High toughness x60 / x65 grade sour resistant line pipe steel and method of manufacturing the same

Abstract

A high-toughness X60 / X65 grade acid-resistant pipeline steel and a manufacturing method thereof, the steel having the following components in percentage by weight: C 0.015-0.040%, Si 0.10-0.30%, Mn 1.18-1.45%, 0P≤0.008%, 0S≤0.0010%, Cu 0.05-0.35%, Ni 0.05-0.30%, Cr 0.15-0.30%, Mo 0.02-0.09%, Nb 0.025-0.055%, Ti 0.005-0.020%, Ca 0.0010-0.0040%, Alt 0.010-0.040%, 0B≤0.0004%, 0O≤0.0025%, 0N≤0.0050%, 0H≤0.0002%, 0Sn≤0.0050%, 0Sb≤0.0025%, 0Bi≤0.0030%, 0Pb≤0.0050%, 0As≤0.0030%, the balance comprising Fe and inevitable impurities, and simultaneously satisfying: Nb+Ti≤0.07%; Ca / S≥2; Cu+Ni+Cr+Mo≤0.80%; 0N+O+H≤0.0070%; 0.7≤(Nb+27.5*Ti) / (5C+27*N)≤5.0; hydrogen-induced cracking sensitivity coefficient Ks≤0.23, Ks=8.0C+24S-Alt-2.3Nb+2.8Ti+117B+0.34Ni-0.30Mo+24.5N-0.19. The obtained pipeline steel can reach X60 / X65 strength grade, and has good low-temperature toughness and excellent HIC and SSC resistance.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials and their manufacturing, specifically relating to a high-toughness X60 / X65 grade acid-resistant pipeline steel and its manufacturing method. Background Technology

[0002] As conventional oil and gas fields are continuously developed and reserves are decreasing, the proportion of H2S-containing acidic oil and gas fields being developed is further increasing. This necessitates that pipelines possess acid-resistant properties. Different acid-resistant pipeline designs will develop customized acid-related indicators based on the pipeline's service conditions, including NACE standard resistance to hydrogen-induced cracking (HIC) and hydrogen sulfide stress corrosion cracking (SSC), as well as stringent low-magnification ratings and flaw detection requirements.

[0003] Chinese patent CN202010552764.7 discloses "a magnesium-containing X65 pipeline steel with excellent acid resistance and its production method." The composition and weight percentage of this pipeline steel are as follows: C: 0.03–0.05%, Si: 0.17–0.20%, Mn: 0.40–0.70%, Ni: 0.12–0.15%, Cr: 0.45–0.48%, Mo: 0.10–0.15%, Mg: 0.002–0.004%, Nb: 0.06–0.09%, Ti: 0.01–0.02%, P≤0.008%, S≤0.010%, with the remainder being Fe and unavoidable impurities. This patent uses a Mg treatment method to remove inclusions. Since the Mn content is relatively low, a higher amount of Nb, Cr, and a certain amount of Mo are added to ensure strength, thus obtaining an X65 strength-grade acid-resistant pipeline steel. However, the method of treating inclusions with Mg is difficult to control stably, and the control effect on inclusions such as MnS is poor. It has an adverse effect on acid resistance and toughness. The resulting acid-resistant pipeline steel has a Charpy impact energy of ≥200J at -20℃ and a DWTT SA% of ≥90% at -15℃.

[0004] Chinese patent CN201510870777.8 discloses "A thick-gauge acid-resistant X60MS hot-rolled coil and its manufacturing method." The composition of the steel plate, by weight percentage, is as follows: C: 0.050-0.070%, Si: 0.15-0.25%, Mn: 1.35-1.45%, Nb: 0.025-0.045%, Ti: 0.008-0.025%, Mo: 0.06-0.10%, Cr: 0.08-0.18%, Als: 0.015-0.045%, P≤0.015%, S≤0.0015%, N≤0.006%. The increased C content significantly increases the segregation degree in the steel, increasing the material's resistance to HIC and reducing its low-temperature toughness to some extent. The resulting material has a Charpy impact energy ≥300J at -40℃ and a DWTT at -30℃. SA% ≥ 85%.

[0005] Chinese patent CN202011197059.6 discloses "an X60 / X65 grade acid-resistant pipeline steel and its preparation method", the chemical composition and mass percentage of which are as follows: C: 0.02~0.06%, Si: 0.10~0.20%, Mn≤0.90%, Al: 0.020~0.050%, Nb: 0.060~0.080%, Ti≤0.020%, Cr≤0.30%, Ni≤0.20%, Cu≤0.40%, P≤0.012%, S≤0.002%, N≤0.0060%, B≤0.0005%, Mn+Cr≤1.2%, and the remainder is iron and unavoidable impurity elements. The patented alloy design incorporates a significant amount of Nb (0.060–0.080%). The high Nb content increases the tendency for Nb particles to agglomerate in the core of the steel, thereby increasing the probability of HIC cracks appearing in the core of the steel plate.

[0006] A comparison with existing patents reveals that the existing acid-resistant pipeline steels contain higher levels of easily segregating elements such as C and Nb, which can easily lead to segregation and inclusions in the core, increasing the material's resistance to HIC and the likelihood of hydrogen-induced cracking. Furthermore, the material's low-temperature toughness is difficult to improve further. Summary of the Invention

[0007] The purpose of this invention is to provide a high-toughness X60 / X65 grade acid-resistant pipeline steel and its manufacturing method. The resulting pipeline steel can achieve the X60 / X65 strength level and has good low-temperature toughness and excellent resistance to HIC and SSC. t0.5 Yield strength 430~570MPa, tensile strength 520~690MPa, yield strength ratio ≤0.92, elongation ≥30% at A50mm, hardness HV10≤220, Charpy impact energy ≥400J at -49℃, DWTT SA%≥90% at -35℃.

[0008] To achieve the above objectives, the technical solution of the present invention is as follows:

[0009] A high-toughness X60 / X65 grade acid-resistant pipeline steel has the following chemical composition by weight percentage: C: 0.015–0.040%, Si: 0.10–0.30%, Mn: 1.18–1.45%, 0 < P ≤ 0.008%, 0 < S ≤ 0.0010%, Cu: 0.05–0.35%, Ni: 0.05–0.30%, Cr: 0.15–0.30%, Mo: 0.02–0.09%, Nb: 0.025–0.055%, Ti: 0.005 ~0.020%, Ca: 0.0010~0.0040%, Alt: 0.010~0.040%, 0 < B ≤ 0.0004%, 0 < O ≤ 0.0025%, 0 < N ≤ 0.0050%, 0 < H ≤ 0.0002%, 0 < Sn ≤ 0.0050%, 0 < Sb ≤ 0.0025%, 0 < Bi ≤ 0.0030%, 0 < Pb ≤ 0.0050%, 0 < As ≤ 0.0030%, with the balance including Fe and unavoidable impurities, and simultaneously satisfying:

[0010] Nb+Ti≤0.07%;

[0011] Ca / S≥2;

[0012] Cu+Ni+Cr+Mo≤0.80%;

[0013] 0 < N + O + H ≤ 0.0070%;

[0014] 0.7≤(Nb+27.5*Ti) / (5C+27*N)≤5.0;

[0015] The hydrogen-induced cracking susceptibility coefficient Ks≤0.23, Ks=8.0C+24S-Alt-2.3Nb+2.8Ti+117B+0.34Ni-0.30Mo+24.5N-0.19.

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

[0017] The microstructure of the pipeline steel described in this invention is uniform and refined granular bainite with a small amount of blocky ferrite, wherein the blocky ferrite content is 3-10%, the grain size is ≥10, and there are no obvious segregation bands or hard phase structures.

[0018] The R of the pipeline steel described in this invention t0.5Yield strength 430~570MPa, tensile strength 520~690MPa, yield strength ratio ≤0.92, elongation ≥30% at A50mm, hardness HV10≤220, Charpy impact energy ≥400J at -49℃, DWTT SA%≥90% at -35℃.

[0019] In the chemical composition design of the acid-resistant pipeline steel described in this invention:

[0020] Carbon (C): The most economical strengthening element in steel, enhancing its strength through interstitial solid solution strengthening. Increasing the carbon content can significantly improve the hardenability of steel, reduce the amount of other precious alloys added, and lower production costs. However, increased C content is detrimental to the ductility, toughness, weldability, and corrosion resistance of steel, especially exacerbating core segregation and significantly impacting acid resistance. Therefore, this invention employs an ultra-low C design, with its content controlled between 0.015% and 0.040%.

[0021] Si is a solid solution strengthening element and also a deoxidizing element in steel. However, excessive Si content can adversely affect surface quality and weldability. If the Si content exceeds 0.30%, toughness may decrease. Therefore, the Si content in this invention is controlled between 0.10% and 0.30%.

[0022] Mn: Mn strengthens steel through solid solution treatment, making it the most important and economical strengthening element for compensating for strength loss caused by reduced carbon content. Mn helps obtain fine phase transformation products and also aids in oxygen and sulfur control during steelmaking. However, low Mn content makes it difficult to achieve the target strength level, but it also exacerbates central segregation. Therefore, in this invention, the Mn content is controlled between 1.18% and 1.45%.

[0023] Cr (Cr): It has a certain solid solution strengthening effect and can effectively improve the hardenability of steel. When the Cr content is above 0.10%, it can effectively improve the corrosion resistance of steel and form a relatively dense protective layer on the surface of the steel, thus protecting the substrate. However, if the Cr content in the steel is too high, it is not conducive to the weld quality and is prone to forming gray spot defects. Therefore, in this invention, the Cr content is controlled at 0.15-0.30%.

[0024] Nitrogen (Nb) is an important element in low-carbon microalloyed steel. During hot rolling, Nb dissolved in solid solution precipitates under strain to form Nb carbonitrides, which pin grain boundaries and inhibit the growth of deformed austenite. Controlled rolling and controlled cooling transform the deformed austenite into fine products with high dislocation density. After the steel strip is coiled, dissolved Nb precipitates as NbC particles dispersed in the matrix, playing a role in precipitation strengthening. Excessive Nb makes the slab prone to cracking, thus affecting surface quality and deteriorating weldability. Therefore, the Nb content in this invention is controlled at 0.025–0.055%.

[0025] Ti: A good deoxidizing and degassing agent and an effective element for fixing nitrogen and carbon. Undissolved Ti carbonitrides can inhibit austenite grain growth during steel heating. TiN and TiC precipitated during rough rolling in the high-temperature austenitic region can effectively suppress austenite grain growth, thereby refining the grains. Simultaneously, it can increase the solid solubility of Nb, reducing the microcrack sensitivity of Nb-containing steel. It is generally added in combination with Nb. Furthermore, precipitation during welding can also inhibit high-temperature grain growth, thereby improving weldability. Therefore, the Ti content in this invention is controlled at 0.005–0.020%.

[0026] Nb+Ti: The addition of more Nb and Ti can easily lead to the aggregation of coarse Nb / Ti particles in the steel plate. These particles tend to form long strip-shaped agglomerates distributed along the rolling direction in the core. As a hard phase with a hardness higher than the matrix, this increases the susceptibility of the microstructure to hydrogen-induced cracking. Hydrogen pinning around these particles prevents further diffusion, making them easy sites for hydrogen accumulation and initiation points for hydrogen-induced cracking. This significantly increases the tendency for HIC cracking in the core and deteriorates the acid resistance of the material. Therefore, in this invention, Nb+Ti is controlled to be ≤0.07%.

[0027] Furthermore, TiN and NbC are important second phases in steel, significantly impacting strength and toughness. When (Nb+27.5*Ti) / (5C+27*N) > 5, the second phase particles are relatively large and unevenly distributed, easily accumulating at core segregation points, leading to stress concentration, brittle fracture, and a sharp decrease in low-temperature toughness. This significantly reduces the toughness of the base metal and weld hot zone, and can also become a hard phase causing hydrogen-induced cracking. When (Nb+27.5*Ti) / (5C+27*N) < 0.7, the number of precipitated phases is small, and their size is too fine, making them easily dissolved back into the base metal. The grain boundary pinning effect is weak, also negatively impacting toughness. Therefore, in this invention, 0.7 ≤ (Nb+27.5*Ti) / (5C+27*N) ≤ 5.0 is controlled.

[0028] Mo (Mo) is a highly hardenable element that significantly delays the ferrite phase transformation, inhibits the formation of ferrite and pearlite, and effectively promotes bainite transformation, thereby strengthening the matrix and resulting in a finer microstructure. A certain amount of Mo has significant benefits for improving the microstructure and properties of materials; however, excessive Mo content reduces the plasticity of steel, and Mo is also expensive. Therefore, in this invention, the Mo content is controlled at a relatively low level of 0.02–0.09%.

[0029] Cu and Ni can improve the strength of steel through solid solution strengthening and enhance its resistance to atmospheric corrosion. However, high content can lead to hot cracking in the steel plate. Ni, on the other hand, can refine the grain size and improve the hot brittleness that Cu easily causes in steel, significantly contributing to improved low-temperature toughness. Therefore, in this invention, the Cu content is controlled at 0.05–0.35%, and the Ni content is controlled at 0.05–0.30%.

[0030] Cu+Ni+Cr+Mo: The addition of Cu, Ni, Cr, and Mo elements increases production costs. Furthermore, if the content of Cu, Ni, Cr, and Mo exceeds a certain range, the properties of the steel will be affected, increasing the material's tendency for hot brittleness. It also promotes the formation of MA particles, increases the proportion of hard phases in the steel, and makes it prone to hydrogen traps, leading to a series of problems such as hydrogen embrittlement and brittle phases, thereby reducing resistance to hydrogen embrittlement (HIC) and low-temperature toughness. To ensure the comprehensive properties of the steel, including strength and toughness, and to maintain controllable production costs, the content of Cu+Ni+Cr+Mo is controlled to be ≤0.80% in this invention.

[0031] S and P: These are the main impurity elements in steel. Phosphorus easily causes cold brittleness in steel, while sulfur easily causes hot brittleness, leading to unstable steel properties. In particular, as the S content increases, the number of MnS inclusions increases, which significantly increases the susceptibility to hydrogen-induced cracking in steel and reduces acid resistance. Therefore, the phosphorus and sulfur content in steel should be minimized. Thus, in this invention, S is controlled to be ≤0.0010% and P ≤0.0080%.

[0032] Ca: Ca treatment can control the morphology of sulfides, improve the anisotropy of steel plates, and enhance low-temperature toughness. Additionally, when the S content is high, ensuring an appropriate Ca / S ratio ensures the material's resistance to HIC (High-Intensity Concentration) corrosion. However, excessively high Ca content can lead to an increase in the size and density of inclusions such as CaO in the steel. In this invention, the Ca content is controlled between 0.0010 and 0.0040%, while maintaining a Ca / S ratio ≥ 2.

[0033] Al: Al is used for deoxidation of steel. Appropriate amounts of Al also help refine grains and improve strength and toughness. However, if the Al content exceeds 0.05%, coarse precipitates may form, thereby reducing the steel's acid resistance and low-temperature toughness. Therefore, in this invention, the Al content is controlled between 0.010% and 0.040%, and Alt is controlled to be ≤0.08% during the LF / RH process.

[0034] N, O, H: For low-temperature toughness, O, N, and H can all lead to brittle fracture, especially at low temperatures. The presence of O and N forms oxides and nitrogen compounds, which may accumulate at grain boundaries, causing intergranular brittleness. Simultaneously, H may penetrate into the grains, causing lattice deformation and void formation, thus leading to brittle fracture. Therefore, the low-temperature toughness of steel is related to the content of O, N, and H; excessive or inappropriate content can lead to brittle fracture. The effects of O, N, and H on acid resistance are also significant. In acidic environments, O reacts with iron in the steel to cause oxidation, reducing the steel's corrosion resistance. N may react with chromium in the steel to form nitrogen compounds, decreasing the steel's corrosion resistance. H promotes stress corrosion cracking in steel, also affecting its acid resistance. Therefore, in this invention, 0 < O ≤ 0.0025%, 0 < N ≤ 0.0050%, 0 < H ≤ 0.0002%, and 0 < N + O + H ≤ 0.0070%.

[0035] B: The main role of B in steel is to increase its hardenability and strength, thereby saving on other rarer and more expensive metals. However, the addition of B can have a significant adverse effect on the low-temperature toughness of the material. Therefore, in this invention, B is controlled to be ≤0.0004%.

[0036] Sn, Sb, Bi, Pb, and As: When the content of these elements in steel exceeds a certain limit, it will reduce the strength of the steel and easily cause severe segregation, forming agglomerates at the grain boundaries. This leads to brittle cracks starting from the grain boundaries and extending along them until complete fracture, significantly reducing the low-temperature toughness of the material. Furthermore, a high Sn content in steel significantly reduces its resistance to hydrogen-induced cracking (HIC), because tin promotes hydrogen diffusion and aggregation, thereby accelerating HIC. Pb has a similar effect. Therefore, in this invention, the contents are controlled as follows: 0 < Sn ≤ 0.0050%, 0 < Sb ≤ 0.0025%, 0 < Bi ≤ 0.0030%, 0 < Pb ≤ 0.0050%, and 0 < As ≤ 0.0030%.

[0037] Hydrogen-induced cracking susceptibility coefficient Ks: Resistance to HIC is affected by various elements in the steel, such as C and S, which exhibit significant segregation; Nb and Ti, which participate in the precipitation of second-phase particles; and inclusion elements such as N and B. Based on the segregation tendency of each element and the relationship between each element and HIC resistance, this invention proposes a hydrogen-induced cracking susceptibility coefficient Ks: Ks = 8.0C + 24S - Alt - 2.3Nb + 2.8Ti + 117B + 0.34Ni - 0.30Mo + 24.5N - 0.19 (element unit: wt%). The correlation between the HIC crack length ratio (CLR) and the coefficient Ks is as follows: Figure 1As shown, with increasing Ks, the material's susceptibility to hydrogen-induced cracking (HIC) increases, the probability of HIC cracks increases, and the crack length also increases. When Ks ≤ 0.23, no cracks were observed in the HIC test; when Ks > 0.23, the CLR (Closing Rate) begins to increase significantly. The API 5L SPEC standard requires a CLR of ≤ 15%, and some engineering projects require a CLR of ≤ 10% or even more stringent requirements. Based on the relationship between CLR and Ks, to ensure the material's acid resistance, this invention controls Ks ≤ 0.23.

[0038] Overall, this invention adopts the composition system designed above, comprehensively considering the effects of easily segregating elements such as C and S in steel, microalloying elements such as Cu, Ni, Cr, Mo and Nb, Ti, as well as impurity elements such as N, O, H, B and Sn, Sb, Bi, Pb, As on the acid resistance and low-temperature toughness of the material. Based on the degree of segregation of each element and its relationship with HIC resistance, the invention proposes control requirements for the hydrogen-induced cracking sensitivity coefficient Ks coefficient to improve the acid resistance of the material. Based on this, the Nb+Ti content was controlled to be ≤0.07%, and 0.7≤(Nb+27.5*Ti) / (5C+27*N)≤5.0. The influence of second-phase particles such as TiN and NbC on the material's acid resistance and low-temperature toughness was considered. Simultaneously, by adding Cu+Ni+Cr+Mo≤0.80%, the inhibitory effect on bulk ferrite was reduced, resulting in a small amount of bulk ferrite in the steel microstructure (bulk ferrite content 3-10%), which inhibits crack formation. Therefore, the pipeline steel possesses both high strength and acid resistance, as well as good low-temperature toughness. The resulting pipeline steel R... t0.5 Yield strength 430~570MPa, tensile strength 520~690MPa, yield strength ratio ≤0.92, elongation ≥30% at A50mm, hardness HV10≤220, Charpy impact energy ≥400J at -49℃, DWTT SA%≥90% at -35℃.

[0039] The manufacturing method of the high-toughness X60 / X65 grade acid-resistant pipeline steel of the present invention includes the following steps:

[0040] 1) Hot metal pretreatment

[0041] The sulfur content of molten iron before pretreatment should be ≤0.050%, and the sulfur content of molten iron entering the furnace should be ≤0.0020%.

[0042] 2) Smelting

[0043] When smelting with the above-mentioned composition, the C content in the tapped steel should be controlled to be ≤0.035%, and the S content in the tapped steel should be ≤0.0080%.

[0044] 3) Refining

[0045] LF and RH refining are adopted. During the LF refining process, the stirring time after all the alloy is added is controlled to be ≥3min. The amount of calcium wire fed in RH is 200-350m. After the calcium wire is fed, the argon blowing time is ≥5min. The target Ca content is 10-40ppm, and the Ca / S ratio is controlled to be ≥2.

[0046] 4) Continuous casting

[0047] The continuous casting settling time is ≥3 min, and the superheat is controlled between 10 and 40℃.

[0048] 5) Reheating

[0049] Heating temperature: 1100~1200℃, holding time: 200~350min;

[0050] 6) Controlled rolling

[0051] Final rolling temperature: 750~900℃;

[0052] 7) Cooling after rolling

[0053] Cooling start temperature: 700~800℃, cooling stop temperature: 300~550℃, cooling rate ≤30℃ / s.

[0054] Preferably, during the smelting process, the bottom blowing flow rate during the converter bottom calming stage is ≥900 Nm³. 3 / h, control the free oxygen level during shutdown to ≤900ppm.

[0055] In the manufacturing method of the present invention:

[0056] Controlling the composition and internal properties is crucial for improving the acid resistance and toughness of materials. Increased sulfur content during the process and in the finished product leads to the precipitation and aggregation of inclusions such as MnS in the steel. MnS inclusions generated in the core are elongated during rolling and easily form gaps after cooling. MnS is one of the most likely sites for HIC (high-temperature oxidative cracking), significantly reducing low-temperature toughness and HIC resistance. To ensure low sulfur content in the finished product and during the process, this invention requires controlling the sulfur content of molten iron to ≤0.050% before pretreatment, ensuring that the sulfur content of the molten iron entering the furnace is ≤0.0020% during KR (Killing Reduction) treatment, and thoroughly removing slag from the ladle after desulfurization.

[0057] During the smelting process, the bottom blowing flow rate during the converter bottom calming stage is ≥900 Nm³. 3 / h, control the free oxygen at the stop blowing level to ≤900ppm. Increased C content will exacerbate the segregation in the steel, causing MnS and other inclusions to further accumulate in the core, and significantly increasing the tendency for hydrogen-induced cracking. Therefore, C and S in the ladle steel need to be strictly controlled. In this invention, the ladle steel is controlled to have C ≤0.035% and S ≤0.0080%.

[0058] In this invention, the LF process, based on the pre-desulfurization and pre-deoxidation conditions after the converter furnace, uses aluminum slag for slag deoxidation. Desulfurization is achieved through bottom stirring with argon gas, with a pure stirring time ≥3 minutes. The target feed rate of the RH silicon-calcium wire is 200-350m. After RH wire feeding, a small flow rate of argon gas is blown into the ladle for ≥5 minutes to improve the purity of the molten steel. The LF refining process further desulfurizes and reduces the S content in the steel. Combined with the RH calcium treatment process, the target Ca is controlled at 10-40ppm, and the Ca / S ratio is controlled at ≥2, modifying inclusions such as MnS and Al2O3 in the steel. By increasing the effective calcium content in the steel, high-melting-point CaS is formed earlier during the solidification process, inhibiting the total amount of MnS generated during this process and modifying it entirely or partially into CaS. On the other hand, large Al2O3 inclusions are transformed into low-melting-point composite inclusions, promoting their flotation and purifying the internal quality of the molten steel. Both the LF and RH processes use low-carbon alloys.

[0059] The continuous casting process employs full-process protection and constant-speed pouring to ensure that the slab has good internal quality and segregation level. The continuous casting calming time is ≥3 minutes. Higher superheat will aggravate the degree of segregation, while too low superheat will prevent inclusions from floating to the surface. Both of these will have an adverse effect on acid resistance and low-temperature toughness. In this invention, the superheat is controlled at 10-40℃ to control center segregation, and the low-magnification rating is ≤2 grade according to the Mannesmann standard.

[0060] Slab heating temperature: 1100~1200℃; slab holding time: 200~350min. Sufficient heating temperature and time ensure complete solution dissolution of the alloy and good uniformity, which is beneficial to obtaining a uniform microstructure and slab shape.

[0061] The rolling process employs TMCP. The final rolling temperature control range is 750–900℃. When the final rolling temperature is too high, the austenite grains coarsen significantly, which adversely affects the final microstructure and properties of the steel plate. Conversely, when the final rolling temperature is too low, mixed-grain structures are easily formed in the steel, which adversely affects the toughness, plasticity, and processing properties of the material.

[0062] After rolling, a DQ+ACC cooling mode is adopted. By limiting the addition levels of Mo and Nb elements to a small amount, their inhibitory effect on blocky ferrite is appropriately reduced. This allows the steel to undergo phase transformation under conditions of an initial cooling temperature of 700–800℃, a final cooling temperature of 300–550℃, and a cooling rate controlled at ≤30℃ / s. Under these conditions, a small amount of ferrite first precipitates along the original austenite grain boundaries and has sufficient time to transform into relatively regular-shaped blocky ferrite, thus obtaining a small amount of blocky ferrite (≤10%) without affecting the bainite transformation. As a ductile phase, blocky ferrite can increase the interfacial area and cause greater plastic deformation at the crack tip, relieving the triaxial tensile stress at the crack tip and hindering crack propagation. At the same time, the presence of granular bainite inhibits the coordinated deformation of ferrite and increases the deformation resistance. Most cracks need to frequently cross the interface between bainite and polygonal ferrite during propagation. When there is a small proportion of blocky ferrite in the microstructure, crack propagation is effectively suppressed, thus exhibiting good low-temperature DWTT toughness.

[0063] At the same time, a faster cooling rate is conducive to rapid and uniform transformation of the microstructure, reducing core microstructure segregation and inclusion aggregation, and avoiding local hard spot defects on the surface. This results in a microstructure with uniform and refined grains throughout the thickness direction and the entire plate, ensuring the overall performance of the steel plate is uniform and stable. It also ensures that the steel plate has good low-temperature toughness and excellent acid resistance.

[0064] Compared with the prior art, the present invention has the following advantages:

[0065] In terms of composition design, this invention employs ultra-low C, ultra-low S, and low inclusion control. Based on the segregation tendency of each element and its relationship with HIC resistance, a control requirement for the hydrogen-induced cracking sensitivity coefficient Ks is proposed to improve the material's acid resistance. Furthermore, Nb+Ti ≤ 0.07%, 0.7 ≤ (Nb + 27.5*Ti) / (5C + 27*N) ≤ 5.0 is controlled, with a focus on the influence of second-phase particles such as TiN and NbC on the material's acid resistance and low-temperature toughness. Simultaneously, by adding Cu+Ni+Cr+Mo ≤ 0.80%, the inhibitory effect on bulk ferrite is reduced, resulting in a small amount of bulk ferrite in the steel microstructure (bulk ferrite content 3-10%), inhibiting crack formation. This allows the pipeline steel to possess both high strength and acid resistance, as well as good low-temperature toughness. In contrast, conventional acid-resistant pipeline steels have higher C, Mo, and Nb content and do not consider the control of inclusion elements.

[0066] Based on the composition design, this invention employs a DQ+ACC cooling mode after rolling in the manufacturing process design, controlling the starting cooling temperature to 700–800℃, the stopping cooling temperature to 300–550℃, and the cooling rate to ≤30℃ / s. This appropriately reduces the inhibitory effect of elements such as Mo on bulk ferrite, forming a microstructure of uniform and refined granular bainite with a small amount of bulk ferrite. The bulk ferrite content is 3–10%, with a grain size ≥10, and no obvious segregation bands or hard phase structures. As a toughening phase, the bulk ferrite increases the interfacial area and causes significant plastic deformation at the crack tip, alleviating the triaxial tensile stress at the crack tip and hindering crack propagation. This results in excellent low-temperature toughness for the acid-resistant pipeline steel. In contrast, traditional X60 / X65 grade acid-resistant steel does not contain bulk ferrite.

[0067] The R of the pipeline steel of this invention t0.5 The steel plate exhibits a yield strength of 430–570 MPa, tensile strength of 520–690 MPa, yield-to-tensile ratio ≤0.92, elongation ≥30% over a 50mm diameter, hardness HV10 ≤220, Charpy impact energy ≥400 J at -49℃, and DWTT SA% ≥90% at -35℃. It possesses excellent acid resistance (no cracking in HIC and SSCC tests (90% SYMS) under international NACE standard A solution), fully meeting the API 5L evaluation standards. Furthermore, it demonstrates superior low-temperature toughness compared to existing patents, with a Charpy impact energy ≥400 J at -49℃ and DWTT SA% ≥90% at -35℃. In contrast, conventional acid-resistant pipeline steels typically exhibit the best low-temperature toughness with a Charpy impact energy ≥300 J at -40℃ and DWTT SA% ≥85% at -30℃. Attached Figure Description

[0068] Figure 1 This is a scatter plot showing the relationship between the crack length ratio CLR and the coefficient Ks in HIC.

[0069] Figure 2 This is a micrograph of the steel from Example 1 of the present invention. Detailed Implementation

[0070] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0071] The chemical composition of the steel in the embodiments of the present invention is shown in Tables 1 and 2, with the balance including Fe and unavoidable impurities; the specific process parameters are shown in Table 3.

[0072] The mechanical properties and microstructure of the steel plates in the embodiments and comparative examples of this invention are shown in Table 4, and the results of resistance to HIC and SSC are shown in Table 5.

[0073] Figure 2 The image shows the microstructure of pipeline steel obtained in Example 1 of this invention. Figure 1It can be seen that the steel plate obtained by using the composition and process described in this invention has a microstructure of uniform and refined granular bainite and a small amount of blocky ferrite; wherein, the blocky ferrite content is 3-10%, the grain size is ≥10, and there are no obvious segregation bands or hard phase structures.

[0074] As can be seen from Table 4, the R of pipeline steel obtained by this invention t0.5 Yield strength: 430~570MPa, tensile strength: 520~690MPa, yield ratio ≤0.92, elongation at A50mm ≥30%, hardness HV10≤220, and good low-temperature toughness, Charpy impact energy at -49℃ ≥400J, and DWTT SA%≥90% at -35℃.

[0075] Hydrogen-induced cracking (HIC) resistance test: The hydrogen-induced cracking resistance of steel for piping and pressure vessels was measured in a wet hydrogen sulfide (H2S) environment according to the NACE TM0284-2016 standard "Test method for evaluating the hydrogen-induced cracking resistance of steel for piping and pressure vessels". The obtained steel was continuously immersed in standard samples and solution A for 96 hours. The measurement results are shown in Table 5. No cracks were found in the samples obtained in the embodiments of the present invention.

[0076] Hydrogen sulfide stress corrosion cracking (SSC) performance test: According to NACE TM 0177-2016 standard "Test method for resistance of metallic materials to stress corrosion cracking of sulfides in pressurized S environment", the resistance to hydrogen sulfide stress corrosion cracking (SSC) performance was measured. The four-point bending method was used, the stress load was 90% SYMS, and after 720 hours of immersion in solution A, the measurement results are shown in Table 5. None of the sample obtained in the example of this invention fractured.

[0077] In the pipeline steel of Comparative Example 1, the C content was high, and Ks reached 0.311 (Ks ≤ 0.23 in this invention), (Nb + 27.5 * Ti) / (5C + 27 * N) was 0.61 (controlled between 0.7 and 5.0 in this invention), Cu + Ni + Cr + Mo reached 0.93% (controlled ≤ 0.80% in this invention), large cracks appeared in the HIC test, two of the three samples cracked, the length ratio CLR exceeded 20%, cracks also appeared in the SSC test, and the DWTT and impact properties were also lower than those of the example.

[0078] In the pipeline steel of Comparative Example 2, the C content is relatively high, and Ks reaches 0.289. The length of the calcium-feeding wire is relatively low, and the Ca / S ratio is only 1.0 (this invention controls Ca / S≥2). In addition, the B content reaches 0.0005% (this invention controls B≤0.0004%), and O+N+H reaches 0.0072% (this invention controls ≤0.0070%). The resistance to HIC and toughness are both poor. One sample has a CLR of 18.56%, cracks also appear in the SSC test, the DWTT is only 70% / 75%, the average impact energy is 279J, and one single value is only 208J.

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Claims

1. A high-toughness X60 / X65 grade acid-resistant pipeline steel, with the following chemical composition by weight percentage: C: 0.015–0.040%, Si: 0.10–0.30%, Mn: 1.18–1.45%, 0 < P ≤ 0.008%, 0 < S ≤ 0.0010%, Cu: 0.05–0.35%, Ni: 0.05–0.30%, Cr: 0.15–0.30%, Mo: 0.02–0.09%, Nb: 0.025–0.055%, Ti: 0.0 0.05~0.020%, Ca: 0.0010~0.0040%, Alt: 0.010~0.040%, 0<B≤0.0004%, 0<O≤0.0025%, 0<N≤0.0050%, 0<H≤0.0002%, 0<Sn≤0.0050%, 0<Sb≤0.0025%, 0<Bi≤0.0030%, 0<Pb≤0.0050%, 0<As≤0.0030%, with the balance being Fe and unavoidable impurities, and simultaneously satisfying the following: Nb+Ti≤0.07%; Ca / S≥2; Cu+Ni+Cr+Mo≤0.80%; 0 < N + O + H ≤ 0.0070%; 0.7≤(Nb+27.5*Ti) / (5C+27*N)≤5.0; The hydrogen-induced cracking susceptibility coefficient Ks≤0.23, Ks=8.0C+24S-Alt-2.3Nb+2.8Ti+117B+0.34Ni-0.30Mo+24.5N-0.19; The microstructure of the pipeline steel is uniform and refined granular bainite with a small amount of blocky ferrite, wherein the blocky ferrite content is 3-10%, the grain size is ≥10, and there are no obvious segregation bands or hard phase structures.

2. The high-toughness X60 / X65 grade acid-resistant pipeline steel as described in claim 1, characterized in that, The R of the pipeline steel t0.5 Yield strength 430~570MPa, tensile strength 520~690MPa, yield strength ratio ≤0.92, elongation ≥30% at A50mm, hardness HV10≤220, Charpy impact energy ≥400J at -49℃, DWTT SA%≥90% at -35℃.

3. The manufacturing method of high-toughness X60 / X65 grade acid-resistant pipeline steel as described in claim 1 or 2, characterized in that, Includes the following steps: 1) Hot metal pretreatment The sulfur content of molten iron before pretreatment should be ≤0.050%, and the sulfur content of molten iron entering the furnace should be ≤0.0020%. 2) Smelting Smelting according to the composition of claim 1, controlling the C content of the steel to be ≤0.035% and the S content to be ≤0.0080%; 3) Refining LF and RH refining are adopted. During the LF refining process, the stirring time after all the alloy is added is controlled to be ≥3min. The amount of calcium wire fed in RH is 200-350m. After the calcium wire is fed, the argon blowing time is ≥5min. The target Ca content is 10-40ppm, and the Ca / S ratio is controlled to be ≥2. 4) Continuous casting The continuous casting settling time is ≥3 min, and the superheat is controlled between 10 and 40℃. 5) Reheating Heating temperature: 1100~1200℃, holding time: 200~350min; 6) Controlled rolling Final rolling temperature: 750~900℃; 7) Cooling after rolling Cooling start temperature: 700~800℃, cooling stop temperature: 300~550℃, cooling rate ≤30℃ / s.

4. The manufacturing method as described in claim 3, characterized in that, During the smelting process, the bottom blowing flow rate during the converter bottom calming stage is ≥900 Nm³. 3 / h, control the free oxygen level during shutdown to ≤900ppm.

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