Pipeline steel and its manufacturing method

Abstract

A pipeline steel and a method for manufacturing the same, wherein the pipeline steel has the following composition in weight percent: C: 0.03-0.10%, Si: 0.1-0.5%, Mn: 1.51-1.85%, P≦0.015%, S≦0.002%, Cr: 0.05-0.3%, Mo: 0.05-0.20%, Cu: 0.06-0.3%, Ni: 0.17-0.50%, Nb: 0.05-0.10%, Ti: 0.005-0.02%, Ca: 0.001-0.005%, Al: 0.02-0.045%, N≦0.006%, B≦0.0002%, O≦0.005%, and the balance being Fe and unavoidable impurities. C×Mn ≦0.14, where the carbon-manganese product parameter J C×Mn =C*Mn*10 4 The pipeline steel of the present invention has a yield strength of 450 to 635 MPa, a tensile strength of 520 to 780 MPa, a full-size Charpy impact work (AKv) at -20°C greater than 275 J, a full-wall thickness DWTT shear fracture area (SA) at -20°C greater than 85%, and a longitudinal uniform elongation (Uel) of ≥ 8%. The pipeline steel can be used for submarine pipelines, transverse pipelines, and polar pipeline structures, and is primarily used for long-distance transportation of natural gas.

Description

[Technical field]

[0001] Technical Field The present invention relates to a pipeline steel, in particular to a pipeline steel and a method for producing the same. [Background technology]

[0002] background With the growing demand for oil and gas in the global consumer market, the pipeline transportation of oil and gas tends to develop from land to sea and from small to large volumes, so the safety of the pipeline is highly required. Subsea pipeline structures usually adopt J-type or S-type methods to lay, and the pipeline needs to cross the canyon and overcome the fluid movement and other harsh environments during the service, so that the subsea pipeline needs high strength and high toughness, and also needs thick walls to enhance the structural stability. At the same time, the subsea pipeline needs high plastic deformation capacity to ensure the safety of the service.

[0003] Based on the research results of existing literature, companies such as Europipe in Germany, VSW in Russia and JFE in Japan already have a long history in the development of submarine pipeline steel and pipeline pipe products. In addition to the development research of composition design, rolling process control, microstructure control and other aspects of optimal design, the key to development is the enhancement of steel strength and toughness through equipment improvement. For example, Dillinger can produce extra-thick continuous casting slabs, which have a thickness greater than 500mm, thus laying the foundation for the production of heavy gauge pipeline steel plates with high internal quality and large reduction ratio. In addition, the internal quality of pipeline steel plates is high, which is due to the control of the content of segregation and inclusions. Thus, the purity, strength and toughness of steel can be improved, and Dillinger can provide raw steel plates to Europipe, VSW and other companies. These raw steel plates can be used to produce welded pipes with a thickness of up to 41mm. Japan's JFE uses an in-line heat treatment device and adopts the HOP process to improve the through-thickness structure uniformity of steel plates, which can improve the toughness and obtain uniform mechanical properties of heavy gauge pipeline steels, and improve the ovality of steel pipes at the same time. At present, domestic and foreign research on submarine pipeline steels mainly focuses on their high strength and high toughness. However, there is little research on the high plasticity of pipeline steels.

[0004] Regarding improving the plasticity of pipeline steel, currently, high uniform elongation is required for strain-engineered land pipelines. The combination of soft and hard phases results in pipeline steel with arched stress-strain curve and good work hardening rate, thus the uniform deformation ability of steel can be enhanced, and steel with dual-phase microstructure can be obtained. Representative technologies include ferrite and lower bainite dual-phase microstructure obtained using relaxation and controlled cooling combined microstructural process adopted by some Chinese companies, and martensite-austenite island and bainite dual-phase microstructure obtained using online heat treatment HOP process adopted by JFE. Both of the above-mentioned microstructures can obtain steel with high uniform elongation. However, due to the existence of dual-phase interface, the low-temperature impact toughness of steel is significantly reduced, which does not lead to the arrest crack safety of steel.

[0005] European patent EP2105513B1 discloses a method for producing a heavy gauge, high toughness pipeline steel with a yield strength of 450 MPa grade by using a low C, low Mn and relatively low Nb micro-alloying design in combination with a low temperature heating process and a controlled rolling and cooling process, and fine Polygonal A ferrite dominated microstructure is obtained, the volume fraction of ferrite is 40-90%, and the grain size of ferrite is ≦10 μm. The pipeline steel produced by the invention has excellent toughness.

[0006] Chinese patent CN101611163A discloses an aging-resistant dual-phase pipeline steel, which has yield strength ≥ 400MPa, tensile strength ≥ 500MPa, and yield ratio ≤ 0.90. By adopting an alloying composition design based on C-Mn and using two-stage cooling rate control after rolling, a steel can be obtained, which has a first phase of ferrite and one or more microstructural second phases including pearlite, upper bainite, lower bainite, granular bainite and martensite. Moreover, the uniform elongation of the steel can reach more than 8%.

[0007] US Patent US20120247606A1 discloses the composition and manufacturing method of 80ksi, 6-16mm thin gauge pipeline steel. By using a composition design with low C, high Nb and Mo alloying and adopting low temperature final rolling at 675-715℃ and air cooling rate of 1-2℃ / s, it is possible to obtain pipeline steel with strength graded up to 80Ksi and good weldability.

[0008] Australian patent AU2006305841A1 discloses a manufacturing method for a dual-phase steel with tensile strength of 900MPa grade and longitudinal yield ratio ≦0.85. By using a composition design of low carbon, high Mn and Nb-Mo alloying and adopting a rational process control, a steel with 10-60% fine ferrite with grain size ≦5μm can be obtained. The remaining structure of the steel is one or more mixed structures such as fine martensite, lower bainite, annealed upper bainite, granular bainite, etc.

[0009] Chinese patent CN109023068A discloses a method for producing NbC nano-precipitation strengthened X80 high plasticity pipeline steel plate, which adopts a composition design of medium C, low Mn and high Nb micro-alloying. The production method of pipeline steel plate includes controlled rolling, controlled cooling, followed by solid solution treatment at 1180-1220℃ and isothermal treatment at 670-710℃. By adopting the above composition design and production method, the volume fraction of NbC precipitation is controlled at 0.05-0.20%, and therefore X80 pipeline steel with high plasticity and high toughness can be obtained.

[0010] Chinese patent CN101343715B discloses a manufacturing method for large strain pipeline steel pipe with 650MPa grade yield strength, which adopts a composition design of alloying medium C, low Mn and high B. Combined with the manufacturing method of controlled rolling, controlled cooling and online tempering process, a steel with a composite microstructure of ferrite, acicular ferrite and martensite-austenite islands can be obtained, which can be used to manufacture steel pipe with yield strength of 650-680MPa and uniform elongation of 12-15%. Summary of the Invention [Means for solving the problem]

[0011] summary The object of the present invention is to provide a high plasticity heavy gauge pipeline steel and a manufacturing method thereof. The pipeline steel has a yield strength R of 450-635 MPa. t0.5 , tensile strength R of 520 to 780 MPa m , in particular, having a full size Charpy impact work AKv at -20°C greater than 275 J, a full wall thickness DWTT shear fracture area SA at -20°C greater than 85%, and a longitudinal uniform elongation Uel ≧8%. The pipeline steel can be used for submarine pipelines, transverse pipelines and polar pipeline structures, and is mainly used for long distance transportation of natural gas.

[0012] In order to achieve the above objectives, the technical solutions of the present invention are as follows: Composition by weight percentage: C: 0.03-0.10%, Si: 0.1-0.5%, Mn: 1.51-1.85%, P≦0.015%, S≦0.002%, Cr: 0.05-0.3%, Mo: 0.05-0.20%, Cu: 0.06-0.3%, Ni: 0.17-0.50%, Nb: 0.05-0.10%, Ti: 0.005-0.02%, Ca: 0.001-0.005%, Al: 0.02-0.045%, N≦0.006%, B≦0.0002%, O≦0.005%, and the balance is Fe and unavoidable impurities; and 0.06≦J C×Mn ≦0.14, where the carbon-manganese product parameter J C×Mn = [C]*[Mn] (where [C] and [Mn] are the respective contents (weight percent) of C and Mn.) Satisfying the above, pipeline steel.

[0013] The pipeline steel according to the present invention is Polygonal having a microstructure of ferrite and acicular ferrite, Polygonal The ratio of the ferrite phase is 15 to 39%.

[0014] The pipeline steel of the present invention has a yield strength R of 450 to 635 MPa. t0.5 , tensile strength R of 520 to 780 MPa m , a full size Charpy impact work at -20°C AKv greater than 275 J, a full wall thickness DWTT shear fracture area fraction SA at -20°C greater than 85%, and a longitudinal uniform elongation Uel ≧8%.

[0015] In the composition design of the pipeline steel of the present invention: Carbon: C is the most basic strengthening element, which can dissolve in steel to form interstitial solid solution, and show the effect of solid solution strengthening. In addition, carbon can form strong carbide-forming elements and carbide precipitates, resulting in the effect of precipitation strengthening. However, excessive C is harmful to the toughness and weldability performance of steel, and at the same time leads to a decrease in the plasticity of steel. However, if the C content is too low, the strength of steel will decrease. Therefore, the C content is controlled at 0.03-0.10%.

[0016] Silicon: Si is a solid solution strengthening element and also a deoxidizing element in steel. However, excessive Si in steel will deteriorate the weldability performance of steel and reduce plasticity, while it will not lead to the removal of hot-rolled iron oxide in the rolling process, so the content of Si is controlled at 0.1-0.5%.

[0017] Manganese: Mn element can improve the strength of steel by solid solution strengthening, and is the most important and economical strengthening element to compensate for the strength loss of steel resulting from the reduction of C content in steel. Mn is also an element to expand the γ phase region, and can reduce the γ→α phase transformation temperature of steel, which can help to obtain fine phase transformation products, resulting in the increase of toughness of steel. However, Mn element easily segregates in steel. When the Mn content is high, Mn easily segregates in the center of the plate during the casting process, resulting in the formation of hard phase martensite after the completion of rolling, and thus the decrease of the plasticity and low temperature toughness of the material. Therefore, the Mn content is limited to 1.51-1.85% in this invention. In addition, since both C and Mn are solid solution strengthening elements, and have a high tendency to segregate, the product parameter of carbon and manganese (J C×Mn = [C]*[Mn] (where [C] and [Mn] are the respective contents (weight percent) of C and Mn.) ) is 0.06≦J C×Mn The requirement of ≦0.14 must be satisfied. C×Mn When J is less than 0.6, due to insufficient C and Mn content, the solid solution strengthening effect is slight and the strength of the steel is low. C×Mn When is larger than 0.14, the interval range of δ-austenite is narrowed, which does not lead to the diffusion of C and Mn, resulting in an increased tendency to segregate.

[0018] Chromium: Cr is an important element to improve the hardenability of steel, which can ensure the structure and performance uniformity of heavy gauge full-layer steel plate, and can effectively improve the corrosion resistance of steel. However, if the Cr content in steel is too high, the strength and hardness of the steel will increase, while the elongation and area reduction will decrease. When Cr is added to steel with high Mn content, their compounds will be easily formed and cause the occurrence of cracks, which will seriously deteriorate the welding performance of steel. Therefore, the Cr content in this invention should be limited to 0.05-0.3%.

[0019] Molybdenum: Mo is an element that can expand the γ phase region and reduce the γ→α phase transformation temperature of steel, and then a finer phase transformation structure can be obtained, which can improve the toughness of steel. At the same time, a small amount of Mo can increase the hardenability of steel and improve the structure uniformity in the thickness direction. However, with an increase in Mo content, the proportion of low-temperature phase transformation products increases, which is unfavorable for the low-temperature toughness of steel and harmful to the plasticity of steel. Therefore, the Mo content in this invention is controlled at 0.05~0.20%.

[0020] Copper: Cu can improve the strength of steel by solid solution strengthening and improve the corrosion resistance of steel in the atmosphere. However, too much Cu will cause copper embrittlement and have a negative effect on the hot formability of steel. Therefore, the Cu content in this invention is controlled at 0.06-0.3%.

[0021] Nickel: Ni can improve the strength of steel by solid solution strengthening. The addition of Ni can improve the thermal embrittlement caused by Cu in steel. In addition, Ni can expand the austenite region and therefore increase the stability of austenite, which is beneficial to the plasticity and toughness of steel. Therefore, the content of Ni content in this invention is controlled at 0.17-0.50%.

[0022] Niobium: Nb is one of the important elements of micro-alloyed steel with low carbon content. The solid solution of Nb can form Nb(N,C) particles by strain-induced precipitation during the hot rolling process, and can also pin the grain boundaries to inhibit the growth of deformed austenite and the occurrence of austenite recrystallization. The deformed austenite phase is transformed into fine products with high dislocation density by the controlled rolling process and controlled cooling process. After cooling, the Nb dissolved in the solid diffusely precipitates in the matrix as the second phase of NbC, which results in the effect of precipitation strengthening. For heavy gauge pipeline steel, if the Nb content is too low, the diffuse precipitation effect is not obvious, and then Nb cannot play its role of refining grains or strengthening the matrix. However, due to the inhibition of recrystallization in the steel plate core, high Nb content does not lead to grain refinement. In addition, the solid solution of Nb is related to the C content. The amount of Nb solid solution is small when the C content is too high, and then there is no precipitation strengthening and grain refinement effect. Low C content will weaken the grain boundary. The precipitation strengthening effect is not obvious when Nb content is too low. Therefore, the Nb content in this invention should be limited to 0.05-0.10%.

[0023] Titanium: Ti is a strong carbonitride forming element, and the insoluble carbonitride of Ti can prevent the growth of austenite grains when the steel is heated. And the TiN precipitated in the high temperature austenite region during the rough rolling process can effectively inhibit the growth of austenite grains. Moreover, in the welding process, the TiN particles in the steel can significantly prevent the grain growth in the heat affected zone, thus improving the welding performance of the steel plate, and at the same time has obvious effect on improving the impact toughness of the welded heat affected zone. Therefore, the Ti content in this invention is controlled at 0.005-0.02%.

[0024] Nitrogen: In microalloyed steel, appropriate N content can improve the strength and toughness of steel by forming TiN particles with high melting point, which can play a role in inhibiting the grain coarsening of slabs during the reheating process. However, if the N content is too high, the high concentration of free N atoms after aging treatment can pin dislocations, resulting in a significant increase in yield strength and a significant decrease in plasticity and toughness. Therefore, the N content in the present invention is controlled to ≦0.006%.

[0025] Oxygen: For the refining of low alloy pure steel, deoxidation is necessary at the end of the refining process to reduce the bubbles and oxide inclusions generated during the casting process, which can improve the internal quality and low temperature impact toughness of the steel, as well as the dynamic tear resistance of the finished steel plate. When the oxygen content is higher than 50ppm, the inclusions, voids and other internal defects will increase significantly, resulting in the deterioration of the plasticity and toughness of the steel. Therefore, the O content in this invention is controlled to ≦0.005%.

[0026] Sulfur and phosphorus: They are inevitable impurity elements in steel, and it is expected that their contents can be as low as possible. The inclusion form of sulfide is controlled by ultra-low sulfur (less than 20 ppm) and Ca treatment. At the same time, the P content should be controlled below 150 ppm to ensure that the steel in this invention can have good low-temperature impact toughness.

[0027] Calcium: Ca treatment can control the form of sulfur and improve the anisotropy and low temperature toughness of the steel plate. The Ca content is controlled at 0.0010-0.0050% to achieve the best results.

[0028] Aluminum: Al is an element added to steel for deoxidation. By adding an appropriate amount of Al, the toughness of steel can be improved and the grains can be refined. Therefore, the content in this invention is controlled at 0.02-0.045%.

[0029] Boron: B is a strong hardening element, which easily precipitates at grain boundaries, resulting in a decrease in the plasticity and toughness of the material. Therefore, the B content in this invention is controlled to B≦0.0002%.

[0030] Therefore, the thickness of 28 to 40 mm and the yield strength R of the grade of 450 to 635 MPa t0.5 The heavy gauge pipeline steel with low C and high Nb micro-alloying composition design is based on material theory including grain refinement and phase transformation control. Combined with manufacturing processes such as low temperature rough rolling and finish rolling, the deformation-induced phase transformation mechanism is fully exerted to promote ferrite phase transformation. By proper control of cooling rate and stop cooling temperature, the grain refinement can be achieved. Polygonal The microstructure of ferrite and acicular ferrite can be obtained. Thus, the pipeline steel with comprehensive mechanical properties of high strength, high toughness and high plasticity as well as good deformation ability is obtained.

[0031] A method for producing a highly plastic heavy gauge pipeline steel according to the present invention, comprising the steps of: 1) A process of refining and casting, in which It is refined according to the above composition and continuously cast into slabs. 2) Reheating the slab at a heating temperature of 1100-1200°C. 3) hot rolling, where Starting temperature of rough rolling: 940-1000℃; Single pass reduction rate in the final pass of rough rolling: ≥12%; Intermediate slab thickness: 3t~5t, where t is the thickness of the pipeline steel in mm; and Starting temperature of finish rolling: 750~810℃; Final temperature of finish rolling: 740~800℃; 4) controlling cooling, Water cooling start temperature T start :620~720℃; Water cooling stop temperature T stop : 150~530℃; and Water cooling rate V in °C / s c =72-T start / 10+T stop / 20; and 5) The process of natural air cooling after water cooling.

[0032] Preferably, the heating temperature in step 2) reheating the slab is 1110 to 1150°C.

[0033] Preferably, in step 3), the starting rolling temperature of rough rolling is 960-990°C. The single pass reduction in the final pass of rough rolling is 14% or more. The thickness of the intermediate slab is 4t-4.5t. The starting temperature of finish rolling is 770-800°C, and the final temperature of finish rolling is 750-780°C.

[0034] Preferably, in step 4) controlled cooling, the water cooling start temperature T start is 660-700°C, and the water cooling stop temperature T stop is 200 to 350 degrees Celsius.

[0035] According to the above technical solutions, a heavy gauge high plasticity pipeline steel with a thickness of 28-40mm can finally be obtained.

[0036] In the manufacturing process design of the present invention:

[0037] In the controlled cooling process: Water cooling start temperature T start is controlled at 620~720℃; water cooling stop temperature T stop is controlled between 150 and 530°C; the water cooling rate V is in °C / s. c =72-T start / 10+T stop / 20.

[0038] The cooling after rolling is a key process that determines the phase transformation structure. Polygonal The ferrite and acicular ferrite phase transformation microstructures are characterized by the water cooling start temperature T start , water cooling stop temperature T stop and water cooling rate V c All these cooling parameters must satisfy the above conditions. Here, the water cooling start temperature T startWhen is higher than 720℃, the soft phase Polygonal The formation force of ferrite precipitation is small, which will result in high strength of the steel. start When the temperature is lower than 620℃, the soft phase Polygonal The ferrite is coarse and its proportion is very high, which will result in a significantly lower strength of the steel. stop The hardness of the hard phase acicular ferrite is mainly determined by T stop When T is higher than 530°C, the dislocation density is small and the hardness and strength are low. stop If the water cooling rate V is lower than 150°C, martensite easily occurs, resulting in a very high dislocation density, high hardness and reduced plasticity. c is a key parameter for controlling the type of phase transformation structure and their phase ratio by matching the water cooling start and stop temperatures. c is the water cooling stop temperature T stop is positively related to the water-cooling start temperature T start is negatively related to

[0039] The following compares the present invention with the prior art.

[0040] Different from that disclosed in European Patent EP2105513B1, the present invention is designed mainly for high plasticity pipeline steel products with 450MPa grade yield strength. By adopting the composition design of low C, high Mn and Nb micro-alloying and the manufacturing process of low temperature controlled rolling in the recrystallization rolling stage, the grain size of the original austenite can be suppressed. By controlling the water cooling rate at a relatively low level, the grain size can be refined. Polygonal Ferrite particles and acicular ferrite phase transformation particles can be obtained. Polygonal The ratio of ferrite phase is controlled to be lower than 40%, and the steel can have good plasticity and toughness.

[0041] Different from that disclosed in Chinese patent CN101611163A, the present invention adopts a composition design of high Nb and low B to fully utilize the effect of grain refinement and suppress the low temperature transformation structure. Combined with the low temperature rolling process and the low cooling rate process, the refinement Polygonal A final microstructure of ferrite and acicular ferrite can be obtained. The steel has a low bending strength ratio, high uniform deformation capacity and good low temperature toughness.

[0042] Unlike that disclosed in US Patent US20120247606A1, the present invention is mainly characterized by a yield strength R of 450 to 635 MPa. t0.5 , tensile strength R of 520 to 780 MPa m and for pipeline steel products with thicknesses of 28 mm or more. By adopting composition design of low C, high Mn and Nb micro-alloying and low-temperature controlled rolling process in the recrystallization rolling stage, the grain size of the original austenite can be suppressed. By controlling the water cooling rate at a relatively low level, the grain size can be refined. Polygonal Ferrite grains and acicular ferrite phase transformation grains can be obtained, and the steel can have good plasticity and toughness.

[0043] Different from that disclosed in Australian patent AU2006305841A1, the present invention is mainly designed for high plasticity pipeline steel products with longitudinal uniform elongation Uel≧8% and yield strength of 450MPa grade. By adopting low C and relatively low Nb composition design and low temperature controlled rolling process in recrystallization rolling stage, the grain size of original austenite can be suppressed. By controlling the water cooling rate at a relatively low level, the grain size of the original austenite can be suppressed. Polygonal Ferrite particles + acicular ferrite phase transformation particles can be obtained, and the steel can have good deformability.

[0044] Different from that disclosed in Chinese Patent CN109023068A, the present invention mainly adopts the composition design of low C and higher Mn and Nb micro-alloying. By adopting the above-mentioned composition and the manufacturing method including low recrystallization rolling temperature, low temperature non-recrystallization rolling and reasonable cooling rate control, the refinement can be achieved. Polygonal A microstructure of ferrite + acicular ferrite can be obtained, which can ensure high strength, high plasticity and toughness of the steel. There is no need for solid solution and isothermal heat treatment, therefore the total cost is low.

[0045] Different from what is disclosed in Chinese patent CN101343715B: the present invention mainly adopts the composition design of low C and higher Mn and Nb micro-alloying. By adopting the above composition and the manufacturing method including low recrystallization rolling temperature, low temperature non-recrystallization rolling and reasonable cooling rate control, the refinement can be achieved. Polygonal A ferrite + acicular ferrite microstructure can be obtained, which can ensure high strength, high plasticity and toughness of the steel. The alloy additions are low and there is no need for in-line heat treatment, therefore the total cost is low.

[0046] Advantages of the present invention include: 1. The present invention adopts a low-temperature heating process to inhibit the growth of austenite grains during the reheating process, which can control the grain size at the source. If the heating temperature is too high, the high-temperature precipitate phase of the micro-alloying element Ti will be in solid solution, which will weaken the effect of fixing the grain boundaries, and these grain boundaries will migrate and fuse, resulting in significantly coarsened grains, which is not conducive to the low-temperature toughness of the steel.

[0047] 2. The present invention adopts a low-temperature rolling process in the recrystallization stage to inhibit the growth of recrystallized grains. If the recrystallization temperature is too high, the Gibbs free energy at the grain boundary is high, and therefore the driving force of the recrystallized grain boundary movement of the grains is high, which can reduce the Gibbs free energy at the grain boundary by promoting the grains to fuse, and therefore cause the recrystallized grains to become coarse.

[0048] 3. The present invention adopts an appropriate amount of Nb alloy design, combined with recrystallization rolling at low temperature, to refine the recrystallized grains. High Nb content in steel causes the recrystallization temperature to increase, which does not lead to the occurrence of recrystallization. However, low Nb content in steel reduces the recrystallization temperature and increases the deformation resistance of recrystallization rolling at low temperature, which puts high requirements on equipment capacity.

[0049] 4. The pipeline steel of the present invention is finely grained Polygonal With a microstructural design of ferrite + acicular ferrite. The plastic deformation capacity of pipeline steel is improved by the soft phase Polygonal It can be strengthened by microstructural design of ferritic structure, so that the pipeline steel has a longitudinal uniform elongation Uel≧8%. The dynamic tear resistance of steel can be effectively improved by using high density high angle grain boundaries to increase the resistance of crack propagation. [Brief description of the drawings]

[0050] [Figure 1] FIG. 1 shows the microstructure (at half thickness) of a steel according to an embodiment of the present invention; and [Diagram 2] FIG. 2 shows the microstructure (at the 1 / 4 plate thickness position) of a steel according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0051] Detailed Description of the Preferred Embodiments The invention will be further described with reference to the following embodiments and drawings.

[0052] As shown in Table 1, the chemical composition of the embodiment is designed according to the chemical composition requirements of the steel in the present invention. The manufacturing process of the embodiment is shown in Table 2. The mechanical property values ​​obtained in each embodiment are listed in Table 3. These mechanical properties are obtained based on the above-mentioned chemical composition and manufacturing process. Specifically, the tensile properties of the steel in the present invention are tested on a tensile tester Zwick Z330 under the test standard of ASTM A370. The impact toughness of the steel is tested on an impact tester Zwick PSW750 under the test standard of ASTM A370. The full wall thickness DWTT shear fracture area ratio SA at -20°C is tested on a 40,000J impact tester ZBC2404 under the test standard of API RP 5L3.

[0053] It can be seen that the steel obtained according to the chemical composition and manufacturing process designed in the present invention can meet the target performance requirements. Specifically, the steel has good overall mechanical properties and low carbon equivalent, which leads to improved steel pipe forming welding and in-situ girth welding performance. In addition, the chemical composition of the steel in the present invention is simple, and the process window is wide, which results in the steel having strong manufacturability.

[0054] The high plasticity heavy gauge pipeline steel of this invention is mainly used for submarine pipelines, cross-seismic zone pipelines and other types of pipelines used in harsh environments with special requirements. The plastic deformation capacity of the pipeline can be enhanced by controlling the microstructure on the basis of ensuring sufficient steel strength and toughness, while the self-weight support and motion resistance capacity of the pipeline can be improved, which can ensure the safety of service. With the development of oil and gas resources from inland to the sea, other regions such as polar permafrost, and areas with frequent geological movements, the high plasticity heavy gauge pipeline steel will have good application prospects.

[0055] [Table 1]

[0056]

Table 2

[0057]

Table 3

Claims

1. Composition in weight percent as follows: C: 0.03-0.10%, Si: 0.1-0.5%, Mn: 1.51-1.85%, P≦0.015%, S≦0.002%, Cr: 0.05-0.3%, Mo: 0.05-0.20%, Cu: 0.06-0.3%, Ni: 0.17-0.50%, Nb: 0.05-0.10%, Ti: 0.005-0.02%, Ca: 0.001-0.005%, Al: 0.02-0.045%, N≦0.006%, B≦0.0002%, O≦0.005%, and the balance being Fe and unavoidable impurities; having 0.06≦J Ｃ×Ｍｎ ≦0.14, where the carbon and manganese product parameter J Ｃ×Ｍｎ = [C] * [Mn] (where [C] and [Mn] are the respective contents (weight percent) of C and Mn.) Satisfied, having a microstructure of polygonal ferrite and acicular ferrite, wherein the proportion of polygonal ferrite phase is 15-39%, Yield strength R of 450 to 635 MPa ｔ０．５ , tensile strength R of 520 to 780 MPa ｍ , a full size Charpy impact work at −20° C. AKv of greater than 275 J, a DWTT shear fracture area ratio SA of the full wall thick steel plate specimen at −20° C. of greater than 85%, and a longitudinal uniform elongation Uel≧8%, having a thickness of 28 to 40 mm; Steel plate for pipelines.

2. A method for manufacturing a steel plate for pipelines, comprising the steps of: 1) A process of refining and casting, smelting according to the composition of claim 1 and continuously casting into slabs; 2) Reheating the slab at a heating temperature of 1100-1200°C; 3) A process of hot rolling, in which Starting temperature of rough rolling: 940-1000°C; Single pass reduction in the final pass of rough rolling: ≧12%; Intermediate slab thickness: 3t-5t, where t is the thickness of the pipeline steel plate in mm; and Starting temperature of finish rolling: 750-810°C; Final temperature of finish rolling: 740-800°C; 4) controlled cooling, Water cooling start temperature T ｓｔａｒｔ :620~720℃; Water cooling stop temperature T ｓｔｏｐ : 150 to 530 ° C; and Water cooling rate V in °C / s ｃ =72-T ｓｔａｒｔ / 10+T ｓｔｏｐ / 20; and 5) Water cooling followed by natural air cooling; The steel plate for pipelines is Composition in weight percent as follows: C: 0.03-0.10%, Si: 0.1-0.5%, Mn: 1.51-1.85%, P≦0.015%, S≦0.002%, Cr: 0.05-0.3%, Mo: 0.05-0.20%, Cu: 0.06-0.3%, Ni: 0.17-0.50%, Nb: 0.05-0.10%, Ti: 0.005-0.02%, Ca: 0.001-0.005%, Al: 0.02-0.045%, N≦0.006%, B≦0.0002%, O≦0.005%, and the balance being Fe and unavoidable impurities; having 0.06≦J Ｃ×Ｍｎ ≦0.14, where the carbon and manganese product parameter J Ｃ×Ｍｎ = [C] * [Mn] (where [C] and [Mn] are the respective contents (weight percent) of C and Mn.) Satisfied, having a microstructure of polygonal ferrite and acicular ferrite, wherein the proportion of polygonal ferrite phase is 15-39%, Yield strength R of 450 to 635 MPa ｔ０．５ , tensile strength R of 520 to 780 MPa ｍ , a full size Charpy impact work at −20° C. AKv of greater than 275 J, a DWTT shear fracture area ratio SA of the full wall thick steel plate specimen at −20° C. of greater than 85%, and a longitudinal uniform elongation Uel≧8%, having a thickness of 28 to 40 mm; Manufacturing method.

3. The method for producing a steel plate for pipelines according to claim 2, wherein the heating temperature for reheating the slab in step 2) is 1110 to 1150°C.

4. The method for producing a steel plate for pipelines according to claim 2, wherein in step 3), the starting rolling temperature of the rough rolling is 960-990°C, and the single pass reduction rate in the final pass of the rough rolling is ≧14%; the thickness of the intermediate slab is 4t-4.5t; the starting temperature of the finish rolling is 770-800°C, and the final temperature of the finish rolling is 750-780°C.

5. In the step 4) of controlling the cooling, the water cooling start temperature T ｓｔａｒｔ is 660 to 700 ° C., and the water cooling stop temperature T ｓｔｏｐ The method for producing a steel plate for pipelines according to claim 2, wherein the temperature is 200 to 350°C.

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