Smelting method of low-sulfur copper-containing corrosion-resistant alloy

Abstract

The invention discloses a smelting method of a low-sulfur copper-containing corrosion-resistant alloy, and aims to solve the technical difficulties that the yield of alloy elements is unstable and components are easy to segregate in the smelting process. The core of the technical scheme provided by the invention is that a'large-tonnage vacuum induction furnace + vacuum consumable electrode furnace 'double-vacuum combined smelting system is innovatively constructed by relying on the first 30-ton ALD imported vacuum induction furnace in China; and engineering mass production of the products is successfully realized through double precise regulation and control of alloy components and microstructures. The method has the beneficial effects that large-scale and engineering mass production of the corrosion-resistant alloy is successfully realized; chemical components are precisely regulated and controlled, so that the element yield stability is greatly improved; by means of refined management and control of a double-vacuum smelting system, the fluctuation range of alloy components is remarkably narrowed, high-quality steel ingots with uniform and consistent chemical components are finally produced by optimizing the process, follow-up product flaw detection standard reaching and performance stability are effectively supported, and macrostructures of finally forged bars are detected to meet relevant standards.

Description

Technical Field

[0001] This invention pertains to special alloy smelting processes, specifically relating to a method for smelting a low-sulfur copper-containing corrosion-resistant alloy. Background Technology

[0002] Alloys such as N09925, N09935, N09945, and N09946 are recommended by the API (American Petroleum Institute) system as primary materials for gas drilling and production equipment. The composition design of these alloys closely addresses the core requirements of corrosion resistance and age-hardening. They utilize a high-nickel matrix to construct a stable austenitic structure, ensuring a corrosion-resistant substrate. Sulfur content is strictly controlled to extremely low levels to avoid impurities causing grain boundary embrittlement and corrosion risks. A specific amount of copper is added to significantly improve the alloy's resistance to chloride ion corrosion. Simultaneously, appropriate amounts of aluminum, titanium, and niobium are added to create favorable conditions for the strengthening phases to function, ultimately achieving a synergistic optimization of corrosion resistance and high strength. Currently, there are few reports on this type of alloy both domestically and internationally, and they are limited to studies on the precipitated phases and corrosion resistance in specific media. There is no relevant literature documenting the smelting and preparation methods for these alloys. To achieve the domestic production and certification of these alloys, independent research and development of smelting processes are necessary. Summary of the Invention

[0003] This invention discloses a method for smelting low-sulfur copper-containing corrosion-resistant alloys, aiming to solve the technical difficulties of unstable yield of alloying elements such as chromium, molybdenum, aluminum, titanium, niobium and copper during the smelting of alloys such as N09925, N09935, N09945 and N09946, as well as the easy segregation of components, and finally to prepare high-quality steel ingots with uniform composition, dense structure and meeting the requirements for use.

[0004] Technical solution of the present invention: 1. Process Flow Pre-mixing → Material preparation → Vacuuming → Loading → Power supply → Full melting → Composition adjustment → Stirring → Refining → Casting → Electrode annealing → Electrode finishing → Cap cutting → Welding → Consumable smelting → Ingot removal → Ingot finishing → Forging.

[0005] 2. Specific technological measures 2.1 The chemical composition (mass fraction) of N09925, N09935, N09945, and N09946 shall conform to the provisions of Table 1, Table 2, Table 3, and Table 4, respectively.

[0006] Table 1 Chemical composition of N09925

[0007] Table 2 Chemical Composition of N09935

[0008] Table 3 Chemical Composition of N09945

[0009] Table 4 Chemical Composition of N09946

[0010] 2.2 Based on the established composition control indicators of the alloy, the loss characteristics of chromium, molybdenum, niobium and copper elements during the smelting process are fully considered, and the input of raw materials is systematically calculated. By accurately matching the element loss law, the batching scheme is optimized to minimize the frequency of composition adjustment and the amount of material added during the feeding and refining process, so as to ensure that the total amount of molten steel and the composition meet the standards and achieve precise control of chemical composition.

[0011] 2.3 Select low-carbon (carbon content not greater than 0.01%) and low-sulfur (sulfur content not greater than 0.001%) high-quality steel, and combine it with raw materials such as nickel, chromium, niobium, aluminum, titanium, molybdenum, copper plates, and graphite electrodes; all materials entering the furnace must have their chemical composition strictly controlled, and the cleanliness of the raw materials must be checked before entering the furnace to ensure that there is no oil, no rust, and no excess impurities, so as to avoid contaminating the molten pool and affecting the quality of the alloy.

[0012] 2.4 During the melting stage, the high vacuum system is started, and the furnace is maintained at a vacuum level not exceeding 20 Pa. The material is loaded in batches. After the vacuum level meets the smelting requirements, the first batch of material is put into the furnace, and then the power is turned on for melting.

[0013] 2.5 In the early and middle stages of melting, high-melting-point elements such as molybdenum and niobium are added in the order of addition, along with strict temperature control throughout the process.

[0014] 2.6 During the refining period, the vacuum level should be controlled between 0.1 Pa and 5 Pa. After the refining period exceeds 120 min, the gas content in the furnace should be analyzed to ensure that the oxygen content is less than 20 × 10⁻⁶. -6 Nitrogen content less than 30×10 -6 The refining temperature is controlled between 1500℃ and 1530℃.

[0015] 2.7 After the content of the furnace gas meets the smelting requirements, the temperature of the molten pool is adjusted to a precise range of 1480℃~1500℃. The alloy raw materials are added in batches in the order of "metallic aluminum → metallic titanium → copper plate". After each batch of raw materials is added, the electromagnetic stirring device is started and stirred for 10min~20min to ensure that the elements are fully diffused and the chemical composition is homogenized. After the sample results are analyzed, argon gas is immediately introduced into the furnace to stabilize the furnace pressure at 20000Pa~25000Pa.

[0016] 2.8 Electrode casting was carried out under argon purging conditions at a casting temperature of 1470℃~1490℃.

[0017] 2.9 After the electrodes are demolded, they should be quickly transferred to an annealing furnace for processing, ensuring that the time from demolding to furnace entry is controlled within 30 minutes.

[0018] 2.10 The electrode is subjected to cap removal treatment to specifically eliminate the shrinkage problem at the filling end, and at the same time remove the impurity accumulation area that is prone to occur at the cap, thereby improving the purity of the core part of the electrode.

[0019] 2.11 By precisely controlling the key melting parameters (current 4KA~6KA, speed 4kg / min~8kg / min, vacuum degree 0.1pa~0.5pa), the melting process is simultaneously equipped with helium charging cooling and self-consumable melting forced cooling functions.

[0020] Description of the inventive points of this invention: The core advantage of this process lies in its innovative construction of a dual-vacuum smelting system combining a large-tonnage vacuum induction furnace and a vacuum arc furnace, utilizing the first 30-tonnage ALD imported vacuum induction furnace introduced by Fushun Special Steel Co., Ltd. Through precise dual control of alloy composition and microstructure, the process has successfully achieved mass production of this type of product, simultaneously improving both technical level and production efficiency. During the early stages of melting, high-melting-point elements such as molybdenum and niobium are added in the correct order, coupled with strict temperature control throughout the process. This ensures the complete melting of these elements while effectively reducing the risk of microsegregation. Molybdenum and niobium easily form complex intermediate phases at high temperatures. If the smelting temperature is insufficient or the element addition order is unreasonable, these intermediate phases may remain, negatively impacting the overall performance of the alloy. After the sample analysis is completed, argon gas is immediately introduced into the furnace, ensuring the stable recovery of aluminum, titanium, and copper elements while effectively reducing the formation of inclusions in the molten pool. The electrode utilizes a timely annealing process to specifically eliminate the thermal and structural stresses generated during electrode formation, ensuring the stability of the electrode during subsequent vacuum arc remelting. Capping the electrode ensures that the internal structure of the electrode is dense and free of obvious defects, laying the foundation for the stable operation of subsequent smelting processes and thus optimizing the final purity and overall performance of the alloy. Synchronous helium-charging cooling and consumable melting forced cooling functions in the melting process can effectively eliminate macroscopic segregation generated in the preceding process, significantly reduce the content of inclusions, refine the grain structure, and further improve the uniformity and density of the internal structure of the material.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows: Relying on a 30-tonnage imported ALD vacuum induction furnace, the limitations of existing small-batch production have been successfully overcome, enabling large-scale, engineered mass production of this type of corrosion-resistant alloy. Addressing the technical challenge of unstable yields of key alloying elements such as chromium, aluminum, titanium, niobium, and copper, the stability of element yields has been significantly improved through precise control of chemical composition. Utilizing the refined management of a dual-vacuum smelting system, the range of alloy composition fluctuations has been significantly reduced. Through process optimization, high-quality steel ingots with uniform chemical composition are ultimately produced, effectively supporting subsequent product flaw detection and performance stability. Finally, the low-magnification microstructure of the forged bars meets relevant standards. Attached Figure Description

[0022] Figure 1 This is a low-magnification microstructure image of the head of the bar in Example 1; Figure 2 This is a low-magnification microstructure image of the tail section of the bar from Example 1; Figure 3 This is a low-magnification microstructure image of the head of the bar in Example 2; Figure 4 This is a low-magnification microstructure image of the tail section of the bar in Example 2; Figure 5 This is a low-magnification microstructure image of the head of the bar in Example 3; Figure 6 This is a low-magnification microstructure image of the tail section of the bar in Example 3; Figure 7 This is a low-magnification microstructure image of the head of the bar in Example 4; Figure 8 This is a low-magnification microstructure image of the tail section of the bar in Example 4. Detailed Implementation

[0023] The present invention will be described in detail below through examples.

[0024] Examples 1, 2, 3, and 4 were executed together: Process flow: pre-mixing → material preparation → vacuuming → charging → power supply → full melting → composition adjustment → stirring → refining → casting → electrode annealing → electrode finishing → cap cutting → welding → consumable smelting → ingot removal → ingot finishing → forging.

[0025] Example 1 ①Based on the established composition of N09925 alloy, the sulfur content should not exceed 0.001%, and the loss characteristics of chromium, molybdenum and niobium elements during the smelting process should be fully considered. The actual composition is as shown in Table 5 below. Table 5. Ingredients of N09925

[0026] ② Select low-carbon (carbon content not greater than 0.01%) and low-sulfur (sulfur content not greater than 0.001%) high-quality steel, and combine it with raw materials such as nickel, chromium, niobium, aluminum, titanium, molybdenum, copper plates, and graphite electrodes; all materials entering the furnace must have their chemical composition strictly controlled, and the cleanliness of the raw materials must be checked before entering the furnace to ensure that there is no oil, no rust, and no excess impurities.

[0027] ③ During the melting stage, the high vacuum system is started, and the furnace is maintained at a vacuum level not exceeding 20 Pa. In the early and middle stages of melting, high-melting-point elements such as molybdenum and niobium are added in the order of addition, along with strict temperature control throughout the process.

[0028] ④ The vacuum degree during the refining period is controlled at 0.1Pa~5Pa, the refining period is 150min, the refining temperature is 1525℃, and the gas is sampled and analyzed. The oxygen content is 12×10-6 and the nitrogen content is 20×10-6. The temperature was measured at 1500℃. The alloy raw materials were added in batches in the order of "metallic aluminum → metallic titanium → copper plate". After each batch of raw materials was added, the electromagnetic stirring device was started and stirred for 20 minutes. After the sample results were analyzed, argon gas was immediately introduced into the furnace to stabilize the furnace pressure at 25000Pa.

[0029] ⑤ Cast Φ430mm electrodes at a casting temperature of 1479℃.

[0030] ⑥ After the electrodes are demolded, the time from demolding to entering the furnace is controlled to be 15 minutes.

[0031] ⑦ Perform cap removal on the electrode to ensure that the electrode fills the shrinkage cavities at the end.

[0032] ⑧ In the vacuum self-consumption melting stage, the current is 5.5KA, the speed is 3.8kg / min, the vacuum degree is 0.1pa~0.5pa, the helium gas charging flow rate is 0.08L / min, the forced cooling process is implemented throughout, and the remelted ingot shape is Φ508mm.

[0033] ⑨ Homogenization annealing and surface finishing after cooling of steel ingots.

[0034] The composition results of the N09925 alloy ingot are shown in Table 6.

[0035] ⑪ According to the ASTM A604 low-magnification microstructure rating requirements in API Standard 6ACRA, the low-magnification microstructure of both ends of the bar meets the Grade A rating criteria, with no non-compliance items. See [link / reference]. Figure 1 , Figure 2 .

[0036] Table 6. Composition of Finished Product N09925

[0037] The forging process employs a combination of upsetting and drawing on a 3150-ton fast forging machine and finishing on an 1800-ton precision forging machine to ultimately forge Φ223mm bars.

[0038] Example 2

[0039] ① Based on the established composition of N09935 alloy, the sulfur content should not exceed 0.001%, and the loss characteristics of chromium, molybdenum and niobium elements during the smelting process should be fully considered. The actual composition should be as shown in Table 7 below.

[0040] Table 7 N09935 ingredients

[0041] ② Select low-carbon (carbon content not greater than 0.01%) and low-sulfur (sulfur content not greater than 0.001%) high-quality steel, and combine it with raw materials such as nickel, chromium, niobium, aluminum, titanium, molybdenum, copper plates, and graphite electrodes; all materials entering the furnace must have their chemical composition strictly controlled, and the cleanliness of the raw materials must be checked before entering the furnace to ensure that there is no oil, no rust, and no excess impurities.

[0042] ③ During the melting stage, the high vacuum system is started, and the furnace is maintained at a vacuum level not exceeding 20 Pa. In the early and middle stages of melting, high-melting-point elements such as molybdenum and niobium are added in the order of addition, along with strict temperature control throughout the process.

[0043] ④ The vacuum degree during the refining period is controlled at 0.1Pa~5Pa, the refining period is 150min, the refining temperature is 1525℃, and the gas is sampled and analyzed. The oxygen content is 15×10-6 and the nitrogen content is 20×10-6. The temperature was measured at 1495℃. The alloy raw materials were added in batches in the order of "metallic aluminum → metallic titanium → copper plate". After each batch of raw materials was added, the electromagnetic stirring device was started and stirred for 20 minutes. After the sample results were analyzed, argon gas was immediately introduced into the furnace to stabilize the furnace pressure at 25000Pa.

[0044] ⑤ Cast Φ430mm electrodes at a casting temperature of 1480℃.

[0045] ⑥ After the electrodes are demolded, the time from demolding to entering the furnace is controlled to be 15 minutes.

[0046] ⑦ Perform cap removal on the electrode to ensure that the electrode fills the shrinkage cavities at the end.

[0047] ⑧ In the vacuum self-consumption melting stage, the current is 5.4KA, the speed is 3.6kg / min, the vacuum degree is 0.1pa~0.5pa, the helium gas charging flow rate is 0.08L / min, the forced cooling process is implemented throughout, and the remelted ingot shape is Φ508mm.

[0048] ⑨ Homogenization annealing and surface finishing after cooling of steel ingots.

[0049] ⑩ The composition results of N09925 alloy ingots are shown in Table 8. ⑪ According to the ASTM A604 low-magnification microstructure rating requirements in API Standard 6ACRA, the low-magnification microstructure of both ends of the bar meets the Grade A rating criteria, with no non-compliance items. See [link / reference]. Figure 3 , Figure 4 .

[0050] Table 8. Composition of Finished Product N09935

[0051] The forging process employs a combination of upsetting and drawing on a 3150-ton fast forging machine and finishing on an 1800-ton precision forging machine to ultimately forge Φ170mm bars.

[0052] Example 3

[0053] ① Based on the established composition of N09945 alloy, the sulfur content should not exceed 0.001%, and the loss characteristics of chromium, molybdenum and niobium elements during the smelting process should be fully considered. The actual composition should be as shown in Table 9 below.

[0054] Table 9 N09945 ingredients

[0055] ② Select low-carbon (carbon content not greater than 0.01%) and low-sulfur (sulfur content not greater than 0.001%) high-quality steel, and combine it with raw materials such as nickel, chromium, niobium, aluminum, titanium, molybdenum, copper plates, and graphite electrodes; all materials entering the furnace must have their chemical composition strictly controlled, and the cleanliness of the raw materials must be checked before entering the furnace to ensure that there is no oil, no rust, and no excess impurities.

[0056] ③ During the melting stage, the high vacuum system is started, and the furnace is maintained at a vacuum level not exceeding 20 Pa. In the early and middle stages of melting, high-melting-point elements such as molybdenum and niobium are added in the order of addition, along with strict temperature control throughout the process.

[0057] ④ The vacuum degree during the refining period is controlled at 0.1Pa~5Pa, the refining period is 150min, the refining temperature is 1525℃, and the gas is sampled and analyzed. The oxygen content is 11×10-6 and the nitrogen content is 19×10-6. The temperature was measured at 1489℃. The alloy raw materials were added in batches in the order of "metallic aluminum → metallic titanium → copper plate". After each batch of raw materials was added, the electromagnetic stirring device was started and stirred for 20 minutes. After the sample results were analyzed, argon gas was immediately introduced into the furnace to stabilize the furnace pressure at 25000Pa.

[0058] ⑤ Cast Φ430mm electrodes at a casting temperature of 1476℃.

[0059] ⑥ After the electrodes are demolded, the time from demolding to entering the furnace is controlled to be 20 minutes.

[0060] ⑦ Perform cap removal on the electrode to ensure that the electrode fills the shrinkage cavities at the end.

[0061] ⑧ In the vacuum self-consumption melting stage, the current is 5.3KA, the speed is 3.5kg / min, the vacuum degree is 0.1pa~0.5pa, the helium gas charging flow rate is 0.08L / min, the forced cooling process is implemented throughout, and the remelted ingot shape is Φ508mm.

[0062] ⑨ Homogenization annealing and surface finishing after cooling of steel ingots.

[0063] ⑩ The composition results of N09925 alloy ingots are shown in Table 10.

[0064] ⑪ According to the ASTM A604 low-magnification microstructure rating requirements in API Standard 6ACRA, the low-magnification microstructure of both ends of the bar meets the Grade A rating criteria, with no non-compliance items. See [link / reference]. Figure 5 , Figure 6 .

[0065] Table 10 Composition of Finished Product N09945

[0066] The forging process employs a combination of upsetting and drawing on a 3150-ton fast forging machine and finishing on an 1800-ton precision forging machine to ultimately forge Φ232mm bars.

[0067] Example 4

[0068] ① Based on the established composition of N09946 alloy, the sulfur content should not exceed 0.001%, and the loss characteristics of chromium, molybdenum and niobium elements during the smelting process should be fully considered. The actual composition should be as shown in Table 11 below.

[0069] Table 11 Ingredients of N09946

[0070] ② Select low-carbon (carbon content not greater than 0.01%) and low-sulfur (sulfur content not greater than 0.001%) high-quality steel, and combine it with raw materials such as nickel, chromium, niobium, aluminum, titanium, molybdenum, copper plates, and graphite electrodes; all materials entering the furnace must have their chemical composition strictly controlled, and the cleanliness of the raw materials must be checked before entering the furnace to ensure that there is no oil, no rust, and no excess impurities.

[0071] ③ During the melting stage, the high-vacuum system is activated, and the furnace vacuum level is maintained at a process standard of no more than 20 Pa. In the early and middle stages of melting, high-melting-point elements such as molybdenum and niobium are added in the order of addition, coupled with strict temperature control throughout the process.

[0072] ④ The vacuum degree during the refining period is controlled at 0.1Pa~5Pa, the refining period is 150min, the refining temperature is 1525℃, and the gas is sampled and analyzed. The oxygen content is 16×10-6 and the nitrogen content is 27×10-6. The temperature was measured at 1495℃. The alloy raw materials were added in batches in the order of "metallic aluminum → metallic titanium → copper plate". After each batch of raw materials was added, the electromagnetic stirring device was started and stirred for 20 minutes. After the sample results were analyzed, argon gas was immediately introduced into the furnace to stabilize the furnace pressure at 25000Pa.

[0073] ⑤ Cast Φ430mm electrodes at a casting temperature of 1472℃.

[0074] ⑥ After the electrodes are demolded, the time from demolding to entering the furnace is controlled to be 24 minutes.

[0075] ⑦ Perform cap removal on the electrode to ensure that the electrode fills the shrinkage cavities at the end.

[0076] ⑧ In the vacuum self-consumption melting stage, the current is 5.4KA, the speed is 3.6kg / min, the vacuum degree is 0.1pa~0.5pa, the helium gas charging flow rate is 0.08L / min, the forced cooling process is implemented throughout, and the remelted ingot shape is Φ508mm.

[0077] ⑨ Homogenization annealing and surface finishing after cooling of steel ingots.

[0078] ⑩ The composition of N09925 alloy ingots is shown in Table 12.

[0079] ⑪ According to the ASTM A604 low-magnification microstructure rating requirements in API Standard 6ACRA, the low-magnification microstructure of both ends of the bar meets the Grade A rating criteria, with no non-compliance items. See [link / reference]. Figure 7 , Figure 8 .

[0080] Table 12 Composition of Finished Product N09946

[0081] The forging process employs a combination of upsetting and drawing on a 3150-ton fast forging machine and finishing on an 1800-ton precision forging machine to ultimately forge Φ160mm bars.

Claims

1. A method for smelting a low-sulfur copper-containing corrosion-resistant alloy, characterized in that, The process flow of the smelting method is as follows: pre-batching → material preparation → evacuation → charging → power supply → full melting → composition adjustment → stirring → refining → casting → electrode annealing → electrode finishing → cap cutting → welding → consumable smelting → ingot removal → ingot finishing → forging. The pre-mixed materials, combined with the established composition control indicators of the alloy, fully consider the loss characteristics of chromium, molybdenum, niobium and copper elements during the smelting process, and systematically calculate the input of raw materials; by accurately matching the element loss law to optimize the batching scheme, the frequency of composition adjustment and the amount of material adjustment during the batching and refining process are minimized, ensuring that the total amount of molten steel and the composition meet the standards, and achieving precise control of chemical composition. The raw materials used are high-quality steel with a low carbon content of no more than 0.01% and a low sulfur content of no more than 0.001%, combined with metallic nickel, metallic chromium, metallic niobium, metallic aluminum, metallic titanium, metallic molybdenum, copper plates, and graphite electrode raw materials. The chemical composition of all materials entering the furnace must be strictly controlled. Before entering the furnace, the cleanliness of the raw materials must be checked to ensure that there is no oil, no rust, and no excess impurities, so as to avoid contaminating the molten pool and affecting the quality of the alloy. The process involves evacuation, charging, power supply, full melting, composition adjustment, stirring, and refining. During the melting stage, a high vacuum system is activated to maintain a vacuum level of no more than 20 Pa in the furnace. Charging is carried out in batches, and after the vacuum level meets the smelting requirements, the first batch of material is fed into the furnace, followed by power supply for melting. During the early stages of melting, high-melting-point elements such as molybdenum and niobium are added in the order of addition, coupled with strict temperature control throughout the process; The vacuum level during the refining period is controlled between 0.1 Pa and 5 Pa. After the refining period exceeds 120 minutes, the gas content in the furnace is analyzed to ensure that the oxygen content is less than 20 × 10⁻⁶. -6 Nitrogen content less than 30×10 -6 The refining temperature is controlled between 1500℃ and 1530℃; After the content of the furnace gas meets the smelting requirements, the temperature of the molten pool is adjusted to a precise range of 1480℃~1500℃. Alloy raw materials are added in batches in the order of "metallic aluminum → metallic titanium → copper plate". After each batch of raw materials is added, the electromagnetic stirring device is started and stirred for 10min~20min to ensure that the elements are fully diffused and the chemical composition is homogenized. After the sample results analysis is completed, argon gas is immediately introduced into the furnace to stabilize the furnace pressure at 20000Pa to 25000Pa. The casting process is carried out under argon purging conditions at a casting temperature of 1470℃~1490℃. After the electrode is demolded, it is quickly transferred to the annealing furnace for processing, ensuring that the time from demolding to furnace entry is controlled within 30 minutes. The cap-cutting process involves removing the cap from the electrode to specifically eliminate the shrinkage problem at the filling end, while also removing areas prone to impurity accumulation at the cap, thereby improving the purity of the core part of the electrode. The self-consumable melting process involves precise control of key melting parameters: current 4KA~6KA, speed 4kg / min~8kg / min, and vacuum degree 0.1pa~0.5pa, in conjunction with helium purging cooling and forced cooling functions during the melting process.

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