A method for preparing a 1.5mm hot-rolled 800MPa-grade weather-resistant photovoltaic support steel

Abstract

The application discloses a preparation method of a 1.5mm hot-rolled 800MPa-grade weather-resistant photovoltaic support steel in the field of hot-rolling steel production, and the method comprises the following steps: adopting a steel grade component with 800MPa-grade weather resistance, controlling the thickness and length of the steel casting blank; adopting a hot feeding and hot charging mode for heating; rough rolling and finish rolling are carried out; laminar cooling is carried out on the steel strip after finish rolling; and finally, coiling is carried out. The application adopts a plate blank with a proper length, which is beneficial to reducing the temperature difference of the intermediate blank and the head and tail of the steel coil, the hot feeding and hot charging mode is adopted for heating, the core temperature of the plate blank can be improved, the rolling is stable, the rough rolling is carried out for four passes, the temperature drop at the entrance of the finish rolling is further reduced, and through reasonable control of the rolling speed, the plate crown, the loop amount, the bending force and the roll shifting amount and other processes in the finish rolling process, the 1.5mm thick hot-rolled steel plate has the advantages of good rolling stability, good weather resistance and excellent comprehensive performance, and the production efficiency and the yield of the extremely thin gauge hot-rolled high-strength weather-resistant steel plate can be improved.

Description

Technical Field

[0001] This invention relates to the field of hot-rolled steel production, and more particularly to a method for preparing 1.5mm hot-rolled 800MPa grade weather-resistant photovoltaic bracket steel. Background Technology

[0002] Currently, photovoltaic (PV) mounting systems are mainly made of plain carbon steel or aluminum alloy, then hot-dip galvanized with aluminum-magnesium alloy or anodized to improve corrosion resistance. The disadvantages of this method include increased processing costs, environmental pollution, and the coating being easily damaged during transportation or processing, leading to accelerated corrosion and premature failure of the system at the damaged areas. Alternatively, some PV mounting systems are made of high-strength weathering steel. When exposed to the atmosphere, weathering steel significantly improves corrosion resistance by forming a protective rust layer on its surface, achieving "rust prevention with rust." This rust layer also has self-healing capabilities, eliminating the need for pickling, painting, and other anti-corrosion treatments, as well as subsequent maintenance, thus shortening the construction cycle, reducing maintenance costs, and extending service life. For high-strength weathering steel, there are some mature products in the existing technology. For example, invention patent CN113278879A discloses a thin-gauge high-strength weathering steel plate with atmospheric corrosion resistance of 800MPa and its production method. However, this patent mainly studies the composition and does not further investigate the rolling process. Currently, conventional hot rolling processes can only produce thin-gauge high-strength weather-resistant steel plates ranging from 2.0 to 10 mm. The thinner the steel plate, the more difficult the rolling process becomes, resulting in lower production efficiency and yield. Given the requirements of photovoltaic brackets for lightweight, thin, and high-strength materials, reducing the steel plate thickness as much as possible, such as replacing 2.0 mm cold-rolled plates with 1.5 mm plates, would further reduce user costs and construction labor intensity. Summary of the Invention

[0003] To overcome the shortcomings of existing ultra-thin high-strength weather-resistant steel plates, such as high production difficulty, low production efficiency, and low yield, the technical problem to be solved by this invention is: to provide a method for preparing 1.5mm hot-rolled high-strength weather-resistant photovoltaic bracket steel using conventional hot rolling mills to improve production efficiency and yield.

[0004] The technical solution adopted by this invention to solve its technical problem is:

[0005] A method for preparing 1.5mm hot-rolled 800MPa grade weather-resistant photovoltaic bracket steel, characterized in that:

[0006] The chemical composition of the steel billet for rolling, by weight percentage, includes: C: ≤0.08%, Si: 0.35~0.50%, Mn: 0.80~1.00%, P: 0.07~0.12%, S≤0.005%, Cu: 0.25%~0.45%, Cr: 1.10%~1.30%, Ti: 0.075%~0.100%, N≤0.005%, with the remainder being Fe and unavoidable impurity elements. The rolling process includes the following steps:

[0007] Step 1: Control the thickness of the steel billet to be 200-250mm and the length of the billet to be 8.5m-9.5m;

[0008] Step 2: The steel billet is loaded into the slab heating furnace using a hot charging method, and the slab exit temperature is 1220~1280℃.

[0009] Step 3: Rough rolling of the steel billet is carried out using two sets of rolling mills. The first set of rolling mills performs one pass, and the second set of rolling mills performs three passes. The inlet temperature of the rough rolling mill is 1160-1200℃, the outlet temperature of the rough rolling mill is 1120-1160℃, and the outlet speed of the rough rolling mill is 2-5m / s.

[0010] Step 4: Perform finish rolling on the intermediate billet obtained in Step 3. The finish rolling inlet temperature is 1060–1120℃, the finish rolling outlet temperature is 860–900℃, and the finish rolling outlet speed is 10.0–11.5 m / s. Control the thickness of the finished strip to 1.5 mm. During the finish rolling process, the number of cooling water groups between stands should not exceed 1, and the number of lubrication rolling groups should not be less than 3. The finish rolling mill includes 7 sets of finish rolling rolls, namely F1–F7, and 6 sets of looper rolls, namely L1–L6. The finish rolling rolls and looper rolls are controlled according to the following requirements:

[0011] F1: 40~50μm, F2: 25~30μm, F3: 20~25μm, F4: 10~15μm, F5: 9~13μm, F6: 7~9μm, F7: 7~9μm;

[0012] Finishing roll gap: F1: 15mm, F2: 7mm, F3: 3mm, F4~F7: 2mm;

[0013] Finishing roll speeds: F1: 0.8~0.9m / s, F2: 1.6~1.8m / s, F3: 2.5~3.0m / s, F4: 4.0~4.5m / s, F5: 5.0~6.0m / s, F6: 6.5~7.5m / s, F7: 10.0~11.5m / s;

[0014] Looping roller tension: L1: 650~750N / mm 2 L2: 800~900N / mm 2L3: 900~1050N / mm 2 L4: 900~1150N / mm 2 L5: 1350~1450N / mm 2 L6: 1600~1800N / mm 2 ;

[0015] Bending force of finishing roll: F1: 300 N / mm 2 F2~F4: 250N / mm 2 F5~F6: 220N / mm 2 F7: 160 N / mm 2 ;

[0016] Finishing roll shifting amount: F1: 50~90mm, F2: 15~30mm, F3: 20~35mm, F4: 5~-20mm, F5: -100~-105mm, F6: -60~-65mm, F7: -40~-55mm;

[0017] Step 5: Perform laminar flow cooling on the finished steel strip. The laminar flow cooling rate is 10-30℃ / s, and the cooling flow ratio between the lower and upper manifolds is 1.2-1.4. The laminar flow cooling model adopts sparse cooling. The coiling temperature is controlled according to U-shaped coiling, that is, the coiling temperature of the first and last 30m of the steel coil is controlled at 650-700℃, and the coiling temperature of the middle part of the steel coil is controlled at 600-650℃.

[0018] Furthermore, in step three, the thickness of the intermediate billet after rough rolling is controlled to be 30-35 mm.

[0019] Furthermore, an insulation cover and a hot coil box are installed on the transfer roller conveyor between steps three and four, and the intermediate billet enters the finishing mill after passing through the insulation cover and the hot coil box in sequence.

[0020] The beneficial effects of this invention are as follows: The use of slabs of appropriate length helps reduce the temperature difference between intermediate slabs and the head and tail of the coil. The use of hot-feeding and hot-charging for heating increases the core temperature of the slab, facilitating stable rolling. Four passes of roughing further reduce the temperature drop at the finishing mill inlet. During finishing, by rationally controlling rolling speed, plate crown, looper amount, bending roll force, and roll shifting, a 1.5mm thick hot-rolled steel plate is obtained, possessing advantages such as good rolling stability, good weather resistance, and excellent comprehensive performance. This avoids defects such as strip tail rolling defects, waviness, and scrap that occur in existing technologies during strip preparation. It can improve the yield of ultra-thin hot-rolled high-strength weather-resistant steel plates, achieving a yield of over 95%. Detailed Implementation

[0021] The present invention will be further described below with reference to embodiments.

[0022] The present invention discloses a method for preparing 1.5mm hot-rolled 800MPa grade weather-resistant photovoltaic bracket steel, characterized in that:

[0023] The chemical composition of the steel billet for rolling, by weight percentage, 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%~0.45%, Cr: 0.75%~1.00%, Ti: 0.040%~0.060%, N≤0.005%, with the remainder being Fe and unavoidable impurity elements. The rolling process includes the following steps:

[0024] Step 1: Control the thickness of the steel billet to be 200-250mm and the length of the billet to be 8.5m-9.5m;

[0025] Step 2: The steel billet is loaded into the slab heating furnace using a hot charging method, and the slab exit temperature is 1220~1280℃.

[0026] Step 3: Rough rolling of the steel billet is carried out using two sets of rolling mills. The first set of rolling mills performs one pass, and the second set of rolling mills performs three passes. The inlet temperature of the rough rolling mill is 1160-1200℃, the outlet temperature of the rough rolling mill is 1120-1160℃, and the outlet speed of the rough rolling mill is 2-5m / s.

[0027] Step 4: Perform finish rolling on the intermediate billet obtained in Step 3. The finish rolling inlet temperature is 1060–1120℃, the finish rolling outlet temperature is 860–900℃, and the finish rolling outlet speed is 10.0–11.5 m / s. Control the thickness of the finished strip to 1.5 mm. During the finish rolling process, the number of cooling water groups between stands should not exceed 1, and the number of lubrication rolling groups should not be less than 3. The finish rolling mill includes 7 sets of finish rolling rolls, namely F1–F7, and 6 sets of looper rolls, namely L1–L6. The finish rolling rolls and looper rolls are controlled according to the following requirements:

[0028] F1: 30~40μm, F2: 15~20μm, F3: 10~15μm, F4: 5~10μm, F5: 4~8μm, F6: 4~6μm, F7: 4~6μm;

[0029] Finishing roll gap: F1: 15mm, F2: 7mm, F3: 3mm, F4~F7: 2mm;

[0030] Finishing roll speeds: F1: 0.8~0.9m / s, F2: 1.6~1.8m / s, F3: 2.5~3.0m / s, F4: 4.0~4.5m / s, F5: 5.0~6.0m / s, F6: 6.5~7.5m / s, F7: 10.0~11.5m / s;

[0031] Looping roller tension: L1: 600~700N / mm 2 L2: 750~850N / mm 2 L3: 850~1000N / mm 2 L4: 1000~1250N / mm 2 L5: 1250~1350N / mm 2 L6: 1500~1700N / mm 2 ;

[0032] Bending force of finishing roll: F1: 250 N / mm 2 F2~F4: 200N / mm 2 F5~F6: 180N / mm 2 F7: 120 N / mm 2 ;

[0033] Finishing roll shifting amount: F1: 50~90mm, F2: 15~30mm, F3: 20~35mm, F4: 5~-20mm, F5: -100~-105mm, F6: -60~-65mm, F7: -40~-55mm;

[0034] Step 5: Perform laminar flow cooling on the finished steel strip. The laminar flow cooling rate is 10-30℃ / s, and the cooling flow ratio between the lower and upper manifolds is 1.2-1.4. The laminar flow cooling model adopts sparse cooling. The coiling temperature is controlled according to U-shaped coiling, that is, the coiling temperature of the first and last 30m of the steel coil is controlled at 650-700℃, and the coiling temperature of the middle part of the steel coil is controlled at 600-650℃.

[0035] In step three, in order to improve the efficiency of finishing rolling and the quality of steel strip forming, the thickness of the intermediate billet after rough rolling is preferably controlled within 30 to 35 mm.

[0036] To prevent the intermediate billet from cooling too quickly during the transition from the roughing mill to the finishing mill, thus lowering the finishing mill inlet temperature, a heat insulation cover and a hot coil box can be installed on the transfer roller conveyor between steps three and four. The intermediate billet passes through the heat insulation cover and the hot coil box sequentially before entering the finishing mill. The heat insulation cover and the hot coil box can ensure a uniform and stable temperature for the intermediate billet, while also helping to reduce burn-off and the formation of secondary iron oxide scale, thereby improving the surface quality of the steel.

[0037] The steel plate of this invention has a microstructure consisting of ferrite with a volume fraction of ≥95% and pearlite or cementite with a volume fraction of ≤5%, a tensile strength of 650–800 MPa, an elongation of ≥25%, a yield strength ratio of ≤0.86, and a weathering index I of ≥8.0. Furthermore, the steel plate of this invention has the advantages of low rolling load, high rolling yield, good plate shape, and good formability.

[0038] The reasons for the restrictions on the main alloying elements in the steel described in this invention are explained below.

[0039] Carbon (C) is an important strengthening element in steel, which can improve the strength of steel through solid solution strengthening and precipitation strengthening. However, the C content should not be too high, otherwise it is easy to form Ti-containing alloy cementite with elements such as Ti during subsequent phase transformations. The cementite phase is not only large in size, but also tends to accumulate at grain boundaries, affecting the formability of the material. Therefore, this invention controls the C content at a relatively low level of 0.04% to 0.08%.

[0040] Si is an important interstitial solid solution strengthening element in steel, and Si can improve corrosion resistance. However, the concentration should not be too high, as excessive Si can easily form a red iron scale that is difficult to remove.

[0041] Mn plays a role in solid solution strengthening and improving toughness in steel. However, when the Mn content is too high, it is not only easy to form compositional segregation in the center of the thickness and reduce the uniformity of the structure, but also easy to increase the resistance to hot deformation and increase the rolling load.

[0042] S and N are impurity elements in steel, which easily form inclusions and reduce the toughness and plasticity of steel. When the content of S and N is too high, they will form sulfide and nitride inclusions with Ti in the steel. Among them, the liquid-precipitated TiN has a cubic morphology. If it exists in large quantities in the steel, it is very easy to form crack initiation sites, which has a significant impact on toughness and plasticity. Therefore, this invention limits the S content to ≤0.010% and the N content to ≤0.005%.

[0043] P is one of the impurity elements in steel, but adding an appropriate amount of P can improve corrosion resistance. However, it should not be added too much, otherwise the steel will become brittle.

[0044] Cr and Cu are important elements for improving corrosion resistance, and should be added appropriately based on the weather resistance index and alloy price.

[0045] In steel, Ti can combine with C to form nanoscale TiC precipitates, which can play a strong precipitation strengthening role. At the same time, Ti inhibits the coarsening of the original austenite structure during the reheating of the slab, which is beneficial to grain refinement. However, the Ti content should not be too high to avoid the formation of large-sized, high-density liquid-precipitated TiN inclusions.

[0046] The reasons for the limitations of the production process are explained below in conjunction with the requirements for steel strip forming control described in this invention.

[0047] Firstly, regarding the heating regime. Since the slab heating furnace uses nozzle heating, the main heat transfer method is thermal radiation. Heat is transferred from the slab surface to the core. Therefore, when the slab is cold-charged, the core temperature is low, and the heating rate is slow. An improper heating regime can easily lead to a low core temperature, resulting in insufficient deformation transfer to the core during subsequent rough rolling. This not only increases the rolling load and rolling difficulty but also easily leads to coarse core microstructure. In contrast, when hot-feeding and hot-charging are used, the slab is directly kept warm at a high temperature and transported to the heating furnace for heating. Because the slab cools naturally in the air after continuous casting, the core temperature is higher than the surface temperature. After slab heating, the core temperature is higher, making it easier to achieve stable rolling. Therefore, this invention requires that the slab obtained after continuous casting be hot-feeded and hot-charged into the slab heating furnace, while the furnace outlet temperature is controlled at a relatively high level of 1220–1280℃.

[0048] Secondly, regarding the roughing process. Conventional roughing uses a "3+3" mode, meaning R1 and R2 are rolled in three passes each. This invention uses a "1+3" mode, reducing two rolling passes, thus minimizing temperature drop. The design of the temperature and deformation regime considers rolling in the austenite recrystallization zone. Recrystallization refines the austenite structure, improving the material's strength and plasticity. Therefore, the roughing temperature is controlled at a relatively high level, and the rolling deformation is controlled at a relatively large level, meaning the intermediate slab thickness is controlled at a relatively low level. Secondly, considering the shape control in roughing, a higher roughing temperature is used to reduce the rolling load. Therefore, this invention requires: a "1+3" roughing process with an inlet temperature of 1160–1200℃, an outlet temperature of 1120–1160℃, an outlet speed of 2–5 m / s, and an intermediate slab thickness of 30–35 mm after roughing.

[0049] Third, heat insulation covers and hot coil boxes are installed on the roller table between the roughing mill and the finishing mill. On the one hand, this is to increase the finishing rolling temperature and reduce the rolling load. On the other hand, it is to remove the surface iron oxide scale by coiling the high-temperature intermediate billet, so that the temperature of the steel plate is uniform at both ends, and the stable rolling of thin-gauge steel plates can be achieved.

[0050] Fourth, regarding the finishing rolling process. Firstly, the main function of finishing rolling is to provide a large number of nucleation sites for subsequent phase transformations through high compression ratio rolling, promoting the formation of a fine and uniform microstructure, and improving the material's strength and toughness through fine grain strengthening. This invention employs a relatively high finishing rolling temperature, specifically an inlet temperature of 1060–1120℃ and an outlet temperature of 860–900℃. This is primarily to ensure that the early stage of finishing rolling (F1–F4) remains within the austenite recrystallization region, forming a uniform and fine equiaxed grain structure through recrystallization. The later stage of finishing rolling (F5–F7) is within the austenite non-recrystallization region, providing sufficient nucleation work and a large number of nucleation sites for subsequent phase transformations through the flattening of the austenite microstructure. Additionally, this invention employs a relatively high finishing rolling exit speed of 10.0–11.5 m / s, mainly to enable the steel plate to complete finishing rolling quickly, reducing the temperature difference between the beginning and end of the plate caused by natural temperature drop, thereby mitigating the performance differences between the beginning and end of the steel plate.

[0051] Secondly, the finishing rolling process also considers the issue of shape control for thin-gauge steel plates. The steel plates described in this invention are relatively thin, with a large finishing rolling compression ratio, making shape control difficult. Therefore, this invention requires: First, that the number of cooling water groups opened between stands not exceed one group to reduce the temperature drop during the finishing rolling process; Second, that the number of lubrication rolling groups open not less than three groups, using oil-water lubrication to reduce friction on the steel plate surface and reduce the finishing rolling load; Third, that the finishing rolling process rationally controls rolling speed, plate crown, looper amount, bending roll force, and roll shifting amount, resulting in hot-rolled steel strips with good rolling stability and excellent overall performance.

[0052] Setting the crown of the finishing rolls to a reasonable value is beneficial to improving the strip shape accuracy throughout the process and avoiding defects such as waviness in the strip. Taking into account the thickness of the intermediate billet and finished product of thin strip and the temperature drop during the rolling process, the finishing roll gap design can achieve a reasonable rolling load distribution while ensuring the amount of finishing deformation. Too high a looper roll tension can easily cause the strip to be stretched too narrow, while too low a looper roll tension can easily cause insufficient stretching of extremely thin strip between the two stands, resulting in poor stability of the strip between the stands and changes in strip shape. In addition, there is also a risk of strip breakage due to looper lifting. The setting of the finishing roll bending force and roll shifting amount is mainly to adapt to the crown of the intermediate billet after roughing, while realizing the control of strip crown, straightness, and shape, and avoiding defects such as waviness in the strip.

[0053] The exit speed of the finishing mill is mainly affected by the temperature drop of the strip during the finishing rolling process. A certain rolling speed is required to ensure that the temperature drop of the strip during the finishing rolling process can meet the requirements of the entry and exit temperatures. The thinner the strip, the greater the temperature drop. In addition, for materials such as automotive structural steel and high-strength weathering steel, the addition of alloying elements such as Ti, Cr, and Cu leads to an increased rate of temperature drop in the strip, thus requiring further increases in rolling speed.

[0054] Fifth, regarding laminar flow cooling: First, to avoid the formation of coarse ferrite or cementite structures, a relatively high laminar flow cooling rate of 10–30°C / s is required to refine the grains and improve the strength and plasticity of the material. Second, the coiling temperature is controlled at 600–650°C; appropriately increasing the coiling temperature is beneficial for improving the elongation of thin-gauge steel plates. Third, due to the influence of gravity, the cooling water in the lower manifold sprays upwards to the surface of the steel plate at a relatively low rate. Therefore, to ensure that the cooling rates of the upper and lower surfaces are similar, the flow rate of the cooling water in the lower manifold should be appropriately higher than that in the upper manifold. Based on industrial production experience, this invention controls the flow rate ratio of the lower and upper manifolds at 1.2–1.4. Fourth, since the steel plate undergoes a phase transformation during laminar flow cooling, internal stress is easily generated during the phase transformation, and the internal stress increases with a higher cooling rate. Therefore, this invention appropriately reduces the cooling rate of the steel plate while meeting the performance requirements of the steel plate, i.e., adopting a sparse cooling water mode, closing one manifold after every two manifolds are opened. Fifth, after the steel plate is coiled, the beginning and end parts are in direct contact with the air, and the temperature drop at the beginning and end of the coil is higher than that inside the coil. Therefore, this invention requires the coiling temperature to be controlled in a U-shape, that is, the coiling temperature at the beginning and end is higher, at 650-700℃, and the coiling temperature in the middle is lower, at 600-650℃. Because the temperature drops faster in the inner and outer rings of the steel coil, the U-shape control is beneficial to reducing the internal stress and uniformity of the steel coil.

[0055] The specific embodiments of the present invention will be further described below with reference to examples, but the present invention is not limited to the scope of the embodiments described herein.

[0056] Example 1

[0057] A steel billet with a thickness of 230 mm and a slab length of 9.0 m was prepared using a converter-LF electric heating-RH vacuum refining-continuous casting process. The billet composition was as follows: 0.05% C, 0.41% Si, 0.91% Mn, 0.08% P, 0.003% S, 0.36% Cu, 1.17% Cr, 0.087% Ti, 0.0033% N, with the remainder being Fe and unavoidable impurities. The billet was then heated, rough-rolled, hot-rolled, finished, laminar flow cooled, and coiled to produce a 1.5 mm thick steel plate. The specific process was as follows: the billet was hot-charged into the slab heating furnace at an exit temperature of 1265℃. The rough-rolling inlet temperature was 1195℃, the rough-rolling outlet temperature was 1148℃, the rough-rolling outlet speed was 3.3 m / s, and the intermediate slab thickness obtained after rough rolling was 33 mm. After rough rolling, the steel sheet is conveyed via roller conveyors, insulated by heat shields, and then coiled in a hot coil box before being fed into the finishing mill. The finishing mill inlet temperature is 1088℃, the finishing mill outlet temperature is 878℃, and the finishing mill outlet speed is 10.0 m / s. One set of interstand cooling water is activated, and three sets are activated for lubrication rolling. The finished steel sheet undergoes laminar flow cooling at a rate of 25℃ / s. The laminar flow cooling model uses one set of two pipes, with a water flow ratio of 1.35 between the lower and upper manifolds. The coiling temperature is 632℃.

[0058] The steel described in Example 1 has a thickness of 1.5 mm, a tensile strength of 825 MPa, an elongation of 24%, a yield strength ratio of 0.86, and a weather resistance index of 8.4.

[0059] Example 2

[0060] A steel billet with a thickness of 200 mm and a slab length of 9.1 m was prepared using a converter-LF electric heating-RH vacuum refining-continuous casting process. The billet composition was as follows: 0.07% C, 0.36% Si, 0.82% Mn, 0.11% P, 0.004% S, 0.098% Ti, 0.27% Cu, 1.25% Cr, 0.0032% N, with the remainder being Fe and unavoidable impurities. The billet was then heated, rough-rolled, hot-rolled, finished, laminar flow cooled, and coiled to produce a 1.5 mm thick steel plate. The specific process was as follows: the billet was hot-charged into the slab heating furnace, with an exit temperature of 1273℃. The rough-rolling inlet temperature was 1191℃, the rough-rolling outlet temperature was 1160℃, the rough-rolling outlet speed was 4.1 m / s, and the intermediate slab thickness obtained after rough rolling was 35 mm. After rough rolling, the steel sheet is conveyed via roller conveyor, insulated by an insulation cover, and then coiled in a hot coil box before being sent to the finishing mill. The finishing mill inlet temperature is 1109℃, the finishing mill outlet temperature is 890℃, and the finishing mill outlet speed is 11.1m / s. The interstand cooling water is not turned on, while five sets of lubrication rolling water are turned on. The finished steel sheet undergoes laminar flow cooling at a rate of 30℃ / s. The laminar flow cooling model consists of two sets of pipes and one set of pipes. The water flow ratio between the lower and upper manifolds is 1.27, and the coiling temperature is 645℃.

[0061] The steel described in Example 2 has a thickness of 1.5 mm, a tensile strength of 935 MPa, an elongation of 25%, a yield strength ratio of 0.87, and a weather resistance index of 8.5.

[0062] Example 3

[0063] A steel billet with a thickness of 200 mm and a slab length of 8.5 m was prepared using a converter-LF electric heating-RH vacuum refining-continuous casting process. The billet composition was as follows: 0.07% C, 0.49% Si, 0.98% Mn, 0.08% P, 0.002% S, 0.076% Ti, 0.44% Cu, 1.12% Cr, 0.0034% N, with the remainder being Fe and unavoidable impurities. The billet was then heated, rough-rolled, hot-rolled, finished, laminar flow cooled, and coiled to produce a 1.5 mm thick steel plate. The specific process was as follows: the billet was hot-charged into the slab heating furnace at an exit temperature of 1268℃. The rough-rolling inlet temperature was 1195℃, the rough-rolling outlet temperature was 1160℃, the rough-rolling outlet speed was 3.2 m / s, and the intermediate slab thickness obtained after rough rolling was 30 mm. After rough rolling, the steel sheet is conveyed via roller conveyor, insulated by an insulation cover, and then coiled in a hot coil box before being sent to the finishing mill. The finishing mill inlet temperature is 1100℃, the finishing mill outlet temperature is 890℃, and the finishing mill outlet speed is 11.5m / s. The interstand cooling water is not turned on, while five sets of lubrication rolling water are turned on. The finished steel sheet undergoes laminar flow cooling at a rate of 30℃ / s. The laminar flow cooling model consists of two sets of pipes and one set of pipes. The water flow ratio between the lower and upper manifolds is 1.32, and the coiling temperature is 605℃.

[0064] The steel described in Example 3 has a thickness of 1.5 mm, a tensile strength of 895 MPa, an elongation of 27%, a yield strength ratio of 0.85, and a weather resistance index of 8.4.

[0065] Comparative Example 1

[0066] A steel billet with a thickness of 200 mm and a slab length of 11 m was prepared using a converter-LF electric heating-RH vacuum refining-continuous casting process. The mass percentages were as follows: 0.07% C, 0.49% Si, 0.98% Mn, 0.08% P, 0.002% S, 0.076% Ti, 0.44% Cu, 1.12% Cr, 0.0034% N, with the remainder being Fe and unavoidable impurities. The billet was then heated, rough-rolled, hot-rolled in a coiler, finished, laminar flow cooled, and coiled to produce a 1.5 mm thick steel plate. The specific process was as follows: the billet was cold-charged into the slab heating furnace, with an exit temperature of 1235℃. The rough-rolling inlet temperature was 1167℃, the rough-rolling outlet temperature was 1130℃, the rough-rolling outlet speed was 3.2 m / s, and the thickness of the intermediate slab obtained after rough rolling was 30 mm. After rough rolling, the steel sheet is conveyed via roller conveyor, insulated by heat preservation covers, and then coiled in a hot coil box before being sent to the finishing mill. The finishing mill is rolled using conventional processes, with an inlet temperature of 1073℃, an outlet temperature of 851℃, and an outlet speed of 7.5m / s. The interstand cooling water is not turned on, while five sets of lubrication rolling are turned on. The finished steel sheet undergoes laminar flow cooling, and the coiling temperature is 585℃.

[0067] The steel described in Example 1 has a thickness of 1.5 mm, poor plate shape, tensile strength of 795 MPa, elongation of 20%, yield strength ratio of 0.89, and weather resistance index of 8.4.

[0068] Therefore, by adopting the production process of this invention, the rolling problem and material forming performance of ultra-thin high-strength weather-resistant steel plates can be solved using conventional hot rolling mills, providing a new approach for the preparation of ultra-thin high-strength weather-resistant photovoltaic bracket steel, and possessing good operability and scalability.

Claims

1. A method for preparing 1.5mm hot-rolled 800MPa grade weather-resistant photovoltaic bracket steel, characterized in that: The chemical composition of the steel billet for rolling, by weight percentage, includes: C: ≤0.08%, Si: 0.35~0.50%, Mn: 0.80~1.00%, P: 0.07~0.12%, S≤0.005%, Cu: 0.25%~0.45%, Cr: 1.10%~1.30%, Ti: 0.075%~0.100%, N≤0.005%, with the remainder being Fe and unavoidable impurity elements. The rolling process includes the following steps: Step 1: Control the thickness of the steel billet to be 200~250mm and the length of the billet to be 8.5m~9.5m; Step 2: The steel billet is loaded into the slab heating furnace using a hot charging method, and the slab exit temperature is 1220~1280℃. Step 3: Rough rolling of the steel billet is carried out using two sets of rolling mills. The first set of rolling mills performs one pass rolling, and the second set of rolling mills performs three passes rolling. The inlet temperature of the rough rolling mill is 1160~1200℃, the outlet temperature of the rough rolling mill is 1120~1160℃, the outlet speed of the rough rolling mill is 2~5m / s, and the thickness of the intermediate billet after rough rolling is controlled to be 30~35mm. Step 4: The intermediate billet obtained in Step 3 is finished rolled. A heat insulation cover and a hot coil box are installed on the transfer roller table between Step 3 and Step 4. The intermediate billet passes through the heat insulation cover and the hot coil box sequentially before entering the finishing mill. The finishing mill inlet temperature is 1060~1120℃, the finishing mill outlet temperature is 860~900℃, and the finishing mill outlet speed is 10.0~11.5m / s. The thickness of the finished steel plate is controlled to be 1.5mm. During the finishing rolling process, the number of cooling water groups between stands opened does not exceed one group, and the number of lubrication rolling groups opened is not less than three groups. The finishing mill includes 7 sets of finishing rolls, F1~F7, and 6 sets of looper rolls, L1~L6. The finishing rolls and looper rolls are controlled according to the following requirements: Finishing roll crown: F1: 40~50μm, F2: 25~30μm, F3: 20~25μm, F4: 10~15μm, F5: 9~13μm, F6: 7~9μm, F7: 7~9μm; Finishing roll gap: F1: 15mm, F2: 7mm, F3: 3mm, F4~F7: 2mm; Finishing roll speeds: F1: 0.8~1.1m / s, F2: 1.6~2.2m / s, F3: 2.5~4.0m / s, F4: 4.5~6.2m / s, F5: 6.6~8.6m / s, F6: 8.4~10.1m / s, F7: 10.0~11.5m / s; Looping roll tension: L1: 650~750 N / mm 2 L2: 800~900 N / mm 2 L3: 900~1050 N / mm 2 L4: 900~1150 N / mm 2 L5: 1350~1450 N / mm 2 L6: 1600~1800 N / mm 2 ; Bending force of finishing roll: F1: 300 N / mm 2 F2~F4: 250N / mm 2 F5~F6: 220N / mm 2 F7: 160 N / mm 2 ; Finishing roll shifting amount: F1: 50~90mm, F2: 15~30mm, F3: 20~35mm, F4: 5~-20mm, F5: -100~-105mm, F6: -60~-65mm, F7: -40~-55mm; Step 5: Perform laminar flow cooling on the finished steel strip. The laminar flow cooling rate is 10~30℃ / s, and the cooling flow ratio between the lower manifold and the upper manifold is 1.2~1.

4. The laminar flow cooling model adopts sparse cooling. The coiling temperature is controlled according to U-shaped coiling, that is, the coiling temperature of the first and last 30m of the steel coil is controlled at 650~700℃, and the coiling temperature of the middle part of the steel coil is controlled at 600~650℃.