JP2599535B2 - Hot-dip galvanized steel sheet with smooth and glossy spangle pattern and excellent intergranular corrosion resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-dip galvanized steel sheet.

[0002]

2. Description of the Related Art There is an increasing need for a hot-dip galvanized steel sheet having a beautiful plated surface appearance mainly for building materials and decorative applications. Conventionally, a hot-dip galvanized steel sheet having a spangle pattern as disclosed in JP-B-56-47946 has been used as one of decorative applications.

[0003] In the publication, Sb 0.02 to 0.15
%, Pb 0.02% or less, Al 0.2 to 17%, using a hot-dip Zn plating bath and adding an appropriate amount of Sb to spangle (crystals with a flower pattern peculiar to the hot-dip Zn-plated steel sheet)
A hot-dip galvanized steel sheet is disclosed in which the formation is promoted, and by adding an appropriate amount of Al, the corrosion resistance is improved, the Pb content is reduced to a small amount, and the intergranular corrosion resistance is enhanced.

[0004]

However, the surface of the ordinary spangle is too large and irregularly reflected even when irradiated with light, including the publication, and the surface is inferior in glossiness and high quality. There has been a need for the development of spangles which have disadvantages and are smooth and glossy. Also, in particular,
It is also known that, on a spangled plating surface containing Pb, Pb is easily segregated at crystal grain boundaries, grain boundary corrosion is liable to occur, and the plating layer is liable to peel off over time. However, although it was improved over the Pb-added bath, it was not complete yet, and an improvement was desired. Accordingly, the present inventors have increased the smoothness and gloss of spangles while taking advantage of the features of the hot-dip galvanized steel sheet having spangles due to the Pb or Sb addition bath described above.
Various plating layers which are less likely to cause intergranular corrosion were examined.

As a result, an appropriate amount of N
It has been found that when i is contained, or when a Ni plating layer is applied to the ground iron interface, a plated surface having a smooth and glossy spangle is obtained and grain boundary corrosion hardly occurs. The present invention provides a hot-dip Zn alloy-plated steel sheet having a smooth and glossy spangle as described above and having excellent intergranular corrosion resistance.

[0006]

Means for Solving the Problems First, the present inventors perform hot-dip plating by adding various trace metals to a Pb-added molten Zn-Al bath, which is a usual spangle forming bath, and form spangles by air cooling. The change in appearance was examined. As a result, it has been found that a glossy and smooth spangle is obtained in the plating layer obtained by adding a trace amount of Ni into the bath. This is considered to be due to the change in the crystal structure due to the addition of a small amount of Ni. In addition, it was confirmed that intergranular corrosion hardly occurred. Further studies have found that the presence of the Ni plating layer at the ground iron interface of the plating layer further improves the intergranular corrosion resistance, in particular, and have completed the present invention described below.

[0007] That is, Ni 0.01 to 1
%, And 0.1 to 1% of Al, with the balance having a molten Zn alloy plating layer of 0.01 to 0.5% of Pb or Sb, alone or in combination, having a smooth spangle. Zn with excellent intergranular corrosion resistance
N as the lower layer of the plated steel sheet and the Zn alloy plating layer
A hot-dip galvanized steel sheet having a smooth spangle, which is characterized by having an i-plated layer, and having excellent intergranular corrosion resistance.

Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a diagram showing the relationship between Ni% (% by weight) in a plating layer and spangle appearance.

Pb is applied to an Al-killed steel sheet (sheet thickness 1.6 mm).
Plating was performed for 3 seconds in a hot-dip Zn plating bath in which the Ni content was changed while 0.15% and Al 0.25% were constant. The coating weight of the hot-dip Zn plating layer was 135 g / m ² .
Both the case where the pre-Ni plating layer was applied to the steel sheet at 0.5 g / m ² and the case where the pre-Ni plating layer was not applied are shown. The spangle appearance was evaluated in four steps in the order of smoothness and glossiness.

◎: Smooth and maximum gloss…: Smooth and large gloss…: Irregularity and small gloss, current Pb or Sb addition bath level ×: Unclear or spangle too small From this figure, plating layer within the scope of the present invention Medium Ni0.01
In the range of １1%, spangles are smoothed and have a glossy and beautiful appearance. When the Ni content in the plating layer is less than 0.01%, the spangle has a low level of gloss due to ordinary unevenness.
Exceeds 1%, the shape of the spangle is lost, and a clear spangle cannot be obtained. Also, when a pre-Ni plating layer is provided as a lower layer, the region having a smooth spangle is almost the same as the case without the pre-Ni plating layer.

FIG. 2 is a graph showing the relationship between the Ni% in the plating layer and the intergranular corrosion resistance under the same conditions as in FIG. As the corrosion test, a constant temperature and humidity test (60 ° C., relative humidity RH 90%) was performed for 7 days, the state of intergranular corrosion was examined by observing the cross section of the plating layer, and the degree of corrosion was evaluated in five levels.

[0012] The state of peeling over time was also examined. The evaluation criteria are as follows. Rating Grain boundary corrosion state Peeling over time 5 ... None None 4 ... Almost none. Very small Very small 3 ... Locally generated (normal Sb added bath level) Very small 2 ... Mostly generated (normally Pb added bath level) Mostly generated 1 ... Fully generated Ni in plating layer within the range of the present invention 0.01 to 0.01 In the range of 1%, almost no intergranular corrosion is observed. For this reason, it is difficult to cause peeling over time. In addition, it can be seen that when a pre-Ni plating layer is provided, the intergranular corrosion resistance is further improved.

FIG. 3 is a diagram showing the relationship between Pb% in the plating layer and spangle appearance. Ni 0.03%, A
l Using a 0.25% constant bath, both cases with and without pre-Ni plating were examined. Pb 0.0 within the scope of the present invention
A smooth and beautiful spangle is obtained in the range of 1 to 0.5%. If Pb is less than 0.01%, spangles are not formed, and irregularities exceeding 0.5% become severe, resulting in a fine shape. The same was true for the Sb-added bath.

Further, the reason why the content of Al is set to 0.1% or more is as follows.
If the content is less than 0.1%, the plating adhesion of the processed portion becomes insufficient. In addition, the reason for setting the content to 1% or less is that A
If l exceeds 1%, Al segregated in the plating layer forms a local battery, and the corrosion resistance deteriorates.

The amount of pre-Ni plating when a Ni plating layer is provided as a lower layer is preferably ² g / m ² or less. If it exceeds ² g / m ² , the plating adhesion will be deteriorated.

There is no particular limitation on the amount of Zn plating attached, but it is preferably 10 g / m ² or more from the viewpoint of corrosion resistance and 350 g / m ² or less from the viewpoint of workability.

As the base steel sheet, both hot-rolled steel sheet and cold-rolled steel sheet can be used, and Al-killed steel sheet, Al-Si-killed steel sheet,
Ti-added, P-Ti-added and Cu-added ultra-low carbon steel sheet,
Various materials such as high-Si, Mn-based high-tensile steel sheets can be applied.

The method for producing the coated steel sheet is not particularly limited, and various methods can be applied, such as a normal non-oxidizing furnace-type hot-dip plating method, a Ni pre-plating method, a hot-dip Zn plating method for performing pretreatment such as flux, and the like. The steel sheet may be immersed in a hot-dip Zn plating bath containing Pb, Pb (or Sb), and Al. In particular, as a method of manufacturing the plated steel sheet of claim 2, the steel sheet proposed by the present inventors is pre-Ni-plated with 0.1%.
It is preferable that the coating is performed at a rate of ^{2 to 2} g / m ² , followed by rapid low-temperature heating in a non-oxidizing or reducing atmosphere, followed by hot-dip plating.

Further, the form of the spangle changes depending on the cooling rate after hot-dip plating. Desirably, air cooling or cooling at 1 to 20 ° C./s is appropriate. Cooling rate is 1 ℃ / s
If it is smaller than this, the spangle becomes too large and becomes unclear. On the other hand, if it is higher than 20 ° C./s, the spangle becomes too small.

[0020]

The reason why the spangle is smoothed and the luster increases by adding a small amount of Ni in the plating layer is not clear yet, but is presumed as follows. The mechanism of spangle formation is that Zn solidifies, crystal nuclei are generated by supercooling, and Pb or S
b, Zn—Pb or Zn—Sb becomes Z
Since it has a eutectic point lower than the solidification point of n, a dendrite crystal grows with a crystal orientation because a melt exists around the crystal nucleus. At that time, Pb or Sb is exuded into the melt around the crystal nucleus and concentrated, and segregates at the dendrite crystal grain boundary or in the grains. The reason that the spangle is smoothed and the gloss increases due to the presence of a trace amount of Ni is as follows.
It is presumed that the dendrite and the crystal orientation are affected.

Further, the reason why the intergranular corrosion is likely to occur in the Pb-added bath and the separation with time is liable to occur is that the segregation of Pb at the crystal grain boundary during solidification makes it easy to form a local battery with Zn, and the corrosion is This is because it is easy to reach the interface. The reason that the intergranular corrosion resistance is improved by adding a small amount of Ni is that
It is considered that the local presence of Ni between Pb and Zn at the grain boundaries is alleviated by the uniform presence of Ni. Also, changes in the dendrite crystal structure may have an effect on corrosion resistance. The reason why the intergranular corrosion and peeling over time are further suppressed by the presence of the Ni plating layer at the ground iron interface is that the local circuit action by the microcircuit at the grain boundary is alleviated by the macro circuit due to the potential difference between the Ni layer and the Zn layer This is considered to be due to the fact that the Ni layer becomes a kind of barrier layer and that the intergranular corrosion hardly reaches the ground iron.

[0022]

EXAMPLES Table 1 shows examples of the plated steel sheet of the present invention. *
The marks are steel plates other than the present invention. Hot rolled Al pickled on base
A killed steel plate (1.6 mm) was used. As preprocessing,
In the case without pre-Ni plating, after reducing at 600 ° C. for 30 s in a 15% H ₂ + N ₂ atmosphere, it was cooled to 450 ° C. When pre-Ni plating was performed, a Ni plating layer was applied in advance at 0.5 g / m ² by electroplating, and then heated to 450 ° C. at 40 ° C./s by energizing heating. These were changed to 45 with varying amounts of Ni, Pb, Sb and Al.
Hot-dip plating was performed for 3 seconds in a Zn plating bath at 0 ° C., and the amount of coating was adjusted to 135 g / m ² by N ₂ wiping, followed by cooling at about 2 ° C./s with air to form spangles. The appearance of the spangle was evaluated based on the evaluation criteria described above. As a corrosion test, a constant temperature and constant humidity test (60 ° C., relative humidity RH 90%) was performed for 7 days, and the evaluation was performed based on the evaluation criteria described above.

No. 2-5, No. 7-9, No. 11
-14 and No. As shown in FIGS.
A plating layer containing 0.01 to 1% and Al of 0.1 to 1%, and having a molten Zn alloy plating layer composed of 0.01 to 0.5% of Pb or Sb alone or in combination and a Ni plating layer as a lower layer thereof The plated steel sheet according to the present invention having the above-mentioned structure provides a smooth and glossy spangle and is excellent in intergranular corrosion resistance. It is also clear that when the Ni plating layer is provided as the lower layer, the intergranular corrosion resistance is further improved.

On the other hand, when the composition of the plating layer deviates from the range of the present invention (No. 1, 6, 10, 15) and the conventional materials (No. 22 and No. 23), the steel sheet of the present invention is compared with the steel sheet of the present invention. As a result, a smooth and glossy spangle cannot be obtained, and the intergranular corrosion resistance is poor.

[0025]

[Table 1]

[0026]

As described above, the present invention is a hot-dip galvanized steel sheet having an unprecedented smooth and glossy spangle and excellent in intergranular corrosion resistance, and is useful for building materials, home electric appliances and the like. Therefore, its industrial significance is extremely large.

[Brief description of the drawings]

FIG. 1 is a view showing a relationship between Ni% (% by weight) in a plating layer and spangle appearance.

FIG. 2 is a view showing a relationship between Ni% in a plating layer and intergranular corrosion resistance under the same conditions as in FIG. 1;

FIG. 3 is a diagram showing a relationship between Pb% in a plating layer and a spangle appearance.

Continuation of front page (56) References JP-A-3-229846 (JP, A) JP-A-3-243749 (JP, A) JP-A-4-147953 (JP, A) JP-A-4-147954 (JP) , A) JP-A-4-147955 (JP, A) JP-A-4-333551 (JP, A)

Claims (2)

(57) [Claims] 1. The method according to claim 1, wherein the surface of the steel sheet has a Ni content of 0.01 to 1%.
l Smooth and glossy spangle, characterized by having a hot-dip Zn alloy plating layer containing 0.1 to 1%, 0.01 to 0.5% of Pb or Sb alone or in combination, and Hot-dip galvanized steel sheet with excellent intergranular corrosion. 2. A smooth and glossy spangle characterized by having a further excellent performance by having a Ni plating layer under the Zn alloy plating layer according to claim 1,
Hot-dip galvanized steel sheet with excellent intergranular corrosion resistance.