Pressure vessel steel plate used for low-temperature spherical tank and having good property uniformity, and manufacturing method therefor

Abstract

The present invention relates to the technical field of steel, in particular to a pressure vessel steel plate used for a low-temperature spherical tank and having good property uniformity, and a manufacturing method therefor. The chemical components of the steel plate, in percentages by weight, are as follows: C: 0.070%-0.100%, Si: 0.15%-0.40%, Mn: 1.45%-1.60%, P≤0.015%, S≤0.003%, Nb: 0.030%-0.050%, Ti: 0.015%-0.025%, V: 0.030%-0.040%, Ni: 0.20%-0.25%, Mo: 0.06%-0.11%, Cu: 0.10%-0.18%, and Alt: 0.015%-0.045%, and with the balance of being Fe and inevitable impurities. In the present invention, by means of a process including nickel-molybdenum niobium-vanadium-titanium microalloying, the smelting of a continuously cast slab, controlled rolling and controlled cooling, differential temperature rolling, and a tempering treatment, the resulting product has well-balanced strength and toughness and a flat plate shape, and the phenomena of a large difference in mechanical properties in the thickness direction and scattering in low-temperature impact toughness at the core are greatly ameliorated.

Description

A pressure vessel steel plate for cryogenic spherical tanks with good performance uniformity and its manufacturing method. Technical Field

[0001] This invention relates to the field of steel technology, and more particularly to a pressure vessel steel plate for low-temperature spherical tanks with good performance uniformity and its manufacturing method. Background Technology

[0002] With the rapid development of modern industrial technology, pressure vessel equipment is constantly evolving towards larger size, thicker walls, and higher parameters. Cryogenic spherical tanks, as indispensable key equipment in the field of cryogenic storage and transportation, are generally designed with a thickness exceeding 40mm to meet increasing storage demands and higher safety standards. This presents even more stringent challenges to the manufacturing of steel plates for cryogenic spherical tanks, requiring materials to not only possess excellent mechanical properties but also to achieve efficient and low-cost production during the manufacturing process.

[0003] To shorten the process flow, improve production efficiency, and reduce production costs, companies tend to adopt online controlled rolling and cooling processes to produce low-temperature spherical tank steel plates. However, when the steel plate thickness is large, especially exceeding 40mm, traditional controlled rolling and cooling processes struggle to achieve uniform deformation and cooling between the steel plate surface and the core (i.e., the half-thickness section). The deformation conditions near the surface and the core differ significantly, resulting in large differences in mechanical properties along the thickness direction for medium-thick plates. Although this can be improved by adjusting subsequent heat treatment processes, it easily leads to a deterioration in the low-temperature impact toughness of the core. Summary of the Invention

[0004] The purpose of this invention is to overcome the above-mentioned problems and deficiencies, and to provide a pressure vessel steel plate for cryogenic spherical tanks with good performance uniformity and a method for manufacturing the same.

[0005] This invention employs a process of nickel-molybdenum-niobium-vanadium-titanium microalloying + continuous casting billet smelting + controlled rolling and cooling + differential temperature rolling + tempering treatment, which enables the product to have good strength and toughness matching and a straight plate shape, and greatly improves the phenomenon of large differences in mechanical properties in the thickness direction and low-temperature impact dispersion in the core.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] This invention provides a pressure vessel steel plate for cryogenic spherical tanks with good performance uniformity. The chemical composition of the steel plate, by weight percentage, is as follows:

[0008] C: 0.070%–0.100%, Si: 0.15%–0.40%, Mn: 1.45%–1.60%, P≤0.015%, S≤0.003%, Nb: 0.030–0.050%, Ti: 0.015%–0.025%, V: 0.030%–0.040%, Ni: 0.20%–0.25%, Mo: 0.06%–0.11%, Cu: 0.10%–0.18%, Alt: 0.015%–0.045%, with the remainder being Fe and unavoidable impurities.

[0009] The reasons for using the above-mentioned components are as follows:

[0010] (1) C: C is the main component element of steel. The strength of steel mainly depends on the C content. Excessive C content will lead to poor toughness, plasticity and weldability of steel; low C content will lead to lower strength and performance after simulated stress relief treatment. In order to ensure that the steel plate has a good match of low-temperature impact toughness, strength and weldability during use, the C content in the steel of this invention is controlled at 0.070% to 0.100%.

[0011] (2) Si: Si is a common solid solution strengthening alloying element in steel. It is essential for the strength, toughness, hardenability and even the deoxidation of steel. However, a high content will also lead to a decrease in the toughness of steel. Therefore, the Si content in the steel of this invention is controlled at 0.15% to 0.40%.

[0012] (3) Mn: Mn can strengthen pearlite in steel through solid solution strengthening. C-Mn strengthening is also the main way to improve the strength of low carbon steel. However, if the Mn content is too high, it will increase the production cost. Mn is easy to combine with S to form MnS, which reduces the material's resistance to hydrogen-induced cracking. At the same time, the Mn content will reduce the activity of carbon. Therefore, the Mn content in the steel of this invention is controlled at 1.45% to 1.60%.

[0013] (4) P: Phosphorus is a harmful element in steel, increasing its cold brittleness, worsening its weldability, reducing its plasticity, and worsening its cold bending performance. Furthermore, P is particularly sensitive to radiation embrittlement. Therefore, the lower the P content in the steel of this invention, the better, ideally controlled below 0.015%.

[0014] (5) S: Sulfur is a harmful element under normal circumstances. S usually forms brittle sulfides with alloying elements in steel, causing hot brittleness and reducing the ductility and toughness of the steel. At the same time, S also tends to accelerate radiation embrittlement. Therefore, the S content in the steel of this invention is controlled below 0.003%.

[0015] (6) V: V is a microalloying element. V microalloying in steel can form fine second phase particles, which can play the role of pinning grain boundaries and precipitation strengthening. It can effectively refine grains and greatly improve the comprehensive mechanical properties of steel such as strength, toughness, ductility and thermal fatigue resistance. Therefore, the V content in the steel of this invention is controlled at 0.030% to 0.040%.

[0016] (7) Nb: As a strong carbide-forming element, Nb forms a highly dispersed NbC phase with good high-temperature stability in steel, playing a precipitation strengthening role. Through multi-stage rolling, it can effectively refine the grains and improve the reduction in toughness caused by precipitation strengthening, thereby enabling the steel plate to obtain comprehensive properties of high strength and high toughness. In addition, in Nb-Mo composite steel, Mo can also agglomerate at the NbC matrix interface, preventing the coarsening of NbC particles, thereby greatly improving the high-temperature strength of the steel. Therefore, the Nb content in this invention is controlled at 0.030% to 0.050%.

[0017] (8) Ti: Adding an appropriate amount of Ti can form a large number of dispersed fine TiN or Ti2O3 particles, which can serve as heterogeneous nucleation sites for acicular pearlite during solidification, thereby refining the microstructure. Ti also has a deoxidizing effect, ensuring that B is not oxidized or nitrided. B can lower the transformation temperature from austenite to pearlite, promoting the formation of acicular pearlite within the grains and refining the grains. However, when w(Ti)≥0.09%, the content of acicular pearlite will decrease, causing the low-temperature toughness of the steel plate to deteriorate. Therefore, the Ti content in the steel of this invention is controlled at 0.015% to 0.025%.

[0018] (9) Alt: Adding a small amount of Al to steel can effectively refine the austenite grains, thereby refining the ferrite grains and microstructure, and improving the impact toughness of the steel. However, Al has the disadvantage of affecting the hot working performance, weldability, and machinability of the steel. Therefore, the Alt content in the steel of this invention is controlled at 0.015% to 0.045%.

[0019] (10) Ni: Ni is a solid solution strengthening element in steel that can improve the strength of steel. Ni reduces the resistance to dislocation movement in steel, relaxes stress, and changes the substructure of the matrix, thereby improving the toughness of steel, especially the low-temperature toughness. However, excessively high Ni content in medium carbon steel will increase the phase transformation temperature. Therefore, the Ni content in this invention is controlled at 0.20% to 0.25%.

[0020] (11) Mo: Mo mainly relies on solid solution strengthening and grain boundary strengthening to improve the strength of steel; secondly, Mo increases the stability of supercooled austenite, causing the austenite to pearlite transformation curve to shift to the right, resulting in a finer pearlite structure after phase transformation; in addition, Ti and Mo combine to precipitate a large amount of nano-sized Ti-Mo(CN) carbides in the steel, and the refined carbides pin dislocations, greatly improving the strength and toughness of the steel. Therefore, the Mo content in the steel of this invention is controlled at 0.06% to 0.11%.

[0021] (12) Cu: The prominent role of Cu in steel is to improve the corrosion resistance of plain carbon low-alloy steel, and it can also increase the strength and yield strength ratio of steel, without adversely affecting the weldability. At the same time, its role is similar to that of nickel, which can play a certain role in saving nickel and reducing costs. However, when the content is high, it will lead to copper embrittlement during hot deformation processing. Therefore, the Cu content in the steel of this invention is controlled at 0.10% to 0.18%.

[0022] In the above technical solution, the thickness of the finished steel plate is further 40-60mm.

[0023] In the above technical solution, the tensile strength in the transverse direction at 1 / 4 and 1 / 2 of the thickness of the finished steel plate is 650-700 MPa, the yield strength is 530-580 MPa, the elongation after fracture is 19-24%, the impact energy at -50℃ is 150-250 J, the welding heat energy is 140-150 KJ / cm, and the impact energy at -50℃ in the heat-affected zone after welding is 100-200 J.

[0024] Another aspect of the present invention provides a method for manufacturing the above-mentioned steel plate, comprising the following steps:

[0025] (1) Continuous casting billet smelting:

[0026] The RH inlet temperature is 1590–1620℃, and the RH oxygen blowing rate is 25–45 m³ / h. 3 / h, circulating oxygen blowing time 10-20min, effectively preventing secondary oxidation of molten steel. At the same time, a two-step aluminum addition method is adopted for deoxidation, namely, adding aluminum cakes for pre-deoxidation during the converter tapping process and adding aluminum wire for deoxidation during the LF furnace refining process. Continuous casting adopts multi-stage argon-protected casting and adopts continuous casting dynamic light reduction process, with a reduction range of 4.0-7.0mm, a reduction rate of 0.3-0.5mm / s, and a continuous casting billet thickness of 250-300mm;

[0027] (2) Controlled rolling and controlled cooling:

[0028] After heating, the continuously cast billet undergoes rough rolling. The core temperature of the billet during rough rolling is 1100–1130℃, and the core temperature during the final rough rolling is 1050–1080℃. After rough rolling, the billet enters a laminar flow cooling device located between the roughing and finishing mills, where high-pressure cooling water is sprayed to cool it down. The core temperature of the billet during finishing rolling is 885–915℃, and the core temperature during the final finishing rolling is 815–855℃. After finishing rolling, the billet is ejected at a high speed of 4–5 m / s to reduce the size of the steel plate. The temperature drops, followed by pre-straightening, and then ACC laminar flow cooling. The initial cooling temperature is 780-830℃ in the core of the steel plate, which allows the austenite grains to retain more stored energy. The cooling rate is 30-50℃ / s water cooling, and the steel plate turns red at the core temperature of 480-580℃. At the same time, the head and tail of the pre-straightened steel plate are shielded during ACC laminar flow cooling, so that the temperature difference between the head and tail of the steel plate is less than 20℃. After ACC laminar flow cooling, the flatness of the steel plate is ≤8mm / m.

[0029] (3) Tempering heat treatment:

[0030] The tempering temperature is 620-640℃, the heating rate is 1.5-2.5 min / mm, the net holding time is 40-150 min, and after the holding time is completed, the temperature is air-cooled to room temperature.

[0031] (4) Straightening: The flatness of the finished steel plate after straightening is ≤5mm / m.

[0032] In the above technical solution, further, in step (1), during converter smelting, the size of the furnace charge is 50-100mm, the hot charging temperature of the furnace charge is 1150-1200℃, and the net heat preservation time is 3-6min;

[0033] The amount of aluminum ingot added is 5-10 kg / ton, and the amount of aluminum wire added is 5-10 kg / ton.

[0034] In the above technical solution, further, in step (1), the mold protective slag is composed of the following components by weight percentage: CaO 35%~50%, Al2O3 30%~40%, MnO 0~10%, SiO2 5%~10%, with the balance being unavoidable impurities. The thickness of the slag film is 0.1~1.5mm, ensuring that the inclusions are fully floated and removed, and preventing the generation of linear defects in the billet.

[0035] In the above technical solution, further, in step (1), the casting temperature is 1530~1550℃ and the casting speed is 0.9~1.0m·min. -1 The steel billet is placed in a slow cooling pit after it leaves the production line. The slow cooling temperature is 300-400℃ and the holding time is 12-24 hours.

[0036] In the above technical solution, further, in step (1), the multi-stage argon-protected casting method is as follows: argon gas is introduced from the ladle to the tundish for protection, with an argon gas pressure of 0.1-0.3 MPa, an argon gas flow rate of 2-5 L / min, and a free O2 content of 0.1%-0.5% in the protective atmosphere; argon gas is introduced from the tundish to the crystallizer for protection, with an argon gas pressure of 0.3-0.5 MPa, an argon gas flow rate of 1-3 L / min, and a free O2 content of 0.5%-1% in the protective atmosphere.

[0037] In the above technical solution, further, in step (2), the heating adopts a four-stage heating method: the temperature of heating stage I is 600-650℃, the temperature of heating stage II is 1000-1050℃, the temperature of heating stage III is 1170-1210℃, the temperature of the heat soaking stage is 1150-1190℃, the heat soaking stage time is 30-40 min, and the total heating time is H*(0.9-1.2) min·mm. -1 H represents the thickness of the continuously cast billet, in mm; appropriate heating regime effectively controls the original austenite grain size and ensures sufficient solid solution of alloying elements, thus guaranteeing good final performance of the product.

[0038] In the above technical solution, further, in step (2), rolling is carried out on a double-stand rolling mill, with a reduction rate of 15% to 25% per pass in roughing, the thickness of the intermediate billet being 3 to 4 times the thickness of the finished steel plate, and a reduction rate of 5% to 15% per pass in finishing.

[0039] In the above technical solution, further, in step (2), the parameters of ACC laminar flow cooling are: 10-12 sets of manifolds, roller speed 1.2-1.4 m / s, and water volume 240-260 mm. 3 / h, water temperature 14~20℃.

[0040] In the above technical solution, further, in step (2), the roll gap setting parameters of the pre-straightening machine system are adjusted according to the thickness of the finished steel plate: the tilt correction range is 0 to +1.5 mm, the roll gap correction range is -1.5 to +0.5 mm, and the load reduction range is +0.5 to +1.0 mm, so that the steel plate after pre-straightening is straight and can smoothly enter the ACC area, avoiding a series of problems such as uneven temperature and plate warping caused by local water accumulation, asymmetrical cooling, and uneven cooling during the water cooling process.

[0041] In the above technical solution, further, in step (2), according to the water outlet plate type of the online quenched steel plate, the head and tail of the steel plate are shielded and controlled, and the shielding flow rate is controlled at 5% to 10%.

[0042] Shielding control principle: The head and tail of the steel plate are close to the body temperature, reducing the number of cooling manifold groups that use shielding control and shortening the flow adjustment time; with the use of ACC head and tail shielding function, the temperature uniformity of the steel plate is good, and the online quenched steel plate is straight, which meets the requirements of the tempering furnace for the plate shape of the steel plate entering the furnace.

[0043] The beneficial effects of this invention are as follows:

[0044] 1. The purpose of using niobium-vanadium-titanium-nickel-molybdenum microalloying is to control the expected transformation of the microstructure and lay the foundation for the strength and toughness of the steel plate.

[0045] 2. By using multi-stage Ar gas-protected casting and continuous casting dynamic light reduction process, the grade of inclusions and content of gas elements in the billet are greatly reduced, the center segregation and center porosity of the billet are reduced, the internal quality of the billet is improved, and thus the purity of steel is improved.

[0046] 3. Based on controlled rolling and controlled cooling, differential temperature rolling process is adopted. Before rolling, cooling is used to create a gradient temperature difference in the thickness direction of the steel plate. The surface temperature is lower and the deformation resistance is greater. The temperature is higher at 1 / 2 of the plate thickness and it is easier to deform. The deformation can be transferred to the core of the steel plate, increasing the amount of core deformation, refining the austenite grains in the core, eliminating the performance difference in the thickness direction of the steel plate, and ensuring that the core of the thick plate still has a good strength and toughness match.

[0047] 4. Tempered bainite + ferrite microstructure is obtained through tempering heat treatment. The second phase is dispersed and uniform, resulting in a product with good strength and toughness. Attached Figure Description

[0048] Figure 1 is a metallographic photograph of the steel plate of Example 1. Detailed Implementation

[0049] The following examples are intended to enable those skilled in the art to more fully understand the present invention, but do not limit the invention in any way.

[0050] Examples 1-7

[0051] The chemical composition of the steel plates in Examples 1-7 of this invention is shown in Table 1.

[0052] Table 1. Chemical composition (wt%) of the steel plates in Examples 1-7 of this invention.

[0053] Table 2 Main process parameters for continuous casting billet smelting in Examples 1-7 of the present invention

[0054] Table 3 Chemical composition and slag film thickness of crystallizer protective slag in Examples 1-7 of the present invention

[0055] Table 4 Casting process parameters for Examples 1-7 of the present invention

[0056] Table 5. Low-magnification evaluation grades and gaseous element content of continuously cast billets in Examples 1-7 of the present invention. Note: 1. GB / T 226-2015 "Acid Etching Test Method for Low Magnification Structure and Defects of Steel" 2. YB / T 4307-2012 "Determination of Oxygen, Nitrogen and Hydrogen Content in Iron and Steel and Alloys - Pulse Heating Inert Gas Fusion-Infrared and Thermal Conductivity Method"

[0057] Table 6 Heating process parameters for continuous casting billets in Examples 1-7 of the present invention

[0058] Table 7 Steel plate rolling process parameters of Examples 1-7 of the present invention

[0059] Table 8. Steel plate pre-straightening process parameters in Examples 1-7 of the present invention.

[0060] Table 9. ACC laminar flow cooling process parameters for steel plates in Examples 1-7 of the present invention.

[0061] Table 10 Steel heat treatment process parameters of the embodiments of the present invention

[0062] Table 11 Comprehensive mechanical properties of steel plates in Examples 1-7 of the present invention Note: 3. GB-T713.6-2023-Steel plates and strips for pressure equipment - Part 6: Quenched and tempered high-strength steel

[0063] The entire steel plate shall be ultrasonically tested in accordance with NB / T 47013.3. The scanning method is as follows: the probe shall be used to scan parallel lines perpendicular to and parallel to the rolling direction of the steel plate at intervals of 190-200 mm. 100% scanning shall be performed within a range of 40-50 mm on both sides of the predetermined line of the steel plate bevel. The defect assessment shall be T1 grade as specified in NB / T 47013.3 as qualified, with a pass rate of 100%.

[0064] Table 12 Results of flaw detection performance tests on steel plates in delivery condition in Examples 1-7 of the present invention

[0065] Table 13 Non-metallic inclusions in steel plates delivered in Examples 1-7 of the present invention 4 Note: 4. GB / T 10561-2023 "Determination of Non-metallic Inclusion Content in Steel - Standard Rating Chart Microscopic Examination Method"

[0066] As shown in Figure 1, the steel plate has a microstructure of 60%-70% tempered bainite and 30%-40% ferrite.

[0067] To illustrate the present invention, the present invention has been appropriately and sufficiently described above through embodiments. The above embodiments are only for illustrating the present invention and are not intended to limit the present invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Any modifications, equivalent substitutions, improvements, etc., should be included within the protection scope of the present invention. The patent protection scope of the present invention should be defined by the claims.

Claims

1. A pressure vessel steel plate for cryogenic spherical tanks with good performance uniformity, characterized in that, The chemical composition of the steel plate, by weight percentage, is as follows: C: 0.070%–0.100%, Si: 0.15%–0.40%, Mn: 1.45%–1.60%, P≤0.015%, S≤0.003%, Nb: 0.030–0.050%, Ti: 0.015%–0.025%, V: 0.030%–0.040%, Ni: 0.20%–0.25%, Mo: 0.06%–0.11%, Cu: 0.10%–0.18%, Alt: 0.015%–0.045%, with the remainder being Fe and unavoidable impurities.

2. The pressure vessel steel plate for cryogenic spherical tanks with good performance uniformity according to claim 1, characterized in that, The thickness of the finished steel plate is 40-60mm.

3. The pressure vessel steel plate for cryogenic spherical tanks with good performance uniformity according to claim 1, characterized in that, The tensile strength in the transverse direction at 1 / 4 and 1 / 2 of the thickness of the finished steel plate is 650-700 MPa, the yield strength is 530-580 MPa, the elongation after fracture is 19-24%, the impact energy at -50℃ is 150-250 J, the welding heat energy is 140-150 KJ / cm, and the impact energy at -50℃ in the heat-affected zone after welding is 100-200 J.

4. A method for manufacturing a pressure vessel steel plate for a cryogenic spherical tank with good performance uniformity as described in any one of claims 1-3, characterized in that, The method includes the following steps: (1) Continuous casting billet smelting: The RH inlet temperature is 1590–1620℃, and the RH oxygen blowing rate is 25–45 m³ / h. 3 / h, circulating oxygen blowing time 10-20min, and a two-step aluminum addition method for deoxidation is adopted, namely, adding aluminum cake for pre-deoxidation during the converter tapping process and adding aluminum wire for deoxidation during the LF furnace refining process. The continuous casting adopts multi-stage argon-protected casting and adopts a continuous casting dynamic light reduction process with a reduction range of 4.0-7.0mm, a reduction rate of 0.3-0.5mm / s, and a continuous casting billet thickness of 250-300mm; (2) Controlled rolling and controlled cooling: After heating, the continuously cast billet undergoes rough rolling. The core temperature of the rough-rolled billet is 1100–1130℃, and the core temperature of the final rough-rolled billet is 1050–1080℃. After rough rolling, the billet enters a laminar flow cooling device located between the roughing mill and the finishing mill, where it is cooled by spraying high-pressure cooling water. The core temperature of the finish-rolled billet is 885–915℃, and the core temperature of the final finish-rolled billet is 815–855℃. After finishing rolling, the billet is ejected at a high speed of 4–5 m / s. After pre-straightening, the steel plate undergoes ACC laminar flow cooling. The initial cooling temperature is 780–830℃, which helps the austenite grains retain more stored energy. The cooling rate is 30–50℃ / s water cooling. The steel plate's red-hot temperature is 480–580℃ at the core of the steel plate. During ACC laminar flow cooling, the head and tail of the pre-straightened steel plate are shielded to ensure that the red-hot temperature difference between the head and tail is less than 20℃. After ACC laminar flow cooling, the flatness of the steel plate is ≤8mm / m. (3) Tempering heat treatment: The tempering temperature is 620-640℃, the heating rate is 1.5-2.5 min / mm, the net holding time is 40-150 min, and after the holding time is completed, the temperature is air-cooled to room temperature. (4) Straightening; The flatness of the finished steel plate after straightening is ≤5mm / m.

5. The manufacturing method according to claim 4, characterized in that, In step (1), during converter smelting, the furnace charge size is 50-100mm, the furnace charge hot charging temperature is 1150-1200℃, and the net heat preservation time is 3-6min; The amount of aluminum ingot added is 5-10 kg / ton, and the amount of aluminum wire added is 5-10 kg / ton.

6. The manufacturing method according to claim 4, characterized in that, In step (1), the pouring temperature is 1530–1550℃ and the casting speed is 0.9–1.0 m / min. -1 The steel billet is placed in a slow cooling pit after it leaves the production line. The slow cooling temperature is 300-400℃ and the holding time is 12-24 hours. The mold flux is composed of the following components by weight percentage: CaO 35%–50%, Al2O3 30%–40%, MnO 0–10%, SiO2 5%–10%, with the balance being unavoidable impurities. The thickness of the flux film is 0.1–1.5 mm.

7. The manufacturing method according to claim 4, characterized in that, In step (2), a four-stage heating method is adopted: the temperature of heating stage I is 600-650℃, the temperature of heating stage II is 1000-1050℃, the temperature of heating stage III is 1170-1210℃, the temperature of the soaking stage is 1150-1190℃, the soaking stage time is 30-40 min, and the total heating time is H*(0.9-1.2) min·mm. -1 H represents the thickness of the continuously cast billet, in mm.

8. The manufacturing method according to claim 4, characterized in that, In step (2), rolling is carried out on a two-stand rolling mill. The reduction rate of each pass in roughing is 15% to 25%, the thickness of the intermediate billet is 3 to 4 times the thickness of the finished steel plate, and the reduction rate of each pass in finishing is 5% to 15%.

9. The manufacturing method according to claim 4, characterized in that, In step (1), the multi-stage argon-protected casting method is as follows: Argon gas is introduced from the ladle to the tundish for protection, with an argon gas pressure of 0.1-0.3 MPa, an argon gas flow rate of 2-5 L / min, and a free O2 content of 0.1%-0.5% in the protective atmosphere; Argon gas is introduced from the tundish to the crystallizer for protection, with an argon gas pressure of 0.3-0.5 MPa, an argon gas flow rate of 1-3 L / min, and a free O2 content of 0.5%-1% in the protective atmosphere.

10. The manufacturing method according to claim 4, characterized in that, In step (2), during ACC laminar flow cooling, the number of manifolds is 10-12 sets, the roller speed is 1.2-1.4 m / s, and the water flow rate is 240-260 mm. 3 / h, water temperature 14~20℃.

Images