EP0572780A2 - Process and device for cleaning of metal strip surfaces by gas-washing in hydrogen-rich athmospheres - Google Patents

Abstract

The invention relates to a process and to a device for thermal cleaning of metal strip surfaces during heating in continuous annealing lines, in particular for removing oil-containing coatings, while dispensing with alkaline and acidic cleaning solutions, chlorinated hydrocarbons, caustic alkalis and the like. The oil film is caused to evaporate by blowing with a hot hydrogen-nitrogen gas mixture, and the oil vapours are converted to CO and CO2 by stoichiometrically adequate feeding of steam, so that the carburisation of the low-carbon annealing material, otherwise unavoidable in view of the existing high carbon potential, is avoided. By means of a circulation fan and of nozzles directed onto the strip surface, an impact flow is generated under which the vapour pressure of the oil film increases proportionally to the heating rate and the evaporating hydrocarbon compounds react with the steam fed in accordance with the heterogeneous and homogeneous water gas equilibrium. In this case, the circumstance is exploited that the blanketing gas temperature is already very high, but the temperature of the strip running in is still comparatively low. Because of this, and owing to the short residence time of the strip in the cleaning section, oxidation reactions on the strip surface and diffusion processes in the strip material are not yet activated. The evaporating oil film, whose maximum boiling point is not reached until just before the exit from the cleaning section, provides additional protection. The boiling curve and the distribution of the evaporation rate over the temperature are known, so that strip heating, blanketing gas throughput and circulation can be matched to the boiling properties of the oil residue. Pretreatment of the strip in tristeam degreasing units or electrolytic degreasing units or other precautions against carburisation of the annealing material can be dispensed with; an appropriate blanketing gas circulation ensures that no carbon epitaxy occurs in the furnace chamber. The process is designed for the lowest possible pollution of the environment and is particularly suitable for the continuous heat treatment of thin strip cross-sections of ferrous materials such as fine sheet and very fine sheet, low-carbon grain-oriented and non-grain-oriented magnetic steel sheets, stainless steel sheets, strip of iron/nickel alloys and of non-ferrous metals such as alloys of titanium, copper, zinc and aluminium, whenever residues of fabrication aids such as oil and emulsion must be removed before annealing. After the evaporation of the oil film, pigments such as abraded metal and dust are blown off from the strip surface by the gas jet, and this counteracts the formation of pimples on the furnace support rollers and the formation of stratches on the underside of the strip. <IMAGE>

Description

The invention relates to a method and a device for cleaning metal strip surfaces, in particular for removing oil-containing deposits when the strip is heated in continuous annealing lines, and with the further generic features mentioned in claim 1.

For example, there is usually a wafer-thin, greasy coating of about 0.1 to 1 µm thick from residues of the rolling emulsion or rolling oil, mixed with a small amount of iron abrasion in a covering density of between 5 and 50 mg Fe per m² surface, on cold-rolled thin sheet , and other solids - mainly iron oxides, but also oxides of the alloying elements. A typical composition of the residues after rolling can be given as follows: Covering in g / m² Percentage ownership % Oil residue 0.387 89.3 Solids 0.0463 10.7 of which iron 0.0138 3,185

The oil residue can also contain dissolved iron, depending on the alkalinity of the oil - 0.05 to 0.5% can be found.

If a grease oil with rust protection function is applied to the freshly pickled strip before rolling and this oil contains saponifiable constituents and polar substances, residues of the oil film can be found even after rolling, due to its anchoring to the strip surface, the saponifiable constituents dominating in the oil and the free fatty acids are approximately twice as rich as fresh oil.

In the pressure and temperature field of the roll gap, the ester compounds hydrolyzed to fatty acids from the roll emulsion in the event of excess water can form metal soaps (carboxylates) with the metal hydroxide on the strip surface. This process only begins at strip temperatures above 100 ° C; iron and manganese tend to form metal soaps, but not copper and nickel. There is also no evidence of metal soap formation on stainless steel strips (chrome / nickel / iron).

During the subsequent heat treatment of the cold-rolled strip, carboxylates are very difficult to distill off and leave dark shades. According to current knowledge, they are the main cause of carbon residues on the belt. When the tape is heated, the volatile paraffinic mineral oil components evaporate between 100 and 300 ° C; their thermal stability decreases sharply with chain length. This is followed by naphthenic and aromatic hydrocarbons, at around 400 ° C only the higher-boiling esters are detectable.

The surface condition of cold-rolled strip before and after annealing and before and after cleaning has been extensively described in the specialist literature. There is also sufficient evidence that the amount of solid increases with the amount of oil on the strip, and that the amount of metal pigments on the annealed strip increases with the amount of residual carbon. The oil residues that evaporate during heating can condense with soot separation in the furnace chamber, preferably on the furnace walls in the inlet part, but also on the jet pipes. Polymorphic carbon (soot and graphite) favors the build-up of pimples on the oven support rollers. Pimples leave damage in the form of impressions and scratches on the underside of the belt and therefore lead to quality devaluations and scrap. Another negative side effect is the carburization of the strip surface, which must be avoided in view of the susceptibility to aging of such materials, particularly in the case of materials with the lowest carbon contents, such as alloyed electrical steel.

Continuous annealing lines are necessary upstream of belt cleaning systems to remove the oil-containing residues on the strip surface. Processes such as spray cleaning with strongly alkaline cleaning solutions, steam or spray degreasing using trichlorethylene, the various electrolytic cleaning processes and combined processes that work in the steam and liquid phases are known. One or more brushing devices are installed in almost all systems. On the other hand, the spraying power can be increased so that the cleaning is only effected by the high pressure.

Alkaline cleaning solutions contain organic solvents such as trichlorethylene or perchlorethylene, and inorganic solvents such as soda, caustic alkalis, as well as complexing agents, silicates, pyrophosphates, tripolyphosphates, and surface-active substances, mostly low-foam biodegradable surfactants.

The choice of cleaning method depends on the desired surface condition of the strip before it is processed further, for example whether it should be painted, hot-dip galvanized, chrome-plated or otherwise surface-coated after cleaning, or whether it should be subjected to an annealing treatment, the result of which determined Material properties are expected - as in the case of electrical sheet, for example, minimal magnetic loss and high polarization with minimal aging.

The preparation of the solvents enriched with the oily residue is critical in all processes, whether immersion, spraying, brushing or electrolytic processes. First, the residue must be separated from the solvent in complex process steps. Only then can it be disposed of appropriately.

In addition to the disposal problems and the high operating costs of belt cleaning systems, it is disadvantageous that adequate degreasing of the belt surface and complete removal of the mechanically adhering abrasion particles is not always possible. It is known, for example, that excellent degreasing is achieved with trichlorethylene, but the removal of the solid particles adhering to the surface is inadequate. Conversely, spraying systems with brushing devices mainly remove solid particles, but do not completely degrease, which, for example, impairs the decarburization process in the subsequent heat treatment.

Other methods do not work more reliably either; during electrolytic cleaning, the adsorption of silicates on the belt surface interferes, even with repeated cleaning no more favorable result can be achieved.

This experience often forces the use of combined cleaning processes with correspondingly higher system, operating and maintenance costs. But even with this, the complete removal of manufacturing aids and abrasion on metal strip surfaces cannot always be achieved; the efficiency of the process usually falls short of expectations. Measured in terms of the initial occupancy density, cleaning is considered above average if the remaining occupancy reaches a rate of less than ten percent.

Particularly clean surfaces are achieved with annealing in a pure hydrogen atmosphere. Representative of the numerous publications that describe the advantages of bell annealing under hydrogen are Hubert, G. and R. Maass: Journal "Gas warmer international". Volume 38 (1989), No. 9, pages 515 to 522. It was recognized that hydrogen influences the boiling behavior of a rolling oil in that the boiling maxima are shifted to lower temperatures.

A method for annealing thin steel sheet with a thickness of less than 0.5 mm with oil residues in a protective gas atmosphere containing at least 20% hydrogen is described in European Patent Application 0405092 A 1 (1990). Thereafter, good cleaning quality can be achieved if, when the coil is heated, a temperature of 450 ° C. is only exceeded after a period of 5 hours or longer.

Another method for annealing cold-rolled steel sheet in a non-oxidizing nitrogen / hydrogen protective gas atmosphere is known from JP patent 63255326 (1988). The carbon compounds CO, CO₂, CH₄, C₂H₄, C₂H₆, C₃H₃ and C₃H₆ originating from the thermal cracking of the oil vapors are removed by catalytic treatment of the protective gas.

Finally, in European patent application 0416758 A 1 (1990), a process for the thermal degreasing of aluminum sheet and aluminum parts is presented, which does not require solvents by the parts to be degreased at a temperature between 300 and 400 ° C over a period of 10 to 30 minutes exposed to a reactive atmosphere, preferably air or oxygen. The organic impurities on the sheet should react with the gas atmosphere without attacking the underlying oxide skin in order not to impair the solderability. Based on the analytical investigation of the surface coverings found after various cleaning processes, it is concluded that thermal degreasing in air is a viable alternative to degreasing under vacuum or with solvent vapors. In addition to the global finding that the annealing parameters are tailored to the type of oil and the reactive components of the gas atmosphere, see page 3 of the patent specification, lines 35 and 36, there is no indication as to how this should be done, such as gas temperature and annealing material temperature, boiling curve and the glow time are to be coordinated with each other to make a complete Evaporation of the oil coating and quantitatively ensure the implementation of the oil vapors with the oxidizing atmosphere. The missing regulation explains the relatively high carbon content in the residue, which has been shown to be over 30%, after thermal degreasing has been completed, see page 5 of the patent specification, Table III (A1-8 to A1-10), columns 7 and 14.

All these processes have in common that there is sufficient time for the evaporation of oily deposits on the annealing material, i.e. the annealing time is not a critical variable for the evaporation process. With continuous annealing lines, on the other hand, you are bound to comparatively short times, which result from the length of the heating part and the maximum strip speed. These times are in the range of seconds and require additional measures to achieve a complete evaporation of the oil residue.

The annealing of metal strip in continuous annealing lines differs from the stationary annealing process in bell-type furnaces in particular in that when untreated strip is passed through, suitable measures have to be taken to prevent the otherwise inevitable carbon deposits in the furnace chamber. In order to convert the oil that evaporates when the strip is heated in the furnace into CO and CO₂, an atmosphere that oxidizes against carbon must be set. On the other hand, however, there is a danger that the metal surface will also be oxidized and, in the worst case, even the surface layers may be detected. Sensitive materials in this regard, such as vacuum-degassed, ultra-pure electrical steel grades with the lowest carbon and oxygen contents, can only be annealed under gas atmospheres that have a reducing effect on the metal surface and the alloying elements.

FR-A-2308436 describes a method for removing oil residues on the inner wall of copper pipe. This consists in that the tube is heated to a temperature during the annealing process which is sufficient to generate the vapor pressure required for the evaporation of the oil residues. The resulting oil vapors are removed by flushing gas throughput.

The patents EP 0 069 009 B1 and EP 0 157 708 B1 describe processes for the continuous degreasing of cold-rolled metal strips (before electroplating), which use the non-oxidizing potential of a flame, or stoichiometric or even by additional injection of air or oxidizing exhaust gas produce slightly oxidizing conditions, cf. R. Wang: "New process for continuous hot-dip strip galvanizing. Galvanizing Into the Next Century" - First International Conference on Galvanizing in South Africa, 3-6 November, 1986. Publisher: Council for Scientific and Industrial Research, PO Box 395, Pretoria 0001, South Africa, Vol. 8801-72-0049, 13 pages, on the other R. Wang: "Direct-fired heating in continous hot-dip galvanizing lines", Verlag Stahleisen mbH, Düsseldorf (1990), pages 75-87, ISBN 3-514-00416-1.

The essential features of these processes are not suitable for solving the technical problem which consists in the fact that not only the oil-containing residues have to be removed from the belt, but also that the belt is recooled by carbon which deposits, as is the case with the substoichiometric conversion of the oil vapors would inevitably occur, but can occur even under slightly oxidizing conditions in the specified temperature ranges, must be prevented. The disadvantage of the above methods can only be overcome by one Technical solution to be eliminated, which ensures that at least as much reactive oxygen is available for the conversion of the oil vapors both when they are formed directly above the strip surface and when they are mixed with the gas atmosphere as for the complete oxidation of the carbon from the Oil residue is needed.

bzw.

ergeben, sind schädlich.Sub-stoichiometric ratios with regard to the carbon conversion to CO and Co₂, as they result from the band and gas temperature ranges specified for the processes and also the prescribed fluctuation ranges of the hydrogen and steam supply (H₂ between 2 and 30%, H₂O between 5 and 30%) and the resulting partial pressure ratios

respectively.

are harmful.

It has also been shown that a clean belt that meets high quality requirements can only be achieved (if pre-cleaning should be avoided) if appropriate technical means are used to ensure an intensive exchange of the reaction gas on the belt surface and sufficient swirling of the gas atmosphere .

According to the invention, such a technical means is offered by the impingement flow, which is generated by blowers, pumps and the like, and slot or round nozzles. This is characterized in that a gas jet emerging from a nozzle with a high outflow velocity strikes the strip surface vertically or with the jet axis slightly inclined to the normal. The turbulent free jet hitting the belt turns into a wall parallel when it hits it Flow redirected. Because of the high gas velocities, much higher heat transfers can be achieved with the impingement flow than with a gas flowing along the material to be heated. The characteristics of the impingement flow have been sufficiently described in the specialist literature; R. Gardon, JC Akfirat: "The rate of turbulence in determining the heat-transfer characteristics of impinging jets", Int. Journal Heat Mass Transfer, Vol. 8 (1965), pp. 1261-1272.

A decisive disadvantage of the methods described in the abovementioned patents is that there is no indication whatsoever for the measurement of the protective gas throughput as a function of the strip temperature and gas temperature and the gas composition, so that economic operation is questioned. In particular, there is no reference to the evaporation behavior of the oil residues on the belt.

As is known, the differentiation of the boiling curve over time gives the course of the evaporation rate, which is the mass fraction of the oil evaporating per unit of time over temperature. At certain boiling temperatures that characterize the oil type, a maximum or several maxima of the evaporation rate can occur. This leads to the idea of coordinating the supply of protective gas and the oxidation potential of the gas with the evaporation behavior of the oil residue at the characteristic boiling temperatures. None of this is mentioned in the specialist literature and in the cited patents.

Finally, it should be noted that the belt entering the cleaning section is exposed to an atmosphere that oxidizes not only carbon but also iron. The negative effects However, they remain tolerably small, as long as the belt stays in the cleaning section only for a short time and the belt surface is protected for as long as possible by the oil film, which has not yet completely evaporated - provided the temperature is appropriate. Under these conditions, the oxidation is of no importance kinetically.

An essential prerequisite for the applicability of such a process and the required process reliability is knowledge of the technological fundamentals and the process parameters in their mutual dependency, for example the temperature gradient under the gas jet in its influence on the vapor pressure of the oil coating on the belt surface. The relevant technical and patent literature does not provide specific technological knowledge for the design of the process.

Overall, the attracted patent specifications and the relevant specialist literature do not convey any teaching that enables the person skilled in the art to use the cleaning processes described above without further or after obvious changes to the continuous annealing process. It is therefore an object of the invention to provide a cleaning method which takes into account the special conditions of the continuous annealing of metal strip, with which oil-containing deposits on the strip surface are available reliably and without in the period of time available, i.e. within a few seconds, without organic and inorganic solvents disruptive side effects such as carbon deposits in the furnace and carburization or oxidation of the annealing material can be removed. The costs for operating upstream cleaning systems using volatile or caustic solvents such as trichlorethylene or soda, and the associated environmental pollution as well as the toxicological hazard to people working on the system and in its surroundings should be eliminated.

This object is achieved according to the invention by a cleaning process integrated in the continuous annealing process with the features mentioned in the characterizing part of claims 1 and 2. This has a particular advantage that the band in the area of the impingement is heated up quickly by superimposing heat radiation and convection to a temperature (T₁) which corresponds to the boiling point of the oil residue, which in connection with the proportional to the heating rate (Ṫ ) increasing vapor pressure (p _D ) allow the thermal conditions for a complete vaporization of even polar adhering oil components to be set. At the same time, the nozzles directed to the belt produce a blow-off effect, with it proving to be an additional advantage that the pressure energy (p _g ) of the gas jet via the outflow speed (u _d ) and thus via the adjustable blower pressure (p 1) to the degree of surface contamination can be adjusted.

Die Temperatur (T₁) des Bandes unter dem Gasstrahl errechnet sich bei gegebener Temperatur (T₀), mit der das Band in den Bereich der Prallströmung eintritt, und gegebener Schutzgastemperatur (T_g), als Funktion des Wärmeübergangskoeffizienten (α_d), der halben Breite (R) der vom auftreffenden Gasstrahl erfaßten Fläche, sowie der Dichte (ρ), Dicke (h), Geschwindigkeit (u) und Wärmekapazität (c) des Bandes gemäß:

Letzteres stellt die Lösung einer Differentialgleichung dar, die sich aus der ortsfesten Betrachtung des Wärmeüberganges und des Wärmetransportes an einem Streifenelement des mit der Geschwindigkeit (u) unter dem Gasstrahl bewegten Bandes ableitet. Sie gilt unter der Voraussetzung, daß der Wärmeentzug infolge Verdampfung des Ölfilms und die Wärmeleitung in Bandlaufrichtung vernachlässigt werden können; Bedingungen, die für den Fall eines sehr dünnen Ölfilms und für dünne Bänder mit vergleichsweise großen Breiten zutreffen. Diese Bedingungen sind für das Glühgut in kontinuierlichen Glühlinien im allgemeinen erfüllt: Ölfilmdicke zu Banddicke etwa 10⁻³, Banddicke zu Bandbreite etwa 5·10⁻³ bis 5·10⁻⁴.The advantage of being able to influence the evaporation process by heating the strip surface under the gas jet is based on the fact that the strip temperature (T₁) and the evaporation rate (Φ) of the oil residue are linked via the boiling curve, and on the observation that the evaporation rate ( φ) is normally distributed over the temperature (T):

The temperature (T₁) of the strip under the gas jet is calculated at a given temperature (T₀) with which the strip enters the area of the impingement flow and given protective gas temperature (T _g ) as a function of the heat transfer coefficient (α _d ), half the width (R) the area covered by the impinging gas jet, as well as the density (ρ), thickness (h), speed (u) and heat capacity (c) of the strip according to:

The latter represents the solution of a differential equation, which is derived from the fixed consideration of the heat transfer and the heat transport on a strip element of the belt moving at speed (u) under the gas jet. It applies provided that the heat removal due to evaporation of the oil film and the heat conduction in the direction of the belt run can be neglected; Conditions that apply in the case of a very thin oil film and for thin strips with comparatively large widths. These conditions are generally met for the annealing material in continuous annealing lines: oil film thickness to strip thickness about 10⁻³, strip thickness to bandwidth about 5 · 10⁻³ to 5 · 10⁻⁴.

It has been shown that the temperature calculation according to the above equation can be used particularly advantageously for strips in the thickness range below 3 mm; the temperature deviation between the surface of the strip and the middle of the strip cross-section is in the range of tenths of a Kelvin.

Ein gutes Reinigungsergebnis stellt sich vor allem dann ein, wenn die Siedetemperatur (T_s), die das Maximum des pro Kelvin Temperaturerhöhung abdampfenden Masseanteils des Ölrückstandes (φ_s) kennzeichnet, unter dem Gasstrahl, d.h. nach Eintritt des Bandes in den Bereich der Prallströmung, erreicht und vor Verlassen dieses Bereiches soweit überschritten wird, daß der Abdampfvorgang beendet ist.Make sure that the belt in the cleaning section reaches the temperature (T _e ), because the oil residue should evaporate completely within the temperature limits (T _a ) and (T _e ). This requirement is met if the evaporation rate (Φ) assumes a numerical value close to or equal to one:

A good cleaning result arises especially when the boiling temperature (T _s ), which characterizes the maximum of the mass of oil residue (φ _s ) evaporating per Kelvin temperature increase, under the gas jet, ie after the belt enters the area of the impingement flow, reached and exceeded before leaving this area that the evaporation process has ended.

Für die Steuerung des Abdampf- und Abblasvorganges lassen sich vorteilhaft die folgenden Zusammenhänge nutzen:

Die Druckenergie des Gasstrahls wächst mit dem Quadrat der Ausströmgeschwindigkeit (u_d). Von Vorteil ist, daß nach den Gesetzen der Prallströmung zugleich auch der mittlere Wärmeübergangskoeffizient (α_d) und mithin die Bandtemperatur (T₁) ansteigt, gemäß:

Somit wirkt sich die Änderung des Gebläsedrucks sowohl auf die Abdampfquote wie auf den Abblaseffekt aus.The rows of nozzles are positioned within the cleaning part, hence T₀> T _a and T₁ <T _e . The temperature (T₁) at which the boiling point occurs is defined using an empirically determined, substance-specific numerical value of the oil residue (γ):

The following relationships can advantageously be used to control the evaporation and blow-off process:

The pressure energy of the gas jet increases with the square of the outflow speed (u _d ). It is advantageous that according to the laws of the impingement flow, the mean heat transfer coefficient (α _d ) and thus the strip temperature (T₁) also increase, according to:

The change in fan pressure thus affects both the evaporation rate and the blow-off effect.

Φ_g

ist der im Reinigungsteil unter dem Gasstrahl bei Temperaturerhöhung von T₀ auf T₁ abdampfende Masseanteil des Ölrückstandes;

φ_s

ist die maximale Abdampfrate des Ölrückstandes pro Kelvin Temperaturzunahme;

T

ist die unter dem Gasstrahl von T₀ auf T₁ ansteigende Bandtemperatur;

T_s

ist die Siedetemperatur, bei der das Maximum φ_s des pro Kelvin Temperaturerhöhung abdampfenden Masseanteils auftritt;

T_a, T_e

sind die Anfangstemperatur bzw. Endtemperatur des Bandes im Reinigungsteil;

γ

ist ein stoffspezifischer Zahlenwert des Ölrückstandes, aus dem sich die Temperatur errechnet, bei der der Abdampfvorgang beendet ist;

P_D

ist die Dampfdruckänderung des Ölrückstandes;

Ṫ

ist der Anstieg der Bandtemperatur pro Zeiteinheit unter dem Gasstrahl;

u

ist die Bandgeschwindigkeit unter dem Gasstrahl;

R

ist die halbe Breite der vom Gasstrahl auf dem Band erfaßten Fläche;

α_d

ist der mittlere effektive Wärmeübergangskoeffizient in dem vom Gasstrahl erfaßten Bereich der Bandoberfläche;

ρ

ist die Dichte des Bandwerkstoffes;

ρ_g

ist die Dichte des Gasgemisches bei der Gastemperatur T_g;

p_g

ist die Druckenergie des Gasstrahls;

c

ist die spezifische Wärmekapazität des Bandwerkstoffes;

c_g

ist die spezifische Wärmekapazität des Gasgemisches bei der Gastemperatur T_g;

λ_g

ist die Wärmeleitfähigkeit des Gasgemisches bei der Temperatur T_g;

ν_g

ist die kinematische Viskosität des Gasgemisches bei der Temperatur T_g;

d

ist der Düsendurchmesser;

k₀

ist eine Konstante, die sich aus der mittleren Nusseltzahl, der Prandtlzahl, der Reynoldszahl und dem Quotienten aus der vom Gasstrahl erfaßten Bandoberfläche und der Düsenquerschnittsfläche errechnet, gemäß:

k₁, k₂

sind die dazugehörigen Exponenten, für die praktische Rechnung ist k₁ = 0,42 und k₂ = 0,707 einzusetzen.

The symbols - letters and indices - used in the above equations have the following meanings:

Φ _g

is the mass portion of the oil residue evaporating in the cleaning section under the gas jet when the temperature rises from T₀ to T₁;

φ _p

is the maximum evaporation rate of the oil residue per Kelvin temperature increase;

T

is the band temperature rising under the gas jet from T₀ to T₁;

T _s

is the boiling temperature at which the maximum φ _{s of} the mass fraction evaporating per Kelvin temperature increase occurs;

T _a , T _e

are the start or end temperature of the belt in the cleaning section;

γ

is a substance-specific numerical value of the oil residue from which the temperature is calculated at which the evaporation process has ended;

P _D

is the change in vapor pressure of the oil residue;

Ṫ

is the increase in strip temperature per unit time under the gas jet;

u

is the belt speed under the gas jet;

R

is half the width of the area covered by the gas jet on the belt;

α _d

is the mean effective heat transfer coefficient in the area of the strip surface covered by the gas jet;

ρ

is the density of the strip material;

ρ _g

is the density of the gas mixture at the gas temperature T _g ;

p _g

is the pressure energy of the gas jet;

c

is the specific heat capacity of the strip material;

c _g

is the specific heat capacity of the gas mixture at the gas temperature T _g ;

λ _g

is the thermal conductivity of the gas mixture at temperature T _g ;

ν _g

is the kinematic viscosity of the gas mixture at temperature T _g ;

d

is the nozzle diameter;

k₀

is a constant which is calculated from the mean Nusselt number, the Prandtl number, the Reynolds number and the quotient from the strip surface covered by the gas jet and the nozzle cross-sectional area, according to:

k₁, k₂

are the associated exponents, for the practical calculation k₁ = 0.42 and k₂ = 0.707 should be used.

Die Gleichgewichtskonstanten der Wassergasreaktionen können dem einschlägigen Fachschrifttum entnommen werden, siehe zum Beispiel D'ANS·LAX, Taschenbuch für Chemiker und Physiker, Springer-Vlg. (1967), Band 1, S. 895, oder W. GUMZ, Kurzes Handbuch d. Brennstoff- u. Feuerungstechnik, Springer-Vlg. (1962). Mit bekannten Schutzgasvolumenanteilen Wasserstoff und Wasserdampf lassen sich die Volumenanteile CO und CO₂ wie folgt berechnen:

Bei bekanntem Kohlenstoffbelag auf dem Band, der für praktische Zwecke hinreichend genau mit der Masse des Ölbelages auf dem Band gleichgesetzt werden kann, läßt sich eine Gleichgewichtsbedingung formulieren, aus der für einen gegebenen Kohlenstoffbelag multipliziert mit der Abdampfquote entweder das erforderliche Mindestoxidationspotential bei gegebenem Gasangebot der Düse (V̇_d) ermittelt werden kann, oder, bei gegebenem Oxidationspotential des Schutzgases das Mindestgasangebot der Düse:

${\dot{\text{V}}}_{\text{d}}{\text{· µ}}_{\text{Ox}}{\text{≧ m}}_{\text{C}}\text{· 2R · u · Φ}$

In diese Bedingung ist definitionsgemäß die Abdampfquote (Φ), das ist der im Reinigungsteil von der Anfangstemperatur (T_a) bis zur Endtemperatur (T_e) des Bandes insgesamt abdampfende Masseanteil des Ölrückstandes, einzusetzen.As is well known, the use of hydrogen-enriched protective gas is advantageous for heat transfer and tape cleanliness. According to the invention, at least as much water vapor is added to it as is required for the conversion of the carbon from the oil residue according to the law of mass action when the heterogeneous and homogeneous water gas equilibrium is established. Given for a protective gas Hydrogen / nitrogen composition and temperature (T _g ) the oxidation potential (µ _Ox ) compared to carbon is calculated from the equilibrium constants of the water gas reactions and the partial pressures or volume fractions of hydrogen and water vapor:

The equilibrium constants of the water gas reactions can be found in the relevant specialist literature, see for example D'ANSLAX, paperback for chemists and physicists, Springer-Vlg. (1967), Volume 1, p. 895, or W. GUMZ, Kurzes Handbuch d. Fuel and Combustion technology, Springer-Vlg. (1962). With known protective gas volume fractions of hydrogen and steam, the volume fractions CO and CO₂ can be calculated as follows:

With a known carbon coating on the belt, which can be equated with the mass of the oil coating on the belt for practical purposes, an equilibrium condition can be formulated, from which for a given carbon coating multiplied by the evaporation rate, either the required minimum oxidation potential for a given gas supply from the nozzle (V̇ _d ) can be determined, or, given the oxidation potential of the protective gas, the minimum gas supply of the nozzle:

${\dot{\text{V}}}_{\text{d}}{\text{· Μ}}_{\text{Ox}}{\text{≧ m}}_{\text{C.}}\text{· 2R · u · Φ}$

The evaporation rate (Φ), which is the mass fraction of the oil residue which evaporates in the cleaning section from the initial temperature (T _a ) to the final temperature (T _e ) of the belt, is to be used in this condition.

${\dot{\text{m}}}_{\text{C}}{\text{= m}}_{\text{C}}\text{· 2R · u}$

Die in vorgenannten Gleichungen benutzten Symbole - Buchstaben und Indizes - haben die folgende Bedeutung:
K₁, K₂  sind die Konstanten des heterogenen bzw. homogenen Massenwirkungsgesetzes bei der Schutzgastemperatur T_g;

sind die Schutzgasvolumenanteile bei heterogenem bzw. homogenem Wassergasgleichgewicht;
M_C  ist die Molmasse des Kohlenstoffs, 0,012011 kg/mol;
V $\genfrac{}{}{0ex}{}{\text{⊖}}{\text{m}}$

  ist das Standardmolvolumen der Gase, 0,02241384 m $\genfrac{}{}{0ex}{}{\text{3}}{\text{n}}$

/mol;
µ_Ox  ist das Oxidationspotential der Schutzgaszusammensetzung gegenüber Kohlenstoff;
V̇_d  ist der Gasvolumenstrom durch die Düse;
m_C  ist der Kohlenstoffbelag auf dem Band pro Flächeneinheit.In practice, it is advantageous to conduct the process so that the quotient (ζ) from the gas supply of the nozzle (V̇ _d ) multiplied by the oxidation potential of the gas jet (µ _Ox ) on the one hand, and the product of that from the gas jet on the The carbon surface (erf _C ) with the evaporation rate (Φ), on the other hand, reaches and exceeds the numerical value one:

${\dot{\text{m}}}_{\text{C.}}{\text{= m}}_{\text{C.}}\text{· 2R · u}$

The symbols - letters and indices - used in the above equations have the following meanings:
K₁, K₂ are the constants of the heterogeneous or homogeneous law of mass action at the protective gas temperature T _g ;

are the protective gas volume fractions with heterogeneous or homogeneous water gas equilibrium;
M _C is the molar mass of carbon, 0.012011 kg / mol;
V $\genfrac{}{}{0ex}{}{\text{⊖}}{\text{m}}$

is the standard molar volume of the gases, 0.02241384 m $\genfrac{}{}{0ex}{}{\text{3rd}}{\text{n}}$

/ mol;
µ _Ox is the oxidation potential of the protective gas composition compared to carbon;
V̇ _d is the gas volume flow through the nozzle;
m _C is the carbon coating on the belt per unit area.

Another advantage of the method according to the invention is that the protective gas temperature and therefore the temperature of the gas jet is very much higher than the temperature of the strip entering the cleaning part. As the band heating progresses, the temperature difference decreases, but the band temperature at the outlet of the cleaning section remains clearly, up to several hundred Kelvin, below the protective gas temperature.

Once the oil residues have been converted into oil vapor, the conversion to CO and CO₂ in the hot gas atmosphere will take place very quickly, because the reaction rates are very high in the upper temperature range. On the other hand, oxidation reactions on the belt surface and diffusion processes in the belt material are of little importance in terms of reaction kinetics due to the low belt temperature and the short residence time of the belt in the cleaning section. The measure offers additional safety by heating the belt in the cleaning section via the distribution of the radiant heating pipes or electrical heating elements and the positioning of the rows of nozzles so that the boiling temperature (T _s ) of the oil residue is only reached shortly before leaving the cleaning section, so that it is not yet completely evaporated oil film the belt surface as long as possible before oxidation protects. It is therefore only exposed to the oxidizing atmosphere for a very short time. After leaving the cleaning section through the outlet lock, the belt enters an atmosphere that reduces iron, manganese, silicon and aluminum.

Hierbei ist der Schutzgasdurchsatz (V̇_g) als Funktion des zeitlich umzusetzenden Kohlenstoffbelages, der Abdampfquote (Φ) und des Oxidationspotentials des Schutzgases (µ_Ox) wie folgt definiert:

Die in vorgenannten Gleichungen benutzten Symbole - Buchstaben und Indizes - haben die folgende Bedeutung:

V̇_g

ist der Schutzgasdurchsatz im Reinigungsteil pro Zeiteinheit;

n

ist die Zahl der Zugeschalteten Düsen;

b

ist die Bandbreite;

Φ

ist der im Reinigungsteil bei Temperaturerhöhung von T_a auf T_e insgesamt abdampfende Masseanteil des Ölrückstandes.

In order to prevent carbon separation, as can be observed in a furnace atmosphere that is at rest or only slightly moved, even when water vapor is fed in at the colder points in the furnace chamber, especially in the area of the strip inlet, but also at the radiant heating pipes, the protective gas is circulated and so for sufficient temperature compensation. According to the invention, this is ensured if the gas supply of the connected nozzles exceeds the protective gas throughput required for converting the oil vapors to CO and CO₂ by a multiple, but at least twice:

The shielding gas throughput (V̇ _g ) is defined as a function of the carbon to be converted over time, the evaporation rate (Φ) and the oxidation potential of the shielding gas (µ _Ox ) as follows:

The symbols - letters and indices - used in the above equations have the following meanings:

V̇ _g

is the protective gas throughput in the cleaning section per unit of time;

n

is the number of activated nozzles;

b

is the bandwidth;

Φ

is the total mass fraction of the oil residue evaporating in the cleaning section when the temperature rises from T _a to T _e .

Oil residues are generally washed off the strip surface with a volatile solvent, for example trichlorofluorocarbon, known under the trade names Freon, Kaltron, Frigen, or n-heptane (light petrol). When examining the boiling behavior of these residues by means of a thermo balance, for example the Mettler or Perkin-Elmer system, it was found that in addition to unimodal distributions of the evaporation rate (φ) over the temperature (T), bimodal or multimodal distributions can also occur. The latter occur whenever the residue consists of a mixture of different types of oil, for example mineral oil and synthetic or animal / vegetable esters. The oils differ in their structure and molar mass and consequently also in their boiling behavior.

These conditions are explained below using thermograms. In Fig. 1, the right part of the picture shows the boiling curve, the left part of the picture shows the first derivative of the boiling curve after time. Since the heating rate is kept constant, for example 1 Kelvin per minute, the boiling loss per unit of time is equivalent to the boiling loss per Kelvin temperature increase. If the boiling loss is divided by the initial weight, the evaporation rate (φ) results. The evaporation rate (Φ) is the definite integral below the evaporation rate in the temperature range under consideration.

In the present case, it is a unimodal distribution of the evaporation rate with the maximum at the temperature (T _s ) of 189.7 ° C, corresponding to 462.9 Kelvin. It has been found that the evaporation rate over temperature with good Approximation can be represented as a normal distribution with a spread (standard deviation) of 34.1 Kelvin. The evaporation rate reaches the value of 0.0117 per Kelvin at the maximum boiling point.

Auch bei bimodaler Verteilung der Abdampfrate über der Temperatur gilt, daß sich ein gutes Reinigungsergebnis dann einstellt, wenn die Siedetemperaturen

und

nach Eintritt des Bandes in den Bereich der Prallströmung erreicht und vor Verlassen dieses Bereiches soweit überschritten sind, daß der Abdampfvorgang beendet ist. Vorteilhaft ist hierbei die Positionierung von mindestens zwei Düsenreihen in der Weise, daß das Maximum der Abdampfrate

der bei der niedrigeren Temperatur siedenden Hauptkomponente des Ölrückstandes unter der Prallströmung der in Bandlaufrichtung zuerst positionierten Düsenreihe erreicht wird, das Maximum der Abdampfrate

der bei der höheren Temperatur siedenden Hauptkomponente unter der Prallströmung der folgenden Düsenreihe. Zur Gewährleistung der vollständigen Verdampfung des Ölrückstandes hat es sich als ausreichend erwiesen, die Temperatur (T_e), die das Band vor Verlassen des Reinigungsteils erreichen soll, am Siedeende der höhersiedenden Hauptkomponente zu orientieren, gemäß:

Die in den vorgenannten Gleichungen benutzten Symbole - Buchstaben und Indizes - haben die folgende Bedeutung:

k_A, k_B

sind die Masseanteile der dominanten Komponenten A und B im Ölrückstand;

φ_A, φ_B

sind die über der Temperatur T normalverteilten Abdampfraten der Hauptkomponenten A und B im Ölrückstand;

sind die maximalen Abdampfraten der Hauptkomponenten A und B im Ölrückstand;

sind die Siedetemperaturen der Hauptkomponenten A und B im Ölrückstand, bei denen das Maximum der Abdampfraten

und

auftritt;

γ_B

ist ein stoffspezifischer Zahlenwert des Ölrückstandes, aus dem sich die Temperatur errechnet, bei der die Verdampfung der Hauptkomponente B beendet ist.

In Fig. 2 boiling curve and evaporation rate are shown one above the other. The evaporation rate follows a multimodal distribution function, which is however so dominated by the normally distributed evaporation rates of the two main components of the oil residue that it can be regarded as a bimodal function to a good approximation. The boiling maxima of the two dominant distributions occur at 229.4 ° C, corresponding to 502.6 Kelvin, and at 378.5 ° C, corresponding to 651.7 Kelvin, with a spread of 53.9 and 18.1 Kelvin. The maximum evaporation rates are 0.0037 and 0.0110 per Kelvin. The evaporation rate (Φ) can thus be calculated on the basis of the bimodality from the sum of the two normally distributed evaporation rates (φ _A ) and (φ _B ) of the main components of the oil residue:

Even with a bimodal distribution of the evaporation rate over the temperature, a good cleaning result is achieved when the boiling point

and

reached after the belt entered the area of the impingement flow and, before leaving this area, exceeded so far that the evaporation process has ended. The positioning of at least two rows of nozzles is advantageous in such a way that the maximum of the evaporation rate

the one at the lower one Temperature boiling main component of the oil residue is reached under the impact flow of the row of nozzles first positioned in the direction of belt travel, the maximum of the evaporation rate

the main component boiling at the higher temperature under the impact flow of the following row of nozzles. In order to ensure the complete evaporation of the oil residue, it has proven sufficient to orient the temperature (T _e ) which the belt is to reach before leaving the cleaning part at the boiling end of the higher-boiling main component, in accordance with:

The symbols - letters and indices - used in the above equations have the following meanings:

k _A , k _B

are the mass fractions of the dominant components A and B in the oil residue;

φ _A , φ _B

are the normally distributed evaporation rates of the main components A and B in the oil residue;

are the maximum evaporation rates of the main components A and B in the oil residue;

are the boiling temperatures of the main components A and B in the oil residue, at which the maximum evaporation rates

and

occurs;

γ _B

is a substance-specific numerical value of the oil residue, from which the temperature is calculated at which the evaporation of the main component B has ended.

Said prediction of the evaporation rate (Φ) of the oil residue requires, in addition to the boiling curve, knowledge of the basic composition of the rolling oil and / or fatty oil used, or at least knowledge of the quantity distribution of the main components. The supplier must provide this information, but also information about changing the recipe. By periodically monitoring the boiling behavior of the oils both at the supplier and The user must ensure the stability of the relevant product properties and ensure the reproducibility of the process. It is also common, for example, to alloy the oil with polar additives. Such a measure proves to be particularly effective when rolling with an emulsion. With the appropriate recipe setting, the oil residue on the cold-rolled strip, unaffected by the rolling parameters, will consist almost exclusively of residues of the fatty oil, not residues of the emulsion oil. The composition of the oil residue and thus the boiling behavior show no significant fluctuations over longer periods.

This fact is confirmed by the thermograms in Fig. 3. The results of examinations are compared, between which there is a period of 16 months.

In addition to the above-described method features of the invention according to claim 1 and claim 2, further details and features of the invention result from the description of the device with reference to the drawing.

4 shows the side view of the inlet part of the glow line in longitudinal section as the first special exemplary embodiment of the cleaning device according to the invention. The belt 1 is guided into the cleaning part 4 separated by the inlet lock 2 and outlet lock 3 against the ambient air or against the adjacent furnace atmosphere. The latter is equipped with nozzle bars 11 and radiant heating tubes 7 arranged successively above and below the belt. The belt is moved on driven support rollers 6 through the cleaning part. From the rest of the heating part 5, dry protective gas, for example a hydrogen / nitrogen mixture in a ratio of four to one, flows in countercurrent through the outlet lock 3 into the cleaning part 4. The protective gas is fed through a steam feed 13, which in the rear third of the cleaning section opens below the belt into the furnace, mixed with water vapor. The protective gas outlet 18 is located near the inlet lock also below the belt. The protective gas in the cleaning part is circulated with a plurality of blowers 8 installed on the furnace and driven by electric motor 15, by suction through a suction line 14 leading through the furnace vault and through the pressure line and nozzle bars 11 and nozzles 12 arranged above and below the belt onto the belt surface is blown. To measure the furnace chamber temperature, two thermocouples 17 are installed in the rear third of the cleaning part, which are expediently guided from above through the vault into the furnace chamber.

In addition, Fig. 5 shows the cross section of the cleaning part with blower 8 and associated electric drive 15, suction line 14, pressure line 9 and the throttle valve 10 built therein for quantity regulation as well as the positioning of the dew point control 16, nozzle bar 11 and nozzles 12, support roller 6 and volume 1 and the steam feed 13.

The features of claim 8 provide a modified version of the cleaning part according to claim 7 insofar as it is divided into several chambers by locks, whereby the cleaning distance can be shortened or lengthened depending on the belt speed by means of fans and rows of nozzles as well as required in the individual chambers Steam can be switched off or on.

The features of claim 2 have already been discussed above in connection with the method features of claim 1.

With the features of claim 3, the areas of the important process parameters time duration, strip temperature and heating speed under the impingement flow are defined for the procedure is essentially designed.

In claim 5, a temperature below the decomposition temperature of the oil residue is given as the upper limit which the belt temperature may reach before leaving the cleaning part. The latter is the temperature at which the vapor pressure, even under isothermal conditions, i.e. without further temperature rise, becomes a time-dependent variable and the vapor pressure increase is by definition 1.87 Pascal per second. By specifying a maximum permissible strip temperature in the cleaning section, cracking phenomena and thus carburization of the strip through graphitic deposits are counteracted.

The features of claims 9 and 11 to 14 provide refinements of the cleaning device which ensure a homogeneous and effective protective gas atmosphere and regulated heating of the belt.

With the features of claim 10, an embodiment of the nozzle holder in the nozzle bar 11 is specified, through which the nozzles 12 are brought out of the danger zone in the simplest way when threading the beginning of the tape.

In claim 5 is given as an alternative process feature to claim 4, the tape after leaving the cleaning part through the outlet lock 3 in a decarburizing furnace atmosphere with a correspondingly high dew point, which extends the scope of the method to materials that, for example, due to the susceptibility to aging must be subjected to decarburization annealing

In claim 15, as an alternative to the belt transport in the cleaning part with the aid of driven idlers 6, the transport in suspension, via a gas cushion generated by blower pressure and nozzles 12 below the belt 1 is specified, which means that for cleaning the usually dirtier belt underside anyway pressure energy brought to a higher level on the underside of the belt is advantageously used for the adjustment of the floating state, so that postponements, scratches and pimples are reliably avoided at least in this stage of heating.

The features of claim 6 provide a feasible with simple technical means, the environmentally friendly exhaust gas combustion or exhaust gas recovery device, which ensures that only CO₂, H₂O and N₂ occur in the exhaust gas.

Further details and features of the invention result from the following calculation and spatial representation of the various process parameters for a special exemplary embodiment of the invention.

Embodiment

The average oil residue on the belt surface has been determined to be 0.5 g / m². Also specified are the nozzle diameter with d = 0.031 m, the impact diameter of the gas jet equal to the width of the area covered by the gas jet on the belt 2R = 0.30 m, the belt thickness h = 0.0005 m and belt width b = 1.240 m, the belt speed in the cleaning section u = 1.75 m / s and the density of the strip material ρ = 7860 kg / m³.

und

sowie mit den maximalen Abdampfraten bei diesen Temperaturen von

und

Fig. 6 zeigt die dem Ausführungsbeispiel zugrundeliegende Siedekurve. Ihre Übertragung auf das Abdampfverhalten unter der realen Ofenatmosphäre erfolgt unter Berücksichtigung des jeweils eingestellten H₂/N₂-Verhältnisses. Die Verschiebung der Siedemaxima zu niedrigeren Temperaturen mit steigendem H₂-Anteil errechnet sich mit für technische Zwecke hinreichender Genauigkeit aus der Beziehung:

Für die Glühung unter 100 % Wasserstoff wäre demgemäß mit einer Erniedrigung der Siedemaxima von 46 Kelvin zu rechnen.The boiling curve of the oil residue - weighed in at 5.796 mg, heating rate 10 Kelvin per minute - recorded under technically pure nitrogen with a thermal balance - results in a bimodal distribution of the evaporation rate with the boiling maxima, dominated by the two main components of the oil residue

and

as well as with the maximum evaporation rates at these temperatures of

and

6 shows the boiling curve on which the exemplary embodiment is based. They are transferred to the evaporation behavior under the real furnace atmosphere, taking into account the respectively set H₂ / N₂ ratio. The shift of the boiling maxima to lower temperatures with increasing H₂ content is calculated from the relationship with sufficient accuracy for technical purposes:

A lowering of the boiling maxima of 46 Kelvin would therefore have to be expected for annealing under 100% hydrogen.

und

In the example, an H₂ / N₂ ratio is set to four; the boiling maxima result accordingly

and

Für das vorliegende Beispiel mit der Spreizung σ_A = 19,3 Kelvin und σ_B = 56,0 Kelvin ergibt die Gegenüberstellung: Hauptkomponente A B φ_s aus der Siedekurve abgelesen 0,0033 0,0100 φ_s aus der Spreizung σ errechnet 0,0036 0,0103 It is now easy to check whether the evaporation rates (φ) are normally distributed over the temperature (T). The latter applies and is used for the calculation of the evaporation rate (Φ) if the boiling maximum (φ _s ) and spread (standard deviation σ) of the distribution function correspond at least approximately:

For the present example with the spread σ _A = 19.3 Kelvin and σ _B = 56.0 Kelvin, the comparison shows: Main component A B φ _s read from the boiling curve 0.0033 0.0100 φ _s calculated from the spread σ 0.0036 0.0103

Thus, the evaporation rate (Φ) can be a good approximation as the sum of the integrals of the two evaporation rates (φ _A ) and (φ _B ) normally distributed over the temperature (T) within the limits given by the strip start temperature (T _a ) and the end strip temperature (T _e ) being represented. For this purpose, the mass fractions of the main components A and B of the oil residue have to be named. The rolling oil supplier provides the relevant information; As a rule, component A will be the mineral oil component boiling at a lower temperature and component B will be the fatty acid / fatty ester component boiling at a higher temperature. Since the information refers to the fresh oil delivered, it is advisable to check it on the oil residue by determining the saponification number according to DIN 51 559. For the exemplary embodiment, the mass fractions A and B are taken into account in the calculation of the evaporation rate with k _A = 1/3 and k _B = 2/3.

Die Temperaturzunahme unter der Prallströmung hängt bei gegebener Schutzgastemperatur (T_g) maßgeblich von der Temperatur des Bandes (T₀) bei Eintritt unter die Prallströmung ab. Da die Banderwarmung im Reinigungsteil ohne Düsen nahezu linear verläuft, d.h die Temperatur des sich mit konstanter Geschwindigkeit (u) durch den Reinigungsteil bewegenden Bandes mit der Ofenlänge linear ansteigt, hängt die Temperaturerhöhung unter der Prallströmung letztlich von der Position der Düse im Reinigungsteil ab. Hierbei sind Einflüsse wie Abstand Düse/Bandoberfläche, Öffnungswinkel des Gasstrahls, Strahlverdrallung, Düsenform und -öffnung, im Ausführungsbeispiel aus Gründen der Übersichtlichkeit nicht berücksichtigte Optimierungsgrößen. Entsprechend ändert sich die Bandtemperatur (T₁) bis zum Austritt aus dem Bereich der Prallströmung bei gleichbleibender Schutzgastemperatur T_g = 1173 Kelvin und unverändertem Gebläsedruck p = 1040 mbar (entspricht 0,104 MPa) im wesentlichen nur mit der Position der Düse über die Länge des Reinigungsteils: Düsenposition, m 2 6 10 14 18 22 26 T₀, Kelvin 332 400 473 548 625 703 783 T₁, Kelvin 357 421 491 563 637 713 791 On the other hand, the final strip temperature (T _e ) must be defined; For this purpose, the material constant (γ _B ) of the oil residue, with the aid of which the temperature at which the evaporation of component B has ended, must be known. In the present case, this quantity was determined experimentally to γ _B = 2.187, so that the end band temperature (T _e ) is as follows:

The temperature increase under the impingement flow for a given protective gas temperature (T _g ) depends largely on the temperature of the belt (T₀) when entering under the impingement flow. Since the belt heating in the cleaning section is almost linear without nozzles, ie the temperature of the belt moving through the cleaning section at a constant speed (u) increases linearly with the length of the furnace, the temperature increase under the impact flow ultimately depends on the position of the nozzle in the cleaning section. Influences such as the distance nozzle / belt surface, opening angle of the gas jet, jet swirl, nozzle shape and opening are optimization variables which have not been taken into account in the exemplary embodiment for reasons of clarity. Correspondingly, the belt temperature (T₁) changes until it leaves the area of the impingement flow with constant protective gas temperature T _g = 1173 Kelvin and unchanged blower pressure p = 1040 mbar (corresponds to 0.104 MPa) essentially only with the position of the nozzle over the length of the cleaning part: Nozzle position, m 2nd 6 10th 14 18th 22 26 T₀, Kelvin 332 400 473 548 625 703 783 T₁, Kelvin 357 421 491 563 637 713 791

The above information applies to a water vapor feed of 10 percent by volume. As the proportion of water vapor in the protective gas increases, the heat transfer coefficient (α _d ) decreases. However, this change is hardly significant. This is shown below for a selected position of the nozzle, but otherwise the process parameters remain unchanged except for the water vapor content; 22 m nozzle position: Water vapor share,% 1 5 10th 15 20th T₀, Kelvin 703 703 703 703 703 T₁, Kelvin 716 715 713 713 712 α _d , W / (m² · Kelvin) 205 182 163 149 139

The position of the nozzle in the cleaning section changes the heating speed (Ṫ) of the strip under the gas jet and, in proportion to the heating speed, the vapor pressure (p _D ) of the oil film on the surface of the strip. The change in vapor pressure with the heating rate in the boiling range of oil components A and B corresponds to their evaporation rates at these temperatures; they component B will be about a factor of 0.0100 / 0.0033 ≈ 3 larger than component A. This is important insofar as the heating rate (Ṫ) decreases with increasing strip temperature (T₀); Water vapor feed 10 volume percent: Nozzle position, m 2nd 6 10th 14 18th 22 26 Ṫ, Kelvin / second 146 123 105 90 75 60 46

Thereafter, the heating speed is reduced, for example, to around half if the value at the nozzle position 14 m is compared with that at the nozzle position 26 m. This disadvantage is compensated for by the larger gradient of the vapor pressure rise with the heating speed at the nozzle position 26 m.

Fig. 7 shows how the minimum shielding gas requirement changes depending on the water vapor content and the nozzle position, indicated by the strip temperature (T₀) when entering under the gas jet. The conditions in the temperature range between 350 and 750 Kelvin and with a steam input of 10 to 20 percent by volume are shown. A constant carbon coating m _C = 0.5 g / m² surface of the belt entering the cleaning section is assumed on average. Under these conditions, otherwise as stated above, the shielding gas requirement increases with the strip temperature and decreases with the proportion of water vapor in the shielding gas. High strip temperatures (T₀), from 664 to 700 Kelvin, and high water vapor proportions between 16 and 20 percent by volume from the point of view of protective gas consumption are considered favorable for the cleaning effect.

On the other hand, with regard to undesired oxidation reactions on the strip surface, the water vapor feed will be limited to the minimum necessary for the oxidation of the carbon. The ones illustrated in the illustration also provide this Connections according to the invention the basis.

Fig. 8 shows the carbon that can be converted per standard cubic meter of protective gas as a function of the water vapor content and the protective gas temperature (T _g ). The relationships in the temperature range between 800 and 1200 Kelvin and with a steam feed of 1 to 20 percent by volume are shown. The oxidation potential (µ _Ox ) compared to carbon grows exponentially with the gas temperature and practically linear with the water vapor content in the protective gas.

The inert gas circulation should be such that temperature differences in the gas atmosphere are reduced and carbon deposits are avoided. The turbulence is intended to promote the conversion of carbon to CO and CO₂ until equilibrium is reached. It is therefore important to ensure that the sum of the volume flows of all nozzles connected exceeds the shielding gas throughput at least twice. On the other hand, with increasing gas circulation, the heat exchange with the belt becomes more intense, the gas will cool down. As the gas temperature drops, the conversion of the carbon according to FIG. 8 will decrease. It is therefore necessary to limit the amount circulated. For the exemplary embodiment, it is specified to limit the circulation in such a way that the protective gas in the cleaning part does not cool down more than 30 Kelvin.

When determining the circulating capacity, the strip cleanliness and freedom from aging achieved will be used as a benchmark, and the special features of the furnace system and the annealing program such as length of the cleaning part, strip speed, strip dimensions, material, surface condition must be taken into account. For example, the circulation rate in the inlet part can be increased without causing the oil film to evaporate there by rotating the nozzles of the front nozzle bars from the vertical position, as in the threading process, so that the gas jet away from the belt surface, for example towards the radiant heating pipes or horizontally.

/h 1040 700 480 For the exemplary embodiment, the minimum circulation capacity as a function of the water vapor feed-in is determined as follows, in standard cubic meters per hour: Water vapor share,% 10th 15 20th Minimum circulation capacity, m $\genfrac{}{}{0ex}{}{\text{3rd}}{\text{n}}$

/H 1040 700 480

5 20 30 34 35 Quotient ζ 3 12 18 20 21 The condition ζ ≧ 1 is checked for a blower pressure p = 1040 mbar and the nozzle position 22 m, which corresponds to a strip temperature when entering under the gas jet of T₀ = 703 Kelvin, as well as for different water vapor content, with the following result: Water vapor share,% 1 5 10th 15 20th Oxidation potential µ _Ox , g / m $\genfrac{}{}{0ex}{}{\text{<figure-callout id="5" label="3rd n" filenames="imgf0004.png,imgf0007.png" state="{{state}}">3rd </figure-callout>}}{\text{n}}$

5 20th 30th 34 35 Quotient ζ 3rd 12th 18th 20th 21

With the fatty esters used for rolling oils and greasing oils after pickling, signs of decomposition must be expected from around 700 Kelvin, with mineral oil content from around 600 Kelvin. The structural decomposition only becomes a concern if cracking residues remain in the form of graphitized carbon. According to the experience available, this phenomenon does not occur as long as the band heating in the cleaning section is regulated in such a way that the boiling point has just been reached when leaving the cleaning section, but has not been exceeded. Moreover, the water vapor fed in counteracts the pyrogenic decomposition by lowering the boiling point of the oil residue.

Claims (15)

Process for cleaning metal strip, in particular for removing oily residues on the strip surface, in the front area of the heating part of a continuous annealing line, which is separated from the rest of the heating part by an inlet lock and an outlet lock as a cleaning part, by heated protective gas, which is used to convert the evaporating carbonaceous material Residues water vapor is mixed in,
characterized in that hydrogen-rich shielding gas is blown vertically or with a low tendency to normal on the top and bottom of the belt and the resulting impingement flow is used in combination with the following features to blow off and evaporate the oil residue as well as to remove, circulate and oxidize the oil vapors: A) The belt entering the cleaning section is first heated primarily by heat radiation to a fixed temperature (T₀) that can be predetermined over the path length and then further regulated under the gas jet at a higher temperature (T _g ) by superimposed convection to a defined surface temperature (T₁ ) heated, at which the evaporation process is finished, so that the evaporation rate (Φ), that is from the start temperature (T _a ) to the end temperature (T _e ) of Volume evaporating mass fraction of the oil residue, assuming a numerical value close to or equal to one; B) the protective gas consisting of a hydrogen-nitrogen mixture, preferably containing 31 to 100% by volume of hydrogen, water vapor is added to the extent that all of the carbon from the oily residues evaporating from the strip surface is converted into CO and CO₂, characterized in that that the dimensionless ratio (ζ) between the volume flow of the gas jet (V̇ _d ) multiplied by its oxidation potential (µ _ox ) on the one hand, and the carbon detected at the same time under the impingement flow on the belt surface (ṁ _c ) multiplied by the evaporation rate (Φ) on the other hand , reaches and exceeds the numerical value one; C) the inert gas circulation (ξ) is dimensioned such that a carbon separation, as would inevitably occur in a furnace atmosphere at rest even if water vapor was fed in, is prevented by the circulated inert gas volume which is the inert gas throughput necessary for the conversion of the oil vapors to CO and CO₂ (V̇ _g ) by a multiple, at least twice; D) when heating the tape is heated in such a way that said surface temperature (T₁), at which the evaporation ends, is only reached shortly before leaving the cleaning part and the oil film which has not yet been completely evaporated protects the tape surface, so that the latter is only exposed to the oxidizing atmosphere for a very limited period of time and the oxidation has no significance in terms of reaction kinetics. Process for cleaning metal strip according to claim 1,
characterized in that the belt surface under the impingement flow in the presence of an oil-containing coating, the oil portion of which consists of a mixture of different oils, is heated to such an extent that the boiling temperature of the higher-boiling component is reached and the evaporation process is ended, so that the evaporation rate (Φ) unites Takes a numerical value close to or equal to one. Process for cleaning metal strip according to claims 1 and 2,
characterized in that the strip entering the cleaning section is heated up to 250 to 510 ^o C by heat radiation in 10 to 30 seconds and heating speeds of between 10 and 100 K / s are achieved in the area of the impingement flow by the protective gas heated to 750 to 950 ^o C . Process for cleaning metal strip according to claims 1 to 3,
characterized in that the cleaned strip is passed through a lock into an atmosphere which reduces iron, manganese, silicon and aluminum. Process for cleaning metal strip according to claims 1 to 3,
characterized in that the cleaned belt is passed through a lock into a decarburizing furnace atmosphere with a high dew point, for example 62 ^o C. Process for cleaning metal strip according to claims 1 to 3,
characterized in that the flowing in the cleaning part against the direction of the belt, loaded with CO and CO₂ hydrogen-water vapor-nitrogen gas mixture of a complete combustion with air in a post-combustion system or, for example, the combustion in jacket radiant tubes, so that the end product in the exhaust gas is carbon dioxide, water vapor and nitrogen accumulate. Device for cleaning metal strip, in particular for removing oily residues from the strip surface, when the strip is heated in the front area of the heating part of a continuous annealing line, which is separated from the rest of the heating part by an inlet lock and an outlet lock as a cleaning part,
characterized in that the shielding gas heated in the cleaning part is circulated with the aid of pumps, blowers and the like and is blown vertically or at a slight inclination to the normal onto the top and bottom of the band by means of nozzle rows or slot nozzles arranged transversely or obliquely to the direction of strip travel. Device for cleaning metal strip according to claim 7,
characterized in that the cleaning rope is divided into several chambers by means of locks, so that the cleaning distance is shortened at a reduced belt speed, and the cleaning distance can be extended at a higher belt speed by switching off or on fans or rows of nozzles and steam as required in the individual chambers. Device for cleaning metal strip according to claim 7 or 8,
characterized in that heat sources with a high heat flow density, for example jacket radiant heating pipes or electrical heating elements, which do not impair the protective gas atmosphere , are used for the indirect heating of the cleaning part. Device for cleaning metal strip according to claims 7 to 9,
characterized in that the nozzle bars (11) arranged above and below the belt are rotatably mounted about their axis so that the nozzles (12) can be rotated out of the danger zone during threading operations and the beginning of the belt slides freely through the cleaning part. Device for cleaning metal strip according to claim 7 to 10,
characterized in that the steam feed (13) takes place either in the suction line (14) of the circulating fan (8) or directly in the furnace space of the cleaning part (4), and in that the latter is separated from the rest of the heating part (5) in the direction of belt travel in such a way that the low oxygen potential present there to avoid oxidation of the cleaned strip surface is not impaired. Device for cleaning metal strip according to claims 7 to 11,
characterized in that the cleaning part (4) is equipped with a suitable dew point measurement and control (16) for the dimensioning, monitoring and setting of the predetermined water vapor content in the protective gas. Device for cleaning metal strip according to claim 7 to 12,
characterized in that the cleaning part (4) is equipped with a suitable temperature measurement and control (17), for example with a temperature control computer for adapting the heating power to the respective mass flow of the strip to be heated. Device for cleaning metal strip according to claims 7 to 13,
characterized in that the shielding gas volume flow is measured via speed-controlled drive motors (15) for the circulating fans (8) and / or via throttle valves (10) installed in the pressure line (9). Device for cleaning metal strip according to claims 7 to 14,
characterized in that the belt is passed through the cleaning part on a gas cushion generated by the blower pressure and nozzles.

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