A continuous composite plating method for solderable tinplate with chromium undercoat and trace tin plating

Abstract

The application discloses a kind of chromium priming trace tin solderable tinplate continuous composite plating, it is related to metal strip surface treatment technical field.The method is formed chromium primer layer on the surface of continuous operation's can-making steel strip by electrodeposition, and before entering trace tin plating section, according to the residual liquid film state and tin nucleation adaptation state after chromium primer layer leaves chromium plating section, surface transition treatment is carried out;Subsequently, trace tin surface layer is formed, and welding resistance information and the relative distribution state of tin residue and chromium exposed area in contact area after welding are obtained by welding contact verification.When there is continuous chromium exposed area, it is determined that trace tin surface layer does not form hot-pressing conduction reserved area, and subsequent process is corrected in the order of adjusting surface transition treatment first, then adjusting first layer nucleation condition, and finally adjusting total deposition condition.The method is conducive to reducing the amount of tin while improving the stability of welding contact.

Description

Technical Field

[0001] This application relates to the field of surface treatment technology for metal strips, and in particular to a continuous composite plating method for solderable tinplate with chromium undercoat and micro-tin plating. Background Technology

[0002] Tinplate typically refers to thin steel sheets or strips with a tin coating on their surface, widely used in packaging materials such as food cans, beverage cans, and chemical packaging cans. The tin coating improves the weldability and processing adaptability of the steel strip surface, and also improves its corrosion resistance to some extent.

[0003] Traditional tinplate typically uses a full-surface tin plating process. This process can meet most can-making welding and forming requirements, but it consumes a large amount of tin. In mass production of packaging materials, fluctuations in tin prices affect production costs. Therefore, reducing tin consumption while maintaining weldability is a direction that requires continuous improvement in the production of coated steel strips for can making.

[0004] Ordinary chrome-plated iron has the advantages of low cost, good corrosion resistance, and good coating adaptability, but its surface welding contact performance is generally not as good as that of traditional tin-plated tinplate. For tank body materials that require resistance welding, ordinary chrome-plated iron is prone to problems such as high welding contact resistance, poor weld continuity, local incomplete welds, or poor weld appearance, making it difficult to directly replace traditional tin-plated tinplate.

[0005] To balance cost and solderability, a chromium layer could be used for basic protection, while a smaller tin layer would handle the welding contact. However, in continuous production, if a small amount of tin is directly applied to the surface of the chromium-plated steel strip for can making, the state of the residual liquid film after the chromium underlayer leaves the chromium plating section, the tin nucleation and adaptation state, and the nucleation state of the subsequent small tin layer will all affect the formation of the small tin surface layer.

[0006] Because the amount of tin on the surface layer is low, uneven initial nucleation or insufficient tin residue after hot pressing can lead to continuous chromium exposure in the post-soldering contact area, causing fluctuations in the soldering contact resistance. Directly increasing the amount of tin deposition after abnormal soldering contact may still not eliminate the localized continuous chromium exposure caused by mismatch between the chromium post-contact surface condition and insufficient tin initial nucleation, and will also increase tin consumption.

[0007] Therefore, there is a need for a method that can link the chromium post-surface transition treatment, micro-tin first-layer nucleation, post-soldering tin residue and chromium exposure state verification, and subsequent process correction sequence during continuous composite plating, so that the correction of abnormal soldering contact no longer depends solely on increasing the amount of tin deposited. Summary of the Invention

[0008] This application provides a continuous composite plating method for weldable tinplate with chromium underlay and micro-tin plating, which improves the welding contact stability of the chromium underlay and micro-tin composite plating layer during the resistance welding process of can making while reducing the amount of tin used.

[0009] To achieve the above objectives, this application adopts the following technical solution: This application provides a continuous composite plating method for weldable tinplate with a chromium-based underlayer and micro-tin plating, comprising: electrodepositing a chromium underlayer onto the surface of a continuously operating can-making steel strip; before the chromium underlayer enters the micro-tin plating section, performing a surface transition treatment on the chromium underlayer based on the residual liquid film state and tin nucleation adaptation state after the chromium underlayer leaves the chromium plating section, and forming a micro-tin surface layer on the surface-transition treated chromium underlayer through micro-tin plating; performing welding contact verification on the can-making steel strip or sampling section with the micro-tin surface layer to obtain welding resistance information and tin residue in the post-weld contact area. The relative distribution of the chromium exposed area; when the welding resistance information is not within the preset welding resistance range, and the relative distribution indicates that there is a continuous chromium exposed area in the post-weld contact area, it is determined that the trace tin surface layer has not formed a hot-pressed conductive retention area. The hot-pressed conductive retention area is the welding contact area where tin residue is still retained after welding hot pressing and the continuous chromium exposed area is blocked; based on the determination result that the trace tin surface layer has not formed a hot-pressed conductive retention area, the subsequent continuous composite plating process is corrected in the order of first adjusting the surface transition treatment, then adjusting the first layer nucleation conditions of the trace tin surface layer, and finally adjusting the total deposition conditions of the trace tin surface layer.

[0010] The continuous composite plating method for weldable tinplate with chromium-based micro-tin plating provided in this application involves electrodepositing a chromium base layer on the surface of a steel strip used for can making, allowing the chromium base layer to provide basic protection. By selecting a surface transition treatment based on the state of the residual liquid film after the chromium base layer leaves the chromium plating section and the tin nucleation adaptation state, the subsequent micro-tin surface layer formation process is adapted to the post-chromium surface state. By verifying the relative distribution of tin residue and chromium exposed areas in the post-weld contact area, it can be determined whether the micro-tin surface layer still has a conductive contact area to prevent continuous chromium exposure after welding hot pressing. In this way, when welding contact abnormalities occur, the surface transition treatment and the first layer nucleation conditions can be adjusted first, and then the overall deposition conditions can be adjusted, which is beneficial for improving welding contact stability under low tin usage conditions.

[0011] In one possible implementation, the residual liquid film state is determined based on at least one of the following: liquid volume per unit area, residual liquid film conductivity, residual liquid film pH, and residual chromium ion content. The tin nucleation adaptation state is determined based on at least one of the following: the transfer time of the chromium undercoat from the chromium plating section to the micro-tin plating section, the surface dry spot state, the tin nucleation point distribution after a short-term trial plating, the initial tin plating current response, and the open-circuit potential change. Based on the residual liquid film state and the tin nucleation adaptation state, at least one of the following—residual liquid film conditioning, tin-containing liquid film replacement, and short-term activation—is selected as the surface transition treatment. This allows the post-chromium surface state to be linked to the subsequent micro-tin deposition process, avoiding the use of a single transition treatment method to handle different surface states.

[0012] In one possible implementation, when the chromium substrate surface has an excessive amount of residual liquid film, the residual liquid film is adjusted; when the chromium substrate surface is in a wet nucleation and adaptation state, the chromium substrate is replaced with a tin-containing liquid film; and when the chromium substrate surface is in an inactive state, the chromium substrate is briefly activated. This allows for the separate treatment of residual liquid film interference, insufficient tin-containing nucleation liquid film, and decreased surface activity.

[0013] In one possible implementation, residual liquid film conditioning of the chromium undercoat includes: squeezing or short-pass cleaning of the can-making steel strip after leaving the chromium plating section to reduce the residual chromium plating liquid film on the surface of the chromium undercoat; maintaining the surface of the chromium undercoat in a continuously wetted state after reducing the residual chromium plating liquid film; and sending the continuously wetted chromium undercoat into a tin-containing transition environment. This reduces interference from the residual chromium plating liquid film on tin deposition while preventing the chromium undercoat from drying out completely before entering the micro-tin plating section.

[0014] In one possible implementation, the chromium underlayer is replaced with a tin-containing liquid film, comprising: contacting the tin-containing transition liquid with the surface of the chromium underlayer; replacing at least a portion of the residual chromium plating liquid film or cleaning liquid film on the surface of the chromium underlayer with the tin-containing transition liquid; and after a tin-containing liquid film is formed on the surface of the chromium underlayer, feeding a can-making steel strip into the micro-tin plating section. This ensures that the surface of the chromium underlayer has tin-containing ion contact conditions before entering the micro-tin plating section, facilitating the deposition of tin at nucleation sites on the surface of the chromium underlayer.

[0015] In one possible implementation, short-term activation of the chromium underlayer includes: cathodic activation of the chromium underlayer surface in an acidic transition environment containing tin ions; reducing the barrier state of the chromium underlayer surface to tin deposition through cathodic activation and distributing tin ions at nucleation sites on the chromium underlayer surface; and allowing the chromium underlayer to enter the micro-tin plating stage in a undried state after cathodic activation. In this way, if the chromium underlayer becomes inactive due to dwell time, cleaning, or transfer, it can still recover a surface state suitable for tin first-layer nucleation before entering the micro-tin plating stage.

[0016] In one possible implementation, a trace tin surface layer is formed on a chromium underlayer after surface transition treatment by trace tin plating. This includes: in the initial stage of trace tin plating, tin deposition preferentially occurs at nucleation sites on the surface of the chromium underlayer to form dispersed tin first-layer nucleation regions; after the formation of the tin first-layer nucleation regions, at least partially adjacent tin first-layer nucleation regions are used to form a continuous welding contact region through subsequent tin deposition; after the formation of the continuous welding contact region, the surface tin content of the trace tin surface layer is controlled within a preset trace tin content range. In this way, the formation process of the trace tin surface layer not only focuses on the total tin content but also on whether a continuous region for welding contact can be formed between the tin first-layer nucleation regions.

[0017] In one possible implementation, adjusting the initial nucleation conditions of the trace tin surface layer includes: adjusting at least one of the following: the deposition current state, the residence time of the steel strip used for can making, and the contact state of the tin-containing plating solution during the initial stage of trace tin plating; transforming the tin deposition from a locally concentrated growth state to a dispersed nucleation state; and improving the continuity of the tin initial nucleation zone distribution on the chromium underlayer surface before adjusting the overall deposition conditions of the trace tin surface layer. In this way, when a continuous chromium exposed area exists in the post-soldering contact zone, the initial nucleation state of the trace tin surface layer can be improved first, and then the overall deposition conditions can be determined based on the surface tin content or welding resistance information.

[0018] In one possible implementation, weld contact verification is performed on a can-making steel strip or sampling section with a trace amount of tin on its surface. This includes: preparing a lap welded sample from the steel strip or sampling section with a trace amount of tin; performing resistance welding on the lap welded sample to obtain weld resistance information; and observing the post-weld contact area of ​​the lap welded sample to obtain the relative distribution of tin residue and exposed chromium area. Specifically, when the exposed chromium area extends continuously along the weld contact direction, or forms an exposed channel uninterrupted by tin residue in the weld contact width direction, and the tin residue cannot divide the exposed channel into multiple discontinuous exposed chromium areas, the corresponding exposed chromium area is defined as a continuous exposed chromium area. In this way, weld contact verification is based not only on resistance information but also on the relative distribution of post-weld tin residue and exposed chromium area.

[0019] In one possible implementation, the subsequent continuous composite plating process is modified in the following order: first, adjusting the surface transition treatment; then, adjusting the first-layer nucleation conditions of the trace tin surface layer; and finally, adjusting the total deposition conditions of the trace tin surface layer. This includes: when it is determined that the trace tin surface layer has not formed a thermo-pressed conductive retention area, first adjusting the residual liquid film adjustment, tin-containing liquid film replacement, or short-term activation in the surface transition treatment; if, after adjusting the surface transition treatment, subsequent welding contact verification still shows the existence of a continuous chromium exposed area, then adjusting the first-layer nucleation conditions of the trace tin surface layer; if, after adjusting the first-layer nucleation conditions of the trace tin surface layer, the surface tin content is determined by surface tin content detection to be not within the preset trace tin content range, or if there is no continuous chromium exposed area in the post-weld contact area but the welding resistance information is not within the preset welding resistance range, then adjusting the total deposition conditions of the trace tin surface layer. The total deposition conditions include at least one of the following: total charge of the trace tin plating section, effective deposition time, steel strip linear speed for can making, or tin ion concentration in the tin-containing plating solution. In this way, post-soldering contact anomalies can be traced back to the chromium post-surface transition treatment and the tin first-layer nucleation process. If the above adjustments are insufficient to meet the welding contact requirements, the overall deposition conditions can then be adjusted. Attached Figure Description

[0020] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments of this application and their descriptions are used to explain this application and do not constitute an undue limitation of this application.

[0021] Figure 1 This is a schematic diagram of a continuous composite plating production line provided in an embodiment of this application; Figure 2 This application provides a partial cross-sectional schematic diagram of a chromium-based trace tin composite plating layer as an embodiment of the present application. Figure 3 This is a schematic diagram of a surface transition processing path selection process provided in an embodiment of this application; Figure 4 This is a schematic diagram of a residual liquid film conditioning process provided in an embodiment of this application; Figure 5 A schematic diagram of a tin-containing liquid film displacement process provided in an embodiment of this application; Figure 6 A schematic diagram of a short-term activation process provided in an embodiment of this application; Figure 7 A schematic diagram of the first-layer nucleation and continuous deposition process of a trace amount of tin surface layer provided in this application embodiment; Figure 8 This is a schematic diagram of a welding contact verification process provided in an embodiment of this application; Figure 9 A comparative schematic diagram of a continuous chromium exposed area and a hot-pressed conductive retention area provided for embodiments of this application; Figure 10 This is a schematic diagram illustrating a correction sequence for a subsequent continuous composite plating process, as provided in an embodiment of this application.

[0022] Reference numerals: 100-Steel strip for can making; 101-Steel substrate; 110-Chromium undercoat; 120-Chromium plating residual liquid film; 130-Cleaning liquid film; 140-Tin-containing liquid film; 150-Trace tin surface layer; 151-Tin first layer nucleation zone; 152-Continuous welding contact zone; 160-Post-weld contact zone; 161-Tin residue; 162-Chromium exposed area; 163-Continuous chromium exposed area; 164-Hot-pressing conductive retention zone; 200-Continuous composite plating production line; 210-Pre-treatment section; 220-Chromium plating section; 230-Post-chromium transition section; 231-Extrusion assembly; 232-Short-range cleaning assembly; 233-Tin-containing liquid film replacement assembly; 234-Short-time activation assembly; 240-Trace tin plating section; 250-Welding contact verification section; 260-Process correction unit; 300-Overlap welding sample. Detailed Implementation

[0023] The technical solutions of the embodiments of this application will now be described with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them.

[0024] In the following description, the terms "first," "second," etc., are used for descriptive convenience only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this application, unless otherwise stated, "a plurality of" means two or more.

[0025] In the embodiments of this application, directional terms such as "upper", "lower", "inner", and "outer" may be defined relative to the schematic placement of components in the accompanying drawings. It should be understood that these directional terms can be relative concepts used for relative description and clarification, and they may change accordingly based on changes in the placement of components in the accompanying drawings.

[0026] In this embodiment, the trace tin surface layer refers to a tin layer or tin-rich layer formed on the side of the chromium underlayer away from the steel strip used for can making, used for welding contact. The term "trace" means that the surface tin content of this tin layer is lower than the amount of tin used in traditional full-surface tin-plated tinplate, which mainly serves the dual functions of protection and welding, and is within a preset trace tin content range. For example, the preset trace tin content range can be from 0.05 g / m² to 0.80 g / m², and more specifically, from 0.10 g / m² to 0.50 g / m².

[0027] In this embodiment, the tin first-layer nucleation region refers to the dispersed deposition area where tin preferentially forms on the surface of the chromium underlayer during the initial stage of trace tin plating. The tin first-layer nucleation region serves to provide a starting point for subsequent tin deposition. The tin first-layer nucleation region can be distributed in a dotted, island-like, or short-sheet pattern; this embodiment does not impose any special limitations on this distribution.

[0028] In this embodiment, the continuous welding contact area refers to the region formed by at least a portion of the tin nucleation area in the post-plating trace tin surface layer through subsequent tin deposition, which can provide tin to participate in the contact during subsequent welding. The continuous welding contact area is the post-plating region. In contrast, the hot-pressed conductive retention area refers to the region where, after welding hot pressing, tin residue remains in the post-weld contact area, blocking continuous chromium exposure. The hot-pressed conductive retention area is the post-weld region.

[0029] In this embodiment, a continuous chromium exposed area refers to a chromium exposed area within the post-solder contact area that extends continuously along the welding contact direction, or forms an exposed channel in the welding contact width direction that is not interrupted by tin residue. If tin residue can divide the exposed channel into multiple discontinuous chromium exposed areas, it is not defined as a continuous chromium exposed area.

[0030] In this embodiment, a state with excessive residual liquid film refers to a state where, after the chromium underlay leaves the chromium plating section, the residual chromium plating liquid film on its surface is sufficient to affect the contact of the tin-containing transition liquid or the initial deposition of tin ions. For example, when there is a visible dripping liquid film on the surface of the steel strip used for can making, or the amount of liquid carried per unit area is greater than 0.30 g / m², or the conductivity, pH, or chromium ion content of the residual liquid film indicates that it will significantly dilute the tin-containing transition liquid, it can be determined that the surface of the chromium underlay is in a state with excessive residual liquid film.

[0031] In this embodiment, the wet nucleation adaptation state refers to a state where the surface of the chromium plating underlayer is not completely dry, and the surface liquid film does not significantly hinder the formation of dispersed nucleation points of tin ions on the surface of the chromium plating underlayer. For example, when there are no obvious dry spots on the surface of the steel strip used for can making, the liquid content per unit area is 0.05 g / m² to 0.30 g / m², and dispersed tin nucleation points can be formed after a short trial plating, it can be determined that the surface of the chromium plating underlayer is in the wet nucleation adaptation state.

[0032] In this embodiment, the insufficient activity state refers to a state where the initial tin ion deposition nucleation points are reduced or the nucleation distribution is discontinuous due to chromium plating layer dwell time, excessive cleaning, or localized drying. For example, when the transfer time of the chromium plating layer from the chromium plating section to the micro-tin plating section exceeds a preset transfer time, or when the tin nucleation points are significantly sparse after a short trial plating, or when the initial tin plating current response decreases, or when the surface open-circuit potential shows an enhanced surface passivation trend, it can be determined that the chromium plating layer surface is in an insufficient activity state.

[0033] In this embodiment, the liquid content per unit area can be determined by offline weighing or online liquid film detection. In one example, a sample of steel strip for can manufacturing with a fixed area can be taken, and the liquid content can be measured without damaging the surface liquid film. The surface liquid film can then be removed, and the sample dried before weighing the dry weight. The difference between the two weights and the sample area is used to determine the liquid content per unit area. In another example, the liquid content per unit area can be calculated from the results of online liquid film thickness detection.

[0034] In this embodiment, the short-time trial plating can be performed in a tin-containing plating solution that is the same as or similar to the initial stage of trace tin plating. For example, the short-time trial plating time can be 0.2 s to 3 s, and the short-time trial plating current density can be 0.1 A / dm² to 2 A / dm². After the short-time trial plating, whether the tin nucleation sites are dispersed can be determined by magnified observation, elemental distribution detection, or surface electrochemical response.

[0035] In this embodiment, the preset trace tin content range can be determined based on the welding requirements of the target packaging material, the thickness of the steel strip used for can making, the type of welding equipment, and the target cost. For example, the surface tin content of the trace tin layer can be from 0.05 g / m² to 0.80 g / m², and more specifically from 0.10 g / m² to 0.50 g / m². The preset trace tin content range in this embodiment is not limited to the above numerical range and can also be determined based on the test welding results of qualified samples from the same batch. Surface tin content detection can be performed using X-ray fluorescence detection, electrostatic stripping detection, or elemental analysis after chemical dissolution.

[0036] In this embodiment, the preset welding resistance range can be determined based on the qualified welding window of the target can-making welding equipment. For example, at least 10 qualified welded samples from the same batch can be selected, and their average welding resistance information can be obtained. The preset welding resistance range can be defined as a percentage fluctuation around this average value; the preset percentage can be 10% to 30%. In another example, the preset welding resistance range can be determined based on the welding current window of the can-making welding machine and the weld quality assessment result.

[0037] In this embodiment, the tin residue and chromium exposed areas in the post-soldering contact area can be determined by scanning electron microscopy, energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), or other detection methods capable of distinguishing the distribution of tin and chromium. For example, an observation field with a magnification of 200x to 2000x can be selected within the post-soldering contact area, and 3 to 10 detection lines can be set along the welding contact direction or along the welding contact width direction within this observation field.

[0038] In one example, when the chromium signal on the detection line is continuously distributed and the tin signal does not form a lateral interruption on the detection line, it can be considered that there is a continuous chromium exposure at the corresponding location of the detection line. When at least one detection line has a continuous chromium exposure, or when the chromium exposure area continuously spans two or more adjacent observation positions, it can be determined that there is a continuous chromium exposure area in the post-weld contact area.

[0039] In one example, if elemental surface scanning is used for determination, areas where the relative tin signal is below a preset tin residue threshold and the relative chromium signal is above a preset chromium exposure threshold can be identified as chromium exposed areas. The preset tin residue threshold and preset chromium exposure threshold can be determined by calibration based on the elemental signals of unsoldered samples, fully chromium-plated areas, and tin-rich areas from the same batch. If the chromium exposed area forms a continuous region uninterrupted by tin residue areas in the weld contact direction or width direction, this continuous region is identified as a continuous chromium exposed area.

[0040] This application provides a continuous composite plating production line. For example... Figure 1 As shown, the continuous composite plating production line 200 may include a pretreatment section 210, a chromium plating section 220, a post-chromium transition section 230, a trace tin plating section 240, a welding contact verification section 250, and a process correction unit 260. The can-making steel strip 100 runs continuously along the continuous composite plating production line 200, passing through the above sections in sequence, to form a chromium underlayer 110 and a trace tin surface layer 150 on the surface of the can-making steel strip 100.

[0041] For example, the steel strip 100 for can making cans can be a low-carbon cold-rolled thin steel strip or other thin steel strips suitable for can making. Before entering the chrome plating section 220, the steel strip 100 for can making cans can undergo degreasing, pickling, water washing, or surface activation treatment in the pretreatment section 210 to remove oil, oxides, or surface impurities that affect electrodeposition. In the embodiments of this application, the specific method of surface pretreatment of the steel strip 100 for can making cans can be selected according to the state of the steel strip material.

[0042] like Figure 2 As shown, in some embodiments, a chromium underlayer 110 can be electrodeposited on the surface of the steel substrate 101, and a trace tin surface layer 150 can be formed on the side of the chromium underlayer 110 away from the steel substrate 101 by trace tin plating. The chromium underlayer 110 serves as a basic protective layer and provides a bearing surface for the trace tin surface layer 150. The trace tin surface layer 150 is used to participate in subsequent welding contact processes.

[0043] In some embodiments, the chromium underlayment 110 may include metallic chromium, or it may include a small amount of chromium oxide or chromium hydrate formed during continuous production. This application does not specifically limit this aspect. Exemplarily, the metallic chromium surface area of ​​the chromium underlayment 110 may be from 20 mg / m² to 100 mg / m², and more specifically from 30 mg / m² to 70 mg / m².

[0044] like Figure 3 As shown, in some embodiments, after the chromium underlayer 110 leaves the chromium plating section 220, a surface transition treatment can be performed on the chromium underlayer 110 according to the state of the residual liquid film and the tin nucleation adaptation state. The surface transition treatment may include at least one of residual liquid film adjustment, tin-containing liquid film replacement, and short-term activation. This selection process does not rely on a weighted calculation model, but rather enters the corresponding process path based on the actual state of the post-chromium surface.

[0045] In some embodiments, when there is a large amount of residual chromium plating liquid film 120 on the surface of the chromium underlayment 110, and the residual chromium plating liquid film 120 dilutes the subsequent tin-containing transition solution or obscures the tin nucleation sites, residual liquid film adjustment can be selected. When the surface of the chromium underlayment 110 remains wet and does not form a dry passivation state that significantly hinders tin deposition, tin-containing liquid film replacement can be selected. When the surface of the chromium underlayment 110 undergoes a long transfer, excessive cleaning, or localized drying, and its adaptability to tin deposition decreases, short-term activation can be selected.

[0046] like Figure 4 As shown, in some embodiments, when the surface of the chromium plating underlayer 110 has a high residual liquid film, the steel strip 100 for can making can be treated by the squeezing assembly 231 or the short-range cleaning assembly 232 to reduce the residual chromium plating liquid film 120 on the surface of the chromium plating underlayer 110. The squeezing assembly 231 may include a squeezing roller, an air knife, or a scraper; the short-range cleaning assembly 232 may include a sprayer, an immersion tank, or a flowing cleaning fluid contact structure.

[0047] In some embodiments, after reducing the residual chromium plating film 120, the surface of the chromium underlayment 110 is not allowed to dry completely, but rather is kept continuously wet. This continuous wetness means that there are no obvious dry spots on the surface of the chromium underlayment 110, and that the subsequent tin-containing transition solution can continuously spread on the surface of the chromium underlayment 110. Subsequently, the chromium underlayment 110, maintaining a continuous wetness, can be placed into a tin-containing transition environment.

[0048] like Figure 5As shown, in some embodiments, when the surface of the chromium plating underlayer 110 is in a wet nucleation and adaptation state, the tin-containing liquid film replacement component 233 can bring the tin-containing transition liquid into contact with the surface of the chromium plating underlayer 110. The tin-containing transition liquid can replace at least a portion of the residual chromium plating liquid film 120 or cleaning liquid film 130 on the surface of the chromium plating underlayer 110, so that a tin-containing liquid film 140 is formed on the surface of the chromium plating underlayer 110.

[0049] For example, the tin-containing transition solution can be an acidic transition solution containing stannous ions, or a tin-containing pre-wetting solution compatible with the subsequent trace tin plating solution. The concentration of stannous ions in the tin-containing transition solution can be from 0.5 g / L to 15 g / L, and the pH can be from 0.5 to 3.0. Different production lines can adjust the composition of the tin-containing transition solution according to the composition of the trace tin plating solution and the running speed of the steel belt used for can making.

[0050] like Figure 6 As shown, in some embodiments, when the surface of the chromium substrate 110 is in an insufficiently active state, the surface of the chromium substrate 110 can be cathodically activated in an acidic transition environment containing tin ions by the short-time activation component 234. Cathodic activation can reduce the barrier state of the chromium substrate 110 surface to tin deposition and allow tin ions to be distributed at the nucleation sites on the surface of the chromium substrate 110.

[0051] For example, the cathode activation time can be from 0.1 s to 5 s, and the activation current density can be from 0.1 A / dm² to 5 A / dm². After cathode activation, the can-making steel strip 100 can enter the micro-tin plating section 240 in a undried state. This can reduce the impact of re-passivation after activation on the nucleation of the first tin layer.

[0052] like Figure 7 As shown, in some embodiments, the chromium underlayer 110, after surface transition treatment, enters the micro-tin plating stage 240. In the initial stage of micro-tin plating, tin deposition preferentially occurs at nucleation sites on the surface of the chromium underlayer 110 to form dispersed tin first-layer nucleation regions 151. After the tin first-layer nucleation regions 151 are formed, subsequent tin deposition can form a continuous solder contact region 152 between at least partially adjacent tin first-layer nucleation regions 151.

[0053] In some embodiments, the initial stage of micro-tin plating can employ low current density deposition, short-time deposition, pulse deposition, or segmented deposition. For example, the current density in the initial stage of micro-tin plating can be from 0.2 A / dm² to 3 A / dm², and the dwell time can be from 0.2 s to 6 s. The current density in subsequent continuous deposition stages can be higher than that in the initial stage, or a different energizing method can be used to form a continuous solder contact region 152 between the tin first-layer nucleation regions 151.

[0054] In some embodiments, if the trace tin surface layer 150 exhibits localized concentrated tin deposition while other areas lack the initial tin nucleation zone 151, at least one of the following can be adjusted during the initial stage of trace tin plating: the deposition current state, the residence time of the steel strip for can making, and the contact state of the tin-containing plating solution. This can transform the tin deposition from a locally concentrated growth state to a dispersed nucleation state. The deposition current state may include current density, energizing time, energizing continuity, or a pulsed state. The contact state of the tin-containing plating solution may include plating solution spraying, stirring, circulation, or wetting of the steel strip surface for can making.

[0055] In some embodiments, the overall deposition conditions of the trace tin surface layer 150 may include at least one of the following: total current flow of the trace tin plating section 240, deposition current density, effective deposition time, tin ion concentration in the tin-containing plating bath, plating bath temperature, can-making steel strip linear speed, and the distance between the anode and the can-making steel strip. These overall deposition conditions are used to influence the overall surface tin content of the trace tin surface layer 150.

[0056] like Figure 8 As shown, in some embodiments, weld contact verification can be performed on the can-making steel strip 100 or sampling section having a trace tin surface layer 150. Weld contact verification may include preparing a lap weld specimen 300, performing resistance welding on the lap weld specimen 300, and observing the post-weld contact area 160. The lap weld specimen 300 may be formed by cutting and stacking pieces of the can-making steel strip 100 having a trace tin surface layer 150, or it may be made from the sampling section during continuous production.

[0057] In some embodiments, after resistance welding the lap welded specimen 300, welding resistance information can be obtained. The welding resistance information may include at least one of the following: changes in contact resistance during the welding process, the applicable range of welding current, and resistance fluctuations at the weld point or weld seam. Subsequently, the welded lap welded specimen 300 can be observed in the post-weld contact area 160 to obtain the relative distribution of tin residue 161 and exposed chromium area 162. The post-weld contact area 160 observation can be performed through cross-sectional observation, surface magnification observation, elemental distribution detection, or other methods capable of distinguishing between tin residue 161 and exposed chromium area 162.

[0058] like Figure 9 As shown, in some embodiments, when the exposed chromium area 162 in the post-solder contact area 160 extends continuously along the welding contact direction, or forms an exposed channel in the welding contact width direction that is not separated by the tin residue 161, and the tin residue 161 cannot divide the exposed channel into multiple discontinuous exposed chromium areas 162, the exposed chromium area 162 that satisfies the above conditions can be defined as a continuous exposed chromium area 163.

[0059] In one example, the post-weld contact area 160 can be observed along the weld contact direction. If the exposed chromium area 162 continuously spans two or more adjacent inspection positions, and no residual tin band exists between the adjacent inspection positions that can interrupt the exposed chromium channel, a continuous exposed chromium area 163 can be identified. In another example, 3 to 10 observation lines can be set along the weld contact width direction. If the exposed chromium area 162 is continuous along at least one observation line and is not divided by residual tin 161, a continuous exposed chromium area 163 can be identified. If a residual tin band can cross the exposed channel and divide the exposed chromium area 162 into two or more non-connected areas, the residual tin 161 can be considered to interrupt the exposed channel.

[0060] In some embodiments, when the welding resistance information is not within the preset welding resistance range, and the relative distribution of tin residue 161 and chromium exposed area 162 indicates that there is a continuous chromium exposed area 163 in the post-weld contact area 160, it can be determined that the trace amount of tin surface layer 150 has not formed a thermo-pressed conductive retention area 164. The thermo-pressed conductive retention area 164 can be understood as tin residue 161 still remaining after welding thermo-pressing, and capable of blocking the continuous chromium exposed welding contact area.

[0061] like Figure 10 As shown, in some embodiments, when it is determined that the trace tin surface layer 150 has not formed a thermo-pressed conductive retention area 164, the residual liquid film adjustment, tin-containing liquid film replacement, or short-term activation in the surface transition treatment can be adjusted first. Specifically, if it is determined that the residual chromium plating liquid film 120 still interferes with tin deposition, the residual liquid film adjustment can be strengthened; if it is determined that the tin-containing liquid film 140 is insufficient before the chromium underlayer 110 enters the trace tin plating section 240, the tin-containing liquid film replacement can be adjusted; if it is determined that the surface activity of the chromium underlayer 110 is insufficient, the short-term activation can be adjusted.

[0062] In some embodiments, if subsequent welding contact checks still show the presence of continuous exposed chromium areas 163 after adjusting the surface transition treatment, the initial nucleation conditions of the trace tin surface layer 150 can be adjusted. This adjustment can be made in the initial stage of trace tin plating, focusing on improving the formation location and distribution continuity of the tin initial nucleation area 151, rather than directly improving the overall deposition conditions of the trace tin surface layer 150.

[0063] In some embodiments, if after adjusting the initial nucleation conditions of the trace tin surface layer 150, the surface tin content of the trace tin surface layer 150 is determined by surface tin content detection to be outside the preset trace tin content range, or if the post-soldering contact area 160 does not have a continuous exposed chromium area 163 but the welding resistance information is outside the preset welding resistance range, then the overall deposition conditions of the trace tin surface layer 150 can be adjusted. The overall deposition conditions may include at least one of the following: the total current carrying capacity of the trace tin plating section 240, the effective deposition time, the linear speed of the steel strip used for can making, or the tin ion concentration in the tin-containing plating solution.

[0064] In the embodiments of this application, the correction of the subsequent continuous composite plating process is not to directly increase the amount of tin deposition after any welding abnormality occurs, but to determine the priority correction target based on the correspondence between welding resistance information, tin residual state and chromium exposed state.

[0065] For example, when the welding resistance information is not within the preset welding resistance range, and there is a large area of ​​continuous exposed chromium in the post-weld contact area, it is primarily determined that the post-chromium surface transition treatment is insufficient. In this case, priority can be given to enhancing residual liquid film adjustment, extending the tin-containing liquid film replacement contact time, or enhancing short-term activation. When there are intermittent exposed chromium areas in the post-weld contact area, and the tin residue distribution is locally concentrated, it is primarily determined that the tin first-layer nucleation area is unevenly distributed. In this case, priority can be given to adjusting the deposition current state, the residence time of the steel strip for can making, or the contact state of the tin-containing plating solution in the initial stage of micro-tin plating. When there is no continuous exposed chromium area in the post-weld contact area, but the welding resistance information is still not within the preset welding resistance range, or the surface tin amount is not within the preset micro-tin amount range, then the overall deposition conditions are adjusted.

[0066] In one example, different anomalies and priority correction targets can be determined as follows: When a continuous chromium-exposed area is formed in the post-soldering contact zone and tin residue is scarce, the surface transition treatment is corrected first, and the adjustment direction includes enhancing residual liquid film conditioning, extending the tin-containing liquid film replacement time, or improving short-term activation sufficiency; When a local chromium-exposed area is formed in the post-soldering contact zone and tin residue is locally agglomerated or island-like concentrated, the first-layer nucleation conditions are corrected first, and the adjustment direction includes reducing the initial deposition current density, shortening the single energizing time, or improving the contact uniformity of the tin-containing plating solution; When a continuous chromium-exposed area is not formed in the post-soldering contact zone but the welding resistance information is still not within the preset welding resistance range, the total deposition conditions are corrected, and the adjustment direction includes adjusting the total current charge, effective deposition time, steel strip linear speed for can making, or tin ion concentration in the tin-containing plating solution; When the tin content on the surface of the trace tin is lower than the preset trace tin content range, the total deposition conditions are corrected, and the adjustment direction includes increasing the total current charge of the trace tin plating section or extending the effective deposition time.

[0067] Example 1: This example provides a continuous composite plating method under conditions of excessive residual liquid film. The steel strip 100 for can making is a low-carbon cold-rolled thin steel strip with a thickness of 0.18 mm. After degreasing, water washing, and pickling, the steel strip 100 enters the chromium plating section 220, where a chromium underlayer 110 is electrodeposited on the surface of the steel strip 100, with a chromium surface area of ​​45 mg / m².

[0068] After the steel strip 100 for can making leaves the chrome plating section 220, a continuous liquid film was observed on the surface of the chrome plating underlayer 110. Sampling revealed a liquid content of approximately 0.42 g / m², indicating a high residual liquid film. Subsequently, the liquid was squeezed out using a squeezing roller and cleaned with a short-range spray, reducing the liquid content per unit area to approximately 0.16 g / m², while maintaining a continuously moist surface on the chrome plating underlayer 110.

[0069] A continuously moistened chromium underlayer 110 is introduced into a tin-containing transition environment, allowing the tin-containing transition solution to contact the surface of the chromium underlayer 110 and form a tin-containing liquid film 140. Subsequently, it enters the micro-tin plating stage 240, where a tin nucleation zone 151 is formed in the initial stage of micro-tin plating, and a micro-tin surface layer 150 is formed in subsequent deposition stages. The surface tin content of the micro-tin surface layer 150 is 0.22 g / m².

[0070] A lap weld specimen 300 was prepared from the sampled section and subjected to resistance welding. After welding, tin residue 161 in the contact area 160 divided the exposed chromium area 162 into multiple discontinuous regions; no continuous exposed chromium area 163 was observed extending along the welding contact direction. The welding resistance information was within the preset welding resistance range, and the weld appearance was continuous.

[0071] Example 2: This example provides a continuous composite plating method under wet nucleation adaptation conditions. After pretreatment, the steel strip 100 for can making enters the chromium plating section 220, forming a chromium underlayer 110 with a chromium surface area of ​​40 mg / m². After the steel strip 100 leaves the chromium plating section 220, the surface of the chromium underlayer 110 shows no obvious dry spots or obvious dripping film, with a liquid content of approximately 0.12 g / m² per unit area. A short-term trial plating test shows the formation of dispersed tin nucleation points, indicating a wet nucleation adaptation state.

[0072] In this embodiment, instead of performing additional short-term activation, the tin-containing transition liquid is directly contacted with the surface of the chromium underlayer 110 to replace at least part of the cleaning liquid film 130 and form a tin-containing liquid film 140. Subsequently, the can-making steel strip 100 is fed into the micro-tin plating section 240 to form a tin first-layer nucleation region 151 on the surface of the chromium underlayer 110, and a continuous welding contact region 152 is formed through subsequent deposition.

[0073] In this embodiment, the surface tin content of the trace tin layer 150 is 0.18 g / m². Solder contact verification shows that tin residue 161 exists in the post-soldering contact area 160, the exposed chromium area 162 does not form a through-hole exposed channel, and the trace tin layer 150 forms a thermo-pressed conductive retention area 164. The soldering resistance information is within the preset soldering resistance range.

[0074] Example 3 provides a continuous composite plating method under insufficient activity conditions. After the steel strip 100 for can making forms a chromium underlayer 110 in the chromium plating section 220, the transfer time of the chromium underlayer 110 from the chromium plating section 220 to the micro-tin plating section 240 is prolonged due to changes in the production cycle. After short-term trial plating observation, the tin nucleation points are sparsely distributed and there are local blank areas, indicating that the surface of the chromium underlayer 110 is in an insufficient activity state.

[0075] In this embodiment, the chromium underlayer 110 surface is cathodically activated in an acidic transition environment containing tin ions for 1.5 s, with an activation current density of 1.0 A / dm². After cathodic activation, the can-making steel strip 100 enters the micro-tin plating section 240 in a undried state.

[0076] In the initial stage of micro-tin plating, a low current density deposition is used to form a dispersed tin nucleation zone 151. Subsequent continuous deposition stages form a micro-tin surface layer 150 with a surface tin content of 0.24 g / m². Solder contact verification shows that the tin residue 161 in the post-solder contact area 160 effectively blocks the continuous extension of the exposed chromium area 162, preventing the formation of a continuous exposed chromium area 163. The solder resistance information is within the preset solder resistance range.

[0077] Comparative Example 1: This comparative example provides a method for direct micro-tin plating after chromium plating. The steel strip 100 for can making, after pretreatment and chromium plating, does not undergo surface transition treatment based on the residual liquid film state and tin nucleation compatibility; instead, it directly enters the micro-tin plating section 240. The surface tin content of the micro-tin surface layer 150 is controlled at 0.22 g / m².

[0078] Welding contact verification revealed a continuous exposed chromium area 163 extending along the welding contact direction within the post-weld contact area 160, with localized tin residue 161 failing to block the exposed chromium channel. The welding resistance information was not within the preset welding resistance range, and some welds exhibited contact instability. This result indicates that, with the same trace amount of tin, without surface transition treatment, it is difficult for the trace amount of tin surface layer 150 to stably form a thermo-pressed conductive retention area 164.

[0079] Comparative Example 2 provides a method for directly increasing the total deposition conditions after a welding anomaly. The steel strip 100 for can making is chromium-plated and then directly subjected to micro-tin plating. After a continuous chromium-exposed area 163 is discovered during welding contact inspection, instead of adjusting the surface transition treatment and the first-layer nucleation conditions, the total deposition conditions of the micro-tin surface layer 150 are directly increased, raising the surface tin content from 0.22 g / m² to 0.36 g / m².

[0080] Welding contact verification showed an increase in tin residue 161, but localized continuous chromium exposed areas 163 still existed in some post-weld contact areas 160, and the fluctuation in welding resistance information remained significant. This result indicates that, under conditions where the post-chromium surface condition or the tin first-layer nucleation condition is incompatible, simply improving the overall deposition conditions of the trace tin surface layer 150 cannot stably eliminate the continuous chromium exposed areas 163, and will increase tin consumption.

[0081] Example 4 provides a method for subsequent process correction based on the state of the post-solder contact area. During continuous composite plating production, a sampling section is subjected to weld contact verification to obtain weld resistance information and the relative distribution of tin residue 161 and exposed chromium area 162 in the post-solder contact area 160. If the weld resistance information is not within the preset weld resistance range, and there is a continuous exposed chromium area 163 in the post-solder contact area 160, it is determined that the trace tin surface layer 150 has not formed a thermo-pressed conductive retention area 164.

[0082] In this embodiment, the surface transition treatment after chromium plating is first checked. If it is found that there is too much residual liquid film after the chromium underlayer 110 leaves the chromium plating section 220, the squeezing or short-range cleaning is prioritized. If it is found that the tin-containing liquid film 140 is not formed sufficiently, the contact time or spraying state of the tin-containing transition liquid is adjusted. If it is found that the surface activity of the chromium underlayer 110 is insufficient, the short-time activation time or activation current density is adjusted.

[0083] If, after adjusting the surface transition treatment, the welding contact check still shows a continuous chromium exposed area 163, then the initial nucleation conditions of the first layer in the initial stage of micro-tin plating should be adjusted. For example, the initial deposition current density can be reduced, the single energizing time can be shortened, or the contact uniformity of the tin-containing plating solution on the surface of the can-making steel strip 100 can be improved, so that the tin deposition changes from a local concentrated growth state to a dispersed nucleation state.

[0084] After adjusting the first-layer nucleation conditions, if the surface tin content of the trace tin layer 150 is not within the preset trace tin content range as determined by the surface tin content detection, or if the post-soldering contact area 160 does not have a continuous exposed chromium area 163 but the welding resistance information is not within the preset welding resistance range, then the overall deposition conditions are adjusted. For example, the total current carrying capacity, effective deposition time, can-making steel strip linear speed, or tin ion concentration of the trace tin plating section 240 can be adjusted.

[0085] Example 5 provides a comparative verification method. Steel strip 100 from the same batch for can making was selected, and a chromium underlayer 110 was formed under the same chromium plating conditions. Samples A, B, C, and D were prepared respectively. Sample A is a comparative example of direct micro-tin plating without surface transition treatment; Sample B is a comparative example of improving the overall deposition conditions only after welding anomalies; Sample C is an example of tin-containing liquid film replacement and two-stage micro-tin plating; Sample D is an example of modification following the order of surface transition treatment, first-layer nucleation conditions, and overall deposition conditions.

[0086] In one example, sample A had a solder amount of 0.22 g / m², a standard deviation of solder resistance of 13.8%, a continuous chromium exposure rate of 32%, and a cold solder joint rate of 6.5%; sample B had a solder amount of 0.36 g / m², a standard deviation of solder resistance of 10.9%, a continuous chromium exposure rate of 18%, and a cold solder joint rate of 3.8%; sample C had a solder amount of 0.22 g / m², a standard deviation of solder resistance of 5.6%, a continuous chromium exposure rate of 6%, and a cold solder joint rate of 1.1%; and sample D had a solder amount of 0.24 g / m², a standard deviation of solder resistance of 4.8%, a continuous chromium exposure rate of 3%, and a cold solder joint rate of 0.6%.

[0087] The above comparison results show that, under the same or similar tin content conditions, tin-containing liquid film replacement, short-time activation, and adjustment of the first-layer nucleation conditions can reduce the probability of continuous chromium exposure areas. After welding contact abnormalities occur, the correction should be carried out in the order of first adjusting the surface transition treatment, then adjusting the first-layer nucleation conditions, and finally adjusting the total deposition conditions. Compared with only improving the total deposition conditions, this can improve the welding resistance stability and welding qualification rate at a lower tin content.

[0088] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A continuous composite plating method for solderable tinplate with chromium underlay and micro-tin plating, characterized in that, include: A chromium underlayer is formed by electrodeposition on the surface of a continuously operating steel strip used for can making; Before the chromium base layer enters the micro-tin plating section, the chromium base layer is subjected to surface transition treatment based on the residual liquid film state and tin nucleation adaptation state after the chromium base layer leaves the chromium plating section. Then, a micro-tin surface layer with a surface tin content within a preset micro-tin content range is formed on the chromium base layer after the surface transition treatment by micro-tin plating. Welding contact verification was performed on the steel strip or sampling section of the can-making plant with the aforementioned trace tin surface layer to obtain welding resistance information and the relative distribution of tin residue and chromium exposed area in the post-weld contact area. When the welding resistance information is not within the preset welding resistance range, and the relative distribution state indicates that there is a continuous chromium exposed area in the post-weld contact area, it is determined that the trace amount of tin surface layer has not formed a hot-pressed conductive retention area. The hot-pressed conductive retention area is the welding contact area where tin residue is still retained after welding hot pressing and the continuous chromium exposed area is blocked. Based on the determination that the trace tin surface layer did not form a hot-pressed conductive retention area, the subsequent continuous composite plating process was modified in the following order: first, the surface transition treatment was adjusted; then, the first-layer nucleation conditions of the trace tin surface layer were adjusted; and finally, the overall deposition conditions of the trace tin surface layer were adjusted. The surface transition treatment of the chromium underlayer, based on the residual liquid film state and tin nucleation adaptation state after the chromium underlayer leaves the chromium plating section, includes: The state of the residual liquid film is determined based on at least one of the following: liquid volume per unit area, residual liquid film conductivity, residual liquid film pH, and residual chromium ion content. The tin nucleation adaptation state is determined based on at least one of the following: the transfer time from the chromium plating section to the micro-tin plating section, the surface dry spot state, the distribution of tin nucleation points after short-term trial plating, the initial tin plating current response, and the open circuit potential change. Based on the residual liquid film state and the tin nucleation adaptation state, at least one of residual liquid film conditioning, tin-containing liquid film replacement, and short-term activation is selected as the surface transition treatment.

2. The continuous composite plating method for weldable tinplate with chromium underlay and micro-tin plating according to claim 1, characterized in that, The step of selecting at least one of residual liquid film conditioning, tin-containing liquid film replacement, and short-term activation as the surface transition treatment based on the residual liquid film state and the tin nucleation adaptation state includes: When the surface of the chromium-plated underlayer is in a state of excessive residual liquid film, the residual liquid film of the chromium-plated underlayer is adjusted. When the chromium substrate surface is in a wet nucleation and adaptation state, the chromium substrate is replaced with a tin-containing liquid film. When the chromium substrate surface is in an inactive state, the chromium substrate is activated for a short time.

3. The continuous composite plating method for weldable tinplate with chromium underlay and micro-tin plating according to claim 2, characterized in that, The residual liquid film conditioning of the chromium underlayer includes: The steel strip used for can making is squeezed or short-range cleaned after leaving the chromium plating section to reduce the residual chromium plating film on the surface of the chromium plating underlayer. After reducing the residual chromium plating film, the surface of the chromium underlayer is kept continuously wet. The chromium underlayer, which is kept continuously moist, is then placed into a tin-containing transition environment.

4. The continuous composite plating method for weldable tinplate with chromium underlay and micro-tin plating according to claim 2, characterized in that, The process of replacing the chromium underlayer with a tin-containing liquid film includes: The tin-containing transition liquid is brought into contact with the chromium base layer surface; The tin-containing transition solution is used to replace at least a portion of the residual chromium plating liquid film or cleaning liquid film on the surface of the chromium underlayer. After a tin-containing liquid film is formed on the surface of the chromium-plated underlayer, the steel strip for can making is fed into the micro-tin plating section.

5. The continuous composite plating method for weldable tinplate with chromium underlay and micro-tin plating according to claim 2, characterized in that, The short-term activation of the chromium underlayer includes: The chromium substrate surface is cathodically activated in an acidic transition environment containing tin ions; The cathode activation reduces the barrier state of the chromium substrate surface to tin deposition and allows tin ions to be distributed at the nucleation sites on the chromium substrate surface. After the cathode is activated, the chromium underlayer is allowed to enter the micro-tin plating section while still undried.

6. The continuous composite plating method for weldable tinplate with chromium underlay and micro-tin plating according to any one of claims 1-5, characterized in that, The process of forming a micro-tin surface layer on a chromium underlayer after surface transition treatment by micro-tin plating includes: In the initial stage of micro-tin plating, tin deposition is preferentially carried out at the nucleation sites on the surface of the chromium underlayer to form a dispersed tin first-layer nucleation zone. After the tin first layer nucleation region is formed, at least some of the adjacent tin first layer nucleation regions are made to form a continuous welding contact region through subsequent tin deposition; After the continuous welding contact area is formed, the surface tin amount of the trace tin layer is controlled within a preset trace tin amount range.

7. The continuous composite plating method for weldable tinplate with chromium underlay and micro-tin plating according to claim 6, characterized in that, The adjustment of the first-layer nucleation conditions of the trace tin surface layer includes: Adjust at least one of the following: the deposition current state in the initial stage of micro-tin plating, the residence time of the steel strip for can making, and the contact state of the tin-containing plating solution; This transforms tin deposition from a locally concentrated growth state to a dispersed nucleation state; Before adjusting the overall deposition conditions of the trace tin surface layer, improve the distribution continuity of the tin first-layer nucleation region on the chromium-based underlayer surface.

8. The continuous composite plating method for weldable tinplate with chromium underlay and micro-tin plating according to claim 1, characterized in that, The welding contact verification of the can-making steel strip or sampling section with the trace tin surface layer includes: Prepare lap-welded specimens from can-making steel strips or sampling sections with the aforementioned trace amount of tin surface layer; The lap welded sample was subjected to resistance welding to obtain the welding resistance information. The post-weld contact area of ​​the welded lap welded sample was observed to obtain the relative distribution of the tin residue and the exposed chromium area. Specifically, when the exposed chromium area extends continuously along the welding contact direction, or forms an exposed channel that is not interrupted by tin residue in the welding contact width direction, and the tin residue cannot divide the exposed channel into multiple discontinuous exposed chromium areas, the corresponding exposed chromium area is defined as the continuous exposed chromium area.

9. The continuous composite plating method for weldable tinplate with chromium underlay and micro-tin plating according to claim 8, characterized in that, The process of correcting subsequent continuous composite plating, following the sequence of first adjusting the surface transition treatment, then adjusting the first-layer nucleation conditions of the trace tin surface layer, and finally adjusting the overall deposition conditions of the trace tin surface layer, includes: When it is determined that the trace tin surface layer has not formed a thermo-pressed conductive retention area, the residual liquid film adjustment, tin-containing liquid film replacement, or short-term activation in the surface transition treatment are first adjusted. If, after adjusting the surface transition treatment, subsequent welding contact verification still shows the presence of continuous exposed chromium areas, then the first-layer nucleation conditions of the trace tin surface layer should be adjusted. If, after adjusting the first-layer nucleation conditions of the trace tin surface layer, the surface tin content is determined by surface tin content detection to be outside the preset trace tin content range, or if there is no continuous exposed chromium area in the post-soldering contact area but the welding resistance information is outside the preset welding resistance range, then the total deposition conditions of the trace tin surface layer are adjusted. The total deposition conditions include at least one of the following: total charge on the trace tin plating section, effective deposition time, steel strip linear speed for can making, or tin ion concentration in the tin-containing plating solution.

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