JPS5856026B2 - Manufacturing method of aluminum coated stainless steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing aluminum coated stainless steel.

Recent advances in petrochemistry, synthetic chemistry, and other fields of engineering that deal with reactions and heat have been remarkable, and energy conservation and the use of inexpensive energy are being considered.

Examples of such considerations include raising the operating temperature and pressure, recovering and utilizing waste heat, and using low-grade fuel to improve the thermal efficiency of equipment.

All of these make the conditions for use of the materials constituting the equipment harsh.

Therefore, there is a strong demand for the development of inexpensive materials with excellent properties.

In particular, reaction towers, heat exchangers, connecting parts, etc. used in petrochemistry and synthetic chemistry are exposed to hydrogen sulfide and sulfuric acid gas atmospheres at high temperatures, causing problems with high-temperature strength and material corrosion.

Conventionally, the pipes used for these are medium and high temperature carbon steel (STB pipe), low chromium molybdenum steel (2a Cr
Although aluminum-coated materials have been adopted for I Mo), it is anticipated that operating conditions will become harsher in the future, especially as the operating temperature increases.
Considering the pressure increase, these materials lack strength.

Also, stainless steel, especially austenitic stainless steel (18Cr-8Ni steel, 18Cr-1ONi-2Mo)
18C is widely used for reaction vessels and piping because it has high temperature strength, excellent workability, and corrosion resistance.
In the case of r-8Ni steel, high temperature (700°C to 800'C
), the surface will undergo oxidation scaling and form a thick oxide film.

The oxide film easily peels off, and if used repeatedly, the base material will become thin and unusable.

Furthermore, since the base material contains Ni, it becomes nickel sulfide (NiS) in a high-temperature hydrogen sulfide atmosphere and corrodes.
It cannot be used in a hydrogen sulfide atmosphere because its corrosion resistance is significantly lower than that of stainless steel or mild steel that does not contain Ni.

Further, although ferritic stainless steel that does not contain Ni has many excellent properties, it has poor workability and its uses are limited.

This problem can be solved by coating the surface of austenitic stainless steel, which has excellent high-temperature strength, with aluminum, which has excellent corrosion resistance in sulfur atmospheres such as hydrogen sulfide.

There are two methods for aluminum coating, each with its own advantages and disadvantages.

First, in the solid diffusion method (kawarizing method), in which a material is buried in powdered aluminum or aluminum alloy and then heated to a high temperature for diffusion treatment, less than 30 wt% of iron is deposited on the surface of the material.
An Al diffusion layer is formed to a thickness of about 100 μm.

Since this diffusion layer is only allowed to elongate by about 1%, subsequent processing is impossible.

If the material is handled with excessive force, cracks will occur and not only will the corrosion resistance be drastically reduced, but the cracks will become sharp notches, which will significantly reduce the toughness of the material.

Therefore, care must be taken during transportation and installation after surface treatment.

Furthermore, processing requires high temperatures (800°C to 1000°C) and long periods of time (6 to 10 hours or more), which not only increases processing costs but also causes coarsening of the material's crystal grains and impairs its mechanical properties. be inferior.

Another method, the molten immersion method (aluma method), in which the material is immersed in molten aluminum and aluminum is attached to the surface, takes only a short time, but
In addition to 1, an iron-A1 alloy layer is formed, and no elongation of this alloy layer can be expected, making subsequent processing impossible.

Therefore, the only way to provide workability after processing is to make the alloy layer thinner or eliminate it altogether.

Therefore, conventionally, about 10% of Si or the like is added to the molten aluminum bath to prevent the formation of an alloy layer.

Addition of other elements such as Si makes it difficult to manage the aluminum bath, and increases the amount of material dissolved into the bath during immersion, leading to problems such as an increase in the amount of impurities in the bath and the occurrence of dross (precipitation of impurities).

By the way, when stainless steel SUS 304 (18Cr-8Ni) is immersed in an aluminum bath (97% Al), the influence of the immersion temperature on the amount of alloy layer formed and the amount of dissolution is as shown in FIG.

According to the figure, the temperature is between 660°C (melting temperature of Al) and 690°C.
Until then, an alloy layer is formed, and the amount of thinning of the base metal is large.

However, at temperatures above 690°C, no alloy layer is formed and the amount of thinning of the base material decreases, but as the temperature rises further, the amount of thinning of the base material increases.

Therefore, in order to form an l' layer without an alloy layer on stainless steel, a temperature of at least 690°C is required, and in order to suppress the amount of stainless steel dissolved and reduce impurities in the bath, the immersion temperature is 750°C. 'C or lower is desirable.

In view of this fact, the present invention proposes a method for manufacturing aluminum-coated stainless steel, which improves the latter method and completely eliminates the problems of this method.

Hereinafter, one embodiment of the present invention will be described based on FIGS. 2 and 3.

In Figure 2, 1 is 5US304 (18Cr-8Ni
) thin plate made of austenitic stainless steel, 2
is an uncoiler, 3 is a cutting and welding device, 4 is a storage loop car, 5 is a washing tank, 6 is a drying device, 7 is a preheating tank, 8 is a molten aluminum tank, 9 is a cold air blowing nozzle, 10 is a storage loop car, 11 is a cutting The device 12 is a winding device.

In the above configuration, the thin plate 1 is washed with water in the washing tank 5,
After removing oil from the surface, it is dried using a drying device 6, and then this thin plate 1 is immersed in a preheating tank 7 at a temperature of 690 to 750°C.
Preheat to.

The reason for preheating here is that when the cold thin plate 1 is immersed in the molten aluminum tank 8, the temperature rises to 660~690 during heating.
This is because an alloy layer is formed on the thin plate 1 in the temperature range of °C, and when the temperature is further increased, this alloy layer dissolves in the bath, increasing the amount of impurities in the bath and reducing the base material, that is, the thin plate 1. be.

As the preheating tower of the preheating tank 7, a flux bath (for example, KC
1, NaC1, Na1AlF6, AlF3, etc.) is desirable.

By immersing the thin plate 1 preheated to about 700° C. into the molten aluminum bath 8 also at about 700° C., it is possible to eliminate the formation of an alloy layer and create a state in which the base material is less likely to melt.

Next, after being immersed for a required period of time (the shorter the better), the thin plate 1 is quickly pulled up from the molten aluminum tank 8 and rapidly cooled to between 690 and 660°C by cold air blown from the nozzle 9.

Blowing compressed gas is preferable in terms of controlling the amount of Al deposited, and has advantages such as uniform distribution of Al deposited on the plane.

However, at temperatures below 660°C where the surface Al solidifies,
Since there is little benefit from blowing cold air and internal residual stress is expected to occur due to rapid cooling, it is sufficient to pull up the thin plate 1, and the necessary cooling rate can be obtained even without blowing cold air. .

FIG. 3 shows a microscopic photograph of the cross-sectional structure of an aluminum-coated stainless steel sheet obtained according to an example of the present invention.

In the figure, A is a 5US304 (18Cr8Ni) stainless steel plate, the opening is an aluminum layer, and no alloy layer is seen between the aluminum layer opening and the stainless steel A.

Although the above embodiment has been explained using the thin plate 1 as an example, it goes without saying that the present invention can be applied to a variety of stainless steel materials in general, such as pipes and structural members.

As described above, according to the method for producing aluminum-coated stainless steel of the present invention, austenitic stainless steel of a predetermined shape is preheated to 690 to 750°C, and then heated to 690 to 750°C.
After immersing the stainless steel in molten aluminum at 90 to 750°C to coat the surface of the stainless steel with aluminum, the steel is pulled out from the molten aluminum and heated to 690 to 66°C.
Since rapid cooling is performed between 0°C, no alloy layer is formed between the stainless steel and the aluminum layer, and melting of the stainless steel can be reduced.

Therefore, it exhibits excellent strength and workability, which are characteristics of austenitic stainless steel, and excellent corrosion resistance in sulfur atmospheres such as hydrogen sulfide, which is a characteristic of aluminum.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between immersion temperature, base metal thickness reduction, and alloy layer thickness when stainless steel is immersed in an aluminum bath, and FIGS. 2 and 3 show an embodiment of the present invention. Figure 2 is a schematic explanatory diagram showing the coating procedure;
The figure is a micrograph of the cross-sectional structure of aluminum-coated stainless steel. 1... Thin plate made of stainless steel, 7...
... Preheating tank, 8... Molten aluminum tank.

Claims (1)

[Claims] 1 Austenitic stainless steel of a predetermined shape is made of 690
The stainless steel is preheated to ~750°C, then immersed in molten aluminum at 690~750°C to coat the surface of the stainless steel with aluminum, and then pulled out of the molten aluminum and immersed in molten aluminum at 690~660°C. A method for producing aluminum-coated stainless steel, characterized by rapidly cooling the aluminum-coated stainless steel.