Collision-resistant pressure vessel steel with yield strength of 355MPa and production method of collision-resistant pressure vessel steel

Abstract

The invention discloses an anti-collision pressure vessel steel with yield strength of 355MPa and a production method thereof. The anti-collision pressure vessel steel comprises the following components in percentage by weight: 0.06-0.13% of C, 0.15-0.40% of Si, 1.10-1.50% of Mn, 0.025-0.045% of Al, less than or equal to 0.015% of P, less than or equal to 0.005% of S, 0.010-0.020% of Nb and the balance of Fe and inevitable impurities. 0.085 < = Nb / C < = 0.28, 230 < = Mn / S < = 1490, and 2.74 < = Mn / (C + Si) < = 4.74. According to the steel for the pressure vessel, the static yield strength ReL ranges from 380 MPa to 490 MPa, the dynamic yield strength Re ranges from 470 MPa to 685 MPa, the tensile strength ranges from 490 MPa to 630 MPa, the static percentage elongation after fracture A ranges from 22% to 26%, the dynamic percentage elongation after fracture Ad ranges from 20% to 25%, and the product of strength and elongation ranges from 13100 MPa.% to 15000 MPa. The drop weight tear (DWTT) energy is 15 to 19 kJ; when the collision speed is 6-50 m / s, the collision displacement is 1.5 m, and the strain rate is 0.028-161 / s, the absorption energy of the steel plate after collision is 2.58-3.35 KJ, and the performance requirements that the 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 355 MPa and its production method. Background Technology

[0002] Hazardous chemicals such as diethyl ether are typically transported in pressure vessels. During transport, accidents such as tank tipping, overturning, and collisions are unavoidable. 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 hazardous chemicals, such as diethyl ether gas, can severely irritate the respiratory tracts of people near the transport vehicle, pollute the surrounding environment, and may even pose an explosion hazard due to the hazardous chemicals 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 355 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 380–490 MPa. e The static tensile strength is 470–685 MPa, the static elongation after fracture (A) is 490–630 MPa, and the dynamic elongation after fracture (A) is 22–26%. d The strength is 20-25%, the strength-ductility product is 13100-15000 MPa·%, the drop hammer tear (DWTT) energy is 15-19 kJ, and when the impact velocity is 6-50 m / s, the impact displacement is 1.5 m, and the strain rate is 0.028-16 1 / s, the energy absorbed by the steel plate after the impact is 2.58-3.35 kJ, which meets the performance requirements of the steel plate absorbing energy during impact without large deformation and without steel plate breakage. It is particularly suitable for pressure vessels transporting hazardous chemicals such as ether.

[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 355 MPa, comprising the following weight percentages: C: 0.06–0.13%, Si: 0.15–0.40%, Mn: 1.10–1.50%, Al: 0.025–0.045%, P≤0.015%, S≤0.005%, Nb: 0.010–0.020%, with the balance including Fe and other unavoidable impurities; and also satisfying the following limiting relationships:

[0012] 0.085≤Nb / C≤0.28, 230≤Mn / S≤1490, 2.74≤Mn / (C+Si)≤4.74.

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

[0014] Static yield strength R eL At 380–490 MPa, the dynamic yield strength R e The static tensile strength is between 470 and 685 MPa, the static elongation after fracture (A) is between 490 and 630 MPa, and the dynamic elongation after fracture (A) is between 22 and 26%. d At 20-25%, the strength-ductility product is 13100-15000 MPa·%; the drop hammer tear (DWTT) energy is 15-19 kJ.

[0015] When the collision velocity is 6–50 m / s, the collision displacement is 1.5 m, and the strain rate is 0.028–16 1 / s, the energy absorbed by the steel plate after the collision is 2.58–3.35 kJ. (Collision absorption energy was tested according to BS / EN15227-2020 "Collision protection requirements for railway facilities and railway vehicle bodies").

[0016] Preferably, the balance is Fe and other unavoidable impurities.

[0017] Preferably, the Si content is 0.16–0.39 wt%.

[0018] Preferably, the Nb content is 0.011–0.017 wt%.

[0019] The thickness of the pressure vessel steel described in this invention is 6 to 35 mm.

[0020] The microstructure of the pressure vessel steel described in this invention is tempered sorbite + bainite, wherein the volume percentage of tempered sorbite is 75-82%, the length of tempered sorbite laths is 11-29 μm, and the width of tempered sorbite laths is 0.32-0.70 μm; the percentage of bainite is 19-24%; and the original austenite grain size is 10-19 μm.

[0021] The carbide precipitates in the steel have a size of 42–74 nm and a volume percentage of 0.006–0.0115%. When the deformation of the steel plate is 10–15%, the geometrically required dislocation density in the steel is 3.38 × 10⁻⁶. 14 ~3.84×10 14 / m 2 The dislocation recovery rate coefficient of tempered sorbite laths is 0.55 to 0.90.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] The dynamic yield strength R defined in this invention e At 470–685 MPa and dynamic elongation after fracture A d The values ​​of 20% to 25% represent the dynamic yield strength and dynamic elongation at fracture obtained from extreme road condition tests conducted in simulated existing transportation environments. For values ​​exceeding the defined dynamic yield strength R... e Range 470–685 MPa and dynamic elongation after fracture A dWithin 20-25%, the dynamic mechanical properties of the material will exhibit rapid plastic instability, leading to fracture or breakage. To ensure the aforementioned dynamic yield strength and dynamic elongation after fracture, the speed of mobile container vehicles transporting such hazardous chemicals needs to be restricted, and appropriate protective barriers or structures should be installed around the tank material to buffer the collision energy and momentum during impact. 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 significant alterations in the material's mechanical properties after the impact.

[0026] In the composition design of the pressure vessel steel described in this invention:

[0027] 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 0.06% and 0.13%.

[0028] 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 0.15–0.40%, preferably 0.16–0.39 wt%.

[0029] 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 1.10% and 1.50%.

[0030] 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 0.025% and 0.045%.

[0031] 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 ferrite 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 0.010–0.020%, preferably 0.011–0.017 wt%.

[0032] 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%.

[0033] The composition design of this invention also specifically requires:

[0034] 0.085≤Nb / C≤0.28, Nb is a microalloying element that can refine grains. The present invention controls the ratio of Nb to C to ensure that while refining grains, the ratio 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 83°, ensuring the realization of the dynamic mechanical properties and impact resistance of the steel.

[0035] 230≤Mn / S≤1490. 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.

[0036] 2.74≤Mn / (C+Si)≤4.74, Mn, C, and Si are the main solid solution strengthening elements in steel. Limiting the ratio of Mn to C and Si elements to 2.74 to 4.74 is mainly to ensure that the yield strength, tensile strength, and elongation after fracture of the steel plate remain within a certain range. On the other hand, it is to ensure the stability of the dynamic yield strength, dynamic elongation after fracture, and strength-ductility product in the dynamic mechanical properties of the steel plate, and to ensure the stability of the solid solution strengthening effect of Mn in the steel without causing the microstructure, dislocation density, and dynamic collision performance caused by microstructure segregation.

[0037] The present invention also provides a method for producing the aforementioned impact-resistant pressure vessel steel, which includes the following steps:

[0038] 1) Smelting and casting

[0039] Smelting and casting into billets according to the above-described components;

[0040] 2) Heating of the billet

[0041] The billet is heated to 1150–1250℃ at a heating rate of 6–13℃ / min.

[0042] 3) Rolling

[0043] The initial rolling temperature of roughing shall not be lower than 1050℃; the cumulative reduction rate of roughing shall not be lower than 34%.

[0044] The finishing rolling temperature shall not exceed 840℃, and the cumulative reduction rate of the last three finishing rolling passes shall not be less than 32%.

[0045] 4) Cooling

[0046] Cooling temperature is 315~610℃, and cooling rate is 6~25℃ / s;

[0047] 5) Quenching and tempering heat treatment

[0048] Quenching temperature is 860~915℃, quenching holding time: (10~25min)+t×1min / mm;

[0049] Tempering temperature is 580~650℃, tempering holding time: (20~40min)+t×1min / mm;

[0050] Where: t is the thickness of the hot-rolled plate, in mm;

[0051] 6) Stress relief

[0052] The stress relief temperature is controlled at 585–640℃, and held at this temperature for 35–120 minutes.

[0053] Preferably, in step 4), the cooling temperature is 320–590°C and the cooling rate is 8–23°C / s.

[0054] Preferably, in step 5), the quenching temperature is 870–910°C and the tempering temperature is 595–635°C.

[0055] In the method for producing impact-resistant pressure vessel steel according to the present invention

[0056] The present invention controls the heating rate at 6-13℃ / min to heat the billet to 1150-1250℃. 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.

[0057] 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 34%. 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.

[0058] This invention controls the finishing rolling temperature to be no higher than 840℃ and the cumulative reduction rate of the last three finishing rolling 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.

[0059] The present invention controls the cooling to 315-610℃ at a cooling rate of 6-25℃ / 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 range, thereby ensuring the required original austenite grain size of the finished steel plate and precisely controlling the rolled properties within a certain range.

[0060] This invention controls the quenching temperature at 860–915℃, with a quenching holding time of (10–25 min) + t × 1 min / mm; and controls the tempering temperature at 580–650℃, 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.

[0061] This invention controls the stress-relief temperature at 585–640℃ 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.

[0062] Compared with the prior art, the advantages of this invention are:

[0063] 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.

[0064] The composition of this invention is based on C-Mn, with the addition of an appropriate amount of Nb, and the Nb / C ratio is limited to the range of 0.085 to 0.28. The Mn / S ratio in the steel is controlled to be in the range of 230 to 1490, and the Mn / (C+Si) ratio in the steel is controlled to be in the range of 2.74 to 4.74. 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 the 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.

[0065] 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.

[0066] The present invention employs controlled rolling and cooling, quenching and tempering, and stress-relieving heat treatment to achieve impact-resistant steel plates. The basic and dynamic mechanical properties of impact-resistant steel plates produced by existing controlled rolling and cooling processes fluctuate significantly, making it impossible to guarantee that the basic and dynamic mechanical properties, as well as the collision absorption energy and collision tearing energy parameters, are controlled within a certain range. This is the essential 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, thus enabling the target steel plate to meet the performance requirements of new application scenarios. Detailed Implementation

[0067] The present invention will be further described below with reference to the embodiments.

[0068] The chemical composition of the embodiments and comparative examples of the present invention is shown in Table 1, with the balance being Fe and other unavoidable impurities; Table 2 shows the process parameters of the embodiments and comparative examples of the present invention; Table 3 shows the performance parameters of the embodiments and comparative examples of the present invention; Tables 4 and 5 show the microstructure of the embodiments and comparative examples of the present invention; Table 6 shows the dynamic performance testing parameters of the embodiments and comparative examples of the present invention.

[0069] 1. Regarding the performance testing in Table 3:

[0070] (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".

[0071] (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".

[0072] 2. Regarding the performance testing in Table 4:

[0073] (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".

[0074] (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.

[0075] (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".

[0076] (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.

[0077] 3. Regarding the performance tests in Table 6:

[0078] (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.

[0079] (2) DWTT tear test: The DWTT tear performance was tested using SY / T6476 "Drop hammer tear test method for pipeline steel pipe".

[0080] (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".

[0081] Table 3 shows that the impact-resistant pressure vessel steel plate of the finished product embodiment has moderate mechanical properties and strong resistance to deformation.

[0082] 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 4.

[0083] The relevant performance testing conditions are as follows:

[0084] 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".

[0085] 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".

[0086] 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".

[0087] 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".

[0088] Comparative analysis of the chemical composition design examples shows that:

[0089] Comparative Example 1 Compared with the embodiments of the present invention, the C content of the steel plate in Comparative Example 1 is lower than the design lower limit of the present invention, with a value of 0.05%. Other chemical elements are not significantly different from those in the embodiments of the present invention. This results in the Mn / (C+Si) ratio 5 exceeding the actual control range of 2.7–4.75 in 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 production process parameters such as controlled rolling and cooling, quenching + tempering, and stress-relieving heat treatment, it can be seen that reducing the C content leads to a decrease in the static yield strength R of the steel. eL The actual value is 370 MPa, which is lower than the required range of 420-490 MPa of the present invention. The strength-ductility product of the steel plate in Comparative Example 1 is 12360 MPa·%. Reducing the C content in the steel reduces the volume fraction of tempered sorbite while increasing the volume fraction of other microstructures, but increases the dislocation recovery rate parameter of the tempered sorbite laths. Analysis of the data in Tables 4 and 5 shows that the tempered sorbite fraction in the steel plate of Comparative Example 1 is 74%, lower than the tempered sorbite volume fraction of 75-82% described in the present invention. The bainite volume fraction in the steel plate of Comparative Example 1 is 21%, which is higher than the bainite volume fraction described in the present invention. Therefore, the tempered sorbite volume fraction of Comparative Example 1 is lower than the control range of the embodiments of the present invention, with a value of 74%. The dislocation recovery rate parameter of the tempered sorbite lath of Comparative Example 1 is 0.93, which is higher than the control range of the embodiment of the present invention. Reducing the C content in the steel will also make the strength-ductility product of the steel worse. The strength-ductility product of the steel plate of Comparative Example 1 is 12360 MPa·%, which is lower than the strength-ductility product range of 13100~15000 MPa·% described in the present invention. Reducing the C content in the steel will also reduce the geometrically required dislocation density in the steel. The dislocation density value of the steel plate of Comparative Example 1 is lower than the control range of the embodiment of the present invention. From the comprehensive mechanical property analysis of the steel plate of Comparative Example 1, it can be seen that the composition-process-microstructure-mechanical properties (including basic mechanical properties and dynamic mechanical properties) of the steel plate of Comparative Example 1 cannot fully meet the requirements of the present invention.

[0090] Comparative Example 2Compared to the embodiments of the present invention, the C content of the steel plate in Comparative Example 2 is higher than the upper limit of the C content range of 0.06-0.13% described in the present invention, with a value of 0.14%. This results in the Mn / (C+Si) ratio 5 being higher than the control range of 2.7-4.75 described in the present invention. Increasing the C content in steel will increase the strength of the steel, and indirectly increase the dislocation density in the steel, reducing the plasticity or impact toughness of the steel. 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 embodiment of the present invention and there is no significant difference. The increased C content in Comparative Example 2 causes the yield strength and tensile strength of the steel to exceed the control range of the present invention, while the static elongation after fracture is lower than the control range of the embodiment of the present invention. Further analysis of the dynamic properties of the steel plate in Comparative Example 2 shows that the C content in Comparative Example 2 exceeds the control range of the present invention, causing the dynamic yield strength, dynamic elongation after fracture, and DWTT absorbed energy values ​​of the steel plate in Comparative Example 2 to be 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.

[0091] Comparative Example 3 Comparative 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 0.15-0.40% in the embodiments of the present invention, and its value is 0.08%. Si plays a solid solution strengthening role in steel. Reducing the Si content in steel will reduce the yield strength and tensile strength of the steel. As shown in Tables 3 and 6, the Si content of the steel plate in Comparative Example 3 is lower than the lower limit of the control range of this invention, while 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 DWTT energy absorption to be lower than the lower limit of the control range of this invention, while the dynamic elongation after fracture is higher than the lower limit of the control range of this invention. Combined with the microstructure data shown in Tables 4 and 5, it can be seen that Si is an element that promotes 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 also causes the dislocation recovery rate coefficient of the tempered sorbite lath to be lower than the lower limit of the control of the example, while the dislocation density value is higher than the upper limit of 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.

[0092] Comparative Example 4Compared with the embodiments of the present invention, 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. Since Si is a solid solution strengthening element, the increase of Si content will reduce the elongation after fracture of the steel. Si is an element that promotes ferrite phase transformation. The effect of increasing Si content in tempered sorbitic or tempered martensitic steel produced by quenching and tempering heat treatment process is to slow down the recovery rate of tempered sorbite. As can be seen from the microstructure analysis shown in Tables 5 and 6, the 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 and have significant differences. Combined with the mechanical properties and impact performance analysis in Tables 3 and 6, it can be seen that the increase of Si content reduces the dynamic elongation after fracture and impact absorption energy of the steel. 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.

[0093] Comparative Example 5 Compared with the embodiments of the present invention, the main difference is that the Mn content of the steel plate in Comparative Example 5 is lower than the lower limit of the control range of the present invention. Mn is a solid solution strengthening element. Reducing the Mn content will reduce the yield strength and tensile strength of the steel. The Mn content of the steel plate in Comparative Example 5 reaches 0.95%, and the Mn / (C+Si) ratio is lower than the lower limit of the control range of the present invention. Under the premise that the production process is no different from that of the embodiments of the present invention, after quenching and tempering heat treatment and stress relief heat treatment, the yield strength and tensile strength of the steel plate are both lower than the lower limit of the control range of the present invention, and the dynamic yield strength of the steel plate is lower than the lower limit of the control range of the present invention, while its dynamic elongation after fracture is higher than the upper limit of the control range of the present invention. Further analysis of the effect of the reduction of Mn content on microstructure shows that the increase of Mn content makes the tempered sorbite content lower than the lower limit of the control range of the embodiments of the present invention, while its bainite content is higher than the upper limit of the control range of the embodiments. Moreover, the contents of each item in the microstructure exceed the control range of the present invention, and its dislocation density value is lower than the lower limit of the control range of the present invention, as can be seen from the microstructure data shown in Tables 4 and 5. The reduction of Mn content increases the elongation after fracture and dynamic elongation after fracture of steel, as well as the strength-ductility product of steel. In the impact performance test, the DWTT performance is lower than the lower limit of the control range of the present invention, which proves that the Mn content exceeds the upper limit of the control range of the present invention. As a result, the proportions of various microstructures, multiple dynamic mechanical properties and impact performance parameters of the steel plate are lower than the lower limit of the control range of the present invention, and cannot meet the microstructure, mechanical properties and impact performance design requirements of the present invention.

[0094] Comparative Example 6Compared with the embodiments of the present invention, the main difference is that the Mn content of the steel plate in Comparative Example 6 is higher than the upper limit of the control range of the present invention. Mn is a solid solution strengthening element. Increasing the Mn content will increase the yield strength and tensile strength of steel. However, excessive Mn content will increase the center segregation of the continuously cast billet. This center segregation will form microstructure center segregation in the core of the steel plate after rolling and heat treatment, resulting in non-uniformity of the mechanical properties of the steel plate. The Mn content of the steel plate in Comparative Example 6 reaches 1.70%, and the Mn / S ratio exceeds the upper limit of the control range of the present invention. Under the premise that the production process is no different from that of the embodiments of the present invention, after quenching and tempering heat treatment and stress relief heat treatment, the yield strength of the steel plate exceeds the upper limit of the control range of the present invention, and the dynamic yield strength of the steel plate exceeds the upper limit of the control range of the present invention, while its dynamic elongation after fracture is lower than the lower limit of the control range of the present invention. Further analysis of the impact of increased Mn content on the microstructure revealed that the increased Mn content resulted in a higher content of tempered sorbite and bainite compared to the embodiments of the present invention. Furthermore, all contents within the microstructure exceeded the control range of the present invention, with the dislocation density exceeding the upper limit of the control range, as shown in Tables 4 and 5. Increased Mn content reduces the elongation after fracture and dynamic elongation after fracture of the steel, as well as the strength-ductility product. In the impact performance test, the DWTT performance was lower than the lower limit of the control range of the present invention, demonstrating that the Mn content exceeded the upper limit of the control range. This resulted in the proportions of various microstructure components, multiple dynamic mechanical properties, and impact performance parameters of the steel plate falling below the lower limit of the control range of the present invention, failing to meet the design requirements for microstructure, mechanical properties, and impact performance as designed in this invention.

[0095] Comparative Example 7 Compared with the embodiments of the present invention, the difference in composition lies in the fact that the sulfur content in the steel is higher than the upper limit of the control range of the present invention. Excessive sulfur content in the steel will form large or numerous MnS inclusions during the continuous casting process, impairing the high-temperature bonding strength of the billet. Furthermore, during subsequent rolling, the MnS inclusions will be elongated and deformed, damaging the plasticity, fracture toughness, and impact resistance of the finished steel plate. Analysis of the mechanical property results shown in Tables 3 and 6 reveals that the sulfur content in the steel of Comparative Example 5 is higher than the upper limit of the sulfur content controlled by the present invention, reducing the elongation after fracture of the finished steel plate, thereby further reducing the dynamic elongation after fracture. In the impact performance test, the DWTT performance and impact absorption energy of the steel plate are both far below the lower limit of the control range of the present invention, failing to meet the impact-resistant steel plate design requirements of the present invention.

[0096] Comparative Example 8Compared with the composition of the embodiments of the present invention, the Nb content exceeds the upper limit of the control range of the embodiments of the present invention. The addition of Nb in steel mainly forms Nb carbides or nitrides in the steel, thereby playing a precipitation strengthening role. When the Nb / C ratio is higher than the lower limit of the control range of the present invention, the effect on the microstructure is that the volume fraction of carbides in the steel exceeds the upper limit of the control range of the present invention. Excessive precipitation of carbides will damage the elongation after fracture of the steel. As can be seen from the mechanical property analysis in Table 3, the excessively high Nb / C ratio causes the elongation after fracture of the steel to be lower than the lower limit of the control range of the present invention, which in turn causes the strength-ductility product to be lower than the lower limit of the control range of the present invention. At the same time, the excessively high Nb / C ratio causes the dynamic elongation after fracture and DWTT performance to be lower than the lower limit of the control range of the present invention, which cannot meet the impact resistance performance index requirements designed in the present invention.

[0097] Comparative Example 9 Compared with the composition of the embodiments of the present invention, the Nb content is lower than the upper limit of the control range of the embodiments of the present invention, and the Nb / C ratio is lower than the lower limit of the control range of the present invention. The Nb / C ratio is too low, resulting in insufficient grain refinement effect of Nb in the rolling deformation and heat treatment processes, causing the original austenite grains to grow, which in turn affects mechanical properties, dynamic mechanical properties, and impact properties. As can be seen from the analysis of the original austenite grain size data of the microstructure shown in Table 4, the original austenite grain size of the steel plate of Comparative Example 8 exceeds the upper limit of the control range of the present invention. The unreasonable control of Nb / C cannot effectively refine the grains, thus failing to effectively improve the yield strength, tensile strength, and fracture toughness of the steel plate. As can be seen from the analysis of the mechanical property data in Tables 3 and 6, the dynamic elongation after fracture (DWTT) and impact properties of the steel plate of Comparative Example 9 are both lower than the lower limit of the control range of the present invention, and cannot meet the design requirements of the present invention.

[0098] Analysis of comparative data on production processes shows that:

[0099] Comparative Example 10Compared 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, followed by quenching + tempering and stress relief heat treatment process. However, the difference is that the quenching temperature is lower than the lower limit of the control range of the present invention. The quenching process can cause diffusionless martensitic transformation when austenite in steel transforms into ferrite, and improve the strength of martensite. When the quenching temperature is lower than 860℃, the transformation of quenched martensite is incomplete, and there is still a considerable amount of bainite and a small amount of ferrite in the microstructure. From the microstructure data analysis shown in Tables 4 and 5, it can be seen that the tempered sorbite of the steel plate of Comparative Example 10 is lower than the lower limit of the control range of the embodiments of the present invention, while its bainite content is lower. The integral score is higher than the upper limit of the control range of this invention. Further analysis of the mechanical property data in Tables 3 and 6 shows that when the quenching temperature is lower than the lower limit of the control range of this invention, the yield strength and tensile strength of the steel are both higher than the upper limit of the control range of this invention, while its elongation after fracture is lower than that of the embodiment of this invention. The higher yield strength leads to a further increase in dynamic yield strength, and the lower elongation after fracture leads to a further decrease in dynamic elongation after fracture. Compared with embodiment 10 of this invention, the dynamic elongation after fracture is lower than the lower limit of the control range of this invention. Based on the comprehensive data of composition, process, microstructure and mechanical properties, it can be seen that an unreasonable quenching process design can also cause significant differences in the mechanical properties and impact properties of steels with similar composition and process.

[0100] Comparative Example 11 Compared with the embodiments of the present invention, the chemical composition is within the control range of the present invention, the controlled rolling and cooling process and the quenching + tempering heat treatment process are within the control range of the present invention, while the stress relief heat treatment temperature is lower than the lower limit of the control range of the present invention. Stress relief heat treatment can optimize the matching of the strength and toughness of the steel plate. When the stress relief heat treatment temperature is too low, the internal stress in the steel plate is high and the dislocation density is high, making it impossible to optimize the strength and toughness of the steel plate. By analyzing the microstructure, as shown in Tables 4 and 5, the dislocation density of the steel plate of Comparative Example 11 exceeds the upper limit of the control range of the present invention. Further analysis of the mechanical property data shows that the stress relief temperature is too low, and the strength and elongation after fracture of the steel plate do not achieve optimal matching. This results in the dynamic yield strength of the steel plate of Comparative Example 11 being higher than the upper limit of the control range of the present invention, while its dynamic elongation after fracture is lower than the lower limit of the control range of the present invention, and its DWTT performance is also lower than the lower limit of the control range of the present invention.

[0101] As can be seen from Tables 3 to 6, 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.

[0102] The static yield strength R of the impact-resistant pressure vessel steel plate according to an embodiment of the present invention elAll are between 380 and 490 MPa, with dynamic yield strength R e All are between 470 and 685 MPa, with static tensile strengths between 490 and 630 MPa, and static elongation after fracture (A) between 22% and 26%, and dynamic elongation after fracture (A) between 26% and 26%. d Between 20% and 25%, the strength-ductility product is in the range of 13100 to 15000 MPa·%; the drop hammer tear (DWTT) energy is in the range of 15 to 19 kJ.

[0103] 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 microstructure of the impact-resistant pressure vessel steel plate in the embodiments is tempered sorbite + bainite, wherein the volume percentage of carbide precipitation is 0.006 to 0.0115%, the size range of carbide precipitates is between 42 and 74 μm, and the original austenitic ferrite grain size is between 10 and 19 μm.

[0104] 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 355 MPa, comprising the following composition by weight percentage: C: 0.06–0.13%, Si: 0.15–0.40%, Mn: 1.10–1.50%, Al: 0.025–0.045%, P ≤ 0.015%, S ≤ 0.005%, Nb: 0.010–0.020%, with the balance including Fe and other unavoidable impurities, and also satisfying the following limiting relationships: 0.085≤Nb / C≤0.28, 230≤Mn / S≤1490, 2.74≤Mn / (C+Si)≤4.74; 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 380–490 MPa. e The static tensile strength is 470–685 MPa, the static elongation after fracture (A) is 490–630 MPa, and the dynamic elongation after fracture (A) is 22–26%. d At 20-25%, the strength-ductility product is 13100-15000 MPa·%; the drop hammer tear (DWTT) energy is 15-19 kJ. When the collision velocity is 6–50 m / s, the collision displacement is 1.5 m, and the strain rate is 0.028–16 1 / s, the energy absorbed by the steel plate after the collision is 2.58–3.35 kJ.

2. The impact-resistant pressure vessel steel with a yield strength of 355 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 355 MPa as described in claim 1, characterized in that: The Si content is 0.16–0.39 wt%.

4. The impact-resistant pressure vessel steel with a yield strength of 355 MPa as described in claim 1, characterized in that: The Nb content is 0.011–0.017 wt%.

5. The impact-resistant pressure vessel steel with a yield strength of 355 MPa as described in any one of claims 1 to 4, characterized in that: The thickness of the pressure vessel steel is 6 to 35 mm.

6. The impact-resistant pressure vessel steel with a yield strength of 355 MPa as described in any one of claims 1 to 5, characterized in that: The microstructure of the pressure vessel steel is tempered sorbite + bainite, wherein, The volume percentage of tempered sorbite is 75-82%, the length of tempered sorbite laths is 11-29 μm, and the width of tempered sorbite laths is 0.32-0.70 μm; the percentage of bainite is 19-24%; and the original austenite grain size is 10-19 μm. The carbide precipitates in the steel have a size of 42–74 nm and a volume percentage of 0.006–0.0115%. When the deformation of the steel plate is 10–15%, the geometrically required dislocation density in the steel is 3.38 × 10⁻⁶. 14 ~3.84×10 14 / m 2 The dislocation recovery rate coefficient of tempered sorbite laths is 0.55 to 0.

90.

7. A method for producing impact-resistant pressure vessel steel with a yield strength of 355 MPa as described in any one of claims 1 to 6, characterized in that, Includes the following steps: 1) Smelting and casting Smelting and casting into billets according to the composition described in claim 1, 2, 3 or 4; 2) Heating of the billet The billet is heated to 1150–1250℃ at a heating rate of 6–13℃ / 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 34%. The finishing rolling temperature shall not exceed 840℃, and the cumulative reduction rate of the last three finishing rolling passes shall not be less than 32%. 4) Cooling Cooling temperature is 315~610℃, and cooling rate is 6~25℃ / s; 5) Quenching and tempering heat treatment Quenching temperature is 860~915℃, quenching holding time: (10~25min)+t×1min / mm; Tempering temperature is 580~650℃, 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 585–640℃, and held at this temperature for 35–120 minutes.

8. The method for producing impact-resistant pressure vessel steel with a yield strength of 355 MPa as described in claim 7, characterized in that, In step 4), the cooling temperature is 320–590℃ and the cooling rate is 8–23℃ / s.

9. The method for producing impact-resistant pressure vessel steel plates with a yield strength of 355 MPa as described in claim 7, characterized in that, In step 5), the quenching temperature is 870–910℃ and the tempering temperature is 595–635℃.

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