Ferritic stainless steel with ultra-low carbon content, its preparation process and applications

Abstract

This invention discloses a ferritic stainless steel with ultra-low carbon content, its preparation process, and its applications, relating to the field of stainless steel technology. The composition of the ultra-low carbon content ferritic stainless steel is: C≤0.015%, N≤0.015%, S≤0.010%, P≤0.025%, Mn≤0.6%, Si 0.2~1.0%, Cr 9.0~30.0%, Mo 0.50~8.0%, Cu 0.05~2.0%, Sn 0.05~0.8%, Ni 0.05~5.0%, stabilizing elements 0.2~3.2%, Co 0~0.30%, with the balance being Fe and unavoidable impurities. The material obtained by the technical solution of this invention has a pure ferritic stainless steel microstructure, exhibiting superior corrosion resistance compared to low-alloy 09Cr2AlMoRE and 08Cr2AlMo, and a significantly higher cost-performance ratio than duplex stainless steel.

Description

Technical Field

[0001] This invention relates to the field of stainless steel technology, and in particular to a ferritic stainless steel with ultra-low carbon content, its preparation process, and its applications. Background Technology

[0002] The stainless steel materials widely used in petrochemical wet hydrogen sulfide corrosive environments are low-alloy steels with grades 09Cr2AlMoRE and 08Cr2AlMo. When the wet hydrogen sulfide corrosive environment contains other complex corrosive media such as trace amounts of chloride ions and microorganisms, the service life of 09Cr2AlMoRE and 08Cr2AlMo low-alloy steels will be significantly shortened.

[0003] While conventional ferritic stainless steels such as 0Cr13 (410) and 0Cr17 (430) possess inherent advantages in resistance to stress corrosion, their poor weldability due to martensite residue in the heat-affected zone means they are generally not used as materials for heat exchanger tubes and tube sheets in heat exchangers and air coolers. Austenitic-ferritic duplex stainless steel is the best choice for resistance to stress corrosion and uniform corrosion, but it is expensive and has a low cost-performance ratio.

[0004] T91 offers better value for money than austenitic-ferritic duplex stainless steel. It contains 0.07~0.14wt% carbon, 8.0~9.5wt% Cr, and approximately 1wt% Mo, and does not contain the expensive metal Ni. Compared to conventional stainless steel, T91 has a lower content of alloying elements. After normalizing and tempering, this steel exhibits a tempered sorbite or tempered martensite microstructure (see attached). Figure 1 (As shown). In ASME's SA-213 / SA-213M standard "Seamless Ferritic and Austenitic Alloy Steel Boiler, Superheater and Heat Exchanger Tubes", T91 specifies a tensile strength ≥585MPa and a yield strength ≥415MPa.

[0005] Although the T91 standard sets the upper limit for the sum of Cr and Mo content at 10.5%, the actual Cr and Mo content in the material is below 10.5%. Furthermore, due to the high carbon content, a dense stainless steel passivation film of Cr and Mo does not form on the metal surface, affecting the material's corrosion resistance. In addition, because T91 is a martensitic heat-resistant steel, although it has high strength, its weldability is poor, making it unsuitable for use in stress corrosion and intergranular corrosion environments. Summary of the Invention

[0006] This invention aims to solve at least one of the technical problems existing in the prior art. To this end, this invention proposes a ferritic stainless steel with ultra-low carbon content, its preparation process, and its applications.

[0007] This invention proposes a ferritic stainless steel with ultra-low carbon content. Its composition, by weight percentage, is as follows: C (carbon) ≤ 0.015 wt%, N (nitrogen) ≤ 0.015 wt%, S (sulfur) ≤ 0.010 wt%, P (phosphorus) ≤ 0.025 wt%, Mn (manganese) ≤ 0.6 wt%, Si (silicon) 0.2~1.0 wt%, Cr (chromium) 9.0~30.0 wt%, Mo (molybdenum) 0.50~8.0 wt%, Cu (copper) 0.05~2.0 wt%, Sn (tin) 0.05~0.8 wt%, Ni (nickel) 0.05~5.0 wt%, stabilizing elements 0.2~3.2 wt%, Co (cobalt) 0~0.30 wt%, with the balance being Fe (iron) and unavoidable impurities.

[0008] The stabilizing elements include W (tungsten), Zr (zirconium), Ti (titanium), V (vanadium), Nb (niobium), and Al (aluminum).

[0009] Optionally, the sum of the contents of C and N is ≤0.02wt%.

[0010] Optionally, the sum of the contents of Cr and Mo is 10.5 to 35.0 wt%.

[0011] Optionally, the stabilizing elements include W (0.05-0.8 wt%), Zr and Ti (0.05-0.8 wt%), V and Nb (0.05-0.8 wt%), and Al (0.05-0.8 wt%).

[0012] Preferably, the ferritic stainless steel with ultra-low carbon content has the following composition by weight percentage: C (carbon) 0.005~0.01wt%, N (nitrogen) 0.008~0.014wt%, S (sulfur) ≤0.005wt%, P (phosphorus) ≤0.020wt%, Mn (manganese) 0.3~0.5wt%, Si (silicon) 0.2~1.0wt%, Cr (chromium) 9.0~22.5wt%, Mo (molybdenum) 0.50~8.0wt%, Cu (copper) 0.4~2.0wt%. %, Sn (tin) 0.05~0.8wt%, Ni (nickel) 0.1~1.2wt%, W (tungsten) 0.05~0.75wt%, Zr (zirconium) 0.03~0.25wt%, Ti (titanium) 0.02~0.47wt%, V (vanadium) 0.04~0.25wt%, Nb (niobium) 0.04~0.51wt%, Al (aluminum) 0.05~0.80wt%, Co (cobalt) 0.10~0.30wt%, with the balance being Fe (iron) and unavoidable impurities.

[0013] Optionally, the equivalent of Cr is Cr eq The equivalent of Ni is Ni eq And Creq with Ni eq The ratio is ≥7.0;

[0014] Among them, Cr eq =W(Cr)+W(Mo)+1.5W(Si)+0.75∑W(Xi);

[0015] Ni eq =W(Ni)+30[W(C)+W(N)]+0.5[W(Mn)+W(Co)]+0.25W(Cu);

[0016] In the formula, W(Cr) is the weight content of Cr, W(Mo) is the weight content of Mo, W(Si) is the weight content of Si, ∑W(Xi) is the sum of the weight contents of each element in the stabilizing element, W(Ni) is the weight content of Ni, W(C) is the weight content of C, W(N) is the weight content of N, W(Mn) is the weight content of Mn, W(Co) is the weight content of Co, and W(Cu) is the weight content of Cu.

[0017] Optionally, the ultra-low carbon content ferritic stainless steel further includes 0.02~0.08wt% RE (rare earth elements), wherein RE is selected from at least one element selected from La (lanthanum), Ce (cerium), Pr (praseodymium), Nd (neodymium), Y (yttrium) and Sc (scandium).

[0018] Preferably, the ultra-low carbon content ferritic stainless steel further includes 0.03 wt% La.

[0019] This invention also proposes a preparation process for ferritic stainless steel with ultra-low carbon content, comprising the following steps:

[0020] The raw material of the ultra-low carbon content ferritic stainless steel is smelted in an electric furnace or medium frequency furnace, and then subjected to AOD (argon-oxygen decarburization), VOD (vacuum oxygen decarburization), and LF (ladle refining furnace) refining to obtain casting liquid.

[0021] The casting liquid is poured, and the resulting billet is kept at 1000~1200℃ for more than 60 minutes, then forged into forgings, rolled into plates, or rolled / drawn into pipes.

[0022] The forgings, plates, or pipes are annealed at 700~950℃ for 1~3 minutes per millimeter of thickness. After being removed from the furnace, they are air-cooled to obtain the ultra-low carbon content ferritic stainless steel.

[0023] Optionally, obtaining the casting liquid includes the following steps:

[0024] The raw materials Fe, Cr, Ni, Mo, Mn, and Cu are smelted in an electric furnace. The slag basicity is controlled at 2.6~3.2, and the molten steel temperature is 1540~1590℃. Argon is blown from the bottom to stir the molten steel, and oxygen is blown from the top to dephosphorize, controlling the P content in the molten steel to below 0.035wt%. The temperature of the molten steel is adjusted to 1650~1700℃ for pre-decarburization, controlling the C content to below 1.0wt%. Ferrosilicon is added to reduce the Cr2O3 oxidized at high temperature, so that the Cr2O3 content in the steel slag is reduced to below 1wt%, and more than 50wt% of high-phosphorus oxide slag is removed.

[0025] The molten steel obtained from the electric furnace smelting is refined using AOD (Alternating Current Deoxidation). P (phosphorus) and C (carbon) are removed by top-blown oxygen and bottom-blown argon, controlling the C content to below 0.035 wt%. Deep deoxidation is achieved by adding aluminum, silicon, and calcium-based deoxidizers, controlling the oxygen content to below 30 ppm. Deep desulfurization is then performed by adding calcium, reducing the S content to below 0.010 wt%. The contents of Cr, Ni, Mo, Mn, and Cu are adjusted to target levels. Then, raw materials containing W, V, Nb, and Co are added, and the mixture is stirred with argon to ensure complete dissolution and mixing of the added elements. Finally, more than 50% of the high-phosphorus slag is removed, reducing the P content to the target level.

[0026] The molten steel obtained by AOD refining is subjected to VOD vacuum smelting. The vacuum degree in the furnace is controlled to be ≤67Pa. Vacuum removal of C and N is carried out to reduce the C and N content in the molten steel to the target content. The pressure in the furnace is restored to normal pressure. Under the condition of bottom blowing argon gas and stirring, CaO and CaF2 are added to reduce the S content to below 0.005wt%.

[0027] The molten steel obtained by the VOD vacuum smelting is subjected to LF refining. The temperature of the molten steel is controlled at 1520~1580℃. Raw materials of Si, Al, Zr, Ti and Sn are added in sequence and stirred thoroughly with argon gas to obtain casting liquid.

[0028] Optionally, casting includes the following steps:

[0029] The casting mold is baked at a temperature of 900~1200℃ until the temperature of the inner lining of the casting mold reaches 800~1000℃.

[0030] The casting mold is wrapped with a heat preservation device, and the casting liquid is poured using continuous casting or ingot casting, with the casting temperature controlled at 1520~1580℃.

[0031] The present invention also proposes the application of the above-mentioned ultra-low carbon content ferritic stainless steel, or the ultra-low carbon content ferritic stainless steel prepared by the above-mentioned preparation process, in heat exchangers, air coolers and composite plate containers.

[0032] Specifically, the ultra-low carbon content ferritic stainless steel of the present invention can be used to make heat exchange tubes for heat exchangers or air coolers; it can also be used as a cladding material for composite plates, with carbon steel materials such as Q345 as the base layer, and the composite plates can be applied in composite plate containers through explosive bonding or vacuum rolling bonding.

[0033] In the technical solution of this invention, C and N have extremely low solubility in ferrite and are harmful precipitating phase elements. Furthermore, both C and N are strong austenite-forming elements. If the composition ratio is not well controlled, the proportion of ferrite transforming into austenite at high temperatures will increase significantly, and martensitic transformation will occur during cooling. Therefore, the C content is controlled to be ≤0.015wt%, and the N content to be ≤0.015wt%.

[0034] Chromium (Cr) is a major ferrite-forming element and one of the most important alloying elements in stainless steel. Higher chromium content results in a more stable passivation film on the metal surface and better resistance to uniform corrosion. Chromium is a strong ferrite-forming element and can inhibit the formation of austenite phase.

[0035] Mo is an important ferrite-forming element. The form is enriched in the passivation film, and the resulting chromium-molybdenum composite oxide is more abundant than that of the single form. More dense, effectively blocking chloride ions ( ( ) Penetration. Furthermore, molybdenum can enhance the repair ability of passivation films, especially in environments containing... In environments such as seawater and saltwater, it can significantly inhibit pitting and crevice corrosion. Mo can form a synergistic effect with Cr in corrosion resistance, with Cr providing basic passivation and Mo further enhancing resistance to localized corrosion.

[0036] Si is an important ferrite-forming element. Adding Si to ferritic stainless steel can promote the diffusion of Cr and increase the formation of Cr-based oxides. Si can work synergistically with Cr and Mo to enhance corrosion resistance, thereby strengthening the formation of oxide coatings. However, excessive Si can negatively affect weldability.

[0037] Ni is an austenite-forming element that can increase the toughness of materials, but excessive content can destroy the single-phase stability of ferrite. Therefore, its content needs to be controlled appropriately.

[0038] W is a carbide and nitride forming element. When other nitride forming elements are added to steel, tungsten nitride usually does not form. After the molten steel is cast and solidified, cellular precipitates containing nanoscale carbides, including the main alloying elements, will precipitate at high temperatures, thus hindering the migration of carbides to grain boundaries and playing a strengthening role. Zr and Ti are strong nitride and weak carbide precipitate forming elements, and also have a strong affinity for oxygen. V and Nb are mixed precipitate forming elements with carbides as the main component and nitrides as the secondary component. The inventors found that V forms finer precipitates in steel, indicating that V has a better grain-refining effect. Al is a strong oxide and strong nitride forming element. The alumina formed on the surface can increase the corrosion resistance of the material, but excessive Al can cause brittleness in the weld heat-affected zone.

[0039] Co acts as a binder for carbides and nitrides, forming a composite phase in which carbides and nitrides coexist, which contains the main alloying element, thereby improving the toughness and high-temperature performance of the material. The addition of Co makes the second-phase precipitates more stable and also helps to form a surface oxide film, making it more corrosion resistant and stronger in high-temperature oxidation resistance.

[0040] Cu can improve the plasticity and toughness of ferritic stainless steel, in the presence of... In neutral or weakly acidic media, Cu can preferentially dissolve and form at localized damage sites in the passivation film (pitting corrosion initiation sites). These ions will react with the solution Combined generation The deposits form a "barrier" within the pitting corrosion pits, hindering... It further penetrates into the substrate, slowing down the rate of pitting corrosion propagation.

[0041] Sn is a low-melting-point element, and its atoms tend to cluster at grain boundaries (the Sn content at grain boundaries can be 5 to 10 times that of the matrix). Its clustering ability is comparable to that of C, and it can compete with C for grain boundary sites, reducing the bonding between C and Cr (i.e., reducing...). Sn precipitation reduces the width of the Cr-depleted zone (from 50-100 nm to 20-30 nm). For ultra-low carbon ferritic stainless steel, Sn grain boundary segregation can also enhance grain boundary bonding and reduce the penetration of corrosive media along grain boundaries. When the passivation film is locally damaged, Sn will preferentially dissolve into... and combines with oxygen in the solution to form It deposits at the damaged area to form a temporary protective layer, providing... Re-enrichment of new The membrane buys time. This "self-healing mechanism" is particularly significant in weakly acidic media (such as industrial wastewater with pH=4~6), which can greatly reduce the pitting corrosion initiation rate.

[0042] RE is a modifier for non-metallic inclusions. It can not only improve the morphology and size of non-metallic inclusions, but also reduce the total number of non-metallic inclusions. In addition, it can significantly increase the melting point of non-metallic inclusions, thereby improving the weldability of ferritic stainless steel.

[0043] The technical solution of this invention improves the corrosion resistance of materials by reducing the carbon and nitrogen content, adding Cr, Mo, Cu and Sn, and adding W, Zr, Ti, V, Nb and Al as stabilizing elements for carbides and nitrides, and Co as a binder for stabilizing elements. This disperses and anchors carbon and nitrogen in the material matrix, preventing them from diffusing and agglomerating at grain boundaries, thereby refining the grains and improving the corrosion resistance of the material. This ensures the formation of a dense and stable stainless steel passivation film on the metal surface, and also plays a beneficial role in improving weldability. The high-melting-point compounds formed by these stabilizing elements can prevent grain growth in the heat-affected zone.

[0044] The material produced by the technical solution of this invention has a microstructure of pure ferritic stainless steel, exhibiting superior corrosion resistance compared to low-alloy steels 09Cr2AlMoRE and 08Cr2AlMo, and a significantly higher cost-performance ratio than duplex stainless steel. Low-alloy steels are used in mild stress corrosion environments, duplex stainless steels are used in extremely severe stress corrosion environments, while the new material provided by this invention fills the gap in its application in moderate stress corrosion environments. Attached Figure Description

[0045] Figure 1 Metallographic diagram of T91 stainless steel in the prior art;

[0046] Figure 2 The image shows the metallographic structure of the stainless steel prepared in Comparative Example 1.

[0047] Figure 3 This is a metallographic diagram of the stainless steel obtained in Example 1 of the present invention.

[0048] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0049] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0050] It should be noted that if the embodiments of the present invention involve directional indicators (such as up, down, left, right, front, back, etc.), the directional indicators are only used to explain the relative positional relationship and movement of the components in a specific posture. If the specific posture changes, the directional indicators will also change accordingly.

[0051] Furthermore, if the embodiments of this invention involve descriptions such as "first" or "second," these descriptions are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined with "first" or "second" may explicitly or implicitly include at least one of those features. Additionally, the use of "and / or" or "and / or" throughout the text includes three parallel solutions. For example, "A and / or B" includes solution A, solution B, or a solution where both A and B are satisfied simultaneously. Furthermore, the technical solutions of the various embodiments can be combined with each other, but this must be based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or impossible to implement, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection claimed by this invention.

[0052] This invention provides a ferritic stainless steel with ultra-low carbon content, whose composition by weight percentage is: C≤0.015wt%, N≤0.015wt%, S≤0.010wt%, P≤0.025wt%, Mn≤0.6wt%, Si 0.2~1.0wt%, Cr 9.0~30.0wt%, Mo 0.50~8.0wt%, Cu 0.05~2.0wt%, Sn 0.05~0.8wt%, Ni 0.05~5.0wt%, stabilizing elements 0.2~3.2wt%, Co 0~0.30wt%, with the balance being Fe and unavoidable impurities; wherein the stabilizing elements include W, Zr, Ti, V, Nb and Al.

[0053] In the technical solution of this invention, the material is optimized and modified by significantly increasing the ratio of chromium equivalent to nickel equivalent, and stabilizing elements containing carbides and nitrides such as W, Zr, Ti, V, Nb and Al are added. Cu and Sn elements are added to stabilize the passivation film on the metal surface, thereby forming an ultra-low carbon ferritic stainless steel with low alloy content and high cost performance. At the same time, in order to prevent the ferrite grain boundaries from being loose, resulting in low yield and poor welding performance, the P content is reduced to 0.025wt% or even lower, and elements such as Co are added to improve the grain boundary bonding force, so that the overall performance of the ferritic stainless steel is better.

[0054] Furthermore, the sum of C and N content should be ≤0.02wt%. Both C and N are strong austenite-forming elements. If the composition ratio is not well controlled, the proportion of ferrite transforming into austenite at high temperature will increase significantly, and martensite transformation will occur during cooling. Therefore, the content of C and N needs to be controlled at a low level.

[0055] Furthermore, the combined content of Cr and Mo is 10.5~35.0 wt%. Mo can form a synergistic effect with Cr in corrosion resistance. Cr provides basic passivation capability, while Mo further enhances the resistance to localized corrosion, thereby ensuring the formation of a dense and stable stainless steel passivation film on the metal surface.

[0056] Furthermore, the stabilizing elements comprise 0.05-0.8 wt% W, 0.05-0.8 wt% Zr and Ti, 0.05-0.8 wt% V and Nb, and 0.05-0.8 wt% Al. The addition of stabilizing elements pins C and N within the grains, refining the grains while also hindering grain boundary slip, thereby improving the material's strength and weldability.

[0057] Furthermore, the equivalent of Cr is Cr eq The equivalent of Ni is Ni eq And Cr eq with Ni eq The ratio is ≥7.0;

[0058] Among them, Cr eq =W(Cr)+W(Mo)+1.5W(Si)+0.75∑W(Xi);

[0059] Ni eq =W(Ni)+30[W(C)+W(N)]+0.5[W(Mn)+W(Co)]+0.25W(Cu);

[0060] In the formula, W(Cr) is the weight content of Cr, W(Mo) is the weight content of Mo, W(Si) is the weight content of Si, ∑W(Xi) is the sum of the weight contents of each element in the stabilizing element, W(Ni) is the weight content of Ni, W(C) is the weight content of C, W(N) is the weight content of N, W(Mn) is the weight content of Mn, W(Co) is the weight content of Co, and W(Cu) is the weight content of Cu.

[0061] By adopting the above technical solution, the ratio of chromium equivalent to nickel equivalent is increased to over 7.0, ensuring that the microstructure of the material is ferrite.

[0062] Furthermore, the ultra-low carbon content ferritic stainless steel also includes 0.02~0.08 wt% RE, wherein RE is selected from at least one element selected from La, Ce, Pr, Nd, Y and Sc. The addition of rare earth elements can improve grain boundary bonding and avoid low yield and poor weldability caused by loose ferrite grain boundaries.

[0063] This invention also proposes a preparation process for ferritic stainless steel with ultra-low carbon content, comprising the following steps:

[0064] The raw material of the ultra-low carbon content ferritic stainless steel is smelted in an electric furnace or medium frequency furnace, and then subjected to AOD smelting, VOD vacuum smelting and LF refining to obtain a casting liquid.

[0065] The casting liquid is poured, and the resulting billet is kept at 1000~1200℃ for more than 60 minutes, then forged into forgings, rolled into plates, or rolled / drawn into pipes.

[0066] The forgings, plates, or pipes are annealed at 700~950℃ for 1~3 minutes per millimeter of thickness. After being removed from the furnace, they are air-cooled to obtain the ultra-low carbon content ferritic stainless steel.

[0067] It should be noted that "rolling / drawing into pipes" refers to pipes being manufactured using rolling or drawing methods.

[0068] Furthermore, obtaining the casting liquid includes the following steps:

[0069] The raw materials Fe, Cr, Ni, Mo, Mn, and Cu are smelted in an electric furnace. The slag basicity is controlled at 2.6~3.2, and the molten steel temperature is 1540~1590℃. Argon is blown from the bottom to stir the molten steel, and oxygen is blown from the top to dephosphorize, controlling the P content in the molten steel to below 0.035wt%. The temperature of the molten steel is adjusted to 1650~1700℃ for pre-decarburization, controlling the C content to below 1.0wt%. Ferrosilicon is added to reduce the Cr2O3 oxidized at high temperature, so that the Cr2O3 content in the steel slag is reduced to below 1wt%, and more than 50wt% of high-phosphorus oxide slag is removed.

[0070] The molten steel obtained from the electric furnace smelting is refined using AOD (Alternating Current Deoxidation). P (phosphorus) and C (carbon) are removed by top-blown oxygen and bottom-blown argon, controlling the C content to below 0.035 wt%. Deep deoxidation is achieved by adding aluminum, silicon, and calcium-based deoxidizers, controlling the oxygen content to below 30 ppm. Deep desulfurization is then performed by adding calcium, reducing the S content to below 0.010 wt%. The contents of Cr, Ni, Mo, Mn, and Cu are adjusted to target levels. Then, raw materials containing W, V, Nb, and Co are added, and the mixture is stirred with argon to ensure complete dissolution and mixing of the added elements. Finally, more than 50% of the high-phosphorus slag is removed, reducing the P content to the target level.

[0071] The molten steel obtained by AOD refining is subjected to VOD vacuum smelting. The vacuum degree in the furnace is controlled to be ≤67Pa. Vacuum removal of C and N is carried out to reduce the C and N content in the molten steel to the target content. The pressure in the furnace is restored to normal pressure. Under the condition of bottom blowing argon gas and stirring, CaO and CaF2 are added to reduce the S content to below 0.005wt%.

[0072] The molten steel obtained by the VOD vacuum smelting is subjected to LF refining. The temperature of the molten steel is controlled at 1520~1580℃. Raw materials of Si, Al, Zr, Ti and Sn are added in sequence and stirred thoroughly with argon gas to obtain casting liquid.

[0073] By adopting the above technical solutions, ferritic stainless steel is particularly sensitive to the content of sulfur (S), phosphorus (P), and impurities. Desulfurization by adding calcium during smelting is relatively easy, while dephosphorization is more complex because the loss of chromium (Cr) must be considered in the high-temperature oxidizing environment. That is, how to minimize Cr loss and maximize P removal involves a balance of costs and technological optimization. Therefore, to achieve the best smelting results with high purity, thorough and effective dephosphorization and desulfurization are necessary during the smelting stage.

[0074] Furthermore, the casting process includes the following steps:

[0075] The casting mold is baked at a temperature of 900~1200℃ until the temperature of the inner lining of the casting mold reaches 800~1000℃.

[0076] The casting mold is wrapped with a heat preservation device, and the casting liquid is poured using continuous casting or ingot casting, with the casting temperature controlled at 1520~1580℃.

[0077] By adopting the above technical solution, the hydrogen (H) dissolving capacity of ferritic stainless steel is only about 1 / 3 that of austenitic stainless steel. Furthermore, unlike austenitic stainless steel where hydrogen easily diffuses, ferritic stainless steel readily absorbs hydrogen and is sensitive to hydrogen embrittlement. Therefore, before casting, the casting mold should be baked, with the baking lining temperature reaching 800~1000℃. High-temperature baking ensures a uniform and stable temperature in the casting mold, preventing uneven solidification caused by excessive local temperature differences during molten steel injection, which can lead to surface cracks and segregation in the billet. Especially since ferritic stainless steel has low high-temperature strength and high viscosity, the thermal state of the billet before casting is crucial for ingot formation. Wrapping the casting mold with asbestos or other insulation devices can provide insulation and slow cooling after casting, ensuring the overall performance of the resulting stainless steel material.

[0078] This invention also proposes the application of the above-mentioned ultra-low carbon content ferritic stainless steel in heat exchangers, air coolers and composite plate containers.

[0079] The technical solution of the present invention will be further described in detail below with reference to specific embodiments and accompanying drawings. It should be understood that the following embodiments are only used to explain the present invention and are not intended to limit the present invention.

[0080] Examples 1-4

[0081] Examples 1-4 provide a ferritic stainless steel with ultra-low carbon content, the chemical composition and content of which are shown in Table 1 below. The preparation process of an ultra-low carbon content ferritic stainless steel includes the following steps:

[0082] (1) Electric furnace smelting: Fe, Cr, Ni, Mo, Mn and Cu raw materials are smelted in an electric furnace as a charge for 10,000 kg of molten steel. Based on the Si content in the smelting composition, the SiO2 content in the slag after silicon oxidation during the metallurgical reaction is calculated. Sufficient CaO and CaF2 are added to maintain the basicity of the high basicity slag system between 2.8 and 3.0. The temperature of the molten steel is controlled between 1540℃ and 1590℃. Argon is blown from the bottom to stir the molten steel, and oxygen is blown from the top to dephosphorize. The P content in the molten steel is controlled to be reduced to below 0.035wt%. The temperature of the molten steel is adjusted to 1650℃ to 1700℃ for pre-decarburization, so that the C content is reduced to below 1.0wt%. After decarburization, ferrosilicon is added to perform preliminary reduction treatment on the Cr2O3 oxidized at high temperature. The Cr2O3 content in the steel slag is reduced to below 1wt%. At the same time, P2O5 in the steel slag is fixed and it is not easy to return to phosphorus. More than 50% of the steel slag is removed.

[0083] (2) AOD smelting: The molten steel obtained from electric furnace smelting is refined using AOD. AOD refining shares the same thermodynamic basis as electric furnace smelting. P and C are removed by top blowing oxygen and bottom blowing argon, controlling the C content to below 0.035 wt%. Deep deoxidation is carried out by adding aluminum, silicon, and calcium deoxidizers, and chromium oxide formed by high-temperature oxidation is partially reduced to Cr, controlling the oxygen content to below 30 ppm. Deep desulfurization is carried out by adding calcium, reducing the S content to below 0.010 wt%. The contents of Cr, Ni, Mo, Mn, and Cu are adjusted to the target contents. Then, raw materials of W, V, Nb, and Co are added. Argon is used to stir to ensure that the added elements are fully dissolved and mixed. Then, more than 50% of the high-phosphorus slag is removed, reducing the P content to below 0.020 wt%.

[0084] (3) VOD vacuum smelting: The molten steel obtained from AOD smelting is subjected to VOD vacuum smelting. The vacuum degree in the furnace is controlled to be ≤67Pa. Vacuum removal of C and N is carried out to reduce the C and N content in the molten steel to the target content. Under the condition of controlling the furnace pressure to return to normal pressure and stirring with argon gas blowing from the bottom, CaO and CaF2 are added to reduce the S content to below 0.005wt%.

[0085] (4) The steel liquid obtained by VOD vacuum smelting is refined by LF. The temperature of the steel liquid is controlled at 1520℃~1580℃. Various chemical elements (excluding Si, Al, Zr, Ti and Sn) that are insufficient are added according to the target content. Then, raw materials of Si, Al, Zr, Ti and Sn are added in sequence and stirred thoroughly with argon to obtain casting liquid with higher purity.

[0086] (5) The casting mold is baked at 1100℃ until the temperature of the inner lining of the casting mold reaches 900℃. The casting mold is wrapped with asbestos. Under the protection of argon atmosphere, the casting liquid is cast by mold casting. During the casting process, the temperature of the casting liquid is controlled at 1520~1580℃. The casting billet is cut off to obtain the forging billet.

[0087] (6) Cut the forging billet into blanks, heat the billet in the furnace, and hold it at 1100~1150℃ for 60 minutes. Remove the billet from the furnace and forge it into a square forging with a thickness of 100mm × width of 150mm × length of 200mm. Then cut it into plates with a thickness of 20mm × width of 150mm × length of 200mm. Anneal the plates at 750℃ for 40 minutes. Immediately air-cool the plates to room temperature after removing them from the furnace. Then finish them to the dimensions specified in the drawings to obtain the shaped plates. If the temperature of the billet drops to about 850℃ during the forging process, the billet should be reheated to above 950℃ to continue forging.

[0088] Table 1. Chemical composition and content of Examples 1-4 (unit: wt%)

[0089] Example 1 Example 2 Example 3 Example 4 C 0.01 0.006 0.005 0.005 N 0.01 0.014 0.009 0.008 Mn 0.4 0.4 0.4 0.4 Si 0.4 0.2 0.7 1.0 Cr 12.0 18.5 9.0 22.5 Mo 0.5 2.4 4.9 7.8 Cu 0.4 0.5 0.9 2.0 Sn 0.23 0.05 0.53 0.8 Ni 0.1 0.8 0.3 1.2 W 0.18 0.05 0.38 0.75 Zr 0.08 0.03 0.16 0.25 Ti 0.16 0.02 0.29 0.47 V 0.08 0.04 0.12 0.25 Nb 0.14 0.04 0.29 0.51 Al 0.2 0.05 0.4 0.8 Co 0.1 0 0.2 0.3 Fe and impurities margin margin margin margin Example 5

[0090] This embodiment is based on embodiment 1, the difference being that: the chemical composition of the ferritic stainless steel with ultra-low carbon content also includes 0.03wt% La (lanthanum), wherein in step (5), the raw material of La is added to the ladle before casting, and then the casting liquid is cast. Comparative Example 1

[0091] This comparative example is based on Example 1, the difference being that the content of some chemical components of stainless steel is different. Specifically, in this comparative example, the content of C is 0.030 wt%, the content of N is 0.025 wt%, the content of Si is 0.13 wt%, the content of Cr is 9.24 wt%, the content of Mo is 1.09 wt%, and the others remain unchanged.

[0092] Performance Test 1

[0093] Round bars were taken from the shaped sheet materials prepared in Examples 1-5 and Comparative Example 1 as test specimens, and mechanical property tests were conducted in accordance with GB / T 228.1-2021 Metallic Materials - Tensile Testing - Part 1: Test Method at Room Temperature. Taking round bar specimens from the shaped sheet material included: taking strip-shaped specimens from the sheet material and machining them into round bar specimens with a diameter of 10 mm and a length of 200 mm according to standard requirements.

[0094] Table 2 Test results of mechanical properties of the specimens

[0095] <![CDATA[Tensile strength R m (MPa)]]> <![CDATA[Yield strength R P0.2 (MPa)]]> Elongation A (%) Example 1 425 232 40.5 Example 2 471 260 40.0 Example 3 435 263 38.0 Example 4 528 338 36.5 Example 5 429 234 40.3 Comparative Example 1 727 639 20.4

[0096] As can be seen from the test results in Table 2, the plate material obtained by this application has a certain strength and sufficient plasticity reserve, and can be processed into plates and pipes to make heat exchange tubes for heat exchangers or air coolers; the plate material obtained by this application can also be used as the cladding material of composite plates, with carbon steel material as the base layer, and the composite plate can be applied in composite plate containers through explosive bonding or vacuum rolling bonding.

[0097] Performance Test 2

[0098] The metallographic structure of the stainless steel sheet prepared in Comparative Example 1 was observed under an optical microscope as follows: Figure 2 As shown; the metallographic structure of the stainless steel sheet obtained in Example 1 is as follows. Figure 3 As shown. By Figure 2 It can be seen that the microstructure of the stainless steel corresponding to Comparative Example 1 is tempered martensite + ferrite, and the proportion of tempered martensite is higher than that of ferrite. From Figure 3It can be seen that the microstructure of the stainless steel corresponding to Example 1 is ferrite + fine dispersed carbides, with no martensite residue, and has achieved the effect of complete ferritization.

[0099] Performance Test 3

[0100] The plate material prepared in Example 1 was sampled by wire cutting, the sample was ground smooth, and stress corrosion test and intergranular corrosion test were performed.

[0101] (1) Stress corrosion test: According to ASTM G36-24 "Evaluation of stress corrosion crack resistance of metals and their alloys in boiling magnesium chloride solution": 45% magnesium chloride four-point bending stress corrosion test, sample size 69.0mm (length) × 5.0mm (width) × 1.5mm (thickness), test temperature 155℃, boiling for 96h, 3 samples.

[0102] Test results: No microcracks were found in any of the three samples.

[0103] (2) Intergranular corrosion test: According to GB / T 31935-2015 "Corrosion of metals and alloys - Test method for intergranular corrosion of low-chromium ferritic stainless steel": 0.5% sulfuric acid + 24% copper sulfate solution (slightly boiling), boil for 15 hours, sample size 31.0 mm (length) × 20.0 mm (width) × 3.6 mm (thickness), 2 samples are kept at 650℃ for 0.5 hours, and then air-cooled to room temperature after being taken out of the furnace.

[0104] Test results: Both samples were examined under 200x microscopic magnification and no intergranular corrosion cracks were found.

[0105] The corrosion tests above show that the stainless steel sheet produced in this application has good resistance to stress corrosion and intergranular corrosion.

[0106] The above are merely preferred embodiments of the present invention and do not limit the patent scope of the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the patent protection scope of the present invention.

Claims

1. A ferritic stainless steel with ultra-low carbon content, characterized in that, By weight percentage, its composition is as follows: C≤0.015wt%, N≤0.015wt%, S≤0.010wt%, P≤0.025wt%, Mn≤0.6wt%, Si 0.2~1.0wt%, Cr 9.0~30.0wt%, Mo 0.50~8.0wt%, Cu0.05~2.0wt%, Sn 0.05~0.8wt%, Ni 0.05~5.0wt%, stabilizing elements 0.2~3.2wt%, Co 0~0.30wt%, with the balance being Fe and unavoidable impurities; The stabilizing elements include W, Zr, Ti, V, Nb, and Al; The equivalent of Cr is Cr eq The equivalent of Ni is Ni eq And Cr eq with Ni eq The ratio is ≥7.0; Among them, Cr eq =W(Cr)+W(Mo)+1.5W(Si)+0.75∑W(Xi); Ni eq =W(Ni)+30[W(C)+W(N)]+0.5[W(Mn)+W(Co)]+0.25W(Cu); In the formula, W(Cr) is the weight content of Cr, W(Mo) is the weight content of Mo, W(Si) is the weight content of Si, ∑W(Xi) is the sum of the weight contents of each element in the stabilizing element, W(Ni) is the weight content of Ni, W(C) is the weight content of C, W(N) is the weight content of N, W(Mn) is the weight content of Mn, W(Co) is the weight content of Co, and W(Cu) is the weight content of Cu.

2. The ferritic stainless steel with ultra-low carbon content as described in claim 1, characterized in that, The sum of C and N content is ≤0.02wt%.

3. The ferritic stainless steel with ultra-low carbon content as described in claim 1, characterized in that, The combined content of Cr and Mo is 10.5–35.0 wt%.

4. The ferritic stainless steel with ultra-low carbon content as described in claim 1, characterized in that, The stabilizing elements contain 0.05–0.8 wt% W, 0.05–0.8 wt% Zr and Ti, 0.05–0.8 wt% V and Nb, and 0.05–0.8 wt% Al.

5. The ferritic stainless steel with ultra-low carbon content as described in claim 1, characterized in that, The ultra-low carbon content ferritic stainless steel also includes 0.02 to 0.08 wt% RE, wherein RE is selected from at least one element selected from La, Ce, Pr, Nd, Y and Sc.

6. A process for preparing ferritic stainless steel with ultra-low carbon content as described in any one of claims 1 to 5, characterized in that, Includes the following steps: The raw material of the ultra-low carbon content ferritic stainless steel is smelted in an electric furnace or medium frequency furnace, and then subjected to AOD smelting, VOD vacuum smelting and LF refining to obtain a casting liquid. The casting liquid is poured, and the resulting billet is kept at 1000-1200℃ for more than 60 minutes, then forged into forgings, rolled into plates, or rolled / drawn into pipes. The forgings, plates, or pipes are annealed at 700–950°C for 1–3 minutes per millimeter of thickness. After being removed from the furnace, they are air-cooled to obtain the ultra-low carbon content ferritic stainless steel.

7. The preparation process of ultra-low carbon content ferritic stainless steel as described in claim 6, characterized in that, Obtaining the casting liquid includes the following steps: The raw materials Fe, Cr, Ni, Mo, Mn, and Cu are smelted in an electric furnace. The slag basicity is controlled at 2.6–3.2, and the molten steel temperature is controlled at 1540–1590℃. Argon is blown from the bottom to stir the molten steel, and oxygen is blown from the top to dephosphorize, controlling the P content in the molten steel to below 0.035 wt%. The temperature of the molten steel is adjusted to 1650–1700℃ for pre-decarburization, controlling the C content to below 1.0 wt%. Ferrosilicon is added to reduce the Cr2O3 oxidized at high temperature, so that the Cr2O3 content in the steel slag is reduced to below 1 wt%, and more than 50 wt% of high-phosphorus oxide slag is removed. The molten steel obtained from the electric furnace smelting is refined using AOD (Alternating Current Deoxidation). P and C are removed by top-blown oxygen and bottom-blown argon, controlling the C content to below 0.035 wt%. Deep deoxidation is achieved by adding aluminum, silicon, and calcium-based deoxidizers, controlling the oxygen content to below 30 ppm. Deep desulfurization is then performed by adding calcium, reducing the S content to below 0.010 wt%. The contents of Cr, Ni, Mo, Mn, and Cu are adjusted to the target levels. Then, raw materials containing W, V, Nb, and Co are added, and the mixture is stirred with argon to ensure complete dissolution and mixing of the added elements. Finally, more than 50% of the high-phosphorus slag is removed, reducing the P content to the target level. The molten steel obtained by AOD refining is subjected to VOD vacuum smelting. The vacuum degree in the furnace is controlled to be ≤67Pa. Vacuum removal of C and N is carried out to reduce the C and N content in the molten steel to the target content. The pressure in the furnace is restored to normal pressure. Under the condition of bottom argon blowing and stirring, CaO and CaF2 are added to reduce the S content to below 0.005wt%. The molten steel obtained by the VOD vacuum smelting is subjected to LF refining. The temperature of the molten steel is controlled at 1520-1580℃. Raw materials of Si, Al, Zr, Ti and Sn are added in sequence and stirred thoroughly with argon gas to obtain casting liquid.

8. The preparation process of ultra-low carbon content ferritic stainless steel as described in claim 7, characterized in that, Casting includes the following steps: The casting mold is baked at a temperature of 900-1200℃ until the temperature of the mold lining reaches 800-1000℃. The casting mold is wrapped with a heat preservation device, and the casting liquid is poured using continuous casting or ingot casting, with the casting temperature controlled at 1520~1580℃.

9. The application of an ultra-low carbon content ferritic stainless steel as described in any one of claims 1 to 5, or an ultra-low carbon content ferritic stainless steel prepared by the preparation process described in any one of claims 6 to 8, in heat exchangers, air coolers, and composite plate containers.

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