JP2018162491A - Method for producing hot-dip Zn-Al plated steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To manufacture a hot-dip Zn-Al based plated steel sheet excellent in plating appearance.SOLUTION: In a plating bath having Al of 1-15 mass%, Mg of 0-6 mass% (where, including a case without including Mg) and the remainder consisting of Zn and inevitable impurities, hot-dip Zn-Al based plating is performed by controlling a plating bath temperature so as to obtain a temperature higher by 60°C or more than the solidification starting temperature of the plating bath estimated from an Al and Mg content in the plating bath. An oxide adhering to an iron base-plating interface by the inclusion of a bath surface oxide in the snout of a continuous type hot-dip plating facility causes the occurrence of pit defects, but since the growth of an alloy layer on the iron base-plating interface is promoted by increasing the plating bath temperature, and the adhering oxide film is pushed up by the grown interface alloy layer to be released in the plating bath, the oxide is removed from the iron base-plating interface to suppress the occurrence of pit defects.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a method for producing a hot-dip Zn—Al-based plated steel sheet used in fields such as electrical machinery and building materials.

  A hot-dip Zn-Al-based plated steel sheet containing Al: 1 to 15 mass% in the plating layer has excellent corrosion resistance as compared with a hot-dip Zn-plated steel sheet, and is therefore widely used mainly in the electric and building materials fields. As a typical hot-dip Zn—Al-plated steel sheet, galfan (GF) containing Al: about 5 mass% has been manufactured since the 1980s and has been used a lot. On the other hand, recently, hot-melted Zn—Al-based plated steel sheets that have been enhanced in functionality by containing elements such as Mg have been developed and used.

As such a highly functional hot-dip Zn-Al-plated steel sheet, for example, Al: 1.0 to 10 mass% and Mg: 0.2 to 1 mass% are contained in the plating layer, which causes a problem with galfan. The molten Zn-Al-based plated steel sheet (for example, Patent Document 1) that suppresses the generation of coarse spangles, Al: 2 to 19 mass%, and Mg: 1 to 10 mass% are contained in the plating layer to further improve the corrosion resistance. Further, there is a hot-dip Zn—Al-based plated steel sheet (for example, Patent Document 2).
Moreover, in patent document 3, Al: 4-10mass% and Mg: 1.0-4.0mass% are contained in a plating layer, and adjustment of the cooling rate after plating bath temperature and plating WHEREIN: A hot-dip Zn—Al-based plated steel sheet that suppresses the occurrence of uneven color tone by controlling the type of Mg—Zn compound is disclosed.

JP 2008-138285 A JP 2000-104154 A Japanese Patent Laid-Open No. 10-226865 JP-A-7-150320

However, while the hot-dip Zn-Al-based plated steel sheet has high performance, especially when producing a plated steel sheet with a plating adhesion amount of 100 g / m ² or less per one surface, “pits” that are considered to be caused by foreign matter adhesion There is a problem that a surface defect in which a part of the plating called “dent” is formed occurs and the appearance quality of the plating is lowered. Even if the manufacturing method of the hot dip Zn-Al system plated steel sheet of patent documents 1-3, especially patent documents 1 and patent documents 3 improved paying attention to appearance quality, the above-mentioned pit defect may occur. .

In Patent Document 4, the dew point in the snout is controlled to be −10 ° C. or lower (preferably −40 ° C. or higher and −10 ° C. or lower), and the oxygen concentration is controlled to 10 ppm or lower, whereby dross (Fe ₂ Al ₅ ) or ash ( A method of manufacturing a hot-dip Zn-plated steel sheet that suppresses the generation of nitride (evaporated Zn and its oxide), and nitrides, oxides, and hydroxides and suppresses foreign matter adhesion to the steel sheet is disclosed. However, even when such a technique is applied to the production of a hot-dip Zn—Al-based plated steel sheet, particularly when a hot-dip Zn—Al-plated steel sheet having a coating adhesion amount per side of 100 g / m ² or less is produced. The occurrence of pit defects cannot be completely suppressed. This is considered to be because the defect described in Patent Document 4 is different from the pit defect generated by hot-dip Zn—Al-based plating, and the type and size of the foreign material that is the starting point of the defect is different.

  Therefore, the objective of this invention is providing the manufacturing method which can manufacture the hot dip Zn-Al system plating steel plate which solves the subject of the above conventional techniques and has the outstanding plating external appearance stably.

As a result of repeated studies to solve the above problems, the present inventors have found that (i) the cause of the occurrence of pit defects is the presence of a bath surface oxide containing Al or Mg that occurs in the snout of a continuous hot dip plating facility. (Ii) an alloy layer formed at the base metal-plating interface by controlling the plating bath to a relatively high bath temperature defined by the relationship with the solidification start temperature of the plating bath. It has been found that by promoting the growth of the above, it is possible to effectively suppress the occurrence of such pit defects and to stably obtain an unprecedented excellent plating appearance.
The present invention has been made based on the above findings, and the gist thereof is as follows.

[1] A method for producing a hot-dip Zn-Al-based plated steel sheet in a continuous hot-dip plating facility, in which Al is 1 to 6 mass% and Mg is 0 to 3.5 mass% (including cases where Mg is not contained) The molten Zn-, characterized in that the balance is made of Zn and inevitable impurities and the bath temperature t is hot-dip plated in a plating bath satisfying the following formulas (1A), (2A) and (3): A method for producing an Al-based plated steel sheet.
T _Al = −6.4 × [Al] +419.6 (1A)
T _Mg = −15.9 × [Mg] +419.6 (2A)
t−T ≧ 60 (3)
Where t: bath temperature (° C)
[Al]: Al content in the plating bath (mass%)
[Mg]: Mg content in plating bath (mass%)
T _Al : Estimated solidification start temperature (° C.) of Al—Zn binary alloy with Al content of [Al]
T _Mg : Estimated solidification start temperature (° C) of Mg-Zn binary alloy with Mg content of [Mg]
T: The higher temperature (° C) of T _Al and T _Mg

[2] A method for producing a hot-dip Zn-Al-based plated steel sheet in a continuous hot-dip plating facility, wherein Al is 1 to 6 mass%, Mg is more than 3.5 mass% and less than 6 mass%, and the balance is Zn and inevitable impurities And a hot-dip Zn-Al-based plated steel sheet, wherein the steel sheet is hot-dip plated in a plating bath having a bath temperature t satisfying the following formulas (1A), (2B) and (3):
T _Al = −6.4 × [Al] +419.6 (1A)
T _Mg = 43.2 × [Mg] +212.8 (2B)
t−T ≧ 60 (3)
Where t: bath temperature (° C)
[Al]: Al content in the plating bath (mass%)
[Mg]: Mg content in plating bath (mass%)
T _Al : Estimated solidification start temperature (° C.) of Al—Zn binary alloy with Al content of [Al]
T _Mg : Estimated solidification start temperature (° C) of Mg-Zn binary alloy with Mg content of [Mg]
T: The higher temperature (° C) of T _Al and T _Mg

[3] A method for producing a hot-dip Zn—Al-based plated steel sheet in a continuous hot-dip plating facility, in which Al is more than 6 mass% and 15 mass% or less, and Mg is 0 to 3.5 mass% (provided that Mg is not contained) And the remainder is composed of Zn and inevitable impurities, and the hot plate is hot-dip plated with a plating bath satisfying the following formulas (1B), (2A) and (3): A method for producing a Zn—Al-based plated steel sheet.
T _Al = 7.1 × [Al] +338.3 (1B)
T _Mg = −15.9 × [Mg] +419.6 (2A)
t−T ≧ 60 (3)
Where t: bath temperature (° C)
[Al]: Al content in the plating bath (mass%)
[Mg]: Mg content in plating bath (mass%)
T _Al : Estimated solidification start temperature (° C.) of Al—Zn binary alloy with Al content of [Al]
T _Mg : Estimated solidification start temperature (° C) of Mg-Zn binary alloy with Mg content of [Mg]
T: The higher temperature (° C) of T _Al and T _Mg

[4] A method for producing a hot-dip Zn—Al-based plated steel sheet in a continuous hot-dip plating facility, wherein Al is more than 6 mass% and less than 15 mass%, Mg is more than 3.5 mass% and less than 6 mass%, and the balance is Zn and inevitable A method for producing a hot-dip Zn-Al-based plated steel sheet, comprising hot-plating a steel sheet in a plating bath comprising impurities and having a bath temperature t satisfying the following formulas (1B), (2B) and (3): .
T _Al = 7.1 × [Al] +338.3 (1B)
T _Mg = 43.2 × [Mg] +212.8 (2B)
t−T ≧ 60 (3)
Where t: bath temperature (° C)
[Al]: Al content in the plating bath (mass%)
[Mg]: Mg content in plating bath (mass%)
T _Al : Estimated solidification start temperature (° C.) of Al—Zn binary alloy with Al content of [Al]
T _Mg : Estimated solidification start temperature (° C) of Mg-Zn binary alloy with Mg content of [Mg]
T: The higher temperature (° C) of T _Al and T _Mg

[5] In the production method of any one of [1] to [4], the mass ratio of Mg content [Mg] and Al content [Al] in the plating bath is [Mg] / [Al] ≦ 5. A method for producing a hot-dip Zn-Al-based plated steel sheet.
[6] In the manufacturing method according to any one of [1] to [4], the mass ratio of the Mg content [Mg] and the Al content [Al] in the plating bath is [Mg] / [Al] ≦ 1. A method for producing a hot-dip Zn-Al-based plated steel sheet.
[7] In the manufacturing method according to any one of [1] to [6], the plating bath further includes at least one of Ni: 0.01 to 0.5 mass% and Si: 0.01 to 0.5 mass%. The manufacturing method of the hot dip Zn-Al system plating steel plate characterized by including.
[8] The method for producing a hot-dip Zn—Al-based plated steel sheet according to any one of the above [1] to [7], wherein the plating adhesion amount per side is 100 g / m ² or less.

  ADVANTAGE OF THE INVENTION According to this invention, the hot-dip Zn-Al type plated steel plate which has the outstanding plating external appearance can be manufactured stably. The hot-dip Zn-Al-plated steel sheet produced according to the present invention can be used in a wide range of fields including electrical machinery and building materials, and is particularly used for applications where the plating surface is exposed to the human eye without being coated. For example, the present invention can be suitably applied to wall materials and back plates of household electrical appliances.

Explanatory drawing schematically showing the shape of pit defects Depth-direction elements at the iron-plating interface by transmission electron microscope-energy dispersive X-ray spectroscopy with respect to the cross section of the pit defect portion generated in the hot-dip Zn-Al-based plated steel sheet (containing Mg in the plating layer) Drawing showing analysis results Drawing explaining the presence or absence of detachment of oxides attached to the iron-plating interface in the conventional plating bath with a low bath temperature and the plating bath according to the present invention with a high bath temperature Al-Zn binary equilibrium calculation phase diagram Mg-Zn binary equilibrium calculation phase diagram The graph which shows (1A) type | formula, (1B) type | formula, (2A) type | formula, (2B) type | formula which approximated each melting curve of Al-Zn binary alloy and Mg-Zn binary alloy prescribed | regulated in this invention with a straight line

  First, the pit defect which generate | occur | produces in manufacture of a hot-dip Zn-Al type plated steel plate is demonstrated. FIG. 1 schematically shows the shape of a pit defect. The pit defect has a shape in which a part of the plating is thinned and a point B immediately above the traveling direction is thickened as indicated by point A in FIG. A cross section of a pit defect portion generated in a hot-dip Zn-Al-based plated steel sheet (with a plating layer containing Mg) is cut into a thin film by FIB processing, and transmission electron microscope-energy dispersive X-ray spectroscopy (TEM-EDS) ) Elemental analysis in the depth direction of the iron-plating interface was conducted. The analysis result is shown in FIG. According to this, it can be seen that the iron-plating interface of the pit defect portion has a high concentration of Al, Mg, and O, and an oxide having a thickness of several nm containing Al and Mg is present. This oxide is formed by oxidizing Al or Mg contained in the plating bath on the bath surface, and since it exists at the base metal-plating interface, the steel plate is immersed in the plating bath within the snout of the continuous hot-dip plating facility. It is thought that it was involved in doing.

In general, oxides have poor wettability with a plating bath. Therefore, even in the production of hot-dip Zn-Al-based plated steel sheets in continuous hot-dip plating equipment, the plating repelling phenomenon occurs starting from the oxide entrained in the snout as described above, and there is a local difference in the plating film thickness. It is considered that a pit defect occurs by solidifying as it is.
Such pit defects are particularly likely to occur at the time of production in which the plating adhesion amount per side is 100 g / m ² or less. This is due to the fact that the repelling phenomenon is particularly likely to occur when the plating adhesion amount on one side is 100 g / m ² or less.

  The plating bath used in the production method of the present invention has a bath composition mainly composed of Zn and containing 1 to 15 mass% of Al. Al in the plating bath has the effect of improving the corrosion resistance of the molten Zn—Al-based plated steel sheet and the effect of suppressing the generation of dross when Mg is further contained in the plating bath. When the Al content is less than 1 mass%, the effect of improving the corrosion resistance is not sufficient, and the effect of suppressing the generation of oxide-based dross containing Mg is low. On the other hand, when the Al content exceeds 15 mass%, not only the effect of improving the corrosion resistance is saturated, but also the Fe—Al alloy layer grows remarkably at the base metal-plating interface, and the plating adhesion is lowered.

  Moreover, in a plating bath, Mg: 6 mass% or less can be further contained as needed, and such addition of Mg is preferable from a corrosion-resistant viewpoint. Mg has the effect of stabilizing the corrosion products and significantly improving the corrosion resistance when the hot-dip Zn-Al-based plated steel sheet corrodes. However, when the Mg content exceeds 6 mass%, such corrosion resistance is improved. The effect is almost saturated. When Mg is contained in the plating bath, if the Mg content is less than 0.1 mass%, the effect of improving the corrosion resistance cannot be obtained sufficiently, so the Mg content is preferably 0.1 mass% or more.

  When Mg is contained in the plating bath, the mass ratio of the Mg content [Mg] and the Al content [Al] in the plating bath is preferably [Mg] / [Al] ≦ 5, More preferably, Mg] / [Al] ≦ 1. When [Mg] / [Al]> 5, the effect of suppressing the generation of dross (oxide containing dross containing Mg) by Al is reduced, so that dross defects to which granular dross adheres are likely to occur. Appearance deterioration of the steel sheet is likely to occur. That is, the occurrence of dross defects can be suppressed by setting [Mg] / [Al] ≦ 5, and the generation of dross defects can be suppressed more stably by setting [Mg] / [Al] ≦ 1. be able to.

Moreover, in a plating bath, 1 or more types of Ni: 0.01-0.5mass% and Si: 0.01-0.5mass% can be further contained as needed. When Ni or Si is contained in the plating bath, an interface alloy layer containing Ni or Si is formed at the ground iron-plating interface of the molten Zn-Al-based plated steel sheet, so that the plating adhesion is improved. In particular, the interfacial alloy layer containing Ni is formed in a needle shape in the thickness direction of the plating, so that the adhesion with the plating upper layer is improved by developing the anchor effect. If the content of Ni or Si is less than 0.01 mass%, the effect of improving plating adhesion cannot be obtained sufficiently. On the other hand, when the content of Ni or Si exceeds 0.5 mass%, not only the addition effect is saturated, but also dross is easily generated during bathing.
In addition, the plating film (plating layer on the upper surface of the interface alloy layer) of the hot-dip Zn-Al-based plated steel sheet produced in a plating bath containing Ni or Si was not taken into the interface alloy layer as an inevitable impurity. Ni and Si may be included.

Next, control of the plating bath temperature, which is an important requirement in the present invention, will be described.
As described above, the cause of the pit defect is the entrainment of the bath surface oxide containing Al and Mg generated in the snout. When the steel plate passes through the pot while the oxide is attached to the base iron-plating interface, the plating is repelled when passing through the gas wiping for adjusting the amount of plating attached, resulting in pit defects. Therefore, it is effective for suppressing the pit defects that the oxide adhering to the steel plate is separated from the base iron-plating interface before passing through the gas wiping.

  In the production method of the present invention, the plating bath temperature is increased, and the growth of the alloy layer (interface alloy layer) formed at the base metal-plating interface is promoted, so that it is caught in the snout and enters the base metal-plating interface. The deposited oxide is detached from the base metal-plating interface in the plating bath. FIG. 3 schematically shows the principle, and in the plating bath of the conventional method having a low bath temperature and the plating bath of the method of the present invention having a high bath temperature, detachment of the oxide adhering to the iron-plating interface. Indicates the presence or absence. As shown in FIG. 3, when the plating bath temperature is low, the growth of the alloy layer at the base metal-plating interface is small, and therefore the oxide attached to the base metal-plating interface is easily held as it is. On the other hand, when the plating bath temperature is high, the growth of the alloy layer at the base metal-plating interface is promoted, so that the attached oxide film is pushed up by the grown interface alloy layer and is separated into the plating bath. It is considered a thing. As a result, it is considered that the oxide is removed from the base metal-plating interface, and the occurrence of pit defects is suppressed.

In the production method of the present invention, the plating bath temperature is controlled so as to be 60 ° C. or more higher than the solidification start temperature of the plating bath. In the case of hot-dip Zn-Al plating targeted by the present invention, the solidification start temperature of the plating bath can be estimated by the Al content [Al] and Mg content [Mg] in the plating bath. Based on the formulas (1A), (2A), (1B), (2B) and (3), the control range of the bath temperature can be clarified. That is, the following equation (3) is satisfied with respect to the solidification start temperature T (° C.) of the plating bath determined based on the following equations (1A), (2A), (1B), and (2B). Control plating bath temperature.
・ When 1 mass% ≦ [Al] ≦ 6 mass% T _Al = −6.4 × [Al] +419.6 (1A)
・ When 6 mass% <[Al] ≦ 15 mass% T _Al = 7.1 × [Al] +338.3 (1B)
・ When 0 mass% ≦ [Mg] ≦ 3.5 mass% T _Mg = −15.9 × [Mg] +419.6 (2A)
・ When 3.5 mass% <[Mg] ≦ 6 mass% T _Mg = 43.2 × [Mg] +212.8 (2B)
・ Plating bath temperature control t-T ≧ 60 (3)
Where t: plating bath temperature (° C)
[Al]: Al content in the plating bath (mass%)
[Mg]: Mg content in plating bath (mass%)
T _Al : Estimated solidification start temperature (° C.) of Al—Zn binary alloy with Al content of [Al]
T _Mg : Estimated solidification start temperature (° C) of Mg-Zn binary alloy with Mg content of [Mg]
T: The higher temperature of T _Al and T _Mg

  Here, the above formulas (1A) and (1B) are calculated based on the data of the Al—Zn binary system equilibrium calculation phase diagram shown in FIG. 4 and the Al—Zn binary having the same Al content as the plating bath. It is the formula which approximated the melting curve which shows the estimated solidification start temperature (degreeC) of an alloy with a straight line. In addition, the above formulas (2A) and (2B) are the Mg—Zn binary alloys having the same Mg content as the plating bath, calculated based on the data of the Mg—Zn binary equilibrium state diagram shown in FIG. It is the type | formula which approximated the melting curve which shows the estimated solidification start temperature (degreeC) of this with a straight line. FIG. 6 shows the above formulas (1A), (2A), (1B), and (2B) in which the melting curves of the Al—Zn alloy and Mg—Zn are approximated by straight lines as described above.

The solidification start temperature of the molten Zn—Al-based plating bath can be estimated to be the higher temperature T (° C.) of the temperature T _Al (° C.) and the temperature T _Mg (° C.) according to the above equations. The actual solidification start temperature may be slightly lower than the temperature T (° C.) depending on the balance of the Al and Mg contents, but the temperature T (° C.) is not exceeded in any balance. Since the molten Zn—Al-based plating bath handled in the present invention can always exist in a liquid if it is T (° C.) or higher, the temperature T (° C.) is set as the solidification start temperature of the plating bath. Therefore, the solidification start temperature T (° C.) of the molten Zn—Al-based plating bath is based on the contents of Al and Mg in the plating bath, and the above formulas (1A), (1B), (2A), and ( 2B) Calculated using the formula.

And in order to suppress generation | occurrence | production of a pit defect, as shown in said (3) type | formula, it controls so that plating bath temperature t (degreeC) may become a temperature 60 degreeC or more higher than solidification start temperature T (degreeC). It is necessary. By increasing the plating bath temperature t (° C.) by 60 ° C. or more with respect to the solidification start temperature T (° C.), the growth of the alloy layer at the base metal-plating interface is sufficiently promoted as shown in FIG. Oxide adhering to the base metal-plating interface in the snout can be separated into the plating bath, and as a result, generation of pit defects can be suppressed. On the other hand, when the plating bath temperature cannot be increased by 60 ° C. or more with respect to the solidification start temperature, the base metal-plating interface necessary to develop the effect of releasing the oxide attached to the base metal-plating interface. The alloy layer does not grow.
As described above, in order to suppress the occurrence of pit defects, the plating bath temperature is controlled to satisfy the above formulas (1A), (1B), (2A), (2B) and (3). This is very important.
Although there is no particular upper limit for the plating bath temperature, it is preferable not to raise the bath temperature higher than necessary because the manufacturing cost increases as the bath temperature increases. That is, the bath temperature is preferably set to a lower temperature while satisfying the above formulas (1A), (1B), (2A), (2B) and (3).

  The production method of the present invention does not require special conditions other than the above-described control of the plating bath composition and the plating bath temperature, and may be carried out in a conventional manner. However, in order to further enhance the effect of suppressing the occurrence of pit defects, it is preferable to positively control the atmosphere in the snout of the continuous hot dip plating facility. Specifically, it is preferable to control the atmosphere in the snout so that the dew point is −50 ° C. or lower and the oxygen concentration is 20 ppm or lower. By controlling the atmosphere in the snout to such conditions, it is possible to reduce moisture and oxygen that cause bath surface oxidation in the snout and to suppress bath surface oxidation.

Further, in the production method of the present invention, there is no particular restriction on the type of the base steel plate to be plated, for example, a hot-rolled steel plate or steel strip that has been pickled and descaled, or obtained by cold rolling them. A cold-rolled steel plate or a steel strip can be used.
In the manufacturing method of the present invention, since the plating bath having the bath composition as described above is used, the plating film (plating layer on the interfacial alloy layer with the base steel plate) of the molten Zn-Al-based steel plate to be manufactured is , Al is 1 to 15 mass%, Mg is 0 to 6 mass% (including a case where Mg is not contained), and the balance is Zn and inevitable impurities. The reasons for limiting the contents of Al and Mg in the plating film are the same as described for the plating bath composition.

  In practicing the present invention, the composition of the plating bath and the plating film can be measured by any method. The composition of the plating bath is confirmed (measured) by, for example, pumping a part of the plating bath, solidifying it, then immersing it in hydrochloric acid, etc., and dissolving the solution by ICP emission spectral analysis or atomic absorption analysis. Can do. In addition, the composition of the plating film (plating layer on the interface alloy layer with the base steel plate) is, for example, after peeling only the plating layer existing on the interface alloy layer with the base steel plate by the low potential electrolytic stripping method. The stripping solution can be confirmed (measured) by ICP emission spectroscopic analysis or atomic absorption analysis.

A cold-rolled steel sheet with a thickness of 1.0 mm manufactured by a conventional method is used as a base steel sheet. In a continuous hot dipping system, a line speed of 60 mpm and a target plating adhesion amount per side of 70 to 80 g / m ² (target plating adhesion on both sides) A molten Zn—Al-based plated steel sheet was produced under the conditions of an amount of 140 to 160 g / m ² ). Table 1 shows the production conditions (the composition of the plating bath, the solidification start temperature of the plating bath, the bath temperature, the atmosphere in the snout, the actual amount of plating coating, the coating film composition) and the performance (plating appearance, corrosion resistance, plating adhesion). To Table 4.

The plating bath composition and the plating film composition were confirmed (measured) as follows.
(I) Measurement of plating bath composition A part of a plating bath was pumped from a pot of a continuous hot dipping plating equipment and solidified, and then a face was collected with a metal drill was used as a sample. This sample was immersed and dissolved in hydrochloric acid, and the composition was confirmed (measured) by ICP emission spectroscopic analysis.
(Ii) Measurement of plating film composition A molten Zn-Al-based plated steel sheet as a sample was punched out to 100 mmφ and immersed in fuming nitric acid to peel off the plating film (plating layer excluding the interface alloy layer). After adding hydrochloric acid to the stripping solution to completely dissolve the remaining Al, the composition was confirmed (measured) by ICP emission spectroscopic analysis.

The performance evaluation of the manufactured hot-dip Zn—Al-based plated steel sheet was performed as follows.
(1) Evaluation of plating appearance: pit defects Five large plate samples of 1000 mm in total width × longitudinal direction (through plate direction) were randomly sampled from the coil of the manufactured molten Zn-Al plated steel sheet. The appearance quality of the front and back was evaluated according to the following criteria.
Excellent: No pit defects are observed on the entire surface of the sample.
Good: A level where pit defects are observed in a part of the sample, but does not cause a problem in appearance.
Inferior: Pit defects are observed on the entire surface of the sample.
(2) Evaluation of plating appearance: Dross defect Five large plate samples of 1000 mm in total width x length (longitudinal direction) were randomly sampled from the coil of the manufactured hot-dip Zn-Al plated steel sheet. The appearance quality of the front and back was evaluated according to the following criteria.
Excellent: Adherence of granular dross is not recognized.
Good: A level at which a small amount of granular dross is observed, but does not cause a problem in appearance.
Inferior: A considerable amount of granular dross is observed.

(3) Corrosion resistance A sample obtained by shearing a molten Zn-Al-based plated steel sheet to a size of 70 mm x 150 mm and then sealing the end surface 5 mm of the evaluation surface and the non-evaluation surface (back surface) with a tape was used as a sample. Using this sample for evaluation, a salt spray test (SST): JIS Z2371 was performed, and the corrosion resistance was evaluated according to the following criteria according to the time until red rust was generated on the surface of the sample.
Excellent: Red rust occurrence time ≧ 800 hours Good: 300 hours ≦ Red rust occurrence time <800 hours Inferior: Red rust occurrence time <300 hours

(4) Plating adhesion The sample obtained by shearing a molten Zn-Al-based plated steel sheet to a size of 30 mm x 30 mm is used as a sample, and the striker diameter: 3/8 inch, weight mass: 1.0 kg, drop height: 1000 mm A DuPont impact test was conducted. Cellotape (registered trademark) was strongly pasted on the outer surface of the overhang after the test, and then peeled off. The state of the outer surface of the overhang and the appearance of the cellotape were visually confirmed, and the plating adhesion was evaluated according to the following criteria.
Excellent: Neither crack generation nor plating peeling was observed.
Good: Cracks are observed, but plating peeling is not observed.
Inferior: Peeling of plating is observed.

According to Tables 1 to 4, pit defects and dross defects are generated in the comparative example, whereas in the present invention example, molten Zn- having an excellent plating appearance in which generation of pit defects and dross defects is suppressed. An Al-based plated steel sheet is obtained.
Further, among the examples of the present invention, those containing an appropriate amount of Mg in the plating bath (plating film) have particularly excellent corrosion resistance in addition to the excellent plating appearance. Furthermore, among the examples of the present invention, those containing an appropriate amount of Ni or Si in the plating bath have particularly excellent plating adhesion in addition to an excellent plating appearance.

Claims (8)

A method for producing a hot-dip Zn-Al-based plated steel sheet in a continuous hot-dip plating facility,
Al is 1 to 6 mass%, Mg is 0 to 3.5 mass% (including the case where Mg is not included), the balance is made of Zn and inevitable impurities, and the bath temperature t is expressed by the following formula (1A), (2A ) And hot dip plating of a steel sheet in a plating bath satisfying the expression (3).
T _Al = −6.4 × [Al] +419.6 (1A)
T _Mg = −15.9 × [Mg] +419.6 (2A)
t−T ≧ 60 (3)
Where t: bath temperature (° C)
[Al]: Al content in the plating bath (mass%)
[Mg]: Mg content in plating bath (mass%)
T _Al : Estimated solidification start temperature (° C.) of Al—Zn binary alloy with Al content of [Al]
T _Mg : Estimated solidification start temperature (° C) of Mg-Zn binary alloy with Mg content of [Mg]
T: The higher temperature (° C) of T _Al and T _Mg A method for producing a hot-dip Zn-Al-based plated steel sheet in a continuous hot-dip plating facility,
Al is 1 to 6 mass%, Mg is more than 3.5 mass% and less than 6 mass%, the balance consists of Zn and inevitable impurities, and the bath temperature t satisfies the following formulas (1A), (2B) and (3) A method for producing a hot-dip Zn-Al-plated steel sheet, comprising hot-dip plating a steel sheet in a plating bath.
T _Al = −6.4 × [Al] +419.6 (1A)
T _Mg = 43.2 × [Mg] +212.8 (2B)
t−T ≧ 60 (3)
Where t: bath temperature (° C)
[Al]: Al content in the plating bath (mass%)
[Mg]: Mg content in plating bath (mass%)
T _Al : Estimated solidification start temperature (° C.) of Al—Zn binary alloy with Al content of [Al]
T _Mg : Estimated solidification start temperature (° C) of Mg-Zn binary alloy with Mg content of [Mg]
T: The higher temperature (° C) of T _Al and T _Mg A method for producing a hot-dip Zn-Al-based plated steel sheet in a continuous hot-dip plating facility,
Al is more than 6 mass% and 15 mass% or less, Mg is 0 to 3.5 mass% (including the case where Mg is not contained), the balance is made of Zn and inevitable impurities, and the bath temperature t is expressed by the following formula (1B): A method for producing a hot-dip Zn-Al-based plated steel sheet, comprising subjecting a steel sheet to hot-dip plating in a plating bath satisfying the formulas (2A) and (3).
T _Al = 7.1 × [Al] +338.3 (1B)
T _Mg = −15.9 × [Mg] +419.6 (2A)
t−T ≧ 60 (3)
Where t: bath temperature (° C)
[Al]: Al content in the plating bath (mass%)
[Mg]: Mg content in plating bath (mass%)
T _Al : Estimated solidification start temperature (° C.) of Al—Zn binary alloy with Al content of [Al]
T _Mg : Estimated solidification start temperature (° C) of Mg-Zn binary alloy with Mg content of [Mg]
T: The higher temperature (° C) of T _Al and T _Mg A method for producing a hot-dip Zn-Al-based plated steel sheet in a continuous hot-dip plating facility,
Al is more than 6mass% and less than 15mass%, Mg is more than 3.5mass% and less than 6mass%, the balance consists of Zn and inevitable impurities, and bath temperature t is the following formulas (1B), (2B) and (3) A method for producing a hot-dip Zn-Al-based plated steel sheet, comprising subjecting a steel sheet to hot-dip plating in a plating bath that satisfies the requirements.
T _Al = 7.1 × [Al] +338.3 (1B)
T _Mg = 43.2 × [Mg] +212.8 (2B)
t−T ≧ 60 (3)
Where t: bath temperature (° C)
[Al]: Al content in the plating bath (mass%)
[Mg]: Mg content in plating bath (mass%)
T _Al : Estimated solidification start temperature (° C.) of Al—Zn binary alloy with Al content of [Al]
T _Mg : Estimated solidification start temperature (° C) of Mg-Zn binary alloy with Mg content of [Mg]
T: The higher temperature (° C) of T _Al and T _Mg   The molten Zn according to claim 1, wherein the mass ratio of Mg content [Mg] and Al content [Al] in the plating bath is [Mg] / [Al] ≦ 5. -Manufacturing method of Al system plating steel plate.   The molten Zn according to claim 1, wherein the mass ratio of Mg content [Mg] and Al content [Al] in the plating bath is [Mg] / [Al] ≦ 1. -Manufacturing method of Al system plating steel plate.   The plating bath further contains at least one of Ni: 0.01 to 0.5 mass% and Si: 0.01 to 0.5 mass%, according to any one of claims 1 to 6. A method for producing a hot-dip Zn-Al plated steel sheet. The method for producing a hot-dip Zn-Al-based plated steel sheet according to any one of claims 1 to 7, wherein a plating adhesion amount per side is 100 g / m ² or less.

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