JPH0860323A - Method for producing hot dip galvanized steel sheet - Google Patents

Abstract

(57) [Summary] [Object] The present invention provides a method for producing a hot-dip Zn-plated steel sheet which is excellent in coating film adhesion and plating adhesion in the processed portion. [Structure] Pre-N on the surface of a steel plate with a thickness of 2.3 mm or more
After i-plating is applied at 0.2 to 2.0 g / m ^2, it is heated to 430 to 550 ° C. in a non-oxidizing or reducing atmosphere and immediately placed in a Zn plating bath containing 0.1 to 0.4% Al. After dipping, hot dip galvanizing, gas wiping, and allowing to cool naturally, the plating layer is alloyed, and F in the plating layer
Molten Zn, characterized in that the e content is 5 to 14%
Manufacturing method of plated steel sheet.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a hot-dip galvanized steel sheet using pre-Ni plating.

[0002]

2. Description of the Related Art The applicant of the present invention has previously disclosed a method for producing a hot-dip galvanized steel sheet using pre-Ni plating, as disclosed in Japanese Patent Laid-Open No. 4-147.
It has already been proposed in Japanese Patent No. 954. This JP-A-4
According to the method described in Japanese Patent No. 147954, the surface of a steel sheet is plated with Ni at 0.2 to 2.0 g / m ² and then H ₂ at 0.1 to 15
% Up to 430 to 500 ° C. in an N ₂ atmosphere containing 30%
After rapid heating at a temperature rising rate of not less than C / sec, the zinc plating is performed by immersing in a molten Zn bath containing 0.1 to 1.0% of Al without exposing to the atmosphere. After hot-dip plating, the amount of adhesion is usually adjusted by gas wiping. Further, generally, immediately after gas wiping, gas or water cooling is performed to solidify the plating layer to prevent Zn from adhering to the guide roll or the like.

[0003]

Recently, in the field of building materials and civil engineering, expectations are high for a hot-dip galvanized steel sheet which is excellent in coating film adhesion and adhesion of a processed part at a low cost. Although the hot-dip galvanized steel sheet produced by the above-mentioned method is extremely excellent in the adhesiveness of the processed part, the coating film adhesiveness,
In particular, since the coating film swelling property on the flaw portion is insufficient, improvement thereof has been desired.

Therefore, the present inventors reviewed the manufacturing conditions of the hot-dip Zn plating method by the pre-Ni plating method in the above-mentioned proposed method and made various studies. After performing hot dip Zn plating, it is also possible to alloy the plating layer by using an alloying furnace in the same manner as in the hot dip Zn plating method by a normal non-oxidation-reduction method to secure coating film adhesion. Since a new alloying furnace is required, it is disadvantageous in terms of cost, and a hot-dip Zn-plated steel sheet that does not use an alloying furnace and has excellent coating film adhesion as well as plating adhesion in the processed part at low cost. We aimed to develop a method for manufacturing.

Therefore, the present inventors have proposed the pre-N
In order to find a method for producing a hot-dip Zn-plated steel sheet having excellent coating film adhesion and plating adhesion at the processed portion while taking advantage of the characteristics of the i plating method, the original plate thickness, pretreatment heating plate temperature, hot-dip Zn
The present invention has been achieved as a result of an examination while paying attention to the Al concentration in the plating bath, the cooling conditions after the molten Zn plating bath has come out, and the like.

[0006] The present invention is a hot-dip Zn plating which is excellent in coating film adhesion by alloying the plating layer without using a costly alloying furnace as described above and also excellent in plating adhesion of the processed portion. It is an object to provide a method for manufacturing a steel sheet.

[0007]

Means for Solving the Problems As a result of various studies, the present inventors have found that in the pre-Ni plating method, the plate thickness of the original plate is set to a certain thickness or more, the pretreatment heating temperature, and the Zn plating bath Keeping the Al concentration within a specific range, and Z
After leaving the n plating bath, it was confirmed that the plating layer was alloyed by allowing to cool naturally.

As a result of evaluating the plating layer structure and performance, it was also confirmed that the Fe content in the plating layer of 5 to 14% has excellent performance in both coating film adhesion and plating adhesion in the processed part. . That is, the gist of the present invention is
Pre-Ni plating of 0.2 to 2.0 g / m ² was preliminarily applied to the surface of a steel sheet having a thickness of 2.3 mm or more, and then heated to 430 to 550 ° C. in a non-oxidizing or reducing atmosphere, and immediately Al.
It is immersed in a Zn plating bath containing 1 to 0.4% for hot-dip Zn plating, gas wiping, and then allowed to cool naturally to alloy the plated layer, and the Fe content in the plated layer is 5 to 14%. And a method for producing a hot-dip Zn-plated steel sheet.

The present invention will be described in detail below based on experimental results. First, the present inventors plated a hot-rolled Al-killed steel sheet (SPHC) having a thickness of 1.6 to 6.0 mm with a pre-Ni plating layer of 0.5 g / m ² and then H ₂ 1% + N ₂ , O ₂ 6
4 at a heating rate of 40 ° C / sec in an atmosphere of 0 ppm
It is heated to 80 ° C., immersed in a molten Zn plating bath at 470 ° C. containing 0.2% of Al at 470 ° C. for 3 sec, the amount of adhesion is adjusted to 60 g / m ² by N ₂ gas wiping, and naturally cooled for 25 sec. With respect to the hot-dip Zn-coated steel sheet produced by cooling with water, the Fe content in the plating layer and the performance (coating film adhesion, plating adhesion of the processed part) were investigated.

FIG. 1 shows the relationship between the original plate thickness and the Fe content in the plating layer. FIG. 2 shows the relationship between the Fe content in the plating layer and the coating adhesion and the plating adhesion of the processed part. In addition, as the coating film adhesion, a cross cut is put after chemical conversion treatment and ED coating, and after the salt spray test (SST), depending on the degree of swelling of the coating film, ◎ (less than 2 mm), ○ (2 mm to 4 mm)
(less than mm), Δ (4 mm to less than 5 mm), and × (5 mm or more). Further, the plating adhesion of the processed portion was examined by a tape peeling test after bending by 60 degrees V. The evaluation criteria are as follows, and a score of 3 or more was passed.

Score 60 degree V-bending tape test Blackness (%) 5 ... less than 20% 4 ... less than 20-30 3 ... less than 30-40 2 ... 40-50 Less than 1 ... 50% or more From FIG. 2, it is that the Fe content in the plating layer is 5 to 14%, and the coating adhesion is 1 if less than 5%.
If it exceeds 4%, the plating adhesion of the processed part may deteriorate.

As shown in FIG. 1, alloying of the plating layer gradually progressed until the plate thickness of the original plate was 2.3 mm or more and 6 mm used in the experiment, and the Fe content was the coating film adhesion and the plating adhesion of the processed part. It can be seen that the optimal range is 5 to 14%, which is excellent in both sex. Next, the present inventors plated a pre-Ni plating layer of 0.5 g / m ² on a 4.3 mm hot rolled Al killed steel plate (SPHC), and then, in an atmosphere of H ₂ 1% + N ₂ and O ₂ 60 ppm. , Heated to various temperatures at a heating rate of 40 ° C./sec, immersed in a molten Zn plating bath at 450 ° C. containing Al0.25% for 3 sec, and 60 g by N ₂ gas wiping.
The Fe content in the plating layer was investigated for the hot-dip Zn-coated steel sheet produced by adjusting the adhesion amount to / m ² and allowing it to cool naturally for 25 seconds and then cooling with steam. Figure 3 shows the survey results. From FIG. 3, it can be seen that the Fe% in the plating layer is optimum in the heating plate temperature range of 430 to 550 ° C.

Next, the present inventors have found that 4.3 mm of hot rolled Al is used.
0.5g pre-Ni plating layer on killed steel plate (SPHC)
/ M ² after _{plating, H 2 1% + N 2} , O 2 60ppm
In an atmosphere of 40 ° C./sec at a heating rate of 480 ° C., immersed in a molten Zn plating bath having various Al concentrations at 450 ° C. for 3 sec, and 60 g / m ₂ with N ₂ gas wiping.
^The amount of adhesion was adjusted to ² , the Fe content in the plating layer of the hot-dip Zn-plated steel sheet produced by natural cooling after 25 seconds of natural cooling was investigated. Figure 4 shows the survey results. From FIG. 4, it is possible to secure Fe of 5 to 14% in the plating layer within the range of Al concentration of 0.1 to 0.4%, and if the Al concentration is less than 0.1%, alloying progresses too much and exceeds 0.4%. It turns out that alloying becomes insufficient.

Next, the present inventors investigated the relationship between the cooling time after wiping and the Fe content in the plating layer. As a result, for example, the plate thickness was 4.3 mm, Al in the bath was 0.25%, and bath temperature was 450. ℃, when the coating weight is 135 g / m ² , when the bath penetration plate temperature is 500 ℃, it is allowed to cool for about 25 seconds to 46
At 0 to 480 ° C., the Fe content in the plating layer becomes 5% in about 46 seconds, the plate thickness is large, the bath entry plate temperature is high, the Al concentration in the bath is low, and the bath temperature is high, the coating adhesion amount is high. It was found that the smaller the amount, the shorter the cooling time required to secure the Fe content of 5 to 14% in the plating layer.

Further, the bath temperature of 430 to 550 ° C. and the amount of deposited Zn of 45 to 150 g / m, which are the usual hot dipping conditions.
² , the plate thickness of the present invention 2.3-6.0 mm, heating temperature 43
If hot-dip plating is performed within a condition range of 0 to 550 ° C. and an Al concentration of 0.1 to 0.4% in the bath, even if the plating layer is naturally left to cool until the plating layer completely solidifies after gas wiping, Fe in the plating layer
It was found that the concentration did not exceed 14% and was 5% or more. Therefore, when there is no facility restriction downstream of the gas wiping, after the hot-dip plating within the above condition range, natural cooling may be performed until the plating layer is completely solidified.

Next, the present inventors have found that 4.3 mm of hot rolled Al is used.
0.5g pre-Ni plating layer on killed steel plate (SPHC)
And / m ² obtained by plating, to prepare those not Ni-plated, both in _{H 2 1% + N 2,} O 2 in 60ppm atmosphere, heated to 550 ° C. at a heating rate of 40 ° C. / sec, Immersed in a molten Zn plating bath of Al 0.25%, 450 ° C. for 3 seconds, wiped with N ₂ gas to adjust the adhesion amount to 60 g / m ² , allowed to cool naturally for 25 seconds, and then cooled with steam to produce molten Zn plating Regarding the steel sheet, the difference in the plating adhesion of the processed part was examined.

As a result, in the case of pre-Ni plating,
In the case where there is no pre-Ni plating, the rating is 5 according to the evaluation criteria described above.
In the case of, the score is 2 and it is better to have pre-Ni plating
It was found that the adhesion was good. Regarding the amount of Ni deposited
0.2 to 2 g / m²I investigated in the range of
The plating adhesion was good in all cases. Also economical
From a viewpoint, Ni is 2 g / m. ²The following is desirable.

The pretreatment heating rate after pre-Ni plating
We also investigated the influence of the temperature of 7 to 70 ° C / sec.
For adhesion of plating under 60 degree V bending condition
Regarding this, no significant changes were seen. Before
For the atmosphere during processing and heating, see N₂H than atmosphere₂
Containing N ₂The atmosphere is better in terms of plating appearance
A trend was recognized.

The above results have been described for the case of only the Zn plating bath, but the results are also obtained for those containing a trace amount of 3% or less of Ni, Mg, Sb and Pb as alloy elements in the plating bath. Was similar. The pretreatment heating method before hot dipping is not particularly limited, but various methods such as a method of directly electrically heating a steel sheet and an induction heating method can be applied.

The amount of Zn plating deposited is 45 to 450 g.
Although the range of / m ² can be adopted, it is preferably 150 g / m ² or less from the viewpoint of workability. Regarding the bath temperature, the usual condition of 430 to 500 ° C. can be used regardless of whether it is a Zn bath or a case where Zn contains a trace amount of alloying elements. As the base steel sheet, not only Al-killed steel, but also hot-rolled steel sheets such as Ti-added ultra-low C steel, Si-Mn-based high-strength steel sheets, and cold-rolled steel sheets can be used.

As a pretreatment on the steel sheet, when roll brush grinding, skin pass, shot blasting or the like is applied,
The alloying rate of the Zn plating layer is fast and uniform, which is more effective.

[0022]

The feature of the present invention is that a thick steel plate is used to alloy the plating layer with sensible heat and no alloying furnace after plating is required. It is considered that the thicker the plate, the higher the sensible heat becomes, and the heat is retained for a long time, so that the alloying of the plating layer proceeds further.

It is also important to carry out pre-Ni plating. Ni covers the surface of the base iron uniformly with a thin layer to prevent oxidation of the base iron, and the base metal itself has a high activity and is active on the steel plate surface. In order to prevent the formation of a strong Fe-Al-based barrier layer by reacting Ni with Al in the bath due to the fact that the points are evenly present, during the hot-dip Zn plating and at the time of cooling, the It is considered that the reactivity is improved.

Further, Al in the bath seems to have an effect on the thickness of the barrier layer in the Zn-Fe alloying reaction.

[0025]

EXAMPLES Table 1 shows the molten Zn obtained by the production method of the present invention.
The example of the sample of a plated steel plate is shown. * Marks other than the present invention
It is a comparative material prepared by the manufacturing method. The base has different thickness.
The following Al-killed hot rolled steel sheet was used. Pre Ni
0.5g / m by electroplating in a sulfuric acid bath²Done,
Temperature rising rate 40 ° C / sec, pretreatment heating (O₂60 pp
m, H₂1% + N₂Atmosphere) by changing the temperature,
In both cases, hot-dip plating was performed at a bath temperature of 450 ° C for 3 seconds,
N₂Adhesion amount of 60 g / m by gas wiping ²age,
After cooling naturally, it was cooled with steam to prepare a sample. Plating
Fe% in layer, plating adhesion of processed part and after ED coating
Adhesion of coating film at cross-cut area (coating with SST4W
The star property) was evaluated. The evaluation criteria are as described above.
It was

[0026]

[Table 1]

No. 1 in Table 1 1-4, 6-10, 12-1
As shown in Nos. 5, 17 to 21, and 23, it is apparent that a predetermined plating layer can be obtained under the manufacturing conditions of the present invention, and the coating film adhesion and the plating adhesion of the processed portion are excellent. If it deviates from the scope of the present invention (No. 5, 11, 16, 22), the performance is inferior.

[0028]

As described above, according to the manufacturing method of the present invention, molten Zn having excellent coating film adhesion and plating adhesion in the processed portion is obtained.
Since the plated steel sheet can be obtained inexpensively without using an alloying furnace, its industrial significance is extremely large.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between an original plate thickness and an Fe content rate in a plating layer.

FIG. 2 is a diagram showing the relationship between the Fe content in the plating layer, the coating film adhesion and the plating adhesion of the processed part.

FIG. 3 is a diagram showing the relationship between the pretreatment heating temperature before entering the plating bath and the Fe content in the plating layer.

FIG. 4 is a diagram showing the relationship between the amount of Al in the plating bath and the Fe content in the plating layer.

Claims (1)

[Claims] 1. A surface of a steel sheet having a thickness of 2.3 mm or more is pre-plated with Ni of 0.2 to 2.0 g / m ² and then heated to 430 to 550 ° C. in a non-oxidizing or reducing atmosphere,
Immediately, it is immersed in a Zn plating bath containing Al 0.1 to 0.4% to perform hot-dip Zn plating, gas wiping, and then allowed to cool naturally to alloy the plating layer, so that the Fe content in the plating layer is 5%. The manufacturing method of the hot-dip Zn-coated steel sheet characterized by making it into 14%.