A guard ring forge piece for a variable-speed pumped storage generator unit and a preparation method thereof

Abstract

The application discloses a variable-speed pumped storage generator set guard ring forge piece and a preparation method thereof, and belongs to the field of large-size high-nitrogen austenitic stainless steel forge piece manufacturing, and solves the manufacturing problem of the large-size variable-speed pumped storage generator set guard ring forge piece in the prior art. The components of the variable-speed pumped storage generator set guard ring forge piece include the following in percentage by mass: C 0.08%-0.11%, Si 0.40%-0.80%, Mn 19.0%-21.0%, Cr 17.5%-19%, Ni 1.2%-3.0%, V 0.05%-0.3%, N 0.65%-0.72%, Nb 0.001%-0.005%, Cu≤0.01%, Al≤0.01%, P≤0.020%, S≤0.010%, and the balance of Fe and inevitable trace impurities. The variable-speed pumped storage generator set guard ring forge piece has excellent performance, and the yield of finished products is high in the preparation process.

Description

Technical Field

[0001] This invention relates to the field of manufacturing large-size high-nitrogen austenitic stainless steel forgings, and particularly to a retaining ring forging for variable-speed pumped storage generator sets and its preparation method. Background Technology

[0002] With the development of new power systems, advanced variable-speed pumped-storage generator sets are receiving increasing attention. The retaining ring forging is a core component of variable-speed pumped-storage generator sets. It belongs to the category of large, thin-walled ring forgings. Because it is located at both ends of the generator set, the material needs to be non-magnetic, and austenitic stainless steel forgings are typically used. The outer diameter of retaining ring forgings used in variable-speed pumped-storage generator sets exceeds 5 meters. Compared to retaining ring forgings used in thermal and nuclear power generator sets, the size of retaining ring forgings is significantly larger, increasing from a maximum outer diameter of 2 meters to 5 meters, and the weight of the forgings from 2 tons to over 10 tons. During manufacturing, problems such as difficulty in controlling nitrogen composition, coarse grains, and forging cracks arise. In particular, these forgings require deformation strengthening after solution treatment. How to achieve the production of large-size austenitic stainless steel and obtain the high-strength performance requirements of thin-walled retaining ring forgings is an urgent problem to be solved. Summary of the Invention

[0003] In view of the above, the present invention aims to provide a retaining ring forging for variable speed pumped storage generator sets and its preparation method, so as to solve the manufacturing problem of existing large-size retaining ring forgings for variable speed pumped storage generator sets.

[0004] The objective of this invention is mainly achieved through the following technical solutions:

[0005] This invention provides a retaining ring forging for a variable speed pumped storage generator set. The components of the retaining ring forging for a variable speed pumped storage generator set, by mass percentage, include: C 0.08%–0.11%, Si 0.40%–0.80%, Mn 19.0%–21.0%, Cr 17.5%–19%, Ni 1.2%–3.0%, V 0.05%–0.3%, N 0.65%–0.72%, Nb 0.001%–0.005%, Cu≤0.01%, Al≤0.01%, P≤0.020%, S≤0.010%, with the balance being Fe and unavoidable trace impurities.

[0006] Furthermore, the composition of the retaining ring forging for variable speed pumped storage generator sets, by mass percentage, is as follows: C 0.08%–0.1%, Si 0.50%–0.80%, Mn 19.0%–20.5%, Cr 18%–19%, Ni 1.5%–2.5%, V 0.07%–0.2%, N 0.65%–0.70%, Nb 0.001%–0.004%, Cu≤0.008%, Al≤0.009%, P≤0.020%, S≤0.010%, with the balance being Fe and unavoidable trace impurities.

[0007] Furthermore, the microstructure of the retaining ring forgings for variable speed pumped storage generator sets is austenitic, with uniform grains and a grain size of grade 2 or higher.

[0008] The present invention also provides a method for preparing the above-mentioned retaining ring forging for variable speed pumped storage generator sets, comprising:

[0009] Step 1: Melt and cast electrode billets, and then obtain steel ingots by electroslag remelting.

[0010] Step 2: The steel ingot is subjected to multi-stage gradient heating treatment;

[0011] Step 3: Forging and rolling are combined to produce hot-forged blanks;

[0012] Step 4: After the hot-forged blank is air-cooled, it is rough-machined and then solution-treated.

[0013] Step 5: Warm forging deformation strengthening treatment;

[0014] Step 6: Perform stress-relief annealing on the forgings after warm forging deformation strengthening treatment to obtain the retaining ring forgings for variable speed pumped storage generator sets.

[0015] Furthermore, in step 2, the multi-stage gradient heating includes the following steps:

[0016] S201. Keep the steel ingot at a furnace loading temperature of 300-350℃.

[0017] S202, first heat up to 600-650℃, then maintain the temperature;

[0018] S203, heat to 850-900℃ for the second time, and maintain the temperature;

[0019] S204, heat to 1050-1100℃ for the third time, and hold for heat;

[0020] S205, the temperature is raised to 1180-1220℃ for the fourth time, and then kept warm.

[0021] Furthermore, the fourth heating rate v4 in S205, the third heating rate v3 in S204, the second heating rate v2 in S203, and the first heating rate v1 in S202 satisfy the following relationship: v4 > v3 > v2, v3 > v1.

[0022] Furthermore, v1 is 30–60 °C / h.

[0023] Furthermore, in step 3, the forging process is carried out at a temperature of 1180–1220°C.

[0024] Furthermore, in step 4, a solution treatment is performed at 1050–1080°C.

[0025] Furthermore, in step 6, the stress-relief annealing process is as follows: heat to 350-370℃ and hold for 6-10 hours, then water-cool.

[0026] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0027] a) The retaining ring forging of the variable speed pumped storage generator set of the present invention precisely controls the content of carbon, manganese, chromium, vanadium, niobium, nitrogen and nickel to avoid the precipitation of harmful phases, ensure that a uniform austenitic structure is obtained, the grains are uniform and the grain size reaches level 2 or above, which can improve the plasticity and toughness of the forging and ensure the excellent strength and toughness of the retaining ring forging of the variable speed pumped storage generator set.

[0028] (b) In the preparation method of the retaining ring forging for variable speed pumped storage generator set of the present invention, high-temperature diffusion is carried out by multi-stage gradient heating to ensure homogenization and reduce dendrite segregation; combined with precise forging and rolling process, solution treatment, warm forging deformation and stress relief annealing process, the probability of cracking during the preparation process is small, the yield is high, and the retaining ring forging for variable speed pumped storage generator set with the required performance is obtained.

[0029] c) The retaining ring forging for the variable-speed pumped-storage generator set of the present invention has excellent performance and a tensile strength σ b The yield strength is above 870 MPa, for example, 875–925 MPa; 0.2 It has a strength of 700 MPa or higher, for example, 704 to 752 MPa; an elongation of 35% or higher, for example, 38% to 45%; and a shrinkage of area of ​​60% or higher, for example, 60% to 63%.

[0030] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of what is particularly pointed out in the written description and the accompanying drawings. Attached Figure Description

[0031] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0032] Figure 1 The image shows the grain size and microstructure of the retaining ring forging in Example 1.

[0033] Figure 2 Photos of the solution treatment process for the retaining ring forgings at the site;

[0034] Figure 3 Macroscopic view of the retaining ring forging in Comparative Example 1 showing cracks;

[0035] Figure 4 This is a microscopic image of the retaining ring forging in Comparative Example 2. Detailed Implementation

[0036] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of the present invention and, together with the embodiments of the present invention, serve to illustrate the principles of the present invention.

[0037] For the outer diameter of the retaining ring forgings used in variable speed pumped storage generator sets exceeds 5m and the weight of the forgings exceeds 10 tons, problems such as difficulty in controlling nitrogen composition, easy coarse grains, and forging cracks may occur during the manufacturing process. In particular, such forgings have deformation strengthening requirements after solution treatment. How to achieve the manufacturing of large-size austenitic stainless steel and obtain the performance requirements of thin-walled retaining ring forgings with high strength is a challenge.

[0038] This invention provides a retaining ring forging for a variable speed pumped storage generator set. The components of the retaining ring forging for the variable speed pumped storage generator set, by mass percentage, include: C 0.08%–0.11%, Si 0.40%–0.80%, Mn 19.0%–21.0%, Cr 17.5%–19%, Ni 1.2%–3.0%, V 0.05%–0.3%, N 0.65%–0.72%, Nb 0.001%–0.005%, Cu ≤0.01%, Al ≤0.01%, P ≤0.020%, S ≤0.010%, with the balance being Fe and unavoidable trace impurities.

[0039] Specifically, the outer diameter of the retaining ring forgings used in the aforementioned variable speed pumped storage generator sets is 5m or more.

[0040] The following details the function and dosage selection of the components contained in this invention:

[0041] Nitrogen (N): Nitrogen possesses strong solid solution strengthening capabilities; as the N content increases, the strength of the steel continuously improves, but its fracture toughness does not decrease significantly. Nitrogen is an austenite stabilizing element, inhibiting the formation of ferrite and strain-induced martensite in steel. Furthermore, Nitrogen is highly beneficial for improving the pitting and stress corrosion resistance of steel. Increasing the N content is a challenge in the smelting of high-nitrogen steel, especially under normal pressure. This invention, through alloying, can increase the N content to 0.65%–0.72%.

[0042] Cr: Cr is one of the most important alloying elements in austenitic stainless steel. Adding Cr to high-nitrogen, chromium-manganese austenitic stainless steel and dissolving it into the iron matrix can effectively increase the electrode potential of the matrix. Under the action of an oxidizing medium, a Cr-rich oxide film forms on the surface, preventing anodic reactions and thus improving its corrosion resistance. Cr can increase the solubility of C and reduce Cr depletion; therefore, increasing the Cr content is beneficial to the resistance of austenitic stainless steel to intergranular corrosion. Cr can effectively improve the steel's resistance to pitting and crevice corrosion; this effectiveness is greatly enhanced when N is also present in the steel. Increasing the Cr content in austenitic stainless steel can lower the martensitic transformation temperature (M). s The Cr content decreases, thus increasing the stability of the austenitic matrix. Therefore, high-chromium austenitic stainless steel is difficult to martensite even after cold working and low-temperature treatment. However, the Cr content cannot be too high, because Cr is a strong ferrite-forming / stabilizing element, i.e., it can shrink the austenitic region. With the increase of Cr content in steel, ferrite (δ) structure can appear in austenitic stainless steel. In austenitic stainless steel, with the increase of Cr content, the tendency for the formation of some intermetallic phases (such as σ phase) increases. If the steel contains Mo, the increase of Cr content will also promote the formation of χ phase. The precipitation of these intermetallic phases not only significantly reduces the plasticity and toughness of the steel, but also reduces the corrosion resistance of the steel under some conditions. When stainless steel maintains a completely austenitic structure without the formation of δ ferrite, simply increasing the Cr content will not have a significant impact on its mechanical properties. Therefore, after in-depth research, this invention controls the Cr content to be 17.5% to 19%.

[0043] Mn: In high-nitrogen, chromium-manganese austenitic stainless steel, the main role of Mn is to stabilize austenite and increase the solubility of N. In Ni-saving stainless steel, Mn is a very important alloying element, and it is mainly added to the steel in combination with austenite-forming elements. Mn is a relatively weak austenite-forming element, but it has a strong austenite-stabilizing effect; Mn itself contributes very little to improving the corrosion resistance of steel. To increase the solubility of N, this invention controls the Mn content at 19.0%–21.0%.

[0044] C: C is a strong austenite-forming and stabilizing element in stainless steel, and it significantly improves the strength of austenitic stainless steel through interstitial solid solution strengthening. C can also improve the stress corrosion resistance of austenitic stainless steel in high-concentration chloride solutions (such as 42% MgCl2 boiling solution). However, C is sometimes considered a harmful element in austenitic stainless steel. For example, C can form high-chromium M with Cr in the steel. 23 C6-type carbides lead to localized chromium depletion in the steel, reducing its corrosion resistance, especially its resistance to intergranular corrosion. Therefore, not only should the C content be controlled as low as possible during the smelting of austenitic stainless steel, but subsequent hot and cold working and heat treatment processes should also prevent C accumulation on the stainless steel surface to avoid the precipitation of Cr carbides. Therefore, this invention, through in-depth research, controls the C content to 0.08%–0.11% primarily to improve the strength of austenitic stainless steel. 23 C6 type carbides are eliminated by solid solution treatment.

[0045] V (V): V has an atomic size comparable to Fe, high solid solubility, easy-to-control addition amount, and is insensitive to segregation bands. In high-nitrogen chromium-manganese austenitic stainless steel, V can play a nitrogen-fixing role because V has a strong affinity for nitrogen in steel. The formation of V(C,N) significantly reduces the content of "free N" in the steel. V mainly affects the microstructure and properties of steel by precipitating V(C,N). During hot working, it can inhibit austenite recrystallization and prevent grain growth, thereby refining the grains and improving the strength and toughness of the steel. In addition, it can effectively avoid strain aging of steel. Nitrogen in steel can enhance the strengthening effect of V. The stronger the V(C,N) precipitation, the more obvious the strengthening effect becomes as the nitrogen content in the steel increases. At higher temperatures (above 1000℃), V (C,N) can dissolve in γ-Fe. Therefore, vanadium mainly strengthens the steel through interphase precipitation during the γ-α transformation and precipitation in ferrite. During hot working, it can inhibit austenite recrystallization and prevent grain growth, thereby refining the grains and improving the strength and thermal stability of the steel. This invention, through in-depth research, controls the V content to be 0.05%–0.3%.

[0046] Nb: Nb has a larger atomic size than Fe and a larger diffusion coefficient in austenite, so it tends to segregate at high-energy sites such as grain boundaries and dislocation lines. This strongly drags down dislocation climb and grain boundary movement, inhibiting recrystallization nucleation and thus increasing the material's recrystallization time. Nb is an austenite stabilizing element and can form carbides / nitrides with elements such as C / N in steel, which pin dislocations and prevent grain boundary migration, refining austenite grains. The complete solution temperature of Nb compounds is relatively high, generally above 1100℃. Usually, the higher the amount added, the higher the complete solution temperature. Therefore, when formulating the processing technology for Nb microalloyed steel, the ratio of Nb dissolved to precipitated amounts, as well as the size range of precipitated phase particles, should be strictly controlled to achieve the effect of grain refinement. As a microalloying element, this invention, through in-depth research, controls the Nb content to be between 0.001% and 0.005%.

[0047] Si: Si is a strong ferrite-forming element. As the Si content increases, the ferrite content increases, and the formation of intermetallic phases also accelerates and increases. Therefore, the Si content in high-nitrogen chromium-manganese austenitic stainless steel is usually below 1%. This invention, through in-depth research, controls the Si content to be between 0.40% and 0.80%.

[0048] Mo: Some high-nitrogen, chromium-manganese austenitic stainless steels typically contain a small amount of Mo, whose main function is to improve the steel's resistance to reducing media corrosion, pitting corrosion, and crevice corrosion. However, Mo is an element that forms and stabilizes ferrite and expands the ferrite region, and promotes the formation of intermetallic phases, thus reducing the steel's plasticity and toughness. This invention, through in-depth research, strictly limits the addition of Mo.

[0049] Ni: Ni is a relatively important alloying element in ring-protected steel. Its main function is to form and stabilize austenite, improving the thermodynamic stability of the steel. Ni, along with Si, can also influence the precipitation temperature of δ-Fe at high temperatures. Nickel and iron are infinitely soluble, and nickel expands the austenite region of iron, making it a major alloying element for austenite formation and stabilization. Nickel and carbon do not form carbides; nickel can improve the strength of steel without affecting its plasticity and toughness. It can also improve the corrosion resistance of steel. Although increasing the N content can replace nickel in high-nitrogen steel, raising the nitrogen content to 0.65%–0.72% requires special methods such as pressurization, necessitating a certain amount of nickel. This invention, through in-depth research, controls the Ni content to 1.2%–3.0%.

[0050] As harmful elements, P and S are controlled to ≤0.020% and ≤0.010% in this invention, taking into account actual manufacturing costs and processes. At the same time, Cu content is controlled to ≤0.010%, and Al content is also strictly controlled to avoid the formation of AlN inclusions in high-nitrogen steel, which are prone to cracking during hot forging. The Al content must also be ≤0.010%.

[0051] To further improve the overall performance of the retaining ring forgings for the aforementioned variable-speed pumped-storage generator sets, the composition of the retaining ring forgings for the aforementioned variable-speed pumped-storage generator sets, by mass percentage, is as follows: C 0.08%–0.1%, Si 0.50%–0.80%, Mn 19.0%–20.5%, Cr 18%–19%, Ni 1.5%–2.5%, V 0.07%–0.2%, N 0.65%–0.70%, Nb 0.001%–0.004%, Cu≤0.008%, Al≤0.009%, P≤0.020%, S≤0.010%, with the balance being Fe and unavoidable trace impurities.

[0052] The present invention also provides a method for preparing the above-mentioned retaining ring forging for variable speed pumped storage generator sets, comprising:

[0053] Step 1: Melt and cast electrode billets, and then obtain steel ingots by electroslag remelting.

[0054] Step 2: The steel ingot is subjected to multi-stage gradient heating treatment;

[0055] Step 3: Forging and rolling are combined to produce hot-forged blanks;

[0056] Step 4: After the hot-forged blank is air-cooled, it is rough-machined and then solution-treated.

[0057] Step 5: Warm forging deformation strengthening treatment;

[0058] Step 6: Perform stress-relief annealing on the forgings after warm forging deformation strengthening treatment to obtain the retaining ring forgings for variable speed pumped storage generator sets.

[0059] Specifically, in step 1 above, the electrode billet is melted and cast in an electric arc furnace, and then the steel ingot is obtained by electroslag remelting.

[0060] Specifically, in step 1 above, the electric arc furnace melting and casting of electrode billets may include the following steps: furnace melting, slag formation, and rough steelmaking in an electric arc furnace; then the molten steel is introduced into a refining ladle for refining, adjusting the alloy composition, adjusting the N content by using nitriding metal Cr and nitriding metal Mn, the vacuum degree of refining is less than 2 Torr, and the tapping temperature is 1460℃~1490℃; the electrode billet casting is completed under argon protection.

[0061] Specifically, in step 1 above, electroslag remelting may include the following steps: electroslag remelting a set of electrode blanks in a crystallizer, selecting CaF2 45%~75%, Al2O3 15%~30%, CaO 0%~20% for slag formation, and the electroslag remelting melting rate is 0.8T / h~1.2T / h.

[0062] Specifically, in step 1 above, the surface of the steel ingot needs to be peeled.

[0063] Specifically, in step 1 above, the peeling process includes the following steps: machining 5mm along the overall surface of the steel ingot's taper, and then locally machining the areas with severe slag grooves to ensure that the slag layer, micro-cracks, and slag grooves on the surface of the steel ingot with poor quality are cleaned.

[0064] Specifically, in step 2 above, the multi-stage gradient heating includes the following steps:

[0065] S201. Keep the steel ingot at a furnace loading temperature of 300-350℃.

[0066] S202, first heat up to 600-650℃, then maintain the temperature;

[0067] S203, heat to 850-900℃ for the second time, and maintain the temperature;

[0068] S204, heat to 1050-1100℃ for the third time, and hold for heat;

[0069] S205, the temperature is raised to 1180-1220℃ for the fourth time, and then kept warm.

[0070] Specifically, in step 2 above, the fourth heating rate v4 in S205, the third heating rate v3 in S204, the second heating rate v2 in S203, and the first heating rate v1 in S202 conform to the following relationship: v4 > v3 > v2, v3 > v1.

[0071] Specifically, in step 2 above, v1 is 30-60℃ / h, v2 is 30-60℃ / h, v3 is 70-100℃ / h, and v4 is above 100℃ / h.

[0072] It should be noted that the values ​​of v1 and v2 are relatively small because the large-diameter steel ingot is in a low-temperature zone during the first two heating processes. Due to the low heat transfer coefficient of the retaining ring forging of this invention, the heat transfer from the outer surface to the core of the ingot is slow. It is necessary to avoid excessively rapid heating, which could lead to large internal temperature differences, generating internal tensile stress and easily causing cracking. Appropriately reducing the heating rate can relatively prolong the first and second heating times of the ingot, thereby ensuring the temperature uniformity of the ingot after the first heating and preventing crack formation. The purpose of the first and second temperature steps is also to achieve a slow heating. The heat transfer coefficient increases at the steepest angle between 850 and 900°C; at this point, slight heat preservation is necessary to maintain temperature consistency between the inside and outside. Furthermore, and most importantly, this temperature range belongs to M... 23 In the C6 carbide and ε-phase sensitive precipitation region, a large amount of harmful phase precipitates at the grain boundaries and within the grains of the as-cast large grains. Normally, this region should not remain for extended periods to avoid excessive precipitation. However, in this invention, we do not restrict precipitation, allowing it to precipitate appropriately at the grain boundaries and within the grains of the as-cast state, promoting elemental dispersion in the segregation region. v3 > v2 because after the previous two heating and holding cycles, the internal temperature of the steel ingot is more uniform, and the heat transfer coefficient is also relatively larger, which can increase the heating rate.

[0073] Specifically, in the above-mentioned S204, the purpose of holding at a temperature range of 1050℃~1100℃ for a certain period of time is to perform solid solution treatment on the low-temperature carbides and ε phase during the heating process, increase the plasticity of the high-temperature austenitic billet, promote the remelting of the precipitated phase at the grain boundaries and within the grains, and perform the first homogenization treatment.

[0074] Specifically, in S205, v4 > v3 because the high-temperature plasticity and thermal conductivity are increasing, allowing for a higher heating rate. In S205, a prolonged holding temperature of 1180–1220℃ is performed, representing the second homogenization treatment of the as-cast microstructure. The main purpose is to promote the high-temperature diffusion of alloying elements such as Mn and Cr, reducing the impact of segregation. While prolonged high-temperature holding in this austenitic steel can easily cause abnormal grain growth, since the as-cast grains are already large, grain growth is not significant and tends to stabilize after prolonged holding. Therefore, the adverse effects on grain size do not need to be considered during the high-temperature homogenization of the as-cast microstructure.

[0075] Specifically, the heat preservation time in S201 above is 1h to 1.2h / 100mm.

[0076] Specifically, the heat preservation time in S202 is 1h to 1.2h / 100mm.

[0077] Specifically, the heat preservation time in S203 is 0.6h to 0.8h / 100mm.

[0078] Specifically, the heat preservation time in S204 is 1.6h to 2h / 100mm.

[0079] Specifically, the heat preservation time in S205 is 2h to 2.5h / 100mm.

[0080] Specifically, in step 3 above, a large press is used to forge the steel ingot into a billet, and a large ring mill is used for hot rolling to form the billet. The forging and rolling process is combined to produce a hot-forged billet.

[0081] Specifically, in step 3 above, the forging blank is completed through light drawing of the ingot, upsetting and lengthening, punching, and pre-expansion of the hole using a large press. The forging blank is carried out at a temperature of 1180-1220℃, and the final forging temperature is controlled above 950℃.

[0082] Specifically, in step 3 above, considering the poor high-temperature plasticity of the retaining ring forging material, narrow forging temperature range, and severe cracking tendency, a multi-stage upsetting and drawing method with small deformation is adopted. Considering that excessive deformation may lead to mixed grain structure in the large deformation zone and cracking, while insufficient deformation may result in incomplete grain deformation and failure to achieve the purpose of grain breakage and recrystallization, the deformation amount for each upsetting is controlled at 30%–50%, and the deformation amount for each drawing is controlled at 25%–40%.

[0083] Specifically, in step 3 above, the hot rolling process using a large ring rolling mill is as follows: the ring billet, which is kept at 1150-1200℃, is rolled and expanded, and the deformation of each rolling is controlled at about 25%-30%, and the final rolling temperature is not lower than 950℃.

[0084] Specifically, in step 4 above, the hot-forged blank is subjected to solution treatment at 1050-1080℃ after rough machining to remelt the harmful phases, obtain a uniform austenitic structure, and improve the plasticity and toughness of the forging, providing a suitable basic strength for strengthening treatment.

[0085] Specifically, in step 4 above, the solution treatment process is as follows: heat to 1050–1080℃ and hold for 3–5 hours, then water cool. Specifically, the temperature should start at ≤500℃, accelerating the heating rate to prevent the precipitation of harmful phases at 500–1000℃. Considering that an excessively rapid heating rate would lead to a large temperature difference between the inside and outside of the alloy, the holding time should be at least 3 hours. At the same time, excessively high temperatures or excessively long holding times would cause grain growth, which is detrimental to alloy properties. Conversely, excessively low temperatures or excessively short holding times would prevent the remelting of the σ and ε phases. Therefore, the heating rate is controlled at 70℃ / h or higher (e.g., 70–100℃ / h) to reach 1050–1080℃, and held for 3–5 hours.

[0086] Specifically, in step 5 above, the solution-treated forging is heated at 550–650°C to perform warm forging deformation.

[0087] Specifically, in step 5 above, warm forging deformation includes: using a large-tonnage ring rolling mill with a rolling expansion process, multiple passes of small reduction deformation, with the reduction per pass controlled at 0.5 to 4 mm, and the total deformation per pass controlled at 8% to 12%, to complete the final forming size of the forging, improve the yield strength and tensile strength of the forging, and make it meet the performance requirements.

[0088] It should be noted that, generally, traditional high-nitrogen austenitic stainless steel forgings with a diameter of around 1000mm can be cold-strengthened using mechanical bulging. This involves using a large press to convert vertical pressure into horizontal pressure via wedges, gradually increasing the inner and outer diameters of the ring-shaped forging while gradually decreasing its height. Hydraulic bulging or externally supplied liquid bulging can also be used, employing internal liquid media to pressurize and slowly deform the forging to the required dimensions. However, the retaining ring forging of this invention, with its finished size reaching the 5m range, requires a large wedge mold and associated molds using wedge bulging technology, resulting in a mold cost exceeding several million. Furthermore, the bulging process is slow, consuming significant press time, and exhibits poor uniformity in deformation, resulting in concave interiors and convex ends. While hydraulic bulging and externally supplied liquid bulging offer better deformation uniformity, they also require substantial mold space to hold the pressurized water medium, which is inconvenient under large presses or frame structures. Therefore, this invention utilizes a large ring rolling mill with a radial pressure exceeding 5000T and a working surface hardness of 60HSD or higher for the rolling rolls.

[0089] Specifically, in step 6 above, the stress-relief annealing process involves heating to 350–370℃ and holding for 6–10 hours, followed by water cooling. Stress-relief annealing can eliminate residual stress from work hardening.

[0090] Specifically, during the preparation of the retaining ring forgings for variable speed pumped storage generator sets obtained in step 6 above, the probability of cracks is small and the yield is high, for example, the yield reaches more than 95%.

[0091] Specifically, the microstructure of the retaining ring forging for the variable speed pumped storage generator set obtained in step 6 above is austenitic, with uniform grains and a grain size of grade 2 or above, for example, a grain size of grade 2 to 4.

[0092] Specifically, the retaining ring forgings for variable-speed pumped storage generator sets obtained in step 6 above exhibit excellent performance and a tensile strength σ b The yield strength is above 870 MPa, for example, 875–925 MPa; 0.2It has a strength of 700 MPa or higher, for example, 704 to 752 MPa; an elongation of 35% or higher, for example, 38% to 45%; and a shrinkage of area of ​​60% or higher, for example, 60% to 63%.

[0093] Examples 1-4

[0094] Embodiments 1-4 of the present invention provide a retaining ring forging for a variable-speed pumped-storage generator set and its preparation method. The chemical composition of the embodiments is shown in Table 1. The microstructure of the retaining ring forgings of Embodiments 1-4 is shown in Table 2. Figure 1 The image shows the grain size and microstructure of the retaining ring forging in Example 1.

[0095] Figure 2 Photographs of the solution treatment of retaining ring forgings for variable-speed pumped-storage generator sets. The main performance test results of the embodiment are shown in Table 3.

[0096] The preparation method of Example 1 includes:

[0097] Step 1: Electrode billets are melted and cast using an electric arc furnace, and then steel ingots are obtained by electroslag remelting. The surface of the steel ingots is then peeled off.

[0098] The process of casting electrode billets in an electric arc furnace includes the following steps: furnace smelting, slag formation, and rough steelmaking are carried out in the electric arc furnace; then, the molten steel is introduced into a refining ladle for refining, adjusting the alloy composition, and adjusting the N content to above 0.68% by using chromium nitride and Mn nitride. The vacuum degree of refining is less than 2 Torr, and the tapping temperature is 1470℃; argon gas is used for protection at a flow rate of 100 NL / min. After refining, the electrode billet is cast.

[0099] The electroslag remelting process includes the following steps: a set of electrode blanks are electroslag remelted in a crystallizer, and slag is made by selecting CaF2 60%, Al2O3 30%, and CaO 10%. The melting rate of the electroslag remelting is 1T / h.

[0100] The surface peeling process of steel ingots includes the following steps: machining 5mm along the tapered surface of the electroslag ingot, with the diameter controlled within the range of 1400-1450mm, and then locally machining the areas with severe slag grooves to ensure that the poor-quality slag layer, micro-cracks, and slag grooves on the surface of the steel ingot are cleaned. At the same time, the locally machined areas should have a certain chamfer to avoid damage during forging.

[0101] Step 2: The steel ingot is subjected to multi-stage gradient heating treatment, specifically including:

[0102] The steel ingot is heated from the charging temperature of 330℃ for 15 hours, then heated for the first time at a rate of 40℃ / h, and then heated to 650℃ and held for 15 hours. The second heating is carried out at a rate of 40℃ / h, from 650℃ to 870℃ and held for 10 hours. The third heating is carried out at a rate of 70℃ / h, from 870℃ to 1050℃ and held for 24 hours. The fourth heating is carried out at a rate of more than 100℃ / h, from 1050℃ to 1220℃ and held for 30 hours.

[0103] Step 3: The steel ingot is forged and billeted using a large press, and then hot-rolled using a large ring mill to produce a hot-forged billet. Specifically, this includes: hot forging is carried out at 1200℃, and the final forging temperature is controlled at 960℃; the ingot body is lightly drawn with a reduction of no more than 30mm; the upsetting deformation is 50% each time, and the elongation deformation is 30% each time; after upsetting, the ingot is punched with a diameter of 700mm; after punching, the ingot is enlarged on a large press with a lever to increase the inner diameter to 1400mm.

[0104] The ring billet, which is kept at 1180℃, is rolled and enlarged on a ring mill. The rolling amount per pass is 20mm, and the total rolling amount per pass is controlled at 30%. After rolling, the outer diameter of the ring reaches 4800mm, and the final forging temperature cannot be lower than 950℃.

[0105] Step 4, Solution treatment: Place the billet in a furnace at a temperature ≤500℃ and rapidly heat it at 100℃ / h until it reaches 1050℃. Hold it at this temperature for 3 hours and immediately cool it with water after removing it from the furnace.

[0106] Step 5: Heat the solution-treated forging at 650℃; use a large-tonnage ring rolling mill with a rolling expansion process, control the reduction per pass to 1mm, control the total deformation per pass to 9%, expand the outer diameter to 5100mm, and complete the final forming size of the large outer diameter forging.

[0107] Step 6: Perform stress-relief annealing on the warm-forged final-formed forging at 350℃, heat to 350℃ and hold for 6 hours, then water-cool.

[0108] The preparation method of Example 2 is largely the same as that of Example 1, except that:

[0109] In step 1, slag is formed using 70% CaF2, 25% Al2O3, and 5% CaO, and the melting rate of electroslag remelting is 1.1 T / h.

[0110] Step 2: The steel ingot is subjected to multi-stage gradient heating treatment, specifically including:

[0111] The steel ingot is heated from its initial charging temperature of 350℃ for 16 hours, then heated for the first time at a rate of 50℃ / h, and then raised to 650℃ and held for 16 hours. A second heating process is then performed at a rate of 50℃ / h, raising the temperature from 650℃ to 850℃ and holding for 9 hours. A third heating process is performed at a rate of 80℃ / h, raising the temperature from 850℃ to 1060℃ and holding for 26 hours. Finally, a fourth heating process is performed at a rate of 110℃ / h or higher, raising the temperature from 1060℃ to 1210℃ and holding for 35 hours.

[0112] Step 3: The steel ingot is forged and billeted using a large press, and then hot-rolled using a large ring mill to produce a hot-forged billet. Specifically, this includes: hot forging is carried out at 1190℃, and the final forging temperature is controlled at 950℃; the ingot body is lightly drawn with a reduction of no more than 30mm; the upsetting deformation is 30% each time, and the elongation deformation is 30% each time; after upsetting, the ingot is punched with a diameter of 700mm; after punching, the ingot is enlarged on a large press with a lever to increase the inner diameter to 1500mm.

[0113] The ring billet, which is kept at 1160℃, is rolled and enlarged on a ring mill. The rolling amount per pass is 15mm, and the total rolling amount per pass is 27%. After rolling, the outer diameter of the ring reaches 4800mm, and the final forging temperature cannot be lower than 950℃.

[0114] Step 4, Solution treatment: Place the billet in a furnace at a temperature ≤500℃ and rapidly heat it at 90℃ / h until it reaches 1050℃. Hold it at this temperature for 3 hours and immediately cool it with water after removing it from the furnace.

[0115] Step 5: Heat the solution-treated forging at 630℃; use a large-tonnage ring rolling mill with a rolling expansion process, control the reduction per pass to 2mm, control the total deformation per pass to 9%, expand the outer diameter to 5100mm, and complete the final forming size of the large outer diameter forging.

[0116] Step 6: Perform stress-relief annealing on the warm-forged final-formed forging at 350℃, heat to 350℃ and hold for 6 hours, then water-cool.

[0117] The preparation method of Example 3 is largely the same as that of Example 1, except that:

[0118] In step 1, slag is formed using 60% CaF2, 20% Al2O3, and 20% CaO, and the melting rate of electroslag remelting is 1.2 T / h.

[0119] Step 2: The steel ingot is subjected to multi-stage gradient heating treatment, specifically including:

[0120] The steel ingot is heated from its initial charging temperature of 350℃ for 16.5 hours, then heated for the first time at a rate of 60℃ / hour, and then raised to 600℃ and held for 16.5 hours. A second heating process is then performed at a rate of 60℃ / hour, raising the temperature from 600℃ to 880℃ and holding for 9 hours. A third heating process is performed at a rate of 80℃ / hour, raising the temperature to 1070℃ and holding for 27 hours. A fourth heating process is performed at a rate of 110℃ / hour, raising the temperature to 1220℃ and holding for 35 hours.

[0121] Step 3: The steel ingot is forged and billeted using a large press, and then hot-rolled using a large ring mill to produce a hot-forged billet. Specifically, this includes: hot forging is carried out at 1180℃, and the final forging temperature is controlled at 950℃; the ingot body is lightly drawn with a reduction of no more than 30mm; the upsetting deformation is 40% each time, and the elongation deformation is 40% each time; after upsetting, the ingot is punched with a diameter of 700mm; after punching, the ingot is enlarged on a large press with a lever to increase the inner diameter to 2000mm.

[0122] The ring billet, which is kept at 1150℃, is rolled and enlarged on a ring mill. The rolling amount is 10mm each time, and the total rolling amount per heat is 25%. After rolling, the outer diameter of the ring reaches 4790mm, and the final forging temperature is 950℃.

[0123] Step 4, Solution treatment: Place the billet in a furnace at a temperature ≤500℃ and rapidly heat it at 80℃ / h until it reaches 1060℃. Hold it at this temperature for 4 hours and immediately cool it with water after removing it from the furnace.

[0124] Step 5: Heat the solution-treated forging at 620℃; use a large-tonnage ring rolling mill with a rolling expansion process, control the reduction per pass to 1.5mm, control the total deformation per pass to 10%, expand the outer diameter to 5100mm, and complete the final forming size of the large outer diameter forging.

[0125] Step 6: Perform stress-relief annealing on the warm-forged final-formed forging at 350℃, heat to 350℃ and hold for 6 hours, then water-cool.

[0126] The preparation method of Example 4 is largely the same as that of Example 1, except that:

[0127] In step 1, slag is formed using 65% CaF2, 20% Al2O3, and 15% CaO, and the melting rate of electroslag remelting is 0.8 T / h.

[0128] Step 2: The steel ingot is subjected to multi-stage gradient heating treatment, specifically including:

[0129] The steel ingot is heated from the furnace loading temperature of 350℃ for 17 hours, then heated for the first time at a rate of 60℃ / h, and then heated to 600℃ and held for 17 hours. The second heating is carried out at a rate of 60℃ / h, from 600℃ to 880℃ and held for 10 hours. The third heating is carried out at a rate of 80℃ / h, and heated to 1070℃ and held for 27 hours. The fourth heating is carried out at a rate of 105℃ / h, and heated to 1220℃ and held for 36 hours.

[0130] Step 3: The steel ingot is forged and billeted using a large press, and then hot-rolled using a large ring mill to produce a hot-forged billet. Specifically, this includes: hot forging is carried out at 1210℃, and the final forging temperature is controlled at 970℃; the ingot body is lightly drawn with a reduction of no more than 30mm; the upsetting deformation is 35% each time, and the elongation deformation is 25% each time; after upsetting, the ingot is punched with a diameter of 700mm; after punching, the ingot is enlarged on a large press with a lever to increase the inner diameter to 2500mm.

[0131] The ring billet, which is kept at 1180℃, is rolled and enlarged on a ring mill. The rolling amount per pass is 10mm and the rolling amount per heat is 28%. After completion, the outer diameter of the ring reaches 4830mm and the final forging temperature is 950℃.

[0132] Step 4, Solution treatment: Place the billet in a furnace at a temperature ≤500℃ and rapidly heat it at 80℃ / h until it reaches 1070℃. Hold it at this temperature for 4 hours and immediately cool it with water after removing it from the furnace.

[0133] Step 5: Heat the solution-treated forging at 600℃; use a large-tonnage ring rolling mill with a rolling expansion process, control the reduction per pass to 1mm, control the total deformation per pass to 8%, expand the outer diameter to 5100mm, and complete the final forming size of the large outer diameter forging.

[0134] Step 6: Perform stress-relief annealing on the warm-forged final-formed forging at 370℃, hold at 370℃ for 6 hours, and then water-cool.

[0135] During the preparation process of Examples 1-4, the probability of cracks is small, and the yield rate reaches over 95%.

[0136] Table 1 Chemical composition, wt%

[0137] 1 0.081 0.61 20.24 0.002 0.015 18.4 2.3 0.0021 0.12 0.68 0.008 0.008 2 0.092 0.54 19.51 0.002 0.014 18.2 2.2 0.0015 0.08 0.66 0.008 0.009 3 0.1 0.8 19.27 0.002 0.013 18.2 2.1 0.0028 0.16 0.7 0.008 0.006 4 0.091 0.53 19.59 0.002 0.012 18.31 1.9 0.0035 0.08 0.65 0.008 0.007

[0138] Table 2 Microstructure

[0139] 1 Level 4 austenite 2 Level 2 austenite 3 Level 2 austenite 4 Level 3 austenite

[0140] Table 3 Performance test results

[0141]

[0142]

[0143] The inventors conducted extensive experimental research during the research process, and some unsuccessful solutions are now presented as comparative examples.

[0144] Comparative Example 1

[0145] This comparative example provides a retaining ring forging, the composition of which is shown in Table 4 below. The preparation method is the same as that in Example 1, and will not be repeated here. The retaining ring forging of this comparative example exhibited severe cracking during hot forging, such as... Figure 3 As shown.

[0146] As can be seen from the components in Table 4, the Mn content is less than 19%, the Cr content is more than 19%, the V content is less than 0.05%, the N content is less than 0.65%, the P content is more than 0.02%, the Cu content is more than 0.01%, and the Al content is more than 0.03%. During the hot deformation process, there is high-temperature δ precipitation, as well as AlN precipitation and other harmful phases, which leads to severe cracking and affects the yield.

[0147] Table 4 Chemical composition, wt%

[0148] 1 0.09 0.6 18.5 0.005 0.03 21 1.9 0.005 0.02 0.59 0.03 0.03

[0149] Comparative Example 2

[0150] This comparative example provides a retaining ring forging with the same composition as Example 1. In the preparation method, the forging temperature in step 3 is selected as 1250℃, and other aspects remain unchanged.

[0151] The retaining ring forging in this comparative example exhibits severe grain growth, with incomplete recrystallization at coarse grain boundaries, significant mixed grains, and poor uniformity. Figure 4 As shown.

[0152] Comparative Example 3

[0153] This comparative example provides a retaining ring forging with the same composition as Example 1. In the preparation method, the total deformation amount in the rolling expansion strengthening deformation in step 5 is 7.5%, and other methods remain unchanged.

[0154] The yield strength of the retaining ring forging in this comparative example did not meet the performance requirements.

[0155] The main performance test results of the comparative steel are shown in Table 5.

[0156] Table 5 Performance test results of Comparative Example 3

[0157]

[0158] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A retaining ring forging for a variable-speed pumped-storage generator set, characterized in that, The composition of the retaining ring forging for the variable speed pumped storage generator set, by mass percentage, includes: C 0.08%~0.11%, Si 0.40%~0.80%, Mn 19.0%~21.0%, Cr 17.5%~19%, Ni 1.2%~3.0%, V 0.05%~0.3%, N 0.65%~0.72%, Nb 0.001%~0.005%, Cu ≤0.01%, Al ≤0.01%, P≤0.020%, S≤0.010%, with the balance being Fe and unavoidable trace impurities; The method for preparing the retaining ring forging for the variable speed pumped storage generator set includes: Step 1: Melt and cast electrode billets, and then obtain steel ingots by electroslag remelting. Step 2: The steel ingot is subjected to multi-stage gradient heating treatment; Step 3: Forging and rolling are combined to produce hot-forged blanks; Step 4: After the hot-forged blank is air-cooled, it is rough-machined and then solution-treated. Step 5: Warm forging deformation strengthening treatment; Step 6: Perform stress-relief annealing on the forgings after warm forging deformation strengthening treatment to obtain the retaining ring forgings for variable speed pumped storage generator sets. Step 2, the multi-stage gradient heating includes the following steps: S201. Keep the steel ingot at a furnace loading temperature of 300~350℃; S202, first heat up to 600~650℃, then keep warm; S203, raise the temperature to 850~900℃ for the second time, and maintain the temperature; S204, heat to 1050~1100℃ for the third time, and keep warm; S205, heat up to 1180~1220℃ for the fourth time, and keep warm; The fourth heating rate v4 in S205, the third heating rate v3 in S204, the second heating rate v2 in S203, and the first heating rate v1 in S202 conform to the following relationship: v4 > v3 > v2, v3 > v1.

2. The retaining ring forging for a variable-speed pumped-storage generator set according to claim 1, characterized in that, The composition of the retaining ring forging for the variable speed pumped storage generator set, by mass percentage, is as follows: C 0.08%~0.1%, Si 0.50%~0.80%, Mn 19.0%~20.5%, Cr 18%~19%, Ni 1.5%~2.5%, V 0.07%~0.2%, N 0.65%~0.70%, Nb 0.001%~0.004%, Cu ≤0.008%, Al ≤0.009%, P≤0.020%, S≤0.010%, with the balance being Fe and unavoidable trace impurities.

3. The retaining ring forging for a variable-speed pumped-storage generator set according to claim 1, characterized in that, The microstructure of the retaining ring forging for the variable speed pumped storage generator set is austenitic, with uniform grains and a grain size of grade 2 or higher.

4. A method for preparing a retaining ring forging for a variable-speed pumped-storage generator set according to any one of claims 1 to 3, characterized in that, include: Step 1: Melt and cast electrode billets, and then obtain steel ingots by electroslag remelting. Step 2: The steel ingot is subjected to multi-stage gradient heating treatment; Step 3: Forging and rolling are combined to produce hot-forged blanks; Step 4: After the hot-forged blank is air-cooled, it is rough-machined and then solution-treated. Step 5: Warm forging deformation strengthening treatment; Step 6: Perform stress-relief annealing on the forgings after warm forging deformation strengthening treatment to obtain the retaining ring forgings for variable speed pumped storage generator sets. Step 2, the multi-stage gradient heating includes the following steps: S201. Keep the steel ingot at a furnace loading temperature of 300~350℃; S202, first heat up to 600~650℃, then keep warm; S203, raise the temperature to 850~900℃ for the second time, and maintain the temperature; S204, heat to 1050~1100℃ for the third time, and keep warm; S205, heat up to 1180~1220℃ for the fourth time, and keep warm; The fourth heating rate v4 in S205, the third heating rate v3 in S204, the second heating rate v2 in S203, and the first heating rate v1 in S202 conform to the following relationship: v4 > v3 > v2, v3 > v1.

5. The preparation method according to claim 4, characterized in that, Step 2, the multi-stage gradient heating includes the following steps: S201. Keep the steel ingot at a furnace loading temperature of 330~350℃; S202, first heat up to 600~650℃, then keep warm; S203, raise the temperature to 850~900℃ for the second time, and maintain the temperature; S204, heat to 1050~1100℃ for the third time, and keep warm; S205, the temperature is raised to 1180~1220℃ for the fourth time, and then kept warm.

6. The preparation method according to claim 5, characterized in that, The v1 is 30~60℃ / h.

7. The preparation method according to claim 4, characterized in that, In step 3, the forging process is carried out at a temperature of 1180~1220℃.

8. The preparation method according to claim 4, characterized in that, In step 4, a solution treatment is performed at 1050~1080℃.

9. The preparation method according to claim 4, characterized in that, In step 6, the stress-relief annealing process is as follows: heat to 350~370℃ and hold for 6~10 hours, then water cool.

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