A method of bell annealing invar cold rolled steel coils

Abstract

This invention discloses a bell-type annealing method for Invar alloy cold-rolled steel coils, comprising the following steps: S1, stacking Invar alloy cold-rolled steel coils in a bell-type annealing furnace, with metal heat transfer pads arranged between adjacent Invar alloy cold-rolled steel coils for isolation, the heat transfer pads having symmetrical groove structures; S2, during the bell-type annealing process, reducing sponge iron is placed in the bell-type annealing furnace, hydrogen is introduced, and the temperature is raised to 500-700℃ at a heating rate of 60-150℃ / h, then raised to 850-1000℃ at a heating rate of 20-50℃ / h and held for 20-40h, then cooled at a cooling rate of 20-40℃ / h, and removed from the furnace when the temperature inside the bell-type annealing furnace drops to less than 100℃; the flatness of the Invar alloy cold-rolled steel coils after bell-type annealing is ≤2.0mm / m. This invention can solve the problems of uneven stress release and easy oxidation of the edges due to uneven heating of Invar alloy strip.

Description

Technical Field

[0001] This invention relates to the field of Invar alloy technology, and more specifically, to a bell-type annealing method for cold-rolled Invar alloy steel coils. Background Technology

[0002] In today's society, there is an increasing variety of steel types to meet different requirements. Some steel grades, after being rolled into strip steel alone, cannot meet the needs of subsequent processing and require annealing before they can be cut and cold-deformed. Therefore, it is necessary to reduce their hardness and increase their plasticity. Annealing can homogenize the microstructure of the steel, improve its performance, and make the crystal particles within the steel finer, facilitating various subsequent processing methods. After annealing, the stress in the steel coil disappears, and the coil will no longer crack or deform.

[0003] Invar alloy is an Fe-Ni alloy with a Ni content of approximately 36wt%. Its coefficient of thermal expansion at room temperature is only about 1 / 10 that of most metals, and it remains approximately constant within the temperature range of -200 to 200°C. It is widely used in practical engineering applications requiring high dimensional accuracy and stability within a specified service temperature range, such as liquefied natural gas production, storage, transportation, and temperature control devices. However, Invar alloy is highly susceptible to corrosion. For example, 0.7mm Invar alloy strip used in the manufacture of insulated cargo tanks for membrane-type LNG carriers develops rust spots upon contact with fingers or even after the application of sweat, posing a critical safety hazard to LNG carriers. The production process for Invar alloy cold-rolled strip generally involves multiple annealing, leveling, grinding, and rolling processes on hot-rolled pickled coils according to finished specifications, followed by cleaning, annealing, and tension leveling to obtain the finished cold-rolled strip steel coil. In production practice, it has been found that when color difference defects occur on the surface of Invar alloy cold-rolled strip, surface corrosion is more likely to occur during storage, transportation, or use. During bell-type annealing, easily oxidizable elements such as Mn, Si, Cr, and Al (especially Si and Mn) in Invar alloy strips readily aggregate as oxides on the steel surface, exhibiting selective oxidation under extremely low oxygen partial pressures. These oxides can form corrosion microcells with the Fe matrix, increasing the activity of Fe atoms on the steel surface and accelerating the corrosion process through their catalytic effect, macroscopically manifesting as easy rusting of the steel. Therefore, it is urgent to find methods to suppress the selective oxidation behavior on the surface of Invar alloy strips.

[0004] Furthermore, LNG marine Invar alloys have high requirements for the uniformity and flatness of cold-rolled strips. The annealing process of cold-rolled strips has a significant impact on the material's shape. Therefore, effectively controlling the uniformity of the strip shape after annealing is crucial. Currently, the main annealing processes for cold-rolled strips are continuous bright annealing furnaces and bell-type annealing furnaces. While continuous bright annealing furnaces offer more uniform heating and better shape control than bell-type annealing, their higher production costs and lower efficiency limit their application to the annealing of high-value-added metal materials. Bell-type annealing furnaces are widely used due to their lower production costs and higher efficiency, but they offer less advantage in terms of heating uniformity and shape control. Moreover, the reducing atmosphere inside the furnace is difficult to guarantee, making the material prone to edge oxidation and surface color defects during high-temperature annealing. This severely limits the use of bell-type annealing furnaces for Invar alloys, and there is an urgent need to solve the problems existing in the use of bell-type annealing furnaces for Invar alloy cold-rolled strips.

[0005] Chinese patent publication CN119710149 A proposes a bell-type furnace hot-rolled coil annealing process, specifying the furnace purging temperature and density inspection method. This method achieves the goal of changing the purging temperature before the heating and homogenization processes in the bell-type furnace annealing process, avoiding the generation of annealing marks due to reaching the upper limit of the program stoppage, thus reducing downtime. It also solves the problem of annealing marks caused by density inspection failures during the heating and homogenization processes in the bell-type furnace annealing process, reducing the impact of the annealing process on the quality of the steel coil. Furthermore, it improves the one-click bottom-up rate and product qualification rate of the bell-type furnace annealing process, and reduces energy consumption while hot-rolled coil stacking, thus playing a role in energy conservation and environmental protection. However, this technology does not solve the technical problems of uneven heating of cold-rolled strip during the bell-type annealing process, poor strip shape after stress release, and the problem of easy oxidation of the edges during the bell-type annealing process.

[0006] Chinese Patent Publication No. CN117587222A discloses a method for preventing interlayer adhesion of austenitic stainless steel coils during bell-type furnace annealing. The method includes: mixing boric acid and alcohol to prepare a supersaturated boric acid-alcohol solution and loading it into the spray box of the uncoiler unit; uncoiling the steel coil on the uncoiler unit; and uniformly spraying the supersaturated boric acid-alcohol solution from the spray box onto the surface of the steel strip as the coil unfolds. This technology can effectively prevent interlayer adhesion of austenitic stainless steel coils during bell-type furnace annealing, resulting in high-quality finished austenitic stainless steel coils that meet customers' specific performance and usage requirements. However, this technology cannot solve problems such as uneven heating and edge oxidation in the bell-type annealing furnace.

[0007] Therefore, it is necessary to develop a bell-type annealing method suitable for Invar alloy cold-rolled steel coils to solve the problems of edge oxidation and uneven heating of Invar alloy in bell-type annealing furnaces, and to further expand the application range of Invar alloy annealing process. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the present invention aims to provide a bell-type annealing method for Invar alloy cold-rolled steel coils, which can solve the problems of uneven stress release and easy oxidation of the edges during high-temperature annealing of Invar alloy strips due to uneven heating, thereby reducing production costs and improving annealing efficiency.

[0009] To achieve the above objectives, the present invention adopts the following technical solution:

[0010] This invention provides a bell-type annealing method for Invar alloy cold-rolled steel coils, comprising the following steps:

[0011] S1, stacking Invar alloy cold-rolled steel coils and placing them into a bell-type annealing furnace, with metal heat transfer pads arranged between adjacent Invar alloy cold-rolled steel coils for isolation, the heat transfer pads having symmetrical groove structures.

[0012] S2, during the bell-type annealing process, reduced sponge iron is placed in the bell-type annealing furnace, and then hydrogen is introduced to raise the temperature to 500-700℃ at a heating rate of 60-150℃ / h. The temperature is then raised to 850-1000℃ at a heating rate of 20-50℃ / h and held for 20-40h. After that, the temperature is lowered at a cooling rate of 20-40℃ / h until the temperature inside the bell-type annealing furnace drops to less than 100℃ before the iron is removed from the furnace.

[0013] The flatness of the Invar alloy cold-rolled steel coil after the bell-type annealing is ≤2.0mm / m.

[0014] Preferably, in step S1, the composition of the Invar alloy cold-rolled steel coil by weight percentage is as follows: C≤0.08%, Si≤1.2%, Mn≤0.9%, P≤0.080%, S≤0.10%, 30%≤Ni≤37%, Mg≤0.4%, Al≤0.60%, with the remainder being Fe and unavoidable residual elements.

[0015] Preferably, in step S1, the thickness of the Invar alloy cold-rolled steel coil / strip is ≤4mm and the width is ≤1200mm.

[0016] If the thickness of the Invar alloy cold-rolled steel coil / strip is greater than 4mm, the Invar alloy cold-rolled steel coil / strip is very prone to uneven heating of the inner and outer layers, incomplete stress release in the core of the cold-rolled steel coil, resulting in poor sheet shape and other quality problems; at the same time, the width is required to be ≤1200mm, otherwise the height of the coil in the bell-type furnace will be too high, affecting the flow of heat in the furnace and causing uneven heating of the cold-rolled steel coil.

[0017] Preferably, in step S1, there is a gap between the layers of the Invar alloy cold-rolled steel coil, and the gap is ≤0.1mm.

[0018] In Invar alloy cold-rolled steel coils, controlling the gap between layers of cold-rolled steel coils to ≤0.1mm can further ensure that the cold-rolled steel coils are heated evenly during the bell-type annealing process, and avoid large differences in heating between the center and the edge of the cold-rolled steel coil in the width direction, which would cause the plate shape to fail to meet the requirements.

[0019] Preferably, in step S1, 1 to 4 rolls of Invar alloy cold-rolled steel are arranged in each furnace.

[0020] If the number of Invar alloy cold-rolled steel coils is greater than 4, the heat flow in the furnace will be obstructed, affecting the heat flow field in the furnace, which will easily lead to uneven heating of the material and the plate shape will not meet the requirements. If there are more than 2 coils of these Invar alloy cold-rolled steel coils, they can be placed vertically stacked, with heat transfer pads used to isolate the coils from each other.

[0021] Preferably, in step S1: the heat transfer pad is made of aluminum metal; the groove structure has eight symmetrically distributed grooves, the width of the grooves is 3-5cm and the depth is 2-4cm.

[0022] Aluminum heat transfer pads offer superior thermal conductivity, facilitating heat transfer within the coil and improving the uniformity of heating in cold-rolled steel coils. The grooved structure, meeting the aforementioned distribution and dimensional requirements, effectively balances heat distribution inside and outside the coil, preventing excessive heat gradient differences and ensuring uniform heating throughout the coil.

[0023] In step S2, the bell-type annealing process follows a "fast at first, slow later" heating rate, raising the Invar alloy to 850–1000°C. This ensures a small temperature gradient difference between the inside and outside of the Invar alloy cold-rolled steel coil, keeping internal thermal stress within a reasonable range. Uniform heating in the high-temperature region ensures sufficient recrystallization, guaranteeing complete removal of rolling stress. If the heating rate is less than 60°C / h, the heating time is too long, causing the refined recrystallized grains to coarsen and grow again, resulting in the material's mechanical properties failing to meet standard requirements. If the heating rate is greater than 100°C / h, the temperature difference between the inside and outside of the coil is large, leading to high residual stress and poor coil shape after cooling. To ensure both material mechanical properties and that residual stress in the cold-rolled coil meets requirements, the initial heating rate is set to 60–150°C / h; subsequent heating is carried out at a rate of 20–50°C / h to further reduce residual stress caused by temperature differences.

[0024] Preferably, in step S2, during the bell-type annealing process, the bell-type annealing furnace adopts a full hydrogen atmosphere, and the purity of hydrogen in the furnace is ≥99.5%.

[0025] If the hydrogen content in the furnace is less than 99.5%, the oxygen mixed in the furnace will selectively oxidize the active elements such as Si and Mn at the edge of the cold-rolled steel coil during the heating process, generating active oxides such as SiO2 and MnO, resulting in obvious oxidation color difference at the edge; controlling the purity of hydrogen in the furnace can effectively solve the problem of edge oxidation of Invar alloy.

[0026] Preferably, in step S2, the reduced sponge iron is placed at the hydrogen inlet of the bell-type annealing furnace.

[0027] Preferably, the diameter of the reduced sponge iron particles is less than 0.15 mm (preferably ≤0.1 mm), and the amount of reduced sponge iron added in each furnace is not less than 0.1 times the effective volume of the bell-type annealing furnace.

[0028] Before heating in the bell-type annealing process, reduced sponge iron is placed into the bell-type annealing furnace, positioned at the H2 inlet. The particle diameter of this reduced sponge iron must be less than 0.15 mm, and the amount of reduced sponge iron added per furnace must be no less than 0.1 times the effective volume of the bell-type annealing furnace (i.e., 0.1 × L, where L is the effective volume of the bell-type annealing furnace in m³). 3 Granular reduced sponge iron has a larger specific surface area in contact with air, which is more conducive to contacting and adsorbing oxygen in the gas of the bell-type furnace, effectively avoiding edge oxidation color difference caused by edge oxidation during the annealing process of cold-rolled steel coils.

[0029] Preferably, the flatness of the Invar alloy cold-rolled steel coil after the bell-type annealing is 1.6 to 2.0 mm / m, with no oxidation or waviness on the edges.

[0030] The bell-type annealing method for Invar alloy cold-rolled steel coils provided by this invention has the following beneficial effects:

[0031] This invention effectively solves the problem of uneven stress release caused by uneven heating in Invar alloy strip. It innovatively designs an aluminum heat transfer pad with symmetrical heat transfer grooves, and innovatively designs a reasonable annealing heating rate and annealing process for Invar alloy cold-rolled steel coils. This controls the interlayer size of the cold-rolled steel coil and effectively controls the difference in thermal stress between the inside and outside of the cold-rolled steel coil by controlling the coil thickness and width range. Furthermore, this invention innovatively places granular sponge iron in the bell-type annealing furnace, which effectively adsorbs oxygen in the reducing atmosphere, ensuring that the edges do not oxidize during annealing and thus avoiding edge color differences. This solves the technical problem of easy oxidation of the edges of Invar alloy cold-rolled steel coils during high-temperature annealing. Compared with this annealing method using a continuous bright annealing furnace, this invention, while ensuring the surface and sheet quality of the cold-rolled steel coil after annealing, is highly efficient and has a low production cost per ton of steel, greatly improving the annealing efficiency of Invar alloy cold-rolled steel coils. This has significant practical implications for the development of Invar alloy production technology and the improvement of production efficiency. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the temperature curve of the bell-type annealing method for Invar alloy cold-rolled steel coils of the present invention.

[0033] Figure 2 This is a schematic diagram of the structure of the heat transfer pad of the present invention;

[0034] Figure 3 This is a morphological image of the Invar alloy cold-rolled steel coil after annealing in Example 1;

[0035] Figure 4 This is a schematic diagram of the edge oxidation color difference after annealing in Comparative Example 4;

[0036] Figure 5 This is a morphological image of the Invar alloy cold-rolled steel coil after annealing, as shown in Comparative Example 4. Detailed Implementation

[0037] To better understand the above-mentioned technical solutions of the present invention, the technical solutions of the present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0038] Example 1

[0039] The composition of the Invar alloy cold-rolled steel coil in this embodiment is shown in Table 1. The thickness of the Invar alloy cold-rolled steel coil strip is 4 mm and the width is 1200 mm.

[0040] like Figure 1 and Figure 2 As shown, the bell-type annealing method for Invar alloy cold-rolled steel coils in this embodiment is as follows:

[0041] According to the process parameters shown in Table 2, two coils of Invar alloy cold-rolled steel were vertically placed into a bell-type annealing furnace. Each coil was separated by an aluminum ingot-based heat transfer plate, which featured eight symmetrically distributed grooves, each 3 cm wide and 2 cm deep. The gap between layers of the Invar alloy cold-rolled steel was controlled at 0.1 mm to ensure uniform heating during annealing. Reduced sponge iron with a particle diameter of 0.10 mm was placed inside the bell-type annealing furnace, which had a furnace volume of 1000 L and an added amount of 100 kg of sponge iron. After the cold-rolled steel was placed in the furnace and the reduced sponge iron was added, a full hydrogen atmosphere was used. Hydrogen gas (purity ≥99.5%) was introduced into the furnace, and the temperature was increased to 500 °C at a rate of 100 °C / h, then increased to 900 °C at a rate of 40 °C / h and held for 30 hours. After the holding time is reached, the furnace is cooled at a rate of 20℃ / h until the temperature inside the furnace drops to less than 100℃ before being removed from the furnace.

[0042] Example 2

[0043] The composition of the Invar alloy cold-rolled steel coil in this embodiment is shown in Table 1. The thickness of the Invar alloy cold-rolled steel coil strip is 3 mm and the width is 1150 mm.

[0044] like Figure 1 and Figure 2 As shown, the bell-type annealing method for Invar alloy cold-rolled steel coils in this embodiment is as follows:

[0045] According to the process parameters shown in Table 2, four coils of Invar alloy cold-rolled steel were placed in a bell-type annealing furnace. Each coil was separated by an aluminum ingot-based heat transfer plate, which featured eight symmetrically distributed grooves, each 4 cm wide and 3 cm deep. The gap between layers of the Invar alloy cold-rolled steel was controlled at 0.05 mm to ensure uniform heating during annealing. Reduced sponge iron with a particle diameter of 0.080 mm was added to the bell-type annealing furnace, which had a furnace volume of 1000 L and an added amount of 100 kg of sponge iron. After the cold-rolled steel coils were placed in the furnace and the reduced sponge iron was added, a full hydrogen atmosphere was used. Hydrogen gas (with a purity ≥99.5%) was introduced, and the furnace temperature was increased to 500 °C at a rate of 100 °C / h, then increased to 900 °C at a rate of 40 °C / h and held for 30 hours. After the holding time is reached, the furnace is cooled at a rate of 20℃ / h until the temperature inside the furnace drops to less than 100℃ before being removed from the furnace.

[0046] Example 3

[0047] The composition of the Invar alloy cold-rolled steel coil in this embodiment is shown in Table 1. The thickness of the Invar alloy cold-rolled steel coil strip is 2 mm and the width is 1150 mm.

[0048] like Figure 1 and Figure 2 As shown, the bell-type annealing method for Invar alloy cold-rolled steel coils in this embodiment is as follows:

[0049] According to the process parameters shown in Table 2, four coils of Invar alloy cold-rolled steel were placed in a bell-type annealing furnace. Each coil was separated by an aluminum ingot-based heat transfer plate, which featured eight symmetrically distributed grooves, each 5 cm wide and 4 cm deep. The gap between layers of the Invar alloy cold-rolled steel was controlled at 0.05 mm to ensure uniform heating during annealing. Reduced sponge iron with a particle diameter of 0.080 mm was added to the bell-type annealing furnace, which had a volume of 2000 L and an added amount of 200 kg. After the cold-rolled steel coils were placed in the furnace and the reduced sponge iron was added, a full hydrogen atmosphere was used. Hydrogen gas (purity ≥99.5%) was introduced into the furnace, and the temperature was increased to 500 °C at a rate of 60 °C / h, then increased to 900 °C at a rate of 20 °C / h and held for 30 hours. After the holding time is reached, the furnace is cooled at a rate of 20℃ / h until the temperature inside the furnace drops to less than 100℃ before being removed from the furnace.

[0050] Example 4

[0051] The composition of the Invar alloy cold-rolled steel coil in this embodiment is shown in Table 1. The thickness of the Invar alloy cold-rolled steel coil strip is 4 mm and the width is 1200 mm.

[0052] like Figure 1 and Figure 2 As shown, the bell-type annealing method for Invar alloy cold-rolled steel coils in this embodiment is as follows:

[0053] According to the process parameters shown in Table 2, two coils of Invar alloy cold-rolled steel were placed in a bell-type annealing furnace. Each coil was separated by an aluminum ingot-based heat transfer plate, which featured eight symmetrically distributed grooves, each 5 cm wide and 2 cm deep. The gap between layers of the Invar alloy cold-rolled steel was controlled at 0.1 mm to ensure uniform heating during annealing. Reduced sponge iron with a particle diameter of 0.10 mm was added to the bell-type annealing furnace, which had a volume of 1000 L and an amount of 100 kg of sponge iron. After the cold-rolled steel was placed in the furnace and the reduced sponge iron was added, a full hydrogen atmosphere was used. Hydrogen gas (purity ≥99.5%) was introduced into the furnace, and the temperature was increased to 500 °C at a rate of 100 °C / h, then increased to 900 °C at a rate of 40 °C / h and held for 30 hours. After the holding time is reached, the furnace is cooled at a rate of 20℃ / h until the temperature inside the furnace drops to less than 100℃ before being removed from the furnace.

[0054] Example 5

[0055] The composition of the Invar alloy cold-rolled steel coil in this embodiment is shown in Table 1. The thickness of the Invar alloy cold-rolled steel coil strip is 3.5 mm and the width is 1100 mm.

[0056] like Figure 1 and Figure 2 As shown, the bell-type annealing method for Invar alloy cold-rolled steel coils in this embodiment is as follows:

[0057] According to the process parameters shown in Table 2, three coils of Invar alloy cold-rolled steel were placed in a bell-type annealing furnace. Each coil was separated by an aluminum ingot-based heat transfer plate, which featured eight symmetrically distributed grooves, each 5 cm wide and 2 cm deep. The gap between layers of the Invar alloy cold-rolled steel was controlled at 0.08 mm to ensure uniform heating during annealing. Reduced sponge iron with a particle diameter of 0.12 mm was added to the bell-type annealing furnace, which had a furnace volume of 1000 L and an added amount of 100 kg of sponge iron. After the cold-rolled steel coils were added and the reduced sponge iron was introduced, a full hydrogen atmosphere was used. Hydrogen gas (purity ≥99.5%) was introduced into the furnace, and the temperature was increased to 700 °C at a rate of 150 °C / h, then increased to 1000 °C at a rate of 50 °C / h and held for 20 hours. After the holding time is reached, the furnace is cooled at a rate of 40℃ / h until the temperature inside the furnace drops to less than 100℃ before being removed from the furnace.

[0058] Example 6

[0059] The composition of the Invar alloy cold-rolled steel coil in this embodiment is shown in Table 1. The thickness of the Invar alloy cold-rolled steel coil strip is 3.6 mm and the width is 1150 mm.

[0060] like Figure 1 and Figure 2 As shown, the bell-type annealing method for Invar alloy cold-rolled steel coils in this embodiment is as follows:

[0061] According to the process parameters shown in Table 2, three coils of Invar alloy cold-rolled steel were placed in a bell-type annealing furnace. Each coil was separated by an aluminum ingot-based heat transfer plate, which featured eight symmetrically distributed grooves, each 3.2 cm wide and 2.6 cm deep. The gap between layers of the Invar alloy cold-rolled steel was controlled at 0.05 mm to ensure uniform heating during annealing. Reduced sponge iron with a particle diameter of 0.14 mm was added to the bell-type annealing furnace, which had a furnace volume of 1000 L and an added amount of 100 kg of sponge iron. After the cold-rolled steel coils were placed in the furnace and the reduced sponge iron was added, a full hydrogen atmosphere was used. Hydrogen gas (purity ≥99.5%) was introduced into the furnace, and the temperature was raised to 600 °C at a rate of 110 °C / h, then increased to 850 °C at a rate of 30 °C / h and held for 40 hours. After the holding time is reached, the furnace is cooled at a rate of 30℃ / h until the temperature inside the furnace drops to less than 100℃ before being removed from the furnace.

[0062] Comparative Example 1

[0063] The composition of the Invar alloy cold-rolled steel coil in this comparative example is shown in Table 1. The thickness of the Invar alloy cold-rolled steel coil strip is 5 mm and the width is 1200 mm.

[0064] This comparative example does not use a heat transfer pad. The bell-type annealing method for the Invar alloy cold-rolled steel coil in this comparative example is as follows:

[0065] According to the process parameters shown in Table 2, two coils of Invar alloy cold-rolled steel were placed in a bell-type annealing furnace. No heat transfer pads were added between the Invar alloy cold-rolled steel coils, and there were no obvious gaps between the layers of steel coils. Reduced sponge iron with a particle diameter of 0.10 mm was placed in the annealing furnace, which had a volume of 1000 L and a weight of 100 kg. After the cold-rolled steel coils were placed in the furnace and the reduced sponge iron was added, a full hydrogen atmosphere was used. Hydrogen gas (with a purity of ≥99.5%) was introduced into the furnace, and the temperature was raised to 500 °C at a rate of 100 °C / h. Then, the temperature was raised to 900 °C at a rate of 40 °C / h and held for 30 h. After the holding time was reached, the temperature was lowered at a rate of 20 °C / h until the furnace temperature dropped below 100 °C before the coils were removed from the furnace.

[0066] Comparative Example 2

[0067] The composition of the Invar alloy cold-rolled steel coil in this comparative example is shown in Table 1. The thickness of the Invar alloy cold-rolled steel coil strip is 3 mm and the width is 1150 mm.

[0068] The bell-type annealing method for Invar alloy cold-rolled steel coils in this comparative example is as follows:

[0069] According to the process parameters shown in Table 2, four coils of Invar alloy cold-rolled steel were placed in a bell-type annealing furnace. Each coil was separated by an aluminum ingot-based heat transfer plate, which featured eight symmetrically distributed grooves, each 4 cm wide and 3 cm deep. The gap between layers of the Invar alloy cold-rolled steel was controlled at 0.05 mm to ensure uniform heating during annealing. Reduced sponge iron with a particle diameter of 0.080 mm was placed in the annealing furnace, which had a volume of 1000 L and a weight of 100 kg. After the cold-rolled steel was placed in the furnace and the reduced sponge iron was added, a full hydrogen atmosphere was used. Hydrogen gas (purity ≥99.5%) was introduced, and the temperature was raised to 800 °C at a rate of 200 °C / h, then increased to 900 °C at a rate of 60 °C / h and held for 40 hours. After the holding time is reached, the furnace is cooled at a rate of 20℃ / h until the temperature inside the furnace drops to less than 120℃ before being removed from the furnace.

[0070] Comparative Example 3

[0071] The composition of the Invar alloy cold-rolled steel coil in this comparative example is shown in Table 1. The thickness of the Invar alloy cold-rolled steel coil strip is 2 mm and the width is 1150 mm.

[0072] This comparative example does not use reducing sponge iron. The bell-type annealing method for the Invar alloy cold-rolled steel coil in this comparative example is as follows:

[0073] According to the process parameters shown in Table 2, four coils of Invar alloy cold-rolled steel were placed in a bell-type annealing furnace. Each coil was separated by a heat transfer plate made of aluminum ingot material. The heat transfer plate was designed with eight symmetrically distributed grooves, each 5 cm wide and 4 cm deep. The gap between layers of the Invar alloy cold-rolled steel was controlled at 0.05 mm to ensure uniform heating of the cold-rolled steel coils during annealing. After the cold-rolled steel coils were placed in the furnace, a full hydrogen atmosphere was used, with hydrogen gas (purity ≥99.5%) introduced into the furnace. The temperature was increased to 500℃ at a rate of 60℃ / h, then increased to 900℃ at a rate of 20℃ / h and held for 30 hours. After reaching the holding time, the temperature was decreased at a rate of 20℃ / h until the furnace temperature dropped below 100℃ before the coils were removed from the furnace.

[0074] Comparative Example 4

[0075] This comparative example uses the method of Example 1, the difference being that the thickness of the Invar alloy cold-rolled steel coil strip is 5mm and the width is 1250mm.

[0076] Comparative Example 5

[0077] This comparative example uses the method of Example 1, the difference being that the gap between layers of Invar alloy cold-rolled steel coil is controlled to be 0.15mm.

[0078] Comparative Example 6

[0079] This comparative example uses the method of Example 1, the difference being the following annealing process:

[0080] The furnace is heated to 480℃ at a rate of 50℃ / h, then to 800℃ at a rate of 15℃ / h and held for 45 hours. After the holding time is reached, the furnace is cooled down at a rate of 15℃ / h until the temperature inside the furnace drops below 100℃.

[0081] Comparative Example 7

[0082] This comparative example uses the method of Example 1, the difference being the following annealing process:

[0083] The temperature is increased to 800℃ at a rate of 160℃ / h, and then increased to 1100℃ at a rate of 55℃ / h and held for 18 hours. After the holding time is reached, the temperature is decreased at a rate of 45℃ / h until the furnace temperature drops to below 100℃ before the furnace is removed from the furnace.

[0084] Figure 3 This is a morphological image of the Invar alloy cold-rolled steel coil after annealing in Embodiment 1 of the present invention. It has a good plate shape and no obvious color difference marks on the edge, which fully meets the user's requirements.

[0085] Figure 4 , 5 The images shown are morphological images of the Invar alloy cold-rolled steel coil after annealing using the unimproved process, as shown in Comparative Example 4. The coil has a poor shape (wavy) and obvious oxidation color difference marks on the edges.

[0086] Table 1: Chemical composition (wt%) of each example and comparative example

[0087]

[0088] Table 2: Relevant technical parameters of each embodiment and comparative example

[0089]

[0090] Table 3. Quality Comparison of Invar alloy cold-rolled steel coils after annealing

[0091]

[0092] As shown in Table 3, the flatness of the Invar alloy cold-rolled steel coil after annealing in the embodiments of the present invention ranges from 1.0 to 2.0 mm / m, the plate shape is good, the surface is free of waviness, and there are no obvious color difference marks on the edges.

[0093] Based on Example 1 and Comparative Example 1, it can be seen that no heat transfer pad was used in the annealing of Comparative Example 1, resulting in poor heat transfer between cold-rolled coils and a large temperature gradient between the inner and outer layers of the cold-rolled coils, which in turn led to a large residual stress in the cold-rolled coils. The wavy shape and flatness of the cold-rolled steel coils prepared in Comparative Example 1 were worse than those in Example 1.

[0094] Based on Example 2 and Comparative Example 2, as well as Example 1 and Comparative Examples 6 and 7, it can be seen that in Comparative Examples 2 and 6 and 7, after introducing hydrogen, the process parameters of step S2 were not implemented according to the relevant range: the temperature was raised to 500-700℃ at a heating rate of 60-150℃ / h, then raised to 850-1000℃ at a heating rate of 20-50℃ / h and held for 20-40h, and then cooled at a cooling rate of 20-40℃ / h until the temperature inside the bell-type annealing furnace dropped to less than 100℃ before being taken out of the furnace. This resulted in poor flatness of the cold-rolled coils after annealing in Comparative Examples 2, 6 and 7, which further affected the flatness control requirements of the subsequent finishing rolling, and the edges of the cold-rolled coils had obvious waviness.

[0095] As can be seen from Example 3 and Comparative Example 3, if reducing sponge iron is not added during the annealing process, the oxygen entering the bell-type annealing furnace cannot be further removed, resulting in obvious oxidation residues on the edge of the cold-rolled strip during the annealing process.

[0096] Based on Example 1 and Comparative Example 4, the Invar alloy cold-rolled coil has a thickness of 5 mm and a width of 1250 mm. This does not meet the requirements of strip thickness ≤ 4 mm and width ≤ 1200 mm. The Invar alloy cold-rolled steel coil used in Comparative Example 4 has a relatively thick strip, which leads to uneven temperature distribution between the inner and outer layers during annealing. Incomplete stress release in the core of the cold-rolled steel coil during cooling results in poor flatness and severe edge waviness. Furthermore, the strip width in Comparative Example 4 is greater than 1200 mm, resulting in a high total height of the coil in the bell-type furnace, which affects the heat flow within the furnace, leading to uneven heating of the cold-rolled steel coil and a poor sheet shape after cooling.

[0097] As can be seen from Example 1 and Comparative Example 5, the interlayer gap of the cold-rolled coil is 0.15mm, which does not meet the requirement of gap ≤ 0.1mm, resulting in obvious loosening and scratches of the coil during the annealing process, and the surface of the strip does not meet the requirements.

[0098] In summary, compared with existing technologies, this invention effectively solves the technical problem of uneven heating of Invar alloys in bell-type annealing furnaces, resulting in poor sheet shape, by adding an aluminum heat transfer pad between coils and innovatively creating eight grooves in the heat transfer pad to increase heat flow inside and outside the cold-rolled coils, while leaving certain gaps between the cold-rolled coil layers. Simultaneously, the innovative addition of reducing sponge iron to the bell-type furnace effectively solves the technical problems of impure reducing atmosphere, easy introduction of oxygen atmosphere, and easy oxidation of the edges during high-temperature annealing. This invention, while ensuring annealing quality, offers higher annealing efficiency and lower production cost per ton of steel compared to continuous bright annealing furnaces, significantly improving the production efficiency of the Invar alloy annealing process.

[0099] The Fe-Ni based Invar alloy prepared by this invention can be widely used in aerospace, defense, energy transportation and other fields; the bell-type annealing furnace process of this invention can improve the surface quality of Invar alloy cold-rolled strip and reduce the process cost, which is conducive to further promoting the application fields of Invar alloy.

[0100] Those skilled in the art should recognize that the above embodiments are merely illustrative of the present invention and are not intended to limit the present invention. Any variations or modifications to the above embodiments that are within the spirit and essence of the present invention will fall within the scope of the claims of the present invention.

Claims

1. A bell annealing method for a cold rolled Invar alloy steel coil, characterized in that: It comprises the following steps: S1, the invar alloy cold-rolled steel coil is placed into a cover annealing furnace, and heat transfer pads of metal material are arranged between adjacent invar alloy cold-rolled steel coils to isolate them, and the heat transfer pads have symmetrical groove structures; The thickness of the invar alloy cold-rolled steel coil is less than or equal to 4 mm, and the width is less than or equal to 1200 mm. There is a gap between the layers of the invar alloy cold-rolled steel coil, and the gap is less than or equal to 0.1 mm. S2, during the cover annealing process, reducing sponge iron is placed into the cover annealing furnace, then hydrogen is introduced to heat it to 500-700℃ at a heating rate of 60-150℃ / h, and then it is heated to 850-1000℃ at a heating rate of 20-50℃ / h for 20-40h, and then it is cooled at a cooling rate of 20-40℃ / h, and the cover annealing furnace is discharged when the temperature is less than 100℃. The flatness of the invar alloy cold-rolled steel coil after cover annealing is less than or equal to 2.0 mm / m.

2. The method of bell annealing of Invar alloy cold rolled steel coils according to claim 1, characterized in that: In step S1, the composition of the invar alloy cold-rolled steel coil is as follows: C≤0.08%, Si≤1.2%, Mn≤0.9%, P≤0.080%, S≤0.10%, 30%≤Ni≤37%, Mg≤0.4%, Al≤0.60%, and the rest is Fe and unavoidable residual elements.

3. The method of bell annealing of Invar cold rolled steel coils according to claim 1, characterized in that: In step S1, 1-4 invar alloy cold-rolled steel coils are arranged in each furnace.

4. The method of bell annealing of Invar cold rolled steel coils as claimed in claim 1 wherein: In step S1, the heat transfer pad is made of aluminum metal material, and the groove structure has eight symmetrical grooves with a width of 3-5 cm and a depth of 2-4 cm.

5. The method of bell annealing of Invar alloy cold rolled steel coils as claimed in claim 1 wherein: In step S2, during the cover annealing process, the cover annealing furnace uses a full hydrogen atmosphere, and the purity of hydrogen in the furnace is greater than or equal to 99.5%.

6. The method of bell annealing of Invar alloy cold rolled steel coils as claimed in claim 1 wherein: In step S2, the reducing sponge iron is placed at the hydrogen inlet of the cover annealing furnace.

7. The method of bell annealing of Invar cold rolled steel coils according to claim 6, characterized in that: The diameter of the reducing sponge iron particles is less than 0.15 mm, and the amount of reducing sponge iron added in each furnace is not less than 0.1 times the effective volume of the cover annealing furnace.

8. The method of bell annealing of Invar alloy cold rolled steel coils as claimed in claim 1 wherein: The flatness of the invar alloy cold-rolled steel coil after cover annealing is 1.6-2.0 mm / m, the edge is not oxidized, and there is no wave shape.

Images