A 304l stainless steel with low ferrite content and a method for producing the same

Abstract

The present application relates to a kind of low ferrite content 304L stainless steel and its preparation method, belong to stainless steel material technical field, solve the technical problem that the low temperature toughness of insufficient due to ferrite content being too high in existing 304L stainless steel.The 304L stainless steel, chemical composition is by mass percentage, including the following components: C:0.017-0.023%, Si≤0.50%, Mn≤1.50%, Cr 18.23-18.38%, Ni 8.68-11.70%, N 0.014-0.090%, P≤0.030%, S≤0.005%, the balance is Fe and unavoidable impurities.The ferrite content of 304L stainless steel prepared by the present application is 0.18%-1.60%, impact energy reaches 150J above at-196 ℃, effectively solve the technical problem that the low temperature toughness of insufficient due to ferrite content being too high in existing 304L stainless steel.

Description

Technical Field

[0001] This invention relates to the field of stainless steel materials technology, and in particular to a 304L stainless steel with low ferrite content and its preparation method. Background Technology

[0002] With the widespread application of clean energy, the demand for storage and transportation of cryogenic liquefied gases such as liquefied natural gas (LNG), liquid hydrogen, and liquid helium is increasing. 300 series austenitic stainless steel is widely used in the manufacture of cryogenic pressure vessels due to its good corrosion resistance, processing performance, and relatively economical cost. Among them, 304L austenitic stainless steel, with its low carbon content (≤0.03%), exhibits good resistance to intergranular corrosion and a certain degree of toughness at low temperatures, accounting for more than 80% of the total usage of austenitic stainless steel.

[0003] However, during solidification or hot working, 304L austenitic stainless steel is prone to the formation of a small amount of body-centered cubic ferrite phase due to improper alloy composition (high chromium-nickel equivalent ratio) or process control. This ferrite phase not only significantly impairs the low-temperature toughness of austenitic stainless steel but also fails to contribute strength at low temperatures, making it a harmful phase that needs to be restricted in austenitic stainless steel used in low-temperature environments. In existing technologies, ferrite is typically suppressed by increasing the Ni content (an austenite-forming element), but Ni is a precious metal and expensive. In recent years, nitrogen (N) has gained attention as a strong austenite-forming element, and replacing Ni with N has become an important direction for cost reduction; however, excessively high N content also leads to deterioration in hot working performance and increased costs. Furthermore, the amount of hot deformation (forging ratio, rolling ratio) also has a significant impact on the mechanical breakage and redissolution of ferrite, but existing technologies lack a clear technical solution for achieving precise control of ferrite content through the synergistic control of Ni and N content and hot deformation.

[0004] Therefore, how to control the ferrite content in 304L stainless steel to a low level that meets the requirements for low-temperature toughness through reasonable composition design and hot deformation process while ensuring economic efficiency is an urgent technical problem to be solved. Summary of the Invention

[0005] Based on the above analysis, the present invention aims to provide a 304L stainless steel with low ferrite content and its preparation method, in order to solve the technical problem of insufficient low-temperature toughness caused by excessive ferrite content in existing 304L stainless steel.

[0006] The objective of this invention is mainly achieved through the following technical solutions: This invention provides a 304L stainless steel with low ferrite content, the chemical composition of which, by mass percentage, includes the following components: C: 0.017-0.023%, Si≤0.50%, Mn≤1.50%, Cr 18.23-18.38%, Ni 8.68-11.70%, N0.014-0.090%, P≤0.030%, S≤0.005%, with the balance being Fe and unavoidable impurities.

[0007] Furthermore, the ferrite suppression equilibrium coefficient K of the 304L stainless steel satisfies: 0.518≤K≤0.634, where K=(Ni+24N) / Cr.

[0008] Furthermore, the Ni content and ferrite content in the 304L stainless steel satisfy the following nonlinear relationship: When the nitrogen content is 0.021-0.025%: In the undeformed as-cast state, the ferrite content (%) = -0.25×Ni² + 3.46×Ni - 3.96; When the hot deformation ratio is 1-3, the ferrite content (%) = A1-A2×(1-e^(-Ni / A3))-A4×(1-e^(-Ni / A5)), where 22700≤A1≤22720, 10245≤A2≤10255, 0.032≤A3≤0.034, 12460≤A4≤12472, 1.03≤A5≤1.06; When the hot deformation ratio is greater than 3, the ferrite content (%) is calculated as B1 - B2 × Ni + B3 × Ni², where 40 ≤ B1 ≤ 50, 7 ≤ B2 ≤ 8.5, and 0.27 ≤ B3 ≤ 0.40. Wherein, Ni represents the mass percentage of Ni.

[0009] Furthermore, the N content and ferrite content in the 304L stainless steel satisfy the following nonlinear relationship: When the Ni content is 9.18-9.37%: In the undeformed as-cast state, the ferrite content (%) = 8.00 - 66.13 × N + 41.17 × N²; When the heat distortion ratio is 1-3, the ferrite content (%) = C1×e^(-N / C2)+C3, where 4.1≤C1≤4.3, 0.025≤C2≤0.029, and 4.60×10 -4 ≤C3≤5.00×10 -4 ; When the heat distortion ratio is greater than 3, the ferrite content (%) is calculated as D1×e^(-N / D2)-D3, where 3.50≤D1≤3.90, 0.026≤D2≤0.029, and 0≤D3≤0.20. Where N is the mass percentage of N.

[0010] This invention also provides a method for preparing 304L stainless steel with low ferrite content, which includes the following steps: S1: Prepare raw materials according to the composition requirements of the 304L stainless steel, and melt the raw materials to obtain steel ingots; S2: Forging and hot rolling of steel ingots to obtain hot-rolled plates; S3: Hot-rolled sheet metal is solution treated to obtain 304L stainless steel.

[0011] Furthermore, in step S1, vacuum induction melting is employed, with the vacuum level controlled at 1.0 × 10⁻⁶. - Below ³Pa.

[0012] Furthermore, in step S2, the cumulative total hot deformation ratio of the forging and hot rolling is ≥3.

[0013] Furthermore, in step S2, the forging start temperature is 1100℃-1180℃, and the final forging temperature is ≥900℃.

[0014] Furthermore, in step S2, the hot rolling heating temperature is 1100℃-1150℃, and the final rolling temperature is ≥900℃.

[0015] Furthermore, in step S3, the solution temperature is 1030℃-1080℃, and the holding time is 30-60min.

[0016] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects: 1. This invention controls the content of elements such as C, Cr, Ni, and N within a specific range. Utilizing the characteristic that N's austenite stabilization ability is approximately 24 times that of Ni, a portion of the high-valence Ni is replaced with low-valence N, effectively reducing raw material costs while ensuring the stability of the austenite structure. Simultaneously, precise control of the Cr content avoids excessive Cr inducing ferrite precipitation. Combined with the synergistic effect of elements such as Mn, Si, P, and S, the formation of harmful ferrite phases is suppressed from the source of composition design. The 304L stainless steel prepared by this invention can have its ferrite content controlled between 0.18% and 1.60%, achieving an impact energy of over 150J at -196℃, effectively solving the technical problem of insufficient low-temperature toughness caused by excessive ferrite content in existing 304L stainless steel.

[0017] 2. This invention achieves optimal equivalent matching between Ni, N and Cr elements by constructing a ferrite suppression balance coefficient K=(Ni+24N) / Cr and optimizing the range of K values. This avoids insufficient austenite stabilization ability and weakened ferrite suppression effect due to an excessively small K value, or relatively high N content, intensified work hardening and nitride precipitation due to an excessively large K value.

[0018] 3. This invention controls the hot deformation ratio to ≥3 and combines it with a solution treatment at 1030℃-1080℃ to mechanically break down and fully dissolve the dendritic segregation and primary ferrite in the as-cast structure, thereby obtaining a single austenitic structure with clear grain boundaries and uniform size, thus avoiding the damage to the low-temperature toughness of ferrite.

[0019] 4. This invention provides a nonlinear quantitative relationship between Ni and N content, heat distortion ratio and ferrite content, and gives multiple sets of fitting formulas. The combination of Ni and N content and heat distortion ratio can be flexibly selected to meet the target ferrite content and low temperature toughness requirements, and has good industrial applicability.

[0020] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0021] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0022] Figure 1 is a metallographic image of the microstructure of the 304L stainless steel prepared in Example 1; Figure 2 Metallographic image of the microstructure of 304L stainless steel prepared in Comparative Example 2. Figure 3 The graph shows the effect of Ni content and hot deformation ratio on ferrite content. Figure 4 The graph shows the effect of N content and heat distortion ratio on ferrite content. Detailed Implementation

[0023] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0024] This invention provides a 304L stainless steel with low ferrite content, whose chemical composition, by mass percentage, includes the following components: C: 0.017-0.023%, Si≤0.50%, Mn≤1.50%, Cr 18.23-18.38%, Ni 8.68-11.70%, N 0.014-0.090%, P≤0.030%, S≤0.005%, with the balance being Fe and unavoidable impurities.

[0025] The reasons for limiting the ingot composition of the 304L stainless steel and its preparation method described above in this invention will be explained. Hereinafter, only the percentage of mass in the composition will be used.

[0026] C: As an austenite stabilizing element, C can effectively expand the austenite phase region and inhibit the precipitation of ferrite phase. However, when the C content is too low, the austenite stability is insufficient, and the ferrite inhibition effect is weakened; when the C content is too high, it easily combines with Cr to form Cr. 23 C6 precipitation reduces resistance to intergranular corrosion and exacerbates grain boundary embrittlement, impairing low-temperature toughness. Therefore, this invention controls the C content to 0.017%-0.023%.

[0027] Si: Si can help stabilize the austenitic structure, improve the fluidity of molten steel, and optimize metallurgical quality. However, excessive Si content (>0.50%) will promote ferrite formation, increase the ferrite content in the microstructure, and impair low-temperature toughness. Therefore, this invention controls the Si content to ≤0.50%.

[0028] Mn: Mn is an auxiliary element for austenite stability, helping to expand the austenite phase region, effectively inhibiting ferrite precipitation, and enhancing the stability of the austenite structure. Simultaneously, Mn can deoxidize and desulfurize, reducing the number of harmful inclusions in the material. It can also slightly improve material strength through solid solution strengthening and enhance cold and hot working properties, ensuring the overall service life of the material. However, when the Mn content is too high (>1.50%), it exacerbates central segregation, forming banded structures and reducing the material's low-temperature toughness and corrosion resistance. Furthermore, excessive Mn increases the tendency for quenched martensite formation, adversely affecting austenite stability. Therefore, this invention controls the Mn content to ≤1.50%.

[0029] Cr: As a ferrite-forming element, precise control of Cr content can effectively balance the phase composition in austenitic stainless steel, preventing excessive Cr from inducing ferrite precipitation and ensuring the stability of the austenitic structure. When the Cr content is too low, austenite stability decreases; when the Cr content is too high, the tendency for ferrite formation increases significantly, and harmful δ-ferrite phases may appear in the microstructure, severely impairing low-temperature toughness and impact performance. Therefore, this invention precisely controls the Cr content at 18.23%-18.38%.

[0030] Ni: Ni is a core austenite-forming element that effectively expands and stabilizes the austenite phase region, inhibits ferrite precipitation and the formation of harmful phases, and avoids low-temperature brittleness and performance deterioration. Simultaneously, it increases stacking fault energy, inhibits deformation-induced martensitic transformation, significantly improves room-temperature and low-temperature toughness, reduces work hardening tendency, and improves cold-working formability. However, Ni resources are scarce and costly. This invention, through the synergistic design of Ni and N, achieves the same austenite stabilization effect at lower Ni contents, balancing material performance and manufacturing economy. However, when the Ni content is too low, austenite stability is insufficient, and martensitic transformation easily occurs at low temperatures, leading to a sharp decrease in toughness. When the Ni content is too high, although austenite stability is improved, it significantly increases alloy cost, and excessive Ni reduces the solid solubility of N, weakening the strengthening effect of N. Therefore, this invention controls the Ni content at 8.68%-11.70%.

[0031] Nitrogen (N) is a strong austenite-forming element that effectively expands the austenite phase region, significantly inhibits ferrite precipitation, greatly improves the stability of the austenite microstructure, and simultaneously hinders martensitic transformation. Furthermore, the interstitial solid solution strengthening effect of N can enhance material strength. N can replace some Ni, balancing mechanical properties and production costs while maintaining a stable austenite microstructure. However, if the N content is too low, austenite stability is insufficient, and the ferrite inhibition effect is weakened; if the N content is too high, it easily leads to increased work hardening and nitride precipitation, impairing hot working properties. Therefore, this invention controls the N content between 0.014% and 0.090%. This content range avoids Cr₂N nitride precipitation, ensuring a healthy matrix microstructure.

[0032] P: When the phosphorus (P) content is too high, P tends to segregate at grain boundaries, reducing grain boundary bonding and causing the material to exhibit brittle fracture at low temperatures, severely impairing its low-temperature impact toughness. Strictly limiting the P content can effectively prevent microstructure segregation, inhibit abnormal ferrite precipitation, ensure the uniformity and stability of the austenitic microstructure, and reduce grain boundary embrittlement defects. This invention controls the P content in 304L austenitic stainless steel to ≤0.030%.

[0033] S: When the sulfur content is too high, plastic sulfide inclusions (such as MnS) will form. These inclusions are stretched along the deformation direction during hot working, becoming microcrack initiation sources, and significantly reducing the impact toughness and elongation of the material under low-temperature loading. Strictly controlling the sulfur content can effectively reduce the precipitation of harmful inclusions, reduce microstructure segregation, inhibit ferrite phase precipitation, ensure the stability of the austenitic structure, and improve the overall metallurgical quality of the material. This invention controls the sulfur content in 304L austenitic stainless steel to ≤0.005%.

[0034] Preferably, the ferrite suppression equilibrium coefficient K, K=(Ni+24N) / Cr], is controlled within the range of 0.518-0.634. Ni and N, as austenite-forming elements, have a synergistic effect in suppressing ferrite precipitation, with N's ability to stabilize the austenite phase being approximately 24 times that of Ni. Based on this, the ferrite suppression equilibrium coefficient K=(Ni+24N) / Cr is defined to characterize the equivalent balance relationship between austenite-forming elements (Ni, N) and ferrite-forming element (Cr). When the K value is too low, the austenite stabilization ability is insufficient, failing to effectively suppress the precipitation of the ferrite phase, leading to deterioration of low-temperature toughness; when the K value is too high, the corresponding N content is too high, easily causing intensified work hardening and nitride precipitation, thus damaging hot working properties. Through regression analysis of experimental data, this invention controls the K value within the range of 0.518-0.634. Within this range, the austenitic structure exhibits optimal stability, achieving optimal elemental matching while effectively suppressing ferrite precipitation, ensuring excellent low-temperature toughness, and avoiding problems such as deterioration in processing performance. The ferrite content can be controlled below 3%, while the impact energy at -196℃ remains above 150J. More preferably, when the Ni content is controlled at 9.18%-9.37% and the N content at 0.075%-0.090%, the K value is in the range of 0.597-0.634, and the ferrite content can be stably controlled below 1%.

[0035] The present invention also provides a method for preparing the above-mentioned 304L stainless steel, which includes the following steps: S1: Prepare raw materials according to the composition requirements of the 304L stainless steel, and melt the raw materials to obtain steel ingots.

[0036] Specifically, vacuum induction melting can be used, with the vacuum level controlled at 1.0 × 10⁻⁶. - Below ³Pa. Vacuum degree below 1.0 × 10⁻⁶. - At a vacuum level of 3Pa, it is beneficial for the dissolution and uniform distribution of nitrogen, and can effectively remove gases such as O and H, as well as harmful impurities such as S from the molten steel, reducing the adverse effects of non-metallic inclusions on austenite nucleation and growth; if the vacuum level is higher than 1.0×10 - If the pressure is below 3Pa, the degassing effect is insufficient, the dissolution efficiency of N decreases, and the accuracy of composition control is reduced. Vacuum induction melting can precisely control the content of key austenite stabilizing elements such as N and C, ensuring uniform and stable composition of the molten steel. This lays a pure and controllable compositional foundation for the subsequent formation of a fully austenitic structure and the inhibition of ferrite precipitation.

[0037] S2: The steel ingot is forged and hot-rolled to obtain hot-rolled sheet.

[0038] Specifically, the steel ingot is heated to 1100℃-1180℃ (exemplary, 1100℃, 1120℃, 1130℃, 1140℃, 1150℃, 1160℃, 1170℃, 1180℃) for forging, and the final forging temperature is not lower than 900℃ (exemplary, 900℃, 910℃, 920℃, 930℃, 940℃, 950℃, 960℃, 970℃, 980℃, 990℃, 1000℃, 1010℃, 1020℃, 1030℃, 1040℃, 1050℃, 1060℃, 1070℃, 1080℃); subsequently, it is hot rolled at a heating temperature of 1100℃. -1150℃ (exemplary, 1100℃, 1120℃, 1130℃, 1140℃, 1150℃), final rolling temperature ≥900℃ (exemplary, 900℃, 910℃, 920℃, 930℃, 940℃, 950℃, 960℃, 970℃, 980℃, 990℃, 1000℃, 1010℃, 1020℃, 1030℃, 1040℃, 1050℃, 1060℃, 1070℃, 1080℃), cumulative total hot deformation ratio of forging and hot rolling ≥3 (exemplary, total hot deformation ratio is 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15).

[0039] During the forging heating process of steel ingots, as the temperature rises above 1100℃, the austenite phase is fully formed, and the coarse dendrites and segregation in the as-cast structure are gradually eliminated. Alloying elements diffuse and homogenize at high temperatures. When the heating temperature is below 1100℃, the core of the steel ingot lacks sufficient heat penetration, resulting in high deformation resistance and a tendency to forging cracks. When the heating temperature is above 1180℃, the austenite grains grow rapidly, and local overheating may occur at the grain boundaries, deteriorating the hot working properties of the material. A final forging temperature of not less than 900℃ ensures that the final forging is still within the austenite recrystallization zone, avoiding final forging within the ferrite precipitation temperature range and preventing abnormal microstructure caused by mixed grains or ferrite precipitation.

[0040] Following hot rolling, if the initial rolling temperature is below 1100℃, the billet's deformation resistance increases, and the temperature drop between passes is too rapid. In the later stages of hot rolling, it may enter the two-phase region, leading to mixed grain or ferrite precipitation. If the initial rolling temperature is above 1150℃, the austenite grains significantly coarsen during heating, weakening the fine-grain strengthening effect. If the final rolling temperature is below 900℃, hot rolling deformation occurs within the austenite-to-ferrite transformation range, easily forming a ferrite / austenite mixed-phase structure, impairing low-temperature toughness. A final rolling temperature not lower than 900℃ ensures that deformation is always completed within the austenite recrystallization region, resulting in complete dynamic recrystallization and a fine-grained, uniform austenite structure. Hot rolling in the 1100~1150℃ range, with a final rolling temperature not lower than 900℃, ensures that the rolling process remains within the austenite single-phase region, avoiding ferrite precipitation induced by residence in the two-phase region. This process can further refine the grains, achieve a uniform distribution of austenite structure, improve austenite stability, and provide good preconditions for subsequent solid solution treatment to eliminate residual ferrite.

[0041] When the cumulative total hot deformation ratio of forging and hot rolling is ≥3, the coarse austenite grains in the as-cast structure are fully broken up, the interdendritic segregation is dispersed, and the as-cast δ-ferrite undergoes mechanical breakage. At the same time, under the action of dynamic recrystallization, fine equiaxed austenite grains are gradually formed, and the uniformity and density of the structure are significantly improved. If the total hot deformation ratio is less than 3, the as-cast structure is not fully broken up, dendritic segregation remains, the uniformity of the structure is insufficient, the grains are coarse, and the ferrite is difficult to eliminate, which ultimately affects the low-temperature toughness and impact performance of the material.

[0042] S3: Hot-rolled sheet metal is solution treated to obtain 304L stainless steel.

[0043] Specifically, the solution temperature is 1030℃-1080℃ (exemplary values: 1030℃, 1035℃, 1040℃, 1045℃, 1050℃, 1055℃, 1060℃, 1065℃, 1070℃, 1075℃, 1080℃), and the holding time is 30-60 min (exemplary values: 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min). After holding, the solution is rapidly cooled to room temperature. When the solution temperature is below 1030℃, the residual ferrite phase cannot be fully dissolved, and the carbonitrides and σ phase cannot be completely dissolved. The second-phase particles become crack initiation sources at low temperatures. When the solution temperature is above 1080℃, the austenite grains are significantly coarsened, the fine-grain strengthening effect is weakened, and local element depletion may occur at the grain boundaries. When the holding time is less than 30 minutes, the diffusion of alloying elements is insufficient, resulting in inadequate microstructure homogenization. When the holding time is greater than 60 minutes, abnormal grain growth occurs due to prolonged high-temperature holding, leading to excessive grain size and decreased impact toughness. Solution treatment within the temperature range of 1030–1080℃ allows for the complete re-dissolution of trace ferrite phases remaining during hot working, forming a uniformly composed, supersaturated austenite. Rapid water cooling after holding fixes the single-phase austenite structure at high temperature to room temperature, inhibiting the re-precipitation of ferrite phases during cooling, thereby obtaining a stable, single-phase austenite structure and significantly improving the material's low-temperature toughness.

[0044] It should be noted that the present invention can control the ferrite content in 304L stainless steel by adjusting the Ni content, N content, and heat distortion ratio. Specifically, under the composition system and preferred preparation process conditions defined by the present invention, there is a significant non-linear (curved) relationship between the Ni content, N content, heat distortion ratio, and ferrite content in 304L stainless steel.

[0045] When the N content is fixed at 0.021-0.025%, the ferrite content, Ni content, and hot deformation ratio satisfy the following: In the undeformed as-cast state (hot deformation ratio of 0), the ferrite content (%) = -0.25×Ni²+3.46×Ni-3.96; When the heat distortion ratio is 1-3, the ferrite content (%) = A1-A2×(1-e^(-Ni / A3))-A4×(1-e^(-Ni / A5)), where 22700≤A1≤22720, 10245≤A2≤10255, 0.032≤A3≤0.034, 12460≤A4≤12472, and 1.03≤A5≤1.06. For example, when the heat distortion ratio is 3, the ferrite content (%) = 22717.00-10250.57×(1-e^(-Ni / 0.033)) -12466.16×(1-e^(-Ni / 1.04)).

[0046] When the heat deformation ratio is greater than 3, the ferrite content (%) is calculated as B1 - B2 × Ni + B3 × Ni², where 40 ≤ B1 ≤ 50, 7 ≤ B2 ≤ 8.5, and 0.27 ≤ B3 ≤ 0.40. For example, when the heat deformation ratio is 13, the ferrite content (%) is calculated as 43.88 - 7.67 × Ni + 0.34 × Ni².

[0047] Based on the above rules, if the ferrite content is required to be controlled below 3%, a Ni content greater than 11.25% can be selected, or a Ni content greater than 8.76% with a hot deformation ratio of not less than 3 can be selected; if the ferrite content is required to be further controlled below 1%, a Ni content greater than 10.13% with a hot deformation ratio of not less than 3 can be selected, or a Ni content greater than 9.99% with a hot deformation ratio of not less than 13 can be selected. Figure 3 The graph shows the effect of Ni content and hot deformation ratio on ferrite content.

[0048] When the Ni content is fixed at 9.18-9.37%, the ferrite content, Ni content, and hot deformation ratio satisfy the following: In the undeformed as-cast state, the ferrite content (%) = 8.00 - 66.13 × N + 41.17 × N²; When the heat distortion ratio is 1-3, the ferrite content (%) = C1×e^(-N / C2)+C3, where 4.1≤C1≤4.3, 0.025≤C2≤0.029, and 4.60×10 -4 ≤C3≤5.00×10 -4 For example, when the heat distortion ratio is 3, the ferrite content (%) = 4.20 × e^(-N / 0.027) + 4.86 × 10^(-N / 0.027) -4 .

[0049] When the heat distortion ratio is greater than 3, the ferrite content (%) is calculated as D1×e^(-N / D2)-D3, where 3.50≤D1≤3.90, 0.026≤D2≤0.029, and 0≤D3≤0.20. For example, when the heat distortion ratio is 13, the ferrite content (%) is calculated as 3.58×e^(-N / 0.028)-0.10.

[0050] Based on the above principles, if the ferrite content is to be controlled below 3%, a hot deformation ratio of not less than 3 can be selected (in which case the N content can be selected within the full range of 0.014% to 0.090%), or an N content greater than 0.080% can be selected (in which case the hot deformation ratio can be less than 3, including the as-cast state). If the ferrite content is to be further controlled below 1%, then the following conditions must be met simultaneously: an N content greater than 0.039% and a hot deformation ratio of not less than 3, or an N content greater than 0.033% and a hot deformation ratio of not less than 13. Figure 4 The graph shows the effect of N content and heat distortion ratio on ferrite content.

[0051] A preferred embodiment of the present invention involves controlling the Ni content to 9.18-9.37%, the N content to 0.075-0.090%, and maintaining a hot deformation ratio of not less than 3. This approach can stably control the ferrite content below 1%, while avoiding excessively high Ni content, thus offering good economic efficiency. Based on the aforementioned nonlinear characteristics, those skilled in the art can flexibly select combinations within a wide range of composition and process windows according to actual process capabilities and cost requirements to achieve precise control of the ferrite content.

[0052] It should be noted that this invention achieves its effect through reasonable component design and precise control of process parameters during preparation, resulting in a synergistic effect between component design and preparation process. First, this invention achieves thermodynamic stability of the austenitic structure through the synergistic configuration of elements such as Cr, Ni, N, and Mn. N, as a strong austenite-forming element, works with Ni and Mn to expand the austenite phase region, suppress ferrite precipitation, and significantly improve the stability of the austenitic structure. Cr, as a ferrite-forming element, is precisely controlled to achieve a balanced phase composition, preventing ferrite precipitation and laying the structural foundation for low-temperature high toughness.

[0053] Secondly, the purity of the molten steel is controlled through vacuum induction melting during the smelting process. The vacuum environment can effectively remove harmful impurities such as O and S from the molten steel, reduce the number of non-metallic inclusions, avoid stress concentration caused by inclusions, prevent grain boundary embrittlement, and ensure that the material has good low-temperature impact toughness.

[0054] Furthermore, this invention achieves uniform and fine austenite grains through a suitable cumulative total heat deformation ratio (≥3) between forging and hot rolling, and solution treatment. When the cumulative total heat deformation ratio is ≥3, the coarse grains in the as-cast microstructure are mechanically broken up, and interdendritic segregation is dispersed, providing favorable conditions for subsequent recrystallization. Combined with solution treatment at 1030℃~1080℃, the austenite undergoes complete recrystallization, resulting in equiaxed austenite grains with clear grain boundaries and uniform size. Grain refinement improves low-temperature impact toughness through the Hall-Petch relationship, avoiding the increase in brittle transition temperature caused by coarse grains.

[0055] Finally, through precise composition design and rapid water cooling after solution treatment, alloying elements such as Cr, N, and Ni are fully dissolved in the austenitic matrix, suppressing the re-precipitation of ferrite during cooling. The material maintains a single, stable austenitic single-phase structure, avoiding damage to the relative toughness of ferrite, thus ensuring excellent low-temperature toughness while effectively controlling the ferrite content.

[0056] Compared with existing technologies, this invention replaces part of the precious Ni with an appropriate amount of N, thereby reducing costs while ensuring controllable ferrite content.

[0057] The microstructure of the 304L stainless steel prepared by this invention is as follows: The microstructure is a single, uniform austenitic phase, with most austenite grains being equiaxed, uniformly distributed in size, and with straight and continuous grain boundaries. There are no precipitates of second phases such as ferrite, carbonitrides, or intermetallic compounds. There are no obvious segregations, inclusions, or banded defects.

[0058] The 304L stainless steel prepared by this invention has a ferrite content of ≤2% (e.g., 0.18-1.6%) and an impact energy KV2 ≥150J (e.g., 150-188J) at -196℃.

[0059] The advantages of precise control of the elemental chemical composition, content, and preparation process parameters of the present invention will be demonstrated below with specific examples.

[0060] Example 1 This embodiment provides a 304L stainless steel, whose chemical composition by mass percentage includes the following components: C: 0.021%, Si: 0.42%, Mn: 1.14%, Cr: 18.3%, Ni: 9.26%, N: 0.081%, P: 0.027%, S: 0.0033%, with the balance being Fe and unavoidable impurities.

[0061] Its preparation method includes the following steps: S1: Prepare raw materials according to the composition requirements of the 304L stainless steel, and perform vacuum induction melting on the raw materials, controlling the vacuum degree at 1.0×10. - Steel ingots are obtained at pressures below ³Pa. S2: Forging and hot rolling of steel ingots to obtain hot-rolled plates; Specifically, the steel ingot is heated to 1150°C for forging and the final forging temperature is 960°C; then it is hot rolled at a heating temperature of 1120°C and a final rolling temperature of 960°C. The cumulative total thermal deformation ratio of forging and hot rolling is 9.

[0062] S3: The hot-rolled sheet is solution treated at a temperature of 1050℃ for 40 minutes. After the solution treatment, it is quickly cooled to room temperature to obtain 304L stainless steel.

[0063] In this embodiment, the ferrite suppression balance coefficient K=(Ni+24N) / Cr is 0.612, which satisfies 0.518≤K≤0.634.

[0064] The metallographic image of the microstructure of the prepared 304L stainless steel is shown below. Figure 1 As shown: a single austenitic grain structure without ferrite.

[0065] Example 2 This embodiment provides a 304L stainless steel, whose chemical composition by mass percentage includes the following components: C: 0.018%, Si: 0.35%, Mn: 0.95%, Cr: 18.25%, Ni: 10.80%, N: 0.025%, P: 0.022%, S: 0.0028%, with the balance being Fe and unavoidable impurities.

[0066] The preparation method is the same as in Example 1.

[0067] In this embodiment, the ferrite suppression balance coefficient K=(Ni+24N) / Cr is 0.625, which satisfies 0.518≤K≤0.634.

[0068] Example 3 This embodiment provides a 304L stainless steel, whose chemical composition by mass percentage includes the following components: C: 0.022%, Si: 0.48%, Mn: 1.30%, Cr: 18.35%, Ni: 10.00%, N: 0.050%, P: 0.025%, S: 0.0040%, with the balance being Fe and unavoidable impurities.

[0069] The preparation method is the same as in Example 1.

[0070] In this embodiment, the ferrite suppression balance coefficient K=(Ni+24N) / Cr is 0.610, which satisfies 0.518≤K≤0.634.

[0071] Example 4 This embodiment provides a 304L stainless steel with the same chemical composition as in Example 1.

[0072] The preparation method differs from that of Example 1 in that the total thermal distortion ratio is 13, while the rest is the same as that of Example 1.

[0073] In this embodiment, the ferrite suppression balance coefficient K=(Ni+24N) / Cr is 0.612, which satisfies 0.518≤K≤0.634.

[0074] Example 5 This embodiment provides a 304L stainless steel, whose chemical composition, by mass percentage, includes the following components: C: 0.020%, Si: 0.40%, Mn: 1.20%, Cr: 18.38%, Ni: 9.18%, N: 0.030%, P: 0.028%, S: 0.0035%, with the balance being Fe and unavoidable impurities.

[0075] The preparation method differs from that of Example 1 in that the total thermal distortion ratio is 6, while the rest is the same as that of Example 1.

[0076] In this embodiment, the ferrite suppression balance coefficient K=(Ni+24N) / Cr is 0.539, which satisfies 0.518≤K≤0.634.

[0077] Comparative Example 1 This comparative example provides a 304L stainless steel with the following chemical composition by mass percentage: C: 0.021%, Si: 0.42%, Mn: 1.14%, Cr: 18.30%, Ni: 8.26%, N: 0.008%, P: 0.027%, S: 0.0033%, with the balance being Fe and unavoidable impurities.

[0078] The preparation method is the same as in Example 1.

[0079] In this comparative example, the ferrite suppression equilibrium coefficient K=(Ni+24N) / Cr is 0.470, which does not satisfy 0.518≤K≤0.634.

[0080] Comparative Example 2 This comparative example provides a 304L stainless steel with the same chemical composition as Example 1 and a similar preparation method, except that: The preparation method differs from that of Example 1 in that the total thermal distortion ratio is 2, while the rest is the same as that of Example 1.

[0081] In this comparative example, the ferrite suppression equilibrium coefficient K=(Ni+24N) / Cr is 0.612, which satisfies 0.518≤K≤0.634, but the hot deformation ratio is only 2, which does not meet the requirement of ≥3 of this invention.

[0082] The metallographic image of the microstructure of the prepared 304L stainless steel is shown below. Figure 2 As shown: Contains residual ferrite.

[0083] Comparative Example 3 This comparative example provides a 304L stainless steel with the following chemical composition by mass percentage: C: 0.021%, Si: 0.42%, Mn: 1.14%, Cr: 18.30%, Ni: 9.26%, N: 0.008%, P: 0.027%, S: 0.0033%, with the balance being Fe and unavoidable impurities.

[0084] The preparation method is the same as in Example 1.

[0085] In this comparative example, the ferrite suppression equilibrium coefficient K=(Ni+24N) / Cr is 0.516, which does not satisfy 0.518≤K≤0.634.

[0086] Comparative Example 4 This comparative example provides a 304L stainless steel with the following chemical composition by mass percentage: C: 0.021%, Si: 0.42%, Mn: 1.14%, Cr: 18.30%, Ni: 8.70%, N: 0.01%, P: 0.027%, S: 0.0033%, with the balance being Fe and unavoidable impurities.

[0087] The preparation method is the same as in Example 1.

[0088] In this comparative example, the ferrite suppression equilibrium coefficient K=(Ni+24N) / Cr is 0.489, which does not satisfy 0.518≤K≤0.634.

[0089] Comparative Example 5 This comparative example provides a 304L stainless steel with the same chemical composition as in Example 1.

[0090] The preparation method is similar to that of Example 1, except that the heat distortion ratio is 2, the solution temperature is 1000℃, the holding time is 30min, and the rest is the same as that of Example 1.

[0091] The preparation method is the same as in Example 1, except that the solution temperature is reduced to 1000℃, the holding time is 30min, and water cooling is used.

[0092] In this comparative example, the ferrite suppression equilibrium coefficient K=(Ni+24N) / Cr is 0.612, which satisfies 0.518≤K≤0.634. However, the solution treatment temperature is 1000℃ (lower than the lower limit of 1030℃), and the solution treatment parameters deviate from the scope of this invention.

[0093] Comparative Example 6 This comparative example provides a 304L stainless steel with the following chemical composition by mass percentage: C: 0.032%, Si: 0.65%, Mn: 1.14%, Cr: 18.30%, Ni: 9.26%, N: 0.081%, P: 0.027%, S: 0.0033%, with the balance being Fe and unavoidable impurities.

[0094] The preparation method is the same as in Example 1.

[0095] In this comparative example, the ferrite suppression equilibrium coefficient K=(Ni+24N) / Cr is (9.26+1.944) / 18.30=0.612, which satisfies 0.518≤K≤0.634. However, the C content is 0.032% (exceeding the upper limit of 0.023% of this invention) and the Si content is 0.65% (exceeding the upper limit of 0.50% of this invention), both of which deviate from the scope of this invention.

[0096] The chemical compositions of each embodiment and comparative example are shown in Table 1, and the ferrite content and -196℃ impact energy test results are shown in Table 2. Ferrite content was measured using metallographic methods (statistical by area percentage) or magnetic methods. The test methods are as follows: After polishing, the metallographic sample was exposed using a suitable etching agent (such as ferric chloride hydrochloric acid solution). Ten fields of view were randomly selected under a microscope to statistically analyze the ferrite area fraction, and the average value was taken. The -196℃ impact energy was determined by Charpy V-notch impact test according to GB / T 229. Table 1 Chemical composition (wt, %) of 304L stainless steel in the examples and comparative examples

[0097] Table 2 Ferrite content and -196°C impact energy of 304L stainless steel in the examples and comparative examples

[0098] As can be seen from Table 2, the ferrite content of Examples 1-5 was controlled below 3%, and that of Examples 1-4 was controlled below 1%. Their impact energy at -196℃ all reached above 150J (Examples 1-4 reached above 163J), demonstrating excellent low-temperature toughness.

[0099] In the 304L stainless steel prepared in Comparative Example 1, the ferrite content was 2.20%, and the impact energy was only 99J. The Ni and N contents of this comparative example were both below the lower limit of this invention, and the K value deviated significantly from the range of 0.518-0.634, indicating a severely insufficient austenite stabilization capacity. Although the ferrite content did not exceed 3%, the impact energy had already deteriorated significantly. This demonstrates that when both Ni and N are insufficient, even if the ferrite content is still acceptable, the low-temperature toughness is severely compromised.

[0100] In the 304L stainless steel prepared in Comparative Example 2, the ferrite content was 2.70%, and the impact energy was 136 J. The chemical composition of this comparative example steel meets the requirements of this invention, but the hot deformation ratio is low, which does not meet the requirements of this invention. The as-cast structure was not fully broken down, and there was a lot of ferrite residue. The impact energy decreased significantly, indicating that excellent low-temperature toughness cannot be obtained when the composition is qualified but the hot deformation ratio is insufficient.

[0101] In the 304L stainless steel prepared in Comparative Example 3, the ferrite content was 3.60%, and the impact energy was 129 J. The nitrogen content was below the lower limit of this invention, the K value did not meet the requirement of 0.518-0.634, and the ferrite content exceeded 3%, resulting in deteriorated low-temperature toughness. This indicates that insufficient nitrogen content weakens the austenite stabilizing ability and reduces the ferrite suppression effect.

[0102] In the 304L stainless steel prepared in Comparative Example 4, the ferrite content was 4.20% and the impact energy was 112 J. The Ni and N contents were both lower than the lower limit of this invention, and the K value deviated significantly from the range of 0.518-0.634. The ferrite content was the highest and the impact energy was the lowest, indicating that when the N content is insufficient, even if the Ni content barely meets the requirements, the austenite stability is still severely lacking.

[0103] In the 304L stainless steel prepared in Comparative Example 5, the ferrite content was 3.40%, and the impact energy was 115 J. The chemical composition of this comparative example steel meets the requirements of this invention, but the solution treatment temperature is below the lower limit of 1030℃, and the residual ferrite cannot be fully dissolved. At the same time, the grain boundary carbonitrides cannot be completely dissolved, resulting in a significant decrease in impact energy. This indicates that when the solution treatment parameters deviate from the range of this invention, even if the composition is qualified, excellent performance cannot be obtained.

[0104] In Comparative Example 6, the 304L stainless steel had a ferrite content of 3.50% and an impact energy of 100J. This comparative example had a C content exceeding the upper limit of this invention (>0.023%) and a Si content exceeding the upper limit of this invention (>0.50%). Although the K value met the range of 0.518-0.634, the excessively high C content led to increased precipitation of grain boundary carbides and grain boundary embrittlement, while the excessively high Si content promoted ferrite formation. The combined effect of these two factors severely deteriorated the low-temperature toughness. This comparative example illustrates that even if the K value meets the preferred range, deviations from other elements can still lead to performance degradation; all elements must be synergistically controlled within the limits defined by this invention.

[0105] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A 304L stainless steel with low ferrite content, characterized in that, The chemical composition, by mass percentage, includes the following components: C: 0.017-0.023%, Si≤0.50%, Mn≤1.50%, Cr 18.23-18.38%, Ni 8.68-11.70%, N 0.014-0.090%, P≤0.030%, S≤0.005%, with the balance being Fe and unavoidable impurities.

2. The 304L stainless steel according to claim 1, characterized in that, The ferrite suppression equilibrium coefficient K of the 304L stainless steel satisfies: 0.518≤K≤0.634, where K=(Ni+24N) / Cr.

3. The 304L stainless steel according to claim 1, characterized in that, The Ni content and ferrite content in the 304L stainless steel satisfy the following nonlinear relationship: When the nitrogen content is 0.021-0.025%: In the undeformed as-cast state, the ferrite content (%) = -0.25×Ni² + 3.46×Ni - 3.96; When the hot deformation ratio is 1-3, the ferrite content (%) = A1-A2×(1-e^(-Ni / A3))-A4×(1-e^(-Ni / A5)), where 22700≤A1≤22720, 10245≤A2≤10255, 0.032≤A3≤0.034, 12460≤A4≤12472, 1.03≤A5≤1.06; When the hot deformation ratio is greater than 3, the ferrite content (%) is calculated as B1 - B2 × Ni + B3 × Ni², where 40 ≤ B1 ≤ 50, 7 ≤ B2 ≤ 8.5, and 0.27 ≤ B3 ≤ 0.

40. Wherein, Ni represents the mass percentage of Ni.

4. The 304L stainless steel according to claim 1, characterized in that, The nitrogen content and ferrite content in the 304L stainless steel satisfy the following nonlinear relationship: When the Ni content is 9.18-9.37%: In the undeformed as-cast state, the ferrite content (%) = 8.00 - 66.13 × N + 41.17 × N²; When the hot deformation ratio is 1-3, the ferrite content (%) = C1 x e^(-N / C2) + C3, wherein 4.1 ≤ C1 ≤ 4.3, 0.025 ≤ C2 ≤ 0.029, 4.60 x 10 -4 ≤ C3 ≤ 5.00 x 10 -4 ; When the heat distortion ratio is greater than 3, the ferrite content (%) is calculated as D1×e^(-N / D2)-D3, where 3.50≤D1≤3.90, 0.026≤D2≤0.029, and 0≤D3≤0.

20. Where N is the mass percentage of N.

5. A method for preparing 304L stainless steel with low ferrite content, characterized in that, Used to prepare the 304L stainless steel according to any one of claims 1-4 Includes the following steps: S1: Prepare raw materials according to the composition requirements of the 304L stainless steel, and melt the raw materials to obtain steel ingots; S2: Forging and hot rolling of steel ingots to obtain hot-rolled plates; S3: Hot-rolled sheet metal is solution treated to obtain 304L stainless steel.

6. The preparation method according to claim 5, characterized in that, In step S1, vacuum induction melting is used, and the vacuum degree is controlled to be 1.0 x 10 - ³ Pa or lower.

7. The preparation method according to claim 5, characterized in that, In step S2, the cumulative total hot deformation ratio of the forging and hot rolling is ≥3.

8. The preparation method according to claim 5, characterized in that, In step S2, the initial forging temperature is 1100℃-1180℃, and the final forging temperature is ≥900℃.

9. The preparation method according to claim 5, characterized in that, In step S2, the hot rolling heating temperature is 1100℃-1150℃, and the final rolling temperature is ≥900℃.

10. The preparation method according to claim 5, characterized in that, In step S3, the solution temperature is 1030℃-1080℃, and the holding time is 30-60min.

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