Medium-carbon high-Cr steel continuous casting round billet surface and internal quality cooperative control method

Abstract

The invention discloses a cooperative control method for the surface quality and the internal quality of a medium-carbon high-Cr steel continuous casting round billet. The cooperative control method is suitable for production of 26Cr3Mo steel with the specification of phi 250-350 mm. According to the method, quality collaborative improvement is achieved through molten steel component fine adjustment, continuous casting process dynamic control, online quality monitoring and precise slow cooling. In the continuous casting process, the superheat degree of a tundish is controlled in a segmented mode according to the condition that the temperature of a first furnace is 26-36 DEG C and the temperature of a continuous furnace is 16-26 DEG C and is dynamically adjusted along with the content of Cr, high-titanium anti-invasion casting powder is adopted, the crystallizer electromagnetic stirring and solidification tail end soft reduction technology is matched, the water volume of primary cooling water is 150-180 m / h, the specific water volume of secondary cooling water is 0.30-0.36 L / kg, and according to the slow cooling process, the pit entering temperature is larger than or equal to 620 DEG C, the slow cooling time is larger than or equal to 78 h, and the pit exiting temperature is smaller than or equal to 260 DEG C. According to the method, the surface crack rate of a casting blank is smaller than or equal to 0.1%, the center segregation index is smaller than or equal to 1.1, the inner folding rate of a rolled tube is smaller than or equal to 0.2%, and the percent of pass reaches 99.9%

Description

Technical Field

[0001] This invention relates to the field of surface quality control technology for medium-carbon high-Cr steel round billets, specifically to a method for coordinated control of the surface and internal quality of medium-carbon high-Cr steel continuously cast round billets. Background Technology

[0002] 26Cr3Mo steel, a typical medium-carbon high-chromium alloy structural steel, with a Cr content of 4.00%~6.00% and a Mo content of 0.45%~0.65%, has become a core material for manufacturing oil well pipes in high-pressure, high-temperature, and corrosive environments due to its excellent corrosion resistance, high-temperature strength, and wear resistance. Among them, the φ310mm round billet has broad market prospects because it is suitable for the demand for large-diameter oil well pipes; however, it faces multiple quality challenges in continuous casting production. 1. Surface quality issues: High Cr and Mo elements synergistically increase the hardenability of steel, and during the cooling process of the billet, it is easy to form a hard and brittle structure of martensite / bainite. With the superposition of thermal stress and phase transformation stress, the surface crack incidence rate is as high as 5% or more. At the same time, high Cr molten steel is easy to reduce the components of the protective slag, which leads to the deformation and failure of the protective slag, causing defects such as billet shell adhesion and vibration marks.

[0003] 2. Internal quality issues: Large-sized round billets have long solidification paths, and the segregation tendency of elements such as Cr and Mo is significant. The center segregation index often exceeds 1.3, accompanied by severe porosity defects. After being rolled into tubes, the internal folding rate reaches 8%, which seriously restricts the yield. High-Cr steel has poor thermal conductivity, well-developed columnar crystals, and its transverse toughness is only 60% of that of ordinary carbon steel.

[0004] Existing technologies mostly focus on surface quality control, such as adjusting the cooling regime or the composition of the protective slag, but they do not solve the problems of internal segregation and uneven microstructure. Moreover, the process parameters are mostly fixed values, which cannot adapt to the working conditions of Cr content fluctuations (4%~6%), resulting in poor adaptability. Therefore, developing a synergistic control method that takes into account both surface and internal quality is of great significance for improving the quality and market competitiveness of 26Cr3Mo steel round billets. Summary of the Invention

[0005] To address the problems of high surface crack rate, severe internal segregation, and poor parameter adaptability in existing processes, a method for synergistic control of surface and internal quality of medium-carbon high-Cr steel continuous casting round billets is proposed.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A method for synergistic control of the surface and internal quality of medium-carbon high-Cr steel continuously cast round billets includes several core steps such as fine-tuning of molten steel composition, dynamic control of continuous casting process parameters, and slow cooling treatment. The specific steps are as follows: 1. Optimization and adjustment of molten steel composition Based on the basic composition of 26Cr3Mo steel (C: 0.24%-0.28%, Si: 0.25%-0.50%, Mn: 0.50%-0.60%, Cr: 4.00%~6.00%, Mo: 0.45%~0.65%, P≤0.035%, S≤0.035%, Ni≤0.60%), 0.02%~0.04% Nb element is added, while the S content is controlled at ≤0.015%; the precipitation of Nb(C,N) refines the grains, reduces the risk of grain boundary weakening, and synergistically improves the toughness of the billet with high Cr.

[0007] 2. Dynamic control of continuous casting process parameters Segmented control of tundish superheat: The superheat of the tundish in the first batch is 26~36℃ to ensure smooth pouring; in subsequent batches, it is dynamically adjusted according to the Cr content. For every 1% increase in Cr content, the superheat is reduced by 2-4℃, with a control range of 16~26℃, to suppress element segregation through low-temperature solidification.

[0008] Application of high-impact protective slag: The chemical composition of the protective slag by mass percentage is CaO 26-30wt%, SiO2 24-26wt%, Al2O3 7-10wt%, MgO 5-7wt%, F 1.5-3.5wt%, TC 18-20wt%, and Cr content is increased to 20wt% when Cr content is ≥5%. The high-titanium composite preferentially reacts with [Cr] to form a stable phase, resisting the erosion of molten steel.

[0009] The casting speed and cooling regime are matched. The target casting speed is adjusted according to the different specifications of 26Cr3Mo steel. The primary cooling water volume is 150-180 m³ / h, and the secondary cooling water volume is 0.30-0.36 L / kg. For every 0.1 m / min increase in casting speed, the primary cooling water volume increases by 10 m³ / h. The secondary cooling water volume is adjusted according to the Cr content. For every 1% increase in Cr content, the volume decreases by 0.02 L / kg, so as to achieve "strong and stable billet shell in primary cooling and weak suppression of phase transformation in secondary cooling".

[0010] Crystallizer and vibration parameter settings: crystallizer taper deviation ≤0.1%, equipped with electromagnetic stirring device, stirring current 180~250A, frequency 3~5Hz; vibration parameters are A1:5, A2:0, f1:105, f2:75, As:0.22, breaking columnar crystals by electromagnetic force to reduce vibration mark depth.

[0011] Light reduction control at the end of solidification: Light reduction of 2-5 mm is applied at the end of solidification to compensate for solidification shrinkage and inhibit solute migration.

[0012] 3. Slow cooling treatment: The temperature upon entering the pit is ≥620℃, the temperature upon exiting the pit is ≤260℃, and the slow cooling time is ≥78h, so as to fully release internal stress and promote hydrogen diffusion.

[0013] Preferably, the protective slag has a viscosity of 0.70-0.80 Pa·S, an alkalinity of 1.05-1.15, a moisture content of ≤0.18%, a consumption of 1.02-1.12 kg / t, and a liquid slag layer thickness of 9-11 mm.

[0014] Preferably, the method is applicable to 26Cr3Mo steel continuous casting round billets with a diameter of φ250~350mm, wherein the slow cooling time for φ350mm billets is ≥120h.

[0015] Preferably, the light reduction is performed in the 6th to 8th stages of the continuous casting machine, corresponding to a liquid core ratio of 30% to 50% and a reduction rate of 0.5 to 1 mm / s; the reduction amount for φ250mm specification is 2 to 3 mm, the reduction amount for φ310mm specification is 3 to 4 mm, and the reduction amount for φ350mm specification is 4 to 5 mm.

[0016] Preferably, the target pulling speed for φ250mm 26Cr3Mo steel is 0.95~1.05m / min, the target pulling speed for φ310mm 26Cr3Mo steel is 0.85~0.95m / min, and the target pulling speed for φ350mm 26Cr3Mo steel is 0.75~0.85m / min.

[0017] Preferably, the method also includes an online quality monitoring step, wherein the online monitoring adopts an AI vision inspection system linked with an infrared thermal imager. When the infrared thermal imager detects a surface temperature fluctuation exceeding 5℃ / s or the AI ​​vision inspection system detects a crack ≥0.1mm, a tension speed adjustment command is triggered.

[0018] Preferably, it also includes abnormal condition handling steps: when the superheat of molten steel is >36℃, the casting speed is reduced by 0.1m / min and the consumption of protective slag is increased by 0.1kg / t; when the liquid level fluctuation is >±5mm, the speed increase is paused and the liquid level is stabilized by manual control bar; if the fluctuation continues for more than 5s, the billet is marked for inspection.

[0019] Preferably, the billet is hoisted into the slow cooling pit within 10 minutes after cutting, and a composite insulation method of heat preservation cover + end heat preservation felt is used, with the temperature upon exiting the pit ≤260℃.

[0020] The beneficial effects of this invention are as follows: the control method of this invention achieves a surface crack rate of ≤0.1% for φ250~350mm 26Cr3Mo round billets, a center segregation index of ≤1.1, an internal folding rate of ≤0.2% for rolled tubes, and a pass rate of over 99.9%. The corrosion resistance and high temperature strength are improved by 25% and 15% respectively compared with conventional processes, and it is suitable for high Cr content fluctuation conditions. Detailed Implementation

[0021] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below through examples.

[0022] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0023] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments. Example 1

[0024] This embodiment provides a method for the coordinated control of surface and internal quality of a φ250mm high-carbon Cr 26Cr3Mo steel continuous casting round billet.

[0025] The composition and mass percentage of 26Cr3Mo steel are as follows:

[0026] The method is as follows: 1. Process parameter setting confirmation: Based on the characteristics of 26Cr3Mo steel and the φ250mm cross-sectional size, the primary cooling water flow rate is set to 160m³ / h, using a weak cooling process, and the secondary cooling water specific flow rate is 0.36L / kg; the crystallizer vibration parameters are set as A1:5, A2:0, f1:105, f2:75, As:0.22 (where A1 represents stroke constant parameter 1, A2 represents stroke constant parameter 2, f1 represents frequency constant parameter 1, f2 represents frequency constant parameter 2, and As represents the non-sinusoidal factor). The chemical composition (mass percentage) of the high-impact protective slag is CaO: 26wt%, SiO2: 26wt%, Al2O3: 8wt%, MgO: 6wt%, F: 2wt%, TC: 18wt%, viscosity 0.75 Pa·S, basicity 1.12, and moisture 0.18%. The crystallizer is equipped with an electromagnetic stirring device, with a stirring current set to 180-200A and a frequency of 4-5Hz. The light reduction at the end of solidification (section 6 of the continuous casting machine) is 2-2.5mm, and the reduction rate is 0.7-0.9mm / s.

[0027] 2. Temperature control of refined molten steel: Before each batch of refined molten steel leaves the station, the temperature of the steel in the refining process is determined to be 1560-1580℃ based on the thickness of the ladle lining bricks (≥150mm) and the temperature drop during the transfer process (approximately 15-20℃), to ensure that the superheat of the tundish meets the requirements: the superheat of the tundish for the first heat is 33~36℃, and for subsequent heats it is 23~26℃.

[0028] 3. Pre-casting inspection: The steelworker in the tundish checks the fit between the dummy bar and the crystallizer to ensure there are no gaps or slippage, and that the dummy bar locking device is intact; checks that the inner wall of the copper tube of the crystallizer is free of scratches and that the taper deviation is ≤0.1%, and confirms that the immersion nozzle diameter φ50mm is free of debris.

[0029] 4. Intermediate Ladle Pretreatment Operation: After confirmation, notify the shutdown. Operators at each station should raise the ladle baking machine (baking temperature ≥1000℃), raise the slag baffle, and slowly lower the stopper rod; install the intermediate ladle argon blowing pipe, inserting it to a depth of 1 / 3 of the bottom of the intermediate ladle, and start the argon blowing operation with an argon flow rate of 0.3-0.5 m³ / h. The argon blowing time should be ≥10 min. Turn on the argon gas 4 minutes before shutdown, and start pouring directly after the ladle is cleaned.

[0030] 5. Ladle opening operation: Open the ladle sliding gate to 20% opening degree at once to receive the diverting sand. The diverting sand should cover the impact zone with a thickness of ≥50mm. After the steel flow stabilizes, control the flow to drop the ladle by 150-200mm. If the steel tipping height in the impact zone exceeds 100mm, reduce the opening degree of the ladle cylinder to 15%. After the steel tipping is reduced (steel tipping height ≤50mm), open the ladle cylinder completely (opening degree 100%).

[0031] 6. Initial pouring operation in the intermediate tank: Before pouring, the pouring speed is set to 0.42-0.47 m / min, with a target value of 0.45 m / min; a protective slag with a thickness of 5-8 mm is pre-laid in the crystallizer to ensure initial lubrication of the meniscus area.

[0032] 7. Casting start control: After the ladle starts casting, when the weight of molten steel in the tundish reaches 18t, the pressure bar is used to start casting; after the molten steel has submerged the side hole of the nozzle (about 10s), protective slag is evenly pushed into the crystallizer. The amount added is controlled at 1.08kg / t to ensure that the thickness of the liquid slag layer is stable at 10mm and to avoid secondary oxidation of the exposed molten steel.

[0033] 8. Pulling speed and liquid level adjustment: After receiving the flow start instruction from the machine operator, the intermediate batching operator starts the flow with the pressure bar, controlling the injection flow to be in a "trumpet" shape, matching the set pulling speed; within 10-15 seconds, raise the liquid level in the crystallizer to 180mm from the top, and simultaneously start the straightening machine, crystallizer vibration and electromagnetic stirring; then within the next 30 seconds, gradually raise the liquid level to 90-100mm from the top of the crystallizer, and steadily increase the pulling speed to 1.6 times the starting pulling speed (approximately 0.72m / min). At this time, the pulling speed is the pulling speed during the liquid level adjustment period. Manually control the bar to stabilize the liquid level for more than 15 seconds, and after confirming that the liquid level fluctuation is ≤±2mm, put it into the automatic liquid level control system.

[0034] 9. Stabilization process control: After the liquid level is automatically controlled, the casting speed is increased by 0.05 m / min every 2 minutes. After all the castings have been opened (about 5 minutes), the tundish cover is lowered and the casting speed is gradually adjusted to the target casting speed of 1.00~1.05 m / min. The online AI vision inspection system monitors the surface condition in real time. The infrared thermal imager shows that the surface temperature fluctuation is ≤3℃ / s. If there is no abnormality, the parameters are kept stable.

[0035] 10. Billet Cutting and Slow Cooling Treatment: The billet is flame-cut to a fixed length of 12m with a cut width ≤5mm. It is then hoisted into the slow cooling pit within 8 minutes after cutting. The temperature inside the pit is ≥630℃. A hot billet pad with a temperature ≥500℃ is pre-laid in the slow cooling pit. The billet stacking height is ≤3 layers. The pit is covered with an insulation cover and the ends are wrapped with insulation felt. The slow cooling time is ≥78h, the cooling rate is controlled at 2.8-3.3℃ / h, and the temperature outside the pit is ≤250℃.

[0036] 11. Quality Inspection and Acceptance: After the billet is removed from the pit, it is shot blasted (shot blasting strength 0.3MPa). The surface is checked for cracks and the depth of vibration marks is ≤0.25mm. Samples are taken for low magnification testing. The center segregation index is 1.06 and the proportion of equiaxed crystal regions is 40%. Mechanical property testing shows that the yield strength is ≥580MPa and the tensile strength is ≥720MPa, which meets the requirements of API 5CT P110 grade. The first-time inspection pass rate is 99.92%. Example 2

[0037] This embodiment provides a method for stabilizing the surface quality of φ310mm medium-carbon high-Cr steel 26Cr3Mo continuously cast round billets.

[0038] The composition and mass percentage of 26Cr3Mo steel are as follows:

[0039] The method is as follows: 1. Confirmation of process parameter settings: Based on the high Cr characteristics of 26Cr3Mo steel and the φ310mm cross-sectional size, the primary cooling water flow rate is set to 180m³ / h, and the secondary cooling water specific volume is set to 0.32L / kg (weak cooling enhancement); the crystallizer vibration parameters are the same as in Example 1; the chemical composition (mass percentage) of the protective slag is CaO: 30wt%, SiO2: 24wt%, Al2O3: 10wt%, MgO: 7wt%, F: 1.5wt%, TC: 20wt%, viscosity 0.80Pa·S, basicity 1.15, and moisture 0.17%; the electromagnetic stirring current is 220-230A, and the frequency is 3-4Hz; the light reduction at the end of solidification (section 7 of the continuous casting machine) is 3.0-3.5mm, and the reduction rate is 0.6-0.8mm / s.

[0040] 2. Temperature control of molten steel: The temperature of molten steel in refining is determined to be 1570-1590℃. The superheat of the tundish is 26-30℃ for the first heat and 16-20℃ for subsequent heats.

[0041] 3. Pre-cast preparation and inspection: Focus on checking the sealing of the ingot head, using refractory mud + asbestos rope for double sealing to ensure no risk of molten steel leakage; apply protective coating (graphite content ≥95%) to the inner wall of the crystallizer copper tube to reduce frictional resistance.

[0042] 4. Tundish pretreatment operation: Argon blowing flow rate 0.4-0.6 m³ / h, argon blowing time ≥12 min, tundish baking temperature ≥1100℃, to ensure stable temperature drop of molten steel.

[0043] 5. Ladle opening operation: The ladle opening degree is initially 15%. After receiving the guide sand, control the flow and drop the ladle by 200-250mm. If the steel overturning in the impact zone is severe, reduce the opening degree to 10%. After the steel flow stabilizes, restore it to 100% opening degree.

[0044] 6. Start-up operation of intermediate tank: Before starting the pouring, set the pulling speed to 0.45-0.50m / min, with a target of 0.48m / min, and lay a protective slag layer with a thickness of 6-8mm in the crystallizer.

[0045] 7. Casting start control: When the weight of molten steel in the tundish reaches 22t, the pressure bar starts casting. After the molten steel has submerged the side hole of the nozzle, protective slag is pushed in at a rate of 1.12kg / t, with a liquid slag layer thickness of 11mm, to avoid slag film rupture caused by Cr element.

[0046] 8. Pulling speed and liquid level adjustment: Raise the liquid level to 180mm from the top within 15-20 seconds. After starting the relevant equipment, adjust the liquid level to 90-110mm within 30 seconds. Increase the pulling speed to 1.6 times the starting pulling speed (about 0.77m / min). The pulling speed at this time is the pulling speed during the liquid level adjustment. Manually stabilize the liquid level for more than 12 seconds and then switch to automatic control. The liquid level fluctuation should be controlled to ≤±3mm.

[0047] 9. Stabilization process control: Increase the casting speed by 0.05 m / min every 3 minutes. After all the casting streams are stable, adjust the casting speed to the target casting speed of 0.85~0.95 m / min. Electromagnetic stirring and light pressure are started simultaneously. Online monitoring shows that the surface temperature is uniform and there is no risk of cracks or adhesion.

[0048] 10. Billet cutting and slow cooling treatment: Cut to a fixed length of 12m, and hoist it into the slow cooling pit within 6 minutes after cutting. The temperature in the pit is ≥650℃. The slow cooling pit adopts the method of "hot billet bottom pad + layered heat preservation". The slow cooling time is ≥80h, the cooling rate is ≤3.0℃ / h, and the temperature after exiting the pit is ≤240℃.

[0049] 11. Quality Inspection and Acceptance: After shot blasting, the surface crack rate is 0.05%, and the vibration mark depth is ≤0.3mm; the center segregation index is 1.08 under low magnification, and the equiaxed crystal region accounts for 42%; after being rolled into φ310mm oil well pipe, the internal fold rate is 0.15%, the CO2 corrosion resistance is improved by 25% compared with the conventional process, and the first-time inspection pass rate is 99.91%. Example 3

[0050] This embodiment provides a method for stabilizing the surface quality of φ350mm medium-carbon high-Cr steel 26Cr3Mo continuously cast round billets.

[0051] The composition and mass percentage of 26Cr3Mo steel are as follows:

[0052] The method is as follows: 1. Process parameter setting confirmation: Based on the solidification characteristics of large cross-section, the primary cooling water flow rate is set to 170 m³ / h, and the secondary cooling water specific volume is set to 0.34 L / kg; the crystallizer vibration parameters are the same as before, with a taper deviation ≤0.08%; the chemical composition (mass percentage) of the protective slag is CaO: 28 wt%, SiO2: 25 wt%, Al2O3: 7 wt%, MgO: 5 wt%, F: 3.5 wt%, TC: 19 wt%, viscosity 0.70 Pa·S, basicity 1.05, and moisture 0.16%; the electromagnetic stirring current is 240-250 A, and the frequency is 3-3.5 Hz; the light reduction at the end of solidification (section 8 of the continuous casting machine) is 4.0-4.5 mm, and the reduction rate is 0.5-0.7 mm / s.

[0053] 2. Temperature control of molten steel: The temperature of steel being refined is 1580-1600℃. The superheat of the tundish is 30-33℃ for the first furnace and 20-23℃ for subsequent furnaces. The temperature drop is reduced by keeping the tundish covered.

[0054] 3. Pre-casting preparation and inspection: The ingot derrick device adopts a double insurance of "hydraulic locking + mechanical reinforcement", and a support roller is installed at the bottom of the crystallizer to prevent the large cross-section billet from falling; check the concentricity of the submerged entry nozzle and the crystallizer, and the deviation is ≤2mm.

[0055] 4. Tundish pretreatment operation: Argon blowing flow rate 0.5-0.7 m³ / h, argon blowing time ≥15 min, and alkaline lining bricks are used in the tundish to reduce nitrogen absorption by molten steel.

[0056] 5. Ladle opening operation: Initial opening degree is 10%. After receiving the diverting sand, control the flow to fall into the ladle by 250-300mm. Set up a slag-blocking weir in the impact zone to reduce the risk of slag entrapment. After the steel is turned over and stabilized, adjust the opening degree to 100%.

[0057] 6. Start-up operation of intermediate tank: Before starting the pouring, set the pulling speed to 0.40-0.45m / min, with a target of 0.42m / min, and the bottom slag thickness of the crystallizer to 8-10mm.

[0058] 7. Casting start control: When the weight of molten steel in the tundish reaches 25t, the pressure bar is used to start casting. The amount of protective slag added is 1.02kg / t, the thickness of the liquid slag layer is 9mm, and a dedicated person is assigned to monitor the slag layer status in real time and replenish the protective slag every 2 minutes.

[0059] 8. Pulling speed and liquid level adjustment: Raise the liquid level to 180mm from the top within 20-25 seconds. After starting the equipment, adjust the liquid level to 90-110mm within 40 seconds. Increase the pulling speed to 1.5 times the starting pulling speed (about 0.63m / min). Manually stabilize the liquid level for more than 20 seconds and then switch to automatic control. The liquid level fluctuation should be ≤±4mm.

[0060] 9. Stable process control: Increase the pulling speed by 0.05 m / min every 5 minutes, and finally stabilize the pulling speed at the target pulling speed of 0.75~0.85 m / min; the secondary cooling zone adopts the "segmented controlled cooling" mode to avoid excessive temperature difference between the inside and outside of the large cross section, and the online infrared monitoring shows that the temperature difference between the center and the surface is ≤450℃.

[0061] 10. Billet cutting and slow cooling treatment: Cut to a fixed length of 10m (to reduce deformation due to self-weight), and hoist into the slow cooling pit within 5 minutes after cutting. The temperature in the pit is ≥620℃. The stacking height of the billet is ≤2 layers. The slow cooling time is ≥120h. The cooling rate is ≤2.5℃ / h. The temperature after exiting the pit is ≤220℃.

[0062] 11. Quality Inspection and Acceptance: No surface cracks, vibration mark depth ≤ 0.35mm; low magnification test center segregation index 1.05, porosity grade 1; internal folding rate after rolling 0.1%, high temperature strength (500℃) ≥ 650MPa, meeting the requirements for high pressure oil well pipe use, and the first-time inspection pass rate is 99.90%.

[0063] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for coordinated control of surface and internal quality of medium-carbon high-Cr steel continuously cast round billets, characterized in that, The process includes steps such as fine-tuning of molten steel composition, dynamic control of continuous casting process, and slow cooling treatment. Specifically, the medium-carbon high-Cr steel is 26Cr3Mo steel, with 0.02%~0.04% Nb added after composition adjustment, and S content ≤0.015%; the superheating temperature of the tundish in the first heat is 26~36℃, and in subsequent heats it is 16~26℃, and the superheating temperature decreases by 2-4℃ for every 1% increase in Cr content; a high-impact protective slag is applied: the chemical composition of the protective slag by mass percentage is: CaO 26-30wt%, SiO2 24-26wt%, Al2O3 7-10wt%, MgO 5-7wt%, F 1.5-3.5wt%, TC 18-20wt%, when Cr content ≥5%, TC content is increased to 20wt%; the target casting speed is adjusted according to different specifications of 26Cr3Mo steel, the primary cooling water flow rate is 150-180m³ / h, the secondary cooling water flow rate is 0.30-0.36L / kg, for every 0.1m / min increase in casting speed, the primary cooling water flow rate increases by 10m³ / h, the secondary cooling water flow rate is adjusted according to Cr content, for every 1% increase in Cr content, it decreases by 0.02L / kg; the crystallizer taper deviation is ≤0.1%, the crystallizer is equipped with electromagnetic stirring, current 180~250A, frequency 3~5Hz, vibration parameters are A1:5, A2:0, f1:105, f2:75, As:0.22; light reduction is implemented at the end of solidification, reduction amount is 2~5mm; the slow cooling process is: pit entry temperature ≥620℃, pit exit temperature ≤260℃, slow cooling time ≥78h.

2. The method for coordinated control of surface and internal quality of medium-carbon high-Cr steel continuously cast round billets according to claim 1, characterized in that, The protective slag has a viscosity of 0.70-0.80 Pa·S, an alkalinity of 1.05-1.15, a moisture content of ≤0.18%, a consumption of 1.02-1.12 kg / t, and a liquid slag layer thickness of 9-11 mm.

3. The method for coordinated control of surface and internal quality of medium-carbon high-Cr steel continuously cast round billets according to claim 1, characterized in that, The method is applicable to 26Cr3Mo steel continuous casting round billets with a diameter of φ250~350mm, wherein the slow cooling time for φ350mm billets is ≥120h.

4. The method for coordinated control of surface and internal quality of medium-carbon high-Cr steel continuously cast round billets according to claim 1, characterized in that, The online monitoring system uses an AI visual inspection system linked with an infrared thermal imager. When the surface temperature fluctuates by more than 5°C / s or a crack of ≥0.1mm appears, a speed adjustment command is triggered.

5. The method for coordinated control of surface and internal quality of medium-carbon high-Cr steel continuously cast round billets according to claim 3, characterized in that, The light reduction is performed in stages 6 to 8 of the continuous casting machine, corresponding to a liquid core ratio of 30% to 50% and a reduction rate of 0.5 to 1 mm / s; the reduction amount for φ250mm specifications is 2 to 3 mm, for φ310mm specifications it is 3 to 4 mm, and for φ350mm specifications it is 4 to 5 mm.

6. The method for coordinated control of surface and internal quality of medium-carbon high-Cr steel continuously cast round billets according to claim 3, characterized in that, The pulling speed control is as follows: the target pulling speed for φ250mm 26Cr3Mo steel is 0.95~1.05m / min, the target pulling speed for φ310mm 26Cr3Mo steel is 0.85~0.95m / min, and the target pulling speed for φ350mm 26Cr3Mo steel is 0.75~0.85m / min.

7. The method for coordinated control of surface and internal quality of medium-carbon high-Cr steel continuously cast round billets according to claim 1, characterized in that, It also includes abnormal condition handling procedures. When the superheat of molten steel is >36℃, the casting speed is reduced by 0.1m / min and the consumption of protective slag is increased by 0.1kg / t. When the liquid level fluctuation is >±5mm, the speed increase is suspended and the liquid level is stabilized by manual control bar. If the fluctuation continues for more than 5s, the billet is marked for inspection.

8. The method for coordinated control of surface and internal quality of medium-carbon high-Cr steel continuously cast round billets according to claim 1, characterized in that, Within 10 minutes of cutting the billet, it is hoisted into the slow cooling pit and insulated with a composite insulation of heat-insulating cover and end heat-insulating felt, with a cooling rate of ≤3.3℃ / h.

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