KR0128131B1 - Manufacturing method for hot dip zinc coated steel sheet with minimized spangle - Google Patents

Abstract

Mini-spangle flame scaling steel plate is produced by (a) spraying 1-2wt% of phosphate system aqueous solution firstly just before the fused zinc which is attached to the steel plate delivered from the flame scaling bath is solidified, (b) spraying the aqueous solution including iron secondly.

Description

Manufacturing method of mini-spangle molten zinc plated steel with excellent weldability and paintability

The present invention relates to a method for manufacturing a mini-spangle hot-dip galvanized steel sheet used as a material for home appliances or automobiles, and more particularly, to a method for manufacturing a mini-spangle hot-dip galvanized steel sheet having excellent weldability as well as paintability. .

Initially, hot-dip galvanized steel sheet has been used mostly as a building material. Recently, as the mass production of hot-dip galvanized steel sheet capable of withstanding harsh processing conditions is possible, its use has been widely expanded to various home appliances or automobile materials. .

As such, as the use range of home appliances or automobile materials is widened, there is an increasing demand for improving quality characteristics such as surface gloss, corrosion resistance, weldability and paintability.

However, in general, since the spangles are formed on the surface of the hot-dip galvanized steel sheet, the steel sheets with such spangles have poor adhesiveness of the coating material during coating, and the surface irregularities caused by the spangles are not removed even after the coating. Pangle is seen because there is a disadvantage that the appearance is poor.

Therefore, in order to prevent the defects caused by such spangles, mini-spangle hot-dip galvanized steel sheets which have been miniaturized in spangles have recently been developed.

That is, various methods are currently used to dissipate spangles formed on the surface of the hot-dip galvanizing layer in the hot-dip galvanizing process, and an aqueous solution spray method is widely used. This method is to refine the spangle by spraying and quenching an aqueous solution containing at least one type of water-soluble inorganic salt to the molten zinc plated surface attached to the steel sheet, and this method is melted in addition to the cooling effect by the latent heat of evaporation of water. This is to use the principle of enlarging the fineness of zinc crystals (spangle) by making the fine particles of the salt collide with the surface of the zinc plating layer to be a myriad zinc crystal nucleus.

As an example, as disclosed in Japanese Patent Laid-Open Nos. 45-26961 and 50-92831, salts such as copper sulfate, sodium acetate, sodium phosphate, and ammonium phosphate are sprayed onto the molten zinc surface layer to spread them. A method of miniaturizing is proposed. Among the methods using the spraying method, phosphates are mainly used as the spraying solution, which is characterized by endothermic reaction characteristics due to thermal decomposition when spraying hot-dip galvanized steel sheets, and chemical conversion such as phosphate treatment and chromium treatment followed by salts remaining on the zinc surface. It is because it is preferable for a process. However, in the case of using the phosphate, for example, in the case of the aqueous solution of ammonium dihydrogen phosphate which is most effective in miniaturizing the spangle, the ammonia gas decomposed at about 190 ° C. is thermally decomposed by the galvanized steel sheet temperature to the galvanized steel sheet. As it remains, it causes fine pinholes in the coating during subsequent coating, thereby causing deterioration of the film adhesion. Therefore, in order to make use of the advantages such as the beautiful surface appearance obtained by miniaturizing the spangles during the production of the mini-spangle solution zinc plated steel sheet, it is necessary to improve such coating adhesion.

On the other hand, galvanized galvanized steel sheet for the purpose of improving the corrosion resistance is relatively deteriorated weldability due to the plated layer and has a great difficulty in setting the welding conditions. For example, when welding a galvanized steel sheet by spot welding, which is one of the resistance welding methods mainly used for thin plate welding, it is difficult to produce nugget. This is because the melt melts first to increase the contact area between the plate and the electrode, thereby lowering the current density. This phenomenon generally occurs because Fe does not exist in the surface layer of the pure hot-dip galvanized steel sheet, and in the past, a method of retaining the Fe component in the hot-dip plating layer has been attempted to prevent such defects and improve weldability. That is, a method of improving the weldability by increasing the melting point of the plating layer while maintaining the Fe content in the plating layer by 8-10% through alloying heat treatment has been widely used. However, this alloying heat treatment method requires an additional alloying heat treatment and has a disadvantage in that a powdering phenomenon occurs during excessive alloying heat treatment. Therefore, the method of improving the welding characteristics of the hot-dip galvanized steel sheet is a problem and a very important problem. In addition, as hot-dip galvanized steel sheet is mostly used as a material for automobiles or home appliances that require welding, such a problem is urgently required. In particular, in the case of mini-spangle hot-dip galvanized steel sheet having a beautiful surface appearance, there is a need for a method of improving paintability as well as weldability by a simple method. Accordingly, the present inventors have conducted studies and experiments on a simple treatment method capable of improving weldability and paintability without requiring expensive equipment, and have therefore proposed the present invention.

That is, in the present invention, the mini-spangle molten zinc-plated steel sheet is first prepared by mini-spangling the hot-dip galvanized layer, and then sprayed with an aqueous solution containing Fe, so that the mini-spangle melt has excellent weldability and paintability. To provide a method for producing a galvanized steel sheet, the purpose is.

Hereinafter, the present invention will be described.

In the present invention, a method for manufacturing a mini-spangle hot-dip galvanized steel sheet, spraying 1-2 wt% aqueous solution of phosphate salt immediately before the molten zinc adhered to the steel plate lifted from the hot-dip galvanizing bath solidifies, and then 2 The present invention relates to a method for producing a mini-spangle hot-dip galvanized steel sheet excellent in weldability and paintability, including spraying an aqueous solution containing Fe.

Hereinafter, the present invention will be described in detail.

First of all, the present invention sprays an appropriate amount of a phosphate-based aqueous solution having excellent endothermic properties immediately before the molten zinc adhered to a steel plate lifted from a hot-dip galvanizing bath to give a good surface gloss, such as ammonium phosphate or sodium phosphate. It is preferable that an aqueous solution is used, and the titer of the solution has 1-2 wt%. In addition, the smaller the particle diameter of the spray particles contained in the aqueous solution is preferable, the present invention preferably has a spray particle diameter of 30㎛ or less.

When the phosphate-based aqueous solution is first sprayed, the supercooled by the endothermic reaction of the phosphate ions and the phosphate ions act as nucleation sites of the myriad zinc crystal grains to minimize minispangling and at the same time minimize the impact of the collision of spray particles. Decreases to increase the surface glossiness. At this time, the conditions such as the injection temperature, the atomizing nozzle pressure, the injection time and the like during the minispangling treatment may be appropriately adjusted in a conventional manner. In addition, spraying at the temperature just before the molten zinc solidifies, that is, spraying at a temperature of 420-422 ℃ is preferable. The spraying of the primary aqueous solution in this way impinges the injection solution on the molten zinc surface to obtain a mini-spangle hot-dip galvanized steel sheet having a high gloss, the surface roughness of the plated layer is kept to less than 0.6㎛ without white spots (pitting).

Then, the secondary spun effect by spraying the aqueous solution containing Fe to the second mini-spangled steel sheet by the endothermic reaction and the weldability and paintability of the mini-spangle galvanized steel sheet by the iron component remaining on the galvanized surface In this case, as the Fe-containing aqueous solution used in the present invention, an aqueous solution of iron phosphate (FePO ₄ ), iron sulfate (FeSO ₄ ), iron chloride (FeCl ₂ ), or the like is used. Among them, the phosphate solution is suitable in consideration of the recyclability, endothermic cooling, and corrosion resistance of the waste water when mixed discharge with the primary spray solution. And, the concentration of the injection solution is preferably in the range of 1-5wt%, the reason is that no significant improvement in weldability and paintability when spraying at a concentration of 1wt% or less, and when it exceeds 5wt% increase in weldability and paintability is On the other hand, the nozzle clogging caused by excessive sludge generation due to the iron component is severely generated, which causes inconvenience to the product production and is not advantageous in terms of cost.

The reason why the weldability of the steel sheet is improved by spraying Fe-containing aqueous solution on the mini-spangle hot-dip galvanized steel sheet is because the iron component contained in the secondary injection solution remains on the zinc surface, thereby improving the melting point of the hot-dip galvanized layer. . In addition, the paintability is improved by Fe on the zinc surface, which shows less thermal breakdown of the coating crystal during phosphate treatment, which is a pre-painting step, and excellent binding property with the phosphophyllite (Zn ₂ Fe (PO ₄ ) ₂ ). It is considered that this is because the formation of 4H ₂ O) -based phosphate crystal film is promoted. That is phosphating when the poor coating adhesion to the crystal structure of needle-like form arc phosphite of the pure galvanized steel sheet: containing, iron components while forming only _{(Hopeit Zn 3 (PO 4)} 2 · 4H 2 O) phosphate crystal film It is presumed that the phosphate crystals produced and grown on the zinc surface are inhibited from growth and form a more dense and finer grained phosphoprite phosphate film.

On the other hand, the spraying method of the spray solution according to the present invention can use a two-stage spray nozzle to change the spray solution in a conventional mini-spangle generator. That is, it is possible to spray the aqueous solution containing phosphate salt having excellent heat absorption primarily through the lower partial nozzle, and then spraying the spray solution containing Fe secondly from the upper injection nozzle.

Hereinafter, the present invention will be described in detail through examples.

(Example 1)

Hot-dip galvanized using cold-rolled steel sheet, and immediately before the molten zinc solidifies, spray the 2wt% NH ₄ H ₂ PO ₄ solution using the lower nozzle of the mini-spangle apparatus, and then first minispangle it, Secondly, a mini-spangle galvanized steel sheet prepared by spraying a ₂ wt% sodium phosphate (Na ₂ HPO ₄ ) solution from the upper nozzle and spraying 2 wt% FePO ₄ solution with A (Comparative Example) Minispangle galvanized steel sheet prepared by spraying 2wt% FeSO ₄ solution with B (Inventive Example 1) was prepared by spraying 2wt% FeCl ₂ solution with C (invention example 2). Weldability and paintability were evaluated as D (Invention Example 3), and the results are shown in Table 1 below. At this time, the weldability and paintability were evaluated, and the results are shown in Table 1 below. At this time, evaluation criteria for weldability and paintability were as follows.

1) weldability

After the welding current was fixed to 760A using an AC spot welder, spot welding was performed continuously. At this time, the RBI method repeated 20 batches of RBIs for 1 second and then rested for 40 seconds. Shear tensile test pieces were taken every 100 RBIs to measure the strength. Weldability with continuous RBI, including RBI at the point when shear tensile strength is less than Class B strength of industrial standard and RBI at break type converted from plug type to interface type Was evaluated.

2) Paintability

Phosphate treatment was carried out in a commercial solution having a acidity of 15, a free acidity of 15, a treatment time of 8 seconds, and a solution temperature of 70 degrees. The lower primer paint was coated with a commercial paint having a viscosity of 130 μm, and then subjected to a bake hardening treatment at 280 ° C. for 30 seconds. The top coat was coated with a commercial paint having a viscosity of 130 μm to about 20 μm. After coating, the plate was hardened at 280 ° C. for 60 seconds to prepare a coated steel sheet. The coating film adhesiveness was evaluated by the following method about the raw material coated as mentioned above.

A) cross cut test

100 crosscuts were cut out to the plated surface at 1 mm intervals of the coating film, and the peeling state of the coating film was observed using a scotch tape.

B) impact deformation test

After dropping the weight from the height of 50 cm using the DuPont impact tester, the peeling state of the coating layer on the surface of the plating layer was observed. The weight used at this time was 500g, and the tip diameter of the weight was 12.7 mm.

C) bending test

Using a bending tester, the curvature radius of the bent portion was deformed to be twice the thickness of the specimen, and then the tape was adhered to the curved surface to be peeled off.

D) Water tightness test

After immersion in distilled water at 40 ° C. for 240 hours, the resultant was dried, a cross-shaped cross cut (2 mm interval) was drawn, the scotch tape was adhered thereon, and then peeled off.

The coating film adhesion was evaluated relative to the five-point method according to the following criteria.

5: Excellent (no peeling and cracking)

4: Good (cracking and partial peeling observation: less than 5%)

3: Normal (cracking and peeling observation: less than 15%)

2: deterioration (cracking and peeling observation: within 35%)

1: Serious (Excessive degree of film peeling: 35% or more)

As shown in Table 1, in the case of Inventive Example (1-3) that satisfies the condition range of the present invention, it is possible to manufacture a mini-spangle hot-dip galvanized steel sheet excellent in weldability and coating film adhesion, whereas in the comparative example the surface of the galvanized layer It can be seen that there is no Fe remaining in the weldability and coating film adhesion. However, the invention examples (2) and (3) have excellent weldability and paintability, but there is a possibility of deterioration of corrosion resistance when SO and Cl ions remain on the surface of the plating layer, and the endothermic property is an phosphate-based aqueous solution (1). It is inferior to) and is mixed with the primary use solution, so there is a problem such as recyclability when discharging.

Example 2

In the same manner as in Example 1, hot-dip galvanized using a cold-rolled steel sheet and immediately before the molten zinc solidified, sprayed 2wt% NHHPO solution using a mini-spangle device's lower nozzle to firstly minispangle it. After that, spray the 2wt% sodium phosphate (NaHPO) solution in the upper nozzle to make the spangles finer (a Comparative Example 1), and the minispangle galvanized steel sheet prepared by spraying the 2wt% FePO solution b. Inventive Example 4, the welder and paintability of each steel sheet were evaluated in the same manner as in Example 1, using c (Comparative Example 2) as a mini-spangle galvanized steel sheet prepared by spraying 2 wt% FeSO solution. The results are shown in Table 2 below.

As shown in Table 2, at a concentration of 1% or less, a marked improvement in weldability and paintability was not observed. When the concentration is greater than 5%, weldability and paintability are increased, while nozzle clogging due to excessive sludge generation by iron is severe. It can be seen that it is not good in terms of cost, because it occurs, causing disruption to the production of the product.

As described above, the present invention primarily refines the spangle by a strong endothermic reaction of phosphate ions, and at the same time, Fe contained in the secondary injection solution remains on the surface of the galvanized steel sheet, thereby performing conventional alloying heat treatment. An excellent mini-spangle hot-dip galvanized steel sheet is provided by a simple method without doing it, and this mini-spangle hot-dip galvanized steel sheet can be used as a material for automobiles or home appliances which are mostly welded, and its industrial value is high.

Claims (2)

In the method of manufacturing a mini-spangle hot-dip galvanized steel sheet, spraying 1-2 wt% aqueous solution of phosphate salt immediately before the molten zinc adhered to the steel plate lifted from the hot-dip galvanizing bath solidifies, and then secondly Fe The method of manufacturing a mini-pangle hot-dip galvanized steel sheet having excellent weldability and paintability, characterized in that it comprises spraying an aqueous solution containing. The method of claim 1, wherein the Fe-containing aqueous solution is characterized in that 1-5wt% FePO ₄ solution.