High-speed continuous casting production method for small square billets of 635MPa grade high-strength rebar

Abstract

This invention discloses a high-speed continuous casting production method for 635MPa grade high-strength rebar using small billets. The method involves pouring molten 635MPa grade high-strength rebar into a tundish, with billet dimensions of 170mm × 170mm. At a casting speed of 4.0m / min, the superheat of the molten steel is controlled at 10-20℃. A crystallizer with multiple grooves on its surface is used, and the cooling water flow rate of the crystallizer is controlled at 210m³ / min. 3 The continuous casting billet operates at a speed of 4.0 m / min or higher. After exiting the crystallizer, the billet enters the secondary cooling zone, where it is cooled by full water pressure of 1.8–2.0 MPa and a specific water volume of 1.3–1.5 L / kg. The billet temperature is controlled at 1150–1250℃ when it enters the straightening machine. This invention uses a 170mm × 170mm small square billet. Under the condition that the casting speed reaches 4.0 m / min, the crack incidence rate of the continuous casting billet is controlled at an extremely low level, meeting the needs of enterprises for high-efficiency production, energy saving, consumption reduction, and green low-carbon development.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, specifically relating to a continuous casting production method for 635MPa grade high-strength rebar. Background Technology

[0002] Reinforced concrete structures dominate construction projects in my country, resulting in a large consumption of steel reinforcement. The promotion of high-strength steel reinforcement can reduce the amount of steel used in projects. It is estimated that using steel reinforcement with a strength of 600MPa or higher in high-rise or large-span buildings can save approximately 30% of steel reinforcement usage. Correspondingly, this can alleviate the supply pressure on energy sources such as coal and electricity in my country, achieving the goal of energy conservation and emission reduction.

[0003] For enterprises, adopting high-speed production of rebar can not only improve production efficiency and match the production rhythm of the rolling mill, but also increase the billet temperature to meet the requirements of hot delivery and charging, save production energy, and reduce emissions.

[0004] To meet the performance requirement of a yield strength of over 635 MPa for high-strength rebar HRB635(E), strengthening elements vanadium, niobium, and nitrogen are added to its composition. Requirements: the mass percentage of vanadium (V) in the steel is controlled at 0.120–0.25%, the mass percentage of niobium (Nb) is controlled at 0.010–0.020%, and the mass percentage of nitrogen (N) is controlled at 0.010–0.025%. Due to the high levels of V, Nb, and nitrogen added to HRB635(E), NbN, Nb(C,N), VN, and V(C,N) precipitate at the initial grain boundaries during continuous casting solidification. Their pinning effect reduces grain boundary fluidity, inhibits dynamic recrystallization, and intensifies stress concentration at austenite grain boundaries, leading to grain boundary embrittlement and voids. This weakens the bonding force at grain boundaries, reduces toughness, and makes the billet prone to cracking under stress.

[0005] Currently, there are very few domestic enterprises capable of producing high-strength rebar HRB635(E), and there are no reports of using 170mm×170mm small square billets to produce high-strength rebar HRB635(E) at high casting speeds. For example, the paper "Improvement and Production Practice of High Casting Speed ​​Technology for Small Square Billets at 6.0 m / min" describes a method for producing rebar using 160mm×160mm cross-section small square billets, increasing the casting speed from 4.1 m / min to 6.0 m / min. Its crystallizer cross-section is only 160mm×160mm, and it produces ordinary rebar (HRB400E, etc.). Patent 202210305153.1, "A High-Speed ​​Production Method for Small Square Billet Continuous Casting," provides a high-speed production method for small square billet continuous casting, achieving a maximum casting speed of 6.0 m / min for rebar and a normal production speed of 5.0 m / min. However, its cross-section is a small square billet of 160mm×160mm~165mm×165mm, and the steel grade produced is ordinary rebar HRB400 series. Patent 201911407523.7, "A 645MPa grade high-strength earthquake-resistant ribbed steel bar and its production method," although its yield strength reaches 645MPa, requires a casting speed of only 1.7~2.0m / min when using 180mm square billets. Patent 202211295829.X, "A 700MPa grade high-strength low yield strength ratio hot-rolled steel bar and its production method," although its yield strength reaches 700MPa, uses 150mm×150mm~170mm×170mm square billets, but the continuous casting billet casting speed is 2.2~2.6m / min. Summary of the Invention

[0006] The purpose of this invention is to provide a high-speed continuous casting method for producing high-strength rebar HRB635(E) using 170mm×170mm small square billets. Under the condition of achieving a casting speed of 4.0m / min, the method controls the crack incidence rate of the continuous casting billet to an extremely low level, thereby meeting the needs of enterprises for efficient production, energy conservation, consumption reduction, and green low-carbon development.

[0007] To achieve the above objectives, the following technical solution is adopted:

[0008] The high-speed continuous casting production method for small square billets of 635MPa grade high-strength rebar includes the following steps:

[0009] (1) Molten steel of 635MPa grade high-strength rebar is poured into the tundish. The billet size is 170mm×170mm. When the casting speed reaches 4.0m / min, the superheat of the molten steel is controlled at 10-20℃. A crystallizer with multiple grooves on the surface is used, and the cooling water volume of the crystallizer is controlled at 210m³. 3 / h or more;

[0010] (2) After the billet exits the crystallizer, it enters the secondary cooling zone and is cooled by full water pressure of 1.8-2.0 MPa. The specific water volume for secondary cooling is 1.3-1.5 L / kg.

[0011] (3) The temperature of the billet entering the straightening machine is controlled at 1150-1250℃.

[0012] According to the above scheme, the mass percentages of vanadium, niobium, and nitrogen in the molten steel of the 635MPa grade high-strength rebar are controlled at 0.120-0.25%, 0.010-0.020%, and 0.010-0.025%, respectively.

[0013] According to the above scheme, the length of the copper tube of the crystallizer is 1m.

[0014] According to the above scheme, the crystallizer vibration adopts a high-frequency non-sinusoidal vibration mode, and the vibration frequency is controlled at 250-300 cpm.

[0015] According to the above scheme, the total width of the groove is controlled at 76-84mm, the number of grooves is 7-10, and the depth of the groove is 7-9mm.

[0016] In the optimized scheme, step 1 controls the superheat of molten steel to be between 10-18℃.

[0017] In the optimized scheme, the water pressure is controlled at 1.85-1.95MPa in step 2, and the specific water volume for secondary cooling is 1.35-1.45L / kg.

[0018] In the optimized scheme, the temperature of the billet entering the straightening machine in step 3 is controlled at 1180-1220℃.

[0019] The reason for using a 1m long copper tube with multiple grooves on its surface in this invention is that when high-strength rebar HRB635(E) is cast at high speeds, the molten steel spends a short time in the crystallizer, resulting in a thinner billet shell. This shell contains higher levels of crack-sensitive elements such as Nb and V, making it more prone to leakage accidents compared to ordinary rebar HRB400E. Traditional crystallizers have a 4mm water gap between the copper tube and its external water jacket. Furthermore, the machining precision of conventional crystallizer copper tube water jackets is not very high, and unevenness can easily occur during the installation and fixing of the water gap. This leads to uneven temperature distribution around the copper tube in the meniscus area of ​​the crystallizer, with the corner billet shell being thicker than the edges. As the billet continues to cool inside the copper tube, the shrinkage at the corners is greater, further increasing the unevenness in temperature and billet shell thickness between the corners and the center, easily leading to surface cracks, delamination, and even leakage accidents. The crystallizer, employing 1m long copper tubes with multiple grooves on its surface, features a unique three-dimensional cavity shape and taper. This improved copper tube-billet shell fit enhances heat transfer at the edges and corners of the crystallizer, increasing the cooling area of ​​the copper tubes and resulting in higher heat exchange efficiency, ensuring uniform thickness of the primary solidified shell within the crystallizer. The use of recessed water channels of varying widths and depths on the copper tube surface further enhances cooling uniformity at the corners and center of the crystallizer, effectively suppressing the high heat flow at the meniscus and transferring it to the lower middle part of the copper tube, ensuring a nearly constant heat flow distribution along the crystallizer's axial direction. This also results in more uniform friction between the copper wall and the billet surface, achieving uniform heat exchange between the inner wall of the copper tube and the billet. Simultaneously, a 210m... 3 A cooling water flow rate of over / h for the crystallizer can meet the initial solidified shell thickness in the crystallizer and will not cause cracking and leakage at high casting speeds of 4.0m / min.

[0020] In this invention, a high-frequency non-sinusoidal vibration mode is adopted for the crystallizer vibration during the production of high-strength rebar HRB635(E) at a high casting speed of 4.0 m / min, with the vibration frequency controlled between 150-200 cpm. This is mainly because this vibration mode has the characteristics of short downward vibration time and high speed within the vibration cycle, and long upward vibration time and slow speed. The upward vibration is relatively gentle, which can significantly reduce the surface tensile stress of the billet shell. At the same time, the positive slip time increases, which increases the consumption of mold flux in the crystallizer. Controlling the vibration frequency between 150-200 cpm can appropriately reduce the negative slip time of the billet in the crystallizer, making it easier to reduce the depth of the vibration mark, which is beneficial to the demolding of the billet and the healing of torn billet shells.

[0021] In this invention, when producing high-strength rebar HRB635(E) at a high casting speed of 4.0 m / min, low superheat casting is adopted, with the superheat of the molten steel in the tundish controlled at 10-20℃. This is mainly because if the superheat of the molten steel in the tundish is too high, the solidified shell exiting the crystallizer will be thin, easily leading to steel leakage; while if the superheat of the tundish is too low, it can easily cause the molten steel to become blocked in the nozzle or even stop flowing, resulting in a casting interruption.

[0022] In this invention, when producing high-strength rebar HRB635(E) at a high drawing speed of 4.0 m / min, full-water intensive cooling is used in the secondary cooling zone, with a water pressure of 1.8–2.0 MPa and a specific water volume of 1.3–1.5 L / kg. This is mainly because at a high drawing speed of 4.0 m / min, after the billet exits the crystallizer and enters the secondary cooling zone, the surface temperature of the billet is high, and the billet shell is relatively thin, making it prone to bulging under the static pressure of molten steel. By controlling the specific water volume of the secondary cooling to between 1.3–1.5 L / kg, it is possible to ensure that the billet does not bulge in the secondary cooling zone, while simultaneously ensuring that the temperature of the billet entering the straightening machine is 1150–1250°C. This avoids cracking during straightening in the brittle transition zone of the billet and also increases the temperature of the billet, which is beneficial for the hot charging temperature of the billet and reduces the gas consumption during rolling.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0024] This invention enables high-strength rebar HRB635(E) to be produced via high-speed continuous casting at 4.0 m / min, while maintaining an extremely low rate of cracking in the cast billet. Without the need for additional equipment such as densely packed rollers in the secondary cooling zone, it achieves an upgrade from producing ordinary rebar HRB400E to high-strength rebar HRB635(E). This meets the needs of enterprises for product upgrading, high-efficiency production, energy conservation, emission reduction, and green low-carbon development. Attached Figure Description

[0025] Figure 1 Cross-sectional view of the product obtained in Example 6.

[0026] Figure 2 Cross-sectional view of the product obtained in Example 7. Detailed Implementation

[0027] The following embodiments further illustrate the technical solution of the present invention, but are not intended to limit the scope of protection of the present invention.

[0028] A specific implementation provides a high-speed continuous casting method for producing high-strength rebar HRB635(E) using a 170mm×170mm small square billet, comprising the following steps:

[0029] The qualified molten steel for high-strength rebar HRB635(E) after being smelted in a converter, refined in an argon station or LF furnace, has the mass percentages of vanadium, niobium and nitrogen in its composition controlled at 0.120-0.25%, 0.010-0.020% and 0.010-0.025%, respectively.

[0030] Molten steel is poured into the tundish for casting. The billet size is 170mm × 170mm. When the casting speed reaches 4.0m / min, the superheat of the molten steel is controlled at 10-20℃. A crystallizer with a 1m long copper tube and multiple grooves on its surface is used. The total width of the grooves is controlled at 76-84mm, the number of grooves is 7-10, and the depth of the grooves is 7-9mm. The cooling water flow rate of the crystallizer is controlled at 210m³. 3 / h or more; the crystallizer vibration adopts a high-frequency non-sinusoidal vibration mode, and the vibration frequency is controlled at 250-300cpm.

[0031] After exiting the crystallizer, the billet enters the secondary cooling zone, where it is cooled by full water pressure of 1.8–2.0 MPa and the specific water volume for secondary cooling is 1.3–1.5 L / kg.

[0032] The temperature of the billet entering the straightening machine is controlled at 1150-1250℃.

[0033] The process parameters of specific embodiments 1-9 and the number of cracks in the quality inspection of the resulting products are shown in Table 1.

[0034] Table 1

[0035]

[0036] Figure 1-2 The figures shown are cross-sectional views of the products obtained in Examples 6 and 7, respectively. In the specific implementation process of this invention, a total of 111 heats of HRB635(E) billets, weighing over 12,000 tons, were produced. Only one heat of billets had slight cracks at low magnification, and it still met the rolling requirements.

Claims

A high-speed continuous casting production method for small square billets of 1.635MPa grade high-strength rebar, characterized in that... Includes the following steps: (1) Molten steel of 635MPa grade high-strength rebar is poured into the tundish. The billet size is 170mm×170mm. When the casting speed reaches 4.0m / min, the superheat of the molten steel is controlled at 10-20℃. The mass percentage content of vanadium, niobium and nitrogen in the molten steel of 635MPa grade high-strength rebar is controlled at 0.120~0.25%, 0.010-0.020% and 0.010-0.025% respectively. A crystallizer with multiple grooves on the surface is used, and the cooling water volume of the crystallizer is controlled at 210m³. 3 For a frequency of 10 ... (2) After the billet exits the crystallizer, it enters the secondary cooling zone and is cooled by full water pressure of 1.8-2.0 MPa. The specific water volume for secondary cooling is 1.3-1.5 L / kg. (3) The temperature of the billet entering the straightening machine is controlled at 1150-1250℃.

2. The high-speed continuous casting production method for small square billets of 635MPa grade high-strength rebar as described in claim 1, characterized in that... The copper tube of the crystallizer is 1m long.

3. The high-speed continuous casting production method for small square billets of 635MPa grade high-strength rebar as described in claim 1, characterized in that... The total width of the grooves is controlled at 76-84mm, the number of grooves is 7-10, and the depth of the grooves is 7-9mm.

4. The high-speed continuous casting production method for small square billets of 635MPa grade high-strength rebar as described in claim 1, characterized in that... In step 1, the superheat of the molten steel is controlled at 10-18℃.

5. The high-speed continuous casting production method for small square billets of 635MPa grade high-strength rebar as described in claim 1, characterized in that... In step 2, the water pressure is controlled at 1.85-1.95 MPa, and the specific water volume for secondary cooling is 1.35-1.45 L / kg.

6. The high-speed continuous casting production method for small square billets of 635MPa grade high-strength rebar as described in claim 1, characterized in that... Step 3: The temperature of the billet entering the straightening machine is controlled at 1180-1220℃.

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