EP2010690B1 - Hot dip coating process for a steel plate product made of high strengthheavy-duty steel - Google Patents

Abstract

A method for coating a flat steel product manufactured from a high strength steel with a metallic coating, wherein the flat steel product is initially subjected to a heat treatment, in order then, in the heated state, to be hot-dip galvanized with the metallic coating in a melting bath containing overall at least 85% zinc and/or aluminum. The heat treatment includes heating the steel product in a reducing atmosphere, followed by converting a surface of the flat product to an iron oxide layer by a heat treatment lasting 1 to 10 secs in an oxidizing atmosphere, followed by annealing in a reducing atmosphere over a period of time which is longer than the duration of the formation of the iron oxide layer such that the iron oxide layer is reduced at least on its surface to pure iron, followed by cooling the product to a melting bath temperature.

Description

The invention relates to a method for coating a flat steel product made of high-strength, different alloying constituents, in particular Mn, Al, Si and / or Cr, containing steel, such as steel strip or sheet, with a metallic coating, in which the flat steel product is subjected to a heat treatment, to then be provided in the heated state in a total of at least 85% zinc and / or aluminum melt bath by hot dip coating with the metallic coating.

In automotive body construction, hot-rolled or cold-rolled sheets made of steel are used, which are surface-finished for reasons of corrosion protection. The demands placed on such sheets are many. They should on the one hand be well deformable and on the other hand have a high strength. The high strength is achieved by adding certain alloying constituents, such as Mn, Si, Al and Cr, to the iron.

In order to optimize the property profile of high-strength steels, it is customary to anneal the sheets in the molten bath immediately before coating with zinc and / or aluminum. During the hot dip coating of steel strips containing only small amounts of the mentioned Alloy constituents contained, is problematic, there are in the hot dip coating of steel sheet with higher alloy levels in conventional approach difficulties. This results in areas in which the coating adheres insufficiently to the respective steel sheet or remain completely uncoated.

There are many attempts in the prior art to avoid these difficulties. However, an optimal solution of the problem does not seem to exist yet.

In a known process for hot dip coating a steel strip with zinc, the strip to be coated passes through a directly heated pre-heater (DFF = Direct Fired Furnace). By changing the gas-air mixture, an increase in the oxidation potential in the atmosphere surrounding the band can be produced at the gas burners used. The increased oxygen potential leads to oxidation of the iron at the strip surface. In a subsequent furnace section, the iron oxide layer thus formed is reduced. A targeted adjustment of the oxide layer thickness at the strip surface is very difficult. At high belt speed, it is thinner than at low belt speed. Consequently, in the reducing atmosphere, no clearly defined condition of the tape surface can be produced. This in turn can lead to adhesion problems of the coating on the strip surface.

In modern hot-dip coating lines with a RTF (RTF = Radiant Tube Furnace) are in contrast used for the above known system no gas-fired burner. A pre-oxidation of the iron via a change in the gas-air mixture can therefore not take place. In these systems, rather, the complete annealing of the strip takes place in a protective gas atmosphere. In such an annealing of a strip of steel with higher alloying constituents, however, these alloying constituents can diffuse to the strip surface and form non-reducible oxides here. These oxides hinder proper coating with zinc and / or aluminum in the molten bath.

The patent literature also describes various processes for hot dip coating a steel strip with various coating materials.

So is out of the DE 689 12 243 T2 a method for continuous hot dip coating of a steel strip with aluminum, in which the strip is heated in a continuous furnace. In a first zone, surface contaminants are removed. But the furnace atmosphere has a very high temperature. However, as the belt passes through this zone at high speed, it is only heated to about half the temperature of the atmosphere. In the subsequent second zone, which is under protective gas, the strip is heated to the temperature of the coating material aluminum.

Furthermore, from the DE 695.07 977 T2 a two-stage hot dip coating method of a chromium-containing steel alloy strip is known. According to this method, the strip is annealed in a first stage to be at the Band surface to obtain an iron enrichment. Subsequently, the tape is heated in a non-oxidizing atmosphere to the temperature of the coating metal.

From the JP 02285057 A It is also known to galvanize a steel strip in a multi-stage process. For this, the previously cleaned band is treated in a non-oxidizing atmosphere at a temperature of about 820 ° C. Then, the tape is treated at about 400 ° C to 700 ° C in a weak oxidizing atmosphere before being reduced on its surface in a reducing atmosphere. Finally, the cooled to about 420 ° C to 500 ° C strip is galvanized in the usual way.

The document JP 02 285057 discloses a method of hot dip coating a steel strip that includes a preheat treatment. The heat treatment includes heating the tape in a reducing atmosphere, a second step of heating in an oxidizing atmosphere, and further heating up to 800 ° C in a reducing atmosphere.

The document US 2004/177903 discloses a process for hot dip coating a high strength steel strip with various oxidizable alloying constituents. The method includes heating in a reducing atmosphere, and further, the heat treatment furnace comprises a region consisting of an oxidizing atmosphere.

The invention had the object of specifying a method for hot dip coating of a high-strength steel produced flat steel product with zinc and / or aluminum, with which a steel strip can be produced with an optimally finished surface in a RTF plant.

1. a) Das Band wird in einer reduzierenden Atmosphäre mit einem H₂-Gehalt von mindestens 2 % bis 8 % auf eine Temperatur von > 750 °C bis 850 °C erwärmt.
2. b) Die überwiegend aus Reineisen bestehende Oberfläche wird durch eine 1 bis 10 sec dauernde Wärmebehandlung des Bandes bei einer Temperatur von > 750 °C bis 850 °C in einer im Durchlaufofen integrierten Reaktionskammer mit einer oxidierenden Atmosphäre mit einem O₂-Gehalt von 0,01 % bis 1 % in eine Eisenoxidschicht umgewandelt.
3. c) Das Stahlflachprodukt wird anschließend in einer reduzierenden Atmosphäre mit einem H₂-Gehalt von 2 % bis 8 % durch Erwärmung bis auf maximal 900 °C über einen Zeitraum geglüht, der um so viel länger ist als die Dauer der zur Bildung der Eisenoxidschicht durchgeführten Wärmebehandlung (Verfahrensschritt b), dass die zuvor gebildete Eisenoxidschicht mindestens an ihrer Oberfläche in Reineisen reduziert wird.
4. d) Das Stahlflachprodukt wird anschließend bis auf Schmelzbadtemperatur abgekühlt.

This object has been achieved on the basis of a method of the type specified at the outset by following the following method steps in the course of the heat treatment preceding the hot-dip coating:

1. a) The strip is heated in a reducing atmosphere with an H ₂ content of at least 2% to 8% to a temperature of> 750 ° C to 850 ° C.
2. b) The predominantly made of pure iron surface is characterized by a 1 to 10 sec continuous heat treatment of the strip at a temperature of> 750 ° C to 850 ° C in a continuous furnace integrated reaction chamber with an oxidizing atmosphere with an O ₂ content of 0, 01% to 1% converted into an iron oxide layer.
3. c) The flat steel product is then annealed in a reducing atmosphere with an H ₂ content of 2% to 8% by heating up to a maximum of 900 ° C for a period of time much longer than the duration of the iron oxide layer formation Heat treatment (process step b) that the previously formed iron oxide layer is reduced at least on its surface in pure iron.
4. d) The flat steel product is then cooled down to the molten bath temperature.

The temperature control according to the invention in step a) prevents that during the heating essential alloying constituents diffuse to the surface of the flat steel product. Surprisingly, it has been found here that by adjusting relatively high temperatures above 750.degree. C. and up to a maximum of 850.degree. C., the diffusion of alloy constituents to the surface is particularly effectively suppressed so that an effective iron oxide layer can be formed in the following step , This prevents further alloying constituents from diffusing to the surface during the subsequently increased annealing temperature. Thus, during the annealing treatment in the reducing atmosphere, a pure iron layer can be formed which is suitable for a full-surface and firmly adhering coating of zinc and / or aluminum is very suitable.

The work result can be optimized by completely reducing the iron oxide layer produced in the oxidizing atmosphere to pure iron. In this state, the coating also has optimum properties with regard to its deformability and strength.

According to one embodiment of the invention, in the treatment of the flat steel product on the route with the oxidizing atmosphere, the thickness of the forming oxide layer is measured and adjusted depending on this thickness and dependent on the flow rate of the flat steel product treatment time of O ₂ content such that the Oxide layer can then be completely reduced. The change in the flow rate of the flat steel product z. B. as a result of disturbances can be considered in this way without detriment to the surface quality of the hot dip coated flat steel product.

Good results have been achieved in carrying out the method when an oxide layer with a maximum thickness of 300 nanometers is produced.

A diffusion of alloy constituents to the surface of the flat steel product can also be counteracted by the heating in step a) of the process according to the invention taking place as rapidly as possible. Good work results are in particular then if the duration of the heating upstream of the oxidation of the flat steel product to more than 750 ° C to 850 ° C to max. 300 s, in particular max. 250 s, is limited.

Accordingly, it is favorable if the heating rate in the case of the heating of the flat steel product preceding the oxidation according to the invention is at least 2.4 ° C./s, in particular in the range from 2.4 to 4.0 ° C./s.

By contrast, the heat treatment followed by oxidation followed by cooling of the flat steel product should take more than 30 seconds, in particular more than 50 seconds, in order to ensure a sufficiently sufficient reduction of the previously formed iron oxide layer to pure iron.

As alloy constituents, the high-strength steel may contain at least one of the following constituents: Mn> 0.5%, A1> 0.2%, Si> 0.1%, Cr> 0.3%. Other ingredients such. Mo, Ni, V, Ti, Nb and P can be added.

When carrying out the process according to the invention, the heat treatment of the flat steel product in the reducing atmosphere, both during warm-up and later annealing, lasts many times longer than the heat treatment in the oxidizing atmosphere. In this way it is achieved that the volume of the oxidizing atmosphere is very small compared to the remaining volume of the reducing atmosphere. This has the Advantage that can be reacted quickly to changes in the treatment process, in particular the flow rate and the formation of the oxidation layer. In practice, therefore, the inventive heat treatment of the flat steel product in the reducing atmosphere can be carried out in a continuous furnace, which is equipped with a chamber containing the oxidizing atmosphere, wherein the volume of the chamber can be many times smaller than the remaining volume of the continuous furnace.

• Z: 99 % Zn
• ZA: 95 % Zn + 5 % A1
• AZ: 55 % A1 + 43,4 % Zn + 1,6 % Si
• AS: 89 - 92 % A1 + 8 - 11 % Si

The inventive method is particularly well suited for hot dip galvanizing. The molten bath may also consist of zinc-aluminum or aluminum with silicon additives. Regardless of which melt composition is selected, the total present in the melt zinc and / or aluminum content should be at least 85% in total. Such composite melts are z. B .:

• Z: 99% Zn
• ZA: 95% Zn + 5% A1
• AZ: 55% A1 + 43.4% Zn + 1.6% Si
• AS: 89 - 92% A1 + 8 - 11% Si

In the case of a pure Zinküberzügs (Z) this can be converted by heat treatment (diffusion annealing) in a ductile zinc-iron layer (galvanized coating).

The invention will be explained in more detail with reference to a drawing illustrating an exemplary embodiment.

The single figure shows schematically a galvanizing plant with a continuous furnace 5 and a melt bath 7. In addition, the temperature profile over the cycle time is plotted in the figure for the continuous furnace.

The galvanizing plant is intended for continuous coating of a flat steel product in the form of hot rolled or cold rolled steel strip 1, which is made of higher strength steel containing at least one alloying element of the Mn, Al, Si and Cr group and optionally further alloying elements containing certain alloying elements. The steel may in particular be a TRIP steel.

The steel strip 1 is withdrawn from a coil 2 and passed through a pickling 3 and / or another system 4 for surface cleaning.

The cleaned belt 1 then passes through a continuous furnace 5 in a continuous operation and is passed from there via a sealed relative to the surrounding atmosphere trunk 6 in a hot dip bath 7. The hot-dip bath 7 is presently formed by a molten zinc.

The emerging from the hot dip 7, provided with the zinc coating steel strip 1 passes through a Cooling section 8 or a device for heat treatment to a winding station 9, in which it is wound into a coil.

If necessary, the steel strip 1 is meander-shaped passed through the continuous furnace 5 in order to achieve sufficiently long treatment times with practical length of the continuous furnace 5 can.

The RTF (RTF = Radiant Tube Furnace) continuous furnace 5 is divided into three zones 5a, 5b, 5c. The middle zone 5b forms a reaction chamber and is atmospherically closed with respect to the first and last zones 5a, 5c. Their length is only about 1/100 of the total length of the continuous furnace 5. For better illustration, the drawing is not to scale extent.

According to the different lengths of the zones and the treatment times of the continuous belt 1 in the individual zones 5a, 5b, 5c are different.

In the first zone 5a there is a reducing atmosphere. A typical composition of this atmosphere consists of 2% to 8% H ₂ , typically 5% H ₂ , and balance N ₂ .

In the zone 5a of the continuous furnace 1, the strip is heated to more than 750 to 850 ° C, typically 800 ° C. The heating takes place at a heating rate of at least 3.5 ° C / s. At this temperature and heating rate diffuse in the steel strip. 1 containing alloying ingredients in only small amounts at the surface.

In the central zone 5b of the continuous furnace 5, the steel strip 1 is kept substantially only at the temperature reached in the first zone 5a. However, the atmosphere of the zone 5b is oxygen-containing, so that oxidation of the surface of the steel strip 1 occurs. The O ₂ content of the atmosphere prevailing in zone 5b is between 0.01% to 1%, typically 0.5%. In this case, the oxygen content of the atmosphere prevailing in the zone 5b can be adjusted, for example, as a function of the treatment time and the thickness of the oxide layer to be produced on the steel bath 1. If the treatment time is short, for example, a high O ₂ content is set, while with a long treatment time, for example, a lower oxygen content can be selected in order to produce an oxide layer of the same thickness.

As a result of the surface of the steel strip 1 being exposed to an oxygen-containing atmosphere, the desired iron oxide layer forms on the surface of the strip. The thickness of this iron oxide layer can be detected optically, the result of the measurement being used to set the respective oxygen content of the zone 5b.

Since the central zone 5b is very short in comparison to the entire furnace length, the chamber volume is correspondingly small. Therefore, the reaction time for a change in the composition of the atmosphere is small, so that on a Changing the belt speed or to a different thickness of the target thickness of the oxide layer by a corresponding adjustment of the oxygen content of the prevailing atmosphere in the zone 5b can be reacted quickly. The small volume of Zone 5b allows for short control times.

In the subsequent to the zone 5b zone 5c of the continuous furnace 5, the steel strip 1 is heated to an annealing temperature of about 900 ° C. The annealing carried out in zone 5c takes place in a reducing nitrogen atmosphere which has an H ₂ content of 5%. On the one hand, during this annealing treatment, the iron oxide layer prevents alloying constituents from diffusing to the strip surface. On the other hand, since the annealing treatment is performed in a reducing atmosphere, the iron oxide layer is converted into a pure iron layer.

The steel strip 1 is further cooled on its further way in the direction of the hot dip bath 7, so that it has a temperature when leaving the continuous furnace 5, which is higher by up to 10% than the temperature of the hot dip bath 7 of about 480 ° C. Since the strip 1 is made of pure iron after leaving the continuous furnace 5 on its surface, it provides an optimum basis for a firmly adhering connection of the zinc coating applied in the hot dip bath 7.

Claims (11)

1. Method for the coating of a flat steel product manufactured from a higher strength steel containing different alloy constituents, in particular Mn, Al, Si and/or Cr, with a metallic coating, wherein the flat steel product is initially subjected to a heat treatment, in order then, in the heated state, to be hot-dip coated with the metallic coating in a melting bath containing overall at least 85% zinc and/or aluminium, characterised in that the heat treatment comprises the following method steps:

   a) The flat steel product is heated in a reducing atmosphere with an H₂ content of at least 2% to 8% to a temperature of > 750°C to 850°C.

   b) The surface, consisting predominantly of pure iron, is converted into an iron oxide layer by a heat treatment of the flat steel product lasting 1 to 10 secs. at a temperature of > 750°C to 850°C in a reaction chamber integrated into the continuous furnace, with an oxidising atmosphere with an O₂ content of 0.01% to 1%.

   c) The flat steel product is then annealed in a reducing atmosphere with an H₂ content of 2% to 8% by heating to a maximum of 900°C over a period of time which is that much longer than the duration of the heat treatment carried out for the formation of the iron oxide layer (method step b) such that the iron oxide layer formed previously is reduced at least on its surface to pure iron.

   d) The flat steel product is then cooled to melting bath temperature.
2. Method according to Claim 1, characterised in that the iron oxide layer produced is completely reduced to pure iron.
3. Method according to Claim 2, characterised in that, during the treatment of the flat steel product on the stretch with the oxidising atmosphere, the thickness of the oxide layer being formed is measured and, as a function of this thickness and of the treatment time, dependent on the run-through speed of the flat steel product, the O₂ content is adjusted in such a manner that the oxide layer is then completely reduced.
4. Method according to Claim 3, characterised in that an oxide layer is produced with a thickness of max 300 nm.
5. Method according to any one of the preceding claims, characterised in that the heating of the flat steel product upstream of the oxidation to more than 750°C to 850°C lasts for a max. 300 secs.
6. Method according to any one of the preceding claims, characterised in that the further heat treatment downstream of the oxidation with following cooling of the flat steel product lasts longer than 30 secs.
7. Method according to any one of the preceding claims, characterised in that the higher strength steel contains at least a selection of the following alloy constituents: Mn > 0.5 %, Al > 0.2 %, Si > 0.1 %, Cr > 0.3 %.
8. Method according to any one of the preceding claims, characterised in that the heat treatment of the flat steel product in the reducing atmosphere takes place in a continuous furnace with an integrated chamber with the oxidising atmosphere, wherein the volume of the chamber is many times smaller than the remaining volume of the continuous furnace.
9. Method according to any one of the preceding claims, characterised in that the flat steel product is heat treated after the hot-dip galvanizing.
10. Method according to any one of the preceding claims, characterised in that the heating-up speed during the heating of the flat steel product upstream of the oxidation amounts to at least 2.4°C/s.
11. Method according to Claim 10, characterised in that the heating-up speed amounts to 2.4 - 4.0°C/s.

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