Method of galvanising a steel strip in a continuous hot dip galvanising line

Description

[0001] The invention relates to the continuous galvanizing of steel strips especially high strength steels with high content of elements like silicon, manganese, aluminium and chromium and, in particular, to the facilities comprising a direct fire or non oxidising furnace and radiant tubes furnace.

[0002] New steel grades with a very high yield point having a high elongation capacity have been developed over the years to meet the demands of higher safety and lower weight in the car industry. Such steels, sometimes referred to as Advanced High Strength Steels (AHSS) comprise families of steel such as "DP" or Dual Phase steels (DP) and or TRansformation Induced Plasticity steels (TRIP). Unfortunately these steels raise some problems for steel manufacturers because some of their alloying elements such as manganese, silicon, aluminium, chromium may result in a thin layer of oxides on the steel surface during the annealing operation preceding the dipping in the galvanizing bath. This (selective) oxidation harms the zinc "wettability" and thus the quality of the coating. These phenomena are due to diffusion processes of the highly oxidisable alloy components towards the strip surface where they can oxidize even in the furnaces radiant tubes zones wherein the atmosphere is yet reducing for the iron oxides.

[0003] A solution which has been proposed is to subject the strips surface to temperatures and atmosphere conditions fit for quickly and deeply oxidizing the alloy components in the direct fired (DFF) part of the annealing furnace, thereby avoiding later migration of the oxidisable elements towards the surface followed by reducing the iron oxide back to iron in the radiant tube section (RTF). For this oxidation to take place it is necessary that direct fired furnace zones are used.

[0004] However, the galvanizing furnaces do not comprise all the required DFF zones to easily perform the oxidizing and many are only using radiant tubes. Now these furnaces, despite their controlled atmosphere, do not prevent the selective oxidizing of the alloy components. Patent WO 2005/017214 recommends two possibilities to solve the problem. The first one consists in using a direct flame combustion chamber separated from the RTF annealing furnace and from which the burnt gasses are collected in order to inject them in the furnace. This method requires adjusting the air to combustible gas ratio to provide excess oxygen after combustion which is then subsequently used for oxidation of the steel strip surface. The second one consists in setting up a direct flame burner in a section of the furnace enclosure. In both cases, the burnt gasses supply the necessary oxidizing atmosphere. The subsequent reduction of the oxides is then commonly obtained by going passing the strip through a nitrogen and hydrogen mixture. These two possibilities require a modification of the existing facilities.

[0005] FR-A1-2920438 or US-B-3936543 disclose the control of prior art of air or of oxygen to fuel in DFF in galvanizing lives. A radial tube furnace after DFF are also foreseen.

[0006] The object of this invention is to provide an improved method of avoiding selective oxidation of alloying elements in AHSS in a direct fire or non oxidising furnace and radiant tube fired continuous hot dip galvanizing line.

[0007] One or more of these objects are reached by a method of galvanizing a steel strip in a continuous hot dip galvanizing line comprising a direct fired furnace or non oxidising furnace section and a subsequent radiant tube furnace section, the method comprising injecting a gas mixture of nitrogen and air or a gas mixture of nitrogen and oxygen into the galvanizing furnace by a nozzle system to cause one or both of the steel strip surfaces to oxidise in a controlled manner 1. in the direct fired furnace section or non oxidising furnace, wherein the direct fired furnace section is operated in a substantially non-oxidising manner or in the connection chamber between the direct fired furnace section or non-oxidising section and radiant tube section and 2. before the radiant tube section, the method further comprising at least partly reducing the oxide back to iron in the radiant tube furnace section and the method further comprising hot dip galvanizing the steel strip in the hot dip galvanizing line, wherein the gas mixture comprises an oxygen content of 0.5 to 10%.

[0008] The invention consists in projecting an oxidizing medium consisting of a gas mixture of nitrogen and oxygen or a gas mixture of nitrogen and air onto one or both of the surfaces of the uncoated strip exiting the non-oxidising or direct fired furnace section. In the prior art the air to combustible gas ratio had to be adjusted in the DFF-section which leads to a compromise between the conditions in the DFF section and the subsequent oxidation of the strip. This leads to control problems and stability problems, leading in turn to bad oxide homogeneity across the strip width caused by the burner pattern. By using a separate nozzle system dedicated to injecting either 1). a gas mixture of nitrogen and air or 2). a gas mixture of nitrogen and oxygen so that a controlled oxygen content is injected into the furnace to cause the oxidation of the hot steel strip to take place in a controlled manner and to prevent selective oxidizing of the steel alloy components. To that end the nozzles are designed such as to distribute the gas mixture evenly thereby causing the hot steel strip surface to oxidise evenly and reproducibly.

[0009] The gas mixture must have an oxygen content such that the steel surface can be oxidised in a controlled way and a controlled oxide thickness. This allows alloy components like silicon, manganese, aluminium and chromium to be oxidized and to not have the possibility to migrate towards the surface anymore during further annealing. To reach this goal, the inventors found that the mixture had to be 1). a mixture of nitrogen and air or 2). a mixture of nitrogen and oxygen. In the first case there is additional nitrogen from the air as well as smaller amounts of other gases present in air. These smaller amounts do not affect the oxidation process. In the second case residual amounts of other gases may be present as a result of their presence in the oxygen and nitrogen to be mixed. These residual amounts also do not affect the oxidation process. It was found that the gas mixture resulting from the combustion of an overstoichiometric air or of an oxygen enriched air or of an oxygen/fuel in a burner like those proposed in the prior art was inadequate to provide a controlled oxidation.

[0010] The inventors found that the oxygen content of the gas mixture needed to be between 0.5 and 10% in volume. At oxygen values above 10% the oxide layer did not have the desired composition and the growth rate of the oxide layer is too high, resulting in thick oxide layers. At oxygen values below 0.5% the oxidation process was too slow and the oxide layer remained too thin.

[0011] In an embodiment a spray bar with specifically designed nozzle is mounted, preferably in the DFF or in the connection chamber between the DFF and RTF, on one or on both sides of the strip which sprays a jet of oxidising medium onto the strip surface so that the strip surface is evenly oxidised. An illustration of the system is provided in a schematic drawing in figure 1.

[0012] In preferred embodiment the maximum oxygen content is 7%. This results in an oxide layer of consistent composition, thickness and homogeneity to provide a good quality galvanised coating. In a preferable embodiment the gas mixture comprises an oxygen content of between 2 to 4.5%.

[0013] In an embodiment a method is provided wherein the control of the oxidation of the steel strip surface or surfaces is based on the measurement of the oxygen content in the mixture. This measurement can be performed by oxygen transducers set up a fixed way and running in closed loop with the flow control valves regulating the flow rate of the mixture injected by the nozzles. This results in an oxide layer of consistent composition, thickness and homogeneity to provide a good quality galvanised coating.

[0014] In an embodiment the oxidation of the steel strip surface or surfaces takes place between 650°C and 900°C. In a preferred embodiment the oxidation of the steel strip surface or surfaces takes place at a temperature of at most 800°C and more preferably of at most 750°C.

[0015] By means of non-limiting examples commercial trials were performed on 1 and 1.5 mm thick and 1200 mm wide coiled strip material of the DP600 and DP800 type. The dew point during the trials was between -30 and -24°C. The oxygen content of the gas mixture was varied between 2.26 to 3.61%. The results in terms of strip wettability and coating adhesion after the annealing and coating were excellent.

[0016] In figure 2 a schematic indication is given where the oxidation of the steel substrate, for instance using the system in figure 1, is performed in a furnace comprising a direct fired furnace and a radiant tube furnace.

Claims

1. Method of galvanizing a steel strip in a continuous hot dip galvanizing line comprising a direct fired furnace section or non oxidising furnace and a subsequent radiant tube furnace section, the method comprising injecting a gas mixture of nitrogen and air or a gas mixture of nitrogen and oxygen into the galvanizing furnace by a nozzle system to cause one or both of the steel strip surfaces to oxidise in a controlled manner 1. in the direct fired furnace section or non oxidising furnace, wherein the direct fired furnace section is operated in a substantially non-oxidising manner or in the connection chamber between the direct fired furnace section or non-oxidising section and radiant tube section and 2. before the radiant tube section, the method further comprising at least partly reducing the oxide back to iron in the radiant tube furnace section and the method further comprising hot dip galvanizing the steel strip in the hot dip galvanizing line, wherein the gas mixture comprises an oxygen content of 0.5 to 10%.

2. Method according to claim 1 wherein the gas mixture comprises an oxygen content of at most 7%.

3. Method according to claim 1 wherein the gas mixture comprises an oxygen content of between 2 to 4.5%.

4. Method according to any one of the preceding claims wherein the oxidation of the steel strip surface or surfaces takes place between 650°C and 900°C.

5. Method according to any one of the preceding claims wherein the oxidation of the steel strip surface or surfaces takes place at a temperature of at most 750°C.

6. Method according to any one of the preceding claims wherein the control of the oxidation of the steel strip surface or surfaces is based on the measurement of the oxygen content of the gas mixture.

Ansprüche

1. Verfahren zum Verzinken eines Stahlbandes in einer kontinuierlichen Feuerverzinkungsanlage, die einen direkt befeuerten Ofenabschnitt oder nichtoxidierenden Ofen und einen anschließenden Strahlrohrheizungsofenabschnitt umfasst, wobei das Verfahren das Einspeisen einer Gasmischung aus Stickstoff und Luft oder einer Gasmischung aus Stickstoff und Sauerstoff in den Verzinkungsofen durch ein Düsensystem umfasst, um zu bewirken, dass eine oder beide Stahlbandflächen in kontrollierter Weise oxidiert werden,

1. im direkt befeuerten Ofenabschnitt oder nichtoxidierenden Ofen, wobei der direkt befeuerte Ofenabschnitt in einer im Wesentlichen nichtoxidierenden Weise betrieben wird, oder in der Verbindungskammer zwischen dem direkt befeuerten Ofenabschnitt oder dem nichtoxidierenden Abschnitt und dem Strahlrohrabschnitt, und

2. vor dem Strahlrohrabschnitt, wobei das Verfahren ferner mindestens teilweise das Oxid im Strahlrohrofenabschnitt zurück zu Eisen reduziert, und wobei das Verfahren ferner das Feuerverzinken des Stahlbandes in der Feuerverzinkungsanlage umfasst, wobei die Gasmischung einen Sauerstoffgehalt von 0,5 bis 10 % umfasst.

2. Verfahren nach Anspruch 1, wobei die Gasmischung einen Sauerstoffgehalt von höchstens 7 % umfasst.

3. Verfahren nach Anspruch 1, wobei die Gasmischung einen Sauerstoffgehalt von 2 bis 4,5 % umfasst.

4. Verfahren nach einem der vorherigen Ansprüche, wobei die Oxidation der Fläche oder der Flächen des Stahlbandes zwischen 650 °C und 900 °C stattfindet.

5. Verfahren nach einem der vorherigen Ansprüche, wobei die Oxidation der Fläche oder der Flächen des Stahlbandes bei einer Temperatur von höchstens 750 °C stattfindet.

6. Verfahren nach einem der vorherigen Ansprüche, wobei die Steuerung der Oxidation der Fläche oder der Flächen des Stahlbandes auf der Messung des Sauerstoffgehalts der Gasmischung beruht.

Revendications

1. Procédé de galvanisation d'un acier en bandes sur une ligne de galvanisation à chaud au trempé continue comprenant une section directe de four chauffé ou un four non oxydant et une section directe de four à tube radiant, le procédé comprenant l'injection d'un mélange de gaz d'azote et d'air ou d'un mélange de gaz d'azote et d'oxygène dans le four de galvanisation par un système de tuyère pour que l'une ou les deux des surfaces d'acier en bandes s'oxyde d'une manière contrôlée

1. dans la section du four chauffé, où la section du four chauffé est utilisée d'une manière essentiellement non-oxydante ou dans la chambre de connexion entre la section directe de four chauffé ou la section non-oxydante et la section à tube radiant et

2. avant la section à tube radiant, le procédé comprenant en outre au moins la réduction partielle de l'oxyde en fer dans la section de four à tube radiant et le procédé comprenant en outre une galvanisation à chaud au trempé de l'acier en bandes dans la ligne de galvanisation à chaud au trempé, où le mélange de gaz comprend une teneur en oxygène de 0.5 à 10%.

2. Procédé selon la revendication 1, où le mélange de gaz comprend une teneur en oxygène d'au maximum 7%.

3. Procédé selon la revendication 1, où le mélange de gaz comprend une teneur en oxygène entre 2 et 4.5%.

4. Procédé selon l'une quelconque des revendications précédentes où l'oxydation de la surface ou des surfaces d'acier en bandes a lieu entre 650°C et 900°C.

5. Procédé selon l'une quelconque des revendications précédentes où l'oxydation de la surface ou des surfaces d'acier en bandes a lieu à une température d'au maximum 750°C.

6. Procédé selon l'une quelconque des revendications précédentes où le contrôle de l'oxydation de la surface ou des surfaces d'acier en bandes se base sur la mesure de la teneur en oxygène du mélange de gaz.

Drawing