Plated steel

Abstract

The plated steel material is a plated steel material having a steel material and a plated layer, the average chemical composition of the plated layer containing Al: 0.2% to less than 4.0%, Mg: more than 4.0% to less than 12.5%, Ca: 0.15% to less than 3.00%, Zn: 65% or more, and impurities, the metal structure in any of the perpendicular sections (thickness direction) of the plated layer contains, in terms of area fraction, MgZn2 phase: 10 to 40%, Al-Zn phase: 0 to 15%, Al phase (Zn < 10%) phase: 0 to 5%, CaZn 13 phase: 1.0 to 15%, and the total of the [Al / MgZn2 / Zn ternary eutectic structure] and the [MgZn2 / Zn binary eutectic structure]: 30% or more.

Description

TECHNICAL FIELD

[0001] The present application relates to plated steel materials.

[0002] This application claims priority based on Japanese Patent Application No. 2022-025405 filed on February 22, 2022, and the contents thereof are hereby incorporated by reference. BACKGROUND

[0003] Plated steel materials on which hot-dip Zn plating is performed are widely used for automobiles, building materials, steel structures, home electric appliances, and the like because of excellent corrosion resistance and economy. Furthermore, in recent years, due to the demand for hot-dip Zn steel materials to be used in severe corrosion environments, in order to further improve durability, for example, a hot-dip Zn-Al-Mg system in which aluminum (Al) and magnesium (Mg) are added to Zn, as shown in Patent Literature 1 and Patent Literature 2, has been developed. The hot-dip Zn-Al-Mg system is applied to uses requiring high corrosion resistance.

[0004] In particular, since most of steel structures are required to have certain corrosion resistance, most of the steel structures are subjected to plating treatment. There are two methods for obtaining plated steel structures. One of them is a method in which a steel plate or the like is formed, welded, or the like to produce a steel structure, and then the steel structure is dipped in a plating bath. Hereinafter, this method is referred to as an immersion plating method. As another method, there is a method in which a plated steel plate having a plated layer formed on the surface in advance is formed, welded, or the like to produce a steel structure. Hereinafter, this method is referred to as a pre-plating method.

[0005] In the case of the immersion plating method, in addition to the generation of thermal strain after forming and the generation of appearance defects at the time of plating solidification, equipment becomes large-scale in order to directly dip the steel structure in the plating bath, which sometimes causes an increase in cost. Therefore, in most cases where the production of steel structures can be automated, the case of adopting the latter pre-plating method is increasing.

[0006] In addition, in the construction of steel structures using plated steel materials, welding work is performed. The welding in steel structures can be applied to various welding methods, such as arc welding, spot welding, laser welding, and the like. Among them, spot welding is attracting attention. The reason is that spot welding has the following features: no flux is required, the welding speed is fast, the amount of slag / smoke is small, it is labor-saving and not easily affected by the skill of the operator, since it can be accurately welded, the heat-affected zone around the welded portion is small, and it can also be joined to dissimilar materials. However, when spot welding is performed, it is necessary to study the properties of the welding target material, the appropriate current value for each welding target material, the selection of the electrode, and the like.

[0007] However, the plated layer of the plated steel material is sometimes difficult to spot weld. The reason is that the plated layer is easily melted at the time of spot welding. In particular, in the case of the plated steel material having a plated layer with a thickness exceeding 10 μm, the electric conduction / heat conduction becomes unstable and the current is unstable, and sometimes an appropriate weld nugget diameter is not obtained. That is, the range of appropriate current becomes small. Further, when the welding electrode contacts the plated layer at the time of welding, the metal such as Zn or Al contained in the plated layer reacts with Cu which is the main component of the welding electrode, and the welding electrode is consumed, and sometimes the electrode life is shortened. These reasons make the spot welding of the plated steel material difficult.

[0008] Further, the Zn-Al-Mg-based plating has a disadvantage that the weldability is difficult because of the low continuous spot welding property at the time of spot welding and the narrow range of appropriate current at the time of welding. This hinders the use of the Zn-Al-Mg-based plated steel material in applications where welding is frequently used, such as steel structures.

[0009] Therefore, for example, in the field of automobiles and the like, spot welding is used for plated steel materials having less unevenness in properties and a small plate thickness, such as alloyed Zn plated steel sheets, hot-dipped Zn steel sheets, but in the case of spot welding of plated materials other than these, it is difficult to set the welding conditions. Therefore, for example, in the case of spot welding of a Zn-Al-Mg-based plated steel sheet of an alloy system as shown in Patent Document 1, Patent Document 2, it is used only in limited applications.

[0010] As the use in applications where weldability is required, for example, examples of Patent Document 3, Patent Document 4 can be cited. However, in Patent Document 3, the continuous spot welding property at the time of welding is insufficient, and in Patent Document 4, the manufacturing is by evaporation due to the influence of the composition, and therefore, compared to hot dipping, there is a tendency that the cost becomes high.

[0011] Prior Art Documents

[0012] Patent Documents

[0013] Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 10-226865

[0014] Patent Document 2: Japanese Patent Application Laid-Open (JP-A) No. 2000-104154

[0015] Patent Document 3: Japanese Patent Application Laid-Open (JP-A) No. 2018-506644

[0016] Patent Document 4: Japanese Patent Application Laid-Open (JP-A) No. 2020-504781 SUMMARY

[0017] PROBLEMS TO BE SOLVED BY THE INVENTION

[0018] The present application has been made in view of the above circumstances, and an object is to provide a hot-dip plated steel material excellent in continuous spot welding property at the time of spot welding, capable of expanding a proper current range at the time of spot welding, and further excellent in corrosion resistance around a welded portion.

[0019] Means for solving the problem

[0020] To solve the above problem, one aspect of the present application adopts the following configuration.

[0021] [1] The plated steel material of one aspect of the present application is a plated steel material having a steel material and a plated layer possessed by the surface of the above steel material, and the average chemical composition of the above plated layer contains, in mass%:

[0022] Al: 0.2% to less than 4.0%,

[0023] Mg: more than 4.0% to less than 12.5%,

[0024] Ca: 0.15% to less than 3.00%,

[0025] Sn: 0% to less than 3.0%,

[0026] Bi: 0% to less than 1.0%,

[0027] In: 0% to less than 1.0%,

[0028] Sc: 0% to less than 0.50%,

[0029] Y: 0% to less than 0.50%,

[0030] La: 0% to less than 0.50%,

[0031] Ce: 0% to less than 0.50%,

[0032] Sr: 0% to less than 0.50%,

[0033] Si: 0% to less than 2.50%,

[0034] B: 0% to less than 0.50%,

[0035] P: 0% to less than 0.50%,

[0036] Cr: 0% to less than 0.25%,

[0037] Ti: 0% to less than 0.25%,

[0038] Ni: 0% to less than 1.0%,

[0039] Co: 0% to less than 0.25%,

[0040] V: 0% to less than 0.25%,

[0041] Nb: 0% to less than 0.25%,

[0042] Cu: 0% to less than 1.0%,

[0043] Mn: 0% to less than 0.25%,

[0044] Mo: 0% to less than 0.25%,

[0045] W: 0% to less than 0.25%

[0046] Zr: 0% to less than 0.25%,

[0047] Fe: 0% to less than 5.0%,

[0048] Ag: 0% to less than 1.0%,

[0049] Li: 0% to less than 0.50%,

[0050] Na: 0% to less than 0.05%,

[0051] K: 0% to less than 0.05%,

[0052] Sb: 0% to less than 0.5%,

[0053] Pb: 0% to less than 0.5%,

[0054] Zn: 65% or more,

[0055] and impurities,

[0056] In the cross section in the thickness direction in the plating layer, the metal structure when observed with a field of view of a scanning electron microscope contains, in terms of area fraction:

[0057] MgZn2 phase: 10 to 40%,

[0058] Al-Zn phase: 0 to 15%,

[0059] Al phase: 0 to 5%,

[0060] CaZn 13 phase: 1.0 to 15%,

[0061] the total of the [Al / MgZn2 / Zn ternary eutectic structure] and the [MgZn2 / Zn binary eutectic structure]: 30% or more.

[0062] [2] The plated steel material according to the above [1], wherein, in the CaZn 13 phase in the metal structure in the cross section in the thickness direction in the plating layer of the above-described plated steel material, the CaZn13 CaZn phase of 80% or more 13 The average equivalent circle diameter of the phase can also be 10 μm or more.

[0063] [3] The plated steel sheet according to any one of the above-mentioned [1] or [2], wherein, when the mass concentration % of the element X in the average chemical composition of the plated layer is set as [X], the average chemical composition of the plated layer can also satisfy the following formula (1).

[0064] 2.5 x [Al] + 2 x [Ca] + [Y] + [Sr] + [La] + [Ce] > [Mg] (1)

[0065] Effects of the Invention

[0066] According to the above-mentioned aspect of the present application, it is possible to provide a plated steel sheet which is excellent in continuous spot welding property at the time of spot welding, which can expand the appropriate current range at the time of spot welding, and which is excellent in corrosion resistance. BRIEF DESCRIPTION OF DRAWINGS

[0067] Figure 1 is a reflection electron image of the metal structure in the plated layer cross section of Example 22. DETAILED DESCRIPTION

[0068] As described above, the Zn-Al-Mg-based plated layer is a plated layer of the Zn alloy system, and is generally a material which deteriorates the spot welding property. On the other hand, the Zn-Al-Mg-based plated layer is high in corrosion resistance compared to the usual Zn plated layer. Therefore, if the spot welding property of the plated layer can be improved, the plated steel sheet having the Zn-Al-Mg-based plated layer becomes a plated steel sheet which is promising as a material of a steel structure.

[0069] The present inventors have conducted intensive studies in order to improve the weldability and corrosion resistance of the plated steel sheet. As a result, it has been found that the Al phase which exists in a bulk state in the plated layer is a factor which destabilizes the spot welding property. The "Al phase" referred to here is an Al phase having a Zn concentration of less than 10%. If the Al phase having a Zn concentration of less than 10% is generated in a large amount, the appropriate current range at the time of welding is narrowed. On the contrary, it has been found that by reducing the Al phase, a plated layer which is easy to weld with a wider appropriate current range is obtained.

[0070] Further, it has been found that by appropriately containing Mg, Ca and the like in the plated layer, the appropriate current value becomes wider. Furthermore, it has been found that by appropriately containing Mg, Ca and the like in the plated layer, these elements form an oxide film of the Ca-Mg system or the like on the surface of the Cu electrode at the time of spot welding, and this oxide film hinders the reaction of the Cu electrode with Al in the plated layer, with the result that the life of the electrode is improved.

[0071] Further, the plated steel sheet of the present embodiment is particularly excellent in corrosion resistance of the welded portion. This is because, by the CaZn phase which occupies a large volume fraction in the plated layer13 and the average crystal grain size of the MgZn2 phase becomes large, and thus the area fraction increases, so that the plating layer itself becomes less likely to react with the Cu electrode at the time of welding, and thus the plating layer itself becomes less likely to dissolve. If the plating layer itself becomes less likely to dissolve, the damage around the welded portion of the plating layer becomes minimal, and the residual amount of the plating layer can be increased, and as a result, the corrosion resistance of the welded portion can be improved.

[0072] Hereinafter, a plated steel material as an embodiment of the present application will be described.

[0073] The plated steel material of the present embodiment is a plated steel material having a steel material and a plating layer possessed by the surface of the steel material, and the average chemical composition of the plating layer contains, in terms of mass%, Al: 0.2% to less than 4.0%, Mg: more than 4.0% to less than 12.5%, Ca: 0.15% to less than 3.00%, Sn: 0% to less than 3.0%, Bi: 0% to less than 1.0%, In: 0% to less than 1.0%, Sc: 0% to less than 0.50%, Y: 0% to less than 0.50%, La: 0% to less than 0.50%, Ce: 0% to less than 0.50%, Sr: 0% to less than 0.50%, Si: 0% to less than 2.50%, B: 0% to less than 0.50%, P: 0% to less than 0.50%, Cr: 0% to less than 0.25%, Ti: 0% to less than 0.25%, Ni: 0% to less than 1.0%, Co: 0% to less than 0.25%, V: 0% to less than 0.25%, Nb: 0% to less than 0.25%, Cu: 0% to less than 1.0%, Mn: 0% to less than 0.25%, Mo: 0% to less than 0.25%, W: 0% to less than 0.25%, Zr: 0% to less than 0.25%, Fe: 0% to less than 5.0%, Ag: 0% to less than 1.0%, Li: 0% to less than 0.50%, Na: 0% to less than 0.05%, K: 0% to less than 0.05%, Sb: 0% to less than 0.5%, Pb: 0% to less than 0.5%, Zn: 65% or more, and impurities, and the metal structure in the cross section in the thickness direction of the plating layer contains, in terms of area fraction, the MgZn2 phase: 10 to 40%, the Al-Zn phase: 0 to 15%, the Al phase (phase with Zn < 10%): 0 to 5%, the CaZn 13 phase: 1.0 to 15.0%, and the total of the [Al / MgZn2 / Zn ternary eutectic structure] and the [MgZn2 / Zn binary eutectic structure]: 30.0% or more.

[0074] Further, in the above metal structure in the cross section in the thickness direction of the plating layer, the CaZn 13 phase, the average equivalent circle diameter of the CaZn 13 phase in the upper 80% in order of the equivalent circle diameter is preferably 10 μm or more.

[0075] Further, in a case where the mass concentration % of the element X in the average chemical composition of the plated layer is set as [X], the average chemical composition of the plated layer preferably satisfies the following formula (1).

[0076] 2.5 x [Al] + 2 x [Ca] + [Y] + [Sr] + [La] + [Ce] > [Mg] (1)

[0077] In the following description, "%" of the content of each element of the chemical composition means "mass %". In addition, the numerical range indicated using "~" means a range including the numerical values recited before and after the "~" as lower limit values and upper limit values. Note that the numerical range in a case where the numerical values recited before and after the "~" are annotated with "more than" or "less than" means a range not including these numerical values as the lower limit value or the upper limit value.

[0078] "Corrosion resistance" indicates a property that the plated layer itself is not easily corroded. The plated layer of the Zn system corrodes and whitewashes before the steel material corrodes due to the sacrificial corrosion protection effect with respect to the steel material, and after the plated layer that whitewashed disappears, the steel material corrodes and red rust is generated, which is the corrosion process of the plated steel material.

[0079] "Appropriate current range" is a welding term, and for example, in a case where the thickness of the plated steel material is set as t (mm), it is the difference between the current value in a case where the diameter of the nugget formed in the central portion of the two overlapping steel materials on which spot welding is performed is 4 x Vt and the current value before spatter is generated. The larger the appropriate current range, the more the material is evaluated as being easier to weld, and the narrower the appropriate current range, the more the nugget of an appropriate size is not formed, and thus the material is evaluated as being more difficult to weld. The appropriate current range is judged from the so-called weld lobe curve.

[0080] "Continuous spot weldability" in spot welding is one of the indices of weldability evaluated at a certain welding condition with a number of spots not less than a prescribed nugget diameter. Specifically, in a case where welding is continuously performed without replacing the electrode with a welding current set as the central value of the appropriate current range, the more the number of weldings until the prescribed nugget diameter cannot be obtained, the more the continuous spot weldability is evaluated as being more excellent. The plated steel material with excellent continuous spot weldability becomes advantageous in terms of manufacturing cost.

[0081] The shape of the steel material is not particularly limited, and the steel material can be exemplified by, in addition to a steel sheet, a steel pipe, civil engineering / construction materials (fence, corrugated pipe, drain cover, flying sand prevention plate, bolt, wire mesh, guardrail, water stop wall, etc.), roof material, home appliance member (housing of an outdoor unit of an air conditioner, etc.), automobile outer panel, member (traveling member, etc.), and the like.

[0082] The steel material is not particularly limited. The steel material can be, for example, a general steel, an Al-killed steel, an extra-low carbon steel, a high carbon steel, various high-tension steels, a part of high-alloy steels (a steel containing a strengthening element such as Ni or Cr, or the like), or the like. Furthermore, the steel material is not particularly limited in terms of the manufacturing method of the steel material, the manufacturing method of the steel sheet (hot-rolling method, pickling method, cold-rolling method, or the like), or the like. Furthermore, the steel material can be a steel material formed with a metal film or an alloy film of Zn, Ni, Sn, or the like having a thickness of less than 1 μm.

[0083] Next, the plated layer will be described.

[0084] The plated layer of the present embodiment contains a Zn-Al-Mg alloy layer. Furthermore, the plated layer can contain an Al-Fe alloy layer. The Zn-Al-Mg alloy layer has corrosion resistance equivalent to or higher than that of a Zn plated layer. Therefore, the plated layer of the present embodiment also has corrosion resistance equivalent to or higher than that of a Zn plated layer.

[0085] The Al-Fe alloy layer is an interfacial alloy layer between the steel material and the Zn-Al-Mg alloy layer. That is, the plated layer of the present embodiment can be a single-layer structure of the Zn-Al-Mg alloy layer, or can be a layered structure containing the Zn-Al-Mg alloy layer and the Al-Fe alloy layer. Furthermore, an oxide film of the plated layer constituent elements is formed on the outermost surface of the plated layer with a thickness of about less than 1 μm, but this oxide film is thin relative to the thickness of the entire plated layer, and thus is negligible from the main body of the plated layer.

[0086] The thickness of the entire plated layer is preferably set to 5 to 80 μm. In general, the thickness of the plated layer is a factor in spot welding properties, and generally, a plated layer of 20 μm or more is not suitable for welding. However, the plated steel material of the present embodiment can be welded even if the thickness of the plated layer is a maximum of 80 μm, because the range of appropriate current during spot welding can be expanded. Thus, the thickness of the plated layer is preferably set to 80 μm or less. Furthermore, if the thickness of the plated layer becomes less than 5 μm, the corrosion resistance decreases, and thus the plated layer is preferably 5 μm or more.

[0087] The thickness of the Al-Fe alloy layer in the case where the plated layer contains the Al-Fe alloy layer is about 10 nm to 5 μm, which is about 1 / 10 or less of the thickness of the upper Zn-Al-Mg alloy layer. The Al-Fe alloy layer improves the peeling resistance of the plated layer by combining the steel material and the Zn-Al-Mg alloy layer. The thickness of the interfacial alloy layer can be controlled by various conditions such as the plating bath temperature, plating bath immersion time, or the like during the manufacturing of the plated steel material. Generally, the interfacial alloy layer has a higher melting point than the upper Zn-Al-Mg alloy layer, and thus has a small influence on the overall weldability, and there is no problem in forming the Al-Fe alloy layer having this degree of thickness.

[0088] The thickness of the entire plated layer is not necessarily limited to the range of 5 to 80 μm because it is dependent on the plating conditions. The thickness of the entire plated layer is influenced by the viscosity and specific gravity of the plating bath in the case of the usual hot-dip plating method. Further, the thickness of the plated layer is adjusted by the drawing speed of the steel material (plating base sheet) and the strength of wiping.

[0089] The Al-Fe alloy layer is formed on the surface of the steel material (specifically, between the steel material and the Zn-Al-Mg alloy layer) as a layer in which Al5Fe phase is the main phase. The Al-Fe alloy layer is formed by the atomic diffusion of the base metal (steel material) and the plating bath. In the case where the hot-dip plating method is used as the production method, the Al-Fe alloy layer is easily formed in the plated layer containing Al element. Since Al is contained in the plating bath at a certain concentration or more, Al5Fe2 phase is formed at the maximum. However, the atomic diffusion takes time, and in addition, there is a portion in which the Fe concentration becomes high in the vicinity of the base metal. Therefore, the Al-Fe alloy layer sometimes partially contains AlFe phase, Al3Fe phase, Al5Fe phase, and the like in small amounts. Further, since Zn is also contained in the plating bath at a certain concentration, Zn is also contained in the Al-Fe alloy layer in small amounts.

[0090] In the case where Si is contained in the plated layer, Si is particularly easily taken into the Al-Fe alloy layer, and Al-Fe-Si intermetallic compound phase is sometimes formed. As the identified intermetallic compound phase, there are AlFeSi phase, and as the isomers, there are α-AlFeSi phase, β-AlFeSi phase, q1-AlFeSi phase, and q2-AlFeSi phase, and the like. Therefore, as the Al-Fe alloy layer, these AlFeSi phases and the like are sometimes detected. The Al-Fe alloy layer containing these AlFeSi phases and the like is also referred to as Al-Fe-Si alloy layer.

[0091] Next, the average chemical composition of the entire plated layer is described.

[0092] Note that the "average chemical composition of the entire plated layer" is the average chemical composition of the Zn-Al-Mg alloy layer in the case where the plated layer is a single layer structure of the Zn-Al-Mg alloy layer. Further, in the case where the plated layer is a stacked structure of the Al-Fe alloy layer and the Zn-Al-Mg alloy layer, the "average chemical composition of the entire plated layer" is the average chemical composition of the total of the Al-Fe alloy layer and the Zn-Al-Mg alloy layer.

[0093] Generally, in the hot-dip plating method, since the formation reaction of the plated layer is substantially completed within the plating bath, the chemical composition of the Zn-Al-Mg alloy layer becomes substantially equivalent to that of the plating bath. Further, in the hot-dip plating method, the Al-Fe alloy layer is formed and grown instantaneously upon just dipping in the plating bath. Moreover, the formation reaction of the Al-Fe alloy layer is completed within the plating bath, and the thickness thereof is also mostly sufficiently small relative to the Zn-Al-Mg alloy layer. Therefore, as long as no special heat treatment such as a post-plating heat alloying treatment is performed, the average chemical composition of the plated layer as a whole is substantially equivalent to that of the Zn-Al-Mg alloy layer, and the components such as the Al-Fe alloy layer can be ignored.

[0094] Hereinafter, the elements contained in the plated layer are described.

[0095] Zn: 65% or more

[0096] Zn is a low-melting metal, and exists as a main phase of the plated layer on the steel material. The reason why the weldability of the plated steel material deteriorates compared to the steel material without the plated layer is because Zn reacts with the electrode, and changes the electric conduction state of the electrode and the plated layer. Generally, the electrode for spot welding mostly uses a copper electrode, but if Zn and copper (Cu) are exposed to high heat, a reaction occurs. This reactivity is smaller compared to the case of Cu and Al. It is presumed that this is related to the fact that Al-Cu forms in a eutectic composition. On the other hand, Zn is an element necessary to ensure corrosion resistance and obtain a passivation corrosion protection effect with respect to the steel material. If the Zn content is less than 65%, the passivation corrosion resistance is insufficient. Therefore, the Zn content is set to 65% or more. It is more preferable to set it to 70% or more. Note that the upper limit of the Zn content is the amount of the remaining portion other than Zn, elements other than Zn, and impurities.

[0097] Al: 0.2% to less than 4.0%

[0098] Al is an element that constitutes the main body of the plated layer, like Zn. Although the passivation corrosion protection effect of Al is small, the planar portion corrosion resistance and the corrosion resistance of the welded portion are improved by containing Al in the plated layer. Further, if Al is not present in the plated layer, Mg cannot be stably maintained in the plating bath. Therefore, Al is added to the plating bath as an indispensable element in manufacturing.

[0099] The Al contained in the plated layer reacts with the copper electrode at the time of spot welding. The reaction product becomes an Al-Cu-based intermetallic compound, deteriorates the conductivity, and deteriorates the electrode life. In the present embodiment, as a means for minimizing the influence of Al, the Al-Zn phase is made by solid-solution of relatively much Zn in the Al phase in the metal structure of the plated layer, and the crystal grain diameter of the CaZn 13 phase is increased, and the reactivity with the copper electrode is reduced. Details are described below.

[0100] The Al content is set to 0.2% or more because it is necessary to contain the amount of Mg described later in a large amount, and if it is less than this amount, it becomes difficult to form a bath as a plating bath. That is, if the bath containing Mg is maintained in a molten state, it reacts with oxygen in the air to form a large amount of MgO-based oxide, and becomes difficult to use as a plating bath. Al has the effect of forming a dense Al2O3 film on the surface of the bath, and suppressing oxidation of Mg. Therefore, in a plating bath containing Mg, it is preferable to add Al. Furthermore, Al is also an element that contributes to improvement of the corrosion resistance of the welded portion. In the case where the Al content in the plated layer is excessively small, the corrosion resistance of the welded portion sometimes deteriorates. Thus, the Al content is set to 0.2% or more.

[0101] The Al content is set to less than 4.0% because if it is more than this amount, a large amount of Al phase becomes likely to precipitate in the plated layer. If a large amount of Al phase precipitates, spot weldability and continuous spot weldability decrease, or electrical conductivity and thermal conductivity increase. Furthermore, if the Al content is excessively large, Al2O3 becomes likely to form on the surface of the plated layer, and reactivity with the electrode at the time of spot welding becomes active, the current value becomes unstable, and the proper current range becomes narrow. Furthermore, if the Al content is excessively large, an Al-Ca-Si intermetallic compound layer is preferentially formed, and it is sometimes not possible to secure a desired amount of Ca-Zn-based alloy phase, particularly CaZn 13 phase. As described in detail below, by forming a prescribed amount of CaZn 13 phase, enlargement of the proper current range and an increase in the number of continuous spots can be achieved. Therefore, taking into account the reaction with the electrode at the time of spot welding, securing of the amount of precipitation of the CaZn 13 phase, the upper limit is set to less than 4.0%. A more preferable Al content is more than 1.5% and 3.5% or less, and can also be more than 2.0% and 3.0% or less.

[0102] Furthermore, as elements that suppress oxidation of Mg as with Al, there are Ca, Y, La, Ce, and Sr. Therefore, by satisfying the following formula (1) when [X] is set to the mass concentration (%) of each element X, it is possible to suppress oxidation of Mg in the plating bath. As a result, the average chemical composition of the plated layer does not deviate from the target composition, furthermore, it is possible to form a large amount of MgZn2 phase in the plated layer, it is possible to improve the corrosion resistance of the plated layer, and it is possible to increase the proper current range at the time of spot welding.

[0103] 2.5 x [Al] + 2 x [Ca] + [Y] + [Sr] + [La] + [Ce] > [Mg] (1)

[0104] Mg: more than 4.0% to less than 12.5%

[0105] Mg is an element that has a sacrificial corrosion prevention effect and improves the corrosion resistance of the plated layer. By having an amount of Mg or more in the plated layer, an MgZn2 phase is formed in the plated layer. The higher the Mg content in the plated layer, the more the MgZn2 phase is formed, and the more the corrosion resistance is improved. In addition, the melting point of the MgZn2 phase is high compared to the melting point of the Zn phase, and if the MgZn2 phase is contained in a large amount in the plated layer, the reactivity of Zn with the electrode becomes insufficient. That is, Zn bonded to Mg has a low reactivity with the electrode because the melting point is increased, and thus the long life of the electrode can be achieved. In addition, Mg is easily oxidized, and thus when spot welding is performed, the slightly melted Mg is immediately oxidized to form a certain thickness of MgO oxide. The tendency of such oxidation of Mg is greater than that of Al and Zn. Therefore, if spot welding is performed on a plated layer containing an appropriate amount of Mg, the surface of the electrode is covered with a thin Mg oxide film, and the state between the plated layer surface and the electrode is always stable, and the current value is stable. In addition, the Mg oxide film becomes a barrier to the reaction of the electrode with the plated layer, and the life of the copper electrode can be greatly improved.

[0106] The content of Mg is set to be more than 4.0% in order to sufficiently ensure the corrosion resistance. In addition, when the content of Mg is 4.0% or less, the reactivity of Zn with the copper electrode becomes active, and the appropriate current value becomes narrow. On the other hand, if the content of Mg becomes excessive, the manufacture of the plated steel material becomes difficult, and the plated layer is powdered during processing, and thus the upper limit is less than 12.5%. More preferably, the content of Mg is more than 5.0% and 10.0% or less, and can be more than 5.0% and 8.0% or less.

[0107] Ca: 0.15% to less than 3.00%

[0108] Ca is an element that most contributes to the spot weldability. Ca is an element that is most easily oxidized in the atmosphere, and like Mg, has the following effect: a small amount of melted Ca in the plated layer immediately forms an oxide film, covers the surface of the copper electrode, and becomes a barrier to the reaction of the electrode with the Zn phase and the electrode with the Al phase. That is, by having Ca in the plated layer, the tendency of the appropriate current range to become wide and the number of continuous spots to become more is produced. In addition, as described above, Ca has the effect of suppressing the oxidation of Mg in the plating bath and improving the stability of the plating bath.

[0109] In addition, when Ca is contained in the plated layer, a Zn-Ca-based or Al-Ca-Si-based compound is formed. These compounds become primary crystals at the time of solidification of the plated layer, become the starting point of the growth of the MgZn2 phase, and promote the formation of coarse MgZn2 phases.

[0110] The content of Ca is set to 0.15% or more, which is a necessary content to ensure a proper current value and continuous dotting. If the Ca concentration becomes lower than 0.15%, the reactivity of Zn with the copper electrode becomes active, the proper current value becomes narrow, and the number of continuous dots also becomes low. Further, if the Ca content is too small, sometimes the above-mentioned Zn-Ca-based compound, particularly CaZn 13 The amount of phase formation becomes insufficient. On the other hand, if the Ca content exceeds the upper limit, there is a tendency that the bath building of the plating bath becomes difficult. Further, if the Ca content is too much, there is a tendency that the attachment of dross and the like becomes more, and the weldability also deteriorates, and there are cases where the proper current value becomes narrow, and the number of continuous dots becomes low. Thus, Ca is set to 0.15% or more to less than 3.00%, preferably 0.20% or more and less than 2.00%, more preferably 0.20% or more and less than 1.50%.

[0111] Element Group A

[0112] Sn: 0% to less than 3.0%,

[0113] Bi: 0% to less than 1.0%,

[0114] In: 0% to less than 1.0%

[0115] In the plating layer, one or two or more of the elements of Element Group A can also be contained. The effects of Sn, Bi, and In of Element Group A at the time of spot welding are not large. On the other hand, these elements have the effect of improving the dezincification corrosion resistance. However, there is a tendency that the bonding with Mg is stronger than that with Zn, and the effect of the contained Mg becomes small, and thus there is an upper limit to the content of these elements. If the content of each of Sn, Bi, and In exceeds the upper limit, there is a tendency that the attachment of dross and the like becomes more, and the weldability also deteriorates, and there are cases where the proper current value becomes narrow, and the number of continuous dots becomes low. Thus, Sn is set to 0 to less than 3.0%, more preferably more than 0% and less than 3.0%. Bi is set to 0% to less than 1.0%, more preferably more than 0% and less than 1.0%. In is set to 0% to less than 1.00%, more preferably more than 0% and less than 1.00%. Sn is set to 0 to less than 3.0%, more preferably more than 0% and less than 3.0%. Bi is set to 0% to less than 1.0%, more preferably more than 0% and less than 1.0%. In is set to 0% to less than 1.0%, more preferably more than 0% and less than 1.0%.

[0116] Element Group B

[0117] Sc: 0% to less than 0.50%,

[0118] Y: 0% to less than 0.50%,

[0119] La: 0% to less than 0.50%,

[0120] Ce: 0% to less than 0.50%,

[0121] Sr: 0% to less than 0.50%

[0122] In the plating layer, any one or two or more of the elements group B can also be contained. Sc, Y, La, Ce, Sr of the elements group B are elements that contribute to the spot welding performance. Sc, Y, La, Ce, Sr are oxidized in the atmosphere as with Ca, and have the effect of becoming a reaction barrier for the Cu electrode and the Zn phase, the Al phase. Further, they have the effect of suppressing oxidation of Mg in the plating bath, and improving the stability of the plating bath. In order to appropriately exert such effects, the content of these elements is set to more than 0%, and more preferably 0.01% or more. However, there is an upper limit to the content of each element. Sc, Y, La, Ce, Sr are each set to 0% to less than 0.50%, preferably more than 0% and less than 0.50%, and more preferably 0.01% or more and less than 0.50%.

[0123] Element group C

[0124] Si: 0% to less than 2.50%,

[0125] B: 0% to less than 0.50%,

[0126] P: 0% to less than 0.50%

[0127] In the plating layer, any one or two or more of the elements group C can also be contained. Si, B, and P of the elements group C are elements that belong to semimetals. These elements also generally form intermetallic compounds containing Zn, Al in the plating layer, and as a result, the plating layer becomes less likely to melt, and the reactivity with the plating layer of the electrode becomes low. That is, by containing the elements group C in the plating layer, there is a tendency for the appropriate current range to become wide. However, with the elements group C, there is no effect of forming a film on the electrode surface as with Mg, Ca. There is an upper limit to the content of each element, and if the upper limit of the content is exceeded, there is a tendency for the attachment of dross and the like to become more, and the weldability also deteriorates in total. Thus, Si is set to 0% to less than 2.50%, and preferably more than 0% and less than 1.00%, and B and P are each set to 0% to less than 0.50%, and preferably more than 0% and less than 0.50%.

[0128] Element group D

[0129] Cr: 0% to less than 0.25%,

[0130] Ti: 0% to less than 0.25%,

[0131] Ni: 0% to less than 1.0%,

[0132] Co: 0% to less than 0.25%,

[0133] V: 0% to less than 0.25%,

[0134] Nb: 0% to less than 0.25%,

[0135] Cu: 0% to less than 1.0%,

[0136] Mn: 0% to less than 0.25%,

[0137] Mo: 0% to less than 0.25%,

[0138] W: 0% to less than 0.25%,

[0139] Zr: 0% to less than 0.25%,

[0140] Fe: 0% to less than 5.0%,

[0141] Ag: 0% to less than 1.0%

[0142] In the plating layer, one or two or more of the elements of Group D can also be contained. Cr, Ti, Ni, Co, V, Nb, Cu, Mn, Mo, W, Zr, Ag, and Fe of Group D are metal elements, and by being taken into the plating layer, a substitutional solid solution or a new high-melting intermetallic compound is formed. Thus, the plating layer becomes less likely to melt and the reactivity with the plating layer of the copper electrode becomes low. That is, by containing the elements of Group D in the plating layer, there is a tendency for the appropriate current range to become wide. However, with respect to Group D, there is no effect of forming a film on the electrode surface as with Mg and Ca, and the effect of improving spot weldability is small compared with Mg and Ca. In particular, in the case where at least one or more of the elements of Group B is present, the effect brought about by Group D can not be confirmed. On the other hand, if Group B, Group C, and Group D are used in combination, the appropriate current range during spot welding is further widened. There is an upper limit to the content of each of the elements of Group D, and if the upper limit is exceeded, there is a tendency for the adhesion of dross and the like to increase and for the weldability to deteriorate in its entirety. Therefore, Cr, Ti, Co, V, Nb, Mn, Mo, W, and Zr are each set to 0% to less than 0.25%, and preferably to more than 0% to less than 0.25%. Ni and Cu are each set to 0% to less than 1.0%, and preferably to more than 0% to less than 1.0%. Ag is set to 0% to less than 1.0%. Furthermore, Fe is sometimes unavoidably contained in the plating layer. This is because it sometimes diffuses from the base metal to the plating layer during plating production. Thus, the content of Fe is 0% to less than 5.00%, and can also be more than 0% and less than 5.0%.

[0143] Group E of Elements

[0144] Sb: 0% to less than 0.5%,

[0145] Pb: 0% to less than 0.5%

[0146] Sb and Pb as the element group E are elements similar in properties to Zn. Therefore, by containing these elements, no particular effect is substantially exerted in spot weldability, but there is an effect of making it easy to form zinc flower patterns and the like on the appearance of plating. However, if Sb and Pb are contained in excess, sometimes corrosion resistance after spot welding is reduced. Therefore, Sb and Pb are each set to 0% to less than 0.5%.

[0147] Element group F

[0148] Li: 0% to less than 0.5%,

[0149] Na: 0% to less than 0.05%,

[0150] K: 0% to less than 0.05%

[0151] Li and Na, K as the element group F are elements belonging to alkali metals. These elements have a property of being oxidized very easily, but substantially no particular effect is exerted in spot welding. However, if these elements are contained in a large amount, they are oxidized on the surface of the plating bath to become dregs, and thus the plating bath becomes difficult to build. Therefore, Li is set to 0% to less than 0.5%, and Na and K are each set to 0% to less than 0.05%.

[0152] Remaining portion: Zn and impurities of 65% or more

[0153] Zn in the remaining portion is as described above. In addition, the impurities in the plated layer mean components contained in raw materials or components mixed in the process of manufacturing, and are components not intentionally contained. For example, in the plated layer, by atomic diffusion of the steel material (base metal) and the plating bath with each other, sometimes components other than Fe are also mixed in a trace amount as impurities.

[0154] The identification of the average chemical composition of the plated layer can be performed by the following method.

[0155] First, the plated layer is peeled and dissolved with an acid containing an inhibitor that suppresses corrosion of the base metal (steel material) to obtain an acid solution. Next, the obtained acid solution is measured by ICP emission spectrometry or ICP-MS method. Thereby, the average chemical composition of the plated layer can be obtained. The kind of acid is not particularly limited as long as it is an acid capable of dissolving the plated layer. If the area and weight before and after peeling are measured, the plating adhesion amount (g / m 2 ) can also be obtained at the same time.

[0156] Next, the organizational form of the plated layer will be described.

[0157] The proportion and size of the phases contained in the plated layer greatly affect the spot weldability of the plated layer. Even if the plated layer is of the same composition, depending on the method of production, the phases or structures contained in the metal structure thereof change, and the properties become different. The metal structure of the plated layer can be easily confirmed by a scanning electron microscope (SEM-EDS) with an energy dispersive X-ray analyzer. Specifically, in a cross section of the plated layer mirror-finished, by obtaining a reflection electron image, for example, the approximate state of the metal structure of the plated layer can be confirmed. The "cross section of the plated layer" referred to here is a cross section of the plated layer in the thickness direction, and refers to a cross section perpendicular to the surface of the plated layer.

[0158] Since the thickness of the plated layer of the present embodiment is about 5 to 80 μm, in the SEM, it is preferable to confirm the metal structure thereof at a field of view of 500 to 5000 times. For example, in the case where the cross section of a plated layer having a thickness of 25 μm is confirmed at a magnification of 2000 times, the cross section of the plated layer in an area of 25 μm (plating thickness) x 40 μm (SEM field of view width) = 1000 μm 2 of each field of view can be confirmed. In the case of the present embodiment, the field of view of the SEM with respect to the plated layer is set so that the field of view of the local part can be observed, and therefore, in order to obtain average information relating to the structure of the plated layer, it is only necessary to select a field of view of 25 points from an arbitrary cross section and set it as average information. That is, by observing the metal structure in a field of view of 25000 μm 2 , the area ratio and size of the phases or structures constituting the metal structure of the plated layer can be determined.

[0159] The reflection electron image using the SEM is preferable in that it can easily discriminate the phases or structures contained in the plated layer. Since elements having a small atomic number such as Al are imaged darker, and elements having a large atomic number such as Zn are imaged lighter, the proportions of these structures can be easily read.

[0160] In order to confirm each phase in the plated layer, in the EDS analysis, the composition of the phase is accurately confirmed, and by element mapping or the like, the approximately same composition phase is read and the phase is determined. In the case where the EDS analysis can be used, by obtaining an element mapping image, the phase of approximately the same composition can be discriminated. If the phase of approximately the same composition can be determined, the area of the crystalline phase in the observation field of view can be known. If the area is grasped, the equivalent circle diameter can be calculated, and the average crystalline grain size can be calculated.

[0161] As a method for calculating the "equivalent circle diameter", the following method can be used. First, binary image processing is performed on the EDS element mapping image using a commercially available image processing software, and only the phase for which the equivalent circle diameter is to be calculated is extracted. After removing noise, the area of each crystal phase is measured. By calculating a circle having an area equal to the measured area, the equivalent circle diameter can be calculated. The average crystal grain size can be derived by arithmetic averaging the equivalent circle diameters of each crystal phase.

[0162] Further, from the area of each phase in the observation field of view, the phase ratio in the plated layer can be calculated. Note that in the present embodiment, the area ratio of a specific phase in the plated layer is regarded as the volume ratio of the phase in the plated layer.

[0163] The area of each phase is calculated using the following method. First, the phases of the same composition are extracted from the EDS element mapping image using a commercially available image processing software by binary image processing. After removing noise, the area of the extracted phase is measured, and the area of the phase of the same composition can be calculated. Note that the interfacial alloy layer in the present embodiment is formed at the boundary between the plated layer and the steel material, and is Al-Fe-based. Therefore, in the EDS element mapping image, the area where Al and Fe overlap on the steel material can be extracted. Further, in the determination of the interfacial alloy phase, a reflection electron image using a SEM can also be used as an aid, in which case, since Al is a light element, the interfacial alloy layer can be confirmed as a dark area on the steel material.

[0164] Next, the phases and structures contained in the plated layer will be described.

[0165] Figure 1 is an example of a reflection electron image of the metal structure of the cross section of the plated layer of Example 22, obtained using a SEM. In Figure 1 , symbol 1 indicates the MgZn2 phase, symbol 2 indicates the CaZn 13 phase, symbol 3 indicates the Al-Zn phase, symbol 4 indicates the eutectic structure, and symbol 5 indicates the interfacial alloy layer.

[0166] The plated layer of the present embodiment contains, in a case where the metal structure thereof is observed by a SEM over a total of 25000 μm 2 of the field of view, 10 to 40 area% of the MgZn2 phase, 0 to 15 area% of the Al-Zn phase having a Zn content of 10% or more, 0 to 5 area% of the Al phase having a Zn content of less than 10%, 1 to 15 area% of the CaZn 13 phase, and 30 area% or more of the eutectic structure (the total of the ternary eutectic structure of [Al / MgZn2 / Zn] and the binary eutectic structure of [MgZn2 / Zn]). The above phases and structures are preferably 90% or more in terms of the area ratio of the plated layer.

[0167] MgZn2 phase

[0168] The MgZn2 phase of the present embodiment is a region in which the Mg concentration becomes 16% (±5%) and the Zn concentration becomes 84 (±5%) in the plated layer. The MgZn2 phase is mostly photographed in a gray color of an intermediate color of Al and Zn in the SEM backscattered electron image. In the SEM backscattered electron image, the MgZn2 phase can be distinguished from the Al-Zn phase, the Al phase, the ternary eutectic structure of Al / MgZn2 / Zn, the binary eutectic structure of MgZn2 / Zn, and the like.

[0169] In the component composition of the plated layer in the present embodiment, a large number of block-shaped MgZn2 phases are generated. By the presence of a large amount of MgZn2 phases in the plated layer, there is a tendency that the appropriate current range at the time of spot welding becomes large. Further, by the presence of a large amount of MgZn2 phases in the plated layer, the proportion of the Zn phase contained in the ternary eutectic structure of Al / MgZn2 / Zn, the binary eutectic structure of MgZn2 / Zn can be relatively reduced, and the reactivity of the plated layer with the electrode of the spot welding machine is reduced. Further, at the time of spot welding, a small amount of the MgZn2 phase is dissolved, Mg is oxidized in the atmosphere, and an Mg-based oxide film is formed on the surface of the copper electrode. This Mg-based oxide film is mostly formed within the first 10 spots of the electrode, and due to the presence of this Mg-based oxide film, the appropriate current value after that is also stabilized, and the reaction of the electrode becomes less likely to proceed.

[0170] The area ratio of the MgZn2 phase in the plated layer is 10% or more. As the Mg content in the plated layer increases, the volume ratio of the MgZn2 phase increases and the spot weldability improves. It is preferable that it be 15% or more, more preferably 20% or more, or 30% or more is further preferable. The upper limit of the area ratio of the MgZn2 phase is set to 40% or less. Within the range of the average chemical composition of the plated layer which is the object of the present embodiment, it is difficult to set the area ratio of the MgZn2 phase to more than 40%. Note that, in the present embodiment, the MgZn2 contained in the ternary eutectic structure of Al / MgZn2 / Zn and the binary eutectic structure of MgZn2 / Zn is not included in the area ratio of the MgZn2 phase.

[0171] By making the particle diameter of the MgZn2 phase in the plated layer grow greatly and thereby increasing the area ratio thereof, the MgZn2 phase becomes less likely to be dissolved due to the heat input of spot welding and becomes less likely to react with the electrode. That is, the continuous spot weldability can be further improved. Further, since a phase having excellent corrosion resistance remains after spot welding, the corrosion resistance is also improved. In order to make the MgZn2 phase grow and thereby increase the area ratio thereof, it is preferable to contain one or more elements selected from Ca and the element group B.

[0172] Al phase

[0173] The Al phase in the present embodiment is a region in which the Al concentration in the plated layer is 90 mass% or more. In this Al phase, Zn can also be contained, but the Zn concentration in the Al phase in this case is less than 10%. The Al phase is distinguished from the Al-Zn phase by this difference in Zn concentration. The Al phase can be distinguished from other phases and structures in SEM backscattered electron images. That is, the Al phase is mostly displayed as the darkest in SEM backscattered electron images. In the present embodiment, the Al phase takes various forms such as a block shape or a dendritic shape such as a circular / flat shape in any cross section. In the case where the area fraction of the Al phase is calculated, the Al phase that is the object is set to an Al phase having a crystal grain diameter of 1 μm or more. That is, in the present embodiment, the Al phase having a crystal grain diameter of 1 μm or more is counted as the "Al phase", and the Al phase having a crystal grain diameter of less than 1 μm is not included in the area fraction of the Al phase.

[0174] The Al phase can reduce spot weldability. If the Al phase appears on the surface of the plated layer, an insulating film such as an Al2O3 film is formed, the proper current range is reduced, and in addition, the electrode life is greatly reduced due to the reaction with the copper electrode and the formation of an Al-Cu intermetallic compound between the electrode, and the continuous spot welding property is reduced. If the area fraction of the Al phase is set to 5% or less, there is a tendency for the electrode life to be extended in spot welding. Therefore, the Al phase is set to 5 area% or less, and preferably 0 area%.

[0175] Al-Zn phase

[0176] The Al-Zn phase in the present embodiment is a phase containing 10 mass% or more of Zn and Al. The Al-Zn phase is a collection of a fine Zn phase (hereinafter referred to as a fine Zn phase) having a particle diameter of about 1 μm and a fine Al phase (hereinafter referred to as a fine Al phase) having a particle diameter of less than 1 μm. In the plated layer in a molten state, Al becomes a structure different from the crystal structure at room temperature, and can dissolve a large amount of Zn, and exists as a high-temperature stable phase containing about 50% of Zn. On the other hand, at room temperature, the solid solubility of Zn in this high-temperature stable phase is extremely reduced, and Al and Zn are separated from each other and exist as an Al-Zn phase containing the fine Al phase and the fine Zn phase. That is, the Al-Zn phase is a phase containing the fine Zn phase at a proportion of 10 to 80 mass%. This Al-Zn phase is different in properties from both the Al phase and the Zn phase contained in the plated layer, and is distinguished in a backscattered electron SEM image or wide-angle X-ray diffraction. The Al-Zn phase is, for example, Al 0.403 Zn 0.597 (JCPDS card #00-052-0856, JCPDS: Joint Committee on Powder Diffraction Standards), Al 0.71 Zn0.29 (PDF card #00-019-0057, PDF: Powder Diffraction File) and the like have inherent diffraction peaks. Therefore, in the present embodiment, the phase having an Al concentration of 90 to 20 mass% and a Zn concentration of 10 to 80 mass% is set as an Al-Zn phase. By surrounding the region of the Al-Zn phase with a closed space, it is also possible to define the crystal size of the Al-Zn phase.

[0177] The Al phase has extremely high reactivity with respect to the welding electrode, and significantly destabilizes the weldability. On the other hand, the reactivity of the Al-Zn phase with the electrode is lower than that of the Al phase described above, by virtue of the fact that the fine Al phase is contained in the phase together with the fine Zn phase. Furthermore, the surface of the plating layer becomes free of thin films that adversely affect the weldability, such as Al203oxide films, and as a result, the weldability is improved.

[0178] In the present embodiment, since Al is contained in the plating layer at a concentration of at least a certain level, depending on the manufacturing conditions, an Al phase having an area fraction of 0 to 15% is formed. However, by strict selection of the manufacturing method, it is possible to suppress the formation of massive, tree-like Al phases, and to cause a large amount of Al to exist as an Al-Zn phase. That is, the more the Al-Zn phase is increased, the more the Al phase can be reduced. Furthermore, the Al-Zn phase is less likely to react with a copper electrode or the like than the Al phase, and furthermore, has a tendency to expand the appropriate current range. Therefore, from the viewpoint of weldability, it is more preferable for Al to be incorporated into Zn as an Al-Zn phase in the Al phase as much as possible, rather than existing in the plating layer as an Al phase.

[0179] The area fraction of the Al-Zn phase in the plating layer is set to a range of 0 to 15%. Within the range of the average chemical composition of the plating layer in the present embodiment described above, it is difficult to set the area fraction of the Al-Zn phase to more than 15%, and therefore the upper limit of the area fraction of the Al-Zn phase is set to 15% or less.

[0180] CaZn 13 Phase

[0181] The CaZn 13 Phase in the present embodiment is a region in which the Ca concentration becomes 5% (±3%) and the Zn concentration becomes 95 (±3%) in the plating layer. The CaZn 13 Phase is displayed in white in the SEM backscattered electron image, and therefore it is difficult to distinguish it from the Zn phase in the state of the photographed image. Therefore, by taking an EDS image, it is possible to distinguish the CaZn 13 Phase from other constituent phases. The CaZn 13The phase has the following effects: a small amount of dissolution at the time of welding, and the dissolved Ca immediately forms an oxide film, covering the surface of the copper electrode, thereby becoming a potential barrier to the reaction of the Zn phase, the Al phase, and the copper electrode. Therefore, by containing the CaZn 13 phase in the plating layer, there is a tendency for the appropriate current range to widen, and the number of continuous dots also increases. Furthermore, the CaZn 13 phase is precipitated as a primary crystal at the time of solidification of plating, and functions as a solidification nucleus of the MgZn2phase described above, having an effect of promoting the growth of the MgZn2phase at the time of solidification of plating. The MgZn2phase that is coarse increases the weldability.

[0182] Therefore, the CaZn 13 phase is set to a range of 1.0 to 15 area %. Within the range of the average chemical composition of the plating layer, it is difficult to set the area ratio of the CaZn 13 phase to more than 15 %, and therefore the CaZn 13 phase is set to an upper limit of 15 area % or less. Furthermore, by setting the CaZn 13 phase to 1.0 area % or more, the appropriate current range widens, and the number of continuous dots also increases.

[0183] Furthermore, by setting the CaZn 13 phase in the plating layer to 1.0 area % or more, the particle diameter of the CaZn 13 phase grows greatly, and therefore it is difficult to dissolve due to the heat input of spot welding, and it becomes difficult to react with the electrode. That is, the continuous dotting property can be further improved. Therefore, the CaZn 13 phase, in order from the largest equivalent circle diameter to the upper 80 %, is preferably 1.0 μm or more. 13 The average of the equivalent circle diameters of the CaZn 13 phase is preferably 10 μm or more. For example, in the case where there are 100 CaZn 2 phases in the observation field, it is only necessary to average the equivalent circle diameters of the CaZn 13 phases in order from the largest equivalent circle diameter to the upper 80 %. 2 That is, the equivalent circle diameters of the CaZn 13 phases in the metal structure in the field of 25000 μm 13 of the cross section of the plating layer are each found, and the average of the equivalent circle diameters of the CaZn 13 phases in order from the largest equivalent circle diameter to the upper 80 % is found.

[0184] As the compound of Ca and Zn, in addition to the CaZn 11 phase, a CaZn 13The proportion of the phase other than the phase is small, and thus has substantially no influence on the weldability and corrosion resistance. As a representative compound of Al, Al2CaZn2 phase is sometimes formed, but they also have substantially no influence on the weldability and corrosion resistance because of the small proportion of formation in the present embodiment. CaZn 13 The phase and the phase other than the phase can be distinguished on a wide-angle X-ray diffraction.

[0185] Further, as a compound of Ca and Al and Si, Al2CaSi2 phase is formed. The Al2CaSi2 phase and the CaZn 13 Similarly, a small amount of the Al2CaSi2 phase is dissolved at the time of welding, covers the electrode, and thus has an effect of improving the weldability. However, the Al2CaSi2 phase has a tendency to be precipitated at the interface between the plated layer and the steel material, and thus has a poor effect of improving the weldability as compared with the CaZn 13 phase. That is, it is preferable that the CaZn 13 phase be precipitated in the form of the phase.

[0186] 〔Eutectic structure〕

[0187] In the component composition of the present embodiment, the〔Al / MgZn2 / Zn ternary eutectic structure〕 and the〔MgZn2 / Zn binary eutectic structure〕 are formed by eutectic reaction. The Zn phase in these eutectic structures, although it has a small degree of influence, reacts with the copper electrode at the time of spot welding, and has an adverse effect on the continuous spot life.

[0188] The eutectic structure is observed as a lamellar structure in the SEM image, and thus can be clearly distinguished from other structures.

[0189] 〔Al / MgZn2 / Zn ternary eutectic structure〕

[0190] The〔Al / MgZn2 / Zn ternary eutectic structure〕 is a eutectic structure including the Al phase, the MgZn2 phase, and the Zn phase, and is clearly distinguished from the MgZn2 phase and the Al phase described above in the reflection electron SEM image.

[0191] The Zn phase is included in the〔Al / MgZn2 / Zn ternary eutectic structure〕. The Zn phase is a region in which the Zn concentration is 95 mass% or more and the Ca concentration is less than 2%. The Zn phase in the〔Al / MgZn2 / Zn ternary eutectic structure〕 is mostly photographed as the whitest in the SEM reflection electron image. The Al phase included in the〔Al / MgZn2 / Zn ternary eutectic structure〕 is included as a fine Al phase together with the MgZn2 phase and the Zn phase. Thus, as compared with the Al phase described above, the reactivity with the electrode is low, and further, a thin film such as an Al2O3 oxide film is not formed on the plated layer surface, which has an adverse effect on the weldability, and as a result, the weldability is improved.

[0192] 〔MgZn2 / Zn binary eutectic structure〕

[0193] 〔MgZn2 / Zn binary eutectic structure〕 is a eutectic structure including MgZn2 phase and Zn phase, and is distinguished from MgZn2 phase or Al phase, and from the〔Al / MgZn2 / Zn ternary eutectic structure〕 in a reflection electron SEM image.

[0194] The Zn phase is included in the〔MgZn2 / Zn binary eutectic structure〕. The Zn phase is a region in which the Zn concentration is 95 mass% or more and the Ca concentration is less than 2%. The Zn phase in the〔Al / MgZn2 / Zn ternary eutectic structure〕 is mostly captured as the whitest in the SEM reflection electron image. The Al phase is not included in the〔MgZn2 / Zn binary eutectic structure〕. Therefore, the adverse effect on the continuous dotting property caused by the〔MgZn2 / Zn binary eutectic structure〕 is smaller than that of the〔Al / MgZn2 / Zn ternary eutectic structure〕.

[0195] On the other hand, if the area ratio of the total of the〔Al / MgZn2 / Zn ternary eutectic structure〕 and the〔MgZn2 / Zn binary eutectic structure〕 becomes less than 30.0 area%, the Zn phase becomes less as the area ratio decreases, the patination corrosion resistance of the plated layer decreases, and the corrosion resistance of the vicinity of the welded portion cannot be maintained. Therefore, it is effective that the area ratio of the total of the〔Al / MgZn2 / Zn ternary eutectic structure〕 and the〔MgZn2 / Zn binary eutectic structure〕 is set to 30.0 area% or more. That is, by allowing a certain degree of the〔Al / MgZn2 / Zn ternary eutectic structure〕 and the〔MgZn2 / Zn binary eutectic structure〕 including the Zn phase to exist in the plated layer, the patination corrosion resistance can be ensured to improve the corrosion resistance of the vicinity of the welded portion. Further, by allowing the Al in the plated layer to be incorporated as much as possible into the〔Al / MgZn2 / Zn ternary eutectic structure〕, the area ratio of the Al phase not included in the〔Al / MgZn2 / Zn ternary eutectic structure〕 is reduced, and the continuous dotting property can be improved. There is no particular limitation on the upper limit of the area ratio of the〔Al / MgZn2 / Zn ternary eutectic structure〕 and the〔MgZn2 / Zn binary eutectic structure〕, but it can be set to 85.0 area% or less, it can be set to 80.0 area% or less, it can be set to 75.0 area% or less, it can be set to 70.0 area% or less, or it can be set to 65.0 area% or less.

[0196] On the other hand, by containing elements other than Zn, Mg, and Al in the plated layer, other metal phases are sometimes formed. For example, Si forms a Mg2Si phase or the like. Of the other metal phases, although effects of improving the weldability and corrosion resistance are obtained, the effects are not significant. According to the composition of the plated layer of the present embodiment, it is difficult to set the area ratio of the other metal phases in total to exceed 10 area%, and thus the area ratio of the other metal phases is preferably 10 area% or less.

[0197] Next, a case where the plated steel material of the present embodiment is manufactured by a hot-dip plating method will be described. The plated steel material of the present embodiment can be manufactured by either of an immersion-type plating method (batch type) and a continuous-type plating method.

[0198] The size, shape, surface morphology, and the like of the steel material to be plated are not particularly restricted. A general steel material, a high-tension steel, a stainless steel, and the like can be used as long as it is a steel material. A steel strip of a general structure steel is most preferable. It is also possible to perform plating after performing surface finishing using a sandblasting, a grinding brush, or the like in advance to attach a metal film or an alloy film of 1 μm or less of Ni, Fe, Zn, Sn, a plated layer, or the like to the surface. Furthermore, as a pretreatment of the steel material, it is preferable to sufficiently wash the steel material by degreasing and pickling.

[0199] After sufficiently heating / reducing the surface of the steel material by a reducing gas such as H2, the steel material is immersed in a plating bath in which the components are adjusted to prescribed components. A high-tension steel or the like is also generally humidified in an atmosphere at the time of annealing, and plating adhesion is ensured for a high-Si, Mn steel or the like by an internal oxidation method or the like, and by performing such a treatment, a plated steel material having less non-plating and appearance defects can be plated as with a general steel material. Such a steel material has a fine steel material surface and an internal oxidation film layer of the crystalline grain system on the side of the base metal, but does not affect the performance of the present embodiment.

[0200] In the case of the hot-dip plating method, the components of the plated layer can be controlled by the components of the plating bath of the built bath. The built bath of the plating bath is made by mixing pure metals in prescribed amounts, and for example, an alloy of the components of the plating bath is made by a melting method under an inert atmosphere. In the case of the present embodiment, the components of the plating bath are set to be substantially the same as the chemical components of the plated layer that is desired to be manufactured.

[0201] By immersing the surface-reduced steel material in a plating bath maintained at a prescribed concentration, a plating layer of approximately the same composition as the plating bath is formed. In the case where the immersion time is long and a long time is taken until solidification is completed, the formation of the interfacial alloy layer becomes active, and thus there are cases where the Fe concentration in the plating layer becomes high. In such cases, by lowering the bath temperature, the Fe content in the plating layer can be suppressed. Specifically, by setting the bath temperature to be lower than 500°C, the reaction with the plating layer can be rapidly slowed down, and thus even in cases where active formation of the interfacial alloy layer is feared, the Fe content contained in the plating layer can generally be suppressed to be lower than 5.0%.

[0202] In order to form the hot-dip plating layer, it is preferable to maintain the plating bath at 450°C to 550°C. Then, it is preferable to immerse the reduced steel material in the plating bath for several seconds. Sometimes on the surface of the reduced steel material, Fe diffuses into the plating bath, reacts with the plating bath, and forms an interfacial alloy layer (mainly an Al-Fe intermetallic compound layer) at the interface between the plating layer and the steel material. In the case where the interfacial alloy layer is formed, the steel material below the interfacial alloy layer and the plating layer above are more firmly combined chemically.

[0203] After immersing the steel material in the plating bath for a prescribed time, the steel material is lifted from the plating bath, and the plating layer is adjusted to a prescribed thickness by performing N2 wiping while the metal attached to the surface is in a molten state. The thickness of the plating layer is preferably adjusted to be 3 to 80 μm. If converted into the amount of plating layer attached, it becomes 20 to 500 g / m 2 Further, the thickness of the plating layer can also be adjusted to be 5 to 70 μm. If converted into the amount of plating layer attached, it becomes about 30 to 400 g / m 2 .

[0204] After adjusting the amount of plating layer attached, the attached molten metal is solidified to form the plating layer. The cooling means at the time of solidification of the plating layer can be performed by blowing of nitrogen, air, or a mixed gas of hydrogen and helium, can be sparging, or can be water submersion. It is preferable to be sparging, and more preferable to be sparging in which nitrogen containing water is included. The cooling speed at the time of solidification of the plating layer is preferably adjusted by the proportion of water contained.

[0205] Under the plating solidification conditions of the usual operating conditions, sometimes the desired structure cannot be controlled, and thus it is ascertained that sometimes the prescribed performance is not satisfied. Therefore, below, the cooling process by which the plating layer of the present embodiment can be obtained is described.

[0206] Average cooling speed from the bath temperature to 400°C: lower than 20°C / sec

[0207] The temperature region from the bath temperature to 400°C is CaZn 13temperature region in which the MgZn2 phase is actively formed. In this temperature region, solidification of the plated layer slowly begins. In the case of the average chemical composition of the plated layer in the present embodiment, the nucleus from which the hot-dip plated layer initially solidifies is CaZn 13 , and generally CaZn 13 becomes a primary crystal. Next, CaZn 13 is precipitated, the MgZn2 phase is precipitated, and the MgZn2 phase becomes a main phase around 380°C.

[0208] Furthermore, in the temperature region of the bath temperature to 400°C, the Al-Zn phase, the Al phase, and the Zn phase as other phases are not substantially formed. On the other hand, in the case where the average cooling rate in the temperature region of the bath temperature to 400°C is large, since solidification proceeds non-equilibrally, even in the plating composition of the present embodiment, the Al-Zn phase, the Al phase in which the Zn concentration is less than 10 mass% is sometimes formed in a small amount. In particular, in the case where the Al content ratio of the plating bath is high, there is a tendency to form the Al phase, and as described above, if the Al phase becomes excessive, the weldability is impaired. Furthermore, the Al-Zn phase and the Al phase, although in a small amount, solid-solve Ca. Therefore, if the Al-Zn phase and the Al phase are formed, there is a tendency that the CaZn 13 phase decreases. Therefore, the average cooling rate in the bath temperature to 400°C is set to be less than 20°C / sec.

[0209] If cooling is performed at an average cooling rate of 20°C / sec or more in the bath temperature to 400°C, the Al phase is generated by non-equilibrium solidification, the CaZn 13 phase decreases, and the weldability of the plated layer is deteriorated. On the other hand, in the case where cooling is performed to 400°C at less than 20°C / sec, the plating solidification approaches equilibrium solidification, Al is precipitated as the Al-Zn phase, and therefore, the Al phase which is non-equilibrium solidification is not generated.

[0210] Furthermore, if the average cooling rate in the bath temperature to 400°C is cooled at more than 10°C / sec, sometimes the particle diameter of the CaZn 13 becomes small, and the improvement of the continuous dotting property becomes insufficient. Therefore, in order to set the equivalent circle particle diameter of the CaZn 13 phase of 80% or more to be 10 μm or more, it is desirable that the average cooling rate in the bath temperature to 400°C be set to be 10°C / sec or less, and more preferably 5°C / sec or less. Furthermore, if the average cooling rate in the bath temperature to 400°C exceeds 10°C / sec, sometimes the MgZn2 phase does not grow, the proportion of the MgZn2 phase contained in the eutectic structure becomes large, and the spot weldability is deteriorated. Therefore, it is desirable that the average cooling rate in the bath temperature to 400°C be 10°C / sec or less. In order to improve the spot weldability, as described above, it is preferable that the MgZn2 phase grow. Thus, it is preferable that the average cooling rate in the bath temperature to 400°C be as small as possible.

[0211] Average cooling rate in the temperature range of 400 to 380°C: 2°C / sec or less

[0212] Further, if the cooling rate in the temperature range of 400 to 380°C is set to exceed 2°C / sec, the MgZn2 phase in the plated layer sometimes becomes coarse, and therefore it is preferable to set the average cooling rate in the temperature range from 400°C to 380°C to 2°C / sec or less. Further, in the cooling in the temperature range of 400 to 380°C, solidification nucleus sites of MgZn2 are formed by blowing a high-temperature gas containing fine particles of ZnO or Al2O3 or the like having a diameter of 0.5 μm or less against the surface of the plated layer, and the growth of the MgZn2 phase can be promoted.

[0213] Average cooling rate in the temperature range of 380 to 300°C: 20°C / sec or more

[0214] In terms of the average bath composition of the plating bath of the present embodiment, the temperature range of 380 to 300°C is a temperature range in which Al is easily released from the Zn phase. If Al is released from the Zn phase, there is a tendency that the volume fraction of the Al-Zn phase as a high-temperature stable phase becomes large, and the volume fraction of the ternary eutectic of the Zn-Al-MgZn2 phase decreases. Therefore, in order to further improve the corrosion resistance, it is effective to accelerate the average cooling rate in the temperature range of 380 to 300°C to suppress the release of Al from the Zn phase. Specifically, the average cooling rate in the temperature range of 380 to 300°C is set to 20°C / sec or more. It is preferably set to 30°C / sec or more, and further preferably to 50°C / sec or more.

[0215] Average cooling rate in the temperature range of 300 to 150°C: 20°C / sec or more

[0216] The temperature range of 300 to 150°C is a region in which the Al phase is stable. That is, if the plated layer is kept in the temperature range of 300 to 150°C, the fine Zn phase incorporated in the Al-Zn phase can be abruptly expelled from the Al-Zn phase. Further, the fine Al phase in the eutectic structure can grow. The higher the temperature, the more violent the atomic movement, and therefore it is preferable to rapidly cool in this temperature range. If the average cooling rate in the temperature range of 300 to 150°C becomes 20°C / sec or less, the Al-Zn phase separates into the fine Al phase and the fine Zn phase. This tendency is particularly strong when the Al concentration is high. In order to suppress the generation of massive or dendritic Al phases from the fine Al phase, it is effective to set the area fraction of the Al phase having a Zn concentration of less than 10 mass% to 5% or less, and to set the average cooling rate in the temperature range of 300 to 150°C to 20°C / sec or more. It is preferably set to 30°C / sec or more, and further preferably to 50°C / sec or more.

[0217] Temperature range of 150°C or less

[0218] The cooling rate in the temperature region below 150°C during solidification does not greatly affect the constituent phases in the plated layer in most cases. Thus, it is not necessary to restrict the cooling conditions in the temperature region below 150°C, and natural cooling can also be performed.

[0219] After the cooling of the plated layer, various chemical conversion treatments, coating treatments can also be performed. Furthermore, in order to further improve the corrosion resistance, repair coating, sputtering treatment and the like can also be performed at the welded portions, processed portions and the like.

[0220] In the plated steel material of the present embodiment, a film can also be formed on the plated layer. The film can be formed in one layer or two or more layers. As the kind of the film directly above the plated layer, for example, chromate film, phosphate film, chromium-free film can be listed. The chromate treatment, phosphate treatment, chromium-free treatment for forming these films can be performed by a known method. However, the chromate treatment sometimes deteriorates the weldability on the plated layer surface in most cases. Thus, in order to sufficiently bring out the effect of improving the weldability in the plated layer, the thickness of the chromate film is preferably set to be lower than 1 μm.

[0221] As the chromate treatment, there are electrolytic chromate treatment in which a chromate film is formed by electrolysis, reaction type chromate treatment in which the film is formed by a reaction with a raw material and then the excess treatment liquid is washed away, and coating type chromate treatment in which the film is formed by drying the treatment liquid applied to the coated object without water washing. Any of the treatments can be employed in the present embodiment in which the chromate treatment is performed.

[0222] As the electrolytic chromate treatment, electrolytic chromate treatment using chromic acid, silica gel, resin (phosphoric acid, acrylic resin, vinyl ester resin, vinyl acetate acrylic emulsion, carboxylated styrene butadiene latex, diisopropanolamine-modified epoxy resin, and the like), and hard silica can be exemplified.

[0223] As the phosphate treatment, for example, zinc phosphate treatment, calcium zinc phosphate treatment, manganese phosphate treatment can be exemplified.

[0224] As the chromium-free treatment, there are electrolytic chromium-free treatment in which a chromium-free film is formed by electrolysis, reaction type chromium-free treatment in which the film is formed by a reaction with a raw material and then the excess treatment liquid is washed away, and coating type chromium-free treatment in which the film is formed by drying the treatment liquid applied to the coated object without water washing. Any of the treatments can be employed.

[0225] If the film on the plated layer has a certain degree of thickness, the appearance of the concentration of the main component elements of each treatment on the plated layer can be confirmed by elemental mapping, and the plated layer can be distinguished from the film. In addition, in the cross-sectional SEM reflection electron image, the film on the plated layer can be distinguished from the plated layer as a film having different contrast, and thus the SEM reflection electron image can be used as an auxiliary.

[0226] The film on the plated layer is thinner than the plated layer, and thus even if the plated layer is subjected to acid dissolution together with the film and component analysis is performed, the composition of the plated layer does not change greatly. In addition, as a method of analyzing the composition of the plated layer from which the film is removed, there is a method in which a plated steel material in which the surface of the plated steel material is ground to remove 1 μm, that is, a plated steel material from which the film is removed by grinding, is used as an analysis object. Specifically, the plated layer is dissolved and separated by an acid containing an inhibitor that suppresses corrosion of the steel material (base metal) to obtain an acid solution. Then, by measuring the obtained acid solution using ICP emission spectrometry or ICP-MS method, the chemical composition of the plated layer can be obtained.

[0227] Further, on the film directly above the plated layer, one or two or more layers of an organic resin film can be provided. As the organic resin, it is not limited to a specific type, and for example, polyester resin, polyurethane resin, epoxy resin, acrylic resin, polyolefin resin, or a modified body of these resins, or the like can be listed. Here, the modified body refers to a resin obtained by reacting a reactive functional group contained in the structure of these resins with another compound (monomer, crosslinking agent, or the like) having a functional group that can react with the functional group in the structure.

[0228] As such an organic resin, one or two or more kinds of organic resins (unmodified resins) can be used in mixture, or an organic resin obtained by modifying at least one other organic resin in the presence of at least one organic resin can be used in mixture of one or two or more kinds. In addition, any coloring pigment, rust-preventive pigment can be contained in the organic resin film. A resin that is water-based by being dissolved or dispersed in water can also be used.

[0229] These organic resin films are mainly composed of light elements such as C and O, and are darkly captured on the cross-sectional SEM reflection electron image compared to the plated layer, and thus can be easily distinguished from the plated layer.

[0230] The composition analysis of the plated layer of the plated steel material with the organic resin coating film has a method of taking as the analysis object the plated steel material whose surface is ground by 1 μm after the organic resin coating film is removed with a peeling agent, i.e., the plated steel material after the coating film is removed by grinding. Specifically, the plated layer is dissolved and removed with an acid containing an inhibitor that inhibits corrosion of the steel material (base metal) to obtain an acid solution. Next, by measuring the obtained acid solution using ICP emission spectrometry or ICP-MS method, the chemical composition can be obtained.

[0231] The evaluation method of the spot weldability of the plated steel material and the evaluation method of the corrosion resistance around the spot weld are described.

[0232] <Evaluation of the appropriate current range>

[0233] For the evaluation of the appropriate current range of the welding current for spot welding, a plurality of test pieces are prepared to be implemented. As the test pieces, for example, plated steel materials in which a mild steel plate with a plate thickness of 0.8 mm is used as the steel material can be used. In the case where a chemical conversion treatment coating film is formed on the surface of the plated layer, the welding scheduled portion is polished with sandpaper or the like to peel off the chemical conversion treatment coating film. The adhesion amount or thickness of the plated layer in each sample is preferably fixed, and there is a tendency that the spot weldability most easily produces a difference in the case where the thickness of the plated layer is 20 μm or thereabout, and therefore the thickness of the plated layer is set to 20 μm. The test pieces use test pieces in which plated layers are formed on both surfaces of the steel plate. As the test pieces, test pieces with a size of 30 x 50 mm are cut out from the plated steel material.

[0234] Next, the search for the recommended welding conditions is performed using a spot welder. The electrode material is set to Cu-Cr alloy. The shape of the electrode is set to a half-spherical type. The welding time, the pressing force, the cooling capacity, the Sq. time, the Up. time, the Down. time, and the like are adjusted for each test piece to search for the basic welding conditions that can be spot welded. The basic welding conditions are preferably the welding conditions that become fixed even if the plated steel plate is changed, which are searched as much as possible. Once the basic welding conditions are found, initially, in order to fix the electrode surface state, temporary spot welding of 10 hits is performed, and then welding is performed while the welding current value is varied from the low side to the high side. The current value is preferably increased in increments of 0.1 kA. After spot welding at each welding current, the superimposed test pieces are resin-embedded, and the nugget diameter at the center of the spot weld is measured. The current value that satisfies the nugget diameter of 4 x Vt (t = the plate thickness of the plated steel plate used) of the prescribed value is set as the lower limit value of the appropriate current range. In addition, the current value that causes spatter to occur is set as the upper limit value of the appropriate current range. The difference between the upper limit value and the lower limit value is the appropriate current range. The larger the appropriate current range of the plated steel plate, the easier it is to perform spot welding, and on the contrary, the narrower the plated steel plate, the more difficult it is to perform spot welding.

[0235] <Evaluation of continuous spot weldability>

[0236] The life of the spot welding electrode is also affected by the plated steel. That is, if a large number of spot welds can be performed without replacing the electrode surface with a current value within the appropriate current range, the manufacturing cost and manufacturing time can be reduced, and it can be said that the plated steel is more preferable. Specifically, the central value of the appropriate current range is set as the welding current, and spot welding is continuously performed. If spot welding is repeatedly performed, the metal elements on the surface of the plating layer, i.e., Ca, Mg, Al, Zn, etc., react with the electrode, and the electrode shape gradually changes. The electrode shape can be easily measured with pressure-sensitive paper or the like at the time of spot welding. As the number of spot welds increases, the nugget diameter of the spot welds gradually decreases. The number of spots when the nugget diameter becomes less than 2 x Vt (t is the thickness of the steel) for 3 consecutive times is set as the continuous spot number. The larger this value, the more it can be said that the plated steel is less wasteful of the electrode and has more excellent continuous spot performance.

[0237] <Corrosion resistance around the spot weld>

[0238] Two plated steel sheets of 70 x 150 mm and 30 x 75 mm were prepared, the central portions of each were overlapped, and two spot welds were formed at a position 20 mm from the central portions. The nugget diameter was adjusted so as to be 4 Vt or more (t is the thickness of the steel). In this way, an overlap test piece was prepared. Only the end surface portion of the plated steel sheet of 70 x 150 mm was subjected to a repair treatment with an epoxy resin paint, and was directly put into a corrosion testing machine. After a predetermined time in the corrosion testing machine, since the overlapped portion is most susceptible to corrosion, it was cut open and the red rust generation condition was confirmed. The longer the period until red rust appears, the higher the corrosion resistance around the weld, and the more suitable it is as a spot welding material.

[0239] Next, the evaluation method for the stability of the plating bath will be described.

[0240] <Stability of the plating bath>

[0241] As a condition for the Zn-Al-Mg plating bath to satisfy the stability of the bath during operation, it can be cited that Mg does not cause abnormal oxidation in the bath. That is, in the case where Mg causes abnormal oxidation in the bath, a black oxide is generated on the surface of the bath, and operation becomes difficult. On the other hand, since Zn-based oxides are white, they can be clearly distinguished in appearance from the black oxides caused by abnormal oxidation of Mg.

[0242] Bath stability can be evaluated by leaving the plating bath in a molten state in the atmosphere. Specifically, a raw material metal is melted in a manner adjusted to a prescribed bath composition, with the bath amount of the plating bath being 10 kg. The raw material metal used is not limited if it is of a known composition, and for example, a Zn raw material metal, an Al raw material metal, an Mg raw material metal, an Al-Ca alloy raw material metal, or the like can be used. The melted plating bath is left in the atmosphere at a bath temperature of 30°C higher than the melting point thereof for 24 hours. The bath surface after the leaving is observed, and if a black oxide is generated, the plating bath is judged to be unstable. On the other hand, if no black oxide is generated, it is judged to have bath stability satisfying the operation.

[0243] Examples

[0244] As shown in Tables 1A to 3C, plated steels Nos. 1 to 57 were manufactured, and the properties were evaluated.

[0245] For the adjustment of the plating bath, a pure metal was adjusted to build the bath. The composition of the plated alloy was analyzed after the bath was built, and Fe powder was added so that there was no increase in the Fe concentration in the test. As for the composition of the plating bath, a metal sheet obtained by solidifying the plating bath was dissolved in acid, and the solution was analyzed by ICP emission spectrometry. Further, as for the composition of the plated layer, the plated layer was peeled off with hydrochloric acid to which a peptizer was added, and the composition analysis after the peeling was performed to confirm it. Except for the Fe composition, the composition of most of the plated layer agreed with the plating bath composition value within a range of ±0.5%. Note that the Zn content of the plated layer was all 65% or more.

[0246] The original plate of the plated steel was set to an original plate cut out in a size of 180 mm x 100 mm from a cold-rolled steel sheet of a thickness of 0.8 mm. All were SS400 (ordinary steel).

[0247] Next, using a batch-type hot dip plating simulation device (manufactured by RHESCA Corporation), a K thermocouple was installed on a part of the steel sheet, and the steel sheet surface was sufficiently reduced at 800°C in a reducing atmosphere of N2 containing 5% of H2. Thereafter, the plated original plate after the annealing was immersed in the plating bath for 3 seconds, after which it was lifted, and the plating thickness was adjusted in a manner so as to become 20 μm (±1 μm) by N2 gas wiping. The thickness of the plated layer on the original plate surface was the same on either side. After the plated original plate was lifted from the plating bath, plated steels were manufactured under various cooling conditions of A to F described below.

[0248] Condition A: After the steel was lifted from the plating bath, the average cooling rate between the bath temperature and 400°C was set to 10°C / sec, the average cooling rate between 400°C and 380°C was set to 2°C / sec, and the average cooling rates between 380°C and 300°C and between 300°C and 150°C were set to 25°C / sec, respectively. Below 150°C, it was set to natural cooling.

[0249] Condition A-2: After the steel material is taken out of the plating bath, the average cooling rate between the bath temperature and 400°C is set to 5°C / sec, the average cooling rate between 400°C and 380°C is set to 2°C / sec, the average cooling rate between 380°C and 300°C and between 300°C and 150°C is set to 50°C / sec, respectively. Below 150°C, natural cooling is set.

[0250] Condition A-3: After the steel material is taken out of the plating bath, the average cooling rate between the bath temperature and 400°C is set to 5°C / sec, the average cooling rate between 400°C and 380°C is set to 2°C / sec, the average cooling rate between 380°C and 300°C is set to 30°C / sec, and the average cooling rate between 300°C and 150°C is set to 50°C / sec. Below 150°C, natural cooling is set.

[0251] Condition A-4: After the steel material is taken out of the plating bath, the average cooling rate between the bath temperature and 400°C is set to 5°C / sec, the average cooling rate between 400°C and 380°C is set to 2°C / sec, the average cooling rate between 380°C and 300°C is set to 50°C / sec, and the average cooling rate between 300°C and 150°C is set to 30°C / sec. Below 150°C, natural cooling is set.

[0252] Condition A-5: After the steel material is taken out of the plating bath, the average cooling rate between the bath temperature and 400°C is set to 5°C / sec, the average cooling rate between 400°C and 380°C is set to 2°C / sec, and high-temperature gas containing φ0.5 μm of Al2O3 is blown during the cooling between 400°C and 380°C. The average cooling rate between 380°C and 300°C and between 300°C and 150°C is set to 50°C / sec, respectively. Below 150°C, natural cooling is set.

[0253] Condition B (comparative condition): After the steel material is taken out of the plating bath, the average cooling rate between the bath temperature and 150°C is set to 20°C / sec. Below 150°C, natural cooling is set.

[0254] Condition C (comparative condition): After the steel material is taken out of the plating bath, the average cooling rate between the bath temperature and 150°C is set to 2°C / sec. Below 150°C, natural cooling is set.

[0255] Condition D (comparative condition): After the steel material is taken out of the plating bath, the average cooling rate between the bath temperature and 150°C is set to 10°C / sec. Below 150°C, natural cooling is set.

[0256] Condition E (Comparative Condition): After the steel material is lifted from the plating bath, the average cooling rate between the bath temperature and 400°C is set to 10°C / sec, the average cooling rate between 400°C and 380°C is set to 15°C / sec, and the average cooling rates between 380°C and 300°C and between 300°C and 150°C are set to 15°C / sec, respectively. Below 150°C, natural cooling is set.

[0257] Condition F (Comparative Condition): After the steel material is lifted from the plating bath, the average cooling rate between the bath temperature and 400°C is set to 30°C / sec, the average cooling rate between 400°C and 380°C is set to 2°C / sec, and the average cooling rates between 380°C and 300°C and between 300°C and 150°C are set to 30°C / sec, respectively. Below 150°C, natural cooling is set.

[0258] Condition G (Example Condition): After the steel material is lifted from the plating bath, the average cooling rate between the bath temperature and 400°C is set to 15°C / sec, the average cooling rate between 400°C and 380°C is set to 15°C / sec, and the average cooling rates between 380°C and 300°C and between 300°C and 150°C are set to 30°C / sec, respectively. Below 150°C, natural cooling is set.

[0259] From each of the manufactured plated steel sheets, a sample for spot welding evaluation and a sample for corrosion test after spot welding were cut out.

[0260] (Spot welding conditions)

[0261] A fixed spot welder of servo pressurization type (air pressurization type) was used. The power source was set to a single-phase alternating current type, and the frequency of the power source was set to 50 Hz. As the electrode, a semi-spherical chrome copper 40R (φ 6 mm) was used. The basic conditions were set to a pressurization force of 250 kg, 30 cycles of pre-pressing, 10 cycles of rising, 10 cycles of power feeding time, 10 cycles of holding time, and a cooling water flow rate of 15 liters / minute. The number of cycles was set to be changed by ±10 cycles as necessary. The weldability curve was plotted in 0.2 kA scales as necessary. Initially, in order to fix the electrode surface state, 10 points of temporary dotting were performed at 10 kA in advance.

[0262] (Appropriate current range)

[0263] After temporary spot welding of 10 spots under the basic welding conditions, welding was performed while varying the welding current value from the low side to the high side. The current value was increased in 0.1 kA increments. After welding at each welding current point, the overlap test piece was resin-embedded, polished, and the sample etched with 3% nitric acid ethanol was observed by optical microscopy to measure the nugget diameter. The range of current values from the current value at which the nugget diameter from the welded portion was 4 x Vt = 3.58 or more to the current value at which spatter occurred was set as the appropriate current range. The evaluation of the appropriate current range was set as described below. "B" was set as unqualified, and "A" to "S" were set as qualified. The results are shown in Tables 3A to 3C.

[0264] Less than 1.2 kA: "B"

[0265] 1.2 kA to less than 1.5 kA: "A"

[0266] 1.5 kA to less than 1.7 kA: "AA"

[0267] 1.7 kA to less than 1.9 kA: "AAA"

[0268] 1.9 kA or more: "S"

[0269] (Spot welding continuous spot performance)

[0270] The current value was set to the center value of the appropriate current range, and spot welding was continuously performed with two samples overlapped. Every 50 spots, the welded portion of the overlap plate was cut out, resin-embedded, and polished, and the sample etched with 3% nitric acid ethanol was observed by optical microscopy to confirm the nugget diameter. In the case where the nugget diameter continuously decreased to less than 2 x Vt three times in a row, the number of spots at which the nugget diameter first decreased to less than 2 x Vt among the three in a row was set as the number of continuous spots. "B" was set as unqualified, and "A" to "S" were set as qualified. The results are shown in Tables 3A to 3C.

[0271] Less than 150 spots: "B"

[0272] 150 spots to less than 250 spots: "A"

[0273] 250 spots to less than 500 spots: "AA"

[0274] 500 spots to less than 750 spots: "AAA"

[0275] 750 spots or more: "S"

[0276] (Spot welding portion corrosion resistance evaluation)

[0277] Two 70 x 150 mm, 30 x 75 mm plated steel sheets were overlapped at the center portion, and spot-welded at a distance of 20 mm from the center, 2 shots (nugget diameter of 4Vt or more) to make an overlap test piece. The test piece was put into a composite cyclic corrosion testing machine, and subjected to a corrosion test according to JIS H 609-91. From 150 cycles, the overlap portion was peeled off every 30 cycles, and the red rust occurrence was confirmed (red rust was generated at a red rust area ratio of 1% or more).

[0278] The evaluation of corrosion resistance was set as described below. "B" was set as unqualified, and "A" to "S" were set as qualified. The results are shown in Tables 3A to 3C.

[0279] Red rust occurrence in 150 cycles or less: "B"

[0280] Red rust occurrence in 150, 180 cycles: "A"

[0281] Red rust occurrence in 210, 240 cycles: "AA"

[0282] Red rust occurrence in 270, 300 cycles: "AAA"

[0283] No red rust occurrence in 300 cycles: "S"

[0284] (Stability of plating bath)

[0285] A plating bath of a prescribed composition was prepared by melting raw material metals in a graphite crucible in an atmosphere so that the bath amount became 10 kg. After the plating bath was prepared, the bath temperature was set to a temperature 30°C higher than the melting point. The bath temperature was measured by immersing an alumina protection tube in which a K thermocouple was inserted into the plating bath. After the bath temperature was stabilized at the set temperature ± 5°C, the scum on the surface of the bath was removed, and the bath was left to stand in the atmosphere for 24 hours. After the bath was left to stand for 24 hours, the surface of the bath was observed, and the generation of black oxide was confirmed. The evaluation of bath stability was set as described below. "B" was set as unqualified, and "A" was set as qualified. The results are shown in Tables 3A to 3C.

[0286] Generation of black oxide: "B"

[0287] No generation of black oxide: "A"

[0288] As shown in Tables 1A to 3C, Nos. 10 to 48, 57 are excellent in the spot-welding continuity because the average chemical composition of the plated layer and the metal structure satisfy the range of the present application, in addition to the fact that the cooling condition at the time of production is appropriate, the range of the proper current at the time of spot-welding is wide, in addition to the fact that the corrosion resistance of the spot-welded portion is excellent. Note that the eutectic structure of Tables 3A to 3C is the total of the ternary eutectic structure of [Al / MgZn2 / Zn] and the binary eutectic structure of [MgZn2 / Zn]. In addition, the Zn2Mg phase of Tables 3A to 3C is the MgZn2 phase.

[0289] As shown in Tables 1A to 3C, Nos. 1 to 9 are poor in the spot-welding continuity, the range of the proper current at the time of spot-welding is narrow, and the corrosion resistance of the spot-welded portion is also poor because the average chemical composition of the plated layer departs from the range of the present application, in addition to the fact that the metal structure departs from the range of the present application in some of the plated steel materials.

[0290] As shown in Tables 1A to 3C, Nos. 49 to 53 are poor in the spot-welding continuity, the range of the proper current at the time of spot-welding is narrow, and the corrosion resistance of the spot-welded portion is also poor because the metal structure departs from the range of the present application due to the fact that the cooling condition at the time of production is inappropriate.

[0291] As shown in Tables 1A to 3C, Nos. 54 to 55, 56 are poor in the spot-welding continuity, the range of the proper current at the time of spot-welding is narrow, and the corrosion resistance of the spot-welded portion is also poor because the average chemical composition of the plated layer departs from the range of the present application, the metal structure of the plated layer departs from the range of the present application, and the cooling condition at the time of production is inappropriate.

[0292]

[0293]

[0294]

[0295]

[0296]

[0297]

[0298]

[0299]

[0300]

[0301] Industrial Applicability

[0302] According to the present application, a steel sheet which can suppress generation of surface unevenness at the time of forming and which is high in strength can be obtained.

Claims

1. A plated steel material, characterized by, The plated steel material has: a steel material, a plated layer possessed by a surface of the steel material, the average chemical composition of the plated layer contains, in mass%: Al: 0.2% or more and less than 4.0%, Mg: more than 4.0% and less than 12.5%, Ca: 0.15% or more and less than 3.00%, Sn: 0% or more and less than 3.0%, Bi: 0% or more and less than 1.0%, In: 0% or more and less than 1.0%, Sc: 0% or more and less than 0.50%, Y: 0% or more and less than 0.50%, La: 0% or more and less than 0.50%, Ce: 0% or more and less than 0.50%, Sr: 0% or more and less than 0.50%, Si: 0% or more and less than 2.50%, B: 0% or more and less than 0.50%, P: 0% or more and less than 0.50%, Cr: 0% or more and less than 0.25%, Ti: 0% or more and less than 0.25%, Ni: 0% or more and less than 1.0%, Co: 0% or more and less than 0.25%, V: 0% or more and less than 0.25%, Nb: 0% or more and less than 0.25%, Cu: 0% or more and less than 1.0%, Mn: 0% or more and less than 0.25%, Mo: 0% or more and less than 0.25%, W: 0% or more and less than 0.25%, Zr: 0% or more and less than 0.25%, Fe: 0% or more and less than 5.0%, Ag: 0% or more and less than 1.0%, Li: 0% or more and less than 0.50%, Na: 0% or more and less than 0.05%, K: 0% or more and less than 0.05%, Sb: 0% or more and less than 0.5%, Pb: 0% or more and less than 0.5%, Zn: 65% or more, and impurities, in a cross section along a thickness direction in the plated layer, the metal structure contains, in area fraction, when observed with a field of view of a scanning electron microscope: MgZn2 phase: 10 to 40%, Al-Zn phase: 0 to 15%, Al phase: 0 to 5%, CaZn 13 Phase: 1.0-15.0%, a total of a ternary eutectic structure of Al / MgZn2 / Zn and a binary eutectic structure of MgZn2 / Zn: 30.0% or more.

2. The plated steel material according to claim 1, wherein CaZn of the metal structure in the cross section in the thickness direction of the plating layer 13 CaZn of the metal structure in the cross section in the thickness direction of the plating layer 13 The average equivalent circle diameter of the phase is 10 μm or more.

3. The plated steel material according to claim 1 or claim 2, characterized in that, In a case where a mass concentration % of an element X in the average chemical composition of the plated layer is set as [X], the average chemical composition of the plated layer satisfies the following formula (1), 2.5×[Al]+2×[Ca]+[Y]+[Sr]+[La]+[Ce]>[Mg] (1).

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