A continuous casting method for G115 heat-resistant steel round billets

Abstract

The application discloses a G115 heat-resistant steel round billet continuous casting method, wherein according to the G115 component, steelmaking raw materials are subjected to electric arc furnace or converter steelmaking, LF furnace refining and RH or VD vacuum degassing to obtain molten steel, the overheat degree of the continuous casting process is controlled to be 40-55 DEG C, the pulling speed is 0.40-1.50 m / min, the specific water quantity of the secondary cooling is 1.10-1.50 L / kg, three-stage partition cooling, mold electromagnetic stirring, secondary cooling zone electromagnetic stirring and solidification end electromagnetic stirring are adopted, the quality of the continuous casting billet can be effectively controlled, the light press-down and the heavy press-down are combined at the solidification end of the continuous casting billet to inhibit the center porosity and segregation, the G115 special protective slag and the double-layer gradient pore size ceramic filter are used, the molten steel is subjected to continuous casting to obtain a φ600-φ1200 mm continuous casting large round billet, the surface crack and the center crack defects of the G115 continuous casting round billet can be avoided, and the performance requirements of the G115 steel used in advanced ultra-supercritical power stations can be met.

Description

Technical Field

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

[0002] With rapid economic development, the demand for energy is increasing. Thermal power generation is an important part of my country's energy mix. However, the environmental pollution problems caused by thermal power generation are increasingly conflicting with my country's ecological civilization construction and environmental protection concepts. Therefore, the demand for high-efficiency, low-emission, high-parameter thermal power units is becoming increasingly urgent. The higher the steam temperature and pressure parameters of coal-fired power generation, the lower the coal consumption and the less pollutant emissions, but the higher the performance requirements for materials. The P92 used in 600℃ ultra-supercritical units can no longer meet the requirements of higher parameter units; and to build ultra-supercritical units with parameters of 630℃ or higher, the high-temperature resistance of materials must reach 650℃.

[0003] G115 is a new type of martensitic heat-resistant steel, as shown in Chinese patent number CN103045962B. This heat-resistant steel adopts a composite strengthening principle, containing 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's creep strength and oxidation resistance at 650℃ are superior to P92's performance at 600℃. The existing process route of ingot casting + (electroslag) + forging + pipe making is basically mature. G115 has also achieved its first engineering application in the world's first 630℃ ultra-supercritical demonstration project.

[0004] However, due to limitations in existing processes, especially the die casting process, the yield is low and there is room for performance improvement, which is not conducive to the large-scale promotion and application of G115.

[0005] Chinese patent CN103045962B mainly involves the composition design, strengthening concept, manufacturing method, and performance characteristics of G115, but the technology does not involve the specific smelting and continuous casting methods 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 include smelting and continuous casting processes.

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

[0008] Chinese patent CN102019389A discloses a continuous casting method for P91 steel round billets, which mainly involves a secondary cooling method to avoid surface cracks and center cracks in the P91 continuous casting round billets. The specifications of the produced P91 continuous casting billets range from φ280 to 350mm. It does not involve the continuous casting production of G115 round billets, nor does it involve the production process of continuous casting billets with a diameter of φ350 or larger.

[0009] Chinese patent CN115044823A discloses a production process for P92 steel continuous casting large round billets, which adopts EAF+LF+VD smelting and continuously casts P92 large round billets with diameters of φ450~φ700mm. It does not involve the continuous casting production of G115 round billets, nor does it involve the production process of continuous casting billets with diameters of φ700 or larger.

[0010] Chinese patent CN115216686A discloses a method for manufacturing P92 steel continuously cast round billets, which adopts a converter + LF furnace + RH furnace steelmaking process to continuously cast round billets with a diameter of φ800mm or less. It does not involve the continuous casting production of G115 round billets, nor does it involve the production process of continuously cast billets with a diameter of φ800 or more.

[0011] Chinese patent CN116083781A discloses a method for manufacturing large-size continuous casting round billets of P92 heat-resistant steel without high-temperature ferrite structure. It adopts EAF+LF+VD smelting and the continuous casting specification is φ690mm round billet. It does not involve the continuous casting production of G115 round billets, nor does it involve the production process of continuous casting billets with a diameter of φ690mm or larger.

[0012] The aforementioned patents related to continuous casting of martensitic heat-resistant steel all concern the steelmaking and continuous casting processes of P91 and P92. The alloy content of P91 and P92 is significantly lower than that of G115, making their steelmaking and continuous casting processes far less difficult. Furthermore, G115 requires the production of large-diameter pipes, necessitating large-diameter continuously cast round billets with a diameter of φ600 or larger. The larger the billet, the more complex the solidification process, the more severe the segregation, and the more prone it is to central shrinkage cavities, porosity, segregation, and cracking, thus increasing the difficulty of continuous casting. G115 steel is characterized by its "multi-element composite strengthening" design, with high alloy content (Cr 9%, W 3%, Co 3%), leading to increased segregation tendency, well-developed columnar crystals, and complex precipitates that reduce high-temperature plasticity and result in large solidification shrinkage. In addition, the molten steel is viscous, has poor fluidity, and poor solidification feeding, making it difficult to control the internal quality of the billet. Existing continuous casting technology cannot meet the production requirements of large-diameter continuously cast round billets of G115 steel with a diameter of φ600~1200mm. Therefore, there is an urgent need to develop innovative processes suitable for continuous casting of large-size round billets of G115 steel. Summary of the Invention

[0013] The purpose of this invention is to provide a continuous casting method for G115 heat-resistant steel round billets. This method addresses the unique characteristics of G115 steel by avoiding central cracks and porosity in the billet, reducing segregation and shrinkage cavities, and preventing surface cracks. It achieves high-quality production of large-diameter round billets (φ600~φ1200mm). Using continuously cast round billets for large-diameter pipe production can increase the yield by 30%, significantly reduce compositional segregation, and improve strength and impact resistance. The performance meets the requirements of standards such as CSTM 00017-2021, Q / OAPD 2753-2022, and Q / OAPD2253-2022.

[0014] To achieve the above objectives, the technical solution of the present invention is as follows: A continuous casting method for G115 heat-resistant steel round billets specifically includes the following steps: 1) Steelmaking in electric arc furnaces or converters The C content in the molten steel is ≤0.04%, the P content is ≤0.005%, and the steel temperature is ≥1630℃. 2) LF furnace refining Argon gas is used for stirring during the refining process, with an argon gas flow rate of 100~300NL / min; ferrotungsten is added in batches, with each batch ≤600kg, and the temperature of the molten steel is controlled to ≥1630℃ when adding ferrotungsten. 3) RH or VD vacuum degassing Vacuum degree ≤67Pa, vacuum holding time ≥20min, after vacuum breaking, add ferroboron to ensure boron content is 0.012~0.018%; 4) Continuous casting When the molten steel is transferred to the tundish, it is protected by argon blowing through a long nozzle. The molten steel in the tundish is injected into the crystallizer through an immersion nozzle. A double-layer gradient pore size ceramic filter is installed in the tundish. Protective slag is added to the crystallizer for full protective casting. Multiple electromagnetic stirring is performed in the crystallizer, secondary cooling zone, and solidification end. The continuous casting billet casting speed is 0.40~1.50m / min, and the continuous casting billet straightening temperature is ≥850℃. The continuous casting secondary cooling zone adopts a three-stage cooling system. The total specific water volume of the secondary cooling zone is 1.10~1.50L / kg. The cooling water volume of the first stage accounts for 37~47% of the total water volume, the cooling water volume of the second stage accounts for 30~38% of the total water volume, and the cooling water volume of the third stage accounts for 25~33% of the total water volume. 5) Annealing Before entering the annealing furnace, the surface temperature of the continuously cast billet is ≥600℃. The annealing furnace heats the billet to 780±10℃ at a rate of ≤80℃ / h and holds it for 25~40h. Then, the billet is cooled down to 450±10℃ with the furnace and then cooled down to below 200℃ at a rate of ≤40℃ / h before being taken out of the furnace.

[0015] Preferably, in step 4), the dual-layer gradient pore size ceramic filter includes an upper pre-filtration layer and a lower deep purification layer; The upper pre-filter layer is made of MgO-Al2O3 honeycomb ceramic with a pore size of 1.2-1.5mm and has a hexagonal boron nitride nano-coating on its surface. The lower deep purification layer is made of ZrO2-CaO porous ceramic with a pore size of 0.3-0.5mm and contains La2O3 modifier.

[0016] Preferably, in step 4), the chemical composition of the protective slag is as follows by weight percentage: CaO: 29~38%, SiO2: 26~34%, Al2O3: 5~8%, Na2O: 5~8%, NaF: 6~11%, Li2O: 2~4%, MgO≤2%, Fe2O3≤1%, MnO: 3~6%, B4C: 1.8~2.7%, C≤0.1%, with the balance being unavoidable impurities, and the particle size of the B4C being 45~75μm.

[0017] Preferably, in step 4), the continuous casting superheat is 40~55℃.

[0018] Preferably, in step 4), the fluctuation range of the continuous casting billet pulling speed is ≤5%.

[0019] Preferably, in step 4), For continuous casting billet sizes Φ600≤Φ700mm, the casting speed is 1.29~1.50 m / min; For continuous casting billet sizes Φ700≤Φ800mm, the casting speed is 1.06~1.28 m / min; For continuous casting billet sizes Φ800≤Φ900mm, the casting speed is 0.83~1.05 m / min; For continuous casting billet sizes Φ900≤Φ1000mm, the casting speed is 0.60~0.82 m / min; For continuous casting billet sizes Φ1000≤Φ1100mm, the casting speed is 0.50~0.60 m / min; For continuous casting billet sizes Φ1100≤Φ1200mm, the casting speed is 0.40~0.50 m / min.

[0020] Preferably, in step 4), the electromagnetic stirring of the crystallizer uses a low-frequency rotating magnetic field with a current of 200~500A and a frequency of 2.0~4.0Hz, and the direction is reversed every 30~35 seconds; The electromagnetic stirring in the secondary cooling zone uses a traveling wave magnetic field with an axial component strength of 50~80mT and a static magnetic field with a radial component strength of 150~200mT, a current of 150~500A, and a frequency of 5.0~10.0Hz. The electromagnetic stirring at the end of solidification uses a high-frequency pulsed magnetic field with an oscillation angle of 5~10°, a current of 800~1100A, and a frequency of 5.0~10.0Hz.

[0021] Preferably, in step 4), a combination of light and heavy pressing is used at the end of the solidification of the billet, wherein the light pressing amount is 4~6mm and the heavy pressing amount is 8~10mm.

[0022] The content of each element in the G115 heat-resistant steel of this invention, by weight percentage, 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. 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 impurity elements.

[0023] In the continuous casting method described in this invention: In step 1), the carbon content in the molten steel at the tapping endpoint of the electric arc furnace or converter should be controlled to be ≤0.04%, and the tapping carbon should be as low as possible to prevent subsequent carbon spikes that could lead to excessive carbon levels. Carbon is one of the key controlled elements for G115. Carbon can form carbides with Cr, W, V, Nb, etc. Appropriately dispersed carbides can improve the creep resistance of G115, but excessive carbides, especially large-scale non-dispersed carbides, will locally consume too much solid solution element, negatively impacting creep resistance, strength, and corrosion resistance. Furthermore, excessively high carbon levels will negatively affect weldability. Controlling P ≤0.005% and molten steel temperature T ≥1630℃ is due to the high alloy content of G115, which facilitates alloy melting, especially ferrotungsten.

[0024] In step 2), ferrotungsten (FeW80-C) is added in batches during LF refining, with each batch containing ≤600kg. The temperature during addition should be greater than 1630℃ to help the ferrotungsten melt fully. G115 contains 3% W, which is one of the most important strengthening elements. W has a large atomic radius and a high melting point. In order to ensure that W is fully dissolved and evenly distributed, it is necessary to control the amount and temperature of addition when adding ferrotungsten.

[0025] In step 2), when the alloy is added during LF refining, argon gas should be stirred at a moderate intensity with a flow rate of 100~300 NL / min to ensure uniform composition of G115 molten steel, which helps to control the composition precisely.

[0026] In step 3), during the RH or VD vacuum degassing stage, the vacuum level should be controlled to ≤67 Pa, and the vacuum holding time should be ≥20 min to ensure effective degassing and inclusion flotation. G115 employs multi-element composite strengthening, resulting in a variety of alloying elements that can form complex inclusions, such as brittle inclusions Al2O3 and AlN. These inclusions can severely affect the performance of G115, easily leading to severe stress concentration around them and becoming initiation points for fatigue cracks, significantly reducing the creep rupture performance and fracture toughness of G115. Similarly, while finely dispersed inclusions like TiO2 and Ti(C,N) can improve the strength and toughness of G115, coarse particles can disrupt the matrix continuity, negatively impacting the steel's plasticity and toughness.

[0027] In step 3), after breaking the vacuum with RH or VD, ferroboron (FeB18C0.5B) is added. Based on a yield of 60-70%, the boron content is controlled at 0.012-0.018%.

[0028] In step 4), when the molten steel is transferred to the tundish, argon blowing through a long nozzle is used to protect it from oxygen and nitrogen in the air.

[0029] In step 4), a dual-layer gradient pore size ceramic filter is installed in the intermediate package. The upper pre-filtration layer is made of MgO-Al2O3 honeycomb ceramic with a pore size of 1.2-1.5 mm and has a hexagonal boron nitride nano-coating on its surface. The lower deep purification layer is made of ZrO2-CaO porous ceramic with a pore size of 0.3-0.5 mm and contains a La2O3 modifier. It adopts a specially designed dual-layer gradient pore size filter structure consisting of an upper pre-filtration layer and a lower deep purification layer. By modifying the ceramic materials used in the upper pre-filtration layer and the lower deep purification layer, and coating the surface of the MgO-Al2O3 honeycomb ceramic used in the upper pre-filtration layer with a hexagonal boron nitride (BN) nano-coating, the wetting and adsorption capacity for non-metallic inclusions is significantly enhanced. The ZrO2-CaO porous ceramic used in the lower deep purification layer incorporates La2O3 (lanthanum oxide) modifier, thereby actively altering the surface chemical properties of the ZrO2-CaO porous ceramic. This attracts and fixes high-melting-point fine inclusions such as TiN and Al2O3, achieving a synergistic effect of chemical adsorption and physical interception. This effectively improves the inclusion removal rate, and the inclusion content in the G115 billet obtained using this filter can be reduced by about 30%, significantly improving the internal quality of the billet.

[0030] The filter of this invention features a larger pore size (1.2-1.5mm) in the upper layer, responsible for intercepting large inclusions and protecting the lower layer; and a denser pore size (0.3-0.5mm) in the lower layer, responsible for capturing small and harmful inclusions, thus achieving graded filtration of "coarse selection-fine selection". Simultaneously, the two-layer design effectively prevents large inclusions from clogging the ceramic layer, significantly extending the service life of the entire filter, reducing replacement frequency and production costs, and improving the continuity and stability of continuous casting production.

[0031] In step 4), a low-carbon boron-containing protective slag specifically for G115 is added to the crystallizer for fully protective casting.

[0032] The chemical composition of the G115 special protective slag by weight percentage is as follows: CaO: 29~38%, SiO2: 26~34%, Al2O3: 5~8%, Na2O: 5~8%, NaF: 6~11%, Li2O: 2~4%, MgO≤2%, Fe2O3≤1%, MnO: 3~6%, B4C: 1.8~2.7%, C≤0.1%, with the balance being unavoidable impurities. The particle size of the B4C is 45~75μm.

[0033] This invention addresses the characteristics of G115 steel by designing a specialized protective slag for the continuous casting process. By strictly controlling the carbon content to ≤0.1%, it avoids carbon enrichment in the molten steel. Through B4C particle size control, it achieves stable boron replenishment, excellent heat preservation, and optimized lubrication. Adding a small amount of boron carbide to the protective slag meets the boron replenishment requirements of high-boron-content G115 steel, solving the cracking problem in G115 and other steel grades, and providing strong support for improving the internal and surface quality of the cast billet. Furthermore, by adding an appropriate amount of MnO, the melting point, solidification temperature, and high-temperature viscosity of the protective slag can be lowered, thereby improving its lubricity and heat transfer stability.

[0034] This invention uses a low-carbon, boron-containing protective slag to maintain the content of key elements such as C and B in molten steel. Combined with the control of other components, the basicity of the protective slag is controlled at 1.1~1.3 and the viscosity at 0.15~0.25 Pa·s, which provides good lubrication, heat transfer and stability for the continuous casting of G115 steel, reduces the generation of longitudinal cracks on the surface of large-size continuous casting billets of G115 steel and improves the quality of the billets.

[0035] In step 4), the casting speed of the continuous casting billet is 0.40~1.50m / min, and constant speed casting is adopted to further control the casting speed to prevent large fluctuations (fluctuation range ≤5%).

[0036] In step 4), the secondary cooling zone of continuous casting adopts a multi-stage air-water spray cooling method. The total specific water volume of the secondary cooling zone is 1.10~1.50L / kg. The secondary cooling zone is cooled in three stages. The first stage has the strongest cooling, with the sprayed water volume accounting for 37~47% of the total water volume. The solidified billet shell thickness increases from 22%R to 50%R, ensuring that the surface billet shell strength is sufficient to bear the core molten steel and the tension of the straightening machine, preventing steel leakage. The cooling intensity of the second stage is appropriately reduced, with the sprayed water volume accounting for 30~38% of the total water volume, reducing the thermal stress caused by the temperature drop, and the solidified billet shell thickness reaches 70%R. The cooling intensity of the third stage is further reduced, with the sprayed water volume accounting for 25~33% of the total water volume, allowing the billet surface to warm up and preventing surface cracks. After adopting weak cooling, the liquid core length is extended, which helps to ensure sufficient feeding. 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, and prevent cracks from forming.

[0037] Furthermore, in step 4), in order to reduce the center segregation of the G115 continuous casting billet and increase the proportion of equiaxed crystals, multiple electromagnetic stirring technology is adopted, including three parts: the crystallizer (M-EMS), the secondary cooling zone (S-EMS), and the solidification end (F-EMS).

[0038] The crystallizer is electromagnetically stirred (M-EMS) with a low-frequency rotating magnetic field of 2.0~4.0Hz (magnetic induction intensity 80~120mT). The low-frequency magnetic field can effectively penetrate the thick billet shell, break the dendrites at the solidification front, form a large number of crystal nuclei rain, and significantly increase the equiaxed crystal ratio. The current is 200~500A, and the stirring mode is unidirectional rotation with periodic reversal (reversal once every 30~35 seconds). This prevents the molten steel from scouring the solidified billet shell and causing local excessive thinning, while also avoiding slag entrapment of the protective slag. The electromagnetic stirring in this section provides strong stirring in the early stage of solidification when the liquidus cavity is widest. This can break up the primary boron-containing inclusions (such as BN) and disperse them, effectively suppressing the macroscopic segregation of boron.

[0039] The second cooling zone uses electromagnetic stirring (S-EMS), applying a 5.0~10.0Hz traveling wave magnetic field (axial component intensity 50~80mT) and a static magnetic field (radial component intensity 150~200mT), with a current of 150~500A. This creates a coupling effect between Lorentz force and thermo-electromagnetic force, effectively penetrating the thick solidified shell and strongly stirring the liquid core. This breaks up the well-developed columnar crystals and inhibits their longitudinal growth. This section is the 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 their interdendritic segregation and laying a good foundation for subsequent homogenization heat treatment.

[0040] Electromagnetic stirring (F-EMS) at the end of solidification involves applying a pulsed magnetic field (frequency 5.0~10.0Hz, current 800~1100A, peak intensity 300~500mT) for high-frequency, weak stirring with small-angle oscillation (5~10°). This induces liquid reciprocating between dendrites in the mushy zone, promoting solute homogenization. This section is in the final stage of solidification, with a very narrow liquid core. The high-frequency magnetic field is effective, gently stirring to push the solute-rich molten steel back to the center, compensating for solidification shrinkage and significantly reducing central porosity and V-type segregation. G115 has extremely high requirements for center quality; any central defect will become a crack initiation point during subsequent high-temperature service. F-EMS is crucial for ensuring the quality of the continuously cast billet core.

[0041] In step 4), a combination of light and heavy reduction is used at the end of the billet solidification process to suppress central porosity and segregation. The reduction amount for light reduction is 4-6 mm, and for heavy reduction it is 8-10 mm. Due to the high alloy content of G115, more severe shrinkage cavities and segregation channels form in the center of the billet during solidification compared to general heat-resistant steels such as P91 and P92. Light reduction compensates for solidification shrinkage, while heavy reduction closes the channels in the mushy area, preventing the molten steel rich in impurities from flowing back to the center, thus significantly suppressing central porosity and segregation.

[0042] In step 4), the straightening temperature of the continuously cast billet should be ≥850℃. A final forging temperature of 850℃ or higher during hot working deformation can ensure that cracking does not occur during straightening.

[0043] Furthermore, in step 4), G115 steel has a high alloy content and is viscous. To reduce the sensitivity to center segregation, shrinkage cavities, and cracks, casting with a higher degree of superheat should be used, with the superheat controlled at 40~55℃. Induction heating technology in the ladle should be adopted to strictly control the superheat. A high degree of superheat can reduce the viscosity of the molten steel, promote the fluidity of the molten steel, and help with feeding.

[0044] In step 5), the round billet is hot-charged and annealed in a timely manner after continuous casting. Before entering the annealing furnace, the surface temperature of the continuously cast billet is ≥600℃. The annealing furnace heats the billet to 780±10℃ at a rate of ≤80℃ / h and holds it for 25~40h. Then, it is cooled down to 450±10℃ with the furnace and then cooled down to 200℃ at a rate of ≤40℃ / h before being taken out of the furnace, thus obtaining the annealed large round billet. Through this process, the large round billet is fully annealed, the structural stress is completely released, and the risk of cracking of the continuously cast billet is avoided.

[0045] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention addresses the challenges of G115 steel, which has a high alloy content, complex inclusions, and significantly different solidification characteristics compared to traditional heat-resistant steel. Consequently, the control requirements for issues such as central shrinkage cavities, central cracks, porosity, and segregation in continuously cast billets differ from those for traditional heat-resistant steel.

[0046] This invention designs the entire process from electric arc furnace or converter steelmaking, LF furnace refining, RH or VD vacuum degassing, to continuous casting of round billets. By controlling key process parameters such as chemical composition, smelting process parameters, inclusions, gas content, and continuous casting process parameters (superheat, protective slag, double-layer gradient pore size ceramic filter, casting speed, electromagnetic stirring, cooling parameters, light reduction + heavy reduction), it avoids the formation of central cracks, surface cracks, central porosity, and reduces segregation and shrinkage cavities in the continuously cast billets, thereby producing high-quality φ600~φ1200mm G115 large-size round billets.

[0047] Using continuously cast round billets for the production of large-diameter pipes can increase the yield by more than 30%, significantly reduce component segregation, improve the uniformity of the microstructure of the inner and outer walls of thick-walled pipes, and improve strength and impact resistance. The performance meets the requirements of standards such as CSTM 00017-2021, Q / OAPD2753-2022, and Q / OAPD 2253-2022.

[0048] Compared with existing continuous casting technology, traditional round billet continuous casting process is only suitable for steel grades such as P91 and P92 with low alloy content, and the specifications are generally below φ800mm. It cannot meet the requirements of G115, which has high alloy content, complex smelting and solidification process, and large-size continuous casting round billet with specifications of φ600~φ1200mm. Attached Figure Description

[0049] Figure 1 This is a typical low-magnification photograph of a large-size continuous casting round billet (G115) obtained in Embodiment 1 of the present invention. Figure 2 This is a photograph of the microstructure of steel pipes produced from G115 continuously cast billets obtained in Example 1 of the present invention. Detailed Implementation

[0050] The present invention will be further described below with reference to embodiments and accompanying drawings. The following embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any way. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0051] The composition of the steels in the embodiments and comparative examples of this invention is shown in Table 1, with the balance being Fe and unavoidable impurity elements.

[0052] The process parameters for the steelmaking process in the embodiments and comparative examples of this invention are shown in Tables 2, 3, 4, 5, and 6. The composition of the protective slag used in the embodiments of this invention is shown in Table 5, with the remainder being unavoidable impurities.

[0053] The results of low-magnification inspection of the continuously cast billets in the embodiments and comparative examples of this invention are shown in Table 7. After the continuously cast billets are forged into tube blanks (forging ratio greater than 6), inclusion rating is performed according to GB / T 10561, and the results are shown in Table 8. The mechanical properties after production into steel pipes are shown in Table 9.

[0054] Figure 1 This is a typical low-magnification photograph of a large-size G115 continuously cast round billet obtained in Example 1 of the present invention. It can be seen that the continuously cast billet has no surface cracks or central cracks, a shrinkage cavity rating of 1, a general porosity rating of 0.5, and an equiaxed crystal ratio of approximately 30%, indicating good overall quality.

[0055] Figure 2 The image shows the microstructure of the steel pipe produced from the G115 continuously cast billet obtained in Example 1 of this invention. It can be seen that the microstructure is tempered martensite, with uniform martensite laths and a primary austenite grain size of 1.0~3.0. Comparative Example

[0056] The composition of the 13.5-ton steel ingot produced using the EAF+LF+VD+ingot casting method is detailed in Table 1 (wt%). The results of the low-magnification horizontal inspection of the ingot are shown in Table 7. After the ingot is forged into a tube blank (forging ratio greater than 6), inclusions are graded according to GB / T 10561, and the results are shown in Table 8. The mechanical properties of the produced steel pipe are shown in Table 9.

[0057] As can be seen from Table 1, the chemical composition of the present invention and the comparative example both meet the requirements of G115 composition, with no significant differences.

[0058] As can be seen from Table 7, the low-magnification inspection results of the continuously cast billet of the present invention are better than those of the ingot cast in the comparative example. The general porosity of the present invention is grade 0.5, while that of the comparative example is grade 1.0. The intermediate porosity and central crack of the present invention are both grade 0, while those of the comparative example are grade 0.5. The shrinkage cavity of the present invention is grade 1, while that of the comparative example is grade 2. It is evident that the low-magnification inspection results of the present invention are superior to those of the ingot cast comparative example.

[0059] As can be seen from Table 8, the coarse inclusions of the present invention are all grade 0, and the fine inclusions are grade 0 to 0.5. In contrast, the coarse inclusions of the comparative example are all grade 0 to 0.5, and the fine inclusions are grade 0 to 1.0. Among them, the fine inclusions of type D are grade 1.0. It can be seen that the inclusion control of the present invention is better than that of the comparative example in the mold casting.

[0060] As can be seen from Table 9, the yield strength Rp0.2 of the present invention is 598~604MPa, and the tensile strength Rm is 743~754MPa, which is better than that of the comparative example Rp0.2=563MPa and Rm=712MPa. The impact energy KV2 of the present invention is 98~113J, while that of the comparative example is 86J. The impact performance of the present invention is better than that of the comparative example. In terms of hardness, the present invention is basically the same as that of the comparative example.

[0061] 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".

[0062] 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.

[0063]

[0064]

[0065]

[0066]

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[0070]

[0071]

Claims

1. A method for continuous casting of G115 heat-resistant steel round billets, characterized in that, Includes the following steps: 1) Steelmaking in electric arc furnaces or converters The C content in the molten steel is ≤0.04%, the P content is ≤0.005%, and the steel temperature is ≥1630℃. 2) LF furnace refining Argon gas is used for stirring during the refining process, with an argon gas flow rate of 100~300NL / min; ferrotungsten is added in batches, with each batch ≤600kg, and the temperature of the molten steel is controlled to ≥1630℃ when adding ferrotungsten. 3) RH or VD vacuum degassing Vacuum degree ≤67Pa, vacuum holding time ≥20min, after vacuum breaking, add ferroboron to ensure boron content is 0.012~0.018%; 4) Continuous casting When the molten steel is transferred to the tundish, it is protected by argon blowing through a long nozzle. The molten steel in the tundish is injected into the crystallizer through an immersion nozzle. A double-layer gradient pore size ceramic filter is installed in the tundish. Protective slag is added to the crystallizer for full protective casting. Multiple electromagnetic stirring is performed in the crystallizer, secondary cooling zone, and solidification end. The continuous casting billet casting speed is 0.40~1.50m / min, and the continuous casting billet straightening temperature is ≥850℃. The continuous casting secondary cooling zone adopts a three-stage cooling system. The total specific water volume of the secondary cooling zone is 1.10~1.50L / kg. The cooling water volume of the first stage accounts for 37~47% of the total water volume, the cooling water volume of the second stage accounts for 30~38% of the total water volume, and the cooling water volume of the third stage accounts for 25~33% of the total water volume. 5) Annealing Before entering the annealing furnace, the surface temperature of the continuously cast billet is ≥600℃. The annealing furnace heats the billet to 780±10℃ at a rate of ≤80℃ / h and holds it for 25~40h. Then, the billet is cooled down to 450±10℃ with the furnace and then cooled down to below 200℃ at a rate of ≤40℃ / h before being taken out of the furnace.

2. The continuous casting method for G115 heat-resistant steel round billets as described in claim 1, characterized in that, In step 4), the dual-layer gradient pore size ceramic filter includes an upper pre-filtration layer and a lower deep purification layer; The upper pre-filter layer is made of MgO-Al2O3 honeycomb ceramic with a pore size of 1.2-1.5mm and has a hexagonal boron nitride nano-coating on its surface. The lower deep purification layer is made of ZrO2-CaO porous ceramic with a pore size of 0.3-0.5mm and contains La2O3 modifier.

3. The continuous casting method for G115 heat-resistant steel round billets as described in claim 1 or 2, characterized in that, In step 4), the chemical composition of the protective slag by weight percentage is as follows: CaO: 29~38%, SiO2: 26~34%, Al2O3: 5~8%, Na2O: 5~8%, NaF: 6~11%, Li2O: 2~4%, MgO≤2%, Fe2O3≤1%, MnO: 3~6%, B4C: 1.8~2.7%, C≤0.1%, with the balance being unavoidable impurities. The particle size of the B4C is 45~75μm.

4. The continuous casting method for G115 heat-resistant steel round billets as described in claim 1, 2, or 3, characterized in that, In step 4), the superheating temperature of continuous casting is 40~55℃.

5. The continuous casting method for G115 heat-resistant steel round billets as described in claim 1, 2, 3, or 4, characterized in that, In step 4), the fluctuation range of the continuous casting billet pulling speed is ≤5%.

6. The continuous casting method for G115 heat-resistant steel round billets as described in any one of claims 1 to 5, characterized in that, In step 4), For continuous casting billet sizes Φ600≤Φ700mm, the casting speed is 1.29~1.50 m / min; For continuous casting billet sizes Φ700≤Φ800mm, the casting speed is 1.06~1.28 m / min; For continuous casting billet sizes Φ800≤Φ900mm, the casting speed is 0.83~1.05 m / min; For continuous casting billet sizes Φ900≤Φ1000mm, the casting speed is 0.60~0.82 m / min; For continuous casting billet sizes Φ1000≤Φ1100mm, the casting speed is 0.50~0.60 m / min; For continuous casting billet sizes Φ1100≤Φ1200mm, the casting speed is 0.40~0.50 m / min.

7. The continuous casting method for G115 heat-resistant steel round billets as described in any one of claims 1 to 6, characterized in that, In step 4), the electromagnetic stirring of the crystallizer uses a low-frequency rotating magnetic field with a current of 200~500A and a frequency of 2.0~4.0Hz, and the direction is reversed every 30~35 seconds; The electromagnetic stirring in the secondary cooling zone uses a traveling wave magnetic field with an axial component strength of 50~80mT and a static magnetic field with a radial component strength of 150~200mT, a current of 150~500A, and a frequency of 5.0~10.0Hz. The electromagnetic stirring at the end of solidification uses a high-frequency pulsed magnetic field with an oscillation angle of 5~10°, a current of 800~1100A, a frequency of 5.0~10.0Hz, and a peak intensity of 300~500mT.

8. The continuous casting method for G115 heat-resistant steel round billets as described in any one of claims 1 to 7, characterized in that, In step 4), a combination of light and heavy pressure is used at the end of the solidification of the billet. The light pressure is 4-6 mm and the heavy pressure is 8-10 mm.

9. The continuous casting method for G115 heat-resistant steel round billets as described in claim 1, characterized in that, The weight percentage composition of the G115 heat-resistant steel 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%, with the balance being Fe and unavoidable impurity elements.

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