EP2611946B1 - Method for hot-dip coating a flat steel product - Google Patents

Description

The invention relates to a process for hot dip coating a steel flat product made of a stainless steel containing more than 5% by weight and up to 30% by weight of Cr, with a metallic, corrosion protective protective coating. When it comes to "flat steel products", it means steel bands or sheets.

Steels of the type in question here with a clear, more than 5 wt .-% lying and typically up to 30 wt .-% reaching chromium content are characterized by a particularly good chemical resistance and high corrosion resistance. This product property is based on the formation of a stable chromium oxide layer, which passivates the steel surface effectively against external influences even at higher temperatures. Therefore, steel grades with a Cr content> 10.5 wt .-% are also referred to as rust, heat and acid resistant (RHS) steels or short as stainless steels. Other alloying elements such as nickel or molybdenum can support this passivation.

Despite these excellent specific material properties against environmental influences can the use of chromium-alloyed steels for particularly stressed components or components makes the application of an additional protective coating technically necessary and / or economically viable.

The chemical passivity of the opaque chromium oxide layer proves to be problematic. This layer impedes both wetting and adhesion reactions when coated with a metallic coating. Therefore, the coating of steels with at least 5.0 wt .-% Cr is a special challenge.

From the AT 392089 B It is known that stainless steels can be electroplated on both sides in the continuous strip process and on both sides. However, this method is comparably costly and has therefore not prevailed in practice.

A more cost-effective alternative to electrolytic coating is the continuous hot dipping of steel strips. In this method, a steel strip, after it has been recrystallized in a continuous furnace, is immersed briefly in a molten metal bath, which is typically based on zinc, aluminum or their alloys.

The hot-dip refinement of alloyed steels requires special care, since these steels can oxidize oxygen-affine alloy constituents selectively on the steel surface during the annealing phase. He follows the selective oxidation externally, ie with oxygen of the ambient atmosphere, must be calculated with wetting disorders and liability deficiencies.

For high / high strength multi-phase steels, which have a comparatively low, typically 0.3 to 2.0 wt .-% amounts of Cr alloy content, a in the EP 2 010 690 B1 proven method in which the respective flat steel product is heated in a first step in a reducing atmosphere with an H ₂ content of at least 2 vol .-% to 8 vol .-% to a temperature of> 750 ° C to 850 ° C. , in which then the predominantly made of pure iron surface by a 1 to 10 sec lasting heat treatment of the flat steel product 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 vol .-% to 1 vol .-% is converted into an iron oxide layer and finally the flat steel product in a reducing atmosphere with an H ₂ content of 2 vol .-% to 8 vol .-% by heating to maximum 900 ° C is annealed over a period which is so much longer than the duration of the heat treatment carried out to form the iron oxide layer that the previously formed iron oxide layer is reduced at least on its surface in pure iron. The thus pretreated flat steel product can then be hot-dip-coated in the heated state in a melt bath containing at least 85% by weight of zinc and / or aluminum with the metallic coating.

From the EP 2 184 376 A1 Furthermore, it is known that a flat aluminum aluminum product for exhaust systems is used. However, it is not clear from this document how the hot dip coating can be carried out in practice. However, the possibility of pre-coating with iron is indicated, which would greatly facilitate the fire aluminizing, but is more costly.

For the hot-dip finishing of steels with more than 5% by weight Cr, in particular more than 10% by weight Cr, basically two types of processes are known, each of which assumes that the steel strip to be coated can be prepared by an annealing treatment in such a way that an optimal coating result is achieved. The first type of process involves annealing under a strongly reducing atmosphere.

A variant of this type of process is in each case in US 4,675,214 ( EP 0 246 418 B1 ) US 5,066,549 and US 4,883,723 described. This variant assumes that the flat steel product to be coated is heated in a non-oxidative atmosphere and then maintained at more than 677 ° C in a highly reductive atmosphere, the more than 95 vol .-% H2 / N2 for 6 steel , 0 - 14.5 wt .-% Cr. The coating then takes place in an aluminum or aluminum / silicon melt bath.

Another variant of the first type of process is known from US 5,023,113 known. This variant is from Flat steel products with a Cr content> 10 wt .-% of.

These flat steel products are heated to 650 ° C without free oxygen and then maintained at 845-955 ° C under a> 95 vol .-% H2 / N2 containing atmosphere. In addition, in the trunk, over which the respective steel strip is led out of the furnace into the melt bath,> 97% by volume H2 / N2 atmosphere with a dew point of <-29 ° C prevails.

A third variant of the first type of method is known from US 5,591,531 known. According to this variant, steel strips of up to 30% by weight Cr are subjected to bell annealing, in which an iron-rich surface layer is produced. The actual annealing should then take place according to one of the two variants of the first type of method explained above.

That from the EP 0 467 749 B1 ( DE 691 04 789 T2 ) known processes bypasses these annealing conditions by preheating to temperatures of less than 500 ° C under a non-oxidizing atmosphere, which may still contain <3 vol .-% O2. This is followed by further heating to a holding temperature of less than 950 ° C in a non-oxidizing, non-reactive N 2 or H 2 / N 2 atmosphere with a dew point below -40 ° C. For the hot-dip coating also an Al or AlSi melt is used.

The second known type of process is based on the application of the oxidation / reduction technique ("pre-oxidation").

A first variant of this second type of process is in the JP 3111546 A described. According to this known method, a steel strip alloyed with 10.0-25.0% by weight Cr is oxidized in a direct-fired preheater at temperatures of 400-600 ° C. The resulting FeO layer is then reduced during a holding phase at 700-950 ° C under a reducing atmosphere. The treated steel strip is then subjected to a fire aluminizing.

According to the JP 5311380 A For example, according to a second variant of the second type of process, a steel strip containing 10.0-25.0% by weight of Cr is similarly flame-treated. The pre-oxidation also takes place during a direct heating up to a temperature between 550 - 750 ° C by regulating the λ value to 0.9 - 1.5. The reduction of the FeO layer then takes place under a reducing atmosphere at a holding temperature which is about 800 ° C or up to max. 1050 ° C is enough.

The first type of process can only be implemented with great effort in everyday operations at a hot-dip coating plant designed for conventionally alloyed steel. The necessary high annealing temperatures and the high H2 consumption cause significantly higher operating costs. Likewise, the large-scale practice shows that a dew point <-40 ° C in the holding zone of the continuous furnace is not to comply with process reliability.

Although the variants belonging to the second type of process can be implemented much more easily in the context of large-scale hot-dip coating. However, here, too, the operational practice shows that wetting disturbances in steel flat products made of steels with high Cr contents can not be reliably avoided. Especially in the JP 3111546 A given low pre-oxidation temperatures prove to be particularly critical under the operating conditions prevailing in practice.

Another disadvantage common to the above-described types of method is that these methods each relate only to the fire aluminization.

In addition to the above-described prior art is from the EP 1 829 983 A1 In general, a process for hot dip coating steel flat products from steel grades containing alloying constituents that oxidize more readily than iron is described. Here, in particular, Si-containing steels are used. However, this prior art does not address the problems associated with hot dip coating of flat steel products made of stainless steel containing more than 5% by weight of Cr.

Also in the WO 2006/061151 A1 No reference is made to the particular problems encountered in hot dip coating a high Cr stainless steel flat product. So should be with the one described in this document Although process flat steel products with Cr contents> 0.3 wt .-% coat. However, this note is linked to the indication that the steels concerned are higher-strength steels. In these steels, the Cr content is known to contribute to hardenability. However, this has nothing to do with stainless steels having Cr contents of more than 5% by weight.

Also in the WO 2007/124781 A1 It is merely mentioned that the process described there is suitable for higher-strength steels with Cr contents of more than 0.3% by weight, without taking into account the special challenges associated with hot-dip coating of non-rusting,> 5% by weight. % Cr containing steel flat products.

Against this background, the object of the invention to provide a method which allows it in a cost-effective and environmentally friendly manner, provided for particularly corrosive applications claimed steel flat products containing more than 5.0 wt .-% and up to 30 wt .-% chromium to be provided with a hot-dip coating.

According to the invention, this object has been achieved by the method specified in claim 1.

Advantageous embodiments of the invention are set forth in the dependent claims and, like the general inventive idea are explained in detail below.

According to the invention, a supplied high-alloy steel flat product is first heat-treated in a continuous furnace in a process completed in a continuous successive sequence of operations and then immediately surface-finished in-line. Depending on the intended use, a zinc, zinc / aluminum, zinc / magnesium, aluminum or aluminum / silicon hot-dip coating can be applied according to the invention.

The process according to the invention for hot dip coating a flat steel product made of a stainless steel containing more than 5% by weight and up to 30% by weight of Cr, with a metallic, corrosion protective protective coating, comprises for this purpose the steps specified in claim 1.

According to the invention, a particularly good wetting and good adhesion of the hot-dip coating are thus reliably achieved even at high degrees of deformation by a targeted temperature and atmosphere control in the continuous furnace that a two-stage heating as a combination of rapid heating (first heating step - step a)) and a conventional heat-up (second heating stage - step b)) up to the holding temperature is performed. This procedure allows a particularly homogeneous and thus particularly effective pre-oxidation during the second heating stage, which is easy to control. As a result, a uniform on the flat steel product to be coated produces opaque FeO layer, which acts as a diffusion barrier of Cr oxidation.

Optimum work results are obtained when the temperature of the flat steel product at the end of the heating phase (step a)) is in the range of 200-500 ° C.

The heating phase (step a)) should preferably take only 1 to 5 seconds.

In practice, the inventive rapid heating (step a)) with the help of a so-called "booster heater" perform, as they are for example in the DE 10 2006 005 063 A1 is described. In this known booster device, the burner is operated with a fuel, in particular a fuel gas, and an oxygen-containing gas. The flat steel product to be heated is brought into direct contact with the flame generated by the burner, wherein the air ratio λ is set within the flame as a function of the starting temperature and / or the target temperature. To carry out the method according to the invention, the temperature, atmosphere and λ value of the booster flames are adjusted so that non-reactive or reducing thermodynamic conditions prevail over the metal / metal oxide equilibria of the alloying elements. Oxidation of the steel surface during the operation a) is mandatory to avoid.

The heating atmosphere during step a) contains, in addition to N ₂ and technically unavoidable impurities, optionally 1 to 50% by volume of H ₂ .

Both the heating atmosphere and the pre-oxidation atmosphere may contain, for example, H ₂ O, CO or CO ₂ as inevitable impurities due to production.

While the heating atmosphere maintained in operation a) should be free of oxygen, ie in its O ₂ possibly present in technically unavoidable, ineffective amounts, the Voroxidationsatmosphäre in addition to N ₂ and technically unavoidable impurities 0.1 - 3.0 vol .-% O ₂ at a dew point of -20 ° C to +25 ° C to achieve the desired oxidation result.

The pre-oxidation (step b) typically takes 1 to 15 seconds. It can be carried out, for example, in a directly heated furnace of the DFF type ("DFF" = Direct Fired Furnace). In a DFF furnace, the oxidation potential can be generated on the gas burners used by adjusting the air ratio λ in the atmosphere surrounding the band. The heating in the DFF furnace has the additional advantage that on the surface of the flat steel product existing organic pollutants are removed by combustion. Alternatively, it is also conceivable to use a furnace of the RTF type (RTF = Radiant Tube Furnace), in which only jet pipes are used and the pre-oxidation of the iron via an adjustment of the Oxygen partial pressure of the Voroxidationsatmosphäre takes place.

Optimally, the steel flat product is thereby oxidized to avoid an external chromium oxide layer on the steel surface in an oxidation temperature range of 550 - 800 ° C, ideally at an oxidation temperature of 600 - 700 ° C, over a period which is typically 1-15 s. For this purpose, on the furnace section, over which the relevant region of the oxidation temperature is present, the predetermined N ₂ / H ₂ -Glühatmosphäre additionally be charged with 0.1 to 3.0 vol .-% O ₂ , while in front of and behind the furnace area in each case a largely oxygen-free atmosphere is maintained. This oxidizing atmosphere can be specifically adjusted in a DFF system by setting a λ-value> 1 in the respective furnace section. In contrast, in the case of an RTF plant, a furnace zone which is sealed off from the preceding and the subsequently passed through region can be formed, in which the oxygen-containing atmosphere exists. Alternatively, the pre-oxidation can also take place via an additional intermediate booster device.

In the course of the pre-oxidation carried out according to the invention, an iron oxide layer with a thickness which is less than 300 nm, ideally in the range from 20 to 200 nm, is formed on the steel surface. The thickness of this optimally opaque layer should be formed as homogeneously as possible over the surface of the flat steel product considered in each case in order to produce an effective Diffusion barrier to effect the external selective Cr oxidation. The dew point of the atmosphere maintained in the oxidation section of the furnace point may be between -20 to +25 ° C.

Optimal process times with simultaneously simple process management result when the successively completed working steps of the method according to the invention are carried out in a heat treatment line in which a booster device, a DFF furnace and / or a RTF furnace are combined with each other and in which the furnace part a holding or cooling zone connects, which merges into a trunk zone, which leads into the respective Schmelztauchbad.

In the course of step b), the steel flat product is further heated to the desired holding temperature of 750-950 ° C., starting from the heating temperature reached after step a), 100-600 ° C. In the event that the processed flat steel product has already been subjected to the softening of a recrystallizing annealing prior to step a), the holding temperature can be limited to 750-850 ° C. On the other hand, if the steel flat product enters the working step a) in the hard-rolling state, it has proved expedient to set the holding temperature to 800-850 ° C. in order to recrystallize during holding.

Upon reaching a holding temperature that is heated in two stages according to the invention and thereby preoxidized steel flat product maintained at the respective holding temperature for a sufficient duration (step c)). In addition to the recrystallization of the microstructure, if necessary, during the holding phase (step c)), the previously produced FeO layer is again reduced to metallic iron under a correspondingly adjusted holding atmosphere.

The recent formation of external Cr oxides can be effectively avoided by promoting internal Cr oxidation. This can be achieved by keeping the dew point of the holding atmosphere at -30 to +25 ° C, in particular at more than -25 ° C. Such a dew point ensures such a high H ₂ O / H ₂ ratio that a sufficient amount of oxygen is available. Accordingly, optimum holding performance at the hold temperature results when the hold atmosphere during holding contains N ₂ and technically unavoidable impurities 1.0-50.0 vol% H ₂ and a dew point of -30 ° C to + 25 ° C has. By the dew point of the holding atmosphere is at least -30 ° C, in particular in the range of -25 to 0 ° C, as mentioned, the external Cr oxidation is additionally inhibited. The duration of the holding phase will typically be 10-120 s in practice, with a holding time of 30-60 s having been found to be optimal in systems available today.

Following the holding (step c)) and the optional overaging treatment (step d)), the flat steel product is cooled to the respective melt bath temperature and over a per se known trunk structure in the respective melt bath passed (step e)). It has proved to be particularly advantageous for the wetting result when the trunk atmosphere has a dew point of -80 to -25 ° C, in particular less than -40 ° C, has.

Such a deep dew point can be realized in practice by an additional N ₂ - or H ₂ feed directly into the trunk zone.

The melt bath filled in a suitable melt bath vessel of a type known per se is subsequently passed through the steel flat product prepared in the manner according to the invention in a continuous pass, whereby in practice a immersion time of 0.5-10 s, in particular 1-3 s, has proved successful. In the melt bath kettle, the molten bath wets the steel surface and a chemical reaction between the metallic iron of the steel strip and the molten bath results in an intermetallic boundary layer, which ensures the good coating adhesion. The tape immersion and melt bath temperatures result depending on the Schmelzenbadzusammensetzung. Table 1 shows typical temperature ranges for coatings on Zn (eg Zn, ZnAl, ZnMg or ZnMgAl coatings) and Al based (eg AlZn, AlSi coatings) the immersed the flat steel product in the respective melt bath, as well as the appropriate range of the temperature of the respective melt bath specified. Table 1 melting bath Strip immersion temperature melt temperature Zn-based 430-650 ° C 420-600 ° C Al-based 650-800 ° C 650-780 ° C

In the case where the hot dip coating is performed as a fire aluminizing and over aging of the flat steel product is performed, the aging temperature can be set to 650 - 780 ° C to obtain further optimized adhesion of the coating.

After leaving the melt bath, if necessary, the coating thickness is adjusted by means of wiping nozzles and the resulting hot-dip-coated Cr-alloyed flat steel product is cooled. Optionally, post-forming (skin pass rolling), passivation, lubrication and coiling of the flat steel product into a coil can be connected to the cooling.

Depending on the coating applied in each case, the flat steel product coated according to the invention is suitable for one-, two- or multi-stage cold or hot forming into one component. Advantages compared to conventional flat steel products and non-hot-dip coated Cr-alloyed flat steel products result in particular in the significantly improved corrosion resistance of components that are used in an environment with high corrosion potential. This proves to be particularly advantageous if there are elevated temperatures at the site in question.

A particular versatility of the usability of coated steel flat products according to the invention results This is also due to the fact that organic coatings or adhesives optimized for galvanized surfaces can now be used effectively for components made of stainless Cr-alloyed steels. This extends the range of applications of Cr-alloyed steel products, eg. B. for structural applications in automotive body construction or chemical apparatus and equipment.

The stainless steel from which the flat steel product of the present invention is produced contains, in addition to iron and unavoidable impurities (in% by weight) Cr: 5.0-30.0%, Mn: less than 6.0%, Mo: less than 5.0%, Ni: up to 30.0%, Si: less than 2.0%, Cu: less than 2.0%, Ti: less than 1.0%, Nb: less than 1.0% , V: less than 0.5%, N: less than 0.2%, Al: less than 0.2%, C: less than 0.1%. By alloying up to 30.0% by weight of Ni, it is possible to produce an austenitic or ferritic-austenitic duplex structure which further increases the formability of the flat steel product. Likewise, this additionally increases the corrosion resistance and improves the formability of the flat steel product.

Particularly suitable for the process according to the invention are steel sheets or strips which are produced from a steel based on the abovementioned alloy specification which contains (in% by weight) Cr: 10.0-13.0%, Ni: less than 3, 0%, Mn: less than 1.0%, Ti: less than 1.0%, C: less than 0.03%.

If steel flat products prepared according to the invention are to be hot-dip galvanized, melt baths suitable for this, in addition to zinc and unavoidable optionally Contains traces of Si and Pb impurities (in wt.) 0.1 - 60.0% Al and up to 0.5% Fe. Likewise, a galvanizing bath may be used, which in the manner of the prior art, in the EP 1 857 566 A1 , of the EP 2 055 799 A1 and the EP 1 693 477 A1 is respectively documented, the contents of which are included in the content of the present application. Accordingly, in addition to zinc and unavoidable impurities (in% by weight), the melt bath may contain 0.1-8.0% Al, 0.2-8.0% Mg, <2.0% Si, <0.1% Pb, <0.2% Ti, <1% Ni, <1% Cu, <0.3% Co, <0.5% Mn, <0.1% Cr, <0.5% Sr, <3.0% Fe, <0.1% B, <0.1% Bi with the proviso that for the Al Al content% Al and the Mg content% Mg of the melt formed ratio% Al /% Mg applies:% Al /% Mg <1. Regardless of the composition of the melt bath, optimum hot dip galvanizing results will be obtained when the melt bath temperature is 420-600 ° C.

If steel flat products prepared according to the invention are to be flame-aluminized, melt baths which, in addition to aluminum and unavoidable impurities (if present) containing traces of Zn, have up to 15% Si and up to 5% Fe.

Optimum coating results are obtained when the melt bath temperature is 660 - 680 ° C. The immersion time during hot aluminizing is typically 0.5 to 10 s, in particular 1 to 3 s. The invention will be explained in more detail with reference to an embodiment.

The FIGURE schematically shows a hot-dip coating plant 1 intended for the coating of a steel strip S according to the invention.

The hot-dip finishing plant 1 comprises a booster zone 2, in which the steel strip S is heated rapidly from room temperature to a temperature of 100-600 ° C. In the booster device, which is shielded by a housing from the environment, the steel strip is kept under an oxygen-free atmosphere which, in addition to nitrogen, optionally contains up to 5% by volume of H ₂ and whose dew point is kept at -20 ° C. to + 25 ° C. Heated rapidly to a strip temperature of 100-950 ° C. for 1 to 30 seconds (step a)).

Subsequent to the booster zone 2, the steel strip S runs without interruption and without coming into contact with the ambient atmosphere U into a pre-oxidation zone 3. There, the steel strip is heated to a strip temperature of up to 950 ° C. under an atmosphere which is formed from nitrogen and up to 50% by volume of H ₂ and 0.1-3% by volume of O ₂ and whose dew point is. 15 ° C to +25 ° C is maintained. Here, too, DFF firing devices are used as the heating means, their λ value being set here> 1 in order to specifically oxidize the surface of the steel strip S.

Subsequently, the steel strip S passes through a likewise shielded from the environment holding zone 4, in which the steel strip S is maintained at the previously achieved, lying in the range of 750 - 950 ° C belt temperature. The atmosphere in the holding zone 4 is next to nitrogen and unavoidable impurities from 1 to 50% by volume of H ₂ in order to achieve a reduction of the steel strip S in addition to the recrystallization. The dew point of the holding zone atmosphere is kept between -30 ° C and +25 ° C.

Connected to the holding zone 4 is a cooling zone 5, in which the steel strip S is cooled under the unchanged holding zone atmosphere to the respective inlet temperature, with which it passes into the melt bath 5.

The introduction of the steel strip S in the melt bath 6 via a trunk 7, the steel strip S coming without interruption coming from the cooling zone 5 and further shielded from the environment U passes. In the trunk 7 a proboscis atmosphere is maintained, which consists either of nitrogen or of hydrogen or a mixture of these two gases. The dew point of the trunk atmosphere is kept at -80 ° C to -25 ° C.

Table 2 shows the composition of a steel used for the production of the steel strip S (data in% by weight, remainder iron and unavoidable impurities). Table 2 Cr C Si Mn Not a word Ni Ti Nb Cu al 11.52 0,015 0.55 0.39 0.01 0.12 0.212 0.01 0.03 0.02

TB a) =

Bandtemperatur am Ende der Boosterzone 2,

TB b) =

Bandtemperatur am Ende der Voroxidationszone 3,

Atm b) =

Zusammensetzung der Atmosphäre in der Voroxidationszone 3,

TB c) =

max. Bandtemperatur in der Haltezone 4,

Atm c) =

Zusammensetzung der Atmosphäre in der Haltezone 4,

TP c) =

Taupunkt der Atmosphäre in der Haltezone 4,

TB e) =

Bandtemperatur in der Rüsselzone 7,

Atm e) =

Zusammensetzung der Atmosphäre in der Rüsselzone 7,

TP e) =

Taupunkt der Atmosphäre in der Rüsselzone 7 und

die Zusammensetzung des jeweils verwendeten Schmelzenbades sind in Tabelle 3 aufgelistet.Six samples of the steel strip S have been passed through the hot dip finishing plant 1 for six experiments V1 - V6. The initial state of each processed sample, the process parameters set

TB a) =

Belt temperature at the end of the booster zone 2,

TB b) =

Strip temperature at the end of the pre-oxidation zone 3,

Atm b) =

Composition of the atmosphere in the pre-oxidation zone 3,

TB c) =

Max. Strip temperature in the holding zone 4,

Atm c) =

Composition of the atmosphere in the holding zone 4,

TP c) =

Dew point of the atmosphere in the holding zone 4,

TB e) =

Strip temperature in the trunk zone 7,

Atm e) =

Composition of the atmosphere in the trunk zone 7,

TP e) =

Dew point of the atmosphere in the trunk zone 7 and

the composition of the melting bath used in each case are listed in Table 3.

The assessments of the coating results for the six experiments V1-V6 are summarized in Table 4. It turns out that the samples coated according to the invention have optimum coating results coupled with an equally optimal behavior of the coating during the deformation of the respective sample into a component, whereas the samples not processed according to the invention do not achieve this property combination. Table 3 attempt initial state TB a) [° C] TB b) [° C] Atm b) [Vol .-%] TB c) [° C] Atm c) [Vol. -%] TP c) [° C] TB e) [° C] Atm e) [Vol .-%] TP e) [° C] Composition melt bath [% by weight] V1 annealed 170 656 0.85% O ₂ 818 5% H ₂ -21 480 N ₂ -51 0.9% Al Remainder N ₂ Remainder N ₂ 0.9% Mg Rest Zn V2 annealed 200 711 0.62% O ₂ 810 7% H ₂ -24 715 N ₂ -50 10% Si Remainder N ₂ Remainder N ₂ Rest al V3 annealed 200 657 1.16% O ₂ 750 5% H ₂ -10 678 N ₂ -57 10% Si Remainder N ₂ Remainder N ₂ Rest al V4 annealed 200 685 1.18% O ₂ 765 5% H ₂ -10 709 N ₂ -57 10% Si Remainder N ₂ Remainder N ₂ Rest al V5 annealed *) 660 N ₂ 900 10% H ₂ -29 680 N ₂ -50 10% Si Remainder N ₂ Rest al V6 annealed *) 730 0.10% O ₂ 804 5% H ₂ -24 462 N ₂ -53 0.9% Al Remainder N ₂ Remainder N ₂ 0.9% Mg Rest Zn *) Step a) (rapid heating in the booster zone 2) is omitted attempt coating Mechanical setpoints According to the invention V1 Well Well Fulfills YES V2 Well Well Fulfills YES V3 Well Well Fulfills YES V4 Well Well Fulfills YES V5 Disturbed Disturbed Not fulfilled NO V6 Disturbed Disturbed Not fulfilled NO

Claims (10)

1. Method for hot-dip coating a flat steel product which has been produced from a stainless steel containing (in % by weight) Cr: 5.0 - 30.0 %, Mn: < 6.0 %, Mo: < 5.0 %, Ni: < 30.0 %, Si: < 2.0 %, Cu: < 2.0 %, Ti: < 1.0 %, Nb: < 1.0 %, V: < 0.5 %, N: < 0.2 %, Al: < 0.2 %, C: < 0.1 %, remainder iron and unavoidable contaminants, with a metallic protective coating protecting against corrosion, comprising the following working steps which are completed in a continuously successive sequence:

   a) heating the flat steel product within a period of 1 - 30 seconds to a heating temperature of 100 - 600°C under an oxygen-free, except for operational contaminants, heating atmosphereruling out an oxidation of the surface of the flat steel product, wherein the heating atmosphere optionally contains 1 - 50 % by volume of H₂ in addition to N₂ and technically unavoidable contaminants;

   b) continuing the heating of the flat steel product up to a holding temperature of 750 - 950°C, wherein the heating procedure

   - is carried out up to a pre-oxidation temperature window of 550 - 800°C under an inert or reducing heating atmosphere

   - is carried out within the pre-oxidation temperature window for 1 to 15 seconds under an oxidising pre-oxidation atmosphere to cause a pre-oxidation of the surface of the flat steel product, the pre-oxidation atmosphere containing, in addition to N₂ and technically unavoidable contaminants, 0.1 - 3.0 % by volume of O₂ and optionally 1 - 50 % by volume of H₂, and having a dew point of -20°C to +25°C, and

   - after leaving the pre-oxidation temperature window, is again carried out under an inert or reducing atmosphere

   until the holding temperature is reached;

   c) keeping the pre-oxidised flat steel product at the holding temperature for 10 - 120 seconds under a reducing holding atmosphere;

   d) optionally: overaging the flat steel product for 1 - 30 seconds under an inert or reducing overaging atmosphere at an overaging temperature of 430 - 780°C;
   wherein the holding atmosphere contains during the holding procedure, or the overaging atmosphere contains during the optionally performed overaging procedure in each case 1.0 - 50.0 % by volume of H₂ in addition to N₂ and technically unavoidable contaminants, and has a dew point of -30°C to +25°C;

   e) passing the flat steel product through a nozzle zone and thereafter through a molten bath in which the flat steel product is hot-dip coated with the metallic coating, wherein the flat steel product is held under an inert or reducing nozzle atmosphere in the nozzle zone until it passes into the molten bath, which nozzle atmosphere has a dew point of -80°C to -25°C and either optionally contains 1 - 50 % by volume of H₂ in addition to N₂ and technically unavoidable contaminants, or completely consists of H₂ in addition to technically unavoidable contaminants, and the temperature of the flat steel product while passing through the nozzle zone is 430-780°C.
2. Method according to Claim 1, characterised in that working step a) is completed within 1 to 5 seconds.
3. Method according to any of the preceding claims,
   characterised in that
   the heating temperature in working step a) is 200 - 500°C.
4. Method according to any of the preceding claims,
   characterised in that
   before working step a), the flat steel product is subjected to a recrystallising annealing procedure, and the holding temperature is 750 - 850°C.
5. Method according to any of Claims 1 to 3,
   characterised in that
   the flat steel product passes into working step a) in a hard rolled state, and the holding temperature is 800 - 850°C.
6. Method according to any of the preceding claims,
   characterised in that
   the hot-dip coating process is carried out as a hot-dip galvanising process, and the overaging temperature which is set during the optionally performed overaging procedure is 430 - 650°C.
7. Method according to any of Claims 1 to 5,
   characterised in that
   the hot-dip coating process is carried out as a hot-dip aluminising process, and the overaging temperature which is set during the optionally performed overaging procedure is 650 - 780°C.
8. Method according to any of the preceding claims,
   characterised in that
   the hot-dip coating process of the flat steel product is carried out as a hot-dip galvanising process, and the temperature of the molten bath is 420 - 600°C.
9. Method according to any of Claims 1 to 7,
   characterised in that
   the hot-dip coating process of the flat steel product is carried out as a hot-dip aluminising process, and the temperature of the molten bath is 650 - 780°C.
10. Method according to any of the preceding claims,
    characterised in that
    the steel contains (in % by weight) Cr: 10.0 - 13.0 %, Ni: < 3.0 %, Mn: < 1.0 %, Ti: < 1.0 %, C: < 0.03 %.

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