A low-cost, ultra-high-strength steel for hydropower and its production method

Abstract

This invention relates to a low-cost, ultra-high-strength hydropower steel and its production method. The chemical composition of the steel, by weight percentage, is as follows: C 0.06%–0.11%; Si 0.1%–0.3%; Mn 1.65%–2.1%; P≤0.010%; S≤0.005%; Cu 0.26%–0.55%; Mo 0.1%–0.39%; V 0.02%–0.08%; Nb 0.061%–0.12%; B 0.001%–0.003%; N 0.01%–0.025%; Ti 0.025%–0.05%; Al 0.025%–0.055%, and (Ti+Al) / N≥4; Ca 0.001%–0.004%; the balance being Fe and unavoidable impurities. Not only are the production costs low and the production cycle short, but the steel plates also have excellent performance and good welding properties, which can fully meet the needs of large-scale hydropower projects.

Description

Technical Field

[0001] This invention relates to the field of metallic materials, and in particular to a low-cost, ultra-high-strength steel for hydropower and its production method. Background Technology

[0002] With the development of China's new power system, primarily based on new energy sources, the demand for hydropower is constantly increasing. Hydropower stations are becoming increasingly larger and have higher heads, thus requiring higher strength grades of steel for their construction. Currently, the highest strength grade has reached 1000MPa. The production of 1000MPa grade hydropower steel generally employs a high alloy composition design combined with offline quenching and tempering (quenching + high-temperature tempering) heat treatment. However, this production method is not only costly but also involves complex processes and long production cycles. How to save production costs while obtaining high-performance steel plates has become a crucial challenge that professionals in this field must address.

[0003] Existing domestic invention patents for this strength level of steel mostly employ high alloy composition designs and offline quenching and tempering heat treatment processes, which not only result in high production costs but also long production cycles.

[0004] Example 1: Invention patent CN 111455269 A discloses "Very high strength marine engineering steel plate with yield strength of 960MPa and its manufacturing method". The invented steel contains 1.0-2.0% Ni, 0.35-0.80% Cr and 0.4-0.7% Mo. The production process adopts a two-stage controlled rolling + offline quenching and tempering heat treatment method, which not only has high alloy cost, but also long production cycle.

[0005] Example 2: Invention patent CN 108504960 A discloses "a 1000MPa grade low-crack hydropower steel plate for large-scale hydropower projects and its production method". The steel plate has a thickness of 10-50mm and contains 1.0-2.0% Ni, 0.3-1.5% Cr, and 0.4-0.7% Mo. The steel plate production adopts a process of casting large steel ingots, first heating + controlled rolling + second heating + controlled rolling and cooling + offline tempering. The production cycle is long and the production cost is high.

[0006] Example 3: Invention patent CN 114058960 A discloses "a 25-60mm thick 1000MPa grade high strength and high toughness easy-to-weld nano steel and its manufacturing method". The steel described in this invention contains 1.5-2.5% Cu, 6.0-8.0% Ni, 0.5-0.8% Cr, and 0.45-0.6% Mo. The production process adopts two-stage controlled rolling + offline ultra-fast cooling + tempering heat treatment, which results in high alloy cost and long production cycle. Summary of the Invention

[0007] This invention provides an ultra-high strength hydropower steel with a thickness of no more than 60mm and its production method. The produced steel plate has a thickness of no more than 60mm and a tensile strength of 1000MPa. It not only has low production cost and short production cycle, but also has excellent steel plate performance, with good low-temperature toughness, cold bending performance and welding performance, which can fully meet the needs of large-scale hydropower projects.

[0008] To achieve the above objectives, the present invention employs the following technical solution:

[0009] A low-cost, ultra-high-strength hydropower steel has the following chemical composition by weight percentage: C 0.06%–0.11%; Si 0.1%–0.3%; Mn 1.65%–2.1%; P≤0.010%; S≤0.005%; Cu 0.26%–0.55%; Mo 0.1%–0.39%; V 0.02%–0.08%; Nb 0.061%–0.12%; B 0.001%–0.003%; N 0.01%–0.025%; Ti 0.025%–0.05%; Al 0.025%–0.055%, and (Ti+Al) / N≥4; Ca 0.001%–0.004%; with the balance being Fe and unavoidable impurities.

[0010] The reasons for using the above-mentioned components are as follows:

[0011] C: It forms fine carbides with elements such as V, Nb, and Ti, thereby playing an effective precipitation strengthening role during heating, rolling, and cooling, and improving the strength and toughness of the steel plate. If the carbon content is too low, the strength of the steel plate cannot be guaranteed; if it is too high, it will promote the formation of martensite, and the fully bainitic structure required by the steel of this invention cannot be obtained. Therefore, the C content of this invention is designed to be 0.06-0.11%.

[0012] Si: In the steel of this invention, an appropriate amount of Si is added to dissolve in bainite, thereby increasing the strength of the steel plate. It also inhibits the precipitation of brittle cementite between lath bainite during phase transformation and promotes the precipitation of V, Nb, and Ti carbonitrides, thus improving the strength and toughness of the steel plate. Excessive Si content impairs the plasticity and toughness of the steel plate and reduces its weldability; therefore, the Si content in the steel of this invention is designed to be 0.1–0.3%.

[0013] Mn: Expands the austenite phase region, improves the rolling and cooling process window of the steel of this invention; lowers the martensite transformation temperature, improves hardenability, and promotes the formation and refinement of lath bainite. The Mn content in the steel of this invention is 1.65–2.1%.

[0014] P and S: Both are harmful elements. P causes cold brittleness in steel, and S causes hot brittleness in steel. They are also prone to segregation and have a significant adverse effect on the low-temperature toughness and weldability of steel. Therefore, the lower the content, the better. However, considering the production cost, this invention controls the P content in steel to be ≤0.010% and the S content to be ≤0.005% as needed.

[0015] Cu: Used to form nano-Cu-rich precipitates, which improves strength without sacrificing ductility and toughness, and can also improve the weldability of steel plates; however, excessively high copper content will lead to a significant decrease in ductility and toughness. Therefore, the Cu content in the steel of this invention is designed to be 0.26–0.55%.

[0016] Mo: Lowers the critical cooling rate for bainitic transformation and promotes lath bainite formation over a wide range of cooling rates; when used in combination with manganese, it suppresses self-temper embrittlement of steel plates during post-rolling cooling and improves the stability of fine carbonitrides, promoting their dispersed distribution. The steel of this invention is designed with a Mo content of 0.1–0.39%.

[0017] V: Forms fine carbonitrides, significantly improving the strength of steel and enhancing its ductility and toughness through precipitation strengthening and grain refinement; when added in combination with Nb and Ti, it has a synergistic effect, greatly improving the strength and toughness of steel plates. The V content in this invention is designed to be 0.02–0.08%.

[0018] Nb: Forms fine carbonitrides, increases the recrystallization temperature of austenite, and refines grains; when added in combination with Nb and Ti, it has a synergistic effect, which can significantly improve the strength and toughness of steel plates; when added together with B, it inhibits the formation of Fe(C,B)6 at grain boundaries by preventing the combination of solid-solution boron segregating at grain boundaries with carbon, ensuring that B exists in solid-solution form in the steel and guaranteeing the application effect of B. In this invention, the Nb content in the steel is controlled to be 0.061–0.12% as needed.

[0019] Ti: Used to form a large number of finely dispersed TiC and TiN during the solidification of molten steel, refining the as-cast microstructure and reducing the formation of coarse columnar crystals and dendrites. To avoid the formation of coarse TiN at high temperatures, which would damage the impact toughness of the steel plate, this invention controls (Ti+Al) / N≥4 to achieve the dispersed precipitation of a large number of fine TiN particles, thereby improving the strength, toughness, and weldability of the steel plate. Therefore, the Ti content in the steel of this invention is designed to be 0.025–0.05%.

[0020] B: The steel of this invention contains an appropriate amount of boron (B). When used in conjunction with Nb, Ti, and Al, it promotes B solid solution, improves the hardenability of the steel plate, and optimizes its strength and toughness. By enriching B near austenite grain boundaries, subgrain boundaries, and dislocations, it effectively reduces grain boundary energy, inhibits the nucleation of ferrite and high-temperature bainite, and promotes the formation of fine bainite lath structures. If the B content is too high, it will increase the tendency for austenite grain coarsening; therefore, the B content of this invention is designed to be 0.001–0.003%.

[0021] Nitrogen (N) expands the austenite phase region of steel, stabilizes the austenite microstructure, improves the morphology and structure of lath bainite, and enhances the strength and toughness of steel plates. It forms fine nitrides with V, Nb, and Ti, resulting in good precipitation strengthening and grain refinement. Furthermore, by controlling (Ti+Al) / N ≥ 4, the formation of coarse TiN in the steel is suppressed, thereby improving the strength, toughness, and weldability of the steel plate. As required, the N content in this invention is designed to be 0.01–0.025%.

[0022] Al: By controlling (Ti+Al) / N≥4, a large amount of fine TiN can be dispersedly precipitated, improving the strength, toughness, and weldability of the steel plate. As needed, the Al content in the steel of this invention is designed to be 0.025–0.055%.

[0023] Ca: Used to transform the strip-shaped manganese sulfide and alumina inclusions generated in steel during smelting into spherical calcium sulfide or calcium aluminate composite inclusions. Combined with the process control of this invention, the transformed spherical inclusions are dispersed throughout the steel, thereby improving the impact performance of the steel plate. Therefore, the Ca content in the steel of this invention is controlled at 0.001–0.004%.

[0024] A low-cost, ultra-high-strength steel production method for hydropower, comprising the following steel plate manufacturing process: smelting—continuous casting—heating—controlled rolling and controlled cooling—ultrasonic flaw detection—performance testing, specifically including the following methods:

[0025] The steel of this invention is smelted using a converter + ladle refining (LF+RH) process. The RH process involves vacuum treatment with a degassing time of 11–17 min. After the RH process breaks the vacuum, a calcium wire is fed in at a rate of 0.6–1 m / s, with the calcium mass fraction controlled at 0.001–0.004%. After feeding the calcium wire, a soft blowing process is performed for 7–13 min before the steel leaves the ladle, at a flow rate of 20–27 L / min. The steel is then allowed to stand for 5–9 min. This process reduces [O] content to ≤8 ppm and [H] content to ≤3 ppm, while simultaneously reducing the content of non-metallic inclusions. It also promotes the complete transformation of residual oxides and sulfides in the molten steel into spherical shapes, improving the cleanliness of the molten steel and the dispersion of inclusions. Furthermore, it reduces the size of non-metallic inclusions in the steel to ≤1.0 grade, thereby improving the overall performance of the steel plate.

[0026] During casting, the tundish is superheated to 25–35°C. Electromagnetic stirring is introduced when the unsolidified portion of the billet is 42%–47%. The electromagnetic stirring current is controlled at 380–420A and the frequency is 6–10Hz. The billet pulling speed is controlled at 0.8–1.1m / min. Through electromagnetic stirring and billet pulling speed control, columnar crystal growth is suppressed and the central equiaxed crystal ratio is increased, thereby controlling the central segregation and central porosity to be no greater than grade 1.0.

[0027] To ensure the penetration effect of steel plate rolling and improve production efficiency, the thickness of the continuously cast billet is preferably 180-250mm.

[0028] The continuous casting billet is heated to 1150–1220℃ and held for 1–2.5 hours. The steel plate is rolled in a two-stage controlled rolling process. The first stage rolling temperature is 1020–1110℃, with a single-pass reduction rate ≥13%, and the thickness of the intermediate billet is greater than twice the thickness of the finished steel plate. The second stage rolling temperature is 870–930℃, and the final rolling temperature is 720–770℃. In the final rolling 3–5 passes, the reduction rate of two consecutive passes is guaranteed to be greater than 15%, and the total reduction rate is greater than 70%. The thickness of the rolled finished steel plate is not greater than 60 mm.

[0029] The cumulative deformation of two stages and multiple passes not only refines the austenite grains, but also generates a large number of dislocation substructures and deformation bands within the austenite grains, promoting a large number of nucleations during the bainite transformation.

[0030] Based on the compositional characteristics and performance requirements of the steel of this invention, the above heating and two-stage controlled rolling processes can fully utilize the precipitation strengthening and grain refinement effects of Nb, Ti, and V in the steel of this invention, and promote the homogenization of austenite grains, thereby effectively improving the strength and toughness of the steel plate.

[0031] The steel plate after controlled rolling is immediately water-cooled at a controlled cooling rate of 11-19℃ / s and a final cooling temperature of 320-370℃, and then air-cooled to room temperature.

[0032] Controlling the cooling rate and final cooling temperature ensures that while lath bainite is formed in the steel plate, the lath substructure is refined, promoting microstructure homogenization and further dispersion of second-phase particles, while retaining the high-density dislocations formed during rolling, thereby improving the strength, toughness, and weldability of the steel plate. Furthermore, controlling the cooling rate prevents residual stress from forming on the steel plate surface, which could affect its cold bending performance.

[0033] The thickness of the hydropower steel plate of this invention is ≤60mm. The microstructure of the hydropower steel is lath bainite, with lath widths of 200-600nm. The steel plate has a room temperature tensile strength >950MPa, yield strength >900MPa, elongation after fracture >15%; impact absorption energy at -40℃ >80J; and passes 180° cold bending. The carbon equivalent Ceq in the steel is ≤0.5%, the weld crack sensitivity coefficient Pcm is ≤0.26%, and the impact absorption energy of the welded joint at -20℃ is >80J.

[0034] Compared with the prior art, the beneficial effects of the present invention are:

[0035] 1. The steel of this invention is designed with low C, medium Mn, and low alloy composition, without adding precious metals such as Ni and Cr, resulting in lower alloy cost;

[0036] 2. Through innovative design of chemical composition and production process, this invention achieves a short-process production of 1000MPa grade high-performance hydropower steel. The steel plate does not require offline tempering heat treatment, shortening the production cycle and greatly reducing production costs.

[0037] 3. This invention provides a low-cost, ultra-high-strength hydropower steel with a thickness of no more than 60mm. The steel plate has a room temperature tensile strength >950MPa, a yield strength >900MPa, an elongation after fracture >15%, an impact absorption energy of -40℃ >80J, and is qualified for 180° cold bending.

[0038] 4. The steel plate described in this invention has high strength and toughness, as well as low carbon equivalent (Ceq≤0.5%) and welding crack sensitivity coefficient (Pcm≤0.26%), and the impact absorption energy of the welded joint at -20℃ is >80J. Attached Figure Description

[0039] Figure 1 This is a typical lath bainitic metallographic structure diagram of the present invention. Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of this invention clearer, the specific implementation methods of this invention will be further described below in conjunction with the embodiments. The following embodiments are used to specifically illustrate the content of this invention. These embodiments are only general descriptions of the content of this invention and do not limit the content of this invention.

[0041] Examples are shown in Tables 1-3, where Table 1 shows the chemical composition of each example; Table 2 shows the process parameters of the steel in each example; and Table 3 shows the properties and microstructure of the steel plates in each example.

[0042] Table 1. Chemical composition (%) of steel smelting in each embodiment

[0043]

[0044]

[0045] Table 2 Process parameters of steel in each embodiment

[0046]

[0047]

[0048] Table 3. Steel plate properties and microstructure in the examples.

[0049]

Claims

1. A low-cost, ultra-high-strength steel for hydropower, characterized in that, The chemical composition of the steel, expressed as a percentage by weight, is: C 0.06%–0.11%; Si 0.1%–0.3%; Mn 1.65%–2.1%; P≤0.010%; S≤0.005%; Cu0.26%–0.55%; Mo0.1%–0.39%; V0.02%–0.08%; Nb0.061%–0.12%; B0.001%–0.003%; N0.015%–0.025%; Ti0.025%–0.05%; Al0.025%–0.055%, and (Ti+Al) / N≥4; Ca0.001%–0.004%; balance Fe and unavoidable impurities. The aforementioned low-cost, ultra-high-strength steel production method for hydropower includes the following: During casting, the tundish is superheated to 25-35°C. Electromagnetic stirring is introduced when the unsolidified portion of the billet is 42%-47%, and the billet pulling speed is controlled to be 0.8-1.1 m / min. The continuous casting billet is heated to 1150–1220℃ and held for 1–2.5 hours. The steel plate is rolled in two stages. The first stage rolling temperature is 1020–1110℃, the single-pass reduction rate is ≥13%, and the thickness of the intermediate billet is greater than twice the thickness of the finished steel plate. The second stage rolling temperature is 870–930℃, the final rolling temperature is 720–770℃, and the reduction rate of two consecutive passes in the final rolling is greater than 15%, with a total reduction rate greater than 70%. The steel plate after controlled rolling is immediately water-cooled at a controlled cooling rate of 11-19℃ / s and a final cooling temperature of 320-370℃, and then air-cooled to room temperature.

2. The low-cost ultra-high strength hydropower steel according to claim 1, characterized in that, The thickness of steel plates used for hydropower is ≤60mm.

3. The low-cost ultra-high strength hydropower steel according to claim 1, characterized in that, The microstructure of steel used in hydropower is lath bainite, with lath widths ranging from 200 to 600 nm.

4. The low-cost ultra-high strength hydropower steel according to claim 1, characterized in that, The steel plate has a room temperature tensile strength > 950 MPa, a yield strength > 900 MPa, and an elongation after fracture > 15%; the impact absorption energy at -40℃ is > 80 J; and it passes the 180° cold bending test.

5. The low-cost ultra-high strength hydropower steel according to claim 1, characterized in that, The carbon equivalent Ceq in the steel is ≤0.5%, the weld crack sensitivity coefficient Pcm of the steel plate is ≤0.26%, and the impact absorption energy of the welded joint at -20℃ is >80J.

6. A method for producing low-cost ultra-high-strength hydropower steel as described in any one of claims 1-5, characterized in that, Includes the following: During casting, the tundish is superheated to 25-35°C. Electromagnetic stirring is introduced when the unsolidified portion of the billet is 42%-47%, and the billet pulling speed is controlled to be 0.8-1.1 m / min. The continuous casting billet is heated to 1150–1220℃ and held for 1–2.5 hours. The steel plate is rolled in two stages. The first stage rolling temperature is 1020–1110℃, the single-pass reduction rate is ≥13%, and the thickness of the intermediate billet is greater than twice the thickness of the finished steel plate. The second stage rolling temperature is 870–930℃, the final rolling temperature is 720–770℃, and the reduction rate of two consecutive passes in the final rolling is greater than 15%, with a total reduction rate greater than 70%. The steel plate after controlled rolling is immediately water-cooled at a controlled cooling rate of 11-19℃ / s and a final cooling temperature of 320-370℃, and then air-cooled to room temperature.

7. A method for producing low-cost ultra-high-strength hydropower steel according to claim 6, characterized in that, The smelting process is carried out using a converter and ladle refining method, with vacuum treatment of RH and a circulation degassing time of 11-17 minutes.

8. A method for producing low-cost ultra-high-strength hydropower steel according to claim 7, characterized in that, After the RH vacuum is broken, the Ca line is fed in at a speed of 0.6-1 m / s and the calcium mass fraction is controlled at 0.001%-0.004%. After feeding the calcium line, it is gently blown for 7-13 minutes before leaving the station at a flow rate of 20-27 L / min. After the gentle blowing, it is left to stand for 5-9 minutes.

9. A method for producing low-cost ultra-high-strength hydropower steel according to claim 6, characterized in that, The electromagnetic stirring current is controlled at 380–420A, and the frequency is 6–10Hz.

10. A method for producing low-cost ultra-high-strength hydropower steel according to claim 6, characterized in that, The thickness of the continuously cast billet is 180–250 mm.

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