Deep-sea collapse-resistant x70mo steel pipe and manufacturing method therefor

Abstract

The present invention provides a deep-sea collapse-resistant X70MO steel pipe and a manufacturing method therefor. In addition to Fe and inevitable impurities, the X70MO steel pipe further comprises the following chemical components in percentage by weight: C: 0.032-0.07%, Si: 0.10-0.28%, Mn: 1.51-1.95%, P: ≤0.0065%, S: ≤0.002%, Ni: 0.05-0.45%, Cr: 0.1-0.5%, Mo: 0.01-0.25%, Nb: 0.025-0.05%, Ti: 0.005-0.018%, Ca: 0.001-0.004%, acid-soluble aluminum (Als): 0.010-0.040%, B: ≤0.0004%, N: ≤0.006%, and Co: 0.0005-0.005%.

Description

A deep-sea pressure collapse resistant X70MO steel pipe and a manufacturing method thereof TECHNICAL FIELD

[0001] The present application belongs to the field of metal materials and its manufacturing, and particularly relates to a deep-sea pressure collapse resistant X70MO steel pipe and a manufacturing method thereof. BACKGROUND

[0002] With the extension of energy exploitation to deep-sea areas, higher requirements are put forward for the service safety of pipes. Specifically, the deep-sea pipes need to bear large compression, tensile load and bending deformation during the laying process. Moreover, during the service period, harsh environmental conditions such as deep water high pressure, swell, strong bottom current and seabed movement may cause the pipe to produce large plastic deformation or even breakage, thereby causing serious environmental disasters. Therefore, the submarine pipeline needs to meet more stringent requirements than the onshore pipeline.

[0003] The submarine pipeline needs to have higher transverse and longitudinal strength, and it is difficult to achieve good strength and toughness matching. At the same time, with the increase of water depth, in order to resist the compression stress generated in the circumferential direction of the pipe due to external water pressure, the pipe needs to have good pressure collapse resistance. In the final pipe-making process of the steel pipe, the steel pipe will undergo an expansion process, i.e. tensile deformation along its circumferential direction, and then the steel pipe is laid on the seabed and subjected to the action of external water pressure, and the pipe circumference is compressed. This compression in the circumferential direction leads to a significant problem, i.e. the reduction of compression yield strength, and this problem is mainly caused by the Bauschinger effect. The Bauschinger effect is a phenomenon commonly found in polycrystalline materials, which usually does not occur in single-crystal materials. This effect is believed to be caused by the residual stress between the grain boundaries of polycrystalline materials, and its existence makes the material exhibit anisotropic properties when subjected to external forces. Specifically, the Bauschinger effect leads to a significant reduction in the yield strength of the steel pipe when it is compressed. In addition, the temperature in the deep water area is low, and higher requirements are put forward for the low-temperature toughness of the pipe. Therefore, the deep-sea service environment requires the pipe to have excellent strength and toughness matching, pressure collapse resistance and low-temperature toughness.

[0004] For example, Chinese Patent 1 (application number CN201610960456.1) discloses a thick-walled plate for a quenched and tempered deep-sea pipeline and a production method thereof. The plate composition is as follows in terms of weight percentage: C: 0.056-0.080%, Si: 0.20-0.35%, Mn: 1.35-1.54%, Nb: 0.03-0.05%, Ti: 0.011-0.029%, Cr: 0.21-0.40%, Als: 0.015-0.040%, N: 0.0020-0.0049%, P≤0.010%, S≤0.002%, H≤0.00015%, O≤0.0020%, Ti / N: 3.42-6.00, and further containing one or both of Ni and Mo elements, wherein Ni: 0-0.10%, Mo: 0-0.15%, (Mo+Ni+Cr): 0.26%-0.45%, and the balance being iron and unavoidable impurities. The patent has the following shortcomings: 1) the quenched and tempered process is not economical; 2) the control requirements for the microstructure are not specified; and 3) although the strength grade reaches X70 grade, the toughness only meets the requirement of -15°C DWTT≥85%, and the toughness and the pressure collapse resistance at lower temperatures are not described.

[0005] For example, Chinese Patent 2 (application number CN200710052601.7) discloses a steel plate for making a submarine pipeline and a rolling method thereof. The chemical composition of the steel plate is as follows in terms of weight percentage: C: 0.030-0.075%, Si: 0.10-0.30%, Mn: 1.40-1.60%, P≤0.015%, S≤0.003%, Cu≤0.20%, Ni: 0.10-0.25%, Mo: 0.07-0.20%, Nb: 0.03-0.05%, V≤0.060%, Ti: 0.001-0.02%, Al: 0.01-0.06%, Cr≤0.20%, N≤0.009%, Ca: 0.001-0.005%, B≤0.0005%, Sn≤0.010%, and As≤0.030%. The obtained steel plate has high cleanliness, high strength, high toughness, and high strength in hot continuous rolling, and can be used for making a submarine pipeline straight seam submerged arc welded pipe. However, the patent has the following shortcomings: 1) the control requirements for the microstructure are not specified; and 2) the strength only reaches X65 grade, the toughness only meets the requirements of -15°C DWTT and impact, and the pressure collapse resistance of the steel plate is not studied, which is difficult to meet the requirements of deep-sea pipeline steel.

[0006] From the analysis of the prior art, although the strength grade of the deep-sea pipeline steel can reach X70 grade, the low-temperature toughness and the pressure collapse resistance of the deep-sea pipeline steel are difficult to meet the requirements. SUMMARY

[0007] To solve the above problems, the present application aims to provide a deep-sea pressure collapse resistant X70MO steel pipe and a manufacturing method thereof. The steel pipe of the present application can reach X70 strength level, and has excellent low-temperature toughness and pressure collapse resistance. The X70MO steel pipe has a transverse / longitudinal yield strength R t0.5 ≥485MPa, a transverse / longitudinal tensile strength R m ≥570MPa, and a transverse / longitudinal uniform elongation ≥8%; the X70MO steel pipe has a transverse compression strength ≥465MPa; and the X70MO steel pipe has a -35℃ DWTT SA% ≥85%.

[0008] To achieve the above-mentioned purposes, the technical scheme of the present application is as follows:

[0009] The first aspect of the present application provides an X70MO steel pipe, which, in addition to Fe and inevitable impurities, contains the following chemical components in the following weight percentages: C: 0.032-0.07%, Si: 0.10-0.28%, Mn: 1.51-1.95%, P≤0.0065%, S≤0.002%, Ni: 0.05-0.45%, Cr: 0.1-0.5%, Mo: 0.01-0.25%, Nb: 0.025-0.05%, Ti: 0.005-0.018%, Ca: 0.001-0.004%, acid-soluble aluminum Als: 0.010-0.040%, B≤0.0004%, N≤0.006%, Co: 0.0005-0.005%, and simultaneously satisfies the following relationship:

[0010] 5Ni+Mn-3C-0.6(Cr+Mo)≥1.4%,

[0011] (Nb+Ti-Cr / 10) / (C+N+5×B)≥0.1,

[0012] In the formula, each element symbol is substituted with the weight percentage of the corresponding element.

[0013] Preferably, the X70MO steel pipe contains the following chemical components in the following weight percentages: C: 0.032-0.07%, Si: 0.10-0.28%, Mn: 1.51-1.95%, P≤0.0065%, S≤0.002%, Ni: 0.05-0.45%, Cr: 0.1-0.5%, Mo: 0.01-0.25%, Nb: 0.025-0.05%, Ti: 0.005-0.018%, Ca: 0.001-0.004%, acid-soluble aluminum Als: 0.010-0.040%, B≤0.0004%, N≤0.006%, Co: 0.0005-0.005%, and the balance is Fe and inevitable impurities.

[0014] The chemical composition of the X70MO steel pipe also satisfies the following relationship:

[0015] 1.4%≤5Ni+Mn-3C-0.6(Cr+Mo)≤3.36%,

[0016] 0.1≤(Nb+Ti-Cr / 10) / (C+N+5xB)≤0.79%.

[0017] Preferably, the balance is Fe and other unavoidable impurities.

[0018] Preferably, the microstructure of the X70MO steel pipe is a complex phase structure of bainite + ferrite + martensite-austenite constituent M / A, wherein the bainite is granular, the ferrite is polygonal, the volume content of the ferrite is ≤12%, preferably 2-12%, the volume content of the martensite-austenite constituent M / A is ≤8%, preferably 1.5-8%, the balance is bainite structure, the average particle size of the martensite-austenite constituent M / A is ≤1.2 μm, preferably 0.53-1.2 μm, and the average particle size of the ferrite is ≤7 μm, preferably 4-7 μm.

[0019] Preferably, the transverse / longitudinal yield strength R t0.5 of the X70MO steel pipe is ≥485 MPa, the transverse / longitudinal tensile strength R m is ≥570 MPa, and the transverse / longitudinal uniform elongation is ≥8%.

[0020] Preferably, the transverse compressive strength of the X70MO steel pipe is ≥465 MPa.

[0021] Preferably, the -35°C DWTT SA% of the X70MO steel pipe is ≥85%, and the ovality is ≤6 mm.

[0022] In the chemical composition design of the X70MO steel pipe according to the present application:

[0023] C: Carbon is the most cost-effective strengthening element in steel, which can significantly increase the strength of the steel through interstitial solid solution strengthening. Increasing the carbon content can greatly enhance the hardenability of the steel and reduce the addition of other expensive alloy elements, thereby reducing the production cost. However, too high C content will have a negative impact on the ductility, toughness, weldability and corrosion resistance of the steel. Therefore, the C content in the present application is controlled between 0.032-0.07%.

[0024] Si: Silicon is a solid solution strengthening element and also a deoxidizing element in steel, which is essential for improving the performance of steel. However, excessive Si content can adversely affect the surface quality and weldability of steel, especially when the Si content exceeds 0.28%, the toughness of the steel may decrease. Therefore, the Si content of the present invention is controlled between 0.10-0.28%.

[0025] Mn: Manganese is an important strengthening element that improves the strength of steel through solid solution strengthening mechanism. Mn is one of the most economical and primary methods to compensate for the loss of strength due to the reduction of C content. During production, Mn can replace part of the C element to reduce the C content of steel, thereby improving its toughness. In addition, Mn helps to obtain fine phase transformation products and control the content of oxygen and sulfur during steelmaking. However, too low Mn content may make it difficult to achieve the target strength level, while too high Mn content may aggravate central segregation. Therefore, the Mn content of the present invention is controlled between 1.51-1.95%.

[0026] Ni: Nickel can promote the transformation of ferrite to austenite in steel, which helps to reduce the plastic strain of steel under impact load, thereby improving the toughness of steel. In addition, Ni can refine ferrite grains, improve the plasticity and toughness of steel while maintaining the same strength, especially at low temperatures. Therefore, the Ni content of the present invention is controlled between 0.05-0.45%.

[0027] Cr: Chromium is an important alloying element that has solid solution strengthening effect and can effectively improve the hardenability of steel. When the Cr content exceeds 0.10%, a protective layer can be formed on the surface of the steel, which plays a protective role for the matrix and significantly improves the corrosion resistance of the steel. However, excessive Cr content can lead to a decrease in the weldability of steel. Therefore, in order to balance the relationship between its strengthening and weldability, the Cr content of the present invention is controlled between 0.1-0.5%.

[0028] Mo: Molybdenum is an element that effectively promotes bainite transformation, which can strengthen the matrix of steel and make the structure more fine. However, excessive Mo content can lead to a decrease in the plasticity of steel, and the cost of Mo is relatively high. Therefore, in order to maintain the strengthening effect while reducing the cost and avoiding the decrease in plasticity, the Mo content of the present invention is controlled at a lower level of 0.01-0.25%.

[0029] However, with the increase of carbon content and Cr, Mo in the steel, the toughness shows a downward trend, and Mn, Ni are austenite stabilizing elements that promote the formation of austenite. Ni and Mn are common austenite forming elements, which can stabilize the austenite phase or promote the transformation of austenite to other beneficial phases, reduce the formation of M / A, and can significantly improve the low temperature toughness and improve the residual stress; while C, Cr, Mo will increase the M / A content. In addition, the binding force of non-carbide forming element Ni and weak carbide forming element Mn and C is not much different from that of Fe, so it has no obvious effect on the decomposition of M / A; the binding force of strong carbide forming elements Cr and Mo and C is stronger, which increases the diffusion activation energy of C in martensite, hinders the diffusion of C in martensite, and thus slows down the decomposition rate of M / A. It is found through research that when 5Ni+Mn-3C-0.6(Cr+Mo)<1.4%, the steel lacks low-temperature mobile dislocations, and it is very easy to form more coarse M / A hard phase. Therefore, in the present application, 5Ni+Mn-3C-0.6(Cr+Mo)≥1.4% is controlled to realize more low-temperature mobile dislocations, which is beneficial to cross slip, obtain low ductile-brittle transition temperature, reduce M / A formation, improve residual stress to reduce Bauschinger effect, thereby avoiding the significant reduction of compression strength after pipe making due to Bauschinger effect, and ensuring the anti-collapse performance.

[0030] Ca: Calcium treatment has a positive effect on improving the performance of the steel, which can modify inclusions and improve the internal quality and anisotropy of the steel. However, when the content of Ca in the steel is too high, it may have adverse effects. Excessive Ca may cause segregation in the steel, that is, uneven distribution of Ca elements in the steel, which may affect the mechanical properties and processing performance of the steel, and may form larger inclusions, which may be unevenly distributed and have poor stability, and have adverse effects on the toughness of the steel. Therefore, the content of Ca in the present application is controlled between 0.001-0.004%.

[0031] S, P: Sulfur and phosphorus are common impurity elements in steel. Phosphorus easily leads to increased cold brittleness of the steel, while sulfur easily causes hot brittleness, resulting in unstable performance of the steel. Especially when the content of S increases, the number of MnS inclusions increases, which significantly reduces the toughness thereof. Therefore, in the present application, S≤0.002% and P≤0.0065% are controlled to improve the performance stability of the steel.

[0032] Co: Cobalt has a significant impact on the performance of steel. The atomic radius of Co is larger than other metal elements, and the crystal lattice formed by Co and other metal elements becomes more compact, thereby increasing the strength and toughness of the steel. When the steel is subjected to compressive force, this compact structure can resist greater pressure, thereby improving the compressive strength. At the same time, the steel is susceptible to oxidation and corrosion, which reduces its compressive crushing performance, and the addition of Co can form a dense cobalt oxide film that effectively blocks the invasion of oxygen and moisture, thereby improving the compressive crushing performance of the steel. However, when the Co content exceeds a certain range, the plasticity and toughness of the steel will decrease, and the brittleness will increase. This is because although Co can increase the hardness and strength of the steel, Co content exceeding 0.005% will reduce its ductility and impact toughness, and Co will also weaken the stability of austenite, causing the Ms point to rise, and Co is a non-carbide forming element, which will slow down the aggregation speed of carbides, which is not conducive to M / A decomposition. Therefore, the Co content in the present application is controlled at 0.0005-0.0050%.

[0033] N: Nitrogen plays a key role in the low-temperature performance of steel. In particular, in a low-temperature environment, excessive N content can increase the risk of brittle fracture. This is because N in the steel will form nitrides, which tend to accumulate at grain boundaries, thereby causing intergranular brittleness. The low-temperature toughness of the steel is closely related to its N content, and excessive or inappropriate N content can promote the occurrence of brittle fracture. In order to ensure the low-temperature performance of the steel, the present application controls the N content to be ≤0.006%.

[0034] Nb: Niobium is a key element in low-carbon micro-alloyed steel. During hot rolling, Nb can form a solid solution and precipitate Nb carbonitride with strain-induced precipitation. These precipitates can pin the grain boundaries, effectively inhibiting the excessive growth of deformed austenite. By precisely controlling the rolling and controlled cooling process, the deformed austenite can be transformed into a product with high dislocation density and fine structure. Further, the solid solution of Nb will subsequently precipitate in the form of second-phase particles NbC in the matrix, which plays a role in precipitation strengthening. However, excessive Nb content can cause cracks in the slab, thereby affecting the surface quality and possibly deteriorating the welding performance. Therefore, the Nb content in the present application is controlled at 0.025-0.05%.

[0035] Ti: Titanium is an extremely effective deoxidizing and degassing agent, and it can also effectively fix nitrogen and carbon in steel. When the steel is heated, the undissolved carbonitride of Ti can prevent the excessive growth of austenite grains. Especially in the rough rolling stage of the high temperature austenite zone, the precipitated TiN and TiC can effectively inhibit the growth of austenite grains, thereby refining the grain structure of the steel. This refinement not only improves the strength of the steel, but also increases the solid solubility of Nb, thereby reducing the micro-crack sensitivity of the Nb-containing steel. Ti is usually added to the steel together with Nb to improve its performance. In addition, during the welding process, the precipitation of Ti can also inhibit the growth of high temperature grains, thereby improving the welding performance of the steel. Therefore, the Ti content of the present application is controlled between 0.005-0.018%.

[0036] Als: Aluminum is mainly used as a deoxidizing agent in steel. Appropriate amount of Al can refine the grain of the steel, thereby improving its strength and toughness. However, when the Al content is too high, coarse precipitates may be formed, which will reduce the low temperature toughness of the steel. Therefore, when controlling the Al content, the effects of grain refinement and low temperature toughness need to be balanced. The Als content of the present application is controlled between 0.010-0.040%.

[0037] B: The main role of boron in steel is to increase its hardenability and strength, thereby helping to save other more expensive metal elements. However, the addition of B will also increase the brittleness of the steel, significantly reduce the toughness, and the content needs to be strictly controlled. Therefore, in order to ensure that the hardenability and strength of the steel are improved, while avoiding the increase of its brittleness, the B content of the present application is controlled ≤0.0004%.

[0038] Nb and Ti can form carbon-nitrogen-boron compounds uniformly distributed when combined with C, N and B, enhance the performance of the material through solid solution strengthening effect, and act as external crystal nucleus in subsequent processing, which helps to refine the grain size of the structure. However, in order to maintain the toughness and strength of the material, the effect of the toughness and strength reduction caused by the formation of carbon-nitrogen-boron compounds by Cr and C, N and B should be avoided. When controlling the proportion of these elements, there is a key parameter that needs to be controlled, that is, the ratio of (Nb+Ti-Cr / 10) / (C+N+5xB). When this ratio is less than 0.1, the number of precipitated phases is small and the size is too small. These small phases are easy to be re-dissolved into the base material, resulting in weak grain boundary pinning effect and difficulty in effectively dispersing the internal stress of the structure, which leads to easy cracking induced by micro-cracks due to external complex load in deep sea environment, and crushing failure due to uneven deformation stress. Therefore, in order to obtain ideal material performance, it is necessary to strictly control the proportion of Nb, V, Ti, C, N and B, that is, (Nb+Ti-Cr / 10) / (C+N+5xB)≥0.1 in the present application, to refine the grain size of the structure and effectively disperse and reduce the internal stress of the material, thereby reducing the effect of Bauschinger effect and ensuring the compressive strength and improving the anti-crushing performance.

[0039] Generally, the design idea of the present application using the above components is as follows:

[0040] 1. Based on the C-Mn strengthening design, appropriate amounts of Ni, Cr and Mo elements are added to optimize the strength and toughness matching of the steel. In addition, in order to further improve the performance of the material, the present application precisely controls 5Ni+Mn-3C-0.6(Cr+Mo)≥1.4% to achieve more low-temperature mobile dislocations, obtain low ductile-brittle transition temperature, reduce M / A formation, improve residual stress to reduce Bauschinger effect, thereby avoiding the significant reduction of compressive strength due to Bauschinger effect after pipe making, and ensuring the anti-crushing performance.

[0041] 2. In terms of micro-alloying of the material, the present application introduces Nb and Ti two elements, and strictly controls the proportion of (Nb+Ti-Cr / 10) / (C+N+5xB) to avoid the effect of toughness and strength reduction caused by the formation of carbon-nitrogen-boron compounds by Cr and C, N and B, form enough refined grain size, effectively disperse and reduce the internal stress of the material, thereby reducing the effect of Bauschinger effect, ensuring the compressive strength and improving the anti-crushing performance.

[0042] 3. The application also strictly controls the content of elements such as S, P, Co, B and the like in the steel material to improve the mobile dislocation characteristics of the steel material at low temperature, further improve the toughness thereof, and reduce the organizational stress. Through the strict control of the content and proportion relationship of the alloying elements, the application successfully realizes the high strength and toughness matching, not only improves the compression strength of the material, but also significantly improves the toughness performance thereof at low temperature environment.

[0043] The second aspect of the application provides a method for manufacturing the X70MO steel pipe, which comprises the following steps in sequence:

[0044] 1) smelting and casting

[0045] The smelting and refining are carried out based on the chemical composition of the X70MO steel pipe, and then the slab is cast;

[0046] 2) heating the slab:

[0047] The heating temperature is 1080-1190 DEG C, and the heating time is 3-6 h;

[0048] 3) rolling the heated slab, and the rolling process is rough rolling and finish rolling:

[0049] The finish rolling temperature of the rough rolling is greater than or equal to 950 DEG C, and the single pass reduction rate of the last two passes of the rough rolling is greater than or equal to 15 %;

[0050] The start rolling temperature of the finish rolling is less than or equal to 900 DEG C, preferably 779-900 DEG C, the total reduction rate of the finish rolling is greater than or equal to 70 %, and the finish rolling temperature is greater than or equal to (A r3 + 35 DEG C);

[0051] 4) cooling the rolled slab, and the cooling process is air cooling, quenching stage, accelerated cooling stage and self tempering treatment:

[0052] After rolling, the slab is first air cooled to A r3 temperature below;

[0053] Quenching stage, the open cooling temperature of the quenching stage is 700-760 DEG C, the stop cooling temperature of the quenching stage is 530-650 DEG C, and the cooling rate of the quenching stage is 20-45 DEG C / s;

[0054] Accelerated cooling stage, the open cooling temperature of the accelerated cooling stage is 530-650 DEG C, the final cooling temperature of the accelerated cooling stage is 320-550 DEG C, and the cooling rate of the accelerated cooling stage is 10-25 DEG C / s;

[0055] Self tempering treatment, the temperature of the self tempering treatment is greater than or equal to 250 DEG C, preferably 250-350 DEG C, more preferably 250-291 DEG C, and the time of the self tempering treatment is greater than or equal to 100 min;

[0056] 5) cooling the slab to make pipe:

[0057] The X70MO steel pipe is obtained by using the process path of C forming→U forming→O forming→welding→expanding.

[0058] Preferably, in step 4), the air cooling time is ≥10s.

[0059] In step 4), the cooling rate of the quenching stage and the final cooling temperature of the accelerated cooling stage satisfy the following relationship:

[0060] In the formula:

[0061] T is the final cooling temperature of the accelerated cooling stage, in ℃;

[0062] R is the cooling rate of the quenching stage, in ℃ / s.

[0063] Preferably, in step 5), the compression rate of the O forming stage is controlled to be 0.15-0.21%.

[0064] Preferably, in step 5), the expanding rate of the expanding stage is controlled to be 0.72-1.18%.

[0065] Preferably, the obtained X70MO steel pipe has an ovality ≤6mm.

[0066] In the manufacturing method described in the present application:

[0067] The temperature of the slab heating is controlled to be 1080-1190℃, and the heating time is 3-6h; the sufficient heating temperature can make Nb and Ti fully solid-solute, and the austenite grain growth is inhibited by low-temperature heating and fine and dispersed carbonitride precipitation, so that uniform and fine austenite grains are obtained.

[0068] In the rolling process, rough rolling is first carried out and then finish rolling. Firstly, the final rolling temperature of the rough rolling stage is required to be ≥950℃, which ensures that the recrystallization and deformation of the rough rolling stage can be alternately carried out. At the same time, in order to further refine the austenite grains, the reduction distribution of the subsequent passes needs to be increased, so as to ensure that the single-pass reduction rate of the last two passes of the rough rolling is ≥15%.

[0069] After entering the finish rolling stage, the rolling should be carried out in the non-recrystallization temperature range, and the opening rolling temperature of the finish rolling needs to be controlled to be ≤900℃. In this stage, the cumulative reduction should be ≥70%, so as to ensure sufficient reduction deformation, so that the austenite grains are fully flattened. Such a deformation process can increase the nucleation ability of the austenite transformation, and promote the deformation-induced ferrite phase transformation. Thus, in the subsequent cooling and phase transformation process, fine and uniform ferrite grains can be obtained. In addition, in order to ensure the quality of the steel, the final rolling temperature must be maintained at ≥(A r3+35℃), wherein A r3 is the temperature at which austenite starts to precipitate free ferrite upon cooling. This temperature control is to avoid excessive hard phase organization in the local part of the steel material, so as to reduce the generation of residual stress, while maintaining good toughness of the steel material at low temperature.

[0070] A small amount of residual stress in the mutual transition between various organizations can reduce local strain concentration, provide dynamic strain partitioning, and thus improve plasticity, so the complex phase organization is beneficial to obtain smaller residual stress and inhibit the Bauschinger effect. If the direct rapid cooling method is used after rolling, although the strength can be ensured, the organization is single bainite, the toughness is insufficient, and the residual stress is high. Therefore, the three-stage controlled cooling process is adopted in the present application, aiming to realize high uniformity of the plate surface temperature control and low residual stress complex phase organization through homogenization of the cooling path.

[0071] After rolling, cooling is carried out, and the process of the cooling is air cooling, quenching stage, accelerated cooling stage and self-tempering treatment.

[0072] Firstly, an air cooling stage is passed, and the air cooling time after rolling is set to be greater than or equal to 10 s, so that the steel material is cooled to below A r3 below the temperature, so as to promote the formation of a certain content of polygonal ferrite in the steel material, which has an important influence on the toughness and plasticity of the steel material. The volume fraction of the polygonal ferrite is less than or equal to 12 %, which can coordinate the deformation of the matrix, improve the toughness of the organization and reduce the residual stress.

[0073] Then, a quenching stage is entered, and the rolled steel plate is quenched on an accelerated cooling device with high-speed cooling capacity, the quenching open cooling temperature is 700-760 DEG C, the quenching final cooling temperature is 530-650 DEG C, and the quenching cooling rate is 20-45 DEG C / s. The purpose of this stage is to make the remaining unconverted supercooled austenite transform into granular bainite in the process of rapid cooling. Granular bainite is a kind of microstructure with excellent mechanical properties, which can ensure high strength and improve the toughness and welding performance of the steel plate under the condition of low carbon content and carbon equivalent. In the case of low C content (i.e. not more than 0.07 %), the hot-rolled steel plate will not undergo the traditional two-phase decomposition process of austenite to ferrite and cementite during the cooling process. Instead, the austenite will directly transform into ferrite with a small amount of C-rich residual austenite. These residual austenites will be transformed into M / A components in the subsequent stage. It is worth noting that M / A components are common and difficult to avoid brittle phases in bainite organization.

[0074] During the formation of M / A constituent, the volume expansion caused by martensitic transformation will accumulate residual stress around the M / A constituent. These residual stresses are difficult to release, and the brittle M / A phase will also destroy the continuity of the matrix, causing lattice distortion in the matrix around the M / A islands. Since the strength and stiffness of the M / A constituent are higher than those of the matrix, this lattice distortion is mainly concentrated in the matrix, forming a high-energy region around the M / A. This high-energy region makes it easy for micro-cracks to occur between the matrix and the M / A constituent, and these micro-cracks are easy to expand along the interface between the two phases, eventually leading to a significant decrease in the toughness of the material.

[0075] The cooling rate during the quenching stage at an open cooling temperature of 700-760°C and a final cooling temperature in the range of 530-650°C has an important influence on the content and size of the M / A constituent. When the cooling rate is <20°C / s, the content of the M / A constituent is higher, and the average particle size increases significantly to 1.2 μm or even 1.5 μm or more. With the increase of the cooling rate, both the content and size of the M / A constituent decrease, but when the cooling rate is >45°C / s, the temperature difference in the thickness direction of the steel plate increases significantly, the surface is undercooled and the core is reheated, and the residual stress of the steel plate is larger.

[0076] Subsequently, an accelerated cooling stage is entered, in which the open cooling temperature is controlled to be 530-650°C, the final cooling temperature is 320-550°C, and the cooling rate is 10-25°C / s. The cooling rate of this stage has less influence on the M / A, so a relatively low cooling rate of 10-25°C / s can be controlled, thereby implementing more fine temperature control, obtaining high temperature uniformity in the plate surface and thickness direction, and greatly improving the residual stress. The final cooling temperature of this stage is also critical. When the final cooling temperature is higher (>550°C), the granular bainite is coarse, and the M / A constituent as an associated phase is also coarse. When the final cooling temperature is lowered, both the granular bainite and the M / A are refined, but when the final cooling temperature is too low (<320°C), lath bainite appears in the structure, and the M / A increases instead.

[0077] After the end of the accelerated cooling stage, a self-tempering treatment is performed, the temperature of the self-tempering treatment is ≥250°C, preferably 250-350°C, more preferably 250-291°C, and the time of the self-tempering treatment is ≥100 min, preferably 100-250 min. During the self-tempering treatment, the M / A with a large size can be decomposed, thereby reducing the content and size of the M / A, further eliminating the residual stress in the steel, improving the plasticity and toughness of the steel, and improving the stability of the structure performance.

[0078] Preferably, in the process range of the present application, based on the comprehensive influence of the above key cooling process parameters on M / A, it is found that there is a certain correlation between the final cooling temperature of the accelerated cooling stage and the cooling rate of the quenching stage for obtaining fine and less content M / A structure, and the present application proposes the control requirement of parameter K, and the specific formula of K is as follows:

[0079] In the formula, T represents the final cooling temperature (℃) of the accelerated cooling stage, and R represents the cooling rate (℃ / s) of the direct quenching stage.

[0080] In the present application, controlling K≤12 can make the M / A content in the final steel pipe structure ≤8%, and the average particle size of M / A ≤1.2μm. The smaller the value of K, the more conducive to obtaining fine and less content M / A structure.

[0081] In the pipe making process, the process path of C forming→U forming→O forming→welding→expanding is adopted to obtain the steel pipe, and the final ovality of the steel pipe has a significant influence on the anti-crushing performance. Precise size can ensure that the steel pipe can uniformly distribute load when subjected to external pressure, thereby improving its anti-crushing ability. It is found in the research that when the ovality increases from 6mm to 10mm, the anti-crushing performance of the steel pipe decreases significantly. Therefore, in the present application, first, the compression rate of O forming is preferably controlled between 0.15-0.21%, and the expansion process further adjusts the size of the steel pipe, and the expansion rate is controlled between 0.72-1.18%, to ensure the stability and accuracy of the size of the steel pipe, and realize the ovality of the steel pipe ≤6mm.

[0082] Compared with the prior art, the present application has the following advantages:

[0083] On the component design of the X70MO steel pipe of the present application, based on the C-Mn strengthening design, the elements of Ni, Cr and Mo are added to optimize the strength and toughness matching of the steel material, by precisely controlling 5Ni+Mn-3C-0.6(Cr+Mo)≥1.4%, the proportion of the above elements is optimized, the M / A component in the structure is effectively reduced, the residual stress of the material is improved, the compression crushing performance is improved, and the Bauschinger effect is inhibited. In addition, through micro-alloying of Nb and Ti, and strictly controlling (Nb+Ti-Cr / 10) / (C+N+5×B)≥0.1, the formation of Cr carbonitride boride is avoided, and the structure grain is refined, the stress in the material is effectively dispersed and reduced, thereby reducing the influence of the Bauschinger effect, ensuring the compression strength, and improving the compression crushing performance. Further, the content of elements such as S, P, Co and B in the steel material is strictly controlled to improve the mobile dislocation characteristics of the steel material at low temperature, further improve the toughness, and reduce the structure stress. Through the strict control of the content and proportion relationship of the alloying elements, the high strength and toughness matching is successfully realized, not only the compression strength of the material is improved, but also the toughness at low temperature environment is significantly improved. Although the existing steel pipe for seabed can reach X70 level in strength grade, the low temperature toughness and compression crushing performance are difficult to meet the requirements, especially the compression crushing performance is rarely studied.

[0084] On the component design of the X70MO steel pipe of the present application, by adopting three-stage controlled cooling process in the cooling stage after rolling, the high uniformity plate surface temperature control and low residual stress complex structure are realized through uniform cooling path. The content of granular bainite in the structure is increased, and a certain amount of polygonal ferrite is ensured to be generated, while the content and size of M / A component are reduced, the stability of the structure performance is improved, the residual stress in the steel is eliminated, the plasticity and toughness of the steel material are improved, and the Bauschinger effect is inhibited. In the existing process, the direct fast cooling mode after rolling is usually adopted, although the strength can be ensured, the structure is single bainite, the toughness is insufficient, the residual stress is high, and the low temperature toughness and compression crushing performance of the steel are affected.

[0085] Through the component and process design of the present application, the microstructure of the X70MO steel pipe is a complex structure of granular bainite+polygonal ferrite+M / A, the content of polygonal ferrite is ≤12%, the content of M / A is ≤8%, the rest is granular bainite structure, and the average particle size of M / A is ≤1.2μm. The transverse / longitudinal yield strength R t0.5 of the obtained X70MO steel pipe is ≥485MPa, the transverse / longitudinal tensile strength R m is ≥570MPa, and the transverse / longitudinal uniform elongation is ≥8%. The transverse compression strength of the obtained X70MO steel pipe is ≥465MPa, and the-35℃DWTT SA% is ≥85%. The ovality of the obtained X70MO steel pipe is ≤6mm. DETAILED DESCRIPTION

[0086] The application will be further described in connection with specific examples. It should be understood that the specific examples described herein are intended to be illustrative only and are not limiting of the application.

[0087] The performance parameters of the application are determined as follows.

[0088] Chemical composition of the steel pipe: determined by a spectroscopic analyzer and a nitrogen-oxygen analyzer.

[0089] Microstructure of the steel pipe: observed by taking a photo with an optical microscope (manufacturer: ZEISS, model: Axio Imager.M2m).

[0090] In the microstructure of the steel pipe, the volume content of ferrite, M / A and bainite is determined / calculated by a metallographic microscope method, and the average particle size of M / A and ferrite is determined / calculated.

[0091] The transverse / longitudinal yield strength Rt0.5 of the steel pipe is determined according to the ASTM A370-2021 standard.

[0092] The transverse / longitudinal tensile strength Rm of the steel pipe is determined according to the ASTM A370-2021 standard.

[0093] The transverse / longitudinal uniform elongation of the steel pipe is determined according to the ASTM A370-2021 standard.

[0094] The transverse compression strength of the steel pipe is determined according to the ASTM E9-2014 standard.

[0095] The -35℃ DWTT SA% of the steel pipe is determined according to the API RP 5L3-2014 standard.

[0096] The ovality of the steel pipe is determined by a mechanical measurement method.

[0097] Examples and comparative examples

[0098] The manufacturing method of the steel pipe of the examples and comparative examples comprises the following steps in sequence:

[0099] 1) Smelting and casting:

[0100] Smelting and refining according to the chemical element composition of the steel pipe, and then casting into a slab;

[0101] 2) Heating: heating the slab;

[0102] 3) Rolling: rolling the heated slab, and the rolling process is rough rolling and finish rolling;

[0103] 4) Cooling: cooling the rolled slab, and the cooling process is air cooling, quenching stage, accelerated cooling stage and self-tempering treatment;

[0104] 5) pipe making: using the process path of C forming→U forming→O forming→welding→expanding diameter to make the pipe, to obtain the steel pipe.

[0105] The chemical components of the steel pipes of the examples and the comparative examples of the present application are shown in Table 1 and Table 2, and the balance includes Fe and inevitable impurities.

[0106] The specific process parameters for making the steel pipes of the examples and the comparative examples of the present application are shown in Table 3 and Table 4.

[0107] The mechanical properties and the microstructure of the steel pipes of the examples and the comparative examples of the present application are shown in Table 5. As can be seen from Table 5, the microstructure obtained is a complex phase structure of granular bainite + polygonal ferrite + M / A, wherein the volume content of the polygonal ferrite is ≤12%, the volume content of M / A is ≤8%, the remaining volume content is granular bainite structure, the average particle size of M / A is ≤1.2 μm, the average particle size of ferrite is ≤7 μm, and the ovality of the steel pipe is ≤6 mm. The tensile properties satisfy: transverse / longitudinal yield strength R t0.5 ≥ 485 MPa, transverse / longitudinal tensile strength R m ≥ 570 MPa, transverse / longitudinal uniform elongation ≥ 8%; transverse compression strength ≥ 465 MPa; and low temperature toughness satisfies: -35℃ DWTT SA% ≥ 85%.

[0108] In the steel pipe of Comparative Example 1, the content of Ni is low, the contents of B and Co are high, the overall content of 5Ni+Mn-3C-0.6(Cr+Mo) is low, the ratio of (Nb+Ti-Cr / 10) / (C+N+5xB) is low, and the heating time, the rough rolling reduction, the air cooling time after rolling, the cooling speed in the quenching stage, the final cooling temperature in the accelerated cooling stage, the cooling speed in the accelerated cooling stage, and the K value in the process do not satisfy the requirements of the present application.

[0109] In the steel pipe of Comparative Example 2, the content of Ni is low, the content of Ti is high, the content of B is high, the content of Co is low, the overall content of 5Ni+Mn-3C-0.6(Cr+Mo) is low, and the final pass reduction in rough rolling, the final rolling temperature in rough rolling, the total reduction in finish rolling, the air cooling time after rolling, the final cooling temperature in the quenching stage, the open cooling temperature in the accelerated cooling stage, the final cooling temperature in the accelerated cooling stage, and the K value in the process do not satisfy the requirements of the present application.

[0110] The M / A size and content in the microstructure of the steel pipes of Comparative Example 1 and Comparative Example 2 are large, and the content of polygonal ferrite is high, the ovality is high, and the pipe precision is poor, which results in poor compression strength, uniform elongation, and toughness, and the transverse compression strength is < 465 MPa, and -35℃ DWTT SA% is < 85%.

Claims

1. An X70MO steel pipe, characterized in that, The X70MO steel pipe, in addition to Fe and unavoidable impurities, also contains the following chemical components by weight percentage: C: 0.032–0.07%, Si: 0.10–0.28%, Mn: 1.51–1.95%, P≤0.0065%, S≤0.002%, Ni: 0.05–0.45%, Cr: 0.1–0.5%, Mo: 0.01–0.25%, Nb: 0.025–0.05%, Ti: 0.005–0.018%, Ca: 0.001–0.004%, acid-soluble aluminum (Als): 0.010–0.040%, B≤0.0004%, N≤0.006%, Co: 0.0005–0.005%, and simultaneously satisfies the following relationship: 5Ni+Mn-3C-0.6(Cr+Mo)≥1.4%, (Nb+Ti-Cr / 10) / (C+N+5×B)≥0.1, Substitute the symbols of each element in the formula with the corresponding weight percentage content of each element.

2. The X70MO steel pipe according to claim 1, characterized in that, The X70MO steel pipe contains the following chemical composition by weight percentage: C: 0.032-0.07%, Si: 0.10-0.28%, Mn: 1.51-1.95%, P≤0.0065%, S≤0.002%, Ni: 0.05-0.45%, Cr: 0.1-0.5%, Mo: 0.01-0.25%, Nb: 0.025-0.05%, Ti: 0.005-0.018%, Ca: 0.001-0.004%, acid-soluble aluminum (Als): 0.010-0.040%, B≤0.0004%, N≤0.006%, Co: 0.0005-0.005%, with the balance being Fe and unavoidable impurities.

3. The X70MO steel pipe according to claim 1 or 2, characterized in that, The chemical composition of the X70MO steel pipe also satisfies the following relationship: 1.4%≤5Ni+Mn-3C-0.6(Cr+Mo)≤3.36%, 0.1≤(Nb+Ti-Cr / 10) / (C+N+5×B)≤0.79%.

4. The X70MO steel pipe according to any one of claims 1-3, characterized in that, The microstructure of the X70MO steel pipe is a multiphase structure of bainite + ferrite + martensite-austenite component M / A, wherein the bainite is granular, the ferrite is polygonal, the volume content of ferrite is ≤12%, preferably 2-12%, the volume content of martensite-austenite component M / A is ≤8%, preferably 1.5-8%, and the remaining volume content is bainite. The average particle size of martensite-austenite component M / A is ≤1.2μm, preferably 0.48-1.2μm, and the average particle size of ferrite is ≤7μm, preferably 4-7μm.

5. The X70MO steel pipe according to any one of claims 1-4, characterized in that, The X70MO steel pipe has a transverse / longitudinal yield strength Rt0.5≥485MPa, a transverse / longitudinal tensile strength Rm≥570MPa, and a transverse / longitudinal uniform elongation ≥8%. The transverse compressive strength of the X70MO steel pipe is ≥465MPa; The X70MO steel pipe has a -35℃ DWTT SA% ≥ 85% and an ellipticity ≤ 6mm.

6. A method for manufacturing the X70MO steel pipe according to any one of claims 1-5, characterized in that, The method includes the following steps in sequence: 1) Smelting and casting: Based on the chemical composition of the X70MO steel pipe according to any one of claims 1-3, it is smelted and refined, and then cast into slabs; 2) Heating the slab: The heating temperature is 1080–1190℃, and the heating time is 3–6 hours; 3) The heated slab is rolled, and the rolling process includes rough rolling and finish rolling: The final rolling temperature of roughing is ≥950℃, and the single-pass reduction rate of the last two passes of roughing is ≥15%. The initial rolling temperature of the finishing mill is ≤900℃, preferably 779~900℃; the total reduction rate of the finishing mill is ≥70%; and the final rolling temperature of the finishing mill is ≥(A). r3 +35℃); 4) The rolled slab is cooled, and the cooling process includes air cooling, quenching, accelerated cooling, and self-tempering. After rolling, air cool to A r3 Below the temperature; During the quenching stage, the starting temperature for quenching is 700–760℃, the stopping temperature for quenching is 530–650℃, and the cooling rate for quenching is 20–45℃ / s. The accelerated cooling stage has an initial cooling temperature of 530–650℃, a final cooling temperature of 320–550℃, and a cooling rate of 10–25℃ / s. Self-tempering treatment, the temperature of self-tempering treatment is ≥250℃, preferably 250~350℃, and the time of self-tempering treatment is ≥100min; 5) The cooled slab is then processed into tubes: X70MO steel pipes are obtained by adopting the process path of C-forming → U-forming → O-forming → welding → diameter expansion.

7. The method according to claim 6, characterized in that, In step 4), the air cooling time is ≥10s.

8. The method according to claim 6 or 7, characterized in that, In step 4), the cooling rate during the quenching stage and the final cooling temperature during the accelerated cooling stage satisfy the following relationship: And K≤12, In the formula: T represents the final cooling temperature during the accelerated cooling stage, in °C. R represents the cooling rate during the quenching stage, expressed in °C / s.

9. The method according to claim 6, characterized in that, In step 5), the compression rate during the O-forming stage is controlled between 0.15% and 0.21%.

10. The method according to claim 6 or 9, characterized in that, In step 5), the diameter expansion rate during the diameter expansion stage is controlled between 0.72% and 1.18%.

11. The method according to claim 6, characterized in that, The ellipticity of the obtained X70MO steel pipe is ≤6mm.

Images