500 mpa grade green low-carbon high-toughness easily weldable wind power steel and production method therefor

Abstract

A 500 MPa grade green low-carbon high-toughness easily weldable wind power steel, comprising the following alloy components in percentage by mass: C: 0.05-0.10%; Si: 0.20-0.30%; Mn: 1.00-2.00%; Cr: 0.2-0.4%; Nb: 0.04-0.10%; V: 0.004-0.01%; Ti: 0.01-0.02%; Ni: 0.20-0.30%; Cu: 0.01-0.02%; Als: 0.03-0.05%; Zr≤0.03%; Mo≤0.01%; P≤0.008%; S≤0.003%; and the balance being Fe and other inevitable impurity elements. The present invention adopts a low-carbon-equivalent Nb+V+Ti microalloying composition design in combination with a two-phase zone controlled rolling process, thereby enabling the production of the 500MPa grade high-toughness easily weldable wind power steel. A steel plate has a thickness of 20-50 mm, a yield strength ≥ 500 MPa, a tensile strength ≥ 600 MPa, an elongation rate ≥ 20%, and an impact energy at -60ºC > 200 J.

Description

A 500MPa grade green, low-carbon, high-strength, tough, and easy-to-weld wind power steel and its production method

[0001] This application claims priority to Chinese Patent Application No. 2025102026485, filed on February 24, 2025, entitled "A 500MPa Grade Green Low Carbon High Strength Toughness Easy Welding Steel for Wind Power and Its Production Method", the entire contents of which are incorporated herein by reference. Technical Field

[0002] This invention relates to the field of steel production technology, and more specifically, to a 500MPa grade green, low-carbon, high-strength, tough, and easy-to-weld wind power steel and its production method. Background Technology

[0003] With the proposal and implementation of the "dual-carbon" strategic goal, new energy industries such as wind power and photovoltaic power generation are playing an important role in supporting the adjustment of the energy structure. After a brief rush to install wind power in 2020, the wind power industry entered the era of grid parity in 2021. In 2023, the newly installed wind power capacity reached 75.9 GW, with a cumulative installed capacity exceeding 400 million kW, maintaining its position as the world's largest installed capacity. In April 2023, the National Energy Administration issued the "Guiding Opinions on Energy Work in 2023," proposing to promote the grid connection and commissioning of the first batch of large-scale wind and solar power base projects, focusing on desert, Gobi, and arid areas, and to construct the second and third batches; and to steadily construct offshore wind power bases. In June of the same year, the "Notice on Issuing the <Management Measures for the Transformation, Upgrading and Decommissioning of Wind Farms>" was also issued, encouraging wind farms that have been connected to the grid for more than 15 years or whose single unit capacity is less than 1.5 MW to carry out transformation and upgrading. Large-scale onshore wind power bases in barren desert areas are progressing steadily, with the China General Nuclear Power Group's 3 million kilowatt wind power project in Xing'an League fully connected to the grid. Offshore wind power is expanding from nearshore to deep-sea areas, breaking through barriers in terms of distance from shore and water depth. The world's first 16-megawatt ultra-large-capacity offshore wind turbine was successfully connected to the grid at an offshore wind farm in Fujian.

[0004] As the wind power industry moves towards larger turbines and more complex operating environments, the wind turbine tower, as a key component, is also entering a new era. Currently, the strength grade of steel plates used for wind turbine towers is mainly 355MPa, which can no longer meet the development needs of wind turbines at this stage. The scaling up of wind turbines is not simply a matter of increasing power; it requires collaborative innovation in materials, design, and other fields. While continuously pushing technological boundaries, strictly controlling the quality of materials used in the turbines is paramount for the long-term and stable development of wind turbines.

[0005] Patent CN118147412A discloses a method for preparing Q500ME high-strength and high-toughness wind power steel plate. Its chemical composition by weight percentage is: C: 0.10-0.12%, Si: 0.20-0.30%, Mn: 1.55-1.70%, P≤0.015%, S≤0.005%, Nb: 0.040-0.050%, V: 0.035-0.045%, Ti: 0.012-0.022%, Cr: 0.20-0.30%, Als: 0.020-0.035%, with the remainder being Fe and unavoidable impurities. The steel plate is produced using the TMCP process, with a finished thickness of 14-50 mm. The steel plate has a yield strength ≥500 MPa, a tensile strength of 610-770 MPa, an elongation after fracture ≥17%, and an impact energy at -40℃ ≥150 J. However, the patent only protects the basic mechanical properties of the steel plate and does not take into account fatigue performance, crack arrest performance, and welding performance.

[0006] Patent CN116536580A discloses an easily weldable, high-strength and tough 500MPa grade wind power steel plate and its preparation method. The specific alloy composition, by mass percentage, is: C: 0.02–0.05%, Si: 0.1–0.3%, Mn: 1.00–1.50%, Mo: 0.15–0.5%, Cr: 0.15–0.5%, Nb: 0.04–0.1%, Ni: 0.2–0.5%, Ti: 0.01–0.05%, Alt: 0.01–0.05%, B: 0.0 The carbon content is 0.01-0.002%, V: 0-0.08%, N≤0.005%, P≤0.01%, S≤0.006%, with the remainder being Fe and unavoidable impurities. Based on an ultra-low carbon + microalloying composition design, the steel plate is treated with TMCP + critical annealing to prepare a microstructure composed of ferrite, bainite, and reversed austenite. The steel plate has a yield strength ≥580MPa, tensile strength ≥750MPa, elongation after fracture ≥17%, impact energy at -60℃ ≥130J, and post-weld thermal simulation impact energy at -60℃ ≥96J. Although this patent significantly reduces the carbon content, it increases the content of microalloying elements such as Ni and Mo, and also requires offline annealing of the hot-rolled steel plate, increasing production costs; moreover, the protection of the post-weld performance of the steel plate is not complete.

[0007] Although the aforementioned existing technologies all involve 500MPa-grade high-strength and high-toughness wind power steel plates and production methods, they are not perfect in terms of composition design, production process and mechanical performance requirements. They have high production costs, complex processes, and insufficient attention to the crack arrest performance and weldability of the steel plates. Summary of the Invention

[0008] In view of this, in order to meet the current development needs of large wind turbine units and the continued upgrading of some, this invention provides a 500MPa grade green, low-carbon, high-strength, tough, and easy-to-weld wind power steel plate and its production method. Based on compliance with relevant standards, it adopts a low-carbon composition design and strictly controls the rolling and cooling process parameters. While improving the strength, plasticity, and toughness of the steel plate, it focuses on the weldability of the steel plate. The high comprehensive mechanical performance index of the steel plate ensures the steady development of the wind power industry in the future.

[0009] This invention provides a 500MPa grade green, low-carbon, high-strength, tough, and easily weldable wind power steel, comprising the following alloy composition by mass percentage: C: 0.05–0.10%; Si: 0.20–0.30%; Mn: 1.00–2.00%; Cr: 0.2–0.4%; Nb: 0.04–0.10%; V: 0.004–0.01%; Ti: 0.01–0.02%; Ni: 0.20–0.30%; Cu: 0.01–0.02%; Als: 0.03–0.05%; Zr≤0.03%; Mo≤0.01%; P≤0.008%; S≤0.003%.

[0010] The remainder consists of Fe and other unavoidable impurity elements.

[0011] Preferably, the steel has a Ceq < 0.42% and a Pcm < 0.20%.

[0012] Preferably, the microstructure of the steel is heterogeneous, mainly composed of coarse quasi-polygonal ferrite, fine acicular ferrite and a small amount of bainite, with an average effective grain size of less than 5 μm.

[0013] Preferably, the steel has a thickness of 20mm to 50mm, a yield strength ≥500MPa, a tensile strength ≥600MPa, and an elongation after fracture ≥20%.

[0014] Preferably, the steel has an impact energy of >200J at -60℃ and a critical crack length >25mm when fatigue failure occurs.

[0015] Preferably, when the welding heat input is ≥20kJ / cm, the tensile strength of the welded joint is >450MPa, the impact energy of the heat-affected zone at -40℃ is >120J, and the CTOD value at -40℃ is >0.25mm.

[0016] This invention also provides a method for producing the 500MPa grade green, low-carbon, high-strength, tough, and easy-to-weld wind power steel described in the above technical solution, comprising the following steps:

[0017] The alloy components are sequentially smelted, continuously cast, heated, rolled, and cooled to obtain the product.

[0018] Preferably, the thickness of the heated billet is 100mm to 300mm, the holding temperature is 1150℃ to 1200℃, and the furnace time is 500min to 700min.

[0019] Preferably, the thickness of the rolled intermediate billet is 50mm to 150mm, the initial rolling temperature is 890℃ to 910℃, the final rolling temperature is 850℃ to 870℃, and the total number of finishing rolling passes is 6 to 10.

[0020] Preferably, the thickness of the cooled finished product is 20mm to 50mm, the initial cooling temperature is 840℃ to 860℃, and the final cooling temperature is 440℃ to 550℃.

[0021] This invention provides a 500MPa grade green, low-carbon, high-strength, tough, and easily weldable wind power steel and its production method. The steel comprises the following alloy composition by mass percentage: C: 0.05-0.10%; Si: 0.20-0.30%; Mn: 1.00-2.00%; Cr: 0.2-0.4%; Nb: 0.04-0.10%; V: 0.004-0.01%; Ti: 0.01-0.02%; Ni: 0.20-0.30%; Cu: 0.01-0.02%; Als: 0.03-0.05%; Zr≤0.03%; Mo≤0.01%; P≤0.008%; S≤0.003%; the remainder being Fe and other unavoidable impurity elements. Compared with existing technologies, this invention adopts a low carbon equivalent and Nb+V+Ti microalloying composition design, supplemented by a two-phase zone controlled rolling process, to achieve the production of 500MPa grade high-strength, tough, and easily weldable wind power steel. The microstructure of the steel plate is heterogeneous, mainly composed of quasi-polygonal ferrite, acicular ferrite, and a small amount of bainite. The steel plate thickness is 20-50mm, yield strength ≥500MPa, tensile strength ≥600MPa, elongation ≥20%, impact energy at -60℃ >200J, and critical crack length at fatigue failure >25mm. Simultaneously, when the welding heat input is ≥20kJ / cm, the tensile strength of the welded joint is >450MPa, the impact energy at -40℃ in the heat-affected zone is >120J, and the CTOD value at -40℃ is >0.25mm. Attached Figure Description

[0022] Figure 1 shows the microstructure of a 20mm thick 500MPa grade wind power steel plate from Example 1.

[0023] Figure 2 shows the fracture morphology of a 20mm thick 500MPa grade wind power steel plate impact specimen at -60℃ from Example 1.

[0024] Figure 3 shows the fracture morphology of the -40℃ impact specimen of the welded heat-affected zone of the 20mm thick 500MPa grade wind power steel plate of Example 1.

[0025] Figure 4 shows the fracture morphology of the CTOD sample of a 20mm thick 500MPa grade wind power steel plate welded joint at -40℃ in Example 1.

[0026] Figure 5 shows the microstructure of a 40mm thick 5000MPa grade wind power steel plate from Example 2.

[0027] Figure 6 shows the fracture morphology of a 40mm thick 500MPa grade wind power steel plate impact specimen at -60℃ from Example 2.

[0028] Figure 7 shows the fracture morphology of the -40℃ impact specimen of the welded heat-affected zone of a 40mm thick 500MPa grade wind power steel plate from Example 2.

[0029] Figure 8 shows the fracture morphology of the -40℃ CTOD sample of the welded joint of a 40mm thick 460MPa wind power steel plate from Example 2. Detailed Implementation

[0030] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0031] To address the aforementioned design and production issues related to wind power steel composition and promote the high-quality and healthy development of the wind power industry, this invention provides a 500MPa grade green, low-carbon, high-strength, tough, and easily weldable wind power steel, comprising the following alloy composition by mass percentage: C: 0.05–0.10%; Si: 0.20–0.30%; Mn: 1.00–2.00%; Cr: 0.2–0.4%; Nb: 0.04–0.10%; V: 0.004–0.01%; Ti: 0.01–0.02%; Ni: 0.20–0.30%; Cu: 0.01–0.02%; Als: 0.03–0.05%; Zr≤0.03%; Mo≤0.01%; P≤0.008%; S≤0.003%.

[0032] The remainder consists of Fe and other unavoidable impurity elements.

[0033] The preferred composition is: C: 0.09–0.10%; Si: 0.20–0.22%; Mn: 1.60–1.66%; Cr: 0.30–0.32%; Nb: 0.04–0.06%; V: 0.004%; Ti: 0.01–0.02%; Ni: 0.22–0.25%; Cu: 0.01%; Als: 0.033–0.040%; Zr: 0.02%; Mo: 0.002%; P: 0.008%; S: 0.002–0.003%.

[0034] The remainder consists of Fe and other unavoidable impurity elements.

[0035] In this invention, the role of the alloy composition (alloying elements) of the steel is as follows:

[0036] Carbon (C) is the main alloying element for improving the strength and toughness of low-alloy steel. Theoretically, the higher the strength grade of the steel plate, the higher the required C content. However, when the C content is high, C and Mn tend to segregate in the core during continuous casting, and this segregation is difficult to eliminate in subsequent processing steps. Furthermore, high-carbon equivalent low-alloy steels generally have poor weldability, resulting in coarse microstructure in the welded joint and significant differences in performance compared to the base material, directly affecting the service performance of the steel plate. Therefore, while ensuring the strength of the steel plate, the C content should be reduced as much as possible to obtain a high-performance acicular ferrite microstructure within a wider cooling rate range. Considering the above factors, the reasonable range for the C content in this invention is 0.05%–0.10%.

[0037] Si: Si is a ferrite solid solution strengthening element. By increasing the ferrite transformation initiation temperature, Si can expand the ferrite transformation region, broaden the metastable austenite region, and improve the strength of ferrite grains. However, Si also has a significant impact on the weldability of low-alloy steel sheets, especially under high welding heat input. During welding, Si promotes the formation of large MA islands, thus deteriorating the toughness of the weld joint. Therefore, in this invention, the Si content in the steel should be controlled between 0.20% and 0.30%.

[0038] Mn: Mn is the most important alloying element in low alloy steel. Mn can expand the austenite phase region and improve the grain size of TMCP low alloy steel plates. Through the grain refinement strengthening effect, it can improve the mechanical properties of steel plates such as strength, low-temperature toughness and fatigue performance. However, Mn is prone to segregation during the solidification process of molten steel. Especially when the Mn content is high, the conjugate segregation with elements such as C, P and S forms abnormal structures in the thick TMCP and welding process, which leads to a decrease in the low-temperature toughness of steel plates and welded joints. For TMCP high strength and toughness steel plates, excessively high Mn content will also lead to a sharp increase in the anisotropy of the yield strength ratio and longitudinal and transverse strength. Therefore, the appropriate Mn content in this invention is 1.00 to 2.00%.

[0039] Cr: Appropriate addition of Cr can improve the hardenability of steel plates, thereby ensuring the uniformity of the microstructure in the thickness direction during controlled cooling. Furthermore, Cr can form a dense Cr(OH)3 film with moisture in the air, protecting the steel substrate and thus improving its corrosion resistance. When the Cr content is low, it contributes little to the strength, toughness, and corrosion resistance of low-alloy steel plates. However, when the Cr content is too high, the microstructure of the heat-affected zone at the weld joint after welding is dominated by coarse bainite or Widmanstätten, severely damaging the weldability of the steel plate. Therefore, in this invention, the Cr content is controlled between 0.20% and 0.40%.

[0040] Nb: Nb is a strong carbide-forming element. At high temperatures, Nb forms Nb(C,N) or NbC carbonitrides with C and N atoms, which can effectively pin austenite grain boundary movement, thereby refining the steel plate microstructure and improving performance. When the amount of Nb added is low, its pinning effect is limited, and the refinement of the steel plate microstructure is not significant. When the amount of Nb added exceeds 0.10%, it not only causes anisotropy in the transverse and longitudinal mechanical properties of the steel plate, but also leads to secondary precipitation of Nb(C,N) in the post-weld microstructure, which impairs the low-temperature toughness of the weld heat-affected zone. Considering both production costs and the strength and toughness requirements of the steel plate, the reasonable Nb content in this invention should be 0.04% to 0.10%.

[0041] V: The role of vanadium (V) is similar to that of nitrogen (Nb), refining the grains and increasing the strength of the steel plate by forming volatile organic compound (VC) particles that pin the austenite grain boundaries. At low V contents, its contribution to strength improvement is not significant. When the V content exceeds 0.010%, excessive V combines with carbon atoms to form more VC particles, leading to a decrease in the strength of the steel plate. Considering alloy cost and practical application, the V content is typically between 0.004% and 0.010%.

[0042] Ti: The addition of Ti preferentially combines with N to form dispersed TiN particles, similar to the effects of NbC and VC, inhibiting austenite grain growth during TMCP, refining the final microstructure of the steel plate, and improving various mechanical properties such as strength, low-temperature toughness, and fatigue performance. However, when the Ti content is too high, the number of TiN particles formed increases significantly, leading to a decrease in the number of BN particles, which fails to improve the low-temperature toughness of the heat-affected zone after welding. Therefore, the suitable Ti content range in this invention is 0.01–0.02%.

[0043] Mo: Mo can significantly improve the hardenability of steel plates, promote the bainitic transformation of supercooled austenite, and significantly improve the strength of steel plates, but it impairs the low-temperature toughness and elongation of TMCP steel plates. Furthermore, when the Mo content is too high, it can also affect the weldability of the steel plate and increase its production cost. Considering the combined effects of Mo on supercooled austenitic transformation, low-temperature toughness, and weldability of steel plates, the Mo content in this invention should be controlled at ≤0.01%.

[0044] Ni: Ni is an indispensable element for improving the low-temperature toughness of TMCP steel plates, and it has little impact on the weldability of the steel plates, which is beneficial to ensuring the stability of the weld-heat-affected zone-matrix properties after welding. Theoretically, within a certain range, the higher the Ni content, the better. However, Ni is a high-cost alloying element. Considering production costs, industrial production, and high-energy welding, the appropriate No addition amount in this invention should be 0.20% to 0.30%.

[0045] Cu: Cu can work with other microalloying elements to achieve composite strengthening, improving the strength and low-temperature toughness of steel plates; however, adding too much Cu not only increases manufacturing costs, but may also lead to Cu precipitation during the TMCP process, impairing the low-temperature toughness of the steel plate. Therefore, in this invention, the Cu addition amount is more suitable in the range of 0.01% to 0.02%.

[0046] Zr: Zr has a strong chemical affinity for elements such as O, N, S, and C, and can refine and modify inclusions in low-alloy Ti-containing steels. Appropriate amounts of Zr, with its strong deoxidizing ability and high yield, can effectively inhibit grain coarsening in the heat-affected zone and promote acicular ferrite nucleation, thereby improving the weldability of low-alloy high-strength steel plates. Therefore, the reasonable range for Zr content in this invention should be ≤0.03%.

[0047] P: P inclusions have a significant impact on the mechanical properties of steel plates, especially low-temperature toughness, and the P content should be strictly controlled to be as low as possible. However, considering steelmaking processes and costs, the reasonable range for P content in this invention is ≤0.008%.

[0048] S: Similar to P, S inclusions also significantly impair the low-temperature toughness of steel plates. More importantly, S readily combines with Mn to form MnS inclusions. During subsequent hot rolling, MnS can extend along the rolling direction of the steel plate, severely damaging its low-temperature toughness. Theoretically, the lower the S content, the better. However, considering steelmaking processes and costs, the S content in this invention needs to be controlled at ≤0.003%.

[0049] In this invention, the Ceq (carbon equivalent) of the 500MPa grade green low-carbon high-strength tough and easy-to-weld wind power steel is preferably <0.42%, Ceq=C+Mn / 6+Si / 24+Ni / 40+Mo / 4+V / 14; the Pcm (crack sensitivity coefficient) is preferably <0.20%, Pcm=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10.

[0050] In this invention, the microstructure of the 500MPa grade green low-carbon high-strength tough and easy-to-weld wind power steel is preferably a heterogeneous structure, mainly composed of coarse quasi-polygonal ferrite, fine acicular ferrite and a small amount of bainite, with the average effective grain size preferably below 5μm.

[0051] In this invention, the thickness of the 500MPa grade green low-carbon high-strength tough and easy-to-weld wind power steel is preferably 20mm to 50mm, specifically 20mm or 40mm.

[0052] In this invention, the yield strength of the 500MPa grade green low-carbon high-strength tough and easy-to-weld wind power steel is preferably ≥500MPa, the tensile strength is preferably ≥600MPa, the elongation after fracture is preferably ≥20%, the impact energy at -60℃ is preferably ≥200J, and the critical crack length when fatigue failure occurs is preferably >25mm.

[0053] In this invention, the 500MPa grade green low-carbon high-strength tough and easy-to-weld wind power steel has the following characteristics when the welding heat input is ≥20kJ / cm: the tensile strength of the welded joint is preferably >450MPa, the impact energy of the heat-affected zone at -40℃ is preferably >120J, and the CTOD value at -40℃ is preferably >0.25mm.

[0054] This invention also provides a method for producing the 500MPa grade green, low-carbon, high-strength, tough, and easy-to-weld wind power steel described in the above technical solution, comprising the following steps:

[0055] The alloy components are sequentially smelted, continuously cast, heated, rolled and cooled to obtain 500MPa grade green low carbon high strength tough easy weldable wind power steel.

[0056] This invention employs a controlled rolling and cooling process, which shortens the production process. This invention does not impose special restrictions on the smelting and continuous casting processes, and can use techniques well known to those skilled in the art to form alloy components into billets.

[0057] The main processes for producing steel plates according to this invention include: heating process (i.e. heating of the billet), controlled rolling process and controlled cooling process (i.e. controlled rolling and controlled cooling). The controlled rolling process is divided into two stages: rough rolling and finish rolling. The cumulative reduction rate of the rough rolling stage is >50% to fully refine the original austenite grains in the billet.

[0058] In this invention, the thickness of the heated billet is preferably 100mm to 300mm, more preferably 200mm to 250mm. During the heat treatment, the holding temperature is preferably 1150℃ to 1200℃, more preferably 1200℃. The furnace time is T = αt, where α is the homogenization coefficient, typically 3 to 4 min / mm, and t is the billet thickness. Sufficient heating and holding time ensures complete austenitization of the billet, allowing for full dissolution of microalloying elements and preventing the precipitation of strong carbides or localized enrichment of carbon. Based on this, the furnace time is preferably 500min to 700min, more preferably 524min to 680min.

[0059] In this invention, the thickness of the rolled intermediate billet is preferably 50mm to 150mm, more preferably 80mm to 120mm; in the roughing rolling process, the cumulative reduction rate is preferably >50%, and a higher total reduction rate in the roughing rolling stage can refine the size of the non-recrystallized austenite grains, thereby refining the microstructure of the final steel plate. In the finishing rolling process, the initial finishing rolling temperature is preferably 890℃ to 910℃, more preferably 900℃ to 907℃, the final rolling temperature is preferably 850℃ to 870℃, more preferably 861℃ to 862℃, and the total number of finishing rolling passes is preferably 6 to 10, more preferably 7.

[0060] In this invention, the thickness of the cooled finished product is preferably 20mm to 50mm, more preferably 20mm to 40mm. The post-rolling controlled cooling process preferably uses water cooling. The initial cooling temperature is preferably 840℃ to 860℃, more preferably 841℃ to 845℃, and the final cooling temperature is preferably 440℃ to 550℃, more preferably 449℃ to 550℃. The steel plate is then air-cooled to room temperature. The lower final cooling temperature increases the supercooling of untransformed austenite, promotes phase transformation in the steel, increases the proportion or density of small-angle grain boundaries in the microstructure, and improves the low-temperature toughness of the steel plate.

[0061] This invention provides a 500MPa grade green, low-carbon, high-strength, tough, and easily weldable wind power steel and its production method. The steel comprises the following alloy composition by mass percentage: C: 0.05-0.10%; Si: 0.20-0.30%; Mn: 1.00-2.00%; Cr: 0.2-0.4%; Nb: 0.04-0.10%; V: 0.004-0.01%; Ti: 0.01-0.02%; Ni: 0.20-0.30%; Cu: 0.01-0.02%; Als: 0.03-0.05%; Zr≤0.03%; Mo≤0.01%; P≤0.008%; S≤0.003%; the remainder being Fe and other unavoidable impurity elements. Compared with existing technologies, this invention adopts a low carbon equivalent and Nb+V+Ti microalloying composition design, supplemented by a two-phase zone controlled rolling process, to achieve the production of 500MPa grade high-strength, tough, and easily weldable wind power steel. The microstructure of the steel plate is heterogeneous, mainly composed of quasi-polygonal ferrite, acicular ferrite, and a small amount of bainite. The steel plate thickness is 20-50mm, yield strength ≥500MPa, tensile strength ≥600MPa, elongation ≥20%, impact energy at -60℃ >200J, and critical crack length at fatigue failure >25mm. Simultaneously, when the welding heat input is ≥20kJ / cm, the tensile strength of the welded joint is >450MPa, the impact energy at -40℃ in the heat-affected zone is >120J, and the CTOD value at -40℃ is >0.25mm.

[0062] To further illustrate the present invention, the following embodiments will be described in detail.

[0063] Example 1

[0064] The thickness of the 500MPa grade green low-carbon easy-to-weld wind power steel plate in this embodiment is 20mm, and its chemical composition and percentage are shown in Table 1.

[0065] The production method of 500MPa grade green low-carbon easy-to-weld wind power steel plate in this embodiment includes smelting, continuous casting, heating, rolling and cooling processes. The specific parameters of the heating, rolling and cooling processes are as follows:

[0066] (1) Heating process: billet thickness 200mm, holding temperature 1200℃, furnace time 680min;

[0067] (2) Rolling process: The thickness of the intermediate billet is 80mm, the number of roughing passes is 5, the cumulative reduction rate is >50%, the starting temperature of finishing rolling is 907℃, the final rolling temperature is 861℃, and the total number of passes in the finishing stage is 7.

[0068] (3) Cooling process: The finished product thickness is 20mm, the initial cooling temperature is 841℃, and the final cooling temperature is 449℃.

[0069] The microstructure of the 500MPa grade green low-carbon high-strength and tough easy-to-weld wind power steel plate at half its thickness prepared in this embodiment is shown in Figure 1. It can be seen that the microstructure is mainly composed of quasi-polygonal ferrite and acicular ferrite. The average effective grain size is 3.54μm. The mechanical properties of the steel plate are shown in Table 2. The fracture morphology of the standard V-shaped impact specimen of the steel plate at -60℃ is shown in Figure 2. The steel plate was welded using SAW welding with a heat input of 25kJ / cm, no preheating before welding, and a weld layer / weld pass ratio of 6 / 2. The tensile strength at the weld is 684MPa, the yield strength is 599MPa, the elongation is 25.7%, and the impact energy at -40℃ is 165J. The fracture morphology of the impact specimen is shown in Figure 3. The CTOD value at -40℃ is 0.320mm. The fracture morphology of the specimen is shown in Figure 4.

[0070] Example 2

[0071] In this embodiment, the thickness of the 500MPa grade green low-carbon easy-to-weld wind power steel plate is 40mm, and its chemical composition and percentage are shown in Table 1.

[0072] The production method of 500MPa grade green low-carbon easy-to-weld wind power steel plate in this embodiment includes smelting, continuous casting, heating, rolling and cooling processes. The specific parameters of the heating, rolling and cooling processes are as follows:

[0073] (1) Heating process: billet thickness 250mm, holding temperature 1200℃, furnace time 524min;

[0074] (2) Rolling process: The thickness of the intermediate billet is 120mm, the number of roughing passes is 5, the cumulative reduction rate is >50%, the starting temperature of finishing rolling is 900℃, the final rolling temperature is 862℃, and the total number of finishing passes is 7.

[0075] (3) Cooling process: The finished product thickness is 40mm, the initial cooling temperature is 845℃, and the final cooling temperature is 550℃.

[0076] The microstructure of the 500MPa grade green low-carbon high-strength and tough easy-to-weld wind power steel plate at half its thickness prepared in this embodiment is shown in Figure 5. It can be seen that the microstructure is mainly composed of quasi-polygonal ferrite and acicular ferrite. The average effective grain size is 4.35μm. The mechanical properties of the steel plate are shown in Table 2. The fracture morphology of the standard V-shaped impact specimen of the steel plate at -60℃ is shown in Figure 6. The steel plate was welded using SAW welding method with a heat input of 30kJ / cm and no preheating before welding. The weld layer / weld pass ratio was 14 / 2. The tensile strength at the weld was 654MPa, the yield strength was 581MPa, the elongation was 23.7%, and the impact energy at -40℃ was 122J. The fracture morphology of the impact specimen is shown in Figure 7. The CTOD value at -40℃ was 0.284mm. The fracture morphology of the specimen is shown in Figure 8.

[0077] Table 1. Chemical composition and percentage (wt%) of the 500MPa grade green, low-carbon, high-strength, tough, and easy-to-weld wind power steel plates provided in Examples 1 and 2.

[0078] Table 2 shows the mechanical properties of the 500MPa grade green, low-carbon, high-strength, tough, and easy-to-weld wind power steel plates provided in Examples 1 and 2.

[0079] Experimental results show that:

[0080] Compared with other steel plates of the same strength grade, the product provided by this invention has the following advantages:

[0081] (1) The composition design of low carbon equivalent and welding crack sensitivity coefficient of steel plate improves weldability and hardenability of steel plate on the basis of reducing alloy cost, and ensures the uniformity of weld, heat-affected zone and matrix structure after steel plate welding.

[0082] (2) Strictly control the rolling and cooling process parameters of the billet, and no offline heat treatment is required after rolling, shortening the process flow, obtaining a microstructure with a fine average grain size, and ensuring the strength and excellent low-temperature toughness of the steel plate.

[0083] (3) The weldability of the steel plate was highlighted. The comprehensive mechanical properties of the welded joint after SAW welding are still excellent. The tensile strength of the welded joint is >450MPa, the impact energy of the heat-affected zone at -40℃ is >120J, and the CTOD value is >0.25mm.

[0084] (4) Adding a small amount of precious alloying element Cu can refine the austenite grains by utilizing Cu precipitates during the rolling stage; on the other hand, it can improve the corrosion resistance of wind power steel in humid service environments such as ocean and deep sea.

[0085] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A 500MPa grade green, low-carbon, high-strength, tough, and easily weldable wind power steel, comprising the following alloy composition by weight percentage: C：0.05～0.10％； Si: 0.20–0.30%; Mn: 1.00~2.00%; Cr：0.2～0.4％； Nb: 0.04–0.10%; V：0.004～0.01％； Ti: 0.01–0.02%; Ni: 0.20–0.30%; Cu: 0.01–0.02%; Als: 0.03–0.05%; Zr≤0.03％； Mo ≤ 0.01%; P≤0.008％； S≤0.003％； The remainder consists of Fe and other unavoidable impurity elements.

2. The 500MPa grade green low-carbon high-strength toughness easy-to-weld wind power steel according to claim 1, characterized in that, The steel has a Ceq < 0.42% and a Pcm < 0.20%.

3. The 500MPa grade green low-carbon high-strength toughness easy-to-weld wind power steel according to claim 1, characterized in that, The microstructure of steel is heterogeneous, mainly composed of coarse quasi-polygonal ferrite, fine acicular ferrite, and a small amount of bainite, with an average effective grain size of less than 5 μm.

4. The 500MPa grade green low-carbon high-strength toughness easy-to-weld wind power steel according to claim 1, characterized in that, The steel has a thickness of 20mm to 50mm, a yield strength ≥500MPa, a tensile strength ≥600MPa, and an elongation after fracture ≥20%.

5. The 500MPa grade green low-carbon high-strength toughness easy-to-weld wind power steel according to claim 1, characterized in that, The impact energy of steel at -60℃ is >200J, and the critical crack length when fatigue failure occurs is >25mm.

6. The 500MPa grade green low-carbon high-strength toughness easy-to-weld wind power steel according to claim 1, characterized in that, When the welding heat input of steel is ≥20kJ / cm, the tensile strength of the welded joint is >450MPa, the impact energy of the heat-affected zone at -40℃ is >120J, and the CTOD value at -40℃ is >0.25mm.

7. A method for producing 500MPa grade green low-carbon high-strength toughness easy-to-weld wind power steel according to any one of claims 1 to 6, comprising the following steps: The alloy components are sequentially smelted, continuously cast, heated, rolled and cooled to obtain the product.

8. The production method according to claim 7, characterized in that, The thickness of the heated billet is 100mm to 300mm, the holding temperature is 1150℃ to 1200℃, and the furnace time is 500min to 700min.

9. The production method according to claim 7, characterized in that, The thickness of the intermediate billet being rolled is 50mm to 150mm, the initial rolling temperature for finishing is 890℃ to 910℃, the final rolling temperature is 850℃ to 870℃, and the total number of finishing passes is 6 to 10.

10. The production method according to claim 7, characterized in that, The thickness of the cooled finished product is 20mm to 50mm, the initial cooling temperature is 840℃ to 860℃, and the final cooling temperature is 440℃ to 550℃.

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