Preparation method of large-size ultralow-expansion alloy electroslag steel ingot

Abstract

The invention provides a preparation method of a large-size ultralow-expansion alloy electroslag steel ingot, vacuum induction melting is firstly adopted, then electroslag remelting forming is adopted, and a large electrode bar adopted in the electroslag remelting forming step is formed by welding three small electrode bars in parallel; and the at least two large electrode bars are alternately remelted. The large-size ultralow-expansion alloy electroslag steel ingot with the diameter being 1200 mm and the piece weight being 20 tons or above can be successfully prepared, the steel ingot is good in quality, the surface of the steel ingot is free of defects such as air holes, cracks and slag grooves, the hot forging performance of the steel ingot is good, and the problem that the large-size ultralow-expansion alloy electroslag steel ingot is difficult to prepare from ultralow-expansion alloy through a single small-specification electrode bar is solved.

Description

Technical Field

[0001] The invention belongs to the technical field of metal materials, and in particular relates to a method for preparing a large-size ultra-low expansion alloy electroslag steel ingot. Background Art

[0002] Ultra-low expansion alloy steel ingots are usually formed by direct casting by vacuum induction melting or by vacuum induction melting plus electroslag remelting. When vacuum induction melting is directly cast, the utilization rate of the material is often very low due to defects such as shrinkage cavities and looseness in the ingot. Therefore, the manufacturing method of vacuum induction melting plus electroslag remelting is widely used: first, vacuum induction melting is used to cast the ingot, and the ingot is cleaned and used as an electrode rod for electroslag remelting, and then the electrode rod is remelted to make a large-sized ultra-low expansion alloy electroslag steel ingot. With the application and promotion of ultra-low expansion alloy materials, large optical platforms require larger-sized ultra-low expansion alloy materials, which requires the preparation of 20-ton ultra-large ingot-shaped ultra-low expansion alloy steel ingots. However, due to the limitation of equipment capacity, it is impossible to prepare a 20-ton electrode rod at one time, and it is difficult to weld multiple electrode rods in series into one electrode rod.

[0003] Therefore, those skilled in the art are committed to developing a method for preparing a large-size ultra-low expansion alloy electroslag steel ingot with a diameter of 1200 mm and a single weight of more than 20 tons. Summary of the invention

[0004] The purpose of the present invention is to provide a method for preparing large-size ultra-low expansion alloy electroslag steel ingots with a diameter of 1200 mm and a single weight of 20 tons or more in view of the corresponding deficiencies of the prior art.

[0005] The objective of the present invention is achieved by adopting the following scheme: a method for preparing a large-sized ultra-low expansion alloy electroslag ingot, which first adopts vacuum induction melting and then adopts electroslag remelting to form. The large electrode rod used in the electroslag remelting forming step is formed by welding three small electrode rods in parallel; at least two of the large electrode rods are remelted in succession.

[0006] Further, the following steps are included:

[0007] S1. Vacuum induction melting:

[0008] S1.1. Batching: weigh each component according to the chemical composition batching table of the large-size ultra-low expansion alloy electroslag steel ingot;

[0009] S1.2. High temperature refining: Add some alloy elements into the furnace and refine at a temperature of 1600-1620℃ for 70-80 minutes;

[0010] S1.3. Low temperature refining: Add the remaining alloying elements into the furnace, perform sufficient electromagnetic stirring after adding the materials, and refine at a temperature of 1480-1520°C for 30-40 minutes;

[0011] S1.4. Casting: Casting into cast bars;

[0012] S2. Electroslag remelting:

[0013] S2.1. Preparation of slag for electroslag remelting: The proportion of slag for electroslag remelting is as follows: CaF 2 : 50~60, CaO: 20~25, Al 2 O 3 : 10-15, MgO: 5-15;

[0014] S2.2. Prepare a large electrode rod: weld three small electrode rods in parallel to form a large electrode rod, and arrange the three small electrode rods in a nearly equilateral triangle;

[0015] S2.3. Preheating the large electrode rod: heating the large electrode rod to 1000-1200°C;

[0016] S2.4. Remelting: Use a crystallizer of corresponding size, place the cast rod and slag into the crystallizer, insert the large electrode rod into the slag, and after powering on and arcing, adjust the remelting voltage to 70-100V and the current to 15-22kA; when one of the large electrode rods is melted, move the other large electrode rod to the smelting station to achieve electrode remelting, and obtain a large-sized ultra-low expansion alloy electroslag ingot;

[0017] Furthermore, in step S2.4, the replacement time of the replacement electrode is less than or equal to 1 minute.

[0018] Furthermore, in step S1.4, before casting, the molten steel needs to be left to stand in an argon environment for 10 minutes and the temperature is adjusted to 1460°C.

[0019] Furthermore, in step S1.4, cast rods of the same length and a diameter of 420-450 mm are cast, and the number of the cast rods is 15.

[0020] Furthermore, in step S1.2, refining is performed under a vacuum degree better than 3Pa.

[0021] Furthermore, in step S1.3, refining is performed under a vacuum degree better than 1 Pa.

[0022] Furthermore, the number of the large electrode rods is 5.

[0023] Furthermore, the large-size ultra-low expansion alloy electroslag steel ingot has a diameter of 1200 mm and a weight of 20 to 22 tons.

[0024] Furthermore, the chemical composition of the large-size ultra-low expansion alloy electroslag steel ingot is, by weight percentage: C: ≤0.05, Si: ≤0.10, Mn: ≤0.30, S: ≤0.010, P: ≤0.010, Al: 0-0.40, Ti: 0-0.40, Nb: 0.4-0.8, Ni: 31.5-34.0, Co: 3.20-4.50, and the balance is Fe and unavoidable impurities.

[0025] The advantages of the present invention are:

[0026] (1) In the process of manufacturing a large-sized ultra-low expansion alloy electroslag steel ingot using the ultra-low expansion alloy steel, three small electrode rods with a diameter of 420 to 450 mm obtained by vacuum induction melting are welded in parallel to obtain a large electrode rod, and then an electroslag remelting process of replacing the electrode remelting is used to successfully prepare a large-sized ultra-low expansion alloy electroslag steel ingot with a diameter of 1200 mm and a single weight of more than 20 tons;

[0027] (2) The slag material formula of the present invention can ensure that the temperature, fluidity, thermal conductivity, etc. of the slag pool during the electroslag remelting process reach the optimal state, which is beneficial to the melting, filtering, and resolidification of the electrode rod;

[0028] (3) Preheating the electrode rods can prevent the temperature of the electrode rods from being too low during the exchange of the electrode rods, which will lead to a decrease in the temperature of the slag pool, thereby quickly entering the smelting state and avoiding the formation of slag grooves;

[0029] (4) The steel ingot has good quality, with no defects such as pores, cracks, slag grooves, etc. on the surface, and the hot forging performance of the steel ingot is good, which solves the problem that it is difficult to prepare large-size ultra-low expansion alloy electroslag steel ingots with ultra-low expansion alloys;

[0030] (5) By matching the electrode rod size with the crystallizer size of the present invention, an optimal filling ratio can be achieved, ensuring a stable electroslag remelting process. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a schematic diagram of the structure of the large electrode of the present invention;

[0032] Figure 2 The invention is a large-size ultra-low expansion alloy electroslag steel ingot weighing 22 tons and φ1200mm;

[0033] Figure 3 The large-sized ultra-low expansion alloy forging of the present invention weighs 22 tons and has a diameter of 1200 mm. DETAILED DESCRIPTION

[0034] In order to enable those skilled in the art to understand the characteristics and effects of the present application, the following is a general description and definition of the terms and expressions mentioned in the specification and claims. Unless otherwise specified, all technical and scientific terms used herein have the common meanings understood by those skilled in the art for the present application. In case of conflict, the definitions in this specification shall prevail.

[0035] The theories or mechanisms described and disclosed herein, whether correct or incorrect, should not limit the scope of the present application in any way, that is, the content of the present application can be implemented without being limited by any specific theory or mechanism.

[0036] As used herein, "this application" means "the present invention" or "the present disclosure".

[0037] The use of "one", "an", "a kind" or similar expressions to describe the components and technical features described in this application is merely for the convenience of expression and to provide a general meaning to the scope of this application. Therefore, such description should be understood to include one or at least one, and the singular also includes the plural, unless it is obvious that it refers to another meaning.

[0038] In this document, "or its combination" means "or any combination thereof", and "any one", "any one" means "any one", "any one" or "any one".

[0039] As used herein, the terms "comprises," "including," "having," "containing," or any other similar terms are open-ended transitional phrases that are intended to cover non-exclusive inclusions. For example, a composition or article containing multiple elements is not limited to the elements listed herein, but may also include other elements that are not explicitly listed but are generally inherent to the composition or article. In addition, unless otherwise expressly stated, the term "or" refers to an inclusive "or" rather than an exclusive "or." For example, any of the following situations satisfies the condition "A or B": A is true (or exists) and B is false (or does not exist), A is false (or does not exist) and B is true (or exists), and both A and B are true (or exist). In addition, in this article, the terms "comprising", "including", "having", and "containing" should be interpreted as being specifically disclosed and simultaneously covering closed connections such as "consisting of", "consisting of", "the remainder of", and "substantially consisting of", "mainly consisting of", "mainly consisting of", "basically containing", "essentially consisting of", "essentially consisting of", "essentially containing", and other connecting words.

[0040] In this article, all features or conditions such as numerical values, quantities, contents and concentrations defined in the form of numerical ranges or percentage ranges are only for simplicity and convenience. Accordingly, the description of numerical ranges or percentage ranges should be deemed to have covered and specifically disclosed all possible sub-ranges and individual values ​​within the range (including integers and fractions), especially integer values. For example, the range description of "1.0 to 8.0" or "between 1.0 and 8.0" or "between 1.0 and 8.0" should be deemed to have specifically disclosed all sub-ranges such as 1.0 to 8.0, 1.0 to 7.0, 2.0 to 8.0, 2.0 to 6.0, 3.0 to 6.0, 4.0 to 8.0, 3.0 to 8.0, etc., and should be deemed to cover endpoint values, especially sub-ranges defined by integer values, and should be deemed to have specifically disclosed individual values ​​such as 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, etc. Unless otherwise indicated, the foregoing method of interpretation applies to all contents of the entire application, regardless of whether the scope is broad or not.

[0041] If the quantity, concentration or other numerical value or parameter is expressed as a range, a preferred range (or a better range) or a series of upper and lower limits, it should be understood that all ranges consisting of any pair of the upper limit or preferred value (or a better value) of the range and the lower limit or preferred value (or a better value) of the range have been specifically disclosed herein, regardless of whether these ranges are disclosed separately. In addition, if a numerical range is mentioned herein, unless otherwise specified, the range should include its endpoints and all integers and fractions within the range.

[0042] In this document, numerical values ​​should be understood to have the accuracy of the number of significant digits of the numerical value, provided that the purpose of the invention can be achieved. For example, the number 40.0 should be understood to cover the range of 39.50 to 40.49.

[0043] It should be understood that the features disclosed in the various embodiments herein may be arbitrarily combined to form the technical solution of the present application, as long as there is no contradiction in the combination of these features.

[0044] The present application will be described below with specific implementations and examples. It should be understood that these specific implementations and examples are merely illustrative and are not intended to limit the scope of the present application and its use.

[0045] Unless otherwise stated, the methods, reagents and conditions used in the following preparation examples, comparative examples and examples are conventional methods, reagents and conditions in the art.

[0046] Example 1

[0047] The diameter of the large-size ultra-low expansion alloy electroslag steel ingot is 1200 mm, and its chemical composition is, by weight percentage: C: 0.05, Si: 0.10, Mn: 0.30, S: 0.010, P: 0.010, Al: 0.2, Ti: 0.4, Nb: 0.8, Ni: 31.5, Co: 4.5, and the balance is Fe and unavoidable impurities.

[0048] The preparation process includes the following steps:

[0049] S1. Vacuum induction melting:

[0050] S1.1. Batching: weigh each component according to the chemical composition batching table of the large-size ultra-low expansion alloy electroslag steel ingot;

[0051] S1.2. High temperature refining: put some alloy elements Fe, Co, Nb and Ni into the furnace, refine at a temperature of 1600-1620℃ for 80 minutes, and refine under a vacuum degree better than 3Pa;

[0052] S1.3. Low temperature refining: Add the remaining alloying elements, namely C, Si, Mn, Ti, and Al, into the furnace, perform sufficient electromagnetic stirring after adding the materials, and refine at a temperature of 1480-1520°C for 30 minutes under a vacuum degree better than 1Pa;

[0053] S1.4. Casting: Before casting, the molten steel must be allowed to stand in an argon environment for 10 minutes and the temperature must be adjusted to 1460°C. Finally, the cast rods are cast into 15 rods of the same length and a diameter of 420 mm.

[0054] S2. Electroslag remelting:

[0055] S2.1. Preparation of slag for electroslag remelting: The proportion of slag for electroslag remelting is as follows: CaF 2 :60、CaO:25、Al 2 O 3 : 10, MgO: 5;

[0056] S2.2. Prepare a large electrode rod: weld three small electrode rods 1 in parallel to form a large electrode rod 2, the structure of which is as follows: Figure 1 As shown, three small electrode rods 1 are arranged in an equilateral triangle, and the number of the large electrode rods 2 is five.

[0057] S2.3. Preheating of the large electrode rod: heating the large electrode rod to 1200°C.

[0058] S2.4. Remelting: Use a crystallizer of corresponding size, put the cast rod and slag into the crystallizer, insert the large electrode rod into the slag, and after power on and arcing, adjust the remelting voltage to 90-100V and the current to 17-20kA; the self-melting electrode rod is slowly melted by the resistance heat, and the melted self-melting electrode rod droplets pass through the molten slag layer and react with the slag to be purified, and recrystallize at the bottom of the crystallizer; when one of the large electrode rods is melted, move the other large electrode rod to the smelting station, and the replacement time of the replacement electrode is less than or equal to 1 minute, so as to realize the remelting of the replacement electrode, and obtain a large-sized ultra-low expansion alloy electroslag steel ingot with a weight of 20 tons, a size of φ1200mm, a dense and uniform structure, purity, and a smooth surface. The appearance of the large-sized ultra-low expansion alloy electroslag steel ingot is as follows: Figure 2 As shown,

[0059] S2.5. Forgings: The large-sized ultra-low expansion alloy electroslag steel ingot is forged into large-sized ultra-low expansion alloy forgings. The large-sized ultra-low expansion alloy forgings are as follows: Figure 3 As shown, no forging cracks appeared on the surface, which verified the successful production of large-sized ultra-low expansion alloy electroslag steel ingots.

[0060] Example 2

[0061] The diameter of the large-size ultra-low expansion alloy electroslag steel ingot is 1200 mm, and its chemical composition is as follows by weight percentage: C: 0.04, Si: 0.092, Mn: 0.28, S: 0.0085, P: 0.0092, Al: 0.4, Ti: 0.2, Nb: 0.4, Ni: 34.0, Co: 3.2, and the balance is Fe and unavoidable impurities.

[0062] The preparation process includes the following steps:

[0063] S1. Vacuum induction melting:

[0064] S1.1. Batching: weigh each component according to the chemical composition batching table of the large-size ultra-low expansion alloy electroslag steel ingot;

[0065] S1.2. High temperature refining: put some alloy elements Fe, Co, Nb and Ni into the furnace, refine at a temperature of 1600-1620℃ for 70 minutes, and refine under a vacuum degree better than 3Pa;

[0066] S1.3. Low temperature refining: Add the remaining alloying elements, namely C, Si, Mn, Ti, Al, P, S, into the furnace, perform sufficient electromagnetic stirring after adding the materials, and refine at a temperature of 1480-1520°C for 30-40 minutes under a vacuum degree better than 1Pa;

[0067] S1.4. Casting: Before casting, the molten steel must be allowed to stand in an argon environment for 10 minutes and the temperature must be adjusted to 1460°C. Finally, the molten steel must be cast into 15 rods of the same length and a diameter of 420 to 450 mm.

[0068] S2. Electroslag remelting:

[0069] S2.1. Preparation of slag for electroslag remelting: The proportion of slag for electroslag remelting is as follows: CaF 2 :60、CaO:20、Al 2 O 3 : 10, MgO: 10;

[0070] S2.2. Prepare a large electrode rod: weld three small electrode rods 1 in parallel to form a large electrode rod 2, the structure of which is as follows: Figure 1 As shown, three small electrode rods 1 are arranged in an equilateral triangle, and the number of the large electrode rods 2 is five.

[0071] S2.3. Preheating the large electrode rod: heat the large electrode rod to 1000-1200°C.

[0072] S2.4. Remelting: Use a crystallizer of corresponding size, put the cast rod and slag into the crystallizer, insert the large electrode rod into the slag, and after power on and arcing, adjust the remelting voltage to 70-90V and the current to 15-20kA; the self-melting electrode rod is slowly melted by the resistance heat, and the melted self-melting electrode rod droplets pass through the molten slag layer and react with the slag to be purified, and recrystallize at the bottom of the crystallizer; when one of the large electrode rods is melted, move the other large electrode rod to the smelting station, and the replacement time of the replacement electrode is less than or equal to 1 minute, so as to realize the remelting of the replacement electrode, and obtain a large-sized ultra-low expansion alloy electroslag steel ingot with a weight of 22 tons, a size of φ1200mm, a dense and uniform structure, purity, and a smooth surface. The appearance of the large-sized ultra-low expansion alloy electroslag steel ingot is as follows: Figure 2 As shown,

[0073] S2.5. Forgings: The large-sized ultra-low expansion alloy electroslag steel ingot is forged into large-sized ultra-low expansion alloy forgings. The large-sized ultra-low expansion alloy forgings are as follows: Figure 3 As shown, no forging cracks appeared on the surface, which verified the successful production of large-sized ultra-low expansion alloy electroslag steel ingots.

[0074] Example 3

[0075] The diameter of the large-size ultra-low expansion alloy electroslag steel ingot is 1200 mm, and its chemical composition is as follows by weight percentage: C: 0.03, Si: 0.081, Mn: 0.25, S: 0.0083, P: 0.0091, Al: 0.4, Ti: 0.2, Nb: 0.6, Ni: 32.5, Co: 4.0, and the balance is Fe and unavoidable impurities.

[0076] The preparation process includes the following steps:

[0077] S1. Vacuum induction melting:

[0078] S1.1. Batching: weigh each component according to the chemical composition batching table of the large-size ultra-low expansion alloy electroslag steel ingot;

[0079] S1.2. High temperature refining: put some alloy elements Fe, Co, Nb and Ni into the furnace, refine at a temperature of 1600-1620°C for 80 minutes, and refine under a vacuum degree of 2Pa;

[0080] S1.3. Low temperature refining: Add the remaining alloying elements, namely C, Si, Mn, Ti, Al, P, S, into the furnace, perform sufficient electromagnetic stirring after adding the materials, and refine at a temperature of 1480-1520°C for 40 minutes under a vacuum degree of 1Pa;

[0081] S1.4. Casting: Before casting, the molten steel must be allowed to stand in an argon environment for 10 minutes and the temperature must be adjusted to 1460°C. Finally, the molten steel must be cast into 15 rods of the same length and a diameter of 420 to 450 mm.

[0082] S2. Electroslag remelting:

[0083] S2.1. Preparation of slag for electroslag remelting: The proportion of slag for electroslag remelting is as follows: CaF 2 :60、CaO:20、Al 2 O 3 : 10, MgO: 10;

[0084] S2.2. Prepare a large electrode rod: weld three small electrode rods 1 in parallel to form a large electrode rod 2, the structure of which is as follows: Figure 1 As shown, three small electrode rods 1 are arranged in an equilateral triangle, and the number of the large electrode rods 2 is five.

[0085] S2.3. Preheating the large electrode rod: heat the large electrode rod to 1000-1200°C.

[0086] S2.4. Remelting: Use a crystallizer of corresponding size, put the cast rod and slag into the crystallizer, insert the large electrode rod into the slag, and after power on and arcing, adjust the remelting voltage to 70-90V and the current to 18-20kA; the self-melting electrode rod is slowly melted by the resistance heat, and the melted self-melting electrode rod droplets pass through the molten slag layer and react with the slag to be purified, and recrystallize at the bottom of the crystallizer; when one of the large electrode rods is melted, move the other large electrode rod to the smelting station, and the replacement time of the replacement electrode is less than or equal to 1 minute, so as to realize the remelting of the replacement electrode, and obtain a large-sized ultra-low expansion alloy electroslag steel ingot with a weight of 22 tons, a size of φ1200mm, a dense and uniform structure, purity, and a smooth surface. The appearance of the large-sized ultra-low expansion alloy electroslag steel ingot is as follows: Figure 2 As shown,

[0087] S2.5. Forgings: The large-sized ultra-low expansion alloy electroslag steel ingot is forged into large-sized ultra-low expansion alloy forgings. The large-sized ultra-low expansion alloy forgings are as follows: Figure 3 As shown, no forging cracks appeared on the surface, which verified the successful production of large-sized ultra-low expansion alloy electroslag steel ingots.

[0088] Example 4

[0089] The diameter of the large-size ultra-low expansion alloy electroslag steel ingot is 1200 mm, and its chemical composition is as follows by weight percentage: C: 0.03, Si: 0.081, Mn: 0.25, S: 0.0083, P: 0.0091, Nb: 0.6, Ni: 32.5, Co: 4.0, and the balance is Fe and unavoidable impurities.

[0090] The preparation process includes the following steps:

[0091] S1. Vacuum induction melting:

[0092] S1.1. Batching: weigh each component according to the chemical composition batching table of the large-size ultra-low expansion alloy electroslag steel ingot;

[0093] S1.2. High temperature refining: put some alloy elements Fe, Co, Nb and Ni into the furnace, refine at a temperature of 1600-1620°C for 90 minutes, and refine under a vacuum degree of 2Pa;

[0094] S1.3. Low temperature refining: Add the remaining alloying elements, namely C, Si, Mn, P, S, into the furnace, perform sufficient electromagnetic stirring after adding the materials, and refine at a temperature of 1480-1520°C for 40 minutes under a vacuum degree of 1Pa;

[0095] S1.4. Casting: Before casting, the molten steel must be allowed to stand in an argon environment for 10 minutes and the temperature must be adjusted to 1460°C. Finally, the molten steel must be cast into 15 rods of the same length and a diameter of 420 to 450 mm.

[0096] S2. Electroslag remelting:

[0097] S2.1. Preparation of slag for electroslag remelting: The proportion of slag for electroslag remelting is as follows: CaF 2 :60、CaO:20、Al 2 O 3 : 10, MgO: 10;

[0098] S2.2. Prepare a large electrode rod: weld three small electrode rods 1 in parallel to form a large electrode rod 2, the structure of which is as follows: Figure 1 As shown, three small electrode rods 1 are arranged in an equilateral triangle, and the number of the large electrode rods 2 is five.

[0099] S2.3. Preheating the large electrode rod: heat the large electrode rod to 1000-1200°C.

[0100] S2.4. Remelting: Use a crystallizer of corresponding size, put the cast rod and slag into the crystallizer, insert the large electrode rod into the slag, and after power on and arcing, adjust the remelting voltage to 70-90V and the current to 18-20kA; the self-melting electrode rod is slowly melted by the resistance heat, and the melted self-melting electrode rod droplets pass through the molten slag layer and react with the slag to be purified, and recrystallize at the bottom of the crystallizer; when one of the large electrode rods is melted, move the other large electrode rod to the smelting station, and the replacement time of the replacement electrode is less than or equal to 1 minute, so as to realize the remelting of the replacement electrode, and obtain a large-sized ultra-low expansion alloy electroslag steel ingot with a weight of 22 tons, a size of φ1200mm, a dense and uniform structure, purity, and a smooth surface. The appearance of the large-sized ultra-low expansion alloy electroslag steel ingot is as follows: Figure 2 As shown,

[0101] S2.5. Forgings: The large-sized ultra-low expansion alloy electroslag steel ingot is forged into large-sized ultra-low expansion alloy forgings. The large-sized ultra-low expansion alloy forgings are as follows: Figure 3 As shown, no forging cracks appeared on the surface, which verified the successful production of large-sized ultra-low expansion alloy electroslag steel ingots.

[0102] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modification made to the present invention by those skilled in the art without departing from the spirit of the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for preparing a large-size ultra-low expansion alloy electroslag steel ingot, characterized in that: First, vacuum induction melting is adopted, and then electroslag remelting is adopted. The large electrode rod adopted in the electroslag remelting step is formed by welding three small electrode rods in parallel; at least two of the large electrode rods are remelted in succession.

2. The preparation method according to claim 1, characterized in that: The following steps are involved: S1. Vacuum induction melting: S1.

1. Batching: weigh each component according to the chemical composition batching table of the large-size ultra-low expansion alloy electroslag steel ingot; S1.

2. High temperature refining: Add some alloy elements into the furnace and refine at a temperature of 1600-1620℃ for 70-80 minutes; S1.

3. Low temperature refining: Add the remaining alloying elements into the furnace, perform sufficient electromagnetic stirring after adding the materials, and refine at a temperature of 1480-1520°C for 30-40 minutes; S1.

4. Casting: Casting into cast bars; S2. Electroslag remelting: S2.

1. Preparation of slag for electroslag remelting: The proportion of slag for electroslag remelting is as follows: CaF2: 50-60, CaO: 20-25, Al2O3: 10-15, MgO: 5-15 in parts by weight; S2.

2. Prepare a large electrode rod: weld three small electrode rods in parallel to form a large electrode rod, and arrange the three small electrode rods in a nearly equilateral triangle; S2.

3. Preheating the large electrode rod: heating the large electrode rod to 1000-1200°C; S2.

4. Remelting: Use a crystallizer of corresponding size, place the cast rods and slag into the crystallizer, insert the large electrode rod into the slag, turn on the power and start the arc, then adjust the remelting voltage to 70-100V and the current to 15-22kA; when one of the large electrode rods is melted, move the other large electrode rod to the smelting station to achieve electrode replacement remelting to obtain a large-sized ultra-low expansion alloy electroslag ingot.

3. The method for preparing a large-size ultra-low expansion alloy electroslag steel ingot according to claim 2, characterized in that: In step S2.4, the replacement time of the replacement electrode is less than or equal to 1 minute.

4. The method for preparing a large-size ultra-low expansion alloy electroslag steel ingot according to claim 2, characterized in that: In step S1.4, before casting, the molten steel needs to be left to stand in an argon environment for 10 minutes and the temperature is adjusted to 1460°C.

5. The method for preparing a large-size ultra-low expansion alloy electroslag steel ingot according to claim 2, characterized in that: In step S1.4, cast rods of the same length and a diameter of 420-450 mm are cast, and the number of the cast rods is 15.

6. The method for preparing a large-size ultra-low expansion alloy electroslag steel ingot according to claim 2, characterized in that: In step S1.2, refining is performed under a vacuum condition of better than 3 Pa.

7. The method for preparing a large-size ultra-low expansion alloy electroslag steel ingot according to claim 2, characterized in that: In step S1.3, refining is performed under a vacuum condition of better than 1 Pa.

8. The method for preparing a large-size ultra-low expansion alloy electroslag steel ingot according to claim 2, characterized in that: The number of the large electrode rods is 5.

9. The method for preparing a large-size ultra-low expansion alloy electroslag steel ingot according to claim 1 or 2, characterized in that: The large-size ultra-low expansion alloy electroslag steel ingot has a diameter of 1200 mm and a weight of 20 to 22 tons.

10. The method for preparing a large-size ultra-low expansion alloy electroslag steel ingot according to claim 1 or 2, characterized in that: The chemical composition of the large-size ultra-low expansion alloy electroslag steel ingot is, by weight percentage: C: ≤0.05, Si: ≤0.10, Mn: ≤0.30, S: ≤0.010, P: ≤0.010, Al: 0-0.40, Ti: 0-0.40, Nb: 0.4-0.8, Ni: 31.5-34.0, Co: 3.20-4.50, and the balance is Fe and unavoidable impurities.

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