TWI686510B - Coated steel - Google Patents

Abstract

Provided is a plated steel material that is stable and has high corrosion resistance of a planar portion and a method for manufacturing the same. A plated steel material and a manufacturing method thereof, the plated steel material has: a steel material, and a plated layer disposed on the surface of the steel material and containing a Zn-Al-Mg alloy layer; the plated layer has a predetermined chemical composition; the Zn-Al- After the surface of the Mg alloy layer is polished to 1/2 of the layer thickness, Al crystals are present in the backscattered electron image of the Zn-Al-Mg alloy layer obtained by observation with a scanning electron microscope at a magnification of 100 times. And the average value of the cumulative circumference of the Al crystal is 88 to 195 mm/mm ² .

Description

Plated steel

Field of invention This disclosure is about a plated steel.

Background of the invention For example, in the field of building materials, various plating steels are being used. Most of them are Zn-plated steel. From the perspective of the long life of building materials, research on the high corrosion resistance of Zn-plated steel has been carried out for a long time, and various plated steels have been developed. The first high-corrosion-resistant plated steel used for building materials is a Zn-5%Al plated steel (Galfan plated steel) that adds Al to the Zn-based plated layer to improve corrosion resistance. Adding Al to the plating layer to improve corrosion resistance is a well-known fact; by adding 5% Al, Al crystals will be formed in the plating layer (specifically, the Zn phase), and the corrosion resistance will be improved. Zn-55%Al-1.6%Si plated steel (Galvalume steel) is basically a plated steel for the same reason to improve corrosion resistance. Therefore, once the Al concentration is increased, basically the corrosion resistance of the planar portion is improved. However, the increase in Al concentration will result in decreased sacrificial corrosion resistance.

Here, the charm of Zn-based plated steel lies in the sacrificial corrosion prevention effect on the base steel. That is, the cut end portion of the plated steel material, the cracked portion of the plating layer during processing, and the exposed portion of the base steel material exposed due to the peeling of the plated layer, etc., before the base steel material is corroded, the surrounding plating layer is dissolved out The dissolved components of the plating will form a protective film. In this way, red rust from the base steel can be prevented to some extent.

For this effect, generally speaking, the Al concentration should be low and the Zn concentration should be high. Therefore, in recent years, this kind of highly corrosion-resistant plated steel material that suppresses the Al concentration to a low concentration of about 5% to 25% is actually being used. In particular, this kind of plated steel material that suppresses the Al concentration to a low value and further contains about 1 to 3% of Mg has superior corrosion resistance and sacrificed corrosion resistance of the planar portion than Galfan plated steel material. Therefore, it has become a market trend as plated steel and is widely known in the market today.

For such plated steel materials containing a certain amount of Al and Mg, for example, the plated steel materials disclosed in Patent Document 1 have also been developed.

Specifically, Patent Document 1 discloses a molten Zn-Al-Mg-Si plated steel material, which has a plated layer on the surface of the steel material; the plated layer is made of Al: 5-18% by mass, Mg: 1~ 10% by mass, Si: 0.01 to 2% by mass, the remainder of Zn and inevitable impurities, and more than 200 Al phases per 1 mm ² on the surface of the plated steel material.

Prior technical literature Patent Literature Patent Document 1: Japanese Patent Laid-Open No. 2001-355053

Summary of the invention Problems to be solved by the invention However, in a plated steel material containing a certain amount of Al concentration, the plated layer (specifically, the Zn-Al-Mg alloy layer) will locally corrode, and the initial tendency to reach the base steel material is high. As a result, the corrosion resistance of the planar portion deteriorates, and the unevenness of the corrosion resistance of the planar portion may increase. Therefore, as far as the current situation is concerned, a stable plated steel material with high corrosion resistance in a planar portion is being sought.

Therefore, the subject of one aspect of the present disclosure is to provide a plated steel material that is stable and has high corrosion resistance of the planar portion.

Means to solve the problem The above problems are solved by the following means. which is:

>1> A plated steel material comprising: a base steel material, and a plated layer disposed on the surface of the base steel material and containing a Zn-Al-Mg alloy layer; the plated layer has a chemical composition consisting of: In terms of mass %, Zn: greater than 65.0%, Al: greater than 5.0% to less than 25.0%, Mg: greater than 3.0% to less than 12.5%, Sn: 0.1% to 20.0%, Bi: 0% to less than 5.0%, In: 0 %~less than 2.0%, Ca:0%~3.00%, Y:0%~0.5%, La:0%~less than 0.5%, Ce:0%~less than 0.5%, Si:0%~less than 2.5%, Cr : 0%~less than 0.25%, Ti:0%~less than 0.25%, Ni:0%~less than 0.25%, Co:0%~less than 0.25%, V:0%~less than 0.25%, Nb:0%~less than 0.25%, Cu: 0% to less than 0.25%, Mn: 0% to less than 0.25%, Fe: 0% to 5.0%, Sr: 0% to less than 0.5%, Sb: 0% to less than 0.5%, Pb: 0 % ~ Less than 0.5%, B: 0% ~ less than 0.5%, and impurities; after grinding the surface of the Zn-Al-Mg alloy layer to 1/2 of the layer thickness, and observing through a scanning electron microscope at a magnification of 100 times In the backscattered electron image of the obtained Zn-Al-Mg alloy layer, there are Al crystals, and the average value of the cumulative circumference of the Al crystals is 88 to 195 mm/mm ² . >2> The plated steel material according to >1>, wherein the plated layer has an Al-Fe alloy layer with a thickness of 0.05 to 5 μm between the base steel material and the Zn-Al-Mg alloy layer.

Invention effect According to one aspect of the present disclosure, it is possible to provide a plated steel material that is stable and has high corrosion resistance in a planar portion.

Forms for carrying out the invention An example of this disclosure will be described below. In addition, in this disclosure, the "%" indicated by the content of each element of the chemical composition means: "mass %". The numerical range indicated by "~" means that the values described before and after "~" are defined as the range covered by the lower limit and the upper limit. If the numerical values described before and after "~" are marked with "greater than" or "less than", the numerical range means that these values are set as the lower limit or upper limit and are not included in the range. The element content of the chemical composition is sometimes marked as element concentration (eg, Zn concentration, Mg concentration, etc.). The term "step" refers not only to an independent step, even if it cannot be clearly distinguished from other steps, as long as it can achieve the intended purpose of the step, it is covered in this term. "Corrosion resistance of the planar portion" means that the plating layer (specifically, the Zn-Al-Mg alloy layer) itself is difficult to corrode. "Sacrifice corrosion resistance" means: suppressing corrosion of the base steel material at the exposed part of the base steel material (for example: the cut end portion of the plated steel material, the cracked portion of the plating layer during processing, and the exposed area of the base steel material due to the peeling of the plating layer) nature.

The plated steel material of the present disclosure is a plated steel material having a base steel material and a plating layer disposed on the surface of the base steel material and containing a Zn-Al-Mg alloy layer. Moreover, as for the plated steel material of the present disclosure, the plated layer has a predetermined chemical composition; after grinding the surface of the Zn-Al-Mg alloy layer to 1/2 of the layer thickness, it is carried out at a magnification of 100 times through a scanning electron microscope In the backscattered electron image of the Zn-Al-Mg alloy layer obtained during the observation, Al crystals are present, and the average cumulative circumference of the Al crystals is 88 to 195 mm/mm ² .

The plated steel material of the present disclosure is a plated steel material that is stable and has high corrosion resistance of the planar portion through the above-mentioned configuration. With regard to the plated steel material of the present disclosure, the following insights were found.

The inventors analyzed the initial corrosion behavior of the plating layer containing the Zn-Al-Mg alloy layer. As a result, the following insights were obtained: Corrosion of the plating layer (specifically, the Zn-Al-Mg alloy layer) would proceed locally in the form of an ant nest, and corrosion would preferentially occur around the Al crystal. The principle is presumed as follows. Potential difference corrosion occurs between Al crystals with higher potential and surrounding tissues with lower potential. Therefore, the larger the contact area between the Al crystal and the phase around the Al crystal, the more likely the corrosion around the Al crystal will occur, the corrosion resistance of the planar portion will deteriorate, and the unevenness of the corrosion resistance of the planar portion will also increase.

Therefore, in order to minimize the contact area of Al crystals and the phases around the Al crystals, the inventors conceived the following method: when manufacturing the plating layer, the cooling conditions after immersion in the plating bath are controlled to cause the Al crystals to coarsely precipitate. As a result, the following insights were obtained. The cumulative circumference of the Al crystal obtained from the image analysis as the Al crystal size index has a high correlation with the corrosion resistance of the planar portion. Then, when the average value of the cumulative circumference of the Al crystal is set within a predetermined range, the contact area between the Al crystal and the phase around the Al crystal decreases. As a result, preferential corrosion around the Al crystal is suppressed, and stable planar portion corrosion resistance is obtained. However, if the average value of the cumulative circumferences of Al crystals is excessively reduced, the workability will decrease.

As can be seen from the above, the plated steel material of the present disclosure is a stable and highly plated steel material with high corrosion resistance in the planar portion.

Hereinafter, the plated steel material of the present disclosure will be described in detail.

The base steel material to be plated will be described. The shape of the base steel is not particularly limited. In addition to the steel plate, the base steel can also include steel pipes, civil engineering construction materials (gutters, corrugated steel pipes, drain cover, anti-flying sand board, bolts, wire mesh, guardrails, cut-off Water base, etc.), home appliances components (housing of outdoor unit of air conditioner, etc.), automobile parts (chassis components, etc.) and other base steel after forming. For forming processing, various plastic processing methods such as pressing processing, roll forming, and bending processing can be used.

The material of the base steel is not particularly limited. The base steel can be applied, for example: general steel, pre-plated steel, Al killed steel, extremely low carbon steel, high carbon steel, various high-tensile steels, and some high alloy steels (including Ni, Cr and other strengthening elements) Steel, etc.) and other base steels. With respect to the base steel material, conditions such as the method of manufacturing the base steel material and the method of manufacturing the base steel sheet (hot rolling method, pickling method, cold rolling method, etc.) are not particularly limited. In addition, as the base steel material, hot-rolled steel sheets, hot-rolled steel belts, cold-rolled steel sheets, and cold-rolled steel belts described in JIS G 3302 (2010) can also be applied.

The base steel may also be pre-plated steel that has been pre-plated. The pre-plated steel material can be obtained by, for example, an electrolytic treatment method or a replacement plating method. In the electrolytic treatment method, the base steel material is immersed in a sulfuric acid bath or a chloride bath containing metal ions of various pre-plating components and subjected to electrolytic treatment, thereby obtaining a pre-plated steel material. In the replacement precipitation plating method, the base steel material is immersed in an aqueous solution containing various pre-plating components metal ions and adjusted to pH with sulfuric acid, and the metal is replaced and precipitated to obtain a pre-plated steel material. As a pre-plated steel material, a pre-plated Ni steel material can be cited as a representative example.

Next, the plating layer will be described. The plating layer contains a Zn-Al-Mg alloy layer. In addition to the Zn-Al-Mg alloy layer, the plating layer may contain an Al-Fe alloy layer. There is an Al-Fe alloy layer between the base steel material and the Zn-Al-Mg alloy layer.

That is, the plating layer may have a single-layer structure of a Zn-Al-Mg alloy layer, or may have a laminated structure including a Zn-Al-Mg alloy layer and an Al-Fe alloy layer. In the case of a laminated structure, the Zn-Al-Mg alloy layer may be the layer that constitutes the surface of the plating layer. However, although the oxide film of the plating layer constituent element is formed on the surface of the plating layer at about 50 nm, the thickness is relatively thin compared to the entire thickness of the plating layer, and it is regarded as the main body that does not constitute the plating layer.

In this case, the thickness of the Zn-Al-Mg alloy layer is, for example, 2 μm or more and 95 μm or less (preferably 5 μm or more and 75 μm or less).

On the other hand, the overall thickness of the plating layer is, for example, about 100 μm or less. The overall thickness of the plating layer is affected by the plating conditions. Therefore, the upper and lower limits of the overall thickness of the plating layer are not particularly limited. For example, the overall thickness of the plating layer is related to the viscosity and specific gravity of the plating bath in the general hot melt plating method. In addition, the amount of plating can be adjusted by the pulling speed of the base steel and the strength of wiping (wiping) to adjust the weight per unit area. Therefore, it is considered that the lower limit of the entire thickness of the plating layer may be about 2 μm. On the other hand, based on the weight and uniformity of the plating metal, the upper limit of the thickness of the plating layer that can be produced by the molten plating method is about 95 μm. The thickness of the plating layer can be freely changed by the speed of pulling from the plating bath and the welding conditions. Therefore, it is not particularly difficult to manufacture a plating layer with a thickness of 2 to 95 μm.

Next, the Al-Fe alloy layer will be described.

The Al-Fe alloy layer is formed on the surface of the base steel material (specifically, between the base steel material and the Zn-Al-Mg alloy layer), and has a structure in which the Al ₅ Fe phase is the main phase. The Al-Fe alloy layer is formed because atoms of the base steel and the plating bath diffuse together. When a hot-dip plating method is used in the manufacturing method, an Al-Fe alloy layer is easily formed in the plating layer containing Al element. Since the plating bath contains Al above a certain concentration, the Al ₅ Fe phase will form the most. However, it takes time for the atoms to diffuse, and there are also parts where the Fe concentration becomes higher among the parts close to the base steel. Therefore, the Al-Fe alloy layer may partially contain a small amount of AlFe phase, Al ₃ Fe phase, and Al ₅ Fe ₂ . In addition, since the plating bath also contains a certain concentration of Zn, the Al-Fe alloy layer also contains a small amount of Zn.

In terms of corrosion resistance, there is not much difference in any of the Al ₅ Fe phase, Al ₃ Fe phase, AlFe phase, and Al ₅ Fe ₂ phase. Here, the corrosion resistance is the corrosion resistance in the part not affected by welding.

In this case, when Si is contained in the plating layer, Si is sometimes easily incorporated into the Al-Fe alloy layer and becomes an Al-Fe-Si intermetallic compound phase. For the intermetallic compound phase to be identified, there is an AlFeSi phase; for isomers, there are α, β, q1, q2-AlFeSi equal. Therefore, the Al-Fe alloy layer will detect that these AlFeSi are equal. The Al-Fe alloy layer containing these AlFeSi equivalents is also called Al-Fe-Si alloy layer. In addition, the thickness of the Al-Fe-Si alloy layer is smaller than that of the Zn-Al-Mg alloy layer, so the effect of corrosion resistance on the entire plating layer is also small.

In addition, when various kinds of pre-plated steel materials are used as base steel materials (base steel plates, etc.), the structure of the Al-Fe alloy layer changes depending on the amount of pre-plating adhesion. Specifically, the pure metal layer used for pre-plating may remain around the Al-Fe alloy layer, and sometimes the constituent components of the Zn-Al-Mg alloy layer and the pre-plating component combine to form an intermetallic compound phase (For example, Al ₃ Ni is equal) an alloy layer is formed, and sometimes an Al-Fe alloy layer that replaces part of Al atoms and Fe atoms is formed, or sometimes an Al-Fe- that replaces part of Al atoms, Fe atoms, and Si atoms The Si alloy layer will form. No matter what, the thickness of these alloy layers is smaller than that of the Zn-Al-Mg alloy layer, so the effect of corrosion resistance on the entire plating layer is small.

That is, the Al-Fe alloy layer is a layer including the alloy layer of the above-mentioned various aspects in addition to the alloy layer mainly composed of the Al ₅ Fe phase.

In addition, among various pre-plated steel materials, when the Ni pre-plated steel material forms a plating layer, an Al—Ni—Fe alloy layer is formed as an Al—Fe alloy layer. Compared with the Zn-Al-Mg alloy layer, the Al-Ni-Fe alloy layer is also relatively small in thickness, so it has little effect on the corrosion resistance in the entire plating layer.

The thickness of the Al-Fe alloy layer is, for example, 0 μm or more and 5 μm or less. That is, the Al-Fe alloy layer may not be formed. From the viewpoint of improving the adhesion of the plating layer (specifically, the Zn-Al-Mg alloy layer) and ensuring the workability, the thickness of the Al-Fe alloy layer is preferably 0.05 μm or more and 5 μm or less.

However, when the plating layer of the chemical composition specified in the present disclosure is generally formed by a molten plating method, an Al-Fe alloy layer of 100 nm or more is likely to be formed between the base steel material and the Zn-Al-Mg alloy layer. The lower limit of the thickness of the Al-Fe alloy layer is not particularly limited, but it is clear that when forming the Al-containing molten plating layer, the Al-Fe alloy layer is necessarily formed. Then, empirically, about 100 nm is the most common thickness after suppressing the formation of the Al-Fe alloy layer, and it is determined as a thickness that can ensure sufficient adhesion between the plating layer and the base steel material. As long as no special measures are taken, it is difficult to form an Al-Fe alloy layer thinner than 100 nm due to the high Al concentration by the hot-dip plating method. However, it is speculated that even if the thickness of the Al-Fe alloy layer is less than 100 nm or the Al-Fe alloy layer is not formed, it will not have much impact on the plating performance.

On the other hand, if the thickness of the Al-Fe alloy layer is greater than 5 μm, the Al component of the Zn-Al-Mg alloy layer formed on the Al-Fe alloy layer will be insufficient, and the adhesion and workability of the plating layer will be extreme The tendency to deteriorate. Therefore, the thickness of the Al-Fe alloy layer should be limited to 5 μm or less. In addition, the Al-Fe alloy layer is also closely related to the Al concentration and the Sn concentration. Generally speaking, the higher the Al concentration and the Sn concentration, the faster the growth rate tends to be.

The Al-Fe alloy layer is mostly composed of Al ₅ Fe phase, so the chemical composition of the Al-Fe alloy layer can be exemplified to include the following composition: Fe: 25-35%, Al: 65-75%, Zn: 5 % And below, and the rest: impurities.

In general, the thickness of the Zn-Al-Mg alloy layer is generally thicker than that of the Al-Fe alloy layer. Therefore, the Al-Fe alloy layer contributes more to the corrosion resistance of the flat part of the plated steel than the Zn-Al-Mg The alloy layer is still small. However, as estimated from the composition results, the Al-Fe alloy layer contains Al and Zn, which are corrosion resistance elements above a certain concentration. Therefore, the Al-Fe alloy layer has a certain degree of sacrificial corrosion resistance and shielding corrosion effect for the base steel.

In this case, the thin Al-Fe alloy layer alone contributes to the corrosion resistance, and it is difficult to confirm by quantitative measurement. However, for example, when the thickness of the Al-Fe alloy layer is sufficient, the Zn-Al-Mg alloy layer on the Al-Fe alloy layer can be finely removed from the surface of the plating layer by end milling, etc., and then subjected to a corrosion test In this way, the corrosion resistance of the Al-Fe alloy layer alone can be evaluated. The Al-Fe alloy layer contains Al components and a small amount of Zn components. Therefore, when the Al-Fe alloy layer is present, red rust is generated in the form of dots, and it does not occur as if the base steel material is not exposed without the Al-Fe alloy layer. Red rust throughout.

Also, in the corrosion test, when observing the cross section of the plating layer before the base steel is red rust, even if the upper Zn-Al-Mg alloy layer is eluted and rusted, only the Al-Fe alloy layer will remain and the base can be confirmed The steel is protected against corrosion. This is because, electrochemically, the potential of the Al-Fe alloy layer is higher than that of the Zn-Al-Mg layer but lower than that of the base steel. From this, it can be judged that the Al-Fe alloy layer also has a certain corrosion resistance.

From the corrosion point of view, the thicker the Al-Fe alloy layer, the better and it has the effect of delaying the generation of red rust. However, the thick Al-Fe alloy layer is the cause of the significant deterioration of the plating processability, so the thickness should be below a certain thickness. From the viewpoint of workability, the thickness of the Al-Fe alloy layer is preferably 5 μm or less. If the thickness of the Al-Fe alloy layer is 5 μm or less, the amount of cracks and pulverization generated from the plating Al-Fe alloy layer as a starting point due to the V-bending test or the like will be reduced. The thickness of the Al-Fe alloy layer is more preferably 2 μm or less.

Next, the chemical composition of the plating layer will be described. Regarding the composition of the Zn-Al-Mg alloy layer contained in the plating layer, the Zn-Al-Mg alloy layer also substantially retains the composition composition ratio of the plating bath. In the molten plating method, the formation of the Al-Fe alloy layer is to complete the reaction in the plating bath, so the formation of the Al-Fe alloy layer causes the Al component and Zn component of the Zn-Al-Mg alloy layer to decrease, usually only some micro.

Therefore, in order to achieve stable corrosion resistance of the planar portion, the chemical composition of the plating layer is set as follows.

That is, the chemical composition of the plating layer is a chemical composition composed of: in mass %, Zn: greater than 65.0%, Al: greater than 5.0%~less than 25.0%, Mg: greater than 3.0%~less than 12.5%, Sn: 0.1%~20.0%, Bi: 0%~less than 5.0%, In: 0%~less than 2.0%, Ca: 0%~3.00%, Y: 0%~0.5%, La: 0%~less than 0.5%, Ce: 0%~less than 0.5%, Si: 0%~less than 2.5%, Cr: 0%~less than 0.25%, Ti: 0%~less than 0.25%, Ni: 0%~less than 0.25%, Co: 0%~less than 0.25%, V: 0%~less than 0.25%, Nb: 0%~less than 0.25%, Cu: 0%~less than 0.25%, Mn: 0%~less than 0.25%, Fe: 0%~5.0%, Sr: 0%~less than 0.5%, Sb: 0%~less than 0.5%, Pb: 0%~less than 0.5%, B: 0%~less than 0.5%, and Impure.

In the chemical composition of the plating layer, Bi, In, Ca, Y, La, Ce, Si, Cr, Ti, Ni, Co, V, Nb, Cu, Mn, Fe, Sr, Sb, Pb and B are arbitrary components . In other words, the plating layer may not contain these elements. When such arbitrary components are contained, the content of each arbitrary element is preferably within the range described below.

In this case, the chemical composition of the plating layer is the average chemical composition of the entire plating layer (when the plating layer is a single-layer structure of the Zn-Al-Mg alloy layer, it is the average of the Zn-Al-Mg alloy layer Chemical composition; when the plating layer is a laminated structure of an Al-Fe alloy layer and a Zn-Al-Mg alloy layer, it is the total average chemical composition of the Al-Fe alloy layer and the Zn-Al-Mg alloy layer).

In general, in the case of the molten plating method, since the reaction for forming the plating layer is almost completed in the plating bath, the chemical composition of the Zn-Al-Mg alloy layer is roughly equivalent to that of the plating bath. In addition, in the hot-dip plating method, the Al-Fe alloy layer is formed and grows instantly after being immersed in the plating bath. Then, the Al-Fe alloy layer will complete the formation reaction in the plating bath, and its thickness will be significantly smaller than that of the Zn-Al-Mg alloy layer. Therefore, after plating, as long as no special heat treatment such as heat alloying is performed, the average chemical composition of the entire plating layer is substantially equivalent to the chemical composition of the Zn-Al-Mg alloy layer, and the Al-Fe alloy layer can be ignored Ingredients.

Hereinafter, each element of the plating layer will be described.

>Zn: greater than 65.0%> Zn is an element necessary to obtain corrosion resistance at the flat portion and to sacrifice corrosion resistance. In terms of Zn concentration, when considering the atomic composition ratio, since the plating layer is formed together with elements having a low specific gravity such as Al and Mg, the atomic composition ratio must also be determined as the main body of Zn. Therefore, the Zn concentration is set to be greater than 65.0%. The Zn concentration should be above 70%. In addition, the upper limit of the Zn concentration is the concentration of the remaining elements other than Zn and impurities.

>Al: greater than 5.0%~less than 25.0%> Al forms Al crystals, but is an element necessary to ensure the corrosion resistance of the planar portion and sacrifice the corrosion resistance at the same time. Then, Al improves the adhesion of the plating layer, so it is also an essential element in terms of ensuring workability. Therefore, the lower limit of the Al concentration is set to more than 5.0% (preferably 10.0% or more). On the other hand, if the Al concentration increases, the sacrificial corrosion resistance tends to deteriorate. Therefore, the upper limit of Al concentration is set to less than 25.0% (preferably 23.0% or less).

>Mg: greater than 3.0%~less than 12.5%> Mg is an element necessary to ensure the corrosion resistance of the planar portion and sacrifice the corrosion resistance at the same time. Therefore, the lower limit of the Mg concentration is set to be greater than 3.0% (preferably greater than 5.0%). On the other hand, if the Mg concentration increases, the processability tends to deteriorate. Therefore, it is set to less than 12.5% (preferably 10.0% or less).

>Sn: 0.1%~20.0%> Sn is an element necessary to impart high sacrificial corrosion resistance. Therefore, the lower limit of Sn concentration is set to 0.1% or more (preferably 0.2% or more). On the other hand, if the Sn concentration increases, the corrosion resistance of the planar portion tends to deteriorate. Therefore, the upper limit of Sn concentration is set to 20.0% or less (preferably 5.0% or less).

>Bi: 0%~less than 5.0%> Bi is an element that contributes to sacrifice of corrosion resistance. Therefore, the lower limit of Bi concentration should be greater than 0% (preferably 0.1% or more, more preferably 3.0% or more). On the other hand, if the Bi concentration increases, the corrosion resistance of the planar portion tends to deteriorate. Therefore, the upper limit of the Bi concentration is set to less than 5.0% (preferably 4.8% or less).

>In: 0%~less than 2.0%> In is an element that contributes to sacrifice of corrosion resistance. Therefore, the lower limit of In concentration should be greater than 0% (preferably 0.1% or more, more preferably 1.0% or more). On the other hand, if the In concentration increases, the corrosion resistance of the planar portion tends to deteriorate. Therefore, the upper limit of In concentration is set to less than 2.0% (preferably 1.8% or less).

>Ca: 0%~3.00%> Ca is an element that can adjust the amount of Mg elution appropriate for imparting corrosion resistance and sacrificing corrosion resistance to the planar portion. Therefore, the lower limit of Ca concentration should be greater than 0% (preferably above 0.05%). On the other hand, if the Ca concentration increases, the corrosion resistance and workability of the planar portion tend to deteriorate. Therefore, the upper limit of Ca concentration is set to 3.00% or less (preferably 1.00% or less).

>Y: 0%~0.5%> Y is an element that contributes to sacrifice of corrosion resistance. Therefore, the lower limit of Y concentration should be greater than 0% (preferably above 0.1%). On the other hand, if the Y concentration increases, the corrosion resistance of the planar portion tends to deteriorate. Therefore, the upper limit of the Y concentration is set to 0.5% or less (preferably 0.3% or less).

>La and Ce: 0%~less than 0.5%> La and Ce are elements that contribute to sacrificing corrosion resistance. Therefore, the lower limit of La concentration and Ce concentration should be greater than 0% (preferably 0.1% or more). On the other hand, if the La concentration and the Ce concentration increase, the corrosion resistance of the planar portion tends to deteriorate. Therefore, the upper limits of La concentration and Ce concentration are each set to less than 0.5% (preferably 0.4% or less).

>Si: 0%~less than 2.5%> Si suppresses the growth of the Al-Fe alloy layer and is an element that helps improve corrosion resistance. Therefore, the Si concentration should be greater than 0% (preferably 0.05% or more, more preferably 0.1% or more). On the other hand, if the Si concentration is increased, the corrosion resistance at the planar portion, sacrificial corrosion resistance, and workability tend to deteriorate. Therefore, the upper limit of the Si concentration is set to less than 2.5%. In particular, from the viewpoint of the corrosion resistance of the planar portion and the sacrifice of corrosion resistance, the Si concentration is preferably 2.4% or less, more preferably 1.8% or less, and even more preferably 1.2% or less.

>Cr, Ti, Ni, Co, V, Nb, Cu and Mn: 0%~less than 0.25%> Cr, Ti, Ni, Co, V, Nb, Cu, and Mn are elements that contribute to sacrificing corrosion resistance. Therefore, the lower limit of the concentrations of Cr, Ti, Ni, Co, V, Nb, Cu and Mn should be greater than 0% (preferably 0.05% or more, more preferably 0.1% or more). On the other hand, if the concentrations of Cr, Ti, Ni, Co, V, Nb, Cu, and Mn increase, the corrosion resistance of the planar portion tends to deteriorate. Therefore, the upper limit values of the concentrations of Cr, Ti, Ni, Co, V, Nb, Cu, and Mn are each set to less than 0.25%. The upper limit of the concentration of Cr, Ti, Ni, Co, V, Nb, Cu and Mn should be 0.22% or less.

>Fe: 0%~5.0%> When the plating layer is formed by the molten plating method, the Zn-Al-Mg alloy layer and the Al-Fe alloy layer will contain a certain Fe concentration. It has been confirmed that if the Fe concentration is up to 5.0%, even if it is contained in the plating layer (especially the Zn-Al-Mg alloy layer), the performance will not be adversely affected. Most of the Fe is contained in the Al-Fe alloy layer. Therefore, the larger the thickness of the layer, the higher the Fe concentration in general.

>Sr, Sb, Pb and B: 0%~less than 0.5%> Sr, Sb, Pb, and B are elements that contribute to sacrificing corrosion resistance. Therefore, the lower limit of the concentration of Sr, Sb, Pb and B should be greater than 0% (preferably 0.05% or more, more preferably 0.1% or more). On the other hand, if the concentrations of Sr, Sb, Pb, and B increase, the corrosion resistance of the planar portion tends to deteriorate. Therefore, the upper concentration limits of Sr, Sb, Pb, and B are each set to less than 0.5%.

>Impurities> Impurities are ingredients contained in raw materials or ingredients mixed in the manufacturing process, and refer to ingredients that are not intentionally contained. For example, in the plating layer, since the atoms of the base steel material and the plating bath diffuse with each other, a component other than Fe may be mixed in a trace amount as an impurity.

The chemical composition of the plating layer is measured as follows. First, an acid solution is obtained by peeling and dissolving the plating layer through an acid containing an inhibitor that inhibits corrosion of the base steel. Next, the obtained acid solution is measured by ICP analysis to obtain the chemical composition of the plating layer (when the plating layer is a single-layer structure of the Zn-Al-Mg alloy layer, it is the chemical of the Zn-Al-Mg alloy layer Composition; when the plating layer is a laminated structure of Al-Fe alloy layer and Zn-Al-Mg alloy layer, it is the total chemical composition of Al-Fe alloy layer and Zn-Al-Mg alloy layer). The type of acid is not particularly limited as long as it can dissolve the plating layer. In addition, the chemical composition is measured by the average chemical composition.

Next, the metal structure of the Zn-Al-Mg alloy layer will be described.

Al crystals exist in the metal structure of the Zn-Al-Mg alloy layer, and the average value of the cumulative circumference of the Al crystals is 88 to 195 mm/mm ² .

If the average value of the cumulative circumference of the Al crystal is less than 88 mm/mm ² , the Al crystal will be too coarse and the workability will deteriorate. On the other hand, if the average value of the cumulative circumference of the Al crystal is greater than 195 mm/mm ² , the Al crystal will be finer, and the contact area between the Al crystal and the phase around the Al crystal will increase. As a result, the larger the contact area between the Al crystal and the phases around the Al crystal, the more easily the corrosion around the Al crystal will occur, the corrosion resistance of the planar portion will deteriorate, and the unevenness of the corrosion resistance of the planar portion will also increase. Therefore, the average value of the cumulative circumference of Al crystals is set to 88 to 195 mm/mm ² . The lower limit of the average value of the cumulative circumference of Al crystals is preferably 95 mm/mm ² or more, and more preferably 105 mm/mm ² or more. The upper limit of the average value of the cumulative circumference of Al crystals is preferably 185 mm/mm ² or less, and more preferably 170 mm/mm ² or less.

The metal structure of the Zn-Al-Mg alloy layer has Al crystals. The metal structure of the Zn-Al-Mg alloy layer may have a Zn-Al phase in addition to the Al crystal.

The Al crystal is equivalent to "the α phase in which Zn concentration is 0 to 3% in solid solution". On the other hand, the Zn-Al phase is equivalent to the "β phase, which contains more than 70% and up to 85% of the Zn phase (η phase) and the α phase and the Zn phase (η phase) have been finely separated."

In this case, Figures 1 to 3 show an example of SEM backscattered electron images of the Zn-Al-Mg alloy layer in the polished surface after grinding the surface of the Zn-Al-Mg alloy layer to 1/2 of the layer thickness. Figure 1 is a backscattered electron image of an SEM with a magnification of 100 times, Figure 2 is a magnification of 500 times, and Figure 3 is a magnification of 10,000 times. In addition, in FIGS. 1 to 3, Al represents Al crystal, Zn-Al represents Zn-Al phase, MgZn ₂ represents MgZn ₂ phase, and Zn-Eu represents Zn eutectic phase.

In the backscattered electron image of the Zn-Al-Mg alloy layer, although the area fraction of each structure is not particularly limited, from the viewpoint of improving the corrosion resistance of a stable planar portion, the area fraction of Al crystal is preferably 8 ~45%, preferably 15~35%. That is, the Al crystal preferably exists within the range of the above area fraction.

The remaining structure other than the Al crystal and the Zn-Al phase includes MgZn ₂ phase, Zn eutectic phase (specifically, Zn-Al-MgZn ₂ -Mg ₂ Sn, etc.) and the like.

Here, the method of measuring the average value of the cumulative circumference of Al crystals and the area fraction of Al crystals will be described.

The average value of the cumulative circumference of Al crystals and the area fraction of Al crystals are obtained by grinding the surface of the Zn-Al-Mg alloy layer to 1/2 of the layer thickness and observing it with a magnification of 100 times through a scanning electron microscope The backscattered electron image of the Zn-Al-Mg alloy layer was measured using this backscattered electron image. The details are as follows.

First, a sample is taken from the plated steel material to be measured. However, the sample was taken from the vicinity of the punched end portion of the plated steel material (2 mm from the end surface) and the plated layer had no recessed portion.

Next, the surface of the plating layer (specifically, Zn-Al-Mg alloy layer) of the sample was polished in the thickness direction of the plating layer (hereinafter also referred to as "Z-axis direction"). The Z-axis direction polishing of the surface of the plating layer is to grind the surface of the Zn-Al-Mg alloy layer to 1/2 of the layer thickness. In this polishing, the surface of the Zn-Al-Mg alloy layer was dry-polished with an abrasive sheet with a number of 1200, and then a precision solution containing an alumina with an average particle diameter of 3 μm and a slurry containing an alumina with an average particle diameter of 1 μm were used in order. Seiko fluid, and Seiko fluid containing colloidal silicon dioxide are used for precision grinding. In addition, before and after polishing, the Zn intensity of the surface of the Zn-Al-Mg alloy layer was measured by XRF (X-ray fluorescence analysis), and when the Zn intensity after polishing reached 1/2 of the Zn intensity before polishing, it was determined as Zn-Al -1/2 of the thickness of the Mg alloy layer.

Next, the ground surface of the Zn-Al-Mg alloy layer of the sample was observed through a scanning electron microscope (SEM) at a magnification of 100 times to obtain a backscattered electron image of the Zn-Al-Mg alloy layer (hereinafter also referred to as "SEM") Backscattered electron image"). The SEM observation conditions were: acceleration voltage: 15 kV, irradiation current: 10 nA, field of view: 1222.2 μm×927.8 μm.

In order to identify each phase included in the Zn-Al-Mg alloy layer, FE-SEM or TEM (transmission electron microscope) equipped with EDS (energy dispersive X-ray analyzer) is used. When using TEM, FIB (focused ion beam) is applied to the polished surface of the Zn-Al-Mg alloy layer of the sample of the same measurement object. After FIB processing, a TEM electron diffraction image of the polished surface of the Zn-Al-Mg alloy layer was obtained. Then, the metal contained in the Zn-Al-Mg alloy layer was identified.

Next, the results of the SEM backscattered electron image and the FE-SEM or TEM electron diffraction image were compared. In the SEM backscattered electron image, each phase of the Zn-Al-Mg alloy layer was identified. In addition, when identifying each phase of the Zn-Al-Mg alloy layer, EDS point analysis can be performed, and the EDS point analysis results can be compared with the TEM electron diffraction image identification results. In addition, the EPMA device can also be used when identifying each phase.

Next, in the backscattered electron image of the SEM, the three values of the brightness, hue, and contrast value of the gray scale shown by the respective phases of the Zn-Al-Mg alloy layer were determined. The three values of lightness, hue, and contrast value shown in each phase will reflect the atomic order of the elements contained in each phase, so there is usually a tendency that the phase with a small amount of Al and a large amount of Mg will appear black The phase with a large amount of Zn is white.

From the comparison results of the above EDS, in order to integrate the backscattered electron image of the SEM, image processing (binarization) is performed so that only the Al crystal contained in the Zn-Al-Mg alloy layer shows the above-mentioned 3 value range Color change (for example, only a specific phase is displayed as a white image, and the area (number of pixels) of each phase in the field of view, etc. are calculated. See FIG. 4). By performing this image processing, the area fraction of Al crystals in the Zn-Al-Mg alloy layer in the SEM backscattered electron image is obtained. 4 is an example of an image in which Al crystals can be recognized by performing image processing (binarization) on the backscattered electron image (backscattered electron image of SEM) of the Zn-Al-Mg alloy layer. In FIG. 4, Al represents Al crystal.

Then, the Al crystal area fraction of the Zn-Al-Mg alloy layer was determined as the average value of the Al crystal area fractions obtained by the above operation in three fields of view. In addition, when it is difficult to distinguish Al crystals, electron beam diffraction or EDS point analysis is performed by TEM.

As an example, it is a binary processing function using two thresholds of the Miro Corporation WinROOF2015 (image analysis software), and records the backscattered electron image of the SEM (grayscale image saved in 8bit, displaying 256 colors) The method of identifying the Al crystal. In addition, grayscale images saved in 8bit display black when the luminosity is 0 and white when the maximum value is 255. From the identification results obtained by FE-SEM and TEM, it is clear that if the backscattered electron image of the SEM is set as described above, the photometric thresholds are set to 10 and 95, and Al crystals can be distinguished with high accuracy. Among them, the image is processed to change the range of these luminosities from 10 to 95 to identify the Al crystal. In addition, binary analysis can also use image analysis software other than WinROOF2015.

Next, using the automatic shape feature measurement function of WinRoOF2015 (image analysis software) made by Mitani Corporation, the perimeter of the Al crystal identified by the above image processing is accumulated, and the cumulative perimeter of the Al crystal is obtained. Then, the cumulative perimeter of the Al crystal is divided by the field of view area to calculate the cumulative perimeter of the Al crystal per unit area (mm ² ). This operation was carried out in three fields of view, and the arithmetic average of the cumulative circumference of Al crystals per unit area (mm ² ) was determined as the “average cumulative circumference of Al crystals”.

In addition, the area fraction of Al crystals can also be obtained using the automatic shape characteristic measurement function of WinROOF2015 (image analysis software) manufactured by Mitani Corporation. Specifically, in the backscattered electron image of the Zn-Al-Mg alloy layer, this function is used to calculate the area fraction (area fraction relative to the field of view area) of the Al crystal identified by the binarization. Then, this operation was carried out in three fields of view, and the calculated average was determined as the area fraction of Al crystals.

The thickness of the Al-Fe alloy layer is measured as follows. After embedding the sample into the resin, it is ground, and the backscattered electron image of the SEM of the cross section of the plating layer (cut section along the thickness direction of the plating layer) (but at a magnification of 5000 times, the field of view size: 50 μm in length × 200 μm in width) In the field of view for observing the Al-Fe alloy layer), the thickness is measured at any five points of the identified Al-Fe alloy layer. Then, the arithmetic average of the five points was determined as the thickness of the interface alloy layer.

Next, an example of the manufacturing method of the plated steel material of the present disclosure will be described.

The plated steel material of the present disclosure is obtained by forming a plating layer having the above-mentioned predetermined chemical composition and metal structure on the surface (ie, one side or both sides) of the base steel material (base steel plate, etc.) by a molten plating method Got.

Specifically, the hot-dip plating process is performed under the following conditions as an example. First, the temperature of the plating bath is set to the melting point of the plating bath + 20° C. or more, and after the base steel material is pulled up from the plating bath, in the temperature range from the temperature of the plating bath to the temperature at which the plating solidification starts, use The cooling is performed at the following average cooling rate, which is greater than the average cooling rate in the temperature range from the plating solidification start temperature to the plating solidification start temperature of -30°C. Next, in the temperature range from the plating solidification start temperature to the plating solidification start temperature of -30°C, cooling is performed at an average cooling rate of 12°C/s or less. Next, in the temperature range from the plating solidification start temperature of -30°C to 300°C, cooling is performed using the following average cooling rate from the plating solidification start temperature to the plating solidification start temperature -30 The average cooling rate in the temperature range up to ℃ is still large.

That is, an example of a method for manufacturing a plated steel material according to the present disclosure is to set the temperature of the plating bath to the melting point of the plating bath +20° C. or more, and after pulling up the base steel material from the plating bath, use the following three-stage cooling conditions The method of applying molten plating treatment to the base steel; the three-stage cooling conditions are as follows: the average cooling rate in the temperature region from the plating bath temperature to the plating solidification start temperature is set to A, and the plating solidification start temperature is up to The average cooling rate in the temperature range from the plating solidification start temperature to -30°C is defined as B, and the average cooling rate from the plating solidification start temperature from -30°C to 300°C is defined as C. At this time, A>B, B ≦12℃/s, C>B.

When the temperature of the plating bath is set to the melting point of the plating bath + 20° C. or more, and the base steel material is pulled up from the plating bath, Al crystals are generated. Then, in the temperature range from the plating solidification start temperature to the plating solidification start temperature -30°C, the cooling is performed at an average cooling rate of 12°C/s or less, whereby the Zn-Al-Mg alloy layer is formed Said metal structure: Al crystals are present and the average value of the cumulative circumference of Al crystals is within the above range. The cooling at this average cooling rate is performed by, for example, air cooling that blows the atmosphere with weak wind. However, from the viewpoint of preventing plating from sticking to the top roll, etc., the lower limit value of the average cooling rate in the temperature region from the plating solidification start temperature to the plating solidification start temperature -30°C is set to 0.5 ℃/s above.

In addition, the plating solidification start temperature can be measured by the following method. After taking the sample from the plating bath and heating the sample to the melting point of the plating bath +20°C or more by DSC, and cooling at 10°C/min, the earliest temperature at which the peak value of differential heat appears is the plating solidification start temperature.

In the method of manufacturing a plated steel material of the present disclosure, the average cooling rate in the temperature region from the temperature when the plating bath pulls the base steel material (ie, the temperature of the plating bath) to the temperature at which the plating solidification starts, although not particularly limited However, from the viewpoint of preventing the plating from adhering to the upper roll or suppressing the appearance of wind, etc., the appearance may be 0.5°C/s to 20°C/s. However, the average cooling rate in the temperature region from the plating bath temperature to the plating solidification start temperature is set to be lower than the average cooling rate in the temperature region from the plating solidification start temperature to the plating solidification start temperature -30°C Large average cooling rate. With this, the nucleation point of the Al crystal can be easily increased, and excessive coarsening of the Al crystal can be suppressed.

In addition, the average cooling rate in the temperature range from the plating solidification start temperature of -30°C to 300°C is not particularly limited, but from the viewpoint of preventing the plating from sticking to the upper roll, etc., it is set to 0.5°C/s~ 20℃/s is enough. However, the average cooling rate in the temperature range from the plating solidification start temperature of -30°C to 300°C is set to: the average cooling rate in the temperature range from the plating solidification start temperature to the plating solidification start temperature of -30°C The average cooling rate is still large. This can suppress excessive coarsening of Al crystals and ensure workability.

In addition, the Al-Fe alloy layer formed with the base steel material will rapidly form and grow within a period of less than 1 second after the immersion plating. The growth rate increases with the temperature of the plating bath and increases with the time of immersion in the plating bath. However, if the temperature of the plating bath is less than 500°C, it hardly grows. Therefore, it is sufficient to shorten the immersion time or move to the cooling process immediately after solidification.

In addition, in the plated steel material, if it is once solidified and then heated to re-melt the plated layer, all the constituent phases disappear and become a liquid phase state. Therefore, for example, even if the plated steel material is subjected to quenching or the like once, it can be reheated off-line and subjected to a proper heat treatment step, whereby the organization control specified in the present disclosure can also be implemented. At this time, the reheating temperature of the plating layer should be controlled in the vicinity of the melting point of the plating bath and in a temperature region where the Al-Fe alloy layer will not grow excessively.

Hereinafter, the post-processing applicable to the plated steel material of the present disclosure will be described.

The plated steel material of the present disclosure can also form a film on the plated layer. The film can be formed in one layer or more than two layers. Examples of the type of film directly above the plating layer include chromate film, phosphate film, and chromate-free film. The chromate treatment, phosphate treatment, and chromate-free treatment used to form these films can be performed by known methods.

The chromate treatment is as follows: electrolytic chromate treatment using electrolysis to form a chromate film, reactive chromate treatment using a reaction with the material to form a film and then washing off excess treatment liquid, and applying the treatment liquid to the coating The material is dried without washing with water to form a coating chromate treatment. Either treatment can be used.

Examples of electrolytic chromate treatment: use of chromic acid, silica sol, resin (acrylic resin, vinyl ester resin, vinyl acetate acrylic emulsion, carboxylated styrene butadiene latex, diisopropanolamine modified epoxy resin Etc.), and electrolytic chromate treatment of hard silica.

Examples of phosphate treatment include zinc phosphate treatment, zinc calcium phosphate treatment, and manganese phosphate treatment.

The chromate-free treatment is preferably not particularly harmful to the environment. The chromate-free treatment is as follows: electrolytic chromate-free treatment using electrolysis to form a chromate-free film; reactive chromate-free treatment using a reaction with the material to form a film to wash off excess treatment liquid; It is a coating-type chromate-free treatment that coats the object to be dried without washing with water to form a film. Either treatment can be used.

In addition, one or more organic resin coatings may be provided on the coating directly above the plating layer. The organic resin is not limited to a specific type, and examples thereof include polyester resin, polyurethane resin, epoxy resin, acrylic resin, polyolefin resin, or modified products of these resins. Here, the modified substance is a resin obtained by reacting the reactive functional groups contained in the resin structure with other compounds (monomers or cross-linking agents, etc.); the other compounds contained in the structure can proceed with the aforementioned functional groups The functional group of the reaction.

The organic resin may be mixed with one or more organic resins (unmodified ones), or may be mixed with one or more than two organic resins modified in the presence of at least one organic resin Organic resin obtained. Moreover, the organic resin film may contain arbitrary coloring pigments and anti-rust pigments. It is also possible to use substances which can be hydrated by dissolving or dispersing in water. [Example]

The embodiments of the disclosure are described. The conditions in the embodiments are examples of conditions adopted to confirm the implementability and effects of the disclosure. The disclosure is not limited to the examples of conditions. As long as it does not deviate from the gist of this disclosure and can achieve the purpose of cost disclosure, this disclosure can adopt various conditions.

(Example) In order to obtain a plating layer of the chemical composition shown in Table 1 to Table 2, a predetermined amount of pure metal ingot was used, and after melting the ingot in a vacuum melting furnace, a plating bath was established in the atmosphere. For the production of plated steel plates, batch-type hot-dip plating equipment is used. As for the base steel, a 2.3 mm general-purpose hot-rolled carbon steel sheet (C concentration><0.1%) is used, and degreasing and pickling are performed before the plating step. In addition, in some examples, the base steel material is a Ni pre-plated steel plate, which is pre-plated with Ni on a 2.3 mm general-material hot-rolled carbon steel plate. The amount of Ni deposited was 2g/m ² . In addition, using Ni pre-plated steel as an example of base steel, the column of "base steel" in the table is marked as "pre-plated Ni".

In the preparation of some samples, the steps up to the time of immersion in the plating bath are to implement the same reduction treatment method on the base steel. That is, in an N ₂ -H ₂ (5%) (dew point below -40°C, oxygen concentration less than 25ppm) environment, the base steel is heated from room temperature to 800°C by energized heating and held for 60 seconds , Blow N ₂ gas to cool to the plating bath temperature +10℃, and then immediately immerse in the plating bath. In addition, in all plated steel sheets, the immersion time in the plating bath is the time shown in the table. The N ₂ gas wiping pressure was adjusted so that the plating thickness was 30 μm (±1 μm) to produce a plated steel plate.

The temperature of the plating bath is basically set at the melting point +20°C, and the plating is performed after the temperature is further increased at a certain level. The plating bath immersion time was set to 2 seconds. After the base steel material was pulled up from the plating bath, the average cooling rate in the following first to third stages shown in Table 1 to Table 2 was used as the conditions shown in Table 1 to Table 2 to perform a cooling procedure to obtain a plating layer. ・1st stage average cooling rate: the average cooling rate in the temperature range from the plating bath temperature to the plating solidification start temperature ・The second stage average cooling rate: the average cooling rate in the temperature range from the plating solidification start temperature to the plating solidification start temperature of -30℃ ・The third stage average cooling rate: the average cooling rate in the temperature range from the plating solidification start temperature of -30℃ to 300℃

-Various measurements- Samples were cut out from the obtained plated steel plates. Then, the following items were measured according to the method described in detail. ・The average value of the cumulative circumference of Al crystals (marked as "Al crystal circumference" in the table) ・Al crystal area fraction ・Thickness of the Al-Fe alloy layer (However, in the case of using a pre-plated Ni steel plate as the base steel, it is expressed as the thickness of the Al-Ni-Fe alloy layer.)

-Corrosion resistance of the flat surface- In order to compare the corrosion resistance of the stable flat surface, the manufacturing samples were supplied to the corrosion promotion test (JASO M609-91) for 120 cycles, and then immersed in a normal temperature 30% chromic acid aqueous solution to remove white rust. The corrosion resistance was evaluated by the amount of corrosion. The test is carried out 5 times, and when the average corrosion amount is less than 80g/m ² and the maximum and minimum corrosion amount in n=5 is within ±100% of the average value, the evaluation is "A+"; when the average corrosion amount is When the maximum value and the minimum value of the corrosion amount in n=5 within 100g/m ² or less are within ±100% of the average value, the evaluation is "A"; those other than these are evaluated as "NG".

-Sacrifice corrosion resistance (corrosion resistance of the end surface of the cut part)- In order to compare the sacrificial corrosion resistance (corrosion resistance of the cut end face), the sample was cut to 50 mm × 100 mm, and the upper and lower end faces were sealed and supplied to the corrosion promotion test (JASO M609-91) for 120 cycles. The average value of the area ratio where red rust was generated at the exposed portion of the end surface was evaluated. Those with a red rust area ratio of 50% or less are evaluated as "A+", those with 70% or less are evaluated as "A", and those with more than 70% are evaluated as "NG".

-Processability- In order to evaluate the workability of the plated layer, the plated steel sheet was bent at 90° V, and then cellophane tape with a width of 24 mm was pressed against the V-bend valley and peeled off, and pulverization was evaluated visually. Those with no powdered peeling powder attached to the tape were rated "A", those with slight attachments were rated "A-", and those with attachments were rated "NG".

-Overall evaluation- The evaluation results of the flat part corrosion resistance, sacrificial corrosion resistance and workability evaluation are all "A", "A+" or "A-", and the evaluation is "A"; as long as there is one "NG", it is evaluated as "NG" ".

The list of examples is shown in Table 1 to Table 2.

[Table 1]

[Table 2]

From the above results, it can be seen that the embodiment of the plated steel material according to the present disclosure has a stable planar portion corrosion resistance compared to the comparative example. In particular, it can be seen that even in the comparative example (Test No. 70) which satisfies the chemical composition of the disclosed plating layer but the average cooling rate is unchanged at 15°C/s, the average cumulative circumference of Al crystals becomes too large to be obtained Stable flat surface corrosion resistance. On the other hand, it can be seen that the comparative example (Comparative Example No. 71) in which the average cooling rate in the second stage is too low, the comparative example (Test No 72) in which the average cooling rate is changed only by 2 stages, and the average cooling rate is 6 ℃/s without change In the comparative example (Test No. 73), the average value of the cumulative circumferences of the Al crystals becomes too small and the workability deteriorates.

In the above, suitable embodiments of the present disclosure are described in detail with reference to the accompanying drawings, but the present disclosure is not limited to related examples. It should be understood that those who have ordinary knowledge in the technical field of the present disclosure can naturally think of various modifications or amendments within the scope of the technical ideas described in the patent application scope, and of course these are also within the scope of the disclosed technology.

Symbol description is as follows. Al Al crystal Zn-Al Zn-Al phase MgZn ₂ MgZn ₂ phase Zn-Eu Zn eutectic phase

In addition, this specification refers to the entire disclosure of Japanese Patent Application No. 2018-094481 and is incorporated therein. All documents, patent applications, and technical specifications described in this specification are incorporated into this specification by reference to the same extent as if specific and individual documents, patent applications, and technical specifications were incorporated by reference.

Al... Al crystal Zn-Al...Zn-Al phase MgZn ₂ ...MgZn ₂ phase Zn-Eu...Zn system eutectic phase

FIG. 1 is an SEM backscattered electron image (magnification 100 times), showing an example of the Zn-Al-Mg alloy layer of the plated steel material of the present disclosure. FIG. 2 is an SEM backscattered electron image (magnification: 500 times), showing an example of the Zn-Al-Mg alloy layer of the plated steel material of the present disclosure. FIG. 3 is an SEM backscattered electron image (magnification 10,000 times), showing an example of the Zn-Al-Mg alloy layer of the plated steel material of the present disclosure. FIG. 4 is a diagram showing an example of the following image, which is a backscattered electron image (SEM backscattered electron image) of the Zn-Al-Mg alloy layer of the plated steel material of the present disclosure. Image processing (binarization) so that Al crystals can be identified.

Al …Al crystal

Claims (2)

A plated steel material comprising: a base steel material and a plated layer disposed on the surface of the base steel material and containing a Zn-Al-Mg alloy layer; the plated layer has a chemical composition consisting of: in mass% , Zn: greater than 65.0%, Al: greater than 5.0% to less than 25.0%, Mg: greater than 3.0% to less than 12.5%, Sn: 0.1% to 20.0%, Bi: 0% to less than 5.0%, In: 0% to less than 2.0%, Ca: 0% to 3.00%, Y: 0% to 0.5%, La: 0% to less than 0.5%, Ce: 0% to less than 0.5%, Si: 0% to less than 2.5%, Cr: 0% ~ Less than 0.25%, Ti: 0%~ Less than 0.25%, Ni: 0%~ Less than 0.25%, Co: 0%~ Less than 0.25%, V: 0%~ Less than 0.25%, Nb: 0%~ Less than 0.25%, Cu: 0% to less than 0.25%, Mn: 0% to less than 0.25%, Fe: 0% to 5.0%, Sr: 0% to less than 0.5%, Sb: 0% to less than 0.5%, Pb: 0% to less than 0.5%, B: 0% to less than 0.5%, and impurities; after grinding the surface of the Zn-Al-Mg alloy layer to 1/2 of the layer thickness and observing it through a scanning electron microscope at a magnification of 100 times, it is obtained In the backscattered electron image of the Zn-Al-Mg alloy layer, Al crystals are present, and the average value of the cumulative circumference of the Al crystals is 88 to 195 mm/mm ² . The plated steel material according to claim 1, wherein the plated layer has an Al-Fe alloy layer with a thickness of 0.05 to 5 μm between the base steel material and the Zn-Al-Mg alloy layer.

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