Sn-zn plated steel sheet

Abstract

The Sn-Zn series plated steel material has a steel material, an alloy layer on the surface of the steel material, and a plated layer on the alloy layer, the alloy layer has a Ni-Fe alloy layer, an FeSn2 alloy layer, and a Ni2Zn 11 phase in order from the surface of the steel material, the coverage of the Ni-Fe alloy layer with respect to the surface of the steel material is 50% or more, the coverage of the FeSn2 alloy layer with respect to the surface of the steel material is 90% or more, the coverage of the Ni2Zn 11 phase with respect to the surface of the steel material is 30% to 80%, the plated layer is a Sn plated layer containing 0.5 to 10.0 at% of Zn, and the adhesion amount of the plated layer is 20 g / m 2 to 50 g / m 2 .

Description

Technical Field

[0001] This disclosure relates to Sn-Zn based coated steel.

[0002] This application claims priority based on Japanese Patent Application No. 2024-077101 filed on May 10, 2024, the contents of which are incorporated herein by reference. Background Technology

[0003] Previously, Pb-Sn alloy-coated steel sheets, known for their excellent corrosion resistance, machinability, brazing properties (soft solder properties), and weldability, were widely used as materials for automotive fuel tanks. On the other hand, Sn-Zn alloy-coated steel sheets are primarily manufactured using electroplating methods, such as those described in Patent Document 1, which involve electrolysis in an aqueous solution containing Zn and Sn ions. Such Sn-Zn alloy-coated steel sheets, with Sn as the main component, exhibit excellent corrosion resistance and brazing properties and are frequently used in electronic components.

[0004] In recent years, Sn-Zn coated steel sheets have been found to have excellent properties for use in automotive fuel tanks, and hot-dip galvanized Sn-Zn steel sheets are disclosed in Patent Documents 2, 3, 4, 5, and 6. In automotive fuel tanks, numerous accessories and pipes require brazing, or surrounding areas need to be welded to prevent fuel leaks; therefore, materials with good and stable bonding properties that do not hinder continuous production are required. Furthermore, recently, in electric vehicle battery casings, especially large water-cooled battery casings, there is a demand for long-term high brazing strength (fatigue strength) comparable to that of the coolant passageway and the vehicle life of brazed sections of water passage components.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 52-130438

[0008] Patent Document 2: Japanese Patent Application Publication No. 08-269733

[0009] Patent Document 3: Japanese Patent Application Publication No. 08-269734

[0010] Patent Document 4: Japanese Patent Application Publication No. 2004-360019

[0011] Patent Document 5: Japanese Patent Application Publication No. 2006-348365

[0012] Patent Document 6: Japanese Patent Application Publication No. 60-013098

[0013] Patent Document 7: Japanese Patent Application Publication No. 60-169587 Summary of the Invention

[0014] The problem that the invention aims to solve

[0015] One embodiment of this disclosure aims to provide Sn-Zn coated steel with excellent corrosion resistance, processability, and spot weldability, and high brazing strength (solder filler metal strength).

[0016] Methods for solving problems

[0017] To address the aforementioned issues, one embodiment of this disclosure proposes the following solution.

[0018] <1> The invention is characterized by comprising steel, an alloy layer on the surface of the steel, and a plating layer on the alloy layer, wherein the alloy layer comprises, in sequence from the surface of the steel, a Ni-Fe alloy layer, a FeSn2 alloy layer, and a Ni2Zn alloy layer. 11 In the phase, the Ni-Fe alloy layer has a coverage rate of more than 50% relative to the surface of the steel, the FeSn2 alloy layer has a coverage rate of more than 90% relative to the surface of the steel, and the Ni2Zn... 11 The coating has a coverage rate of 30% to 80% relative to the surface of the steel, and is a Sn coating containing 0.5 to 10.0 at% Zn, with an adhesion weight of 20 g / m². 2 ~50g / m 2 .

[0019] <2> The coating is characterized in that the area fraction of the Sn-Zn eutectic structure is less than 50%, and the area fraction of the Sn crystal is more than 50%.

[0020] <3> The coating is characterized in that it contains one or more of Mg, Al, W and Mo in a total of 0.5 to 5.0 at% of each.

[0021] <4> The coating is characterized in that it has a film containing Si and P on its surface.

[0022] <5> The coating is characterized in that it has a film containing Si and P on its surface.

[0023] <6> The characteristic feature is that the FeSn2 alloy layer exists on the Ni-Fe alloy layer in at least one of the following forms: a layered morphology and columnar crystals with a major diameter of less than 1 μm.

[0024] Invention Effects

[0025] According to one embodiment of this disclosure, Sn-Zn based coated steels with excellent corrosion resistance, processability, and spot weldability, as well as high brazing strength, can be provided. In particular, according to one embodiment of this disclosure, Sn-Zn based coated steels suitable for automotive fuel tank materials and battery casing materials can be provided. Attached Figure Description

[0026] Figure 1 This is a cross-sectional view showing the thickness direction of the Sn-Zn coated steel in this embodiment.

[0027] Figure 2 This is a cross-sectional view in the thickness direction of the alloy layer structure of the Sn-Zn coated steel in this embodiment.

[0028] Figure 3 This is a cross-sectional view showing the state of foreign matter (flux residue, dust, etc.) adhering to the steel before immersion in the hot-dip Sn-Zn plating bath, in the thickness direction of the alloy layer structure of the Sn-Zn coated steel according to this embodiment.

[0029] Figure 4 This is a diagram showing a cross-section of Sn-Zn coated steel with a FeSn2 alloy layer containing coarse FeSn2 crystals.

[0030] Figure 5 This is a schematic diagram showing the plane of the test material for corrosion resistance testing.

[0031] Figure 6 This is a schematic diagram of the cross-section of a tensile test specimen. Detailed Implementation

[0032] The inventors of this invention have discovered that in Sn-Zn coated steel 1, in order to ensure corrosion resistance, workability, spot weldability and brazing strength, it is particularly important to control the alloy layer structure near the interface between the coated body and the side of the steel 10.

[0033] In Fe-Ni alloy electroplating, as disclosed in Patent Documents 6 and 7, in order to generate Fe at the anode during electrolysis 3+ To stabilize ions, additive solutions containing organic compounds such as aminosulfonic acid, cyclic compounds retaining hydroxyl and sulfonic acid groups, tartaric acid, and citric acid are sometimes used. The inventors of this invention have found that, although using the above-mentioned additive solutions in electroplating affects Fe... 3+Ion stabilization is effective, but it increases strain within the coating. Therefore, if hot-dip Sn-Zn plating is performed on a Fe-Ni alloy coating, Fe and Sn in the Fe-Ni alloy coating readily undergo an alloying reaction, easily forming a FeSn2 alloy layer on the Fe-Ni alloy coating. If the Sn at the interface is consumed due to the formation of the FeSn2 alloy layer, the Zn in the coating locally concentrates on the FeSn2 alloy layer, thus leading to the widespread formation of Ni2Zn on top of the FeSn2 alloy layer. 11 Equal Zn-based alloys. In that case, due to the difference in thermal expansion / contraction ratio between the FeSn2 alloy layer and the Zn-based alloy during heating and cooling during brazing, microcracks are generated in the brittle FeSn2 alloy layer, making it difficult to obtain sufficient brazing strength.

[0034] Therefore, the inventors of this invention have found it important that by not adding organic matter to the Fe-Ni alloy electroplating solution, the strain in the Fe-Ni coating is reduced, and the formation of FeSn2 crystals is controlled to reduce Ni2Zn. 11 The coverage area ratio of the phase is set in the range of 30% to 80%.

[0035] This implementation is based on the discovery that is described in the following summary.

[0036] The steel 10 comprises an alloy layer 20 on the surface of the steel 10 and a Sn-Zn based coating 30 on the alloy layer 20. The alloy layer 20 comprises, from the surface of the steel 10, a Ni-Fe alloy layer 21, an FeSn2 alloy layer 22, and a Ni2Zn alloy layer 30 in sequence. 11 Phase 23, the Ni-Fe alloy layer 21 has a coverage rate of 50% or more on the surface of the steel 10, the FeSn2 alloy layer 22 has a coverage rate of 90% or more on the surface of the steel 10, and Ni2Zn 11 Phase 23 has a coverage rate of 30% to 80% relative to the surface of the steel 10. The Sn-Zn coating 30 is a Sn coating containing 0.5 to 10.0 at% Zn, and the adhesion weight of coating 30 is 20 g / m. 2 ~50g / m 2 .

[0037] The specific structure of the Sn-Zn coated steel 1 of this embodiment will be described below.

[0038] In this description, the range indicated by "~" is used as a general rule, with the values ​​at both ends being the lower and upper limits of the range. However, values ​​expressed as "exceeding" or "below" are not included in the range.

[0039] exist Figure 1The cross-section (schematic diagram) of the Sn-Zn based coated steel 1 of this embodiment is shown.

[0040] like Figure 1 As shown, the Sn-Zn based coated steel 1 of this embodiment has a steel 10, an alloy layer 20 formed on at least one surface of the steel 10, and a Sn-Zn based coating 30 formed on the alloy layer 20.

[0041] <Steel 10>

[0042] The steel material 10 that will be plated will be described.

[0043] There are no particular restrictions on the dimensions of steel material 10. Steel material 10 can be, for example, steel plate, steel wire, or steel wire.

[0044] There are no particular limitations on the composition of the steel 10, but it is preferred to have a composition system that can be processed into complex shapes such as fuel tanks and battery boxes.

[0045] As for steel type 10, in addition to IF steel (interstitial free steel), Al-k steel (aluminum killed steel), Cr-containing steel, stainless steel, and high-strength steel with added Ti, Nb, B, etc., Ti-added steel can also be used from the perspective of improving heat resistance. In building materials applications, Al-k or stainless steel are preferred; in exhaust systems, Ti-IF and Ti-added steels are preferred; in household appliances, Al-k and N-free added steels are preferred; and in fuel tank applications, B-added IF steel is preferred.

[0046] <Alloy Layer 20>

[0047] exist Figure 2 The image shows an enlarged cross-sectional view of alloy layer 20. It should be noted that... Figure 2 The illustration of Sn-Zn coating 30 (hereinafter also referred to as coating 30) is omitted.

[0048] like Figure 2 As shown, the alloy layer 20 comprises, from the surface of the steel 10 toward the coating 30 side, a Ni-Fe alloy layer 21, an FeSn2 alloy layer 22, and a Ni2Zn alloy layer 23. 11 Phase 23.

[0049] Furthermore, the alloy layer 20 at the interface between the steel 10 and the coating 30 is preferably thin and capable of preventing coating peeling; and is a compositional system for inhibiting the progression of corrosion in the internal and external environments of the fuel tank.

[0050] exist Figure 3The image shows a cross section of steel 10 before it is immersed in a hot-dip Sn-Zn bath, with foreign matter F (fluxing solution residue, dust, etc.) adhering to it.

[0051] Next, the layers that make up alloy layer 20 will be described.

[0052] [Ni-Fe alloy layer 21]

[0053] The Ni-Fe alloy layer 21 refers to a layer containing a Ni-Fe alloy. The chemical composition of the Ni-Fe alloy layer 21 is preferably 10% to 45% by mass, based on the Ni content. The remaining portion of the chemical composition of the Ni-Fe alloy layer 21 is Fe, and impurities to a degree that do not impair the function of the Ni-Fe alloy layer 21 are permissible. The Ni-Fe alloy layer 21 may also contain W and Co, totaling less than 1% by mass.

[0054] The Ni-Fe alloy layer on the steel 10 disclosed herein has a coverage rate of 50% or more relative to the surface of the steel 10. The coverage rate of the Ni-Fe alloy layer relative to the surface of the steel 10 becomes less than 50% in cases such as over-alloying with Sn-Zn plating or uneven Ni adhesion. In these cases, the processability deteriorates due to the excessive formation of brittle, coarse FeSn2 crystals 22b, described later.

[0055] The chemical composition and coverage of the Ni-Fe alloy layer 21 were determined as follows.

[0056] Using the low-temperature FIB micro-sampling method (registered trademark), samples from any surface, ranging from steel 10 to coating 30, were cut. Figure 1 The surface section shown was thinned to create a sample. The Sn-Zn coated steel section 1 sample was analyzed using a 200kV-FE-TEM. Fe, Ni, Sn, and Zn were mapped using EDS (1nm probe), and the chemical composition was determined by electron X-ray diffraction. Figure 2 As shown, on the mapped image, for any length L1 of steel 10 with a thickness of 10 μm or more, the total length L2 of the Ni-Fe alloy layer 21 that is continuously or discontinuously present on its upper part (corresponding length position) is calculated. The value obtained by dividing the total length L2 of the Ni-Fe alloy layer 21 calculated in this way by the length L1 on the surface of steel 10 is taken as the coverage rate. The coverage rate is calculated at 3 or more randomly selected points with an interval of 50 μm or more, and the arithmetic mean of these is taken as the coverage rate of Ni-Fe alloy layer 21 on the surface of steel 10.

[0057] Thin-film formation of samples can be achieved using, for example, the "NB5000" manufactured by Hitachi High-Tech. For transmission electron microscopy (TEM) observation, for example, the "HD-2700" (accelerating voltage: 200kV) manufactured by Hitachi High-Tech can be used.

[0058] [FeSn2 alloy layer 22]

[0059] The FeSn2 alloy layer 22 refers to a layer formed of FeSn2. The FeSn2 alloy layer 22 may contain other components (so-called impurities) to the extent that it does not impede the barrier function described later.

[0060] The FeSn2 alloy layer 22 is a layer that effectively blocks corrosive agents such as chloride ions and water. Therefore, if the coverage of the FeSn2 alloy layer 22 relative to the surface of the steel 10 decreases, the corrosion resistance decreases; however, if the coverage is 90% or more, sufficient corrosion resistance can be ensured. Therefore, the lower limit of the coverage of the FeSn2 alloy layer 22 relative to the surface of the steel 10 is 90%. The coverage of the FeSn2 alloy layer 22 relative to the surface of the steel 10 is preferably 95% or more, and more preferably 98% or more.

[0061] The coverage of the FeSn2 alloy layer 22 relative to the surface of the steel 10 was determined by the same method as the determination of the coverage of the Ni-Fe alloy layer 21 relative to the surface of the steel 10 described above.

[0062] In this embodiment, the FeSn2 alloy layer 22 exists in at least one of the following forms: layered and columnar crystals with a major diameter of 1 μm or less.

[0063] exist Figure 4 The diagram shows a cross-section (schematic diagram) of Sn-Zn coated steel 1 with FeSn2 alloy layer 22 containing coarse FeSn2 crystals 22b.

[0064] The morphology of FeSn2 contained in the FeSn2 alloy layer 22 can generally be divided into two patterns: layered and columnar. Specifically, as... Figure 4As shown, FeSn2 can take the following forms: a layered form formed on the Ni-Fe alloy layer 21; and a columnar crystal form grown from the Ni-Fe alloy layer 21 toward the surface of the Sn-Zn plating layer 30. However, coarse columnar crystals (coarse FeSn2 crystals 22b) with a major diameter exceeding 1 μm in the columnar FeSn2 may hinder the sliding properties of the Sn-Zn plating steel 1. Therefore, in this embodiment, it is preferable to suppress the formation of coarse FeSn2 crystals 22b. That is, when the FeSn2 alloy layer 22 in this embodiment contains columnar FeSn2, it is preferable to have ultrafine columnar crystals (fine FeSn2 crystals 22a) with a major diameter of 1 μm or less. It should be noted that fine FeSn2 crystals 22a and coarse FeSn2 crystals 22b can be distinguished by the major diameter of the columnar crystals.

[0065] [Ni2Zn 11 Phase 23]

[0066] If Ni2Zn 11 Phase 23 has a coverage rate of over 80% relative to the surface of steel 10. Therefore, during heating and cooling in brazing, the FeSn2 alloy layer 22 and Ni2Zn... 11 The difference in the thermal expansion / contraction ratio of phase 23 leads to the difference between FeSn2 alloy layer 22 and Ni2Zn. 11 At the interface of phase 23, microcracks are generated within the brittle FeSn2 alloy layer 22. These microcracks within the FeSn2 alloy layer 22 reduce the brazing strength of the Sn-Zn coated steel 1. Therefore, Ni2Zn 11 The coverage of phase 23 relative to the surface of steel 10 is controlled to be below 80%. Ni2Zn 11 The upper limit of the coverage of phase 23 relative to the surface of steel 10 is preferably 70% or less, more preferably 60% or less.

[0067] On the other hand, Ni2Zn 11 The coating coverage of phase 23 relative to the surface of steel 10 becomes less than 30% when the single-sided adhesion of Ni-Fe coating is less than 0.01 g / m (calculated as Ni). 2 Or, excessive heat input during hot-dip plating can lead to the formation of coarse columnar FeSn2 crystals 22b. Therefore, if Ni2Zn 11 If the coverage of phase 23 on the surface of steel 10 is less than 30%, the coating's slip properties decrease, and the workability of the Sn-Zn coated steel 1 decreases. Therefore, Ni2Zn... 11 The coating rate of phase 23 relative to the surface of steel 10 is controlled to be above 30%. Ni2Zn 11 The lower limit of the coverage of phase 23 relative to the surface of steel 10 is preferably 40% or more, and more preferably 50% or more.

[0068] Ni2Zn 11 The coating rate of the phase 23 relative to the surface of the steel material 10 is carried out by the same method as the measurement of the coating rate of the Ni-Fe alloy layer 21 relative to the surface of the steel material 10 described above.

[0069] <Sn-Zn based coating layer 30>

[0070] The Sn-Zn based coating layer 30 is disposed above the alloy layer 20 (on the surface on the side opposite to the steel material 10).

[0071] The coating layer 30 contains a Sn-Zn eutectic structure 31 and Sn crystals 32.

[0072] {Area ratio of the Sn-Zn eutectic structure 31: 50% or less}

[0073] When the area ratio of the Sn-Zn eutectic structure 31 in the coating layer 30 exceeds 50%, there is a tendency for the corrosion resistance to decrease. Therefore, the area ratio of the Sn-Zn eutectic structure 31 in the coating layer 30 is preferably 50% or less.

[0074] The reason for the decrease in the corrosion resistance of the plated steel material 1 when the area ratio of the Sn-Zn eutectic structure 31 in the coating layer 30 exceeds 50% can be explained as follows. The Zn phase of the Sn-Zn eutectic structure 31 in the coating layer 30 is in the form of broken lamella. Therefore, if the area ratio of the Sn-Zn eutectic structure 31 in the cross-sectional field of view of the coating layer 30 in the plate thickness direction of the steel material 10 increases to 50% or more, the probability of the corrosion part penetrating from the upper layer to the lower layer of the coating layer 30 due to the corrosion of a small amount of Zn increases, resulting in a decrease in the corrosion resistance of the plated steel material 1. The cross-section of the coating layer 30 here refers to the part from the outermost layer to the alloy layer 20 of an arbitrary cross-section in the plate thickness direction. In order to exert the function of sacrificial anticorrosion, the area ratio of the Sn-Zn eutectic structure 31 is preferably set to 20% - 40%.

[0075] The area ratio of the Sn-Zn eutectic structure 31 is calculated as follows.

[0076] Using the electron beam scanning method of FE-EPMA (for example, JXA-8530 manufactured by JEOL Ltd.) with an acceleration voltage of 15.0 kV and an irradiation current of 3.0×10 -8A. With an irradiation time of 50 ms and an electron beam diameter of approximately 0.1 μm (minimum probe diameter), Zn was mapped and measured at 0.1 μm intervals in a cross-section of the Sn-Zn coating 30, targeting areas with a width of 25 μm or more and a height of 5 μm or more. The area of ​​the Zn-dispersed region was defined as the Sn-Zn eutectic structure 31. The area of ​​the Zn-dispersed region measured in this manner was divided by the measured area and multiplied by 100 to obtain the area ratio of the Sn-Zn eutectic structure 31.

[0077] {Sn crystal 32 area ratio: over 50%}

[0078] Since Sn crystal 32 is the structure that ensures the barrier properties of coating 30, its area fraction is preferably 50% or more. From the viewpoint of improving the barrier properties of coating 30, the area fraction of Sn crystal 32 is preferably 70% or more.

[0079] The area ratio of Sn crystal 32 is calculated by subtracting the area ratio of Sn-Zn eutectic structure 31 and the area ratio of alloy structures with other additives from 100. The area ratio of alloy structures with other additives is determined by the same method as that used for Ni-Fe alloy layer 21 described above.

[0080] Next, the chemical composition of the Sn-Zn coating 30 will be explained.

[0081] {Zn content: 0.5~10.0at%}

[0082] To ensure corrosion resistance, the Sn-Zn based coating 30 needs to contain at least two structures: primary Sn crystals 32 and Sn-Zn eutectic. Therefore, the Zn content of the Sn-Zn based coating needs to be controlled between 0.5% and 10.0 at%. When the Zn content of the Sn-Zn based coating is below 0.5 at%, Zn is almost entirely replaced by Ni2Zn. 11 The formation of phase 23 consumes so little that a Sn-Zn eutectic will crystallize. Therefore, Zn's role in rust prevention and corrosion protection of the base metal (steel 10) and the protection provided by Zn corrosion products are ineffective, and corrosion resistance cannot be ensured. Therefore, the Zn content of the Sn-Zn coating is set to 0.5 at% or more. Preferably, it is 3.0 at% or more, and more preferably 5.0 at% or more. Furthermore, if the Zn content of the Sn-Zn coating exceeds 10.0 at%, primary crystals form Zn, and due to the small amount of Zn corrosion, the probability of corrosion extending from the upper layer to the lower layer of the coating 30 increases. Therefore, the Zn content of the Sn-Zn coating is set to 10.0 at% or less. Preferably, it is 8.0 at%, and more preferably 7.0 at% or less.

[0083] The total amount of at least one of Mg, Al, W, and Mo is 0.5–5 at%.

[0084] Furthermore, to improve corrosion resistance, the Sn-Zn plating bath may contain at least one of Mg, Al, W, and Mo in a total of 0.5 to 5.0 at% concentration. When at least one of Mg, Al, W, and Mo is present, it is difficult to achieve an improvement in corrosion resistance if the total concentration is less than 0.5 at%; therefore, a concentration of 0.5 at% or more is preferred. More preferably, 1.0 at% or more is preferred, and even more preferably, 2.0 at% or more is preferred. On the other hand, if the concentration exceeds 5.0 at% concentration, the melting point becomes excessively high, resulting in the excessive formation of columnar FeSn2 crystals 22b during Sn-Zn plating. If coarse FeSn2 crystals 22b are formed, the lubricity of the Sn-Zn plating steel 1 decreases, thereby deteriorating its workability. Therefore, even when the above-mentioned elements are present, a total concentration of 5.0 at% or less is preferred. More preferably, 4.0 at% or less is preferred, and even more preferably, 3.0 at% or less is preferred.

[0085] It should be noted that the element content in the Sn-Zn plating bath is determined, for example, by scooping out a portion of the plating bath every 4 hours, dissolving the rapidly solidified ingot on the copper plate with an acid such as nitric acid, and then analyzing it using ICP. Preferably, the concentration is adjusted using a Sn alloy based on the results of the above determination.

[0086] {Coating adhesion amount}

[0087] To ensure corrosion resistance of the inner surface of the fuel tank and the water-cooled interior of the battery box, the Sn-Zn coating needs to have an adhesion of 20 g / m² per single side. 2 That's all. Additionally, if the adhesion amount of the Sn-Zn plating exceeds 50 g / m² per single side... 2 During spot welding, the Cu on the electrode alloys with the Sn in the plating layer 30. Consequently, during continuous spot welding, the electrode surface area increases in the early stages, making it impossible to ensure the current density required for weld nugget formation, resulting in poor continuous spot welding performance. Therefore, the preferred adhesion amount of the Sn-Zn plating is 50 g / m² per single side. 2 the following.

[0088] Regarding the adhesion amount of Sn-Zn system coating and the chemical composition of Sn-Zn system coating 30, the coating 30 was peeled off using the following method. The peeled water was then quantitatively analyzed for each element using ICP determination. The adhesion amount was calculated based on the measured area along with the composition ratio.

[0089] Regarding coating peeling, for example, the 10mm × 10mm measurement surface and lead wire connection of a 15mm × 50mm sample are masked with beeswax, silicone coating, etc., and then the sample is peeled off using a three-electrode method (working electrode, counter electrode) in 50mL of 5% NaOH aqueous solution. <pt>Anodic electrolysis is performed using a reference electrode. The current value is set to the maximum value that will hardly cause water decomposition. As bubbles based on water decomposition become visible as the coating decreases, the current value is gradually reduced. Finally, the current is increased to 1-2 mA. If no change is observed on the surface, the current is set to stop.

[0090] <Skin>

[0091] Over time, an oxide film forms on the surface of the Sn-Zn plating 30 due to air oxidation, which deteriorates the wettability and diffusion of the solder and the adhesion of the coating. Therefore, it is preferable to apply a coating of 50–1500 mg / m² to the outermost surface of the plating 30. 2 A film containing Si and P. If the coating adhesion is less than 50 mg / m³. 2 If the concentration exceeds 1500 mg / m², it will be difficult to achieve a coating effect. 2 If this occurs, the wettability and diffusion of the solder deteriorate, and the film resistance increases, thus reducing the solderability. The preferred Si content (the amount of Si in the film) is 20 mg / m³. 2 ~1000mg / m 2 As for elemental phosphorus (the content of phosphorus in the film (adhesion amount)), it is preferably 5 mg / m³. 2 ~300mg / m 2 As a source of Si, colloidal silica, silane coupling agents, etc., are preferred. As a source of P, phosphoric acid, polyphosphoric acid, phosphonic acid, hypophosphonic acid or their salts, as well as phosphine oxides, phosphine, etc., are preferred. The film may also contain trivalent Cr compounds, V (vanadium) compounds, Mg compounds, Zr compounds, Ti compounds. Furthermore, to improve corrosion resistance and coatability, a water-soluble resin is preferred as an additional component. There are no particular limitations on the water-soluble resin, but a water-soluble acrylic resin is preferred.

[0092] There are no specific regulations for the coating method, but after applying the aqueous solution containing the above-mentioned reagents to the coating surface using a roller coating method, it is baked in a hot air drying oven at a temperature of 60-180°C upon reaching the plate. The adhesion amount is adjusted by the clamping pressure between the rollers and between the roller plates.

[0093] Regarding the determination of Si and P adhesion amounts, the intensity of each element was measured using a fluorescence X-ray analysis device (wavelength dispersion type), and quantification was performed using a calibration method. When the intensity of Si and P contained in the base metal affects the coating, the adhesion amount of the coated steel 1 without a coating was measured, and the value obtained by subtracting the adhesion amount of the steel 10 portion was taken as the coating adhesion amount.

[0094] Next, the manufacturing method of the Sn-Zn based coated steel 1 of this embodiment will be described.

[0095] <Manufacturing Method>

[0096] For annealed steel 10 obtained by a series of processes including hot rolling, pickling, cold rolling, annealing, and tempering rolling of steel billets, Ni-Fe alloy plating (pre-plating) is performed using electroplating to obtain pre-plated steel. A flux (e.g., ZnCl2-NH4Cl system) is applied to the pre-plated steel. The pre-plated steel with flux is then hot-dip coated with Sn-Zn system plating. After immersing the pre-plated steel in a Sn-Zn system plating bath, it is removed, and the adhesion is controlled by air wiping. It is then cooled below the solidification point by air blowing. Subsequently, a Si and P-containing liquid is applied and dried to form a Si and P-containing film.

[0097] (Ni-Fe alloy plating)

[0098] In this embodiment, the Ni-Fe alloy plating is performed by electroplating. It should be noted that the Ni-Fe alloy plating is not limited to electroplating, but from the viewpoint of controlling the amount of Ni-Fe alloy plating, electroplating is preferred.

[0099] In Ni-Fe alloy electroplating solutions, in order to ensure that Fe is generated at the anode during electrolysis... 3+ The stability of the ions requires the use of an additive-free solution without added organic substances such as aminosulfonic acid, cyclic compounds retaining hydroxyl and sulfonic acid groups, tartaric acid, and citric acid. That is, the organic content in the Ni-Fe alloy electroplating solution is controlled to below 1 ppm. If organic substances are present in the plating solution, the strain in the generated Ni-Fe alloy layer 21 increases. This strain acts as a driving force, promoting the alloying of Sn in the Sn-Zn plating solution with Fe in the Ni-Fe alloy layer 21. The resulting FeSn2 alloy layer 22 becomes layered, but in large quantities. Therefore, the Zn released through the above alloying process, as described later, is treated as Ni2Zn. 11 Phase 23 crystallizes in a layered manner over a large area, Ni2Zn 11 Phase 23 has a surface coverage of over 80% relative to the steel 10, resulting in insufficient brazing strength. Therefore, in order to... 11 To control the coverage of phase 23 to below 80%, the amount of organic matter in the Ni-Fe alloy electroplating solution needs to be controlled to below 1 ppm. Furthermore, in order to achieve Ni2Zn... 11 The coating rate of phase 23 should be controlled to be above 30%, and the Ni-Fe alloy coating amount should be set to 0.02 g / m² per single side based on the adhesion amount calculated in Ni conversion. 2 That's all. It should be noted that the composition ratio of the Ni-Fe alloy electroplating solution is not particularly limited, but from the viewpoint of the robustness of Ni% in the coating 30 (the robustness considering the Ni supply), it is preferable, for example, that the Ni content is 15% to 25% and the Fe content is 75% to 85%.

[0100] Furthermore, in the absence of a Ni-Fe alloy coating, during the molten Sn-Zn coating described later, coarse columnar FeSn2 crystals 22b grow towards the surface of the Sn-Zn coating 30, resulting in poor coating slipability and insufficient processability. Thus, the inventors of this invention discovered that, without setting the coverage of the Ni-Fe alloy layer 21 to 50% or more, coarse columnar FeSn2 crystals 22b, which are detrimental to processability, grow in the uncoated portions of the Ni-Fe alloy layer 21.

[0101] The amount of Ni applied is 0.02 g / m. 2 In the following cases, it is difficult to set the coverage of the Ni-Fe alloy layer 21 to be above 50%, which is 3.0 g / m. 2 At this point, the aforementioned effect saturates, thus becoming economically disadvantageous. Therefore, the lower limit for the Ni-Fe alloy coating amount, calculated in Ni equivalents, is preferably set to 0.02 g / m². 2 Set the upper limit to 3.0 g / m 2 .

[0102] To ensure that the area ratio of the Sn-Zn eutectic structure 31 in the coating 30 is set to 50% or less, it is necessary to appropriately control the surface layer of the pre-plated steel. For example, when immersed in a Sn-Zn plating bath, if the surface layer of the pre-plated steel is a Ni monolayer, a Ni-Zn alloy layer is formed at the plating interface. In this case, since the Ni monolayer is smooth, the crystallization of Sn as a primary crystal is delayed, and more than 98% of the Sn-Zn coating 30 becomes the Sn-Zn eutectic structure 31. On the other hand, by forming the Ni-Fe alloy layer 21 through Ni-Fe alloy plating (pre-plating), fine FeSn2 crystals 22a are formed immediately after immersion in the Sn-Zn plating bath, making it easier for primary Sn crystals to crystallize from the edges, vertices, and other corners of these fine FeSn2 crystals 22a. At this point, the area ratio of the Sn-Zn eutectic structure 31 is also affected by the cooling rate. Therefore, in order to set the area ratio of the Sn-Zn eutectic structure 31 to below 50%, it is necessary not only to control the pre-plated steel but also to control the cooling rate of the Sn-Zn plating. Details regarding the control of the cooling rate of the Sn-Zn plating will be discussed later.

[0103] (Sn-Zn based plating)

[0104] In this disclosure, Sn-Zn based plating is preferably performed by hot-dip plating. Hot-dip plating ensures sufficient plating adhesion. The plating adhesion per single side in this embodiment ranges from 20 to 50 g / m², which is a relatively thick unit area weight range. 2 Therefore, hot-dip plating is the optimal method. For example, even with electroplating, as long as electrolysis is performed for a long time, the coating adhesion can be ensured, but it is not economical. Furthermore, when the potential difference between the plating elements is large, it is difficult to properly control the composition using electroplating. Therefore, for Sn-Zn system plating, hot-dip plating is the best method.

[0105] The Sn-Zn plating bath has a Sn content of 90.0–99.5 at and a Zn content of 0.5–10.0 at.

[0106] The FeSn2 alloy layer 22 is formed when the Ni-Fe alloy layer 21 comes into contact with the Sn-Zn hot-dip galvanizing solution, resulting in a reaction between Fe and Sn, which have high affinity. Therefore, as... Figure 3 As shown, as long as there is no foreign matter F (thick oxide film, impurities, etc.) adhering to the interface between the Ni-Fe alloy layer 21 and the Sn-Zn hot-dip galvanizing bath, the FeSn2 alloy layer 22 will be formed. Foreign matter F can include, for example, flux-based compounds (including post-reaction residues) that cannot detach from the surface during flux plating, or foreign matter F adhering to the rollers during plate passing through the plating production line. Therefore, in order to remove the adhering substances from the rollers in the plating production line and promote the peeling of flux from the steel 10 in the plating bath, measures such as applying the plating bath using a metal pump are effective for the plate just before it comes into contact with the rollers in the bath.

[0107] As described above, the FeSn2 contained in the FeSn2 alloy layer 22 is preferably layered. It should be noted that FeSn2 may grow in a columnar shape along the coating thickness direction. In this case, if the columnar crystals of FeSn2 grow excessively towards the surface in the direction of composition with the coating thickness direction, becoming coarse FeSn2 crystals 22b with a major diameter exceeding 1 μm, it will hinder the sliding properties of the Sn-Zn coated steel 1. Therefore, when FeSn2 contains columnar crystals in addition to layered FeSn2, it is preferable that the major diameter of the columnar crystals of FeSn2 is 1 μm or less. Here, the major diameter refers to the longest side among the edges of the columnar crystal. The major diameter is determined by observing a section of the plate thickness with a length of 5 mm or more orthogonal to the plate thickness direction at a randomly selected location, and is set as the length of the longest side among the edges of the columnar crystals observed in that section of the plate thickness.

[0108] The conditions for Sn-Zn system plating are explained below.

[0109] The Sn-Zn plating bath temperature is set below 350°C. If the Sn-Zn plating bath temperature is below 350°C, the formation of coarse FeSn2 crystals 22b can be suppressed. There is no particular limitation on the lower limit of the Sn-Zn plating bath temperature, but it is preferably above 240°C. If the Sn-Zn plating bath temperature is above 260°C, further surface purification (oxide removal) by the flux can be expected.

[0110] In addition, the plate temperature of the pre-plated steel when entering the Sn-Zn system plating process is preferably 10℃~90℃.

[0111] Under the above conditions, the pre-plated steel is immersed in a Sn-Zn plating bath.

[0112] The preferred immersion time for pre-plated steel in a Sn-Zn plating bath is 3 to 20 seconds. If the immersion time for pre-plated steel in a Sn-Zn plating bath is 8 to 16 seconds, it ensures the surface cleaning (oxide removal) caused by the flux and the removal time of the flux after the reaction.

[0113] After the hot-dip galvanized steel is retrieved from the Sn-Zn plating bath, the coating adhesion is adjusted using a gas wiping method. The coating adhesion is set to 20–50 g / m² per single side. 2 By setting the coating adhesion amount to 20-50 g / m 2 This ensures corrosion resistance and weldability.

[0114] Next, the hot-dip galvanized steel with adjusted coating adhesion is cooled. Cooling is preferably performed at an average cooling rate of 10–30 °C / sec within a plate temperature range of 160–210 °C. This temperature range (160–210 °C on the surface of the steel 10) is approximately solid in the equilibrium state diagram, but it loses equilibrium and becomes liquid within the aforementioned cooling rate range, which promotes the precipitation of Sn crystals 32. Therefore, if cooling is performed under these conditions, the area fraction of the Sn-Zn eutectic structure 31 can be controlled to be below 50%. More preferably, the cooling rate is set to 15–25 °C / sec within a plate temperature range of 200 °C–160 °C.

[0115] Example

[0116] A cold-rolled steel sheet (sheet thickness: 0.8 mm) with the steel composition shown in Table 1 was used as the material, and pre-plating was carried out under the conditions shown in Table 2 to produce a pre-plated steel sheet. After that, the produced pre-plated steel sheet was heated to the sheet temperature shown in Table 3. Then, using the heated pre-plated steel sheet, hot-dip Sn-Zn plating was carried out under the conditions shown in Table 3. Specifically, after pre-plating, Sn-Zn plating was carried out by the flux method (flux solution: 750 g / L ZnCl2 + 250 g / L NH4Cl). After Sn-Zn plating, the plating adhesion amount was adjusted by the gas wiping method, and then cooling and coiling were carried out. In the Sn-Zn system plating, as the bath composition, Sn and Zn were appropriately changed to produce various specimens. Specimens added with elements for improving corrosion resistance were also produced. Furthermore, as shown in Table 4, specimens with a film containing P and S were also produced. The characteristics of these were evaluated through the tests shown below.

[0117] [Table 1]

[0118]

[0119] [Table 2]

[0120]

[0121] [Table 3]

[0122]

[0123] [Table 4]

[0124]

[0125] <Characteristic evaluation>

[0126] (Corrosion resistance (outer surface corrosion resistance))

[0127] As Figure 5 shown in the schematic diagram (top view) of the corrosion resistance test specimen, the specimens obtained by spot welding a 70×150 mm flat plate and a 35×100 mm plate on a 70×150 mm plate were evaluated by JASO (Automobile Standards based on the Japan Society of Automotive Engineers) M610-92 "Automobile Component Appearance Corrosion Test Method". Here, in the 70×150 mm flat plate, its end face and back face were sealed. It should be noted that no sealing part was provided on the 35×100 mm plate. The evaluation was carried out based on the rust area ratio of the flat part and the end face part of the 35×100 mm plate. For the end face part, specimens evaluated as A and B were judged as qualified. In addition, for the case of C evaluation, since it can be used well as long as it is painted, it was also judged as qualified.

[0128] [Evaluation criteria]

[0129] During the test: 360 cycles (120 days)

[0130] [Evaluation criteria] (evaluated by the rust area ratio)

[0131] A: Red rust generation is less than 0.1%

[0132] B: Red rust generation is 0.1% or more and less than 1.0% or white rust occurs (white rust is less than 20%)

[0133] C: Red rust generation is 1.0% or more and less than 3.0% or white rust is obvious (white rust is 20% or more and less than 40%)

[0134] D: Red rust generation is 3.0% or more and less than 5.0% or white rust is very obvious (white rust is 40% or more and less than 70%)

[0135] X: Red rust generation is 5.0% or more or white rust is significant (white rust exceeds 70%)

[0136] (Workability)

[0137] Workability is evaluated by a cylindrical deep drawing test. Using a flat-bottom cylindrical die with a punch diameter of φ50mm, cylindrical deep drawing is performed. Noxrust53**0-F40 (manufactured by Nihon Parkerizing) is used as the lubricant, and the anti-wrinkle pressure is carried out at 700 kgf. It is evaluated by the maximum drawing ratio (blank diameter ÷ punch diameter) that can be drawn at this time and the plating appearance of the processed part. The specimens evaluated as A and B are regarded as qualified.

[0138] [Evaluation criteria]

[0139] A: It can be formed, and there are no defects in the coating, and the drawing ratio is 2.3 or more

[0140] B: It can be formed, and there are no defects in the coating, and the drawing ratio is 2.2 or more

[0141] C: It can be formed, and there are no defects in the coating, and the drawing ratio is 2. **0 or more

[0142] X: It can be formed, but the drawing ratio is less than 2.0 or adhesive wear occurs in the coating

[0143] Regarding weldability, two items of spot weldability and brazing strength are evaluated.

[0144] (Spot weldability)

[0145] For Sn-Zn coated steel sheets (test material 100) (material: ultra-low carbon steel, thickness: 0.8 mm), the applied pressure was set to 200 kgf, the welding current to 8.0 kA, the welding energizing time to 10 cycles (60 Hz region), and the spacing between the upper and lower electrodes before welding was set to 30 mm. Spot welding 130 was performed on flat plates 110 and 120. The electrodes used were both Cr-Cu DR-shaped, with a 6φ40R front end. Continuous spot welding tests were conducted at 95% of the current generated by the spatter.

[0146] Regarding the electrode life of continuous dotting, the welded part was peeled off using the peeling method every 25 points to determine the weld nugget diameter. When the weld nugget diameter was less than 3.6 mm, it was set as NG. The number of dots before the 25 dots with a diameter less than 3.6 mm was taken as the continuous dotting under this test level.

[0147] [Evaluation Criteria]

[0148] A: More than 200 points

[0149] B: 150-200 points

[0150] C: 50-149 points

[0151] X: Continuous scoring less than 50 points

[0152] (Bragging strength)

[0153] Prepare four specimens 210, each measuring 10mm × 70mm × 0.8mm, with the edges trimmed. Overlap two specimens 210 and immerse them in 50ml of flux solution (Sparkle flux T-5: manufactured by Senju Metal Industries) up to the bottom 10mm of the specimens 210. Next, slowly insert the specimens into Sn-Ag based solder (Sn-3.5 wt% Ag-0.5 wt% Cu) maintained at 300°C until the bottom 15mm of the overlapping specimens 210 is reached, and immerse for 20 seconds. Perform brazing by air cooling the specimens 210 by retrieving them and fixing the top. Then, by clamping the brazed portion 220 of the specimens 210 into a vise, bend the opposite side of the brazed side into a T-shape in a cross-sectional view to create a tensile test piece 200. The length of the bent portion is set to 40mm.

[0154] exist Figure 6 The diagram shows a cross-sectional schematic of tensile test specimen 200. Two such tensile test specimens are prepared to evaluate the brazing strength.

[0155] Using a tensile testing machine, the brazed joint is stretched vertically along opposite sides, and the maximum strength (N / 10mm) is measured until the brazed joint peels off. However, if the brazed joint cracks and peels off completely during the initial stage of the stretching, the strength is set to 0N / 10mm.

[0156] [Evaluation Criteria]

[0157] A: Exceeding 250N / 10mm

[0158] B: 150 ~ Over 250 N / 10 mm

[0159] C: 50~149N / 10mm

[0160] X: Less than 50N / 10mm

[0161] [Table 5]

[0162]

[0163] [Table 6]

[0164]

[0165] As shown in Invention Example D4, if the Sn-Zn eutectic area ratio in the coating exceeds 50.0%, there is a tendency for reduced corrosion resistance.

[0166] [Table 7]

[0167]

[0168] A comparison of Invention Example E1 with Invention Examples E2 to E10 shows that corrosion resistance is improved by adding a specified amount of corrosion-enhancing elements such as Mg, Al, W, and Mo.

[0169] [Table 8]

[0170]

[0171] Comparing Invention Example F1 with Invention Examples F2 to F8, which have a Si and P-containing film applied to the coating surface, it can be seen that applying a film to the coating surface optimizes corrosion resistance, processability, and weldability. However, as shown in Invention Example F6, if the film becomes thicker, the surface resistivity increases, making it easier for spatter to occur and reducing weldability.

[0172] It should be noted that the FeSn2 alloy layers in Invention Examples C1-C21, D1-D4, E1-E10, and F1-F8 are formed by fine FeSn2 crystals (layered morphology, tiny columnar crystals with a major diameter of less than 1 μm), and no coarse FeSn2 crystals are formed.

[0173] Industrial availability

[0174] Sn-Zn coated steel 1 has excellent corrosion resistance, processability and spot weldability, and has high brazing strength. Therefore, it can be suitably applied to automotive fuel tank materials and battery housing materials, especially according to one embodiment of the present disclosure.

[0175] Symbol Explanation

[0176] 1 Sn-Zn coated steel

[0177] 10. Steel

[0178] 20 alloy layers

[0179] 21 Ni-Fe alloy layer

[0180] 22 FeSn2 alloy layer

[0181] 23 Ni2Zn 11 Mutually

[0182] 30 Sn-Zn coating

[0183] 31 Sn-Zn eutectic structure

[0184] 32 Sn crystal< / pt>

Claims

1. A Sn-Zn based coated steel, characterized in that, have: steel The alloy layer located on the surface of the steel, and The Sn-Zn plating layer located on the alloy layer The alloy layer comprises, sequentially from the surface of the steel, a Ni-Fe alloy layer, an FeSn2 alloy layer, and a Ni2Zn alloy layer. 11 Mutually, The Ni-Fe alloy layer covers more than 50% of the surface of the steel. The FeSn2 alloy layer covers more than 90% of the surface of the steel. The Ni2Zn 11 The surface coverage of the steel is 30% to 80%. The Sn-Zn coating is a Sn coating containing 0.5 to 10.0 at% Zn. The adhesion amount of the Sn-Zn system coating is 20 g / m. 2 ~50g / m 2 .

2. The Sn-Zn based coated steel according to claim 1, characterized in that, In the Sn-Zn system coating, the area ratio of the Sn-Zn eutectic structure is less than 50%, and the area ratio of Sn crystals is more than 50%.

3. The Sn-Zn based coated steel according to claim 1 or 2, characterized in that, The Sn-Zn coating contains one or more of Mg, Al, W, and Mo in a total of 0.5 to 5.0 at% of each.

4. The Sn-Zn based coated steel according to claim 1 or 2, characterized in that, The Sn-Zn coating has a film containing Si and P on its surface.

5. The Sn-Zn based coated steel according to claim 3, characterized in that, The Sn-Zn coating has a film containing Si and P on its surface.

6. The Sn-Zn based coated steel according to claim 1, characterized in that, The FeSn2 alloy layer exists on the Ni-Fe alloy layer in at least one of the following forms: a layered morphology and columnar crystals with a major diameter of less than 1 μm.

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