JP7464012B2 - Zn-Al-Mg alloy plated steel sheet and manufacturing method thereof - Google Patents

Description

The present invention relates to Zn-Al-Mg alloy-plated steel sheets, and in particular to Zn-Al-Mg alloy-plated steel sheets with excellent corrosion resistance that are used in the fields of automobiles, construction, civil engineering, home appliances, etc., and to a method for manufacturing the same.

Zn-Al alloy-plated steel sheets have superior corrosion resistance compared to Zn plating, and have been widely used in the fields of automobiles, architecture, civil engineering, home appliances, etc. Representative examples of Zn-Al alloy-plated steel sheets include hot-dip galvanized steel sheets with an Al content of 0.30 mass% or less in the plating layer, hot-dip zinc-5% aluminum alloy-plated steel sheets with an Al content of about 5 mass%, and hot-dip 55% aluminum-zinc alloy-plated steel sheets with an Al content of about 55 mass%. On the other hand, in recent years, Mg has been added to Zn-Al alloy plating to improve corrosion resistance, and Zn-Al-Mg alloy-plated steel sheets with various Al and Mg contents and metal structures have been developed.

For example, Patent Document 1 discloses a hot-dip Zn-Al alloy-plated steel sheet in which a hot-dip Zn-Al alloy plating layer containing 1.0 to 10 mass % Al and 0.2 to 1.0 mass % Mg contains 10 to 30 area % of a ternary eutectic of Zn-Al-MgZn ₂ in the cross section of the plating layer. Patent Document 2 discloses a hot-dip Zn-Al-Mg plating steel sheet having excellent corrosion resistance, in which a hot-dip Zn-Al-Mg plating layer containing 4.0 to 10.0 wt % Al and 1.0 to 4.0 wt _% Mg has a metal structure in which a primary crystal Al phase or a primary crystal Al phase and a Zn single phase are mixed in a base of a ternary eutectic structure of Al/Zn/Zn 2 Mg. Patent Document 3 discloses a Zn-Al-Mg plated steel material in which a single phase of Zn ₂ Mg and/or Zn ₁₁ Mg ₂ is precipitated with a grain size of 0.5 μm or more in a hot-dip Zn-Al-Mg plating layer containing 3 to 15 wt % Al and 4 to 10 wt % Mg, thereby improving the corrosion resistance of a processed portion and the corrosion resistance of a painted end surface. Patent Document 4 discloses a Zn-Al-Mg plated steel sheet having excellent corrosion resistance in a repeated dry-wet environment, in which Mg-Zn compounds contained in a Zn-Al-Mg plating layer containing 4 to 15 wt % Al and 2 to 8 wt % Mg grow in a columnar shape from the interface between the plating layer and the base steel toward the plating surface, and the exposed area ratio on the plating layer surface is 15 to 60%.

JP 2008-138285 A Japanese Patent Application Laid-Open No. 10-306357 JP 2001-20050 A JP 2010-100897 A

In this way, Zn-Al-Mg alloy plated steel sheet exhibits excellent corrosion resistance due to the formation of Zn and Mg intermetallic compounds, Zn ₂ Mg and Zn ₁₁ Mg _2. Furthermore, it is considered that the corrosion retardation effect of Zn by Mg becomes larger and excellent corrosion resistance is exhibited when Zn ₂ Mg or Zn ₁₁ Mg ₂ is present in the plating layer as a single phase of a certain size, rather than being finely distributed in the ternary eutectic as in Patent Documents 3 and 4.

However, it was found that the conventional techniques described in Patent Documents 3 and 4 have the following problems:

In the manufacturing method of Zn-Al-Mg plated steel material described in Patent Document 3, the location of the Zn-Mg alloy single phase in the plated layer cannot be controlled, so the exposed area of the Zn-Mg alloy single phase on the plated layer surface may be small. In this case, the corrosion retardation effect of Zn by Mg is not sufficiently obtained from the early stage of corrosion, resulting in poor corrosion resistance.

On the other hand, in the Zn-Al-Mg plated steel sheet described in Patent Document 4, the Mg-Zn compounds contained in the plating layer are formed in columns from the interface between the plating layer and the base steel toward the surface of the plating layer. These Mg-Zn compounds corrode preferentially over their surroundings, so in areas where the Mg-Zn compounds are present, corrosion of the base steel progresses early and red rust is formed.

The present invention was made in consideration of these circumstances, and aims to provide a Zn-Al-Mg alloy-plated steel sheet with excellent long-term corrosion resistance, i.e., red rust resistance, by controlling the form of the Zn-Mg intermetallic compound, and a manufacturing method thereof.

In order to achieve the above object, the present inventors have conducted extensive research and obtained the following findings.
(1) When the area ratio of the Zn--Mg intermetallic compound phase to the entire surface of the plating layer of a Zn--Al--Mg alloy-plated steel sheet exceeds 80%, the steel sheet exhibits excellent corrosion resistance.
(2) If the average thickness of the Zn--Mg intermetallic compound phase present on the surface of the Zn--Al--Mg based alloy plating layer in the thickness direction cross section is 0.5 μm or more, the corrosion resistance is further improved.
(3) The corrosion resistance is further improved by dissolving Al in the Zn--Mg intermetallic compound phase present on the surface of the Zn--Al--Mg based alloy plating layer.
(4) When the Al concentration in the lower plating layer from the base steel/plating interface to 1/2 of the thickness direction of the Zn-Al-Mg based alloy plating layer is 1.5 times or more the Al concentration in the entire plating layer, the corrosion resistance is further improved.
(5) As a method for making the Zn-Mg intermetallic compound phase exist on the surface of a Zn-Al-Mg based alloy plating layer in an area ratio exceeding 80% relative to the entire plating layer surface, a method of forming an Mg or Mg-Zn layer on the plating layer of a Zn-Al based alloy plated steel sheet and performing a heat treatment is preferred.
(6) When an Mg or Mg-Zn layer of 0.5 g/ ^m2 or more is formed on the surface of a Zn-Al alloy plating layer and heat-treated at a temperature lower than the melting point of the Zn-Al plating layer, a Zn-Mg intermetallic compound phase is easily present on the surface of the plating layer in an area ratio of more than 80% relative to the entire surface of the plating layer, which is preferable.

The gist of the present invention consists of the following configuration.
[1] A Zn-Al-Mg based alloy plated steel sheet having a Zn-Al-Mg based alloy plating layer on at least one surface of the steel sheet,
The Zn-Al-Mg based alloy plated steel sheet is characterized in that an area ratio of a Zn-Mg intermetallic compound phase present on a surface of the Zn-Al-Mg based alloy plating layer to an entire surface of the Zn-Al-Mg based alloy plating layer is more than 80%.
[2] The Zn-Al-Mg alloy plated steel sheet according to [1], characterized in that the Zn-Mg intermetallic compound phase present on the surface of the Zn-Al-Mg alloy plating layer is mainly composed of Zn ₂ Mg with Al as a solid solution and/or Zn ₁₁ Mg ₂ with Al as a solid solution, and has an average thickness of 0.5 μm or more in a cross section in the thickness direction.
[3] The Zn-Al-Mg based alloy plated steel sheet according to claim 1 or 2, characterized in that an Al concentration in a lower plating layer from a base steel/plating interface to 1/2 of the way in a thickness direction of the Zn-Al-Mg based alloy plating layer is 1.5 times or more of an Al concentration in the entire plating layer.
[4] The Zn-Al-Mg based alloy plated steel sheet according to any one of [1] to [3], characterized in that the Zn-Al-Mg based alloy plating layer contains 0.1 to 70 mass % of Al.
[5] The Zn-Al-Mg based alloy plated steel sheet according to [4], characterized in that the Zn-Al-Mg based alloy plating layer further contains 0.005 to 0.1 mass % of Ni.
[6] A method for producing a Zn-Al-Mg-based alloy-plated steel sheet, comprising forming an Mg or Mg-Zn layer on a Zn-Al-based alloy plating layer of a Zn-Al-based alloy-plated steel sheet having a Zn-Al-based alloy plating layer, and performing a heat treatment by heating at a temperature lower than a melting point of the Zn-Al-based alloy plating layer.
[7] The method for producing a Zn-Al-Mg based alloy plated steel sheet according to [6], characterized in that the coating weight of the Mg or Mg-Zn layer is 0.5 g/ ^m2 or more per side.
[8] The method for producing a Zn-Al-Mg based alloy plated steel sheet according to [6] or [7], characterized in that the Zn-Al based alloy plating layer contains 0.1 to 70 mass% of Al and 0 to 10 mass% of Mg.
[9] The method for producing a Zn-Al-Mg based alloy plated steel sheet according to [8], characterized in that the Zn-Al based alloy plating layer further contains 0.005 to 0.1 mass % of Ni.

According to the present invention, it is possible to provide a Zn-Al-Mg alloy-plated steel sheet that has excellent long-term corrosion resistance, i.e., red rust resistance. The Zn-Al-Mg alloy-plated steel sheet of the present invention is suitable for a variety of applications, including automotive applications, home appliance applications, and building materials.

The following describes an embodiment of the present invention. Note that the following description shows one preferred embodiment of the present invention, and the present invention is not limited in any way by the following description.

The Zn-Al-Mg alloy plated steel sheet of the present invention has a Zn-Al-Mg alloy plated layer on at least one surface. In order to obtain excellent corrosion resistance, the coating weight of the Zn-Al-Mg alloy plated layer in the present invention is preferably 10 g/ ^m2 or more per one surface, and more preferably 60 g/ ^m2 or more.

The Zn-Al-Mg alloy plating layer in the present invention is characterized in that it is composed of Zn, Al, Mg and unavoidable impurities, and that the Zn-Mg intermetallic compound phase is present on the surface of the plating layer in an area ratio of more than 80% of the entire surface of the plating layer. The reasons for this limitation are explained below.

Zn-Mg intermetallic compounds are alloys of electrochemically active metals Zn and Mg, and dissolution reactions occur preferentially in corrosive environments. The dissolved Mg ions are mainly taken up into basic zinc chloride, a highly protective corrosion product. As a result, basic zinc chloride is stabilized, and exerts the effect of suppressing the progress of corrosion thereafter. Furthermore, the dissolved Mg exists as Mg(OH) ₂ and Mg ions in a corrosive environment, and the coexistence of these has the effect of exhibiting a pH buffer effect against both acids and alkalis, so that corrosion is further delayed.

The present inventors have found that when the area ratio of the Zn-Mg intermetallic compound phase to the entire surface of the plating layer exceeds 80%, the corrosion resistance is significantly improved. Here, the corrosion resistance refers to the corrosion resistance in the flat portion excluding the end face of the plated steel sheet. Here, the Zn-Mg intermetallic compound phase includes intermetallic compounds containing Zn ₂ Mg and/or Zn ₁₁ Mg ₂ as the main component, and further containing Al and unavoidable components such as ZnMg, Zn ₃ Mg ₂ and Mg ₃₂ (AlZn) _49. Here, the main component means that the content of Zn ₂ Mg and/or Zn ₁₁ Mg ₂ in the Zn-Mg intermetallic compound phase is 60 mass% or more in total. In addition, fine Zn-Mg intermetallic compounds contained in the Zn/Al/Zn ₂ Mg ternary eutectic structure and the Zn/Al binary eutectic structure are excluded. When the area ratio of the Zn-Mg intermetallic compound phase to the entire surface of the plating layer exceeds 80%, the above-mentioned stabilizing effect of basic zinc chloride and pH buffering effect of Mg ions are exerted over almost the entire surface of the plating layer from the early stage of corrosion, so that corrosion is uniformly suppressed over almost the entire surface of the plated steel sheet. Due to the above effects, in the Zn-Al-Mg alloy plated steel sheet of the present invention, the area ratio of the Zn-Mg intermetallic compound phase to the entire surface of the plating layer is set to exceed 80%, and more preferably to be 90% or more.

The present inventors also found that better corrosion resistance can be obtained when the Zn-Mg intermetallic compound phase present on the surface of the plating layer has an average thickness of 0.5 μm or more in the cross section in the thickness direction of the plating layer, and further contains Zn ₂ Mg in which Al is dissolved and/or Zn ₁₁ Mg ₂ in which Al is dissolved as the main component. As described later, the Zn-Mg intermetallic compound phase can be formed by forming an Mg or Mg-Zn layer on the plating layer and performing heat treatment, thereby causing mutual diffusion of Zn in the plating layer and Mg in the Mg-Zn layer formed on the plating layer, and in this case, Zn ₂ Mg and Zn ₁₁ Mg ₂ , which have high thermodynamic stability, are mainly generated. Since Zn ₂ Mg and Zn ₁₁ Mg ₂ alloys have higher corrosion resistance than ZnMg alloy and Zn ₃ Mg ₂ alloy, it is preferable to use these phases as the main components.

If the average thickness of the Zn-Mg intermetallic compound phase in the cross section in the thickness direction of the plating layer is 0.5 μm or more, the stabilizing effect of the basic zinc chloride and the pH buffering effect of the Mg ions described above will be sustained for a longer period, and corrosion will be further delayed, resulting in excellent corrosion resistance. It is preferably 3.0 μm or more, and more preferably 6.0 μm or more.

Furthermore, if Al is dissolved in the Zn-Mg intermetallic compound phase present on the surface of the plating layer, when Al is exposed to the surface as the corrosion of the Zn-Mg intermetallic compound phase progresses, a dense, poorly soluble passive film is formed on the surface, which exhibits a barrier effect, further improving corrosion resistance. The upper limit of the amount of Al in solid solution is determined by the solid solubility limit. If the amount of Al in solid solution exceeds the solid solubility limit, an alloy phase containing Al as the main component is formed, and the area ratio of the Zn-Mg intermetallic compound phase on the surface of the plating layer decreases. To obtain the above-mentioned effect, the amount of Al in solid solution in the Zn-Mg intermetallic compound phase is preferably 0.1 mass% or more.

The inventors also discovered that corrosion resistance is further improved when the Al content in the lower plating layer from the base steel/plating interface (the interface between the steel sheet and the plating layer) to half the thickness of the plating layer is at least 1.5 times the Al concentration in the entire plating layer. This is because, even after all of the Zn-Mg intermetallic compound phase present on the surface of the plating layer has corroded, if the Al concentration in the lower plating is high, the barrier effect of the dense and strong Al passive film is continuously exerted.

As described above, the Zn-Mg intermetallic compound phase present on the surface of the plating layer can be formed by forming an Mg or Mg-Zn layer on the plating layer of a Zn-Al alloy-plated steel sheet having a Zn-Al alloy plating layer, and then performing heat treatment, thereby causing mutual diffusion of Zn in the Zn-Al alloy plating layer and Mg in the Mg or Mg-Zn layer, as described later. Generally, Zn has a lower melting point and a smaller atomic radius than Mg, so that the rate at which Zn diffuses into Mg is faster than the rate at which Mg diffuses into Zn, and therefore Zn ₂ Mg with a low Zn ratio is generated first, followed by Zn ₁₁ Mg ₂ with a high Zn ratio. When the amount of Mg attached is small, almost the entire Mg or Mg-Zn layer is diffused and transformed to Zn ₁₁ Mg ₂ , but as the amount of Mg attached increases, the thickness of the diffusion layer increases, and the time required for complete diffusion transformation to Zn ₁₁ Mg ₂ becomes longer. Therefore, when comparing with the same heating time, the ratio of Zn ₂ Mg in the Mg or Mg-Zn layer increases as the Mg adhesion amount increases. In addition, when the Mg adhesion amount is the same, the lower the heating temperature and the shorter the heat treatment time, the smaller the amount of Zn diffused into the Mg or Mg-Zn layer, and therefore the ratio of Zn ₂ Mg in the Mg or Mg-Zn layer increases. Conversely, the higher the heating temperature and the longer the heat treatment time, the larger the amount of Zn diffused into the Mg or Mg-Zn layer, and therefore the ratio of Zn ₁₁ Mg ₂ in the Mg or Mg-Zn layer increases. Therefore, the formed Zn-Mg intermetallic compound phase has a configuration in which Zn ₁₁ Mg ₂ exists on the plating layer side and Zn ₂ Mg exists on the surface side. In addition, during heat treatment, Al in the plating layer is concentrated in the lower part of the plating layer. At this time, the higher the heating temperature and the longer the heat treatment time, the higher the concentration of Al becomes.

The Al content in the plating layer of the Zn-Al-Mg alloy-plated steel sheet of the present invention is preferably 0.1 to 70 mass%. If the Al content is 0.1 mass% or more, Al dissolves in the Zn-Mg intermetallic compound phase, further improving corrosion resistance. More preferably, it is 2.5 mass% or more. On the other hand, if the Al content exceeds 70 mass%, the sacrificial corrosion protection effect of Zn in the plating layer on the base steel decreases, making red rust more likely to occur and possibly reducing corrosion resistance. More preferably, it is 60 mass% or less.

The plating layer of the Zn-Al-Mg alloy-plated steel sheet of the present invention may contain Ni in order to improve resistance to blackening. When Ni is contained, the Ni content in the plating layer is preferably 0.005 to 0.1 mass%. If the Ni content is less than 0.005 mass%, excellent resistance to blackening cannot be obtained. On the other hand, if the Ni content exceeds 0.1 mass%, as described below, Al-based dross containing Ni is generated in the plating bath when forming the Zn-Al alloy plating layer before forming the Mg or Mg-Zn layer, and the adhesion of the dross may impair the plating appearance, which is not preferable.

The Zn-Al-Mg alloy-plated steel sheet of the present invention is manufactured by forming an Mg or Mg-Zn layer on the Zn-Al alloy plating layer of a Zn-Al alloy-plated steel sheet having the Zn-Al alloy plating layer, and then performing a heat treatment at a temperature lower than the melting point of the Zn-Al alloy plating layer.

The steel sheet used in the present invention is not particularly limited, and cold-rolled steel sheet or hot-rolled steel sheet can be used.

The plating layer of the Zn-Al based alloy plated steel sheet before the formation of the Mg or Mg-Zn layer preferably contains 0.1 to 70 mass% Al and 0 to 10 mass% Mg. The balance other than the above components preferably consists of Zn and unavoidable impurities. The plating layer of the Zn-Al based alloy plated steel sheet contains a Zn phase and a Zn/Al eutectic. When the plating layer of the Zn-Al based alloy plated steel sheet contains Mg, the plating layer contains a Zn phase, a Zn/Al eutectic, and a Zn/Al/Zn ₂ Mg ternary eutectic.

The Al content in the plating layer of the Zn-Al alloy plated steel sheet before the formation of the Mg or Mg-Zn layer is preferably 0.1 to 70 mass%. More preferably, it is 2.5 mass% or more. The reasons for setting the upper and lower limits of the Al content are as described above. Furthermore, if the Al content exceeds 70 mass%, top dross containing Al as the main component is likely to occur in the plating bath, which can cause problems such as impairing the plating appearance, so the upper limit is preferably 70%.

The plating layer of the Zn-Al alloy-plated steel sheet may contain Mg. In this case, the upper limit of the Mg content is preferably set to 10 mass%. If the content exceeds 10 mass%, the generation of dross becomes significant and the appearance of the plating deteriorates, so it is preferable to set the upper limit at 10 mass%.

The plating layer of the above Zn-Al alloy plated steel sheet may further contain Ni. In this case, the Ni content is preferably 0.005 to 0.1 mass%. The reasons for setting the upper and lower limits of the Ni content are as described above.

In the present invention, when forming an Mg or Mg-Zn layer on a Zn-Al alloy plating layer, the formation method can be selected from any method such as vacuum deposition, thermal evaporation, electromagnetic levitation induction heating evaporation, sputtering, electron beam evaporation, ion plating, or electromagnetic levitation physical vapor deposition.

In the present invention, when an Mg or Mg-Zn layer is formed on a Zn-Al alloy plating layer, the coating weight of the Mg or Mg-Zn layer per side is preferably 0.5 g/ ^m2 or more. The coating weight of the Mg or Mg-Zn layer is preferably 0.5 to 10 g/ ^m2 per side. If the coating weight is 0.5 g/ ^m2 or more, the Zn-Mg intermetallic compound phase is easily present on the plating layer surface with a surface area ratio of more than 80%. The coating weight is preferably 2 g/ ^m2 or more, more preferably 4 g/ ^m2 or more. In addition, the Mg content in the Mg-Zn layer is preferably 10 mass% or more. In addition, it is preferable to form the Mg-Zn layer so that the Mg coating weight in the Mg-Zn layer is 0.5 g/ ^m2 or more per side.

The Zn-Al-Mg alloy plated steel sheet of the present invention is prepared by forming an Mg or Mg-Zn layer on the Zn-Al alloy plated steel sheet, and then performing a heat treatment at a temperature lower than the melting point of the Zn-Al plated layer. The melting point here refers to the temperature at which a liquid phase appears when heated. When the heat treatment is performed at a temperature lower than the melting point of the plated layer, a solid phase transformation occurs due to interdiffusion between the Zn phase in the plated layer or the Zn in the Zn/Al eutectic and the Mg or Mg in the Mg-Zn layer formed on the plated layer, and a Zn-Mg intermetallic compound phase is formed on the surface of the plated layer. In addition, by performing the heat treatment at a temperature lower than the melting point of the plated layer, Al is dissolved in the Zn-Mg intermetallic compound phase, and a Zn-Al-Mg alloy plated steel sheet with higher corrosion resistance can be obtained.

If the plating layer is heated at a temperature equal to or higher than its melting point, a liquid phase appears and a Zn-Mg intermetallic compound phase is formed inside the plating layer, so that a structure in which the area ratio of the Zn-Mg intermetallic compound phase to the entire surface of the plating layer exceeds 80% cannot be obtained. For this reason, the heat treatment temperature is set to a temperature lower than the melting point of the plating layer. In general, the melting point of a Zn-Al alloy plating layer with a composition of Al: 0.1 to 15 mass% and Mg: 0.5 to 10 mass% is 340°C to 420°C. Also, the melting point of a Zn-Al alloy plating layer with a composition of Al: 50 to 70 mass% and Mg: 0 to 0.5 mass% is 550 to 600°C. On the other hand, if the heat treatment temperature is lower than 240°C, the diffusion of Zn and Mg is insufficient and a long heat treatment time is required, so the heat treatment temperature is preferably 240°C or higher. More preferably, it is 280°C or higher, and even more preferably, it is 300°C or higher.

The heating method in the heat treatment is not particularly limited, but it is preferable to use a heating furnace. The heat treatment time is also not particularly limited, but considering the time required for the sheet temperature to reach the target temperature and to be maintained at the target temperature, it is preferable that the time from when the sheet is put into the furnace until when it is taken out is 10 minutes or more. In order to suppress oxidation of Mg and Al in the plating layer, heating may be performed under an inert gas atmosphere such as _N2 or Ar.
There are no particular limitations on the cooling method after heating, and any method such as air cooling, cooling using a mold, gas cooling, mist cooling, etc. can be selected.

The present invention will be specifically described below based on examples. The following examples do not limit the present invention, and appropriate modifications within the scope of the gist and configuration of the present invention are considered to be included in the scope of the present invention.

A hot-dip Zn-Al alloy plating layer (except for No. 12, which is a hot-dip Zn alloy plating layer that does not contain Al) was formed on the surface of the steel sheet so as to have the composition and coating weight shown in Table 1. An Mg or Mg-Zn layer was formed on the above plating layer by vacuum deposition in the amount shown in Table 1, and heat treatment was performed in a heating furnace at the specified temperature and time shown in Table 1 to produce a Zn-Al-Mg alloy-plated steel sheet (except for No. 12, which is a Zn-Mg alloy-plated steel sheet that does not contain Al).

Ten samples with a size of 20 mm x 20 mm were taken from any position of the prepared plated steel sheet, and the Zn-Mg intermetallic compound phase was identified by thin film X-ray diffraction. Next, a backscattered electron image (BSE image) of a field of view of 125 μm x 95 μm was obtained for each sample surface using a scanning electron microscope (SEM) at an acceleration voltage of 15 kV, and the same area was subjected to SEM-EDX area analysis to identify the Zn-Mg intermetallic compound phase in the BSE image. Next, the area ratio of the Zn-Mg intermetallic compound (area ratio to the entire surface of the plating layer) excluding Zn ₂ Mg in the Zn/Al/Zn ₂ Mg ternary eutectic was calculated using image analysis software, and the average value of 3 fields of view x 10 samples was taken as the area ratio of the Zn-Mg intermetallic compound.

Next, for the 10 samples whose surfaces had been observed, cross sections were embedded in resin and polished, and a BSE image of a width of 50 μm was obtained at an acceleration voltage of 15 kV. SEM-EDX surface analysis was also performed on the same area to identify the Zn-Mg intermetallic compound phase in the BSE image, and the thickness of the Zn-Mg intermetallic compound phase (thickness in the cross section in the thickness direction of the plating layer) was measured at 10 points, and the average value of 3 fields of view x 10 samples was determined as the thickness of the Zn-Mg intermetallic compound phase.

The amount of Al dissolved in the Zn-Mg intermetallic compound phase was determined by determining the Al concentration in an arbitrary 30 μm wide region of the Zn-Mg intermetallic compound phase within one field of view of the cross-sectional BSE image above using SEM-EDX area analysis, and the average value of the Al concentration for 3 fields of view x 10 samples was taken as the amount of Al dissolved in the phase. The ratio of the Al concentration in the lower plating layer from the base steel/plating interface to 1/2 of the way in the thickness direction to the Al concentration in the entire plating layer was calculated as follows:

First, perpendicular lines were drawn at 12 locations at intervals of approximately 10 μm within one field of view of the cross-sectional BSE image to connect the base steel/plating interface and the surface layer (surface), and the midpoints of each perpendicular line were connected with a line. The plating layer below this line was defined as the lower plating layer. SEM-EDX surface analysis was performed on the lower plating layer and the entire plating layer at an arbitrary location within the plating layer with a width of 30 μm, respectively, to determine the Al concentrations of the lower plating layer and the plating layer, and the ratio of the Al concentration in the lower plating layer to the Al concentration of the entire plating layer was calculated. The above process was performed on 3 fields of view x 10 samples, and the average value was defined as the lower Al enrichment (the ratio of the Al concentration in the lower plating layer from the base steel/plating interface to 1/2 of the way in the thickness direction to the Al concentration of the entire plating layer).

(1) Corrosion Resistance A sample with a size of 150 mm × 50 mm was cut out from the prepared plated steel sheet, and the end faces and the back face were sealed with tape. Then, a salt spray test was carried out in accordance with JIS Z 2371 standard, and the time until red rust appeared on the surface was evaluated according to the following criteria.
<Criteria>
◎: Red rust occurrence time is 4000 hours or more. 〇: Red rust occurrence time is 3500 hours or more and less than 4000 hours. 〇-: Red rust occurrence time is 3000 hours or more and less than 3500 hours. 〇=: Red rust occurrence time is 2500 hours or more and less than 3000 hours. △: Red rust occurrence time is 2000 hours or more and less than 2500 hours. ×: Red rust occurrence time is less than 2000 hours.

(2) Blackening Resistance A sample of 50 mm x 50 mm size cut out from the prepared plated steel sheet was placed in a thermo-hygrostat controlled at a temperature of 80°C and a relative humidity of 98% for 24 hours, and evaluated based on the change in lightness (L value) (ΔL = L value after test - L value before test) and visual appearance observation according to the following criteria. The L value was measured in SCI mode (including regular reflection light) using an SR2000 manufactured by Nippon Denshoku Industries Co., Ltd.
<Criteria>
◯: -10<ΔL and uniform appearance without unevenness ◯-: -14<ΔL≦-10 and uniform appearance without unevenness △: -14<ΔL≦-10 and slight unevenness in appearance ×: ΔL≦-14 or noticeable unevenness in appearance

(3) Appearance A sample measuring 230 mm × 350 mm was cut out from the prepared plated steel sheet, and the number of defects such as bare spots, peeled plating, fading, cracks, etc. was visually counted and evaluated according to the following criteria.
<Criteria>
◯: No defects ◯-: 1 to 2 defects △: 3 to 10 defects ×: 11 or more defects

When the Zn-Mg intermetallic compound phase is present at a surface area ratio of more than 80% on the surface of the Zn-Al-Mg alloy plating layer, excellent corrosion resistance is exhibited. Furthermore, when the Zn-Mg intermetallic compound phase has an average thickness of 0.5 μm or more in the cross section in the thickness direction and further contains solid solution Al, corrosion resistance is further improved. Furthermore, when the Al concentration in the lower plating layer from the base steel/plating interface to 1/2 of the way in the cross section in the thickness direction is 1.5 times or more the Al concentration of the entire plating layer, corrosion resistance is further improved.

The Zn-Al-Mg alloy-plated steel sheet of the present invention has excellent corrosion resistance and can be applied to a wide range of fields, including automobiles, electrical equipment, and building materials.

Claims (8)

A Zn-Al-Mg based alloy plated steel sheet having a Zn-Al-Mg based alloy plating layer on at least one surface of the steel sheet,
an area ratio of a Zn-Mg intermetallic compound phase present on a surface of the Zn-Al-Mg alloy plating layer to an entire surface of the Zn-Al-Mg alloy plating layer is more than 80%;
The Zn-Mg intermetallic compound phase present on the surface of the Zn-Al-Mg based alloy plating layer contains Zn ₂ Mg and/or Zn ₁₁ Mg ₂ in total at 60 mass% or more and has an average thickness of 0.1 μm or more in a cross section in the thickness direction;
The Zn-Al-Mg based alloy plated steel sheet is characterized in that the Zn-Al-Mg based alloy plated layer contains 0.1 to 70 mass % of Al . 2. The Zn-Al-Mg alloy plated steel sheet according to claim 1, wherein the Zn-Mg intermetallic compound phase present on the surface of the Zn-Al-Mg alloy plating layer contains 60 mass% or more in total of Zn ₂ Mg in which Al is dissolved and/or Zn ₁₁ Mg ₂ in which Al is dissolved, and has an average thickness of 0.5 μm or more in a cross section in the thickness direction. The Zn-Al-Mg alloy-plated steel sheet according to claim 1 or 2, characterized in that the Al concentration in the lower plating layer from the base steel/plating interface to 1/2 of the thickness direction of the Zn-Al-Mg alloy plating layer is 1.5 times or more the Al concentration in the entire plating layer. The Zn-Al-Mg alloy plated steel sheet according to any one of claims 1 to 3 , characterized in that the Zn-Al-Mg alloy plating layer further contains 0.005 to 0.1 mass % of Ni. A method for producing a Zn-Al-Mg-based alloy-plated steel sheet , comprising: forming an Mg or Mg-Zn layer having a coating weight of 0.4 to 10 g/ ^m2 per side on a Zn-Al-based alloy plating layer of a Zn-Al-based alloy-plated steel sheet having a Zn-Al-based alloy plating layer containing 0.1 to 70 mass% of Al; and performing a heat treatment for 5 minutes or more at a temperature lower than the melting point of the Zn-Al-based alloy plating layer and not lower than 200°C . 6. The method for producing a Zn-Al-Mg based alloy plated steel sheet according to claim 5 , wherein the coating weight of the Mg or Mg-Zn layer is 0.5 g/ ^m2 or more per side. 7. The method for producing a Zn-Al-Mg based alloy plated steel sheet according to claim 5 , wherein the Zn-Al based alloy plating layer contains 0.1 to 70 mass % of Al and 0 to 10 mass % of Mg. The method for producing a Zn-Al-Mg based alloy plated steel sheet according to claim 7 , wherein the Zn-Al based alloy plating layer further contains 0.005 to 0.1 mass % of Ni.