Method for manufacturing a 460mpa grade steel for pressure vessels, steel for pressure vessels

Abstract

The application provides a preparation method of a 460MPa-grade pressure container steel and the pressure container steel, and belongs to the field of steel preparation.The method comprises the following steps: obtaining a casting blank;sequentially performing heating, descaling, rolling, pre-straightening and post-rolling rapid cooling on the casting blank to obtain a hot-rolled plate;and performing normalizing, air cooling and slow cooling on the hot-rolled plate to obtain the 460MPa-grade pressure container steel.Through chemical composition design and straight rolling + post-rolling rapid cooling + normalizing and slow cooling process, it is ensured that 100% of the precipitation strengthening particles of the steel plate are V(C,N) and the size is controlled within 20nm, good precipitation strengthening effect is achieved, the low-temperature toughness of the steel plate is not deteriorated, the content of solid solution N in the steel is reduced by controlling the precipitation of V, the influence of N element on the strain age sensitivity of the steel plate is avoided, the strength and toughness matching of the steel plate after normalizing is realized, and the post-weld heat treatment and strain age performance are good.

Description

Technical Field

[0001] This application relates to the field of steel preparation technology, and in particular to a method for preparing 460MPa grade pressure vessel steel and the pressure vessel steel. Background Technology

[0002] With the rapid development of the national economy, the gas industry has grown rapidly, increasing by about 10% annually, leading to a surge in demand for mobile pressure vessel tank trucks. Mobile pressure vessel tank trucks place extremely high demands on the safety of their storage and transportation, further requiring higher standards for the strength, low-temperature toughness, yield strength ratio, strain aging performance, and simulated post-weld performance of the tank truck materials. High-strength steel with a strength of Rm≥570MPa is widely used abroad. Its tank walls are thin and its weight coefficient is low, thus improving the tank truck's density ratio and carrying efficiency, laying the foundation for the development of larger tank trucks. The transported materials are mostly cryogenic liquid fuels, therefore higher requirements are placed on the low-temperature toughness of the steel plates, with an impact energy of ≥60J at -60℃. Since the tank material needs to be bent into rings, the strain aging sensitivity of the steel plates is required to be high, with a strain aging sensitivity index <30%. The rings are connected by welding, and the welded workpieces require post-weld heat treatment, thus placing requirements on the post-weld heat treatment performance of the steel plates. The steel plate heads need to be hot-worked, and the performance of the steel plates must still meet the requirements after heating and natural cooling; therefore, the steel plates must be delivered in a normalized state.

[0003] Current technologies utilize v (V) precipitation to ensure the strength of steel plates after normalizing, but they fail to effectively control the precipitation behavior and size of the precipitated phases, thus failing to fully leverage the precipitation strengthening effect of v. Furthermore, the v precipitates in existing technologies are relatively coarse, necessitating the addition of nitrogen (Ni) to ensure the low-temperature toughness of the steel plate, thereby increasing alloy costs. Additionally, current production technologies neglect the strain aging performance of the steel plate. With high nitrogen content, without process control, the strain aging sensitivity of the steel plate will significantly increase, leading to a rapid deterioration of its low-temperature impact toughness after cold deformation, posing a significant threat to its service safety. Therefore, meeting the strain aging performance requirements while ensuring a balance between strength and toughness in pressure vessel steel is a pressing technical problem that needs to be solved. Summary of the Invention

[0004] This application provides a method for preparing 460MPa grade pressure vessel steel and the pressure vessel steel itself. By controlling the chemical composition, size and distribution of the V precipitate phase, it solves the technical problem in the prior art that it is difficult to meet the strain aging performance requirements while ensuring the strength and toughness matching of pressure vessel steel.

[0005] In a first aspect, this application provides a method for preparing 460MPa grade pressure vessel steel, the method comprising:

[0006] A cast billet is obtained; the chemical composition of the cast billet, by mass fraction, includes: C: 0.16%–0.18%, Si: 0.20%–0.30%, Mn: 1.50%–1.70%, P≤0.010%, S≤0.003%, Cr: 0.10%–0.30%, V: 0.055%–0.075%, N: 0.010%–0.015%, with the balance being Fe and unavoidable impurities;

[0007] The billet is sequentially heated, descaled, rolled, pre-straightened, and rapidly cooled after rolling to obtain a hot-rolled plate;

[0008] The hot-rolled plate is normalized, air-cooled, and slow-cooled to obtain 460MPa grade pressure vessel steel.

[0009] Optionally, the heating temperature is 1130℃~1160℃, and the heating time is 150min~440min.

[0010] Optionally, the two-stage rolling process adopts a direct rolling process, wherein the initial rolling temperature is 1050℃~1080℃ and the final rolling temperature is 900℃~950℃.

[0011] Optionally, during the rolling process, the single-pass reduction rate of at least 3 passes is ≥25%, and the number of reduction passes with a reduction rate of <5% in the last 5 passes is less than 2.

[0012] Optionally, the post-rolling rapid cooling adopts UFC+ACC combined cooling or UFC cooling alone, the final temperature of the post-rolling rapid cooling is 250℃~400℃, and the cooling rate is 15℃ / s~30℃ / s.

[0013] Optionally, the heating rate of the normalizing is 1.5 min / mm to 2.0 min / mm, and the heating endpoint temperature T of the normalizing satisfies: T = Ac3 + t, where Ac3 is the actual phase transformation temperature of carbon steel during heating, and t is the increase temperature, which is 2℃ to 10℃; the holding time of the normalizing is 5 min to 20 min.

[0014] Optionally, the final temperature of the air cooling is 550℃~600℃; the slow cooling adopts a stacking cooling process, and the stacking cooling time is ≥24h.

[0015] Optionally, the chemical composition, by mass fraction, further includes: Ti ≤ 0.005%, Nb ≤ 0.005%, and Al ≤ 0.005%.

[0016] Secondly, this application provides a 460MPa grade pressure vessel steel prepared by the method described in any one of the embodiments of the first aspect, wherein, by volume fraction, the precipitated phase in the metallographic structure of the pressure vessel steel is V(C,N) particles, and the size of the V(C,N) particles is ≤20nm; the metallographic structure comprises: ferrite: 65% to 80%, pearlite: 20% to 35%, and the average grain size of the ferrite is ≤8µm.

[0017] Optionally, the pressure vessel steel shall meet at least one of the following properties: yield strength ≥ 460 MPa, tensile strength ≥ 570 MPa, elongation after fracture ≥ 23%, low temperature toughness at -60℃ ≥ 100 J, and strain aging sensitivity coefficient ≤ 30%.

[0018] The technical solutions provided in this application have the following advantages compared with the prior art:

[0019] This application provides a method for preparing 460MPa grade pressure vessel steel. Through chemical composition design and a process of direct rolling + rapid cooling after rolling + normalizing and slow cooling, the method ensures that the precipitation strengthening particles in the steel plate are 100% V(C,N) with a size controlled within 20nm, achieving a good precipitation strengthening effect without deteriorating the low-temperature toughness of the steel plate. By controlling the precipitation of V, the content of dissolved N in the steel is reduced, avoiding the influence of N element on the strain aging sensitivity of the steel plate, achieving a balance between strength and toughness after normalizing, and demonstrating good simulated post-weld heat treatment and strain aging performance. The pressure vessel steel prepared by this method has a yield strength ≥460MPa, tensile strength ≥570MPa, elongation after fracture ≥23%, and impact toughness at -60℃ is stably controlled above 100J. Simulated post-weld performance is also good, with a strain aging sensitivity coefficient ≤30%. This solves the technical problem in the prior art of failing to meet strain aging performance requirements while ensuring a balance between strength and toughness in pressure vessel steel. Attached Figure Description

[0020] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0021] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 A schematic flowchart illustrating a method for preparing 460MPa grade pressure vessel steel provided in this application embodiment;

[0023] Figure 2 This is a metallographic diagram of the pressure vessel steel at 1 / 4 of its thickness, provided in Embodiment 2 of this application.

[0024] Figure 3 The morphology of the V(C,N) precipitates in the pressure vessel steel provided in Embodiment 2 of this application is shown. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0026] Various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the referred range.

[0027] Furthermore, in the description of this application, the terms "comprising," "including," etc., mean "including but not limited to." In this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. In this document, "and / or" describes the relationship between related objects, indicating that three relationships can exist; for example, A and / or B can represent: A alone, A and B simultaneously, or B alone. A and B can be singular or plural. In this document, "at least one" means one or more, and "more than" means two or more. "At least one," "at least one of the following," or similar expressions refer to any combination of these items, including any combination of single or plural items. For example, "at least one of a, b, or c" or "at least one of a, b, and c" can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be a single or multiple.

[0028] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0029] Figure 1 This is a schematic flowchart illustrating a method for preparing 460MPa grade pressure vessel steel according to an embodiment of this application.

[0030] Please see Figure 1 This application provides a method for preparing 460MPa grade pressure vessel steel, the method comprising:

[0031] S1. Obtain a cast billet; the chemical composition of the cast billet, by mass fraction, includes: C: 0.16%–0.18%, Si: 0.20%–0.30%, Mn: 1.50%–1.70%, P≤0.010%, S≤0.003%, Cr: 0.10%–0.30%, V: 0.055%–0.075%, N: 0.010%–0.015%, with the balance being Fe and unavoidable impurities;

[0032] In some embodiments, the chemical composition, by mass fraction, further includes: Ti ≤ 0.005%, Nb ≤ 0.005%, and Al ≤ 0.005%.

[0033] The positive effects of controlling the carbon content to 0.16%–0.18% are: ensuring the steel plate has a certain solid solution strengthening effect, and the carbon content causes a certain proportion of pearlite phase transformation in the microstructure, providing a certain guarantee for the strength of the steel plate after normalizing. For example, the carbon content can be 0.16%, 0.165%, 0.17%, 0.175%, 0.18%, etc.

[0034] The positive effects of controlling the Si content to 0.20%–0.30%: As an essential element for deoxidation in steelmaking, Si strongly inhibits and delays the decomposition of carbides in supercooled austenite, improves the stability of austenite, and increases the strength of the steel plate. However, the Si content also affects the low-temperature toughness of the steel plate. For example, the Si content can be 0.20%, 0.22%, 0.24%, 0.26%, 0.28%, 0.30%, etc.

[0035] The positive effects of controlling the Mn content to 1.50%–1.70% include: lower Mn alloy cost, increased steel strength, toughness, and hardness, strong austenite stabilizing element, and guaranteed steel strength through solid solution strengthening, although limited by the carbon equivalent of the steel plate. For example, the Mn content can be 1.50%, 1.52%, 1.54%, 1.56%, 1.58%, 1.70%, etc.

[0036] The positive effects of controlling the phosphorus (P) content to ≤0.010% and the sulfur (S) content to ≤0.003%: P and S are harmful elements in steel, adversely affecting the plasticity, low-temperature toughness, and weldability of steel plates. Therefore, to ensure the comprehensive mechanical properties of steel plates, the P and S content in the steel plates should be strictly controlled. For example, the P content can be 0.005%, 0.006%, 0.007%, 0.008%, 0.009%, 0.010%, etc., and the S content can be 0.001%, 0.002%, 0.003%, etc.

[0037] The positive effects of controlling the Cr content to 0.10%–0.30% include: the addition of Cr element plays a role in solid solution strengthening, while improving the hardenability of the material, thereby ensuring the tensile strength of the steel plate. For example, the Cr content can be 0.10%, 0.15%, 0.18%, 0.20%, 0.22%, 0.25%, 0.28%, 0.30%, etc.

[0038] The positive effects of controlling the V content to 0.055%–0.075% are as follows: V is a strong carbide-forming element, similar to Nb, and has a strong binding ability with carbon. The fine, dispersed V(C,N) particles formed can play a dispersion strengthening role, significantly increasing the strength of the steel. However, if the V content exceeds 0.10%, the toughness of the steel will decrease. For example, the V content can be 0.055%, 0.060%, 0.065%, 0.070%, 0.075%, etc.

[0039] The positive effects of controlling the nitrogen (N) content to be between 0.010% and 0.015% include: N expands the austenite region, and adding N to steel can enhance the precipitation strengthening effect of the microalloying element vanadium (V), thereby effectively improving the strength of the steel. However, when the N content exceeds a certain level, the steel's toughness will decrease sharply due to the aggregation and growth of nitrides. Furthermore, if N is not completely dissolved in the steel as a second phase, it will accumulate at dislocations after cold deformation, resulting in a strong work hardening effect that reduces toughness and increases the strain aging sensitivity of the steel plate. For example, the N content can be 0.010%, 0.011%, 0.012%, 0.013%, 0.014%, 0.015%, etc.

[0040] The positive effects of controlling the Ti content to ≤0.005% and Nb to ≤0.005% are as follows: To avoid competition for nitrogen by niobium-titanium carbonitrides, the Nb and Ti contents are strictly controlled within 0.005% in the composition design. For example, the Ti content can be 0.001%, 0.002%, 0.003%, 0.004%, 0.005%, etc., and the Nb content can be 0.001%, 0.002%, 0.003%, 0.004%, 0.005%, etc.

[0041] The positive effects of controlling the Al content to ≤0.005%: To avoid the impact of added Al on the surface quality of the cast billet, the Al content is strictly controlled within 0.005% in the composition design. For example, the Al content can be 0.001%, 0.002%, 0.003%, 0.004%, 0.005%, etc.

[0042] Based on the above composition design, to avoid the reaction between nitrogen (N) in the steel and Nb, Ti, and Al, which are prone to forming nitrides, Nb, Ti, and Al were not added to the composition. By controlling the size of the V(C,N) precipitates, the formation of precipitates smaller than 20 nm will not have a deteriorating effect on the low-temperature toughness of the steel plate. Therefore, it is not necessary to add Ni to ensure the low-temperature toughness of the steel plate. In addition, to ensure the hardness of the steel plate, Cr, an economical element that improves hardenability, was added to increase the carbon equivalent and ensure the tensile strength of the steel plate.

[0043] In some embodiments, the process before step S1 includes: LF refining after molten iron smelting, followed by RH or VD vacuum treatment to obtain molten steel with the same chemical composition as the pressure vessel steel. Specifically, molten iron and / or scrap steel are smelted in a primary furnace, i.e., through converter blowing or electric furnace smelting, with C% controlled to 0.02%–0.03% and P% controlled to below the target value of 0.003%. During tapping, deoxidizers and required alloys are added for deoxidation and alloying. The steel is then transported to the LF furnace for refining. During LF refining, the steel composition, except for N and manganese, is adjusted to the target values, ensuring that the LF outlet temperature is not less than 50°C–80°C above the liquidus. After LF refining, RH or VD vacuum degassing is performed to ensure that the hydrogen content in the steel is less than 2 ppm after vacuum treatment. Simultaneously, N and Mn elements are adjusted to the target values ​​using a nitrogen-manganese alloy. Soft blowing is then performed after vacuum treatment for 8–15 minutes. Control the refining gap of the casting machine rolls to be no greater than ±0.5mm, control the superheat of the molten steel in the tundish to be between 10℃ and 30℃, maintain a stable casting speed, and ensure that the fluctuation of the liquid level in the crystallizer is no greater than ±3mm, so that the center segregation results of the billet under low magnification inspection are better than Class C 1.0 grade.

[0044] S2. The billet is sequentially heated, descaled, rolled, pre-straightened, and rapidly cooled after rolling to obtain a hot-rolled plate;

[0045] In some embodiments, the heating temperature is 1130°C to 1160°C, and the heating time is 150 min to 440 min.

[0046] By controlling the heating temperature and time of the billet, drastic growth of austenite grain size can be avoided during the heating process. For example, the heating temperature can be 1130℃, 1135℃, 1140℃, 1145℃, 1150℃, 1155℃, 1160℃, etc., and the time can be 150min, 200min, 250min, 300min, 350min, 400min, 440min, etc.

[0047] In some embodiments, the rolling process is a direct rolling process, wherein the initial rolling temperature is 1050℃~1080℃ and the final rolling temperature is 900℃~950℃.

[0048] In this application, direct rolling is defined as continuous rolling without waiting for a specific temperature. To avoid the precipitation of large-sized VC that is detrimental to low-temperature toughness induced by rolling below 900℃, the direct rolling process is designed to ensure that the final rolling temperature is above 900℃.

[0049] To refine the austenite through recrystallization during the rolling stage, the initial rolling temperature is controlled. For example, the initial rolling temperature can be 1050℃, 1055℃, 1060℃, 1065℃, 1070℃, 1075℃, or 1080℃. To avoid rolling in the mixed crystal zone between the austenite recrystallization and non-recrystallization regions during the low-temperature stage, the final rolling temperature is controlled. For example, the final rolling temperature can be 900℃, 910℃, 920℃, 930℃, 940℃, or 950℃.

[0050] In some embodiments, during the rolling process, at least three passes have a single-pass reduction rate of ≥25%, and the number of passes with a reduction rate of <5% in the last five passes is no more than two.

[0051] During rolling, a higher reduction rate is used in non-final passes to ensure the density of the steel sheet and the uniformity of the original austenite grains. Fewer low-reduction passes are used in the last five rolling passes to avoid the formation of mixed grains. For example, single-pass reduction rates of 25%, 25.2%, 25.5%, 25.8%, 26%, 26.2%, 26.5%, and 27% can be used for 3, 4, or 5 passes, respectively. The number of reduction passes with rates of 1%, 2%, 3%, 4%, and 4.5% in the last five passes is 0, 1, and 2, respectively.

[0052] In some embodiments, the pre-straightening includes: hot-rolled steel plates being pre-straightened using seven rolls, with the roll gap designed to be the target thickness of the finished product minus 0.5 mm.

[0053] After rolling, the steel plate is fed into a pre-straightening machine to improve the head and tail warping of the steel plate caused by the biting and throwing process of the rolling mill, thereby improving the water cooling uniformity of the steel plate and solving the problem of hard bending at the head and tail of the steel plate.

[0054] In some embodiments, straightening the hot-rolled plate includes: the hot-rolled plate is fed into a hot straightening machine for hot straightening, the roll gap is designed to be the target thickness, and the straightening is performed 1 to 3 times depending on the plate shape improvement. After straightening, the steel plate is removed from the line and placed in a slow cooling pit for cooling.

[0055] In some embodiments, the post-rolling rapid cooling employs UFC+ACC combined cooling or UFC cooling alone, with the final temperature of the post-rolling rapid cooling being 250℃~400℃ and the cooling rate being 15℃ / s~30℃ / s.

[0056] By employing a rapid post-rolling cooling process, the precipitation of large-sized V(C,N) compounds can be avoided during the post-rolling cooling process. For example, the endpoint temperature of rapid post-rolling cooling can be 250℃, 280℃, 300℃, 320℃, 350℃, 380℃, 400℃, etc., and the cooling rate can be 15℃ / s, 18℃ / s, 20℃ / s, 22℃ / s, 25℃ / s, 28℃ / s, 30℃ / s, etc.

[0057] S3. The hot-rolled plate is normalized, air-cooled, and slow-cooled to obtain 460MPa grade pressure vessel steel.

[0058] In some embodiments, the heating rate of the normalizing is 1.5 min / mm to 2.0 min / mm, and the heating endpoint temperature T of the normalizing satisfies: T = Ac3 + t, where Ac3 is the actual phase transformation temperature of carbon steel during heating, and t is the increase temperature, which is 2℃ to 10℃; the holding time of the normalizing is 5 min to 20 min.

[0059] To avoid austenite growth during normalizing, a relatively low normalizing temperature should be used. For example, the normalizing heating rate can be 1.5 min / mm, 1.6 min / mm, 1.7 min / mm, 1.8 min / mm, 1.9 min / mm, 2.0 min / mm, etc.; the temperature increase t can be 2℃, 4℃, 6℃, 8℃, 10℃, etc.; and the normalizing holding time can be 5 min, 8 min, 10 min, 12 min, 15 min, 18 min, 20 min, etc.

[0060] In some embodiments, the final temperature of the air cooling is 550°C to 600°C; the slow cooling adopts a stacking cooling process, and the stacking cooling time is ≥24 hours.

[0061] After normalizing, the steel plate is stacked and cooled offline to promote the full precipitation of fine V (C,N) particles, ensuring the precipitation strengthening effect of the steel plate, while avoiding free N elements in the steel, thus ensuring the strain aging performance of the steel plate. In order to promote precipitation within the optimal precipitation temperature range of V, the final temperature of air cooling and the stacking cooling time are controlled.

[0062] Based on a general inventive concept, this application provides a 460MPa grade pressure vessel steel prepared by the method described in any one embodiment of the first aspect, wherein, by volume fraction, the precipitated phase in the metallographic structure of the pressure vessel steel is V(C,N) particles, and the size of the V(C,N) particles is ≤20nm; the metallographic structure comprises: ferrite: 65%~80%, pearlite: 20%~35%, and the average grain size of the ferrite is ≤8um.

[0063] Ensuring that the precipitation strengthening particles in the steel plate are 100% V(C,N) and their size is controlled within 20nm achieves a good precipitation strengthening effect without deteriorating the low-temperature toughness of the steel plate. By controlling the chemical composition and preparation process of the steel, the metallographic structure of the steel is achieved as ferrite + pearlite. For example, the size of the V(C,N) particles can be 10nm, 12nm, 14nm, 16nm, 18nm, 20nm, etc., the ferrite content can be 65%, 68%, 70%, 72%, 75%, 78%, 80%, etc., the pearlite content can be 20%, 25%, 28%, 30%, 35%, etc., and the average grain size of the ferrite can be 4µm, 5µm, 6µm, 7µm, 8µm, etc.

[0064] In some embodiments, the pressure vessel steel satisfies at least one of the following properties: yield strength ≥ 460 MPa, tensile strength ≥ 570 MPa, elongation after fracture ≥ 23%, low temperature toughness at -60℃ ≥ 100 J, and strain aging sensitivity coefficient ≤ 30%.

[0065] The performance of the steel plate is ensured by controlling the chemical composition, size, and distribution of the V precipitate. For example, the yield strength can be 460 MPa, 470 MPa, 480 MPa, 490 MPa, 500 MPa, 510 MPa, 520 MPa, etc., the tensile strength can be 570 MPa, 580 MPa, 600 MPa, 620 MPa, 640 MPa, 650 MPa, etc., and the elongation after fracture can be 23% or 24%.

[0066] 25%, 26%, 28%, 30%, 31%, 32%, etc.; low-temperature toughness at -60℃ can be 100J, 110J, 120J, 140J, 150J, 170J, 180J, etc.; strain aging sensitivity coefficient can be 3%, 5%, 8%, 10%, 15%, 20%, 25%, 30%, etc.

[0067] The pressure vessel steel is made based on the above-described preparation method for pressure vessel steel. The specific steps of the preparation method for pressure vessel steel can be referred to the above embodiments. Since the pressure vessel steel adopts some or all of the technical solutions of the above embodiments, it has at least all the beneficial effects brought about by the technical solutions of the above embodiments, which will not be elaborated here.

[0068] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to industry standards. If there is no corresponding industry standard, then common international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0069] Example 1

[0070] This embodiment provides a 10mm normalized 460MPa grade vanadium-nitrogen strengthened pressure vessel steel, which is prepared by the following method:

[0071] The chemical composition of the molten steel, by mass fraction, includes: C: 0.15%, Si: 0.25%, Mn: 1.65%, P: 0.008%, S: 0.0012%, V: 0.06%, N: 0.012%, Cr: 0.15%, Nb: 0.001%, Nb: 0.003%, Ti: 0.002%, Al: 0.003%.

[0072] After hot metal treatment, converter refining, LF refining, VD vacuum treatment, and continuous casting, a 237mm thick continuous casting billet is produced. After being fully heated to 1160℃ in a walking beam furnace, it is descaled in a descaling machine. During the rolling stage, a low-speed, high-reduction process is used, with a maximum single-pass reduction of 26.5% and a final rolling temperature of 930℃. After rolling, it is pre-straightened in a pre-straightening machine with a roll gap of 9.5mm. After pre-straightening, it undergoes accelerated cooling in a UFC (Ultra-Fuel Cooling Machine) at a final cooling temperature of 380℃ and a cooling rate of 27℃ / s. After water cooling, the steel plate is fed into a hot straightening machine, straightened once, and then quickly removed from the production line via a roller conveyor. The steel plate undergoes normalizing treatment at 875℃, with a heating coefficient of 1.8 min / mm. After reaching the normalizing temperature, a 10-minute holding time is added. After normalizing, it is air-cooled to 600℃ and then stacked for slow cooling over 24 hours.

[0073] Example 2

[0074] This embodiment provides a 20mm normalized 460MPa grade vanadium-nitrogen strengthened pressure vessel steel, which is prepared by the following method:

[0075] The chemical composition of the molten steel, by mass fraction, includes: C: 0.15%, Si: 0.25%, Mn: 1.65%, P: 0.008%, S: 0.0012%, V: 0.07%, N: 0.012%, Cr: 0.15%, Nb: 0.001%, Ti: 0.002%, Al: 0.003%.

[0076] After hot metal treatment, converter refining, LF refining, VD vacuum treatment, and continuous casting, a 237mm thick continuous casting billet is produced. After being fully heated to 1160℃ in a walking beam furnace, it is descaled in a descaling machine. During the rolling stage, a low-speed, high-reduction process is used, with a maximum single-pass reduction of 27.5% and a final rolling temperature of 925℃. After rolling, it is pre-straightened in a pre-straightening machine with a roll gap of 19.5mm. After pre-straightening, it undergoes accelerated cooling in a UFC (Ultra-Fuel Cooling Machine) at a final cooling temperature of 370℃ and a cooling rate of 26℃ / s. After water cooling, the steel plate is fed into a hot straightening machine, straightened once, and then quickly removed from the production line via a roller conveyor. The steel plate undergoes normalizing treatment at 875℃, with a heating coefficient of 1.8 min / mm. After reaching the normalizing temperature, a 10-minute holding time is added. After normalizing, it is air-cooled to 580℃ and then stacked for slow cooling over 24 hours.

[0077] Example 3

[0078] This embodiment provides a 30mm normalized 460MPa grade vanadium-nitrogen strengthened pressure vessel steel, which is prepared by the following method:

[0079] The chemical composition of the molten steel, by mass fraction, includes: C: 0.16%, Si: 0.27%, Mn: 1.63%, P: 0.008%, S: 0.0010%, V: 0.075%, N: 0.013%, Cr: 0.17%, Nb: 0.001%, Ti: 0.002%, Al: 0.003%.

[0080] After hot metal treatment, converter refining, LF refining, VD vacuum treatment, and continuous casting, a 237mm thick continuous casting billet is produced. After being fully heated to 1160℃ in a walking beam furnace, it is descaled in a descaling machine. During the rolling stage, a low-speed, high-reduction process is used, with a maximum single-pass reduction of 27.5% and a final rolling temperature of 942℃. After rolling, it is pre-straightened in a pre-straightening machine with a roll gap of 29.5mm. After pre-straightening, it undergoes accelerated cooling in a UFC (Ultra-Fuel Cooling Machine) at a final cooling temperature of 350℃ and a cooling rate of 25℃ / s. After water cooling, the steel plate is fed into a hot straightening machine, straightened once, and then quickly removed from the production line via a roller conveyor. The steel plate undergoes normalizing treatment at 878℃, with a heating coefficient of 1.8 min / mm. After reaching the target temperature, a 10-minute holding time is added. After normalizing, it is air-cooled to 560℃ and then stacked for slow cooling over 24 hours.

[0081] Example 4

[0082] This embodiment provides a 60mm normalized 460MPa grade vanadium-nitrogen strengthened pressure vessel steel, which is prepared by the following method:

[0083] The chemical composition of the molten steel, by mass fraction, includes: C: 0.17%, Si: 0.27%, Mn: 1.70%, P: 0.008%, S: 0.0010%, V: 0.085%, N: 0.014%, Cr: 0.18%, Nb: 0.001%, Ti: 0.002%, Al: 0.003%.

[0084] After hot metal treatment, converter refining, LF refining, VD vacuum treatment, and continuous casting, a 400mm thick continuous casting billet is produced. After being fully heated to 1160℃ in a walking beam furnace, it is descaled in a descaling machine. During the rolling stage, a low-speed, high-reduction process is used, with a maximum single-pass reduction of 22.5% and a final rolling temperature of 935℃. After rolling, it is pre-straightened in a pre-straightening machine with a roll gap of 59.5mm. After pre-straightening, it undergoes accelerated cooling in a UFC (Ultra-Fuel Cooling Machine) at a final cooling temperature of 270℃ and a cooling rate of 15℃ / s. After water cooling, the steel plate is fed into a hot straightening machine, straightened once, and then quickly removed from the production line via a roller conveyor. The steel plate undergoes normalizing treatment at 882℃, with a heating coefficient of 1.8 min / mm. After reaching the normalizing temperature, a 10-minute holding time is added. After normalizing, it is air-cooled to 570℃ and then stacked for slow cooling for 36 hours.

[0085] The pressure vessel steels obtained in Examples 1-4 were subjected to mechanical property tests, and the results are shown in Table 1 below, where tensile strength and impact strength are both transverse.

[0086] Table 1 Mechanical properties of steel for pressure vessels

[0087] Example Thickness / mm ReH / MPa Rm / MPa A / ％ <![CDATA[KV2@-60℃ / J]]> 1 10 519 641 27.5 100、160、108 2 20 497 615 27.5 131、117、117 3 30 475 594 30.5 170、165、160 4 60 473 600 31.5 182、171、155

[0088] In Table 1, KV2@-60℃ / J represents the transverse impact energy at -60℃. Table 1 shows that the steel plate has a yield strength ≥460MPa, tensile strength ≥590MPa, elongation after fracture ≥27.5%, and impact energy at -60℃ ≥100J.

[0089] The pressure vessel steels obtained in Examples 1-4 were subjected to simulated post-weld mechanical property tests, and the results are shown in Table 2. The simulated post-weld heat treatment process was as follows: the heat treatment temperature was 540±10℃, the simulated post-weld heat treatment holding time was 30min, and when the temperature was above 425℃, the heating and cooling rate was controlled to be ≤200℃ / h; when the temperature was below 425℃, the steel was cooled and air-cooled after being removed from the furnace.

[0090] Table 2 Simulated post-weld mechanical properties of steel for pressure vessels

[0091] Example Thickness / mm ReH / MPa Rm / MPa A / ％ <![CDATA[KV2@-60℃ / J]]> 1 10 486 610 30.0 147、105、110 2 20 471 601 29.5 160、159、152 3 30 448 592 27.5 151、190、170 4 60 463 610 30.5 168、179、157

[0092] As shown in Table 2, the simulated post-weld performance of the steel plate is good.

[0093] The pressure vessel steels obtained in Examples 1-4 were subjected to strain-aged mechanical property tests, and the results are shown in Table 3. The strain aging process was 5% strain followed by holding at 250℃ for 1 hour.

[0094] Table 3 Strain-Time Mechanical Properties of Steel for Pressure Vessels

[0095]

[0096] As shown in Table 3, the steel for pressure vessels exhibits good low-temperature toughness after strain aging.

[0097] Figure 2 This is a metallographic diagram of the pressure vessel steel at 1 / 4 of its thickness, provided in Embodiment 2 of this application.

[0098] Depend on Figure 2 It can be seen that the microstructure of the steel plate is pearlite + ferrite, and the ferrite grain size is small, with an average grain size of 7 μm and a ferrite content of 82%.

[0099] Figure 3 The morphology of the V(C,N) precipitates in the pressure vessel steel provided in Embodiment 2 of this application is shown.

[0100] Depend on Figure 3 It can be seen that the precipitated phase of the steel plate is V(C,N), and there are a large number of fine second phases with a diameter of <20nm.

[0101] Furthermore, one or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

[0102] (1) In the embodiments of the present invention, no precious metal elements such as Nb, Ti, Al and Ni are added, and the alloy cost is low.

[0103] (2) In this embodiment of the invention, the direct rolling process is adopted, which has high mill production efficiency and can fully release the potential of rolling capacity.

[0104] (3) In the embodiments of the present invention, the production technology based on the control of V(C,N) precipitate composition and size ensures that a large number of <20nm V precipitates are precipitated in the steel plate, so as to achieve both the 460MPa strength and -60℃ low temperature toughness of the steel plate after normalizing.

[0105] (4) In this embodiment of the invention, the N element in the steel is fully precipitated, thereby avoiding the problem of increased strain aging sensitivity caused by free nitrogen and ensuring the performance of the user during use.

[0106] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for preparing 460MPa grade pressure vessel steel, characterized in that, The method includes: A cast billet is obtained; the chemical composition of the cast billet, by mass fraction, includes: C: 0.16%~0.18%, Si: 0.20%~0.30%, Mn: 1.50%~1.70%, P≤0.010%, S≤0.003%, Cr: 0.10%~0.30%, V: 0.055%~0.075%, N: 0.010%~0.015%, with the balance being Fe and unavoidable impurities; The billet is sequentially heated, descaled, rolled, pre-straightened, and rapidly cooled after rolling to obtain a hot-rolled plate; The hot-rolled plate was normalized, air-cooled, and slow-cooled to obtain 460MPa grade pressure vessel steel. The rolling process adopts a direct rolling process, and the initial rolling temperature is 1050℃~1080℃, and the final rolling temperature is 900℃~940℃. The rapid cooling after rolling adopts UFC+ACC combined cooling or UFC cooling alone. The final temperature of the rapid cooling after rolling is 250℃~400℃, and the cooling rate is 15℃ / s~30℃ / s. The heating rate of the normalizing process is 1.5 min / mm to 2.0 min / mm, and the heating endpoint temperature T of the normalizing process satisfies: T = Ac3 + t, where Ac3 is the actual phase transformation temperature of carbon steel during heating, and t is the increase temperature, which is 2℃ to 10℃; the holding time of the normalizing process is 5 min to 20 min. The final temperature of the air cooling is 550℃~600℃; the slow cooling adopts a stacking cooling process, and the stacking cooling time is ≥24h; The precipitated phase in the metallographic structure of the 460MPa grade pressure vessel steel is V(C,N) particles, and the size of the V(C,N) particles is ≤20nm. The 460MPa grade pressure vessel steel has a yield strength ≥460MPa, tensile strength ≥570MPa, elongation after fracture ≥23%, low temperature toughness at -60℃ ≥100J, and strain aging sensitivity coefficient ≤30%.

2. The method according to claim 1, characterized in that, The heating temperature is 1130℃~1160℃, and the heating time is 150min~440min.

3. The method according to claim 1, characterized in that, During the rolling process, the single-pass reduction rate of at least 3 passes is ≥25%, and the number of reduction passes with a reduction rate of <5% in the last 5 passes is less than 2.

4. The method according to claim 1, characterized in that, The chemical composition, by mass fraction, also includes: Ti ≤ 0.005%, Nb ≤ 0.005%, and Al ≤ 0.005%.

5. A 460MPa grade pressure vessel steel prepared by the method according to any one of claims 1 to 4, characterized in that, The metallographic structure of the pressure vessel steel, by volume fraction, comprises: ferrite: 65%~80%, pearlite: 20%~35%, and the average grain size of the ferrite is ≤8µm.

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