High-efficiency rolled low-yield-ratio 345MPa-grade steel plate for low-temperature pressure vessel and preparation method of high-efficiency rolled 345MPa-grade steel plate

Abstract

The invention relates to the technical field of low-carbon microalloyed steel production, in particular to an efficient rolling low-yield-ratio 345 MPa-grade low-temperature pressure vessel steel plate and a preparation method thereof.The steel plate comprises, by weight, 0.15%-0.18% of C, 0.20%-0.25% of Si, 1.05%-1.55% of Mn, smaller than or equal to 0.020% of P, smaller than or equal to 0.015% of S, 0.01%-0.02% of Nb, 0.015%-0.04% of Al, 0.003%-0.006% of N and the balance Fe and inevitable impurities, and the total amount of impurity elements is smaller than 0.05%; the preparation method of the steel plate comprises the following technological processes of molten steel smelting, external refining and degassing, continuous casting, casting blank heating, controlled rolling, controlled cooling and air cooling to the room temperature. According to the method, the continuous casting billet with the thickness of 170-250 mm is used for efficiently producing the 345 MPa grade steel plate with the thickness of 10-50 mm and the low yield ratio for the low-temperature pressure vessel, the steel plate has the advantages of low cost, high strength and toughness, the low yield ratio and high straightness, and the service safety of the steel plate for the low-temperature pressure vessel can be guaranteed.

Description

Technical Field

[0001] This invention relates to the field of low-carbon microalloyed steel production technology, and in particular to a method for efficiently rolling a low yield strength ratio 345MPa grade steel plate for cryogenic pressure vessels and its preparation. Background Technology

[0002] The yield strength ratio is the ratio of a steel's yield strength to its tensile strength, reflecting the steel's ability to resist stress concentration during plastic deformation. A lower yield strength ratio results in a higher uniform elongation of the steel, allowing it to distribute plastic deformation more evenly over a wider area, thus making it safer as a structural component. Currently, steels with high yield strength ratio requirements are mainly used in bridges, buildings, and pipelines, primarily operating at room temperature. However, with increasing societal progress and rising demands on structural materials, many low-temperature pressure vessel steels are also facing more stringent requirements for yield strength ratio. Pressure vessel steel has been widely used in all aspects of life and production. Among them, 345MPa grade cryogenic pressure vessel steel is commonly used in the storage and transportation of cryogenic liquids such as liquid nitrogen, liquid oxygen, liquid hydrogen, liquid helium, and liquefied natural gas. Its application range is wide and the market demand is large. As users have increasingly higher requirements for the performance of this type of steel, pressure vessel steel with a lower yield strength ratio and higher strength is becoming more popular due to its greater safety and better processing performance. Developing a high-efficiency and economical 345MPa grade cryogenic pressure vessel steel with a lower yield strength ratio is crucial for steel mills to improve their profitability.

[0003] Currently, to ensure the low-temperature toughness and yield strength ratio of 345MPa grade pressure vessel steel, higher alloy compositions (Ni, rare earth elements) 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. Simultaneously, to maintain the yield strength ratio, the rolled steel plate undergoes a relaxation process (heat cooling), significantly increasing the rolling difficulty. Firstly, mill load, steel plate temperature uniformity, and plate shape control all face severe challenges. Secondly, improving the steel plate's performance primarily 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, the key to developing low-yield-strength-ratio 345MPa grade low-temperature pressure vessel steel lies in solving the aforementioned problems in producing this steel, while simultaneously reducing alloy costs, improving rolling efficiency, and ensuring excellent toughness and a low yield strength ratio.

[0004] Compared with existing technologies:

[0005] To date, there are very few reports, both domestically and internationally, on the manufacture of high-efficiency rolled low-yield-strength-ratio 345MPa grade low-temperature pressure vessel steel plates and their preparation methods. Prior to this invention, patent application number 201611152263.X disclosed a 345MPa grade pressure vessel steel plate, 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%, O≤0.003%. This patent mainly achieves low yield-strength ratio and 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 conventional 345MPa grade pressure vessel is usually used in an environment of -40-60℃).

[0006] 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%, 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, its strength is high but its low-temperature impact toughness is poor, which cannot meet the requirements of low-temperature use environments.

[0007] Patent application number 201110320517.5 discloses a method for manufacturing high-toughness steel plates with a yield strength of 345-390MPa. It prepares high-toughness steel plates with excellent mechanical properties through composition design and process improvement. However, the patent adds Ti element, and the yield strength ratio of the actual steel plate is ≥0.70, and the elongation is poor.

[0008] Japanese Patent JP2005194607 discloses a high-strength steel plate with excellent crack-arresting toughness and its manufacturing method. The weight percentage of the components in this patent 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%, Mg: 0.0005%~0.005%. However, this steel contains Mo and Mg, resulting in high cost.

[0009] Although the steels disclosed in the above patent documents have achieved high strength and toughness, they are either rolled plates or have high production costs and low rolling efficiency, and the yield strength ratio is not clearly defined. Therefore, they are not suitable for the efficient production of low-cost, low-yield strength ratio, and excellent strength and toughness 345MPa grade cryogenic pressure vessels. Summary of the Invention

[0010] This invention provides a method for efficiently rolling low-yield-strength ratio 345MPa grade steel plates for cryogenic pressure vessels and its preparation. It enables the efficient production of 10-50mm thick, low-yield-strength ratio 345MPa grade steel plates for cryogenic pressure vessels using continuously cast billets with a thickness of 170-250mm. This solves problems such as low rolling efficiency, high rolling resistance, and difficulty in controlling plate shape. The steel plate has low cost, high strength and toughness, low yield-strength ratio, and high flatness, which can ensure the service safety of steel plates for cryogenic pressure vessels.

[0011] To achieve the above objectives, the present invention employs the following technical solution:

[0012] A high-efficiency rolled low-yield-to-tensile ratio 345MPa grade low-temperature pressure vessel steel plate, wherein the chemical composition and weight percentage of the steel plate are as follows: C: 0.15%~0.18%, Si: 0.20%~0.25%, Mn: 1.05%~1.55%, P≤0.020%, S≤0.015%, Nb: 0.01%~0.02%, Al: 0.015%~0.04%, N: 0.003%~0.006%, with the balance being Fe and unavoidable impurities, the total amount of impurity elements being less than 0.05%.

[0013] Furthermore, the target thickness of the steel plate is 10–50 mm.

[0014] Furthermore, the steel plate has the following properties: a yield strength of 360-390 MPa in the transverse tensile test, a tensile strength of 550-590 MPa, a yield-to-tensile ratio of less than 0.7, an elongation of ≥25%, and a transverse Charpy impact energy of ≥150 J at -40℃.

[0015] A method for preparing high-efficiency rolled low-yield-strength ratio 345MPa grade low-temperature pressure vessel steel plates, the method comprising the following processes:

[0016] Steel smelting → ladle refining and degassing → continuous casting → billet heating → controlled rolling → controlled cooling → air cooling to room temperature, specifically:

[0017] 1) Steel smelting: Smelting is carried out according to composition. The raw materials are pretreated with KR hot metal to control the S content to be less than 0.015%. After slag removal, the raw materials are fed into the converter. In the converter smelting, the double slag method is used to remove P to control the P content to ≤0.02%. At the end of the converter smelting, the C content is controlled to be 0.15-0.18%. Argon gas is blown for 25-35 minutes when tapping the steel.

[0018] 2) Ladle refining and degassing: Perform LF refining and RH vacuum degassing, maintaining RH vacuum for more than 20 minutes;

[0019] 3) Continuous casting: The superheat of continuous casting is 8-14℃, and the casting speed is 0.7-0.9m / min; in the horizontal fan-shaped section, that is, at the end of solidification, heavy pressure is applied and the continuous casting billet is reduced by 15-20mm. After the billet is removed from the line, it is stacked and cooled slowly. The stacking temperature is not lower than 800℃ and the cooling time is not lower than 48h.

[0020] 4) 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 range of the preheating section is 900-1050℃, the temperature range of the heating section is 1210-1230℃, and the temperature range of the soaking section is 1130-1160℃. The time in the heating and soaking sections is not less than 3.5-4 hours, and the time in the soaking section is not less than 0.5 hours.

[0021] 5) Controlled rolling: Before rolling, use high-pressure water to descale the billet after it exits the furnace for 1-2 minutes, with the descaling machine pressure being 20-25 MPa;

[0022] The rolling process is divided into two stages: the first stage is recrystallization rolling, i.e. rough rolling, with an initial rolling temperature range of 1050 to 1100°C and a final rolling temperature range of 960 to 1010°C. The rough rolling process consists of no more than seven passes, and the reduction rate is guaranteed to be above 20% in two of the first four passes. The thickness of the intermediate billet is 1.5 to 1.8 times that of the finished product.

[0023] The second stage is non-recrystallization rolling, i.e. finishing rolling. The initial rolling temperature range is 920-950℃, and the final rolling temperature range is 850-900℃. There are no more than four finishing rolling passes. The reduction system for each pass is to ensure a reduction rate of more than 20% in the first two passes. The rolling speed is increased, and the steel is quickly ejected after rolling at a speed of 5-6.5m / s. Pre-straightening is also introduced.

[0024] 6) Controlled cooling: Laminar flow cooling is adopted, with an initial cooling temperature range of 800–830℃ and a final cooling temperature range of 700–750℃. The cooling rate is controlled at 8–12℃ / s. After the steel plate exits controlled cooling, the side spray and air purging are activated. The side spray pressure and water volume are 2–5MPa and 80–100m³ / s, respectively. 3 / h, the air purging pressure is 5~10MPa, the hot straightening is followed by three straightening processes, the inlet roller position is -1.3mm~-2.2mm, the outlet roller position is -2.2mm~-3.5mm, and then it is air cooled to room temperature.

[0025] Furthermore, the thickness of the continuously cast billet is 170-250 mm, and it is produced on a medium-thick plate reciprocating rolling mill, with water as the cooling medium.

[0026] Compared with the prior art, the beneficial effects of the present invention are:

[0027] 1) After deep desulfurization pretreatment of KR hot metal, the slag is thoroughly removed. The converter uses a double-slag method for P removal, resulting in lower P and S content in the billet. By controlling the argon blowing time and maintaining the RH vacuum degassing time, defects such as central segregation, inclusions, and excessive H and O content in the billet caused by high Mn and C content are overcome. This is beneficial to improving the plasticity and toughness of the steel plate, reducing superheat, reducing continuous casting speed, and placing it under heavy pressure. This can improve the macro segregation of the continuous casting billet, reduce the spacing of secondary dendrite arms in the solidification structure of the continuous casting billet, help reduce billet segregation, reduce internal structural defects, refine austenite grains, and reduce the width of ferrite strips in the banded structure after rolling. At the same time, the billets after the line are stacked and slowly cooled, and the stacking temperature and slow cooling time are limited, which helps to promote the diffusion of Mn, C, and H elements and reduce their impact on the microstructure and properties due to compositional segregation.

[0028] 2) The billet is heated by limiting the temperature and time of the preheating, heating, and soaking sections. This ensures that Nb carbides and nitrides dissolve rapidly and fully into the matrix and diffuse sufficiently, while promoting the diffusion of alloying elements Mn and C, reducing their impact on microstructure and properties due to compositional segregation. Furthermore, controlling the temperature (low-temperature heating) and time of the heating and soaking sections suppresses excessive growth of the original austenite grains, increasing the contribution of fine-grain strengthening to the steel plate's strength. In addition, low-temperature heating in the soaking section reduces the waiting time for the second-stage rolling of the steel plate, improving rolling efficiency.

[0029] 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 and adding a small amount of micro-alloying element Nb, the cost of alloy and the resistance to high temperature deformation in roughing and finishing stages are greatly reduced. This is conducive to increasing the reduction per pass and ensuring the comprehensive performance of the super steel plate.

[0030] 4) A two-stage controlled rolling process is adopted to control the reduction in roughing and finishing passes, optimize the thickness of intermediate slabs, and simultaneously control the starting temperature of finishing rolling to reduce the thickness of intermediate slabs waiting to be heated, accelerate the temperature drop of intermediate slabs, improve rolling efficiency, and reduce dislocation density during the two-stage rolling process. The final rolling temperature is controlled to ensure the proportion of large-angle grain boundaries, reduce dislocation density, increase the degree of matrix recovery, improve the low-temperature toughness of the steel plate, and reduce the yield strength ratio. Moreover, no relaxation waiting is required after rolling. Through rapid steel throwing after rolling, pre-straightening, and control of cooling rate, the final cooling temperature of the steel plate is controlled at 700-750℃, ensuring that the pearlite ratio is below 30%, and ensuring that the yield strength ratio and other properties of the steel plate meet the technical requirements. The use of side spraying, air blowing, and hot straightening is conducive to the control of steel plate shape, improves the uniformity of steel plate performance, reduces the probability of head and tail shape problems, and saves the investment cost of subsequent cold straightening equipment.

[0031] 5) This invention reduces alloy costs through simple composition design and achieves efficient and low-cost manufacturing of a 10-50mm thick, low yield strength ratio 345MPa grade low-temperature pressure vessel steel plate by controlling the steelmaking, continuous casting, heating, and controlled rolling and cooling processes. The microstructure is a multiphase structure mainly composed of pearlite and polygonal ferrite, with the pearlite proportion below 30%. The rolling efficiency is increased by 20-40s per piece, and it has good low-temperature toughness and high flatness. The specific properties are: the yield strength in the transverse tensile test is between 360-390MPa, the tensile strength is between 550-590MPa, the strength range is narrow, the yield strength ratio is less than 0.7, the elongation is ≥25%, and the transverse Charpy impact energy at -40℃ is ≥150J. Detailed Implementation

[0032] The specific embodiments of the present invention will be further described below:

[0033] This invention relates to a high-efficiency rolled low yield strength ratio 345MPa grade low-temperature pressure vessel steel plate, which is a high-efficiency, low-cost, low yield strength ratio 345MPa grade low-temperature pressure vessel hot-rolled steel plate with a target thickness of 10-50mm. It is produced on a medium-thick plate reciprocating rolling mill using a continuous casting billet with a thickness of 170-250mm, and the cooling medium is water.

[0034] The chemical composition and weight percentage of the steel plate are as follows: C: 0.15%–0.18%, Si: 0.20%–0.25%, Mn: 1.05%–1.55%, P≤0.020%, S≤0.015%, Nb: 0.01%–0.02%, Al: 0.015%–0.04%, N: 0.003%–0.006%, with the balance being Fe and unavoidable impurities, the total amount of impurity elements being less than 0.05%.

[0035] The roles of the main elements in the chemical composition of the steel plate of this invention are as follows:

[0036] 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, the present invention sets the C content range to 0.15% to 0.18%.

[0037] Mn: It can improve the strength of steel through solid solution strengthening, while compensating for the strength loss of steel plate caused by the reduction of C content; in addition, it can lower the γ-α phase transformation temperature, thereby refining the ferrite grains, which helps to obtain fine low-temperature phase transformation products and improve its toughness; however, increasing the Mn content will aggravate the center segregation of the continuously cast billet, which is not conducive to improving the low-temperature toughness of the steel plate, and cannot guarantee the uniformity of the cross-sectional structure of the steel plate; therefore, the Mn content range of this invention is designed to be 1.05% to 1.55%.

[0038] Si has the function of deoxidation in steelmaking and improving the strength of the matrix; however, excessive Si will reduce the toughness of the heat-affected zone of the base material. Increasing the Si content can purify ferrite and reduce the content of pearlite, which is beneficial to reducing the Bauschinger effect of the matrix material. Therefore, the Si content is set to 0.20% to 0.25% in this invention.

[0039] Nitrogen (Nb) is one of the commonly used elements in modern microalloyed pipeline steel. It has good grain refinement and precipitation strengthening effects and can also delay austenite recrystallization. However, excessive Nb will increase production costs and make continuous casting process control more difficult. This invention selects an Nb content range of 0.01% to 0.02% and, with a reasonable TMCP process, can obtain a uniform composite phase mainly composed of pearlite and polygonal ferrite, which gives it good toughness.

[0040] 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%.

[0041] Al: It is usually used as a deoxidizer in steel. If it forms AlN, it can also refine the microstructure. When the Al content exceeds 0.04%, the excessive alumina inclusions will reduce the cleanliness of the steel. If the Al content is too low, the deoxidation will be insufficient, and easily oxidized elements such as Ti will form oxides. Therefore, the lower limit of Al content is set at 0.015%.

[0042] P and S are unavoidable impurity elements in steel, and the lower the better; however, due to considerations of smelting costs and processes, they cannot be kept indefinitely low; therefore, this invention sets the upper limit of P and S content to 0.020% and 0.015%, respectively.

[0043] This invention discloses a method for preparing high-efficiency rolled steel plates for low-yield-to-tensile ratio 345MPa grade cryogenic pressure vessels. The method for preparing the steel plates includes the following process: steel smelting → ladle refining and degassing → continuous casting → billet heating → controlled rolling → controlled cooling → air cooling to room temperature, specifically:

[0044] 1) Steelmaking and continuous casting: The steel is smelted according to the following composition by weight percentage: C: 0.15%–0.18%, Si: 0.20%–0.25%, Mn: 1.05%–1.55%, P≤0.020%, S≤0.015%, Nb: 0.01%–0.02%, Al: 0.015%–0.04%, 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 S content to be less than 0.015%, and then fed into the converter after slag removal. During converter smelting, a double-slag method is used to remove P, controlling the P content... The content is ≤0.02%, and the C content is controlled at 0.15-0.18% 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 RH vacuum is maintained for more than 20 minutes. After that, slab continuous casting is carried out, with a continuous casting superheat of 8-14℃ and a continuous casting drawing speed of 0.7-0.9m / min. In the horizontal fan section, that is, at the end of solidification, heavy pressure is applied, and the continuous casting billet is reduced by 15-20mm. After the billet is removed from the line, it is stacked and slowly cooled. The stacking temperature is not lower than 800℃ and the slow cooling time is not lower than 48h to promote the diffusion of Mn, C and H elements and reduce their influence on microstructure and properties due to component segregation.

[0045] 2) Billet Heating: Billets with a thickness of 170–250 mm are fed into a walking beam furnace for heating. The billets pass through a preheating section, a heating section, and a soaking section before exiting the furnace. The temperature range of the preheating section is 900–1050℃, which promotes the rapid and complete dissolution of Nb carbides and nitrides into the matrix and facilitates their diffusion. The temperature range of the heating section is 1210–1230℃, and the temperature range of the soaking section is 1130–1160℃. The time spent in the heating and soaking sections is no less than 3.5–4 hours, with the soaking section lasting no less than 0.5 hours. Increasing the heating temperature and extending the time spent in the furnace further promotes the diffusion of alloying elements Mn, C, and H, reducing their impact on microstructure and properties due to compositional segregation. At the same time, the temperature and time of the heating and soaking sections are controlled to suppress excessive growth of the original austenite grains and ensure that fine grain strengthening contributes to the strength of the steel plate. In addition, the soaking section adopts low-temperature heating to reduce the waiting time for the second-stage rolling of the steel plate and improve rolling efficiency.

[0046] 3) High-pressure water descaling and controlled rolling: Before rolling, the billet after exiting the furnace is descaled with high-pressure water for 1-2 minutes, with a descaling machine pressure of 20-25 MPa; rolling is carried out in two stages: the first stage is recrystallization rolling (rough rolling), with an initial rolling temperature range of 1050-1100℃ and a final rough rolling temperature range of 960-1010℃. The rough rolling process has no more than seven passes, and the reduction ratio for each pass is such that two of the first four passes ensure a reduction rate of more than 20%. The thickness of the resulting intermediate billet is the finished product thickness. The thickness is 1.5 to 1.8 times that of the intermediate billet, reducing the intermediate billet's waiting thickness, accelerating the intermediate billet's temperature drop, improving rolling efficiency, and simultaneously reducing the contribution of dislocation strengthening to the steel plate's strength during the second-stage rolling process, ensuring the yield strength ratio; the second stage is non-recrystallization rolling (finishing rolling) with an initial rolling temperature range of 920–950℃ and a final rolling temperature range of 850–900℃. Finishing rolling has no more than four passes, and the pass reduction regime ensures that two of the first four passes have a reduction rate of over 20%, guaranteeing core penetration during rolling. The process promotes dynamic recrystallization of austenite grains through accelerated rolling and two-stage rolling to refine the grain structure at different stages and introduce a certain degree of pre-deformation to reduce the grain size after phase transformation. Controlling the final rolling temperature ensures that the proportion of large-angle grain boundaries is above 30%, reducing dislocation density, increasing matrix recovery, improving the low-temperature toughness of the steel plate, and lowering the yield strength ratio. Rapid post-rolling steel blasting at a speed of 5–6.5 m / s is employed, along with pre-straightening to ensure the initial plate shape upon entering the water, while controlling the steel plate's water entry temperature to suppress proprecipitated ferrite. Laminar flow cooling is then used, with an initial cooling temperature range of 800–830℃ and a final cooling temperature range of 700–750℃, controlled at a cooling rate of 8–12℃ / s. Low cooling rates ensure that the pearlite proportion is below 30%, guaranteeing that the steel plate's yield strength ratio and other performance indicators meet technical requirements. After the steel plate exits controlled cooling, side spraying and air purging are activated, with side spraying pressure and water volume at 2–5 MPa and 80–100 m³ / s, respectively. 3 / h, the air purging pressure is 5~10MPa, the hot straightening is followed by three straightening processes, the inlet roller position is -1.3mm~-2.2mm, the outlet roller position is -2.2mm~-3.5mm, and then it is air cooled to room temperature.

[0047] 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 cryogenic pressure vessel steel plates with thicknesses ranging from 10 to 50 mm and a low yield strength ratio of 345 MPa. The steel plate's microstructure is a multiphase structure dominated by pearlite and polygonal ferrite, with pearlite accounting for less than 30%. This steel plate possesses low cost, high strength and toughness, a low yield strength ratio, and high flatness, ensuring the safe service of cryogenic pressure vessel steel plates.

[0048] The following embodiments are implemented based on the technical solution of the present invention, providing detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the following embodiments. Unless otherwise specified, the methods used in the following embodiments are conventional methods.

[0049] Example:

[0050] Table 1 shows the chemical composition of the steel in the examples; Table 2 shows the smelting and stacking process of the steel in the examples; Table 3 shows the billet heating process and high-pressure water descaling process before rolling of the continuously cast billet in the examples; Table 4 shows the rolling parameters of the steel in the examples; Table 5 shows the main process parameters of controlled cooling of the steel in the examples; Table 6 shows the performance and efficiency indicators of the steel plates in the examples.

[0051] Table 1: Chemical composition (wt, %) of embodiments of the present invention

[0052]

[0053] Note: Impurity elements in steel: P≤0.02%; S≤0.015%; O≤0.0050%; total amount of other impurity elements is less than 0.05%.

[0054] Table 2: Smelting and Stacking Process Regulations for Steel in Examples

[0055]

[0056] Table 3: Heating regime of steel billet and high-pressure water descaling process before rolling of continuous casting billet in the examples

[0057]

[0058] Table 4: Rolling parameters of the steel in the examples

[0059]

[0060] Table 5: Main process parameters for controlled cooling of steel in embodiments of the present invention

[0061]

[0062] Table 6: Steel Plate Performance and Efficiency Indicators of Embodiments of the Invention

[0063]

[0064] 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, achieving efficient and low-cost manufacturing of a 10-50mm thick, 345MPa grade low-yield-strength steel plate for cryogenic pressure vessels. The microstructure of the steel plate is a multiphase structure mainly composed of pearlite and polygonal ferrite, with the pearlite proportion below 30%. This steel plate possesses low cost, high strength and toughness, low yield-strength ratio, and high flatness, ensuring the safe service of cryogenic pressure vessel steel plates.

[0065] 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 high-efficiency rolled low-yield-to-tensile strength ratio 345MPa grade low-temperature pressure vessel steel plate, characterized in that, The chemical composition and weight percentage of the steel plate are as follows: C: 0.15%–0.18%, Si: 0.20%–0.25%, Mn: 1.05%–1.55%, P≤0.020%, S≤0.015%, Nb: 0.01%–0.02%, Al: 0.015%–0.04%, N: 0.003%–0.006%, with the balance being Fe and unavoidable impurities, the total amount of impurity elements being less than 0.05%.

2. The high-efficiency rolled low-yield-strength ratio 345MPa grade low-temperature pressure vessel steel plate according to claim 1, characterized in that, The target thickness of the steel plate is 10-50 mm.

3. The high-efficiency rolled low-yield-strength ratio 345MPa grade low-temperature pressure vessel steel plate according to claim 1, characterized in that, The steel plate has the following properties: yield strength in the transverse tensile test is between 360 and 390 MPa, tensile strength is between 550 and 590 MPa, yield-to-tensile ratio is less than 0.7, elongation is ≥25%, and transverse Charpy impact energy at -40℃ is ≥150 J.

4. A method for preparing high-efficiency rolled low-yield-strength ratio 345MPa grade low-temperature pressure vessel steel plates as described in any one of claims 1 to 3, characterized in that, The method for preparing the steel plate includes the following process: Steel smelting → ladle refining and degassing → continuous casting → billet heating → controlled rolling → controlled cooling → air cooling to room temperature, specifically: 1) Steel smelting: Smelting is carried out according to composition. The raw materials are pretreated with KR hot metal to control the S content to be less than 0.015%. After slag removal, the raw materials are fed into the converter. In the converter smelting, the double slag method is used to remove P to control the P content to ≤0.02%. At the end of the converter smelting, the C content is controlled to be 0.15-0.18%. Argon gas is blown for 25-35 minutes when tapping the steel. 2) Ladle refining and degassing: Perform LF refining and RH vacuum degassing, maintaining RH vacuum for more than 20 minutes; 3) Continuous casting: The superheat of continuous casting is 8-14℃, and the casting speed is 0.7-0.9m / min; in the horizontal fan-shaped section, that is, at the end of solidification, heavy pressure is applied and the continuous casting billet is reduced by 15-20mm. After the billet is removed from the line, it is stacked and cooled slowly. The stacking temperature is not lower than 800℃ and the cooling time is not lower than 48h. 4) 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 range of the preheating section is 900-1050℃, the temperature range of the heating section is 1210-1230℃, and the temperature range of the soaking section is 1130-1160℃. The time in the heating and soaking sections is not less than 3.5-4 hours, and the time in the soaking section is not less than 0.5 hours. 5) Controlled rolling: Before rolling, use high-pressure water to descale the billet after it exits the furnace for 1-2 minutes, with the descaling machine pressure being 20-25 MPa; The rolling process is divided into two stages: the first stage is recrystallization rolling, i.e. rough rolling, with an initial rolling temperature range of 1050 to 1100°C and a final rolling temperature range of 960 to 1010°C. The rough rolling process consists of no more than seven passes, and the reduction rate is guaranteed to be above 20% in two of the first four passes. The thickness of the intermediate billet is 1.5 to 1.8 times that of the finished product. The second stage is non-recrystallization rolling, i.e. finishing rolling. The initial rolling temperature range is 920-950℃, and the final rolling temperature range is 850-900℃. There are no more than four finishing rolling passes. The reduction system for each pass is to ensure a reduction rate of more than 20% in the first two passes. The rolling speed is increased, and the steel is quickly ejected after rolling at a speed of 5-6.5m / s. Pre-straightening is also introduced. 6) Controlled cooling: Laminar flow cooling is adopted, with an initial cooling temperature range of 800–830℃ and a final cooling temperature range of 700–750℃. The cooling rate is controlled at 8–12℃ / s. After the steel plate exits controlled cooling, the side spray and air purging are activated. The side spray pressure and water volume are 2–5MPa and 80–100m³ / s, respectively. 3 / h, the air purging pressure is 5~10MPa, the hot straightening is followed by three straightening processes, the inlet roller position is -1.3mm~-2.2mm, the outlet roller position is -2.2mm~-3.5mm, and then it is air cooled to room temperature.

5. The method for preparing a high-efficiency rolled low-yield-strength ratio 345MPa grade low-temperature pressure vessel steel plate according to claim 4, characterized in that, The thickness of the continuously cast billet is 170-250 mm, and it is produced on a medium-thick plate reciprocating rolling mill with water as the cooling medium.

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