A steel material for easily formed refining pipe fittings resistant to 600°C high temperature and wet hydrogen sulfide environment and its manufacturing method

Abstract

The invention discloses a steel plate material resistant to 600°C and wet hydrogen sulfide environment. The steel plate material comprises, by weight, C: 0.08-0.12%, Si: 0.20-0.50%, Mn: 0.30-0.60%, Cr: 7.0-9.0%, Mo: 0.80-1.00%, Nb: 0.06-0.10%, V: 0.15-0.25%, and N: 0.030-0.070%. The preparation method comprises: smelting, casting, heating, rolling, heat treatment, namely, normalizing and then tempering heat treatment to decompose martensite, and stabilization treatment at 730-750°C. The steel plate prepared by the present invention has a tensile strength of 405-440 MPa at a high temperature of 600°C, a transverse elongation after fracture of 34.0-37.5%, and a longitudinal elongation after fracture of 35.0-38.0%. It does not break after continuous loading for 720 hours in the SSCC four-point bending test.

Description

Technical Field

[0001] The present invention belongs to the technical field of metal materials, and in particular relates to a steel plate for easily formed refining pipe fittings capable of withstanding high temperatures of 600°C and wet H2S environments, and a manufacturing method thereof. Background Art

[0002] The interior of the equipment in an oil refinery and between the equipment are covered with crisscrossing and complex pipeline systems. These pipeline systems are the main arteries of its production, playing the role of decomposition and cracking, process connection, and finished product and semi-finished product transportation in the entire production process, connecting the entire production line to form a whole.

[0003] The pipeline system actually consists of two major components: pipes and fittings. During operation, these pipes are not only subjected to external conditions such as high temperatures and loading pressures imposed by the refining process, but also come into contact with sulfides contained in crude oil and various additives added during the refining process, exposing them to the risk of corrosion from high-temperature, wet hydrogen sulfide. These complex factors severely impact the long-term, safe production of the pipeline system. In particular, the pipe fittings used for pipe bends experience large and uneven plastic deformation during the processing and forming process, which can easily lead to stress concentration in the bends. Long-term operation in this area at temperatures of 600°C and in a wet hydrogen sulfide environment can easily cause corrosion perforation, resulting in significant losses to production safety.

[0004] Prior to the present invention, a Chinese patent (publication number CN101760687A) disclosed a high-temperature alloy pipe fitting and a method for producing the steel and pipe fittings used therein. The material involved in the patent is a nickel-based alloy material, which does not fall within the scope of steel, and the final product of the invention is a bent pipe and pipe fitting, which does not solve the technical problem of high temperature resistance.

[0005] A Chinese patent (publication number CN 109161790 A) discloses a high-grade, high-toughness pipe steel for use under acidic conditions and its production method. The patent disclosure mainly involves pipe steel for long-distance transportation that is resistant to low-temperature corrosion environments and cannot meet the requirements of high-temperature corrosion environments.

[0006] A Chinese patent (publication number CN108085593A) discloses a steel and manufacturing method for elbows and fittings suitable for oil and gas transportation in low-temperature environments. The steel mainly addresses the strength and low-temperature toughness issues of elbows and fittings used in low-temperature stations.

[0007] A Chinese patent (publication number CN102912250A) discloses an economical low-yield ratio steel pipe fitting for oil and gas transportation and its production method. The pipe fittings involved in this patent are mainly used as pipeline steel materials required for long-distance oil and gas transportation. The contact medium is crude oil and natural gas, and the external environment is high-altitude cold and seismic zones. Therefore, the main issues to be addressed are the room temperature strength and low-temperature toughness of the material.

[0008] A Chinese patent (publication number CN 112267068 A) discloses a high-temperature corrosion-resistant steel for pipelines and its production process. This patent mainly uses the addition of rare earth elements and external spraying of graphene for corrosion protection. It is not only expensive and complex in process, but also does not reflect the high temperature and corrosion resistance effects among the beneficial effects.

[0009] It can be seen that in the existing technology, steel for pipe fittings is mainly used for the transportation of crude oil and natural gas before refining, which mainly involves low-temperature environments. Even for pipe fittings used in high-temperature corrosive environments, their corrosion resistance principle is to add expensive strategic rare earth resources or spray graphene on the outside. However, steel for easy-to-form pipe fittings that is truly suitable for a high-temperature, wet hydrogen sulfide environment of 600°C is still in a blank state of invention. Summary of the Invention

[0010] The purpose of the present invention is to provide a steel plate for refining pipe fittings that is easy to form and can withstand 600°C and wet H2S environment, and a manufacturing method thereof, so as to solve the problem that pipeline steel in the existing technology is difficult to withstand high temperature and corrosion, and the corrosion resistance cost is too high.

[0011] To achieve the above objectives, the present invention provides a steel plate material that is resistant to 600°C and a wet hydrogen sulfide environment, the components of which include, by weight percentage, C: 0.08-0.12%, Si: 0.20-0.50%, Mn: 0.30-0.60%, Cr: 7.0-9.0%, Mo: 0.80-1.00%, Nb: 0.06-0.10%, V: 0.15-0.25%, N: 0.030-0.070%, and the balance is Fe.

[0012] The present invention provides a method for preparing a steel plate material resistant to 600° C. and wet hydrogen sulfide environment, comprising the following steps:

[0013] S1. Smelting: Control the molten iron content at 50-70%, the tapping temperature at 1600-1640℃, feed calcium silicon at a speed of 2-4m / s, and a feed rate of 3-5m / t. After desulfurization, impurity removal, and vacuum degassing, tapping is completed.

[0014] S2. Casting: continuous casting after breaking vacuum, superheat 20-30℃, casting speed 0.8-1.2m / min; the billets are stacked and slowly cooled after being cast, and the stacking slow cooling time is 36-48h;

[0015] S3. Heating: Control the heating temperature of the steel billet at 1200-1250℃ to dissolve the alloy elements and inhibit the growth of the original austenite grains;

[0016] S4, rolling: rough rolling end temperature 1000-1100 ℃, total deformation rate 60-70%, intermediate billet thickness 2.5-3.0 times the thickness of the finished steel plate; finishing rolling start temperature 860-900 ℃, finishing rolling end temperature 820-850 ℃, cumulative deformation rate 60-70%;

[0017] S5. Heat treatment: Normalizing heat treatment is used to obtain a martensite structure with fine lath bundles, and then tempering heat treatment is used to decompose the martensite. After tempering, stabilization treatment is performed at 730-750℃ and the temperature is kept for 20-30h.

[0018] Preferably, the components added during smelting include, by weight percentage: C: 0.08-0.12%, Si: 0.20-0.50%, Mn: 0.30-0.60%, Cr: 7.0-9.0%, Mo: 0.80-1.00%, Nb: 0.06-0.10%, V: 0.15-0.25%, and N: 0.030-0.070%.

[0019] Preferably, the weight percentages are controlled such that the P content is ≤0.010%, and the S content is ≤0.002%.

[0020] Preferably, the desulfurization method includes blowing argon gas from the bottom of the ladle to stir the molten steel for desulfurization.

[0021] Preferably, the vacuum degassing method comprises evacuating the vacuum in a VD furnace for 10-15 minutes, and blowing in N2 after breaking the vacuum until the concentration reaches 0.030-0.070%.

[0022] Preferably, the normalizing temperature is 1040-1080° C., the holding time is 1.0-3.0 min / mm, and the cooling rate is 0.5-1.5° C. / s.

[0023] Preferably, the tempering temperature is 750-780° C., and the holding time is 5.0-9.0 min / mm.

[0024] Preferably, the lath bundles in the martensite structure with fine lath bundles are 200-300 nm.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] (1) The present invention is based on the chemical elements of C, Mn and Si, strictly controls the harmful elements P and S, and adds high contents of Cr and Mo, Nb, V and N alloy elements to play the role of solid solution strengthening and precipitation strengthening, thereby obtaining a stable and dense tempered martensite structure, preventing hydrogen penetration, and ensuring the comprehensive mechanical properties and high-temperature corrosion resistance of the steel for refining and chemical bending pipes.

[0027] (2) The high-temperature resistant and easy-to-form steel plate for pipe bending is obtained through a special process of smelting, rolling, ultra-high temperature normalizing air cooling, high temperature tempering, and high temperature long-term stabilization treatment. Its mechanical properties are as follows: tensile strength at high temperature of 600℃ is 405-440MPa, transverse elongation after fracture is 34.0-37.5%, longitudinal elongation after fracture is 35.0-38.0%, HBW is 165-175, yield strength ratio is between 0.74-0.79, D=2a, 180° transverse and longitudinal cold bending results show that the steel plate surface is intact and has no cracks. The above excellent comprehensive properties ensure the cold forming processability and long-term high-temperature service performance of the steel plate in the pipe bending manufacturing process.

[0028] (3) The steel prepared by the present invention has a CSR% of 0 in the HIC test (solution A) at a high temperature of 600°C, and does not break after continuous loading for 720 hours in the SSCC four-point bending test.

[0029] In summary, the present invention uses a unique smelting, rolling and heat treatment production process to produce steel for refining pipe fittings that can meet the needs of high-temperature corrosion environments of 600°C and is easy to cold-form, which effectively supports the long-term safe development of large-scale, integrated refining equipment in my country. DETAILED DESCRIPTION

[0030] The present invention will be further described below with reference to specific examples, but the present invention is not limited thereto in any way.

[0031] Unless otherwise specified, the instruments, reagents, and materials used in the following examples are all conventional instruments, reagents, and materials available in the prior art and can be obtained through regular commercial channels. The experimental methods and detection methods used in the following examples are all conventional experimental methods and detection methods available in the prior art, unless otherwise specified.

[0032] The present invention provides a steel plate for easily formable refinery pipe fittings that is resistant to 600°C and wet H2S environments. The steel plate has a chemical composition range of: C: 0.08-0.12%, Si: 0.20-0.50%, Mn: 0.30-0.60%, P ≤ 0.010%, S ≤ 0.002%, Cr: 7.0-9.0%, Mo: 0.80-1.00%, Nb: 0.06-0.10%, V: 0.15-0.25%, N: 0.030-0.070%, and the balance is Fe and unavoidable inclusions. The reasons for limiting the amounts of each element are detailed as follows:

[0033] The reasons for limiting the amounts of alloying elements C, Si, Mn, P, S, Cr, Mo, Nb, V, and N in steel plates are detailed as follows:

[0034] Carbon is the most important and inexpensive element for ensuring the strength of steel at both room and high temperatures. It improves the strength of the matrix through solid solution and carbide formation. However, excessive carbon content can affect the steel's weldability, while too low a carbon content cannot guarantee the steel's performance in high-temperature environments. Therefore, the present invention sets the carbon content range to 0.08-0.12%.

[0035] Si is a beneficial element in steel that protects against high-temperature corrosion. At high temperatures, it can form a highly protective, dense SiO2 film on the steel surface. When the Si content in steel is between 0.20-0.50%, it has a significant antioxidant effect. However, when the silicon content exceeds 1.0%, it can lead to deterioration of mechanical properties. Therefore, the silicon content in steel generally does not exceed 1.0%. Furthermore, alloying silicon with molybdenum has a significant effect on improving the oxidation resistance of steel, increasing its oxidation resistance to 800°C. Therefore, the present invention sets the Si content range to 0.20-0.50%.

[0036] Mn can increase the toughness, strength and hardness of steel. It is an element that strongly stabilizes austenite, can effectively reduce the decomposition rate of austenite, and improve the hardenability of steel. A high Mn content will enhance temper brittleness. Therefore, the present invention sets the Mn content range to 0.30-0.60%.

[0037] S and P are both harmful elements in steel, which increase the brittleness of steel. Therefore, the content of phosphorus and sulfur in steel should be reduced as much as possible, S≤0.002%, P≤0.010%.

[0038] Cr is a strong carbide-forming element. It forms alloy carbides with C in steel, which are not only stable but also dense. It not only has a strong effect of consuming carbon to prevent it from combining with H2 to generate CH4, which causes hydrogen embrittlement or hydrogen-induced cracking, but also the produced carbides are stable and dense, which can effectively prevent the penetration of hydrogen and the separation of C.

[0039] In the present invention, the Cr content is set within the range of 7.0-9.0% in order to play a high-temperature strengthening role while also having a high-temperature anti-corrosion role to ensure the long-term high-temperature use requirements of the pipe fittings.

[0040] Mo is a strong carbide-forming element. It forms stable alloy carbides with carbon in steel and has excellent resistance to temper brittleness. It plays an important role in the high-temperature resistance of the elbow. Too high a content will affect the processing performance of the steel. Therefore, the present invention sets the Mo content range to 0.80-1.00%.

[0041] Nb is a strong carbon and nitride-forming element, forming a dispersed precipitate at grain boundaries, which significantly hinders the slip and climb of dislocations, resulting in excellent high-temperature strengthening effects. Its grain-refining effect can also reduce the steel's overheating sensitivity and temper brittleness. Under certain conditions, it can also improve toughness and creep resistance. Therefore, the present invention sets the Nb content range to 0.06-0.10%.

[0042] V is a refractory metal with a melting point as high as 1910°C. Vanadium is an alloying element that reduces the γ-Fe region and expands the α-Fe phase. The carbon and nitrides formed are dispersed within the grains and at the grain boundaries, significantly hindering the slip and climb of dislocations, thereby contributing to high-temperature thermal strength. However, excessive V content increases the yield ratio of the steel, hindering the forming and manufacturing of pipe fittings. Therefore, the present invention sets the V content range to 0.15-0.25%.

[0043] Like carbon, nitrogen can be dissolved in iron to form an interstitial solid solution, and can form stable nitrides with elements such as Cr, V, and Nb, thereby strengthening the steel. It can also improve the strength and impact toughness of high-chromium steel without reducing its plasticity, thereby increasing the high-temperature endurance strength of the steel. Therefore, the present invention sets the nitrogen content range to 0.030-0.070%.

[0044] A method for manufacturing a steel plate for easily formed refining pipe fittings resistant to a 600°C wet H2S environment is as follows:

[0045] 1. Electric arc furnace - LF furnace - VD furnace - slab continuous casting - slab finishing - heating - rolling - straightening - heat treatment - performance evaluation.

[0046] 2. Smelting Process: The composition designed in this invention contains 7.0%-9.0% Cr and refractory molybdenum. Therefore, smelting must be carried out in an electric arc furnace with high temperatures and thermal efficiency. To ensure steel purity, high-quality scrap steel and molten iron are used as raw materials, with the molten iron content controlled at 50-70%. Since the carbon content in this invention is 0.08-0.12%, chromium is added in the form of low-carbon ferrochrome and metallic chromium. To prevent chromium oxidation, the addition period is selected at the initial reduction stage. Ferromolybdenum and ferrotungsten are also added during this initial stage. The tapping temperature is controlled between 1600°C and 1640°C. An LF furnace is used for fine-tuning the composition, adding alloying elements to achieve the target chemical composition. Ferroniobium and ferrovanadium are then added. Silicon calcium is fed at a linear speed of 2-4 m / s and a feed rate of 3-5 m / t. Argon is blown through the bottom of the ladle to agitate the molten steel for further desulfurization, controlling inclusions in the steel to extremely low levels, ensuring steel purity and high-temperature performance. The vacuum degassing treatment is carried out in VD, and the vacuum time is 10-15 minutes. After breaking the vacuum, N2 is blown until it reaches 0.030-0.070%, and then the steel is ready to be tapped.

[0047] 3. Casting process: After breaking the vacuum, continuous casting is used, with an overheat temperature of 20-30°C and a casting speed of 0.8-1.2 m / min. The billets are stacked and slowly cooled after leaving the production line. The stacking slow cooling time is 36-48 hours.

[0048] 4. Heating process: By controlling the heating process of the steel billet, ensure that the alloy elements are fully dissolved and effectively inhibit the growth of the original austenite grains. The slab heating temperature is controlled at 1200-1250℃, and the total heating time is 6-8 hours.

[0049] 5. Rolling process parameters: Rough rolling end temperature 1000-1100°C, total deformation rate ≥ 60%, intermediate bar thickness 2.5-3.0 times the finished plate thickness; finishing rolling start temperature 860-900°C, finishing rolling end temperature 820-850°C, cumulative deformation rate ≥ 60%. These parameters are designed based on the steel grade's composition and performance requirements. Because the material is intended for high-temperature environments, an optimized controlled rolling process is used to maximize the refinement of the as-rolled microstructure, ensuring excellent high-temperature strength after final heat treatment. Rolling thickness ranges from 10-40mm.

[0050] 6. Heat treatment process: A relatively refined original rolled structure is obtained through controlled rolling. However, since the steel contains a large amount of alloying elements such as Cr, Mo, W, V, Nb, and N, in order to give full play to their synergistic strengthening effect, ultra-high temperature normalizing treatment is required to completely dissolve and homogenize the alloying elements. After normalizing, water medium is used for accelerated cooling to obtain a fine martensite structure with a lath bundle of 200-300nm. The normalizing temperature is 1040-1080℃, the holding time is 1.0-3.0min / mm, and the cooling rate is 0.5-1.5℃ / s. Although martensite has excellent high temperature strength, its poor plasticity and high hardness are extremely unfavorable for forming elbows. The present invention promotes the decomposition of martensite through high temperature tempering to obtain (Fe, Mn, Cr, Mo, W) with excellent precipitation strengthening effect. 23 The tempered martensite structure of C6 and (Nb, V)CN second phases is tempered at a temperature of 750-780°C for a holding time of 5.0-9.0 min / mm. To further eliminate retained austenite and stress in the steel and prevent stress cracking during cold forming, the present invention adds a long-term stabilization treatment at 730-750°C for 20-30 hours after the high-temperature tempering treatment. The resulting structure exhibits three key characteristics: First, internal stress is completely eliminated. Second, unstable retained austenite is completely converted to tempered martensite. Finally, more stable second-phase particles form in the steel, aggregate, grow, and stabilize during this process, ultimately becoming non-transformable. This eliminates free carbon atoms in the steel from interacting with hydrogen generated in the medium, potentially causing hydrogen-induced cracking. The resulting structure obtained through this heat treatment is fine, uniform, and dense, exhibiting excellent high-temperature resistance, corrosion resistance, and cold forming properties.

[0051] Table 1 shows the chemical compositions of the steel plates prepared in Examples 1-10 of the present invention, Table 2 shows the process parameters of the steel plates prepared in Examples 1-10 of the present invention, Table 3 shows the mechanical property test results of the steel plates prepared in Examples 1-10 of the present invention, Table 4 shows the hydrogen induced cracking (HIC) test results of Examples 1-10 of the present invention at 600°C (Solution A), and Table 5 shows the SSCC four-point bending test results of the steels prepared in Examples 1-10 of the present invention at 600°C.

[0052] Table 1 Chemical compositions of the steel plates prepared in Examples 1-10 of the present invention (wt, %)

[0053]

[0054] Table 2 Production process parameters of the steel plates prepared in Examples 1-10 of the present invention

[0055]

[0056] Table 3 Mechanical properties test results of steel plates prepared in Examples 1-10 of the present invention

[0057]

[0058] Table 4 HIC test results of the steel plates prepared in Examples 1-10 of the present invention at 600°C (Solution A)

[0059]

[0060] Table 5 SSCC four-point bending test results of the steel plates prepared in Examples 1-10 of the present invention at 600°C

[0061]

[0062] The steel plate for easily formed bendable pipes provided by the present invention, which is resistant to a high-temperature, wet hydrogen sulfide working environment at 600°C, has excellent properties such as a transverse elongation after break of 34.0-37.5%, a longitudinal elongation after break of 35.0-38.0%, a Brinell hardness of 165-175, a yield strength ratio of 0.75-0.79, and a high-temperature tensile strength at 600°C in the range of 405-440 MPa. Transverse and longitudinal cold bending results at D=2a and 180° show that the surface of the steel plate is intact and free of cracks. The results of an HIC test (solution A) for hydrogen-induced cracking resistance at a high temperature of 600°C show that the CSR / % is 0. An SSCC four-point bending test shows no fracture after continuous loading for 720 hours, and other excellent properties.

[0063] Anyone skilled in the art will be able to utilize the above-disclosed technical content to make many possible changes and modifications to the technical solution of the present invention, or to modify it into equivalent embodiments with equivalent changes, without departing from the scope of the technical solution of the present invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention that do not depart from the content of the technical solution of the present invention shall still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A steel plate material resistant to 600°C and wet hydrogen sulfide environment, characterized in that: The composition includes, by weight percentage: C: 0.08-0.12%, Si: 0.20-0.50%, Mn: 0.30-0.60%, Cr: 7.0-9.0%, Mo: 0.80-1.00%, Nb: 0.06-0.10%, V: 0.15-0.25%, N: 0.030-0.070%, and the balance is Fe; The preparation method comprises the following steps: S1. Smelting: Control the molten iron content at 50-70%, the tapping temperature at 1600-1640℃, feed calcium silicon at a speed of 2-4m / s, and a feed rate of 3-5m / t. After desulfurization, impurity removal, and vacuum degassing, tapping is completed. S2. Casting: continuous casting after breaking vacuum, superheat 20-30℃, casting speed 0.8-1.2m / min; the billets are stacked and slowly cooled after being cast, and the stacking slow cooling time is 36-48h; S3. Heating: Control the heating temperature of the steel billet at 1200-1250℃ to dissolve the alloy elements and inhibit the growth of the original austenite grains; S4, rolling: rough rolling end temperature 1000-1100 ℃, total deformation rate 60-70%, intermediate billet thickness 2.5-3.0 times the thickness of the finished steel plate; finishing rolling start temperature 860-900 ℃, finishing rolling end temperature 820-850 ℃, cumulative deformation rate 60-70%; S5. Heat treatment: Normalizing heat treatment obtains martensite structure with fine lath bundles, and then tempering heat treatment is used to decompose the martensite. After tempering, stabilization treatment is carried out at 730-750℃ and heat preservation for 20-30h. Normalizing temperature is 1040-1080℃, holding time is 1.0-3.0min / mm, cooling rate is 0.5-1.5℃ / S; Tempering temperature 750-780℃, holding time 5.0-9.0min / mm.

2. The steel plate material resistant to 600°C and wet hydrogen sulfide environment according to claim 1, characterized in that: The components added during smelting include, by weight percentage, C: 0.08-0.12%, Si: 0.20-0.50%, Mn: 0.30-0.60%, Cr: 7.0-9.0%, Mo: 0.80-1.00%, Nb: 0.06-0.10%, V: 0.15-0.25%, and N: 0.030-0.070%.

3. The steel plate material resistant to 600°C and wet hydrogen sulfide environment according to claim 1, characterized in that: Control weight percentage: P content ≤ 0.010%, S content ≤ 0.002%.

4. The steel plate material resistant to 600°C and wet hydrogen sulfide environment according to claim 1, characterized in that: The desulfurization method includes blowing argon gas from the bottom of the ladle to stir the molten steel for desulfurization.

5. The steel plate material resistant to 600°C and wet hydrogen sulfide environment according to claim 1, characterized in that: The vacuum degassing treatment method includes evacuating the vacuum in a VD furnace for 10-15 minutes, and blowing in N2 after breaking the vacuum until the concentration reaches 0.030-0.070%.

6. The steel plate material resistant to 600°C and wet hydrogen sulfide environment according to claim 1, characterized in that: The lath bundles in the martensite structure are fine and have a size of 200-300 nm.

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