JPS6133070B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention has excellent corrosion resistance and does not cause "wrinkling" on the plating surface.
This invention relates to a method for producing hot-dip galvanized steel sheets that are free from "hair" and have good gloss. Hot-dip galvanized steel sheets have rapidly grown due to their excellent corrosion-resistant properties, and are used as corrosion-resistant materials in a wide range of fields such as building materials, home appliance materials, and automobile body materials.
It has reached 6 million tons, accounting for approximately 30% of cold-rolled steel sheets. Zinc is an inexpensive and chemically active metal, and at the same time, the compound produced by the reaction is dense, so that a moderate corrosion rate can be obtained, making it a metal suitable for corrosion protection of steel materials. Generally, the sacrificial corrosion protection ability of zinc against steel materials in a neutral environment is in an over-corrosion protective state, and even if the corrosion rate of zinc is further suppressed, sufficient sacrificial corrosion protection ability can be exhibited. For example, when measured in 3% saline, sacrificial corrosion protection against steel is effective even if the corrosion rate of pure zinc is suppressed to 1/20 to 1/50. Therefore, in a neutral environment, if the corrosion rate of zinc can be suppressed to 1/20 to 1/50 by some means, a life of 20 to 50 times longer than that of pure zinc with the same basis weight can be achieved, and the current It is possible to lower the basis weight to 1/20 to 1/50 to obtain the same performance. The present invention focuses on this point and is a method for manufacturing a plated steel strip, which is as follows. (1) In a method for producing hot-dip galvanized steel strip that includes plating at least one side in a zinc bath and controlling the amount of plating, 0.1 to 2.0% Mg and 2.0% Mg and Al are added to the zinc bath.
A bath consisting of 0.1 to 0.5% Sn, 0.1 to (1.07-1.33Al%)%, and the balance being zinc and unavoidable impurities is used. A part of the bath is surrounded by a seal box, and the oxygen concentration on the bath surface side including the wiping nozzle is set to 50 to 1000 ppm, and the oxygen concentration on the solidification area side of the plating metal above it is set so that the plating weight is less than 50 g/ ^m2. If the plating weight is 50g/ ^m2 or more and less than 200g/ ^m2 , the plating weight is 100ppm or more.
A method for producing a hot-dip galvanized steel strip, which comprises solidifying the unsolidified galvanized metal on the surface of the steel strip by controlling the concentration to 100 ppm to less than the atmospheric concentration when the concentration is 200 g/m ² or more. (2) The method for producing a hot-dip galvanized steel strip according to item 1 above, characterized in that the bath surface side is a space of 1 m at most above the wiping nozzle from the plating bath surface. DESCRIPTION OF THE PREFERRED EMBODIMENTS The mode of carrying out the present invention will be described below in detail with reference to the drawings. In recent years, the applications of galvanized steel sheets have expanded from the traditional field of building materials to home appliances, automobiles, and steel furniture, and they must have performance that matches the application. In other words, it has excellent corrosion resistance, and
Requirements include (1) good plating adhesion, (2) good appearance and no discoloration, and (3) good topcoat performance (including chemical conversion treatment). The object of the present invention is to provide a galvanized steel sheet with quality that meets the above-mentioned requirements. In order to achieve this object, the present invention is carried out by limiting the composition of the zinc bath and by maintaining the unsolidified region of the plated metal attached to the surface of the steel strip under specific conditions. Figure 1 shows electrolytic zinc (purity 99.97%), electrolytic zinc and mixed zinc (Al 0.22%, other impurities including Pb 0.1%, Cd 0.01%, Fe 0.02%) and Mg.
The corrosion resistance of the hot-dipped steel plate was tested for three days using a Sendzimer pilot line using three types of baths containing the following: This is a record of the corrosion loss measured by _Mg .
Additive-free electrolytic zinc bath, curves N ₁ and A ₁ are the results of baths in which Mg was added to electrolytic zinc and blended zinc as shown. From Figure 1, the amount of corrosion _M1 of hot-dip electrogalvanized steel sheets is significantly improved by the addition of Mg, as shown by the curve _N1 , and is about 1/10 at 0.5% Mg compared to the case without Mg addition.
The weight loss is 1/20 at Mg1.0%. However, adding a large amount of Mg is meaningless and the performance is saturated at 2.0% Mg. Therefore, for the purpose of improving corrosion resistance, the effective amount of Mg added is 0.1 to 2.0%. The above is the reason for adding Mg in the present invention, but in order to further improve the adhesion of plating,
It is preferable to add a small amount of Al together with Mg. Figure 2 shows 0.05% Al added to an electrolytic zinc bath containing 1.0% Mg.
%, 0.1%, 0.15%, 0.2%, 0.3%, 0.4%, 0.5%
The plating adhesion when added was evaluated using a ball impact test. The ball impact test was conducted by hand using a ball with a diameter of 25 mm. After the test, it was forcibly peeled off with an adhesive tape, and the peeling was expressed as an area percentage. As is clear from FIG. 2, the adhesion is improved when the Al content is 0.1% or more. Also, Figure 3 shows Mg0.2% + Al 0.2%
(○ mark), Mg0.5% + Al0.2% (△ mark), Mg1.0% +
Plated steel plates with different plating amounts were created by controlling the GJC wiping gas pressure with a bath composition of 0.2% Al (□) and 0.2% Mg (●), and their adhesion was evaluated using a ball impact test. Evaluation was made in the same manner as in Figure 2. Those with 0.2% Al added exhibit good plating adhesion over a wide range of basis weights, but those without Al have insufficient plating adhesion. Plating adhesion can be improved by adding Al at 0.1% or more, but if it is too large, intergranular corrosion will occur due to the combination with Sn or the influence of impurities such as Pb, which will be described later, so the upper limit should be set at 0.5%. . Plated steel strips treated with a plating bath containing Mg or Mg and Al may turn black over time.
Blackening is a phenomenon in which Mg and other plating metals oxidize over time and the surface turns black. In order to prevent such a blackening phenomenon, it has been found through experiments by the present inventors that it is extremely effective to further add Sn to the above-mentioned Zn bath. Table 1 shows the effect of the bath composition of the invention on blackening.

[Table] Plated steel strips with each composition shown in Table 1 above (JIS H2107 ordinary zinc ingot purity 99.97% was used) were tested with chromate treatment and without treatment at 38°C. This shows the period at which blackening occurs when the samples are stacked in a thermostatic room (humidity is not controlled) and stored in waterproof packaging. The chromate treatment was carried out by applying a 2% aqueous solution of chromic anhydride to a total Cr deposition amount of 10 to 25 mg/m ² and drying with hot air. The numbers in the table indicate the number of months in which black discoloration occurred. ○ indicates that no black discoloration was observed for 6 months. Black discoloration does not occur in normally produced regular materials. Black discoloration is likely to occur in compositions containing Mg and Al. In particular, the higher the Mg content, the earlier the occurrence time. However, the composition containing Sn of the present invention hardly causes blackening and is equivalent to regular materials. Figure 4 shows the same evaluation as the exposure test for untreated materials in Table 1, organized by Mg% and Sn%. Note that 0.2% of Al is added. The time of occurrence of black discoloration is marked with ● (within 1 month), ○†

Claims (1)

[Scope of Claims] 1. A method for producing a hot-dip galvanized steel strip, which includes a step of plating at least one side in a zinc bath and controlling the amount of plating, wherein 0.1 to 2.0% Mg and 0.1 to 0.1% Al are plated in the zinc bath.
A bath consisting of 0.5% Sn, 0.1~(1.07-1.33Al%)%, and the balance being zinc and unavoidable impurities is used, and at least part of the time while the plated metal adhering to the steel strip surface solidifies from the bath surface. The area is surrounded by a seal box, and the oxygen concentration on the bath surface side including the wiping nozzle is set to 50~50.
1000ppm, and the oxygen concentration on the solidification area side of the plating metal above it is set to 50g/m ²
If the plating weight is less than 50 g/m ² or more and less than 200 g/m ² , no control is applied.
A method for producing a hot-dip plated steel strip, characterized by solidifying the unsolidified plated metal on the surface of the steel strip by controlling it to 100 ppm or more and, if the plating weight is 200 g/m ² or more, to less than 100 ppm to the atmospheric concentration. . 2. Claim 1, in which the bath surface side is a space of 1 m at most above the wiping nozzle from the plating bath surface.
A method for producing a hot-dip galvanized steel strip as described in Section 1.