A 355mpa grade high ductility very low magnetic steel and its production method

Abstract

This invention discloses a 355MPa grade high-ductility extremely weak magnetic steel and its production method. The extremely weak magnetic steel comprises the following chemical composition by weight percentage: C 0.15%~0.35%, Si 0.20%~0.70%, Mn 17.0%~22.0%, Cr 2.5%~4.5%, V 0.01%~0.04%, P ≤0.030%, S ≤0.005%, with the remainder being Fe and other unavoidable impurities; wherein, A=33.5×C+Mn, A≥25.0. This extremely weak magnetic steel can maintain a single austenitic structure at room temperature, has an optimal match between strength and yield strength ratio (yield strength ratio below 0.55), and exhibits good elongation performance (elongation exceeding 45%) and a maximum force total elongation exceeding 40%). It also possesses good weak magnetic properties, with remanence below 0.3nT before and after processing.

Description

Technical Field

[0001] This invention belongs to the field of steel technology, specifically relating to a 355MPa grade high ductility extremely weak magnetic steel and its production method. Background Technology

[0002] With the development of science and technology, the application of extremely weak magnetic materials is expanding, leading to a continuously increasing market size and higher requirements for remanence. Currently, the industry's highest requirement for remanence in extremely weak magnetic materials is <0.5 nT, and this requirement must be maintained even after magnetization in a 300 Gs magnetic field, which presents a very high level of technical difficulty. In some specialized fields, certain buildings require the use of extremely weak magnetic steel profiles to achieve an extremely weak magnetic environment or to protect the building's safety in strong magnetic field scenarios.

[0003] Currently, there is very little research on extremely weak magnetic steel in the industry, mainly focusing on extremely weak magnetic threaded steel and plate steel. Therefore, research on extremely weak magnetic steel is of great significance for extremely weak magnetic steel structures, providing reliable extremely weak magnetic steel materials for steel structures in extremely weak magnetic environments and strong magnetic field environments.

[0004] Chinese patent CN 117802392 A discloses a method for preparing zero-magnetic steel bars for special non-magnetic concrete structures, including the following steps: S1, smelting: melting raw materials and adjusting the chemical composition; S2, rolling: changing the shape and size by rolling to form steel bars; S3, post-treatment of non-magnetic steel: removing surface magnetic materials through physical pretreatment, chemical pretreatment, and powder spraying. The smelting process in step S1 is electric furnace steelmaking + LF furnace refining + ingot casting. The zero-magnetic steel bars produced through these steps for special non-magnetic concrete structures ensure a remanence of ≤0.5nT (measured at zero distance of 2cm). The resulting zero-magnetic steel bars have multiple advantages such as zero magnetism, high strength, good ductility, and corrosion resistance, and are reasonably priced, making them suitable for industrial production and large-scale use. However, the zero-magnetic steel bars produced by this patent are produced by ingot casting, resulting in low yield and high cost.

[0005] In addition, Chinese patent CN 120006163 A discloses a 500MPa grade low-temperature resistant and seawater corrosion resistant high-manganese non-magnetic steel bar and its production method; Chinese patent CN 117512308 A discloses a non-magnetic steel bar and its preparation method; Chinese patent CN 115449598A discloses a non-magnetic steel bar preparation method.

[0006] Most of the disclosed technologies are for non-magnetic or zero-magnetic steel bars, with fewer disclosures for extremely weak magnetic or zero-magnetic steel bars. However, in practical applications, non-magnetic or zero-magnetic steel bars cannot replace non-magnetic, zero-magnetic, or extremely weak magnetic steel bars. Therefore, research on low-cost extremely weak magnetic steel bars is of great significance. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention provides a 355MPa grade high-ductility extremely weak magnetic steel and its production method, which can be used in steel structures in extremely weak magnetic field buildings or strong magnetic field environments to obtain an extremely weak magnetic environment or protect the safety of steel structures in strong magnetic field environments.

[0008] The technical solution adopted in this invention is as follows:

[0009] The present invention provides a 355MPa grade high ductility extremely weak magnetic steel, comprising the following chemical composition by weight percentage: C 0.15%~0.35%, Si 0.20%~0.70%, Mn 17.0%~22.0%, Cr 2.5%~4.5%, V 0.01%~0.04%, P ≤0.030%, S ≤0.005%, with the remainder being Fe and other unavoidable impurities;

[0010] Where, A = 33.5 × C + Mn, A ≥ 25.0; in formula A, the value of each component is their content in steel × 100.

[0011] The preferred chemical composition and weight percentage of the 355MPa grade high-ductility extremely weak magnetic steel are: C 0.20%~0.30%, Si 0.35%~0.45%, Mn 17.0%~20.0%, Cr 3.0%~4.0%, V 0.025%~0.035%, P ≤0.015%, S ≤0.005%, with the remainder being Fe and other unavoidable impurities.

[0012] The metallographic structure of the 355MPa grade high ductility extremely weak magnetic steel is a single austenitic structure.

[0013] The 355MPa grade high-ductility extremely weak magnetic steel has a yield strength ≥355MPa, tensile strength ≥760MPa, yield-to-tensile ratio ≤0.55, elongation A ≥45.0%, and elongation at maximum force A gt ≥40.0%, remanence ≤0.3nT, relative permeability ≤1.005.

[0014] The 355MPa grade high ductility extremely weak magnetic steel is one of [10~[20 channel steel, I14~I20 I-beam steel, and ∠10~∠18 angle steel.

[0015] The present invention also provides a method for producing the 355MPa grade high ductility extremely weak magnetic steel, the method comprising the following steps: converter smelting → LF refining → billet continuous casting → heating → medium section rolling → straightening → finished product → pickling → spraying.

[0016] In the converter smelting step, the final converter concentration is C≤0.06% and P≤0.020%; the final temperature is controlled to be above 1660℃.

[0017] In the LF refining step, argon gas is blown into the ladle throughout the process; lime is added to form slag, and the basicity R of the refining slag is 1.5-2.5.

[0018] In the billet continuous casting step, the entire casting process is protected; a weak cooling process is adopted, with a crystallizer cooling water flow rate of 105~120m³. 3 / h, secondary cooling water volume 0.50~0.65l / kg; pulling speed controlled at 2.0~2.5m / min, straightening temperature controlled at 1020~1100℃.

[0019] In the rolling process of the medium-sized profile rolling mill, the heating temperature is controlled at 1180~1250℃, the heating time is ≥110min, of which the soaking time is ≥25min to ensure that Mn, Nb and V elements are fully dissolved; the heated billet is descaled by high-pressure water with a descaling pressure ≥12MPa; then it enters BD1 billet rolling, the BD1 initial rolling temperature is ≥1050℃, and the rolling speed is ≤5.0m / s; finally, it enters the continuous rolling mill for rolling, the finishing rolling initial rolling temperature is ≥900℃, and the final rolling temperature should be controlled at 850℃~950℃. The higher final rolling temperature can ensure the uniformity of composition and structure.

[0020] In the pickling step, a hydrochloric acid solution with a volume concentration of 20% to 28% is used for pickling, the pickling temperature is 30℃ to 80℃, and the pickling time is 90 to 150 minutes.

[0021] In the spraying step, the dry film thickness of the coating is controlled at 40~80μm, and the film weight is 2~5g / m³. 2 The coating is a zinc protective layer with a zinc content of 96% or higher.

[0022] The functions and controls of each component in the 355MPa grade high-ductility extremely weak magnetic steel provided by this invention are as follows:

[0023] Mn (manganese): Manganese plays a role in expanding the austenite phase region in steel, and is the sole decisive element for austenite formation and the main element affecting austenite stability. Both manganese and carbon can improve the stability of austenite, and their combination yields a single-phase austenite structure. Mn not only promotes austenite formation but also has a solid solution strengthening effect. Only within a suitable range can the manganese content meet the requirements for austenite formation and stabilize the single-phase austenite structure of high-manganese steel. For Fe-Mn binary alloys, a stable austenite structure with excellent low-temperature toughness can be obtained when the Mn content is not less than 28%, but the strength is relatively low. When the Mn content is not less than 35%, Mn tends to segregate at grain boundaries, leading to grain boundary brittle fracture at low temperatures, which is detrimental to strength and toughness. Furthermore, as the manganese content in the steel increases, it promotes austenite dendrite growth, reduces the thermal conductivity of the steel, increases the uneven cooling of the continuously cast billet during continuous casting, and easily generates hot cracks, increasing the difficulty of continuous casting. Therefore, considering the carbon content, the manganese content in this invention is controlled between 17.0% and 22.0%.

[0024] C: Carbon has a significant impact on the mechanical properties of high-manganese steel. Carbon plays two roles in high-manganese steel: firstly, it promotes the formation of a single-phase austenite structure; secondly, it strengthens through solid solution, ensuring high mechanical properties. If the carbon content in the steel is too low, the steel's strength is low; if the carbon content is too high, the steel's plasticity and toughness are significantly reduced. With increasing carbon content, the number of carbides in the steel increases, even forming a continuous casting network of carbides at grain boundaries, reducing grain boundary strength and plasticity, and also affecting the remanence of weakly magnetic steel. Therefore, considering the manganese content, the carbon content in this invention is controlled at 0.15%–0.35%.

[0025] A value: Both carbon and manganese can improve the stability of austenite, and their combination can obtain a single-phase austenitic structure. To ensure that the steel of this invention has a single austenitic structure at room temperature, A = 33.5 × C + Mn ≥ 25.0.

[0026] Si: The solid solution strengthening effect of Si improves the mechanical properties of manganese steel, but Si reduces the carbon content of the eutectic, thus leading to more low-melting-point eutectic structures. In addition, the aggregation of Si at grain boundaries will generate low-melting-point liquid films, which exacerbates the tendency for solidification cracking. The Si content should be controlled between 0.20% and 0.70%.

[0027] Chromium (Cr) has a significant impact on the mechanical properties of high-manganese steel. High-manganese steel has an austenitic microstructure and relatively low yield strength. To improve yield strength, chromium is added to increase strength. With increasing chromium content, the steel's strength, hardness, wear resistance, and corrosion resistance are all improved. However, excessively high chromium content may lead to a decrease in the steel's plasticity and toughness. Chromium can also form a passivation film on the steel surface, significantly improving corrosion resistance; however, when chromium-containing carbides precipitate, the steel's corrosion resistance decreases. Considering the application environment of extremely weak magnetic steel, and taking into account the carbon content to achieve appropriate mechanical strength, while chromium enhances solid solution strengthening, excessive content reduces austenite stability. Therefore, the chromium content is controlled between 2.5% and 4.5%.

[0028] Vanadium (V) can improve the strength of steel. Vanadium has a strong affinity for carbon, nitrogen, and oxygen, and in steel, it mainly exists in the form of carbides, nitrides, or oxides, exhibiting a strong precipitation strengthening effect. During phase transformation, the diffusion of V is inhibited, resulting in a large amount of V dissolved in the steel. The dissolved V can significantly inhibit the diffusion of C during phase transformation, thus refining the grains and improving the strength of the steel. However, excessive V content can lead to an excessively high yield strength ratio, causing the R-section of the steel to... eL 0 / R eL A content of ≤1.30 is unacceptable and increases costs; therefore, the V content is controlled between 0.01% and 0.04%.

[0029] Sulfur (S) readily reacts with Mn to form MnS inclusions, which segregate at grain boundaries, increasing the steel's susceptibility to delayed fracture. Therefore, the sulfur content in the steel is controlled to be low. The S content is controlled to be ≤0.005%.

[0030] Phosphorus, a common harmful element in high-manganese steel, is directly related to its segregation. Higher carbon content in austenite results in lower phosphorus solubility, making it more prone to precipitating as a phosphorus eutectic after solidification. Due to severe phosphorus segregation, it easily accumulates in localized areas of the steel, forming a high-phosphorus eutectic structure. Excessive phosphorus content also affects other elements, such as increasing the segregation of carbon and manganese. Therefore, the phosphorus content is controlled to be ≤0.030%.

[0031] The 355MPa grade low-cost extremely weak magnetic steel produced by the method of this invention has a yield strength ≥355MPa, tensile strength ≥760MPa, yield-to-tensile ratio ≤0.55, elongation A ≥45.0%, and elongation at maximum force A gt ≥40.0%, remanence ≤0.3nT (10mm from the surface of the steel section), remanence ≤0.3nT after magnetization for 5 minutes under a magnetic field strength of 300Gs (10mm from the surface of the steel section). Relative permeability ≤1.005.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] This invention employs a high-manganese, high-nitrogen composition design and a rational alloy composition matching to achieve an austenitic microstructure at room temperature and excellent mechanical properties, while maintaining the stability of the austenitic microstructure under severe cold work hardening conditions. The addition of microalloying elements such as chromium and vanadium improves the room-temperature mechanical properties of the steel, while simultaneously enhancing the stability of the austenitic microstructure.

[0034] This invention employs a production process involving converter smelting, LF refining, continuous casting of small square billets, and rolling of medium-sized profiles. The process is rationally structured and has low production costs. By controlling the heating and cooling processes, a stable austenitic structure at room temperature is obtained, resulting in excellent room-temperature mechanical properties and weak magnetic properties.

[0035] The 355MPa grade high-ductility extremely weak magnetic steel provided by this invention can maintain a single austenitic structure at room temperature, with an austenite ratio of over 99.9%. Through microalloying and controlled cooling during rolling, the austenite grains are refined to achieve the best match between strength and yield strength ratio, with a yield strength ratio below 0.55. It also has good elongation properties, with an elongation of over 45% and a total elongation at maximum force of over 40%. In addition, it has good weak magnetic properties, with remanence below 0.3nT before and after processing. Attached Figure Description

[0036] Figure 1 The image shows the metallographic structure of the 355MPa grade high-ductility extremely weak magnetic steel in Example 1. Detailed Implementation

[0037] This invention provides a 355MPa grade high-ductility extremely weak magnetic steel, comprising the following chemical composition by weight percentage: C 0.15%~0.35%, Si 0.20%~0.70%, Mn 17.0%~22.0%, Cr 2.5%~4.5%, V 0.01%~0.04%, P ≤0.030%, S ≤0.005%, with the remainder being Fe and other unavoidable impurities;

[0038] Where, A = 33.5 × C + Mn, A ≥ 25.0; in formula A, the value of each component is their content in steel × 100.

[0039] The production method of the 355MPa grade high-ductility, extremely weakly magnetic steel is as follows: converter smelting → LF refining → small billet continuous casting → heating → medium section rolling mill rolling → straightening → finished product → pickling → spraying. Specific operational points are as follows:

[0040] (1) Converter smelting: molten iron and scrap steel are added for smelting. The steel output is controlled at 75.0% of the normal steel output per furnace. The final carbon content of the converter is ≤0.06%, and the phosphorus content is ≤0.020%. The final temperature is controlled above 1660℃. Slag is blocked before tapping. When about 1 / 3 of the steel has been tapped, deoxidizers and alloys are added in the following order: carbon powder → ferrosilicon → ferroniobium → vanadium nitrogen → electrolytic manganese. All ferrosilicon, ferroniobium, carbon powder, and vanadium nitrogen are added. The manganese content is added by electrolytic manganese. Electrolytic manganese with a manganese content of 1% is added during the tapping process. After tapping, 100 kg of refining slag is added to the slag surface according to the amount of slag discharged. The target carbon content of the molten steel after tapping is 0.20%.

[0041] (2) LF furnace refining: The molten steel after tapping is heated and refined in the LF furnace. Argon is blown into the ladle throughout the process. The argon flow rate is based on the molten steel not splashing out of the ladle. Lime is added to form slag, and the basicity R is 1.5-2.5. The remaining electrolytic manganese and low carbon ferrochrome are gradually added while heating until the manganese and chromium content meets the requirements. Before the end of the LF furnace refining, the contents of C, Si, Mn, V, Cr and Nb are adjusted. After the refining is completed, a small amount of argon is used for soft blowing for 10-20 minutes.

[0042] (3) Small billet continuous casting: Small billet continuous casting is adopted. Full-process protective casting is used. A protective sleeve + argon seal is used between the ladle and the tundish. The tundish is protected by molten steel covering agent and argon blowing. An integral submerged nozzle is used between the tundish and the crystallizer. A weak cooling process is adopted. The flow rate of the crystallizer cooling water is 105~120m³. 3 / h, secondary cooling water volume 0.50~0.65l / kg; because the steel of this invention has high high-temperature strength, in order to ensure that the billet can be straightened and to avoid large fluctuations in the liquid level of the crystallizer, the casting speed is controlled at 2.0~2.5m / min, and the straightening temperature is controlled at 1020~1100℃. Because the steel of this invention is medium carbon high manganese steel, MnO floating in the molten steel will enter the protective slag during the casting process. MnO, as a fluxing agent, will reduce the melting point and viscosity of the protective slag. In order to prevent the sensitivity of longitudinal cracks caused by the solidification shrinkage of the austenitic primary billet shell, a special crystallizer protective slag with appropriately high basicity, melting point and viscosity is selected. The basicity of the protective slag (CaO / SiO2) is controlled at 0.8~1.0, the melting point is controlled at 1000~1160℃, and the viscosity at 1300℃ is controlled at 0.55~0.75Pa.s.

[0043] (4) Rolling of medium-sized profiles: This invention can realize the rolling of [10~[20 channel steel, I14~I20 I-beam steel and ∠10~∠18 angle steel. In order to meet the needs of the rolling process and to dissolve the carbides and nitrides of V and Nb in austenite, the heating temperature is controlled at 1180~1250℃, the heating time is ≥110 minutes, of which the soaking time is ≥25 minutes, to ensure that Mn, Nb and V elements are fully dissolved. The heated billet is descaled by high pressure water, the descaling pressure is ≥12MPa; then it enters BD1 billet rolling, the BD1 initial rolling temperature is ≥1050℃, and the rolling speed is ≤5.0m / s. The continuous rolling mill is used for rolling, the finishing rolling initial rolling temperature is ≥900℃, and the final rolling temperature should be controlled in the range of 850℃~950℃. The higher final rolling temperature ensures the uniformity of composition and structure.

[0044] (5) Pickling: Soak in hydrochloric acid with a concentration of 20% to 28%, a temperature of 30°C to 80°C, and a pickling time of 90 to 150 minutes.

[0045] (6) Spraying zinc protective layer: After pickling, steel bars are very easy to rust. Therefore, a zinc protective layer with a zinc content of more than 96% is sprayed on the surface of the steel bars, and the dry film thickness is controlled at 40~80μm to prevent rust.

[0046] The present invention will now be described in detail with reference to the embodiments.

[0047] The chemical composition and weight percentage of the steel sections in each embodiment and comparative example are shown in Table 1, with the balance being iron and unavoidable impurities.

[0048] Table 1 Chemical composition (wt%)

[0049]

[0050] The steelmaking, refining, and continuous casting processes of the steel sections in each embodiment and comparative example are shown in Table 2.

[0051] Table 2 Steelmaking, refining, and continuous casting processes in the embodiments

[0052]

[0053] The rolling processes and hot-rolled microstructures of the steel sections in the examples and comparative examples are shown in Table 3. Comparative Examples 1, 2, and 4 have an austenitic + ferrite microstructure, with the austenite content not exceeding 99.9%.

[0054] Table 3. Steel rolling processes of embodiments and comparative examples of the present invention.

[0055]

[0056] The pickling and spraying processes of the finished products in the examples and comparative examples are shown in Table 4.

[0057] Table 4 Pickling and Spraying Processes

[0058]

[0059] The mechanical and magnetic properties of the steel profiles produced in the test examples and comparative examples before sawing and bending, and their magnetic properties after sawing and bending, were examined. Room temperature mechanical properties were tested according to GB / T 228.1-2021 Metallic Materials - Tensile Testing - Part 1: Test Methods at Room Temperature. Remanence was measured using a high-precision fluxgate magnetometer in a non-magnetic environment, and relative permeability was measured using a high-precision permeability meter. Note: Remanence testing conditions: distance from the test surface < 20 mm, magnetic field fluctuation in the test area < 0.1 nT. The test results are shown in Table 5.

[0060] Table 5 Mechanical and Magnetic Properties

[0061]

[0062] Note: Remanence detection conditions: distance from the detection surface < 10 mm, magnetic field fluctuation in the test area < 0.1 nT. In the tables above, underlined data indicates data that does not meet the requirements of this invention.

[0063] As can be seen from the above, the extremely weak magnetic steel produced by this method maintains an austenitic structure at room temperature, with an austenite content exceeding 99.9%. Through microalloying and controlled cooling during rolling, the austenite grains are refined to achieve an optimal match between strength and yield strength ratio, with a yield strength ratio below 0.55. It also exhibits good elongation properties, with an elongation exceeding 45% and a total elongation at maximum force exceeding 40%. Furthermore, it possesses good weak magnetic properties, with remanence below 0.3 nT before and after processing. The microstructure of the steel in Example 1 is shown in [image missing]. Figure 1 The structure is austenitic at room temperature. The mechanical properties of the various example steel sections are shown in Table 5. The room temperature mechanical properties are as follows: yield strength ≥ 355 MPa, tensile strength ≥ 760 MPa, yield-to-tensile ratio ≤ 0.55, elongation A ≥ 45.0%, and elongation at maximum force A... gt ≥40.0%, remanence ≤0.3nT (10mm from the surface of the steel section), remanence ≤0.3nT after magnetization for 5 minutes under a magnetic field strength of 300Gs (10mm from the surface of the steel section). Relative permeability ≤1.005.

[0064] In Comparative Example 1, because the manganese in the composition does not meet the requirements of this invention, the A value cannot meet the requirements of this invention, a stable austenitic structure cannot be obtained, the austenitic ratio is less than 99.9%, and the yield strength ratio, remanence before and after processing, and relative permeability cannot meet the performance requirements of this invention.

[0065] In Comparative Example 2, the C content does not meet the requirements of this invention, the A value does not meet the requirements of this invention, a stable austenitic structure cannot be obtained, the austenitic proportion is less than 99.9%, and the yield strength ratio, remanence before and after processing, and relative permeability do not meet the performance requirements of this invention.

[0066] Comparative Example 3 has a low V content and, when produced according to the process of this invention, has a low yield strength at room temperature, which does not meet the performance requirements of this invention.

[0067] Comparative Example 4 has a low chromium content and, when produced according to the process of this invention, has a low yield strength at room temperature, which does not meet the performance requirements of this invention.

[0068] Although the composition of Comparative Example 5 meets the requirements of this invention, the final rolling temperature during production does not meet the requirements of this invention, and the yield strength at room temperature is low, which does not meet the performance requirements of this invention.

[0069] Although the composition of Comparative Example 6 meets the requirements of this invention, the pickling time in production does not meet the requirements of this invention. The short pickling time results in the iron oxide scale on the surface of the steel not being cleaned properly. Because the iron oxide scale is magnetic, the residual magnetism before and after processing does not meet the requirements of this invention.

[0070] The above detailed description of a 355MPa grade high-ductility extremely weak magnetic steel and its production method, with reference to the embodiments, is illustrative rather than limiting. Several embodiments can be listed according to the defined scope. Therefore, changes and modifications without departing from the overall concept of the present invention should be within the protection scope of the present invention.

Claims

1. A 355MPa grade high-ductility extremely weak magnetic steel, characterized in that, The chemical composition includes the following weight percentages: C 0.15%–0.35%, Si 0.20%–0.70%, Mn 17.0%–22.0%, Cr 2.5%–4.5%, V 0.01%–0.04%, P ≤0.030%, S ≤0.005%, with the remainder being Fe and other unavoidable impurities; Where, A = 33.5 × C + Mn, A ≥ 25.0; in formula A, the value of each component is their content in steel × 100.

2. The 355MPa grade high-ductility extremely weak magnetic steel according to claim 1, characterized in that, The metallographic structure of the 355MPa grade high ductility extremely weak magnetic steel is a single austenitic structure.

3. The 355MPa grade high-ductility extremely weak magnetic steel according to claim 1, characterized in that, The 355MPa grade high-ductility extremely weak magnetic steel has a yield strength ≥355MPa, tensile strength ≥760MPa, yield-to-tensile ratio ≤0.55, elongation A ≥45.0%, and elongation at maximum force A gt ≥40.0%, remanence ≤0.3nT, relative permeability ≤1.

005.

4. A method for producing 355MPa grade high-ductility extremely weak magnetic steel as described in claim 1, characterized in that, The production method includes the following steps: converter smelting → LF refining → billet continuous casting → heating → medium profile rolling → finished product → pickling → spraying.

5. The production method according to claim 4, characterized in that, In the converter smelting step, the final converter concentration is C≤0.06% and P≤0.020%; the final temperature is controlled to be above 1660℃.

6. The production method according to claim 4, characterized in that, In the LF refining step, argon gas is blown into the ladle throughout the process; lime is added to form slag, and the basicity R of the refining slag is 1.5-2.

5.

7. The production method according to claim 4, characterized in that, In the billet continuous casting step, the entire casting process is protected; a weak cooling process is adopted, with a crystallizer cooling water flow rate of 105~120m³. 3 / h, secondary cooling water volume 0.50~0.65l / kg; pulling speed controlled at 2.0~2.5m / min, straightening temperature controlled at 1020~1100℃.

8. The production method according to claim 4, characterized in that, In the rolling process of the medium-sized profile rolling mill, the heating temperature is controlled at 1180~1250℃, the heating time is ≥110min, of which the soaking time is ≥25min; the heated billet is descaled by high pressure water, and the descaling pressure is ≥12MPa. Then it enters the BD1 billet rolling process, with a BD1 initial rolling temperature ≥1050℃ and a rolling speed ≤5.0m / s; finally, it enters the continuous rolling mill for rolling, with a finishing rolling initial rolling temperature ≥900℃ and a final rolling temperature controlled between 850℃ and 950℃.

9. The production method according to claim 4, characterized in that, In the pickling step, a hydrochloric acid solution with a volume concentration of 20% to 28% is used for pickling, the pickling temperature is 30℃ to 80℃, and the pickling time is 90 to 150 minutes.

10. The production method according to claim 4, characterized in that, In the spraying step, the dry film thickness of the coating is controlled at 40~80μm.

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