A method for manufacturing a high-efficiency low-cost 345mpa-grade low-temperature pressure vessel steel plate

Abstract

The application discloses a manufacturing method of a high-efficiency low-cost 345MPa-grade low-temperature pressure container steel plate and belongs to the technical field of low-carbon microalloy steel production. The manufacturing method comprises the following steps: smelting molten steel, LF refining, RH vacuum degassing, continuous casting, casting blank heating, rolling, controlled cooling and straightening. The application solves the problems of low rolling efficiency, large rolling resistance and difficult plate shape control of the steel plate, and the steel plate has low cost, high strength and toughness and high flatness, so that the service safety of the low-temperature pressure container steel plate can be ensured.

Description

Technical Field

[0001] This invention belongs to the field of low-carbon microalloyed steel production technology, and specifically relates to a low-cost and high-efficiency method for manufacturing steel plates for low-temperature pressure vessels with a thickness of 10-30mm and a yield strength of 345MPa. Background Technology

[0002] Faced with the severe market situation in the steel industry, reducing manufacturing costs while ensuring product quality and maximizing production line capacity has become particularly important. Under the premise of ensuring production and equipment safety, increasing the output per unit time of the rolling line, i.e., the number of rolled plates per hour, can ultimately achieve high-efficiency production with maximum efficiency and minimum energy and alloy cost consumption, while ensuring the performance and technical requirements of the steel plates. This is especially true for medium and heavy plates of the 345MPa level (such as Q345R), where vicious market competition has become increasingly apparent in recent years. This type of steel is mainly based on carbon-manganese steel, with the addition of microalloying elements such as Al, Ti, and Nb, which are carbide and nitride-forming elements, as well as a high content of Mn, to improve the strength and toughness of the steel plate through solid solution strengthening, precipitation strengthening, and grain refinement strengthening. Statistics show that medium and heavy plates of this strength level account for more than 30% of the annual production. Q345R cryogenic pressure vessel steel, as a high-value-added grade of 345MPa medium-thick plate, is widely used in the storage and transportation of cryogenic liquids such as liquid nitrogen, liquid oxygen, liquid hydrogen, liquid helium, and liquefied natural gas. As users have increasingly higher requirements for the performance of this type of steel, high-strength and high-toughness pressure vessel steel with better safety and processing performance is becoming more popular. Developing a high-efficiency, low-cost, high-strength and high-toughness 345MPa grade cryogenic pressure vessel steel is crucial for steel mills to improve their profitability.

[0003] Currently, to ensure the low-temperature toughness of 345MPa grade pressure vessel steel, higher-cost alloy compositions (Ni, Nb, etc.) are typically used, along with a relatively thick intermediate billet for heating. Furthermore, two-stage or even three-stage controlled rolling processes are employed, resulting in the final rolling temperature of the steel plate potentially being controlled below 820℃, or even lower. This significantly increases the difficulty of rolling. Firstly, mill load, steel plate temperature uniformity, and plate shape control all face severe challenges. Secondly, improving the steel plate's performance mainly depends on the content of impurities such as phosphorus and sulfur, the content of alloying elements, and the control of key process parameters in smelting and rolling. Therefore, solving the aforementioned problems in producing 345MPa grade low-temperature pressure vessel steel, achieving reduced alloy costs, improved rolling efficiency, and simultaneously ensuring excellent strength and toughness, is crucial for the development of economical 345MPa grade low-temperature pressure vessel steel.

[0004] Compared with existing technologies:

[0005] To date, there are very few reports, both domestically and internationally, on the manufacture of high-efficiency, low-cost rolled steel plates for 345MPa-grade cryogenic pressure vessels and their preparation methods. Prior to this invention, patent application number 201611152263.X disclosed a steel plate for pressure vessels with a yield strength of 345MPa, whose main components include: C 0.15~0.18%, Si 0.15~0.35%, Mn 1.40~1.50%, P≤0.008%, S≤0.002%, Al 0.020~0.040%, Nb 0.020~0.035%, and O≤0.003%. This patent mainly achieves high plasticity by adding a small amount of Nb and combining it with a heat treatment process, but it can only meet the performance requirements of the -10℃ operating environment (the typical extreme service environment for 345MPa-grade pressure vessels is -40℃). Patent application number 200810200100.3 discloses a high-strength, low-yield-strength-ratio steel plate, whose main components include: C 0.15-0.20%, Si 1.0-2.0%, Mn 1.8-2.0%, Al≤0.036%, V 0.05-0.10%, P≤0.01%, S≤0.005%, and Cr 0.8-1.0%. This patent uses low-alloying treatment and traditional TMCP rolling process to obtain a hot-rolled steel plate with high strength and low yield-strength ratio; however, while its strength is high, its low-temperature impact toughness is poor, making it unsuitable for low-temperature operating environments. Patent application number 201110320517.5 discloses a method for manufacturing a high-toughness steel plate with a yield strength of 345-390 MPa, which prepares a high-toughness steel plate with excellent mechanical properties through composition design and process improvement; however, this patent adds Ti, resulting in a yield-strength ratio ≥0.70 and poor elongation of the actual steel plate. JP2005194607 discloses a high-strength steel sheet for line pipe superior in high-speed ductile fracture resistance and its manufacturing method. The patented composition by weight percentage is: C 0.02%–0.08%, Si 0.01%–0.5%, Mn 0.5%–1.8%, P≤0.01%, S≤0.002%, Nb 0.005%–0.05%, Mo 0.005%–0.5%, Ti 0.005%–0.04%, N 0.002%–0.007%, Al≤0.07%, and Mg 0.0005%–0.005%. This steel contains Mo and Mg, resulting in high cost, and is manufactured using a two-stage depth controlled rolling process.

[0006] While the steels disclosed in the aforementioned patent documents achieve high strength and toughness, their production costs are high and rolling efficiency is low. Therefore, they are not suitable for the efficient production of low-cost, high-strength and high-toughness 345MPa grade low-temperature pressure vessel steel. The technical solution provided by this invention can effectively overcome these shortcomings, enabling the efficient production of 10-30mm thick, economical 345MPa grade low-temperature pressure vessel steel plates using 150-250mm thick continuously cast billets. Furthermore, the steel plates possess excellent low-temperature toughness and other characteristics. Summary of the Invention

[0007] The purpose of this invention is to overcome the aforementioned technical problems and deficiencies, and to provide an efficient and low-cost manufacturing method for steel plates with a thickness of 10-30mm and a pressure rating of 345MPa for cryogenic pressure vessels. This method solves problems such as low rolling efficiency, high rolling resistance, and difficulty in controlling plate shape. The resulting steel plate is characterized by low cost, high strength and toughness, and high flatness, ensuring the safe operation of steel plates for cryogenic pressure vessels.

[0008] To achieve the above objectives, the technical solution of the present invention is as follows:

[0009] A method for preparing a high-efficiency, low-cost steel plate for 345MPa cryogenic pressure vessels, wherein the chemical composition of the steel plate by weight percentage includes: C 0.15%–0.19%, Si 0.20%–0.25%, Mn 1.05%–1.35%, P≤0.020%, S≤0.015%, Al 0.015%–0.035%, N 0.003%–0.006%, with the balance being Fe and unavoidable impurities, and the total amount of other impurity elements being less than 0.05%.

[0010] The roles of the main elements in the chemical composition of the steel plate of this invention are as follows:

[0011] Carbon (C): The most economical and basic strengthening element in steel. It has a significant effect on improving the strength of steel through solid solution strengthening and precipitation strengthening. However, increasing the C content has a negative impact on the plasticity, toughness, and weldability of steel. Therefore, this invention sets the C content range to 0.15% to 0.19%.

[0012] Mn: Mn strengthens steel through solid solution treatment, compensating for the strength loss caused by the reduction in carbon content. Furthermore, it lowers the γ-α phase transformation temperature, thereby refining ferrite grains and contributing to finer low-temperature transformation products, thus improving toughness. However, increasing the Mn content exacerbates center segregation in continuously cast billets, hindering the improvement of low-temperature toughness and compromising the uniformity of the cross-sectional microstructure. Therefore, the Mn content range in this invention is designed to be 1.05%–1.35%.

[0013] Si (Si): It plays a role in deoxidation in steelmaking and improving the strength of the matrix. Increasing the Si content can purify ferrite and reduce the pearlite content, which helps to reduce the Bauschinger effect in the matrix material. However, excessive Si will reduce the toughness of the heat-affected zone in the weld. Therefore, the Si content is set at 0.20% to 0.25% in this invention.

[0014] N: In steel, the element N has no other significant role besides forming fine NbN particles to refine the austenite grains. Therefore, it needs to be kept at a low content level. The N content range selected in this invention is 0.003% to 0.006%.

[0015] Al: Commonly used as a deoxidizer in steel, it can also refine the microstructure if it forms AlN. When the Al content exceeds 0.035%, excessive alumina inclusions will reduce the cleanliness of the steel. If the Al content is too low, deoxidation will be insufficient, and easily oxidized elements such as Ti will form oxides. Therefore, the lower limit for Al content is set at 0.015%.

[0016] P and S are unavoidable impurity elements in steel, and their content should be as low as possible. However, due to considerations of smelting costs and processes, their content cannot be infinitely low. Therefore, this invention sets the upper limits for P and S content at 0.020% and 0.015%, respectively.

[0017] The high-efficiency and low-cost 345MPa grade low-temperature pressure vessel steel plate of this invention is a hot-rolled steel plate with a target thickness of 10-30mm. It is produced on a medium-thick plate reciprocating rolling mill using a continuous casting billet with a thickness of 150-250mm, and the cooling medium is water.

[0018] The objective of this invention is achieved through the following technical solution:

[0019] This invention provides a high-efficiency, low-cost method for preparing 345MPa grade low-temperature pressure vessel steel plates, comprising: steel smelting → ladle refining (LF refining), RH vacuum degassing → continuous casting → billet reheating → rolling → controlled cooling → straightening; specifically including the following steps:

[0020] 1) Steel smelting to continuous casting: Smelting according to the following composition, with the following chemical composition by weight percentage: C 0.15%~0.19%, Si 0.20%~0.25%, Mn 1.15%~1.35%, P≤0.020%, S≤0.015%, Al 0.015%~0.035%, N 0.003%~0.006%, with the balance being Fe and unavoidable impurities, and the total amount of other impurity elements being less than 0.05%. The continuous casting superheat is 8–15℃, and the continuous casting billet pulling speed is 0.7–0.9 m / min (reducing the superheat and pulling speed can improve the macroscopic segregation of the continuous casting billet, reduce the spacing of secondary dendrite arms in the solidification structure of the continuous casting billet, and help reduce billet segregation and internal structural defects); in the fan-shaped section, strong cooling is used, with a total cooling water volume of 1200–1500 L / min in the first half and 1100–1300 L / min in the second half. At the same time, heavy pressure is applied at the end of solidification, and the reduction of the continuous casting billet is 15–25 mm (strong cooling is used to ensure the temperature gradient in the thickness direction of the billet, and heavy pressure can help reduce billet segregation, reduce internal structural defects, and promote core grain breakage, thus ensuring the strength and toughness of the subsequent steel plate).

[0021] 2) Billet Heating: The billet is fed into a walking beam furnace for heating, passing through a preheating section, a heating section, and a soaking section before exiting the furnace. The preheating section temperature range is 500–750℃ (to promote homogenization of the billet's microstructure and facilitate element diffusion), the heating section temperature range is 1235–1260℃, and the soaking section temperature range is 1100–1120℃. The total time spent in the furnace during heating and soaking is controlled to be 3–3.5 hours, ensuring the temperature difference between the upper and lower surfaces of the billet is within 15℃, and the soaking section time is no less than 0.5 hours (increasing the heating temperature and extending the soaking time). Furnace time further promotes the diffusion of alloying elements Mn and C, mitigating their impact on microstructure and properties due to compositional segregation; simultaneously, the heating section provides high-temperature heating to ensure temperature uniformity across all parts of the billet, improving the uniformity of transverse and longitudinal metal flow on the steel plate surface; combined with the billet composition, by reducing the heating temperature of the billet soaking section, energy consumption is reduced, and the temperature and time of the heating and soaking sections are limited to suppress excessive growth of the original austenite grains, ensuring the contribution of fine grain strengthening to the steel plate strength; in addition, the soaking section adopts low-temperature heating to avoid the waiting time for the second-stage rolling of the steel plate, improving rolling efficiency).

[0022] 3) Rolling and Controlled Cooling: The billet adopts a transverse and longitudinal rolling mode. In the transverse rolling stage, the reduction rate of each of the first two rolling passes is greater than 20%, and the first rolling pass is descaling, with a time of 0.5 to 1 minute and a pressure of 20 to 25 MPa. (The first two rolling passes should make full use of the mill's capacity and adopt a large reduction rate to promote dynamic austenite re-crystallization, refine the original austenite grains, and use high-pressure water descaling in the mill to create a temperature gradient from the surface to the core of the steel plate. This promotes core deformation and uniform microstructure distribution during the subsequent finishing rolling process, thereby improving the strength and toughness of the steel plate.) In the longitudinal rolling stage, the reduction rate of each of the first three passes is greater than 30% (the first three passes in the longitudinal rolling stage use a large reduction rate to further refine the austenite grains and improve the core structure). In the later stage of rolling, due to the temperature drop of the steel plate and the increase in deformation resistance, the last pass uses a small reduction rate (less than 5%) to flatten the shape of the steel plate and reduce the internal stress of the steel plate. Subsequently, laminar flow cooling is adopted, with the starting cooling temperature range being 810-850℃ and the final cooling temperature range being 630-650℃. The number of controlled cooling manifold groups is 3-6, and the water flow rate of a single manifold is 120-150m³. 3 The cooling rate is controlled at 10~15℃ / s per hour. The steel plate is divided into samples every 0.1~0.2 meters from the head. Based on the steel plate thickness, the initial running speed at 1 / 15 of the distance from the head and tail of the steel plate after entering the laminar flow is set. The initial running speed at each of the head and tail is 1.3~1.5 times the normal roller conveyor speed, with a head and tail shielding value of 200~500mm, maintained for 1~2 seconds. Then the roller conveyor speed returns to normal. The normal operating speed of the steel plate is 1.2~1.5m / s, and the acceleration is 0.003~0.005m / s². 2 (Controlling the initial cooling temperature ensures the steel plate's microstructure is austenitic when it enters the water. During cooling, controlling the cooling rate and final cooling temperature suppresses the formation of bainite and martensite phases on the steel plate surface and the amount of bainite phase in the core microstructure, preventing surface bainite or martensite phase transformations from affecting the steel plate's toughness. To achieve temperature uniformity control along the steel plate's length, the sample is divided into regions every 0.1–0.2 meters from the head. Based on this, various models (temperature, flow rate, acceleration, etc.) perform velocity optimization calculations for each sample region, controlling the model based on the longitudinal temperature of the steel plate.) The measured values, target final cooling temperature process requirements, and head and tail shielding value calculation formula (head and tail shielding value = HMI interface set shielding distance - shielding time * roller speed) are used to obtain the optimal operating speed trend for each sample. For the low-temperature zones at the head and tail of the steel plate, when the head of the steel plate enters the laminar flow region or the tail leaves the laminar flow region, the roller speed is appropriately increased and the water flow rate at the head and tail positions is controlled according to the thickness group interval to reduce the excessive cooling of the head and tail of the steel plate by the cooling water. This is beneficial for controlling the shape of the steel plate, improving the uniformity of the steel plate performance, and reducing the probability of head and tail shape problems.

[0023] 4) Straightening: After the steel plate is cooled under controlled cooling, it is hot straightened and then air-cooled to room temperature.

[0024] Further, in step 1), the raw material is pretreated with KR molten iron to control the S content to be below 0.015%, and then enters the converter after slag removal; during converter smelting, the P content is controlled to be ≤0.02%, and the C content is controlled to be between 0.15% and 0.19% at the end of converter smelting; argon gas is blown for 25 to 35 minutes during tapping (argon blowing and calming before continuous casting can promote the removal of inclusions in the molten steel and improve the uniformity of the steel composition); then LF refining and RH vacuum degassing are carried out, and the RH vacuum is maintained for more than 30 minutes; after that, slab continuous casting is carried out.

[0025] Furthermore, during the converter smelting process, a double-slag method is used for phosphorus removal.

[0026] Furthermore, in the continuous casting process, the first half is section 1-4, and the second half is section 5-8.

[0027] Furthermore, in step 3), before rolling, the billet after exiting the furnace is descaled with high-pressure water for 1-2 minutes, with the descaling machine pressure being 20-25 MPa.

[0028] Furthermore, in step 3), the cooling medium for laminar flow cooling is water.

[0029] Furthermore, in step 4), the hot straightening is a single straightening process, with the inlet roller position at -1.5mm to -3.1mm and the outlet roller position at -2.3mm to -3.8mm. The straightening force is between 2000KN and 2500KN (by setting appropriate roller gaps and straightening force, the straightened steel plate is ensured to be straight and have a good shape).

[0030] By employing the aforementioned composition and a steelmaking process involving continuous casting, heating, controlled rolling, and controlled cooling, the shortcomings of existing technologies have been overcome, enabling the efficient and low-cost manufacturing of 10-30mm thick, 345MPa grade low-temperature pressure vessel steel plates. These steel plates possess low cost, high strength and toughness, and high flatness, ensuring the safe operation of low-temperature pressure vessels.

[0031] The beneficial effects of this invention are:

[0032] 1. After deep desulfurization pretreatment of KR molten iron, the slag is thoroughly removed. The converter employs a double-slag method for P removal, resulting in lower P and S content in the billet. Controlling the argon blowing time and maintaining the RH vacuum degassing time overcomes defects such as central segregation, inclusions, and excessive H and O content in the billet caused by high Mn and C content, thus improving the plasticity and toughness of the steel plate. Reducing superheat and continuous casting speed can improve macroscopic segregation in the continuously cast billet, decrease the spacing of secondary dendrite arms in the solidification structure, and help reduce billet segregation and internal structural defects. Strong cooling is used in the fan-shaped section to ensure a temperature gradient along the billet thickness, while heavy pressure promotes core grain breakage, ensuring the subsequent strength and toughness of the steel plate.

[0033] 2. Billet Heating: The temperatures and times of the billet in the preheating, heating, and soaking zones are limited to ensure sufficient diffusion of elements and reduce the impact of component segregation on microstructure and properties. Simultaneously, the heating zone provides high-temperature heating to ensure temperature uniformity across the billet and improve the uniformity of transverse and longitudinal metal flow on the steel plate surface. Based on the billet composition, energy consumption is reduced by lowering the soaking zone heating temperature. Controlling the temperatures and times of the heating and soaking zones inhibits excessive growth of the original austenite grains, increasing the contribution of fine-grain strengthening to the steel plate strength. Furthermore, low-temperature heating in the soaking zone avoids the need for preheating during the second-stage rolling process, improving rolling efficiency.

[0034] 3. The composition of this invention is reasonable and the amount of alloy added is low. By replacing dislocation and precipitation strengthening with fine grain strengthening, no expensive alloying elements are added. The low temperature heating + hot rolling manufacturing mode is adopted, which greatly reduces the alloy cost and the resistance to high temperature deformation in the roughing and finishing rolling stages. This is conducive to ensuring the rolling deformation rate of steel plates and ensuring the comprehensive performance of ultra-high-strength steel plates.

[0035] 4. By adopting a transverse and longitudinal rolling mode, in the transverse rolling stage, descaling water is used in the spray rolling mill to create a temperature gradient from the surface to the core of the steel plate. This promotes core deformation and uniform microstructure distribution during the subsequent finish rolling process, improving the strength and toughness of the steel plate and ensuring the uniformity of microstructure in the thickness direction after rolling. In the finish rolling stage, the first three passes use a large reduction rate to further refine the austenite grains and improve the core microstructure. In the later stages of rolling, due to the temperature drop of the steel plate and the increase in deformation resistance, the last pass uses a small reduction rate to flatten the shape of the steel plate and reduce the internal stress of the steel plate.

[0036] 5. Control the initial cooling temperature to ensure the steel plate is in austenitic state when it enters the water. During the cooling process, control the cooling rate and final cooling temperature to suppress the formation of bainite and martensite phases on the steel plate surface and the amount of bainite phase in the core structure, thus avoiding the impact on the toughness of the steel plate due to bainite or martensite phase transformation on the surface. In order to achieve temperature uniformity control along the length of the steel plate, the samples are divided into samples every 0.1~0.2 meters from the head. Based on this, various models (temperature, flow rate, acceleration, etc.) perform speed optimization calculations for each sample interval. The control model obtains the optimal running speed trend for each sample based on the longitudinal temperature measurement value of the steel plate, the target final cooling temperature process requirements, and the head and tail shading value calculation formula (head and tail shading value = shading distance set by HMI interface - shading time * roller speed). For the low-temperature zones at the head and tail of the steel plate, when the head of the steel plate enters the laminar flow zone or the tail leaves the laminar flow zone, the roller speed should be appropriately increased and the water flow rate at the head and tail positions controlled according to the thickness group spacing to reduce the excessive cooling of the head and tail of the steel plate by the cooling water. This is beneficial to the control of the steel plate shape, improves the uniformity of the steel plate performance, and reduces the probability of head and tail shape problems. By setting appropriate roller gaps and straightening forces, the straightened steel plate can be guaranteed to be straight and have a good shape.

[0037] 6. This invention reduces alloy costs through simple composition design and achieves efficient and low-cost manufacturing of 10-30mm thick, 345MPa grade low-temperature pressure vessel steel plates by controlling the steelmaking, continuous casting, heating, and rolling cooling processes. Rolling efficiency is increased by 30-50 seconds per plate, exhibiting good low-temperature toughness and high flatness. Specific properties include: transverse tensile yield strength between 350-400MPa, tensile strength between 560-590MPa, elongation ≥22%, transverse Charpy impact energy ≥100J at -40℃, and flatness below 5mm / 2m. Detailed Implementation

[0038] The following examples are used to illustrate the content of the present invention. These examples are only general descriptions of the content of the present invention and do not limit the content of the present invention.

[0039] A high-efficiency, low-cost manufacturing method for 345MPa grade cryogenic pressure vessel steel plates includes steel smelting → LF refining, RH vacuum degassing → continuous casting → billet reheating → rolling → controlled cooling → straightening; specifically including the following steps:

[0040] 1) Steel smelting to continuous casting: Smelting according to the following composition, the chemical composition by weight percentage includes C 0.15%~0.19%, Si 0.20%~0.25%, Mn 1.15%~1.35%, P≤0.020%, S≤0.015%, Al 0.015%~0.035%, N 0.003%~0.006%, with the balance being Fe and unavoidable impurities, and the total amount of other impurity elements being less than 0.05%. The raw materials are pretreated with KR hot metal to control the sulfur content to be below 0.015%, and then fed into the converter after slag removal. During converter smelting, a double-slag method is used to remove phosphorus (P), controlling the P content to ≤0.02%. At the end of converter smelting, the carbon content is controlled to be between 0.15% and 0.19%. Argon gas is blown for 25–35 minutes during tapping. Next, LF refining and RH vacuum degassing are performed, with the RH vacuum maintained for at least 30 minutes. Afterward, slab continuous casting is carried out at a superheat of 8–15°C and a casting speed of 0.7–0.9 m / min. In the fan-shaped section, strong cooling is used, with a total cooling water volume of 1200–1500 L / min for sections 1–4 and 1100–1300 L / min for sections 5–8. Simultaneously, heavy pressure is applied at the end of solidification, with a slab reduction of 15–25 mm.

[0041] 2) Billet heating: The billet is fed into a walking beam furnace for heating. The billet passes through the preheating section, heating section and soaking section in sequence before exiting the furnace. The temperature of the preheating section is 500-750℃, the temperature of the heating section is 1235-1260℃, and the temperature of the soaking section is 1100-1120℃. The total time in the furnace for the heating and soaking sections is controlled at 3-3.5 hours to ensure that the temperature difference between the upper and lower surfaces of the billet is within 15℃, and the soaking section time is not less than 0.5 hours.

[0042] 3) High-pressure water descaling, rolling, and controlled cooling: Before rolling, the billet is descaled with high-pressure water for 1-2 minutes after exiting the furnace, with a descaling machine pressure of 20-25 MPa. A transverse and longitudinal rolling mode is adopted. In the transverse rolling stage, the reduction rate of each pass in the first two passes is greater than 20%, and the first pass is used for descaling, with a time of 0.5-1 minute and a pressure of 20-25 MPa. In the longitudinal rolling stage, the reduction rate of each pass in the first three passes is greater than 30%, and the reduction rate of the last pass is less than 5%. Laminar flow cooling is then used, with an initial cooling temperature of 810-850℃ and a final cooling temperature of 630-650℃. The number of controlled cooling manifold groups is 3-6, and the water flow rate per manifold is 120-150 m³. 3The cooling rate is controlled at 10~15℃ / s per hour. The steel plate is divided into sections every 0.1~0.2 meters from the head. Based on the steel plate thickness, the initial running speed at 1 / 15 of the distance from the head and tail of the steel plate after entering the laminar flow is set. The initial running speed at each of the head and tail is 1.3~1.5 times the normal roller conveyor speed, with a head and tail shielding value of 200~500mm, maintained for 1~2 seconds. Then the roller conveyor speed returns to normal. The normal operating speed of the steel plate is 1.2~1.5m / s, and the acceleration is 0.003~0.005m / s². 2 ;

[0043] 4) Straightening: After the steel plate is cooled under controlled conditions, it is hot straightened (single-pass straightening). The position of the inlet roller is -1.5mm to -3.1mm, and the position of the outlet roller is -2.3mm to -3.8mm. The straightening force is between 2000KN and 2500KN. Then it is air-cooled to room temperature.

[0044] The target thickness of the steel plate is 10-30mm, and it is produced on a medium-thick plate reciprocating rolling mill using a continuous casting billet with a thickness of 150-250mm, with water as the cooling medium.

[0045] Examples 1-6

[0046] Table 1 shows the chemical composition of the steel in the examples; Table 2 shows the smelting process of the steel in the examples; Table 3 shows the heating process of the billet and the high-pressure water descaling process before rolling of the continuously cast billet in the examples; Table 4 shows the rolling parameters and partial cooling process of the steel plate in the examples; Table 5 shows the partial controlled cooling and straightening process parameters of the steel in the examples; Table 6 shows the performance and efficiency indicators of the steel plate in the examples.

[0047] Table 1 Chemical composition (wt, %) of the steel in the examples

[0048]

[0049] Note: Impurity elements in steel: P≤0.02%; S≤0.015%; total amount of other impurity elements less than 0.05%.

[0050] Table 2. Smelting process of steel in the examples

[0051]

[0052] Table 3 Heating regime of steel billets and high-pressure water descaling process before rolling of continuously cast billets in the examples

[0053]

[0054] Table 4 Rolling parameters and partial cooling process of the steel in the examples

[0055]

[0056] Table 5. Partial controlled cooling and straightening process parameters for the steel in the examples.

[0057]

[0058] Table 6 Performance and efficiency indicators of the steel plates in the examples

[0059]

[0060] Therefore, compared with existing technologies, the composition design, steelmaking continuous casting, heating, and controlled rolling and cooling scheme of this invention overcomes the shortcomings of existing technologies, providing a high-efficiency and low-cost manufacturing method for 10-30mm thick, 345MPa grade low-temperature pressure vessel steel plates. It solves problems such as low rolling efficiency, high rolling resistance, and difficulty in controlling plate shape. This steel plate possesses low cost, high strength and toughness, and high flatness, ensuring the safe service of low-temperature pressure vessel steel plates.

[0061] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for manufacturing high-efficiency, low-cost steel plates for 345MPa cryogenic pressure vessels, characterized in that, Includes the following steps: 1) Steelmaking to continuous casting: Molten iron undergoes pretreatment, converter smelting, LF refining, RH vacuum degassing, and continuous casting; the superheat of continuous casting is 8-15℃, and the billet pulling speed is 0.7-0.9m / min; in the fan-shaped section, strong cooling is adopted, with a total cooling water volume of 1200-1500L / min in the first half and 1100-1300L / min in the second half. At the same time, heavy pressure is applied at the end of solidification, and the reduction of the continuous casting billet is 15-25mm. 2) Billet heating: The billet is fed into a walking beam furnace for heating. The billet passes through the preheating section, heating section and soaking section in sequence before exiting the furnace. The temperature of the preheating section is 500-750℃, the temperature of the heating section is 1235-1260℃, and the temperature of the soaking section is 1100-1120℃. The total time in the furnace for the heating and soaking sections is controlled at 3-3.5 hours to ensure that the temperature difference between the upper and lower surfaces of the billet is within 15℃, and the soaking section time is not less than 0.5 hours. 3) Rolling and Controlled Cooling: A transverse and longitudinal rolling process is adopted. In the transverse rolling stage, the reduction rate of each pass in the first two passes is greater than 20%, and the first pass is for descaling, with a time of 0.5–1 min and a pressure of 20–25 MPa. In the longitudinal rolling stage, the reduction rate of each pass in the first three passes is greater than 30%, and the reduction rate of the last pass is less than 5%. Laminar flow cooling is then employed, with an initial cooling temperature of 810–850℃ and a final cooling temperature of 630–650℃. The number of controlled cooling manifold groups is 3–6, and the water flow rate per manifold is 120–150 m³. 3 The cooling rate is controlled at 10~15℃ / s per hour. The steel plate is divided into sections every 0.1~0.2 meters from the head. Based on the steel plate thickness, the initial running speed at 1 / 15 of the distance from the head and tail of the steel plate after entering the laminar flow is set. The initial running speed at each of the head and tail is 1.3~1.5 times the normal roller conveyor speed, with a head and tail shielding value of 200~500mm, maintained for 1~2 seconds. Then the roller conveyor speed returns to normal. The normal operating speed of the steel plate is 1.2~1.5m / s, and the acceleration is 0.003~0.005m / s². 2 ; 4) Straightening: After the steel plate is cooled under controlled cooling, it is hot straightened and then air-cooled to room temperature; The chemical composition of the steel plate, by weight percentage, includes: C 0.15%–0.19%, Si 0.20%–0.25%, Mn 1.05%–1.35%, P ≤0.020%, S ≤0.015%, Al 0.015%–0.035%, N 0.003%–0.006%, with the balance being Fe and unavoidable impurities, and the total amount of other impurity elements being less than 0.05%.

2. The manufacturing method according to claim 1, characterized in that, The thickness of the steel plate is 10-30mm.

3. The manufacturing method according to claim 1, characterized in that, The thickness of the cast billet is 150-250 mm.

4. The manufacturing method according to claim 1, characterized in that, The steel plate is obtained by rolling a cast billet on a medium-thick plate reciprocating rolling mill.

5. The manufacturing method according to claim 1, characterized in that, In step 1), the raw material is pretreated with KR molten iron to control the S content to be less than 0.015%, and then enters the converter after slag removal. During converter smelting, the P content is controlled to be ≤0.02%, and the C content is controlled to be 0.15-0.19% at the end of converter smelting. Argon gas is blown for 25-35 minutes when tapping the steel. Then, LF refining and RH vacuum degassing are carried out, and the RH vacuum is maintained for more than 30 minutes.

6. The manufacturing method according to claim 1, characterized in that, During the converter smelting process, the double-slag method is used for phosphorus removal; During the continuous casting process, the first half is sections 1-4, and the second half is sections 5-8.

7. The manufacturing method according to claim 1, characterized in that, In step 3), before rolling, the billet after exiting the furnace is descaled with high-pressure water for 1-2 minutes, with the descaling machine pressure being 20-25 MPa; the cooling medium for laminar flow cooling is water.

8. The manufacturing method according to claim 1, characterized in that, In step 4), the hot straightening is a single straightening process, with the inlet roller position at -1.5mm to -3.1mm and the outlet roller position at -2.3mm to -3.8mm, and the straightening force between 2000KN and 2500KN.

9. The manufacturing method according to claim 1, characterized in that, The steel plate has a transverse tensile yield strength of 350-400 MPa, a tensile strength of 560-590 MPa, an elongation of ≥22%, a transverse Charpy impact energy of ≥100 J at -40℃, and a flatness of ≤5 mm / 2 m.

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