous casting method based on the pre-production prediction are described in detail below.

[0071] (1) Pre-production assessment: The condition of the casting machine is rated. The longitudinal runout of the crystallizer is ≤ ±0.15mm and the lateral runout is ±0.05~±0.12mm, which is grade B;

[0072] The deviation of the outer arc of the sector segment is ≥0.5mm at 3 points (1 point is 0.5mm, 1 point is 0.7mm, and 1 point is 0.8mm), and the deviation at other points is ≤0.5mm, which is Grade B;

[0073] The overall spray pattern is a diffused mist. There are a total of 140 spray nozzles in sections 1 to 8 (70 on the inner arc and 70 on the outer arc). There are 6 nozzles blocked (4 on the inner arc and 2 on the outer arc), with a blockage rate of 4.3%. This meets the requirement of 3% < blockage rate ≤ 5%, which is classified as Grade B.

[0074] The overall equipment rating is predicted to be B.

[0075] (2) Continuous casting process: used to produce structural steel SAPH440, with the following chemical composition and mass percentage: C 0.075%, Si 0.05%, Mn 1.15%, P 0.016%, S 0.008%, Al 0.025%, Nb 0.008%, Ti 0.009%, with the balance being Fe and unavoidable impurities. Based on the carbon equivalent formula, the carbon equivalent of the structural steel product SAPH440 is 0.26. This meets the requirement of 0.1 ≤ carbon equivalent < 0.3 for equipment Class B.

[0076] The SAPH440 cast billet has a cross-section of 1200mm and a thickness of 240mm. Initially, the casting speed is rapidly increased, starting at 0.4m / min through the bottom of the crystallizer, and then decreasing to 0.4m / min. 2 The acceleration rate increased to 0.8 m / min, remained at that level for 3 minutes, and then decreased to 0.2 m / min. 2 The growth rate benchmark was raised to the target casting speed of 1.5 m / min, and the casting speed was kept constant.

[0077] The protective slag for the crystallizer is added every 5 to 7 minutes, with each addition being 4 to 6 kg. It is evenly dispersed on the surface of the molten steel in the crystallizer, with no exposed molten steel, no local redness, and no molten steel sparks. The slag strips are not picked up during the casting process, the slag layer thickness is 8 mm, and the slag consumption is 0.35 kg / t.

[0078] The heat flux of the crystallizer is controlled at ≤0.2MW / m. 2Within this range, the heat flux ratio between the wide and narrow sides is controlled within the range of 1.15 to 1.25, the temperature difference between the inlet and outlet of the cooling water in the crystallizer is 4℃ (in summer), and the superheat of the narrow side is controlled within the range of 17 to 28℃;

[0079] The crystallizer vibration control adopts a smooth sawtooth wave negative slip vibration mode with a skew rate of 60%, smoothness of 25%, a pulling speed of 1.5 m / min, a vibration frequency of 180 / min, a negative slip time of 0.13 s, a negative slip displacement of 2.5 mm, and a negative slip time rate of 39%.

[0080] (3) Product inspection: The obtained billet was cold inspected and no longitudinal crack defects were found on the surface of the billet; the subsequent rolling was followed up and the surface condition of the cold coil was good, with no longitudinal crack defects.

[0081] Example 4: The method for predicting the risk of longitudinal cracks in the billet before production and the continuous casting method based on the pre-production prediction are described in detail below.

[0082] (1) Pre-production assessment: The condition of the casting machine is rated. The longitudinal runout of the crystallizer is ±0.20~±0.23mm and the lateral runout is ≤±0.2mm, which is grade C;

[0083] The deviation of the outer arc of the sector segment is ≥0.5mm at one point (0.9mm), and the deviations at other points are ≤0.5mm, which is Grade B;

[0084] The overall spray pattern is a diffused mist. There are a total of 140 spray nozzles in sections 1 to 8 (70 on the inner arc and 70 on the outer arc). There are 8 nozzles blocked (5 on the inner arc and 3 on the outer arc), with a blockage rate of 5.7%. This meets the requirement of a blockage rate >5% and ≤8%, which is classified as Grade C.

[0085] The overall equipment rating is predicted to be C.

[0086] (2) Continuous casting process: used to produce ordinary low-carbon product DC01, with the following chemical composition and mass percentage: C 0.025%, Mn 0.12%, P 0.02%, S 0.015%, Al 0.03%, and the balance being Fe and unavoidable impurities. Based on the carbon equivalent formula, the carbon equivalent of ordinary low-carbon product DC01 is 0.045, which is a common steel grade that is not prone to longitudinal cracking.

[0087] The SPHC casting has a cross-section of 1600mm and a thickness of 240mm. Initially, the casting speed is rapidly increased, starting at 0.4m / min through the bottom of the crystallizer, then decreasing to 0.35m / min. 2 The acceleration rate increased to 0.8 m / min, remained constant for 2 minutes, and then decreased to 0.2 m / min. 2 The growth rate benchmark was raised to the target casting speed of 1.2 m / min, and the casting speed was kept constant.

[0088] The protective slag for the crystallizer is added every 6-8 minutes, with each addition being 6-7 kg. It is evenly dispersed on the surface of the molten steel in the crystallizer, with no exposed molten steel, no local redness, and no molten steel sparks. The slag strips are not picked up during the casting process, the slag layer thickness is 15 mm, and the slag consumption is 0.5 kg / t.

[0089] The heat flux of the crystallizer is controlled at ≤0.1MW / m. 2 Within the specified range, the heat flux ratio between the wide and narrow sides is controlled within the range of 1.15 to 1.2, the temperature difference between the inlet and outlet of the cooling water in the crystallizer is 5℃ (in summer), and the superheat of the narrow side is controlled within the range of 16 to 25℃;

[0090] The crystallizer vibration control adopts a smooth sawtooth wave negative slip vibration mode with a skewness of 60%, smoothness of 25%, a pulling speed of 1.2 m / min, a vibration frequency of 160 / min, a negative slip time of 0.15 s, a negative slip displacement of 3.2 mm, and a negative slip time rate of 43%.

[0091] (3) Product inspection: The obtained billet was cold inspected and no longitudinal crack defects were found on the surface of the billet; the subsequent rolling was followed up and the surface condition of the cold coil was good, with no longitudinal crack defects.

[0092] Example 5: The method for predicting the risk of longitudinal cracks in the billet before production and the continuous casting method based on the pre-production prediction are described in detail below.

[0093] (1) Pre-production assessment: First, the condition of the casting machine is rated. The longitudinal runout of the crystallizer is ≤ ±0.18mm and the lateral runout is ≤ ±0.04mm, which is Grade A;

[0094] The deviation of the outer arc of the sector segment is ≤0.5mm, which is Grade A;

[0095] The overall spray pattern is a diffused mist. There are a total of 140 spray nozzles in sections 1 to 8 (70 on the inner arc and 70 on the outer arc). There are 2 nozzles blocked (1 on the inner arc and 1 on the outer arc), with a blockage rate of 1.4%, which meets the requirement of a blockage rate of ≤3%, and is classified as Grade A.

[0096] The overall equipment rating is predicted to be Grade A.

[0097] (2) Continuous casting process: Used to produce high-strength steel product 980FG, with the following chemical composition and mass percentage: C 0.13%, Si 0.5%, Mn 1.86%, P 0.01%, S 0.007%, Al 0.035%, Cr 0.3%, Ti 0.02%, with the balance being Fe and unavoidable impurities. Based on the carbon equivalent formula, the carbon equivalent of high-strength steel product 980FG is 0.51. This meets the requirement of equipment class A: 0.3 ≤ carbon equivalent < 0.6.

[0098] The 980FG billet has a cross-section of 1100mm and a thickness of 240mm. Initially, the casting speed is rapidly increased, starting at 0.4m / min through the bottom of the crystallizer, then decreasing to 0.3m / min. 2 The acceleration rate increased to 0.8 m / min, remained constant for 2 minutes, and then decreased to 0.2 m / min. 2 The growth rate benchmark was raised to the target casting speed of 1.6 m / min, and a constant casting speed was maintained.

[0099] The protective slag for the crystallizer is added every 5 to 7 minutes, with each addition being 4 to 6 kg. It is evenly dispersed on the surface of the molten steel in the crystallizer, with no exposed molten steel, no local redness, and no molten steel sparks. The slag strips are not picked up during the casting process, the slag layer thickness is 12 mm, and the slag consumption is 0.4 kg / t.

[0100] The heat flux of the crystallizer is controlled at ≤0.1MW / m. 2 Within the specified range, the heat flow ratio between the wide and narrow sides is controlled within the range of 1.1 to 1.15, the temperature difference between the inlet and outlet of the cooling water in the crystallizer is 4℃ (in summer), and the superheat of the narrow side is controlled within the range of 12 to 20℃;

[0101] The crystallizer vibration control adopts a smooth sawtooth wave negative slip vibration mode with a skew rate of 60%, smoothness of 25%, a pulling speed of 1.6 m / min, a vibration frequency of 180 m / min, a negative slip time of 0.12 s, a negative slip displacement of 2.5 mm, and a negative slip time rate of 39%.

[0102] (3) Product inspection: The obtained billet was cold inspected and no longitudinal crack defects were found on the surface of the billet; the subsequent rolling was followed up and the surface condition of the cold coil was good, with no longitudinal crack defects.

[0103] Example 6: The method for predicting the risk of longitudinal cracks in the billet before production and the continuous casting method based on the pre-production prediction are described in detail below.

[0104] (1) Pre-production assessment: The condition of the casting machine is rated. The longitudinal runout of the crystallizer is ±0.25~±0.28mm and the lateral runout is ≤±0.18mm, which is grade D;

[0105] The deviation of the outer arc of the sector segment is ≥0.5mm at 4 points (1 point is 0.5mm, 2 points are 0.7mm, and 1 point is 0.9mm), and the deviation at other points is ≤0.5mm, which is Grade C;

[0106] The overall spray pattern is a diffused mist. There are a total of 140 spray nozzles in sections 1 to 8 (70 on the inner arc and 70 on the outer arc). There are 9 nozzles blocked (5 on the inner arc and 4 on the outer arc), with a blockage rate of 6.4%. This meets the requirement of a blockage rate >5% and ≤8%, which is classified as Grade C.

[0107] The overall equipment rating is predicted to be D.

[0108] (2) Continuous casting process: used to produce ultra-low carbon product DC05, with the following chemical composition and mass percentage: C 0.0018%, Si 0.003%, Mn 0.097%, P 0.012%, S 0.006%, Al 0.037%, Ti 0.061%, B 0.0005%, N 0.0022%, with the balance being Fe and unavoidable impurities. Based on the carbon equivalent formula, the carbon equivalent of ordinary low-carbon product DC05 is 0.018, making it a common steel grade that is not prone to longitudinal cracking.

[0109] The DC05 casting has a cross-section of 1500mm and a thickness of 240mm. Initially, the casting speed is rapidly increased, starting at 0.4m / min through the bottom of the crystallizer, and then decreasing to 0.4m / min. 2 The acceleration rate increased to 0.8 m / min, remained constant for 2 minutes, and then decreased to 0.2 m / min. 2 The growth rate benchmark was raised to the target casting speed of 1.3 m / min, and the casting speed was kept constant.

[0110] The protective slag for the crystallizer is added every 6-8 minutes, with each addition being 6-7 kg. It is evenly dispersed on the surface of the molten steel in the crystallizer, with no exposed molten steel, no local redness, and no molten steel sparks. The slag strips are not picked up during the casting process, the slag layer thickness is 12 mm, and the slag consumption is 0.45 kg / t.

[0111] The heat flux of the crystallizer is controlled at ≤0.1MW / m. 2 Within the specified range, the heat flux ratio between the wide and narrow sides is controlled within the range of 1.15 to 1.2, the temperature difference between the inlet and outlet of the cooling water in the crystallizer is 5℃ (in summer), and the superheat of the narrow side is controlled within the range of 18 to 26℃;

[0112] The crystallizer vibration control adopts a smooth sawtooth wave negative slip vibration mode with a skew rate of 60%, smoothness of 25%, a pulling speed of 1.3 m / min, a vibration frequency of 130 m / min, a negative slip time of 0.14 s, a negative slip displacement of 3.5 mm, and a negative slip time rate of 41%.

[0113] (3) Product inspection: The obtained billet was cold inspected and no longitudinal crack defects were found on the surface of the billet; the subsequent rolling was followed up and the surface condition of the cold coil was good, with no longitudinal crack defects.

Claims

1. A method for pre-production prediction of longitudinal crack risk in cast billets, characterized in that: The condition rating of casting equipment is evaluated using four levels: A, B, C, and D. Crystallizer vibration: Longitudinal runout < ±0.2mm and lateral runout < ±0.05mm is Grade A; longitudinal runout < ±0.2mm and ±0.05mm ≤ lateral runout < ±0.2mm is Grade B; ±0.2mm ≤ longitudinal runout < ±0.25mm and lateral runout ≤ ±0.2mm is Grade C; ±0.25mm ≤ longitudinal runout ≤ ±0.3mm and lateral runout ≤ ±0.2mm is Grade D. Deviation of the outer arc of the sector segment: Grade A: all points < ±0.5mm; Grade B: ±0.5mm ≤ deviation of 1-3 points on the outer arc < ±1.0mm, and all other deviations ≤ ±0.5mm; Grade C: ±0.5mm ≤ deviation of 4-5 points on the outer arc < ±1.0mm, and all other deviations ≤ ±0.5mm; Grade D: ±0.5mm ≤ deviation of 6-8 points on the outer arc < ±1.0mm, and no more than 2 points with deviations ≥ ±1.0mm, and all other deviations ≤ ±0.5mm. Spraying status: During continuous casting, the blockage rate before the straightening section is ≤3%, which is Grade A; 3% < blockage rate ≤5%, which is Grade B; 5% < blockage rate ≤8%, which is Grade C; 8% < blockage rate, which is Grade D; The equipment rating prediction is based on the weakest link system, using the worst of three factors—crystallizer vibration, deviation of the outer arc of the fan-shaped section, and spray condition—as the comprehensive evaluation standard to predict the risk of longitudinal cracks on the surface of the cast billet: the comprehensive evaluation grade A is the lowest risk of longitudinal cracks, followed by grades B and C, and the comprehensive evaluation grade D is the highest risk of longitudinal cracks.

2. A continuous casting method based on pre-production prediction, wherein the pre-production prediction is performed using the method described in claim 1, characterized in that: The comprehensive evaluation of casting equipment is as follows: Grade A is for producing sensitive steel grades with carbon equivalent of 0.3 ≤ carbon equivalent < 0.6; Grade B is for producing pickled steel and structural steel with carbon equivalent of 0.1 ≤ carbon equivalent < 0.3; and Grades C and D are for producing ultra-low carbon and low carbon steel with carbon equivalent < 0.

1.

3. The continuous casting method based on pre-production prediction according to claim 2, characterized in that, The casting speed control for continuous casting initiation is as follows: after starting, the casting speed is 0.4 m / min through the lower opening of the crystallizer, and then 0.3–0.4 m / min. 2 The acceleration rate increases to 0.8 m / min, remains constant for 2-3 minutes, and then decreases to 0.2 m / min. 2 The growth benchmark was raised to the target acceleration.

4. The continuous casting method based on pre-production prediction according to claim 2, characterized in that: During continuous casting, the slag layer thickness is controlled at 8-15 mm, and the consumption of protective slag is calculated according to formula (Ⅰ); Q=0.8(1 / A 0.3 )(60 / f)(nV 2 ) -0.5 +0.13 (Ⅰ) In the formula, Q: consumption of protective slag, kg / m³ 2 A: Amplitude of crystallizer vibration, mm; f: Frequency of crystallizer vibration, times / min; n: Viscosity of protective slag, Pa·s; V: Target pulling speed, m / min.

5. The continuous casting method based on pre-production prediction according to claim 2, characterized in that: During continuous casting, the heat flow ratio between the wide and narrow sides of the crystallizer is controlled at 1.1 to 1.25, and the superheat of the molten steel is controlled at 15 to 30°C.

6. The continuous casting method based on pre-production prediction according to any one of claims 2-5, characterized in that: During continuous casting, the crystallizer vibration adopts a smooth sawtooth wave negative slip mode with a skew rate of 60%, a smoothness of 25%, a negative slip time controlled at 0.12 to 0.15 s, and a negative slip time rate controlled at 39 to 43%.

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