Preparation method of weather-resistant polyester coating guardrail free of pickling and galvanizing

Abstract

The application provides a preparation method of an acid-free and galvanization-free weather-resistant polyester coating guardrail, and the production process route is as follows: steel member forming, shot blasting, surface spraying and solidification, cooling, inspection and packaging; the production process does not need acid pickling and hot-dip galvanization. The process route of the steel member forming is as follows: blast furnace molten iron smelting, molten iron desulfurization pretreatment, converter molten steel smelting, LF molten steel refining treatment and slab continuous casting and hot rolling. Through reasonable design of weather-resistant elements, the acid-free and galvanization-free weather-resistant plastic spraying guardrail is realized, shot blasting is used to replace acid pickling, acid-free production is realized, pollution and production cost are greatly reduced, and the processing cycle is shortened.

Description

Technical Field

[0001] This invention relates to the field of steel production, specifically to a method for preparing a weather-resistant polyester-coated guardrail that requires no pickling or galvanizing. Background Technology

[0002] As an important component of road traffic safety infrastructure, the highway guardrail industry has shown steady development in recent years, accompanying the continuous advancement of national highway network construction and the constant improvement of traffic safety standards. In modern highway construction, semi-rigid guardrails, represented by corrugated beam steel guardrails, are the most widely used.

[0003] The base material of corrugated beam steel guardrails is mostly ordinary carbon steel Q235B. In order to overcome the corrosion of steel materials during use and extend the service life of steel guardrails, all components of steel guardrails are required to undergo anti-corrosion treatment. At present, the most commonly used anti-corrosion methods for corrugated beam steel guardrails in China are hot-dip galvanizing single coating and hot-dip galvanizing polyester composite coating.

[0004] GB / T18226, "Technical Conditions for Corrosion Protection of Steel Components in Highway Traffic Engineering," requires that hot-dip galvanizing coating be achieved by immersing the steel component in molten zinc, allowing dissolution, chemical reaction, and diffusion between the steel substrate and the molten zinc to form a coating. The minimum thickness of the zinc layer is 84 μm. Hot-dip galvanized polyester composite coating typically involves electrostatic spraying an additional layer of polyester coating onto the hot-dip galvanized component, resulting in a double-layer coating for the steel component consisting of an inner hot-dip galvanized coating and an outer polyester coating. The minimum thickness of the zinc layer is 39 μm, and the minimum thickness of the polyester coating is 76 μm.

[0005] Hot-dip galvanizing of metal steel components causes severe environmental pollution, and has low production efficiency and high energy consumption. Before galvanizing, the components need to be pickled, followed by high-temperature hot-dip galvanizing. The entire production process generates a large amount of HCl, NH3, and zinc oxide dust. The actual production conditions are harsh, causing great environmental pollution.

[0006] In summary, the existing technology has the following problems: highway guardrails require galvanizing during production, which causes significant environmental pollution. Summary of the Invention

[0007] This invention provides a method for preparing weather-resistant polyester-coated guardrails that are free from acid washing and galvanizing, thereby solving the problem of severe environmental pollution caused by the galvanizing production of highway guardrails.

[0008] Therefore, this invention proposes a method for preparing a weather-resistant polyester-coated guardrail that requires no pickling or hot-dip galvanizing. The production process is as follows: steel component forming, shot blasting, surface spraying and curing, cooling, inspection and packaging. The production process does not require pickling or hot-dip galvanizing.

[0009] The process route for forming the steel components is as follows: blast furnace molten iron smelting, molten iron desulfurization pretreatment, converter molten steel smelting, LF molten steel refining treatment, slab continuous casting, and hot rolling.

[0010] The chemical composition of the steel component by weight percentage is as follows: C: 0.05 wt%~0.09 wt%, Si: 0.1 wt%~0.35 wt%, Mn: 0.5 wt%~1.2 wt%, P: 0.08 wt%~0.12 wt%, S: ≤0.01 wt%, Al: 0.020%~0.050 wt%, Nb: ≤0.05 wt%, Ti: 0.015 wt%~0.090 wt%, Ni: ≤0.65 wt%, Cr: 0.3 wt%~2.0 wt%, Cu: 0.2 wt%~0.55 wt%, with the balance being Fe and unavoidable trace elements.

[0011] Furthermore, in the hot rolling process, the roughing stage is carried out in the austenite recrystallization zone, the roughing exit temperature is 1050-1080℃, and the reduction rate of the last two passes of the roughing is >30%.

[0012] Furthermore, in the hot rolling process, the finishing rolling temperature is 820-900℃, and the single-pass reduction rate of finishing rolling is increased to >10%, except for the finishing rolling control plate shape pass.

[0013] Furthermore, in the hot rolling process, laminar flow cooling is adopted after rolling, with a temperature control range of 580-640℃ and a forward rapid cooling mode.

[0014] Furthermore, in the converter steelmaking process, the binary basicity R of the slag is controlled at 2~4 and set according to the P balance correction; the smelting process adopts bottom blowing argon gas throughout, and the converter endpoint C is controlled at 0.06~0.1Wt%, P≤0.090Wt.

[0015] Furthermore, in the slab continuous casting, an automatic slag discharge detection and control system is implemented in the ladle; the superheat of the tundish pouring is controlled at 20-30℃, an alkaline covering agent is used in the tundish, and weathering steel protective slag is adopted; the slab casting speed is 0.9m / min~1.3m / min, and the liquid level fluctuation in the crystallizer is automatically controlled, with the fluctuation range controlled within ±3mm; the reduction range is 0.5~1.2, and the reduction amount is 5.0; the electromagnetic stirring parameters are current 430A and frequency 6.0Hz.

[0016] Furthermore, in the LF steel refining process, deoxidation and Al, Mn, Nb, Ti, Cr, and Cu alloying are performed; the binary basicity R of the refining top slag is controlled at 8~12; the molten steel undergoes Ca treatment, controlled at [Ca] / [Als]=0.10~0.14; after the Ca treatment, the ladle is soft-blown with argon for 8~10 minutes; from the end of soft-blowing with argon to the start of continuous casting, the molten steel calming time is ≥18 minutes.

[0017] Furthermore, after the shot blasting process, the surface cleanliness of the workpiece reaches the Sa 2.5 standard, and the average roughness is controlled at 25-40μm.

[0018] Furthermore, in the hot rolling heating process, the target temperature for heat soaking is 1220~1260℃, the total time in the furnace is 120-180min, and the heat soaking period is ≥50min, thereby limiting the initial heating time and the heating rate.

[0019] Furthermore, the thickness of the continuously cast slab is 220-250mm, and the thickness of the hot-rolled finished product is 2-4mm; the tensile strength Rm of the resulting guardrail ranges from 370 to 800MPa.

[0020] This invention also provides a low-cost, zinc-free, high-weathering steel preparation method. The chemical composition of this steel, by weight percentage, is: C: 0.05 wt%~0.09 wt%, Si: 0.1 wt%~0.35 wt%, Mn: 0.5 wt%~1.2 wt%, P: 0.08 wt%~0.12 wt%, S: ≤0.01 wt%, Al: 0.020%~0.050 wt%, Nb: ≤0.05 wt%, Ti: 0.015 wt%~0.090 wt%, Ni: ≤0.65 wt%, Cr: 0.3 wt%~2.0 wt%, Cu: 0.2 wt%~0.55 wt%, with the balance being Fe and unavoidable trace elements. This steel grade undergoes blast furnace smelting, desulfurization pretreatment of molten iron, converter smelting, LF steel refining, slab continuous casting, and hot rolling to form finished plates. After steel component forming, shot blasting, surface spraying and curing, cooling, inspection and packaging, it can be used to make highway guardrails. The production process of making guardrails does not require pickling or hot-dip galvanizing.

[0021] The present invention also provides a low-cost, galvanized-free, high-weather-resistant steel, produced using the above-described preparation method.

[0022] Furthermore, the low-cost, galvanized-free, high-weather-resistant steel has a thickness of 3-3.5 mm and a tensile strength Rm ranging from 570 to 780 MPa.

[0023] On the other hand, the present invention also provides a weather-resistant polyester-coated guardrail that is free from pickling and galvanizing, and its production process is as follows: steel component forming, shot blasting, surface spraying and curing, cooling, inspection and packaging; the production process does not require pickling or hot-dip galvanizing.

[0024] The chemical composition of the steel components, by weight percentage, is as follows:

[0025] C: 0.05 wt%~0.09 wt%, Si: 0.1 wt%~0.35 wt%, Mn: 0.5 wt%~1.2 wt%, P: 0.08 wt%~0.12 wt%, S: ≤0.01 wt%, Al: 0.020%~0.050 wt%, Nb: ≤0.05 wt%, Ti: 0.015 wt%~0.090 wt%, Ni: ≤0.65 wt%, Cr: 0.3 wt%~2.0 wt%, Cu: 0.2 wt%~0.55 wt%, balance being Fe and unavoidable trace elements.

[0026] Furthermore, the thickness of the hot-rolled finished product is 2-4 mm, and the tensile strength Rm ranges from 370 to 800 MPa.

[0027] Furthermore, the thickness of the hot-rolled finished product is 3-3.5 mm, and the tensile strength Rm ranges from 570 to 780 MPa.

[0028] This invention achieves galvanization-free use of weather-resistant powder-coated guardrails through the rational design of weather-resistant elements. Simultaneously, shot blasting replaces pickling, enabling pickle-free production, significantly reducing pollution and production costs, and shortening the processing cycle. Furthermore, the other production processes for the pickle-free, galvanization-free weather-resistant powder-coated guardrails are consistent with conventional guardrails, allowing them to share production lines with existing powder-coated guardrails without requiring specific design or production line modifications. Ultimately, this results in a complete set of technologies encompassing component design, steel production processes, and guardrail manufacturing processes, with low-cost, pickle-free, galvanization-free weather-resistant polyester-coated guardrails at its core.

[0029] Furthermore, current hot-dip galvanizing methods are often small-scale workshops, making it difficult to guarantee the quality of galvanized products. Issues such as incomplete galvanizing or thin zinc layers frequently occur. The overall production efficiency of galvanizing steel components is also relatively low, leading to significantly extended delivery cycles. This invention eliminates the need for galvanizing, avoiding galvanizing quality problems, shortening the review cycle, and improving production efficiency.

[0030] Furthermore, the guardrail produced by this invention has high tensile strength, providing more adequate protection compared to existing Q235B carbon steel guardrails. It can also be made thinner than existing Q235B carbon steel guardrails, reducing steel consumption and lowering production costs. Attached Figure Description

[0031] Figure 1The image shows the metallographic structure of the weathering steel of Embodiment 1 of the present invention, magnified 500 times.

[0032] Figure 2 The image shows the metallographic structure of the weathering steel of Embodiment 2 of the present invention, magnified 500 times.

[0033] Figure 3 The image shows the metallographic structure of the weathering steel of Example 3 of this invention, magnified 500 times.

[0034] Figure 4 This is a metallographic photograph of the weathering steel of Example 4 of the present invention, magnified 500 times. Detailed Implementation

[0035] To provide a clearer understanding of the technical features, objectives, and effects of this invention, the invention is now described.

[0036] This invention provides a weather-resistant polyester-coated guardrail that requires no acid washing or galvanizing and its preparation method, specifically including three parts: steel alloy design for the weather-resistant powder-coated guardrail, steel production process, and guardrail production process.

[0037] 3.1 Alloy Design

[0038] By employing a low-C, low-S, high-P composition design, matching Ni / Cr / Cu, and Nb-Ti composites, a balance of high strength, high toughness, and high weather resistance is achieved in weather-resistant guardrail steel at a low cost. Furthermore, shot blasting of guardrail samples made from weather-resistant steel improves surface roughness, thereby enhancing the adhesion between the powder coating and the sample, ultimately replacing galvanizing.

[0039] The adopted technical solution is a low-cost, galvanized-free, high weather-resistant steel and its preparation method. The chemical composition of this steel is as follows (weight percentage): C: 0.05 wt%~0.09 wt%, Si: 0.1 wt%~0.35 wt%, Mn: 0.5 wt%~1.2 wt%, P: 0.08 wt%~0.12 wt%, S: ≤0.01 wt%, Al: 0.020%~0.050 wt%, Nb: ≤0.05 wt%, Ti: 0.015 wt%~0.090 wt%, Ni: ≤0.65 wt%, Cr: 0.3 wt%~2.0 wt%, Cu: 0.2 wt%~0.55 wt%, with the balance being Fe and unavoidable trace elements.

[0040] (1) Strengthening elements

[0041] Carbon (C) is the cheapest strengthening element in steel. It promotes the formation of cementite and pearlite, as well as the precipitation of other carbides, increasing the strength of steel through solid solution strengthening, microstructure strengthening, and precipitation strengthening, particularly significantly improving tensile strength and thus reducing the yield strength ratio. However, high carbon content leads to poor microstructure uniformity, causing micro-area galvanic corrosion and reducing weather resistance. Furthermore, C reduces the plasticity of steel, negatively impacting welding performance and low-temperature impact toughness. In particular, the high phosphorus (P) content in the product of this invention further affects the material's plasticity, toughness, and weldability. Therefore, the C content needs to be controlled within a suitable range; in this invention, the C content is 0.05 wt% to 0.09 wt%.

[0042] In addition to improving strength and toughness, manganese (Mn) as an alloying element in steel plates can expand the austenite phase region, lower the Ac1, Ac3, Ar1, and Ar3 temperatures, and refine ferrite grains. However, excessive Mn content can lead to center segregation, reducing the uniformity of mechanical properties and low-temperature toughness of the steel plate, while also decreasing weather resistance. Considering these factors, Mn content should be controlled between 0.5 wt% and 1.2 wt%.

[0043] This invention employs Nb and Ti composite microalloying. By adjusting the content of Nb, Ti, and Mn, different strength levels of weather-resistant highway guardrail steel can be produced. The Nb content of this invention is ≤0.05 wt%, and the Ti content is 0.015 wt%~0.070 wt%.

[0044] Nitrogen (Nb) can increase the TNR temperature, delay the recrystallization of deformed austenite, refine the grains, and improve the strength and toughness of steel plates. If the Nb content is too low, it cannot effectively exert the rolling control effect in the non-recrystallized region and the two-phase region, resulting in insufficient strengthening ability. If the content is too high, it will increase production costs and affect weldability.

[0045] Ti is a strong carbonitride forming element and the most economical and effective grain refiner and precipitation strengthening element. Furthermore, the combined addition of Ti and P can reduce P grain boundary segregation by forming a certain amount of iron-titanium-phosphorus phase, thus promoting the weather-resistant effect of P. However, adding too much Ti will coarsen the TiN particles, reducing the low-temperature toughness of the steel plate.

[0046] (2) Weathering elements

[0047] The present invention comprises P: 0.08%~0.15%, Si: 0.1wt%~0.35wt%, Cr: 0.3wt%~2.0wt%, Cu: 0.2wt%~0.55wt%, Ni: ≤0.65wt%. Through the rational design of weather-resistant elements, the weather-resistant powder-coated guardrail can be used without galvanizing.

[0048] Pollen (P) is one of the most effective alloying elements for improving the atmospheric corrosion resistance of steel. Generally, the best corrosion resistance is achieved when the P content is controlled between 0.08% and 0.15%. For atmospheric corrosion, P can depolarize the anolyte, accelerate the uniform dissolution of steel and the oxidation rate of iron, help form a uniform FeOOH rust layer on the steel surface, and simultaneously contribute to the formation of a dense Fex(OH)3-2x protective film, increasing electrical resistance, hindering the entry of corrosive media into the matrix, and preventing internal corrosion. In the presence of Cu and P, the resulting composite salt simultaneously becomes the crystallization nucleus of FeOOH, refining the grain size of the inner rust layer to achieve density. Given that P is prone to macroscopic segregation, controlling its content in the lower to middle limits and appropriately increasing the Si content to accelerate the solidification rate of the billet can reduce segregation and promote P uphill diffusion, which is beneficial for its enrichment on the billet surface, thereby improving its corrosion resistance.

[0049] Si can play a role in solid solution strengthening and is also a good deoxidizer in the smelting process. Increasing the Si content can refine the α FeOOH in the inner rust layer of weathering steel, increase the rust layer resistance, and help improve the weather resistance of the steel. While increasing the Si content, increasing the P content can also effectively suppress red rust defects on the steel surface through their synergistic effect. On the other hand, it can suppress the enrichment of Cu elements in the low-temperature section during the slab heating process and promote the diffusion of Cu elements in the high-temperature section, thereby reducing the amount of Cu enriched at the unit interface between the iron sheet and the steel matrix and thus suppressing copper embrittlement. However, Si seriously impairs the low-temperature toughness and weldability of the steel plate. Therefore, the Si content should not be controlled too high. Considering the economy and operability of the steelmaking process, the Si content is controlled at 0.1 wt% to 0.35 wt%.

[0050] Cr can form a dense oxide film on the steel surface, significantly improving the steel's passivation ability. The effect is even better when Cr and Cu are added simultaneously. During the corrosion process, Cr also accumulates on the substrate surface, forming a multi-element alloy oxide of iron, chromium, and copper, filling the microcracks and pores in the rust layer, thereby increasing the density of the rust layer. Cr replaces part of the Fe in α-FeOOH, giving the rust layer cation-selective permeability, inhibiting the entry of Cl-, and also inhibiting the reduction of Fe3+ to Fe2+ during the alternating wet and dry process, slowing down the corrosion process and improving the weather resistance of the steel. In addition, among Ni / Cr / Cu weather-resistant alloys, Cr alloys have the lowest cost. This invention specifically incorporates a high-P design, and then optimizes the Cr content to significantly reduce the amount of Ni / Cu precious alloys added. The Cr content is set at 0.3 wt%~2.0 wt%.

[0051] Cu has a significant impact on weather resistance, playing a major role in inhibiting the dissolution rate of iron and slowing down the electrochemical process. Several theories exist regarding the mechanism by which Cu improves weather resistance. One theory suggests that Cu acts as an active cathode during atmospheric corrosion, promoting anodic passivation. Another theory proposes that during rust formation, copper accumulates between the rust layer and the steel substrate, effectively preventing corrosive media from contacting the steel substrate and inhibiting further corrosion. Additionally, the presence of Cu combines with sulfur to form refractory sulfides, effectively preventing hot brittleness. However, when the copper content is too high, due to copper's low melting point (below the billet heating temperature), the precipitated copper accumulates in a liquid state at the austenite grain boundaries. When the precipitated copper content reaches a certain level, cracks are easily generated during heating or hot rolling.

[0052] Ni shifts the self-corrosion potential of steel in the positive direction and can also hinder Cl- penetration into the matrix, enhancing the weather resistance of the steel. Simultaneously, Ni plays a role in solid solution strengthening and improves the low-temperature performance and toughness of steel by refining the grain size. Literature indicates that adding a certain amount of Ni to Cu-containing steel can, on the one hand, increase the solid solubility of Cu in the steel, reducing Cu precipitation and intrusion into grain boundaries; on the other hand, it can increase the melting point of the Cu-Ni alloy, reducing the intrusion of liquid-phase Cu alloy into the matrix. With the increase of the Ni / Cu mass fraction ratio, the incidence of copper embrittlement defects during rolling is significantly reduced. However, Ni is a precious alloy, and adding Ni will lead to a significant increase in cost. This invention is designed to add less or no Ni, controlling copper embrittlement and peeling through the rolling process.

[0053] (3) Other elements

[0054] Al in steel can fix free nitrogen (N), reduce free N in the weld heat-affected zone (HAZ), and promote ferrite precipitation during welding cooling cycles, thereby improving the low-temperature impact toughness of the HAZ. However, excessive Al will form a large number of dispersed needle-like Al₂O₃ inclusions in the steel, impairing the low-temperature impact toughness and weldability of the steel plate. According to the steel plate composition system analysis, the Alt content of this invention is 0.020% to 0.050 wt%.

[0055] S forms sulfide inclusions, reducing the ductility and toughness of steel. It also acts as a source of pitting corrosion, leading to poor weather resistance. In particular, the high P content in the product of this invention further affects the material's ductility, toughness, and weldability. Therefore, the S content needs to be controlled within a low range; in this invention, S ≤ 0.01 wt%.

[0056] 3.2 Steel Production Process

[0057] The steel production process is as follows: blast furnace molten iron smelting → molten iron desulfurization pretreatment → converter molten steel smelting → LF molten steel refining treatment → slab continuous casting → hot rolling → warehousing.

[0058] 3.2.1 Smelting process

[0059] Hot metal desulfurization pretreatment: KR desulfurization is adopted, and the sulfur content of the hot metal entering the furnace is controlled to be ≤0.010Wt.

[0060] Converter steelmaking: The binary basicity R (CaO / Al2O3) of the slag is controlled at 2~4, and adjusted according to the P balance. The smelting process uses bottom blowing argon gas throughout, and the converter endpoint C: 0.06~0.1Wt%, P≤0.090Wt.

[0061] LF steel refining process: Deoxidation and alloying processes such as Al, Mn, Nb, Ti, Cr, and Cu are carried out. The binary basicity R (CaO / Al2O3) of the refining top slag is controlled at 8~12. The molten steel is treated with Ca, and the ratio of [Ca] / [Als] is controlled at 0.10~0.14. After the Ca treatment, the ladle is soft-blown with argon for 8~10 minutes. The quenching time of the molten steel in the ladle between the end of soft-blowing with argon and the start of casting is controlled at ≥18 minutes.

[0062] Slab continuous casting: Automatic slag discharge detection and control in the ladle is required. The superheat temperature in the tundish is 20–30°C. An alkaline covering agent is used in the tundish, along with weathering steel protective slag. The slab casting speed is 0.9 m / min to 1.3 m / min. Automatic control of the liquid level fluctuation in the crystallizer is employed, with the fluctuation range controlled within ±3 mm. Dynamic light reduction is used, with a reduction range of 0.5–1.2 mm and a reduction amount of 5.0 mm. Electromagnetic stirring is employed at 430 A, 6.0 Hz. The continuous casting slab thickness is 220–250 mm.

[0063] 3.2.2 Hot rolling process

[0064] The target temperature for heating and homogenization is set at 1220~1260℃ to ensure that the billet is fully austenitized and most of the alloying elements are fully dissolved, in order to prepare for obtaining a uniform and refined microstructure and second-phase particles. The total time in the furnace is 120-180 min, and the time of the homogenization section is limited to ≥50 min, thereby limiting the time and heating rate of the initial heating stage.

[0065] The roughing stage is performed in the austenite recrystallization zone, with an exit temperature of 1050-1080℃. Increasing the single-pass reduction rate, with the last two passes having a reduction rate >30%, allows for a lower roughing exit temperature design, enabling the addition of more descaling passes. Multiple descaling passes remove the liquid copper-rich phase at the interface between the scale and the steel matrix along with the scale. Increasing the reduction rate in the last three roughing passes reduces the harmful effects of the liquid copper-rich phase at the austenite grain boundaries on the steel matrix surface. The positive effect of a roughing exit temperature of 1050℃~1080℃ is that within this temperature range, the exit temperature of the strip after the roughing mill can be controlled below the melting point of the liquid copper-rich phase, 1085℃.

[0066] The finishing rolling stage mainly involves rolling in the non-recrystallized austenite region, avoiding rolling in the partially recrystallized region or the two-phase region. The main purpose is to control the grain structure of the steel plate and avoid coarse grains and mixed grains, which can cause surface quality problems or performance defects. The finishing rolling temperature is 820-900℃, and the single-pass reduction rate is increased (except for the shape control pass). The single-pass reduction rate is >10%, and the rolling temperature is appropriately reduced for thicker specifications.

[0067] During the post-rolling cooling stage, the laminar flow temperature is controlled within the range of 580-640℃, using a forward rapid cooling mode. If the temperature is too low, the precipitation strengthening effect of Ti is weakened, resulting in low tensile strength. If the temperature is too high, the finished product has coarse grains, leading to poor toughness of the steel plate. The pre-cooling method using laminar flow cooling increases the cooling rate after final rolling, enhancing the supersaturation precipitation of Ti in the ferrite during the coiling stage, forming nanoscale precipitation and improving the precipitation strengthening effect.

[0068] 3.3 Fence Manufacturing Process

[0069] The production process for acid-free, galvanized-free, weather-resistant powder-coated guardrails is as follows: steel component forming → shot blasting → surface spraying and curing → cooling → inspection and packaging. The conventional hot-dip galvanized polyester composite coating guardrail production process is: steel component forming → acid pickling → hot-dip galvanizing → surface spraying and curing → cooling → inspection and packaging. Weather-resistant powder-coated guardrails replace the acid pickling process with shot blasting, eliminating the hot-dip galvanizing process. Shot blasting avoids the contamination caused by acid pickling and improves the surface roughness of the steel components, resulting in stronger adhesion between the substrate and the coating. Eliminating the hot-dip galvanizing process significantly reduces contamination and production costs, while also shortening the processing cycle. After shot blasting, the surface cleanliness of the workpiece must meet the Sa 2.5 standard, with an average roughness controlled between 25-40μm. Other production processes for acid-free, galvanized-free, weather-resistant powder-coated guardrails are consistent with conventional guardrails, requiring no special design or production line modifications.

[0070] Currently, powder-coated steel sheets require semi-galvanization. The weather-resistant steel powder-coated guardrail of this invention does not require semi-galvanization; the coating is applied directly to the steel sheet.

[0071] The surface coating of this invention uses polyester powder; existing semi-galvanized sheets require coating, while full-thickness galvanized sheets do not, but both are relatively expensive and cause pollution.

[0072] IV. Effects

[0073] The low-cost, acid-free, zinc-free weather-resistant steel + polyester-coated guardrail obtained by this invention has a tensile strength (Rm) ranging from 370 to 800 MPa, covering the requirements of guardrail steel of various strengths. It employs high phosphorus (P) and reduces the content of nitrogen (Ni), chromium (Cr), and copper (Cu), lowering the content of precious alloying elements. Through a combination of low carbon content and controlled rolling and cooling, it avoids the cold brittleness caused by high P content. The rational design of weather-resistant elements enables the zinc-free use of the weather-resistant powder-coated guardrail. Simultaneously, shot blasting replaces acid pickling, achieving acid-free production, significantly reducing pollution and production costs, and shortening the processing cycle. Furthermore, the other production processes for the acid-free, zinc-free, weather-resistant powder-coated guardrail are consistent with conventional guardrails, allowing it to share production lines with existing powder-coated guardrails without requiring special design or production line modifications. Ultimately, this invention forms a complete set of technologies for component design, steel production processes, and guardrail production processes, with low-cost, acid-free, zinc-free, weather-resistant polyester-coated guardrails as the core. It achieves good weather resistance and strength while maintaining low cost, reaching a good balance between weather resistance and cost. Specific Implementation

[0074] This invention discloses a weather-resistant polyester-coated guardrail that requires no pickling or galvanizing and its preparation method. It is manufactured using 600MPa-grade low-cost, galvanized-free, high-weather-resistant steel and its preparation method. The 600MPa-grade low-cost, galvanized-free, high-weather-resistant steel and its preparation method employ the following component ratios and specific processes. Table 1 shows the composition of the steel in each embodiment (by weight percentage). Table 2 shows the process parameters corresponding to the steel in the embodiments described in Table 1. Table 3 shows the comprehensive performance corresponding to the steel compositions in each embodiment described in Table 1.

[0075] Table 1. Chemical composition of the product (Wt%)

[0076] Example Finished product thickness / mm C Si Mn P S Alt Nb Ti Cr Cu Ni Example 1 3.15 0.078 0.30 0.51 0.0954 0.0010 0.034 0.002 0.019 0.72 0.31 0.13 Example 2 3.15 0.069 0.21 0.74 0.091 0.0011 0.040 0.016 0.033 0.75 0.32 0.11 Example 3 3.15 0.060 0.24 0.86 0.088 0.0009 0.038 0.025 0.045 0.73 0.33 0.12 Example 4 3.15 0.055 0.28 0.95 0.085 0.0008 0.036 0.030 0.051 0.71 0.32 0.13

[0077] Table 2 Specific smelting process parameters for each embodiment

[0078] Example Finished product thickness / mm C Si Mn P S Alt Nb Ti Cr Cu Ni Example 1 3.15 0.078 0.30 0.51 0.0954 0.0010 0.034 0.002 0.019 0.72 0.31 0.13 Example 2 3.15 0.069 0.21 0.74 0.091 0.0011 0.040 0.016 0.033 0.75 0.32 0.11 Example 3 3.15 0.060 0.24 0.86 0.088 0.0009 0.038 0.025 0.045 0.73 0.33 0.12 Example 4 3.15 0.055 0.28 0.95 0.085 0.0008 0.036 0.030 0.051 0.71 0.32 0.13

[0079] Table 3 Specific rolling process parameters for each embodiment

[0080] Example Finished product thickness / mm Isotropic temperature / ℃ Furnace time Heat equalization time Reduction rate of the last pass in roughing rolling Roughing reduction rate in the last pass Roughing mill exit temperature / ℃ Final rolling temperature / ℃ Final cooling temperature / ℃ Example 1 3.15 1241 146 55 32.1 34.2 1071 883 615 Example 2 3.15 1252 141 58 32.2 34.1 1073 878 608 Example 3 3.15 1255 155 56 32.5 34.2 1065 889 610 Example 4 3.15 1261 143 54 32.5 34.0 1074 895 612

[0081] Table 4 shows the overall performance obtained from each embodiment.

[0082] Serial Number Specifications / mm ReL / MPa Rm / MPa A / % 180° bending test (D=a) Example 1 3.15 458 575 33 qualified Example 2 3.15 543 654 27 qualified Example 3 3.15 604 729 23 qualified Example 4 3.15 658 775 20.5 qualified

[0083] Table 4 illustrates that weather-resistant + powder-coated guardrails are applicable to guardrails of various strengths, making them widely used. They offer significantly improved strength compared to existing Q235B guardrails and allow for lightweight design.

[0084] The metallographic structures corresponding to each example are as follows: Figure 1 , Figure 2 , Figure 3 and Figure 4 As shown. The metallographic structures of the above four embodiments are all ferrite + pearlite structures, with grain sizes of 10, 10.5, 11, and 11, respectively.

[0085] The above description is merely an illustrative embodiment of the present invention and is not intended to limit the scope of the invention. The various components of the present invention can be combined with each other without conflict. Any equivalent changes and modifications made by those skilled in the art without departing from the concept and principles of the present invention should fall within the scope of protection of the present invention.

Claims

1. A method for preparing a weather-resistant polyester-coated guardrail that requires no acid washing or galvanizing, characterized in that, The preparation method of the acid-free, zinc-free, weather-resistant polyester coated guardrail includes: The production process for guardrails is as follows: steel component forming, shot blasting, surface coating and curing, cooling, and inspection and packaging; no pickling or hot-dip galvanizing is required. The production process route for forming steel components is as follows: blast furnace molten iron smelting, molten iron desulfurization pretreatment, converter molten steel smelting, LF molten steel refining treatment, slab continuous casting, and hot rolling. The chemical composition of the steel used in the steel components, by weight percentage, is as follows: C: 0.05 wt%~0.09 wt%, Si: 0.1 wt%~0.35 wt%, Mn: 0.5 wt%~1.2 wt%, P: 0.08 wt%~0.12 wt%, S: ≤0.01 wt%, Al: 0.020%~0.050 wt%, Nb: ≤0.05 wt%, Ti: 0.015 wt%~0.090 wt%, Ni: ≤0.65 wt%, Cr: 0.3 wt%~2.0 wt%, Cu: 0.2 wt%~0.55 wt%, balance being Fe and unavoidable trace elements.

2. The method for preparing the acid-free, zinc-free, weather-resistant polyester coated guardrail as described in claim 1, characterized in that, In the hot rolling process, the roughing stage is carried out in the austenite recrystallization zone, with an exit temperature of 1050-1080℃ and a reduction rate of >30% in the last two passes of the roughing.

3. The method for preparing the acid-free, zinc-free, weather-resistant polyester coated guardrail as described in claim 1, characterized in that, In the hot rolling process, the finishing rolling temperature is 820-900℃, and the single-pass reduction rate of finishing rolling is increased to >10%, except for the finishing rolling control plate shape pass.

4. The method for preparing the acid-free, zinc-free, weather-resistant polyester coated guardrail as described in claim 1, characterized in that, In the hot rolling process, during the post-rolling cooling stage, the laminar flow temperature is controlled within the range of 580-640℃, using a forward rapid cooling mode.

5. The method for preparing the acid-free, zinc-free, weather-resistant polyester coated guardrail as described in claim 1, characterized in that, Converter steelmaking: The binary basicity R of the slag is controlled at 2~4 and adjusted according to the P balance. The smelting process adopts bottom blowing argon gas throughout. The converter endpoint C: 0.06~0.1Wt%, P≤0.090Wt.

6. The method for preparing the acid-free, zinc-free, weather-resistant polyester coated guardrail as described in claim 1, characterized in that, Slab continuous casting: Automatic slag discharge detection and control is implemented in the ladle. The superheat of the tundish pouring is set at 20-30℃. Alkaline covering agent is used in the tundish. Weathering steel protective slag is used. The billet casting speed is 0.9m / min~1.3m / min. Automatic control of liquid level fluctuation in the crystallizer is adopted, with the fluctuation range controlled within ±3mm. The reduction range is 0.5~1.2, and the reduction amount is 5.

0. The electromagnetic stirring current is 430A and the frequency is 6.0Hz.

7. The method for preparing the acid-free, zinc-free, weather-resistant polyester coated guardrail as described in claim 1, characterized in that, In the aforementioned LF steel refining process: deoxidation and Al, Mn, Nb, Ti, Cr, and Cu alloying processes are carried out. The binary basicity R of the refining top slag is controlled at 8~12. The molten steel undergoes Ca treatment, controlled at [Ca] / [Als]=0.10~0.

14. After the Ca treatment, the ladle is subjected to soft argon blowing for 8~10 minutes. The molten steel in the ladle is kept still for ≥18 minutes between the end of soft argon blowing and the start of casting in the continuous casting ladle.

8. The method for preparing the acid-free, zinc-free, weather-resistant polyester coated guardrail as described in claim 1, characterized in that, After shot blasting, the surface cleanliness of the workpiece should meet the Sa 2.5 standard, and the average roughness should be controlled within 25-40μm.

9. The method for preparing the acid-free, zinc-free, weather-resistant polyester coated guardrail as described in claim 1, characterized in that, In the hot rolling heating process: the target temperature for homogenization is 1220~1260℃, the total time in the furnace is 120-180min, and the time for homogenization is limited to ≥50min, thereby limiting the time and heating rate of the initial heating stage.

10. The method for preparing the acid-free, zinc-free, weather-resistant polyester coated guardrail as described in claim 1, characterized in that, The thickness of the continuously cast slab is 220-250mm, the thickness of the hot-rolled finished product is 2-4mm, and the tensile strength Rm ranges from 370 to 800MPa.

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