Hot-rolled high-weather-resistance steel plate with extremely low yield ratio and 850MPa-grade tensile strength as well as manufacturing method and application of hot-rolled high-weather-resistance steel plate

Abstract

The invention discloses a hot-rolled high-weather-resistance steel plate with extremely low yield ratio and 850 MPa-grade tensile strength as well as a manufacturing method and application of the hot-rolled high-weather-resistance steel plate. The steel plate comprises chemical components in percentage by weight as follows: 0.06%-0.08% of C, 0.40%-0.60% of Si, 0.40%-0.60% of Mn, smaller than or equal to 0.015% of P, smaller than or equal to 0.005% of S, 2.50%-3.00% of Cr, 0.25%-0.30% of Cu, 0.13%-0.15% of Ti, 0.020%-0.040% of Alt, smaller than or equal to 0.0040% of N, smaller than or equal to 0.0030% of O and the balance of Fe and inevitable impurities. The high Cr + high Ti design is adopted, the tensile strength reaches 850 MPa, the weather resistance index I value is larger than or equal to 8.0, addition of precious metal Ni is omitted, the addition amount of the strong hardenability element Cr in the steel exceeds 2.50%, a fine grain ferrite and bainite double-phase structure is obtained in combination with the controlled rolling and controlled cooling process, meanwhile, the precipitation strengthening and fine grain strengthening effects of Ti are fully exerted, control over high strength and the extremely-low yield ratio smaller than 0.65 is achieved, and the high-strength and low-yield-ratio steel is obtained. The forming performance is excellent, the corrosion rate is smaller than 40% relative to Q345B, and good atmospheric corrosion resistance is achieved.

Description

Technical Field

[0001] The present invention relates to the field of steel materials and their manufacturing, and in particular to a hot-rolled high-weathering steel plate with an extremely low yield ratio and a tensile strength of 850 MPa, and a manufacturing method and application thereof. Background Art

[0002] As the world pays more and more attention to environmental protection and sustainable development, the photovoltaic industry, as an important part of clean energy, has ushered in unprecedented development opportunities. As the key supporting structure of the photovoltaic system, the selection and performance of the materials of the photovoltaic bracket are directly related to the overall stability and service life of the photovoltaic system.

[0003] Traditionally, photovoltaic brackets are mostly made of ordinary weathering steel. However, this type of material is prone to corrosion due to insufficient corrosion resistance in complex and changeable natural environments, resulting in reduced bracket strength, local peeling, and even safety hazards. In order to overcome this problem, high-strength and high-weathering steel has gradually become the new favorite in the manufacture of photovoltaic brackets. The surface of photovoltaic brackets made of high-strength and high-weathering steel can be used directly bare or lightly painted to achieve very high weather resistance, thereby being able to replace some steel surface anti-corrosion processes, such as pre-galvanizing, zinc-aluminum-magnesium, etc., which can not only reduce the energy consumption and pollution problems caused by metal coating processes, but also reduce manufacturing costs and later anti-corrosion maintenance costs, with significant benefits. Photovoltaic brackets not only need to have high strength to resist deformation, but also need to have a low yield strength ratio when manufactured using cold bending technology. On the one hand, it improves the forming performance, and on the other hand, it can absorb vibration energy through plastic deformation when encountering strong earthquakes, thereby improving the service safety of photovoltaic brackets.

[0004] CN101376953A discloses a high-corrosion-resistant and high-strength weathering steel and a manufacturing method thereof, with a C content of 0.002% to 0.005%, a Mn content of 0.01% to 0.05%, and a yield strength of >700MPa. According to the embodiments, its tensile strength is >800MPa, and the yield strength ratio is >0.85. The yield strength ratio is relatively high, which is not conducive to forming and processing by downstream users. CN111349850A discloses a high corrosion-resistant weathering steel and a manufacturing method thereof, wherein the chemical composition mass percentage content (wt%) is: C: 0.03% to 0.05%, Si: 0.03% to 0.05%, Mn: 0.8% to 1.2%, Cu: 0.1% to 0.2%, Cr: 6.8% to 8.2%, Ni: 0.05% to 0.15%, Mo: 0.15% to 0.25%, Al: 0.03% to 0.05%, yield strength ≥ 700MPa, tensile strength ≥ 900MPa, elongation 10 to 14%. On the one hand, the high content of Ni and Cr increases the welding difficulty and manufacturing cost of the material; on the other hand, the precious alloy Mo is contained, which significantly increases the manufacturing cost. Patent Nos. CN 114411041A and CN 113278879B both disclose a method for producing 800 MPa grade high strength weathering steel, but the addition amount of precious metal Ni is 0.05% to 0.10% and 0.12% to 0.30% respectively, and the yield strength ratio is greater than 0.80, resulting in poor formability. Summary of the invention

[0005] In view of the shortcomings of the prior art, the purpose of the present invention is to propose a hot-rolled high-weathering steel plate with extremely low yield ratio and tensile strength of 850MPa, and its manufacturing method and application. The steel plate achieves a perfect combination of high strength, low yield ratio and excellent weather resistance through high Cr+high Ti design and controlled rolling and controlled cooling process, providing an ideal material solution for high-end application fields such as photovoltaic brackets.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] The invention provides a hot-rolled high-weathering steel plate with an extremely low yield ratio and a tensile strength of 850 MPa. The chemical composition of the steel plate is calculated by weight percentage: C: 0.06%-0.08%, Si: 0.40%-0.60%, Mn: 0.40%-0.60%, P: ≤0.015%, S: ≤0.005%, Cr: 2.50%-3.00%, Cu: 0.25%-0.30%, Ti: 0.13%-0.15%, Alt: 0.020%-0.040%, N≤0.0040%, O≤0.0030%, and the balance is Fe and inevitable inclusions.

[0008] As a further optimization solution of the present invention, the thickness of the steel plate ranges from 2.0 mm to 6.0 mm.

[0009] As a further optimization solution of the present invention, the chemical composition of the steel plate satisfies effective Ti=Ti-3.42N-1.5S≥0.11%.

[0010] As a further optimization scheme of the present invention, the metallographic structure of the steel plate is a dual-phase structure of ferrite + bainite, the volume proportion of ferrite is 10-12%, the ferrite grain size is 12-13 levels, and a high-density nano-scale TiC precipitation phase is dispersed in the ferrite.

[0011] As a further optimization scheme of the present invention, the yield strength of the steel plate is ≥500MPa, the tensile strength is ≥850MPa, the elongation A is ≥12%, the yield strength ratio is <0.65, the weathering index I is ≥8.0, and the corrosion rate is <40% relative to Q345B.

[0012] The present invention also provides a method for preparing a hot-rolled high-weathering steel plate with an extremely low yield ratio and a tensile strength of 850 MPa. The preparation method includes: molten iron pretreatment, converter smelting, LF+RH double refining, continuous casting, slab heating, rough rolling, finish rolling, laminar cooling and coiling process.

[0013] As a further optimization scheme of the present invention, the whole process of molten steel casting adopts a long nozzle and uses special protective slag for weathering steel to protect casting, and is sealed with argon gas; the thickness of the continuous casting slab is 230mm, the slab pulling speed is ≤1.5m / min, and the superheat is controlled at 10℃~25℃; slow cooling treatment is adopted in the crystallizer, and weak cooling method is selected for the secondary cooling water. The production process parameters, pulling speed and abnormal liquid level fluctuations are carried out on the surface of the slab offline for flame cleaning.

[0014] As a further optimization scheme of the present invention, after the slab is taken out of the furnace, high-pressure descaling water is used to blow and remove the surface oxide scale; 3+5 passes of reciprocating rough rolling are adopted, and the temperature of the last rough rolling pass is 1050°C to 1080°C.

[0015] As a further optimization scheme of the present invention, the finishing rolling stage adopts 7 stands for continuous rolling in the austenite non-recrystallization zone, the finishing rolling final rolling temperature is 860-900°C, and after the finishing rolling is completed, laminar water cooling and coiling are carried out, the cooling rate is controlled to be greater than 50°C / s, the coiling temperature is controlled to be 590-640°C, and the hot rolled coil is air-cooled to room temperature.

[0016] The present invention also provides an application of a hot-rolled high-weathering steel plate with an extremely low yield ratio and a tensile strength of 850 MPa in the preparation of a photovoltaic bracket.

[0017] The advantages of chemical composition and production process are as follows:

[0018] C (Carbon): As the carbon content increases, the strength of the steel increases. However, the higher the carbon content, the more obvious the peritectic reaction tendency is, and it is easier for air gaps to form between the shell and the wall of the crystallizer. The premature formation of air gaps will lead to uneven shrinkage and uneven shell thickness, and cracks will easily form in weak places, which will easily cause steel leakage accidents and surface quality defects of the ingot. Therefore, the C content is controlled to 0.06% to 0.08%.

[0019] Si (silicon), Mn (manganese): Si and Mn can both improve the strength of steel, and Si can effectively improve the atmospheric corrosion resistance of steel. However, excessive Si and Mn content will significantly reduce the plasticity and toughness of steel, so the Si and Mn contents are controlled to be 0.40% to 0.60%.

[0020] P (Phosphorus): Although P can increase the strength and hardness of steel, it will cause a significant decrease in plastic toughness. Especially at low temperatures, it makes the steel significantly brittle, a phenomenon called "cold brittleness". Cold brittleness makes the cold working and weldability of steel worse. The higher the phosphorus content, the greater the tendency of cold brittleness, so the phosphorus content in steel is strictly controlled. P and Cu are both low melting point elements, which are prone to precipitating brittle phases along grain boundaries and causing surface defects. Therefore, in this application, the P content is strictly controlled to be ≤0.015%.

[0021] Ti (titanium), N (nitrogen), S (sulfur), O (oxygen): S reduces the plastic toughness of steel, easily causes cracks during rolling, is not conducive to welding performance, and reduces corrosion resistance. TiC particles precipitated during layer cooling and coiling after cooling are small and dispersed, and have a strong precipitation strengthening effect, which is also the main reason for the precipitation strengthening of Ti elements. Since the main reason for Ti to exert precipitation strengthening is the large amount of dispersed precipitation of fine TiC, the N and S that will form large particle precipitates are not conducive to the precipitation strengthening of Ti, affecting the effective Ti content in steel. Ti and other alloying elements O, N and S in steel have a strong affinity. The fluctuation of these chemical components during smelting will directly affect the content of effective Ti, resulting in large fluctuations in performance products. Therefore, Ti is controlled to be 0.13% to 0.15%, N≤0.0040%; O≤0.0030%, S:≤0.005%, and Ti-3.42N-1.5S≥0.11%.

[0022] Cr (chromium): Cr is an important alloying element to improve the atmospheric corrosion resistance of steel. It can be enriched on the surface of steel to promote the formation of a dense protective rust layer with good adhesion to the matrix, and prevent the diffusion of corrosive media such as oxygen and water into the steel. Cr can also improve the hardenability of the material and promote the formation of bainite structure. Too high Cr content will deteriorate the welding performance of steel and increase manufacturing costs, so the Cr content is designed to be 2.50-3.00%.

[0023] Cu (copper): The prominent role of Cu in steel is to improve the atmospheric corrosion resistance of ordinary low alloy steel, but the melting point of Cu is only 1083°C. It is easy to precipitate along the grain boundary to form a brittle phase, which has an adverse effect on the high temperature plasticity of the ingot and the toughness and surface quality of the steel plate. The effect is particularly significant in nickel-free steel, so the Cu content is controlled to be 0.25% to 0.30%.

[0024] The molten iron is pretreated with deep desulfurization to [S] ≤ 0.005%, and the refining adopts "LF+RH" double treatment to further reduce the content of non-metallic inclusions and residual N, S and O, improve the cleanliness of the molten steel, and lay the foundation for the effective Ti content of subsequent products ≥ 0.11%.

[0025] The whole process of molten steel casting adopts a long nozzle and uses special protective slag for weathering steel to protect casting, and is sealed with argon. The thickness of the continuous casting slab is 230mm, the slab pulling speed is ≤1.5m / min, and the superheat is controlled at 10℃~25℃. Slow cooling treatment is adopted in the crystallizer, and weak cooling method is selected for the secondary cooling water. The production process parameters, pulling speed and abnormal liquid level fluctuations are carried out on the surface of the slab off the line for flame cleaning. The above measures can effectively suppress the corner cracks of the slab across the wide and narrow surfaces, thereby preventing the occurrence of sawtooth defects on the edge after rolling into hot coils.

[0026] Control the atmosphere in the heating furnace to a reducing atmosphere, with an excess air coefficient of <1.0 and a target of 0.90-0.95, to reduce the formation of an oxide layer on the surface of the slab.

[0027] After the slab is out of the furnace, high-pressure descaling water is used to blow and remove the surface oxide scale. In order to reduce the rolling load of the rolling mill, "3+5" round-trip rough rolling is adopted, and the temperature of the last rough rolling pass is 1050℃~1080℃ to prevent the low slab temperature from causing a sudden increase in rolling force, and at the same time ensure that the temperature of the intermediate billet after rough rolling is outside the sensitive temperature range of thermal cracks. It is forbidden to use edge heaters after rough rolling.

[0028] In the finishing rolling stage, 7 stands are used for continuous rolling in the austenite non-recrystallization zone. The start rolling temperature of the finishing rolling is controlled at 1000℃~1050℃, and the final rolling temperature of the finishing rolling is 860~900℃. After the finishing rolling, laminar ultra-fast water cooling and coiling are carried out. The cooling rate is controlled to be greater than 50℃ / s, and the coiling temperature is controlled at 590~640℃. The hot rolled coil is air-cooled to room temperature. The type and size of fine Ti precipitates are controlled by deformation and temperature to improve the precipitation strengthening effect of two-phase particles. At the same time, segmented cooling is used to control the soft phase structure to be ferrite with lower strength, and the hard phase structure to be bainite with higher strength, so as to obtain a lower yield strength ratio. The steel plate produced by the above method has a metallographic structure of ferrite + bainite, a ferrite grain size of 12~13, and a large number of nano-scale TiC precipitation phases are distributed in the ferrite.

[0029] The hot-rolled high-weathering steel plate with extremely low yield ratio and tensile strength of 850MPa and its manufacturing process proposed in the present invention have significant advantages and beneficial effects:

[0030] (1) The present invention adopts high Cr+high Ti design, the addition amount of Cr, a strong hardenability element in the steel, exceeds 2.50%, and combined with controlled rolling and controlled cooling process, obtains fine-grained ferrite+bainite dual-phase structure, gives full play to the precipitation strengthening and fine grain strengthening effects of Ti, achieves yield strength ≥500MPa, tensile strength ≥850MPa, A ≥12%, yield strength ratio <0.65, achieves high strength and extremely low yield strength ratio control, and has excellent forming performance. This high strength characteristic enables the steel plate to withstand greater loads in applications such as photovoltaic brackets, improves the stability and safety of the overall structure, and enables the photovoltaic bracket to have high strength and resist deformation. At the same time, the photovoltaic bracket can absorb vibration energy through plastic deformation when encountering strong earthquakes. This is because the extremely low yield strength ratio gives the photovoltaic bracket a higher plastic deformation ability, so that it can consume and disperse energy through deformation when subjected to external forces. In addition, the good elongation ensures that the material has good plasticity and toughness when subjected to external forces, avoids the occurrence of brittle fracture, thereby improving the service safety of the photovoltaic bracket, and provides an ideal material solution for high-end application fields such as photovoltaic brackets.

[0031] (2) The present invention achieves a weathering index I value of ≥8.0 through reasonable chemical composition design, especially high Cr content, which significantly improves the atmospheric corrosion resistance of the steel plate. The Cr element can be enriched on the surface of the steel material, promote the formation of a dense and well-adhesive protective rust layer, and effectively prevent the invasion of corrosive media; and the weathering index I of the steel is calculated with reference to the Legault-Leckie formula modified in the American Society for Testing and Materials standard ASTM G101-01, I = 26.01 (% Cu) + 3.88 (% Ni) + 1.20 (% Cr) + 1.49 (% Si) + 17.28 (% P) - 7.29 (% Cu) × (% Ni) - 9.10 (% Ni) × (% P) - 33.39 (% Cu) 2 ≥8.0; The steel plate was subjected to a periodic immersion corrosion test according to TB / T 2375-93 "Test method for periodic immersion corrosion of weathering steel for railway use". The test time was 72h. The corrosion rate was <40% relative to Q345B, which shows good atmospheric corrosion resistance.

[0032] (3) The strict molten iron pretreatment and refining process of the present invention: Through deep desulfurization of molten iron pretreatment and "LF+RH" double refining, the content of non-metallic inclusions and residual N, S, and O is effectively reduced, the cleanliness of molten steel is improved, and the foundation for the subsequent production of high-quality steel plates is laid. Optimized continuous casting and rolling process: The long nozzle protection casting, slow cooling treatment, weak cooling and secondary cooling water and other measures are adopted to effectively suppress the generation of slab cracks and rolling defects. Through high-pressure descaling, reasonable rolling temperature and pass design, as well as ultra-fast cooling and coiling process, ideal microstructure and mechanical properties are obtained. Fine grain strengthening and precipitation strengthening: Through controlled rolling and controlled cooling process, combined with high Cr + high Ti design, the precipitation strengthening and fine grain strengthening effects of Ti are fully exerted, forming a fine-grained ferrite + bainite dual-phase structure, which significantly improves the strength and toughness of the material.

[0033] (4) The present invention controls the chemical composition of the steel plate so that the chemical composition satisfies effective Ti=Ti-3.42N-1.5S≥0.11%, thereby reducing the adverse effects of N and S on Ti precipitation strengthening and enhancing the precipitation strengthening effect of TiC.

[0034] (5) The present invention eliminates the addition of precious metal Ni without sacrificing weather resistance, thereby reducing material costs and improving economic efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 This is the tensile curve of Example 1 of the present invention.

[0036] Figure 2 The microstructure morphology of Example 1 of the present invention under an optical microscope.

[0037] Figure 3 for Figure 2 Scanning electron microscope image of high-density nano-sized TiC precipitation phase (white round particles) dispersed in the ferrite.

[0038] Figure 4 yes Figure 2 EDS energy spectrum of high-density nano-scale TiC precipitation phase dispersed in ferrite. DETAILED DESCRIPTION

[0039] In order to enable those skilled in the art to better understand the technical solution of the present invention, the preferred implementation scheme of the present invention is described below in conjunction with specific embodiments, but it should not be understood as limiting the present invention and is only used as an example.

[0040] The experimental methods or test methods described in the following examples are conventional methods unless otherwise specified; the reagents and materials are obtained from conventional commercial channels or prepared by conventional methods unless otherwise specified.

[0041] The chemical composition weight percentage of the hot-rolled high-weathering steel plate with extremely low yield ratio and tensile strength of 850MPa provided by the present invention is as follows: C: 0.06%-0.08%, Si: 0.40%-0.60%, Mn: 0.40%-0.60%, P: ≤0.015%, S: ≤0.005%, Cr: 2.50%-3.00%, Cu: 0.25%-0.30%, Ti: 0.13%-0.15%, Alt: 0.020%-0.040%, N≤0.0040%, O≤0.0030%, and the balance is Fe and inevitable inclusions. The process flow is: molten iron pretreatment → converter smelting → "LF+RH" double refining → continuous casting → slab heating → rough rolling → finishing rolling → laminar cooling → coiling.

[0042] The chemical composition of each embodiment is shown in Table 1, the main process parameters and mechanical properties of each embodiment are shown in Table 2, and the atmospheric corrosion resistance of each embodiment is shown in Table 3.

[0043] Table 1 Chemical compositions of various embodiments of the present invention

[0044]

[0045] Note: The chemical composition test is carried out according to GB / T 4336 "Spark Source Atomic Emission Spectrometric Analysis Method for Carbon Steel and Medium and Low Alloy Steel (Conventional Method)".

[0046] Table 2 Main process parameters and mechanical properties of rolling process in each embodiment

[0047]

[0048] Note: Mechanical properties test is carried out according to GB / T 228.1-2010 "Tensile test of metallic materials Part 1: Room temperature test method".

[0049] Table 3 Atmospheric corrosion resistance of various embodiments and comparative examples

[0050] Relative Q345B corrosion rate / % Example 1 36 Example 2 37 Example 3 38 Comparative Example 1 55 Comparative Example 2 43 Q345B 100

[0051] Note: A 72h periodic immersion corrosion test was carried out in accordance with TB / T 2375, with Q345B as the comparison sample.

[0052] Tables 1 to 3 in the above test results show that the high weathering resistant steel plate manufactured using the chemical composition and process parameters of the present invention has a yield strength ≥500MPa, a tensile strength ≥850MPa, an elongation A ≥12%, a yield strength ratio <0.65, a weathering index I ≥8.0, and a corrosion rate of <40% relative to Q345B, achieving a perfect combination of high strength, low yield strength ratio and excellent weathering resistance.

[0053] Figure 1The engineering stress-engineering strain curve of Example 1 shows that the gap between the yield point and the tensile strength of the material of the present invention is very large, and the ratio of the yield strength to the tensile strength is extremely low, which means that the material can withstand greater deformation without breaking during the forming process, and has good forming performance. The microstructure of the steel plate of the present invention is a dual-phase structure of ferrite + bainite, in which the volume fraction of the ferrite structure accounts for about 10%, and the ferrite grain size is 12 to 13. Figure 2 The SEM morphology and EDS spectrum of the high-density nano-scale TiC precipitates dispersed in the ferrite are shown in Figure 3 and Figure 4 The present invention adopts high Cr+high Ti chemical composition and controlled rolling and controlled cooling process design, so that the soft phase in the matrix is ​​a ferrite structure with lower strength and the hard phase is a bainite structure with higher strength, which significantly reduces the yield strength ratio of the material.

[0054] In summary, the present invention achieves a perfect combination of high strength, low yield ratio, excellent weather resistance and high economy through unique chemical composition design and advanced manufacturing process, providing an ideal material solution for photovoltaic brackets and other fields.

[0055] The above description only provides a specific exemplary description of the present invention. It should be noted that the specific implementation of the present invention is not limited to the above method. As long as various non-substantial improvements are made using the technical concept and technical solution of the present invention, or the technical concept and technical solution of the present invention are directly applied to other occasions without improvement, they are all within the protection scope of the present invention.

Claims

1. A hot-rolled high-weathering steel plate with extremely low yield ratio and tensile strength of 850MPa, characterized in that: The chemical composition of the steel plate is calculated by weight percentage: C: 0.06% to 0.08%, Si: 0.40% to 0.60%, Mn: 0.40% to 0.60%, P: ≤0.015%, S: ≤0.005%, Cr: 2.50% to 3.00%, Cu: 0.25% to 0.30%, Ti: 0.13% to 0.15%, Alt: 0.020% to 0.040%, N≤0.0040%, O≤0.0030%, and the balance is Fe and unavoidable inclusions.

2. The hot-rolled high weathering steel plate with extremely low yield ratio and tensile strength of 850 MPa according to claim 1, characterized in that: The thickness of the steel plate ranges from 2.0 mm to 6.0 mm.

3. The hot-rolled high weathering steel plate with extremely low yield ratio and tensile strength of 850 MPa according to claim 1, characterized in that: The chemical composition of the steel plate satisfies effective Ti=Ti-3.42N-1.5S≥0.11%.

4. The hot-rolled high weathering steel plate with extremely low yield ratio and tensile strength of 850 MPa according to claim 1, characterized in that: The metallographic structure of the steel plate is a dual-phase structure of ferrite and bainite, the volume proportion of ferrite is 10-12%, the ferrite grain size is 12-13 levels, and a high-density nano-scale TiC precipitation phase is dispersed in the ferrite.

5. The hot-rolled high weathering steel plate with extremely low yield ratio and tensile strength of 850 MPa according to claim 1, characterized in that: The steel plate has a yield strength of ≥500 MPa, a tensile strength of ≥850 MPa, an elongation A of ≥12%, a yield strength ratio of <0.65, a weathering index I of ≥8.0, and a corrosion rate of <40% relative to Q345B.

6. A method for preparing a hot-rolled high-weathering steel plate with an extremely low yield ratio and a tensile strength of 850 MPa as claimed in any one of claims 1 to 5, characterized in that: The preparation method comprises: molten iron pretreatment, converter smelting, LF+RH double refining, continuous casting, slab heating, rough rolling, finish rolling, laminar cooling and coiling process.

7. The method for preparing the hot-rolled high weathering steel plate with extremely low yield ratio and tensile strength of 850 MPa according to claim 6, characterized in that: The entire process of molten steel casting adopts a long nozzle and uses special protective slag for weathering steel to protect casting, and is sealed with argon gas; the thickness of the continuous casting slab is 230mm, the slab pulling speed is ≤1.5m / min, and the superheat is controlled at 10℃~25℃; slow cooling treatment is adopted in the crystallizer, and weak cooling method is selected for the secondary cooling water. The production process parameters, pulling speed and abnormal liquid level fluctuations will cause the slab to be taken off the line for surface flame cleaning.

8. The method for preparing the hot-rolled high weathering steel plate with extremely low yield ratio and tensile strength of 850 MPa according to claim 6, characterized in that: After the slab is taken out of the furnace, high-pressure descaling water is used to blow away the surface oxide scale; 3+5 passes of reciprocating rough rolling are used, and the temperature of the last rough rolling pass is 1050℃~1080℃.

9. The method for preparing the hot-rolled high weathering steel plate with extremely low yield ratio and tensile strength of 850 MPa according to claim 6, characterized in that: In the finishing rolling stage, 7 stands are used for continuous rolling in the austenite non-recrystallization zone. The final rolling temperature is 860-900°C. After finishing rolling, laminar water cooling and coiling are carried out. The cooling rate is controlled at >50°C / s, the coiling temperature is controlled at 590-640°C, and the hot rolled coil is air-cooled to room temperature.

10. Use of the hot-rolled high-weathering steel plate with extremely low yield ratio and tensile strength of 850 MPa as claimed in any one of claims 1 to 5 in the preparation of photovoltaic brackets.

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