JPS6144946B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method of manufacturing a steel pipe with a molten metal exterior and an organic coating on the interior. Steel pipes plated with molten metal (zinc, aluminum, tin-lead alloys, etc.) are generally plated on the inner and outer surfaces at the same time after pre-plating treatment. It has become increasingly desirable to coat them with organic coatings. Taking galvanized steel pipes as an example, conventional hot-dip galvanized steel pipes were produced by pre-plating the steel pipes by ``pickling, fluxing, and drying'', then immersing them in a molten zinc bath and plating both the inside and outside simultaneously. It is something. However, in recent years, customer demands for galvanized steel pipes have been changing. For example, in the case of steel pipes for water supply, water sources must be found over a wide area in order to meet the increasing demand for tap water, and the quality of the water may deteriorate, resulting in cloudy water or red water. All of these phenomena are caused by the dissolution of zinc. For this purpose, the outer surface of the steel pipe is galvanized, which is advantageous in terms of corrosion resistance, strength, and cost, and the inner surface is coated with organic paint or plastic coating after chemical conversion treatment, which does not cause dissolution even in water conditions.・Products with internal organic coatings are now required. In order to manufacture the above steel pipe with galvanized outer surface and organic coating on the inner surface, the organic coating decomposes at 250 to 300°C, so it is necessary to first galvanize the outer surface and then apply the organic coating to the inner surface. There are the following methods for producing externally plated steel pipes, which are intermediate products before this organic coating treatment. 1 Plating only the outside surface using plating equipment that pours or showers molten zinc. 2. Plug the end of the pipe with a stopper. 3. Double-sided plating is performed using conventional methods, and the galvanizing on the inner surface is removed by acid treatment or physical methods. However, the above method has drawbacks as described below. Method 1 requires investment in new plating equipment, and although this external plating line is advantageous for manufacturing externally plated steel pipes, it is difficult to use it in combination with conventional internally plated steel pipes. In method 2, when the diameter of the steel pipe increases, its buoyancy increases, so a greater force must be applied to immerse the pipe into the plating bath, resulting in poor workability. In method 3, the manufacturing cost increases because the zinc to be removed and the acid consumption for dissolving the zinc or the physical zinc removal treatment are expensive. The present invention eliminates the drawbacks of each of the above-mentioned methods, produces an intermediate product, externally plated steel pipe, at low cost using conventional plating equipment, and furthermore, the inner plated steel pipe can be coated with inner surface without adversely affecting the inner surface coating in the next process. The present invention provides a method for manufacturing a steel pipe with a molten metal plating on the outside and an organic coating on the inside, which can obtain a base material. The method of the present invention involves coating the inner surface of a steel pipe whose outer surface is to be plated with a plating inhibitor mainly consisting of alkali carbonate powder to form a plating preventing layer, and then plating the outer surface of the steel pipe by immersing it in a molten metal bath. Then, the inner surface of the steel pipe is acid-treated with mineral acid (hydrochloric acid or sulfuric acid) to remove the plating prevention layer, and then the inner surface is coated with an organic substance. There is almost no information, such as patent examples or examples, regarding the technology for manufacturing externally plated steel pipes by applying a plating inhibitor to the inner surface of the steel pipes. However, similar examples can be found in the manufacturing method of single-sided galvanized steel sheets developed for automobile steel sheets in the continuous hot-dip galvanized steel sheet manufacturing process, and therefore these will be explained below. For example, as a method of manufacturing a single-sided hot-dip galvanized steel sheet by applying a plating inhibitor to one side of the steel sheet,
Special Publication No. 39-4522, Special Publication No. 24966, Special Publication No. 42-24966, Special Publication No.
As shown in No. 51-8101 and U.S. Pat. No. 3,104,993, there are methods to prevent zinc adhesion by applying water glass, phosphate film, silicone resin, or silica gel to the surface where plating is to be prevented. 52―
No. 151638 describes using water as a plating inhibitor as a medium, and adding SiO ₂ , Al ₂ O ₃ , CaO, K ₂ O, MgO,
Examples have been given in which one or a composite of two or more oxides such as Mg ₂ O, TiO, BeO, etc. are used. The required characteristics of an intermediate product in which one side (or inner surface) is coated with a plating inhibitor and the other side (or outer surface) is plated with molten metal (e.g. zinc) are as follows: 1. The galvanized surface is perfect; There shall be no defects such as smudges. 2. The surface to which the plating inhibitor has been applied must be completely prevented from plating. 3. After plating, it is possible to completely remove the scale and plating inhibitor on the inner surface, and it does not interfere with the formation of the organic coating in the next step. However, even if the various plating inhibitors used in the steel sheet can satisfy characteristics 1) and 2), there remains a problem regarding the removal of the plating inhibitor of characteristic 3). In the case of steel pipes, the amount of plating required to satisfy the standard (JIS) is larger than that of steel plates, and therefore the plating time is longer than in the case of steel plates. The coating layer of inhibitor must be thick. For this purpose, plating inhibitors must be particularly easy to remove. According to the experience of the present inventors, water glass used in the steel sheet, a plating inhibitor that forms a silicate film mainly composed of colloidal silica, or oxides such as SiO ₂ and Al ₂ O ₃ that have low acid solubility The film has poor removability with hydrochloric acid, sulfuric acid, etc. Therefore, if it is used in the production of steel pipes with hot-dip galvanized outer surfaces and organic coated inner surfaces, a problem arises in that the adhesion of the coating in the next step or the adhesion of the chemical conversion coating deteriorates. In addition, in the case of steel plates, when chemical acid treatment is performed as a method to remove scale and plating inhibitors that form during plating on the unplated surface after plating, the acid solution permeates around the surface and removes the zinc on the plating surface. Since it dissolves, if you want to avoid it, you have to use physical grinding methods such as bubbling or brushing. Therefore, in the above-mentioned Japanese Patent Publication No. 52-151638, in order to reduce the amount of zinc dissolved in the galvanized surface during the removal of the scale on the unplated surface and the plating inhibitor, a phosphoric acid aqueous solution was added as a removing solution, which had a higher ionization tendency than zinc phosphate. This shows an example of the development of a chemical solution in which one or more types of metal phosphates are selected and mixed with high metal phosphates. However, such a removing agent is special compared to ordinary mineral acids such as hydrochloric acid and sulfuric acid, and is inevitably expensive. In contrast, in the case of steel pipes, the inner surface is a closed curved surface, so hydrochloric acid and sulfuric acid are used to remove scale and plating inhibitors on the inner surface without dissolving the galvanized outer surface. It is possible to take a method of distributing such items. Next, details of the method of the present invention will be explained. In the present invention, a powdery alkaline carbonate such as CaCO ₃ , K ₂ CO ₃ or Na ₂ CO ₃ (including mixtures thereof) is added to water as a plating inhibitor with good removability. ) is used as a slurry-form plating inhibitor. Adding water glass or colloidal silica as a binder to the above medium (water) to the extent that the SiO ₂ content is 1.5wt% or less improves the adhesion between carbonate particles and the adhesion of the plating inhibitor to the steel pipe surface, thereby inhibiting plating. It has the effect of promoting performance. The carbonate powder is easily decomposed by acids to produce salts of the acids (e.g. CaCO ₃ +2Hcl→
CaCl ₂ +CO ₂ +H ₂ O), the resulting salt has high solubility in acid solution or water, and is suitable for removal with acid solution. Furthermore, CO ₂ is generated during decomposition, so
Promotes peeling of the plating inhibitor from the inner surface of the steel pipe.
Therefore, the plating inhibitor used in the method of the present invention, unlike the plating inhibitor made of oxide used for the steel sheet described above, is extremely easy to remove with mineral acid and is suitable for organic coating in the next step. There is no risk of any problems occurring. The steps of the method for manufacturing a steel pipe with molten metal plating on the outside and organic coating on the inside according to the present invention are shown in FIG. 1.
This process is broadly divided into an external plating process and an internal organic coating process. First, in the degreasing and pickling process of the external plating process, mill scale is removed simultaneously from the inside and outside of the steel pipe. In the next plating inhibitor coating process, a plating inhibitor mainly composed of alkali carbonates as described above is applied by pouring the plating inhibitor onto the inner surface of the pipe or using a coating machine, or by impregnating a sponge or the like with the plating inhibitor and moving it inside the pipe. Apply the agent. In the next flux treatment step, the ends of the steel pipes are plugged and immersed in flux to coat only the outer surface of the tube with flux, and then the plug is removed, or flux is poured and painted only on the outer surface of the tube. (The steps of coating the plating inhibitor and flux treatment may be performed in reverse order.) In the drying step, the plating inhibitor on the inner surface of the tube and the flux on the outer surface of the tube are simultaneously dried. Next, the pipe is plated using ordinary steel pipe plating equipment, and then cooled with hot water. In the inner surface organic coating step that follows the above, in the inner surface acid treatment step, scale and plating inhibitor formed during plating are removed by flowing hydrochloric acid or sulfuric acid into the pipe, and then organic coating is applied or After the chemical conversion treatment, an organic coating is applied to complete the method of the present invention. Next, examples of the method of the present invention will be described. Comparison of the method of the present invention using a forge-welded pipe and an electric resistance welded pipe with a pipe size of 65A and a pipe length of 5.5 m, and a method using a conventional method used in the production of continuous hot-dip galvanized steel sheets, and the method of the present invention. A factory experiment was conducted on the change in the performance of the plating inhibitor. The plating inhibitor in the method of the present invention is 1) CaCO ₃ + water, 2) CaCO ₃ + water + water glass (as 1% SiO ₂ ), 3) CaCO ₃ + water + colloidal silica (as 1% SiO ₂ ). For the conventional method, three types were used: 4) water glass, 5) colloidal silica, and 6) Al ₂ O ₃ ·CaO. The experiment was conducted according to the steps shown in FIG.
A coating machine is used to apply the plating inhibitor on the inside surface, and the flux treatment on the outside surface is performed by plugging the end of the tube and immersing it in a flux bath. I went there and cooled it down. Thereafter, the inner surface was subjected to pickling and chemical conversion treatment. Table 1 shows the comparison results between the method of the present invention and the conventional method. (Similar results were obtained with forge-welded pipes and electric resistance welded pipes.)

[Table] As shown in Table 1, the plating inhibitor used in the conventional method is mainly water glass or colloidal silica, or metal oxide powder such as Al ₂ O ₃ / CaO is used as a medium in water. In these examples, the plating inhibitory properties were good, but the plating inhibitor could not be completely removed by the pickling treatment in the tube after plating, which had an adverse effect on the subsequent chemical conversion treatment.
On the other hand, in the case of the method of the present invention, the plating inhibiting property of the plating inhibitor is good, and no zinc plating is formed on the plating inhibiting surface in the plating process, and the scale formed during plating and the plating inhibitor could be completely removed in the pickling process, and the next process could be carried out in good condition. Figure 2 shows the relationship between the adhesion amount (coating amount) of the plating inhibitor and the plating prevention property in Method 1 of the present invention in Table 1, and the horizontal axis shows the adhesion amount (CaCO ₃ g/m ² ), the vertical axis shows the plating prevention property.
As is clear from the figure, if the coating amount is less than 20 g/m ² , the plating prevention property is insufficient and zinc plating is formed locally. When the amount of adhesion was 20 g/m ² or more, the plating prevention property was good, and the scale and plating inhibitor could be sufficiently removed by the hydrochloric acid washing process on the inner surface of the tube after plating. Figure 3 shows the relationship between the plating inhibitory property and the plating inhibitor removability of the plating inhibitor according to Method ² of the present invention in Table 1, with respect to the amount of water glass added when the amount of CaCO ₃ deposited was 10 g/m2. The horizontal axis is CaCO ₃
In the composition of + water + water glass, the weight ratio (wt%) of SiO ₂ in water + water glass is shown. In addition, on the vertical axis, A indicates the plating prevention property, B indicates the plating inhibitor removability, and the curves A and B indicate the vertical axis A and B.
corresponds to From FIG. 3, it can be seen that the plating inhibitor (CaCO ₃ + water + water glass) of method 2 of the present invention has a CaCO ₃ adhesion amount of 10
In the case of SiO ² content up to 1.5wt%, both plating prevention and plating inhibitor removal performance are good, but if it exceeds 1.5wt%, the plating in the pipe inner surface pickling process after plating will be reduced _. It is clear that the inhibitor removability deteriorates rapidly and adversely affects the next step. Therefore, the SiO ₂ content needs to be 1.5wt% or less. Figure 4 shows that the plating inhibitor according to method 2 of the present invention in Table 1 has a SiO ₂ content of 0.5 to 1.5 wt%.
FIG. 3 is a diagram showing the relationship between the amount of CaCO ₃ deposited and the plating prevention property, the plating inhibitor removability, and the adhesion of a paint or chemically treated film. In the figure, the vertical axis indicates the plating prevention property, the plating inhibitor removability, and the adhesion of the paint or chemically treated film (hereinafter referred to as adhesion).
The horizontal axis is a plating inhibitor (CaCO ₃ + water + water glass)
It shows the amount of _CaCO3 attached inside. Note that C of the curve represents the relationship between plating inhibition and the amount of CaCO ₃ deposited, D of the curve represents the relationship between plating inhibitor removability and the amount of CaCO ₃ deposited, and E of the curve represents the relationship between adhesion and the amount of CaCO ₃ deposited. From FIG. 4, the plating resistance tends to improve rapidly when the CaCO ₃ adhesion amount is around 7 g/m ² and to become slightly worse from around 10 g/m ² . In addition, the removability and adhesion of the plating inhibitor were determined when the CaCO ₃ adhesion amount was 10g/
It shows a tendency to deteriorate rapidly from around ^m2 . In particular, the tendency for adhesion to deteriorate is more remarkable than the tendency for plating inhibitor removability to deteriorate. Note that the SiO ₂ content is 0.5wt%
In the following cases, plating prevention properties deteriorate as the SiO ₂ content decreases. On the other hand, the plating inhibitor removability and adhesion tend to improve. By the way, the third
As shown in the figure, when the SiO ₂ content is less than 0.3wt%,
The plating prevention property deteriorates significantly due to reasons such as increased dispersion in performance due to whether it is practical or not practical in actual operation. However, when the SiO ₂ content is in the range of 0.3 to 0.5 wt%, the plating prevention property is at its best (SiO ₂ content of 0.5 to 1 wt%).
%), but there is no operational problem and the initial objective can be achieved, so it is within the acceptable range.
The content was 0.3 to 1.5 wt%. FIG. 5 shows the plating inhibitor (CaCO ₃ + water + colloidal silica) of method 3 of the present invention in Table 1.
FIG _{. 2} is a diagram showing the relationship between the amount of CaCO ₃ deposited and the plating prevention property, the plating inhibitor removability, and the adhesion property when the SiO 2 content (content in water + colloidal silica) is in the range of 0.5 to 1.5 wt%. The vertical axis is the same as in FIG. 4, and the horizontal axis shows the amount of CaCO ₃ deposited in the plating inhibitor (CaCO ₃ + colloidal silica + water). Note that curve f shows the relationship between plating inhibition and CaCO ₃ adhesion amount, curve G shows the relationship between plating inhibitor removability and CaCO ₃ adhesion amount, and curve H shows the relationship between adhesion and CaCO ₃ adhesion amount. From Figure 5, plating prevention, plating inhibitor removability, and adhesion are determined when the amount of CaCO ₃ attached is 10 to 15 g/m ²
In the range of , the results show similar trends to those shown in FIG. However, the plating inhibitor removability and adhesion tend to be slightly lower than in the case of FIG. 4. In addition, if the SiO ₂ content is 0.5wt% or less,
As the SiO ₂ content decreases, the plating resistance deteriorates. On the other hand, the removability and adhesion of the plating inhibitor tend to improve, but the degree of improvement is small compared to the plating inhibitor of Method 2 of the present invention in Table 1. The reason why the initial objective can be achieved even when the SiO ₂ content is 0.3 to 0.5 wt% is the same as in the case of the plating inhibitor of method 2 of the present invention. Therefore, it is preferable that the plating inhibiting material according to method 3 of the present invention also has a SiO ₂ content in the range of 0.3 to 1.5 wt.

[Brief explanation of the drawing]

FIG. 1 is a flow sheet showing the steps of the method of the present invention, FIG. 2 is a diagram showing the relationship between the plating-inhibiting properties of the plating inhibitor used in the present invention and the amount of the plating inhibitor attached, and FIG. Figures 4 and 5 are diagrams showing the relationship between the plating inhibiting properties and the plating inhibitor removability of the plating inhibitor used in the process and the SiO ₂ content (wt%).
The figure is a diagram showing the relationship between the adhesion amount of CaCO ₃ and the plating-inhibiting property, the plating-inhibiting agent removability, and the adhesion of the plating inhibitor of the present invention. In the figure, A is a curve showing the relationship between plating inhibition and SiO ₂ content (wt%), B is a curve showing the relationship between plating inhibitor removability and SiO ₂ content (wt%),
C and f are curves showing the relationship between plating inhibition and CaCO ₃ adhesion amount, D and G are curves showing the relationship between plating inhibitor removability and
Curves E and H represent the relationship between the amount of CaCO ₃ deposited and the relationship between adhesion and the amount of CaCO ₃ deposited.

Claims (1)

[Scope of Claims] 1. A method for manufacturing an outer surface-plated inner surface organic-coated steel pipe in which the outer surface of the steel pipe is plated with molten metal and then the inner surface of the steel pipe is coated with an organic substance, comprising: (a) preventing plating of an alkali carbonate and water composition; (b) For plating inhibitors whose composition is alkali carbonate, water glass, and water, the amount of alkali carbonate deposited is 20 g/m ² or more. _SiO2 content is 0.3~1.5wt% relative to the total amount of
(c) For plating inhibitors whose composition ^is alkali carbonate, water, and colloidal silica, Among them, the composition ratio is determined so that the SiO ₂ content is 0.3 to 1.5 wt% with respect to the total amount of water and colloidal silica, and the amount of alkali carbonate attached is 10 to 15 g/
m ² to form an alkali carbonate-based plating inhibitor, then the steel pipe is immersed in a molten metal bath to plate its outer surface, and after the plating bath has cooled, the inner surface of the steel pipe is pickled. A method for producing a steel pipe with an outer surface plated and an inner surface organically coated, characterized in that the remaining plating prevention layer is completely removed by treatment, and then the inner surface of the steel pipe is coated with an organic substance.