A high-strength pressure vessel steel resistant to liquid carbon dioxide corrosion and its manufacturing method

Abstract

This invention discloses a high-strength pressure vessel steel resistant to liquid carbon dioxide corrosion, which contains Fe and unavoidable impurity elements, and also contains the following chemical elements in the following mass percentages: C: 0.045-0.065%, Si: 0.15-0.45%, Mn: 0.80-1.25%, Alt: 0.020-0.045%, Cu: 0.15-0.25%, Cr: 0.75-1.05%, Mo: 0.10-0.35%, 0 < Ni ≤ 0.15%, 0 < Sn ≤ 0.0020%; and at least one of V, B, and Nb in the following amounts: 0 < V ≤ 0.030%, B: 0.0005-0.0012%, Nb: 0.015-0.040%. Accordingly, the present invention also discloses a method for manufacturing the above-mentioned high-strength pressure vessel steel, which includes the following steps: (1) smelting and casting; (2) hot charging and hot delivery of continuous casting billet + heating furnace: the charging temperature of the high-temperature continuous casting billet directly into the heating furnace is 820-900℃, the heating temperature in the heating furnace is controlled at 1100-1245℃, and the heating rate is 8-13min / cm; (3) rapid cooling after rolling: the cooling rate is controlled at 12-40℃ / s, and the final cooling temperature is 310-390℃; (4) rapid cooling after rolling; (5) heat treatment.

Description

Technical Field

[0001] This invention relates to a steel plate and its manufacturing method, and more particularly to a steel plate for pressure vessels and its manufacturing method. Background Technology

[0002] In recent years, with the rapid development of industrial production, pressure vessels have been increasingly used in various industries. Currently, traditional pressure vessels such as large fixed spherical storage tanks and mobile pressure vessels often need to be made of high-strength pressure vessel steel. However, these pressure vessels are often used to hold some special corrosive substances and are easily corroded, especially when subjected to stress and corrosion, which can lead to cracks in the pressure vessel steel.

[0003] As is well known, although pure carbon dioxide does not have corrosive properties, it exhibits extremely strong corrosiveness when dissolved in water. At the same pH value, the total acidity of carbon dioxide is higher than that of hydrochloric acid. Therefore, in practical applications, carbon dioxide is more corrosive to steel than hydrochloric acid.

[0004] Currently, in actual use, when pressure vessels contain liquid carbon dioxide, carbon dioxide corrosion can cause the service life of the pressure vessel to be much shorter than the design life. Its corrosion rate on low carbon steel is as high as 7 mm / a, and sometimes even higher. This corrosion rate of low carbon steel is considered extremely severe corrosion.

[0005] Research has shown through actual equipment testing that the higher the strength of the steel, the more susceptible it is to cracking under the corrosive effects of carbon dioxide. Therefore, there is an urgent need to obtain a pressure vessel steel with both high strength and good resistance to liquid carbon dioxide corrosion.

[0006] Based on this, the inventors aim to overcome the problems of excessively high strength and excessive hardness of existing pressure vessel steel plates and welded joints, and to solve the problem of carbon dioxide corrosion of the steel plates and welded joints, thereby improving the service life of such steel containers holding liquid carbon dioxide. Therefore, this invention aims to provide a new high-strength pressure vessel steel with excellent resistance to liquid carbon dioxide corrosion and a method for manufacturing the same.

[0007] Although steel plates with resistance to carbon dioxide corrosion already exist in the existing technology, they still cannot meet the performance requirements of specific strength and manufacturing process parameters, and there are still shortcomings in performance.

[0008] For example, Chinese patent document CN101928888A, published on December 29, 2010, entitled "A Low Alloy Steel for Carbon Dioxide Corrosion Resistance and Its Manufacturing Method", discloses a low alloy steel for carbon dioxide corrosion resistance and its manufacturing method. It adopts the design concept of C and Cr-Mo-V-Al alloying. However, the chemical composition designed by this scheme will make the strength or hardness of the finished steel plate too high. It is very easy to crack in the liquid carbon dioxide environment, and cannot guarantee the safe service performance of the material.

[0009] For example, Chinese patent document CN103320705A, published on September 25, 2013, entitled "CO2 Corrosion Resistant Pipeline Steel for Ground Gathering and Transportation and its Preparation Method," discloses a CO2 corrosion resistant pipeline steel for ground gathering and transportation and its preparation method. This method employs a low-C and Cr-Mo-Cu-Ni alloy design, with the steel plate being hot-rolled and then directly cooled. However, the strength and performance parameters of the steel plate produced by this method differ significantly from those of the present invention, and it cannot meet the service environment and performance requirements of the present invention.

[0010] For example, Chinese patent document CN107904496A, published on April 13, 2018, entitled "A Carbon Dioxide Corrosion Resistant Pipeline Steel and Its Manufacturing Method," discloses a carbon dioxide corrosion resistant pipeline steel and its manufacturing method. This technical solution uses a Cr-Mo-Nb-Ti alloy system, and its supply state is controlled rolling. However, the strength and performance parameters of this steel plate are still far from those of the present invention, and it cannot meet the service environment and service performance requirements of the present invention. Summary of the Invention

[0011] One objective of this invention is to provide a high-strength pressure vessel steel resistant to liquid carbon dioxide corrosion. This high-strength pressure vessel steel plate is a steel plate using a low-C-Mn-Cr-Mo-Cu alloy system. By precisely controlling the chemical composition of the steel during the steel plate production process, excellent comprehensive mechanical properties can be obtained. This pressure vessel steel plate not only possesses good strength and toughness, resistance to cold and hot welding cracking, and excellent resistance to liquid carbon dioxide corrosion, but also exhibits good weldability and weldability. It can be used to manufacture pressure vessels, thereby addressing the comprehensive requirements of existing technologies for high-strength pressure vessels with a tensile strength of 600MPa, including resistance to carbon dioxide stress corrosion cracking, high strength reduction and lightweighting, and high parameterization.

[0012] To achieve the above objectives, the present invention provides a high-strength pressure vessel steel resistant to liquid carbon dioxide corrosion, which contains Fe and unavoidable impurity elements, and further contains the following chemical elements in the following mass percentages:

[0013] C: 0.045–0.065%, Si: 0.15–0.45%, Mn: 0.80–1.25%, Alt: 0.020–0.045%, Cu: 0.15–0.25%, Cr: 0.75–1.05%, Mo: 0.10–0.35%, 0 < Ni ≤ 0.15%, 0 < Sn ≤ 0.0020%; and at least one of the following contents of V, B, and Nb: 0 < V ≤ 0.030%, B: 0.0005–0.0012%, Nb: 0.015–0.040%.

[0014] Furthermore, in the high-strength pressure vessel steel plate described in this invention, the mass percentage content of each chemical element is as follows:

[0015] C: 0.045–0.065%, Si: 0.15–0.45%, Mn: 0.80–1.25%, Alt: 0.020–0.045%, Cu: 0.15–0.25%, Cr: 0.75–1.05%, Mo: 0.10–0.35%, 0 < Ni ≤ 0.15%, 0 < Sn ≤ 0.0020%; and at least one of the following contents of V, B, and Nb: 0 < V ≤ 0.030%, B: 0.0005–0.0012%, Nb: 0.015–0.040%; the balance being Fe and unavoidable impurity elements.

[0016] In the above-described technical solution of this invention, a new steel grade is designed using a low-C-Mn-Cr-Mo-Cu alloy system through a rational chemical composition design. This steel plate requires the addition of at least one of V, B, and Nb. On one hand, this utilizes the toughening effect of the NbC precipitate, improving the grain refinement and precipitation strengthening effect of the Nb microalloy. On the other hand, this invention can also add an appropriate amount of V. V can combine with C to form a VC precipitate, which further refines the grains, improves the low-temperature fracture toughness of the steel, and lowers the ductile-brittle transition temperature.

[0017] Meanwhile, in the design of the chemical composition, this invention also strictly controls the added content of elements such as Alt, Mn, and Ni in the steel, and controls the impurity element O to avoid the single or combined effects of these elements damaging the toughness of the steel. It also prevents these elements from segregating and agglomerating during the continuous casting cooling process, especially during the cooling of steel from high temperature to low temperature (980℃ to 700℃), thus preventing further segregation of the continuously cast billet from solidification to low temperature cooling and from rolling into finished steel plates. This prevents the steel plates from experiencing high-probability stress corrosion cracking when used in a liquid carbon dioxide stress corrosion environment.

[0018] The design principles of each chemical element in the high-strength pressure vessel steel plate described in this invention are as follows:

[0019] C: In the high-strength pressure vessel steel plate described in this invention, C is one of the essential elements for improving the strength of the steel. With the increase of C content in the steel, the Fe3C content increases, hardenability also increases, and the yield strength and tensile strength of the steel improve, while elongation and notched impact toughness decrease. Specifically, for every 0.1% increase in C content, the tensile strength increases by approximately 90 MPa, and the yield strength increases by approximately 40-50 MPa. However, it should be noted that the C content in the steel should not be too high. As the C content increases, the elongation and impact toughness of the steel decrease, especially the low-temperature toughness. Moreover, when welding steel with a high C content, hardening will occur in the heat-affected zone, which will exacerbate the tendency for cold cracking during welding. When operating in a carbon dioxide corrosive environment, the steel plate and welded joints will become weak points for stress corrosion cracking. Therefore, considering the influence of carbon element on the properties of pressure vessel steel, the mass percentage of carbon element in the high-strength pressure vessel steel plate of the present invention is controlled between 0.045% and 0.065%. This ensures that the strength stability of the steel is within a suitable range and is suitable for production operations, thereby improving its applicability and feasibility in industrial production.

[0020] Si: In the high-strength pressure vessel steel plate described in this invention, Si can improve the strength of the steel in a solid solution state. When the Si content increases from 0.15% to 0.45%, the strength of the steel remains basically unchanged or increases slightly, while the toughness of the steel is significantly improved. Appropriately increasing the Si content will increase the substitution solid solution strengthening effect in the steel and refine the grains, thereby improving the toughness of the steel. However, it should be noted that the Si content in the steel should not be too high. If the Si content is too high, SiO2 inclusions are easily formed during the steelmaking process. Therefore, in the high-strength pressure vessel steel plate described in this invention, the mass percentage of Si is controlled between 0.15% and 0.45%.

[0021] Mn: In the high-strength pressure vessel steel plate described in this invention, the element Mn plays a significant role in improving the strength of low-carbon steel. Adding 1% Mn to the steel can increase the tensile strength of the steel by approximately 100 MPa. Generally speaking, controlling the mass percentage of Mn in the steel to below 1.70% is beneficial for improving the toughness of the weld metal. In low-carbon high-strength steel, the Mn content can reach up to 1.80%, but in the steel of this invention, excessive Mn will adversely affect the performance of the steel. In addition, Mn will also increase the solubility of elements such as Nb and V in steel. Based on this, considering the influence of Mn content on the performance of steel, the Mn content must be strictly controlled. In this invention, the mass percentage of Mn is controlled between 0.80% and 1.25%. This ensures that the steel obtains suitable tensile strength without compromising the low-temperature impact toughness and elongation of the steel.

[0022] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of Mn element can be preferably controlled between 0.85% and 1.24%.

[0023] Alt: In the high-strength pressure vessel steel plate described in this invention, Alt is added as a deoxidation balancing element during the steelmaking process. Specifically, in the early refining stage, the mass percentage of Alt in the molten steel must be controlled below 0.045%. In the later refining stage, the oxygen content in the steel is already controlled to a low level; if Alt is added again, large-sized chain-like alumina inclusions will form in the molten steel, severely impairing the low-temperature toughness of the finished steel plate. Furthermore, adding Alt in the later refining stage will form a large amount of AlN in the steel. AlN easily precipitates in the continuously cast billet cooling range of 800–950°C, reducing the hot plasticity of the continuously cast billet and also forming corner cracks or intergranular cracks on the surface or corners of the billet. Therefore, considering the impact of Alt on the performance of the pressure vessel steel plate in this technical solution, the mass percentage of Alt in the high-strength pressure vessel steel plate described in this invention is controlled between 0.020% and 0.045%.

[0024] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage of Alt can be preferably controlled between 0.021% and 0.040%.

[0025] Cu: In the high-strength pressure vessel steel plate described in this invention, Cu mainly plays a role in precipitation strengthening. Adding an appropriate amount of Cu to the steel is beneficial for obtaining good low-temperature toughness and increasing the corrosion resistance of the steel plate. However, it should be noted that the Cu content in the steel should not be too high. When the Cu content in the steel is too high, it will not only reduce the toughness of the weld heat-affected zone of the steel plate, but also reduce the solid solubility of Cu in the steel due to the presence of residual elements such as Sn during continuous casting. This will form a low-melting-point copper-rich phase in the iron oxide scale on the surface of the billet, causing surface cracks in the billet under the bending and straightening stress of continuous casting, leading to network cracking of the steel plate during rolling. Based on this, in order to ensure the quality of the steel, the mass percentage of Cu in the high-strength pressure vessel steel plate described in this invention is controlled between 0.15% and 0.25%.

[0026] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage of Cu can be preferably controlled between 0.16% and 0.23%.

[0027] Cr: In the high-strength pressure vessel steel plate described in this invention, Cr is both an element that shrinks the austenite region and a medium-strength carbide-forming element. It can form carbides in steel and can also be dissolved in ferrite. Simultaneously, Cr can improve the corrosion resistance of the pressure vessel steel plate. Cr has a high atomic binding energy between metal and oxygen, and it readily forms dense Cr-containing corrosion products such as Cr(OH)3 and Cr2O3 on the steel surface, preventing corrosive ions in the corrosive medium from entering the iron matrix, thereby improving the corrosion resistance of the steel. Cr is also an effective element for improving the hardenability of steel, but the Cr content in the steel should not be too high, as excessive Cr will increase the steel's sensitivity to cold cracking during welding. Therefore, to ensure the performance of the steel, the mass percentage of Cr is controlled between 0.75% and 1.05% in this invention. This ensures that the steel obtains suitable tensile strength and excellent resistance to carbon dioxide corrosion, while also not compromising the steel's low-temperature impact toughness and elongation.

[0028] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of Cr element can be controlled between 0.80% and 1.00%.

[0029] Mo: In the high-strength pressure vessel steel plate described in this invention, Mo can improve the strength of the steel, especially its high-temperature strength. Its ability to improve the high-temperature strength of steel is higher than that of Mn and Cr. Simultaneously, Mo can improve the steel's resistance to carbon dioxide corrosion. It should be noted that adding 0.50% Mo to steel can increase the high-temperature creep strength of the steel by 75%. A small amount of Mo can also improve the toughness of the weld metal. However, adding Mo also increases the hardenability of the steel, thereby increasing the steel's susceptibility to cold cracking during welding. On the other hand, sufficient Mo content can also ensure the stability of the steel plate after tempering, ensuring that the tempered steel plate still has sufficient strength and toughness. Based on this, considering the influence of Mo on the properties of steel, in the high-strength pressure vessel steel plate described in this invention, the mass percentage of Mo is controlled between 0.10% and 0.35%.

[0030] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of Mo element can be controlled between 0.12% and 0.34%.

[0031] Ni: In the high-strength pressure vessel steel plate described in this invention, Ni has a certain strengthening effect. Adding 1.00% Ni to the steel can increase the steel strength by approximately 20 MPa. Simultaneously, Ni can significantly improve the toughness of the steel, especially the low-temperature toughness of low-carbon bainitic and low-carbon martensitic steels. Adding Ni to the steel can significantly improve the low-temperature toughness of both the base material and the weld heat-affected zone. However, it should be noted that the Ni content in the steel should not be too high. When the Ni content is too high, Ni will promote the carbon dioxide corrosion rate of the steel, reducing its resistance to carbon dioxide corrosion. Therefore, in the high-strength pressure vessel steel plate described in this invention, the mass percentage of Ni is controlled to meet the following condition: 0 < Ni ≤ 0.15%.

[0032] Sn: In the high-strength pressure vessel steel plate described in this invention, the addition of Sn element increases the strength and hardness of the steel, but reduces its toughness. When there is excessive Sn in the steel, the solubility of Cu element in the austenite decreases sharply, causing the formation of a low-melting-point Cu-rich phase in the iron oxide scale on the steel surface. This results in surface cracking during hot working and cold forming of the steel plate, severely affecting the metallurgical and surface quality of the steel. However, by controlling the Sn element in the steel within a certain range, the aforementioned adverse coupling effect of Cu and Sn will not occur. Therefore, in order to ensure the internal metallurgical and surface quality of the high-strength pressure vessel steel plate described in this invention, the Sn element content must be strictly controlled, and the mass percentage of Sn element must be controlled to meet the following condition: 0 < Sn ≤ 0.0020%.

[0033] V: In the high-strength pressure vessel steel plate described in this invention, vanadium (V) is a strong carbonitride forming element. It can refine grains by forming carbides to prevent austenite grain growth. Vanadium carbonitrides can form hydrogen traps in steel, preventing hydrogen-induced cracking. It should be noted that while adding V to steel can greatly improve its strength, the V content should not be too high. Excessive V content will increase the number and size of precipitates, leading to a decrease in steel toughness. Therefore, considering the various strengthening and toughening effects of V in steel, the mass percentage of V in the high-strength pressure vessel steel plate of this invention is controlled to 0 < V ≤ 0.030%.

[0034] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of element V can be controlled between 0.010 and 0.025%.

[0035] Nb: In the high-strength pressure vessel steel plate described in this invention, Nb can promote grain refinement of the steel rolling microstructure, improving the strength and toughness of the steel. During controlled rolling, Nb can effectively refine the microstructure by inhibiting austenite recrystallization and can reduce the steel's overheat sensitivity and temper brittleness. During welding, the segregation and precipitation of Nb can hinder the coarsening of austenite grains during heating, ensuring a relatively fine heat-affected zone microstructure after welding, thereby improving weldability. Therefore, considering the beneficial effects of Nb, the mass percentage of Nb in the high-strength pressure vessel steel plate described in this invention is controlled between 0.015% and 0.040%.

[0036] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of Nb element can be controlled between 0.017% and 0.038%.

[0037] B: In the high-strength pressure vessel steel plate described in this invention, the addition of element B can compensate for the insufficient hardenability and strength caused by insufficient carbon content in the steel. However, excessive amounts of element B should not be added to the steel. As the B content in the steel increases, B will undergo severe segregation at the grain boundaries, and there is a tendency to reduce the strength and toughness of the steel. Therefore, in order to maximize the beneficial effects of element B, the mass percentage of element B in the high-strength pressure vessel steel described in this invention is controlled between 0.0005% and 0.0012%.

[0038] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of element B can be controlled between 0.0006 and 0.0010%.

[0039] Furthermore, in the high-strength pressure vessel steel plate of the present invention, among the unavoidable impurity elements, the content of each impurity element satisfies at least one of the following: P≤0.015%, S≤0.005%, O≤0.0018%.

[0040] In the above technical solution, P, S, and O are all impurity elements in the high-strength pressure vessel steel plate described in this invention. Where technical conditions permit, to obtain steel with better performance and superior quality, the content of impurity elements in the pressure vessel steel plate should be reduced as much as possible. Only by smelting pure steel can the performance of the steel of this invention be guaranteed; therefore, the content of P, S, and O elements in the steel must be controlled within a low range. Thus, in this invention, the P element content is controlled to P≤0.015%, the S element content is controlled to S≤0.005%, and the O element content is controlled to O≤0.0018%.

[0041] Furthermore, in the high-strength pressure vessel steel plate described in this invention, each chemical element satisfies at least one of the following formulas:

[0042] Cu + 6Sn ≤ 0.27;

[0043] Cu / Sn≥20;

[0044] Cu / Ni > 2; all elements in the formula should be substituted with the value before the percentage sign of the element's mass percentage content.

[0045] In the above-described technical solution of this invention, while controlling the mass percentage content of a single chemical element in the steel, this invention can also control the elements in the steel to satisfy the limiting relationship Cu+6Sn≤0.27. Controlling Cu+6Sn≤0.27 is to prevent the melting point of Cu in the steel from being lowered by Sn in the presence of Sn when the continuous casting billet is around 960-1100℃, thus avoiding the formation of a low-melting-point enriched Cu liquid phase in the surface shell of the billet, and preventing corner cracks and surface cracks in the billet under the straightening stress of continuous casting.

[0046] Accordingly, while controlling the mass percentage of a single chemical element in the steel, this invention can also control the elements in the pressure vessel steel plate to satisfy the constraint relationship Cu / Sn≥20. Controlling Cu / Sn≥20 is to limit the harmful effects of Sn segregation at grain boundaries and reduce grain boundary strength, so that Cu in the steel will not have a strong coupling effect with Sn, allowing Cu to play a beneficial role independently in the steel.

[0047] Furthermore, this invention, while controlling the mass percentage content of a single chemical element, can also control the elements in the pressure vessel steel plate to satisfy the constraint relationship Cu / Ni > 2. Controlling Cu / Ni > 2 is to reduce the addition of the precious metal Ni while ensuring the low-temperature toughness of the steel plate; on the other hand, adding Ni to steel weakens the carbon dioxide corrosion resistance of added Cu. Therefore, by limiting the Ni content in the steel while adding Cu, the carbon dioxide corrosion resistance of the Cu in the steel remains unchanged or is enhanced.

[0048] Furthermore, in the high-strength pressure vessel steel plate described in this invention, the mass percentage content of each chemical element also satisfies at least one of the following conditions:

[0049] Mn: 0.85–1.24%;

[0050] Alt: 0.021~0.040%;

[0051] V: 0.010~0.025%;

[0052] Nb: 0.017–0.038%;

[0053] Cu: 0.16–0.23%;

[0054] Cr: 0.80–1.00%;

[0055] Mo: 0.12–0.34%;

[0056] B: 0.0006~0.0010%.

[0057] Furthermore, in the high-strength pressure vessel steel plate described in this invention, its microstructure is bainite or tempered bainite.

[0058] Furthermore, in the high-strength pressure vessel steel plate described in this invention, its performance meets the following requirements: the yield strength of the steel plate is ≥510MPa, the tensile strength is ≥620MPa, the transverse KV2 at -60℃ is ≥185J, the elongation is ≥18%, and the liquid carbon dioxide corrosion rate is 0.013~0.017mm / a; the tensile strength of its welded joint is ≥625MPa, the transverse KV2 at -60℃ is ≥190J, the NDTT transition temperature of the weld heat-affected zone is ≤-65℃, and the liquid carbon dioxide corrosion rate is 0.014~0.018mm / a.

[0059] Accordingly, another objective of the present invention is to provide a method for manufacturing the high-strength pressure vessel steel plate described above. The high-strength pressure vessel steel obtained by this manufacturing method has high strength and excellent resistance to liquid carbon dioxide corrosion, as well as good resistance to cold and hot welding cracks, good weldability, and good weldability.

[0060] To achieve the above objectives, the present invention provides a method for manufacturing the aforementioned high-strength steel plate for pressure vessels, comprising the following steps:

[0061] (1) Smelting and casting;

[0062] (2) Hot charging and hot delivery of continuous casting billet + heating furnace: The charging temperature of the high-temperature continuous casting billet is 820-900℃, and the heating temperature in the heating furnace is controlled at 1100-1245℃, and the heating rate is 8-13min / cm.

[0063] (3) Rolling;

[0064] (4) Rapid cooling after rolling: control the cooling rate to be 12-40℃ / s, and the final cooling temperature to be 310-390℃;

[0065] (5) Heat treatment.

[0066] Furthermore, in the manufacturing method described in this invention, in step (1), the dynamic light reduction of the billet in the second cooling water zone of continuous casting is controlled to be 0.7-2.3% so as to obtain a ferrite grain size range of 115-215μm for the continuous casting billet.

[0067] In this invention, by precisely controlling the chemical composition of the steel during the steel plate production process and ensuring that the secondary cooling water production stage of the continuous casting billet for the pressure vessel steel meets the above-mentioned process parameters, the thermoplasticity of the surface and interior of the continuous casting billet can be guaranteed, thus avoiding the production of defective continuous casting billets.

[0068] In the above technical solution of the present invention, in the smelting and casting process of step (1), vanadium microalloying can be achieved by converter smelting, heating by blowing nitrogen gas at the bottom of the ladle furnace, removing gas inclusions by vacuum and controlling the level of N content in the steel; in the casting process, the dynamic light reduction of the billet in the continuous casting secondary cooling water zone can preferably be controlled to be 0.7 to 2.3% so that the ferrite grain size of the continuously cast billet is between 115 and 215 μm.

[0069] Furthermore, in the manufacturing method described in this invention, in step (2), the heating temperature in the heating furnace is controlled to be 1156-1245°C.

[0070] In step (2) above, in some preferred embodiments, the heating temperature inside the furnace can be further controlled between 1156 and 1245°C. Through this optimized control, not only can energy saving and consumption reduction be achieved, but also the degree of austenite grain coarsening can be reduced while ensuring sufficient reaustification of the billet.

[0071] Furthermore, in the manufacturing method described in this invention, in step (3), the roughing rolling start temperature is controlled to be no less than 1000°C, the roughing rolling interval time is controlled to control the steel plate surface temperature to be no greater than 960°C, the finishing rolling finish temperature is controlled to be 800-940°C, and the cumulative reduction rate of the last three passes is controlled to be no less than 30%.

[0072] In step (3) above, non-crystallization controlled rolling technology can be used for rolling, that is, the finishing rolling temperature is controlled not lower than 800°C, thereby significantly reducing the rolling force. Therefore, in this invention, to avoid edge cracking of the steel plate, the finishing rolling temperature can preferably be controlled within the range of 800 to 940°C.

[0073] In this invention, under the condition of reasonable distribution of pass reduction rate, the deformed austenite is guaranteed to recrystallize above the recrystallization temperature to ensure the grain refinement of the steel plate. When the austenite transforms into ferrite, rapid cooling (controlling the cooling rate of rapid cooling to 12-40℃ / s) is used to ensure the refinement of the martensite or bainite grains after the phase transformation. Under the action of rapid cooling, the steel can form a sufficient number of precipitates of appropriate size at the recrystallization temperature, thereby further refining the martensite or bainite grains.

[0074] It should be noted that after the rolling process in step (3), the controlled cooling process of rapid cooling after rolling in step (4) has the effect of adjusting the size of the precipitated phase and improving the toughness of the material.

[0075] Furthermore, in the manufacturing method described in this invention, in step (5), a tempering heat treatment is performed, the tempering temperature is 600-650℃, and the holding time is (15-50) min + t × 1 min / mm, where t represents the plate thickness and its unit parameter is mm.

[0076] In this invention, after the steel is rapidly cooled following rolling, it undergoes a heat treatment process. Appropriate heat treatment can eliminate uneven deformation and internal stress in the formed steel plate generated during hot forming. As shown in the above technical solution, in some preferred embodiments, the heat treatment process can specifically be tempering heat treatment.

[0077] The high-strength pressure vessel steel plate resistant to liquid carbon dioxide corrosion and its manufacturing method described in this invention have the following advantages and beneficial effects compared to the prior art:

[0078] In this invention, when high-strength pressure vessel steel is used in a service environment where liquid carbon dioxide corrosion cracking occurs, in order to ensure the smooth progress of resistance to liquid carbon dioxide corrosion cracking and to ensure that the shape and performance of the steel plate after welding meet the usage requirements, fine control can be made on the composition of the steel plate, the preparation process of the steel plate, and the parameter design, so that this type of pressure vessel steel can achieve the goal of smooth production in the factory, qualified manufacturing of finished products for downstream users, and ultimately safe use of the product.

[0079] Pressure vessels made using the technical solution of this invention can effectively meet the requirements of spherical tanks or mobile containers for high strength and lightweight steel, resistance to liquid carbon dioxide corrosion cracking and high parameterization, and ensure that the performance of the steel plate in the liquid carbon dioxide medium environment is comparable to that of the steel plate in the delivery state.

[0080] The high-strength pressure vessel steel of this invention employs a low-C-Mn-Cr-Mo-Cu alloy system. Through chemical composition design, a new steel grade is prepared based on the Cr-Mo alloy system, which can improve the hardenability and resistance to carbon dioxide stress corrosion cracking. In this Cr-Mo alloy system, Cr increases the electrode potential of the iron matrix and causes passivation. Cr can effectively improve the steel material's resistance to carbon dioxide corrosion on the steel matrix surface and prevent the contact of wet liquid carbon dioxide medium with the iron matrix. Mo can promote the formation of surface passivation in the steel in a reducing wet carbonic acid environment. The combined effect of Cr and Mo can improve the steel's resistance to pitting corrosion.

[0081] Accordingly, in the design of the chemical composition, this invention also controls the content of Cu and Sn elements in the steel. The Cu added to the steel further increases the electrode potential of the steel based on the Cr-Mo alloy system, improving the steel's resistance to carbon dioxide pitting corrosion. However, if a certain amount of Sn appears in the steel, it will cause the Cu in the steel to form a low-melting-point copper-rich phase on the surface of the steel matrix, which will have a detrimental effect on the metallurgical quality of the steel, such as cracks caused by the low-melting-point Cu phase on the surface of the steel plate, and will not effectively play the role of Cu in increasing the electrode potential of the steel matrix. Therefore, the inventors have strictly controlled the content of Cu and Sn elements in the steel, and in some preferred embodiments, controlled Cu+6Sn≤0.27; Cu / Sn≥20.

[0082] Furthermore, the addition of Ni to steel after adding Cu reduces the steel's resistance to carbon dioxide corrosion. This is because the addition of Ni lowers the electrode potential of the steel matrix, accelerating carbon dioxide corrosion. Therefore, this invention also rationally controls the content of Cu and Ni elements, and in some preferred embodiments, further controls the Cu / Ni ratio to be greater than 2.

[0083] This invention, through comprehensive control of alloy composition and manufacturing process, yields a high-strength pressure vessel steel plate with excellent comprehensive mechanical properties. The yield strength is ≥510MPa, tensile strength is ≥620MPa, transverse KV2 at -60℃ is ≥185J, and elongation is ≥18%. Simultaneously, the welded joints of this high-strength pressure vessel steel plate also exhibit good mechanical properties, with a tensile strength ≥625MPa, transverse KV2 at -60℃ ≥190J, and NDTT transition temperature of the weld heat-affected zone ≤-65℃. Furthermore, both the high-strength pressure vessel steel plate and its corresponding welded joints possess good resistance to carbon dioxide corrosion. The liquid carbon dioxide corrosion rate of the high-strength pressure vessel steel plate is 0.013~0.017mm / a, and the liquid carbon dioxide corrosion rate of its welded joints is 0.014~0.018mm / a.

[0084] Accordingly, in this invention, by combining controlled rolling and cooling processes with heat treatment processes, the size of martensite or bainite grains in the steel can be refined, resulting in steel plates with good strength and toughness, resistance to cold and hot welding cracks, resistance to stress corrosion cracking by liquid carbon dioxide, and excellent weldability and weldability. This high-strength pressure vessel steel plate can be used to meet the comprehensive requirements of large-scale, high-strength, thinner, lighter, and more parameterized spherical tanks or mobile containers, which has significant practical implications. Attached Figure Description

[0085] Figure 1 This is a metallographic photograph of the steel plate for a high-strength pressure vessel in Example 3. Detailed Implementation

[0086] The high-strength pressure vessel steel plate resistant to liquid carbon dioxide corrosion and its manufacturing method described in this invention will be further explained and described below with reference to specific embodiments and accompanying drawings. However, this explanation and description do not constitute an improper limitation on the technical solution of this invention.

[0087] Examples 1-8 and Comparative Examples 1-6

[0088] The high-strength pressure vessel steel plates of Examples 1-8 and the comparative steel plates of Comparative Examples 1-6 of this invention were all prepared using the following steps:

[0089] (1) Smelting and casting according to the chemical composition shown in Tables 1-1 and 1-2: Vanadium microalloying is achieved by converter smelting, nitrogen blowing to raise the temperature in the ladle furnace bottom, vacuum removal of gas inclusions and control of the N content in the steel to achieve vanadium-nitrogen microalloying; during the casting process, the dynamic light reduction of the billet in the continuous casting secondary cooling water zone is controlled to be 0.7-2.3% so that the ferrite grain size of the continuous casting billet is between 115 and 215 μm.

[0090] (2) Hot charging and hot delivery of continuous casting billet + heating furnace: The charging temperature of the high-temperature continuous casting billet is 820-900℃, and the heating temperature in the heating furnace is controlled at 1100-1245℃, preferably between 1156-1245℃, and the heating rate is controlled at 8-13min / cm.

[0091] (3) Rolling: The roughing rolling start temperature is controlled to be no less than 1000℃, the roughing rolling interval is controlled to control the steel plate surface temperature to be no more than 960℃, the finishing rolling finish temperature is controlled to be 800~940℃, and the cumulative reduction rate of the last three passes is controlled to be no less than 30%.

[0092] (4) Rapid cooling after rolling: control the cooling rate to 12-40℃ / s and control the final cooling temperature to 310-390℃.

[0093] (5) Heat treatment: Perform tempering heat treatment, control the tempering temperature to 600~650℃, and control the holding time to (15~50)min+t×1min / mm, where t represents the thickness of the finished steel plate, and its unit parameter is mm.

[0094] It should be noted that the chemical composition design and related processes of the high-strength pressure vessel steel plates in Examples 1-8 all meet the specifications designed in this invention. Although the comparative steel plates in Comparative Examples 1-6 are also prepared using the above steps (1)-(5), there are parameters in the chemical composition design and related processes of the comparative steel plates in Comparative Examples 1-6 that do not meet the design requirements of this invention.

[0095] Tables 1-1 and 1-2 list the mass percentage of each chemical element in the high-strength pressure vessel steel plates of Examples 1-8 and the comparative steel plates of Comparative Examples 1-6.

[0096] Table 1-1. (wt%, balance Fe and other unavoidable impurities besides P, S and O)

[0097]

[0098] Table 1-2.

[0099] serial number Cu+6Sn Cu / Sn Cu / Ni Example 1 0.17 114.3 2.13 Example 2 0.24 115 3.83 Example 3 0.18 94.44 3.4 Example 4 0.19 200 18 Example 5 0.21 153.85 6.67 Example 6 0.23 137.5 44 Example 7 0.22 210 10.5 Example 8 0.25 175 2.04 Comparative Example 1 0.10 <![CDATA[ 0.67 ]]> <![CDATA[ 0.50 ]]> Comparative Example 2 <![CDATA[ 0.36 ]]> <![CDATA[ 13.89 ]]> <![CDATA[ 1.04 ]]> Comparative Example 3 0.12 <![CDATA[ 4.17 ]]> <![CDATA[ 10 ]]> Comparative Example 4 0.17 <![CDATA[ 2.5 ]]> <![CDATA[ 0.33 ]]> Comparative Example 5 0.19 <![CDATA[ 1.6 ]]> <![CDATA[ 0.4 ]]> Comparative Example 6 0.02 <![CDATA[ 6.25 ]]> <![CDATA[ 1 ]]>

[0100] Note: In the table above, for the three relationships Cu+6Sn, Cu / Sn and Cu / Ni, the values ​​of the elements in the formulas are all substituted with the values ​​before the percentage sign of the mass content of the element, and the calculated values ​​are all rounded to two decimal places.

[0101] Tables 2-1 and 2-2 list the specific process parameters for the high-strength pressure vessel steel plates of Examples 1-8 and the comparative steel plates of Comparative Examples 1-6.

[0102] Table 2-1.

[0103]

[0104] Table 2-2.

[0105]

[0106]

[0107] Samples of the high-strength pressure vessel steel plates of Examples 1-8 and the comparative steel plates of Comparative Examples 1-6 obtained through the above process steps were taken for observation, analysis and mechanical property testing. The observation results and mechanical property test results are listed in Table 3.

[0108] The relevant performance testing methods are as follows:

[0109] (1) Tensile property test: The tensile test was conducted in accordance with GB / T 228.1 "Metallic materials - Tensile test - Part 1: Test method at room temperature" to test the yield strength, tensile strength and elongation of the steel plates in each embodiment and comparative example.

[0110] (2) Impact performance test: The transverse impact performance KV2 values ​​of the steel plates of each embodiment and comparative example were tested according to GB / T 229 "Metallic materials Charpy pendulum impact test method". The test results are shown in Table 3.

[0111] Accordingly, in this invention, the metallographic structure evaluation reference standard for the high-strength pressure vessel steel plates of Examples 1-8 and the comparative steel plates of Comparative Examples 1-6 is based on GB / T 13298 "Metallic Microstructure Examination Method".

[0112] Table 3 lists the observation results and mechanical property test results of the high-strength pressure vessel steel plates of Examples 1-8 and the comparative steel plates of Comparative Examples 1-6.

[0113] Table 3.

[0114]

[0115]

[0116] Microscopic observation of the high-strength pressure vessel steel plates of Examples 1-8 shows that the microstructure of the high-strength pressure vessel steel plates of Examples 1-8 can be bainite or tempered sorbite.

[0117] As shown in Table 3, compared with the comparative steel plates of Comparative Examples 1-6, the high-strength pressure vessel steel plates of Examples 1-8 of this case have better comprehensive mechanical properties. In particular, the transverse impact energy KV2 of the steel plates of this case at -60℃ is much greater than that of the comparative steels of Comparative Examples 1-6.

[0118] In this invention, the yield strength of the high-strength pressure vessel steel plates in Examples 1-8 is between 512-543 MPa, its tensile strength is between 625-665 MPa, its transverse -60℃ impact energy KV2 is between 214-303 J, and its elongation is between 18-21%.

[0119] Accordingly, in order to demonstrate that the high-strength pressure vessel steel plates of Examples 1-8 of the present invention have good weldability and welding performance, welding process tests were also conducted on the high-strength pressure vessel steel plates of Examples 1-8 and the comparative steel plates of Comparative Examples 1-6, and the performance of the welded joints after welding was tested. The relevant test results are listed in Table 4 below.

[0120] Welding process test conditions: The welding heat input was controlled at 18–45 kJ / cm, and the time t for the weld pool temperature to drop from 800 degrees to 500 degrees was controlled. 8 / 5 Cooling time should be controlled within the range of 15 to 48 seconds.

[0121] In this invention, the tensile strength of the welded steel plate joints in Examples 1-8 and Comparative Examples 1-6 was measured using GB / T228.1 "Metallic materials, tensile testing - Part 1: Room temperature test method", the impact energy KV2 of the welded joint at -60℃ was measured using GB / T 229 "Metallic materials, Charpy pendulum impact test method", and the non-plastic transformation temperature (NDTT) of the weld heat-affected zone was tested using GB / T6803 "Ferritic steel, non-plastic transformation temperature drop hammer test method".

[0122] Table 4 lists the mechanical properties of the welded joints of the high-strength pressure vessel steel plates of Examples 1-8 and the comparative steel plates of Comparative Examples 1-6.

[0123] Table 4.

[0124]

[0125] Therefore, as can be seen from Tables 3 and 4 above, the high-strength pressure vessel steel plates of Examples 1-8 have significantly better overall performance compared to the comparative steel plates of Comparative Examples 1-6. The high-strength pressure vessel steel plates of Examples 1-8 of the present invention not only possess excellent comprehensive mechanical properties and resistance to cold and hot welding cracks, but also exhibit good weldability and weldability.

[0126] As shown in Table 4, in this invention, the welded joints corresponding to the steel plates of Examples 1-8 also have excellent mechanical properties. The tensile strength of their respective welded joints is between 625-670MPa, the transverse impact energy KV2 at -60℃ is between 193-258J, and the non-plastic transformation temperature of the weld heat-affected zone is between -75 and -80℃.

[0127] In addition, to further illustrate that the high-strength pressure vessel steel plates of Examples 1-8 of the present invention have good resistance to liquid carbon dioxide corrosion, it is necessary to conduct carbon dioxide corrosion tests on the steel plates of Examples 1-8 and Comparative Examples 1-6 of the present invention and their corresponding welded joints, and the results of the corrosion tests are listed in Table 6 below.

[0128] In this invention, the test standards for the resistance of steel plates and welded joints to liquid carbon dioxide corrosion are: ISO 11945 "Corrosion of metals and alloys - General principles of corrosion testing" and ISO 9224 "Corrosion of metals and alloys - Corrosion of climate - Guiding numerical values ​​for classification of corrosion" and NACERP-0775-91 standard regarding the degree of CO2 corrosion.

[0129] Table 5 lists the test parameters for the steel plates and their corresponding welded joints in Examples 1-8 and Comparative Examples 1-6 during the carbon dioxide corrosion test.

[0130] Table 5.

[0131] Inspection standards ISO 11945 Experimental temperature 35℃ Total gas pressure 3.5MPa <![CDATA[Partial pressure of CO2]]> 1.5MPa Flow rate Dynamic 1.5m / s test cycle 432 hours (approximately 2.6 weeks) test solution 98% (wt,%) liquid carbon dioxide + 2% (wt,%) water pH value 3.0

[0132] Table 6 lists the carbon dioxide corrosion test results of the steel plates and their corresponding welded joints in Examples 1-8 and Comparative Examples 1-6.

[0133] Table 6.

[0134] serial number Steel plate corrosion rate (mm / a) Corrosion rate of welded joints (mm / a) Example 1 0.0135 0.014 Example 2 0.016 0.017 Example 3 0.013 0.015 Example 4 0.017 0.016 Example 5 0.014 0.015 Example 6 0.015 0.016 Example 7 0.014 0.015 Example 8 0.017 0.016 Comparative Example 1 <![CDATA[ 0.16 ]]> <![CDATA[ 0.18 ]]> Comparative Example 2 <![CDATA[ 2.5 ]]> <![CDATA[ 2.8 ]]> Comparative Example 3 <![CDATA[ 3.0 ]]> <![CDATA[ 3.3 ]]> Comparative Example 4 <![CDATA[ 0.64 ]]> <![CDATA[ 0.78 ]]> Comparative Example 5 <![CDATA[ 4.0 ]]> <![CDATA[ 4.7 ]]> Comparative Example 6 <![CDATA[ 1.0 ]]> <![CDATA[ 2.4 ]]>

[0135] As can be seen from Table 6 above, compared with Comparative Examples 1-6, the steel plates and their corresponding welded joints of Examples 1-8 exhibit lower corrosion rates and superior resistance to liquid carbon dioxide corrosion. In this invention, the liquid carbon dioxide corrosion rate of the high-strength pressure vessel steel plates of Examples 1-8 is between 0.013 and 0.017 mm / a, and the liquid carbon dioxide corrosion rate of the welded joints corresponding to the steel plates of Examples 1-8 is between 0.014 and 0.017 mm / a.

[0136] In summary, the pressure vessel steel provided by this invention is a high-strength pressure vessel steel with good strength, toughness and resistance to liquid carbon dioxide corrosion. It not only has good strength and toughness, resistance to cold and hot welding cracks and resistance to liquid carbon dioxide corrosion, but also has good weldability and weldability.

[0137] Meanwhile, this high-strength pressure vessel steel plate and its corresponding welded joints have good resistance to carbon dioxide corrosion. The carbon dioxide corrosion rate of this high-strength pressure vessel steel plate is 0.013 to 0.017 mm / a, which can effectively meet the engineering application requirements of pressure vessel steel for containing liquid carbon dioxide, and has very important practical significance.

[0138] Figure 1 This is a metallographic photograph of the steel plate for a high-strength pressure vessel in Example 3.

[0139] like Figure 1 As shown, in this embodiment, the metallographic structure of the high-strength pressure vessel steel plate of Example 3 is tempered bainite, and its original austenite grain size is grade 8.

[0140] It should be noted that the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0141] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. A high-strength pressure vessel steel resistant to liquid carbon dioxide corrosion, characterized in that, Its mass percentage content of each chemical element is as follows: C: 0.045–0.065%, Si: 0.15–0.45%, Mn: 0.80–1.25%, Alt: 0.020–0.045%, Cu: 0.15–0.25%, Cr: 0.75–1.05%, Mo: 0.10–0.35%, 0 < Ni ≤ 0.15%, 0 < Sn ≤ 0.0020%; and at least one of the following contents of V, B, and Nb: 0 < V ≤ 0.030%, B: 0.0005–0.0012%, Nb: 0.015–0.040%; the balance being Fe and unavoidable impurity elements; each of its chemical elements satisfies at least one of the following formulas: Cu + 6Sn ≤ 0.27; Cu / Sn≥20; Cu / Ni > 2; The liquid carbon dioxide corrosion rate of its steel plate is 0.013–0.017 mm / a; the liquid carbon dioxide corrosion rate of its welded joint is 0.014–0.018 mm / a.

2. The high-strength pressure vessel steel as described in claim 1, characterized in that, Among the unavoidable impurity elements, the content of each impurity element must satisfy at least one of the following conditions: P≤0.015%, S≤0.005%, O≤0.0018%.

3. The high-strength pressure vessel steel as described in claim 1, characterized in that, The mass percentage content of each chemical element also satisfies at least one of the following conditions: Mn: 0.85–1.24%; Alt: 0.021~0.040%; V：0.010～0.025％； Nb: 0.017–0.038%; Cu: 0.16–0.23%; Cr：0.80～1.00％； Mo: 0.12–0.34%; B：0.0006～0.0010％。 4. The high-strength pressure vessel steel as described in claim 1, characterized in that, Its microstructure is bainite or tempered bainite.

5. The high-strength pressure vessel steel as described in claim 1, characterized in that, Its performance meets the following requirements: the yield strength of its steel plate is ≥510MPa, the tensile strength is ≥620MPa, the transverse KV2 at -60℃ is ≥185J, the elongation is ≥18%, and the liquid carbon dioxide corrosion rate is 0.013~0.017mm / a; the tensile strength of its welded joint is ≥625MPa, the transverse KV2 at -60℃ is ≥190J, the NDTT transition temperature of the weld heat-affected zone is ≤-65℃, and the liquid carbon dioxide corrosion rate is 0.014~0.018mm / a.

6. The method for manufacturing high-strength pressure vessel steel according to any one of claims 1-5, characterized in that, It includes the following steps: (1) Smelting and casting; (2) Hot charging and hot delivery of continuous casting billet + heating furnace: The charging temperature of the high-temperature continuous casting billet is 820-900℃, and the heating temperature in the heating furnace is controlled at 1100-1245℃, and the heating rate is 8-13min / cm. (3) Rolling; (4) Rapid cooling after rolling: control the cooling rate to be 12-40℃ / s, and the final cooling temperature to be 310-390℃; (5) Heat treatment.

7. The manufacturing method as described in claim 6, characterized in that, In step (1), the dynamic light reduction of the billet in the second cooling water zone of continuous casting is controlled to be 0.7-2.3% so as to obtain a ferrite grain size range of 115-215μm for the continuous casting billet.

8. The manufacturing method as described in claim 6, characterized in that, In step (2), the heating temperature is controlled at 1156 to 1245°C in the heating furnace.

9. The manufacturing method as described in claim 6, characterized in that, In step (3), the roughing rolling start temperature is controlled to be no lower than 1000℃, the roughing rolling interval is controlled to control the steel plate surface temperature to be no higher than 960℃, and the finishing rolling finish temperature is controlled to be 800~940℃. ， The cumulative reduction rate of the last three passes should not be less than 30%.

10. The manufacturing method as described in claim 6, characterized in that, In step (5), tempering heat treatment is performed at a temperature of 600-650℃ and a holding time of (15-50) min + t × 1 min / mm, where t represents the plate thickness and its unit parameter is mm.

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