Marine atmosphere corrosion-resistant high-toughness steel and preparation method thereof

Abstract

The invention discloses marine atmosphere corrosion-resistant high-toughness steel and a preparation method thereof, and relates to the technical field of steel production, and the marine atmosphere corrosion-resistant high-toughness steel comprises the following chemical components in percentage by mass: 0.025-0.035% of C, 1.29-1.51% of Si, less than or equal to 0.010% of P, less than or equal to 0.0010% of S, 1.6-2.9% of Cr, 0.41-0.65% of Cu, 0.15-0.45% of Al and 0.95-2.25% of Ti. And the balance of Fe and inevitable impurities. The high-toughness steel resistant to marine atmosphere corrosion has good seawater corrosion resistance and marine organism corrosion resistance, the safety of sailors, ships, transported goods and marine ecology is guaranteed, and the service life of the whole steel structure engineering is prolonged.

Description

Technical Field

[0001] This invention relates to the field of steel production technology, and in particular to a high-toughness steel resistant to marine atmospheric corrosion and its preparation method. Background Technology

[0002] In recent years, with the booming ocean shipping industry, the demand for large ocean-going vessels has been increasing year by year. The requirements for the corrosion resistance of shipbuilding steel to marine environments have become increasingly important. In order to improve the service life of ships, increase transportation efficiency, and increase economic benefits, the corrosion resistance of shipbuilding steel is required to be better than that of existing steels such as 10CrMoAl, AH36, and DH36, thereby reducing the time cycle for ship maintenance, upkeep, and repair in dry dock.

[0003] Chinese invention patent application (application number 202311831268.5) discloses "a production method of 80-120mm thick seawater corrosion resistant steel 10CrMoAl". The chemical composition of the steel, by weight percentage, is as follows: C: 0.09-0.11, Si: 0.35-0.45, Mn: 0.50-0.60, P: ≤0.010, S≤0.002, Cr: 1.0-1.1, Mo: 0.28-0.32, Alt: 0.65-0.75, Cu: 0.15-0.25, with the remainder being Fe and residual elements; yield strength ≥320MPa, tensile strength 570-620MPa, elongation ≥23%, 0℃ impact value ≥100J, and the steel plate's resistance to lamellar tearing meets the Z35 performance requirements. The steel produced by this patented technology has low strength and requires offline tempering heat treatment after rolling, which increases production costs. Furthermore, it only tests the impact value at 0℃ ≥100J, making it unsuitable for building large cryogenic ships such as those carrying LPG, liquid ammonia, or liquefied ethylene.

[0004] Chinese invention patent application (application number 201811580228.7) discloses "a seawater corrosion-resistant steel and its manufacturing method," wherein the chemical composition of the corrosion-resistant steel is as follows (weight percentage): C: 0.03-0.05%, Si: 0.04-0.08%, Mn: 0.8-1.2%, Cu: 0.1-0.2%, Cr: 2.5-5.5%, Ni: 0.05-0.15%, Mo: 0.15-0.35%, Al: 1.5-3.5%, Ti: 0.01-0.02%, Ca: 0.0015-0.003%, with the balance being Fe and other unavoidable impurities. Among the other unavoidable impurities, the mass percentage content of P, S, and N elements satisfies at least one of the following: P ≤ 0.015%, S ≤ 0.004%, N ≤ 0.005%. Furthermore, this invention also discloses a method for manufacturing the aforementioned seawater corrosion-resistant steel, comprising the steps of: (1) smelting and casting; (2) reheating: the billet is reheated to 1200℃-1260℃; (3) rough rolling; (4) finish rolling; (5) coiling; and (6) cooling to room temperature. This seawater corrosion-resistant steel possesses both excellent seawater corrosion resistance and superior mechanical properties. This patented technology intentionally incorporates expensive alloying elements Ni and Mo, resulting in higher production costs. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a high-toughness steel resistant to marine atmospheric corrosion and its preparation method.

[0006] To solve the above technical problems, the technical solution of the present invention is as follows: A high-toughness steel resistant to marine atmospheric corrosion has the following chemical composition and mass percentages: C: 0.025~0.035%, Si: 1.29~1.51%, P≤0.010%, S≤0.0010%, Cr: 1.6~2.9%, Cu: 0.41~0.65%, Al: 0.15~0.45%, Ti: 0.95~2.25%. The balance is Fe and unavoidable impurities.

[0007] As a preferred embodiment of the high-toughness steel resistant to marine atmospheric corrosion described in this invention, the mass percentage of the C element is 0.025~0.03%.

[0008] As a preferred embodiment of the high-toughness steel resistant to marine atmospheric corrosion described in this invention, the mass percentage of the Si element is 1.29~1.34%.

[0009] As a preferred embodiment of the high-toughness steel resistant to marine atmospheric corrosion described in this invention, the mass percentage of the Cr element is 2.65~2.90%.

[0010] As a preferred embodiment of the high-toughness steel resistant to marine atmospheric corrosion described in this invention, the mass percentage of Cu element is 0.41~0.55%.

[0011] As a preferred embodiment of the high-toughness steel resistant to marine atmospheric corrosion described in this invention, the mass percentage of the Ti element is 1.08~2.15%.

[0012] This invention also provides a method for preparing high-toughness steel resistant to marine atmospheric corrosion, comprising the following steps in sequence: smelting, continuous casting, rolling, and cooling; The smelting process includes: The raw materials are added to a converter or electric furnace for melting, oxygen is blown to decarburize, and the carbon content is controlled to be below 0.012% when sampled and analyzed. When the temperature of molten steel is in the range of 1600-1690℃, desulfurization is carried out in an LF ladle furnace, alloying is performed and the content of alloying elements is adjusted within a preset range, then aluminum blocks are added for further deoxidation, and then aluminum wire is fed to alloy aluminum. A vacuum degassing cycle is used to control the vacuum level to ≤5 mbar, and argon gas is blown for stirring.

[0013] In a preferred embodiment of the method for preparing the high-toughness steel resistant to marine atmospheric corrosion described in this invention, the average casting speed of the continuous casting process is controlled to be 1.51-2.2 m / min.

[0014] In a preferred embodiment of the method for preparing the high-toughness steel resistant to marine atmospheric corrosion described in this invention, the rolling process includes: The billet heating temperature is controlled at 1115~1200℃, and the holding time is 3.1-5.0 hours; The billet is rolled immediately after exiting the furnace, with the initial rolling temperature controlled at 980-1040℃ and the final rolling temperature at 815-860℃.

[0015] As a preferred embodiment of the method for preparing high-toughness steel resistant to marine atmospheric corrosion according to the present invention, the rolling process includes eleven rolling passes, with seven passes completed at 980-1040℃ and four passes completed at 815-860℃.

[0016] In a preferred embodiment of the method for preparing the high-toughness steel resistant to marine atmospheric corrosion described in this invention, the cooling control includes rapid cooling after rolling and natural cooling. The post-rolling rapid cooling includes: using a blower to blow water mist for cooling after rolling, with the cooling rate controlled at 3.5-4.5℃ / s and the reheating temperature at 455-660℃; The natural cooling process includes: cooling to room temperature in air using the residual heat after rolling.

[0017] The beneficial effects of this invention are: (1) The present invention adopts an ultra-low carbon composition design and controls the C content in the range of 0.025~0.035% during the smelting process. The purpose is to solidify the carbon atoms in the Fe matrix to improve the strength of the steel. However, excessive carbon content will form carbides such as Fe3C and Cr3C during the steel production process. These carbides form a potential difference with the Fe matrix to generate a battery and cause electrochemical corrosion, resulting in pitting corrosion on a macroscopic scale and reducing the steel's resistance to seawater corrosion. Therefore, the upper limit of C content is controlled at 0.035%. The alloying element Ti is added and controlled in the range of 0.95~2.25%. On the one hand, the alloying element Ti is solidified in the Fe matrix to form a shielding effect, which prevents Cl ions in the atmosphere from penetrating into the matrix and avoids corrosion of the matrix metal. On the other hand, it forms a dense, enriched layer with Cu and Cr, exhibiting excellent corrosion resistance. Furthermore, a composite oxide passivation layer (SiO2, Cr2O3, Al2O3, etc.) containing Ti3O5 and Ti2O3 is formed on the surface. This composite oxide passivation layer is tightly bonded to the substrate, improving corrosion resistance. Controlling the Cr content at a relatively high level of 1.6~2.9% allows Cr to form a dense, enriched layer with Cu, Ti, and Al, creating a protective barrier against corrosion. The resulting composite oxides (SiO2, Cr2O3, Al2O3, Ti3O5, Ti2O3) also prevent corrosion of the base steel from the external environment. Additionally, adding an appropriate amount of Cr lowers the phase transformation temperature of the steel. The microstructure of the steel produced by the method described in this application is carbide-free bainite, a homogeneous structure, avoiding electrochemical corrosion caused by potential differences resulting from multiphase structures. The Si alloy forms a substitutional solid solution with Fe atoms in the steel, creating lattice distortion and increasing the steel's strength. In addition, Si, along with added alloying elements such as Cr, Ti, and Al, forms SiO2, Cr2O3, Al2O3, Ti2O3, and Ti3O5 on the surface of the steel. Together with the solid-solution anti-corrosion agents such as Cr, Cu, and Ti, it forms a dense passivation layer, which can effectively prevent Cl in seawater from entering the steel. - To improve the steel's resistance to seawater corrosion, the Si alloying element is incorporated into the Fe matrix. Therefore, the Si alloying element is set in the range of 1.29% to 1.51%.

[0018] (2) This invention utilizes the residual heat after rolling to cool to room temperature in air, thereby forming a composite oxide (Ti3O5, Ti2O3, SiO2, Cr2O3, Al2O3) passivation layer on the surface of the angle steel. This passivation layer has high density and strong adhesion to the substrate, forming a gradient metallurgical transition layer, which effectively prevents Cl from forming. - The penetration of ions hinders the corrosion behavior caused by chemical reactions, thus exhibiting excellent resistance to marine atmospheric corrosion. Simultaneously, natural cooling to room temperature within this temperature range eliminates residual internal stress generated during the controlled rolling process of the angle steel, preventing stress corrosion.

[0019] (3) The high-toughness steel resistant to marine atmospheric corrosion prepared by the present invention has good resistance to seawater corrosion and marine biological corrosion, ensuring the safety of crew members, ships, transported goods and marine ecology, and improving the service life of the overall steel structure project. Detailed Implementation

[0020] To make the content of this invention easier to understand, the invention will be further described in detail below based on specific embodiments.

[0021] This application provides a high-toughness steel resistant to marine atmospheric corrosion, with the following chemical composition and mass percentages: C: 0.025~0.035%, Si: 1.29~1.51%, P≤0.010%, S≤0.0010%, Cr: 1.6~2.9%, Cu: 0.41~0.65%, Al: 0.15~0.45%, Ti: 0.95~2.25%. The balance is Fe and unavoidable impurities.

[0022] The principles governing the control of key alloying elements are explained below: The most significant feature of this patent application is that its chemical composition system is an ultra-low carbon Cr-Ti series seawater corrosion resistant steel.

[0023] C (Carbon): This application adopts an ultra-low carbon composition design, and the C content is controlled within the range of 0.025~0.035% during the smelting process. The purpose is to allow carbon atoms to dissolve in the Fe matrix and improve the strength of the steel. However, excessive carbon content will form carbides such as Fe3C and Cr3C during the steel production process. These carbides form a potential difference with the Fe matrix, which will generate a cell and cause electrochemical corrosion, resulting in pitting corrosion on a macroscopic scale and reducing the steel's resistance to seawater corrosion. Therefore, the upper limit of C content is controlled at 0.035%. If the C content is lower than the lower limit of 0.025%, it will be difficult to guarantee the lower limit of 720MPa for the strength of the angle steel produced.

[0024] Silicon (Si): In steel, the alloying element Si forms a substitutional solid solution with Fe atoms, creating lattice distortion and increasing the steel's strength. Furthermore, Si, along with added alloying elements such as Cr, Ti, and Al, forms SiO2, Cr2O3, Al2O3, Ti2O3, and Ti3O5 on the steel surface. Together with the solid-solution Cr, Cu, and Ti, these corrosion-resistant elements form a dense passivation layer, effectively preventing the formation of chloride ions (Cl) in seawater. - Infiltrating into the Fe matrix improves the steel's resistance to seawater corrosion. Therefore, the Si content is set in the range of 1.29~1.51%. However, too high a Si content leads to a decrease in low-temperature impact toughness, while too low a Si content makes it difficult to achieve a yield strength of 720MPa.

[0025] P (phosphorus): P in steel is not an intentionally added alloying element, but a harmful element brought in from the raw materials, especially reducing the low-temperature impact toughness of steel. Some P can be removed during the smelting process. However, controlling P to even lower levels significantly increases production costs. Based on current general smelting operations, controlling the residual harmful element P to ≤0.010% is appropriate.

[0026] S (sulfur): S is a harmful element in steel. During the steel production process, it easily forms inclusions such as FeS and MnS, which can cleave the base metal, produce microcracks, reduce the low-temperature impact toughness of steel, and also reduce the corrosion resistance of steel. Therefore, the content of S is controlled in a low range: S≤0.001%.

[0027] Ti (Titanium): my country has abundant Ti resources, which are widely used in the iron and steel metallurgy field. Ti alloys also have excellent resistance to seawater and marine corrosion. This patent application intentionally adds the alloying element Ti, controlling it within the range of 0.95~2.25%. On the one hand, the alloying element Ti is dissolved in the Fe matrix, forming a shielding effect, preventing atmospheric Cl ions from penetrating into the matrix and avoiding corrosion of the base metal. On the other hand, it forms a rich and dense layer with Cu and Cr, which has excellent corrosion resistance. Moreover, a composite oxide passivation layer containing Ti3O5 and Ti2O3 (SiO2, Cr2O3, Al2O3, etc.) is formed on the surface. This composite oxide passivation layer is tightly bonded to the matrix, improving corrosion resistance.

[0028] Cr (chromium); This patent application is for an ultra-low carbon Cr-Ti series seawater corrosion-resistant steel. Therefore, the content of the alloying element Cr is controlled at a relatively high level of 1.6~2.9%. On the one hand, Cr can form a rich and dense layer with Cu, Ti, Al, etc., to create a protective barrier and protect the matrix from corrosion. Moreover, the formed composite oxides (SiO2, Cr2O3, Al2O3, Ti3O5, Ti2O3) prevent the base steel from being corroded by the external environment. On the other hand, adding an appropriate amount of Cr can lower the phase transformation temperature of the steel. The microstructure of the steel produced by the preparation method of this application is carbide-free bainite, which is a homogeneous microstructure, avoiding electrochemical corrosion caused by potential differences due to multiphase microstructure.

[0029] Al (aluminum): The alloying element Al reacts with residual N in steel to form AlN, which refines the grains and improves the overall mechanical properties of the steel. Al is an easily oxidized element, and the Al2O3 produced by reacting with [O] has good corrosion resistance. On the other hand, the angle steel produced by the process described in this application forms a dense passivation layer of Ti2O3, Ti3O5, SiO2, Cr2O3, and Al2O3 on the surface with Ti, Cr, Si, and Al, protecting the steel and steel structural components from corrosion. Although Al is a relatively inexpensive alloying element, excessively high Al content makes the continuous casting process difficult. Therefore, the Al composition range is set at 0.15~0.45%.

[0030] Cu (copper): Adding a certain amount of the alloying element Cu to seawater-resistant steel can effectively prevent Cl from forming. - Ions penetrate into the matrix to improve resistance to seawater corrosion. On the one hand, they can form a rich composite dense layer with Cr, Ti, Al, etc., which can significantly enhance the corrosion resistance of steel. On the other hand, the copper oxides CuO, Cu2O, etc. formed on the surface by the preparation process of this patent application have good resistance to marine microbial erosion. However, excessive Cu content is prone to thermal cracking during production. Therefore, the Cu content range is set at 0.41~0.65%.

[0031] Preferably, the present application further controls the mass percentage of C element to 0.025~0.03%, the mass percentage of Si element to 1.29~1.34%, the mass percentage of Cr element to 2.65~2.90%, the mass percentage of Cu element to 0.41~0.55%, and the mass percentage of Ti element to 1.08~2.15%.

[0032] This application also provides a method for preparing high-toughness steel resistant to marine atmospheric corrosion, the method comprising the following steps in sequence: smelting, continuous casting, rolling, and cooling.

[0033] The smelting process is as follows: CaO, iron oxide (FexOy), blast furnace hot metal, scrap steel, low-carbon alloy materials such as CrFe, SiFe, TiFe, and Cu are charged into a converter or electric furnace and melted. Oxygen is blown to decarburize the steel, and the carbon content is controlled below 0.012% during this process. When the steel temperature is in the range of 1600-1690℃, LF ladle furnace desulfurization is performed. Alloying is then carried out, and the content of alloying elements is adjusted within the control range. After further deoxidation with Al blocks, Al wire is fed to alloy Al. Vacuum degassing circulation is used, with a vacuum degree ≤5 mbar, and argon gas is blown to stir and remove harmful inclusions from the steel.

[0034] In the continuous casting process, the molten steel obtained from smelting is continuously cast into a rectangular billet of 180mm×280mm, and the average casting speed is controlled within the range of 1.51-2.2 meters / minute.

[0035] The rolling process is as follows: the billet heating temperature is controlled at 1115–1200℃, and the holding time is 3.1–5.0 hours. The billet is rolled immediately after exiting the furnace, with the initial rolling temperature controlled at 980–1040℃ and the final rolling temperature at 815–860℃. The rolling process employs eleven passes, with seven passes completed at 980–1040℃ and four passes completed at 815–860℃. The web thickness of the finished product is 18 ± 0.5 mm.

[0036] Cooling control includes rapid cooling after rolling and natural cooling. Rapid cooling after rolling involves using a blower to blow water mist after rolling, with a cooling rate controlled at 3.5-4.5℃ / s, and a reheat temperature of 455-660℃. The microstructure of the finished steel is 60-70% carbide-free bainite + 30-40% ferrite. Natural cooling utilizes the residual heat after rolling to cool to room temperature in air.

[0037] During the process of cooling the angle steel to room temperature in air at the residual temperature of 455-660℃ after rolling, a composite oxide (Ti3O5, Ti2O3, SiO2, Cr2O3, Al2O3) passivation layer is formed on the surface. This passivation layer has high density and strong adhesion to the substrate, forming a gradient metallurgical transition layer, which effectively prevents Cl from forming. - The penetration of ions hinders the corrosion behavior caused by chemical reactions, thus exhibiting excellent resistance to marine atmospheric corrosion. Simultaneously, natural cooling to room temperature within this temperature range eliminates residual internal stress generated during the controlled rolling process of the angle steel, preventing stress corrosion.

[0038] The above solution will be further illustrated below through specific embodiments and comparative examples.

[0039] According to the chemical element composition, mass percentage, and production method requirements of this invention, five examples were prepared, namely Example 1, Example 2, Example 3, Example 4, and Example 5. To verify the influence of the chemical element composition and mass percentage content, as well as process parameters such as finishing rolling temperature, cooling rate after finishing rolling, and red-hot temperature on performance parameters, three comparative examples were prepared, namely Comparative Example 1, Comparative Example 2, and Comparative Example 3, i.e., eight batches of steel were smelted and prepared. Among them, the mass percentage content of the chemical components in Comparative Example 1 is outside the scope of this invention, while the process parameters of the preparation process are within the scope of this invention; the mass percentage content of the chemical components in Comparative Example 2 is within the scope of this invention, while the process parameters of the preparation process are outside the scope of this invention; and the mass percentage content of the chemical components and the process parameters of the preparation process in Comparative Example 3 are both outside the scope of this invention. The weight percentages of the chemical element composition of the five examples and the three comparative examples are shown in Table 1, where the balance is Fe and unavoidable impurities. The control parameters of the preparation process and the performance and quality of the angle steel are shown in Table 2.

[0040] Artificial simulation of marine atmospheric environment corrosion test methods: 1. The example samples and comparative samples of this application are processed into block corrosion samples with a size of 50mm×50mm×5mm. The samples are polished on all six sides with 800 grit metallographic sandpaper, the surfaces are cleaned with pure alcohol and dried. 2. Preparation of corrosive solution: Place a 10% HCl + 20% NaCl + 70% H2O solution in a 50-liter container, control the temperature in the container at 30±5℃, and place the test sample on top of the solution in the sealed container without contacting the solution. 3. Samples were taken out at corrosion times of 24 hours, 168 hours, 240 hours and 720 hours respectively. The samples were cleaned with alcohol and dried. The depth of the corrosion pits was measured with a thickness gauge. The number of pits was ≥50. The arithmetic mean of the measured corrosion depths was calculated and extrapolated to the annual (365 days) corrosion rate.

[0041] Table 1. Comparison of chemical composition (wt%) of the embodiments and comparative embodiments of this application

[0042] Table 2. Effects of Production Process Control on Steel Performance in Embodiments and Comparative Embodiments of the Invention

[0043] Tables 1 and 2 show that the angle steel produced by the chemical composition and mass percentage of Examples 1-5 of the present invention, as well as the parameters for controlling the production process, all have a room temperature yield strength higher than 720 MPa, an impact energy at -100℃ higher than 80 J, and an average annual corrosion rate lower than 0.01 mm / year. In contrast, the comparative steel produced by Comparative Examples 1, 2, and 3, whose steel composition range and / or production process are not within the scope of this application, all have a room temperature yield strength lower than 488 MPa, an impact energy at -100℃ lower than 41 J, and an average annual corrosion rate higher than 0.18 mm / year. Among them, the angle steel prepared in Example 2 of this application has a room temperature yield strength of 731 MPa, an impact energy of 268 J at -100℃, and an average annual corrosion rate of 0.002 mm / year. It has excellent comprehensive mechanical properties and excellent resistance to marine environmental corrosion. It is safe and reliable for manufacturing keels and reinforcing ribs of large ocean-going low-temperature ships, as well as marine engineering structural components such as large deep-sea drilling platforms and marine power generation facilities. It can significantly improve the operational safety of steel structure engineering, greatly extend service life, and has huge economic and social benefits. It is the best embodiment.

[0044] In addition to the above embodiments, the present invention may have other implementation methods; all technical solutions formed by equivalent substitution or equivalent transformation fall within the protection scope claimed by the present invention.

Claims

1. A high-toughness steel resistant to marine atmospheric corrosion, characterized in that: Its chemical composition and mass percentage are as follows: C: 0.025~0.035%, Si: 1.29~1.51%, P≤0.010%, S≤0.0010%, Cr: 1.6~2.9%, Cu: 0.41~0.65%, Al: 0.15~0.45%, Ti: 0.95~2.25%. The balance is Fe and unavoidable impurities.

2. The high-toughness steel resistant to marine atmospheric corrosion according to claim 1, characterized in that: The mass percentage of the C element is 0.025~0.03%.

3. The high-toughness steel resistant to marine atmospheric corrosion according to claim 1, characterized in that: The mass percentage of Si is 1.29~1.34%.

4. The high-toughness steel resistant to marine atmospheric corrosion according to claim 1, characterized in that: The mass percentage of Cr element is 2.65~2.90%.

5. The high-toughness steel resistant to marine atmospheric corrosion according to claim 1, characterized in that: The mass percentage of Cu is 0.41~0.55%.

6. The high-toughness steel resistant to marine atmospheric corrosion according to claim 1, characterized in that: The mass percentage of Ti is 1.08~2.15%.

7. A method for preparing high-toughness steel resistant to marine atmospheric corrosion according to any one of claims 1 to 6, characterized in that: The process includes the following steps in sequence: smelting, continuous casting, rolling, and cooling; The smelting process includes: The raw materials are added to a converter or electric furnace for melting, oxygen is blown to decarburize, and the carbon content is controlled to be below 0.012% when sampled and analyzed. When the temperature of molten steel is in the range of 1600-1690℃, desulfurization is carried out in an LF ladle furnace, alloying is performed and the content of alloying elements is adjusted within a preset range, then aluminum blocks are added for further deoxidation, and then aluminum wire is fed to alloy aluminum. A vacuum degassing cycle is used to control the vacuum level to ≤5 mbar, and argon gas is blown for stirring.

8. The method for preparing high-toughness steel resistant to marine atmospheric corrosion according to claim 7, characterized in that: The rolling process includes: The billet heating temperature is controlled at 1115~1200℃, and the holding time is 3.1-5.0 hours; The billet is rolled immediately after exiting the furnace, with the initial rolling temperature controlled at 980-1040℃ and the final rolling temperature at 815-860℃.

9. The method for preparing high-toughness steel resistant to marine atmospheric corrosion according to claim 8, characterized in that: The rolling process includes eleven rolling passes, with seven passes completed at 980-1040℃ and four passes completed at 815-860℃.

10. The method for preparing high-toughness steel resistant to marine atmospheric corrosion according to claim 7, characterized in that: The cooling control includes post-rolling rapid cooling and natural cooling; The post-rolling rapid cooling includes: using a blower to blow water mist for cooling after rolling, with the cooling rate controlled at 3.5-4.5℃ / s and the reheating temperature at 455-660℃; The natural cooling process includes: cooling to room temperature in air using the residual heat after rolling.

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