EP2824216B1 - Method for manufacturing a flat steel product having a protective metal coating produced by means of hot-dip coating and continuous furnace for a hot-dip coating system - Google Patents

Description

The invention relates to a process for producing a flat steel product provided by hot dip coating with a metallic protective layer, in particular a high-strength steel flat product having a tensile strength of at least 500 MPa or a high-strength steel flat product having a tensile strength of at least 1000 MPa. The invention further relates to a DFF-type continuous furnace for a hot-dip coating apparatus having a pre-oxidation section in which a flat steel product to be coated is exposed to an oxidizing atmosphere to form a covering FeO layer on the surface of the flat steel product, with burners arranged in the pre-oxidation section are operated with excess oxygen, and wherein at least one of the burner is associated with the top of the flat steel product and at least one of the other burner of the underside of the flat steel product. When referring to flat steel products in the following, this means any cold or hot rolled steel strip.

High-strength and high-strength flat steel products are in demand due to their advantageous combination of strength and formability in increasing quantities. This applies in particular to sheet metal applications in automotive body construction. The outstanding mechanical properties of such flat steel products are based on a multiphase microstructure of the material, possibly supported by induced plasticity of austenitic phase components (TRIP, TWIP or SIP effect). In order to obtain such a complex microstructure, the flat steel products in question usually have appreciable contents of certain alloying elements, typically manganese (Mn), aluminum (Al), silicon (Si). or chromium (Cr) count. A surface finish in the form of a metallic protective layer not only increases the resistance of the steel flat products to corrosion and, consequently, their product lifetime, but also improves their visual appearance.

Various methods of applying a metallic protective layer are known. These include electrolytic deposition and hot dip coating. In addition to an electrolytically produced processing, the hot dipping refinement has established itself as an economically and ecologically particularly favorable process. In hot-dip coating, the flat steel product to be coated is immersed in a molten metal bath.

Hot dip refinement proves to be particularly cost-effective if a flat-rolled steel semi-finished product delivered in the hard-rolled state is subjected to the process steps of purification, recrystallization annealing, hot dip coating, cooling, optional thermal, mechanical or chemical aftertreatment and coiling.

The annealing treatment can be used to activate the steel surface. For this purpose, an N ₂ -H ₂ -Glühgasatmosphäre typically with unavoidable traces of H ₂ O and O ₂ is usually maintained in the continuous flow in the annealing furnace.

The presence of oxygen in the annealing atmosphere has the disadvantage that the oxygen-affine alloying elements (for example Mn, Al, Si, Cr,...) Contained in the particular steel flat product to be treated form selectively passive, non-wettable oxides on the steel surface, thereby improving the coating quality or adhesion on the steel substrate can be sustainably worsened. Therefore, various attempts have been made to carry out the annealing of high and high strength steels of the type in question so that the selective oxidation of the steel surface is largely suppressed.

A first method of this kind is from the DE 10 2006 039 307 B3 known. In this process for hot dip finishing of steels with 6 to 30% by weight of Mn, the hot-dip coated steel flat product is bright annealed under particularly reductive atmosphere conditions (low H ₂ O / H ₂ ratio of the annealing atmosphere and high annealing temperature).

In the EP 1 936 000 A1 and the JP 2004 315 960 A In each case, process concepts are described in which the atmospheric conditions in the continuous furnace are set within certain limits and as a function of the temperature of the respectively processed flat steel product. In this way, in each case the internal oxidation of the oxygen-affine alloying elements should be promoted without the formation of FeO on the surface of the flat steel product. However, a prerequisite for this is a precisely coordinated interaction of the various influencing factors on the annealing gas-metal reaction, such as annealing gas composition, annealing gas moisture content or annealing temperature. As a rule, these are distributed in an inhomogeneous manner over the entire furnace space due to the plant. This inhomogeneity makes it difficult to effectively use these processes on an industrial scale.

Another possibility of preparation of a hot-rolled flat steel product in the course of an annealing treatment is that preoxidations are carried out in a continuous annealing zone used for the annealing within a DFF type ("Direct Fired Furnace") preheating zone. In a DFF furnace, flames emitted by gas burners act directly on the flat steel product to be treated. By operating the burners with an O ₂ excess (balanced to an air ratio λ> 1), the oxidation potential of the atmosphere surrounding the steel flat product is adjusted in such a way that a covering FeO layer forms on the surfaces of the flat steel product. This FeO layer inhibits the selective oxidation of the oxygen-affine alloying elements of the flat steel product. In a subsequent one Holding zone carried out second annealing step, the FeO layer is completely reduced back to metallic iron back.

A method approach of this kind has long been from the DE 25 22 485 A1 known. The advantage of preheating the flat steel product in a preheating furnace designed in DFF design consists in addition to the above-mentioned effects that can be achieved particularly high heating rates of the steel strip, which significantly reduces the duration of the annealing cycle and thus coupled with the output of a corresponding continuous furnace Can significantly increase hot dip coating plant.

However, due to the burners usually arranged on the longitudinal sides of the preheating furnace, a uniform furnace atmosphere is not achieved with regard to the oxygen content and the temperature distribution. In practice it has been shown that the oxygen content decreases over the furnace or bandwidth. Furthermore, a non-uniform temperature distribution over the bandwidth was found, which can lead to a different degree of oxidation tendency and also to overheating of the band edges. To even out the distribution of temperature and oxygen in the prior art, inter alia, a trimming of the DFF burner flames is known (see DE 10 2011 051 731 A1 ). The setting of an optimal FeO layer thickness of 20-200 nm in a homogeneous, uniform distribution over the bandwidth, however, is difficult to control only by trimming the DFF burner flames. Both too low and too thick a FeO layer can lead to wetting and adhesion problems.

Furthermore, from the EP 1 829 983 A1 It is known to use line burners for equalizing the temperature and oxygen distribution, with a burner flame being directed directly onto the strip surface. As a result, however, the surface quality may deteriorate due to the formation of micro-notches. In These micro-scores can accumulate, for example, organic residues and lead to unwetted places in a bead-string arrangement.

A very uniform pre-oxidation due to the direct band contact to a Hüllflamme allows a so-called "DFI-Booster"("DFI" - Direct Flame Impingement), as in the DE 10 2006 005 063 A1 is described. However, the use of such a DFI booster is possible only under certain structural conditions, as they are not given in many existing hot dip coating equipment.

From the EP 2 010 690 B1 and the DE 10 2004 059 566 B3 Furthermore, methods are known in which a FeO layer on the surface of each processed flat steel product by feeding 0.01 - 1 vol .-% O ₂ over a period of 1 - 10 seconds is generated in a closed reaction chamber. The installation of such a reaction chamber, however, is only possible in an indirectly heated RTF furnace, in which the heating of the flat steel product takes place via thermal radiation ("RTF": Radiant Tube Furnace).

Against the background of the prior art described above, the object of the invention was to provide a continuous furnace or a method of the type mentioned above, in a large-scale hot dip coating plant as uniform as possible pre-oxidation of steel strip, the significant alloying parts of oxygen-affine alloying elements (Mn , Al, Si, Cr, ...), can achieve over the bandwidth. As a result, an improvement of the wetting image and the coating adhesion over the entire width of the steel strip to be achieved.

This object is achieved by a method having the features of claim 1 or by a continuous furnace with the features of claim 7. Preferred and advantageous embodiments of the method according to the invention and of the continuous furnace according to the invention are specified in the subclaims.

1. a) Bereitstellen eines kalt- oder warmgewalzten Stahlflachprodukts, das neben Fe und unvermeidbaren Verunreinigungen (in Gew.-%) bis zu 35,0 % Mn, bis zu 10,0 % Al, bis zu 10,0 % Si, bis zu 5,0 % Cr, bis zu 2,0 % Ni, jeweils bis zu 0,5 % an Ti, V, Nb, Mo, jeweils bis zu 0,1 % S, P und N, bis zu 1,0 % C sowie optional 0,0005 - 0,01 % B enthält;
2. b) Aufheizen des Stahlflachprodukts, vorzugsweise auf eine Temperatur im Bereich von 600 - 1000°C innerhalb einer Aufheizzeit von 5 - 60 Sekunden, in einem Vorwärmofen des DFF-Typs, in welchem ein Voroxidationsabschnitt ausgebildet ist und in welchem das Stahlflachprodukt einer oxidierenden Atmosphäre ausgesetzt wird, um auf der Oberfläche des Stahlflachprodukts eine deckende FeO-Schicht zu bilden, wobei in dem Voroxidationsabschnitt Brenner angeordnet sind, die mit Sauerstoffüberschuss betrieben werden, und wobei mindestens einer der Brenner der Oberseite des Stahlflachproduktes und mindestens ein anderer der Brenner der Unterseite des Stahlflachproduktes zugeordnet ist;
3. c) rekristallisierendes Glühen des Stahlflachprodukts in einem Glühofen, der im Anschluss an den Vorwärmofen durchlaufen wird, um eine Rekristallisierung des Stahlflachprodukts zu bewirken, wobei in dem Glühofen eine gegenüber FeO reduzierend wirkende Glühatmosphäre herrscht;
4. d) Abkühlen des Stahlflachprodukts auf eine Badeintrittstemperatur im Bereich von 430 bis 800°C in einer Schutzgasatmosphäre;
5. e) Einleiten des Stahlflachprodukts in ein Schmelzenbad, dessen Temperatur im Bereich von 420 bis 780°C liegt; und
6. f) Durchleiten des Stahlflachprodukts durch das Schmelzenbad und Einstellen der Dicke der auf dem aus dem Schmelzenbad austretenden Stahlflachprodukt vorhandenen metallischen Schutzschicht.

A method according to the invention for producing a flat steel product provided by hot-dip coating with a metallic protective layer accordingly comprises at least the following working steps:

1. a) providing a cold or hot rolled flat steel product, which in addition to Fe and unavoidable impurities (in wt .-%) up to 35.0% Mn, up to 10.0% Al, up to 10.0% Si, up to 5 , 0% Cr, up to 2.0% Ni, up to 0.5% each of Ti, V, Nb, Mo, up to 0.1% S, P and N, up to 1.0% C, respectively optionally contains 0.0005 - 0.01% B;
2. b) heating the flat steel product, preferably to a temperature in the range of 600-1000 ° C within a heating time of 5-60 seconds, in a preheating furnace of the DFF type in which a pre-oxidation section is formed and in which the flat steel product is exposed to an oxidizing atmosphere to form a covering FeO layer on the surface of the flat steel product, wherein burners are operated in the Voroxidationsabschnitt operated with excess oxygen, and wherein at least one of the burner of the upper side of the flat steel product and at least one other of the burner of the underside of the flat steel product assigned;
3. c) recrystallizing annealing of the flat steel product in an annealing furnace, which is passed after the preheating furnace to cause a recrystallization of the steel flat product, wherein in the annealing furnace there is a refractory to FeO reducing annealing atmosphere prevails;
4. d) cooling the steel flat product to a bath inlet temperature in the range of 430 to 800 ° C in a protective gas atmosphere;
5. e) introducing the steel flat product into a melt bath whose temperature is in the range of 420 to 780 ° C; and
6. f) passing the flat steel product through the melt bath and adjusting the thickness of the metallic protective layer present on the steel flat product leaving the melt bath.

In addition, the inventive method is characterized in that as a burner in the pre-oxidation of the preheating furnace flameless burner be used, by means of which fuel, preferably fuel gas, and oxygen-containing gas are introduced separately from each other at a flow rate of at least 60 m / s in the preheating furnace, wherein in addition to at least one of the top of the flat steel product associated flameless burner and at least one of the underside of the flat steel product associated flameless burner per at least one gas line for supplying at least one additional gas stream is provided, by means of which the fuel and the oxygen-containing gas are additionally mixed, and wherein the additional gas stream is directed obliquely to the plane of the flat steel product introduced into the preheating furnace.

The inventive use of flameless burners in combination with at least one gas line for supplying an additional gas stream, by means of which the fuel and the oxygen-containing gas are additionally mixed, a very homogeneous temperature and oxygen distribution over the entire width of the steel strip to be coated is achieved. As a result, a local overheating of the refractory lining of the preheating furnace is avoided. In conjunction with the superstoichimetric operation of the preheat furnace, a uniformly oxidizing furnace atmosphere is created which produces a uniformly thick oxide layer on the steel strip containing oxygen affinity alloying elements to be coated. Since a uniformly thick oxide layer is reliably produced on the steel strip by the method according to the invention, advantageously, relatively thin pre-oxidation layers of less than 1 μm, preferably less than 0.3 μm, in particular less than 0.2 μm, can be realized.

The quality of subsequent hot dip coating in the subsequent process can be significantly improved because of the intermediate reduction step, a clean, free of unwanted alloy oxides band surface is achieved, which has a very good wettability and thus avoids coating defects in the form of uncoated sites or at least significantly reduced.

An advantageous embodiment of the method according to the invention is characterized in that the flameless burners are operated with an oxygen excess of at least 1.1, preferably at least 1.2, more preferably at least 1.3. In experiments, it was found that in this case, the oxide layer with uniform layer thickness is very reliable, with the nitrogen oxide emissions decrease from a lambda value of about 1.05 with increasing oxygen excess.

A further advantageous embodiment of the method according to the invention is characterized in that at least two, preferably at least three flameless burners the top of the flat steel product and at least two, preferably at least three flameless burners are assigned to the bottom of the flat steel product, wherein the top of the flat steel product associated flameless burner in Passing direction of the flat steel product offset to be arranged on the underside of the flat steel product associated flameless burners. This embodiment favors the setting of a homogeneous as possible oxidizing furnace atmosphere, in particular homogeneous temperature distribution.

According to a further advantageous embodiment of the method according to the invention at least two, preferably at least three flameless burner the top of the flat steel product and at least two, preferably at least three flameless burners the underside of the Stahlflachprodukts assigned, wherein the top of the flat steel product associated flameless burner in a side wall of the preheating furnace integrated, while the underside of the flat steel product associated flameless burners are integrated in the opposite side wall of the preheating furnace. This embodiment also favors the setting of a most homogeneous oxidizing furnace atmosphere.

A further advantageous embodiment of the method according to the invention is that the oxygen-containing gas is introduced preheated in the preheating furnace. This makes it possible to better use the fuel and reduce fuel consumption. The oxygen-containing gas, typically air, is preheated for this purpose, for example, by means of at least one heat exchanger, which is coupled to the annealing furnace, the cooling device following the annealing furnace and / or the molten bath vessel.

A further advantageous embodiment of the method according to the invention provides that the oxygen-containing gas and / or the fuel inert gas, for example, exhaust gas is added. As a result, the dimensions of the combustion cloud and in particular the combustion temperature can be influenced in a targeted manner.

In order to achieve optimal pre-oxidation of the flat steel product (steel strip) with the lowest possible emission values, in particular low NO _x emissions, it is also advantageous if, according to a further embodiment of the method, inert gas and / or exhaust gas, preferably preheated inert gas and / or heated exhaust gas for the additional gas flow is used, by means of which the fuel and the oxygen-containing gas are additionally mixed.

A further advantageous embodiment of the method according to the invention is characterized in that the dew point of the annealing atmosphere over the entire path of the steel flat product through the annealing furnace between -40 ° C and + 25 ° C is maintained by losses or irregularities by supplying moisture by means of at least one moistening device the distribution of the humidity of the atmosphere can be compensated. The dew point is on the one hand -40 ° C or more to minimize the driving force of the external oxidation of the alloying elements (eg Mn, Al, Si, Cr). On the other hand, by the dew point of a maximum of + 25 ° C a unwanted oxidation of iron avoided. This embodiment thus contributes to the fact that the surface of the flat steel product is largely free of interfering oxides when entering the Schmelztauschbad.

Accordingly, the continuous furnace according to the invention is characterized in that at least some of the burners in the pre-oxidation section are designed as flameless burners, by means of which fuel, preferably fuel gas, and oxygen-containing gas can be introduced separately into the preheating furnace at a flow rate of at least 60 m / s at least one flameless burner associated with the upper side of the flat steel product and at least one gas line for supplying at least one additional gas stream are provided adjacent to at least one underside of the flat steel product, the end of the respective gas line being oriented in such a way that the additional emerging therefrom Gas flow crosses or tangent to the emerging from the flameless burner fuel flow and / or oxygen-containing gas stream.

The continuous furnace according to the invention offers the advantages already mentioned above in connection with the method according to the invention.

In the case of pre-oxidation in a DFF preheating furnace of hot dip coating systems, there is the problem in principle that unfavorably high accumulation of oxygen in the furnace zone upstream of the preoxidation zone occurs due to gas flows in the furnace, which can lead to a negative influence on the coating. It has been shown that this oxygen accumulation can be significantly reduced when using flameless burners, in that in addition to the material flows of the flameless burner another gas stream is fed via at least one so-called jet tube. This is especially true with respect to the first superstoichiometric flameless burner in the pre-oxidation zone. In this case, the jet tube is aligned with respect to the burner nozzle (s) so that the further gas flow is a spiral-like swirling causes. For this purpose, the jet pipe is oriented obliquely to the outflow axis of the burner nozzle and also inclined relative to the plane of the flat steel product.

Fig. 1

eine zur Durchführung des erfindungsgemäßen Verfahrens geeignete Schmelztauchbeschichtungsanlage;

Fig. 2

einen in der Schmelztauchbeschichtungsanlage gemäß Fig. 1 eingesetzten flammenlosen Brenner in Kombination mit einem Jet-Rohr, in einer Schnittansicht;

Fig. 3

eine Prinzipdarstellung der Stoff- bzw. Gasströme an einem flammenlosen Brenner während des flammenlosen Betriebs; und

Fig. 4

einen Querschnitt durch einen Durchlaufofen (Vorwärmofen) der Schmelztauchbeschichtungsanlage gemäß Fig. 1 im Bereich der mit Brennern gemäß Fig. 2 ausgerüsteten Vorwärmzone.

The invention will be explained in more detail with reference to an exemplary embodiments illustrative drawing. Each show schematically:

Fig. 1

a suitable for performing the method according to the invention hot dip coating equipment;

Fig. 2

one in the hot dip coating plant according to Fig. 1 flameless burner used in combination with a jet pipe, in a sectional view;

Fig. 3

a schematic representation of the material or gas flows on a flameless burner during flameless operation; and

Fig. 4

a cross section through a continuous furnace (preheating furnace) of the hot dip coating plant according to Fig. 1 in the area with burners according to Fig. 2 equipped preheating zone.

In the Fig. 1 shown hot dip coating plant A has in the conveying direction F of present as steel strip to be coated flat steel product S in direct connection consecutively one for preheating the Stahlflachprodukts S provided DFI booster 1, connected to its input 2 to the DFI booster preheating furnace 3, in which a pre-oxidation section 4 is formed, an annealing furnace 6, which is connected to a transition region 7 to the output 8 of the preheating furnace 3, connected to the output 9 of the annealing furnace 6 cooling zone 10, connected to the cooling zone 10 proboscis 11, to the output 12 of the cooling zone 10 is connected and immersed with its free end in a melt bath 13, a arranged in the melt bath 13 first deflecting means 14, a means 15 for adjusting the thickness of the on the Flat steel product S in the molten bath 13 applied metallic coating and a second deflecting 16 on.

The preheating furnace 3 is of the DFF type (Direct Fired Furnace). In it, distributed over the conveyor line of the flat steel product S or at least in the pre-oxidation section 4 a plurality of flameless burners 17 are arranged. In these burners, the fuel (B), preferably fuel gas, and oxygen-containing gas (L), typically air is introduced unmixed or largely unmixed at high flow rate in the preheating furnace 3 (see. Fig. 3 ). The inflow rate of the fuel B and the oxygen-containing gas L is at least 60 m / s, preferably at least 120 m / s. The essential difference to conventional burners in flame operation is the intensive internal recirculation of the exhaust gases AG in the furnace chamber and their mixing with the combustion air or the oxygen-containing gas L (see. Fig. 3 ). As a result, and by the delayed mixing of fuel B and oxygen L no visible flame front can form more. At sufficiently high temperatures of, for example, at least 700 ° C., preferably at least 800 ° C., the fuel B oxidizes in the entire furnace chamber volume. As a result, a very homogeneous temperature distribution arises over the entire width of the flat steel product S.

For pre-oxidation of the flat steel product S, the flameless burners 17 are operated in a superstoichiometric range, i. with a lambda value greater than 1, creating an oxidizing furnace atmosphere. Preferably, a lambda value of at least 1.05, particularly preferably at least 1.1, in particular at least 1.2 or at least 1.3 is set.

The formation of thermal nitrogen oxides, which takes place in burners in flame operation, especially at the flame boundary with their high peak temperatures, is largely avoided in the flameless burners 17. With the more even temperature distribution not only the NO _x emissions decrease, it can also be a maintained higher average furnace chamber temperature. In particular, the NO _x emissions decrease with increasing lambda value.

The structure of a flameless burner 17 for use in a continuous furnace (preheating furnace) 3 of the DFF type for a hot-dip coating machine is shown in FIG Fig. 2 shown. The burner 17 has a pipe section 17.1 with an elongated gas nozzle 17.2. The pipe section 17.1 is provided with a connecting flange 17.3 and connected via a fuel gas line, not shown, to a fuel gas supply, also not shown. Furthermore, the burner 17 has a hollow chamber 17.4 for supplying oxygen-containing gas, preferably air, which surrounds a longitudinal section of the pipe section 17.1 with the fuel gas nozzle 17.2. The hollow chamber 17.4 is provided with a connecting flange 17.5 and connected via a supply line, not shown, to a likewise not shown oxygen or air supply. The gas nozzle 17.2 opens with a central opening 17.6 and a coaxial annular opening (ring nozzle) 17.7 in the preheating furnace 3. Further, in the furnace chamber facing end face of the burner 17, an annular nozzle or more arranged on a common pitch circle, preferably evenly spaced from each other Nozzles 17.8 provided for the supply of oxygen or air. The oxygen or air nozzles 17.8 are designed so that the oxygen or air jets emerging therefrom cross the fuel gas stream leaving the gas nozzle. On its front side, the burner 17 is also provided with a nozzle stone 17.9. The nozzle block 17.9 has a channel 17.10, into which the nozzles 17.2, 17.6, 17.7, 17.8 open. In addition, the nozzle block 17.9 is provided with a pilot burner 17.11, which is received in a smaller, branched off from the channel 17.10 transverse channel 17.12. 17. In addition, the nozzle block 17.9 has a plurality of jet pipes (so-called jet pipes) 17.13 for the supply of oxygen-containing gas, which are connected to the hollow chamber 17.4 and whose longitudinal axes extend substantially parallel to the fuel inflow direction defined by the fuel gas nozzle 17.2. Preferably, each of the flameless burners 17 has three or more of these jet pipes 17, 13 equally spaced on a channel 17.10 surrounding pitch are arranged. The nozzle block 17.9 is inserted in a form-fitting manner in a burner block 3.1 having a passage opening. The burner block 3.1 is provided with a gas line 5 for supplying an additional gas flow ZG. The end opening into the preheating furnace 3 (jet pipe) of the gas line 5 is oriented such that the additional gas flow ZG the fuel stream B emerging from the flameless burner 17 and oxygen-containing gas stream L crosses or tangent.

The burner air L required for the combustion or the oxygen supplied for the combustion is preferably preheated. For this purpose, the respective flameless burner 17 may be preceded by a device (not shown here) for heating the oxygen-containing gas or the burner air L. Likewise, the fuel gas supply line can also be provided with a device (not shown here) for heating the fuel gas B. Additionally or alternatively, also the additional jet pipe 5 or the gas line connected thereto for supplying the at least one additional gas stream ZG can be preceded by a device (not shown here) for heating the additional gas stream. Optionally, in addition separately or mixed with the fuel and / or air / oxygen stream, preferably, heated, inert gas and / or exhaust gas are introduced. For this purpose, an inert gas or exhaust pipe (not shown) is connected to the connected to the respective connecting flange 17.3 or 17.5 supply line (feed), which is provided with a metering valve (control valve).

The separate introduction of fuel B and air / oxygen L at high speed, a mixture of media in the preheating furnace 3 over the entire width of the flat steel product S. At least the introduced oxygen-containing gas and / or the optionally admixed inert gas / exhaust gas are preheated to a temperature which, after mixing the media in the pre-oxidation furnace, gives a sufficient reaction energy (ignition temperature) at which the combustion of the fuel B takes place. The formation of a visible flame is on this way largely avoided. Rather, a "combustion cloud" W is generated over the entire width of the steel strip S or furnace 3.

The flameless burners 17 are preferably integrated in at least one of the side walls 3.2, 3.3 of the preheating furnace 3, wherein at least two, preferably at least three flameless burners 17 the top of the flat steel product S and at least two, preferably at least three flameless burners 17 associated with the bottom of the flat steel product S. are. To even out the temperature distribution and the oxygen distribution, the flameless burners 17 are preferably arranged offset from one another (cf. Fig. 1 ). By way of example, the flameless burners assigned to the upper side of the flat steel product S can be arranged offset in the direction of passage of the flat steel product S to the flameless burners associated with the underside of the flat steel product S. In particular, it is favorable for equalizing the temperature and oxygen distribution over the width of the steel strip S, if - as in Fig. 4 sketched - the top of the flat steel product S associated flameless burner 17 are integrated in a side wall (3.2) of the preheating furnace 3, while the underside of the flat steel product S associated flameless burner in the opposite side wall (3.3) of the preheating furnace 3 are integrated.

By the installation of the jet pipes (jet pipes) 17.13, which are arranged with respect to the Brenngaseinströmstrahl and the channel 17.10 at a distance, and is injected via the additionally oxygen-containing gas L, preferably oxygen or air in the fuel gas stream B, already a homogenization the fuel gas distribution causes. About the opening into the preheating furnace 3 gas line 5 (jet pipe 5) is preferably introduced inert gas and / or exhaust gas in the pre-oxidation. This additional gas flow ZG crosses or touches the fuel flow. This is in the FIGS. 3 and 4 shown schematically. In Fig. 4 is the above the flat steel product S arranged flameless burner 17, which is arranged in the side wall 3.2 of the preheating furnace 3, combined with a jet pipe 5. This jet tube 5 is above or (not shown here) preferably laterally adjacent to the flameless one Burner 17 is arranged and in each case aligned so that the emerging therefrom additional gas flow ZG crossed by the flameless burner 17 introduced fuel flow B or tangent. Furthermore, the flameless burner 17 arranged below the flat steel product S, which is arranged in the side wall 3.3 of the preheating furnace, is also combined with a jet pipe 5. This jet pipe 5 is arranged below or (not shown here) preferably laterally next to the flameless burner 17 and in each case aligned so that the additional gas flow ZG emerging therefrom crosses or touches the fuel flow B introduced by means of the flameless burner 17. This combination of flameless burner 17 and jet pipe 5 delimits the transition to the flameless zone in the preheating furnace.

At the transition region 7 to the annealing furnace 6, a device not shown in detail for the targeted feeding of oxygen or air is provided in the transition region 7. The purpose of this feed is the setting of hydrogen, which possibly passes in the transition region 7 as a result of flowing in the annealing furnace 6 from the outlet 9 in the direction of its inlet gas flow G. At the same time a suction device 24 is arranged in the region of the entrance of the annealing furnace 6, which sucks the reaching to the entrance of the annealing gas flow G.

Adjacent to the outlet 9 of the annealing furnace 6, two moistening devices 25, 26 are arranged, one of which is assigned to one of the upper side and the other of the underside of the flat steel product S to be coated. The moistening devices 25, 26 are designed as slotted or perforated tubes oriented transversely to the conveying direction F of the flat steel product S and connected to a supply line 27, via which the moistening devices 25, 26 with steam or a moistened carrier gas, such as N ₂ or N ₂ /. H ₂ , to be supplied.

The cooling zone 10 may be designed so that the cooled to the respective bath inlet temperature flat steel product S before its entry into the trunk 11 yet in the cooling zone 10 undergoes an overaging treatment at the bath inlet temperature.

In the molten bath 13, the flat steel product S is deflected at the first deflection device 14 in the vertical direction and passes through the device 15 for adjusting the thickness of the metallic protective layer. Subsequently, the steel flat product S provided with the metallic protective layer is deflected again in the horizontal conveying direction F at the second deflection device 16 and, if appropriate, subjected to further treatment steps in plant parts not shown here.

A hot-dip coated flat steel product according to the invention is excellently suited for further processing by means of one-stage, two-stage or multi-stage cold or hot forming into a high-strength / high-strength sheet metal component. This applies primarily to applications in the automotive industry, but also for apparatus, machine or household appliance construction as well as the construction industry. In addition to the outstanding mechanical component properties, such a sheet metal component continues to be distinguished by its particular resistance to environmental influences. The use of a hot-dip coated steel flat product according to the invention thus not only raises lightweight potential, but also prolongs the product life.

In summary, it can be said that the method according to the invention achieves a very homogeneous pre-oxidation of a steel strip to be provided with a metallic protective layer by hot-dip coating, over the entire bandwidth in a large-scale DFF preheating furnace. This results in an improvement of the wetting pattern and the coating adhesion over the entire width of the flat steel product. Coating defects at the strip edges can thus be avoided even with relatively wide insert strips. Another advantage lies in the optimized combustion, which differs significantly characterized by reduced pollutant emissions and reduced fuel consumption.

Claims (14)

1. Method for producing a flat steel product (S) provided with a metallic protective layer by hot-dip coating, comprising the following production steps:

   a) providing a cold-rolled or hot-rolled flat steel product (S) which in addition to Fe and unavoidable impurities contains (in % wt.) up to 35.0 % Mn, up to 10.0 % Al, up to 10.0 % Si, up to 5 % Cr, up to 2.0 % Ni, up to 0.5 % of Ti, V, Nb, Mo respectively, up to 0.1 % S, P and N respectively, up to 1.0 % C and optionally 0.0005 - 0.01 % B;

   b) heating the flat steel product (S) in a DFF-type preheating furnace (3) in which a preoxidation section (4) is formed and in which the flat steel product (S) is exposed to an oxidising atmosphere, in order to form a covering FeO layer on the surface of the flat steel product, wherein burners are arranged in the preoxidation section which are operated with excess oxygen, and wherein at least one of the burners (17) is assigned to the upper side of the flat steel product and at least another of the burners (17) is assigned to the underside of the flat steel product;

   c) recrystallising annealing of the flat steel product (S) in an annealing furnace (6) which is passed through following the preheating furnace (3) in order to bring about a recrystallisation of the flat steel product, wherein an annealing atmosphere having a reducing effect with respect to the FeO prevails in the annealing furnace (6);

   d) cooling the flat steel product down to a bath entry temperature in the range from 430 °C to 800 °C in a protective gas atmosphere;

   e) introducing the flat steel product into a molten bath (13), the temperature of which is in the range from 420 °C to 780 °C; and

   f) passing the flat steel product through the molten bath (13) and setting the thickness of the metallic protective layer present on the flat steel product coming out of the molten bath, characterised in that flameless burners are used as burners (17) in the preoxidation section of the preheating furnace (3), by means of which fuel (B), preferably burnable gas, and oxygen-containing gas (L) are introduced into the preheating furnace (3) separately from one another at a flow rate of at least 60 m/s, wherein in addition to at least one of the flameless burners (17) assigned to the upper side of the flat steel product (S) and in addition to at least one flameless burner (17) assigned to the underside of the flat steel product (S), in each case at least one gas pipe (5) is provided for supplying at least one additional gas flow (ZG), by means of which the fuel (B) and the oxygen-containing gas (L) are mixed in a complementary manner, and wherein the additional gas flow (ZG) is introduced directed obliquely to the plane of the flat steel product (S) into the preheating furnace (3).
2. Method according to Claim 1, characterised in that the flameless burners (17) are operated with an oxygen excess of at least 1.1, preferably at least 1.2, particularly preferably at least 1.3.
3. Method according to Claim 1 or 2, characterised in that the oxygen-containing gas (L) is introduced preheated into the preheating furnace (3).
4. Method according to any one of Claims 1 to 3, characterised in that inert gas is added to the oxygen-containing gas (L) and/or to the fuel (B).
5. Method according to any one of Claims 1 to 4, characterised in that inert gas and/or flue gas, preferably preheated inert gas and/or heated flue gas, is used for the additional gas flow (ZG).
6. Method according to any one of Claims 1 to 5, characterised in that the dew point of the annealing atmosphere is held between -40 °C and +25 °C over the entire passage of the flat steel product (S) through the annealing furnace (6) by balancing losses or irregularities in the distribution of the humidity in the atmosphere by supplying moisture by means of at least one humidifier (25, 26).
7. DFF-type continuous furnace (3) for a hot-dip coating installation, having a preoxidation section (4), in which a flat steel product (S) to be coated is exposed to an oxidising atmosphere in order to form a covering FeO layer on the surface of the flat steel product, wherein burners (17) are arranged in the preoxidation section (4) which are operated with excess oxygen, and wherein at least one of the burners (17) is assigned to the upper side of the flat steel product (S) and at least another of the burners (17) is assigned to the underside of the flat steel product (S), characterised in that at least some of the burners (17) in the preoxidation section (4) are designed as flameless burners, by means of which fuel (B), preferably burnable gas, and oxygen-containing gas (L) can be introduced into the preheating furnace (3) separately from one another at a flow rate of at least 60 m/s, wherein in addition to at least one of the flameless burners (17) assigned to the upper side of the flat steel product (S) and in addition to at least one flameless burner (17) assigned to the underside of the flat steel product (S), in each case at least one gas pipe (5) is provided for supplying at least one additional gas flow (ZG), wherein the end of the respective gas pipe (5) is aligned in such a way that the additional gas flow (ZG) escaping there crosses or is tangent to the fuel flow (B) and/or oxygen-containing gas flow (L) escaping from the flameless burner (17).
8. Continuous furnace according to Claim 7, characterised in that at least two, preferably at least three, flameless burners (17) are assigned to the upper side of the flat steel product (S) and at least two, preferably at least three, flameless burners (17) are assigned to the underside of the flat steel product (S), wherein the flameless burners (17) assigned to the upper side of the flat steel product (S) are offset in the direction of passage (F) of the flat steel product (S) in relation to the flameless burners (17) assigned to the underside of the flat steel product (S).
9. Continuous furnace according to Claim 7 or 8, characterised in that at least two, preferably at least three, flameless burners (17) are assigned to the upper side of the flat steel product (S) and at least two, preferably at least three, flameless burners (17) are assigned to the underside of the flat steel product (S), wherein the flameless burners (17) assigned to the upper side of the flat steel product (S) are integrated in a side wall (3.2) of the preheating furnace (3), while the flameless burners assigned to the underside of the flat steel product are integrated in the opposite side wall (3.3) of the preheating furnace (3).
10. Continuous furnace according to any one of Claims 7 to 9, characterised in that a device for heating the oxygen-containing gas (L) is connected upstream of the respective flameless burner (17).
11. Continuous furnace according to any one of Claims 7 to 10, characterised in that a device for heating the gas flow (ZG) is connected upstream of the gas pipe (5) for supplying the at least one additional gas flow (ZG).
12. Continuous furnace according to any one of Claims 7 to 11, characterised in that the respective flameless burner (17) has at least one fuel nozzle (17.2) which is encompassed by a ring nozzle or a plurality of nozzles (17.8), which are arranged on a common pitch circle and are uniformly spaced apart from one another, for supplying oxygen-containing gas (L).
13. Continuous furnace according to Claim12, characterised in that the respective flameless burner (17) is provided with a plurality of jet pipes (17.13) for supplying oxygen-containing gas (L), the longitudinal axes of which run essentially parallel to the fuel inflow direction defined by the fuel nozzle (17.2).
14. Continuous furnace according to Claim 12 or 13, characterised in that the respective flameless burner (17)) is provided with at least three jet pipes (17.13) for supplying oxygen-containing gas (L) which are arranged uniformly spaced apart from one another on a common pitch circle.

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