304 stainless steel with excellent sensitization resistance and austenite stability and application thereof

Abstract

The invention relates to 304 stainless steel with sensitization resistance and austenite stability and application of the 304 stainless steel. By accurately controlling chemical components, the mass percentage content range of carbon is 0.05-0.08, the equivalent weight of chromium is 19-21, the equivalent weight of nickel is 13-15, and the balance is iron and inevitable impurities. The composition range not only ensures the formation of a single austenite structure in a standard solid solution process, but also can maintain good intergranular corrosion resistance after 650 DEG C / 1h sensitization treatment. The stainless steel is suitable for structural parts in a medium-temperature corrosion environment, such as a bipolar plate or an end plate of a proton exchange membrane fuel cell, has excellent performance in a working environment of a fluorine-containing dilute sulphuric acid solution, and can be directly used without post-welding solution treatment. Compared with the prior art, the invention provides a more accurate component range, solves the contradiction between the formability and the corrosion resistance after welding of the traditional 304 stainless steel, and has important industrial application value.

Description

Technical Field

[0001] This invention relates to the field of stainless steel materials technology, specifically to a 304 stainless steel with both excellent resistance to sensitization and austenitic stability, and its applications. Background Technology

[0002] Austenitic stainless steel 304 (also known as 18-8 stainless steel) is one of the most widely used stainless steel grades. In this alloy, chromium (Cr) is the core element that provides corrosion resistance, while nickel (Ni) and carbon (C) are key austenite stabilizing elements, used to ensure the formation and maintenance of a single face-centered cubic austenite structure at room temperature to high temperatures, thereby obtaining good formability, toughness and corrosion resistance.

[0003] However, carbon plays a double-edged sword role in 304 stainless steel. On the one hand, carbon is a strong austenite stabilizing element, crucial for maintaining a single-phase austenite structure and inhibiting ferrite precipitation; on the other hand, excessive carbon content can combine with chromium in the sensitization temperature range (e.g., 450 ~ 850 ℃), promoting Cr... 23 Chromium-rich carbides such as C6 preferentially precipitate at grain boundaries, leading to chromium depletion in the vicinity of the grain boundaries. This triggers intergranular corrosion (i.e., "sensitization"), severely impairing the material's corrosion resistance. Therefore, there exists an optimal window for carbon content that requires precise balance.

[0004] Currently, the national standard GB / T 20878-2024 specifies that the upper limit of carbon content (by mass percentage) for 304 stainless steel is ≤0.08%, and internationally accepted standards such as ASTM A240 also generally set it at ≤0.07%. These standards provide a broad safety range, but do not provide a precise lower limit for carbon content that optimally balances austenitic stability and sensitization resistance, nor the corresponding chromium and nickel equivalent matching range. In applications requiring both formability and post-weld corrosion resistance, high-performance 304 stainless steel materials with clearly defined compositional boundaries are still lacking.

[0005] In actual production, this imprecision leads to vague product quality control objectives, making it difficult to consistently achieve optimal performance. This poses a significant challenge, especially for demanding applications requiring both high formability (stable austenite) and resistance to sensitization (such as post-weld or mid-temperature service). Therefore, there is an urgent need for a composition design method to accurately determine the optimal carbon content range for 304 stainless steel. Summary of the Invention

[0006] The purpose of this invention is to overcome the technical challenge of balancing austenitic stability and sensitization resistance in existing 304 stainless steel, and to provide a 304 stainless steel with both excellent sensitization resistance and austenitic stability, as well as its applications. By rationally controlling the carbon content and the synergistic matching of chromium and nickel equivalents, the material exhibits a single austenitic structure in the solid solution state, and can still effectively inhibit intergranular corrosion after sensitization treatment, thereby achieving a significant improvement in overall performance.

[0007] Specifically, the 304 stainless steel, which combines excellent resistance to sensitization and austenitic stability, has the following composition by mass percentage:

[0008] The carbon content is 0.05 ~ 0.08. The chromium equivalent is 19 to 21. The nickel equivalent is 13 to 15. The equivalent mentioned above is calculated using the Schaeffler equivalent formula, where the chromium equivalent is Cr. eq = Cr + 1.5Si, nickel equivalent Ni eq = Ni + 30C + 0.5Mn.

[0009] The balance consists of iron and unavoidable impurities. The "unavoidable impurities" conform to the standard requirements of GB / T 20878-2024 for residual elements in 304 stainless steel, such as P ≤ 0.045 wt.% and S ≤ 0.030 wt.%.

[0010] Further, in a preferred embodiment, the composition of the 304 stainless steel, by mass percentage, is as follows: The carbon content is 0.05 ~ 0.07. The chromium equivalent is 19.2 ~ 20.0. The nickel equivalent is 13.5 ~ 13.9. The balance consists of iron and unavoidable impurities.

[0011] The 304 stainless steel described in this invention, after standard solution treatment (e.g., 1150℃ / 2h / water cooling), can obtain a single austenitic structure. Even after standard sensitization treatment (e.g., 650℃ / 1h / furnace cooling), it still maintains excellent resistance to intergranular corrosion, demonstrating a synergistic optimization of austenitic stability and resistance to sensitization. This invention clarifies and narrows the optimized range of carbon mass percentage content from ≤0.08% in the national standard to 0.05~0.08%, and further recommends an optimal range of 0.05~0.07%, providing a precise target composition range for the stable production of high-quality stainless steel. Experimental results show that when the carbon content is below 0.05%, the nickel equivalent is insufficient, and the austenitic stability decreases; when the chromium equivalent is below 19.2%, even with a high carbon content, the material is still prone to intergranular corrosion after sensitization. Only when the above three parameters simultaneously fall within the aforementioned narrow range can the simultaneous improvement of the two properties be achieved. This technical effect is difficult for those skilled in the art to directly anticipate based on existing knowledge.

[0012] Furthermore, the 304 stainless steel described in this invention can be used in medium-temperature corrosive environments where there is a risk of sensitization.

[0013] In a broad sense, it is suitable for industrial components that operate at temperatures of 400 to 800°C and come into contact with acidic media containing sulfur or halogens. Preferably, it is suitable for bipolar plates or end plates of proton exchange membrane fuel cells, which are in long-term contact with a dilute sulfuric acid containing fluorine at 60~90℃. Particularly preferred is that, as a structural component that needs to be directly put into service after welding or hot working, such as a flow field plate in a fuel cell stack, it can still maintain excellent resistance to intergranular corrosion without post-weld solution treatment.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The compositional design method can efficiently determine the equivalent range of chromium and nickel with high corrosion resistance and no ferrite phase precipitation, and give the preferred equivalent range. Based on the sensitization test results of a series of alloy samples designed according to the aforementioned composition formula, corrosion resistance was tested by electrochemical experiments, and phase composition was determined by XRD. This revealed that 304 stainless steel, possessing both high corrosion resistance and austenitic stability (no ferrite precipitation), meets the chromium equivalent Cr requirement. eq = Cr + 1.5Si = 19.2 ~ 20.0, Nickel equivalent Ni eq = Ni+30C+0.5Mn = 13.5 ~13.9, which constitutes the preferred equivalent range.

[0015] (2) The aforementioned preferred equivalent range can define the preferred carbon content range. Without sufficient carbon, austenite stability is insufficient, and ferrite is easily formed. Using the aforementioned nickel equivalent, the lower limit of carbon mass percentage content can be calculated to be 0.05%. Considering that the carbon content cannot exceed the standard upper limit of 0.08%, the preferred carbon content range is 0.05 ~ 0.07.

[0016] (3) Recommended carbon content and equivalent range that take into account both optimal laboratory performance and the feasibility and economy of industrial production. Considering the difficulty in precisely controlling the actual industrial alloy composition and related equivalents, and that slightly higher chromium and nickel equivalents are beneficial with their increase limited only by cost, it is recommended that the carbon content of industrial 304 stainless steel be 0.05~0.08%, with the equivalent range extended to 19~21 for chromium and 13~15 for nickel. This range provides reasonable tolerance for compositional fluctuations while retaining core performance advantages, making it more suitable for engineering applications. Attached Figure Description

[0017] Figure 1 The composition distribution in the Schaeffer equivalent diagram of this invention is shown in Table 1, with corresponding carbon mass percentages of 0.02 to 0.08, chromium equivalents of 17 to 20, and nickel equivalents of 12 to 14. This region is located at the lower limit of the austenite phase region and corresponds to the high austenite stability and anti-sensitization properties verified in subsequent experiments.

[0018] Figure 2 The X-ray diffraction (XRD) patterns after standard solution treatment (a) and sensitization (b) are used to demonstrate its austenitic stability.

[0019] Figure 3 Comparison of metallographic structures of 304 stainless steel with different carbon contents after standard sensitization treatment (650℃ / 1h) is shown to visually assess their tendency for intergranular corrosion (sensitization). The numbers 1 to 9 correspond to the example numbers in Table 1.

[0020] Figure 4 This is a comparison of the potentiodynamic polarization curves of 304 stainless steel with different carbon contents in acidic media, used to quantitatively compare their corrosion resistance differences. The Nyquist plot reflects the impedance behavior and is divided into (a) the solution state and (b) the sensitized state, where Z... and Z These correspond to the real and imaginary parts of the impedance, respectively. Detailed implementation method: The following embodiments will further illustrate the present invention. In these embodiments, the smelting, heat treatment, microstructure characterization, and corrosion performance testing of the alloy were all performed using conventional methods in the art. The raw materials used were industrially pure metals, and smelting was completed in a non-consumable vacuum arc furnace; heat treatment was carried out in a vacuum box furnace; phase structure analysis was performed using X-ray diffraction (XRD), metallographic microstructure observation was performed using an optical microscope, and electrochemical testing was performed using a standard three-electrode system. Specific process parameters and testing conditions are described below.

[0021] The chromium and nickel equivalents mentioned in this article are calculated using the Schaeffler equivalent formula, i.e., chromium equivalent Cr eq = Cr + 1.5Si, nickel equivalent Ni eq = Ni + 30C + 0.5Mn.

[0022] Those skilled in the art can make equivalent substitutions for the equipment model, reagent type, or process parameters in the above method according to actual conditions, without affecting the realization of the effect of the present invention.

[0023] The composition design of the 304 stainless steel in the embodiment is shown in Table 1.

[0024] Table 1. Composition table of 304 stainless steel in the examples

[0025] According to the alloy composition given in Table 1, each metal raw material was weighed separately using a precision electronic balance. The error for each metal raw material was controlled within 0.0005 g, and the total mass of each ingot sample was approximately 30 g. Clean gloves were required when weighing the metal raw materials using the balance, and the tweezers were wiped with alcohol swabs to ensure the cleanliness of the raw materials. Melting was carried out in a non-consumable vacuum arc furnace ingot mold rotary non-consumable vacuum arc melting system. The position of the tungsten tip was adjusted so that it was aligned with the central axis of the electromagnetic stirrer. The copper mold was lightly brushed with an iron brush until it was the natural color of copper. After polishing, the copper mold was cleaned with anhydrous ethanol before placing the metal material inside. High-melting-point metal raw materials (Cr, Fe) were placed in the center of the copper mold to melt first, preventing them from being encased by lower-melting-point materials. Titanium granules were also placed in other molds. Titanium granules have a lower melting point and will melt first compared to the metal raw materials during the melting process; their function is to absorb oxygen. A layer of sealing grease was applied around the furnace door, and the furnace door was closed. When the vacuum degree inside the furnace reached the required 6 × 10⁻⁶... -3 After the pressure reaches -0.02 MPa, the vacuum gauge is closed, and argon gas is introduced for protection. The argon purging valve is opened and argon gas is introduced to -0.02 MPa, and then the argon purging valve is closed. In order to obtain a uniform ingot during melting, each sample needs to be turned over and melted five times after cooling for 10 minutes, and finally a button-shaped alloy ingot with a diameter of 40 mm × 10 mm is obtained.

[0026] The alloy ingot was solution treated at 1150℃ for 2 hours and then water quenched in a vacuum box furnace. Finally, the alloy ingot was sensitized by holding at 650℃ for 1 hour. The specific operation procedure was as follows: set the heating program, input the temperature and holding time required for the heat treatment; evacuate the furnace until the pressure reading can no longer drop, while continuously tightening the furnace door during the evacuation process. At this time, it is necessary to dilute the oxygen with argon gas to reduce the pressure, thereby reducing the oxygen concentration and preventing oxidation. Introduce argon gas until the pressure reading reaches approximately -0.05 MPa; repeat the above operation 2-3 times until the furnace pressure reading reaches approximately -0.1 MPa, then start the program; after the heat treatment is completed, continue to introduce argon gas into the furnace to keep the furnace in a vacuum environment.

[0027] The phase structure characterization method for 304 stainless steel involved in this application is as follows: Bruker D8 Focus X-ray diffractometer (Cu target) was used K The phase structure of the alloy was tested using X-ray tubes with α, λ = 0.15406 nm. The tube voltage was 40 kV and the tube current was 40 mA. The results are as follows: Figure 2 As shown. Figure 2 The XRD diffraction patterns of the samples listed in Table 1 are as follows: (a) solid solution state (1150℃ / 2h / water cooling) and (b) sensitized state (650℃ / 1h / furnace cooling); from Figure 2 (a) It can be seen that all embodiments exhibit a single face-centered cubic austenite phase in the solid solution state; while Figure 2 In (b), only when the mass percentage of carbon is between 0.05 and 0.08 and the chromium equivalent Cr eq When the content is ≥ 19.2, the alloy maintains a stable austenitic structure after sensitization, and no obvious ferrite or carbide precipitation occurs, further verifying the effectiveness of the preferred composition window.

[0028] The microstructure characterization method for 304 stainless steel involved in this application is as follows: The metallographic sample preparation chemical etching agent consisted of 20% HF + 10% HNO3 + 70% H2O (volume fraction). Appropriate etching was applied to the sample surface, and the metallographic morphology of the sample was observed using an optical microscope. Metallographic images were then captured and preserved. The results are as follows: Figure 3 As shown. Figure 3 The results show that the grain boundary corrosion susceptibility of the material changes significantly with increasing carbon content. Specifically, when the carbon content is below 0.05%, the material is prone to ferrite phase formation after sensitization; while when the carbon content exceeds 0.08%, although the austenite has good stability, chromium-rich carbides precipitate at the grain boundaries, leading to an increased risk of intergranular corrosion. In contrast, the carbon content is between 0.05 and 0.08%, and the Cr content is significantly higher. eqSamples with a concentration of ≥ 19.2 exhibited the best resistance to sensitization, with clear grain boundaries and no obvious signs of corrosion, further confirming the unique advantages of the preferred composition range.

[0029] The corrosion performance testing method for 304 stainless steel involved in this application is as follows: Electrochemical tests were performed on the samples using an electrochemical workstation. A standard three-electrode, two-loop system was used, with Ag / AgCl (saturated KCl) as the reference electrode and a Pt mesh electrode as the auxiliary electrode. The corrosive medium simulated the actual working environment of the proton exchange membrane fuel cell cathode / anode (0.5 mol / L H2SO4 + 2ppm HF aqueous solution). The experimental temperature was room temperature. After the open circuit potential stabilized, an AC voltage of 5 mV was used to scan from 10 kHz to 0.01 Hz, with a settling time of 2 s.

[0030] After solution treatment (1150℃ / 2h / water cooling), the X-ray diffraction patterns of all embodiments showed a single face-centered cubic austenite phase, such as Figure 2 As shown in (a). Sensitized (650℃ / 1h / furnace cooling, Figure 2 (b) Following this, in Examples 4 (0.067 wt.% C), 5 (0.084 wt.% C), and 6 (0.084 wt.% C) containing higher carbon content, no significant ferrite formation was observed. Even with carbide precipitation (significant in the high-carbon Examples 5 and 6), the austenite retained sufficient stability, with an equivalent of Cr. eq =19.2 ~ 20.0, Ni eq = 13.5 ~ 13.9.

[0031] After sensitization treatment (650℃ / 1h / furnace cooling), its microstructure and corrosion resistance showed significant differences. For example... Figure 3 As shown, Examples 4 (0.067 wt.% C), 5 (0.084 wt.% C), and 6 (0.084 wt.% C), which contain higher carbon content, only exhibit moderate sensitization; other high-carbon samples formed broad grain boundary corrosion networks, exhibiting strong sensitization; although the low-carbon samples showed weak sensitization, the austenite was unstable and easily formed ferrite.

[0032] Figure 4 The potentiodynamic polarization curves further show that Examples 4, 5, and 6 exhibit the largest Nyquist impedance spectrum radius in both the solid solution and sensitized states, indicating that they have the best corrosion resistance. Figure 4 (a) and (b) show the Nyquist plots of samples with different carbon contents in the solution and sensitized states, respectively. The results show that all samples exhibit good corrosion resistance in the solution state; however, after sensitization treatment, only samples with carbon contents of 0.05–0.08% and Cr...eq Samples with an impedance ≥ 19.2 still maintained a high impedance value, indicating excellent resistance to intergranular corrosion under practical application conditions. This result is consistent with... Figure 3 The metallographic observations were consistent, jointly verifying the effectiveness and reliability of the optimized composition window. For example... Figure 1 As shown, the compositions of Examples 4-6 are located in the region typically considered suitable for austenite formation on the Schaeffer diagram. However, it is noteworthy that not all compositions in this region exhibit equivalent properties—for example, Examples 7-9, although also within the predicted austenite range, showed significant intergranular corrosion after sensitization due to their low chromium equivalent. This further illustrates that relying solely on phase diagram positioning is insufficient to ensure comprehensive performance; a carbon content of 0.05-0.08% and a chromium content of Cr must also be met. eq 19.2 ~ 20.0 and Ni eq Only a synergistic constraint of 13.5 to 13.9 can achieve simultaneous optimization of austenite stability and sensitization resistance.

[0033] Experimental results show that when the composition of 304 stainless steel is controlled within the following ranges: carbon mass percentage range of 0.05 ~ 0.08, chromium equivalent Cr eq The nickel equivalent is 19.2 ~ 20.0. eq When the austenite content is 13.5 to 13.9, this alloy material can obtain a single austenitic structure after solution treatment at 1150℃ for 2 hours followed by water quenching. Even after sensitization treatment at 650℃ for 1 hour, it still maintains excellent resistance to intergranular corrosion, exhibiting excellent austenitic stability and resistance to sensitization (see [reference]). Figure 2 ~ 4 and Table 1). In the solution-treated state, it exhibits a single austenitic structure, while after sensitization treatment, it exhibits weaker intergranular corrosion.

[0034] Further analysis of the microstructure and properties of the various embodiments shown in Table 1 reveals that when the carbon content is below 0.05% (e.g., 0.017~0.034% in Examples 1 and 2), although the material exhibits some resistance to sensitization, the nickel equivalent is insufficient (Ni... eq ≤ 12.5), which leads to a decrease in austenite stability and makes the solid solution state prone to ferrite phase; while when the chromium equivalent is less than 19.2 (as in Examples 7 ~ 9, Cr eq When the carbon content reaches 0.084 (=17.6 ~ 18.9), the material still exhibits a significant tendency for grain boundary corrosion after sensitization, and its resistance to sensitization deteriorates significantly.

[0035] Based on the above experimental results, considering that the carbon content cannot exceed the standard carbon content upper limit of 0.08, the 304 stainless steel with both sensitization resistance and high austenitic stability should meet the preferred carbon content range of 0.05 ~ 0.07, chromium equivalent of 19.2 ~ 20.0, and nickel equivalent of 13.5 ~ 13.9 (as in Examples 4 ~ 6). The alloy exhibits a single austenitic structure in the solid solution state, and can still effectively inhibit intergranular corrosion after sensitization treatment at 650℃ / 1h, demonstrating the synergistic optimization of austenitic stability and sensitization resistance.

[0036] Considering the difficulty in precisely controlling the actual industrial alloy composition and related equivalents, and that slightly higher chromium and nickel equivalents are beneficial (their increase is only limited by cost), it is recommended that the carbon content of industrial 304 stainless steel be 0.05~0.08, with the equivalent range extended to 19~21 for chromium and 13~15 for nickel. It is worth noting that in traditional 304 stainless steel composition design, it is generally believed that increasing the carbon content exacerbates sensitization, while decreasing the carbon content is beneficial for resistance to sensitization; simultaneously, austenitic stability mainly depends on nickel content control. However, this invention has found that, under specific chromium and nickel equivalent matching conditions, moderately increasing the carbon content to above 0.05 not only does not lead to worsening of sensitization, but also synergistically achieves a simultaneous improvement in resistance to sensitization and austenitic stability in conjunction with high chromium equivalents—a phenomenon that breaks through conventional understanding and cannot be achieved by simply adjusting the content of a single element. This indicates that the composition window has nonlinear synergistic characteristics, and its technical effects are difficult for those skilled in the art to directly predict based on existing knowledge.

[0037] The embodiments described above are merely illustrative of the implementation methods of the present invention, but should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the protection scope of the present invention.

Claims

1. A 304 stainless steel possessing both excellent resistance to sensitization and austenitic stability, characterized in that, Its composition by mass percentage is as follows: Carbon content: 0.05 ~ 0.08 Chromium equivalent: 19 ~ 21 Nickel equivalent: 13 ~ 15 The balance consists of iron and unavoidable impurities.

2. The 304 stainless steel according to claim 1, characterized in that, The carbon content is 0.05 to 0.07, the chromium equivalent is 19.2 to 20.0, and the nickel equivalent is 13.5 to 13.

9.

3. The application of the 304 stainless steel as described in claim 1 or 2 in the manufacture of structural components for use in a medium-temperature corrosive environment, wherein the medium-temperature corrosive environment is a temperature of 400°C to 800°C and is an acidic medium containing sulfur or halogens.

4. The application according to claim 3, characterized in that, The structural component is a bipolar plate or end plate of a proton exchange membrane fuel cell, and the working environment is a fluorinated dilute sulfuric acid solution at 60℃ to 90℃.

5. The application according to claim 3, characterized in that, The structural components can be put into service directly after welding or hot working without post-weld solution treatment.

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