Melting analysis and mechanical properties of the material for assistance

Low-alloy rare earth steel resistant to hydrogen sulfide corrosion. The present invention relates to a corrosion-resistant iron-based low-alloy rare earth steel, which is primarily used in petroleum and chemical plants for resisting corrosion caused by H2S, HCN, NH3, CO2, and Cl-. The low-alloy steels currently in use fail to exhibit good performance in terms of resistance to H2S corrosion. Although typical low-alloy Cr, Mo steels do possess a certain degree of corrosion resistance, the brittleness and hardness induced by Cr and Mo in these steels result in poor cold-forming and welding properties. For example, the 12Cr2AlMoV steel developed by Shanghai Iron and Steel Plant No. 3, which is designed to resist stress corrosion, suffers from reduced overall performance due to the presence in it of not only Cr and Mo elements that contribute to brittleness and hardness, but also V, which further enhances these tendencies as well as the risk of welding cold cracks. As a result, its widespread application is limited. Similarly, Cr4NiAlCuMo developed by Chongqing Special Steel and Xining Special Steel Companies in the 1980s also failed to be widely adopted due to its poor cold working and welding properties. None of the aforementioned steel grades take into account the beneficial effect of RE (rare earths) on the corrosion resistance, as well as on the welding and cold working properties of steel. The objective of this invention is to utilize China’s abundant rare earth resources to develop a low-alloy rare earth steel product that is resistant to high-temperature oxidation, as well as to H2S and Cl– induced stress corrosion at low temperatures, and possesses high resistance to uniform corrosion. To achieve the objectives of the present invention, the technical solution adopted is one composed of C, Si, Mn, Cr, Mo, Al, S, P, and RE, with the remainder balanced by Fe. The chemical composition of this rare earth alloy steel (based on the proportions in the molten steel) is as follows: C: 0.02–0.15%, Si: 0.15–0.90%, Mn: 0.30–1.50%, Cr: 1.00–5.50%, Al: 0.20–1.20%, Mo: 0.20–1.20%, RE: ≤0.150%, S: ≤0.035%, P: ≤0.035%; the remainder being iron (Fe). Depending on the intended use, elements such as Nb, V, and Ti can also be added to this invention, with the addition levels being Nb ≤ 0.40%, Ti ≤ 0.40%, and V ≤ 0.15%. In the component design of this invention, Cr, Al, and Mo are the main corrosion-resistant elements. At the same time, Al and V also play a role in refining the grain structure. Although there is no evidence indicating that RE (rare earth elements) have a direct and independent corrosion-resistant effect, their alloying effect in steel can significantly improve the steel’s corrosion resistance and machinability. Due to its high reactivity, RE has a strong affinity for the impurity elements C, S, P in steel, as well as for harmful inclusions; it can thereby improve the purity of the steel and control the shape of these inclusions, transforming linear MnS inclusions into small, hammer-shaped or spherical particles that are evenly distributed throughout the steel. When RE is present in an optimal amount, the steel exhibits the best corrosion resistance and superior impact toughness. Furthermore, RE (rare earths) are abundant in China; their price is only about 1/8 of that of V, and the amount used is also only around 1/10 of that used for V. The addition amount of RE shall not exceed 0.15%. The residual amount of RE in the steel must be at least as high as the content of S, and at most 6 times higher than that value; that is, RE/S = 1.0–6.0. Generally, a RE/S ratio of 2.0–3.0 is optimal. Since the technology of this invention uses RE (rare earths), it not only improves its corrosion resistance but also enhances the overall machinability of the steel. Within the range of components specified in this invention, the resulting alloy steels exhibit resistance to high-temperature oxidation and high-temperature sulfidic corrosion, as well as resistance to uniform corrosion caused by H2S, HCN, and CO2. They are also resistant to stress corrosion induced by H2S and Cl-. Furthermore, they can be used as high-strength hydrogen-resistant steels. Examples of its implementation are as follows: Example 1. Production of steel resistant to H2S stress corrosion and H2S uniform corrosion; the composition of this steel is as follows (by weight percentage): C≤0.10%, Si: 0.20~0.65%, Mn: 0.30~0.90%, Cr: 1.80~2.50%, Al: 0.30~0.80%, Mo: 0.30~0.60%, RE: 0.005~0.030%, S≤0.015%, P≤0.025%, with the remainder being Fe. The designation for the rare earth alloy steel produced by this method is temporarily set as 08Cr2AlMoRE or 09Cr2AlMoRE. Laboratory tests have shown that the stress corrosion resistance under constant load for both the base material and the weld is σth≥0.75σs and σth≥0.70σs respectively, which is well above the requirement of σth≥0.45σs specified in NACE MRO175-97 by the American Society of Corrosion Engineers. The comparison data on the uniform corrosion resistance of 12Cr2AlMoV and 08Cr2AlMo without RE are as follows: Comparison of corrosion tests under H2S simulated conditions (immersion time: 144 hours). H2S concentration in the gas at the top of the catalytic system: 979 ppm; temperature: 100°C. 08Cr2AlMoRE, 12Cr2AlMoV, 08Cr2AlMo, 10# steel. Corrosion rate: V10–2 mg/cm2·h; values: 1.58, 2.28, 2.42, 9.28. Example 2: Steel with high resistance to high-temperature sulfur and high-temperature oxidation corrosion, as well as high strength against hydrogen (H) attack. The composition of this steel is as follows (by weight percentage): C ≤ 0.12%, Si: 0.30–0.90%, Mn: 0.50–1.20%, Cr: 3.50–5.50%, Al: 0.30–0.80%, Mo: 0.60–1.20%, V: 0.05–0.15%, RE: 0.005–0.060%, S ≤ 0.020%, P ≤ 0.025%; the remainder is Fe. Low-alloy rare earth steel produced by this method, with an increased Cr content and added V, not only enhances the material’s resistance to high-temperature sulfur and high-temperature oxidation corrosion, but its high strength also allows it to be used as a high-strength hydrogen-resistant steel; it can also be employed in steels for molds and cutting tools. The steel produced using the chemical compositions in Example 1 and Example 2 above can be rolled into sheets, pipes, sections, and bars; it can also be used to forge forgings from billets or round bars. The low-alloy rare earth steel of the present invention possesses various corrosion resistance properties; it exhibits good corrosion resistance compared to similar materials such as 12Cr2AlMoV and 08Cr2AlMo without rare earth elements. Moreover, the presence of RE elements in the steel, which have deoxidizing and desulfurizing effects, enables a significant improvement in the weldability of the steel. The present invention uses the same steelmaking and rolling techniques as those for conventional low-alloy steels without rare earths; during steelmaking, RE can be added either during the tapping process or into the ladle after tapping. RE is abundant in the domestic market and relatively inexpensive. The low-alloy rare earth steel of this invention can be widely used in corrosion-resistant environments in industries such as petroleum, chemicals, and power generation.