High-performance uniform subsea pipeline steel and manufacturing method thereof

Abstract

The invention discloses high-performance uniform subsea pipeline steel and a manufacturing method thereof. The high-performance uniform subsea pipeline steel comprises the following chemical components in percentage by weight: 0.02 to 0.06 percent of C, 0.10 to 0.30 percent of Si, 1.20 to 1.65 percent of Mn, less than or equal to 0.30 percent of Cu, less than or equal to 0.30 percent of Ni, 0.15 to 0.30 percent of Cr, less than or equal to 0.15 percent of Mo, 0.03 to 0.05 percent of Nb, less than or equal to 0.05 percent of V, 0.008 to 0.020 percent of Ti, 0.0012 to 0.0030 percent of Ca, 0.015 to 0.045 percent of Al and the balance of Fe and inevitable impurities. In addition, impurity elements such as P < = 0.01%, S < = 0.001%, B < = 0.0004%, N < = 0.005%, O < = 0.0025% and H < = 0.0002% need to be controlled. According to the manufacturing method of the subsea pipeline steel, an austenite recrystallization zone and non-recrystallization zone two-stage controlled rolling technology, broadening ratio control and controlled cooling are adopted, and the obtained subsea pipeline steel has good performance uniformity on the basis of having high performance.

Description

Technical Field

[0001] The invention belongs to the field of pipeline steel manufacturing, and in particular relates to high-performance uniform submarine pipeline steel and a manufacturing method thereof. Background Art

[0002] The laying of submarine pipelines has a long history of development in the world. Since the first submarine pipeline was laid by Brown & Root Marine Engineering Company in the Gulf of Mexico in 1954, a large number of submarine pipelines of various types and diameters have been successfully laid in various offshore waters around the world, including the North Sea, the Gulf of Mexico, the Mediterranean, Australia and the South China Sea. In particular, in recent years, with the gradual depletion of terrestrial oil and gas resources, the exploitation of oil and gas resources has extended to areas with harsh geological conditions such as the ocean and the polar regions, which has put forward a stronger demand for high-performance submarine pipeline steel.

[0003] Submarine pipelines will be subjected to large compression and tensile loads and even bending deformation during laying; during service, harsh working conditions such as deep-water high pressure, swells, strong bottom currents, and seabed movement will also cause large plastic deformation and even fracture, causing serious environmental disasters. These complex working conditions place more stringent requirements on deepwater pipelines than onshore pipelines, and as the water depth increases, the requirements for raw materials continue to increase.

[0004] Submarine pipeline construction is characterized by high risk and high investment. Compared with onshore pipeline construction, it has very high requirements for construction quality, construction environment, construction technology, etc. At the same time, in order to avoid the risk of difficulty in repair and environmental pollution caused by the failure of submarine pipelines, more stringent technical requirements are often put forward to improve the quality of submarine pipelines, and process management is strengthened to improve product reliability. Therefore, requirements are put forward for the isotropy of steel plates (more uniform strength / toughness distribution in the length / thickness direction), and steel plates are also required to have good welding performance, low-temperature fracture toughness, strain aging performance, etc. In addition, since submarine pipelines are subjected to large axial forces and displacements during laying and service, their bearing capacity and strain capacity can be guaranteed by controlling longitudinal performance, so longitudinal tensile performance, especially longitudinal low yield ratio, is also required for steel plates.

[0005] Chinese patent CN201210123639.X discloses "a thick-walled submarine pipeline steel plate and its production method", which is composed of the following components in weight percentage: 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 the balance is Fe and unavoidable impurities. It has the characteristics of high low-temperature impact toughness, good tear resistance, high strength, moderate yield strength ratio and good elongation. However, the pipeline steel in this patent can only meet the low temperature toughness requirement of -20℃, but cannot meet the toughness requirement at extremely low temperature, and it is difficult to meet the tensile performance requirements after aging, especially the low yield strength ratio requirement. Most importantly, the realization of performance uniformity is not considered.

[0006] Chinese patent CN201310457780.8 discloses "a low-cost thick-gauge submarine pipeline steel plate and its manufacturing method", which is composed of the following components by weight percentage: 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 the rest is iron and unavoidable impurities. The produced steel plate has a dual-phase microstructure, achieves high strength and low yield ratio, is suitable for the large deformation resistance requirements of submarine pipelines, and has good toughness. However, the pipeline steel in this patent can only meet the low temperature toughness requirement of -30°C, cannot meet the toughness requirement at extremely low temperatures, and is difficult to meet the tensile performance requirements after aging, especially the low yield ratio requirement, and this patent does not consider the realization of performance uniformity.

[0007] Chinese patent CN201210492187.2 discloses "thick wall high DWTT performance X65-70 submarine pipeline steel and manufacturing method", whose weight percentage is: 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%, the rest is Fe and inevitable 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, the rest is iron and inevitable impurities. The steel plate has thick specifications, high strength, excellent low-temperature impact toughness and weldability and good DWTT performance. However, the pipeline steel in this patent can only meet the -20℃ low-temperature toughness requirements, cannot meet the low yield ratio requirements and the tensile performance requirements after aging, and does not consider the realization of performance uniformity.

[0008] Chinese patent CN201410515400.6 discloses "a kind of extra-thick specification X70 pipeline steel and its manufacturing method", whose chemical composition is 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 the balance is Fe and unavoidable impurity elements. A pipeline steel with large thickness, high strength and narrow strength range, excellent low-temperature toughness, low-temperature aging resistance and HIC resistance is obtained. However, the pipeline steel in this patent can only meet the low-temperature toughness requirements of -30°C, cannot meet the toughness requirements at extremely low temperatures, and does not consider the realization of performance uniformity.

[0009] Chinese patent CN201510383013.6 discloses "a method for producing low yield strength ratio acid-resistant submarine pipeline steel", with chemical composition: 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 the balance is Fe and unavoidable impurity elements. The patent adopts advanced smelting and continuous casting process technology to control the narrow fluctuation range of carbon equivalent and the number, size and shape of inclusions, and adopts relaxation phase transformation process to control the organizational morphology of steel plate and adopts new water cooling control mode to improve the water cooling uniformity of steel plate, effectively ensuring that the steel plate has low yield strength ratio and excellent acid resistance. However, the patent does not take into account the realization of performance uniformity of the pipeline steel.

[0010] Chinese patent CN202011039510.1 discloses "a submarine pipeline steel plate coil with excellent low-temperature CTOD performance and its production process", whose chemical composition and mass percentage are: 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 the rest are Fe and unavoidable impurity elements. This makes the submarine pipeline steel plate coil have excellent CTOD performance, but does not consider the performance uniformity of pipeline steel.

[0011] Existing patent research shows that although existing submarine pipeline steels take strength and low-temperature toughness into consideration, not much research has been done on how to ensure that pipeline steels have good performance uniformity on the basis of high performance. Summary of the invention

[0012] The object of the present invention is to provide a high-performance uniform submarine pipeline steel and a manufacturing method thereof, wherein the pipeline steel has high strength and good low-temperature toughness, and at the same time, can ensure the uniformity of the performance of the pipeline steel.

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

[0014] A high-performance uniform submarine pipeline steel, the chemical composition weight percentage of which is: 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%, the balance includes Fe and other inevitable impurities, and the impurity elements P≤0.01%, S≤0.001%, B≤0.0004%, N≤0.005%, O≤0.0025%, H≤0.0002% need to be controlled; and at the same time, the following are satisfied:

[0015] Ti / N≥2;

[0016] (Nb / 7.7C+V / 4.3C)≥0.10;

[0017] 1.5≤Ca / S≤4;

[0018] 0.30≤Ceq≤0.39, Ceq=C+Mn / 6+(Cr+V+Mo) / 5+(Cu+Ni) / 15;

[0019] Welding cold crack sensitivity index Pcm≤0.17, Pcm=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B.

[0020] Furthermore, the balance is Fe and other inevitable impurities.

[0021] The microstructure of the submarine pipeline steel of the present invention is ferrite+bainite, wherein the ferrite content is 20-40%.

[0022] The grain size grade of the submarine pipeline steel of the present invention is ≥9.

[0023] Transverse tensile properties of the submarine pipeline steel of the present invention: yield strength R t0.5 : 415~555MPa, tensile strength R m : 515~670MPa, yield strength ratio R t0.5 / R m ≤0.88; the head-to-tail strength difference of the steel plate is ≤50MPa;

[0024] Longitudinal tensile strength: yield strength R t0.5 : 405~530MPa, tensile strength R m : 505~650MPa, yield strength ratio R t0.5 / Rm≤0.86; the strength difference between the head and tail of the steel plate is ≤50MPa;

[0025] After 3% pre-strain and 250℃ heat preservation for 1 hour, the longitudinal tensile properties of the steel plate are: yield strength R t0.5 : 405~530MPa, tensile strength R m : 505~650MPa, yield strength ratio R t0.5 / Rm≤0.88.

[0026] The submarine pipeline steel of the present invention has a Charpy impact energy of ≥250J at -45°C, and a difference in Charpy impact energy between the center and near the surface of the steel plate thickness of ≤50J; a DWTT test SA of ≥85% at -30°C, and a difference in SA between a single-sided thinned DWTT specimen and a full-thickness DWTT specimen of ≤5%; and a hardness of ≤230HV10, wherein a difference between the surface hardness of the steel plate and the hardness at the center of the steel plate is ≤25HV10.

[0027] The characteristics of the submarine pipeline steel composition design of the present invention are:

[0028] 1. Adopt low C and low Mn alloy composition system, control 0.30≤Ceq≤0.39, Pcm≤0.17, ensure good welding performance under the premise that the strength and toughness of the material meet the requirements, and at the same time improve the center segregation and banded structure to ensure the isotropy of material properties.

[0029] 2. Impurity elements P, S, B, N, O, and H are controlled at extremely low levels to reduce the number of inclusions, lower the degree of segregation, and ensure strength and toughness, which is especially beneficial to ensure the excellent low-temperature toughness of pipeline steel.

[0030] 3. Adding alloy elements Cu, Ni, Cr, and Mo plays a role in solid solution strengthening, while affecting the phase change behavior of the material during cooling and improving the isotropy of the steel plate performance.

[0031] 4. Add trace amounts of V-Nb-Ti, and strictly limit Ti / N≥2, (Nb / 7.7C+V / 4.3C)≥0.10, to form microalloy carbonitrides, which refine the grains, strengthen through precipitation, and fix C and N atoms, thus reducing the effects of strain aging on material properties.

[0032] C: The most basic strengthening element. Carbon dissolves in steel to form interstitial solid solution, which plays a role of solid solution strengthening. It forms carbide precipitation with strong carbide-forming elements, which plays a role of precipitation strengthening. However, as the C content increases, the ductility, toughness and welding performance of the steel are significantly reduced, and at the same time, the center segregation of the continuous casting billet is caused, which is not conducive to the uniformity of the steel plate performance in the thickness direction; in addition, after the steel plate is strain-aged, the free C atoms will be entangled with the dislocations, hindering the movement of the dislocations, increasing the yield strength, which is not conducive to the maintenance of the low yield strength ratio. Therefore, the present invention controls the C content to 0.02-0.06%.

[0033] Mn: solid solution strengthening element, is the most important and economical strengthening element in steel to compensate for the strength loss caused by the reduction of C content. Mn is also an element that expands the γ phase region, which can improve the hardenability of steel, reduce the γ→α phase transformation temperature of steel, help to obtain fine phase transformation products, and improve the toughness of steel. However, Mn is also an element that is easy to segregate. When the content is too much, it is easy to produce center segregation in the continuous casting billet, and it is easy to generate hard phase structure during the cooling process after rolling, which reduces the low-temperature toughness of the material and is not conducive to the uniformity of the thickness direction of the steel plate performance. Therefore, in the present invention, the Mn content is controlled to be 1.20-1.65%.

[0034] Si: a solid solution strengthening element and also a deoxidizing element in steel. However, if the content is too high, the welding performance of the steel will be deteriorated, especially the toughness of the welding heat affected zone will be deteriorated, and it will be unfavorable to remove the iron oxide scale during hot rolling. Therefore, in the present invention, the Si content is controlled to be 0.10-0.30%.

[0035] Cu, Ni: solid solution strengthening elements, which can improve the toughness of steel and also improve the hardenability of steel and delay the transformation of pearlite during the cooling of steel plate. However, excessive addition is not good for the toughness and welding performance of steel plate. Therefore, in the present invention, the content of Cu and Ni is controlled to ≤0.30%.

[0036] Cr, Mo: solid solution strengthening elements, important elements for improving the hardenability of steel, promoting the formation of acicular ferrite, improving the toughness of steel, and helping to ensure the uniformity of the structure and performance of the steel plate throughout its thickness; however, excessive content will promote the formation of low-temperature phase transformation structure, which is not conducive to low-temperature toughness. Therefore, in the present invention, the Cr content is controlled to be 0.15-0.30%, and the Mo content is controlled to be ≤0.15%.

[0037] Nb: one of the important elements of low-carbon microalloy steel. The Nb strain-induced precipitation of solid-dissolved Nb during hot rolling forms Nb (C, N) particles, which can delay austenite recrystallization, increase the austenite recrystallization temperature of steel, make two-stage controlled rolling possible, avoid the formation of mixed crystals, and help obtain uniform and fine structures, while improving the toughness of the material, ensuring the isotropy of the steel plate performance. In addition, the solid-dissolved Nb disperses and precipitates in the matrix as the second phase particle NbC during the cooling process, which plays a precipitation strengthening role. However, too high a content of Nb cannot be completely dissolved, not only does not play a role but also increases production costs, and causes NbC to precipitate prematurely at high temperatures, and is prone to forming larger-sized NbC precipitation at the center segregation, which is unfavorable for toughness. Therefore, the Nb content is controlled at 0.03-0.05% in the present invention.

[0038] V: One of the important microalloying elements, it is easy to combine with C in steel to form VC, which has a high precipitation strengthening effect and a weak grain refining effect. However, if the V content is too high, the precipitation particles will be obviously coarsened, which will damage the low-temperature toughness of the steel. In the present invention, V mainly compensates for the strength reduction caused by other alloy deficiencies by playing the role of precipitation strengthening. Therefore, in the present invention, the V content is controlled to be ≤0.05%.

[0039] In addition, in order to give full play to the carbon fixation effect of Nb and V and reduce the influence of free carbon on strain aging properties, it is necessary to control (Nb / 7.7C+V / 4.3C)≥0.10.

[0040] Ti: It is a strong carbonitride-forming element that can play a role in nitrogen fixation. It can form a fine, high-temperature stable TiN precipitate phase during slab continuous casting, thereby improving the toughness of the material. In addition, the undissolved carbonitrides of Ti can prevent the growth of austenite grains when the steel is heated, and the TiN and TiC precipitated during rough rolling in the high-temperature austenite zone can effectively inhibit the growth of austenite grains. In addition, during the welding process, the TiN and TiC particles in the steel can significantly prevent the growth of grains in the heat-affected zone, thereby improving the welding performance of the steel plate. Therefore, in the present invention, the Ti content is controlled at 0.008-0.020%; at the same time, in order to give full play to the nitrogen fixation effect of Ti, it is necessary to control Ti / N≥2.

[0041] Ca: It is mainly used to modify MnS inclusions, making the inclusions spheroidized and evenly distributed, which is beneficial to improving low-temperature toughness. However, excessive content is prone to form CaO inclusions, which is detrimental to toughness. Therefore, in the present invention, the Ca content is controlled to be 0.0012-0.0030%. At the same time, in order to give full play to the sulfide modification effect of Ca, 1.5≤Ca / S≤4 is controlled to transform the long strip MnS inclusions into dispersed spherical CaS inclusions, thereby improving the low-temperature toughness of the material.

[0042] Al: It is mainly used for deoxidation of steel. Adding an appropriate amount of Al is beneficial to grain refinement and improving the strength and toughness of steel. However, the increase in Al content will cause Al oxides to form in the steel, thereby reducing the toughness of the parent material and the welding heat-affected zone. Therefore, in the present invention, the Al content is controlled to be 0.015-0.045%.

[0043] S, P, B, N, O, H: inevitable impurity elements in steel, P is an element that is easy to segregate in steel; S is easy to form sulfides in steel, which will seriously reduce the low-temperature toughness of steel; B is easy to precipitate at the grain boundary, resulting in a decrease in the plasticity and toughness of the material; excessive N and H content is easy to form cracks in the continuous casting billet; excessive O content forms various oxides, which reduces hot workability, corrosion resistance, toughness, etc. Therefore, the present invention controls P≤0.01%, S≤0.001%, B≤0.0005%, N≤0.005%, O≤0.0025%, and H≤0.0002%.

[0044] Pcm: An indicator of welding cold crack sensitivity. When the Pcm value exceeds a certain value, the toughness of the weld zone is significantly reduced due to the excessive hardenability of the steel. The present invention controls Pcm≤0.17.

[0045] The present invention adopts a low C and low Mn alloy component system, reduces the center segregation under the premise of meeting the strength requirements of the steel plate, combines the effects of Cr, Mo, Cu, and Ni in improving hardenability and inhibiting ferrite transformation, controls the ferrite content to 20-40%, further reduces the center segregation, and meets the good uniformity in the thickness direction of the steel plate. The SA% difference between the single-sided thinning DWTT sample and the full-thickness DWTT sample is ≤5%, and the difference between the surface hardness of the steel plate and the center hardness of the steel plate is ≤25HV10. In addition, through the combination of C, Mn and Cu, Ni, Cr, Mo, and V, 0.30≤Ceq≤0.39 is controlled to ensure the strength performance of the steel plate.

[0046] Usually, microalloying elements Nb and V are added to pipeline steel to play the role of fine grain strengthening and precipitation strengthening. However, in the present invention, the fixing effect of Nb and V on C atoms is utilized, and (Nb / 7.7C+V / 4.3C)≥0.10 is designed to maximize the fixing effect of Nb and V on C atoms, reduce the influence of free carbon on strain aging performance, and meet the requirements of submarine pipeline steel for the unconventional performance of tensile performance after strain aging. After the strain aging test after 3% pre-strain and 250℃ insulation for 1 hour, the longitudinal tensile properties of the steel plate are: yield strength R t0.5 : 405~530MPa, tensile strength R m : 505~650MPa, yield strength ratio R t0.5 / Rm≤0.88.

[0047] The present invention controls the ferrite content to be between 20% and 40%. Obtaining a suitable proportion of ferrite is the key to meeting the performance requirements of submarine pipeline steel. If the ferrite content is too low, the low yield ratio requirement cannot be met, and if the ferrite content is too high, the high strength is insufficient. Therefore, in terms of component design, Cu, Ni, Cr, and Mo alloy elements are added to obtain a certain austenite stability, and the ferrite transformation amount can be controlled within a desired range.

[0048] The method for manufacturing high-performance uniform submarine pipeline steel of the present invention comprises the following steps:

[0049] 1) Smelting and continuous casting

[0050] The above components are smelted and continuously cast into billets; during the continuous casting process, the superheat of the tundish is controlled at 15-40°C, and the pulling speed of the continuous casting billet is controlled at 0.40-1.10 m / min to ensure that the solidification end of the continuous casting billet is in the horizontal section of the continuous casting machine; the continuous casting machine crystallizer adopts an electromagnetic stirring process;

[0051] 2) Controlled rolling

[0052] The heating temperature is 1120-1190°C, the rough rolling start temperature is 1000-1100°C, the rough rolling final rolling temperature is 930-980°C, the rough rolling reduction rate is ≥50%, and the width ratio is 1.20≤≤1.80; the finishing rolling start temperature is 790-880°C, the finishing rolling final rolling temperature is 780-850°C, and the finishing rolling reduction rate is ≥70%;

[0053] 3) Controlled cooling

[0054] Relaxation time ≥30s, cooling start temperature 730~780℃, cooling rate 10~40℃ / s, cooling stop temperature 350~550℃, temperature difference between upper and lower surfaces of steel plate after cooling stop ≤30℃;

[0055] 4) After the steel plate stops cooling, air cool or pile cool it to room temperature.

[0056] Preferably, in step 4), when the cooling stop temperature is ≤450°C, the steel plate is tempered at 450-650°C for 10-30 min.

[0057] Preferably, in step 1), the fluctuation value of superheat degree of adjacent slabs is ≤10°C and / or the fluctuation value of drawing speed of adjacent slabs is ≤10%.

[0058] Preferably, in step 1), a dynamic light reduction process is adopted at the end of solidification of the continuous casting billet, and the reduction amount is 2 to 3 mm.

[0059] Preferably, in step 3), the temperature difference between the head and tail of the steel plate entering water is ≤30°C.

[0060] Preferably, in step 4), when the cooling stop temperature is ≤450°C, the steel plate is tempered at 450-650°C for 10-30 min.

[0061] Preferably, in step 4), after the steel plate is cooled to room temperature, the head and tail parts of the steel plate are cut off, and the head and tail cutting amount is ≥250mm.

[0062] In the manufacturing method of the present invention:

[0063] In order to reduce the central segregation of the continuous casting billet and improve the low-temperature toughness and isotropy of the steel plate, the continuous casting process needs to meet the following requirements:

[0064] ① The superheat of the tundish is controlled at 15-40℃, and the superheat fluctuation of adjacent slabs is ≤10℃;

[0065] ② The continuous casting billet pulling speed is controlled at 0.40~1.10m / min, with constant pulling speed control, and the pulling speed fluctuation of adjacent slabs is ≤10%, ensuring that the solidification end of the continuous casting billet is in the horizontal section of the continuous casting machine;

[0066] ③ Use electromagnetic stirring technology in the continuous casting mold section;

[0067] ④ Dynamic light reduction process is adopted at the end of solidification of continuous casting billet.

[0068] The present invention ensures the stability of the continuous casting process by controlling the tundish superheat, the continuous casting billet pulling speed and strictly limiting the fluctuation range, and can ensure that the solidification end of the continuous casting billet is in the horizontal section of the continuous casting machine, and maximize the dynamic light pressure effect, thereby ensuring good control of the center segregation of the slab. At the same time, the continuous casting process adopts electromagnetic stirring, which can effectively make the element distribution uniform, thereby reducing the segregation tendency and ensuring the uniformity of pipeline steel performance.

[0069] The heating temperature of the slab is 1120-1190°C. If the heating temperature is too low, the alloy carbides will not be fully dissolved, and it will be difficult to fully exert the effects of solid solution strengthening and precipitation strengthening, and it will be unfavorable for the control of center segregation. If the heating temperature is too high, the grains will coarsen, which is unfavorable for the low-temperature toughness of the steel plate.

[0070] The starting temperature of rough rolling is 1000-1100°C, the finishing temperature of rough rolling is 930-980°C, the rough rolling reduction rate is ≥50%, the width ratio is 1.20≤≤1.80, and the rough rolling is carried out in the complete recrystallization zone of austenite to avoid the appearance of mixed crystals. Sufficient rough rolling reduction rate can fully refine the original austenite grains, which is beneficial to the strength and toughness of the steel plate; at the same time, precise control of the width ratio is beneficial to the control of the plate shape and reduces the difference in the transverse and longitudinal properties of the steel plate.

[0071] The start temperature of finishing rolling is 790-880°C, the final rolling temperature of finishing rolling is 780-850°C, and the finishing reduction rate is ≥70%; finishing rolling is carried out in the austenite non-recrystallization zone, and a sufficient finishing reduction rate can fully flatten the austenite grains, provide more nucleation sites for subsequent phase transformation, thereby refining the final structure and improving the strength and toughness of the steel plate.

[0072] The relaxation time is ≥30s, the cooling start temperature is 730-780℃, the cooling rate is 10-40℃ / s, and the cooling stop temperature is 350-550℃. The appropriate cooling rate and cooling stop temperature can ensure the formation of a dual-phase structure of ferrite + bainite, ensuring the strength and toughness of the steel plate while obtaining the characteristics of a low yield strength ratio; a certain relaxation time and cooling start temperature can ensure the generation of a certain amount of proeutectoid ferrite in the steel plate structure, thereby reducing the yield strength ratio while ensuring strength. The temperature difference between the head and tail of the steel plate entering the water is ≤30℃. Reducing the temperature difference between the head and tail entering the water is conducive to ensuring the isotropy of the steel plate performance. After cooling, the temperature difference between the upper and lower surfaces of the steel plate is ≤30℃. Reducing the temperature difference between the upper and lower surfaces after cooling is conducive to improving the isotropy of the thickness direction performance.

[0073] Higher final rolling temperature, lower final cooling temperature and faster cooling rate are beneficial to inhibiting the enrichment of alloy elements to the center of the plate thickness during phase transformation, thereby further reducing the impact of center segregation.

[0074] The steel plate is cooled to room temperature after the cooling is stopped. Preferably, the steel plate can be cooled to room temperature by stack cooling.

[0075] Preferably, for a steel plate whose cooling stop temperature is ≤ 450°C, tempering may be performed at 450-650°C for 10-30 minutes.

[0076] After the steel plate stops cooling, it can usually be cooled to room temperature on a cooling bed; or the rolled steel plates can be stacked together and cooled to room temperature.

[0077] The stack cooling method has a slow cooling speed, which can keep the steel plate at high temperature for a long time, which is conducive to the precipitation of alloy carbides and gives full play to the precipitation strengthening and carbon fixation effects. For steel plates with low cooling temperature, the precipitation time of alloy carbides is insufficient. The use of short-term high-temperature tempering can give full play to the precipitation strengthening and carbon fixation effects of alloys, and is also conducive to the decomposition of hard phase MA components, which is conducive to improving low-temperature strength and toughness.

[0078] The head and tail cut-off amount of the finished steel plate after rolling is ≥250mm. Due to the large differences in the deformation conditions, cooling conditions and the steel plate body at the head and tail of the steel plate, the organizational properties are greatly different from the steel plate body. Ensuring a certain head and tail cut-off amount eliminates the influence of uncontrolled deformation and cooling positions, which is conducive to achieving isotropy of steel plate performance.

[0079] Compared with the prior art, the present invention has the following advantages:

[0080] In terms of composition design, the present invention adds relatively low content of C and Mn elements, controls the impurity elements P, S, B, N, O and H at extremely low levels, combines the addition of alloy elements Cr, Mo, Cu and Ni, and controls 0.30≤Ceq≤0.39 and Pcm≤0.17, thereby improving central segregation, affecting the phase transformation behavior during the cooling process of the material, increasing the stability of austenite, delaying the phase transformation process of ferrite, and ensuring good uniformity of the performance of the steel plate while ensuring the mechanical properties of the steel plate.

[0081] The present invention utilizes the addition of trace V-Nb-Ti and strictly limits Ti / N≥2, (Nb / 7.7C+V / 4.3C)≥0.10 to form microalloy carbonitride, refine the grains, strengthen through precipitation, and fix C and N atoms, thereby weakening the influence of strain aging on material properties; the performance remains relatively high after strain aging. After the steel plate is subjected to 3% pre-strain and 250°C heat preservation 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, yield strength ratio R t0.5 / Rm≤0.88.

[0082] On the basis of composition design, the continuous casting process strictly controls the tundish superheat, continuous casting speed and its fluctuation range, and uses the dynamic light pressure reduction process to ensure that the solidification end of the continuous casting billet is in the horizontal section of the continuous casting machine, so as to maximize the dynamic light pressure reduction effect, thereby reducing the segregation tendency and ensuring the uniformity of pipeline steel performance. However, in the conventional continuous casting process, there are no special regulations or low requirements for the tundish superheat, continuous casting speed and its fluctuation range, and the parameters are set mainly considering the matching of the production rhythm of the previous and next processes. Therefore, the solidification end of the continuous casting billet is often in the arc section of the continuous casting machine, and the dynamic light pressure applied in the horizontal section cannot fully play the role of improving the internal quality of the slab and reducing the center segregation, and thus cannot meet the requirements for high performance and high uniformity.

[0083] In addition, the two-stage controlled rolling technology and width ratio control in the austenite recrystallization zone and non-recrystallization zone are used to ensure sufficient compression ratio in the two-stage rolling process, fully refine the grains, avoid mixed crystal structure, and control the ferrite phase transformation process from both thermodynamic and kinetic aspects by adjusting the rolling temperature, relaxation time, and cooling temperature to form a dual-phase structure of ferrite + bainite. The ferrite ratio is accurately controlled at 20-40%, ensuring that the steel plate has excellent mechanical properties such as strength and toughness and isotropy, solving the problem of mismatch between strength and performance uniformity of submarine pipeline steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0084] Figure 1 This is a microstructure photograph of the steel according to Example 1 of the present invention. DETAILED DESCRIPTION

[0085] The present invention will be further described below in conjunction with the accompanying drawings.

[0086] The chemical composition of the steel of the embodiment of the present invention is shown in Table 1 and Table 2, and the balance includes Fe and unavoidable impurities; the specific process parameters are shown in Table 3 and Table 4.

[0087] Tensile property test: At room temperature, according to ASTM A370-2021, the tensile property test of the steel plate samples obtained in each embodiment of the present invention was carried out to measure the yield strength, tensile strength and yield strength ratio of the steel plate of each embodiment, and the longitudinal tensile properties of the steel plate after 3% pre-strain and 250°C insulation for 1 hour of strain aging were measured. The specific results are shown in Table 5.

[0088] Low temperature toughness test: At -45°C, using a V-notch specimen, the test steel plate was subjected to an impact test in accordance with ASTM E23-2018, and the -45°C Charpy impact energy of the steel plates of each embodiment was measured, and the specific results are shown in Table 6. At -30°C, using a full-thickness V-notch specimen and a single-sided thinned V-notch specimen, the test steel plate was subjected to a drop weight tear test in accordance with API RP 5L3-2014 to measure the DWTT test SA value of the steel plates of each embodiment at -30°C, and the specific results are shown in Table 7.

[0089] Hardness test: According to ASTM E92, at 10 kgf, the hardness values ​​of the steel plate near the upper surface, the center of the thickness, and the near lower surface were measured. The specific results are shown in Table 8.

[0090] Depend on Figure 1 It can be seen that the microstructure of the steel plate obtained by using the composition and process of the present invention is a ferrite + bainite structure, wherein the ferrite content is 20-40%.

[0091] As can be seen from Table 5, the transverse tensile properties of the steel of the present invention: yield strength R t0.5 : 415~555MPa, tensile strength Rm: 515~670MPa, yield strength ratio R t0.5 / Rm0.88, and the strength difference between the head and tail of the steel plate is ≤50MPa.

[0092] Longitudinal tensile properties: yield strength R t0.5 : 405~530MPa, tensile strength Rm: 505~650MPa, yield strength ratio R t0.5 / Rm0.86, and the strength difference between the head and tail of the steel plate is ≤50MPa.

[0093] After 3% pre-strain and 250℃ insulation for 1 hour, the longitudinal tensile properties of the steel plate are: yield strength R t0.5: 405~530MPa, tensile strength Rm: 505~650MPa, yield strength ratio R t0.5 / Rm0.88.

[0094] It can be seen from Table 6 that the -45°C Charpy impact energy of the steel plate obtained by the present invention is ≥250J, and the difference in Charpy impact energy between the center of the steel plate thickness and the near surface is ≤50J.

[0095] It can be seen from Table 7 that the SA of the steel plate obtained in the present invention at -30°C DWTT test is ≥85%. In particular, the SA difference between the single-side thinned DWTT specimen (tested at a correspondingly lower temperature according to API RP 5L3) and the full-thickness DWTT specimen is ≤5%.

[0096] It can be seen from Table 8 that the hardness of the steel plate obtained by the present invention is ≤230HV10, and in particular, the difference between the hardness near the surface of the steel plate and the hardness at the center of the steel plate is ≤25HV10.

[0097] In Comparative Example 1, Ti / N=1.5, Ca / S=1.12, both of which do not meet the requirements of Ti / N≥2, 1.5≤Ca / S≤4 of the present invention, and the P, S, and N contents are relatively high. The pulling wave velocity in the process control is not within the control range of the present invention, and cannot meet the requirement that the solidification end of the continuous casting billet is in the horizontal section of the continuous casting machine, and the superheat fluctuation value, the temperature difference between the head and tail of the steel plate entering the water, and the temperature difference between the upper and lower surfaces of the steel plate after cooling are not within the control range of the present invention. Although the strength of the steel plate finally obtained can meet the requirements, the strength difference between the head and tail is too large, exceeding 50MPa, and the yield strength is relatively high. The Charpy impact energy difference between the center and near the surface of the steel plate thickness is 66, the SA difference between the single-sided thinning DWTT sample and the full-thickness DWTT sample is 14, and the difference between the surface hardness of the steel plate and the center hardness of the steel plate is 35. Obviously, the performance uniformity of the steel plate obtained in Comparative Document 1 is not good.

[0098] In Comparative Example 2, Nb / 7.7C+V / 4.3C=0.09, which does not meet the requirement of the present invention that (Nb / 7.7C+V / 4.3C)≥0.10, and the B and O contents are relatively high. The heating temperature, the rough rolling and final rolling temperatures and the relaxation time in the process control are not within the range specified in the present invention. The final obtained -45°C Charpy impact energy is less than 250J, the DWTT test SA at -30°C is less than 85%, and the low-temperature toughness of the steel plate is poor.

[0099] In Comparative Example 3, Ceq=0.40, Pcm=0.18, and Nb / 7.7C+V / 4.3C=0.07, all of which fail to meet the requirements of the present invention of 0.30≤Ceq≤0.39, Pcm≤0.17, and (Nb / 7.7C+V / 4.3C)≥0.10. The SA difference between the single-sided thinned DWTT specimen and the full-thickness DWTT specimen of the steel plate finally obtained is 13, and the Charpy impact energy difference between the thickness center and the near-surface is 115J. The performance uniformity of the steel plate obtained in Comparative Example 3 is not good.

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Claims

1. A high-performance uniform submarine pipeline steel, the chemical composition weight percentage of which is: 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%, the balance includes Fe and other inevitable impurities, and the impurity elements P≤0.01%, S≤0.001%, B≤0.0004%, N≤0.005%, O≤0.0025%, H≤0.0002% need to be controlled; and at the same time, the following are satisfied: Ti / N≥2; (Nb / 7.7C+V / 4.3C)≥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+5B.

2. The high-performance uniform submarine pipeline steel according to claim 1, characterized in that: The balance is Fe and other inevitable impurities.

3. The high performance uniform submarine pipeline steel according to claim 1 or 2, characterized in that: The microstructure of the submarine pipeline steel is ferrite+bainite, wherein the ferrite content is 20-40%.

4. The high performance uniform submarine pipeline steel as claimed in claim 1, 2 or 3, characterized in that: The grain size grade of the submarine pipeline steel is ≥9.

5. The high-performance uniform submarine pipeline steel according to claim 1, 2, 3 or 4, characterized in that: Transverse tensile properties of submarine pipeline steel: yield strength R t0.5 : 415~555MPa, tensile strength R m : 515~670MPa, yield strength ratio R t0.5 / R m ≤0.88; the head-to-tail strength difference of the steel plate is ≤50MPa; Longitudinal tensile strength: yield strength R t0.5 : 405~530MPa, tensile strength R m : 505~650MPa, yield strength ratio R t0.5 / Rm≤0.86; The strength difference between the head and tail of the steel plate is ≤50MPa; After 3% pre-strain and 250℃ heat preservation for 1 hour, the longitudinal tensile properties of the steel plate are: yield strength R t0.5 : 405~530MPa, tensile strength R m : 505~650MPa, yield strength ratio R t0.5 / Rm≤0.

88.

6. The high-performance uniform submarine pipeline steel according to claim 1, 2, 3, 4 or 5, characterized in that: The submarine pipeline steel has a Charpy impact energy of ≥250J at -45°C, and a Charpy impact energy difference between the center and near-surface of the steel plate thickness of ≤50J; a DWTT test SA of ≥85% at -30°C, and a SA difference between a single-sided thinned DWTT specimen and a full-thickness DWTT specimen of ≤5%; and a hardness of ≤230HV10, wherein the difference between the surface hardness of the steel plate and the hardness at the center of the steel plate is ≤25HV10.

7. The method for manufacturing high-performance uniform submarine pipeline steel according to any one of claims 1 to 6, characterized in that: The steps include: 1) Smelting and continuous casting The composition according to claim 1 or 2 is smelted and continuously cast into billets; during the continuous casting process, the superheat of the tundish is controlled at 15-40°C, and the pulling speed of the continuous casting billet is controlled at 0.40-1.10 m / min to ensure that the solidification end of the continuous casting billet is in the horizontal section of the continuous casting machine, and the continuous casting machine crystallizer adopts an electromagnetic stirring process; 2) Controlled rolling The heating temperature is 1120-1190°C, the rough rolling start temperature is 1000-1100°C, the rough rolling final rolling temperature is 930-980°C, the rough rolling reduction rate is ≥50%, and the width ratio is 1.20≤≤1.80; the finishing rolling start temperature is 790-880°C, the finishing rolling final rolling temperature is 780-850°C, and the finishing rolling reduction rate is ≥70%; 3) Controlled cooling Relaxation time ≥30s, cooling start temperature 730~780℃, cooling rate 10~40℃ / s, cooling stop temperature 350~550℃, temperature difference between upper and lower surfaces of steel plate after cooling stop ≤30℃; 4) After the steel plate stops cooling, air cool or pile cool it to room temperature.

8. The manufacturing method according to claim 7, characterized in that: In step 1), the fluctuation value of superheat degree of adjacent slabs is ≤10°C and / or the fluctuation value of drawing speed of adjacent slabs is ≤10%.

9. The manufacturing method according to claim 7 or 8, characterized in that: In step 1), a dynamic light reduction process is adopted at the end of solidification of the continuous casting billet, with a reduction amount of 2 to 3 mm.

10. The manufacturing method according to claim 7, characterized in that: In step 3), the temperature difference between the head and tail of the steel plate entering water is ≤30°C.

11. The manufacturing method according to claim 7, characterized in that: In step 4), when the cooling stop temperature is ≤450°C, the steel plate is tempered at 450-650°C for 10-30 min.

12. The manufacturing method according to claim 7 or 11, characterized in that: In step 4), after the steel plate is cooled to room temperature, the head and tail parts of the steel plate are cut off, and the head and tail cutting amount is ≥250mm.

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