Stainless steel for stress-annealing-free and high-efficiency brazing processing and preparation method thereof

Abstract

The invention belongs to the technical field of stainless steel, and particularly relates to stainless steel free of stress annealing and used for high-efficiency brazing machining and a preparation method thereof.The stainless steel comprises, by mass, 0.010%-0.035% of C, 0.01%-0.5% of Si, 0.1%-1.2% of Mn and Plt; 0.04%, Slt; the high-strength steel comprises the following components in percentage by weight: 0.03% of Cr, 17.2-20.0% of Cr, 7.5-10.0% of Ni, 1.0-3.5% of Cu, 0.010-0.045% of N, 0.035% < C + N < = 0.065%, 0.01-0.4% of Mo, one or more of B, V, Ti, Nb, Zr and rare earth element RE, and the balance of Fe and inevitable impurities, and the Cr + Mo is more than or equal to 17.8% and less than or equal to 3.3% and less than or equal to 20%. According to the stainless steel, stress annealing can be omitted, and the brazing machining efficiency can be improved.

Description

Technical Field

[0001] This invention belongs to the field of stainless steel technology, specifically relating to a stainless steel for stress-free annealing and high-efficiency brazing, and its preparation method. Background Technology

[0002] With copper prices remaining high, production costs in the air conditioning, refrigerator, and other refrigeration industries have increased. At the same time, copper pipes are prone to corrosion in many service environments, leading to leaks. Austenitic stainless steel, on the other hand, has excellent corrosion resistance and is relatively inexpensive. Replacing copper with stainless steel in the refrigerant transmission pipes of heat exchangers would significantly reduce production costs in the refrigeration industry.

[0003] Austenitic stainless steel SUS304 or SUS304L is widely used in civil and industrial production due to its excellent formability, corrosion resistance, and low life-cycle cost. It can be joined using conventional welding processes and is also suitable for various conventional processing methods such as stamping, bending, and spinning. However, in certain applications, such as the air conditioning heat exchange industry, the processing requirements for heat exchange materials are higher, and welding of dissimilar metals, such as stainless steel and copper, is involved. Brazing is often used to connect these metals, requiring stainless steel to have brazing filler metal spreadability comparable to copper to facilitate rapid brazing between stainless steel and copper or between stainless steel components. SUS304 or SUS304L stainless steel does not meet the high brazing spreadability requirements. When used as connecting pipes in heat exchange systems, it is necessary to first braze stainless steel and copper pipes in a tunnel furnace using a copper-phosphorus brazing filler metal with a high silver content before brazing the stainless steel and copper sleeve. In the subsequent air conditioning component assembly stage, the copper pipes at both ends of the two stainless steel fittings are then brazed together, ultimately achieving a connection where steel replaces copper pipes. Tunnel furnace brazing is also performed concurrently with the stress-relief annealing process of SUS304 or SUS304L stainless steel forming. This process partially achieves the substitution of copper with stainless steel, but the brazing process is time-consuming, requiring tunnel furnace brazing to complete the connection between the stainless steel and the copper sleeve, which greatly reduces production efficiency and diminishes the cost reduction effect of stainless steel replacing copper.

[0004] Tunnel furnace brazing can simultaneously achieve stress-relief annealing of stainless steel pipe fittings, reducing stress corrosion cracking during air conditioning use and improving the installation accuracy of the fittings. However, if tunnel furnace brazing uses an ammonia decomposition hydrogen reduction annealing protection mode, the high-temperature heating (1000-1080℃) inevitably leads to nitriding on the stainless steel surface, especially the Cr2N precipitates formed by grain boundary nitriding, which can also cause local chromium depletion in the stainless steel, affecting its corrosion resistance. If tunnel furnace welding uses full hydrogen protection, the cost of prolonged brazing also increases significantly. SUS304 or SUS304L stainless steel has poor brazing spreadability and can only be brazed in tunnel furnaces using special brazing filler metals such as tin bronze or high-silver-content brazing filler metals. The welding process needs to last 80-100 minutes to ensure the weld seal and meet the pressure leak test requirements. Tunnel furnace brazing results in low welding efficiency, high carbon emissions, and the risk of subsequent intergranular sensitization corrosion failure. Therefore, it is necessary to find a new type of stainless steel material that can achieve brazing wettability comparable to that of copper, while also reducing the deformation stress generated during the processing of stainless steel pipes, such as bending, without the need for stress-relief annealing to eliminate deformation stress, thus achieving rapid brazing processing that integrates high efficiency, low cost, and environmental friendliness.

[0005] Currently, some manufacturers have begun designing and developing soft stainless steels, such as SUS304ES developed in Japan and low-strength stainless steel developed by Baosteel Desheng. These materials, through the addition of nickel and copper, exhibit typical low strength and low hardness characteristics. Because these new materials employ ultra-low carbon and nitrogen design, with carbon content as low as 0.001%-0.005%, their production process requires an electric furnace + AOD + VOD production mode. Compared to the conventional SUS304 or SUS304L stainless steel production mode, this adds a VOD smelting process, leading to increased costs, longer production cycles, higher carbon emissions, and higher manufacturing costs.

[0006] The invention with application number 202410284959.6 (hereinafter referred to as Patent 1) discloses an ultra-soft austenitic stainless steel and its preparation method. The mass percentage of each component in the ultra-soft austenitic stainless steel is as follows: C 0.005%-0.02%, N 0.05%-0.2%, Si 0.01%-0.1%, Mn 1.5%-6.5%, Cr 15.5%-19.6%, Ni 7.5%-13.5%, Cu 1.8%-5.3%, with the balance being iron.

[0007] The invention with application number 202410284786.8 (hereinafter referred to as Patent 2) discloses a corrosion-resistant ultra-soft stainless steel for low-temperature environments and its preparation method. The mass percentage of each component in the corrosion-resistant ultra-soft stainless steel is as follows: C 0.005%-0.02%, N 0.05%-0.2%, Si 0.01%-0.1%, Mn 1.5%-6.5%, Cr 15.5%-19.6%, Ni 7.5%-13.5%, Mo 0.5%-3.0%, Cu 1.8%-5.3%, with the balance being iron.

[0008] Patent application No. 202510658849.6 (hereinafter referred to as Patent 3) discloses a stainless steel with low strength, low hardness, no phase transformation during deformation, and high corrosion resistance. The stainless steel's mass percentage composition is: C: 0.001-0.02%, Si: 0.01-0.5%, Mn: 0.01-1.5%, P<0.04%, S<0.01%, Cr: 17.0-20.0%, Ni: 9.0-14.0%, Cu: 2.5-4.5%, N≤0.025%, C+N≤0.035%, Mo: 0.01-0.3%, Al: 0.005-0.05%, Sn: 0.01-0.30%, with the balance being Fe and unavoidable impurities. Compared with Patents 1 and 2, Patent 3 further reduces the lower limit of carbon content, with C+N≤0.035%.

[0009] The ultra-low carbon design routes in the three patents mentioned above require a three-step ultra-pure production process, using vacuum smelting to reduce the content of the dissolved element C. However, vacuum smelting generates more carbon emissions, resulting in a longer production cycle and higher costs. Furthermore, the addition of low-melting-point metals such as tin in Patent 3 can easily lead to the segregation of low-melting-point metals during ingot casting or continuous casting, weakening grain boundary strength during heating and even causing hot-rolled edge cracking defects. Compared to the conventional production methods for SUS304 or SUS304L stainless steel, while achieving lower strength, these patents also introduce problems such as extended smelting cycles, segregation of low-melting-point alloy components, increased emissions, and increased costs. Summary of the Invention

[0010] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a stainless steel for stress-free annealing and high-efficiency brazing, as well as its preparation method. The stainless steel prepared by this invention possesses a combination of characteristics including high corrosion resistance, high brazing spreadability and wettability, low deformation hardening properties, low carbon emission production design, and the ability to achieve rapid brazing.

[0011] To achieve the above technical objectives, the technical solution adopted in the embodiments of the present invention is as follows: In a first aspect, embodiments of the present invention provide a stainless steel for stress-free annealing and high-efficiency brazing, comprising the following components by mass percentage: C 0.010%-0.035%, Si 0.01%-0.5%, Mn 0.1%-1.2%, P<0.04%, S<0.03%, Cr 17.2%-20.0%, Ni 7.5%-10.0%, Cu 1.0%-3.5%, N: 0.010%-0.045%, 0.035%<C+N≤0.065%, Mo 0.01%-0.4%, and satisfying 17.8%≤Cr+3.3Mo≤20%, and also including one or more of microalloying elements B, V, Ti, Nb, Zr and rare earth elements RE, with the balance being Fe and unavoidable impurities.

[0012] To refine grain size, strengthen grain boundary strength, and avoid the infiltration corrosion problem caused by copper alloying, the alloy design simultaneously adds one or more of the microalloying elements B, V, Ti, Nb, Zr, and RE (rare earth elements).

[0013] Furthermore, to ensure low phase transformation hardening behavior under high forming conditions and reduce processing stress to eliminate the need for conventional tunnel furnace stress-relief annealing after deformation of SUS304 or SUS304L austenitic stainless steel, the MD of the stainless steel is 551-462*(W C +W N )-8.2*W Si -8.5*W Mn -30*(W Ni +W Cu )-14.0*W Cr -18.0*W Mo ≤-60, where W C W N W Si W Mn W Ni W Cu W Cr and W Mo This indicates the mass content of the corresponding element, expressed in percent.

[0014] The absolute difference between the MD value of the stainless steel of the present invention and the MD value of existing conventional stainless steel (such as Comparative Example 3) is not less than 60.

[0015] Furthermore, the addition of low-melting-point copper to austenitic stainless steel often leads to intergranular segregation. To suppress intergranular segregation, the content of alloying elements Mn, Cu, and S should be optimized. This can enable copper to precipitate more within the grains with manganese sulfide precipitates as nuclei, thus avoiding copper precipitation at grain boundaries. For this purpose, the mass percentage of manganese to sulfur should be controlled between 40-500, preferably between 60-300, and the copper to manganese ratio should be controlled between 2-10.

[0016] Furthermore, the required addition amounts of microalloying elements B, V, Ti, Nb, Zr, and RE are as follows: B 0.001%-0.05%, Ti 0.001%-0.20%, Nb 0.001%-0.20%, V 0.001%-0.20%, Zr 0.001%-0.20%, and RE 0.001%-0.20%.

[0017] Furthermore, when adding trace elements B, Zr, or RE, they should be added individually, and the amount added should not exceed the above requirements; when adding one or more of the trace alloying elements V, Nb, and Ti, the total amount should not exceed 0.30%; when adding microalloying elements RE and Zr, they should be added in combination with V, Nb, and Ti, and the total amount should be 0.0015%-0.30%.

[0018] The roles of each element in stainless steel are as follows: Carbon (C): Carbon dissolved in stainless steel ensures a certain strength, but excessive amounts can lead to high strength but reduced machinability, affecting deep drawing, flanging, and other processing properties. Large deformations result in strain hardening and deformation, while also reducing corrosion resistance. C is an austenitizing element, beneficial for forming a single austenitic structure, preventing the formation of high-temperature ferrite, and improving processing performance. This invention controls the C content to 0.010%-0.035%, preferably 0.021%-0.035%.

[0019] Nitrogen (N): Nitrogen can improve corrosion resistance and increase the strength of austenitic stainless steel. However, excessive nitrogen can lead to form hardening and affect formability. Therefore, the nitrogen content should be controlled between 0.010% and 0.045%, with 0.015% to 0.035% being optimal.

[0020] C and N exist as interstitial atoms; too low a concentration will increase smelting costs, while too high a concentration will affect corrosion resistance and strength. A target of 0.035% < C + N ≤ 0.065% was specifically proposed.

[0021] Silicon (Si): Silicon can play a deoxidizing role in stainless steel smelting. Silicon in the stainless steel matrix increases the strength of the steel to a certain extent and reduces the processability of the material. In this invention, the silicon content is controlled at 0.01%-0.5%, preferably 0.05%-0.2%.

[0022] Manganese (Mn): A certain manganese content is beneficial for pickling of steel coils and also for the stability of austenite. Manganese has a significant impact on increasing the recrystallization temperature of austenitic stainless steel. Excessive manganese content leads to an increased recrystallization temperature, making it difficult to achieve homogenized annealing microstructure. On the other hand, excessive manganese content can affect the function of other beneficial elements in stainless steel. Changes in manganese content also significantly affect the passivation film performance of austenitic stainless steel. High manganese content weakens the quality and stability of the passivation film, reduces its protective ability, and increases the risk of corrosion. This makes it more susceptible to corrosion in acidic, alkaline, and chloride-containing environments, increasing the probability of pitting corrosion, crevice corrosion, and other corrosion problems. In this invention, the manganese content is controlled at 0.1%-1.2%, preferably 0.3%-1.2%.

[0023] Phosphorus (P) and sulfur (S): Phosphorus and sulfur are considered harmful elements in stainless steel and should be controlled as low as possible. In this invention, P < 0.04% and S < 0.03%. To improve the segregation of copper at grain boundaries, it is necessary to appropriately optimize the content and ratio of manganese and sulfur, with a preferred sulfur content of 0.001%-0.015%.

[0024] Chromium (Cr): Chromium is the most important alloying element in stainless steel. It readily comes into contact with oxygen to form a dense passivation film of Cr2O3, which improves the corrosion resistance of steel. Too low a chromium content will affect the corrosion resistance, while too high a chromium content will cause the precipitation of ferrite phase, resulting in increased strength and hardness. In this invention, the chromium content is 17.2%-20.0%.

[0025] Molybdenum (Mo): The addition of molybdenum improves corrosion resistance, particularly resistance to atmospheric chloride ion corrosion. Under certain stress conditions, the addition of molybdenum significantly improves chloride ion corrosion resistance. Too low a molybdenum content will not achieve the desired improvement in corrosion resistance, while too high a content will increase costs and lead to the formation of precipitates during hot rolling, affecting both corrosion resistance and processability. In this invention, the Mo content is controlled at 0.01%-0.4%, preferably 0.05%-0.3%. This Mo content ensures that the corrosion resistance and processability of the stainless steel meet requirements while also reducing manufacturing costs.

[0026] Copper (Cu): In high-copper austenitic low-strength stainless steel, copper alloying elements play multiple important roles. First, it enhances corrosion resistance. Copper promotes the formation of a denser and more protective passivation film on the stainless steel surface, hindering the contact between corrosive media and the base metal, thus enhancing its resistance to pitting corrosion, crevice corrosion, and other localized corrosion. Second, it regulates mechanical properties. Copper helps reduce the strength of stainless steel, giving it low-strength characteristics, which meets the needs of applications where high strength is not required but formability and flexibility are important. Simultaneously, copper can improve the toughness and ductility of stainless steel to a certain extent, making the material easier to deform. Copper also affects the stability of the microstructure, stabilizing the austenitic structure and ensuring that the stainless steel has a uniform microstructure and stable performance. A certain amount of copper can also significantly improve the brazing spreadability and wettability of the stainless steel surface, facilitating the flow and spread of the filler metal during brazing. This allows for complete brazing in a short time (5-30 seconds) using only flame welding or induction welding, without the need for long-term brazing in a tunnel furnace, meeting the requirements of pressure testing and sealing tests (18 MPa pressure, 3 minutes holding time). However, the amount of copper added needs to be properly controlled. Excessive copper can lead to segregation of low-melting-point compounds at grain boundaries. The appropriate copper content needs to be determined comprehensively based on specific application requirements and production processes. Too low a content will not meet the requirements for improving spreadability, will not achieve effective brazing, and may even result in defects such as incomplete soldering and porosity. Too high a content will cause segregation of low-melting-point copper and its compounds. Therefore, the copper content in this invention is controlled at 1.0%-3.5%, preferably 1.5%-3.0%.

[0027] Nickel (Ni): In low-strength austenitic stainless steel, nickel plays a crucial role in alloying. Firstly, it stabilizes the austenitic structure. As a strong austenite-forming element, nickel significantly expands the austenite phase region, ensuring a stable austenitic structure in stainless steel at room temperature and over a wide temperature range, thus achieving phase transformation-free strengthening during processing. Secondly, it helps regulate mechanical properties. Similar to copper, it reduces the strength of stainless steel to meet low-strength requirements, while improving toughness and ductility, making it easier to deform during processing. Nickel enhances the resistance of stainless steel to various corrosive media, especially in complex environments with both oxidizing and reducing properties. Nickel works synergistically with other alloying elements such as chromium to improve the stability of the passivation film on the stainless steel surface, making it denser and stronger, effectively blocking corrosion. However, nickel is a relatively expensive alloying element, and excessive use increases material costs. Therefore, it is essential to control its addition amount to balance performance and cost. In this invention, the nickel content is controlled between 7.5% and 10.0%.

[0028] In addition to the elements mentioned above, the following microalloying elements also play an important role.

[0029] Vanadium (V): In steel, it has a strong affinity for carbon, nitrogen, and oxygen, forming carbides or nitrides. These stable carbides (such as VC) can refine the grain size, reduce the steel's overheating sensitivity, and thus improve strength and toughness. A small amount of vanadium can improve toughness through grain refinement, but excessive amounts can lead to carbide aggregation, which in turn reduces room temperature toughness. At high temperatures, dispersed vanadium carbides can also improve the steel's creep strength and creep resistance. Furthermore, vanadium can fix carbon, which helps improve the resistance of stainless steel to intergranular corrosion.

[0030] Niobium (Nb): Niobium is a strong carbide-forming element, enhancing grain boundary strength and benefiting the brazing process of stainless steel. Adding niobium to nitrogen-containing austenitic stainless steel leads to the formation of Z-phase precipitates (such as NbN or complex nitrides and carbides). These precipitates effectively pinnate grain boundaries during high-temperature aging, significantly inhibiting grain growth. Studies show that niobium-containing alloys, after aging at 1050°C for a certain time (e.g., 200 minutes), maintain a grain size of approximately 11 μm, while the grain size of niobium-free alloys coarsens from 17 μm to 33 μm. Grain refinement (increased grain boundaries) directly contributes to strength improvement according to the Hall-Petch relationship. Niobium preferentially forms carbides, inhibiting harmful precipitation. Niobium has a stronger affinity for carbon than chromium, preferentially forming stable NbC, thereby inhibiting chromium-rich carbides (such as Cr). 23 C6 precipitates at grain boundaries; this avoids chromium depletion zones near grain boundaries and is a key mechanism for preventing intergranular corrosion and improving grain boundary strength.

[0031] Titanium (Ti): Titanium's role is similar to niobium, serving as a stabilizer and preventing intergranular corrosion. Titanium has an extremely strong affinity for carbon, nitrogen, and oxygen. In austenitic stainless steel, titanium preferentially combines with carbon to form TiC, preventing carbon from combining with chromium to form Cr. 23 C6 effectively prevents intergranular corrosion caused by chromium depletion at grain boundaries and improves intergranular bonding. To fully utilize the role of titanium, titanium-containing stainless steel can undergo stabilization treatment (heating at 850-1080°C) to promote the decomposition of chromium carbides and form stable TiC. Grain refinement and strengthening phase formation: Titanium can refine the grain structure of steel, improving strength and toughness. Under certain conditions, titanium can also disperse and precipitate intermetallic compounds such as Fe2Ti, enhancing the high-temperature strength of stainless steel and inhibiting hot-rolling cracking. However, the addition of titanium may also introduce inclusions such as TiO2 or TiN, affecting the purity and surface finish of the steel; therefore, the amount added needs to be controlled.

[0032] Zirconium (Zr): As a strong carbide-forming element, zirconium enhances grain boundary strength and high-temperature structural stability primarily through the formation of highly thermally stable precipitates. In Fe-Cr-Ni austenitic stainless steel, the addition of trace amounts of Zr (e.g., 0.20 wt.%) promotes the dispersed precipitation of MC-type (e.g., ZrC) nanoparticles, effectively inhibiting the precipitation of coarse Cr. 23C6 and brittle σ phases precipitate at grain boundaries, thus maintaining microstructural stability and toughness at high temperatures. In Fe-Cr-Ni austenitic stainless steels, Zr forms Fe, which has higher thermal stability. 23 Zr6 phase effectively pins grain boundaries and inhibits high-temperature grain coarsening: Adding Zr can also increase the recrystallization temperature of the alloy, further inhibiting grain growth at high temperatures.

[0033] Boron (B): Boron is a surface-active element that readily segregates at grain boundaries. This segregation fills grain boundary vacancies, lowers grain boundary energy, and thus strengthens the grain boundaries. The addition of trace amounts of boron (e.g., 0.003%) can significantly improve the thermoplasticity and hot workability of austenitic stainless steel, indirectly contributing to a more complete and robust grain structure. For copper-alloyed austenitic stainless steel, the addition of low-melting-point copper weakens intergranular bonding at high temperatures; the addition of boron can precisely compensate for the adverse effects of low-melting-point copper.

[0034] Rare earth elements (RE, Ce, La, etc.): The main role of rare earth elements is to purify molten steel and remove impurities. They have a strong affinity for oxygen and other elements, forming high-melting-point rare earth oxides, reducing the segregation of harmful impurities at grain boundaries. Simultaneously, the addition of rare earth elements improves the hot workability of steel. Purer molten steel and less segregation of grain boundary impurities such as phosphorus contribute to stronger and more stable grain boundaries.

[0035] Secondly, embodiments of the present invention provide a method for preparing stainless steel for stress-free annealing and high-efficiency brazing, comprising the following steps: (1) Smelting and forging: According to the required components in the first aspect, the raw materials are prepared and smelted, and the ingot is obtained by mold casting. The ingot is cooled at a cooling rate of ≥30℃ / s. After the ingot is cooled, it is heated in the furnace at a heating temperature of 1000-1250℃ and a holding time of 50-250min. The ingot is then forged after being taken out of the furnace. The initial forging temperature is not higher than 1180℃ and the final forging temperature is greater than 900℃. After forging, it is air-cooled. (2) Hot rolling: After forging, the steel billet is milled to remove the surface oxide scale and then hot rolled. It is heated at 1000-1250℃ and held for 60-150 minutes before rolling begins. The initial rolling temperature does not exceed 1200℃ and the final rolling temperature is controlled above 850℃. After rolling, it is air-cooled or water-cooled. (3) Homogenization annealing: The hot-rolled steel plate is annealed at a temperature of 930-1100℃ for 1-5 minutes and pickled to obtain a white surface with a clean surface free of oxide scale residue. Pickling is performed using the same pickling solution and process as conventional SUS304 / SUS304L to obtain a white surface. Typical process parameters are: acid solution ratio is usually 10-13% nitric acid by mass, hydrofluoric acid by mass, 1.0%-1.3% hydrofluoric acid by mass, treatment temperature is 55±5℃, and treatment time is 1-5 minutes.

[0036] For the neutral salt electrolytic pickling process in a continuous annealing pickling line, the pickling temperature is 70±10℃, the pickling concentration is 1.14±0.1g / L, the pH value is controlled at 6.5±0.5, and the current linear density is 7000±500A / dm. This process requires strict temperature control and should not exceed 88℃ during operation.

[0037] (4) Cold rolling: The hot-rolled steel plate after pickling and surface grinding is cold rolled, and after cold rolling, it is annealed and pickled to obtain stainless steel cold-rolled strip.

[0038] Further, in step (1), molten steel with qualified composition is obtained by sequentially refining through electric furnace smelting, AOD furnace (argon oxygen decarburization furnace, abbreviated as AOD furnace) and LF furnace (Ladle refining furnace, abbreviated as LF furnace), and then continuously cast to obtain a continuously cast slab with a thickness of 160-220mm.

[0039] Furthermore, in step (2), for the continuously cast slab, the continuously cast slab is placed in a heating furnace at 1000-1250℃ for heating, and after holding at that temperature for 150-230 minutes, rolling begins.

[0040] Furthermore, in step (4), single-pass cold rolling or continuous rolling is used, and the total reduction rate of cold rolling is not less than 50%-90%.

[0041] Furthermore, in step (4), a reducing atmosphere bright annealing furnace is used for annealing. The bright annealing temperature is 930-1100℃, the annealing time is 1-5min, the annealing TV value (product of thickness and pulling speed) is controlled at 30-100 m*min, the grain size of the stainless steel plate is controlled at level 5-8, and the surface roughness Ra is guaranteed to be 0.003-0.3mm.

[0042] Compared with the prior art, the advantages of the present invention are as follows: This invention optimizes the alloy formulation and production process to produce stainless steel that combines high corrosion resistance, high brazing spread and wettability, low strain hardening strength, no need for tunnel furnace brazing, and the ability to achieve rapid brazing such as induction welding or flame welding. The added copper and nickel elements work synergistically to reduce the strength and hardness of the metal matrix, lowering the yield strength by 50-100 MPa and the tensile strength by 80-150 MPa compared to conventional SUS304 / SUS304L stainless steel, while maintaining high elongation and low work hardening ability, with an elongation of not less than 50%. The synergistic effect of copper and nickel further stabilizes the austenitic structure of the stainless steel, achieving austenite stabilization at lower temperatures. Even under high forming conditions, it maintains low strain stress and strain hardening effect, eliminating the need for tunnel furnace heating and stress-relief annealing as required for conventional SUS304 / SUS304L stainless steel to meet the requirements of subsequent component installation and use, thus achieving anneal-free, low-carbon manufacturing of components. The resulting austenitic stainless steel exhibits a yield strength ≤220MPa, tensile strength ≤580MPa, elongation ≥50%, hardness not exceeding 140HV, grain size controlled between 5-10, and pitting potential ≥260mV. Welded pipe products manufactured using this cold-rolled sheet possess comprehensive properties including high corrosion resistance, high brazing wettability, and low strain hardening strength. Rapid welding processes such as induction welding or flame welding allow stainless steel to replace copper materials (plates or pipes) or to connect stainless steel and copper pipes. Compared to the conventional SUS304 / SUS304L stainless steel with a copper sleeve, this invention saves on expensive welding processing costs, achieving efficient manufacturing and low carbon emissions.

[0043] This invention utilizes chemical composition design and specific production and processing technology to obtain components with better brazing spreadability, relatively low yield and tensile strength, high corrosion resistance, high elongation, and low hardening behavior. It can achieve efficient brazing without the need for tunnel furnace brazing. In heat exchange components, including air conditioning, AI heat exchangers, wind turbines, automobiles, etc., flame brazing or induction brazing can be used to directly replace copper materials. Alternatively, stainless steel with copper sleeves can be used for brazing, but without the need for long-term tunnel furnace brazing. Brazing can be completed in less than 10 minutes, greatly reducing the production cycle and component cost. Attached Figure Description

[0044] Figure 1 This is a metallographic photograph of the stainless steel in Embodiment 1 of the present invention.

[0045] Figure 2 This is the result of the stainless steel brazing filler metal spreadability test in Embodiment 1 of the present invention.

[0046] Figure 3 This is the result of the stainless steel brazing filler metal spreadability test in Comparative Example 3 of this invention. Detailed Implementation

[0047] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0048] Example 1 A type of stainless steel for stress-free annealing and high-efficiency brazing, comprising the following components by mass percentage: C 0.022%, Si 0.42%, Mn 0.85%, P 0.012%, S 0.003%, Cr 17.8%, Ni 8.4%, Cu 2.4%, N 0.033%, Mo 0.1%, B 0.0015%, Ti 0.06%, with the balance being Fe and unavoidable impurities.

[0049] The MD of stainless steel is 551 - 462 * (W) C +W N )-8.2*W Si -8.5*W Mn -30*(W Ni +W Cu )-14.0*W Cr -18.0*W Mo =551-462*(0.022+0.033)-8.2*0.42-8.5*0.85-30*(8.4+2.4)-14.0*17.8-18.0*0.1=-60.079 Among them W C W N W Si W Mn W Ni W Cu W Cr and W Mo This indicates the mass content of the corresponding element, expressed in percent.

[0050] The above-mentioned method for preparing stainless steel for stress-free annealing and high-efficiency brazing includes the following steps: (1) Melting and forging: According to the required ingredients, the raw materials are mixed and melted, and the mold is used to cast the ingot. The ingot is cooled at a cooling rate of 56℃ / s. After the ingot is cooled, it is heated in the furnace. The heating temperature is 1208℃ and the holding time is 150min. The ingot is then forged. The initial forging temperature is 1170℃ and the final forging temperature is 1015℃. After forging, it is air-cooled. (2) Hot rolling: The oxide scale on the surface of the forged steel billet is milled off, and then hot rolling is carried out. The billet is heated at 1206℃ and held for 150 minutes before rolling begins. The initial rolling temperature is 1185℃ and the final rolling temperature is 940℃. After rolling, the billet is air-cooled or water-cooled. (3) Homogenization annealing: The hot-rolled steel plate is annealed at a temperature of 990℃ for 3 minutes. Pickling is carried out using the same pickling solution and pickling process as conventional SUS304 / SUS304L to obtain a clean surface free of oxide scale residue. (4) Cold rolling: The pickled and polished steel plate is cold rolled by single-pass cold rolling or continuous rolling. The total reduction rate of cold rolling is 75%. After cold rolling, the same annealing (see Comparative Example 3) and pickling process as conventional SUS304L stainless steel white skin can be used for pickling. The surface roughness Ra of the stainless steel plate is controlled to be 0.003-0.3mm, so as to obtain a low-carbon emission stainless steel cold-rolled strip that improves the brazing spreadability of stainless steel and improves the brazing efficiency.

[0051] Example 2 A type of stainless steel for stress-free annealing and high-efficiency brazing, comprising the following components by mass percentage: C 0.025%, Si 0.5%, Mn 0.74%, P 0.023%, S 0.011%, Cr 17.6%, Ni 8.4%, Cu 2.7%, N 0.035%, Mo 0.09%, Zr 0.02%, V 0.06%, with the balance being Fe and unavoidable impurities.

[0052] The MD of stainless steel is 551 - 462 * (W) C +W N )-8.2*W Si -8.5*W Mn -30*(W Ni +W Cu )-14.0*W Cr -18.0*W Mo =551-462*(0.025+0.035)-8.2*0.5-8.5*0.74-30*(8.4+2.7)-14.0*17.6-18.0*0.09=-68.13 Among them W C W N W Si W Mn W Ni W Cu W Cr and W Mo This indicates the mass content of the corresponding element, expressed in percent.

[0053] The above-mentioned method for preparing stainless steel for stress-free annealing and high-efficiency brazing includes the following steps: (1) Melting and forging: According to the required ingredients, the raw materials are mixed and melted, and the mold is used to cast the ingot. The ingot is cooled at a cooling rate of 64℃ / s. After the ingot is cooled, it is heated in the furnace at a heating temperature of 1188℃ and a holding time of 135min. The ingot is then forged at a starting temperature of 1168℃ and a final forging temperature of 1010℃. After forging, it is air-cooled. (2) Hot rolling: After forging, the steel billet is milled to remove the surface oxide scale and then hot rolled. It is heated at 1210℃ and held for 130 minutes before rolling begins. The initial rolling temperature is 1185℃ and the final rolling temperature is 940℃. After rolling, it is air-cooled or water-cooled. By extending the holding time, the plasticity of the steel billet can be improved, the deformation resistance can be reduced, the composition can be homogenized, and the processing performance can be improved.

[0054] (3) Homogenization annealing: The hot-rolled steel plate is annealed at a temperature of 1083℃ for 1.5 min. Pickling is carried out using the same pickling solution and pickling process as conventional SUS304 / SUS304L to obtain a clean surface free of oxide scale residue. (4) Cold rolling: The hot-rolled steel plate after pickling and grinding is cold rolled by single-pass cold rolling or continuous rolling. The total reduction rate of cold rolling is 80%. After cold rolling, it is annealed in a reducing atmosphere bright annealing furnace with nitrogen protection. The bright annealing temperature is 1085℃ and the annealing time is 4min. The annealing TV value (product of thickness and pulling speed) is controlled at 80 m*min. The surface roughness Ra of the stainless steel plate is controlled at 0.003-0.3mm. A low-carbon emission stainless steel cold-rolled strip with improved brazing spreadability and improved brazing efficiency is obtained.

[0055] Example 3 A type of stainless steel for stress-free annealing and high-efficiency brazing, comprising the following components by mass percentage: C 0.033%, Si 0.23%, Mn 1.06%, P 0.025%, S 0.005%, Cr 17.8%, Ni 8.2%, Cu 3.0%, N 0.021%, Mo 0.12%, B 0.009%, with the balance being Fe and unavoidable impurities.

[0056] The MD of stainless steel is 551 - 462 * (W) C +W N )-8.2*W Si -8.5*W Mn -30*(W Ni +W Cu )-14.0*W Cr -18.0*W Mo=551-462*(0.033+0.021)-8.2*0.23-8.5*1.06-30*(8.2+3.0)-14.0*17.8-18.0*0.12=-72.204 Among them W C W N W Si W Mn W Ni W Cu W Cr and W Mo This indicates the mass content of the corresponding element, expressed in percent.

[0057] The above-mentioned method for preparing stainless steel for stress-free annealing and high-efficiency brazing includes the following steps: (1) Melting and forging: According to the required ingredients, the raw materials are mixed and melted, and the mold is used to cast the ingot. The ingot is cooled at a cooling rate of 58℃ / s. After the ingot is cooled, it is heated in the furnace. The heating temperature is 1185℃ and the holding time is 180min. The ingot is then forged. The initial forging temperature is 1173℃ and the final forging temperature is 1015℃. After forging, it is air-cooled. (2) Hot rolling: The oxide scale on the surface of the forged steel billet is milled off, and then hot rolling is carried out. The billet is heated at 1180℃ and held for 120 minutes before rolling begins. The initial rolling temperature is 1158℃ and the final rolling temperature is above 880℃. After rolling, the billet is air-cooled or water-cooled. (3) Homogenization annealing: The hot-rolled steel plate is annealed at a temperature of 1075℃ for 3.5 min. The same pickling solution and pickling process as conventional SUS304 / SUS304L are used for pickling to obtain a white surface with a clean surface and no oxide scale residue. (4) Cold rolling: The hot-rolled steel plate after pickling and grinding is cold rolled by single-pass cold rolling or continuous rolling. The total reduction rate of cold rolling is 65%. After cold rolling, the same annealing (see Comparative Example 3) and pickling process as conventional SUS304L are used for pickling. The surface roughness Ra of the stainless steel plate is controlled to be 0.003-0.3mm. A low-carbon emission stainless steel cold-rolled strip with improved brazing spreadability and improved brazing efficiency is obtained.

[0058] Example 4 A type of stainless steel for stress-free annealing and high-efficiency brazing, comprising the following components by mass percentage: C 0.023%, Si 0.37%, Mn 1.11%, P 0.016%, S 0.015%, Cr 17.9%, Ni 9.4%, Cu 2.8%, N 0.030%, Mo 0.05%, RE 0.035%, with the balance being Fe and unavoidable impurities.

[0059] The MD of stainless steel is 551 - 462 * (W) C +W N )-8.2*W Si -8.5*W Mn -30*(W Ni +W Cu )-14.0*W Cr -18.0*W Mo =551-462*(0.023+0.030)-8.2*0.37-8.5*1.11-30*(9.4+2.8)-14.0*17.9-18.0*0.05=-103.455 Among them W C W N W Si W Mn W Ni W Cu W Cr and W Mo This indicates the mass content of the corresponding element, expressed in percent.

[0060] The above-mentioned method for preparing stainless steel for stress-free annealing and high-efficiency brazing includes the following steps: (1) Melting and forging: According to the required ingredients, the raw materials are mixed and melted, and the mold is used to cast the ingot. The ingot is cooled at a cooling rate of 72℃ / s. After the ingot is cooled, it is heated in the furnace at a heating temperature of 1210℃ and a holding time of 145min. The ingot is then forged at a starting temperature of 1178℃ and a final forging temperature of 990℃. After forging, it is air-cooled. (2) Hot rolling: The oxide scale on the surface of the forged steel billet is milled off, and then hot rolling is carried out. The billet is heated at 1208℃ and held for 110 minutes before rolling begins. The initial rolling temperature is 1180℃ and the final rolling temperature is above 905℃. After rolling, the billet is air-cooled or water-cooled. (3) Homogenization annealing: The hot-rolled steel plate is annealed at a temperature of 1070℃ for 2.5 min. The same pickling solution and pickling process as conventional SUS304 / SUS304L are used for pickling to obtain a white surface with a clean surface and no oxide scale residue. (4) Cold rolling: The hot-rolled steel plate after pickling and surface grinding is cold rolled. Single-pass cold rolling or continuous rolling is adopted. The total reduction rate of cold rolling is 70%. After cold rolling, a bright annealing furnace with a reducing atmosphere is used for annealing. The atmosphere is a mixture of N2 and H2. The higher the H2 content, the better the surface reduction effect, but the cost will also be higher. In this embodiment, nitrogen and 4% H2 protection (mole fraction) are used. The bright annealing temperature is 1080℃, the annealing time is 2.5min, and the annealing TV value (product of thickness and pulling speed) is controlled at 70 m*min. The surface roughness Ra of the stainless steel plate is controlled at 0.003-0.3mm to obtain a low-carbon emission stainless steel cold-rolled strip that improves the brazing spreadability of stainless steel and improves the brazing efficiency.

[0061] Example 5 A type of stainless steel for stress-free annealing and high-efficiency brazing, comprising the following components by mass percentage: C 0.023%, Si 0.48%, Mn 0.96%, P 0.017%, S 0.016%, Cr 18.2%, Ni 9.2%, Cu 2.98%, N 0.036%, Mo 0.21%, Nb 0.08%, Ti 0.07%, with the balance being Fe and unavoidable impurities.

[0062] The MD of stainless steel is 551 - 462 * (W) C +W N )-8.2*W Si -8.5*W Mn -30*(W Ni +W Cu )-14.0*W Cr -18.0*W Mo =551-462*(0.023+0.036)-8.2*0.48-8.5*0.96-30*(9.2+2.98)-14.0*18.2-18.0*0.21=-112.334 Among them W C W N W Si W Mn W Ni W Cu W Cr and W Mo This indicates the mass content of the corresponding element, expressed in percent.

[0063] The above-mentioned method for preparing stainless steel for stress-free annealing and high-efficiency brazing includes the following steps: (1) Melting and forging: According to the required ingredients, the raw materials are mixed and melted, and the mold is used to cast the ingot. The ingot is cooled at a cooling rate of 61℃ / s. After the ingot is cooled, it is heated in the furnace. The heating temperature is 1192℃ and the holding time is 120min. The ingot is then forged. The initial forging temperature is 1170℃ and the final forging temperature is 1023℃. After forging, it is air-cooled. (2) Hot rolling: The oxide scale on the surface of the forged steel billet is milled off, and then hot rolling is carried out. The steel billet is placed in a heating furnace at 1210℃ for heating and held for 130 minutes before rolling begins. The initial rolling temperature is 1198℃ and the final rolling temperature is 930℃. After rolling, it is air-cooled or water-cooled.

[0064] (3) Homogenization annealing: The hot-rolled steel plate is annealed at a temperature of 1069℃ for 3.0 min. The same pickling solution and pickling process as conventional SUS304L are used for pickling to obtain a white surface with a clean surface and no oxide scale residue. (4) Cold rolling: After annealing, the hot-rolled steel plate is pickled and ground, and then cold-rolled. Single-pass cold rolling or continuous rolling is adopted. The total reduction rate of cold rolling is 85%. After cold rolling, the same annealing (see Comparative Example 3) and pickling process as conventional SUS304L are adopted for pickling. The surface roughness Ra of stainless steel plate is controlled to be 0.003-0.3mm, so as to obtain a low-carbon emission stainless steel cold-rolled strip that improves the brazing spreadability of stainless steel and improves the brazing efficiency.

[0065] Comparative Example 1 A type of stainless steel for stress-free annealing and high-efficiency brazing, comprising the following components by mass percentage: C 0.028%, Si 0.28%, Mn 0.34%, P 0.017%, S 0.01%, Cr 17.6%, Ni 7.2%, Cu 1.7%, N 0.029%, Mo 0.26%, V 0.05%, with the balance being Fe and unavoidable impurities.

[0066] The MD of stainless steel is 551 - 462 * (W) C +W N )-8.2*W Si -8.5*W Mn -30*(W Ni +W Cu )-14.0*W Cr -18.0*W Mo =551-462*(0.028+0.029)-8.2*0.28-8.5*0.34-30*(7.2+1.7)-14.0*17.6-18.0*0.26=1.4 Among them W C W N W SiW Mn W Ni W Cu W Cr and W Mo This indicates the mass content of the corresponding element, expressed in percent.

[0067] The above-mentioned method for preparing stainless steel for stress-free annealing and high-efficiency brazing includes the following steps: (1) Melting and forging: According to the required ingredients, the raw materials are mixed and melted, and the mold is used to cast the ingot. The ingot is cooled at a cooling rate of 62℃ / s. After the ingot is cooled, it is heated in the furnace. The heating temperature is 1196℃ and the holding time is 110min. The ingot is then forged. The initial forging temperature is 1167℃ and the final forging temperature is 1026℃. After forging, it is air-cooled. (2) Hot rolling: The oxide scale on the surface of the forged steel billet is milled off, and then hot rolling is carried out. The billet is heated at 1200℃ and held for 120 minutes before rolling begins. The initial rolling temperature is 1170℃ and the final rolling temperature is 955℃. After rolling, the billet is air-cooled or water-cooled. (3) Homogenization annealing: The hot-rolled steel plate is annealed at a temperature of 1078℃ for 4 minutes. The same pickling solution and pickling process as conventional SUS304 / SUS304L are used for pickling to obtain a white surface with a clean surface and no oxide scale residue. (4) Cold rolling: After annealing, the hot-rolled steel sheet is pickled and ground, and then cold-rolled. Single-pass cold rolling or continuous rolling is adopted, and the total reduction rate of cold rolling is 68%. After cold rolling, the same annealing (see Comparative Example 3) and pickling process as conventional SUS304L are adopted for pickling. The surface roughness Ra of the stainless steel sheet is controlled to be 0.003-0.3mm to obtain stainless steel cold-rolled strip.

[0068] Comparative Example 2 A type of stainless steel for stress-free annealing and high-efficiency brazing, comprising the following components by mass percentage: C 0.021%, Si 0.28%, Mn 0.34%, P 0.017%, S 0.01%, Cr 17.6%, Ni 7.1%, Cu 1.7%, N 0.029%, Mo 0.26%, V 0.05%, with the balance being Fe and unavoidable impurities.

[0069] The MD of stainless steel is 551 - 462 * (W) C +W N )-8.2*W Si -8.5*W Mn -30*(W Ni +W Cu )-14.0*W Cr -18.0*W Mo=551-462*(0.021+0.029)-8.2*0.28-8.5*0.34-30*(7.1+1.7)-14.0*17.6-18.0*0.26=7.634 Among them W C W N W Si W Mn W Ni W Cu W Cr and W Mo This indicates the mass content of the corresponding element, expressed in percent.

[0070] The above-mentioned method for preparing stainless steel for stress-free annealing and high-efficiency brazing includes the following steps: (1) Smelting and forging: The steel is smelted in an electric furnace, refined in an AOD furnace and an LF furnace in sequence to obtain molten steel with qualified composition. The molten steel is then continuously cast to obtain a continuous casting slab with a thickness of 200 mm. (2) Hot rolling: The continuous casting slab is heated at 1200℃. The continuous casting slab is placed in a heating furnace for heating. After holding at the temperature for 210 minutes, rolling begins. The initial rolling temperature is 1153℃ and the final rolling temperature is 884℃. After rolling, it is air-cooled or water-cooled. (3) Homogenization annealing: The hot-rolled steel plate is annealed at a temperature of 1080℃ for 1-5 minutes. Pickling is carried out using the same pickling solution and pickling process as conventional SUS304L to obtain a white surface with a clean surface and no oxide scale residue. (4) Cold rolling: After pickling and surface grinding, the annealed hot-rolled steel sheet is cold rolled using single-pass or continuous rolling with a total reduction of 75%. After cold rolling, it is annealed in a reducing atmosphere bright annealing furnace with nitrogen protection. The bright annealing temperature is 1070℃, the annealing time is 3.5min, the annealing TV value (product of thickness and drawing speed) is controlled at 80 m*min, and the surface roughness Ra of the stainless steel sheet is controlled at 0.003-0.3mm to obtain stainless steel cold-rolled strip.

[0071] Comparative Example 3 An austenitic SUS304L stainless steel comprises, by weight percentage: C 0.028%, Si 0.38%, Mn 1.03%, P 0.024%, S 0.006%, Cr 18.3%, Ni 8.1%, N 0.039%, with the balance being Fe and unavoidable impurities.

[0072] The MD of stainless steel is 551 - 462 * (W) C +W N )-8.2*W Si -8.5*W Mn-30*(W Ni +W Cu )-14.0*W Cr -18.0*W Mo =551-462*(0.028+0.039)-8.2*0.38-8.5*1.03-30*(8.1+0)-14.0*18.3-18.0*0=8.975 Among them W C W N W Si W Mn W Ni W Cu W Cr and W Mo This indicates the mass content of the corresponding element, expressed in percent.

[0073] The above-mentioned method for preparing stainless steel includes the following steps: (1) Smelting and forging: The steel is smelted in an electric furnace, refined in an AOD furnace and an LF furnace to obtain molten steel with qualified composition. The molten steel is then continuously cast to obtain a continuous casting slab with a thickness of 210 mm.

[0074] (2) Hot rolling: The continuous casting slab is heated at 1200℃. The continuous casting slab is placed in a heating furnace for heating. After holding for 210 minutes, rolling begins. The initial rolling temperature is 1161℃ and the final rolling temperature is 903℃. By extending the holding time, the plasticity of the steel billet can be improved, the deformation resistance can be reduced, the composition can be homogenized, and the processing performance can be improved.

[0075] (3) Homogenization annealing: The hot-rolled steel plate is annealed at a temperature of 1060℃ for 3 minutes. The same pickling solution and pickling process as conventional SUS304 / SUS304L are used for pickling to obtain a white surface with a clean surface and no oxide scale residue. (4) Cold rolling: The annealed hot-rolled steel sheet is cold rolled using single-pass cold rolling or continuous rolling, with a total reduction of 70%. After cold rolling, the sheet is annealed or pickled. The annealing temperature is 1086℃, the annealing time is 2min, the annealing TV value (product of thickness and drawing speed) is controlled at 90 m*min, and the surface roughness Ra of the stainless steel sheet is controlled at 0.003-0.3mm to obtain stainless steel cold-rolled strip.

[0076] The metallographic structure of Example 1 is as follows Figure 1 As shown, from Figure 1 It can be seen that the stainless steel in Example 1 has a single austenite structure and twinning characteristics are visible.

[0077] The test results comparing the brazing spreadability of stainless steel in Example 1 of this invention with those of stainless steel brazing filler metal in Comparative Example 3 are shown below. Figure 2 and Figure 3 .from Figure 2 and Figure 3 The comparison clearly shows that the brazing filler metal placed on the surface of Example 1 melts at high temperature and forms a good spread on the material surface (the fan-shaped part at the top of the photo), while the brazing filler metal on the surface of Comparative Example 3 only retains a circular feature at high temperature (no spread feature). Good spread characteristics mean that the brazing filler metal can fill the weld well and ensure the weld sealing.

[0078] The results of measuring the grain size grade, mechanical properties, corrosion potential, etc. of the obtained samples of the examples and comparative examples are listed in Table 1.

[0079] Table 1 Performance parameters of the examples and comparative examples (bright annealing)

[0080] The results in Table 1 show that the stainless steel obtained in Examples 1-5 of this invention can meet the following performance requirements: yield strength ≤220MPa, tensile strength ≤580MPa, hardness ≤140 (load 300-1000g), elongation ≥50%, pitting potential ≥260mV, and grain size grade: 5-8. The comparative examples, because their MD value cannot meet the requirement of ≤-60, also show that their mechanical properties cannot meet the design requirements.

[0081] The stainless steel obtained in Examples 1-5 of this invention can be brazed in 10-30 seconds using induction welding or flame welding, which significantly shortens the brazing time. In particular, it can ensure that the product still meets the requirements after deformation and stress-free annealing. Unlike conventional SUS304 / SUS304L stainless steel, it does not require tunnel furnace brazing for about 60-90 minutes to achieve stress-free annealing, thereby improving production efficiency, reducing carbon emissions, lowering costs, and achieving a win-win situation for economic and social benefits.

[0082] Finally, it should be noted that the above specific 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 examples, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A type of stainless steel for stress-free annealing and high-efficiency brazing, characterized in that, By mass percentage, it includes the following components: C 0.010%-0.035%, Si 0.01%-0.5%, Mn 0.1%-1.2%, P<0.04%, S<0.03%, Cr 17.2%-20.0%, Ni 7.5%-10.0%, Cu 1.0%-3.5%, N 0.010%-0.045%, 0.035%<C+N≤0.065%, Mo 0.01%-0.4%, and satisfies 17.8%≤Cr+3.3Mo≤20%, and also includes one or more of the microalloying elements B, V, Ti, Nb, Zr and rare earth elements RE, with the balance being Fe and unavoidable impurities.

2. The stainless steel for stress-free annealing and high-efficiency brazing as described in claim 1, characterized in that, The MD of the stainless steel is 551 - 462 * (W) C +W N )-8.2*W Si -8.5*W Mn -30*(W Ni +W Cu )-14.0*W Cr -18.0*W Mo ≤-60, where W C W N W Si W Mn W Ni W Cu W Cr and W Mo This indicates the mass content of the corresponding element, expressed in percent.

3. The stainless steel for stress-free annealing and high-efficiency brazing as described in claim 1, characterized in that, The mass percentage ratio of manganese (Mn) to sulfur (S) is controlled between 40 and 500, and the mass percentage ratio of copper (Cu) to manganese (Mn) is controlled between 2 and 10.

4. The stainless steel for stress-free annealing and high-efficiency brazing as described in claim 1, characterized in that, The required addition amounts of microalloying elements B, V, Ti, Nb, Zr, and RE are as follows: B 0.001%-0.05%, Ti 0.001%-0.20%, Nb 0.001%-0.20%, V 0.001%-0.20%, Zr 0.001%-0.20%, and RE 0.001%-0.20%.

5. The stainless steel for stress-free annealing and high-efficiency brazing as described in claim 4, characterized in that, When adding trace elements B, Zr, or RE, add them individually, and the amount added shall not exceed the above requirements; when adding one or more of trace alloying elements V, Nb, and Ti, the total amount shall not exceed 0.30%; when adding microalloying elements RE and Zr, add them together with V, Nb, and Ti, and the total amount shall be 0.0015%-0.30%.

6. A method for preparing stainless steel for stress-free annealing and high-efficiency brazing, characterized in that, Includes the following steps: (1) Smelting and forging: The required ingredients are prepared and smelted according to claim 1, and the ingot is obtained by mold casting. The ingot is cooled at a cooling rate of ≥30℃ / s. After the ingot is cooled, it is heated in the furnace at a heating temperature of 1000-1250℃ and a holding time of 50-250min. The ingot is then forged after being taken out of the furnace. The initial forging temperature is not higher than 1180℃ and the final forging temperature is greater than 900℃. After forging, it is air-cooled. (2) Hot rolling: After forging, the steel billet is milled to remove the surface oxide scale and then hot rolled. It is heated at 1000-1250℃ and held for 60-150 minutes before rolling begins. The initial rolling temperature does not exceed 1200℃ and the final rolling temperature is controlled above 850℃. After rolling, it is air-cooled or water-cooled. (3) Homogenization annealing: The hot-rolled steel plate is annealed at a temperature of 930-1100℃ for 1-5 minutes and pickled to obtain a white surface with a clean surface free of oxide scale residue. (4) Cold rolling: The hot-rolled steel plate after pickling and surface grinding is cold rolled, and after cold rolling, it is annealed and pickled to obtain stainless steel cold-rolled strip.

7. The method for preparing stainless steel for stress-free annealing and high-efficiency brazing according to claim 6, characterized in that, In step (1), molten steel with qualified composition is obtained by smelting in an electric furnace, refining in an AOD furnace and an LF furnace in sequence, and then continuously cast to obtain a continuously cast slab with a thickness of 160-220mm.

8. The method for preparing stainless steel for stress-free annealing and high-efficiency brazing according to claim 6, characterized in that, In step (2), for the continuously cast slab, the continuously cast slab is placed in a heating furnace at 1000-1250℃ for heating, and after holding at the temperature for 150-230 minutes, rolling begins.

9. The method for preparing stainless steel for stress-free annealing and high-efficiency brazing according to claim 6, characterized in that, In step (4), single-pass cold rolling or continuous rolling is used, and the total reduction rate of cold rolling is not less than 50%-90%.

10. The method for preparing stainless steel for stress-free annealing and high-efficiency brazing according to claim 6, characterized in that, In step (4), a reducing atmosphere bright annealing furnace is used for annealing. The bright annealing temperature is 930-1100℃, the annealing time is 1-5min, the annealing TV value is controlled at 30-100 m*min, the grain size of the stainless steel plate is controlled at 5-8, and the surface roughness Ra is guaranteed to be 0.003-0.3mm.

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