DE102018107435A1 - Process for the pre-oxidation of strip steel in a reaction chamber arranged in a furnace chamber - Google Patents

Abstract

The invention relates to an improved process for the pre-oxidation of high-strength strip steel in a reaction chamber arranged in a furnace chamber.
The reaction chamber is sealed at a belt inlet and a belt outlet against gas exchange between the furnace chamber and the reaction chamber, and a gas forming an oxidizing atmosphere in the reaction chamber is introduced and the gas is thereby continuously circulated within the reaction chamber.

Description

The invention relates to an improved process for the preoxidation of oxidation-sensitive steel strip in a reaction chamber arranged in a furnace chamber, in order to thereby set suitable surface properties of the strip steel to be coated for a directly following hot dip coating.

Conventional high-strength strip steels contain manganese, silicon and / or aluminum as alloying elements. During the possible recrystallizing annealing prior to the hot dip coating, these alloying elements diffuse towards the strip surface. Since these alloying elements are very oxygen-affine, they are almost inevitably oxidized as far as they are at the strip surface or at a shallow depth in the strip. However, the basic material iron is not oxidized. This phenomenon is also known as selective oxidation. However, the surface, by the selective oxidation formed manganese, silicon, and / or aluminum oxides affect the wettability of the strip surface with a molten coating metal (for example zinc), with the result of defects (so-called bare spots) or a poor adhesion the coating with the tape surface. The alloy composition is decisive for the coating problems on high-strength steel, in particular the tendency to form non-reducible oxides at the surface.

This applies, for example, to the following steel grades: group C max [%] Si max [%] Mn max [%] Cr + Mo max [%] DP 0.14-0.23 0.5 - 1.0 1.8 - 2.9 1.0 - 1.4 CP 0.18 - 0.23 1.0 2.5 - 2.9 1.0 TRIP 0.23-0.25 1.8 - 2.2 2.1 - 2.5 0.2 Q & P 0.10 - 0.30 1.0 - 2.0 1.5 - 3.0

1. 1. Erwärmen des Bandes unter reduzierender Atmosphäre, mit 2 bis 3 % Wasserstoffanteil, bis auf 650 bis 750°C;
2. 2. Oxidieren der weitgehend aus Reineisen bestehenden Bandoberfläche in einer Reaktionskammer mit einer Atmosphäre mit 0,01 bis 1 % Sauerstoffanteil. Dabei wird eine Eisenoxidschicht gebildet, welche die vorher gebildeten Legierungsoxide abdeckt. Die Behandlungsdauer soll 1 bis 10 sec und die Dicke der gebildeten Oxidschicht soll 300 nm betragen;
3. 3. Glühen des Bandstahls unter reduzierender Atmosphäre mit 2 bis 8% Wasserstoffanteil bis auf maximal 900°C. Dabei wird die Eisenoxidschicht wieder zu Reineisen reduziert, auf der dann das Überzugsmetall gut und sicher haftet.

In order to improve the adhesion of the coating to the strip surface, is in the DE 102 004 059 566 a method is described in which the strip is pre-oxidized. The method described in this document can be summarized as follows:

1. 1. heating the ribbon under reducing atmosphere, with 2 to 3% hydrogen content, up to 650 to 750 ° C;
2. 2. Oxidizing the existing largely pure iron strip surface in a reaction chamber with an atmosphere with 0.01 to 1% oxygen content. In this case, an iron oxide layer is formed, which covers the previously formed alloy oxides. The treatment time should be 1 to 10 sec and the thickness of the oxide layer formed should be 300 nm;
3. 3. annealing the strip steel under a reducing atmosphere with 2 to 8% hydrogen content up to a maximum of 900 ° C. The iron oxide layer is reduced again to pure iron, on which then adheres the coating metal well and safely.

Here, the reaction chamber, with a strongly oxidizing atmosphere in the interior, in the furnace chamber of a continuous furnace with a hydrogen-containing, reducing atmosphere. Bandeinlauf and belt outlet in the reaction chamber must be sealed as possible against gas exchange. A gas transfer from the furnace into the reaction chamber causes the penetrating hydrogen at least partially consumes the oxygen required for the oxidation and impairs the nature of the desired oxide layer on the strip surface. This problem is exacerbated by the lower the oxygen content in the reaction chamber. Conversely, a gas transfer from the reaction chamber into the furnace causes a higher water content (dew point) in the furnace and thus an increased oxidation potential. This is disadvantageous in particular for very high strength steels with a higher content of oxygen affinity alloying elements.

Experiments have shown that for setting a desired oxide layer, the strip temperature is the decisive parameter for process control. This is preferably between 650 and 750 ° C. As long as the oxygen content is> 1% and the treatment time> 1 s, their influence on the thickness of the oxide layer formed is negligibly small. With oxygen contents in the range of 2 to 5%, an insensitive process can be assumed.

It is therefore an object of the present invention to provide an improved process for the pre-oxidation of high strength steel strip in a reaction chamber within a furnace chamber during the recrystallizing annealing prior to a hot dip coating.

According to the teachings of the invention, this object is achieved by the features specified in claim 1, in particular in that the reaction chamber is sealed at a belt inlet and a belt outlet against gas exchange between the furnace chamber and the reaction chamber and a gas, the oxidizing atmosphere in the reaction chamber is formed, introduced and the gas within the reaction chamber in a closed circuit is permanently circulated, the composition of which is regulated and losses due to leakage and consumption are compensated.

In this way, it is possible to produce a particularly uniformly formed oxide layer on the strip surface, so that defects in the subsequent hot-dip coating are avoided, thus improving the quality of the final product and reducing rejects.

The reaction chamber is basically sealed towards the furnace chamber and in particular at the belt inlet and belt outlet against gas exchange.

The atmosphere is constantly being circulated. The gas is sucked out of the reaction chamber, cooled, fed to a fan, enriched with fresh air and fed back into the chamber. This achieves good homogeneity of the atmosphere.

Another desired effect is that controlled by nozzle systems (at least one nozzle system) and evenly gas with high kinetic energy density with the aid of nitrogen as the carrier gas of the strip surface is supplied. This is necessary to avoid laminar boundary layer effects.

In order to achieve a sufficient buffer against penetrating hydrogen, the oxygen content of the atmosphere in the reaction chamber is a minimum of 1.5 to a maximum of 5 vol%.

To compensate for volume changes, the reaction chamber has a trigger. Preferably, this trigger is controlled so that the internal pressure of the reaction chamber corresponds to the pressure of the surrounding furnace atmosphere and so the gas exchange over the unavoidable leaks is minimal.

As a result of these measures, a good-natured, controllable oxidation process is achieved and impairment of the furnace atmosphere surrounding the reaction chamber is prevented.

The oxidation-sensitive steel may contain at least a selection of the following alloy constituents: Mn> 0.5%, Al> 0.2%, Si> 0.1%, Cr> 0.3%.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102004059566 [0004]

Claims (7)

A method of pre-oxidation of oxidation-sensitive steel strip in a reaction chamber located in a furnace chamber, characterized in that the reaction chamber is sealed at a belt inlet and a belt outlet against gas exchange between the furnace chamber and the reaction chamber and a gas, which forms an oxidizing atmosphere in the reaction chamber, initiated and the gas within the reaction chamber is continuously circulated in a closed circuit, the composition of which is regulated and losses due to leakage and consumption are compensated. Method for pre-oxidation after Claim 1 , characterized in that the oxidizing gas is sucked from the reaction chamber, cooled, fed to a fan, enriched with air and fed back into the reaction chamber in order to achieve a good homogeneity of the atmosphere. Method for pre-oxidation after Claim 2 , characterized in that controlled by at least one of the reaction chamber associated nozzle system and evenly the gas with high kinetic energy density is supplied with the aid of nitrogen as a carrier gas to the surface of the strip to avoid laminar boundary layer effects on the strip surface. Method for pre-oxidation after Claim 3 , characterized in that the oxygen content of the atmosphere in the reaction chamber is kept at a minimum of 1.5 to a maximum of 5 vol%, thereby to achieve a sufficiently large buffer against penetrating from the furnace chamber into the reaction chamber hydrogen. Method for pre-oxidation according to one or more of Claims 1 to 4 , characterized in that the reaction chamber to compensate for volume changes a deduction is assigned. Method for pre-oxidation after Claim 5 , characterized in that the withdrawal is preferably controlled so that the internal pressure of the reaction chamber corresponds to the pressure of the surrounding furnace atmosphere and so the gas exchange is kept minimal via unavoidable leaks. Method according to one or more of Claims 1 to 6 , characterized in that the oxidation-sensitive steel contains at least a selection of the following alloy constituents: Mn> 0.5%, Al> 0.2%, Si> 0.1%, Cr> 0.3%.