1000MPa-grade hydroelectric steel with optimized low-temperature toughness and preparation method of 1000MPa-grade hydroelectric steel

Abstract

The invention discloses 1000MPa-grade hydroelectric steel with optimized low-temperature toughness and a preparation method of the 1000MPa-grade hydroelectric steel, and belongs to the technical field of steel production. The 1000MPa-grade hydroelectric steel with the optimized low-temperature toughness is prepared from the following chemical components in percentage by weight: 0.03 to 0.09 percent of C, 0.80 to 1.40 percent of Mn, 0.10 to 0.15 percent of Si, 0.05 to 0.14 percent of Cu, less than or equal to 0.003 percent of S, less than or equal to 0.01 percent of P, 0.01 to 0.04 percent of Al, 0.03 to 0.10 percent of V, 0.03 to 0.06 percent of Cr, 0.3 to 0.65 percent of Mo, 0.001 to 0.002 percent of B, 0.01 to 0.018 percent of Ti, 0.015 to 0.025 percent of Nb, 0.18 to 0.60 percent of Ni and the balance of Fe and other inevitable impurities. The preparation method comprises a steel-making process, a heating process, a rough phosphorus removal process, a rolling process and a heat treatment process. By optimizing the chemical components and the preparation process of the 1000MPa-grade hydroelectric steel, the low-temperature toughness of the 1000MPa-grade hydroelectric steel is remarkably improved on the basis that the 1000MPa-grade hydroelectric steel with high strength is obtained, and the 1000MPa-grade hydroelectric steel is suitable for industrial application.

Description

Technical Field

[0001] The invention belongs to the technical field of steel production, and more specifically relates to a 1000MPa-grade hydroelectric steel with optimized low-temperature toughness and a preparation method thereof. Background Art

[0002] As hydropower stations develop towards large capacity and ultra-high head, the performance requirements for hydropower steel are increasing, especially in terms of strength, plasticity, toughness and welding performance. It involves multiple aspects such as energy demand, engineering development, material performance, economy, construction convenience, corrosion resistance, technological innovation, standard formulation, and national major engineering needs. However, although the 800MPa-grade hydropower steel in the existing technology has been applied in some fields, its performance still cannot fully meet the needs of high head and large capacity hydropower station construction. In order to promote the rapid development of the hydropower industry, it is particularly urgent to develop 1000MPa-grade hydropower steel with higher strength and better welding performance. Domestic enterprises such as Nanjing Iron and Steel and Baosteel have successfully developed 1000MPa-grade high-strength hydropower steel that meets the requirements through years of technical research and have achieved mass production. However, most of the existing 1000MPa-grade high-strength hydropower steel plates have poor low-temperature toughness, which leads to increased sensitivity to welding cracks and is difficult to apply industrially. In addition, although some 1000MPa high-strength hydropower steel plates have low cold crack sensitivity and good low-temperature toughness, their strength is low, which is far from the market demand for 1000MPa hydropower steel. Therefore, it is of great significance to develop a 1000MPa hydropower steel with high strength and excellent low-temperature toughness. Summary of the invention

[0003] The purpose of the present invention is to provide a 1000MPa grade hydropower steel with optimized low-temperature toughness and a preparation method thereof, so as to solve the problems existing in the above-mentioned prior art and realize the preparation of 1000MPa grade hydropower steel with higher strength and excellent low-temperature toughness.

[0004] To achieve the above object, the present invention provides the following solutions:

[0005] One of the technical solutions of the present invention is to provide a 1000MPa grade hydroelectric steel with optimized low temperature toughness, which comprises the following chemical components by weight percentage:

[0006] C: 0.03~0.09%, Mn: 0.80~1.40%, Si: 0.10~0.15%, Cu: 0.05~0.14%, S≤0.003%, P≤0.01%, Al: 0.01~0.04%, V: 0.03~0.10%, Cr: 0.03~0.06%, Mo: 0.3~0.65%, B: 0.001~0.002%, Ti: 0.01~0.018%, Nb: 0.015~0.025%, Ni: 0.18~0.60%, the balance is Fe and other inevitable impurities.

[0007] The technical principle of the chemical composition design of the present invention is as follows:

[0008] The present invention controls the carbon content to 0.03-0.09%. The role of C is to provide a solid solution strengthening effect, and to form carbonitrides in combination with elements such as Nb to ensure the tensile strength of the material. Therefore, the present invention stipulates that the minimum limit of the carbon content is 0.03%. However, if the C content is too high, it will not only promote the formation of martensite (MA), but also lead to the formation of pearlite, which will cause the impact properties of the material at a specific temperature to decrease. Therefore, the present invention stipulates that the maximum limit of the carbon content is 0.09%.

[0009] The present invention controls the content of silicon to 0.1-0.15%. The role of Si is to assist Al in deoxidation of molten steel and is a key element to ensure yield strength and tensile strength, so the present invention stipulates that the minimum value of the silicon content is not less than 0.1%. However, too high Si content will hinder the diffusion of C and promote the formation of martensite (MA), which will lead to a decrease in impact properties under low temperature conditions, so the present invention limits the maximum value of the silicon content to 0.15%.

[0010] The present invention controls the Al content to 0.01-0.04%. Al plays a major deoxidation role in steel, so in order to meet the solubility requirements, at least 0.01% Al needs to be added. However, if too much aluminum is added, the number and size of aluminum oxide (Al2O3) inclusions may increase, which will reduce the low-temperature toughness of the material. Similar to Si, excessive Al will further reduce the low-temperature toughness by promoting the formation of martensite (MA) phase in the matrix and the heat-affected zone of the weld. Therefore, the present invention limits the Al content to no more than 0.04% in a dissolved state.

[0011] The present invention controls the content of Mn to 0.80-1.40%. As an element that improves the strength of materials through solid solution strengthening, the minimum content of Mn is set to 0.80% to achieve the above-mentioned strengthening effect. However, if the content of Mn is too high, it may lead to the formation of manganese sulfide (MnS) inclusions and reduce the toughness of the material due to the center segregation phenomenon. Therefore, the present invention sets the maximum value of the Mn content to 1.40%.

[0012] The present invention controls the content of Nb to 0.015-0.025%. Nb plays a role in preventing recrystallization during rolling and cooling by precipitation strengthening or forming carbonitrides, thereby refining the organizational structure and enhancing the rigidity of the material. In order to achieve this effect, the present invention stipulates that the minimum content of Nb shall not be less than 0.015%. However, if the content of Nb is too high, due to its strong affinity with C, it may cause local concentration of C, thereby promoting the formation of martensite (MA) phase, which will have an adverse effect on low-temperature toughness and the overall performance of the material. Therefore, the present invention sets the maximum content of Nb to 0.025%.

[0013] The present invention controls the Ni content to 0.18-0.60%. Ni is an element that can improve the stiffness of the material without sacrificing impact toughness, and it also helps to form acicular ferrite. In order to achieve these effects, the present invention stipulates that the minimum Ni content shall not be less than 0.18%. However, if the Ni content is too high, it may cause the Ar3 temperature to decrease, thereby promoting the formation of bainite, which may reduce the impact toughness of the material in some cases. Therefore, the present invention sets the maximum Ni content to 0.60% to avoid the risk of reduced impact toughness when forming bainite.

[0014] Preferably, the contents of C, Si and Al in the chemical composition of the 1000MPa grade hydropower steel with optimized low-temperature toughness satisfy the relationship: 0.23%≤[C]+[Si]+10×[Al]≤0.64%, wherein [C], [Si] and [Al] represent the content of each chemical component in weight percentage.

[0015] When [C]+[Si]+10×[Al]<0.23%, the yield strength of the obtained hydroelectric steel is less than 900MPa, and a higher yield strength cannot be achieved. When [C]+[Si]+10×[Al]>0.64%, the formation of martensite (MA) in the obtained hydroelectric steel increases, reducing the low-temperature toughness of the material, thereby reducing the impact resistance of the material. The present invention aims to control the relative contents of C, Si and Al so that the relative contents of C, Si and Al satisfy the relationship: 0.23%≤[C]+[Si]+10×[Al]≤0.64%, thereby ensuring that the obtained steel has excellent strength and low-temperature toughness.

[0016] Preferably, the contents of Mn, Ni and Nb in the chemical composition of the 1000MPa grade hydropower steel with optimized low-temperature toughness satisfy the relationship: 1.31%≤[Mn]+2×[Ni]+10×[Nb]≤2.85%, wherein [Mn], [Ni] and [Nb] represent the content of each chemical component in weight percentage.

[0017] The present invention controls the contents of Mn, Ni and Nb to satisfy the relationship: 1.31%≤[Mn]+2×[Ni]+10×[Nb]≤2.85%, ensuring that the content of acicular ferrite is 20-30% (area percentage). The organization of acicular ferrite is a chain structure, which can well prevent the expansion of cracks and make the steel have good toughness. The content of 20-30% (area percentage) can ensure the strength while making the hydropower steel have stronger fracture resistance when it is impacted or vibrated, reduce the risk of brittle fracture of components during use, and improve the safety of hydropower equipment operation.

[0018] In summary, the present invention can optimize the low temperature plasticity and toughness of the steel while ensuring the strength by controlling the contents of the above chemical components. In addition, the low carbon content helps to improve the welding performance, while trace amounts of V and Ti can improve the hardenability of the steel.

[0019] The second technical solution of the present invention is to provide a method for preparing the 1000MPa grade hydroelectric steel with optimized low temperature toughness, comprising the following steps:

[0020] S1, steelmaking process: batching and weighing the raw materials according to the mass percentage of the above chemical components, smelting to obtain molten steel, and continuously casting the molten steel to obtain a continuous casting billet;

[0021] S2, heating process: heating the continuous casting billet;

[0022] S3, rough dephosphorization: removing the furnace-born iron oxide scale on the surface of the continuous casting billet after the heating treatment to obtain a steel billet;

[0023] S4, rolling process: rolling the steel billet using a two-stage controlled rolling process to obtain a steel plate;

[0024] S5. Heat treatment process: heat-treating the steel plate by a double quenching + tempering heat treatment process to obtain the 1000MPa grade hydropower steel.

[0025] Preferably, in step S1, the smelting includes converter blowing, LF refining and VD treatment, specifically including: firstly performing converter blowing with a tapping temperature ≥1590°C, then performing LF refining, ensuring that the white slag retention time is greater than 30 minutes during LF refining, and finally performing VD treatment with a VD treatment time of more than 30 minutes to remove gas impurities such as nitrogen and hydrogen in the molten steel and improve the purity of the steel; the continuous casting includes: pouring the molten steel into a continuous casting billet with a thickness of 150 to 240 mm through a continuous casting machine, controlling the pulling speed and superheat during this process, and using argon to seal the butt joint to ensure the quality of the continuous casting billet.

[0026] Furthermore, during the converter blowing process, the carbon content and temperature in the molten steel are precisely controlled to meet the requirements of subsequent processing steps, ensuring that the steel tapping temperature is ≥1590°C while preventing slag from being generated during the steel tapping process.

[0027] Furthermore, in the LF furnace, a further refining treatment stage is carried out to purify the molten steel more deeply and adjust its chemical composition by methods such as heating, slagging, controlling arc operation, adjusting the opening size of the dust removal cover and the bottom blowing gas mode, ensuring that the white slag is retained for more than 30 min.

[0028] Furthermore, when LF refining is nearing the end, it also includes the step of calcium treatment; the calcium treatment includes: when calcium treatment is carried out in LF refining, first of all, preparations must be made to confirm that the temperature of the molten steel reaches 1600±10℃ and the composition meets the requirements, and at the same time, the wire feeder, argon supply system and other equipment are checked to ensure that the length of the wire feeding conduit is ≥1.4m and the conduit diameter is within 0.1-0.15m. Then, the slag oxidation is reduced. When the LF furnace is heated, white ash is added in batches to make slag. When the LF furnace is heated for the first time, an appropriate amount of aluminum particles is added to remove oxygen from the slag. During the subsequent stirring, deoxidation and desulfurization, the changes in the slag are observed and the slag deoxidizer is added in time to maintain the white slag to ensure that FeO+MnO in the slag is less than 1%. Then, the calcium feeding line operation is carried out. In the early stage of wire feeding, the argon flow rate is controlled at 100-150L / min for the double line, and in the later stage, it is controlled at 30-80L / min. The wire feeding speed of the high calcium wire is 1.4-1.6m / s, and the wire feeding speed of the iron calcium wire is 1.8-2.2m / s. The wire feeding amount is determined according to the steel type and the composition of the molten steel. After the wire feeding is completed, the soft blowing operation is carried out, and the argon flow rate is adjusted to 16-200L / min for the double line. The molten steel is not exposed when the slag surface moves slightly. The soft blowing time is maintained for 7-9 minutes, so that the calcium reacts fully with the inclusions in the molten steel, promotes the floating of the inclusions and is adsorbed by the refined slag. During the entire calcium treatment process, the calcium content in the molten steel and the changes in inclusions are monitored in real time using a spectrometer. After the soft blowing is completed, sampling and analysis are carried out to detect the composition of the molten steel, the shape and quantity of inclusions, etc., to determine whether the treatment effect meets the standard, and provide a basis for subsequent production operations. One of the main purposes of calcium treatment near the end of LF refining is to change the morphology of inclusions in the steel. The alumina (Al2O3) inclusions originally present in the molten steel have a high melting point and high hardness. During the subsequent processing and use of the steel, these sharp inclusions are prone to cause stress concentration, resulting in a decrease in steel performance. Calcium treatment can transform alumina inclusions into liquid calcium aluminates (such as 12CaO·7Al2O3). This type of inclusion is spherical and has good fluidity. It is easy to float up and be removed in the molten steel, thereby improving the purity of the steel. Another purpose is to prevent nozzle blockage: During the continuous casting process, alumina inclusions in the molten steel tend to gather and solidify at the nozzle of the tundish, causing nozzle blockage. After the alumina inclusions are denatured by calcium treatment, this situation can be effectively avoided and the smooth progress of the continuous casting process can be ensured.

[0029] Preferably, in step S2, the temperature of the heating treatment is 1150-1250°C, and the heating coefficient is 10-14 min / cm. The heating treatment conditions can ensure that the continuous casting billet is fully austenitized, the grain size is uniform, and the heating uniformity of the continuous casting billet is ensured.

[0030] Preferably, in step S3, the removal method includes water jet erosion.

[0031] Furthermore, the water jet erosion method includes: using high-pressure water generated by a high-pressure water pump to form a high-speed jet through a nozzle to hit the surface of the heated continuous casting billet, and under the erosion and thermal explosion effect of the jet, the furnace-born iron oxide scale on the surface of the heated continuous casting billet is removed.

[0032] Preferably, in step S4, the two-stage controlled rolling process includes a rough rolling stage and a finishing rolling stage; the parameters of the rough rolling stage are controlled as follows: the starting rolling temperature is ≥1150°C, the final rolling temperature is ≥1000°C, and the total reduction rate is 40-50%; the parameters of the finishing rolling stage are controlled as follows: the starting rolling temperature is 840-880°C, the final rolling temperature is 800-850°C, and the total reduction rate is 15-30%.

[0033] Furthermore, after the rolling is completed, the step of air cooling to room temperature is also included.

[0034] Preferably, in step S5, the double quenching + tempering heat treatment process includes the first quenching, the second quenching and tempering; the temperature of the first quenching and the second quenching is 850-930°C, and the holding time is 1.5-2.5 min / mm; the temperature of the tempering is 600-650°C, and the holding time is 2.0-3.5 min / mm. The corresponding organization is obtained by quenching treatment to ensure the strength and low-temperature impact resistance of the steel. The tempering treatment can greatly reduce the internal stress generated by quenching, reduce the surface hardness of the steel, and improve the low-temperature toughness, so that the steel has good strength and toughness.

[0035] The technical principle of the production process of the present invention is as follows:

[0036] 1. Use advanced smelting technologies such as converter smelting and LF refining to ensure the purity and uniformity of molten steel.

[0037] 2. Through the two-stage controlled rolling process, the first stage (rough rolling stage) achieves a large total reduction rate to obtain fine grains, and the second stage (finishing rolling stage) uses low-temperature rolling technology to further refine the grains. This rolling method helps to improve the strength and low-temperature toughness of steel.

[0038] 3. The heat treatment process includes offline quenching and tempering heat treatment, among which quenching and tempering processes are crucial to obtain the desired microstructure and properties. Precise control of quenching temperature and time, as well as adjustment of tempering temperature and time, can optimize the yield strength, tensile strength and impact toughness of steel.

[0039] The microstructure of the 1000MPa-grade hydropower steel with optimized low-temperature toughness described in the present invention includes tempered troostite, tempered martensite and ferrite; the yield strength of the 1000MPa-grade hydropower steel with optimized low-temperature toughness is 880-930MPa, the tensile strength is 980-1050MPa, and the impact energy at -60°C is ≥80J.

[0040] The third technical solution of the present invention is to provide the application of the 1000MPa grade hydropower steel with optimized low-temperature toughness in the hydropower industry.

[0041] The fourth technical solution of the present invention: provides a method for optimizing the low-temperature toughness of 1000MPa grade hydropower steel, specifically: preparing 1000MPa grade hydropower steel by the above-mentioned preparation method.

[0042] The present invention discloses the following technical effects:

[0043] 1. The steel plate has obtained a high strength of 1000MPa and good toughness through quenching and tempering (double quenching + tempering), which is much higher than traditional hydropower steel and can meet the strict requirements for material properties in hydropower projects. Especially in the case of large thickness, it still maintains good mechanical properties and is suitable for withstanding high water pressure and other complex working conditions. In addition, the steel still maintains good toughness in low temperature environments and can effectively resist low-temperature brittle fracture. This is very important for extreme climatic conditions that may be encountered in hydropower projects (such as winter construction in cold areas), ensuring the safety and reliability of the material.

[0044] 2. The present invention optimizes the chemical composition of steel, especially controls the carbon equivalent (C eq ), the content of alloying elements (such as Mn, Ni, Cr, Mo, etc.), to ensure the carbon equivalent C eq ≤0.53%, crack sensitivity P cm ≤0.25%, so that the steel plate has low cold crack sensitivity and good low-temperature toughness while ensuring strength. This helps to improve the adaptability of the steel plate in complex environments. At the same time, the present invention adds Al and Ti elements to synergistically slow down the nitridation process of B, hinder the precipitation of BN, give full play to the precipitation of free boron at the original austenite grain boundary, and improve the hardenability of the steel plate.

[0045] 3. The present invention adds V to the composition design. Due to the stronger carbide-forming ability of V, it will preferentially combine with carbon to form VC (vanadium carbide) carbides. The addition of VC particles helps to improve the mechanical properties of steel, including bending strength and hardness. Under a specific VC ratio and insulation time, the maximum material strength can be achieved. At the same time, the addition of VC particles promotes the formation of M2C carbides, which will undergo a decomposition reaction during the sintering process to generate more stable M6C and MC carbides, thereby improving the strength and low-temperature toughness of the steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 The flowchart of the preparation of the 1000MPa grade hydroelectric steel with optimized low temperature toughness according to the present invention;

[0047] Figure 2 This is a schematic diagram of the metallographic structure at 1 / 4 of the plate thickness in Example 1;

[0048] Figure 3 This is a schematic diagram of the metallographic structure at 1 / 4 of the plate thickness in Example 2;

[0049] Figure 4 This is a schematic diagram of the metallographic structure at 1 / 4 of the plate thickness in Example 3. DETAILED DESCRIPTION

[0050] 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.

[0051] It should be understood that the terms described in the present invention are only for describing a particular embodiment 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. The intermediate value in any stated value or stated range, and each smaller range between 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.

[0052] 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.

[0053] 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.

[0054] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0055] Most existing 1000MPa high-strength hydropower steel plates have poor low-temperature toughness, which increases welding crack sensitivity and makes them difficult to apply industrially. In addition, although some 1000MPa high-strength hydropower steel plates have low cold crack sensitivity and good low-temperature toughness, their strength is low, which is far from the market demand for 1000MPa hydropower steel.

[0056] Compared with the prior art, the present invention optimizes the chemical composition and preparation process of 1000MPa-grade hydropower steel, thereby obtaining 1000MPa-grade hydropower steel with higher strength and significantly improving its low-temperature toughness, making it suitable for industrial application.

[0057] The room temperature in the following examples and comparative examples of the present invention is 25±5° C. unless otherwise specified.

[0058] Unless otherwise specified, the raw materials used in the following examples and comparative examples of the present invention are all commercially available products.

[0059] The calcium treatment described in the following embodiments and comparative examples of the present invention is specifically as follows: when calcium treatment is carried out in LF refining, first of all, preparations must be made to confirm that the temperature of the molten steel reaches 1600±10°C and the composition and the like meet the requirements, and at the same time, the wire feeder, argon supply system and other equipment are checked to ensure that the length of the wire feeding conduit is ≥1.4m and the conduit diameter is within 0.1-0.15m. Subsequently, the slag oxidation reduction operation is carried out, white ash is added in batches to make slag during the heating of the LF furnace, an appropriate amount of aluminum particles are added for the first heating to remove oxygen from the slag, and the slag changes are observed during subsequent stirring, deoxidation and desulfurization, and slag deoxidizer is added in time to maintain white slag, ensuring that FeO+MnO in the slag is less than 1%. Then, the calcium wire feeding operation is carried out, and the argon flow rate is controlled at 100-150L / min for the double wire in the early stage of wire feeding, and controlled at 30-80L / min in the later stage, the wire feeding speed of the high calcium wire is 1.4-1.6m / s, and the wire feeding speed of the iron calcium wire is 1.8-2.2m / s, and the wire feeding amount is determined according to the steel type and the composition of the molten steel. After the wire feeding is completed, the soft blowing operation is carried out, and the argon flow rate is adjusted to 16-200L / min for two lines. The molten steel is not exposed when the slag surface is slightly moved. The soft blowing time is maintained for 7-9 minutes to allow calcium to fully react with inclusions in the molten steel, promote the inclusions to float up and be adsorbed by the refined slag. During the entire calcium treatment process, the spectrometer is used to monitor the calcium content and inclusion changes in the molten steel in real time. After the soft blowing is completed, sampling and analysis are carried out to detect the composition of the molten steel, the shape and quantity of inclusions, etc., to determine whether the treatment effect meets the standard, and provide a basis for subsequent production operations.

[0060] Figure 1 This is a flow chart for preparing the 1000MPa grade hydropower steel with optimized low-temperature toughness according to the present invention.

[0061] Example 1

[0062] This embodiment provides a 1000MPa grade hydropower steel with optimized low temperature toughness, and the thickness of the produced steel plate is 40mm. Its chemical composition by weight percentage is: C: 0.03%, Mn: 1.40%, Si: 0.15%, Cu: 0.14%, S: 0.002%, P: 0.007%, Al: 0.04%, V: 0.09%, Cr: 0.05%, Mo: 0.60%, B: 0.002%, Ti: 0.016%, Nb: 0.025%, Ni: 0.60%, and the balance is Fe and other inevitable impurities.

[0063] After calculation, [C]+[Si]+10×[Al]=0.58%, wherein [C], [Si] and [Al] represent the content of each chemical component in weight percentage, satisfying the range corresponding to the relationship.

[0064] After calculation, [Mn]+2×[Ni]+10×[Nb]=2.85%, wherein [Mn], [Ni] and [Nb] represent the content of each chemical component in weight percentage, satisfying the range corresponding to the relationship.

[0065] The preparation method is as follows:

[0066] S1. Steelmaking process: The raw materials are batched and weighed according to the mass percentage of the above chemical components, and then smelted. First, converter steelmaking is carried out, which is the initial stage of steel production. Blowing is carried out in the converter to accurately control the carbon content and temperature in the molten steel to meet the needs of subsequent processing steps. The tapping temperature is 1590°C; at the same time, slag is prevented from being generated during the tapping process. Next is the LF refining stage, in which further refining treatment is carried out in the LF furnace. By heating, slag removal, arc operation control, adjustment of the opening size of the dust removal cover and bottom blowing gas mode, the molten steel is purified at a deeper level and its chemical composition is adjusted to ensure that the white slag is maintained for 35 minutes. Calcium treatment is carried out when the LF refining is nearing the end. VD treatment is then carried out, that is, degassing treatment is carried out in a high vacuum environment for 35 minutes to remove gas impurities such as nitrogen and hydrogen in the molten steel and improve the purity of the steel. The last step is continuous casting, where the refined molten steel is cast into billets through a continuous casting machine. During this process, the pulling speed and superheat are controlled, and argon is used to seal the joints to ensure the quality of the billet, which is then cast into a continuous casting billet with a thickness of 150 mm.

[0067] S2. Heating process: The heating temperature of the continuous casting billet in the heating furnace is controlled at 1150°C, and the heating coefficient is controlled at 1.4min / mm to ensure the heating uniformity of the continuous casting billet.

[0068] S3, rough dephosphorization: high-pressure water generated by a high-pressure water pump is used to form a high-speed jet through a nozzle to hit the surface of the steel billet obtained in step S2. Under the erosion and thermal explosion effect of the jet, the furnace-generated iron oxide scale on the surface of the steel billet obtained in step S2 is removed.

[0069] S4. Rolling process: A two-stage controlled rolling process is adopted, which is divided into a rough rolling stage and a finishing rolling stage. In the first stage (rough rolling stage), the slab starting rolling temperature is 1150°C, the final rolling temperature is 1012°C, and the total reduction rate is 45%; the second stage (finishing rolling stage) adopts low-temperature rolling technology, with a starting rolling temperature of 840°C, a final rolling temperature of 800°C, and a total reduction rate of 25%; the rolled steel plate is cooled to room temperature in air.

[0070] S5. Heat treatment process: After rolling, the steel plate is heat treated by double quenching + tempering, that is, the first quenching is performed first, then the second quenching, and finally the tempering. The temperature of the first quenching and the second quenching is 850℃, and the holding time is 1.6min / mm; the tempering temperature is 610℃, and the holding time is 2.3min / mm.

[0071] The metallographic structure of the steel plate obtained in this embodiment at 1 / 4 of the thickness is as follows Figure 2 As shown. Figure 2 It can be seen that the microstructure of the steel plate is tempered troostite (S), a small amount of tempered martensite (MA) and a small amount of ferrite (F).

[0072] Example 2

[0073] This embodiment provides a 1000MPa grade hydropower steel with optimized low temperature toughness, and the thickness of the produced steel plate is 60mm. Its chemical composition by weight percentage is: C: 0.06%, Mn: 1.20%, Si: 0.12%, Cu: 0.10%, S: 0.001%, P: 0.006%, Al: 0.02%, V: 0.06%, Cr: 0.04%, Mo: 0.45%, B: 0.0015%, Ti: 0.015%, Nb: 0.02%, Ni: 0.40%, and the balance is Fe and other inevitable impurities.

[0074] After calculation, [C]+[Si]+10×[Al]=0.38%, wherein [C], [Si] and [Al] represent the content of each chemical component in weight percentage, satisfying the range corresponding to the relationship.

[0075] After calculation, [Mn]+2×[Ni]+10×[Nb]=2.2%, wherein [Mn], [Ni] and [Nb] represent the content of each chemical component in weight percentage, satisfying the range corresponding to the relationship.

[0076] The preparation method is as follows:

[0077] S1. Steelmaking process: The raw materials are batched and weighed according to the mass percentage of the above chemical components, and then smelted. First, converter steelmaking is carried out, which is the initial stage of steel production. Blowing is carried out in the converter to accurately control the carbon content and temperature in the molten steel to meet the needs of subsequent processing steps. The tapping temperature is 1600°C; at the same time, slag is prevented from being generated during the tapping process. Next is the LF refining stage, in which further refining treatment is carried out in the LF furnace. By heating, slag removal, controlling arc operation, adjusting the opening size of the dust removal cover and the bottom blowing gas mode, the molten steel is purified at a deeper level and its chemical composition is adjusted to ensure that the white slag is maintained for 33 minutes. Calcium treatment is carried out when the LF refining is nearing the end. VD treatment is then carried out, that is, degassing treatment is carried out in a high vacuum environment for 32 minutes to remove gas impurities such as nitrogen and hydrogen in the molten steel and improve the purity of the steel. The last step is continuous casting, where the refined molten steel is cast into billets through a continuous casting machine. During this process, the pulling speed and superheat are controlled, and argon is used to seal the joints to ensure the quality of the billet, which is then cast into a continuous casting billet with a thickness of 200 mm.

[0078] S2. Heating process: The heating temperature of the continuous casting billet in the heating furnace is controlled at 1200°C, and the heating coefficient is controlled at 1.3min / mm to ensure the heating uniformity of the continuous casting billet.

[0079] S3, rough dephosphorization: high-pressure water generated by a high-pressure water pump is used to form a high-speed jet through a nozzle to hit the surface of the steel billet obtained in step S2. Under the erosion and thermal explosion effect of the jet, the furnace-generated iron oxide scale on the surface of the steel billet obtained in step S2 is removed.

[0080] S4. Rolling process: A two-stage controlled rolling process is adopted, which is divided into a rough rolling stage and a finishing rolling stage. In the first stage (rough rolling stage), the slab starting rolling temperature is 1200°C, the final rolling temperature is 1020°C, and the total reduction rate is 40%; the second stage (finishing rolling stage) adopts low-temperature rolling technology, with a starting rolling temperature of 863°C, a final rolling temperature of 825°C, and a total reduction rate of 20%; the rolled steel plate is cooled to room temperature in air.

[0081] S5. Heat treatment process: After rolling, the steel plate is heat treated by double quenching + tempering, that is, the first quenching is performed first, then the second quenching, and finally the tempering. The temperature of the first quenching and the second quenching is 889℃, and the holding time is 1.9min / mm; the tempering temperature is 620℃, and the holding time is 2.5min / mm.

[0082] The metallographic structure of the steel plate obtained in this embodiment at 1 / 4 of the thickness is as follows Figure 3 As shown. Figure 3 It can be seen that the microstructure of the steel plate is tempered troostite (S), a small amount of tempered martensite (MA) and a small amount of ferrite (F).

[0083] Example 3

[0084] This embodiment provides a 1000MPa grade hydropower steel with optimized low temperature toughness, and the thickness of the produced steel plate is 60mm. Its chemical composition by weight percentage is: C: 0.09%, Mn: 1.00%, Si: 0.10%, Cu: 0.05%, S: 0.001%, P: 0.003%, Al: 0.01%, V: 0.03%, Cr: 0.03%, Mo: 0.30%, B: 0.001%, Ti: 0.012%, Nb: 0.015%, Ni: 0.20%, and the balance is Fe and other inevitable impurities.

[0085] After calculation, [C]+[Si]+10×[Al]=0.29%, wherein [C], [Si] and [Al] represent the content of each chemical component in weight percentage, satisfying the range corresponding to the relationship.

[0086] After calculation, [Mn]+2×[Ni]+10×[Nb]=1.55%, wherein [Mn], [Ni] and [Nb] represent the content of each chemical component in weight percentage, satisfying the range corresponding to the relationship.

[0087] The preparation method is as follows:

[0088] S1. Steelmaking process: The raw materials are batched and weighed according to the mass percentage of the above chemical components, and then smelted. First, converter steelmaking is carried out, which is the initial stage of steel production. Blowing is carried out in the converter to accurately control the carbon content and temperature in the molten steel to meet the needs of subsequent processing steps. The tapping temperature is 1610°C; at the same time, slag is prevented from being generated during the tapping process. Next is the LF refining stage, in which further refining treatment is carried out in the LF furnace. By heating, slag removal, controlling arc operation, adjusting the opening size of the dust removal cover and the bottom blowing gas mode, the molten steel is purified at a deeper level and its chemical composition is adjusted to ensure that the white slag is maintained for 34 minutes. Calcium treatment is carried out when the LF refining is nearing the end. VD treatment is then carried out, that is, degassing treatment is carried out in a high vacuum environment for 30 minutes to remove gas impurities such as nitrogen and hydrogen in the molten steel and improve the purity of the steel. The last step is continuous casting, where the refined molten steel is cast into billets through a continuous casting machine. During this process, the pulling speed and superheat are controlled, and argon is used to seal the joints to ensure the quality of the billet, which is then cast into a continuous casting billet with a thickness of 240 mm.

[0089] S2. Heating process: The heating temperature of the continuous casting billet in the heating furnace is controlled at 1250°C, and the heating coefficient is controlled at 1.3min / mm to ensure the heating uniformity of the continuous casting billet.

[0090] S3, rough dephosphorization: high-pressure water generated by a high-pressure water pump is used to form a high-speed jet through a nozzle to hit the surface of the steel billet obtained in step S2. Under the erosion and thermal explosion effect of the jet, the furnace-generated iron oxide scale on the surface of the steel billet obtained in step S2 is removed.

[0091] S4. Rolling process: A two-stage controlled rolling process is adopted, which is divided into a rough rolling stage and a finishing rolling stage. In the first stage (rough rolling stage), the slab starting rolling temperature is 1250°C, the final rolling temperature is 1024°C, and the total reduction rate is 40%; the second stage (finishing rolling stage) adopts low-temperature rolling technology, with a starting rolling temperature of 880°C, a final rolling temperature of 845°C, and a total reduction rate of 15%; the rolled steel plate is cooled to room temperature in air.

[0092] S5. Heat treatment process: After rolling, the steel plate is heat treated by double quenching + tempering, that is, the first quenching is performed first, then the second quenching, and finally the tempering. The temperature of the first quenching and the second quenching is 923℃, and the holding time is 2.3min / mm; the tempering temperature is 630℃, and the holding time is 3.1min / mm.

[0093] The metallographic structure of the steel plate obtained in this embodiment at 1 / 4 of the thickness is as follows Figure 4 As shown. Figure 4 It can be seen that the microstructure of the steel plate is tempered troostite (S), a small amount of tempered martensite (MA) and a small amount of ferrite (F).

[0094] Comparative Example 1

[0095] The difference from Example 1 is that the dosage of chemical components is adjusted, specifically:

[0096] The chemical composition of the 1000MPa grade hydropower steel described in this comparative example is as follows by weight percentage: C: 0.02%, Mn: 1.40%, Si: 0.09%, Cu: 0.14%, S: 0.002%, P: 0.007%, Al: 0.01%, V: 0.09%, Cr: 0.05%, Mo: 0.60%, B: 0.002%, Ti: 0.016%, Nb: 0.025%, Ni: 0.60%, and the balance is Fe and other inevitable impurities.

[0097] After calculation, [C]+[Si]+10×[Al]=0.21%, wherein [C], [Si] and [Al] represent the content of each chemical component in weight percentage, which is lower than the specified lower limit of [C]+[Si]+10×[Al].

[0098] After calculation, [Mn]+2×[Ni]+10×[Nb]=2.85%, wherein [Mn], [Ni] and [Nb] represent the content of each chemical component in weight percentage, satisfying the range corresponding to the relationship.

[0099] The rest is the same as in Example 1.

[0100] Comparative Example 2

[0101] The difference from Example 1 is that the dosage of chemical components is adjusted, specifically:

[0102] The chemical composition of the 1000MPa grade hydropower steel described in this comparative example is as follows by weight percentage: C: 0.03%, Mn: 1.45%, Si: 0.15%, Cu: 0.14%, S: 0.002%, P: 0.007%, Al: 0.04%, V: 0.09%, Cr: 0.05%, Mo: 0.60%, B: 0.002%, Ti: 0.016%, Nb: 0.025%, Ni: 0.70%, and the balance is Fe and other inevitable impurities.

[0103] After calculation, [C]+[Si]+10×[Al]=0.58%, wherein [C], [Si] and [Al] represent the content of each chemical component in weight percentage, satisfying the range corresponding to the relationship.

[0104] After calculation, [Mn]+2×[Ni]+10×[Nb]=3.1%, wherein [Mn], [Ni] and [Nb] represent the content of each chemical component in weight percentage, which is higher than the specified upper limit of [Mn]+2×[Ni]+10×[Nb].

[0105] The rest is the same as in Example 1.

[0106] Comparative Example 3

[0107] The difference from Example 3 is that the dosage of chemical components is adjusted, specifically:

[0108] The chemical composition of the 1000MPa grade hydropower steel described in this comparative example is as follows by weight percentage: C: 0.09%, Mn: 1.00%, Si: 0.20%, Cu: 0.05%, S: 0.001%, P: 0.003%, Al: 0.05%, V: 0.03%, Cr: 0.03%, Mo: 0.30%, B: 0.001%, Ti: 0.012%, Nb: 0.015%, Ni: 0.20%, and the balance is Fe and other inevitable impurities.

[0109] After calculation, [C]+[Si]+10×[Al]=0.79%, wherein [C], [Si] and [Al] represent the content of each chemical component in weight percentage, which is higher than the specified upper limit of [C]+[Si]+10×[Al].

[0110] After calculation, [Mn]+2×[Ni]+10×[Nb]=1.55%, wherein [Mn], [Ni] and [Nb] represent the content of each chemical component in weight percentage, satisfying the range corresponding to the relationship.

[0111] The rest is the same as Example 3.

[0112] Comparative Example 4

[0113] The difference from Example 3 is that the dosage of chemical components is adjusted, specifically:

[0114] The chemical composition of the 1000MPa grade hydropower steel described in this comparative example is as follows by weight percentage: C: 0.09%, Mn: 0.70%, Si: 0.10%, Cu: 0.05%, S: 0.001%, P: 0.003%, Al: 0.01%, V: 0.03%, Cr: 0.03%, Mo: 0.30%, B: 0.001%, Ti: 0.012%, Nb: 0.015%, Ni: 0.10%, and the balance is Fe and other inevitable impurities.

[0115] After calculation, [C]+[Si]+10×[Al]=0.29%, wherein [C], [Si] and [Al] represent the content of each chemical component in weight percentage, satisfying the range corresponding to the relationship.

[0116] After calculation, [Mn]+2×[Ni]+10×[Nb]=1.05%, wherein [Mn], [Ni] and [Nb] represent the content of each chemical component in weight percentage, which is lower than the specified lower limit of [Mn]+2×[Ni]+10×[Nb].

[0117] The rest is the same as Example 3.

[0118] Comparative Example 5

[0119] The difference from Example 2 is that the preparation process is adjusted, and the double quenching-tempering mode is changed to a single quenching-tempering mode, specifically:

[0120] When performing the heat treatment process in the preparation method, the steel plate is heat treated by a single quenching + tempering process after rolling, that is, only one quenching is performed, the quenching temperature is 889°C, and the holding time is 1.9 min / mm; the tempering temperature is 620°C, and the holding time is 2.5 min / mm.

[0121] The rest is the same as in Example 2.

[0122] Performance Verification:

[0123] The yield strength, tensile strength and impact energy at -60°C of the steels obtained in Examples 1 to 3 and Comparative Examples 1 to 5 after tempering were tested using GB / T 228.1-2021 and GB / T 229-2020, and the results are shown in Table 2. In addition, in order to better compare the differences in the amounts of the chemical components in Examples 1 to 3 and Comparative Examples 1 to 4, the amounts of the chemical components in Examples 1 to 3 and Comparative Examples 1 to 4 are summarized in Table 1.

[0124] Table 1 Amount of each chemical component used in Examples 1 to 3 and Comparative Examples 1 to 4 (weight percentage, %)

[0125]

[0126] Table 2 Mechanical properties of steels obtained in Examples 1 to 3 and Comparative Examples 1 to 5 after tempering

[0127]

[0128]

[0129] It can be seen from Table 2 that the yield strength of the steels obtained in Comparative Examples 1 to 5 is lower than 900MPa, and the impact performance is reduced. The surface impact performance and 1 / 4 impact performance mostly do not meet the requirements, which ultimately leads to the low-temperature toughness of the prepared steel not meeting industrial needs and is not suitable for industrial production. In comparison, the 1000MPa-grade hydropower steel plate of the present invention with a maximum thickness of no more than 100mm has a metallographic structure of tempered troostite, a small amount of tempered martensite and a small amount of ferrite, a yield strength of 900-945MPa, a tensile strength of 1000-1070MPa, an impact energy of ≥80J at -60°C, and stable performance. The 1000MPa-grade hydropower steel obtained by the present invention has higher strength and better low-temperature toughness, and can meet the application of hydropower steel with higher requirements.

[0130] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0131] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A 1000MPa grade hydropower steel with optimized low temperature toughness, characterized in that: The chemical components are as follows by weight percentage: C: 0.03~0.09%, Mn: 0.80~1.40%, Si: 0.10~0.15%, Cu: 0.05~0.14%, S≤0.003%, P≤0.01%, Al: 0.01~0.04%, V: 0.03~0.10%, Cr: 0.03~0.06%, Mo: 0.3~0.65%, B: 0.001~0.002%, Ti: 0.01~0.018%, Nb: 0.015~0.025%, Ni: 0.18~0.60%, the balance is Fe and other inevitable impurities.

2. The 1000MPa grade hydropower steel with optimized low temperature toughness according to claim 1, characterized in that: The contents of C, Si and Al in the chemical composition of the 1000MPa grade hydropower steel with optimized low-temperature toughness satisfy the relationship: 0.23%≤[C]+[Si]+10×[Al]≤0.64%, wherein [C], [Si] and [Al] represent the contents of each chemical component in weight percentage.

3. The 1000MPa grade hydropower steel with optimized low temperature toughness according to claim 1, characterized in that: The contents of Mn, Ni and Nb in the chemical composition of the 1000MPa grade hydropower steel with optimized low-temperature toughness satisfy the relationship: 1.31%≤[Mn]+2×[Ni]+10×[Nb]≤2.85%, wherein [Mn], [Ni] and [Nb] represent the contents of each chemical component in weight percentage.

4. The method for preparing the 1000MPa grade hydroelectric steel with optimized low temperature toughness according to any one of claims 1 to 3, characterized in that: The steps include: S1, steelmaking process: batching and weighing the raw materials according to the mass percentage of the above chemical components, smelting to obtain molten steel, and continuously casting the molten steel to obtain a continuous casting billet; S2, heating process: heating the continuous casting billet; S3, rough dephosphorization: removing the furnace-born iron oxide scale on the surface of the continuous casting billet after the heating treatment to obtain a steel billet; S4, rolling process: rolling the steel billet using a two-stage controlled rolling process to obtain a steel plate; S5. Heat treatment process: heat-treating the steel plate by a double quenching + tempering heat treatment process to obtain the 1000MPa grade hydropower steel.

5. The preparation method according to claim 4, characterized in that: In step S1, the smelting includes converter blowing, LF refining and VD treatment, specifically including: firstly performing converter blowing, with the tapping temperature ≥1590°C, then performing LF refining, during the LF refining process, ensuring that the white slag retention time is greater than 30 minutes, and finally performing VD treatment, with the VD treatment time being maintained for more than 30 minutes; the continuous casting includes: pouring the molten steel into a continuous casting billet with a thickness of 150 to 240 mm through a continuous casting machine.

6. The preparation method according to claim 4, characterized in that: In step S2, the temperature of the heat treatment is 1200-1250°C, and the heating coefficient is 10-14 min / cm; and / or, in step S3, the removal method includes water jet erosion.

7. The preparation method according to claim 4, characterized in that: In step S4, the two-stage controlled rolling process includes a rough rolling stage and a finishing rolling stage; the parameters of the rough rolling stage are controlled as follows: the starting rolling temperature is ≥1150°C, the final rolling temperature is ≥1000°C, and the total reduction rate is 40-50%; the parameters of the finishing rolling stage are controlled as follows: the starting rolling temperature is 840-880°C, the final rolling temperature is 800-850°C, and the total reduction rate is 15-30%.

8. The preparation method according to claim 4, characterized in that: In step S5, the double quenching + tempering heat treatment process includes a first quenching, a second quenching and tempering; the temperature of the first quenching and the second quenching is 850-930°C, and the holding time is 1.5-2.5 min / mm; the temperature of the tempering is 600-650°C, and the holding time is 2.0-3.5 min / mm.

9. Application of the 1000MPa grade hydropower steel with optimized low temperature toughness as claimed in any one of claims 1 to 3 in the hydropower industry.

10. A method for optimizing the low temperature toughness of 1000MPa grade hydropower steel, characterized in that: 1000MPa grade hydroelectric steel is prepared by the preparation method described in any one of claims 4 to 8.

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