Thin metal strip preparation method and thermal bimetal composite strip

Abstract

The invention discloses a thin metal strip preparation method and a thermal bimetal composite strip, and the method comprises the following steps: adopting a vacuum induction melting process to smelt all raw materials in a preset proportion to obtain alloy liquid with preset components, and pouring in a protective atmosphere to obtain an alloy ingot; the alloy cast ingot is sequentially subjected to forging, hot rolling, acid pickling and surface coping treatment, and an intermediate steel coil is obtained; the middle steel coil is subjected to multi-pass cold rolling, a strip is obtained, multi-pass cold rolling is executed through a preset deformation sequence, and at least one time of middle solution annealing treatment is conducted in the multi-pass cold rolling process; and the strip is subjected to final cold rolling and finished product solution annealing treatment, and the metal thin strip for the passive layer of the thermal bimetal composite strip is obtained. The alloy components and the purity are accurately controlled by adopting vacuum induction melting, and the corrosion resistance and the stability of thermal expansion and resistivity of the passive layer thin strip are improved by combining the multi-pass cold rolling with the preset deformation sequence and the intermediate solution annealing process.

Description

Technical Field

[0001] This invention relates to the field of precision alloy material manufacturing and processing technology, and in particular to a method for preparing metal strips and a thermal bimetallic composite strip. Background Technology

[0002] Thermo-bimetallic composite strips utilize the difference in thermal expansion coefficients between the active and passive layers to induce directional bending upon temperature changes, thereby achieving precise temperature control and overload protection. The passive layer must be made of a material with a low coefficient of thermal expansion and good linearity of change within a specific temperature range. Currently, the passive layer typically uses an iron-nickel alloy with a maximum carbon content controlled below 0.08%, and its properties are adjusted by adding elements such as nickel (Ni) and chromium (Cr). This alloy, as a passive layer, has a stable coefficient of thermal expansion and can work synergistically with the high-expansion active layer upon temperature changes to drive the element to produce precise and controllable deformation, thus achieving automatic temperature control and overload protection. Furthermore, it exhibits no harmful phase transformations within the typical operating temperature range, ensuring reliable control. In addition, this alloy typically contains approximately 7.0-7.5% chromium, giving it a certain degree of corrosion resistance, and is therefore widely used in automatic temperature control switches in household appliances, overload protectors for motors and transformers in industrial applications, and sensing elements in thermometers in the instrumentation field.

[0003] However, with increasingly demanding application environments and ever-increasing requirements for device reliability, existing technologies have revealed two prominent problems. First, as the core material of precision temperature control switches, the coefficient of linear expansion and resistivity of the passive layer alloy are key parameters determining its temperature control accuracy and response characteristics. These are closely related to the material's chemical composition, purity, and microstructure uniformity. Current manufacturing processes still face challenges in precisely controlling the alloy composition and effectively reducing gas and impurity content to stably obtain the target coefficient of expansion and resistivity. Second, when such components operate under long-term high temperature, high humidity, or corrosive media (such as chloride-containing environments), the passive layer alloy is still prone to corrosion. This not only affects the long-term stability of its thermal expansion behavior but also significantly shortens the lifespan of the entire temperature control element, restricting its reliable application in more demanding environments. Therefore, developing a passive layer material and its preparation method that combines excellent and stable thermal expansion and electrical properties with significantly improved corrosion resistance has become an urgent technical problem to be solved in this field. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing metal strips and a hot bimetallic composite strip. By using vacuum induction melting to precisely control the alloy composition and purity, and combining it with a multi-pass cold rolling and intermediate solution annealing process with a preset deformation sequence, the corrosion resistance, thermal expansion and resistivity stability of the passive layer strip are improved. This solves the problems of existing materials being prone to corrosion in high temperature, high humidity and chloride ion-containing environments, and having large fluctuations in performance parameters, resulting in insufficient temperature control accuracy and service life.

[0005] To address the aforementioned technical problems, a first aspect of this invention provides a method for preparing a metal strip, comprising the following steps: Step S100: Using a vacuum induction melting process, nickel, chromium, graphite, iron, molybdenum, manganese, silicon and titanium in a preset proportion are melted to obtain an alloy liquid with a preset composition, and then cast into an alloy ingot under a protective atmosphere. Step S200: The alloy ingot is subjected to forging, hot rolling, pickling and surface grinding in sequence to obtain intermediate steel coil; Step S300: The intermediate steel coil is subjected to multiple cold rolling passes to obtain strip. The multiple cold rolling passes are performed using a preset deformation sequence, and at least one intermediate solution annealing treatment is performed during the multiple cold rolling passes. Step S400: Perform final cold rolling and solution annealing on the strip to obtain a thermal bimetallic composite strip passive layer metal strip.

[0006] Furthermore, in step S300, the preset deformation sequence ensures that the multi-pass cold rolling has an increasing and then decreasing deformation distribution, and the deformation of the first pass is less than the deformation of the subsequent main deformation passes.

[0007] Further, in step S300, the multi-pass cold rolling includes five cold rolling passes, and the deformation amounts of the five passes in the preset deformation amount sequence are as follows: the deformation amount of the first pass is about 5%, the deformation amount of the second pass is about 18%, the deformation amount of the third pass is about 15%, the deformation amount of the fourth pass is about 12%, and the deformation amount of the fifth pass is about 5%.

[0008] Furthermore, the intermediate solution annealing treatment is performed when the multi-pass cold rolling reaches a first preset thickness, which is greater than the final target thickness of the strip.

[0009] Furthermore, the intermediate solution annealing process includes: heating the strip to a temperature range of 900℃-920℃ under a protective atmosphere and holding it at that temperature, followed by rapid cooling at a cooling rate of not less than 1000℃ / min; The protective atmosphere has a dew point of less than or equal to -45°C and an oxygen content of less than or equal to 10 ppm.

[0010] Furthermore, the process employs a vacuum induction melting process to melt nickel plates, metallic chromium, graphite carbon, industrial pure iron, molybdenum bars, metallic manganese, industrial silicon, and industrial pure titanium according to a preset ratio, to obtain an alloy liquid with a preset composition, including: Nickel plates, metallic chromium, graphite carbon, industrial pure iron, and molybdenum bars are loaded into the furnace under an environment where the vacuum degree is lower than the first preset vacuum degree value. After the vacuum degree is reduced to the second preset vacuum degree value, electricity is supplied for heating to melt the furnace charge. After the furnace charge is melted and cleared, the temperature of the molten steel is raised to the first preset temperature and the refining period begins, with the refining time being greater than or equal to the first preset duration. After refining, metallic manganese, industrial silicon, and industrial pure titanium are added to the molten steel under the protective atmosphere to obtain the alloy liquid.

[0011] Furthermore, the first preset vacuum level is 5 Pa; The second preset vacuum level is 3 Pa; The first preset temperature is 1510℃-1530℃; The first preset duration is 40 minutes; The protective atmosphere comprises argon gas, and the pressure value of the protective atmosphere is 8000 Pa.

[0012] Further, in step S200, the heating regime for forging is as follows: first, preheating and holding at 840℃-860℃, then raising the temperature to 1190℃-1210℃ and holding before forging begins, with the initial forging temperature not lower than 1150℃ and the final forging temperature not lower than 850℃. And / or, The heating process for hot rolling is as follows: first, preheating and holding at 840℃-860℃, then raising the temperature to 1170℃-1190℃ and holding before starting rolling, with the initial rolling temperature not lower than 1150℃ and the final rolling temperature not lower than 850℃.

[0013] Further, the preset components, by mass percentage, are: Ni: 37.4%-38.2%, Cr: 7.0%-7.5%, C≤0.08%, Si≤0.3%, S≤0.020%, P≤0.020%, Ti: 0.2%-0.4%, Mo: 1.0%-2.0%, with the balance being Fe and unavoidable impurities.

[0014] Accordingly, a second aspect of the present invention provides a thermal bimetallic composite strip, which includes a high expansion layer and a passive layer, wherein the passive layer is a metal strip prepared by the above-described metal strip preparation method.

[0015] The above-described technical solutions of the embodiments of the present invention have the following beneficial technical effects: 1. By precisely controlling the alloy composition, especially the synergistic addition of 0.2-0.4% Ti and 1.0-2.0% Mo, not only are the stable low-expansion characteristics required for the iron-nickel-chromium alloy as a passive layer maintained, but the corrosion resistance of the material is also significantly improved; the Ti element effectively fixes carbon and nitrogen atoms and inhibits the corrosion of Cr. 23 Harmful carbides such as C6 precipitate at grain boundaries, preventing the formation of chromium-depleted regions at grain boundaries; the addition of Mo enhances the stability and density of the surface passivation film in corrosive media containing chloride ions; the synergistic effect of the two significantly enhances the material's resistance to pitting corrosion, crevice corrosion, and intergranular corrosion when it is in long-term high-temperature, high-humidity, or harsh environments, thus solving the technical problem that the insufficient corrosion resistance of traditional passive layer alloys affects the service life and reliability of components. 2. By adopting a vacuum induction melting process and optimizing its key parameters (such as high vacuum charging, refining at 1520±10℃, and alloying under specific pressure), precise control over the purity and compositional uniformity of the alloy melt was achieved. This process effectively reduced gas content and harmful impurities, ensuring the accuracy and stability of the alloy composition, especially the content of key elements nickel and chromium. It fundamentally guarantees the precise controllability and excellent consistency of the core physical properties of the final product. It solves the problem of batch-to-batch instability in material properties caused by metallurgical fluctuations, enabling the prepared strips to meet the stringent requirements of precision temperature control elements for highly stable and predictable thermo-electrical performance parameters. 3. By designing and implementing a complete collaborative process chain of "hot working-cold rolling-heat treatment," including optimized forging / hot rolling temperature regimes, specific multi-pass cold rolling deformation sequences (such as 5%, 18%, 15%, 12%, 5%), and rapid cooling solution annealing introduced at critical thickness nodes, precise control of the material's microstructure (such as grain size, texture, and dislocation density) was achieved. This effectively avoided the risk of cracking during processing, promoted the formation of favorable microstructures, and suppressed the precipitation of brittle and harmful phases. It not only ensured a high yield under complex process flows but also enabled the material to achieve a uniform and refined microstructure, thereby comprehensively improving the dimensional accuracy, surface quality, mechanical properties, and overall service performance of the strip as a passive layer element. Attached Figure Description

[0016] Figure 1 This is a flowchart of the metal strip preparation method provided in the embodiments of the present invention. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments and the accompanying drawings. It should be understood that these descriptions are merely exemplary and not intended to limit the scope of the invention. Furthermore, descriptions of well-known structures and techniques are omitted in the following description to avoid unnecessarily obscuring the concept of the invention.

[0018] As a key thermally sensitive actuating element, the typical structure of the thermal bimetallic composite strip consists of a high-expansion layer with a high coefficient of thermal expansion and a passive layer with a low coefficient of thermal expansion, bonded together through metallurgy. The passive layer is usually made of an iron-nickel-based low-expansion alloy. In this invention, it specifically refers to a thin metal strip prepared by vacuum melting, plastic processing, and heat treatment of an alloy system containing specific elements such as nickel, chromium, titanium, and molybdenum. This passive layer has an extremely low and linearly stable coefficient of thermal expansion within a set operating temperature range, while also possessing good electrical conductivity, suitable hardness, and excellent corrosion resistance imparted by titanium and molybdenum alloying. When the temperature changes, the passive layer expands much less than the high-expansion layer, thereby constraining and guiding the entire composite material to produce directional and controllable bending deformation, ultimately achieving precise temperature sensing, control, or protection functions.

[0019] Please refer to Figure 1 The first aspect of this invention provides a method for preparing a metal strip, comprising the following steps: In step S100, a vacuum induction melting process is used to melt nickel, chromium, graphite, iron, molybdenum, manganese, silicon and titanium in a preset proportion to obtain an alloy liquid with a preset composition, and then cast it under a protective atmosphere to obtain an alloy ingot.

[0020] In step S100, the vacuum induction melting process plays a decisive role in the initial stage of alloy preparation. This step is carried out in a highly sealed vacuum induction furnace, and its core application is the preparation of special alloys with extremely high requirements for compositional purity, uniformity, and precision. During operation, based on the preset chemical composition of the target alloy (specifically exemplified in this embodiment as Ni: 37.4-38.2%, Cr: 7.0-7.5%, Ti: 0.2-0.4%, Mo: 1.0-2.0%, etc.), high-purity nickel plates, metallic chromium, graphite carbon (for carbon content adjustment), industrial pure iron, and molybdenum bars are accurately weighed and prepared as the main furnace charge, while metallic manganese, industrial silicon, and industrial pure titanium are used as subsequent alloying additives. The charging process is completed in an environment with a vacuum degree below 5 Pa, and then the vacuum is further reduced to below 3 Pa before electric heating is applied. This is to maximize the removal of air from the furnace chamber and prevent oxidation of the raw materials in the early stages of melting.

[0021] The smelting process is typically carried out in stages. Initially, a lower power (e.g., 160kW) is used to preheat the furnace charge to avoid splashing caused by rapid temperature increases. Subsequently, the power is increased (e.g., 420-430kW) to completely melt the charge. After the molten steel is formed, its temperature is precisely raised to 1520±10℃ and maintained at this temperature for at least 40 minutes for refining. The refining period is continued under vacuum or low pressure, which is beneficial for removing gases (such as H and N) and non-metallic inclusions from the melt through boiling and flotation, while also promoting preliminary homogenization of the composition. After refining, high-purity nitrogen is introduced into the furnace to a pressure of approximately 8000Pa to establish an inert protective atmosphere. Under this protective atmosphere, easily oxidizable metallic manganese, industrial silicon, and industrial pure titanium as a key microalloying element are added sequentially and stirred thoroughly to ensure uniform melting. Finally, when the molten steel reaches a temperature of approximately 1510±10℃, it is poured under continuous nitrogen protection to obtain an alloy ingot with highly precise composition and excellent cleanliness. This process ensures that all subsequent processing and treatment are based on high-quality raw materials that meet design requirements.

[0022] In step S200, the alloy ingot is sequentially forged, hot-rolled, pickled, and surface-ground to obtain an intermediate steel coil.

[0023] First, the ingot is forged into a billet, employing a segmented heating process: for example, the ingot is preheated to 850±10℃ and held for 2 hours to ensure uniform heat penetration and reduce thermal stress, then heated to 1200±10℃ and held for another 2 hours to fully austenitize and soften the alloy. Forging begins at this high temperature, with the initial forging temperature typically not lower than 1150℃ and the final forging temperature controlled above 850℃. The forging process not only deforms the ingot to the required dimensions (e.g., 8120×400×5000mm or 8110×630×5500mm), but more importantly, it breaks down the coarse dendrite structure through intense plastic deformation, welds the internal porosity, and significantly improves the material's density and isotropic mechanical properties. The forged billet then undergoes surface grinding to thoroughly remove defects such as oxide scale, surface cracks, and pits formed during the high-temperature forging process, providing a clean surface for subsequent hot rolling. After grinding, the billet is then hot rolled. The heating process is similar to that of forging. It is usually preheated at 850±10℃ and then heated to 1180±10℃ for homogenization. The initial rolling temperature is controlled above 1150℃ (e.g., 1160-1165℃), and the final rolling temperature also needs to be higher than 850℃ (e.g., 855-861℃) to ensure that the material is deformed within the temperature range of complete recrystallization and to obtain a fine and uniform recrystallized austenitic structure.

[0024] Furthermore, instead of air cooling after hot rolling, a laminar flow system is immediately activated to force-cool the high-temperature rolled piece. This controlled rapid cooling can suppress excessive grain growth at high temperatures and the precipitation of certain brittle phases during slow cooling, which is beneficial for refining the microstructure and optimizing subsequent processing performance. The hot-rolled coil is then pickled (using hydrochloric acid or sulfuric acid solution, soaking for about 1 hour) to remove the thicker iron oxide scale formed during hot rolling. After pickling, a fine surface grinding is performed again to thoroughly remove residual oxide scale and any minor surface defects that may have been exposed after pickling, ultimately obtaining a smooth-surfaced, dimensionally regular intermediate steel coil, which is ready for the high-precision cold rolling process.

[0025] Step S300: The intermediate steel coil is subjected to multiple cold rolling passes to obtain strip. The multiple cold rolling passes are performed using a preset deformation sequence, and at least one intermediate solution annealing treatment is performed during the multiple cold rolling process.

[0026] Multi-pass cold rolling does not employ a uniform deformation amount, but rather a pre-set, non-uniform deformation sequence. For example, in a specific implementation, five passes are used, with deformation amounts designed sequentially as approximately 5%, 18%, 15%, 12%, and 5%. This sequence has a clear engineering intent: the small deformation amount of approximately 5% in the first pass aims to ensure the strip smoothly enters the mill and achieves initial, gentle deformation, which facilitates the smooth progress of subsequent large deformation rolling; the subsequent second, third, and fourth passes use relatively larger deformation amounts (18%, 15%, and 12%) to efficiently thin the strip, accumulate sufficient dislocation density and deformation energy to provide driving force for subsequent recrystallization, and promote the formation of specific textures; the final pass again uses a small deformation amount of approximately 5%, mainly to precisely control the final thickness and shape of the strip, and to finely adjust its surface quality and internal stress state.

[0027] In this cold rolling process, when the strip is rolled to a preset intermediate thickness (e.g., from the initial thickness to 82.0 mm), rolling is interrupted for at least one intermediate solution annealing treatment. This annealing is carried out in a dedicated continuous annealing furnace, which is purged with a high-purity protective atmosphere (such as a nitrogen-hydrogen mixture) with a dew point not exceeding -45°C and an oxygen content not exceeding 10 ppm to prevent oxidation of the strip at high temperatures. The strip is rapidly heated to the austenitic single-phase region temperature, strictly controlled within the range of 910 ± 10°C, and held at that temperature for a sufficient time. The purpose of this process is to allow the deformation structures generated during the previous cold rolling deformation (such as high-density dislocations and deformation bands) to undergo complete recrystallization, while simultaneously dissolving any carbides that may have precipitated during rolling and those inherent to the material (especially chromium-rich carbides) into the austenitic matrix.

[0028] After the heat treatment is completed, the strip must be cooled at an extremely rapid rate (not less than 1000℃ / min), for example, by blowing high-speed cooling gas or entering a rapid cooling section with extremely high cooling efficiency. The purpose is to suppress the re-precipitation of brittle phases such as carbides during the cooling process and to "freeze" the supersaturated solid solution structure at high temperature to room temperature to the maximum extent. After this intermediate solution annealing, the work hardening of the strip is eliminated, plasticity is restored, the internal structure is reset to a uniform and clean supersaturated solid solution, and the chromium distribution at the grain boundaries is optimized, thus enabling subsequent cold rolling deformation and ultimately obtaining excellent comprehensive properties.

[0029] Step S400: Perform final cold rolling and solution annealing on the strip to obtain a metal strip for the passive layer of the hot bimetallic composite strip.

[0030] Step S400 is the final shaping and performance determination stage of the manufacturing process. The strip, after intermediate solution annealing, undergoes final cold rolling deformation to precisely achieve the final target thickness required for the product (e.g., rolling from 82.0 mm to 80.7 mm or 80.5 mm). This final stage involves relatively small cold rolling deformation, and its main task is to perform final calibration of the strip's thickness tolerance, surface finish, and flatness. After rolling to the target dimensions, the strip undergoes finished product solution annealing. This heat treatment is also carried out under a strictly controlled protective atmosphere, and its heating temperature is typically similar to or the same as that of intermediate solution annealing, for example, set at 910 ± 10 °C, to ensure the matrix structure is in an optimal solution state. However, the process parameters for finished product annealing may be specifically adjusted. For example, in actual operation, the strip's running speed in the finished product annealing furnace may be set to a higher 6 m / min or 7 m / min (compared to 3 m / min for intermediate annealing), which affects its thermal cycle period. Its core objectives include: First, to eliminate the slight work hardening introduced by the final cold rolling, enabling the product to achieve a suitable soft-state delivery hardness (typically required to be ≤130HV); second, to perform a final stabilization treatment on the microstructure of the material, ensuring the uniformity of the austenitic matrix; and third, and most importantly, to precisely control this annealing and subsequent cooling process, ultimately locking in the intrinsic physical properties of the material, especially stabilizing its average coefficient of linear expansion within a preset precision range (e.g., 25-100℃: 5.92-6.67×10⁻⁶). -6 / ℃, 25-150℃: 6.70-7.20×10 -6 The resistivity (91.0-98.6 μΩ·cm at 20℃) is simultaneously increased to meet specified requirements. This step solidifies the strip's dimensions, microstructure, and properties, resulting in a metal strip that meets design requirements and can be directly used for composite fabrication of thermal bimetallic passive layers.

[0031] The technical benefits of this preparation method are reflected in multiple aspects: at the composition and microstructure level, a high-purity Ti and Mo-rich specific composition system was obtained through vacuum melting and precise alloying, laying a material foundation for the material's high corrosion resistance; at the processing and structural control level, through optimized hot forging, hot rolling, and a unique "small-large-small" multi-pass cold rolling deformation sequence, combined with rapid solution annealing with strictly controlled parameters introduced at key nodes, effective management of material deformation texture, dislocation structure, grain size, and second-phase precipitation behavior was achieved, avoiding processing cracks and obtaining a uniform and fine ideal microstructure; at the final performance level, the above comprehensive measures ensure that the prepared passive layer strip not only has a highly stable and linearly good low coefficient of thermal expansion and a specific resistivity, meeting the core requirements of thermal bimetallic elements for driving characteristics, but more importantly, its corrosion resistance, especially its resistance to pitting and intergranular corrosion in chloride-containing environments, has been substantially improved compared to traditional composition alloys, thus significantly enhancing the long-term service reliability and service life of the thermal bimetallic elements composed of it under harsh working conditions. The entire process is interconnected, forming a complete, controllable, and reproducible high-performance passive layer thin strip fabrication technology solution.

[0032] Specifically, in step S300, the preset deformation sequence ensures that the deformation amount of the multi-pass cold rolling has an increasing and then decreasing distribution, and the deformation amount of the first pass is less than the deformation amount of the subsequent main deformation passes. Furthermore, in step S300, the multi-pass cold rolling includes five cold rolling passes, and the deformation amounts of the five passes in the preset deformation sequence are as follows: the deformation amount of the first pass is approximately 5%, the deformation amount of the second pass is approximately 18%, the deformation amount of the third pass is approximately 15%, the deformation amount of the fourth pass is approximately 12%, and the deformation amount of the fifth pass is approximately 5%.

[0033] The preset deformation sequence is specifically designed based on the processing characteristics, microstructure evolution, and final performance requirements of the specific alloys prepared by this method (Fe-based alloys with Ni: 37.4-38.2%, Cr: 7.0-7.5%, Ti: 0.2-0.4%, Mo: 1.0-2.0%). Taking the five-pass cold rolling explicitly described in the specific implementation as an example, the sequence shows an increasing and then decreasing distribution, with specific values ​​of approximately 5%, 18%, 15%, 12%, and 5% respectively. At the beginning of cold rolling, facing the intermediate steel coil obtained after hot rolling, pickling, and grinding, the small deformation of about 5% in the first pass is of crucial significance. Its main purpose is to achieve smooth and controllable bite of the strip in the rolling mill and establish a stable rolling process, avoiding bite difficulties or instantaneous load impacts caused by excessive initial deformation. Simultaneously, this small deformation amount applies a gentle pre-deformation to the strip, which helps to partially release residual stress within the billet and induces initial, uniform dislocation multiplication, preparing the microstructure and stress for subsequent, more extensive plastic deformation. In the subsequent second, third, and fourth passes, the deformation amounts increase to approximately 18%, 15%, and 12%, respectively, constituting the main stage of cold rolling thinning. In practice, for example, the thickness is rolled from approximately 5.5 mm or 5.0 mm after hot rolling to the target thickness before intermediate annealing (e.g., 82.0 mm) through these passes. These relatively large deformation amounts aim to efficiently achieve thickness reduction while simultaneously accumulating a high density of dislocations and significant deformation energy storage within the material. This specific energy storage distribution and accumulation rate are crucial for triggering a uniform and complete recrystallization process during subsequent intermediate solution annealing, directly affecting the size, morphology, and final texture strength and type of recrystallized grains. These microstructural characteristics are closely related to the material's coefficient of thermal expansion, anisotropy, and other final performance characteristics. In the fifth and final pass, the deformation amount returns to a small amount of approximately 5%. The main objective at this stage shifts from significant thinning to ultimate control over the strip's geometric accuracy and surface condition. In practice, this pass is performed after intermediate solution annealing, rolling the strip from an intermediate thickness (e.g., 82.0 mm) to the final thickness (e.g., 80.7 mm). This small deformation allows for fine-tuning of strip thickness tolerances, shape (straightness), and surface roughness, ensuring dimensional accuracy while avoiding excessive residual stress or surface damage caused by excessive final deformation. This provides a semi-finished product with ideal dimensions and condition for the final solution annealing.

[0034] Furthermore, the intermediate solution annealing is performed when the strip reaches a first preset thickness during multi-pass cold rolling, which is greater than the final target thickness of the strip. The intermediate solution annealing includes: heating the strip to a temperature range of 900°C-920°C under a protective atmosphere and holding it at that temperature, followed by rapid cooling at a cooling rate of not less than 1000°C / min.

[0035] In the process flow set in step S300, the intermediate solution annealing treatment is initiated when the multi-pass cold rolling reaches a specific "first preset thickness," which is explicitly defined as greater than the final target thickness of the strip. Specifically, taking Embodiment 1 of this application as an example, the multi-pass cold rolling first rolls the intermediate steel coil from the thickness after hot rolling (approximately 5.5 mm) to 82.0 mm. This thickness can be set as the "first preset thickness" to trigger intermediate solution annealing. After annealing, the strip is further rolled to the final target thickness of 80.7 mm. When the cold rolling deformation accumulates to a certain extent but has not yet reached the final finished product size, sufficient distortion energy is stored inside the material due to plastic deformation, the dislocation density increases significantly, and a specific deformation texture may have initially formed, with a tendency for plasticity to decrease due to work hardening. The primary purpose of annealing at this moment (e.g., at a thickness of 82.0 mm) is to utilize high-temperature heating and holding to allow the deformed structure that has undergone a certain degree of cold working to undergo complete recrystallization. This process eliminates the high-density dislocation structure and transforms the elongated and broken grains into new, equiaxed, fine recrystallized grains, thereby completely eliminating work hardening, restoring and significantly improving the material's plasticity, enabling it to withstand further cold rolling deformation without cracking.

[0036] Secondly, for the specific alloy composition of this invention (containing Cr, which readily forms carbides, and Ti, as a stabilizing element), the deformation stress during cold rolling may cause carbides to precipitate prematurely or unfavorably at grain boundaries or dislocation lines. High-temperature solution treatment (910±10℃) at the "first preset thickness" can facilitate the precipitation of these precipitated or precipitating carbides (especially chromium-rich carbides, such as Cr). 23 C6) redissolves into the austenitic matrix, and then its supersaturated state is retained to room temperature through rapid cooling at a rate of not less than 1000℃ / min. This not only optimizes the chromium distribution near the grain boundaries and reduces the formation of chromium-depleted regions at grain boundaries (directly improving resistance to intergranular corrosion), but also lays the foundation for obtaining a uniform and singular supersaturated solid solution microstructure in the final product. The selection of this thickness node requires balancing two aspects: if annealing is too early (thickness much greater than the first preset thickness), the accumulated deformation energy may be insufficient, resulting in incomplete or uneven recrystallization; if annealing is too late (thickness close to the final target thickness), the risk of cracking due to over-hardening increases dramatically, and the process window for subsequent final rolling and performance adjustment is too narrow. Therefore, setting the "first preset thickness" at an appropriate position before the final target thickness (e.g., 82.0 mm versus 80.7 mm) is the optimal balance between safe and efficient processing and high performance goals.

[0037] Furthermore, the dew point of the protective atmosphere is less than or equal to -45°C, and the oxygen content is less than or equal to 10 ppm.

[0038] In the intermediate solution annealing and finished product solution annealing processes involved in steps S300 and S400 of this invention, the strict specifications for the "protective atmosphere"—a dew point not exceeding -45°C and an oxygen content not exceeding 10 ppm—are key process control measures to prevent oxidation and ensure surface quality and compositional stability during high-temperature heat treatment of alloys with specific compositions (containing high levels of Cr and easily oxidized reactive elements such as Ti and Mo) prepared by this method. This protective atmosphere is typically established and maintained in a continuous annealing furnace or a bell-type annealing furnace. Its application scenario is to provide a highly pure and inert environment during the heating of cold-rolled alloy strip to a high-temperature single-phase region of 910±10°C for holding and subsequent rapid cooling. Dew point is a direct indicator of the water vapor content in an atmosphere. A dew point ≤ -45℃ means that the partial pressure of water vapor in the atmosphere is extremely low, equivalent to a water content of less than about 10 ppm(v). This effectively prevents water molecules from reacting with the alloy surface at high temperatures (especially above 400℃), avoiding the formation of uncontrollable oxide films or selective oxidation of alloying elements (especially Cr and Si). Such selective oxidation can deplete the surface alloying elements and may form a loose oxide layer with poor adhesion, damaging the surface finish of the strip and the subsequent composite bonding strength. At the same time, the stringent requirement of oxygen content ≤ 10 ppm reduces the concentration of free oxygen in the atmosphere to an extremely low level, fundamentally inhibiting the direct oxidation reaction of the alloy matrix metal (Fe, Ni) and its key alloying elements (such as Ti and Mo) at high temperatures. In practice, to achieve and maintain such stringent atmospheric purity, highly purified high-purity nitrogen, high-purity argon, or nitrogen-hydrogen mixtures (such as a nitrogen-based atmosphere containing 5% hydrogen) are typically used. A closed-loop atmosphere circulation system, coupled with a high-efficiency catalyst and molecular sieve adsorption device, continuously removes trace amounts of air seeping into the furnace seal, as well as water vapor and oxygen-containing gases generated from the decomposition of residual grease and other organic matter on the strip surface. During the annealing and holding stage, this low dew point and low-oxygen atmosphere ensures that the strip surface maintains its metallic color or forms only an extremely thin, dense, and controllable passivation film. In the subsequent rapid cooling stage (cooling rate ≥1000℃ / min), the same protective atmosphere serves as a cooling medium (e.g., in a gas jet rapid cooling system) or as a barrier to isolate air, preventing secondary oxidation of the strip due to exposure to oxygen in the high to medium temperature range. For example, in the embodiments, both intermediate solution annealing and finished solution annealing are performed in an atmosphere that meets these dew point and oxygen content requirements, ensuring the uniformity and cleanliness of the strip surface before and after heat treatment.

[0039] Further, in step S100, a vacuum induction melting process is used to melt nickel plates, metallic chromium, graphite carbon, industrial pure iron, molybdenum bars, metallic manganese, industrial silicon, and industrial pure titanium according to a preset ratio to obtain an alloy liquid with a preset composition, including: Step S110: Nickel plates, metallic chromium, graphite carbon, industrial pure iron, and molybdenum bars are loaded into the furnace under a vacuum level lower than the first preset vacuum value. After the vacuum level decreases to the second preset vacuum value, electric heating is applied to melt the furnace charge. Specifically, the first preset vacuum value is 5 Pa; the second preset vacuum value is 3 Pa.

[0040] The vacuum loading and initial melting stage is the starting point for alloy preparation using this method. It establishes an ultra-high purity initial environment and a controllable melting process for subsequent smelting, and is carried out within the melting chamber of a vacuum induction furnace. During operation, the main furnace charge is first precisely weighed according to the alloy composition design: including nickel plates (serving as the matrix and main alloying elements), industrial pure iron, metallic chromium, and molybdenum bars, as well as graphite carbon for precise carbon content control. The process of loading these raw materials into the furnace feeder or placing them directly into the crucible must be completed under a vacuum environment with an absolute pressure below 5 Pa within the furnace. This vacuum requirement aims to maximize the removal of residual air (mainly N2 and O2) from the furnace, thereby greatly reducing the possibility of oxidation due to contact with oxygen before the raw materials begin to be heated. This is a fundamental prerequisite for obtaining a low-gas-content, high-purity alloy liquid. After loading is complete, the vacuum system is activated again to further reduce the furnace pressure to below 3 Pa before power is supplied to the induction coil to begin the heating process. The sequence of "evacuating to a high vacuum before powering on," along with the more stringent vacuum threshold of 3 Pa, is crucial. This ensures that the furnace charge remains in an environment with extremely low oxygen and nitrogen partial pressures throughout the entire process, from initial heating and softening to complete melting. Heating and melting under this high vacuum not only effectively prevents the oxidation and burning loss of elements such as Ni, Cr, and Fe, ensuring precise control of the alloy's main components, but also promotes the removal of volatile impurities such as moisture and oil adsorbed on the surface of the raw materials under high temperature and vacuum, which are then discharged from the furnace through the vacuum system. In specific embodiments, such as embodiments one and two, the vacuum levels during charging are recorded as 3.2 Pa and 2.3 Pa, respectively, both meeting the initial charging requirement of less than 5 Pa and the power-on heating requirement of less than 3 Pa, ensuring the strict implementation of the process. This step, through the strict stepwise control of the vacuum environment (<5 Pa during charging, <3 Pa during heating), lays an irreplaceable environmental foundation for obtaining initial molten steel with accurate composition and extremely low gas and impurity content.

[0041] Step S120: After the furnace charge is completely melted, the temperature of the molten steel is raised to the first preset temperature and the refining period begins. The refining time is greater than or equal to the first preset duration. Specifically, the first preset temperature is 1510℃-1530℃; the first preset duration is 40 minutes.

[0042] The refining stage is a deep purification and preliminary homogenization process carried out after the furnace charge has completely melted and formed molten steel. Its application occurs in the middle of the vacuum induction melting process. After the electric heating converts all solid furnace charge into a liquid state, forming an initial molten steel with an uneven composition and potentially containing a significant amount of gas and inclusions, the molten steel needs to be heated and refined. Specifically, after the furnace charge has completely melted and the surface of the molten steel is observed to be calm and free of solids, the input power of the induction furnace is further increased to precisely raise and stabilize the temperature of the molten steel within the range of 1510°C to 1530°C (for example, controlled at 1520°C and 1530°C respectively in this embodiment), and this refining temperature is maintained at this high temperature for at least 40 minutes. The refining temperature was set in this relatively high range (1510-1530℃) primarily based on two considerations: First, higher temperatures significantly reduce the viscosity of molten steel and improve its fluidity, which is highly beneficial for convection generated by electromagnetic stirring within the molten pool, thereby accelerating the diffusion and initial homogenization of chemical components. Second, high temperatures increase the solubility gradient of gases (especially hydrogen [H] and nitrogen [N]) in molten steel and enhance the kinetics of the carbon-oxygen reaction, facilitating the formation and escape of dissolved gases through recombination and precipitation into bubbles. Simultaneously, it promotes the aggregation, flotation, and absorption of some non-metallic inclusions by the slag. Maintaining a refining time of 40 minutes provides sufficient time for the aforementioned physicochemical reactions. During this period, the molten steel undergoes intense electromagnetic stirring under vacuum or low pressure (possibly maintaining a certain degree of vacuum or introducing a small amount of inert gas according to the process), allowing for thorough degassing, inclusion removal, and macroscopic and microscopic homogenization of alloying elements (especially the added Ni, Cr, Mo, Fe, and C). This refining step is crucial for obtaining clean and homogeneous molten steel, directly affecting the internal quality of the ingot and the levels of macroscopic segregation, inclusions, and gas content that may exist in subsequent processed materials. Therefore, step S120 systematically achieves deep purification and preliminary compositional homogenization of the molten steel by strictly controlling the two core parameters of refining temperature and time, preparing for subsequent precise alloying.

[0043] Step S130: After refining, metallic manganese, industrial silicon, and industrial pure titanium are added to the molten steel under a protective atmosphere to obtain an alloy liquid. Specifically, the protective atmosphere includes argon gas, and the pressure of the protective atmosphere is 8000 Pa.

[0044] The alloying stage is a crucial step after refining, where easily oxidizable alloying elements are precisely added to the molten steel, which has achieved a high degree of purity and homogeneity. Its application occurs in the later stages of vacuum melting and before casting. At this point, after refining in step S120, the main components of the molten steel (Fe, Ni, Cr, Mo, C) are relatively homogeneous, and the content of gases and inclusions has been significantly reduced. To add reactive elements such as metallic manganese (Mn), industrial silicon (Si), and industrial pure titanium (Ti), which readily react with oxygen and nitrogen, a reliable inert gas protective environment must be created to prevent their burn-off. Specifically, after the refining stage, a high-purity inert gas, such as argon (Ar), is first introduced into the furnace, which has been evacuated to a high vacuum, until the furnace pressure reaches and stabilizes at approximately 8000 Pa (about 0.08 atmospheres). The purpose of establishing this specific pressure protective atmosphere is twofold: firstly, the pressure is sufficient to effectively prevent outside air from seeping in through any possible tiny leaks in the furnace, providing a stable inert environment for the alloying operation; secondly, this pressure value is optimized to provide sufficient atmosphere protection without being too high, which could lead to difficulties in controlling the molten steel flow or excessive splashing during subsequent pouring. After the atmosphere pressure stabilizes, predetermined amounts of metallic manganese, industrial silicon, and industrial pure titanium are sequentially added to the molten steel through a specialized feeding system. Due to their strong chemical reactivity, these elements would oxidize violently and produce a large number of oxide inclusions if added directly in a vacuum or air environment, severely affecting the accuracy of the alloy composition and the cleanliness of the molten steel. However, adding them under an 8000 Pa argon protective atmosphere can maximally suppress their oxidation reaction. After addition, these added elements are rapidly melted and evenly diffused using electromagnetic stirring or manual stirring. Afterwards, samples are typically taken for rapid compositional analysis to ensure that the content of all elements falls within the preset "target alloy composition" range (e.g., Ni: 37.4-38.2%, Cr: 7.0-7.5%, Ti: 0.2-0.4%, etc.). Once the composition is confirmed to be qualified, a final alloy liquid with precise composition and high purity is obtained, which can be immediately used for the next casting step. This step, through precise control of the protective atmosphere type (argon) and pressure (8000 Pa), successfully solves the problem of efficiently and with low burn-off of active alloying elements, ensuring the precise control of key components such as Ti, Mn, and Si. This is the material guarantee for achieving specific material properties (such as Ti improving corrosion resistance, and Si and Mn affecting deoxidation and mechanical properties).

[0045] Optionally, in step S200, the heating regime for forging is as follows: first, preheat at 840℃-860℃ and hold, then raise the temperature to 1190℃-1210℃ and hold before forging begins, with the initial forging temperature not lower than 1150℃ and the final forging temperature not lower than 850℃.

[0046] The forging heating process is a specific heat treatment procedure implemented before the alloy ingot obtained after vacuum melting undergoes billet deformation. Its application involves heating the ingot to a state suitable for plastic deformation in a forging furnace (such as a bogie-type gas furnace or electric furnace). This process is clearly divided into two key stages: First, the ingot at room temperature is placed into the furnace and heated to a temperature range of 840°C to 860°C at a controlled heating rate (i.e., the preheating stage), and held at this temperature for a sufficient time (e.g., 2 hours as described in the example) for thorough heat preservation. The purpose of this preheating stage is to achieve slow and uniform heat penetration for ingots with large cross-sectional dimensions, avoiding the formation of internal cracks due to excessive thermal stress caused by a large temperature difference between the surface and the core. It also helps to partially eliminate any residual casting stress that may exist in the ingot. After the ingot has been homogenized at the preheating temperature, it is further heated to a higher temperature range of 1190°C to 1210°C at a set heating rate (i.e., the high-temperature homogenization stage), and held at this temperature again (e.g., 2 hours). The purpose of this high-temperature stage is to completely transform the alloy microstructure into a homogeneous single-phase austenite, and to allow alloying elements (especially Ni, Cr, Mo, etc.) to fully diffuse within the austenite, achieving microstructure homogenization and softening. This significantly reduces the material's deformation resistance, improves its high-temperature plasticity, and prepares the microstructure and properties for subsequent forging processes that withstand large deformations. In specific forging operations, the initial forging temperature after removing the ingot from the furnace, i.e., the initial forging temperature, must not be lower than 1150℃. This is to ensure that the material remains above its recrystallization temperature throughout the main forging deformation process, allowing the work hardening generated during deformation to be softened in time through dynamic recrystallization, thus maintaining excellent plasticity and preventing cracking. Simultaneously, the final forging temperature must not be lower than 850℃. This restriction aims to prevent deformation of the material at excessively low temperatures, as low temperatures cause a sharp increase in deformation resistance and a decrease in plasticity, easily leading to forging cracks and potentially retaining unrecrystallized work hardened structures or unfavorable textures in subsequent processes. In the examples, the final forging temperatures were recorded as 860°C and 855°C, both of which met the requirements. The entire heating process, through precise control of the preheating temperature, high-temperature homogenization temperature, and forging temperature window, effectively avoided casting heating defects (such as overheating and burning) while providing the forging process with an ideal billet with high plasticity and low deformation resistance.

[0047] Optionally, the heating regime for hot rolling is as follows: first, preheat at 840℃-860℃ and hold at that temperature, then raise the temperature to 1170℃-1190℃ and hold at that temperature before starting rolling, with the initial rolling temperature not lower than 1150℃ and the final rolling temperature not lower than 850℃.

[0048] The hot rolling heating regime is suitable for heating forged and surface-ground billets to a state suitable for hot rolling. Its application is typically achieved in continuous or walking beam slab heating furnaces. This regime is conceptually similar to the forging heating regime, but the parameters are specifically adjusted according to the characteristics of rolling deformation. First, the billet is heated to a preheating temperature of 840°C to 860°C and held at that temperature (e.g., for 2 hours). The purpose is also to achieve uniform heat penetration, eliminate internal stresses that may be generated due to post-forging cooling or transportation, and prevent thermal shock caused by subsequent rapid heating. After preheating and homogenization, the billet is further heated to a temperature range of 1170°C to 1190°C and held for a period of time (e.g., for 1 hour). This temperature range is slightly lower than the high-temperature homogenization range of forging and is optimized by comprehensively considering the recrystallization temperature of the alloy, deformation resistance, and preventing excessive grain growth and melting of certain low-melting-point phases (such as sulfides). The aim is to obtain a uniform austenitic structure and good thermoplasticity in the billet. In hot rolling operations, the initial rolling temperature (the first pass) must not be lower than 1150°C to ensure that rolling deformation occurs above the material's full recrystallization temperature. This allows for sufficient dynamic recrystallization during deformation, resulting in fine and uniform recrystallized grains, which is crucial for the microstructure and properties of the final product. Simultaneously, the final rolling temperature (the last pass) must not be lower than 850°C. Controlling the final rolling temperature above this value serves two purposes: firstly, it ensures the material retains sufficient plasticity at the end of rolling, preventing cracking or severe edge defects due to excessively low temperatures and a surge in deformation resistance in the final stage; secondly, maintaining a higher final rolling temperature range (e.g., 855°C and 861°C as recorded in the examples) facilitates the formation of uniform and fine austenite grains after rolling and provides an ideal phase transformation initiation microstructure for subsequent laminar water forced cooling, effectively suppressing the unfavorable precipitation of second phases such as carbides during slow cooling. Therefore, this hot rolling heating system provides crucial thermal process assurance for the stable and efficient acquisition of high-quality hot-rolled coils through precise regulations on preheating, homogenization, and rolling temperature ranges.

[0049] Furthermore, the preset composition by mass percentage is as follows: Ni: 37.4%-38.2%, Cr: 7.0%-7.5%, C≤0.08%, Si≤0.3%, S≤0.020%, P≤0.020%, Ti: 0.2%-0.4%, Mo: 1.0%-2.0%, with the balance being Fe and unavoidable impurities.

[0050] The Ni content is precisely controlled within a narrow range of 37.4-38.2% to ensure that the alloy has an extremely low and linear coefficient of thermal expansion within the target operating temperature range (e.g., from room temperature to approximately 150°C). This Ni content range corresponds to the vicinity of the optimal Invar effect region for Fe-Ni alloys, where the magnetostriction and lattice thermal expansion of the alloy cancel each other out, resulting in significantly low expansion characteristics. This is the physical basis for the passive layer to work synergistically with the high-expansion active layer to achieve precise thermal bending. The Cr content is set at 7.0-7.5%. Firstly, Cr dissolved in the matrix effectively increases the electrode potential of the alloy, enhancing its basic electrochemical corrosion resistance. Secondly, it forms an extremely thin but dense Cr₂O₃ oxide film (passivation film) on the alloy surface, providing a basic protective barrier. Furthermore, Cr forms a solid solution with Fe and Ni, which has a certain solid solution strengthening effect on the matrix and fine-tunes the physical properties of the alloy. The C content is strictly limited to ≤0.08% (preferably even lower, such as 0.02% or 0.016% in the examples) in order to minimize the formation of carbides, especially Cr-rich carbides (such as Cr...). 23 C6), because the precipitation of these carbides at grain boundaries consumes Cr in the matrix, leading to "chromium depletion" in the grain boundary region, becoming a weak point in resistance to intergranular corrosion. The addition of Ti (0.2-0.4%) is one of the key design features of this invention for corrosion resistance. Ti has a strong affinity for C and N, preferentially forming stable compounds such as TiC and TiN. During the specific smelting and heat treatment processes, this "stabilizing" effect of Ti effectively "fixes" C and N atoms in the molten steel, thereby greatly reducing the formation of harmful Cr by combining with C. 23 The addition of C6 inhibits the formation of chromium-depleted regions at grain boundaries from the source, significantly improving the alloy's resistance to intergranular corrosion. The addition of Mo (1.0-2.0%) is another key innovation, its main function being to significantly improve the alloy's resistance to chloride ion (Cl-) formation. - The addition of Mo enhances the resistance to localized corrosion (such as pitting and crevice corrosion) in corrosive media. It also improves the stability and repair ability of the surface passivation film in corrosive environments, making the passivation film denser and less susceptible to corrosion by Cl. -This allows for penetration, effectively delaying the initiation and spread of corrosion. Si and Mn (although not listed separately in the final composition, they are added during smelting as deoxidizers and alloying elements, with a residual Si content ≤ 0.3%) primarily function as deoxidizers and beneficial alloying agents, contributing to clean molten steel and some solid solution strengthening. The content of S and P, two common harmful impurity elements, is strictly limited to ≤ 0.020% to prevent the segregation at grain boundaries to form low-melting-point brittle sulfide or phosphide phases. These brittle phases not only deteriorate hot working properties (increasing hot brittleness) but also impair the material's toughness, fatigue performance, and corrosion resistance. Therefore, this composition system is a comprehensive design based on Fe-Ni-Cr, ensuring core thermal expansion performance through precise control of the main alloying elements (Ni, Cr), and innovatively synergistically adding Ti and Mo to specifically address corrosion resistance shortcomings, while strictly limiting harmful impurities to ensure processing and service reliability.

[0051] The technical solution of the present invention will be further described below with reference to two embodiments: Example 1 A method for preparing a metal strip for a passive layer of a thermal bimetallic composite strip: (mass fraction %): Ni: 37.8, Cr: 7.2, C: 0.02, Si: 0.2, S≤0.010, P≤0.010, Ti: 0.23, Mo: 1.5, balance Fe.

[0052] 1) Prepare the furnace charge: nickel plate, metallic chromium, metallic manganese, industrial silicon, graphite carbon, molybdenum bars, and industrial pure iron. Nickel plate, metallic chromium, graphite carbon, molybdenum bars, and industrial pure iron are added during furnace loading, with a vacuum degree of 3.2 Pa. Metallic manganese, industrial silicon, and industrial pure titanium are added during alloying under a protective atmosphere.

[0053] 2) During the alloy preheating period, power is supplied when the vacuum degree is 5.5pa. First, a low power of 160kw is used for preheating, and then a medium-low power of 420kw is used for material melting. After the furnace charge is melted and cleared, the temperature is increased. When the temperature of the molten steel reaches 1520℃, the refining period begins and is maintained for 40 minutes.

[0054] 3) After the refining period, the alloying period begins. After purging the argon gas channel at 8000 Pa, industrial silicon, metallic manganese, and industrial pure titanium are added in sequence. After melting and cleaning, the mixture is stirred. After taking a sample from the molten steel, the composition is found to be qualified.

[0055] 4) When the temperature of the molten steel reaches 1510℃, it is poured under an argon protective atmosphere.

[0056] 5) Forge the steel ingot obtained in step 4. The forging specifications are δ120×400×5000. The forging heating regime is 850±10℃ in the preheating section and hold for 2 hours, then raise the temperature to 1200±10℃ and hold for another 2 hours. The initial forging temperature is ≥1150℃ and the final forging temperature is 860℃.

[0057] 6) Grind the surface of the forging billet obtained in step 5 to remove defects such as oxide scale, cracks, and pits.

[0058] 7) The billet obtained in step 6 is hot rolled with a hot rolling size of δ5.5×400. The hot rolling heating regime is as follows: preheating section 850±10℃ and holding for 2h, heating to 1180±10℃ and holding for 1h, starting rolling temperature 1165℃, finishing rolling temperature 855℃, and then turning on the laminar flow water for forced cooling.

[0059] 8) Pickle the hot-rolled coil obtained in step 7 for 1 hour.

[0060] 9) Grind the steel coils after pickling in step 8 to remove surface oxide scale, pits and other defects.

[0061] 10) The steel coil obtained in step 9 is cold rolled to a thickness of δ2.0 mm, then subjected to intermediate solution annealing at a holding temperature of 910℃ and a speed of 3 m / min, followed by rapid cooling. After that, it is rolled to δ0.7 mm and subjected to finished product solution annealing at a holding temperature of 910℃ and a speed of 6 m / min, followed by rapid cooling. The expansion coefficient of the strip is as follows: average linear expansion coefficient (10-6 / ℃): 25-100℃: 6.4, 25-150℃: 7.0; resistivity at a test temperature of 20℃: 93 μΩ·cm; hardness: 113 HV; the corrosion resistance of the alloy is 50% higher than that of the alloy without Ti and Mo.

[0062] Example 2 A method for preparing a metal strip for a passive layer of a thermal bimetallic composite strip: (mass fraction %): Ni: 37.8, Cr: 7.4, C: 0.016, Si: 0.22, S≤0.010, P≤0.010, Ti: 0.25, Mo: 1.65, balance Fe.

[0063] 1) Prepare the furnace charge: nickel plate, metallic chromium, metallic manganese, industrial silicon, graphite carbon, molybdenum bars, and industrial pure iron. Nickel plate, metallic chromium, graphite carbon, molybdenum bars, and industrial pure iron are added during furnace loading, with a vacuum degree of 2.3 Pa. Metallic manganese, industrial silicon, and industrial pure titanium are added during alloying under a protective atmosphere.

[0064] 2) During the alloy preheating period, power is supplied when the vacuum degree is 5.6 Pa. First, a low power of 160 kW is used for preheating, and then a medium-low power of 430 kW is used for material melting. After the furnace charge is melted and cleared, the temperature is increased. When the temperature of the molten steel reaches 1530℃, the refining period begins and is maintained for 40 minutes.

[0065] 3) After the refining period, the alloying period begins. After argon gas is purged to 8000 Pa, industrial silicon, metallic manganese, and industrial pure titanium are added in sequence. After melting and cleaning, the mixture is stirred. Samples are taken from the molten steel and the composition is found to be qualified.

[0066] 4) When the temperature of the molten steel reaches 1509℃, it is poured under an argon protective atmosphere.

[0067] 5) Forge the steel ingot obtained in step 4. The forging specifications are δ110×630×5500. The forging heating regime is 850±10℃ in the preheating section and held for 2 hours, then the temperature is raised to 1200±10℃ and held for another 2 hours. The initial forging temperature is 1153℃ and the final forging temperature is 855℃.

[0068] 6) Grind the surface of the forging billet obtained in step 5 to remove defects such as oxide scale, cracks, and pits.

[0069] 7) The billet obtained in step 6 is hot rolled to a size of δ5.0×650. The hot rolling heating regime is as follows: preheating section 850±10℃ and holding for 2h, heating to 1180±10℃ and holding for 1h, starting rolling temperature 1160℃, finishing rolling temperature 861℃, and then turning on the laminar flow water for forced cooling.

[0070] 8) Pickle the hot-rolled coil obtained in step 7 for 1 hour.

[0071] 9) Grind the steel coils after pickling in step 8 to remove surface oxide scale, pits and other defects.

[0072] 10) The steel coil obtained in step 9 is cold rolled to a thickness of δ2.0 mm, then subjected to intermediate solution annealing at a temperature of 910℃ and a speed of 3 m / min, followed by rapid cooling. After that, it is rolled to δ0.5 mm and subjected to finished product solution annealing at a temperature of 910℃ and a speed of 7 m / min, followed by rapid cooling. The expansion coefficient of the strip is as follows: average linear expansion coefficient (10-6 / ℃): 25-100℃: 6.3, 25-150℃: 7.10; resistivity at a test temperature of 20℃: 95 μΩ·cm; hardness: 118 HV. The corrosion resistance of the alloy is more than 50% higher than that of the alloy without Ti and Mo.

[0073] Accordingly, a second aspect of the present invention provides a thermal bimetallic composite strip, which includes a high expansion layer and a passive layer, wherein the passive layer is a metal strip prepared by the above-described thermal bimetallic strip preparation method.

[0074] The embodiments of this invention aim to protect a method for preparing metal strips and a thermal bimetallic composite strip, which have the following effects: 1. By precisely controlling the alloy composition, especially the synergistic addition of 0.2-0.4% Ti and 1.0-2.0% Mo, not only are the stable low-expansion characteristics required for the iron-nickel-chromium alloy as a passive layer maintained, but the corrosion resistance of the material is also significantly improved; the Ti element effectively fixes carbon and nitrogen atoms and inhibits the corrosion of Cr. 23 Harmful carbides such as C6 precipitate at grain boundaries, preventing the formation of chromium-depleted regions at grain boundaries; the addition of Mo enhances the stability and density of the surface passivation film in corrosive media containing chloride ions; the synergistic effect of the two significantly enhances the material's resistance to pitting corrosion, crevice corrosion, and intergranular corrosion when it is in long-term high-temperature, high-humidity, or harsh environments, thus solving the technical problem that the insufficient corrosion resistance of traditional passive layer alloys affects the service life and reliability of components. 2. By adopting a vacuum induction melting process and optimizing its key parameters (such as high vacuum charging, refining at 1520±10℃, and alloying under specific pressure), precise control over the purity and compositional uniformity of the alloy melt was achieved. This process effectively reduced gas content and harmful impurities, ensuring the accuracy and stability of the alloy composition, especially the content of key elements nickel and chromium. It fundamentally guarantees the precise controllability and excellent consistency of the core physical properties of the final product, namely the coefficient of linear expansion and resistivity. It solves the problem of batch-to-batch instability in material properties caused by metallurgical fluctuations, enabling the prepared strips to meet the stringent requirements of precision temperature control elements for highly stable and predictable thermo-electrical performance parameters. 3. By designing and implementing a complete collaborative process chain of "hot working-cold rolling-heat treatment," including optimized forging / hot rolling temperature regimes, specific multi-pass cold rolling deformation sequences (such as 5%, 18%, 15%, 12%, 5%), and rapid cooling solution annealing introduced at critical thickness nodes, precise control of the material's microstructure (such as grain size, texture, and dislocation density) was achieved. This effectively avoided the risk of cracking during processing, promoted the formation of favorable microstructures, and suppressed the precipitation of brittle and harmful phases. It not only ensured a high yield under complex process flows but also enabled the material to achieve a uniform and refined microstructure, thereby comprehensively improving the dimensional accuracy, surface quality, mechanical properties, and overall service performance of the strip as a passive layer element.

[0075] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a metal strip, characterized in that, Includes the following steps: Step S100: Using a vacuum induction melting process, nickel, chromium, graphite, iron, molybdenum, manganese, silicon and titanium in a preset proportion are melted to obtain an alloy liquid with a preset composition, and then cast into an alloy ingot under a protective atmosphere. Step S200: The alloy ingot is subjected to forging, hot rolling, pickling and surface grinding in sequence to obtain intermediate steel coil; Step S300: The intermediate steel coil is subjected to multiple cold rolling passes to obtain strip. The multiple cold rolling passes are performed using a preset deformation sequence, and at least one intermediate solution annealing treatment is performed during the multiple cold rolling passes. Step S400: Perform final cold rolling and solution annealing on the strip to obtain a metal strip for the passive layer of the hot bimetallic composite strip.

2. The method for preparing metal strips according to claim 1, characterized in that, In step S300, the preset deformation sequence causes the multi-pass cold rolling to have an increasing and then decreasing deformation distribution, and the deformation of the first pass is less than the deformation of the subsequent main deformation passes.

3. The method for preparing metal strips according to claim 2, characterized in that, In step S300, the multi-pass cold rolling includes five cold rolling passes, and the deformation amounts of the five passes in the preset deformation amount sequence are as follows: the deformation amount of the first pass is about 5%, the deformation amount of the second pass is about 18%, the deformation amount of the third pass is about 15%, the deformation amount of the fourth pass is about 12%, and the deformation amount of the fifth pass is about 5%.

4. The method for preparing metal strips according to claim 3, characterized in that, The intermediate solution annealing process is performed when the multi-pass cold rolling reaches a first preset thickness, which is greater than the final target thickness of the strip.

5. The method for preparing metal strips according to claim 4, characterized in that, The intermediate solution annealing process includes: heating the strip to a temperature range of 900℃-920℃ under a protective atmosphere and holding it at that temperature, followed by rapid cooling at a cooling rate of not less than 1000℃ / min; The protective atmosphere has a dew point of less than or equal to -45°C and an oxygen content of less than or equal to 10 ppm.

6. The method for preparing metal strips according to claim 1, characterized in that, The process employs a vacuum induction melting technique, melting nickel plates, metallic chromium, graphite carbon, industrial pure iron, molybdenum bars, metallic manganese, industrial silicon, and industrial pure titanium according to a preset ratio to obtain an alloy liquid with a preset composition, including: Nickel plates, metallic chromium, graphite carbon, industrial pure iron, and molybdenum bars are loaded into the furnace under an environment where the vacuum degree is lower than the first preset vacuum degree value. After the vacuum degree is reduced to the second preset vacuum degree value, electricity is supplied for heating to melt the furnace charge. After the furnace charge is melted and cleared, the temperature of the molten steel is raised to the first preset temperature and the refining period begins, with the refining time being greater than or equal to the first preset duration. After refining, metallic manganese, industrial silicon, and industrial pure titanium are added to the molten steel under the protective atmosphere to obtain the alloy liquid.

7. The method for preparing metal strips according to claim 6, characterized in that, The first preset vacuum level is 5 Pa; The second preset vacuum level is 3 Pa; The first preset temperature is 1510℃-1530℃; The first preset duration is 40 minutes; The protective atmosphere comprises argon gas, and the pressure value of the protective atmosphere is 8000 Pa.

8. The method for preparing metal strips according to claim 7, characterized in that, In step S200, the heating regime for forging is as follows: first, preheating and holding at 840℃-860℃, then raising the temperature to 1190℃-1210℃ and holding before forging begins, with the initial forging temperature not lower than 1150℃ and the final forging temperature not lower than 850℃. And / or, The heating process for hot rolling is as follows: first, preheating and holding at 840℃-860℃, then raising the temperature to 1170℃-1190℃ and holding before starting rolling, with the initial rolling temperature not lower than 1150℃ and the final rolling temperature not lower than 850℃.

9. The method for preparing metal strips according to any one of claims 1-8, characterized in that, The preset components, by mass percentage, are: Ni: 37.4%-38.2%, Cr: 7.0%-7.5%, C≤0.08%, Si≤0.3%, S≤0.020%, P≤0.020%, Ti: 0.2%-0.4%, Mo: 1.0%-2.0%, with the balance being Fe and unavoidable impurities.

10. A thermal bimetallic composite strip, characterized in that, The thermal bimetallic composite strip includes a high expansion layer and a passive layer, wherein the passive layer is a metal strip prepared by any one of the metal strip preparation methods described in claims 1-9.

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