EP2767602B1 - Cold rolled steel flat product for deep drawing applications and method for its production - Google Patents

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 In addition, the weight of steels can be achieved by alloying larger amounts of light Al. 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 Fe-Al ferritic steel with high Al contents 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 high Al content Fe-Al steel.

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 Al (cf. U. Brüx "Thermoformable iron-aluminum lightweight steels", construction April 4, 2002 ).

In addition to the above-described prior art from the EP 0 826 787 A2 a higher-strength lightweight structural steel known for vehicle parts and cladding, which has a purely ferritic microstructure and an Al content of> 5 to 9% by weight Al, a Si content of less than 0.2% by weight Si and a Mn Content of 0.03 to 0.2 wt .-%, without in the alloy of this steel rare earth metals are present in alloying levels.

Furthermore, from the JP 2010-121213 A a method for producing a high-strength steel sheet is known, which should have a low specific gravity of <7.2, tensile strength of ≥ 440 MPa and an elongation of ≥ 25%. The steel sheet is made of a steel containing (in% by weight) 0.001-0.01% C, ≦ 3.0% Si,> 0.2-3.0% Mn, ≦ 0.02% P, ≦ 0 , 02% S, 5.0-10.0% Al, 0.001-0.05% N, and the remainder contains Fe and unavoidable impurities, the Mn and S content of the slab having the condition up to 20 <(Mn / S). Optionally, rare earth metals may be present in the alloy of the steel to aid in the desulfurization of the steel by sulfide formation.

Finally, out of the JP 2001 271148 A a steel sheet is known, which should have improved high-temperature oxidation resistance. For this purpose, this known steel sheet consists of a steel containing (in% by weight) ≤ 0.01% C, 0.01-2.0% Si, 0.05-2.5% Mn, ≤ 0.02% P , 5 - 10% Al, ≤ 0.010% S and ≤ 0.010% N, balance iron and unavoidable impurities. Optionally, in total 0.01-0.3% by weight of Ti, Nb, Mo and V, in total 0.01-0.2% by weight of La, Ce, Nd and Y, 1-8% Cr and 0.0002 - 0.002% B be present in the steel. By adding the Rare earth metals should form a protective Al oxide film on the surface of the flat steel product to ensure high resistance of the steel sheet to high temperature oxidation.

Against the background of the prior art explained above, the object of the invention was to provide a flat steel product which, with a significant weight reduction, 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 solution according to the invention of the above-mentioned object with regard to the method consists in the fact that in the production of flat steel products according to the invention the steps specified in claim 8 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 steel flat 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: up to 0.1%, Al: 6.5-11%, at least one rare earth metal from the group "Ce "La" with the proviso the content of rare earth metals is 0.02-0.2%, 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, Nb, Ti, Mo, Cr, Zr, V, W, Co, Ni, B, Cu, Ca, N" with the proviso contains, Mn: up to 6%, Si: up to 1%, Nb: up to 0.3%, Ti: up to 0.3%, 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 cold rolled flat steel product according to the invention has an r-value which is at least 1, and a structure which is largely free of κ-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.

In the alloy rule provided according to the invention for a flat steel product according to the invention, only Al and at least one element assigned from the group of rare earth elements are compulsory constituents except iron. Accordingly, in addition to iron and unavoidable impurities (in% by weight), the steel processed according to the invention contains at least 6.5-11% Al, up to 0.1% C and a content of 0.02-0.2% of one or more Elements of the group of rare earth metals.

The cold-rolled steel strip according to the invention is distinguished by r values of at least 1, with flat steel products according to the invention regularly achieving r values greater than 1. The high r-value stands for a good thermoformability of the cold-rolled flat steel product according to the invention, since with increasing r-value, the tendency to thinning during deep drawing is reduced and, consequently, greater degrees of deep drawing 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 achieves an elongation A50 of regularly more than 15%, in particular at least 18%. It is characteristic of the structure of a flat steel product according to the invention that it is completely ferritic and, as stated above, typically largely free of κ-carbides (Fe-Al-C-carbides).

The high aluminum content of flat steel products according to the invention causes not only a decrease in density and weight but also an increase in the energy absorption capacity and, consequently, an improvement in the crash behavior. The invention provides such reduced-density flat steel products with improved crash properties and a comparatively high modulus of elasticity, which can be produced in a simple manner and offer optimum conditions for use in vehicle construction.

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, Nb, Ti, Mo, Cr, Zr, V, W, Co, Ni, B, Cu, Ca, N". Each of these optionally added Alloying elements can be present in the steel according to the invention or completely absent, wherein the respective element is to be regarded as "not present" if it is present in the steel flat product according to the invention in an amount in which it is ineffective and therefore attributable to the production-unavoidable impurities.

Aluminum is present in the steel of the present invention at levels of 6.5-11 wt.%, With Al contents of greater than 6.5 wt.%, More preferably greater than 6.7 wt.% Or greater than 7 wt. -%, are advantageous in view of the desired density reduction. The presence of high Al contents reduces the density of the steel and significantly improves its corrosion and oxidation resistance. At the same time, Al increases the tensile strength at these levels. However, excessive contents of Al 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 Al, therefore, the Al content is limited to a maximum of 11 wt .-%. An optimally balanced ratio of reduced density and processability arises when 8-11 wt.% Al, in particular at least 9 wt.% Al, are present in the steel according to the invention.

The C content in steel according to the invention is limited to at most 0.1% by weight, in particular 0.07% by weight, low C contents of less than 0.05%, in particular 0.01% by weight, or less, especially cheap. Above 0.1 wt.% C contents, the formation of undesirable brittle kappa carbides ("κ carbides") on the Grain limits and a consequent reduction in hot and cold workability cause. In practice, it has proven expedient in this regard to adjust the C content of the steel according to the invention in the range of up to 0.05% by weight, with a steel according to the invention typically containing up to 0.008% by weight.

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 early in the processing of generic steels during hot processing at high temperatures on the grain boundaries and cause embrittlement of the material. By minimizing the C content according to the invention and by adding carbide-forming alloying elements in accordance with the invention, the lowest possible C content is set.

It has proven to be particularly effective with regard to the desired ductility of the steel according to the invention that the steel according to the invention comprises at least one element from the group of rare earth metals in amounts of 0.02-0.2% by weight, in particular up to 0.15 Wt .-%, wherein the rare earth metal content is typically at least 0.03 wt .-%. Cerium and lanthanum are comparably inexpensive and available in sufficient quantities. The presence of rare earth metals contributes to improved oxidation resistance and strength of a flat steel product according to the invention and acts desulfurizing as well as deoxidizing. The positive effects of rare earth metals can be particularly targeted in the steel according to the invention, when the contents of rare earth metals are at least 0.03 wt .-%, wherein in the range of 0.06 - 0.12 wt .-%, in particular 0.06 - 0.10 wt .-%, lying rare earth

Contents allow a particularly reliable production of cold rolled steel flat products according to the invention.

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 of 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.

Ti, Nb, V, Zr, W and Mo can each be added individually or in different combinations to the steel according to the invention as carbide formers in order to bind off the existing C content. In addition, the carbides formed by the addition of one or more of the elements Ti, Nb, V, Zr, W, Mo additionally contribute to increasing the strength of the steel according to the invention.

For this purpose, Ti and Nb in amounts of up to 0.3 wt .-%, in particular in each case up to 0.1 wt .-%, V, W and Zr in amounts of up to 1 wt .-%, in particular in each case up to 0.5% by weight, and Mo in contents of up to 1% by weight in the steel according to the invention.

Mo also contributes to increasing the tensile strength, creep resistance and fatigue strength of a flat steel product of the present invention. In addition, 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.

By adding Mn in amounts of up to 6% by weight, in particular up to 3% by weight or up to 1% 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.

Si in amounts of up to 1 wt .-%, in particular up to 0.5 wt .-%, supported during the melting also the deoxidation and increases the strength and corrosion resistance of the steel according to the invention. At too high a content, however, the presence of Si reduces the ductility of the steel and its weldability.

Also by the addition of Cr in amounts of up to 3 wt .-% existing carbon in the invention steel can be bonded to carbides. simultaneously 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 order to avoid an increase in the recrystallization temperature, the Co content of the steel according to the invention is limited to max. 1% by weight, preferably max. 0.5% 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.

The addition of B can also lead to the formation of a fine, the deformability of the steel according to the invention favoring structure. However, excessive contents of B may impair cold workability and oxidation resistance. Therefore, the B content of the steel of the present invention is limited to 0.05% by weight, especially up to 0.01% 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 is limited to at most 1 wt .-%.

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

Due to the manufacturing process, oxygen is taken up in the steel according to the invention and forms precipitates with the rare earth metals present in the strip. If the rare earth metal is Ce, then cerium oxide precipitates are present in the steel flat product produced according to the invention.

erfüllt, für das bevorzugt gilt $$\mathrm{0,6}\le \left(\%\mathrm{Ce}+\%\mathrm{La}\right)/\%\mathrm{O}\le \mathrm{0,7},$$

mit %Ce = jeweiliger Cer-Gehalt, %La = jeweiliger Lanthan-Gehalt und %O jeweiliger Sauerstoff-Gehalt des Stahls, jeweils angegeben in Atom-%. Diese Oxide haben einen Durchmesser kleiner 5 µm.According to the invention, Ce or La are used as rare earth metals, the atomic ratio of the contents of Ce, La and O _{2 being} the condition $$0.5\le \left(\%\mathrm{Ce}+\%\mathrm{La}\right)/\%\mathrm{O}\le 0.8.$$

fulfilled, for the preferred applies $$0.6\le \left(\%\mathrm{Ce}+\%\mathrm{La}\right)/\%\mathrm{O}\le 0.7.$$

with% Ce = respective cerium content,% La = respective lanthanum content and% O respective oxygen content of the steel, each in atomic%. These oxides have a diameter less than 5 μm .

• 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.

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.
• Pouring the molten steel to 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 final casting can be done by using known per se for this purpose conventional casting facilities. 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 die 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 820 - 1000 °C, insbesondere 850 - 1000 °C, eingestellt werden. Bei praktischen Versuchen haben sich oberhalb von 920 °C liegende Warmwalzendtemperaturen als besonders günstig herausgestellt.
• Im ungeglühten Warmband ist eine mittlere Ferritkornlänge im Bandkern zu finden, die in Bandrichtung gemessen größer 100 µm ist.
• Das erhaltene Warmband wird zu einem Coil gehaspelt, wobei die Haspeltemperatur bis zu 850 °C betragen kann, insbesondere 450 - 750 °C beträgt.
• 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, 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 also possible to show that steels according to the invention can be cast into blocks which are then rolled out into slabs by pre-blocking become.

• 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. In the case where the precursor is a slab, the duration over which the preheating takes place is, for example, 120-240 minutes.
• The precursor is, if appropriate after the optional heating carried out to the preheating temperature, hot rolled into a hot strip, the rolling end temperature more than 820 ° C, in particular more than 850 ° C, and in practice hot rolling end temperatures of 820 - 1000 ° C, in particular 850 - 1000 ° C. In practical experiments, hot rolling end temperatures above 920 ° C. have proven to be particularly favorable.
• In unannealed hot strip is a middle Ferritkornlänge available on the tape core, the measured in band direction is greater than 100 μ m.
• The resulting hot strip is coiled into a coil, wherein the coiler temperature can be up to 850 ° C, in particular 450 - 750 ° C.
• 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, in particular 700 - 900 ° C amounting annealing temperature. Annealing temperatures of about 850 ° C, in particular 850 ° C +/- 20 ° C, have proven to be particularly practical. The annealing times provided for this purpose typically amount to 1-50 h in the case of this annealing, which is usually carried out as a bell annealing.

As a result of the annealing carried out in the predetermined temperature range according to the invention, the hot-rolled strip can be cold-rolled despite its high Al contents, without severe edge tears or even ribbon tears occurring. The hot strip annealing serves to produce a sufficiently recrystallized recovered core band area, lowering the cold rolling resistance and increasing the maximum achievable 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.

• 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 zwei- oder mehrstufig erfolgen, wobei der Kaltwalzgrad mindestens 30 % betragen muss, insbesondere mindestens 40 % beträgt. Kaltwalzgrade von mehr als 40 % haben sich als besonders vorteilhaft herausgestellt. Kaltwalzgrade von mindestens 30 %, bevorzugt mehr als 40 %, werden benötigt, um Versetzungen in ausreichender Zahl in das Material einzubringen. Diese Versetzungsdichte ist die treibende Kraft für die nach dem Kaltwalzen durchgeführte rekristallisierende Schlussglühung, die die gewünschte rekristallisierte Mikrostruktur und Textur des fertigen erfindungsgemäßen Stahlflachprodukts einstellt.

The hot strip annealing causes a stronger recovery of the hot strip and together with the effects achieved by the presence of rare earth metal in the steel according to the invention a very good, safe cold rolling.

• If necessary, after annealing, pickling of the hot strip may be performed on the hot strip remove sticky residues.
• The annealed and optionally pickled hot strip is then cold rolled to a cold rolled flat steel product. The cold rolling can take place in one stage or in two or more stages, wherein the degree of cold rolling must be at least 30%, in particular at least 40%. Kaltwalzgrade of more than 40% have been found to be particularly advantageous. Kaltwalzgrade of at least 30%, preferably more than 40%, are required to introduce dislocations in sufficient numbers in the material. This dislocation density is the driving force for post-cold recrystallization annealing which sets the desired recrystallized microstructure and texture of the finished flat steel product of the present invention.

• 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 vorteilhafter Textur aus.

In the case where the cold rolling is performed in two or more cold rolling stages, an intermediate annealing may be performed between the cold rolling stages.

• 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 an advantageous texture is formed.

• Optional kann das erhaltene Kaltband beispielsweise zur Verbesserung seiner Korrosionsbeständigkeit mit einer metallischen Schutzschicht belegt werden, die beispielsweise auf Al oder Zn basiert. Hierzu eignen sich die an sich bekannten Beschichtungsverfahren.

The respective annealing of the cold rolled strip can be found in the Continuously run annealing plants with annealing temperatures of 750 - 850 ° C for a typical duration of 1 - 20 minutes carried out, with annealing temperatures of more than 780 ° C, especially 800 - 850 ° C, and an annealing time of 2 - 5 min as particularly have proven practice. 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 based, for example, on Al or Zn. For this purpose, the coating methods known per se are suitable.

To test the invention, four inventive melts E1, E2, E3, E4 and three comparative melts V1, V2, V3, have been melted, whose compositions are given in Table 1.

The steel melts E1 - E3 have been cast into precursors in the form of blocks. The blocks have then been heated through a preheating period VD to a preheating temperature VT and converted into 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 in each case into a coil.

From the molten steel E4, a cast strip was produced as a precursor via a two-roll strip casting plant, 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 E4 has been coiled after hot rolling at a reel temperature HT to form a coil.

After reeling, the hot strips produced in each case, unless stated otherwise in Table 2, have been subjected to annealing in a tempering annealing plant at an annealing temperature GT over an annealing time GD.

The so annealed hot strips were cold rolled with a cold rolling grade KWG each to a cold rolled steel strip.

The resulting cold-rolled steel strips were then each subjected to a final annealing at a final annealing temperature SGT and a final annealing time SGD. The final annealing has been carried out either as a continuous annealing or as a bell annealing.

The respective preheating time VD, preheating temperature VT, hot rolling end temperature WET, coiling temperature HT, annealing temperature GT, annealing time GD, the respective cold rolling degree KWG, the respective final annealing temperature SGT, the respective closing annealing time SGD and the plant used for the final annealing ("hood" = annealing annealing plant). Conti "= continuous annealing system completed in a continuous cycle) are given in Table 2.

The mechanical properties "yield strength Rp", "tensile strength Rm", "elongation A50", "r-value r" determined in the rolling direction and "n-value n" determined in the direction of rolling are shown in Table 3 on the cold-rolled steel strips produced in this way.

It turns out that the cold-rolled steel strips produced from the steels E1-E4 produced according to the invention in accordance with the invention have yield strengths which are regularly greater than 400 MPa, in particular greater than 420 MPa and thereby reach values of 500 MPa and more, and tensile strengths which are regularly greater 500 MPa, in particular greater than 520 MPa, while achieving values of 600 MPa and more, and having elongation values A50 of at least 16%, always having r values of 1 or greater.

The cold-rolled steel strips produced from the steels according to the invention in a manner according to the invention contain, in addition to a Fe (Al) mixed-crystal matrix, a hardening precursor state. In common hot rolling parameters, rolling is carried out in the fully ferrite phase region and hot strip is obtained with a typical three-layer structure, the again characterized by recrystallized globulitic margins and the only recovered core region with stem crystals. However, owing to the Ce content and the manner of processing according to the present invention, a texture favorable for thermoformability which ensures r values of more than 1 is achieved here. At rare earth metal contents below 200 ppm, this effect does not occur, which can be used particularly reliably with rare-earth metal contents of at least 300 ppm. The hot strip annealing performed according to the invention reduces the dislocation density in the recovered area and facilitates subsequent cold rolling processing. Thus, the hot strips which are assembled according to the invention are not only hot-rollable in the full-ferrite phase region, but can be cold-rolled in spite of the existence of the Fe3Al intermetallic phase at room temperature, in contrast to the non-inventive rare earth-metal steels V1-V3. By means of suitable final annealing parameters, an extremely solid and density-reduced steel can be produced, which has high r-values and correspondingly optimized forming properties.

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. Accordingly, the steel strips produced according to the invention have, despite their high Al contents, a superior deep-drawing capability, without the need for elaborate alloying or process technology Measures are required. The cold-rolled steel strips produced from the steels V1, V2, V3 which are not composed according to the invention also contain a hardening precursor state in addition to a Fe (Al) mixed-crystal matrix. A hot strip annealing also facilitates cold rolling processing here. However, the cold-rolled steel strips not assembled according to the invention do not achieve the r-values required for a good deep drawing behavior. Although precursors produced from the non-inventive steel S3 are hot-rollable in the fully ferrite phase region, they can not be cold-cracked at room temperature due to the existence of the intermetallic phase Fe3Al. Table 1 stole C Si Mn P S Cr Mo Ni al Ce La Ce + La N Ti Nb V E1 0,008 0.09 0.15 0,003 0.005 0.01 0.00 0.01 8.20 0.073 0,040 .1130 0.0032 0.001 0,003 0,002 E2 0,007 0.09 0.25 0,003 0.005 0.40 0.01 0.02 8.30 0.048 0.019 0.0670 0.0510 0,003 0,002 0,002 E3 0,004 0.09 0.15 0,003 0,004 0.01 0.00 0.01 10.10 0.067 0.034 .1010 0.0048 0.001 0.001 0,003 E4 0.026 0.43 0.38 0.011 <0.001 1.16 0.06 0.35 6.7 0.0258 0.0152 0.0410 0.0009 0.22 0.12 0.009 V1 0,004 0.14 0.09 0,007 0,003 0.04 0.00 0.03 8.10 0.0004 0.0002 0.0006 0.0048 0,004 0,004 0.016 V2 0.005 0.11 0.11 0,004 0,003 0.03 0.01 0.03 8.20 0.0009 0.0005 0.0014 0.0018 0.001 0.001 0.005 V3 0,006 0.15 0.11 0,018 0,002 0.03 0.00 0.11 9.70 0.0010 0.0006 0.0015 0.0031 0,003 0,004 0,010 Data in wt .-%, balance iron and unavoidable impurities stole VT [° C] VD [h] WET [° C] HT [° C] GT [° C] GD [h] KWG [%] SGT [° C] SGD investment E1 1300 2 985 650 740 2 66 720 24 hours Hood E2 1300 2 960 800 740 2 66 820 3 min Konti E3 1300 2 1000 600 740 2 66 720 24 hours Hood E3 1300 2 1000 600 740 2 66 820 3 min Konti E4 - - 910 600 850 30 50 720 24 hours Hood V1 1300 2 930 800 740 2 66 720 24 hours Hood V1 1300 2 930 800 740 2 66 820 3 min Konti V2 1300 2 960 800 No annealing not cold-rolled V2 1300 2 960 800 740 2 66 820 3 min Konti V2 1300 2 960 800 850 2 66 820 3 min Konti V2 1300 2 960 800 740 2 80 820 3 min Konti V3 1300 2 980 800 740 2 not cold-rolled stole Rp [MPa] Rm [MPa] A50 [%] r *) n *) E1 422 527 22 1.21 0.14 E2 438 541 23 1.02 0.14 E3 529 627 18 1.05 0.12 E3 520 609 19 1.25 0.12 E4 553 634 16 1.13 0.12 V1 469 563 24 0.71 0.15 V1 466 562 22 0.72 0.15 V2 - V2 433 538 25 0.80 0.14 V2 428 533 21 0.85 0.15 V2 410 520 16 0.83 0.14 V3 - *) in the rolling direction

Claims (13)

1. Cold-rolled flat steel product for
   deep-drawing applications,

   - comprising a steel, which includes, in addition to iron and unavoidable impurities (in wt%),

   C: up to 0.1%,

   Al: 6.5 - 11%,

   P: up to 0.1%,

   S: up to 0.03%,

   N: up to 0.1%,

   at least one rare earth metal from the group "Ce, La" with the proviso that the content of rare earth metals amounts to 0.02 - 0.2%, whereby for the contents of Ce and La in relation to the content of oxygen taken up in the steel as a result of the production the following applies: $$0.5\le \left(\%\mathrm{Ce}+\%\mathrm{La}\right)/\%\mathrm{O}\le 0.8,$$

   

   with %Ce = respective cerium content, %La = respective lanthanum content and %O respective oxygen content of the steel, in each case given in at%, and optionally one or more elements from the group "Mn, Si, Nb, Ti, Mo, Cr, Zr, V, W, Co, Ni, B, Cu, Ca, N" with the proviso,

   Mn: up to 6 %,

   Si: up to 1 %,

   Nb: up to 0.3%,

   Ti: up to 0.3%,

   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%,

   - whereby the cold-rolled flat steel product has an r-value which is at least 1, and

   - the structure of the cold-rolled flat steel product contains 0 to 0.1 vol% of κ-carbides.
2. Flat steel product according to Claim 1, characterised in that its Al content amounts to more than 6.7 wt%.
3. Flat steel product according to Claim 2, characterised in that its Al content amounts to 8 - 11 wt%.
4. Flat steel product according to any one of the preceding claims, characterised in that its C content amounts to at most 0.05 wt%.
5. Flat steel product according to any one of the preceding claims, characterised in that its content of rare earth metals amounts to 0.06 - 0.12 wt%.
6. Method for the production of a cold-rolled flat steel product intended for deep-drawing applications comprising the following steps:

   - melting a molten steel, which includes, in addition to iron and unavoidable impurities (in wt%),

   C: up to 0.1 %,

   Al: 6.5 - 11%,

   P: up to 0.1 %,

   S: up to 0.03 %,

   N: up to 0.1 %

   at least one rare earth metal from the group "Ce, La" with the proviso that the content of rare earth metals amounts to 0.02 - 0.2 %, whereby for the content of Ce and La in relation to the content of oxygen taken up in the steel as a result of the production the following applies: $$0.5\le \left(\%\mathrm{Ce}+\%\mathrm{La}\right)/\%\mathrm{O}\le 0.8,$$

   

   with %Ce = respective cerium content, %La = respective lanthanum content and %O respective oxygen content of the steel, in each case given in at%, and optionally one or more elements from the group "Mn, Si, Nb, Ti, Mo, Cr, Zr, V, W, Co, Ni, B, Cu, Ca, N" with the proviso,

   Mn: up to 6%,

   Si: up to 1%,

   Nb: up to 0.3%,

   Ti: up to 0.3%,

   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%,

   - casting the molten steel into a precursor;

   - optional warming or holding of the precursor at a preheating temperature amounting to 1000 - 1300 °C;

   - hot rolling the precursor into a hot-rolled strip, whereby the hot rolling end temperature amounts to 820-1000 °C;

   - coiling of the hot-rolled strip into a coil, whereby the coiling temperature is in the range of the ambient temperature up to 850 °C;

   - annealing of the hot-rolled strip at an annealing temperature greater than 650 °C and up to 1200 °C over an annealing time of 1 - 50 h;

   - optional pickling of the hot-rolled strip;

   - cold rolling of the annealed and optionally pickled hot-rolled strip into a cold-rolled flat steel product having a cold-rolling degree that amounts to at least 30%;

   - Final annealing of the cold-rolled flat steel product at a final annealing temperature of 650 - 850 °C.
7. Method according to Claim 6, characterised in that the precursor is a cast strip.
8. Method according to any one of Claims 6 or 7, characterised in that the annealing temperature during annealing of the hot-rolled strip amounts to at least 700 °C.
9. Method according to any one of Claims 6 to 8, characterised in that the cold-rolling degree amounts to at least 40%.
10. Method according to any one of Claims 6 to 9, characterised in that the cold rolling is carried out in two or more rolling stages and between the stages of cold rolling an annealing of the cold-rolled flat steel product is performed.
11. Method according to any one of Claims 6 to 10, characterised in that the respective annealing of the cold-rolled flat steel product is carried out as a continuous annealing at an annealing temperature of 750 - 850 °C and an annealing time of 1-20 min.
12. Method according to any one of Claims 6 to 10, characterised in that the respective annealing of the cold-rolled flat steel product is carried out as a bell annealing at an annealing temperature of 700 - 800 °C and an annealing time of 1 - 30 h.
13. Method according to any one of Claims 6 to 12, characterised in that the hot-rolled strip coiling temperature amounts to 450 - 750 °C.