Large-scale thick 800 MPa-grade high-toughness hydroelectric steel plate and preparation method thereof

Abstract

The invention discloses a large-size thick-specification 800MPa-grade high-strength and high-toughness hydroelectric steel plate and a preparation method thereof, and belongs to the technical field of steel production, the large-size thick-specification 800MPa-grade high-strength and high-toughness hydroelectric steel plate comprises the following chemical components in percentage by mass: 0.06 to 0.12 percent of C, 0.80 to 1.50 percent of Mn, 0.12 to 0.30 percent of Si, 0.2 to 0.3 percent of Cu, 0.3 to 0.6 percent of Cr, 0.3 to 0.6 percent of Mo, 0.0012 to 0.0020 percent of B, 0.008 to 0.016 percent of Ti, 0.015 to 0.040 percent of Nb, 0.025 to 0.060 percent of Alt, 1.80 to 2.50 percent of Ni, less than or equal to 0.01 percent of P, less than or equal to 0.003 percent of S and the balance of Fe and other inevitable By limiting the short-process preparation process of blank smelting, continuous casting, homogenization, two-stage controlled rolling, online quenching and tempering water cooling, the steel plate has the performance of high strength, high low-temperature impact toughness and high elongation at the same time; the requirements of the hydroelectric steel for high strength, high low-temperature impact toughness and high ductility can be met, the application scene of the hydroelectric steel is considered, and good corrosion resistance is achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of steel production, and in particular relates to a large-scale thick specification 800MPa grade high-strength and toughness hydropower steel plate and a preparation method thereof. Background Art

[0002] With the transformation of the global energy structure and the growing demand for clean energy, hydropower, as a renewable and clean form of energy, has been widely used and promoted around the world. Hydropower projects, especially the construction of large hydropower stations and pumped storage power stations, have increasingly higher requirements for materials, especially the significant increase in demand for high-strength and high-toughness hydropower steel plates. In hydropower projects, steel plates are mainly used to manufacture key core components such as water diversion pressure pipes, bifurcated pipes, volutes and units. With the development of hydropower station construction towards higher heads and larger unit capacities, higher requirements are placed on the strength level of steel plates. Achieving a tensile strength of 800MPa and good strength and toughness properties under large-size and thick specifications has become a key technical challenge.

[0003] However, traditional 800MPa hydropower steel plates often lack toughness while increasing strength, or it is difficult to ensure internal structural uniformity and performance stability when thick specifications are produced. In order to solve these problems, domestic and foreign research institutions and enterprises have conducted a lot of research and development, striving to explore more efficient production methods to improve production efficiency and reduce costs.

[0004] Chinese patent CN111826580A discloses high-strength, easy-to-weld steel plates for ultra-thick large hydropower stations and an efficient and low-cost production method. The steel plates are subjected to offline tempering heat treatment after shot blasting. The production process cost is high and the strength of the steel plates only reaches 600MPa level, which cannot guarantee the safety and durability of the structure and is difficult to meet the needs of large-capacity hydropower station construction.

[0005] Chinese patent CN116904847A discloses a thick-gauge 800MPa hydropower steel with excellent core toughness and a manufacturing method thereof. The patent solves the problem of core toughness of thick-gauge 800MPa hydropower steel by designing a specific chemical composition and optimizing the production process. However, hydropower projects may have different performance requirements for steel depending on different structural parts and use environments. For some special parts that require higher corrosion resistance, this steel cannot meet the requirements.

[0006] Chinese patent CN114293110A discloses a thick-gauge 800MPa grade hydroelectric steel and its efficient and low-cost production method. This method requires long-term (not less than 48 hours) stacking and slow cooling after rolling, and then offline tempering treatment. Quenching requires special settings and a complex structure. Therefore, although the strength of the prepared steel plate can reach 800MPa, the surplus is small, the thickness is only 40 to 65mm, and the production efficiency is low.

[0007] Chinese patent CN118389945A discloses an 800MPa grade hydroelectric steel for pumped storage power stations and a preparation method thereof. The method adopts a low alloy composition design, but the operation is complicated in the LF refining and continuous casting steps and is not conducive to mass production. The strength-plasticity product is low, the low-temperature impact energy is also low, and only a low-temperature impact test at -40°C is performed, and the performance at lower temperatures cannot be judged.

[0008] Chinese patent CN106521358A discloses a method for producing hydroelectric steel with a tensile strength of 800MPa. The method requires the design of the billet shape, and rough rolling and finish rolling are performed after homogenization treatment. The total pass reduction rate of rough rolling is high, the total pass reduction rate of finish rolling is low, and the cooling rate of water cooling is also low. That is, although it has undergone quenching and tempering processes, the average tensile strength is less than 800MPa, the average elongation is 19%, the strength-plasticity product is low, and the low-temperature impact energy is also low.

[0009] Therefore, there is an urgent need to provide a large-scale, thick-specification 800MPa high-strength and tough hydropower steel plate and its preparation method to solve the problems of insufficient steel plate strength, complex process, low production efficiency and other problems that are not conducive to industrial production practice in existing hydropower steel. Summary of the invention

[0010] In response to the above technical problems, the present invention proposes a large-scale, thick, 800MPa-grade high-strength and toughness hydropower steel plate and a preparation method thereof. The steel plate prepared by the chemical composition and preparation method defined by the present invention has the properties of high strength, high toughness and high elongation, and the preparation method has high production efficiency and a wide range of applications.

[0011] To achieve the above object, the present invention provides the following technical solutions:

[0012] One of the technical solutions of the present invention:

[0013] A large-size, thick-specification, 800MPa-grade high-strength and toughness hydropower steel plate, whose chemical composition, calculated by mass percentage, is as follows: C: 0.06-0.12%, Mn: 0.80-1.50%, Si: 0.12-0.30%, Cu: 0.2-0.3%, Cr: 0.3-0.6%, Mo: 0.3-0.6%, B: 0.0012-0.0020%, Ti: 0.008-0.016%, Nb: 0.015-0.040%, Alt: 0.025-0.060%, Ni: 1.80-2.50%, P≤0.01%, S≤0.003%, and the remainder is Fe and other inevitable impurity elements.

[0014] Beneficial effect: The present invention adds an appropriate amount of alloying elements chromium, nickel, and molybdenum. These alloying elements can not only improve the toughness of the steel plate, but also form a stable oxide layer with the iron in the steel, improve the corrosion resistance of the steel, and be better applied to the actual scene of hydropower engineering. Among them, the chemical composition of the high-strength and tough hydropower steel plate disclosed by the present invention contains the alloying element B, which can improve the hardenability, and its mechanism of action is: trace amounts of B are solid-dissolved in high-temperature austenite, and are segregated at the austenite grain boundaries during cooling, inhibiting the nucleation of grain boundary ferrite, thereby improving the hardenability of the steel, especially greatly improving the hardenability under low cooling rate conditions. However, B in steel is an active element and is easily combined with N to form stable BN. These BNs are insoluble at the quenching temperature, and the effective B content of solid solution in austenite is reduced, which reduces the effect of B on improving hardenability. Therefore, it is necessary to add nitride-forming elements, fix the N element, and maintain the solid solution amount of B in austenite, thereby improving hardenability. Among them, strong nitride-forming elements such as Ti and Al can play a role in fixing N to varying degrees. Alloy elements Ti, Al, and B are strong nitride-forming elements in steel. Their ability to bind to N is TiN, BN, and AlN from strong to weak. When Ti is added to B-containing steel, the N in the steel will preferentially precipitate as TiN or Ti(C,N), and the precipitation start temperature is generally as high as 1400°C or above, which is much higher than the precipitation start temperature of BN. As the temperature decreases, the proportion of solid N in TiN will increase, thereby greatly fixing the N in the steel, preventing the formation of BN, and increasing the effective B solid solution content in austenite, thereby achieving the purpose of improving hardenability.

[0015] Optionally, the large-size, thick-gauge 800MPa-grade high-strength and toughness hydropower steel plate has a chemical composition, calculated by mass percentage, of: C: 0.06%, Mn: 1.5%, Si: 0.12%, Cu: 0.27%, Cr: 0.3%, Mo: 0.51%, B: 0.0020%, Ti: 0.009%, Nb: 0.015%, Alt: 0.051%, Ni: 1.80%, and the remainder is Fe and other inevitable impurities.

[0016] Optionally, the chemical composition of the 800MPa grade high-strength and toughness hydropower steel plate with a certain size and thickness, calculated by mass percentage, is: C: 0.09%, Mn: 1.25%, Si: 0.2%, Cu: 0.3%, Cr: 0.6%, Mo: 0.3%, B: 0.0017%, Ti: 0.008%, Nb: 0.04%, Alt: 0.060%, Ni: 2.16%, P≤0.01%, S≤0.003%, and the remainder is Fe and other inevitable impurities.

[0017] Optionally, the chemical composition of the 800MPa grade high-strength and toughness hydropower steel plate with a certain size and thickness, calculated by mass percentage, is: C: 0.12%, Mn: 0.80%, Si: 0.30%, Cu: 0.2%, Cr: 0.46%, Mo: 0.6%, B: 0.0012%, Ti: 0.016%, Nb: 0.029%, Alt: 0.025%, Ni: 2.50%, P≤0.01%, S≤0.003%, and the remainder is Fe and other inevitable impurities.

[0018] Optionally, the chemical composition of the 800MPa grade high-strength and toughness hydropower steel plate with a certain size and thickness, calculated by mass percentage, is: C: 0.10%, Mn: 1.12%, Si: 0.23%, Cu: 0.22%, Cr: 0.39%, Mo: 0.47%, B: 0.0015%, Ti: 0.013%, Nb: 0.033%, Alt: 0.047%, Ni: 2.36%, P≤0.01%, S≤0.003%, and the remainder is Fe and other inevitable impurities.

[0019] Optionally, the thickness of the thick-sized 800MPa grade high-strength and toughness hydropower steel plate is 90 to 140 mm; its metallographic structure is tempered troostite, and the average effective grain size is less than 8.5 μm.

[0020] Optionally, the large-size, thick-gauge 800MPa-grade high-strength and toughness hydropower steel plate has a yield strength of 792-843MPa, a tensile strength of 837-882MPa, an elongation of 19.6-22.6%, a strength-plasticity product of 16.512-18.396GPa·%, an impact energy at -40°C of >210J, and an impact energy at -60°C of >170J.

[0021] The second technical solution of the present invention:

[0022] A method for preparing a large-size, thick, 800MPa-grade high-strength and tough hydropower steel plate comprises the following steps:

[0023] According to the above chemical composition dosage, raw materials (such as metal-containing ores, waste materials or intermediate products, wherein the intermediate products are molten steel, steel billets, intermediate compounds of microalloying elements or recycled products of scrap steel) are batched and weighed;

[0024] After the raw materials are smelted to obtain molten iron, heat treatment, converter smelting and slab continuous casting are sequentially performed to obtain hydropower steel continuous casting slabs;

[0025] The hydropower steel continuous casting billet is then subjected to a homogenization treatment, two-stage temperature-controlled hot rolling and water-cooling quenching (online quenching) in sequence, followed by tempering and rapid cooling, to prepare the large-size, thick-specification, 800MPa-grade high-strength and tough hydropower steel plate;

[0026] The converter smelting includes SGRS process dephosphorization, LF refining and RH vacuum degassing.

[0027] Further, the specific process of dephosphorization by the SGRS process is:

[0028] The molten iron is charged into the converter and dephosphorization blowing is carried out. As the blowing is carried out, part of the slag is poured out before the temperature rises to a level that is unfavorable for dephosphorization (i.e., 1450°C ~ 1550°C), and then the slag is added to the next round of dephosphorization blowing process, and the cycle is repeated.

[0029] Further, the LF refining process is:

[0030] (1) Early stage of temperature raising and slag removal (10-15 min): Use medium-high-grade slag removal, pay attention to arc burial and control the dust cover opening (controlled at 40%-60%), and use the full-process argon control mode for bottom blowing;

[0031] (2) Before the foamy slag is formed, a small current (20-25 kA) and a small voltage (150-200 V) are operated: then the current (30-40 kA) and the voltage (200-300 V) are gradually increased;

[0032] (3) Alloying operation after refining white slag in LF furnace: add aluminum wire, ferrotitanium alloy, ferrozirconium alloy and ferroboron alloy in turn, adjust the content of elements in molten steel, and continue to supply power for 3 to 5 minutes; Among them, in the above steps, although the raw material batching and weighing can roughly control the composition of molten steel, in the actual smelting process, due to factors such as raw material composition fluctuations and burn-in loss, the final composition of molten steel may deviate from the target value. The LF furnace refining stage is the last opportunity to adjust the composition of molten steel. By adding alloys (aluminum wire, ferrotitanium alloy, ferrozirconium alloy and ferroboron alloy), the content of alloy elements in molten steel can be accurately controlled to ensure that the composition of molten steel meets the requirements; take samples multiple times during the smelting process, use a spectrometer to quickly analyze the composition of molten steel, and make timely adjustments based on the analysis results;

[0033] (4) Soft argon blowing treatment (20 min): promotes the floating of inclusions and ensures the purity of molten steel;

[0034] (5) Final inspection and bag hanging: After confirming that all indicators are qualified, prepare to hang the bag.

[0035] Further, the RH vacuum degassing process is:

[0036] (1) Preparation stage: Ensure that the slag surface in the ladle is surging and the molten steel surface is not exposed, and check the status of the RH device;

[0037] (2) Molten steel enters the RH device: the molten steel is transferred to the RH treatment position and the riser and downcomer are inserted;

[0038] (3) Vacuuming and clean circulation: Start the vacuum pump and maintain the clean circulation time for not less than 10 minutes to remove the gas in the molten steel;

[0039] (4) Composite deoxidizer treatment: Add composite deoxidizer during RH treatment to perform deep deoxidation and inclusion modification treatment;

[0040] (5) Breaking the air and standing: slowly restore atmospheric pressure and stand for at least 15 minutes before pouring (the starting step of the continuous casting process) to allow inclusions to float up;

[0041] (6) Final inspection and bagging: Carry out component testing and temperature adjustment, and prepare for bagging after confirmation of compliance.

[0042] Furthermore, the conditions during the slab continuous casting process are: the superheat of the molten steel is below 30° C., and the continuous casting slab drawing speed is 1.0 m / min.

[0043] Optionally, the conditions during the homogenization treatment are: the heat treatment temperature is 1200-1250° C., and the time is 1.5-3 hours.

[0044] Furthermore, after the homogenization treatment, hard particles and water are mixed to remove phosphorus and iron oxide.

[0045] Furthermore, the dephosphorization process of mixing hard particles with water requires the use of a continuous casting billet dephosphorization device, which includes a dephosphorization unit 1, a water tank 3 (for holding the continuous casting billet 4) and a dephosphorization unit 2. The dephosphorization unit 1 and the dephosphorization unit 2 are respectively located above and below the water tank;

[0046] The dephosphorization unit 1 includes a stirrer 5, which is used to stir the hard particles and water contained in the dephosphorization unit 1 into a uniform beam, and spray the hard particles and water onto the continuous casting billet to remove phosphorus; the dephosphorization unit 2 contains water. The specific working principle is: the hard particles have a smooth surface, and the hard particles carried by high-pressure water can increase the impact force of the medium, increase the dephosphorization effect and reduce the water consumption, and reduce the temperature drop of the continuous casting billet. After dephosphorization, the water flow in the lower area of ​​the water tank is used to blow away hard particles such as fine sand and steel shots. No other additional equipment is required, and the process is simple.

[0047] Optionally, the two-stage temperature-controlled hot rolling includes a rough rolling stage and a finishing rolling stage.

[0048] Furthermore, the rough rolling stage is carried out in two passes in the austenite recrystallization zone, the cumulative reduction rate in the rough rolling stage is 50-60%, the final rolling temperature is 1000-1100°C, and the thickness of the intermediate billet obtained by the rough rolling is 160-250mm.

[0049] Furthermore, the finishing rolling stage is carried out in two passes in the austenite non-recrystallization zone, the cumulative reduction rate in the finishing rolling stage is 44-50%, the final rolling temperature is 850-900°C, and the thickness of the intermediate billet obtained by finishing rolling is 90-140mm.

[0050] Beneficial effect: The present invention adopts two-stage temperature-controlled hot rolling, with rough rolling with a reduction of no more than 60% in the first stage and finishing rolling with a reduction of more than 44% in the second stage, which can lay a solid foundation for the subsequent high-surplus tensile strength, high and low temperature impact toughness, and high elongation.

[0051] Optionally, the water-cooled quenching adopts an ultra-fast cooling technology, and its parameter conditions are: the starting cooling temperature is 780-830°C, the final cooling temperature is 600-640°C, and the cooling rate is 15-35°C / s.

[0052] Optionally, the tempering rapid cooling process is: heating the steel plate to 600-680° C., keeping the steel plate warm for 90-120 minutes after the core of the steel plate reaches the furnace temperature; and rapidly cooling the steel plate to room temperature with water after it is taken out of the furnace.

[0053] Beneficial effects: The present invention adopts ultra-fast cooling technology for water cooling, and its principle is to transform the austenite structure in the steel into martensite or bainite structure through rapid cooling, thereby improving the strength and hardness of the steel. Achieving a large cooling rate in a short time can effectively inhibit the growth of austenite grains, refine the structure, and enable the steel to obtain a better match between strength and toughness. Online quenching improves the stacking slow cooling method, reduces the intermediate turnover links, saves manpower, material resources and time. In the tempering water cooling process, rapid water cooling after tempering helps to maintain the state of the structure after tempering. This heat treatment process can greatly improve the impact toughness of the core of thick-gauge hydropower steel plates, and can significantly improve the impact performance and stability of the core of thick-gauge steel plates.

[0054] Compared with the prior art, the present invention has the following advantages and technical effects:

[0055] Compared with other traditional methods, the present invention achieves the performance of high strength, high and low temperature impact toughness and high elongation of steel plates by limiting the short process of "blank smelting + continuous casting + homogenization + two-stage controlled rolling + online quenching + tempering water cooling". It can meet the requirements of high strength, high and low temperature impact toughness and high elongation for hydropower steel and take into account the application scenarios of hydropower steel, and has good corrosion resistance. In addition, the preparation method of the present invention has high production efficiency, simple influencing factors, and a wide range of applications, which is conducive to large-scale industrial production and promotion. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] The accompanying drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the accompanying drawings:

[0057] Figure 1 A schematic diagram of the process flow for preparing a large-scale, thick, 800MPa-grade high-strength and tough hydropower steel plate according to the present invention;

[0058] Figure 2 The metallographic structure diagram of the hydroelectric steel plate prepared in Example 1 of the present invention;

[0059] Figure 3 The engineering stress-strain curve diagram of the hydropower steel plate prepared in Example 1 of the present invention;

[0060] Figure 4 This is a dephosphorization device used in an embodiment of the present invention; wherein 1 is a dephosphorization unit 1, 2 is a dephosphorization unit 2, 3 is a water tank, 4 is a continuous casting billet, and 5 is a stirrer. DETAILED DESCRIPTION

[0061] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0062] It should be understood that the terms described in the present invention are only for describing special embodiments and are not intended to limit the present invention. In addition, for the numerical range in the present invention, it should be understood that each intermediate value between the upper and lower limits of the scope is also specifically disclosed. Each smaller range between the intermediate value in any stated value or stated range and any other stated value or intermediate value in the described range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0063] Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as those generally understood by those skilled in the art. Although the present invention describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of a conflict with any incorporated document, the content of this specification shall prevail.

[0064] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention description without departing from the scope or spirit of the present invention. Other embodiments derived from the present invention description will be apparent to those skilled in the art. The present invention description and examples are exemplary only.

[0065] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0066] The embodiment of the present invention discloses a large-size, thick-gauge, 800MPa-grade high-strength and toughness hydropower steel plate, which has the following chemical compositions, measured by mass percentage: C: 0.06-0.12%, Mn: 0.80-1.50%, Si: 0.12-0.30%, Cu: 0.2-0.3%, Cr: 0.3-0.6%, Mo: 0.3-0.6%, B: 0.0012-0.0020%, Ti: 0.008-0.016%, Nb: 0.015-0.040%, Alt: 0.025-0.060%, Ni: 1.80-2.50%, P≤0.01%, S≤0.003%, and the remainder is Fe and other inevitable impurity elements.

[0067] The above elemental composition also satisfies the following relationship:

[0068] Boron equivalent BE = [(0.259×Als+0.457×Ti)-N]≥0.005% (where the elements represent their percentage content). This ensures that the effective B content in the steel is ≥0.0012%, giving full play to the strong hardenability of B. At the same time, Als (acid-soluble aluminum) and Ti are precipitated in a fine dispersed state, improving the high strength and toughness of the steel plate.

[0069] The calculation formula of the boron equivalent can provide a basis for limiting the element content and more fully explain the properties of steel. Among them, ALs is acid-soluble aluminum, AL=ALt (total aluminum)=ALs (acid-soluble aluminum)+acid-insoluble aluminum; N is the percentage of nitrogen, N≤0.018%, which plays a role in solid solution strengthening. Nitrogen has a wide source and solubility characteristics, and is an inevitable other impurity element.

[0070] The inventive principle of the technical solution of the present invention is that in the quenched and tempered high-strength steel, adding alloy element B can improve the hardenability, and its mechanism of action is: trace amounts of B are dissolved in high-temperature austenite, and are segregated at the austenite grain boundaries during cooling, inhibiting the nucleation of grain boundary ferrite, thereby improving the hardenability of the steel, especially greatly improving the hardenability under low cooling rate conditions. However, B in steel is an active element and is easily combined with N to form stable BN. These BNs are insoluble at the quenching temperature, and the effective B content of solid solution in austenite is reduced, which reduces the effect of B on improving hardenability. Therefore, it is necessary to add nitride-forming elements to fix the N element and maintain the solid solution amount of B in austenite, thereby improving hardenability. Among them, strong nitride-forming elements such as Ti and Al can play a role in fixing N to varying degrees. Alloy elements Ti, Al, and B are strong nitride-forming elements in steel, and their ability to combine with N is TiN, BN, and AlN from strong to weak. When Ti is added to B-containing steel, N in the steel will preferentially precipitate as TiN or Ti(C,N), and the precipitation start temperature is generally as high as 1400℃, which is much higher than the precipitation start temperature of BN. As the temperature decreases, the proportion of solid N in TiN will increase, thereby greatly fixing the N in the steel, preventing the formation of BN, and increasing the effective B solid solution content in austenite, thereby achieving the purpose of improving hardenability.

[0071] The large-size, thick-specification, 800MPa-grade high-strength and tough hydropower steel plate defined in the present invention has a thickness of 90 to 140 mm, and the structure is all tempered troostite. The average effective grain size is less than 8.5 μm, and the average lath width of the recovered martensite is 211-260 nm. Since there is no re-austenitization process, the original austenite grains present a flat deformation feature, and the average original austenite grain size is less than 30 μm.

[0072] In addition, the embodiment of the present invention also discloses a method for preparing a large-size, thick, 800 MPa grade high-strength and tough hydropower steel plate, comprising the following steps:

[0073] 1. Billet selection: the raw materials are batched and weighed according to the chemical mass percentage of the large-size, thick-gauge, 800MPa-grade high-strength and tough hydropower steel plate;

[0074] 2. Billet smelting: After the raw materials are smelted to obtain high-purity molten iron, the molten iron is heat treated, followed by converter smelting, LF refining, and RH vacuum degassing to obtain molten steel that meets the requirements, and the molten steel is continuously cast into slabs to obtain hydropower steel continuous casting billets;

[0075] 3. Homogenization treatment: The hydropower steel continuous casting billet is subjected to homogenization treatment at a temperature of 1200-1250°C for 1.5-3 hours in a walking beam heating furnace, thereby eliminating casting stress and reducing segregation to obtain a homogenized continuous casting billet;

[0076] 4. Temperature-controlled rolling: The steel billet is subjected to two-stage temperature-controlled hot rolling. The rough rolling stage is in the austenite recrystallization zone and is rolled in two passes. The cumulative reduction rate in the rough rolling stage is 50-60%, the final rolling temperature is 1000-1100°C, and the thickness of the intermediate billet obtained by rough rolling is 160-250mm; the finishing rolling stage is in the austenite non-recrystallization zone and is rolled in two passes. The cumulative reduction rate in the finishing rolling stage is 44-50%, the final rolling temperature is 850-900°C, and the thickness of the intermediate billet obtained by finishing rolling is 90-140mm;

[0077] 5. Water-cooled quenching: using ultra-fast cooling technology, the starting cooling temperature is 780-830°C, the final cooling temperature is 600-640°C, and the cooling rate is 15-35°C / s;

[0078] 6. Tempering and rapid cooling: The steel plate is heated to 600-680℃, and the core of the steel plate is kept warm for 90-120 minutes after reaching the furnace temperature. After being taken out of the furnace, it is quickly water-cooled to room temperature to obtain a large-size, thick-gauge, 800MPa-grade high-strength and tough hydropower steel plate.

[0079] In some optional embodiments, during the billet smelting process, the SGRS process is used for dephosphorization treatment, utilizing the basic thermodynamic principle that low temperature is beneficial to the dephosphorization reaction. At the end of the converter blowing, due to the high temperature and low phosphorus content in the molten steel, the slag no longer has the dephosphorization ability, and the converter final slag remains in the furnace. In the early stage of the next furnace blowing, due to the low temperature and high phosphorus content in the molten iron, the slag regains the dephosphorization ability. As the blowing proceeds, part of the slag is poured out before the temperature rises to a level that is unfavorable for dephosphorization, and then the slag is re-made for the decarburization stage of blowing.

[0080] In some optional embodiments, during the billet smelting process, LF and RH are used to refine the molten steel outside the furnace. The time for the early stage of LF refining and slag removal is 10 to 15 minutes, during which medium and high-grade slag removal is used. At the same time, the entire refining process pays attention to arc burial. The opening of the dust removal cover of the LF process is controlled at 40% to 60%, and the bottom blowing gas adopts the full-process argon control mode. Before the foam slag is formed, a small current and a small voltage are used for operation. As the foam slag is formed, the current and voltage are increased in turn. After the LF furnace refines the white slag, aluminum wire, titanium iron alloy, zirconium iron alloy and boron iron alloy are added in sequence, and the power is continued for 3 to 5 minutes, and then the argon is soft-blown for more than 20 minutes. Ensure that the LF furnace produces white slag, ensure that S in the steel is ≤ 0.001%, strictly control the gas content of H, O, N in the steel, require that the RH net circulation shall not be less than 10 minutes, and the static time before pouring shall not be less than 15 minutes. Keep the slag surface in the ladle in a surging state without exposing the molten steel surface. At the same time, the inclusions in the molten steel are modified through composite deoxidizer treatment to maximize the purity of the steel.

[0081] In some optional embodiments, during the billet smelting process, protective casting is used throughout the continuous casting process to prevent the molten steel from being oxidized again. The superheat of the molten steel is controlled below 30°C to minimize the occurrence of defects such as center porosity and center segregation. In order to ensure the intrinsic quality of the continuous casting billet, the continuous casting billet pulling speed is controlled to 1.0m / min, and the casting is performed at a constant pulling speed. An ultra-low carbon covering agent is used to enhance the adsorption of inclusions, while achieving good gas protection and improving the sealing of the submerged nozzle. The light pressure reduction technology is used at the end of the continuous casting to fully improve the intrinsic quality of the billet, with center segregation of Class C 1.0 level and below, and center porosity of 1.0 level and below.

[0082] In some optional embodiments, after the homogenization treatment, hard particles are mixed with water to remove phosphorus and iron oxide scale, and high-pressure water is used to remove phosphorus during rolling. In addition, the present invention adopts a method of dephosphorization by mixing hard particles with water, which requires the use of a continuous casting billet dephosphorization device, and its dephosphorization working area sprays the hard particles and water in the upper area of ​​the water tank onto the continuous casting billet to remove phosphorus. The upper area of ​​the water tank contains hard particles and water, and the lower area contains water. A stirring device is provided in the upper area of ​​the water tank to stir the hard particles and water into a uniform beam. In this method, the hard particles have a smooth surface, and the hard particles carried by the high-pressure water can increase the impact force of the medium, increase the dephosphorization effect and reduce the water consumption, thereby reducing the temperature drop of the continuous casting billet. After dephosphorization, the water flow in the lower area of ​​the water tank is used to sweep hard particles such as fine sand and steel shots, without the need for other additional equipment, and the process is simple.

[0083] After testing, the large-size, thick-gauge, 800MPa-grade high-strength and tough hydropower steel plate prepared by the preparation method of the present invention has a yield strength of 792-843MPa, a tensile strength of 837-882MPa, an elongation of 19.6-22.6%, a strength-ductility product of 16.512-18.396GPa·%, an impact energy of -40°C>210J, and an impact energy of -60°C>170J, which meets the requirements of relevant protocol standards.

[0084] The "room temperature" in the present invention refers to 20-30°C unless otherwise specified.

[0085] The raw materials used in the present invention are all purchased from the market. The composite deoxidizer used in the following examples is purchased from Henan Xinchuang Metallurgical Materials Co., Ltd.; the ultra-low carbon covering agent is purchased from Shandong Guomao Metallurgical Materials Co., Ltd.; the hard particles are fine sand and steel shot (the mass ratio of the two is 1:1.5-3).

[0086] The technical solution of the present invention is further illustrated by the following embodiments.

[0087] Example 1

[0088] A large-size, thick-gauge, 800MPa-grade high-strength and tough hydropower steel plate, whose chemical composition, by weight percentage, is: C: 0.06%, Mn: 1.5%, Si: 0.12%, Cu: 0.27%, Cr: 0.3%, Mo: 0.51%, B: 0.0020%, Ti: 0.009%, Nb: 0.015%, Alt: 0.051%, Ni: 1.80%, P≤0.01%, S≤0.003%, and the balance is Fe and other inevitable impurities;

[0089] like Figure 1 As shown, the preparation method of the above-mentioned large-size thick specification 800MPa grade high-strength and toughness hydropower steel plate comprises the following steps:

[0090] 1. Billet selection: Raw materials are batched and weighed according to the chemical composition of large-size, thick-size, 800MPa-grade high-strength and tough hydropower steel plates;

[0091] 2. Billet smelting: After the raw materials are smelted to obtain molten iron, the molten iron is heat treated, followed by converter smelting (including SGRS process dephosphorization, LF refining and RH vacuum degassing in sequence) to obtain molten steel that meets the requirements, and then the molten steel is continuously cast into slabs to obtain hydropower steel continuous casting billets. The specific operations of converter smelting are as follows:

[0092] ①Use SGRS process for dephosphorization:

[0093] The molten iron is loaded into the converter and dephosphorization blowing is carried out; as the blowing is carried out, part of the slag is poured out before the temperature rises to a level that is unfavorable for dephosphorization, and then the next round of dephosphorization blowing is carried out, and the cycle is repeated;

[0094] ② Use LF and RH to refine the molten steel after dephosphorization outside the furnace; the time of the early stage of LF refining temperature and slag removal is 10 minutes, during which medium and high-grade slag removal is used, and the whole refining process pays attention to arc submersion, the opening of the dust removal cover of the LF process is controlled at 40%, and the bottom blowing gas adopts the full-process argon control mode. Before the formation of foamy slag, use small current and small voltage (current and voltage are 20kA and 170V respectively). With the formation of foamy slag, increase the current and voltage in turn (increase to 32kA and 225V respectively. After the LF furnace refines the white slag, add aluminum wire, titanium-iron alloy, zirconium-iron alloy and boron-iron alloy in sequence, and continue to supply power for 4 minutes, and then softly blow argon for 21 minutes to hang the bag;

[0095] Ensure that the LF furnace produces white slag, ensure that S in the steel is ≤ 0.001%, strictly control the gas content of H, O, N and other gases in the steel, require RH net circulation for 10 minutes, and the static time before pouring is 17 minutes. Keep the slag surface in the ladle in a surging state without exposing the molten steel surface. At the same time, through the treatment of composite deoxidizer, the inclusions in the steel are modified to maximize the purity of the steel;

[0096] ③ The entire continuous casting process uses protective casting to prevent the molten steel from being secondary oxidized:

[0097] Control the superheat of molten steel below 30℃ to minimize the occurrence of defects such as center porosity and center segregation;

[0098] The continuous casting billet drawing speed is controlled at 1.0m / min, and casting is performed at a constant drawing speed to ensure the internal quality of the continuous casting billet;

[0099] Use ultra-low carbon covering agent to enhance the adsorption of inclusions, while achieving good gas protection and improving the sealing of submerged nozzles;

[0100] The light reduction technology is used at the end of continuous casting to fully improve the internal quality of the ingot, with the center segregation C class 1.0 and below, and the center looseness 1.0 and below;

[0101] 3. Heat treatment: The hydroelectric steel continuous casting billet obtained by smelting is subjected to heat treatment at a temperature of 1200°C for 1.5 hours in a walking beam heating furnace to eliminate casting stress and reduce segregation to obtain a homogenized continuous casting billet; after the heat treatment, a dephosphorization method is used to remove iron oxide scale by mixing hard particles and water (the mass ratio of hard particles to water is 1:1; the hard particles are fine sand and steel shots, and the mass ratio of the two is 1:2.5);

[0102] 4. Temperature controlled rolling: The above steel billet is subjected to two stages of temperature controlled hot rolling, including rough rolling and finishing rolling stages:

[0103] The rough rolling stage is carried out in two passes in the austenite recrystallization zone. The cumulative reduction rate in the rough rolling stage is 54%, the final rolling temperature is 1000°C, and the thickness of the intermediate billet obtained by rough rolling is 193mm.

[0104] The finishing rolling stage is carried out in two passes in the austenite non-recrystallization zone. The cumulative reduction rate in the finishing rolling stage is 50%, the final rolling temperature is 850℃, and the thickness of the intermediate billet obtained by the finishing rolling is 96.5mm. In the whole rolling process, high-pressure water is used to remove phosphorus.

[0105] 5. Use ultra-fast cooling technology for water-cooling quenching. The specific parameter conditions are: the starting cooling temperature is 830°C, the final cooling temperature is 627°C, and the cooling rate is 20°C / s;

[0106] 6. Tempering water cooling: Heat the steel plate to 680℃, and keep it warm for 100 minutes after the core of the steel plate reaches the furnace temperature. After it comes out of the furnace, it is quickly water-cooled to room temperature to obtain a large-size, thick-gauge, high-strength and tough hydropower steel plate with a tensile strength of 829MPa.

[0107] The large-size thick specification 800MPa grade high-strength and tough hydropower steel plate prepared in this embodiment has a thickness of 96.5mm and a metallographic structure as shown in FIG. Figure 2As shown, the microstructure is tempered troostite with an average effective grain size of 7.6 μm. The average lath width of the recovered martensite is 232 nm. Since there is no re-austenitization process, the original austenite grains show flat deformation characteristics, and the average original austenite grain size is 28.3 μm.

[0108] Figure 3 The engineering stress-strain curve of the hydropower steel plate prepared in Example 1 is shown. It can be seen that within the range of relatively small engineering strain (about 0% to 5%), the engineering stress and engineering strain are linearly related, indicating that the steel plate is in the elastic stage; when the engineering strain reaches about 5%, the engineering stress reaches a platform value of about 850MPa.

[0109] Example 2

[0110] A large-size, thick-gauge, 800MPa-grade high-strength and tough hydropower steel plate, whose chemical composition, by weight percentage, is: C: 0.09%, Mn: 1.25%, Si: 0.2%, Cu: 0.3%, Cr: 0.6%, Mo: 0.3%, B: 0.0017%, Ti: 0.008%, Nb: 0.04%, Alt: 0.060%, Ni: 2.16%, P≤0.01%, S≤0.003%, and the balance is Fe and other inevitable impurities;

[0111] The method for preparing the above-mentioned large-size, thick-gauge, 800MPa-grade high-strength and toughness hydropower steel plate comprises the following steps:

[0112] 1. Billet selection: the raw materials are batched and weighed according to the chemical mass percentage of the large-size, thick-gauge, 800MPa-grade high-strength and tough hydropower steel plate;

[0113] 2. Billet smelting: After the raw materials are smelted to obtain molten iron, the molten iron is heat treated, followed by converter smelting (including SGRS process dephosphorization, LF refining and RH vacuum degassing in sequence) to obtain molten steel that meets the requirements, and then the molten steel is continuously cast into slabs to obtain hydropower steel continuous casting billets. The specific operations of converter smelting are as follows:

[0114] ①Use SGRS process for dephosphorization:

[0115] The molten iron is loaded into the converter and dephosphorization blowing is carried out; as the blowing is carried out, part of the slag is poured out before the temperature rises to a level that is unfavorable for dephosphorization, and then the next round of dephosphorization blowing is carried out, and the cycle is repeated;

[0116] ② Use LF and RH to refine the molten steel after dephosphorization. The time of the early stage of LF refining is 12 minutes, and medium and high-grade slag is used during this period. At the same time, pay attention to arc submersion during the whole refining process. The opening of the dust cover of the LF process is controlled at 45%, and the bottom blowing gas adopts the full-process argon control mode. Before the formation of foamy slag, use small current and low voltage (current and voltage are 23kA and 160V respectively). With the formation of foamy slag, increase the current and voltage in turn (increase to (current and voltage are 40kA and 240V respectively)). After the LF furnace refines the white slag, add aluminum wire, titanium iron alloy, zirconium iron alloy and boron iron alloy in sequence, and continue to supply power for 4 minutes, and then soft blow argon for 20 minutes to hang the bag. Ensure that the LF furnace produces white slag, ensure that S in the steel is ≤ 0.001%, strictly control the gas content of H, O, N in the steel, require RH net circulation for 12 minutes, and stand for 15 minutes before pouring. Keep the slag surface in the ladle in a surging state without exposing the molten steel surface. At the same time, modify the inclusions in the steel through composite deoxidizer treatment to maximize the purity of the steel.

[0117] ③ The continuous casting process is the same as in Example 1;

[0118] 3. Heat treatment: The hydroelectric steel continuous casting billet obtained by smelting is subjected to heat treatment at a temperature of 1238°C for 2.5 hours in a walking beam heating furnace to eliminate casting stress and reduce segregation to obtain a homogenized continuous casting billet. After the heat treatment, a dephosphorization method is used to remove the iron oxide scale by mixing hard particles and water (the mass ratio of hard particles to water is 1:1.5; the hard particles are fine sand and steel shots, and the mass ratio of the two is 1:3).

[0119] 4. Temperature controlled rolling: The above steel billet is subjected to two stages of temperature controlled hot rolling, including rough rolling and finishing rolling stages:

[0120] The rough rolling stage is carried out in two passes in the austenite recrystallization zone. The cumulative reduction rate in the rough rolling stage is 60%, the final rolling temperature is 1100°C, and the thickness of the intermediate billet obtained by rough rolling is 250mm.

[0121] The finishing rolling stage is carried out in two passes in the austenite non-recrystallization zone. The cumulative reduction rate in the finishing rolling stage is 44%, the final rolling temperature is 900°C, and the thickness of the intermediate billet obtained by finishing rolling is 140mm. In the whole rolling process, high-pressure water is used to remove phosphorus.

[0122] 5. Use ultra-fast cooling technology for water-cooling quenching. The specific parameter conditions are: the starting cooling temperature is 780°C, the final cooling temperature is 640°C, and the cooling rate is 15°C / s;

[0123] 6. Tempering and water cooling: Heat the steel plate to 600℃, and keep it warm for 120 minutes after the core of the steel plate reaches the furnace temperature. After it comes out of the furnace, it is quickly water-cooled to room temperature to obtain a large-size, thick-gauge, high-strength and tough hydropower steel plate with a tensile strength of 817MPa.

[0124] The large-size, thick-specification, 800MPa-grade high-strength and tough hydropower steel plate prepared in this embodiment has a thickness of 140mm, and the structure is tempered troostite. The average effective grain size is 8.5μm, and the average lath width of the recovered martensite is 260nm. Since there is no re-austenitization process, the original austenite grains show a flat deformation feature, and the average original austenite grain size is 27.4μm.

[0125] Example 3

[0126] A large-size, thick-gauge, 800MPa-grade high-strength and tough hydropower steel plate, whose chemical composition, by weight percentage, is: C: 0.12%, Mn: 0.80%, Si: 0.30%, Cu: 0.2%, Cr: 0.46%, Mo: 0.6%, B: 0.0012%, Ti: 0.016%, Nb: 0.029%, Alt: 0.025%, Ni: 2.50%, P≤0.01%, S≤0.003%, and the balance is Fe and other unavoidable impurities;

[0127] The method for preparing the above-mentioned large-size, thick-gauge, 800MPa-grade high-strength and toughness hydropower steel plate comprises the following steps:

[0128] 1. Stock selection: the raw materials are batched and weighed according to the chemical mass percentage of the large-size, thick-specification 800MPa-grade high-strength and tough hydropower steel plate;

[0129] 2. Billet smelting: After the raw materials are smelted to obtain molten iron, the molten iron is heat treated, followed by converter smelting (including SGRS process dephosphorization, LF refining and RH vacuum degassing in sequence) to obtain molten steel that meets the requirements, and then the molten steel is continuously cast into slabs to obtain hydropower steel continuous casting billets. The specific operations of converter smelting are as follows:

[0130] ①Use SGRS process for dephosphorization:

[0131] The molten iron is loaded into the converter and dephosphorization blowing is carried out; as the blowing is carried out, part of the slag is poured out before the temperature rises to a level that is unfavorable for dephosphorization, and then the next round of dephosphorization blowing is carried out, and the cycle is repeated;

[0132] ② Use LF and RH to refine the molten steel after dephosphorization. The time of the early stage of LF refining is 13 minutes, and medium and high-grade slag is used during this period. At the same time, pay attention to arc submersion during the whole refining process. The opening of the dust cover of the LF process is controlled at 60%, and the bottom blowing gas adopts the full-process argon control mode. Before the formation of foamy slag, use small current and low voltage operation (current and voltage are 22kA and 150V respectively). With the formation of foamy slag, increase the current and voltage in turn (current and voltage are 35kA and 280V respectively). After the LF furnace refines the white slag, add aluminum wire, titanium iron alloy, zirconium iron alloy and boron iron alloy in sequence, and continue to supply power for 5 minutes, and then soft blow argon for 22 minutes to hang the bag. Ensure that the LF furnace produces white slag, ensure that S in the steel is ≤ 0.001%, strictly control the gas content of H, O, N and other gases in the steel, require RH net circulation for 12 minutes, and stand for 16 minutes before pouring, keep the slag surface in the ladle in a surging state without exposing the molten steel surface, and at the same time modify the inclusions in the steel through composite deoxidizer treatment to maximize the purity of the steel;

[0133] ③ The continuous casting process is the same as in Example 1;

[0134] 3. Heat treatment: The hydroelectric steel continuous casting billet obtained by smelting is subjected to heat treatment at a temperature of 1250°C and a time of 1.5 in a walking beam heating furnace to eliminate casting stress and reduce segregation to obtain a homogenized continuous casting billet; after the heat treatment, a dephosphorization method is used to remove iron oxide scale by mixing hard particles and water (the mass ratio of hard particles to water is 1:2; the hard particles are fine sand and steel shots, and the mass ratio of the two is 1:1.5);

[0135] 4. Temperature controlled rolling: The above steel billet is subjected to two stages of temperature controlled hot rolling, including rough rolling and finishing rolling stages:

[0136] The rough rolling stage is carried out in two passes in the austenite recrystallization zone. The cumulative reduction rate in the rough rolling stage is 50%, the final rolling temperature is 1056°C, and the thickness of the intermediate billet obtained by the rough rolling is 160mm.

[0137] The finishing rolling stage is carried out in two passes in the austenite non-recrystallization zone. The cumulative reduction rate in the finishing rolling stage is 44%, the final rolling temperature is 875°C, and the thickness of the intermediate billet obtained by the finishing rolling is 90mm. In the whole rolling process, high-pressure water is used to remove phosphorus.

[0138] 5. Use ultra-fast cooling technology for water-cooling quenching. The specific parameter conditions are: the starting cooling temperature is 808°C, the final cooling temperature is 600°C, and the cooling rate is 35°C / s;

[0139] 6. Tempering water cooling: Heat the steel plate to 645℃, and keep it warm for 90 minutes after the core of the steel plate reaches the furnace temperature. After it comes out of the furnace, it is quickly water-cooled to room temperature to obtain a large-size, thick-gauge, high-strength and tough hydropower steel plate with a tensile strength of 831MPa.

[0140] The large-size, thick-specification 800MPa-grade high-strength and toughness hydropower steel plate prepared in this embodiment has a thickness of 90mm, and the structure is tempered troostite. The average effective grain size is 6.8μm, and the average lath width of the recovered martensite is 211nm. Since there is no re-austenitization process, the original austenite grains show a flat deformation feature, and the average original austenite grain size is 27.7μm.

[0141] Example 4

[0142] A large-size, thick-gauge, 800MPa-grade high-strength and tough hydropower steel plate, whose chemical composition, by weight percentage, is: C: 0.10%, Mn: 1.12%, Si: 0.23%, Cu: 0.22%, Cr: 0.39%, Mo: 0.47%, B: 0.0015%, Ti: 0.013%, Nb: 0.033%, Alt: 0.047%, Ni: 2.36%, P≤0.01%, S≤0.003%, and the balance is Fe and other inevitable impurities;

[0143] The method for preparing the above-mentioned large-size, thick-gauge, 800MPa-grade high-strength and toughness hydropower steel plate comprises the following steps:

[0144] 1. Billet selection: the raw materials are batched and weighed according to the chemical mass percentage of the large-size, thick-gauge 800MPa-grade high-strength and tough hydropower steel plate.

[0145] 2. Billet smelting: After the raw materials are smelted to obtain molten iron, the molten iron is heat treated, followed by converter smelting (including SGRS process dephosphorization, LF refining and RH vacuum degassing in sequence) to obtain molten steel that meets the requirements, and then the molten steel is continuously cast into slabs to obtain hydropower steel continuous casting billets. The specific operations of converter smelting are as follows:

[0146] ①Use SGRS process for dephosphorization:

[0147] The molten iron is loaded into the converter and dephosphorization blowing is carried out; as the blowing is carried out, part of the slag is poured out before the temperature rises to a level that is unfavorable for dephosphorization, and then the next round of dephosphorization blowing is carried out, and the cycle is repeated;

[0148] ② Use LF and RH to refine the molten steel after dephosphorization. The time of the early stage of LF refining is 15 minutes, and medium and high-grade slag is used during this period. At the same time, pay attention to arc submersion during the whole refining process. The opening of the dust cover of the LF process is controlled at 55%, and the bottom blowing gas adopts the full-process argon control mode. Before the formation of foamy slag, use small current and low voltage operation (current and voltage are 25kA and 200V respectively). With the formation of foamy slag, increase the current and voltage in turn (current and voltage are 30kA and 300V respectively). After the LF furnace refines the white slag, add aluminum wire, titanium iron alloy, zirconium iron alloy and boron iron alloy in sequence, and continue to supply power for 4 minutes, and then soft blow argon for 24 minutes to hang the bag. Ensure that the LF furnace produces white slag, ensure that S in the steel is ≤ 0.001%, strictly control the gas content of H, O, N in the steel, require RH net circulation for 13 minutes, and stand still for 18 minutes before pouring. Keep the slag surface in the ladle in a surging state without exposing the molten steel surface. At the same time, modify the inclusions in the steel through composite deoxidizer treatment to maximize the purity of the steel.

[0149] ③ The continuous casting process is the same as in Example 1;

[0150] 3. Heat treatment: The hydropower steel continuous casting billet is subjected to heat treatment at a temperature of 1250°C for 2 hours in a walking beam heating furnace to eliminate casting stress and reduce segregation to obtain a homogenized continuous casting billet; after the heat treatment, a dephosphorization method is used to remove iron oxide scale by mixing hard particles and water (the mass ratio of hard particles to water is 1:1.2; the hard particles are fine sand and steel shots, and the mass ratio of the two is 1:2);

[0151] 4. Temperature controlled rolling: The above steel billet is subjected to two stages of temperature controlled hot rolling, including rough rolling and finishing rolling stages:

[0152] The rough rolling stage is carried out in two passes in the austenite recrystallization zone. The cumulative reduction rate in the rough rolling stage is 54%, the final rolling temperature is 1078°C, and the thickness of the intermediate billet obtained by the rough rolling is 182mm.

[0153] The finishing rolling stage is carried out in two passes in the austenite non-recrystallization zone. The cumulative reduction rate in the finishing rolling stage is 47%, the final rolling temperature is 883°C, and the thickness of the intermediate billet obtained by the finishing rolling is 96mm. In the whole rolling process, high-pressure water is used to remove phosphorus.

[0154] 5. Use ultra-fast cooling technology for water-cooling quenching. The specific parameter conditions are: the starting cooling temperature is 797°C, the final cooling temperature is 630°C, and the cooling rate is 30°C / s;

[0155] 6. Tempering water cooling: Heat the steel plate to 675℃, and keep it warm for 100 minutes after the core of the steel plate reaches the furnace temperature. After it comes out of the furnace, it is quickly water-cooled to room temperature to obtain a large-size, thick-gauge, high-strength and tough hydropower steel plate with a tensile strength of 829MPa.

[0156] The large-size, thick-gauge 800MPa-grade high-strength and toughness hydropower steel plate prepared in this embodiment has a thickness of 96mm, and the structure is tempered troostite. The average effective grain size is 7.9μm, and the average lath width of the recovered martensite is 256nm. Since there is no re-austenitization process, the original austenite grains show a flat deformation feature, and the average original austenite grain size is 28.8μm.

[0157] Effect verification

[0158] The production standards and performance testing methods of the large-size, thick-gauge, 800MPa-grade high-strength and tough hydropower steel plates prepared in Examples 1-4 were carried out with reference to the national standard GB / T 31946-2015 "Steel Plates for Hydropower Station Penstocks", and room temperature tensile tests, hot compression tests, -40°C low-temperature impact tests, and -60°C low-temperature impact tests were performed. The specific test results are shown in Table 1.

[0159] Table 1

[0160]

[0161] In summary, the method of the present invention optimizes the short-process preparation method of billet smelting + continuous casting + homogenization + two-stage controlled rolling + online quenching + tempering water cooling to achieve the steel plate with high strength, high and low temperature impact toughness and high elongation. It can meet the requirements of hydropower steel for high strength, high and low temperature impact toughness, high elongation and take into account the application scenarios of hydropower steel, and has good corrosion resistance. The method is efficient, has simple influencing factors, and has a wide range of applications, which is conducive to large-scale industrial production and promotion.

[0162] The above are only preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A large-size, thick, 800MPa grade high-strength and tough hydropower steel plate, characterized in that: The chemical composition, by mass percentage, is: C: 0.06-0.12%, Mn: 0.80-1.50%, Si: 0.12-0.30%, Cu: 0.2-0.3%, Cr: 0.3-0.6%, Mo: 0.3-0.6%, B: 0.0012-0.0020%, Ti: 0.008-0.016%, Nb: 0.015-0.040%, Alt: 0.025-0.060%, Ni: 1.80-2.50%, P≤0.01%, S≤0.003%, and the balance is Fe and other inevitable impurity elements.

2. The large-size, thick-gauge, 800MPa-grade high-strength and tough hydropower steel plate according to claim 1 is characterized in that: The large-size, thick-gauge, 800MPa-grade high-strength and tough hydropower steel plate has a thickness of 90 to 140 mm; its metallographic structure is all tempered troostite, and the average effective grain size is less than 8.5 μm.

3. The large-size, thick-gauge, 800MPa-grade high-strength and tough hydropower steel plate according to claim 1 is characterized in that: The large-size, thick-gauge, 800MPa-grade high-strength and tough hydropower steel plate has a yield strength of 792-843MPa, a tensile strength of 837-882MPa, an elongation of 19.6-22.6%, a strength-ductility product of 16.512-18.396GPa·%, an impact energy of -40°C>210J, and an impact energy of -60°C>170J.

4. A method for preparing a large-size, thick, 800MPa grade high-strength and tough hydropower steel plate, characterized in that: The following steps are involved: Mix and weigh the raw materials according to their chemical composition and mass percentage; After the raw materials are smelted to obtain molten iron, heat treatment, converter smelting and slab continuous casting are sequentially performed to obtain hydropower steel continuous casting slabs; The hydropower steel continuous casting billet is then subjected to homogenization treatment, two-stage temperature-controlled hot rolling, water-cooling quenching and tempering rapid cooling in sequence to prepare the large-size thick specification 800MPa grade high-strength and toughness hydropower steel plate as described in any one of claims 1 to 3; The converter smelting includes SGRS process dephosphorization, LF refining and RH vacuum degassing.

5. The method for preparing a large-size, thick, 800MPa grade high-strength and tough hydropower steel plate according to claim 4, characterized in that: The conditions in the LF refining process are: the time of the early stage of heating and slag removal is 10 to 15 minutes, the dust cover opening is 40% to 60%; and / or, The conditions in the RH vacuum degassing process are: the RH net cycle is not less than 10 minutes, and a composite deoxidizer is added during this process to perform deep deoxidation and inclusion modification treatment; the standing time before pouring shall not be less than 15 minutes.

6. The method for preparing a large-size, thick, 800MPa high-strength and tough hydropower steel plate according to claim 4, characterized in that: The conditions during the slab continuous casting process are: the superheat of the molten steel is below 30° C., and the continuous casting slab drawing speed is 1.0 m / min.

7. The method for preparing a large-size, thick, 800MPa grade high-strength and tough hydropower steel plate according to claim 4, characterized in that: The conditions during the homogenization treatment are as follows: the heat treatment temperature is 1200-1250°C and the time is 1.5-3h; After the homogenization treatment, the hard particles are mixed with water to remove phosphorus and iron oxide.

8. The method for preparing a large-size, thick, 800MPa high-strength and tough hydropower steel plate according to claim 4, characterized in that: The two-stage temperature-controlled hot rolling includes a rough rolling stage and a finishing rolling stage; The rough rolling stage is carried out in two passes in the austenite recrystallization zone, the cumulative reduction rate in the rough rolling stage is 50-60%, the final rolling temperature is 1000-1100°C, and the thickness of the intermediate billet obtained by the rough rolling is 160-250mm; The finishing rolling stage is carried out in two passes in the austenite non-recrystallization zone, the cumulative reduction rate in the finishing rolling stage is 44-50%, the final rolling temperature is 850-900°C, and the thickness of the intermediate billet obtained by the finishing rolling is 90-140mm.

9. The method for preparing a large-size, thick, 800MPa grade high-strength and tough hydropower steel plate according to claim 4, characterized in that: The water-cooled quenching adopts ultra-fast cooling technology, and the parameter conditions in the process are: the starting cooling temperature is 780-830°C, the final cooling temperature is 600-640°C, and the cooling rate is 15-35°C / s.

10. The method for preparing a large-size, thick, 800MPa grade high-strength and tough hydropower steel plate according to claim 4, characterized in that: The tempering rapid cooling process is as follows: heating the steel plate to 600-680° C., keeping the steel plate warm for 90-120 minutes after the core of the steel plate reaches the furnace temperature; and rapidly cooling the steel plate to room temperature with water after the steel plate is taken out of the furnace.

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