A tinplate for milk powder can and a method for manufacturing the same and an application thereof

Abstract

The present application belongs to the technical field of alloy materials, and particularly relates to a tin-plated plate for milk powder cans and a preparation method and application thereof. The profile skewness of the inner surface of the tin-plated plate for milk powder cans is -0.8 to -0.01, and the profile skewness of the outer surface is -0.2 to 0.5. The profile parameters of the inner surface and the outer surface of the tin-plated plate are differentially regulated, so that the surface topographies of the two surfaces are formed to match the use requirements. The specific profile skewness of the inner surface can optimize the surface bearing state, improve the oil storage performance, and thus significantly improve the scratch and mar resistance of the inner surface. The specific profile skewness of the outer surface can improve the coating spreadability and adhesion performance, effectively inhibit coating problems such as shrinkage cavity and poor adhesion, and ensure the outer coating forming quality and use reliability.

Description

Technical Field

[0001] This invention belongs to the field of alloy materials technology, specifically relating to a tin-plated sheet for milk powder cans, its preparation method, and its application. Background Technology

[0002] Tinplate is used as a raw material for food and beverage cans, and its surface quality is crucial to performance, appearance, and even food safety. Tinplate typically requires coating treatment to form a coating layer on the tin layer to further enhance the film's resistance to acids and alkalis. However, tinplate used in milk powder cans is usually only coated on the outer surface, with the inner surface in direct contact with the milk powder. This places higher demands on the cleanliness and environmental protection of tinplate used in milk powder cans.

[0003] In addition, when the outer surface is coated, the inner surface (which is not coated) comes into direct contact with the flower rack and is sent to the drying oven through the flower rack for the curing of the outer surface coating. Sometimes, the friction between the flower rack and the inner surface at high temperature can cause visible scratches, color differences, and burns, resulting in surface defects that are unacceptable to customers.

[0004] CN118086781A solves the problem of scratch defects in tin-plated sheets during transportation and processing by forming a dense tin-iron alloy layer through alkaline washing, continuous annealing, leveling, and electroplating. This improves scratch resistance and yield, meeting the high-quality requirements of food cans. However, this invention mainly improves the scratch resistance of the inner surface by increasing the content and density of the alloy layer, and cannot simultaneously meet the requirements of high cleanliness, high environmental protection, and high surface finish for tin-plated sheets used in milk powder cans. Summary of the Invention

[0005] Therefore, the technical problem to be solved by the present invention is to overcome the defects of the existing tin-plated sheet, which has poor scratch resistance on the inner surface and even worse scratch resistance at high temperature, and insufficient coating on the outer surface, thereby providing a tin-plated sheet for milk powder cans, its preparation method and application.

[0006] Therefore, the present invention provides the following technical solution: The first aspect of this invention protects a tin-plated sheet for milk powder cans, wherein the profile skewness of the inner surface of the tin-plated sheet for milk powder cans is -0.8 to -0.01, and the profile skewness of the outer surface is -0.2 to 0.5.

[0007] As an example, the profile skewness of the inner surface of the tin-plated sheet used for the milk powder can be -0.8, -0.7, -0.6, -0.5, -0.4, -0.3, -0.2, -0.1, -0.01, or within any of the above values; the profile skewness of the outer surface can be -0.2, -0.1, 0, 0.1, 0.2, 0.3, 0.4, 0.5, or within any of the above values; furthermore, a combination of inner surface profile skewness of -0.5 and outer surface profile skewness of 0.4 is presented, a combination of inner surface profile skewness of -0.4 and outer surface profile skewness of 0.5 is presented, and a combination of inner surface profile skewness of -0.6 and outer surface profile skewness of 0.3 is presented.

[0008] In this invention, the inner and outer surface contours are differentiated. The inner surface adopts a specific contour skew, which can effectively reduce the proportion of surface peaks. This allows for effective storage and retention of the oil after coating (such as DOS-A), significantly enhancing the scratch and abrasion resistance of the inner surface of the tinplate and meeting the surface stability requirements under high-temperature baking and flower stand contact conditions. The outer surface adopts a specific contour skew, which can improve surface wettability and coating leveling, enhance coating uniformity, effectively avoid coating defects such as pinholes and poor adhesion, and ensure stable outer coating quality.

[0009] Optionally, the profile skewness of the inner surface of the tin-plated sheet used for the milk powder can is -0.5 to -0.4. Optionally, the profile skewness of the outer surface is 0.1-0.15.

[0010] As an example, the profile skewness of the inner surface of the tin-plated sheet used for the milk powder can be -0.5, -0.45, -0.4, or within any range of the above values; the profile skewness of the outer surface can be 0.10, 0.12, 0.14, or within any range of the above values; furthermore, a combination of inner surface profile skewness of -0.53 and outer surface profile skewness of 0.10 is presented, a combination of inner surface profile skewness of -0.45 and outer surface profile skewness of 0.14 is presented, and a combination of inner surface profile skewness of -0.4 and outer surface profile skewness of 0.13 is presented.

[0011] In one optional embodiment, the arithmetic mean roughness of the inner surface of the tin-plated sheet for the milk powder can is 0.45-0.75 μm, and the peak density is 80-150 peaks / cm². As an example, the arithmetic mean roughness of the inner surface of the tin-plated sheet for the milk powder can can be 0.45 μm, 0.6 μm, 0.65 μm, 0.7 μm, 0.75 μm, or within any range of the above values; the peak density can be 80 peaks / cm², 100 peaks / cm², 120 peaks / cm², etc. The arithmetic mean roughness is 0.6-0.7 μm, and the peak density is 100-150 peaks / cm. For example, the arithmetic mean roughness can be 0.6 μm, 0.65 μm, 0.7 μm, or within the range of any of the above values; the peak density can be 100 peaks / cm, 120 peaks / cm, 140 peaks / cm, or within the range of any of the above values.

[0012] In one optional embodiment, the arithmetic mean roughness of the outer surface of the tin-plated sheet for the milk powder can is 0.40-0.75 μm, and the peak density is 80-150 peaks / cm². As an example, the arithmetic mean roughness of the outer surface of the tin-plated sheet for the milk powder can can be 0.40 μm, 0.6 μm, 0.65 μm, 0.7 μm, 0.75 μm, or within any range of the above values; the peak density can be 80 peaks / cm², 100 peaks / cm², 120 peaks / cm², etc. The arithmetic mean roughness is 0.6-0.7 μm, and the peak density is 100-150 peaks / cm. For example, the arithmetic mean roughness can be 0.6 μm, 0.65 μm, 0.7 μm, or within any of the above values; the peak density can be 100 peaks / cm, 120 peaks / cm, 140 peaks / cm, or within any of the above values.

[0013] In one optional embodiment, the tin-plated plate for the milk powder can includes a first passivation layer, a first tin plating layer, a first alloy layer, a substrate layer, a second alloy layer, a second tin plating layer, and a second passivation layer disposed sequentially. The surface of the first passivation layer that is far from the first tin plating layer is the inner surface; The surface of the second passivation layer that is away from the second tin plating layer is the outer surface.

[0014] The second aspect of this invention protects a method for preparing the aforementioned tin-plated plate for milk powder cans, wherein the preparation method includes the following steps: annealing, flattening, surface activation, electroplating, remelting, water quenching, passivation, oiling, and packaging of the substrate layer; The substrate layer is leveled using two-frame work rollers. The profile skewness of the upper work roller of the frame is -0.2 to 0.8, and the profile skewness of the lower work roller is -1.0 to 0.2. The profile skewness of the upper working roll on the second frame is -0.2 to 0.8, and the profile skewness of the lower working roll is -1.0 to 0.2.

[0015] As an example, the profile skewness of the work roll on one stand can be -0.2, -0.1, 0, 0.2, 0.4, 0.6, 0.8, or within any range of the above values; the profile skewness of the work roll on the lower stand can be -1.0, -0.8, -0.6, -0.4, -0.2, 0, 0.2, or within any range of the above values; the profile skewness of the work roll on two stands can be -0.2, -0.1, 0, 0.2, 0.4, 0.6, 0.8, or within any range of the above values. Within the range; the profile skewness of the lower working roll of the two frames can be -1.0, -0.8, -0.6, -0.4, -0.2, 0, 0.2, or within any range of the above values; furthermore, a combination is presented where the profile skewness of the upper working roll of one frame is 0.5 and the profile skewness of the lower working roll of one frame is -0.6, a combination is presented where the profile skewness of the upper working roll of two frames is 0.5 and the profile skewness of the lower working roll of two frames is -0.8, and a combination is presented where the profile skewness of the upper working roll of one frame is 0 and the profile skewness of the lower working roll of one frame is 0.1.

[0016] In this invention, the contact surfaces of the upper working rollers on the first and second frames correspond to the inner surface of the tin-plated sheet used in milk powder cans, and the contact surfaces of the lower working rollers correspond to the outer surface of the tin-plated sheet used in milk powder cans. Furthermore, the surface profile of the working roller on the first frame is obtained through an electrical discharge machining process, resulting in a granular surface, while the surface profile of the working roller on the second frame is obtained through grinding, resulting in a striped surface. The manufacturing method precisely controls and replicates the differential microscopic profile morphology of the inner and outer surfaces of the tin-plated sheet by limiting the skewness of the profiles on the first and second frames. This ensures that the inner surface possesses excellent scratch resistance, abrasion resistance, and high-temperature friction resistance, while simultaneously ensuring good paint wetting properties, coating uniformity, and paint film adhesion on the outer surface. This stable guarantee of the surface quality and performance of the finished sheet from the source of the process ensures optimal performance.

[0017] Optionally, the profile skewness of the upper working roll of a frame is 0-0.8, and the profile skewness of the lower working roll is -1.0 to 0. Optionally, the profile skewness of the upper working roll on the second frame is 0-0.8, and the profile skewness of the lower working roll is -1.0 to 0.

[0018] As an example, the profile skewness of the work roll on one frame can be 0, 0.2, 0.4, 0.5, 0.6, 0.8, or within any range of the above values; the profile skewness of the work roll on the lower frame can be -1.0, -0.8, -0.6, -0.4, -0.2, 0, or within any range of the above values; the profile skewness of the work roll on two frames can be 0, 0.2, 0.4, 0.5, 0.6, 0.8, or within any range of the above values; the profile skewness of the work roll on the lower frame can be -1.0, -0.8, -0.6, -0.4, -0.2, 0, or within any range of the above values.

[0019] In one optional embodiment, the arithmetic mean roughness and peak density of the upper and lower work rolls of a frame are the same, wherein the arithmetic mean roughness is 1.3-1.8 μm and the peak density is 150-200 peaks / cm; as an example, the arithmetic mean roughness of the upper and lower work rolls of a frame can be 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, or within any range of the above values; the peak density can be 150 peaks / cm, 160 peaks / cm, 170 peaks / cm, 18 ... The peak density is 0, 190, 200, or any value within the range of the above values; optionally, the arithmetic mean roughness is 1.4-1.6 μm, and the peak density is 170-200 peaks / cm; as an example, the arithmetic mean roughness can be 1.4 μm, 1.5 μm, 1.6 μm, or any value within the range of the above values; the peak density can be 170, 180, 190, 200 peaks / cm, or any value within the range of the above values.

[0020] In one optional embodiment, the arithmetic mean roughness and peak density of the upper and lower work rolls of the two stands are the same, with an arithmetic mean roughness of 0.6-0.9 μm and a peak density of 150-200 peaks / cm. As an example, the arithmetic mean roughness of the upper and lower work rolls of the two stands can be 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, or within any range of the above values; the peak density can be 150 peaks / cm, 160 peaks / cm, 170 peaks / cm, 180 peaks / cm, 190 peaks / cm, etc. The peak density is 0, 200, or any value within the range of the above values; optionally, the arithmetic mean roughness is 0.7-0.8 μm, and the peak density is 170-200 peaks / cm; as an example, the arithmetic mean roughness can be 0.7 μm, 0.75 μm, 0.8 μm, or any value within the range of the above values; the peak density can be 170, 180, 190, 200 peaks / cm, or any value within the range of the above values.

[0021] In one optional embodiment, the electroplating uses an MSA (methanesulfonic acid) plating solution, wherein the concentration of MSA is 120-130 g / L, the concentration of free acid is 25-35 mL / L, and the Sn in the solution... 2+ The concentration of ions is 30-40 g / L, Fe 2+ The concentration of ions is 0-80 ppm, and the lead content is less than 20 ppm.

[0022] In this invention, free acid refers to MSA that exists freely without being bound to metal ions; part of the methanesulfonic acid in the plating solution is bound to metal ions to form salts, while the other part is unbound and remains free methanesulfonic acid; the free acid is titrated with standard NaOH solution using an acid-base titration method, and the volume of NaOH consumed is defined as the concentration of the free acid, hence the unit is mL / L, which is different from the unit g / L for the concentration of methanesulfonic acid.

[0023] In one alternative embodiment, the tin content in the anode used for electroplating is 99.99%-100%.

[0024] In one optional embodiment, the electroplating bath temperature is 40-45°C, and the plating bath current density is controlled at 15-25 A / dm³. 2 .

[0025] In this invention, strict control over the purity of the solution and anode during electroplating helps to reduce the lead content of the coating and improve its environmental friendliness.

[0026] In one optional embodiment, the softening conditions include: a softening temperature of 235-270°C and a softening time of 0.8-1.8 seconds.

[0027] In one optional embodiment, the softening atmosphere is a N2 and H2 volume ratio of 1:(0.3-0.8) with a dew point of -40°C to -30°C; as an example, the volume ratio of N2 to H2 in the softening atmosphere can be 1:0.3, 1:0.4, 1:0.5, 1:0.6, 1:0.7, 1:0.8, or within any range of the above values; the dew point can be -40°C, -38°C, -36°C, -34°C, or -32°C. The temperature is -30°C, or within any range of the above values; optionally, the softening atmosphere is N2 and H2 in a volume ratio of 1:(0.7-0.75), with a dew point of -37°C to -35°C; as an example, the volume ratio of N2 to H2 can be 1:0.7, 1:0.72, 1:0.75, or within any range of the above values; the dew point can be -37°C, -36°C, -35°C, or within any range of the above values.

[0028] In one alternative embodiment, quenching is performed 0.6-1.2 s after the softening process ends; as an example, the interval between the end of softening and quenching can be 0.6 s, 0.7 s, 0.8 s, 0.9 s, 1.0 s, 1.1 s, 1.2 s, or within any of the above values; quenching is a conventional operation in the art, and demineralized water is generally used for quenching, with the conductivity of the demineralized water being κ, 0 < κ ≤ 8 μS / cm.

[0029] In one optional embodiment, the quenching conditions include: a quenching water temperature of 80-82°C and a quenching time of 0.6-1.0 s.

[0030] In this invention, specific softening temperature and softening time, combined with a reducing softening atmosphere, and rapid water quenching after softening, help reduce the thickness of the oxide film in the coating and reduce the external oxidation power of residual lead in the coating. In particular, the control of the softening atmosphere and dew point reduces the degree of oxidation of the tin layer during the softening process, reduces the formation of oxide film on the coating surface, further inhibits the segregation of impurity elements and the migration of harmful metals in the coating, and improves the uniformity, density and food contact safety of the coating, especially the food contact safety of the inner surface that comes into direct contact with milk powder.

[0031] In one optional embodiment, the thickness of the substrate layer is 0.18-0.20 mm; and the substrate layer comprises, by mass percentage of chemical composition, C: 0.045-0.075%; Si: 0-0.002%; Mn: 0.15-0.20%; P: 0-0.010%; S: 0-0.01%; Cr: 0-0.01%; Ni: 0-0.1%; Cu: 0-0.1%; Al: 0.03-0.05%; with the remainder being iron and unavoidable impurity elements.

[0032] In one optional embodiment, the surface activation includes alkaline washing and acid washing. Alkaline washing is a conventional method in the art, typically and non-limitingly using a mixed solution of NaOH and Na₂CO₃. The concentration of NaOH in the mixed solution is 30-40 g / L, and the concentration of Na₂CO₃ is 10-20 g / L. The amount of the mixed solution is not limited, as long as it completely submerges the smoothed tin-plated substrate. The alkaline washing time is 1-1.5 seconds. Sodium hydroxide, as a strong alkali, has a strong saponification ability, enabling the saponification and removal of grease, while also possessing a certain emulsifying ability. Sodium carbonate has a buffering effect on pH, helping to maintain the alkalinity of the solution, thereby stabilizing the degreasing ability and avoiding instability in the degreasing effect due to pH fluctuations.

[0033] In one optional embodiment, the pickling step includes: first performing electrochemical pickling, then performing chemical pickling. Performing electrochemical pickling first and then chemical pickling can quickly and deeply remove the dense oxide film on the substrate surface, greatly improve the activation uniformity of the substrate surface, and create a surface state with higher reactivity for subsequent chemical pickling. Electrochemical pickling is performed using an H2SO4 solution with a concentration of 20-40 g / L. The amount of H2SO4 solution is not limited, as long as it completely submerges the tin-plated substrate after the first pickling step. The current density is 12-20 A / dm³. 2 The pickling time is 1-1.2 seconds; Chemical pickling is performed using a 60-70 g / L H₂SO₄ solution and a 20-30 g / L H₂C₂O₄ solution. The amounts of H₂SO₄ and H₂C₂O₄ solutions are not limited, as long as they completely submerge the tin-plated substrate after alkaline pickling. The pickling time is 1-1.5 seconds. The coordinated use of H₂SO₄ and H₂C₂O₄ pickling removes the surface oxide film, increases the surface activity of the substrate, and improves the substrate's plating susceptibility, thereby ensuring the uniformity and density of the plating layer.

[0034] In one alternative embodiment, chromate is used for passivation. Since chromate passivation films are resistant to abrasion and high temperatures, a higher passivation amount can improve the passivation film coverage and thickness, which is beneficial for further improving the scratch and abrasion resistance of the coating surface. Typically, and not specifically, passivation is performed using a concentration of 20-25 g / L Na₂Cr₂O₇.

[0035] In this invention, the oiling method is a conventional method in the art. Typically, and not specifically, vertical electrostatic oiling is used, and the reagent is DOS-A oil (dioctyl sebacate). The amount of oil applied is 2.5-7 mg / m³ based on DOS-A oil. 2 When applying the oil, the oil temperature should be 40-45℃ and the voltage 18-30kV. By controlling the type and amount of DOS-A oil, the wettability of the tinplate surface can be improved. Using DOS-A oil with better wettability is beneficial to improving the oil retention effect of the inner surface, thereby improving the scratch and abrasion resistance of the inner surface. At the same time, since the outer surface contour itself has poor oil retention capacity and DOS-A oil has good wettability, it also improves the coating effect of the outer surface and avoids problems such as pinholes and poor adhesion.

[0036] In this invention, packaging is carried out within 30 minutes to 2 hours after oiling.

[0037] The technical solution of this invention has the following advantages: 1. This invention protects a tinplate for milk powder cans, wherein the contour skewness of the inner surface of the tinplate is -0.8 to -0.01, and the contour skewness of the outer surface is -0.2 to 0.5. This invention implements differentiated control of the contour parameters of the inner and outer surfaces of the tinplate, so that both surfaces form surface morphologies that match the usage requirements. The specific contour skewness of the inner surface optimizes the surface bearing capacity, increases the effective bearing contact area, and reduces the proportion of surface peaks. Peaks are easily removed under high-temperature friction conditions, thereby significantly reducing color difference defects caused by friction, improving surface stability, enhancing oil retention performance, and thus significantly improving the scratch and abrasion resistance of the inner surface. The specific contour skewness of the outer surface improves the coating spreadability and adhesion, effectively suppressing coating problems such as pinholes and poor adhesion, ensuring the quality of the outer coating and its reliability in use.

[0038] 2. The present invention provides a method for preparing the aforementioned tin-plated sheet for milk powder cans, wherein the preparation method includes the following steps: annealing, leveling, surface activation, electroplating, remelting, quenching, passivation, oiling, and packaging the substrate layer; using two-stand work rolls to level the substrate layer, wherein the profile skewness of the upper work roll of the first stand is -0.2 to 0.8, and the profile skewness of the lower work roll is -0.1 to 0.2; the profile skewness of the upper work roll of the second stand is -0.2 to 0.8, and the profile skewness of the lower work roll is -1.0 to 0.2; the profile skewness of the first and second stand work rolls is limited, which can precisely impart different microscopic profile features to the inner and outer surfaces of the steel plate, making the inner surface more wear-resistant and scratch-resistant, and the outer surface more suitable for coating and less prone to shrinkage cavities and poor adhesion, thus achieving a stable differentiated design of inner and outer surface properties from the source of the rolling process. Detailed Implementation

[0039] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0040] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0041] Example 1 This embodiment provides a tin-plated sheet for milk powder cans, comprising the following preparation method: (1) The thickness of the substrate layer is 0.185 mm. Based on the mass percentage of chemical composition, the substrate layer includes C: 0.046%; Si: 0.001%; ​​Mn: 0.18%; P: 0.009%; S: 0.008%; Cr: 0.009%; Ni: 0.08%; Cu: 0.07%; Al: 0.04%; the remainder is iron and unavoidable impurity elements; (2) Annealing: The annealing temperature is 648°C, the annealing time is 25s, the annealing dew point is -34°C, and the annealing atmosphere is a mixture of N2 and H2. The concentration of H2 is 17%, the concentration of N2 is 83%, the concentration of impurity gas is 0.7ppmv, and the concentration of O2 is 1ppmv, based on the mass of the mixture. (3) Smoothing: The arithmetic mean roughness of the upper and lower working rollers of the first frame is 1.5 μm, the peak density is 180 peaks / cm, the profile skewness of the upper working roller is 0.5, and the profile skewness of the lower working roller is -0.6; The arithmetic mean roughness of the upper and lower working rollers of the second frame is 0.8 μm, the peak density is 195 peaks / cm, the profile skewness of the upper working roller is 0.5, and the profile skewness of the lower working roller is -0.8. (4) Surface activation includes alkaline washing and acid washing; The alkaline washing solution was a mixture of NaOH and Na2CO3. The concentration of NaOH was 32 g / L and the concentration of Na2CO3 was 19 g / L. The mixed solution completely submerged the flattened substrate layer. The alkaline washing temperature was 65℃ and the washing time was 1 s. After alkaline washing, the substrate was rinsed with deionized water. The conductivity of deionized water was 1.9 μS / cm and the rinsing time was 1.2 s. Pickling is performed in two steps. The first step is electrochemical pickling, using a 38 g / L H2SO4 solution. The H2SO4 solution completely submerges the alkaline-washed substrate layer, and the current density is 17 A / dm³. 2 The first pickling step was 1.15s; the second pickling step was chemical pickling, using a 24g / L H2C2O4 solution. The H2C2O4 solution completely submerged the substrate layer after the first pickling, and the pickling time was 1.6s. After pickling, it was rinsed with deionized water, which has a conductivity of 2.1μS / cm, and the rinsing time was 1.2s. (5) Electroplating: The electroplating solution is an MSA plating solution with an MSA concentration of 124 g / L and a free acid concentration of 26 mL / L. 2+ The ion concentration is 36 g / L, Fe 2+ The ion concentration was 45 ppm, the electroplating solution temperature was 43°C, the anode was a tin anode (tin content 99.998%), the lead content of the electroplating solution was 10 ppm, and the current density was 22 A / dm³. 2 ; (6) Soft melting and quenching: High-frequency pure inductive soft melting is adopted. The soft melting temperature is 265°C. The soft melting atmosphere is a mixture of N2 and H2 gas, with the remainder being impurity gases. The volume ratio of N2 to H2 is 1:0.7, the concentration of impurity gases is 2ppm, the dew point is -35°C, the soft melting time is 1.1s, and quenching is carried out immediately 0.6s after the soft melting ends. The quenching temperature is 82°C, the quenching time is 0.9s, and the quenching solution is demineralized water with a conductivity of 3.1μS / cm. (7) Passivation: Passivation was performed using a Na2Cr2O7 solution with a concentration of 23 g / L, and the pH value was adjusted to 4.1 using a CrO3 aqueous solution with a concentration of 240 g / L. The passivation temperature was 39°C, and the passivation charge was 0.65 C / dm³. 2 The passivation current density is 7 A / dm² 2 The passivation amount, based on the mass of Na2Cr2O7, is 7.5 mg / m³. 2 ; (8) Oiling: Vertical electrostatic oiling was used, with DOS-A oil as the reagent and an oiling amount of 7 mg / m². 2 The oil temperature is 44°C and the voltage is 29kV. (9) Packaging: Seal the package 1.5 hours after applying the oil, with an ambient temperature of 25°C and a humidity of 60%.

[0042] Example 2 This embodiment provides a tin-plated sheet for milk powder cans, following the method of embodiment 1, except that in step (3), the arithmetic mean roughness of the upper and lower working rollers of the first frame is 1.3 μm, the peak density is 155 peaks / cm, the profile deviation of the upper working roller is 0, and the profile deviation of the lower working roller is 0.1; the arithmetic mean roughness of the upper and lower working rollers of the second frame is 0.6 μm, the peak density is 150 peaks / cm, the profile deviation of the upper working roller is 0.1, and the profile deviation of the lower working roller is 0.

[0043] Example 3 This embodiment provides a tin-plated sheet for milk powder cans, which is the same as in Embodiment 1, except that the melting atmosphere is a mixture of N2 and H2, with the remainder being impurity gases. The volume ratio of N2 to H2 is 1:0.6, the concentration of impurity gases is 2 ppm, and the dew point is -34°C.

[0044] Example 4 This embodiment provides a tin-plated sheet for milk powder cans, following the method of Embodiment 1, except that the melting atmosphere is a mixture of N2 and H2 gases, with the remainder being impurity gases. The volume ratio of N2 to H2 is 1:0.5, the concentration of impurity gases is 2 ppm, and the dew point is -40°C.

[0045] Comparative Example 1 This comparative example provides a tin-plated sheet for milk powder cans, which is the same as in Example 1, except that in step (3), the arithmetic mean roughness of the upper and lower working rollers of the first frame is 1 μm, the peak density is 100 peaks / cm, the profile deviation of the upper working roller is 0.5, and the profile deviation of the lower working roller is -0.5; the arithmetic mean roughness of the upper and lower working rollers of the second frame is 0.45 μm, the peak density is 110 peaks / cm, the profile deviation of the upper working roller is -0.1, and the profile deviation of the lower working roller is 0.35.

[0046] Test case The arithmetic mean roughness, peak density, and profile skewness of tin-plated sheets used for milk powder cans were tested using a contact surface roughness tester. The test results are shown in Table 1; Table 1. Arithmetic mean roughness, peak density, and profile skewness of tinplate used for milk powder cans

[0047] The lead content of the tinplate used for milk powder cans was tested using inductively coupled plasma optical emission spectrometry (ICP-OES). Specifically, water and hydrochloric acid (mass fraction of 36%) were mixed in a volume ratio of 3:1 to obtain a hydrochloric acid solution. The tinplate used for milk powder cans was then completely immersed in the hydrochloric acid solution at 80°C until the solution turned green before testing. The test results are shown in Table 2; Table 2 Lead content of tinplate used in milk powder cans

[0048] Adhesion rating White phthalide was used to coat the outer surface of the tin-plated sheet of the packaged milk powder can, with a coating amount of 10g / m². 2 The baking temperature was 180℃ and the time was 15min. The adhesion level of the coated tinplate for milk powder cans was tested according to GB / T41899-2022. Surface black and gray level Double-ring filter paper (12.5cm diameter, produced by Hangzhou Wohua Filter Paper Co., Ltd.) was used to wipe a certain area of ​​the test piece surface under constant pressure. The difference in lightness (-AL) before and after wiping was then measured using a YQ-Z-48A whiteness colorimeter from Hangzhou Qingtong Instrument Development Co., Ltd. -AL was used to quantitatively characterize the degree of blackness / grayness. Three parallel test pieces were used for each sample. During the lightness difference measurement, the transfer of test paper on each test piece was measured three times. After each measurement, the test paper was rotated horizontally by 120°, and the average value was taken. The degree of blackness / greyness was determined based on the color on the test strips and divided into 6 levels. Test strips classified as blackness / greyness levels 2-6 by on-site technicians were measured using the above method, and the results are shown in Table 3. The lightness difference corresponding to each blackness / greyness level should be a range. The average value in Table 3 must fall within the corresponding lightness difference range. To determine this range, the average value in Table 3 was used as a benchmark, and the midpoint between two benchmarks was used as the grading point. The same level excludes the left endpoint but includes the right endpoint, resulting in Table 4. Table 3

[0049] Table 4

[0050] Scratch resistance test The edges of the tin-plated sheets from the milk powder cans manufactured in each embodiment and comparative example were cut into 125mm × 50mm samples. The surface scratch resistance was tested using a coating scratch tester. The test method is as follows: Take a scribing needle (the needle tip is a 1mm diameter hardened steel ball, included with the coating scratch tester), and inspect the needle tip with a 30x magnifying glass. The needle tip should be a smooth, hemispherical shape, without obvious wear or contaminants; otherwise, it should be replaced with a new scribing needle. Fix the scribing needle to the chuck and position it perpendicular to the tinplate plating. Cut a 125mm x 50mm flat template from tinplate used for milk powder cans, with the long side facing the rolling direction. The sampling position is 1 / 4 of the way across the transverse surface (perpendicular to the rolling direction). Place the template with the inner surface facing up and clamp it onto the sliding plate of the coating scratch tester, with the long side of the template parallel to the direction of the scratch. Place the weight on the support above the scratch needle to obtain the given load. The load can range from 100g to 2000g depending on the experimental requirements; 500g is used in this experiment. Start the instrument and move the sliding plate horizontally at a constant speed (30-40mm / s). Scratches will be made on the surface of the template. Remove the template and check the back side for scratches. Observe and measure the scratch width under a metallographic microscope. Under a certain load, the wider the scratch, the lower the scratch hardness and the worse the scratch resistance; the narrower the scratch, the higher the scratch hardness and the better the scratch resistance. Note: A flange will be generated on both sides of the scratch. When measuring, the flange and outer edge of the scratch should not be measured, but the width of the actual scratch should be measured. When observing, an appropriate magnifying glass or optical microscope can be used for inspection, but the magnification should be noted in the test report. In this application, the observation was carried out using an optical microscope with a magnification of 100x. The test results are shown in Table 5. Table 5. Surface Coating Quality and Processing Stability of Tinplate for Milk Powder Cans

[0051] This invention implements differentiated control of the contour parameters of the inner and outer surfaces of tin-plated sheets, enabling each surface to form a surface morphology that matches the application requirements. The inner surface, through a specific contour deviation, optimizes its surface load-bearing capacity and improves its oil retention performance, thereby significantly enhancing its scratch and abrasion resistance. The outer surface, also through a specific contour deviation, improves coating spreadability and adhesion, effectively suppressing coating problems such as pinholes and poor adhesion, ensuring the quality of the outer coating and its reliability in use.

[0052] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A tin-plated sheet for milk powder cans, characterized in that, The profile skewness of the inner surface of the tin-plated sheet used for the milk powder can is -0.5 to -0.4, and the profile skewness of the outer surface is 0.1 to 0.

15. The arithmetic mean roughness of the inner surface of the tin-plated plate used for the milk powder can is 0.45-0.75 μm, and the peak density is 80-150 peaks / cm. The arithmetic mean roughness of the outer surface of the tin-plated sheet used for the milk powder can is 0.40-0.75 μm, and the peak density is 80-150 peaks / cm. The tin-plated plate for the milk powder can includes a first passivation layer, a first tin plating layer, a first alloy layer, a substrate layer, a second alloy layer, a second tin plating layer, and a second passivation layer arranged sequentially. The surface of the first passivation layer that is far from the first tin plating layer is the inner surface; The surface of the second passivation layer that is furthest from the second tin plating layer is the outer surface; The method for preparing the tin-plated sheet for milk powder cans includes the following steps: annealing, flattening, surface activation, electroplating, soft melting, water quenching, passivation, oiling, and packaging of the substrate layer; The substrate layer is leveled using two-frame work rollers. The profile skewness of the upper work roller of the frame is -0.2 to 0.8, and the profile skewness of the lower work roller is -1.0 to 0.

2. The profile skewness of the upper working roll on the second frame is -0.2 to 0.8, and the profile skewness of the lower working roll is -1.0 to 0.

2.

2. A method for preparing a tin-plated sheet for a milk powder can as described in claim 1, characterized in that, The preparation method includes the following steps: annealing, planing, surface activation, electroplating, soft melting, water quenching, passivation, oiling, and packaging of the substrate layer; The substrate layer is leveled using two-frame work rollers. The profile skewness of the upper work roller of the frame is -0.2 to 0.8, and the profile skewness of the lower work roller is -1.0 to 0.

2. The profile skewness of the upper working roll on the second frame is -0.2 to 0.8, and the profile skewness of the lower working roll is -1.0 to 0.

2.

3. The preparation method according to claim 2, characterized in that, The profile skewness of the upper working roll of a machine frame is 0-0.8, and the profile skewness of the lower working roll is -1.0 to 0. And / or, the profile skewness of the upper working roll on the second frame is 0-0.8, and the profile skewness of the lower working roll is -1.0 to 0; And / or, the arithmetic mean roughness and peak density of the upper and lower working rollers of a frame are the same, wherein the arithmetic mean roughness is 1.3-1.8μm and the peak density is 150-200 peaks / cm; And / or, the arithmetic mean roughness and peak density of the upper and lower working rollers of the two stands are the same, with an arithmetic mean roughness of 0.6-0.9μm and a peak density of 150-200 peaks / cm.

4. The preparation method according to claim 2 or 3, characterized in that, The electroplating uses a methanesulfonic acid electroplating solution, wherein the concentration of methanesulfonic acid is 120-130 g / L, the concentration of free acid is 25-35 mL / L, and the Sn in the solution... 2+ The concentration of ions is 30-40 g / L, Fe 2+ The concentration of ions is 0-80 ppm, and the lead content is less than 20 ppm; And / or, the tin content in the anode used for electroplating is 99.99%-100%; And / or, the electroplating bath temperature is 40-45°C, and the plating bath current density is controlled at 15-25 A / dm³. 2 .

5. The preparation method according to claim 2, characterized in that, The conditions for softening include: a softening temperature of 235-270℃ and a softening time of 0.8-1.8s; And / or, the atmosphere for the softening process is N2 and H2 in a volume ratio of 1:(0.3-0.8), with a dew point of -40°C to -30°C; And / or, quenching is performed 0.6-1.2 seconds after the softening process ends; And / or, the quenching conditions include: a quenching water temperature of 80-82℃ and a quenching time of 0.6-1.0s.

6. The preparation method according to claim 2, characterized in that, The thickness of the substrate layer is 0.18-0.20 mm; based on the mass percentage of chemical composition, the substrate layer has the following composition: C: 0.045-0.075%; Si: 0-0.002%; Mn: 0.15-0.20%; P: 0-0.010%. S: 0-0.01%; Cr: 0-0.01%; Ni: 0-0.1%; Cu: 0-0.1%; Al: 0.03-0.05%; the remainder is iron and unavoidable impurity elements.

7. The preparation method according to claim 2, characterized in that, The surface activation includes alkaline washing and acid washing.

8. The preparation method according to claim 2, characterized in that, The passivation was performed using chromate.

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