Steel for low-temperature U-shaped bolt and smelting method thereof

Abstract

The invention belongs to the technical field of steel smelting, and relates to steel for a low-temperature U-shaped bolt and a smelting method of the steel. The steel comprises the following chemical components in percentage by weight: 0.33%-0.38% of C, 0.20%-0.30% of Si, 0.60%-0.70% of Mn, less than or equal to 0.010% of P, less than or equal to 0.005% of S, 0.95%-1.10% of Cr, 0.18%-0.22% of Mo, 0.020%-0.050% of Al, less than or equal to 0.0010% of O and the balance of Fe and inevitable residual elements. The tensile strength of produced steel is larger than or equal to 980 MPa, the yield ratio Rel / Rm is larger than or equal to 0.9, the percentage elongation after fracture A is larger than or equal to 12%, the percentage reduction of area Z is larger than or equal to 45%, the impact absorbing energy AKV at the temperature of-20 DEG C is larger than or equal to 80 J, and the austenite grain size reaches the grade 7 or above.

Description

Technical Field

[0001] This invention belongs to the field of steel smelting technology and relates to a low-temperature U-bolt steel and its smelting method. Background Technology

[0002] U-bolts, named for their unique U-shaped design, are threaded at both ends for easy engagement with nuts. They are widely used to secure tubular objects such as water pipes, and sheet-like objects such as leaf springs in automobiles. These bolts play a vital role in building installation, mechanical component connections, vehicles, ships, bridges, tunnels, and railways. Their high strength and load-bearing capacity, along with their flexibility in adapting to irregular structures, make them indispensable fasteners in engineering. With the rapid development of new energy infrastructure (such as wind power and photovoltaics) and automated equipment, the demand for high-performance cryogenic fasteners continues to grow. This provides reliable protection for engineering projects in extremely cold environments, driving related industries to expand into more demanding environments and promoting advancements in materials science and manufacturing processes. As industrial modernization accelerates, the performance of cryogenic U-bolts will continue to improve, and their application areas will further expand.

[0003] In low-temperature environments, the properties of ordinary bolt materials deteriorate significantly, making them prone to brittle fracture. Cast iron almost completely loses its toughness at low temperatures, and ordinary rubber seals harden and fail. Therefore, low-temperature bolts need to meet special requirements. These bolts typically use nickel-chromium-molybdenum composite reinforced (Ni-Cr-Mo) materials, and their low-temperature lattice stability is improved through special heat treatment processes. In addition, surface treatment processes such as hot-dip galvanizing or Dacromet coating can significantly improve corrosion resistance and extend service life.

[0004] The development of cryogenic U-bolts has gone through several key stages. First, in terms of material selection, the technology has evolved from ordinary carbon steel to chromium-molybdenum alloy steel, and then to nickel-chromium-molybdenum composite reinforced materials, significantly improving low-temperature toughness. Second, in terms of manufacturing processes, technologies such as hot forging, precision thread rolling, and intelligent detection have been adopted to ensure product quality. These technological breakthroughs enable cryogenic U-bolts to meet the requirements for use in extreme environments.

[0005] Currently, U-bolts in my country are mainly made of Mn and CrMo series steels, with different steel grades used depending on the application. Therefore, based on the quality requirements of a leading U-bolt steel manufacturer in China, we have developed a medium-carbon alloy structural steel for low-temperature U-bolts and its smelting and production method, which is of great significance. Summary of the Invention

[0006] To achieve the above objectives, this invention provides a steel for low-temperature U-bolts and its smelting method. By controlling the content of C, Mn, Cr, and Mo elements and reducing the content of harmful elements such as P and S, the toughness of the material is improved. The process includes converter smelting, LF furnace refining, RH vacuum treatment, continuous casting, and two-fire rolling (350*430mm→150*150mm→round steel) to produce finished round steel with a diameter of 20-30mm suitable for processing into low-temperature U-bolts.

[0007] The technical solution adopted by the present invention is as follows: Firstly, the present invention provides a steel for low-temperature U-bolts, wherein the chemical composition of the steel by weight percentage is C: 0.33%–0.38%, Si: 0.20%–0.30%, Mn: 0.60%–0.70%, P≤0.010%, S≤0.005%, Cr: 0.95%–1.10%, Mo: 0.18%–0.22%, Al: 0.020%–0.050%, O≤0.0010%, with the remainder being Fe and unavoidable residual elements; the tensile strength of the steel is ≥980MPa, the yield strength ratio Rel / Rm≥0.9, the elongation after fracture A≥12%, the reduction of area Z≥45%, the impact absorption energy AKV at -20℃≥80J, and the austenite grain size reaches level 7 or above.

[0008] Furthermore, the steel has a diameter of 20-30 mm and a metallographic structure that is a composite structure of ferrite, pearlite, and acicular ferrite.

[0009] Secondly, the present invention also provides a method for producing steel for low-temperature U-bolts, specifically including the following steps:

[0010] (1) Initial smelting of molten iron: smelting is carried out in a top and bottom blown converter. Molten iron and scrap steel are added in a ratio of not less than 3.0. Lime and lightly calcined dolomite are added for slag formation. The phosphorus content at the end of smelting is controlled at ≤0.010%, and the carbon content at the end is not less than 0.06%. During the tapping process, silicon manganese alloy and high carbon ferrochrome alloy are used for alloying. Aluminum blocks are used as the deoxidizer. After tapping, 350±50 kg of lime is added to wash the slag. The slag discharge after tapping is detected by infrared and automatically blocked by a sliding plate.

[0011] (2) Steel refining: Add lime and high-alumina pre-melted slag to make white slag for refining. The total slag amount is controlled at 13-15 kg / ton of steel. During the refining process, aluminum particles and silicon carbide are used for diffusion deoxidation. After white slag is formed and deoxidation is ensured, the composition is finely adjusted according to the target composition and the aluminum adjustment is ensured in one go. After refining, feed pure calcium wire for 50-100m for inclusion modification treatment.

[0012] (3) Vacuum treatment: Vacuum treatment is carried out using an RH furnace. The vacuum degree is controlled below 65 Pa and held for 12-15 min. After breaking the vacuum, soft blowing is performed for 15-25 min. After soft blowing, continuous casting is performed.

[0013] (4) Continuous casting of steel: 350mm×430mm billet cross section is used for production. The sand is removed by tool when pouring the ladle. The liquid level in the ladle is controlled at 1000-1200mm during normal pouring. The typical pouring speed is controlled at 0.58m / min. The water flow rate in the crystallizer is 2700L / min. Electromagnetic stirring and light pressure are used at the end of solidification. Infrared detection and automatic slag blocking by sliding plate are used in the later stage of ladle pouring.

[0014] (5) Rolling: The first rolling process of the billet is heating + initial rolling + intermediate rolling. The cross-section is rolled from 350*430mm to 150*150mm. After the surface is fully ground, the second rolling is carried out. The second rolling process is heating + initial rolling + intermediate rolling + pre-finishing rolling + finishing rolling. The rolling temperature of the first and second rolling is controlled at 1100-1150℃. The round steel is placed in a densely packed, slow-cooled cooling bed with an insulation cover after it is taken off the line.

[0015] Furthermore, in the initial smelting process of molten iron, the total slag amount is controlled at 35-45 kg / t steel, and the amount of precipitating deoxidizer added is 0.8-1.2 kg / ton steel.

[0016] Principle of this invention:

[0017] Purity: High-purity alloy structural steel mainly refers to steel with low contents of phosphorus, sulfur, oxygen, nitrogen, and hydrogen. Oxygen content is usually the most important factor, as lower oxygen content reduces oxide inclusions. Experiments have shown the effect of oxygen content on the fatigue life of gears in carburized chromium-molybdenum steel; when the oxygen content decreased from 25 ppm to 11 ppm, the contact fatigue strength increased fourfold. Currently, the oxygen content in alloy steel is generally controlled below 15 ppm, which effectively reduces inclusion content and improves the steel's impact toughness and wear resistance.

[0018] Grain size: Grain size is an important indicator for alloy steels. A uniform and fine austenite grain size is crucial for stabilizing the end-hardenability of the steel, reducing deformation after heat treatment, and improving the brittle fracture resistance of carburized steel. Grain refinement is mainly achieved by adding certain amounts of grain-refining elements such as Al, Ti, and Nb. The steel grade of this invention primarily utilizes Al to refine the grains, resulting in a finer grain structure with an austenite grain size reaching level 7 or higher.

[0019] Two-stage rolling process: Through staged processing, two-stage rolling can more effectively control the grain structure and mechanical properties of steel. The first rolling mainly completes roughing with large deformation, while the second rolling is used for finishing and performance adjustment. This staged processing method is particularly suitable for high-end steels with strict performance requirements. By staged deformation, precise temperature control, and optimized cooling process, the two-stage rolling process can suppress the precipitation of proeutectoid ferrite, promote the formation of acicular ferrite and fine-grained bainite, and ultimately form a composite structure of ferrite + pearlite + acicular ferrite, thus improving low-temperature toughness.

[0020] The beneficial effects of this invention are as follows: This invention closely combines the usage characteristics and processing performance requirements of the product. Based on medium carbon alloy structural steel, it controls the content of C, Mn, Cr, and Mo elements and reduces the content of harmful elements such as P and S to improve the toughness of the material. Through a process including converter smelting, LF furnace refining, RH vacuum treatment, continuous casting, and two-fire rolling (350*430mm→150*150mm→round steel), it produces finished round steel with a diameter of 20-30mm, suitable for processing into low-temperature U-bolts. Its tensile strength is ≥980MPa, yield strength ratio Rel / Rm≥0.9, elongation after fracture A≥12%, reduction of area Z≥45%, and impact absorption energy AKV≥80J at -20℃. It has good low-temperature impact performance, strength, and toughness. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is a metallographic image of the round steel produced by the method in Embodiment 1 of the present invention. Detailed Implementation

[0023] To facilitate understanding by those skilled in the art, the present invention will be further described below with reference to embodiments and accompanying drawings. The content mentioned in the embodiments is not intended to limit the present invention.

[0024] Example 1:

[0025] The steel used for the low-temperature U-bolts in this embodiment has a diameter of 24 mm and its chemical composition by weight percentage is C: 0.36%, Si: 0.20%, Mn: 0.66%, P: 0.008%, S: 0.002%, Cr: 0.98%, Mo: 0.20%, Al: 0.033%, O: 0.0007%, with the remainder being Fe and unavoidable residual elements.

[0026] The process steps include:

[0027] (1) Initial smelting of molten iron: The ratio of molten iron to scrap steel is 3.2. Lime and lightly calcined dolomite are added during the smelting process to form slag. The total slag amount is 42 kg / ton of steel. The phosphorus content at the end of the smelting process is 0.007%, and the carbon content at the end of the process is 0.07%. During the tapping process, silicon manganese alloy and medium carbon ferrochrome are used for alloying. High aluminum manganese ferroprecipitate is used as the deoxidizer, and the amount added is 1.0 kg / ton of steel. The amount of lime added for slag washing is 370 kg, which is added along the steel flow when about 1 / 4 of the steel is tapped. The slag discharge after tapping is detected by infrared and automatically blocked by a sliding plate.

[0028] (2) Steel refining: Add lime and high-alumina pre-melted slag to make white slag for refining. The total slag amount is 14.5 kg / ton of steel. During the refining process, aluminum particles and silicon carbide are used for diffusion deoxidation. The amount of diffusion deoxidizer is controlled at 1.0 kg / ton of steel. After white slag is formed and deoxidation is ensured, the composition is finely adjusted according to the target composition and the aluminum is adjusted in one go. After refining, feed 60 m of pure calcium wire for inclusion modification treatment.

[0029] (3) Vacuum treatment: Vacuum treatment is carried out using an RH furnace, and the vacuum degree is controlled below 65 Pa for 17 minutes. After breaking the vacuum, soft blowing is performed for 20 minutes. After soft blowing, continuous casting is performed.

[0030] (4) Continuous casting of steel: 350mm×430mm billet cross section is used for production. The sand is removed by tool when pouring from the ladle. The liquid level in the ladle is controlled at 1100mm during normal pouring. The typical pouring speed is controlled at 0.58m / min. The water flow rate in the crystallizer is 2700L / min. Electromagnetic stirring and light pressure are used to control the core segregation at the end of solidification. Infrared detection and automatic slag blocking by sliding plate are used in the later stage of ladle pouring.

[0031] (5) Continuous rolling: The billet first-fire rolling process is heating + primary rolling + intermediate rolling. The first-fire rolling temperature is 1115℃, and the cross-section is rolled from 350*430mm to 150*150mm. After the surface is fully ground, the second-fire rolling is carried out. The second-fire rolling process is heating + primary rolling + intermediate rolling + pre-finishing rolling + finishing rolling. The first-fire rolling temperature is controlled at 1135℃. The cooling bed is covered with a heat insulation cover for slow cooling. The round steel is kept warm and cooled slowly after leaving the line.

[0032] The metallographic structure of the steel produced by the above process is as follows: Figure 1 As shown in Table 1, the performance test results are as follows.

[0033] Example 2:

[0034] The steel used for the low-temperature U-bolts in this embodiment has a diameter of 23 mm and its chemical composition by weight percentage is C: 0.36%, Si: 0.26%, Mn: 0.66%, P: 0.006%, S: 0.003%, Cr: 0.99%, Mo: 0.19%, Al: 0.035%, O: 0.0006%, with the remainder being Fe and unavoidable residual elements.

[0035] The process steps include:

[0036] (1) Initial smelting of molten iron: The ratio of molten iron to scrap steel is 3.3. Lime and lightly calcined dolomite are added during the smelting process to form slag. The total slag amount is 44 kg / ton of steel. The phosphorus content at the end of the smelting process is 0.005%, and the carbon content at the end of the process is 0.08%. During the tapping process, silicon manganese alloy and medium carbon ferrochrome are used for alloying. High aluminum manganese ferroprecipitate is used as the deoxidizer, and the amount added is 0.9 kg / ton of steel. The amount of lime added for slag washing is 380 kg, which is added along the steel flow when about 1 / 4 of the steel is tapped. The slag discharge after tapping is detected by infrared and automatically blocked by a sliding plate.

[0037] (2) Steel refining: Add lime and high-alumina pre-melted slag to make white slag for refining. The total slag amount is 14.3 kg / ton of steel. During the refining process, aluminum particles and silicon carbide are used for diffusion deoxidation. The amount of diffusion deoxidizer is controlled at 1.0 kg / ton of steel. After white slag is formed and deoxidation is ensured, the composition is finely adjusted according to the target composition and the aluminum is adjusted in one go. After refining, feed 70 m of pure calcium wire for inclusion modification treatment.

[0038] (3) Vacuum treatment: Vacuum treatment is carried out using an RH furnace, and the vacuum degree is controlled below 65 Pa for 16 minutes. After breaking the vacuum, soft blowing is performed for 22 minutes. After soft blowing, continuous casting is performed.

[0039] (4) Continuous casting of steel: 350mm×430mm billet cross section is used for production. The sand is removed by tool when pouring from the ladle. The liquid level in the ladle is controlled at 1100mm during normal pouring. The typical pouring speed is controlled at 0.58m / min. The water flow rate in the crystallizer is 2700L / min. Electromagnetic stirring and light pressure are used to control the core segregation at the end of solidification. Infrared detection and automatic slag blocking by sliding plate are used in the later stage of ladle pouring.

[0040] (5) Continuous rolling: The billet first-fire rolling process is heating + primary rolling + intermediate rolling. The first-fire rolling temperature is 1128℃, and the cross-section is rolled from 350*430mm to 150*150mm. After the surface is fully ground, the second-fire rolling is carried out. The second-fire rolling process is heating + primary rolling + intermediate rolling + pre-finishing rolling + finishing rolling. The first-fire rolling temperature is controlled at 1120℃. The cooling bed is covered with a heat insulation cover for slow cooling. The round steel is kept warm and cooled slowly after leaving the line.

[0041] The performance test results of the steel produced by the above process are shown in Table 1.

[0042] Example 3:

[0043] The steel used for the low-temperature U-bolts in this embodiment has a diameter of 25mm and its chemical composition by weight percentage is C: 0.35%, Si: 0.27%, Mn: 0.69%, P: 0.008%, S: 0.003%, Cr: 0.97%, Mo: 0.21%, Al: 0.030%, O: 0.0008%, with the remainder being Fe and unavoidable residual elements.

[0044] The process steps include:

[0045] (1) Initial smelting of molten iron: The ratio of molten iron to scrap steel is 3.7. Lime and lightly calcined dolomite are added during the smelting process to form slag. The total slag amount is 40 kg / ton of steel. The final phosphorus content is 0.006% and the final carbon content is 0.07%. During the tapping process, silicon manganese alloy and medium carbon ferrochrome are used for alloying. High aluminum manganese ferroprecipitate is used as the deoxidizer, and the amount added is 0.9 kg / ton of steel. The amount of lime added for slag washing is 360 kg, which is added along the steel flow when about 1 / 4 of the steel is tapped. The slag discharge after tapping is detected by infrared and automatically blocked by a sliding plate.

[0046] (2) Steel refining: Add lime and high-alumina pre-melted slag to make white slag for refining. The total slag amount is 14.0 kg / ton of steel. During the refining process, aluminum particles and silicon carbide are used for diffusion deoxidation. The amount of diffusion deoxidizer is controlled at 0.9 kg / ton of steel. After white slag is formed and deoxidation is ensured, the composition is finely adjusted according to the target composition and the aluminum is adjusted in one go. After refining, feed 50 m of pure calcium wire for inclusion modification treatment.

[0047] (3) Vacuum treatment: Vacuum treatment is carried out using an RH furnace, and the vacuum degree is controlled below 65 Pa for 18 minutes. After breaking the vacuum, soft blowing is performed for 25 minutes. After soft blowing, continuous casting is performed.

[0048] (4) Continuous casting of steel: 350mm×430mm billet cross section is used for production. The sand is removed by tool when pouring from the ladle. The liquid level in the ladle is controlled at 1100mm during normal pouring. The typical pouring speed is controlled at 0.58m / min. The water flow rate in the crystallizer is 2700L / min. Electromagnetic stirring and light pressure are used to control the core segregation at the end of solidification. Infrared detection and automatic slag blocking by sliding plate are used in the later stage of ladle pouring.

[0049] (5) Continuous rolling: The billet first-fire rolling process is heating + primary rolling + intermediate rolling. The first-fire rolling temperature is 1135℃, and the cross-section is rolled from 350*430mm to 150*150mm. After the surface is fully ground, the second-fire rolling is carried out. The second-fire rolling process is heating + primary rolling + intermediate rolling + pre-finishing rolling + finishing rolling. The first-fire rolling temperature is controlled at 1140℃. The cooling bed is covered with a heat insulation cover for slow cooling. The round steel is kept warm and cooled slowly after leaving the line.

[0050] The performance test results of the steel produced by the above process are shown in Table 1.

[0051] Table 1 Product Performance Testing Indicators of Examples

[0052]

[0053] Comparative Example 1:

[0054] The steel used for the low-temperature U-bolts in this embodiment has a diameter of 24 mm and its chemical composition by weight percentage is C: 0.34%, Si: 0.25%, Mn: 0.67%, P: 0.012%, S: 0.006%, Cr: 0.95%, Mo: 0.18%, Al: 0.021%, O: 0.00010%, with the remainder being Fe and unavoidable residual elements.

[0055] The process steps differ from those in Example 1 in that the rolling process changes from 150*150 to φ24mm.

[0056] Its performance was tested: -20℃ low temperature impact 38 / 43 / 38J.

[0057] Comparative Example 2:

[0058] The steel used for the low-temperature U-bolts in this embodiment has a diameter of 24 mm and its chemical composition by weight percentage is C: 0.35%, Si: 0.26%, Mn: 0.70%, P: 0.009%, S: 0.005%, Cr: 0.98%, Mo: 0.19%, Al: 0.025%, O: 0.0009%, and Fe and unavoidable residual elements.

[0059] The process steps differ from those in Example 1 in that the rolling process changes the cross-section from 240*240 to 150*150 to φ24mm.

[0060] Its performance was tested: -20℃ low temperature impact test: 55 / 58 / 62J.

[0061] Comparative Example 3:

[0062] The steel used for the low-temperature U-bolts in this embodiment has a diameter of 24 mm and its chemical composition by weight percentage is C: 0.36%, Si: 0.25%, Mn: 0.55%, P: 0.008%, S: 0.007%, Cr: 0.97%, Mo: 0.20%, Al: 0.033%, O: 0.0008%, with the remainder being Fe and unavoidable residual elements.

[0063] The process steps are the same as in Example 1.

[0064] Its performance was tested: -20℃ low temperature impact test: 63 / 60 / 67J.

[0065] In summary, by adopting the technical solution of this invention, the content of C, Mn, Cr, and Mo elements is controlled, and the content of harmful elements such as P and S is reduced to improve the toughness of the material. Through a process including converter smelting, LF furnace refining, RH vacuum treatment, continuous casting, and two-fire rolling (350*430mm→150*150mm→round steel), finished round steel with a diameter of 20-30mm is produced, which is suitable for processing into low-temperature U-bolts. Its tensile strength is ≥980MPa, yield strength ratio Rel / Rm≥0.9, elongation after fracture A≥12%, reduction of area Z≥45%, and impact absorption energy AKV≥80J at -20℃.

[0066] The above embodiments are preferred implementations of the present invention. In addition, the present invention can be implemented in other ways. Any obvious substitutions without departing from the concept of the present technical solution are within the protection scope of the present invention.

[0067] To facilitate understanding by those skilled in the art of the improvements of this invention over the prior art, some of the accompanying drawings and descriptions have been simplified, and for clarity, some other elements have been omitted from this application. Those skilled in the art should realize that these omitted elements may also constitute the content of this invention.

Claims

1. A type of steel for low-temperature U-bolts, characterized in that, The steel's chemical composition by weight percentage is: C: 0.33%–0.38%, Si: 0.20%–0.30%, Mn: 0.60%–0.70%, P≤0.010%, S≤0.005%, Cr: 0.95%–1.10%, Mo: 0.18%–0.22%, Al: 0.020%–0.050%, O≤0.0010%, with the remainder being Fe and unavoidable residual elements; the steel has a tensile strength ≥980MPa, a yield strength ratio Rel / Rm ≥0.9, an elongation after fracture A ≥12%, a reduction of area Z ≥45%, an impact energy absorbed at -20℃ AKV ≥80J, and an austenite grain size of grade 7 or higher.

2. The steel for low-temperature U-bolts as described in claim 1, characterized in that, The steel has a diameter of 20-30 mm and a metallographic structure that is a composite structure of ferrite, pearlite and acicular ferrite.

3. A method for producing steel for low-temperature U-bolts as described in claim 1 or 2, characterized in that, Specifically, the following steps are included: (1) Initial smelting of molten iron: smelting is carried out in a top and bottom blown converter. Molten iron and scrap steel are added in a ratio of not less than 3.

0. Lime and lightly calcined dolomite are added for slag formation. The phosphorus content at the end of smelting is controlled at ≤0.010%, and the carbon content at the end is not less than 0.06%. During the tapping process, silicon manganese alloy and high carbon ferrochrome alloy are used for alloying. Aluminum blocks are used as the deoxidizer. After tapping, 350±50 kg of lime is added to wash the slag. The slag discharge after tapping is detected by infrared and automatically blocked by a sliding plate. (2) Steel refining: Add lime and high-alumina pre-melted slag to make white slag for refining. The total slag amount is controlled at 13-15 kg / ton of steel. During the refining process, aluminum particles and silicon carbide are used for diffusion deoxidation. After white slag is formed and deoxidation is ensured, the composition is finely adjusted according to the target composition and the aluminum adjustment is ensured in one go. After refining, feed pure calcium wire for 50-100m for inclusion modification treatment. (3) Vacuum treatment: Vacuum treatment is carried out using an RH furnace. The vacuum degree is controlled below 65 Pa and held for 12-15 min. After breaking the vacuum, soft blowing is performed for 15-25 min. After soft blowing, continuous casting is performed. (4) Continuous casting of steel: 350mm×430mm billet cross section is used for production. The sand is removed by tool when pouring the ladle. The liquid level in the ladle is controlled at 1000-1200mm during normal pouring. The typical pouring speed is controlled at 0.58m / min. The water flow rate in the crystallizer is 2700L / min. Electromagnetic stirring and light pressure are used at the end of solidification. Infrared detection and automatic slag blocking by sliding plate are used in the later stage of ladle pouring. (5) Rolling: The first rolling process of the billet is heating + initial rolling + intermediate rolling. The cross-section is rolled from 350*430mm to 150*150mm. After the surface is fully ground, the second rolling is carried out. The second rolling process is heating + initial rolling + intermediate rolling + pre-finishing rolling + finishing rolling. The rolling temperature of the first and second rolling is controlled at 1100-1150℃. The round steel is placed in a densely packed, slow-cooled cooling bed with an insulation cover after it is taken off the line.

4. The method for producing steel for low-temperature U-bolts as described in claim 3, characterized in that, In the initial smelting process of molten iron, the total slag amount is controlled at 35-45 kg / t steel, and the amount of precipitating deoxidizer added is 0.8-1.2 kg / ton steel.

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