A control process for improving quality of continuous casting head-end billet

Abstract

The application discloses a control process for improving the quality of a continuous casting starting head blank, which comprises the following processes: when a continuous casting machine starts casting, the pulling speed is first increased to V1 at an acceleration a0 and kept for t1; then the pulling speed is increased to V2 at an acceleration a1 and kept for t2; finally, the pulling speed is increased to a target pulling speed at an acceleration a2; according to the target pulling speed, the blank head cutting size is calculated by using the following formula, and the blank head is cut according to the calculated blank head cutting size; L 坯头 =V1t1+V1+0.5a1+V2t2+V2×((V 目标 ‑V2) / a2)+0.5×a2((V 目标 ‑V2) / a2) 2 ; wherein: L 坯头 is the blank head cutting size, m; V 目标 is the target pulling speed, m / min. The process can accurately determine the non-steady state blank head size, effectively improve the quality of the continuous casting starting head blank, reduce the occurrence of inclusion defects after the head blank is rolled, and improve the metal yield of the continuous casting; the application significantly improves the quality of the starting head blank, reduces the occurrence rate of inclusion defects of the starting head blank from 21% of the conventional method to below 4.6%, greatly reduces the production cost, and improves the product quality.

Description

Technical Field

[0001] This invention relates to the field of continuous casting technology, and in particular to a control process for improving the quality of the first-cast billet in continuous casting. It is applicable to the continuous casting production of various steel grades, including low-carbon steel, ultra-low-carbon steel, and alloy steel, such as SPHC, DC06, and Q345R. Background Technology

[0002] The first billet in continuous casting refers to the billet produced after the initial casting stage, where the billet head is removed. During the initial casting stage, the molten steel comes into contact with air and refractory materials in the tundish, resulting in severe secondary oxidation and contamination, leading to a high oxygen content in the first billet. Simultaneously, frequent adjustments to the casting speed during this stage cause instability in the flow field within the crystallizer, resulting in large fluctuations in the liquid level and a high slag inclusion rate. This leads to downgrading and re-judgment after rolling, resulting in significant waste of production costs. Therefore, it is necessary to develop targeted processes for the initial casting stage to improve the quality of the first billet.

[0003] Chinese patent application CN117505795A discloses a method for improving the quality of non-quenched automotive steel billets. This method mainly controls the billet quality by adding a covering agent to the tundish before casting and controlling the timing and amount of mold flux. This is a common industry practice, but it does not involve speed gradient optimization or precise billet cutting, and it is only applicable to non-quenched automotive steel, resulting in poor versatility. Chinese patent application CN116984600A discloses a method for controlling the quality of continuous casting billets before casting. This method involves argon and oxygen replacement in the tundish 10 minutes before casting, starting casting after the tundish liquid level reaches half, and controlling the rate of increase in molten steel weight in the tundish to improve billet quality. The specific process control is relatively broad. Chinese patent application CN106270421A discloses a method for improving the cleanliness of aluminum-killed steel billets in the first casting. This method involves argon blowing in the tundish before casting and adding MgAl alloy to promote inclusion removal. However, the argon blowing time is too short, and the uniformity and absorption rate of the alloy addition are unstable. Chinese patent CN113664171A discloses a method for controlling the quality of continuous casting billets. It mainly controls the quality through operations such as adding refining slag-forming agents, vacuum control, baking of continuous casting tundish and argon replacement. It belongs to the cleanliness control of the whole process, but does not conduct in-depth research on the specific process of continuous casting opening and control. The effect of controlling the non-steady-state defects in the opening stage is limited. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a control process for improving the quality of continuous casting start billets, so as to effectively improve the quality of continuous casting start billets, accurately control the size of unsteady billet heads, and reduce inclusion defects.

[0005] To solve the above-mentioned technical problems, the technical solution adopted by the present invention includes the following process: when the continuous casting machine starts casting, the casting speed is first increased to V1 with acceleration a0 and held for t1; then increased to V2 with acceleration a1 and held for t2; finally increased to the target casting speed with acceleration a2; wherein a0 is 5-6 m / min. 2 V1 is 0.4–0.5 m / min, t1 is 70–90 s, and a1 is 0.4–0.5 m / min. 2 V2 is 0.8–0.85 m / min, t2 is 40–60 s, and a2 is 0.1–0.15 m / min. 2 ; Based on the target drawing speed, the blank cutting size is calculated using the following formula (1), and the blank is cut according to the calculated blank cutting size; L 坯头 =V1t1+V1+0.5a1+V2t2+V2×((V 目标 -V2) / a2)+0.5×a2×((V 目标 -V2) / a2) 2 (1); In the formula: L 坯头 V represents the blank cutting dimensions, in meters (m); 目标 The target pulling speed is measured in m / min; t1 and t2 are measured in min.

[0006] Furthermore, the superheat of the molten steel in the first tundish is controlled at 35-48℃ to avoid excessive superheating, which would lead to increased secondary oxidation of the molten steel and difficulty in the floating of inclusions. At the same time, it is also to prevent excessive superheating, which would result in poor fluidity of the molten steel and uneven growth of the billet shell in the crystallizer.

[0007] Furthermore, the tundish is baked before pouring. During the baking process, the atmosphere inside the tundish is replaced with inert gas. The tundish is baked until the inner wall temperature is ≥1200℃, and pouring is started within 10 minutes after the tundish is baked.

[0008] Furthermore, during the tundish casting process, when the weight of the molten steel in the tundish reaches 1 / 2 to 3 / 5 of the nominal capacity of the tundish, the injection of molten steel into the tundish is stopped and allowed to stand to ensure that inclusions in the molten steel float to the surface; then the tundish stopper rod is lifted and the molten steel is poured into the crystallizer through the submerged entry nozzle, and then molten steel is injected into the tundish again.

[0009] The beneficial effects of adopting the above technical solution are as follows: By controlling the continuous casting start-up process and removing the unsteady billet during acceleration, the size of the unsteady billet head can be accurately determined, effectively improving the quality of the start-up billet, reducing the occurrence of inclusion defects after the start-up billet is rolled, and increasing the yield of continuous casting metal; the present invention significantly improves the quality of the start-up billet, reducing the incidence of inclusion defects in the start-up billet from 21% of the conventional method to below 4.6%, greatly reducing production costs and improving product quality.

[0010] This invention can accurately determine the dimensions of unsteady billets, and the specific principle is as follows: 1. Gradient optimization of acceleration parameters: A three-stage acceleration path is adopted: "high acceleration start-uniform speed stabilization-medium acceleration increase-uniform speed stabilization-low acceleration to achieve target", with a0 set at 5-6 m / min. 2 This allows for a rapid initial stable casting speed V1 (0.4–0.5 m / min), preventing excessively slow start-up that could lead to prolonged slag retention and inclusion buildup in the crystallizer; t1 is controlled at 70–90 s to provide sufficient time for the molten steel to form a billet shell, preventing the risk of steel leakage due to an excessively thin shell; a1 is controlled at 0.4–0.5 m / min. 2 Slowly increase the speed to V2 (0.8–0.85 m / min) and maintain it at t2 (40–60 s) to avoid sudden changes in the crystallizer liquid level causing slag entrapment; control a2 at 0.1–0.15 m / min. 2 Slowly increase the speed to the target speed to ensure uniform billet growth and reduce defects such as slag entrapment and inclusions.

[0011] 2. Precise derivation of the billet cutting formula: Formula (1) is based on the three-stage motion law of speed increase. The unsteady billet lengths of “initial acceleration segment + uniform speed segment + secondary acceleration segment + uniform speed segment + final acceleration segment” are calculated respectively. V1t1 corresponds to the length of the initial uniform speed segment, V1+0.5a1 corresponds to the length of the initial acceleration segment, V2t2 corresponds to the length of the secondary uniform speed segment, and the latter two terms correspond to the uniform speed component and acceleration component of the final acceleration segment respectively. It accurately covers the unsteady region of the entire process from the start to the stability of the speed, ensuring that the billet cut off completely includes the oxidation, slag inclusion and variable speed unsteady part, while avoiding excessive cutting to avoid metal waste, thereby improving the metal yield.

[0012] 3. Full-process oxidation and inclusion control: Further, inert gas replacement during tundish baking, inner wall temperature control, and aging control during casting reduce the contact between molten steel and oxygen and low-temperature refractory materials from the source, reducing the risk of secondary oxidation; the superheat of molten steel in the tundish is controlled at 35-48℃, taking into account both the fluidity of molten steel and the flotation efficiency of inclusions; the molten steel residence period in the tundish provides sufficient time for oxides and refractory material inclusions in the molten steel to float, reducing the total amount of inclusions brought into the crystallizer, and fundamentally reducing inclusion defects in the billet. Attached Figure Description

[0013] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0014] Figure 1 This is a schematic diagram of the structure of the single-tube argon blowing device for the intermediate package described in this invention; Figure 2 This is the curve of the continuous casting machine's casting speed increase as described in this invention.

[0015] In the diagram: 1. Intermediate container; 2. Argon blowing single tube; 3. Support frame; 4. Argon blowing valve. Detailed Implementation

[0016] The control process for improving the quality of the first-cast billet in continuous casting includes the following steps: (1) Superheat of molten steel at the start of pouring: The superheat of molten steel leaving the station after pouring the first batch, i.e. the superheat of molten steel in the tundish, is 35-48℃.

[0017] (2) Tundish baking stage: The tundish is baked. 3-5 minutes before the end of baking, the atmosphere inside the tundish is replaced with an inert gas, preferably with argon. When the baking is finished, the inner wall temperature of the tundish is guaranteed to be ≥1200℃.

[0018] Figure 1 As shown, a single-tube argon blowing device is used to replace the atmosphere inside the tundish. The structure of the single-tube argon blowing device is as follows: it includes an argon blowing tube 2, made of high-temperature resistant metal; one end of the argon blowing tube 2 passes through the cover of the tundish 1 and extends into the interior of the tundish 1, with the outlet of the argon blowing tube 2 5-10 cm from the bottom of the tundish 1; the other end is connected to an argon gas supply device, and an argon blowing valve 4 is provided on the argon blowing tube 2; a support frame 3 is provided on the cover of the tundish 1, which can be used to fix the argon blowing tube 2; when the argon blowing tube 2 is fixed on the support frame 3, the outlet of the argon blowing tube 2 extends downwards 5-10 cm from the cover. When replacing the atmosphere inside the tundish, the argon blowing valve 4 is opened, and the outlet of the argon blowing tube 2 is 5-10 cm from the bottom of the tundish 1, so that argon gas is injected into the interior of the tundish 1, ensuring that the oxidizing atmosphere inside the tundish is completely replaced before casting.

[0019] (3) Ladle pouring stage: When opening the ladle sliding gate for pouring, ensure that the time from the end of the tundish baking to the start of pouring, i.e., the time from the end of the baking to the start of pouring, is ≤10 minutes; otherwise, reheat. Within 1 to 2 minutes after pouring, lift and fix the argon blowing pipe to the metal bracket 3 on the tundish cover, control the outlet of the argon blowing single pipe 2 to be 5 to 10 cm below the ladle cover, adjust the argon blowing valve 4, and perform soft argon blowing for the entire pouring process to ensure a low oxidation atmosphere in the tundish and prevent secondary oxidation of the molten steel.

[0020] (4) Tundish opening stage: When the weight of the molten steel in the tundish reaches 1 / 2 to 3 / 5 of the nominal capacity of the tundish, stop pouring molten steel into the tundish and stay for 1 to 2 minutes to ensure that the inclusions in the molten steel in the ladle float to the surface; then the tundish stopper rod is lifted and the molten steel is poured into the crystallizer through the submerged entry nozzle, and at the same time the ladle slide plate is reopened to pour molten steel into the tundish.

[0021] (5) Continuous casting machine start-up stage: After the continuous casting machine starts up, the casting speed is as follows: Figure 2 The acceleration curve shown first accelerates to V1 with an acceleration of a0 and holds for t1; then accelerates to V2 with an acceleration of a1 and holds for t2; finally, accelerates to the target pulling speed with an acceleration of a2; where a0 ranges from 5 to 6 m / min. 2 V1 is 0.4–0.5 m / min, t1 is 70–90 s, and a1 is 0.4–0.5 m / min. 2 V2 ranges from 0.8 to 0.85 m / min, t2 ranges from 40 to 60 s, and a2 ranges from 0.1 to 0.15 m / min. 2 .

[0022] (6) Billet cutting stage: After the continuous casting machine starts casting, the billet cutting size is calculated according to the target casting speed using the following formula (1), and the billet is cut according to the calculated billet cutting size; L 坯头 =V1t1+V1+0.5a1+V2t2+V2×((V 目标 -V2) / a2)+0.5×a2×((V 目标 -V2) / a2) 2 (1); In the formula: L 坯头 V represents the blank cutting dimensions, in meters (m); 目标 The target pulling speed is expressed in m / min; a1 and a2 are both accelerations of the pulling speed, expressed in m / min. 2 The values ​​are obtained from step (5); V1 and V2 are both pulling speeds, m / min, and the values ​​are obtained from step (5); t1 and t2 are both pulling speed holding times, min, and the values ​​are obtained from step (5).

[0023] (7) After adopting this control process, the incidence of inclusion defects in the first billet is reduced to below 4.6%.

[0024] Example 1: Production of low-carbon steel grade SPHC, crystallizer cross section 1350mm, target pulling speed 1.4m / min, the process is as follows.

[0025] (1) Superheat of molten steel at the start of pouring: 58°C for molten steel leaving the station.

[0026] (2) Tundish baking stage: 3 minutes before the end of the tundish baking, the outlet of the argon blowing tube is 5 cm away from the bottom of tundish 1, and the oxidizing atmosphere inside the tundish is replaced with argon. After the baking is completed, the temperature of the inner wall of tundish 1 is measured to be 1290℃.

[0027] (3) Ladle pouring stage: When the ladle sliding gate is opened for pouring, the time from the end of the tundish baking to the start of pouring is 8 minutes. Soft argon blowing begins 2 minutes after pouring. The outlet of the argon blowing single tube is located 5cm below the ladle cover to ensure a low oxidation atmosphere in the tundish and prevent secondary oxidation of the molten steel.

[0028] (4) Tundish opening stage: When the molten steel in the tundish reaches 3 / 5 of the nominal capacity of the tundish, close the ladle sliding gate to stop injecting molten steel into the tundish and stay for 2 minutes to ensure that the inclusions in the molten steel in the ladle float to the surface; then the tundish stopper rod is lifted and the molten steel in the tundish starts to flow into the crystallizer through the submerged entry gate, and at the same time the ladle slide plate is reopened to inject molten steel into the tundish.

[0029] (5) Continuous casting machine start-up stage: After the continuous casting machine starts up, it will first accelerate at 6m / min 2 Towards 0.4 m / min, hold for 90 seconds, then at 0.4 m / min 2 Accelerate to 0.8 m / min, maintain for 60 seconds, and finally accelerate to 0.1 m / min. 2 The acceleration began to increase to the target pulling speed of 1.4 m / min.

[0030] (6) Billet head cutting stage: After the continuous casting machine starts casting, the billet head cutting length is calculated according to the target casting speed of 1.4m / min, using formula (1) to calculate the billet head cutting size: L 坯头 =0.4×90 / 60+0.4+0.5×0.4+0.8×60 / 60+0.8×((1.4-0.8) / 0.1)+0.05×0.1×((1.4-0.8) / 0.1) 2 =6.98m. Based on the calculation results, cut a blank head with a length of 6.98m.

[0031] According to production tracking, the first billet in the continuous casting process was free of inclusions after rolling.

[0032] Example 2: Production of ultra-low carbon steel DC06, crystallizer cross section 1700mm, target pulling speed 1.2m / min, the process is as follows.

[0033] (1) Superheat of molten steel at the start of pouring: 45°C superheat of molten steel leaving the station.

[0034] (2) Tundish baking stage: 5 minutes before the end of the tundish baking, the outlet of the argon blowing tube is 10 cm away from the bottom of tundish 1, and the oxidizing atmosphere inside the tundish is replaced with argon. After the baking is completed, the temperature of the inner wall of tundish 1 is measured to be 1200℃.

[0035] (3) Ladle pouring stage: When the ladle sliding gate is opened for pouring, the time from the end of the tundish baking to the start of pouring is 10 minutes. One minute after pouring, soft argon blowing begins. The outlet of the argon blowing single tube is located 10 cm below the ladle cover to ensure a low oxidation atmosphere in the tundish and prevent secondary oxidation of the molten steel.

[0036] (4) Tundish opening stage: When the weight of molten steel in the tundish reaches 1 / 2 ton of the nominal capacity of the tundish, close the ladle sliding gate to stop injecting molten steel into the tundish and stay for 1 minute to ensure that the inclusions in the molten steel in the ladle float to the surface. Then, lift the tundish stopper rod and let the molten steel enter the crystallizer through the submerged entry nozzle. At the same time, reopen the ladle slide plate to inject molten steel into the tundish.

[0037] (5) Continuous casting machine start-up stage: After the continuous casting machine starts up, it will first accelerate at 5m / min 2 Towards 0.5 m / min, hold for 70 seconds, then at 0.5 m / min 2 Accelerate to 0.85 m / min, maintain for 40 seconds, and finally accelerate to 0.15 m / min. 2 The acceleration began to increase to the target pulling speed of 1.2 m / min.

[0038] (6) Billet head cutting stage: After the continuous casting machine starts casting, the billet head cutting length is calculated according to the target casting speed of 1.2m / min, using formula (1) to calculate the billet head cutting size: L 坯头 =0.5×70 / 60+0.5+0.5×0.5+0.85×40 / 60+0.85×((1.2-0.85) / 0.15)+0.05×0.15×((1.2-0.85) / 0.15) 2 =3.92m. Based on the calculation results, cut a blank head with a length of 3.92m.

[0039] According to production tracking, the first billet in the continuous casting process was free of inclusions after rolling.

[0040] Example 3: Production of alloy steel grade Q345R, crystallizer cross section 1900mm, target casting speed 1.0m / min, the process is as follows.

[0041] (1) Superheat of molten steel at the start of pouring: 50°C superheat of molten steel leaving the station.

[0042] (2) Tundish baking stage: 4 minutes before the end of the tundish baking, the outlet of the argon blowing tube is 6 cm away from the bottom of tundish 1, and the oxidizing atmosphere inside the tundish is replaced with argon. After the baking is completed, the temperature of the inner wall of tundish 1 is measured to be 1260℃.

[0043] (3) Ladle pouring stage: When the ladle sliding gate is opened for pouring, the time from the end of the tundish baking to the start of pouring is 9 minutes. 1.5 minutes after pouring, soft argon blowing begins. The outlet of the argon blowing single tube is located 8cm below the ladle cover to ensure a low oxidation atmosphere in the tundish and prevent secondary oxidation of the molten steel.

[0044] (4) Tundish opening stage: When the weight of the molten steel in the tundish reaches 11 / 20 tons of the nominal capacity of the tundish, close the ladle sliding gate to stop injecting molten steel into the tundish and stay for 1.5 minutes to ensure that the inclusions in the molten steel in the ladle float to the surface. Then, lift the stopper rod of the tundish and let it flow down. The molten steel in the tundish starts to enter the crystallizer through the submerged entry gate. At the same time, reopen the ladle slide plate to inject molten steel into the tundish.

[0045] (5) Continuous casting machine start-up stage: After the continuous casting machine starts up, it will first accelerate at 5.5 m / min. 2 Towards 0.45 m / min, hold for 80 seconds, then at 0.45 m / min 2 Accelerate to 0.82 m / min, maintain for 50 seconds, and finally accelerate to 0.12 m / min. 2 The acceleration begins to increase to the target pulling speed of 1.0 m / min.

[0046] (6) Billet head cutting stage: After the continuous casting machine starts casting, the billet head cutting length is calculated according to the target casting speed of 1.0m / min, using formula (1) to calculate the billet head cutting size: L 坯头 =0.45×80 / 60+0.45+0.5×0.45+0.82×50 / 60+0.82×((1.0-0.82) / 0.12)+0.05×0.12×((1.0-0.82) / 0.12) 2 =3.2m. Based on the calculation results, a 3.2m billet head was cut. Production tracking showed that the first billet in the continuous casting cycle had no inclusions or defects after rolling.

Claims

1. A control process for improving the quality of the first-cast billet in continuous casting, characterized in that, The process includes the following steps: When the continuous casting machine starts casting, the casting speed is first increased to V1 with acceleration a0 and held at t1; then increased to V2 with acceleration a1 and held at t2; finally, increased to the target casting speed with acceleration a2; where a0 is 5-6 m / min. 2 V1 is 0.4–0.5 m / min, t1 is 70–90 s, and a1 is 0.4–0.5 m / min. 2 V2 is 0.8–0.85 m / min, t2 is 40–60 s, and a2 is 0.1–0.15 m / min. 2 ; Based on the target drawing speed, the blank cutting size is calculated using the following formula (1), and the blank is cut according to the calculated blank cutting size; L 坯头 =V1t1+V1+0.5a1+V2t2+V2×((V 目标 -V2) / a2)+0.5×a2×((V 目标 -V2) / a2) 2 (1) In the formula: L 坯头 V represents the blank cutting dimensions, in meters (m); 目标 The target pulling speed is measured in m / min; t1 and t2 are measured in min.

2. The control process for improving the quality of the first-cast billet in continuous casting according to claim 1, characterized in that: The superheat of the first batch of molten steel in the tundish should be controlled at 35-48℃.

3. The control process for improving the quality of the first-cast billet in continuous casting according to claim 1, characterized in that: Before pouring, the tundish is baked. During the baking process, the atmosphere inside the tundish is replaced with inert gas. The tundish is baked until the inner wall temperature is ≥1200℃, and pouring is started within 10 minutes after the tundish is baked.

4. A control process for improving the quality of the first-cast billet in continuous casting according to claim 1, 2, or 3, characterized in that: During the initial casting process in the tundish, when the weight of the molten steel in the tundish reaches 1 / 2 to 3 / 5 of the nominal capacity of the tundish, the injection of molten steel into the tundish is stopped and allowed to stand to ensure that inclusions in the molten steel float to the surface. Then, the stopper rod of the tundish is raised and the molten steel is poured into the crystallizer through the submerged entry nozzle, and then molten steel is injected into the tundish again.

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