A threaded steel bar with a yield strength of ≥830 MPa and a production method thereof

Abstract

A type of rebar with a yield strength ≥830MPa and its production method, wherein the composition and wt% are: C: 0.35~0.50%, Si: 0.50~0.90%, Mn: 0.40~0.90%, P≤0.035%, S≤0.035%, Al: 0.050~0.120%, V: 0.01~0.07%, N: 0.0045~0.010%, Zr: 0.40~0.60%; the production method includes: hot metal pretreatment; converter smelting; LF furnace refining; heating of the billet; roughing and intermediate rolling; finishing rolling in two stages; and cooling to room temperature. This invention ensures that the hot-rolled yield strength Rel≥830MPa, tensile strength Rm≥980MPa, elongation A≥6%, and total elongation Att≥3.5% under maximum force. The process is simple, the performance of the finished threaded steel product is stable, and by increasing the heating temperature by 25%, the mill load can be reduced by 20% and the roll consumption cost can be reduced by about 18%, which is conducive to improving production efficiency. The process has strong adaptability. In addition, due to the wide process window, it is beneficial to on-site operation and smooth production.

Description

Technical Field

[0001] This invention relates to a type of threaded steel and a method for producing it, specifically to a type of threaded steel with a yield strength ≥ 830 MPa and a method for producing it, which is particularly suitable for producing 830 MPa precision rolled threaded steel and a method for producing it. 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 have extremely stringent requirements regarding their grade and quality. Since the influence of 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 connecting 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 obtain 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 groove 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 grades, including five grades: PSB785, PSB830, PSB930, PSB1080, and PSB1200. Among them, PSB830 is the most widely used grade of fine-rolled threaded steel bars. The commonly used process in production is the post-rolling residual heat treatment process of core self-tempering to improve the strength grade. In this process, after the steel bars come out of the last stand of the rolling mill, they pass through a special water cooling system to quench the surface layer of the steel bars into martensite, while the core remains in an austenitic state. During the subsequent cooling, the heat from the core is transferred to the surface of the steel bars, resulting in the tempering of the surface martensite.

[0008] Search results:

[0009] Chinese patent application CN201911119120.2 (publication number CN 110760735A) discloses "A PSB830 precision-rolled threaded steel bar and its production method," with the following chemical composition and mass percentages: C: 0.43%–0.48%, Si: 1.60%–1.80%, Mn: 0.90%–1.00%, (Cr+Ni+Cu)≤0.10%, Nb: 0.01%–0.025%, V: 0.125%–0.155%, P≤0.020%, S≤0.015%, N: 75–110ppm. The production method involves electric arc furnace smelting, LF furnace refining, VD treatment, continuous casting, slab heating, rolling, and post-rolling controlled cooling. The process is as follows: Steelmaking: Electric furnace smelting is used, with C ≥ 0.04% and P ≤ 0.013% controlled at the end of the process; High-purity silicon carbide is used for diffusion deoxidation during the LF refining process, with a refining time of more than 30 minutes; Vacuum treatment is performed using VD, with a vacuum degree ≤ 1 mbar and a holding time of more than 12 minutes; After vacuum treatment, manganese nitride wire is fed in to adjust the nitrogen content, followed by 50-150 meters of pure calcium rod cored wire, and then static stirring is performed for more than 15 minutes; Continuous casting: The temperature of the first ladle is 1535-1550℃, the temperature of the second ladle in the continuous casting furnace is 1525-1540℃, the liquidus temperature is 1480℃, and the superheat is 10-40℃; The cross-section is 150*150mm. 2The casting speed is 2.0–2.4 m / min. Continuous casting employs full protective casting with electromagnetic stirring to ensure stable liquid levels in the tundish and crystallizer, and to guarantee the alignment and insertion depth of the immersion nozzle. Rolling: Preheating furnace temperature ≤900℃, heating furnace temperature 1120–1180℃, soaking furnace temperature 1120–1200℃; initial rolling temperature 1000–1050℃, reheating temperature 600–630℃, high-pressure water descaling pressure ≥15MPa; controlled cooling after rolling. This steel is smelted in an electric furnace, treated with VD vacuum, continuously cast into slabs, and then rolled into precision threaded steel bars. However, its relatively complex process and high production cost hinder its widespread application.

[0010] Chinese patent application CN202011293465.2 (publication number CN 112410669A) discloses "A corrosion-resistant PSB830 precision-rolled threaded steel and its production process". The process involves first hot-rolling to PSB400MPa grade, followed by high-temperature quenching and medium-temperature tempering heat treatment to finally obtain a corrosion-resistant PSB830MPa grade or higher precision-rolled threaded steel. Its composition and mass percentage are as follows: C 0.10-0.15%, Si 0.50-0.80%, Mn 1.30-1.60%, P≤0.030%, S≤0.030%, Cr 0.50~0.80%, Ni 0.50-0.65%, Cu 0.50-0.65%, Nb 0.015~0.035%, with the balance being Fe and unavoidable impurities. The parameters for the hot rolling and heat treatment processes are: initial rolling temperature: 1120℃; finishing rolling temperature: 1010℃; upper cooling bed temperature ≥950℃; quenching temperature: 925℃; tempering temperature: 440℃. This steel is produced using an online hot rolling + offline quenching and tempering heat treatment process. Compared to the online controlled rolling and controlled cooling process, it has the disadvantages of a complex process and the need for a separate heat treatment production line or corresponding plant.

[0011] Chinese patent application CN202210329814.4 (publication number CN 114686764A) discloses "A Low-Relaxation Ultra-High Strength Threaded Steel Bar and Its Preparation Method," whose chemical composition, by 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 0.1%-0.2%, Zr The content of the steel bars is 0.02%-0.05%, P≤0.035%, S≤0.035%, with the balance being iron and unavoidable impurities, and satisfying the relationship: 0.23%≤0.55[Ni]+[Mo]+0.27[Hf]+0.1[Zr]≤0.25%. This literature describes precision-rolled threaded steel bars that can effectively improve corrosion resistance while maintaining both strength and toughness. The yield strength is ≥785MPa, tensile strength ≥980MPa, elongation after fracture ≥12%, and total elongation at maximum force ≥4.5%. After continuous rolling, the steel bars are not directly water-cooled. Instead, they are 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, and the water cooling quenches the surface layer into martensite. During the cooling process, residual heat is transferred from the core, allowing the surface to be tempered, forming tempered martensite. In addition to the conventional C, Si, and Mn, this literature also adds V, Nb, B, Ni, Mo, Hf, Zr and other alloying elements in specific amounts. The chemical composition is complex, the composition hit rate in the smelting process is not high, and the total amount of alloy is not low. Using the controlled process of air cooling-water cooling-air cooling, the strength of the developed steel bars is only 785 MPa.

[0012] Chinese patent application number CN201110347552.6 (publication number CN) Document 102363832A discloses a production process for precision-rolled threaded steel bars. The process route is as follows: 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 step. The self-tempering temperature for PSB785 and PSB830 is controlled at 480-550℃. Natural cooling on a cooling bed is performed at a cooling rate of 1-5℃ / s. In the stacking cooling process, the steel is stacked at 150-350℃ for at least 48 hours. However, this process suffers from drawbacks such as long heating time, high energy consumption, poor cooling effect, and low production efficiency. Summary of the Invention

[0013] This invention addresses the shortcomings of existing technologies, such as high alloy content, high energy consumption, poor cooling effect, and low production efficiency in steel bars. It provides a method for producing rebar with a yield strength ≥830MPa, ensuring hot-rolled yield strength Rel≥830MPa, tensile strength Rm≥980MPa, elongation A≥6%, and total elongation Att≥3.5% under maximum force. This method simplifies the process, achieves a product performance qualification rate of over 98%, and provides stable performance.

[0014] Measures to achieve the above objectives:

[0015] A type of rebar with a yield strength ≥830MPa has the following composition and weight percentage content: C: 0.35~0.50%, Si: 0.50~0.90%, Mn: 0.40~0.90%, P≤0.035%, S≤0.035%, Al: 0.050~0.120%, V: 0.01~0.07%, N: 0.0045~0.010%, Zr: 0.40~0.60%, with the remainder being Fe and unavoidable impurities.

[0016] Preferably, the weight percentage content of C is 0.38% to 0.45%.

[0017] Preferably, the weight percentage content of Si is 0.55% to 0.85%.

[0018] Preferably, the weight percentage content of Mn is 0.50 to 0.80%.

[0019] Preferably, the weight percentage content of Al is between 0.060 and 0.110%.

[0020] Preferably, the weight percentage content of V is 0.01 to 0.06%.

[0021] Preferably, the weight percentage content of N is 0.0055 to 0.009%.

[0022] Preferably, the weight percentage content of Zr is 0.45% to 0.55%.

[0023] A method for producing rebar with a yield strength ≥ 830 MPa, comprising the following steps:

[0024] 1) Perform hot metal pretreatment. After desulfurization, the hot metal should have the following composition: P < 0.020% and S < 0.020%.

[0025] 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.

[0026] 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.

[0027] 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 size of the square billet is 160-170mm.

[0028] 5) Heat the billet, with the temperature of the soaking zone controlled at 1080-1200℃ and the total heating time at 60-90 minutes;

[0029] 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%;

[0030] 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;

[0031] 8) Cool the bed by controlling the temperature of the bed to be between 570 and 620°C and cooling it to room temperature at a cooling rate of 6 to 9°C / s.

[0032] The role and mechanism of each component and main process in this invention

[0033] C: C is the cheapest and most effective strengthening element in steel materials. 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%, preferably between 0.38 and 0.45%.

[0034] 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 the 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 the steel. Therefore, it should be controlled at 0.50-0.90%, preferably 0.55-0.85%.

[0035] 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, the content is controlled at 0.40-0.90%, preferably 0.50-0.80%.

[0036] Al: Al is the most efficient deoxidizer, effectively improving the purity of molten steel. Simultaneously, AlN particles can refine grains and strengthen the steel through precipitation. However, excessive Al content can deteriorate the mechanical properties of the steel; therefore, it is controlled within the range of 0.05–0.12%, preferably 0.06–0.11%.

[0037] 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 0.01 to 0.07%, preferably 0.01 to 0.06%.

[0038] 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. Insufficient addition makes it difficult to guarantee the aforementioned strengthening effect, while excessive addition reduces the toughness of the steel reinforcement and increases the difficulty of smelting operations. Therefore, the content is controlled between 0.0045% and 0.0100%. The preferred N content is 0.0055% to 0.09%.

[0039] 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 is controlled at 0.40-0.60%, preferably 0.45-0.55%.

[0040] The reason why this invention controls the converter endpoint C to be between 0.06% and 0.30%, and adds all the alloys and carburizing agents at once when the molten steel has been poured to 2 / 3, is to ensure precise control of the content of alloying elements Si, Mn, Al, V, Zr and C. If added in advance, the alloying elements will oxidize prematurely, resulting in a low yield. If added in a later manner, the alloying lumps will not have enough time to melt into the molten steel, resulting in uneven composition and even the presence of foreign metals.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] The reason why the temperature of the steel entering the cooling bed is controlled at 570-620℃ 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.

[0045] Compared with the prior art, this invention has the advantages of simple process, stable performance of fine-rolled threaded steel products, and the ability to ensure hot-rolled yield strength Rel≥830MPa, tensile strength Rm≥980MPa, elongation A≥6%, and total elongation Att≥3.5% under maximum force. In addition, by increasing the heating temperature by 25%, the mill load can be reduced by 20% and the roll consumption cost can be reduced by about 18%, which is conducive to improving production efficiency. It has strong process adaptability. Furthermore, due to the wide process window, it is beneficial to on-site operation and smooth production. Attached Figure Description

[0046] Figure 1 This image shows the metallographic structure of the threaded surface layer of the present invention as tempered martensite.

[0047] Figure 2 The image shows the metallographic structure of the threaded core of this invention, which is pearlite + ferrite. Detailed Implementation

[0048] The present invention will now be described in detail:

[0049] Table 1 is a list of chemical composition values ​​for each embodiment and comparative example of the present invention;

[0050] Table 2 is a list of process parameters for the smelting process in each embodiment and comparative example of the present invention;

[0051] Table 3 is a list of rolling process parameters for each embodiment and comparative example of the present invention;

[0052] Table 4 shows the experimental results of various embodiments and comparative examples of the present invention.

[0053] The various embodiments of the present invention are produced according to the following steps.

[0054] 1) Perform hot metal pretreatment. After desulfurization, the hot metal should have the following composition: P < 0.020% and S < 0.020%.

[0055] 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.

[0056] 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.

[0057] 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 size of the square billet is 160-170mm.

[0058] 5) Heat the billet, with the temperature of the soaking zone controlled at 1080-1200℃ and the total heating time at 60-90 minutes;

[0059] 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%;

[0060] 7) Perform finishing rolling in two stages, during which: control the temperature entering finishing rolling stage I at 810–850℃, and the temperature entering finishing rolling stage II at…

[0061] 790~830℃, rolling speed of the last stand is 20~45m / s;

[0062] 8) Cool the bed by controlling the temperature of the bed to be between 570 and 620°C and cooling it to room temperature at a cooling rate of 6 to 9°C / s.

[0063] Table 1. List of chemical components (wt%) of various embodiments and comparative examples of the present invention.

[0064]

[0065]

[0066] Table 2. List of process parameters for the smelting process of various embodiments and comparative examples of the present invention.

[0067]

[0068] Table 3. List of rolling process parameters for each embodiment and comparative example of the present invention.

[0069]

[0070]

[0071] Note: The dimensional accuracy of the square billet is ±3mm.

[0072] Table 4. Experimental Results of Various Embodiments and Comparative Examples of the Invention

[0073]

[0074] 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 reducing production costs and increasing output.

[0075] 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 ≥ 830 MPa, comprising the following components and weight percentages: C: 0.35–0.50%, Si: 0.50–0.90%, Mn: 0.40–0.90%, P ≤ 0.035%, S ≤ 0.035%, Al: 0.05–0.12%, V: 0.01–0.07%, N: 0.0045–0.010%, Zr: 0.40–0.60%, B: 0.0008% or B: 0 0.0012% or B:0.0023% or B:0.0020% or B:0.0009% or B:0.0025% or B:0.0028% or B:0.0029% or B:0.0030% or B:0.0027%, with the remainder being Fe and unavoidable impurities; its mechanical properties: yield strength Rel≥830MPa, tensile strength Rm≥980MPa, elongation A≥6%, total elongation at maximum force Agt≥3.5%.

2. The rebar with a yield strength ≥ 830 MPa as described in claim 1, characterized in that: The weight percentage content of C is 0.38% to 0.45%.

3. A rebar with a yield strength ≥ 830 MPa as described in claim 1, characterized in that: The weight percentage content of Si is between 0.55% and 0.85%.

4. A rebar with a yield strength ≥ 830 MPa as described in claim 1, characterized in that: The weight percentage content of Mn is between 0.50% and 0.80%.

5. A rebar with a yield strength ≥ 830 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 ≥ 830 MPa as described in claim 1, characterized in that: The weight percentage content of V is 0.01 to 0.06%.

7. A rebar with a yield strength ≥ 830 MPa as described in claim 1, characterized in that: The weight percentage of N is between 0.0055 and 0.009%.

8. A rebar with a yield strength ≥ 830 MPa as described in claim 1, characterized in that: The Zr content is between 0.45% and 0.55% by weight.

9. A method for producing a rebar with a yield strength ≥ 830 MPa as described in claim 1, comprising the following steps: 1) Hot metal pretreatment is performed. After desulfurization, the hot metal has the following properties: P < 0.020% and 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 size of the square billet is 160-170mm. 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 by controlling the temperature of the bed to be between 570 and 620°C and cooling it to room temperature at a cooling rate of 6 to 9°C / s.

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