EP2964792B1 - Method for producing a cold-rolled flat steel product for deep-drawing and ironing applications, flat steel product and use of such a flat steel product - Google Patents

Description

The invention relates to a method for producing a cold-rolled, up to 0.5 mm thick flat steel product for deep-drawing and ironing applications. Moreover, the invention relates to a flat steel product produced according to such a method and to an advantageous use of a corresponding flat steel product.

Processes of the type in question are carried out on so-called "casting-rolling plants", abbreviated "GWA" or "CSP", in which the casting of the steel into a strand and the subsequent rolling processes in hot strip production are coordinated in that a continuous sequence of casting and rolling operations is possible. In this way, the costs incurred in conventional slab production for reheating and rough rolling can be avoided.

In cast-rolling plants, the steel is poured into a continuously drawn strand, from which thin slabs are then separated in-line, which are then hot-rolled in-line into hot-rolled strip. The Experience gained in the operation of cast-rolling plants and the advantages of casting-rolling are, for example, in W. Bald et al. "Innovative Technology for Banding", Stahl and Eisen 119 (1999) No. 9, pages 77-85 , or C. Hendricks et al. "Commissioning and first results of the casting rolling mill of Thyssen Krupp Stahl AG", Stahl and Eisen 120 (2000) Nr. 2, pages 61 - 68 , documented. Hot-rolled strip with hot strip thicknesses of less than 3 mm can be produced with today available cast-rolling plants.

Despite the process advantages offered by conventional cast-rolling plants, it has not been possible with the requisite reliability since the large-scale introduction of such plants, steels which have sufficient isotropy of their deformation properties for thermoforming and ironing applications on thin slab continuous casting or the associated casting-rolling plants to produce. It was found that standard thin slabs cast from aluminum-killed steels and hot-rolled strips made from them are not suitable for products with the highest levels of purity and surface requirements. Hot strip, which is intended in particular for the production of tinplate with a typical thickness of at most 0.5 mm, in particular at most 0.251 mm, therefore, could not be produced in the past on a casting-rolling plant. Tinplates of this thickness are needed for example for the production of beverage cans or the like. The situation is even more critical if primary material for up to 0.1 mm, in particular up to 0.06 mm, thin packaging steel is to be produced via a cast-rolling plant.

The reasons for the problems of producing very thin thermoforming and ironing applications of cold rolled steel flat products over a GWA are well known. In thin slab continuous casting or strip casting of Al-killed steels, with Al contents typically in the range of 0.010 - 0.060 wt%, to avoid clogging, the dip tubes required for potting are provided by alumina inclusions ("clogging"). ) a calcium treatment of molten steel in the steelworks required. In this process, liquid calcium aluminates with contents of approximately 50% CaO and 50% Al ₂ O ₃ must be reproducibly produced in the molten steel.

If this inclusion composition is not sufficiently accurately struck and there is a significant shortage or excess of CaO in the non-metallic inclusions or spinel inclusions (with MgO content), continuous clogging with enhanced bath level fluctuations occurs in continuous casting despite Ca treatment the mold on. Such a situation results in the infusion of foundry slag into the cast strand which causes a generally deteriorated degree of purity and increased shell defects on the strand surface. As a result, insufficient adjustment of the CaO and Al ₂ O ₃ inclusions in the production of hot strip via a caster-rolling plant, therefore, results in deterioration of the internal and surface condition of the thin slabs separated from the cast strand and thereby , from each of these thin slabs hot-rolled hot strip. The same problem arises in strip casting plants, in which the molten steel is cast into cast strip and then rolled in-line to form a hot strip.

In thin slab continuous casting or strip casting, it is therefore important to achieve a very good non-metallic degree of purity already in secondary metallurgy. Unlike conventional slab casting, the inclusions (oxides, sulfides) contained in the cast molten steel can neither rise during continuous casting nor strip casting due to the significantly higher casting speeds in the mold and can be deposited in the foundry slag. Unlike conventional alumina inclusions, the calcium aluminate inclusions resulting from the calcium-treated melts and remaining in the slab in the slab or thin slab are not comminuted during hot rolling, but retain their size. The same applies to strip casting. Macroscopic Ca-aluminate inclusions can therefore, for example, cause shell defects on the product surface during cold-rolling or forming processes or, in the case of very thin end material, holes in the rolled material.

Against this background is in the WO 2011/012242 A1 a method for producing a steel strip or sheet from a ULC steel has been proposed, in which a molten steel is cast into a slab or a cast strip containing (in% by weight) ≤ 0.003% C, 0.05-0, 35% Mn, <0.025% P, <0.020% S, <0.004% Si, ≤ 0.002 Al, <0.004% N, in total ≤ 0.1% Cr, Cu, Ni, Sn and Mo, ≤ 0.004% N, each ≤ 0.005% Nb, Ti, Zr and V, ≤ 0.0030% B and balance Fe and unavoidable impurities.

In order to produce an alloy of this purity, the steel melt according to the known method after its melting is first subjected to a vacuum treatment and then a ladle furnace treatment. The purpose of the ladle furnace treatment is in particular the setting of a minimized oxygen and aluminum content in each obtained after casting thin slab or cast strip obtained after casting. Here, the oxygen activity of the molten steel for continuous casting or strip casting should be as low as possible in order to avoid pore formation in the cast product and casting defects. The adjustment of the oxygen content or the oxygen activity is carried out by a targeted addition of aluminum in an amount that is determined depending on the result of monitoring the current oxygen activity of the melt according to the target that the oxygen content of the melt at the end of the pans Treatment is below 100 ppm.

Apart from the high technical requirements involved in permanently monitoring the oxygen content of a melt, practical experience in producing very thin cold-rolled steel flat products for deep-drawing and ironing applications ("tinplate") can be expected beyond the prior art discussed above In addition, measures are required in order to produce a cast-rolling plant or a Strip casting plant to ensure the very good non-metallic purity of the molten steel for a flat steel product with optimal thermoforming and ironing suitability.

Therefore, the object to be solved by the invention was to provide a method by which can be produced in a reliable manner from thin slabs or cast strip a thin, maximum 0.5 mm thick cold-rolled steel flat product, which also meets the highest demands on his thermoforming and Ironing suitability is fair. In addition, a correspondingly procured flat steel product and a particularly expedient use of such a flat steel product should be specified.

With regard to the method, this object has been achieved according to the invention, that in the production of cold-rolled, up to 0.5 mm thick flat steel products for thermoforming and ironing the specified in claim 1 operations are passed.

With respect to the flat steel product, the above-mentioned object has accordingly been achieved in that such a flat steel product is produced in accordance with the invention.

A flat steel product produced in accordance with this invention is particularly suitable for thermoforming applications in which the tip heights determined according to ISO 11531 at a deep drawing ratio β of 1.8 and a cup diameter of 33 mm are in the range of 0.2 - 0.7 mm. Such conditions are particularly in so-called "twist-off closures" (cam latches) and "DRD Cans", but also in general before thin-walled beverage cans.

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

According to the process of the invention for producing a cold-rolled up to 0.5 mm thick flat steel product for deep-drawing and ironing applications, a steel melt is produced in step a) which contains (in% by weight) up to 0.008% C, up to 0.005% Al , up to 0.043% Si, 0.15-0.5% Mn, up to 0.02% P, up to 0.03% S, up to 0.020% N, and each optionally up to 0.03% Ti and up to to 0.03% Nb and balance iron and unavoidable impurities, with unavoidable impurities containing up to 0.08% Cr, up to 0.08% Ni, up to 0.08% Cu, up to 0.02 % Sn, up to 0.01% Mo, up to 0.0020% V, up to 0.007% B, up to 0.05% Co and up to 0.0060% Ca. In practice, the S-contents of the melt according to the invention are typically in the range of 0.005-0.03 wt%. At the same time, when the invention is practiced in practice, the Al content of the melt is typically at least 0.001% by weight. With regard to the desired result according to the invention, optimum Al contents of the molten steel ready for casting are in the range of 0.001-0.002% by weight.

In order to ensure, on the one hand, good castability and, on the other hand, optimum purity of the strand or strip to be cast from this molten steel, the molten steel is subjected to a secondary metallurgical treatment, which, in addition to a conventional vacuum treatment, results in a ladle furnace. Treatment includes. In the ladle furnace treatment, the molten steel is held according to the invention under a slag, for whose Mn content% Mn and Fe content% Fe is% Mn +% Fe <15 wt .-%, in particular <9 wt .-%.

The measures provided for in the production of the molten steel according to the invention are based on the finding that the ladle slag must be kept well fluid for a good absorption of non-metallic inclusions in the melt. This can not be achieved by a conventional vacuum treatment in a RH or DH plant. In the case of the ladle furnace treatment according to the invention, however, the ladle slag can be intensively liquefied via the heating with electrodes. As a result, it is very well suited to absorb non-metallic inclusions rising to the bath surface and thus further improve the degree of purity of the molten steel after the vacuum treatment.

Of particular importance for the success of the method according to the invention is also that in the vacuum and the subsequent ladle furnace treatment a slag is kept in contact with the molten steel, in which a certain oxygen potential is set before the vacuum treatment. This Oxygen potential "a _O -Slag" of the ladle slag must be matched to the required oxygen activity "a _O -Melt" of the molten steel. If the oxygen activity a ₀ -Slag is too high, there is the unfavorable situation that due to the tendency to equilibrium between slag and molten steel, too much oxygen is transported from the slag into the molten steel. This exchange would result in an excessively high oxygen activity a _O -Melt of z. B. 120 ppm, in particular 100 ppm, result, so that increasingly formed via reaction products with the molten steel alumina or alumina manganese oxide inclusions. As a result, the degree of purity of the molten steel would be worsened as a result. In addition, the problem arises when an excessive oxygen uptake of the melt that it is the optimal oxygen activity a _O -Melt can not be set without violating the guidelines "Ensuring lowest levels of dissolved Al _sol", ie target levels for Al _sol of particular less as 0.0020 wt .-%, on the one hand and "inducing a sufficiently partially calmed state without pore formation during continuous casting" on the other hand to violate. This can be explained from the fact that for the adjustment of a recognized as optimal target area of the oxygen activity a _O -Melt from 40 - would be as high 60 ppm required Al addition amount such that in the molten steel is too high Al content and consequently, a unfavorable non-metallic degree of purity would result. Through this, the deep drawing and ironing suitability of the flat steel product to be produced would be impaired in impermissible, the requirements of modern forming processes, such as the DWI process, no longer sufficient.

As an indirect measure of the oxygen activity a _O -Slag the Fe content% Fe and Mn content% Mn of the ladle slag can be used. By according to the invention the sum% Fe +% Mn of the Fe and Mn contents of the ladle slag is set to less than 15% by weight, in particular <9% by weight, it is ensured that the oxygen activity "a _O -Melt" can be set in the optimum range of 40-60 ppm, without having to continuously perform a measurement of the oxygen content of the slag. This is especially true if the Fe content% Fe of the ladle slag is:% Fe <10 wt .-%, in particular% Fe <6 wt .-%.

The molten steel produced in the manner according to the invention is continuously cast into a strand in step b), from which one or more thin slabs are then separated in a conventional manner, which are then fed to further processing in a continuous process sequence. Alternatively, the melt produced in accordance with the invention can be cast into a cast strip, for example by means of a two-roll strip casting device or according to the DSC process.

The thus obtained cast precursor in the form of a thin slab or a cast strip is then hot rolled in step c) in a conventional manner into a hot strip having a thickness of less than 2.5 mm, in particular less than 2.3 mm. has, with hot strip thicknesses of less than 2 mm prove to be particularly favorable in terms of further processing. If necessary is, the respective precursor can be brought to a temperature of 1000 - 1250 ° C optimal for the further process sequence before hot rolling. This can be done for example by a targeted cooling of the case in this case for hot rolling still hot respectively cast precursor or by a targeted heating of the excessively cooled in this case precursor. Optionally, it may also be expedient to subject the respective cast precursor to a heat treatment in order to equalize its temperature distribution before the hot rolling begins. The hot rolling itself is optimally started with a hot rolling start temperature which is in the range of 950 - 1200 ° C and finished with a hot rolling end temperature which is in the range of 800 - 950 ° C.

After hot rolling, the hot strip obtained is conventionally wound into a coil at a coiler temperature typically of 500 - 750 ° C.

After cooling in the coil, the hot strip is cold rolled to the thickness of up to 0.5 mm, in particular at most 0.26 mm thick, cold-rolled steel flat product. Cold rolling may be preceded by a surface treatment which mechanically or chemically removes scale and other contaminants adhering to the hot strip in a conventional manner.

The cold rolling itself can be carried out in one or more stages. In a multi-stage cold rolling a recrystallizing intermediate annealing can be carried out between the cold rolling steps.

In the case of two-stage cold rolling, the first stage of cold rolling should be carried out with a degree of deformation of more than 85%, in particular more than 90%, and the second stage of cold rolling with a degree of deformation of 0.4-50%, in particular at least 1% , where degrees of deformation of 4 - 42% are particularly practical.

Finally, the resulting cold-rolled steel flat product can be provided with a protective coating for protection against corrosive attacks. For this purpose, the cold-rolled steel flat product according to the invention can be coated with a metallic protective layer. For this purpose, it can for example undergo an electrolytic tinning.

With the method according to the invention can thus be avoided by the purity grade-based disadvantages of the production of particularly thin, designed for deep drawing and ironing cold rolled steel flat products via thin slab continuous casting and other near-net casting or casting rolling process, characterized in that the flat steel products based on a Alloy concept with minimized Al levels are generated. At such low Al contents can be dispensed with a Ca treatment of the melt, so that the formation of the deformation properties interfering calcium aluminates is excluded.

Accordingly, flat steel products produced according to the invention fulfill the highest demands on their formability. So they are suitable for all forming applications, for one according to ISO 11531 certain tension of less than 0.86 mm is required. In particular, flat steel products according to the invention are suitable for uniformity-critical forming applications and sophisticated deep-drawing and ironing applications in which the di-minidity determined according to ISO 11531 is less than 0.7 mm.

Owing to their particularly good deformability achieved by the method according to the invention, flat steel products produced according to the invention are particularly suitable for the production of packaging for loose goods. These packages are typically cans and similar containers used for the packaging of food, beverages, pet food and other pourable, free-flowing or pourable goods and products. These goods and products also include, for example, general chemical or biological products, such as gases or aerosols. Likewise, flat steel products according to the invention can be used for the production of closures for such containers, bottle caps for closing bottles or spray cans.

On the basis of the method according to the invention for producing the molten steel, a very good non-metallic purity of the hot strip is achieved, which is the prerequisite for an optimally procured cold-rolled flat steel product of the type according to the invention. For example, in the manner according to the invention produced, for example, 0.13 mm thick tinplate for the particularly purity critical use purpose "production of twist-off closures" in tests by eddy current and Magnetic powder only a minimum number of inclusions with a diameter of more than 70 microns. The thus obtained flat steel product material certainly met the stringent purity requirements for this critical use. By contrast, flat steel products made by comparison with conventional Al-killed LC steel with an Al content of 0.033 wt.% Had an unsuitable purity for tinplate.

At the same time, the comparative investigations showed that during the casting of an Al-free ULC molten steel produced according to the invention, clogging effects were only weakly pronounced in thin slab continuous casting, so that not only the degree of purity, but also the surface quality of the hot strips cast from the thin slabs meet the high requirements are sufficient, which are made of suitable for the production of tinplate hot strips.

As a result of the metallurgical treatment according to the invention, the composition of the oxide limestone shorts remaining in the molten steel (size range <10 μm) changes compared with aluminum tempering and production via a conventional continuous casting plant. The inventively achieved minimization of the proportion of hard Al ₂ O ₃ particles in the structure of a flat steel product according to the invention results in the production of deep drawn or ironed products from a cold rolled flat steel product according to the invention not only to an optimal forming behavior of the material, but also to a significant increase in the service life of each forming tool used. Due to the low Al content, the nitrogen in the steel is not bound as AIN, but is essentially interstitially dissolved. This results in a significantly higher solidification potential.

To demonstrate the effect of the invention, three experiments E1, E2, E3 have been carried out, in which the respectively cast to thin slab melt has been treated according to the invention secondary metallurgy. For comparison, three further experiments V1, V2, V3 were performed, in which the ladle furnace treatment according to the invention has been omitted in each case.

The composition of the respective processed molten steel, the parameters taken into account during hot and cold rolling and the parameters essential for thermoforming are shown in Table 1.

In addition, in Table 1, an evaluation of the internal degree of purity of the examined samples is recorded. The degree of purity is determined before the refining process, which may for example be in the form of the order of a metallic coating, such as tinning or chrome plating, based on the number of non-metallic inclusions with an expansion> 70 microns by electromagnetic measurement method over the entire volume. The classification was based on the number of inclusions per m ² according to the following conditions: rating ID Number of inclusions per m ² Very well + <0.5 satisfactory 0 0.6 - 3.0 Insufficient - > 3.0

For example, "very good" rated samples can be used without restriction for all packaging steel applications. "Satisfactory" rated samples can be used for certain non-critical packaging steel applications. In general, samples rated "Inadequate" are not suitable for packaging steel applications.

In each of experiments E1 - E3 and V1 - V3, the hot rolled strips obtained after pickling had undergone a pickling and were then cold rolled in two stages. After a first cold rolling, the flat steel products were recrystallized in a continuous furnace at a temperature of 700 ° C. in each case and then cold-rolled to a final thickness of 0.13 mm with a cold rolling degree of 38%. Finally, the so cold-rolled steel flat products have been electrolytically tinned. Table 1 E1 E2 E3 v1 V2 V3 C [Ppm] 34 34 34 30 30 30 N [Ppm] 23 23 23 20 20 20 Mn [Ppm] 2400 2400 2400 2500 2500 2500 al [Ppm] 20 20 20 20 20 20 Al sol. [Ppm] <10 <10 <10 <10 <10 <10 Chemical Analysis Si [Ppm] 80 80 80 100 100 100 O [Ppm] 50 50 50 55 55 55 Cr [Ppm] 240 240 240 210 210 210 Ni [Ppm] 130 130 130 140 140 140 Cu [Ppm] 130 130 130 130 130 130 P [Ppm] 70 70 70 80 80 80 S [Ppm] 62 62 62 78 78 78 hot rolling hot strip [Mm] 1.8 2.0 1.6 1.6 1.8 2.0 final temperature [° C] 871 880 861 870 880 865 coiling temperature [° C] 623 584 584 601 603 590 cold strip final thickness [Mm] 0.13 0.13 0.13 0.13 0.13 0.13 characteristics Yield strength (200 ° C, 20 min) [MPa] 570 580 565 569 575 580 Tensile strength (200 ° C, 20 min) [MPa] 580 585 572 579 580 588 Tip height (ß = 1.8, cup = 33 mm) [Mm] <0.86 <0.86 <0.86 <0.86 <0.86 <0.86 Inner purity: + 0 + - - -

Claims (15)

1. Method for producing a cold-rolled flat steel product of up to 0.5 mm in thickness for deep-drawing and ironing applications, comprising the following working steps:

   a) producing a metal melt which (in wt%) comprises up to 0.008 % C, up to 0.005 % Al, up to 0.043 % Si, 0.15 - 0.5 % Mn, up to 0.02 % P, up to 0.03 % S, up to 0.020 % N and in each case optionally up to 0.03 % Ti and up to 0.03 % Nb and, as a remainder, iron and unavoidable impurities, the contents of which are to be attributed to up to 0.08 % Cr, up to 0.08 % Ni, up to 0.08 % Cu, up to 0.02 % Sn, up to 0.01 % Mo, up to 0.0020 % V, up to 0.007 % B, up to 0.05 % Co and up to 0.0060 % Ca, wherein the steel melt is, with the omission of a Ca treatment, subjected to a secondary metallurgical treatment which, in addition to a vacuum treatment, comprises a ladle furnace treatment and during which the steel melt to be treated is kept under a slag, the Mn content %Mn and Fe content %Fe are defined by %Mn + %Fe < 15 wt%;

   b) continuously casting the metal melt to form a strand, and severing a thin slab from the strand, or to form a cast strip;

   c) hot-rolling the thin slab or the cast strip to form a hot strip with a thickness of less than 2.5 mm;

   d) winding the hot strip to form a coil;

   e) cold-rolling the hot strip to form the cold-rolled flat steel product of up to 0.5 mm in thickness.
2. Method according to Claim 1, characterized in that the Al content of the steel melt amounts to at most 0.002 wt%.
3. Method according to one of the preceding claims, characterized in that the Fe content %Fe of the slag under which the steel melt is kept during the ladle furnace treatment amounts to less than 10 wt%.
4. Method according to one of the preceding claims, characterized in that the oxygen content of the steel melt at the end of the ladle furnace treatment lies below 100 ppm.
5. Method according to one of the preceding claims, characterized in that the thin slab is, before the hot rolling, brought to a temperature of 1000 - 1250°C.
6. Method according to one of the preceding claims, characterized in that the hot-rolling start temperature of the thin slab at the start of the hot-rolling process is 950 - 1200 °C.
7. Method according to one of the preceding claims, characterized in that the hot-rolling end temperature of the hot strip at the end of the hot-rolling process is 800 - 950 °C.
8. Method according to one of the preceding claims, characterized in that the hot strip is wound at a winding temperature of 500 - 750°C.
9. Method according to one of the preceding claims, characterized in that the thickness of the cold-rolled flat steel product amounts to less than 0.26 mm.
10. Method according to one of the preceding claims, characterized in that the cold rolling is performed in at least two stages, and the cold-rolled flat steel product is subjected to recrystallization annealing between the stages of the cold-rolling process.
11. Method according to Claim 10, characterized in that the degree of deformation achieved by way of the first stage of the cold-rolling process is greater than 85%, and the degree of deformation achieved by way of the second stage of the cold-rolling process amounts to 0.4 - 50%.
12. Method according to one of the preceding claims, characterized in that the cold-rolled flat steel product is subjected to electrolytic tin plating.
13. Flat steel product produced through the use of the method configured according to one of the preceding claims.
14. Use of a flat steel product according to Claim 13 for the production of cans for foodstuffs, animal feed, beverages or other filling materials, such as chemical or biological products, or for the production of aerosol cans, closures, crown caps or spray cans.
15. Use of a flat steel product according to Claim 13 for deformation applications in which the earing, as defined in accordance with ISO 11531, is < 0.86 mm.