High-weatherability enamel steel and preparation method thereof

Abstract

The invention belongs to the field of metal materials, and particularly relates to high-weather-resistance enamel steel and a preparation method thereof.The high-weather-resistance enamel steel comprises a steel plate base material and an enamel layer combined to the surface of the steel plate base material; the chemical components of the steel plate base material comprise trace elements such as C, Cr, Ni, Ti and B, so that the formation of a compact oxide film and the refinement of grains are facilitated, and the corrosion resistance and the binding force with an enamel layer are improved. And enamel glaze of the enamel layer contains SiO2, Al2O3, B2O3, Na2O, K2O, CaO, MgO and a weather-resistant additive, so that the ultraviolet resistance and the chemical stability are enhanced. A base material is prepared through steelmaking, continuous casting, hot rolling, cold rolling and continuous annealing, and then surface pretreatment, glaze spraying and firing are conducted. According to the invention, through regulation and control of base material components, glaze additives and a whole-flow process, the product is free of corrosion after being exposed for 500 hours in a simulated outdoor severe environment, the fading, pulverization and peeling resistance is remarkably improved, and the service life is prolonged.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, and particularly relates to a high weather-resistant enamel steel and its preparation method. Background Technology

[0002] Enameled steel is a composite material made by coating an inorganic enamel glaze onto the surface of a steel plate and then firing it at high temperatures. It combines the strength of steel with the excellent properties of enamel, such as corrosion resistance and aesthetics, and is widely used in construction, home appliances, chemicals, and many other fields. However, in actual use, especially in harsh outdoor environments, existing enamel steels face the problem of insufficient weather resistance. Long-term exposure to environmental factors such as the atmosphere, ultraviolet radiation, and rainwater can easily cause the enamel layer to fade, powder, and peel off, leading to a decline in its protective performance and aesthetics, and seriously affecting the outdoor stability of enamel steel products.

[0003] Currently, methods to improve the weather resistance of enamel steel mainly focus on improving the enamel glaze, such as adding pigments or additives with better weather resistance. However, these methods have certain limitations. On the one hand, the improvement effect is limited and cannot fundamentally solve the problem of insufficient weather resistance; on the other hand, they may adversely affect other properties of enamel steel, such as the bonding strength with steel plates and processing performance. Meanwhile, research on the weather resistance of the steel plate substrate itself is relatively limited, and no effective technical solution has yet been developed to comprehensively improve the overall weather resistance of enamel steel. Furthermore, existing technologies do not pay sufficient attention to the process flow and details of steelmaking, hot rolling, cold rolling, and continuous annealing in the preparation of enamel steel, resulting in poor performance stability of the steel plate substrate, which in turn affects the overall quality and weather resistance of the enamel steel.

[0004] In the prior art, patent application number CN202510332325.8 discloses an ultra-low carbon, high sulfur, and high nitrogen enamel steel and its manufacturing method. The manufacturing process involves multiple stages, including hot metal desulfurization, converter smelting, and RH vacuum segmented treatment. The parameters for each stage are precisely and complexly controlled. For example, RH vacuum treatment requires segmented control of different gas flow rates and vacuum levels to adjust carbon and nitrogen content, placing high demands on production equipment and operational technology, increasing the difficulty and cost of production process control. Patent application number CN202410852463.4 discloses a polymer pre-coated cold-rolled enamel steel sheet and its manufacturing method. While it has advantages in green and environmentally friendly pretreatment, its performance improvement is limited. In high-end applications requiring high weather resistance and comprehensive performance, this technology has relatively weak product competitiveness and a relatively narrow application range. Its main improvement lies in the pretreatment process of the cold-rolled enamel steel sheet surface; optimization of other process stages is relatively insufficient, and the systematic nature of process improvement is weak, which may affect the overall performance improvement of the product. Patent application number 201810853530.9 discloses a cold-rolled enamel steel and its manufacturing method. Its composition design (Cr only 0.02%-0.08%, no weather-resistant glaze additives) does not involve improving weather resistance. In outdoor exposure scenarios, the enamel layer is prone to fading and powdering due to ultraviolet aging and atmospheric corrosion, which limits its application in building curtain walls and other fields. Moreover, its Ti content (0.005%-0.03%) is low and no effective element synergy mechanism is formed. Its anti-scaling performance relies only on the "hydrogen trap" effect of MnS and TiN, and the hydrogen diffusion inhibition effect is limited in long-term high temperature and high humidity environments. Patent application number CN201110091114.8 discloses a method for preparing enamel steel plates, but does not mention the control of weather resistance by the composition of the substrate. The steel plate substrate does not contain weather-resistant elements such as Cr and Ni. Long-term exposure to outdoor environments can easily cause substrate corrosion, which in turn leads to the peeling of the enamel layer. Moreover, its glaze formula is mainly composed of SiO2 and Na2O, which can easily cause glaze degradation under long-term ultraviolet radiation, resulting in a decrease in gloss. This technology is mainly suitable for indoor decoration and low-corrosion environments.

[0005] Therefore, there is an urgent need to develop a high weather-resistant enamel steel and its preparation method to solve the above problems. Summary of the Invention

[0006] To overcome the shortcomings of existing technologies, the purpose of this invention is to provide a high weather-resistant enamel steel and its preparation method. Through optimized design of the steel plate substrate composition, refined improvement of key processes such as steelmaking, and innovation of enamel preparation process, the enamel steel can maintain good performance in long-term harsh outdoor environments, significantly improve weather resistance, and meet the needs of long-term outdoor use.

[0007] To achieve the above objectives, the present invention provides the following technical solution: A high weather-resistant enamel steel includes a steel plate substrate and an enamel layer bonded to its surface; The chemical composition of the steel plate substrate, expressed as a percentage by mass, is as follows: C: 0.001%~0.005%, Si: 0.01%~0.05%, Mn: 0.2%~0.5%, P≤0.01%, S≤0.005%, Cr: 0.5%~1.5%, Ni: 0.2%~0.8%, Ti: 0.01%~0.05%, B: 0.0005%~0.002%, balance being Fe and unavoidable impurities; The raw material for the enamel layer includes enamel glaze, and the chemical composition of the enamel glaze by mass percentage is: SiO2: 45%–60%, Al2O3: 10%–20%, B2O3: 8%–15%, Na2O+K2O: 5%–10%, CaO+MgO: 3%–8%, weather-resistant additives: 1%–5%.

[0008] Tensile strength of enamel steel: 350–420 MPa; Yield strength: 180–240 MPa; Elongation is calculated using A... 50 Gauge length: ≥30%; Work hardening index n: 0.20~0.28; Plastic strain ratio R: 1.4~2.0; Under simulated outdoor conditions: Irradiance 500W / m 2 After 1000 hours of continuous testing at 40℃ and 80% relative humidity, the enamel layer showed no fading, powdering, or peeling.

[0009] Composition design of high weather-resistant enamel steel: The main functions of the components in the steel plate substrate are: C: 0.001%~0.005%, with the carbon content strictly controlled at a low level. If the carbon content exceeds 0.005%, the probability of reaction between carbon and enamel glaze increases during firing, reducing the bonding strength between the enamel layer and the substrate, and increasing the brittleness of the steel. If it is below 0.001%, it will increase steelmaking costs and make it difficult to guarantee the basic strength of the steel. This content range can reduce the adverse reaction between carbon and enamel glaze, reduce the brittleness of the steel, and improve its toughness.

[0010] Mn: 0.2%–0.5%. Manganese improves the strength and toughness of steel and combines with sulfur to form manganese sulfide. If the content is below 0.2%, the desulfurization effect is poor, and the adverse effects of sulfur on steel properties are difficult to eliminate; if it exceeds 0.5%, it will increase the hardenability of steel, easily leading to uneven steel structure and affecting subsequent processing performance. This range can effectively reduce the harm of sulfur and improve the structural stability of enamel steel in complex environments.

[0011] Si (Silicon): 0.01%–0.05%. Silicon can enhance the strength of steel and adjust the coefficient of thermal expansion to match the enamel glaze. If the content is below 0.01%, the strength enhancement effect is not obvious, the coefficient of thermal expansion is poorly matched, and it is prone to cracking of the enamel layer. If it exceeds 0.05%, it will increase the brittleness of the steel and may react with the components in the enamel glaze, reducing the bonding strength. This range can reduce enamel layer defects caused by temperature changes and improve weather resistance.

[0012] P: ≤0.01%. Phosphorus can easily cause cold brittleness in steel. If the content exceeds 0.01%, phosphorus is prone to segregation at low temperatures, which will degrade the performance of steel and affect the overall weather resistance of enamel steel. Therefore, it needs to be strictly controlled.

[0013] S: ≤0.005%. Sulfur easily forms low-melting-point sulfides. If the content exceeds 0.005%, the sulfides are prone to melting during high-temperature firing and use, resulting in defects and reducing the hot working performance and weather resistance of steel. Therefore, it needs to be strictly limited.

[0014] Cr: 0.5%–1.5%. Chromium is a key element for improving weather resistance and can form a dense oxide film. If the content is below 0.5%, the oxide film is not dense and the protective effect is poor; if it exceeds 1.5%, it will increase the cost of steel and may lead to excessive carbides in the steel structure, affecting processing performance. This range can effectively prevent environmental corrosion and enhance the adhesion between the enamel layer and the steel plate.

[0015] Ni: 0.2%–0.8%. Nickel improves the strength and toughness of steel and synergistically enhances oxide film stability with chromium. If the content is below 0.2%, the synergistic effect is not significant, and the improvement in weather resistance is limited; if it exceeds 0.8%, it will significantly increase costs and has no significant benefit to the steel's processing performance. This range broadens the application environment of enamel steel and improves its acid and alkali resistance.

[0016] Ti: 0.01%–0.05%. Titanium can form stable compounds with nitrogen and carbon, refining the grain size. If the content is below 0.01%, the grain refinement effect is poor, and the content of free nitrogen and carbon in the steel is high, which easily leads to bubbles in the enamel layer. If it exceeds 0.05%, too many compounds will be formed, increasing the brittleness of the steel. This range can improve the performance of the steel and improve the surface quality of enamel steel.

[0017] B: 0.0005%~0.002%, boron can improve the hardenability of steel and improve its microstructure. If the content is below 0.0005%, the improvement in hardenability is not significant; if it exceeds 0.002%, it can easily lead to grain boundary embrittlement of steel, affecting toughness. This range can enhance the performance of steel and improve the bonding strength between the enamel layer and the steel plate.

[0018] In addition, Cr, Ni, Ti and B have a synergistic effect: Cr and Ni together optimize the density and stability of the oxide film, Ti refines the grains to provide a uniform substrate for the formation of the oxide film, and B improves the microstructure and enhances the overall performance of the substrate. The four work together to significantly improve the weather resistance of enamel steel.

[0019] The main functions of the components in enamel glaze are: SiO2: 45%–60%, as the main component, imparts hardness, wear resistance, and chemical stability to the enamel layer. If the content is below 45%, the enamel layer has poor performance and is prone to wear and corrosion; if it exceeds 60%, the glaze becomes more difficult to melt, resulting in poor fluidity and difficulty in forming a uniform coating. This range is fundamental to ensuring the weather resistance of enamel steel.

[0020] Al2O3: 10%–20%, can improve the high-temperature viscosity and chemical stability of enamel. If the content is below 10%, the high-temperature performance of the enamel is poor, and the mechanical strength and thermal shock resistance of the enamel layer are insufficient; if it exceeds 20%, the melting temperature of the enamel is too high, increasing the difficulty of firing. This range can enhance the enamel layer's resistance to environmental damage and improve its weather resistance.

[0021] B2O3: 8%–15%, can lower the melting point of enamel and improve its fluidity and sintering performance. If the content is below 8%, the enamel melting point is high, the fluidity is poor, and the coating is uneven; if it exceeds 15%, it will reduce the chemical stability of the enamel layer and make it susceptible to environmental corrosion. This range can make the enamel layer uniform and dense, and enhance its adhesion to the steel plate.

[0022] Na₂O + K₂O: 5%–10%. Alkali metal oxides can lower the melting temperature of the enamel and regulate its chemical activity. If the content is below 5%, the melting temperature of the enamel is high, resulting in poor bonding with the steel plate; if it exceeds 10%, it will significantly reduce the chemical stability of the enamel layer, making it prone to fading and powdering. This range ensures good bonding between the enamel layer and the steel plate while maintaining stability.

[0023] CaO + MgO: 3%–8%, which can improve the high-temperature performance and chemical stability of enamel. If the content is below 3%, the water and acid resistance is poor; if it exceeds 8%, the enamel is prone to crystallization, affecting the appearance and performance of the coating. This range can enhance the water and acid resistance of the enamel layer and improve its weather resistance.

[0024] Weather-resistant additives: 1%–5%, a composite additive of nano-titanium dioxide and nano-zinc oxide, possessing excellent UV shielding properties. If the content is below 1%, the UV shielding effect is poor, and the enamel layer is prone to aging; if it exceeds 5%, it increases the cost of the glaze and may affect the glaze melting and coating uniformity. This range effectively prevents UV damage and also provides antibacterial and self-cleaning functions.

[0025] A method for preparing high weather-resistant enamel steel includes the following steps: 1) The preparation of steel plate substrate includes steelmaking, continuous casting, hot rolling, cold rolling, and continuous annealing; 2) Pretreatment: The steel plate substrate undergoes surface degreasing, rust removal, and cleaning. The steel plate substrate is cut to the required dimensions. A combination of alkaline and acid pickling is used to degrease and remove rust from the steel plate surface. First, the steel plate substrate is immersed in a 5%–10% sodium hydroxide solution at 60–80°C for 10–15 minutes to remove surface oil. Then, the steel plate substrate is transferred to a 10%–15% hydrochloric acid solution and pickled at room temperature for 5–10 minutes to remove surface rust and oxide scale. After pickling, it is rinsed thoroughly with clean water, and then immersed in a 2%–5% sodium carbonate solution for neutralization. Finally, it is rinsed with deionized water and dried. 3) Enameling: Enamel glaze is evenly sprayed onto the surface of the pretreated steel plate substrate to form a glaze coating with a thickness of 0.2-0.3 mm; The spraying voltage is 60-80kV, the distance between the spray gun and the steel plate substrate is 150-200mm, and the spraying time is 3-5 minutes, so that the enamel glaze forms a uniform coating on the steel plate surface. 4) Firing: The enameled steel plate is placed in a high-temperature furnace for firing. The furnace temperature is raised to 800-850℃ at a heating rate of 5-10℃ / min and held for 10-15 minutes to allow the enamel glaze to fully melt and react chemically with the steel plate to form a strong bonding layer. Then, the furnace temperature is lowered to 500-550℃ at a cooling rate of 3-5℃ / min and allowed to cool naturally to room temperature. During the firing process, the atmosphere inside the furnace is strictly controlled, with the oxygen content kept at 2%-5% to maintain a weakly oxidizing atmosphere and prevent excessive oxidation of the steel plate.

[0026] The method for preparing the steel plate substrate includes: 1) Steelmaking: In converter steelmaking, molten iron is poured into the converter, and slag-forming agents (such as lime, fluorite, etc.) are added to carry out decarburization, dephosphorization, and desulfurization. When the carbon content in the molten steel drops to 0.02% to 0.03% by mass, the steel is tapped. During the tapping process, argon gas is blown into the ladle to stir the steel, and alloys (such as ferrochrome, ferronickel, ferrotitanium, ferroboron, etc.) are added to the ladle. After the molten steel is left to stand in the ladle for 5 to 10 minutes to allow the inclusions to float to the surface, it is then continuously cast. 2) Continuous casting: Control the pulling speed to 1.0–1.5 m / min, and the crystallizer water flow rate to 150–200 m³ / min. 3 / h, to ensure slab quality; 3) Hot rolling: Heat the continuously cast slab to 1150-1250℃ and hold for 2-3 hours to ensure uniform slab temperature. Multi-pass rolling is adopted, with the reduction rate of the first pass being 33.3%~50%, and the reduction rate of each subsequent pass gradually decreasing, with the total reduction rate controlled at 80%~90%. The final rolling temperature is controlled at 850-900℃, and then laminar flow cooling is performed at a rate of 10-20℃ / s. After the steel plate is cooled to 550-600℃, it is coiled to obtain a hot-rolled coil. 4) Cold rolling: The hot-rolled coil is pickled to remove the surface oxide scale. The pickling is carried out using a hydrochloric acid solution with a mass fraction of 10% to 15%, at a temperature of 60 to 80°C, and for a pickling time of 5 to 10 minutes. After pickling, the hot-rolled plate is cold-rolled using a multi-roll mill, with the total reduction rate controlled at 60% to 80%, and the thickness accuracy of the cold-rolled steel plate controlled at ±0.01mm. 5) Continuous annealing: The cold-rolled sheet is fed into a continuous annealing furnace for annealing treatment. The annealing temperature is 750-850℃ and the holding time is 30-60 seconds. During the annealing process, the atmosphere inside the furnace is a mixture of hydrogen and nitrogen, with hydrogen accounting for 5% to 10% by volume, to prevent the steel plate from oxidizing. After annealing, the material is cooled. First, it is rapidly cooled to 300-400°C at a rate of 20-30°C / s, and then slowly cooled to room temperature at a rate of 5-10°C / s.

[0027] The tensile strength of the steel plate substrate is 340–400 MPa, the yield strength is 170–230 MPa, and the elongation is A. 50 ≥32%; the metallographic structure is single-phase ferrite, with equiaxed ferrite grains, grain size grade I~II9, average grain diameter 10~15μm, and inclusion grades BT1.0, DT0.5, and DTTiN1.0. The substrate is subjected to simulated outdoor conditions with a light intensity of 500W / m². 2 At a temperature of 40℃ and a relative humidity of 80%, no rust was observed after 500 hours of exposure. The metallographic structure is single-phase ferrite with equiaxed ferrite grains of grade I to II 9, an average grain diameter of 10–15 μm, and inclusion grades of BT1.0, DT0.5, and DTTiN1.0. The microstructure is uniform and the inclusion content is low, providing a good substrate for subsequent enamel preparation.

[0028] Enamel glaze preparation: Preparation of the enamel glaze: Weigh each raw material according to the chemical composition of the enamel glaze, mix them evenly, and then feed them into a ball mill. Add water and grinding media accounting for 20% to 30% of the total mass of the raw materials. Zirconia balls can be used as the grinding media. The ball-to-material ratio is 3:1 to 5:1. The ball milling time is 10 to 15 hours to fully grind and refine the raw materials to obtain a uniform and delicate enamel glaze slurry. Spray dry the enamel glaze slurry under the conditions of an inlet air temperature of 180 to 220°C and an outlet air temperature of 80 to 100°C to prepare a dry powder enamel glaze. The particle size of the dry powder is controlled at 50 to 100 μm.

[0029] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention achieves a breakthrough in weather resistance through three aspects of synergistic innovation: First, in terms of the steel plate substrate composition, the precise ratio of elements such as Cr, Ni, Ti, and B forms a dense oxide film and refines the grains, effectively preventing the erosion of environmental media while enhancing the bonding force between the enamel layer and the steel plate; second, the introduction of nano-level weather-resistant additives into the glaze enhances ultraviolet shielding and chemical stability, effectively slowing down the aging and damage of the enamel layer caused by ultraviolet rays; third, the synergistic control of process parameters throughout the entire process, from steelmaking to firing, ensures the performance matching between the substrate and the enamel layer. Under these combined effects, the enamel steel of this invention can effectively resist the erosion of the atmosphere, ultraviolet rays, and rainwater in long-term outdoor environments, reducing fading, powdering, and peeling, significantly improving weather resistance and meeting the needs of long-term outdoor use.

[0030] 2. Excellent Comprehensive Performance: While improving weather resistance, the strength, toughness, and processing performance of the steel are ensured through reasonable control of the content of various elements and refinement of processes such as steelmaking, hot rolling, cold rolling, and continuous annealing. The hardness, wear resistance, and chemical stability of the enamel layer have also been optimized, enabling the enamel steel to meet the application requirements of different fields while possessing high weather resistance. Its mechanical properties are: tensile strength: 350~420MPa, yield strength: 180~240MPa, elongation (A50 gauge length): ≥30%, n-value (work hardening index): 0.20~0.28, R-value (plastic strain ratio): 1.4~2.0.

[0031] 3. The preparation method of the present invention optimizes and adjusts the process parameters such as steelmaking, hot rolling, cold rolling and continuous annealing, adopts conventional equipment and operation procedures, and is easy to promote and apply on existing production lines, with good economic efficiency and practicality. Attached Figure Description

[0032] Figure 1 The image shows the metallographic microstructure of the longitudinal section of the cold-rolled steel sheet in Example 1 after etching with 4% nitric acid alcohol.

[0033] Figure 2This is a 100x metallographic microstructure image of the longitudinal section of the cold-rolled steel sheet in Example 1 after being etched with 4% nitric acid alcohol. Detailed Implementation

[0034] The present invention will now be described in detail with reference to the accompanying drawings, but it should be noted that the implementation of the present invention is not limited to the following embodiments.

[0035] Example 1: The preparation of the steel plate substrate for high weather-resistant enamel steel includes the following steps: 1) Steelmaking: A converter is used for steelmaking. Slagging agents are added after the molten iron is fed into the furnace, and the steel is decarburized to 0.025% before tapping. Alloying materials are added during tapping to ensure the steel plate base material composition (mass percentage) reaches: C: 0.003%, Mn: 0.3%, Si: 0.03%, P: 0.008%, S: 0.003%, Cr: 1.0%, Ni: 0.5%, Ti: 0.03%, B: 0.001%, with the balance being Fe and unavoidable impurities. The continuous casting speed is 1.2 m / min, and the crystallizer water flow rate is 180 m³ / min. 3 / h, to obtain the slab.

[0036] 2) Hot rolling: The slab is heated to 1200℃, held for 2.5 hours, rolled in multiple passes, with a total reduction of 85%, a final rolling temperature of 880℃, a laminar flow cooling rate of 15℃ / s, and cooled to 580℃ before being coiled.

[0037] 3) Cold rolling: The hot-rolled coil is pickled in 12% hydrochloric acid solution (70℃) for 8 minutes, and then cold-rolled with a total reduction of 70% to obtain the steel plate of the required thickness.

[0038] 4) Continuous annealing: The cold-rolled sheet is held at 780°C for 40 seconds in a continuous annealing furnace with a hydrogen content of 8%, and then rapidly cooled to 350°C before being slowly cooled to room temperature.

[0039] Enamel steel coating and firing methods: 1) Preparation of enamel glaze: Raw materials are prepared according to the following mass percentages: SiO2: 50%, Al2O3: 15%, B2O3: 12%, Na2O+K2O: 8%, CaO+MgO: 5%, weather-resistant additive (a composite additive of nano-titanium dioxide and nano-zinc oxide in a mass ratio of 1:1): 2%, and the remainder is kaolin, feldspar, and other additives. The raw materials are mixed and added to a ball mill, along with water and zirconia balls (ball-to-material ratio 4:1) accounting for 25% of the total mass of the raw materials. The mixture is ball-milled for 12 hours, and then spray-dried at an inlet air temperature of 200℃ and an outlet air temperature of 90℃ to produce a dry powder enamel glaze (particle size 70-80μm).

[0040] 2) Steel plate pretreatment: The steel plate substrate is subjected to alkaline washing, acid washing, neutralization and drying treatment.

[0041] 3) Enameling and firing: The enamel glaze is sprayed onto the pretreated steel plate surface using an electrostatic spraying method, and then fired according to the above firing process.

[0042] 4) Post-processing and inspection: The fired enamel steel was inspected, and the surface quality was good with no obvious defects. The product was then subjected to a weathering resistance test in a simulated outdoor environment (light intensity 500W / m²). 2 After 1000 hours of continuous testing at 40℃ and 80% relative humidity, the enamel layer showed no obvious fading, powdering, or peeling.

[0043] Figure 1 The microstructure of the longitudinal section of the cold-rolled steel sheet in Example 1, after being etched with 4% nitric acid alcohol, is shown at 200x magnification. Figure 1 It can be seen that the microstructure of the sample is single-phase ferrite (F), with ferrite grains distributed in an equiaxed manner, clear and continuous grain boundaries, and no obvious second-phase microstructures such as pearlite (P) and bainite (B). Combined with the grain size test results (grades I~II9), it can be seen that the ferrite grain size is uniform, with an average grain diameter of about 10~15μm, which meets the characteristics of grade 9 grain size in the ferrite grain size evaluation standard for cold-rolled thin plates of low carbon steel. This indicates that the cold rolling process has a significant effect on grain refinement and good microstructure stability. Figure 2 The metallographic structure of the same sample under 100x magnification shows that the ferrite grains are uniformly distributed overall, with no obvious abnormal grain growth or local distortion, indicating uniform deformation during cold rolling and no subsequent obvious recrystallization anomalies. Furthermore, no Widmanstätten or ledeburite structures were observed in the field of view, further verifying the rationality of the material composition control and rolling process. Table 1 shows the microstructure, grain size, and inclusion statistics under an optical microscope. The ferrite grain size grade assessment for cold-rolled thin plates of low-carbon steel is GB / T4335~2013 (Method A); the non-metallic inclusion grade assessment for steel is GB / T10561~2023 (Method A).

[0044] Table 1. Microstructure and Grain Size style organize Grain size, grade Inclusions, level Example 1 F I~II 9 <![CDATA[BT1.0;DT0.5;DT TiN 1.0]]> Table 2 shows the scanning electron microscope (SEM) characterization results of non-metallic inclusions in the steel plate of Example 1, demonstrating the distribution characteristics of inclusions in local areas. Based on the test data (BT1.0; DT0.5; DTTiN1.0) and the evaluation standard for non-metallic inclusions in steel (GB / T10561~2023, Method A), it can be seen that there are three main types of inclusions in the samples: BT type inclusions (grade 1.0): irregular angular shape, size about 5~10μm, verified by energy dispersive spectroscopy (EDS) as silicate inclusions, mainly distributed near grain boundaries, few in number and well dispersed; DT type inclusions (grade 0.5): spherical sulfide inclusions, diameter about 2~5μm, uniformly distributed in the ferrite matrix, without forming chains or aggregates; DTTiN type inclusions (grade 1.0): fine particles (1~3μm), titanium nitride (TiN), with high hardness and stability, mainly existing in an isolated state, without damaging the continuity of the matrix. Overall, the levels of all three types of inclusions were low, and the total content met the requirements for inclusion control in cold-rolled low-carbon steel sheets. This indicates that the deoxidation and desulfurization processes during steelmaking were effective, and the inclusions were removed thoroughly, meeting the purity requirements of the substrate for subsequent enamel steel preparation. The energy dispersive spectroscopy (EDS) analysis results were consistent with the inclusion level assessment obtained from metallographic testing, providing direct evidence for the analysis of inclusion types and their causes. This indicates that the inclusions in the material are of a single composition, without complex composite inclusions, and their adverse effects on the mechanical properties of the steel sheet and the adhesion of the enamel layer are negligible.

[0045] Table 2: Statistical Analysis of Inclusions in Scanning Electron Microscopy The above microstructure and inclusion characterization results were obtained using Leco metallographic cutting machine, Buhler metallographic polishing machine, and ZEISS Imager.D1m microscope. The testing process conformed to the GB / T13298~2015 sample preparation standard, and the data are reliable. The uniformity of the microstructure and the low-level distribution of inclusions provide a good substrate foundation for the subsequent enameling and firing processes of enamel steel, and help reduce defects such as bubbles and peeling in the enamel layer.

[0046] Example 2: The preparation of the steel plate substrate for high weather-resistant enamel steel includes the following steps: 1) Steelmaking: A converter steelmaking process is adopted. After molten iron is poured into the converter, appropriate amounts of lime and fluorite are added as slag-forming agents to carry out decarburization, dephosphorization, and desulfurization operations. When the carbon content in the molten steel drops to 0.02%, tapping begins. During tapping, argon gas is blown from the bottom of the ladle for stirring. At the same time, alloy materials such as ferrochrome, ferronickel, ferrotitanium, and ferroboron are added to the ladle to ensure that the composition of the steel plate substrate reaches the following mass percentages: C: 0.002%, Mn: 0.4%, Si: 0.02%, P: 0.006%, S: 0.002%, Cr: 1.2%, Ni: 0.6%, Ti: 0.04%, B: 0.0015%, with the balance being Fe and unavoidable impurities. The molten steel is allowed to stand in the ladle for 8 minutes to allow inclusions to float to the surface before continuous casting. During continuous casting, the casting speed is controlled at 1.0 m / min, and the crystallizer water flow rate is 150 m³ / min. 3 / h, to obtain a slab of acceptable quality.

[0047] 2) Hot rolling: The continuously cast slab is placed in a heating furnace and heated to 1150℃, held for 3 hours to ensure uniform slab temperature. Then, multiple rolling passes are performed. The thickness of the first pass is half the slab thickness, and the reduction rate gradually decreases in subsequent passes, with the total reduction rate controlled at 80%. The final rolling temperature is controlled at 850℃. After rolling, laminar cooling is performed at a rate of 10℃ / s. The steel plate is cooled to 550℃ and then coiled to obtain a hot-rolled coil.

[0048] 3) Cold rolling: The hot-rolled coil is fed into a pickling tank, and a 10% hydrochloric acid solution is used as the pickling medium. The pickling is carried out at 60°C for 10 minutes to remove the surface oxide scale. The pickled hot-rolled plate is then cold-rolled on a multi-roll mill, with the total reduction rate controlled at 60%. The thickness accuracy of the cold-rolled steel plate is controlled within ±0.01mm.

[0049] 4) Continuous Annealing: The cold-rolled sheet is fed into a continuous annealing furnace, and the annealing temperature is set to 750℃ for 60 seconds. During annealing, the furnace atmosphere is a mixture of hydrogen and nitrogen, with a hydrogen content of 5% to prevent oxidation of the steel sheet. After annealing, the sheet is first rapidly cooled to 300℃ at a rate of 20℃ / s, and then slowly cooled to room temperature to obtain a high-performance steel sheet substrate.

[0050] Enamel steel coating and firing methods: 1) Steel plate pretreatment: The prepared steel plate was cut to the required size and first immersed in a 5% sodium hydroxide solution at 60°C for 15 minutes to remove surface oil. Then, it was transferred to a 10% hydrochloric acid solution and pickled at room temperature for 10 minutes to remove surface rust and scale. After pickling, it was rinsed thoroughly with water and then immersed in a 2% sodium carbonate solution for neutralization. Finally, it was rinsed with deionized water and dried.

[0051] 2) Preparation of enamel glaze: Weigh the following raw materials according to their mass percentages: SiO2: 50%, Al2O3: 15%, B2O3: 12%, Na2O+K2O: 8%, CaO+MgO: 5%, weather-resistant additive (a composite additive of nano-titanium dioxide and nano-zinc oxide in a mass ratio of 1:1): 2%, with the remainder being other additives. Mix the raw materials thoroughly and add them to a ball mill. Add water and zirconia balls (28% of the total raw material mass) as grinding media, with a ball-to-material ratio of 4:1. Ball mill for 12 hours to obtain a uniform and fine enamel slurry. Spray dry the enamel slurry to produce a dry powder enamel glaze for later use.

[0052] 3) Enameling and firing: The dry powder enamel glaze was uniformly sprayed onto the pretreated steel plate surface using an electrostatic spraying method. The spraying voltage was controlled at 60kV, the distance between the spray gun and the steel plate was 150mm, and the spraying time was 5 minutes, so that the enamel glaze formed a uniform coating with a thickness of 0.2mm on the steel plate surface.

[0053] The enameled steel sheet is placed in a high-temperature furnace for firing. First, the furnace temperature is raised to 800°C at a heating rate of 5°C / minute and held for 15 minutes. Then, the furnace temperature is lowered to 500°C at a cooling rate of 3°C / minute, and then allowed to cool naturally to room temperature. During the firing process, a weak oxidizing atmosphere is maintained inside the furnace to prevent excessive oxidation of the steel sheet.

[0054] Post-processing and testing: After firing, the enamel steel is inspected and found to have a smooth and even surface, free from obvious defects such as bubbles, pinholes, and peeling. After 18 months of exposure to heavy rain and strong ultraviolet radiation in a real outdoor environment, the enamel layer showed only very slight color changes, with no powdering or peeling. Its impact resistance meets the highest level requirements of GB / T2520-2017 standard, fully satisfying the needs of use in harsh outdoor environments.

[0055] Example 3: The preparation of the steel plate substrate for high weather-resistant enamel steel includes the following steps: 1) Steelmaking: Steel is decarburized in a converter to 0.03% before tapping. Alloying materials are added to achieve the following composition (mass percentage): C: 0.004%, Mn: 0.25%, Si: 0.04%, P: 0.007%, S: 0.004%, Cr: 0.8%, Ni: 0.4%, Ti: 0.02%, B: 0.0008%, with the balance being Fe and unavoidable impurities. Continuous casting speed is 1.5 m / min, and the crystallizer water flow is 200 m³ / min. 3 / h.

[0056] 2) Hot rolling: The slab is heated to 1250℃, held for 2 hours, with a total reduction of 90%, a final rolling temperature of 900℃, a cooling rate of 20℃ / s, and then cooled to 600℃ before being coiled.

[0057] 3) Cold rolling: Pickled in 15% hydrochloric acid solution (80℃) for 5 minutes, with a total cold rolling reduction of 80%.

[0058] 4) Continuous annealing: Hold at 850℃ for 30 seconds with 10% hydrogen content, then cool slowly to 400℃.

[0059] Enamel steel coating and firing methods: The same steel plate pretreatment, enamel glaze preparation (weather-resistant additives adjusted to 3%, and nano-titanium dioxide to nano-zinc oxide mass ratio 2:1), coating and firing processes as in Example 1 were used.

[0060] Post-treatment and testing: After 500 cycles of spraying in a simulated acid rain environment (a mixed solution with pH=3.5), the enamel layer showed no corrosion spots, and the substrate showed no rust. Acid resistance was significantly improved, and according to GB / T21866-2008 "Determination of Antibacterial Properties and Antibacterial Effects of Antibacterial Coatings (Films)", the surface antibacterial rate (against Escherichia coli) reached 99.5%. Suitable for special environments such as chemical and food processing industries.

[0061] Example 4: The preparation of the steel plate substrate for high weather-resistant enamel steel includes the following steps: 1) Steelmaking: Steel is decarburized in a converter to 0.028% before tapping. Alloying materials are added to achieve the following composition (mass percentage): C: 0.0015%, Mn: 0.35%, Si: 0.015%, P: 0.005%, S: 0.003%, Cr: 1.1%, Ni: 0.7%, Ti: 0.035%, B: 0.0012%, with the balance being Fe and unavoidable impurities. Continuous casting speed is 1.3 m / min, and the crystallizer water flow rate is 170 m³ / min. 3 / h.

[0062] 2) Hot rolling: The slab is heated to 1220℃, held for 2.2 hours, with a total reduction of 88%, a final rolling temperature of 870℃, a cooling rate of 18℃ / s, and then cooled to 570℃ before being coiled.

[0063] 3) Cold rolling: Pickled in 13% hydrochloric acid solution (75℃) for 7 minutes, with a total cold rolling reduction of 75%.

[0064] 4) Continuous annealing: Hold at 800℃ for 45 seconds with 7% hydrogen content, then cool slowly to 370℃.

[0065] Enamel steel coating and firing methods: Subsequent steps: steel plate pretreatment, enamel glaze preparation (weather-resistant additive 4%, nano-titanium dioxide to nano-zinc oxide mass ratio 1:2), coating and firing process are the same as in Example 1.

[0066] Post-processing and testing: After being placed in a high temperature and high humidity environment (60℃, 95% relative humidity) for 1000 hours, the enamel steel showed no blistering or cracking. After the thermal shock stability test (50 cycles from -40℃ to 120℃), its performance did not degrade, which can meet the usage requirements of tropical climates and high temperature equipment housings.

[0067] The mechanical properties of each embodiment are shown in Table 3 below: Table 3: Mechanical properties of the examples Example Tensile strength (MPa) Yield strength MPa <![CDATA[Elongation A 50 (%)]]> n value R value 1 373 191 34 0.23 1.6 2 367 202 32.5 0.24 1.6 3 352 184 35 0.22 1.6 4 382 215 32 0.25 1.7 The aforementioned properties are achieved through composition optimization (synergistic effects of alloys such as Cr, Ni, and Ti) and process control (precise rolling and annealing). This ensures good formability of the steel plate (high elongation and suitable n / R value), meets the requirements of complex coating and enamel processing, and also has the strength level to match outdoor structural components. This, combined with high weather resistance, creates a synergistic advantage and meets the licensing requirements for the practicality of the technical solution.

[0068] Four embodiments were conducted to verify the effectiveness of the proposed solution under different environmental conditions (strong outdoor ultraviolet radiation, rainy and humid conditions, acid corrosion, and high-temperature alternating conditions). The results showed that the enamel steel exhibited significantly improved weather resistance and excellent overall performance, including bonding strength and impact resistance. The process of this invention is compatible with existing production lines and can be widely applied in fields such as building curtain walls, appliance housings, and chemical equipment, yielding significant economic and social benefits.

[0069] This invention achieves a breakthrough in weather resistance through multi-dimensional optimization: In the steel substrate composition, the content of basic elements such as C, Mn, and Si is precisely controlled, and alloying elements such as Cr, Ni, Ti, and B are added to form a dense oxide film and optimize the microstructure; nano-level weather-resistant additives are introduced into the enamel glaze to enhance ultraviolet shielding and chemical stability. In terms of the manufacturing process, the parameters of the entire process—steelmaking (converter + continuous casting with precise speed control), hot rolling (gradient reduction + precise temperature control), cold rolling (efficient pickling + precise thickness control), and continuous annealing (atmosphere conditioning + segmented cooling)—are refined to ensure stable substrate performance. Through electrostatic spraying and gradient firing processes, a strong bond is formed between the enamel layer and the steel plate.

Claims

1. A high weather-resistant enamel steel, characterized in that, Includes a steel plate substrate and an enamel layer bonded to its surface; The chemical composition of the steel plate substrate, expressed as a percentage by mass, is as follows: C: 0.001%~0.005%, Si: 0.01%~0.05%, Mn: 0.2%~0.5%, P≤0.01%, S≤0.005%, Cr: 0.5%~1.5%, Ni: 0.2%~0.8%, Ti: 0.01%~0.05%, B: 0.0005%~0.002%, balance being Fe and unavoidable impurities; The raw material for the enamel layer includes enamel glaze, and the chemical composition of the enamel glaze by mass percentage is: SiO2: 45%–60%, Al2O3: 10%–20%, B2O3: 8%–15%, Na2O+K2O: 5%–10%, CaO+MgO: 3%–8%, weather-resistant additives: 1%–5%.

2. The high weather-resistant enamel steel according to claim 1, characterized in that, The weather-resistant additive is a composite additive of nano-titanium dioxide and nano-zinc oxide.

3. The high weather-resistant enamel steel according to claim 1, characterized in that, The tensile strength of the enamel steel is 350–420 MPa; the yield strength is 180–240 MPa; and the elongation is determined by A. 50 Gauge length: ≥30%; Work hardening index n: 0.20~0.28; Plastic strain ratio R: 1.4~2.0; Under simulated outdoor conditions: Irradiance 500W / m 2 After 1000 hours of continuous testing at 40℃ and 80% relative humidity, the enamel layer showed no fading, powdering, or peeling.

4. A method for preparing a high weather-resistant enamel steel according to any one of claims 1-3, characterized in that, Includes the following steps: 1) The preparation of steel plate substrate includes steelmaking, continuous casting, hot rolling, cold rolling, and continuous annealing; 2) Pretreatment: The steel plate substrate undergoes surface degreasing, rust removal, and cleaning. 3) Enameling: Enamel glaze is evenly sprayed onto the surface of the pretreated steel plate substrate to form a glaze coating with a thickness of 0.2-0.3 mm; 4) Firing: The enameled steel plate is kept at 800-850℃ for 10-15 minutes and then cooled to form an enamel layer on the surface of the steel plate substrate.

5. The method for preparing a high weather-resistant enamel steel according to claim 4, characterized in that, The method for preparing the steel plate substrate includes: 1) Steelmaking: In the process of steelmaking using a converter, molten iron is poured into the converter, and slag-forming agents are added to carry out decarburization, dephosphorization, and desulfurization. When the carbon content in the molten steel drops to 0.02% to 0.03% by mass, the steel is tapped. During the tapping process, argon gas is blown into the ladle to stir the steel, and alloys are added to the ladle. The molten steel is allowed to stand in the ladle for 5 to 10 minutes before continuous casting. 2) Continuous casting: Control the pulling speed to 1.0–1.5 m / min, and the crystallizer water flow rate to 150–200 m³ / min. 3 / h; 3) Hot rolling: Heat the continuously cast slab to 1150-1250℃ and hold for 2-3 hours; Multi-pass rolling is adopted, with the reduction rate of the first pass being 33.3%~50%, and the reduction rate of each subsequent pass gradually decreasing, with the total reduction rate controlled at 80%~90%. The final rolling temperature is controlled at 850-900℃, and then laminar flow cooling is carried out at a cooling rate of 10-20℃ / s. The steel plate is then coiled after being cooled to 550-600℃. 4) Cold rolling: The hot-rolled coil is pickled using a hydrochloric acid solution with a mass fraction of 10% to 15%, at a temperature of 60 to 80°C, for a pickling time of 5 to 10 minutes. After pickling, the hot-rolled plate is cold-rolled using a multi-roll mill, with the total reduction rate controlled at 60% to 80%, and the thickness accuracy of the cold-rolled steel plate controlled at ±0.01mm. 5) Continuous annealing: The annealing temperature is 750–850℃, and the holding time is 30–60 seconds; During the annealing process, the atmosphere inside the furnace is a mixture of hydrogen and nitrogen, with hydrogen accounting for 5% to 10% by volume. After annealing, the material is cooled. First, it is rapidly cooled to 300-400°C at a rate of 20-30°C / s, and then slowly cooled to room temperature at a rate of 5-10°C / s.

6. The method for preparing a high weather-resistant enamel steel according to claim 4, characterized in that, The steel plate substrate has a tensile strength of 340–400 MPa, a yield strength of 170–230 MPa, and an elongation of A. 50 ≥32%; the metallographic structure is single-phase ferrite, with equiaxed ferrite grains, grain size grade I~II9, average grain diameter 10~15μm, and inclusion grades BT1.0, DT0.5, and DTTiN1.

0. The substrate is subjected to simulated outdoor conditions with a light intensity of 500W / m². 2 At a temperature of 40℃ and a relative humidity of 80%, no rust was observed after 500 hours of exposure.

7. The method for preparing a high weather-resistant enamel steel according to claim 4, characterized in that, The pretreatment involves a combination of alkaline and acid pickling to degrease and remove rust from the steel plate surface. First, the steel plate substrate is immersed in a 5%–10% sodium hydroxide solution at 60–80°C for 10–15 minutes. Then, the substrate is transferred to a 10%–15% hydrochloric acid solution and pickled at room temperature for 5–10 minutes. After pickling, it is rinsed thoroughly with clean water, then immersed in a 2%–5% sodium carbonate solution for neutralization. Finally, it is rinsed with deionized water and dried.

8. The method for preparing a high weather-resistant enamel steel according to claim 4, characterized in that, Step 3) The spraying voltage is 60-80kV, the distance between the spray gun and the steel plate substrate is 150-200mm, and the spraying time is 3-5 minutes.

9. The method for preparing a high weather-resistant enamel steel according to claim 4, characterized in that, In step 4), the coated steel plate is placed in a high-temperature furnace, the furnace temperature is raised to the firing temperature at a heating rate of 5-10℃ / min and held at that temperature, and then the furnace temperature is lowered to 500-550℃ at a cooling rate of 3-5℃ / min, and then naturally cooled to room temperature; the oxygen content in the furnace is controlled at 2%-5%.

10. The method for preparing a high weather-resistant enamel steel according to claim 4, characterized in that, Preparation of the enamel glaze: Weigh each raw material according to the chemical composition of the enamel glaze, mix them evenly and send them into a ball mill, then add water and grinding media accounting for 20% to 30% of the total mass of the raw materials, and ball mill for 10 to 15 hours to obtain enamel glaze slurry; spray dry the enamel glaze slurry under the conditions of inlet air temperature of 180 to 220℃ and outlet air temperature of 80 to 100℃ to make dry powder enamel glaze, and the particle size of the dry powder is controlled at 50 to 100 μm.

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