EP2767601A1 - Cold rolled steel flat product for deep drawing applications and method for its production - Google Patents

Abstract

The cold-rolled flat product made of steel, is claimed. The steel contains iron, unavoidable impurities, carbon, aluminum, niobium, titanium, phosphorus, sulfur, nitrogen, and optionally elements including manganese, rare earth metal, silicon, zirconium, vanadium, tungsten, molybdenum, chromium, cobalt, nickel, boron, copper and calcium. The product has deep drawability (r-value) of 1.3, and contains 0-0.1 vol.% of carbides. A ratio of a grain length in a rolling direction to a width in a transverse direction of a grain of the flat steel product is less than 1.5. The cold-rolled flat product made of steel, is claimed. The steel contains iron, unavoidable impurities, 0.05 wt.% of carbon, 6.8 wt.% of aluminum, 0.1-0.15 wt.% of niobium, 0.15-0.3 wt.% of titanium, less than 0.1 wt.% of phosphorus, less than 0.03 wt.% of sulfur, less than 0.1 wt.% of nitrogen, and optionally elements including 0-0.1 wt.% of manganese, 0-0.2 wt.% of rare earth metal, 0-2 wt.% of silicon, 0-1 wt.% of zirconium, 0-1 wt.% of vanadium, 0-1 wt.% of tungsten, 0-1 wt.% of molybdenum, 0-3 wt.% of chromium, 0-1 wt.% of cobalt, 0-2 wt.% of nickel, 0-0.1 wt.% of boron, 0-3 wt.% of copper and 0-0.015 wt.% of calcium. A ratio of titanium to niobium is 1.5-2.5. The product has deep drawability (r-value) of 1.3, and contains 0-0.1 vol.% of carbides. A ratio of a grain length in a rolling direction to a width in a transverse direction of a grain of the flat steel product is less than 1.5 An independent claim is included for a method of producing a cold-rolled flat product.

Description

The invention relates to a cold-rolled steel flat product for thermoforming applications, which has a reduced weight as a result of a density reduction with optimized mechanical properties and an optimized deformability. Likewise, the invention relates to a method for producing such a flat steel product.

When it comes to flat-rolled steel products, it refers to steel strips obtained by rolling processes, steel sheets and blanks, blanks and the like obtained therefrom.

If information about the content of an alloying element is given in connection with an alloying regulation, these relate to the weight, unless expressly stated otherwise.

Particularly in the case of flat steel products used in the field of vehicle construction, in addition to the ratio of strength to formability, physical properties such as stiffness and density are of particular importance with regard to the generally desired weight saving and improvement of the natural frequencies of the respective vehicle. A significant minimization of the density and thus As a result of the weight, steels can be obtained by alloying larger contents of light A1. Moreover, at sufficiently high Al contents, the precursor phase (K-state) or the order phase Fe3Al (D03) occurs, which is particle-hardening, strength-increasing and reduces ductility.

The application-related advantages of high Al ferritic Fe-Al steels of the type in question are faced with difficulties in production and processing. Thus, practical experience shows that a non-recrystallized strip core area must be reduced on the hot strip produced from such steels, otherwise difficulties in trimming and cold rolling of the hot strip may occur. In addition, in the prior art, elaborate processes must be run to avoid anisotropic cold-rolled properties due to inadequate cold-rolled texture. Such anisotropies are characterized by low r and n values and entail a low elongation at break. This results in a problematic forming and processing behavior of cold-rolled flat steel products produced from Fe-Al steels with high Al contents.

The problems summarized above increase with increasing Al content and therefore limit the previously achievable reduction in density. In practice, for example, Al-containing deep-drawing steels may contain a maximum of 6.5% by weight of A1 (cf. U. Brüx "Thermoformable iron-aluminum lightweight steels", construction April 4, 2002 ).

Against the background of the above-mentioned state The object of the invention was to provide a flat steel product which, with a significant reduction in weight, has optimized deformation suitability and likewise optimized mechanical properties.

In addition, a method for producing such a flat steel product should be specified.

According to the invention, this object is achieved with regard to the cold-rolled flat steel product by providing a product having the features specified in claim 1.

The inventive solution of the above-mentioned object with respect to the method is that in the production of flat steel products according to the invention the steps specified in claim 10 are completed.

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

A cold-rolled flat steel product according to the invention for deep-drawing applications consists of a steel which, in addition to iron and unavoidable impurities (in% by weight) C: 0.008-0.1%, Al: 6.5-12%, Nb: 0.1-0, 2%, Ti: 0.15-0.5%, P: up to 0.1%, S: up to 0.03%, N: up to 0.1%, and optionally one or more elements from the group " Mn, Si, rare earth metals, Mo, Cr, Zr, V, W, Co, Ni, B, Cu, Ca, N "with the proviso contains, Mn: up to 1%, rare earth metals: up to 0.2%, Si : up to 2%, Zr: up to 1%, V: up to 1%, W: up to 1%, Mo: up to 1%, Cr: up to 3%, Co: up to 1%, Ni: up to 2%, B: up to 0 , 1%, Cu: up to 3%, Ca: up to 0.015%. In this case, the ratio% Ti /% Nb of the Ti content% Ti and the Nb content% Nb
2.5 ≥% Ti /% Nb ≥ 1.5,
especially
2.2 ≥% Ti /% Nb ≥ 1.8.

In the alloy rule provided according to the invention for a flat steel product according to the invention, only A1 and titanium and niobium are compulsory constituents apart from iron.

The cold-rolled steel strip according to the invention is distinguished by r values of at least 1.3, with flat steel products according to the invention regularly achieving r values of greater than 1.3. The high r-value stands for a good deep drawability of the cold-rolled steel flat product according to the invention, since with increasing r-value the tendency to thinning during deep-drawing is reduced and consequently stronger deep-drawing degrees are made possible. Otherwise there would be a risk of component failure at the thinned area.

A cold-rolled flat steel product according to the invention not only has high r values, but also reaches an elongation A50 of more than 18% on a regular basis. Steel flat products produced under optimum processing conditions have elongations A50 of 25% or more.

At the same time, it is characteristic of the structure of a flat steel product according to the invention that it is completely ferritic and largely free from κ-carbides (Fe-Al-C-carbides). Accordingly, the κ-carbide content of a flat steel product according to the invention is from 0% by volume (completely κ-carbide-free state) to at most 0.1% by volume. Due to the minimized κ-carbide content, the processability of the flat steel product according to the invention is reliably ensured.

A composite steel flat product according to the invention is further distinguished by the fact that the grains are globulitically pronounced in their microstructure. The ratio of the grain length in the rolling direction to the grain width in the transverse direction of the band is generally less than 1.5, in particular less than 1.2. That is, the length of the grains is at most 50%, in particular at most 20%, greater than their width.

In addition to the mandatory components, the steel according to the invention may contain a large number of further alloying elements in order to set certain properties. The relevant elements are summarized in the group "Mn, Si, rare earth metal, Mo, Cr, Zr, V, W, Co, Ni, B, Cu, Ca, N". Each of these optionally added alloying elements may be present in the steel according to the invention or completely absent, the respective element is also considered "not present" when it is present in the flat steel product according to the invention in an amount in which it is ineffective and therefore the production unavoidable impurities attributable to.

Aluminum is present in the steel of the present invention at levels of 6.5-12 wt%, with Al contents greater than 6.8 wt% being advantageous in view of the desired density reduction. Typical Al contents of flat steel products according to the invention are in the range from 6.5 to 10% by weight, in particular from 6.8 to 9% by weight. The presence of high Al contents reduces the density of the steel and significantly improves its corrosion and oxidation resistance. At the same time A1 increases the tensile strength in these contents. Excessive contents of A1, however, can lead to a deterioration of the forming behavior, which is expressed in a decrease in the r value. In order to minimize the negative effects of A1, therefore, the Al content is limited to a maximum of 12 wt .-%. An optimally balanced ratio of reduced density and processability arises when 6.5 to 10% by weight of Al, in particular at least 6.8% by weight of Al, are present in the steel according to the invention.

The C content is limited to at most 0.1% by weight in steel according to the invention, with C contents of 0.015-0.05% by weight, in particular 0.008-0.05% by weight, being particularly favorable. C contents above 0.1 wt.% Can cause the formation of undesirable brittle kappa carbides ("K carbides") at the grain boundaries and consequent reduction in hot and cold workability.

The avoidance of the formation of κ-carbides (Fe-Al-C compounds) is of particular importance in the steel according to the invention. κ-carbides are formed in the Processing of generic steels early during hot processing at high temperatures on the grain boundaries and cause embrittlement of the material. As a result of the addition of carbide-forming alloying elements in accordance with the invention, the lowest possible free C content is set, thus largely preventing the formation of κ carbides.

In the steel according to the invention, 0.15-0.5% by weight of Ti and 0.1-0.2% by weight of Nb are present in the first place for this purpose. The effect of titanium can then be used particularly reliably if the Ti content is 0.15-0.3% by weight. The same applies to niobium when Nb is present in amounts of 0.1-0.15% by weight in the steel according to the invention. At the same time, the respective contents of Ti and Nb must be adjusted in such a way that they fulfill the condition prescribed according to the invention for the ratio of these contents. Ti and Nb contents which fulfill these requirements cause the formation of finely dispersed Ti and Nb carbides in the steel according to the invention, which promote the formation of a fine structure which supports the deformability of the flat steel product. At the same time free carbon is bound, which could otherwise lead to the formation of deformability hindering the risk of embrittlement bring Fe-Al-C-carbides. Too high levels of Ti and Nb, however, may form undesirable precipitates of these elements in the steel, which could cause a decrease in toughness and ductility.

V, Zr and W are also effective carbide formers and In amounts of up to 1% by weight each can supplement the effect of the Nb and Ti required contents provided according to the invention. The effect of V, Zr and W can be used particularly purposefully if their content is limited to in each case up to 0.5% by weight, in particular 0.3% by weight.

By adding Mn in amounts of up to 1% by weight, in particular up to 0.5% by weight, the hot workability and weldability of the steel according to the invention can be improved. In addition, Mn aids in deoxidation during melting and contributes to increasing the strength of the steel. These positive effects of Mn can be used particularly effectively when the Mn content is 0.05-0.5 wt%.

Mo can be present in amounts of up to 1% by weight in the steel according to the invention. Mo also forms carbides and contributes to increasing the tensile strength, creep resistance and fatigue strength of a flat steel product of the present invention. The carbides formed by Mo with C are particularly fine and thus improve the fineness of the structure of the flat steel product according to the invention. High levels of Mo, however, degrade the hot and cold workability. In order to avoid this particularly reliably, the optionally present Mo content of a steel according to the invention can be limited to 0.5% by weight.

To avoid negative effects of sulfur and phosphorus on the properties of the steel processed according to the invention, the S content to a maximum of 0.03 wt .-%, preferably at most 0.01 wt .-%, and the P content to a maximum 0.1 wt .-%, preferably at most 0.05 wt .-%, limited.

The N content of the flat steel product according to the invention is limited to at most 0.1% by weight, in particular at most 0.02% by weight, preferably at most 0.001% by weight, in order to avoid the formation of relatively large amounts of Al nitrides. These would degrade the mechanical properties.

The presence of rare earth metals in amounts of up to 0.2% by weight contributes to improved resistance to oxidation and increased strength of a flat steel product of the present invention. At the same time, contents of rare earth metals are desulfurizing and deoxidizing. The oxides formed by the respective rare earth metals also have a fine grain and promote positive texture selection for improved technological properties. Particularly suitable rare earth metals are Ce and La. The positive influences of rare earth metals in the steel according to the invention can be used particularly purposefully if the contents of rare earth metals are in the range of up to 0.05% by weight.

Basically, the carbides formed by the presence of one or more of the elements Ti, Nb, V, Zr, W, Mo contribute to increasing the strength of the steel of the present invention.

Si in amounts of up to 2 wt .-%, in particular up to 0.5 wt .-%, supported in the melting also the deoxidation and increases the strength and corrosion resistance of the steel according to the invention. Too high levels are due to the presence of Si however, reduces the ductility of the steel and its weldability. Typical Si contents of steels according to the invention are in the range of 0.1-0.5% by weight, in particular 0.1-0.2% by weight.

Also by the addition of Cr in amounts of up to 3 wt .-% existing carbon in the invention steel can be bonded to carbides. At the same time, the presence of Cr increases the corrosion resistance. The advantageous properties of Cr in the steel according to the invention are achieved with particular accuracy when Cr is present in amounts of up to 1% by weight, in particular up to 0.5% by weight.

In order to avoid an increase in the recrystallization temperature, the Co content of the steel according to the invention is limited to max. 1 wt .-%, in particular max. 0.5% by weight, preferably max. 0.3% by weight, limited.

Nickel in amounts of up to 2 wt .-%, in particular 1 wt .-%, also contributes in the inventive steel to increase the strength and toughness. In addition, Ni improves the corrosion resistance and reduces the proportion of primary ferrite in the structure of the steel according to the invention. Ni can be used in the steel according to the invention at levels of up to 0.5% by weight in a particularly practical manner.

The addition of B can also lead to the formation of a fine, the deformability of the steel according to the invention favoring structure. Too high levels of B, however, the cold workability and the Impair oxidation resistance. Therefore, the B content of the steel according to the invention is limited to 0.1% by weight, in particular up to 0.01% by weight, preferably 0.005% by weight.

Cu in amounts of up to 3% by weight improves corrosion resistance in the steel of the present invention, but at higher levels may also deteriorate hot workability and weldability. If present, therefore, the Cu content in a practical embodiment of the invention to at most 1 wt .-%, in particular 0.5 wt .-%, limited.

Ca in amounts of up to 0.015 wt .-%, in particular 0.005 wt .-% or 0.003 wt .-%, binds sulfur in the steel according to the invention, which could reduce the corrosion resistance.

• Erschmelzen einer entsprechend den voranstehend erläuterten Maßgaben erfindungsgemäß zusammengesetzten Stahlschmelze.
• Vergießen der Stahlschmelze zu einem Vorprodukt, wie einem Block, einer Bramme, einer Dünnbramme oder einem gegossenen Band. Hier hat sich insbesondere das Vergießen zu einem endabmessungsnah gegossenen Band als vorteilhaft herausgestellt. Das endabmessungsnahe Gießen kann dabei durch Einsatz von an sich zu diesem Zweck bekannten konventionellen Gießeinrichtungen erfolgen. Hierzu zählt z. B. die "Zwei-Rollen-Bandgießmaschine". Da dieses Verfahren mit einer mitlaufenden Kokille operiert, besteht keine Relativbewegung zwischen Kokille und erstarrender Bandschale. Auf diese Weise können diese Verfahren ohne Gießpulver arbeiten und sind daher grundsätzlich gut geeignet, das Vormaterial für die Herstellung von erfindungsgemäßen Stahlflachprodukten zu erzeugen. Beim Bandgießen ebenfalls positiv wirkt sich aus, dass das gegossene Band bis zu seiner Abkühlung allenfalls geringen mechanischen Spannungen ausgesetzt ist, so dass die Gefahr der Entstehung von Rissen im Hochtemperaturbereich minimiert ist.

In the production of a cold-rolled steel flat product according to the invention, the following work steps are carried out according to the invention:

• Melting a according to the above-explained provisos according to the invention composite molten steel.
• Casting the molten steel into a precursor such as a block, a slab, a thin slab or a cast strip. In particular, the casting to a near-net cast tape has been found to be beneficial. The close to the final casting can by using known per se for this purpose Conventional pouring done. This includes z. As the "two-roll strip casting machine". Since this method operates with a co-rotating mold, there is no relative movement between mold and solidifying band shell. In this way, these methods can work without casting powder and therefore are generally well suited to produce the starting material for the production of flat steel products according to the invention. Another positive aspect of strip casting is the fact that the cast strip is exposed to at most low mechanical stresses until it cools, so that the risk of cracking in the high-temperature region is minimized.

When melting the molten steel poured according to the invention, a waiting time of at least about 15 minutes should elapse between the last addition of alloy and the casting, in order to ensure thorough mixing of the molten steel. Typical effluent temperatures are in the range of about 1590 ° C.

• Das Vorprodukt wird erforderlichenfalls auf eine 1000 - 1300 °C betragende Vorwärmtemperatur gebracht oder in diesem Temperaturbereich gehalten, wobei sich hier Vorwärmtemperaturen von 1200 - 1300 °C, insbesondere 1200 - 1280 °C, als besonders praxisgerecht erwiesen haben. Im Fall, dass das Vorprodukt eine Bramme ist, beträgt die Dauer, über die diese Vorerwärmung abläuft, beispielsweise 120 - 240 Minuten.
• Das Vorprodukt wird, gegebenenfalls nach der optional durchgeführten Erwärmung auf die Vorwärmtemperatur, zu einem Warmband warmgewalzt, wobei die Walzendtemperatur mehr als 820 °C, insbesondere mehr als 850 °C, betragen soll und in der Praxis Warmwalzendtemperaturen von 830 - 960 °C eingestellt werden. Bei praktischen Versuchen haben sich im Bereich von 840 - 880 °C liegende Warmwalzendtemperaturen als besonders günstig herausgestellt.
• Das erhaltene Warmband wird zu einem Coil gehaspelt, wobei die Haspeltemperatur bis zu 750 °C, insbesondere bis zu 650 °C, betragen kann. In der Praxis werden typischerweise Haspeltemperaturen von 450 - 750 °C, insbesondere 500 °C +/- 20 °C, eingestellt. Das so erhaltene Warmband hat eine mittlere Ferritkornlänge im Bandkern, die in Bandrichtung gemessen größer 100 µm ist.
• Nach dem Haspeln wird das Warmband geglüht. Diese Glühung ist von besonderer Bedeutung für die Eigenschaften des erfindungsgemäß erzeugten Stahlflachprodukts. Die Warmbandglühung wird bei einer oberhalb von 650 °C liegenden, bis 1200 °C reichenden, insbesondere 700 - 900 °C betragenden Glühtemperatur durchgeführt. Glühtemperaturen von etwa 850 °C, insbesondere 850 °C +/-20 °C, haben sich dabei als besonders praxisgerecht erwiesen. Die hierfür vorgesehenen Glühzeiten betragen bei dieser üblicherweise als Haubenglühung durchgeführten Glühung typischerweise 1 - 50 h.

On the basis of practical experiments it was possible to show that steels according to the invention can also be cast into blocks, which are then rolled out into slabs by pre-blocking.

• If necessary, the precursor is brought to a preheating temperature of 1000-1300 ° C. or kept within this temperature range, with preheating temperatures of 1200-1300 ° C., in particular 1200-1280 ° C., proving to be particularly practical to have. In the case where the precursor is a slab, the duration over which this preheating takes place is, for example, 120-240 minutes.
• The precursor is, if appropriate after the optionally performed heating to the preheating temperature, hot rolled into a hot strip, the rolling end temperature should be more than 820 ° C, in particular more than 850 ° C, and be set in practice Heißwalzendtemperaturen 830-960 ° C. , In practical experiments, hot rolling temperatures in the range of 840-880 ° C have proven to be particularly favorable.
• The resulting hot strip is coiled into a coil, wherein the coiler temperature can be up to 750 ° C, in particular up to 650 ° C. In practice, reel temperatures of 450-750 ° C., in particular 500 ° C. +/- 20 ° C., are typically set. The hot strip thus obtained has an average ferrite grain length in the strip core which, measured in the strip direction, is greater than 100 μm.
• After reeling, the hot strip is annealed. This annealing is of particular importance for the properties of the steel flat product produced according to the invention. The hot strip annealing is carried out at a temperature above 650 ° C, reaching up to 1200 ° C, in particular 700 - 900 ° C amount annealing temperature. Annealing temperatures of about 850 ° C, in particular 850 ° C +/- 20 ° C, have proven to be particularly practical. The intended annealing times are usually carried out as a bell annealing at this Annealing typically 1 - 50 h.

• Erforderlichenfalls kann nach dem Glühen ein Beizen des Warmbands durchgeführt werden, um auf dem Warmband haftende Rückstände zu entfernen.
• Das geglühte und optional gebeizte Warmband wird dann zu einem kaltgewalzten Stahlflachprodukt kaltgewalzt. Das Kaltwalzen kann in einer Stufe oder zweistufig erfolgen. Beim zweistufigen Kaltwalzen kann in an sich bekannter Weise zwischen den Kaltwalzstufen eine Zwischenglühung durchgeführt werden. Durch zweistufiges Kaltwalzen mit Zwischenglühung wird eine positive Texturauslese gefördert.

As a result of the annealing carried out in the predetermined temperature range according to the invention, the hot strip can be cold-rolled despite its high Al contents, without severe edge cracks or even ribbon tears occurring. The hot strip annealing serves to produce a sufficiently recovered band core area, lowering the cold rolling resistance and increasing the maximum attainable degree of cold rolling. A texture selection effected by the hot-band annealing and a high degree of cold deformation promote the formation of a suitable cold-rolled texture with the desired property profile. For the hot strip annealing, in particular the crucible annealing process with peak temperatures above 650 ° C. set in accordance with the variants explained above is suitable.

• If necessary, pickling of the hot strip may be performed after annealing to remove any residue left on the hot strip.
• The annealed and optionally pickled hot strip is then cold rolled to a cold rolled flat steel product. Cold rolling can be done in one stage or two stages. In the two-stage cold rolling an intermediate annealing can be carried out in a conventional manner between the cold rolling stages. Two-stage cold rolling with intermediate annealing promotes positive texture selection.

In any case, when cold rolling before the end of the Cold rolling completed rolling stage with the highest possible degree of cold deformation. In the case of single-stage cold rolling, this means that the hot strip is cold rolled to a degree of cold rolling of at least 65%, or a cold rolling degree of at least 65% is also achieved in the two- and multi-stage cold rolling after the intermediate annealing. In order to obtain optimum rolling results, the two-stage cold rolling can be carried out in such a way that the degree of cold rolling in the first stage is at least 40% and the last stage at least 65%, in particular more than 70%, for example at least 80%.

• Nach dem Kaltwalzen wird das erhaltene Kaltband einer Glühung unterzogen, die im kontinuierlichen Glühprozess oder batchweise als Haubenglühung ausgeführt wird. Sowohl die Schlussglühung als auch die optional beim Kaltwalzen durchgeführten Zwischenglühungen können in konventioneller Weise bei Temperaturen und Glühdauern durchgeführt werden, die an sich bekannt sind. Bei der abschließenden Schlussglühung des Kaltbandes bildet sich ein Material mit rekristallisierter Mikrostruktur und vorteilhafter Textur aus. Die erhaltene Textur ist gekennzeichnet durch eine geringe Belegung der α-Fasern von weniger als 4 und einer starken Belegung der γ-Fasern von mehr als 4, was zu r-Werten größer 1,3 führt. Die jeweilige Glühung des kaltgewaltzen Bandes kann in im kontinuierlichen Durchlauf durchlaufenen Glühanlagen mit Glühtemperaturen von 750 - 850 °C über eine typische Dauer von 1 - 20 min erfolgen, wobei sich Glühtemperaturen von mehr als 780 °C, insbesondere 800 - 850 °C, und eine Glühdauer von 2 - 5 min als besonders praxisgerecht erwiesen haben. Alternativ kann die jeweilige Glühung auch in einer Haubenglühanlage durchgeführt werden, bei der die Glühtemperatur mehr als 650 °C, insbesondere 650 - 850 °C, und die Glühdauer 1 - 50 h beträgt. In der Praxis haben sich für das Haubenglühen Glühtemperaturen von 700 - 800 °C und eine Glühdauer von 1 - 30 h besonders bewährt.
• Optional kann das erhaltene Kaltband beispielsweise zur Verbesserung seiner Korrosionsbeständigkeit mit einer metallischen Schutzschicht belegt werden, die beispielsweise auf A1 oder Zn basiert. Hierzu eignen sich die an sich bekannten Beschichtungsverfahren.

The high degree of cold rolling of at least 65% in the respective last cold rolling step promotes the formation of a suitable cold-rolled texture. The effect is particularly pronounced in the alloyed according to the invention Ti / Nb-alloyed materials.

• After cold rolling, the cold strip obtained is subjected to an annealing, which is carried out in a continuous annealing process or batchwise as a bell annealing. Both the final annealing and the optional intermediate anneals carried out during cold rolling can be carried out in a conventional manner at temperatures and annealing times which are known per se. In the final annealing of the cold strip, a material with a recrystallized microstructure and an advantageous texture is formed. The obtained texture is characterized by a low occupancy of the α-fibers of less than 4 and a strong occupancy of the γ-fibers of more than 4, which leads to r-values greater than 1.3. The respective annealing of the cold-rolled strip can be carried out in continuously continuous annealing plants with annealing temperatures of 750 - 850 ° C over a typical period of 1 - 20 min, with annealing temperatures of more than 780 ° C, in particular 800 - 850 ° C, and An annealing time of 2 - 5 min have proven to be particularly practical. Alternatively, the respective annealing can also be carried out in a bell annealing plant in which the annealing temperature is more than 650 ° C., in particular 650-850 ° C., and the annealing time is 1-50 h. In practice, annealing temperatures of 700 ° to 800 ° C. and an annealing time of 1 to 30 h have proven particularly suitable for bell annealing.
• Optionally, the cold strip obtained, for example, to improve its corrosion resistance can be covered with a metallic protective layer, which is based for example on A1 or Zn. For this purpose, the coating methods known per se are suitable.

To test the invention, three inventive melts E1, E2, E3 and two comparative melts V1, V2 have been melted, whose compositions are given in Table 1.

The molten steels E1 and E2 have been cast into precursors in the form of blocks. The blocks were then heated through a preheating of two hours in each case to a preheat temperature VWT and vorgeblockt to slabs.

Subsequently, the reheated slabs are hot rolled at a hot rolling end temperature WET to a hot strip and the resulting hot strip was wound at a reel temperature HT each to form a coil.

From the molten steel E3, a cast strip was produced as a precursor via a two-roll strip caster, which was then also hot-rolled into a hot strip with a hot rolling end temperature WET. The processing to the hot strip was carried out in a continuous process sequence without interruption following the strip casting, so that the precursor already had a temperature lying in the range of inventively predetermined preheating temperatures when entering the hot rolling device and the preheating could be omitted. Also, the hot strip produced from the steel E3 has been coiled after hot rolling at a reel temperature HT to form a coil.

After coiling, the hot strips produced in each case, unless otherwise indicated in Table 2, have been subjected to annealing in a bell annealing system at an annealing temperature GT over an annealing time of eight hours each time.

The so annealed hot strips were cold rolled in one or two stages with cold rolling degrees KWG1 (cold rolling degree of the first cold rolling stage) and KWG2 (cold rolling degree of the respective second cold rolling stage) each to a cold rolled steel strip. If two-stage cold rolling has been used, an intermediate annealing at an intermediate annealing temperature ZGT is in each case between the cold rolling stages Have been carried out. After cold rolling, the cold-rolled steel flat products have undergone a final annealing at an annealing temperature SGT. The intermediate annealing and the final annealing have each been completed in continuous operation.

The respective preheat temperature VWT, hot rolling end temperature WET, coiler temperature HT, annealing temperature GT, the respective cold rolling degree KWG1, KWG2, and the respective intermediate annealing temperature ZGT and final annealing temperature SGT, are given in Table 2.

The mechanical properties "yield strength Rp0.2", "tensile strength Rm", "elongation A50", "r value r" and "n value n" determined on the cold-rolled steel strips thus produced are given in Table 3. All mechanical technological parameters were determined in the transverse direction. In addition, Table 3 shows the maximum values of the occupancy of the α and γ fibers.

It can be seen that the cold-rolled steel strips produced from the steels E1 and E2 produced according to the invention in accordance with the invention have yield strengths which are regularly greater than 300 MPa, in particular greater than 320 MPa and thereby reach values of 380 MPa and more, and tensile strengths which are regularly greater 460 MPa, in particular greater than 480 MPa, while achieving values of 530 MPa and more, and having elongation values A50 of at least 18%, which regularly exceed 21%, in particular greater than 25%, and always have r values of 1 , 3 or greater.

Cold-rolled steel strips not assembled according to the invention do not achieve such r-values even if these steel strips have been produced taking into account production parameters that are closely related to the parameters set in the production of the cold-rolled steel flat products according to the invention. Also according to the invention composite, but not according to the invention processed flat steel products do not reach the properties of steel flat products produced according to the invention or can not even be cold-rolled.

Accordingly, the steel strips produced according to the invention have, despite their high Al contents, a superior deep-drawing capability, without the need for expensive alloying or process-engineering measures.

A flat steel product with optimum deformation properties (r ≈ 2, n ≈ 0.2, A50 ≈ 30%) is achieved by a combination of alloy according to the invention, high degree of cold deformation and low hot rolling temperature (about 850 ° C).

The cold-rolled steel strips produced from the steels according to the invention in accordance with the invention contain, in addition to a Fe (Al) mixed-crystal matrix, locally occurring hardening precursor phase. In standard hot rolling parameters, rolling is carried out in the full-ferrite phase region and hot strip with typical three-layer structure is obtained, which in turn is characterized by recrystallized globulitic margins and the only recovered core area with stem crystals is marked. The hot strip annealing performed according to the invention reduces the dislocation density in the recovered area and facilitates subsequent cold rolling processing. Without the hot strip annealing, the alpha fiber texture component is strong but weak with hot strip annealing. A low maximum cold rolling degree of up to 50% results in weak gamma fiber texture components, one-stage cold rolling with a high cold rolling degree of at least 65%, especially at least 80%, or two-stage cold rolling with correspondingly high deformation in the last rolling stage results in one strong gamma fiber component. These dependencies are more pronounced at lower hot rolling end temperatures, which are in the range of 830-960 ° C, especially 840-880 ° C.

The deformation behavior of the resulting cold-rolled steel flat product is significantly influenced by the texture. High r and n values as well as a high elongation at break A50 are particularly noticeable when the gamma fast texture component dominates over the alpha fiber texture component. A lying in the context of the invention combination of Nb and Ti contents, the inventively predetermined hot strip annealing and the inventively provided parameters of cold rolling ensure that this goal is achieved. Table 1 C Si Mn P S Cr Not a word Ni al N Ti Nb V % Ti /% Mb E1 0,018 0.09 0.08 0,006 0,003 0.04 0.00 0.03 7.1 0.0048 0,180 0,100 0,004 1.8 E2 0,017 0.11 0.09 0.005 0,003 0.09 0.00 0.03 8.5 0.0039 0.210 0,110 0,003 1.91 E3 0,012 0.33 0.21 0,010 0,003 1.11 0.04 0.35 6.93 0.0020 0,262 0,120 0,010 2.18 V1 0,007 0.18 0.09 0,050 0,003 0.03 0.01 0.03 7.2 0.0056 0,060 0,002 0,003 30 V2 0,006 0.15 0.11 0,006 0,002 0.03 0.00 0.05 9.7 0.0051 0,070 0,004 0,004 17.5 Content in wt .-%, balance iron and unavoidable impurities stole VWT [° C] WET [° C] HT [° C] GT [° C] KWG1 [%] ZGT [° C] KWG2 [%] SGT [° C] According to the invention? E1 1250 850 500 - Non-crack cold-rollable NO E1 1250 850 500 850 50 - - 830 NO E1 1250 860 500 850 50 830 70 830 YES E1 1250 870 500 850 80 - - 830 YES E1 1250 955 700 - 50 - - 830 NO E1 1250 940 700 850 50 - - 830 NO E1 1250 940 700 - 50 830 70 830 NO E1 1250 935 700 850 50 830 70 830 YES E1 1250 930 700 - 80 - - 830 NO E1 1250 955 700 850 80 - - 830 YES E2 1250 880 500 - Non-crack cold-rollable E2 1250 880 500 850 80 830 YES E2 1250 870 700 850 50 830 70 830 YES E3 - 860 600 850 80 - - 830 YES V1 1250 930 700 - Non-crack cold-rollable NO V1 1250 930 700 850 80 - - 830 NO V2 1250 980 700 850 Non-crack cold-rollable NO stole Mechanical-technological properties Maximum value texture component (S = 0.1) According to the invention? Rp0.2 [MPa] Rm [MPa] A50 [%] r n α-fiber y fiber {111} <011> {111} <112> E1 Non-crack cold-rollable NO E1 353 507 28.0 0.48 0.17 4 4 NO E1 346 502 27.0 1.36 0.18 3 6 YES E1 329 488 29.5 2.05 0.19 1 5 YES E1 421 521 19.0 0.8 0.13 12 2 NO E1 368 503 19.9 0.86 0.15 2 1.5 NO E1 363 523 21.9 1.03 0.17 12 6 NO E1 324 471 18.9 1.73 0.19 2 4 YES E1 373 529 23.4 1.09 0.17 8th 5 NO E1 325 461 21.1 1.70 0.17 3 5 YES E1 Non-crack cold-rollable NO E2 406 556 18.3 1.93 0.17 2 5 YES E2 391 537 21.8 1.56 0.14 3 5 YES E3 451 588 18.2 1.71 0.18 1 5 YES V1 Non-crack cold-rollable NO V1 408 532 22.0 0.72 0.15 9 2 NO V2 Non-crack cold-rollable NO

Claims (15)

Cold rolled flat steel product for thermoforming applications, consisting of a steel containing iron and unavoidable impurities (in% by weight)
C: 0.008 - 0.1%,
Al: 6.5-12%,
Nb: 0.1-0.2%,
Ti: 0.15-0.5%,
P: up to 0.1%,
S: up to 0.03%,
N: up to 0.1%
and optionally one or more elements from the group "Mn, Si, rare earth metals, Mo, Cr, Zr, V, W, Co, Ni, B, Cu, Ca, N" with the proviso contains
Mn: up to 1%,
Rare earth metals: up to 0.2%,
Si: up to 2%,
Zr: up to 1%,
V: up to 1%,
W: up to 1%,
Mo: up to 1%,
Cr: up to 3%,
Co: up to 1%,
Ni: up to 2%,
B: up to 0.1%,
Cu: up to 3%,
Ca: up to 0.015%, - where the ratio% Ti /% Nb of the Ti content% Ti and the Nb content% Nb applies
2.5 ≥% Ti /% Nb ≥ 1.5. Flat steel product according to claim 1, characterized in that its Al content is 6.5-10% by weight. Flat steel product according to one of the preceding claims, characterized
characterized in that its Al content is more than 6.8 wt .-%. Flat steel product according to one of the preceding claims, characterized
characterized in that its C content is at most 0.05 wt .-%. Flat steel product according to one of the preceding claims, characterized
characterized in that its Nb content is 0.1-0.15% by weight. Flat steel product according to one of the preceding claims, characterized
characterized in that its Ti content is 0.15-0.3% by weight. Flat steel product according to one of the preceding claims, characterized
characterized in that its microstructure contains 0 to 0.1 vol .-% κ-carbides. Flat steel product according to one of the preceding claims, characterized
characterized in that its r-value is at least 1.3. Flat steel product according to one of the preceding claims, characterized
characterized in that in its structure, the grains have a ratio of the grain lengths in the rolling direction to the grain width in the transverse direction of the flat steel product <1.5. A method of producing a cold rolled steel flat product intended for thermoforming applications, comprising the following steps Melting of a molten steel, in addition to iron and unavoidable impurities (in% by weight) C: 0.008 - 0.1%,

Al: 6, 5 - 12%,
Nb: 0.1-0.2%,
Ti: 0.15-0.5%,
P: up to 0.1%,
S: up to 0.03%,
N: up to 0.1%
and optionally one or more elements from the group "Mn, Si, rare earth metals, Mo, Cr, Zr, V, W, Co, Ni, B, Cu, Ca, N" with the proviso contains
Mn: up to 1%,
Rare earth metals: up to 0.2%,
Si: up to 2%,
Zr: up to 1%,
V: up to 1%,
W: up to 1%,
Mo: up to 1%,
Cr: up to 3%,
Co: up to 1%,
Ni: up to 2%,
B: up to 0.1%,
Cu: up to 3%,
Ca: up to 0.015%, - wherein for the ratio% Ti /% Nb of the Ti content% Ti and the Nb content% Nb is 2.5 ≥% Ti /% Nb ≥ 1.5; - casting the molten steel into a precursor; - optionally reheating or holding the pre-product to a preheating temperature of 1000 - 1300 ° C; Hot rolling the precursor to a hot strip, the hot rolling end temperature being 820-1000 ° C; - Coiling the hot strip into a coil, wherein the reel temperature is in the range of room temperature to 750 ° C; - Annealing of the hot strip at a more than 650 ° C and up to 1200 ° C amount annealing temperature over an annealing time of 1 - 50 h; - optional pickling of the hot strip; - cold rolling the annealed and optionally pickled hot strip into a cold rolled flat steel product in one or more stages with a total cold rolling degree of at least 65%; - Final annealing of the cold-rolled steel flat product at a final annealing temperature of 650 - 850 ° C. A method according to claim 10, characterized in that the precursor is a cast strip. A method according to claim 10 or 11, characterized in that the hot rolling end temperature is 830-960 ° C. Method according to one of claims 10 to 12,
characterized in that the reel temperature is 450 - 750 ° C. Method according to one of claims 10 to 13,
characterized in that the hot strip annealing is performed as a bell annealing. Method according to one of claims 9 to 13,
characterized in that the cold rolling is carried out in two or more stages and between the stages of cold rolling an intermediate annealing takes place.