A large thickness sx610cf hydroelectric steel and a production method thereof

Abstract

This invention discloses a thick SX610CF hydroelectric steel and its production method, comprising the following chemical composition by mass percentage (wt%): C: 0.08–0.10, Si: 0.10–0.15, Mn: 1.2–1.6, P ≤ 0.010, S ≤ 0.003, Als: 0.020–0.040, Nb: 0.030–0.040, V: 0.03–0.040, Cr: 0.40–0.50, Ni: 0.50–0.80, Cu: 0.22–0.26, Mo: 0.3–0.40, B: 0.0010–0.0020 Ti: 0.030-0.0040, Ba: 0.0010-0.0020, with the balance being Fe and residual elements; after converter smelting and LF+VD refining, especially with the addition of high-barium alloy in the LF refining stage, the total oxygen content of the molten steel and the melting point of inclusions can be significantly reduced, the shape and size of inclusions can be improved, inclusions can be promoted to aggregate and float, and the hazards of large inclusions in the steel can be eliminated. The grain size of the product reaches grade 10 or above. Combined with subsequent controlled rolling and cooling and quenching and tempering treatment, the final microstructure is mainly composed of tempered bainite, which improves the toughness index of the steel plate, enhances the weldability of the product, and achieves a perfect match of high strength and high toughness.

Description

Technical Field

[0001] This invention belongs to the field of thick plate production, specifically involving a 220-250mm thick SX610CF hydroelectric steel and its production method. Background Technology

[0002] The steel used in hydropower stations mainly includes the following four types: steel for foundation construction, steel for metal structures, steel for pressure pipelines, and steel for turbines. Steel for pressure pipelines is used for manufacturing the water intake pressure pipelines within the power plant and dam, as well as auxiliary equipment such as ribs, branch pipes, and spiral casings. The water intake pressure pipes and spiral casings withstand enormous water pressure heads and are pressure vessel structures. They have special requirements for materials, design methods, and welding processes that differ from general hydraulic structures, and are generally made from pressure vessel steel plates.

[0003] There are two main types of 600MPa grade steel plates for pressure pipelines: WDB620 and HT610CF. While WDB620 and HT610CF steels have comparable strength and ductility, 610CF steel exhibits superior weldability and low-temperature toughness. Because 610CF effectively addresses cracking issues in high-strength steel applications and improves toughness reserves, it is more beneficial for enhancing the safety of pressure vessels. Therefore, 610CF steel is widely used in pressure pipes and spiral casings for hydropower stations. In recent years, domestically developed HT610CF steel plates have been increasingly used in pressure pipes and spiral casings for hydropower stations, particularly in the Three Gorges Dam right bank project, where all 12 generating units (12000t) utilize domestically produced 600MPa grade steel plates. However, with the increasing installed capacity and head of hydropower stations, higher requirements are being placed on hydropower steel plates, including greater thickness, better low-temperature toughness, and improved weldability. Therefore, developing steel for thicker pressure pipelines is of great significance.

[0004] Patent document CN 109972032 A discloses a thick 610CF steel plate with low weld crack sensitivity and its production method. Its chemical composition (mass percentage) is 0.10–0.19, Si: 0.17–0.30, Mn: 1.00–1.45, P: ≤0.018, S: ≤0.005, Cr+MO+Ni: ≤3.5, V+Nb+Ti: ≤0.1, B: 0.001–0.002, Als: 0.015–0.050, with the remainder being Fe and trace impurities. The 150mm 610CF produced using the composition described in this document and the high-temperature recrystallization rolling + two-phase sub-temperature quenching process meets the performance requirements. However, this method results in a limited thickness range for the produced steel plate, a complex heat treatment process, a long production cycle, and high production costs.

[0005] Patent document CN 115717219 A discloses a high-strength steel Q610CF for hydropower engineering and its production method. Its chemical composition (mass percentage) is 0.060–0.12%, Si: 0.15–0.40%, Mn: 1.10–1.60%, P: ≤0.012%, S: ≤0.005%, Cr: 0.4–1.00%, Ni: 0.25–0.60%, Nb: 0.030–0.050%, Mo: 0.15–0.65%, Ti: 0.015–0.022%, B: 0.0010–0.0020%, Als: ≤0.050%, with the remainder being Fe and residual elements. The 610CF produced using the composition, clean steel smelting, continuous casting, controlled cooling, and slow cooling process described in this document meets the performance requirements, but the thickness range of the produced steel plates is only 20–50 mm. Summary of the Invention

[0006] To address the aforementioned technical deficiencies, the present invention aims to provide a thick SX610CF hydroelectric steel plate. The internal quality and thickness of this hydroelectric steel plate can reach the level of forgings, achieving a perfect match of high strength and high toughness, and can meet the requirements for long-term service in low-temperature environments.

[0007] Another objective of this invention is to provide a method for producing thick SX610CF hydroelectric steel.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is: a thick SX610CF hydroelectric steel, wherein the thickness of the hydroelectric steel is 220-250mm, and it contains the following chemical composition by mass percentage (unit, wt%): C: 0.08~0.10, Si: 0.10~0.15, Mn: 1.2~1.6, P≤0.010, S≤0.003, Als: 0.020~0.040, Nb : 0.030~0.040, V: 0.03-0.040, Cr: 0.40-0.50, Ni: 0.50-0.80, Cu: 0.22-0.26, Mo: 0.3~0.40, B: 0.0015-0.0025, Ti: 0.030-0.0040, Ba: 0.0010-0.0020, others are Fe and residual elements, carbon equivalent Ceq≤0.6%;

[0009] The microstructure of the hydroelectric steel is a uniform and fine tempered bainite structure, with non-metallic inclusions A+B+C+D ≤ grade 2.0 and banded structure less than grade 1.0; its yield strength is ≥ 450 MPa, tensile strength is 590-730 MPa, elongation after fracture is ≥ 17%, longitudinal and transverse impact energy at -20℃ is ≥ 47 J, and transverse impact energy after 5% strain aging is ≥ 47 J; the hydroelectric steel is in a quenched and tempered state.

[0010] The production method of the aforementioned thick SX610CF hydroelectric steel includes clean steel smelting, involving converter smelting, LF refining, and VD refining, as detailed below:

[0011] a. Converter smelting: tapping temperature ≥1600℃, 0.04% ≤ C ≤ 0.07%, P ≤ 0.007%. After tapping, slag is blocked by a slag-blocking cone. If the slag blocking fails, the furnace must be lifted in advance to ensure that the thickness of the slag in the converter is controlled below 20mm to avoid the slag returning to P.

[0012] b. LF Refining II follows the large slag volume process standard for slag making, with lime added at 1000-1200 kg and basicity controlled at 4.0-6.0. During the refining process, the white slag is required to be maintained for 30-35 minutes. After alloying, the oxygen content of the molten steel is determined, with an oxygen content ≤10ppm. After the oxygen content is determined, 0.6-0.8 kg / t of high barium alloy is added for precipitation deoxidation, with an oxygen content ≤5ppm. The molten steel is then vacuum treated by VD.

[0013] The high-barium alloy is an aluminum-barium alloy with a barium content of 30% to 40%.

[0014] c. VD refining: The VD holding time should be controlled at ≥18min. Under a vacuum of 67Pa, the molten steel should have good turbulence during the holding process, and the H content should be ≤1.0PPm.

[0015] Furthermore, the production method of the hydroelectric steel also includes casting 930-960mm thick water-cooled steel ingots, heating, rolling, and heat treatment, as detailed below:

[0016] a. Heating process: The steel ingot is kept at a temperature of 1200-1220℃ in the heating pit for 12-13 hours;

[0017] b. Rolling process: The initial rolling temperature is ≥1020℃, and the final rolling temperature is ≤950℃. High-pressure water is used to cool the steel plate during the rolling process. After the steel plate surface is fully reddened, rolling continues. The total rolling time is controlled within 4-5 minutes. After rolling, the ACC is turned on and the steel plate is cooled twice in water to quickly reduce the surface and core temperature to the non-recrystallization zone. The water temperature is 900-950℃ and the reddening temperature is 800-830℃.

[0018] c. Heat treatment process: High-temperature quenching + tempering process is adopted. The steel plate is heated to 920-940℃ in an external mechanized furnace and held for 2.4 min / mm. After the hot steel plate is taken out of the furnace, it is cooled to room temperature in a quenching machine within 1 minute. The steel plate temperature is 890-900℃ when it is immersed in water and the water temperature is 15-18℃. After quenching, it is put into a roller hearth tempering furnace for tempering at 620-650℃ for 3.5-4.0 min / mm.

[0019] The beneficial effects of this invention are as follows: The use of a top-and-bottom blown converter for smelting, followed by LF+VD refining, solves the problem of cleanliness in thick hydroelectric steel. In particular, the high-barium alloy treatment effectively reduces the total oxygen content of the steel, decreases the amount of oxide inclusions, and improves the shape and size of inclusions. The barium oxides appear as dots and spheres with a diameter of less than 5μm, eliminating the hazards of large inclusions in the steel. Furthermore, some barium remains unoxidized, ensuring the lowest possible level of oxide inclusions, thereby improving the cleanliness of the steel, with an oxygen content ≤5ppm.

[0020] This invention employs controlled rolling and cooling, along with quenching and tempering, to form a microstructure dominated by tempered bainite. The grain size of the product reaches grade 10 or higher, improving the toughness of the steel plate and enhancing its weldability. Attached Figure Description

[0021] The technical features of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0022] Figure 1 This is a schematic diagram of the metallographic structure of the thickness section of Embodiment 1 of the present invention (metallographic microscope, ×100).

[0023] Figure 2 This is a schematic diagram of the metallographic structure of the thickness section of Embodiment 1 of the present invention (metallographic microscope, ×200).

[0024] Figure 3 This is a schematic diagram of the metallographic structure of the thickness section of Embodiment 1 of the present invention (metallographic microscope, ×500). Detailed Implementation

[0025] The present invention will be further described below with reference to the embodiments.

[0026] This invention provides a method for producing thick SX610CF hydroelectric steel, comprising the following process flow: hot metal pretreatment, converter smelting, LF refining, VD refining, casting, heating, rolling, and heat treatment, specifically completed through the following steps;

[0027] The slag before the molten iron arrives at the KR pretreatment station must be completely removed to ensure that the slag layer thickness on the liquid surface is ≤25mm. After KR treatment, the sulfur content of the molten iron is controlled below 0.005%, and the desulfurization temperature drop is ≤25℃.

[0028] Converter smelting: The molten iron charged into the furnace has S≤0.005% and P≤0.050%, and the molten iron temperature≥1280℃. The scrap steel added to the converter must be dry, high-quality offcuts and nickel-alloyed special scrap steel. The converter charging amount is controlled according to the casting residue of 6-8 tons. The tapping temperature is ≥1600℃, 0.04%≤C≤0.07%, and P≤0.007%. After tapping, slag is blocked by a slag-blocking cone. If the slag blocking fails, the furnace must be lifted in advance to ensure that the slag thickness at the bottom of the converter is controlled below 20mm to avoid slag back to P.

[0029] LF refining follows the large slag volume process standard for slag preparation. Lime is added at a rate of 1000-1200 kg, and the basicity is controlled at 4.0-6.0. After the first heating for 3 minutes, 50 kg of calcium carbide is added, followed by 20-40 kg of aluminum granules. Every 2 minutes thereafter, 2-4 shovelfuls of aluminum granules are added to the ladle using an iron shovel to ensure the slag turns white. During the second heating, 10-30 kg of calcium carbide is added each time, depending on the submerged arc effect, while 1-3 shovelfuls of aluminum granules are added to the ladle every 2 minutes to maintain white slag throughout the second heating process. During the third heating, the deoxidizer is added according to the slag color, maintaining white slag. The white slag must be maintained for 30-35 minutes during refining. After alloying, oxygen determination is performed in the molten steel, with an oxygen content ≤10 ppm. After oxygen determination, 0.7 kg / t of aluminum-barium alloy is added for precipitation deoxidation, with an oxygen content ≤5 ppm. The molten steel is then vacuum-treated using a vacuum deoxidizer (VD).

[0030] VD refining: The VD holding time should be controlled at ≥18min. Under a vacuum of 67Pa, the molten steel should have good turbulence during the holding process, and the H content should be ≤1.0PPm. After breaking the vacuum, add rice husks for soft blowing for 5-8min in time, and then prepare to lift the ladle to leave the station. The temperature at the station should be controlled at 1570-1575℃.

[0031] Casting: The temperature of the heat-insulating cap must be greater than 150℃. Use a 45-ton fixed-width, adjustable-thickness water-cooled steel ingot mold. The thickness of the steel ingot body is 930-960mm. During the mold placement process, ensure that all auxiliary materials are dry and that the gating system is dry and clean. The water-cooled mold must have a taper (30mm) and a side extrusion mechanism to compress the wide edge of the ingot in the later stage of solidification, further forming a dense structure. Take proper heat-insulating measures at the ingot cap opening to form a feeding channel that is larger at the top and smaller at the bottom inside the ingot during solidification. The casting temperature should be controlled at 1560-1565℃. After the molten steel arrives at the station, the temperature should be within the upper limit of the standard to the upper limit +3℃. After temperature measurement and confirmation that it meets the casting requirements, casting can begin. If the temperature of the molten steel arriving at the station does not meet the aforementioned standard, soft argon blowing should be performed immediately. After argon blowing, it is required to calm for 5 minutes before casting can begin. After casting, the heat-insulating effect of the cap opening must be ensured.

[0032] Heating: The target temperature of the high-temperature section of the heat spreader is 1200-1220℃, and the total heat preservation time is 12-13 hours.

[0033] Rolling: The initial rolling temperature is ≥1020℃, and the final rolling temperature is ≤950℃. High-pressure water is used to cool the steel plate during the rolling process. Rolling can only begin after the steel plate surface has fully turned red. The total rolling time is controlled within 4-5 minutes. After rolling, the ACC is turned on and the steel plate is cooled twice in water to quickly reduce the surface and core temperature to the non-recrystallization zone. The water temperature is 900-950℃ and the reddening temperature is 800-830℃.

[0034] Heat treatment: To effectively improve strength and ensure low-temperature impact toughness, a high-temperature quenching + tempering process is adopted. The external mechanized furnace is heated to 920-940℃ for high-temperature quenching, and the holding time is equal to the actual thickness of the steel plate x 2.4 min / mm. After the hot steel plate is taken out of the furnace, it is cooled to room temperature for 120 minutes in a quenching machine within 1 minute. When immersed in water, the steel plate temperature is 890-900℃ and the water temperature is 15-18℃. After quenching, it is placed in a roller hearth tempering furnace at 620-650℃ for tempering, and the tempering time is 3.5-4.0 min / mm.

[0035] Examples 1 and 2 respectively provide 230mm thick and 240mm thick high-strength, low-weld-crack-susceptibility SX610CF steel plates, whose chemical compositions are shown in Table 1 below:

[0036] Table 1. Chemical composition (wt%) of the steel in the examples

[0037]

[0038] Wherein: the carbon equivalent Ceq of Example 1 is C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14=0.51, and the carbon equivalent Ceq of Example 2 is C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14=0.50.

[0039] Testing and Analysis: The sampling location and specimen preparation for the mechanical properties of the steel plates were carried out in accordance with the provisions of GB / T 2975 for mechanical property testing. The low-temperature impact toughness test was carried out according to GB / T 229, and the tensile property test was carried out according to GB / T 228. A comprehensive inspection of the mechanical properties and metallographic structure of the steel plates in the supplied state was conducted. The mechanical property test results of the steel plates in Example 1 and Example 2 are shown in Table 2 below:

[0040] Table 2. Test results of mechanical properties of the steel in the examples.

[0041]

[0042] Based on the data above, the steel plate has sufficient strength reserves, excellent low-temperature impact toughness and strain-aged impact, and the mechanical properties of the steel plate are relatively similar at the beginning and end, with all performance indicators being excellent.

[0043] The metallographic structure of the 230mm thick SX610CF steel plate section obtained in Example 1 is shown in the attached figure. Figure 1-3 As shown.

[0044] Metallographic test results: (1) Inclusions A+B+C+D ≤ 2.0 grade; (2) Structure: tempered bainite structure;

[0045] External inspection and flaw detection: The external inspection pass rate of the developed steel plate is 100%, and the final flaw detection of the steel plate meets the Class I flaw detection requirements of GB / T 2970 "Inspection Method for Thick Steel Plates".

[0046] The above description is only a preferred embodiment of the present invention. The above specific embodiments are not intended to limit the present invention. Any modifications, alterations or equivalent substitutions made by those skilled in the art based on the above description shall fall within the protection scope of the present invention.

Claims

1. A method for producing thick SX610CF hydroelectric steel, characterized in that: The steel has a thickness of 220-250 mm and contains the following chemical composition by mass percentage: C: 0.08-0.10, Si: 0.10-0.15, Mn: 1.2-1.6, P≤0.010, S≤0.003, Als: 0.020-0.040, Nb: 0.030-0.040, V: 0.03-0.040, Cr: 0.40-0.50, Ni: 0.50-0.80, Cu: 0.22-0.26, Mo: 0.3-0.40, B: 0.0015-0.0025, Ti: 0.030-0.040, Ba: 0.0010-0.0020, with the remainder being Fe and residual elements, and a carbon equivalent (Ceq) ≤0.6%. The metallographic structure of the steel is a uniform and fine tempered bainite structure, with non-metallic inclusions A+B+C+D ≤ grade 2.0 and banded structure less than grade 1.0; its yield strength is ≥ 450 MPa, tensile strength is 590-730 MPa, elongation after fracture is ≥ 17%, longitudinal and transverse impact energy at -20℃ is ≥ 47 J, and transverse impact energy after 5% strain aging is ≥ 47 J; the steel is in a quenched and tempered state. The production method of the aforementioned thick SX610CF hydroelectric steel also includes casting 930-960mm thick water-cooled steel ingots, heating, rolling, and heat treatment, as detailed below: a. Heating process: The steel ingot is kept at a temperature of 1200-1220℃ in the heating pit for 12-13 hours; b. Rolling process: The initial rolling temperature is ≥1020℃, and the final rolling temperature is ≤950℃. High-pressure water is used to cool the steel plate during the rolling process. After the steel plate surface is fully reddened, rolling continues. The total rolling time is controlled within 4-5 minutes. After rolling, the ACC is turned on and the steel plate is cooled twice in water to quickly reduce the surface and core temperature to the non-recrystallization zone. The water temperature is 900-950℃ and the reddening temperature is 800-830℃. c. Heat treatment process: High-temperature quenching + tempering process is adopted. The steel plate is heated to 920-940℃ in an external mechanized furnace and held for 2.4 min / mm. After the hot steel plate is taken out of the furnace, it is cooled to room temperature in a quenching machine within 1 minute. The steel plate temperature is 890-900℃ when it is immersed in water and the water temperature is 15-18℃. After quenching, it is put into a roller hearth tempering furnace for tempering at 620-650℃ for 3.5-4.0 min / mm.

2. The production method of the thick SX610CF hydroelectric steel according to claim 1, characterized in that: The steel production includes clean steel smelting, involving converter smelting, LF refining, and VD refining, as detailed below: a. Converter smelting: tapping temperature ≥1600℃, 0.04%≤C≤0.07%, P≤0.007%. After tapping, slag is blocked by a slag-blocking cone. If the slag blocking fails, the furnace must be lifted in advance to ensure that the thickness of the slag in the converter is controlled below 20mm to avoid the slag returning to P. b. LF Refining: Slag making is carried out according to the large slag volume process standard. The amount of lime added is 1000-1200 kg, and the basicity is controlled at 4.0-6.

0. During the refining process, the white slag is required to be maintained for 30-35 minutes. After alloying, the oxygen content of the molten steel is determined. The oxygen content is ≤10ppm. After the oxygen determination is completed, 0.6-0.8 kg / t of high barium alloy is added for precipitation deoxidation. The oxygen content is ≤5ppm. The molten steel is then vacuum treated by VD. c. VD refining: The VD holding time should be controlled at ≥18min, and the molten steel should have good turbulence during the holding process under a vacuum of 67Pa. The H content should be ≤1.0ppm.

3. The production method of thick SX610CF hydroelectric steel according to claim 2, characterized in that: The high-barium alloy is an aluminum-barium alloy with a barium content of 30% to 40%.

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