JPS648068B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention aims to improve the corrosion resistance of hot-dip galvanized products,
This invention relates to a magnesium-containing hot-dip galvanized coating intended to improve adhesion between an iron base and a zinc layer. Generally, galvanizing is applied to protect iron materials from corrosion, and they are widely used as hot-dip galvanized and electroplated products. The corrosion resistance of galvanized products is due to the fact that zinc itself forms a protective film when exposed to corrosion from the outside world.For example, the corrosion loss in the atmosphere is 1/10 of that of iron.
~1/25. Furthermore, since zinc is electrochemically base, it has a so-called sacrificial anticorrosion effect and protects the iron when a part of the iron base is exposed. Therefore, the most important factors for the anticorrosion effect of zinc are the thickness of the zinc layer and the uniformity of the film. Therefore, it is said that the corrosion resistance of galvanized products does not depend on the plating method. Nowadays, it is common for galvanized products to be commercialized with added value through some kind of post-processing such as surface treatment, coloring such as painting, and processing.
Needs are also diversifying. For this reason, calls for improvements in corrosion resistance, adhesion between the zinc layer and iron base, chemical conversion treatment of zinc, paint adhesion, and workability are increasing day by day, and various improved plated products are emerging. . In hot-dip galvanizing, technologies related to steel sheets, steel materials, coatings, and plating methods have been developed to improve the above properties. Among these, there are many techniques that change the amount of Al in the galvanized coating or add new additive elements. Many reports have been made on the effect of Al in hot-dip galvanizing baths, but the main effect is that Fe-Zn at 0.1-0.3% by weight of Al in the bath.
By suppressing the alloy layer and improving the adhesion between the zinc layer and the iron base,
Further effects include improved fluidity in the bath, prevention of oxidation, and improved surface properties such as gloss and roughness of the plated surface. This effect is mainly utilized in the manufacturing process of continuous hot-dip galvanized steel sheets, but since flux is used in the hot-dip galvanizing process of steel materials, wire rods, and iron parts, _AlCl3
There is also consumption of Al, and the amount of Al in the bath is generally small enough to not suppress the alloy layer sufficiently. Particularly in silicon-containing steels, which have recently been increasing in number, the development of the alloy layer is remarkable, so it has been announced that it is necessary to limit the Al content in the bath to 0.01 to 0.5% by weight. It seems likely that more will be added. In the continuous manufacturing process of hot-dip galvanized steel sheets, the plating line speed has increased to improve productivity, and as a result, the amount of Al introduced into the product has increased compared to conventional products.
It has become difficult to maintain the amount of Al in the bath at the above-mentioned 0.1 to 0.3% by weight, and it has become necessary to increase the amount of Al in the coating material. In addition, due to such changes in bath composition and changes in operating methods, there have been cases in which the corrosion resistance of galvanized products and the adhesion between the zinc layer and the iron base deteriorate. On the other hand, in a normal melting bath, the effect of improving the fluidity of the bath, the effect of lowering the surface tension,
Considering the effect of suppressing the occurrence of wrinkles on the plated surface and smoothing the surface, the effect of reducing scratches, and the effect of protecting the pot when using a cast iron pot, Pb is set to 0.2~
It was contained in the bath at 1.0% by weight. However, there are reports pointing out that Pb reduces the adhesion between the zinc layer and the iron base, and under certain conditions, it reduces corrosion resistance. Taking these points into consideration, the inventors investigated the effects of the hot-dip coating composition on various properties in order to suppress the harmful effects without losing the conventional effects of Al and Pb. As a result, a molten zinc coating consisting of 0.45 to 0.50% by weight of Al, 0.01 to 0.10% by weight of Mg, 0.10 to 0.50% by weight of Cu, 0.15 to 0.25% by weight of Pb, and the balance being Zn apart from impurities. It was discovered that without losing the conventional effects of Al and Pb, the effect of improving the adhesion between the zinc layer and the iron base and corrosion resistance was even greater. Note that if the Al content in the film is less than 0.45% by weight, the aforementioned bath problem in high-speed line operation will occur.
On the other hand, if the Al content exceeds 0.50% by weight, it becomes difficult to control the increase in dross in the bath and the Al content. Mg in the film
If it is less than 0.01% by weight, the adhesion and corrosion resistance will not be effective, and if it exceeds 0.10% by weight, the plated surface will lose its luster and become gray, the surface roughness will increase, and the amount of dross in the bath will decrease. This is not desirable as it may increase the amount of water. In addition, if the Pb content in the film is less than 0.15% by weight, surface defects due to build-up tend to occur due to the increase in the surface tension of the bath, and conversely, even if the Pb content exceeds 0.25% by weight, the effect of Pb does not change. , the unevenness increases due to the development of spangles on the plated surface. Also in the film
If Cu is less than 0.10% by weight, there is no effect of adding Cu, and if it exceeds 0.50% by weight, the effect of Cu will not increase, so it is useless. In the present invention, impurities refer to one or more of Fe<0.025% by weight, Cd<0.40% by weight, and Sn<0.10% by weight, which may be contained in the zinc base metal. (Compliant with JIS H 2107) Patents related to the present invention include USP 4029478 (June 14, 1977) of Inland Steel Corporation, USP 3245765 (April 12, 1966) of Armco Steel Corporation, and
No. 11205 and Asarco USP 3320040
(May 16, 1967), but the scope of the present invention is defined for the reasons stated above, and the effects within this scope are completely different from those of the related patents. Also
A composition containing 1 to 4% by weight of Mg has also been proposed, but this is aimed at improving corrosion resistance by using a Zn-Mg system, and is the main focus of the present invention, which also takes surface properties and adhesion into consideration. are completely different. According to the present invention as described above, the corrosion resistance of a galvanized coating and the adhesion between the coating and the substrate can be improved without changing the conventional plating method or equipment or losing the conventional effects. Not only does this make it possible to improve the amount of Al, Mg, and Pb, but the amounts of Al, Mg, and Pb can also be changed by a small amount, making it possible to supply large quantities of inexpensive alloys for compounding, making it extremely practical. Examples of the present invention are shown below. Example A low carbon SPCC cold-rolled steel plate 70 x 100 mm with a plate thickness of 0.4 mm was used as a base steel plate, and the surface appearance and corrosion resistance were evaluated after plating in a matting bath by melting the alloy prepared to obtain the coating of the present invention or for comparison. , adhesion was determined. Table 1 shows the plating layer composition and the above characteristic results.
The alloy used in the plating bath is made of zinc base metal and high-purity aluminum (Al>
99.99), high purity magnesium (Mg>99.99), electrolytic copper (Cu>99.99), and electrolytic lead (Pb>99.99). The base steel sheet is degreased using sodium orthosilicate or trichlorethylene, pickled with dilute sulfuric acid or dilute hydrochloric acid, then fluxed in a mixed solution of NH ₄ Cl and ZNCl ₂ , and dried at around 250°C. Immersed in a tightening bath. Although the so-called flux method was used in this example, a so-called non-flux method that does not use flux or a continuous plating method using an atmosphere conditioning furnace may also be used, and the present invention limits the plating pretreatment and plating method. It's not something you should receive. The plating operation was carried out in a graphite crucible set in a small electric furnace. The previously prepared master alloy was placed in a crucible and kept in a molten state at 450 to 470°C. Sufficient stirring was performed to ensure uniform composition within the bath. Immediately before immersion, the dross on the hot water surface was removed. The base steel plate that had undergone the above-mentioned pretreatment was immersed in the bath and tightened. The immersion time was kept at 30 seconds. After removing the dross on the molten metal surface again, the steel plate was pulled up and left to cool in the atmosphere. The plating bath was changed every 10 steel plates so that the compositions of the zinc alloy, bath, and coating matched. Regarding the surface appearance, spangle size, ripples, unevenness, and gloss were determined visually. The plating amount and film thickness were determined from the comprehensive results of the gravimetric method, antimony method, and simple film thickness meter. Corrosion resistance and adhesion were determined after a steam test. The steam test was carried out under conditions such as sealed storage in a humid box maintained at 80°C for 10 days. Corrosion resistance was determined by the presence or absence of microscopic blisters and cracks that appeared on the surface of the plated sample after the steam test. In addition, after the steam test, a general cross-cut tape peeling test was performed on the coated steel plate directly on the plated surface to determine the peeling ratio of the plated layer after the tape was peeled off, and a 500g steel ball was applied from the plated surface using the Dupont impact test. The adhesion between the zinc layer and the iron base was determined by dropping it from directly above 50 cm and observing the state of peeling of the plating layer on the hemispherically protruding mating surface.

【table】

【table】

[Table] In the above table, Nos. 1 to 28 are comparative examples for the present invention, and Nos. 29 to 30 and Nos. 35 to 40 are outside the scope of the claims of the present invention. As can be seen from the judgment results of Nos. 31 to 34, the compositions within the range of the present invention have sufficiently satisfactory properties in appearance, corrosion resistance, and adhesion after plating, even when compared with comparative examples, and the coating of the present invention is effective. It is clear that this is the case. Further, as is clear from the attached drawing showing the results of the cross-cut test, the coating of the present invention has excellent adhesion (broken line in the drawing), and it can be seen that the effect is further improved.

[Brief explanation of drawings]

The drawing is a relationship diagram showing an example of the adhesion improvement effect of Mg and Cu obtained from the results of the cross-over test.

Claims (1)

[Claims] 1 Aluminum 0.45-0.50% by weight, Magnesium 0.01-0.10% by weight, Copper 0.10-0.50% by weight, Lead 0.15%
Hot-dip galvanized coating consisting of ~0.25% by weight and the remainder zinc apart from impurities.