Acid-resistant pipeline steel continuous casting billet and preparation method and application thereof

Abstract

The present application relates to the technical field of steelmaking continuous casting, and particularly relates to a continuous casting billet for acid-resistant pipeline steel and a preparation method and application thereof. The preparation method of the continuous casting billet for acid-resistant pipeline steel comprises sequentially performing the processes of molten iron pretreatment, converter smelting, LF refining, RH refining and continuous casting, and controlling the chemical composition of the continuous casting billet; in the processes of converter smelting and LF refining, the argon blowing intensity of the ladle bottom is controlled according to different operation stages; the continuous casting process adopts a crystallizer protective slag with specific basicity, melting point and viscosity; the secondary cooling zone is divided into 9 cooling zones, the water distribution of each zone is accurately controlled, and the light and heavy press-down process of the fan-shaped section is combined. After acid etching detection, the central segregation of the continuous casting billet prepared by the present application is not higher than C0.5 level, there is no central loose and shrinkage cavity, no intermediate crack, the surface quality is good, and the inclusions are not higher than level 1.

Description

Technical Field

[0001] This invention relates to the field of continuous casting technology in steelmaking, specifically to a continuous casting billet for acid-resistant pipeline steel, its preparation method, and its application. Background Technology

[0002] The corrosive media such as H2S in acidic oil and gas fields pose a severe challenge to the safety and service life of pipelines, necessitating the development of high-quality acid-resistant pipeline steel with excellent resistance to hydrogen-induced cracking (HIC). Among these processes, continuous casting is a key factor affecting the HIC resistance of pipeline steel, and the type of inclusions, surface quality, and internal quality of the continuously cast billet directly determine the overall performance of the final product.

[0003] Currently, quality control of continuously cast billets mainly focuses on parameters such as the roll gap accuracy of the sector section, the superheat of molten steel in the tundish, casting speed, the specific water volume in the secondary cooling zone, the reduction range, and the reduction amount, in order to improve the central quality of the continuously cast billet. However, most existing technologies focus on controlling single indicators such as central segregation or internal cracks, failing to fully consider the synergistic control of the surface quality and inclusion type of the continuously cast billet. This makes it difficult to simultaneously meet the comprehensive quality requirements of acid-resistant pipeline steel for surface quality control, central density, and high cleanliness of the continuously cast billet. Summary of the Invention

[0004] This invention provides a continuous casting billet for acid-resistant pipeline steel, its preparation method, and its application, in order to solve the problem in the prior art that it is difficult to simultaneously control the surface quality, central segregation, and inclusions of continuous casting billets for acid-resistant pipeline steel, resulting in the overall quality of the continuous casting billet failing to meet the production requirements of high-grade acid-resistant pipeline steel.

[0005] In a first aspect, the present invention provides a method for preparing a continuously cast billet for acid-resistant pipeline steel, comprising sequential hot metal pretreatment, converter smelting, LF refining, RH refining and continuous casting processes, wherein the chemical composition of the continuously cast billet, in mass percentage, is controlled as follows: C: 0.03~0.07%, Si: 0.10~0.30%, Mn: 0.50~1.00%, Ni: 0.2~0.4%, Nb: 0.03~0.05%, V: 0.05~0.07%, P≤0.010%, S≤0.0010%, Al: 0.02~0.04%, Mo≤0.02%, Ti≤0.0020%, O≤0.0010%, N≤0.0030%, H≤0.00015%, with the balance being Fe and other unavoidable impurities;

[0006] During the steel tapping process in the converter smelting, when the steel tapping rate is 60t, the intensity of bottom blowing argon gas in the ladle is controlled at 5.0~6.0L / (min·t); when the steel tapping rate is 120t, the intensity of bottom blowing argon gas in the ladle is controlled at 4.1~4.9L / (min·t).

[0007] During the LF refining process, the intensity of bottom-blown argon gas in the ladle is controlled according to the operation type: when adding alloys to fine-tune the composition, the intensity of bottom-blown argon gas is controlled at 3.1~3.9 L / (min·t); when adding lime and fluorite for slag formation, the intensity of bottom-blown argon gas is controlled at 2.1~2.9 L / (min·t); when adding calcium carbide (CaC2) for deoxidation, the intensity of bottom-blown argon gas is controlled at 1.5~1.9 L / (min·t); when heating with electricity, the intensity of bottom-blown argon gas is controlled at 1.0~1.4 L / (min·t); and when there are no operations as described above, the intensity of bottom-blown argon gas is controlled at 0.3~0.7 L / (min·t).

[0008] In the continuous casting process, a mold flux is used for casting. The binary basicity of the flux, CaO / SiO2, is 1.00~1.15, the melting point is 1030~1090℃, and the viscosity at 1300℃ is 0.12~0.22 Pa·s.

[0009] In the continuous casting process, the secondary cooling zone is divided into 9 cooling zones: zone 1 corresponds to the foot roll section, zones 2-4 correspond to the zero section, zone 5 corresponds to sector sections 1-2, zone 6 corresponds to sector sections 3-4, zone 7 corresponds to sector sections 5-7, zone 8-9 corresponds to sector section 8, and zone 9 corresponds to sector sections 10-12. The water volume of zones 1-9 is controlled to account for the following proportions of the total water volume of the secondary cooling zone: zone 1 13%-16%, zone 2 14%-17%, zone 3 15%-18%, zone 4 10%-13%, zone 5 7%-10%, zone 6 6%-9%, zone 7 6%-9%, zone 8 5%-7%, and zone 9 10%-13%.

[0010] In the continuous casting process, the 9#~11# sector segments are controlled to perform light reduction, with a total reduction of 5~7mm, while the 12# sector segment is controlled to perform heavy reduction, with a reduction of 20~25mm.

[0011] In one optional embodiment, in the hot iron pretreatment process, a desulfurizing agent is used for desulfurization. The mass ratio of CaO to CaF in the desulfurizing agent is (8~10):1, the consumption of the desulfurizing agent per ton of iron is 5~7kg, the mass percentage of the desulfurizing agent particles with a size of 3~5mm is ≥95%, the mass percentage of particles smaller than 3mm is ≤3%, and the mass percentage of particles larger than 5mm is ≤2%. After desulfurization, the hot iron temperature is ≥1300℃, S≤0.0010%, and the slag removal rate is ≥98%. Preferably, the amount of hot iron before desulfurization is 175~185t, the temperature is 1430~1450℃, S≤0.03%, C: 4.2%~4.6%, Si: 0.30%~0.65%, P≤0.10%, and the remainder is Fe and other unavoidable impurities.

[0012] In one optional embodiment, during the converter smelting process, clean scrap steel and desulfurized molten iron are added for smelting, and the scrap steel ratio is controlled to be ≤23wt%; high-quality lime and lightly calcined dolomite are used for slag formation, and top and bottom combined blowing is used for decarburization and heating, with a top-blown oxygen flow rate of 38,000 to 40,000 m³ / h. 3 / h, oxygen supply time 12~14min, bottom blowing gas flow rate 650~670m³ / h 3 / h, bottom blowing carbon dioxide time 30~32min; control the molten steel temperature ≥1630℃, P≤0.010%, S≤0.0015%, C:0.01%~0.03% during tapping; turn on bottom blowing in the ladle during tapping, and add aluminum blocks, ferrosilicon, metallic manganese and nickel plates into the ladle to adjust the composition of the molten steel when the tapping volume is 60t.

[0013] In one optional embodiment, the composition of the clean scrap steel, by mass percentage, is Si≤0.3%, Al≤0.08%, P≤0.010%, S≤0.0060%, and Fe and other unavoidable impurity components.

[0014] And / or, the aluminum block contains Al ≥ 99%, P ≤ 0.015%, S ≤ 0.010%, and other unavoidable impurity components;

[0015] And / or, the ferrosilicon contains 76-80% Si, ≤0.012% P, ≤0.003% S, and Fe and other unavoidable impurity components;

[0016] And / or, the metallic manganese contains Mn≥99%, P≤0.010%, S≤0.004%, and other unavoidable impurity components;

[0017] And / or, the nickel plate contains Ni ≥ 99%, P ≤ 0.008%, S ≤ 0.005%, and other unavoidable impurity components.

[0018] In one optional embodiment, during the LF refining process, bottom blowing is continuously activated in the ladle after the molten steel enters the station. The flow rate of bottom blowing argon is adjusted according to the operation type, and the temperature is increased by electricity. Alloys, lime, and synthetic slag are added to adjust the composition of the molten steel and to form slag. The total amount of slag in the refining process is 12~15 kg / t of steel. The slag composition, by mass percentage, is: CaO: 53%~57%, Al2O3: 25%~28%, SiO2: 5%~7%, MgO: 5%~8%, CaF2: 3%~6%, T.Fe+MnO≤1.0%, and other unavoidable impurities. After the LF refining is completed, the tapping temperature is controlled at 1615~1625℃.

[0019] And / or, after the LF refining is completed, calcium treatment is performed. The calcium wire feed rate Q and the Al content of the molten steel satisfy the following relationship: Q=70+(Al-0.02)×5000, where Al is the mass percentage of aluminum in the molten steel, and the unit of Q is m; the calcium core weight of the pure calcium wire used for calcium treatment is 60~70g / m, the sheet thickness is 2.3~2.7mm, the wire feeding speed is 1.5~2.5m / s, and the Ca content of the molten steel after calcium treatment is 0.0013%~0.0018%.

[0020] In one optional embodiment, during the RH refining process, the vacuum chamber pressure is ≤1.5 mbar, the molten steel circulation time is ≥15 min, and the gas flow rate is increased to 150~200 m³ / min. 3 / min; After breaking the void, the molten steel is subjected to temperature measurement, sampling and hydrogen determination. The Ca content of the molten steel is 0.0005%~0.0008%, H≤0.00018%, and the temperature is 1575~1585℃.

[0021] In one optional embodiment, during the continuous casting process, the ladle is hoisted to the continuous casting platform and left to stand for ≥8 minutes; a high-alkalinity covering agent is used in the tundish, with a consumption of 0.3~0.4 kg per ton of steel, a particle size of 2~4 mm accounting for ≥97%, and the covering agent composition by mass percentage is SiO2: 10%~15%, CaO: 45%~50%, MgO: 10%~15%, Al2O3: 25%~35%, Fe2O3: 1%~3%; argon is blown throughout the tundish, with an argon flow rate of 150~250 L / min at the long nozzle, 3~5 L / min at the stopper rod and submersible nozzle, 400~500 L / min at the casting zone, and 100~150 L / min at the impact zone;

[0022] And / or, in the continuous casting process, the superheat of the molten steel in the tundish is controlled to be 30~40℃, the casting speed of the continuous casting machine is 0.9~1.1m / min, the water content in the secondary cooling zone is 0.6~0.7L / kg, and the cross-sectional size of the continuously cast billet is 250mm×(2100~2700)mm.

[0023] In one optional embodiment, in the continuous casting process, the mold flux is a low-carbon steel flux, and its chemical composition by mass percentage is: SiO2: 27%~29%, CaO: 29%~31%, MgO: 1.0%~3.0%, Fe2O3: 1%~3%, Al2O3: 5%~7%, Na2O: 8%~10%, F: 8%~10%, C: 9%~11%, with the remainder being unavoidable impurities;

[0024] And / or, in the continuous casting process, the thickness of the protective slag layer in the crystallizer is 80~130mm, the thickness of the liquid slag layer is 8~13mm, and the insertion depth of the tundish submersible nozzle is 130~210mm; preferably, the thickness of the protective slag layer in the crystallizer is 100~130mm, the thickness of the liquid slag layer is 10~13mm, and the insertion depth of the tundish submersible nozzle is 150~210mm;

[0025] And / or, in the continuous casting process, the insertion depth of the tundish submersible nozzle is adjusted according to the cross-sectional width of the continuous casting machine: 150-170mm when the cross-sectional width is 2100-2300mm, 170-190mm when the cross-sectional width is 2300-2500mm, and 190-210mm when the cross-sectional width is 2500-2700mm.

[0026] In one optional embodiment, during the continuous casting process, water is distributed in Zone 1 for the narrow side and wide side of the continuously cast billet, with the narrow side accounting for 50%~60% of the total water volume in Zone 1 and the wide side accounting for 40%~50% of the total water volume in Zone 1; water is distributed in Zones 2, 3, and 4 for the center and edge of the wide side of the continuously cast billet, with the center of the wide side accounting for 30%~40% of the total water volume in Zones 2, 3, and 4 and the edge of the wide side accounting for 60%~70% of the total water volume in Zones 2, 3, and 4; the corner temperature of the continuously cast billet before entering the No. 5 sector section is ≥910℃, and the difference between the highest and lowest surface temperatures is ≤18℃; the center length of the wide side is 1000mm; preferably, the corner temperature of the continuously cast billet before entering the No. 5 sector section is ≥930℃, and the difference between the highest and lowest surface temperatures is ≤15℃;

[0027] And / or, in the continuous casting process, the roll gap shrinkage of the foot roll section and the zero section is 0.05~0.10mm, the roll gap shrinkage of the 1#~4# sector sections is 0.10~0.15mm, and the roll gap shrinkage of the 5#~8# sector sections is 0.15~0.20mm; the reduction of the 9# section accounts for 10%~15% of the total light reduction, the 10# section accounts for 20%~25%, and the 11# section accounts for 60%~70%; the reduction of each roll in the 12# sector section is controlled separately, and the maximum reduction of a single roll is 6mm; the solidification endpoint at the center of the wide face of the continuous casting billet is located within the range of the 11# sector section; after being flame-cut to length, the continuous casting billet is hot-charged and hot-sent for rolling or stacked for cooling.

[0028] In one optional embodiment, the length of the foot roll section is 0.7m, with 3 pairs of rolls distributed on the wide side and 4 pairs of rolls distributed on the narrow side; the length of the zero section is 3m, with 10 pairs of rolls distributed on the wide side; the length of the 1#~12# sector sections is 2.2m, with 7 pairs of rolls distributed on the wide side of each sector section; the 1#~6# sector sections are located in the arc section of the continuous casting machine, and the 7#~12# sector sections are located in the horizontal section of the continuous casting machine; the 5#~6# sector sections are used for straightening the continuous casting billet.

[0029] In one optional embodiment, the continuous casting machine is a straight-arc slab continuous casting machine with an arc radius of 10m.

[0030] Secondly, the present invention also provides a continuous casting billet for acid-resistant pipeline steel, which is prepared by the above-mentioned method for preparing a continuous casting billet for acid-resistant pipeline steel.

[0031] Thirdly, the present invention also provides the application of the above-mentioned continuous casting billet for acid-resistant pipeline steel in the preparation of acid-resistant pipeline steel.

[0032] The technical solution of this invention has the following advantages:

[0033] 1. This invention achieves precise control of stirring intensity by adjusting the intensity of bottom-blowing argon gas in the ladle during different operational stages of converter tapping and LF refining. This prevents secondary oxidation of the molten steel while effectively removing inclusions, providing a high-purity steel foundation for subsequent processes. The continuous casting process employs a mold flux with specific basicity, melting point, and viscosity, combined with water distribution in the nine cooling zones of the secondary cooling area and light / heavy pressure reduction processes. This ensures uniform billet shell growth and good lubrication, significantly reducing thermal stress in the billet and guaranteeing the consistency of the solidification endpoint in the width direction of the continuously cast billet. This fundamentally solves the problem of controlling center segregation in traditional processes. The synergistic effect of these three aspects simultaneously solves the problems of surface quality control, center segregation, and inclusion control in continuously cast billets. The resulting acid-resistant pipeline steel continuously cast billets have center segregation no higher than grade C0.5, no central porosity or shrinkage cavities, no intermediate cracks, good surface quality, and inclusions of types A, B, C, and D all no higher than grade 1. The overall quality fully meets the production requirements of high-grade acid-resistant pipeline steel.

[0034] 2. This invention optimizes the molten steel flow and temperature field within the crystallizer by further controlling the protective slag layer thickness to 100-130 mm, the liquid slag layer thickness to 10-13 mm, and the submerged entry nozzle insertion depth to 150-210 mm. This results in a uniform and stable liquid slag layer at the meniscus of the molten steel, leading to more uniform billet shell growth, improved lubrication performance, and a significant reduction in thermal stress and surface cracking tendency in the cast billet. By controlling the temperature at the corner of the continuously cast billet before entering the No. 5 sector section to ≥930℃ and the surface temperature difference to ≤15℃, the temperature distribution in the straightening section is optimized, avoiding surface transverse cracks caused by excessively low corner temperatures or excessive temperature differences. The implementation of the above-mentioned preferred scheme effectively improves the surface quality of the continuously cast billet. While ensuring central segregation of C0.5 grade and the absence of central porosity and shrinkage cavities, it achieves effective control over surface longitudinal cracks and corner transverse cracks, making the overall quality of the continuously cast billet more stable and reliable, and better meeting the stringent requirements of high-grade acid-resistant pipeline steel. Detailed Implementation

[0035] The following embodiments are provided to better understand the present invention, but the following embodiments do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the scope of protection of the present invention.

[0036] Unless otherwise specified, all experimental steps or conditions in the examples were performed according to conventional experimental procedures and conditions in the art. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0037] The following specific embodiments further illustrate the present invention. The examples given do not represent all embodiments of the invention; only some embodiments are described as examples. The composition of the clean scrap steel used, by mass percentage, is: Si: 0.25%, Al: 0.06%, P: 0.008%, S: 0.005%, and Fe and other unavoidable impurities; the main components of the aluminum block are: Al: 99.2%, P: 0.012%, S: 0.008%, and other unavoidable impurities; the main components of the ferrosilicon are: Si: 78%, P: 0.010%, S: 0.002%, and Fe and other unavoidable impurities; the main components of metallic manganese are: Mn: 99.3%, P: 0.008%, S: 0.003%, and other unavoidable impurities; the main components of the nickel plate are: Ni: 99.2%, P: 0.006%, S: 0.004%, and other unavoidable impurities. The continuous casting machine parameters are as follows: The continuous casting machine is a straight-arc slab continuous casting machine with a cross-sectional size of 250mm × (2100~2700)mm and an arc radius of 10m. The secondary cooling zone includes a foot roll section, a zero section, and 12 sector sections. The foot roll section corresponds to secondary cooling zone 1, the zero section corresponds to secondary cooling zones 2, 3, and 4, sector sections 1 and 2 correspond to zone 5, sector sections 3 and 4 correspond to zone 6, sector sections 5 to 7 correspond to zone 7, sector sections 8 to 9 correspond to zone 8, and sector sections 10 to 12 correspond to zone 9. The water distribution in zone 1 is divided into the narrow side and wide face of the continuously cast slab, and the water distribution in zones 2, 3, and 4 is divided into the center and edge of the wide face of the continuously cast slab. The center length of the wide face is 1000mm. mm; the length of the foot roller section is 0.7m, with 3 pairs of rollers distributed on the wide side and 4 pairs of rollers distributed on the narrow side; the length of the zero section is 3m, with 10 pairs of rollers distributed on the wide side; the length of the fan-shaped sections numbered 1#~12# is 2.2m, with 7 pairs of rollers distributed on the wide side of each fan-shaped section; the fan-shaped sections numbered 1#~6# are located in the arc section of the continuous casting machine, and the fan-shaped sections numbered 7#~12# are located in the horizontal section of the continuous casting machine; the fan-shaped sections numbered 5#~6# are used for straightening the continuous casting billet; the fan-shaped sections numbered 9#~11# are used for light pressing; the fan-shaped section numbered 12# is used for heavy pressing, and the pressing amount of each roller can be controlled individually, with a maximum pressing amount of 6mm per roller; the solidification endpoint at the center of the wide side of the continuous casting billet is located within the range of the fan-shaped section numbered 11#.

[0038] Example 1

[0039] This embodiment provides a method for preparing a continuous casting billet for acid-resistant pipeline steel, the specific steps of which are as follows:

[0040] (1) Hot metal pretreatment process: 175t of hot metal with a temperature of 1430℃, S content of 0.025%, C content of 4.2%, Si content of 0.65%, P content of 0.09%, and the remainder being Fe and other unavoidable impurities, was mechanically stirred for desulfurization. The desulfurizing agent used had a CaO to CaF mass ratio of 9:1, a desulfurizing agent consumption of 7kg per ton of hot metal, and a desulfurizing agent particle size of 3-5mm accounting for 96% of the mass, particles smaller than 3mm accounting for 2.5% of the mass, and particles larger than 5mm accounting for 1.5% of the mass. After desulfurization, the hot metal temperature was 1340℃, the S content was 0.0010%, and the slag removal rate was 98%.

[0041] (2) Converter smelting process: Desulfurized molten iron and clean scrap steel are added to the converter. The scrap steel loading is 50t, and the scrap steel ratio is 22.2wt%. Top and bottom combined blowing is used for decarburization and heating, with a top blowing oxygen flow rate of 38,000 m³ / h. 3 / h, oxygen supply time 12min, bottom blowing gas flow rate 650m³ / h 3 / h, bottom blowing carbon dioxide time 30min. During tapping, the molten steel temperature is controlled at 1630℃, with P content of 0.009%, S content of 0.0015%, and C content of 0.03%. Bottom blowing of the ladle is activated during tapping. When the tapping rate is 60t, the argon intensity of bottom blowing is controlled at 5.0L / (min·t), and aluminum blocks, ferrosilicon, metallic manganese, and nickel plates are added to the ladle to adjust the molten steel composition. When the tapping rate is 120t, the argon intensity of bottom blowing is controlled at 4.1L / (min·t).

[0042] (3) LF refining process: After the molten steel enters the station, the bottom blowing of the ladle is turned on throughout the process. The intensity of the bottom blowing argon gas in the ladle is controlled according to the operation type: when adding alloy to fine-tune the composition, the intensity of the bottom blowing argon gas is controlled at 3.1 L / (min·t); when adding lime and fluorite to form slag, the intensity of the bottom blowing argon gas is controlled at 2.1 L / (min·t); when adding calcium carbide (CaC2) for deoxidation, the intensity of the bottom blowing argon gas is controlled at 1.5 L / (min·t); when energizing and heating, the intensity of the bottom blowing argon gas is controlled at 1.0 L / (min·t); when there is no operation as described above, the intensity of the bottom blowing argon gas is controlled at 0.3 L / (min·t). Simultaneously, electricity was applied to raise the temperature, and alloys, lime, and synthetic slag were added to adjust the steel composition and slag formation. The total slag amount for refining was 12 kg / t of steel. The slag composition, by mass percentage, was: CaO: 53%, Al2O3: 28%, SiO2: 7%, MgO: 8%, CaF2: 3%, T.Fe+MnO: 0.5%, and other unavoidable impurities. After refining, the tapping temperature was controlled at 1615℃, followed by calcium treatment. The Al content of the molten steel was 0.023%, and the amount of calcium wire fed was 85 m. The calcium treatment used pure calcium wire with a calcium core weight of 60 g / m, a sheet thickness of 2.3 mm, and a feeding speed of 1.5 m / s. After calcium treatment, the Ca content in the molten steel was 0.0013%.

[0043] (4) RH refining process: The ladle is hoisted to the RH refining station for vacuum treatment, and the gas flow rate is increased to 150m³. 3 The vacuum chamber pressure was 1.5 mbar, and the molten steel circulation time was 15 min. After the vacuum chamber was broken, the molten steel was subjected to temperature measurement, sampling, and hydrogen determination. The composition of the molten steel was C: 0.07%, Si: 0.10%, Mn: 0.50%, Ni: 0.2%, Nb: 0.03%, V: 0.05%, P: 0.010%, S: 0.0010%, Al: 0.02%, Ca: 0.0005%, Mo: 0.01%, Ti: 0.0018%, O: 0.0009%, N: 0.0030%, H: 0.00015%, with the balance being Fe and other unavoidable impurities. The temperature of the molten steel was 1575℃.

[0044] (5) Continuous casting process: The ladle is hoisted to the continuous casting platform and left to stand for 8 minutes before casting begins. A high-alkalinity covering agent is used in the tundish, with a consumption of 0.3 kg per ton of steel. The particle size of the covering agent is 2-4 mm, accounting for 98% of the total mass. The composition of the covering agent, by mass percentage, is SiO2: 12%, CaO: 48%, MgO: 12%, Al2O3: 26%, and Fe2O3: 2%. Argon is blown throughout the tundish process. The argon blowing flow rate is 150 L / min for the long nozzle, 3 L / min for the stopper rod and submerged entry nozzle, 400 L / min for the casting zone, and 100 L / min for the impact zone. The superheat of the molten steel in the tundish is controlled at 30°C, the casting speed of the continuous casting machine is 1.1 m / min, the water content in the secondary cooling zone is 0.6 L / kg, the cross-sectional size of the continuously cast billet is 250 mm × 2100 mm, and the insertion depth of the submerged entry nozzle is 150 mm. Casting was performed using a mold flux. The chemical composition of the mold flux, by mass percentage, was: SiO2: 29%, CaO: 29%, MgO: 3.0%, Fe2O3: 3.0%, Al2O3: 7.0%, Na2O: 8.0%, F: 8.0%, C: 9.0%, with the remainder being unavoidable impurities. The binary basicity of the mold flux, CaO / SiO2, was 1.00, the melting point was 1030℃, and the viscosity at 1300℃ was 0.22 Pa·s. The thickness of the mold flux layer inside the mold was 100 mm, and the thickness of the liquid slag layer was 10 mm. The secondary cooling zone is divided into 9 cooling zones: zone 1 corresponds to the foot roller section, zones 2-4 to the zero section, zone 5 to the 1#-2# fan-shaped section, zone 6 to the 3#-4# fan-shaped section, zone 7 to the 5#-7# fan-shaped section, zone 8 to the 9# fan-shaped section, and zone 9 to the 10#-12# fan-shaped section. The water volume of zones 1-9 is controlled to account for the following proportions of the total water volume of the secondary cooling zone: zone 1 13%, zone 2 16%, zone 3 18%, zone 4 13%, zone 5 7%, zone 6 6%, zone 7 9%, zone 8 5%, and zone 9 13%. Zone 1 distributes water to the narrow side and wide face of the continuous casting billet, with the narrow side accounting for 50% of the water volume in zone 1 and the wide face accounting for 50%. Zones 2, 3, and 4 distribute water to the center and edge of the wide face of the continuous casting billet, with the center of the wide face accounting for 30% of the water volume and the edge of the wide face accounting for 70%. The temperature of the continuously cast billet entering the front corner of sector #5 is controlled at 930℃, with a surface temperature difference of 15℃ between the highest and lowest temperatures. The roll gap shrinkage is controlled at 0.05mm for the foot roll section and zero roll section, 0.10mm for sector #1-4, and 0.15mm for sector #5-8. Sector #9-11 is controlled to undergo light reduction, with a total reduction of 5mm, of which sector #9 accounts for 15%, sector #10 for 25%, and sector #11 for 60%. Sector #12 is controlled to undergo heavy reduction, with a reduction of 20mm. After flame cutting to length, the continuously cast billet undergoes stack cooling.

[0045] According to GB / T 226-2015, low-magnification samples of the continuously cast billet prepared in this embodiment were subjected to surface and cross-sectional pickling (the etching solution was a 1:1 (volume ratio) hydrochloric acid aqueous solution, the etching temperature was 75℃, and the etching time was 15min). The microstructure was rated according to YB / T 4003-2016. The results showed that the continuous casting billet had a center segregation of C0.5 grade, no central porosity or shrinkage cavities, and no surface cracks or intermediate cracks. Inclusion analysis samples were taken from the half-width position of the low-magnification cross-section sample and rated according to GB / T 10561-2005. The inclusion ratings for categories A, B, C, and D were 1.0, 1.0, 0.5, and 1.0, respectively.

[0046] Example 2

[0047] This embodiment provides a method for preparing a continuous casting billet for acid-resistant pipeline steel, the specific steps of which are as follows:

[0048] (1) Hot metal pretreatment process: 180t of hot metal with a temperature of 1440℃, S content of 0.020%, C content of 4.4%, Si content of 0.45%, P content of 0.08%, and the remainder being Fe and other unavoidable impurities, was mechanically stirred for desulfurization. The desulfurizing agent used had a CaO to CaF mass ratio of 9:1, a desulfurizing agent consumption of 6kg per ton of hot metal, and a desulfurizing agent particle size of 3-5mm accounting for 97% of the mass, particles smaller than 3mm accounting for 2% of the mass, and particles larger than 5mm accounting for 1% of the mass. After desulfurization, the hot metal temperature was 1350℃, the S content was 0.0009%, and the slag removal rate was 98%.

[0049] (2) Converter smelting process: Desulfurized molten iron and clean scrap steel are added to the converter. The scrap steel loading is 48t, and the scrap steel ratio is 21.1wt%. Top and bottom combined blowing is used for decarburization and heating. The top blowing oxygen flow rate is 39,000 m³ / h, the oxygen supply time is 13 min, and the bottom blowing gas flow rate is 660 m³ / h. 3 / h, bottom blowing carbon dioxide time 31min. During tapping, the molten steel temperature is controlled at 1640℃, with P content of 0.0085%, S content of 0.0012%, and C content of 0.02%. Bottom blowing of the ladle is activated during tapping. When the tapping rate is 60t, the argon intensity of bottom blowing is controlled at 5.5L / (min·t), and aluminum blocks, ferrosilicon, metallic manganese, and nickel plates are added to the ladle to adjust the molten steel composition. When the tapping rate is 120t, the argon intensity of bottom blowing is controlled at 4.5L / (min·t).

[0050] (3) LF refining process: After the molten steel enters the station, the bottom blowing of the ladle is turned on throughout the process. The intensity of the bottom blowing argon gas in the ladle is controlled according to the operation type: when adding alloy to fine-tune the composition, the intensity of the bottom blowing argon gas is controlled at 3.5L / (min·t); when adding lime and fluorite to form slag, the intensity of the bottom blowing argon gas is controlled at 2.5L / (min·t); when adding calcium carbide (CaC2) for deoxidation, the intensity of the bottom blowing argon gas is controlled at 1.7L / (min·t); when energizing and heating, the intensity of the bottom blowing argon gas is controlled at 1.2L / (min·t); when there is no operation as described above, the intensity of the bottom blowing argon gas is controlled at 0.5L / (min·t). Simultaneously, electricity was applied to raise the temperature, and alloys, lime, and synthetic slag were added to adjust the steel composition and slag formation. The total slag amount for refining was 13 kg / t of steel. The slag composition, by mass percentage, was: CaO: 55%, Al2O3: 27%, SiO2: 5%, MgO: 7%, CaF2: 5%, T.Fe+MnO: 1.0%, and other unavoidable impurities. After refining, the tapping temperature was controlled at 1620℃, followed by calcium treatment. The Al content of the molten steel was 0.033%, and the amount of calcium wire fed was 135 m. The calcium treatment used pure calcium wire with a calcium core weight of 65 g / m, a sheet thickness of 2.4 mm, and a feeding speed of 2.0 m / s. After calcium treatment, the Ca content in the molten steel was 0.0015%.

[0051] (4) RH refining process: The ladle is hoisted to the RH refining station for vacuum treatment, and the gas flow rate is increased to 180m³. 3 The vacuum chamber pressure was 1.1 mbar, and the molten steel circulation treatment time was 17 min. After the vacuum chamber was broken, the molten steel was subjected to temperature measurement, sampling, and hydrogen determination. The composition of the molten steel was C: 0.05%, Si: 0.20%, Mn: 0.70%, Ni: 0.3%, Nb: 0.04%, V: 0.06%, P: 0.009%, S: 0.0009%, Al: 0.03%, Ca: 0.0007%, Mo: 0.01%, Ti: 0.0018%, O: 0.0008%, N: 0.0028%, H: 0.00015%, with the balance being Fe and other unavoidable impurities. The temperature of the molten steel was 1580℃.

[0052] (5) Continuous casting process: The ladle is hoisted to the continuous casting platform and left to stand for 9 minutes before casting begins. A high-alkalinity covering agent is used in the tundish, with a consumption of 0.35 kg per ton of steel. The particle size of the covering agent is 2-4 mm, accounting for 98% by mass. The composition of the covering agent, by mass percentage, is SiO2: 12%, CaO: 48%, MgO: 12%, Al2O3: 26%, and Fe2O3: 2%. Argon is blown throughout the tundish process. The argon blowing flow rate is 200 L / min for the long nozzle, 4 L / min for the stopper rod and submerged entry nozzle, 450 L / min for the casting zone, and 130 L / min for the impact zone. The superheat of the molten steel in the tundish is controlled at 35°C, the casting speed of the continuous casting machine is 1.0 m / min, the water content in the secondary cooling zone is 0.65 L / kg, the cross-sectional size of the continuously cast billet is 250 mm × 2400 mm, and the insertion depth of the submerged entry nozzle is 180 mm. Casting was performed using a mold flux. The chemical composition of the mold flux, by mass percentage, was: SiO2: 28%, CaO: 30%, MgO: 2.0%, Fe2O3: 2.0%, Al2O3: 6%, Na2O: 9%, F: 9%, C: 10%, with the remainder being unavoidable impurities. The binary basicity of the mold flux, CaO / SiO2, was 1.07, the melting point was 1060℃, and the viscosity at 1300℃ was 0.17 Pa·s. The thickness of the mold flux layer inside the mold flux was 120 mm, and the thickness of the liquid slag layer was 12 mm. The secondary cooling zone is divided into 9 cooling zones: the foot roller section corresponds to zone 1, the zero section corresponds to zones 2-4, the 1#-2# fan-shaped sections correspond to zone 5, the 3#-4# fan-shaped sections correspond to zone 6, the 5#-7# fan-shaped sections correspond to zone 7, the 8#-9# fan-shaped sections correspond to zone 8, and the 10#-12# fan-shaped sections correspond to zone 9. The water volume of zones 1-9 is controlled to account for the following proportions of the total water volume of the secondary cooling zone: zone 1 15%, zone 2 14%, zone 3 16%, zone 4 12%, zone 5 9%, zone 6 9%, zone 7 7%, zone 8 6%, and zone 9 12%. Zone 1 distributes water to the narrow side and wide face of the continuous casting billet, with the narrow side accounting for 55% of the water volume in zone 1 and the wide face accounting for 45%. Zones 2, 3, and 4 distribute water to the center and edge of the wide face of the continuous casting billet, with the center of the wide face accounting for 35% and the edge of the wide face accounting for 65%. The temperature of the continuously cast billet entering the front corner of sector 5 is controlled at 940℃, with a surface temperature difference of 13℃ between the highest and lowest temperatures. The roll gap shrinkage is controlled at 0.07mm for the foot roll section and zero roll section, 0.12mm for sector sections 1-4, and 0.17mm for sector sections 5-8. Sector sections 9-11 are controlled to undergo light reduction, with a total reduction of 6mm, of which sector 9 accounts for 15%, sector 10 for 20%, and sector 11 for 65%. Sector 12 is controlled to undergo heavy reduction, with a reduction of 23mm. After flame cutting to length, the continuously cast billet undergoes stack cooling.

[0053] According to GB / T 226-2015, low-magnification samples of the continuously cast billet prepared in this embodiment were subjected to surface and cross-sectional pickling (the etching solution was a 1:1 (volume ratio) hydrochloric acid aqueous solution, the etching temperature was 75℃, and the etching time was 15min). The microstructure was rated according to YB / T 4003-2016. The results showed that the continuous casting billet had a center segregation of C0.5 grade, no central porosity or shrinkage cavities, and no surface cracks or intermediate cracks. Inclusion analysis samples were taken from the 1 / 2 width position of the low-magnification cross-section sample and rated according to GB / T 10561-2005. The inclusion ratings for categories A, B, C, and D were 1.0, 1.0, 0.5, and 0.5, respectively.

[0054] Example 3

[0055] This embodiment provides a method for preparing a continuous casting billet for acid-resistant pipeline steel, the specific steps of which are as follows:

[0056] (1) Hot metal pretreatment process: 185t of hot metal with a temperature of 1440℃, S content of 0.015%, C content of 4.6%, Si content of 0.30%, P content of 0.07%, and the remainder being Fe and other unavoidable impurities, was mechanically stirred for desulfurization. The desulfurizing agent used had a CaO to CaF mass ratio of 9:1, a desulfurizing agent consumption of 5kg per ton of hot metal, and a desulfurizing agent particle size of 3-5mm accounting for 98% of the mass, particles smaller than 3mm accounting for 1.5% of the mass, and particles larger than 5mm accounting for 0.5% of the mass. After desulfurization, the hot metal temperature was 1350℃, the S content was 0.0008%, and the slag removal rate was 99%.

[0057] (2) Converter smelting process: Desulfurized molten iron and clean scrap steel are added to the converter. The scrap steel loading is 45t, and the scrap steel ratio is 19.6wt%. Top and bottom combined blowing is used for decarburization and heating. The top blowing oxygen flow rate is 40,000 m³ / h, the oxygen supply time is 14 min, and the bottom blowing gas flow rate is 670 m³ / h. 3 / h, bottom blowing carbon dioxide time 32min. During tapping, the molten steel temperature is controlled at 1650℃, with P content of 0.008%, S content of 0.0010%, and C content of 0.01%. Bottom blowing of the ladle is activated during tapping. When the tapping rate is 60t, the argon intensity of bottom blowing is controlled at 6.0L / (min·t), and aluminum blocks, ferrosilicon, metallic manganese, and nickel plates are added to the ladle to adjust the molten steel composition. When the tapping rate is 120t, the argon intensity of bottom blowing is controlled at 4.9L / (min·t).

[0058] (3) LF refining process: After the molten steel enters the station, the bottom blowing of the ladle is turned on throughout the process. The intensity of the bottom blowing argon gas in the ladle is controlled according to the operation type: when adding alloy to fine-tune the composition, the intensity of the bottom blowing argon gas is controlled at 3.9 L / (min·t); when adding lime and fluorite to form slag, the intensity of the bottom blowing argon gas is controlled at 2.9 L / (min·t); when adding calcium carbide (CaC2) for deoxidation, the intensity of the bottom blowing argon gas is controlled at 1.9 L / (min·t); when energizing and heating, the intensity of the bottom blowing argon gas is controlled at 1.4 L / (min·t); when there is no operation as described above, the intensity of the bottom blowing argon gas is controlled at 0.7 L / (min·t). Simultaneously, electricity was applied to raise the temperature, and alloys, lime, and synthetic slag were added to adjust the steel composition and slag formation. The total slag amount for refining was 15 kg / t of steel. The slag composition, by mass percentage, was: CaO: 57%, Al2O3: 25%, SiO2: 6%, MgO: 5%, CaF2: 6%, T.Fe+MnO: 0.7%, and other unavoidable impurities. After refining, the tapping temperature was controlled at 1625℃, followed by calcium treatment. The Al content of the molten steel was 0.043%, and the amount of calcium wire fed was 185 m. The calcium treatment used pure calcium wire with a calcium core weight of 70 g / m, a sheet thickness of 2.7 mm, and a feeding speed of 2.5 m / s. After calcium treatment, the Ca content in the molten steel was 0.0018%.

[0059] (4) RH refining process: The ladle is hoisted to the RH refining station for vacuum treatment, and the gas flow rate is increased to 200 m³ / h. 3 The vacuum chamber pressure was 0.67 mbar, and the molten steel circulation time was 20 min. After the vacuum chamber was broken, the molten steel was subjected to temperature measurement, sampling, and hydrogen determination. The composition of the molten steel was C: 0.03%, Si: 0.30%, Mn: 1.00%, Ni: 0.4%, Nb: 0.05%, V: 0.07%, P: 0.008%, S: 0.0008%, Al: 0.04%, Ca: 0.0008%, Mo: 0.01%, Ti: 0.0015%, O: 0.0008%, N: 0.0025%, H: 0.00012%, with the balance being Fe and other unavoidable impurities. The temperature of the molten steel was 1585℃.

[0060] (5) Continuous casting process: The ladle is hoisted to the continuous casting platform and left to stand for 10 minutes before casting begins. A high-alkalinity covering agent is used in the tundish, with a consumption of 0.4 kg per ton of steel. The particle size of the covering agent is 2-4 mm, accounting for 98% of the total mass. The composition of the covering agent, by mass percentage, is SiO2: 12%, CaO: 48%, MgO: 12%, Al2O3: 26%, and Fe2O3: 2%. Argon is blown throughout the tundish process. The argon blowing flow rate is 250 L / min for the long nozzle, 5 L / min for the stopper rod and submerged entry nozzle, 500 L / min for the casting zone, and 150 L / min for the impact zone. The superheat of the molten steel in the tundish is controlled at 40°C, the casting speed of the continuous casting machine is 0.9 m / min, the water content in the secondary cooling zone is 0.7 L / kg, the cross-sectional size of the continuously cast billet is 250 mm × 2700 mm, and the insertion depth of the submerged entry nozzle is 210 mm. Casting was performed using a mold flux. The chemical composition of the mold flux, by mass percentage, was: SiO2: 27%, CaO: 31%, MgO: 1.0%, Fe2O3: 1.0%, Al2O3: 5%, Na2O: 10%, F: 10%, C: 11%, with the remainder being unavoidable impurities. The binary basicity of the mold flux, CaO / SiO2, was 1.15, the melting point was 1090℃, and the viscosity at 1300℃ was 0.12 Pa·s. The thickness of the mold flux layer inside the mold flux was 130 mm, and the thickness of the liquid slag layer was 13 mm. The secondary cooling zone is divided into 9 cooling zones: the foot roller section corresponds to zone 1, the zero section corresponds to zones 2-4, the 1#-2# fan-shaped sections correspond to zone 5, the 3#-4# fan-shaped sections correspond to zone 6, the 5#-7# fan-shaped sections correspond to zone 7, the 8#-9# fan-shaped sections correspond to zone 8, and the 10#-12# fan-shaped sections correspond to zone 9. The water volume of zones 1-9 is controlled to account for the following proportions of the total water volume of the secondary cooling zone: zone 1 16%, zone 2 17%, zone 3 15%, zone 4 10%, zone 5 10%, zone 6 8%, zone 7 6%, zone 8 7%, and zone 9 11%. Zone 1 distributes water to the narrow side and wide face of the continuous casting billet, with the narrow side accounting for 60% of the water volume in zone 1 and the wide face accounting for 40%. Zones 2, 3, and 4 distribute water to the center and edge of the wide face of the continuous casting billet, with the center of the wide face accounting for 40% of the water volume and the edge of the wide face accounting for 60%. The temperature of the continuously cast billet entering the front corner of sector #5 is controlled at 950℃, with a surface temperature difference of 10℃ between the highest and lowest temperatures. The roll gap shrinkage is controlled at 0.10mm for the foot roll section and zero roll section, 0.15mm for sector #1-4, and 0.20mm for sector #5-8. Sector #9-11 undergoes light reduction, with a total reduction of 7mm, of which sector #9 accounts for 10%, sector #10 for 20%, and sector #11 for 70%. Sector #12 undergoes heavy reduction, with a reduction of 25mm. After flame cutting to length, the continuously cast billet undergoes stack cooling.

[0061] According to GB / T 226-2015, low-magnification samples of the continuously cast billet prepared in this embodiment were subjected to surface and cross-sectional pickling (the etching solution was a 1:1 (volume ratio) hydrochloric acid aqueous solution, the etching temperature was 75℃, and the etching time was 15min). The microstructure was rated according to YB / T 4003-2016. The results showed that the central segregation of the continuously cast billet was grade C0.5, with no central porosity or shrinkage cavities, and no surface cracks or intermediate cracks. Inclusion samples were taken from the half-width position of the low-magnification cross-section for inclusion analysis. According to GB / T 10561-2005, the inclusion grades for categories A, B, C, and D were 0.5, 0.5, 0.5, and 0.5, respectively.

[0062] Example 4

[0063] This embodiment provides a method for preparing a continuously cast billet for acid-resistant pipeline steel. The difference between this method and Embodiment 1 is that, in the continuous casting process, the thickness of the protective slag layer in the crystallizer is controlled to be 80 mm, the thickness of the liquid slag layer is 8 mm, and the insertion depth of the submerged entry nozzle is 130 mm. The remaining steps are the same as in Embodiment 1.

[0064] According to GB / T 226-2015, low-magnification samples of the continuously cast billet prepared in this embodiment were subjected to surface and cross-sectional pickling (the etching solution was a 1:1 (volume ratio) hydrochloric acid aqueous solution, the etching temperature was 75℃, and the etching time was 15min). The microstructure was rated according to YB / T 4003-2016. The results showed that the continuous casting billet had a C0.5 grade segregation at the center, no central porosity or shrinkage cavities, and slight longitudinal cracks and no intermediate cracks on the surface. Inclusion samples were taken from the half-width position of the low-magnification cross-section for inclusion analysis. According to GB / T 10561-2005, the inclusions of categories A, B, C, and D were rated as 1.0, 1.0, 0.5, and 1.0, respectively.

[0065] Example 5

[0066] This embodiment provides a method for preparing a continuously cast billet for acid-resistant pipeline steel. The difference between this method and Embodiment 1 is that, in the continuous casting process, the corner temperature of the billet before entering the No. 5 sector section is 910℃, and the difference between the highest and lowest surface temperatures is 18℃. The remaining steps are the same as in Embodiment 1.

[0067] According to GB / T 226-2015, low-magnification samples of the continuously cast billet prepared in this embodiment were subjected to surface and cross-sectional pickling (the etching solution was a 1:1 (volume ratio) hydrochloric acid aqueous solution, the etching temperature was 75℃, and the etching time was 15min). The microstructure was rated according to YB / T 4003-2016. The results showed that the continuous casting billet had a center segregation of C0.5 grade, no central porosity or shrinkage cavities, and slight corner transverse cracks and no intermediate cracks on the surface of the continuous casting billet. Inclusion samples were taken from the half-width position of the low-magnification cross-section for inclusion analysis. According to GB / T10561-2005, the inclusion grades of A, B, C, and D were 1.0, 1.0, 0.5, and 1.0, respectively.

[0068] Comparative Example 1

[0069] This comparative example provides a method for preparing a continuously cast billet for acid-resistant pipeline steel. The difference between this method and Example 1 is that during the tapping process in the converter smelting, the bottom-blown argon gas intensity in the ladle is maintained at 4.1 L / (min·t) throughout the process; and during the LF refining process, the bottom-blown argon gas intensity in the ladle is maintained at 1.5 L / (min·t) throughout the process. Other conditions are the same as in Example 1.

[0070] According to GB / T 226-2015, low-magnification samples of the continuously cast billet prepared for this comparative example were subjected to surface and cross-sectional pickling (the etching solution was a 1:1 (volume ratio) hydrochloric acid aqueous solution, the etching temperature was 75℃, and the etching time was 15min). The microstructure was rated according to YB / T 4003-2016. The results showed that the continuous casting billet had a center segregation of C0.5 grade, no central porosity or shrinkage cavities, and no surface cracks or intermediate cracks. Inclusion samples were taken from the half-width position of the low-magnification cross-section for inclusion analysis. According to GB / T 10561-2005, the inclusion grades for categories A, B, C, and D were 1.0, 2.0, 0.5, and 2.0, respectively.

[0071] Comparative Example 2

[0072] This comparative example provides a method for preparing a continuously cast billet for acid-resistant pipeline steel. The difference between this method and Example 1 is that in the continuous casting process, conventional pipeline steel protective slag is used. The conventional protective slag has a binary basicity (CaO / SiO2) of 0.89, a melting point of 980℃, and a viscosity of 0.45 Pa·s at 1300℃. Other conditions are the same as in Example 1.

[0073] According to GB / T 226-2015, low-magnification samples of the continuously cast billet prepared for this comparative example were subjected to surface and cross-sectional pickling (the etching solution was a 1:1 (volume ratio) hydrochloric acid aqueous solution, the etching temperature was 75℃, and the etching time was 15min). The microstructure was rated according to YB / T 4003-2016. The results showed that the continuous casting billet had a C0.5 grade segregation at the center, no central porosity or shrinkage cavities, and severe longitudinal cracks on the surface of the continuous casting billet but no intermediate cracks. Inclusion samples were taken from the half-width position of the low-magnification cross-section for analysis. According to GB / T 10561-2005, the inclusion ratings for categories A, B, C, and D were 1.0, 1.0, 0.5, and 1.0, respectively.

[0074] Comparative Example 3

[0075] This comparative example provides a method for preparing a continuously cast billet for acid-resistant pipeline steel. The difference between this method and Example 1 is that the secondary cooling zone in the continuous casting process adopts a conventional uniform water distribution mode, meaning the water volume in each cooling zone is evenly distributed according to length. Specifically, the water volume in zones 1-9 accounts for the following proportions of the total water volume in the secondary cooling zone: Zone 1 11%, Zone 2 11%, Zone 3 11%, Zone 4 11%, Zone 5 11%, Zone 6 11%, Zone 7 11%, Zone 8 11%, and Zone 9 12%. Other conditions are the same as in Example 1.

[0076] According to GB / T 226-2015, low-magnification samples of the continuously cast billet prepared for this comparative example were subjected to surface and cross-sectional pickling (the etching solution was a 1:1 (volume ratio) hydrochloric acid aqueous solution, the etching temperature was 75℃, and the etching time was 15min). The microstructure was graded according to YB / T 4003-2016. The results showed that the central segregation of the continuously cast billet was grade B1.0, the central porosity was grade 0.5, there were no shrinkage cavities, and there were no surface cracks or intermediate cracks grade 0.5. Inclusion samples were taken from the half-width position of the low-magnification cross-section for inclusion analysis. According to GB / T 10561-2005, the inclusion grades for categories A, B, C, and D were 1.5, 1.0, 0.5, and 1.0, respectively.

[0077] Comparative Example 4

[0078] This comparative example provides a method for preparing a continuously cast billet for acid-resistant pipeline steel. The difference between this method and Example 1 is that: in the continuous casting process, a uniform roll gap shrinkage is used (0.10 mm from the foot roll section to the 8# sector section); and the light and heavy reduction processes are not performed (no reduction is applied to the 9#~12# sector sections). All other conditions are the same as in Example 1.

[0079] According to GB / T 226-2015, low-magnification samples of the continuously cast billets prepared for this comparative example were subjected to surface and cross-sectional pickling (the etching solution was a 1:1 (volume ratio) hydrochloric acid aqueous solution, the etching temperature was 75℃, and the etching time was 15min). The microstructure was graded according to YB / T 4003-2016. The results showed that the central segregation of the continuously cast billet was grade A1.0, the central porosity was grade 0.5, the shrinkage cavity was grade 1.0, and there were no surface cracks or intermediate cracks. Inclusion samples were taken from the half-width position of the low-magnification cross-section for analysis. According to GB / T 10561-2005, the inclusion grades for categories A, B, C, and D were 1.5, 1.0, 0.5, and 1.0, respectively.

[0080] Comparative Example 5

[0081] This comparative example provides a method for preparing a continuously cast billet for acid-resistant pipeline steel. The difference between this method and Example 1 is that the water distribution pattern in the secondary cooling zone during the continuous casting process is as follows: Zone 1 8%, Zone 2 26%, Zone 3 25%, Zone 4 18%, Zone 5 8%, Zone 6 6%, Zone 7 3%, Zone 8 3%, and Zone 9 3%. Other conditions are the same as in Example 1.

[0082] According to GB / T 226-2015, low-magnification samples of the continuously cast billet prepared for this comparative example were subjected to surface and cross-sectional pickling (the etching solution was a 1:1 (volume ratio) hydrochloric acid aqueous solution, the etching temperature was 75℃, and the etching time was 15min). The microstructure was rated according to YB / T 4003-2016. The results showed that the central segregation of the continuously cast billet was grade B1.5, the central porosity was grade 1.0, and there were no shrinkage cavities. The surface of the continuously cast billet had severe transverse cracks and no intermediate cracks. Inclusion samples were taken from the half-width position of the low-magnification cross-section for analysis. According to GB / T10561-2005, the inclusion grades for categories A, B, C, and D were 1.5, 1.0, 0.5, and 1.0, respectively.

[0083] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing a continuously cast billet for acid-resistant pipeline steel, characterized in that, The process includes sequential hot metal pretreatment, converter smelting, LF refining, RH refining, and continuous casting. The chemical composition of the continuously cast billet is controlled as follows (by mass percentage): C: 0.03~0.07%, Si: 0.10~0.30%, Mn: 0.50~1.00%, Ni: 0.2~0.4%, Nb: 0.03~0.05%, V: 0.05~0.07%, P≤0.010%, S≤0.0010%, Al: 0.02~0.04%, Mo≤0.02%, Ti≤0.0020%, O≤0.0010%, N≤0.0030%, H≤0.00015%, with the balance being Fe and other unavoidable impurities. During the steel tapping process in the converter smelting, when the steel tapping rate is 60t, the intensity of bottom blowing argon gas in the ladle is controlled at 5.0~6.0L / (min·t); when the steel tapping rate is 120t, the intensity of bottom blowing argon gas in the ladle is controlled at 4.1~4.9L / (min·t). During the LF refining process, the intensity of bottom-blown argon gas in the ladle is controlled according to the operation type: when adding alloys to fine-tune the composition, the intensity of bottom-blown argon gas is controlled at 3.1~3.9 L / (min·t); when adding lime and fluorite for slag formation, the intensity of bottom-blown argon gas is controlled at 2.1~2.9 L / (min·t); when adding calcium carbide for deoxidation, the intensity of bottom-blown argon gas is controlled at 1.5~1.9 L / (min·t); when heating with electricity, the intensity of bottom-blown argon gas is controlled at 1.0~1.4 L / (min·t); when there are no operations as described above, the intensity of bottom-blown argon gas is controlled at 0.3~0.7 L / (min·t). In the continuous casting process, a mold flux is used for casting. The binary basicity of the flux, CaO / SiO2, is 1.00~1.15, the melting point is 1030~1090℃, and the viscosity at 1300℃ is 0.12~0.22 Pa·s. In the continuous casting process, the secondary cooling zone is divided into 9 cooling zones: zone 1 corresponds to the foot roll section, zones 2-4 correspond to the zero section, zone 5 corresponds to sector sections 1-2, zone 6 corresponds to sector sections 3-4, zone 7 corresponds to sector sections 5-7, zone 8-9 corresponds to sector section 8, and zone 9 corresponds to sector sections 10-12. The water volume of zones 1-9 is controlled to account for the following proportions of the total water volume of the secondary cooling zone: zone 1 13%-16%, zone 2 14%-17%, zone 3 15%-18%, zone 4 10%-13%, zone 5 7%-10%, zone 6 6%-9%, zone 7 6%-9%, zone 8 5%-7%, and zone 9 10%-13%. In the continuous casting process, the 9#~11# sector segments are controlled to perform light reduction, with a total reduction of 5~7mm, while the 12# sector segment is controlled to perform heavy reduction, with a reduction of 20~25mm.

2. The preparation method according to claim 1, characterized in that, In the hot iron pretreatment process, a desulfurizing agent is used for desulfurization. The mass ratio of CaO to CaF in the desulfurizing agent is (8~10):

1. The consumption of the desulfurizing agent per ton of iron is 5~7kg. The mass percentage of desulfurizing agent particles with a size of 3~5mm is ≥95%, the mass percentage of particles smaller than 3mm is ≤3%, and the mass percentage of particles larger than 5mm is ≤2%. After desulfurization, the hot iron temperature is ≥1300℃, S≤0.0010%, and the slag removal rate is ≥98%.

3. The preparation method according to claim 1 or 2, characterized in that, In the converter smelting process, clean scrap steel and desulfurized molten iron are added for smelting, and the scrap steel ratio is controlled to be ≤23wt%; top and bottom combined blowing is used for decarburization and heating, with a top-blown oxygen flow rate of 38,000~40,000 m³ / h. 3 / h, oxygen supply time 12~14min, bottom blowing gas flow rate 650~670m³ / h 3 / h, bottom blowing carbon dioxide time 30~32min; control the molten steel temperature ≥1630℃, P≤0.010%, S≤0.0015%, C:0.01%~0.03% during tapping; turn on bottom blowing in the ladle during tapping, and add aluminum blocks, ferrosilicon, metallic manganese and nickel plates into the ladle to adjust the composition of the molten steel when the tapping volume is 60t.

4. The preparation method according to claim 1 or 2, characterized in that, In the LF refining process, after the molten steel enters the station, bottom blowing is continuously activated in the ladle. The flow rate of bottom blowing argon is adjusted according to the operation type, and electricity is applied to raise the temperature. Alloys, lime, and synthetic slag are added to adjust the composition of the molten steel and to form slag. The total amount of slag in refining is 12~15 kg / t of steel. The slag composition, by mass percentage, is: CaO: 53%~57%, Al2O3: 25%~28%, SiO2: 5%~7%, MgO: 5%~8%, CaF2: 3%~6%, T.Fe+MnO≤1.0%, and other unavoidable impurities. After LF refining, the tapping temperature is controlled at 1615~1625℃. And / or, after the LF refining is completed, calcium treatment is performed. The calcium wire feed rate Q and the Al content of the molten steel satisfy the following relationship: Q=70+(Al-0.02)×5000, where Al is the mass percentage of aluminum in the molten steel, and the unit of Q is m; the calcium core weight of the pure calcium wire used for calcium treatment is 60~70g / m, the sheet thickness is 2.3~2.7mm, the wire feeding speed is 1.5~2.5m / s, and the Ca content of the molten steel after calcium treatment is 0.0013%~0.0018%.

5. The preparation method according to claim 1 or 2, characterized in that, In the RH refining process, the vacuum chamber pressure is ≤1.5 mbar, the molten steel circulation time is ≥15 min, and the gas flow rate is increased to 150~200 m³ / min. 3 / min; after breaking the void, the Ca content of the molten steel is 0.0005%~0.0008%, H≤0.00018%, and the temperature is 1575~1585℃.

6. The preparation method according to claim 1 or 2, characterized in that, In the continuous casting process, the ladle is hoisted to the continuous casting platform and left to stand for ≥8 minutes; a high-alkalinity covering agent is used in the tundish, with a consumption of 0.3~0.4 kg per ton of steel, a particle size of 2~4 mm accounting for ≥97%, and the composition of the covering agent by mass percentage is SiO2: 10%~15%, CaO: 45%~50%, MgO: 10%~15%, Al2O3: 25%~35%, Fe2O3: 1%~3%; argon is blown throughout the tundish, with an argon blowing flow rate of 150~250 L / min at the long nozzle, 3~5 L / min at the stopper rod and submersible nozzle, 400~500 L / min at the casting zone, and 100~150 L / min at the impact zone; And / or, in the continuous casting process, the superheat of the molten steel in the tundish is controlled to be 30~40℃, the casting speed of the continuous casting machine is 0.9~1.1m / min, the water content in the secondary cooling zone is 0.6~0.7L / kg, and the cross-sectional size of the continuously cast billet is 250mm×(2100~2700)mm.

7. The preparation method according to claim 1 or 2, characterized in that, In the continuous casting process, the chemical composition of the mold flux, by mass percentage, is as follows: SiO2: 27%~29%, CaO: 29%~31%, MgO: 1.0%~3.0%, Fe2O3: 1%~3%, Al2O3: 5%~7%, Na2O: 8%~10%, F: 8%~10%, C: 9%~11%, with the remainder being unavoidable impurities; And / or, in the continuous casting process, the thickness of the protective slag layer in the crystallizer is 80~130mm, the thickness of the liquid slag layer is 8~13mm, and the insertion depth of the tundish immersion nozzle is 130~210mm. And / or, in the continuous casting process, the insertion depth of the tundish submersible nozzle is adjusted according to the cross-sectional width of the continuous casting machine: 150-170mm when the cross-sectional width is 2100-2300mm, 170-190mm when the cross-sectional width is 2300-2500mm, and 190-210mm when the cross-sectional width is 2500-2700mm.

8. The preparation method according to claim 1 or 2, characterized in that, In the continuous casting process, water is distributed in Zone 1 on the narrow side and wide side of the continuous casting billet, with the narrow side accounting for 50%~60% of the water volume in Zone 1 and the wide side accounting for 40%~50% of the water volume in Zone 1; water is distributed in Zones 2, 3, and 4 on the center and edge of the wide side of the continuous casting billet, with the center of the wide side accounting for 30%~40% of the water volume in Zones 2, 3, and 4 and the edge of the wide side accounting for 60%~70% of the water volume in Zones 2, 3, and 4; the corner temperature of the continuous casting billet before entering the No. 5 sector section is ≥910℃, and the difference between the highest and lowest surface temperatures is ≤18℃; And / or, in the continuous casting process, the roll gap shrinkage of the foot roll section and the zero section is 0.05~0.10mm, the roll gap shrinkage of the 1#~4# sector sections is 0.10~0.15mm, and the roll gap shrinkage of the 5#~8# sector sections is 0.15~0.20mm; the reduction of the 9# section accounts for 10%~15% of the total light reduction, the 10# section accounts for 20%~25%, and the 11# section accounts for 60%~70%; the reduction of each roll in the 12# sector section is controlled separately, and the maximum reduction of a single roll is 6mm; the solidification endpoint at the center of the wide face of the continuous casting billet is located within the range of the 11# sector section; after being flame-cut to length, the continuous casting billet is hot-charged and hot-sent for rolling or stacked for cooling.

9. A continuously cast billet for acid-resistant pipeline steel, characterized in that, It is prepared by the method for preparing continuously cast billets for acid-resistant pipeline steel as described in any one of claims 1 to 8.

10. The application of the continuous casting billet for acid-resistant pipeline steel according to claim 9 in the preparation of acid-resistant pipeline steel.