A pressure vessel steel plate with good ductility and toughness and its manufacturing method

Abstract

This invention relates to a pressure vessel steel plate with good plasticity and toughness and its manufacturing method. The chemical composition of the steel plate is: C: 0.60%~0.70%, Si: 0.80%~0.90%, Mn: 1.00%~1.60%, P≤0.012%, S≤0.005%, Cr: 0.7%~0.8%, Ni: 0.50%~1.35%, Mo: 0.2%~0.8%, Nb: 0.03%~0.06%, Ti: 0.02%~0.03%, Cu: 0.15%~0.25%, V: 0.07%~0.08%, B: 0.001%~0.002%, with the remainder being Fe and impurities. By employing vanadium-silicon microalloying, continuous casting billet smelting, electroslag remelting, controlled rolling and cooling, and normalizing heat treatment, a network carbide control technology is used to refine the microstructure, enabling medium carbon steel to not only have high room temperature strength but also good low-temperature plasticity and toughness; thus, it has broad application prospects in the field of pressure vessel equipment manufacturing.

Description

Technical Field

[0001] This invention relates to the field of pressure vessel steel plate production technology, and in particular to a pressure vessel steel plate with good plasticity and toughness for hydrogen tank trucks and its manufacturing method. Background Technology

[0002] Traditionally, it is generally believed that high-carbon steel can achieve high hardness and strength, making it suitable for use as tool steel and die steel in applications where toughness requirements are not high. However, due to its high susceptibility to crack induction and short fatigue life, it cannot be used for equipment such as pressure vessels that operate for extended periods in complex environments. With continuous technological advancements, the development of various equipment is trending towards larger size, higher performance, lighter weight, and longer service life. This places higher demands on the raw materials used in equipment manufacturing. The strength and toughness of traditional low-carbon alloy container steel plates can no longer meet the manufacturing requirements of high-end pressure vessel equipment.

[0003] As is well known, the properties of a material depend on its microstructure. For medium-carbon hypoeutectoid steel, reducing the proportion of network carbides and refining the carbide grade to ensure their even distribution within the microstructure will significantly improve the material's overall mechanical properties, achieving a high degree of balance between strength and toughness. Currently, microstructure refinement has been successfully achieved for low-carbon steel, but research on microstructure refinement for medium- and high-carbon steel is relatively limited. This invention employs network carbide control technology to achieve sufficient carbide refinement, enabling medium-carbon steel to possess not only high room-temperature strength but also good low-temperature toughness and room-temperature plasticity. Through a low-temperature rolling + controlled cooling process, it can also achieve higher elongation after fracture and reduction of area. These excellent processability and mechanical properties make it a promising candidate for widespread application in the manufacturing of pressure vessels.

[0004] This invention employs a continuous casting billet smelting process (V, Si microalloying) + electroslag remelting + controlled rolling and cooling + normalizing heat treatment. This process can be divided into five stages: The first stage involves using controlled oxygen and controlled aluminum methods during continuous casting to fully improve the purity of the steel. V, Si microalloying refines the austenite grain size, pearlite cluster size, and lamellar spacing of the steel. In the second stage, during electroslag remelting, electroslag water cooling strengthening technology is used to improve cooling intensity, enhance the density of the low-magnification microstructure of the electroslag ingot, reduce crack defects and compositional segregation, and lay the foundation for reducing the level of network carbides. In the third stage of rough rolling, proeutectoid ferrite forms as fine particles in the austenite grain boundaries and high-density dislocation regions within the grains. After rough rolling, the roller table is heated to 840–860°C for continuous multi-pass finishing rolling. The thermal deformation during this process helps to break up the proeutectoid ferrite precipitated from the austenite, obtaining fine proeutectoid grains while refining pearlite pellets. The fourth stage is ACC laminar flow cooling at a cooling rate of 40–60°C / s, followed by water cooling to room temperature. This refines the grains while effectively controlling the precipitation of network carbides, improving the toughness and plasticity of the product. The fifth stage is normalizing heat treatment at a normalizing temperature of 900–930°C, with a net holding time of 20–40 minutes. After exiting the furnace, the product is air-cooled to room temperature, followed by water cooling at a rate of 10–30°C / s. This effectively controls the precipitation of network carbides and increases the proportion of large-angle grain boundaries, with the proportion of grain boundaries above 15° reaching 1–2%. After the above treatment, a sufficiently refined microstructure can be obtained, with austenite grain size of 14-20 μm, proeutectoid ferrite volume fraction of 19%-21%, pearlite cluster size of 5-7 μm and lamellar spacing of 0.14-0.15 μm, and network carbide grade of 1-2.

[0005] The following two patent applications are related to this invention:

[0006] (1) Chinese patent application No. 201710037112.8 discloses a "manufacturing process for large-size 42CrMo4 quenched and tempered steel for the outer main shaft of a wind turbine speed increaser". Its chemical composition is: carbon: 0.38-0.45, silicon: not more than 0.04, manganese: 0.60-0.80, chromium: 0.90-1.20, molybdenum: 0.15-0.30, phosphorus: not more than 0.025, sulfur: not more than 0.035, oxygen: not more than 0.0025, hydrogen: not more than 0.0002. Its composition system is fundamentally different from that of this invention. At the same time, it adopts the die casting + 3500t fast forging machine forging process, which has high production cost, high energy consumption, slow production pace, and poor plate shape and performance uniformity of the product.

[0007] (2) Chinese patent application No. 201710919045.2 discloses a "normalizing method for refining the microstructure of low-activation pearlite / martensitic steel". The normalizing process involves heating at 50-60°C / s to 900-930°C, holding for 1-2 seconds, and then air-cooling to room temperature (20-25°C) to refine the microstructure of duplex stainless steel. This method differs from the chemical composition system and steel plate type of this invention. Furthermore, it only relates to the heat treatment process and does not describe the specific rolling process or address the issue of microstructure refinement. Summary of the Invention

[0008] This invention provides a pressure vessel steel plate with good plasticity and toughness and its manufacturing method. It employs a network carbide control technology, including vanadium-silicon microalloying, continuous casting billet smelting, electroslag remelting, controlled rolling and cooling, and normalizing heat treatment, to refine the microstructure. This results in medium-carbon steel possessing not only high room temperature strength but also good low-temperature plasticity and toughness. After the above treatment, a sufficiently refined microstructure is obtained, with grain boundaries above 15° reaching 1%–2%, austenite grain size of 14–20 μm, proeutectoid ferrite volume fraction of 19%–21%, pearlite cluster size of 5–7 μm with lamellar spacing of 0.14–0.15 μm, and a network carbide grade of 1–2. This achieves effective control of the network carbide grade and sufficient refinement of the microstructure, improving the material's toughness and plasticity. The finished steel plate exhibits good formability and mechanical properties, showing broad application prospects in the field of pressure vessel equipment manufacturing.

[0009] To achieve the above objectives, the present invention employs the following technical solution:

[0010] A pressure vessel steel plate with good plasticity and toughness, wherein the chemical composition of the steel plate by weight percentage is: C: 0.60%–0.70%, Si: 0.80%–0.90%, Mn: 1.00%–1.60%, P≤0.012%, S≤0.005%, Cr: 0.7%–0.8%, Ni: 0.50%–1.35%, Mo: 0.2%–0.8%, Nb: 0.03%–0.06%, Ti: 0.02%–0.03%, Cu: 0.15%–0.25%, V: 0.07%–0.08%, B: 0.001%–0.002%, with the remainder being Fe and unavoidable impurities.

[0011] A method for manufacturing pressure vessel steel plates with good plasticity and toughness includes a process route of continuous casting billet smelting, electroslag remelting, controlled rolling and controlled cooling, and normalizing heat treatment, with the specific control of the following processes:

[0012] (1) Continuous casting billet smelting;

[0013] During converter smelting, the oxygen blowing time is 10–16 minutes, the furnace charge temperature is controlled at 1150–1200℃, and the net holding time is 3–6 minutes; the smelting time is 20–25 minutes, and the oxygen consumption per ton of steel is controlled at 40–55 Nm³. 3 / t; A two-step aluminum addition method is adopted for deoxidation, namely, aluminum cakes are added for pre-deoxidation during the converter tapping process, and aluminum wire is added during the LF furnace refining process to complete the final deoxidation; The Ds-type inclusion grade in the continuous casting billet is controlled at 0.5 to 1.0.

[0014] (2) Electroslag remelting;

[0015] The inlet water temperature of the crystallizer is controlled at 15-35℃, the outlet water temperature is controlled at 20-40℃, and the grade of carbides precipitated from the electroslag billet is controlled at 0-1.

[0016] (3) Controlled rolling and controlled cooling;

[0017] The roughing rolling temperature is 1100-1130℃, the roughing rolling finishing temperature is 910-940℃, and the reduction rate per pass is 15%-25%. After roughing, the temperature is lowered to 840-860℃ for finishing rolling, with a reduction rate of 5%-15% per pass and a finishing rolling temperature of 790-820℃. Then, ACC laminar flow cooling is performed, with an initial cooling temperature of 750-800℃ and a cooling rate of 40-60℃ / s. Finally, the mixture is water-cooled to room temperature.

[0018] (4) Normalizing heat treatment;

[0019] The normalizing heat treatment temperature is 870-900℃, the net holding time is 20-40 minutes, and after power is cut off, the furnace is cooled to 600-650℃; after being taken out of the furnace, it is air-cooled to room temperature at a rate of 20-40℃ / s.

[0020] Furthermore, in step (1), the size of the furnace charge is controlled to be 50-100mm during converter smelting.

[0021] Furthermore, in step (3), the electroslag billet is stacked and slowly cooled for more than 48 hours, and then rolled after being cut, ground, sprayed and reheated.

[0022] Furthermore, in step (3), the thickness of the intermediate billet is 2 to 3 times the thickness of the finished steel plate.

[0023] Furthermore, the finished steel plate exhibits a microstructure with a grain boundary ratio of 1% to 2% above 15°, an austenite grain size of 14 to 20 μm, a proeutectoid ferrite volume fraction of 19% to 21%, a pearlite cluster size of 5 to 7 μm with a lamellar spacing of 0.14 to 0.15 μm, and a network carbide grade of 1 to 2.

[0024] Furthermore, the finished steel plate has a tensile strength of 1150–1280 MPa, a yield strength of 1040–1170 MPa, an elongation after fracture of 40%–50%, and a reduction of area of ​​40%–50%; an impact energy of 260–360 J at -50℃ and a lateral expansion value (LE) of 1.26–1.48 mm; and an impact energy of 210–310 J at -50℃ for the heat-affected zone after welding.

[0025] Furthermore, the thickness of the continuously cast billet is 250–300 mm, the thickness of the electroslag billet is 330–800 mm, and the thickness of the finished steel plate is 10–80 mm.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] 1) The present invention adopts the process route of "continuous casting billet smelting (V, Si microalloying) + electroslag remelting + controlled rolling and controlled cooling + normalizing heat treatment", which can obtain a fully refined microstructure. The product not only has high room temperature strength, but also good low temperature plasticity and toughness.

[0028] 2) By refining the austenite grain size, pearlite cluster size and lamellar spacing of the steel through V and Si microalloying, the proportion of ferrite is increased;

[0029] 3) The oxygen-aluminum control method is used in the continuous casting billet smelting process to fully improve the purity of steel, reduce the content of inclusions in steel, and control component segregation;

[0030] 4) During the electroslag remelting process, electroslag water cooling strengthening and improvement technology is adopted to improve the cooling intensity, improve the density of the low-magnification structure of the electroslag ingot, reduce crack defects and component segregation, and further reduce the network carbide grade.

[0031] 5) By adopting a controlled rolling and cooling process, low-temperature rolling can obtain fine proto-austenite grains and refined pearlite clusters; finally, normalizing heat treatment is used to obtain a fully refined and uniform microstructure, with the proportion of grain boundaries above 15° reaching 1% to 2%, austenite grain size of 14 to 20 μm, proeutectoid ferrite volume fraction of 19% to 21%, pearlite cluster size of 5 to 7 μm and lamellar spacing of 0.14 to 0.15 μm, and network carbide grade of 1 to 2.

[0032] 6) The finished steel plate has a tensile strength of 1150-1280MPa, a yield strength of 1040-1170MPa, an elongation after fracture of 40%-50%, a reduction of area of ​​40%-50%, an impact energy of 260-360J at -50℃, and a lateral expansion value (LE) of 1.26-1.48mm; the impact energy of the heat-affected zone after welding at -50℃ is 210-310J. Attached Figure Description

[0033] Figure 1This is a metallographic photograph (magnified 200x) of the proeutectoid ferrite / pearlite microstructure of the finished steel plate in Embodiment 1 of the present invention.

[0034] Figure 2 This is a SEM image (1000x magnification) of the pearlite sheets of the finished steel plate in Embodiment 1 of the present invention.

[0035] Figure 3 This is a metallographic photograph (magnified 200x) of the austenite grain morphology of the finished steel plate in Embodiment 1 of the present invention. Detailed Implementation

[0036] The pressure vessel steel plate with good plasticity and toughness described in this invention has the following chemical composition by weight percentage: C: 0.60%–0.70%, Si: 0.80%–0.90%, Mn: 1.00%–1.60%, P≤0.012%, S≤0.005%, Cr: 0.7%–0.8%, Ni: 0.50%–1.35%, Mo: 0.2%–0.8%, Nb: 0.03%–0.06%, Ti: 0.02%–0.03%, Cu: 0.15%–0.25%, V: 0.07%–0.08%, B: 0.001%–0.002%, with the remainder being Fe and unavoidable impurities.

[0037] The design rationale for using the above-mentioned components in this invention is as follows:

[0038] (1) C: C is the main component element in steel. The strength of steel mainly depends on the C content. Too high a C content will lead to poor toughness, plasticity and weldability of steel; too low a C content will lead to lower strength and performance after simulated stress relief treatment. In order to ensure that the steel plate has a good match of low-temperature impact toughness, strength and weldability during use, the C content in the steel of this invention is controlled within the range of 0.60% to 0.70%.

[0039] (2) Si: Si will appropriately reduce the volume fraction of ferrite, but it can significantly refine the interlamellar spacing of pearlite and, to some extent, refine the size of austenite grains and pearlite clusters. Si can also promote the precipitation of VC second-phase particles in steel, further pinning grain boundaries and refining grains. Si can also improve the yield strength and tensile strength of steel through solid solution strengthening. Therefore, the Si content in steel is controlled at 0.80% to 0.90% in this invention.

[0040] (3) Mn: Mn can strengthen pearlite in steel through solid solution strengthening. C-Mn strengthening is also the main way to improve the strength of low carbon steel. However, if the Mn content is too high, it will not only increase the production cost, but Mn will also easily combine with S to form MnS, which will reduce the material's resistance to hydrogen-induced cracking. At the same time, the Mn content will reduce the activity of carbon. Therefore, this invention controls the Mn content in steel to 1.00% to 1.60%.

[0041] (4) P: Phosphorus is a harmful element in steel. It increases the cold brittleness of steel, which worsens the weldability, reduces the plasticity, which worsens the cold bending performance, and P is also particularly sensitive to radiation embrittlement. Therefore, the lower the P content in steel, the better. Taking into account the production cost, this invention controls the P content in steel to below 0.012%.

[0042] (5) S: Sulfur is a harmful element under normal circumstances. S readily forms brittle sulfides with alloying elements in steel, causing hot brittleness and reducing the ductility and toughness of the steel; at the same time, S also tends to accelerate irradiation embrittlement. Therefore, the present invention controls the S content in steel to below 0.005%.

[0043] (6) B: B can lower the transformation temperature of austenite to pearlite phase, promote the formation of acicular pearlite within the grains, and play a role in refining the grains. In this invention, the B content in the steel is controlled at 0.001% to 0.002%.

[0044] (7) V: V is a microalloying element. In steel, V microalloying can form fine second-phase particles, which act as grain boundary pinning and precipitation strengthening, effectively refining the grain size, austenite grain size, pearlite cluster size, and lamellar spacing. It also increases the ferrite volume fraction, significantly improving the overall mechanical properties of the steel, such as strength, toughness, ductility, and resistance to thermal fatigue. The precipitation strengthening effect of V second-phase particles can further improve the strength and toughness of the steel plate, especially its toughness and plasticity. Therefore, this invention adds V to the steel and controls its content to be between 0.07% and 0.08%.

[0045] (8) Ni: Ni is a solid solution strengthening element in steel, which can improve the strength of steel. Ni relaxes stress by reducing the resistance to dislocation movement in steel, thereby changing the substructure of the matrix structure and improving the toughness of steel, especially the low temperature toughness. However, excessively high Ni content in medium carbon steel will increase the phase transformation temperature. Therefore, this invention controls the Ni content in steel to 0.50% to 1.35%.

[0046] (9) Cr: Chromium is an element that stabilizes carbides, and the addition of chromium will reduce the dissolution rate of carbides. Therefore, when using the hot deformation microstructure refinement process, even if the heating temperature is increased or the heating time is extended, the eutectoid transformation can still be avoided, and a refined microstructure can be obtained. Chromium can also inhibit the graphitization of ultra-high carbon steel containing silicon and aluminum, increase the hardenability of steel and have a secondary hardening effect, which can improve the hardness and wear resistance of high carbon steel without making the steel brittle, and can improve fatigue life. Therefore, the Cr content in the steel is controlled at 0.7% to 0.8% in this invention.

[0047] (10) Cu: The prominent role of Cu in steel is to improve the corrosion resistance of plain carbon low-alloy steel, and it can also increase the strength and yield strength ratio of steel, without adversely affecting the weldability. When the copper content exceeds 0.75%, it can produce an age-hardening effect after solution treatment and aging. At the same time, its effect is similar to that of nickel, which can play a certain role in saving nickel and reducing costs. However, when its content is high, it will lead to copper embrittlement during hot deformation processing. Therefore, this invention controls the Cu content in steel at 0.15% to 0.25%.

[0048] (11) Nb: As a strong carbide-forming element, Nb forms a highly dispersed NbC phase with good high-temperature stability in steel, playing a precipitation strengthening role. Through multi-stage rolling, it can effectively refine the grains and improve the reduction in toughness caused by precipitation strengthening, thereby enabling the steel plate to obtain comprehensive properties of high strength and high toughness. In addition, in Nb-Mo composite steel, Mo can also agglomerate at the NbC matrix interface, preventing the coarsening of NbC particles, thereby greatly improving the high-temperature strength of the steel; therefore, the present invention controls the Nb content in the steel at 0.03% to 0.06%.

[0049] (12) Mo: Mo mainly relies on solid solution strengthening and grain boundary strengthening to improve the strength of steel; Mo can increase the stability of supercooled austenite, shifting the curve of austenite to pearlite transformation to the right, resulting in a finer pearlite structure after phase transformation; In addition, Ti and Mo combine to precipitate a large amount of nano-sized Ti-Mo(CN) carbides in steel, and the refined carbides pin dislocations, which greatly improves the strength and toughness of steel. Therefore, the present invention controls the Mo content in steel to 0.2% to 0.8%.

[0050] (13) Ti: Adding an appropriate amount of Ti to steel can form a large number of dispersed fine TiN or Ti2O3 particles, which can serve as heterogeneous nucleation sites for acicular pearlite during solidification, thereby refining the microstructure. Ti also has a deoxidizing effect, ensuring that B is not oxidized or nitrided. B can lower the transformation temperature from austenite to pearlite, promoting the formation of acicular pearlite within the grains and refining the grains. However, when w(Ti)≥0.09%, the content of acicular pearlite will decrease, causing the low-temperature toughness of the steel plate to deteriorate. Therefore, this invention controls the Ti content in the steel to be between 0.02% and 0.03%.

[0051] The present invention discloses a method for manufacturing pressure vessel steel plates with good toughness and plasticity. The main production process includes continuous casting billet smelting, electroslag remelting, controlled rolling and cooling, and normalizing heat treatment. The reasons for selecting the process and parameters are as follows:

[0052] (1) Continuous casting billet smelting:

[0053] The preferred method is to use continuously cast billets with a cross-sectional size of 250×300mm for smelting. A controlled oxygen-controlled aluminum method is employed to significantly improve steel purity, reduce inclusion content, and control component segregation, laying the foundation for lowering the grade of network carbides. Specific process parameters are as follows: during converter smelting, the charge size is controlled at 50–100mm; the oxygen blowing time is shortened to 10–16 minutes; the charge hot charging temperature is controlled at 1150–1200℃; the net holding time is 3–6 minutes; the smelting time is shortened to 20–25 minutes; and the oxygen supply intensity is reduced, i.e., the oxygen consumption per ton of steel is controlled at 40–55 Nm³. 3 Within a certain range ( / t), a two-step aluminum addition method is employed for deoxidation: aluminum cakes are added for pre-deoxidation during the converter tapping process, and aluminum wire is added during the LF furnace refining process to complete the final deoxidation. The amount of Al deoxidizer added is controlled to reduce Ds inclusions. Ultimately, the Ds inclusion level in the continuously cast billet is controlled to 0.5–1.0, and Ds inclusions above level 1.5 are completely eliminated.

[0054] (2) Electroslag remelting:

[0055] The preferred method for smelting is using electroslag ingots with a cross-sectional dimension of 330mm × 800mm. During the electroslag remelting process, electroslag water cooling strengthening technology is employed to improve cooling intensity, enhance the density of the low-magnification microstructure of the electroslag ingot, and reduce crack defects and component segregation, laying the foundation for further reducing the network carbide grade. By adding an external circulating water temperature regulating variable frequency fan, the inlet temperature of the cooling circulating water is controlled, with the crystallizer inlet temperature controlled at 15–35℃ and the outlet temperature controlled at 20–40℃, and the grade of carbides precipitated from the electroslag ingot controlled at 0–1.

[0056] (3) Controlled rolling and controlled cooling:

[0057] After being stacked and slowly cooled for more than 48 hours, the electroslag billets are slit, ground, sprayed, and reheated before being rolled on a two-stand mill. First, the continuously cast billets are rough-rolled at 1100–1130℃ and 910–940℃, ensuring a reduction of 15%–25% per pass. The intermediate billet thickness is 2–3 times the thickness of the finished steel plate. Rough rolling causes proeutectoid cementite to form as fine particles at austenite grain boundaries and in high-density dislocation regions within the grains. After rough rolling, the intermediate billets are air-cooled by reciprocating oscillation on the roller table until they reach 840–860℃, then subjected to continuous multi-pass rapid finish rolling, ensuring a reduction of 5%–15% per pass, until the final finished thickness is reached. The finish rolling temperature is 790–820℃. The finishing rolling process uses low-temperature rolling to obtain fine original austenite grains and refined pearlite clusters; then ACC laminar flow cooling is performed, with an initial cooling temperature of 750-800℃ and a cooling rate of 40-60℃ / s, followed by water cooling to room temperature. This process refines the grains while effectively controlling the precipitation of network carbides, thereby improving the toughness and plasticity of the product.

[0058] (4) Normalizing heat treatment:

[0059] The normalizing heat treatment temperature is 870-900℃, the net holding time is 20-40 min, and after power is cut off, it is cooled in the furnace to 600-650℃. After being removed from the furnace, it is air-cooled to room temperature at a rate of 20-40℃ / s. The normalizing treatment, which is slightly higher than the Ac3 temperature, can refine the original austenite grains, pearlite clusters, and lamellar spacing, increase the proportion of large-angle interfaces, and the large-angle grain boundaries strongly hinder dislocation movement, which is beneficial to improving the toughness and plasticity of the material, especially the reduction of area.

[0060] After the above treatment, a completely refined and uniform microstructure can be obtained, with the proportion of grain boundaries above 15° reaching 1% to 2%, austenite grain size of 14 to 20 μm, proeutectoid ferrite volume fraction of 19% to 21%, pearlite cluster size of 5 to 7 μm and lamellar spacing of 0.14 to 0.15 μm, network carbide grade of 1 to 2, and the steel plate has a good strength and toughness match.

[0061] In this invention, the thickness of the continuously cast billet is preferably 250–300 mm, the thickness of the electroslag billet is preferably 330–800 mm, and the thickness of the finished steel plate is 10–80 mm.

[0062] To more intuitively illustrate the present invention, the embodiments of the present invention will be further described in conjunction with the examples. The following examples are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any technical solutions that can be obviously obtained by those skilled in the art within the scope of the technology disclosed in the present invention, including simple variations or equivalent substitutions, are all within the scope of protection of the present invention.

[0063]

Example

[0064] The chemical composition of the steel plates in each embodiment is shown in Table 1. The process parameters for continuous casting billet smelting and electroslag remelting in each embodiment are shown in Table 2. The process parameters for controlled rolling and controlled cooling in each embodiment are shown in Table 3. The process parameters for normalizing heat treatment of the steel plates in each embodiment are shown in Table 4. The grain size of the finished steel plates in each embodiment is shown in Table 5. The comprehensive mechanical properties of the finished steel plates in each embodiment are shown in Table 6.

[0065] Table 1 Chemical composition of steel plates (wt%)

[0066]

[0067] Table 2 Process parameters for continuous casting billet smelting and electroslag remelting

[0068]

[0069]

[0070] Table 3. Process parameters for controlled rolling and cooling of steel plates

[0071]

[0072] Table 4. Process parameters for normalizing heat treatment of steel plates

[0073]

[0074] Table 5 Grain Size of Finished Steel Plates

[0075]

[0076] Table 6 Comprehensive Mechanical Properties of Finished Steel Plates

[0077]

[0078] Example 1: Metallographic photograph (200x magnification) of the proeutectoid ferrite / pearlite microstructure of the finished steel plate. Figure 1 As shown. SEM image (1000x magnification) of the pearlite layers in the finished steel plate of Example 1. Figure 2 As shown. Metallographic photograph (200x magnification) of the austenite grain morphology of the finished steel plate in Example 1. Figure 3 As shown.

[0079] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A method for manufacturing pressure vessel steel plates with good plasticity and toughness, characterized in that, The chemical composition of the steel plate, by weight percentage, is: C: 0.60%–0.70%, Si: 0.80%–0.90%, Mn: 1.00%–1.60%, P≤0.012%, S≤0.005%, Cr: 0.7%–0.8%, Ni: 0.50%–1.35%, Mo: 0.2%–0.8%, Nb: 0.03%–0.06%, Ti: 0.02%–0.03%, Cu: 0.15%–0.25%, V: 0.07%–0.08%, B: 0.001%–0.002%, with the remainder being Fe and unavoidable impurities. The process route includes continuous casting billet smelting, electroslag remelting, controlled rolling and cooling, and normalizing heat treatment, specifically controlled as follows: (1) Continuous casting billet smelting; During converter smelting, the oxygen blowing time is 10–16 minutes, the furnace charge temperature is controlled at 1150–1200℃, and the net holding time is 3–6 minutes; the smelting time is 20–25 minutes, and the oxygen consumption per ton of steel is controlled at 40–55 Nm³. 3 / t; A two-step aluminum addition method is adopted for deoxidation, namely, aluminum cakes are added for pre-deoxidation during the converter tapping process, and aluminum wire is added during the LF furnace refining process to complete the final deoxidation; The Ds-type inclusion grade in the continuous casting billet is controlled at 0.5 to 1.

0. (2) Electroslag remelting; The inlet water temperature of the crystallizer is controlled at 15-35°C, the outlet water temperature is controlled at 20-40°C, and the grade of carbides in the electroslag billet is controlled at 0-1. (3) Controlled rolling and controlled cooling; The roughing rolling temperature is 1100-1130℃, the roughing rolling finishing temperature is 910-940℃, and the reduction rate per pass in the roughing rolling is 15%-25%. After roughing rolling, the temperature is lowered to 840-860℃ for finishing rolling, with a reduction rate of 5%-15% per pass in the finishing rolling, and a finishing rolling temperature of 790-820℃. Subsequently, ACC laminar flow cooling is performed, with an initial cooling temperature of 750-800℃ and a cooling rate of 40-60℃ / s. Finally, it is water-cooled to room temperature. (4) Normalizing heat treatment; The normalizing heat treatment temperature is 870-900℃, the net holding time is 20-40 minutes, and after power is cut off, the furnace is cooled to 600-650℃; after being taken out of the furnace, it is air-cooled to room temperature at a rate of 20-40℃ / s.

2. The method for manufacturing a pressure vessel steel plate with good plasticity and toughness according to claim 1, characterized in that, In step (1), the size of the furnace charge is controlled to be 50-100mm during converter smelting.

3. The method for manufacturing a pressure vessel steel plate with good plasticity and toughness according to claim 1, characterized in that, In step (3), the electroslag billet is stacked and slowly cooled for more than 48 hours, and then rolled after being cut, ground, sprayed and reheated.

4. The method for manufacturing a pressure vessel steel plate with good plasticity and toughness according to claim 1, characterized in that, In step (3), the thickness of the intermediate billet is 2 to 3 times the thickness of the finished steel plate.

5. The method for manufacturing a pressure vessel steel plate with good plasticity and toughness according to claim 1, characterized in that, The finished steel plate has a microstructure with a grain boundary ratio of 1% to 2% above 15°, an austenite grain size of 14 to 20 μm, a proeutectoid ferrite volume fraction of 19% to 21%, a pearlite cluster size of 5 to 7 μm and a lamellar spacing of 0.14 to 0.15 μm, and a network carbide grade of 1 to 2.

6. The method for manufacturing a pressure vessel steel plate with good plasticity and toughness according to claim 1, characterized in that, The finished steel plate has a tensile strength of 1150–1280 MPa, a yield strength of 1040–1170 MPa, an elongation after fracture of 40%–50%, and a reduction of area of ​​40%–50%; an impact energy of 260–360 J at -50°C and a lateral expansion value (LE) of 1.26–1.48 mm; and an impact energy of 210–310 J at -50°C in the heat-affected zone after welding.

7. The method for manufacturing a pressure vessel steel plate with good plasticity and toughness according to claim 1, characterized in that, The thickness of the continuously cast billet is 250-300 mm, the thickness of the electroslag billet is 330-800 mm, and the thickness of the finished steel plate is 10-80 mm.

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