X65mos grade deep-sea acid-resistant pipeline steel and a preparation method thereof

Abstract

The application discloses X65MOS-grade deep-sea acid-resistant pipeline steel and a preparation method thereof, and belongs to the technical field of pipeline steel production. t0.50 The mechanical properties of the X65MOS-grade deep-sea acid-resistant pipeline steel are as follows: the transverse yield strength R m ≥ 460 MPa, the tensile strength R t0.50 ≥ 535 MPa, the yield strength ratio R m ≤ 0.89, the elongation A 50mm ≥ 47%, the Charpy impact energy at -38 DEG C ≥ 300 J, and the CTOD characteristic value δm at -28 DEG C ≥ 1 mm. The application adopts a composition system mainly with high V precipitation strengthening and auxiliary Nb-Ti composite micro-alloying fine-grain, strictly controls the addition amount of expensive alloys such as Ni, Cr, Mo and Cu, has good economy, and simultaneously, by strictly limiting the content of P and S, the formation of harmful inclusions is reduced, so that the acid resistance of the pipeline steel is improved.

Description

Technical Field

[0001] This invention belongs to the field of pipeline steel production technology, specifically relating to an X65MOS grade deep-sea acid-resistant pipeline steel and its preparation method. Background Technology

[0002] With the accelerated development of deep-sea oil and gas resources globally, subsea pipelines face severe challenges from harsh corrosive environments characterized by high pressure, low temperature, and high humidity hydrogen sulfide content. This places extremely high demands on the comprehensive performance of pipeline materials. X65MOS grade acid-resistant pipeline steel is designed to meet the requirements of such acidic service environments, and must possess high strength, excellent low-temperature toughness, good weldability, and outstanding resistance to hydrogen-induced cracking and hydrogen sulfide stress corrosion cracking.

[0003] However, existing production technologies for thick-gauge X65MOS steel plates face a series of significant challenges: traditional processes typically require the addition of expensive alloying elements such as copper, nickel, and molybdenum to ensure corrosion resistance, leading to a significant increase in production costs; more importantly, as the thickness of the steel plate increases, simultaneously ensuring the uniformity of the core structure, the matching of strength and toughness throughout the thickness, and stable acid corrosion resistance becomes extremely difficult, especially controlling crack arrest toughness under low-temperature conditions, which is significantly more challenging; furthermore, the complex alloy system and stringent rolling process make process control difficult, hindering further improvements in yield and performance stability. Therefore, how to achieve a balance and reliable guarantee of high strength, high toughness, and excellent acid resistance in thick-gauge X65MOS steel plates through more optimized and stable production processes while effectively controlling alloy costs remains a key technological bottleneck restricting the localization and cost reduction of high-end subsea pipelines. Summary of the Invention

[0004] To address the aforementioned technical problems in the existing technology, this invention provides an X65MOS-grade deep-sea acid-resistant pipeline steel and its preparation method.

[0005] In a first aspect of the invention, an X65MOS grade deep-sea acid-resistant pipeline steel is provided, the chemical composition of which is controlled by mass percentage as follows: 0.05%≤C≤0.08%, 0.10%≤Si≤0.30%, 1.00%≤Mn≤1.40%, P≤0.018%, S≤0.0014%, 0.015%≤Al≤0.060%, 0.030%≤Nb≤0.050%, 0.030%≤V≤0.050%, 0.005%≤Ti≤0.020%, 0.13%≤Cr≤0.25%, 0.08%≤Mo≤0.11%, 0.15%≤Ni≤0.25%, 0.13%≤Cu≤0.18%, N≤0.008%, with the balance being Fe and unavoidable impurities.

[0006] Furthermore, the chemical composition of the aforementioned X65MOS grade deep-sea acid-resistant pipeline steel is controlled by mass percentage as follows: 0.05%≤C≤0.06%, 0.11%≤Si≤0.17%, 1.21%≤Mn≤1.30%, P≤0.010%, S≤0.0008%, 0.021%≤Al≤0.030%, 0.042%≤Nb≤0.047%, 0.035%≤V≤0.041%, 0.008%≤Ti≤0.013%, 0.17%≤Cr≤0.21%, 0.09%≤Mo≤0.10%, 0.18%≤Ni≤0.21%, 0.14%≤Cu≤0.15%, N≤0.006%, with the balance being Fe and unavoidable impurities.

[0007] Furthermore, the mechanical properties of the aforementioned X65MOS grade deep-sea acid-resistant pipeline steel are as follows: transverse yield strength R t0.50 ≥460MPa, tensile strength R m ≥535MPa, yield strength ratio R t0.50 / R m ≤0.89, Elongation after fracture A 50mm ≥47%, Charpy impact energy at -38℃ ≥300J, CTOD characteristic value δm at -28℃ ≥1mm.

[0008] In a second aspect of the present invention, the method for preparing X65MOS grade deep-sea acid-resistant pipeline steel includes the following steps: (1) Converter smelting: The pretreated molten iron is sent into the converter for smelting, and slag-forming materials and deoxidizers are added during the tapping process; (2) LF refining: The molten steel after converter smelting is sent to the LF furnace for heating, deep desulfurization and composition fine adjustment; Argon blowing protection is carried out throughout the refining process to avoid oxidation of the molten steel. The total processing time of the LF furnace is ≥35min, and the weak stirring time before leaving the refining station is ≥3min. (3) RH vacuum treatment: The molten steel after LF refining is sent into the RH vacuum device for vacuum degassing treatment. The cyclic treatment is carried out for ≥20 minutes under the condition of vacuum degree ≤5mbar. After wire feeding alloying, weak stirring is carried out for ≥10 minutes. (4) Continuous casting: The molten steel after vacuum treatment is continuously cast and poured. Electromagnetic stirring is used in the continuous casting process to obtain the continuous casting billet. (5) Controlled rolling and controlled cooling: After the continuous casting billet is heated and homogenized, it is subjected to controlled rolling. The final rolling temperature is controlled at 850℃~910℃. After rolling, water cooling is carried out and the final cooling temperature is controlled at 400℃~550℃. Finally, X65MOS grade deep-sea acid-resistant pipeline steel with acicular ferrite structure with a thickness of 15~27mm is obtained.

[0009] Furthermore, in the above-mentioned preparation method of X65MOS grade deep-sea acid-resistant pipeline steel, in step (1), the molten iron before entering the furnace must meet the pretreatment endpoint conditions: sulfur content mass percentage ≤ 0.005%, slag thickness ≤ 30 mm, and molten iron temperature ≥ 1250℃.

[0010] Furthermore, in the above-mentioned method for preparing X65MOS grade deep-sea acid-resistant pipeline steel, in step (2), 500~800 kg of lime and 100~300 kg of fluorite are used for slag formation during the LF refining process. From 15 minutes before the start of weak stirring, no aluminum deoxidizer is added until all subsequent processes.

[0011] Furthermore, in the above-mentioned method for preparing X65MOS grade deep-sea acid-resistant pipeline steel, in step (3), the weak stirring time after wire alloying is ≥15min.

[0012] Furthermore, in the above-mentioned preparation method of X65MOS grade deep-sea acid-resistant pipeline steel, in step (4), the tundish is protected by argon blowing throughout the continuous casting process, the nitrogen increase in the molten steel is controlled to be ≤5ppm, the temperature of the molten steel in the tundish is controlled to be 1520℃~1560℃, the casting speed is matched to 0.75~1.45m / min, and the electromagnetic stirring is roller electromagnetic stirring.

[0013] Furthermore, in the above-mentioned preparation method of X65MOS grade deep-sea acid-resistant pipeline steel, in step (5), the heating temperature of the continuous casting billet is 1120℃~1220℃, the furnace dwell time is controlled at 8~11min / 10mm plate thickness, and the controlled rolling is divided into two stages: rough rolling and finish rolling, with the rough rolling start temperature ≥1000℃.

[0014] Furthermore, in the above-mentioned preparation method of X65MOS grade deep-sea acid-resistant pipeline steel, in step (5), the water inlet temperature for post-rolling water cooling is 750℃~820℃, the heating and soaking temperature of the continuous casting billet is 1200℃~1210℃, the rough rolling start temperature is 1010℃~1080℃, the finishing rolling finish temperature is 880℃~900℃, and the final cooling temperature is 440℃~470℃.

[0015] The X65MOS grade deep-sea acid-resistant pipeline steel and its preparation method of the present invention have the following advantages and beneficial effects: In summary, this invention specifically employs a composition system that primarily utilizes high-V precipitation strengthening and secondarily uses Nb-Ti composite microalloying for fine grains. It strictly controls the addition of expensive alloys such as Ni, Cr, Mo, and Cu, resulting in good economic efficiency. At the same time, by strictly limiting the content of P and S, the formation of harmful inclusions is reduced, thereby improving the acid resistance of pipeline steel. By rationally applying the process of converter smelting + LF + RH vacuum treatment ladle refining + continuous casting, high steel purity and uniform microstructure are obtained. By optimizing the preheating billet process and the controlled rolling and cooling process, the core deformation rate and microstructure density of the steel plate are improved, and the resistance to HIC and SSC is enhanced. By precisely controlling the heating, final rolling, water immersion and final cooling temperatures, a microstructure dominated by acicular ferrite is obtained, avoiding the formation of hard phases and significantly improving the toughness and acid corrosion resistance of the steel plate. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only for further understanding of the embodiments of this application and constitute a part of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. In the drawings: Figure 1 Microstructure diagrams of X65MOS grade deep-sea acid-resistant pipeline steel prepared according to embodiments of this application, wherein (a) is the microstructure diagram of Example 1; (b) is the microstructure diagram of Example 2; and (c) is the microstructure diagram of Example 3. Figure 2 The following are HIC test charts of X65MOS grade deep-sea acid-resistant pipeline steel prepared according to the embodiments of this application, wherein (a) is the HIC test chart of Example 1; (b) is the HIC test chart of Example 2; and (c) is the HIC test chart of Example 3. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0018] In a first aspect, the present invention provides an X65MOS grade deep-sea acid-resistant pipeline steel. The X65MOS grade deep-sea acid-resistant pipeline steel of the present invention, through precise composition design and process control, reduces alloy costs while ensuring the excellent acid corrosion resistance of the steel plate.

[0019] Specifically, the chemical composition of the X65MOS grade deep-sea acid-resistant pipeline steel of the present invention is controlled by mass percentage as follows: 0.05%≤C≤0.08%, 0.10%≤Si≤0.30%, 1.00%≤Mn≤1.40%, P≤0.018%, S≤0.0014%, 0.015%≤Al≤0.060%, 0.030%≤Nb≤0.050%, 0.030%≤V≤0.050%, 0.005%≤Ti≤0.020%, 0.13%≤Cr≤0.25%, 0.08%≤Mo≤0.11%, 0.15%≤Ni≤0.25%, 0.13%≤Cu≤0.18%, N≤0.008%, with the balance being Fe and unavoidable impurities.

[0020] Preferably, the chemical composition of the X65MOS grade deep-sea acid-resistant pipeline steel of the present invention is controlled by mass percentage as follows: 0.05%≤C≤0.06%, 0.11%≤Si≤0.17%, 1.21%≤Mn≤1.30%, P≤0.010%, S≤0.0008%, 0.021%≤Al≤0.030%, 0.042%≤Nb≤0.047%, 0.035%≤V≤0.041%, 0.008%≤Ti≤0.013%, 0.17%≤Cr≤0.21%, 0.09%≤Mo≤0.10%, 0.18%≤Ni≤0.21%, 0.14%≤Cu≤0.15%, N≤0.006%, with the balance being Fe and unavoidable impurities.

[0021] Specifically, the mechanical properties of the X65MOS grade deep-sea acid-resistant pipeline steel of the present invention are: transverse yield strength R t0.50 ≥460MPa, tensile strength R m ≥535MPa, yield strength ratio R t0.50 / R m ≤0.89, Elongation after fracture A 50mm ≥47%, Charpy impact energy at -38℃ ≥300J, CTOD characteristic value δm at -28℃ ≥1mm.

[0022] The function of the required elemental and content control ranges for the X65MOS grade deep-sea acid-resistant pipeline steel of this invention is described below: Carbon, as an important interstitial solid solution strengthening element in steel, can significantly improve the strength of steel through lattice distortion. However, excessive carbon content will aggravate carbon segregation during the solidification process of the billet, increase HIC sensitivity and increase the failure rate of corrosion cracking. At the same time, it is easy to form coarse carbides, which will aggravate local corrosion. It will also reduce the low-temperature toughness and weldability of steel. Taking all factors into consideration, the carbon content should be controlled at 0.05% to 0.08% by mass.

[0023] Silicon, as an important deoxidizing and solid solution strengthening element in acid-resistant pipeline steel, can not only effectively deoxidize during the smelting process, ensuring the purity of the molten steel and reducing the adverse effects of oxide inclusions on hydrogen-induced cracking, but also inhibit the excessive precipitation and coarsening of carbides, improve the uniformity of the matrix structure, and reduce the tendency for localized corrosion. Within a reasonable content range, it can also improve the passivation ability and acid resistance of the steel. Taking all factors into consideration, the mass percentage of Si should be controlled at 0.10% to 0.30%.

[0024] Manganese is a key alloying element in acid-resistant pipeline steel. It can significantly expand the austenite phase region, lower the γ→α phase transformation temperature, effectively refine ferrite grains and promote the formation of acicular ferrite in conjunction with the TMCP process, and improve the strength and toughness of the steel through solid solution strengthening. It can also combine with sulfur to form MnS inclusions, avoiding the formation of low-melting-point FeS at grain boundaries that could cause hot brittleness, and improving the hot working properties of the steel. However, excessive manganese content can easily lead to significant segregation in the core of the cast billet, resulting in uneven microstructure and hardness, forming localized hard areas and harmful hydrogen traps, reducing acid corrosion resistance. At the same time, excessive manganese can also increase the hardenability of the steel, deteriorating low-temperature toughness and weldability. Therefore, in X65MOS acid-resistant pipeline steel, manganese must be controlled within an appropriate range. Taking all factors into consideration, the mass percentage of Mn is controlled at 1.00% to 1.40%.

[0025] Chromium significantly improves the corrosion resistance of steel, especially in acidic environments (such as oil and gas environments containing H2S or CO2). Its mechanism of action is through the formation of a dense Cr2O3 oxide film, which inhibits further corrosion. In addition, chromium can improve the strength and high-temperature oxidation resistance of steel. However, excessive addition (e.g., >1.0%) may increase costs and affect weldability (e.g., by promoting carbide precipitation). Considering all factors, the chromium content should be controlled at 0.13%–0.25% by mass.

[0026] Vanadium is a highly efficient precipitation strengthening element in acid-resistant pipeline steel. During post-rolling cooling, it can disperse nano-sized V(C,N) particles in the ferrite matrix, significantly improving yield strength and tensile strength. It also has a weak inhibitory effect on austenite recrystallization, which is beneficial for grain refinement in the recrystallization zone. However, excessive vanadium content can lead to coarsening of the V(C,N) precipitates, reducing the precipitation strengthening effect and increasing the carbon equivalent of the steel, thus exacerbating hardening and cold cracking tendencies in the weld heat-affected zone. Considering all factors, the vanadium content is controlled at a mass percentage of 0.030%–0.050%.

[0027] Niobium is a typical grain-refining strengthening element in pipeline steel, strongly inhibiting austenite recrystallization. Combined with the TMCP process, it effectively refines the grains, significantly improving toughness and resistance to HIC while enhancing strength. Its strain-induced precipitates can further strengthen the matrix. However, excessive niobium will increase the recrystallization temperature, increasing rolling load and process difficulty. Considering all factors, the niobium content is controlled at a mass percentage of 0.030%–0.050%. Titanium can form stable TiN with nitrogen, fixing free nitrogen, preventing grain coarsening, and improving inclusion morphology and resistance to HIC (high-intensity cracking). However, excessive titanium easily forms coarse TiN inclusions, becoming crack initiation sites. It also excessively consumes carbon and nitrogen, weakening the strengthening effects of V and Nb, and reducing the purity and toughness of the steel. Considering all factors, the titanium content is controlled at 0.005%–0.020% by mass. Molybdenum promotes the formation of acicular ferrite, optimizes the microstructure, and improves acid resistance and resistance to HIC / SSCC. However, excessive molybdenum significantly increases hardenability, hardens the weld heat-affected zone, increases cracking tendency, exacerbates segregation, and increases alloy cost. Considering all factors, the molybdenum content is controlled at 0.08%–0.11% by mass.

[0028] In a second aspect of the present invention, a method for preparing X65MOS grade deep-sea acid-resistant pipeline steel is provided, comprising the following steps: (1) Converter smelting: The pretreated molten iron is sent into the converter for smelting. Slag-forming material and deoxidizer are added during the tapping process, and aluminum is not added in the first 15 minutes before and after the weak stirring process. (2) LF refining: The molten steel after converter smelting is sent to the LF furnace for heating, deep desulfurization and composition fine adjustment; Argon blowing protection is carried out throughout the refining process to avoid oxidation of the molten steel. The total processing time of the LF furnace is ≥35min, and the weak stirring time before leaving the refining station is ≥3min. (3) RH vacuum treatment: The molten steel after LF refining is sent into the RH vacuum device for vacuum degassing treatment. The cyclic treatment is carried out for ≥20 minutes under the condition of vacuum degree ≤5mbar. After wire feeding alloying, weak stirring is carried out for ≥10 minutes. (4) Continuous casting: The molten steel after vacuum treatment is continuously cast and poured. Electromagnetic stirring is used in the continuous casting process to obtain a continuous casting billet. At least once for each casting, a sample is taken from the tail of the first billet of each flow for low magnification inspection. The billet is rated according to the Mannesmann center segregation standard, and the acceptance standard is ≤2. (5) Controlled rolling and controlled cooling: After the continuous casting billet is heated and homogenized, it is subjected to controlled rolling. The final rolling temperature is controlled at 850℃~910℃. After rolling, water cooling is carried out and the final cooling temperature is controlled at 400℃~550℃. Finally, X65MOS grade deep-sea acid-resistant pipeline steel with acicular ferrite structure with a thickness of 15~27mm is obtained.

[0029] Preferably, in the preparation method of X65MOS grade deep-sea acid-resistant pipeline steel of the present invention, in step (1), the molten iron before entering the furnace must meet the pretreatment endpoint conditions: sulfur content mass percentage ≤ 0.005%, slag thickness ≤ 30 mm, and molten iron temperature ≥ 1250 °C.

[0030] Preferably, in the preparation method of X65MOS grade deep-sea acid-resistant pipeline steel of the present invention, in step (2), 500~800 kg of lime and 100~300 kg of fluorite are used for slag formation during the LF refining process. From 15 minutes before the start of weak stirring, no aluminum deoxidizer is added in subsequent processes.

[0031] Preferably, in the preparation method of X65MOS grade deep-sea acid-resistant pipeline steel of the present invention, in step (3), the weak stirring time after wire feeding alloying is ≥15min.

[0032] Preferably, in the preparation method of X65MOS grade deep-sea acid-resistant pipeline steel of the present invention, in step (4), argon blowing protection is adopted in the tundish throughout the continuous casting process, the nitrogen increase of molten steel in the continuous casting process is controlled to be ≤5ppm, the temperature of molten steel in the tundish is controlled to be 1520℃~1560℃, the casting speed is matched to 0.75~1.45m / min, and the electromagnetic stirring is roller electromagnetic stirring.

[0033] Preferably, in the preparation method of X65MOS grade deep-sea acid-resistant pipeline steel of the present invention, in step (5), the homogenization temperature of the continuously cast billet is 1120℃~1220℃, the furnace dwell time is controlled at 8~11min / 10mm plate thickness, and the controlled rolling is divided into two stages: rough rolling and finish rolling, with the rough rolling start temperature ≥1000℃. More preferably, in step (5), the water temperature for post-rolling water cooling is 750℃~820℃, the heating and homogenization temperature for continuous casting billet is 1200℃~1210℃, the roughing rolling start temperature is 1010℃~1080℃, the finishing rolling finish temperature is 880℃~900℃, and the final cooling temperature is 440℃~470℃.

[0034] The following detailed description of the X65MOS-grade deep-sea acid-resistant pipeline steel and its preparation method is illustrated in conjunction with specific embodiments.

[0035] Table 1 Mechanical performance of X65MOS grade deep-sea acid-resistant pipeline steel in Examples 1-3

[0036] Table 2. HIC and SSC test performance of X65MOS grade deep-sea acid-resistant pipeline steel in Examples 1-3.

[0037] Table 3 Rolling process of X65MOS grade deep-sea acid-resistant pipeline steel in Examples 1-3

[0038] Example 1 The chemical composition of the X65MOS grade deep-sea acid-resistant pipeline steel in Example 1, by mass percentage, is as follows: C: 0.055%, Si: 0.11%, Mn: 1.21%, P: 0.008%, S: 0.0008%, Al: 0.028%, Nb: 0.047%, V: 0.041%, Ti: 0.013%, Cr: 0.21%, Mo: 0.10%, Ni: 0.18%, Cu: 0.15%, N: 0.002%, with the balance being Fe and unavoidable impurities.

[0039] The specific process of preparing X65MOS-grade deep-sea acid-resistant pipeline steel in Example 1 includes: (1) Converter smelting The pretreated molten iron is sent to the converter for smelting at a temperature of 1260℃. During the tapping process, 1500 kg of slag-forming lime and an appropriate amount of fluorite are added, along with aluminum shot as a deoxidizer.

[0040] (2) LF refining The molten steel after converter smelting is sent to the LF furnace for heating, deep desulfurization and composition fine-tuning; argon blowing protection is carried out throughout the refining process to avoid oxidation of the exposed molten steel. The total processing time in the LF furnace is 40 minutes, and the weak stirring time before leaving the refining station is 3 minutes.

[0041] (3) RH vacuum treatment The molten steel refined by LF is sent to the RH vacuum device for vacuum degassing treatment, and the vacuum degree is controlled to be ≤5mbar. Under this condition, it is circulated for 20 minutes. After wire feeding and alloying, it is weakly stirred for 10 minutes.

[0042] (4) Continuous casting The vacuum-treated molten steel is continuously cast. The continuous casting inlet temperature is 1540℃, the rejection temperature is 1566℃, and the casting speed is 1.20m / min. Roller electromagnetic stirring is used in the continuous casting process to produce continuously cast billets.

[0043] (5) Controlled rolling and controlled cooling The continuously cast billet is fed into a heating furnace for homogenization at a temperature of 1210°C and a holding time of 16 minutes. The initial rolling temperature is 1030°C. By adjusting parameters such as rolling speed, cooling water pressure and flow rate during rolling, the finishing rolling temperature is controlled at 880°C. After rolling, rapid water cooling is implemented with an initial water temperature of 750°C and a final cooling temperature of 460°C, ultimately obtaining X65MOS grade deep-sea acid-resistant pipeline steel with acicular ferrite structure.

[0044] The transverse mechanical properties test results of the X65MOS grade deep-sea acid-resistant pipeline steel prepared in this embodiment are as follows: Transverse yield strength R t0.50 The tensile strength is 475 MPa, and the tensile strength R is... m It has a strength of 630 MPa and a yield strength ratio of R. t0.50 / R m The elongation after fracture is 0.75, and the elongation after fracture is A. 50mm The percentage of the results is 47%; the Charpy impact energy at -38℃ is 337J, 354J, and 373J respectively; the CTOD characteristic value δm at -28℃ is 1.25mm; the cold bending test is qualified; the drop hammer shear area at -35℃ is 100%; and all performance indicators meet the requirements.

[0045] Example 2 The chemical composition of the X65MOS grade deep-sea acid-resistant pipeline steel in Example 2, by mass percentage, is as follows: C: 0.060%, Si: 0.15%, Mn: 1.27%, P: 0.010%, S: 0.0007%, Al: 0.021%, Nb: 0.043%, V: 0.035%, Ti: 0.008%, Cr: 0.19%, Mo: 0.09%, Ni: 0.19%, Cu: 0.15%, N: 0.004%, with the balance being Fe and unavoidable impurities.

[0046] The specific process of preparing X65MOS-grade deep-sea acid-resistant pipeline steel in Example 2 includes: (1) Converter smelting The pretreated molten iron is sent to the converter for smelting at a temperature of 1255℃. During the tapping process, 1480 kg of slag-forming lime and an appropriate amount of fluorite are added, along with aluminum shot as a deoxidizer.

[0047] (2) LF refining The molten steel after converter smelting is sent to the LF furnace for heating, deep desulfurization and composition fine-tuning; argon blowing protection is carried out throughout the refining process to avoid oxidation of the exposed molten steel. The total processing time in the LF furnace is 42 minutes, and the weak stirring time before leaving the refining station is 3.5 minutes.

[0048] (3) RH vacuum treatment The molten steel refined by LF was sent to the RH vacuum device for vacuum degassing treatment, and the vacuum degree was controlled to be ≤4mbar. Under this condition, the treatment was circulated for 22 minutes. After wire feeding and alloying, weak stirring was performed for 11 minutes.

[0049] (4) Continuous casting The vacuum-treated molten steel was continuously cast. The continuous casting inlet temperature was 1538℃, the rejection temperature was 1563℃, and the casting speed was 1.15m / min. Roller electromagnetic stirring was used in the continuous casting process to produce continuously cast billets.

[0050] (5) Controlled rolling and controlled cooling The continuously cast billet is fed into a heating furnace for homogenization at a temperature of 1200°C and a holding time of 17 minutes. The initial rolling temperature is 1010°C. By adjusting parameters such as rolling speed, cooling water pressure and flow rate during rolling, the finishing rolling temperature is controlled at 900°C. After rolling, rapid water cooling is implemented with an initial water temperature of 756°C and a final cooling temperature of 441°C, ultimately obtaining X65MOS grade deep-sea acid-resistant pipeline steel with acicular ferrite structure.

[0051] The transverse mechanical properties test results of the X65MOS grade deep-sea acid-resistant pipeline steel prepared in this embodiment are as follows: Transverse yield strength R t0.50 The tensile strength is 490 MPa, and the tensile strength R is... m It has a strength of 625 MPa and a yield strength ratio of R. t0.50 / R m The elongation after fracture is 0.78, and the elongation after fracture is A. 50mm The percentage of the results is 52%; the Charpy impact energy at -38℃ is 349J, 338J, and 403J respectively; the CTOD characteristic value δm at -28℃ is 1.18mm; the cold bending test is qualified; the drop hammer shear area at -35℃ is 100%; and all performance indicators meet the requirements.

[0052] Example 3 The chemical composition of the X65MOS grade deep-sea acid-resistant pipeline steel in Example 3, by mass percentage, is as follows: C: 0.050%, Si: 0.17%, Mn: 1.30%, P: 0.009%, S: 0.0006%, Al: 0.030%, Nb: 0.042%, V: 0.039%, Ti: 0.008%, Cr: 0.17%, Mo: 0.09%, Ni: 0.21%, Cu: 0.14%, N: 0.006%, with the balance being Fe and unavoidable impurities.

[0053] The specific process of preparing X65MOS-grade deep-sea acid-resistant pipeline steel in Example 3 includes: (1) Converter smelting The pretreated molten iron is sent to the converter for smelting at a temperature of 1258℃. During the tapping process, 1510 kg of lime and an appropriate amount of fluorite are added as slag-forming material, and aluminum shot is added as a deoxidizer for deoxidation.

[0054] (2) LF refining The molten steel after converter smelting is sent to the LF furnace for heating, deep desulfurization and composition fine-tuning; argon blowing protection is carried out throughout the refining process to avoid oxidation of the exposed molten steel. The total processing time in the LF furnace is 41 minutes, and the weak stirring time before leaving the refining station is 3 minutes.

[0055] (3) RH vacuum treatment The molten steel refined by LF was sent to the RH vacuum device for vacuum degassing treatment, and the vacuum degree was controlled to be ≤4.5mbar. Under this condition, the treatment was circulated for 21 minutes. After wire feeding and alloying, weak stirring was performed for 10.5 minutes.

[0056] (4) Continuous casting The vacuum-treated molten steel was continuously cast. The continuous casting inlet temperature was 1539℃, the rejection temperature was 1565℃, and the casting speed was 1.18m / min. Roller electromagnetic stirring was used in the continuous casting process to produce continuously cast billets.

[0057] (5) Controlled rolling and controlled cooling The continuously cast billet is fed into a heating furnace for homogenization at a temperature of 1205°C and a holding time of 16.5 min. The initial rolling temperature is 1080°C. By adjusting parameters such as rolling speed, cooling water pressure and flow rate during rolling, the finishing rolling temperature is controlled at 884°C. After rolling, rapid water cooling is performed at an initial water temperature of 780°C and the final cooling temperature is controlled at 463°C, ultimately obtaining X65MOS grade deep-sea acid-resistant pipeline steel with acicular ferrite structure.

[0058] The transverse mechanical properties test results of the X65MOS grade deep-sea acid-resistant pipeline steel prepared in this embodiment are as follows: Transverse yield strength R t0.50 The tensile strength is 475 MPa, and the tensile strength R is... m It has a strength of 630 MPa and a yield strength ratio of R. t0.50 / R m The elongation after fracture is 0.75, and the elongation after fracture is A. 50mm The Charpy impact energy at -38℃ is 425J, 334J, and 394J respectively; the CTOD characteristic value δm at -28℃ is 1.32mm; the cold bending test is qualified; the drop hammer shear area at -35℃ is 100%; and all performance indicators meet the requirements.

[0059] In summary, this invention specifically employs a composition system that primarily utilizes high-V precipitation strengthening and secondarily uses Nb-Ti composite microalloying for fine grains. It strictly controls the addition of expensive alloys such as Ni, Cr, Mo, and Cu, resulting in good economic efficiency. At the same time, by strictly limiting the content of P and S, the formation of harmful inclusions is reduced, thereby improving the acid resistance of pipeline steel. By rationally applying the process of converter smelting + LF + RH vacuum treatment ladle refining + continuous casting, high steel purity and uniform microstructure are obtained. By optimizing the preheating billet process and the controlled rolling and cooling process, the core deformation rate and microstructure density of the steel plate are improved, and the resistance to HIC and SSC is enhanced. By precisely controlling the heating, final rolling, water immersion and final cooling temperatures, a microstructure dominated by acicular ferrite is obtained, avoiding the formation of hard phases and significantly improving the toughness and acid corrosion resistance of the steel plate.

[0060] In the description of this specification, the term "embodiment" means a specific feature, structure, material, or characteristic described in connection with that embodiment is included in at least one embodiment of the invention. The illustrative expressions of the above terms in this specification do not necessarily refer to the same embodiment. Furthermore, those skilled in the art can combine or combine the different embodiments and features described in this specification without creating contradiction.

[0061] It should be noted that, unless otherwise specified, the terms used herein have the meanings commonly understood by those skilled in the art. Furthermore, when a numerical range is disclosed herein, the range is considered continuous and includes the minimum and maximum values ​​of the range, as well as every value between such minimum and maximum. Further, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Moreover, when multiple ranges are provided to describe features, the ranges may be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0062] It should also be noted that, in this document, the term "comprising" or any other variation thereof is intended to cover non-exclusive inclusion, such that an article or device that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such an article or device.

[0063] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the present invention.

Claims

1. An X65MOS grade deep-sea acid-resistant pipeline steel, characterized in that, The chemical composition of the X65MOS grade deep-sea acid-resistant pipeline steel is controlled by mass percentage as follows: 0.05%≤C≤0.08%, 0.10%≤Si≤0.30%, 1.00%≤Mn≤1.40%, P≤0.018%, S≤0.0014%, 0.015%≤Al≤0.060%, 0.030%≤Nb≤0.050%, 0.030%≤V≤0.050%, 0.005%≤Ti≤0.020%, 0.13%≤Cr≤0.25%, 0.08%≤Mo≤0.11%, 0.15%≤Ni≤0.25%, 0.13%≤Cu≤0.18%, N≤0.008%, with the balance being Fe and unavoidable impurities.

2. The X65MOS grade deep-sea acid-resistant pipeline steel according to claim 1, characterized in that, The chemical composition of the X65MOS grade deep-sea acid-resistant pipeline steel is controlled by mass percentage as follows: 0.05%≤C≤0.06%, 0.11%≤Si≤0.17%, 1.21%≤Mn≤1.30%, P≤0.010%, S≤0.0008%, 0.021%≤Al≤0.030%, 0.042%≤Nb≤0.047%, 0.035%≤V≤0.041%, 0.008%≤Ti≤0.013%, 0.17%≤Cr≤0.21%, 0.09%≤Mo≤0.10%, 0.18%≤Ni≤0.21%, 0.14%≤Cu≤0.15%, N≤0.006%, with the balance being Fe and unavoidable impurities.

3. The X65MOS grade deep-sea acid-resistant pipeline steel according to claim 2, characterized in that, The mechanical properties of the X65MOS grade deep-sea acid-resistant pipeline steel are as follows: transverse yield strength R t0.50 ≥460MPa, tensile strength R m ≥535MPa, yield strength ratio R t0.50 / R m ≤0.89, Elongation after fracture A 50mm ≥47%, Charpy impact energy at -38℃ ≥300J, CTOD characteristic value δm at -28℃ ≥1mm.

4. A method for preparing X65MOS grade deep-sea acid-resistant pipeline steel, characterized in that, Includes the following steps: (1) Converter smelting: The pretreated molten iron is sent into the converter for smelting, and slag-forming materials and deoxidizers are added during the tapping process; (2) LF refining: The molten steel after converter smelting is sent to the LF furnace for heating, deep desulfurization and composition fine adjustment; Argon blowing protection is carried out throughout the refining process to avoid oxidation of the molten steel. The total processing time of the LF furnace is ≥35min, and the weak stirring time before leaving the refining station is ≥3min. (3) RH vacuum treatment: The molten steel after LF refining is sent into the RH vacuum device for vacuum degassing treatment. The cyclic treatment is carried out for ≥20 minutes under the condition of vacuum degree ≤5mbar. After wire feeding alloying, weak stirring is carried out for ≥10 minutes. (4) Continuous casting: The molten steel after vacuum treatment is continuously cast and poured. Electromagnetic stirring is used in the continuous casting process to obtain the continuous casting billet. (5) Controlled rolling and controlled cooling: After the continuous casting billet is heated and homogenized, it is subjected to controlled rolling. The final rolling temperature is controlled at 850℃~910℃. After rolling, water cooling is carried out and the final cooling temperature is controlled at 400℃~550℃. Finally, X65MOS grade deep-sea acid-resistant pipeline steel with acicular ferrite structure with a thickness of 15~27mm is obtained.

5. The method for preparing X65MOS-grade deep-sea acid-resistant pipeline steel according to claim 4, characterized in that, In step (1), the molten iron before entering the furnace must meet the following pretreatment endpoint conditions: sulfur content by mass percentage ≤ 0.005%, slag thickness ≤ 30 mm, and molten iron temperature ≥ 1250℃.

6. The method for preparing X65MOS-grade deep-sea acid-resistant pipeline steel according to claim 4, characterized in that, In step (2), 500-800 kg of lime and 100-300 kg of fluorite are used for slag formation during the LF refining process. Starting 15 minutes before the start of weak stirring, no aluminum deoxidizer is added in any subsequent process.

7. The method for preparing X65MOS-grade deep-sea acid-resistant pipeline steel according to claim 4, characterized in that, In step (3), the weak stirring time after wire alloying is ≥15min.

8. The method for preparing X65MOS-grade deep-sea acid-resistant pipeline steel according to claim 4, characterized in that, In step (4), argon blowing protection is used in the tundish throughout the continuous casting process. The nitrogen increase in the molten steel during the continuous casting process is controlled to be ≤5ppm. The temperature of the molten steel in the tundish is controlled to be 1520℃~1560℃. The casting speed is matched to 0.75~1.45m / min. The electromagnetic stirring is a roller electromagnetic stirring.

9. The method for preparing X65MOS grade deep-sea acid-resistant pipeline steel according to claim 4, characterized in that, In step (5), the heating temperature of the continuously cast billet is 1120℃~1220℃, and the furnace dwell time is controlled at 8~11min / 10mm plate thickness. The controlled rolling is divided into two stages: rough rolling and finish rolling. The rough rolling start temperature is ≥1000℃.

10. The method for preparing X65MOS grade deep-sea acid-resistant pipeline steel according to claim 9, characterized in that, In step (5), the water temperature for post-rolling water cooling is 750℃~820℃, the heating and soaking temperature for continuous casting billet is 1200℃~1210℃, the roughing rolling start temperature is 1010℃~1080℃, the finishing rolling finish temperature is 880℃~900℃, and the final cooling temperature is 440℃~470℃.

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