Large-size ultralow ferrite 0Cr17Ni4Cu4Nb stainless steel forged material and preparation method thereof

Abstract

The invention relates to a stainless steel hardening and tempering technology, belongs to the field of stainless steel smelting and forging forming, and particularly relates to martensitic stainless steel. The invention relates to a stainless steel forging material, in particular to a large-size ultralow ferrite 0Cr17Ni4Cu4Nb stainless steel forging material and a preparation method thereof. By strictly controlling the content of elements contained in the large-size ultra-low ferrite 0Cr17Ni4Cu4Nb stainless steel forging material and through a specific heat treatment and forming method, under the condition that the size of the forging material is within the range of about 400-1000 mm of the maximum size dimension of the geometric shape of the cross section, the uniform structure can still be achieved, and the tensile strength of the forging material is greatly improved. And the ferrite content measured by the most serious field-of-view method is only about 2% by area or less, even about 1% by area or less.

Description

Technical Field

[0001] This invention relates to a stainless steel quenching and tempering technology, belonging to the field of stainless steel smelting and forging, specifically to martensitic stainless steel. More specifically, this invention relates to a large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forging and its preparation method. Background Technology

[0002] 0Cr17Ni4Cu4Nb stainless steel is a precipitation-hardening, ultra-low-carbon martensitic stainless steel with high strength, hardness, and corrosion resistance. Its chemical composition includes nickel (Ni), chromium (Cr), carbon (C), and certain amounts of copper (Cu) and niobium (Nb). The Chinese national standard GB / T 1220-2007, "Stainless Steel Bars," specifies the following weight percentages of the chemical composition of 0Cr17Ni4Cu4Nb stainless steel: C ≤ 0.07, Si (silicon) ≤ 1.00, Mn (manganese) ≤ 1.00, P (phosphorus) ≤ 0.040, S (sulfur) ≤ 0.030, Cr = 15.00-17.50, Ni = 3.00-5.00, Cu = 3.00-5.00, Nb = 0.15-0.45. The content and proportion of these elements determine the properties of 0Cr17Ni4Cu4Nb stainless steel.

[0003] In terms of applications, 0Cr17Ni4Cu4Nb stainless steel is widely used in aerospace, automotive, petrochemical, medical, electronics, and food industries due to its high strength and corrosion resistance. For example, it is used to manufacture aircraft parts, engine parts, automotive drive shafts, brackets, petrochemical equipment, medical devices, electronic components, and food processing equipment.

[0004] The microstructure of 0Cr17Ni4Cu4Nb stainless steel is mainly martensite with a small amount of ferrite. The Chinese national standard GB / T 1220-2007 specifies the weight percentage of the chemical composition of 0Cr17Ni4Cu4Nb stainless steel, but does not specify requirements for ferrite content.

[0005] Although research on 0Cr17Ni4Cu4Nb stainless steel is becoming increasingly in-depth in this field, the ferrite content of this material is generally required to be less than about 5% area or less than about 10% area (most severe field of view), and this indicator only applies to materials with a size not greater than about 250 mm.

[0006] As the demand for 0Cr17Ni4Cu4Nb stainless steel in this field expands, the required specifications are also increasing, and the requirements for the material's microstructure and segregation are becoming more stringent. Due to compositional segregation and improper forging process control, a large amount of ferrite is generated during the production of 0Cr17Ni4Cu4Nb stainless steel, which can lead to failure when using the material. Summary of the Invention

[0007] Purpose of the invention In view of the problems existing in the prior art described in the background section above, the purpose of this invention is to provide a large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forging and its preparation method.

[0008] Technical solution To achieve the above objectives, the present invention adopts the following technical solution: Option 1: A large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forging, wherein the maximum dimension of the cross-sectional geometry is in the range of approximately 400 to approximately 1000 mm, and the ferrite content is approximately less than 2% by area, wherein the 0Cr17Ni4Cu4Nb stainless steel forging is composed of the following elements: based on the total weight of the 0Cr17Ni4Cu4Nb stainless steel forging... Cr element in the range of approximately 15.00 to approximately 15.50% by weight. Ni element in the range of approximately 4.00 to approximately 5.00 wt%. Cu element in the range of approximately 3.00 to approximately 3.50% by weight. Nb element in the range of approximately 0.20 to approximately 0.35 wt%. N element in the range of approximately 0.03 to approximately 0.08 wt%. The balance consists of Fe and unavoidable impurity elements.

[0009] Option 2: The large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forging as described in Option 1 above, wherein the unavoidable impurity element is: The content of element C is controlled to be below approximately 0.05% by weight. The Si content is controlled to be below approximately 0.50% by weight. The Mn content is controlled to be below approximately 0.50% by weight. The content of phosphorus (P) is controlled to be below approximately 0.030% by weight, and The sulfur content is controlled to be below approximately 0.003% by weight.

[0010] Option 3: The ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forging described in Option 2 above, wherein the Nb content is more than 5 times the C content.

[0011] Option 4: Ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forgings according to any one of Options 1 to 3 above, wherein the 0Cr17Ni4Cu4Nb stainless steel forgings are round bars with a cross-sectional diameter in the range of about 400 to about 1000 mm, or square bars or flat bars with a maximum cross-sectional width in the range of about 400 to about 1000 mm.

[0012] Option 5: Ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forgings according to any one of Options 1 to 4 above, wherein the ferrite content of the 0Cr17Ni4Cu4Nb stainless steel forgings is less than about 1% by area, or even less than about 0.3% by area.

[0013] Scheme 6: A method for preparing large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forgings according to any one of Schemes 1 to 5 above, wherein the preparation method includes the following steps: Step 1: Provide molten steel, The elemental composition and content of the molten steel conform to the elemental composition and content of the 0Cr17Ni4Cu4Nb stainless steel forging as defined in Scheme 1 or 2 above, i.e., based on the total weight of the molten steel. Cr element in the range of approximately 15.00 to approximately 15.50% by weight. Ni element in the range of approximately 4.00 to approximately 5.00 wt%. Cu element in the range of approximately 3.00 to approximately 3.50% by weight. Nb element in the range of approximately 0.20 to approximately 0.35 wt%. N element in the range of approximately 0.03 to approximately 0.08 wt%. The balance consists of Fe and unavoidable impurity elements; The preferred choice is the element that is considered an unavoidable impurity: The content of element C is controlled to be below approximately 0.05% by weight. The Si content is controlled to be below approximately 0.50% by weight. The Mn content is controlled to be below approximately 0.50% by weight. The content of phosphorus (P) is controlled to be below approximately 0.030% by weight, and The sulfur content is controlled to be below approximately 0.003% by weight; Step 2: Molten steel casting The molten steel obtained from step 1 above is poured into a mold at a temperature of about 40 to about 60°C above the liquidus and then cooled to obtain an ingot. Step 3: Red-hot annealing of the ingot. The ingot obtained from step 2 above is subjected to red annealing at a temperature of about 850 to about 950°C for about 15 to about 30 hours. Step 4: Electroslag remelting of the ingot. The ingot obtained from step 3 above after red heat annealing is subjected to gas-supported electroslag remelting under inert gas protection, and then cooled in a crystallizer to obtain a steel ingot. Step 5: High-temperature homogenization of steel ingots. The steel ingot obtained from step 4 above after electroslag remelting is subjected to a second or more high-temperature homogenization treatment, wherein the temperature of the high-temperature homogenization treatment is in the range of about 1100 to about 1250°C, and the total high-temperature homogenization time is about 20 hours or more. Step 6: Forging of steel ingots, The steel ingot obtained from step 5 above, which has undergone high-temperature homogenization, is directly forged in a furnace, wherein the initial forging temperature is at least about 1050°C, the final forging temperature is at least about 850°C, and the total compression ratio of the forging is not less than about 5:1, thereby obtaining the large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forging.

[0014] Option 7: According to the preparation method described in Option 6 above, the provision of molten steel in step 1 includes smelting the raw materials using one or more smelting processes including electric furnace smelting, induction furnace mixing, oxygen-argon decarburization refining or vacuum oxygen blowing decarburization refining, ladle refining, and vacuum degassing refining to obtain molten steel; and controlling the chemical composition of the smelted molten steel to conform to the elemental composition and content of 0Cr17Ni4Cu4Nb stainless steel forging as defined in Option 1 above, or conforming to the elemental composition and content of molten steel as defined in Option 6 above.

[0015] Option 8: The preparation method described in Option 7 above, wherein the raw materials include one or more of stainless steel recycled materials, scrap steel, ferrosilicon, ferromanganese, ferrochrome, nickel plates, copper plates, and ferroniobium.

[0016] Option 9: The preparation method according to any one of Options 6 to 8 above, wherein the ingot obtained in step 2 is about 5 to about 10 tons, and the ingot melting rate in the gas-supported electroslag remelting process in step 4 is about 650 to about 800 kg per hour.

[0017] Option 10: The preparation method according to any one of Options 6 to 8 above, wherein the ingot obtained in step 2 is about > 10 tons to about 20 tons, and the ingot melting rate in the gas-supported electroslag remelting process in step 4 is about 750 to about 1100 kg per hour.

[0018] Option 11: The preparation method according to any one of Options 6 to 10 above, wherein in step 4, the specifications of the steel ingot obtained are such that the maximum dimension of the cross-sectional geometry is in the range of about 600 to about 1400 mm.

[0019] Option 12: The preparation method according to any one of Options 6 to 11 above, wherein in step 4, the electroslag remelting process includes purging the bottom of the crystallizer with argon gas for about 10 to about 25 minutes before arc ignition.

[0020] Scheme 13: The preparation method according to any one of Schemes 6 to 12 above, wherein in step 4, an arc-starting block of the same steel grade as the 0Cr17Ni4Cu4Nb stainless steel forging is used for arc starting during the electroslag remelting process.

[0021] Option 14: The preparation method according to any one of Options 6 to 13 above, wherein in step 6, the forging process includes three or more upsetting and drawing processes, and the upsetting reduction height in each upsetting and drawing process is not less than about 1 / 3 of the ingot body.

[0022] Option 15: The preparation method according to any one of Options 6 to 14 above, wherein the preparation method further includes heat treatment, inspection, finishing, flaw detection, packaging and warehousing of the large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forging obtained from step 6.

[0023] Technical effect The large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forgings of the present invention achieves uniform microstructure even with a maximum cross-sectional geometry ranging from approximately 400 to approximately 1000 mm by strictly controlling the elemental content and employing specific heat treatment and forming methods. Furthermore, the ferrite content of the forgings, measured by the most severe field-of-view method, is less than approximately 2% by area, preferably less than approximately 1% by area, and even reaches less than approximately 0.3% by area; the grain size is at least grade 5.

[0024] The preparation method of large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forgings of the present invention has a wide range of applications, and the stainless steel forgings prepared can be in the form of round steel, square steel, and flat steel, etc. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the specific embodiments of the present invention, the accompanying drawings used in the specific embodiments will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0026] Figure 1 Metallographic image of the outer periphery of a large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel circular forging provided in Embodiment 1 of the present invention.

[0027] Figure 2 Metallographic image of the cross section (1 / 2R, 1 / 2 radius) of the large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel circular forging provided in Embodiment 1 of the present invention.

[0028] Figure 3 Metallographic image of the center of the cross-section of a large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel circular forging provided in Embodiment 1 of the present invention. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Those skilled in the art should understand that the embodiments described are merely for the purpose of aiding understanding of this invention and should not be considered as specific limitations on this invention. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention. Process parameters in the following embodiments that are not specifically specified are generally performed under conventional conditions.

[0030] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. The term "about" as used in this invention means that the number it modifies may fluctuate within ±20%, ±15%, ±10%, ±5%, or ±2% of that number. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and the individual point values ​​contained within them, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0031] According to a first aspect of the invention, the present invention provides a large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forging. The large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forging according to the first aspect of the invention has the following specifications: the maximum dimension of the cross-sectional geometry is in the range of about 400 to about 1000 mm, for example, about 500 mm, about 600 mm, about 700 mm, about 800 mm, or about 900 mm, and wherein the ferrite content measured by the most severe field-of-view method is less than about 2% by area, preferably less than about 1% by area, and even less than about 0.3% by area.

[0032] In some exemplary preferred embodiments, the large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forgings according to the first aspect of the invention can be round bars with a cross-sectional diameter in the range of about 400 to about 1000 mm (e.g., about 500 mm, about 600 mm, about 700 mm, about 800 mm or about 900 mm), or square or flat bars with a maximum cross-sectional width in the range of about 400 to about 1000 mm (e.g., about 500 mm, about 600 mm, about 700 mm, about 800 mm or about 900 mm).

[0033] The large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forgings according to the first aspect of the present invention comprises or is composed of the following elements within defined content ranges: Based on the total weight of the 0Cr17Ni4Cu4Nb stainless steel forgings Cr element in the range of approximately 15.00 to approximately 15.50% by weight. Ni element in the range of approximately 4.00 to approximately 5.00 wt%. Cu element in the range of approximately 3.00 to approximately 3.50% by weight. Nb element in the range of approximately 0.20 to approximately 0.35 wt%. N element in the range of approximately 0.03 to approximately 0.08 wt%. The balance consists of Fe and unavoidable impurity elements.

[0034] In some preferred embodiments, the unavoidable impurity element can be: The content of element C is controlled to be below approximately 0.05% by weight. The Si content is controlled to be below approximately 0.50% by weight. The Mn content is controlled to be below approximately 0.50% by weight. The content of phosphorus (P) is controlled to be below approximately 0.030% by weight, and The sulfur content is controlled to be below approximately 0.003% by weight.

[0035] In the large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forging of the first aspect of the present invention, the design of the range of element content is mainly based on the following concept: Since the main composition of the material required by the Chinese national standard GB / T1220-2007 is relatively wide and ferrite is present in the microstructure, the composition design takes into account the effects of different elements on ferrite formation. Specifically, the Cr and Si element content in the forging is designed according to the lower limit specified in the Chinese national standard GB / T1220-2007, and the Ni element content is designed according to the upper limit specified in the standard. In addition, an appropriate amount of N element is also used in the forging to further reduce ferrite formation.

[0036] Furthermore, in the large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forgings of the first aspect of this invention, the C content should be as low as possible, preferably less than about 0.05% by weight, for example less than about 0.04% by weight or less than about 0.03% by weight. Reducing the C content helps to reduce carbide precipitation in stainless steel, thereby improving its corrosion resistance, high-temperature resistance, and resistance to intergranular corrosion.

[0037] Furthermore, Cu primarily functions as precipitation strengthening in the large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forgings of the first aspect of this invention. The specific Cu content may be fine-tuned according to the specific application environment and performance requirements of the material. The Cu used in the 0Cr17Ni4Cu4Nb stainless steel forgings of this invention can range from about 3.00 to about 3.50% by weight, preferably between about 3.2 and about 3.5% by weight, for example, about 3.3% by weight or about 3.4% by weight. The addition of Cu significantly improves the strength and hardness of the forgings of this invention. In addition, Cu also helps to improve the corrosion resistance of the 0Cr17Ni4Cu4Nb stainless steel forgings of this invention, giving them better performance in humid and corrosive environments.

[0038] Furthermore, in the large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forgings of the first aspect of this invention, Nb, through interaction with other elements, forms precipitates in the stainless steel, thereby achieving precipitation hardening and further enhancing the material's strength. The Nb used in the 0Cr17Ni4Cu4Nb stainless steel forgings of this invention can be in the range of about 0.20 to about 0.35% by weight, for example, about 0.25% by weight or about 0.30% by weight. Nb within this range imparts better wear resistance to the large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forgings of the first aspect of this invention.

[0039] In some further preferred embodiments, the Nb content can be designed to be about 5 times or more the C content, as is conventional in the art.

[0040] In addition, in some further preferred embodiments, the content of Mn, as an unavoidable impurity element, can be controlled to below about 1% by weight, preferably below about 0.50% by weight, for example below about 0.40% by weight or even below about 0.30% by weight, in accordance with the requirements of the Chinese National Standard GB / T1220-2007.

[0041] As an unavoidable impurity element, phosphorus (P) content should be as low as possible. However, since alloy raw materials typically introduce P, excessively low P content can lead to a significant increase in cost. Furthermore, P tends to segregate at grain boundaries, increasing grain boundary brittleness, leading to enhanced crack susceptibility, and deteriorating weldability and low-temperature reliability. According to the requirements of the Chinese National Standard GB / T1220-2007, the P content can be controlled as needed to be below approximately 0.040 wt%, preferably below approximately 0.030 wt%, for example, below approximately 0.025 wt% or even below approximately 0.020 wt%.

[0042] As an unavoidable impurity element, sulfur (S) exists primarily in steel as iron sulfide (FeS). Iron sulfide can form a low-melting-point eutectic with iron, with a melting point of approximately 989°C, lower than the hot working temperature of steel (typically around 1000°C to 1200°C). This causes grain boundary melting during processing, disrupting metal integrity and leading to "hot brittleness," severely deteriorating hot working performance. Therefore, a lower S content is better. Typically, according to the requirements of the Chinese national standard GB / T1220-2007, the S content can be controlled to below approximately 0.003% by weight, for example, below approximately 0.0025% by weight or even below approximately 0.002% by weight.

[0043] According to a second aspect of the present invention, the present invention provides a method for preparing large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forgings according to the first aspect of the present invention, wherein the preparation method includes steps 1 to 6.

[0044] Step 1: Provide molten steel The elemental composition and content of the molten steel provided in step 1 conform to the elemental composition and content of the 0Cr17Ni4Cu4Nb stainless steel forging as defined above in the first aspect of the present invention.

[0045] In some preferred embodiments, step 1 may include smelting the raw materials to obtain molten steel using one or more smelting processes including electric furnace smelting, induction furnace mixing, argon oxygen decarburization (AOD) refining or vacuum oxygen decarburization (VOD) refining, ladle furnace refining, and vacuum degassing (VD) refining; and controlling the chemical composition of the molten steel to conform to the elemental composition and content of the 0Cr17Ni4Cu4Nb stainless steel forging as defined above in the first aspect of the present invention, that is, the molten steel contains: Based on the total weight of the molten steel Cr element in the range of approximately 15.00 to approximately 15.50% by weight. Ni element in the range of approximately 4.00 to approximately 5.00 wt%. Cu element in the range of approximately 3.00 to approximately 3.50% by weight. Nb element in the range of approximately 0.20 to approximately 0.35 wt%. N element in the range of approximately 0.03 to approximately 0.08 wt%. The balance consists of Fe and unavoidable impurity elements; The preferred choice is the element that is considered an unavoidable impurity: The content of element C is controlled to be below approximately 0.05% by weight. The Si content is controlled to be below approximately 0.50% by weight. The Mn content is controlled to be below approximately 0.50% by weight. The content of phosphorus (P) is controlled to be below approximately 0.030% by weight, and The sulfur content is controlled to be below approximately 0.003% by weight.

[0046] In some further preferred embodiments, the raw materials for smelting or refining may include one or more of stainless steel reclaimed material, scrap steel, ferrosilicon, ferromanganese, ferrochrome, nickel plate, copper plate, and ferroniobium.

[0047] Step 2: Ingot Casting Step 2 involves pouring the molten steel obtained from Step 1 into a mold at a temperature of about 40 to about 60°C above the liquidus line and cooling it to obtain an ingot.

[0048] In step 2, the casting temperature should be between approximately 40 and 60°C (e.g., approximately 50°C) above the liquidus line. Specifically, the casting temperature should not exceed approximately 60°C above the liquidus line; otherwise, high superheat in the molten steel will result in slow solidification, potentially leading to well-developed columnar crystals, coarse grains, and increased central segregation. Furthermore, the casting temperature should not be lower than approximately 40°C above the liquidus line; otherwise, poor fluidity in the molten steel may result in premature solidification, easily causing defects such as shrinkage porosity, surface inclusions, and cracks.

[0049] Step 3: Red annealing Step 3 involves subjecting the ingot obtained from Step 2 to hot annealing at a temperature of approximately 850 to approximately 950°C for approximately 15 to approximately 30 hours. The purpose of hot annealing is to eliminate internal stress in the ingot, i.e., to eliminate residual stress generated during the forming process, reducing the risk of workpiece deformation or cracking; to refine the grains and homogenize the microstructure: improving and refining the internal grain structure of the metal, making the composition more uniform, thereby enhancing the overall mechanical properties; at the same time, it also makes the preparation process energy-efficient and efficient, and can also prevent cracks from forming during the subsequent cooling process.

[0050] In step 3, the holding temperature for the hot annealing should be in the range of about 850 to about 950°C, for example, about 900°C. Here, the upper limit of the holding temperature is generally not more than about 950°C in the art; the holding temperature should also not be too low, for example, it should not be lower than about 850°C, otherwise it may be detrimental to the complete elimination of internal stress.

[0051] Furthermore, in step 3, the holding time for the red annealing should be in the range of approximately 15 to approximately 30 hours, for example, approximately 20 hours or approximately 25 hours. Here, the holding time should not be too long, for example, not exceeding approximately 30 hours, otherwise it may lead to unnecessary energy waste; the holding time should also not be too short, for example, not less than approximately 15 hours, otherwise the purpose of the red annealing may not be well achieved.

[0052] Step 4: Electroslag Remelting Step 4 involves performing electroslag remelting (ESR) on the ingot obtained from step 3, which has undergone red-heat annealing, under an inert gas (e.g., nitrogen or argon) atmosphere, followed by cooling in a crystallizer to obtain a steel ingot. In the preparation method of this invention, the primary purpose of ESR is to purify the ingot and obtain a clean, uniformly dense steel ingot. ESR-remelted steel ingots have high purity, low sulfur content, few non-metallic inclusions, a smooth surface, are clean, uniform, and dense, and have a uniform metallographic structure and chemical composition. Furthermore, a crucial advantage of ESR is that the diameter of the ingot material is typically large. Using these ingots could compromise the quality of the core of the resulting forging. The steel ingots obtained after ESR typically have a maximum cross-sectional geometry ranging from approximately 400 to approximately 1400 mm, for example, approximately 600 or approximately 1000 mm.

[0053] In some preferred embodiments, the electroslag remelting process further includes purging the bottom of the crystallizer with argon gas for approximately 10 to 25 minutes, such as approximately 15 minutes or approximately 20 minutes, before arc ignition. The core purpose of purging the bottom of the crystallizer with argon gas is to establish a "barrier" between the molten steel and the air, a crucial protective measure for producing high-quality steel. The argon purging time should not be too long, for example, not exceeding approximately 25 minutes, otherwise it may lead to unnecessary waste of time and cost; nor should the argon purging time be too short, for example, not less than approximately 10 minutes, otherwise it may fail to provide the necessary protection.

[0054] In some other preferred embodiments, in order to prevent Cr element burn-off and ensure the consistency of composition at the beginning and end of the steel ingot, an arc-starting block of the same steel grade as the prepared 0Cr17Ni4Cu4Nb stainless steel forging is used for arc starting during the electroslag remelting process.

[0055] In some preferred embodiments, to reduce component segregation within the steel ingot, on the one hand, for ingots obtained in step 2 weighing approximately 5 to 10 tons, the ingot melting rate during the gas-supported electroslag remelting process in step 4 is approximately 650 to 800 kg per hour, for example, approximately 700 kg or approximately 750 kg per hour. Here, the ingot melting rate should not be too high, the purpose being to allow the ingot to "cool slowly." For example, the ingot melting rate should not exceed approximately 800 kg per hour; otherwise, although production efficiency will increase, it will give harmful structures and segregation sufficient time to grow, leading to a decrease in internal quality. On the other hand, the ingot melting rate should not be too low, the purpose being to allow the ingot to "cool rapidly." For example, the ingot melting rate should not be lower than approximately 650 kg per hour, which is beneficial for obtaining a fine and uniform structure, resulting in high internal quality of the forging. An excessively low ingot melting rate will reduce production efficiency and may affect surface finish.

[0056] On the other hand, in order to reduce component segregation inside the steel ingot, for the case where the ingot obtained in step 2 is > 10 to 20 tons, the ingot melting rate in the gas-supported electroslag remelting process in step 4 can be adjusted to about 750 to about 1100 kg per hour, for example about 800 kg per hour, about 900 kg per hour or about 1000 kg per hour.

[0057] Step 5: High-temperature homogenization Step 5 includes subjecting the steel ingot obtained from step 4 after electroslag remelting to at least two (i.e., two or more) high-temperature homogenization treatments, wherein the temperature of the high-temperature homogenization treatment is in the range of about 1100 to about 1250°C, and the total time of the high-temperature homogenization is about 20 hours or more. This high-temperature homogenization process can avoid quality problems that may occur during the slow cooling and reheating of the steel ingot, while also saving energy and shortening the production cycle.

[0058] The two homogenization processes in step 5 address different issues. The time control can be basically the same or different, but the temperature control is different. Generally, one process uses a relatively high temperature, and the other uses a relatively low temperature, to address the segregation problem of different elements.

[0059] In step 5, the holding temperature for high-temperature homogenization should be within the range of approximately 1100 to approximately 1250°C, for example, approximately 1150°C or approximately 1200°C. Here, the holding temperature should not be too high, for example, it should not exceed approximately 1250°C, otherwise the grain boundaries of the steel ingot may melt, potentially forming a large amount of irreversible high-temperature ferrite; the holding temperature should also not be too low, for example, it should not be below approximately 1100°C, otherwise the homogenization effect may not be achieved.

[0060] Furthermore, in step 6, the total holding time for high-temperature homogenization should be at least 20 hours, for example, about 25 hours, 30 hours, 35 hours, or 40 hours. Here, the total holding time should not be too short, for example, not less than about 20 hours, otherwise the homogenization effect may be poor, and many elements may not have enough time to diffuse fully. In fact, the element diffusion caused by high-temperature homogenization is the core of this invention. In the forging material of this invention, the more uniform the composition, the lower the probability of ferrite formation.

[0061] Step 6: Forging Step 6 involves directly forging the high-temperature homogenized steel ingot obtained from Step 5, wherein the initial forging temperature is at least approximately 1050°C, the final forging temperature is at least approximately 850°C, and the total compression ratio of the forging is not less than approximately 5:1, thereby obtaining the large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forging. The purpose of forging in Step 6 is to ensure that the obtained forging has a consistent mass across its entire cross-section.

[0062] It is known in the art that "forging compression ratio" generally refers to the degree of deformation of a material in the height direction during forging, i.e., the upsetting ratio or compression ratio, which is defined as the ratio of the height before forging to the height after forging, or the ratio of the cross-sectional area after forging to the cross-sectional area before forging. Since both the upsetting and finishing processes used in forging have their own compression ratios, their sum is the total compression ratio.

[0063] In step 6, forging and the preceding high-temperature homogenization are a continuous process, and the temperature of the steel ingot after high-temperature heating will only decrease. When the ingot temperature drops below approximately 900°C, the risk of surface cracking increases. Therefore, in this invention, the final forging temperature is set to at least approximately 850°C, preferably at least approximately 900°C. In step 6, the total compression ratio of the forging should be greater than approximately 5:1, for example, approximately 5.5:1, approximately 6:1, or approximately 6.5:1. Here, the total compression ratio should not be too low, for example, not lower than approximately 5:1, otherwise it may result in the ingot not being fully forged and having poor core properties. A forging compression ratio of approximately 5:1 or higher can promote the fragmentation and refinement of ferrite.

[0064] In addition, in step 6, in order to achieve a total compression ratio of about 5:1 or more in the forging process, the forging process preferably includes three or more upsetting processes, wherein after each upsetting process, since the steel may cool naturally, the forging may optionally be reheated to the forging temperature in the furnace.

[0065] Furthermore, in some preferred embodiments of step 6, the upsetting reduction height during each upsetting process is not less than about 1 / 3 of the ingot body. Here, the upsetting reduction height should not be less than about 1 / 3 of the ingot body; otherwise, the deformation will be too small and the objective will not be achieved.

[0066] In a further preferred embodiment, the preparation method according to the second aspect of the present invention, after step 6, further includes subjecting the large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forging obtained from step 6 to further heat treatment, inspection, finishing, flaw detection, packaging, and warehousing. The heat treatment process here is not specifically limited, and conventional heat treatment methods for this steel grade can be used.

[0067] The present invention will now be described in further detail with reference to specific embodiments and comparative examples.

[0068] Example 1 Step 1: Using approximately 40 tons of recycled 0Cr17Ni4Cu4Nb stainless steel, scrap steel, ferrosilicon, ferromanganese, ferrochrome, nickel plates, copper plates, and ferroniobium as raw materials, molten steel is obtained through an electric arc furnace smelting + induction furnace mixing + VOD refining + LF refining + VD refining process. The smelting process is largely based on commonly used techniques in the field, with the refining furnace performing fine-tuning of the composition to ensure that elements such as C, Cr, and Ni are within the designed range. Additionally, vacuum degassing is performed during the smelting process to ensure extremely low oxygen content. The obtained molten steel contains, based on the total weight of the molten steel (analyzed according to ASTM A751): Approximately 0.04% by weight of carbon element, Approximately 0.32% by weight of Si element, Approximately 0.47% by weight of Mn element, Approximately 0.023% by weight of P element, Approximately 0.001% by weight of sulfur. Approximately 15.25% by weight of Cr element, Approximately 4.47% by weight of Ni element, Approximately 3.25% by weight of Cu element, Approximately 0.23% by weight of Nb element, Approximately 0.0478% by weight of nitrogen. The balance consists of Fe and other unavoidable impurity elements; Step 2: Molten steel casting The molten steel obtained from step 1 above is poured into a mold at a temperature of about 50°C above the liquidus and then cooled to obtain four ingots, each weighing about 10 tons. Step 3: Red-hot annealing of the ingot. The ingot obtained from step 2 above is subjected to red annealing at a temperature of about 925°C for about 20 hours. Step 4: Electroslag remelting of the ingot. The ingot obtained from step 3 above after red-heat annealing is subjected to gas-supported electroslag remelting under argon gas protection. This includes purging the bottom of the crystallizer with argon gas for about 20 minutes before arc initiation, and obtaining a round steel ingot after cooling in the crystallizer. The cross-sectional diameter of the obtained round steel ingot is about 885 mm. The electroslag remelting uses a whole 0Cr17Ni4Cu4Nb base pad for arc initiation, and the ingot remelting rate is about 740 kg per hour. Step 5: High-temperature homogenization of steel ingots. The steel ingot obtained from step 4 above is subjected to two high-temperature homogenization treatments, wherein the temperature of the first high-temperature homogenization treatment is about 1150°C and the high-temperature homogenization time is about 35 hours, and the temperature of the second high-temperature homogenization treatment is about 1180°C and the high-temperature homogenization time is about 4 hours. Step 6: Forging of steel ingots, The high-temperature homogenized steel ingot obtained from step 5 above is directly subjected to a three-stage upsetting and drawing process at an initial temperature of about 1180°C and a total compression ratio of about 6.5:1. Each upsetting process involves a reduction of at least 1 / 3 of the ingot body. After the upsetting and drawing process, the temperature of the forging material is reduced to about 1160°C, thereby obtaining the large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forging material with a cross-sectional diameter of about 600 mm.

[0069] Example 2 Step 1: Using approximately 40 tons of recycled 0Cr17Ni4Cu4Nb stainless steel, scrap steel, ferrosilicon, ferromanganese, ferrochrome, nickel plates, copper plates, and ferroniobium as raw materials, molten steel is obtained through an electric arc furnace smelting + induction furnace mixing + VOD refining + LF refining + VD refining process. The smelting process is largely based on commonly used techniques in the field, with the refining furnace performing fine-tuning of the composition to ensure that elements such as C, Cr, and Ni are within the designed range. Additionally, vacuum degassing is performed during the smelting process to ensure extremely low oxygen content. The obtained molten steel contains, based on the total weight of the molten steel (analyzed according to ASTM A751): Approximately 0.03% by weight of carbon element, Approximately 0.37% by weight of Si element, Approximately 0.44% by weight of Mn element, Approximately 0.021% by weight of P element, Approximately 0.001% by weight of sulfur. Approximately 15.13% by weight of Cr element, Approximately 4.33% by weight of Ni element, Approximately 3.14% by weight of Cu element, Approximately 0.21% by weight of Nb element, and Approximately 0.0441% by weight of nitrogen. The balance consists of Fe and other unavoidable impurity elements; Step 2: Molten steel casting The molten steel obtained from step 1 above is poured into a mold at a temperature of about 55°C above the liquidus and then cooled to obtain two ingots, each weighing about 20 tons. Step 3: Red-hot annealing of the ingot. The ingot obtained from step 2 above is subjected to red annealing at a temperature of about 900°C for about 30 hours. Step 4: Electroslag remelting of the ingot. The ingot obtained from step 3 above after red-heat annealing is subjected to gas-supported electroslag remelting under argon gas protection. This includes purging the bottom of the crystallizer with argon gas for about 20 minutes before arc initiation, and obtaining a round steel ingot after cooling in the crystallizer. The cross-sectional diameter of the obtained round steel ingot is about 1175 mm. The electroslag remelting uses a whole 0Cr17Ni4Cu4Nb base pad for arc initiation, and the ingot remelting rate is about 1000 kg per hour. Step 5: High-temperature homogenization of steel ingots. The steel ingot obtained from step 4 above is subjected to two high-temperature homogenization treatments, wherein the temperature of the first high-temperature homogenization treatment is about 1150°C and the high-temperature homogenization time is about 35 hours, and the temperature of the second high-temperature homogenization treatment is about 1180°C and the high-temperature homogenization time is about 4 hours. Step 6: Forging of steel ingots, The steel ingot obtained from step 5 above, which has undergone high-temperature homogenization, is directly subjected to a three-stage upsetting process at an initial temperature of approximately 1180°C and a total compression ratio of approximately 5:1. Each upsetting process involves a reduction of at least 1 / 3 of the ingot body. After the upsetting process, the temperature of the forging material is reduced to approximately 1160°C, thereby obtaining the large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forging material with a cross-sectional diameter of approximately 1000 mm.

[0070] Comparative Example 1 (similar to Example 2, except that high-temperature homogenization was not performed) Step 1: Using approximately 40 tons of recycled 0Cr17Ni4Cu4Nb stainless steel, scrap steel, ferrosilicon, ferromanganese, ferrochrome, nickel plates, copper plates, and ferroniobium as raw materials, molten steel is obtained through an electric arc furnace smelting + induction furnace mixing + VOD refining + LF refining + VD refining process. The smelting process is largely based on commonly used techniques in the field, with the refining furnace performing fine-tuning of the composition to ensure that elements such as C, Cr, and Ni are within the designed range. Additionally, vacuum degassing is performed during the smelting process to ensure extremely low oxygen content. The obtained molten steel contains, based on the total weight of the molten steel (analyzed according to ASTM A751): Approximately 0.03% by weight of carbon element, Approximately 0.37% by weight of Si element, Approximately 0.44% by weight of Mn element, Approximately 0.021% by weight of P element, Approximately 0.001% by weight of sulfur. Approximately 15.13% by weight of Cr element, Approximately 4.33% by weight of Ni element, Approximately 3.14% by weight of Cu element, Approximately 0.21% by weight of Nb element, and Approximately 0.0441% by weight of nitrogen. The balance consists of Fe and other unavoidable impurity elements; Step 2: Molten steel casting The molten steel obtained from step 1 above is poured into a mold at a temperature of about 55°C above the liquidus and then cooled to obtain two ingots, each weighing about 20 tons. Step 3: Red-hot annealing of the ingot. The ingot obtained from step 2 above is subjected to red annealing at a temperature of about 900°C for about 30 hours. Step 4: Electroslag remelting of the ingot. The ingot obtained from step 3 above after red-heat annealing is subjected to gas-supported electroslag remelting under argon gas protection. This includes purging the bottom of the crystallizer with argon gas for about 20 minutes before arc initiation, and obtaining a round steel ingot after cooling in the crystallizer. The cross-sectional diameter of the obtained round steel ingot is about 1175 mm. The electroslag remelting uses a whole 0Cr17Ni4Cu4Nb base pad for arc initiation, and the ingot remelting rate is about 1000 kg per hour. Step 5: Forging of steel ingots, The steel ingot obtained from step 4 above is heated to about 1180°C in a heating furnace and then sent for forging. It undergoes three upsetting and drawing processes with a total compression ratio of about 5:1, wherein the height of each upsetting is not less than about 1 / 3 of the ingot body. After the upsetting and drawing process, the temperature of the forging material is reduced to about 1160°C, thereby obtaining the large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forging material with a cross-sectional diameter of about 1000 mm.

[0071] Comparative Example 2 (similar to Example 2, except that the upsetting step is not performed). Step 1: Using approximately 40 tons of recycled 0Cr17Ni4Cu4Nb stainless steel, scrap steel, ferrosilicon, ferromanganese, ferrochrome, nickel plates, copper plates, and ferroniobium as raw materials, molten steel is obtained through an electric arc furnace smelting + induction furnace mixing + VOD refining + LF refining + VD refining process. The smelting process is largely based on commonly used techniques in the field, with the refining furnace performing fine-tuning of the composition to ensure that elements such as C, Cr, and Ni are within the designed range. Additionally, vacuum degassing is performed during the smelting process to ensure extremely low oxygen content. The obtained molten steel contains, based on the total weight of the molten steel (analyzed according to ASTM A751): Approximately 0.03% by weight of carbon element, Approximately 0.37% by weight of Si element, Approximately 0.44% by weight of Mn element, Approximately 0.021% by weight of P element, Approximately 0.001% by weight of sulfur. Approximately 15.13% by weight of Cr element, Approximately 4.33% by weight of Ni element, Approximately 3.14% by weight of Cu element, Approximately 0.21% by weight of Nb element, and Approximately 0.0441% by weight of nitrogen. The balance consists of Fe and other unavoidable impurity elements; Step 2: Molten steel casting The molten steel obtained from step 1 above is poured into a mold at a temperature of about 55°C above the liquidus and then cooled to obtain two ingots, each weighing about 20 tons. Step 3: Red-hot annealing of the ingot. The ingot obtained from step 2 above is subjected to red annealing at a temperature of about 900°C for about 30 hours. Step 4: Electroslag remelting of the ingot. The ingot obtained from step 3 above after red-heat annealing is subjected to gas-supported electroslag remelting under argon gas protection. This includes purging the bottom of the crystallizer with argon gas for about 20 minutes before arc initiation, and obtaining a round steel ingot after cooling in the crystallizer. The cross-sectional diameter of the obtained round steel ingot is about 1175 mm. The electroslag remelting uses a whole 0Cr17Ni4Cu4Nb base pad for arc initiation, and the ingot remelting rate is about 1000 kg per hour. Step 5: High-temperature homogenization of steel ingots. The steel ingot obtained from step 4 above is subjected to two high-temperature homogenization treatments, wherein the temperature of the first high-temperature homogenization treatment is about 1150°C and the high-temperature homogenization time is about 35 hours, and the temperature of the second high-temperature homogenization treatment is about 1180°C and the high-temperature homogenization time is about 4 hours. Step 6: Forging of steel ingots, The high-temperature homogenized steel ingot obtained from step 5 above is directly drawn at a temperature of 1160°C to obtain the large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forging with a cross-sectional diameter of approximately 1000 mm.

[0072] Comparative Example 3 (similar to Example 2, except that the high-temperature homogenization and upsetting steps are not performed). Step 1: Using approximately 40 tons of recycled 0Cr17Ni4Cu4Nb stainless steel, scrap steel, ferrosilicon, ferromanganese, ferrochrome, nickel plates, copper plates, and ferroniobium as raw materials, molten steel is obtained through an electric arc furnace smelting + induction furnace mixing + VOD refining + LF refining + VD refining process. The smelting process is largely based on commonly used techniques in the field, with the refining furnace performing fine-tuning of the composition to ensure that elements such as C, Cr, and Ni are within the designed range. Additionally, vacuum degassing is performed during the smelting process to ensure extremely low oxygen content. The obtained molten steel contains, based on the total weight of the molten steel (analyzed according to ASTM A751): Approximately 0.03% by weight of carbon element, Approximately 0.37% by weight of Si element, Approximately 0.44% by weight of Mn element, Approximately 0.021% by weight of P element, Approximately 0.001% by weight of sulfur. Approximately 15.13% by weight of Cr element, Approximately 4.33% by weight of Ni element, Approximately 3.14% by weight of Cu element, Approximately 0.21% by weight of Nb element, and Approximately 0.0441% by weight of nitrogen. The balance consists of Fe and other unavoidable impurity elements; Step 2: Molten steel casting The molten steel obtained from step 1 above is poured into a mold at a temperature of about 55°C above the liquidus and then cooled to obtain two ingots, each weighing about 20 tons. Step 3: Red-hot annealing of the ingot. The ingot obtained from step 2 above is subjected to red annealing at a temperature of about 900°C for about 30 hours. Step 4: Electroslag remelting of the ingot. The ingot obtained from step 3 above after red-heat annealing is subjected to gas-supported electroslag remelting under argon gas protection. This includes purging the bottom of the crystallizer with argon gas for about 20 minutes before arc initiation, and obtaining a round steel ingot after cooling in the crystallizer. The cross-sectional diameter of the obtained round steel ingot is about 1175 mm. The electroslag remelting uses a whole 0Cr17Ni4Cu4Nb base pad for arc initiation, and the ingot remelting rate is about 1000 kg per hour. Step 5: Forging of steel ingots, The steel ingot obtained from step 4 above is heated to about 1160°C in a heating furnace and then drawn once to obtain the large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forging with a cross-sectional diameter of about 1000 mm.

[0073] The ferrite content of the head and tail four-quadrant surfaces, 1 / 2R (1 / 2 radius), and core of the multiple forgings obtained in Examples 1 and 2 was evaluated according to the most severe field of view method described in AMS2315.

[0074] Instruction manual attached Figures 1 to 3 The metallographic images of the 600 mm forging 1 obtained in Embodiment 1 of the present invention, including the outer periphery, 1 / 2R, and core in the first quadrant, are shown as an example. It can be seen from these metallographic images that the ferrite content of the forging is 0% in all areas. Furthermore, the grain size of the 600 mm forging 1 obtained in Embodiment 1, as determined by the rating method described in ASTM E112, is grade 6-5.

[0075] The ferrite content (area %) of the forging material in Example 1, including the four quadrants at the head and tail, 1 / 2R, and the core, is shown in the table below: The head of forging material 1 in Example 1 Tail end of forging 1 in Example 1 The head of forging material 2 in Example 1 Tail end of forging material 2 in Example 1 The ferrite content and grain size of the 0Cr17Ni4Cu4Nb stainless steel forgings obtained in Examples 1 and 2 and Comparative Examples 1 to 3, as determined by the rating method described in AMS 2315, are as follows: The ferrite content of the 0Cr17Ni4Cu4Nb stainless steel forgings obtained in Examples 1 and 2 was much less than 1% by area, and the grain size was grade 5.5.

[0076] The ferrite content of the 0Cr17Ni4Cu4Nb stainless steel forging obtained in Comparative Example 1 was 4% and the grain size was 4.5. The ferrite content of the 0Cr17Ni4Cu4Nb stainless steel forging obtained in Comparative Example 2 was 3% and the grain size was 4.5%. The ferrite content of the 0Cr17Ni4Cu4Nb stainless steel forging obtained in Comparative Example 3 was 6% by area and the grain size was grade 4.

[0077] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions claimed by the present invention.

Claims

1. A large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forging, wherein the 0Cr17Ni4Cu4Nb stainless steel forging has a maximum cross-sectional geometry dimension in the range of 400 to 1000 mm, and a ferrite content of less than 2% by area, characterized in that, The 0Cr17Ni4Cu4Nb stainless steel forging is composed of the following elements: based on the total weight of the 0Cr17Ni4Cu4Nb stainless steel forging. Cr element in the range of 15.00 to 15.50 wt%, Ni element in the range of 4.00 to 5.00 wt%, Cu element in the range of 3.00 to 3.50 wt%, Nb element in the range of 0.20 to 0.35 wt%, N element in the range of 0.03 to 0.08 wt%, The balance consists of Fe and unavoidable impurity elements. The preferred choice is the element that is considered an unavoidable impurity: The content of element C is controlled below 0.05% by weight. The Si content is controlled below 0.50% by weight. The Mn content is controlled below 0.50% by weight. The content of phosphorus (P) is controlled below 0.030% by weight, and The sulfur content is controlled to be below 0.003% by weight.

2. The ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forging according to claim 1, characterized in that, The content of Nb element is more than 5 times that of C element.

3. The ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forging according to claim 1, characterized in that, The 0Cr17Ni4Cu4Nb stainless steel forgings are round bars with a cross-sectional diameter in the range of 400 to 1000 mm, or square or flat bars with a maximum cross-sectional width in the range of 400 to 1000 mm.

4. The ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forging according to any one of claims 1 to 3, characterized in that, The ferrite content of the 0Cr17Ni4Cu4Nb stainless steel forging is below 1% by area, or even below 0.3% by area.

5. A method for preparing large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forgings according to any one of claims 1 to 4, characterized in that, The preparation method includes the following steps: Step 1: Provide molten steel, The elemental composition and content of the molten steel conform to the elemental composition and content of the 0Cr17Ni4Cu4Nb stainless steel forging as defined in claim 1. Step 2: Molten steel casting The molten steel obtained from step 1 above is poured into a mold at a temperature of 40 to 60°C above the liquidus and then cooled to obtain an ingot. Step 3: Red-hot annealing of the ingot. The ingot obtained from step 2 above is subjected to red annealing at a temperature of 850 to 950°C for 15 to 30 hours. Step 4: Electroslag remelting of the ingot. The ingot obtained from step 3 above after red heat annealing is subjected to gas-supported electroslag remelting under inert gas protection, and then cooled in a crystallizer to obtain a steel ingot. Step 5: High-temperature homogenization of steel ingots. The steel ingot obtained from step 4 above after electroslag remelting is subjected to a second or more high-temperature homogenization treatment, wherein the temperature of the high-temperature homogenization treatment is in the range of 1100 to 1250°C and the total high-temperature homogenization time is more than 20 hours. Step 6: Forging of steel ingots, The steel ingot obtained from step 5 above after high-temperature homogenization is directly forged in a furnace, wherein the initial forging temperature is at least 1050°C, the final forging temperature is at least 850°C, and the total compression ratio of the forging is not less than 5:1, thereby obtaining the large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forging.

6. The preparation method according to claim 5, characterized in that, The provision of molten steel in step 1 includes smelting the raw materials using one or more smelting processes, including electric furnace smelting, induction furnace mixing, oxygen-argon decarburization refining or vacuum oxygen blowing decarburization refining, ladle refining, and vacuum degassing refining, to obtain molten steel; and controlling the chemical composition of the smelted molten steel to conform to the elemental composition and content of the 0Cr17Ni4Cu4Nb stainless steel forging as defined in claim 1.

7. The preparation method according to claim 6, characterized in that, The raw materials include one or more of the following: recycled stainless steel, scrap steel, ferrosilicon, ferromanganese, ferrochrome, nickel plate, copper plate, and ferroniobium.

8. The preparation method according to claim 5, characterized in that, The ingot obtained in step 2 is 5 to 10 tons, and the ingot melting rate during the gas-supported electroslag remelting process in step 4 is 650 to 800 kg per hour; or The ingots obtained in step 2 are >10 to 20 tons, and the ingot melting rate in the gas-supported electroslag remelting process in step 4 is 750 to 1100 kg per hour.

9. The preparation method according to claim 5, characterized in that, In step 4, The obtained steel ingots have the following specifications: the maximum dimension of the cross-sectional geometry is in the range of 600 to 1400 mm; and / or The electroslag remelting process includes purging the bottom of the crystallizer with argon gas for 10 to 25 minutes before arc ignition, and / or During the electroslag remelting process, an arc-starting block of the same steel grade as the 0Cr17Ni4Cu4Nb stainless steel forging is used for arc starting. and / or In step 6, the forging process includes three or more upsetting and drawing processes, and the upsetting reduction height in each upsetting and drawing process is not less than 1 / 3 of the ingot body.

10. The preparation method according to any one of claims 5 to 9, characterized in that, The preparation method also includes further heat treatment, inspection, finishing, flaw detection, packaging and warehousing of the large-size ultra-low ferritic 0Cr17Ni4Cu4Nb stainless steel forgings obtained from step 6.

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