EP1203106B1 - Method and installation for hot dip galvanizing hot rolled steel strip - Google Patents

Abstract

A method for hot dip galvanizing of hot-rolled steel strip, wherein in a first method step, the strip (50) is introduced into a pickling station (10-13) and a layer of scale and reaction products are removed from the strip surface in the pickling station. In another method step, the strip (50) is introduced into a rinsing station (21-23) and residues of the pickle and pickling products are removed from the strip surface in the rinsing station, and subsequently the strip is introduced into a drying station and is dried. And from there, in another method step, the strip is introduced into a furnace (40) and is adjusted to galvanizing temperature under a protective gas atmosphere. In a last method step, the strip is guided through a galvanizing bath and the surface of the (50) is coated with a hot dip galvanizing layer in the galvanizing bath, wherein the strip temperature in the furnace (40) is adjusted at most to 50° K. above immersion temperature of the strip (50) into the zinc bath.

Description

The invention relates to a method and a system for hot dip galvanizing hot-rolled steel strip, the strip being in a first process step introduced into a pickling station and therein a layer of scale and reaction products be removed from the belt surface. In a further process step the belt is inserted into a rinsing station and the Belt surface washed away from residues of the pickling and pickling products, and then inserted into a drying station and dried in it. From there in a further process step, the strip is introduced into an oven and set in a protective gas atmosphere to the galvanizing temperature, and in a final process step through a galvanizing bath and the surface of the strip is coated with a hot-dip galvanizing layer, the belt temperature in the furnace is a maximum of 50 ° K above the immersion temperature is placed in the zinc bath.

The hot-dip coating, in particular the hot-dip galvanizing of hot-rolled steel strip, so-called hot strip, is becoming more and more important economically compared to conventional cold-rolled strip coating.
Through the development of thin slab technology in hot strip, there is the technical possibility of producing hot strips in the thickness range below 1.2 mm from the casting heat. There is also the possibility, depending on customer requirements, to replace cold strip with comparatively cheaper hot strip.

For hot-dip coating, especially hot-dip galvanizing steel strips, different processes and systems are known. This is what it is about are mainly plant types in which cold-rolled strips are used come.

In such systems, the actual coating process is an annealing furnace upstream, in which at high temperatures a structural transformation to achieve the desired mechanical properties. The existing one Temperature difference between molten bath, preferably zinc or Zinc alloys, and the maximum strip temperature can be up to 400 ° C. With this tape overheating, however, a fire coating is not feasible, which is why the tape cools down before coating Temperatures close to the melting bath temperature must be made.

Hot strip or pre-annealed cold strip, however, do not require annealing in order to influence the mechanical properties, rather the Band temperature only adjusted to that of the melt pool to the desired Reaction of the steel strip surface with the alloy components to reach the weld pool. In contrast is a high temperature annealing often even disadvantageous for the mechanical properties of the belt.

By way of example, the present invention relates exclusively to the process variants for hot strip hot finishing or hot strip hot galvanizing.

The desired temperature level, especially for hot-dip hot-dip galvanizing, lies with previously operated systems for fire coating still higher than the required 450 ° C of the zinc bath. reason for that is the required removal of all oxidation products and their precursors the steel strip surface. Oxidation products inevitably arise in the transition area from the pickling stage via the rinsing and drying stage into the oven entrance by exposure to atmospheric oxygen. The amount and training of in Oxidation products entering the furnace and those brought in from the belt Atmospheric oxygen determine the necessary process parameters of the treatment procedure, characterized by a required reduction potential, Temperature level and holding time. The temperature level used is often high so high that the strip is additionally cooled before entering the zinc bath must become.

Another way of working is by significantly increasing the temperature level marked in the zinc bath to values above 460 ° C. Especially a disadvantage of this procedure is the increased occurrence of zinc-containing Slag. On the one hand, this leads to increased material and operating costs for the Zinc bath, as well as loss of quality on the assembly line.

Das Dokument US 4,143,183 beschreibt bei einem Verfahren zum kontinuierlichen Galvanisieren eines Stahlbandes die folgenden Schritte:

• Vorheizen bis zu einer Temperatur, die geeignet ist zum Erreichen erwünschter mechanischer Eigenschaften des Bandes. Die Vorheiztemperatur wird zwischen 400° C und 950° C eingestellt. Abschrecken des Bandes in einem wässrigen Bad bei dessen Siedetemperatur. Herausnehmen des Bandes aus dem Bad. Dabei soll eine Oxydschicht von weniger als 2 g/m² ausgebildet sein. Eliminieren der Oxydschicht von wenigstens einem Oberflächenbereich des Bandes und Einstellen der Temperatur im Bereich von 420° und 550° C und Aufrechterhalten dieser Temperatur.

The documented state of the art also shows the following:

The document US 4,143,183 describes the following steps in a method for the continuous electroplating of a steel strip:

• Preheat to a temperature suitable for achieving the desired mechanical properties of the belt. The preheating temperature is set between 400 ° C and 950 ° C. Quench the tape in an aqueous bath at its boiling temperature. Take the tape out of the bathroom. An oxide layer of less than 2 g / m ^{2 should be} formed. Eliminate the oxide layer from at least one surface area of the tape and adjust the temperature in the range of 420 ° and 550 ° C and maintain that temperature.

Document WO 83/00885 discloses a metal band, of which the iron oxides were removed by reduction in a protective gas atmosphere, and its Temperature brought a little higher than the melting temperature of the zinc bath with the metal strip immersed in the conventional zinc bath becomes. The bath contains mixed metal in a ratio between 0.01% and 0.5 % preferably between 0.02% and 0.08%.

Starting from the aforementioned prior art, the object of the invention based on specifying a method and a hot strip galvanizing plant, which overcomes the aforementioned disadvantages and difficulties and hot-dip galvanized with an economical outlay on material and operating costs Steel strip delivers high and flawless surface quality.

To solve the problem is in a method in the preamble of claim 1 mentioned type with the invention that the method steps between the last rinsing stage of the rinsing station via the drying station to the inlet of the furnace under hermetic shielding against air and Oxygen can be carried from the environment.

A corresponding plant for performing the method according to the invention accordingly provides that the outlet of the last rinsing stage of the rinsing station with the inlet of the dryer and its outlet with the inlet of the Furnace connected by locks and facing the surrounding Atmosphere are hermetically sealed.

Further expedient refinements of the method on the one hand and on the other hand the hot strip galvanizing plant are in accordance with the characteristics of Subclaims provided.

• direkte Kopplung zumindest der letzten Spülstufe der Spülstation über die Trocknungsstufe mit dem Ofeneingang unter Abschirmung von Luftsauerstoff,
• Auftragen eines wasserbindenden Mediums, bevorzugt NH₃, oder einer Lösung davon, auf das Band in der Spülstufe, wonach sich in der anschließenden Trocknungsstufe das wasserbindende Medium schnell und rückstandsfrei, das heißt ohne Eintrag von Sauerstoff oder flüssiges Reinigungsmedium vom Band entfernen lässt,
• alternativ durch einen Betrieb der Trockenstufe mit einer reduzierend wirkenden Atmosphäre, zum Beispiel N₂/H₂-Gasgemisch.

Advantageously, the method and the system according to the invention ensure that the optimal surface condition of the strip which has been achieved after the strip has passed through the pickling station and rinsing station is preserved in the subsequent drying stage and during the transition in the furnace areas and from there into the galvanizing bath.
This is achieved through:

• direct coupling of at least the last rinsing stage of the rinsing station via the drying stage to the oven entrance with shielding from atmospheric oxygen,
• Applying a water-binding medium, preferably NH ₃ , or a solution thereof, to the belt in the rinsing stage, after which the water-binding medium can be removed from the belt quickly and without residue in the subsequent drying stage, that is to say without oxygen or liquid cleaning medium being introduced,
• alternatively by operating the drying stage with a reducing atmosphere, for example N ₂ / H ₂ gas mixture.

The above-mentioned measures preserve the optimal strip condition after pickling into the oven and achieve an optimal setting of the strip temperature when immersed in the zinc bath. The access of oxygen and the associated surface reactions, especially oxidation, are prevented. This enables furnace operation at temperatures in the range of the melt bath temperature. Overheating of the belt and an extension of the holding time in the oven are not necessary. A belt cooler becomes superfluous. Overall, the procedure according to the invention and the corresponding system allow a much more compact design of the furnace element and lower investment and operating costs. At the same time, furnace operation with lower H ₂ contents in the protective gas is possible. The disadvantages in the aforementioned conventional method with an increased zinc bath temperature are advantageously avoided.
According to the invention, the strip is namely set to a temperature which is at most 50 ° K higher than the immersion temperature in the zinc bath.

Figur 1

ein Layout einer Feuerverzinkungsanlage nach dem Stand der Technik,

Figur 2

ein Layout einer Feuerverzinkungsanlage nach der Erfindung.

Further details, features and advantages of the invention result from the following explanation of an exemplary embodiment shown schematically in the drawings. Show it:

Figure 1

a layout of a hot-dip galvanizing plant according to the prior art,

Figure 2

a layout of a hot-dip galvanizing plant according to the invention.

According to the layout of a conventional hot-dip galvanizing plant shown in FIG in a first process step, a strip 50 is transferred to a pickling station 10 introduced with three pickling stages 11 to 13 and in it a scale layer as well as reaction products removed from the belt surface. Usually pickling in the pickling station 10 or in the pickling stages 11, 12, 13 using hydrochloric acid (HCL).

In the following process step, the belt 50 is brought into the rinsing station 20 the rinse stages 21 to 23 introduced and therein the belt surface of residues the stain and the stain products exempt. Then the tape is in the drying station 30 inserted and dried therein. From there the tape 50 in a further method step into an oven 40, comprising a preheating stage 41 and an integrated temperature control stage 42, introduced and therein preferably heated to a tempering temperature in a protective gas atmosphere, and passed through a galvanizing bath in a last process step. The surface of the strip 50 is coated with a hot-dip galvanizing layer overdrawn.

In contrast to the conventional galvanizing plant according to Figure 1 according to the layout of the hot-dip galvanizing plant according to the invention according to FIG 2 the process steps between the last rinsing stage 23 of the rinsing station 20 via the drying station 30 to the inlet 43 of the tempering furnace 40 carried out under hermetic shielding of atmospheric oxygen from the environment.

By expanding the rinsing station 20 by a rinsing stage 23 or by partitioning the rinsing stage 23 with the aid of a partition 24 from the previous rinsing stages 21, 22, a water-repellent or binding medium 25 is introduced into the rinsing stage 23. For example, NH ₃ or a solution of NH ₃ can be used as the medium.

A preferred embodiment of the method provides that the belt 50 is rinsed in the rinsing station 20 in the first stages 21, 22 with deionized water and in the third stage 23 with or with the addition of NH ₃ as the drying medium.

The belt 50 is dried in the drying station 30 without air being supplied. According to the invention, drying is carried out by means of heat radiation with the addition of a mixture of nitrogen, hydrogen and ammonia gas (N ₂ / NH ₃ ) or H ₂ .

The drying station 30 is on both sides with locks 70, 80 following the neighboring stations 20 and 40 hermetically against the entry of atmospheric oxygen completed, the outlet of the last rinsing stage 23 of the rinsing station 20 with the inlet of the drying station 30 and its outlet with the inlet 43 of the tempering furnace 40 connected by locks 70, 80 and are hermetically sealed from the surrounding atmosphere.

The measures according to the invention maintain the optimum strip state after pickling into the tempering furnace because the introduction of atmospheric oxygen is prevented. As a result, as can be seen from the illustration of the tempering furnace 40 in FIG. 2, its design can be simplified due to the lower heating power required and the cooling section eliminated and can be realized with more favorable investment and operating costs. Oven operation is also possible with comparatively low H ₂ contents in the protective gas.

Claims (6)

1. Method of hot-galvanising hot-rolled steel strip, wherein:

   in a first step the strip (50) is introduced into a pickling station (10-13) and a scale layer as well as reaction products are removed from the strip surface,

   in a further step the strip (50) is introduced into a washing station (21-23) and the strip surface is freed therein from residues of the pickling and pickling products, and subsequently

   the strip is introduced and dried in a drying station and from there

   introduced in a further step into an oven (40) and adjusted therein to galvanising temperature under a protective gas atmosphere, and

   in a last step the strip is conducted through a galvanising bath and the strip surface (50) is then covered with a hot-galvanising layer, wherein the strip temperature in the oven (50) is set to at most 50° K above immersion temperature in the zinc bath,

   characterised in that the method steps between the last washing stage (23) of the washing station (20) through the drying station (20) up to the inlet (43) of the oven (40) are carried out with hermetic shielding against air and oxygen from the environment.
2. Method according to claim 1, characterised in that in the last washing stage (23) of the washing station (20) a water-repelling or water-binding medium (25) wetting the strip (50) is fed.
3. Method according to claim 2, characterised in that the medium (25) fed in the third washing stage (23) is NH₃ or a solution containing NH₃.
4. Method according to one or more of claims 1 to 3, characterised in that the drying of the strip (50) in the drying station (30) is carried out without feed of air from the outside by means of heat radiation with addition of a mixture of nitrogen, hydrogen and ammonia gas (N₂/NH₃) + H₂ or a mixture of two of the said gases.
5. Hot strip galvanising plant, comprising a pickling station (10), a washing station (20), a drier (30), an oven (40) as well as a downstream hot-galvanising bath (60), characterised in that the outlet of the last washing stage (23) of the washing station (20) is connected with the inlet of the drier (39), and the outlet thereof is connected with the inlet (43) of the furnace (40), by locks (70, 80) and are hermetically sealed off relative to the ambient atmosphere.
6. Plant according to claim 5, characterised in that the washing stages (21, 23) and the heating stages (41) or the temperature-controlling stage (42) are partitioned off relative to one another by intermediate walls (24).

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