EP0959145B1 - Method and apparatus for carrying out the heating of a galvannealed process - Google Patents

Abstract

During annealing the product heating stage is interrupted by at least one further holding stage, resulting in a stepped increase of the temperature with time. The apparatus including a zone for inductive heating of the product and a zone for holding it at the end temperature has a holding zone (3a,3b) which interrupts the inductive heating zone (2a,2b).

Description

The invention relates to a method and an apparatus for Execution of the annealing of a galvannealing process, at the strips and sheets, especially of steel, after a Hot galvanizing are subjected to annealing Heating the coated material with the following Keep at glowing end temperature.

Becomes hot-dip galvanized sheet or strip after hot dipping annealed (at temperatures above the melting point of Zinc), that's what the product is called galvannealed sheet or strip, the process of galvanizing ("galvanizing") Hot-dip galvanizing of metals and "annealing" = annealing).

The coating of the coated tape treated in this way exists only from iron-zinc compounds with approx. 10-12% Fe.

The annealing of a galvannealing process goes a conventional one Hot-dip galvanizing process ahead. Here, the Steel surface cleaned first. Then one recrystallizing annealing of the hard-rolled input material performed in an oven under a protective gas atmosphere. The The strip is then cooled to the galvanizing temperature and hot-dip galvanized using an aluminum-containing zinc melt. Finally, the excess zinc melt stripped with air or nitrogen.

With such a coated steel strip, the galvannealing process by a subsequent annealing in completed an additional oven.

Such a galvannealing process is known from JP-08-165550 A. This describes a hot dip galvanizing process with two warming up steps during the Subsequent annealing treatment to a zinc galvanized sheet to improve that the plating layers when the Do not lift off the sheet (so-called "flaking").

Here runs between the steel matrix and the zinc coating a diffusion-controlled process. Depending on the the temperature and the glow time set for the glow Different FeZn phases are formed according to the zinc-iron state diagram out. Determine the respective phase proportions the total iron content of the coating.

The phase structure set for this annealing is for Quality of the coating as well as the usability of the galvannealing-treated Base material crucial, for example during the later deep-drawing process in the press shop.

This galvannealing furnace is used in conventional systems consisting of two zones: on the one hand, the zone for inductive heating of the tape and on the other hand the adjoining zone to keep at the desired final temperature. The stop zone is usually via resistance-heated or gas-fired Heated parts.

Galvannealing annealing and thus achieving a defined one The phase structure of the coating material is special depending on the parameters temperature and time. These important Parameters can be determined by the system parameters, the strip infeed temperature in the zinc smelt, the temperature of the Zinc melt, the aluminum concentration in the zinc melt and the thickness of the coating can be influenced. The most essential The influencing variable is the base material, i.e. the alloy composition of the steel and its condition.

Galvannealing-treated sheet is mainly found in the Automotive industry use and is characterized by good Weldability and paintability.

So far, have been used almost exclusively for this area of application IF steels as base material for deep-drawing, special deep-drawing and extra deep drawing grades for galvannealing treatment used.

IF steels (= abbreviation for interstitial free) are are steels that are not interstitial in the iron grid have dissolved atoms. The C and N atoms are through selectively added carbonitride formers (Ti, Nb, V). IF steels have no noteworthy levels strength-increasing elements such as P, Mn or B. The Element Si, however, can improve the adhesion of the Galvanealed coating can be added (up to approx. 0.10%).

To meet the demand for weight reduction in cars increasingly thin sheets are used, which but the same strength properties as conventional ones Must have sheets. This requirement can only be met by Use of high-strength steels, including high-strength IF steels, be fulfilled. Higher strength IF steels have noteworthy Stop the above items. Be subsequent higher-strength steels mentioned, are therefore also higher-strength IF steels, BH steels and TRIP steels are meant.

The two steel groups, IF steels and higher strength steels, have, however, in relation to the applied zinc coating a clearly different alloy behavior, especially with regard to their speed. in this connection Alloy formation in the high-strength steels runs significantly slower than with the IF steels.

The present invention is therefore based on the object a method and an apparatus for the annealing of a galvannealing process to propose the sheets and strips different base materials, in particular high-strength steels without being subjected to performance losses can

This task is accomplished by means of the features of the process claim 1 and the device features of claim 3 solved. Advantageous designs are in the subclaims disclosed.

The core of the invention is the adaptation of the glow cycle with regard to the parameters of temperature and time for the base materials, here in particular high-strength steels, to be taken into account the material-specific progress of the alloy. The proposed process engineering in the form of a gradual Annealing treatment is an opportunity for controlled adjustment the properties between base material and coating material as well as the covering material itself.

This annealing treatment should advantageously be carried out in this way be that the heating process with subsequent hold on Final temperature by a second holding process at one temperature interrupted below the final temperature.

Regarding the device features to build a suitable one It is proposed that this be a zone for inductive Heating the belt and another zone for Maintaining the tape at the warm-up temperature includes between the zone for inductive heating of the strip and the End stop zone at least one further stop zone is provided is.

In an advantageous embodiment, which is particularly for this oven is made of high-strength steel strips four zones, namely a first induction zone with one subsequent first stop zone and a second itself subsequent induction zone with subsequent second holding zone.

The proposed procedure of the gradual Annealing treatment in a galvannealing process as well as the proposed one The structure of the annealing furnace shows the following advantages:

By gradually increasing the temperature will adjust the annealing treatment to the slower ones Diffusion processes and thus alloying speed in higher strength Steels reached. The alloying process is controlled and adjustable. This makes it even Product quality under controllable production conditions possible. This gradual heating shows no disadvantages for the IF steels.

The annealing parameters, especially the heating temperature and - speed, are the alloy sequence of the combination Steel / coating material adapted. This is not the point overheating in the coating material without a Alloy formation occurs. Furthermore, one becomes possible counteracted increased evaporation of zinc. This is a major advantage for both the operation of the Galvannealed furnace as well as for the morphology of the Galvannealed coating.

To avoid overheating the zinc coating in conventional Galvannealing treatment ovens, only one Heating zone consisting of several or only one induction coil have, as well as for setting a controlled Alloying process would have the performance of the induction zone be lowered. But at the desired galvannealing temperature to achieve it is necessary to speed up the plant to diminish. But this is with a loss of performance connected to the hot-dip galvanizing plant.

In contrast to this, the method according to the invention means and the proposed furnace no loss in performance of the hot-dip galvanizing plant.

Figur 1

schematisch den Aufbau einer Ausführungsform des erfindungsgemäßen Ofenaufbaus zur Durchführung einer Glühung während eines Galvannealing-Prozesses;

Figuren 2a und b

schematisch den Aufbau von herkömmlichen Öfen zur Durchführung einer Glühung während eines Galvannealing-Prozesses;

Figur 3

den Bandtemperaturverlauf über der Zeit bei den verschiedenen Galvannealing-Ofenvarianten nach Figur 1 und Figuren 2a und b.

Further details and advantages of the invention emerge from the claims and the following description. Here show:

Figure 1

schematically the structure of an embodiment of the furnace structure according to the invention for performing an annealing during a galvannealing process;

Figures 2a and b

schematically the structure of conventional furnaces for performing annealing during a galvannealing process;

Figure 3

the strip temperature curve over time in the different galvannealing furnace variants according to FIG. 1 and FIGS. 2a and b.

Figure 1 shows schematically the structure of the invention Galvannealing furnace variant with interrupted heating zone. The galvanealing furnace 1 comprises a first zone 2a for inductive heating. This is followed by a holding zone 3a on. After this holding zone 3a, the belt is passed through again led a heating zone 2b. Then in one second holding zone 3b the coated strip to the final temperature held.

Figure 3 shows an example of such Step-by-step heating curve resulting from furnace build-up (Curve course c). The line speed is here 90m / min. The belt runs at an initial temperature of 420 ° C in the oven and will quickly open in a first stage Heated to 470 ° C. The belt then runs into the first holding zone (3a) and is kept at the intermediate temperature for about 7s. Then another heating process takes place on the Final glowing temperature of 520 ° C instead.

FIGS. 2a and b schematically show the structure of conventional ones Galvannealing ovens shown. Both variants exist from a first zone for inductive heating 2 as well a second, adjoining zone 3, for holding the Band to final temperature. Known systems with inductive Band heating have either in the inductive heating section 2 several induction coils 2a, b, c, d, usually 4-7 coils, on (Figure 2a) or include only a single induction coil 2 (Figure 2b). This single coil 2 has the same installed capacity as the previous several Coils together. The difference is in the essential smaller band area in the inductor, which makes the specific Performance or power density is significantly increased, what manifests itself in a higher heating rate.

The temperature-time profiles of the annealing treatments after Oven variants of FIGS. 2a and b are also shown in FIG. 3 shown. In contrast to the furnace according to the invention quickly reached the final glowing temperature. This is convenient for IF steels, the alloying point of which is already shorter Time is reached.

In the case of higher-strength steels, the alloying point also becomes due to higher alloy contents in the steel only after prolonged glow reached. To overheat the coating to avoid and a controlled alloy process between base material and coating material as well as in Setting the coating material yourself will be a Intermediate annealing introduced with subsequent heating up Glowing temperature. Furthermore, one is hereby possible Evaporation of zinc counteracted in the galvannealing furnace.

Claims (6)

1. Method of carrying out the annealing of a galvannealing process, in which strips and sheets of steel after a hot galvanising are subjected to an annealing by heating, in steps, the coated material with subsequent maintenance at annealing temperature, characterised in that for treatment of steels of different alloy compositions, which have a different alloying behaviour in relation to the applied zinc coating, during the annealing the heating steps of at least one further maintaining step are interrupted, wherein the hot-galvanised strip or sheet of steel is rapidly heated in a first heating step and thereafter enters into a first holding zone for performance of a maintaining step with a duration longer in relation to the heating step and subsequently is subjected to a second rapid heating step, wherein the heating takes place inductively during the heating steps and the heating takes place by means of resistance or gas heating during the respective maintaining steps.
2. Method according to claim 1, characterised in that the heating process with subsequent maintenance at final temperature is interrupted by a maintaining stage at a temperature below the final temperature.
3. Device for performance of the annealing of a galvannealing process according to claim 1, in which strips and sheets of steel after a hot-galvanising are subjected to an annealing by heating, in steps, of the coated material with subsequent maintenance at annealing temperature, comprising a zone for inductive heating of the strip or sheet of steel and a further zone for maintenance of the strip or sheet at final temperature, characterised in that the zone for inductive heating of the strip (2a, 2b) is interrupted by at least one further maintaining zone (3a), with a first heating zone (2a) for rapid heating of the hot-galvanised strip or sheet of steel, with a subsequent maintaining zone for performance of a maintaining step with a duration longer in relation to the heating step and with a second heating zone (2b) for rapid heating of the strip or sheet, wherein the heating zones (2a, 2b) are inductively heated and wherein the respective maintaining zones (3a) are resistance-heated or gas-heated.
4. Device according to claim 3, characterised in that a first induction zone (2a) with a downstream first maintaining zone (3a) as well as a second induction zone (2b), which adjoins thereat, with a downstream second maintaining zone (3b) are provided.
5. Device according to claim 4, characterised in that the zones for inductive heating (2a, 2b) consist of several induction coils.
6. Device according to claim 5, characterised in that the zones for inductive heating (2a, 2b) consist of an induction coil.

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