KR101585748B1 - HOT DIP Zn-Al-Mg BASED ALLOY PLATED STEEL SHEET HAVING EXCELLENT ANTI-CORROSION AND EXCELLETN COATING ADHESION AND METHOD FOR MEHOTD FOR MANUFACTURING THEREOF - Google Patents

Abstract

The present invention relates to a high corrosion resistant coated steel sheet having excellent adhesion between a plating layer and a post-treatment film without complicated control for controlling the plating structure, and a method of manufacturing the same, 5% by weight, Be: 0.001% by weight or more and less than 0.01% by weight, and the remainder Zn and inevitable impurities, wherein the content of Al and Mg satisfies a relationship of Mg > Al, A highly corrosion-resistant plated steel sheet having excellent post-treatment film adhesion including a phosphate bonded to an Al-Mg-Zn oxide. And the balance of Zn and unavoidable impurities, wherein the content of Al and Mg satisfies the relationship of Mg > Al. A step of depositing a steel sheet on a plating bath for plating; And a step of cooling the plated steel sheet and spraying an aqueous phosphate solution before the plating solution is solidified. The present invention also provides a method of manufacturing a high corrosion resistant coated steel sheet having excellent aftercoat film adhesion.

Description

FIELD OF THE INVENTION [0001] The present invention relates to a Zn-Al-Mg high corrosion resistant plated steel sheet having excellent chrome-free post-treatment film adhesion and a method for manufacturing the same. THEREOF}

The present invention relates to a high corrosion resistant coated steel sheet having excellent adhesion between a plating layer and a chrome-free after-treatment film without complicated control for controlling the plating structure, and a method for manufacturing the same.

Generally, in order to improve the physical properties of the plating layer, an additional post-treatment coat layer is formed on the plating layer of the coated steel sheet. Examples of such a post-treatment film include a chromate film and a Cr-free film.

The double chromium-free coating is thicker than the chromate coating, and a phenomenon in which the post-treatment coating is pushed by the friction between the mold and the coating during peeling of the customer or peeling is observed. It is presumed that such a phenomenon of peeling of the post-treatment film during processing is caused by insufficient adhesion between the post-treatment film and the plating layer. Therefore, when the adhesion between the plating layer and the post-treatment film is insufficient, there is a problem in processing such as peeling of the post-treatment film at the time of processing the coated steel sheet. In the case of forming the post-treatment film, It is required to have a firm adhesion with the coating film.

As a technique for improving the adhesion between the post-treatment film and the plating layer, JP-A-2012-134807 discloses a technique for improving the adhesion between the post-treated film and the plating layer, comprising 1.0 to 22.0 mass% of aluminum and 1.3 to 10.0 mass% of magnesium, Is not more than 60 in terms of L * value.

Japanese Patent Application Laid-Open No. 2002-212699 discloses that aluminum is contained in an amount of 3.5 mass% or more and 10 mass% or less and magnesium is contained in an amount of 0.01 mass% or more and 0.50 mass% or less and inevitable impurities, and the average diameter of the sequins is 2.0 mm or less , And on the other hand, the orientation index of the Zn (00 占) plane parallel to the plated surface is less than 3.5.

The above conventional techniques all relate to a technique for controlling the texture of the plating layer to ensure satisfactory plating characteristics. Actually, the process factors affecting the plating structure in the process of manufacturing the coated steel sheet include the thickness of the steel sheet, the deposition amount of plating, There are so many. Therefore, it is very difficult to obtain the desired plating structure by controlling all of them.

An object of the present invention is to provide a high corrosion resistant coated steel sheet having an excellent aftercoat film adhesion with a plating layer having an improved adhesion to a post-treated coating by appropriately controlling the composition of the plating bath and forming a film of a phosphate compound on the plating layer formed thereby.

The present invention relates to a high corrosion resistant alloy coated steel sheet excellent in adhesion to a chrome-free post-treated film, which is formed on a steel sheet and comprises 1 to 4% by weight of Al, 2 to 5% by weight of Mg, And the balance of Zn and inevitable impurities, wherein the contents of Al and Mg include a plating layer satisfying the relation of Mg > Al, and the phosphate contained in the outermost layer of the Al-Mg-Zn oxide of the plating layer A highly corrosion-resistant coated steel sheet excellent in adhesion to a post-treated film.

The phosphate may be contained at an adhesion amount of 1 mg / m 2 or more and 30 mg / m 2 or less.

In the high corrosion resistant coated steel sheet having excellent after-treatment film adhesion, the plating layer may further contain Ca in an amount of 0.005 wt% to less than 0.1 wt%.

The coating layer may further comprise at least one post-treatment coating selected from an inorganic coating, an organic coating, and an organic-inorganic hybrid coating on the plating layer.

The present invention relates to a method for producing a high corrosion resistant coated steel sheet having excellent adhesion to a chrome-free post-treated coating film, which comprises 1 to 4% by weight of Al, 2 to 5% by weight of Mg, 0.001 to less than 0.01% , And the remainder Zn and inevitable impurities, plating the steel sheet by immersing the steel sheet in a plating bath where the content of Al and Mg satisfies Mg > Al, cooling the coated steel sheet, and before the plating solution is solidified A method for manufacturing a high corrosion-resistant plated steel sheet excellent in adhesion of a post-treated film including spraying an aqueous phosphoric acid solution.

The cooling is preferably performed at a cooling rate of 5 to 20 DEG C / sec.

It is also preferable that the plated steel sheet is sprayed with the aqueous phosphate solution at a temperature of 380 ° C or higher.

Further, it is preferable that the phosphate aqueous solution is sprayed so that the phosphate is deposited at 1 mg / m 2 or more and 30 mg / m 2 or less.

And forming at least one post-treatment coating film selected from the inorganic coating, the organic coating and the organic / inorganic composite coating on the phosphate coating.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to improve the adhesion between the plating layer and the post-treatment film, and it is possible to reduce the phenomenon of peeling of the coating film during processing of the coated steel sheet.

Fig. 1 is a photograph of a cross-section of a cross-sectional structure of a plated steel sheet obtained according to the respective conditions in Example 11, magnified 1000 times by an electron microscope.

An object of the present invention is to provide an excellent coating adhesion between a plating layer and a post-treatment coating which impart a high corrosion resistance in carrying out a post-treatment coating in order to impart additional properties to a steel sheet requiring high corrosion resistance.

The hot-dip zinc-alloy plating solution of the present invention is a hot-dip galvannealed steel sheet comprising a plated layer having a high corrosion resistance including Zn-Al-Mg on the surface of the base steel sheet. The plating layer includes 1 to 4 wt% of Al and 2 to 5 wt% of Mg.

The Al is added to improve the corrosion resistance of the plating layer and to prevent the generation of dross on the surface layer of the plating bath due to the oxidation of magnesium. The Al is contained in a range of 1 to 4 wt% of the total weight of the plating liquid do. When the content of Al is less than 1% by weight, the effect of improving the corrosion resistance is not sufficient and the effect of preventing magnesium oxidation in the surface layer of the plating bath is insufficient. On the other hand, when the content of Al exceeds 4% by weight, the iron elution of the base steel sheet is increased, and the weldability and the phosphate treatment property of the plating layer may be lowered. Do.

In addition, the plating bath of the present invention contains 2-5% by weight of Mg. When the content of Mg is less than 2% by weight, the effect of improving the corrosion resistance is not sufficient. When the content of Mg is more than 5% by weight, Mg causes the corrosion resistance of the plating bath upper layer The occurrence of dross may increase, which may make management of the plating bath difficult. Therefore, Mg is preferably contained in an amount of 2 to 5% by weight

It is preferable that the content of Al and Mg is larger than the content of Al in view of corrosion resistance. That is, it is necessary to add Mg to satisfy the relation of Mg > Al. When the Zn-Al-Mg-based coated steel sheet is exposed to the corrosive environment, Mg in the plating layer is actively dissolved and the generation of simon clit on the surface of the plating layer may be promoted when the above conditions are satisfied. The simon clit is a kind of zinc oxide. When the simon clit is formed on the surface of the plating layer, the generated oxide acts as a corrosion barrier, thereby providing an additional corrosion inhibiting effect to the plating layer. However, when the relationship of Mg > Al is not satisfied, the cause is not clear, but the dissolution of Mg is suppressed by Al and does not provide the additional corrosion inhibiting effect as described above.

Further, the plating bath of the present invention comprises Be and optionally Ca.

The Be is used for inhibiting growth of the magnesium oxide coating on the surface of the plating layer due to oxidation of magnesium, and is preferably contained in an amount of 0.001 wt% or more and less than 0.01 wt%. If the content of Be is less than 0.001% by weight, the effect of inhibiting excessive oxidation of magnesium in the plating bath and the plating layer is insufficient due to the addition of Be. If the content of Be is more than 0.01% by weight, As a matter of course, there may arise a plating layer unplated problem. In particular, in a high-tensile steel sheet having a high concentration of Si and Mn, when the amount of Be is excessively added, the wettability is deteriorated, and thus such an unplating problem may occur.

In addition, Ca plays a role of inhibiting the oxidation of Mg similarly to Be, and also has an effect of forming an Al-Ca intermetallic compound of Al ₄ Ca by binding with Al to miniaturize the plating structure. At this time, Ca is preferably contained in an amount of 0.005 wt% or more and 0.1 wt% or less. When the content of Ca is less than 0.005% by weight, the effect of adding Ca is hardly obtained. When the content of Ca exceeds 0.01% by weight, Ca is supersaturated in the plating bath to exist in a fine dross form, It is preferably added within the above range.

Further, Ca and Be contribute to smooth formation of the surface of the plating layer. If the surface of the plating layer is smooth, the thickness of the post-treatment coating formed thereon can be uniformly formed, thereby reducing the problem of peeling off the post-treatment coating during processing of the steel sheet. On the contrary, if the plating layer is not smooth, the post-treatment coating may be formed non-uniformly when the post-treatment solution is coated on the surface of the plating layer by a roll coating or a roll squeezing method. If the thickness of the post-treatment film is unevenly formed, the friction between the metal mold and the post-treatment film at the time of forming the steel sheet becomes non-uniform in each part, and the post-treatment film may peel off at a portion where friction is severe.

The plating bath of the present invention is a hot-dip coating bath of a zinc alloy containing the above components and containing the remainder Zn and unavoidable impurities. The plating layer can be formed on the surface of the steel sheet by depositing and drawing the steel sheet into the plating bath. At this time, by performing gas wiping using N ₂ gas on the steel sheet drawn out from the plating bath, the amount of coating on the surface of the steel sheet can be controlled.

The plating adhesion amount is not particularly limited, and can be plated with an adhesion amount of 30 g / m 2 or more and 200 g / m 2 or less on a one-side basis generally used in hot-dip coating.

The plating layer can be formed by adjusting the plating amount as described above and cooling the steel sheet to solidify the molten amount adhering to the surface of the steel sheet. On the other hand, the cooling can be performed by a conventional method, and is not particularly limited. For example, an air cooling method in which air is cooled can be applied.

At this time, the cooling rate of the steel sheet may be in the range of 5-20 ° C / sec. When the cooling rate of the steel sheet is less than 5 ° C / sec, the cooling rate of the steel sheet is too slow to grow the steel sheet coarsely. When the cooling rate exceeds 20 ° C / sec, the further improvement effect is not clear. There is a problem that the equipment investment is increased due to a large scale of the equipment, and it is preferable to cool the equipment within the above range.

However, the present invention aims to improve the adhesion between the plating layer and the post-treatment film by forming a phosphate film on the plating layer. For this purpose, it is preferable to spray the phosphate during the process of solidifying the plating solution by cooling the steel plate with the plating solution (after the initiation of solidification) and before the plating layer is solidified.

By spraying the phosphate aqueous solution onto the surface of the non-coagulated plated layer, the cooling rate during solidification of the plated layer is increased, which has the effect of making the plating structure finer. Therefore, the Al-Ca intermetallic compound of fine structure formed by the Ca addition acts as the solidification nucleus, and the solidification nuclei can be inhibited from growing so fast by increasing the cooling rate by spraying the aqueous phosphate solution. Particularly, the aqueous phosphate solution adhered to the surface of the plating layer has an effect of endothermic not only by evaporation of water but also by condensation reaction of phosphate.

It is assumed that the phosphate sprayed on the surface of the non-solidified plating layer reacts with the oxide present in the extreme surface layer of the plating layer, thereby forming a composite structure of Zn-Al-Mg oxide and phosphate compound, thereby improving the adhesion of the post- .

The spraying of the aqueous phosphate solution is preferably carried out before the steel sheet is cooled to less than 380 ° C. When the steel sheet temperature is lowered to less than 380 ° C, solidification of the Zn-Al-MgZn _{2 component} , which is a member of the surface oxide, proceeds and the reaction between the phosphate and the oxide can not be sufficiently obtained. Can not be improved.

It is preferable that the phosphate coating is formed on the surface of the plating layer at an adhesion amount of 5 mg / m 2 or more and 30 mg / m 2 or less. When the coating amount of the phosphate coating is less than 5 mg / m 2, the effect of improving the adhesion with the post-treatment coating is small and the effect of improving the adhesion with the post-treatment coating due to the formation of the phosphate coating is not additionally observed even if the coating amount exceeds 30 mg / .

In forming the phosphate film, there is no particular limitation, but an aqueous phosphate solution containing phosphate in a concentration of 0.01 wt% or more and 1 wt% or less can be used. When the phosphate content of the phosphate aqueous solution is less than 0.01% by weight, the concentration is too low to act as a phosphate aqueous solution. If the phosphate concentration is more than 1% by weight, phosphate is precipitated in the injection hole of the solution injection nozzle, As a clogging problem, a phosphate solution having a phosphate concentration of 0.01 wt% to 1.0 wt% can be used.

In addition, the aqueous phosphate solution may be suitably used in the present invention without particular limitation, as long as it has water solubility such as ammonium hydrogen phosphate, ammonium phosphate monobasic or ammonium dibasic ammonium compound.

On the other hand, the spraying of the phosphate aqueous solution is not particularly limited, and a conventionally performed method can be applied. For example, spray coating or electrostatic spraying may be used.

As described above, the adhesion between the plating layer and the post-treatment film can be improved by spraying the phosphate aqueous solution onto the surface of the non-solidified plating layer. The reason for this effect is not clearly understood, but it is presumed that the reason is as follows.

That is, when the phosphate aqueous solution is injected onto the surface of the non-solidified plating layer, the cooling rate during solidification of the plating layer is increased, which has the effect of making the plating structure finer. Therefore, the Al-Ca intermetallic compound of fine structure formed by the Ca addition acts as the solidification nucleus, and the solidification nuclei can be inhibited from growing so fast by increasing the cooling rate by spraying the aqueous phosphate solution. Particularly, the aqueous phosphate solution adhered to the surface of the plating layer has an effect of endothermic not only by evaporation of water but also by condensation reaction of phosphate.

Further, it is presumed that a composite structure is formed between the Zn-Al-Mg oxide and the phosphate compound in the outermost surface layer of the plating layer, thereby enhancing the adhesion between the plating layer and the post-treatment coating.

The steel sheet of the present invention can form a Cr coating, an inorganic or organic post-treatment coating on the phosphate coating as described above, and the adhesion between the plating layer and the post-treatment coating can be strengthened by the phosphate coating.

At this time, the after-treatment film may be suitably formed in the present invention as long as it is an inorganic film or organic film as long as it is ordinarily carried out. The post-treatment coating is also not particularly limited, but in the case of a chrome-free coating, it may be formed with an adhesion amount of 0.1 g / m 2 or more and 2 g / m 2 or less.

According to the present invention, a phosphate coating is formed on the high corrosion resistant plating layer, and a post-treatment coating is formed on the phosphate coating, whereby a high corrosion resistant coated steel sheet having excellent adhesion between the plating layer and the post treatment layer can be obtained.

Example

Hereinafter, the present invention will be described more specifically with reference to examples. However, the following examples are representative examples of the present invention, and the present invention is not limited thereto.

Comparative Example  1 to 8

Rolled steel sheet having a thickness of 1.6 mm was subjected to a pickling treatment with a pickling solution having a hydrochloric acid concentration of 15% for 3 minutes, followed by washing with water and drying.

The steel sheet was heated to a temperature of 600 ° C in a reducing atmosphere of N ₂ + H _{2 having} a hydrogen concentration of 10%, and then immersed in a plating pot containing a plating solution having the composition shown in Table 1 and having a temperature of 450 ° C.

Subsequently, the amount of the plating solution adhered to the surface of the steel sheet by gas wiping was adjusted to 70 g / m < 2 > and the steel sheet was cooled by the air cooling method under the same conditions as in Table 1 to solidify the plating solution, .

The coated steel sheet thus obtained was evaluated for corrosion resistance and post-treatment film adhesion.

The corrosion resistance evaluation was carried out by using a 5% saline solution until the redness of the surface was reached, and the saline spraying time required until the redness occurred was measured. The results are shown in Table 1.

The after-treatment film adhesion is generally evaluated by applying an acrylic resin solution used for the chrome free coating to the surface of the steel sheet to form a coating having an adhesion amount of 0.8-1.2 g / m 2, and drawing the coated steel sheet into a cup shape And the surface rubbed against the mold was visually observed to evaluate the adhesion.

Observation results are shown in Table 1. In this case, a case in which the trace of blackened resin is clearly displayed on the surface of the coating film is indicated by?, A case where there is a trace of being pushed onto the surface of the resin coating film but a trace thereof is indicated by?, And no trace of the resin is observed, Is indicated as "& cir &".

Comparative Example
No. Plating bath composition (% by weight) Cooling rate
(° C / sec) After treatment film
Adhesiveness Red rush hour
(hr) Al Mg Be Ca One 0.5 3 0.002 0.01 10 - - 2 5 2 0.004 0.015 15 △ 196 3 4 One 0.004 0.015 5 △ 144 4 4 6 0.01 0.01 10 - - 5 2.5 3 0 0.03 15 △ 120 6 2 3 0.02 0.005 10 △ 480 7 1.5 2 0.004 0.2 0.3 - 360 8 2.5 3 0.005 0.05 One △ 480

As shown in Table 1, in Comparative Examples 1 and 4, plated layers were not formed due to excessive dross formation in the plating bath, resulting in extremely poor workability. In the case of Comparative Example 1, the content of Al was too small to sufficiently inhibit the oxidation of Mg. In Comparative Example 4, the content of Mg was too high, resulting in a large amount of dross in the plating bath.

In Comparative Example 2, when the content of Al was too high and the content of Al was higher than the content of Mg, the coating film was peeled off due to insufficient adhesion with the after-treatment film, and the corrosion resistance was also insufficient have. In Comparative Example 3, the content of Mg was so small that the corrosion resistance was insufficient, and the content of Al was larger than the content of Mg, so that a peeling phenomenon of a post-treatment film was observed as in Comparative Example 2. [

In Comparative Examples 5, 7, and 8, the content of Mg and the content of Al both satisfied the range of the present invention and were evaluated to exhibit good corrosion resistance. However, in Comparative Example 5, there was a phenomenon in which the post-treatment film peeled off during cup drawing when Be was not added. This is because the flatness of the surface of the plating is poor due to the non-addition of Be, and uniform post-treatment coating is not obtained. In Comparative Example 7, when the Ca content was high, since excessive Ca addition could not ensure the uniformity of the post-treatment film due to the formation of dross on the surface of the steel sheet, peeling of the post-treatment film occurred at the time of cup drawing Respectively. In Comparative Example 8, the cooling rate was slow. In this case, the corrosion resistance was satisfactory, but the after-treatment film peeling phenomenon was observed.

On the other hand, in Comparative Example 6, when Be was added in an excessive amount, there was a phenomenon that the post-treatment coating was peeled off by the cup drawing process, as well as unplated in the plating layer, and redness occurred in the salt water spraying time of 120 hours .

Example  1 to 5

Rolled steel sheet having a thickness of 1.6 mm was subjected to a pickling treatment with a pickling solution having a hydrochloric acid concentration of 15% for 3 minutes, followed by washing with water and drying.

The steel sheet was heated to a temperature of 600 ° C in a reducing atmosphere of N ₂ + H _{2 with} a hydrogen concentration of 10%, and then immersed in a plating pot containing a plating solution having the composition shown in Table 2 and having a temperature of 450 ° C.

Subsequently, the steel sheet from the above-mentioned plating port was adjusted to an adhesion amount of 70 g / m 2 of the plating solution adhered to the surface of the steel sheet by gas wiping and cooled by the air cooling method under the conditions shown in Table 2.

The obtained coated steel sheet was evaluated for corrosion resistance and post-treatment film adhesion.

The corrosion resistance evaluation and the post-treatment film adhesion evaluation were carried out in the same manner as in Comparative Example 1, and the results are shown in Table 2.

Example
No. Plating bath composition (% by weight) Cooling rate
(° C / sec) After treatment film
Adhesiveness Red rush hour
(hr) Al Mg Be Ca One 4 2 0.004 0.015 5 ○ 360 2 4 5 0.01 0.01 10 ○ 600 3 2.5 3 0.005 0.08 15 ○ 480 4 2 3 0.001 0.005 10 ○ 480 5 1.5 2 0.004 0.1 20 ○ 360

As can be seen from the above Table 2, all of the corrosion resistance was satisfactory, and the trace of the post-treatment film pushed by the cup drawing processing was faint, but the peeling of the film was not observed.

Comparative Example  9-13

As shown in Table 3 below, the steel sheets were immersed in the plating pots in the same manner as in Comparative Examples 1 to 8 (except for Comparative Examples 1, 4, and 7, which were not operable due to the generation of dross) And the steel sheet was cooled by the air cooling method. At this time, the cooling was performed until the temperature of the steel sheet reached 400 캜.

Subsequently, a phosphate aqueous solution (aqueous ammonium phosphate solution) having a concentration as shown in Table 3 was sprayed onto the surface of the steel sheet using a twin-screw injection nozzle to form a phosphate coating. The amount of phosphate coating deposited was as shown in Table 3.

The coated steel sheet thus obtained was evaluated for corrosion resistance and post-treatment film adhesion.

Comparative Example
No. Plating bath composition (% by weight) Phosphate spray After treatment
film
Adhesiveness Red
Occurrence time (hr) Al Mg Be Ca density
(%) Coating weight
(mg / m 2) 9 5 2 0.004 0.015 0.1 5 △ 196 10 4 One 0.004 0.015 0.5 20 △ 144 11 2.5 3 0 0.03 0.01 One △ 120 12 2 3 0.02 0.005 0.2 10 △ 480 13 2.5 3 0.005 0.05 One 30 △ 480

From Table 3, it can be seen that Comparative Examples 9 to 13 are each formed by forming a phosphate coating on the plating layer formed by using the same composition as the plating compositions of Comparative Examples 2, 3, 5, 6 and 8 by spraying an aqueous phosphate solution. The results are shown in Table 1.

Furthermore, even if a phosphate coating is formed on such a plating layer, the adhesion of the post-treatment film is not improved, and the post-treatment film peels off due to the shrinkage of the post-treatment coating by the cup drawing process.

Example  6 to 10

As shown in Table 4 below, the steel sheets were immersed in the plating pots in the same manner as in Examples 1 to 5, and the steel sheets were cooled by the air cooling method. At this time, the cooling was performed until the temperature of the steel sheet reached 400 캜.

Subsequently, a phosphate aqueous solution (aqueous ammonium phosphate solution) having a concentration as shown in Table 4 was sprayed onto the surface of the steel sheet using a twin-screw injection nozzle to form a phosphate coating. The amount of phosphate coating deposited was as shown in Table 4.

The coated steel sheet thus obtained was evaluated for corrosion resistance and post-treatment film adhesion.

Example No. 2. Plating bath composition (% by weight) Phosphate spray After treatment
film
Adhesiveness Red
Occurrence time (hr) Al Mg Be Ca density
(%) Coating weight
(mg / m 2) 6 4 2 0.004 0.015 0.1 5 ◎ 360 7 4 5 0.01 0.01 0.5 20 ◎ 600 8 2.5 3 0.005 0.08 0.01 One ◎ 480 9 2 3 0.001 0.005 0.2 10 ◎ 480 10 1.5 2 0.004 0.1 One 30 ◎ 360

As can be seen from Table 4, in Examples 6 to 10, a plating layer was formed by using a plating bath having the same composition as in Examples 1 to 5, and a phosphate coating was formed thereon. Were the same as the results of Examples 1 to 5 shown in Fig.

However, no traces were observed in the film due to the cup drawing processing in terms of the post-treated film adhesion, and thus, a better surface state was exhibited as compared with Examples 1 to 5.

From the above results, it can be seen that simply forming the phosphate film according to the present invention can not improve the adhesion between the plating layer and the post-treatment film, and can form a phosphate film on the plating layer satisfying the plating composition proposed in the present invention It was found that the adhesion to the post-treatment film was further improved.

Example  11

As shown in the following (a) to (c), the effect of phosphate aqueous solution treatment on the surface smoothness of the steel sheet was examined by varying the treatment temperature conditions in the absence and in the phosphate treatment.

(a) A coated steel sheet was prepared in the same manner as in Example 6, except that the phosphate aqueous solution was not sprayed.

(b) A coated steel sheet was prepared in the same manner as in Example 6, except that 0.05% (NH ₄ ) H ₂ PO ₄ aqueous solution was sprayed at a temperature of 370 ° C.

(c) A coated steel sheet was prepared in the same manner as in Example 6, except that 0.05% (NH ₄ ) H ₂ PO ₄ aqueous solution was sprayed at a temperature of 420 ° C.

The sectional structure of the plated steel sheet obtained under the respective conditions was magnified 1000 times by an electron microscope and the sectional structure thereof was photographed and shown in Fig.

It can be seen from FIG. 1 that (a) in which 0.05% of (NH ₄ ) H ₂ PO ₄ aqueous solution was sprayed at the temperature of 370 ° C as compared with (b) . Further, when comparing the smoothness of the plating layer in the cross-sectional bath, it can be seen that (a) in which the aqueous phosphate solution is sprayed is significantly smoother.

On the other hand, in the case of (c) in which an aqueous phosphate solution was sprayed at a temperature of 420 ° C, the plating texture and plating smoothness were similar to those of (b). From this, it can be seen that the phosphate aqueous solution is not effective in improving the plating smoothness before the start of solidification of the plating liquid.

In the case where the phosphate aqueous solution is not sprayed or the phosphate aqueous solution is sprayed before the solidification of the plating bath, the main reaction at the initial stage of solidification is precipitation of the Zn superstrate and intermetallic compounds such as CaZl ₄ and MgZn ₂ may precipitate.

That is, when the cooling rate is slow, the Zn-sec phase is sufficiently grown, and the uncoagulated plated layer forms a solidification pool between the Zn-superstructure, where the concentrations of Al and Mg become high. When the solidification of the Zn superconducting phase progresses, it grows while consuming the molten Zn around the supercritical phase, and as a result, the Zn superconducting state protrudes upward from other portions. In addition, the molten metal pool portion having a high concentration of Mg and Al in which the final solidification occurs is thinner than the other portions, and the smoothness is lowered due to variations in the plating thickness.

In order to prevent this phenomenon, it is necessary to suppress the growth of Zn 2 -secondary phase. For this purpose, it is effective to spray an aqueous phosphate solution. This is because the water in the aqueous solution of the phosphate aqueous solution adhering to the surface of the steel sheet is heated while being evaporated, and further, the condensed endothermic reaction of the phosphate compound occurs, so that the plating structure can be miniaturized by increasing the cooling rate.

Claims (10)

delete delete delete delete Al and Mg in an amount of 1 to 4% by weight, Mg 2 to 5% by weight, Be in an amount of 0.001 to 0.01% by weight, Ca in an amount of 0.005 to 0.1% by weight and the balance of Zn and inevitable impurities, Plating a steel sheet to a plating bath satisfying the relationship And
The plated steel sheet is cooled, and the phosphate aqueous solution of 0.1-1 wt% ammonium phosphate aqueous solution or ammonium phosphate aqueous solution is added at a concentration of 5 mg / m 2 or more and 30 mg / m 2 or less before the plating solution is solidified And a step of spraying the coated film on the substrate.
6. The method according to claim 5, wherein the cooling is performed at a cooling rate of 5 to 20 DEG C / sec.
6. The method of claim 5, wherein the phosphate aqueous solution is sprayed at a temperature of 380 DEG C or higher.
delete delete The method according to any one of claims 5 to 7, further comprising the step of forming at least one post-treatment coating film selected from the inorganic coating film, the organic coating film and the organic-inorganic hybrid coating film on the plating layer, A method for manufacturing a corrosion resistant coated steel sheet.

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