A low-cost hot-rolled substrate for color coating and its simplified manufacturing process

Abstract

This invention discloses a low-cost hot-rolled substrate for color coating. The chemical composition of the hot-rolled substrate, in terms of its mass percentage, is as follows: C 0.18%–0.22%, Si 0.1%–0.2%, Mn 0.1%–0.6%, P ≤ 0.020%, S ≤ 0.008%, Als 0.01%–0.04%, N ≤ 0.006%, with the balance being Fe and unavoidable impurities; and simultaneously satisfying: Als × N ≤ 18 × 10⁻⁶. ‑9 The Mn / S ratio is greater than 30. This invention, through control of the chemical composition of the hot-rolled substrate and key process points in a simplified manufacturing process, produces hot-rolled substrates with high strength, yield strength of 290–460 MPa, tensile strength of 440–600 MPa, and elongation exceeding 23%; the thickness is 1.2–6.0 mm, resulting in low production costs and easier control over plate shape and thickness accuracy.

Description

Technical Field

[0001] This invention relates to the field of steel product manufacturing, specifically to a low-cost hot-rolled substrate for color coating and its simplified manufacturing process. Background Technology

[0002] With economic development and the improvement of people's living standards, color-coated steel sheets are widely used in the construction industry due to their excellent corrosion resistance and decorative properties. In 2019, my country's consumption of color-coated steel sheets was approximately 12 million tons, a year-on-year increase of 3%. Their application in the construction industry is mainly divided into two parts: external and internal applications. External applications primarily use them as roof panels, roofs, exterior panels of steel structure workshops or buildings, rainwater harvesting systems, etc., typically after cold bending. Because color-coated steel sheet structures offer advantages such as fast assembly and flexible construction, many buildings both domestically and internationally are constructed using large quantities of galvanized and color-coated steel sheets, with construction costs approaching those of reinforced concrete structures.

[0003] However, the steel sheets currently used for color-coated steel sheets still have the following problems:

[0004] (1) Existing technologies for producing thin-gauge substrates all use a total cold rolling reduction rate of 70%-80%. In the cold rolling process, not only is the rolling load large and the energy consumption high, but the plate shape and thickness accuracy are also difficult to control, especially the rolling of high-strength steel.

[0005] (2) Currently, high-strength steels of 200-550MPa grade are all designed with niobium-containing alloys. As the strength level increases, the amount of alloy added also increases, which leads to a corresponding increase in production costs.

[0006] (3) Currently, the strength of steel used for color coating in construction is low, the thickness of the steel plate is relatively thick, and the coating thickness is also relatively thick to improve corrosion resistance. This not only makes hot-dip galvanizing production difficult, but also increases costs.

[0007] Therefore, it is of great significance to develop low-cost hot-rolled substrates for color coating that are high in strength, thin in thickness, and easy to control in terms of shape and thickness precision. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to provide a low-cost hot-rolled substrate for color coating and a simplified manufacturing method thereof, which addresses the shortcomings of the prior art. The hot-rolled substrate adopts a low-cost design, has high strength and thinness, and can reduce the production cost of subsequent processes. At the same time, the control of the plate shape and thickness accuracy is easy.

[0009] The technical solution adopted by the present invention to solve the above-mentioned problems is as follows:

[0010] A low-cost hot-rolled substrate for color coating, wherein the chemical composition of the hot-rolled substrate and its mass percentage are as follows: C 0.18%–0.22%, Si 0.1%–0.2%, Mn 0.1%–0.6%, P ≤ 0.020%, S ≤ 0.008%, Als 0.01%–0.04%, N ≤ 0.006%, with the balance being Fe and unavoidable impurities; and simultaneously satisfying: Als × N ≤ 18 × 10⁻⁶. -9 Mn / S>30.

[0011] In the above scheme, the thickness of the hot-rolled substrate is 1.2 to 6.0 mm.

[0012] In the above scheme, the yield strength of the hot-rolled substrate is 290-460 MPa, the tensile strength is 440-600 MPa, and the elongation is ≥23%.

[0013] This invention also claims protection for the simplified process manufacturing method of the above-mentioned low-cost hot-rolled substrate for color coating, comprising the following steps:

[0014] (1) Converter smelting: control the sulfur content in the molten steel to ≤0.025% at the end point.

[0015] (2) LF furnace refining: Off-site control of Als≤0.025%, N≤0.006%, and ensuring Als×N≤18×10 -9 .

[0016] (3) Slab casting, key points of process control:

[0017] ① The thickness of the billet is 65-85mm, and the casting speed is 4.6-5.2m / min.

[0018] ② Secondary cooling water reduction: The water flow in zones 4.1 to 7.1 and 4.2 to 7.2 is reduced to a protective water flow of 120 to 130 L / min, and the water flow in zone 3.2 is reduced to 255 to 280 L / min.

[0019] (4) Heating and rolling, key points of process control:

[0020] ① The slab should be in the furnace for 20 to 50 minutes and the furnace temperature should be 1160 to 1200℃ to ensure that the surface quality of the edges of the slab with medium carbon content meets the requirements.

[0021] ② The arrangement of the cooling manifold and nozzles of the F2-F4 frame work rolls is optimized. The width of the waterless area between two adjacent drain nozzles of the frame work roll cooling manifold is 38mm, the spray fan angle of the water nozzle is 60°, and the water flow ratio of the inlet side to the outlet side is 11:89, so as to ensure the cooling effect of the F2-F4 frame work rolls, and achieve the purpose of protecting the oxide film on the roll surface and thus controlling the iron oxide scale of the roll system.

[0022] ③ Control of the cross-section of the finished strip: the crown is controlled at 20-60μm, the wedge shape is controlled at ≤30μm, and the local high point is ≤10μm; the automatic crown change model is put into use, and the crown target value in the entire rolling unit is set by a negative parabola, and the crown target value is set as (40~45)→(22~27)→(35~40)μm.

[0023] The design concept of this invention regarding the chemical composition of the hot-rolled substrate and the key process points in the production process is as follows:

[0024] Control of C content: In this invention, the C content is controlled at 0.18% to 0.22%. The medium carbon content design can improve the strength of the hot-rolled substrate and reduce costs.

[0025] Mn composition control: To improve crack resistance and ensure slab quality, Mn / S ratio is controlled to be greater than 30; while ensuring strength, the Mn content is designed to be relatively small to minimize costs. Therefore, this invention controls the Mn content to be between 0.1% and 0.6%.

[0026] Control of phosphorus (P) content: P is highly sensitive to longitudinal cracking, reducing the plasticity of steel and making it brittle. P readily forms low-melting-point eutectics in steel, such as Fe3P and Fe2P, which accumulate at grain boundaries, significantly reducing grain boundary strength, lowering the high-temperature strength and plasticity of the steel, and increasing the tendency for longitudinal cracking during cooling. Therefore, this invention controls the P content to ≤0.020%.

[0027] Control of sulfur (S) composition: Sulfur is a readily segregating element in steel, lowering its zero plasticity temperature. S segregation at grain boundaries reduces specific surface energy, decreasing the bonding strength between precipitates and the bulk, thus promoting grain boundary slip. Stress concentration at grain boundaries leads to the formation of pores between sulfides and grain boundaries, resulting in further stress concentration and propagation into grain boundary cracks. The formation and growth of micropores at grain boundaries are also accelerated by sulfur segregation, ultimately leading to microcrack formation. Therefore, practical control requirements necessitate S ≤ 0.008% and Mn / S > 30.

[0028] Als and N content control: This invention aims to reduce AlN precipitation at grain boundaries during slab cooling, thereby reducing grain boundary crack formation, decreasing steel brittleness, and improving slab edge quality. Therefore, the invention controls the Als content to 0.01%–0.04% and the N content to ≤0.006%, while simultaneously satisfying Als×N≤18×10⁻⁶. -9 .

[0029] Matching the casting speed and billet thickness in continuous casting: This process can improve crack defects in medium carbon steel. Practice has shown that: too low a casting speed affects production capacity and weakens the upward flow of molten steel from the submerged entry nozzle, resulting in a thicker and less uniform initial billet shell. Simultaneously, the protective slag melts poorly due to low temperature, easily leading to cracks. Conversely, at a high casting speed, under a certain level of superheat, the solidification of the molten steel in the crystallizer is delayed, the billet shell becomes thinner, and the surface temperature of the billet shell increases. Under constant stress, the overall temperature of the billet shell shifts towards the first brittle zone of steel, exacerbating the tendency for surface cracks in the billet. Practice has shown that using a casting speed of 4.6–5.2 m / min matched with a corresponding billet thickness provides the best control of longitudinal cracks in medium carbon steel. Furthermore, a billet thickness of 65–85 mm also helps increase the accuracy of slab profile control during the rolling process.

[0030] Secondary cooling with weakest edge cooling: Using extremely weak cooling and reducing the intensity of edge cooling can reduce the longitudinal and transverse temperature unevenness of the slab, prevent overcooling of the edges, and reduce edge cracking of the slab.

[0031] Control of the heating furnace acceleration roller and the furnace exit temperature: In conjunction with the process control of thin slab continuous casting, it promotes the full dissolution of AlN precipitated when the edge temperature is too low, and improves the edge brittleness of the slab.

[0032] Automatic variable crown control for plate profile in precision rolling: When the initial roll crown is too small, and the edge drop intensifies with the increase of rolling tonnage per unit, the cross-section becomes flatter, affecting the cross-section control after subsequent cold rolling and galvanizing. Therefore, an automatic variable crown control for plate profile has been developed.

[0033] Compared with existing technologies, the beneficial effects of this invention are:

[0034] (1) The present invention provides a low-cost hot-rolled substrate for color coating. The hot-rolled substrate has high strength, with a yield strength of 290-460 MPa, a tensile strength of 440-600 MPa, and an elongation of more than 23%. The thickness is 1.2-6.0 mm. The production cost is low and the plate shape and thickness accuracy are easy to control.

[0035] (2) This invention provides a simplified process manufacturing method for low-cost hot-rolled substrates for color coating. This method reduces the unit cost by adopting a medium carbon composition design to reduce the amount of Mn and other alloys added; improves the longitudinal crack defect problem of medium carbon composition color coating products by adopting a low P, low S and Mn / S design, combined with continuous casting speed and billet thickness control; improves the edge crack problem of medium carbon composition color coating products by adopting an Als, N and Als×N design, combined with weak cooling at the secondary cooling edge, acceleration rollers in the heating furnace and furnace exit temperature control; and ensures that the cross-sectional profile of the hot-rolled substrate meets the color coating requirements by adopting an automatic variable crown control for precision rolling. Attached Figure Description

[0036] Figure 1This is a picture of the finished product after color coating of the low-cost hot-rolled substrate for color coating prepared according to the present invention. Detailed Implementation

[0037] The technical solution of the present invention will be described in full and clear below with reference to the embodiments. However, the described embodiments are only some embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0038] A low-cost hot-rolled substrate for color coating, wherein the chemical composition of the hot-rolled substrate and its mass percentage are as follows: C 0.18%–0.22%, Si 0.1%–0.2%, Mn 0.1%–0.6%, P ≤ 0.020%, S ≤ 0.008%, Als 0.01%–0.04%, N ≤ 0.006%, with the balance being Fe and unavoidable impurities; and simultaneously satisfying: Als × N ≤ 18 × 10⁻⁶. -9 Mn / S > 30. The thickness of the hot-rolled substrate is 1.2–6.0 mm.

[0039] The above-mentioned simplified process manufacturing method for low-cost hot-rolled substrate for color coating includes the following steps:

[0040] (1) Converter smelting: control the sulfur content in the molten steel to ≤0.025% at the end point.

[0041] (2) LF furnace refining: Off-site control of Als≤0.025%, N≤0.006%, and ensuring Als×N≤18×10 -9 .

[0042] (3) Slab casting, key points of process control:

[0043] ① The thickness of the billet is 65-85mm, and the casting speed is 4.6-5.2m / min, not exceeding 5.2m / min.

[0044] ② Secondary cooling water reduction: The water flow in zones 4.1 to 7.1 and 4.2 to 7.2 is reduced to a protective water flow of 120 to 130 L / min, and the water flow in zone 3.2 is reduced to 255 to 280 L / min.

[0045] (4) Key points for controlling the heating and rolling processes:

[0046] ① The slab is in the furnace for 20 to 50 minutes and the furnace exit temperature is 1160 to 1200℃.

[0047] ② The width of the waterless area between two adjacent drain nozzles of the working roller cooling manifold of frame F2-F4 is 38mm, the spray fan angle of the water nozzle is 60°, and the flow ratio of the inlet side to the outlet side is 11:89.

[0048] ③ Control of the cross-section of the finished strip: the crown is controlled at 20-60μm, the wedge shape is controlled at ≤30μm, and the local high point is ≤10μm; the automatic crown change model is put into use, and the crown target value in the entire rolling unit is set by a negative parabola, and the crown target value is set as (40~45)→(22~27)→(35~40)μm.

[0049] A batch of hot-rolled substrates was prepared according to the composition design and production process parameters described above, and all hot-rolled substrate samples were subjected to relevant performance tests. The specific chemical composition of the hot-rolled substrates is shown in Table 1, and the relevant performance data of the prepared hot-rolled substrates are shown in Table 2.

[0050] Table 1 Chemical composition (wt%) of various embodiments of the present invention

[0051] Example C Si Mn P S Als N 1 0.182 0.12 0.13 0.0198 0.001 0.0189 0.0058 2 0.219 0.15 0.46 0.017 0.003 0.0256 0.0046 3 0.208 0.17 0.24 0.015 0.006 0.0198 0.0035 4 0.198 0.20 0.38 0.014 0.004 0.0186 0.0038 5 0.203 0.16 0.27 0.017 0.005 0.0356 0.0047 6 0.194 0.13 0.58 0.016 0.003 0.0246 0.0054 7 0.213 0.18 0.34 0.013 0.002 0.0291 0.0039

[0052] Table 2. Data on the hot-rolled substrates and their mechanical properties obtained according to the embodiments of the present invention.

[0053] Example Yield strength / MPa Tensile strength / MPa Elongation / % Intensity level Hot-rolled substrate thickness 1 305 461 31.5 450MPa level 2.0 2 425 559 26.6 550Mpa level 4.0 3 297 442 32.9 400Mpa level 4.0 4 380 502 30.6 500Mpa level 3.5 5 330 483 30.8 450Mpa level 4.0 6 452 596 23.7 550Mpa level 3.5 7 411 569 26 550Mpa level 3.5

[0054] As shown in Table 2, the present invention provides a low-cost hot-rolled substrate for color coating. The hot-rolled substrate has high strength, with a yield strength of 290-460 MPa, a tensile strength of 440-600 MPa, and an elongation of over 23%.

[0055] The prepared hot-rolled substrate is first galvanized and then color coated. The chemical composition of the galvanizing solution, by mass percentage, is: Al 1%–3%, Mg 5%–6%, Si 1%–2%, with the balance being Zn and unavoidable impurities. The galvanizing solution temperature is 580–600℃, and the temperature of the hot-rolled substrate entering the zinc bath during galvanizing is 550–570℃. After galvanizing, air cooling is used, with a cooling rate of 100–140℃ / s. The primer used for color coating is an epoxy primer, with a heating and baking temperature of 300–350℃ and a curing time of 1–2 minutes. The topcoat used is a polyester coating, with a heating and baking temperature of 300–350℃ and a curing time of 1–3 minutes.

[0056] Figure 1 The image shows the finished product of the hot-rolled substrate prepared according to an embodiment of the present invention, after being galvanized and then color coated. Figure 1 This indicates that the color-coated products prepared from the hot-rolled substrate of the present invention have good surface quality, with no longitudinal crack defects and no edge crack defects.

[0057] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and its improved concept, should be covered within the scope of protection of the present invention.

Claims

1. A low-cost hot-rolled substrate for color coating, characterized in that, The chemical composition and mass percentage of the hot-rolled substrate are as follows: C 0.182%~0.22%, Si 0.1%~0.2%, Mn 0.1%~0.6%, P≤0.020%, S≤0.008%, Als0.01%~0.04%, N≤0.006%, with the balance being Fe and unavoidable impurities; and simultaneously satisfying: Als×N≤18×10 -9 Mn / S > 30; The method for manufacturing a low-cost hot-rolled substrate for color coating includes the following steps: converter smelting, LF furnace refining, slab casting, heating, and rolling. In the slab casting process, the casting speed is 4.6 to 5.2 m / min; in the secondary cooling zone, the water volume in zones 4.1 to 7.1 and 4.2 to 7.2 is reduced to a protective water volume of 120 to 130 L / min, and the water volume in zone 3.2 is reduced to 255 to 280 L / min; During the heating and rolling process, the slab is in the furnace for 20-50 minutes and the exit temperature is 1160-1200℃; the cross-section control of the finished strip is as follows: the crown is controlled at 20-60μm, the wedge shape is controlled at ≤30μm, and the local high point is ≤10μm; an automatic crowning model is used, and the crowning target value in the entire rolling unit is set using a negative parabola, with the crowning target value set as (40-45) → (22-27) → (35-40)μm; In the slab casting process, the slab thickness is 65–85 mm; In the heating and rolling process, the width of the waterless area between two adjacent drain nozzles of the cooling manifold of the work rolls of the F2-F4 stands is 38mm, the spray fan angle of the water nozzle is 60°, and the water flow ratio between the inlet side and the outlet side is 11:

89.

2. The low-cost hot-rolled substrate for color coating according to claim 1, characterized in that, The thickness of the hot-rolled substrate is 1.2 to 6.0 mm.

3. The low-cost hot-rolled substrate for color coating according to claim 1, characterized in that, The hot-rolled substrate has a yield strength of 290–460 MPa, a tensile strength of 440–600 MPa, and an elongation of ≥23%.

4. The low-cost hot-rolled substrate for color coating according to claim 1, characterized in that, In the converter smelting process, the sulfur content in the molten steel is controlled to be ≤0.025% at the end point.

5. A low-cost hot-rolled substrate for color coating according to claim 1, characterized in that, In the LF furnace refining step, the off-site control parameters are Als≤0.025%, N≤0.006%, and Als×N≤18×10⁻⁶. -9 .

6. A method for manufacturing a low-cost hot-rolled substrate for color coating as described in any one of claims 1 to 5, characterized in that, Includes the following steps: (1) Converter smelting: control the sulfur content in the molten steel to ≤0.025% at the final stage; (2) LF furnace refining: Off-site control of Als≤0.025%, N≤0.006%, and ensuring Als×N≤18×10 -9 ; (3) Slab casting, wherein: ① The billet thickness is 65-85mm, and the casting speed is 4.6-5.2m / min; ② Secondary cooling water reduction: The water flow in zones 4.1 to 7.1 and 4.2 to 7.2 is reduced to a protective water flow of 120 to 130 L / min, and the water flow in zone 3.2 is reduced to 255 to 280 L / min; (4) Heating and rolling, wherein: ① The slab is in the furnace for 20 to 50 minutes and the exit temperature is 1160 to 1200℃; ② Adjust the arrangement of the cooling manifold and nozzles of the working rollers of F2-F4 frames. The width of the waterless area between two adjacent drain nozzles of the cooling manifold of the working rollers of the frames is 38mm, the spray fan angle of the water nozzle is 60°, and the water flow ratio between the inlet side and the outlet side is 11:

89. ③ Control of the cross-section of the finished strip: the crown is controlled at 20-60μm, the wedge shape is controlled at ≤30μm, and the local high point is ≤10μm; the automatic crown change model is put into use, and the crown target value in the entire rolling unit is set by a negative parabola, and the crown target value is set as (40~45)→(22~27)→(35~40)μm.

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