A threaded steel bar with a yield strength of ≥930 MPa and a production method thereof

Abstract

A type of engineering machinery steel with excellent plate shape and yield strength ≥930MPa has the following composition and wt%: C: 0.35~0.50%, Si: 0.60~1.20%, Mn: 0.80~1.40%, P≦0.035%, S≦0.035%, Al: 0.050~0.120%, B: 0.0008~0.0035%, V: 0.02~0.08%, N: 0.0045~0.0100%, Zr: 0.50~0.75%. The steel is prepared by: hot metal pretreatment; converter smelting; LF furnace refining; casting into square billets; heating the billets; roughing and intermediate rolling; finishing rolling in two stages; cooling; cold shearing in a groove; and pit cooling to room temperature. This invention ensures stable product performance while maintaining hot-rolled yield strength Rel≥930MPa, tensile strength Rm≥1080MPa, elongation A≥6%, and total elongation Att≥3.5% under maximum force. Furthermore, by increasing the heating temperature, it reduces the mill load while significantly lowering costs, which is beneficial for improving production efficiency. It also has strong process adaptability and a wide process window, which is conducive to on-site operation and smooth production.

Description

Technical Field

[0001] This invention relates to a type of threaded steel and its production method, specifically to a type of threaded steel with a yield strength ≥930MPa and its production method, which is particularly suitable for producing precision-rolled threaded steel with a diameter of 25-36mm and a yield strength ≥930MPa. Background Technology

[0002] Precision-rolled threaded steel bars, also known as threaded steel bars for prestressed concrete, are straight steel bars with discontinuous external threads on the surface of the entire bar, without longitudinal ribs. These bars are directly connected using nuts and spiral sleeves, eliminating the need for welding and machining of the threads. They offer advantages such as simple connection and anchoring, strong adhesion, reliable tensioning and anchoring, convenient and fast construction, and material savings. Precision-rolled threaded steel bars are widely used in large-scale water conservancy and hydropower projects, continuous beams and large frame structures in industrial and civil buildings, highways, high-speed railways, large bridge cable trays, nuclear power plants, ground anchors, slope anchoring, and other extra-large buildings, frame structures, bridges, and culverts.

[0003] Precision rolled steel bars are considered high-tech steel products in China, with extremely stringent requirements for product grade and quality. Since weldability is not a concern, various micro-alloying or heat treatment processes can be employed to produce this steel. Based on the characteristics of precision rolled threaded steel bars, the connection device and finished product pass of the rolling mill need to be redesigned, and high precision control is required during the rolling process.

[0004] Compared with traditional ordinary bar rolling production lines, high-speed bar production lines can produce higher precision finished products by using a roll ring mill. The finishing rolling zone uses single-strand twist-free low-temperature controlled rolling technology to achieve alloy reduction production. On the cooling bed, single bars are placed in a single slot for cooling and single-strand alignment to obtain a higher yield. Based on these advantages, high-speed bar production technology has developed rapidly in recent years.

[0005] There are currently two main types of high-speed bar production lines: one is a single-line high-speed bar production line, also known as a single high-speed bar production line, in which the roughing, intermediate, pre-finishing, and finishing mills all produce single-bar rolling. After finishing, the bar is fed into a cooling bed by a high-speed steel feeding system. The other is a double-line high-speed bar production line, also known as a double high-speed bar production line, in which the roughing and intermediate mills produce single-bar rolling, the pre-finishing mills produce two separate rolling processes, and after the pre-finishing mills, the bar is fed into two finishing mills simultaneously on two separate lines. Each line produces single-bar rolling in each finishing mill, and after finishing, the two lines are combined and fed into the cooling bed by a double high-speed steel feeding system that can support simultaneous loading of the two lines onto the cooling bed.

[0006] In the national standard GB / T 20065-2016, the chemical composition (smelting composition) of high-strength threaded steel bars is not explicitly specified, except for the control of S and P mass fractions ≤0.035%. The selection of chemical composition and alloying elements is left to the manufacturer to ensure that finished steel bars processed by different methods meet the mechanical performance requirements specified in the standard. High-strength threaded steel bars have added inspection items for relaxation, fatigue, and non-metallic inclusions, making their quality requirements significantly higher than those for ordinary threaded steel bars.

[0007] Fine-rolled threaded steel bars are classified according to their yield strength levels, including five grades: PSB785, PSB830, PSB930, PSB1080, and PSB1200. Among them, PSB785 and PSB830 are used more extensively. PSB930 is more difficult to produce, and currently, the commonly used processes are microalloying + controlled rolling and controlled cooling, or conventional hot continuous rolling + post-rolling quenching and tempering heat treatment. Due to the high yield strength of this steel grade (930MPa) and tensile strength (1080MPa), especially under rapid cooling, the temperature of the steel parts is very low, which places high demands on the ability of long-length flying shears and the mechanical strength of the shear body and arms. At the same time, cold shearing can easily cause chipping and injury. In addition, due to the high alloy content, when the ambient temperature is below 28℃, it is necessary to quickly cut, collect, and bundle the steel, and then slowly cool it in a suitable location.

[0008] Search results:

[0009] Chinese patent application CN201910978574.9 discloses a method for online controlled rolling and cooling of PSB930 precision-rolled threaded steel bars. The method primarily uses 160mm×160mm steel billets as raw material and designs the compression ratio of the rolled piece in different mill units. During the rolling process, the compression ratio is controlled to be ≥6.0 in the roughing mill, ≥1.4 in the intermediate mill, and ≥3.8 in the finishing mill, with a final rolling speed of 7m / s. During cooling, the temperature of the rolled piece after passing through the first set of controlled cooling devices in the finishing mill is 600-640℃, and the temperature after passing through the second set of controlled cooling devices is 530-550℃. The remaining processing steps are carried out according to conventional processes. The surface cooling rate in the first set of controlled cooling devices in the finishing mill is 157-178℃ / s, and the surface cooling rate in the second set of controlled cooling devices is 70-80℃ / s. Because of the online controlled cooling and the use of a quenching + self-tempering controlled cooling water tank, the cooling rate is fast, which easily leads to uneven microstructure on the core surface of the rolled piece. Due to excessive concentrated cooling, the surface is prone to producing brittle martensitic microstructure. During fixed-length shearing, the end of the finished product is prone to chipping and popping out, causing injury.

[0010] Chinese patent application CN201510863527.1 (publication number CN 105296853B) discloses "A 930MPa high-strength precision-rolled rebar and its preparation process." The rebar composition is: C: 0.40-0.50%, Mn: 0.80-1.20%, Si: 1.45-1.85%, V: 0.120-0.220%, S≤0.030%, P≤0.030%, with the balance being Fe. The preparation process employs a rolling production route of "cold charging of billets into a heating furnace + full-process low-temperature controlled rolling + multiple intermittent controlled cooling + pinch roll feeding of tail steel + multiple-length flying shear segmentation + cooling bed cooling + indoor stacking and cooling." A three-stage water cooling system is used, with the final cooling bed tempering temperature at 650-670℃. The paper describes a process that uses low-temperature rolling throughout the entire process, which results in a high mill load current, low steel throughput on the rolls, high roll consumption, and increased roll changing frequency, thus reducing rolling production efficiency.

[0011] Chinese patent application CN201110347552.6 (publication number CN 102363832A) discloses a "Production Process of Precision Rolled Threaded Steel Bars". The process route is: billet heating → rolling → online controlled cooling → natural cooling on a cooling bed → length setting → stacking cooling. The billet uses medium carbon steel with dimensions of 150mm × 150mm. The heating process employs a three-stage heating method: preheating, heating, and homogenization. In the rolling process, the initial rolling temperature is controlled at 1000℃-1100℃. In the online controlled cooling process, the water tank temperature is controlled at 920-1000℃, using a single cooling method, with the PSB930 self-tempering temperature controlled at 430-480℃. Natural cooling on a cooling bed is performed at a cooling rate controlled at 1-5℃ / s. In the stacking cooling process, the steel is stacked offline at 150-350℃ for at least 48 hours. This process suffers from drawbacks such as high cost, long stacking cooling time, and poor cooling effect.

[0012] Chinese patent application CN201910646827.2 (publication number CN 110218952A) describes a "Precision-rolled threaded steel bar and its production method", which includes the following steel composition: carbon: 0.28-0.33%, silicon: 0.55-0.80%, manganese: 1.20-1.60%, chromium: 0.30%-0.45%, vanadium: 0.10%-0.12%, phosphorus: <0.025%, sulfur: <0.025%, boron ≤0.0035%, N: 0.017-0.023%; the balance being iron and unavoidable impurities. The continuously cast billet is slowly cooled for more than 24 hours before rolling; the temperature of the soaking zone of the heating furnace is controlled at 1050℃~1070℃, the heating time is 125min~140min, the initial rolling temperature of the billet is 950℃~980℃, and the final rolling temperature is controlled at 840℃~890℃. This method has disadvantages such as long heating time, high energy consumption, poor cooling effect, and low production efficiency.

[0013] Chinese patent application CN202210329814.4 (publication number CN 114686764A) discloses "A Low-Relaxation Ultra-High Strength Threaded Steel Bar and Its Preparation Method". The chemical composition of the invented low-relaxation ultra-high strength precision-rolled threaded steel bar, in mass percentage, is: C 0.24%-0.50%, Si 1.3-2.0%, Mn 0.5%-1.4%, V 0.07%-0.14%, Nb 0.10-0.21%, B 0.01-0.04%, Ni 0.1%-0.2%, Mo 0.10%-0.15%, Hf The composition is 0.1%-0.2%, Zr 0.02%-0.05%, P≤0.035%, S≤0.035%, with the balance being iron and unavoidable impurities, satisfying the relationship: 0.23%≤0.55[Ni]+[Mo]+0.27[Hf]+0.1[Zr]≤0.25%. The precision-rolled threaded steel bars provided in this literature can effectively improve the corrosion resistance of steel bars while maintaining both strength and toughness; the yield strength of the threaded steel bars is ≥785MPa, tensile strength is ≥980MPa, elongation after fracture is ≥12%, and total elongation at maximum force is ≥4.5%. After continuous rolling, the steel is not directly water-cooled. Instead, it is first air-cooled to a specific temperature of 820-850℃, then water-cooled to 450-460℃, and finally air-cooled to room temperature. The air cooling maintains a certain austenitic state, while water cooling quenches the surface layer into martensite. During cooling, residual heat is transferred from the core, allowing the surface to temper and form tempered martensite. This steel, in addition to conventional C, Si, and Mn, also contains V, Nb, B, Ni, Mo, Hf, and Zr in specific amounts. The chemical composition design is extremely complex, resulting in a low composition hit rate during the smelting process, low production efficiency, and high production costs. Summary of the Invention

[0014] This invention addresses the shortcomings of existing technologies, such as high alloy content in steel bars, high energy consumption, poor cooling effect, low production efficiency, and frequent shearing process failures. It provides a method for producing rebar with a yield strength ≥930MPa, ensuring hot-rolled yield strength Rel≥930MPa, tensile strength Rm≥1080MPa, elongation A≥6%, and total elongation Att≥3.5% under maximum force. This method not only ensures stable product performance and high production efficiency but also simplifies the process. It is suitable for high-rise buildings and other applications requiring rebar with a diameter of 25-36mm and a yield strength ≥930MPa.

[0015] Measures to achieve the above objectives:

[0016] A type of rebar with a yield strength ≥930MPa has the following composition and weight percentage content: C: 0.35~0.50%, Si: 0.60~1.20%, Mn: 0.80~1.40%, P≦0.035%, S≦0.035%, Al: 0.050~0.120%, B: 0.0008~0.0035%, V: 0.02~0.08%, N: 0.0045~0.0100%, Zr: 0.50~0.75%, with the remainder being Fe and unavoidable impurities.

[0017] Preferably, the weight percentage content of C is 0.38% to 0.45%.

[0018] Preferably, the weight percentage content of Si is 0.63 to 1.00%.

[0019] Preferably, the weight percentage content of Mn is 0.90 to 1.28%.

[0020] Preferably, the weight percentage content of Al is between 0.060 and 0.110%.

[0021] Preferably, the weight percentage content of B is between 0.0009% and 0.0029%.

[0022] Preferably, the weight percentage content of V is 0.03% to 0.07%.

[0023] Preferably, the weight percentage content of N is between 0.0055 and 0.0900%.

[0024] Preferably, the weight percentage content of Zr is 0.55% to 0.70%.

[0025] A method for producing rebar with a yield strength ≥ 930 MPa, comprising the following steps:

[0026] 1) Perform hot metal pretreatment. After desulfurization, the hot metal should have the following composition: P < 0.020% and S < 0.020%.

[0027] 2) In converter smelting, control the final carbon content at the converter end point to be between 0.06% and 0.30%; when the molten steel has been poured to 2 / 3 full, all alloys and carburizing agents should be added at once.

[0028] 3) Perform LF furnace refining for no less than 25 minutes, with argon blowing throughout the refining process; after the composition is adjusted, exposed molten steel is prohibited.

[0029] 4) Cast into square billets, with the pouring process protected by a long nozzle in a large ladle and an immersion nozzle in a crystallizer, and the superheat of the molten steel in the tundish is controlled at 20-35℃; the cross-sectional dimensions of the continuously cast square billets are 155-170mm.

[0030] 5) Heat the billet, with the temperature of the soaking zone controlled at 1080-1200℃ and the total heating time at 60-90 minutes;

[0031] 6) Perform roughing and intermediate rolling, and control the initial rolling temperature at 980-1050℃, and control the cumulative deformation rate of roughing and intermediate rolling at 20-60%;

[0032] 7) Perform finishing rolling in two stages, during which: control the temperature of entering finishing rolling stage I at 810-850℃, the temperature of entering finishing rolling stage II at 790-830℃, and the rolling speed of the last stand at 20-45m / s;

[0033] 8) Cool the bed, controlling the temperature at 590–630℃, and cool it to room temperature at a cooling rate of 6.5–11.0℃ / s;

[0034] 9) Perform grooved cold shearing;

[0035] 10) Cool the pit to room temperature, but when the ambient temperature is below 28°C, the cooling time shall not be less than 24 hours.

[0036] The role and mechanism of each component and main process in this invention

[0037] C: C is the cheapest and most effective strengthening element in steel. When dissolved in the matrix, it can play a solid solution strengthening role. If the C content is less than 0.35%, it is difficult to guarantee the yield strength and tensile strength of the steel bars. If the C content is higher than 0.50%, it will worsen the toughness of the steel. Therefore, the C content should be controlled between 0.35% and 0.50%.

[0038] Si: Si exists in steel in a solid solution state to increase strength. It is also a deoxidizing element in steelmaking. Under the premise of ensuring that the toughness and strength of steel bars are up to standard, we should use as much cheap Si as possible, but the content should not be too high to avoid reducing the toughness and plasticity of steel. Therefore, it should be controlled at 0.60 to 1.20%.

[0039] Mn: Mn dissolves in ferrite to increase the strength of steel, lowers the transformation temperature of supercooled austenite, and refines ferrite grains, which is beneficial to improving the strength and toughness of steel bars. However, Mn is also a segregating element; the higher the content, the easier it is for segregation to occur. Compositional segregation can lead to the formation of bainite structure during continuous cooling of steel bars, which is not conducive to ensuring the toughness of steel bars. Therefore, it is controlled at 0.80-1.40%.

[0040] Al: Al is the most efficient deoxidizer, effectively improving the purity of molten steel. At the same time, AlN particles can refine grains and strengthen the steel through precipitation. However, excessive Al content can deteriorate the mechanical properties of steel; therefore, it is controlled within the range of 0.050–0.120%.

[0041] B: In alloy steel, B mainly improves the hardenability of the steel, but the inherent properties of reinforcing bars do not involve hardenability. In this invention, B is a strong nitride-forming element, and the formation of fine and dispersed BN can produce a precipitation strengthening effect, increasing the strength of the steel. It also has a strong affinity for O, which can reduce free oxygen and nitrogen in the steel, thus helping to eliminate the aging phenomenon of reinforcing bars. At the same time, adding trace amounts of hardenability-enhancing elements can compensate for the lack of online controlled cooling capability and accelerate the microstructure transformation. However, if the B content is too high, it will deteriorate the strength and toughness of the steel. Therefore, the content is controlled at 0.0008-0.0035%.

[0042] V: One of the most common microalloying elements. V is a strong nitride and carbide forming element. During heating or rolling, when fine and dispersed second-phase particles are generated, it can produce a strong solid solution and precipitation strengthening effect, and can refine the grains and improve the strength and toughness of steel. When the V content is too high, it will lead to an increase in the size of the precipitates, which may worsen the above effects. Therefore, the content is controlled at 0.02 to 0.08%.

[0043] Nitrogen (N): Nitrogen is a gaseous element that combines with vanadium (V) microalloying elements in steel to form second-phase particles such as vanadium (VN), enhancing precipitation strengthening and grain refinement, significantly improving the strength and toughness of the steel. Adding too little nitrogen makes it difficult to guarantee the aforementioned strengthening effect, while adding too much will reduce the toughness of the steel reinforcement and increase the difficulty of smelting operations. Therefore, the content is controlled between 0.0045% and 0.0100%.

[0044] Zr: Zr can degas and refine grains, but excessive zirconium content will increase smelting difficulty and production cost, and cause local segregation, which will deteriorate the performance of steel. Therefore, the content should be controlled at 0.50-0.75%.

[0045] The reason why the temperature of the soaking zone is controlled at 1080-1200℃ in this invention is that it is conducive to the uniform diffusion of each component in the steel, sufficient solid solution, and the uniformity of microstructure and stable performance.

[0046] The reason why the present invention controls the initial rolling temperature at 980-1050℃ during roughing and intermediate rolling, and controls the cumulative deformation rate of roughing and intermediate rolling at 20-60%, is that using this initial rolling temperature can ensure that the billet is fully austenitized in the heating furnace and undergoes austenitic deformation during subsequent rolling, which is beneficial to the uniformity of composition and structure. Controlling the cumulative deformation rate can ensure the uniform refinement of the microstructure.

[0047] The reason why the temperature of the first stage of finishing mill is controlled at 810-850℃, the temperature of the second stage of finishing mill is controlled at 790-830℃, and the rolling speed of the last stand is controlled at 20-45m / s is that the finishing mill still uses the austenitic region for rolling deformation, which is beneficial to ensuring the appropriate strength and plasticity of the finished steel. The speed of the last stand is controlled at 20-45m / s because different specifications use different rolling speeds, which on the one hand play the role of deformation-induced strengthening, and on the other hand ensure smooth production and increase hourly output.

[0048] The reason why the temperature of the steel entering the cooling bed is controlled at 590-630℃ is to obtain suitable strength and plasticity. If the temperature of the steel entering the cooling bed is too low, the strength will be too high and the plasticity will be insufficient. Conversely, if the temperature of the steel entering the cooling bed is too high, the strength will be insufficient and the plasticity will be too high.

[0049] The reason why the cooling time in this invention is not less than 24 hours when the ambient temperature is below 28°C is that the steel is prone to air quenching when the ambient temperature is below 28°C, which can lead to cracks on the surface of the steel.

[0050] Compared with the prior art, this invention, while ensuring hot-rolled yield strength Rel≥930MPa, tensile strength Rm≥1080MPa, elongation A≥6%, and total elongation Att≥3.5% under maximum force, not only ensures stable product performance, but also reduces mill load by significantly lowering costs through increased heating temperature, which is conducive to improving production efficiency. It has strong process adaptability and a wide process window, which is beneficial to on-site operation and smooth production. Attached Figure Description

[0051] Figure 1 This is an image showing that the surface metallographic structure of the threaded layer of this invention is martensitic;

[0052] Figure 2 The image shows the metallographic structure of the threaded core of this invention, which is bainite + pearlite + ferrite. Detailed Implementation

[0053] The present invention will now be described in detail:

[0054] Table 1 is a list of chemical composition values ​​for each embodiment and comparative example of the present invention;

[0055] Table 2 is a list of process parameters for the smelting process in each embodiment and comparative example of the present invention;

[0056] Table 3 is a list of rolling process parameters for each embodiment and comparative example of the present invention;

[0057] Table 4 shows the experimental results of various embodiments and comparative examples of the present invention.

[0058] The various embodiments of the present invention are produced according to the following steps.

[0059] 1) Perform hot metal pretreatment. After desulfurization, the hot metal should have the following composition: P < 0.020% and S < 0.020%.

[0060] 2) In converter smelting, control the final carbon content at the converter end point to be between 0.06% and 0.30%; when the molten steel has been poured to 2 / 3 full, all alloys and carburizing agents should be added at once.

[0061] 3) Perform LF furnace refining for no less than 25 minutes, with argon blowing throughout the refining process; after the composition is adjusted, exposed molten steel is prohibited.

[0062] 4) Cast into square billets, with the pouring process protected by a long nozzle in a large ladle and an immersion nozzle in a crystallizer, and the superheat of the molten steel in the tundish is controlled at 20-35℃; the cross-sectional dimensions of the continuously cast square billets are 155-170mm.

[0063] 5) Heat the billet, with the temperature of the soaking zone controlled at 1080-1200℃ and the total heating time at 60-90 minutes;

[0064] 6) Perform roughing and intermediate rolling, and control the initial rolling temperature at 980-1050℃, and control the cumulative deformation rate of roughing and intermediate rolling at 20-60%;

[0065] 7) Perform finishing rolling in two stages, during which: control the temperature of entering finishing rolling stage I at 810-850℃, the temperature of entering finishing rolling stage II at 790-830℃, and the rolling speed of the last stand at 20-45m / s;

[0066] 8) Cool the bed, controlling the temperature at 590–630℃, and cool it to room temperature at a cooling rate of 6.5–11.0℃ / s;

[0067] 9) Perform grooved cold shearing;

[0068] 10) Cool the pit to room temperature, but when the ambient temperature is below 28°C, the cooling time shall not be less than 24 hours.

[0069] Table 1. List of chemical components (wt%) of various embodiments of the present invention

[0070]

[0071] Table 2. List of process parameters for the smelting process of various embodiments and comparative examples of the present invention.

[0072]

[0073]

[0074] Table 3. List of rolling process parameters for each embodiment and comparative example of the present invention.

[0075]

[0076] Table 4. Experimental Results of Various Embodiments and Comparative Examples of the Invention

[0077]

[0078]

[0079] Note: The thread diameter in all the above embodiments is in the range of 25-36mm.

[0080] As can be seen from Table 4, compared with the comparative example, the steel of the present invention in the embodiment has higher yield strength, tensile strength and more suitable elongation and other plastic properties, while no cracking occurs on the surface of the steel.

[0081] This specific embodiment is merely a best example and is not intended to limit the implementation of the technical solution of the present invention.

Claims

1. A type of rebar with a yield strength ≥ 930 MPa, comprising the following components and weight percentages: C: 0.35–0.50%, Si: 0.60–1.20%, Mn: 0.80–1.40%, P≦0.035%, S≦0.035%, Al: 0.050–0.120%, B: 0.0008–0.0035%, V: 0.02–0.08%, N: 0.0045–0.0100%, Zr: 0.50–0.75%, with the remainder being Fe and unavoidable impurities; its mechanical properties are: yield strength Rel ≥ 930 MPa, tensile strength Rm ≥ 1080 MPa, elongation A ≥ 6%, and total elongation Att ≥ 3.5% under maximum force.

2. A rebar with a yield strength ≥ 930 MPa as described in claim 1, characterized in that: The weight percentage of C is 0.38~0.45%.

3. A rebar with a yield strength ≥ 930 MPa as described in claim 1, characterized in that: The weight percentage content of Si is 0.63~1.00%.

4. A rebar with a yield strength ≥ 930 MPa as described in claim 1, characterized in that: The weight percentage content of Mn is between 0.90 and 1.28%.

5. A rebar with a yield strength ≥ 930 MPa as described in claim 1, characterized in that: The weight percentage content of Al is between 0.06% and 0.11%.

6. A rebar with a yield strength ≥ 930 MPa as described in claim 1, characterized in that: The weight percentage content of B is between 0.0009 and 0.0029%.

7. A rebar with a yield strength ≥ 930 MPa as described in claim 1, characterized in that: The weight percentage content of V is 0.03~0.07%.

8. A rebar with a yield strength ≥ 930 MPa as described in claim 1, characterized in that: The weight percentage content of Zr is 0.55% to 0.70%.

9. A method for producing a rebar with a yield strength ≥ 930 MPa as described in claim 1, comprising the following steps: 1) Hot metal pretreatment is performed. After desulfurization, the hot metal has the following composition: P < 0.020%, S < 0.020%. 2) In converter smelting, the final carbon content (C) should be controlled between 0.06% and 0.30%; when the molten steel has been poured to 2 / 3 full, all alloys and carburizing agents should be added at once. 3) Perform LF furnace refining for at least 25 minutes, with argon blowing throughout the refining process; after the composition is adjusted, it is forbidden to store molten steel. In the phenomenon of nudity; 4) Cast into square billets, with the pouring process protected by a long nozzle in a large ladle and an immersion nozzle in a crystallizer, and the superheat of the molten steel in the tundish is controlled at 20-35℃; the cross-sectional dimensions of the continuously cast square billets are 155-170mm. 5) Heat the billet, with the temperature of the soaking zone controlled at 1080-1200℃ and the total heating time at 60-90 min; 6) Perform roughing and intermediate rolling, and control the initial rolling temperature at 980–1050℃, and control the cumulative deformation rate of roughing and intermediate rolling at 20–60%; 7) Perform finishing rolling in two stages, during which: control the temperature of entering finishing rolling stage I at 810-850℃, the temperature of entering finishing rolling stage II at 790-830℃, and the rolling speed of the last stand at 20-45m / s; 8) Cool the bed, controlling the temperature at 590–630℃, and cool it to room temperature at a cooling rate of 6.5–11.0℃ / s; 9) Perform grooved cold shearing; 10) The pit should be cooled to room temperature, but when the ambient temperature is below 28°C, the cooling time should not be less than 24 hours.

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