X65MOS-grade seabed acid-resistant pipeline steel plate and manufacturing method thereof

Abstract

The invention discloses an X65MOS-grade seabed acid-resistant pipeline steel plate and a manufacturing method thereof. The X65MOS-grade seabed acid-resistant pipeline steel plate comprises the following chemical components in percentage by weight: 0.01-0.05% of C, 0.05-0.35% of Si, 1.21-1.55% of Mn, less than or equal to 0.010% of P, less than or equal to 0.0015% of S, less than or equal to 0.3% of Cu, less than or equal to 0.3% of Ni, 0.1-0.5% of Cr, 0.02-0.09% of Mo, 0.02-0.049% of Nb, 0.005-0.015% of Ti, 0.001-0.004% of Ca, 0.01-0.05% of Alt, less than or equal to 0.0004% of B, less than or equal to 0.0002% of H, less than or equal to 0.0025% of O and the balance of Fe Alt-Als is less than or equal to 0.008%, and Ca / Alt is more than or equal to 0.1. The thickness of the steel plate reaches 15-40 mm, the yield strength Rt0.50 of the steel plate is larger than or equal to 450 MPa, the tensile strength Rm of the steel plate is larger than or equal to 535 MPa, the yield ratio Rt0.50 / Rm of the steel plate is smaller than or equal to 0.92, the Charpy impact energy at the temperature of-25 DEG C is higher than 250 J, the DWTT test SA% at the temperature of-15 DEG C is larger than or equal to 85%, and the CTOD characteristic value delta m at the temperature of-25 DEG C is larger than or equal to 0.35 mm.

Description

Technical Field

[0001] The invention belongs to the technical field of pipeline steel, and specifically relates to an X65MOS-grade submarine acid-resistant pipeline steel plate and a manufacturing method thereof. Background Art

[0002] From a global perspective, the production and demand of oil and natural gas are showing a trend of continuous growth. However, as high-quality oil and gas fields are exhausted, submarine oil and gas fields containing acidic gas H2S are also being exploited one after another. When pipelines transport media containing wet H2S, hydrogen generated by corrosion enters the steel, which may cause HIC (hydrogen-induced cracking) and / or SSC (sulfide stress cracking). In addition, for submarine pipeline steel, not only the transverse strength but also the longitudinal strength must be guaranteed. The wall thickness is usually thicker and the transmission pressure is higher, so the H2S partial pressure in the medium is also increased. High-strength and high-pressure transmission further increases the risk of hydrogen-induced cracking, so higher requirements are placed on the acid resistance of submarine pipeline steel.

[0003] In recent years, TMCP (thermomechanical control process) technology has been widely used in the production of pipeline steel because it can reduce alloy costs, reduce production processes, and obtain steel plates with excellent strength, toughness and weldability. TMCP acid-resistant pipeline steel has also been used in large quantities. The highest steel grade used in batches in engineering projects reaches the X65 level, but its wall thickness specification is relatively thin and is mainly used for land pipelines. There are relatively few cases of submarine pipeline applications. At the same time, due to the inherent characteristics of TMCP technology, the microstructure and performance distribution of the steel will be uneven during the controlled rolling and controlled cooling process, and even local hard spots will be generated on the surface, thereby reducing the acid resistance of TMCP pipeline steel.

[0004] Chinese patent CN201510135771.6 discloses "an ultra-low carbon high-strength and toughness HIC-resistant pipeline steel plate and its preparation method". The components and weight percentages of the pipeline steel are: C: 0.03-0.05%, Si: 0.15-0.30%, Mn≤1.20%, S≤0.002%, P≤0.010%, Nb: 0.02-0.08%, Ti≤0.05%, and the rest are Fe and unavoidable trace impurities. Through the design of low-carbon, high-Nb content microalloying, Ni alloying or Cr, Ni composite alloying composition, and through the implementation of controlled rolling and controlled cooling process technology, a high-strength and toughness HIC-resistant pipeline steel plate with fine and uniform ferrite + a small amount of degenerate pearlite structure type is obtained.

[0005] Chinese patent CN201310356491.9 discloses "a method for preparing low-carbon acid-resistant pipeline steel", in which the weight percentages of pipeline steel components are: C 0.025-0.045%, Mn 1.0-1.5%, S≤0.0010%, P≤0.010%. It mainly adopts the converter "full three-removal" smelting-LF-RH process. By controlling the C content in the molten steel at an extremely low level in the converter process, the problem of high carbon in the slab caused by converter alloying, LF heating, and continuous casting covering agent is avoided.

[0006] Chinese patent CN201510153515.X discloses "an ultra-thick specification HIC-resistant and SSCC X65 pipeline steel plate and its manufacturing method". The chemical composition of the steel plate is C0.02-0.04%, Mn1.25-1.45%, Si0.1-0.3%, S≤0.0005%, P≤0.010%, Nb0.05-0.07%, Ti0.008-0.03%, V≤0.10%, Al≤0.06%, N≤0.010%, O≤0.006%, Mo≤0.25%, Cu0.18-0.30%, Ni≤0.40%, Cr≤0.30%, Ca0.0005-0.0015%, and the Ca / S content ratio is 1.0-2.0, and the balance is Fe and unavoidable impurity elements. Through improved refining + continuous casting + specific TMCP (a kind of DQ + ACC) process, the steel has excellent HIC and SSCC resistance and excellent low-temperature toughness.

[0007] Chinese patent CN201910053546.6 discloses "A deep-sea acid-resistant pipeline steel and smelting method", whose chemical composition and mass percentage are as follows: C: 0.020%~0.040%, Si: 0.30%~0.40%, Mn: 1.00%~1.20%, P≤0.010%, S≤0.0010%, Nb: 0.030%~0.040%, Ti: 0.010%~0.030%, Ni: 0.20%~0.30%, Cr: 0.10%~0.30%, Mo: 0.10%~0.20%, Cu≤0.010%, V≤0.010%, Al: 0.015%~0.050%, Ceq: 0.25%~0.38%, Pcm: 0.10%~0.17%, and the balance is Fe and impurities. The patent details the specific smelting and continuous casting process, and develops deep-sea acid-resistant pipeline steel and smelting process, but does not introduce the specific rolling process and organizational type.

[0008] Chinese patent CN201710052940.9 discloses "a quenched and tempered acid-resistant pipeline steel and its manufacturing method", and its chemical elements are as follows: C: 0.025-0.090%, Si: 0.1-0.5%, Mn: 0.50-1.35%, P≤0.020%, S≤0.003%, Mo: 0.01-0.09%, Nb: 0.01-0.06%, Ti: 0.005-0.02%, Ca: 0.001-0.006%, Al: 0.02-0.045%, 0<N≤0.010%, and the balance is Fe and other inevitable impurities. The microstructure of the quenched and tempered acid-resistant pipeline steel has a phase ratio of more than 90% tempered bainite, and its grain size reaches more than 11 levels. In addition, the present invention also discloses a method for manufacturing the above-mentioned quenched and tempered acid-resistant pipeline steel. The quenched and tempered pipeline steel of the present invention has a yield strength of 290-550 MPa, a tensile strength of 415-760 MPa, a Charpy impact energy of higher than 120 J at -40°C, and a shear fracture area percentage of greater than 85% in a DWTT (drop weight tear test) test at -25°C.

[0009] In addition to meeting the acid resistance and strength requirements, acid-resistant pipeline steel must also have sufficient low-temperature toughness and good welding performance.

[0010] In the existing technology, the acid-resistant pipeline steel mainly improves the acid resistance of steel by adopting measures such as low-C high-alloying composition design, triple refining to improve purity, and tempering heat treatment process to achieve organizational control. These measures not only increase the cost of steel, but also have an adverse effect on low-temperature toughness.

[0011] When the thickness specification reaches 25mm or above, the existing acid-resistant pipeline steel can meet the acid-resistant performance requirement of CLR≤15%, but the test temperature for meeting the DWTT performance is only as low as -10℃, which is difficult to meet the acid resistance and low-temperature toughness requirements of thicker acid-resistant pipeline steel. Summary of the invention

[0012] The object of the present invention is to provide an X65MOS-grade submarine acid-resistant pipeline steel plate and a manufacturing method thereof, so as to obtain a super-thick submarine acid-resistant pipeline steel having strength, toughness and acid resistance that can meet the requirements, and the thickness of the steel plate reaches 15 to 40 mm, and the yield strength Rt 0.50 ≥450MPa, tensile strength Rm≥535MPa, yield strength ratio Rt 0.50 / Rm≤0.92, Charpy impact energy at -25℃ is higher than 250J, DWTT test SA% at -15℃ is ≥85%, and CTOD characteristic value δm≥0.35mm at -25℃.

[0013] To achieve the above object, the technical solution of the present invention is:

[0014] The invention discloses an X65MOS-grade submarine acid-resistant pipeline steel plate, which has the following chemical components by weight: C: 0.01-0.05%, Si: 0.05-0.35%, Mn: 1.21-1.55%, P≤0.010%, S≤0.0015%, Cu≤0.3%, Ni≤0.3%, Cr: 0.1-0.5%, Mo: 0.02-0.09%, Nb: 0.02-0.049%, Ti: 0.005-0.015%, Ca: 0.001-0.004%, Alt: 0.01-0.05%, B≤0.0004%, H≤0.0002%, O≤0.0025%, and the balance includes Fe and other inevitable impurities; and the following conditions need to be satisfied at the same time: Alt-Als≤0.008%, and Ca / Alt≥0.1.

[0015] Furthermore, the balance is Fe and other inevitable impurities.

[0016] The microstructure of the submarine acid-resistant pipeline steel plate of the present invention is ferrite+acicular ferrite, and the grain size difference between the surface and the core is ≤2 levels.

[0017] The thickness of the submarine acid-resistant pipeline steel plate of the present invention is 15 to 40 mm.

[0018] Yield strength Rt of the submarine acid-resistant pipeline steel plate of the present invention 0.50 ≥450MPa, tensile strength Rm≥535MPa, yield strength ratio Rt 0.50 / Rm≤0.92, Charpy impact energy at -25℃ is higher than 250J, DWTT test SA% at -15℃ is ≥85%, and CTOD characteristic value δm≥0.35mm at -25℃.

[0019] The submarine acid-resistant pipeline steel plate of the present invention has a HIC performance CLR of ≤10% under the A solution condition of the NACE TM0284-2016 standard and a SSC crack-free condition under the 90% SMYS condition.

[0020] In the chemical composition design of the acid-resistant pipeline steel of the present invention:

[0021] C: The most basic strengthening element, which plays a role in solid solution strengthening. It forms carbides with strong carbide elements and then plays a role in precipitation strengthening. In addition, C is also a necessary austenite stabilizing element, which can improve the hardenability of steel. However, C is an element that is easy to segregate. Too high a content will aggravate the center segregation and the segregation of Mn. For acid-resistant pipeline steel, it will seriously reduce its resistance to hydrogen induced cracking (Hydrogen Induced Crack). Too high a C content is also unfavorable to the toughness and welding performance of the steel. Therefore, the present invention adopts an ultra-low C design, and its content is controlled at 0.01-0.05%.

[0022] Si: a solid solution strengthening element and also a deoxidizing element in steel. However, a high content of Si will deteriorate the welding performance of the steel, especially the toughness of the welding heat affected zone, and is not conducive to the removal of iron oxide scale during hot rolling. Therefore, in the present invention, the Si content is controlled at 0.05-0.35%.

[0023] Mn: solid solution strengthening element, is the most important and economical strengthening element besides C. Mn is also an element that expands the γ phase region, which can improve the hardenability of steel, reduce the γ→α phase transformation temperature of steel, help obtain fine phase transformation products, and improve the toughness of steel. However, when the Mn content is too much, it is easy to produce center segregation in the continuous casting billet, reducing the HIC resistance of the steel. Therefore, in the present invention, the Mn content is 1.21-1.55%.

[0024] S and P are inevitable impurity elements in steel. P is an element that is easily segregated in steel. S is easy to form sulfides in steel, which will seriously reduce the HIC performance of steel. Therefore, the lower the S and P contents are controlled, the better. Therefore, the present invention controls P≤0.015% and S≤0.0015%.

[0025] Cu, Ni: solid solution strengthening elements. Cu and Ni can improve the corrosion resistance of steel and prevent softening in hot zones. Ni can also improve the toughness of steel, but excessive addition often leads to damage to plasticity and toughness. Therefore, in the present invention, the Cu content is controlled to be ≤0.30% and the Ni content is ≤0.30%.

[0026] Cr: solid solution strengthening element, an important element for improving the hardenability of steel, promoting the formation of acicular ferrite, and combined with process optimization, can improve the uniformity of the structure and performance of thick gauge steel plates throughout the thickness. Therefore, in the present invention, the Cr content is controlled to 0.1-0.5%.

[0027] Mo: solid solution strengthening element, is an important element to improve the hardenability of steel, promotes the formation of acicular ferrite, refines the structure and improves the strength. For acid-resistant pipeline steel with low C design, it is necessary to improve the strength through phase transformation strengthening of Mo, so in the present invention, the Mo content is controlled to 0.02-0.09%.

[0028] Nb: one of the important elements of low-carbon microalloy steel, Nb strain-induced precipitation in the hot rolling process to form Nb (C, N) particles, can delay austenite recrystallization, increase the austenite recrystallization temperature of steel, is conducive to refining the transformed structure, and improves the strength and toughness of the material. In addition, the Nb dissolved in the cooling process disperses and precipitates in the matrix as the second phase particle NbC, which plays a precipitation strengthening role. However, too high a content of Nb cannot be completely dissolved, not only does not play a role, but also aggravates the segregation tendency, increases production costs, and causes NbC to precipitate prematurely at high temperatures, forming larger-sized NbC, which is not conducive to improving the strength of the material through precipitation strengthening. Therefore, in the present invention, the Nb content is controlled to 0.020-0.049%.

[0029] Ti: It is a strong carbonitride-forming element that can fix nitrogen, thereby improving the toughness of the material. In addition, the undissolved carbonitrides of Ti can prevent the growth of austenite grains when the steel is heated, and the TiN and TiC precipitated during rough rolling in the high-temperature austenite zone can effectively inhibit the growth of austenite grains. In addition, during the welding process, the TiN and TiC particles in the steel can significantly prevent the grain growth of the heat-affected zone, thereby improving the welding performance of the steel plate. Therefore, in the present invention, the Ti content is controlled to 0.005-0.015%.

[0030] Ca: mainly used to modify inclusions, making the inclusions spheroidized and evenly distributed, which is beneficial to improving the HIC resistance. Therefore, in the present invention, the Ca content is controlled to be 0.001-0.004%.

[0031] Al: It is mainly used for deoxidation of steel. Adding an appropriate amount of Al is beneficial to grain refinement and improving the strength and toughness of steel. However, the increase in the Al content will cause the formation of Al oxides in the steel, which will reduce the toughness of the parent material and the heat affected zone of the weld. Al in acid-resistant pipeline steel mainly exists in two forms: acid-soluble aluminum and acid-insoluble aluminum. The acid-soluble aluminum Als is mainly AlN that can be dissolved in acid, and the acid-insoluble aluminum Al2O3 is the main inclusion in the steel. The inclusion is continuously distributed in the steel in a string after rolling deformation, which has an adverse effect on resistance to HIC and low-temperature toughness. Therefore, in the present invention, the content of all-aluminum Alt is controlled to 0.01-0.05% to meet the deoxidation requirements, and the difference between all-aluminum and acid-soluble aluminum is controlled to Alt-Als≤0.008, so as to ensure a low content of aluminum oxide to improve the resistance to hydrogen-induced cracking and improve the low-temperature toughness of thick specifications.

[0032] Ca / Al ratio: Calcium treatment can modify alumina. Ca diffuses in Al2O3 particles and replaces Al to enter the molten steel. The CaO content on the surface of Al2O3 particles gradually increases. When CaO>25%, calcium aluminate is in liquid state and floats out of the molten steel, and only a small number of small-sized spherical particles that do not float remain in the steel. The present invention controls Ca / Al≥0.1, removes large particle inclusions, improves the morphology of inclusions, obtains spherical calcium aluminate inclusions, prevents continuous casting cracks, improves the fluidity of molten steel, and improves the HIC resistance and low-temperature toughness of acid-resistant pipeline steel under thickness specifications.

[0033] B: A strong hardenability element that is easily precipitated at the grain boundary, resulting in a decrease in the plasticity and toughness of the material. Therefore, the present invention controls B≤0.0004%.

[0034] H: gas element in steel. Excessive hydrogen may cause delayed cracking of steel, which is especially disadvantageous to acidic service environment. The present invention controls the hydrogen content to ≤ 0.0002%.

[0035] O: gas element in steel. Too high oxygen content will easily generate oxide inclusions and hydrogen-induced cracks. The present invention controls the hydrogen content to ≤0.0025%.

[0036] According to the composition design described above, the technical solution of the present invention adopts a low-C and low-Mn microalloying composition system, strictly controls the P and S contents in the steel, and removes large-particle inclusions and improves the morphology of inclusions through Ca treatment and Al control of key parameters such as Ca / Al≥0.1 and Alt-Als≤0.008, thereby obtaining spherical calcium aluminate inclusions, which can meet the HIC resistance and low-temperature toughness requirements of extra-thick pipeline steels up to 40 mm thick.

[0037] The manufacturing method of the X65MOS-grade submarine acid-resistant pipeline steel plate of the present invention comprises the following steps:

[0038] 1) Smelting and refining

[0039] Smelting, RH and LF refining according to the above composition;

[0040] 2) Continuous casting

[0041] Continuous casting calming time ≥4min, casting speed fluctuation ≤0.15m / min;

[0042] 3) Reheating

[0043] Heating temperature: 1100~1200℃;

[0044] 4) Controlled rolling

[0045] The total reduction rate of rough rolling is ≥50%, and the end temperature is ≥940℃; the start rolling temperature of finishing rolling is 780~850℃, the final rolling temperature of finishing rolling is 760~810℃, and the final rolling temperature of finishing rolling is ≥Ar3+20℃;

[0046] 5) Cooling after rolling

[0047] Stop cooling temperature: 350~560℃, cooling rate: 8~38℃ / s, stack and slow cool after stop cooling, slow cooling time ≥24h.

[0048] Preferably, during the refining process of step 1), the amount of calcium wire fed is 130-400 m / 300 tons of molten steel, and the argon blowing and stirring time after the calcium wire feeding is completed is ≥5 min.

[0049] Preferably, in step 3), the insulation time is 1.5 to 2.5×t, in min; wherein t is the slab thickness, in mm.

[0050] Preferably, in step 5), the cooling temperature is 750-800°C.

[0051] In the manufacturing method of the present invention:

[0052] During the smelting process, the Si-Ca wire feeding amount for every 300 tons of molten steel is controlled to be 130-400m, and the Ar blowing stirring time is ≥5min; after wire feeding, argon blowing and stirring are used to make the inclusions modified by Ca treatment fully float up. If the argon blowing time is too short, large-sized inclusions will not float up fully, which will increase the HIC tendency and is not conducive to the anti-HIC performance.

[0053] The casting speed in the continuous casting process is a key parameter for determining the position of the end of solidification. The present invention controls the casting speed fluctuation in the continuous casting process to ≤0.15m / min to ensure that the end of solidification is stable in the optimal light pressure reduction area, thereby reducing the segregation degree of the continuous casting billet and controlling the segregation stability to reach the Mannesmann low-multiple standard M1.8 level. When the continuous casting casting speed fluctuation exceeds 0.15m / min, the end of solidification will move back and forth significantly, resulting in the segregation of a large amount of C, S, Nb and other segregation-prone elements at the end of solidification, thereby significantly increasing the segregation and inclusion content of the casting billet, and significantly deteriorating the HIC resistance of the steel.

[0054] The rolling adopts two-stage controlled rolling, the finishing rolling start temperature is controlled to be 780-850℃, the finishing rolling temperature is 760-810℃, and the finishing rolling temperature is ≥Ar3+20℃; when the finishing rolling temperature is lower than Ar3+20℃, the γ+α dual-phase region rolling deformation will occur in the local position of the steel plate, thereby forming a deformation-hardened ferrite deformation structure in the steel. The internal stress of this structure is large, and hydrogen-induced cracking cracks are easily generated, which is not conducive to the anti-HIC performance. At the same time, the uneven deformation will cause differences in the grain size of the full thickness, reducing the low-temperature toughness and performance stability of the steel.

[0055] Controlled cooling starts at a temperature above Ar3+20℃, which can ensure that the initial cooling stage is a single-phase deformed austenite structure, and then cools to 350-560℃ at a relatively low cooling rate of 8-38℃ / s, so that the phase transformation passes through the two phases of ferrite + acicular ferrite, thereby obtaining a ferrite + acicular ferrite microstructure of continuous cooling transformation. This technology avoids the pro-eutectoid ferrite + acicular ferrite microstructure with a clear phase interface obtained by cooling below the Ar3 point, thereby improving acid resistance and low-temperature toughness; at the same time, since cooling starts from single-phase austenite and the cooling rate is not high, the cooling can be conducted to the core of the steel plate, avoiding the temperature return and uneven cooling caused by rapid cooling, making the phase transformation process of the full thickness position relatively consistent, thereby controlling the grain size difference of the full thickness to ≤2 levels.

[0056] Compared with the prior art, the present invention has the following advantages:

[0057] In the composition design of the present invention, a low-C and low-Mn microalloying composition system is adopted to reduce the possibility of central segregation and banded structure; at the same time, the P and S contents in the steel are strictly controlled at a low level to reduce the number of inclusions and the degree of segregation; and through the treatment of Ca and Al, key parameters such as Ca / Al≥0.1 and Alt-Als≤0.008 are controlled to remove large-particle inclusions, improve the morphology of inclusions, and obtain spherical calcium aluminate inclusions, which can meet the HIC resistance and low-temperature toughness requirements of extra-thick pipeline steel up to 40mm thick. Compared with the traditional low-C high-alloy composition design, central segregation and banded structure are significantly improved, and low-temperature toughness is also significantly improved.

[0058] The maximum wall thickness of the X65MOS-grade acid-resistant submarine pipeline steel involved in the present invention can reach 40 mm, and the steel plate has excellent acid resistance. Based on the A solution anti-HIC test specified in the NACE TM0284-2016 standard, its crack length rate CLR≤10%, crack thickness rate CTR≤3%, crack sensitivity rate CSR≤1%, and no SSCC cracks, DWTT test SA%≥85% at -15°C, CTOD characteristic value δm≥0.35mm at -25°C, and other acid resistance and low-temperature toughness. However, the production of extra-thick acid-resistant pipeline steel with a thickness of 40 mm prepared by the existing technology can only meet the requirements of CLR≤15%, DWTT test SA%≥85% at -10°C, CTOD characteristic value δm≥0.35mm at -15°C, and the related index levels are significantly lower than the technical solution of the present invention.

[0059] On the basis of composition design, the manufacturing process design of the present invention controls the segregation to be stable to the Mannesmann low-multiple standard M1.8 level through the pulling speed fluctuation control in the smelting and continuous casting processes, reduces the segregation of elements such as C, S, and Nb, and further controls the finishing rolling temperature ≥Ar3+20°C, and cools to 350-560°C at a relatively low cooling rate of 8-38°C / s, so that the phase transformation passes through the two phase regions of ferrite + acicular ferrite, thereby obtaining a ferrite + acicular ferrite microstructure of continuous cooling transformation; combined with controlled cooling, cooling starts from single-phase austenite, and a lower cooling rate is adopted, so as to avoid the temperature return and uneven cooling caused by rapid cooling, so that the phase transformation process of the full thickness position is relatively consistent, thereby controlling the grain size difference of the full thickness to be ≤2 levels, and improving the acid resistance and low-temperature toughness on the basis of meeting the strength level requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Figure 1 This is a microstructure photograph of the steel according to Example 1 of the present invention. DETAILED DESCRIPTION

[0061] The present invention will be further described below in conjunction with the embodiments and drawings.

[0062] The chemical composition of the steel of the embodiment of the present invention is shown in Table 1, and the balance includes Fe and unavoidable impurities; the specific process parameters are shown in Table 2.

[0063] The performance results of the steel plates of the embodiments of the present invention and the comparative examples are shown in Table 3.

[0064] Figure 1 This is a microstructure photo of the pipeline steel obtained in Example 1 of the present invention. Figure 1 It can be seen that the steel of the present invention is a refined polygonal ferrite + acicular ferrite (PF + AF) structure, with a grain size of 3 to 10 μm and uniform distribution. By introducing the transition soft phase PF on the AF matrix, high strength is obtained while reducing the internal stress in thick gauge steel, which not only significantly reduces the HIC tendency of the steel, but also significantly improves the low-temperature impact toughness of the steel through the hindering effect of the large-angle grain boundary. The near-surface structure of the steel is relatively finer, but the core-surface grain size difference can be controlled within 2 levels.

[0065] It can be seen from Table 3 that the Rt of the pipeline steel obtained by the present invention is 0.50 ≥450MPa, tensile strength Rm≥535MPa, yield strength ratio Rt 0.50 / Rm≤0.92, Charpy impact energy at -25℃ is higher than 250J, DWTT test SA% at -15℃ is ≥85%, and CTOD characteristic value δm≥0.35mm at -25℃.

[0066] Hydrogen induced cracking (HIC) resistance test: The NACE TM0284-2016 standard "Test method for evaluation of hydrogen induced cracking resistance of steel for pipelines and pressure vessels" was used to measure the hydrogen induced cracking (HIC) resistance in a wet hydrogen sulfide (H2S) environment. The obtained steel was immersed in the standard sample and solution A for 96 hours. The measurement results are shown in Table 3. No cracks appeared in the samples obtained in the embodiments of the present invention.

[0067] Hydrogen sulfide stress corrosion cracking (SSC) resistance test: According to the NACE TM 0177-2016 standard "Test method for resistance of metallic materials to sulfide stress corrosion cracking in pressurized sulfur environments", the hydrogen sulfide stress corrosion cracking (SSC) resistance was measured by using a four-point bending method with a stress loading of 90% SMYS. After 720 hours of immersion in solution A, the measurement results are shown in Table 3. None of the samples of the embodiments of the present invention were broken.

[0068] In the pipeline steel of comparative example 1, a composition design with higher C and Mn contents is adopted, and the final cooling temperature of rolling and cooling is low, so that a full AF microstructure is obtained, and the yield strength of the steel reaches the X70 steel grade, and the steel has a large internal stress of the organization. Therefore, there are large-sized cracks in the HIC test, which exceed the requirements of conventional specifications. At the same time, due to the high proportion of the affected organization, the low-temperature toughness is relatively poor.

[0069] In the pipeline steel of Comparative Example 2, although a low C design is adopted and the steel grade reaches X65 level, due to the high S content in the steel and the low Ca / Al ratio, the inclusions in the steel are not fully modified or floated, and the number of inclusions in the finished steel plate is large, which not only causes excessive cracks in the HIC test of the sample, but also significantly reduces the low-temperature toughness of the steel.

[0070] In summary, the composition and process designed according to the present invention can meet the high requirements of 15-40 mm thick X65MOS grade high-strength acid-resistant steel plate for strength, toughness, DWTT performance and acid resistance.

[0071]

[0072]

[0073]

Claims

1. An X65MOS-grade submarine acid-resistant pipeline steel plate, the chemical composition weight percentage of which is: C: 0.01-0.05%, Si: 0.05-0.35%, Mn: 1.21-1.55%, P≤0.010%, S≤0.0015%, Cu≤0.3%, Ni≤0.3%, Cr: 0.1-0.5%, Mo: 0.02-0.09%, Nb: 0.02-0.049%, Ti: 0.005-0.015%, Ca: 0.001-0.004%, Alt: 0.01-0.05%, B≤0.0004%, H≤0.0002%, O≤0.0025%, the balance includes Fe and other inevitable impurities; and It is necessary to simultaneously satisfy Alt-Als≤0.008% and Ca / Alt≥0.

1.

2. The X65MOS-grade submarine acid-resistant pipeline steel plate according to claim 1, characterized in that: The balance is Fe and other inevitable impurities.

3. The X65MOS-grade submarine acid-resistant pipeline steel plate according to claim 1 or 2, characterized in that: The thickness of the submarine acid-resistant pipeline steel plate is 15 to 40 mm.

4. The X65MOS-grade submarine acid-resistant pipeline steel plate according to claim 1, 2 or 3, characterized in that: The microstructure of the submarine acid-resistant pipeline steel plate is ferrite+acicular ferrite, and the grain size difference between the surface and the core is ≤2 levels.

5. The X65MOS-grade submarine acid-resistant pipeline steel plate according to claim 1, 2, 3 or 4, characterized in that: The yield strength Rt of the submarine acid-resistant pipeline steel plate 0.5 ≥450MPa, tensile strength Rm≥535MPa, yield strength ratio Rt 0.50 / Rm≤0.92, -25℃ Charpy impact energy is higher than 250J, DWTT test SA% at -15℃≥85%, -25℃ CTOD characteristic value δm≥0.35mm.

6. The X65MOS-grade submarine acid-resistant pipeline steel plate according to claim 1, 2, 3, 4 or 5, characterized in that: The submarine acid-resistant pipeline steel plate has a HIC performance CLR of ≤10% under the A solution condition of the NACE TM0284-2016 standard and a SSC crack-free condition under the 90% SMYS condition.

7. The method for manufacturing the X65MOS-grade acid-resistant submarine pipeline steel plate according to any one of claims 1 to 6, characterized in that: The steps include: 1) Smelting and refining Smelting according to the composition of claim 1 or 2, RH and LF refining; 2) Continuous casting Continuous casting calming time ≥4min, casting speed fluctuation ≤0.15m / min; 3) Reheating Heating temperature: 1100~1200℃; 4) Controlled rolling The total reduction rate of rough rolling is ≥50%, and the final rolling temperature of rough rolling is ≥940℃; the starting rolling temperature of finishing rolling is 780~850℃, and the final rolling temperature of finishing rolling is 760~810℃ and ≥Ar3+20℃; 5) Cooling after rolling Cooling rate: 8-38℃ / s, cooling stop temperature: 350-560℃; stack and cool slowly after cooling stop, slow cooling time ≥24h.

8. The manufacturing method according to claim 7, characterized in that: Step 1) During the refining process, the amount of calcium wire fed is 130-400 m / 300 tons of molten steel, and the argon blowing and stirring time after the calcium wire feeding is completed is ≥5 min.

9. The manufacturing method according to claim 7, characterized in that: In step 3), the holding time is (1.5-2.5)×t, in min; wherein t is the slab thickness, in mm.

10. The manufacturing method according to claim 7, characterized in that: In step 5), the cooling temperature is 750-800°C.

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