EP0379104B1 - Process for annealing metal work pieces in a hydrogen-enriched protective atmosphere in a continuous furnace - Google Patents

Description

The invention relates to a method for annealing metal parts under a hydrogen-rich protective gas in a continuous furnace.

DE 37 33 884 A1 proposes a method for annealing metal parts under a protective gas atmosphere consisting essentially of hydrogen. The problem here is as follows: Annealing with a hydrogen atmosphere in a continuous furnace is relatively expensive, since hydrogen is a comparatively expensive shielding gas on the one hand, and on the other hand a large amount of it is lost from the continuous furnace due to the continuous process flow with continuously or periodically opened discharge options.

In this regard, it is known from the document mentioned to reduce the leakage losses of protective gas by means of lock-type devices at the furnace inlet and outlet of the system. In nitrogen is continuously introduced into the lock chambers at the inlet and outlet of the continuous furnace, thus further reducing the loss of hydrogen from the furnace.

In addition to the pure hydrogen heat treatment now described, heat treatments and annealing processes are known which work with a protective gas atmosphere generated from ammonia. This atmosphere is generated by cleavage of ammonia (NH₃) and contains a very high proportion of hydrogen with 75 vol% hydrogen and 25 vol% nitrogen. Many heat treatment processes are also carried out with this protective gas and this protective gas production, which in turn requires particularly high amounts of gas in continuous furnaces.

In addition, many heat treatments are known that work on the basis of separate individual gases and in which the user can produce so-called synthetic protective gases of any composition on site. Of these, the heat treatment processes with more than 50% hydrogen content in the protective gas are essentially of importance for the invention in question. In the present context, these processes are to be regarded as processes with hydrogen-rich protective gas atmospheres, and there is also an attempt to keep the losses of protective gas as low as possible.

In particular in the case of processes in which a pure hydrogen atmosphere is sought, a further problem arises in particular again Foreground if locks are arranged on the inlet and outlet sides of the continuous furnace and these are flushed with nitrogen, since nitrogen also penetrates into the main areas of the continuous furnace. The penetration of nitrogen into said zones of such a system may cause the formation of a so-called white dust, which subsequently leads to a reduction in the cooling capacity in the cooling zone due to the blockage of the heat exchangers located there (for this problem, see in detail in the above-mentioned patent application or Stahl und Eisen / 107, March 1987 / No. 6, pages 267 - 273, especially page 271, right column, below). Therefore, with these hydrogen heat treatments there is the problem of either accepting high hydrogen consumption without nitrogen locks or accepting the formation of white dust to an increased extent.

The object of the present invention is therefore to design a method for annealing metal parts under hydrogen gas or hydrogen-rich protective gas in continuous furnaces so that the need for protective gas can be reduced as far as possible without other negative phenomena, such as air entry or the formation of white dust , to cause.

This object is achieved by the features of claim 1. In contrast to the prior art, the gas with the higher density than hydrogen is blown directly into the openings of the continuous furnace, ie without lock chambers being necessary, and the blowing is carried out in the direction from the interior of the furnace to the outside.

(A = Ofenkonstante).This procedure is based on the knowledge that the flow rate Q of a gas through a leak opening from an oven operated with excess pressure Δp depends on the density d of the gas flowing through as follows:

${\text{Q = f (d) = A * (1 /}}_{\text{d}}{\text{* 2 Δp)}}^{\text{1/2}}$

(A = furnace constant).

Since hydrogen-rich atmospheres are gases of low density, there is a very large flow rate for hydrogen and hydrogen-rich gas mixtures and therefore, in the case of heat treatments with such protective gases, large losses of protective gas. If one brings in the vicinity of such leak openings, for example in the cross section of the leak openings, in the sense of the basic idea of the present invention, gases of higher density than hydrogen, for. B. nitrogen, argon or SF₆ and others, the protective gas is mixed in this area with the supplied gas, which increases the density of the resulting gas mixture and the flow rate through the leakage opening correspondingly decreases provided a constant furnace pressure. The result of this is that less protective gas overall leaves the heat treatment system at the same pressure conditions, or there is an increase in pressure in the heat treatment system with the same amount of protective gas added.

This results in a particularly advantageous embodiment of the present invention, namely that the protective gas atmosphere is maintained in terms of composition and pressure by the fact that the addition of protective gas is regulated based on the measurement of the internal furnace pressure and in such a way that a defined pressure curve, for example a constant pressure value, for example between 3 - 5 mbar internal furnace pressure, is maintained.

With this pressure-controlled shielding gas supply, the aim of protecting gas saving according to the invention is achieved in an automated manner, because, as already stated, the furnace pressure rises after the admixing gases have been supplied and the supply quantity of protective gas has remained constant. This supply quantity is now reduced due to the pressure-induced control and thus less protective gas is consumed. The withdrawal of the shielding gas quantity may, however, only extend so far that other parameters important for the shielding gas atmosphere, e.g. the ratio H₂ / H₂O (oxidation equilibrium), should not be undercut. In critical cases, this can be monitored by another sensor and included in the control.

In principle, the addition of the admixing gases directly into the openings and in the direction from the inside of the furnace ensures that practically no admixing gas, in particular no nitrogen, gets into the main areas of the continuous furnace, which limits the formation of white dust.

Figur 1

zeigt das Schema eines Durchlaufofens zum Glühen von Stahlbändern mit erfindungsgemäßen Einrichtungen an Ofenein- und -austritt,

Figur 2

zeigt einen erfindungsgemäßen Ofeneintritt im Detail,

Figur 3

zeigt einen Ofeneintritt mit Führungsrollen.

The method according to the invention is to be explained in more detail below by way of example with reference to the schematic drawings.

Figure 1

shows the diagram of a continuous furnace for annealing steel strips with devices according to the invention at furnace entry and exit,

Figure 2

shows a furnace inlet according to the invention in detail,

Figure 3

shows an oven inlet with guide rollers.

Figure 1 shows a continuous furnace for annealing steel strips. A steel strip 5 is guided through the furnace by means of rollers 6, which successively consists of an inlet area 1, an annealing area 2, a cooling area 3 and an outlet area 4. Between the annealing area 2 and the cooling area 3 there is a protective gas supply line 7, e.g. with a heat treatment with a pure H₂ atmosphere, a hydrogen supply line. Devices 8, 8 ′ for carrying out the method according to the invention are located in the entry area 1 and exit area 4.

In pure hydrogen operation, hydrogen is thus introduced into the continuous furnace via the feed line 7. Compared to operation with ammonia cracked gas or a corresponding synthetic gas mixture of 75% hydrogen and 25% Nitrogen increases the required amount of protective gas to be introduced under otherwise identical conditions solely through the use of pure hydrogen, since pure hydrogen has a lower density than the gas mixture just mentioned. The use of the present method is therefore advantageous in particular in pure hydrogen atmospheres, but also in hydrogen-rich atmospheres.

According to the invention, nitrogen is blown in toward the exterior of the furnace in the area of the essential leak cross-sections through which the protective gas leaves the system. For this purpose, nitrogen feeds 8, 8 'are provided, which are shown in detail in FIG. 2, which represents a section along the line A-A from FIG. 1. Figure 2 shows the inlet area of the continuous furnace with the steel belt 5 schematically in cross section. A tube 9, which is closed at both ends and which is provided with holes 10 on its circumference approximately on a longitudinal line (so-called nozzle assembly), is connected transversely to the nitrogen supply device 8. These bores 10 are directed obliquely downwards with respect to the plane of the drawing, that is to say they are oriented towards the outside of the furnace into the entry gap of the steel strip 5, which represents an essential leak opening. According to FIG. 2, nitrogen is specifically introduced into the column to the side next to the running sheet metal strip 5 and on one side over the entire width of the inlet gap, and the effects desired according to the invention are thus achieved.

In particular, the entry or exit area of a strip annealing furnace can also be designed with guide rollers 21, 22. This is also in one in FIG Cross-sectional top view shown. The sheet metal strip 5 runs between the rollers 21, 22 into the continuous annealing furnace. To seal this arrangement, felt strips 25, 26 are provided between the outer walls 23, 24 and the rollers 21, 22 of this arrangement and the rollers have end plates 27 on their end faces. Both in the gaps 28 formed next to the sheet metal strip and in the areas 29 outside of the end plates 27, admixing gas is blown into the leak cross sections via feed lines 30, 31.

The procedure according to the invention takes place as follows for the overall sequence with regard to the heat treatment to be carried out: The furnace pressure is measured via a pressure sensor 35 installed in the continuous furnace. The admixture of a certain amount of the admixing gas in the leak cross sections causes the pressure in the furnace to rise after the admixture has started. This increase is determined by means of the pressure sensor and the protective gas addition is subsequently reduced by means of a suitably programmed control unit 36, which is coupled to a control valve 37 arranged in the protective gas supply line 7, and the pressure is maintained as desired. Overall, the pressure is expediently set approximately constant at a level to be determined in the control loop method. This means that the amount of protective gas required for the heat treatment is significantly reduced. For example, the need for a hydrogen atmosphere can be reduced below the need for a process with NH₃-cracked gas, in which one works without the inventive or other saving devices. The formation of white dust is also reduced in the case of pure hydrogen atmospheres.

In addition to controlling the pressure of the annealing process via the furnace pressure, it is also possible in particular to control the hydrogen content in the exit gas or the exit speed of the exit gas from the leak opening in question. The amount of admixing gas added is also another variable that can be varied when performing annealing processes.

As a side effect, the process according to the invention additionally increases the safety for processes with hydrogen-rich atmospheres, since the risk of ignition is reduced due to the reduced hydrogen content of the protective gas escaping.

The proposed method is therefore a particularly advantageous embodiment of a heat treatment with hydrogen-rich protective gas, in which substantial savings in treatment gas are achieved through the use of small amounts of a further gas.

Claims (3)

1. A process for annealing metal components in protective gas high in hydrogen in a continuous furnace operated at excess pressure, wherein on one side protective gas is introduced into the furnace apparatus and on the other side protective gas issues from the furnace through apparatus openings, whereas inside the continuous furnace the protective gas atmosphere high in hydrogen is maintained in accordance with the thermal treatment to be carried out, and wherein gas with a higher density than hydrogen is blown into the openings of the continuous furnace at the furnace inlet and outlet, and optionally also the other leakage openings of the furnace, with a direction from the interior of the furnace towards the exterior of the furnace, and thus the gas is admixed precisely with the protective gas present at the leakage openings, and thus precisely at these locations - and only at these locations - the density of the respective protective gas is increased, where the gas mixture formed in this way subsequently flows out, with the result however that the outflowing of protective gas is reduced and the penetration of heavy admixed gas into the interior of the apparatus is likewise prevented.
2. A process as claimed in Claim 1, characterised in that the protective gas atmosphere is maintained in that the addition of protective gas is regulated on the basis of measurement of the furnace inner pressure, and in such manner that a predetermined pressure curve dependent upon operating mode and product is adhered to.
3. A process as claimed in one of Claims 1 to 2, characterised in that sufficient gas at a higher density is blown in to ensure that the proportion of this gas in the outflowing gas amounts to more than 25%.

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