Welded laminate and manufacturing method therefor

Abstract

To provide a welded laminate made by spot-welding a plurality of steel plates including a galvanized steel plate and having high corrosion resistance, and a manufacturing method for the same.SOLUTION: A welded laminate, in which a plurality of steel plates are spot-welded, has a stainless steel plate welded by spot-welding, on an outer side in at least spot-welded weld zone with respect to the plurality of steel plates. The outermost steel plate of the plurality of steel plates includes a galvanized steel plate having a galvanized layer arranged on an outer side of the steel plate and a coating film arranged on an outer side of the galvanized layer, in at least non-spot-welded non-weld zone. A plate thickness of the stainless steel plate is 0.2 mm or more, and a shortest distance between a nugget outer edge part which welds the stainless steel plate and the steel plate, and an outside surface of the stainless steel plate in the weld-zone is 50 μm or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a welded laminate and a method for manufacturing the same. [Background technology]

[0002] Automobiles and other vehicles are generally manufactured by pressing steel sheets, welding them, and then painting them. When painting, an anti-rust electrodeposition coating (primer coating) is first applied, followed by a base coat and a color coat (top coat). A large amount of carbon dioxide is emitted during the painting process. Therefore, if the painting process could be simplified, it would be possible to significantly reduce carbon emissions.

[0003] Zinc-plated steel sheets have high corrosion resistance, so their use in vehicle components is expected to reduce carbon emissions by eliminating or simplifying the need for anti-corrosion electrodeposition coating.

[0004] Steel sheets are welded by methods such as arc welding, spot welding, etc. As described in Patent Documents 1 and 2, various methods for spot welding zinc-based plated steel sheets have been investigated. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2023 / 080076 [Patent Document 2] Patent No. 7355282 Summary of the Invention [Problem to be solved by the invention]

[0006] Methods of spot welding zinc-based plated steel sheets are known, as described in Patent Document 1 and Patent Document 2. However, according to the findings of the present inventors, when zinc-based plated steel sheets are spot welded, the zinc-based plating and the coating film formed on the surface of the zinc-based plating disappear at the welded portion, exposing the steel sheet base material and reducing corrosion resistance.

[0007] In view of these circumstances, an object of the present invention is to provide a welded laminate formed by spot welding a plurality of steel sheets, including a zinc-based plated steel sheet, which has high corrosion resistance, and a method for manufacturing the same. [Means for solving the problem]

[0008] One aspect of the present invention for solving the above problems relates to the following welded laminates [1] to [6]. [1] A welded laminate in which multiple steel plates are spot-welded, a stainless steel plate welded by the spot welding at least on the outer side of the plurality of steel plates in the spot-welded welded portion; the steel sheet arranged outermost among the plurality of steel sheets includes a zinc-based plated steel sheet having, at least in a non-welded portion where the spot welding is not performed, a zinc-based plating layer arranged on the outer side of the steel sheet and a coating film arranged on the outer side of the zinc-based plating layer, The thickness of the stainless steel plate is 0.2 mm or more, In the welded portion, the shortest distance between the outer edge of the nugget welding the stainless steel plate and the steel plate and the outer surface of the stainless steel plate is 50 μm or more. Welded laminate. [2] The plurality of steel plates include steel plates having a tensile strength of 590 MPa or less. [1] The welded laminate described in [1]. [3] The coating film contains a binder resin, conductive particles, and an anti-rust pigment, and has a film thickness of 1 μm or more and 10 μm or less. [1] or [2]. The welded laminate. [4] The coating film contains an acrylic resin, zirconium, divalent or more and tetravalent or more vanadium, phosphorus, and cobalt, and has a film thickness of 0.3 μm or more and 1.0 μm or less. The welded laminate according to any one of [1] to [3]. [5] The zinc-based plated steel sheet has a zinc-based plating layer containing zinc, more than 0% by mass and not more than 30% by mass of aluminum, more than 0% by mass and not more than 10% by mass of magnesium, and more than 0% by mass and not more than 2% by mass of silicon. The welded laminate according to any one of [1] to [4]. [6] A component that constitutes the vehicle frame. The welded laminate according to any one of [1] to [5].

[0009] Another aspect of the present invention for solving the above problems relates to the method for producing a welded laminate described below in [7]. [7] stacking a plurality of steel sheets including zinc-based plated steel sheets having coating films so that the coating films of the zinc-based plated steel sheets are arranged on the outermost side; a step of placing a stainless steel plate on the outer side of the zinc-based plated steel plate; and spot welding the plurality of steel plates together at the locations where the stainless steel plates are arranged, In the spot welding step, the electrode pressure is set to 1.4 kN or more and 4.9 kN or less, and the product of the welding current (kA) and the welding time (cycles) is set to 80 (kA cycles) or more and 560 (kA cycles) or less. Method for manufacturing welded laminates. [Effects of the Invention]

[0010] According to the present invention, there are provided a welded laminate formed by spot welding a plurality of steel sheets including a zinc-based plated steel sheet, the welded laminate having high corrosion resistance, and a method for manufacturing the same. [Brief explanation of the drawings]

[0011] [Figure 1] 1A and 1B are schematic cross-sectional views of a welded stack according to one embodiment of the present invention. [Figure 2] 2A and 2B are schematic diagrams showing how stacked steel plates and stainless steel plates are spot-welded. DETAILED DESCRIPTION OF THE INVENTION

[0012] 1. Welded laminate 1A and 1B are schematic cross-sectional views of a welded stack according to one embodiment of the present invention.

[0013] 1A and 1B, the welded stack 100 has a plurality of steel plates (two steel plates 110 and 120 in FIGS. 1A and 1B), which are spot-welded at a weld 130. A nugget 150 is formed between the plurality of steel plates 110 and 120 due to resistance heating between the steel plates during spot welding.

[0014] Resistance heat generated during spot welding is conducted through the steel sheets in the thickness direction. This conducted heat causes the zinc-based plating layer and coating film, which were located on the outermost surface of the stacked steel sheets, to disappear at the welded portion 130. As a result, the welded stack 100 has sufficient corrosion resistance at the non-welded portion 135 where the zinc-based plating layer and coating film remain, but the corrosion resistance at the welded portion 130 is reduced. In this embodiment, a stainless steel plate 140 is placed outside the welded portion 130 during spot welding, and the two plates are spot-welded together. By placing the stainless steel plate 140 outside the welded portion 130 in this manner, the corrosion resistance of the welded portion 130, from which the zinc-based plating layer and coating film have disappeared, can be improved in a simple manner.

[0015] Therefore, in this embodiment, the welded stack 100 has a stainless steel plate 140 on the outer side of the plurality of steel plates (steel plate 110 and steel plate 120) at least in the welded portion 130. The welded stack 100 shown in FIG. 1A has the stainless steel plate 140 on only one surface (upper surface 110a) of the plurality of steel plates. The welded stack 100 shown in FIG. 1B has the stainless steel plate 140a and the stainless steel plate 140b on both surfaces (upper surface 110a and lower surface 120b) of the plurality of steel plates.

[0016] The stainless steel plate 140 (or the stainless steel plate 140a and the stainless steel plate 140b; the same applies below) is welded together by spot welding when multiple steel plates are welded together. A nugget 160 (nuggets 160a and 160b in FIG. 1B; the same applies below) is formed between the steel plate 110 and the stainless steel plate 140 due to resistance heating between the steel plate 110 and the stainless steel plate 140 during spot welding. In this embodiment, the nugget 160 is united with the nugget 150 formed between the steel plate 110 and the steel plate 120 to form a single nugget. The term "outside" refers to the direction in which the surface of the outermost steel plate among the multiple steel plates, to which no other zinc-based plated steel plates are welded, faces.

[0017] According to the findings of the present inventors, stainless steel has a relatively high electrical resistance and generates a large amount of heat during spot welding. Therefore, the nugget 160 formed in the stainless steel plate 140 tends to grow to a relatively large size. If this grown nugget 160 extends to a portion close to the outer surface of the stainless steel plate 140 (the surface opposite the steel plate 110), the effect of improving corrosion resistance provided by the stainless steel plate 140 is reduced, and the corrosion resistance of the welded portion 130 is reduced. To prevent this, in this embodiment, the shortest distance between the outer edge of the nugget 160 welding the steel plate 110 and the stainless steel plate 140 and the outer surface of the stainless steel plate 140 in the welded portion 130 is set to 50 μm or more.

[0018] 1-1. Steel plate The multiple steel sheets can be any steel sheet, such as hot-rolled mild steel sheets and steel strips described in JIS G 3131 (2018), hot-rolled steel sheets and steel strips for automobiles described in JIS G 3113 (2018), cold-rolled steel sheets and steel strips described in JIS G 3141 (2017), and various stainless steel sheets (including austenitic, martensitic, ferritic, and ferritic-martensitic dual-phase).

[0019] The plurality of steel sheets includes zinc-based plated steel sheets having a zinc-based plating layer and a coating film on their surfaces. The plurality of steel sheets may all be zinc-based plated steel sheets having a coating film, or may be a combination of zinc-based plated steel sheets having a coating film and other steel sheets (e.g., zinc-based plated steel sheets without a coating film, plated steel sheets having a coating layer other than a zinc-based plating layer, or steel sheets having no coating film). However, the plurality of steel sheets are stacked such that the zinc-based plating layer and the coating film are disposed on their outermost surfaces. Note that in FIGS. 1A, 1B, 2A, and 2B, the plating layer and the coating film are omitted for the purpose of simplifying the explanation of the basic configuration of the present application, particularly the state of nugget formation and welding. Although not shown, at least one of the illustrated steel sheets 110 and 120 has a plating layer (zinc-based plating layer) and a coating film on its surface. Furthermore, in the weld 130 formed by spot welding, the plating layer (zinc-based plating layer) and the coating film are lost due to the heat generated during welding.

[0020] The plurality of steel plates preferably includes steel plates having a tensile strength of 270 MPa or more and 980 MPa or less, and more preferably includes steel plates having a tensile strength of 270 MPa or more and 590 MPa or less. Steel plates with high tensile strength have a large resistance heat generation due to the additive elements contained in the steel plate. As a result, the nugget 160 formed between the steel plate and the stainless steel plate 140 becomes large, and is likely to form in a region close to the outer surface of the stainless steel plate 140, resulting in a decrease in corrosion resistance at the weld 130. From the above perspective, the steel plate with low tensile strength is preferably arranged as the outermost of the plurality of steel plates that contacts the stainless steel plate 140.

[0021] The thickness of the steel plates may be the same or different, and may be selected appropriately depending on the parts to be manufactured. For example, the thickness is 0.6 mm or more and 2.5 mm or less.

[0022] 1-1-1. Zinc-based plating layer The zinc-based plating layer is a plating layer disposed on the surface of a steel sheet and containing zinc (Zn) as its main component. The term "containing zinc as its main component" means that the zinc-based plating layer contains 50% by mass or more of Zn. The zinc-based plating layer may be a plating layer manufactured by any method, such as electroplating, hot-dip plating, or vapor deposition plating. The zinc-based plating layer may be formed on only one surface (the upper surface 110a) of the steel sheet having the zinc-based plating layer, or on both surfaces (the upper surface 110a and the lower surface 120b). The zinc-based plating layer may be a zinc-based plating layer, or a plating layer made of an alloy containing Zn (for example, an alloy such as Zn-Al, Zn-Mg, Zn-Ni, Zn-Al-Mg, or Zn-Al-Mg-Si).

[0023] For example, the zinc-based plating layer may contain aluminum (Al) from more than 0% by mass to 30% by mass, preferably 0.1% by mass to 22% by mass. Al passivates the zinc-based plating layer, improving its corrosion resistance and suppressing the generation of dross during production. By keeping the Al content at 30% by mass or less, it is possible to suppress a decrease in the adhesion of the zinc-based plating layer and a decrease in the sacrificial corrosion protection effect of Zn.

[0024] The zinc-based plating layer may also contain more than 0% by mass and 10% by mass or less, preferably 1.5% by mass or more and 10% by mass or less, of magnesium (Mg). Mg uniformly forms dense corrosion products on the surface of the zinc-based plating layer, preventing erosion by corrosion factors and improving the corrosion resistance of the zinc-based plating layer. By setting the Mg content to 10.0% by mass or less, it is possible to suppress a decrease in the sacrificial corrosion protection effect of Zn and also to suppress the generation of dross.

[0025] The zinc-based plating layer may also contain silicon (Si) of more than 0% by mass to 2% by mass, preferably 0.005% by mass to 2% by mass. Si inhibits the growth of an Fe-Zn alloy layer and an Fe-Al alloy layer at the interface between the steel sheet and the zinc-based plating layer, thereby improving adhesion between the steel sheet and the zinc-based plating layer.

[0026] Furthermore, when Zn-based plating layers contain Mg, the Zn content is reduced, which has a negative effect on appearance and corrosion resistance. 11 To suppress the formation and growth of the Mg2 phase, titanium (Ti), boron (B), a Ti-B alloy, a Ti-containing compound, or a B-containing compound may be contained. The content of these compounds is preferably in the range of 0.001% by mass to 0.1% by mass for Ti and in the range of 0.0005% by mass to 0.045% by mass for B.

[0027] The zinc-based plating layer may also contain optional trace additives and unavoidable impurities. For example, when the zinc-based plating layer contains Mg, Zn has a negative effect on the appearance and corrosion resistance. 11 To suppress the formation and growth of the Mg2 phase, the zinc-based coating layer may contain titanium (Ti), boron (B), a Ti-B alloy, a Ti-containing compound, or a B-containing compound. Furthermore, to enhance the corrosion resistance of the coating layer, the zinc-based coating layer may contain elements such as Sn, Bi, In, Ca, Y, La, Ce, Si, Cr, Ti, Ni, Co, V, Nb, Cu, Mn, Fe, Sr, Sb, Pb, B, Li, Zr, Mo, W, Ag, and P. The content of these trace additives can be from the detection limit to 1% by mass.

[0028] 1-1-2.Paint film The coating is an organic or inorganic coating disposed on the surface of the zinc-based plating layer.

[0029] The coating film may be, for example, a coating film containing a binder resin, conductive particles, and an anti-rust pigment (hereinafter simply referred to as a "first coating film"), or a coating film containing an acrylic resin, zirconium (Zr), divalent to tetravalent or more vanadium (V), phosphorus (P), and cobalt (Co) (hereinafter simply referred to as a "second coating film").

[0030] (First coating film) The first coating film includes a binder resin, conductive particles, and an anti-rust pigment.

[0031] The type of binder resin is not particularly limited. Examples of binder resins include polyester resin, urethane resin, acrylic resin, epoxy resin, phenolic resin, and mixtures thereof. These resins may be aqueous resins that dissolve or disperse in water, or solvent-based resins that dissolve or disperse in an organic solvent. These resins may be used alone or in combination of two or more. Of these, urethane resins are preferred from the viewpoints of improving the corrosion resistance, workability, and conductivity of the welded laminate 100, as well as adhesion to the undercoat and topcoat.

[0032] From the viewpoint of improving the weldability and corrosion resistance of the welded laminate 100, as well as adhesion to the undercoat and topcoat, the content of the binder resin is preferably 25% by mass or more and 70% by mass or less, and more preferably 30% by mass or more and 60% by mass or less, relative to the total mass of the first coating film.

[0033] The type of conductive particles is not particularly limited. Examples of conductive particles include non-oxide ceramic particles, iron alloy particles, stainless steel particles, oxide particles such as NiO and ZnO, and carbon particles such as carbon black, graphite, and carbon nanotubes.

[0034] Examples of non-oxide ceramic particles include boride ceramics, carbide ceramics, nitride ceramics, and silicide ceramics of elements such as transition metals of Group IV (Ti, Zr, Hf), Group V (V, Nb, Ta), and Group VI (Cr, Mo, W) of the periodic table, Mn, Fe, Co, and Ni, as well as boride ceramic particles of rare earth elements or elements of Group II (Ca, Sr, Ba).

[0035] Examples of iron alloy particles include particles of an alloy of iron with Si, V, Mn, W, Mo, Ti, Ni, or Nb. Specific examples of iron alloy particles include ferrosilicon, ferrovanadium, ferromanganese, ferrotungsten, ferromolybdenum, ferrotitanium, ferronickel, ferroboron, and ferroniobium. Of these, ferrovanadium, ferrosilicon (e.g., FSi2 ferrosilicon with an Si content of 75 to 80 mass%), and ferromanganese are preferred from the viewpoint of improving the weldability of the zinc-based plated steel sheet and the corrosion resistance of the welded laminate 100.

[0036] The stainless steel particles are alloy particles of an alloy in which 10.5 mass % or more of Cr is contained in Fe (however, the C content is 1.2 mass % or less).

[0037] From the viewpoint of improving the weldability of the welded laminate 100 and the adhesion to the intermediate coat and top coat, the content of the conductive particles is preferably 5% by mass to 30% by mass, and more preferably 10% by mass to 20% by mass, of the total mass of the first coating film. The conductive particles increase the conductivity of the first coating film, making it easier for current to pass through the first coating film during spot welding. Therefore, the first coating film containing the conductive particles can be made thicker and therefore more corrosion-resistant.

[0038] The type of the rust-preventive pigment is not particularly limited. Examples of the rust-preventive pigment include phosphate-based rust-preventive pigments such as aluminum tripolyphosphate, phosphate and phosphite salts of Zn, Mg, Al, Ti, Zr, and Ce, and hydrocalumite-treated phosphate compounds, and silica-based rust-preventive pigments such as Ca ion-exchanged silica and amorphous silica.

[0039] From the viewpoint of improving the corrosion resistance of the welded laminate 100, the content of the anti-rust pigment is preferably 5% by mass or more and 40% by mass or less, and more preferably 10% by mass or more and 30% by mass or less, relative to the total mass of the first coating film.

[0040] The thickness of the first coating film is preferably 1 μm or more and 10 μm or less, more preferably 2 μm or more and 10 μm or less, and even more preferably 4 μm or more and 10 μm or less. The thicker the first coating film, the higher the corrosion resistance of the welded stack 100 in the non-welded portion 135.

[0041] (Second coating) The second coating film contains an acrylic resin, zirconium (Zr), divalent or greater and tetravalent or greater vanadium (V), phosphorus (P), and cobalt (Co).

[0042] The type of acrylic resin is not particularly limited, but is preferably a copolymer of styrene, (meth)acrylic acid, (meth)acrylic acid alkyl ester, and acrylonitrile. The copolymer may also be a copolymer of these monomers with other vinyl group monomers. In this specification, (meth)acrylic means either or both of acrylic and methacrylic.

[0043] The structural units derived from styrene improve the corrosion resistance and adhesion to the intermediate coat and top coat of the welded laminate 100. The amount of structural units derived from styrene is preferably 15% by mass or more and 25% by mass or less, and more preferably 17% by mass or more and 23% by mass or less, of the total mass of the copolymer.

[0044] The structural units derived from (meth)acrylic acid improve the adhesion of the second coating film to the zinc-based plating layer, and the amount of the structural units derived from (meth)acrylic acid is preferably from 1 to 10% by mass, more preferably from 2 to 6% by mass, based on the total mass of the copolymer.

[0045] The structural units derived from (meth)acrylic acid alkyl esters enhance the corrosion resistance of the welded laminate 100. The amount of structural units derived from (meth)acrylic acid alkyl esters is preferably 40% by mass or more and 58% by mass or less, and more preferably 40% by mass or more and 55% by mass or less, relative to the total mass of the copolymer. The (meth)acrylic acid alkyl esters may be methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)allylate, 2-methylhexyl (meth)acrylate, and isomers thereof, with ethyl acrylate and butyl acrylate being preferred.

[0046] The Mitsuse units derived from acrylonitrile improve adhesion to the undercoat and topcoat. The amount of the acrylonitrile-derived structural units is preferably 20% by mass or more and 38% by mass or less, and more preferably 20% by mass or more and 35% by mass or less, based on the total mass of the copolymer.

[0047] From the viewpoint of improving corrosion resistance, the content of the acrylic resin is preferably 20% by mass or more and 60% by mass or less, and more preferably 20% by mass or more and 40% by mass or less, relative to the total mass of the second coating film.

[0048] Zirconium crosslinks the acrylic resin, improving the barrier properties of the second coating and the corrosion resistance of the welded laminate 100. Zirconium also bonds with Zn in the zinc-based plating layer via oxygen atoms, improving the adhesion of the second coating to the zinc-based plating layer.

[0049] The content of zirconium is set to 4 mg / m in terms of zirconium atoms in order to improve the corrosion resistance of the welded laminate 100 and the adhesion of the second coating film, while suppressing a decrease in corrosion resistance due to the tendency of cracks to occur in the second coating film caused by an excess of zirconium. 2 More than 400mg / m 2 Preferably, it is 50 mg / m or less. 2 More than 350mg / m 2 More preferably, it is:

[0050] Divalent or more, tetravalent or more vanadium is preferentially eluted in a corrosive environment, suppressing an increase in pH due to dissolution of plating components, thereby improving the corrosion resistance of the welded laminate 100 .

[0051] The vanadium content is preferably an amount such that the mass ratio (V / Zr) of vanadium atoms (V) to zirconium atoms (Zr) is 0.07 or more and 0.69 or less, more preferably 0.14 or more and 0.56 or less, from the viewpoint of increasing the corrosion resistance of the welded laminate 100 while suppressing a decrease in adhesion to the undercoat paint and topcoat paint due to vanadium.

[0052] Phosphorus reacts with Zn in the zinc-based plating layer to form zinc phosphate, which forms a passivation layer on the surface of the zinc-based plating layer, improving the corrosion resistance of the welded laminate 100 and increasing adhesion to the intermediate coat and top coat.

[0053] From the viewpoint of increasing the corrosion resistance of the welded laminate 100 while suppressing a decrease in resistance to blackening due to phosphorus, the phosphorus content is preferably an amount such that the mass ratio (P / Zr) of phosphorus atoms (P) to zirconium atoms (Zr) is 0.04 or more and 0.58 or less, and more preferably an amount such that 0.07 or more and 0.29 or less.

[0054] Cobalt reduces the oxidation (corrosion) rate of Zn in the zinc-based plating layer, and inhibits blackening of the zinc-based plating layer.

[0055] From the viewpoint of improving the blackening resistance of the zinc-based plating layer and the corrosion resistance of the welded laminate 100, the cobalt content is preferably an amount such that the mass ratio (Co / Zr) of cobalt atoms (Co) to zirconium atoms (Zr) is 0.005 or more and 0.08 or less, and more preferably an amount such that the mass ratio is 0.009 or more and 0.03 or less.

[0056] The thickness of the second coating is preferably 0.3 μm or more and 1.0 μm or less, and more preferably 0.7 μm or more and 1.0 μm or less. The thicker the second coating is, the more the corrosion resistance of the welded stack 100 in the non-welded portion 135 can be improved.

[0057] 1-1-3. Steel plate placement The welded stack 100 is formed by stacking and welding multiple steel plates. There is no limit to the number of steel plates; it may be two, or it may be three or more as long as spot welding is possible. In the welded stack 100 shown in FIGS. 1A and 1B, two steel plates 110 and 120 are stacked and welded together. The multiple steel plates are stacked so that a zinc-based plating layer and a coating film are disposed on the outermost surfaces (upper surface 110a and lower surface 120b) of the stacked multiple steel plates. By disposing a zinc-based plating layer and a coating film on the outermost surfaces, the corrosion resistance of the welded stack 100 can be improved.

[0058] Furthermore, by increasing the corrosion resistance of the welded laminate 100 with the zinc-based plating layer and coating film, the subsequent electrocoating process for improving corrosion resistance is no longer necessary, simplifying the painting process and reducing carbon emissions from painting.

[0059] The multiple steel plates are spot-welded at a welded portion 130. A nugget 150 is formed by spot welding so as to straddle the multiple steel plates at the welded portion 130. The nugget 150 is a massive portion made of Fe that is formed when the steel plate is cooled after melting due to resistance heat generated at the contact surface between the steel plates during spot welding, and this welds the multiple steel plates together.

[0060] As described above, the zinc-based plating layer and coating film are removed by the spot welding in the welded portion 130. On the other hand, the zinc-based plating layer and coating film remain in the non-welded portion 135. In the welded stack 100 of this embodiment, the remaining zinc-based plating layer and coating film enhance the corrosion resistance in the non-welded portion 135. On the other hand, the stainless steel plate 140 enhances the corrosion resistance in the welded portion 130 where the zinc-based plating layer and coating film have been removed. However, the zinc-based plating layer and coating film may remain partially intact in the welded portion 130.

[0061] The plurality of stacked steel plates may include steel plates of the same type having the same material and thickness, or may include steel plates of different types having different materials or thicknesses.

[0062] The total thickness of the stacked steel sheets is preferably 1.0 mm to 6.0 mm, more preferably 2.0 mm to 5.0 mm. By keeping the thickness within this range, the occurrence of expulsion during spot welding can be suppressed while ensuring the rigidity of the spot-welded parts.

[0063] 1-2.Stainless steel plate The stainless steel plate 140 is a plate-shaped member made of stainless steel.

[0064] The type of stainless steel is not limited. Examples of stainless steel include various types of stainless steel, such as austenitic, ferritic, and martensitic. Stainless steel has high corrosion resistance and is not easily lost due to the heat generated during spot welding. Therefore, stainless steel remains in the welded stack 100, thereby improving the corrosion resistance of the weld 130.

[0065] From the viewpoint of improving the corrosion resistance of the welded laminate 100, the stainless steel is preferably ferritic or austenitic.

[0066] For example, the stainless steel may be alloys such as SUS304 and SUS430. These alloys are readily available and can provide sufficient corrosion resistance.

[0067] In the welded portion 130, a nugget 160 (nuggets 160a and 160b in FIG. 1B) is formed inside the stainless steel plate 140 by resistance heating at the interface between the steel plate 110 and the stainless steel plate 140. The nugget 160 welds the outermost steel plate 110 and the stainless steel plate 140 together. Stainless steel has high electrical resistance and generates a large amount of heat. Therefore, the diameter of the nugget 160 (the distance between the ends of the nugget in a cross section obtained by cutting the welded stack 100 in a direction perpendicular to the stacking direction) is larger on the stainless steel plate 140 side of the interface between the steel plate and the stainless steel plate 140.

[0068] In this embodiment, the nugget 160 is united with the nugget 150 formed between the steel plates.

[0069] However, in order to improve the corrosion resistance of the welded portion 130, in this embodiment, the shortest distance between the outer edge of the nugget 160 that welds the steel plate 110 and the stainless steel plate 140 and the outer surface of the stainless steel plate 140 in the welded portion 130 is set to 50 μm or more. The shortest distance is preferably 100 μm or more. As shown in FIG. 1B , when two stainless steel plates are welded to both surfaces of a steel plate to form nuggets 160a and 160b, respectively, the shortest distance is the shorter of the shortest distance between the outer edge of the nugget 160a formed on the upper surface 110a and the outer surface of the stainless steel plate 140a, and the shortest distance between the outer edge of the nugget 160b formed on the lower surface 120b and the outer surface of the stainless steel plate 140b.

[0070] The diameter of the nugget 160 used to weld the steel plate 110 and the stainless steel plate 140 is preferably larger than the diameter of the nugget 150 used to weld the steel plate 110 and the steel plate 120. This allows the steel plate 110 and the stainless steel plate 140 to be joined with sufficient strength. Here, the diameter of the nugget 160 refers to the length of a straight line connecting the ends of the nugget 160 along the interface between the steel plate 110 and the stainless steel plate 140 in a cross section of the welded stack 100 cut in a direction perpendicular to the stacking direction. Furthermore, the diameter of the nugget 160 refers to the length of a straight line connecting the ends of the nugget 150 along the interface between the steel plate 110 and the steel plate 120 in a cross section of the welded stack 100 cut in a direction perpendicular to the stacking direction.

[0071] The thickness of the stainless steel plate 140 is 0.2 mm or more, preferably 0.4 mm or more and 3.0 mm or less, and more preferably 1.0 mm or more and 2.5 mm or less. The thicker the stainless steel plate 140, the easier it is to lengthen the shortest distance and improve the corrosion resistance of the welded portion 130. The thinner the stainless steel plate 140, the thinner the thickness of the welded stack 100, making it easier to use the welded stack 100 for various applications.

[0072] Furthermore, when spot welding is performed partially, the size of the stainless steel plate 140 is preferably 20 mm or more in width and 20 mm or more in length, and 100 mm or less in width and 100 mm or less in length. A width of 20 mm or more and a length of 20 mm or more sufficiently covers the areas where corrosion resistance decreases due to the loss of zinc-based plating or the coating film formed on its surface, making it easy to sufficiently increase corrosion resistance and to fix with a fixing jig or the like. A width of 100 mm or less and a length of 100 mm or less does not make the welded stack 100 too heavy, making it less likely to cause a decrease in fuel efficiency when used in an automobile or the like. When spot welding is performed continuously, the size can be appropriately adjusted to a width of 20 mm or more and a length of 20 mm or more so that the weight of the welded stack 100 does not become too heavy.

[0073] The stainless steel plate 140 may cover the entire outer surface of the steel plate 110, or may cover only a portion of the outer surface of the steel plate 110. For example, the stainless steel plate 140 may cover only a partial region of the outer surface of the steel plate 110, including the welds 130. Furthermore, when the welded stack 100 has a plurality of welds 130, the shape of the stainless steel plate 140 in plan view (the shape when viewed from a direction perpendicular to the stacking direction of the steel plates) may be a shape that follows the plurality of welds 130, such as a linear shape extending along the plurality of welds 130, or a planar shape having an outer edge shape that includes the plurality of welds 130.

[0074] Even when the stainless steel plate 140 covers only a portion of the outer surface of the steel plate 110, the zinc-based plating layer and the coating remain on the outer surface of the steel plate 110 that is not covered with the stainless steel plate 140 (the outer surface of the non-welded portion 135). Therefore, the corrosion resistance of the welded stack 100 in the non-welded portion 135 is sufficiently high.

[0075] Furthermore, the stainless steel plate 140 may be welded to only one surface (upper surface 110a) of the multiple steel plates (see FIG. 1A), or may be welded to both surfaces (upper surface 110a and lower surface 120b) (see FIG. 1B). For example, the stainless steel plate 140 does not need to be disposed on the surface that is placed in an environment where the steel plate is less susceptible to corrosion, such as the interior of an automobile body.

[0076] 2. Manufacturing method of welded laminate The welded stack can be manufactured by carrying out the steps of stacking multiple steel plates (step 1), placing a stainless steel plate on the outside of the stacked steel plates (step 2), and spot welding the multiple steel plates together at the locations where the stainless steel plate is placed (step 3).

[0077] 2-1. Process 1: Stacking steel sheets First, multiple steel plates are stacked. There are no particular limitations on the type of steel plate, and for example, the steel plates described above may be used. There is no limitation on the number of steel plates to be stacked, and it may be two, or three or more as long as they can be spot welded. In addition, the coating film may be disposed on only one surface of the stacked multiple steel plates, or on both surfaces.

[0078] However, the plurality of steel sheets includes zinc-based plated steel sheets having a coating film, and the plurality of steel sheets are stacked so that the coating film of the zinc-based plated steel sheet having a coating film is arranged on the outermost side.

[0079] 2-2. Step 2: Placing the stainless steel plate Next, a stainless steel sheet is placed on the outer side of the coating film on the zinc-based plated steel sheet among the stacked steel sheets. The stainless steel sheet is preferably placed so as to be in contact with the coating film. The type of stainless steel sheet is not particularly limited, and for example, the stainless steel sheet described above may be used. The stainless steel sheet may be placed on only one surface of the stacked steel sheets, or on both surfaces.

[0080] The stainless steel plate may be arranged so as to cover the entire outer surface of the steel plate, or may be arranged so as to cover only a portion of the outer surface of the steel plate, including the area to be spot welded.

[0081] A method for fixing (holding) the above-mentioned stacked steel sheets and the stainless steel sheets arranged on their outer surfaces without misalignment between the electrodes may be selected as appropriate, such as a method using a workpiece fixing jig such as a clamp or a vice, or a weld bond method using an adhesive.

[0082] 2-3. Process 3: Spot welding Next, the steel plates are spot-welded together at the locations where the stainless steel plates are placed, and the stainless steel plates are simultaneously welded.

[0083] 2A and 2B are schematic diagrams showing the spot welding of stacked steel plates and stainless steel plates. In Fig. 2A, a stainless steel plate 140 is disposed on only one surface (upper surface 110a) of stacked steel plates 110 and 120. In Fig. 2B, a stainless steel plate 140a and a stainless steel plate 140b are disposed on both surfaces (upper surface 110a and lower surface 120b) of stacked steel plates 110 and 120.

[0084] 2A and 2B, electrodes 210a and 210b are brought into contact with the stacked steel plates 110 and 120 at the position where the stainless steel plate 140 is arranged, so as to sandwich the steel plates 110 and 120 from above and below. In this embodiment, DR-type electrodes are used as electrodes 210a and 210b, in which a radius shape (a curved surface with a larger radius of curvature than the curved surface) is formed at the tip of a D-shaped electrode tip (the tip of the electrode is curved, and the radius of curvature of the curved surface is 1 / 2 the outer diameter of the electrode).

[0085] Then, while electrodes 210a and 210b are pressed in the stacking direction of the steel sheets, a current is passed between these electrodes. This current passage generates resistance heat at the interface between the stacked steel sheets 110 and 120, and this heat forms a nugget 150, welding the steel sheets 110 and 120 together. Resistance heat also occurs at the interface between the steel sheet 110 and the stainless steel sheet 140, and this heat forms a nugget 160 (nugget 160a in FIG. 1B ), welding the steel sheets 110 and 140 together. In FIG. 2B , resistance heat also occurs at the interface between the steel sheet 120 and the stainless steel sheet 140b, and this heat forms a nugget 160b, welding the steel sheets 120 and 140 together. The nuggets 150 and 160 grow in the thickness direction within the steel sheet 110 (and the steel sheet 120) and coalesce into a single nugget overall.

[0086] In this embodiment, the pressure (electrode pressure) applied by electrodes 210a and 210b during energization is set to 1.4 kN or more and 4.9 kN or less, and the product of the current (welding current, kA) during energization and the energization time (cycles) is set to 80 (kA cycles) or more and 560 (kA cycles) or less.

[0087] The greater the electrode pressure, the larger the nuggets 150 and 160 that are formed. Furthermore, the greater the product of the magnitude of the welding current and the current-flow time, the larger the nuggets 150 and 160 that are formed. Therefore, by setting the product of the electrode pressure, the magnitude of the welding current, and the current-flow time within the above ranges, it is possible to form nuggets of sufficient size and sufficiently weld the steel plate 110, the steel plate 120, and the stainless steel plate 140. Furthermore, by setting the product of the electrode pressure, the magnitude of the welding current, and the current-flow time within the above ranges, it is possible to make the shortest distance between the outer edge of the nugget 160 and the outer surface of the stainless steel plate 140 50 μm or more, thereby improving the corrosion resistance of the welded portion 130.

[0088] 3.Applications The welded laminate 100 can be used in applications that use spot-welded steel plates, such as automobile exterior panels, interior panels, and vehicle frames. In particular, when the welded laminate 100 is used in automotive applications that require subsequent painting, it can eliminate the need for electrodeposition coating to improve corrosion resistance, thereby reducing carbon dioxide emissions during electrodeposition coating. For example, the welded laminate 100 can be suitably used in applications where the stainless steel plate 140 is difficult to see from the outside, such as vehicle frames. [Example]

[0089] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

[0090] 1. Prepare ingredients 1-1. Zinc-plated steel sheet The following zinc-plated steel sheets were prepared. GA: Galvannealed steel sheet (Zn-10% Fe by mass, coating weight 45g / m 2 ) (plate thickness 1.2mm, 1.6mm, 2.3mm) Zn1: Hot-dip galvanized steel sheet (Zn-6% by mass Al-3% by mass Mg, coating weight 60 g / m 2 ) (plate thickness 1.6mm) Zn2: Hot-dip galvanized steel sheet (Zn-11% by mass, Al-6% by mass, Mg-0.2% by mass, Si, coating weight 60 g / m 2 ) (plate thickness 1.6mm) Zn3: Hot-dip galvanized steel sheet (Zn-19% by mass Al-6% by mass Mg-0.2% by mass Si, coating weight 60 g / m 2 ) (plate thickness 1.6mm)

[0091] For GA, the tensile strengths of the base steel sheets were 270 MPa, 440 MPa, 590 MPa, and 980 MPa. For Zn1, Zn2, and Zn3, the tensile strength of the base steel sheets was 440 MPa.

[0092] 1-2.Stainless steel plate The following stainless steel plates were prepared: ·SUS304 plate (plate thickness 0.1mm, 0.2mm, 1.2mm, 2.3mm) ·SUS430 plate (plate thickness 0.2mm, 1.2mm, 2.3mm)

[0093] 1-3. Formation of coating film A coating weight of 20 mg / m was applied to both sides of a degreased zinc-based coated steel sheet. 2A primer coating solution containing trivalent chromic acid, silica, and phosphoric acid was applied to the specimens, and the resulting coating was dried at a temperature of 70°C to form a primer layer. The surface of each primer layer was then bar-coated with an organic coating containing polyester-urethane resin, 76% Si-containing ferrosilicon particles as conductive particles (in an amount that would make the coating 30% by volume), and a 1:1 mass ratio mixture of Ca3(PO4)2 and V2O5 as rust-preventive pigments and fumed silica (in an amount that would make the coating 30% by volume), to the desired dry film thickness. The coating was then baked at a temperature of 200°C and immediately water-cooled to form a PU coating.

[0094] A degreased zinc-plated steel sheet was bar-coated on both sides without forming a primer layer. The sheet was coated with a water-based composition containing a non-oxide ceramic VB2 (5.0% by solids), an anti-rust pigment containing amorphous silica (Fuji Silysia Chemical Industries, Ltd., Silomask 02) (5.0% by solids), a magnesium-coated aluminum tripolyphosphate (Teika Corporation, condensed aluminum phosphate K-450H) (25.0% by solids), an epoxy resin emulsion (ADEKA Corporation, Adeka Resin EM-0718) (45.0% by solids) as a binder resin, and aluminum-doped conductive zinc oxide particles (Hakusui Tech, 23-Kt) (20.0% by solids) as oxide particles. The sheet was then baked at 140°C for 8 seconds to form a PE coating.

[0095] Organic coating material 3 was bar coated on both sides of a degreased zinc-based plated steel sheet to a predetermined dry film thickness, and baked at a final sheet temperature of 150°C to form a coating film PA.

[0096] 2. Spot welding Two zinc-based coated steel sheets (both of which had been coated under the same conditions) were stacked on top of each other, and a stainless steel sheet was placed on either the top or both the top and bottom surfaces. The stainless steel sheet was placed so as to cover only the periphery of the spot-welded area.

[0097] Spot welding was performed using a DC inverter spot welding machine under the conditions shown in Tables 1 to 3. The commercial power source used had a power frequency of 50 Hz. The electrode used was a DR-type electrode tip with a diameter of 16 mm, a tip radius of 40 mm, and a tip diameter of 6 mm. The initial pressure application time was 35 cycles, and the hold time after energization was 24 cycles.

[0098] Welded laminates 1 to 33 were produced by changing the type of stainless steel sheet, the type of zinc-based plated steel sheet and coating, and the spot welding conditions.

[0099] 3. Evaluation The welded laminate obtained by spot welding was evaluated as follows.

[0100] 3-1. Cross-sectional observation The welded laminate was cut to expose the cross section, which was then embedded in epoxy resin and polished to allow observation of the center of the weld. After polishing, the specimen was etched with a 3% NaOH aqueous solution, followed by further etching with 3% nitric acid in ethanol. The cross section was then observed under an optical microscope, and the shortest distance between the outer edge of the nugget welding the stainless steel sheet and the zinc-plated steel sheet and the outer surface of the stainless steel sheet (nugget-surface distance) was measured. Furthermore, the maximum diameter of the nugget welding the zinc-plated steel sheets (nugget 150 diameter) and the maximum diameter of the nugget welding the stainless steel sheet and the zinc-plated steel sheet (nugget 160 diameter: W2) were measured.

[0101] 3-2.Corrosion resistance The rust was removed by 90 cycles of corrosion in accordance with JASO-M609-91. After that, 10 locations on the welded parts that were visually confirmed to have deep corrosion were selected, and the corrosion depth was measured using a laser microscope. The corrosion resistance was evaluated based on the average corrosion depth of the 10 locations and the following criteria. For non-welded parts that were not covered with stainless steel plate, the corrosion resistance was evaluated based on the area ratio of red rust and the following criteria. Note that a rating of "AA," "A," or "B" in the corrosion resistance evaluation was considered to have excellent corrosion resistance. Corrosion resistance of welds A: The erosion depth was greater than 0 μm and less than 50 μm. B: The erosion depth was over 50 μm and less than 100 μm. C: The erosion depth was over 100 μm. Corrosion resistance of non-welded parts AA: No red rust was observed A: The area rate of red rust was over 0% and less than 10%. B: The area ratio of red rust was over 10% and less than 20%. C: The area ratio of red rust was over 20%

[0102] Tables 1 to 3 show the manufacturing conditions and evaluation results of the welded laminates 1 to 33.

[0103] [Table 1]

[0104] [Table 2]

[0105] [Table 3]

[0106] As shown in Tables 1 to 3, when spot welding multiple steel sheets together, the corrosion resistance of the welded portion can be improved by simultaneously welding a stainless steel sheet with a thickness of 0.2 mm or more to the outside of the weld and setting the shortest distance between the outer edge of the nugget welding the stainless steel sheets and the outer surface of the stainless steel sheet to 50 μm or more. In addition, by placing a zinc-based plating layer and a paint film on the outermost surface of the laminated steel sheets, the corrosion resistance of the non-welded portions can also be improved. [Industrial Applicability]

[0107] The welded laminate is useful in any application where steel sheets are spot welded. [Explanation of symbols]

[0108] 100 Welded Laminate 110, 120 steel plate 110a top side 120b Bottom side 130 Welded Section 135 Non-welded parts 140, 140a, 140b stainless steel plate 150, 160, 160a, 160b Nuggets 210a, 210b electrode

Claims

1. A welded laminate in which a plurality of steel plates are spot-welded, a stainless steel plate welded by the spot welding at least on the outer side of the plurality of steel plates in the spot-welded welded portion; the steel sheet arranged outermost among the plurality of steel sheets includes a zinc-based plated steel sheet having, at least in a non-welded portion where the spot welding is not performed, a zinc-based plating layer arranged on the outer side of the steel sheet and a coating film arranged on the outer side of the zinc-based plating layer, The thickness of the stainless steel plate is 0.2 mm or more, In the welded portion, the shortest distance between the outer edge of a nugget welding the stainless steel plate and the steel plate and the outer surface of the stainless steel plate is 50 μm or more. Welded laminate.

2. The plurality of steel plates include steel plates having a tensile strength of 590 MPa or less. The welded stack of claim 1.

3. The coating film contains a binder resin, conductive particles, and an anti-rust pigment, and has a film thickness of 1 μm or more and 10 μm or less. The welded stack of claim 1.

4. the coating film contains an acrylic resin, zirconium, divalent or greater and tetravalent or greater vanadium, phosphorus, and cobalt, and has a film thickness of 0.3 μm or greater and 1.0 μm or less; The welded stack of claim 1.

5. The zinc-based plated steel sheet has a zinc-based plating layer containing zinc, more than 0 mass% but not more than 30 mass% aluminum, more than 0 mass% but not more than 10 mass% magnesium, and more than 0 mass% but not more than 2 mass% silicon. The welded stack of claim 1.

6. It is a component that constitutes the vehicle frame. The welded laminate according to any one of claims 1 to 5.

7. a step of stacking a plurality of steel sheets including zinc-based plated steel sheets having coating films so that the coating films of the zinc-based plated steel sheets are arranged on the outermost side; a step of placing a stainless steel plate on the outer side of the zinc-based plated steel plate; and spot welding the plurality of steel plates together at the locations where the stainless steel plates are arranged, In the spot welding step, the electrode pressure is set to 1.4 kN or more and 4.9 kN or less, and the product of the welding current (kA) and the welding time (cycles) is set to 80 (kA cycles) or more and 560 (kA cycles) or less. Method for manufacturing welded laminates.

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