Method for eliminating creases of color-coated sheet for downpipe

Abstract

A crease eliminating method for a color-coated sheet for a downpipe relates to the technical field of manufacturing of color-coated steel sheets, and comprises the following steps: optimizing raw material components (SPHC hot rolled steel comprises 0.03-0.05% of C, less than or equal to 0.03% of Si, less than or equal to 0.25% of Mn, less than or equal to 0.020% of S / P, and 0.02-0.06% of Al), cold rolling (the reduction rate is less than or equal to 85%, and the yield after cold rolling is less than or equal to 850 MPa), aluminum and zinc plating annealing (the direct combustion section is 610-660 DEG C, the RTF section is 780-820 DEG C, and the thermal expansion chamber is 520-560 DEG C); fast cooling and isothermal treatment at 600 DEG C for 15 seconds; and according to the color coating process (curing at the temperature of 216-224 DEG C, slow cooling at the plate temperature of 70-80 DEG C and stretching and straightening elongation of greater than or equal to 0.3%), the yield strength of the product is 270-350 MPa, the elongation of the product is greater than or equal to 25%, and bending creases are eliminated.

Description

Technical Field

[0001] This invention belongs to the field of color-coated steel sheet manufacturing technology, specifically relating to a method for eliminating creases in color-coated steel sheets used for downpipes. Background Technology

[0002] Color-coated steel sheets (color-coated plates) are widely used in the construction materials industry, such as roofing, wall panels, and downpipes. Downpipes, as a critical component of drainage systems, must withstand frequent bending and impacts, thus requiring extremely high formability from the color-coated sheets. While existing color-coated sheet technology can meet general building material needs, customers have reported noticeable creases after bending in specific downpipe applications. This defect not only affects the product's appearance but can also lead to material fatigue cracking and shortened service life, thus necessitating a targeted solution.

[0003] Current manufacturing processes for color-coated steel sheets mainly include hot rolling, cold rolling, plating, and coating. However, traditional methods have limitations in applications such as downpipes: for example, excessively high cold rolling reduction can lead to material hardening, insufficient annealing temperature for aluminum-zinc plating can result in uneven ferrite structure, and rapid cooling after color coating curing can easily trigger quenching effects. Under the combined effect of these factors, color-coated steel sheets are prone to creases during bending tests, exhibit large fluctuations in yield strength (often higher than 350 MPa), and low elongation (often lower than 20%), failing to meet the stringent forming requirements of downpipes.

[0004] To address the aforementioned issues, existing technologies attempt to improve the situation by adjusting chemical composition or adding post-processing steps, but with limited effectiveness. For example, some methods fail to adequately consider stress release during the annealing stage or optimize cooling methods, resulting in residual internal stress and stress concentration during bending, leading to creases. Furthermore, improper setting of straightening process parameters in color-coated sheets fails to effectively eliminate yield plateaus. Therefore, a method integrating raw material control, process parameter optimization, and cooling innovation is needed to fundamentally solve the crease defects in color-coated sheets used for downpipes, thereby improving product reliability and market competitiveness. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the above-mentioned defects of the prior art and provide a method for eliminating creases in color-coated steel sheets for downpipes that integrates raw material control, process parameter optimization and cooling innovation.

[0006] The technical solution adopted by this invention to solve its technical problem is as follows: A method for eliminating creases in color-coated steel sheets used for downpipes, characterized by: SPHC hot-rolled steel raw materials undergoing cold rolling, aluminum-zinc plating, and color coating; wherein the chemical composition of the hot-rolled steel raw materials meets the following requirements: C content 0.03-0.05%, Si content ≤0.03%, Mn content ≤0.25%, S content ≤0.020%, P content ≤0.020%, Al content 0.02-0.06%, and mechanical properties meet the following requirements: yield strength <320MPa, tensile strength 300-400MPa, and elongation ≥35% at 80mm.

[0007] Preferably, the total cold rolling reduction rate is ≤85%, and the yield strength after cold rolling is ≤850MPa.

[0008] Preferably, the aluminized zinc treatment includes an annealing step: the temperature of the direct-fired section is controlled at 610-660℃, the temperature of the radiant tube RTF section is controlled at 780-820℃, and the temperature of the hot tension chamber is controlled at 520-560℃; and a rapid cooling and isothermal process of 600℃ for 15 seconds is used in the cooling stage.

[0009] Preferably, a protective atmosphere of N2 and H2 is introduced during the aluminum-zinc annealing process, with a dew point ≤ -40℃.

[0010] Preferably, after the aluminized zinc treatment, the surface finishing and tension straightening are performed: the surface finishing machine has a finishing pressure of ≥260 tons and an elongation rate controlled at 0.6-0.8%; the tension straightening machine has an elongation rate control value of ≥0.6%.

[0011] Preferably, the color coating process includes: production line speed ≤ 80m / min; paint curing temperature 216-224℃.

[0012] Preferably, the color coating is cooled slowly after curing: a single set of water mist spray is applied at the support roller at the oven outlet, the first 5-6 meters of the water quenching tank are cooled naturally, and then 1-2 sets of water mist spray are applied for double-sided cooling. Hot air at 80-90℃ is applied at the outlet of the water quenching tank to ensure that the plate temperature is 70-80℃.

[0013] Preferably, the color coating straightening step involves: both sets of bending rollers being fully engaged, with an overlap of 12-14 mm, a tension setting of 3500-4500 kg, and an elongation of ≥0.3%.

[0014] Preferably, the yield strength of the final color-coated product is controlled at 270-350 MPa, and the elongation is ≥25%.

[0015] Preferably, the slow cooling process specifically includes: injecting a single set of water mist spray at the support roller at the oven outlet to prevent the paint on the back side from sticking to the roller; stopping water cooling 5-6 meters away from the inlet of the water quenching tank and adopting natural cooling; injecting 1-2 sets of water mist spray after the natural cooling section to cool the steel strip on both sides; using 80-90℃ hot air to blow at the outlet of the water quenching tank, and using an infrared thermometer to monitor the plate temperature in real time to ensure that the plate temperature is stable at 70-80℃.

[0016] The beneficial effects of this invention are as follows: (1) By optimizing the aluminum-zinc annealing (rapid cooling and isothermal treatment at 600℃ for 15 seconds) and color coating stretching (elongation ≥0.3%), the product bending test showed no creases, meeting the customer's testing standards; (2) The yield strength of the product of the present invention is stable at 270-350MPa (270-310MPa in the aluminum-zinc plating stage, and 270-350MPa after color coating), and the elongation is ≥25%, ensuring high plasticity and impact resistance during molding; (3) The high annealing temperature (610-660℃ in the direct combustion section and 780-820℃ in the RTF section) of this invention, combined with a protective atmosphere (dew point ≤ -40℃), homogenizes the ferrite and reduces the internal stress by 40% as measured by experiments, thus avoiding deformation or cracking. (4) The color coating of this invention is slowly cooled (plate temperature 70-80℃) to reduce the quenching effect, the plate temperature fluctuation is ≤5℃, and the surface quality is defect-free. Detailed Implementation

[0017] The present invention will be further described below with reference to the embodiments.

[0018] Unless otherwise specified, the raw materials or chemical reagents used in the embodiments and comparative examples of this invention are obtained through conventional commercial channels. The experimental equipment used in the embodiments and comparative examples of this invention are a cold rolling mill (with reduction rate control), an aluminized zinc coating line (annealing furnace, finishing machine, tension leveling machine), and a color coating line (curing furnace, water quenching tank, tension leveling machine).

[0019] Example 1 of a material for eliminating creases in color-coated steel sheets used in downpipes Example 1 of the raw material for eliminating creases in color-coated steel sheets used for downpipes includes: SPHC hot-rolled steel, 2.75mm thick, with a C content of 0.04%, a Si content of 0.02%, a Mn content of 0.20%, a S content of 0.015%, a P content of 0.018%, and an Al content of 0.04%.

[0020] Example 1 of a method for eliminating creases in color-coated steel sheets used for downpipes According to Example 1 of a material for eliminating creases in color-coated steel sheets used for downpipes, SPHC hot-rolled steel raw materials are first subjected to cold rolling; then they enter the aluminized zinc plating production line: the direct-fired section plate temperature is 630℃, the radiant tube RTF section plate temperature is 800℃, and the hot tension chamber plate temperature is 540℃, with a protective atmosphere of N2 and H2 (dew point -45℃) introduced, and a "rapid cooling and 600℃ isothermal 15-second" process is adopted; after annealing, the sheet is processed by a finishing machine (pressure 280 tons) and a tension leveling machine to obtain aluminized zinc plate; finally, the sheet undergoes color coating treatment: the production line speed is 75m / min, the primer and topcoat curing temperature is 220℃, after curing, slow cooling is adopted, a single set of water mist spray anti-sticking rollers are applied at the support rollers at the oven outlet, the first 5 meters of the water quenching tank are naturally cooled, then two sets of water mist spray double-sided cooling are applied, the outlet is swept with 80-90℃ hot air and the plate temperature is controlled at 75℃ by infrared temperature measurement; both sets of bending rollers of the color coating tension leveling machine are fully engaged to obtain the final product.

[0021] To evaluate the crease-eliminating effect of the method for eliminating creases in color-coated steel sheets for downpipes according to Embodiment 1 of the present invention, the following standardized testing procedure was adopted: First, samples (100mm × 200mm) were prepared according to ASTM D4145 standard and placed in a constant temperature and humidity environment (23±2℃, 50±5%RH) for 24 hours; then, a 180° forced bending test was performed using a universal testing machine with a bending mandrel diameter of 1T (bending rate 5mm / min, fulcrum spacing 50mm), and 5 sets of samples were tested repeatedly; after bending, crease defects on the inner and outer surfaces of the bent part were inspected visually and with a 10x magnifying glass, and the yield strength (GB / T 228.1 standard) and elongation after fracture (gauge length 80mm) were recorded; finally, the surface was deemed qualified when there were no visible creases and no coating peeling when the bending radius was ≤1mm.

[0022] Testing revealed that the main reason why the yield strength of the color-coated steel sheet used in the application embodiment 1 of this invention, after being treated with a method for eliminating creases in downpipes, remained stable at 310 MPa was the synergistic optimization of process parameters: First, the high-temperature control during the aluminized zinc annealing stage (800℃ in the RTF stage) combined with the "rapid cooling and 600℃ isothermal 15-second" process promoted ferrite recrystallization and homogenization, effectively releasing the internal stress of cold rolling (yield strength after cold rolling 800 MPa), thus initially reducing the material's yield strength to 290 MPa; Second, the finishing process (elongation 0.7%)... The plastic deformation of the tension leveler (elongation 0.65%) consumes the yield plateau in advance through micro-stretching, thereby improving the toughness of the material. Finally, the slow cooling process in the color coating stage (water mist spraying anti-sticking rollers at the oven outlet, 5-meter natural cooling section, double-sided water mist cooling, and 75℃ plate temperature control) reduces the quenching effect and avoids the accumulation of thermal stress. At the same time, the tension leveler (elongation 0.35%) further homogenizes the stress distribution through fine-tuning of the bending roller tension. Ultimately, while ensuring an elongation of 27%, the yield strength is precisely controlled within the range of 310MPa.

[0023] Testing revealed that, after treatment with a method for eliminating creases in color-coated steel sheets for downpipes, the elongation rate of Application Example 1 of this invention reached 27%. The main reasons for this include: the use of a high annealing temperature of 800℃ (RTF stage) combined with a "rapid cooling and 600℃ isothermal 15-second" process during the aluminum-zinc plating stage promotes ferrite recrystallization and releases rolling internal stress; the elongation rate of 0.7% on the finishing machine and 0.65% on the tension leveler improves material ductility by consuming the yield platen through plastic deformation; and the slow cooling process for color coating (single set of water mist spray anti-sticking rollers at the oven outlet, 5-meter natural cooling section before the water quenching tank, followed by two sets of water mist double-sided cooling and 75℃ plate temperature control) effectively reduces thermal stress, avoids microcracks caused by quenching, and thus optimizes the uniformity of the microstructure.

[0024] Upon testing, it was found that after the application of the present invention in Example 1, which uses a method to eliminate creases in color-coated steel sheets for downpipes, there were no creases at the bent parts. This is because the high annealing temperature (800℃) combined with rapid cooling and isothermal treatment at 600℃ for 15 seconds promotes the homogenization of ferrite and releases internal stress. The finishing elongation of 0.7% and the tensile elongation of 0.65% eliminate the yield plateau and improve plasticity. The slow cooling of the color coating ensures that the sheet temperature is 75℃, reducing thermal stress and avoiding quenching effects.

[0025] Example 2 of a material for eliminating creases in color-coated steel sheets used in downpipes Example 2 of the material for eliminating creases in color-coated steel sheets used for downpipes includes: the same as in Example 1.

[0026] Example 2 of a method for eliminating creases in color-coated steel sheets used for downpipes According to Example 2 of a raw material for eliminating creases in color-coated steel sheets used for downpipes, the components are SPHC hot-rolled steel raw material (thickness 2.75mm, C 0.04%, Si 0.02%, Mn 0.20%, S 0.015%, P 0.018%, Al...). The plate (0.04%) is first cold-rolled; then it enters the aluminized zinc plating production line: the direct-fired section has a plate temperature of 660℃, the radiant tube RTF section has a plate temperature of 820℃, and the hot tension chamber has a plate temperature of 560℃. A protective atmosphere of N2 and H2 (dew point -45℃) is introduced, and a "rapid cooling and 600℃ isothermal treatment for 15 seconds" process is used. After annealing, it is processed by a finishing machine (pressure 270 tons) and a tension leveler to obtain aluminized zinc plate. Finally, it undergoes color coating: the production line speed is 70m / min, the primer and topcoat curing temperature is 224℃, and after curing, slow cooling is used. A single set of water mist sprays anti-sticking rollers at the oven outlet support rollers, the first 5 meters of the water quenching tank are naturally cooled, followed by two sets of water mist spraying for double-sided cooling, and 80-90℃ hot air blowing at the outlet with infrared temperature measurement to control the plate temperature at 78℃. Both sets of bending rollers of the color coating tension leveler are fully engaged to obtain the final product.

[0027] The evaluation and testing methods are the same as in Example 1.

[0028] Testing revealed that the yield strength of the color-coated sheet used in Example 2 of this invention, after treatment with a method for eliminating creases in downpipes, reached 350 MPa due to the synergistic effect of the following processes: In the aluminizing-zinc plating stage, high-temperature annealing (radiant tube RTF section temperature 820℃, upper limit) combined with rapid cooling followed by a 15-second isothermal process at 600℃ promoted ferrite grain refinement and homogenization, effectively releasing cold-rolled internal stress and increasing the yield strength of the aluminizing-zinc plate to 310 MPa. In the color coating stage, a curing temperature of 224℃ (upper limit) was combined with a slow cooling process—including a single set of water mist spray anti-sticking rollers at the oven outlet, natural cooling of the first 5-6 meters of the water quenching tank to reduce sudden cooling stress, followed by 1-2 sets of water mist spray double-sided cooling, and hot air blowing at 80-90℃ at the outlet to control the plate temperature to 78℃, avoiding quenching effects that could lead to thermal stress concentration. Simultaneously, the high elongation rate of 0.3% in the color coating straightening machine consumed the yield platen through micro-plastic deformation, ultimately resulting in a stable yield strength of 350 MPa under the conditions of uniform material structure and minimized stress.

[0029] Testing revealed that, after processing with a method for eliminating creases in color-coated steel sheets for downpipes, the elongation rate of the 25% in Application Example 2 of this invention is due to the high annealing temperature (820℃ in the RTF section) combined with rapid cooling and isothermal treatment at 600℃ for 15 seconds, which promotes ferrite homogenization and grain refinement. The elongation rate of 0.8% in the polished machine and 0.6% in the tension leveler eliminates the yield plateau and enhances plastic deformation capacity. Furthermore, the slow cooling process of the color coating controls the plate temperature at 78℃, reducing thermal stress and avoiding quenching effects.

[0030] Upon testing, it was found that after the application embodiment 2 of this invention was treated with a method for eliminating creases in color-coated steel sheets used for downpipes, no creases were observed during the bending test. This is because the high annealing temperature of 820℃ in the RTF section of the radiant tube accelerates the refinement of ferrite grains and releases internal stress; the elongation rate of 0.8% in the light-finished machine and 0.6% in the tension leveler consumes yield deformation in advance and improves plasticity; and the slow cooling process ensures that the plate temperature is stable at 78℃. The natural cooling section and water mist spraying reduce the thermal stress gradient and avoid microcracks.

[0031] Example 3 of a material for eliminating creases in color-coated steel sheets used in downpipes Example 3 of the material for eliminating creases in color-coated steel sheets used for downpipes includes: the same as in Example 1.

[0032] Example 3 of a method for eliminating creases in color-coated steel sheets used for downpipes According to Example 3 of a material for eliminating creases in color-coated steel sheets used for downpipes, SPHC hot-rolled steel raw material (thickness 2.75mm, C content 0.04%, Si content 0.02%, Mn content 0.20%, S content 0.015%, P content 0.018%, Al content 0.04%) is provided, which is first subjected to cold rolling; then it enters the aluminized zinc plating line: the plate temperature of the direct-fired section is controlled at 610℃, the plate temperature of the radiant tube RTF section is 780℃, the plate temperature of the hot tension chamber is 520℃, and a protective atmosphere of N2 and H2 (dew point -40℃) is introduced, using "rapid cooling and 600℃ isothermal treatment". The process involves a 15-second process; after annealing, the sheet is processed by a finishing machine (finishing pressure 260 tons) and a straightening machine to obtain aluminized zinc plate; finally, it undergoes color coating: the production line speed is 60m / min, the curing temperature of the primer and topcoat is 216℃, and slow cooling is adopted after curing. A single set of water mist spraying is introduced at the support roller of the oven outlet to prevent the back paint from sticking to the roller. Water cooling is stopped in the first 6 meters of the water quenching tank and natural cooling is adopted. Then, a set of water mist spraying is introduced for double-sided cooling. At the outlet, 80-90℃ hot air is blown and infrared temperature measurement is used to control the plate temperature at 70℃; both sets of bending rollers of the color coating straightening machine are put into operation to obtain the final product.

[0033] The evaluation and testing methods are the same as in Example 1.

[0034] Testing revealed that, after treatment with a method for eliminating creases in color-coated steel sheets for downpipes, the yield strength of Application Example 3 of this invention was 270 MPa. This was due to the lower cold rolling reduction rate (78%) which reduced work hardening, the annealing temperature (780°C for the RTF section of the radiant tube) which inhibited excessive grain growth and promoted ferrite softening, the finishing and tensioning elongation rates (both 0.6%) which consumed yield deformation and released internal stress in advance, and the slow cooling process which ensured a plate temperature of 70°C through natural cooling sections and water mist spraying, reducing the thermal stress gradient and avoiding material quenching hardening.

[0035] Testing revealed that, after treatment with a method for eliminating creases in color-coated steel sheets for downpipes, the elongation rate of Application Example 3 of this invention reached 26%. This is mainly attributed to the precise coordination of process parameters: the 780℃ annealing temperature of the radiant tube RTF section combined with a N2+H2 protective atmosphere (dew point -40℃) promotes the refinement and homogenization of ferrite grains, effectively releasing cold-rolled internal stress; the 0.6% elongation rate of the finishing mill and the 0.6% elongation rate of the tension leveler significantly enhance the material's plastic deformation capacity by consuming the yield platen in advance through micro-plastic deformation; the slow cooling process of the color coating—including a single set of water mist spraying anti-sticking rollers at the oven outlet, a 6-meter natural cooling section before the water quenching tank, followed by a single set of water mist double-sided cooling, and hot air blowing at the outlet at 80-90℃ to control the plate temperature at 70℃—reduces the thermal stress gradient and quenching effect, avoiding the formation of microcracks; the 78% cold rolling reduction rate and low linear speed of 60m / min further optimize the degree of work hardening, ensuring that the material achieves an elongation rate exceeding the 25% threshold under high plasticity and low stress conditions.

[0036] Testing revealed that, after treatment with a crease-eliminating method for color-coated steel sheets used in downpipes according to Example 3 of this invention, no creases were observed during bending tests. This is because the 780℃ annealing temperature of the radiant tube RTF section, combined with a protective atmosphere of N2 and H2 (dew point -40℃), optimizes the uniformity of the ferrite structure and releases internal stress. The elongation of the finished product (0.6%) and the elongation of the tension leveler (0.6%) eliminate the yield plateau through plastic deformation, improving the material's plasticity. Simultaneously, a slow cooling process is employed—a single set of water mist spraying anti-sticking rollers at the oven outlet, natural cooling for the first 6 meters of the water quenching tank to reduce the cooling rate, followed by a set of water mist double-sided cooling, and hot air blowing at 80-90℃ at the outlet to ensure the plate temperature reaches 70℃. This precise control of the thermal stress gradient prevents microcracks during quenching.

[0037] A traditional method for eliminating creases in color-coated steel sheets used for downpipes. (Raw material comparison example 1) The raw materials for the traditional method of eliminating creases in color-coated steel sheets for downpipes, as described in Example 1, include the same as in Example 1.

[0038] Comparative Example 1: A Traditional Method for Eliminating Creases in Color-Coated Steel Sheets for Downpipes According to a conventional method for eliminating creases in color-coated steel sheets used for downpipes, Comparative Example 1 uses the same SPHC hot-rolled steel raw material as in Example 1 (thickness 2.75mm, C content 0.04%, Si content 0.02%, Mn content 0.20%, S content 0.015%, P content 0.018%, Al content 0.04%). First, it undergoes cold rolling (total reduction rate 85%). Then, it enters the aluminizing and zinc plating line: the direct-fired section temperature is controlled at 600℃, the radiant tube RTF section temperature at 770℃, and the hot-tensioning chamber temperature at 510℃, with nitrogen introduced... The sheet is heated in a protective atmosphere of 2 and H2, and the "rapid cooling + isothermal" process is not used. After annealing, it is processed by a finishing machine (finishing pressure 250 tons) and a tension leveler (elongation 0.5%) to obtain aluminized zinc sheet (yield strength 320MPa). Finally, it is coated with color: the production line speed is 90m / min, the curing temperature of the primer and topcoat is 210℃, and the slow cooling process is skipped after curing - no water mist spraying and natural cooling section is used, and rapid water cooling is used directly (the sheet temperature drops to 50℃). The color coating tension leveler is not put into operation (elongation 0%) to obtain the final product.

[0039] The evaluation and testing methods are the same as in Example 1.

[0040] Testing revealed that, after treatment with a traditional method for eliminating creases in color-coated steel sheets used for downpipes, the yield strength of the material in Comparative Example 1 reached 380 MPa. This was primarily due to a systematic deviation from several key process parameters: during the aluminum-zinc annealing process, the RTF section temperature of the radiant tube was only 770°C, and the "rapid cooling + isothermal" treatment was omitted, resulting in uneven coarsening of the ferrite structure and residual internal stress; the finishing pressure was 250 tons, and the finishing / stretching elongation was only 0.5%, which failed to effectively eliminate the yield plateau and improve plastic deformation capacity; the curing temperature of 210°C in the color coating process caused insufficient cross-linking of coating molecules, and the combined effect of a production line speed of 90 m / min exacerbated work hardening; more importantly, the slow cooling process was skipped, and direct rapid water cooling caused the plate temperature to drop sharply to 50°C, inducing a severe quenching effect that produced lattice distortion and dislocation accumulation. These multiple defects synergistically increased the material's yield strength.

[0041] Testing revealed that, after treatment with a traditional method for eliminating creases in color-coated steel sheets used for downpipes, the elongation rate of Comparative Example 1 was only 20%. This was mainly attributed to: the direct-fired section of the aluminum-zinc annealing process reaching 600°C and the radiant tube RTF section reaching 770°C, without the use of rapid cooling + isothermal processes, resulting in uneven ferrite structure and residual internal stress; the elongation rate of 0.5% for both the polished sheet and the straightening machine failing to effectively consume the yield plateau, significantly inhibiting plastic deformation capacity; the color coating line speed of 90m / min and curing temperature of 210°C exacerbating work hardening, while skipping the slow cooling process (directly and rapidly water-cooling to a sheet temperature of 50°C) triggering a severe quenching effect, increasing the thermal stress gradient and causing microcracks; and the absence of a color coating straightening machine (0% elongation) further weakening the material's ductility, ultimately resulting in an elongation rate far below the critical value of 25% at a high yield strength (380MPa), failing to meet the bending requirements of downpipes.

[0042] Testing revealed that, in Comparative Example 1, after treatment with a traditional method for eliminating creases in color-coated steel sheets used for downpipes, significant creases appeared during bending tests. Insufficient annealing temperature led to uneven ferrite structure and residual internal stress. Eliminating rapid cooling and isothermal processes resulted in grain coarsening. The elongation rate of only 0.5% after finishing and straightening was insufficient to eliminate the yield plateau, indicating inadequate plastic deformation capacity. A color coating speed of 90 m / min and a curing temperature of 210°C exacerbated material embrittlement. Skipping slow cooling and directly applying rapid water cooling caused the sheet temperature to plummet to 50°C, triggering quenching effects and microcracks. Furthermore, the absence of color coating straightening resulted in a final elongation rate of only 20%, leading to stress concentration and crease formation during bending.

[0043] In summary, after Comparative Example 1 was treated with a traditional method for eliminating creases in color-coated steel sheets used for downpipes, obvious creases appeared in the bending test, with a yield strength as high as 380 MPa and an elongation of only 20%; while after Example 1 adopted the optimized method of the present invention, there were no crease defects, the yield strength was stable at 310 MPa, and the elongation reached 27%. This significant difference stems from deviations in core process parameters: The comparative example suffers from insufficient annealing temperature (770℃ in the RTF section, below the lower limit of 780℃), resulting in grain coarsening due to the omission of rapid cooling + isothermal process; a finishing and straightening elongation of only 0.5% (below the 0.6% requirement), failing to eliminate the yield plateau; a color coating line speed of 90m / min (above the upper limit of 80m / min) and a curing temperature of 210℃ (below the lower limit of 216℃), exacerbating embrittlement; and skipping slow cooling and directly using rapid water cooling causes the plate temperature to plummet to 50℃ (far below the 70-80℃ range), triggering quenching effects and microcracks. In contrast, Example 1, through high-temperature annealing (800℃ in the RTF section), precise elongation control (0.7% in finishing, 0.65% in straightening), and a slow cooling process—including a natural cooling section before the water quenching tank and water mist spraying—ensures a plate temperature of 75℃, effectively releasing internal stress and improving plastic deformation capacity. Therefore, Example 1 achieves high elongation and low stress, fully meeting the requirements for downpipe forming.

Claims

1. A method for eliminating creases in color-coated steel sheets used for drain pipes, characterized in that: SPHC hot-rolled steel raw materials undergo cold rolling, aluminum-zinc plating, and color coating. The chemical composition of the hot-rolled steel raw materials meets the following requirements: C content 0.03-0.05%, Si content ≤0.03%, Mn content ≤0.25%, S content ≤0.020%, P content ≤0.020%, Al content 0.02-0.06%. The mechanical properties meet the following requirements: yield strength <320MPa, tensile strength 300-400MPa, and elongation ≥35% (A80mm).

2. The method for eliminating creases in color-coated steel sheets for downpipes according to claim 1, characterized in that: The total cold rolling reduction rate is ≤85%, and the yield strength after cold rolling is ≤850MPa.

3. The method for eliminating creases in color-coated steel sheets for downpipes according to claim 1, characterized in that: The aluminized zinc plating process includes an annealing step: the temperature of the direct-fired section is controlled at 610-660℃, the temperature of the radiant tube RTF section is controlled at 780-820℃, and the temperature of the hot tension chamber is controlled at 520-560℃; and a rapid cooling and isothermal process of 600℃ for 15 seconds is used in the cooling stage.

4. The method for eliminating creases in color-coated steel sheets for downpipes according to claim 3, characterized in that: During the aluminum-zinc annealing process, a protective atmosphere of N2 and H2 is introduced, with a dew point ≤ -40℃.

5. The method for eliminating creases in color-coated steel sheets for downpipes according to claim 3, characterized in that: The aluminum-zinc plating process is followed by finishing and straightening: the finishing machine has a finishing pressure of ≥260 tons and an elongation rate controlled at 0.6-0.8%; the straightening machine has an elongation rate control value of ≥0.6%.

6. The method for eliminating creases in color-coated steel sheets for downpipes according to claim 1, characterized in that: The color coating process includes: production line speed ≤ 80m / min; paint curing temperature 216-224℃.

7. The method for eliminating creases in color-coated steel sheets for downpipes according to claim 6, characterized in that: After the color coating is cured, it is cooled slowly: a single set of water mist spray is applied at the support roller at the oven outlet, the first 5-6 meters of the water quenching tank are cooled naturally, and then 1-2 sets of water mist spray are applied for double-sided cooling. Hot air at 80-90℃ is applied at the outlet of the water quenching tank to ensure that the plate temperature is 70-80℃.

8. The method for eliminating creases in color-coated steel sheets for downpipes according to claim 6, characterized in that: The color coating straightening step involves: both sets of bending rollers being fully engaged, with an overlap of 12-14mm, a tension setting of 3500-4500kg, and an elongation rate ≥0.3%.

9. The method for eliminating creases in color-coated steel sheets for downpipes according to claim 1, characterized in that: The final yield strength of the color-coated product is controlled at 270-350MPa, and the elongation is ≥25%.

10. The method for eliminating creases in color-coated steel sheets for downpipes according to claim 7, characterized in that: The slow cooling process specifically includes: injecting a single set of water mist spray at the support roller at the oven outlet to prevent the paint on the back side from sticking to the roller; stopping water cooling 5-6 meters away from the inlet of the water quenching tank and adopting natural cooling; injecting 1-2 sets of water mist spray after the natural cooling section to cool the steel strip on both sides; using 80-90℃ hot air to blow at the outlet of the water quenching tank and using an infrared thermometer to monitor the plate temperature in real time to ensure that the plate temperature is stable at 70-80℃.