JPH0452211A - Method for heat-treating surface of metal body - Google Patents

Abstract

PURPOSE:To achieve high temp. and to efficiently reduce and remove oxide film on surface of a metal body for short time by supplying reducing gas together with working gas into a plasma torch, heating the metal body with plasma jet of the reducing gas and reducing the oxide film on the surface of metal body. CONSTITUTION:A steel strip S recoiled from a recoiler 11 is charged into a non-oxidizing reduction furnace 15 providing the plasma torch after passing through a shearing machine 12, welding machine 13 and looping car 14 to reduce the oxide film on the surface of steel strip S with the plasma torch. In this non-oxidizing reduction furnace 15, plural plasma torches are set at upper and lower parts of the steel strip S and in electrode 36 and the working gas nozzle 37 in the plasma torch 35, negative pole and positive pole in the DC electric source 39 are connected, respectively. Into the working gas nozzle 36, the working gas is supplied and into the reducing gas nozzle 38, the reducing gas (hydrogen gas, etc.) is supplied. Further, tip part of the plasma torch 35 is cooled with cooling water and fused zinc plating is executed to the reduced steel strip S with a plating vessel 17 after cooling in a cooling furnace 16.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for heat treating the surface of a metal body, which heats the metal body and reduces an oxide film on the surface of the metal body.

[Prior Art] A radiant tube method using combustion gas as a heating source for surface treatment of a metal body, for example, a steel plate, has been used.

Since the radiant tube method uses indirect heating, it is inefficient, and radiant tubes are often damaged and maintenance costs are high. For this reason, reduction burners using gas combustion using an open flame method have been considered and are now in use. For example, ``Direct fire reduction heating method for copper strip'' of JP-A No. 60-77929, or JP-A No. 62-5
2310 "Direct Fire Reduction Heating Burner" etc.

However, direct-fired reduction burners reduce the amount of unburned matter in the combustion exhaust gas, so ■ The reduction area is narrow.■ They are unsuitable for reductions that require high temperatures. ■ The reduction effect occurs only after the steel plate is heated to a certain high temperature. There is a problem in that it takes time for the reduction to occur.

[Issue to be solved by the invention] In view of the above points, the present invention provides a metal that can efficiently heat a metal body such as a steel plate to a required temperature in a short time and reduce the surface of the metal body. The present invention provides a method for heat treatment of a body surface.

[Means for Solving the Problem] The method of heat treating the surface of a metal body of the present invention supplies a reducing gas together with a working gas to a plasma torch, heats the metal body with a plasma jet of the reducing gas, and heats the metal body. Reduces the oxide film on the surface.

This invention will be explained in more detail below.

- When fishing on a boat, the heating of an object is expressed by the following equation (1).

dθ3 If the temperature increase rate of the object is dt dθgA = α・□(θ, -0S) ... (1) dt
CW θf: Heat source temperature ("C) θ3: Object temperature ("C) t: Time (h) α: Heat transfer coefficient (kcal/m2ht) A: Effective heat receiving area of the object (m2) C: Specific heat of the object (kcal) /J-℃) W: Specific gravity of object (kg) Heat receiving area A, specific heat C1 weight [IW is determined by the object,
There is no freedom. Therefore, it is necessary to increase the heat transfer coefficient α, but since the heat transfer coefficient α is dominated by radiation heat transfer in high temperature regions, the following formula % Formula % (2) Therefore, in order to increase the heat transfer coefficient α, , it is necessary to increase the temperature θ of the heating source. That is, in order to increase A-temperature rate □, dt can be increased by increasing the temperature of of the heating source.

Furthermore, the reduction mechanism on the surface of the metal body is no worse than the reaction caused by the increase in the temperature of the metal body and the plasma ions H'', and the -F increase in the temperature of the metal body is essential for the reduction of the surface of the metal body.

From the above points, the inventors used a high-temperature plasma torch as a heating source, used argon gas as the working gas of the plasma torch, and used hydrogen gas, methane gas, carbon oxide gas, etc. as the heating/reducing gas. A reduction experiment was conducted on the surface of a steel plate as a metal object.

The experimental results are shown in FIGS. 1 and 2.

According to FIG. 1, it is remarkable that the plasma torch reaches the predetermined temperature (700° C.) level in a short time of about 172 seconds compared to the direct fire type return burner. Further, as is clear from FIG. 2, the initial oxide film thickness of 500 on the surface of the steel plate can be reduced to the final oxide film thickness of 200 (reduced Ji: 300). Although FIGS. 1 and 2 show data for hydrogen gas, similar results are obtained for methane gas and -carbon oxide gas.

The reason why the reduction reaction occurs in a short time is that in direct-fire reduction burners, after the steel plate reaches a certain temperature, unburned reducing gas (hydrogen gas, methane gas, etc.) acts on the steel plate and reduces the surface of the steel plate. On the other hand, with a plasma torch, high-temperature hydrogen gas that has turned into plasma directly hits the surface of the steel plate, so heating and reduction occur almost simultaneously. The reason why the temperature rises particularly quickly is thought to be because the heat transfer coefficient of radiation heat transfer by the high-temperature plasma jet blown out from the plasma torch is large, and the heating time is shorter than that of a direct-fired reduction burner.

In addition, with a direct-fired reduction burner, reduction of the surface of the steel plate is performed by combustion exhaust gas, and since the combustion exhaust gas reacts with oxygen gas in the furnace every moment and burns, the reduction zone and oxidation zone vary depending on the distance from the burner. area is generated. On the other hand, in the case of a plasma torch, only a working gas for the plasma torch (inert gas such as argon) and a heating/reducing gas (hydrogen gas, methane gas, etc.) are present, so the reduction of the steel plate surface is performed stably.

As described above, the experimental results have revealed that it is possible to reduce the oxide film on the surface of a steel sheet using a plasma torch in a time that is approximately twice as long as that of a direct-fired reduction burner.

[Example] Fig. 3 shows a galvanizing line for a steel plate in which all zones were constructed using a horizontal furnace.

The steel plate S rewound from the unwinding machine 11 is passed through a shearing machine 12,
After passing through the welding machine 13 and the loop car 14, it enters a non-oxidation reduction furnace 15 equipped with a plasma torch, and the oxide film on the surface of the steel sheet S is reduced by the plasma torch.

In the conventional method of reducing the steel plate S using a direct-fired reduction burner, a non-oxidation furnace 27 and a reduction furnace 28 are installed in the above-mentioned line instead of the non-oxidation reduction furnace 15, as shown in FIG.

The non-oxidation reduction furnace 15 is provided with a front chamber 31 for the sole as shown in FIG.
There are 3. The seal uses an inert gas such as nitrogen gas.

In the non-oxidation reduction furnace 15, a plurality of plasma torches are arranged above and below the steel plate S. The electrode 36 and working gas nozzle 37 of the plasma torch 35 are connected to the negative and positive electrodes of a DC power source 39, respectively, as shown in FIG. The working gas nozzle 36 is filled with a working gas (argon gas, etc.), and the reducing gas nozzle 38 is filled with a reducing gas (such as argon gas).
hydrogen gas, etc.) is supplied. In addition, plasma torch 35
The tip of the tube is cooled by cooling water (not shown).

The steel sheet S reduced in the non-oxidation reduction furnace 15 is cooled in a cooling furnace 16 and then hot-dipped with zinc in a plating bath 17. The plated steel sheet S passes through an alloying furnace 18, a shed cooler 19, a cooling tower 20, and then a skin bath mill 21.
The skin is bath rolled. Accordingly, steel plate S is leveler 2.
After being straightened in step 2 and subjected to chromate treatment in a chemical treatment device 23, it is wound up in a winder 24.

As shown in FIG. 3, the equipment length of the non-oxidation reduction furnace 15 using a plasma torch is 1/2 compared to the furnace 27.28 equipped with a direct-fired reduction burner, and moreover, the required reduction You can get better performance.

Although this embodiment deals with steel plates, the present invention is not limited thereto, and can be applied to a wide variety of materials such as steel materials such as wire rods, steel bars, and long steel materials, stainless steel materials, cast iron, nonferrous metals, and alloy materials. It is something.

[Effects of the Invention] As explained above, according to the present invention, a high temperature can be achieved using a plasma torch, and an oxide film on the surface of a metal body can be efficiently reduced and removed in a short time. As a result, thermal efficiency and production efficiency can be improved. Also,
Since the equipment length is shortened, equipment costs can be reduced.

[Brief explanation of drawings]

Figure 1 is a diagram comparing the rate of increase in steel plate temperature between this invention and the conventional method, Figure 2 is a diagram illustrating an example of the relationship between reducing gas exposure time and reduction amount, and Figure 3 is a diagram showing this example. An example of equipment for implementing the invention, which is an equipment configuration diagram of a galvanizing line,
FIG. 4 is a longitudinal cross-sectional view of the non-oxidation-reduction furnace provided in the line, and FIG. 5 is a partial cross-sectional view showing the tip of a plasma torch attached to the non-oxidation-reduction furnace. 15... Non-oxidation reduction furnace, 17... Cooling furnace, 27...
- Non-oxidation furnace, 28... Reduction furnace, 31-... Front chamber of non-oxidation and reduction furnace, 33... Sealing device of non-oxidation and reduction furnace, 35
...Plasma torch, 36...: Electrode, 37... Working gas nozzle, 38... Reducing gas nozzle, 39...
・DC power supply, S...steel plate.

Claims (1)

[Claims] 1. A method for heat treatment of a surface of a metal body, which comprises supplying a reducing gas together with a working gas to a plasma torch, heating the metal body with a plasma jet of the reducing gas, and reducing an oxide film on the surface of the metal body.