Low alloy steel resistant to corrosion of H2S-HCl-H2CO3 aqueous solution

Abstract

The invention discloses low alloy steel resistant to corrosion of H2S-HCl-H2CO3 aqueous solution, which comprises the following components in percentage by weight: 0.05-0.18% of C, less than or equal to 0.35% of Si, less than or equal to 0.50% of Mn, less than or equal to 0.015% of S, less than or equal to 0.025% of P, 1.25-3.60% of Cr, 0.15-1.50% of Mo, 0.15-0.62% of Al, less than or equal to 0.010% of N and the balance of Fe and inevitable inclusions, the elements need to meet the condition that Al / N is greater than or equal to 25 at the same time, and Al exists in a simple substance state; under a low alloy steel system, H2S-HCl-H2CO3 coupling aqueous solution corrosion resistance is achieved, the yield strength Re is larger than or equal to 205 MPa, the tensile strength Rm is larger than or equal to 415 MPa, the ductility A is larger than or equal to 25%, and the average corrosion rate in H2S-HCl-H2CO3 coupling aqueous solution corrosion is smaller than or equal to 0.2 mm / a.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of low alloy steel production, and particularly relates to a low alloy steel resistant to H2S-HCl-H2CO3 aqueous solution corrosion. BACKGROUND

[0002] As one of the most important energy sources in the world, the reserves and output of high-quality low-sulfur (S≤0.5%) crude oil cannot meet the demand of global economic operation, especially in China with the energy situation of more coal, less oil and poor gas. In order to meet the demand of China's rapid economic development, since the 1990s, the petrochemical industry in China has blended imported sulfur-containing (0.5%<S≤2%) crude oil, and then evolved into smelting 100% high-sulfur (2%<S) crude oil, and the corrosion of each link in the whole process of petrochemical industry is gradually enhanced. In the process of petrochemical production, in addition to the inherent H2S in the crude oil, other sulfur components will also be converted to H2S, accompanied by HCl and H2CO3, forming an H2S-HCl-H2CO3 aqueous solution corrosion environment, which poses a new challenge to the material selection of petrochemical industry equipment.

[0003] Chinese patent publication No. CN112111698A discloses a steel for refinery exposed pipeline with high corrosion resistance and a preparation method thereof, the chemical composition of the steel is as follows: C: 0.09-0.12%, Si: 0.20-0.50%, Mn: 0.90-1.20%, P≤0.020%, S≤0.005%, Ni: 0.20-0.40%, Cr: 0.80-1.10%, Cu: 0.20-0.40%, Ti: 0.02-0.05%, Zr: 0.01-0.03%, Als: 0.020-0.040%, and the balance is Fe and inevitable inclusions. By adding Cr, Ni, Cu, Ti and Zr, etc. grain refining and corrosion resistance elements, a multi-heat treatment process method of high temperature normalizing + water cooling normalizing + tempering is creatively proposed, starting from the corrosion-resistant steel itself, the corrosion resistance of the material to oil and gas refining conveying medium and industrial atmosphere is improved by means of changing the microstructure, ultra-fining the grain and uniformizing the structure.

[0004] Chinese patent publication No. CN103469094A discloses a pipeline steel resistant to chloride ion corrosion and a preparation method thereof, the chemical composition of the steel is as follows: C: 0.008-0.02%, Si: 0.05-0.15%, Mn: 0.50-0.85%, P≤0.01%, S≤0.010%, Mo: 0.8-2.0%, Als: 0.015-0.050%, Ni: 0.50-1.2%, W: 0.02-0.08%, and the balance is Cr and Fe and inevitable impurities. By adjusting the proportion of alloy elements Cr, Mo, W and C, the steel resistant to chloride ion corrosion is obtained.

[0005] Chinese patent publication No. CN1729306A discloses a high-strength martensitic stainless steel having excellent resistance to carbon dioxide gas corrosion and sulfide stress corrosion cracking, with chemical elements mass percentage of C: 0.005-0.04%, Si: 0.5% or less, Mn: 0.1-3.0%, P: 0.04% or less, S: 0.01% or less, Cr: 10-15%, Ni: 4.0-8%, Mo: 2.8-5.0%, Al: 0.001-0.10%, and N: 0.07% or less, with the balance being Fe and other unavoidable impurities, which can be used in an environment containing carbon dioxide gas, hydrogen sulfide, chloride ions, or two or more of them, and has excellent resistance to carbon dioxide gas corrosion and sulfide stress corrosion cracking.

[0006] Chinese patent publication No. CN1861832A discloses a martensitic stainless steel for oil wells and a method for manufacturing a martensitic stainless steel pipe for oil wells, with chemical elements mass percentage of C: 0.16-0.22%, Si: 0.1-0.8%, Mn: 0.25-1.00%, P: 0.025% or less, S: 0.010% or less, Cr: 12.0-13.5%, Al: 0.010% or less, Ni: 0-0.2%, Cu: 0-0.10%, Mo: 0-0.20%, Ti: 0-0.050%, and N: 0.01-0.1%. The martensitic stainless steel for oil wells has excellent resistance to SSC in the presence of Cl - , wet CO2, and trace amounts of H2S, and also has excellent low-temperature toughness.

[0007] In summary, the current steel types that can resist H2S-HCl-H2CO3 coupling aqueous solution corrosion all belong to the stainless steel system, which has high alloy content and high cost. If a low-alloy steel can be obtained by adjusting the composition to meet the H2S-HCl-H2CO3 coupling aqueous solution corrosion, it will greatly help the development of high-S content process smelting in the petrochemical industry. SUMMARY

[0008] The purpose of the present application is to provide a low-alloy steel that can resist H2S-HCl-H2CO3 aqueous solution corrosion. In the low-alloy steel system, it realizes resistance to H2S-HCl-H2CO3 coupling aqueous solution corrosion, with yield strength Re≥205 MPa, tensile strength Rm≥415 MPa, elongation A≥25%, and average corrosion rate in H2S-HCl-H2CO3 coupling aqueous solution corrosion≤0.2 mm / a.

[0009] To achieve the above purpose, the technical solution of the present application is as follows:

[0010] A low-alloy steel resistant to corrosion by H2S-HCl-H2CO3 aqueous solution comprises the following components by weight percentage: C: 0.05-0.18%, Si≤0.35%, Mn≤0.50%, S≤0.015%, P≤0.025%, Cr: 1.25-3.60%, Mo: 0.15-1.50%, Al: 0.15-0.62%, N≤0.010%, with the balance including Fe and unavoidable inclusions. All the above elements must simultaneously satisfy Al / N≥25, and the Al exists in elemental form.

[0011] Preferably, the low alloy steel composition contains 0.06% to 0.15% C.

[0012] Preferably, the low alloy steel composition contains 1.50-3.30% Cr.

[0013] Preferably, the low alloy steel composition contains 0.25-1.20% Mo.

[0014] Preferably, the low alloy steel composition contains Al: 0.20-0.60%.

[0015] Preferably, in the composition of the low alloy steel, Si ≤ 0.30% and / or Mn ≤ 0.45% and / or S ≤ 0.010% and / or P ≤ 0.020% and / or N ≤ 0.008%.

[0016] Furthermore, the balance consists of Fe and unavoidable inclusions.

[0017] The microstructure of the low-alloy steel described in this invention consists of ferrite and pearlite.

[0018] The low-alloy steel described in this invention has a yield strength Re≥205MPa, a tensile strength Rm≥415MPa, an elongation A≥25%, and an average corrosion rate ≤0.2mm / a in H2S-HCl-H2CO3 coupled aqueous solution corrosion.

[0019] In the composition design of the low-alloy steel resistant to H2S-HCl-H2CO3 aqueous solution corrosion described in this invention:

[0020] Carbon: The oxidation potential of element C is close to that of element Fe, and it is partially retained in molten steel. Element C is not only one of the most common inherent elements in steel, but also one of the most economical elements for ensuring strength. However, excessive carbon content is detrimental to resistance to the corrosion of H2S-HCl-H2CO3 coupled aqueous solutions. Therefore, this invention limits the C content to 0.05-0.18%, preferably C: 0.06-0.15%.

[0021] Silicon: The oxidation potential of element Si is higher than that of element Fe. As the main deoxidizer, it is rarely retained in molten steel, but it is also one of the most common inherent elements in steel. It is beneficial to strength, but not to resistance to corrosion by H2S-HCl-H2CO3 coupled aqueous solution. Moreover, excessive Si content is not conducive to rolling. Therefore, this invention limits Si to ≤0.35%, preferably Si ≤0.30%.

[0022] Manganese: The oxidation potential of element Mn is close to that of element Fe, and it is partially retained in molten steel. It is also one of the most common inherent elements in steel. While beneficial for strength, it is detrimental to resistance to corrosion from H₂S-HCl-H₂CO₃ coupled aqueous solutions. Therefore, this invention limits Mn to ≤0.50%, preferably Mn ≤0.45%.

[0023] Phosphorus: The oxidation potential of element P is close to that of element Fe. It is partially retained in molten steel and is one of the inherent harmful residual elements in conventional steel. It is not conducive to resistance to the corrosion of H2S-HCl-H2CO3 coupled aqueous solution, nor is it conducive to rolling. Therefore, the lower the value, the better. This invention limits P to ≤ 0.025%, preferably P ≤ 0.020%.

[0024] Sulfur: The oxidation potential of element S is close to that of element Fe. It is partially retained in molten steel and is one of the inherent harmful residual elements in conventional steel. It is not conducive to resistance to the corrosion of H2S-HCl-H2CO3 coupled aqueous solution, nor is it conducive to rolling. Therefore, the lower the value, the better. This invention limits S to ≤0.015%, preferably S≤0.010%.

[0025] Chromium: The oxidation potential of element Cr is higher than that of element Fe, which is not only beneficial to mechanical properties such as strength, but also to the formation of an oxide film through self-passivation, which plays a protective role; it not only improves resistance to uniform corrosion, but also helps to resist localized corrosion such as pitting corrosion; however, the effect of Cr on improving resistance to H2S-HCl-H2CO3 coupled aqueous solutions is not very obvious. Cr is also one of the main elements affecting cost. Adding too much will lead to excessive cost and is not conducive to rolling. Therefore, the present invention limits the Cr content to 1.25-3.60%, preferably Cr: 1.50-3.30%.

[0026] Molybdenum (Mo): The oxidation potential of elemental molybdenum (Mo) is similar to that of elemental feldspar (Fe), which is beneficial not only to strength but also to resistance to pitting corrosion. However, Mo's effect on improving resistance to corrosion from H₂S-HCl-H₂CO₃ coupled aqueous solutions is not very significant. Mo is also one of the main elements affecting cost; adding too much will lead to excessively high costs and is also detrimental to rolling. Therefore, this invention limits the Mo content to 0.15–1.50%, preferably 0.25–1.20%.

[0027] Nitrogen: Element N is beneficial to strength and is not required in conventional steel. However, it easily forms nitrides such as XN with alloying elements. Nitrides are easily soluble in H2S-HCl-H2CO3 coupled aqueous solutions, which is not conducive to corrosion resistance. Therefore, this invention limits N to ≤0.010%, preferably N ≤0.008%.

[0028] Aluminum: The oxidation potential of elemental Al is higher than that of elemental Fe. Studies have shown that solid-solid elemental Al significantly improves resistance to corrosion from H2S-HCl-H2CO3 coupled aqueous solutions. At the same content, solid-solid elemental Al improves corrosion resistance far more effectively than Cr and Mo. However, excessively high Al content is detrimental to rolling. Therefore, this invention limits the Al content to 0.15–0.62%, preferably 0.20–0.60%.

[0029] Al is a strong nitrogen compound-forming element, and a high nitrogen content leads to excessive consumption of elemental Al due to the formation of aluminum nitride. On one hand, aluminum nitride is soluble in acidic solutions, reducing resistance to corrosion from H₂S-HCl-H₂CO₃ coupled aqueous solutions. On the other hand, given a fixed Al content, the more aluminum nitride is formed, the lower the content of elemental Al, thus reducing the matrix's resistance to H₂S-HCl-H₂CO₃ coupled aqueous solutions. Therefore, to ensure that elemental Al is more dispersed in the matrix in its elemental form and to guarantee the low-alloy steel's resistance to H₂S-HCl-H₂CO₃ coupled aqueous solutions, this invention limits the Al / N ratio to ≥25, preferably Al / N ≥30.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0031] In this invention, Al is added as a core element to improve the corrosion resistance of low-alloy steel to H2S-HCl-H2CO3 coupled aqueous solutions. By limiting the upper limit of the content of the unfavorable element N (N≤0.010%) and the lower limit of the Al / N ratio (Al / N≥25), the corrosion resistance of Al in elemental form is maximized and dispersed in the matrix. Combined with the addition of elements such as Cr and Mo, the steel's resistance to H2S-HCl-H2CO3 coupled aqueous solutions is achieved in the low-alloy steel system. In contrast, in traditional alloy steel compositions, Al is often added as a deoxidizing element, and Al often exists in the matrix as a compound. It is also one of the inherent residual harmful elements in conventional steels because it easily forms inclusions. Therefore, it is strictly limited in conventional steels and is not intentionally added as a corrosion-resistant element.

[0032] The corrosion resistant H2S-HCl-H2CO3 aqueous solution obtained by this invention is a low alloy steel system. In contrast, the existing technologies that can obtain steels that can withstand H2S-HCl-H2CO3 coupled aqueous solution corrosion are all stainless steel systems, with an alloy content of more than 12%, which is costly.

[0033] The present invention yields low-alloy steel with a yield strength Re≥205MPa, tensile strength Rm≥415MPa, elongation A≥25%, and an average corrosion rate ≤0.2mm / a in H2S-HCl-H2CO3 coupled aqueous solution corrosion. Detailed Implementation

[0034] The present invention will be further described below with reference to embodiments and comparative examples.

[0035] The composition of the steels in the embodiments and comparative examples of the present invention is shown in Table 1, and the tensile properties of the steels obtained in the embodiments and comparative examples of the present invention are shown in Table 2.

[0036] The steel samples of the present invention and the comparative examples were immersed in petrochemical field water (water sample composition is shown in Table 3) at 200℃ for 168h, and the average corrosion rates obtained are shown in Table 4.

[0037] In Comparative Example 1, although the contents of Al and N elements are within the range of this invention, the Al / N ratio is 15, which is much smaller than the Al / N ≥ 25 specified in this invention. This indicates that the Al element exists primarily as AlN rather than in its elemental state. The steel contains 7.12% Cr+Mo, and the (Cr+Mo) / Al ratio is approximately 48. The resulting steel exhibits high yield strength and tensile strength, but its average corrosion rate in the H2S-HCl-H2CO3 coupled aqueous solution is 0.52 mm / a, indicating poor resistance to corrosion from this solution.

[0038] In Comparative Example 2, the Al content was low, while the N content was high, resulting in an Al / N ratio of 7, which is far less than the Al / N ≥ 25 specified in this invention. This indicates that the Al element exists primarily as AlN rather than in its elemental form. The steel contained 6.95% Cr+Mo, and the (Cr+Mo) / Al ratio was 71. The resulting steel exhibited high yield strength and tensile strength, but its average corrosion rate in the H2S-HCl-H2CO3 coupled aqueous solution was 0.57 mm / a, indicating poor resistance to corrosion from this solution.

[0039] In Comparative Example 3, the Al content was low. Although the N content and Al / N ratio met the requirements of this invention, the low Al content resulted in a low content of elemental Al. The steel contained 8.6% Cr+Mo, with a (Cr+Mo) / Al ratio of 173. The resulting steel exhibited high yield strength and tensile strength, but the average corrosion rate in the H2S-HCl-H2CO3 coupled aqueous solution was 0.41 mm / a, indicating poor resistance to corrosion from this solution. The higher Cr content...

[0040] In Comparative Example 4, the Al content was low, with Al / N = 24, which is less than the Al / N ≥ 25 specified in this invention. The steel grade had a Cr+Mo content of 9.25% and a (Cr+Mo) / Al ratio of 78. The resulting steel had high yield strength and tensile strength, but its average corrosion rate in the H2S-HCl-H2CO3 coupled aqueous solution was 0.52 mm / a, indicating poor resistance to corrosion from the H2S-HCl-H2CO3 coupled aqueous solution.

[0041] In Comparative Example 5, the Cr content was high, while the Al content was low. Although the N content and Al / N ratio met the requirements of this invention, the low Al content, particularly the low content of elemental Al, resulted in a low overall Al content. The steel achieved a Cr+Mo content of 8.27% and an (Cr+Mo) / Al ratio of 60, resulting in high yield strength and tensile strength. However, the average corrosion rate in the H2S-HCl-H2CO3 coupled aqueous solution was 0.55 mm / a, indicating poor resistance to corrosion from this solution.

[0042] In Comparative Example 6, the Cr content was high, while the Al content was low, with an Al / N ratio of 10, far less than the Al / N ≥ 25 specified in this invention. This indicates that the Al element existed primarily as AlN rather than in its elemental form. The steel contained 11.2% Cr + Mo, resulting in a (Cr + Mo) / Al ratio of 141. Although the final steel exhibited high yield strength and tensile strength, the extremely low elemental aluminum content led to an average corrosion rate of only 0.64 mm / a in the H₂S-HCl-H₂CO₃ coupled aqueous solution, demonstrating poor resistance to H₂S-HCl-H₂CO₃ corrosion. This proves that even high Cr and Mo contents do not necessarily improve resistance to H₂S-HCl-H₂CO₃ coupled aqueous solutions.

[0043] The correlation between the content of elements Cr, Mo, and Al and corrosion resistance in the embodiments and comparative examples 1-6 of this invention demonstrates that elemental Al significantly improves resistance to H₂S-HCl-H₂CO₃ coupled aqueous solutions than elemental Cr and / or elemental Mo. The correlation between Al / N coupling and corrosion resistance indicates that only a dispersed solid solution distribution of elemental Al can maximize its ability to improve resistance to H₂S-HCl-H₂CO₃ coupled aqueous solutions.

[0044] In the embodiments of this invention, the Cr+Mo content in the steel is ≤4.7%, which is lower than the Cr+Mo content in comparative examples 1 to 6. It also simultaneously satisfies the requirements of Al: 0.15–0.62%, N ≤0.010%, and Al / N ≥25. The average corrosion rate of the obtained steel in petrochemical field water is ≤0.2 mm / a. This demonstrates that the technical solution of this invention, which uses low Cr and / or elemental Mo, adds elemental Al, and ensures the content of elemental dispersed solid solution Al, is effective in improving the corrosion resistance of steel to H2S-HCl-H2CO3 coupled aqueous solutions.

[0045]

[0046]

[0047]

[0048]

Claims

1. A low-alloy steel resistant to corrosion by H2S-HCl-H2CO3 aqueous solution, characterized in that, Its composition by weight percentage includes: C: 0.05-0.18%, Si≤0.35%, Mn≤0.50%, S≤0.015%, P≤0.025%, Cr: 1.25-3.60%, Mo: 0.15-1.50%, Al: 0.15-0.62%, N≤0.010%, with the balance including Fe and unavoidable inclusions. All of the above elements must simultaneously satisfy Al / N≥25, and the Al exists in elemental form.

2. The low-alloy steel resistant to corrosion by H2S-HCl-H2CO3 aqueous solution as described in claim 1, characterized in that, The low alloy steel composition contains 0.06% to 0.15% C.

3. The low-alloy steel resistant to corrosion by H2S-HCl-H2CO3 aqueous solution as described in claim 1, characterized in that, The low alloy steel composition contains Cr: 1.50–3.30%.

4. The low-alloy steel resistant to corrosion by H2S-HCl-H2CO3 aqueous solution as described in claim 1, characterized in that, The low alloy steel composition contains 0.25–1.20% Mo.

5. The low-alloy steel resistant to corrosion by H2S-HCl-H2CO3 aqueous solution as described in claim 1, characterized in that, The low alloy steel composition contains Al: 0.20–0.60%.

6. The low-alloy steel resistant to corrosion by H2S-HCl-H2CO3 aqueous solution as described in claim 1, characterized in that, The low alloy steel composition contains Si ≤ 0.30% and / or Mn ≤ 0.45% and / or S ≤ 0.010% and / or P ≤ 0.020% and / or N ≤ 0.008%.

7. The low-alloy steel resistant to corrosion by H2S-HCl-H2CO3 aqueous solution as described in claim 1, characterized in that, The ratio of Al to N satisfies Al / N≥30.

8. The low-alloy steel resistant to corrosion by H2S-HCl-H2CO3 aqueous solution as described in any one of claims 1 to 7, characterized in that, The balance is Fe and unavoidable inclusions.

9. The low-alloy steel resistant to corrosion by H2S-HCl-H2CO3 aqueous solution as described in any one of claims 1 to 8, characterized in that, The microstructure of the low alloy steel consists of ferrite and pearlite.

10. The low-alloy steel resistant to corrosion by H2S-HCl-H2CO3 aqueous solution as described in any one of claims 1 to 9, characterized in that, The low-alloy steel has a yield strength Re≥205MPa, tensile strength Rm≥415MPa, elongation A≥25%, and an average corrosion rate ≤0.2mm / a in H2S-HCl-H2CO3 coupled aqueous solution corrosion.

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