JPS6327438B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a Zn-based alloy coated steel sheet that has excellent properties required for a rust-proof steel sheet for automobiles, such as corrosion resistance, phosphating properties, performance after painting, especially corrosion resistance after electrodeposition coating, and weldability. . Currently, galvanized steel sheets, particularly single-sided galvanized steel sheets, are used as corrosion-resistant steel sheets for automobiles that can be mass-produced. As is generally known, this galvanized steel sheet has a significant anodic effect on the cold rolled steel sheet because the zinc coating layer of the plating layer is significantly anodic compared to the galvanized base plate such as the cold rolled steel sheet. In addition, it has excellent corrosion resistance, such as a high anodic corrosion protection effect on plating defects such as pinholes and areas where scratches occur during processing, and in particular, is extremely effective in preventing red rust from forming on the plating original plate. However, in recent years, galvanized steel sheets used as corrosion-resistant steel sheets for automobiles are often used in severe corrosive environments, such as salts (NaCl, CaCl ₂ , etc.) such as road antifreeze sprayed in cold regions.
The rate of corrosion of the zinc coating layer is extremely high in an environment where the zinc coating layer is exposed to water, and it is becoming impossible to expect long-term use unless the amount of zinc coating (the amount of plating) is increased considerably. Further, as a negative effect of increasing the amount of plating, not only does it increase the cost, but also tends to cause problems in terms of performance such as deterioration of plating adhesion, workability, or weldability. For this reason, by coating the steel plate with a Zn-based alloy such as Zn-Fe, Zn-Ni, etc., which is more cathodic than the galvanized metal, the coating layer ( It is expected to reduce the corrosion rate of the plating layer) and develop zinc-based alloy-plated steel sheets that have excellent corrosion resistance with a thin plating amount. In addition, some of them have been used on a trial basis so far. Among these, Zn--Fe alloy plated steel sheets have excellent phosphate treatment properties as a pre-painting treatment and also have excellent corrosion resistance after painting. However, on the other hand, the adhesion of the plating layer is not very good, and the protective and anticorrosion effect of the alloy coating layer on the steel plate is not sufficient, and the occurrence of red rust from the original plate is significant, and its corrosion resistance cannot necessarily be said to be perfect. In addition, regarding Zn-Ni alloy plated steel sheets, although the corrosion rate is significantly reduced by alloying with Ni metal, the protective and anticorrosive effect on the steel sheet deteriorates, making it easier for red rust to occur from the original sheet, and its corrosion resistance is reduced. On the contrary, there was a tendency for it to be inferior. Moreover, this tendency naturally becomes more pronounced as the Ni content in the Zn--Ni alloy increases. As mentioned above, conventional Zn-based alloy plated steel sheets have had both advantages and disadvantages. In view of the above-mentioned problems, the present inventors have developed a number of surface-treated steel sheets with the aim of providing a surface-treated steel sheet with excellent corrosion resistance, phosphate treatment properties, and performance after painting, especially corrosion resistance and weldability after electrodeposition coating. We conducted repeated experiments and studies. As a result, the corrosion rate of the coating layer has been significantly reduced, and the occurrence of red rust from the plating original plate (steel plate), which is likely to occur due to the cathodic formation of the coating layer, has been reduced. , and as a method of obtaining a coated steel plate (corrosion-resistant steel plate) with excellent weldability, Ni, Co,
The lower layer is coated with a single metal such as Sn, which reduces the corrosion rate compared to Zn metal, and provides excellent phosphate treatment and painting performance.
It has been found that a plated steel sheet that satisfies the desired properties can be obtained by applying an upper layer coating of a Zn-based alloy plate containing Fe etc. The present invention was constructed based on this knowledge. The gist of this method is to form a coating layer with a thickness of 0.03 to less than 0.50μ on a steel plate using a single metal such as Ni, Co, Sn, or Cu, and then add one or two types of Ni, Fe, Sn, or Mn to the coating layer. This is a Zn-based alloy plating-coated steel sheet with a two-layer coating having a Zn-based alloy coating layer containing 10 to 95%, which has excellent corrosion resistance and phosphoric acid treatment properties. The present invention will be explained in detail below. A steel sheet manufactured through the usual plating original sheet manufacturing process such as rolling, annealing, and surface cleaning treatment is first plated with one of the metals Ni, Co, Sn, and Cu using an electroplating method. These plated metals compensate for the fact that the upper coating layer of the plated metal alone is not sufficient to prevent the generation of red rust from the plated original plate (steel plate) in the defective areas of the coating layer due to its anodic corrosion protection effect. Therefore, in order to suppress the generation of red rust from the plating original plate (steel plate), it is applied in advance as a lower coating layer between the plating original plate and the upper coating layer. Therefore, the metal used as the lower coating layer prevents the generation of red rust on the plated original plate (steel plate) as described above, and the corrosion rate is extremely low in areas corresponding to defects in the upper coating layer.
Metals with good corrosion resistance are used. That is, the potential is more cathodic than the upper coating layer.
Potentially anodic metals that have a high dissolution rate will cause significant corrosion from the parts corresponding to the defects in the upper coating layer, and the lower coating layer will corrode first. This is undesirable because it causes a phenomenon in which the upper coating layer peels off. Furthermore, the metal used for this lower coating layer is
Since zinc-based alloy plating is performed as an upper coating layer on top of this, it is necessary that the upper coating layer be made of a metal that can be easily applied using the normal electroplating method and that can provide good plating adhesion. . Metals that satisfy these conditions include Ni, Co,
Sn and Cu are preferred, and Ni and Co are particularly preferred. In addition, alloy plating made by combining two or more of these metals may be applied as the lower coating layer, or two or more layers of overlapping plating may be applied as the lower coating layer, but this will increase the complexity and economic efficiency of the manufacturing process. Therefore, it is not included in the scope of the present invention. In addition, the thickness of the metal coating layer plated as the lower coating layer is
The thickness should be regulated within the range of 0.03 to 1.50μ in terms of corrosion resistance, adhesion, etc. If it is less than 0.03μ, the lower coating layer will not provide sufficient uniform coverage to the plated original plate (steel plate), and even if the superimposed effect with the upper coating layer is taken into account, a satisfactory effect of preventing the occurrence of red rust will not be obtained. If the thickness is 0.5μ or more, the effect of preventing the occurrence of red rust of the plating base plate is reduced, and the adhesion of the base coating layer also deteriorates as shown in the figure. The evaluation test shown in the figure was conducted as follows. (Note 1) Evaluation test method An evaluation material with a width of 30 mm x length of 100 mm was cut out from a product in which the above coating layer was provided on a base plate with a thickness of 0.8 mm.
A plunger with a tip thickness of 1.6 mm was subjected to bending by applying an impact load of 5 kg from a height of 1 m. Next, a so-called reverse bend test was used in which the material was stretched flat under the same load and then bent in the opposite direction under the same conditions as above to examine the peeling state of the plating layer at the processed portion. (Note 2) Evaluation criteria 5...No abnormality in the plating layer 4...Although some cracks are generated in the plating layer, there is no peeling of the plating layer 3...Peeling of the plating layer is less than 30% 2...Peeling of the plating layer is over 30% ~60% or less 1... Peeling of the plating layer more than 60% ~ Peeling off the entire surface Furthermore, since the base metal coating layer of the present invention is a metal with a lower melting point than the plating original plate, the increase in the thickness of the coating layer causes the electric resistance welding to occur in the melted nugget area. Molten metal overflows (splatters) from the nuggets, creating voids in the nuggets and deteriorating weldability. Therefore, the upper limit of the thickness of the metal coating layer was set to less than 0.50μ. In the present invention, a steel plate is coated with a single metal such as Ni, Co, etc. on the coating layer, and further Ni, Fe, etc.
Contains 10 to 95%, preferably 15 to 75%, particularly preferably 25 to 60% of one or more of Sn and Mn.
Form a Zn-based alloy coating layer. The addition of Ni, Fe, Sn, and Mn to the base component of Zn is more effective than Zn in making its potential cathodic and reducing its corrosion rate. Accordingly, compared to Zn, the anodic corrosion protection effect on the plated original plate (steel plate) is reduced, and red rust is significantly likely to occur from the original plate at defective parts of the Zn-based alloy coating layer. In order to prevent the occurrence of red rust, the above-mentioned lower layer coating of Ni, Co, Sn, Cu, etc. is applied, and the upper coating layer is effective in obtaining a Zn-based alloy coated steel sheet with extremely low corrosion rate and excellent corrosion resistance. There is. In addition, this zinc-based alloy coating layer has a
It significantly improves its phosphate treatment properties and post-painting performance, especially its corrosion resistance after electrodeposition coating. That is, since the corrosion-resistant materials for automobiles, which are the main purpose of the alloy plating products of the present invention, are generally used in many fields after being phosphated and painted, they are required to have excellent performance. Therefore, compared to Zn, the Zn-based alloy coating layer applied in the present invention is effective against the formation of microcells necessary for obtaining uniform and dense phosphate crystals in phosphate treatment. Since the element is added, a large number of local cells are uniformly formed, which is very advantageous, so that the phosphate treatment properties are significantly improved and uniform and dense phosphate crystals are easily generated. For the formation of this microcell, alloying elements such as Ni and Fe added to the above Zn are
When two or more types of metals are used, a potential difference is created between each metal, the amount of local batteries generated is increased, microcells are easily formed, and the effect of improving phosphate treatment properties is remarkable. Furthermore, in combination with the excellent phosphate treatment properties of the upper coating layer, the performance after coating, especially the corrosion resistance after electrodeposition coating, is significantly improved. In other words, the formation of uniformly dense phosphate crystals improves adhesion with the paint film, and in a corrosive environment, the corrosive solution that enters through the paint film reaches the zinc-based alloy plating surface with the uniformly dense phosphate crystals. In order to prevent this, the corrosion resistance after painting is improved. Furthermore, as mentioned above, the corrosion rate of the zinc-based alloy coating layer is extremely low, so even if a corrosive solution reaches the surface through the coating film, the corrosion products will cause blisters (so-called blisters) in the coating film. ) and other defects, and has a remarkable effect of improving corrosion resistance. Among them, alloy-plated steel sheets that have an alloy coating layer consisting of Zn-Fe-Ni system as the upper coating layer have phosphate crystals consisting of Zn ₂ Fe (PO ₄ ) _{2.4H 2} O system crystals.
Phosphophilite crystals are easily generated, and although it is not theoretically certain, it is known in the literature that it has excellent corrosion resistance after chipping of painted materials, especially in terms of phosphate treatment properties and post-painting performance. Good results are obtained. In addition, compared to Zn, the Zn-based alloy coating layer, which is the upper coating layer, allows electric resistance welding at low current due to the effect of adding alloying elements, and there is less pickup from the coating layer to the electrode. etc. It has good weldability with respect to electric resistance weldability. Furthermore, in the coated product of the present invention, the upper coating layer is made of a zinc-based alloy coating layer with a low corrosion rate, and the reduction in the anodic corrosion protection effect of this coating layer prevents red rust from defective parts of the coating layer. Since the occurrence can be prevented by the lower coating layer,
It is possible to reduce the amount of the upper coating layer that has a large effect on weldability. Consequently, from this point of view as well, the product of the present invention brings about good results in its weldability by providing a base metal coating layer of an appropriate thickness. Therefore, in order to constitute the Zn-based alloy coating layer as the upper coating layer applied to the present invention, the content of one or more of Ni, Fe, Sn, and Mn contained in Zn is 10%. If it is less than 95%, the effect of addition as mentioned above will not be observed, and if it is added in excess of 95%, the corrosion rate reducing effect will become supersaturated, and
Since the amount of Zn is reduced to less than 10%, the anodic corrosion protection effect of the Zn-based alloy coating layer further decreases, and the effects of Zn metal are lost, such as marked deterioration of phosphate treatment properties. In addition, the thickness of this upper coating layer is 1μ ~
A thickness of 10μ, preferably in the range of 2.5 to 5μ, is desirable in terms of performance and economy, but is not particularly specified in the present invention. In addition, in the present invention, the coating layer does not contain unavoidable metals included in the single metal or alloyed metal forming the coating layer, such as Co contained in nickel sulfate or nickel carbonate used as a Ni source. However, this does not depart from the purpose of the present invention. The present invention, in which a two-layer coating layer is formed on a steel plate as described above, can prevent red rust on the steel plate with the lower coating layer plated with a single metal such as Ni or Co.
By applying a zinc-based alloy coating layer that has little anodic corrosion protection effect on steel sheets but has a significantly low corrosion rate as the upper coating layer, in combination with the lower coating layer, excellent corrosion resistance can be obtained, as well as other phosphate treatment properties.
It is possible to provide a coated steel sheet with extremely excellent performance and weldability after painting. In addition, the Zn-based alloy plated steel sheet of the present invention as described above is obtained by applying a lower layer coating such as Ni, Co, etc. to the plated original plate using the commonly used electroplating method, and then applying Ni, Fe, etc. using the same electroplating method. It is manufactured by applying an upper layer coating containing one or more types of. Next, examples of the present invention will be described. Ultra-low carbon steel plated base plate (C: 0.04%, Si:
0.01%, Mn: 0.10% balance essentially Fe) was coated with a lower layer by electroplating, and then an upper layer was applied to the lower coating layer by electroplating in the same manner. Table 1 shows the evaluation results regarding its performance.

【table】

[Table] As is clear from the above results, the products of the present invention (1 to 5) exhibit superior performance compared to the comparative materials (7 to 12). Figure 1 shows the effect on the adhesion of Zn-11.5%Ni-0.5%Fe plated steel plate (plate layer thickness 2.6μ).
The influence of the thickness of the Ni coating layer is shown.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the influence of the thickness (μ) of the underlying Ni coating layer on the adhesion of the Zn alloy layer.

Claims (1)

[Claims] 1 Thickness made of a single metal of Ni, Co, Sn, and Cu on a steel plate
A coating layer having a thickness of less than 0.03 to 0.50μ is formed, and a Zn-based alloy coating layer containing 10 to 95% of one or more of Ni, Fe, Sn, and Mn is further formed in the coating layer. A Zn-based alloy coated steel sheet with two coating layers that has excellent corrosion resistance and phosphate treatment properties.