JPS6112987B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to a method for producing hot-dip galvanized steel sheets by the flux method, and more specifically, when hot-dip galvanizing steel sheets containing easily oxidizable elements and having a hard-to-gold property, a plating layer of zinc, nickel, or copper is applied onto the steel sheet before flux application. The present invention relates to a method for producing hot-dip galvanized steel sheets using a flux method, which prevents the occurrence of plating defects caused by easily oxidizable elements. In recent years, due to the demand for weight reduction in vehicles, agricultural machinery, civil engineering construction machinery, etc., conventional steel types (tensile strength 30
~40Kg/ ^mm2 ) High-strength steel plates such as carbon steel, low-alloy steel, and high-alloy steel such as stainless steel, which exhibit the same or higher strength even if they are thinner than the above, have come into use. There is. However, when these high-strength steel plates are used, the plate thickness is made thinner, so there is less margin for rust, so in order to cover this, the surface of the steel plate is plated with a low-melting point metal such as zinc or aluminum. We are trying to improve corrosion resistance by using steel plates. By the way, high-strength steel plates such as those mentioned above generally contain silicon, manganese, chromium, aluminum, niobium, vanadium, titanium,
It contains many easily oxidizable elements such as zirconium, and these elements easily combine with oxygen in the atmosphere during annealing to form a stable and strong oxide film, and are also concentrated in the steel directly below it. There is a tendency to For this reason, even if high-strength steel sheets containing these easily oxidizable elements are plated on a continuous gas reduction plating line that uses a N ₂ - H ₂ mixed gas as a pretreatment, the oxide film and easily oxidizable elements will be removed. Since the concentrate is hardly activated by reduction, plating defects such as pinholes and nicks occur, which poses a major problem in terms of corrosion resistance. On the other hand, even if we try to plate such high-strength steel sheets on a continuous plating line using dry or wet flux methods, the reducing power of flux is limited, so we have no choice but to strengthen the pickling process before flux application. Ta. However, if the pickling process is strengthened, the surface of the steel sheet will become rough and hydrogen absorption will become significant, which will appear as roughness of the product surface and blistering of the plating layer due to hydrogen release, which will not only cause quality problems, but also reduce the amount of acid used. There were problems such as an increase in acid consumption and a decrease in productivity due to a decrease in line speed. Furthermore, even if the pickling process is strengthened, the dry or wet flux method includes a flux drying process (drying at 150 to 250°C), so this drying process removes easily oxidizable elements in the steel during annealing. Similarly, it re-concentrates on the surface of the steel plate, inhibits wettability to molten metal, and causes pinholes and
This caused plating defects such as blemishes. As mentioned above, the gas reduction method, dry flux method, and wet flux method, which have been commonly used as pretreatment for hot-dip galvanizing steel sheets containing easily oxidizable elements, do not allow the easily oxidized elements to combine with oxygen in the atmosphere. , can concentrate on the surface and cause surface defects, so it is essential to prevent these in the pre-plating treatment of steel sheets containing easily oxidizable elements. To achieve this, after removing some of the oxidized film on the steel plate by pickling, etc., coat it with a metal that can be melted in the absence of oxygen, and then melt-weld it. This is to prevent direct melting and adhesion to the oxide film of easily oxidizable elements. Regarding this point, the present inventors conducted intensive research on the production method of hot-dip galvanized steel sheets using the flux method, and found that forming a plating layer of zinc, copper, or nickel on the steel sheet before applying flux produces an excellent effect. I found out what to do. Also, when a plating layer of these metals is formed,
It has also been found that even if the amount of flux deposited is less than that of the conventional method, a hot-dip galvanized steel sheet with good adhesion comparable to that of the conventional method can be obtained. Therefore, an object of the present invention is to provide a steel sheet containing easily oxidizable elements that is free from plating defects such as pinholes, blemishes, roughness of the plating layer, or blisters due to hydrogen gas, and which has a low flux consumption rate. It is an object of the present invention to provide a method for producing a hot-dip galvanized steel plate by a flux method that can reduce the flux. In order to completely prevent plating defects, the thickness of the zinc, copper or nickel plating layer formed in this invention should be at least 0.1 micron, preferably between 0.1 and 10 microns, but less than 0.1 micron, e.g.
If the thickness is 0.05 micron, some plating defects may appear. Preferred plating methods for forming the metal plating layer include electroplating, vacuum evaporation,
There are electroless plating methods, displacement plating methods, etc.
Even if the thickness of the plating layer is made too thick using these plating methods, the effectiveness of preventing plating defects will not change, so in consideration of economic efficiency, it is preferable to keep the thickness at a maximum of 10 microns or less. In the conventional dry flux method, even if the flux concentration is low, it is limited to 15-18Be, and if it is lower than this, pinholes and imperfections occur.
However, if a zinc, nickel, or copper plating layer is formed on the steel plate before flux application according to the method of the present invention, the value can be significantly lowered to 5 to 10 Be. Even if the flux concentration is drastically reduced in this way, there is no need to reduce the line speed, and the quality of the obtained hot-dip galvanized steel sheet remains unchanged from that of the conventional method. Zinc is suitable as the metal to be melt-plated on the plated layer of the metal described above, but other metals such as aluminum, tin, lead, etc. which are melt-plated by the flux method can also be used. Hereinafter, the effects of the present invention will be specifically explained in Examples. Example 1 High-silicon steel was coated with zinc, nickel, and copper using the electroplating method as a pre-plating treatment, and the plating thickness was varied in the range of 0.01 to 20 microns, and hot-dip galvanizing was performed using the dry flux method. It has been marked. The chemical composition and processing conditions of the sample materials were as follows, and the degree of occurrence of the flickering defects was as shown in FIG. 1 Chemical composition of sample material (plate thickness 3.2mm)

[Table] 2 Pickling conditions HCl 100gr/, Fe ion 100gr/,
50℃, 60 seconds immersion 3 Electroplating conditions (only for the method according to the present invention)

[Table] 4 Flux composition ZnCl ₂ and NH ₄ Cl were mixed in a molar ratio of 3:7 and diluted with water to 22Be. 5 Hot-dip galvanizing conditions Plating bath: Zn + 0.15% Al bath Temperature: 460±3°C Immersion time: 20 sec. The evaluation of plating properties in Figure 1 was based on the following criteria. A: Evenly and beautifully plated, with almost no pinholes observed. B: The plate is uniformly plated, but there are minute pinholes and some roughness on the plated surface, which is enough to cause no problem in practical use. It is a passed product as a plated product. C: Some spot defects with a diameter of several mm or less were observed, and the surface roughness was also significant. The product is rejected as a plated product. D: A relatively large defect with a diameter of several mm or more has occurred, and in severe cases, the defect covers more than half of the surface area of the test piece. As shown in the figure, the degree of occurrence of plating defects is the same regardless of the type of plating metal, and when the plating thickness is 0.1 micron or more, a plated product with beautiful skin of grade B or higher can be obtained. It was done. Also,
In a bending test, the workability of the plated layer of this material was the same as that of a healthy plated part, although the plated metal was not plated. Example 2 Targeting ferritic stainless steel, the thickness of zinc plating was reduced by vacuum evaporation as a plating pretreatment.
Hot-dip galvanizing was carried out by the dry flux method, varying the thickness from 0.01 to 20 microns. The chemical composition and processing conditions of the sample materials were as follows, and the degree of occurrence of flickering defects was as shown in Figure 2. Note that the flux composition, hot-dip galvanizing conditions, and criteria for determining plating properties shown in FIG. 2 were the same as in Example 1. 1 Chemical composition of sample material (plate thickness 0.6 mm)

【table】 2 Annealing/pickling conditions

[Table] 〓Pickling (electrolytic pickling)
Electrolyte: 5% HNO ₃ , 60°C Current density: 15 mA/cm ² Electrolysis time: 15 seconds 3 Vacuum deposition conditions Evaporation source temperature: 500 to 550°C Steel plate temperature: 200 to 250°C Degree of vacuum …………1×10 ^-4 ~9×
10 ^-5 Torr. As shown in the figure, those with a zinc plating thickness of 0.1 micron or more had beautiful skin of grade B or higher. The workability of the plated layer of the obtained hot-dip galvanized steel sheet was superior to that of the sound part without zinc plating, even when a contact bending test was performed. Example 3 A steel plate was hot-dip galvanized by a dry flux method under the same conditions as in Example 1 but with varying flux concentrations. In addition, only nickel was used for electroplating, and the plating thickness was kept constant at 1 micron. The plating properties in this case varied as shown in Table 1 depending on the flux concentration, and it was possible to reduce the flux concentration to a greater extent than in the case of the conventional method.

【table】

[Table] According to the present invention, easily oxidizable elements that could not be melt-plated without plating defects by any of the gas reduction method, dry flux method, and wet flux method can be removed. A beautiful molten metal plating without plating defects such as pinholes and nicks can be applied to the steel plate containing the present invention. Furthermore, in the production of molten metal plated steel sheets according to the present invention, the plating properties are not impaired even if the flux concentration is lowered, so that the flux consumption rate can be lowered than in the conventional method.

[Brief explanation of the drawing]

FIG. 1 is a diagram illustrating the relationship between the degree of plating defects in a hot-dip galvanized steel sheet produced by the method of the present invention and the thickness of the metal plating layer formed by electroplating before flux application. FIG. 2 is a diagram for explaining the relationship between the degree of plating defects in a hot-dip galvanized steel sheet produced by the method of the present invention and the thickness of the metal plating layer formed by vacuum evaporation before flux application. .

Claims (1)

[Scope of Claims] 1. A method for producing a hot-dip galvanized steel sheet using a flux method, which is characterized in that a zinc, nickel, or copper plating layer of 0.1 micron or more is formed on the steel sheet before flux application. A method for producing hot-dip galvanized steel sheets. 2. A method for producing a hot-dip galvanized steel sheet by a flux method according to claim 1, wherein the steel sheet contains an easily oxidizable element as a component. 3. A method for producing a hot-dip galvanized steel sheet by a flux method according to claim 1, wherein the hot-dip galvanized metal is zinc. 4. A method for producing a hot-dip galvanized steel sheet by the flux method according to claim 1, characterized in that the thickness of the galvanized layer is 0.1 to 10 microns. 5. Claim 1, wherein the method for forming the plated layer is any one of electroplating, vacuum evaporation, electroless plating, or displacement plating.
A method for producing a hot-dip galvanized steel sheet as described in .