EP1114196A1

EP1114196A1 - Method and device for cleaning metallic surfaces

Info

Publication number: EP1114196A1
Application number: EP99942869A
Authority: EP
Inventors: Hans-Peter Schmidt; Peter Zylla
Original assignee: Messer Griesheim GmbH
Current assignee: Air Liquide Deutschland GmbH; Messer Group GmbH
Priority date: 1998-09-07
Filing date: 1999-08-13
Publication date: 2001-07-11
Anticipated expiration: 2019-08-13
Also published as: YU39999A; PL346466A1; YU49428B; PL193048B1; EP1114196B2; WO2000014289A1; ATE217029T1; DE59901364D1; DE19840778A1; EP1114196B1

Abstract

When metallic parts are annealed in an inert low-hydrogen atmosphere, carbon deposits often remain on the surface of said parts. The inventive method and device allow for controlled dampening of the dry inert low-hydrogen atmosphere and transformation of carbon deposits into volatile carbon oxides. Said method and device provide a soot-free annealing process and substantially prevent any mass transfer with the material.

Description

Method and device for cleaning metal surfaces

The invention relates to a method and a device for cleaning metal surfaces for stationary batch processes in stationary furnace systems and for transient continuous processes in transient furnace systems under low-hydrogen protective gas atmospheres with up to 30% by volume hydrogen.

In many heat treatment processes of metals, special requirements are placed on the composition of the gas atmosphere surrounding the feed in order to avoid chemical reactions with the metal parts to be treated or to bring about them in a targeted manner. These processes take place in furnace systems that enable heating and holding to a certain temperature and subsequent cooling, whereby a distinction is made here between stationary furnace systems for carrying out stationary batch processes and unsteady furnace systems for transient continuous processes.

With the help of technical gases such as nitrogen, argon and hydrogen or their gas mixtures, a relatively high and uniform product quality of the heat-treated metal parts can be achieved. Nevertheless, surface defects often occur during annealing in the form of remaining drawing media, rolling media or other residues that originate from pretreatment processes. Most of these residues are carbon residues distributed in various forms on the surface. Costly post-cleaning, e.g. Pickling or brushing is often necessary after the heat treatment process, since the demands on the quality of the treated surfaces are high.

For stationary batch processes, processes are known which allow sufficient surface cleaning directly in the furnace. For example, in DE 39 21 321 A1 and DE 36 31 551 C1, there is an intermediate stage in the heating phase or a very slow heating and a rinsing process in the heating phase described. However, these methods lead to an extended glow time and a relatively high amount of purge gas.

A method is known from DE 42 41 746 C1 in which a hydrogen atmosphere is used for cleaning during the holding time. The carbon residues are converted to methane with hydrogen and removed from the furnace intermittently after certain limit values have been reached. Excretion of soot is thus avoided. The application of the process ensures optimal shielding gas consumption and high cleanliness of the surfaces. However, the process is limited to stationary batch processes.

In the case of transient continuous processes, an apparatus and a method are known from EP 0 572 780 A2 in which metal parts, preferably metal strips, are cleaned in a cleaning chamber as a preliminary stage of the annealing process. The metal strip surface is acted upon in the cold state with hydrogen-rich hot gas mixtures with a hydrogen content of 30-70 vol.% By impingement flow. In a few seconds, the oil residues are evaporated and discharged from the cleaning chamber. The belt then goes into a continuous furnace, where it is heat treated. The strip is unwound from the coil and goes through a subsequent furnace.

The technical gases used have a high purity, typically a purity of 99.99 vol.%, So that their moisture or residual oxygen content is very low. This high level of purity ensures a relatively constant quality and reliability of the processes and products. This is because contamination of these gases, such as oxygen, carbon dioxide or water vapor, can lead to uncontrolled oxidation reactions that have a negative impact on the quality of the treated surfaces.

The invention has for its object to provide a method which a cleaning with the help of a low-hydrogen protective gas atmosphere during enables the heat treatment process in the holding phase both in stationary furnace systems and in stationary furnace systems and which ensures high surface cleanliness of the heat-treating metal parts, even in the wound state in the form of coils, rolls or coils. Starting from the prior art taken into account in the preamble of claim 1, this object is achieved according to the invention with the features specified in the characterizing part of claim 1. Advantageous developments of the invention are specified in the subclaims.

The term “low-hydrogen protective gas” here means a protective gas with a relatively small proportion, in particular a proportion of less than 30% by volume, preferably less than 5% by volume, of hydrogen, the rest being in particular nitrogen and / or noble gas (s) .

The humidification of the protective gas atmosphere in the furnace initiated by the method according to the invention during the holding phase enables the treated surfaces of the annealing material to be cleaned without causing a mass exchange with the metal parts. For this purpose, a certain amount of water is added to the low-hydrogen protective gas. Depending on the holding temperature, carbon residues are broken down by oxidation. The reaction products of carbon oxidation are volatile and are taken up in the gas phase. This cleaning process is advantageously carried out during the heat treatment in the holding phase and is preferably monitored and regulated.

The reactions described below occur between the dampened protective gas and the surface coverings.

In the heating phase, the adhering, long-chain and unsaturated hydrocarbons are desorbed from the surface into short, volatile fission products: without residue: C _x H _y => z C | H _m with residue: C _x H _y => y C _k H _n + Coke

The term "coke" here means a burned-in, solid covering which essentially contains carbon and oxidic constituents.

This process essentially depends on the melting, gap and boiling temperatures of the rolling lubricants. In practice, the splitting of the substances is observed at temperatures of approx. 400 ° C. Whether the volatile fission products are desorbed from the surface without or with residues essentially depends on the amount of drawing or rolling agents, the surface area treated and the heating rate. With large quantities and rapid heating speeds, the remaining coke is burned into the metal surface as the annealing continues and can only be removed by pickling or brushing. This has a negative impact on the surface quality.

If they are not removed from the furnace early, the gaseous fission products can decompose to soot and hydrogen on the metal surface during further heating and cause further contamination:

CιH _m => Cι (R _Uß ) + m / 2 H ₂

When using dry, technical gases (e.g. hydrogen) in the

Holding time, the surfaces are cleaned by methane formation, as described in DE 42 41 746 C1:

'Coke + 2 H ₂ => CH ₄

The reaction rate is relatively slow here and the absorption capacity of carbon in the gas atmosphere is relatively large. In contrast, when using nitrogen / hydrogen gas mixtures, the absorption capacity decreases significantly with increasing temperature and decreasing hydrogen content. If a nitrogen / hydrogen gas mixture with a proportion of 5% by volume of hydrogen is heated to 700 ° C, for example, a maximum methane content of only 0.034% by volume can be achieved, which is about 320 in comparison with a 100% hydrogen atmosphere times less.

The absorption capacity of carbon in a low-hydrogen protective gas atmosphere is increased by the addition of water vapor to the nitrogen / hydrogen gas mixture according to the invention. This water vapor can effectively remove the burned-in carbon residues.

The protective gas is moistened in a defined manner before or after reaching the holding temperature. The water supplied is fed in in such quantities (e.g. via a lance) that iron oxidation of the treated material does not occur and a conversion of the coke to volatile carbon oxides is initiated. For this reason, monitoring the process is advantageous. The control of the water feed by means of an oxygen probe, e.g. λ probe, preferred.

The surface is cleaned using the following reaction:

H ₂ 0 + C _C oe => CO + H ₂

The carbon coating is converted to volatile carbon monoxide and hydrogen with water vapor. The high holding temperature favors the process of cleaning initiated in this way.

In parallel to the deposit degradation, the water vapor reacts with the volatile hydrocarbons:

C | H _m + x H ₂ 0 => I CO + m / 2 H ₂ + x H ₂ The carbon monoxide formed is further oxidized to carbon dioxide:

I CO + y H ₂ 0 => I C0 ₂ + y H ₂

The amounts of carbon monoxide and carbon dioxide formed are determined by the temperature dependence of the water gas reaction.

This cleaning process is supported by the reaction of the formed carbon dioxide with Coke to carbon monoxide:

'Coke + C0 ₂ 2 CO

Due to the course of the reaction, the water vapor is consumed during cleaning. Therefore, the atmosphere is favorably moistened by new water injections. The amount of water essentially depends on the hydrogen concentration in the protective gas, the material being treated and the free volume of the annealing furnace. It is determined by the oxygen partial pressure or the ratio of the corresponding partial pressures (P _H2O / P _H2 ), the limit values being chosen so that the formation of iron oxides does not occur. Such oxidation of the metal surface is undesirable. It is advantageously avoided by regulating the protective gas composition in order to control the process sequence, preferably in every time period of the process.

An oxygen probe, for example a zirconium dioxide solid-state electrolyte cell or a lambda probe, is advantageously used to measure the oxygen partial pressure or the ratio of the corresponding partial pressures (P _H2O / P _H2 ). With an appropriate control device, the atmosphere is then set as a function of the material being treated, depending on the measured value, so that an oxide-free treatment of the

Material is guaranteed. This is done, for example, by switching the water supply on / off and, if necessary, by a carbon-neutral treatment Appropriate control of the protective gas purging This procedure is particularly advantageous for stationary furnace systems.

The probe voltage of the oxygen probe depends on the furnace temperature and the P _H20 / P _H2 ratio of the furnace gas, as shown in Fig. 1 Depending on the holding temperature, the atmosphere in the furnace is controlled so that a certain probe voltage is kept constant, so that an optimal cleaning the surface can be done. The probe voltage can vary within a certain measuring range without affecting the cleaning effect. The optimal range is indicated by lines b for P _H2O / P _H2 = 0.100 and c for P _H2O / P _H2 = 0.150 when using an oxygen probe arranged in the furnace chamber, and by lines d for P _H2O P _H2 = 0.100 and e for P _H2O / P _H2 = 0.150 when using an external lambda probe located outside the furnace chamber. Line a limits the overall control range upwards and represents a dry gas mixture for P _H2O / P _H2 = 0.015. Within these limits, depending on the application, the furnace gas composition that is set can be regulated and thus the surfaces cleaned, so that none Iron oxidation and / or water condensation occurs in cold areas of the plant, the P _H2 o / P _H2 ratio should be less than 0.15

The stability of the iron oxides depends on the temperature and the ratio of the partial pressures of water vapor to hydrogen. Magnetite (Fe ₃ 0 ₄ ) formation occurs below 560 ° C. and oxidation to FeO occurs above this temperature. For annealing under nitrogen-hydrogen gas mixtures, a P _H2 o / P _H2 ratio of 0.10 has proven to be favorable. For example, if a low-alloy steel is treated with a gas mixture of nitrogen and 5 vol% hydrogen, the water vapor content is reduced to 0. 5 vol% is set, which corresponds to a dew point of the protective gas atmosphere of -2 ° C. This dew point is advantageously measured with the help of internal or external measuring cells and regulated by controlling appropriate valves, for example solenoid valves, so that no water condenses at the cold points The method according to the invention and the device according to the invention for carrying out the method will now be explained in more detail by way of example with reference to an illustration (FIG.) And one exemplary embodiment.

Fig. 2 shows schematically an apparatus for performing the method with a regulated water injection into the furnace.

The device shown in FIG. 2 has a furnace 1 in which a water injector evaporator 2, a gas injection pipe 3 for protective gas and an oxygen measuring probe 4 are arranged. The signal measured in the oxygen measuring probe 4 reaches a control unit 5, in which the current measured value (actual value) in the furnace 1 is continuously compared with a target value (target value) in the holding phase. The oxygen partial pressure or the (P _H2 o / PH ₂ ) ratio can be used as the actual value. If the actual value deviates from the target value, the control unit 5 controls

Solenoid valves 6a and 6b, which are arranged in the water injector evaporator 2 and to which water is supplied from a water reservoir 8 via a line 7. The water is preferably metered in at intervals of preferably about 5 minutes. The water vapor forming in the interior of the furnace 1 is evenly distributed in this way by circulation fans of the furnace 1 in the gas atmosphere of the furnace 1. Further injections are carried out until the actual value and the target value match. The amount of water that is sprayed in per injection is measured by a flow measuring device 9 and adjusted via a control device 17, preferably a valve. The timing of the injection is set by a timer on the control unit 5. After reaching the target value in the furnace 1, the water injection into the protective gas atmosphere is interrupted by closing the solenoid valves 6a and 6b. At the same time, the time signal for clocking the injection is interrupted. For safety reasons, the water injector evaporator 2 is equipped here with two solenoid valves 6a and 6b, which are arranged one behind the other. The protective gas atmosphere is created by supplying a nitrogen and hydrogen-containing gas mixture set. Nitrogen is removed from a storage container 10, set to ambient temperature, for example through the air evaporator 11 shown here and fed via a line 12 to a mixing device 13, which is also supplied with hydrogen from a storage container 15 via a line 14. The gas mixture from the mixing device 13 is fed to the protective gas injection pipe 3 via a line 16, the protective gas supply being controlled with the aid of devices according to the prior art.

example

A nitrogen / hydrogen mixture containing 5% by volume of hydrogen was humidified at 700 ° C. to a dew point of -2 ° C. With a constant P _H2 o / P _H2 ratio of 0.10, an equilibrium composition of the protective gas atmosphere was established, which approx. 12.24 vol.% H ₂ , 1, 22 vol.% H ₂ 0, 6.74 vol % CO, 0.50% by volume CO ₂ , 0.20% by volume CH ₄ , balance N ₂ contained. The measured values of the probes used were -1110 mV for the lambda probe and -1071 mV (H ₂ 0 / H ₂ ) for the oxygen probe. The partial pressure ratio P _H2O / P _H was 0.10.

The sum of the carbon-containing component Cx = (% CO +% C0 ₂ +% CH ₄ ) of the moist gas mixture which is in chemical equilibrium is 7.44% and is therefore about 220 times larger than a dry gas mixture. The factor of 220 shows the strong influence of humidification on the cleaning properties of low-hydrogen protective gas atmospheres. The carbon uptake here is almost comparable to that in a pure hydrogen atmosphere. This applies in particular to gas mixtures with a low hydrogen content, below 30% by volume, preferably below 5% by volume. With increasing H ₂ fractions, this factor decreases and for pure hydrogen with 1.5% moisture it is comparable to cleaning via methane formation. The addition of the heterogeneous and homogeneous water gas reaction results in the so-called Boudouard reaction:

2 CO C0 ₂ + C carbon black

As the temperature drops in the cooling phase, the equilibrium shifts in the direction of soot formation. This results in a strong increase in C0 ₂ on the one hand and a very strong soot separation in the furnace chamber on the other. The C0 ₂ formation below 500 ° C leads to the oxidation of the surface, which can no longer be undone. The soot separation leads to heavy surface pollution. Both can be avoided by a complete exchange of the atmosphere according to the invention at the end of the holding phase, for example by flushing with a dry N ₂ / H ₂ gas mixture. This ensures a clean surface during the cooling phase. This is because the humidified protective gas, which contains the cleaning products it contains, is removed from the oven and replaced by a dry gas mixture. The flushing process can advantageously be programmed into the existing furnace control of the stationary system.

In continuous furnaces for the heat treatment of annealing material, an atmosphere exchange takes place continuously. Therefore, it makes sense to choose the gas feed so that the entire amount of shielding gas flows continuously in countercurrent with the treated annealing material over the entire system, which also means over the cooling section. The humidification is only carried out in the actual kiln section, that is, in the hottest part of the system, and monitored there by a control unit. A sufficient amount of gas flowing from the cooling section against the annealing material ensures that the cooling section is kept free of the carbon oxides taken up in the hot part. In this way, soot formation is avoided in this colder part of the plant. The protective gas therefore leaves the furnace system essentially via the inlet area.

Claims

claims

1. Process for cleaning metal surfaces for stationary batch processes in stationary furnace systems (1) and for transient continuous processes in transient furnace systems (1) under low-hydrogen protective gas atmospheres with the phases of heating, holding and cooling, characterized in that in the holding phase of the protective gas atmosphere water or Steam is supplied for the oxidation of carbon residues.

2. The method according to claim 1, characterized in that the supply of water or steam is carried out in cycles.

3. The method according to claim 2, characterized in that the supply of water or steam takes place in a cycle of 2 to 10 minutes.

4. The method according to any one of claims 1 to 3, characterized in that the oxygen partial pressure of the furnace atmosphere is measured continuously and the supply of water or steam is controlled depending on the measured values.

5. The method according to any one of claims 1 to 4, characterized in that the oxygen partial pressure of the furnace atmosphere is measured continuously and the supply of protective gas in the furnace system (1) is controlled depending on the measured values.

6. The method according to any one of claims 1 to 5, characterized in that the furnace atmosphere is adjusted so that the partial pressure ratio (PH2O / PH2) is in a range between 0, 10 to 0, 15.

7. The method according to any one of claims 1 to 6, characterized in that a low-hydrogen protective gas is supplied to the furnace system (1) at least once during the cooling phase.

8. The method according to any one of claims 1 to 7, characterized in that in unsteady furnace systems (1) water or steam is supplied to the hottest part of the furnace system (1).

9. The method according to any one of claims 1 to 8, characterized in that in unsteady furnace systems (1) the protective gas in the cooling section of the furnace system (1) is fed in countercurrent with the metal material and the furnace system (1) in the entrance area for the Metal goods are discharged again.

10. The method according to any one of claims 1 to 9, characterized in that in stationary furnace systems (1) during the holding phase, the low-hydrogen protective gas is led away at least once, preferably at the end of the holding phase, from the furnace system (1) and fresh protective gas into the furnace system (1) is supplied.

1 1. Device for cleaning metal surfaces for stationary

Batch processes in stationary furnace systems (1) and for unsteady continuous processes in unsteady furnace systems (1) under low-hydrogen protective gas atmospheres with the heating, holding and cooling phases, characterized in that the furnace system (1) has at least one injection pipe (2) for the

Supply of water or water vapor into the protective gas atmosphere.

12. The apparatus according to claim 11, characterized in that at least one oxygen probe (4) or a lambda probe is arranged in the furnace system for the continuous measurement of the oxygen partial pressure of the furnace atmosphere and that the oxygen probe (4) or lambda probe has at least one control device (5 ) is assigned to control the supply of water or water vapor and, if necessary, control the supply of protective gas as a function of the measured values of the oxygen probe or lambda probe.

13. The apparatus according to claim 11 or 2, characterized in that in unsteady furnace systems (1) water or steam via a protective gas / water injection tube (3) in the hottest part of the furnace system (1) is supplied.

14. The apparatus according to claim 1 1 or 12, characterized in that in stationary furnace systems (1) water or steam is supplied via an injection evaporator (2) of the furnace system (1).

15. Use of a method according to one of claims 1 to 10 or a device according to one of claims 11 to 14, characterized in that the low-hydrogen protective gas atmosphere has a content less than 30th

Vol .-%, preferably less than 5 vol .-%, of hydrogen and a residual portion consisting essentially of nitrogen and / or noble gas (s).