JP2783457B2 - Manufacturing method of hot-dip Zn-Al plated steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for producing a hot-dip Zn-Al plated steel sheet using a pre-Ni method.

[Prior art] Conventionally, a method for producing a hot-dip Zn-Al plated steel sheet using a pre-Ni plating method is disclosed in, for example, JP-A-63-213650.
Known as a method of obtaining a plated steel sheet having excellent surface properties, the same publication discloses a method of suppressing the turtle-like surface pattern, pre-plating, heating at 720 ° C. for 30 seconds, and then hot-dip Zn-Al plating. ing.

[Problems to be Solved by the Invention] Recently, when a hot-dip coated steel sheet is used for automobiles or construction, it may be used in a corrosive environment after being subjected to severe processing to produce various complicated shapes. More and more. Therefore, the excellent plating adhesion during processing and the excellent corrosion resistance after processing are required for molten Zn.
It has become an important performance to be provided for plated steel sheets. The method for producing a hot-dip Zn-Al plated steel sheet by a pre-Ni plating method disclosed in JP-A-63-213650 has a higher corrosion resistance than a normal hot-dip Zn-plated steel sheet having a small Al content, A method for obtaining an excellent plated steel sheet that combines the properties of the original hot-dip Zn-Al coated steel sheet, which has excellent plating adhesion due to the thin alloy layer, and the action of preventing the surface of the plated layer from being glazed by pre-plating However, even with this method, there is still room for improvement in terms of plating adhesion when subjected to severe processing currently required and corrosion resistance of the processed portion.

Therefore, the present inventors used a pre-Ni plating method
In order to further improve the plating adhesion and the corrosion resistance of the processed part of the plated steel sheet obtained by the Zn-Al plated steel sheet manufacturing method, we examined the manufacturing method. By hot-dip plating, we succeeded in obtaining a hot-dip galvanized steel sheet with a plating layer configuration that was not available in the conventional method, and had excellent surface appearance and significantly improved plating adhesion and corrosion resistance in the processed part compared to the conventional method I found to do. In addition, it was found that the surface appearance was further improved by performing N ₂ wiping at the time of lifting from the plating bath. As described above, the present invention relates to molten Zn having excellent adhesion, corrosion resistance and surface appearance of a processed portion.
A method for producing an Al-plated steel sheet is provided.

Means for Solving the Problems The present inventors first prepared a plated layer according to a conventional method for producing a Zn-Al plated steel sheet by a Ni pre-plating method described in Japanese Patent Application Laid-Open No. 63-213650, and Was examined. That is,
After Ni pre-plating, with 15% H ₂ -N ₂ gas in the reduction annealing furnace
After heating at 720 ° C. for 30 seconds, Zn—Al (5%) plating was performed. As a result, the residual amount of the pre-plated layer during heating is extremely small, so that there is almost no reaction layer composed of pre-Ni plating and Zn, Al at the plating layer-base iron interface during hot-dip Zn-Al plating, and the thin Fe layer Only the -Zn alloy layer was present, and therefore, it was found that the plating adhesion and the corrosion resistance of the processed portion were almost the same as those of the normal Zn-Al plated steel sheet without pre-plating. The reason why the pre-plated layer is unlikely to remain during heating is that the heating after pre-plating is heating in a furnace and the heating temperature of the steel sheet is as high as 720 ° C.
And the heating rate is low (20 ° C / s or less), so that the pre-Ni plating layer diffuses into the base iron during heating, which makes it difficult to form a reaction layer with the Ni pre-plating during hot-dip Zn-Al plating It has been found. Therefore, the present inventors considered that there is a manufacturing point in the heating temperature and heating rate after electroplating the Ni pre-plated layer, and as a result of various examinations by changing the heating conditions, Ni was 0.2 to 2 g / m ⁽²⁾ After hot-dipping in a Zn-Al plating bath containing 4 to 7% of Al, when rapid heating is performed at a heating rate of 30 ° C / s or more at a temperature rising rate of 30 ° C / s or more within a range of 430 to 500 ° C, it is obtained. Ni-Al-
A reaction layer consisting of a Zn-based ternary alloy layer is obtained, and a Zn
-It has been found that the plating layer has an Al plating layer and the Zn-Fe alloy layer is extremely thin. In order to investigate the workability and corrosion resistance of these steel sheets when subjected to severe processing, the adhesion test and the corrosion resistance test of the processed parts formed by cup drawing were performed by a corrosion cycle test, and they were manufactured by the method of the present invention. Ni-
The Zn plating layer with the Al-Zn based ternary alloy layer has the plating adhesion and the corrosion resistance of the processed part which are Zn-
It has been found that it is significantly improved as compared with Al-plated steel sheets. The present inventors have also found that the surface appearance is improved by setting the heating atmosphere to N ₂ containing 0.1 to 15% of H ₂ . Furthermore, the present inventors have found that in addition to the above heating conditions, the present invention also found the following to Zn-Al plated steel sheet having more excellent surface appearance by the wiping gas after plating in N ₂ to obtain Is completed.

After the Ni on the surface of the steel sheet 0.2 to 2 g / m ² Plating, H ₂ 0.1 to 15%
After performing rapid heating at a content of N ₂ in the atmosphere to 430~500 ℃ 30 ℃ / s or more heating rate, dip Zn bath containing Al4～7% at a sheet temperature four hundred thirty to five hundred ° C. without exposure to air to control the coating weight in N ₂ gas wiping during further pulling the melting Zn-Al plating bath in the manufacturing process and the process of melting Zn-Al plated steel sheet, characterized in that the galvanized by immersion in A method for producing a hot-dip Zn-Al plated steel sheet.

Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 and FIG. 2 are diagrams showing the relationship between the heating plate temperature and the plating adhesion and corrosion resistance of the processed portion.

Hot-rolled Al-killed steel sheet electroplated 0.5 g / m ² Pre-Ni plating layer (thickness _{1.2mm), O 2 60ppm, H} 2 3% content of N ₂ in the atmosphere to 400~720 ℃ 70 ℃ / s , And immediately subjected to plating for 3 seconds in a hot-dip Zn plating bath of Al 5% at a bath temperature of 450 ° C without exposure to the atmosphere. Air wiping for plating weight
135 g / m ² . The plating adhesion was evaluated by measuring the blackening degree of the tape by performing a tape peeling test after subjecting the test piece to cup drawing with a 40 mm overhang. Corrosion resistance after processing is evaluated by corrosion cycle test (CCT) on the specimen after cup drawing.
Was carried out for three weeks, and the occurrence rate of red rust in the processed part was investigated. Each of the plating adhesion and the corrosion resistance of the processed portion was evaluated by a 5-point method. 3
A score greater than or equal to the score was passed. The evaluation criteria are as follows.

According to this figure, when the heating plate temperature before hot-dip plating is in the range of 430 to 500 ° C. which is the range of the present invention, the adhesion and corrosion resistance of the processed portion are extremely excellent. If the temperature is lower than 430 ° C., the adhesion and the corrosion resistance deteriorate. Further, non-plating is also likely to occur. The heating plate temperature is 500
If the temperature exceeds 100 ° C., the adhesion and corrosion resistance deteriorate.
At 720 ° C. in the example of No. 50, good adhesion and corrosion resistance of the processed portion cannot be obtained. Especially at 450 to 480 ° C, the adhesion and corrosion resistance of the processed part are the most excellent.

FIGS. 3 and 4 show the relationship between the processing speed and the plating adhesion and corrosion resistance of the processed portion. Hot rolled Al-killed steel sheet (sheet thickness 1.
Electroplated 0.5 g / m ² Pre-Ni plating layer 2mm), O ₂ 60pp
m, 4% at a heating rate of 10 to 100 ° C / s in an N ₂ atmosphere containing 3% of H ₂
Heat to 50 ° C and immediately bath temperature 450 without touching the atmosphere
Plating was performed for 3 seconds in a hot-dip Zn-Al plating bath of 5% Al at 5 ° C. The coating weight was 135 g / m ² by air wiping. It is clear that when the rapid heating is performed at a temperature rising rate range of 30 ° C./s or more according to the present invention, the plating adhesion and corrosion resistance of the processed portion are good. If the heating temperature is lower than 30 ° C./s, the plating adhesion and corrosion resistance deteriorate, and good plating adhesion and corrosion resistance cannot be obtained at the heating rate corresponding to the conventional technology range. Particularly, at 50 ° C./s or more, the processed part has the best adhesion and corrosion resistance. As described above, in the present invention, the heating temperature after pre-Ni plating is in a specific range, and the heating rate is high, in order to produce a hot-dip galvanized steel sheet excellent in plating adhesion and corrosion resistance of a processed portion. Is a big point. Although there is no particular limitation on the method of rapid heating, various methods such as a method of directly energizing and heating a steel sheet and an induction heating method can be applied.

FIG. 5 shows the relationship between the H ₂ content in the heating atmosphere and the surface appearance of the plated surface. For hot-rolled Al-killed steel sheet (sheet thickness 1.2mm)
Electroplated 0.5 g / m ² Pre-Ni plated layer, containing H ₂ N
₂ Heat to 450 ° C in an atmosphere at 70 ° C / sec and immediately in a hot-dip Zn plating bath of
Plating was performed for seconds. The coating weight was 135 g / m ² by air wiping. The surface appearance was evaluated by a five-point method according to the occurrence of spots on the plating layer surface due to dross floating on the plating bath surface. Five points had no bumps, one point had a front surface, and three or more points were acceptable. H ₂₀ within the scope of the present invention.
When the content is 1% or more, the appearance is improved. When the content is 1% or more, the best is obtained, and the effect is saturated. If the content exceeds 15%, the upper limit of the H ₂ content is set to 15 considering that the steel sheet easily absorbs H ₂ during heating and blisters are easily generated after plating.
%. The results in FIGS. 1 to 5 did not change even if the plating amount adjustment was performed with N ₂ gas.

FIG. 6 shows the relationship between the N ₂ content in the wiping air and the surface appearance of the plated surface. Electroplated 0.5 g / m ² Pre-Ni plating layer hot-rolled Al-killed steel sheet (thickness 1.2 mm), H ₂
In N ₂ atmosphere containing 3% to 450 ° C. and heated at 70 ° C. / sec, as soon 450 ° C. without exposure to the atmosphere, melting of Al5%
Plating was performed in a Zn plating bath for 3 seconds. The plating weight is
135 g / m ² . As for the surface appearance, the surface gloss of the plated surface was evaluated by the degree of metallic luster. The one with the largest metallic luster was ranked A, the medium one was ranked B, and the small one was ranked C. As shown in FIG. 6, even if ordinary air is used as the wiping gas, the metallic luster is reasonable,
Metallic luster is increased by N ₂ wiping, it is clear that appearance is improved.

Zn-Al plating layer obtained by the present invention and conventional pre-Ni
The result of analyzing the structure of the Zn—Al plating layer obtained by the method is schematically shown in FIG. When the heating temperature and the heating temperature are within the range of the present invention, almost no diffusion of the pre-Ni layer into the ground iron is observed during heating, and the pre-Ni layer remains almost as it is. On the other hand, when the heating temperature is high (more than 500 ° C) at 720 ° C and 20 ° C / s as in the conventional method, the heating rate is low (less than 30 ° C / s)
In case (2), most of the Ni diffuses into the ground iron during heating and changes to a Fe-Ni solid solution layer. Also, the heating temperature
When the temperature rise rate is less than 430 ° C. and the rate of temperature rise is less than 30 ° C./s, Ni remains, but non-plating is apt to occur during hot-dip plating, resulting in poor adhesion.

It is considered that the difference in the state of Ni during the heating causes a difference in the plating layer configuration during the subsequent hot-dip plating. That is, the Ni adhesion amount of the present invention 0.2 to 1.5 g /
In m ^2, pre-Ni layer melt was almost remained in the base steel surface
It has a strong bond with Al and Zn during Zn plating, and a Ni-Al-Zn alloy layer (barrier layer) is formed near the iron-metal interface, and almost no Zn-Fe alloy layer is recognized. The growth was extremely thin and suppressed. Zn as its upper layer
-An Al plating layer was formed. Further, when the pre-Ni deposition amount was 1.5 to ² g / m ² , a part of the metal Ni layer remained. On the other hand, as in the conventional method, the heating temperature is high (50
If the temperature rise rate is low (less than 30 ° C./s), the pre-Ni layer hardly remains during heating. of
Ni-Al-Zn based alloy layer was not formed and formed during heating
The structure was such that a thin Zn-Fe layer was formed on the Fe-Ni layer, and a Zn-Al plating layer was formed thereon.
When the heating temperature is low (lower than 430 ° C.) and the heating rate is low (lower than 30 ° C./s), the wettability of the plating is not ensured, and some non-plating is likely to occur.

Although the details are not clear, in the present invention, the plating adhesion and the corrosion resistance of the processed part are remarkably improved because the ternary alloy layer at the base iron interface serves as a kind of binder, and It is considered that this has a barrier effect of suppressing the growth of the Zn—Fe alloy layer.
Further, the stabilization of Zn corrosion products by Ni and Al may also contribute to the improvement of the corrosion resistance of the processed portion.

The pre-Ni plating amount of 0.2 g / m ² or more is considered to be more than this, because interaction with Al and Zn is recognized, a ternary alloy layer is formed, plating adhesion, corrosion resistance of the processed part This is because the improvement effect is large. In addition, the Ni plating adhesion amount is 0.2 g / m ²
If it is less than 1, non-plating is likely to occur. The reason why the upper limit is set to 2 g / m ² is that if it exceeds ² g / m ² , the plating adhesion deteriorates. In this case, a Zn-Ni plating layer was newly formed at the base iron interface, and the formation of the ternary layer having good adhesion was small. As the method of pre-Ni plating, various methods such as electroplating and electroless plating can be adopted.

Further, when the Al content in the bath was 4 to 7%, the corrosion resistance of the processed portion was the best. Zn-Al plating layer is Zn and Al around 5% Al
However, under the heating conditions of the present invention, as a result of analysis, it was found that the Zn solid solution phase (α phase) was the most in this Al% range.
Since the Al solid solution phase (β phase) is likely to be uniformly mixed, it is considered that the corrosion resistance is improved.

When the content of Al in the bath was less than 4%, the corrosion resistance of the processed portion was insufficient. In this case, it is considered that the Zn-enriched phase (α phase) is apt to be unevenly distributed in the plating layer, so that a local battery is easily formed. When the Al content in the bath exceeds 7%, Al
The concentrated phase (β phase) is likely to segregate, and the corrosion resistance of the processed part is deteriorated. It is conceivable that even when the β phase is unevenly distributed, a local battery is easily formed in the plating layer and corrosion resistance is deteriorated.

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

The above results were described only for the case of Zn-Al plating bath.
The same results were obtained for hot-dip galvanized steel sheets containing trace amounts of Ni, Sb, Pb, and Mg, alone or in combination, of less than 0.2%.

Regarding the bath temperature, ordinary conditions of about 430 to 500 ° C. can be used regardless of whether the bath is a Zn—Al bath or a case where the Zn bath contains a trace amount of an alloy element.

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-Sulc, P-Ti
Various things such as Sulc low carbon steel sheet and high tensile steel sheet can be applied.

[Examples] Table 1 shows examples of a method for producing a Zn-Al-plated steel sheet of the present invention and examples of the obtained steel sheet.

The * mark is a comparative material prepared by a method other than the production method of the present invention. Pre-Ni plating was performed by electroplating in a sulfuric acid acid bath using a pickling material of hot rolled steel sheet SGHC (1.2 mm) as a base steel sheet. Pretreatment heating was performed in an N ₂ atmosphere containing 60 ppm of O ₂ and 3% of H ₂ . Hot-dip plating was performed for 3 seconds in a plating bath at a bath temperature of 450 ° C., and N ₂ wiping or air wiping was performed to set the adhesion amount to 135 g / m ² . The performance evaluation was performed based on the evaluation criteria described above.

As shown in No. 1-17 and No. 26-30, the pre-Ni plating layer 0.2 to ² g / m ² , the heating plate temperature 430 to 500 ° C., and the temperature increase rate 30 ° C./s or more under the production conditions of the present invention. The obtained plated steel sheet is excellent in both the plating adhesion and the corrosion resistance of the processed part.

In comparison, without pre-Ni (No.18) pre-Ni
When the adhesion amount of the plating layer, the heating plate temperature, the temperature rising rate, and the Al content in the bath deviate from the range of the present invention (No. 19-25), the plating adhesion of the processed portion or the corrosion resistance of the processed portion is poor.

The person in the case of using N ₂ wiping than when using air to the wiping as No.26 increases metallic luster, surface appearance is improved.

Further, No. 27-30 was a case where the plating bath contained another alloy element, and also exhibited excellent performance in this case.

[Effects of the Invention] As described above, according to the present invention, a hot-dip Zn-Al-plated steel sheet having excellent surface appearance and having both unprecedented plating adhesion and corrosion resistance in a processed portion can be obtained, and used for automobiles or buildings. Since it is useful as a structural material, its industrial significance is extremely large.

[Brief description of the drawings]

1 and 2 show the heating plate temperature and the plating adhesion of the processed part,
FIGS. 3 and 4 are diagrams showing the relationship between the corrosion resistance and the heating rate, and FIG. 5 is a diagram showing the relationship between the plating adhesion of the processed portion and the corrosion resistance.
The figure shows the relationship between the H ₂ content in the heating atmosphere and the surface appearance (dust), and FIG. 6 shows the relationship between the N ₂ content in the wiping gas and the surface appearance (metallic luster). , FIG.
FIG. 3 is a diagram schematically showing the configuration of a Zn—Al plating layer obtained by a production method of the present invention and a state of a pre-Ni plating layer in a pretreatment heating stage, in comparison with a conventional production method.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2-236263 (JP, A) JP-A-2-190462 (JP, A) JP-A-61-44168 (JP, A) JP-A-58-58 117866 (JP, A) JP-A-57-76176 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C23C 2/00-2/40

Claims (2)

(57) [Claims] 1. A later Ni on the surface of the steel sheet 0.2 to 2 g / m ² Plating, H ₂
In N ₂ atmosphere containing 0.1% to 15% up to 430~500 ℃ 30 ℃ / s
A hot-dip Zn-Al coated steel sheet characterized by being rapidly heated at the above-mentioned heating rate and immersed in a hot-dip Zn bath containing 4-7% Al at a plate temperature of 430-500 ° C without being exposed to the atmosphere. Production method. 2. The method according to claim 1, wherein the molten Zn—Al
A method for producing a hot-dip Zn-Al-plated steel sheet, wherein the amount of plating is suppressed by N ₂ gas wiping when the steel sheet is pulled up from a plating bath.