Steel coil for photovoltaic bracket and preparation method of steel coil

Abstract

The invention relates to a steel coil for a photovoltaic support and a preparation method of the steel coil. The preparation method comprises the steps that a continuous casting blank is obtained; a steel coil is obtained through the continuous casting billet; the continuous casting billet is prepared from the following components in percentage by mass: 0.14 percent to 0.16 percent of C, 0.96 percent to 1.10 percent of Si, 0.21 percent to 0.30 percent of Mn, 0.041 percent to 0.055 percent of P, 0.0031 percent to 0.0040 percent of S, 0.13 percent to 0.18 percent of Cu, 3.4 percent to 3.6 percent of Cr, 0.13 percent to 0.16 percent of Alt and the balance of Fe and inevitable impurities. The prepared steel coil can better meet the development requirements of photovoltaic supports.

Description

Technical Field

[0001] The invention belongs to the technical field of hot-rolled steel strip manufacturing, and specifically relates to a steel coil for a photovoltaic support and a preparation method thereof. Background Art

[0002] In recent years, under the guidance of the dual carbon and clean energy transformation goals, the development of renewable energy has been unstoppable. As a renewable energy source, solar energy has significant advantages such as cleanliness, safety, inexhaustible and inexhaustible. Photovoltaic power generation is a form of utilization of solar energy. Photovoltaic power generation is the fastest-growing power generation technology, and its incremental power generation accounts for a steadily increasing proportion of the global total power generation.

[0003] Photovoltaic brackets are special equipment installed in photovoltaic power stations to support, fix and rotate photovoltaic modules. They are the link with the most consumables in the photovoltaic industry chain. On the one hand, photovoltaic power stations are developing in a high-power trend, and the weight of modules is increasing, which leads to an increase in the load of photovoltaic brackets. On the other hand, photovoltaic power generation is mostly built in deserts, Gobi, wastelands, saline-alkali land and other harsh and changeable areas. These two aspects require photovoltaic brackets to have high bearing capacity, corrosion resistance and stability.

[0004] The raw materials of photovoltaic brackets include aluminum alloy, stainless steel, hot-dip galvanized or zinc-aluminum-magnesium plain carbon steel. Aluminum alloy and stainless steel have excellent corrosion resistance, but their alloy costs are high, their strength is low, and they are difficult to promote. At present, the raw materials used for photovoltaic brackets are mainly based on plain carbon steel, which is hot-dip galvanized or zinc-aluminum-magnesium to improve corrosion resistance; but hot-dip galvanizing or zinc-aluminum-magnesium will pollute the environment, increase production processes, extend the process flow, and increase manufacturing costs; and the coating is easily damaged during transportation, processing, and installation, causing corrosion to the substrate, and post-maintenance must be performed to avoid premature failure. Therefore, the above three materials cannot meet the development requirements of high-strength, lightweight, green, and low-cost photovoltaic brackets.

[0005] Photovoltaic brackets made of weathering steel can form a dense protective rust layer on their surface during service, which can greatly improve their corrosion resistance. The rust layer can heal itself after damage, and there is no need for subsequent pickling, painting and maintenance. Therefore, photovoltaic brackets made of weathering steel have the advantages of energy saving and environmental protection. In order to adapt to the development trend of photovoltaic power stations, weathering steel for photovoltaic brackets should have the advantages of high strength, light weight, high corrosion resistance, long life, easy manufacturing and low cost. Existing weathering steel products cannot fully meet these requirements.

[0006] The prior art discloses a high-toughness 800MPa grade photovoltaic support steel, whose chemical composition includes C: ≤0.08%, Si: 0.20%-0.40%, Mn: 0.50%-0.60%, P: ≤0.02%, S: ≤0.01%, Cu: 0.25%-0.30%, Cr: 2.5%-3.0%, Ti: 0.10%-0.15%, Als: 0.02%-0.05%, N: ≤0.0060%; and the thickness of the steel is 1.5-6.0mm, I index is 8.07-8.96, yield strength is 735-821MPa, tensile strength is 849-920MPa, and elongation after fracture is 18-24%. However, the precious alloy element Cu content in the above photovoltaic support steel is relatively high, the product cost is high, and network crack defects are easily generated during hot rolling. The high Ti content will cause the coarse TiN precipitates to become stress concentration points and microcrack sources, which will reduce the forming and fatigue performance of the photovoltaic bracket. At the same time, the recovery rate of Ti is unstable and the mechanical properties fluctuate greatly, which will increase the difficulty of photovoltaic bracket processing and reduce processing efficiency. At the same time, the above-mentioned photovoltaic bracket steel lacks data on 5-year corrosion resistance, making it difficult for users to select materials.

[0007] The prior art discloses a low-cost 800MPa-level antimony-containing weathering steel for a photovoltaic bracket, the chemical composition of which includes: C: ≤0.08%, Si: 0.35%-0.50%, Mn: 0.40%-0.60%, P: 0.08%-0.12%, S: ≤0.010%, Cu: 0.25%-0.40%, Cr: 0.75%-1.00%, Ti: 0.120%-0.170%, N: ≤0.0040%, Sb: 0.05%-0.07%; and the steel has a thickness of 1.5-4.0mm, an I index of 7.61-9.02, a tensile strength of 850-875MPa, and an elongation after fracture of 23.5-24.5%. However, the P content in the above photovoltaic bracket steel is relatively high, which causes serious segregation of the ingot, which will correspondingly reduce the plasticity, low temperature toughness and welding performance of the steel, and easily cause hydrogen-induced cracks, stress corrosion cracks and surface cracks, aggravating intergranular corrosion; at the same time, the content of precious alloy element Cu is relatively high, the product cost is high and it is easy to produce network crack defects during hot rolling; the Ti content is relatively high, and the coarse TiN precipitates formed become stress concentration points and microcrack sources, which will correspondingly reduce the forming and fatigue performance of the photovoltaic bracket, the recovery rate of Ti is unstable, and the mechanical properties fluctuate greatly, which will correspondingly increase the difficulty of photovoltaic bracket processing and reduce processing efficiency. In addition, Sb in the above photovoltaic bracket steel is one of the five harmful elements, which is easy to cause central segregation of the ingot, it is easy to segregate at the grain boundary, weaken the grain boundary, increase the tendency of brittle fracture along the crystal, and correspondingly significantly reduce the impact toughness and forming performance of the steel. It is easy to precipitate and enrich between the steel matrix and the oxide layer, increase the tendency of hot brittleness, and correspondingly deteriorate the surface quality of the steel strip. At the same time, the above photovoltaic bracket steel lacks data such as 25-year corrosion resistance, and it is difficult for users to select materials.

[0008] The prior art also discloses an 800MPa grade high weathering steel for photovoltaic brackets, whose chemical composition includes: C: ≤0.08%, Si: 0.20%~0.30%, Mn: 0.40%~0.60%, P: ≤0.02%, S: ≤0.010%, Cu: 0.25%~0.40%, Cr: 2.50%~3.00%, Ni: 0.20%~0.40%, Nb: 0.015%~0.035%, Ti: 0.100%~0.150%, N: ≤0.0040%; and the steel has a thickness of 1.5mm~4.0mm, an I index of 8.3-9.79, a tensile strength of 855-895MPa, and an elongation after fracture of 23-25.5%. However, the precious alloy element Cu content in the above photovoltaic bracket steel is relatively high, the product cost is high and it is easy to produce network crack defects during hot rolling. At the same time, the addition of precious alloy elements such as Ni and Nb increases the alloy cost. In addition, the high Ti content makes the coarse TiN precipitates formed easily become stress concentration points and microcrack sources, reducing the forming and fatigue performance of the photovoltaic bracket. The recovery rate of Ti is unstable and the mechanical properties fluctuate greatly, which will correspondingly increase the difficulty of photovoltaic bracket processing and reduce processing efficiency. At the same time, the above photovoltaic bracket steel lacks data such as 25-year corrosion resistance, making it difficult for users to select materials. Summary of the invention

[0009] In order to solve all or part of the above problems, the purpose of the present invention is to provide a steel coil for photovoltaic brackets and a preparation method thereof. The difficulty of producing hot-rolled steel coils using the preparation method of the present invention is small; the obtained steel coil has a long service life, high strength and excellent corrosion resistance.

[0010] According to one aspect of the present invention, there is provided a method for preparing a steel coil for a photovoltaic support, comprising:

[0011] Obtaining a continuous casting billet through pretreatment;

[0012] Heating and heat-insulating the continuous casting billet;

[0013] Rough rolling the continuous casting billet using a rough rolling mill to obtain a strip billet;

[0014] Using a finishing mill to finish-roll the strip to obtain a hot-rolled steel strip; and

[0015] Cooling the hot-rolled steel strip, and coiling the cooled hot-rolled steel strip into a steel coil;

[0016] Wherein, the continuous casting billet comprises the following components by mass percentage:

[0017] C: 0.14%-0.16%, Si: 0.96%-1.10%, Mn: 0.21%-0.30%, P: 0.041%-0.055%, S: 0.0031%-0.0040%, Cu: 0.13%-0.18%, Cr: 3.4%-3.6%, Alt: 0.13%-0.16%, and the rest are Fe and unavoidable impurities.

[0018] Furthermore, the continuous casting billet also needs to satisfy C / Cu≤1.2.

[0019] Furthermore, the obtaining of the continuous casting billet by pretreatment specifically comprises: obtaining the continuous casting billet by pretreatment of molten iron for desulfurization, top and bottom composite blowing in converter smelting, LF refining and slab continuous casting;

[0020] Furthermore, the heating and heat preservation treatment of the continuous casting billet further comprises: loading the continuous casting billet into a walking beam heating furnace; and heating and heat preservation treatment of the continuous casting billet in the walking beam heating furnace;

[0021] The furnace discharge temperature of the continuous casting billet is 1170-1190° C., and the cumulative furnace residence time of the continuous casting billet is 180-210 minutes.

[0022] Further, the step of rough rolling the continuous casting billet with a rough rolling mill to obtain a strip billet is as follows: the starting rolling temperature of the rough rolling is controlled to be 1120-1140° C., the final rolling temperature of the rough rolling is controlled to be 1050-1090° C., and the continuous casting billet is rolled with a rough rolling mill to obtain a strip billet;

[0023] Among them, the thickness of the strip obtained by rough rolling is 28-40mm, the rough rolling passes are 7 passes, and the process parameters of each pass are: the rolling speed of the 1st pass is 1.8-2.2m / s, the rolling speed of the 2nd pass is 2.4-2.8m / s, the rolling speed of the 3rd pass is 3.3-3.7m / s, the rolling speed of the 4th pass is 3.8-4.2m / s, the rolling speed of the 5th pass is 3.8-4.2m / s, the rolling speed of the 6th pass is 4.8-5.2m / s, and the rolling speed of the 7th pass is 5.3-5.7m / s.

[0024] Further, the use of a finishing mill to perform finish rolling on the strip to obtain a hot-rolled steel strip is specifically as follows: controlling the start rolling temperature of the finishing rolling to be 990-1040° C., the final rolling temperature of the finishing rolling to be 840-880° C., and using a finishing mill to perform finish rolling on the strip to obtain a hot-rolled steel strip;

[0025] Among them, the thickness of the hot-rolled steel strip obtained by finish rolling is 1.5-4.0 mm. The finish rolling adopts a 7-stand finishing rolling unit. The process parameters of each stand are: the rolling speed of the first stand is 0.96-1.26 m / s, the rolling speed of the second stand is 1.63-2.34 m / s, the rolling speed of the third stand is 2.5-3.82 m / s, the rolling speed of the fourth stand is 3.41-5.5 m / s, the rolling speed of the fifth stand is 4.26-7.37 m / s, the rolling speed of the sixth stand is 5.27-9.07 m / s, and the rolling speed of the seventh stand is 6.22-10.37 m / s.

[0026] Further, the hot-rolled steel strip is cooled and the cooled hot-rolled steel strip is coiled into a steel coil, specifically:

[0027] Cooling the hot-rolled steel strip transported on the laminar cooling roller, wherein the cooling method of the hot-rolled steel strip on the laminar cooling roller is rear-stage cooling, and the flow rate ratio of the upper and lower header cooling water during the cooling process is 8:12; and controlling the coiling temperature to be 530-570°C, and coiling the cooled hot-rolled steel strip into a steel coil by a coiler;

[0028] After cooling the hot-rolled steel strip and coiling the cooled hot-rolled steel strip into a steel coil, the method further comprises:

[0029] The steel coil unloaded from the coiler is hoisted into the slow cooling pit for slow cooling within 15 minutes. The temperature of the steel coil entering the slow cooling pit is ≥480℃; and the steel coil is hoisted out of the slow cooling pit after slow cooling for 60-66 hours.

[0030] The present invention also provides a steel coil for a photovoltaic support, which is prepared by any one of the preparation methods described above.

[0031] Furthermore, the yield strength is ≥690MPa, the tensile strength is ≥800MPa, the elongation after fracture is ≥18%, and the 180°d=a cold bending test is qualified;

[0032] The corrosion rate of the steel coil after 72 hours of periodic immersion in a mixed solution of NaHSO3 with an initial concentration of 0.01 mol / L and 3.5% NaCl is ≤1.8 g / (m 2 h); the uniform corrosion amount of a single side of the steel coil after 25 years of uncoated service under C1, C2, C3 and C4 environments is ≤4μm, ≤12μm, ≤26μm and ≤45μm respectively.

[0033] It can be seen from the above technical solution that the photovoltaic support steel coil and the preparation method thereof provided by the present invention have the following beneficial effects:

[0034] The steel coil prepared by the preparation method of the present invention can better meet the new requirements for the development of photovoltaic brackets such as high strength, light weight, high corrosion resistance, long life, easy manufacturing, low cost, high efficiency, painting-free, maintenance-free, energy-saving, and environmental protection. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The accompanying drawings are incorporated in and constitute a part of this specification and, together with the description, serve to explain the principles of the present invention.

[0036] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0037] Figure 1 The present invention is a flowchart of a preparation method according to an embodiment of the present invention. DETAILED DESCRIPTION

[0038] In order to fully understand the purpose, features and effects of the present invention, the present invention is described in detail through the following specific embodiments. Except for the following contents, the process method of the present invention adopts conventional methods or devices in the art. Unless otherwise specified, the following terms have the meanings commonly understood by those skilled in the art.

[0039] When a numerical range is disclosed herein, the above range is considered to be continuous and includes the minimum and maximum values ​​of the range, as well as every value between such minimum and maximum values. Further, when a range refers to an integer, every integer between the minimum and maximum values ​​of the range is included. In addition, when multiple ranges are provided to describe a feature or characteristic, the ranges can be combined. In other words, unless otherwise indicated, all ranges disclosed herein should be understood to include any and all subranges included therein.

[0040] A method for preparing a steel coil for a photovoltaic support according to an embodiment of the present invention, such as Figure 1 As shown, including:

[0041] Step S001: obtaining a continuous casting billet through pretreatment;

[0042] Step S002: heating and heat-insulating the continuous casting billet;

[0043] Step S003: rough rolling the continuous casting billet using a rough rolling mill to obtain a strip billet;

[0044] Step S004: using a finishing mill to finish-roll the strip to obtain a hot-rolled steel strip; and

[0045] Step S005: cooling the hot-rolled steel strip, and coiling the cooled hot-rolled steel strip into a steel coil;

[0046] Wherein, the continuous casting billet in step S001 includes the following components by mass percentage:

[0047] C: 0.14%-0.16%, Si: 0.96%-1.10%, Mn: 0.21%-0.30%, P: 0.041%-0.055%, S: 0.0031%-0.0040%, Cu: 0.13%-0.18%, Cr: 3.4%-3.6%, Alt: 0.13%-0.16%, and the rest are Fe and unavoidable impurities.

[0048] The steel coil prepared by the preparation method of the embodiment of the present invention has a long service life when used as a photovoltaic support and meets the requirement of 25 years of service in a C1-C4 environment.

[0049] The reasons for limiting the chemical elements in the embodiments of the present invention are as follows:

[0050] It has been found in practice that an effective and economical way to improve the strength of steel is to increase the C content. However, as the C content increases, the corrosion resistance, plasticity, welding performance and forming performance of the steel will decrease. In order to make the steel strip of the embodiment of the present invention have a long life, and comprehensively consider the strength, welding performance and processing performance, while taking into account the economy, the C content of the embodiment of the present invention is controlled within the range of 0.14%-0.16%.

[0051] Si is not a precious alloy element. It has been found in practice that its solid solution strengthening coefficient in ferrite is higher than that of Mn. Si is very effective in improving strength and corrosion resistance. However, too high Si content will reduce material toughness and is not conducive to improving welding performance. In order to make the steel strip of the embodiment of the present invention have a long life, and comprehensively consider the impact toughness and welding performance, while taking into account the economy, the embodiment of the present invention controls the Si content in the range of 0.96%-1.10%.

[0052] It has been found in practice that Mn can form substitutional solid solution in steel, which plays a role in solid solution strengthening and increases the strength linearly. Mn is an austenite-forming element, which has the function of stabilizing austenite, can reduce the austenite transformation temperature (Ar3), increase the ferrite nucleation rate, and reduce the grain growth rate, that is, it has the function of refining grains. However, an increase in the Mn content can increase the C equivalent (Ceq) of the steel, which is not good for welding performance. In order to make the steel strip of the embodiment of the present invention have high strength, and comprehensively consider the welding performance and economy, the embodiment of the present invention controls the Mn content in the range of 0.21%-0.30%.

[0053] It has been found in practice that the solid solution strengthening effect of P in steel is second only to C, and it can effectively improve the atmospheric corrosion resistance of steel. Among the many alloy elements that improve corrosion resistance, P is the most cost-effective element. However, P is easy to form and precipitate Fe3P in steel, which increases the brittleness of steel and is not conducive to improving the welding performance of steel. The diffusion rate of P in γ iron and α iron is low, and it is easy to form segregation, which has an adverse effect on the forming performance of steel. In order to make the steel strip of the embodiment of the present invention have a long life, and comprehensively consider the strength, processing performance and welding performance, while taking into account economy, the embodiment of the present invention controls the P content in the range of 0.041%-0.055%.

[0054] It has been found in practice that S is harmful to the corrosion resistance of steel, and the sulfides in steel can become the source of rust. A higher S content can cause "hot brittleness" defects in steel. Adding Mn and S to the steel to form MnS plastic inclusions can reduce the harmful effects of S. However, during the rolling process, the MnS extending along the rolling direction tends to form a banded structure in the steel strip, which correspondingly reduces the transverse impact toughness and forming properties of the steel strip, and leads to anisotropy of mechanical properties. Therefore, the S content should be reduced as much as possible, but if the S content is controlled too low, the manufacturing cost will increase and the production efficiency will be reduced. Taking into account the corrosion resistance, uniformity of mechanical properties, and processing performance of the steel strip in the embodiment of the present invention, while taking into account economy and production efficiency, the S content in the embodiment of the present invention is controlled within the range of 0.0031%-0.0040%.

[0055] It has been found in practice that Cu forms precipitates on the surface of steel and acts as a cathode to passivate the surface of the steel and accumulate in the rust layer, changing the structure of the rust layer and preventing Cl - Invades the matrix, so Cu is an effective element to improve the corrosion resistance of steel. However, Cu has a low melting point (1083°C), and is easily segregated at the grain boundary during the heating process of the continuous casting billet. Too high a Cu content will cause a network crack defect during hot rolling. Cu is also a relatively precious alloying element, and too high a Cu content will lead to an increase in product cost. Taking into account the corrosion resistance, hot working performance and surface quality of the steel strip in the embodiment of the present invention, while taking into account the economy, the embodiment of the present invention controls the Cu content in the range of 0.13%-0.18%.

[0056] It has been found in practice that Cr has a solid solution strengthening effect, which can improve the hardenability and strength of steel, form a dense and uniform rust layer containing fine α-FeOOH on the surface of the substrate, accelerate the development of electrochemical corrosion products to a thermodynamically stable state, block the further penetration of the corrosive medium into the substrate, and reduce the corrosion rate of steel. However, when the Cr content is too high, carbides will precipitate at the grain boundaries, reduce the plasticity and toughness of the steel, and correspondingly deteriorate the forming and welding properties, and increase the alloy cost. In order to make the steel strip of the embodiment of the present invention have a long life, and comprehensively consider the plasticity and toughness, processing properties, and welding properties, while taking into account economy, the embodiment of the present invention controls the Cr content in the range of 3.4%-3.6%.

[0057] It has been found in practice that Al has great potential in improving the corrosion resistance of steel and reducing costs. Under marine atmospheric conditions, Al and Si alloyed steel have a synergistic effect in improving the corrosion resistance of steel. The generated FeAl2O4 fine-grained spinel oxide can enhance the barrier effect of the rust layer on corrosive media and moisture. The fine AlN formed by Al and N can inhibit the growth of austenite grains during the heating process of the continuous casting billet, has the effect of refining grains, and can improve the strength, toughness and welding performance of steel. However, too high Al content will lead to more Al2O3 inclusions, which will reduce the purity of the steel and the impact performance and surface quality of the steel strip on the one hand, and on the other hand, it is very easy to cause nodules and blockages at the nozzle during continuous casting, increase the difficulty of production, reduce production efficiency, and increase the alloy cost and manufacturing cost. Taking into account the corrosion resistance, toughness, welding performance, processing performance, impact performance and surface quality of the long-life steel strip in the embodiment of the present invention, and taking into account the difficulty of production, production efficiency and economy, the embodiment of the present invention controls the Alt content in the range of 0.13%-0.16%.

[0058] The continuous casting in step S001 also needs to satisfy C / Cu≤1.2.

[0059] It has been found in practice that increasing the C content is an effective and economical method to increase the strength of steel. However, as the C / Cu ratio increases, the solubility of Cu in austenite decreases, the segregation of Cu at the grain boundary increases, and the tendency to generate network crack defects during hot rolling increases. Taking into account the strength, processing performance and surface quality of the long-life steel strip in the embodiment of the present invention, the embodiment of the present invention controls C / Cu≤1.2.

[0060] In the embodiment of the present invention, the selection of the above-mentioned C, Si, Mn, P, and Cr element contents has a synergistic effect. By controlling the content of each element within the above-mentioned range, the solid solution strengthening, fine grain strengthening, and phase transformation strengthening effects can be fully exerted, so that the long-life low-alloy corrosion-resistant steel for the photovoltaic bracket has high strength and excellent cold working properties.

[0061] In the embodiment of the present invention, the selection of the above-mentioned Si, P, S, Cu, Cr, and Alt element contents has a synergistic effect. By controlling the content of each element within the above-mentioned range, a dense inner rust layer can be formed on the surface of the steel matrix, so that the low-alloy corrosion-resistant steel for the photovoltaic bracket has a long life and high corrosion resistance, and can meet the service requirement of 25 years in a C1-C4 environment. There is no need for subsequent pickling, painting and maintenance, which is energy-saving and environmentally friendly.

[0062] In the embodiment of the present invention, the above-mentioned C, Mn, S, Cu, Alt element contents and the selection of C / Cu have a synergistic effect. By controlling the content of each element and C / Cu within the above-mentioned range, defects such as hot-rolled network cracks can be avoided, so that the long-life low-alloy corrosion-resistant steel for the photovoltaic bracket has excellent hot processing performance and high surface quality, which can meet the stringent processing and service requirements of the photovoltaic bracket.

[0063] In the embodiment of the present invention, the selection of all the above-mentioned element contents and C / Cu has a synergistic effect. By controlling all the element contents and C / Cu within the above-mentioned range, the long-life low-alloy corrosion-resistant steel used for the photovoltaic bracket has low processing difficulty, high production efficiency, low manufacturing cost and short delivery cycle.

[0064] Among them, step S001 of obtaining the continuous casting billet through pretreatment specifically comprises: obtaining the continuous casting billet through molten iron pretreatment desulfurization, top and bottom composite blowing converter smelting, LF refining and slab continuous casting.

[0065] Wherein, step S002 of heating and heat-insulating the continuous casting billet further comprises: loading the continuous casting billet into a walking beam heating furnace; and heating and heat-insulating the continuous casting billet in the walking beam heating furnace;

[0066] The furnace discharge temperature of the continuous casting billet is 1170-1190° C., and the cumulative furnace residence time of the continuous casting billet is 180-210 minutes.

[0067] It has been found in practice that if the furnace temperature of the continuous casting billet is lower than 1170°C, the austenite grains can be refined, but the thermoplasticity of the continuous casting billet is poor, the deformation resistance is high, and the thermal deformation is difficult, which will lead to a low subsequent rolling mill operating rate; if the furnace temperature of the continuous casting billet is higher than 1190°C, although the thermoplasticity of the continuous casting billet is improved, the deformation resistance is reduced, and it is easy to be thermally deformed, the corresponding subsequent rolling mill operating rate will be improved, but the austenite grains are easy to coarsen. Taking the above effects into consideration, the furnace temperature of the continuous casting billet in the embodiment of the present invention is controlled to be 1170-1190°C.

[0068] Among them, step S003 uses a rough rolling mill to rough roll the continuous casting billet to obtain a strip billet. Specifically, the starting rolling temperature of the rough rolling is controlled to be 1120-1140°C, the final rolling temperature of the rough rolling is controlled to be 1050-1090°C, and the continuous casting billet is rolled by the rough rolling mill to obtain a strip billet.

[0069] It has been found in practice that the main metallurgical purpose of rough rolling is to destroy the as-cast structure of the continuous casting billet, effectively refine the austenite grains through multiple recrystallizations, and obtain an austenite structure with uniform and fine grains. If the rough rolling start temperature is too high, uneven austenite grains will occur, resulting in differences in the strip structure and affecting the uniformity of the mechanical properties of the finished product. If the rough rolling start temperature is too low, the strip shape control becomes more difficult, the mill load and rolling energy consumption increase, and it is not conducive to smooth rolling. Taking the above influences into consideration, the embodiment of the present invention controls the rough rolling start temperature to 1120-1140°C, and the rough rolling final rolling temperature to 1050-1090°C.

[0070] The thickness of the strip obtained by rough rolling is 28-40 mm, the rough rolling is performed in 7 passes, and the rolling speed of each pass is: 1.8-2.2 m / s for the first pass, 2.4-2.8 m / s for the second pass, 3.3-3.7 m / s for the third pass, 3.8-4.2 m / s for the fourth pass, 3.8-4.2 m / s for the fifth pass, 4.8-5.2 m / s for the sixth pass, and 5.3-5.7 m / s for the seventh pass. Specifically, the rolling speed of each pass is shown in Table 1.

[0071] Table 1 Rolling speed table in the rough rolling process of a photovoltaic support steel coil

[0072]

[0073] It has been found in practice that if the strip is too thick, the cumulative reduction rate and pass reduction rate of rough rolling will be reduced, partial recrystallization is prone to occur, and mixed crystal phenomenon will occur, which will have an adverse effect on the uniformity of the mechanical properties of the finished steel strip. At the same time, it will increase the rolling load and rolling energy consumption of the finishing mill, affecting the stability of finishing rolling. If the strip is too thin, it will not be conducive to improving the quality of the strip shape, and will increase the rolling load and rolling energy consumption of the roughing mill, which is not conducive to the smooth rolling. At the same time, it is impossible to ensure the finishing rolling start temperature required for rolling the 1.5-4.0mm thin finished steel strip of the present invention, and it may even increase the rolling load and rolling energy consumption of the finishing mill. Taking the above influences into consideration, the embodiment of the present invention controls the strip thickness to be 28-40mm. According to the roughing mill force and energy parameters, the continuous casting billet and the strip thickness, it can be calculated that the reasonable rolling pass of the roughing mill is 7 passes.

[0074] If the rolling speed of each pass of the rough rolling mill is lower than the lower limit specified in Table 1, the rolling efficiency of the rough rolling mill will decrease and the output will be reduced; if the rolling speed of each pass is higher than the upper limit specified in Table 1, the deformation resistance and rolling load of the rough rolling mill will increase. Taking the above factors into consideration, the rolling speeds of the 7 rough rolling passes are determined to be 1.8-2.2m / s, 2.4-2.8m / s, 3.3-3.7m / s, 3.8-4.2m / s, 3.8-4.2m / s, 4.8-5.2m / s, and 5.3-5.7m / s.

[0075] Among them, step S004 uses a finishing mill to finish rolling the strip to obtain a hot-rolled steel strip. Specifically, the starting rolling temperature of the finishing rolling is controlled to be 990-1040°C, the final rolling temperature of the finishing rolling is controlled to be 840-880°C, and the finishing mill is used to finish rolling the strip to obtain a hot-rolled steel strip.

[0076] It has been found in practice that if the finishing rolling start temperature is too high, partial recrystallization will easily occur in the first stand (F1) and the second stand (F2), resulting in mixed crystal problems and differences in the finished steel strip structure and abnormal fluctuations in mechanical properties; if the finishing rolling start temperature is too low, the desired final rolling temperature cannot be guaranteed. Taking the above factors into consideration, the finishing rolling start temperature is controlled to be 990-1040°C in the embodiment of the present invention.

[0077] It has been found in practice that if the finishing rolling temperature is too high, the finished steel strip will be coarsened and the toughness will be reduced; if the finishing rolling temperature is too low, the strength of the finished steel strip will be increased and the plasticity will be reduced, and the strength-plasticity matching will be unreasonable. At the same time, the rolling stability of the finishing rolling unit will be reduced, and the difficulty of controlling the thickness accuracy, width accuracy and cross-sectional shape of the steel strip will increase. In serious cases, the scrap steel strip will be rolled and the equipment will be damaged. Taking the above factors into consideration, the finishing rolling temperature of the embodiment of the present invention is controlled to be 840-880℃.

[0078] The thickness of the hot-rolled steel strip obtained by the finishing rolling is 1.5-4.0 mm, and the finishing rolling is carried out by a 7-stand finishing rolling unit, and the rolling speed of each stand is: 0.96-1.26 m / s for the first stand, 1.63-2.34 m / s for the second stand, 2.5-3.82 m / s for the third stand, 3.41-5.5 m / s for the fourth stand, 4.26-7.37 m / s for the fifth stand, 5.27-9.07 m / s for the sixth stand, and 6.22-10.37 m / s for the seventh stand. The rolling speeds of each stand are shown in Table 2.

[0079] Table 2 Rolling speed table of each stand in the finishing rolling process of a steel coil for photovoltaic support

[0080]

[0081] Among them, the finishing rolling adopts 7-stand finishing rolling unit for rolling, which depends on the actual equipment configuration, and the thickness of the finished steel strip is 1.5-4.0mm, which is selected by the user.

[0082] It is found in practice that if the rolling speed of each stand of the finishing mill is lower than the lower limit specified in Table 2, the rolling efficiency will decrease and the output will be reduced; if the rolling speed is higher than the upper limit specified in Table 2, the deformation resistance and rolling load of each stand of the finishing mill will increase. Taking the above factors into consideration, the rolling speeds of each stand of the finishing mill are determined to be 0.96-1.26m / s, 1.63-2.34m / s, 2.5-3.82m / s, 3.41-5.5m / s, 4.26-7.37m / s, 5.27-9.07m / s, and 6.22-10.37m / s.

[0083] Among them, step S005 cools the hot-rolled steel strip and coils the cooled hot-rolled steel strip into a steel coil, specifically: cooling the hot-rolled steel strip transported on the laminar cooling roller, wherein the cooling method of the hot-rolled steel strip on the laminar cooling roller is rear-stage cooling, and the upper and lower header cooling water flow ratio during the cooling process is 8:12; and controlling the coiling temperature to 530-570°C, and using a coiler to coil the cooled hot-rolled steel strip into a steel coil.

[0084] It has been found in practice that compared with other cooling methods, the rear cooling method is conducive to the full precipitation of ferrite, uniform ferrite grain size, reduced solid solution carbon content in ferrite crystals, and increased movable dislocation density, thereby making the strength, toughness, plasticity and cold forming properties of the finished steel strip well matched.

[0085] In addition, the use of the back-end cooling method can make the 1.5-4.0 mm thin finished steel strip of the embodiment of the present invention run smoothly on the laminar cooling roller and be coiled smoothly, avoiding rolling accidents such as coiling and steel piling. Taking the above factors into consideration, the cooling method of the hot-rolled steel strip on the laminar cooling roller in the embodiment of the present invention is back-end cooling.

[0086] It has been found in practice that if the cooling water flow ratio of the upper and lower headers is higher or lower than 8:12, the cooling rates of the upper and lower surfaces of the steel strip will be greatly different, resulting in fluctuations in the microstructure and mechanical properties, and increasing the difficulty of plate shape control. Taking the above factors into consideration, the cooling water flow ratio of the upper and lower headers in the embodiment of the present invention is controlled to be 8:12.

[0087] It has been found in practice that by properly controlling the coiling temperature, the γ→α phase transformation temperature can be appropriately reduced, the α phase nucleation rate can be increased, and the ferrite grains can be refined. At the same time, the proportion of bainite in the structure is appropriately increased. The bainite grain size is finer than that of ferrite, about 1μm, and has a higher dislocation density, which can improve the comprehensive mechanical properties of the steel strip. In addition, reasonable control of the coiling temperature is also a necessary measure to reduce the difficulty of controlling the plate shape and coil shape. Taking the above factors into consideration, the coiling temperature in the embodiment of the present invention is controlled to be 530-570℃.

[0088] Wherein, after cooling the hot-rolled steel strip in step S005 and coiling the cooled hot-rolled steel strip into a steel coil, the preparation method further comprises:

[0089] The steel coil unloaded from the coiler is hoisted into the slow cooling pit for slow cooling within 15 minutes. The temperature of the steel coil entering the slow cooling pit is ≥480℃; and the steel coil is hoisted out of the slow cooling pit after slow cooling for 60-66 hours.

[0090] It has been found in practice that during the laminar cooling process, the steel strip has large internal stress due to phase change, which is not conducive to improving the quality of the finished plate shape. When coiling, the head of the steel strip directly contacts the reel, and the tail is exposed to the air. The cooling rate of the head and tail is significantly higher than that of the inside of the steel coil, resulting in microstructural differences and mechanical property fluctuations in different parts of the steel strip. In order to make up for the above shortcomings, the embodiment of the present invention quickly places the coiled steel coil in the slow cooling pit for slow cooling. If the temperature of the steel coil entering the slow cooling pit is lower than 480°C and the slow cooling time is shorter than 60 hours, the effect of releasing the internal stress of the steel coil and improving the uniformity of the organization is poor. If the slow cooling time is longer than 66 hours, the production efficiency will be reduced. The time from unloading the coil to loading into the slow cooling pit is controlled within 15 minutes, which is a necessary measure to ensure that the temperature of the coil entering the slow cooling pit is ≥480°C. Taking the above factors into consideration, after unloading the coil from the coiler, the embodiment of the present invention needs to hoist the steel coil into the slow cooling pit within 15 minutes. The temperature of the steel coil entering the slow cooling pit is ≥480°C, and the steel coil is hoisted out of the slow cooling pit after 60-66 hours of slow cooling.

[0091] The present invention is further described below by way of examples, but the present invention is not limited to the scope of the examples. The experimental methods without specific conditions in the following examples are based on conventional methods and conditions.

[0092] The preparation methods of Examples 1 to 3 of the present invention include the following steps:

[0093] Firstly, the continuous casting billet is obtained through the desulfurization of molten iron pretreatment, top and bottom composite blowing converter smelting, LF refining and slab continuous casting.

[0094] The comparison of the chemical composition of the continuous casting billets obtained in Examples 1 to 3 of the present invention and the chemical composition of the continuous casting billets obtained in Comparative Examples 1 to 3 is shown in Table 3.

[0095] Table 3 Comparison of chemical composition (mass percentage) of continuous casting slabs of Examples 1-3 of the present invention and continuous casting slabs of Comparative Examples 1-3

[0096]

[0097] Secondly, the continuous casting billet is heated, rough rolled, finished rolled, and cooled to obtain a hot-rolled steel strip, and the hot-rolled steel coil is obtained by coiling and slow cooling. In each process, the process parameters used in Examples 1 to 3 of the present invention and the process parameters used in Comparative Examples 1 to 3 are shown in Tables 4-5. The performance parameters of the steel coils of Examples 1 to 3 obtained according to the chemical composition of Table 3 and the process parameters of Tables 4-5, and the performance parameters of the steel coils of Comparative Examples 1 to 3 obtained according to the chemical composition of Table 3 and the process parameters of Tables 4-5 are shown in Table 6.

[0098] Table 4 Heating and rough rolling process parameters of Examples 1-3 of the present invention and heating and rough rolling process parameters of Comparative Examples 1-3

[0099]

[0100]

[0101] Table 5 Finishing rolling and laminar cooling process parameters of Examples 1-3 of the present invention and Comparative Examples 1-3

[0102]

[0103] Table 6 Slow cooling process parameters and properties of steel coils in Examples 1-3 of the present invention and properties of steel coils in Comparative Examples 1-3

[0104]

[0105]

[0106] As can be seen from Tables 3-6, there are significant differences in the chemical composition, heating, rough rolling, finishing rolling, cooling and other process parameters between Examples 1-3 of the present invention and Comparative Examples 1-3, and there are also significant differences in the various properties of the obtained steel coils. The long-life low-alloy corrosion-resistant steel for photovoltaic brackets of the present invention can better meet the new requirements for the development of photovoltaic brackets such as high strength, light weight, high corrosion resistance, long life, easy manufacturing, low cost, high efficiency, free of painting, maintenance-free, energy saving, and environmental protection.

[0107] The preparation method of the embodiment of the present invention optimizes the content of elements such as C, Si, Mn, P, S, Cu, Cr, Alt, etc., and controls C / Cu, continuous casting billet discharge temperature, furnace dwelling time, rough rolling start and final rolling temperature, rolling pass, strip billet thickness, rolling speed, finishing rolling start and final rolling temperature, rolling pass, finished product thickness, rolling speed, cooling method, upper and lower header cooling water flow ratio, coiling temperature and steel coil cooling method after coiling, so that the yield strength of the obtained finished steel is ≥690MPa, the tensile strength is ≥800MPa, the elongation after fracture is ≥18%, and the 180°d=a cold bending test is qualified; the corrosion rate after periodic immersion in a mixed solution of NaHSO3 and 3.5% NaCl with an initial concentration of 0.01mol / L for 72 hours is ≤1.8g / (m 2 ·h); The single-side uniform corrosion amount of the steel coil prepared by the embodiment of the present invention after 25 years of uncoated service under C1, C2, C3, and C4 environments is ≤4μm, ≤12μm, ≤26μm, and ≤45μm, respectively.

[0108] The steel coil prepared by the preparation method of the embodiment of the present invention has low content of harmful and impurity elements and pure steel. It has high strength and can reduce weight by 33% by replacing Q235B galvanized sheet, with a significant lightweight effect. It has strong corrosion resistance and can meet the requirement of 25 years of service without painting in C1-C4 environment, and has a long service life. It has excellent forming performance and can meet the demanding processing requirements of photovoltaic brackets. The P and Al contents are moderate and the Cu content is low, which avoids its adverse effects on plastic toughness, welding performance, surface quality, etc., has stable mechanical properties, low manufacturing difficulty and high production efficiency. It does not contain precious alloy elements such as W, Ni, Nb, V, Mo, etc., has a low Mn content and a low alloy cost. No need for post-pickling, painting and maintenance, energy saving, environmental protection, short delivery cycle and low cost of use.

[0109] The steel coil prepared by the preparation method of the embodiment of the present invention can better meet the new requirements for the development of photovoltaic brackets such as high strength, light weight, high corrosion resistance, long life, easy manufacturing, low cost, high efficiency, painting-free, maintenance-free, energy-saving, and environmental protection.

[0110] An embodiment of the present invention further provides a steel coil for a photovoltaic support, which is prepared by the preparation method of any one of the above embodiments.

[0111] Among them, the yield strength of the steel coil is ≥690MPa, the tensile strength is ≥800MPa, the elongation after fracture is ≥18%, and the 180°d=a cold bending test is qualified.

[0112] The corrosion rate of the steel coil after 72 hours of periodic immersion in a mixed solution of NaHSO3 with an initial concentration of 0.01 mol / L and 3.5% NaCl is ≤1.8 g / (m 2 h).

[0113] The single-sided uniform corrosion amount of the steel coil after 25 years of coating-free service in C1, C2, C3 and C4 environments is ≤4μm, ≤12μm, ≤26μm and ≤45μm respectively.

[0114] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention. The present invention is not limited to the specific embodiments disclosed herein, but includes all technical solutions that fall within the scope of the claims.

Claims

1. A method for preparing a steel coil for a photovoltaic support, characterized in that: include: Obtaining a continuous casting billet through pretreatment; Heating and heat-insulating the continuous casting billet; Rough rolling the continuous casting billet using a rough rolling mill to obtain a strip billet; The strip is finish-rolled by a finishing mill to obtain a hot-rolled steel strip; as well as Cooling the hot-rolled steel strip, and coiling the cooled hot-rolled steel strip into a steel coil; Wherein, the continuous casting billet comprises the following components by mass percentage: C: 0.14%-0.16%, Si: 0.96%-1.10%, Mn: 0.21%-0.30%, P: 0.041%-0.055%, S: 0.0031%-0.0040%, Cu: 0.13%-0.18%, Cr: 3.4%-3.6%, Alt: 0.13%-0.16%, and the rest are Fe and unavoidable impurities, and the continuous casting billet also needs to satisfy C / Cu≤1.

2.

2. The preparation method according to claim 1, characterized in that: The continuous casting billet obtained by pretreatment specifically comprises: desulfurization through molten iron pretreatment, converter smelting through top and bottom composite blowing, LF refining, and slab continuous casting to obtain the continuous casting billet; the thickness of the continuous casting billet is 180-230 mm.

3. The preparation method according to claim 1, characterized in that: The continuous casting billet is subjected to heating and heat preservation treatment, wherein the furnace-out temperature of the continuous casting billet is 1170-1190° C., and the cumulative furnace-staying time of the continuous casting billet is 180-210 minutes.

4. The preparation method according to claim 1, characterized in that: The step of using a rough rolling mill to perform rough rolling on the continuous casting billet to obtain a strip billet is as follows: controlling the starting rolling temperature of the rough rolling to be 1120-1140° C., the final rolling temperature of the rough rolling to be 1050-1090° C., and using a rough rolling mill to perform rough rolling on the continuous casting billet to obtain a strip billet; Among them, the thickness of the strip obtained by rough rolling is 28-40mm, the rough rolling passes are 7 passes, and the rolling speed of each pass is: 1.8-2.2m / s for the first pass, 2.4-2.8m / s for the second pass, 3.3-3.7m / s for the third pass, 3.8-4.2m / s for the fourth pass, 3.8-4.2m / s for the fifth pass, 4.8-5.2m / s for the sixth pass, and 5.3-5.7m / s for the seventh pass.

5. The preparation method according to claim 1, characterized in that: The method of using a finishing mill to finish-roll the strip to obtain a hot-rolled steel strip comprises: controlling the start rolling temperature of the finishing rolling to be 990-1040° C., the final rolling temperature of the finishing rolling to be 840-880° C., and using a finishing mill to finish-roll the strip to obtain a hot-rolled steel strip; Among them, the thickness of the hot-rolled steel strip obtained by finish rolling is 1.5-4.0 mm, and the finish rolling is carried out by a 7-stand finishing rolling unit, and the rolling speeds of each stand are: 0.96-1.26 m / s for the first stand, 1.63-2.34 m / s for the second stand, 2.5-3.82 m / s for the third stand, 3.41-5.5 m / s for the fourth stand, 4.26-7.37 m / s for the fifth stand, 5.27-9.07 m / s for the sixth stand, and 6.22-10.37 m / s for the seventh stand.

6. The preparation method according to claim 1, characterized in that: The cooling treatment of the hot-rolled steel strip and coiling the cooled hot-rolled steel strip into a steel coil specifically comprises: Cooling the hot-rolled steel strip transported on the laminar cooling roller, wherein the cooling method of the hot-rolled steel strip on the laminar cooling roller is rear-stage cooling, and the flow rate ratio of the upper and lower header cooling water during the cooling process is 8:12; and controlling the coiling temperature to be 530-570°C, and coiling the cooled hot-rolled steel strip into a steel coil by a coiler; After cooling the hot-rolled steel strip and coiling the cooled hot-rolled steel strip into a steel coil, the method further comprises: The steel coil unloaded from the coiler is hoisted into the slow cooling pit for slow cooling within 15 minutes. The temperature of the steel coil entering the slow cooling pit is ≥480℃; and the steel coil is hoisted out of the slow cooling pit after slow cooling for 60-66 hours.

7. A steel coil for photovoltaic support, characterized in that: The method is prepared according to any one of claims 1 to 6.

8. The steel coil according to claim 7, characterized in that: The steel coil has a yield strength of ≥690MPa, a tensile strength of ≥800MPa, an elongation after fracture of ≥18%, and is qualified in a 180°d=a cold bending test.

9. The steel coil according to claim 8, characterized in that: The corrosion rate of the steel coil after 72 hours of periodic immersion in a mixed solution of NaHSO3 with an initial concentration of 0.01 mol / L and 3.5% NaCl is ≤1.8 g / (m 2 h); the uniform corrosion amount of a single side of the steel coil after 25 years of uncoated service under C1, C2, C3 and C4 environments is ≤4μm, ≤12μm, ≤26μm and ≤45μm respectively.

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