fused steel

Abstract

The molten steel in this case comprises: steel and a coating layer disposed on the surface of the steel; the coating layer has the following chemical composition by mass%: Al: greater than 10% and less than 40%, Mg: 4.0% and less than 15.0%, the remainder being: Zn and impurities, and the total amount of V and Mn, ΣTR1: 0.05% and less than 3.0%; in the elemental distribution profile obtained by GDS analysis from the surface of the coating layer toward the steel in the thickness direction of the steel, the depth position where the Fe concentration reaches 95% is defined as the interface between the coating layer and the steel, and the interval from the surface of the coating layer to the interface is defined as the thickness t of the coating layer. At this time, the proportion of depths with a total amount of Fe of 0.1% or more relative to the thickness t is 10% or more.

Description

[Technical Field]

[0001] Field of the Invention This disclosure relates to a molten steel. This case claims priority based on Japanese Patent Application No. 2024-135644 filed on August 15, 2024, the contents of which are incorporated herein by reference. [Previous Technology]

[0002] Background of the Invention When steel is used for a long time, it is advisable to apply some kind of rust-proof treatment that can resist corrosion. As a low-cost method of steel rust prevention, molten Zn plating has been used in various fields such as civil engineering, construction, and automobiles where steel rust prevention is required.

[0003] The corrosion protection provided by the coating layer depends on the inherent corrosion resistance and thickness of the coating layer. For example, Patent Document 1 describes a method for manufacturing coated steel sheets by continuously immersing steel sheets in a molten plating bath, i.e., by a continuous molten plating method. The coated steel sheets are then processed into part shapes. The continuous molten plating method is suitable for forming zinc plating or Zn alloy plating containing Al and Mg. Compared to the later plating methods described, the coating thickness of the continuous molten plating method is relatively thinner.

[0004] Patent Document 2 describes a method for immersing a workpiece, processed into a predetermined shape, in a plating bath, which is called batch melt plating or post-plating. Post-plating, because the workpiece is immersed in the plating bath for more than one minute, tends to result in a very thick plating layer. However, compared to continuous melt plating, post-plating has greater limitations on alloy composition and tends to deteriorate corrosion resistance, and its manufacturability is also significantly worse.

[0005] However, in environments where the time spent in water (wetting time) increases, the corrosion of the coated steel sheet becomes extremely severe. Therefore, increasing the thickness of the coating is important in order to extend the service life of the coated steel sheet. Thus, if a thick Zn alloy coating containing a large amount of Al can be manufactured through a continuous melt coating method, to the same extent as in subsequent coating methods, a steel sheet that can resist corrosion for a long time can be effectively manufactured.

[0006] The coating thickness of steel sheets manufactured by continuous melt plating is mostly around 20-30 μm. This is due to the characteristics of the continuous melt plating process, such as the molten metal being lifted from the plating bath, gas wiping, and gas solidification. For example, if the lifting speed of the steel sheet from the plating bath is increased, the coating thickness will become very thick, but the surface appearance control will become difficult. Therefore, the upper limit of the coating thickness is strictly limited by the adjustment of wiping. That is, when manufacturing a Zn alloy coating containing Al and Mg using continuous melt plating, it is generally difficult to produce a coating thickness of more than 30 μm. This is because the plating bath containing a large amount of Al has a lighter specific gravity and a lower viscosity, resulting in less molten metal adhering when the steel sheet is lifted from the plating bath.

[0007] Furthermore, in recent years, in order to impart properties other than corrosion resistance to the plating layer, various elements other than Al and Mg have been added to the Zn alloy plating bath. As shown in Patent Document 1, Sn is sometimes added to the plating layer. These elements readily bond with Mg, Al, or Zn in the Zn alloy to form high-melting-point intermetallic compounds.

[0008] [Prior Art Documents] [Patent Documents] [Patent Document 1] International Publication No. 2018 / 139619 [Patent Document 2] Japanese Patent Application Publication No. 61-295361 [Summary of the Invention]

[0009] Summary of the Invention The problem to be solved by the invention is the thickening of the coating as one of the means to improve the lifespan of the coating, that is, to improve the corrosion resistance. In order to obtain a coating with a large thickness, it is effective to increase the viscosity of the coating bath, which can be achieved, for example, by making the coating bath contain an element with a relatively heavy specific gravity. It can be expected that if the coating bath contains an element with a heavy specific gravity, the specific gravity will be restored and the coating adhesion will be increased.

[0010] However, when the plating bath contains elements with high specific gravity, the high specific gravity of these elements may alter the reactivity between the elements in the plating bath and the base steel. Therefore, it is difficult, in the prior art, to both confirm the effect of adding elements with high specific gravity and to obtain a thick and robust plating layer by including elements with high specific gravity in the plating bath.

[0011] This disclosure was made in view of the above-mentioned reasons, and its object is to provide a molten steel that can increase the coating thickness even when the coating contains elements with a relatively heavy specific gravity.

[0012] Means for Solving the Problem In order to solve the aforementioned problem, the inventors reviewed techniques for adding heavy elements such as V and Mn (transition metals), which are difficult to add to the plating bath. Furthermore, although transition elements such as V and Mn have relatively high melting points, high-melting-point substances sometimes frequently produce plating defects such as slag. Therefore, a technique was reviewed that allows even high-melting-point metal elements to be stably added to the bath. It was found that even when the plating bath contains heavy and high-melting-point elements, a technique can stably produce plated steel, and a plated steel with a thick plating layer and excellent corrosion resistance was discovered. It was also found that plating layers containing these heavy and high-melting-point elements and exhibiting appropriate morphology can achieve excellent corrosion resistance attributable to V and Mn.

[0013] In order to solve the above-mentioned problems, this disclosure adopts the following structure. [1] The present invention discloses a molten-coated steel sample comprising: steel and a coating disposed on the surface of the steel; the coating having the following chemical composition by mass%: Al: greater than 10% and less than 40%, Mg: 4.0% and more than 15.0% and less, Si: 0% and more than 2.0% and less, V: 0% and more than 3.0% and less, Mn: 0% and more than 3.0% and less, Cr: 0% and more than 3.0% and less, Mo: 0% and more than 3.0% and less, Sn: 0% and more than 0.7% and less, Bi: 0% and more than 0.3% and less, In: 0% and more than 0.3% and less, Ca: 0% and more than 0.6% and less, Y: 0% and more than 0.3% and less, La: 0% and more than 0.3% and less, Ce: 0% and more than 0.3% and less, Sr: 0% and more than 0.3% and less, Li: 0% and more than 0.3% and less. Ni: ≥0% and ≤3.0%, Co: ≥0% and ≤3.0%, Cu: ≥0% and ≤0.25%, Ag: ≥0% and ≤0.25%, Sb: ≥0% and ≤0.25%, Pb: ≥0% and ≤0.25%, B: ≥0% and ≤0.50%, P: ≥0% and ≤0.50%, Ti: ≥0% and ≤0.25%, Nb: ≥0% and ≤0.25%, Zr: ≥0% and ≤0.25%, W: ≥0% and ≤0.25%, Fe: ≥0% and ≤5.0%, Remaining portion: Zn and impurities, V and Mn combined amount ΣTR1: ≥0.05% and ≤3.0%, Cr and Mo combined amount ΣTR1': ≥0% and ≤3.0%. ΣTR1+ΣTR1': 0.05% or more and 3.0% or less; Σα: 0% or more and 0.7% or less; Σβ: 0% or more and 0.6% or less; ΣTR2: 0% or more and 3.0% or less; Σγ: 0% or more and 1.00% or less; In the elemental distribution profile of GDS analysis performed from the surface of the aforementioned coating towards the aforementioned steel in the thickness direction of the aforementioned steel, the depth position where the Fe concentration reaches 95% is defined as the interface between the aforementioned coating and the aforementioned steel, and the interval from the aforementioned surface of the aforementioned coating to the aforementioned interface is defined as the thickness t of the aforementioned coating. At this time, the proportion of the aforementioned total amount ΣTR1 showing a depth of 0.1% or more relative to the aforementioned thickness t is 10% or more.[2] The molten steel of [1] above can also be as follows: when V and Mn in the aforementioned chemical composition are defined as TR1, the aforementioned coating layer contains: intermetallic compounds containing TR1; in the cross section of the aforementioned coating layer, the total area ratio of the aforementioned intermetallic compounds containing TR1 is 1% or more. [3] The molten steel of [1] or [2] above can also be as follows: when Ni and Co in the aforementioned chemical composition are defined as TR2, in the aforementioned elemental distribution cross section, the depth showing the maximum concentration of TR2 is defined as t(TR2), and the depth showing the maximum concentration of TR1 is defined as t(TR1), at which time the following equation (1) is satisfied: t(TR2)-t(TR1)≦0.30t・・・(1). [4] The molten steel of [1] or [2] above can also be as follows: using Cu-Kα line and X-ray output of 50kV and 300mA, the X-ray diffraction pattern of the surface of the aforementioned coating layer measured below, I1 as specified in equation (4) and I2 as specified in equation (5) below satisfy equations (2) and (3) below: 1.5≦I1…(2) 1.5≦I2…(3) I1=(Imax(10.5°~11.0°) / (I(10.5°)+0.2{|I(11.0°)-I(10.5°)|})…(4) I2=(Imax(20.2°~20.5°)) / (I(20.2°)+0.667{|I(20.5°)-I(20.2°)|})…(5) In equations (4) and (5), Imax(k~m°) is the maximum X-ray diffraction intensity between diffraction angles k~m°, I(n°) is the X-ray diffraction intensity at diffraction angle n°, and k, m, and n are the diffraction angles (2θ) shown in equations (4) and (5), respectively.

[0014] Effects of the Invention According to the embodiment of the present invention, a molten-plated steel with a large plating thickness can be provided. Furthermore, since the molten-plated steel according to the present embodiment can increase the thickness of the plating layer, the plating life (corrosion resistance) can be improved.

Implementation Method

[0016] Embodiments of the present invention for carrying out the invention The following describes a melt-coated steel material according to one embodiment of the present invention.

[0017] In addition, in this instruction book, the "%" marking of the content of each element in the chemical composition of the coating layer means "% of mass" as long as it is not otherwise stated. Further, as long as there is no special statement, "%" when labeling the composition of intermetallic compounds means "atomic %". In addition, the numerical range indicated using “~” means: a range that includes the values ​​​​recorded before and after “~” as lower and upper limits. In addition, the range of values ​​​​when the values ​​​​recorded before and after “~” are marked with “greater than” or “less than” means: a range that does not include such values ​​as a lower or upper limit value.

[0018] In this case, the inventors and others concentrated on studying a melt-coated steel with a coating layer containing Al, Mg and Zn and manufactured by a continuous fused zinc plating method.

[0019] The melting point of a Zn alloy melting coating bath obtained by making Zn of a low melting point metal contain elements such as Al or Mg has a tendency to increase with a distance away from the 3-membered eutectic composition or Zn-3%Al-3%Mg (marked % as mass %). Plating baths containing more than 10% Al and more than 4% Mg will have a higher melting point than that of pure Zn. In this Zn alloy plating bath, if the melt plating process is considered, the operation will be carried out at about 500°C above 50°C higher than the melting point.

[0020] The specific gravity of alloying elements such as Al and Mg is less than Zn. The specific gravity of the alloy components is closely related to the viscosity in the molten state. That is, at high concentrations of molten metal containing Al, Mg, etc. its viscosity will become smaller. Therefore, in the continuous melt plating method, when the steel plate is passed through a plating bath of about 500°C at a certain production line speed, the amount of molten plating bath attached to the steel plate and brought up with the steel plate will be significantly reduced than in the case of pure Zn plating. Accordingly, in order to stably manufacture plating layers with thicknesses greater than 20μm, it is necessary to develop wiping techniques and so on. However, the specific gravity of the molten metal that makes up the coating layer is inherently small and therefore light, so when wiping and blowing, the molten metal is easily blown away, resulting in a reduction in the amount of attachment of the molten coating bath attached to the steel plate and brought up.

[0021] Furthermore, when the plating bath contains Al, the reaction between Al and Fe becomes more active, thus easily forming an Al-Fe interfacial alloy layer when steel passes through the plating bath. In this case, during bending tests, cracks originating from the Al-Fe alloy layer propagate within the plating layer, sometimes causing peeling. This peeling becomes more pronounced at higher plating bath temperatures. Due to immersion time and the Al loss within the plating layer caused by Fe, it becomes difficult to manufacture molten coated steel with a highly adhesive plating layer that can be processed into various products when using plating baths with high Al concentrations and high temperatures.

[0022] Therefore, in order to solve the above-mentioned problems, the inventors of this case conducted intensive research. Under the conditions of high Al concentration and high bath temperature in the plating bath, in order to produce a plating layer with a large adhesion, it is necessary to first increase the viscosity of the plating bath, which can be achieved by adding a heavy element to the plating bath. In particular, if transition metal elements such as V and Mn (TR1) are added, the specific gravity and viscosity of the plating bath will increase, thereby increasing the plating adhesion to the steel during plate transfer.

[0023] On the other hand, when these transition metal elements are added to the plating bath, these transition metal elements tend to form suspended dross in the plating bath because they easily bond with elements such as Si, Ca, and Mg. The plating steel containing this dross will have reduced adhesion or plating defects. However, if the concentration of the added transition metal elements is limited to a certain range, these suspended dross will be fine. Therefore, by raising the plating bath temperature to around 600°C, the suspended dross can be re-dissolved in the plating bath.

[0024] On the other hand, in a plating bath at around 600°C, the Al-Fe alloying reaction is active, and the Al-Fe interfacial alloy layer will grow very thick in a short time after immersion in the steel, which may significantly impair the adhesion of the subsequent molten steel plating.

[0025] Furthermore, the added transition metal elements have a similar reaction mode to Fe and diffuse into the steel (Fe substrate). In particular, the accumulation of V and Mn on the steel surface immediately after immersion in the plating bath can lead to a large number of poorly plated areas or deteriorated areas such as unreacted areas (areas not reacted with the steel surface) due to the accumulation of these elements, which are even more difficult to reduce than Fe. This is because in Zn-Al-Mg based coated steel, the fine oxides formed by V and Mn beneath the steel surface (i.e., the steel surface layer) hinder the adhesion of the plating. Therefore, by adding transition metal elements to the plating bath and taking measures to prevent excessive diffusion of the elements within the steel, the integrity of the plating layer can be maintained, resulting in a coated steel with fewer defects and excellent corrosion resistance.

[0026] Hereinafter, a melt-coated steel material according to an embodiment of the present invention obtained based on the above innovative knowledge and insights will be described in detail.

[0027] [Fused-coated steel] A fused-coated steel according to an embodiment of the present invention will be described. The fused-coated steel of this embodiment includes: steel and a coating layer disposed on the surface of the steel.

[0028] The average chemical composition of the plating layer, in mass %, is as follows: Al: greater than 10% and less than 40%, Mg: greater than 4.0% and less than 15.0%, Si: greater than 0% and less than 2.0%, V: greater than 0% and less than 3.0%, Mn: greater than 0% and less than 3.0%, Cr: greater than 0% and less than 3.0%, Mo: greater than 0% and less than 3.0%, Sn: greater than 0% and less than 0.7%, Bi: greater than 0% and less than 0.3%, In: greater than 0% and less than 0.3%, Ca: greater than 0% and less than 0.6%, Y: greater than 0% and less than 0.3%, La: greater than 0% and less than 0.3%, Ce: greater than 0% and less than 0.3%, Sr: greater than 0% and less than 0.3%, Li: greater than 0% and less than 0.3%, Ni: greater than 0% and less than 3.0%, Co: greater than 0% and less than 3.0%. Cu: ≥0% and ≤0.25%, Ag: ≥0% and ≤0.25%, Sb: ≥0% and ≤0.25%, Pb: ≥0% and ≤0.25%, B: ≥0% and ≤0.50%, P: ≥0% and ≤0.50%, Ti: ≥0% and ≤0.25%, Nb: ≥0% and ≤0.25%, Zr: ≥0% and ≤0.25%, W: ≥0% and ≤0.25%, Fe: ≥0% and ≤5.0%, Remaining portion: Zn and impurities, the combined amount of V and Mn ΣTR1: ≥0.05% and ≤3.0%, the combined amount of Cr and Mo ΣTR1': ≥0% and ≤3.0%, ΣTR1+ΣTR1': ≥0.05% and ≤3.0%. The combined amount of Sn, Bi, and In, Σα: 0% or more and 0.7% or less; the combined amount of Ca, Y, La, Ce, Sr, and Li, Σβ: 0% or more and 0.6% or less; the combined amount of Ni and Co, ΣTR2: 0% or more and 3.0% or less; and the combined amount of Cu, Ag, Sb, Pb, B, P, Ti, Nb, Zr, and W, Σγ: 0% or more and 1.00% or less. Furthermore, although V, Mn, Cr, and Mo are all transition metal elements, in this specification, V and Mn are sometimes designated as "transition metal element TR1," and Cr and Mo are sometimes designated as "transition metal element TR1'." Also, V, Mn, Cr, and Mo are sometimes collectively designated as "transition metal element TR."

[0029] Furthermore, in the molten-coated steel of this embodiment, during the elemental distribution profile of GDS analysis performed from the surface of the coating towards the steel in the thickness direction of the steel, the proportion of regions where ΣTR1 shows a depth of 0.1% or more relative to the thickness t of the coating, from the surface of the coating to the interface between the coating and the steel, is 10% or more. That is, the total depth where ΣTR1 shows 0.1% or more is 0.1t or more. In addition, the term "thickness t of the coating" here refers to the interval from the surface of the coating to the interface when the depth position where the Fe concentration relative to the maximum Fe concentration reaches 95% is defined as the interface between the coating and the steel in the aforementioned elemental distribution profile.

[0030] (Steel) The steel to be plated will be described. The steel is primarily steel sheet, but its size is not particularly limited. The steel sheet can be used in the usual molten zinc plating process. Specifically, steel sheets suitable for use in continuous molten zinc plating lines (CGLs) where the metal is immersed in molten metal and solidified are acceptable. The steel sheet size can be, for example, less than 10 mm thick and less than 2000 mm wide, but the size is not limited to these dimensions.

[0031] The type of steel is not particularly limited. Applicable steels include, for example: general steel, pre-plated steel with a thin coating of various metals, aluminum-deoxidized steel, very low carbon steel, high carbon steel, various high-tensile steels, locally high-alloy steel (steel containing corrosion-resistant strengthening elements such as Ni and Cr), mild steel wire, hard steel wire, spring steel, stainless steel cord, bolt steel, steel wire for bridge cables, and various steel plates, steel wires, or steel wires. More specifically, it can be applied to, for example: hot-rolled mild steel sheets and strips as shown in JIS G 3131:2018; cold-rolled steel sheets and strips as shown in JIS G 3141:2021; rolled steel products for general structural use as shown in JIS G 3101:2020; JIS H 8641:2021, JIS G 3302:2022, JIS G 3303:2022, JIS G 3313:2021, JIS G 3314:2022, JIS G 3315:2022, JIS G 3317:2022, JIS G Various coated steels thinly coated with various metals, such as 3321:2022 (hereinafter, the act of coating steel sheets for use as base steel is also called "pre-coating", and the coated steel or coated steel sheet used as base steel is also called "pre-coated steel or pre-coated steel sheet"), rolled steel for building structures as shown in JIS G 3136:2022, various high-tensile steels as shown in JIS G 3113:2018, JIS G 3134:2018, JIS G 3135:2018, etc., and locally high-alloy steels (steels containing corrosion-resistant strengthening elements such as Ni and Cr).

[0032] Alternatively, an Fe plating layer, Ni plating layer, or Co plating layer with an adhesion amount of 0.3~5 g / m2 can be pre-applied to the surface of the steel as a pre-plating layer. Using such steel with a pre-plating layer, the plating layer containing transition elements V and Mn of this embodiment can be stably formed. Details will be provided later.

[0033] Furthermore, the manufacturing steps of steel can include general steps such as ironmaking-steelmaking, hot rolling, pickling, cold rolling, and heat treatment through a blast furnace or electric furnace. However, the steel in this embodiment can undergo any of these steps. Moreover, the processing conditions for each step are not limited.

[0034] (Coating Layer) Next, the coating layer applied to the steel will be described. The coating layer of this embodiment contains a Zn-Al-Mg alloy layer. The presence of alloying elements such as Al and Mg in Zn improves corrosion resistance. Therefore, even a thin film with a Zn-Al-Mg alloy layer, such as about half the thickness of a typical Zn coating, will exhibit the same or better corrosion resistance as a Zn coating, even if it is a thin film. The coating layer may also contain an Al-Fe interface alloy layer.

[0035] The Zn-Al-Mg alloy layer is composed of Zn-Al-Mg alloy. The so-called Zn-Al-Mg alloy refers to a ternary alloy containing Zn, Al and Mg.

[0036] The thickness of the Zn-Al-Mg alloy layer can be 4~100μm. Depending on the requirements, the lower limit of the thickness of the Zn-Al-Mg alloy layer can also be 7μm, 10μm, 15μm or 20μm, and the upper limit of the thickness of the Zn-Al-Mg alloy layer can also be 90μm, 80μm or 70μm.

[0037] Regarding the Al-Fe system interface alloy layer, it is mostly an interface alloy layer located between the steel and the Zn-Al-Mg system alloy layer, and is in contact with the surface of the steel.

[0038] The overall thickness t of the plating layer is affected by the plating conditions, therefore the upper and lower limits of the overall thickness t of the plating layer are not particularly limited. Furthermore, the overall thickness of the plating layer is affected by the extraction speed of the steel from the plating bath and the wiping conditions. That is, the overall thickness t of the plating layer in the continuous melt plating method is affected by the viscosity and specific gravity of the plating bath. The maximum thickness t of the plating layer formed by the continuous melt plating method is mostly below 100 μm, therefore the plating thickness t of the melt-plated steel in this embodiment can also be, for example, below 100 μm, below 80 μm, below 70 μm, or below 60 μm. The thickness t of the plating layer can also be below 30% of the steel plate thickness, but it can also be below 20% of the steel plate thickness as needed. The thickness t of the coating layer can also be set to 15μm or more, 20μm or more, 30μm or more, 35μm or more, 40μm or more, or 45μm or more, depending on the requirements.

[0039] <Chemical Composition> Then, the average chemical composition of the coating layer is explained. If the coating layer is a monolayer structure of the Zn-Al-Mg series alloy layer, the average chemical composition of the overall coating layer is the average chemical composition of the Zn-Al-Mg series alloy layer. Also, if the plating layer is the layer structure of the Al-Fe series interfacial alloy layer and the Zn-Al-Mg series alloy layer, it is the combined average chemical composition of the Al-Fe series interfacial alloy layer and the Zn-Al-Mg series alloy layer. In addition, if a pre-plated application layer is pre-formed on the surface of the steel (mother plate) before plating, the composition of the components also comprises Ni, Co, etc. from the pre-coated coating layer.

[0040] In the coating layer of this embodiment, the thickness of the Al-Fe series interface alloy layer should be as small as 10% or less with respect to the overall thickness of the coating layer. Therefore, the Fe concentration of the coating layer is mostly within 5.0%.

[0041] Al: greater than 10% and less than 40% Al are elements that constitute the main body of the coating layer. When the Al content is greater than 10%, the melting point of the coating layer becomes higher than that of pure Zn. Also, the transition metal elements required to increase the thickness of the coating layer of the present embodiment are almost insoluble in Zn, Mg. Therefore, in order to contain transition metal elements, more than a certain amount of Al is required. The essential concentration for this minimum is greater than 10%, so the Al content is greater than 10%. Al is also the element required to form V-containing intermetallic compounds and / or Mn-containing intermetallic compounds (hereafter, V-containing intermetallic compounds and Mn-containing intermetallic compounds are also collectively referred to as TR1-containing intermetallic compounds). The Al content should be more than 11%, more preferably more than 12%, more preferably more than 18%, and more preferably more than 25%. On the other hand, if the Al content reaches more than 40%, the reactivity of the plating bath and the base iron will increase, becoming difficult to inhibit the Al-Fe system interface alloy layer, ensuring processability under a thick plating thickness will become serious. Furthermore, the thickness of the Zn-Al-Mg series alloy layer also decreases with the thick formation of the Al-Fe series interface alloy layer. Accordingly, the Al content was set to less than 40%. The Al content should be less than 38%, preferably less than 35%.

[0042] Mg: 4.0% to 15.0% Mg is an element that, like Zn, constitutes the main body of the plating layer. Mg is used to ensure the corrosion resistance of the plating layer. Mg enables the formation of the relatively hard and corrosion-resistant MgZn2 phase in the Zn-Al-Mg alloy layer. Mg is also an effective element that can bond with transition metal elements to improve various properties. When the Mg content is less than 4.0%, it becomes impossible to form suitable compounds with transition metal elements, and sometimes it promotes the formation of compounds between Al and transition metal elements, resulting in dross, which in turn prevents the formation of a sound plating layer. Therefore, the Mg content is set to 4.0% or more. The Mg content is preferably 4.5% or more, more preferably 4.8% or more, more preferably 5.0% or more, and even more preferably 6.0% or more. On the other hand, if the Mg content is greater than 15.0%, the amount of dross produced will increase, and the unplated areas without the plating layer may increase. Therefore, the Mg content is set to 15.0% or less. The Mg content should be below 13.0% or 10.0%.

[0043] Si: 0% to 2.0% Si has the function of suppressing the reaction between the plating bath and Fe. Generally speaking, if the plating bath contains a large amount of transition metal element TR, the reaction will become excessive. Based on this, by using Si, a certain degree of diffusion of transition metal element can be suppressed. On the other hand, if transition metal element and a large amount of Si are mixed, oxides will be locally generated at the interface and on the surface of the steel (Fe surface), and sometimes the plating layer will become loose, resulting in so-called "unplated". Accordingly, the Si content is 2.0% or less. The Si content is preferably 1.5% or less, 1.0% or less, or 0.30% or less. If a pre-plating layer is used, the reactivity with the substrate can be sufficiently suppressed even if Si is not present. Since Si can also be absent, the lower limit of the Si content is 0%. The Si content is preferably 0.01% or more, and more preferably 0.1% or more.

[0044] V: 0% or more and 3.0% or less; Mn: 0% or more and 3.0% or less; total amount of V and Mn; ΣTR1: 0.05% or more and 3.0% or less of V and Mn (transition metal element TR1). If added to a Zn-based plating bath containing Al and Mg, the specific gravity of the plating bath will increase, thus increasing the viscosity of the plating bath. As a result, when the steel is lifted from the plating bath, the amount of molten metal adhering to the steel will increase, and the thickness of the plating layer will increase. Furthermore, by containing the transition metal element TR1 in the plating bath, Al-Mg-TR1 intermetallic compounds (intermetallic compounds containing TR1), represented by Al18(TR1)2Mg3, will be formed in the early stage of the solidification of the plating phase, which will further promote the increase of the plating layer thickness. While it's difficult to distinguish which effect—the increased thickness, higher specific gravity, and higher viscosity—results from the formation of intermetallic compounds, both can be considered effects of adding transition metals. When Al and Mg are within appropriate concentration ranges, these effects tend to increase plating adhesion when the transition metal element TR1 is 0.05% or higher. Therefore, the contents of V and Mn (transition metal element TR1) should ideally be 0.05% or higher. More preferably, the contents of each transition metal element TR1 should be 0.1%, 0.3%, 0.5%, 0.7%, or 1.0% or higher.

[0045] On the other hand, intermetallic compounds containing TR1 can form suspended dross in the plating bath. Suspended dross can lead to plating defects such as unplated areas and dross adhesion defects, significantly affecting the product's morphology and performance, and deteriorating its corrosion resistance. Therefore, it is advisable to completely dissolve this suspended dross in the plating bath. Thus, in this embodiment, the plating bath temperature containing the transition metal element TR1 must be set to 570°C or higher. Preferably, it should be 600°C or higher. When the content of the transition metal element TR1 is greater than 3.0%, even with increased plating bath temperature, it is still difficult to completely dissolve this suspended dross; furthermore, the viscosity of the plating bath becomes extremely high, and when the steel is lifted from the plating bath, the amount of molten metal adhering to it decreases, resulting in an extremely thin plating layer. Furthermore, the plating appearance deteriorates significantly. Therefore, the contents of V and Mn (transition metal element TR1) are each set to 3.0% or less. Ideally, the content of each transition metal element TR1 should be below 2.5% and below 2.0%.

[0046] In this embodiment, the chemical composition of the plating layer may contain at least one of V and Mn, or both. In this embodiment, to obtain the aforementioned effects brought about by V and Mn, the total concentration ΣTR1 of V and Mn is set to 0.03~3.0%. It is not necessary to contain both V and Mn; the concentration of either V or Mn may be within this range. Therefore, the lower limit of the concentration of V and Mn is 0%. The total concentration ΣTR1 is preferably 0.2% or more, 0.3% or more, 0.5% or more, 0.7% or more, or 1.0% or more. Furthermore, the total concentration ΣTR1 is preferably 2.5% or less, or 2.0% or less.

[0047] Furthermore, in this embodiment, if the plating layer contains Ca, Al10Ca(TR1)2 may sometimes be formed as an intermetallic compound containing TR1. However, in terms of crystal structure and atomic arrangement, Al10Ca(TR1)2 is difficult to distinguish from Al18Mg3(TR1)2, and it can also be considered as a compound obtained by replacing part of the Mg in Al18Mg3(TR1)2 with Ca. Therefore, in this embodiment, it is not a problem to discuss the two as similar substances.

[0048] Cr: 0% or more and 3.0% or less; Mo: 0% or more and 3.0% or less. Cr and Mo (transition metal element TR1') are elements with effects similar to V and Mn. When V and Mn are present, a portion of V and Mn will substitute with Cr and / or Mo, thus exhibiting the same effects as V and Mn. Accordingly, Cr and Mo are defined as transition metal elements TR1'. The content of each of Cr and Mo is set to 3.0% or less, the same as that of V and Mn. Preferably, the content of each of Cr and Mo is 2.5% or less and 2.0% or less. Alternatively, the content of each of Cr and Mo can be 0%. The content of each of Cr and Mo is preferably 0.05% or more, more preferably 0.1% or more, 0.3% or more, 0.5% or more, 0.7% or more, or 1.0% or more.

[0049] Cr, Mo, V, and Mn, as transition metal elements (TR), all form intermetallic compounds with similar characteristics. However, by examining each element and confirming the properties of their respective intermetallic compounds, slight differences in their properties become apparent. Intermetallic compounds containing Cr and Mo (transition metal element TR1') tend to have a lower sacrificial corrosion protection effect. This can be attributed to the predictable high potential of the formed intermetallic compounds due to the oxide film of Cr and Mo, which tends to slow down the dissolution time point within the plating layer during corrosion. On the other hand, it has been learned that V and Mn (transition metal element TR1) do not have the function of strengthening the oxide film of intermetallic compounds, and therefore dissolve at an appropriate time point during plating corrosion, making a clear contribution to corrosion resistance. It has also been learned that even intermetallic compounds dominated by Cr and / or Mo, by containing a certain amount of Mn and V, can suppress the aforementioned delay in corrosion time point caused by the oxide film. Therefore, by incorporating V and Mn (transition metal element TR1) into intermetallic compounds mainly composed of Cr and / or Mo, the corrosion time of the intermetallic compounds can be controlled.

[0050] Furthermore, in this embodiment, in order to obtain the aforementioned effects brought about by Cr and Mo, the combined concentration of Cr and Mo, ΣTR1', is 0% or more and 3.0% or less. It is not necessary to contain both Cr and Mo; the concentration of either Cr or Mo may be within this range.

[0051] ΣTR1+ΣTR1': 0.05% or more and 3.0% or less As described above, if V, Mn, Cr, and Mo, which are transition metal elements TR, are added to a Zn-based plating bath containing Al and Mg, the specific gravity of the plating bath will increase and the viscosity of the plating bath will increase. Therefore, the thickness of the obtained plating layer can be increased. Therefore, the total content of transition metal elements TR1 and TR1' is 0.05% or more. On the other hand, when the total content of transition metal elements TR1 and TR1' (ΣTR1+ΣTR1') is greater than 3.0%, the viscosity of the plating bath will become extremely high. When the steel is lifted from the plating bath, the amount of molten metal adhering will decrease, and the thickness of the plating layer will become extremely thin. Therefore, the total content of transition metal elements TR1 and TR1' is set to 3.0% or less. It is preferable that the total content of transition metal elements TR1 and TR1' is 2.5% or less and 2.0% or less.

[0052] Element group α: Sn: 0% or more and 0.7% or less; Bi: 0% or more and 0.3% or less; In: 0% or more and 0.3% or less; Total amount of Sn, Bi, and In Σα: 0% or more and 0.7% or less. Each element in element group α (Sn, Bi, In) is an element that, when present in the plating layer, will cause the plating layer to soften. Since Sn, Bi, and In are elements that can be included arbitrarily, their respective content limits are 0%. If Sn is present, Mg9Sn5 will tend to form in the plating layer. Bi will also form Mg3Bi2, and In will also form Mg3In, etc. Element group α can be included as needed for the purpose of improving corrosion resistance, processability, and other properties.

[0053] However, there is an upper limit to the content of each element in element group α. If it is present in large quantities, it will absorb Mg from the intermetallic compound containing TR1, thus preventing the formation of TR1-containing intermetallic compounds and deteriorating the corrosion resistance. Alternatively, the appearance of the plating layer may become inferior due to the formation of intermetallic compounds with element group α. Therefore, the upper limit for element group α is set to 0.7% or less for Sn, and 0.3% or less for Bi and In. In addition, the total amount of Σα is also set to 0.7% or less. The total amount of Σα should preferably be 0.5% or less and 0.4% or less.

[0054] Next, the elements of element groups β and γ described below are all arbitrarily added elements, and their lower limit is set to 0% or more.

[0055] Element group β: Ca: 0%~0.6% Y: 0%~0.3% La: 0%~0.3% Ce: 0%~0.3% Sr: 0%~0.3% Li: 0%~0.3% Total amount of Ca, Y, La, Ce, Sr and Li ΣY: ≥0% and ≤0.6%

[0056] As one of the elements in group β, Ca has various effects in the plating bath, so it is advisable to include it. Ca is the element in the plating bath that most easily bonds with transition metals. Therefore, in a plating bath containing Ca and transition metals, depending on the content of element group β, Al18(Mg,Ca)3(TR1)2 or Al10Ca(TR1)2, in which a portion of Mg is replaced by Ca, will be formed. If such intermetallic compounds are formed, the plating viscosity increases, and a very thick plating layer can be easily obtained.

[0057] If the Ca content is greater than 0.6%, various suspended dross will form in the plating bath, increasing plating defects. Furthermore, the viscosity of the plating bath will become extremely high, resulting in reduced molten metal adhesion to the steel when lifted from the bath, leading to an extremely thin plating layer and deterioration of corrosion resistance. Additionally, the appearance of the plating will be significantly worse. Therefore, the Ca content is set to 0.6% or less. The Ca content should preferably be 0.5% or less, or 0.4% or less.

[0058] Elements in element group β other than Ca can achieve roughly the same effect as Ca and can therefore be included as substitutes for Ca. However, the price of elements other than Ca is high, and if they are included in large quantities as Ca, it may impair economic efficiency; the total content of elements other than Ca should not exceed the content of Ca. Intermetallic compounds formed by elements in element group β other than Ca will form Al10(β)(TR1)2 as a substitute for Al10Ca(TR1)2. If Ca or any one of these elements is included, almost no difference in corrosion resistance or performance can be observed. In the case of high concentrations, just like with Ca, Mg will be absorbed from the intermetallic compounds containing TR1, and the compound will not be formed, resulting in a thinner plating thickness; therefore, there is an upper limit to the concentration. Therefore, the concentration of each element in element group β other than Ca is set to 0~0.3%. The concentration of each element in element group β other than Ca should preferably be 0.2% or less and 0.1% or less. Furthermore, the total concentration Σβ of all elements in element group β, including Ca, is set to 0~0.6%. The total concentration Σβ should preferably be below 0.5% or 0.4%.

[0059] Ni: 0% or more and 3.0% Co: 0% or more and 3.0% Total amount of Ni and Co ΣTR2: 0~3.0% or less Ni and Co are elements that can be used as materials constituting the pre-plating layer. When a Ni layer or Co layer is provided as a pre-plating layer on the steel (base plate) before plating, they will eventually be included in the plating layer. Generally speaking, when a metal layer containing Ni or Co is provided as a pre-plating layer on the plating base plate, Ni or Co in the range of 3.0% or less will mostly be mixed into the plating layer. In addition, these pre-plating layers will eventually exist in the form of a layer at the interface between the plating layer and the steel. Therefore, in terms of the composition concentration in the Zn-Al-Mg alloy layer, the concentration of Ni and Co is low. In addition, Ni and Co can also be added to the plating bath in trace amounts (e.g., 0.25% or less) in addition to the pre-plating layer.

[0060] Element group γ: Cu: 0% or more and 0.25% or less; Ag: 0% or more and 0.25% or less; Sb: 0% or more and 0.25% or less; Pb: 0% or more and 0.25% or less; B: 0% or more and 0.50% or less; P: 0% or more and 0.50% or less; Ti: 0% or more and 0.25% or less; Nb: 0% or more and 0.25% or less; Zr: 0% or more and 0.25% or less; W: 0% or more and 0.25% or less; The total amount of Cu, Ag, Sb, Pb, B, P, Ti, Nb, Zr and W Σ: 0% or more and 1.00% or less. Elements in element group γ can be added arbitrarily. When the concentration of any one of these elements is 0.10% or more, the effect of improving corrosion resistance can be obtained. Therefore, even for any one of these elements, the concentration should preferably be 0.10% or more. However, if the total concentration of these elements, Σγ, becomes excessive, plating defects such as incomplete plating may occur. Therefore, the total concentration of Σγ is set to 1.00% or less. The total concentration of Σγ should preferably be 0.50% or less. From the viewpoint of corrosion resistance, the concentrations of Cu, Ag, Sb, Pb, Ti, Nb, Zr, and W are each set to 0.25% or less. The concentrations of B and P are each set to 0.50% or less.

[0061] Fe: 0% or more and 5.0% or less. Since the molten steel of this embodiment is manufactured using a continuous molten steel plating method, Fe diffuses from the plating material into the plating layer during manufacturing. As mentioned earlier, in this embodiment, the Al concentration in the plating layer is high, sometimes forming an Al-Fe interfacial alloy layer, but its thickness is thin. Furthermore, if an Fe pre-plating layer is used, Fe will inevitably diffuse into the plating layer, thus Fe will be contained in the Zn-Al-Mg alloy layer. As a result, the plating layer may sometimes contain up to 5.0% Fe. While the Fe concentration can also be 0%, if the Fe concentration is 5.0% or less, it will not affect the frequency of cracking in the plating layer. Therefore, the Fe content is set to 5.0% or less. The Fe concentration is preferably 4.0% or less, 3.0% or less, 2.0% or less, or 1.0% or less. The Fe content can also be greater than 0%.

[0062] Residual Portion: Zn and Impurities The residual portion contains Zn and impurities. Since the molten-coated steel of this embodiment is a highly versatile Zn-based coated steel, Zn is the main phase constituting the coating. The Zn concentration does not need to be specifically specified, but it can be set to 30-96% by mass. Depending on the requirements, its upper limit can be set to 90%, 80%, 70%, or 60%, and its lower limit can be 35%, 45%, 50%, or 55%.

[0063] Impurities are components contained in the raw materials or components mixed in during the manufacturing process, and refer to components that are not intentionally present. For example, due to atomic diffusion between the steel (base iron) and the plating bath, components other than Fe may sometimes be mixed in trace amounts as impurities in the plating layer. Furthermore, plating alloys are usually manufactured using metals with a purity of 3N, so the total concentration of impurities can also be set to about 0.03% or less.

[0064] The average chemical composition of the plating layer is identified by using an acid containing an inhibitor that suppresses corrosion of the base iron (steel). The plating layer is peeled and dissolved using this acid to obtain an acid solution. The resulting acid solution is then analyzed by ICP-MS or ICP-luminescence spectroscopy to determine its chemical composition. There are no particular limitations on the type of acid; any acid capable of dissolving the plating layer is acceptable. Pre-measuring the area and weight before and after peeling also allows for the simultaneous determination of the plating adhesion amount (g / m²).

[0065] <Intermetallic Compounds> Next, the intermetallic compounds contained in the plating layer will be explained. Since the plating layer of this embodiment is a Zn-Al-Mg alloy plating, it contains Zn phase, Al phase, and MgZn2 phase. The plating layer of this embodiment also contains: an intermetallic compound containing the transition metal element TR1 (an intermetallic compound containing TR1).

[0066] MgZn2 phase MgZn2 phase is a phase formed in the coating to achieve high corrosion resistance. MgZn2 phase is a hard phase that brings corrosion resistance to the coating.

[0067] Zn Phase (Al-Zn Phase, Zn-Al Phase) The Zn phase exists in the coating layer, mainly in the form of a ternary eutectic structure (Zn / Al / MgZn2 ternary eutectic structure). Furthermore, since the coating layer contains a large amount of Al, Al microcrystals and Zn microcrystals will mix. If the amount of Al phase is high, the Zn phase will become an Al-Zn phase (mixed phase). On the other hand, if the Zn phase contains a high amount of Al phase, it will be a Zn-Al phase (mixed phase). Since the amount of this phase is relatively low compared to the overall coating layer, it is often impossible to confirm any deterioration or improvement in corrosion resistance. In addition, the phase composed of Zn and Al is highly processable.

[0068] The Al phase exists in the form of fine grains in the Zn-Al and Al-Zn phases mentioned above, and it can also exist in ternary eutectic structures. Furthermore, in regions with high Al concentrations, grains will grow, sometimes forming coarse Al phases.

[0069] Intermetallic compounds containing transition metal elements (TR1) (TR1-containing intermetallic compounds) The plating layer of this embodiment contains, for example, (Al,Zn)18(Mg)3(TR1)2, a TR1-containing intermetallic compound. This TR1-containing intermetallic compound is formed in the plating layer because the plating layer contains TR1. Furthermore, when the plating layer contains Ca, TR1 may sometimes bond with Ca and Al to form Al10Ca(TR1)2. Furthermore, when Ca is insufficient, TR1 may bond with Al and Mg to form Al18Mg3(TR1)2. Furthermore, since Al and Zn have similar atomic radii, and Mg and Ca have similar atomic radii, sometimes (Al,Zn)18(Ca,Mg)3(TR1)2, in which a portion of Al is replaced by Zn and a portion of Mg is replaced by Ca, may exist in the plating layer. Furthermore, in areas of insufficient Ca in the plating layer, (Al,Zn)18(Mg)3(TR1)2 may sometimes be formed.

[0070] The crystal structures of these intermetallic compounds are all similar and difficult to distinguish from one another. Even when XRD is used, diffraction peaks appear at almost the same positions for these intermetallic compounds. However, from the viewpoint of enjoying the effects of this embodiment, there is no particular problem in forming any one of the intermetallic compounds, and their properties are substantially equal when included in a plating layer, so there is no need to differentiate them. Details will be provided later.

[0071] By adding TR1 to the plating bath, the viscosity of the plating bath is greatly increased, and the amount of plating bath carried up when the steel is lifted from the plating bath is increased. That is, according to this embodiment, by using TR1 as a constituent element of the plating layer, a very thick plating steel sheet can be manufactured, and a plating steel sheet with excellent plating life can be easily manufactured.

[0072] To achieve this effect, it is effective to have a certain or higher content of the intermetallic compound containing TR1 in the plating layer. Specifically, in a cross-section along the thickness direction of the plating layer, the total area fraction of the TR1-containing compound should preferably be 1% or higher. By increasing the total area fraction of the TR1-containing compound, sufficient plating thickness can be ensured, while obtaining a plating layer with excellent adhesion. In plating layers produced by appropriate manufacturing methods, this area fraction is mainly proportional to the added concentration of the transition metal TR1. The TR1-containing compound can be observed using a scanning electron microscope (SEM). Details of the determination of the area fraction of the intermetallic compound, such as the TR1-containing compound, will be explained later.

[0073] Furthermore, if the plating layer contains a large amount of these TR1-containing intermetallic compounds, the TR1-containing intermetallic compounds will grow to a large size. In this case, the intermetallic compounds can be detected even by XRD.

[0074] Regarding the role of TR1-containing intermetallic compounds, in addition to increasing the thickness of the plating layer, during the corrosion process of the plating layer, transition metals are incorporated into the corrosion products. The effect of the corrosion products is particularly evident in sacrificing corrosion resistance. For example, regarding plating layers containing coarse TR1-containing intermetallic compounds, these coarse TR1-containing intermetallic compounds coexist with Mg in the cut surfaces of the plated steel, thereby stabilizing the formation of corrosion products and inhibiting the formation of red rust from the cut surfaces in the early stages of corrosion. Furthermore, as the TR1-containing intermetallic compounds grow larger, unevenness appears on the plating surface, and the appearance also changes.

[0075] The presence of TR1-containing intermetallic compounds can be indirectly confirmed by a highly sensitive and quantitative detection method, namely GDS (glow discharge emission spectrometry).

[0076] The transition metal elements remaining in the plating layer will preferentially form these intermetallic compounds. On the other hand, since transition metal elements easily diffuse into Fe, if a large amount of transition metal elements diffuse into the interior of the steel (interior of Fe), the amount of transition metal elements in the plating layer will decrease. In addition, even when the concentration of TR1 is low, there are fine grains, and TR1 is distributed throughout the plating layer, it can be easily detected.

[0077] Furthermore, when the plating layer contains a large amount of TR1 intermetallic compound, the cross-section of the plating layer can be observed using a scanning electron microscope (SEM) to directly identify the intermetallic compound. At the same time, by measuring the area fraction of the contained intermetallic compound, the magnitude of the effect of the contained compound can be identified. These measurement methods are described later.

[0078] Other intermetallic compounds containing Ca and Si may form Al-Ca-Si compounds in addition to the aforementioned TR1-containing metal compounds. Furthermore, in TR1-containing metal compounds, a portion of Al may be replaced by Zn. Ca may also be partially replaced by Mg.

[0079] Although Si readily forms Mg2Si, the presence of Ca will lead to the formation of Al-Ca-Si compounds. However, if the Si content exceeds the Ca concentration, Mg2Si will form. However, since Mg2Si reduces corrosion resistance, it is not advisable for it to form in large quantities. However, even trace amounts of Mg2Si result in a low degree of degradation in the overall corrosion resistance of the coating.

[0080] Sn and In tend to bond with Mg, forming Mg2Sn, Mg9Sn5, and Mg3In. As mentioned above, these are sacrificial phases with excellent corrosion resistance. If Ni is present, Al3Ni may also be present. In this embodiment, the elements constituting the plating layer are grouped according to their atomic radius and properties. Therefore, if other elements are present, substitutions of these representative intermetallic compounds may be formed. Generally, these intermetallic compounds do not occupy the main phase in the plating layer, so their impact on performance is relatively low.

[0081] Next, the method for identifying intermetallic compounds containing the transition metal element TR1 in the plating layer of this embodiment will be explained. Furthermore, in the evaluation, to eliminate the influence of the solidification and microstructure characteristics of the plating layer, the plating thickness of the sample used for observing the plating layer profile by SEM, GDS, or XRD should preferably be set to 25 μm (±2 μm). In this manner, a relative evaluation of the various plating components can be performed.

[0082] In the method of compositional analysis in the depth direction within the coating layer, a glow discharge emission spectrometer (GDS) can be used. Although the inventors of this case used a LECO JAPAN 850A as the glow discharge emission spectrometer, the measuring device is not limited to this. When performing depth direction analysis, it is advisable to perform the analysis while performing Ar sputtering; the analysis conditions are set as follows: argon pressure: 0.27 MPa, output power: 30 W, output voltage: 1000 V, discharge area: a circular area with a diameter of 4 mm. The measurement is carried out from the surface of the coating layer towards the depth direction until the Fe concentration reaches 100% (reaching the base iron). Accordingly, the analytical range of depth direction analysis using GDS will be from the coating surface to the Zn-Al-Mg coating layer, and depending on the situation, it will be from the coating surface to the Fe-Al alloy layer and pre-coating layer existing between the coating layer and a portion of the steel. Following GDS analysis, the sputtering depth of the profile was determined using a "surfcom130A" manufactured by Tokyo Seimitsu Co., Ltd. GDS analysis provides an elemental distribution profile along the depth direction of the coating. With the total detected elements set at 100%, the depth-direction content distribution of each element is displayed in the elemental distribution profile. The boundary (interface) between the coating and the steel is defined as the location (t) where the Fe content is greater than 95% by mass; areas deeper than this location are identified as base iron (steel).

[0083] In this embodiment, when performing GDS analysis from the surface of the plating layer toward the steel, the interval from the surface of the plating layer to the interface in the elemental distribution profile is defined as the thickness t of the plating layer. At this time, in the region from the surface of the plating layer to the interface, there exists a region with an intensity of 0.1t or more where the total concentration ΣTR1 of the transition metal element TR1 is 0.1% or more (relative to the total thickness of the plating layer being 10%). In addition, when the plating layer contains two or more transition metal elements TR1, the total concentration ΣTR1 of the transition metal elements TR1 is obtained from the obtained elemental distribution profile as a total (concentration) intensity profile of the transition metal elements TR1.

[0084] Figure 1 shows an example of the results of depth direction analysis using GDS in the plating layer of this embodiment. The graph shown in Figure 1 is an elemental distribution profile. In the example shown in Figure 1, the plating thickness t is 26 μm.

[0085] Within the aforementioned evaluation range, that is, in the region from the surface of the coating to the interface, if an area where ΣTR1 shows 0.1% or more continuously exists in the depth direction for 0.1t or more, then a clear increase in viscosity can be observed during the manufacturing of the coating. Specifically, under the same coating conditions with constant wiping and board-passing speeds, the thickness of the coating will increase. Considering the corrosion of the coating, from a corrosion resistance point of view, an increase in coating thickness is preferable as it leads to an improvement in the corrosion resistance period of the steel.

[0086] Furthermore, as explained above, in order to achieve a proper distribution of transition metal elements in the plating layer, it is effective to properly control the manufacturing conditions. That is, it is effective to properly manage the plating bath temperature, plating substrate, etc.

[0087] Furthermore, when there are pre-plated layers such as Ni and Co on the plating substrate, the presence of Ni and Co layers can be confirmed by GDS even after plating.

[0088] In this embodiment, when Ni and Co in the chemical composition are defined as transition metal elements TR2, in the elemental distribution profile, the depth showing the maximum concentration of TR2 is defined as t(TR2), and the depth showing the maximum concentration of TR1 is defined as t(TR1). At this time, the following equation (1) should be satisfied.

[0089] t(TR2)-t(TR1)≦0.30t・・・(1)

[0090] It has been clarified that when Equation (1) is satisfied, that is, when the residual layers of transition metal elements TR1 and TR2 can be observed near the interface, excellent adhesion and high sacrificial corrosion resistance can be obtained. This can be attributed to the presence of a large amount of intermetallic compounds containing TR1, which can suppress the exposure of the base steel and have excellent sacrificial corrosion resistance near the interface. In order to satisfy Equation (1), it is effective to pre-set a Ni layer or Co layer, etc., and ensure an adhesion amount sufficient to suppress the diffusion of transition metals.

[0091] The method for determining the area ratio of intermetallic compounds containing the transition metal element TR1 (TR1-containing intermetallic compounds) is to use an electron beam microanalyzer (EPMA). The cross-section of the plating layer is exposed, and the vertical cross-section along the depth direction of the plating layer is observed using a scanning electron microscope attached to the EPMA to identify the regions where V and Mn are present. These identified regions are identified as TR1-containing intermetallic compounds. Further quantitative analysis of these identified regions is performed using point analysis or similar methods, and the intermetallic compounds are identified based on atomic percentages. For example, although the intermetallic compounds obtained in this embodiment are Al10Ca(TR1)2, Al18Mg3(TR1)2, etc., atomic ratios similar to these are acceptable. Furthermore, when the concentration of the transition metal element is low, it may become fine grains, making EPMA analysis sometimes difficult. In such cases, TEM or similar methods can be used to identify the intermetallic compounds from electron beam diffraction images. After the intermetallic compounds are identified, the total area fraction of the intermetallic compounds containing TR1 is calculated.

[0092] More specifically, an elemental distribution image is obtained through EPMA with a 1000x magnification field of view. Since the distribution image of the transition metal being tested corresponds to the location of the intermetallic compound, the area fraction of the intermetallic compound in the plating layer can be determined from the elemental distribution image. In this embodiment, the plating layer contains at least 1% TR1-containing intermetallic compound in terms of area fraction, thereby tending to increase the amount of plating layer adhered. Preferably, the area fraction of TR1-containing intermetallic compound is 5% or more, 15% or more, or 25% or more. From the viewpoint of improving sacrificial corrosion resistance, it is preferable to contain at least 15%. Regarding the area fraction within the plating layer, it can be seen that it is related to the concentration of the added transition metal element.

[0093] After obtaining the elemental distribution image of each element, it is analyzed using image analysis software such as ImageJ. For the Zn elemental distribution image (in this embodiment, a trace amount of Zn is distributed in any part of the plating layer), it is binarized, and a threshold value is set in such a way that the entire plating layer is white or black. Then, the number of pixels of Zn distribution is calculated in pixels.

[0094] Next, for the distribution image of the transition metal element TR1, after binarization, the number of TR1 pixels is obtained in a way that can identify the intermetallic compounds containing TR1 (the area where the detection location of TR1 coincides with the location of the intermetallic compounds in the SEM image). The value obtained by dividing the number of TR1 pixels by the number of Zn pixels represents the area fraction of the intermetallic compounds containing TR1. The same operation is performed for 20 fields of view. That is, in the cross-section of the coating layer, 20 fields of view are randomly selected from different fields of view, the area fraction is calculated in each field of view, and the average value is calculated.

[0095] Next, it is described that the index of intermetallic compounds containing TR1 is obtained by X-ray diffraction.

[0096] Regarding X-ray diffraction measurement, since its detection sensitivity is lower than that of GDS analysis, the prerequisite is that the coating contains intermetallic compounds with a certain amount or more of transition metal elements. Specifically, the area ratio of intermetallic compounds containing TR1, as determined by the aforementioned EPMA, should be 5% or more. That is, when crystal sizes showing a specific orientation appear within the coating, these intermetallic compounds can be detected even by XRD. If nuclei grow inside the coating and crystals with a specific orientation grow within the coating, an uneven surface will be formed on the coating surface, and metallic reflections will be suppressed, making it easy to obtain a homogeneous appearance with a concrete-like texture. This is a suitable property for coated steel sheets with moderately reduced gloss and used as building materials in various applications.

[0097] In X-ray diffraction measurement, the X-ray diffraction pattern of the coating surface measured using Cu-Kα line and with X-ray output of 50kV and 300mA shall satisfy I1 as specified in equation (4) and I2 as specified in equation (5) as specified in equation (2) and (3) respectively.

[0098] 1.5≦I1…(2) 1.5≦I2…(3) I1=(Imax(10.5°~11.0°) / (I(10.5°)+0.2{|(I(11.0°)-I(10.5°)|})…(4) I2=(Imax(20.2°~20.5°)) / (I(20.2°)+0.667{|I(20.5°)-I(20.2°)|})…(5)

[0099] Where Imax(k~m°) is the maximum X-ray diffraction intensity between diffraction angles k~m°, I(n°) is the X-ray diffraction intensity at diffraction angle n°, and k, m, and n are the diffraction angles (2θ) shown in equations (4) and (5), respectively.

[0100] I1 and I2 in formulas (4) and (5) refer to the peaks of intermetallic compounds represented by (TR1)2Mg3Al18, including, for example, intermetallic compounds defined in JCPDS Card #00-040-1153 (V2Mg3Al8) or #04-007-9047 (Mn2Mg3Al18), #04-008-7169 (Cr2Mg3Al18). However, even if the elements of (TR1) and (TR1´) are changed, the diffraction peak positions remain almost unchanged. Therefore, these intermetallic compounds can be treated as the same type; moreover, the diffraction positions of I1 and I2 do not overlap with other diffraction peaks, and are therefore suitable for detection. That is, I1 and I2 represent the relevant specifications for X-ray diffraction peaks of V-containing intermetallic compounds and Mn-containing intermetallic compounds. Furthermore, when discussing Mo-containing intermetallic compounds, CaMo₂Al₂O is defined according to JCPDS Card #00-051-1061. Additionally, these intermetallic compounds are difficult to distinguish from each other due to the similarity of their diffraction peaks; that is, Al and Mg are mutually soluble, as are Zn and Ca. Therefore, it can be assumed that their compositions and structures are roughly the same, and their diffraction peaks appear in roughly the same positions. Accordingly, these transition metal-containing intermetallic compounds do not need to be distinguished from each other in X-ray diffraction measurements.

[0101] On the other hand, in the plating layer of this embodiment, most of the diffraction peaks of these TR1-containing intermetallic compounds repeat the diffraction peaks of other major constituent phases, namely MgZn2, Al, and Zn. Accordingly, among most of the diffraction peaks of the TR1-containing intermetallic compounds, attention must be paid to diffraction peaks that do not repeat with other constituent phases. One of these is the diffraction peak around 2θ = 10.6°, and the other is the diffraction peak around 20.4°.

[0102] Equation (4) is a formula focusing on the diffraction peak near 2θ = 10.6°. In Equation (4), Imax (10.5°~11.0°) is the maximum value of X-ray diffraction intensity between the diffraction angles of 10.5° and 11.0°. Imax (10.5°) is the X-ray diffraction intensity at the diffraction angle of 10.5°, and Imax (11.0°) is the X-ray diffraction intensity at the diffraction angle of 11.0°.

[0103] Also, Equation (5) is a formula focusing on the diffraction peak near 2θ = 20.4°. In Equation (4), Imax (20.2°~20.5°) is the maximum X-ray diffraction intensity between the diffraction angles of 20.2° and 20.5°. Imax (20.2°) is the X-ray diffraction intensity at the diffraction angle of 20.2°, and Imax (20.5°) is the X-ray diffraction intensity at the diffraction angle of 20.5°.

[0104] The molecules in formulas (4) and (5) are equivalent to the diffraction peak intensities of compounds containing TR1, and are the maximum diffraction peak intensities including the background intensity. Due to the measurement error of X-ray diffraction, the diffraction peaks may sometimes deviate from 10.6° or 20.4°, so the maximum values ​​between 10.5°~11.0° and 20.2°~20.5° are adopted.

[0105] The denominators of equations (4) and (5) are obtained as follows: the background intensity at a diffraction angle of 10.6° or 20.4° is calculated from the diffraction intensity at a diffraction angle of 10.6° or 20.4°.

[0106] For example, regarding the denominator of equation (4), as shown in Figure 2, draw a straight line connecting the 10.5° diffraction ray and the 11.0° diffraction ray. This straight line is the baseline of the diffraction peak. Next, calculate the absolute value of I(10.5°) - I(11.0°). Also, calculate the ratio (0.1 / 0.5 = 0.2) of the difference between diffraction angles 10.5° and 10.6° (0.1°) to the difference between diffraction angles 10.5° and 11.0° (0.5°). Then, calculate the background intensity of the diffraction angle 10.6° using the formula recorded in the denominator of the above equation (4).

[0107] Regarding the denominator of equation (5), as shown in Figure 2, draw a straight line connecting the 20.2° diffraction ray and the 20.5° diffraction ray. This straight line is the baseline of the diffraction peak. Next, calculate the absolute value of I(20.2°) - I(20.5°). Also, calculate the ratio of the difference between the diffraction angles 20.2° and 20.4° (0.2°) to the difference between the diffraction angles 20.2° and 20.5° (0.3°) (0.2 / 0.3 = 0.667). Then, calculate the background intensity of the diffraction angle 20.4° using the formula recorded in the denominator of equation (5) above.

[0108] The conditions for obtaining X-ray diffraction images are defined as follows.

[0109] X-ray diffraction using Cu as the target material and X-ray source is most suitable because it can obtain average information about the constituent phases in the coating. For example, the X-ray conditions are set to a voltage of 50 kV and a current of 300 mA. There are no particular limitations on the X-ray diffraction apparatus, but a sample-level high-power X-ray diffraction apparatus such as the RINT-TTR III manufactured by Rigaku Corporation can be used.

[0110] [Manufacturing Method of Hot-Dip Coated Steel] Next, the manufacturing method of the hot-dip coated steel according to this embodiment will be described. The hot-dip coated steel of this embodiment is preferably manufactured by a continuous hot-dip coating method. In addition, due to the limitation of steel size, it can also be manufactured by a batch hot-dip coating method as needed.

[0111] <Method A: Sendzimir Method> In this embodiment, as described above, the diffusion of the transition metal element TR1 added to the bath is sufficiently suppressed, and the transition metal element TR1 remains inside the plating layer, thus forming a specific intermetallic compound (an intermetallic compound containing TR1) within the plating layer. However, if the plating layer is formed using the conventional Sendzimir method, the diffusion of the transition metal element TR1 towards the steel surface (Fe surface) becomes active. During immersion in the plating bath, the transition metal element TR1 accumulates on the Fe surface, easily losing its reduction effect and producing an oxide film composed of the transition metal element TR1. This state of plating layer is related to the occurrence of unplated areas or peeling during processing, and is therefore undesirable.

[0112] <Preparation Method B> Therefore, the inventors reviewed the optimal preparation conditions and found that, in order to effectively retain the transition metal element TR1 within the plating layer, it is effective to form a physical diffusion barrier on the plating substrate. Furthermore, the so-called "physical diffusion barrier" is, for example, a surface on the Fe surface of the plating substrate that is mismatched with the Fe crystal structure, precipitated through electroplating. Specifically, an Fe plating layer (e.g., 0.5~2.0 g / m²) is attached to the Fe surface of the plating substrate. This creates a grain continuity between the Fe surface and the Fe plating layer, achieving a temporary anti-diffusion effect. The plating substrate with the Fe plating layer is then immersed in a high-temperature bath, whereby the Fe melts and diffuses into the plating bath. Most of the Fe then becomes the nucleus of the intermetallic compound and promotes its growth, leaving almost no residue, effectively ensuring that the transition metal element TR1 remains within the plating layer. In method B, which simultaneously promotes the melting of the Fe layer, the plating bath temperature should ideally be in the range of 590~620°C. Outside this temperature range, adverse effects may occur on unplated areas and adhesion. Furthermore, since a portion of the transition metal element TR1 diffuses to the Fe surface, the oxidation conditions of the plating substrate must be carefully controlled. Specifically, setting the dew point above 0°C effectively hinders the reaction between oxygen and the transition metal element TR1. On the other hand, in method A (Sendzimir process), the unplated elements Cr, Mo, Mn, and V tend to accumulate instantaneously at the interface during the reaction, easily leading to unplated areas. Moreover, method A (Sendzimir process) is unsatisfactory because it fails to demonstrate the function and effect of the Fe plating layer; the function and effect is to prevent these elements from accumulating at the interface during the reaction and instead remain in the plating bath solution.

[0113] <Preparation Method C> Furthermore, in order to more efficiently retain transition metal elements within the plating layer, it is advisable to first form a Ni pre-plating layer or a Co pre-plating layer before forming the Fe plating layer as a pre-plating layer. The Ni pre-plating layer or the Co pre-plating layer will function as a further barrier to the diffusion of transition metal elements towards the Fe surface. The Fe plating layer present on the Ni pre-plating layer or the Co pre-plating layer, as described in Preparation Method B above, will react with the transition metal elements during plating and then rapidly diffuse into the plating layer. That is, the Fe plating layer and the transition metal elements will react during plating, thereby causing the plating bath that has accumulated at the interface to move into the bath, achieving a repeated reduction effect of obtaining a fresh Fe surface again. This can be considered to be related to the mismatch of the crystal surfaces of the Fe in the base material and the Fe crystals (fine grains) of the Fe plating layer precipitated by electroplating, which is related to the reaction with the transition metal elements. The Ni in the Ni pre-plating layer and the Co in the Co pre-plating layer tend to diffuse towards the Fe surface of the base material rather than towards the plating bath side, and are more likely to remain on the Fe surface after immersion in the plating bath. On the other hand, the Fe in the Fe plating layer formed on this substrate tends to diffuse towards the plating bath side. Furthermore, if the substrate has a Ni pre-plating layer or a Co pre-plating layer, the dew point control method used in Method B is not required. In the case of Method C, the effective Ni or Co adhesion amount is 0.5 g / m² or more. When this adhesion amount is 2.0 g / m² or more, the transition metal elements become more concentrated near the interface, thus being suitable for sacrificial corrosion protection.

[0114] <Manufacturing Method D> On the other hand, if only a Ni pre-plating layer or a Co pre-plating layer is used, the following situation occurs: shortly after immersion in the plating bath, Ni or Co diffuses to the steel side. At the same time, transition metals accumulate on the Ni pre-plating layer or Co pre-plating layer, easily forming an oxide film. As a result, Fe-Ni-transition metal elements are formed, making it difficult to reduce and resulting in an oxide film. In this case, the plating adhesion deteriorates, leading to unplated areas.

[0115] As explained above, the manufacturing method of the plated steel in this embodiment should be the above-mentioned manufacturing method B or manufacturing method C.

[0116] <Other Manufacturing Conditions> Regarding the continuous melt plating method, after preparing the plating master plate in accordance with the methods described in Methods B and C above, the Sendzimir process is then performed. That is, before immersing the steel in the plating bath, the steel plate is heated to a temperature above the plating bath temperature under a nitrogen-hydrogen mixed gas annealing temperature. Typically, the annealing gas environment is a nitrogen gas environment containing hydrogen, with a hydrogen concentration of 5%, and heating is performed at a temperature of approximately 600~800°C to fully reduce the steel surface. If a pre-plating layer such as a Ni pre-plating layer or a Co pre-plating layer is not applied to the plating substrate, as described above, humidification is effective for adjusting the dew point. In addition, when the plating master plate is immersed in the plating bath, the steel temperature is cooled to the plating bath temperature using N2 gas, and the plating bath temperature is controlled during the manufacturing process to prevent fluctuations.

[0117] Next, the steel, whose surface has been fully reduced, is immersed in the plating bath in a reduced state. The temperature of the plating bath is set to 570°C or higher, preferably 600°C or higher.

[0118] The immersion time of the steel in the plating bath is set to a range of 1 to 5 seconds. When the immersion time is greater than 5 seconds, the interfacial alloy layer will be thicker, which is undesirable.

[0119] After immersion in the plating bath, the thickness of the plating layer is adjusted by wiping. After wiping, the plating layer is cooled.

[0120] When the cooling rate is too slow at temperatures above 550°C, more Ni pre-plated layers, Co pre-plated layers, or Fe layers will dissolve into the plating bath, which will affect the plating performance. Therefore, cooling up to 550°C should be completed within 10 seconds.

[0121] Furthermore, after immersion in the plating bath, the Fe in the Fe layer immediately diffuses, and intermetallic compounds containing transition metal elements grow using this as a nucleus. On the other hand, if the cooling rate after immersion is high, the crystal size of each phase will decrease; conversely, if the cooling rate is low, the crystal size will increase. When the crystal size of the plating layer is large, the plating thickness will increase, which is suitable. That is, the cooling rate after immersion should be low. Also, the plating structure is determined before 350°C when the residual liquid phase still exists. Based on the above, the upper limit of the cooling rate from the plating bath to 350°C is set to 100°C / second or less, preferably 50°C / second or less. In addition, from the viewpoint of the limitations of the manufacturing production line equipment after the plating bath is removed, the lower limit of the cooling rate can be set to 1°C / second or more.

[0122] <Preparation Method E> Here, the cooling rate while the liquid phase is still present in the Fe layer, from Fe-based nucleation to growth, has an impact. That is, between 600 and 300°C, the higher the temperature, the easier the growth; the lower the temperature, the more nucleation occurs, making growth difficult. Therefore, the high-temperature region of 600 to 450°C should be cooled slowly to suppress the amount of nucleation. Specifically, for example, the cooling rate in this high-temperature region should be suppressed to between 5 and 20°C / second. A plating master plate that utilizes pre-plated Fe to promote nucleation growth must also be used.

[0123] In addition, after the above-mentioned plating layer is formed, various chemical conversion treatments or coating treatments can also be performed.

[0124] In this embodiment, a film can also be formed on the molten steel coating. One or more films can be formed. Examples of films directly above the coating include chromate films, phosphate films, and chromate-free films. These films can be formed using known methods such as chromate treatment, phosphate treatment, and chromate-free treatment.

[0125] Regarding chromate treatment, there are the following treatments: electrolytic chromate treatment, which forms a chromate film through electrolysis; reactive chromate treatment, which forms a film by reacting with the material and then rinsing off the remaining treatment solution; and coating-type chromate treatment, which forms a film by applying the treatment solution to the object and drying it without washing with water. Any of these treatments may be used.

[0126] Electrolytic chromate treatment can be exemplified by using the following components: chromic acid, silica sol, resin (phosphoric acid, acrylic resin, vinyl ester resin, vinyl acetate acrylic emulsion, carboxylated styrene butadiene latex, diisopropanolamine modified epoxy resin, etc.), and hard silica.

[0127] Phosphate treatment can be exemplified by: zinc phosphate treatment, zinc calcium phosphate treatment, manganese phosphate treatment.

[0128] Chromate-free treatment is particularly suitable due to its lack of environmental impact. Chromate-free treatments include the following: electrolytic chromate-free treatment, which forms a chromate-free film through electrolysis; reactive chromate-free treatment, which forms a film by reacting with the substrate and then rinsing off the remaining treatment solution; and coating-type chromate-free treatment, which forms a film by applying the treatment solution to the substrate and drying it without rinsing. Any of these treatments may be used.

[0129] Furthermore, one or more organic resin films may be present on the film directly above the coating layer. The organic resin is not limited to a specific type, and examples include: polyester resin, polyurethane resin, epoxy resin, acrylic resin, polyolefin resin, or modified forms of such resins. Here, "modified form" refers to a resin obtained by reacting reactive functional groups contained in the structure of such resins with other compounds (monomers or crosslinking agents, etc.), wherein the other compounds contain functional groups in their structure that can react with the reactive functional groups.

[0130] Regarding this organic resin, one or more organic resins (unmodified) may be used in combination, or one or more of the following organic resins may be used in combination: those obtained by modifying at least one other organic resin in the presence of at least one organic resin. Furthermore, the organic resin film may also contain any coloring pigments or anti-rust pigments. Aqueous materials that are dissolved or dispersed in water may also be used.

[0131] As explained above, increasing the Al content of the plating layer makes it difficult to increase the adhesion of the plating layer due to the decrease in the specific gravity of the plating bath. However, according to the molten steel of this embodiment, by increasing the viscosity of the plating bath by containing V or Mn, a molten steel with a large adhesion of the plating layer can be manufactured. Furthermore, since the plating layer contains V-containing intermetallic compounds or Mn-containing intermetallic compounds, corrosion resistance can be improved. Moreover, by containing V or Mn in the plating layer, the thickness of the interfacial alloy layer can be reduced, thereby improving adhesion. Furthermore, because the adhesion of the plating layer is large, corrosion resistance can be further improved. [Example]

[0132] Next, embodiments of the present invention will be described. However, the conditions in the embodiments are examples of conditions used to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples of conditions. Various conditions can be used in the present invention as long as they do not depart from the essential points of the present invention and achieve the purpose of the present invention.

[0133] Details of the embodiments are shown in Tables 1 to 3. First, two cold-rolled steel sheets with different thicknesses of 100mm×200mm×0.8mm and 100mm×200mm×2.3mm (equivalent to JIS G 3141(2017)) are prepared as plating master sheets (base steel) for plating ("0.8mm sheet" and "2.3mm sheet" in the table).

[0134] Using these base steels, continuous melt plating is performed via the Sendzimir process. A K-thermocouple point is fused to the center of the back side of the plating substrate to monitor temperature changes during the plating process. Melt plating is performed using a melt plating simulator manufactured by Rhesca.

[0135] [Manufacturing Conditions for the Plating Layer] The plating layer is manufactured using any of the following methods A to E. In each method, the following steps are taken: A Zn-Al-Mg molten plating bath is prepared by mixing pure metals (purity 3N or higher) of Zn, Al, and Mg in a manner that yields a plating layer with the chemical composition shown in Table 1. Furthermore, the elements listed under "Master Plate" in the table are the constituent elements of the pre-plated layer formed on the plating master plate, and the numerical values ​​indicated for these elements refer to the amount of plating applied (g / m²).

[0136] <Manufacturing Method A> The Sendzimir process is used as the melt plating method. That is, before immersing the plating substrate in the plating bath, the steel is heated to a temperature above the plating bath temperature under a nitrogen-hydrogen mixed gas annealing temperature. The annealing gas environment is set to a nitrogen gas environment containing 5% hydrogen (N2-5%H2), and the steel surface is fully reduced by maintaining a temperature of around 800°C for about 1 minute. Humidification is not performed, and the dew point is set to -40°C. In addition, when immersing the plating substrate in the plating bath, the substrate is cooled with N2 gas until the temperature of the substrate reaches the plating bath temperature, while maintaining the same plating bath temperature as in the manufacturing steps. The plating bath temperature is set to 600°C. The immersion time in the plating bath is set to 3 seconds. By applying a lifting speed and wiping with N2 gas, the plating thickness t is 25μm (±2)μm. Furthermore, after lifting the plated motherboard, the temperature range from 600 to 200°C is cooled at a rate of 10°C / second. Afterward, it is allowed to cool naturally.

[0137] <Preparation Method B> The Sendzimir process is used as the melt plating method; however, an Fe plating layer is pre-formed on the plating substrate. Regarding the Fe plating, it is carried out under the conditions of FeSO4・7H2O: 600 g / L, 6.7 A / dm2, and a bath temperature of 47°C, by adjusting the energizing time to achieve the predetermined Fe amount. Regarding the subsequent plating conditions, except that the dew point is set to 0°C, the plating conditions are the same as in Preparation Method A.

[0138] <Preparation Method C> The Sendzimir process is used as the melt plating method. However, a Ni or Co layer is pre-plated on the plating substrate as a pre-plating layer before Fe plating is performed in the same manner as in Preparation Method B. For the Ni pre-plating, the conditions are NiSO4・6H2O: 340 g / L, NiCl2・6H2O: 70 g / L, H3BO3: 40 g / L, bath temperature 50°C, and 5 A / dm2, with the energizing time adjusted to achieve the desired Ni content. For the Co pre-plating, the conditions are H2SO4: 0.5 g / L, CoSO4・6H2O: 246 g / L, H3BO3: 45 g / L, bath temperature 50°C, and 5 A / dm2, with the energizing time adjusted to achieve the desired Co content. The subsequent plating conditions are the same as in Preparation Method A.

[0139] <Preparation Method D> The Sendzimir process is used as the melt plating method; however, only a Ni layer or a Co layer is formed on the plating substrate as a pre-plating layer. The subsequent plating conditions are set to be the same as those in Preparation Method A.

[0140] <Manufacturing Method E> Manufacturing Method E is similar to Manufacturing Method C. In Manufacturing Method E, plating is performed using Manufacturing Method C. After the motherboard is lifted from the plating bath, it is cooled with water mist and cooled to 30°C at a cooling rate of 30°C / second.

[0141] Coated steel was manufactured using the methods A to E described above. A sample with an area of ​​4 cm² was cut from the center of the obtained coated steel. The coated layer was analyzed by performing GDS, XRD and EPMA analysis on the surface of the coated layer. The results are shown in Table 3. In addition, the "Equation (1)" column in Table 3 shows the calculation result of "(t(TR2)-t(TR1)) / t".

[0142] [Performance] Next, the various properties of the obtained plated steel were evaluated using "0.8mm plate" and "2.3mm plate". Specifically, the effects of plating thickness, adhesion, sacrificial corrosion resistance, and gloss were investigated.

[0143] (Influence of Plating Thickness) The influence on plating thickness was evaluated through the following tests. Before immersing a 100×200×0.8mm sample substrate in the plating bath, it was annealed in a reducing gas environment of N2-5%H2 to reduce the substrate surface. Afterwards, it was immersed in the plating bath for at least 3 seconds and then lifted vertically. Tests were conducted at three lifting speeds: 10mm / s, 50mm / s, and 100mm / s. Wiping was not performed during this process. The plating thickness was calculated based on the amount of adhesion. Specifically, a φ50mm sample was taken from the center of the obtained plating steel sheet, immersed in 10% hydrochloric acid containing an inhibitor, and only the plating layer was peeled off. The amount of plating adhesion was calculated from the weight difference before and after peeling. The plating thickness was calculated by dividing the amount of adhesion by the theoretical density of the composition. These three standards are tested repeatedly with N=3, and the average plating thickness is calculated from the average of all samples. The evaluation criteria are defined as follows: "S", "A", "B", "C", and "D" are considered acceptable, and "E" is considered unacceptable.

[0144] Average plating thickness greater than 31.5 μm… "S" Average plating thickness greater than 30 μm and less than 31.5 μm… "A" Average plating thickness greater than 27 μm and less than 30 μm… "B" Average plating thickness greater than 24 μm and less than 27 μm… "C" Average plating thickness greater than 21 μm and less than 24 μm… "D" Average plating thickness less than 21 μm… "E"

[0145] (Adhesion) The adhesion of the plating layer (unplated) is evaluated through a 1t bending test. First, a sample measuring 40mm × 120mm × 0.8mm is taken from the obtained plated steel, including the plating layer. Next, the sample is bent at the center along its long side (120mm) (60mm on one side). For the sample, adhesive tape is firmly attached to both sides of the plated steel sheet beforehand. 1t (the thickness of one sheet of steel) is sandwiched on the inside, and the sample is bent 180° using a jig to fully press it, thereby creating a bending test piece with a space equal to the thickness of one sheet of steel on the inside. Afterwards, the inner steel sheet is removed, and the adhesive tape is pulled off in one go. At this time, the inner and outer adhesive tapes are attached to black thick paper, and it is checked whether there is plating peeling powder on the processing area. The evaluation criteria are set as follows: "G" is defined as qualified and "B" is defined as unqualified.

[0146] With release powder on both sides… "Bad (B)" No release powder on either side… "Good (G)"

[0147] (Sacrificing Corrosion Resistance) Three cuboid samples (50×100×2.3mm) were taken from different locations on the obtained plated steel. Furthermore, to eliminate the influence of the sheared portion visible at the cut ends, all four ends were mirror-polished. Each sample was completely immersed in a 0.01M NaCl aqueous solution for 10 seconds and placed horizontally for 240 hours at 45% humidity and 30°C. This cycle was repeated three times. The red rust area percentage of the four ends was measured after 240 hours. The same test was performed on all three samples, and the average was defined as the red rust area percentage. The evaluation criteria were defined as follows: "EX", "VG", and "G" were defined as acceptable, and "B" was defined as unacceptable.

[0148] Red rust area rate less than 10%... "Excellent (EX)" Red rust area rate 10% or more but less than 20%... "Very Good (VG)" Red rust area rate 20% or more but less than 30%... "Good (G)" Red rust area rate greater than 30%... "Bad (B)"

[0149] (Gloss) A sample for gloss testing was cut from the center of a 2.3mm plate and used to determine the gloss. Specifically, the 60° gloss (G60) specified in JIS Z 8741:1997 was measured on the sample surface using a gloss meter. A sample made at 0.8mm was used. The 60° gloss was measured at 10 random locations on the plated steel surface, and the average value of these measurements was calculated. The evaluation criteria were set as follows, with "FA" indicating good (i.e., low gloss).

[0150] 60° gloss (G60) below 40… "favorable (FA)" 60° gloss (G60) above 40… "dazzling (DZ)"

[0151] [Table 1]

[0152] [Table 2]

[0153] [Table 3]

[0154] Industrial Applicability According to the above-described form of the present invention, a molten-coated steel can be provided that can still ensure the coating thickness even when the coating layer contains elements with a relatively heavy specific gravity (especially transition metal elements). As a result, a long-life molten-coated steel can be provided that can maintain good corrosion resistance over a long period of time. [Simplified Explanation of the Diagram]

[0015] Figure 1 is a chart showing an example of elemental distribution profile, and this figure shows the results of GDS analysis of the coating layer of the molten coated steel of the embodiment of the present invention. Figure 2 is a diagram showing the XRD analysis method.

Claims

1. A molten-coated steel material, comprising: steel material, and a coating layer disposed on the surface of the steel material; the coating layer having the following chemical composition by mass%: Al: greater than 10% and less than 40%, Mg: 4.0% and more than 15.0% and less, Si: 0% and more than 2.0% and less, V: 0% and more than 3.0% and less, Mn: 0% and more than 3.0% and less, Cr: 0% and more than 3.0% and less, Mo: 0% and more than 3.0% and less, Sn: 0% and more than 0.7% and less, Bi: 0% and more than 0.3% and less, In: 0% and more than 0.3% and less, Ca: 0% and more than 0.6% and less, Y: 0% and more than 0.3% and less, La: 0% and more than 0.3% and less, Ce: 0% and more than 0.3% and less, Sr: 0% and more than 0.3% and less, Li: 0% and more than 0.3% and less. Ni: ≥0% and ≤3.0%, Co: ≥0% and ≤3.0%, Cu: ≥0% and ≤0.25%, Ag: ≥0% and ≤0.25%, Sb: ≥0% and ≤0.25%, Pb: ≥0% and ≤0.25%, B: ≥0% and ≤0.50%, P: ≥0% and ≤0.50%, Ti: ≥0% and ≤0.25%, Nb: ≥0% and ≤0.25%, Zr: ≥0% and ≤0.25%, W: ≥0% and ≤0.25%, Fe: ≥0% and ≤5.0%, Remaining portion: Zn and impurities, total stoichiometry of V and Mn ΣTR1: 0.05% or more and 3.0% or less; Total amount of Cr and Mo ΣTR1': 0% or more and 3.0% or less; ΣTR1 + ΣTR1': The total amount of Sn, Bi and In is Σα: 0% or more and 0.7% or less; the total amount of Ca, Y, La, Ce, Sr and Li is Σβ: 0% or more and 0.6% or less; the total amount of Ni and Co is ΣTR2: 0% or more and 3.0% or less; the total amount of Cu, Ag, Sb, Pb, B, P, Ti, Nb, Zr and W is Σγ: 0% or more and 1.00% or less. In the elemental distribution profile of GDS analysis performed from the surface of the aforementioned plating layer toward the aforementioned steel in the thickness direction of the aforementioned steel, the depth position where the Fe concentration reaches 95% is defined as the interface between the aforementioned plating layer and the aforementioned steel. The interval from the aforementioned surface of the aforementioned plating layer to the aforementioned interface is defined as the thickness t of the aforementioned plating layer. At this time, relative to the aforementioned thickness t, the proportion of the depth with a total measurement ΣTR1 of 0.1% or more is 10% or more.

2. As in request item 1, the fused-coated steel, wherein, When V and Mn in the aforementioned chemical composition are defined as TR1, the aforementioned plating layer contains: an intermetallic compound containing TR1; and in the cross-section of the aforementioned plating layer, the total area fraction of the aforementioned intermetallic compound containing TR1 is 1% or more.

3. For the molten steel as requested in item 1 or 2, wherein, When Ni and Co in the aforementioned chemical composition are defined as TR2, in the aforementioned elemental distribution profile, the depth showing the maximum concentration of TR2 is defined as t(TR2), and the depth showing the maximum concentration of TR1 is defined as t(TR1). At this time, the following equation (1) is satisfied: t(TR2)-t(TR1)≦0.30t・・・(1).

4. For the molten steel as requested in item 1 or 2, wherein, In the X-ray diffraction pattern of the aforementioned surface of the aforementioned coating, measured using Cu-Kα line and with X-ray output of 50kV and 300mA, I1 as defined in equation (4) and I2 as defined in equation (5) satisfy equations (2) and (3) respectively: 1.5≦I1…(2) 1.5≦I2…(3) I1=(Imax(10.5°~11.0°) / (I(10.5°)+0.2{|I(11.0°)-I(10.5°)|})…(4) I2=(Imax(20.2°~20.5°)) / (I(20.2°)+0.667{|I(20.5°)-I(20.2°)|})…(5) In equations (4) and (5), Imax(k~m°) is the maximum X-ray diffraction intensity between diffraction angles k~m°, I(n°) is the X-ray diffraction intensity at diffraction angle n°, and k, m, and n are the diffraction angles (2θ) shown in equations (4) and (5), respectively.

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