EP3783119A1 - Flat steel product with excellent oxidation and hot gas corrosion resistance and method for producing such a flat steel product - Google Patents

Abstract

The invention provides a flat steel product with a fully ferritic structure which, due to its combination of properties, is ideally suited for the production of components for use at high temperatures and which has an equally optimized visual appearance. For this purpose, the flat steel product according to the invention consists of (in% by weight) Al: 2.0-8.0%, Cr: 2.0-10.0%, Nb: 0.1-1.0%, optionally one element each or more elements from the group Mn, Ti, Si, B, Ta, W, V, Zr, Mo, Ni, Cu, Ca, SEM, Co with the proviso that the Mn contents: up to 3.0%, Ti: up to 1.0%, Si: up to 1.0%, B: up to 0.10%, Ta: up to 1.0%, W: up to 1.0%, V: up to 0 , 50%, Zr: up to 1.0%, Mo: up to 2.0%, Ni: up to 2.0%, Cu: up to 2.0%, Ca: up to 0.150%, SEM: up to to 0.50%, Co: up to 2.0%, with the content% Cr of Cr, the content% Al of Al, the optionally present content% Si of Si and the likewise optionally present content% Mn of Mn The following applies:% Cr + 10 x% Si +% Al +% Mn ≤ 20%, and the remainder of Fe and unavoidable impurities, with the unavoidable impurities up to 0.10% C, up to 0.30% S, to count to 0.10% P, up to 0.30% Sb, up to 0.30% As, up to 0.50% N. Flat steel products made according to the invention are particularly suitable for the production of components for exhaust systems at which temperatures of more than 200 ° C. occur. The invention also mentions a method for producing a flat steel product according to the invention.

Description

The invention relates to a flat steel product with excellent oxidation and hot gas corrosion resistance and a method for producing such a flat steel product.

Flat steel products are understood here to mean rolled products, the length and width of which are each significantly greater than their thickness. These include in particular steel strips, steel sheets and blanks obtained therefrom, such as blanks and the like, which are obtained by cold rolling a previously hot-rolled flat steel product.

All information on the contents of the steel compositions given in the present application are based on weight, unless expressly stated otherwise. All "% figures" which are not specified in more detail and in connection with a steel alloy are therefore to be understood as figures in "% by weight".

Information on the content of a constituent of the structure of the steel products considered here, on the other hand, relates to the area of a section of a sample of the respective product (information in area percent "area%"), unless expressly stated otherwise.

Information given in this text on the contents of the constituents of an atmosphere relate to the volume (information in "vol .-%").

Mechanical properties such as tensile strength, yield point, elongation, which are reported here, have been determined in tensile tests in accordance with DIN EN ISO 6892-1: 2009, unless expressly stated otherwise.

If values for oxidation and hot gas corrosion resistance are given for certain flat steel products, the flat steel products in question were heated to 900 ° C and kept for 2 hours under air with a content of 6% water vapor to determine these samples. The samples were then cooled for 30 minutes under a stream of compressed air. Each sample has gone through 500 such cycles. For an initial statement on the oxidation that has occurred during these cycles, the samples were weighed before the beginning and after the end of the cycle, including any flaked oxidation products. In addition, the samples were examined by GD-OES (Glow Discharge Optical Emission Spectroscopy). As a result, these investigations provide information about the oxide layer thickness that has arisen in the course of the cycle and the chemical composition of the oxidic surface.

Information on the thermal conductivity of flat steel products of the type in question was obtained using the laser flash method in accordance with DIN 821-2 using a device of the type LFA 427/3 / D from NETZSCH-Gerätebau GmbH, Selb, Germany (see the URL https://www.netzsch-thermal-analysis.com/de/produkte-loesungen/waerme-und- Temperaturleitfaehigkeitsbestemme/lfa-427/ Brochure available for download "Laser Flash Apparatus LFA 427", imprint "NGB · LFA 427 · DE · 1118 · NWS "). For this purpose, a scale-free, matt sample with a diameter of 12.7 mm and a thickness of two mm was taken from the hot strip to be examined. This sample disc has been briefly heated on one side with a laser flash at various measuring temperatures. Of the The temperature rise over time was measured on the opposite side with an infrared detector and the thermal diffusivity was calculated from this. The specific heat capacity is determined by dynamic differential calorimetry in accordance with DIN 821-3 using a DSC 404 F / SO device, also manufactured by NETZSCH-Gerätebau GmbH (see the URL https://www.netzsch-thermal-analysis.com/de / product solutions / dynamic-differential-calorimetry-differential-thermal-analysis / dsc-404-f3-pegasus / downloadable brochure "Dynamic Differential Calorimetry - DSC 404 F1 / F3 Pegasus®", imprint "NGB · DSC 404 F1 / F3 Pegasus® · DE · 0816 · NWS "). For this purpose, a hot strip sample with a diameter of five mm and a thickness of one mm was heated and the heat capacity was calculated from the required heat flow.

In order to be able to determine the thermal conductivity λ = α xc _p x ρ (with α: _{thermal conductivity, c p} : specific heat capacity, p: density), the temperature-dependent density was calculated from the room temperature density and the technical expansion coefficient of the steel material. The room temperature density according to DIN 66137-2 and the expansion coefficient were determined with a dilatometer from the temperature-related percentage change in length of a 20 mm long sample, for which a dilatometer of the type DIL 402C, also manufactured by NITZSCH-Gerätebau GmbH, was used (see the https://www.netzsch-thermal-analysis.com/de/produkteloesungen/dilatometer/dil-402-expedis-classic/ brochure "Dilatometer-Serie DIL 402", imprint "NGB · Dilatometer Series DIL 402 Expedis® Classic DE 0119 ").

The brittle-ductile transition temperature was determined by pulling cups and in the cup impact test on cups. For this purpose, a round (diameter 80 mm) of the respective flat steel product was deep-drawn into a cup using a cylindrical punch (diameter 50 mm).

In the subsequent cup impact test, the temperature-controlled cup was beaten in a Pellini dropper (drop weight 27 kg, drop height 300 mm). If a crack occurs during the test, the test was deemed to have failed. The lowest temperature at which no crack occurred in the cell was rated as the brittle-ductile transition temperature. The test temperatures of the cells were -80 to +60 ° C and were checked in 20K steps. The test temperatures below room temperature were set in chilled alcohol and the temperatures above room temperature in an oven (storage time each 10 min).

The density of the flat steel products considered here was determined using a helium gas pycnometer (https://de.wikipedia.org/wiki/Gaspyknometer).

The mean roughness values Ra mentioned here of flat steel products according to the invention have been determined in accordance with DIN EN 10049.

Information on cold bending deformability has been determined in the plate bending test in accordance with test instruction VDA 238-100.

Information on the thickness and composition of an oxide layer on a flat steel product has been determined by means of measurement in the GD-OES method, which is described in DIN 11505 and ISO 16962.

Components of automobile exhaust systems are now predominantly formed from flat steel products made of austenitic CrNi steels, e.g. the steel standardized under material number 1.4828, or ferritic Cr steels, such as the steel known under material number 1.4509. These alloy steels are corrosion and oxidation resistant. The ferritic grades are inferior to the austenitic grades in terms of resistance to oxidation and hot gas corrosion, but at the same time significantly more cost-effective.

Ferritic Fe-Al-Cr steels offer an attractive profile of properties, but are not yet commercially available. Due to their lower alloying content, they are more cost-effective than Cr steels and, thanks to the addition of Al, offer excellent, in some cases superior, oxidation and hot gas corrosion resistance with particularly low thermal conductivity.

$$0,2*\mathrm{Nb}\le \mathrm{Si}\le 0,7*\mathrm{Nb},$$

$$\mathrm{19}\%<\mathrm{Cr}+\mathrm{4}*\mathrm{Nb}+\mathrm{21},\mathrm{6}*\mathrm{Min}\left(\mathrm{Si};\mathrm{0},\mathrm{5}*\mathrm{Nb}\right),$$

wobei in diesen Formel Cr, AI, Si, Mo, W und Nb der Legierungsgehalt dieser Elemente in Masse-% und mit Min(Si ; 0,5* Nb) der kleinere Wert von Si und 0,5* Nb ist. Aus einem derart komplex zusammengesetzten Stahl sollen sich insbesondere Bauteile herstellen lassen, die für den Einsatz in einer bis zu 1100 °C heißen Umgebung bestimmt sind.From the EP 2 723 910 B1 an iron-chromium-aluminum alloy with improved heat resistance, low chromium evaporation rate and good processability is known, which consists of (in% by mass) 2.0 - 4.5% Al, 12 - 25% Cr, 1.0 - 4% W, 0.25 - 2.0% Nb, 0.05 - 1.2% Si, 0.001 - 0.70% Mn, 0.001 - 0.030% C, 0.0001 - 0.05% Mg, 0.0001 - 0.03% Ca, 0.001 - 0.030% P, max. 0.03% N, max. 0.01% S, 0.01 - 0.10% Y, 0.01 - 0.10% Hf, 0, 01-0.10% Zr, where W can be replaced by 1-4% Mo, where Y can be completely or partially replaced by 0.01-0.10% of at least one of the elements Sc and / or La and / or Cer and where Hf or Zr can be wholly or partially replaced by 0.01-0.1% Ti, as well as optionally max. 1.0% Ni, max. 1.0% Co, max. 0.5% Cu, max 0.1% V, 0.001-0.010% O and / or 0.0001-0.008% B, the remainder being iron and the usual impurities caused by the melting, to which in particular Pb with a maximum of 0.002%, Zr with a maximum of 0.002% and Sn count with a maximum of 0.002%, with the alloy also fulfills the following formulas: $$\mathrm{36}\%<\mathrm{Cr}+\mathrm{3}*\left(\mathrm{Al}+\mathrm{Si}\right)+\mathrm{4th},\mathrm{6th}*\mathrm{Mon}+\mathrm{5},\mathrm{2}*\mathrm{W.}+\mathrm{10}*\mathrm{Nb},$$

$$0,2*\mathrm{Nb}\le \mathrm{Si}\le 0,\mathrm{7th}*\mathrm{Nb},$$

$$\mathrm{19th}\%<\mathrm{Cr}+\mathrm{4th}*\mathrm{Nb}+\mathrm{21st},\mathrm{6th}*\mathrm{Min}\left(\mathrm{Si};\mathrm{0},\mathrm{5}*\mathrm{Nb}\right),$$

where in these formulas Cr, Al, Si, Mo, W and Nb is the alloy content of these elements in mass% and with Min (Si; 0.5 * Nb) the smaller value of Si and 0.5 * Nb. In particular, components that are intended for use in an environment with a temperature of up to 1100 ° C. should be able to be produced from such a complex steel.

From the EP 3 176 277 A1 Furthermore, a ferritic stainless steel for a fuel cell is known which (in mass%) consists of Cr: 11 to 25%, C: 0.03% or less, Si: 2% or less, Mn: 2% or less, Al: 0.5 to 4.0%, P: 0.05% or less, S: 0.01% or less, N: 0.03% or less, Ti: 1% or less, and the balance of Fe and unavoidable impurities, in addition, in the steel, optionally Ni: 1% or less, Cu: 1% or less, Mo: 2% or less, Sn: 1% or less, Sb: 1% or less, W: 1% or less less, Co: 0.5% or less, Nb: 0.5% or less, V: 0.5% or less, Zr: 0.5% or less, Ga: 0.1% or less, Mg: 0 , 01% or less, B: 0.005% or less, Ca: 0.005% or less, La: 0.1% or less, Y: 0.1% or less, Hf: 0.1% or less, and REM: 0 , 1% or less may be included. At this time, the maximum of the concentration of Al in a surface of the steel should be 30 mass% or more of a cation content excluding O in a depth direction region twice as thick as an oxide film on the steel whose thickness is less than 0.1 mm. In the course of its processing, the steel thus composed is subjected to a heating step in which it is annealed in an atmosphere containing oxygen or hydrogen at a temperature of 700 to 1100 ° C. to form an oxide film on a surface of the steel.

From the WO 2013/178629 A1 a steel is known which is characterized by high strength both at room temperature and at higher operating temperatures. The heat-resistant Fe-Al-Cr steel according to the invention contains iron and unavoidable impurities (in% by weight) 3 - 7% Al, 3-11% Cr, up to 0.15% C, up to 1% Mn, up to 2% Si, up to 0.1% P, up to 0.03% S, up to 2% Mo, optionally at least one element from the group “Zr, V, W, Nb, Ti” in contents of up to in each case to a 1%, up to 1% Co, up to 2% Ni, up to 0.1% B, up to 3% Cu, up to 0.015% Ca, up to 0.2% REM and up to 0.1 % N.

From the EP 2 995 697 A1 there is known a stainless steel substrate for a solar cell which contains (in mass%) Cr: 9% to 25%, C: 0.03% or less, Mn: 2% or less, P: 0.05% or less, S: 0.01% or less, N: 0.03% or less, Al: 0.005% to 5.0%, Si: 0.05% to 4.0% and the remainder Fe and unavoidable impurities, the Si Content is at least 0.4% and / or the Al content is at least 0.5% and the following applies to the contents of% Cr,% Al,% Si and% Mn of Cr, Al, Si and Mn
% Cr + 10% Si +% Mn +% AI> 24.5%. At the same time, an oxide film present on the substrate should be contained according to a first alternative Al ₂ O ₃ with a content of at least 50% or according to a second alternative Al ₂ O ₃ and SiO ₂ in a total amount of at least 50%.

Against the background of the prior art, the task has arisen of specifying a flat steel product which, due to its combination of properties, is optimally suited for the production of components for high-temperature use and which has an equally optimized visual appearance. In addition, a method for the production of such a flat steel product should be specified, which can be carried out on conventional production facilities intended for black plate production. Finally, the invention should also specify a component that can withstand high temperatures in continuous use.

The invention has achieved this object by means of a flat steel product which has at least the features specified in claim 1 and a method which comprises at least the work steps specified in claim 10.

The component that achieves the above object according to the invention is part of an exhaust system of an internal combustion engine which is formed from a flat steel product created according to the invention.

Advantageous embodiments of the invention are specified in the dependent claims and, like the general inventive concept, are explained in detail below.

und als Rest aus Fe und unvermeidbaren Verunreinigungen, wobei zu den unvermeidbaren Verunreinigungen bis zu 0,10 % C, bis zu 0,30 % S, bis zu 0,10 % P, bis zu 0,30 % Sb, bis zu 0,30 % As, bis zu 0,50 % N zählen.A flat steel product according to the invention accordingly has a fully ferrite structure and consists of (in% by weight) Al: 2.0-8.0%, Cr: 2.0-10.0%, Nb: 0.10-1.0% %, optionally one or more elements from the group Mn, Ti, Si, B, Ta, W, V, Zr, Mo, Ni, Cu, CA, SEM, Co with the proviso that the Mn contents: to to 3.0%, Ti: up to 1.0%, Si: up to 1.0%, B: up to 0.10%, Ta: up to 1.0%, W: up to 1.0% , V: up to 0.50%, Zr: up to 1.0%, Mo: up to 2.0%, Ni: up to 2.0%, Cu: up to 2.0%, Ca: up to 0.150%, SEM: up to 0.50%, Co: up to 2.0%, with the content% Cr of Cr, the content% Al of Al, the optionally present content% Si of Si and the likewise optional existing content% Mn of Mn applies: $$\%\mathrm{Cr}+\mathrm{10}\times \%\mathrm{Si}+\%\mathrm{Al}+\%\mathrm{Mn}\le \mathrm{20th}\%,$$

and the remainder of Fe and unavoidable impurities, with the unavoidable impurities up to 0.10% C, up to 0.30% S, up to 0.10% P, up to 0.30% Sb, up to 0, Count 30% As, up to 0.50% N.

A flat steel product according to the invention is characterized by high oxidation and hot gas corrosion resistance and generally high corrosion resistance at high temperatures with an extraordinarily low thermal conductivity, reduced specific weight, good cold formability, which is reflected in high elongation at break and low yield point as well as good values in Erichsen seam deepening tests on a weld seam produced on the flat steel product according to the invention, expresses good weldability and a particularly deep brittle-ductile transition temperature proven in the cup impact test, high heat resistance and high strength at room temperature achieved by solid solution hardening. Flat steel products according to the invention have a low thermal conductivity of a maximum of 18 W / mK at room temperature, a high resistance to oxidation and hot gas corrosion of a maximum of 10 mg / cm ² , a low density of a maximum of 7.3 g / cm ³ , an elongation at break A50 of at least 18%, seam depressions with a through TIG welding produced a weld seam of at least 6 mm, a transition temperature in the cup impact test of no more than 20 ° C, at 600 ° C a high hot yield strength of at least 150 MPa and an equally high hot tensile strength of at least 200 MPa as well as a yield point of at least 300 MPa at room temperature and a maximum of 550 MPa, a tensile strength of at least 450 MPa and a maximum of 700 MPa and an elongation at break A50 longitudinally and transversely of at least 18% and a maximum of 34%. To achieve this, the steel substrate from which a flat steel product according to the invention is made has the following in Individually explained contents of mandatory components.

In order to further develop the special properties of a flat steel product according to the invention, additional alloying elements can optionally be added to these mandatory components, the contents of which are also explained below. At the same time, the invention makes specific regulations with regard to the contents which must not be exceeded, so that the technically unavoidable impurities have no influence on the desired properties of a flat steel product according to the invention.

In addition to iron ("Fe"), mandatory components that are necessary in the flat steel product according to the invention are
Aluminum ("Al") is a mandatory component in the steel of a flat steel product according to the invention in a content of 2.0-8.0% by weight. Al in these contents reduces the density of a flat steel product according to the invention and, with the further mandatory component Cr, promotes the formation of fully ferritic structures. At the same time, the Al contents provided according to the invention result in a minimized thermal conductivity of a flat steel product according to the invention. To use these positive influences safely, the Al content of a flat steel product according to the invention is at least 2.0% by weight, contents of at least 3.0% by weight or in particular at least 4.0% by weight proving to be particularly favorable in this regard. With Al contents of more than 8% by weight, however, the cold formability would be made more difficult and the suitability for welding would be impaired by the formation of stable welding slag that increases the electrical resistance in the welding zone. If the Al contents are too high, there is also the risk that Al would form embrittling phases such as Al nitrides and kappa carbides with the N and C contents that are usually unavoidable as impurities. In addition, if the Al content is too high, the stacking fault energy would be increased, which would make recovery and recrystallization after cold rolling more difficult and result in a structure rich in dislocations and an associated high yield point and low elongation at break. Ultimately, excessively high Al contents would make tape production more difficult because of the low thermal conductivity due to significantly increased dwell times in the furnace and lower the E-module due to the high Al atomic radius. Possible negative influences of the presence of Al in the flat steel product according to the invention can be avoided particularly reliably by limiting the Al content to a maximum of 7.0% by weight, in particular to a maximum of 6.0% by weight.

Chromium ("Cr") is present in the flat steel product according to the invention in contents of 2.0-10.0% by weight. With Al, Cr promotes the formation of a fully ferritic structure, which guarantees high heat resistance. In addition, the presence of Cr improves the corrosion and oxidation resistance and increases the yield strength and tensile strength. Furthermore, Cr in the amounts provided according to the invention supports the formation of an oxidation-resistant Al ₂ O ₃ layer and increases the low-temperature corrosion resistance. In addition, Cr can be contained in small proportions in the Laves phase Fe ₂ Nb. These advantageous influences can be used particularly reliably if the Cr content of a Flat steel product is at least 3.0% by weight, in particular at least 5.5% by weight. In combination with Al, excessively high Cr contents could lead to a reduction in the elongation at break and high yield strengths, which lead to impaired cold formability. These negative influences can be avoided particularly reliably by limiting the Cr content to a maximum of 9.5% by weight, in particular a maximum of 8.4% by weight.

Niobium ("Nb") is a mandatory component in the flat steel product according to the invention in contents of 0.10-1.0% by weight. _{Nb forms the Laves phase Fe 2} Nb, which is not necessarily present in the structure of a flat steel product according to the invention, but with regard to the desired properties, which contributes to the high heat resistance of a flat steel product _{according to the invention, as Fe 2} Nb slows down grain growth through grain boundary pinning and is very temperature-stable is. The Laves phase Fe ₂ Nb is already formed in the flat steel product. However, the development of this phase does not necessarily have to be completed in the flat steel product, but can only be completed when the component is used. If the Nb content is too high, the cold formability and welding properties of a flat steel product according to the invention would be impaired. The positive effects of the presence of Nb can be used particularly reliably if the Nb content is at least 0.20% by weight, in particular at least 0.30% by weight. The negative influences of Nb on the properties of a flat steel product according to the invention can be excluded particularly reliably by limiting the Nb content to a maximum of 0.70% by weight, in particular a maximum of 0.50% by weight.

According to an embodiment optimized with regard to the content of its mandatory components, a flat steel product according to the invention thus contains an Al content of 3.0-7.0% by weight, a Cr content of 3.0-9.5% by weight and an Nb Content of 0.2-0.70% by weight, with an overall optimized Property profile with an Al content of 4.0-6.0% by weight, a Cr content of 5.5-8.4% by weight and an Nb content of 0.30-0.50% by weight. -% results.

The alloy components optionally present in a flat steel product according to the invention and their effects are explained below. "Optionally present" means that in the event that the respective alloy component is present in a content effective for the development of the effect attributed to it, which goes beyond the content up to which the respective element is considered technically unavoidable, but with regard to the Properties of a flat steel product according to the invention ineffective contamination may be present:
A flat steel product according to the invention can optionally contain tantalum ("Ta") or tungsten ("W") in contents of 1.0% by weight each, these elements, if present, in contents of up to 0.50% by weight each , preferably up to 0.30% by weight, develop their effect optimally. Both elements support the strength through the formation of carbide and also contribute to the heat resistance. To use these effects, a minimum content of at least 0.02% by weight Ta and / or at least 0.05% by weight W can be present in the flat steel product according to the invention.

Zirconium (“Zr”) can optionally also be present in the steel flat product according to the invention in contents of up to 1.0% by weight, in particular up to 0.50% by weight, particularly preferably up to 0.30% by weight to increase the strength through the formation of carbides and to contribute to the high-temperature strength. In order to use these effects, a minimum content of at least 0.02 wt.% Zr can be present in the flat steel product according to the invention. Zr contents above the content limits specified according to the invention would impair the ductility and restrict the cold deformability of a flat steel product according to the invention.

Vanadium ("V") can optionally be present in contents of up to 0.50% by weight, in particular up to 0.30% by weight, in the flat steel product according to the invention be present in order to increase the strength also by carbide formation. In order to use this effect, a minimum content of at least 0.05% by weight V can be present in the flat steel product according to the invention.

Molybdenum (“Mo”) is also an alloy element optionally present in the flat steel product according to the invention. Mo increases the tensile strength and heat resistance. In addition, Mo forms fine carbides with the C present as an impurity, which can contribute to a fine structure and can be incorporated into the Laves phase Fe ₂ Nb. These effects can be used safely if a flat steel product according to the invention has a minimum content of at least 0.05% by weight Mo. However, excessively high Mo contents would impair the hot and cold formability of a flat steel product according to the invention and also the ductility in a weld seam produced on a flat steel product according to the invention. The Mo content of a flat steel product according to the invention is therefore limited to at most 2.0% by weight, in particular 1.0% by weight, preferably at most 0.3% by weight.

Nickel ("Ni") can be present in a flat steel product according to the invention in contents of 2.0% by weight, in particular up to 1.0% by weight, preferably up to 0.5% by weight, in order to increase the strength and toughness and, as a result, the cold formability and the ductility in a weld seam produced on the flat steel product according to the invention. Higher contents of Ni would lead to an undesirable increase and to a stabilization of the austenite content in the structure of the flat steel product according to the invention. The advantageous effects of the presence of Ni can safely be used if a flat steel product according to the invention has a minimum content of at least 0.1% by weight Ni.

Copper (“Cu”) can optionally be present in the flat steel product according to the invention in contents of up to 2.0% by weight in order to improve the corrosion resistance. This positive effect can be achieved with certainty if the Cu content is at least 0.05% by weight. However, too high a Cu content can affect the hot rolling behavior Cold formability and weldability deteriorate. These negative influences can safely be avoided by limiting the Cu content of the flat steel product according to the invention to a maximum of 1.0% by weight, in particular a maximum of 0.5% by weight.

Calcium (“Ca”) can optionally be present in the flat steel product according to the invention in contents of up to 0.150% by weight in order to use its desulphurizing and deoxidizing effect in steel production. In addition, the presence of Ca helps to improve oxide adhesion. This positive effect can be achieved with certainty if the Ca content is at least 0.0015% by weight. However, excessively high Ca contents can make it difficult to process the steel from which a flat steel product according to the invention is made. These negative influences can safely be avoided by limiting the Ca content of the flat steel product according to the invention to a maximum of 0.050% by weight.

Rare earth metals ("SEM") can also optionally be present in the flat steel product according to the invention in contents of up to 0.50% by weight in order to use their desulphurising and deoxidising effect and their positive influence on oxide adhesion during steel production. This positive effect can be achieved safely if the SEM content is at least 0.01% by weight. However, SEM contents that are too high can make it more difficult to process the steel from which a flat steel product according to the invention is made. These negative influences can reliably be avoided by limiting the SEM content of the flat steel product according to the invention to a maximum of 0.20% by weight.

Cobalt (“Co”) can optionally be present in the flat steel product according to the invention in contents of up to 2.0% by weight. Co makes a positive contribution to the heat resistance. This positive effect can be achieved with certainty if the Co content is at least 0.1% by weight. However, Co promotes in the course of the production of an inventive Flat steel product the formation of austenite and thus destabilizes the necessary full ferrite. For this reason, the content of optional Co is preferably 2.0 wt% or less. Its optimal effect is shown by the presence of Co in a flat steel product according to the invention at contents of up to 1.0% by weight, in particular up to 0.50% by weight.

Manganese (“Mn”) is optionally present in the flat steel product according to the invention in contents of up to 3.0% by weight. Mn promotes the ductility and toughness of a flat steel product according to the invention. The presence of Mn increases the elongation at break, the r-value and the deformability of a weld seam produced on a flat steel product according to the invention, for example by MIG, MAG, TIG or laser welding. In addition, the transition temperature and the yield strength can be lowered by adding Mn. These positive influences, which can be used in a particularly targeted manner from contents of at least 0.30% by weight, in particular at least 0.50% by weight, are countered by the fact that Mn is an austenite former which increases the ductility if the contents are too high can reduce and the corrosion resistance decreases with increasing contents. These negative effects can be safely avoided by limiting the Mn content to a maximum of 2.0% by weight, in particular less than 2.0% by weight or at most 1.5% by weight.

Titanium ("Ti") is optionally present in the flat steel product according to the invention in contents of up to 1.0% by weight. Ti can help increase strength by forming Ti carbides. In addition, the presence of Ti improves the corrosion resistance. The high temperature strength is also improved as a result of a higher temperature stability, ie higher dissolution temperature of the Laves phase Fe ₂ Nb due to the presence of Ti. Ti is built into the Laves phase and the presence of Ti increases the proportions of the Laves phases in the structure. In the contents optionally provided according to the invention, Ti also leads to a reduced density of the flat steel product. Ti can also be used in the production of the steel according to the invention Flat steel product can be used for deoxidation and forms titanium borides with B, which have a grain-refining effect. These positive properties can be used particularly reliably with Ti contents of at least 0.050% by weight, in particular at least 0.10% by weight. Detrimental effects of the presence of Ti, such as excessive volume contents of Laves phases in the structure, can be avoided particularly reliably by limiting the Ti content to a maximum of 0.60% by weight.

Silicon (“Si”) is optionally present in contents of up to 1.0% by weight in the flat steel product according to the invention. Si increases strength and corrosion resistance. At the same time, the presence of Si improves the heat resistance through higher temperature stability, ie higher dissolution temperature of the Laves phase Fe ₂ Nb. Si also contributes to increasing the proportion of the Laves phase formed and reduces the density of the flat steel product according to the invention. At contents of more than 1% by weight, Si can reduce the weldability and shift the brittle-ductile transition temperature to unfavorable, higher temperatures. These positive influences can be used particularly reliably with Si contents of at least 0.050% by weight, in particular at least 0.10% by weight. Harmful effects of the presence of Si can be avoided particularly reliably by limiting the Si content to a maximum of 0.60% by weight.

Boron ("B") is optionally present in contents of up to 0.10% by weight in the flat steel product according to the invention. In the presence of Ti, B can form titanium borides, which have a fine-graining effect. This positive influence of the presence of B can be used with B contents of at least 0.001% by weight. However, excessively high B contents impair cold formability. Harmful effects of the presence of B can be avoided particularly reliably by limiting the dissolved B content to a maximum of 0.010% by weight.

Elements that may be present as impurities, the contents of which must be kept so low that they are ineffective, are:
Carbon ("C") is only present as an impurity in the flat steel product according to the invention. Its content is therefore limited as far as possible so that it has no effect on the properties of a flat steel product according to the invention. C forms brittle Fe-Al carbides with Al. To avoid this negative effect, C can be bonded with a mandatory component or an added carbide former, such as Ti, Nb, Mo, Zr, W or Ta. To avoid any negative influence of C on the properties of a flat steel product according to the invention, its C content is limited to at most 0.10% by weight, preferably at most 0.050% by weight or, particularly preferably, to at most 0.030% by weight .

Phosphorus (“P”) is also only present as an impurity in the flat steel product according to the invention. P tends to segregate, which is difficult to balance, and deteriorates cold formability, weldability and oxidation resistance. The P content of a flat steel product according to the invention is therefore limited to at most 0.10% by weight so that it has no effect on the properties of a flat steel product according to the invention.

Sulfur ("S") is also one of the impurities and is permitted in the flat steel product according to the invention with a maximum of 0.30% by weight. At higher contents, the hot formability during hot rolling and the corrosion resistance of the flat steel product would be impaired. The S content of a flat steel product according to the invention, like the content of the other impurities, should therefore be set as low as possible.

Nitrogen ("N") is only present as an impurity in the steel according to the invention, the content of which is limited to a maximum of 0.05% by weight in order to avoid harmful influences. The presence of higher contents of N would lead to the formation of disadvantageous Al nitrides, which affect the mechanical Properties and the deformability of a flat steel product according to the invention would worsen. In order to minimize these negative effects of the presence of N, the N content can be limited to a maximum of 0.030% by weight, in particular a maximum of 0.015% by weight.

Antimony ("Sb") is also an impurity and is permitted in contents of no more than 0.30% by weight. Sb is toxic and increases the brittleness and the risk of redness. In order to minimize the negative influence of Sb on the properties of a flat steel product according to the invention, the Sb content is to be limited as low as possible, in particular to a maximum of 0.10% by weight, preferably to a maximum of 0.010% by weight.

Arsenic ("As") is also one of the undesirable impurities, as it tends to segregate like P. It also reduces the weldability, but cannot be removed from the steel metallurgically, so that unavoidable residues of As can regularly be detected in the steel. In order to avoid any negative influence of the presence of As, the As content in the flat steel product according to the invention is limited to at most 0.30% by weight, in particular to at most 0.10% by weight, preferably to at most 0.010% by weight.

erfüllen, wird eine gute Kaltumformbarkeit, Bruchdehnung und Schweißnahtverformbarkeit (Nahttiefung) eines erfindungsgemäßen Stahlflachprodukts sichergestellt. Dies gilt insbesondere dann, wenn der Mn-Gehalt höchstens 2 Gew.-%, insbesondere weniger als 2 Gew.-%, also beispielsweise bis zu 1,5 Gew.-%, beträgt.In addition, the respective content% Cr of Cr, the respective content% Al of Al and, if present, the respective content% Si of Si and the respective content% Mn of Mn the condition $$\%\mathrm{Cr}+10\times \%\mathrm{Si}+\%\mathrm{Al}+\%\mathrm{Mn}\le \mathrm{20th}\%,$$

meet, good cold formability, elongation at break and weld seam deformability (seam depression) of a flat steel product according to the invention is ensured. This applies in particular when the Mn content is at most 2% by weight, in particular less than 2% by weight, that is to say for example up to 1.5% by weight.

A flat steel product according to the invention is characterized by an extremely thin oxide layer on its free surface, the thickness of which is at most 150 nm, in particular at most 110 nm. The working steps and parameters of the production of a flat steel product according to the invention are coordinated in such a way that, despite the presence of Al and Ti as oxide-affine elements, the formation of an excessively thick oxide layer is counteracted. This oxide layer is very dense so that it counteracts further oxidation. As a result, a flat steel product according to the invention has a high surface gloss that is distributed uniformly over the surface of the flat steel product.

In the case of the optional presence of Mn in the steel substrate of a steel flat product according to the invention in contents of up to 3.0% by weight, which is important for many applications due to the improvement in mechanical properties achieved thereby, is the presence on the surface of a steel flat product according to the invention any oxide layer present an Mn content of at most 2 wt .-%. The parameters selected according to the invention for the production of a flat steel product according to the invention ensure that there is no excessive growth of the surface oxide layer even in the case of the presence of effective Mn contents in the steel substrate. In particular, the setting of the atmosphere, explained below, under which the final annealing (step h) of the method according to the invention) is carried out, ie the setting of the dew point to the lowest possible temperatures at the end of the final annealing, but in any case to ≤ -50 ° C, especially ≤ -60 ° C, particularly preferably <60 ° C, is of particular importance.

Because of the, if present, only very thin, namely only a maximum of 150 nm, in particular a maximum of 100 nm thick, oxide layer that is present on its surface, an inventive Flat steel product also has a visual appearance that is similar to that of sheet metal made from corrosion- and oxidation-resistant ferritic Cr steel or CrNi steel.

The fact that a flat steel product according to the invention has a mean roughness Ra of 0.1-1.5 μm, in particular 0.2-1.0 μm, determined in accordance with DIN EN 10049, contributes to an appearance of a flat steel product made from such a steel flat product, mean roughness values Ra of 0.3-0.7 µm are reliably achieved.

A flat steel product according to the invention has a completely ferritic structure.

The addition of Nb as a mandatory component of the steel of a flat steel product according to the invention _{enables the formation of Laves phases Fe 2} Nb. These improve the heat resistance by preventing excessive grain growth at high temperatures through grain boundary pinning. The proportion of the Laves phases Fe ₂ Nb in the structure of a flat steel product according to the invention is advantageously 0.10-1% by volume, in particular 0.20-0.80% by volume, particularly advantageously 0.30-0.60% by volume .-%.

The inventive way of producing a flat steel product according to the invention and its composition according to the invention ensures that more than 20%, in particular more than 40% and particularly preferably more than 70% of the particle number of the Laves phase as Fe ₂ Nb precipitates the grain boundaries of the structure and thus particularly effectively and reliably prevent grain growth even in uses in which flat steel products according to the invention or the components made from them are exposed to high temperatures and thus ensure optimized heat resistance.

Their profile of properties makes flat steel products according to the invention particularly suitable for the production of components for exhaust systems of internal combustion engines in which these components are regularly exposed to temperatures of more than 200 ° C, in particular at least 600 ° C. For example, exhaust manifolds in which temperatures of up to 1000 ° C. occur in practical use can be produced from flat steel products according to the invention. The excellent oxidation and hot gas corrosion resistance of a flat steel product according to the invention proves to be particularly valuable in the case of components of this type that are subject to extreme thermal loads.

Flat steel products according to the invention can be produced on conventional production plants such as are available today for the production of black plate and the like. Accordingly, the production of flat steel products according to the invention can be carried out much more easily and cost-effectively than the production of flat steel products from stainless steels with comparable properties.

1. a) Es wird eine gemäß den voranstehend näher erläuterten Maßgaben der Erfindung legierte Schmelze erschmolzen und in an sich bekannter Weise zu einem Vorprodukt, insbesondere Block, einer Bramme mit einer Dicke von mehr als 70 mm oder einem gegossenen Band vergossen. Die Schmelzerzeugung kann in konventioneller Weise über eine hierfür bewährte Elektroofenroute erfolgen. Ein Elektroschmelzofen ist durch seine Fähigkeit zur Verflüssigung hoher Legierungsmengen hier besonders geeignet. Niedrige Begleitelementgehalte bzw. niedrige Gehalte an bestimmten Elementen in der Vorschmelze (z.B. C, N, O) sind vorteilhaft für die Eigenschaften eines erfindungsgemäßen Werkstoffs. Diese können durch geeignete sekundärmetallurgische Maßnahmen erzielt werden.
2. b) Das jeweilige Vorprodukt wird auf eine 1150 - 1300 °C, bevorzugt 1150 - 1250 °C, betragende Vorwärmtemperatur durcherwärmt oder dort gehalten. Die vergleichbar hohe Vorwärmtemperatur ist erforderlich, da der Stahl, aus dem ein erfindungsgemäßes Stahlflachprodukt besteht, eine geringe Wärmeleitfähigkeit aufweist und das anschließend durchlaufene Walzen bei hohen Temperaturen absolviert werden muss, um die hohen Warmverformungswiderstände zu überwinden, die warmfeste Stähle wie der, aus dem ein erfindungsgemäßes Stahlflachprodukt besteht, aufweisen.
3. c) Das auf die Vorwärmtemperatur durcherwärmte Vorprodukt wird zu einem warmgewalzten Band fertig warmgewalzt, wobei dem Fertigwarmwalzen optional ein Vorwalzen vorgeschaltet sein kann, wobei das optional vorgewalzte Vorprodukt zu Beginn des Fertigwarmwalzens eine Warmwalzstarttemperatur von 1050 - 1200 °C, bevorzugt 1050 - 1150 °C, aufweist und wobei das aus dem Vorprodukt fertig warmgewalzte Warmband bei Ende des Warmwalzens eine Warmwalzendtemperatur von 800 - 950 °C aufweist. Die hohen Warmwalzstarttemperaturen sind wie die hohen Vorwärmtemperaturen erforderlich, um den Verformungswiderstand beim Warmwalzen zu minimieren. Der erfindungsgemäß für die Warmwalzendtemperatur vorgeschriebene Temperaturbereich stellt dabei sicher, dass das erhaltene Warmband eine optimierte mechanischtechnologische Eigenschaft besitzt, die sich insbesondere in einer optimalen Duktilität äußert, da sich bei dieser Warmwalzendtemperatur eine hohe Versetzungsdichte einstellt, die bei anschließenden Glühungen Erholungs- und Rekristallisationsvorgänge unterstützt. Besonders sicher lässt sich dies erreichen, wenn die Warmwalzendtemperatur höchstens 900 °C beträgt. Als Untergrenze für den Bereich der Warmwalzendtemperatur kann gemäß einer besonders praxisgerechten Ausgestaltung der Erfindung eine Mindesttemperatur von 830 °C vorgesehen werden, da auf diese Weise das Warmwalzen auf die jeweilige Zieldicke besonders einfach ist und die für die Ausprägung der Eigenschaften eines erfindungsgemäßen Stahlflachprodukts wichtige Haspeltemperatur sicher eingestellt werden kann.
4. d) Das erhaltene Warmband wird mit einer Abkühlgeschwindigkeit von typischerweise 5 - 200 K/s auf eine Haspeltemperatur abgekühlt, die von der Raumtemperatur bis höchstens 570 °C reichen kann. Bei Überschreiten der für die Haspeltemperatur vorgegebenen Obergrenze würde sich auf dem Warmband eine Zunderschicht bilden, die sich aufgrund einer Anreicherung von Legierungselementen in der an das Stahlsubstrat angrenzenden unteren Zunderschicht nicht mehr durch Beizen vollständig entfernen ließe. Besonders sicher wird dieser Effekt eingestellt, wenn die Haspeltemperatur 520°C nicht überschreitet. Das auf die Haspeltemperatur abgekühlte Warmband wird zu einem Coil gehaspelt und auf Raumtemperatur erkaltet.
5. e) Nach dem Haspeln durchläuft das Warmband optional eine Warmbandglühung, bei der es über eine Dauer von 1 - 100 Stunden bei einer Warmbandglühtemperatur von 600 - 900 °C gehalten wird, um seine Duktilität für die nachfolgenden Prozessschritte zu optimieren. Die optionale Warmbandglühung kann in konventioneller Weise als Haubenglühung durchgeführt werden.
6. f) Das nach dem Haspeln optional warmbandgeglühte Warmband wird vollständig entzundert. Hierzu kann es durch einen Zunderbrecher geleitet werden, um den auf ihm haftenden Zunder aufzubrechen, und indem es, gegebenenfalls nach dem optionalen Zunderbrechen, ein Beizmedium durchläuft, das an der Oberfläche des Warmbands vorhandenen Zunder chemisch entfernt. Als Beizmedium kommt dabei beispielsweise HCl, HF/HNO₃ oder H₂SO₄ in Frage. Die Beizdauer, über die das Warmband dem typischerweise auf eine Temperatur von 60 -100 °C erwärmten Beizmedium ausgesetzt wird, beträgt dabei 50 - 500 s, um eine möglichst vollständige Entfernung des Zunders zu gewährleisten.
7. g) Nach dem Entzundern wird das Warmband in einem oder mehreren Kaltwalzstichen zu einem Kaltband kaltgewalzt. Dabei beträgt der über das Kaltwalzen erzielte Kaltwalzgrad = (Dicke des Warmbands vor dem Kaltwalzen - Dicke des Kaltbands nach dem Kaltwalzen) / (Dicke des Warmbands vor dem Warmwalzen) mindestens 30 %, um die Voraussetzungen für eine ausreichende Erholung und/oder Rekristallisation im nachfolgend durchlaufenen Schlussglühen (Arbeitsschritt h)) zu schaffen. Nach oben hin ist der Kaltwalzgrad aufgrund der in der Praxis heute zur Verfügung stehenden Anlagentechnik typischerweise auf 90 % beschränkt, wobei höhere Kaltwalzgrade zulässig sind, wenn die hierzu erforderlichen Kaltwalzaggregate zur Verfügung stehen.
8. h) Als für die Erfindung wesentlichen Arbeitsschritt durchläuft das Kaltband eine Schlussglühung, bei der es über eine Schlussglühdauer von 1 - 100 h bei einer Schlussglühtemperatur von 650 - 950 °C unter einer SchutzgasAtmosphäre gehalten wird. Die Schlussglühdauer beträgt dabei in der Praxis typischerweise mindestens 15 h. Schlussglühdauern von höchstens 50 h, insbesondere höchstens 30 h, haben sich als besonders praxisgerecht bewährt. Die Einhaltung der Mindest-Schlussglühtemperatur ist für die Rekristallisation und/oder Erholung des Gefüges erforderlich. Höhere Schlussglühtemperaturen würden zur Vergröberung oder Auflösung der Laves-Phase Fe₂Nb führen, was eine Schwächung des Korngrenzen-Pinn-Effektes und eine damit einhergehende Gefügevergröberung zur Folge hätte. Zudem bestünde bei einer über 950 °C liegenden Schlussglühtemperatur die Gefahr von Whisker-Bildung und einer damit einhergehenden Erhöhung der Rauigkeit der Oberfläche. In der Praxis erweisen sich hier Schlussglühtemperaturen von mindestens 750 °C im Hinblick auf die Wirkung der Schlussglühung als besonders vorteilhaft. Eine Beschränkung der Schlussglühtemperatur auf höchstens 900 °C trägt zur wirtschaftlichen Umsetzung des erfindungsgemäßen Verfahrens bei und vermeidet sicher die starke Vergröberung und Auflösung der Laves-Fe₂Nb-Phasen. Als Atmosphäre für die Schlussglühung kommt eine aus mind. 70 Vol.-% Wasserstoff, vorzugsweise mind. 80% Vol.-% Wasserstoff, besonders bevorzugt mind. 95 Vol.-% Wasserstoff, Rest Stickstoff und technisch unvermeidbare Verunreinigungen enthaltende Atmosphäre in Frage. Der hohe Wasserstoffgehalt ist nötig, um eine Reduktion der Oberflächenoxide durch den Wasserstoff sicherzustellen. Zu Beginn der erfindungsgemäß durchgeführten Schlussglühung wird dabei der Taupunkt auf 0 bis -30 °C eingestellt. Während der Schlussglühung wird der Taupunkt dann abgesenkt, so dass er am Ende der Schlussglühung höchstens -50 °C beträgt. Noch niedrigere Taupunkte von höchstens -60 °C oder darunter sind besonders vorteilhaft. Bei höheren Taupunkten am Ende der Schlussglühung bestünde die Gefahr der Entstehung einer zu dicken Oxidschicht auf der Oberfläche des Kaltbands, die sich als grau-brauner Schleier bzw. Anlauffarben zeigen würden und die optische Erscheinung in nicht akzeptabler Weise beeinträchtigen würde. Mögliche Maßnahmen zur Reduzierung des Taupunktes sind eine nahezu oxidfreie Ofenkammer und die Zugabe von 1 bis 20 kg Titanschwamm oder andere Schwämme bzw. Pulver sauerstoffaffiner Elemente in die Ofenkammer.
9. i) Sollte sich die Oberflächenerscheinung des erhaltenen Kaltbands nach dem Schlussglühen als nicht ausreichend gut herausstellen, kann das Kaltband optional einem Nachbeizen unterzogen werden, bei dem es vorzugsweise über eine Beizdauer von 50 - 500 s einem auf 60 - 100 °C erwärmten Beizmedium ausgesetzt wird, bei dem es sich vorzugsweise um HCl handelt.
10. j) Abschließend wird das Kaltband einem Dressierwalzen unterzogen, bei dem es mit einem Dressiergrad = (Dicke des Kaltbands vor dem Dressierwalzen - Dicke des Kaltbands nach dem Dressierwalzen) / (Dicke des Kaltbands vor dem Dressierwalzen) von 0,10 - 1 % kaltgewalzt wird. Für so erfindungsgemäß prozessierte Kaltbänder lassen sich betriebssicher reproduzierbar Mittenrauwerte Ra von 0,1 - 1,5 µm, insbesondere 0,2 - 1,0 µm, gewährleisten, wobei zuverlässig Mittenrauwerte Ra von 0,3 - 0,7 µm erreicht werden.

This is achieved in that at least the working steps explained below are carried out to produce a flat steel product according to the invention. It goes without saying that the person skilled in the art supplements all measures not mentioned here that are known to him as necessary or helpful from the conventional production of cold-rolled flat steel products:

1. a) A melt alloyed in accordance with the provisions of the invention explained in more detail above is melted and cast in a manner known per se to form a preliminary product, in particular a block, a slab with a thickness of more than 70 mm or a cast strip. The melt can be generated in a conventional manner using an electric furnace route that has been tried and tested for this purpose. An electric melting furnace is special here because of its ability to liquefy large amounts of alloy suitable. Low accompanying element contents or low contents of certain elements in the premelt (eg C, N, O) are advantageous for the properties of a material according to the invention. These can be achieved through suitable secondary metallurgical measures.
2. b) The respective preliminary product is heated through to a preheating temperature of 1150-1300 ° C., preferably 1150-1250 ° C., or is kept there. The comparably high preheating temperature is necessary because the steel from which a flat steel product according to the invention is made has a low thermal conductivity and the subsequent rolling must be completed at high temperatures in order to overcome the high resistance to hot deformation that heat-resistant steels like the one made of flat steel product according to the invention consists.
3. c) The pre-product, heated through to the preheating temperature, is finished hot-rolled into a hot-rolled strip, with the finish-hot-rolling optionally being preceded by pre-rolling, the optionally pre-rolled pre-product having a hot-rolling start temperature of 1050-1200 ° C, preferably 1050-1150 ° C, at the beginning of finish hot-rolling , and wherein the hot rolled strip finished from the preliminary product has a hot rolling end temperature of 800-950 ° C. at the end of the hot rolling. The high hot rolling start temperatures, like the high preheating temperatures, are required to minimize the deformation resistance during hot rolling. The temperature range prescribed according to the invention for the final hot-rolling temperature ensures that the hot strip obtained has an optimized mechanical-technological property, which is expressed in particular in an optimal ductility, since a high dislocation density is established at this final hot-rolling temperature, which supports recovery and recrystallization processes during subsequent annealing. This can be achieved particularly reliably if the end temperature of the hot rolling does not exceed 900 ° C. As a lower limit for the range of According to a particularly practical embodiment of the invention, a minimum temperature of 830 ° C can be provided, since hot rolling to the respective target thickness is particularly simple and the coiling temperature, which is important for the development of the properties of a flat steel product according to the invention, can be reliably set.
4. d) The hot strip obtained is cooled at a cooling rate of typically 5-200 K / s to a coiling temperature which can range from room temperature to at most 570 ° C. If the upper limit specified for the coiling temperature is exceeded, a layer of scale would form on the hot strip which, due to an accumulation of alloying elements in the lower scale layer adjacent to the steel substrate, could no longer be completely removed by pickling. This effect is set particularly reliably if the coiling temperature does not exceed 520 ° C. The hot strip, cooled to the coiling temperature, is wound into a coil and cooled to room temperature.
5. e) After coiling, the hot strip optionally goes through a hot strip annealing, in which it is held for a period of 1 - 100 hours at a hot strip annealing temperature of 600 - 900 ° C in order to optimize its ductility for the subsequent process steps. The optional hot strip annealing can be carried out in a conventional manner as hood annealing.
6. f) The hot strip, which is optionally hot-strip annealed after coiling, is completely descaled. For this purpose it can be passed through a scale breaker in order to break up the scale adhering to it, and by running through a pickling medium, if necessary after the optional scale breakage, which chemically removes scale present on the surface of the hot strip. The pickling medium used is, for example, HCl, HF / HNO ₃ or H ₂ SO _{4 are possible} . The pickling time over which the hot strip is exposed to the pickling medium, which is typically heated to a temperature of 60-100 ° C, is 50-500 s in order to ensure that the scale is removed as completely as possible.
7. g) After descaling, the hot strip is cold rolled in one or more cold rolling passes to form a cold strip. The degree of cold rolling achieved via cold rolling = (thickness of the hot strip before cold rolling - thickness of the cold strip after cold rolling) / (thickness of the hot strip before hot rolling) is at least 30% in order to meet the requirements for adequate recovery and / or recrystallization in the following completed final annealing (step h)). At the top, the degree of cold rolling is typically limited to 90% due to the plant technology available in practice today, with higher degrees of cold rolling being permissible if the cold rolling units required for this are available.
8. h) As an essential work step for the invention, the cold strip undergoes final annealing, during which it is kept under a protective gas atmosphere for a final annealing period of 1-100 hours at a final annealing temperature of 650-950 ° C. In practice, the final glow time is typically at least 15 hours. Final glow periods of a maximum of 50 hours, in particular a maximum of 30 hours, have proven to be particularly practical. Compliance with the minimum final annealing temperature is necessary for the recrystallization and / or recovery of the structure. Higher final annealing temperatures would lead to a coarsening or dissolution of the Laves phase Fe ₂ Nb, which would weaken the grain boundary pin effect and result in an associated coarsening of the structure. In addition, at a final annealing temperature above 950 ° C, there is a risk of whisker formation and an associated increase in the roughness of the surface. In practice, final annealing temperatures of at least 750 ° C are found here With regard to the effect of the final annealing as particularly advantageous. Limiting the final annealing temperature to a maximum of 900 ° C. contributes to the economic implementation of the process according to the invention and reliably avoids the great coarsening and dissolution of the Laves Fe ₂ Nb phases. The atmosphere for the final annealing is an atmosphere containing at least 70% by volume of hydrogen, preferably at least 80% by volume of hydrogen, particularly preferably at least 95% by volume of hydrogen, the remainder nitrogen and technically unavoidable impurities. The high hydrogen content is necessary to ensure a reduction of the surface oxides by the hydrogen. At the beginning of the final annealing carried out according to the invention, the dew point is set to 0 to -30 ° C. During the final glow, the dew point is then lowered so that at the end of the final glow it is at most -50 ° C. Even lower dew points of no more than -60 ° C or below are particularly advantageous. At higher dew points at the end of the final annealing there would be the risk of an excessively thick oxide layer forming on the surface of the cold strip, which would show up as a gray-brown haze or tarnish and would impair the visual appearance in an unacceptable manner. Possible measures to reduce the dew point are an almost oxide-free furnace chamber and the addition of 1 to 20 kg of titanium sponge or other sponges or powders of elements with an affinity for oxygen to the furnace chamber.
9. i) If the surface appearance of the resulting cold strip turns out to be insufficiently good after the final annealing, the cold strip can optionally be subjected to a pickling process in which it is preferably exposed to a pickling medium heated to 60-100 ° C for a pickling time of 50 - 500 s , which is preferably HCl.
10. j) Finally, the cold strip is subjected to skin-pass rolling, in which it has a skin-pass degree = (thickness of the cold strip before Skin pass rolling - thickness of the cold strip after skin pass rolling) / (thickness of the cold strip before skin pass rolling) of 0.10 - 1% is cold rolled. For cold strips processed in this way according to the invention, mean roughness values Ra of 0.1-1.5 μm, in particular 0.2-1.0 μm, can be ensured in an operationally reliable, reproducible manner, mean roughness values Ra of 0.3-0.7 μm being reliably achieved.

The invention is explained in more detail below on the basis of exemplary embodiments.

To test the invention, operational tests BV1 - BV3 have been carried out on an industrial scale. The composition of the steel melts melted for this purpose is given in Table 1.

To demonstrate the influence of different process parameters, such as the reel temperature HT and the dew point of the atmosphere under which the final annealing was carried out, the operational test BV1 is in four variants A - D, the operational test BV2 in eight variants A - H, of which the variants A - D were not according to the invention and the variants E - H were according to the invention, and the operational test BV3 was carried out in two variants A, B according to the invention. The operating parameters set in the various variants in the operational tests BV1 - BV3 are given in Table 2.

Flat steel products in the form of cold-rolled steel strips were produced from the melts melted for the experiments BV1 - BV3. For this purpose, in the operational tests BV1 - BV3, the melts were each cast in a conventional manner to form slabs with a thickness of> 70 mm. The slabs were then thoroughly heated to a preheating temperature VWT. The thoroughly heated slabs then went through hot rolling, in which they were first pre-rolled in a conventional manner. After roughing, the slabs have a hot rolling start temperature WST of 1100 ° C was hot rolled into a hot strip in a conventional hot rolling mill.

At the end of the hot rolling, the hot strips obtained each had a hot rolling end temperature WET, from which they were cooled in a conventional manner by air or water cooling at a cooling rate of 5 to 200 K / s to a coiling temperature HT, at which they were wound into a coil been. After cooling to room temperature in the coil, the hot strips were descaled. In some of the variants of the operational tests BV1 - BV3, the scale adhering to the hot strips was first mechanically broken in a scale breaker. The hot strips then passed through a pickling bath in which they were exposed to a pickling medium heated to a pickling temperature BT for a pickling time Bt.

After removing the scale, the hot strips were cold rolled to cold strip in the conventional manner with a cold rolling degree KWG. The cold strips then went through a final annealing during which they were kept with a dew point Tp for an annealing time SGt at an annealing temperature SGT under an atmosphere of 100% hydrogen and technically unavoidable impurities. Finally, the cold strips obtained were pass-rolled with a skin pass degree DWG of 0.3%.

As a reference, a conventionally produced, commercially available steel strip made of 1.4509 material has been purchased.

The presence of an oxide layer on the surface of the cold strips obtained was first assessed visually. The oxide layer thickness and the Mn content in the oxide layer were then determined by means of a GD-OES measurement carried out in accordance with DIN 11505 and ISO 16962. In addition, the mean roughness Ra is 0.5 to 0.7 µm in accordance with DIN EN 10049 has been determined. The results of these investigations are summarized in Table 3.

Furthermore, the yield strength ReL, tensile strength Rm and elongation A50 determined in accordance with DIN EN ISO 6892-1: 2009 - A224 in each case in the longitudinal and transverse directions of the examined cold strip, which are determined in accordance with the explanations given at the beginning, are shown in the cell drawing on the cold strips obtained in the operational tests - and cup impact test determined transition temperature Tü, the low-temperature corrosion determined according to DIN EN ISO 9227 and evaluated according to the German school grading system, the deepening of the seam t-WIG determined in accordance with DIN EN ISO 20482 (2003-12) in the cupping test according to Erichsen with a TIG produced on the respective cold strip Welding and seam deepening t-LASER for a laser weld produced on the respective cold strip, the respective hot yield strengths W-ReL and W-Rm determined in accordance with DIN 50125 at 600 ° C in the longitudinal direction (tensile specimens form H with La = 50 mm; deviating from the standard central measuring range of only 20 mm in length, in which a constant temperature can be guaranteed; on heating rate 15 K / s, 3 min hold at 600 ° C; Drawing speed ε = 0.004 s ^-1 , the density p, oxidation and hot gas corrosion resistance OB as well as thermal conductivity λ of the flat steel product determined in the manner already explained at room temperature. The results of these investigations are summarized in Table 4.

The operational tests show that, with the procedure according to the invention, flat steel products can be produced using a conventional production route, which have a property profile that is particularly suitable for the production of components for high-temperature use, as was previously only possible with high-alloy steels of the type discussed above, which are significantly more complex to process Need to become.

The BV01 operational test was intended to show that a coiling temperature below 570 ° C. and complete descaling are necessary in order to produce a hot strip as an intermediate product that can be processed into a flat steel product according to the invention. During the operational test BV01 A, the reel temperature was too high and no scale breaker was used, so that complete descaling was not possible. A scale breaker was used in operational test BV01 B, but the coiling temperature was too high so that no further processing into cold strip could take place. In the operational test BV01 C, the coiling temperature was set in accordance with the requirements of the invention, but here, too, no scale breaker was used and consequently no adequate descaling was achieved. In the BV01 D operating test, the reel temperature and scale breaker were set according to the invention, but the dew point of the annealing atmosphere was too high.

The BV02 operational test was intended to show the influence of the dew point of the annealing atmosphere on the properties of the flat steel products produced. Accordingly, dew points that were too high were set for variants A - D of the operational test BV02.

As a result of the implementation of operational tests BV01 and BV02 (variants A - D) not according to the invention, the resulting cold-rolled flat steel products (in operational test BV01 a cold-rolled steel strip could only be produced in variant D) did not meet all the requirements placed on a flat steel product according to the invention . They showed visually visible tarnishing and had an oxide layer thickness greater than 150 nm (see Table 3).

In contrast, the variants of the operational test BV03 and variants E - H of the operational test BV02 carried out according to the invention in every respect resulted in reliably cold-rolled steel strips which fully met the requirements formulated by the invention. Table 1 Bez. Al Cr Nb Ti Si Mn C. N P S. B. Others BV1 5.5 6.8 0.51 0.49 0.056 0.19 0.023 0.0046 0.013 0.001 - Mo: 0.011 V: 0.023 Ni: 0.14 Cu: 0.031 Co: 0.009 Ta: 0.02 BV2 5.1 5.6 0.42 0.17 0.49 0.48 0.023 0.0120 0.014 0.001 - Mo: 0.014 V: 0.033 Ni: 0.09 Cu: 0.051 Co: 0.009 Ca: 0.001 W: 0.05 BV3 5.3 5.9 0.4 0.41 0.34 0.21 0.015 0.0110 0.005 0.001 0.061 V: 0.013 Ni: 0.08 Cu: 0.068 SEM: 0.01 Ref. *) 0.005 18.1 0.39 0.14 0.31 0.21 0.019 0.013 0.028 0.001 <0.0004 Zr: 0.04 V: 0.070 Ni: 0.101 Co: 0.04 Contents in% by weight, remainder Fe and unavoidable impurities *) not according to the invention Table 3 attempt variant Visual tarnish Oxide layer thickness Average Mn content in the oxide layer Average roughness Ra [nm] [%] [µm] BV1 A. - - - B. - - - C. - - - D. Yes 400 6 0.5 BV2 A. Yes 400 3 0.6 B. Yes 170 5 0.5 C. Yes 220 3 0.7 D. Yes 170 5 0.6 E. No 90 ≤2 0.5 F. No 100 ≤2 0.6 G No 90 ≤2 0.7 H No 110 ≤2 0.5 BV3 A. No 30th ≤2 0.6 B. No 90 ≤2 0.6 Ref - No 80 ≤2 0.5 Table 2 attempt variant VWT WST WET HT Scale breaker? Bt BT KWG SGt SGT Tp DWG [° C] [s] [° C] [%] [H] [° C] [%] BV01 A *) 1200 1100 850 620 No 210 80 Scale cannot be removed, therefore no further processing B *) 600 Yes 320 C *) 560 No 180 D *) 550 Yes 190 68 24 830 -30 0.3 BV02 A *) 550 Yes 170 65 36 -30 B *) 540 Yes 170 75 15th -45 C *) 570 Yes 190 65 24 -40 D *) 530 Yes 150 70 15th -45 E. 550 Yes 180 70 24 -50 F. 520 Yes 160 75 20th -55 G 530 Yes 200 75 28 -50 H 540 Yes 190 60 36 -50 BV03 A. 560 Yes 180 65 24 -60 B. 510 Yes 180 65 36 -55 Ref - purchased sheet metal Table 4 Bez. variant ReL Rm A50 Tü RT corrosion t-TIG t-LASER W-ReL W-Rm ρ IF λ [MPa] [%] [° C] [mm] [MPa] [g / cm ³ ] [mg / cm ² ] [W / mK] BV01 D. 433/460 578/597 28/25 nb 2 nb nb 214 298 BV02 F. 439/466 581/597 26/25 nb 4th nb 7.1 nb nb 7.2 <1 ≤18 BV03 B. 461/459 624/596 25/26 nb 4th nb nb nb nb Ref Ref 333/350 480/490 34/32 -80 1 9.2 8.8 164 289 7.6 42 21st

Claims (15)

Flat steel product with a fully ferrite structure and consisting of (in% by weight): Al: 2.0 - 8.0%, Cr: 2.0-10.0%, Nb: 0.1 - 1.0%, optionally one or more elements from the group Mn, Ti, Si, B, Ta, W, V, Zr, Mo, Ni, Cu, Ca, SEM, Co with the proviso that the Mn contents: up to 3 , 0%, Ti: up to 1.0%, Si: up to 1.0%, B: up to 0.10%, Ta: up to 1.0%, W: up to 1.0%, V : up to 0.50%, Zr: up to 1.0%, Mo: up to 2.0%, Ni: up to 2.0%, Cu: up to 2.0%, Ca: up to 0.150% , SEM: up to 0.50%, Co: up to 2.0%,
where for the content% Cr of Cr, the content% Al of Al, the optionally available content% Si of Si and the optionally available content% Mn of Mn applies: $$\%\mathrm{Cr}+10\times \%\mathrm{Si}+\%\mathrm{Al}+\%\mathrm{Mn}\le \mathrm{20th}\%,$$

and the remainder of Fe and unavoidable impurities, with the unavoidable impurities up to 0.10% C, up to 0.30% S, up to 0.10% P, up to 0.30% Sb, up to 0, Count 30% As, up to 0.50% N. Flat steel product according to Claim 1, characterized in that it has an oxide layer at most 150 nm thick on its surface. Flat steel product according to Claim 2, characterized in that, in the case of the presence of Mn in the flat steel product, the mean Mn content of the oxide layer formed on the surface of the flat steel product is at most 2% by weight. Flat steel product according to one of the preceding claims, characterized in that its structure contains 0.1-1% by volume of Laves phase Fe ₂ Nb. Flat steel product according to Claim 4, characterized in that more than 20% of the number of particles in the Laves phase Fe ₂ Nb lie on the grain boundaries of its structure. Flat steel product according to one of the preceding claims, characterized in that its Al content is 3.0-7.0% by weight, its Cr content is 3.0-9.5% by weight and its Nb content is 0.20 - 0.70 wt .-%. Flat steel product according to Claim 6, characterized in that its Al content is 4.0-6.0% by weight, its Cr content 5.5-8.4% by weight and its Nb content 0.30-0 , 50 wt .-%. Component for an exhaust system of an internal combustion engine that is exposed to temperatures of more than 200 ° C in use and is made of a one of the preceding claims provided flat steel product is formed. Component according to Claim 8, characterized in that it is exposed to temperatures of at least 600 ° C in use. Method for manufacturing a flat steel product, comprising the following steps: a) Casting a melt consisting of (in% by weight) AI: 2.0-8.0%, Cr: 2.0-10.0%, Nb: 0.1-1.0%, optionally each one or more elements from the group Mn, Ti, Si, B, Ta, W, V, Zr, Mo, Ni, Cu, Ca, SEM, Co with the proviso that the Mn content: ≤ 3.0% ,
Ti: ≤ 1.0%, Si: ≤ 1.0%, B: ≤ 0.10%, Ta: ≤ 1.0%, W: ≤ 1.0%,
V: ≤ 0.50%, Zr: ≤ 1.0%, Mo: ≤ 2.0%, Ni: ≤ 2.0%, Cu: ≤ 2.0%,
Ca: 0.150%, SEM: 0.50%, Co: 2.0%, with% Cr of Cr, the% Al of Al, the optionally present% Si of Si and the same optionally present content% Mn of Mn applies:% Cr + 10 x% Si +% Al +% Mn ≤ 20%, and the remainder consists of Fe and unavoidable impurities, with the unavoidable impurities up to 0.10% C, to to 0.30% S, up to 0.10% P, up to 0.30% Sb, up to 0.30% As, up to 0.50% N, to a preliminary product; b) heating through or holding the preliminary product at a preheating temperature of 1150-1300 ° C; c) Hot rolling of the preliminary product to form a hot strip, the hot rolling comprising an optionally run-through preliminary rolling and a finish hot-rolling which follows the optionally run-through preliminary rolling and is completed in several passes, the optionally pre-rolled intermediate product at the beginning of finish hot rolling has a hot rolling start temperature of 1050-1200 ° C and the hot strip finished hot-rolled from the intermediate product has a hot-rolling end temperature of 800-900 ° C at the end of hot rolling; d) Coiling of the hot strip obtained at a coiling temperature not exceeding 570 ° C to form a coil; e) Optional: hot strip annealing of the hot strip cooled in the coil over a period of 1 - 100 hours at a hot strip annealing temperature of 600 - 900 ° C; f) removing scale present on the hot strip by pickling the hot strip, optionally the scale present on the hot bath being mechanically broken before pickling; g) cold rolling the hot strip to form a cold strip in one or more cold rolling passes, the degree of cold rolling achieved via the cold rolling being at least 30%; h) final annealing of the resulting cold strip over a period of 1 - 100 h at a final annealing temperature of 650 - 950 ° C under a protective gas atmosphere, the dew point of which is set to a maximum of -50 ° C at the end of the final annealing; i) Optional: pickling of the finally annealed cold strip; j) Skin pass rolling of the cold strip with a skin pass degree of 0.1 - 1%. Process according to Claim 10, characterized in that the preliminary product into which the melt is cast in step a) is a slab with a thickness of more than 70 mm or a cast strip. Method according to Claim 10 or 11, characterized in that the coiling temperature is at most 520 ° C. Process according to one of Claims 10 to 12, characterized in that the final hot rolling temperature is at least 830 ° C. Method according to one of Claims 10 to 13, characterized in that in work step f) the hot strip is exposed to a pickling medium heated to 60-100 ° C for 50-500 s to remove the scale. Method according to one of Claims 10 to 14, characterized in that the dew point of the protective gas atmosphere at the end of the final annealing is at most -60 ° C.