Anti-collision pressure vessel steel with yield strength of 790 MPa and production method thereof

Abstract

The invention discloses an anti-collision pressure vessel steel with yield strength of 790 MPa and a production method thereof. The anti-collision pressure vessel steel comprises the following components in percentage by weight: 0.130 to 0.185 percent of C, 0.28 to 0.63 percent of Si, 1.45 to 1.88 percent of Mn, 0.020 to 0.047 percent of Al, less than or equal to 0.015 percent of P, less than or equal to 0.005 percent of S, 0.025 to 0.050 percent of Nb, 0.035 to 0.065 percent of V, 0.30 to 0.50 percent of Cr, 0.25 to 0.52 percent of Mo and the balance of Fe and inevitable impurities. 0.37 < = (Nb + V) / C < = 0.62, 0.50 < = Nb / V < = 1.43, 3.72 < = (Cr + Mo) / C < = 5.60, 330 < = Mn / S < = 1630, and 2.40 < = Mn / (C + Si) < = 3.60. According to the steel for the pressure vessel, the static yield strength ReL ranges from 815 MPa to 890 MPa, the dynamic yield strength Re ranges from 895 MPa to 1260 MPa, the static tensile strength ranges from 870 MPa to 960 MPa, the static percentage elongation after fracture A ranges from 19% to 24%, the dynamic percentage elongation after fracture Ad ranges from 18.5% to 22.5%, and the product of strength and ductility ranges from 18250 MPa.% to 22420 MPa. The drop weight tear (DWTT) energy ranges from 23.5 kJ to 27.8 kJ; when the collision speed is 6-50 m / s, the collision displacement is 1.5 m and the strain rate range is 0.03-16 1 / s, the absorption energy of the steel plate after collision is 4.17-4.73 KJ, and the performance requirements that the static steel plate does not generate large deformation and does not generate steel plate fracture during collision energy absorption are met.

Description

Technical Field

[0001] This invention relates to pressure vessel steel, specifically to an impact-resistant pressure vessel steel with a yield strength of 790 MPa and its production method. Background Technology

[0002] Hazardous chemicals such as propane are typically transported in pressure vessels. During transport, accidents such as tank tipping, overturning, and collisions are inevitable. In the event of an accident, the pressure vessel deforms, leading to a rapid increase in internal pressure. When this pressure exceeds the dynamic yield strength of the pressure vessel steel or the deformation reaches its limit, the pressure vessel will crack. Simultaneously, the leaked propane gas can severely irritate the respiratory tracts of people near the transport vehicle, pollute the surrounding environment, and may even cause an explosion due to the hazardous chemical coming into contact with air. These collision-induced accidents and their cascading effects are characteristics not present in conventional automotive steel.

[0003] Existing pressure vessel steels used for storing hazardous chemicals do not address the dynamic mechanical properties of the steel during collisions, nor do they address the material properties for collision resistance, the resistance to fracture and tearing under high strain rates during collision deformation, or the high energy absorption capacity of the tank within a certain distance after a collision. This severely restricts the improvement of safety in the transportation of mobile pressure vessels and safety design technology.

[0004] Regarding collision avoidance performance and technical performance indicators, most technical literature focuses on the safety of steel used in automobiles. The safety of steel used in automobiles is mainly based on human safety factors, and the collision and energy absorption characteristics of the materials are designed in detail. However, for steel materials used in mobile pressure vessels that transport hazardous chemicals, the design of the materials must not only consider the safety of the personnel in the transport vehicle, but also the safety of the medium transported and stored in such pressure vessels during and after a collision. This is something that cannot be verified in the design of steel materials used in automobiles.

[0005] Chinese Patent Publication No. CN112746222B discloses "a 355MPa grade impact-resistant steel plate", which adopts a low-to-medium carbon alloy system of 0.07-0.10% C, 1.0-1.6% Mn, and micro-0.001-0.1% Ti+Cu+Ni. The steel plate with the target performance is obtained by controlled rolling and controlled cooling process. The microstructure of the steel plate is ferrite, pearlite and a small amount of bainite. The content of large-angle grain boundaries in the microstructure is ≥50%. The material has a yield strength ≥355MPa, a tensile strength ≥490MPa, and an elongation after fracture ≥55%. The unit size of cleavage fracture of the steel plate is ≤10μm, and the fiber content of the fracture surface is ≥80%.

[0006] Chinese Patent Publication No. CN115287431A discloses "a low-temperature marine steel plate with excellent plasticity and its manufacturing method". It uses medium-low carbon 0.08-0.10% C-0.8~1.50% Mn + microalloyed Ti+Nb alloy, and adopts controlled rolling and controlled cooling process to obtain steel plates with target performance, resulting in a ferrite + bainite two-phase structure. The material has a yield strength ≥355MPa, tensile strength >490MPa, impact energy at -60℃ >200J, and elongation after fracture >33%.

[0007] International Patent Publication No. WO2022171081A discloses "A Steel for Impact-Resistant and Crack-Resistant Ship Hull Structures and Its Manufacturing Method." This steel uses low-carbon steel (0.06-0.12% C, 1.3-1.7% Mn, and micro-Ti (0.005-0.012%)), supplemented with Mg+Ca pure steel treatment technology. It employs controlled rolling and controlled cooling, followed by normalizing heat treatment to obtain a steel plate with the target performance. The microstructure of the steel plate is ferrite + pearlite. The material has a yield strength ≥315MPa, tensile strength 440~570MPa, CTOD ≥1.5mm at -60℃, NDTT ≤70℃, uniform elongation ≥18%, and total elongation A5 ≥38%.

[0008] The aforementioned materials can only guarantee the static mechanical properties of the materials under normal transportation conditions, but cannot guarantee the dynamic yield strength or dynamic elongation after fracture. In the event of a collision, the physical properties of the hazardous chemicals being transported in the containers will not change significantly, nor can the dynamic mechanical properties of the materials be guaranteed after the collision, thus failing to guarantee the safety of the moving pressure vessel in a collision. Summary of the Invention

[0009] The purpose of this invention is to provide a pressure vessel steel with a yield strength of 790 MPa and a method for producing the same, wherein the static yield strength R of the pressure vessel steel is... eL The dynamic yield strength R is 815–890 MPa. e The static tensile strength is 870–960 MPa, the static elongation after fracture (A) is 19–24%, and the dynamic elongation after fracture (A) is... d With a strength of 18.5%–22.5%, a strength-ductility product of 18250–22420 MPa·%, and a drop hammer tear (DWTT) energy of 23.5–27.8 kJ, when the impact velocity is 6–50 m / s, the impact displacement is 1.5 m, and the strain rate is 0.03–16 1 / s, the energy absorbed by the steel plate after the impact is 4.17–4.73 kJ, which meets the performance requirements of the steel plate absorbing energy during impact without large deformation or cracking.

[0010] To achieve the above objectives, the technical solution of the present invention is as follows:

[0011] A pressure vessel steel with a yield strength of 790 MPa, comprising the following weight percentages: C: 0.130–0.185%, Si: 0.28–0.63%, Mn: 1.45–1.88%, Al: 0.020–0.047%, P≤0.015%, S≤0.005%, Nb: 0.025–0.050%, V: 0.035–0.065%, Cr: 0.30–0.50%, Mo: 0.25–0.52%, with the balance including Fe and other unavoidable impurities; and also satisfying the following limiting relationships:

[0012] 0.37≤(Nb+V) / C≤0.62, 0.50≤Nb / V≤1.43, 3.72≤(Cr+Mo) / C≤5.60;

[0013] The properties of the steel plate of the pressure vessel steel are as follows:

[0014] Static yield strength R eL The dynamic yield strength R is 815–890 MPa. e The static tensile strength is 870–960 MPa, the static elongation after fracture (A) is 19–24%, and the dynamic elongation after fracture (A) is... d The strength-ductility product is 18250–22420 MPa·% at 18.5–22.5%; the drop hammer tear (DWTT) energy is 23.5–27.8 kJ.

[0015] When the collision velocity is 6–50 m / s, the collision displacement is 1.5 m, and the strain rate is 0.03–16 1 / s, the energy absorbed by the steel plate after the collision is 4.17–4.73 kJ. (Collision absorption energy was tested according to BS / EN15227-2020 "Collision protection requirements for railway facilities and railway vehicle bodies").

[0016] Furthermore, the balance consists of Fe and other unavoidable impurities.

[0017] Furthermore, 2.40≤Mn / (C+Si)≤3.60, 330≤Mn / S≤1630.

[0018] Preferably, the Si content is 0.31–0.60 wt%.

[0019] Preferably, the Nb content is 0.028–0.048 wt%.

[0020] Preferably, the V content is 0.037–0.060 wt%.

[0021] Preferably, the Cr content is 0.30–0.47 wt%.

[0022] Preferably, the Mo content is 0.26–0.50 wt%.

[0023] The thickness of the pressure vessel steel described in this invention is 6 to 35 mm.

[0024] The microstructure of the pressure vessel steel described in this invention is tempered sorbite + bainite, wherein the volume percentage of tempered sorbite is 82-91%, the length of tempered sorbite laths is 12-28 μm, and the width of tempered sorbite laths is 0.34-0.67 μm; the percentage of bainite is 9-18%; and the original austenite grain size is 9-22 μm.

[0025] The carbide precipitates in the steel have a size of 47–78 nm and a volume percentage of 0.0178–0.0252%. When the deformation of the steel plate is 10–15%, the geometrically required dislocation density in the steel is 4.78 × 10⁻⁶. 14 ~5.67×10 14 / m 2 The dislocation recovery rate coefficient of tempered sorbite laths is 0.38–0.77.

[0026] In this invention, the properties of the pressure vessel steel are divided into basic mechanical properties and impact resistance properties. The basic mechanical properties include yield strength, tensile strength, elongation, and elongation after fracture, which are the conventional mechanical properties of the steel plate. Impact resistance properties are divided into the following dimensions: strength-ductility product characterizes the material's ability to undergo uniform plastic strain; drop hammer tear (DWTT) characterizes the material's resistance to tearing after impact; and impact absorption energy is the material's ability to absorb external impact energy when it deforms after impact.

[0027] The dynamic yield strength and dynamic elongation after fracture that this invention focuses on are important properties of steel for collision-resistant mobile pressure vessels.

[0028] In the past, the design of mobile pressure vessels only focused on static yield strength and static elongation after fracture. However, when the steel used in mobile pressure vessels is involved in a collision, the mechanical behavior of the material changes under normal load conditions. When a mobile pressure vessel is involved in a collision, the material of its tank will undergo deformation and strain rate higher than that under static yield strength test conditions, resulting in drastic changes in the material's mechanical properties in a short period of time. By focusing on the important characteristic parameters of the material's dynamic mechanical properties during a collision—dynamic yield strength and dynamic elongation after fracture—it is possible to add collision safety evaluation indicators to the design of mobile pressure vessel tank materials from the initial stage of the design of the mobile vessel tank and transportation vehicle, and to enhance the safety of mobile pressure vessels during transportation, thereby ensuring the safety of the material throughout its entire life cycle. This is something that has been ignored or neglected in the design of this type of material in the past.

[0029] The dynamic yield strength (895–1260 MPa) and dynamic elongation at fracture (18.5–22.5%) defined in this invention are obtained through tests conducted under extreme road conditions in simulated existing transportation environments. For values ​​exceeding these ranges, the material's dynamic mechanical properties will exhibit rapid plastic instability, leading to fracture or breakage. To ensure the aforementioned dynamic yield strength and elongation at fracture, the speed of vehicles transporting such hazardous chemicals must be constrained, and appropriate protective barriers or structures must be installed around the tank material to buffer the collision energy and momentum. This ensures that the physical and chemical properties of the hazardous chemicals within the tank do not change significantly during a collision, thus preventing large deformation and alterations in the material's mechanical properties after impact.

[0030] In the composition design of the pressure vessel steel described in this invention:

[0031] Carbon (C) is an essential element for improving the strength of steel, playing a role in solid solution strengthening. As the C content in steel increases, the Fe3C content increases, leading to improved yield strength and tensile strength. Specifically, for every 0.1% increase in C content, tensile strength increases by approximately 90 MPa, and yield strength increases by approximately 40-50 MPa. However, it is important to note that the C content should not be too high, as increased C content leads to a decrease in elongation and impact toughness. Studies have found that a C content of no more than 0.20% is optimal for both improving steel strength and suitability for industrial production, enhancing its applicability and feasibility in industrial applications. Therefore, considering the impact of C on the performance of pressure vessel steel plates in this invention, and to achieve both improved steel strength and suitability for industrial processes, the C content is controlled between 0.130% and 0.185%.

[0032] Si, a deoxidizing element primarily used in steelmaking, plays a role in solid solution strengthening. It should be noted that when the Si content in steel increases from 0.20% to 0.60%, the strength of the steel remains essentially unchanged or increases slightly, while the toughness improves significantly. Appropriately increasing the Si content in steel will increase the volume fraction of ferrite in the microstructure and refine the grains, thus benefiting the toughness of the steel. Therefore, this invention controls the Si content to be between 0.28% and 0.63%, preferably between 0.31% and 0.60%.

[0033] Mn (Mn) has a significant effect on improving the strength of low-carbon and medium-carbon pearlitic steels. Adding 1% Mn to steel can increase the tensile strength by approximately 100 MPa. Mn has a significant effect on center segregation in as-cast structures; higher Mn levels will result in a higher degree of center segregation in as-cast steel, severely affecting the low-temperature impact toughness, post-impact tear resistance, and elongation at high strain rates. This invention controls the Mn content to be between 1.45% and 1.88%.

[0034] Al (Al) is added as a deoxidizing balancing element in the steelmaking process. In the early stages of refining, the Al content in the molten steel needs to be controlled to no more than 0.035%. In the later stages of refining, the oxygen content in the steel is already controlled to a low level; if Al is added again, large-sized chain-like alumina inclusions will form in the molten steel, severely impairing the low-temperature toughness, impact tear resistance, and high strain rate elongation properties of the finished steel plate. Furthermore, adding Al in the later stages of refining will form a large amount of AlN in the steel. AlN easily precipitates during the casting of the molten steel into a continuous casting billet, reducing the hot plasticity of the billet and causing corner cracks or intergranular cracks on the surface or corners of the billet. Therefore, this invention controls the Al content to be between 0.020% and 0.047%.

[0035] Adding an appropriate amount of nitrogen (Nb) promotes grain refinement in the rolled steel microstructure and increases the proportion of large-angle grain boundaries in the tempered sorbite or bainite grains, with the large-angle grain boundary range being 15° to 180°. This improves the strength, toughness, impact energy absorption, and impact tear resistance of the steel plate. Nb can effectively refine the microstructure during controlled rolling by inhibiting austenite recrystallization; moreover, Nb can effectively reduce the steel's overheat sensitivity and temper brittleness. Therefore, this invention controls the Nb content to 0.025% to 0.050%, preferably 0.028% to 0.048%.

[0036] Adding an appropriate amount of vanadium (V) promotes grain refinement in the rolled steel microstructure and increases the proportion of large-angle grain boundaries in the tempered sorbite or bainite grains. The large-angle grain boundary difference ranges from 15° to 180°, improving the strength, toughness, impact energy absorption, and impact tear resistance of the steel plate. V can effectively refine the microstructure during controlled rolling by inhibiting austenite recrystallization; moreover, V can effectively reduce the steel's susceptibility to overheating cracking and temper brittleness. Therefore, this invention controls the V content to 0.035–0.065%, preferably 0.037–0.060%.

[0037] P and S are both impurity elements in the pressure vessel steel plate described in this invention. Under the condition that the technical conditions permit, in order to obtain steel with better performance and higher quality, the content of impurity elements in the pressure vessel steel plate should be reduced as much as possible. Therefore, the P content is controlled to be P≤0.015% and the S content is controlled to be S≤0.005%.

[0038] Cr is an element that shrinks the austenite region and is a medium-strength carbide-forming element. In steel, it can form carbides or dissolve in ferrite. Cr can improve the impact strength-ductility product and dynamic yield strength of container steel. Cr is also an effective element for improving the hardenability of steel, increasing the strength-ductility product and strength of steel plates. However, adding too much Cr to steel will increase the hardenability of the steel and reduce the strength-ductility product and dynamic elongation after fracture of container steel. In this invention, the Cr content is controlled at 0.30–0.50%, preferably 0.30–0.47%.

[0039] Mo (Mo) is superior to Mn and Cr in enhancing the strength, especially the impact strength and ductility, of steel. It is also one of the main elements that enhance the strength and toughness of steel. A small amount of Mo (around 0.20%) can improve the toughness of steel plates, but adding Mo also increases the hardenability of the steel, thereby increasing its susceptibility to cold cracking during welding. On the other hand, sufficient Mo content ensures the stability of the steel plate after tempering, guaranteeing that the steel plate still possesses sufficient strength and toughness after tempering. Therefore, the Mo content is controlled between 0.25% and 0.52%, preferably between 0.26% and 0.50%.

[0040] The composition design of this invention also specifically requires:

[0041] 0.37≤(Nb+V) / C≤0.62, where Nb and V are microalloying elements that can refine grains. The present invention controls the ratio of Nb, V and C to ensure that while refining grains, the proportion of large-angle grain boundary difference in ferrite grains in the steel can be further optimized. The optimal range of this large-angle grain boundary difference is 31 to 82°, ensuring the realization of the dynamic mechanical properties and impact resistance of the steel.

[0042] 0.50≤Nb / V≤1.43, where Nb and V are microalloying elements. During controlled rolling and tempering processes, Nb and V form Nb or V carbonitrides. The size of Nb carbonitrides is often smaller than that of V carbonitrides, and the grain-refining effect of Nb carbonitride precipitation is greater than that of V carbonitrides. In order to achieve the effect of grain refinement, this invention controls the ratio of Nb to V to ensure that while refining the grains, the proportion of large-angle grain boundary difference in the ferrite grains in the steel can be further optimized. The optimal range of this large-angle grain boundary difference is 32 to 76°, ensuring the optimal matching of the dynamic mechanical properties and impact resistance of the steel.

[0043] 3.72≤(Cr+Mo) / C≤5.60. Cr and Mo are strong carbide-forming elements, forming Cr carbides or Mo carbides in steel. At the same time, Cr and Mo are hardenability and quenching elements during steel quenching. The ratio of Cr+Mo and C in the steel must reach an appropriate level to achieve the best match between the strength and impact absorption energy of the steel plate and the toughness of the steel plate. Otherwise, an excessively high ratio of Cr+Mo and C will result in excessively high strength but excessively low toughness and impact absorption energy. An excessively low ratio of Cr+Mo and C will result in insufficient strength-ductility product of the steel plate and the impact absorption energy of the steel plate will not meet the impact performance requirements of the invented steel.

[0044] 330≤Mn / S≤1630. Mn in steel is the main element for solid solution strengthening in the steel plate of this invention. However, in the later stage of steelmaking refining and during the solidification process of continuous casting billet, Mn in steel will combine with S to form MnS inclusions. Under the conditions that allow for temperature and MnS nucleation and growth, MnS will cause microcracks in the billet and seriously deteriorate the drop hammer tearing performance and impact resistance of the finished steel plate.

[0045] 2.40≤Mn / (C+Si)≤3.60, Mn, C, and Si are the main solid solution strengthening elements in steel. Limiting the ratio of Mn to C and Si elements to 2.40 to 3.60 is mainly to ensure the stability of dynamic yield strength, dynamic elongation after fracture, and strength-ductility product in the dynamic mechanical properties of steel plates, and to ensure the stability of the solid solution strengthening effect of Mn in steel without causing microstructure, dislocation density, and dynamic collision performance caused by microstructure segregation.

[0046] The present invention also provides a method for producing the aforementioned impact-resistant pressure vessel steel, comprising the following steps:

[0047] 1) Smelting and casting

[0048] Smelting and casting into billets according to the above-described components;

[0049] 2) Heating of the billet

[0050] The billet is heated to 1170–1245℃ at a heating rate of 7–12℃ / min.

[0051] 3) Rolling

[0052] The initial rolling temperature of roughing shall not be lower than 1050℃; the cumulative reduction rate of roughing shall not be lower than 33%.

[0053] The initial rolling temperature of finishing mill is not higher than 930℃, the final rolling temperature of finishing mill is not higher than 860℃, and the cumulative reduction rate of the last three finishing mill passes is not less than 32%.

[0054] 4) Cooling

[0055] The cooling temperature is 305–555℃, and the cooling rate is 11–36℃ / s.

[0056] 5) Quenching and tempering heat treatment

[0057] Quenching temperature is 885~940℃, quenching holding time: (10~25min)+t×1min / mm;

[0058] Tempering temperature is 600~645℃, tempering holding time: (20~40min)+t×1min / mm;

[0059] Where: t is the thickness of the hot-rolled plate, in mm;

[0060] 6) Stress relief

[0061] The stress relief temperature is controlled at 580–605℃, and held at this temperature for 35–120 minutes.

[0062] Preferably, in step 4), the cooling temperature is 320–550°C and the cooling rate is 10–33°C / s.

[0063] Preferably, in step 5), the quenching temperature is 890–928°C and the tempering temperature is 605–638°C.

[0064] In the method for producing impact-resistant pressure vessel steel according to the present invention

[0065] The present invention controls the heating rate at 7-12℃ / min to heat the billet to 1170-1245℃. This is because, under a suitable heating rate, it ensures that no heating cracks occur on the surface of the billet, and that the billet can be fully austenitized. Furthermore, the billet is heated without coarsening the austenite grains, thereby providing a heated billet with sufficiently low rolling deformation resistance for subsequent rolling.

[0066] The present invention controls the roughing rolling temperature to be no less than 1050℃ and the cumulative reduction rate of roughing rolling to be no less than 33%. This is because the roughing rolling temperature is designed to allow the billet to deform sufficiently above the non-recrystallization temperature of austenite, and to break down the grains within the range of this roughing rolling temperature and reduction rate, thereby achieving large deformation and smaller grain size in the high-temperature section of austenite, and providing sufficient temperature and thickness control parameters for subsequent finishing rolling temperature control.

[0067] This invention controls the initial rolling temperature of the finishing mill to be no higher than 930℃, the final rolling temperature to be no higher than 860℃, and the cumulative reduction rate of the last three finishing mill passes to be no less than 32%. This is to further reduce the austenite grain size in the rough-rolled steel plate near the austenite non-recrystallization temperature. When the rolling temperature is further reduced to below 900℃, the austenite transforms into ferrite, increasing grain nucleation at the grain boundaries. This achieves the control of the static yield strength and static elongation after fracture values ​​of the basic mechanical properties of the steel plate described in this invention, as well as the dynamic yield strength and dynamic elongation after fracture values ​​in the dynamic mechanical properties, thereby achieving the purpose of controlling the comprehensive mechanical properties of the steel plate.

[0068] The present invention controls the cooling to 305-555°C at a cooling rate of 11-36°C / s because the temperature of the steel plate after finishing rolling is still at A C1 Near the temperature range, controlling the cooling rate and the final cooling temperature range after finishing rolling is to rapidly reduce the temperature of the rolled steel plate to near or below the bainite transformation temperature to martensite transformation temperature, and to allow fine carbide precipitation of Nb and V in the steel, as well as fine carbide precipitation of Cr and Mo in the steel, thereby ensuring the required original austenite grain size of the finished steel plate and precisely controlling the rolled properties within a certain range.

[0069] This invention controls the quenching temperature at 885–940℃, with a quenching holding time of (10–25 min) + t × 1 min / mm; and controls the tempering temperature at 600–645℃, with a tempering holding time of (20–40 min) + t × 1 min / mm. Since the strength and plasticity of the rolled steel plate are not yet optimally matched, this invention achieves an optimal balance between strength and plasticity through a reasonable quenching and tempering heat treatment, and controls the steel plate strength within a more precise range, ensuring the static mechanical properties and impact resistance of the steel plate.

[0070] This invention controls the stress-relief temperature at 580–605℃ and holds it at this temperature for 35–120 minutes. Stress-relief heat treatment can further eliminate the hard phase structure in the quenched and tempered steel plate, appropriately reduce the strength of the steel plate, and reduce the internal stress caused by uneven phase transformation during rolling and cooling, thereby improving the collision safety characteristics of the steel plate during the transportation of hazardous chemicals.

[0071] Compared with the prior art, the advantages of the present invention are as follows:

[0072] Existing technologies employ C-Mn components supplemented with Nb elements or low C-Mn components, and control the composition through modification treatment by adding Ca, Mg, or Mg+Ca.

[0073] The composition of this invention is based on C-Mn, with the addition of appropriate amounts of Nb, and the addition of hardenability elements such as Cr and Mo. The ratio of Cr, Mo, and C is controlled within the range of 3.72 to 5.60, and the Nb / V ratio is limited to the range of 0.50 to 1.43. The Mn / S ratio in the steel is controlled within the range of 330 to 1630, and the Mn and C+Si ratio in the steel is controlled within the range of 2.40 to 3.60. The composition design and control of this invention can precisely control the performance and microstructure of the impact-resistant steel plate, thereby achieving precise control of the dynamic mechanical properties and impact absorption energy parameters in impact performance. In contrast, the dynamic mechanical properties and impact absorption energy parameters of steel plates produced by existing technologies fluctuate greatly. This is the essential difference between the composition design and control of this invention and the composition design and control in existing technologies.

[0074] Existing technology uses controlled rolling and controlled cooling processes to produce impact-resistant steel plates, which achieves the impact resistance of the steel plates by controlling the initial rolling temperature of the finishing mill, the cooling rate of the controlled cooling process, and the final cooling temperature.

[0075] The present invention employs controlled rolling and cooling, quenching and tempering, and stress-relieving heat treatment to achieve impact-resistant steel plates. Existing controlled rolling and cooling processes produce impact-resistant steel plates with significant fluctuations in their basic and dynamic mechanical properties, making it impossible to guarantee that the basic and dynamic mechanical properties, as well as the collision absorption energy and tearing energy parameters during collisions, are controlled within a certain range. This is the fundamental difference between the present invention and existing technologies, and it is also the dynamic mechanical properties and impact resistance performance that existing technologies cannot achieve, enabling the target steel plate to meet performance requirements in new application scenarios. Detailed Implementation

[0076] The present invention will be further described below with reference to the embodiments.

[0077] The chemical composition of the embodiments and comparative examples of the present invention is shown in Tables 1 and 2, with the balance being Fe and other unavoidable impurities; Table 3 shows the process parameters of the embodiments and comparative examples of the present invention; Table 4 shows the performance parameters of the embodiments and comparative examples of the present invention; Tables 5 and 6 show the microstructure of the embodiments and comparative examples of the present invention; Table 7 shows the dynamic performance testing parameters of the embodiments and comparative examples of the present invention.

[0078] 1. Regarding the performance testing in Table 4:

[0079] (1) Tensile property test: Tensile property test: Under the condition of room temperature 20℃, the strain rate is controlled at 0.0067s. -1 The yield strength, tensile strength and elongation A values ​​were tested according to GB228.1 "Metallic materials - Tensile testing - Part 1: Test method at room temperature".

[0080] (2) Ferrite grain size test: The ferrite grain size was tested at room temperature according to GB / T 6394 "Method for determination of average grain size of metal".

[0081] 2. Regarding the performance tests in Table 5:

[0082] (1) Tensile property test: Under room temperature conditions, the strain rate was controlled at 0.0067 s. -1 The yield strength, tensile strength and elongation values ​​were tested according to GB228.1 "Metallic materials - Tensile testing - Part 1: Test method at room temperature".

[0083] (2) Measurement of precipitate size and spacing in steel: The spacing of precipitates was measured using JY / T 0581 "General Rules for Analysis by Transmission Electron Microscopy" and YB / T 4676 "Analysis of Precipitates in Steel by Transmission Electron Microscopy". The microstructure of steel plates is typically determined by optical microscopy using a nitric acid + 3% ethanol etching solution. However, when optical microscopy is insufficient, electron backscatter diffraction scanning electron microscopy (EBSD) can be used for identification. The volume fraction of the microstructure is equivalent to the area fraction in the measured field of view. The orientation difference of ferrite grain boundaries was observed using the EBSD method. The main carbide precipitates in steel are NbC, VC, and Fe3C. Thin-film transmission electron microscopy was used for observation. The size and spacing of precipitates in steel were observed using transmission electron microscopy at 5000x magnification in 30 arbitrary fields of view. Images were obtained, and then image analysis software was used to obtain the precipitate particle size and spacing from the images.

[0084] (3) Testing of different metallographic structures in steel: The different metallographic compositions in steel were tested using DIN 50600 "Inspection of metallic materials - Metallographic micrographs - Image proportions and dimensions".

[0085] (4) Testing of Ferrite Grain Boundary Orientation Difference and Geometrically Required Dislocation Density in Steel, and Dislocation Recovery Rate Coefficient of Tempered Sorbite Laths: Ferrite grain boundary orientation difference and geometrically required dislocation density were tested using or referenced in YB / T 4677 "Determination of Texture in Steel - Electron Backscatter Diffraction (EBSD) Method". Geometrically required dislocations differ from conventionally statistically distributed dislocations; their density is related to the average Burgers vector of deformation-induced dislocations. Geometrically required dislocations adapt to deformation inconsistencies, unlike statistically distributed dislocations, ensuring maximum deformation of the material. Geometrically required dislocation density (ρ) GND) ρ is calculated using the intergranular orientation difference (θ) measured by EBSD, the unit length (u) of the analytical position of dislocation density, and the average Burgers vector (b) of the deformed material.GND =2θ / (ub); The dislocation recovery rate coefficient for tempered sorbite laths is a parameter set for the dislocation recovery change of the microstructure of quenched and tempered steel after loading deformation due to the merging of dislocations or the decrease of dislocation density after unloading. It can characterize the influence of the change of microdislocations in quenched and tempered steel on the strength of quenched and tempered steel from the perspective of microstructure.

[0086] 3. Regarding the performance tests in Table 7:

[0087] (1) Dynamic yield strength and dynamic elongation test: Under room temperature conditions, the strain rate was controlled at 0.01-30s. -1 According to GB228.1 "Metallic materials - Tensile testing - Part 1: Test method at room temperature", the dynamic yield strength and dynamic elongation are tested. The strain rate refers to the change in strain per unit time.

[0088] (2) DWTT tear test: The DWTT tear performance was tested using SY / T6476 "Drop hammer tear test method for pipeline steel pipe".

[0089] (3) Collision energy absorption test: The collision energy absorption test shall be conducted in accordance with BS / EN15227-2020 "Requirements for collision resistance of railway facilities and railway vehicle bodies".

[0090] Table 4 shows that the impact-resistant pressure vessel steel plate of the finished product embodiment has moderate mechanical properties and strong resistance to deformation.

[0091] To illustrate the deformation resistance of the impact-resistant pressure vessel steel plate of the present invention, it is necessary to measure the microstructure and ferrite grain size of the impact-resistant pressure vessel steel plate of the present invention and the comparative steel plate of the comparative example, respectively. The test results are listed in Table 5.

[0092] The relevant performance testing conditions are as follows:

[0093] 1) Tensile property test: conducted at room temperature (20℃) and strain rate (0.0067s). -1 Under the specified conditions, the yield strength, tensile strength and elongation were tested according to GB228.1 "Metallic materials - Tensile testing - Part 1: Test method at room temperature".

[0094] 2) Test of precipitate size and spacing in steel: The spacing of precipitates was tested using JY / T 0581 "General Rules for Analysis by Transmission Electron Microscopy" and YB / T 4676 "Analysis of Precipitates in Steel by Transmission Electron Microscopy".

[0095] 3) Testing of different metallographic structures in steel: The different metallographic compositions in steel are tested using DIN 50600 "Inspection of metallic materials - Metallographic micrographs - Image proportions and dimensions".

[0096] 4) Ferrite grain boundary orientation difference and geometrically required dislocation density test in steel: The ferrite grain boundary orientation difference and geometrically required dislocation density, as well as the dislocation recovery rate coefficient of tempered sorbite laths, are tested using or with reference to YB / T 4677 "Determination of Texture in Steel by Electron Backscatter Diffraction (EBSD) Method".

[0097] Comparative analysis of the chemical composition design examples shows that:

[0098] Comparative Example 1 Comparative analysis with the embodiments of the present invention shows that reducing the C content in steel also worsens the strength-ductility product and changes the proportion of each phase in the microstructure of the steel. The C content of the steel plate of Comparative Example 1 is lower than the design lower limit of the present invention, with a value of 0.11%. Other chemical elements are not significantly different from those of the embodiments of the present invention. This results in the (Cr+Mo) / C ratio of 7.727, which exceeds the actual control range of 3.90 to 5.60 of the embodiments of the present invention. C is a solid solution strengthening element. Reducing the C content will reduce the strength of the steel and appropriately increase the elongation after fracture. Combined with the analysis of the production process parameters of controlled rolling and controlled cooling, quenching and tempering, and stress relief heat treatment, it can be seen that reducing the C content causes the static yield strength and tensile strength of the steel to be lower than the required range of the present invention. Its actual static yield strength is 845 MPa and its tensile strength is 764 MPa. The strength-ductility product of the steel plate of Comparative Example 1 is 18167.5 MPa·%, which is lower than the control range of 18250 to 22420 MPa·% of the embodiments of the present invention. Reducing the carbon content in steel decreases the volume fraction of tempered sorbite while increasing the volume fraction of other microstructures. Analysis of the data in Tables 5 and 6 shows that the bainite volume fraction in the steel plate of Comparative Example 1 is 24%, higher than the 9-18% bainite volume fraction described in this invention; the geometrically necessary dislocation density of Comparative Example 1 is 4.76 × 10⁻⁶. 14 This is lower than the strong plastic volume range of 4.78 × 10⁻⁶ described in this invention. 14 ~5.67×10 14 / m 2The reduction of carbon (C) in the steel resulted in a Nb+V to C ratio exceeding the upper limit of the control range in the embodiments of the present invention. This caused the carbide volume fraction and size in the steel to exceed the control range of the embodiments of the present invention. The carbide volume fraction in Comparative Example 1 was 0.0276%, and the carbide size in Comparative Example 1 was 89 nm, which is lower than the carbide volume fraction control range of 0.0178–0.0252% described in the present invention. The carbide size in Comparative Example 1 was 89 nm, which is higher than the carbide size control range of 47–78 nm described in the present invention. Analysis of the impact performance data in Table 6 shows that reducing the C content in the steel resulted in the DWTT absorption energy and impact absorption energy of the steel plate being lower than the control range of the present invention. Analysis of the comprehensive mechanical properties of the steel plate in Comparative Example 1 shows that the composition, process, microstructure, and mechanical properties (including basic mechanical properties and dynamic mechanical properties) of the steel plate in Comparative Example 1 cannot fully meet the requirements of the present invention.

[0099] Comparative Example 2 Compared with the embodiments of the present invention, the carbon content of the steel plate in Comparative Example 2 is higher than the upper limit of the carbon content range of 0.130-0.185% described in the present invention, with a value of 0.23%. Increasing the carbon content in steel will increase the yield strength and tensile strength of the steel, and indirectly reduce the elongation after fracture, plasticity, strength-ductility product, or impact DWTT absorption energy. Based on the analysis of production process data, the production process of the steel plate in Comparative Example 2 is similar to that of the embodiments of the present invention and there is no significant difference. The increased carbon content in Comparative Example 2 makes the static elongation after fracture of the steel lower than the control range of the embodiments of the present invention. Further analysis of the dynamic performance of the steel plate in Comparative Example 2 shows that the carbon content exceeds the control range described in the present invention, making the dynamic elongation after fracture, DWTT absorption energy, and impact absorption energy of the steel plate in Comparative Example 2 all lower than the lower limit of the control range of the present invention. From the comprehensive mechanical property analysis of the steel plate in Comparative Example 2, it can be seen that the composition-process-microstructure-mechanical properties (including basic mechanical properties and dynamic mechanical properties) of the steel plate in Comparative Example 2 cannot fully meet the requirements of the present invention.

[0100] Comparative Example 3Comparative analysis with the embodiments of the present invention shows that the Si content of the steel plate of Comparative Example 3 is lower than the lower limit of the control range of 0.28-0.63% in the embodiments of the present invention, and its value is 0.21%. Si plays a solid solution strengthening role in steel. Reducing the Si content in steel will decrease the yield strength, tensile strength, static elongation after fracture, and dynamic elongation after fracture. As shown in Tables 4 and 7, the Si content of the steel plate in Comparative Example 3 is lower than the lower limit of the control range of this invention, and the Mn / (C+Si) value is higher than the upper limit of the control range of this invention. This results in the strength-ductility product of the steel plate being lower than the lower limit of the control range of this invention. Furthermore, the reduction in Si content causes the dynamic elongation after fracture and impact absorption energy to be lower than the lower limit of the control range of this invention. Combined with the microstructure data shown in Tables 5 and 6, it can be seen that Si is an element that promotes the ferrite phase transformation. The reduction in Si content causes the proportion of bainite and tempered sorbite in the microstructure of the steel to change and exceed the control range of this invention. This indicates that reducing the Si content in steel and deviating from the control range of this invention will cause the comprehensive mechanical properties of the steel plate, especially the typical dynamic mechanical properties such as impact absorption energy and strength-ductility product, to fail to meet the control requirements described in this invention.

[0101] Comparative Example 4 Compared with the embodiments of the present invention, it can be seen that the Si content of the steel plate in Comparative Example 4 is higher than the upper limit of the control range of the present invention, with a value of 0.65, resulting in the Mn / (C+Si) ratio being lower than the lower limit of the control range of the embodiments of the present invention. Since Si is a solid solution strengthening element, the increase in Si content will reduce the elongation after fracture of the steel, thereby reducing the strength-ductility product of the steel. Si is an element that promotes ferrite phase transformation. The increase in Si content affects the proportion of ferrite produced by the quenching and tempering heat treatment process. As can be seen from the microstructure analysis shown in Tables 6 and 7, the increase in Si content increases the proportion of ferrite in the steel and changes the proportion of bainite and tempered sorbite in the steel. Moreover, the types and proportions of its microstructure phases are different from the control requirements of the present invention, showing significant differences. Combined with the mechanical properties and impact performance analysis in Tables 4 and 7, it can be seen that the increase in Si content reduces the dynamic elongation after fracture and impact absorption energy. Therefore, from the comprehensive analysis of composition, process, microstructure, and mechanical properties, it can be seen that the steel design of Comparative Example 4 cannot fully meet the requirements of the present invention.

[0102] Comparative Example 5Compared with the embodiments of the present invention, it can be seen that the Mn content of the steel plate in Comparative Example 4 is lower than the lower limit of the control range of the present invention, with a value of 1.32, resulting in the Mn / (C+Si) ratio being lower than the lower limit of the control range of the embodiments of the present invention. Since Mn is a solid solution strengthening element, the reduction of Mn content will reduce the yield strength, tensile strength, and elongation after fracture of the steel, thereby reducing the strength-ductility product of the steel. Mn is an element that delays the ferrite phase transformation. As can be seen from the microstructure analysis shown in Tables 6 and 7, the reduction of Mn content leads to the dislocation density value in Comparative Example 5 being lower than the lower limit of the control range of the embodiments of the present invention. Combining the mechanical properties and impact performance analysis in Tables 4 and 7, it can be seen that the increase of Mn content reduces the dynamic elongation after fracture (DWTT) absorption energy and impact absorption energy. Therefore, from the comprehensive analysis of composition, process, microstructure, and mechanical properties, it can be seen that the steel design of Comparative Example 5 cannot fully meet the requirements of the present invention.

[0103] Comparative Example 6 Compared with the embodiments of the present invention, the Mn content of the steel plate in Comparative Example 4 is higher than the upper limit of the control range of the present invention, with a value of 1.94, resulting in an Mn / (C+Si) ratio higher than the upper limit of the control range of the embodiments of the present invention. Since Mn is a solid solution strengthening element, the increase of Mn content will increase the yield strength and tensile strength of the steel but reduce the elongation after fracture, thus reducing the strength-ductility product of the steel. Mn is an element that delays the ferrite phase transformation. As can be seen from the microstructure analysis shown in Tables 6 and 7, the dislocation density value of the increased Mn content is higher than the control range of the embodiments of the present invention, showing a significant difference. Combined with the mechanical properties and impact performance analysis in Tables 4 and 7, it can be seen that the increase of Mn content reduces the dynamic elongation after fracture. The increase of Mn content causes the Mn / S ratio to exceed the upper limit of the control range of the embodiments of the present invention. The increase of Mn increases the segregation of the steel structure on the one hand, and also increases the number of MnS inclusions in the steel on the other hand, which impairs the toughness and impact performance of the steel. Therefore, based on a comprehensive analysis of composition, process, microstructure, and mechanical properties, it can be seen that the steel design of Comparative Example 6 cannot fully meet the requirements of this invention.

[0104] Comparative Example 7 Compared with the embodiments of the present invention, the main difference is that the S content of Comparative Example 7 is higher than the upper limit of the control range of the embodiments of the present invention, and the Mn / S ratio is lower than the lower limit of the control range of the present invention. The excessive S in the steel leads to a significant increase in the number of MnS inclusions in the steel, thereby reducing the static elongation and impact performance of the steel, as can be seen from the data analysis of Tables 4 and 7. From the mechanical property data analysis of Tables 4 and 7, it can be seen that the dynamic elongation after fracture (DWTT) and impact performance of the steel plate of Comparative Example 6 are both lower than the lower limit of the control range of the present invention. Based on the above analysis, it can be concluded that Comparative Example 6 cannot meet the design requirements of the present invention in terms of microstructure, mechanical properties, and impact performance.

[0105] Comparative Example 8Compared with the embodiments of the present invention, the main difference is that the Cr content of the steel plate in Comparative Example 7 is higher than the upper limit of the Cr control range of the steel in the present invention, and the Cr+Mo to C ratio is higher than the lower limit of the control range of the present invention. Cr and Mo are hardenability elements and also carbonitride forming elements. Under the premise that the production process is no different from the embodiments of the present invention, after quenching and tempering heat treatment and stress relief heat treatment, the dynamic elongation after fracture of the steel plate is lower than the lower limit of the control range of the present invention. Further analysis of the effect of the increase in the Cr+Mo to C ratio on the microstructure shows that the increase in the Cr+Mo to C ratio makes the dislocation recovery coefficient of the martensitic lath higher than the upper limit of the control range of the embodiments of the present invention, and the carbide volume fraction and carbide size in the steel both exceed the control range of the embodiments of the present invention, as can be seen from the microstructure data shown in Tables 5 and 6. An increase in the Cr+Mo to C ratio will result in the dynamic elongation after fracture (DWTT) absorbed energy and impact absorbed energy being lower than the lower limit of the control range of this invention during the impact performance test. This proves that the Cr+Mo to C ratio exceeds the upper limit of the control range of this invention, causing the various proportions of the microstructure, multiple dynamic mechanical properties, and impact performance parameters of the steel plate to be lower than the lower limit of the control range of this invention, thus failing to meet the microstructure, mechanical properties, and impact performance design requirements of this invention.

[0106] Analysis of comparative data on production processes shows that:

[0107] Comparative Example 9 Compared with the embodiments of the present invention, the composition is controlled within the range of the embodiments. In the process parameter design, Comparative Example 10 adopts controlled rolling and controlled cooling process + tempering process and stress relief heat treatment process (SR process). The difference is that there is no quenching process in the process. The faster cooling rate in the controlled cooling process can form a medium-temperature transformation structure of bainite or martensite. However, for medium and thick steel plate products, when the steel plate is more than 30mm, the 1 / 4 plate thickness and the core structure cannot be quenched through controlled cooling process to form a martensite structure. Instead, a bainite or bainite + ferrite structure will be formed. Analysis of the mechanical properties in Table 4 and the microstructure data in Table 6 shows that when using controlled cooling + tempering and stress-relieving heat treatment, the tensile strength of the steel cannot reach the strength value after quenching + tempering + stress-relieving heat treatment. This results in the strength-ductility product of the steel plate being lower than the lower limit of the control range of the present invention, and the dislocation recovery coefficient of the martensitic laths in its microstructure being higher than the upper limit of the control range of the present invention. Based on the comprehensive data of composition, process, microstructure and mechanical properties, it can be seen that an unreasonable design of the stress-relieving heat treatment process can also cause significant differences in the mechanical properties and impact performance of steels with similar composition and process.

[0108] Comparative Example 10Compared to the embodiments of the present invention, the composition is controlled within the range of the embodiments. In the process parameter design, Comparative Example 10 adopts a controlled rolling and controlled cooling process, followed by a quenching + tempering process and a stress-relieving heat treatment process (SR process). The difference lies in that the stress-relieving heat treatment temperature is lower than the lower limit of the control range of the present invention. The stress-relieving heat treatment process can further soften the hard phase structure in the tempered steel and improve the toughness of the steel, especially its impact performance. Combining the mechanical properties in Table 4 and the microstructure data in Table 6, it can be seen that when the stress-relieving heat treatment temperature is lower than the control range of the present invention, the tensile strength of the steel after stress-relieving heat treatment is higher than the upper limit of the control range of the present invention, while the dynamic elongation after fracture is lower than the lower limit of the control range of the present invention. Based on the comprehensive data of composition, process, microstructure, and mechanical properties, it can be seen that an unreasonable design of the stress-relieving heat treatment process can also lead to significant differences in the mechanical properties and impact performance of steels with similar compositions and processes. Based on the comprehensive data of composition, process, microstructure, and mechanical properties, it can be seen that an unreasonable design of the tempering process can also lead to significant differences in the mechanical properties and impact performance of steels with similar compositions and processes.

[0109] As can be seen from Tables 4 to 77, the pressure vessel steel plate of this invention exhibits significantly superior overall impact resistance compared to the steel plate in the comparative example. The impact-resistant pressure vessel steel plate of this invention not only possesses excellent comprehensive mechanical properties but also exhibits good resistance to deformation and strain strengthening.

[0110] The static yield strength R of the impact-resistant pressure vessel according to an embodiment of the present invention eL The dynamic yield strength R is 815–890 MPa. e The static tensile strength is 870–960 MPa, the static elongation after fracture (A) is 19–24%, and the dynamic elongation after fracture (A) is... d The strength is 18.5%–22.5%, the strength-ductility product is 18250–22420 MPa·%, and the drop hammer tear (DWTT) energy is 23.5–27.8 kJ.

[0111] Furthermore, it should be noted that, through observation of the microstructure of the pressure vessel steel plate in the embodiments of the present invention, it can be seen that the matrix of the impact-resistant pressure vessel steel plate in the embodiments is tempered sorbite + bainite, and the volume percentage of carbide precipitation is 0.0178 to 0.0252%, the size range of carbide precipitates is between 47 and 78 μm, and the original austenitic ferrite grain size is between 9 and 22 μm.

[0112] In summary, the pressure vessel steel plate described in this invention has excellent performance, not only possessing good resistance to deformation and strain rate strengthening, but also good resistance to impact tearing and impact absorption, and has a very broad application prospect.

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Claims

1. A pressure vessel steel with a yield strength of 790 MPa, comprising the following composition by weight percentage: C: 0.130–0.185%, Si: 0.28–0.63%, Mn: 1.45–1.88%, Al: 0.020–0.047%, P≤0.015%, S≤0.005%, Nb: 0.025–0.050%, V: 0.035–0.065%, Cr: 0.30–0.50%, Mo: 0.25–0.52%, with the balance including Fe and other unavoidable impurities, and also satisfying the following limiting relationships: 0.37≤(Nb+V) / C≤0.62, 0.50≤Nb / V≤1.43, 3.72≤(Cr+Mo) / C≤5.60; The properties of the steel plate of the pressure vessel steel are as follows: Static yield strength R eL The dynamic yield strength R is 815–890 MPa. e The static tensile strength is 870–960 MPa, the static elongation after fracture (A) is 19–24%, and the dynamic elongation after fracture (A) is... d The strength-ductility product is 18250–22420 MPa·% at 18.5–22.5%; the drop hammer tear (DWTT) energy is 23.5–27.8 kJ. When the collision velocity is 6–50 m / s, the collision displacement is 1.5 m, and the strain rate is 0.03–16 1 / s, the energy absorbed by the steel plate after the collision is 4.17–4.73 kJ.

2. The impact-resistant pressure vessel steel with a yield strength of 790 MPa as described in claim 1, characterized in that: The balance consists of Fe and other unavoidable impurities.

3. The impact-resistant pressure vessel steel with a yield strength of 790 MPa as described in claim 1 or 2, characterized in that: 2.40≤Mn / (C+Si)≤3.60, 330≤Mn / S≤1630.

4. The impact-resistant pressure vessel steel with a yield strength of 790 MPa as described in claim 1, 2, or 3, characterized in that: The Si content is 0.31–0.60 wt%.

5. The impact-resistant pressure vessel steel with a yield strength of 790 MPa as described in claim 1, characterized in that: The Nb content is 0.028–0.048 wt%.

6. The impact-resistant pressure vessel steel with a yield strength of 790 MPa as described in claim 1, characterized in that: The V content is 0.037–0.060 wt%.

7. The impact-resistant pressure vessel steel with a yield strength of 790 MPa as described in claim 1, characterized in that: The Cr content is 0.30–0.47 wt%.

8. The impact-resistant pressure vessel steel with a yield strength of 790 MPa as described in claim 1, characterized in that: The Mo content is 0.26–0.50 wt%.

9. The impact-resistant pressure vessel steel with a yield strength of 790 MPa as described in any one of claims 1 to 8, characterized in that: The thickness of the pressure vessel steel is 6 to 35 mm.

10. The impact-resistant pressure vessel steel with a yield strength of 790 MPa as described in any one of claims 1 to 9, characterized in that: The microstructure of the pressure vessel steel is tempered sorbite + bainite, wherein the volume percentage of tempered sorbite is 82-91%, the length of tempered sorbite laths is 12-28 μm, and the width of tempered sorbite laths is 0.34-0.67 μm; the percentage of bainite is 9-18%; and the original austenite grain size is 9-22 μm. The carbide precipitates in the steel have a size of 47–78 nm and a volume percentage of 0.0178–0.0252%. When the deformation of the steel plate is 10–15%, the geometrically required dislocation density in the steel is 4.78 × 10⁻⁶. 14 ~5.67×10 14 / m 2 The dislocation recovery rate coefficient of tempered sorbite laths is 0.38–0.

77.

11. A method for producing impact-resistant pressure vessel steel with a yield strength of 790 MPa as described in any one of claims 1 to 10, characterized in that, Includes the following steps: 1) Smelting and casting Smelting and casting into billets according to the components described in claim 1 or 2 or 3 or 4 or 5 or 6 or 7 or 8; 2) Heating of the billet The billet is heated to 1170–1245℃ at a heating rate of 7–12℃ / min. 3) Rolling The initial rolling temperature of roughing shall not be lower than 1050℃; the cumulative reduction rate of roughing shall not be lower than 33%. The initial rolling temperature of finishing mill is not higher than 930℃, the final rolling temperature of finishing mill is not higher than 860℃, and the cumulative reduction rate of the last three finishing mill passes is not less than 32%. 4) Cooling The cooling temperature is 305–555℃, and the cooling rate is 11–36℃ / s; 5) Quenching and tempering heat treatment Quenching temperature is 885~940℃, quenching holding time: (10~25min)+t×1min / mm; Tempering temperature is 600~645℃, tempering holding time: (20~40min)+t×1min / mm; Where: t is the thickness of the hot-rolled plate, in mm; 6) Stress relief The stress relief temperature is controlled at 580–605℃, and held at this temperature for 35–120 minutes.

12. The method for producing impact-resistant pressure vessel steel with a yield strength of 790 MPa as described in claim 11, characterized in that, In step 4), the cooling temperature is 320–550℃ and the cooling rate is 10–33℃ / s.

13. The method for producing impact-resistant pressure vessel steel plates with a yield strength of 790 MPa as described in claim 11, characterized in that, In step 5), the quenching temperature is 890–928℃ and the tempering temperature is 605–638℃.

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