A multi-field coupling dynamic solidification control method for G115 heat-resistant steel continuous casting large round billet

Abstract

A G115 heat-resistant steel continuous casting large round billet multi-field coupling dynamic solidification control method, comprising: 1) the intermediate ladle molten steel superheat is strictly controlled at 40-55 DEG C; 2) three-section electromagnetic stirring is carried out below the crystallizer; 3) two cooling sections are divided, the first section accounts for 38-48% of the total water, the second section accounts for 28-38% of the total water, and the third section accounts for 24-32% of the total water, to inhibit the excessive growth of columnar crystals; 4) a light press + heavy press combined mode is used at the solidification end of the billet, in the light press stage, the solid phase rate fs=0.3-0.7, the single roller reduction is 0.8-1.2 mm, and the cumulative reduction is 4-6 mm; in the heavy press stage, the solid phase rate fs=0.7-0.9, the single roller reduction is 1.5-2.0 mm, and the cumulative reduction is 8-10 mm.

Description

Technical Field

[0001] This invention belongs to the field of continuous casting technology in iron and steel metallurgy, specifically relating to a multi-field coupled dynamic solidification control method for continuous casting of G115 heat-resistant steel large round billets. Background Technology

[0002] With rapid economic development, the demand for energy is increasing. Thermal power generation is a crucial component of my country's energy mix, and the need for high-efficiency, low-emission, high-parameter thermal power units is becoming increasingly urgent. Higher steam temperature and pressure parameters in coal-fired power generation result in lower coal consumption and fewer pollutant emissions, but also place higher demands on material performance. The P92 material used in 600℃ ultra-supercritical units is no longer sufficient for higher-parameter units; and to construct ultra-supercritical units with parameters below 630℃, the high-temperature resistance of materials must reach 650℃.

[0003] G115 is a new type of martensitic heat-resistant steel (see Chinese patent CN103045962B). This heat-resistant steel adopts a composite strengthening principle and contains multiple strengthening elements such as W, Co, Cu, B, and N. Its high-temperature long-term aging stability, high-temperature creep strength, and oxidation resistance are significantly higher than P92. It is suitable for long-term use under high-pressure conditions at temperatures below 650℃. Comparative studies have found that G115 has better creep strength and oxidation resistance at 650℃ than P92 at 600℃.

[0004] To improve the yield and further refine the process, the G115 continuous casting process was developed. However, due to the high content of G115 alloy, the complexity of alloying elements, the wide solidification range, and the low thermal conductivity, there are two major technical bottlenecks in the continuous casting of large cross-section round billets (φ≥600mm): 1) well-developed columnar crystals and an equiaxed crystal ratio generally below 30%, which significantly increases the risk of hot working cracking; 2) severe central segregation of elements such as W and B, which deteriorates the isotropy of the material.

[0005] In the existing technology, Chinese patent CN103045962B mainly involves the composition design, strengthening concept, manufacturing method and performance characteristics of G115, but this technology does not involve the technology related to continuous casting of G115.

[0006] Chinese patent CN108998650A relates to a method for manufacturing G115 large-diameter thick-walled seamless steel pipes for 630℃ ultra-supercritical units. This patent only relates to the pipe-making process of G115 large-diameter thick-walled seamless steel pipes and does not involve the technology related to G115 continuous casting.

[0007] Chinese patent CN108950148A relates to a method for improving the radial microstructure and performance uniformity of G115 large-diameter thick-walled pipes. However, this technology does not involve the continuous casting protective slag specifically for G115.

[0008] Chinese patent CN108672666A discloses a method for improving center segregation in continuously cast round billets of spring steel. This method controls factors such as the tundish temperature, crystallizer water volume, secondary cooling zone water volume, electromagnetic stirring parameters, and the position of the final electromagnetic stirring point during the continuous casting process. This reduces center segregation in the billet, minimizes plasticity discrepancies caused by segregation, and improves the product yield. However, this method only applies to 60Si2CrVAT spring steel with a diameter of φ380mm. Since G115 has a much higher alloy content than 60Si2CrVAT spring steel, the technology described in this patent is not suitable for improving segregation in large-diameter continuously cast round billets of G115.

[0009] Chinese patent CN111940690A discloses an electromagnetic feeding control method for continuous casting of large-section billets. By installing an electromagnetic stirring and heating device at the end of the continuous casting machine, and applying an alternating magnetic field and Joule heating, the method promotes the flow of molten metal within the billet for feeding, refines the grain structure, and facilitates end-of-solidification feeding, thereby improving the quality of large-section billets. This technology only involves end-of-situ electromagnetic stirring and heating, and the stirring electromagnetic field is a fixed alternating electromagnetic field.

[0010] Chinese patent CN109097681A discloses a high-strength, low-inclusion automotive steel sheet and its electromagnetic stirring process for continuous casting. The process is mainly for continuous casting of dual-phase steel billets for automotive sheets. The electromagnetic stirring only involves crystallizer stirring and solidification end stirring, and the frequency is a fixed frequency. It is not suitable for continuous casting of large-size G115 round billets, as G115 molten steel is viscous and the solidification process is complex.

[0011] Chinese patent CN101244452A discloses a method for determining electromagnetic stirring parameters in a round billet continuous casting mold. The method primarily focuses on quickly determining the current intensity and frequency of electromagnetic stirring under different conditions to improve the billet microstructure and increase the equiaxed crystal ratio. However, this method is only suitable for setting the electromagnetic stirring parameters in the mold and is limited to fixed frequencies. It cannot meet the requirements of G115, which requires alternating magnetic field stirring and necessitates the superposition of multi-field coupled dynamic solidification control technology.

[0012] Therefore, in order to address the problems of high alloy content, low carbon content, and the presence of easily segregating elements such as W and B in G115 steel, resulting in viscous molten steel, well-developed columnar crystals, and severe segregation during continuous casting, which would seriously affect the microstructure and properties of the subsequent finished products, it is urgent to develop a process for controlling the solidification structure during G115 continuous casting to solve the problem of continuous casting of large-sized round billets of G115 steel. Summary of the Invention

[0013] The purpose of this invention is to provide a multi-field coupled dynamic solidification control technology for G115 heat-resistant steel continuously cast large round billets. This technology can effectively control the solidification structure of G115 large-size continuously cast round billets, solving the problems caused by the viscosity, well-developed columnar crystals, and severe segregation of G115 molten steel. This results in an equiaxed crystal ratio of over 40% for G115 large-size continuously cast billets, reducing central cracking, porosity, shrinkage cavities, and surface cracks. Carbon segregation is reduced by more than 5%, tungsten segregation by more than 45%, cobalt segregation by more than 18%, and boron segregation by more than 112%. This technology can produce G115 large-size continuously cast round billets with a diameter of φ600~1200mm. G115 steel pipes / forgings produced using this technology meet the requirements of standards such as CSTM 00017-2021, Q / OAPD 2753-2022, and Q / OAPD 2253-2022.

[0014] To achieve the above objectives, the technical solution of the present invention is as follows: A multi-field coupled dynamic solidification control method for continuous casting large round billets of G115 heat-resistant steel, comprising: 1) The superheat of molten steel in the tundish must be strictly controlled between 40 and 55°C; 2) The continuous casting billet pulling speed is 0.40~1.50m / min; 3) Three-section electromagnetic stirring is performed below the crystallizer, wherein, In section I, at a depth of 0.2 to 1.0 m below the meniscus, the crystallizer is subjected to electromagnetic stirring (M-EMS) with a low-frequency rotating magnetic field of 2 to 4 Hz. In section II, which is 3.5 to 4.5 m from the meniscus, corresponding to the solid fraction fs = 0.4 to 0.6, electromagnetic stirring S-EMS in the secondary cooling zone is carried out. In section III, which is 3-5m before the end of solidification, corresponding to the solid fraction fs=0.7-0.8 region, electromagnetic stirring F-EMS at the end of solidification is performed, and a pulsed magnetic field with a high frequency oscillation of 5-8Hz is applied. 4) The secondary cooling zone adopts segmented cooling, with the first segment accounting for 38-48% of the total water volume, the second segment accounting for 28-38% of the total water volume, and the third segment accounting for 24-32% of the total water volume, to inhibit the excessive growth of columnar crystals; 5) At the end of the solidification of the billet, a combination of light and heavy pressing is adopted. In the light pressing stage, the solid fraction fs = 0.3~0.7, the single roll pressing amount is 0.8~1.2mm, and the cumulative pressing amount is 4~6mm; in the heavy pressing stage, the solid fraction fs = 0.7~0.9, the single roll pressing amount is 1.5~2.0mm, and the cumulative pressing amount is 8~10mm.

[0015] The composition by weight percentage of the G115 heat-resistant steel continuously cast large round billet described in this invention is as follows: C 0.060~0.100%, Si≤0.55%, Mn 0.27~0.73%, P≤0.020%, S≤0.010%, Cr 8.40~9.60%, W 2.33~3.17%, Co 2.80~3.25%, Cu 0.40~1.20%, V 0.13~0.27%, Nb 0.03~0.10%, N 0.005~0.019%, B 0.008~0.022%, Ni≤0.13%, Ti≤0.02%, Al≤0.015%, O≤0.0040%, As≤0.015%, Sb≤0.015%, Bi≤0.005%, Sn≤0.020%, Pb≤0.015%, As+Sb+Bi+Sn+Pb≤0.035%, balance is Fe and unavoidable impurities.

[0016] Preferably, a stopper rod and an immersion-type argon gas curtain are used for protection to prevent secondary oxidation.

[0017] Preferably, the crystallizer is subjected to electromagnetic stirring (M-EMS) at a depth of 0.2-1.0m below the meniscus in section I, i.e., the crystallizer is subjected to a low-frequency rotating magnetic field of 2-4Hz, i.e., a magnetic induction intensity of 80-120mT and a current intensity of 250-350A. The stirring mode is unidirectional rotation with periodic reversal—reversing once every 30 seconds, which forces the primary dendrites to melt and increases the crystal nucleus density.

[0018] Preferably, in section II, which is about 3.5 to 4.5 meters from the meniscus, corresponding to the solid fraction fs=0.4 to 0.6 region, a second-cooling zone electromagnetic stirring S-EMS is performed, applying a 5 to 10 Hz traveling wave magnetic field (axial component intensity 50 to 80 mT) and a static magnetic field (radial component intensity 150 to 200 mT), and a current of 400 to 500 A, to form a Lorentz force and thermo-electromagnetic force coupling effect, which inhibits the longitudinal growth of columnar crystals.

[0019] Preferably, in section III, i.e., 3-5m before the end of solidification, electromagnetic stirring (F-EMS) is performed in the region with a solid fraction fs = 0.7-0.8. A high-frequency oscillating pulsed magnetic field is applied, with a frequency of 5-8Hz, a current of 800-1100A, a peak intensity of 300-500mT, and an oscillation angle of 5-10°, to induce liquid reciprocating between dendrites in the paste-like region and promote solute homogenization.

[0020] Preferably, the water content in the secondary cooling zone is 1.10~1.50L / kg, and the surface temperature of the billet is ≥1050℃ for the foot roll section and ≥850℃ for the straightening point.

[0021] Preferably, different casting speeds correspond to different specifications of continuously cast billets, among which, Continuous casting billet Φ600~800mm, casting speed 1.05~1.50 m / min; Continuous casting billet Φ800~1000mm, casting speed 0.60~1.05 m / min; The continuous casting billet is Φ1000~1200 mm, and the casting speed is 0.40~0.60 m / min.

[0022] In the multi-field coupled dynamic solidification control method for continuous casting large round billets of G115 heat-resistant steel described in this invention: Due to the high content of G115 alloy and the viscosity of the molten steel, in order to ensure smooth casting and control of solidification structure, the superheat of the molten steel in the tundish is strictly controlled at 40~55℃. A stopper rod + immersion nozzle argon curtain protection is used to prevent secondary oxidation.

[0023] This invention employs a three-section electromagnetic stirring system.

[0024] In section I (0.2-1.0 m below the meniscus), electromagnetic stirring (M-EMS) is performed in the crystallizer. A low-frequency, periodically commutating rotating magnetic field is applied for stirring. The low-frequency magnetic field can effectively penetrate the thick billet shell, breaking up the dendrites at the solidification front and forming a large number of free crystal nuclei, significantly increasing the equiaxed crystal ratio. The periodically commutating rotating magnetic field can prevent the molten steel from scouring the solidified billet shell and causing localized excessive thinning, while also avoiding slag entrapment in the protective slag. Since G115 alloy content is high, it will form various complex inclusions. Intense stirring in the early stages of solidification, when the liquidus cavity is widest, can break up primary boron-containing inclusions (such as BN) and disperse them, effectively suppressing macroscopic boron segregation. The low-frequency rotating magnetic field has a frequency of 2-4 Hz (magnetic induction intensity 80-120 mT), a current of 250-350 A, and a unidirectional rotation stirring mode with periodic commutation (commutation every 30 seconds).

[0025] In section II (3.5-4.5 meters from the meniscus, corresponding to a solidity fs = 0.4-0.6), secondary cooling zone electromagnetic stirring (S-EMS) is performed. A composite magnetic field, combining traveling wave and static magnetic fields, is applied to create a Lorentz force coupled with thermo-electromagnetic force. This effectively penetrates the thick solidified shell, strongly stirring the liquid core, breaking up the well-developed columnar crystals, and inhibiting their longitudinal growth. This section is a key area for the transformation of columnar crystals into equiaxed crystals. Strong stirring can maximize the equiaxed crystal ratio. G115 contains 3% W and 0.015% B. W and B are key strengthening elements, and their uniform dispersion will seriously affect the performance of the finished G115 product. Strong electromagnetic stirring in this section can greatly promote the diffusion and redistribution of solute elements such as W and B, effectively reducing interdendritic segregation and laying a good foundation for subsequent homogenization heat treatment. The applied magnetic field parameters are a 5~10Hz traveling wave magnetic field (axial component intensity 50-80mT) and a static magnetic field (radial component intensity 150-200mT), and a current of 400~500A.

[0026] In section III (3-5m before the end of solidification, corresponding to the solidification fraction fs=0.7-0.8 region), electromagnetic stirring (F-EMS) is performed at the end of solidification. A high-frequency oscillating pulsed magnetic field is applied to perform weak stirring with high-frequency, small-angle oscillation, inducing liquid reciprocating flow between dendrites in the pasty zone and promoting solute homogenization. This section has entered the final stage of solidification, and the liquid core is very narrow. The high-frequency magnetic field can effectively act, and through gentle stirring, the solute-rich molten steel is squeezed back to the center, compensating for solidification shrinkage, thereby significantly reducing central porosity and V-type segregation. G115 has a high alloy content and viscous molten steel. In the final stage of solidification, alloying elements such as Cr, W, Co, and B segregate more severely in the center. Moreover, G115 continuous casting billets have extremely high requirements for center quality. Any central defect will become a crack source during subsequent high-temperature service. F-EMS is the key to ensuring the quality of the core of the continuous casting billet. The applied magnetic field parameters are: frequency 5~8Hz, current 800~1100A, peak intensity 300-500mT, and swing angle 5~10°.

[0027] The continuous casting secondary cooling zone adopts a segmented approach, with cooling occurring in three stages. The first stage has the strongest cooling, accounting for 38-48% of the total water volume, increasing the solidified billet shell thickness from 22%R to 50%R. This ensures that the surface shell strength is sufficient to withstand the molten steel in the core and the tension of the straightening machine, preventing steel leakage. The second stage has a slightly reduced cooling intensity, accounting for 28-38% of the total water volume, minimizing thermal stress caused by temperature drop, and achieving a solidified billet shell thickness of 70%R. The third stage has a further reduced cooling intensity, accounting for 24-32% of the total water volume, allowing the billet surface to warm up and preventing surface cracks. The use of weak cooling extends the liquid core length, which helps ensure sufficient feeding.

[0028] G115 is a high-alloy heat-resistant steel with poor plasticity at high temperatures. It requires a weak cooling mode. If strong cooling is used, it will cause an excessive temperature difference between the surface and the center of the billet, generating huge thermal stress, which can easily induce surface and internal cracks. Weak cooling will allow the billet to cool evenly, reduce thermal stress, prevent crack formation, and slow down the solidification rate, providing a longer time window for the growth of equiaxed crystals and the effect of EMS. The specific water content is 1.10~1.50L / kg. The water content in each zone is dynamically adjusted based on surface temperature feedback. The surface temperature of the billet is ≥1050℃ for the foot roll section and ≥850℃ for the straightening point.

[0029] Preferably, different casting speeds correspond to different specifications of continuous casting billets: Φ600~800mm continuous casting billet, casting speed 1.05~1.50 m / min; Φ800~1000mm continuous casting billet, casting speed 0.60~1.05 m / min; Φ1000~1200 mm continuous casting billet, casting speed 0.40~0.60 m / min.

[0030] At the end of the solidification process of the billet, a combination of light and heavy pressure is used to suppress central porosity and segregation. From the perspective of the solidification defect formation mechanism, central porosity and segregation originate from the volume shrinkage and crystal bridge closure effect at the end of solidification. The solidification time of large-size G115 continuously cast round billets is longer, the crystal bridge phenomenon is more obvious, and the defects are more severe. It is necessary to break the crystal bridges by light and heavy pressure to compensate for solidification shrinkage, compact the loose voids in the central area of ​​the billet, prevent or at least greatly suppress the lateral flow of solute-enriched molten steel to the central area, thereby reducing the formation of central porosity and segregation. Light and heavy pressure will induce the reverse flow of G115 solute, promoting homogenization. According to the calculation of the multiphase solidification physics model, when a billet with a certain liquid core is pressed, the strong external force will induce a reverse flow field inside the billet that is opposite to the direction of billet pulling. This reverse flow can agitate the molten steel in the core of the billet that has not yet fully solidified, allowing it to mix thoroughly with the surrounding molten steel with relatively uniform composition, thereby macroscopically diluting and homogenizing the solute concentration gradient caused by selective crystallization. During the light reduction stage (solid fraction fs = 0.3-0.7), the single-roller reduction is 0.8-1.2 mm, and the cumulative reduction is 4-6 mm, to compensate for solidification shrinkage. During the heavy pressure stage (fs=0.7-0.9), the single roller reduction is 1.5-2.0mm, the cumulative reduction is 8-10mm, and the paste-like area channel is forcibly closed.

[0031] Compared with the prior art, the beneficial effects of the present invention are as follows: In response to the challenges of high alloy content, viscous molten steel, complex solidification process, well-developed columnar crystals and severe segregation during continuous casting of G115 steel, as well as the control requirements for central shrinkage cavities, central cracks, porosity, and segregation in large-size continuously cast round billets, this invention achieves synergistic control of high axial crystal ratio and low central segregation in large-size continuously cast round billets of G115 through a multi-field coupled dynamic control method of temperature field, solute field, and stress field.

[0032] In terms of temperature field, it is mainly achieved through the control of superheat and cooling method. Due to the viscosity of G115 molten steel, in order to balance smooth casting and control of solidification structure, the superheat of molten steel in the tundish is strictly controlled at 40~55℃. During the cooling stage, a weak cooling strategy is adopted, and cooling is carried out in three stages. Weak cooling will make the billet cool evenly, reduce thermal stress, prevent cracking, and slow down the solidification rate, providing a longer time window for the growth of equiaxed crystals and the effect of EMS.

[0033] Regarding the solute field, a three-section electromagnetic stirring system is mainly used. Based on the solidification characteristics of G115 at each stage, a dedicated electromagnetic stirring method is designed. The crystallizer electromagnetic stirring (M-EMS) employs a low-frequency, periodically commutating rotating magnetic field, which effectively penetrates thicker billet shells, breaking down dendrites at the solidification front and forming numerous free crystal nuclei, significantly increasing the equiaxed crystal ratio. The periodically commutating rotating magnetic field prevents the molten steel from scouring the solidified billet shell, causing localized excessive thinning, and also avoids slag entrapment in the protective slag. The secondary cooling zone electromagnetic stirring (S-EMS) uses a traveling wave... The composite magnetic field stirring, which couples the magnetic field with the static magnetic field, forms a Lorentz force coupled with thermo-electromagnetic force, which effectively penetrates the thick solidified shell and strongly stirs the liquid core, breaking up the well-developed columnar crystals and inhibiting their longitudinal growth. The electromagnetic stirring at the solidification end (F-EMS) uses a high-frequency oscillating pulsed magnetic field to perform weak stirring with high-frequency, small-angle oscillation, inducing liquid reciprocating flow between dendrites in the pasty region, promoting solute homogenization, and squeezing the solute-enriched molten steel back to the center through gentle stirring, compensating for solidification shrinkage, thereby significantly reducing central porosity and V-shaped segregation.

[0034] Regarding the stress field, a combination of light and heavy pressure is mainly used to suppress central porosity and segregation. Central porosity and segregation originate from volume shrinkage and crystal bridge closure effects at the end of solidification. G115 large-size continuously cast round billets have longer solidification times, more pronounced crystal bridge phenomena, and more severe defects. Therefore, light and heavy pressure are needed to break down the crystal bridges, compensate for solidification shrinkage, and compact the loose voids in the central region of the billet. This prevents or at least greatly inhibits the lateral flow of solute-rich molten steel towards the central region, thereby mitigating the formation of central porosity and segregation. Light and heavy pressure also induce reverse flow of G115 solute, promoting homogenization. According to calculations from a multiphase solidification physics model, when a billet with a certain liquid core is pressed down, the strong external force will induce a reverse flow field inside the billet, opposite to the direction of billet pulling. This reverse flow can agitate the molten steel in the core of the billet that has not yet fully solidified, allowing it to mix thoroughly with the surrounding molten steel, which has a relatively uniform composition. This macroscopically dilutes and homogenizes the solute concentration gradient caused by selective crystallization.

[0035] This invention utilizes the aforementioned dynamic control method involving the coupling of temperature, solute, and stress fields to effectively control central shrinkage cavities, central cracks, porosity, and segregation in large-diameter G115 continuously cast round billets (φ600~φ1200mm). Segregation can be reduced by 35%. Continuously cast billets produced using this method, after subsequent processing into G115 steel pipes, meet the performance requirements of standards such as CSTM 00017-2021 and Q / OAPD2253-2022. Detailed Implementation

[0036] The present invention will be further described below with reference to the embodiments.

[0037] The process parameters of this invention are shown in Tables 1 and 2. The low-magnification inspection results of the produced G115 continuous casting round billets are shown in Table 3. The segregation inspection results are shown in Table 4. The inclusion inspection results are shown in Table 5. The mechanical properties of the rolled tubes are shown in Table 6.

[0038] Example 1 A multi-field coupled dynamic solidification control method for G115 heat-resistant steel continuously cast large round billets includes: electric arc furnace or converter steelmaking, LF furnace refining, RH or VD vacuum degassing, and continuous casting + continuous casting billet annealing; wherein... 1) The superheat of the molten steel in the tundish must be strictly controlled at 55℃; 2) The continuous casting billet pulling speed is 0.40 m / min; 3) Three-section electromagnetic stirring is performed below the crystallizer, wherein, In section I, at a depth of 0.2~1.0m below the meniscus, the crystallizer is subjected to electromagnetic stirring (M-EMS), with a low-frequency rotating magnetic field of 2-4Hz (magnetic induction intensity 120mT), a current intensity of 350A, and a stirring mode of unidirectional rotation with periodic reversal—reversing once every 30 seconds), which forces the primary dendrites to melt and increases the crystal nucleus density. In section II, at a distance of 3.5 to 4.5 m from the meniscus, corresponding to a solid fraction fs≈0.4 to 0.6, electromagnetic stirring (S-EMS) in the second cooling zone is performed. A traveling wave magnetic field (axial component strength 80 mT) and a static magnetic field (radial component strength 200 mT) of 5 to 10 Hz are applied, with a current of 500 A, to form a coupling effect between Lorentz force and thermo-electromagnetic force, which inhibits the longitudinal growth of columnar crystals. In section III, which is 3-5m before the end of solidification, corresponding to the solid fraction fs=0.7-0.8 region, electromagnetic stirring F-EMS is performed at the end of solidification. A high-frequency oscillating pulsed magnetic field (frequency 5-8Hz, current 1100A, peak intensity 500mT, oscillation angle 10°) is applied to induce liquid re-flow between dendrites in the paste-like region and promote solute homogenization. 4) The secondary cooling zone of continuous casting adopts a multi-stage air-water spray cooling method with a specific water volume of 1.50 L / kg. The first stage accounts for 38% of the total water volume, the second stage accounts for 36% of the total water volume, and the third stage accounts for 26% of the total water volume, which inhibits the excessive growth of columnar crystals. 5) At the end of the solidification of the billet, a combination of light and heavy pressing is adopted. In the light pressing stage, the solid fraction fs = 0.3~0.7, the single roll pressing amount is 0.8~1.2mm, and the cumulative pressing amount is 6mm; in the heavy pressing stage, the solid fraction fs = 0.7~0.9, the single roll pressing amount is 1.5~2.0mm, and the cumulative pressing amount is 10mm.

[0039] The comparative example is a G115 continuously cast billet produced according to the P92 continuous casting process, with a specification of φ1000mm. The process parameters are shown in Table 1 and Table 2.

[0040] As can be seen from Table 3, compared with the comparative example, the general porosity of the present invention is grade 0.5, which is better than grade 1.0 of the comparative example; the intermediate porosity and central crack of the present invention are both grade 0, which is better than grade 0.5 of the comparative example; the shrinkage cavity of the present invention is grade 1, which is better than grade 2 of the comparative example; the tungsten surface cracks of the present invention are 20~35mm, while the comparative example has surface cracks; the equiaxed crystal ratio of the present invention is 40.7~48.9%, which is far better than 24.4% of the comparative example.

[0041] As can be seen from Table 4, the center segregation indices of carbon, tungsten, cobalt, and boron in this invention are all superior to those in the comparative example. The segregation of carbon is reduced by more than 5%, the segregation of tungsten is reduced by more than 45%, the segregation of cobalt is reduced by more than 18%, and the segregation of boron is reduced by more than 112%, which significantly reduces the segregation of key elements. The method of this invention has a significant effect on controlling segregation.

[0042] As can be seen from Table 5, the fine inclusions of the present invention are all ≤0.5 grade, and the coarse inclusions are all 0 grade, which is better than the fine inclusions of the comparative example which are 0.5~1.0 grade and the coarse inclusions are 0~0.5 grade.

[0043] As can be seen from Table 6, the mechanical properties of the present invention are comparable to those of the comparative example, and slightly higher. Specifically, the yield strength is 3% higher than that of the comparative example, the tensile strength is 4% higher than that of the comparative example, and the impact energy is 18% higher than that of the comparative example.

[0044] In summary, the forgings, steel pipes, and other products produced from the continuous casting billets of martensitic heat-resistant steel G115 prepared by this invention all meet the CSTM standards and enterprise standards such as "CSTM 00017-2021 Seamless steel pipes of martensitic heat-resistant steel 08Cr9W3Co3VNbCuBN (G115) for power plants", "Q / OAPD 2753-2022 Tube blanks and profiles of new martensitic heat-resistant steel 08Cr9W3Co3VNbCuBN (G115) for power plants" and "Q / OAPD 2253-2022 Seamless steel pipes of new martensitic heat-resistant steel 08Cr9W3Co3VNbCuBN (G115) for power plants".

[0045] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

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Claims

1. A method for controlling the dynamic solidification of G115 heat-resistant steel continuously cast large round billets through multi-field coupling, characterized in that, include: 1) The superheat of the molten steel in the tundish should be controlled at 40~55℃; 2) The continuous casting billet pulling speed is 0.40~1.50m / min; 3) Three-section electromagnetic stirring is performed below the crystallizer, wherein, In section I, at a depth of 0.2 to 1.0 m below the meniscus, the crystallizer is subjected to electromagnetic stirring (M-EMS) with a low-frequency rotating magnetic field of 2 to 4 Hz. In section II, which is 3.5 to 4.5 m from the meniscus, corresponding to the solid fraction fs = 0.4 to 0.6, electromagnetic stirring S-EMS in the secondary cooling zone is carried out. In section III, which is 3-5m before the end of solidification, corresponding to the solid fraction fs=0.7-0.8 region, electromagnetic stirring F-EMS at the end of solidification is performed, and a pulsed magnetic field with a high frequency oscillation of 5-8Hz is applied. 4) The secondary cooling zone adopts segmented cooling, with the first segment accounting for 38-48% of the total water volume, the second segment accounting for 28-38% of the total water volume, and the third segment accounting for 24-32% of the total water volume, to inhibit the excessive growth of columnar crystals; 5) At the end of the solidification of the billet, a combination of light and heavy pressing is adopted. In the light pressing stage, the solid fraction fs = 0.3~0.7, the single roll pressing amount is 0.8~1.2mm, and the cumulative pressing amount is 4~6mm; in the heavy pressing stage, the solid fraction fs = 0.7~0.9, the single roll pressing amount is 1.5~2.0mm, and the cumulative pressing amount is 8~10mm.

2. The multi-field coupled dynamic solidification control method for G115 heat-resistant steel continuous casting large round billets as described in claim 1, characterized in that, A stopper rod and an immersion-type argon gas curtain are used for protection to prevent secondary oxidation.

3. The multi-field coupled dynamic solidification control method for G115 heat-resistant steel continuous casting large round billets as described in claim 1, characterized in that, In section I, i.e., 0.2~1.0m below the meniscus, the crystallizer is subjected to electromagnetic stirring (M-EMS). A low-frequency rotating magnetic field of 2~4Hz (magnetic induction intensity of 80~120mT) and a current intensity of 250~350A are applied. The stirring mode is unidirectional rotation with periodic reversal—reversing once every 30 seconds.

4. The multi-field coupled dynamic solidification control method for G115 heat-resistant steel continuous casting large round billets as described in claim 1, characterized in that, In section II, which is about 3.5 to 4.5 meters from the meniscus, corresponding to the solid fraction fs=0.4 to 0.6 region, electromagnetic stirring S-EMS in the second cooling zone is performed, with a 5 to 10 Hz traveling wave magnetic field (axial component intensity 50 to 80 mT) and a static magnetic field (radial component intensity 150 to 200 mT) and a current of 400 to 500 A applied.

5. The multi-field coupled dynamic solidification control method for G115 heat-resistant steel continuous casting large round billets as described in claim 1, characterized in that, In section III, 3-5m before the end of solidification, electromagnetic stirring (F-EMS) is performed in the region with a solid fraction fs = 0.7-0.

8. A high-frequency oscillating pulsed magnetic field is applied with a frequency of 5-8Hz, a current of 800-1100A, a peak intensity of 300-500mT, and an oscillation angle of 5-10°.

6. The multi-field coupled dynamic solidification control method for G115 heat-resistant steel continuous casting large round billets as described in claim 1, characterized in that, The water content in the secondary cooling zone is 1.10~1.50L / kg, and the surface temperature of the billet is ≥1050℃ at the foot roll section and ≥850℃ at the straightening point.

7. The multi-field coupled dynamic solidification control method for G115 heat-resistant steel continuous casting large round billets as described in claim 1, characterized in that, Different specifications of continuously cast billets correspond to different casting speeds, among which, Continuous casting billet Φ600~800mm, casting speed 1.05~1.50 m / min; Continuous casting billet Φ800~1000mm, casting speed 0.60~1.05 m / min; The continuous casting billet is Φ1000~1200 mm, and the casting speed is 0.40~0.60 m / min.

8. The multi-field coupled dynamic solidification control method for G115 heat-resistant steel continuous casting large round billets as described in claim 1, characterized in that, The composition by weight percentage of the G115 martensitic heat-resistant steel continuously cast large round billet is as follows: C 0.060~0.100%, Si≤0.55%, Mn 0.27~0.73%, P≤0.020%, S≤0.010%, Cr 8.40~9.60%, W 2.33~3.17%, Co 2.80~3.25%, Cu 0.40~1.20%, V 0.13~0.27%, Nb 0.03~0.10%, N 0.005~0.019%, B 0.008~0.022%, Ni≤0.13%, Ti≤0.02%, Al≤0.015%, O≤0.0040%, As≤0.015%, Sb≤0.015%, Bi≤0.005%, Sn≤0.020%, Pb≤0.015%, As+Sb+Bi+Sn+Pb≤0.035%, balance is Fe and unavoidable impurities.

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