Production process of weldable electroplated chromium thin steel plate

Abstract

The invention belongs to the technical field of metal material surface treatment, and discloses a production process of a weldable electroplated chromium plate. According to the process, cluster-shaped peak morphology is transferred on the surface of a cold-rolled steel coil through a laser texturing roller, and staged double-layer chromium electroplating is combined, specifically, a discontinuous chromium layer is formed on the top of a micro convex peak through first-time high-current pulse electroplating, and local iron leakage points are reserved; the second low current direct current plating covers an ultrathin continuous chromium film to maintain macro passivation. According to the invention, a composite functional structure with both macroscopic passivation integrity and microcosmic conductive selectivity is constructed on the surface of chromium-plated iron by accurately controlling the microstructure of the surface of a base material and double-layer chromium plating process parameters, so that the technical bottleneck of the chromium-plated plate in resistance welding application is effectively solved, collaborative optimization of weldability and corrosion resistance is realized, and the application range of the chromium-plated plate in resistance welding is widened. And a reliable technical path is provided for comprehensive replacement of chromium-plated iron in the field of metal packaging.

Description

Technical Field

[0001] This invention belongs to the field of metal material surface treatment technology, and relates to a production process for weldable electroplated chromium thin steel sheets. Background Technology

[0002] In the field of metal packaging materials, electroplated chromium thin steel sheet is a material made by electroplating a layer of metallic chromium and hydrated chromium oxide onto thin steel sheet. Compared to tin-plated thin steel sheet (tinplate), the chromium layer is thinner, and the hydrated chromium oxide can form hydrogen bonds with the coating, eliminating the need for tin resources. It has lower manufacturing costs, better coating adhesion, and sulfur resistance, making it a high-quality metal packaging material to replace tinplate. It has long been widely used in applications such as top and bottom lids and deep-drawn two-piece cans.

[0003] While chrome-plated iron performs well in non-welding applications, its application in three-piece cans has consistently faced obstacles. Currently, most three-piece can welds on the market use resistance welding. The principle of resistance welding is to utilize the heat effect generated by the current flowing through the resistance in the welding circuit, while applying a certain pressure to permanently fuse the metal together. However, metallic chromium has a melting point of approximately 1900℃, far higher than the iron substrate (approximately 1538℃), and chromium oxide has an even higher melting point (approximately 2435℃). The resistivity of the metal plating is as high as 2170 Ω·m. During resistance welding, the chromium plating layer acts as a thermal barrier, preventing heat from being conducted to the iron substrate to achieve steel plate welding. Therefore, it faces significant technical bottlenecks in the welding of three-piece can seams, severely restricting its widespread adoption in mainstream metal packaging structures.

[0004] Existing technologies typically employ three strategies: First, using high-power or special waveform welding equipment to forcibly break through the oxide layer, such as laser welding. However, such equipment is very expensive and difficult to widely promote and use. Second, mechanical grinding or chemical treatment is performed before welding to remove the surface oxide film. However, the welding speed is not high due to the limitations of the grinding process. Third, a tin-plating followed by chromium plating process is used. However, the welding conditions are relatively narrow when welding such products, and adjustments are difficult. Furthermore, the tin-plating followed by chromium plating process also increases the number of steps.

[0005] The contradiction between the functional properties of the chromium plating layer and the requirements of the welding process lies in the following: on the one hand, the corrosion resistance and barrier properties of chromium-plated iron require a complete chromium oxide film; on the other hand, the high resistivity of this film hinders current conduction during the welding process. This contradiction is particularly prominent on chromium-plated plates with traditionally smooth or slightly rough surfaces, because their surface morphology is uniform and continuous, and the oxide film is completely covered, leaving almost no exposed metal for the formation of conductive pathways. Even by optimizing electroplating parameters to control the thickness or crystal morphology of the chromium layer, it is difficult to achieve a substantial breakthrough in welding performance without sacrificing corrosion resistance. Summary of the Invention

[0006] To achieve the aforementioned objectives, this invention provides a manufacturing process for weldable electroplated chromium sheets. This process constructs a cluster-like morphology with a specific microscopic topology on the surface of a cold-rolled steel coil substrate, and combines this with a staged, differentiated parameter-controlled double-layer electroplating process. While maintaining the overall barrier properties and corrosion resistance of the chromium plating layer, it forms controllable conductive pathways at the microscopic scale, thereby significantly improving the weldability of the material during resistance welding. This solves the technical problem in existing technologies where excessively high contact resistance caused by the surface chromium oxide film leads to defects such as incomplete welds and weld failures.

[0007] The manufacturing process of the weldable electroplated chromium plate of the present invention includes the following steps: S1. Roll Surface Pretreatment: The surface of the working roll is modified using laser texturing equipment to create a clustered array of micro-pits on its surface. The clustered distribution refers to multiple micro-pits agglomerated with a geometric center spacing of no more than 20 μm to form a cluster unit. Each cluster unit contains 3 to 9 micro-pits, and the minimum spacing between each cluster unit is no less than 50 μm. The depth of the micro-pits is 8-15 μm, the opening diameter is 3-8 μm, and the sidewall inclination angle is 60°-80°. The metal surface after being rolled by this roll will form a clustered micro-protrusion structure that mirrors the micro-pit array. Its three-dimensional morphological characteristics are that multiple adjacent peaks surround a local high point to form a mountain-like cluster. The height of a single peak is 8-15 μm, and the radius of curvature of the peak is 3-8 μm.

[0008] S2, Surface Transfer: Using rolls treated in S1, cold-rolled steel coils with a thickness of 0.18-0.30mm are leveled and rolled with a rolling reduction rate of 0.8%-1.5%, a rolling speed of 400-800m / min, and a rolling tension of 8-15kN. This completely transfers the clustered micro-protrusion structure to the surface of the cold-rolled steel coil, resulting in a cold-rolled steel coil substrate with a mountain-shaped clustered surface morphology. The surface roughness Ra of this substrate is 0.8-1.5μm, the Rz value is 6-12μm, and the surface is free of oil spots, scratches, or oxide scale residue.

[0009] S3, Alkaline Washing: The cold-rolled steel coil substrate obtained in S2 is continuously fed into an alkaline washing tank for degreasing and cleaning; the alkaline washing solution is a mixed aqueous solution of sodium hydroxide and sodium carbonate, wherein the concentration of sodium hydroxide is 30-50 g / L, the concentration of sodium carbonate is 20-40 g / L, the cleaning temperature is 60-75℃, and the cleaning time is 3-6 s; after cleaning, it undergoes three-stage countercurrent rinsing, the rinsing water temperature is 40-50℃, and the conductivity is less than 50 micro Siemens / cm to ensure that the surface cleanliness meets the pretreatment standard for electroplating.

[0010] S4. First Electroplating of Chromium: The steel coil substrate, cleaned and dried in S3, is continuously introduced into the first electroplating tank for pulse electroplating. The first electroplating tank contains a chromic acid-based plating solution with the following composition: 200-250 g / L chromic anhydride and 1.0-2.5 g / L sulfuric acid; preferably, the chromic anhydride concentration is 210-235 g / L and the sulfuric acid concentration is 1.4-2.0 g / L; the plating solution temperature is controlled at 60-80℃, preferably 65-75℃; the current density is set to 60-80 A / dm³. 2 Preferably 65-73 A / dm 2 The power supply used is a unidirectional pulsed DC power supply with a pulse frequency of 50-200Hz and a duty cycle of 30%-60%. The electroplating time is 8-15s. Under these process conditions, due to the clustered peak structure on the substrate surface, the electric field is first highly concentrated at the top of the micro-protrusions, causing chromium ions to preferentially deposit in the peak area, forming a discontinuous first chromium plating layer with a thickness of 30-60nm. This plating layer covers the top of multiple peaks inside the cluster unit with dense metallic chromium, while in the flat areas between clusters, the current density is low, resulting in sparse deposition or even local exposure of the base iron. After electroplating, the steel coil is cleaned by deionized water spraying at a water temperature of 30-40℃ and a spraying pressure of 0.2-0.4MPa.

[0011] S5. Second Electroplating of Chromium: The steel coil, after S4 electroplating and cleaning, is introduced into the second electroplating tank for direct current electroplating. The second electroplating tank contains a chromic acid-based plating solution with a different formulation: 50-90 g / L chromic anhydride and 0.35-0.55 g / L sulfuric acid. The plating solution temperature is controlled at 40-50℃, and the current density is set to 25-30 A / dm³. 2 The power supply used is a constant DC power supply; the electroplating time is 5-10s; under these low concentration, low temperature and low current density conditions, the chromium deposition rate is significantly reduced, and the newly generated chromium layer uniformly covers the first chromium plating layer and the exposed substrate area, forming a continuous second chromium plating layer with a thickness of 10-25nm; although the second chromium plating layer is thin, it is sufficient to seal the surface pores on a macroscopic scale and restore the overall passivation ability, while retaining the iron leakage point distribution characteristics formed by the first chromium plating layer on a microscopic scale; the final chromium plating plate has a total chromium layer thickness of 40-85nm, of which the metallic chromium content is not less than 95%, and the remainder is trace amounts of chromium oxide and adsorbed water molecules.

[0012] The geometric arrangement of the clustered peak structure satisfies the following spatial constraints: the distance D between the centers of any two adjacent cluster units satisfies 50μm≤D≤150μm; the number N of micro-protrusions in each cluster unit satisfies 3≤N≤9; the ratio H / d of the peak height H of a single micro-protrusion to the base diameter d satisfies 0.3≤H / d≤0.6. This structural design ensures that during the subsequent lap welding of the longitudinal seam of the three tank plates, under pressure, multiple peaks in the cluster unit undergo elastic-plastic deformation and interlock with each other, locally penetrating the thin chromium layer on the surface, exposing the underlying metallic iron, and forming multiple discrete but dense conductive contact points. These contact points generate Joule heat when welding current passes through them, rapidly melting and fusing to form a stable weld nugget. Meanwhile, the inter-cluster region retains an intact passivation film and still possesses excellent corrosion resistance.

[0013] In a preferred embodiment of the present invention, the laser texturing process employs a nanosecond pulsed fiber laser with a wavelength of 1064 nm, a laser power of 80-150 watts, a scanning speed of 500-1200 mm / s, a pulse overlap rate of 70%-90%, and a focused spot diameter of 30-50 μm. By controlling the laser energy density and scanning path, the depth, shape, and cluster distribution density of the micro-pits are precisely controlled, thereby ensuring the consistency and repeatability of the substrate surface morphology after transfer.

[0014] In another preferred embodiment of the present invention, during the first electroplating of chromium, the rise time of the pulse current does not exceed 10 μs and the fall time does not exceed 5 μs, so as to enhance the preferential deposition effect of the peak current on the top of the micro-protrusion; at the same time, 0.1-0.5 g / L of sodium fluorosilicate is added to the plating solution as an auxiliary activator, which decomposes in the high current density region to generate fluoride ions, slightly etch the oxide film at the peak, promote the formation of chromium crystal nuclei, and further enhance the localization characteristics of the discontinuous plating layer.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: On the one hand, a clustered mountain structure with a clear spatial distribution pattern is constructed by laser texturing-rolling transfer technology, providing geometric guidance for subsequent electroplating; On the other hand, by utilizing the differences in current density, plating solution concentration and power supply type between the two electroplating processes, the functions of selective deposition and overall passivation can be achieved respectively.

[0016] The first high-current pulse electroplating forms a discontinuous chromium layer on the top of the micro-protrusion, while preserving the exposed base iron in the inter-cluster area; The second low-current DC electroplating coats an ultra-thin continuous chromium film, restoring macroscopic barrier properties.

[0017] The synergistic effect of the two results in a completely passivated surface in the static state of the material, while under dynamic welding pressure, the clustered protrusions undergo local plastic deformation, break through the thin chromium layer, expose the iron matrix with good conductivity, and form a low-resistance path.

[0018] This invention fundamentally avoids the dilemma in traditional technologies where removing the oxide film destroys the protective properties, and can achieve highly reliable welding without additional grinding or high-energy welding equipment.

[0019] Furthermore, the process described in this invention is fully compatible with existing continuous electroplating production lines, requiring only adaptive adjustments in the roll pretreatment and electroplating parameter control stages. No additional independent processes or expensive equipment are needed, demonstrating excellent industrial feasibility and cost advantages. The resulting weldable electroplated chromium sheet is suitable for manufacturing three-piece metal cans for food, beverage, aerosol, and chemical products, and is particularly suitable for applications in high-speed can manufacturing production lines with stringent requirements for weld stability and can sealing. Detailed Implementation

[0020] This invention provides a production process for weldable electroplated chromium sheets. By constructing a cluster-like mountain-like morphology with a specific microscopic topology on the surface of a cold-rolled steel coil substrate, and combining it with a staged, differentiated parameter-controlled double-layer electroplating chromium process, a controllable distribution of conductive pathways is formed at the microscopic scale while maintaining the overall barrier performance and corrosion resistance of the chromium plating layer. This significantly improves the weldability of the material during resistance welding, solving the technical problem in the prior art where excessive contact resistance caused by the surface chromium oxide film leads to defects such as incomplete welding and desoldering.

[0021] The technical solution of the present invention will be described in detail below with reference to specific embodiments and comparative examples, so as to ensure that those skilled in the art can fully understand and implement the present invention.

[0022] Examples 1-8 are all based on the process of this invention, covering the boundary and intermediate values ​​of key parameters of laser texturing, rolling conditions, and double-layer electroplating. By precisely controlling the morphology of clustered peaks, electroplating current density, and plating solution concentration, the stability of the process and the synergy of product performance are fully verified. All steps strictly control the surface cleanliness and the consistency of the electroplating bath solution.

[0023] Example 1: Cold-rolled steel coil thickness 0.18mm (MR steel); Laser texturing roll uses an 80-watt nanosecond pulsed fiber laser, scanning speed 1200mm / s, pulse overlap rate 70%, focused spot diameter 50μm; roll micro-dimple depth 8μm, opening diameter 3μm, sidewall inclination angle 60°, cluster unit contains 3 micro-dimples, minimum spacing between clusters 50μm; Process: Leveling and rolling with a reduction rate of 0.8%, speed of 400 m / min, and tension of 8 kN; alkaline washing with sodium hydroxide 30 g / L, sodium carbonate 20 g / L, temperature of 60℃, and time of 3 s; first electroplating with chromic anhydride 200 g / L, sulfuric acid 1.0 g / L, temperature of 60℃, and current density of 60 A / dm³. 2 The pulse frequency was 50Hz, duty cycle 30%, and duration 8s, with 0.1g / L of sodium fluorosilicate added; the second electroplating used 50g / L of chromic anhydride and 0.35g / L of sulfuric acid, at a temperature of 40℃ and a current density of 25A / dm³. 2 Time: 5 seconds.

[0024] Example 2: Cold-rolled steel coil thickness 0.24mm (MR steel); Laser texturing roll uses a 110W laser, scanning speed 800mm / s, pulse overlap rate 80%, focused spot diameter 40μm; roll micro-dimple depth 12μm, opening diameter 5μm, sidewall inclination angle 70°, cluster unit contains 6 micro-dimples, inter-cluster spacing 100μm; Process: Leveling and rolling with a reduction rate of 1.2%, speed of 600 m / min, and tension of 12 kN; alkaline washing with sodium hydroxide 40 g / L, sodium carbonate 30 g / L, temperature of 68℃, and time of 4.5 s; first electroplating with chromic anhydride 225 g / L, sulfuric acid 1.8 g / L, temperature of 70℃, and current density of 70 A / dm³. 2 The pulse frequency was 120Hz, duty cycle 45%, and duration 12s, with sodium fluorosilicate added at 0.3g / L. The second electroplating used 70g / L chromic anhydride and 0.45g / L sulfuric acid, at a temperature of 45℃ and a current density of 28A / dm³. 2 Time: 8 seconds.

[0025] Example 3: Cold-rolled steel coil thickness 0.30mm (MR steel); Laser texturing roll uses a 150W laser, scanning speed 500mm / s, pulse overlap rate 90%, focused spot diameter 30μm; roll micro-dimple depth 15μm, opening diameter 7μm, sidewall inclination angle 80°, cluster unit contains 9 micro-dimples, inter-cluster spacing 150μm; Process: Leveling and rolling with a reduction rate of 1.5%, speed of 800 m / min, tension of 15 kN; alkaline washing with sodium hydroxide 50 g / L, sodium carbonate 40 g / L, temperature of 75℃, time of 6 s; first electroplating with chromic anhydride 250 g / L, sulfuric acid 2.5 g / L, temperature of 80℃, current density of 80 A / dm³ 2 The pulse frequency was 200Hz, duty cycle 60%, and duration 15s, with sodium fluorosilicate added at 0.5g / L. The second electroplating used 90g / L chromic anhydride and 0.55g / L sulfuric acid, at a temperature of 50℃ and a current density of 30A / dm³. 2 Time: 10 seconds.

[0026] Example 4: Same as Example 2, substrate thickness 0.24mm, laser texturing intermediate parameters; Process: First electroplating current density 65A / dm 2 The remaining electroplating parameters are the same as in Example 2; the remaining processes (rolling, alkaline washing, and second electroplating) are also the same as in Example 2.

[0027] Example 5: Same as Example 2; Process: First electroplating current density 75A / dm 2 The remaining electroplating parameters are the same as in Example 2; the remaining processes are also the same as in Example 2.

[0028] Example 6: Same as Example 2 (substrate thickness 0.24mm); the roll cluster unit contains 3 micro-dimples, and the remaining laser texturing parameters are the same as in Example 2; Process: Same as Example 2.

[0029] Example 7: Same as Example 2; the roll cluster unit contains 9 micro-dimples, and the remaining laser texturing parameters are the same as in Example 2; Process: Same as Example 2.

[0030] Example 8: Same as Example 2; Process: Second electroplating: 75g / L chromium anhydride, 0.4g / L sulfuric acid, other parameters are the same as in Example 2; all other processes are the same as in Example 2.

[0031] Comparative Example 1: Cold-rolled steel coil thickness 0.24mm (MR steel); rolls without laser roughening (flat surface), rolling parameters the same as in Example 2; alkaline washing process the same as in Example 2; Process: Single-pass DC electroplating, chromic anhydride 200g / L, sulfuric acid 1.8g / L, temperature 70℃, current density 50A / dm³ 2 Time: 15 seconds; No second electroplating.

[0032] Comparative Example 2: Same as Example 2 (laser texturing parameters and rolling process are the same); Process: Only the first pulse electroplating (parameters same as in Example 2) is performed, without the second DC electroplating; the rest of the process is the same as in Example 2.

[0033] Test method: Morphology and coating testing: The surface roughness Ra and Rz of the substrate were tested using laser confocal microscopy; the clustered peak structure and the continuity of the chromium layer were observed using scanning electron microscopy; the iron signal ratio in the inter-cluster region was analyzed using X-ray photoelectron spectroscopy; and the total chromium layer and the thickness of each layer were tested using X-ray fluorescence spectroscopy.

[0034] Welding performance testing: The welding machine was an FH20-90ZD ordinary welding machine, with a welding frequency of 183Hz-215Hz, a welding current of 30A-38A, and an overlap of 0.6mm. The welding quality was tested using a universal testing machine to assess the tensile strength of the weld joint; the formation of the weld nugget was also observed.

[0035] Corrosion resistance test: Neutral salt spray test, using 5% sodium chloride, at 35℃ for 1 hour, observe whether rust spots appear on the surface; Test data comparison table 1: Test data comparison table 2: Examples 1-8 cover the boundaries and intermediate values ​​of core parameters. The total chromium layer of all products is 40-85 nm, and the surface contact resistance is 8-15 mΩ·cm. 2 The welded joint has a tensile strength of 280-320MPa and no red rust after 1 hour of salt spray testing, which proves that the process parameters of this invention are scientifically reasonable and can achieve the required performance under basic conditions.

[0036] Example 2 (intermediate parameters) performed best, with a surface contact resistance of 11 mΩ·cm. 2 The welded joint has a tensile strength of 305 MPa and a weld nugget formation rate of 98%. Due to the optimal matching degree between the cluster morphology and the electroplating parameters, the iron leakage points are evenly distributed, which not only ensures the conductive path of the weld but also maintains the integrity of the passivation film.

[0037] Examples 1-8 all employed laser texturing to construct clustered structures, resulting in contact resistances significantly lower than the 42 mΩ·cm of Comparative Example 1 (flat surface). 2 Comparative Example 1, due to the absence of protruding structures and complete chromium layer coverage, resulted in welding current being unable to penetrate, leading to a nucleus formation rate of only 45% and a tensile strength of only 180 MPa. This confirms that the cluster morphology is the core factor in reducing contact resistance.

[0038] Comparative Example 2 only uses single-pulse electroplating, although the contact resistance is low (7mΩ·cm). 2 However, the chromium layer is discontinuous, and large-area pitting corrosion occurs in the salt spray test; Examples 1-8 form a continuous thin chromium film by a second low-current DC electroplating, which achieves macroscopic passivation while retaining the iron leakage points, thus resolving the contradiction between corrosion resistance and solderability.

[0039] The laser texturing energy is increased (Examples 1→2→3), the substrate roughness increases, the contact resistance decreases, and the welding strength is improved; the first electroplating current density increases, the thickness of the first chromium plating layer increases, the proportion of iron elements between clusters decreases slightly, but it does not affect the conductive path; the number of cluster unit micro-protrusions increases (Examples 6→2→7), the number of conductive contact points increases during welding, the weld nugget is more stable, and the tensile strength is improved.

[0040] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A production process of a solderable electroplated chromium plate, characterized by, The method comprises the following steps: S1, roller surface pretreatment: a laser texturing device is used to perform surface modification treatment on the roller surface of the working roller, and a micro-pit array in a cluster distribution is processed on the surface thereof; S2, surface transfer printing: a cold-rolled low-carbon steel coil is subjected to flat rolling using the roller treated in S1, so that the micro-protrusion structure in the cluster distribution is completely transferred to the surface of the cold-rolled steel coil, and a cold-rolled steel coil base material with a mountain-shaped cluster distribution surface morphology is obtained; S3, alkaline washing: the cold-rolled steel coil base material obtained in S2 is continuously sent into an alkaline washing tank for degreasing cleaning, the alkaline washing liquid is a mixed aqueous solution of sodium hydroxide and sodium carbonate, and after cleaning, three-stage countercurrent rinsing is performed to ensure the surface cleanliness; S4, first chromium electroplating: the steel coil base material cleaned and dried in S3 is introduced into a first electroplating tank for pulse electroplating, thereby forming a discontinuous first chromium plating layer, the plating layer covers dense chromium metal on the top of the plurality of micro-protrusions in the cluster unit, and sparse or locally exposed base iron is deposited on the flat area between the clusters; S5, second chromium electroplating: the steel coil electroplated and cleaned in S4 is introduced into a second electroplating tank for direct current electroplating, thereby forming a continuous second chromium plating layer.

2. The production process of a solderable electroplated chromium plate according to claim 1, characterized in that, The cluster distribution refers to that a plurality of micro-pits are gathered to form a cluster unit in a geometric center spacing of not more than 20 μm, each cluster unit contains 3 to 9 micro-pits, and the minimum spacing between each cluster unit is not less than 50 μm.

3. The production process of a solderable electroplated chromium plate according to claim 2, characterized in that, The depth of the micro-pit is 8-15 μm, the opening diameter is 3-8 μm, and the side wall inclination angle is 60°-80°.

4. The production process of a solderable electroplated chromium plate according to claim 3, characterized in that, The cluster-shaped mountain structure satisfies the following spatial constraint relationship: the distance D between the centers of any two adjacent cluster units satisfies 50 μm≤D≤150 μm; the number N of micro-protrusions in each cluster unit satisfies 3≤N≤9; and the ratio H / d of the peak height H of a single micro-protrusion to the base diameter d satisfies 0.3≤H / d≤0.

6.

5. The production process of a solderable electroplated chromium plate according to claim 4, characterized in that, The peak top curvature radius of the micro-protrusion is 3-8 μm, the height is 8-15 μm, and a plurality of adjacent micro-protrusions form a mountain-shaped cluster group around a local high point.

6. The production process of a solderable electroplated chromium plate according to claim 1, characterized by, In the step S1, a nanosecond pulse fiber laser with a wavelength of 1064 nm is used, the laser power is 80-150 watts, the scanning speed is 500-1200 mm / s, the pulse overlap rate is 70%-90%, and the focused spot diameter is 30-50 μm.

7. The production process of a solderable electroplated chromium plate according to claim 1, characterized by, In the step S4, the rising time of the pulse current is not more than 10 μs, the falling time is not more than 5 μs, and 0.1-0.5 g / L of sodium fluorosilicate is added as an auxiliary activator in the first chromium plating solution.

8. The production process of a solderable electroplated chromium plate according to claim 1, characterized by, In the step S3, the concentration of sodium hydroxide in the alkaline washing liquid is 30-50 g / L, the concentration of sodium carbonate is 20-40 g / L, the cleaning temperature is 60-75 °C, and the cleaning time is 3-6 s; the conductivity of the last-stage rinsing water in the three-stage countercurrent rinsing is less than 50 μsiemens / cm.

9. The production process of a solderable electroplated chromium plate according to claim 1, characterized in that, In the step S4, the chromium metal content in the first chromium plating layer inside the cluster unit is more than 98%, and the iron element signal strength ratio in the inter-cluster area is more than 40%.

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