Forging method for improving plasticity of large invar alloy steel ingot and invar alloy

Abstract

The invention discloses a forging method for improving plasticity of a large invar alloy steel ingot and invar alloy, and the forging method comprises the following steps: smelting molten steel by adopting an electric furnace, and casting to obtain the large invar alloy steel ingot with the weight of 15-25t; the cold-state invar alloy steel ingot is heated to 1000-1200 DEG C for heat preservation, and the heat preservation time is controlled to be longer than or equal to 10 h; the heated invar alloy steel ingot is subjected to three-heating-number upsetting and drawing-out, upsetting is conducted according to the rolling reduction of 15-50% of the total height of the invar alloy steel ingot in each heating number, the invar alloy steel ingot is drawn out to the height of the original invar alloy steel ingot after upsetting, and the invar alloy steel ingot is returned to a heating furnace to be heated and subjected to heat preservation after upsetting and drawing-out in each heating number; the final forging temperature of the last heating number is greater than or equal to 750 DEG C; and the invar alloy steel ingot subjected to heating and heat preservation in the last heating number is forged to the specified size, and then the invar alloy steel ingot is slowly cooled to the room temperature in air to obtain an invar alloy steel forging stock. The thermoplasticity of the large invar alloy steel ingot is improved, and the technical problem that the large invar alloy steel ingot is prone to cracking during forging is solved.

Description

Technical Field

[0001] The present invention relates to the field of metal materials, in particular to a forging method for improving the plasticity of a large Invar alloy steel ingot and an Invar alloy. Background Art

[0002] Invar alloy is a binary austenitic Fe-Ni alloy. It is a precision alloy with abnormal or controllable thermal expansion characteristics. It is widely used in electronic components and other fields that require high material expansion performance. The expansion coefficient of Invar alloy is low, and its expansion coefficient at 20-100℃ is ≤1.5×10 -6 / ℃, which is only 1 / 5 to 1 / 10 of that of ordinary steel. Currently, the commercially available Invar alloys are mainly Fe-Ni alloys, such as 4J36 Invar alloy for aircraft composite molds, Ni36 Invar alloy for thin-film LNG ships, and high-strength H36 Invar alloy for high-voltage double-capacity conductors. In addition to Fe-Ni Invar alloys, Fe-Ni-Co super Invar alloys are widely used in the aerospace field due to their excellent ultra-low expansion properties. Their main components are C≤0.04%, Si≤0.25%, 0.2%≤Mn≤0.4%, S≤0.0015%, P≤0.008%, 35%≤Ni≤36.5%, 3%≤Co≤4%, and the rest is Fe. The expansion performance requirement of this alloy is 1 to 2×10 -6 / ℃. In recent years, demand for cold-rolled Invar alloy strip has increased across a wide range of applications. Conventional small-sized Invar alloy ingots are no longer sufficient for production and processing, necessitating the development of a smelting and forging process for large-sized Invar alloy ingots. However, due to the solidification microstructure of Invar alloy, larger ingots exhibit poorer hot working plasticity, leading to a high risk of forging cracking, which can cause significant economic losses for manufacturers. Therefore, a forging method to improve the plasticity of large Invar alloy ingots is urgently needed.

[0003] Chinese Patent Publication No. CN118854142A proposes a method for preparing thick Invar alloy plates with a diameter of 70 mm or greater for use in aviation molds. The method utilizes a combination of non-vacuum induction furnace smelting, vacuum consumable melting, and vacuum consumable melting. Forging with a large deformation ratio and forming by upsetting and drawing are then performed, followed by heat treatment to re-grow the grains and reduce the grain size difference between the edge and core. This method can produce Invar alloy plates with a width of 200 × 1000 mm, but the steel ingots involved in this technology are smaller in size, resulting in a lower risk of forging cracking. Chinese patent publication number CN119456694A proposes a method for producing Invar alloy 4J36 hot-rolled coil. The patent discloses a method for rolling Invar alloy hot-rolled coil, but the technology only discloses technical parameters related to the hot rolling process and does not involve the steel ingot forging process.

[0004] Chinese patent publication number CN117548519A proposes a forging and rolling hybrid forming process for low expansion coefficient iron-nickel based Invar steel. This method is a hybrid forming process of forging billets and hot rolling, mainly targeting small electroslag ingots, and does not involve the forging process of large Invar alloy steel ingots.

[0005] In summary, there is an urgent need to develop a forging method to improve the plasticity of large Invar alloy steel ingots, which can solve the technical problems such as easy cracking during forging of large Invar alloy steel ingots. Summary of the Invention

[0006] In view of the defects existing in the prior art, the purpose of the present invention is to provide a forging method and Invar alloy for improving the plasticity of large Invar alloy steel ingots, which effectively improves the thermoplasticity of large Invar alloy steel ingots, solves the technical problems such as the easy cracking of large Invar alloy steel ingots during forging, and improves the qualified rate of Invar alloy forging products.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] A first aspect of the present invention provides a forging method for improving the plasticity of a large Invar alloy steel ingot, comprising the following steps: S1, smelting process, using an electric furnace to smelt molten steel that meets the internal control composition requirements of Invar alloy, and after casting, a large Invar alloy steel ingot weighing 15-25 tons is obtained; S2, a heating process, heating the cold Invar alloy steel ingot to 1000°C to 1200°C and keeping the temperature, and controlling the holding time to be ≥10h; S3, a roughing and drawing process, wherein the heated Invar alloy steel ingot is subjected to three rounds of roughing and drawing, wherein each round is subjected to roughing and drawing by a reduction of 15 to 50% of the total height of the Invar alloy steel ingot, and then drawn to the original height of the Invar alloy steel ingot after upsetting, and after each round of roughing and drawing, the ingot is returned to the heating furnace for heating and heat preservation, and the final forging temperature of each round of roughing and drawing is ≥750°C; S4, forging and shaping, forging the Invar alloy steel ingot after the last heating and heat preservation in step S3 to a specified size, and then slowly cooling it to room temperature in air to obtain the Invar alloy steel forging blank.

[0009] Preferably, in step S1, pure nickel plate and pure iron are used as raw materials in the smelting process, the Ni content in the pure nickel plate is greater than 99.5%, and the total content of impurity elements is ≤0.05%, and these impurity elements include Zr, Ti, Cr, Nb, V, etc.

[0010] Preferably, in step S1, the molten steel meets the following internal control composition requirements of Invar alloy: C≤0.08%, Si≤0.8%, Mn≤0.5%, P≤0.020%, S≤0.06%, 28%≤Ni≤35%, Mg≤0.1%, Al≤0.20%, and the balance is iron and unavoidable impurities, calculated in weight percentage.

[0011] Preferably, in step S2, the holding time satisfies the following formula: t≥8d+1.5 Wherein, t is time, unit is h; d is the diameter of the Invar alloy steel ingot, unit is m.

[0012] If the holding time in step S2 does not meet the requirements of the above formula, the steel ingot cannot be kept warm thoroughly, and there is a risk of forging cracking during the subsequent forging process; if the holding time is too long, there is a risk of overburning the steel ingot. The holding temperature is 1100-1200℃ Preferably, in step S2, during the heating process, the heating rate is 20-100°C / h.

[0013] Preferably, the upsetting and stretching process in step S3 specifically includes the following process: S31, first fire upsetting and drawing, upsetting the heated Invar alloy steel ingot by a reduction of 30-50% of the total height of the Invar alloy steel ingot, and then drawing it to the original Invar alloy steel ingot height, with a single-pass reduction of 5-10 mm and a final forging temperature of ≥750°C. Then, returning the ingot to the heating furnace and heating at 1100-1200°C for 1-3 hours; S32, the second fire upsetting and drawing, the upsetting amount is controlled to be 20-40% of the total height of the Invar alloy steel ingot, the single-pass reduction is 10-15mm, the final forging temperature is ≥750℃, and after upsetting and drawing, the steel is returned to the heating furnace and heated and kept at 1100-1200℃ for 1-3h. Since the grain structure is refined after the first fire upsetting and drawing, the single-pass reduction is increased compared with the previous fire to ensure that the grains inside the steel ingot are fully recrystallized and refined; S33, the third fire upsetting and drawing, control the upsetting amount to 15-35% of the total height of the Invar alloy steel ingot, the single-pass reduction is 15-20mm, the final forging temperature is ≥750℃, after upsetting and drawing, return to the heating furnace and heat and keep warm at 1100-1200℃ for 1-3h.

[0014] In the three-time upsetting and drawing process of step S3: the first upsetting and drawing process is to increase the cross-sectional area by axially compressing the metal billet and reduce the height of the ingot, providing a material distribution basis for subsequent drawing. Subsequently, the wide side of the ingot is tapped and drawn to the original ingot height. The drawing process is to break the coarse columnar grains on the surface of the ingot and optimize the material streamline distribution. The single-pass pressing amount cannot exceed 10mm to prevent excessive deformation from causing cracks. In the first upsetting and drawing process, if the upsetting If the amount is too small (less than 30% of the total height of the ingot), the reduction ratio is small, resulting in insufficient crushing of the columnar crystals of the ingot. If the upsetting amount exceeds 50% of the total height of the ingot, the ingot is at risk of forging cracking. Therefore, based on the production experience of the ingot, the upsetting amount of the first fire upsetting and drawing process is set to 30-50% of the total height of the ingot; similarly, the upsetting amount of the second fire upsetting and drawing process and the third fire upsetting and drawing process are set to 20-40% and 15-35% of the total height of the ingot, respectively. The single-pass reduction in the first hot upsetting and drawing process is controlled between 5 and 10 mm. If the single-pass reduction is ≤ 5 mm, the steel ingot undergoes a small amount of deformation and low grain deformation energy, which prevents the ingot from fully crushing and recrystallizing its columnar structure. If the single-pass reduction is ≥ 10 mm, the deformation is excessive, which can easily lead to cracks. Therefore, when the single-pass reduction is controlled between 5 and 10 mm, the steel ingot can ensure sufficient deformation required for crushing the columnar structure and recrystallization while avoiding the risk of cracks caused by excessive deformation. As the number of upsetting and drawing passes increases, the columnar structure gradually breaks down, the grain size gradually decreases, and the single-pass reduction gradually increases, ensuring that the refined grain structure can fully undergo the recrystallization process. In the above-mentioned upsetting and drawing process, the reduction rate and drawing rate are controlled between 0.1 and 10 / s (for example, 1 to 10 / s). If the reduction rate and the drawing rate are too large, the columnar crystal structure in the ingot cannot be fully recrystallized and refined, and the grain structure cannot be fully refined.

[0015] Preferably, in step S4, the thickness of the Invar alloy steel forging blank is 150-300 mm, the length is 4-7 m, and the corners of the forging blank are right angles.

[0016] Preferably, in step S4, the grain size of the Invar alloy steel forging blank is ≥ grade 4, the elongation is ≥ 40%, and the expansion performance at a temperature of -180 to 0°C is -180～0℃ 1×10 -6 / ℃~2×10 -6 / ℃.

[0017] In the forging method of the present invention, the heating furnace used in the heating and temperature raising process of steps S2 and S3 may be a trolley-type heating furnace.

[0018] A second aspect of the present invention provides an Invar alloy prepared by the forging method for improving the plasticity of a large Invar alloy steel ingot as described in the first aspect of the present invention, wherein the composition thereof is as follows in percentage by weight: C ≤ 0.08%, Si ≤ 0.8%, Mn ≤ 0.5%, P ≤ 0.020%, S ≤ 0.06%, 28% ≤ Ni ≤ 35%, Mg ≤ 0.1%, Al ≤ 0.20%, and the balance being iron and unavoidable impurities.

[0019] Preferably, the grain size is ≥4, the elongation is ≥40% (e.g. 40% to 80%), and the expansion performance at -180 to 0°C is a -180～0℃ 1×10 -6 / ℃~2×10 -6 / ℃.

[0020] In the Invar alloy prepared above: Mn: Mn is an austenite-forming element with a strong ability to stabilize the austenite phase and effectively control the low-temperature martensite transformation. Therefore, the present invention requires that the Mn content be ≤ 0.5%.

[0021] Si: Si acts as a deoxidizer in molten steel, strengthening the matrix and improving the steel's high-temperature oxidation resistance. A moderate amount of Si can enhance the steel's resistance to strong acid corrosion, but excessive Si content can reduce the steel's hot workability and toughness, and affect its magnetic permeability. Therefore, in the present invention, the Si content is controlled to ≤ 0.8%.

[0022] Ni: an austenite-forming element that can improve the strength and corrosion resistance of steel. However, if the Ni content is too high, it will reduce the solubility of nitrogen in the steel, inhibit the precipitation of strengthening phase carbon and nitrides, affect the strength, and increase the cost. The influence of Ni content on the expansion properties of Invar alloy in the low-temperature service temperature range and the high-temperature service temperature range is different. As the Ni content decreases, the Curie temperature decreases and is closer to the low-temperature service temperature range. When the Ni content is lower than 28% or higher than 35%, its expansion performance does not meet the user's standard requirements. Therefore, the Ni content in the present invention is controlled at 28%≤Ni≤35%.

[0023] C: C is an austenite-strengthening element. Increasing the C content effectively reduces the saturated solubility of nitrogen in molten steel. Regarding mechanical properties, C improves alloy strength while simultaneously reducing toughness and low-temperature expansion. Therefore, to ensure both strength and toughness, the C content in this invention is controlled to ≤ 0.08%.

[0024] The elemental pair Mg and Al can further remove oxygen from Invar alloy, improving the thermoplasticity of the steel ingot. However, their concentrations should not be too high. Adding Mg to Invar alloy also refines austenite grains, pinning them during hot working and inhibiting their growth. Therefore, in the present invention, Mg is controlled to ≤ 0.1% and Al ≤ 0.20%.

[0025] P and S: are harmful elements. These two elements tend to segregate near the columnar grain boundaries of the steel ingot, which can have a negative impact on the welding performance. Therefore, the P and S elements are controlled in a lower range. Combined with the P and S content levels of the raw materials, P is required to be ≤ 0.020% and S ≤ 0.06%.

[0026] Compared with the prior art, the present invention has the following beneficial effects: 1. The present invention can effectively improve the technical difficulties of large-scale Invar alloy steel ingots, such as poor thermoplasticity and easy cracking during forging, and produce Invar alloy steel ingots with uniform and fine structure and grain size of grade 4 or above, thereby reducing the scrap rate of unqualified steel ingots during forging and further reducing product production costs; 2. The present invention adjusts the composition of Invar alloy and controls the parameters of the heating process and forging process, thereby improving the thermoplasticity of large Invar alloy ingots and solving the technical problem of easy cracking of large Invar alloy ingots during forging; 3. The Invar alloy prepared by the present invention can be widely used in aerospace, national defense, energy transportation and other fields. Due to its excellent hot working performance and excellent expansion performance, the elongation of the forging blank in the high temperature section can reach up to 80%, and the expansion performance in the temperature range of -180 to 0°C is a -180～0℃ Reach 1×10 -6 / ℃~2×10 -6 / ℃; The successful design and development of Invar alloy will bring extensive economic benefits and broad market applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a microstructure morphology diagram of the Invar alloy prepared in Example 1 of the present invention;

[0028] Figure 2 This is the microstructure of the forging blank prepared in Comparative Example 1. DETAILED DESCRIPTION

[0029] The present invention will be described in detail below with reference to specific examples. The following examples will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form.

[0030] Example 1

[0031] The forging method for improving the plasticity of a large Invar alloy steel ingot of this embodiment comprises the following steps:

[0032] Step 1: Smelting: Pure nickel plates and pure iron are used as Invar alloy raw materials. The nickel content of the pure nickel plates is greater than 99.5%, and the total content of impurity elements (such as Zr, Ti, Cr, Nb, and V) is ≤0.05%. A 100-ton electric furnace is used to smelt molten steel that meets the internal control composition requirements of Invar alloy (composition shown in Table 1). Molten steel is then poured into a 20-ton Invar alloy ingot mold. Pouring must be completed within 22 minutes. After cooling for 30 hours, the ingot is demolded to produce a 20-ton Invar alloy ingot with a diameter of 1.1 meters.

[0033] Step 2: Ingot heating process: Place the solidified and cooled Invar alloy ingot into a trolley-type heating furnace and heat it to 1200°C at a heating rate of 20°C / h for insulation. The insulation time is controlled within 12h (satisfying t≥8d+1.5=10.3).

[0034] Step 3: Upsetting and drawing process: Take out the Invar alloy steel ingot that meets the heating requirements and perform three-fire upsetting and drawing; in the first fire upsetting and drawing process, the upsetting amount is upsetting according to the 30% reduction of the total height of the steel ingot. After the upsetting is completed, the wide side of the steel ingot is drawn to the original steel ingot, and the drawing is divided into three passes. The reduction amount of each pass is controlled at 5mm, and the reduction rate is 1 / s. After completing the above process, the steel ingot is returned to the heating furnace and heated and kept warm at 1100℃ for 3h; in the second fire upsetting and drawing process, the upsetting amount is 20% of the original height of the steel ingot. After the upsetting step is completed, the wide side of the steel ingot is drawn to the original ingot length, and the drawing is divided into three passes. The steel ingot is drawn in three passes, with the reduction amount of 10 mm per pass, the reduction rate of 1 / s, and the final forging temperature required to be greater than 800°C. After completing the above processes, the steel ingot is returned to the heating furnace and heated and kept warm at 1100°C for 3 hours. In the third upsetting and drawing process, the upsetting amount is 15% of the original height of the steel ingot. After the upsetting step is completed, the wide side of the steel ingot is drawn to the original ingot length. The drawing is carried out in three passes, with the reduction amount of 15 mm per pass, the reduction rate of 1 / s, and the final forging temperature required to be greater than 750°C. After completing the above processes, the steel ingot is returned to the trolley-type heating furnace and heated and kept warm at 1100°C for 3 hours.

[0035] Step 4: Forging process: forge the Invar alloy steel ingot after the third heating and heat preservation in step 3 to a thickness of 300mm and a length of 4m. The corners of the forging blank must be forged into right angles. Then slowly cool it to room temperature in air to obtain the Invar alloy steel forging blank.

[0036] Example 2

[0037] The forging method for improving the plasticity of a large Invar alloy steel ingot of this embodiment comprises the following steps:

[0038] Step 1: Smelting: Pure nickel plates and pure iron are used as Invar alloy raw materials. The nickel content of the pure nickel plates is greater than 99.5%, and the total content of impurity elements (such as Zr, Ti, Cr, Nb, and V) is ≤0.05%. A 100-ton electric furnace is used to smelt molten steel that meets the internal control composition requirements of Invar alloy (composition shown in Table 1). Molten steel is then poured into a 20-ton Invar alloy ingot mold. Pouring must be completed within 22 minutes. After cooling for 30 hours, the ingot is demolded to produce a 20-ton Invar alloy ingot with a diameter of 1.2 meters.

[0039] Step 2: Ingot heating process: Place the solidified and cooled Invar alloy ingot into a trolley-type heating furnace and heat it to 1100°C at a heating rate of 40°C / h for insulation. The insulation time is controlled within 13h (satisfying t≥8d+1.5=11.1).

[0040] Step 3: Upsetting and drawing process: Take out the Invar alloy steel ingot that meets the heating requirements and perform three-fire upsetting and drawing; in the first fire upsetting and drawing process, the upsetting amount is upsetting according to 30% of the total height of the steel ingot. After the upsetting is completed, the wide side of the steel ingot is drawn to the original height of the steel ingot, and the drawing is divided into three passes. The reduction amount of each pass is controlled to 7mm, and the reduction rate is 3.0 / s. After completing the above process, the steel ingot is returned to the heating furnace and heated and kept warm at 1100℃ for 2h; in the second fire upsetting and drawing process, the upsetting amount is 25% of the original height of the steel ingot. After the upsetting step is completed, the wide side of the steel ingot is drawn to the original length of the steel ingot, and the drawing is divided into three passes. The steel ingot is drawn in three passes, with the reduction amount of each pass controlled at 12 mm, the reduction rate at 1 / s, and the final forging temperature required to be greater than 850°C. After completing the above processes, the steel ingot is returned to the heating furnace and heated and kept warm at 1120°C for 2.5 hours. In the third upsetting and drawing process, the upsetting amount is 25% of the original height of the steel ingot. After the upsetting step is completed, the wide side of the steel ingot is drawn to the original ingot length. The drawing is carried out in three passes, with the reduction amount of each pass controlled at 17 mm, the reduction rate at 3 / s, and the final forging temperature required to be greater than 750°C. After completing the above processes, the steel ingot is returned to the trolley-type heating furnace and heated and kept warm at 1120°C for 2.5 hours.

[0041] Step 4: Forging process: forge the Invar alloy steel ingot after the third heating and heat preservation in step 3 to a thickness of 250mm and a length of 5m. The corners of the forging blank must be forged into right angles. Then slowly cool it to room temperature in air to obtain the Invar alloy steel forging blank.

[0042] Example 3

[0043] The forging method for improving the plasticity of a large Invar alloy steel ingot of this embodiment comprises the following steps:

[0044] Step 1: Smelting: Pure nickel plates and pure iron are used as Invar alloy raw materials. The nickel content of the pure nickel plates is greater than 99.5%, and the total content of impurity elements (such as Zr, Ti, Cr, Nb, and V) is ≤0.05%. A 100-ton electric furnace is used to smelt molten steel that meets the internal control composition requirements of Invar alloy (composition shown in Table 1). Molten steel is then poured into a 20-ton Invar alloy ingot mold. Pouring must be completed within 22 minutes. After cooling for 30 hours, the ingot is demolded to produce a large 20-ton Invar alloy ingot with a diameter of 1.1 meters.

[0045] Step 2: Ingot heating process: Place the solidified and cooled Invar alloy ingot into a trolley-type heating furnace and heat it to 1130°C at a heating rate of 50°C / h for insulation. The insulation time is controlled within 12h (satisfying t≥8d+1.5=10.3).

[0046] Step 3: Upsetting and drawing process: Take out the Invar alloy steel ingot that meets the heating requirements and perform three-fire upsetting and drawing; in the first fire upsetting and drawing process, the upsetting amount is upsetting according to the 30% reduction of the total height of the steel ingot. After the upsetting is completed, the wide side of the steel ingot is drawn to the original height of the steel ingot, and the drawing is divided into three passes. The reduction amount of each pass is controlled to 8mm, and the reduction rate is 5.0 / s. After completing the above process, the steel ingot is returned to the heating furnace and heated and kept warm at 1140℃ for 2.2h; in the second fire upsetting and drawing process, the upsetting amount is 30% of the original height of the steel ingot. After the upsetting step is completed, the wide side of the steel ingot is drawn to the original length of the steel ingot, and the drawing is divided into three passes. The steel ingot is drawn in three passes, with the reduction amount of each pass controlled at 13mm, the reduction rate at 1 / s, and the final forging temperature required to be greater than 850℃. After completing the above processes, the steel ingot is returned to the heating furnace and heated and kept warm at 1140℃ for 2.2h. In the third upsetting and drawing process, the upsetting amount is 30% of the original height of the steel ingot. After the upsetting step is completed, the wide side of the steel ingot is drawn to the original ingot length. The drawing is carried out in three passes, with the reduction amount of each pass controlled at 18mm, the reduction rate at 10 / s, and the final forging temperature required to be greater than 750℃. After completing the above processes, the steel ingot is returned to the trolley-type heating furnace and heated and kept warm at 1140℃ for 2.2h.

[0047] Step 4: Forging process: forge the Invar alloy steel ingot after the third heating and heat preservation in step 3 to a thickness of 220 mm and a length of 5.5 m. The corners of the forging blank must be forged into right angles. Then slowly cool it to room temperature in air to obtain the Invar alloy steel forging blank.

[0048] Example 4

[0049] The forging method for improving the plasticity of a large Invar alloy steel ingot of this embodiment comprises the following steps:

[0050] Step 1: Smelting: Pure nickel plates and pure iron are used as Invar raw materials. The nickel content of the pure nickel plates is greater than 99.5%, and the total content of impurity elements (such as Zr, Ti, Cr, Nb, and V) is ≤0.05%. A 100-ton electric furnace is used to smelt molten steel that meets the internal control composition requirements of Invar (see Table 1 for the composition). The steel is then poured into a 15-ton Invar ingot mold. The pouring process must be completed within 22 minutes. After cooling for 30 hours, the ingot is demolded to produce a large 15-ton Invar ingot with a diameter of 1.1 meters.

[0051] Step 2: Ingot heating process: Place the solidified and cooled Invar alloy steel ingot into a trolley-type heating furnace and heat it to 1150°C at a heating rate of 60°C / h for insulation. The insulation time is controlled within 12h (satisfying t≥8d+1.5=10.3).

[0052] Step 3: Upsetting and drawing process: Take out the Invar alloy steel ingot that meets the heating requirements and perform three-fire upsetting and drawing; in the first fire upsetting and drawing process, the upsetting amount is upsetting according to the 30% reduction of the total height of the steel ingot. After the upsetting is completed, the wide side of the steel ingot is drawn to the original height of the steel ingot, and the drawing is divided into three passes. The reduction amount of each pass is controlled to 10mm, and the reduction rate is 8.0 / s. After completing the above process, the steel ingot is returned to the heating furnace and heated and kept warm at 1160℃ for 2h; in the second fire upsetting and drawing process, the upsetting amount is 40% of the original height of the steel ingot. After the upsetting step is completed, the wide side of the steel ingot is drawn to the original length of the steel ingot, and the drawing is divided into three passes. The steel ingot is drawn for the first time, with the reduction of 14 mm in each pass, the reduction rate of 1 / s, and the final forging temperature required to be greater than 850°C. After completing the above processes, the steel ingot is returned to the heating furnace and heated and kept warm at 1160°C for 2 hours; in the third upsetting and drawing process, the upsetting amount is 30% of the original height of the steel ingot. After the upsetting step is completed, the wide side of the steel ingot is drawn to the original ingot length, and the drawing is carried out in three passes, with the reduction of 15 mm in each pass, the reduction rate of 3 / s, and the final forging temperature required to be greater than 750°C; after completing the above processes, the steel ingot is returned to the trolley-type heating furnace and heated and kept warm at 1160°C for 2 hours.

[0053] Step 4: Forging process: forge the Invar alloy steel ingot after the third heating and heat preservation in step 3 to a thickness of 200mm and a length of 6m. The corners of the forging blank must be forged into right angles. Then slowly cool it to room temperature in air to obtain the Invar alloy steel forging blank.

[0054] Example 5

[0055] The forging method for improving the plasticity of a large Invar alloy steel ingot of this embodiment comprises the following steps:

[0056] Step one: smelting process: pure nickel plate and pure iron as the raw material of invar alloy, the content of Ni element in pure nickel plate is greater than 99.5%, the total content of impurity elements (Zr, Ti, Cr, Nb, V, etc.) is less than or equal to 0.05%. A 100t electric furnace is used to smelt the molten steel (the composition is shown in Table 1) meeting the internal composition requirements of invar alloy, and a 25t invar alloy ingot mold is used for pouring, which needs to be completed within 22min, and the ingot is demolded after cooling for 30h to prepare a 25t large-size invar alloy ingot with a diameter of 1.1m.

[0057] Step two: ingot heating process: the invar alloy ingot after solidification and cooling is placed in a trolley type heating furnace and heated to 1200℃ at a heating rate of 100℃ / h for heat preservation, and the heat preservation time is controlled within 12h.

[0058] Step three: upsetting and drawing process: the invar alloy ingot meeting the heating requirements is taken out for three-fire upsetting and drawing; in the first-fire upsetting and drawing process, the upsetting amount is 50% of the total height of the ingot, and after the upsetting is completed, the ingot is drawn to the original ingot height, and the drawing is divided into three passes, with a reduction of 10mm per pass and a reduction rate of 10 / s; after the above process, the ingot is returned to the heating furnace and heated at 1100℃ for heat preservation, and the heat preservation time is 3h; in the second-fire upsetting and drawing process, the upsetting amount is 40% of the original height of the ingot, and after the upsetting step is completed, the ingot is drawn to the original ingot length, and the drawing is divided into three passes, with a reduction of 15mm per pass and a reduction rate of 1 / s, and the final forging temperature is required to be greater than 850℃; after the above process, the ingot is returned to the heating furnace and heated at 1200℃ for heat preservation, and the heat preservation time is 2h; in the third-fire upsetting and drawing process, the upsetting amount is 30% of the original height of the ingot, and after the upsetting step is completed, the ingot is drawn to the original ingot length, and the drawing is divided into three passes, with a reduction of 15mm per pass and a reduction rate of 3 / s, and the final forging temperature is required to be greater than 750℃; after the above process, the ingot is returned to the trolley type heating furnace and heated at 1200℃ for heat preservation, and the heat preservation time is 1h.

[0059] Step four: forging forming process, the invar alloy ingot after heat preservation in step three is forged to a thickness of 150mm and a length of 7m, and the corner of the forging blank must be forged into a right angle, and then air-cooled to room temperature to obtain an invar alloy forging blank.

[0060] Comparative Example 1

[0061] The forging method of the invar alloy of the comparative example is as follows:

[0062] Step 1: Melting process: Pure nickel plate and pure iron are used as Invar alloy raw materials. A 100-ton electric furnace is used to smelt molten steel that meets the standard internal control composition requirements (composition is shown in Table 1). The steel is then poured into a 20-ton Invar alloy steel ingot mold. The pouring must be completed within 25 minutes. After cooling for 30 hours, the steel ingot is demolded to produce a large-sized Invar alloy steel ingot.

[0063] Step 2: Ingot heating process: The solidified and cooled Invar alloy ingot is placed in a trolley type heating furnace and heated to 1100°C at a heating rate of 120°C / h for 15 hours.

[0064] Step 3: Upsetting and drawing process: Take out the Invar alloy steel ingot that meets the heating requirements. The first upsetting is carried out according to the reduction of 60% of the total height of the ingot. After the upsetting step is completed, the wide side of the ingot is drawn to the original ingot length. The drawing is divided into three steps. The reduction of each step is controlled at 10mm and the reduction rate is 1 / s.

[0065] Step 4: Return to the furnace and keep warm: The steel ingot that was upset and stretched in step 3 is returned to the trolley type heating furnace and heated and kept warm at 1100°C for 3 hours.

[0066] Step 5: Forging process: After the forging blank is kept warm for the specified time in step 4, it is taken out of the furnace and forged to a thickness of 150-300 mm and a length of 4-7 m. The corners of the forging blank must be forged into right angles.

[0067] Step 6: Use air cooling to cool the finished forging blank.

[0068] Comparative Example 2 The comparative example adopts the method of Example 1, except that the upsetting amounts in the three-fire upsetting and drawing process in step 3 are 26%, 17% and 12% of the total height of the steel ingot respectively.

[0069] Comparative Example 3 The method of this comparative example adopts the method of Example 1, except that the upsetting amounts in the three-fire upsetting and drawing process in step 3 are 53%, 43% and 40% of the total height of the steel ingot respectively.

[0070] Comparative Example 4 The method of this comparative example adopts the method of Example 1, except that the reduction amounts in the three-pass upsetting and drawing process in step 3 are 12 mm, 18 mm, and 25 mm respectively.

[0071] Comparative Example 5 The method of this comparative example adopts the method of Example 1, except that the reduction amounts in the three-pass upsetting and drawing process in step 3 are 4 mm, 8 mm, and 12 mm respectively.

[0072] Comparative Example 6 The method of this comparative example adopts the method of Example 1, except that, in the smelting process of step 1, the molten steel meets the internal control composition requirements of the Invar alloy shown in Table 1.

[0073] Comparative Example 7 The method of this comparative example adopts the method of Example 1, except that, in the smelting process of step 1, the molten steel meets the internal control composition requirements of the Invar alloy shown in Table 1: Table 1 Chemical composition of molten steel and Invar alloy forging blanks of Examples and Comparative Examples (wt%)

[0074] Table 2 Performance indicators of Invar alloy forging blank

[0075] Figure 1 The grain structure morphology of the Invar alloy forging blank prepared in Example 1 can be seen from the figure. The grain structure morphology is more refined and uniform, less prone to cracking during forging, and has good hot working plasticity. The grain size of the Invar alloy forging blank reaches level 4, and there are no mixed crystals or abnormally grown grains. The elongation of the forging blank sample at room temperature is 40%.

[0076] Figure 2 This is the microstructure morphology diagram of the forging blank prepared in Comparative Example 1. As can be seen from the figure, the forging blank has a coarse microstructure and is prone to cracking during forging. The grain size of the Invar alloy forging blank is level 2, and part of the grain size is level 1. The elongation of the forging blank sample in the room temperature mechanical properties is 20%.

[0077] The grain size and elongation of the Invar alloy forgings prepared in Examples 1 to 5 and Comparative Examples 1 to 7 were tested. The specific performance is shown in Table 2. The grain size of the Invar alloy forgings prepared in Examples 1 to 5 is 4 to 6, the elongation is 40% to 60%, and the forged slabs are all formed without quality problems such as cracking of the forgings. The expansion performance is -180～0℃ 1.2×10 -6 / ℃~1.6×10 -6 / ℃, meet the use requirements -180～0℃ ≤2×10 -6 / °C. The grain refinement grade and elongation of the Invar alloy forging blank prepared in Example 1 are significantly better than those in Comparative Example 1. This is because the Example performs a three-stage upsetting and drawing process, and the process parameters are more consistent with the hot working characteristics of Invar alloy. During the full upsetting and drawing process, the grain size of the forging blank is fully refined, and its hot working plasticity is greatly improved.

[0078] From the combination of Example 1 and Comparative Examples 2 and 3, it can be seen that the upsetting amount of Comparative Example 2 is obviously less than the range specified in the present application, which results in insufficient fragmentation of the internal structure of the ingot, and although the ingot does not crack in the end, the grain size of the forged blank is poor, the coarse grain is obvious, and the core grain structure reaches level 2. The upsetting amount of each heating cycle of Comparative Example 3 exceeds the range specified in the present application, and because the deformation amount is too large, the forged blank cracks during the second heating deformation, and cannot continue to deform.

[0079] From the combination of Example 1 and Comparative Examples 4 and 5, it can be seen that the deformation amount of Comparative Example 4 obviously exceeds the range specified in the present application, which results in cracking of the ingot during the second heating forging, and the deformation forging cannot continue; the deformation amount of Comparative Example 5 does not exceed the range specified in the present application, but the grain structure reaches level 2, and there is partial mixed grain.

[0080] From the combination of Example 1 and Comparative Examples 6 and 7, it can be seen that the chemical components of Comparative Examples 6 and 7 both exceed the range specified in the present application, which results in significant deterioration of the expansion performance of the alloy, and the alloy cannot meet the performance index requirement of a -180～0℃ ≤2×10 -6 / ℃.

[0081] In summary, the invar alloy prepared by the present application can be widely used in the fields of aerospace, national defense and military industry, energy transportation, etc. Because of its excellent hot working performance and excellent expansion performance, the successful design and development of the alloy will bring extensive economic benefits, and the market application is wide.

[0082] It should be noted that the above examples are only used to illustrate the technical solutions of the present application and are not limiting. Although the present application has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that the technical solutions of the present application can be modified or replaced equivalently without departing from the purpose and scope of the present application, and they should be covered in the scope of the claims of the present application.

Claims

1. A forging method for improving the plasticity of a large Invar alloy steel ingot, characterized in that: The following steps are involved: S1, smelting process, using an electric furnace to smelt molten steel that meets the internal control composition requirements of Invar alloy, and after casting, a large Invar alloy steel ingot weighing 15 to 25 tons is obtained; S2, a heating process, heating the cold Invar alloy steel ingot to 1000°C to 1200°C and keeping the temperature, and controlling the holding time to be ≥10h; S3, a roughing and drawing process, wherein the heated Invar alloy steel ingot is subjected to three rounds of roughing and drawing, wherein each round is subjected to roughing and drawing by a reduction of 15 to 50% of the total height of the Invar alloy steel ingot, and then drawn to the original height of the Invar alloy steel ingot after upsetting, and after each round of roughing and drawing, the ingot is returned to the heating furnace for heating and heat preservation, and the final forging temperature of each round of roughing and drawing is ≥750°C; S4, forging and shaping, forging the Invar alloy steel ingot after the last heating and heat preservation in step S3 to a specified size, and then slowly cooling it to room temperature in air to obtain the Invar alloy steel forging blank.

2. The forging method for improving the plasticity of a large Invar alloy steel ingot according to claim 1, characterized in that: In step S1, pure nickel plate and pure iron are used as raw materials in the smelting process, the Ni content in the pure nickel plate is greater than 99.5%, and the total content of impurity elements is ≤0.05%.

3. The forging method for improving the plasticity of a large Invar alloy steel ingot according to claim 1, characterized in that: In step S1, the molten steel meets the following internal control composition requirements of Invar alloy: C≤0.08%, Si≤0.8%, Mn≤0.5%, P≤0.020%, S≤0.06%, 28%≤Ni≤35%, Mg≤0.1%, Al≤0.20%, and the balance is iron and unavoidable impurities, calculated in weight percentage.

4. The forging method for improving the plasticity of a large Invar alloy steel ingot according to claim 1, characterized in that: In step S2, the holding time satisfies the following formula: t≥8d+1.5 Wherein, t is time, unit is h; d is the diameter of the Invar alloy steel ingot, unit is m.

5. The forging method for improving the plasticity of a large Invar alloy steel ingot according to claim 1, characterized in that: In step S2, during the heating process, the heating rate is 20-100°C / h.

6. The forging method for improving the plasticity of a large Invar alloy steel ingot according to claim 1, characterized in that: Step S3 includes the following steps: S31, first fire upsetting and drawing, upsetting the heated Invar alloy steel ingot by a reduction of 30-50% of the total height of the Invar alloy steel ingot, and then drawing it to the original Invar alloy steel ingot height, with a single-pass reduction of 5-10 mm and a final forging temperature of ≥750°C. Then, returning the ingot to the heating furnace and heating at 1100-1200°C for 2-3 hours; S32, the second fire upsetting and drawing, the upsetting amount is controlled to be 20-40% of the total height of the Invar alloy steel ingot, the single-pass reduction is 10-15mm, the final forging temperature is ≥750℃, after upsetting and drawing, return to the heating furnace and heat at 1100-1200℃ for 1-2h; S33, the third fire upsetting and drawing, control the upsetting amount to 15-35% of the total height of the Invar alloy steel ingot, the single-pass reduction is 15-20mm, the final forging temperature is ≥750℃, after upsetting and drawing, return to the heating furnace and heat and keep warm at 1100-1200℃ for 1-2h.

7. The forging method for improving the plasticity of a large Invar alloy steel ingot according to claim 1, characterized in that: In step S4, the thickness of the Invar alloy steel forging blank is 150-300 mm, the length is 4-7 m, and the corners of the forging blank are right angles.

8. The forging method for improving the plasticity of a large Invar alloy steel ingot according to claim 1, wherein: In step S4, the grain size of the Invar alloy steel forging blank is ≥ grade 4, the elongation is ≥ 40%, and the expansion performance at a temperature of -180 to 0°C is -180～0℃ 1×10 -6 / ℃~2×10 -6 / ℃.

9. An Invar alloy prepared by a forging method for improving the plasticity of a large Invar alloy steel ingot according to any one of claims 1 to 8, characterized in that: The composition of the Invar alloy is as follows by weight percentage: C≤0.08%, Si≤0.8%, Mn≤0.5%, P≤0.020%, S≤0.06%, 28%≤Ni≤35%, Mg≤0.1%, Al≤0.20%, and the balance is iron and inevitable impurities.

10. The Invar alloy according to claim 9, wherein Its properties are as follows: grain size ≥4, elongation ≥40%, expansion performance at -180~0℃ -180～0℃ 1×10 -6 / ℃~2×10 -6 / ℃.

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