High-performance-uniformity subsea pipeline steel and manufacturing method therefor

Abstract

A high-performance-uniformity subsea pipeline steel and a manufacturing method therefor. The chemical components of the high-performance-uniformity subsea pipeline steel comprise, in percent by weight: C: 0.02-0.06%, Si: 0.10-0.30%, Mn: 1.20-1.65%, Cu ≤ 0.30%, Ni ≤ 0.30%, Cr: 0.15-0.30%, Mo ≤ 0.15%, Nb: 0.03-0.05%, V ≤ 0.05%, Ti: 0.008-0.020%, Ca: 0.0012-0.0030%, and Al: 0.015-0.045%, with the balance comprising Fe and inevitable impurities, where it is required to control impurity elements P ≤ 0.01%, S ≤ 0.001%, B ≤ 0.0004%, N ≤ 0.005%, O ≤ 0.0025% and H ≤ 0.0002%. The manufacturing method uses an austenite recrystallization zone and non-recrystallization zone two-stage controlled rolling technology, spread ratio control, and controlled cooling, such that in addition to high performance, obtained subsea pipeline steel also has good performance uniformity.

Description

Technical field

[0001] The present invention relates to a field of pipeline steel manufacturing, in particular to a high-performance-uniformity subsea pipeline steel and manufacturing method therefor.Background Art

[0002] The laying of subsea pipeline has a long history of development internationally. Since the first subsea pipeline was laid by Brown & Root Marine Engineering Company in the Gulf of Mexico, USA, in 1954, a large number of subsea pipelines of various types and diameters have been successfully laid in offshore areas around the world, including the North Sea, the Gulf of Mexico, the Mediterranean Sea, Australia, and the South China Sea. Especially in recent years, with the gradual depletion of onshore oil and gas resources, the exploitation of oil and gas has extended to areas with harsh geological conditions such as oceans and polar region, leading to a stronger demand for high-performance subsea pipeline steels.

[0003] Subsea pipelines are subjected to significant compression, tensile loads, and even bending deformations during the laying of the subsea pipeline. During service, harsh operating conditions such as deep-water high pressure, surging waves, strong bottom currents, and subsea movements will also cause substantial plastic deformations or even fracture in subsea pipelines, resulting in serious environmental disasters. These complex operating conditions impose stricter requirements on deep-water pipelines compared to onshore pipelines. Furthermore, as water depth increases, the demands on raw materials continue to rise.

[0004] Subsea pipeline construction is characterized by high risk and high investment. Compared to onshore pipeline construction, it has high requirements for construction quality, construction environment, and construction technology, etc. To avoid the risks of difficult repair and environmental pollution caused by the failure of subsea pipelines, stricter technical requirements are often put forward to improve the quality of subsea pipelines, and process management is strengthened to improve the reliability of the product. Therefore, isotropy of the steel plate (more uniform strength / toughness distribution in the length / thickness direction) is required, as well as good welding performance, low-temperature fracture toughness, strain aging performance, etc. Furthermore, as the subsea pipeline is subjected to significant axial forces and displacements during laying and service process, its load-bearing capacity and strain capacity can be ensured by controlling longitudinal performance. Therefore, longitudinal tensile properties, especially the longitudinal low yield ratio is required for the steel plate.

[0005] The Chinese patent application CN201210123639.X discloses "A Large Wall Thickness Subsea Pipeline Steel Plate and Production Method therefor". The steel plate consists of the following composition in mass percentage of: C: 0.05∼0.07%, Si: 0.15~0.25%, Mn: 1.42∼1.48%, P≤0.010%, S≤0.002%, Ni: 0.13~0.18%, Nb: 0.043~0.048%, Al: 0.020~0.040%, Ti: 0.014~0.024%, Mo: 0.13~0.18%, and a balance of Fe and unavoidable impurities. The steel plate has characteristics such as high low-temperature impact toughness, good tear resistance, high strength, moderate yield ratio, and good elongation. However, the pipeline steel in this patent can only meet the low-temperature toughness requirement at -20°C but fail to meet the toughness requirement at extremely low temperatures. And it is difficult to meet the tensile properties requirement after aging, especially a low yield ratio requirement. Most importantly, the achievement of performance uniformity of the pipeline steel has not been considered in this patent.

[0006] The Chinese patent application CN201310457780.8 discloses "A Low-cost Thick-gauge Subsea Pipeline Steel Plate and Manufacturing Method therefor", which comprises the following composition in mass percentage of: C: 0.04∼0.06%, Si: 0.15~0.20%, Mn: 1.43~1.47%, P≤0.008%, S≤0.003%, Ni: 0.10~0.15%, Nb: 0.030~0.040%, Ti: 0.012~0.023%, Alt: 0.015~0.025%, N≤0.006%, Nb+V+Ti≤0.15%, and a balance of iron and unavoidable impurities. The steel plate obtained has a dual-phase microstructure, achieves high strength and a low yield ratio, and it is suitable for the high deformation resistance requirements of subsea pipelines and also has good toughness. However, the pipeline steel in this patent can only meet the low-temperature toughness requirement at -30°C but fail to meet the toughness requirement at extremely low temperatures. It is difficult to meet the tensile properties requirement after aging, especially a low yield ratio requirement. Furthermore, the achievement of performance uniformity of the pipeline steel has not been considered in this patent.

[0007] The Chinese patent application CN201210492187.2 discloses "A Thick-walled High DWTT Performance X65-70 Subsea Pipeline Steel and Manufacturing Method Therefor", which comprises a composition in mass percentage of: C 0.03~0.05%, Si≤0.25%, Mn 1.47∼1.70%, P≤0.010%, S≤0.001%, Ti 0.006~0.010%, Cr 0.10~0.20%, Cu 0.12~0.20%, Ni 0.36~0.45%, Al 0.025~0.045%, Ca 0.0008~0.0025%, N≤0.0035%, O≤0.0025%, Nb 0.040~0.050%, and a balance of Fe and unavoidable impurities; and Ceq=0.34~0.040, Pcm=0.13~0.17; wherein, Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15; Pcm=C+Si / 30+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10, with a balance of iron and unavoidable impurities. The steel plate has thick specifications, high strength, excellent low-temperature impact toughness, weldability, and good DWTT performance. However, the pipeline steel disclosed in this patent can only meet a low-temperature toughness requirement at -20°C but fail to meet the requirement for low yield ratio and tensile properties after aging. Furthermore, the achievement of performance uniformity of the pipeline steel has not been considered in this patent.

[0008] The Chinese patent application CN201410515400.6 discloses "A Special Thick Specification X70 Pipeline Steel and Manufacturing Method therefor", which comprises a chemical composition in mass percentage of: C: 0.03~0.06%, Si: 0.1~0.30%, Mn: 1.30∼1.60%, P≤0.010%, S≤0.0050%, Nb: 0.030~0.050%, Ti: 0.008~0.020%, Ni: 0.10~0.30%, Cr: 0.10~0.30%, Cu: 0.10~0.30%, and a balance of Fe and unavoidable impurity elements. A pipeline steel with large thickness, high strength within a narrow range, excellent low-temperature toughness, and superior resistance to low-temperature aging and HIC is obtained. However, the pipeline steel disclosed in this patent can only meet the low-temperature toughness requirement at -30°C but fail to meet the toughness requirement at extremely low temperatures. Furthermore, the achievement of performance uniformity of the pipeline steel has not been considered in this patent.

[0009] The Chinese patent application CN201510383013.6 discloses "A Production Method for Low Yield Ratio Acid-resistant Subsea Pipeline Steel", which comprises a chemical composition of: C: 0.03~0.05%, Si: 0.10~0.30%, Mn: 1.15~1.35%, P≤0.010%, S≤0.0010%, Alt: 0.01~0.06%, Nb: 0.04~0.07%, Ti: 0.008~0.020%, Ni: 0.10~0.25%, Cr: 0.10~0.30%, and a balance of Fe and unavoidable impurity elements. This patent adopts advanced smelting and continuous casting technology to control the narrow fluctuation range of carbon equivalent and the quantity, size and morphology of inclusions. It also employs a relaxation phase transformation process to control the structure morphology of the steel plate and adopts a new water-cooling control mode to improve the uniformity of water cooling of the steel plate, effectively ensuring that the steel plate has a low yield ratio and excellent acid resistance. However, the achievement of performance uniformity of the pipeline steel has not been considered in this patent.

[0010] The Chinese patent application CN202011039510.1 discloses "A Subsea Pipeline Steel Plate Coil with Excellent Low-temperature CTOD Performance and Production Process therefor", which comprises a chemical composition in mass percentage of: C: 0.030~0.070%, Si: 0.10~0.20%, Mn: 1.10~1.60%, Nb: 0.020~0.050%, V: 0.020~0.040%, Ti: 0.010~0.030%, Mo: 0.10~0.20%, Alt≤0.06%, S≤0.0080%, P≤0.018%, and a balance of Fe and unavoidable impurity elements, so that the subsea pipeline steel plate coil has excellent CTOD performance. However, the achievement of performance uniformity of the pipeline steel has not been considered in this patent.

[0011] A review of existing patent research reveals that, while the strength and low-temperature toughness are considered in current subsea pipeline steels, there has been little research on how to ensure that the pipeline steel not only have high performance but also have good performance uniformity.Summary

[0012] The objective of the present invention is to provide a high-performance-uniformity subsea pipeline steel and manufacturing method therefor. The pipeline steel has high strength and good low-temperature toughness, and at the same time, the uniformity of performance of the pipeline steel can also be ensured.

[0013] In order to achieve the above objective, the technical solution of the present invention is as follows: a high-performance-uniformity subsea pipeline steel, comprises a chemical composition in weight percentage of: C: 0.02~0.06%, Si: 0.10~0.30%, Mn: 1.20~1.65%, Cu≤0.30%, Ni≤0.30%, Cr: 0.15~0.30%, Mo≤0.15%, Nb: 0.03~0.05%, V≤0.05%, Ti: 0.008~0.020%, Ca: 0.0012~0.0030%, Al: 0.015~0.045%, and a balance of Fe and other unavoidable impurities; the impurity elements shall be controlled as follow: P≤0.01%, S≤0.001%, B≤0.0004%, N≤0.005%, O≤0.0025%, and H≤0.0002%; and simultaneously satisfying: Ti / N≥2; Nb / 7.7 C + V / 4.3 C ≥ 0.10 ; 1.5 ≤ Ca / S ≤ 4 ; 0.30 ≤ Ceq ≤ 0.39 , Ceq = C + Mn / 6 + Cr + V + Mo / 5 + Cu + Ni / 15 ; a welding cold crack sensitivity index Pcm is ≤0.17, and Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B.

[0014] Further, the balance is Fe and other unavoidable impurities.

[0015] The microstructure of the subsea pipeline steel described in the present invention is ferrite + bainite, wherein, a volume fraction of the ferrite is 20%~40%.

[0016] The subsea pipeline steel described in the present invention has a grain size grade of ≥ grade 9.

[0017] The transverse tensile properties of the subsea pipeline steel described in the present invention are as follows: yield strength R t0.5 :415~555MPa, tensile strength R m : 515~670MPa, and a yield ratio R t0.5 / R m of ≤0.88; and a difference in strength between the head and tail of the steel plate is ≤50MPa; longitudinal tensile: yield strength R t0.5 : 405~530MPa, tensile strength R m : 505~650MPa, and a yield ratio R t0.5 / R m of ≤0.86; and a difference in strength between the head and tail of the steel plate is ≤50MPa; after a strain aging test with a 3% pre-strain and holding at 250°C for 1 hour, the longitudinal tensile properties of the steel plate are as follows: yield strength R t0.5 : 405~530MPa, tensile strength R m :505~650MPa, and a yield ratio R t0.5 / R m of ≤0.88.

[0018] A Charpy impact energy of the subsea pipeline steel described in the present invention at -45°C is ≥250J, and a difference in Charpy impact energy between the thickness center of the steel plate and the near-surface of the steel plate is ≤50J; a SA of DWTT test at -30°C is≥85%, and a difference in SA between a single-sided thinned DWTT sample and full-thickness DWTT sample is ≤5%; a hardness is ≤230HV10, wherein, a difference between the surface hardness of the steel plate and center hardness of the steel plate is ≤25HV10, preferably a difference between the surface hardness of the steel plate and center hardness of the steel plate is ≤15HV10.

[0019] The characteristics of the composition design of the subsea pipeline steel of the present invention are as follow: 1. By adopting an alloy composition system of low C and low Mn, and controlling 0.30≤Ceq≤0.39 and Pcm≤0.17, good weldability is achieved under the premise of the steel meeting the requirement for strength and toughness. At the same time, central segregation and banded structures are improved, and the isotropy of material performance is guaranteed. 2. The amount of impurity elements P, S, B, N, O, and H is controlled at extremely low levels, so that the number of inclusions is decreased, the degree of segregation is reduced, and the strength and toughness is ensured, and it is conducive to ensure excellent low-temperature toughness of the pipeline steel in particularly. 3. The addition of alloy elements such as Cu, Ni, Cr, and Mo exerts a solid solution strengthening effect, simultaneously affects the phase transformation during a cooling process of the material, and improves the isotropy of the steel plate properties. 4. By adding trace amount of V-Nb-Ti and strictly limiting Ti / N≥2 and (Nb / 7.7C+V / 4.3C)≥0.10, micro-alloy carbonitrides are formed, grains are refined, precipitation are strengthened, C and N atoms are fixed , and the effects of strain aging on material performance are reduced.

[0020] C: A most basic strengthening element, carbon dissolves in a steel to form interstitial solid solution and exerts a solid solution strengthening effect. When C forms carbides precipitation with strong carbide-forming elements, it helps to precipitation strengthening. However, with the increase of C content, the ductility, toughness and weldability of steel are decreased significantly, and central segregation in the continuous-casting slab occurs, which is not conducive to the uniformity of the performance of steel plate in the thickness direction. Furthermore, after strain aging of the steel plate, free C atoms may entangle with dislocations, hinder the movement of dislocations, and increase the yield strength, which is not conducive to the maintenance of a low yield ratio. Therefore, the content of C is controlled at 0.02%~0.06% in the present invention.

[0021] Mn: A solid solution strengthening element, Mn is a most important and economical strengthening element in the steel to compensate the loss of strength caused by the decrease of the content of C. Mn is also an element that expands the γ-phase, and Mn may improve the hardenability of steel, reduce the γ→α phase transformation temperature of steel, help to obtain fine phase transformation products, and improve the toughness of steel. However, Mn is also an element that is prone to segregate. When the content of Mn is too high, central segregation in the continuous casting slab tends to occur, hard-phase structures are prone to be formed during a cooling process after rolling, a low-temperature toughness of the material is reduced, and it is also not conductive to the uniformity of the performance of steel plate in the thickness direction. Therefore, the content of Mn is controlled at 1.20~1.65% in the present invention.

[0022] Si: A solid solution strengthening element, and it is also a deoxidation element in the steel. However, when the content of Si is too high, it may deteriorate the welding performance of steel, especially the toughness of a welding heat-affected region, and it is not conducive to the removal of iron oxide scale during a hot rolling process. Therefore, the content of Si is controlled at 0.10~0.30% in the present invention.

[0023] Cu and Ni: Solid solution strengthening elements, and they may improve the toughness of the steel. They also help to improve the hardenability of steel and delaying the transformation of pearlite during a cooling process of the steel plate. However, when the addition of Cu and Ni is too high, it is not conducive to the toughness and welding performance of the steel plate. Therefore, the content of Cu and Ni is each controlled at ≤30% in the present invention.

[0024] Cr and Mo: Solid solution strengthening elements, and important elements for improving the hardenability of the steel. They promote the formation of acicular ferrite, improve the toughness of steel, and are conducive to the uniformity of microstructure and performance across the full thickness of the steel plate; however, when the content is too high, they may promote the formation of low-temperature phase transformation structures, and are not conducive to the low-temperature toughness. Therefore, the content of Cr is controlled at 0.15~0.30%; and Mo is controlled at ≤0.15% in the present invention.

[0025] Nb: One of the important elements in low-carbon micro-alloying steel. Solid-soluted Nb precipitates as Nb(C,N) particles via strain-induced precipitation during a hot rolling process, which may delay austenite recrystallization, increase the austenite recrystallization temperature of steel, make a two-stage controlled rolling possible, prevent the formation of mixed grains, help to obtain a uniform and fine structure, and improve the strength and toughness of the material while the isotropy of the steel plate properties is ensured. Furthermore, during a cooling process, solid-soluted Nb precipitates dispersedly as the second-phase particles NbC in the matrix, contributing to precipitation strengthening. However, when the content of Nb is too high, it cannot be fully solid-solubilized, which not only fails to exert its effect but also increases production costs. And it may lead to premature precipitation of NbC at a high temperature, and larger NbC precipitates at central segregation regions tend to be formed, which is not conducive to the toughness. Therefore, the content of Nb is controlled at 0.03~ 0.05% in the present invention.

[0026] V: One of the important micro-alloying elements, it tends to combine with C in the steel to form VC, having a relatively high precipitation strengthening effect and a relatively weak grain refinement effect. However, when the content of V is too high, the precipitation particles will coarsen significantly, and the low-temperature toughness of steel is reduced. In the present invention, V mainly helps to strength precipitation to make up for strength reduction caused by the deficiency of other alloys. Therefore, the content of V is controlled at ≤0.05% in the present invention.

[0027] In addition, in order to fully utilize the carbon fixation effect of Nb and V, and reduce the impact of free carbon on strain aging properties, it is necessary to control (Nb / 7.7C+V / 4.3C) ≥ 0.10. In some embodiments, (Nb / 7.7C+V / 4.3C) is controlled at 0.10-0.50, such as 0.10-0.47.

[0028] Ti: Ti is a strong carbonitride-forming element that may play a role in nitrogen fixation. During a slab continuous casting process, it may form fine and high-temperature stable TiN precipitates, thereby improving the toughness of the material. In addition, the undissolved carbonitrides of Ti may prevent the growth of austenite grains during heating process of the steel, and TiN and TiC precipitated in high-temperature austenite zone during a rough rolling process may effectively inhibit the growth of austenite grains. Furthermore, TiN and TiC particles in steel may significantly prevent grain growth in the heat-affected region during a welding process, thereby improving the welding performance of the steel plate. Therefore, the content of Ti is controlled at 0.008~0.020% in the present invention; meanwhile, to fully utilize Ti's nitrogen fixation effect, it is necessary to control Ti / N ≥2. In some embodiments, Ti / N is controlled at 2.0~5.0.

[0029] Ca: Ca is mainly used to modify MnS inclusions, resulting in the morphology of the inclusions spheroidized and uniform distributed, helping to improve low-temperature toughness. However, when the content of Ca is too high, it may tend to form CaO inclusions, which is not conductive to toughness. Therefore, the content of Ca is controlled at 0.0012%~0.0030% in the present invention. Meanwhile, to fully utilize the modifying effect of Ca on sulfides, Ca / S is controlled at 1.5≤Ca / S≤4, resulting in elongated MnS inclusions convert into dispersed spherical CaS inclusions, and a low-temperature toughness of the material is improved.

[0030] Al: Al is mainly used for deoxidation of steel. Adding an appropriate amount of Al helps to refine grains and improve the strength and toughness of steel. However, with the increase of the content of Al, oxides of Al will be formed in the steel, thereby reducing the toughness of the base material and the welding heat-affected region. Therefore, the content of Al is controlled at 0.015%~0.045% in the present invention.

[0031] S, P, B, N, O, H: Unavoidable impurity elements in steel. P is an element which is prone to segregation; and S tends to form sulfides in steel, which both may significantly reduce the low-temperature toughness of steel. B tends to precipitate at grain boundaries, resulting in a decrease in the plasticity and toughness of the material. When the content of N and H is too high, cracks tend to occur in a continuous-casting slab. When the content of O is too high, various oxides are formed, thereby reducing hot workability, corrosion resistance, and toughness. Therefore, the contents of the impurity elements are controlled as follows: P≤0.01%, S≤0.001%, B≤0.0005%, N≤0.005%, O≤0.0025%, and H≤0.0002% in the present invention.

[0032] Pcm: An indicator of welding cold crack sensitivity. When Pcm exceeds a certain threshold, the hardenability of steel becomes too great, resulting in a significant decrease in the toughness of the welding region. Pcm is controlled at ≤0.17 in the present invention. In some embodiments, Pcm is controlled at 0.12~0.17.

[0033] By adopting an alloy composition system of low C and low Mn in the present invention, center segregation is reduced under the premise of the steel meeting the requirement for strength. By combining the effects of Cr, Mo, Cu, and Ni on enhancing hardenability and inhibiting ferrite transformation, the ferrite content is controlled at 20~40%, center segregation is reduced further, good uniformity in the thickness direction of the steel plate is ensured, a difference in SA% between a single-sided thinned DWTT sample and a full-thickness DWTT sample is ≤5%, and a difference between the surface hardness and center hardness of the steel plate is ≤25HV10. Furthermore, by combining C, Mn, Cu, Ni, Cr, Mo, and V, the Ceq is controlled at 0.30≤Ceq≤0.39, ensuring the strength performance of the steel plate.

[0034] Typically, micro-alloying elements such as Nb and V are added in pipeline steel to strengthen grain refinement and precipitation. However, in the present invention, by utilizing the fixation effect of Nb and V on C atoms, (Nb / 7.7C+V / 4.3C) ≥ 0.10 is designed. By maximizing the fixation effect of Nb and V on C atoms, the influence of free carbon on strain aging properties is reduced, and the unconventional performance requirement for tensile properties of the subsea pipeline steel after strain aging is satisfied. After a strain aging test with a 3% pre-strain and holding at 250°C for 1 hour for the subsea pipeline steel described in the present invention, the longitudinal tensile properties of the steel plate are as follows: yield strength R t0.5 : 405~530MPa, tensile strength R m :505~650MPa, and a yield ratio R t0.5 / R m of ≤ 0.88.

[0035] The content of ferrite is controlled at 20%~40% in the present invention. it is crucial for a suitable proportion of ferrite obtained to meet the performance requirement of a subsea pipeline steel. When the content of ferrite is too low, a low yield ratio requirement can't be satisfied; when the content of ferrite is too high, strength of the steel is insufficient. Therefore, in terms of composition design, by adding the alloy elements such as Cu, Ni, Cr, and Mo, a certain degree of austenite stability is obtained, and the amount of ferrite transformation may be controlled within a desired range.

[0036] The manufacturing method of the high-performance-uniformity subsea pipeline steel described in the present invention comprises the following steps:1) Smelting and continuous casting

[0037] The above composition is smelt and continuous cast into a slab; during the continuous casting process, a superheat of the tundish is controlled at 15~40°C, and a casting speed of the continuous casting slab is controlled at 0.40-1.1Om / min, ensuring that the solidification end of the continuous casting slab is located in the horizontal section of a continuous casting caster; and an electromagnetic stirring process is adopted in the continuous casting mold;2) Controlled rolling

[0038] A heating temperature is 1120~1190°C, a start rolling temperature of rough rolling is 1000 -1100°C, a finish rolling temperature of rough rolling is 930~980°C, a reduction ratio of rough rolling is ≥50%, and a spread ratio is 1.20-1.80; a start rolling temperature of finish rolling is 790~880°C, a finish rolling temperature of finish rolling is 780-850°C, and a reduction ratio of finish rolling is ≥70%;3) Controlled cooling

[0039] A relaxation time is ≥30s, a start cooling temperature is 730~780°C, a cooling rate is 10~40°C / s, a stop cooling temperature is 350~550°C, and a temperature difference between the upper and lower surfaces of the steel plate after stop cooling is ≤30°C;4) After stop cooling, the steel plate is air-cooled or stack-cooled to room temperature.

[0040] Preferably, in step 4), when a stop cooling temperature is ≤450°C, the steel plate is tempered at 450~650°C for 10~30min.

[0041] Preferably, in step 1), a fluctuation value of the superheat of adjacent slabs is ≤10°C and / or a fluctuation of the casting speed of adjacent slabs is ≤10%.

[0042] Preferably, in step 1), a dynamic soft reduction process is carried out at the solidification end of the continuous casting slab, with a reduction amount of 2~3mm.

[0043] Preferably, in step 3), a temperature difference of water entry between the head and tail of the steel plate is ≤30°C.

[0044] Preferably, in step 4), after the steel plate is cooled to room temperature, the head and tail portions of the steel plate are cropped, with a cropping amount of ≥250mm each, such as 250~350mm. In some embodiments, a cropping amount of the head is 250~350mm, and a cropping amount of the tail is 250~300mm.

[0045] In the manufacturing method described in the present invention: To reduce center segregation in the continuous-casting slab and improve the low-temperature toughness and isotropy of properties in the steel plate, the following requirements should be satisfied in the continuous-casting process as follow: (1) a superheat of the tundish is controlled at 15~40°C, and a fluctuation value of the superheat of adjacent slabs is ≤10°C; (2) a casting speed of the continuous casting slab is controlled at 0.40~1.10m / min, with a casting speed controlled constant, and a fluctuation of the casting speed of adjacent slabs is ≤10%, ensuring that the solidification end of the continuous casting slab is located at the horizontal section of a continuous casting caster; (3) an electromagnetic stirring process is used in the continuous casting mold section ; (4) a dynamic soft reduction technology is adopted at the solidification end of the continuous casting slab.

[0046] A superheat of the tundish and a casting speed of the continuous-casting slab are controlled, and a fluctuation range of the casting speed is strictly limited in the present invention, so that the continuous casting process is stable, and the solidification end of the continuous-casting slab is located in the horizontal section of a continuous casting caster, and the effectiveness of a dynamic soft reduction is maximized, thereby ensuring good control of center segregation in the slab. Meanwhile, electromagnetic stirring is adopted in a continuous casting process, effectively resulting in the uniform distribution of elements, thereby reducing the segregation tendency and ensuring the uniformity of the performance of the pipeline steel.

[0047] A heating temperature of the slab is 1120~1190°C. when a heating temperature is too low, the alloy carbides fail to be fully dissolved, resulting in fail to fully play a role of solid solution strengthening and precipitation strengthening, and it is not conducive to the control of center segregation. When a heating temperature is too high, grain coarsening occurs, and it has an adverse effect on the low-temperature toughness of the steel plate.

[0048] A start rolling temperature of rough rolling is 1000~1100°C, and a finish rolling temperature of rough rolling is 930~980°C. A reduction ratio of rough rolling is ≥50%, and a spread ratio is 1.20~1.80. Rough rolling is carried out in a fully recrystallized austenite zone to avoid the occurrence of mixed grains. A sufficient reduction ratio of rough rolling can fully refine the original austenite grains, and is beneficial to the strength and toughness of the steel plate. At the same time, precise control of the spread ratio is conducive to control of plate shape and reduces the performance difference between the transverse and longitudinal directions of the steel plate. In some embodiments, a reduction ratio of rough rolling is 50~80%, such as 50~74%.

[0049] A start rolling temperature of finish rolling is 790~880°C, a finish rolling temperature of finish rolling is 780~850°C, and a reduction ratio of finish rolling is ≥70%. Finish rolling is carried out in a non-recrystallized austenite zone. A sufficient reduction ratio of finish rolling may fully flatten the austenite grains, and provide more nucleation sites for subsequent phase transformation, thereby refining the final structure and improving the strength and toughness of the steel plate. In some embodiments, a reduction ratio of finish rolling is 70~85%.

[0050] A relaxation time is ≥30s, a start cooling temperature is 730~780°C, a cooling rate is 10~40°C / s, and a stop cooling temperature is 350~550°C. An appropriate cooling rate and stop cooling temperature may ensure the formation of a dual-phase structure of ferrite + bainite, guaranteeing both the strength and toughness of the steel plate while achieving a low yield ratio.. A certain relaxation time and start cooling temperature may ensure that a certain amount of pro-eutectoid ferrite is generated in the structure of steel plate, thereby reducing a yield ratio while maintaining strength. A temperature difference of water entry between the head and tail of the steel plate is ≤30°C, and reducing the temperature difference of water entry between the head and tail of the steel plate is beneficial for ensuring the isotropy of the steel plate properties. A temperature difference between the upper and lower surfaces of the steel plate after cooling is ≤30°C. Reducing the temperature difference between the upper and lower surfaces of the steel plate after cooling is beneficial for improving the isotropy of properties in the thickness direction. In some embodiments, a relaxation time is 30~60s.

[0051] A relatively high finish rolling temperature, a relatively low finish cooling temperature, and a relatively fast cooling rate are beneficial for inhibiting the enrichment of alloy elements towards the Center of thickness of the plate during a phase transformation process, thereby further reducing the impact of center segregation.

[0052] After stops cooling, the steel plate is cooled to room temperature. Preferably, it may be cooled to room temperature by stack-cooled.

[0053] Preferably, for the steel plate with a stop cooling temperature of ≤450°C, tempering may be performed at 450~650°C for 10~30 minutes.

[0054] After stop cooling, the steel plate may usually be cooled to room temperature by air cooling on a cold bed; or the steel plates after rolling are stacked together and cooled to room temperature.

[0055] A cooling rate of the stack-cooled is relatively slow, so that the steel plate may stay at a high temperature for a longer period of time, which is conducive to the precipitation of alloy carbides and fully play a role of precipitation strengthening and carbon fixation. For the steel plate with a lower stop cooling temperature, the precipitation time of alloy carbides is insufficient. By utilizing short-term high-temperature tempering, it gives full play to the role of alloy precipitation strengthening and carbon fixation, and it is beneficial for the decomposition of hard phase MA constituents and improving low-temperature strength and toughness.

[0056] A cropping amount of the head and tail of the finished steel plate after rolling is ≥250mm. Due to significant differences in deformation conditions, cooling conditions between the head / tail section and the main body section of the steel plate, there are significant differences in structure and performance between the head / tail section and the main body section of the steel plate. By ensuring a certain cropping amount of the head and tail, the influence of deformation and cooling independent of positions is eliminated, which is beneficial for achieving the isotropy of the steel plate properties.

[0057] Compared with the prior art, the present invention has the following advantages: In terms of component design, by adding a relatively low amount of C and Mn elements, controlling the amount of impurity elements such as P, S, B, N, O, and H at an extremely low levels, and controlling 0.30≤Ceq≤0.39 and Pcm≤0.17, in combination with the addition of alloy elements such as Cr, Mo, Cu, and Ni in the present invention, central segregation is improved, phase transformation behavior during a cooling process of the material is affected, the stability of austenite is increased, and the ferrite phase transformation process is delayed, and the requirement of good uniformity of the performance in steel plate is satisfied on the basis of ensuring the mechanical performance of the steel plate.

[0058] By utilizing the addition of trace amounts of V-Nb-Ti, and strictly limiting Ti / N≥2 and (Nb / 7.7C+V / 4.3C) ≥0.10, micro-alloying carbonitrides are formed, grains are refined and precipitation are strengthened, C and N atoms are fixed, and the effects of strain aging on material performance are reduced. After strain aging, the steel still has high performance. After a strain aging test with a 3% pre-strain and holding at 250°C for 1 h, the steel plate has the following longitudinal tensile properties: yield strength R t0.5 : 405~530MPa, tensile strength R m : 505~650MPa, and a yield ratio R t0.5 / R m of ≤0.88.

[0059] Based on the composition design, a superheat of the tundish, a continuous casting speed, and a fluctuation range are controlled strictly in a continuous casting process, a dynamic soft reduction process is adopted, so that the solidification end of the continuous casting slab is located in the horizontal section of a continuous casting caster, the effect of dynamic soft reduction is maximized, thereby reducing the segregation tendency and ensuring the uniformity of pipeline steel properties. However, there are no specific requirements or low requirements for a superheat of the tundish, a continuous casting speed, and a fluctuation range in the conventional continuous casting process. Parameter settings are primarily determined by matching production rhythms between the previous and subsequent processes. Therefore, the solidification end of the continuous casting slab is often located in the curved section of the continuous casting caster, so that it limits the effectiveness of dynamic soft reduction applied in the horizontal section to improve the quality of the internal slab and reduce central segregation, ultimately failing to meet the requirements for high performance and uniformity.

[0060] In addition, by using a two-stage controlled rolling technology and spread ratio control in a recrystallized austenite zone and non-recrystallized austenite zone, and ensuring sufficient compression ratio during the two-stage rolling process, grain refinement is fully achieved, and mixed crystal structure is avoided to occur. A rolling temperature, a relaxation time, and a start cooling temperature are controlled to control the ferrite transformation process from both thermodynamic and kinetic aspects, thereby forming a dual phase structure of ferrite and bainite. And the proportion of ferrite is accurately controlled at 20-40%, so that the steel plate has excellent mechanical performance such as strength and toughness, and isotropy, and the issue of mismatched between the strength and uniformity of the performance of subsea pipeline steel is solved.Description of the drawings

[0061] Fig.1 shows the microstructure photograph of the steel in Example 1 of the present invention.Detailed Description

[0062] The present invention will be further explained and illustrated below with reference to the drawings.

[0063] Table 1 and 2 list the chemical composition of the steel in the Examples of the present invention, with a balance of Fe and unavoidable impurities; Table 3 and 4 list the specific process parameters.

[0064] Tensile properties test: Tensile properties tests were conducted on the steel plates obtained from each example of the present invention at room temperature according to ASTM A370-2021. A yield strength, a tensile strength, and a yield ratio of the steel plates of each example were tested. And the longitudinal tensile properties of the steel plates after a strain aging with 3% pre-strain and holding at 250°C for 1 hour were tested. The specific results are shown in Table 5.

[0065] Low-temperature toughness test: According to ASTM E23-2018, impact tests were conducted on the test steel plates at -45°C using V-notched samples, and the Charpy impact energy at -45°C for the steel plate of each example was measured. The specific results are shown in Table 6. According to API RP 5L3-2014, drop weight tear tests (DWTT) were conducted on the test steel plates at -30°C using full-thickness V- notched samples and single-sided thinned thickness V- notched samples respectively, and the SA value of DWTT test at -30°C for the steel plate of each example was tested. The specific results are shown in Table 7.

[0066] Hardness test: According to ASTM E92, the hardness of the steel plate near the upper surface, the hardness of the steel plate at the thickness center, and the hardness of the steel plate near the lower surface were measured under a load of 10kgf. The specific results are shown in Table 8.

[0067] The testing method of grain size was in accordance with ASTM E112.

[0068] As can be seen from Figure 1, the microstructure of the steel plate obtained by using the composition and process described in the present invention is ferrite + bainite, with a ferrite content of 20~40%.

[0069] As shown in Table 5, the transverse tensile properties of the steels of the present invention are as follows: yield strength R t0.5 : 415~555MPa, tensile strength Rm: 515~670MPa, and a yield ratio R t0.5 / Rm of 0.88, and a difference in strength between the head and tail of the steel plate is ≤50MPa.

[0070] Longitudinal tensile properties: yield strength R t0.5 : 405~530MPa, tensile strength Rm: 505~650MPa, and a yield ratios R t0.5 / R m of 0.86, and a difference in strength between the head and tail of the steel plate is ≤50MPa.

[0071] After a strain aging test with a 3% pre-strain and holding at 250°C for 1 hour, the longitudinal tensile properties of the steel plate are as follows: yield strength R t0.5 : 405~530MPa, tensile strength Rm: 505~650MPa, and a yield ratio Rt0.5 / Rm of 0.88.

[0072] As can be seen from Table 6, the steel plate obtained in the present invention has a Charpy impact energy of ≥250J at -45°C, and a difference in Charpy impact energy between the thickness center of the steel plate and the near-surface of the steel plate is ≤50J.

[0073] As can be seen from Table 7, the steel plate obtained in the present invention has a SA of ≥85% in the DWTT test at -30°C. Specifically, a difference in SA between a single-sided thinned DWTT sample (Test was conducted in accordance with API RP 5L3 with the temperature progressively lowered) and a full-thickness DWTT sample is ≤5%.

[0074] As can be seen from Table 8, the steel plate obtained in the present invention has a hardness of ≤230HV10. Specifically, a difference between the near-surface hardness and center hardness of the steel plate is ≤25HV10.

[0075] In Comparative Example 1, Ti / N=1.5 and Ca / S=1.12, neither of which meets the requirements of the present invention: Ti / N≥2 and 1.5≤Ca / S≤4. Moreover, the contents of P, S, and N are relatively high, and the casting speed waveform is not within the control range specified in the present invention during process control, which cannot satisfy the condition that the solidification end of the continuous casting slab is located in the horizontal section of a continuous casting caster. Furthermore, a fluctuation value of the superheat, a temperature difference of water entry between the head and tail of the steel plate, and a temperature difference between the upper and lower surfaces of the steel plate after cooling are all not within the control range specified in the present invention. Although the strength of the steel plate obtained meet the requirements, a difference in strength between the head and tail of the steel plate is too large, exceeding 50 MPa, and the yield ratio is relatively high. Furthermore, a difference in Charpy impact energy between the Center of thickness and near-surface of the steel plate is 66, and a difference in SA between the single-sided thinned DWTT sample and the full-thickness DWTT sample is 14, and a difference between the surface hardness and the center hardness of the steel plate is 35. Obviously, the uniformity of the performance of the steel plate obtained in Comparative Example 1 is poor.

[0076] In Comparative Example 2, Nb / 7.7C+V / 4.3C=0.09, which fails to meet the requirement of the present invention: (Nb / 7.7C+V / 4.3C) ≥ 0.10. Furthermore, the contents of B and O are relatively high, and a heating temperature, a rough rolling temperature of finish rolling, and a relaxation time in process control are not within the range specified in the present invention. As a result, a Charpy impact energy at -45°C is less than 250J, a SA of DWTT test at -30°C is less than 85%, and the low-temperature toughness of the steel plate is poor.

[0077] In Comparative Example 3, Ceq=0.40, Pcm=0.18, and Nb / 7.7C+V / 4.3C=0.07, none of which meets the requirements of the present invention: 0.30≤Ceq≤0.39, Pcm≤0.17, and (Nb / 7.7C+V / 4.3C) ≥0.10. A difference in SA between the single-sided thinned DWTT sample and the full-thickness DWTT sample of the steel plate obtained is 13, and a difference in Charpy impact energy between the Center of thickness and near-surface of the steel plate is 115J. The uniformity of the performance of the steel plate obtained in Comparative Example 3 is poor. Table 1 (unit: weight percentage)CSiMnPSCuNiCrMoVNbTiCaAltBNOHEx.10.020.101.450.00750.00100.300.300.160.110.0200.0300.0100.00150.04500.00040.00500.00250.00020Ex.20.030.151.200.08800.00080.1000.300.150.0300.0390.0080.00260.02800.00030.00300.00210.000170Ex.30.050.181.570.00980.000900.080.24000.0400.0120.00140.03340.00030.00380.00180.00015Ex.40.0250.221.220.00760.00070.250.100.190.040.0150.0330.0180.00210.02200.00040.00430.00170.00017Ex.50.0400.201.380.00830.00080.170.260.150.080.0400.0370.0150.00280.01900.00030.00370.00230.00016Ex.60.0310.191.240.00630.00060.190.150.2600.0370.0450.0110.00190.03300.00030.00400.00190.00015Ex.70.0350.301.650.01000.00080.130.140.180.120.0500.0350.0200.00300.01500.00040.00400.00220.00018Ex.80.0410.231.230.00700.00060.200.170.250.0600.0410.0130.00160.02870.00020.00390.00190.00016Ex.90.0600.271.620.00840.00080.050.060.2000.0100.0500.0160.00120.03100.00030.00330.00200.00019Comp.Ex.10.040.191.380.01450.00250.170.260.150.080.0400.0380.0080.00280.01900.00030.00550.00230.00016Comp.Ex.20.050.181.570.00980.00090.000.080.240.000.0000.0330.0120.00140.0334000060.00380.00350.00015Comp.Ex.30.070.271.700.00840.00080.050.060.2000.0100.0200.0160.00120.03100.00030.00330.00200.00019 Table 2 CeqPcmTi / NNb / 7.7C+V / 4.3CCa / SEx.10.360.132.00.431.50Ex.20.330.122.70.403.25Ex.30.360.153.20.101.56Ex.40.300.124.20.313.00Ex.50.350.154.10.353.50Ex.60.320.132.70.473.17Ex.70.390.155.00.463.75Ex.80.330.143.30.132.67Ex.90.380.174.80.141.50Comp.Ex.10.350.151.50.361.12Comp.Ex.20.360.153.20.091.56Comp.Ex.30.400.184.80.071.50 Note: Ceq=C+Mn / 6+(Cr+V+Mo) / 5+(Cu+Ni) / 15; Pcm= C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B. Table 3 A superheat of the tundish °CA fluctuation value of the superheat °CA casting speed of the continuous casting slab m / minA fluctuation of the casting speed %A heating temperature °CA start rolling temperature of rough rolling °CA finish rolling temperature of rough rolling °CA reduction ratio of finish rolling %A spread ratioA start rolling temperature of finish rolling °CA finish rolling temperature of finish rolling °CA reduction ratio of finish rolling %Ex.14051.00911301070940581.4081081073Ex.22061.10811701010945631.6082082082Ex.3255.50.55511521020960651.2079079075Ex.430100.40711901090975601.8087082070Ex.51570.70411401060970551.4283080078Ex.6234.51.051011801100980701.5088085079Ex.71880.80611201000930501.3080078080Ex.8285.70.65511601040950741.4886084078Ex.93590.905.511381080938691.7085083085Comp.Ex. 125120.701511401060970551.4283080078Comp.Ex. 22550.55512001040910651.2079079075Comp.Ex. 33590.90511381080938691.7085083085 Table 4 A relaxation time sA start cooling temperature °CA temperature difference of water entry between the head and tail of the steel plate°CA cooling rate °C / sA stop cooling temperature °CA temperature difference between the upper and lower surfaces of the steel plate after stop cooling°CA cooling method of the steel plateA tempering temperature °CA tempering time minA cropping amount of the head of the steel plate mmA cropping amount of the tail of the steel plate mmEx.15076018303808stack-cooled--300250Ex.255770231840018stack-cooled55020250250Ex.335745152046010stack-cooled--350300Ex.460730202555020stack-cooled--300250Ex.545750181544525air-cooled65010300250Ex.633775284050012air-cooled--350300Ex.740780301035030stack-cooled45030250250Ex.830765253345015stack-cooled--350300Ex.94274026264205air-cooled--350300Comp.Ex.145750511545042air-cooled--300250Comp.Ex.210760152046010stack-cooled--10050Comp.Ex.34274026264205air-cooled--350300 Table 5 Sample positionTransverse tensileLongitudinal tensileLongitudinal tensile after strain agingYield strength R t0.5 MPaTensile strength Rm MPaA difference in strength between the head and tail of the steel plate MPaYield ratio R t0.5 / R mYield strength R t0.5 MPaTensile strength Rm MPaA difference in strength between the head and tail of the steel plate MPaYield ratio R t0.5 / R mYield strength R t0.5 MPaTensile strength Rm MPaYield ratio R t0.5 / RmEx.1The head of the steel plate46056012 / 200.824575588 / 190.824595660.81The tail of the steel plate4485400.834495390.834665650.82Ex.2The head of the steel plate50059050 / 500.854855755 / 90.844885880.83The tail of the steel plate4505400.834805660.854895770.85Ex.3The head of the steel plate51359717 / 170.8648656414 / 100.864955680.87The tail of the steel plate4965800.864725540.854865660.86Ex.4The head of the steel plate43853023 / 150.8342851828 / 130.834305200.83The tail of the steel plate4155150.814055050.804055050.80Ex.5The head of the steel plate49560028 / 120.8348558350 / 500.834795860.82The tail of the steel plate4675880.794355330.824685680.82Ex.6The head of the steel plate4985683 / 40.884885661 / 20.864995700.88The tail of the steel plate4955720.874895680.864955720.87Ex.7The head of the steel plate55567050 / 200.8353065026 / 60.825306500.82The tail of the steel plate5056500.785046440.785156480.79Ex.8The head of the steel plate51358113 / 120.884935708 / 70.865095880.87The tail of the steel plate5005690.884855630.865035770.87Ex.9The head of the steel plate53566020 / 500.8152863911 / 60.835256480.81The tail of the steel plate5156100.845176330.825306350.83Comp.Ex. 1The head of the steel plate54261055 / 5208952160154 / 500875446110.89The tail of the steel plate4875580.874675510.854955880.84Comp.Ex. 2The head of the steel plate55961757 / 160.9152359735 / 250.885656040.94The tail of the steel plate5026330.794886220.785286330.83Comp.Ex. 3The head of the steel plate59668011 / 310.885786615 / 160875956670.89The tail of the steel plate5856490.905736450.895886500.90 Table 6 Sample positionCVN1 JCVN1 JCVN1 JCVN-avg JDifferenceEx.1Center of thickness25026827826550Near-surface327318301315Ex.2Center of thickness33034531733119Near-surface359340351350Ex.3Center of thickness41141440240943Near-surface465438453452Ex.4Center of thickness31531233031930Near-surface346342358349Ex.5Center of thickness32034333133118Near-surface342352354349Ex.6Center of thickness42942442242513Near-surface453434427438Ex.7Center of thickness33631432232419Near-surface354322354343Ex.8Center of thickness43042741142318Near-surface457439428441Ex.9Center of thickness31430730931036Near-surface358337343346Comp.Ex.1Center of thickness18825522322266Near-surface286279299288Comp.Ex.2Center of thickness19522523121737Near-surface232214241229Comp.Ex.3Center of thickness235247211231115Near-surface325338321346 Table 7 Sample typeSA %SA %SA-avg %DifferenceEx.1A full-thickness sample8988891A single-sided thinned sample899190Ex.2A full-thickness sample8585855A single-sided thinned sample889290Ex.3A full-thickness sample9087892A single-sided thinned sample909191Ex.4A full-thickness sample8686864A single-sided thinned sample899190Ex.5A full-thickness sample9090904A single-sided thinned sample929594Ex.6A full-thickness sample9593942A single-sided thinned sample959696Ex.7A full-thickness sample8688875A single-sided thinned sample958892Ex.8A full-thickness sample9293934A single-sided thinned sample989597Ex.9A full-thickness sample8790892A single-sided thinned sample948791Comp.Ex.1A full-thickness sample67817414A single-sided thinned sample888788Comp.Ex.2A full-thickness sample6977234A single-sided thinned sample787677Comp.Ex.3A full-thickness sample68727013A single-sided thinned sample808683 Table 8 Near the upper surfaceCenter of ThicknessNear the lower surfaceDifferenceGrain sizehardnesshardnesshardnesshardnesshardnesshardnesshardnesshardnesshardnesshardnesshardnessEx.1210203194211208194200196199410E9x.219719519620220320719119920089Ex.3218213214200219212213215218511Ex.420320821521019019620219120159Ex.5198198203214214192196203195810Ex.6201210203197190207202204203611Ex.720520721821723021521521320999Ex.8201205200204201190208207205610Ex.92082112021961952072202122121211Comp.Ex.1197201200233238231195205199359Comp.Ex.222321821920522021621822122378Comp.Ex.3211213215228218215198210205129

Examples

Embodiment Construction

[0062]The present invention will be further explained and illustrated below with reference to the drawings.

[0063]Table 1 and 2 list the chemical composition of the steel in the Examples of the present invention, with a balance of Fe and unavoidable impurities; Table 3 and 4 list the specific process parameters.

[0064]Tensile properties test: Tensile properties tests were conducted on the steel plates obtained from each example of the present invention at room temperature according to ASTM A370-2021. A yield strength, a tensile strength, and a yield ratio of the steel plates of each example were tested. And the longitudinal tensile properties of the steel plates after a strain aging with 3% pre-strain and holding at 250°C for 1 hour were tested. The specific results are shown in Table 5.

[0065]Low-temperature toughness test: According to ASTM E23-2018, impact tests were conducted on the test steel plates at -45°C using V-notched samples, and the Charpy impact energy at -45°C for the st...

Technical field

[0001] The present invention relates to a field of pipeline steel manufacturing, in particular to a high-performance-uniformity subsea pipeline steel and manufacturing method therefor.Background Art

[0002] The laying of subsea pipeline has a long history of development internationally. Since the first subsea pipeline was laid by Brown & Root Marine Engineering Company in the Gulf of Mexico, USA, in 1954, a large number of subsea pipelines of various types and diameters have been successfully laid in offshore areas around the world, including the North Sea, the Gulf of Mexico, the Mediterranean Sea, Australia, and the South China Sea. Especially in recent years, with the gradual depletion of onshore oil and gas resources, the exploitation of oil and gas has extended to areas with harsh geological conditions such as oceans and polar region, leading to a stronger demand for high-performance subsea pipeline steels.

[0003] Subsea pipelines are subjected to significant compression, tensile loads, and even bending deformations during the laying of the subsea pipeline. During service, harsh operating conditions such as deep-water high pressure, surging waves, strong bottom currents, and subsea movements will also cause substantial plastic deformations or even fracture in subsea pipelines, resulting in serious environmental disasters. These complex operating conditions impose stricter requirements on deep-water pipelines compared to onshore pipelines. Furthermore, as water depth increases, the demands on raw materials continue to rise.

[0004] Subsea pipeline construction is characterized by high risk and high investment. Compared to onshore pipeline construction, it has high requirements for construction quality, construction environment, and construction technology, etc. To avoid the risks of difficult repair and environmental pollution caused by the failure of subsea pipelines, stricter technical requirements are often put forward to improve the quality of subsea pipelines, and process management is strengthened to improve the reliability of the product. Therefore, isotropy of the steel plate (more uniform strength / toughness distribution in the length / thickness direction) is required, as well as good welding performance, low-temperature fracture toughness, strain aging performance, etc. Furthermore, as the subsea pipeline is subjected to significant axial forces and displacements during laying and service process, its load-bearing capacity and strain capacity can be ensured by controlling longitudinal performance. Therefore, longitudinal tensile properties, especially the longitudinal low yield ratio is required for the steel plate.

[0005] The Chinese patent application CN201210123639.X discloses "A Large Wall Thickness Subsea Pipeline Steel Plate and Production Method therefor". The steel plate consists of the following composition in mass percentage of: C: 0.05∼0.07%, Si: 0.15~0.25%, Mn: 1.42∼1.48%, P≤0.010%, S≤0.002%, Ni: 0.13~0.18%, Nb: 0.043~0.048%, Al: 0.020~0.040%, Ti: 0.014~0.024%, Mo: 0.13~0.18%, and a balance of Fe and unavoidable impurities. The steel plate has characteristics such as high low-temperature impact toughness, good tear resistance, high strength, moderate yield ratio, and good elongation. However, the pipeline steel in this patent can only meet the low-temperature toughness requirement at -20°C but fail to meet the toughness requirement at extremely low temperatures. And it is difficult to meet the tensile properties requirement after aging, especially a low yield ratio requirement. Most importantly, the achievement of performance uniformity of the pipeline steel has not been considered in this patent.

[0006] The Chinese patent application CN201310457780.8 discloses "A Low-cost Thick-gauge Subsea Pipeline Steel Plate and Manufacturing Method therefor", which comprises the following composition in mass percentage of: C: 0.04∼0.06%, Si: 0.15~0.20%, Mn: 1.43~1.47%, P≤0.008%, S≤0.003%, Ni: 0.10~0.15%, Nb: 0.030~0.040%, Ti: 0.012~0.023%, Alt: 0.015~0.025%, N≤0.006%, Nb+V+Ti≤0.15%, and a balance of iron and unavoidable impurities. The steel plate obtained has a dual-phase microstructure, achieves high strength and a low yield ratio, and it is suitable for the high deformation resistance requirements of subsea pipelines and also has good toughness. However, the pipeline steel in this patent can only meet the low-temperature toughness requirement at -30°C but fail to meet the toughness requirement at extremely low temperatures. It is difficult to meet the tensile properties requirement after aging, especially a low yield ratio requirement. Furthermore, the achievement of performance uniformity of the pipeline steel has not been considered in this patent.

[0007] The Chinese patent application CN201210492187.2 discloses "A Thick-walled High DWTT Performance X65-70 Subsea Pipeline Steel and Manufacturing Method Therefor", which comprises a composition in mass percentage of: C 0.03~0.05%, Si≤0.25%, Mn 1.47∼1.70%, P≤0.010%, S≤0.001%, Ti 0.006~0.010%, Cr 0.10~0.20%, Cu 0.12~0.20%, Ni 0.36~0.45%, Al 0.025~0.045%, Ca 0.0008~0.0025%, N≤0.0035%, O≤0.0025%, Nb 0.040~0.050%, and a balance of Fe and unavoidable impurities; and Ceq=0.34~0.040, Pcm=0.13~0.17; wherein, Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15; Pcm=C+Si / 30+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10, with a balance of iron and unavoidable impurities. The steel plate has thick specifications, high strength, excellent low-temperature impact toughness, weldability, and good DWTT performance. However, the pipeline steel disclosed in this patent can only meet a low-temperature toughness requirement at -20°C but fail to meet the requirement for low yield ratio and tensile properties after aging. Furthermore, the achievement of performance uniformity of the pipeline steel has not been considered in this patent.

[0008] The Chinese patent application CN201410515400.6 discloses "A Special Thick Specification X70 Pipeline Steel and Manufacturing Method therefor", which comprises a chemical composition in mass percentage of: C: 0.03~0.06%, Si: 0.1~0.30%, Mn: 1.30∼1.60%, P≤0.010%, S≤0.0050%, Nb: 0.030~0.050%, Ti: 0.008~0.020%, Ni: 0.10~0.30%, Cr: 0.10~0.30%, Cu: 0.10~0.30%, and a balance of Fe and unavoidable impurity elements. A pipeline steel with large thickness, high strength within a narrow range, excellent low-temperature toughness, and superior resistance to low-temperature aging and HIC is obtained. However, the pipeline steel disclosed in this patent can only meet the low-temperature toughness requirement at -30°C but fail to meet the toughness requirement at extremely low temperatures. Furthermore, the achievement of performance uniformity of the pipeline steel has not been considered in this patent.

[0009] The Chinese patent application CN201510383013.6 discloses "A Production Method for Low Yield Ratio Acid-resistant Subsea Pipeline Steel", which comprises a chemical composition of: C: 0.03~0.05%, Si: 0.10~0.30%, Mn: 1.15~1.35%, P≤0.010%, S≤0.0010%, Alt: 0.01~0.06%, Nb: 0.04~0.07%, Ti: 0.008~0.020%, Ni: 0.10~0.25%, Cr: 0.10~0.30%, and a balance of Fe and unavoidable impurity elements. This patent adopts advanced smelting and continuous casting technology to control the narrow fluctuation range of carbon equivalent and the quantity, size and morphology of inclusions. It also employs a relaxation phase transformation process to control the structure morphology of the steel plate and adopts a new water-cooling control mode to improve the uniformity of water cooling of the steel plate, effectively ensuring that the steel plate has a low yield ratio and excellent acid resistance. However, the achievement of performance uniformity of the pipeline steel has not been considered in this patent.

[0010] The Chinese patent application CN202011039510.1 discloses "A Subsea Pipeline Steel Plate Coil with Excellent Low-temperature CTOD Performance and Production Process therefor", which comprises a chemical composition in mass percentage of: C: 0.030~0.070%, Si: 0.10~0.20%, Mn: 1.10~1.60%, Nb: 0.020~0.050%, V: 0.020~0.040%, Ti: 0.010~0.030%, Mo: 0.10~0.20%, Alt≤0.06%, S≤0.0080%, P≤0.018%, and a balance of Fe and unavoidable impurity elements, so that the subsea pipeline steel plate coil has excellent CTOD performance. However, the achievement of performance uniformity of the pipeline steel has not been considered in this patent.

[0011] A review of existing patent research reveals that, while the strength and low-temperature toughness are considered in current subsea pipeline steels, there has been little research on how to ensure that the pipeline steel not only have high performance but also have good performance uniformity.Summary

[0012] The objective of the present invention is to provide a high-performance-uniformity subsea pipeline steel and manufacturing method therefor. The pipeline steel has high strength and good low-temperature toughness, and at the same time, the uniformity of performance of the pipeline steel can also be ensured.

[0013] In order to achieve the above objective, the technical solution of the present invention is as follows: a high-performance-uniformity subsea pipeline steel, comprises a chemical composition in weight percentage of: C: 0.02~0.06%, Si: 0.10~0.30%, Mn: 1.20~1.65%, Cu≤0.30%, Ni≤0.30%, Cr: 0.15~0.30%, Mo≤0.15%, Nb: 0.03~0.05%, V≤0.05%, Ti: 0.008~0.020%, Ca: 0.0012~0.0030%, Al: 0.015~0.045%, and a balance of Fe and other unavoidable impurities; the impurity elements shall be controlled as follow: P≤0.01%, S≤0.001%, B≤0.0004%, N≤0.005%, O≤0.0025%, and H≤0.0002%; and simultaneously satisfying: Ti / N≥2; Nb / 7.7 C + V / 4.3 C ≥ 0.10 ; 1.5 ≤ Ca / S ≤ 4 ; 0.30 ≤ Ceq ≤ 0.39 , Ceq = C + Mn / 6 + Cr + V + Mo / 5 + Cu + Ni / 15 ; a welding cold crack sensitivity index Pcm is ≤0.17, and Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B.

[0014] Further, the balance is Fe and other unavoidable impurities.

[0015] The microstructure of the subsea pipeline steel described in the present invention is ferrite + bainite, wherein, a volume fraction of the ferrite is 20%~40%.

[0016] The subsea pipeline steel described in the present invention has a grain size grade of ≥ grade 9.

[0017] The transverse tensile properties of the subsea pipeline steel described in the present invention are as follows: yield strength R t0.5 :415~555MPa, tensile strength R m : 515~670MPa, and a yield ratio R t0.5 / R m of ≤0.88; and a difference in strength between the head and tail of the steel plate is ≤50MPa; longitudinal tensile: yield strength R t0.5 : 405~530MPa, tensile strength R m : 505~650MPa, and a yield ratio R t0.5 / R m of ≤0.86; and a difference in strength between the head and tail of the steel plate is ≤50MPa; after a strain aging test with a 3% pre-strain and holding at 250°C for 1 hour, the longitudinal tensile properties of the steel plate are as follows: yield strength R t0.5 : 405~530MPa, tensile strength R m :505~650MPa, and a yield ratio R t0.5 / R m of ≤0.88.

[0018] A Charpy impact energy of the subsea pipeline steel described in the present invention at -45°C is ≥250J, and a difference in Charpy impact energy between the thickness center of the steel plate and the near-surface of the steel plate is ≤50J; a SA of DWTT test at -30°C is≥85%, and a difference in SA between a single-sided thinned DWTT sample and full-thickness DWTT sample is ≤5%; a hardness is ≤230HV10, wherein, a difference between the surface hardness of the steel plate and center hardness of the steel plate is ≤25HV10, preferably a difference between the surface hardness of the steel plate and center hardness of the steel plate is ≤15HV10.

[0019] The characteristics of the composition design of the subsea pipeline steel of the present invention are as follow: 1. By adopting an alloy composition system of low C and low Mn, and controlling 0.30≤Ceq≤0.39 and Pcm≤0.17, good weldability is achieved under the premise of the steel meeting the requirement for strength and toughness. At the same time, central segregation and banded structures are improved, and the isotropy of material performance is guaranteed. 2. The amount of impurity elements P, S, B, N, O, and H is controlled at extremely low levels, so that the number of inclusions is decreased, the degree of segregation is reduced, and the strength and toughness is ensured, and it is conducive to ensure excellent low-temperature toughness of the pipeline steel in particularly. 3. The addition of alloy elements such as Cu, Ni, Cr, and Mo exerts a solid solution strengthening effect, simultaneously affects the phase transformation during a cooling process of the material, and improves the isotropy of the steel plate properties. 4. By adding trace amount of V-Nb-Ti and strictly limiting Ti / N≥2 and (Nb / 7.7C+V / 4.3C)≥0.10, micro-alloy carbonitrides are formed, grains are refined, precipitation are strengthened, C and N atoms are fixed , and the effects of strain aging on material performance are reduced.

[0020] C: A most basic strengthening element, carbon dissolves in a steel to form interstitial solid solution and exerts a solid solution strengthening effect. When C forms carbides precipitation with strong carbide-forming elements, it helps to precipitation strengthening. However, with the increase of C content, the ductility, toughness and weldability of steel are decreased significantly, and central segregation in the continuous-casting slab occurs, which is not conducive to the uniformity of the performance of steel plate in the thickness direction. Furthermore, after strain aging of the steel plate, free C atoms may entangle with dislocations, hinder the movement of dislocations, and increase the yield strength, which is not conducive to the maintenance of a low yield ratio. Therefore, the content of C is controlled at 0.02%~0.06% in the present invention.

[0021] Mn: A solid solution strengthening element, Mn is a most important and economical strengthening element in the steel to compensate the loss of strength caused by the decrease of the content of C. Mn is also an element that expands the γ-phase, and Mn may improve the hardenability of steel, reduce the γ→α phase transformation temperature of steel, help to obtain fine phase transformation products, and improve the toughness of steel. However, Mn is also an element that is prone to segregate. When the content of Mn is too high, central segregation in the continuous casting slab tends to occur, hard-phase structures are prone to be formed during a cooling process after rolling, a low-temperature toughness of the material is reduced, and it is also not conductive to the uniformity of the performance of steel plate in the thickness direction. Therefore, the content of Mn is controlled at 1.20~1.65% in the present invention.

[0022] Si: A solid solution strengthening element, and it is also a deoxidation element in the steel. However, when the content of Si is too high, it may deteriorate the welding performance of steel, especially the toughness of a welding heat-affected region, and it is not conducive to the removal of iron oxide scale during a hot rolling process. Therefore, the content of Si is controlled at 0.10~0.30% in the present invention.

[0023] Cu and Ni: Solid solution strengthening elements, and they may improve the toughness of the steel. They also help to improve the hardenability of steel and delaying the transformation of pearlite during a cooling process of the steel plate. However, when the addition of Cu and Ni is too high, it is not conducive to the toughness and welding performance of the steel plate. Therefore, the content of Cu and Ni is each controlled at ≤30% in the present invention.

[0024] Cr and Mo: Solid solution strengthening elements, and important elements for improving the hardenability of the steel. They promote the formation of acicular ferrite, improve the toughness of steel, and are conducive to the uniformity of microstructure and performance across the full thickness of the steel plate; however, when the content is too high, they may promote the formation of low-temperature phase transformation structures, and are not conducive to the low-temperature toughness. Therefore, the content of Cr is controlled at 0.15~0.30%; and Mo is controlled at ≤0.15% in the present invention.

[0025] Nb: One of the important elements in low-carbon micro-alloying steel. Solid-soluted Nb precipitates as Nb(C,N) particles via strain-induced precipitation during a hot rolling process, which may delay austenite recrystallization, increase the austenite recrystallization temperature of steel, make a two-stage controlled rolling possible, prevent the formation of mixed grains, help to obtain a uniform and fine structure, and improve the strength and toughness of the material while the isotropy of the steel plate properties is ensured. Furthermore, during a cooling process, solid-soluted Nb precipitates dispersedly as the second-phase particles NbC in the matrix, contributing to precipitation strengthening. However, when the content of Nb is too high, it cannot be fully solid-solubilized, which not only fails to exert its effect but also increases production costs. And it may lead to premature precipitation of NbC at a high temperature, and larger NbC precipitates at central segregation regions tend to be formed, which is not conducive to the toughness. Therefore, the content of Nb is controlled at 0.03~ 0.05% in the present invention.

[0026] V: One of the important micro-alloying elements, it tends to combine with C in the steel to form VC, having a relatively high precipitation strengthening effect and a relatively weak grain refinement effect. However, when the content of V is too high, the precipitation particles will coarsen significantly, and the low-temperature toughness of steel is reduced. In the present invention, V mainly helps to strength precipitation to make up for strength reduction caused by the deficiency of other alloys. Therefore, the content of V is controlled at ≤0.05% in the present invention.

[0027] In addition, in order to fully utilize the carbon fixation effect of Nb and V, and reduce the impact of free carbon on strain aging properties, it is necessary to control (Nb / 7.7C+V / 4.3C) ≥ 0.10. In some embodiments, (Nb / 7.7C+V / 4.3C) is controlled at 0.10-0.50, such as 0.10-0.47.

[0028] Ti: Ti is a strong carbonitride-forming element that may play a role in nitrogen fixation. During a slab continuous casting process, it may form fine and high-temperature stable TiN precipitates, thereby improving the toughness of the material. In addition, the undissolved carbonitrides of Ti may prevent the growth of austenite grains during heating process of the steel, and TiN and TiC precipitated in high-temperature austenite zone during a rough rolling process may effectively inhibit the growth of austenite grains. Furthermore, TiN and TiC particles in steel may significantly prevent grain growth in the heat-affected region during a welding process, thereby improving the welding performance of the steel plate. Therefore, the content of Ti is controlled at 0.008~0.020% in the present invention; meanwhile, to fully utilize Ti's nitrogen fixation effect, it is necessary to control Ti / N ≥2. In some embodiments, Ti / N is controlled at 2.0~5.0.

[0029] Ca: Ca is mainly used to modify MnS inclusions, resulting in the morphology of the inclusions spheroidized and uniform distributed, helping to improve low-temperature toughness. However, when the content of Ca is too high, it may tend to form CaO inclusions, which is not conductive to toughness. Therefore, the content of Ca is controlled at 0.0012%~0.0030% in the present invention. Meanwhile, to fully utilize the modifying effect of Ca on sulfides, Ca / S is controlled at 1.5≤Ca / S≤4, resulting in elongated MnS inclusions convert into dispersed spherical CaS inclusions, and a low-temperature toughness of the material is improved.

[0030] Al: Al is mainly used for deoxidation of steel. Adding an appropriate amount of Al helps to refine grains and improve the strength and toughness of steel. However, with the increase of the content of Al, oxides of Al will be formed in the steel, thereby reducing the toughness of the base material and the welding heat-affected region. Therefore, the content of Al is controlled at 0.015%~0.045% in the present invention.

[0031] S, P, B, N, O, H: Unavoidable impurity elements in steel. P is an element which is prone to segregation; and S tends to form sulfides in steel, which both may significantly reduce the low-temperature toughness of steel. B tends to precipitate at grain boundaries, resulting in a decrease in the plasticity and toughness of the material. When the content of N and H is too high, cracks tend to occur in a continuous-casting slab. When the content of O is too high, various oxides are formed, thereby reducing hot workability, corrosion resistance, and toughness. Therefore, the contents of the impurity elements are controlled as follows: P≤0.01%, S≤0.001%, B≤0.0005%, N≤0.005%, O≤0.0025%, and H≤0.0002% in the present invention.

[0032] Pcm: An indicator of welding cold crack sensitivity. When Pcm exceeds a certain threshold, the hardenability of steel becomes too great, resulting in a significant decrease in the toughness of the welding region. Pcm is controlled at ≤0.17 in the present invention. In some embodiments, Pcm is controlled at 0.12~0.17.

[0033] By adopting an alloy composition system of low C and low Mn in the present invention, center segregation is reduced under the premise of the steel meeting the requirement for strength. By combining the effects of Cr, Mo, Cu, and Ni on enhancing hardenability and inhibiting ferrite transformation, the ferrite content is controlled at 20~40%, center segregation is reduced further, good uniformity in the thickness direction of the steel plate is ensured, a difference in SA% between a single-sided thinned DWTT sample and a full-thickness DWTT sample is ≤5%, and a difference between the surface hardness and center hardness of the steel plate is ≤25HV10. Furthermore, by combining C, Mn, Cu, Ni, Cr, Mo, and V, the Ceq is controlled at 0.30≤Ceq≤0.39, ensuring the strength performance of the steel plate.

[0034] Typically, micro-alloying elements such as Nb and V are added in pipeline steel to strengthen grain refinement and precipitation. However, in the present invention, by utilizing the fixation effect of Nb and V on C atoms, (Nb / 7.7C+V / 4.3C) ≥ 0.10 is designed. By maximizing the fixation effect of Nb and V on C atoms, the influence of free carbon on strain aging properties is reduced, and the unconventional performance requirement for tensile properties of the subsea pipeline steel after strain aging is satisfied. After a strain aging test with a 3% pre-strain and holding at 250°C for 1 hour for the subsea pipeline steel described in the present invention, the longitudinal tensile properties of the steel plate are as follows: yield strength R t0.5 : 405~530MPa, tensile strength R m :505~650MPa, and a yield ratio R t0.5 / R m of ≤ 0.88.

[0035] The content of ferrite is controlled at 20%~40% in the present invention. it is crucial for a suitable proportion of ferrite obtained to meet the performance requirement of a subsea pipeline steel. When the content of ferrite is too low, a low yield ratio requirement can't be satisfied; when the content of ferrite is too high, strength of the steel is insufficient. Therefore, in terms of composition design, by adding the alloy elements such as Cu, Ni, Cr, and Mo, a certain degree of austenite stability is obtained, and the amount of ferrite transformation may be controlled within a desired range.

[0036] The manufacturing method of the high-performance-uniformity subsea pipeline steel described in the present invention comprises the following steps:1) Smelting and continuous casting

[0037] The above composition is smelt and continuous cast into a slab; during the continuous casting process, a superheat of the tundish is controlled at 15~40°C, and a casting speed of the continuous casting slab is controlled at 0.40-1.1Om / min, ensuring that the solidification end of the continuous casting slab is located in the horizontal section of a continuous casting caster; and an electromagnetic stirring process is adopted in the continuous casting mold;2) Controlled rolling

[0038] A heating temperature is 1120~1190°C, a start rolling temperature of rough rolling is 1000 -1100°C, a finish rolling temperature of rough rolling is 930~980°C, a reduction ratio of rough rolling is ≥50%, and a spread ratio is 1.20-1.80; a start rolling temperature of finish rolling is 790~880°C, a finish rolling temperature of finish rolling is 780-850°C, and a reduction ratio of finish rolling is ≥70%;3) Controlled cooling

[0039] A relaxation time is ≥30s, a start cooling temperature is 730~780°C, a cooling rate is 10~40°C / s, a stop cooling temperature is 350~550°C, and a temperature difference between the upper and lower surfaces of the steel plate after stop cooling is ≤30°C;4) After stop cooling, the steel plate is air-cooled or stack-cooled to room temperature.

[0040] Preferably, in step 4), when a stop cooling temperature is ≤450°C, the steel plate is tempered at 450~650°C for 10~30min.

[0041] Preferably, in step 1), a fluctuation value of the superheat of adjacent slabs is ≤10°C and / or a fluctuation of the casting speed of adjacent slabs is ≤10%.

[0042] Preferably, in step 1), a dynamic soft reduction process is carried out at the solidification end of the continuous casting slab, with a reduction amount of 2~3mm.

[0043] Preferably, in step 3), a temperature difference of water entry between the head and tail of the steel plate is ≤30°C.

[0044] Preferably, in step 4), after the steel plate is cooled to room temperature, the head and tail portions of the steel plate are cropped, with a cropping amount of ≥250mm each, such as 250~350mm. In some embodiments, a cropping amount of the head is 250~350mm, and a cropping amount of the tail is 250~300mm.

[0045] In the manufacturing method described in the present invention: To reduce center segregation in the continuous-casting slab and improve the low-temperature toughness and isotropy of properties in the steel plate, the following requirements should be satisfied in the continuous-casting process as follow: (1) a superheat of the tundish is controlled at 15~40°C, and a fluctuation value of the superheat of adjacent slabs is ≤10°C; (2) a casting speed of the continuous casting slab is controlled at 0.40~1.10m / min, with a casting speed controlled constant, and a fluctuation of the casting speed of adjacent slabs is ≤10%, ensuring that the solidification end of the continuous casting slab is located at the horizontal section of a continuous casting caster; (3) an electromagnetic stirring process is used in the continuous casting mold section ; (4) a dynamic soft reduction technology is adopted at the solidification end of the continuous casting slab.

[0046] A superheat of the tundish and a casting speed of the continuous-casting slab are controlled, and a fluctuation range of the casting speed is strictly limited in the present invention, so that the continuous casting process is stable, and the solidification end of the continuous-casting slab is located in the horizontal section of a continuous casting caster, and the effectiveness of a dynamic soft reduction is maximized, thereby ensuring good control of center segregation in the slab. Meanwhile, electromagnetic stirring is adopted in a continuous casting process, effectively resulting in the uniform distribution of elements, thereby reducing the segregation tendency and ensuring the uniformity of the performance of the pipeline steel.

[0047] A heating temperature of the slab is 1120~1190°C. when a heating temperature is too low, the alloy carbides fail to be fully dissolved, resulting in fail to fully play a role of solid solution strengthening and precipitation strengthening, and it is not conducive to the control of center segregation. When a heating temperature is too high, grain coarsening occurs, and it has an adverse effect on the low-temperature toughness of the steel plate.

[0048] A start rolling temperature of rough rolling is 1000~1100°C, and a finish rolling temperature of rough rolling is 930~980°C. A reduction ratio of rough rolling is ≥50%, and a spread ratio is 1.20~1.80. Rough rolling is carried out in a fully recrystallized austenite zone to avoid the occurrence of mixed grains. A sufficient reduction ratio of rough rolling can fully refine the original austenite grains, and is beneficial to the strength and toughness of the steel plate. At the same time, precise control of the spread ratio is conducive to control of plate shape and reduces the performance difference between the transverse and longitudinal directions of the steel plate. In some embodiments, a reduction ratio of rough rolling is 50~80%, such as 50~74%.

[0049] A start rolling temperature of finish rolling is 790~880°C, a finish rolling temperature of finish rolling is 780~850°C, and a reduction ratio of finish rolling is ≥70%. Finish rolling is carried out in a non-recrystallized austenite zone. A sufficient reduction ratio of finish rolling may fully flatten the austenite grains, and provide more nucleation sites for subsequent phase transformation, thereby refining the final structure and improving the strength and toughness of the steel plate. In some embodiments, a reduction ratio of finish rolling is 70~85%.

[0050] A relaxation time is ≥30s, a start cooling temperature is 730~780°C, a cooling rate is 10~40°C / s, and a stop cooling temperature is 350~550°C. An appropriate cooling rate and stop cooling temperature may ensure the formation of a dual-phase structure of ferrite + bainite, guaranteeing both the strength and toughness of the steel plate while achieving a low yield ratio.. A certain relaxation time and start cooling temperature may ensure that a certain amount of pro-eutectoid ferrite is generated in the structure of steel plate, thereby reducing a yield ratio while maintaining strength. A temperature difference of water entry between the head and tail of the steel plate is ≤30°C, and reducing the temperature difference of water entry between the head and tail of the steel plate is beneficial for ensuring the isotropy of the steel plate properties. A temperature difference between the upper and lower surfaces of the steel plate after cooling is ≤30°C. Reducing the temperature difference between the upper and lower surfaces of the steel plate after cooling is beneficial for improving the isotropy of properties in the thickness direction. In some embodiments, a relaxation time is 30~60s.

[0051] A relatively high finish rolling temperature, a relatively low finish cooling temperature, and a relatively fast cooling rate are beneficial for inhibiting the enrichment of alloy elements towards the Center of thickness of the plate during a phase transformation process, thereby further reducing the impact of center segregation.

[0052] After stops cooling, the steel plate is cooled to room temperature. Preferably, it may be cooled to room temperature by stack-cooled.

[0053] Preferably, for the steel plate with a stop cooling temperature of ≤450°C, tempering may be performed at 450~650°C for 10~30 minutes.

[0054] After stop cooling, the steel plate may usually be cooled to room temperature by air cooling on a cold bed; or the steel plates after rolling are stacked together and cooled to room temperature.

[0055] A cooling rate of the stack-cooled is relatively slow, so that the steel plate may stay at a high temperature for a longer period of time, which is conducive to the precipitation of alloy carbides and fully play a role of precipitation strengthening and carbon fixation. For the steel plate with a lower stop cooling temperature, the precipitation time of alloy carbides is insufficient. By utilizing short-term high-temperature tempering, it gives full play to the role of alloy precipitation strengthening and carbon fixation, and it is beneficial for the decomposition of hard phase MA constituents and improving low-temperature strength and toughness.

[0056] A cropping amount of the head and tail of the finished steel plate after rolling is ≥250mm. Due to significant differences in deformation conditions, cooling conditions between the head / tail section and the main body section of the steel plate, there are significant differences in structure and performance between the head / tail section and the main body section of the steel plate. By ensuring a certain cropping amount of the head and tail, the influence of deformation and cooling independent of positions is eliminated, which is beneficial for achieving the isotropy of the steel plate properties.

[0057] Compared with the prior art, the present invention has the following advantages: In terms of component design, by adding a relatively low amount of C and Mn elements, controlling the amount of impurity elements such as P, S, B, N, O, and H at an extremely low levels, and controlling 0.30≤Ceq≤0.39 and Pcm≤0.17, in combination with the addition of alloy elements such as Cr, Mo, Cu, and Ni in the present invention, central segregation is improved, phase transformation behavior during a cooling process of the material is affected, the stability of austenite is increased, and the ferrite phase transformation process is delayed, and the requirement of good uniformity of the performance in steel plate is satisfied on the basis of ensuring the mechanical performance of the steel plate.

[0058] By utilizing the addition of trace amounts of V-Nb-Ti, and strictly limiting Ti / N≥2 and (Nb / 7.7C+V / 4.3C) ≥0.10, micro-alloying carbonitrides are formed, grains are refined and precipitation are strengthened, C and N atoms are fixed, and the effects of strain aging on material performance are reduced. After strain aging, the steel still has high performance. After a strain aging test with a 3% pre-strain and holding at 250°C for 1 h, the steel plate has the following longitudinal tensile properties: yield strength R t0.5 : 405~530MPa, tensile strength R m : 505~650MPa, and a yield ratio R t0.5 / R m of ≤0.88.

[0059] Based on the composition design, a superheat of the tundish, a continuous casting speed, and a fluctuation range are controlled strictly in a continuous casting process, a dynamic soft reduction process is adopted, so that the solidification end of the continuous casting slab is located in the horizontal section of a continuous casting caster, the effect of dynamic soft reduction is maximized, thereby reducing the segregation tendency and ensuring the uniformity of pipeline steel properties. However, there are no specific requirements or low requirements for a superheat of the tundish, a continuous casting speed, and a fluctuation range in the conventional continuous casting process. Parameter settings are primarily determined by matching production rhythms between the previous and subsequent processes. Therefore, the solidification end of the continuous casting slab is often located in the curved section of the continuous casting caster, so that it limits the effectiveness of dynamic soft reduction applied in the horizontal section to improve the quality of the internal slab and reduce central segregation, ultimately failing to meet the requirements for high performance and uniformity.

[0060] In addition, by using a two-stage controlled rolling technology and spread ratio control in a recrystallized austenite zone and non-recrystallized austenite zone, and ensuring sufficient compression ratio during the two-stage rolling process, grain refinement is fully achieved, and mixed crystal structure is avoided to occur. A rolling temperature, a relaxation time, and a start cooling temperature are controlled to control the ferrite transformation process from both thermodynamic and kinetic aspects, thereby forming a dual phase structure of ferrite and bainite. And the proportion of ferrite is accurately controlled at 20-40%, so that the steel plate has excellent mechanical performance such as strength and toughness, and isotropy, and the issue of mismatched between the strength and uniformity of the performance of subsea pipeline steel is solved.Description of the drawings

[0061] Fig.1 shows the microstructure photograph of the steel in Example 1 of the present invention.Detailed Description

[0062] The present invention will be further explained and illustrated below with reference to the drawings.

[0063] Table 1 and 2 list the chemical composition of the steel in the Examples of the present invention, with a balance of Fe and unavoidable impurities; Table 3 and 4 list the specific process parameters.

[0064] Tensile properties test: Tensile properties tests were conducted on the steel plates obtained from each example of the present invention at room temperature according to ASTM A370-2021. A yield strength, a tensile strength, and a yield ratio of the steel plates of each example were tested. And the longitudinal tensile properties of the steel plates after a strain aging with 3% pre-strain and holding at 250°C for 1 hour were tested. The specific results are shown in Table 5.

[0065] Low-temperature toughness test: According to ASTM E23-2018, impact tests were conducted on the test steel plates at -45°C using V-notched samples, and the Charpy impact energy at -45°C for the steel plate of each example was measured. The specific results are shown in Table 6. According to API RP 5L3-2014, drop weight tear tests (DWTT) were conducted on the test steel plates at -30°C using full-thickness V- notched samples and single-sided thinned thickness V- notched samples respectively, and the SA value of DWTT test at -30°C for the steel plate of each example was tested. The specific results are shown in Table 7.

[0066] Hardness test: According to ASTM E92, the hardness of the steel plate near the upper surface, the hardness of the steel plate at the thickness center, and the hardness of the steel plate near the lower surface were measured under a load of 10kgf. The specific results are shown in Table 8.

[0067] The testing method of grain size was in accordance with ASTM E112.

[0068] As can be seen from Figure 1, the microstructure of the steel plate obtained by using the composition and process described in the present invention is ferrite + bainite, with a ferrite content of 20~40%.

[0069] As shown in Table 5, the transverse tensile properties of the steels of the present invention are as follows: yield strength R t0.5 : 415~555MPa, tensile strength Rm: 515~670MPa, and a yield ratio R t0.5 / Rm of 0.88, and a difference in strength between the head and tail of the steel plate is ≤50MPa.

[0070] Longitudinal tensile properties: yield strength R t0.5 : 405~530MPa, tensile strength Rm: 505~650MPa, and a yield ratios R t0.5 / R m of 0.86, and a difference in strength between the head and tail of the steel plate is ≤50MPa.

[0071] After a strain aging test with a 3% pre-strain and holding at 250°C for 1 hour, the longitudinal tensile properties of the steel plate are as follows: yield strength R t0.5 : 405~530MPa, tensile strength Rm: 505~650MPa, and a yield ratio Rt0.5 / Rm of 0.88.

[0072] As can be seen from Table 6, the steel plate obtained in the present invention has a Charpy impact energy of ≥250J at -45°C, and a difference in Charpy impact energy between the thickness center of the steel plate and the near-surface of the steel plate is ≤50J.

[0073] As can be seen from Table 7, the steel plate obtained in the present invention has a SA of ≥85% in the DWTT test at -30°C. Specifically, a difference in SA between a single-sided thinned DWTT sample (Test was conducted in accordance with API RP 5L3 with the temperature progressively lowered) and a full-thickness DWTT sample is ≤5%.

[0074] As can be seen from Table 8, the steel plate obtained in the present invention has a hardness of ≤230HV10. Specifically, a difference between the near-surface hardness and center hardness of the steel plate is ≤25HV10.

[0075] In Comparative Example 1, Ti / N=1.5 and Ca / S=1.12, neither of which meets the requirements of the present invention: Ti / N≥2 and 1.5≤Ca / S≤4. Moreover, the contents of P, S, and N are relatively high, and the casting speed waveform is not within the control range specified in the present invention during process control, which cannot satisfy the condition that the solidification end of the continuous casting slab is located in the horizontal section of a continuous casting caster. Furthermore, a fluctuation value of the superheat, a temperature difference of water entry between the head and tail of the steel plate, and a temperature difference between the upper and lower surfaces of the steel plate after cooling are all not within the control range specified in the present invention. Although the strength of the steel plate obtained meet the requirements, a difference in strength between the head and tail of the steel plate is too large, exceeding 50 MPa, and the yield ratio is relatively high. Furthermore, a difference in Charpy impact energy between the Center of thickness and near-surface of the steel plate is 66, and a difference in SA between the single-sided thinned DWTT sample and the full-thickness DWTT sample is 14, and a difference between the surface hardness and the center hardness of the steel plate is 35. Obviously, the uniformity of the performance of the steel plate obtained in Comparative Example 1 is poor.

[0076] In Comparative Example 2, Nb / 7.7C+V / 4.3C=0.09, which fails to meet the requirement of the present invention: (Nb / 7.7C+V / 4.3C) ≥ 0.10. Furthermore, the contents of B and O are relatively high, and a heating temperature, a rough rolling temperature of finish rolling, and a relaxation time in process control are not within the range specified in the present invention. As a result, a Charpy impact energy at -45°C is less than 250J, a SA of DWTT test at -30°C is less than 85%, and the low-temperature toughness of the steel plate is poor.

[0077] In Comparative Example 3, Ceq=0.40, Pcm=0.18, and Nb / 7.7C+V / 4.3C=0.07, none of which meets the requirements of the present invention: 0.30≤Ceq≤0.39, Pcm≤0.17, and (Nb / 7.7C+V / 4.3C) ≥0.10. A difference in SA between the single-sided thinned DWTT sample and the full-thickness DWTT sample of the steel plate obtained is 13, and a difference in Charpy impact energy between the Center of thickness and near-surface of the steel plate is 115J. The uniformity of the performance of the steel plate obtained in Comparative Example 3 is poor. Table 1 (unit: weight percentage)CSiMnPSCuNiCrMoVNbTiCaAltBNOHEx.10.020.101.450.00750.00100.300.300.160.110.0200.0300.0100.00150.04500.00040.00500.00250.00020Ex.20.030.151.200.08800.00080.1000.300.150.0300.0390.0080.00260.02800.00030.00300.00210.000170Ex.30.050.181.570.00980.000900.080.24000.0400.0120.00140.03340.00030.00380.00180.00015Ex.40.0250.221.220.00760.00070.250.100.190.040.0150.0330.0180.00210.02200.00040.00430.00170.00017Ex.50.0400.201.380.00830.00080.170.260.150.080.0400.0370.0150.00280.01900.00030.00370.00230.00016Ex.60.0310.191.240.00630.00060.190.150.2600.0370.0450.0110.00190.03300.00030.00400.00190.00015Ex.70.0350.301.650.01000.00080.130.140.180.120.0500.0350.0200.00300.01500.00040.00400.00220.00018Ex.80.0410.231.230.00700.00060.200.170.250.0600.0410.0130.00160.02870.00020.00390.00190.00016Ex.90.0600.271.620.00840.00080.050.060.2000.0100.0500.0160.00120.03100.00030.00330.00200.00019Comp.Ex.10.040.191.380.01450.00250.170.260.150.080.0400.0380.0080.00280.01900.00030.00550.00230.00016Comp.Ex.20.050.181.570.00980.00090.000.080.240.000.0000.0330.0120.00140.0334000060.00380.00350.00015Comp.Ex.30.070.271.700.00840.00080.050.060.2000.0100.0200.0160.00120.03100.00030.00330.00200.00019 Table 2 CeqPcmTi / NNb / 7.7C+V / 4.3CCa / SEx.10.360.132.00.431.50Ex.20.330.122.70.403.25Ex.30.360.153.20.101.56Ex.40.300.124.20.313.00Ex.50.350.154.10.353.50Ex.60.320.132.70.473.17Ex.70.390.155.00.463.75Ex.80.330.143.30.132.67Ex.90.380.174.80.141.50Comp.Ex.10.350.151.50.361.12Comp.Ex.20.360.153.20.091.56Comp.Ex.30.400.184.80.071.50 Note: Ceq=C+Mn / 6+(Cr+V+Mo) / 5+(Cu+Ni) / 15; Pcm= C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B. Table 3 A superheat of the tundish °CA fluctuation value of the superheat °CA casting speed of the continuous casting slab m / minA fluctuation of the casting speed %A heating temperature °CA start rolling temperature of rough rolling °CA finish rolling temperature of rough rolling °CA reduction ratio of finish rolling %A spread ratioA start rolling temperature of finish rolling °CA finish rolling temperature of finish rolling °CA reduction ratio of finish rolling %Ex.14051.00911301070940581.4081081073Ex.22061.10811701010945631.6082082082Ex.3255.50.55511521020960651.2079079075Ex.430100.40711901090975601.8087082070Ex.51570.70411401060970551.4283080078Ex.6234.51.051011801100980701.5088085079Ex.71880.80611201000930501.3080078080Ex.8285.70.65511601040950741.4886084078Ex.93590.905.511381080938691.7085083085Comp.Ex. 125120.701511401060970551.4283080078Comp.Ex. 22550.55512001040910651.2079079075Comp.Ex. 33590.90511381080938691.7085083085 Table 4 A relaxation time sA start cooling temperature °CA temperature difference of water entry between the head and tail of the steel plate°CA cooling rate °C / sA stop cooling temperature °CA temperature difference between the upper and lower surfaces of the steel plate after stop cooling°CA cooling method of the steel plateA tempering temperature °CA tempering time minA cropping amount of the head of the steel plate mmA cropping amount of the tail of the steel plate mmEx.15076018303808stack-cooled--300250Ex.255770231840018stack-cooled55020250250Ex.335745152046010stack-cooled--350300Ex.460730202555020stack-cooled--300250Ex.545750181544525air-cooled65010300250Ex.633775284050012air-cooled--350300Ex.740780301035030stack-cooled45030250250Ex.830765253345015stack-cooled--350300Ex.94274026264205air-cooled--350300Comp.Ex.145750511545042air-cooled--300250Comp.Ex.210760152046010stack-cooled--10050Comp.Ex.34274026264205air-cooled--350300 Table 5 Sample positionTransverse tensileLongitudinal tensileLongitudinal tensile after strain agingYield strength R t0.5 MPaTensile strength Rm MPaA difference in strength between the head and tail of the steel plate MPaYield ratio R t0.5 / R mYield strength R t0.5 MPaTensile strength Rm MPaA difference in strength between the head and tail of the steel plate MPaYield ratio R t0.5 / R mYield strength R t0.5 MPaTensile strength Rm MPaYield ratio R t0.5 / RmEx.1The head of the steel plate46056012 / 200.824575588 / 190.824595660.81The tail of the steel plate4485400.834495390.834665650.82Ex.2The head of the steel plate50059050 / 500.854855755 / 90.844885880.83The tail of the steel plate4505400.834805660.854895770.85Ex.3The head of the steel plate51359717 / 170.8648656414 / 100.864955680.87The tail of the steel plate4965800.864725540.854865660.86Ex.4The head of the steel plate43853023 / 150.8342851828 / 130.834305200.83The tail of the steel plate4155150.814055050.804055050.80Ex.5The head of the steel plate49560028 / 120.8348558350 / 500.834795860.82The tail of the steel plate4675880.794355330.824685680.82Ex.6The head of the steel plate4985683 / 40.884885661 / 20.864995700.88The tail of the steel plate4955720.874895680.864955720.87Ex.7The head of the steel plate55567050 / 200.8353065026 / 60.825306500.82The tail of the steel plate5056500.785046440.785156480.79Ex.8The head of the steel plate51358113 / 120.884935708 / 70.865095880.87The tail of the steel plate5005690.884855630.865035770.87Ex.9The head of the steel plate53566020 / 500.8152863911 / 60.835256480.81The tail of the steel plate5156100.845176330.825306350.83Comp.Ex. 1The head of the steel plate54261055 / 5208952160154 / 500875446110.89The tail of the steel plate4875580.874675510.854955880.84Comp.Ex. 2The head of the steel plate55961757 / 160.9152359735 / 250.885656040.94The tail of the steel plate5026330.794886220.785286330.83Comp.Ex. 3The head of the steel plate59668011 / 310.885786615 / 160875956670.89The tail of the steel plate5856490.905736450.895886500.90 Table 6 Sample positionCVN1 JCVN1 JCVN1 JCVN-avg JDifferenceEx.1Center of thickness25026827826550Near-surface327318301315Ex.2Center of thickness33034531733119Near-surface359340351350Ex.3Center of thickness41141440240943Near-surface465438453452Ex.4Center of thickness31531233031930Near-surface346342358349Ex.5Center of thickness32034333133118Near-surface342352354349Ex.6Center of thickness42942442242513Near-surface453434427438Ex.7Center of thickness33631432232419Near-surface354322354343Ex.8Center of thickness43042741142318Near-surface457439428441Ex.9Center of thickness31430730931036Near-surface358337343346Comp.Ex.1Center of thickness18825522322266Near-surface286279299288Comp.Ex.2Center of thickness19522523121737Near-surface232214241229Comp.Ex.3Center of thickness235247211231115Near-surface325338321346 Table 7 Sample typeSA %SA %SA-avg %DifferenceEx.1A full-thickness sample8988891A single-sided thinned sample899190Ex.2A full-thickness sample8585855A single-sided thinned sample889290Ex.3A full-thickness sample9087892A single-sided thinned sample909191Ex.4A full-thickness sample8686864A single-sided thinned sample899190Ex.5A full-thickness sample9090904A single-sided thinned sample929594Ex.6A full-thickness sample9593942A single-sided thinned sample959696Ex.7A full-thickness sample8688875A single-sided thinned sample958892Ex.8A full-thickness sample9293934A single-sided thinned sample989597Ex.9A full-thickness sample8790892A single-sided thinned sample948791Comp.Ex.1A full-thickness sample67817414A single-sided thinned sample888788Comp.Ex.2A full-thickness sample6977234A single-sided thinned sample787677Comp.Ex.3A full-thickness sample68727013A single-sided thinned sample808683 Table 8 Near the upper surfaceCenter of ThicknessNear the lower surfaceDifferenceGrain sizehardnesshardnesshardnesshardnesshardnesshardnesshardnesshardnesshardnesshardnesshardnessEx.1210203194211208194200196199410E9x.219719519620220320719119920089Ex.3218213214200219212213215218511Ex.420320821521019019620219120159Ex.5198198203214214192196203195810Ex.6201210203197190207202204203611Ex.720520721821723021521521320999Ex.8201205200204201190208207205610Ex.92082112021961952072202122121211Comp.Ex.1197201200233238231195205199359Comp.Ex.222321821920522021621822122378Comp.Ex.3211213215228218215198210205129

Examples

Embodiment Construction

[0062]The present invention will be further explained and illustrated below with reference to the drawings.

[0063]Table 1 and 2 list the chemical composition of the steel in the Examples of the present invention, with a balance of Fe and unavoidable impurities; Table 3 and 4 list the specific process parameters.

[0064]Tensile properties test: Tensile properties tests were conducted on the steel plates obtained from each example of the present invention at room temperature according to ASTM A370-2021. A yield strength, a tensile strength, and a yield ratio of the steel plates of each example were tested. And the longitudinal tensile properties of the steel plates after a strain aging with 3% pre-strain and holding at 250°C for 1 hour were tested. The specific results are shown in Table 5.

[0065]Low-temperature toughness test: According to ASTM E23-2018, impact tests were conducted on the test steel plates at -45°C using V-notched samples, and the Charpy impact energy at -45°C for the st...

Claims

1. A high-performance-uniformity subsea pipeline steel, wherein chemical components of the high-performance-uniformity subsea pipeline steel, in weight percentage, are: C: 0.02~0.06%, Si: 0.10~0.30%, Mn: 1.20~1.65%, Cu≤0.30%, Ni≤0.30%, Cr: 0.15~0.30%, Mo≤0.15%, Nb: 0.03~0.05%, V≤0.05%, Ti: 0.008~0.020%, Ca: 0.0012~0.0030%, Al: 0.015~0.045%, and a balance of Fe and unavoidable impurities, wherein, the impurity elements shall be controlled as follow: P≤0.01%, S≤0.001%, B≤0.0004%, N≤0.005%, O≤0.0025%, and H≤0.0002%; and simultaneously satisfying: Ti / N ≥ 2 ; Nb / 7.7 C + V / 4.3 C ≥ 0.10 ; 1.5 ≤ Ca / S ≤ 4 ; 0.30 ≤ Ceq ≤ 0.39 , Ceq = C + Mn / 6 + Cr + V + Mo / 5 + Cu + Ni / 15 ; Pcm ≤ 0.17 , Pcm = C + Si / 30 + Mn + Cu + Cr / 20 + Ni / 60 + Mo / 15 + V / 10 + 5 B .

2. The high-performance-uniformity subsea pipeline steel according to claim 1, wherein the balance is Fe and unavoidable impurities.

3. The high-performance-uniformity subsea pipeline steel according to claim 1 or 2, wherein a microstructure of the subsea pipeline steel is ferrite + bainite, wherein a volume fraction of the ferrite is 20-40%.

4. The high-performance-uniformity subsea pipeline steel according to any one of claims 1-3, wherein a grain size grade of the subsea pipeline steel is ≥ grade 9.

5. The high-performance-uniformity subsea pipeline steel according to any one of claims 1-4, wherein: transverse tensile properties of the subsea pipeline steel are as follows: yield strength Rt0.5: 415~555MPa, tensile strength Rm: 515~670MPa, and a yield ratio Rt0.5 / Rm of ≤0.88; and a difference in yield strength between the head and tail of the steel plate and a difference in tensile strength between the head and tail of the steel plate are both ≤50MPa; longitudinal tensile: yield strength Rt0.5: 405~530MPa, tensile strength Rm: 505~650MPa, and a yield ratio Rt0.5 / Rm of ≤0.86; and a difference in yield strength between the head and tail of the steel plate and a difference in tensile strength between the head and tail of the steel plate are both ≤50MPa; after a strain aging test with a 3% pre-strain and holding at 250°C for 1 hour, the longitudinal tensile properties of the steel plate are as follows: yield strength Rt0.5: 405~530MPa, tensile strength Rm: 505~650MPa, and a yield ratio Rt0.5 / Rm of ≤0.88.

6. The high-performance-uniformity subsea pipeline steel according to any one of claims 1-5, wherein a Charpy impact energy of the subsea pipeline steel at -45°C is ≥250J, and a difference in Charpy impact energy between the thickness center of the steel plate and the near-surface of the steel plate is ≤50J; a SA of the DWTT test at -30°C is ≥85%, and a difference in SA between a single-sided thinned DWTT sample and a full-thickness DWTT sample is ≤5%; a hardness is ≤230HV10, wherein, a difference between the surface hardness of the steel plate and center hardness of the steel plate is ≤25HV10.

7. A manufacturing method for the high-performance-uniformity subsea pipeline steel according to any one of claims 1-6, comprising the following steps: 1) smelting and continuous casting the composition according to claim 1 or 2 is smelted and continuously cast into a slab; during the continuous casting process, a superheat of the tundish is controlled at 15~40°C, and a casting speed of the continuous casting slab is controlled at 0.40~1.10m / min, ensuring that the solidification end of the continuous casting slab is located in the horizontal section of a continuous casting caster, and an electromagnetic stirring process is adopted in the continuous casting mold; 2) controlled rolling a heating temperature is 1120~1190°C, a start rolling temperature of rough rolling is 1000~1100°C, a finish rolling temperature of rough rolling is 930~980°C, a reduction ratio of rough rolling is ≥50%, and a spread ratio is 1.20~1.80; a start rolling temperature of finish rolling is 790~880°C, a finish rolling temperature of finish rolling is 780~850°C, and a reduction ratio of finish rolling is ≥70%; 3) controlled cooling a relaxation time is ≥30s, a start cooling temperature is 730~780°C, a cooling rate is 10~40°C / s, a stop cooling temperature is 350~550°C, and a temperature difference between the upper and lower surfaces of the steel plate after stop cooling is ≤30°C; 4) after stop cooling, the steel plate is air-cooled or stack-cooled to room temperature.

8. The manufacturing method according to claim 7, wherein in step 1), a fluctuation value of the superheat of adjacent slabs is ≤10°C and / or a fluctuation of the casting speed of adjacent slabs is ≤10%.

9. The manufacturing method according to claim 7 or 8, wherein in step 1), a dynamic soft reduction process is adopted at the solidification end of the continuous casting slab, with a reduction amount of 2~3mm.

10. The manufacturing method according to claim 7 or 8, wherein in step 1), a reduction ratio of rough rolling is 50~80%, and a reduction ratio of finish rolling is 70~85%.

11. The manufacturing method according to claim 7, wherein in step 3), a temperature difference of water entry between the head and tail of the steel plate is ≤30°C.

12. The manufacturing method according to claim 7, wherein in step 3), a relaxation time is 30~60s.

13. The manufacturing method according to claim 7, wherein in step 4), when a stop cooling temperature is ≤450°C, the steel plate is tempered at 450~650°C for 10~30min.

14. The manufacturing method according to claim 7 or 13, wherein in step 4), after the steel plate is cooled to room temperature, the head and tail portions of the steel plate are cropped, and a cropping amount of both the head and tail is≥250mm.

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