JP2018159107A - PRODUCTION METHOD OF MOLTEN Zn ALLOY PLATING STEEL PLATE - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a production method of a molten Zn alloy plating steel plate capable of easily suppressing blacking of the surface of a plating layer without lowering productivity.SOLUTION: A molten Zn alloy plating layer containing an intermetallic compound MgZnis formed on the surface of a base steel plate by dipping the base steel plate in a molten Zn alloy plating bath containing Al or Mg. Next, an aqueous solution containing one or two polyatomic ions selected from the group consisting of a polyatomic ion containing V, a polyatomic ion containing Si, and a polyatomic ion containing Cris brought into contact with the surface of the molten plating layer. The aqueous solution contains the polyatomic ions of 0.01 g/L or more in terms of one or two atoms selected from the group consisting of V, Si, and Cr.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a method for producing a hot-dip Zn alloy-plated steel sheet having excellent blackening resistance.

  As a plated steel sheet excellent in corrosion resistance, a hot-dip Zn alloy plated steel sheet in which a hot-dip Zn alloy plated layer containing Al and Mg is formed on the surface of a base steel sheet is known.

  The aforementioned hot-dip Zn alloy-plated steel sheet can be continuously produced by the following steps. First, after the base steel plate (steel strip) passed through the furnace is immersed in a molten Zn alloy plating bath containing Al and Mg, the molten metal adhered to the surface of the base steel plate, for example, by passing through a gas wiping device. Is adjusted to a predetermined amount. Next, the molten metal is cooled by passing the steel strip to which a predetermined amount of molten metal is attached through an air jet cooler and an air-water cooling region, and a molten Zn alloy plating layer is formed. Furthermore, the hot-dip Zn alloy-plated steel sheet is obtained by passing the steel strip on which the hot-dip Zn alloy plating layer is formed through the water quench zone and bringing cooling water into contact therewith.

  However, in the hot-dip Zn alloy-plated steel sheet produced in this way, a part of the plating layer surface may change black over time. The black change on the surface of the plating layer occurred in 2 to 3 days after the production in an early case, and may occur after 4 to 7 days depending on the production conditions, which impaired the appearance of the hot-dip Zn alloy plated steel sheet.

  As a method for preventing such black change, a method for adjusting the temperature of the plating layer surface in the water quench zone has been proposed (see, for example, Patent Document 1). In invention of patent document 1, the black change of the plating layer surface is prevented by making the temperature of the plating layer surface at the time of making it contact with cooling water in a water quench zone below 105 degreeC. Alternatively, instead of setting the temperature of the plating layer surface to less than 105 ° C, an easily oxidizable element (rare earth element, Y, Zr or Si) is added to the plating bath and the temperature of the plating layer surface is set to 105 to 300 ° C. , Preventing the black change of the plating layer surface.

Moreover, in patent document 2, as a cooling water used in a water quench zone, 1 selected from the group consisting of polyatomic ions containing V ⁵⁺ , polyatomic ions containing Si ⁴⁺ and polyatomic ions containing Cr ⁶⁺ By using an aqueous solution containing two or more polyatomic ions, the black change of the plating layer surface is prevented.

JP 2002-226958 A Japanese Patent No. 5356616

  In invention of patent document 1, since it was necessary to cool the plating layer surface to predetermined | prescribed temperature before letting it pass through a water quench zone | band, the production | generation of the hot-dip Zn alloy plated steel plate might be restrict | limited. For example, in the case of a plated steel plate having a large plate thickness, it is necessary to slow down the feed rate of the plated steel plate to cool the plated steel plate to a predetermined temperature, and thus productivity has been reduced. Moreover, when an easily oxidizable element is blended in the plating bath, the easily oxidizable element is liable to become dross, and the concentration control of the easily oxidizable element is complicated, so that the manufacturing process becomes complicated.

  This invention is made | formed in view of this point, and can suppress the black change of the plating layer surface easily, without reducing productivity and without performing complicated component management of a plating bath. An object of the present invention is to provide a method for producing a hot-dip Zn alloy-plated steel sheet.

  The present inventors have found that the above problem can be solved by adding a predetermined polyatomic ion at a predetermined concentration to the cooling water to be contacted after forming the molten Zn alloy plating layer. In addition, the present invention has been completed.

That is, this invention relates to the manufacturing method of the following hot dip Zn alloy plating steel plates.
[1] A step of immersing a base steel plate in a hot-dip Zn alloy plating bath containing Al and Mg to form a hot-dip Zn alloy plating layer on the surface of the base steel plate, a polyatomic ion containing V ⁵⁺ , Si Contacting an aqueous solution containing one or more polyatomic ions selected from the group consisting of polyatomic ions containing ⁴⁺ and polyatomic ions containing Cr ⁶⁺ with the surface of the molten Zn alloy plating layer; The aqueous solution contains 0.01 g / L or more of the polyatomic ions in terms of 1 or 2 or more atoms selected from the group consisting of V, Si and Cr, and a molten Zn alloy plating layer includes an intermetallic compound _Mg 2 _{Zn 11,} the manufacturing method of the molten Zn alloy plated steel sheet.
[2] The method for producing a hot-dip Zn alloy-plated steel sheet according to [1], wherein the hot-dip Zn alloy plating layer includes the intermetallic compound Mg ₂ Zn ₁₁ in a surface layer.
[3] When the aqueous solution is brought into contact with the surface of the molten Zn alloy plating layer, the temperature of the surface of the molten Zn alloy plating layer is 100 ° C. or more and not more than the freezing point of the plating layer. 2] The manufacturing method of the hot dip Zn alloy plating steel plate as described in 2].
[4] The molten Zn alloy plating layer contains Al: 1.0 to 22.0 mass%, Mg: 0.1 to 10.0 mass%, and the balance: Zn, and any one of [1] to [3] A method for producing the hot-dip Zn alloy-plated steel sheet according to claim 1.

  According to the present invention, a hot-dip Zn alloy-plated steel sheet excellent in blackening resistance can be easily produced with high productivity.

1A and 1B are diagrams showing an example of a method for bringing a cooling aqueous solution into contact with the surface of a molten Zn alloy plating layer. FIG. 2 is a schematic diagram showing a partial configuration of a production line for a hot-dip Zn alloy-plated steel sheet.

  The method for producing a hot-dip Zn alloy-plated steel sheet (hereinafter also simply referred to as “plated steel sheet”) of the present invention is (1) a hot-dip Zn alloy plated layer (hereinafter simply referred to as “plated layer”) on the surface of the base steel sheet. ) And (2) a second step of bringing a cooling aqueous solution containing polyatomic ions into contact with the surface of the molten Zn alloy plating layer. One feature of the manufacturing method of the present invention is to suppress black change of the plating layer by bringing a predetermined cooling aqueous solution into contact with the surface of the plating layer after forming the molten Zn alloy plating layer.

(1) First Step In the first step, the base steel plate is immersed in a hot-dip Zn alloy plating bath containing Al and Mg to form a hot-dip Zn alloy plating layer on the surface of the base steel plate.

[Base steel sheet]
The kind of base steel plate is not particularly limited. For example, as the base steel plate, a steel plate made of low carbon steel, medium carbon steel, high carbon steel, alloy steel, or the like can be used. When good press formability is required, a steel sheet for deep drawing made of low carbon Ti-added steel, low carbon Nb-added steel, or the like is preferable as the base steel sheet. Moreover, you may use the high strength steel plate which added P, Si, Mn, etc.

[Formation of plating layer]
First, the base steel plate is immersed in a molten Zn alloy plating bath containing Al and Mg, and a predetermined amount of molten metal is adhered to the surface of the base steel plate by using gas wiping or the like.

  As the composition of the plating bath, for example, a plating bath containing Al: 1.0 to 22.0 mass%, Mg: 0.1 to 10.0 mass%, balance: Zn and inevitable impurities may be used. it can. Moreover, the plating bath may further contain Si: 0.001 to 2.0 mass%. Furthermore, the plating bath may further contain less than 0.03% by mass of Ti and less than 0.015% of B. As described in Patent Document 1, the black change of the plating layer can be suppressed by adding Si, but when a plated steel sheet is manufactured by the manufacturing method of the present invention, Si is not added. Also, the black change of the plating layer can be suppressed.

  Next, the molten metal adhering to the surface of the base steel sheet is cooled to solidify the molten metal, thereby obtaining a plated steel sheet in which a plating layer having the same composition as that of the plating bath is formed on the surface of the base steel sheet. be able to. At this time, the cooling rate is preferably set to 4 ° C./s or less, for example.

The hot-dip Zn alloy plating layer having the above composition contains [Al / Zn / Mg ₂ Zn ₁₁ ternary eutectic structure]. [Ternary eutectic structure of Al / Zn / Mg ₂ Zn ₁₁ ] may appear on the surface layer of the plating layer. The surface layer of the plating layer is a region including the surface of the plating layer and including a predetermined depth in the thickness direction of the plating layer, and the predetermined depth can be arbitrarily set as long as the surface of the plating layer is included. For example, the predetermined depth can be a depth at which Mg ₂ Zn ₁₁ (a spot-like portion exhibiting a gloss different from the surroundings) can be visually recognized from the surface direction of the plating layer.

The molten Zn alloy plating layer having the above composition may include [Al / Zn / Zn ₂ Mg ternary eutectic structure]. At this time, when the cross section of the molten Zn alloy plating layer is observed, [Al / Zn / Zn ₂ Mg ternary eutectic structure] shows that each phase of Al, Zn, Zn ₂ Mg is finely distributed in a lamellar shape. Yes. Even when this [Al / Zn / Zn ₂ Mg ternary eutectic structure] appears in the surface layer of the plating layer, each phase of Al, Zn, Zn ₂ Mg is finely distributed.

Although not shown in particular, the ratio of the area occupied by the [Al / Zn / Zn ₂ Mg ternary eutectic structure] in cross-sectional observation differs depending on the plating composition. In the Zn—Al—Mg ternary system, Al is 4% by mass, Mg is 3% by mass, and the balance is the eutectic composition near the composition of Zn. For this reason, when the plating composition is close to this ternary eutectic composition, [Al / Zn / Zn ₂ Mg ternary eutectic structure] shows an area ratio of about 80%, and the widest area in the cross section of the plating layer It becomes the phase which shows a rate. However, as the composition of the plating layer is separated from the ternary eutectic composition, the area ratio of the ternary eutectic structure of Al / _Zn / Zn 2 Mg] is reduced, terpolymers of [Al / _Zn / Zn 2 Mg A phase other than the crystal structure] may be maximized as the area ratio.

Depending on the plating composition, the molten Zn alloy plating layer having the above composition contains an Al phase, a Zn phase or a Zn ₂ Mg phase as the primary crystal, in addition to [Al / Zn / Zn ₂ Mg ternary eutectic structure] When Si is contained in the plating composition, an Mg ₂ Si phase may be included.

  In addition, an oxide film containing Al, Zn, and Mg is formed on the surface of the plating layer. In addition, when a predetermined amount of Si is contained in the plating bath, Si may be contained in the oxide film.

The adhesion amount of the molten Zn alloy plating layer is not particularly limited. For example, the adhesion amount of the molten Zn alloy plating layer is about 60 to 500 g / m ² .

(2) Second Step In the second step, an aqueous solution (cooling aqueous solution) containing predetermined polyatomic ions is brought into contact with the surface of the molten Zn alloy plating layer. From the viewpoint of productivity, the second step is preferably performed as a water quench (water cooling) step. In this case, when the cooling aqueous solution is brought into contact with the surface of the molten Zn alloy plating layer, the temperature of the surface of the molten Zn alloy plating layer is about 100 ° C. or more and below the freezing point of the plating layer.

The polyatomic ions contained in the cooling aqueous solution are selected from the group consisting of polyatomic ions containing V ⁵⁺ , polyatomic ions containing Si ⁴⁺ , and polyatomic ions containing Cr ⁶⁺ . These polyatomic ions can suppress the black change of the plating layer surface. These polyatomic ions may be used alone or in combination of two or more.

The method for preparing the cooling aqueous solution containing polyatomic ions is not particularly limited. For example, when preparing a cooling aqueous solution containing polyatomic ions containing V ⁵⁺ , a predetermined compound (V compound, Si compound or Cr compound; hereinafter also referred to as “additive”) and, if necessary, a dissolution accelerator Can be dissolved in water (solvent). Examples of suitable V compounds include acetylacetone vanadyl, vanadium acetylacetonate, vanadium oxysulfate, vanadium pentoxide, ammonium vanadate. Examples of suitable Si compounds include sodium silicate. Furthermore, examples of suitable Cr compounds include ammonium chromate and potassium chromate.

The concentration of the polyatomic ion containing V ⁵⁺ , the polyatomic ion containing Si ⁴⁺ or the polyatomic ion containing Cr ⁶⁺ is preferably 0.01 g / L or more in terms of V, Si, and Cr. When two or more compounds are used in combination, the total concentration in terms of V, Si, and Cr may be 0.01 g / L or more. When the concentration of these polyatomic ions is less than 0.01 g / L in terms of V, Si, and Cr, there is a possibility that the black change on the plating layer surface cannot be sufficiently suppressed.

  Moreover, when mix | blending a solubility promoter, the compounding quantity of a solubility promoter is not specifically limited. For example, 90-130 parts by mass of a dissolution accelerator may be added to 100 parts by mass of the additive. When the blending amount of the dissolution accelerator is too small, the additive may not be sufficiently dissolved. On the other hand, if the blending amount of the dissolution accelerator is excessive, the effect is saturated, which is disadvantageous in terms of cost.

  Examples of the dissolution accelerator include 2-aminoethanol, tetraethylammonium hydroxide, ethylenediamine, 2,2'-iminodiethanol, 1-amino-2-propanol.

  The method for bringing the cooling aqueous solution into contact with the surface of the molten Zn alloy plating layer is not particularly limited. Examples of the method of bringing the cooling aqueous solution into contact with the surface of the molten Zn alloy plating layer include a spray method and an immersion method.

  FIG. 1 is a diagram showing an example of a method of bringing a cooling aqueous solution into contact with the surface of a molten Zn alloy plating layer. FIG. 1A is a diagram illustrating an example of a method in which a cooling aqueous solution is brought into contact with the surface of a molten Zn alloy plating layer by a spray method. FIG. 1B is a diagram illustrating an example of a method in which a cooling aqueous solution is brought into contact with the surface of a molten Zn alloy plating layer by an immersion method.

  As shown in FIG. 1A, a cooling device 100 used for a spray system covers a plurality of spray nozzles 110, a squeezing roll 120 disposed downstream of the spray nozzles 110 in the feed direction of the steel strip S, and these. And a housing 130. The spray nozzle 110 is arranged on both surfaces of the steel strip S. The steel strip S is cooled inside the housing 130 while supplying an amount of cooling water from the spray nozzle 110 such that a water film is temporarily formed on the surface of the plating layer. Then, the cooling water is removed by the squeeze roll 120.

  Moreover, as shown in FIG. 1B, the cooling device 200 used in the dipping method includes a dipping cup 210 in which cooling water is stored, a dipping roll 220 disposed inside the dipping bowl 210, and a dipping roll 220. It has a squeezing roll 230 that is disposed downstream of the steel strip S in the feed direction and removes excess cooling water adhering to the steel strip S. After the steel strip S is put into the immersion tub 210, the direction is changed by the rotating immersion roll 220 while being in contact with the cooling water, and the steel strip S is pulled upward, and the cooling water is removed by the squeezing roll 230. The squeezing roll can remove all the cooling water.

  It is not clear why the production method of the present invention can suppress the problem that a part of the plated layer surface of the hot-dip Zn alloy-plated steel sheet turns black over time. Hereinafter, after explaining the mechanism that is assumed to cause black change in the molten Zn alloy plating layer, the mechanism that is supposed to suppress black change by the manufacturing method of the present invention will be explained. However, the mechanism of black change suppression is not limited to these hypotheses.

(Mechanism of black change)
According to the study by the present inventors, Zn is not only present as metal Zn but also as hydroxide (Zn (OH) ₂ ) in the normal part that is not blackened, and Zn is Zn. More than (OH) ₂ . In the blackened part, Zn is present not only as metal Zn but also as a hydroxide (Zn (OH) ₂ ), as in the normal part, but Zn (OH) ₂ is more than Zn. Existing. On the other hand, Al is present as metal Al and an oxide (Al ₂ O ₃ ) in the normal part and the blackened part, but Al ₂ O ₃ is more than Al in both the normal part and the blackened part. There is no significant change in the existence ratio between the normal part and the blackened part. Similarly, in the normal part and blackening part, Mg exists as metal Mg, oxide (MgO) and hydroxide (Mg (OH) ₂ ), but the metal Mg, Mg in the normal part and blackening part. There is no significant change in the abundance ratio of (OH) ₂ and MgO. From these results, the bonding state of Zn has an influence on the formation of the blackened portion, and it is suggested that the blackened portion may be formed due to the increase in the abundance ratio of Zn (OH) _2. .

In addition, according to the study by the present inventors, when water in the factory is temporarily formed on the surface of the hot-dip Zn alloy plating layer by an air-water cooling device, a water film with cooling water is produced to produce a hot-dip Zn alloy-plated steel sheet. A blackened portion is formed on the surface of the hot-dip Zn alloy-plated steel sheet, but when a hot-dip Zn alloy-plated steel sheet is produced by contacting the cooling water without forming a water film by the cooling water, hot-dip Zn alloy plating is performed. The surface gloss of the steel sheet is uniform, and formation of dark parts (blackened parts) is not recognized. Furthermore, when the analysis by XPS was performed, in the hot-dip Zn alloy plated steel sheet produced by temporarily forming a water film with cooling water, Zn (OH) ₂ was present more than Zn, but with the cooling water. In the hot-dip Zn alloy-plated steel sheet produced without forming a water film, more Zn was present than Zn (OH) ₂ . From these results, the formation of the Zn (OH) ₂ is affected by the formation of a water film in the cooling process, and when the water film is not temporarily formed, Zn (OH) ₂ is not easily generated. It is presumed that the formation of the abnormal part was suppressed.

As described above, the present inventors have found that 1) Zn (OH) ₂ is generated on the surface of the plating layer depending on the manufacturing conditions (for example, water quenching conditions) regarding the black change of the plating layer of the hot-dip Zn alloy-plated steel sheet. And 2) it was found that black changes are likely to occur in the region where Zn (OH) ₂ was generated among the surfaces of the plating layer. Therefore, the present inventors have inferred the mechanism of the black change of the plating layer as follows.

First, when a water film is temporarily formed with cooling water on the surface of the plating layer at a high temperature (for example, about 160 ° C.), Zn is partially eluted from the oxide film on the surface of the plating layer or the Zn phase of the plating layer. .
Zn → Zn ²⁺ + 2e ⁻ (1)

Zn ²⁺ combines with OH ⁻ in the cooling water to become Zn (OH) _{2 on the} surface of the plating layer.
Zn ²⁺ + 2OH ⁻ → Zn (OH) ₂ (2)

As time passes, a part of Zn (OH) _{2 on} the surface of the plating layer becomes ZnO by a dehydration reaction.
Zn (OH) ₂ → ZnO + H ₂ O (3)

Next, a part of ZnO is deprived of O by Al or Mg of the plating layer to become ZnO _1-X . This ZnO _1-X is the color center and visually shows black.

(Mechanism of black change suppression)
Next, the present inventors use a cooling aqueous solution containing polyatomic ions containing V ⁵⁺ at a concentration of 1.0 g / L instead of factory water, and a molten Zn alloy by spray-type water quenching. A water film with a cooling aqueous solution was temporarily formed on the surface of the plating layer to produce a hot-dip Zn alloy-plated steel sheet. At this time, it was estimated that the surface temperature of the hot-dip Zn alloy-plated steel sheet immediately before coming into contact with the cooling aqueous solution was about 160 ° C.

  When the hot-dip Zn alloy-plated steel sheet stored for one week in a room at room temperature of 20 ° C. and a relative humidity of 60% was observed with the naked eye after formation, formation of a dark part (blackened part) was not observed. Moreover, the glossiness of the steel sheet was almost the same as the normal part in the hot-dip Zn alloy-plated steel sheet produced by temporarily forming a water film using factory water.

Next, when a hot-dip Zn alloy plated steel sheet immediately after forming a water film temporarily using a cooling aqueous solution containing V ⁵⁺ was analyzed by XPS, the strength of Zn and Zn (OH) ₂ was determined. From the ratio, it was found that Zn was present more than Zn (OH) ₂ . From this, it was estimated that when a cooling aqueous solution containing V ⁵⁺ was used, the formation of Zn (OH) ₂ was not promoted even when a temporary water film was formed.

The mechanism of black change suppression considered when an aqueous solution containing polyatomic ions containing V ⁵⁺ , Si ⁴⁺ or Cr ⁶⁺ is used as the cooling water will be described by taking V ⁵⁺ as an example. For example, when a cooling solution comprising polyatomic ions containing V ^5+, V ⁵⁺ is reduced temporarily form a dense passivation film between the oxide film on the plating layer surface and the cooling solution . Thereby, it is suppressed that Zn is eluted from the oxide film into the cooling aqueous solution. Therefore, the production | generation of Zn (OH) ₂ is suppressed and the black change of a plating layer is suppressed as a result. In addition, since the said passive film is removed with cooling water when removing cooling water, it is thought that it hardly remains on the plating layer surface after cooling.

  The above-described method for producing a hot-dip Zn alloy-plated steel sheet of the present invention can be carried out, for example, in the following production line.

  FIG. 2 is a schematic diagram of a part of a production line 300 for hot-dip Zn alloy-plated steel sheet. The production line 300 can continuously produce hot-dip Zn alloy-plated steel sheets by forming a plating layer on the surface of a base steel sheet (steel strip). Moreover, the production line 300 can also produce a chemical conversion treatment plated steel plate continuously by further forming a chemical conversion treatment film on the surface of a plating layer as needed.

  As shown in FIG. 2, the production line 300 includes a furnace 310, a plating bath 320, an air jet cooler 340, an air / water cooling zone 350, a water quench zone 360, a skin pass mill 370, and a tension leveler 380.

  The steel strip S fed out from a feeding reel (not shown) is heated in the furnace 310 through a predetermined process. The molten steel adheres to the front and back surfaces of the steel strip S by immersing the heated steel strip S in the plating bath 320. Next, excess molten metal is removed by a wiping device having the wiping nozzle 330, and a predetermined amount of molten metal is adhered to the surface of the steel strip S.

  The steel strip S to which a predetermined amount of molten metal adheres is cooled to below the freezing point of the molten metal by the air jet cooler 340 and the air / water cooling zone 350. The air jet cooler 340 is a facility for cooling the steel strip S by gas blowing. The air-water cooling zone 350 is a facility intended to cool the steel strip S by spraying a mist-like fluid (for example, cooling water) and gas. Thereby, the molten metal is solidified and a molten Zn alloy plating layer is formed on the surface of the steel strip S. When the steel strip S is cooled by the air / water cooling zone 350, no water film is formed on the surface of the plating layer. The temperature after cooling is not particularly limited, and is, for example, 100 to 250 ° C.

The hot-dip Zn alloy plated steel sheet cooled to a predetermined temperature is further cooled in the water quench zone 360. The water quench zone 360 is equipment for the purpose of cooling the steel strip S by contact with a large amount of cooling water compared to the air-water cooling zone 350, and the amount by which a water film is temporarily formed on the surface of the plating layer. Supply water. For example, in the water quench zone 360, seven rows of 10 flat spray nozzles arranged at intervals of 150 mm in the width direction of the steel strip S are arranged in the feed direction of the base steel plate S. In the water quench zone 360, one or more polyatomic ions selected from the group consisting of polyatomic ions containing V ⁵⁺ , polyatomic ions containing Si ⁴⁺ and polyatomic ions containing Cr ⁶⁺ are converted into atoms. An aqueous solution containing 0.01 g / L or more in total is used as the cooling aqueous solution. In the water quench zone 360, the steel strip S is cooled while supplying a quantity of cooling water that temporarily forms a water film on the surface of the plating layer. For example, the water temperature of the cooling aqueous solution is about 20 ° C., the water pressure is about 2.5 kgf / cm ² , and the amount of water is about 150 m ³ / h.

  The water-cooled hot-dip Zn alloy-plated steel sheet is temper-rolled by a skin pass mill 370, straightened by a tension leveler 380, and wound around a tension reel 390.

  When a chemical conversion treatment film is further formed on the surface of the plating layer, a predetermined chemical conversion treatment solution is applied by the roll coater 400 to the surface of the hot dip Zn alloy plated steel sheet corrected by the tension leveler 380. The hot-dip Zn alloy plated steel sheet that has been subjected to the chemical conversion treatment is dried and cooled in the drying zone 410 and the air cooling zone 420 and then wound around the tension reel 390.

  As described above, the method for producing a hot-dip Zn alloy-plated steel sheet of the present invention is a hot-dip Zn alloy plating that is excellent in resistance to blackening by simply bringing an aqueous solution containing predetermined polyatomic ions into contact with the surface of the hot-dip Zn alloy plating layer. A steel plate can be easily manufactured with high productivity.

(Experiment 1)
In Experiment 1, the blackening resistance of the hot-dip Zn alloy plating layer when the hot-dip Zn alloy-plated steel sheet was cooled using cooling water not containing polyatomic ions was examined.

1. Production of hot-dip Zn alloy-plated steel sheet Using a production line 300 shown in FIG. As a base steel plate (steel strip) S, a hot-rolled steel strip having a thickness of 2.3 mm was prepared. The base steel sheet was plated with the plating bath composition and plating conditions shown in Table 1 to produce three types of hot-dip Zn alloy plated steel sheets having different plating layer compositions. The composition of the plating bath and the composition of the plating layer are almost the same. At this time, the cooling rate after plating is determined by plating no. No. 1 at 7 ° C./s, plating no. 2 and 3 were set to 11 ° C./s. Although not particularly shown, in the observation of the cross section of the plating layer, the plating No. For the hot-dip Zn alloy-plated steel sheet 1, [Al / Zn / Mg ₂ Zn ₁₁ ternary eutectic structure] was confirmed, and in the surface layer of the plated layer, spotted portions exhibiting a gloss different from the surroundings were observed. In addition, plating No. [Al / Zn / Zn ₂ Mg ternary eutectic structure] was confirmed on the hot-dip Zn alloy plated steel sheets 2 and 3, and [Al / Zn / Mg ₂ Zn ₁₁ ternary eutectic structure] was confirmed. There wasn't.

When manufacturing a hot-dip Zn alloy plated steel sheet, the cooling conditions in the air jet cooler 340 and the air / water cooling zone 350 are changed, and the temperature of the steel sheet (plating layer surface) immediately before passing through the water quench zone 360 is 100 ° C., 120 It adjusted so that it might become ° C, 160 ° C, 200 ° C or 250 ° C. As the spray device in the water quench zone 360, a header in which ten flat spray nozzles were arranged at intervals of 150 mm in the width direction was arranged in seven rows in the feed direction of the base steel plate S. The cooling conditions in the water quench zone 360 were cooling water: water (pH 7.6, water temperature 20 ° C.), water pressure: 2.5 kgf / cm ² , and water amount: 150 m ³ / h.

2. Evaluation of hot-dip Zn alloy-plated steel sheet (1) Gloss degradation promotion treatment A test piece was cut out from each hot-dip Zn alloy-plated steel sheet produced. Each test piece was placed in a thermo-hygrostat (LHU-113; ESPEC Co., Ltd.), and the gloss deterioration was promoted under each condition shown in Table 2. No. The test conditions of No. 2 Since the processing time is longer than 1. The test conditions are more severe than 1.

（２）黒変化度の測定
各溶融Ｚｎ合金めっき鋼板について、光沢劣化促進処理の前後におけるめっき層表面の明度（Ｌ^＊値）を測定した。めっき層表面の明度（Ｌ^＊値）は、分光型色差計（ＴＣ−１８００；有限会社東京電色）を用いて、ＪＩＳ Ｋ ５６００に準拠した分光反射測定法で測定した。測定条件を以下に示す。
光学条件：ｄ／８°法（ダブルビーム光学系）
視野：２度視野
測定方法：反射光測定
標準光：Ｃ
表色系：ＣＩＥＬＡＢ
測定波長：３８０〜７８０ｎｍ
測定波長間隔：５ｎｍ
分光器：回折格子 １２００／ｍｍ
照明：ハロゲンランプ（電圧１２Ｖ、電力５０Ｗ、定格寿命２０００時間）
測定面積：７．２５ｍｍφ
検出素子：光電子増倍管（Ｒ９２８；浜松ホトニクス株式会社）
反射率：０−１５０％
測定温度：２３℃
標準板：白色

(2) Measurement of degree of black change For each hot-dip Zn alloy-plated steel sheet, the lightness (L ^* value) of the plating layer surface before and after the gloss deterioration promotion treatment was measured. The lightness (L ^* value) of the plating layer surface was measured by a spectral reflection measurement method based on JIS K 5600 using a spectral color difference meter (TC-1800; Tokyo Denshoku Co., Ltd.). The measurement conditions are shown below.
Optical conditions: d / 8 ° method (double beam optical system)
Field of view: 2 degree field of view Measurement method: Reflected light measurement Standard light: C
Color system: CIELAB
Measurement wavelength: 380 to 780 nm
Measurement wavelength interval: 5 nm
Spectrometer: Diffraction grating 1200 / mm
Lighting: Halogen lamp (voltage 12V, power 50W, rated life 2000 hours)
Measurement area: 7.25mmφ
Detection element: Photomultiplier tube (R928; Hamamatsu Photonics Co., Ltd.)
Reflectance: 0-150%
Measurement temperature: 23 ° C
Standard plate: white

For each plated steel sheet, “○” when the difference (ΔL ^* ) in the L ^* value before and after the gloss deterioration promotion treatment is less than 0.5, “△” when 0.5 or more and less than 3, In the above case, it was evaluated as “×”. In addition, it can be judged that the plated steel plate whose evaluation is “◯” has blackening resistance.

(3) Evaluation Results Table 3 shows the relationship between the gloss deterioration promoting condition and the temperature of the steel plate (plating layer surface) immediately before cooling in the water quench zone 360 and the evaluation result of the degree of black change for each plated steel plate.

  No. When the gloss deterioration accelerating treatment was performed under the conditions of No. 1, the plating No. Since none of the test pieces 1 and 2 contained Si, black change occurred when the steel plate temperature immediately before cooling in the water quench zone was 120 ° C. or higher.

  On the other hand, no. No. 2 in which a plating layer containing Si was formed when the gloss deterioration promotion treatment was performed under the conditions of No. 2. Even in the test piece 3, black change occurred when the steel plate temperature immediately before cooling in the water quench zone was 120 ° C. or higher. In addition, the plating No. having a plating layer containing no Si was formed. In the test pieces 1 and 2, black change occurred even when the steel plate temperature immediately before cooling in the water quench zone was 100 ° C.

  From the above results, it can be seen that when the plating layer does not contain Si, black change cannot be prevented unless the steel plate temperature immediately before cooling in the water quench zone is sufficiently lowered. Further, under severe conditions, it can be seen that even if Si is contained in the plating layer, black change cannot be completely prevented unless the steel plate temperature immediately before cooling in the water quench zone is sufficiently lowered.

(Experiment 2)
In Experiment 2, the blackening resistance of the molten Zn alloy plated layer when the molten Zn alloy plated steel sheet was cooled using a cooling aqueous solution containing polyatomic ions was examined. In this experiment, no. The blackening resistance when the gloss deterioration promotion treatment was performed under the condition 1 was examined.

1. Manufacture of hot-dip Zn alloy-plated steel sheet In the same manner as in Experiment 1, the base steel sheet was plated with the plating bath composition and plating conditions shown in Table 1 to produce three types of hot-dip Zn alloy-plated steel sheets with different plating layer compositions. did.

When manufacturing the hot-dip Zn alloy plated steel sheet, the temperature of the steel sheet (plated layer surface) immediately before passing through the water quench zone 360 is changed to 100 ° C, 120 ° C, 160 ° C, 200 ° C by changing the cooling conditions in the air jet cooler 340. It adjusted so that it might become degreeC or 250 degreeC. In the water quench zone 360, any one of the aqueous solutions shown in Table 4 was used as a cooling aqueous solution. Each cooling aqueous solution was prepared by dissolving the additives shown in Table 4 in water at pH 7.6 and, if necessary, a dissolution accelerator at a predetermined ratio, and then adjusting the water temperature to 20 ° C. The conditions of each cooling aqueous solution supplied from the water quench zone 360 were water pressure: 2.5 kgf / cm ² and water amount: 150 m ³ / h. In addition, the concentration of polyatomic ions in each aqueous solution prepared 5 types shown in Table 5 in terms of atoms shown as ion species.

2. Evaluation of hot-dip Zn alloy-plated steel sheet (1) Measurement of gloss deterioration accelerating treatment and blackness change No. 1 shown in Table 2 for each hot-dip Zn alloy-plated steel sheet. The gloss deterioration accelerating treatment was performed under the condition of 1. Further, for each hot-dip Zn alloy-plated steel sheet, the lightness (L ^* value) of the plated layer surface before and after the gloss deterioration promotion treatment was measured in the same procedure as in Experiment 1.

  The plating No. of the hot-dip Zn alloy-plated steel sheet that was evaluated. Table 6 shows the relationship between the concentration of the additive of the cooling aqueous solution used and the concentration of the additive. Each result is shown in the table of numbers shown in Table 6.

(2) Evaluation result About each plating steel plate, the relationship between the kind of cooling water solution used, the temperature of the steel plate (plating layer surface) just before cooling in the water quench zone 360, and the evaluation result of the black change degree is shown in Tables 7 to 7. Table 18 shows.

  The “test piece No.” appearing in each table is defined by the following rules so that the contents of the experiment can be understood. That is, test piece No. Is “(Gloss degradation promoting condition No .; see Table 2) − (Plating No .; see Table 1) − (Concentration symbol of cooling aqueous solution No. and polyatomic ions; see Table 4 and Table 5)”).

As shown in Tables 8 to 11, in the test piece in which the plating layer containing the intermetallic compound Mg ₂ Zn ₁₁ was formed, even if the plating layer not containing Si was formed, V ⁵⁺ , Si ⁴⁺ or Cr ⁶⁺ was contained. As long as it is cooled using an aqueous solution containing polyatomic ions and having a concentration of polyatomic ions of 0.01 g / L or more in terms of atoms, black resistance is improved regardless of the steel plate temperature immediately before cooling in the water quench zone. Denaturation was good.

Further, as shown in Table 7, the test piece formed with the plating layer containing the intermetallic compound Mg ₂ Zn ₁₁ has a plating layer containing no Si and contains V ⁵⁺ , Si ⁴⁺ or Cr ^6+. Even in the case of using an aqueous solution containing atomic ions, the black change could not be sufficiently suppressed when the concentration of polyatomic ions was 0.001 g / L in terms of atoms.

Plating No. In the hot-dip Zn alloy-plated steel sheet 1, [Al / Zn / Mg ₂ Zn ₁₁ ternary eutectic structure] is confirmed immediately after plating, and the surface of the plated layer has a spot-like portion exhibiting a gloss different from the surroundings. It was recognized. However, if cooling is performed using an aqueous solution containing polyatomic ions containing V ⁵⁺ , Si ⁴⁺ or Cr ⁶⁺ and having a polyatomic ion concentration of 0.01 g / L or more in terms of atoms, in the water quench zone Regardless of the steel plate temperature immediately before cooling, the blackening resistance was good, and spotted portions were not visible in the surface layer of the plating layer after cooling in the water quench zone.

These results show that, as shown in Tables 12 to 16, with respect to the plated steel sheet containing [Al / Zn / Zn ₂ Mg ternary eutectic structure], a polyatom containing V ⁵⁺ , Si ⁴⁺ or Cr ⁶⁺ The result was the same as when cooled using an aqueous solution containing ions and having a polyatomic ion concentration of 0.01 g / L or more in terms of atoms.

  As shown in Table 17, in the test piece in which Al and Mg in the plating layer are within a predetermined concentration range and a plating layer containing Si is formed, cooling is performed in the presence or absence of additives and in the water quench zone. Regardless of the immediately preceding steel plate temperature, the blackening resistance was good.

From the above results, water quenching is achieved by cooling with an aqueous solution containing polyatomic ions containing V ⁵⁺ , Si ⁴⁺ or Cr ⁶⁺ and having a polyatomic ion concentration of 0.01 g / L or more in terms of atoms. It can be seen that the black change can be sufficiently suppressed regardless of the steel plate temperature immediately before cooling in the zone.

As shown in Tables 10 to 14, in the test pieces cooled using an aqueous solution containing polyatomic ions containing Mn ²⁺ , Ca ²⁺ , Mg ²⁺ , or Zn ²⁺ , the concentration of polyatomic ions is converted into atoms. When it was 0.01 g / L or more, the blackening resistance was good to some extent.

(Experiment 3)
In Experiment 3, each hot-dip Zn alloy-plated steel sheet produced in Experiment 2 was replaced with No. 1 in Table 2. The blackening resistance when the gloss deterioration accelerating treatment was performed under the condition 2 was examined and evaluated in the same manner as in Experiment 1.

  The plating No. of the hot-dip Zn alloy-plated steel sheet that was evaluated. Table 18 shows the relationship between the concentration of the additive of the cooling aqueous solution used and the concentration of the additive. Each result is shown in the table of numbers shown in Table 18.

(2) Evaluation result About each plating steel plate, the relationship between the kind of cooling water solution used, the temperature of the steel plate (plating layer surface) just before cooling in the water quench zone 360, and the evaluation result of the black change degree is shown in Table 19 to Table 29 shows.

As shown in Tables 20 to 23, in the test piece in which the plating layer containing the intermetallic compound Mg ₂ Zn ₁₁ was formed, V ⁵⁺ , Si ^4+, or Cr ⁶⁺ was used even when the plating layer not containing Si was formed. If cooling is performed using an aqueous solution containing polyatomic ions and having a concentration of polyatomic ions of 0.01 g / L or more in terms of atoms, regardless of the steel plate temperature immediately before cooling in the water quench zone, Black denaturation was good.

On the other hand, as shown in Table 19, the test piece formed with the plating layer containing the intermetallic compound Mg ₂ Zn ₁₁ has a plating layer not containing Si, and the cooling aqueous solution is V ⁵⁺ , Si ⁴⁺ or Cr ⁶⁺ . Even if it contains polyatomic ions contained, when it was cooled using an aqueous solution having a polyatomic ion concentration of less than 0.01 g / L in terms of atoms, the blackening resistance was poor.

Plating No. In the hot-dip Zn alloy-plated steel sheet 1, [Al / Zn / Mg ₂ Zn ₁₁ ternary eutectic structure] is confirmed immediately after plating, and the surface of the plated layer has a spot-like portion exhibiting a gloss different from the surroundings. It was recognized. However, if cooling is performed using an aqueous solution containing polyatomic ions containing V ⁵⁺ , Si ⁴⁺ or Cr ⁶⁺ and having a polyatomic ion concentration of 0.01 g / L or more in terms of atoms, in the water quench zone Regardless of the steel plate temperature immediately before cooling, the blackening resistance was good, and spotted portions were not visible in the surface layer of the plating layer after cooling in the water quench zone.

These results show that, as shown in Tables 24 to 28, a polyatomic material containing V ⁵⁺ , Si ⁴⁺ or Cr ⁶⁺ with ^respect to a plated steel sheet containing [Al / Zn / Zn ₂ Mg ternary eutectic structure]. The result was the same as when cooled using an aqueous solution containing ions and having a polyatomic ion concentration of 0.01 g / L or more in terms of atoms.

As shown in Table 29, in the test piece in which Al and Mg in the plating layer are within a predetermined concentration range and a plating layer containing Si is formed, V ⁵⁺ , Si ⁴⁺ or Cr ⁶⁺ is contained. If cooling is performed using an aqueous solution containing polyatomic ions and having a polyatomic ion concentration of 0.01 g / L or more in terms of atoms, blackening resistance is maintained regardless of the steel plate temperature immediately before cooling in the water quench zone. Was good. At this time, when the cooling aqueous solution did not contain any of V ⁵⁺ , Si ⁴⁺ or Cr ⁶⁺ , the blackening resistance was not improved.

As described above, Experiment 2 The gloss deterioration promoting process is performed under the condition of 1. In this case, if Al and Mg in the plating layer are within a predetermined concentration range and the plating layer contains Si, blackening resistance is prevented regardless of the steel plate temperature immediately before cooling in the water quench zone 360. Was good. On the other hand, Experiment 3 No. 1 more severe than No. 1. The gloss deterioration promoting treatment is performed under the condition of 2. In Experiment 3, a cooling aqueous solution containing polyatomic ions containing V ⁵⁺ , Si ⁴⁺ or Cr ⁶⁺ was used in the cooling aqueous solution even when the plating layer contained Si, and the concentration of polyatomic ions was 0 in terms of atoms. It became clear that blacking resistance cannot be suppressed regardless of the steel plate temperature immediately before cooling in the water quench zone 360 unless it is cooled at a condition of 0.01 g / L or more. That is, in the method for producing a hot-dip Zn alloy-plated steel sheet of the present invention, polyatomic ions in which Al and Mg in the plating layer are within a predetermined concentration range and the cooling aqueous solution contains V ⁵⁺ , Si ⁴⁺ or Cr ⁶⁺ In the water quench zone 360, whether the concentration of polyatomic ions is cooled under conditions of 0.01 g / L or more in terms of atoms, whether or not Si is contained in the plating layer, This is a manufacturing method capable of suppressing blackening resistance regardless of the steel plate temperature immediately before cooling.

  Since the hot-dip Zn alloy-plated steel sheet obtained by the production method of the present invention is excellent in blackening resistance, it is useful as a plated steel sheet used in, for example, building roofing materials, exterior materials, home appliances, and automobiles.

DESCRIPTION OF SYMBOLS 100,200 Cooling device 110 Spray nozzle 120,230 Drawing roll 130 Case 210 Immersion bowl 220 Immersion roll 300 Production line 310 Furnace 320 Plating bath 330 Wiping nozzle 340 Air jet cooler 350 Air water cooling zone 360 Water quench zone 370 Skin pass mill 380 Tension leveler 390 Tension reel 400 Roll coater 410 Drying zone 420 Air cooling zone S Steel strip

Claims (4)

Immersing the base steel plate in a hot-dip Zn alloy plating bath containing Al and Mg, and forming a hot-dip Zn alloy plating layer on the surface of the base steel plate;
An aqueous solution containing one or more polyatomic ions selected from the group consisting of polyatomic ions containing V ⁵⁺ , polyatomic ions containing Si ⁴⁺ and polyatomic ions containing Cr ⁶⁺ , And contacting the surface of the alloy plating layer,
The aqueous solution contains 0.01 g / L or more of the polyatomic ions in terms of 1 or 2 or more atoms selected from the group consisting of V, Si and Cr,
The molten Zn alloy plating layer contains an intermetallic compound Mg ₂ Zn ₁₁ .
Manufacturing method of hot dip Zn alloy plated steel sheet. The molten Zn alloy plating layer includes the intermetallic compound Mg ₂ Zn ₁₁ on the surface layer, the manufacturing method of the molten Zn alloy plated steel sheet according to claim 1.   The temperature of the surface of the hot-dip Zn alloy plating layer when the aqueous solution is brought into contact with the surface of the hot-dip Zn alloy plating layer is 100 ° C or higher and lower than the freezing point of the plating layer. Manufacturing method of hot dip Zn alloy plated steel sheet.   The said hot-dipped Zn alloy plating layer is any one of Claims 1-3 containing Al: 1.0-22.0 mass%, Mg: 0.1-10.0 mass%, and remainder: Zn. Method for producing a hot-dip Zn alloy-plated steel sheet.

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