Rare earth-containing low-alloy sulfuric acid dew point corrosion-resistant steel component design and manufacturing method thereof

Abstract

The invention discloses a component design and a manufacturing method of rare earth-containing low-alloy sulfuric acid dew point corrosion resistant steel. The rare earth-containing low-alloy sulfuric acid dew point corrosion resistant steel is characterized by comprising the following chemical components in percentage by weight: less than or equal to 0.15% of C; less than or equal to 0.50% of Si; mn < = 1.80%; p is less than or equal to 0.025%, S is less than or equal to 0.025%, and Cu is 0.25-0.50 0.50% to 1.30% of Cr; 0.1 to 0.50 percent of Ni; 0.01 to 0.5 percent of Mo; sb < = 0.15; 0.10% to 0.50% of RE (La, Ce); wherein Cu / S is 30-40, and RE (La, Ce) / S is 10-30. On the basis of weathering resistant steel, Cu, Cr, Ni and RE are mainly added to develop Cu-Cr-Ni-RE series low alloy steel, the inclusion form of Sb in the steel is controlled by adding RE (rare earth), the impact toughness and corrosion resistance of the steel can be further improved, and the steel is mainly used in the high-temperature and high-concentration sulfuric acid corrosion environment. The high-temperature-resistant sulfuric acid dew point corrosion inhibitor has excellent high-temperature-resistant sulfuric acid dew point corrosion resistance, and is widely applied to petroleum and petrochemical equipment, process pipeline corrosion environments and flue gas treatment systems taking coal or heavy oil as main fuel.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials and relates to the composition design and manufacturing method of a rare earth low alloy steel resistant to sulfuric acid dew point corrosion, particularly to a steel with excellent corrosion resistance and manufacturability resistant to sulfuric acid dew point corrosion. Background Technology

[0002] In energy and oil and gas chemical industries, sulfuric acid dew point corrosion is becoming increasingly serious, prompting countries to invest significant resources in research on sulfuric acid dew point corrosion-resistant steels. Sulfuric acid dew point corrosion typically occurs in boiler preheaters and coal-fired equipment, air preheater heat exchange elements, as well as pipelines, flues, and chimneys, causing substantial damage to industrial equipment. This corrosion involves both chemical and electrochemical processes, with complex reaction types. Generally, sulfuric acid dew point corrosion occurs in lower-temperature areas of equipment, specifically where the flue gas temperature is below the dew point temperature of sulfuric acid in that environment. At this point, SO3 in the flue gas combines with water vapor in the air to form sulfuric acid, which condenses in the low-temperature areas, eventually forming droplets that adhere to the metal surface and cause corrosion. Dew point corrosion is prevalent both internally and externally in various petrochemical and coal chemical equipment, with internal corrosion caused by process media and external corrosion caused by a combination of atmospheric and various corrosive media.

[0003] The resistance of steel to H2SO4 corrosion depends primarily on its chemical composition. By adding appropriate amounts of special alloying elements to ordinary carbon steel, it is possible to achieve good sulfuric acid dew point corrosion resistance while reducing production costs. The influence of alloying elements on the corrosion resistance of steel varies significantly depending on the corrosive environment. Various alloying elements are closely related to each other; only with appropriate proportions can good resistance to H2SO4 dew point corrosion be obtained.

[0004] Traditional steels do not exhibit sufficient corrosion resistance for equipment using high-sulfur fuels with a sulfur content exceeding 2%. As the sulfur content increases, the concentration of sulfuric acid in the exhaust gas increases, and with decreasing temperature, the amount of sulfuric acid condensing increases. This creates a more severe corrosive environment compared to environments with low-sulfur fuels. Therefore, steels that exhibit excellent corrosion resistance in relatively mild environments with sulfur content below 2% may not perform well in high-sulfur fuel environments. Currently, there is no high-temperature sulfuric acid dew point corrosion resistant steel, meaning no steel grade that exhibits excellent corrosion resistance in high-sulfur fuel environments. In high-sulfur fuel environments, the exhaust gas condenses at a sulfuric acid dew point of approximately 110-160℃, resulting in a high concentration of sulfuric acid (70-90%) adhering to the steel surface.

[0005] The prominent problems with existing patents include: unreasonable design and proportion of alloying elements that are beneficial to corrosion resistance, resulting in poor resistance to sulfuric acid dew point corrosion; and the high content of some multi-functional elements (such as Cu, Cr, etc.) or unreasonable combination and use of these elements will have significant side effects, affecting the comprehensive properties of steel such as mechanical and weldability. Summary of the Invention

[0006] Purpose of the invention:

[0007] To overcome the problems existing in the prior art, the purpose of this invention is to provide a rare earth low-alloy steel for resisting sulfuric acid dew point corrosion and its manufacturing method. This rare earth low-alloy steel for resisting sulfuric acid dew point corrosion exhibits excellent resistance to high-temperature sulfuric acid dew point corrosion.

[0008] Technical solution:

[0009] The objective of this invention is achieved through the following technical solution:

[0010] A rare-earth low-alloy steel for resisting sulfuric acid dew point corrosion, comprising the following composition design and manufacturing method: The rare-earth low-alloy steel for resisting high-temperature sulfuric acid dew point corrosion contains the following chemical composition by weight percentage:

[0011] A rare earth low-alloy steel resistant to sulfuric acid dew point corrosion, characterized in that: the rare earth low-alloy steel for resisting sulfuric acid dew point corrosion contains the following chemical composition by weight percentage: C≤0.15%; Si≤0.50%; Mn≤1.80%; P≤0.025%; S≤0.025%; Cu0.25%–0.50%; Cr0.50%–1.30%; Ni0.1-0.50%; Mo0.01–0.5%; Sb≤0.15%; RE(La,Ce)0.10-0.50%; wherein Cu / S is 30-40, and RE(La,Ce) / S is 10-30.

[0012] The optimized chemical composition (by weight percentage) of the rare earth low-alloy steel resistant to sulfuric acid dew point corrosion is as follows: C: 0.050–0.100%, Si: 0.20–0.40%, Mn: 1.00–1.50%, P: 0.005–0.020%, S: 0.005–0.020%, Cu: 0.30–0.50%, Cr: 0.70%–1.20%, Ni: 0.10–0.20%, Mo: 0.10–0.20%, Sb: 0.10–0.15%, RE(La,Ce): 0.15–0.30%; the remainder being Fe.

[0013] The main alloying element content of this invention is based on the following principles:

[0014] In oxidizing acids, when the carbon content is <0.4%, the corrosion resistance of steel increases with increasing carbon content. However, when coexisting with copper (Cu), a carbon content exceeding 0.150% deteriorates the resistance to sulfuric acid dew point corrosion, as well as weldability and the toughness of the weld heat-affected zone. Therefore, a carbon content of ≤0.15% is preferred. Low-carbon copper-containing steel exhibits the best corrosion resistance, with a preferred carbon content range of 0.050-0.100%.

[0015] Si is added as a deoxidizer and has the effect of improving the strength of steel; however, when the Si content exceeds 0.80%, the toughness of the steel deteriorates. Si forms an anti-corrosion coating in a sulfuric acid aqueous solution environment, which helps to improve the resistance to sulfuric acid dew point corrosion. To obtain such an improved resistance to sulfuric acid dew point corrosion, the Si content is preferably set at 0.20-0.40%.

[0016] Mn is an element that increases the strength of steel, but excessive Mn content reduces the toughness and weldability of the steel. From the viewpoint of maintaining strength and suppressing the formation of inclusions that deteriorate resistance to sulfuric acid dew point corrosion, it is preferable to set the Mn content in the range of 1.00% to 1.50%. P is a harmful element that causes segregation at grain boundaries and reduces the toughness of steel; therefore, the P content is set to 0.025% or less. Taking into account manufacturing cost control, P is preferably set to 0.005% to 0.020%.

[0017] In the presence of Cu, sulfur (S) helps form a Cu₂S coating, inhibiting corrosion reactions on the steel surface and thus improving resistance to sulfuric acid dew point corrosion. On the other hand, S is also a detrimental element that forms MnS as a non-metallic inclusion, which becomes the starting point for localized corrosion, reducing resistance to localized corrosion. Therefore, from the viewpoint of ensuring resistance to sulfuric acid dew point corrosion, the S content is set to 0.002% or more. Conversely, from the viewpoint of avoiding a decrease in resistance to localized corrosion, the S content is set to 0.005-0.020%.

[0018] Cu plays a crucial role in improving the corrosion resistance of steel, while sulfur (S) and phosphorus (P) are harmful elements in steel, and their lower content is better. However, for resistance to sulfuric acid dew point corrosion, the presence of a certain amount of sulfur can promote the formation of a Cu₂S passivation film on the steel surface, thereby inhibiting the anodic and cathodic electrochemical reactions. If the sulfur content is insufficient, the Cu₂S surface film cannot form, and Cu simply accumulates on the surface, increasing the anodic area and accelerating corrosion. Therefore, the Cu and S content should meet a certain ratio, with Cu / S at 10-30. On the other hand, when the Cu content exceeds 0.50%, the effect of improving acid resistance saturates, and it leads to deterioration in manufacturability, especially hot workability. Therefore, the Cu content is set in the range of 0.30-0.50%.

[0019] Cr is beneficial for improving the corrosion resistance of steel, especially its resistance to sulfuric acid dew point corrosion in environments above 120°C. However, when the Cr content in steel is greater than 1.50%, the steel's resistance to sulfuric acid corrosion will decrease. A chromium content of 0.70%-1.20% is preferred.

[0020] Ni is an element that inhibits the deterioration of hot workability caused by the addition of Cu and Sb. It can be used to prevent the hot workability of copper from deteriorating. However, when its content exceeds 0.5%, it will reduce corrosion resistance and increase costs. The preferred nickel content is set in the range of 0.10% to 0.20%.

[0021] Mo (Mo) is an element that helps improve corrosion resistance in the presence of hydrogen chloride in exhaust gas, but a high content is detrimental to sulfuric acid corrosion resistance. Therefore, a Mo content of 0.03% to 0.25% is preferred. The addition of Sb (Sb) significantly improves the sulfuric acid corrosion resistance of steel. Sb inhibits anodic reactions and, through its combination with Cu, forms a Cu₂Sb film enriched on the steel surface, which also inhibits cathodic reactions. When the Sb content is less than 0.050%, its effect is minimal; when the Sb content exceeds 0.300%, the weldability of the steel deteriorates, and the quality of the cast billet is also affected. Therefore, an Sb content of ≤0.15% is preferred, and an Sb content range of 0.10% to 0.15% is also preferred.

[0022] RE (Rare Earth): In this invention, the commonly used rare earth elements La and Ce are selected as additive elements, which mainly exist in steel in two forms:

[0023] (1) It combines with oxygen and sulfur compounds to form complex inclusions;

[0024] (2) They segregate at interfaces such as grain boundaries, phase boundaries, and free surfaces. Stable rare earth addition technology can significantly improve the yield of rare earths in steel (≥70%) and make the rare earth distribution more uniform.

[0025] Furthermore, the RE (rare earth element) in this invention has the following functions:

[0026] (1) Rare earth elements change the composition of sulfide inclusions and the morphology of impurities in steel, resulting in a decrease in the electrode potential difference between them and the matrix, thereby improving the pitting corrosion resistance of steel; by adding RE (rare earth) to control the morphology of Sb inclusions in steel, the impact toughness and corrosion resistance of steel can be improved.

[0027] (2) Rare earth elements are segregated at interfaces, including grain boundaries, phase boundaries, and free surfaces, which reduces the interface energy and thus avoids local corrosion.

[0028] (3) It has a great affinity for low melting point metals and non-metals in steel, thereby improving the bonding force between the corrosion product film and the substrate on the steel surface, delaying the time for the corrosion product film to fall off the substrate, and thus effectively slowing down the corrosion rate.

[0029] (4) The aggregation of dispersed CeO particles at grain boundaries can significantly hinder the movement of dislocations, thereby increasing the strength of steel.

[0030] (5) Adding rare earth elements to steel can eliminate sulfur segregation, purify grain boundaries, and improve the hot plasticity of steel.

[0031] In this invention, the RE (rare earth) content in the steel is controlled at 0.10-0.50%, and preferably at 0.15-0.30%, and the RE(La,Ce) / S ratio is controlled at 10-30.

[0032] This invention achieves optimal resistance to sulfuric acid dew point corrosion in steel by controlling the weight percentage of various components. Compared with existing technologies, this invention has the following significant advantages:

[0033] (1) Based on weathering steel, this invention mainly adds Cu, Cr, Ni and RE to develop Cu-Cr-Ni-RE series of low alloy steels. By adding RE (rare earth) to control the morphology of Sb inclusions in the steel, the impact toughness and corrosion resistance of the steel can be further improved. It is mainly used in high temperature and high concentration sulfuric acid corrosion environments.

[0034] (2) This invention achieves optimal high-temperature sulfuric acid dew point corrosion resistance in steel by controlling the weight percentage of various components. The material manufactured using this invention underwent immersion tests according to national standard GB10124 at 70% H2SO4–110℃ and 90% H2SO4–160℃. Comparison with other sulfuric acid dew point corrosion resistant steels showed that the high-temperature sulfuric acid dew point corrosion resistance of this invention is significantly superior. This invention possesses excellent high-temperature sulfuric acid dew point corrosion resistance and is widely applicable to corrosive environments in petroleum and petrochemical equipment and process pipelines, as well as flue gas treatment systems using coal or heavy oil as primary fuels. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the corrosion weight loss rate when the present invention is immersed in sulfuric acid solution.

[0036] Figure 2 This is a schematic diagram showing the corrosion weight loss rate of Comparative Example 1 after immersion in sulfuric acid solution.

[0037] Figure 3 This is a schematic diagram showing the corrosion weight loss rate of Comparative Example 2 after immersion in sulfuric acid solution. Detailed Implementation

[0038] A rare-earth low-alloy steel for resisting sulfuric acid dew point corrosion is designed and manufactured using the following process flow:

[0039] Hot metal pretreatment → converter smelting → refining → continuous casting → slab heating → high-pressure water descaling → controlled rolling → laminar flow cooling → coiling.

[0040] The aforementioned molten iron pretreatment requires pre-desulfurization when the sulfur content of the molten iron is greater than 0.040%.

[0041] The converter smelting adopts a low-carbon steel smelting mode, and the converter uses strong bottom blowing with no additional blowing at the end point to ensure that the content of the main chemical components of the molten steel meets the requirements.

[0042] The refining process employs the LF refining clean steel smelting process, involving alloying of all components and fine-tuning of the temperature in a refining furnace. During the refining process, antimony-iron is added after the formation of white slag.

[0043] In the continuous casting process, a special protective slag is used throughout the pouring of molten steel, and the slab thickness is 210mm. During continuous casting, a special protective slag for peritectic steel is used, and the casting speed is generally controlled at 0.8m / min-1.2m / min to ensure the surface and internal quality of the slab.

[0044] During production, hot-charging rolling is preferred for slabs, and the time interval between slab cutting and furnace entry should be ≤4 hours. Rolling should only proceed after confirming that there are no cracks on the surface of the cast slab; if cracks are found, they must be cleaned before rolling.

[0045] In the slab heating process, the slab is heated to 1180-1250℃ for 140-160 minutes. Simultaneously, a reducing atmosphere is maintained inside the heating furnace to minimize the adverse effects of the oxide layer on the slab surface on the surface quality of the rolled steel plate.

[0046] The high-pressure water descaling process ensures the complete removal of iron oxide scale from the surface, meeting surface quality requirements.

[0047] The controlled rolling process includes roughing, hot roll forming, finishing, laminar flow cooling, and coiling.

[0048] The roughing process employs a "3+5" pass reciprocating rolling method, with the final pass exit temperature at 1050-1130℃.

[0049] The finishing rolling process consists of 7 consecutive passes, with a final rolling temperature of 850-890℃. An insulation cover should be used to reduce the temperature difference between the beginning and end of the intermediate billet and prevent excessive temperature drop at the edges.

[0050] The laminar flow cooling is performed by front-end centralized laminar flow cooling after finishing rolling, with a cooling rate of 10-30℃ / s.

[0051] In the winding process, the winding cooling adopts a centralized front-end cooling method, and then the temperature is controlled at 580-650℃ during winding. If the winding temperature is too high or too low, it will affect the oxygen pressure on the surface of the hot-rolled plate and the "copper embrittlement" phenomenon.

[0052] Furthermore, a hot-rolled steel plate with a thickness of 4mm was finally produced, resulting in the corresponding steel product resistant to sulfuric acid dew point corrosion.

[0053] Example

[0054] A rare earth low alloy steel resistant to high-temperature sulfuric acid dew point corrosion according to the present invention, the chemical composition of which is shown in Table 1.

[0055] Table 1: Chemical composition (mass %) of this embodiment

[0056] Ingredients C％ Si% Mn% P％ S％ Cu% Cr％ Sb% Ni% Mo% RE% Example 0.095 0.30 1.45 0.015 0.013 0.40 1.15 0.13 0.17 0.15 0.25

[0057] The experimental materials were manufactured using the alloy composition described in this embodiment. The experimental hanging plates were wire-cut into rectangular pieces of 50mm × 30mm × 4.5mm, and φ3mm holes were drilled in the samples for easy hanging. Immersion tests were conducted for 24h, 48h, 72h, and 96h according to national standard GB10124 under conditions of 70% H2SO4–110℃ and 90% H2SO4–160℃.

[0058] Experimental results are as follows Figure 1 As shown. From Figure 1 It can be observed that under conditions of 70% H₂SO₄–110℃ and 90% H₂SO₄–160℃, the corrosion weight loss rate of this invention is 0.11–0.15 mm / a, which is a low corrosion rate. Furthermore, the corrosion rate gradually decreases and tends to level off with increasing immersion time, indicating the formation of a dense corrosion product film on the metal surface, which inhibits further corrosion. This invention exhibits excellent resistance to high-temperature sulfuric acid dew point corrosion.

[0059] Comparative Example 1

[0060] The alloy composition of the material in Comparative Example 1 is shown in Table 2.

[0061] Table 2: Chemical composition (mass%) of the experimental materials in Comparative Example 1

[0062] Ingredients C％ Si% Mn% P％ S％ Cu% Cr％ Sb% Ni% Mo% RE% Example 0.097 0.30 1.40 0.010 0.012 0.53 0.80 0.11 0.15 0.16 0.20

[0063] The experimental materials were manufactured using the alloy steel described in Comparative Example 1. The experimental hanging plates were wire-cut into rectangular pieces measuring 50mm × 30mm × 4.5mm, and φ3mm holes were drilled in the samples for easy hanging. Immersion tests were conducted for 24h, 48h, 72h, and 96h according to national standard GB10124 under the conditions of 10% H2SO4–30℃, 30% H2SO4–50℃, and 50% H2SO4–0℃.

[0064] Experimental results are as follows Figure 2 As shown. From Figure 2 It can be found that under the conditions of 70% H2SO4–110℃ and 90% H2SO4–160℃, the corrosion weight loss rate of Comparative Example 1 is 0.10–0.18 mm / a, which is relatively high compared with the present invention. Moreover, the corrosion rate gradually increases with the extension of immersion time, indicating that a protective corrosion product film was not formed on the metal surface.

[0065] This is because the Cu / S ratio in Comparative Example 1 was 44, exceeding the copper-sulfur ratio of the present invention. This resulted in insufficient S content, preventing the formation of the Cu₂S surface film. Cu simply accumulated on the surface, increasing the anodic area and accelerating corrosion. Consequently, its resistance to high-temperature sulfuric acid dew point corrosion was poor.

[0066] Comparative Example 2

[0067] The alloy composition of the material in Comparative Example 2 is shown in Table 3.

[0068] Table 3: Chemical composition (mass%) of the experimental materials in Comparative Example 2

[0069] Ingredients C％ Si% Mn% P％ S％ Cu% Cr％ Sb% Ni% Mo% Example 0.098 0.37 1.45 0.020 0.013 0.49 0.95 0.10 0.16 0.18

[0070] The experimental materials were manufactured using the alloy steel described in Comparative Example 2. The experimental hanging plates were wire-cut into rectangular pieces of 50mm × 30mm × 4.5mm, and φ3mm holes were drilled in the samples for easy hanging. Immersion tests were conducted for 24h+48h+72h+96h according to national standard GB101248 at 10% H2SO4–30℃, 30% H2SO4–50℃, and 50% H2SO4–70℃.

[0071] Experimental results are as follows Figure 3 As shown. From Figure 3 It can be found that under the conditions of 70% H2SO4–110℃ and 90% H2SO4–160℃, the corrosion weight loss rate of Comparative Example 2 is 0.10–0.20 mm / a, which is a further increase compared with the present invention and Comparative Example 1. Moreover, the corrosion rate gradually increases with the extension of immersion time, indicating that a protective corrosion product film was not formed on the metal surface.

[0072] This is because Comparative Example 2 did not add rare earth elements (RE), resulting in the inability to control the morphology of Sb inclusions in the steel and thus failing to improve corrosion resistance. Furthermore, the Cu / S ratio in Comparative Example 2 was 38, exceeding the copper-sulfur ratio of this invention, leading to insufficient S content. Consequently, the Cu₂S surface film could not form, and Cu simply accumulated on the surface, increasing the anodic area and accelerating corrosion. This resulted in poor resistance to high-temperature sulfuric acid dew point corrosion.

[0073] The high-temperature sulfuric acid dew point corrosion resistance of this invention is significantly better than that of other sulfuric acid dew point corrosion resistant steels. It has excellent high-temperature sulfuric acid dew point corrosion resistance and is widely applicable to petroleum and petrochemical equipment, process pipeline corrosive environments, and flue gas treatment systems that use coal or heavy oil as the main fuel.

Claims

1. A rare earth low-alloy steel for resisting sulfuric acid dew point corrosion, wherein the rare earth low-alloy steel for resisting sulfuric acid dew point corrosion contains the following chemical composition by weight percentage: C≤0.15%; Si≤0.50%; Mn≤1.80%; P≤0.025%; S≤0.025%; Cu0.25%–0.50%; Cr0.50%–1.30%; Ni0.1–0.50%; Mo:0.01–0.5%; Sb≤0.15%; RE(La,Ce):0.10–0.50%; wherein Cu / S is 30-40 and RE(La,Ce) / S is 10-30.

2. The chemical composition of the rare earth low alloy steel resistant to sulfuric acid dew point corrosion is optimized as follows (weight percentage): C: 0.050-0.100%, Si: 0.20-0.40%, Mn: 1.00-1.50%, P: 0.005-0.020%, S: 0.005-0.020%, Cu: 0.30-0.50%, Cr: 0.70%-1.20%, Ni: 0.10-0.20%, Mo: 0.10-0.20%, Sb: 0.10-0.15%, RE(La,Ce): 0.15-0.30%; the remainder is Fe.

3. A rare earth low alloy steel composition design and manufacturing method for sulfuric acid dew point corrosion resistance, comprising the following process flow: molten iron pretreatment → converter smelting → refining → continuous casting → slab heating → high-pressure water descaling → controlled rolling → laminar flow cooling → coiling.

4. This invention achieves optimal resistance to sulfuric acid dew point corrosion in steel by controlling the weight percentage of various components. Compared with existing technologies, this invention has the following significant advantages: (1) Based on weathering steel, this invention mainly adds Cu, Cr, Ni and RE to develop Cu-Cr-Ni-RE series of low alloy steels. By adding RE (rare earth) to control the morphology of Sb inclusions in the steel, the impact toughness and corrosion resistance of the steel can be further improved. It is mainly used in high temperature and high concentration sulfuric acid corrosion environments. (2) This invention achieves optimal high-temperature sulfuric acid dew point corrosion resistance in steel by controlling the weight percentage of various components. The material manufactured using this invention underwent immersion tests according to national standard GB10124 at 70% H2SO4–110℃ and 90% H2SO4–160℃. Comparison with other sulfuric acid dew point corrosion resistant steels showed that the high-temperature sulfuric acid dew point corrosion resistance of this invention is significantly superior. This invention possesses excellent high-temperature sulfuric acid dew point corrosion resistance and is widely applicable to corrosive environments in petroleum and petrochemical equipment and process pipelines, as well as flue gas treatment systems using coal or heavy oil as primary fuels.

Images