JPS637359A - Heat treatment method for aluminum plated parts - Google Patents

Abstract

PURPOSE:To improve the corrosion resistance of formed parts, by heating formed parts made of Al-plated steel material in a reducing atmosphere and by subjecting Al and steel to mutual thermal diffusion. CONSTITUTION:The steel material on the surface of which an Al plating layer is formed is formed into part shape, and the resulting formed parts are heated to 400-800 deg.C in a reducing-gas atmosphere of ammonia decomposed gas (AX gas(R)), hydrogen, gas etc., to subject Al plating layer and steel material to mutual diffusion, so that an Al-Fe alloy layer is formed and, simultaneously, an Al2O3 film of about 100Angstrom thickness is formed on the surface of the Al-Fe alloy layer by the action of O2 generated by the decomposition of trace amounts of water vapor in the AX gas or hydrogen gas. Owing to the above Al-Fe alloy layer and the Al2O3 film at the above layer surface, corrosion resistance of steel parts can be improved.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for heat treating aluminum plated parts, and in particular to a method for improving the corrosion resistance of molded parts in which an aluminum plating layer is formed on the surface of steel material. The present invention relates to an improvement in a suitable heat treatment method for aluminum plated parts.

[Conventional technology]

Conventionally, parts manufactured by forming steel materials with an aluminum plating layer formed on the surface (hereinafter referred to as g!l! Al-plated parts) have aluminum plated in the parts that have undergone strong forming processing.
Defects (for example, cracks or holes) had occurred in the plating layer.

And, conventionally prepared Al-plated parts are made of such Al-plated parts.
It was used with defects in the plating layer.

The reason for this is that prepared Al-plated parts have traditionally been considered unsuitable for heat treatment.

That is, for example, an aluminum-plated steel plate is heated to 550°C in the atmosphere.
If heated above 700°C, the Al plating layer will easily peel off, and especially when heated above 700°C, the Al plating layer will definitely peel off.
This is because the plating layer peels off.

For this reason, the heat treatment of Al-plated steel sheets itself was considered meaningless, and no specific study results were published in literature or the like.

In addition, when the tensile strain exceeds 10%, cracks occur in the Al plating layer, and in the 90" bending test, the bending radius is R
: If the plate thickness is t, cracks were present in the Al plating layer when R/l was 7 or less.

Therefore, if strong molding is applied, AI! The occurrence of defects in the mesoki layer has been considered unavoidable.

[Problem that the invention seeks to solve]

However, since conventional steel Al-plated parts were used with defects in the A2 plating layer, there was a problem that the defective parts were subject to high-temperature oxidation and wet corrosion.

For this reason, there was a problem in that the anticorrosive performance of All-Plated was not exhibited, and the parts themselves corroded and deteriorated early.

Therefore, an object of the present invention is to improve the pitch resistance and corrosion resistance of Al-plated parts manufactured by molding by eliminating defects that occur in the Al-plated layer.

[Means for solving problems]

Therefore, the heat treatment method for aluminum-plated parts according to the present invention is characterized in that defects occurring in the Al-plated layer are eliminated by mutual thermal diffusion between the Al-plated layer and the base material.

Specifically, after a steel material with an aluminum plating layer formed on its surface is formed into a component shape, the aluminum plating layer and the steel material are heated at high temperature in a reducing gas to cause thermal diffusion between the aluminum plating layer and the steel material. It is something.

Here, the reducing gas needs to have the effect of reducing Fe oxides and oxide films existing in defective areas of the Al plating layer to Fe.

As such a reducing gas, AX gas (ammonia decomposition gas) and hydrogen gas are suitable.

In addition, the temperature at which the high temperature is heated is
A temperature of 400 to 800°C is suitable for interdiffusion of the plating layers.

[Effect]

To explain in detail the present invention according to the above configuration, the steel A
By heating the Al-plated part at high temperature in a reducing gas, the Al-plated layer reacts with the steel material and changes into an Al-Fe alloy layer.

In particular, if heat treated at 550°C or higher, steel Al-plated parts will have a melting point of approximately 1300'C1 and a hardness of Hv300~
1ooo, and the pitch resistance improves. Incidentally, the hardness of the Al plating layer is HV50 to 100.

Also, Aj! On the surface of the -Fe-based alloy layer, an aluminum oxide (hereinafter referred to as Altos) film of about 100% is formed, improving corrosion resistance.

This is done by heating F in AX gas or Hz gas at medium heat.
The oxide of e is reduced to Fe.

However, this is because Al easily combines with oxygen and takes away oxygen from HtO (containing water vapor) in AX gas or H2 gas to form an Al1tOs film on the surface.

〔Example〕

Hereinafter, one embodiment of the present invention will be described in detail based on the drawings.

FIG. 1 is a front view showing the schematic structure of a front exhaust pipe, which is a steel A1 plated part that has been heat treated by the heat treatment method for aluminum plated parts according to the present invention;
Fig. 2 is an enlarged sectional view of part A in Fig. 1, Fig. 3 is a sectional view showing the schematic structure of the portion corresponding to Fig. 2 of a steel Al-plated part before heat treatment, and Fig. 4 The figure is a graph showing heat treatment conditions.

First, a front exhaust pipe 2 for a four-cylinder engine, which is made of electrical resistance welded steel pipe and subjected to a 60" bending process, is prepared.

As shown in FIG. 3, this pipe 2 has a base material 22 and an Al
Front exhaust pipe (hereinafter referred to as a pipe) formed from a plating layer 24, with holes 242 and cracks 244 present in the Al plating layer 24
The specifications of 2 are outer diameter φ = 42.7 m, wall thickness t = 1.6 mm.
, All plating amount (both sides); 8゜g/rd (thickness 2
9μ) Max processing strain of the most elongated part by 1 bending 30
% or more.

This pipe 2 was heat treated in an AX gas furnace (point n; -40°C).

The heat treatment temperature conditions were as shown in Figure 4: the temperature was raised to 6°C, held for 30 minutes, then further raised to 800°C, held for 30 minutes, and then cooled in a reducing atmosphere. .

As a result, Aj? 203 film 28 is formed, and the lower layer is A/! -The Fe alloy layer 26 was formed, and defects such as pores 242 and cracks 244 were eliminated.

The hardness of the Al plating layer after heat treatment is HV300, A
The thickness of the l plating layer is 60μ, the thickness of the /1203 coating is 1
There were 00 people.

In this way, the material of the present invention was manufactured.

(Comparative Example) In order to evaluate the pinch resistance and corrosion resistance of the present invention material, a comparative material without heat treatment was produced.

This comparative material had about 8/- defects such as holes in the AI plating layer 24 of the bent portion of the pipe 2.

Hereinafter, the results of comparative evaluation of pitch resistance and corrosion resistance of each test material of the present invention material and comparative material will be explained.

A bench high-speed durability test was conducted using each of the above-mentioned test materials. Note that the test conditions were a temperature of 700°C and a duration of 200 hours.

As a result, the A1 plating layer 24 peeled off in a range of 30n in width and 5Qm in length near the bending part of the comparative material, and the base material steel 22
was significantly oxidized.

Moreover, at that time, the wall thickness of the vibrator 2 had decreased by 0.2 m. That is, peeling of the A1 plating layer 24 could not be prevented.

On the other hand, in the material of the present invention, peeling of the AN plating layer 24 and oxidation of the base material #22 were not observed at all.

Next, the results of studying the heat treatment conditions will be explained.

That is, the results of a comparative evaluation of the presence or absence of peeling of the Al plating layer when heat treatment is performed while changing the type of atmospheric gas will be explained.

The results are shown in Table 1.

Table 1 Note that Rx gas refers to a carburizing atmosphere gas, and specifically refers to a decomposed gas of butane or propane. The gas atmosphere condition is a carbon potential of 0.8%.

Further, the ratio of the mixed gas of AX gas and H2 gas indicates the flow rate ratio.

As is clear from Table 1, good results were obtained for AX gas, Ht gas, and a mixture of both gases.

Next, the results of studying the heat treatment temperature will be explained.

When using AX gas alone or a mixed gas of AX gas and H2 gas, the temperature is preferably 700 to 800°C from the viewpoint of placing importance on the stability of the gas atmosphere.

In addition, in the case of Hz gas alone, H with an explosion-proof device added,
400-800, provided that gas management methods are used.
Heat treatment at ℃ is possible.

Note that the upper limit of the heat treatment temperature is set to 800° C. because the rate of mutual diffusion of Al and Fe becomes extremely high when it exceeds 800° C.

Further, regarding the heat treatment time, it is important to consider the economic efficiency of equipment operation depending on the length of the heat treatment time.

For this reason, assuming that the thickness of the plating layer is controlled to be about 100 μm, the optimum temperature is 800° C. xi for 5 hours.

Although specific embodiments of the present invention have been described above, the present invention is not limited thereto, and various other embodiments that can be implemented by those skilled in the art are possible within the scope of the claims.

For example, in the above embodiment, the bending part of the front exhaust pipe was targeted, but it can be applied to any similar part other than the front exhaust pipe.

〔Effect of the invention〕

As explained above, the heat treatment method for aluminum plated parts according to the present invention removes defects occurring in the Al plating layer.
A/Since the heat diffusion between the plating layer and the base material is eliminated, the pinch resistance of the Al-plated parts manufactured by molding can be improved.

In addition, corundum-like Al is formed on the surface of Al-plated parts.
Since an oz film is formed, corrosion resistance can be significantly improved.

Incidentally, if a general steel AIT plated part is immersed in 50% hydrochloric acid, the A1 plating layer will melt away in a few minutes and the base steel will start to corrode, but the steel plated part made of the heat treated steel according to the present invention In the case of the Al-plated parts, there was no corrosion of the Al-plated layer even after immersion for 3 hours.

Furthermore, the Al plating layer becomes an α layer of Al-Fe alloy,
Its hardness is also approximately Hv300, which provides an excellent effect of improving pitch resistance.

[Brief explanation of the drawing]

FIG. 1 is a front view showing the schematic structure of a front exhaust pipe, which is a steel Al-plated part that has been heat-treated by the method of heat-treating aluminum-plated parts according to the present invention;
Fig. 2 is an enlarged sectional view of part A in Fig. 1, Fig. 3 is a sectional view showing the schematic structure of the portion corresponding to Fig. 2 of a steel Al-plated part before heat treatment, and Fig. 4 The figure is a graph showing heat treatment conditions. 22-・- Steel material (base material steel)

Claims (1)

[Claims] An aluminum plated part characterized in that a steel material with an aluminum plating layer formed on its surface is formed into a part shape and then heated at high temperature in a reducing gas to cause thermal diffusion between the aluminum plating layer and the steel material. heat treatment method.