Trivalent chromium coated steel sheet, production method and device

Abstract

The application discloses a trivalent chromium coated steel plate, a production method and a device, and belongs to the technical field of metal materials. The application optimizes the S-shaped roller coating process, designs the trivalent chromium passivation agent, the film weight and the plate temperature drying temperature in combination, selects the roller coating mode according to the thickness of the steel plate, and produces the coated product with excellent surface quality, processing forming performance and corrosion resistance and other process performances, and the surface is free of stains, stripes and the like, the surface is free of blackening after roller pressing and deep drawing, and no white rust is generated on the plate surface after 120h of neutral salt spray testing.

Description

Technical Field

[0001] This invention belongs to the field of metal materials technology, and more specifically, relates to a trivalent chromium coated steel plate, its production method, and its apparatus. Background Technology

[0002] Hot-dip galvanized, hot-dip galvanized aluminum-magnesium and other coated steel sheets are widely used in construction, home appliances and automobiles. Users have increasingly higher requirements for the surface quality of coated steel sheets. As an important post-treatment process for hot-dip coated products, the post-coating process has a great impact on the surface quality of the products and their use by users.

[0003] In hot-dip galvanizing production lines, traditional surface passivation modification treatments mainly include two processes: roll coating and spray drying. Roll coating further includes S-type roll coating and vertical roll coating. Existing trivalent chromium surface passivation agents are highly acidic and react violently with the coating during film formation, classifying them as strongly reactive passivation products. When using the S-type roll coating process, the corrosion products generated by this strong reaction easily crystallize on the roll surface and in the circulation system, causing defects such as passivation streaks and passivation spots on the surface. The trivalent chromium passivated finished product has a relatively high coefficient of friction, which can easily cause frictional blackening of the processed areas during rolling and deep drawing processes, affecting the product's appearance.

[0004] A search revealed that patent CN116288304A discloses a trivalent chromium passivation solution and a method for coating steel plates. This invention uses a trivalent chromium passivation solution, the composition of which is 3%-6% trivalent chromium salt, 1%-2% phosphoric acid, 2%-4% hydrofluoric acid, 3%-4% silane, and also includes 1%-2% sealing agent, with the remainder being deionized water. The passivation solution is used to passivate and coat steel plates using a spray-drying passivation process or an S-roll coating passivation process. By designing the film weight, plate temperature, and drying temperature, the coated steel plate is obtained.

[0005] However, this patent does not explain how to ensure uniform film weight. Chinese patent CN113073317A discloses a passivation film weight control system for galvanized steel strip, including a roller coater and a film thickness gauge. The control system is equipped with four strip guide rollers that turn the horizontally running strip into an upwardly convex frame shape. The roller coater is located on the front side of this frame shape, and the film thickness gauge is located on the rear side of this frame shape. This invention also discloses a method for controlling this system. However, this patent uses a film thickness gauge to control film thickness uniformity.

[0006] For example, patent CN113680629A discloses a method for improving the weight uniformity of the passivation film on the lower surface of a horizontal roll coating machine. Based on an existing S-type horizontal roll coating machine, it adds support rollers and rationally controls the position of the support rollers and the strip's moving speed according to the strip thickness, strip density, and unit tension of the strip. This creates a suitable wrap angle between the strip and the lower coating roller, improving the weight uniformity of the passivation film on the lower surface of the strip and reducing wear on the lower coating roller. However, this method is not suitable for steel plates of different thicknesses. Summary of the Invention

[0007] 1. The problem to be solved

[0008] To address the problem that existing coating technologies cannot be applied to steel plates of different thicknesses, this invention provides a trivalent chromium coated steel plate and its production method, which adjusts the coating parameters according to the thickness of the steel plate to achieve uniform coating on steel plates of different thicknesses.

[0009] Another object of the present invention is to provide an apparatus for implementing the production method, which effectively improves the coating effect of steel plates.

[0010] 2. Technical Solution

[0011] To solve the above problems, the technical solution adopted by the present invention is as follows:

[0012] The trivalent chromium passivation solution used in this invention comprises 3%-6% trivalent chromium salt, 3%-6% polybasic acid, 3%-4% silane, 1%-2% sealing agent, 1‰-2‰ lubricant and 2%-4% defoamer, with the remaining component being water.

[0013] The trivalent chromium salt can be one or more of chromium nitrate, chromium halide, and chromium sulfate; the polybasic acid includes 1%-2% phosphoric acid and 2%-4% hydrofluoric acid; the silane is one or more of aminosilane, vinylsilane, and epoxysilane; the sealing agent is one or more of water-based silicone oil, potassium silicate solution, sodium silicate solution, nano silicate solution, and emulsified paraffin wax solution; the lubricant is polyethylene wax; and the defoamer can be one or more of organosilicon and polyether.

[0014] In this invention, a polybasic acid is added to the trivalent chromium passivation solution, using the trivalent chromium salt as a framework. The polybasic acid reacts with the surface zinc layer to form a passivation film, and the reaction formula is Zn. 2+ +Cr 3+ +(2F - )+PO4 3- →ZnCrF2PO4 forms a passivation film with an interatomic bond between the passivation film and the zinc layer, resulting in a tight, dense, and seamless passivation film with a high degree of surface smoothness. The film is relatively thin and exhibits excellent corrosion resistance, compared to the Cr product formed by a single acid. 3+ +PO4 3-→CrPO4 exhibits excellent corrosion resistance. Preferably, the mass ratio of phosphoric acid to hydrofluoric acid is controlled at 1:2. Silanes remain stable in the passivation film, are insoluble in water, and contribute to increased adhesion, improved coating corrosion resistance, increased coating hardness, and scratch resistance. Polyethylene wax acts as a lubricant, reducing the coefficient of friction on the coated steel surface and improving processing and forming performance. Defoamers reduce the surface tension of water, solutions, and suspensions, preventing foam formation; the pH value is 1-2.

[0015] This invention discloses an apparatus for implementing the above-described production method, comprising a guide roller, a coating roller, and a take-up roller. The coating roller includes an upper coating roller acting on the upper surface of the strip and a lower coating roller acting on the lower surface of the strip. The take-up roller includes an upper take-up roller for supplying liquid to the upper coating roller and a lower take-up roller for supplying liquid to the lower coating roller. A liquid storage tank is disposed below the take-up roller. The bottom and top of the liquid storage tank are connected by a circulation pipe, and a circulation tank is disposed on the circulation pipe for storing passivation liquid.

[0016] Furthermore, a filter screen with a mesh size of ≥1000 is added to the circulation tank to filter out and remove the crystallized products.

[0017] This invention targets a hot-dip galvanizing coating production line that employs an S-type roller coating process for coating products such as pure zinc coating and zinc-aluminum-magnesium coating. This process produces coated products with high surface quality and stable, excellent process performance for the highly reactive trivalent chromium inorganic passivation solution.

[0018] For trivalent chromium passivation films with strong reactivity, traditional roll coating methods are vertical roll coating, where both upper and lower coating rollers use a unidirectional coating process, meaning the linear velocity direction at the contact point between the coating roller and the strip is the same as the strip's running direction. This application uses an S-type roll coating method, where the upper surface of the strip contacts the upper coating roller, allowing for more adjustable parameters, including rotational speed, pressure between the upper take-up roller and the upper coating roller, and pressure between the upper coating roller and the guide roller. The lower surface of the strip contacts the idler roller and the lower coating roller, with fewer adjustable parameters, including rotational speed, the effect of the idler roller's vertical movement on the wrap angle between the lower coating roller and the strip, and pressure between the lower take-up roller and the lower coating roller.

[0019] This application aims to reduce the reaction rate between the trivalent chromium passivation solution and the strip steel, minimize the impact of crystallization products on surface quality, and reduce defects such as passivation streaks in roll coating, thereby achieving the production of high-quality coated products. The production process adopted in this application is as follows:

[0020] Control the temperature of the strip steel to the roll coating section to ≤20℃, add a refrigeration system to the circulating tank and pipeline, control the temperature of the passivation liquid to ≤20℃, and reduce the aggravation of crystallization reaction caused by high temperature.

[0021] The thicker the strip, the slower the production line speed. This is reflected in the passivation film, where thicker strips tend to have longer dwell times and higher surface film weight, while thinner strips have shorter dwell times and lower surface film weight, resulting in uneven passivation film coating on the strip surface. To control the uniformity of the passivation film weight, the roll coating mode is classified according to the strip thickness. For products with a thickness ≤1.0mm, a reverse coating process is adopted. To reduce problems such as uneven passivation and wear of the coating rollers, the distance between the coating roller and the strip is controlled at -1mm to 1mm. The upper take-up roller speed is controlled at 70%-120% and the upper take-up roller pressure at 30-60kg, the upper coating roller speed at 100%-180% and the pressure at 30-60kg, the lower take-up roller speed at 50%-90% and the pressure at 130-180kg, and the lower coating roller speed at 100%-160%. As the strip thickness increases and the production line speed decreases, the take-up roller speed needs to be continuously increased to ensure sufficient liquid on both sides of the coating roller. For products with a thickness greater than 1.0mm, a sequential coating process is used. The distance between the coating roller and the strip is controlled at 0-2mm. Low rotation speed is employed to prevent liquid splashing. The upper take-up roller speed is controlled at 40%-60% and pressure at 90-130kg; the upper coating roller speed at 90%-130% and pressure at -70kg to -40kg; the lower take-up roller speed at 40%-60%; the lower coating roller pressure at 130-160kg; and the lower coating roller speed at 80%-120%. The film weight is controlled at 20-40mg / m² per side. 2 If the film weight is too low, the corrosion resistance will be insufficient; if the film weight is too high, the surface color difference will be obvious. The drying temperature of the steel plate surface can meet the low temperature requirement, which is greater than 60℃ and less than 120℃.

[0022] 3. Beneficial effects

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0024] (1) The production method of the present invention optimizes the S-type roller coating process. For products of different thicknesses, the coating method, the distance between the coating roller and the strip steel, and the rotation speed and pressure of the feeding roller and the coating roller are designed. The resulting product has uniform film weight, excellent surface quality, processing and forming performance and corrosion resistance. The surface is free of spots, streaks, etc., and the surface does not turn black after roll pressing and deep drawing. No white rust is generated on the plate surface after 120h neutral salt spray test.

[0025] (2) The device of the present invention removes the crystallized product by adding a filter screen with a mesh size of ≥1000 mesh in the circulation tank, preventing the crystallized product from adhering to the roller surface and causing defects such as white spots due to friction with the plate surface, thereby effectively improving the surface quality of the coated product. Attached Figure Description

[0026] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. However, it should be understood that these drawings are designed for illustrative purposes only and are not intended to limit the scope of the present invention. Furthermore, unless specifically indicated, these drawings are intended only to conceptually illustrate the structural construction described herein and are not necessarily drawn to scale.

[0027] Figure 1 This is a schematic diagram of the S-type roller coating device.

[0028] Figure 2 The surface condition of Example 1;

[0029] Figure 3 The surface condition after rolling in Example 2;

[0030] Figure 4 For the surface condition of Comparative Example 1;

[0031] Figure 5 For the surface condition of Comparative Example 3;

[0032] Figure 6 The surface condition after roller pressing is shown in Comparative Example 4. Detailed Implementation

[0033] The following detailed description of exemplary embodiments of the invention is taken with reference to the accompanying drawings, which form part of the description and illustrate exemplary embodiments in which the invention may be practiced. While these exemplary embodiments have been described in sufficient detail to enable those skilled in the art to practice the invention, it should be understood that other embodiments may be implemented and various changes may be made to the invention without departing from the spirit and scope thereof. The more detailed description of embodiments of the invention below is not intended to limit the scope of the claimed invention, but is merely illustrative and not restrictive of the description of the features and characteristics of the invention, to suggest the best mode for carrying out the invention, and is sufficient to enable those skilled in the art to practice the invention. Therefore, the scope of the invention is defined only by the appended claims.

[0034] The trivalent chromium passivation solution used in the examples and comparative examples includes 3%-6% trivalent chromium salt, 3%-6% polybasic acid, 3%-4% silane, 1%-2% sealing agent, 1‰-2‰ lubricant and 2%-4% defoamer, with the remaining component being water.

[0035] Its specific components are shown in Table 1. The pH value of the trivalent chromium passivation agent is controlled at 1-2, and the solid content is 10% < 20%.

[0036] Table 1. Components and content of trivalent chromium passivation solution

[0037]

[0038] The products were coated with trivalent chromium passivation solutions according to the examples and comparative examples.

[0039] For products with a thickness of ≤1.0mm, a reverse coating process is adopted. To reduce problems such as uneven passivation and wear of the coating roller, the distance between the coating roller and the strip is controlled from -1mm to 1mm. The upper take-up roller speed is controlled at 70%-120% and the upper take-up roller pressure is controlled at 30-60kg. The upper coating roller speed is controlled at 100%-180% and the pressure is controlled at 30-60kg. The lower take-up roller speed is controlled at 50%-90% and the pressure is controlled at 130-180kg. The lower coating roller speed is controlled at 100%-160%.

[0040] For products with a thickness greater than 1.0mm, a forward coating process is adopted, controlling the distance between the coating roller and the strip to 0-2mm, using a low speed to prevent liquid splashing, controlling the speed of the upper take-up roller to 40%-60% and the pressure to 90-130kg, the speed of the upper coating roller to 90%-130% and the pressure to -70kg to -40kg, the speed of the lower take-up roller to 40%-60%, the pressure of the lower coating roller to 130-160kg, and the speed of the lower coating roller to 80%-120%.

[0041] Control the membrane weight to 20-40 mg / m² per side. 2 The drying temperature of the steel plate surface can meet the low-temperature requirements, which are greater than 60℃ and less than 120℃.

[0042] The specific production process parameters are shown in Table 2.

[0043] Table 2 Production process parameters for the examples and comparative examples

[0044]

[0045] The production method of this invention optimizes the S-roll coating process, and incorporates optimizations to the trivalent chromium passivation agent, film weight, plate temperature, and drying temperature.

[0046] Coating methods: Forward coating refers to the linear velocity direction of the coating roller at the contact point with the strip being the same as the strip's running direction. Backward coating refers to the linear velocity direction of the coating roller at the contact point with the strip being opposite to the strip's running direction. The difference in coating effect between forward and backward coating arises from the different frictional torques generated by the different motion patterns between the coating roller and the strip. Backward coating produces a more uniform film, but it causes greater damage to the coating roller, while forward coating is more prone to passivation liquid spillage. The thicker the strip, the thinner the trivalent chromium inorganic passivation film. If backward coating is used, the frictional torque is high, leading to greater torque fluctuations between the upper and lower coating rollers and resulting in greater equipment damage. Therefore, reverse coating is chosen for products with a thickness ≤1.0mm, while forward coating is chosen for products with a thickness >1.0mm.

[0047] Distance between coating roller and strip: The smaller the distance between the coating roller and the strip, the easier it is for the coating roller to wear out, and the shorter the service life of the coating roller. When using reverse coating for products with a thickness ≤1.0mm, the distance between the coating roller and the strip should be controlled from -1mm to 1mm. When using forward coating for products with a thickness >1.0mm, the distance between the coating roller and the strip should be controlled from 0mm to 2mm.

[0048] The speed and pressure of the feed roller and coating roller: The speed of the feed roller affects the amount of passivating agent carried onto the coating roller, while the speed and pressure of the coating roller affect the amount of passivating agent coated on the strip surface. For products with a thickness ≤1.0mm, the production line speed for thin strips is high, the residence time of the strip in the passivation solution is short, and the passivation film weight is relatively thin. To increase the film weight, the speed of the upper feed roller is controlled at 70%-120%, the pressure of the upper feed roller is 30-60kg, the speed of the upper coating roller is 100%-180%, the pressure is 30-60kg, the speed of the lower feed roller is 50%-90%, the pressure is 130-180kg, and the speed of the lower coating roller is 100%-160%. For products with a thickness greater than 1.0mm, the production line speed for thick strip steel is slow, the strip steel stays in the passivation solution for a long time, and the passivation film is heavier and thicker. At the same time, in order to prevent liquid splashing, the speed of the upper take-up roller is controlled at 40%-60% and the pressure at 90-130kg, the speed of the upper coating roller is controlled at 90%-130% and the pressure at -70kg to -40kg, the speed of the lower take-up roller is controlled at 40%-60%, the pressure of the lower coating roller is controlled at 130-160kg, and the speed of the lower coating roller is controlled at 80%-120%.

[0049] An apparatus for implementing the above-described production method includes a guide roller, a coating roller, and a take-up roller. The coating roller includes an upper coating roller acting on the upper surface of the strip and a lower coating roller acting on the lower surface of the strip. The take-up roller includes an upper take-up roller for supplying liquid to the upper coating roller and a lower take-up roller for supplying liquid to the lower coating roller. A liquid storage tank is provided below the take-up roller. The bottom and top of the liquid storage tank are connected by a circulation pipe. A circulation tank is provided on the circulation pipe for storing passivation liquid. Considering that crystals may appear on the surface of the strip during passivation and fall into the liquid storage tank with the passivation liquid, a filter screen with a mesh size of ≥1000 mesh is added to the circulation tank to filter and remove the crystallized products, preventing the crystallized products from adhering to the roller surface and causing defects such as white spots due to friction with the strip surface.

[0050] Table 3 shows the control uniformity of the passivation film on the upper and lower edges and the middle of the steel plate along the width direction, the process properties such as 120h neutral salt spray corrosion resistance and surface friction coefficient, and the surface quality.

[0051] The coating weight was tested using X-ray fluorescence spectroscopy. The neutral salt spray test was conducted according to GB / T 10125 "Artificial Atmosphere Corrosion Test - Salt Spray Test". The white rust corrosion area was evaluated according to GB / T 6461 "Rating of Specimens and Test Pieces with Metallic and Other Inorganic Coatings on Metallic Substrates after Corrosion Testing". The coefficient of friction was measured according to GB / T 13448 "Test Methods for Color-Coated Steel Sheets and Strips". The surface quality was measured according to GB / T 2518 "Continuously Hot-Dip Galvanized Steel Sheets and Strips". The corrosion resistance test was conducted under neutral salt spray conditions, and the test and evaluation methods were performed according to GB 10125.

[0052] Table 3. Process performance and surface quality

[0053]

[0054]

[0055] The resulting coated steel sheet has a surface coating containing, by weight percentage, 0.12%-12% Al, 0%-3% Mg, ≤0.004% Fe, with the remainder being Zn.

[0056] As shown in Table 3, the steel plate prepared using the process described in this application exhibits uniform film weight on one side and small difference in film weight between the upper and lower surfaces, demonstrating good surface quality. In contrast, the steel plate prepared using the comparative example with the same passivation solution has uneven film weight, resulting in poor surface quality, white spots on the surface, and blackening of the surface after processing.

Claims

1. A method for producing trivalent chromium-coated steel sheet, comprising a roll coating step, characterized in that, The steel plate is coated using an S-type roll coating method based on its thickness, and the passivation solution used for roll coating is a trivalent chromium passivation solution. The trivalent chromium passivation solution used for roller coating includes 3%-6% trivalent chromium salt, 1%-2% phosphoric acid, 2%-4% hydrofluoric acid, 3%-4% silane, 1%-2% sealing agent, 1‰-2‰ lubricant, and 2%-4% defoamer, with the remaining components being water; the passivation solution is treated with a filter screen with a mesh size ≥1000; When the steel plate thickness is ≤1.0mm, a reverse coating process is adopted. The distance between the coating roller and the strip is controlled from -1mm to 1mm. The upper take-up roller speed is controlled at 70%-120% and the upper take-up roller pressure is controlled at 30-60kg. The upper coating roller speed is controlled at 100%-180% and the pressure is controlled at 30-60kg. The lower take-up roller speed is controlled at 50%-90% and the pressure is controlled at 130-180kg. The lower coating roller speed is controlled at 100%-160%. When the steel plate thickness is >1.0mm, the unidirectional coating process is adopted. The distance between the coating roller and the strip is controlled at 0-2mm. A low speed is used to prevent liquid splashing. The speed of the upper take-up roller is controlled at 40%-60% and the pressure at 90-130kg. The speed of the upper coating roller is controlled at 90%-130% and the pressure at -70kg to -40kg. The speed of the lower take-up roller is controlled at 40%-60%. The pressure of the lower coating roller is 130-160kg and the speed of the lower coating roller is controlled at 80%-120%. Control the membrane weight to 20-40 mg / m² per side. 2 ; The temperature of the strip steel to the roll coating section shall be controlled to be ≤20℃, and the temperature of the passivation solution shall be controlled to be ≤20℃. The coating on the surface of the steel plate comprises, by weight percentage, 0.12%-12% Al, 0%-3% Mg, ≤0.004% Fe, and the remainder is Zn.

2. The production method according to claim 1, characterized in that, It also includes a drying step, with a drying temperature greater than 60℃ and less than 120℃.

3. A product manufactured using the production method according to any one of claims 1-2, characterized in that, The coating on the surface of the steel plate comprises, by weight percentage, 0.12%-12% Al, 0%-3% Mg, ≤0.004% Fe, and the remainder is Zn.

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