EP0564437A1 - Process of galvanizing a strip and arrangement for carrying out the process - Google Patents

Abstract

In a process for galvanising a strip (1), the strip (1) is continuously coated with zinc, is then subjected, in order to form a Zn-Fe layer, to a heat treatment in a tunnel furnace (10, 11), followed by on-line inspection of the zinc layer, the galvanising process being controlled as a function of the zinc layer. In order to be able to produce a strip (1) having a defined coating structure while ensuring uniform quality, the through-reaction of the Zn-Fe layer is ascertained by measuring the radiation emission of the strip surface during or after the heat treatment by means of at least one pyrometer (14), and through-reaction at the position of the pyrometer is ensured by varying the tunnel furnace (10, 11) heat output which serves as the manipulated variable of a control process which proceeds as a function of the pyrometer indication (Figure 1). <IMAGE>

Description

The invention relates to a method for galvanizing a strip, in particular a steel strip, the strip being continuously coated with zinc in a continuous process either electrolytically with zinc or according to the hot-dip galvanizing process in a zinc bath with zinc, then to form a Zn-Fe layer of a heat treatment in a continuous furnace and further subjected to an on-line control of the zinc layer, the galvanizing process being controlled in dependence on the on-line control, and a plant for carrying out the method.

In modern galvanizing processes of this type (known from EP-A-0 473 154) for the production of so-called "galvanized-old" strip, which is understood to mean a thermally post-treated, metal-coated steel strip, the strip, which has already been galvanized, is continuously subjected to an afterglow (galvanneal) subjected. After passing through the galvanizing part (zinc bath + stripping system in hot-dip galvanizing plants, galvanizing cells in electrolytic galvanizing plants), the strip is passed through a post-annealing furnace (galvannealing furnace and holding furnace). This oven can e.g. operated inductively or with gas.

The post-annealing process converts the pure zinc layer into a Zn-Fe layer by diffusing iron. Depending on the iron content of the Zn-Fe alloy, a product with different mechanical properties (e.g. toughness, hardness) is formed, which decisively determines the area of use (abrasion behavior, weldability, paintability, corrosion resistance, deep-drawing capacity). The Fe content can be measured on-line by appropriate measuring devices (for example by means of X-ray fluorescence, X-ray diffraction or similar methods), that is to say on-line, as described, for example, in EP-A-0 473 154, the measurement result generally about one Represents the mean of the Fe content over the thickness of the Zn-Fe layer.

It is of great importance for the quality of the product that the Zn-Fe layer has reacted completely, i.e. that iron has penetrated to the surface of the Zn-Fe layer and forms a stable phase with the zinc there.

The object of the invention is to further develop the method described at the outset in such a way that a galvanized strip with a defined, fully reacted layer structure can be produced, it being possible to intervene directly and directly in the manufacturing process to ensure a uniform quality of the galvanized strip and the production of rejects is minimized. In particular, the method according to the invention should enable the automatic consideration of intended changes in process parameters as well as their unintentional changes, so that the manufacturing process is continuously optimized and without manual intervention.

This object is achieved in that the radiation emission of the strip surface is measured during or after the heat treatment by means of at least one pyrometer and the through reaction at the point of the pyrometer by changing the as a manipulated variable depending on the purpose of determining a through reaction of the Zn-Fe layer Pyrometer display running control serving heating output of the continuous furnace is ensured.

Preferably, the procedure is such that the point from which the Zn-Fe layer has reacted is determined on the belt passage section by measurement by means of a plurality of pyrometers arranged one behind the other in the belt running direction, and by regulating the heating power of the continuous furnace this point on the belt passage section into a desired one and is brought into a position that acts as a benchmark.

According to a preferred procedure, in addition to determining the through-reaction, a value of the iron content of the Zn-Fe layer is determined as the reference variable, the actual value of the iron content of the Zn-Fe layer is compared with the reference variable and a control deviation is changed by means of a controller by changing the control variable serving heating power of the continuous furnace balanced.

The temperature control in the continuous furnace has a decisive influence on the structure of the galvanized layer and therefore also the mechanical properties of the product, since the diffusion processes of the iron in the zinc layer (diffusion rate, iron content) depend on the temperature and the duration of the heat treatment in the continuous furnace.

At the moment, however, it is not possible to measure the temperature of the strip in the area of the afterglow furnace, since there is no inexpensive, conventional, non-contact temperature measuring device available which could measure the temperature with sufficient accuracy under these conditions. According to the preferred procedure described above, provision is therefore made to set the furnace temperature indirectly via the heating power and thus to ensure a particularly uniform quality of the coated strip.

The control is preferably carried out in a closed control circuit with the aid of a computer which registers the control deviation and controls the heating power of the continuous furnace by means of control commands, the computer advantageously having the strip dimension, the basic material of the strip with regard to its chemical composition and / or structure, the zinc layer thickness, the composition of the zinc bath, such as its Al content, the belt speed and possibly other parameters such as the temperature of the belt at the inlet of the continuous furnace and the ambient temperature are taken into account.

A preferred embodiment is characterized in that the heating power and thus the temperature within the Continuous furnace can be set differently in individual heating zones.

In this case, in order to take into account measurement values of the iron content of the iron-zinc alloy or the radiation emission of the surface which differ over the bandwidth, the heating power in heating zones adjacent to one another in the direction of the bandwidth can be set differently.

According to another embodiment, the heating power can advantageously be set differently in the heating zones lying one behind the other in the direction of the strip passage, as a result of which the heating-up speed of the strip or the holding time of the strip at a specific temperature can be varied in order to achieve optimum strip quality.

The measurement of the radiation emission and / or the Fe content is expediently carried out at locations distributed over the bandwidth.

A system for carrying out the method with a belt guiding device continuously guiding a belt along a belt run, a zinc coating device arranged on the belt run, a subsequent heat treatment device for the belt formed by an afterglow furnace, and a heat treatment device also located on the belt run, arranged in or after the heat treatment device Measuring device for checking the zinc layer, is characterized in that the measuring device is formed by at least one pyrometer, which is coupled to a controller, which is coupled via a control line to the heating device of the heat treatment device.

According to a preferred embodiment, a plurality of pyrometers arranged one behind the other in the strip running direction and coupled to the controller are provided.

In order to take into account a large number of parameters influencing the quality of the coated strip, the controller is advantageously coupled to a process computer.

To determine the iron content of the Zn-Fe layer, a measuring device downstream of the heat treatment device for measuring the iron content of the zinc layer is expediently provided, which is coupled to a controller which is coupled via a control line to the heating device of the heat treatment device.

The invention is explained in more detail below with reference to the drawing, with FIG. 1 schematically illustrating a system for galvanizing a strip. The diagram shown in FIG. 2 shows the dependence of the iron content on the heating power. 3 shows a deviation of the iron content in the Zn-Fe layer as a function of the bandwidth, FIG. 4 shows the dependence of the radiation emission on the holding time.

As can be seen from FIG. 1, a steel strip 1 to be galvanized is guided continuously by means of a strip guide device which has a plurality of strip guide rollers 2 along a strip path 3 from an unwinding station (not shown) to a also not shown winding station. On the belt run 3, the steel strip first reaches a zinc coating device 4, which in the exemplary embodiment shown is designed as a hot-dip galvanizing device. This has a zinc bath 5 and a stripping device 7 arranged downstream in the strip running direction 6 to ensure a constant zinc layer which is of equal thickness over the bandwidth.

Subsequently, the steel strip 1 is introduced via a hot-thickness measuring system 8 for measuring the thickness of the zinc layer and via a temperature measuring device 9 into a heat treatment device 13 having two continuous furnaces 10, 11. Heating takes place primarily in the first continuous furnace 10 of the galvanized steel strip 1 to the required annealing temperature. In the further continuous furnace 11 arranged subsequently, the steel strip 1 is primarily kept at a constant annealing temperature.

After the steel strip 1 has emerged from the second continuous furnace 11, the radiation emission of the completely annealed steel strip 1 is measured by means of a pyrometer 14. Cooling devices 15 are then arranged on the belt guide. At a point downstream of the heat treatment device 13 of the belt path 3, a measuring device 16 is also provided for measuring the iron content of the Zn-Fe layer, which, as indicated by the double arrow 17, can preferably be shifted over the bandwidth, so that the bandwidth can be varied at different points a measurement can be taken. The measuring device preferably works according to the X-ray method.

A controller 19 coupled to a process computer 18 is coupled to heating devices of the two continuous furnaces 10, 11 for the purpose of setting the heating power, as illustrated by the double arrows 20.

The function of the system is as follows:

Due to its higher affinity for iron, the aluminum dissolved in the zinc bath 5 initially forms an iron-aluminum layer (Fe₂Al₅) on the steel strip, which prevents a reaction of the iron substrate of the steel strip 1 and the zinc layer. This system (steel strip 1 + Fe-Al layer + liquid Zn layer) reaches the first continuous furnace 10 and is brought to a temperature of 450 ° C to 700 ° C. In the second continuous furnace 11, the steel strip 1 is kept at a certain temperature or heated even further. The process of diffusion of iron into the zinc layer that occurs converts the pure zinc layer into a zinc-iron layer.

Here, the Fe-Al barrier layer formed in the zinc bath is first broken up by the Zn-Fe growth at the grain boundaries of the base material, and a mushroom-shaped growth of the Zn-Fe complexes begins. Depending on the iron content, different metallurgical phases are formed that have different properties.

The main phases are listed in the table below: phase % Fe Crystal structure Hardness (MPa) Eta .. <0.03 Hexagonal 300-500 Zeta .. 5 - 6 Monoclinic 1800-2700 Delta .. 7-12 Hexagonal 2500 - 4500 Gamma .. 21-28 Cubic 4500 - 5500

As can be seen from the table above, the phases become harder or more brittle with increasing iron content. With subsequent deformation (e.g. deep drawing), this can lead to increased abrasion, which makes the adhesion of the Zn-Fe layer very poor.

The process is analogous in the case of electrolytic galvanizing plants, but the Al content plays a minor role.

It is of great importance for the quality of the product that the Zn-Fe layer has reacted completely, i.e. that the zinc forms a stable phase with the iron on the surface of the coated steel strip 1 due to the advance of the iron during the diffusion process.

Since the emissivity of the coating changes abruptly when the zinc layer is converted into a Zn-Fe layer as soon as the surface of the Zn-Fe layer has iron (see FIG. 4), a radiation emission measurement using a pyrometer 14 can be used for evaluation the galvan-aged layer will. The pyrometer 14 can be arranged after or in the heat treatment device 13 (for example between the galvannealing furnace 10 and the holding furnace 11). This measurement is information about the radiation energy emitted purely from the surface of the steel strip 1, ie its Zn-Fe layer, which is a function of the temperature and the emission number of the surface condition. The emission number of a pure zinc surface is less than 0.2 and that of a fully reacted Fe-Zn surface is approximately 0.6. If the Zn-Fe layer has not yet completely reacted at the point of the pyrometer, the heating power of the continuous furnaces 10, 11 is increased with the aid of the controller 19 connected to a process computer 18, to which the measured value of the pyrometer is input, until a complete reaction with the aid of the pyrometer is detectable. The heating power is the control variable of the control process.

Due to the strong change in emissivity in the event of the so-called through reaction, i.e. If the iron penetrates to the surface of the Zn layer, it is also possible to recognize that point in the strip running direction 6 from which the reaction has been completed. This can e.g. in that two or more pyrometers are arranged one behind the other in the tape running direction 6. By knowing about the jump in emissivity when the Zn-Fe layer reacts, and from the measured radiation intensities of the pyrometers 14, it can be concluded that point of the belt path 3 from which the Zn-Fe layer has already reacted.

The heating power of the continuous furnaces 10, 11 is now regulated with the help of the controller 19 so that the through reaction is completed from a certain desired point. Another way of recognizing the point in the strip running direction at which the through reaction is completed is to compare the pyrometer measurement with a thermal model calculation.

For this purpose, the pyrometer measurement is the empirically determined emissivity for the pure zinc layer and a second time the empirically determined emissivity of the fully reacted layer is used. In terms of calculations, this initially results in two pyrometer temperature values for the running belt that differ according to the different emission numbers.

By comparing these two values with a model calculation of the temperature carried out in parallel for the strip section in question, which can be calculated from the entry temperature of the steel strip into the heat treatment device and the power supplied to it, it is determined which of the two pyrometer temperature values corresponds to the calculated temperature. This temperature value is then regarded as the correct temperature of the running steel strip 1. The associated emissivity indicates whether the steel strip still has a pure zinc coating or whether the coating has already reacted. The heating power of the heat treatment device is controlled so that the through reaction at the point of the pyrometer 14 is completed.

Since the information about the reaction of the coating alone cannot be used to directly and directly deduce the Fe content of the Zn-Fe layer, but the Fe content is of great importance for the properties of the product, it is advantageous that Heat output distribution over the length of the heat treatment device based on a combination of the two information, namely the Fe content of the Zn-Fe layer and the emissivity determination.

In order to set the optimum iron content, ie an iron content which allows the galvanized steel strip 1 to be deformed without difficulty, the iron content of the Fe-Zn layer is advantageously used in addition to determining and regulating the reaction through on-line X-ray fluorescence measurement with the aid of the measuring device 16 determined, preferably over the entire bandwidth and also over the entire band length. This actual value of the iron content of the Zn-Fe layer is determined with the help of the controller 19 with a value of the Iron content of the Zn-Fe layer compared. A possible control deviation is compensated for by the controller 19 by changing the heating output of the first and also the second continuous furnace 10, 11, which serves as a control variable. If, for example, the measured iron content is lower than the desired one, the heating output of the continuous furnace is increased until the control deviation becomes zero or has dropped below a predetermined value (dead band), as explained below with reference to FIG. 2:

The course I indicates the relationship between the Fe content of the Zn-Fe layer and the heating power. This is determined empirically and e.g. made available to the control computer (controller 19) as a formula or as a table.

If the steel strip 1 behaves exactly according to the course I, the desired setpoint of the Fe content Fe 1 (point A) is achieved with the power setting P 1. The band behaves somewhat differently, e.g. according to course II, due to unintentional changes in process parameters, e.g. Drift of the ambient temperature, drift in the transformer output when the continuous furnaces are electrically heated or when other faults occur, this results in an Fe content Fe 2 on the strip which deviates from the set value Fe 1 (point B).

The heating power is now changed, for example depending on the slope dP / dFe in point A 'of the course I, for example by the value $$\text{k.}\frac{\text{dP}}{\text{dFe}}\text{. ΔFe}$$

For the gain factor k = 1, the changed power is shown in the picture with P₂. This results in an improved value of the Fe content (point C). The regulation takes place as long as a system deviation is determined (Fe content ≠ Fe₁).

• 1) ungleichmäßige Beschichtungsdicke
• 2) Querwölbung des Bandes
• 3) Bandprofil
• 4) ungleichmäßige Heizleistung über die Bandbreite
• 5) ungleichmäßige Temperaturverteilung über die Bandbreite beim Eintritt des Bandes in den Galvannealingofen usw.

Uneven Fe content profiles across the range can result from various effects:

• 1) uneven coating thickness
• 2) Cross curvature of the band
• 3) Belt profile
• 4) uneven heating power across the range
• 5) non-uniform temperature distribution over the bandwidth when the strip enters the galvannealing furnace, etc.

Primarily, the coating thickness has to be adjusted within narrow tolerances evenly over the bandwidth (known methods for layer thickness control). Despite the uniform layer thickness across the bandwidth, the causes 2) to 5) etc. cause an uneven Fe content across the bandwidth. The influence of uneven heat input into the steel strip 1 can e.g. lead to the Fe content shown in Fig. 3, although the coating thickness is uniform over the bandwidth.

However, a coating thickness that is as uniform as possible and a uniform Fe content over the bandwidth (and length) are desired.

By reducing the heating power of the continuous furnaces 10, 11 in heating zones 12, which mainly act on the belt center, or by increasing the heating power in heating zones 12 ', which mainly affect the belt edges, the uniformity of the Fe content over the bandwidth can be improved as well Full reaction across the entire range can be ensured.

With the aid of the method according to the invention, the following influences can also be taken into account:
6) The thickness of the zinc layer:

• 7) Der Aluminiumgehalt im Zinkbad:
  Das im Zinkbad 5 gelöste Aluminium lagert sich vor allem in folgenden zwei Schichten ab:
  • a) Grenzschicht zwischen Eisensubstrat und Zinkschicht: Durch seine hohe Affinität zum Eisen bildet das Aluminium im Zinkbad zuerst eine Fe₂Al₅-Sperrschicht aus, die ein vorzeitiges Wachstum der spröden Zn-Fe-Phase verhindert. Diese muß beim Galvannealingprozeß durchbrochen werden, um das Zink-Eisen-Wachstum zu starten.
  • b) Oberflächenoxidschicht:
    Ein Großteil des Aluminiums liegt als Al₂O₃ an der Oberfläche der Zinkbeschichtung vor.
• 8) Die Geschwindigkeit, mit der das Stahlband 1 durch die Anlage bewegt wird:
  Sie beeinflußt die Aufenthaltsdauer (Haltezeit) des Stahlbandes 1 in den Durchlauföfen 11, 12 und somit die Temperaturführung für das Stahlband 1. Dadurch wird die Reaktionszeit verändert, und dies beeinflußt den Eisengehalt.
• 9) Die chemische Zusammensetzung des Stahlbandes 1 und dessen Gefüge:
  Da das Wachstum der Zn-Fe-Komplexe hauptsächlich an den Korngrenzen beginnt, hängt die Reaktionsfähigkeit auch von der Grundmaterialzusammensetzung und dem Gefüge ab.
• 10) Des weiteren beeinflussen die Bandabmessungen die Wärmeeinbringung und damit die Diffusionsbedingungen.

The diffusion paths and therefore also the structure of the coating change with the layer thickness. Under the same furnace performance ratios, a smaller layer thickness (less layer support) leads to a higher iron content. Man can observe this at the strip edges, which may have a different layer thickness than in the strip center area.

• 7) The aluminum content in the zinc bath:
  The aluminum dissolved in zinc bath 5 is mainly deposited in the following two layers:
  • a) Boundary layer between iron substrate and zinc layer: Due to its high affinity for iron, the aluminum in the zinc bath first forms an Fe₂Al₅ barrier layer, which prevents the brittle Zn-Fe phase from growing prematurely. This has to be broken during the galvannealing process in order to start the zinc-iron growth.
  • b) surface oxide layer:
    Much of the aluminum is present as Al₂O₃ on the surface of the zinc coating.
• 8) The speed at which the steel strip 1 is moved through the system:
  It affects the length of stay (holding time) of the steel strip 1 in the continuous furnaces 11, 12 and thus the temperature control for the steel strip 1. This changes the reaction time, and this affects the iron content.
• 9) The chemical composition of the steel strip 1 and its structure:
  Since the growth of the Zn-Fe complexes mainly begins at the grain boundaries, the reactivity also depends on the basic material composition and the structure.
• 10) Furthermore, the strip dimensions influence the heat input and thus the diffusion conditions.

According to the invention, all the factors listed above which influence the iron content can be taken into account by entering the data defining these factors into the process computer 18 and, as a result of the coupling of the process computer 18 to the controller 19, of the latter when determining the Heating power of the heat treatment device 13 are taken into account.

Desired changes in process parameters, e.g. a change in the dimension of the steel strip 1, a change in the chemical composition of the steel strip 1, a change in the zinc layer thickness or a change in the conveying speed of the steel strip 1 are entered into the process computer 18 to take into account the heating power of the heat treatment device.

Each of the control methods described above can be operated in a closed control loop.

The manipulated variables for the heat treatment device 13 are calculated by a computer of the controller 19 from the measured values and the target-actual deviation for the emissivity and possibly for the iron content. The measured values from the hot measurement (layer thickness measurement) and / or a temperature measurement arranged in front of the continuous furnace 10, the belt speed and the heating power supplied in the individual zones of the heat treatment device 13 can be used to increase the accuracy of the control process, as indicated by the arrows 20, 21 is indicated.

The manipulated variables are calculated with the aid of a control model, which can vary depending on the measuring devices and actuating devices available in the specific system. For a specific system configuration, the control model is described by model parameters. These model parameters can be different for different base material, strip dimension, Al content in the zinc bath. Base material, tape dimension, Al content in the zinc bath can be transferred from a higher-level computer (eg production planning computer) or an external input unit to the process computer 18. They can be transferred to the computer of controller 19 with the target value valid for the product to be manufactured, cf. Arrow 22.

The computer of the controller 19 then calculates the corresponding control commands taking into account these model parameters of the control model.

Depending on the design of the continuous furnaces 10, 11, the total output or the performance of parts of the continuous furnaces 10, 11 (zones in the longitudinal direction of the strip) can be set within certain limits. It is particularly advantageous if the distribution of the heat input onto the belt, that is to say the heating power of the continuous furnaces 10, 11, can also be adjusted within certain limits over the width, since this makes it possible to vary the deviation of the Fe shown in FIG. Compensate the content of the Zn-Fe layer, which can occur despite the uniform thickness of the Zn-Fe layer.

Claims (13)

Process for galvanizing a strip (1), in particular a steel strip (1), the strip (1) being continuously coated with zinc in a continuous process either electrolytically with zinc or according to the hot-dip galvanizing process in a zinc bath with zinc, then to form a Zn-Fe layer is subjected to a heat treatment in a continuous furnace (10, 11) and furthermore an on-line control of the zinc layer, the galvanizing process being controlled as a function of the on-line control, characterized in that in order to determine a through reaction of the Zn-Fe Layer, the radiation emission from the strip surface is measured during or after the heat treatment by means of at least one pyrometer (14) and the through-reaction at the point of the pyrometer is ensured by changing the heating power of the continuous furnace (10, 11) which serves as a control variable depending on the pyrometer display . Method according to Claim 1, characterized in that the point from which the Zn-Fe layer has reacted is determined on the strip passage section (3) by measurement by means of a plurality of pyrometers (14) arranged one behind the other in the strip travel direction (6) and by regulating the Heating capacity of the continuous furnace (10, 11) this point on the belt conveyor section (3) is brought into a desired position which acts as a reference variable. A method according to claim 1 or 2, characterized in that a value of the iron content of the Zn-Fe layer is determined as a reference variable, the actual value of the iron content of the Zn-Fe layer is compared with the reference variable and a control deviation via a controller (19) is compensated for by a change in the heating power of the continuous furnace (10, 11) serving as a control variable. Method according to one or more of claims 1 to 3, characterized in that the control is carried out in a closed control circuit with the aid of a computer which registers the control deviation and controls the heating power of the continuous furnace (10, 11) by means of control commands. Method according to claim 4, characterized in that the computer measures the strip dimension, the base material of the strip (1) with regard to its chemical composition and / or structure, the zinc layer thickness, the composition of the zinc bath, e.g. its Al content, the belt speed and possibly other parameters, such as the temperature of the belt (1) at the inlet of the continuous furnace (10, 11) and the ambient temperature are taken into account. Method according to one or more of claims 1 to 5, characterized in that the heating power and thus the temperature within the continuous furnace (10, 11) can be set differently in individual heating zones (12, 12 '). Method according to Claim 6, characterized in that the heating power can be set differently in heating zones (12, 12 ') which are adjacent to one another in the direction of the bandwidth. Method according to Claim 6 or 7, characterized in that the heating power can be set differently in the heating zones located one behind the other in the direction of the strip pass. Method according to one or more of claims 6 to 8, characterized in that the measurement of the radiation emission and / or the Fe content is carried out at locations distributed over the bandwidth. Device for carrying out the method according to one or more of claims 1 to 9, with a tape guiding device (2) guiding a tape (1) along a tape running section (3), a zinc coating device (4) arranged on the tape running section (3), and a subsequent one , A heat treatment device (13) for the strip (1) formed by an afterglow furnace and a measuring device (14) arranged in or after the heat treatment device (13) for checking the zinc layer, which is also located on the strip travel section, characterized in that the measuring device is formed by at least one pyrometer (14) which is coupled to a controller (19) which is coupled to the heating device of the heat treatment device (13) via a control line. Device according to claim 10, characterized in that a plurality of pyrometers arranged one behind the other in the strip running direction (6) and coupled to the controller (19) is provided. Device according to claim 10 or 11, characterized in that the controller (19) is coupled to a process computer (18). Device according to one or more of claims 10 to 12, characterized in that a measuring device (16) downstream of the heat treatment device (13) for measuring the iron content of the zinc layer is additionally provided, which is coupled to a controller (19) which is connected via a control line is coupled to the heating device of the heat treatment device (13).

Images