Duplex stainless steel for concentrated sulfuric acid environment as well as preparation method and application of duplex stainless steel

Abstract

The invention provides duplex stainless steel for a concentrated sulfuric acid environment and a preparation method and application of the duplex stainless steel, and the duplex stainless steel comprises the following components in percentage by weight: less than or equal to 0.03 wt% of C, less than or equal to 1.50 wt% of Mn, less than or equal to 2.0 wt% of Si, less than or equal to 0.03 wt% of P, less than or equal to 0.02 wt% of S, 24.5-27.0 wt% of Cr, 7.0-9.0 wt% of Ni, 3.5-4.5 wt% of Mo, 3.0-4.0 wt% of Cu, 0.50-1.0 wt% of W, 0.20-0.30 wt% of N and the balance of Fe and inevitable impurities. Through alloying of low carbon, high chromium, added nickel, copper, tungsten, nitrogen and other elements and coordination of all the components, the duplex stainless steel has good obdurability, sulfuric acid uniform corrosion resistance, erosion corrosion resistance, cavitation corrosion resistance and the like. The prepared duplex stainless steel can serve as an equipment material which is subjected to concentrated sulfuric acid and long-term scouring of fluid, the service time of the material can be remarkably prolonged, the maintenance cost and the construction period of the material are reduced, and the safety reliability and the production efficiency of fluid equipment operation are improved.

Description

Technical Field

[0001] The present invention relates to the field of anti-corrosion materials, and more particularly, to a duplex stainless steel for concentrated sulfuric acid environment, a preparation method thereof, and an application thereof. Background Art

[0002] In concentrated sulfuric acid environments such as petroleum refining, sulfuric acid industry, and flue gas desulfurization, the concentration of sulfuric acid can reach 85-100%. Concentrated sulfuric acid is a strong oxidizing acid with strong corrosiveness. Currently, steel materials used in concentrated sulfuric acid corrosion media environments include high-silicon cast iron, high-nickel cast iron, high-alloy stainless steel, nickel-molybdenum, and nickel-molybdenum-chromium alloys. The corrosion mechanism of metal materials in concentrated sulfuric acid is very complex.

[0003] Carbon steel and cast iron: Ordinary steel is widely used in 80-100% sulfuric acid, and the temperature can reach 60-80°C. Steel forms a protective film layer on the surface in sulfuric acid of this concentration and temperature range, making it have certain corrosion resistance in sulfuric acid. However, in the applicable concentration and temperature range mentioned above, steel and cast iron can only be used as storage tanks, pipelines, tank trucks, and ship cabins, etc., and are not suitable for high-speed flowing sulfuric acid (flow rate greater than 0.9 m / s) because the surface protective film layer may be damaged. When cast iron and carbon steel are used together in sulfuric acid of about 100%, galvanic corrosion may occur.

[0004] High-silicon cast iron: The typical composition of high-silicon cast iron is 0.95% carbon, 14.5% silicon, and the rest is iron. Judging from the composition, it seems to belong to steel, but from the perspective of structure and properties, it is similar to cast iron. High-silicon cast iron has better corrosion resistance than other engineering alloys for all concentrations and temperatures (up to the boiling point) of sulfuric acid, so it has been widely used in equipment with high temperature and alternating contact with dilute and concentrated sulfuric acid. However, high-silicon cast iron has defects such as high hardness, difficult processing, poor resistance to thermal shock and mechanical shock, is very sensitive to temperature changes, and will crack and damage under rapid cooling and heating, and is also more likely to crack when subjected to mechanical vibration or impact. Therefore, special care must be taken during manufacturing, installation, and use, and sudden heating and cooling should be avoided as much as possible during high-temperature use. If the sulfuric acid contains fluorides, sulfurous acid, etc., high-silicon cast iron will be severely corroded and cannot be used.

[0005] High-nickel cast iron: High-nickel cast iron is suitable for extremely dilute (<10%) non-aerated sulfuric acid and non-oxidizing sulfates at medium temperatures and below. Increasing the temperature, flow rate, and air content will increase corrosion. Its corrosion resistance to concentrated sulfuric acid is similar to that of ordinary cast iron and is used in service conditions with a concentration of 70-100% and a temperature up to 70-100°C.

[0006] Stainless steel: Ordinary stainless steel has poor corrosion resistance to sulfuric acid and is usually not used in equipment that comes into contact with sulfuric acid. All chromium stainless steels are not suitable for sulfuric acid in any concentration without aeration; chromium-nickel stainless steel can be used in dilute sulfuric acid below 5% and concentrated sulfuric acid above 90% at room temperature; chromium-nickel-molybdenum stainless steel containing molybdenum has better corrosion resistance, but the use temperature should not exceed 50 - 70 °C; silicon-containing stainless steel 1Cr18Ni11Si4AlTi has better corrosion resistance than ordinary stainless steel in concentrated sulfuric acid and can be used in the working environment of concentrated sulfuric acid at 60 - 100 °C.

[0007] Chromium-nickel-molybdenum-copper alloy: Chromium-nickel-molybdenum-copper alloy is an alloy specially developed for use in sulfuric acid. Well-known foreign grades include Durimet 20 (Dareme), Esco 20 (Esco), Carpenter 20 (Carpenter) (forged state), FA-20, etc. They are usually collectively referred to as alloy No. 20. These alloys are all austenitic structures and have mechanical properties similar to those of 18-8 chromium-nickel stainless steel. Alloy 20 has good corrosion resistance to sulfuric acid and fuming sulfuric acid in any concentration at room temperature. As the temperature rises, the corrosion rate also increases. It has good corrosion resistance to boiling acid below 10% concentration, acid at 75 - 95%, 50 - 80 °C, and acid at 95 - 100%, 80 - 100 °C. For fuming sulfuric acid, the temperature can reach 200 °C. The corrosion resistance of alloy 20 in sulfuric acid is inferior to that of high-silicon cast iron, and it is much more expensive.

[0008] Nickel-chromium-iron alloy: The corrosion resistance of nickel-chromium-iron alloy to sulfuric acid is similar to that of ordinary stainless steel. It has good corrosion resistance to dilute sulfuric acid below 10%, and the temperature can reach 80 °C. It is also corrosion-resistant to concentrated sulfuric acid above 80% and fuming sulfuric acid. Concentrated sulfuric acid or the presence of oxidants can passivate it, but it is not suitable for high temperatures. Due to its high price, nickel-chromium-iron alloy has limited use in sulfuric acid.

[0009] Nickel-molybdenum alloy and nickel-chromium-molybdenum alloy: Nickel-molybdenum alloy includes cast Chlorimet No. 2 and Hastelloy B alloy that can be cast or forged, etc., which is composed of 2 / 3 nickel and 1 / 3 molybdenum. Excellent corrosion resistance can be obtained after solution heat treatment. Nickel-chromium-molybdenum alloy includes cast Chlorimet No. 3 alloy and Hastelloy C alloy that can be cast and forged, etc., a nickel-based alloy containing 18% chromium and 18% molybdenum. Nickel-molybdenum alloy and nickel-chromium-molybdenum alloy are suitable for sulfuric acid in all concentrations below 96 - 100%, and the temperature can reach 70 °C. The lower the acid concentration, the higher the applicable temperature. Dilute acid below 25% can be used at the boiling point. Generally speaking, nickel-molybdenum alloy is suitable for reducing environments, and nickel-chromium-molybdenum alloy is suitable for oxidizing environments. Due to their high prices, both are only used in working environments with extremely severe corrosion.

[0010] In the application of the above-mentioned steel materials in concentrated sulfuric acid environment, due to the corrosion characteristics of concentrated sulfuric acid, carbon steel and ordinary cast iron can only be used as static sulfuric acid equipment materials such as storage tanks, pipelines, tank trucks and ship cabins, and are not used as fluid equipment materials. High-silicon cast iron has high hardness and good corrosion resistance, but its manufacturing and processing performance is poor, and the installation, maintenance and use requirements are harsh; the manufacturing process performance of high-nickel cast iron is not as good as that of steel, its metallographic structure is austenite, its strength and hardness are low, and its resistance to erosion-corrosion and cavitation corrosion is poor; the corrosion resistance of chromium-nickel-molybdenum-copper alloy is inferior to that of high-silicon cast iron, and its price is much more expensive, but its strength is relatively high and it is widely used; nickel-chromium-iron alloy, nickel-molybdenum alloy and nickel-chromium-molybdenum alloy are expensive, and their wide application is restricted; although the corrosion resistance of stainless steel is not as good as that of the above alloys, its strength and hardness are relatively high, especially silicon-containing stainless steel and duplex stainless steel also have good corrosion resistance to concentrated sulfuric acid.

[0011] Among fluid equipment materials, except for high-silicon cast iron (with a hardness of about HB500), the hardness of the other materials is generally HB120 - 220. As fluid equipment materials, especially impeller materials, their hardness is relatively soft, and their ability to resist uniform corrosion, erosion-corrosion and cavitation corrosion of concentrated sulfuric acid is weak. As a result, the impeller needs to be replaced after running in the concentrated sulfuric acid working condition environment for 1 - 3 years, which seriously affects the working efficiency of fluid equipment and the production efficiency of the entire production system. Although a hardening layer (such as chromium oxide, aluminum oxide, tungsten carbide, silicon carbide, etc.) can be coated on the surface of the impeller to increase the surface hardness of the impeller and enhance its ability to resist fluid erosion and cavitation corrosion, due to problems such as low bonding strength between the hardening layer and the matrix structure, large differences in the ability of the hardening layer to resist oxidation corrosion of concentrated sulfuric acid, poor strength and toughness of the hardening layer, and process defects in the hardening treatment, the surface hardening treatment of the impeller is rarely used in the concentrated sulfuric acid working condition environment.

[0012] CN112522642A discloses a tungsten-containing rare-earth-free economical duplex stainless steel and its preparation method. By adding tungsten to control the proportion of two phases and adjust the proportion relationship of other elements in the steel in the two phases, a solution-state structure with optimal performance is obtained, improving the corrosion resistance and tensile strength. However, when applied to equipment such as impellers that require high-speed rotation, it cannot resist uniform corrosion, erosion-corrosion and cavitation corrosion of concentrated sulfuric acid. Summary of the Invention

[0013] In view of this, the present invention aims to provide a duplex stainless steel for concentrated sulfuric acid environment and its application to solve the problem in the prior art that under the concentrated sulfuric acid working condition environment and fluid erosion conditions, metal materials cannot simultaneously meet the requirements of good strength and toughness and sulfuric acid corrosion resistance.

[0014] To achieve the above object, the technical solution of the present invention is realized as follows:

[0015] A duplex stainless steel for concentrated sulfuric acid environment, comprising components in the following weight percentages: C ≤ 0.03wt%, Mn ≤ 1.50wt%, Si ≤ 2.0wt%, P ≤ 0.03wt%, S ≤ 0.02wt%, Cr: 24.5 - 27.0wt%, Ni: 7.0 - 9.0wt%, Mo: 3.5 - 4.5wt%, Cu: 3.0 - 4.0wt%, W: 0.50 - 1.0wt%, N: 0.20 - 0.30wt%, and the balance being Fe and unavoidable impurities.

[0016] The corrosion resistance mechanisms of the main alloying elements are as follows:

[0017] Carbon (C): It is an element that forms and stabilizes austenite, and its effect is equivalent to 30 times that of nickel, which can significantly improve the strength of stainless steel. However, the carbon content should not be too high. On the one hand, carbon has a large affinity with chromium and is prone to form complex chromium carbides, increasing the intergranular corrosion tendency of stainless steel and leading to an increase in chromium consumption, thus affecting the corrosion resistance of stainless steel. On the other hand, a high carbon content will result in the generation of more brittle phases during the welding process, increasing the embrittlement tendency and cracking risk of the welded joint.

[0018] Manganese (Mn): It is an element that forms austenite and can replace nickel. Together with nitrogen, it serves as an alloying element for economic duplex stainless steel. However, the manganese element content should not be too high. The manganese sulfide inclusions formed by the combination of manganese and sulfur are mostly distributed along the grain boundaries and become pitting corrosion sensitive points. Adding molybdenum to the steel can, to a certain extent, eliminate the adverse effects of manganese, thus enabling the role of manganese in increasing the nitrogen solubility in duplex stainless steel to be exerted.

[0019] Silicon (Si): It strongly forms ferrite in duplex stainless steel, promotes the formation of σ phase, and this effect of silicon becomes more intense with the increase in the chromium and molybdenum contents in the steel. In order to delay the precipitation of σ phase and obtain better toughness for chromium-containing and high-molybdenum duplex stainless steel, the silicon content should not exceed 2%. The influence of silicon on corrosion resistance is very similar to that of silicon on austenitic stainless steel.

[0020] Nickel (Ni): An element that strongly forms and expands the γ phase, it can be considered the primary microstructure-controlling element in duplex stainless steel. Appropriately increasing the nickel content in duplex steel stabilizes the γ phase, expands the γ region, and lowers the martensitic transformation temperature, significantly improving the steel's cold working properties and resistance to pitting, crevice corrosion, and pitting corrosion, as well as sulfuric acid and phosphoric acid corrosion. Studies have shown that for duplex stainless steel containing 5%-7% nickel, variations in nickel have little effect on the steel's crevice corrosion resistance in the solid solution state, but have a significant impact on the steel's crevice corrosion resistance in the sensitized state. Within the duplex range, increasing nickel content increases the γ phase content. The steel reaches its maximum yield strength at approximately 5% nickel, and its maximum tensile strength at approximately 10% nickel. However, in an oxidizing concentrated sulfuric acid environment, especially above 60°C, excessive nickel can negatively impact corrosion resistance. Taking into account the influence of nickel on yield strength, tensile strength, corrosion resistance, and the advantage of tensile strength, the nickel content is controlled at 7.0-9.0%

[0021] Chromium (Cr): A strong element that forms and stabilizes the ferrite phase and reduces the austenite phase. It is also the only industrially valuable element that passivates steel and imparts excellent corrosion resistance and stainless properties. Chromium improves stainless steel's resistance to both reducing and oxidizing media. As the chromium content in steel increases, atmospheric corrosion resistance improves, with the chromium content that causes a sudden change in corrosion resistance being around 12%. In dilute nitric acid, a chromium content of 17%-18% provides satisfactory corrosion resistance. Chromium is a very effective element in improving the pitting corrosion resistance of duplex stainless steel. As the nickel content in steel increases, the chromium content required for good pitting corrosion resistance also increases, which benefits duplex stainless steel's resistance to sulfuric acid. As the chromium content increases, the yield strength of duplex stainless steel also increases. Chromium enhances steel's oxidation resistance, with this performance increasing significantly with increasing chromium content. Therefore, chromium is the most important alloying element for duplex stainless steel for sulfuric acid service, and high chromium content enhances its synergistic effect with nickel, molybdenum, and copper.

[0022] Molybdenum (Mo): As an element that strongly forms ferrite and narrows the γ region. In duplex stainless steel, it is beneficial to the formation of the α phase and also greatly promotes the precipitation of some intermetallic phases, such as the σ phase, χ phase, etc., accelerating the precipitation rate of the σ phase, widening the precipitation range, and shifting the precipitation temperature upward, thereby increasing the embrittlement tendency of duplex stainless steel. For super duplex stainless steel with a molybdenum content > 3.5%, the increase in molybdenum content has a more serious impact on brittleness. This is currently the main obstacle affecting the development and application of high-molybdenum duplex stainless steel with a molybdenum content > 3.5%. However, since molybdenum can significantly improve the pitting corrosion resistance of duplex stainless steel, its addition should be beneficial under stress corrosion conditions originating from pitting corrosion. Molybdenum is an essential alloying element in dilute reducing sulfuric acid; but in concentrated sulfuric acid environment, molybdenum is prone to passivation, and a certain molybdenum content should be present, but too much should not be added.

[0023] Copper (Cu): Adding Cu to Cr-Ni austenitic stainless steel can improve the corrosion resistance of the steel in reducing acids. This effect is also reflected in duplex stainless steel. On the premise of sufficient chromium presence, the combined addition of Cu and Mo is more beneficial to improving the sulfuric acid corrosion resistance of duplex stainless steel. The solubility of Cu in austenite is higher than that in ferrite. During aging treatment, nano-scale copper-rich ε-Cu phases will precipitate in the supersaturated ferrite phase. The ε-Cu phases are nano-scale and are dispersed, which will hinder the movement of dislocations, thereby increasing the hardness of the material and the resistance to erosion-corrosion and cavitation corrosion.

[0024] Tungsten (W): It is a ferrite-forming element, and its role is similar to that of molybdenum. Tungsten can effectively improve the crevice corrosion resistance of duplex stainless steel, which is related to the corrosion inhibition effect of tungsten ions in the solution. Tungsten can also improve the pitting corrosion resistance of duplex stainless steel. Tungsten is added to duplex stainless steel as a substitute for molybdenum. The brittle phases precipitated by it in duplex stainless steel are η, χ, and σ phases, which are different from molybdenum (Mo only has the σ phase). Moreover, since the precipitation temperature of the σ phase is in the upper part of the TTT curve, the nucleation driving force decreases, and the amount of σ phase precipitation becomes very small. In addition, tungsten can also increase the hardness of duplex stainless steel and improve the impeller's resistance to erosion-corrosion and cavitation corrosion.

[0025] Nitrogen (N): It is an element that strongly forms and expands the γ-phase region, and its ability far exceeds that of nickel. Nitrogen can significantly improve the pitting corrosion resistance of duplex stainless steel. The enrichment of nitrogen at the surface film and the interface between the film and the substrate improves the stability of the surface film. Nitrogen plays an important role in preventing single-phase ferrite from appearing after welding in duplex stainless steel, thus leading to the birth of the second-generation nitrogen-containing duplex stainless steel. N above 0.1% can improve the mechanical properties of the steel. When the content of the N element exceeds 0.3%, the δ→γ phase transformation is greatly reduced. At this time, the heat treatment temperature has little effect on the phase ratio of duplex stainless steel. However, for cast duplex stainless steel, such a high nitrogen content is also prone to porosity, but the solubility of nitrogen in duplex stainless steel can be increased by increasing the manganese content.

[0026] Furthermore, the ferrite content in the duplex stainless steel is 40-60%.

[0027] Furthermore, the duplex stainless steel is smelted by the process of pure materials + intermediate frequency furnace.

[0028] Furthermore, the duplex stainless steel is smelted by the process of intermediate frequency furnace / electric arc furnace + AOD furnace / VOD furnace.

[0029] The present invention also provides a preparation method of the above-mentioned duplex stainless steel, including the following steps:

[0030] Add various raw materials in the furnace and melt them by any one of the methods of pure materials + intermediate frequency furnace, or intermediate frequency furnace / electric arc furnace + AOD furnace / VOD furnace;

[0031] Control the content of each alloy element, tap the furnace after the composition is qualified, and tap and pour the steel after reaching the tapping temperature;

[0032] Perform high-temperature solution treatment and aging treatment on the samples obtained by tapping and pouring.

[0033] Furthermore, the temperature of the aging treatment is 350-550°C.

[0034] Furthermore, before tapping, add calcium silicate alloy and aluminum blocks for final deoxidation.

[0035] The present invention also provides an application of the above-mentioned duplex stainless steel for concentrated sulfuric acid environment in equipment materials subjected to the dual effects of concentrated sulfuric acid and fluid erosion.

[0036] Furthermore, the equipment subjected to the dual effects of concentrated sulfuric acid and fluid erosion is an impeller.

[0037] Compared with the prior art, the present invention's duplex stainless steel for concentrated sulfuric acid environment, its preparation method and application have the following advantages:

[0038] (1) The tensile strength of the duplex stainless steel is ≥690 MPa, the yield strength is ≥480 MPa, the elongation is ≥20%, AkV ≥80 J, HRC ≥28, and the ferrite content is ≥40%.

[0039] (2) The duplex stainless steel used in the concentrated sulfuric acid environment has good strength and toughness, and the abilities of resisting uniform corrosion, erosion corrosion, cavitation corrosion, etc. of sulfuric acid, which can significantly increase the service time of fluid equipment, reduce the maintenance cost and construction period of fluid equipment, and improve the safety reliability and production efficiency of fluid equipment operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The drawings forming a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments and descriptions thereof of the present invention are used to explain the present invention and do not constitute an improper limitation to the present invention. In the drawings:

[0041] Figure 1 is the element distribution coefficient of the duplex stainless steel after solution water quenching at 1040 - 1090 °C. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0042] The present invention will be further described below in conjunction with the detailed description of the embodiments. First, it should be noted that the data in the following experimental examples are obtained by the inventor through a large number of experiments. Due to space limitations, only a part of them are shown in the specification, and those of ordinary skill in the art can understand and implement the present invention based on this data. These embodiments are only used to illustrate the present invention and are not used to limit the scope of the present invention. In addition, it should be understood that after reading the content of the present invention, those skilled in the art can make various changes or modifications to the present invention, and these changes or modifications also fall within the scope protected by this application.

[0043] It should be noted that, without conflict, the embodiments in the present invention and the features in the embodiments can be combined with each other.

[0044] The present invention will be described in detail below with reference to the drawings and in conjunction with the embodiments.

[0045] A kind of duplex stainless steel for concentrated sulfuric acid environment of the present invention, its chemical composition is in weight percentage as shown in Table 1.

[0046] Table 1 Chemical composition of duplex stainless steel (wt%)

[0047]

[0048] The balance is Fe and inevitable impurities.

[0049] Remarks: PREN = %Cr + 3.3(%Mo + 0.5%W) + 16%N ≥ 40.

[0050] The duplex stainless steel for concentrated sulfuric acid environment of the present invention is alloyed by means of low carbon, high chromium and addition of elements such as nickel, copper, tungsten, nitrogen, etc. In particular, nickel helps to improve acid corrosion resistance, yield strength and tensile strength. At the same time, as an austenite-forming element, it can balance the increase in ferrite content caused by high chromium content. Molybdenum can improve the pitting corrosion resistance of duplex stainless steel in concentrated sulfuric acid environment. Therefore, the content of nickel in the present invention is selected to be 7.0-9.0 wt%, and the content of molybdenum is selected to be 3.5-4.5 wt%. The chromium content is selected in a relatively high range of 24.5-27.0 wt%. On the one hand, the oxide film formed in concentrated sulfuric acid environment is utilized to improve the corrosion resistance and stainlessness of duplex stainless steel. On the other hand, it makes up for the adverse effect on pitting corrosion resistance caused by high nickel content and the influence on brittleness caused by molybdenum content. The carbon content is selected to be ≤0.03 wt% to reduce the formation of chromium carbide and the brittle phase generated during the welding process.

[0051] The coordinated action of each component of the present invention enables the duplex stainless steel to have good strength and toughness and the abilities of resisting uniform corrosion, erosion corrosion, cavitation corrosion, etc. of sulfuric acid. The duplex stainless steel prepared by the present invention can be used as the equipment material that undergoes the dual actions of long-term erosion by concentrated sulfuric acid and fluid, which can significantly increase the service time of the material, reduce the maintenance cost and construction period of the material, and improve the safety reliability and production efficiency of the fluid equipment operation. Specifically, the equipment that undergoes the dual actions of long-term erosion by concentrated sulfuric acid and fluid is an impeller.

[0052] In the concentrated sulfuric acid working condition environment, as the core power component of the fluid equipment, the impeller not only has a complex stress environment, but also has to bear the corrosion of concentrated sulfuric acid. And the service performance of the impeller directly affects the safety reliability and service time of the fluid equipment operation. Using the duplex stainless steel of the present invention as the preparation material of the impeller enables the impeller to have relatively high strength, hardness, good toughness and excellent sulfuric acid corrosion resistance.

[0053] When designing the above alloying elements and their contents, the roles of ferrite-forming elements (Si, Cr, Mo, W) and austenite-forming elements (C, Mn, Ni, Cu, N) and the combined effects between alloying elements, as well as the PREN value balance requirements of ferrite phase and austenite phase are fully considered. And the above alloy ranges are designed through Schaeffler diagram, Delong diagram, WRC-92 diagram, austenite line equation and Thermo-Calc numerical simulation to ensure that the ferrite content in the duplex structure is controlled in the range of 40-60%, and the PREN value of ferrite phase ≈ PREN value of austenite phase.

[0054] As a preferred example of the present invention, the duplex stainless steel for concentrated sulfuric acid environment is smelted by the process of pure materials + intermediate frequency furnace.

[0055] As another preferred example of the present invention, the duplex stainless steel for concentrated sulfuric acid environment is smelted by the process of intermediate frequency furnace / electric arc furnace + AOD furnace / VOD furnace.

[0056] The process parameters of the intermediate frequency furnace, electric arc furnace, AOD furnace and VOD furnace can be determined by those skilled in the art according to empirical values.

[0057] As a preferred example of the present invention, the samples of tapped steel for casting are subjected to solution heat treatment and aging treatment at high temperature. The temperature of the solution heat treatment is 1100 - 1200 °C, and the treatment time is 100 - 300 min. The temperature of the aging treatment is 350 - 550 °C.

[0058] The distribution of alloying elements in the ferrite and austenite phases is different. The ferrite phase is enriched with ferrite-forming elements, while the austenite is enriched with austenite-forming elements. The distribution coefficient of alloying elements in the two phases (the ratio of the content of an element in the ferrite phase to the content of the element in the austenite phase) is as Figure 1 shown. This distribution coefficient is similar for most duplex stainless steels in the solution state (1040 - 1090 °C), but the distribution coefficient of alloying elements in duplex stainless steel is not constant, but changes with the heating temperature and phase ratio. With the increase of the solution temperature, the distribution of alloying elements between the two phases gradually tends to be uniform, that is, the contents of chromium, molybdenum and silicon in the ferrite phase gradually decrease, and the contents of nickel and copper gradually increase.

[0059] The present invention eliminates the intermetallic phases such as η, χ, σ, etc. existing in the as-cast structure through solution heat treatment at high temperature, so that the metallographic structure is austenite + ferrite. Through aging treatment at 350 - 550 °C and combined with a copper content of 3.0 - 4.0 wt%, ε-Cu phase is precipitated after aging treatment, improving the hardness of the material and the resistance to erosion-corrosion and cavitation corrosion.

[0060] The preparation method of the duplex stainless steel of the present invention includes the following steps:

[0061] Add various raw materials in the furnace, and use pure materials + intermediate frequency furnace, or any one of the methods of intermediate frequency furnace / electric arc furnace + AOD furnace / VOD furnace for melting;

[0062] Control the content of each alloying element, tap the furnace after the composition is qualified, and tap and cast the steel after reaching the tapping temperature;

[0063] Subject the samples of tapped steel for casting to solution heat treatment and aging treatment at high temperature.

[0064] Example 1

[0065] The smelting method of "pure materials + medium-frequency furnace" is adopted. Raw materials such as pure iron, electrolytic manganese, crystalline silicon, ferrochrome, electrolytic nickel, ferromolybdenum, cathode copper, ferrotungsten, ferrochromium nitride, etc. are added into the furnace for melting. The content of each alloy element of the steel is controlled according to the composition range in Table 1. After the composition is qualified, calcium-silicon alloy and aluminum blocks are added for final deoxidation before tapping. After reaching the tapping temperature, tapping and pouring are carried out.

[0066] Spectral analysis is carried out on the samples cast with the furnace. The analysis results are shown in Table 2. The samples are subjected to high-temperature solution treatment at 1100 °C for 100 min and aging treatment at 500 °C. The mechanical properties and ferrite content are shown in Table 3. The concentrated sulfuric acid corrosion test is carried out according to JB / T 7901 "Test Method for Laboratory Immersion Corrosion of Metallic Materials in Uniform Conditions". The test results are shown in Table 4.

[0067] Example 2

[0068] The smelting method of "medium-frequency furnace + AOD furnace" is adopted. Raw materials such as returned charge, electrolytic manganese, industrial silicon, ferrochrome, electrolytic nickel, ferromolybdenum, cathode copper, ferrotungsten, etc. are added into the medium-frequency furnace for melting. After the molten steel is melted, the composition and temperature of the molten steel are detected, and then the molten steel is transferred to the AOD furnace for refining. The content of each alloy element of the molten steel is controlled according to the composition range in Table 1. After the composition is qualified, tapping and pouring are carried out after reaching the tapping temperature.

[0069] Spectral analysis is carried out on the samples cast with the furnace. The analysis results are shown in Table 2; the samples are subjected to high-temperature solution treatment at 1200 °C for 300 min and aging treatment at 450 °C. The mechanical properties and ferrite content are shown in Table 3; the concentrated sulfuric acid corrosion test is carried out according to JB / T 7901 "Test Method for Laboratory Immersion Corrosion of Metallic Materials in Uniform Conditions". The test results are shown in Table 4.

[0070] Table 2 Duplex stainless steel for concentrated sulfuric acid environment (wt / %)

[0071]

[0072] Table 3 Measured mechanical properties and ferrite content

[0073]

[0074] Table 4 Corrosion rate of concentrated sulfuric acid (mm / a)

[0075]

[0076]

[0077] It can be seen from Table 3 and Table 4 that the duplex stainless steel of the present invention shows excellent corrosion resistance in the concentrated sulfuric acid environment, and has good mechanical properties. It can be used as the equipment material that is subjected to the dual effects of concentrated sulfuric acid and fluid erosion, and can ensure the operation safety and improve the service time during long-term use.

[0078] Although the present invention is disclosed as above, the present invention is not limited thereto. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the scope defined by the claims.

Claims

1. A duplex stainless steel for concentrated sulfuric acid environment, characterized in that, Comprising components in the following weight percentages: C ≤ 0.03 wt%, Mn ≤ 1.50 wt%, Si ≤ 2.0 wt%, P ≤ 0.03 wt%, S ≤ 0.02 wt%, Cr: 24.5 - 27.0 wt%, Ni: 7.0 - 9.0 wt%, Mo: 3.5 - 4.5 wt%, Cu: 3.0 - 4.0 wt%, W: 0.50 - 1.0 wt%, N: 0.20 - 0.30 wt%, the balance being Fe and unavoidable impurities.

2. The duplex stainless steel for concentrated sulfuric acid environment according to claim 1, characterized in that, The ferritic content in the duplex stainless steel is 40 - 60%.

3. The duplex stainless steel for concentrated sulfuric acid environment according to claim 1, characterized in that, The duplex stainless steel is smelted by the process of pure materials + medium frequency furnace.

4. The duplex stainless steel for concentrated sulfuric acid environment according to claim 1, characterized in that, The duplex stainless steel is smelted by the process of medium frequency furnace / electric arc furnace + AOD furnace / VOD furnace.

5. The preparation method of the duplex stainless steel according to any one of claims 1-4, characterized in that, Including the following steps: Adding each raw material in the furnace, and melting by the method of pure materials + medium frequency furnace, or medium frequency furnace / electric arc furnace + AOD furnace / VOD furnace; Controlling the content of each alloy element, tapping after the composition is qualified, and tapping and casting after reaching the tapping temperature; Performing high-temperature solution treatment and aging treatment on the samples obtained by tapping and casting.

6. The preparation method of the duplex stainless steel for concentrated sulfuric acid environment according to claim 5, characterized in that, The temperature of the aging treatment is 350 - 550 °C.

7. The preparation method of the duplex stainless steel for concentrated sulfuric acid environment according to claim 5, characterized in that, Adding calcium silicate alloy and aluminum blocks for final deoxidation before tapping.

8. Application of a duplex stainless steel for concentrated sulfuric acid environment in equipment materials subjected to the dual actions of concentrated sulfuric acid and fluid erosion.

9. The application according to claim 8, characterized in that, The equipment subjected to the dual actions of concentrated sulfuric acid and fluid erosion is an impeller.

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