Thick gauge 800MPa hydroelectric steel with excellent core toughness and its manufacturing method

Abstract

The invention discloses a thick specification 800MPa hydroelectric steel with excellent toughness in the core and a manufacturing method thereof. The components are C: 0.11-0.13%, Si: 0.1-0.15%, Mn: 1.05-1.15%, Cr: 0.4-0.5%, Ni: 0.55-0.65%, Mo: 0.35-0.40%, Cu: 0.35-0.40%, Nb: 0.01-0.03%, V: 0.055-0.065%, Ti: 0.006-0.012%, Als: 0.01-0.02%, B: 15-30ppm, Mg: 8-18ppm, N: 60-80ppm, and the rest is Fe. The steel plate is produced by continuous casting billet, and a 120-150mm thick steel plate is produced by controlling the heating temperature, rolling water cooling process, and quenching and tempering process, and the toughness at the center of the thickness is excellent.

Description

Technical Field

[0001] The present invention belongs to the technical field of metal material production, and specifically relates to a thick 800MPa hydroelectric steel with excellent core toughness and a manufacturing method thereof, and adopts continuous casting billets to produce extra thick 800MPa hydroelectric steel with excellent core toughness. Background Art

[0002] With the rapid development of my country's economy and society, the continuous optimization of industrial structure and the gradual improvement of people's living standards, in recent years, the power load has continued to grow, the peak-to-valley difference of the power system has gradually increased, and the demand for flexible power adjustment of the power system has increased significantly. However, new energy sources such as photovoltaics and wind power have the characteristics of volatility, intermittency and randomness. They are unstable power sources and affect the stability of the power system.

[0003] As the most mature and largest energy storage technology, pumped storage has multiple functions such as peak load regulation, valley filling, frequency regulation, phase regulation, energy storage, emergency standby and black start, and has many advantages such as large project capacity, good safety and low cost per kilowatt-hour. It has become an important way for countries around the world to ensure the safe and stable operation of power systems. It is also a hot spot for hydropower project construction in my country.

[0004] Hydropower steel is mainly used for the manufacture of pressure pipes in hydropower stations, and a small amount is used to manufacture ribs, bifurcated pipes, unit volutes and other equipment. In recent years, as the pressure pipes of pumped storage power stations have developed towards large diameters and thick pipe walls, the demand for 800MPa-grade high-toughness thick steel plates with a thickness of ≥120mm has gradually increased. However, due to the limitation of the compression ratio of extra-thick steel plates and the thickness effect of the steel plates themselves, it is difficult to ensure the low-temperature toughness of the center position in the thickness direction. Whether the existing production technology uses large-thickness steel ingots or adds expensive alloy elements, or multi-process heat treatment, there are certain technical or cost deficiencies. For example, Publication No. CN115323251A proposes a design idea of ​​using low carbon, high manganese (4-8%) and rare earth addition to replace the traditional CrNiMo alloy composite toughening idea, and uses steel ingots to produce 120-300mm ultra-thick homogeneous steel plates. However, the process of this solution is too complicated, the yield rate of steel ingot production is too low, and industrial mass production is difficult; Publication No. CN101962741A, a quenched and tempered steel plate and its production method, uses high CrNiMo alloying and steel ingot large compression ratio to produce 100-150m m steel plate, the content of precious alloys such as Ni and Mo in this scheme is relatively high, and the cost is relatively high. On the other hand, under the conditions of steel ingot production and compression ratio of >4 times, the steel plate does not reflect the core toughness level and aging performance, and the uniformity of performance in the thickness direction of the steel plate needs to be verified; Publication No. CN101709432A, thick quenched and tempered steel for offshore platforms and its production method, this scheme adopts high C-high NiCrMo composition design, uses electroslag remelting steel to produce 152mm thick steel plates, the alloy cost is high, the process cost is high, and the welding crack sensitivity index reaches 0.37, cannot meet the actual application needs in the hydropower field; Publication No. CN110791713A, a low compression ratio 690MPa grade extra thick plate, adopts a low C, high Mn, high NiMo composition design, and produces 120-160mm steel plates in the form of continuous casting billet double-fire rolling. The process cost is increased, the toughness of the thickness core is not reflected, and the intermediate billet is heated to 850-930℃ for secondary rolling, which is difficult to be widely promoted in actual production; Publication No. CN102605 282A, 80 kg ultra-high toughness, ultra-thick steel plate and its manufacturing method, the scheme adopts low C-high NiMo composition design, uses steel ingots with a compression ratio of >5 times, and produces 100-190mm thick steel grids through two quenchings. The production process has high energy consumption and high cost, and does not reflect the toughness of the core of the thick steel plate; Publication No. CN105734401A, a 80-150mm thick low crack sensitivity SX780CF steel plate for hydropower and its production method, the scheme adopts low carbon, low manganese- High NiMo component design, the production process includes: molten iron pretreatment, converter, argon station, VD vacuum, LF refining, VD vacuum, die casting, ingot cleaning, heating, rolling, quenching and tempering heat treatment, etc. The program process is relatively complicated, the die casting production efficiency is not high, and under the condition of large compression ratio, the average impact energy of 150mm steel plate thickness 1 / 4 position -60℃ is only 132J, which does not reflect the performance level of the thickness core; Publication No. CN109504897A, an 80kg grade low carbon equivalent low crack High-sensitivity thick hydroelectric steel and its manufacturing method, adopt low-carbon, high-NiMo design, use continuous casting billet low compression ratio to produce 120-150mm hydroelectric steel plate, although the claim C: 0.07-0.12%, Ni: 0.5-1.8%, but from the embodiment, the actual C ≤ 0.1%, Ni at least > 0.8%, under the economic production conditions of higher carbon content and lower nickel content, it is unknown whether the performance in the embodiment can be achieved, and the toughness at the center of the steel plate thickness is not reflected. .

[0005] In summary, it can be seen that the existing technology for producing extra-thick hydropower steel plates has the disadvantages of complex processes, low efficiency, and high costs. There is little work done on the uniformity of toughness in the thickness direction. Moreover, from the actual production experience, it is indeed very difficult to improve the performance of the core structure of the extra-thick steel plates and achieve homogeneity along the thickness direction of the steel plates. The uniformity of the performance of the core is also of great significance to the safe and stable operation of large pressure vessels and pipelines. Therefore, in response to the above problems, a method of producing extra-thick 800MPa hydropower steel with excellent core toughness using continuous casting billets and a manufacturing method thereof are invented. This method is relatively simple, efficient, and low in cost. In particular, the transverse impact energy of the 120-150mm thick 800MPa steel plate at the center of the thickness at -40℃ is ≥130J, and the transverse impact energy at the center of the thickness at -20℃ after 5% strain aging is ≥115J. All performances meet the application requirements of hydropower projects and have good application prospects. Summary of the invention

[0006] The object of the present invention is to provide a thick-gauge 800MPa hydroelectric steel with excellent core toughness and a production method thereof, so as to solve the problems in the prior art.

[0007] The present invention is achieved through the following technical solutions:

[0008] A thick-gauge 800MPa hydroelectric steel with excellent core toughness, characterized in that it comprises the following components in mass ratio: C: 0.11-0.13%, Si: 0.1-0.15%, Mn: 1.05-1.15%, P≤0.015%, S≤0.002%, Cr: 0.4-0.5%, Ni: 0.55-0.65%, Mo: 0.35-0.40%, Cu: 0.35-0.40%, Nb: 0.01-0.03%, V: 0.055-0.065%, Ti: 0.006-0.012%, Als: 0.01-0.02%, B: 15-30ppm, Mg: 8-18ppm, N: 60-80ppm, and the rest are Fe and unavoidable impurities.

[0009] The manufacturing method of the thick-gauge 800MPa hydroelectric steel with excellent core toughness as described above comprises the steps of converter - refining outside the furnace - vacuum degassing - thick slab continuous casting - heating - rolling - water cooling - quenching and tempering treatment, and is characterized in that:

[0010] 1) Nitrogen is used as the lifting gas throughout the RH vacuum treatment. The net cycle time is ≥8 minutes at a vacuum degree of ≤60Pa. After the RH high vacuum is completed, the pump is withdrawn to 5-7Kpa and the cycle is ≥10 minutes. After breaking the air, the N content is tested. According to the chemical composition, vanadium iron nitride or vanadium iron alloy is added to adjust V and N within the target composition range. After adjusting the chemical composition, the molten steel is cleanly blown for 3 minutes, and Si-Mg cored wire is fed for ≥200 meters, and Mg is controlled to be 8-18ppm. After feeding the wire, the molten steel is cleanly blown for 3-5 minutes before being put into casting machine;

[0011] 2) Continuous casting adopts low superheat casting, controls superheat between 15-20℃, keeps constant pulling speed, adopts single-roller stirring mode for electromagnetic stirring in the secondary cooling zone, stirring current is 570A, current frequency is 5Hz, and large pressing mode is adopted for pressing at the end of solidification, with cumulative pressing amount ≥15mm. Ensure that the center segregation of slab is ≤C1.0 level;

[0012] 3) The walking beam heating furnace is used for heating. The billet entering the furnace temperature is ≤200℃, the temperature of the three heating sections is 1150-1180℃, the temperature of the soaking section is 1130-1150℃, and the total time of the billet in the furnace is (1.2-1.4) min / mm*T, of which the heating time of the three heating sections is controlled at (0.25-0.35) min / mm*T, and the heating time of the soaking section is controlled at (0.20-0.3) min / mm*T; T is the thickness of the billet, in mm;

[0013] 4) The rolling adopts three-stage controlled rolling, in which the final rolling temperature of the recrystallization rolling stage is ≥980℃, the rolling speed is ≤1.2m / s, the reduction of at least 5 passes is ≥30mm, of which the reduction of 2 passes is ≥40mm, and the thickness of the intermediate billet after rough rolling is ≥1.6h; the intermediate billet is forced to cool the surface by the intermediate billet cooling device before the finishing mill, and 2 passes are rolled after the surface temperature is ≤850℃ and the temperature is evenly maintained for 10-20 seconds, with a cumulative reduction of ≥35mm; the non-recrystallization zone rolling is continued at a temperature of 780-800℃, and the final rolling temperature is 760-780℃; where h is the thickness of the finished steel plate, in mm;

[0014] 5) ACC uses a reciprocating walking mode to cool the steel plate. For the first cooling, the water inlet pressure of the gap nozzle is set to 0.5MPa, the upper and lower water ratio is 2.1, the water inlet pressure of the high-density nozzle is set to 0.15MPa, the upper and lower water ratio is 2.4, 30 sets of headers are fully open, and the roller speed is 0.35-0.45 m / min; after cooling, the steel plate body is completely out of the cooling device, and after 10-20 seconds of equalization, it enters the cooling device for the next cooling. The steel plate is reciprocated and water-cooled 3 times, and the gap nozzle is closed for the second and third times; the final cooling temperature of the steel plate is ≤400℃; the steel plate is stacked offline for ≥24 hours after cooling;

[0015] 6) The steel plate is subjected to offline quenching + tempering treatment, wherein the quenching temperature is controlled at 865-875°C, the time in the furnace is 3.0 min / mm*h, and the holding time is ≥0.5 min / mm*h. The steel plate is water quenched after being taken out of the furnace, and the surface temperature of the steel plate after quenching is lower than 50°C; the tempering temperature is 600-630°C, the time in the furnace is 3.5 min / mm*h, and the holding time is ≥1.0 min / mm*h; wherein h is the thickness of the finished steel plate, in mm.

[0016] Further: 475mm thick continuous casting billet is used to produce 120-150mm thick hydropower steel plates. The yield strength of the delivered steel plates is ≥660MPa, the tensile strength is 760-900MPa, the elongation is ≥16%, the transverse impact energy at -40℃ at the center of the thickness is ≥135J, and the transverse impact energy at -20℃ at the center of the thickness after 5% strain aging is ≥130J.

[0017] Advantages of the present invention: Compared with the prior art, the method of the present invention is suitable for production by enterprises with continuous casting equipment. The 120-150mm thick hydropower steel plate produced by this method has a high yield rate, high efficiency and low cost. While meeting the 800MPa strength requirement, it has excellent low-temperature toughness at the core of the thickness. When used in projects such as hydropower and low-temperature pressure vessels, it has significant performance advantages over conventional products. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a schematic diagram of the distribution of the reduction amount and reduction rate of each pass in the rough rolling stage;

[0019] Figure 2 The core structure of 120mm quenched and tempered steel plate;

[0020] Figure 3 The core structure of 150mm quenched and tempered steel plate. Implementation

[0021] The invention provides a thick specification 800MPa hydroelectric steel with excellent core toughness, wherein the chemical composition mass percentage is as follows: C: 0.11-0.13%, Si: 0.1-0.15%, Mn: 1.05-1.15%, P≤0.015%, S≤0.002%, Cr: 0.4-0.5%, Ni: 0.55-0.65%, Mo: 0.35-0.40%, Cu: 0.35-0.40%, Nb: 0.01-0.03%, V: 0.055-0.065%, Ti: 0.006-0.012%, Als: 0.01-0.02%, B: 15-30ppm, Mg: 8-18ppm, N: 60-80ppm, and the rest is Fe and unavoidable impurities.

[0022] The above composition range is mainly based on the following considerations:

[0023] C: Carbon plays a major role in solid solution strengthening in steel. In particular, the target of the present invention is 800MPa grade steel plates with a thickness of 120-150mm. Too low carbon content can reduce the cold crack sensitivity of the steel plates, but it can easily lead to insufficient strength of the steel plates. Therefore, C is controlled to be 0.11-0.13%.

[0024] Si: Silicon can improve the strength of quenched and tempered steel plates. However, under low cooling rate conditions, high silicon content tends to promote the formation of large-sized MA, which is not conducive to the stability of the impact performance of the core of thick-gauge steel plates. Therefore, the present invention limits Si to 0.10-0.15%.

[0025] Mn: Manganese is an austenite stabilizing element, which is beneficial to improving the toughness of the core of thick-gauge steel plates. However, under continuous casting conditions, high manganese tends to aggravate the central segregation of the continuous casting billet and cannot be completely diffused and eliminated at high temperatures. Taking into account the carbon equivalent and welding cold crack sensitivity of the finished steel plate, Mn is limited to 1.05-1.15% under the premise that the C content ranges from 0.11-0.13%.

[0026] P, S: Phosphorus and sulfur are harmful elements in steel, especially for thick steel plates requiring low-temperature toughness. In theory, the lower the phosphorus and sulfur content, the better. However, considering production economy, the present invention limits P≤0.015% and S≤0.002%.

[0027] Ni, Cu: Nickel has the function of increasing the hardenability of quenched and tempered steel, and improving the strength and low-temperature toughness of steel plates. Conventional 800MPa grade thick quenched and tempered high-strength steel generally adds more than 0.8% nickel, but the high nickel content greatly increases the production cost on the one hand, and increases the risk of iron oxide scale being pressed into the surface of the steel plate on the other hand. At the same time, copper is also an austenite stabilizing element, and can also improve the hardenability of the steel plate. In particular, the composite addition of copper and nickel can not only improve the toughness of the steel plate, but also is not easy to produce surface crack defects caused by copper. In addition, another major role of copper is the precipitation strengthening effect of solid solution copper during the tempering process, which has a significant effect on improving the yield strength of the steel plate. Therefore, the present invention limits Ni: 0.55-0.65%, Cu: 0.35-0.40%.

[0028] Cr: Chromium can improve the hardenability of steel plates, is conducive to obtaining martensite / bainite during quenching, and also has a certain effect on improving the toughness of quenched and tempered steel. However, too high chromium affects the welding crack sensitivity of steel plates and easily leads to reduced toughness of welded joints. Therefore, Cr is set to 0.4-0.5%.

[0029] Mo: Mo is a strong carbide-forming element. On the one hand, it can greatly improve the hardenability of steel plates, especially the core performance of thick-gauge steel plates. On the other hand, it can precipitate intermetallic compounds during tempering, which can significantly improve the strength of steel plates. However, if the Mo content is too high, on the one hand, it will seriously increase the cost. On the other hand, under low cooling rate conditions, it is easy to form a coarse MA structure, which is not easy to decompose through tempering, resulting in deterioration of low-temperature toughness. In consideration of economy and core low-temperature toughness, the present invention sets the Mo content to 0.35-0.40%.

[0030] Nb: The microalloying element niobium is mainly matched with the controlled rolling process in the subsequent steel plate rolling process. Through the effect of Nb inhibiting the recovery and recrystallization of deformed austenite, a certain cumulative deformation effect can still be obtained when the core temperature of the thick gauge steel plate is high, and the core post-phase transformation structure is refined. However, due to the thick steel plate, the total deformation amount of the non-recrystallized zone is small, and excessive Nb addition cannot further improve the strain accumulation effect, but leads to poor economic efficiency. Therefore, the present invention sets Nb: 0.01-0.03%.

[0031] Als, Ti: Aluminum is the main deoxidizing element in steel, but under the premise of VN strengthening in the present invention, too high aluminum content is easy to cause AlN precipitation at high temperature, weakening the austenite grain boundary to cause surface quality problems of continuous casting billet, and also affects the low-temperature impact performance. Therefore, under the premise of meeting the deoxidation requirements in steel, the present invention reduces the Als content as much as possible, and the target controls Als: 0.01-0.02%; Ti in steel is easy to combine with N. Under normal circumstances, TiN can play a role in inhibiting excessive growth of austenite during heating. However, when the N content in steel is high, TiN is easy to precipitate in molten steel or at the solidification front. Coarse TiN particles are not conducive to the stability of low-temperature impact in the core of the steel plate. Therefore, the present invention sets Ti: 0.006-0.012%.

[0032] V, N: The use of VN microalloying is one of the key measures for the present invention to achieve a thick specification 800MPa steel plate, and the two elements must be controlled to meet the range requirements at the same time, wherein the V requirement is: 0.055-0.065%. Too little cannot meet the strength requirements of the steel plate, and too much is not conducive to the low-temperature toughness and welding performance of the steel plate; N is a harmful element in steel. In addition to precipitating nitrides during the high-temperature continuous casting process to cause the surface quality of the billet to deteriorate, N in the steel can also produce a strong aging effect and deteriorate the impact properties of the steel. Therefore, the present invention controls the addition of Als and Ti, greatly reducing the surface quality risk of the billet caused by high N content. In addition, through the reasonable control of V and N content and heat treatment process, V and N elements in the steel are precipitated in the form of VN compounds, avoiding the aging effect caused by high N content. Therefore, the present invention controls N: 60-80ppm.

[0033] B: Adding boron to quenched and tempered steel can significantly improve the hardenability of steel. The main mechanism is that boron is segregated at the austenite grain boundary, which can reduce the interfacial energy, inhibit the nucleation of ferrite on the grain boundary, and promote the formation of low-temperature phase transformation structure. Considering that the N content in the present invention is high, it is easy to combine with B. The effective B content is reduced, which affects the hardenability effect. Therefore, the present invention sets B slightly higher than conventional quenched and tempered steel: 15-30ppm.

[0034] Mg: Mg treatment is one of the key measures for the present invention to obtain stable low-temperature toughness of the thick core. Since Mg is a strong deoxidizing element, adding an appropriate amount of Mg to the ladle after RH treatment can play a role in deformation and spheroidization of inclusions. It can modify large-sized calcium-aluminate inclusions in steel into Mg-containing inclusions or MgO inclusions. Such inclusions are small and not easy to aggregate and grow, which can effectively purify the steel quality and improve the low-temperature toughness of the matrix. Moreover, such inclusions are uniform and small. When the billet is heated, they have the effect of inhibiting the migration of austenite, and promote the strength, toughness and welding performance of the rolled steel plate. If the Mg content is too low, the above effects cannot be achieved. However, since the oxidizing property of Mg is too strong, setting an excessively high Mg content greatly increases the difficulty of addition. Therefore, the Mg content is set in the chemical composition of the present invention: 8-18ppm.

[0035] The manufacturing method of the thick specification 800MPa hydroelectric steel with excellent core toughness of the present invention mainly comprises the following process flow: converter - refining outside the furnace - vacuum degassing - thick slab continuous casting - heating - rolling - water cooling - quenching and tempering treatment, wherein:

[0036] 1) Nitrogen is used as the lifting gas throughout the RH vacuum treatment. The net cycle time of high vacuum (vacuum degree ≤ 60Pa) is ≥ 8 minutes. After the RH high vacuum is finished, the pump is withdrawn to 5-7Kpa and circulated for ≥ 10 minutes. After breaking the air, the N content is tested. A small amount of vanadium iron nitride or vanadium iron alloy is added according to the chemical composition to adjust V and N within the target composition range; after adjusting the chemical composition, the molten steel is clean blown for 3 minutes, and Si-Mg cored wire is fed ≥ 200 meters at a feeding speed of 250-350m / min. Mg is controlled at 8-18ppm. After feeding the wire, the molten steel is clean blown for 3-5 minutes before being put into casting. This part is one of the core control points of the smelting process of the present invention, which is crucial to achieving the strength and toughness of the thick 800MPa steel plate. The use of nitrogen as the lifting gas and the low vacuum nitrogen addition method can not only complete the RH cycle treatment and purify the steel, but also reduce the purity loss caused by the large amount of alloy added after the RH treatment to adjust the composition; magnesium is added in the form of feeding Si-Mg cored wire, and the feeding speed is also very important for obtaining a stable Mg content. If the speed is too low, the cored wire is fed shallowly and the burning loss is serious. If the feeding is too fast, the oxidation reaction is violent and it is not conducive to improving the yield. After feeding the wire, the net blowing is 3-5 minutes, mainly considering the completion of the modification and uniform distribution of inclusions;

[0037] 2) Continuous casting adopts low superheat casting, controls the superheat between 15-20℃, keeps the pulling speed constant, and adopts single-roll stirring mode for electromagnetic stirring in the second cooling zone, with stirring current of 570A and current frequency of 5Hz. The large pressing mode is adopted for the final pressing at the solidification end, and the cumulative pressing amount is ≥15mm. Ensure that the center segregation of the slab is ≤C1.0 level. The main purpose of this part is to obtain ultra-thick continuous casting slabs with excellent internal quality. Since the continuous casting slab is used to roll 120-150mm thick steel plates, the compression ratio is naturally inferior to that of steel ingots. In order to obtain excellent performance, the internal quality of the continuous casting slab must meet the above requirements;

[0038] 3) Use a walking beam heating furnace for heating, the billet entry temperature is ≤200℃, the temperature of the three heating sections is 1150-1180℃, the temperature of the soaking section is 1130-1150℃, and the total time of the billet in the furnace is (1.2-1.4) min / mm*T (T is the thickness of the billet, in mm), of which the heating time of the three heating sections is controlled at (0.25-0.35) min / mm*T, and the heating time of the soaking section is controlled at (0.20-0.3) min / mm*T. The core purpose of this part is to ensure uniform heating of the billet and full solid solution of various alloy elements, but too high a heating temperature is not conducive to obtaining stable low-temperature impact properties of the core under low compression ratio conditions. Therefore, the mode of long-term low-temperature heating of thick continuous casting billets is adopted, and the time in the high-temperature zone in the furnace is strictly controlled;

[0039] 4) The rolling adopts three-stage controlled rolling, in which the final rolling temperature of the recrystallization rolling stage is ≥980℃, the rolling speed is ≤1.2m / s, the reduction is ≥30mm for at least 5 passes, of which the reduction is ≥40mm for 2 passes, and the thickness of the intermediate billet after rough rolling is ≥1.6h (h is the thickness of the finished steel plate, unit: mm); the intermediate billet is forced to cool the surface by the intermediate billet cooling device before the finishing mill, and 2 passes of rolling are carried out after the surface temperature is ≤850℃ and the temperature is evenly maintained for 10-20 seconds, with a cumulative reduction of ≥35mm; the non-recrystallization zone rolling is continued at a temperature of 780-800℃, and the final rolling temperature is 760-780℃. The rolling process is mainly controlled to refine the recrystallized austenite in the core of the thick plate. The core structure is refined through multiple passes of low-speed and high-reduction roughing and two passes of differential temperature rolling in the second stage. Then, the non-recrystallized area is cumulatively deformed through low-temperature rolling, laying the foundation for obtaining a refined core structure after phase transformation.

[0040] 5) ACC uses a reciprocating walking form to cool the steel plate. The first cooling is set to a slit nozzle water inlet pressure of 0.5MPa, an upper and lower water ratio of 2.1, a high-density nozzle water inlet pressure of 0.15MPa, an upper and lower water ratio of 2.4, 30 sets of headers are fully open, and the roller speed is 0.35-0.45 m / min; after cooling, the steel plate body is completely out of the cooling device, and after 10-20 seconds of average temperature, it enters the cooling device for the next cooling. The steel plate is reciprocated by water cooling 3 times, and the slit nozzle is closed for the second and third times. The final cooling temperature of the steel plate is ≤400℃. After cooling, the steel plate is stacked offline for ≥24 hours. Traditional thick-gauge quenched and tempered steels are usually controlled by air cooling or high final cooling temperature. The present invention adopts a reciprocating cooling + low-temperature final cooling mode, aiming to obtain a relatively uniform and fine low-temperature phase transformation structure in the thickness direction of the steel plate, especially in the core, by controlling the appropriate cooling rate, so as to provide more nucleation sites for the steel plate in the final quenched and tempered austenitization process, and refine the quenched austenite structure;

[0041] 6) The steel plate is subjected to off-line quenching + tempering treatment, wherein the quenching temperature is controlled at 865-875°C, the furnace time is 3.0 min / mm*h (h is the thickness of the finished steel plate, unit: mm), and the holding time is required to be ≥0.5 min / mm*h. The steel plate is water quenched after being taken out of the furnace, and the surface temperature of the steel plate after quenching is lower than 50°C; the tempering temperature is 600-630°C, the furnace time is 3.5 min / mm*h, and the holding time is required to be ≥1.0 min / mm*h. The quenching temperature and holding time are the key to the quenching and tempering treatment of the present invention. If the quenching temperature is set too high, the austenite in the core of the steel plate will be coarse, which will have a significant impact on the low-temperature toughness. The quenching temperature set in the present invention meets the austenitization requirements of the steel. By extending the furnace time, the austenite size is small on the basis of sufficient diffusion of the alloy elements.

[0042] Through the above key process design, 475mm thick continuous casting billets are used to produce 120-150mm thick hydropower steel plates. The yield strength of the delivered steel plates is ≥660MPa, the tensile strength is 760-900MPa, the elongation is ≥16%, the transverse impact energy at -40℃ at the center of the thickness is ≥135J, and the transverse impact energy at -20℃ at the center of the thickness after 5% strain aging is ≥130J.

[0043] Advantages of the present invention: Compared with the prior art, the method of the present invention is suitable for production by enterprises with continuous casting equipment. The 120-150mm thick hydropower steel plate produced by this method has a high yield rate, high efficiency and low cost. While meeting the 800MPa strength requirement, it has excellent low-temperature toughness at the core of the thickness. When used in projects such as hydropower and low-temperature pressure vessels, it has significant performance advantages over conventional products.

[0044] The present invention is further described below by means of specific examples.

[0045] According to the above method, two furnaces of 475mm cross-section continuous casting billets were produced. The process production data are as follows:

[0046] Table 1 Actual smelting process of the embodiment

[0047] serial number RH vacuum degree / pa RH lifting gas and flow Net cycle time / min Low vacuum degree / pa Low vacuum cycle time / min N content after breaking air / ppm Added alloy / kg Si-Mg cored wire / m Feeding speed / m / min Net blowing time / min 1 52 <![CDATA[N2 gas 50m 3 / h]]> 8 6000 10 64.5 5 (Ferrovanadium Nitride) 250 300 4 2 50 <![CDATA[N2 gas 50m 3 / h]]> 9 6000 10 68.7 5 (Ferrovanadium Nitride) 250 300 5

[0048] Two furnaces of steel were continuously cast (No. 1 was used in Example 1 and Example 2, and No. 2 was used in Example 3), the superheat was controlled at 18°C ​​and 20°C respectively, the casting speed was constant, the electromagnetic stirring in the second cooling zone adopted a single-roll stirring mode, the stirring current was 570A, the current frequency was 5Hz, the solidification end reduction adopted a large reduction mode, the reduction was 16mm, the actual sampling slab center segregation was C0.5 level, and there were no other internal defects. The actual chemical composition of the smelting is as follows:

[0049] Table 2 Actual chemical composition of the examples (weight percentage, wt / %)

[0050] serial number C Si Mn P S Als Ni Cr 1 0.118 0.12 1.10 0.012 0.0018 0.015 0.58 0.45 2 0.125 0.14 1.12 0.012 0.0016 0.018 0.62 0.45 serial number Cu Mo Nb Ti V B(ppm) N (ppm) Mg (ppm) 1 0.36 0.35 0.015 0.008 0.06 22 68 12 2 0.37 0.38 0.022 0.010 0.065 20 73 10

[0051] After inspection and cleaning, the smelted billet is cold-sent to a 5m wide thick plate mill for thick plate rolling. The steel plate rolling information is as follows:

[0052] Table 3 Steel plate rolling information

[0053] - Specification / mm Actual thickness of steel billet / mm Temperature of the third stage / ℃ Total time of three stages / min Soaking section temperature / ℃ Total time of soaking section / min Total time of billet in furnace / min Rough rolling and final rolling temperature / ℃ Example 1 120 450 1165-1178 138 1140 126 585 998 Example 2 130 450 1160-1180 142 1140 130 597 987 Example 3 150 450 1158-1178 148 1140 138 612 1002 - Rough rolling speed / m / s Intermediate billet thickness / mm Whether to use intermediate billet cooling After the intermediate cooling device, the rolling temperature is restarted again / ℃ Cumulative reduction in two passes / mm Rolling temperature in non-recrystallization zone / ℃ Final rolling temperature in non-recrystallization zone / ℃ Final cooling temperature / ℃ Example 1 0.95 243 yes 840 45 790 778 365 Example 2 0.95 243 yes 838 42 785 767 387 Example 3 1.05 243 yes 845 35 795 762 372

[0054] The actual thickness of the intermediate billet during the rough rolling stage is consistent, all 243mm. The reduction of different specifications during the rough rolling stage is basically the same. The reduction amount and reduction rate of each pass are shown in Figure 1 .

[0055] After rolling, the steel plate is directly cooled by ACC, and the cooling is carried out in a reciprocating walking mode. The first cooling is set to 0.5MPa of the gap nozzle water inlet pressure, 2.1 of the upper and lower water ratio, 0.15MPa of the high-density nozzle water inlet pressure, 2.4 of the upper and lower water ratio, 30 sets of headers are fully open, and the roller speed is 0.35-0.45 m / min; after cooling, the steel plate body is completely out of the cooling device, and after 10-20 seconds of equalization, it enters the cooling device for the next cooling. The steel plate is reciprocated by water cooling 3 times, and the second and third times the gap nozzle is closed. The setting of the cooling water volume is based on the final cooling temperature of the steel plate ≤400℃. The steel plate after water cooling is stacked and slowly cooled, and the stacking time is greater than 24 hours.

[0056] The unstacking steel plates are subjected to quenching and tempering treatment. The actual process execution of each steel plate is as follows:

[0057] Table 4 Steel plate heat treatment information

[0058] - Specification / mm Quenching target temperature / ℃ Time in furnace / min Insulation time / min Quenching method Surface temperature after quenching Tempering temperature / ℃ Time in furnace / min Insulation time / min Example 1 120 870 360 75 water quenching Room temperature 620 420 135 Example 2 130 870 390 84.5 water quenching Room temperature 620 455 142 Example 3 150 870 450 95 water quenching Room temperature 610 525 165

[0059] The quenched and tempered steel plates were sampled for mechanical property tests at 1 / 4 thickness and the center of thickness, and artificial aging tests were performed at 250°C for 1 hour after 5% pre-strain. The performance of each steel plate and each position is as follows:

[0060]

[0061] The samples of Example 1 and Example 3 were observed by electron microscope. Figure 2 and Figure 3 As shown in the figure, it can be seen that the microstructure is tempered bainite structure, the structure is uniform, and there is no obvious segregation structure in the center.

[0062] From the above, it can be seen that the 120-150mm thick hydropower steel plate produced by using 475mm thick continuous casting billet has a yield strength of ≥668MPa, a tensile strength of ≥794MPa, an elongation of ≥17.2%, a transverse impact energy of ≥135J at -40℃ in the center of the thickness, and a transverse impact energy of ≥130J at -20℃ after 5% strain aging in the center of the thickness. The quality of the steel plate thickness core described in this scheme fully meets the demand for hydropower steel.

Claims

1. A method for manufacturing a thick-gauge 800MPa hydroelectric steel with excellent core toughness, characterized in that: The thick-gauge 800MPa hydroelectric steel with excellent core toughness comprises the following components in mass ratio: C: 0.11-0.13%, Si: 0.1-0.15%, Mn: 1.05-1.15%, P≤0.015%, S≤0.002%, Cr: 0.4-0.5%, Ni: 0.55-0.65%, Mo: 0.35-0.40%, Cu: 0.35-0.40%, Nb: 0.01-0.03%, V: 0.055-0.065%, Ti: 0.006-0.012%, Als: 0.01-0.02%, B: 15-30ppm, Mg: 8-18ppm, N: 60-80ppm, and the rest is Fe and unavoidable impurities; The manufacturing method comprises the steps of converter - refining outside the furnace - vacuum degassing - thick slab continuous casting - heating - rolling - water cooling - quenching and tempering treatment, wherein: 1) Nitrogen is used as the lifting gas throughout the RH vacuum treatment. The net cycle time is ≥8 minutes at a vacuum degree ≤60Pa, and then the cycle time is ≥10 minutes at a vacuum degree of 5-7KPa. After breaking the air, the N content is tested, and vanadium iron nitride or vanadium iron alloy is added according to the chemical composition to adjust V and N within the target composition range; after adjusting the chemical composition, the molten steel is clean blown for 3 minutes, and Si-Mg cored wire is fed to control Mg within the target composition range. After feeding the wire, the molten steel is clean blown for 3-5 minutes before being put into casting; 2) Continuous casting adopts low superheat casting, controls the superheat between 15-20℃, keeps the pulling speed constant, and adopts single-roller stirring mode for electromagnetic stirring in the secondary cooling zone, with stirring current of 570A and current frequency of 5Hz. The large pressing mode is adopted for the pressing at the end of solidification, and the cumulative pressing amount is ≥15mm; ensure that the center segregation of the slab is ≤C1.0 level; 3) The walking beam heating furnace is used for heating. The billet entering the furnace temperature is ≤200℃, the temperature of the three heating sections is 1150-1180℃, the temperature of the soaking section is 1130-1150℃, and the total time of the billet in the furnace is (1.2-1.4) min / mm*T, of which the heating time of the three heating sections is controlled at (0.25-0.35) min / mm*T, and the heating time of the soaking section is controlled at (0.20-0.3) min / mm*T; T is the thickness of the billet, in mm; 4) The rolling adopts three-stage controlled rolling, in which the final rolling temperature of the recrystallization rolling stage is ≥980℃, the rolling speed is ≤1.2m / s, the reduction of at least 5 passes is ≥30mm, and the reduction of 2 passes in 5 passes is ≥40mm, and the thickness of the intermediate billet after rough rolling is ≥1.6h; the intermediate billet is forced to cool the surface by the intermediate billet cooling device before the finishing mill, and 2 passes are rolled after the surface temperature is ≤850℃ and the temperature is averaged for 10-20 seconds, with a cumulative reduction of ≥35mm; the non-recrystallization zone rolling is continued at a temperature of 780-800℃, and the final rolling temperature is 760-780℃; where h is the thickness of the finished steel plate, in mm; 5) ACC uses a reciprocating walking mode to cool the steel plate. For the first cooling, the water inlet pressure of the gap nozzle is set to 0.5MPa, the upper and lower water ratio is 2.1, the water inlet pressure of the high-density nozzle is set to 0.15MPa, the upper and lower water ratio is 2.4, 30 sets of headers are fully open, and the roller speed is 0.35-0.45 m / min; after cooling, the steel plate body is completely out of the cooling device, and after 10-20 seconds of equalization, it enters the cooling device for the next cooling. The steel plate is reciprocated and water-cooled 3 times, and the gap nozzle is closed for the second and third times; the final cooling temperature of the steel plate is ≤400℃; the steel plate is stacked offline for ≥24 hours after cooling; 6) The steel plate is subjected to offline quenching + tempering treatment, wherein the quenching temperature is controlled at 865-875°C, the time in the furnace is 3.0 min / mm*h, and the holding time is ≥0.5 min / mm*h. The steel plate is water quenched after being taken out of the furnace, and the surface temperature of the steel plate after quenching is lower than 50°C; the tempering temperature is 600-630°C, the time in the furnace is 3.5 min / mm*h, and the holding time is ≥1.0 min / mm*h; wherein h is the thickness of the finished steel plate, in mm.

Images