Continuous casting billet for sulfur-resistant pipe and preparation method of continuous casting billet

Abstract

The invention relates to a continuous casting billet for a sulfur-resistant pipe and a preparation method thereof, and belongs to the technical field of preparation of continuous casting billets. In order to solve the problems that an existing continuous casting billet for the sulfur-resistant pipe is obvious in banded structure, the surface is sunken and cracked, the internal density is poor, and the control rate of aluminum element entering a furnace is low, the preparation method of the continuous casting billet for the sulfur-resistant pipe comprises the steps of converter primary smelting, refining treatment, vacuum treatment and continuous casting forming. Through converter, refining and vacuum whole-process cooperative regulation and control and matching with special refining slag, accurate control over molten steel components is achieved, component segregation and inclusion aggregation are eradicated, the banded structure of a formed blank pipe is smaller than or equal to 2.0 level, and the performance requirement of the sulfur-resistant pipe is met. By combining continuous casting process closed-loop regulation and control, the problem of peritectic steel surface depression is solved, the casting blank surface quality and the yield are greatly improved, the internal quality qualification rate of the casting blank reaches 100%, structural guarantee is provided for corrosion resistance of a sulfur-resistant pipe, and the method is suitable for severe working conditions of a high-acidity oil and gas field.

Description

Technical Field

[0001] This invention belongs to the field of continuous casting billet preparation technology, and particularly relates to a sulfur-resistant pipe continuous casting billet and its preparation method. Background Technology

[0002] Sulfur-resistant pipes are special industrial pipelines adapted to highly corrosive media such as hydrogen sulfide. They are primarily used in oil and gas extraction, chemical production, and other fields, serving as key components in acidic oil and gas field development and media transportation systems. Their core value lies in resisting corrosion failure modes induced by hydrogen sulfide, such as stress corrosion cracking, hydrogen-induced cracking, and sulfide stress corrosion cracking, ensuring the long-term safe and stable operation of pipeline systems under extremely harsh conditions.

[0003] Currently, sulfur-resistant pipes are mostly made of low-carbon and ultra-low-carbon low-alloy steel. The corrosion resistance is optimized by adding alloying elements such as chromium, molybdenum and nickel, and the content of impurities such as carbon, sulfur and phosphorus is strictly controlled to reduce corrosion sensitivity. However, the existing smelting, continuous casting and forming processes have many inherent shortcomings, and the performance of the pipes is difficult to meet the stringent requirements of high acidity oil and gas fields, and the core technical problems are prominent.

[0004] Under conventional production processes, obvious banded structures are prone to appear inside the tube blank. Affected by component segregation and uneven distribution of inclusions, ferrite and pearlite are distributed in layers along the rolling direction, which destroys the uniformity of the matrix, widens the difference in longitudinal and transverse mechanical properties of the tube, and becomes a channel for hydrogen atom aggregation and crack initiation and propagation, exacerbating hydrogen-induced cracking and sulfide stress corrosion sensitivity. Moreover, these hereditary structural defects are difficult to completely eliminate through subsequent heat treatment.

[0005] Sulfur-resistant billet steel is mostly peritectic steel. During continuous casting, the peritectic reaction easily leads to uneven solidification and shrinkage of the initial billet shell and heat transfer imbalance in the crystallizer, directly causing defects such as surface depressions and longitudinal cracks in the continuously cast billet. These defects will be further amplified during subsequent rolling, forming surface flaws in the finished pipe and creating stress concentration, which becomes a breakthrough point for corrosion failure. At the same time, the conventional continuous casting process parameters are poorly controlled, making it difficult to manage defects such as porosity, shrinkage cavities, and center segregation inside the continuously cast billet. The insufficient density of the microstructure reduces the overall strength and toughness of the pipe and provides a pathway for corrosive media to penetrate.

[0006] In addition, aluminum is an easily oxidized element during the smelting process, and it is very easy to combine with oxygen to form alumina inclusions. The effective aluminum element control rate in the furnace is low, which not only fails to give full play to its role in refining grains and purifying grain boundaries, but also produces hard inclusions that damage the continuity of the matrix, further affecting the corrosion resistance and service life of the sulfur-resistant pipe. Summary of the Invention

[0007] To address the problems of obvious banded structure, surface depressions and cracks, poor internal density, and low aluminum element control rate in existing continuous casting billets for sulfur-resistant pipes, this invention provides a continuous casting billet for sulfur-resistant pipes and its preparation method.

[0008] The technical solution of the present invention:

[0009] A method for preparing a continuous casting billet for sulfur-resistant pipes includes the following steps:

[0010] Step 1: Initial refining in the converter:

[0011] Molten steel and clean scrap steel are selected as raw materials. The molten iron is controlled with As ≤ 0.015%, S ≤ 0.040%, and P ≤ 0.130%, and the scrap steel is free of harmful elements exceeding the standard. Oxygen top and bottom blowing converter is used for smelting. The steel has a carbon content of 0.04~0.06%, a tapping temperature of not less than 1600℃, and a P content ≤ 0.008%. Aluminum ingots, alloys, lime, and refining slag are added in stages during the tapping process to ensure that the Al content of the molten steel is stable at 0.040~0.050wt% upon arrival. Slag addition is strictly prohibited.

[0012] Step 2, Refining Process:

[0013] The molten steel is heated to 1580℃, and after temperature measurement and sampling, it undergoes two-stage electric refining. The total refining time is not less than 60 minutes, the white slag holding time is not less than 30 minutes, calcium wire is fed in during the later stage of refining, and then vacuum treatment is carried out.

[0014] Step 3: Vacuum treatment:

[0015] Control the deep vacuum degree to ≤67Pa, maintain the vacuum time for no less than 15min, and the soft blowing time after breaking the vacuum should be no less than 15min;

[0016] Step 4: Continuous casting:

[0017] The casting speed was matched to the superheat of the molten steel in the tundish, with the superheat stabilized at 25±3℃, the casting speed at 1.2m / min, and the crystallizer water flow rate at 110m³ / min. 3 The continuous casting billet for sulfur-resistant pipes was prepared by using electromagnetic stirring at 250A / 3Hz in the crystallizer and 50A / 8Hz at the end of solidification.

[0018] Furthermore, the internal control parameters for molten iron in step one are S≤0.030%, As≤0.014%, and Cr≤0.20%.

[0019] Furthermore, in step one, the converter tapping temperature is not lower than 1610℃, the tapping volume is 100 tons, 160 kg of aluminum ingots are added when the tapping volume reaches 20 tons, 2.6 kg / t of ferrosilicon and 11.5 kg / t of low-carbon ferromanganese are added when the tapping volume reaches 30 tons, and 600 kg of lime and 400 kg of deoxidation and desulfurization refining slag are added when the tapping volume reaches 50 tons.

[0020] Furthermore, in step two, 300-400 kg of lime, 0-100 kg of fluorite, 40-60 kg of aluminum granules, 20-30 kg of ferrosilicon powder, and 10-20 kg of silicon carbide are added during a single power-on process.

[0021] Furthermore, in step two, 100-200 kg of lime and 0-50 kg of fluorite are added during the second power-on process, along with 10-20 kg of aluminum granules and 30-40 kg of silicon carbide or ferrosilicon powder; the length of the calcium feed line is 40-80 m.

[0022] Furthermore, in step four, a six-strand continuous casting machine is used to produce φ210mm round billets.

[0023] Furthermore, in step four, double argon sealing is used to protect the pouring process, and the intermediate package is kept full of liquid level during package change.

[0024] Furthermore, in step four, the number of firing cycles used for the copper tubes in the crystallizer must be strictly ≤100.

[0025] A continuously cast slab for sulfur-resistant pipe prepared by any one of the preparation methods according to claims 1-8, comprising, by weight percentage: C 0.10~0.13%, Si 0.20~0.35%, Mn 0.95~1.10%, P≤0.015%, S≤0.003%, Alt 0.015~0.040%, Cr≤0.30%, N≤0.0120%, O≤0.0010%, Ni≤0.30%, Cu≤0.20%, Mo≤0.10%, As≤0.0150%, Sn≤0.0150%, Pb≤0.0030%, Bi≤0.0030%, Sb≤0.0100%, Ti≤0.04%, V≤0.05%, Ca≤0.006%, B≤0.0005%, with the remainder being Fe and unavoidable impurities; among which, Alt / N is not less than 2:1, V+Nb≤0.06%, and V+Nb+Ti≤0.15%.

[0026] The beneficial effects of this invention are:

[0027] This invention achieves precise control of steel composition through a comprehensive process of strict impurity control at the source of converter primary refining, step-by-step deoxidation and alloying and deep desulfurization in LF refining, and deep vacuum purification of molten steel. Combined with specialized low-carbon peritectic steel pre-melting deoxidation and desulfurization refining slag, it achieves 100% composition control throughout the process, with the control rate of easily oxidized alumina elements consistently above 98%. This fundamentally eliminates compositional segregation and abnormal aggregation of inclusions. The final billet tube's banded microstructure rating is strictly controlled below grade 2.0, and all mechanical and corrosion resistance test results meet standard requirements. The overall stability and consistency of the material are significantly improved, fully meeting the core indicators of the special technical specifications for sulfur-resistant pipes.

[0028] Relying on the guaranteed purity of molten steel in the early stage, and combined with the closed-loop control of the continuous casting process for peritectic steel characteristics, such as optimizing the crystallizer water volume, strictly controlling the use of copper tubes, and precisely adapting special protective slag, along with the pre-process support of slag-free steel tapping in the converter, long-term maintenance of refined white slag in LF, and precise feeding of calcium wire, the problems of uneven solidification shrinkage of billet shell and heat transfer imbalance in the crystallizer caused by peritectic reaction are effectively alleviated. The steel fluidity, cooling uniformity, and billet shell lubricity are optimized simultaneously from multiple dimensions, completely solving the problem of surface depression of peritectic steel continuous casting billet. Throughout the production process, the billet appearance is regular, the surface is uniform and smooth, and there are no obvious depressions or cracks. The surface quality is fundamentally improved, and the yield is significantly increased.

[0029] Based on the achievement of compositional standards and optimization of surface quality, the process employs micro-stirring for oxidation prevention in the later stages of LF refining, deep vacuum pressure holding for degassing, and soft blowing to promote the full flotation of inclusions. Combined with the precise control of the linkage between continuous casting superheat and casting speed, and the coupling effect of dual electromagnetic stirring in the crystallizer and solidification end, the abnormal dendrite growth during the solidification process of the billet is effectively suppressed, internal component segregation is dispersed, and matrix grains are refined. The final continuous casting billet has excellent internal microstructure and properties, with the central porosity controlled within the range of ≤1, the shrinkage porosity strictly controlled within the level of ≤0.5, and the segregation index stably maintained within the ideal range of 0.95 to 1.05. The internal quality qualification rate reaches 100%, providing a solid microstructure guarantee for the subsequent anti-sulfur pipe to resist hydrogen-induced cracking and sulfide stress corrosion cracking, and fully adapting to the stringent service requirements of high acidity oil and gas field conditions. Attached Figure Description

[0030] Figure 1 This is a photograph of the continuous casting billet for sulfur-resistant pipes prepared in Example 2;

[0031] Figure 2 A physical image of the sulfur-resistant pipe continuous casting billet prepared for Comparative Example 1. Detailed Implementation

[0032] The technical solution of the present invention will be further described below with reference to embodiments, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention. In the following embodiments, the process equipment or apparatus not specifically specified are all conventional equipment or apparatus in the art. Unless otherwise specified, the raw materials used in the embodiments of the present invention are all commercially available; unless otherwise specified, the technical means used in the embodiments of the present invention are all conventional means well known to those skilled in the art.

[0033] Example 1

[0034] This embodiment provides a method for preparing sulfur-resistant pipe continuous casting billets, the process route being converter → refining → vacuum → six-strand continuous casting machine.

[0035] The chemical composition of the continuously cast billet for sulfur-resistant pipes in this embodiment, by weight percentage, includes: C 0.10~0.13%, Si 0.20~0.35%, Mn 0.95~1.10%, P≤0.015%, S≤0.003%, Alt 0.015~0.040%, Cr≤0.30%, N≤0.0120%, O≤0.0010%, Ni≤0.30%, Cu≤0.20%, Mo≤0.10%, As≤0.0150%, Sn≤0.0150%, Pb≤0.0030%, Bi≤0.0030%, Sb≤0.0100%, Ti≤0.04%, V≤0.05%, Ca≤0.006%, B≤0.0005%, with the remainder being Fe and unavoidable impurities; among which, Alt / N is not less than 2:1, V+Nb≤0.06%, and V+Nb+Ti≤0.15%.

[0036] The preparation method of the continuous casting billet for sulfur-resistant pipe in this embodiment includes the following steps:

[0037] Step 1: Initial refining in the converter:

[0038] Raw material control strictly adheres to composition standards: molten iron must meet the following requirements: As≤0.015%, S≤0.040%, P≤0.130%, with optimized internal control indicators of S≤0.030%, As≤0.014%, and Cr≤0.20%. This rigorous control of harmful impurities and residual elements from the source prevents subsequent impacts on the purity of molten steel and the corrosion resistance of the pipes. Scrap steel is strictly prohibited from being mixed with materials exceeding the standards for harmful elements. High-quality, clean scrap steel is selected and rationally combined with different types to precisely adjust the content of carbon, silicon, and manganese basic elements in the molten steel, ensuring initial compositional balance and laying the foundation for subsequent refining.

[0039] The oxygen top-and-bottom blowing converter is used for smelting. By controlling the oxygen flow rate, lance position and slag formation system, carbon, silicon and manganese oxidation and preliminary removal of phosphorus and sulfur are completed. The core parameters for steel tapping are: C content 0.04%~0.06%, tapping temperature ≥1600℃, internal control preferred ≥1610℃, P content ≤0.008%, and a fixed tapping volume of 100t. Slag feeding is strictly prohibited throughout the process. If slag feeding occurs, it must be dumped immediately to prevent harmful elements in the slag from contaminating the molten steel.

[0040] The alloy is added in stages following the deoxidation alloying logic: when the steel reaches 20 tons, 160 kg of aluminum ingot is added to implement high aluminum control and ensure that the Al content of the molten steel is stable at 0.040%~0.050%, achieving strong deoxidation in the early stage; when the steel reaches 30 tons, 2.6 kg / t ferrosilicon and 11.5 kg / t low-carbon ferromanganese are added to ensure that the Si element is added to the target range at one time, avoiding abnormal increase in calcium content due to later addition; when the steel reaches 50 tons, 600 kg of lime and 400 kg of deoxidation and desulfurization refining slag are added to quickly form slag to cover the molten steel, isolate it from the air and prevent secondary oxidation.

[0041] Step 2, Refining Process:

[0042] After the molten steel arrives, the temperature is raised to 1580℃. After temperature measurement and sampling, refining is carried out. During the first power-on stage, 300-400 kg of lime, 0-100 kg of composite slag, 40-60 kg of aluminum granules, 20-30 kg of ferrosilicon powder, and 10-20 kg of silicon carbide are added to simultaneously create reducing slag for in-depth deoxidation and desulfurization. It is strictly forbidden to add lime at low temperature to prevent the slag from becoming less fluid and the reaction from being delayed. In the early stage, high-power stirring can be used to assist desulfurization and improve the efficiency of sulfur removal to ensure that the final S is ≤0.002% and achieve desulfurization in one step.

[0043] During the secondary power-on stage, the amount of lime used is reduced to 100-200 kg, and 0-50 kg of fluorite (composite slag) can be added to adjust the slag fluidity. If the slag condition is not good, about 100 kg of fluorite can be added to optimize the slag phase. Based on the composition test results, 10-20 kg of aluminum particles and 30-40 kg of silicon carbide or ferrosilicon powder are added to fine-tune the composition to the internal control range. The deoxidation rhythm is controlled throughout the process to avoid excessive oxidation of molten steel.

[0044] During the later stages of refining, add 20-30 kg of silicon carbide or ferrosilicon powder to raise the temperature of the molten steel to the appropriate temperature for continuous casting. Simultaneously, precisely adjust the composition of each element to the target range. In the later stages, it is strictly forbidden to stir with a large flow of argon gas. Only maintain a micro-stirring state to prevent slag from being rolled into the molten steel and secondary oxidation, and ensure the cleanliness of the molten steel. The oxygen content should be controlled at ≤10 ppm and the nitrogen content at ≤100 ppm.

[0045] The total refining time is ≥60min, and the white slag holding time is ≥30min. The white slag atmosphere can enhance the deoxidation and desulfurization effect and inhibit the formation of inclusions. After refining, 40~80m of calcium wire is fed into the molten steel. Calcium treatment of variable alumina inclusions improves the fluidity and castability of the molten steel, ensures continuous and stable casting, and avoids nozzle blockage.

[0046] Step 3: Vacuum treatment:

[0047] Deep vacuum refining is performed with a vacuum degree ≤67Pa and a vacuum holding time ≥15min to deeply remove harmful gases such as hydrogen and nitrogen from the molten steel, reduce the risk of hydrogen embrittlement, and meet the requirements of anti-sulfur pipes to resist hydrogen-induced cracking. After breaking the vacuum, the soft blowing time is ≥15min. During soft blowing, the slag surface floats slightly and is strictly prohibited from turning over, so as to promote the full floating and removal of fine inclusions, further purifying the steel. After soft blowing is completed, it is transferred to the continuous casting process.

[0048] Step 4: Continuous casting:

[0049] The six-strand continuous casting machine is used to produce φ210mm round billets. The entire continuous casting process is protected by double argon seals. When changing ladles, the tundish is kept full to completely prevent air from entering and causing secondary oxidation and slag entrapment problems in the molten steel, thus ensuring the cleanliness of the molten steel throughout the process.

[0050] The superheat of the molten steel in the tundish is linked and matched with the casting speed, and the superheat is strictly controlled to be stable within the optimal range of 25℃±3℃; the casting speed is 1.20m / min, stabilizing the casting speed and heat transfer rhythm to suppress component segregation; the water flow rate in the crystallizer is fixed at 110m³. 3 / h, adapted to the characteristics of peritectic steel to reduce the intensity of primary cooling, combined with the crystallizer electromagnetic stirring (M-EMS) 250A / 3Hz and the solidification end electromagnetic stirring (F-EMS) 50A / 8Hz, the two-stage stirring refines the grains and disperses dendrites, solves the problems of segregation and porosity in the center of the continuous casting billet, and ensures the uniformity of the internal structure.

[0051] The copper tubes used in the crystallizer are strictly tested for ≤100 heats to ensure the dimensional accuracy and cooling uniformity of the crystallizer cavity, and to avoid uneven cooling due to copper tube wear. Given that peritectic steel is prone to peritectic reactions and surface depressions, a special protective slag for peritectic steel is selected to enhance lubrication and reduce adhesion between the billet and the copper tubes. The billet surface is monitored in real time during production; if depressions are found, they are immediately and meticulously repaired and micro-cracks are investigated to completely eliminate potential surface quality issues. The uniformity of the secondary cooling water spray is simultaneously controlled to prevent uneven cooling in certain areas from causing billet deformation and excessive segregation.

[0052] Example 2

[0053] This embodiment provides a method for preparing sulfur-resistant pipe continuous casting billets, the process route being converter → refining → vacuum → six-strand continuous casting machine.

[0054] The chemical composition of the continuously cast billet for sulfur-resistant pipes in this embodiment, by weight percentage, includes: C 0.11%, Si 0.28%, Mn 1.02%, P 0.012%, S 0.002%, Alt 0.030%, Cr 0.15%, N 0.0085%, O 0.0008%, Ni 0.12%, Cu 0.10%, Mo 0.05%, As 0.0120%, Sn 0.0100%, Pb 0.0015%, Bi 0.0015%, Sb 0.0060%, Ti 0.02%, V 0.03%, Ca 0.0035%, B 0.0003%; Alt / N = 3.5:1, V+Nb = 0.03%, V+Nb+Ti = 0.05%.

[0055] The preparation method of the continuous casting billet for sulfur-resistant pipe in this embodiment includes the following steps:

[0056] Step 1: Initial refining in the converter:

[0057] Raw material control: The composition of molten iron is controlled as As 0.012%, S 0.025%, P 0.110%, and Cr 0.15%; high-quality, clean, low-carbon scrap steel is selected, free of harmful elements exceeding the standard, and the basic composition is adjusted according to the ratio.

[0058] Converter smelting: oxygen top and bottom blowing converter, steel tapping C content is 0.05%, tapping temperature is 1620℃, steel tapping P content is 0.006%, steel tapping volume is fixed at 100t, slag feeding is strictly prohibited throughout the process, and if slag feeding occurs, the slag must be dumped immediately.

[0059] Alloy addition: When the steel reaches 20 tons, add 160 kg of aluminum ingot; when the steel reaches 30 tons, add 260 kg of ferrosilicon and 1150 kg of low-carbon ferromanganese; when the steel reaches 50 tons, add 600 kg of lime and 400 kg of pre-melted deoxidation and desulfurization refining slag. The main components of the pre-melted deoxidation and desulfurization refining slag include 1.00 wt% SiO2, 42.17 wt% Al2O3, 47.41 wt% CaO and 3.73 wt% MgO.

[0060] Step 2, Refining Process:

[0061] After the molten steel is in place, the temperature is raised to 1580℃. Then, 350kg of lime, 50kg of composite slag, 50kg of aluminum granules, 25kg of ferrosilicon powder, and 15kg of silicon carbide are added at one time. High-power stirring is used in the early stage to assist desulfurization and ensure that the final sulfur content is 0.002%.

[0062] During the second power-on process, add 150 kg of lime and 30 kg of fluorite, and supplement with 15 kg of aluminum granules and 35 kg of ferrosilicon powder. In the later stage of refining, add 25 kg of ferrosilicon powder. Stir with micro-argon gas throughout the process, and strictly prohibit large-flow stirring.

[0063] The total refining time is 65 minutes, and the white residue retention time is 35 minutes. After refining, 60m of calcium line is fed in, with oxygen content controlled at 8ppm and nitrogen content controlled at 75ppm.

[0064] Step 3: Vacuum treatment:

[0065] The deep vacuum degree is 50Pa, and the vacuum holding time is 20min; after breaking the vacuum, the soft blowing time is 20min. During the soft blowing, the slag surface floats slightly and there is no churning phenomenon. After the soft blowing is completed, it is transferred to continuous casting.

[0066] Step 4: Continuous casting:

[0067] The six-strand continuous casting machine produces φ210mm round billets, with full-process double argon sealing protection during casting. The tundish is kept full during ladle changes; the tundish superheat is 25℃, and the casting speed is fixed at 1.20m / min; the crystallizer water flow rate is 110m³. 3 / h, the electromagnetic stirring of the crystallizer is 250A / 3Hz, and the electromagnetic stirring at the solidification end is 50A / 8Hz.

[0068] In this embodiment, the copper tube of the crystallizer uses ≤80 heat treatments and selects BPJ-21 type special crystallizer protective slag for peritectic steel round billets. The main components of the crystallizer protective slag include 40% SiO2, 39% Al2O3, 35% CaO, and 4% R2O. The melting rate is 48~52s at a temperature of 1350℃, and the uniformity of the secondary cooling water is strictly controlled.

[0069] Example 3

[0070] This embodiment provides a method for preparing sulfur-resistant pipe continuous casting billets, the process route being converter → refining → vacuum → six-strand continuous casting machine.

[0071] The chemical composition of the continuously cast billet for sulfur-resistant pipes in this embodiment, by weight percentage, includes: C 0.10%, Si 0.22%, Mn 0.98%, P 0.010%, S 0.002%, Alt 0.025%, Cr 0.18%, N 0.0090%, O 0.0007%, Ni 0.15%, Cu 0.12%, Mo 0.06%, As 0.0110%, Sn 0.0090%, Pb 0.0010%, Bi 0.0010%, Sb 0.0050%, Ti 0.01%, V 0.02%, Ca 0.0030%, B 0.0002%; Alt / N = 2.8:1, V+Nb = 0.02%, V+Nb+Ti = 0.03%.

[0072] The preparation method of the continuous casting billet for sulfur-resistant pipe in this embodiment includes the following steps:

[0073] Step 1: Initial refining in the converter:

[0074] Raw material control: The composition of molten iron is controlled as As 0.011%, S 0.022%, P 0.105%, and Cr 0.18%; the scrap steel is clean and free of impurities, and is mixed in proportion.

[0075] Converter smelting: The carbon content of the tapped steel is 0.04%, the tapping temperature is 1610℃, the phosphorus content of the tapped steel is 0.005%, the tapping volume is 100t, and slag discharge is strictly prohibited.

[0076] Alloy additions: 160 kg of aluminum ingots for every 20 tons of steel produced; 260 kg of ferrosilicon and 1150 kg of low-carbon ferromanganese for every 30 tons of steel produced; 600 kg of lime and 400 kg of pre-melted deoxidizing and desulfurizing refining slag for every 50 tons of steel produced. The main components of the pre-melted deoxidizing and desulfurizing refining slag include 1.00 wt% SiO2, 42.17 wt% Al2O3, 47.41 wt% CaO, and 3.73 wt% MgO.

[0077] Step 2: Refining Process

[0078] When the molten steel is heated to 1580℃, 300kg of lime, 40kg of aluminum granules, 20kg of ferrosilicon powder and 10kg of silicon carbide are added during the first power-on process; 100kg of lime and 20kg of fluorite are added during the second power-on process, and 10kg of aluminum granules and 30kg of silicon carbide are added as supplementary materials.

[0079] 20 kg of silicon carbide was added during the later stage of refining, and the mixture was stirred with a small amount of argon. The total refining time was 60 min, and the white residue retention time was 30 min. 40 m of calcium wire was fed in, with an oxygen content of 7 ppm and a nitrogen content of 70 ppm.

[0080] Step 3: Vacuum treatment

[0081] The vacuum degree is 60 Pa, and the vacuum holding time is 15 min; after breaking the vacuum, the soft blowing time is 15 min, the slag surface is free of blemishes, and the molten steel is continuously cast after purification.

[0082] Step 4: Continuous casting

[0083] Six-strand, six-stream φ210mm round billet casting, double argon-sealed casting, full ladle change; tundish superheat 22℃, casting speed 1.20m / min; crystallizer water flow 110m³. 3 / h, the electromagnetic stirring of the crystallizer is 250A / 3Hz, and the electromagnetic stirring at the solidification end is 50A / 8Hz.

[0084] In this embodiment, the copper tube of the crystallizer uses ≤80 heat treatments and selects BPJ-21 type special crystallizer protective slag for peritectic steel round billets. The main components of the crystallizer protective slag include 40% SiO2, 39% Al2O3, 35% CaO, and 4% R2O. The melting rate is 48~52s at a temperature of 1350℃, and the uniformity of the secondary cooling water is strictly controlled.

[0085] Example 4

[0086] This embodiment provides a method for preparing sulfur-resistant pipe continuous casting billets, the process route being converter → refining → vacuum → six-strand continuous casting machine.

[0087] The chemical composition of the continuously cast billet for sulfur-resistant pipes in this embodiment, by weight percentage, includes: C 0.13%, Si 0.34%, Mn 1.08%, P 0.014%, S 0.002%, Alt 0.038%, Cr 0.22%, N 0.0080%, O 0.0009%, Ni 0.18%, Cu 0.15%, Mo 0.08%, As 0.0130%, Sn 0.0120%, Pb 0.0020%, Bi 0.0020%, Sb 0.0080%, Ti 0.03%, V 0.04%, Ca 0.0050%, B 0.0004%; Alt / N = 4.8:1, V+Nb = 0.04%, V+Nb+Ti = 0.07%.

[0088] The preparation method of the continuous casting billet for sulfur-resistant pipe in this embodiment includes the following steps:

[0089] Step 1: Initial refining in the converter:

[0090] Raw material control: The composition of molten iron is controlled as As 0.013%, S 0.028%, P 0.120%, and Cr 0.20%; high-quality clean scrap steel is used, with no harmful impurities mixed in.

[0091] Converter smelting: The carbon content of the tapped steel is 0.06%, the tapping temperature is 1630℃, the phosphorus content of the tapped steel is 0.007%, the tapping volume is 100t, and slag is prevented throughout the process.

[0092] Alloy additions: 160 kg of aluminum ingots are added for 20 tons of steel; 260 kg of ferrosilicon and 1150 kg of low-carbon ferromanganese are added for 30 tons of steel; 600 kg of lime and 400 kg of pre-melted deoxidation and desulfurization refining slag are added for 50 tons of steel. The main components of the pre-melted deoxidation and desulfurization refining slag include 1.00 wt% SiO2, 42.17 wt% Al2O3, 47.41 wt% CaO and 3.73 wt% MgO.

[0093] Step 2: Refining Process

[0094] When the molten steel is heated to 1580℃, 400kg of lime, 100kg of composite slag, 60kg of aluminum granules, 30kg of ferrosilicon powder, and 20kg of silicon carbide are added during the first power-on process. During the second power-on process, 200kg of lime and 50kg of fluorite are added, along with 20kg of aluminum granules and 40kg of ferrosilicon powder.

[0095] Add 30 kg of ferrosilicon powder during the later stage of refining and stir with a little argon; the total refining time is 70 min and the white slag retention time is 40 min; feed in 80 m of calcium wire with an oxygen content of 9 ppm and a nitrogen content of 80 ppm.

[0096] Step 3: Vacuum treatment

[0097] The vacuum degree is 40 Pa, and the vacuum holding time is 25 min; after breaking the vacuum, the soft blowing time is 25 min, the slag surface is slightly floating, there is no slag entanglement, and it is transferred after purification.

[0098] Step 4: Continuous casting

[0099] Six-strand, six-stream φ210mm round billet casting, double argon-sealed casting, full ladle changeover; tundish superheat 28℃, casting speed 1.20m / min; crystallizer water flow 110m³. 3The electromagnetic stirring in the crystallizer is 250A / 3Hz, and the electromagnetic stirring at the end of solidification is 50A / 8Hz. The copper tubes are used for ≤90 heat treatments. BPJ-21 type mold flux, specifically for peritectic steel round billets, is selected. The main components of the mold flux include 40% SiO2, 39% Al2O, 35% CaO, and 4% R2O. The melting rate at 1350℃ is 48~52s, and the uniformity of the secondary cooling water is strictly controlled.

[0100] Comparative Example 1

[0101] This comparative example provides a method for preparing sulfur-resistant pipe continuous casting billets, with the process route being converter → refining → vacuum → six-strand continuous casting machine.

[0102] The chemical composition of the continuous casting billet for sulfur-resistant pipes in this comparative example, by weight percentage, includes: C 0.11%, Si 0.28%, Mn 1.02%, P 0.015%, S 0.005%, Alt 0.022%, Cr 0.25%, N 0.0120%, O 0.0020%, Ni 0.12%, Cu 0.10%, Mo 0.05%, As 0.0150%, Sn 0.0120%, Pb 0.0030%, Bi 0.0030%, Sb 0.0100%, Ti 0.02%, V 0.03%, Ca 0.0015%, B 0.0003%; Alt / N = 1.8:1, V+Nb = 0.03%, V+Nb+Ti = 0.05%.

[0103] The comparative method for preparing continuously cast billets for sulfur-resistant pipes includes the following steps:

[0104] Step 1: Initial refining in the converter:

[0105] Raw material control: The composition of molten iron is controlled as As 0.015%, S 0.040%, P 0.130%, and Cr 0.30%; scrap steel is not strictly screened and contains a small amount of harmful elements, and the basic composition is simply adjusted.

[0106] Converter smelting: oxygen top and bottom blowing converter, steel tapping C content is 0.05%, tapping temperature is 1600℃, steel tapping P content is 0.010%, steel tapping volume is fixed at 100t, slag discharge is not strictly controlled, and occasionally slag is mixed into molten steel.

[0107] Alloy addition: Add 120 kg of aluminum ingot when the steel reaches 20 tons; add 200 kg of ferrosilicon and 1000 kg of low-carbon ferromanganese when the steel reaches 30 tons; add 500 kg of lime and 400 kg of ordinary pre-melted refining slag when the steel reaches 50 tons.

[0108] Step 2: After the molten steel is in place, the temperature is raised to 1570℃. 250kg of lime, 100kg of composite slag, 30kg of aluminum granules, 20kg of ferrosilicon powder, and 10kg of silicon carbide are added initially. The initial stirring power is low, resulting in insufficient desulfurization; the final sulfur content is 0.005%. A second stirring is performed, adding 200kg of lime and 50kg of fluorite, along with 20kg of aluminum granules and 20kg of ferrosilicon powder. In the later stages of refining, 15kg of ferrosilicon powder is added, and high-flow-rate argon gas is used for stirring. The total refining time is 50 minutes, with a white slag holding time of 20 minutes. At the end of refining, 30m of calcium wire is fed in, controlling the oxygen content to 15ppm and the nitrogen content to 120ppm.

[0109] Step 3: Vacuum treatment with a deep vacuum of 100 Pa and a vacuum holding time of 10 min; soft blowing time after vacuum breaking is 10 min. During soft blowing, there is a slag surface ripple phenomenon. After soft blowing is completed, transfer to continuous casting.

[0110] Step 4: The six-strand continuous casting machine (six-strand casting machine) produces φ210mm round billets. Double argon sealing protection was not used during casting, resulting in significant fluctuations in the tundish level during ladle changes. The tundish superheat was 35℃, and the casting speed was fixed at 1.20m / min. The crystallizer water flow rate was 130m³. 3 / h, the electromagnetic stirring of the crystallizer and the electromagnetic stirring at the end of solidification were not activated. The copper tube of the crystallizer in this comparative example used 120 flares, ordinary low carbon steel crystallizer protective slag was selected, the surface quality was not monitored in real time, and there were local unevenness in the secondary cooling water spray.

[0111] Figure 1 This is a photograph of the continuous casting billet for sulfur-resistant pipes prepared in Example 2; Figure 2 This is a photograph of the actual continuously cast billet for sulfur-resistant pipes prepared in Comparative Example 1. Figure 1 and Figure 2 The comparison shows that the peritectic steel continuous casting billet of Example 1, without optimized process, has a rough, uneven surface with obvious pits, pitting, oxide scale / rust marks, and signs of localized cracking tendency. These are typical surface defects caused by peritectic reaction. In contrast, the continuous casting billet prepared using the process of this invention in Example 2 has a smooth, flat surface without obvious pits, cracks, or pitting. The overall texture is uniform, oxide / rust marks are basically gone, and the appearance is regular. This demonstrates that this invention, through synergistic control of the entire process, can fundamentally suppress surface defects caused by peritectic reaction, improving the appearance quality and yield of the casting billet.

Claims

1. A method for preparing a continuously cast billet for sulfur-resistant pipes, characterized in that, Includes the following steps: Step 1: Initial refining in the converter: Molten steel and clean scrap steel are selected as raw materials. The molten iron is controlled with As ≤ 0.015%, S ≤ 0.040%, and P ≤ 0.130%, and the scrap steel is free of harmful elements exceeding the standard. Oxygen top and bottom blowing converter is used for smelting. The steel has a carbon content of 0.04~0.06%, a tapping temperature of not less than 1600℃, and a P content ≤ 0.008%. Aluminum ingots, alloys, lime, and refining slag are added in stages during the tapping process to ensure that the Al content of the molten steel is stable at 0.040~0.050wt% upon arrival. Slag addition is strictly prohibited. Step 2, Refining Process: The molten steel is heated to 1580℃, and after temperature measurement and sampling, it undergoes two-stage electric refining. The total refining time is not less than 60 minutes, the white slag holding time is not less than 30 minutes, calcium wire is fed in during the later stage of refining, and then vacuum treatment is carried out. Step 3: Vacuum treatment: Control the deep vacuum degree to ≤67Pa, maintain the vacuum time for no less than 15min, and the soft blowing time after breaking the vacuum should be no less than 15min; Step 4: Continuous casting: The casting speed was matched to the superheat of the molten steel in the tundish, with the superheat stabilized at 25±3℃, the casting speed at 1.2m / min, and the crystallizer water flow rate at 110m³ / min. 3 The continuous casting billet for sulfur-resistant pipes was prepared by using electromagnetic stirring at 250A / 3Hz in the crystallizer and 50A / 8Hz at the end of solidification.

2. The method for preparing a sulfur-resistant pipe continuous casting billet according to claim 1, characterized in that, In step one, the internal control parameters for molten iron are S≤0.030%, As≤0.014%, and Cr≤0.20%.

3. The method for preparing a continuously cast billet for sulfur-resistant pipes according to claim 1 or 2, characterized in that, In step one, the converter tapping temperature is not lower than 1610℃, the tapping volume is 100 tons, 160 kg of aluminum ingots are added when the tapping volume reaches 20 tons, 2.6 kg / t of ferrosilicon and 11.5 kg / t of low-carbon ferromanganese are added when the tapping volume reaches 30 tons, and 600 kg of lime and 400 kg of deoxidation and desulfurization refining slag are added when the tapping volume reaches 50 tons.

4. The method for preparing a continuously cast billet for sulfur-resistant pipes according to claim 3, characterized in that, In step two, add 300-400 kg of lime, 0-100 kg of fluorite, 40-60 kg of aluminum granules, 20-30 kg of ferrosilicon powder, and 10-20 kg of silicon carbide in one step after powering on.

5. The method for preparing a sulfur-resistant pipe continuous casting billet according to claim 4, characterized in that, In step two, add 100-200 kg of lime and 0-50 kg of fluorite during the second power-on process, supplement with 10-20 kg of aluminum granules and 30-40 kg of silicon carbide or ferrosilicon powder; feed in a calcium wire of 40-80 m in length.

6. The method for preparing a continuously cast billet for sulfur-resistant pipes according to claim 5, characterized in that, Step four uses a six-strand continuous casting machine to produce custom-made φ210mm round billets.

7. The method for preparing a continuously cast billet for sulfur-resistant pipes according to claim 6, characterized in that, In step four, double argon sealing is used for pouring protection, and the intermediate package is kept full of liquid level when changing packages.

8. The method for preparing a continuously cast billet for sulfur-resistant pipes according to claim 7, characterized in that, In step four, the number of firing cycles for the copper tubes in the crystallizer must be strictly ≤100.

9. A continuously cast slab for sulfur-resistant pipe prepared by the preparation method according to any one of claims 1-8, characterized in that, The chemical composition by weight percentage includes: C 0.10~0.13%, Si 0.20~0.35%, Mn 0.95~1.10%, P≤0.015%, S≤0.003%, Alt 0.015~0.040%, Cr≤0.30%, N≤0.0120%, O≤0.0010%, Ni≤0.30%, Cu≤0.20%, Mo≤0.10%, As≤0.0150%, Sn≤0.0150%, Pb≤0.0030%, Bi≤0.0030%, Sb≤0.0100%, Ti≤0.04%, V≤0.05%, Ca≤0.006%, B≤0.0005%, with the remainder being Fe and unavoidable impurities; wherein the Alt / N ratio is not less than 2:1, V+Nb≤0.06%, and V+Nb+Ti≤0.15%.

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