Plated steel material

Abstract

This plated steel material has a steel material, a base plating layer disposed on the surface of the steel material, and a Zn-based plating layer disposed on the surface of the base plating layer. The Zn-based plating layer has a prescribed average chemical composition. The base plating layer contains Cr or Ni, and if the maximum value of the Cr concentration or the Ni concentration is defined as 100%, in terms of mass%, the maximum oxygen concentration is less than 1% in a range where the Cr concentration or the Ni concentration is 5% or more, in the element distribution profile as quantitatively analyzed by glow discharge optical emission spectrometry from the surface of the Zn-based plating layer to the steel material.

Description

Plated steel

[0001] This disclosure relates to plated steel materials. This application claims priority under Japanese Patent Application No. 2025-011612, filed in Japan on January 27, 2025, the contents of which are incorporated herein by reference.

[0002] When steel materials are to be used for a long period of time, it is necessary to apply some kind of rust prevention treatment to them so that they can withstand corrosion. Hot-dip galvanizing is a widely known method of such rust prevention treatment.

[0003] Steel materials coated with a zinc-based (Zn-based) plating layer by the hot-dip galvanizing method can achieve long-term corrosion resistance at low cost due to the sacrificial corrosion protection effect of the plating layer. Therefore, they are used in various fields where rust prevention of steel materials is required, such as civil engineering, construction, and the automotive industry.

[0004] As for the composition system that makes up a hot-dip zinc plating layer, an alloy component in which Al, Mg, etc. are added to the main component Zn is known, and plated steel materials having a zinc-based alloy plating layer with such components have been put into practical use as plated steel materials with excellent corrosion resistance.

[0005] On the other hand, molten Zn-based plating layers, which have zinc-based alloy components as described above, have poor ductility, and therefore, when subjected to processes such as bending and drawing, the plating layer cracks, exposing the base steel. In the base steel exposed by cracking, an anodic and cathodic reaction occurs between it and the surrounding plating layer, and the plating layer oxidizes and dissolves, providing sacrificial protection for the base steel itself. However, compared to parts that have not been subjected to bending or drawing, i.e., sound parts where cracking has not occurred, a deterioration in the corrosion resistance of the exposed parts is unavoidable.

[0006] One way to solve these problems is to apply a corrosion-resistant coating to the surface of the base steel, and then apply a Zn-based plating layer to the surface of that coating. Examples of coatings to be applied between the base steel and the plating layer include a Cr-based pre-plating layer and a Ni-based pre-plating layer.

[0007] Patent Document 1 discloses a method of forming a Cr plating layer on the surface of a steel sheet and then forming various Zn-based plating layers thereon by a molten salt method, an electroplating method, a hot-dip plating method, or a vapor deposition plating method.

[0008] Patent Document 2 discloses a method of improving the wettability with respect to a molten Zn-based plating layer by forming a Cr plating layer on the surface of an Mn-containing steel.

[0009] Japanese Patent Application Laid-Open No. 6-33263, Japanese Patent Application Laid-Open No. 2010-144190

[0010] However, as described in Patent Document 1, when forming a molten Zn-based plating layer on a Cr plating layer, since an oxide film exists on the surface of the Cr plating layer, the wettability with respect to the molten Zn-based metal is very poor. As a result, it is difficult to obtain sufficient wetting only by heating in a reducing atmosphere, which is usually performed as a pretreatment for hot-dip plating. Further, when forming a molten Zn-based plating layer on a Cr plating layer, problems such as the occurrence of non-plating and poor adhesion of the plating layer occur, but Patent Document 1 does not mention these problems.

[0011] Further, in the method described in Patent Document 2, annealing is performed after forming the Cr plating layer to form a composite oxide with Mn in the base steel, and then hot-dip Zn plating is performed. In such a method, when the formation amount of the Cr plating layer becomes excessive, the wettability of the Zn-based plating layer may decrease, and the adhesion of the plating layer may deteriorate.

[0012] The present invention has been made in view of the above circumstances, and an object thereof is to provide a plated steel material having excellent corrosion resistance in a portion where the base steel is exposed and excellent adhesion of the plating layer.

[0013] The present invention has been made based on the above findings, and the gist thereof is as follows.

[0014] (1) A plated steel material according to one aspect of the present invention is a plated steel material having a steel material, an undercoat plating layer provided on the steel material, and a Zn-based plating layer provided on the undercoat plating layer, wherein the average chemical composition of the Zn-based plating layer is, in mass%, Al: 0.01% or more and less than 45.0%, Mg: 0% or more and 15.0% or less, Si: 0% or more and 2.0% or less, Cr: 0% or more and 3.00% or less, Ni: 0% or more and 3.0% or less, Sn: 0% or more and 0.7% or less, Bi: 0% or more and 0.3% or less, In: 0% or more and 0.3% or less, Ca: 0% or more and 0.6% or less, Y: 0% or more and 0.3% or less, La: 0% or more and 0.3% or less, Ce: 0% or more and 0.3% or less Sr: 0% or more, 0.3% or less, Li: 0% or more, 0.3% or less, Mo: 0% or more, 1.0% or less, Fe: 0% or more, 5.0% or less, Sb: 0% or more, 0.25% or less, Pb: 0% or more, 0.25% or less, B: 0% or more, 0.50% or less, P: 0% or more, 0.50% or less, Ti: 0% or more, 0.25% or less, Co: 0% or more, 0.25% or less, V: 0% or more, 0.25% or less, Nb: 0% or more, 0.25% or less, Mn: 0% or more, 0.25% or less, Zr: 0% or more, 0.25% or less, W: 0% or more, 0.25% or less, Cu: 0% or more, 0.25% or less, Ag: 0% or more, 0.25% or less, with the remainder being Zn and impurities, and the undercoat plating layer is a Cr plating layer, a Cr-Mo plating layer, a Ni-Mo plating layer, a Ni-Co plating layer, or a Ni-W plating layer as the first plating layer, and the amount of adhesion of the first plating layer is 0.01 to 1.0 g / m² per side. ２and in the element distribution profile obtained by quantitative analysis by glow discharge optical emission spectrometry from the surface of the Zn-based plating layer toward the steel material, when the maximum value of the Cr or Ni concentration is 100% by mass%, the maximum oxygen concentration in the range where the Cr concentration or Ni concentration is 5% or more is less than 1%. (2) In the plated steel material according to (1) above, the first plating layer may be the Cr plating layer, and a Ni plating layer may be provided as the second plating layer between the surface of the steel material and the Cr plating layer. (3) In the plated steel material according to (1) above, the first plating layer may be the Cr-Mo plating layer, and a Ni plating layer may be provided as the second plating layer between the surface of the steel material and the Cr-Mo plating layer.

[0015] According to the above aspect of the present invention, it is possible to provide a plated steel material excellent in corrosion resistance in a portion where the base steel is exposed and adhesion of the plating layer. Here, the "adhesion of the plating layer" refers to the wettability of the Zn-based plating layer, and the "adhesion of the plating layer" can be enhanced by improving the wettability of the Zn-based plating layer.

[0016] FIG. 1 is a cross-sectional schematic view of a Cr-based plating layer and a molten plating metal when a steel material provided with a Cr-based plating layer is immersed in a plating bath by a conventional method. FIG. 2 is an example of an element distribution profile when a Zn-based plating layer is formed on the surface of a steel material while applying ultrasonic vibration in a plating bath. FIG. 3 is an example of an element distribution profile when a Zn-based plating layer is formed on the surface of a steel material while applying ultrasonic vibration in a plating bath. FIG. 4 is a schematic diagram for explaining an example of a method for manufacturing a molten-plated steel material by the vacuum plating method of the present embodiment. FIG. 5 is a schematic diagram for explaining an example of a method for manufacturing a molten-plated steel material by the ultrasonic plating method of the present embodiment. FIG. 6 is a schematic diagram for explaining an example of a method for manufacturing a molten-plated steel material by the ultrasonic plating method of the present embodiment.

[0017] The plated steel material according to this embodiment and a preferred method for manufacturing it will be described below. In the following description, the case in which a Cr-based plating layer is mainly applied as an example of a pre-plating layer will be described, but a Ni-based pre-plating layer can also be applied as the pre-plating layer according to this embodiment. That is, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications are possible without departing from the spirit of the invention. In addition, the numerical limit ranges described below, separated by "~", include both a lower limit and an upper limit. Numerical values ​​indicated as "less than" or "greater than" do not include the numerical range. In the following description, the percentages for the chemical composition of the Zn-based plating layer are mass percent unless otherwise specified.

[0018] First, we will explain the new findings obtained by the present inventors regarding the corrosion resistance and adhesion of exposed parts in plated steel materials having a Zn-based plating layer (also called hot-dip Zn-based plated steel materials).

[0019] Conventionally, when a chromium plating layer is applied as a pre-plating layer (under-plating layer) on a base steel, and then a molten zinc-based plating layer is formed on top of it, there was a problem in that the wettability to the molten zinc-based metal was very poor because an oxide film was present on the surface of the chromium plating layer.

[0020] Therefore, the inventors investigated the relationship between the adhesion of the Zn-based plating layer to the Cr-based plating layer (hereinafter also referred to as plating adhesion) and the structure of the plating layer in molten Zn-based plated steel. As a result, they found that plating adhesion can be improved by reducing the amount of oxide present at the interface between the Zn-based plating layer and the Cr-based plating layer. Specifically, when quantitatively analyzed by glow discharge emission spectrometry (GDS), which is used as an indicator of the amount of oxide generated, they found that good plating adhesion can be obtained when the oxygen concentration near the Zn-based plating layer / Cr-based plating layer interface is less than 1% by mass. The notation "Zn-based plating layer / Cr-based plating layer interface" refers to the interface between the Zn-based plating layer and the Cr-based plating layer.

[0021] The oxygen near this Zn-based plating layer / Cr-based plating layer interface mainly originates from oxides of the metal components constituting the Cr-based plating layer and gaps present at the interface with the Zn-based plating layer.

[0022] Figure 1 is a schematic cross-sectional view of the Cr-based plating layer and the hot-dip galvanized metal when a steel material, which has been prepared with a Cr-based plating layer as a pre-plating layer (under-plating layer) using a conventional method, is immersed in a plating bath. Although fine irregularities exist on the surface of the steel material, the hot-dip galvanized metal has difficulty conforming to these irregularities due to its surface tension. Therefore, when the steel material is immersed in the plating bath, a gas layer exists between the steel material and the hot-dip galvanized metal (see Figure 1).

[0023] As described above, a gas layer exists between the Cr-based plating layer and the molten metal plating, preventing good wetting. Therefore, we investigated methods to remove this gas layer and identified two methods that can achieve good wetting. One is plating under reduced pressure (hereinafter also called the reduced pressure plating method), and the other is a method in which ultrasonic vibrations are applied to the plating bath when the steel material is immersed, generating cavitation around the immersed steel material (hereinafter also called the ultrasonic plating method). Here, "cavitation" refers to bubbles that are generated in the molten metal.

[0024] The vacuum plating method reduces the absolute amount of gas present between the Cr-based plating layer and the molten Zn-based metal when a steel material with a Cr-based plating layer is immersed in a Zn-based molten plating bath. As a result, improved wetting is achieved. Furthermore, if the pressure is reduced from an atmospheric environment, the reduction in oxygen partial pressure suppresses the growth of the oxide layer formed on the surface of the Zn-based molten plating bath. This increases the reactivity between the Cr-based plating layer and the Zn-based molten plating metal, making it easier to obtain wetting.

[0025] The mechanism by which good wetting is obtained by ultrasonic vibration is hypothesized as follows: When ultrasonic vibration is applied to the plating bath, cavitation is generated and eliminated even in the Zn-based molten metal (plating bath). When the cavitation is eliminated, the impact causes the Zn-based molten metal to lose its surface shape due to surface tension, and the gas layer is dispersed and removed. Wetting is then achieved when the Zn-based molten metal comes into contact with the oxide layer on the surface of the Cr-based plating layer.

[0026] The wetting achieved by these methods is a physical wetting resulting from the removal of the gas layer due to acceleration generated by reduced pressure and cavitation cessation. In other words, in this embodiment, removal of the oxide layer on the surface of the Cr-based plating layer is not essential to ensure the wettability of the plating. For example, even if the plating substrate is a ceramic that does not react at all with the molten plating metal, wetting can be obtained even in the atmosphere by removing the gas layer on the surface by applying appropriate cavitation. Similarly, even with steel materials equipped with a Cr-based plating layer, good wetting can be obtained even when immersed in a plating bath in the atmosphere without reducing or removing the oxide film on the surface of the Cr-based plating layer.

[0027] After this physical wetting is achieved, if the material is subsequently immersed in the plating bath, the Al contained in the Zn-based hot-dip galvanizing metal reduces the oxide film on the surface of the Cr-based plating layer, further reducing the oxygen concentration near the Zn-based plating layer / Cr-based plating layer interface. In this state, when the oxide film on the surface of the Cr-based plating layer is reduced by Al, quantitative analysis by GDS shows that the oxygen concentration near the Zn-based plating layer / Cr-based plating layer interface is less than 1% by mass.

[0028] Figures 2 and 3 show the elemental distribution profiles when a Zn-based plating layer is formed on the surface of a steel material while applying ultrasonic vibrations in the plating bath. Figure 2 shows the profile when using steel material with a Cr-based plating layer, and Figure 3 shows the profile when using steel material with both a Ni-based and a Cr-based plating layer.

[0029] As shown in Figures 2 and 3, when the maximum Cr concentration is set to 100%, it can be seen that the oxygen concentration is less than 1% by mass in the range between D1 and D2, where the Cr concentration is 5% or more relative to this maximum value. In other words, when a Zn-based plating layer is formed on the surface of a steel material while applying ultrasonic vibrations in the plating bath, no oxide layer is formed at or near the Zn-based plating layer / Cr-based plating layer interface, resulting in good plating adhesion.

[0030] Furthermore, in the ultrasonic plating method of this embodiment, it is effective to apply optimal ultrasonic vibrations over a wide area in the plating bath in order to obtain plated steel having the above-mentioned oxygen concentration. Conventional ultrasonic plating methods often involved placing the steel material in close proximity to the tip of a horn placed in the plating bath. This is because the amplitude change is greatest at the tip of the horn, and therefore the cavitation generated there is the strongest. However, in such conventional methods, the cavitation intensity decreases rapidly at locations away from the tip of the horn, so it was necessary to limit the placement of the steel material to close proximity to the tip of the horn. For example, it was necessary to provide a plane at the tip of the horn, place the steel material parallel to that plane, and limit the distance between the steel material and the horn to about a few millimeters.

[0031] In contrast to such conventional methods, in this embodiment, the position of the steel material is not limited to the vicinity of the horn tip. In other words, by deeply immersing the horn in the plating bath, good wetting can be obtained not only at the horn tip but also at the steel material positioned opposite the side of the horn.

[0032] The horn expands and contracts in the longitudinal direction, and its cross-sectional area also changes. In this embodiment, good wetting is obtained by utilizing the cavitation generated by such longitudinal expansion and contraction and changes in cross-sectional area. Specifically, when the horn is resonantly vibrating, the part where the change in cross-sectional area is greatest is located at a position 1 / 4 wavelength away from the tip of the horn. Immersing the horn in the plating bath so that at least the portion up to this position is immersed in the bath is effective in increasing cavitation.

[0033] Generally, in ultrasonic cleaning of aqueous solutions, ultrasonic vibrations propagating through the bath reflect off the walls of the cleaning tank and the water surface, generating standing waves and causing cavitation at various points within the tank. A similar phenomenon is thought to occur in zinc-based hot-dip galvanized metals (plating baths), which is believed to allow for good wetting even when the steel material is located far from the tip of the horn.

[0034] The hot-dip Zn-plated steel material according to this embodiment was made based on the above-mentioned findings obtained by the inventors.

[0035] [Hot-dip Zn-plated steel material] The plated steel material according to this embodiment (hereinafter also referred to as hot-dip Zn-plated steel material or hot-dip plated steel material) comprises a steel material, a base plating layer disposed on the surface of the steel material, and a Zn-plated layer disposed on the surface of the base plating layer. The average chemical composition of the Zn-based plating layer is as follows (in mass%): Al: 0.01% or more, less than 45.0%, Mg: 0% or more, 15.0% or less, Si: 0% or more, 2.0% or less, Cr: 0% or more, 3.0% or less, Ni: 0% or more, 3.0% or less, Sn: 0% or more, 0.7% or less, Bi: 0% or more, 0.3% or less, In: 0% or more, 0.3% or less, Ca: 0% or more, 0.6% or less, Y: 0% or more, 0.3% or less, La: 0% or more, 0.3% or less, Ce: 0% or more, 0.3% or less, Sr: 0% or more, 0.3% or less, Li: 0% or more, 0.3% or less, Mo: 0% or more, 1.0% or less, Fe: 0% or more, 5.0% or less. Sb: 0% or more, 0.25% or less; Pb: 0% or more, 0.25% or less; B: 0% or more, 0.50% or less; P: 0% or more, 0.50% or less; Ti: 0% or more, 0.25% or less; Co: 0% or more, 0.25% or less; V: 0% or more, 0.25% or less; Nb: 0% or more, 0.25% or less; Mn: 0% or more, 0.25% or less; Zr: 0% or more, 0.25% or less; W: 0% or more, 0.25% or less; Cu: 0% or more, 0.25% or less; Ag: 0% or more, 0.25% or less; the remainder consists of Zn and impurities. Furthermore, in the plated steel material according to this embodiment, the undercoat plating layer is a Cr plating layer, a Cr-Mo plating layer, a Ni-Mo plating layer, a Ni-Co plating layer, or a Ni-W plating layer as the first plating layer. In addition, in the elemental distribution profile obtained by quantitative analysis of the steel material from the surface of the Zn-based plating layer by glow discharge emission spectrometry, when the maximum value of the Cr concentration or Ni concentration is taken as 100% by mass%, the maximum oxygen concentration in the range where the Cr concentration or Ni concentration is 5% or more is less than 1%. That is, the maximum oxygen concentration in the range where the Cr concentration is 5% or more relative to the maximum value of the Cr concentration, or where the Ni concentration is 5% or more relative to the maximum value of the Ni concentration, is less than 1%.Furthermore, if the first plating layer in this embodiment is a Cr plating layer, a Ni plating layer may be provided as a second plating layer between the surface of the steel material and the Cr plating layer. Also, if the first plating layer in this embodiment is a Cr-Mo plating layer, a Ni plating layer may be provided as a second plating layer between the surface of the steel material and the Cr-Mo plating layer.

[0036] (Steel Materials) First, let's explain the steel materials (base plates) that are to be plated. The steel materials are mainly steel plates, but there are no particular restrictions on their size. The steel plates can be any type that is applicable to a normal hot-dip galvanizing process. Specifically, this includes steel plates that can be used in processes where the material is immersed in molten metal and solidified, such as continuous hot-dip galvanizing lines (CGL). As for the size of the steel plates, for example, plates with a thickness of 10 mm or less and a width of 2000 mm or less can be used, but the size of the steel plates is not limited to these.

[0037] The material of the steel is not particularly limited. Applicable steels include, for example, general steel, pre-plated steel with thin layers of various metals, Al-killed steel, ultra-low carbon steel, high carbon steel, various high-tensile steels, some high-alloy steels (such as steels containing corrosion-resistant strengthening elements like Ni and Cr), bolt steel, and steel wire for bridge cables. More specifically, steel materials include, for example, hot-rolled steel sheets as defined in JIS G 3131 (2018), cold-rolled steel sheets as defined in JIS G 3141 (2017), general structural rolled steel materials corresponding to so-called SS materials as defined in JIS G 3101 (2015), pre-plated steel with various metals thinly plated on it, as described in JIS H 8641 (2021), JIS G 3302 (2019), 3303 (2019), 3313 (2017), 3314 (2019), 3315 (2019), 3317 (2019), and 3321 (2019), rolled steel materials for building structures as described in JIS G 3136 (2012), and JIS G Al-killed steel, ultra-low carbon steel, and high carbon steel as described in JIS 3126 (2015), various high-tensile steels as described in JIS G 3113 (2018), 3134 (2018), and 3135 (2018), and some high-alloy steels (such as steels containing corrosion-resistant strengthening elements like Ni and Cr) are applicable.

[0038] (Plating Layer) Next, the undercoat plating layer and the Zn-based plating layer provided on the steel material will be described. In this embodiment, the undercoat plating layer is a Cr plating layer, a Cr-Mo plating layer, a Ni-Mo plating layer, a Ni-Co plating layer, or a Ni-W plating layer as the first plating layer. The term "Cr plating layer" here refers to a layer in which the Cr plating layer reacts with Fe in the underlying steel material that has been heat-diffused. The term "Cr plating layer" here refers to a layer in which the Cr concentration is 0.1% or more. The term "Cr-Mo plating layer" refers to a layer in which the Cr concentration is 0.1% or more and the Mo concentration is 0.01% or more. The term "Ni-Mo plating layer" refers to a layer in which the Ni concentration is 0.1% or more and the Mo concentration is 0.01% or more. The term "Ni-Co plating layer" refers to a layer in which the Ni concentration is 0.1% or more and the Co concentration is 0.01% or more. The term "Ni-W plating layer" refers to a layer in which the Ni concentration is 0.1% or higher and the W concentration is 0.01% or higher.

[0039] If the first plating layer is a Cr plating layer, the underlayment plating layer may be a two-layer plating layer having a Ni plating layer as the second plating layer below the Cr plating layer. In other words, the plated steel material may have a Ni plating layer between the surface of the steel material and the Cr plating layer. The "Ni plating layer" referred to here may be an Fe-Ni alloy layer formed by the reaction of the Ni plating layer with Fe in the underlying steel material that has been heat-diffused, a Ni-Cr alloy layer formed by the reaction with the Cr plating layer, or an Fe-Ni-Cr alloy layer.

[0040] When the first plating layer is a Cr-Mo plating layer, the underlayer plating layer may be a two-layer plating layer having a Ni plating layer as the second plating layer below the Cr-Mo plating layer, similar to the one described above. In other words, the plated steel material may have a Ni plating layer between the surface of the steel material and the Cr plating layer. As described above, when the underlayer plating layer is a two-layer plating layer, the thickness of the underlayer plating layer itself increases, resulting in better corrosion resistance of the processed part.

[0041] The Zn-based plating layer according to this embodiment may include a Zn-Al alloy layer. The inclusion of alloying elements such as Al in the Zn phase improves corrosion resistance. Therefore, in the case of a plating layer containing such a Zn phase, even if it is a thin film (for example, about half the thickness of a normal Zn plating layer), it can exhibit corrosion resistance equivalent to that of a normal Zn plating layer. Similarly, even when the plating layer of this embodiment is a thin film, corrosion resistance equivalent to or better than that of a conventional Zn plating layer is ensured.

[0042] The Zn-Al alloy layer consists of a Zn-Al alloy. A Zn-Al alloy refers to a binary alloy containing Zn and Al. It is preferable that the Zn-Al alloy layer contains Mg from the viewpoint of improving corrosion resistance.

[0043] There are no particular upper or lower limits to the thickness of the Zn-based plating layer. Furthermore, the thickness is affected by the steel material withdrawal speed from the plating bath and the wiping conditions. That is, the overall thickness of the Zn-based plating layer is affected by the viscosity and specific gravity of the plating bath in the case of continuous hot-dip galvanizing. The maximum thickness of the plating layer formed by continuous hot-dip galvanizing is often 80 μm or less. Therefore, the thickness of the Zn-based plating layer on the hot-dip galvanized steel material in this embodiment may be, for example, 80 μm or less. There are also no particular limits to the amount of Zn-based plating layer applied, but for example, 480 g / m² per side. ２ The following is acceptable.

[0044] Next, we will explain the average chemical composition of the Zn-based plating layer.

[0045] Al: 0.01% or more, less than 45.0%. Al, along with Zn, is a main component of the Zn-based plating layer. Including 0.01% or more improves plating properties and appearance. While including Al is effective in improving corrosion resistance, if the Al concentration exceeds 45.0%, sufficient sacrificial corrosion protection may not be ensured in cut and bent areas. Therefore, the Al concentration is less than 45.0%.

[0046] Mg: 0% or more, 15.0% or less. Mg has the effect of improving the sacrificial corrosion protection of the plating layer. However, if Mg is present in excess, a rapid oxidation reaction may occur on the surface of the plating bath, making it impossible to perform plating stably. In the range where the Al concentration is less than 45.0%, Mg can be included up to 15%, so the upper limit of the Mg concentration in this embodiment is 15.0% or less.

[0047] Si: 0% or more, 2.0% or less. If the plating bath contains Mg, then the intermetallic compound Mg is present in the plating layer. ２ Forms Si. Mg ２ As the Si content increases, the Si phase becomes coarser, leading to a decrease in the corrosion resistance and workability of plated steel. Therefore, when Si is included, the upper limit of the Si content is 2.0% or less.

[0048] Sn: 0% or more, 0.7% or less Bi: 0% or more, 0.3% or less In: 0% or more, 0.3% or less Sn, Bi, and In are elements that, when included in a Zn-based plating layer, promote softening of the plating layer and impart sacrificial corrosion protection. Since Sn, Bi, and In are elements that can be included arbitrarily, their respective contents should be 0% or more. By including at least one of Sn, Bi, and In, an effect of improving the corrosion resistance of the processed part can be obtained.

[0049] Ca: 0% to 0.6% Y: 0% to 0.3% La: 0% to 0.3% Ce: 0% to 0.3% Sr: 0% to 0.3% Li: 0% to 0.3% Ca, Y, La, Ce, Sr, and Li are elements that can be included within the above ranges. Ca, Y, La, Ce, Sr, and Li all form intermetallic compounds with Zn, Al, etc. However, if the content of these elements is within the above ranges, it will not affect the initial corrosion of the plating layer. On the other hand, if these elements are included in excess, a potential difference will be generated in the plating layer, and a lot of initial white rust may occur. Therefore, if these elements are included, it is best to keep them within the above ranges.

[0050] Ni: 0% to 3.0% Cr: 0% to 3.00% Ni and Cr also form intermetallic compounds with Zn, Al, etc. However, if the content of these elements is within the above range, it will not affect the initial corrosion of the plating layer. On the other hand, if these elements are present in excess, a potential difference will be generated in the plating layer, and a large amount of initial white rust may occur. Therefore, if these elements are present, it is best to keep them within the above range. Note that Ni and Cr are constituent elements of the undercoat plating layer provided between the Zn-based plating layer and the steel material. In other words, Ni or Cr originating from the undercoat plating layer may diffuse into the Zn-based plating layer, but the amount is extremely small.

[0051] Mo: 0% or more, 1.0% Sb: 0% or more, 0.25% Pb: 0% or more, 0.25% B: 0% or more, 0.50% or less P: 0% or more, 0.50% or less Ti: 0% or more, 0.25% or less Co: 0% or more, 0.25% or less V: 0% or more, 0.25% or less Nb: 0% or more, 0.25% or less Mn: 0% or more, 0.25% or less Zr: 0% or more, 0.25% or less W: 0% or more, 0.25% or less Mo, Sb, Pb, B, P, Ti, Co, V, Nb, Mn, Zr, and W also form intermetallic compounds with Zn, Al, etc. However, if the content of these elements is within the above range, it will not affect the initial corrosion of the plating layer. On the other hand, if these elements are included in excess, a potential difference may occur in the plating layer, which may lead to the formation of a large amount of initial white rust. Therefore, if they are included, it is best to keep them within the above range.

[0052] Fe: 0% or more, 5.0% or less. In this embodiment, Fe may diffuse from the raw material to the plating layer during manufacturing, but the amount is extremely small. If the Fe concentration in the Zn-based plating layer is 5.0% or less, there will be no adverse effects such as an increased frequency of crack occurrence in the Zn-based plating layer.

[0053] Remainder: Zn and impurities. Zn is the element that constitutes the main phase of the Zn-based plating layer. Since the hot-dip galvanized steel material of this embodiment is a highly versatile Zn-based galvanized steel material, the element that constitutes the main phase of the plating layer is Zn.

[0054] Impurities refer to components contained in the raw materials or components introduced during the manufacturing process. For example, in the plating layer, trace amounts of components other than Fe may be introduced as impurities due to atomic diffusion between the steel (base metal) and the plating bath. Also, since metals of 3N purity are usually used to manufacture the plating alloy, the total concentration of impurities can be approximately 0.03% or less.

[0055] To identify the average chemical composition of the plating layer, an acid solution is obtained by stripping and dissolving the plating layer with an acid containing an inhibitor that suppresses corrosion of the base metal (steel). Next, the chemical composition can be obtained by measuring the obtained acid solution using ICP emission spectrometry or ICP-MS. There are no particular restrictions on the type of acid, as long as it is an acid that can dissolve the plating layer. If the area and weight before and after stripping are measured, the amount of plating adhesion (g / m²) can be determined. ２ ) can also be obtained at the same time.

[0056] Next, we will explain the oxygen concentration distribution near the interface between the Zn-based plating layer and the undercoat plating layer. To understand the component distribution in the depth direction of the Zn-based plating layer and the undercoat plating layer, glow discharge emission spectrometry (GDS) is used. Specifically, GDS is used to quantitatively analyze the concentrations of each metal component and oxygen that make up each plating layer. The concentration of each component can be calculated using a calibration curve obtained from the correlation between the time integral value of the emission intensity and the concentration of standard samples with known concentrations.

[0057] The plated steel material according to this embodiment improves plating adhesion by reducing the oxide layer formed near the interface between the Zn-based plating layer and the undercoat plating layer. Specifically, in quantitative analysis of GDS, it is effective to keep the maximum oxygen concentration detected at and near the interface between the Zn-based plating layer and the undercoat plating layer below a predetermined value. Specifically, as an example, if the undercoat plating layer is a Cr plating layer, and the maximum value of the Cr concentration is taken as 100% by mass, then in the range where the Cr concentration is 5% or more relative to this maximum value (corresponding to the area between D1 and D2 in Figures 2 and 3), the maximum oxygen concentration is less than 1% by mass. On the other hand, if the maximum oxygen concentration is 1% or more in this range where the Cr concentration is 5% or more, the adhesion between the Zn-based plating layer and the Cr-based plating layer will decrease, and there is a risk of peeling. Preferably, the maximum oxygen concentration in this range is 0.5% or less. The lower limit of the maximum oxygen concentration in this range may be 0% or greater than 0%.

[0058] In this embodiment, a LECO Japan 850A glow discharge emission spectrometer is used, but the measuring device is not limited to this. When performing depth-direction analysis, it is preferable to perform the analysis while Ar sputtering, and the analysis conditions are as follows: argon pressure: 0.27 MPa, output power: 30 W, output voltage: 1000 V, discharge area: within a circular area with a diameter of 4 mm. The measurement is performed from the surface of the plating layer toward the depth until the Fe concentration reaches 100% (reaching the base metal). Therefore, the analysis range of the depth-direction analysis by GDS is the range from the surface of the Zn-based plating layer to the Cr-based plating layer and part of the steel material. After GDS analysis, the sputtering depth of the cross-section is measured using a surfcom 130A manufactured by Tokyo Seimitsu Co., Ltd. The elemental distribution profile in the depth direction of the plating layer is obtained by GDS analysis. In the elemental distribution profile, the distribution of the content of each element in the depth direction is shown, with the total amount of detected elements set to 100%. For calibration curve verification during concentration measurement, use "RH18 / 55" (Ulich Nell) for Co, "RN14 / 60" (Ulich Nell) for Cr, and "JK49" (Jernkontoret) for Ni, Mo, and W.

[0059] [Method for manufacturing hot-dip galvanized steel sheet] Next, the method for manufacturing the hot-dip Zn-based galvanized steel sheet of the present embodiment will be described. As a method for forming the Zn-based plating layer and the Cr-based plating layer as described above, first, a base plating layer is provided on the steel sheet (pretreatment step), and then a Zn-based plating layer is provided on the base plating layer. Hereinafter, this method will be described in detail.

[0060] (Pretreatment step) As a pretreatment step for the hot-dip Zn-based plating step, a base plating layer is formed in advance on the surface of a plating base material (steel sheet) such as a cold-rolled steel sheet or a hot-rolled steel sheet. Examples of the means include electroplating and vapor deposition, but electroplating is preferable from the viewpoint of simplicity.

[0061] When Cr plating is performed by electroplating, there are no particular restrictions, and a general mixed bath containing chromium trioxide and sulfuric acid may be used. Also, sodium molybdate may be added here to perform Cr-Mo plating. As a plating bath for forming a Ni plating layer as a pretreatment step for forming a Cr plating layer or a Cr-Mo plating layer, a chloride bath, a sulfuric acid bath, a Watts bath, etc. can be exemplified. As a base plating other than Cr-based plating, there is Ni-based plating. Ni-based alloy plating obtained by adding salts such as Co, W, and Mo to a nickel sulfate bath may also be used. Examples of Ni-based alloy plating include Ni-Mo plating, Ni-Co plating, or Ni-W plating. In any bath case, a desired plating adhesion amount can be obtained by adjusting the current density and the cumulative energization time.

[0062] The adhesion amount of the base plating layer is 0.01 to 1.0 g / m ２ shall be set to. That is, the plating adhesion amount of each of the Cr-based plating layer (Cr plating layer, Cr-Mo plating layer) and the Ni-based plating layer (Ni-Mo plating layer, Ni-Co plating layer, or Ni-W plating layer) is 0.01 to 1.0 g / m ２ shall be set to. Also, when the base plating layer has a two-layer structure, the adhesion amount of each of the first plating layer and the second plating layer may be 0.01 to 1.0 g / m ２ shall be set to. Note that when the adhesion amount of each layer is small (for example, 0.01 g / m ２Although an Al-Fe alloy layer (interfacial alloy layer) may be formed between the plating layer and the steel material, this does not affect the effects of the present invention.

[0063] The amount of Cr-based plating layer is 0.01 g / m². ２ If the amount is less than 1.0 g / m², the entire surface of the steel material may not be sufficiently covered by the Cr-based plating layer. Furthermore, because the Cr-based plating layer formed by electroplating is harder and has greater internal stress compared to bulk Cr, cracking tends to occur as the plating thickness increases, leading to a decrease in adhesion. To suppress peeling due to cracking, the amount of Cr-based plating layer should be 1.0 g / m². ２ The following applies:

[0064] Furthermore, even if the Cr-based plating layer formed by electroplating does not peel off, fine cracks may develop. To avoid corrosion of the steel material due to these fine cracks, a treatment to seal these cracks with an inorganic compound may be performed.

[0065] Furthermore, a Ni-based plating layer may be formed as a lower layer, i.e., a second plating layer, beneath the Cr-based plating layer. Generally, Ni-based plating methods include Ni-W plating, Ni-Co plating, and Ni-Mo plating as electroplating methods, and Ni-P plating as an electroless plating method; all of these are applicable.

[0066] The amount of Ni-based plating layer to be applied is 0.01 to 1.0 g / m², similar to the Cr-based plating layer, considering the coverage rate of the steel surface and the prevention of cracking due to processing. ２ Let's assume that.

[0067] The amount of each layer deposited can be obtained by observing the cross-section of the plating layer using a transmission electron microscope (TEM). Specifically, 10 locations can be arbitrarily selected from the cross-sectional image obtained by TEM observation, the thickness of each layer can be measured, and the average thickness of the 10 locations can be calculated. Then, the amount of each layer deposited can be converted by multiplying this by the specific gravity of each metal element.

[0068] Furthermore, in addition to the electroplating methods described above, Cr-based and Ni-based plating methods may also utilize dry processes such as vapor deposition, sputtering, and ion plating. In the case of Cr or Ni plating by vapor deposition, it is preferable to use Cr or Ni with a purity of 3N or higher as the evaporation source and to perform vapor deposition in a manner that provides an appropriate amount of adhesion from the perspective of coverage and cracking during processing. Also, in the case of dry processes, the plating layer is softer and less prone to cracking compared to electroplating, so the upper limit of the plating thickness should be 1.0 g / m². ２ It is not limited to this; you should choose a thickness that does not cause cracking or delamination.

[0069] (Hot-dip plating process) Next, a Zn-based plating layer is formed on the steel material (pre-plated material) that has the Cr-based plating layer and Ni-based plating layer formed as described above, by hot-dip Zn plating.

[0070] In this embodiment, a Zn-based plating layer is formed on the surface of a Cr-based or Ni-based plating layer as a pre-plating layer (undercoat plating layer) by hot-dip plating under reduced pressure (reduced pressure plating method) or ultrasonic hot-dip plating method. As a pre-treatment for hot-dip plating, it is sufficient to remove oil, moisture, and dust from the surface of the pre-plated material. This improves the wettability of the Zn-based molten metal to the Cr-plated layer under reduced pressure. With ultrasonic plating, even in so-called "dip plating" where the pre-plated material at room temperature is immersed in a plating bath under atmospheric pressure, applying ultrasonic vibrations to the bath makes it possible to produce hot-dip Zn-plated steel with good adhesion without using flux.

[0071] Furthermore, the pre-plated material may be subjected to hot-dip plating while heated in a reducing atmosphere. Heating in a reducing atmosphere promotes the reduction of the Cr-based plating layer surface and the oxide film on the Cr-based plating layer surface, and by raising the temperature of the steel material, good wetting of the plating can be obtained in a shorter time than with the dip-dip plating method. In addition, annealing has the effect of suppressing cracking of the Cr-based plating layer and Ni-based plating layer due to processing, and it is also expected that Cr and Ni will be heated and diffused into the steel material, partially forming stainless steel.

[0072] (Hot-dip plating method) The composition of the plating bath to be prepared may be substantially the same as the chemical composition of the Zn-based plating layer described above. Since the Zn-based plating layer according to this embodiment is formed on the steel material via an undercoat plating layer, the influence of the steel material components on the Zn-based plating layer is very small. Therefore, the composition of the plating bath should be adjusted to match the target chemical composition of the Zn-based plating layer.

[0073] In vacuum plating, reducing the gas layer is more effective when the ambient pressure during plating is low. Preferably, the ambient pressure is 1 kPa (1000 Pa) or less. However, excessive vacuum increases the evaporation rate from the surface of the Zn-based molten plating bath, resulting in Zn-based molten metal adhering to the inside of the vacuum tank and reducing the workability of the plating. Therefore, the ambient pressure should be determined based on the effect of improving wettability and workability. If high ambient pressure makes it difficult to obtain performance such as adhesion and corrosion resistance, it is effective to temporarily replace the atmosphere inside the vacuum container with a non-oxidizing atmosphere before reducing the pressure.

[0074] As shown in Figure 4, the basic structure of the plating apparatus is one in which a molten plating tank and a device for immersing steel plates in the plating bath are arranged inside a vacuum chamber. In addition, mechanisms used in conventional gas reduction plating tests and dipping plating tests, such as a mechanism for heating the steel plates before and after plating and a mechanism for adjusting the amount of plating deposited, may be added. A vacuum environment can be achieved by using a vacuum pump that can achieve a vacuum level of about 1 Pa. Common examples include oil rotary vacuum pumps and dry vacuum pumps that do not use oil, such as scroll pumps.

[0075] In ultrasonic molten metal plating, a horn is immersed in the plating bath to apply ultrasonic vibrations, and the steel material to be plated is immersed around the horn to perform the plating. Immersing the horn to a length of at least 1 / 4 wavelength of the resonance length is effective in increasing cavitation. The position and number of horns are adjusted as appropriate to ensure proper wetting of the material to be plated. Using multiple horns can shorten the time required for plating wetting.

[0076] Furthermore, applying a vacuum plating method to a continuous plating method for steel strips results in a complex mechanism. Therefore, applying an ultrasonic plating method that does not require vacuum is easier. As an example of an ultrasonic plating method, the configuration shown in Figure 5 can be illustrated. In Figure 5, three positions ("Position 1" to "Position 3") are shown as examples of ultrasonic horn positions, but the positions of the ultrasonic horns in the manufacturing method of this embodiment are not particularly limited, and the positions and number of horns may be adjusted as appropriate to achieve plating wettability. In addition, if the shape of the steel material to be plated is not a continuous steel strip, but rather a batch process of processed parts, for example, the application of a vacuum plating method is relatively easy. If an ultrasonic plating method is to be applied, as shown in Figure 6, the steel material to be plated can be placed in a container such as a basket and ultrasonic horns can be arranged around it. In such cases, uniformity of plating wettability can be achieved by rotating the basket or moving around the horns.

[0077] The plating time must be at least long enough for the temperature of the steel to be plated to reach the plating bath temperature in order to achieve good plating adhesion. Preheating the steel can shorten the time until wetting is achieved. After wetting is achieved, the plating time can be adjusted as appropriate based on the reactivity between the Cr plating layer and the Zn-based metal. Furthermore, maintaining a reduced pressure state or applying ultrasonic vibration is not essential after wetting is achieved. The timing of returning to atmospheric pressure and stopping ultrasonic vibration can be determined based on the characteristics of the resulting plated product. In ultrasonic plating, if non-plating occurs in the same location on the material to be plated, or if a decrease in adhesion is observed, it is possible that cavitation uneven distribution due to standing waves is the cause. In such cases, this can be resolved by moving the material to be plated or the horn during plating. It is also effective to place multiple horns driven by independent ultrasonic oscillators into the bath.

[0078] (Ultrasonic device) A rod-shaped object called an ultrasonic horn is connected to an ultrasonic transducer, and the tip of the ultrasonic horn is immersed in the plating bath to cause resonant vibration, thereby generating cavitation in the plating bath. The length of the ultrasonic horn is adjusted so that it resonates at the vibration frequency of the ultrasonic transducer. The length (one wavelength) at which the resonant frequency matches the frequency of the ultrasonic transducer is determined from the Young's modulus (modulus of elasticity) and density of the horn material, and resonance is obtained by using half that length as a base and then making integer multiples of that length. In other words, with the resonant length being one wavelength, resonance can be obtained by setting the length to 0.5 wavelengths, 1 wavelength, 1.5 wavelengths, 2 wavelengths, etc. By causing resonance, the amplitude is maximized, and cavitation can be efficiently generated in the plating bath. To enhance cavitation, a booster that amplifies the amplitude may be inserted between the ultrasonic transducer and the ultrasonic horn.

[0079] The material of the ultrasonic horn is preferably ceramic from the viewpoint of the stability of the resonant frequency with respect to temperature changes, resistance to erosion in the plating bath, and the influence of the molten horn material on the plating bath. Specifically, it is preferable that the rate of change of the Young's modulus when the temperature of the horn rises is small. More specifically, in the manufacturing method of this embodiment, the ultrasonic horn is preferably made of a material in which the absolute value of the rate of change of the Young's modulus from 25°C to 600°C is 0.050 GPa / °C or less. Generally, as the temperature rises, the ultrasonic horn softens, the Young's modulus decreases, and the resonant frequency decreases. If this decreased resonant frequency falls outside the range of frequencies in which the ultrasonic transducer can resonate, resonant vibration cannot occur, and cavitation cannot be generated in the plating bath. However, in this embodiment, by using a material as the ultrasonic horn in which the absolute value of the rate of change of the Young's modulus is 0.050 GPa / °C or less in the above temperature range, the change in the resonant frequency due to temperature changes is small and does not fall outside the vibrable frequency range of the ultrasonic transducer, so stable resonant vibration can be achieved at any stage, from room temperature to bath immersion.

[0080] The ultrasonic oscillator that drives the ultrasonic transducer preferably has a constant amplitude function and a function that automatically tracks the resonant frequency. Examples of ultrasonic devices having such functions include welding equipment, welding equipment, processing equipment, and dispersion / emulsifying machines (homogenizers). Furthermore, these devices have a structure that allows a horn of any shape to be attached by screw connection.

[0081] The resonant frequency varies depending on the depth to which the ultrasonic horn is immersed in the bath and the intensity of cavitation. Therefore, it is preferable to use an ultrasonic oscillator that has the function of maintaining the resonant state by changing the oscillation frequency in accordance with this variation. For example, ultrasonic oscillators and homogenizers for welding and processing have the function of automatically tracking the oscillation frequency in accordance with such changes in the resonant frequency and the function of keeping the amplitude constant even when the load on the horn fluctuates, making them suitable for stabilizing cavitation during plating. The conditions for such ultrasonic equipment can be a vibration frequency of 15 kHz to 100 kHz and an ultrasonic output of 10 W to several kW, and the conditions that can be appropriately selected to obtain the desired wettability should be selected.

[0082] Various methods used in conventional hot-dip galvanizing can be applied to adjust the amount of plating deposited. There are no particular limitations on the upper and lower limits of the plating thickness. Generally, factors that affect the plating thickness include the viscosity and specific gravity of the plating bath, the speed at which the steel is drawn out of the plating bath, and the wiping conditions. In addition, in ultrasonic plating, the improvement in the fluidity of the plating metal due to vibration also has an effect. In the dipping method, the speed at which the material is pulled out of the plating bath, the angle between the plating surface and the plating bath surface, and, in the case of dipping small parts, the centrifugal separation conditions to remove excess plating deposits should be considered. In the continuous hot-dip galvanizing method, the maximum thickness of the formed plating layer is often 100 μm or less, so in this embodiment as well, the plating thickness of the hot-dip galvanized steel material may be 100 μm or less.

[0083] Cooling after plating can be done using conventional methods. In the case of vacuum plating, if the steel sheet is removed from the plating bath after wetting is achieved by opening to the atmosphere, conventional methods such as water cooling and air cooling can be applied. For continuous plating of steel sheets in CGL, air cooling or water spraying can be applied.

[0084] Post-treatment similar to that in the past may be applied to the hot-dip galvanized steel obtained by the above manufacturing method. For example, an inorganic coating such as a chromate coating, a chromate-free coating, or a phosphate coating, or an organic resin coating may be formed on the surface of the hot-dip galvanized steel.

[0085] The present invention will be described in more detail with reference to examples.

[0086] <Pre-plating treatment> The steel material to be plated was prepared by the following method. First, a hot-rolled material measuring 3.2 × 150 × 300 mm (C: 0.045%, Si: 0.015%, Mn: 0.182%, P: 0.014%, S: 0.0082%, Al: 0.030%) was electrolytically degreased and surface-cleaned in a 3% NaOH aqueous solution, and then plated at 7 vol.%H at 50°C. ２ SO ４ It was soaked in acid water for 5 minutes and then rinsed.

[0087] Subsequently, a Cr-based pre-plating layer (undercoat) and / or a Ni-based pre-plating layer (undercoat) were formed by electroplating to achieve the deposition amounts shown in Tables 1A, 2A, and 3A. The deposition amount was measured by gravimetric method, and the electrodeposition time and flow rate were controlled to achieve the predetermined deposition amount.

[0088] The conditions for electroplating when forming a Cr plating layer are CrO ３ =150g / L,NH ４ F: 3.5 g / L, plating bath temperature 40°C, current density 1 A / dm ２ That's what I decided.

[0089] The electroplating conditions for forming the Cr-Mo plating layer are: CrO ３ = 300g / L, H ２ SO ４ = 3 g / L, Na ２ MoO ４ ・2H ２O = 75 g / L, HOOCCOOH (oxalic acid): 5 g / L, plating bath temperature 50°C, current density 50 A / dm ２ That's what I decided.

[0090] The electroplating conditions for forming the Ni plating layer are NiSO ４ 6H ２ O: 340g / L, NiCl ２ 6H ２ O: 70g / L, H ３ BO ３ : 40 g / L, plating bath temperature 50°C, current density 5 A / dm ２ That's what I decided.

[0091] The electroplating conditions for forming the Ni-Mo plating layer are NiSO ４ 6H ２ O: 30g / L, Na ２ MoO ４ ・2H ２ O=25g / L, HOCH ２ (CHOH) ４ OONa (sodium gluconate): 65 g / L, plating bath temperature 60°C, current density 2 A / dm ２ That's what I decided.

[0092] The electroplating conditions for forming the Ni-W plating layer are NiSO ４ 6H ２ O: 26g / L, Na ２ WO ４ ・2H ２ O: 33g / L, C ６ H １４ N ２ O ７ (Diammonium citrate): 50 g / L, plating bath temperature 60°C, current density 20 A / dm ２ That's what I decided.

[0093] The electroplating conditions for forming the Ni-Co plating layer are NiSO ４ 6H ２ O: 100g / L, CoSO ４ 7H ２ O: 200g / L, H ３ BO ３ : 45g / L, NaCl: 65g / L, bath temperature 60°C, current density 20A / dm ２ That's what I decided.

[0094] <Example 1: Vacuum Plating Method> (Plating Apparatus) The plating apparatus in the vacuum plating method has a structure in which a Zn-based molten plating bath and a lifting device for immersing steel plates in the plating layer are arranged inside a vacuum container (see Figure 4). By attaching a thermocouple to the steel plate, the bath temperature of the steel plate immersed in the plating bath can be measured.

[0095] (Plating Method) A thermocouple was attached to the center of a plated steel sheet having an undercoat, and the sheet was mounted on a steel sheet lifting device. The pressure was reduced to a predetermined pressure by closing the vacuum chamber. In some parts, the pressure inside the vacuum chamber was reduced to 1 Pa, then nitrogen was introduced until it reached atmospheric pressure to replace the pressure, and then the pressure was reduced again to the predetermined pressure. While maintaining the predetermined pressure, the steel sheet was immersed in the Zn-based molten plating bath shown in Table 1A. After the temperature of the steel sheet reached the temperature of the plating bath, it was held for 60 seconds. After that, air was introduced into the vacuum chamber to return it to atmospheric pressure, and 180 seconds after the start of immersion, the steel sheet was removed from the plating bath and air-cooled. The results are shown in Table 1B.

[0096] <Example 2: Ultrasonic Plating Method> (Ultrasonic Application Conditions) Vibrations generated by a 20 kHz ultrasonic transducer (TELSONIC) connected to an ultrasonic power supply were amplified by a Ti alloy booster (amplitude magnification 2.5 times: BRANSON) and transmitted to a Sialon ultrasonic horn. The rate of change of the Young's modulus of Sialon from 25°C to 600°C was -0.014 GPa / °C. The shape of the horn was φ48 × 485 mm. This length corresponds to one wavelength at 20 kHz and resonates at 20 kHz.

[0097] The tip of this horn was immersed 200 mm perpendicularly to the plating bath surface to apply vibration to the plating bath. The vibration conditions were a frequency of 20 kHz and an effective voltage of 600 V applied to the ultrasonic transducer. This resulted in a power of approximately 30 W (effective value) and an amplitude of approximately 35 μm at the tip of the horn in air. When the tip of the horn in this state was immersed in the plating bath to generate cavitation, the power was approximately 840 W (effective value) due to the increased load on the horn.

[0098] <Example 2-1: Gas Reduction Ultrasonic Plating> A thermocouple was attached to the center of a plated steel sheet having an undercoat, and hot-dip plating was performed in a hot-dip plating simulator under the conditions shown in Tables 2A and 2B.

[0099] First, as a pre-annealing step for the plated steel sheet, H ２ Concentration 5%-N ２ The plate was heated to a temperature of 600°C at a heating rate of 30°C / second using an atmosphere of 95% humidity and an electric heating method.

[0100] After heating, the plate was immersed in various plating baths with ultrasonic vibration applied once its temperature had dropped to (plating bath temperature + 30)°C. The distance from the horn cylindrical surface was 15 mm. The immersion time was 6 seconds.

[0101] Immediately after removal, the nitrogen gas flow rate was adjusted using a nitrogen gas wiping device so that the plating thickness was approximately 10 μm. Nitrogen gas blowing cooling was performed immediately after wiping, and the temperature was cooled from the bath temperature to 200°C at an average cooling rate of 10 to 20°C / second. Further cooling to room temperature was then performed by natural radiative cooling.

[0102] <Example 2-2: Ultrasonic dip plating> As shown in Tables 3A and 3B, a Zn-based plating layer was formed on a plated steel sheet having an undercoat (pre-plating layer) by so-called "dip plating". Specifically, a plating bath in the atmosphere was used, and after scraping off the oxides from the surface of the plating bath, the plated steel sheet at room temperature was immersed in a plating bath to which ultrasonic vibrations were applied.

[0103] The plating substrate was immersed perpendicularly in the plating bath with a length of 300 mm. The minimum distance from the horn cylindrical surface was 15 mm. After 15 seconds of immersion, it was lifted at a speed of 300 mm / min and air-cooled with a fan. The average cooling rate until solidification was approximately 5°C / second.

[0104] <Evaluation of plated material> Next, the hot-dip Zn-plated steel materials obtained in Examples 1, 2-1, and 2-2 were evaluated as follows: non-plating, plating adhesion, oxygen concentration by GDS, and corrosion resistance of the bent portion.

[0105] <Unplated> The presence or absence of unplated surfaces was visually checked for 150 x 250 mm (longitudinal direction). A rating of A was considered acceptable.

[0106] A: No unplated parts. E: Unplated parts are present.

[0107] <Plating Adhesion> A 100 (length) x 45 (width) sample was taken from the obtained hot-dip Zn-plated steel material, ensuring that it included the plating layer. After conducting a 180-degree bending (full bending) test, a tape peel test was performed on the outer bent portion (bend apex). Specifically, the tape that had been pre-applied to the bend apex was forcefully pulled off, and the peeled tape was attached to black cardboard to check for the presence or absence of plating peeling powder. This tape peel test was performed on 10 samples. An evaluation of A was considered a pass.

[0108] A: No plating removal powder adhered to the tape surface in any of the 10 samples. E: Plating removal powder adhered to the tape surface in one or more of the 10 samples.

[0109] <Oxygen concentration at the plating layer interface> In quantitative analysis (mass%) of the depth direction of the plating layer by GDS, the maximum oxygen concentration was determined in the range where the Cr concentration is 5% or higher, with the maximum value of the Cr concentration set to 100% by mass.

[0110] A: When the maximum oxygen concentration is less than 1% E: When the maximum oxygen concentration is 1% or more

[0111] <Corrosion Resistance of Bent Sections (Corrosion Resistance of Processed Sections)> To confirm the corrosion resistance of the bent section of the plated layer, a 180-degree bending test and a combined cycle corrosion test (CCT, JASO M609) are used for evaluation. First, a bending test specimen is prepared. A sample measuring 50 x 150 mm is taken, and the sample, with all four end faces sealed, is bent 180 degrees in the center of a 50 mm width. Next, the bending test specimen is placed upright in a CCT apparatus and subjected to a combined cycle corrosion test (corrosion test mode of JIS H 8502 (1999) (JASO M609-91)) to observe the occurrence of red rust. The corrosion test is performed for 180 cycles, and the percentage of red rust area at the 180-cycle mark is evaluated. Note that the top of the bend is subjected to severe processing, and cracks occur in the plated layer, inevitably resulting in the exposure of the base steel. In this example, the corrosion resistance of the bent portion of the test specimen was evaluated at 180 cycles by measuring the percentage of red rust area on the evaluation surface viewed from the vertical direction. Ratings A+, A, B, and C were considered acceptable.

[0112] A+: Red rust area ratio is less than 3%. A: Red rust area ratio is 3% or more but less than 5%. B: Red rust area ratio is 5% or more but less than 15%. C: Red rust area ratio is 15% or more but less than 30%. D: Red rust area ratio is 30% or more. E: Evaluation was not possible due to peeling of the plating layer.

[0113]

[0114]

[0115]

[0116]

[0117]

[0118]

[0119] <Example 3> Next, the effect of the immersion depth of the ultrasonic horn on adhesion was evaluated. Specifically, vibrations generated by a 20 kHz ultrasonic transducer (manufactured by TELSONIC) connected to an ultrasonic power supply were amplified by a Ti alloy booster (amplitude multiplier 2.5 times: manufactured by BRANSON) and transmitted to a ceramic (Sialon) ultrasonic horn. The shape of the horn was φ48 × 485 mm. Next, the tip of the horn was immersed in a plating bath in the atmosphere while maintaining resonant vibration.

[0120] The steel material to be plated was prepared as described above, as follows: A hot-rolled steel sheet measuring 3.2 × 150 × 300 mm was electrolytically degreased and surface-cleaned in a 3% NaOH aqueous solution, and then plated at 50°C with 7 vol.% H ２ SO ４ It was soaked in acid water for 5 minutes and then rinsed.

[0121] The plating bath and pre-plating conditions were the same as those for No. B7 in Table 3. For the plating method, the steel to be plated was immersed in the bath to a depth of 250 mm parallel to the length of the horn. The distance (spacing) between the cylindrical surface of the horn and the steel to be plated was 15 mm. The immersion time in the bath was 15 seconds. The steel was then lifted at a speed of 300 mm / min and air-cooled with a fan.

[0122] Plating was performed by changing the immersion depth of the horn in the plating bath in six steps from 0 mm to 40 mm increments, as shown in Table 4. For each of the obtained hot-dip Zn-plated steel materials, the non-plating, plating adhesion, oxygen concentration by GDS, and corrosion resistance of the bent section were evaluated in the same manner as described above.

[0123] It was found that immersing the horn to a depth of 120 mm (1 / 4 wavelength) or more improves the bending adhesion of the plating layer, and also reduces the oxygen concentration near the pre-plated layer to 1% or less in GDS analysis.

[0124]

[0125] <Example 4> The material of the horn was evaluated. First, titanium alloy (JIS Class 60), carbon steel (S45C), ceramic (alumina (Al)) were used as the material for the horn. ２ O ３) and SiAlON (Si ３ N ４ Al ２ O ３ We prepared the following four types of materials. Using these four materials, we fabricated horns with a diameter of 50 mm that have a wavelength of 1 at 20 kHz.

[0126] Each of the obtained horns was connected to a 20 kHz transducer, and it was confirmed that they resonated and vibrated at approximately 20 kHz using an ultrasonic power supply that automatically sets the resonant frequency. With an applied voltage of 600 V, an amplitude of 14 μm was obtained for all horns.

[0127] Next, these horns were immersed in a Zn-based plating bath at 500°C, with a length of 1 / 4 wavelength extending from the tip of the horn, and their vibration state was observed. Immediately after immersion, all horns maintained resonant vibration and emitted noise due to cavitation. However, as the temperature of the horns rose, the resonant frequency gradually decreased, and in the case of the titanium alloy horn and the carbon steel horn, the vibration stopped because it fell outside the lower limit of the oscillator's vibrating frequency range of 19.5 kHz.

[0128] In contrast, while the resonant frequency of the alumina horn and the Sialon horn decreased, it never dropped below the lower limit of 19.5 kHz, maintaining a resonant state and continuing to generate noise due to cavitation.

[0129] Next, these four types of horns were plated using the same conditions as in <Example 3> for the plated steel material, plating bath, and pre-plating. As a result, the titanium alloy horn and the carbon steel horn could not be driven resonantly because their resonant frequencies were lower than the lower limit of the oscillator's vibrating frequency of 19.5 kHz. Consequently, as shown in Table 5, the power consumption of the oscillator was about 0.1 W, resulting in many unplated areas and poor plating adhesion. On the other hand, the alumina horn and the Sialon horn were driven resonantly at approximately 20 kHz, resulting in a power consumption of 500 W or more, no unplated areas, and good bending adhesion.

[0130]

[0131] According to the above embodiment of the present invention, a plated steel material can be obtained that has excellent corrosion resistance in the portion where the base steel is exposed, as well as excellent adhesion of the plating layer.

Claims

1. A plated steel material comprising: a steel material; an undercoat plating layer provided on the steel material; and a Zn-based plating layer provided on the undercoat plating layer, wherein the average chemical composition of the Zn-based plating layer is, in mass%,: Al: 0.01% or more and less than 45.0%, Mg: 0% or more and 15.0% or less, Si: 0% or more and 2.0% or less, Cr: 0% or more and 3.00% or less, Ni: 0% or more and 3.0% or less, Sn: 0% or more and 0.7% or less, Bi: 0% or more and 0.3% or less, In: 0% or more and 0.3% or less, Ca: 0% or more and 0.6% or less, Y: 0% or more and 0.3% or less, La: 0% or more and 0.3% or less, Ce: 0% or more and 0.3% or less, Sr: 0% or more and 0.3% or less Li: 0% or more, 0.3% or less, Mo: 0% or more, 1.0% or less, Fe: 0% or more, 5.0% or less, Sb: 0% or more, 0.25% or less, Pb: 0% or more, 0.25% or less, B: 0% or more, 0.50% or less, P: 0% or more, 0.50% or less, Ti: 0% or more, 0.25% or less, Co: 0% or more, 0.25% or less, V: 0% or more, 0.25% or less, Nb: 0% or more, 0.25% or less, Mn: 0% or more, 0.25% or less, Zr: 0% or more, 0.25% or less, W: 0% or more, 0.25% or less, Cu: 0% or more, 0.25% or less, Ag: 0% or more, 0.25% or less, the remainder being Zn and impurities. The aforementioned undercoat plating layer is a Cr plating layer, a Cr-Mo plating layer, a Ni-Mo plating layer, a Ni-Co plating layer, or a Ni-W plating layer as the first plating layer, and the amount of the first plating layer deposited is 0.01 to 1.0 g / m² per side. ２ The plated steel material wherein, in the elemental distribution profile obtained by quantitative analysis from the surface of the Zn-based plating layer toward the steel material by glow discharge emission analysis, when the maximum value of the Cr concentration or Ni concentration is taken as 100% by mass, the maximum oxygen concentration in the range where the Cr concentration or Ni concentration is 5% or more is less than 1%.

2. The plated steel material according to claim 1, wherein the first plating layer is the Cr plating layer, and a Ni plating layer is provided as a second plating layer between the steel material and the Cr plating layer.

3. The plated steel material according to claim 1, wherein the first plating layer is the Cr-Mo plating layer, and a Ni plating layer is provided as a second plating layer between the steel material and the Cr-Mo plating layer.

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