EP2611946A1

EP2611946A1 - Method for hot-dip coating a flat steel product

Info

Publication number: EP2611946A1
Application number: EP11745783.8A
Authority: EP
Inventors: Marc Blumenau; Hans-Joachim Heiler; Fred Jindra; Rudolf Schönenberg; Hans-Joachim Krautschick
Original assignee: ThyssenKrupp Steel Europe AG
Current assignee: ThyssenKrupp Steel Europe AG
Priority date: 2010-08-31
Filing date: 2011-08-18
Publication date: 2013-07-10
Anticipated expiration: 2031-08-18
Also published as: CN103080363A; EP2611946B1; DE102010037254B4; ES2701756T3; WO2012028465A1; US20140144550A1; US9279175B2; DE102010037254A1; CN103080363B

Abstract

The aim of the invention is the hot-dip coating of a flat steel product that consists of a stainless steel comprising more than 5 wt.% Cr with a protective metal overlay that is anti-corrosive. According to the invention, this is achieved by carrying out the following work steps: - the flat steel product is heated to 100 - 600 °C within a time span of 1 - 30 seconds in an oxygen-free heating atmosphere that eliminates an oxidation of the surface of the flat steel product; - the heating is continued to a holding temperature of 750 - 950 °C, said heating being carried out to a temperature window of 550 - 800 °C in an inert or reducing heating atmosphere, in an oxidizing pre-oxidation atmosphere within said temperature window for 1 to 15 seconds, and again in an inert or reducing atmosphere after leaving said temperature window until the holding temperature is reached; - the flat steel product that is pre-oxidized in this manner is held at the holding temperature for 10 - 120 seconds in a reducing holding atmosphere; and - the flat steel product is ran through a nozzle zone into a melt bath in which the flat steel product is hot-dip coated with the metal overlay, wherein the product is held in an inert or reducing nozzle atmosphere until entering the melt bath in the nozzle zone, and the temperature of the flat steel product is 430 - 780 °C while passing through the nozzle zone.

Description

METHOD FOR THE MELT EXTRACTION STYLE

A STEEL FLAT PRODUCT

The invention relates to a method for

Hot-dip coating a steel flat product made of a stainless steel containing more than 5% by weight, in particular at least 10.5% by weight of Cr, with a metallic, corrosion-protecting

Protective coating. When it comes to "flat steel products", it means steel bands or sheets.

Steels of the type in question with a

distinct, 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. Further

Alloy elements such as nickel or molybdenum can support this passivation.

Despite this outstanding specific

Material properties against environmental influences can the use of chromium-alloyed steels for particularly stressed components or components the application of an additional protective coating technically

necessary and / or make economic sense.

The chemical passivity of the opaque chromium oxide layer proves to be problematic. Through this layer, both the wetting and the adhesion reaction in the coating with a metallic coating

with special needs. Therefore, the coating of steels with at least 5.0 wt .-% Cr is a special challenge.

From AT 392089 B it is known that stainless steels in the continuous strip process and

galvanize on both sides electrolytically. This

However, the process is comparably costly and has therefore not prevailed in practice.

As a cheaper alternative to the electrolytic coating offers the continuous

Hot dip finishing of steel strips. In this process, a steel strip, after being in a

Continuous furnace has been annealed recrystallizing, immersed briefly in a metallic molten bath, typically on zinc, aluminum or their

Alloys based.

The hot-dip refinement of alloyed steels requires particular care, since in the case of these steels, oxygen-deficient alloy constituents can oxidize 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 disturbances and liability deficiencies.

For high / ultrahigh-strength multiphase steels, which have a comparatively low Cr content, typically 0.3-2.0% by weight, a process described in EP 2 010 690 B1 has proved successful, in which the respective flat steel product in a first step in a reducing atmosphere with an H ₂ content of at least 2 vol .-% to 8 vol .-% is heated to a temperature of> 750 ° C to 850 ° C, in which then the predominantly pure iron surface by a 1 to 10 sec lasting

Heat treatment of the flat steel product in a

Temperature of> 750 ° C to 850 ° C in an im

Continuous furnace integrated reaction chamber with an oxidizing atmosphere with a 0 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 up to maximum

900 ° C is annealed over a period which is much longer than the duration of the formation of the

Heat treatment performed iron oxide layer that the previously formed iron oxide layer is reduced at least on its surface in pure iron. That so

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 EP 2 184 376 AI is further a with

Aluminum hot dip coated flat steel product known for exhaust systems. 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 described in each case in US Pat. No. 4,675,214 (EP 0 246 418 B1), US Pat. No. 5,066,549 and US Pat. No. 4,883,723. 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 steels with 6 , 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 Pat. No. 5,023,113. This variant is from Flat steel products with a Cr content> 10% by weight. These Stahlfiachprodukte 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

each steel strip is passed from the furnace into the melt bath,> 97 vol .-% H2 / N2 - atmosphere with a dew point of <-29 ° C prevail.

A third variant of the first type of process is known from US 5,591,531. 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.

The method known from EP 0 467 749 B1 (DE 691 04 789 T2) circumvents these annealing conditions by a

Preheating to temperatures of less than 500 ° C under a non-oxidizing atmosphere, which may still contain <3 vol .-% 02. 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, an Al or AlSi melt is also used.

The second known type of method is based on

Application of the oxidation / reduction technique

("Pre-oxidation"). A first variant of this second type of process is described in JP 3111546 A. According to this known method, a steel strip alloyed with 10.0-25.0 wt.% Cr is added in a directly fired preheater

Temperatures of 400 - 600 ° C oxidized. 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 JP 5311380 A, according to a second variant of the second type of method, a similar manner is used

10.0 - 25.0 wt .-% Cr containing steel strip

hot-dip aluminum. 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 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 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 not process reliable

is to be adhered to. 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 company practice shows that

Wetting disturbances in steel flat products made of steels with high Cr contents can not be reliably avoided. In particular, the low pre-oxidation temperatures specified in JP 3111546 A prove to be particularly critical under the operating conditions prevailing in practice.

Another, the above explained

A common disadvantage of types of proceedings is that these methods only apply to the

Refer to fire aluminization.

Against this background, the object of the invention was to provide a method which it

cost-effective and environmentally friendly way, for particularly corrosive claimed applications

provided flat steel products containing more than 5.0 wt .-% chromium, to provide 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 provided high Cr content alloyed steel flat product will be in a continuously sequential manner

Process sequence completed process first heat-treated in a continuous furnace and then immediately surface-refined in-line. Depending on the desired

Use according to the invention can be a zinc, zinc / aluminum, zinc / magnesium, aluminum or

Aluminum / silicon hot dip coating can be applied.

The inventive method for

Hot-dip coating a flat steel product made of a stainless steel containing more than 5% by weight, in particular at least 10.5% by weight of Cr,

For this purpose, the process is carried out with a metallic protective coating which protects against corrosion and comprises, for this purpose, the following steps, which are carried out in a continuous successive sequence: a) heating of the product within 1 to 30 seconds

Flat steel product to a heating temperature of 100-600 ° C under an up to operational impurities oxygen-free, an oxidation of the surface of the steel flat product excluding heating atmosphere; b) Continuing the heating of the flat steel product up to a 750-950 ° C holding temperature, wherein

- up to a pre-oxidation temperature window of

550-800 ° C, the heating is carried out under an inert or reducing heating atmosphere, - within the pre-oxidation temperature window for 1 - 15 s, the heating is carried out under an oxidizing pre-oxidation atmosphere to cause a pre-oxidation of the surface of the flat steel product, and

- After leaving the pre-oxidation temperature window, the heating is again carried out under an inert or reducing atmosphere until the holding temperature is reached; c) holding the pre-oxidized steel flat product at the holding temperature for 10 - 120 s below a

reducing holding atmosphere; d) Optional: over aging of the flat steel product for

1 - 30 s under an inert or reducing over-aging atmosphere at 430 - 780 ° C

amount of aging temperature; e) passing the flat steel product through a

Grousszone and subsequently by a melt bath, in which the flat steel product with the metallic coating is dip-coated, wherein the

Steel flat product until entry into the molten bath in the trunk zone under an inert or

is kept reducing trunk atmosphere and the temperature of the flat steel product during the

Pass through the trunk zone is 430 - 780 ° C.

According to the invention, therefore, a particularly good wetting and good adhesion of the hot-dip coating, even at high degrees of deformation by a targeted temperature and atmospheric control in the continuous furnace process reliable achieved by a two-stage heating as

Combination of a rapid warming (first

Heating step - step a)) and a

conventional reheating (second heating stage - step b)) up to the holding temperature

is carried out. This procedure allows a particularly homogeneous and thus particularly effective

Pre-oxidation during the second heating stage, which is easy to control. This will lead to the

coating flat steel product evenly

produces opaque FeO layer, which acts as a diffusion barrier of Cr oxidation.

Optimal work results arise when the

Temperature of the flat steel product at the end of the

Heating phase (step a)) in the range of

200 - 500 ° C is located.

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

For example, in DE 10 2006 005 063 Äl is described. In this known booster device, the burner is operated with a fuel, in particular a fuel gas, and an oxygen-containing gas. That too

heating steel flat product is brought into direct contact with the flame generated by the burner, wherein set within the flame, the air ratio λ depending on the starting temperature and / or the target temperature becomes. 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 Aufhei zatmosphäre during step a), in addition to N ₂ and technically unavoidable impurities optionally 1 to 50 vol .-% H ₂ included.

Both the heating atmosphere and the

Voroxidationsatmosphäre can contain, for example, H ₂ O, CO or CO ₂ as inevitable impurities due to production.

While the heating atmosphere maintained in step a) should be oxygen-free, ie in its 0 ₂ possibly present in technically unavoidable, ineffective amounts, the Voroxidationsatmosphäre addition of ₂ and technically unavoidable impurities 0.1 - 3.0 vol .-% 0 ₂ at a dew point of -20 ° C to +25 ° C in order to achieve the desired oxidation result.

The pre-oxidation (step b) typically takes 1 to 15 seconds. For example, it can be used in a direct heated DFF type furnace ("DFF" = Direct Fired

Furnace). In the case of a DFF furnace, the oxidation potential at the gas burners used can be adjusted by adjusting the air ratio λ in the strip surrounding atmosphere are generated. 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 a setting of the

Oxygen partial pressure of the Voroxidationsatmosphäre takes place.

Optimally, the steel flat product is 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, oxidized over a period of time, the

typically 1-15 s. This can be done on the

Oven section, over which the area of the

Oxidation temperature is present, the predetermined N ₂ / H ₂ - glow atmosphere additionally with 0.1 - 3.0 vol .-% O ₂

be acted upon, while in front of and behind the furnace area each one largely

Oxygen-free atmosphere is maintained. This oxidizing atmosphere can be specifically adjusted in a DFF plant that in the respective

Furnace section a λ-values> 1 is set. 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 trained layer should be formed as homogeneously as possible over the respective considered surface of the flat steel product to an effective

Diffusion barrier to effect the external selective Cr oxidation. The dew point of the

Oxidation portion of the furnace point maintained atmosphere may be between -20 - +25 ° C to.

Optimal process times while simplifying

Process management arise when the consecutively completed steps of the

inventive method in one

Heat treatment line are performed in which a booster device, a DFF furnace and / or a RTF furnace are combined with each other and in which the

Oven part of a holding or cooling zone connects, which merges into a trunk zone, which in the respective

Hot dip bath leads.

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, the steel flat product occurs in the hardwood Condition in the step a), it has proved to be useful to set the holding temperature to 800 - 850 ° C in order to hold a

To cause recrystallization.

When reaching a holding temperature, the in

According to the invention two-stage heated and thereby pre-oxidized steel flat product maintained over a sufficient period of time at the respective holding temperature

(Step c)). In addition to the recrystallization of the microstructure, if necessary, during the holding phase (step c)), the previously produced FeO layer is adjusted to a corresponding value

Holding atmosphere again reduced to metallic iron. The recent formation of external Cr oxides can be effectively avoided by promoting internal Cr oxidation. This can be achieved by the fact that

Dew point of holding atmosphere at -30 to +25 ° C,

especially at more than -25 ° C, is maintained. Such a dew point ensures such a high H ₂ 0 / H ₂ ratio that a sufficient amount of oxygen to

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

having. 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 in practice typically be 10-120 s, with today available Plants a holding period of 30 - 60 s as optimal

has proved.

Following holding (step c)) and optional overaging treatment

(Step d)), the flat steel product is cooled to the respective melt bath temperature and passed through a per se known trunk structure in the respective melt bath (step ej). It has proved to be particularly advantageous for the wetting result, if the trunk atmosphere has a dew point of -80 to -25 ° C, especially less than -40 ° C. Such a deep dew point can in practice by an additional N ₂ - or H ₂ feed directly into the

Enigma are realized.

The melt bath charged in a suitable melt bath vessel of a type known per se is subsequently prepared from the one according to the invention

Steel flat product in a continuous pass happens, which in practice a dipping time of

0.5 - 10 s, in particular 1 - 3 s, has proven. in the

Schmelzenbadkessel wets the melt the

Steel surface and there is a chemical reaction between the metallic iron of the steel strip and the melt bath to an intermetallic boundary layer, which ensures the good coating adhesion. The

Bandedauch- and melt bath temperatures arise depending on the Schmelzenbadzusammenset tion. 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

In the case of hot dip coating as

Fire aluminizing is performed and a overaging of the flat steel product is performed, the

Aging temperature to 650 - 780 ° C can be adjusted to achieve a further optimized adhesion of the coating.

After leaving the melt bath, if necessary, the coating thickness is adjusted by means of stripping nozzles and the resulting hot-dip-coated, Cr-alloyed

Cooled flat steel product. 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 over conventional flat steel products and not

hot-dip coated Cr-alloyed flat steel products result in particular in terms of the significantly improved corrosion resistance of components that in an environment with high corrosion potential

be used. 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 also results from the fact that organic coatings or adhesives, which are optimized for galvanized surfaces, can now be used effectively also for components made of stainless Cr-alloyed steels. This extends the range of applications of Cr-alloyed steel products, eg. For example, for structural applications in automobiles

Body construction or chemical apparatus and plant construction.

A stainless steel, from which the flat steel product according to the invention is produced, typically contains besides 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 thereby becomes the

Corrosion resistance additionally increased and the

Formability of the flat steel product improved.

Particularly suitable for the process according to the invention are steel sheets or strips, which consist of a on the alloy (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 galvanized, then they are suitable for this purpose

Melt baths, which in addition to zinc and unavoidable, possibly containing traces of Si and Pb

Impurities (in wt.) 0.1 - 60.0% Al and up to 0.5% Fe. Likewise, a galvanizing bath can be used which, according to the type of prior art documented in EP 1 857 566 A1, EP 2 055 799 A1 and EP 1 693 477 A1, the contents of which are in this respect in the content of the present registration

be included. 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% A1 and the Mg content% Mg of the melt formed ratio% Al /% Mg is:% Al /% Mg <1. Regardless of the

The composition of the melt bath gives optimum coating results in hot dip galvanizing when the melt bath temperature is 420-600 ° C.

If steel flat products prepared according to the invention are to be flame-aluminized, then they are suitable for this purpose

Melt baths, in addition to aluminum and unavoidable, possibly traces of Zn-containing impurities (in wt.) 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 is during the fire aluminizing

typically 0.5 to 10 seconds, especially 1 to 3 seconds.

The invention is based on a

Embodiment explained in more detail.

The figure shows schematically a specific for the coating of a steel strip S according to the invention

Hot dip finishing plant 1.

The hot dip finishing plant 1 comprises a

Boosterzone 2, in which the steel band S swiftly from

Room temperature is heated to a temperature of 100 - 600 ° C. In the shielded by a housing from the environment booster device is the

Steel strip under an oxygen-free atmosphere, containing, in addition to nitrogen, optionally up to 5% by volume of H2 and keeping its dew point at -20 ° C to + 25 ° C, within 1-30 s, rapidly to 100-950 ° C Belt temperature heated (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. As heating device, DFF combustion devices are also used here, their λ value being here > 1 is set 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. In addition to nitrogen and unavoidable impurities, the atmosphere in the holding zone 4 consists of 1-50% by volume of H ₂ in order, in addition to the recrystallization, to reduce the

To achieve steel bands S The dew point of

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 under the unchanged

Holding zone atmosphere on the respective

Is cooled inlet temperature, with which it enters the melt 5.

The introduction of the steel strip S in the melt bath 6 via a trunk 7, the steel strip S

uninterrupted 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.

In Table 2, the composition of one for the

Production of steel strip S used steel specified (In% by weight, balance iron and unavoidable impurities).

Table 2

Six samples of the steel strip S have been passed through the hot dip finishing plant 1 for six experiments VI - V6. The initial state of the processed sample, which was set

process parameters

TB a) = belt temperature at the end of the booster zone 2,

TB b) = strip temperature at the end of the pre-oxidation zone atm b) = composition of the atmosphere in the

Pre-oxidation zone 3,

TB c) = max. Band 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) = belt 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 the composition of the respectively used

Melt baths are listed in Table 3. The evaluations of the coating results for the six experiments VI-V6 are summarized in Table 4. It turns out that the samples coated according to the invention have optimum coating results combined with an equally optimal behavior of the coating in the case of

Deformation of the respective sample to a component

while the samples not processed according to the invention do not achieve this property combination.

*) Step a) {Fast heating in booster zone 2) is eliminated

Table 3

Table 4

Claims

P A T E N T S P R O C H E

1. A process for hot dip coating a

Steel flat product made of a stainless steel containing more than 5% by weight of Cr, with a metallic, against corrosion

protective protective coating, comprising the following operations carried out in a continuous sequence: (a) heating the flat steel product to a heating temperature of 100-600 ° C within one to 30 seconds, with the exception of any operational impurities

Oxygen-free, excluding oxidation of the surface of the flat steel product

Aufheizatmosphäre; b) Continuing the heating of the flat steel product up to 750 - 950 ° C

Holding temperature, with heating

up to a pre-oxidation temperature window of 550-800 ° C under an inert or reducing heating atmosphere, within the pre-oxidation temperature window, heating for 1 to 15 seconds under one

Oxidizing Voroxidationsatmosphäre to pre-oxidation of the surface of the

To produce flat steel product, and

- After leaving the pre-oxidation temperature window again under an inert or reducing atmosphere is carried out until the holding temperature

is reached; c) holding the preoxidized steel flat product

the holding temperature for 10-120 s under a reducing holding atmosphere; d) Optional: over aging of the flat steel product for

1 - 30 s under an inert or reducing overaging atmosphere at a

430 - 780 ° C over-aging temperature; e) passing the flat steel product through a

Groin zone and then by a

Melting bath in which the flat steel product is hot-dip coated with the metallic coating, the flat steel product remaining until the

Entry into the molten bath in the trunk zone under an inert or reducing

Whole atmosphere is maintained and the temperature of the flat steel product during the passage through the trunk zone is 430 - 780 ° C. Method according to one of the preceding claims, characterized in that during the working step a) the heating atmosphere in addition to N ₂ and technically unavoidable

Contains impurities optionally 1 - 50 Vol .-% H2.

Method according to one of the preceding claims, characterized in that the process step a) within 1 - 5 s

Oabsolviert.

Method according to one of the preceding claims, characterized in that the heating temperature in step a) is 200-500 ° C, d a.

Method according to one of the preceding claims, characterized in that the pre-oxidation atmosphere in addition to N ₂ and technically unavoidable impurities 0.1 - 3.0 vol .-% 0 ₂ and optionally 1 - contains 50 vol .-% H ₂ and a dew point of -20 ° C to +25 ° C has.

Method according to one of the preceding claims, characterized in that the holding atmosphere during holding or the overaging atmosphere during the optionally performed Überaiterns next to N ₂ and technically unavoidable impurities 1.0-50.0 vol .-% H ₂ and may have a dew point of -30 ° C to +25 ° C.

7. Method according to one of the preceding claims, characterized in that the flat steel product prior to step a) has been subjected to a recrystallizing annealing and the holding temperature is 750-850 ° C.

8. The method according to any one of claims 1 to 6,

The steel flat product in the hard-rolling state enters stage a) and the holding temperature is 800-850 ° C.

9. Method according to one of the preceding claims, characterized in that the hot dip coating is carried out as hot dip galvanizing and the set during the optionally performed Überaiterns

Over-aging temperature is 430 - 650 ° C.

10. The method according to any one of claims 1 to 8,

The hot dip coating is performed as a fire aluminizing and is set during the optional overhaul

Over-aging temperature is 650 - 780 ° C.

11. The method according to any one of the preceding claims, characterized in that the trunk atmosphere has a dew point of -80 ° C to -25 ° C and either in addition to N ₂ and technically unavoidable impurities optional

Contains 1 - 50 vol .-% H ₂ or consists entirely of H2 in addition to technically unavoidable impurities.

12. Method according to one of the preceding claims, characterized in that the hot dip coating of the flat steel product is carried out as hot dip galvanizing and the

Melt bath temperature is 420 - 600 ° C.

13. The method according to any one of claims 1 to 11,

The hot-dip coating of the flat steel product is carried out as a fire-aluminizing process, and the

Melt bath temperature is 650 - 780 ° C.

14. Method according to one of the preceding claims, characterized in that the stainless steel from which the flat steel product is produced, in addition to iron and unavoidable impurities (in% by weight), is used.

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

AI: <0, 2

C: <0, 1 contains.

The method of claim 14, d a d u r c h

characterized in that the steel is {in% by weight) Cr: 10.0 - 13.0%, Ni: <3.0%, Mn: <1.0%, Ti: <1.0%, C: < Contains 0.03%.