EP2163659B1 - Stainless steel, cold strip made of same and method for producing cold strip from same - Google Patents

Description

The invention relates to a cold-rolled flat steel product made of a stainless steel, such as a steel strip or steel sheet, and a method for producing such a flat steel product.

A proven in practice stainless steel is known under the name X5CrNi18-10 and is listed under the EN material number 1.4301. This material is a relatively soft, non-ferromagnetic austenitic steel, from which, for example, pots, cutlery, sinks, parts of household appliances, so-called "white goods", such as washing machines, clothes dryers, dishwashers, etc., are manufactured. It contains according to DIN EN 10088 in addition to iron and unavoidable impurities typically (in wt .-%) up to 0.07% C, 17.0 - 19.5% Cr, 8.0 - 10.5% Ni, max. 1.0% Si, max. 2.0% Mn, max. 0.045% P, max. 0.015% S and max. 0.110% N. The high nickel content ensures the austenitic structure of the steel, which is a prerequisite for its good formability. The high Cr content ensures the good corrosion resistance of this steel. A disadvantage of the steel 1.4301, however, is that it can only be produced at a comparatively high cost, since its alloy components, in particular the high contents of nickel, require high prices.

Due to the high alloying costs of 1.4301 steel, there are numerous attempts to substitute for this material. The common goal of these efforts is to reduce the nickel content.

An example of such a development is in the EP 0 969 113 A1 described. The austenitic steel known from this publication has, in addition to iron and unavoidable impurities (in% by weight) 0, 01-0.08% C, 0.1-1% Si, 5-11% Mn, 15-17.5% Cr, 1 - 4% Ni, 1 - 4% Cu, 0.1 - 0.3% N, and further defined levels of sulfur, calcium, aluminum, phosphorus, boron and oxygen.

Another example of a steel of the type discussed here is from the JP 56 146862 known. This austenitic steel contains (in% by weight) up to 0.03% C, up to 0.5% Si, 2.2-3.0% Mn, 14-18% Cr, 6-9% Ni, bis at 0.03% N, 0.15-0.50% Mo, 1-3% Cu and balance iron and unavoidable impurities. Particular emphasis is placed on a good forming behavior, which is set by the controlled setting of the so-called MD30 value, which according to a in the JP 56 146862 calculated special formula.

The term "M _d30 " generally refers to the temperature at after a cold reduction of 30%, the conversion of austenite into martensite has gone to 50%. Above this temperature, conversely, a reduced conversion occurs (see Material Science Stahl, Volume 2, publisher: Verein Deutscher Eisenhüttenleute, 1985, Springer-Verlag Berlin Heidelberg New York Tokyo, Verlag Stahleisen mbH Dusseldorf, Chapter D 10.3.2).

In the European patent specification EP 1 431 408 B1 Further, a low Ni austenitic austenitic CrNiMnCu steel having the following composition has been proposed (in wt%): 0.03-0.064% C, 0.2-1.0% Si, 7.5-10 , 5% Mn, 14.0-16.0% Cr, 1.0-5.0% Ni, 0.04-0.25% N, 1.0-3.5% Cu, traces of molybdenum and as Remaining iron and unavoidable impurities. In order to obtain hot rolling capability, it is prescribed for the δ-ferrite content ("delta-ferrite content") that its according to a in the EP 1 431 408 B1 calculated formula is less than 8.5%. The steel produced in this way shows comparable mechanical properties to the well-known steel 1.4301.

Also from the EP 0 593 158 A1 is a stainless, belonging to the genus of the steel considered austenitic CrNiMnCuN steel known. In addition to iron and unavoidable impurities (in% by weight), this steel has <0.15% C, <1% Si, 6.4-8.0% Mn, 16.5-17.5% Cr, 2.50 - 5.0% Ni, <0.2% N and 2.0 - 3.0% Cu. In this steel, a good hot rolling ability, in particular the avoidance of edge cracks in hot rolling, has been achieved with at the same time acceptable mechanical properties and corrosion resistance.

In order to ensure this combination of properties, the Cr content of the steel is in each case adjusted so that it does not exceed 17.5% by weight.

From the EP 1 319 091 B1 a possibility for the cost-effective production of a predominantly made of Mn austenite steel strip or sheet is known, which has a relation to the prior art increased strength. For this purpose, a steel is melted which contains (in% by weight) at least the following alloy constituents: 15.00- 24.00% Cr, 5.00-12.00% Mn, 0.10-0.60% N, 0.01-0.2% C, max. 3.00% Al and / or Si, max. 0.07% P, max. 0.05% S, max. 0.5% Nb, max. 0.5% V, max. 3.0% Ni, max. 5.0% Mo, max. 2.0% Cu and the remainder iron and unavoidable impurities. Such a steel is in the casting gap formed between two rotating rollers of a two-roll caster to a thin strip with a thickness of max. Poured 10 mm. Meanwhile, the rolls or rolls are cooled to such an extent that the thin strip in the casting gap cools at a cooling rate of at least 200 K / s. The known method makes use of the generally known technique of a strip casting plant in this way by casting the steel in the casting gap formed between the rolls or rolls, for example a double roller casting machine, cooling it to such an extent that it cools there is a shift from a primary ferritic towards a primary austenitic solidification. This allows the dissolved in the melt nitrogen in the steel too austenite has a high solubility for nitrogen. Due to the intensive, with high cooling speed cooling is thereby ensured that in the solidifying melt possibly resulting nitrogen gas bubbles remain small and the pressure directed against them is large. This prevents outgassing of the high nitrogen contents in the course of solidification.

From the EP 1 352 982 B1 Finally, a cost-effectively producible stainless steel is known, which is insensitive to the formation of stress cracks even with conventional cold forming. In this steel, instead of the usually desired, single-phase, purely austenitic microstructure, a two-phase mixed microstructure is set in which by alloying Si and / or Mo and partially lowering the Ni content or by exchanging Ni with Cu, the austenite (A ) and ferrite (F) levels are set. The austenite is stabilized to such an extent that martensite formation occurring during deformation no longer leads to stress cracks. In wt .-% indicated is the chromium content of the EP 1 352 982 B1 known steel between 16 - 20%, the manganese content between 6 - 12%, the nickel content is less than or equal to 9.05% and the copper content is less than or equal to 3%. Nitrogen is added between 0.1 - 0.5%. The alloy is composed such that the t-factor (ratio of ferrite-forming elements to austenite-forming elements having respective pre-factors) is in a range of more than 1.3 to less than 1.8. At the same time, the MD30 temperature of the alloy must meet a certain condition.

From the article "Effects of Alloying Elements on Solidification Structures in Cr-Mn-Ni Austenitic Stainless Steels" Authors T. Oshima et al., Published in Tetsu To Hagane, Iron and Steel Institute of Japan, Tokyo, JP, ISSN: 0021-1575, Vol. 92, No. 6, January 1, 2006 , Pages 372/7, are finally known base compositions of five steels with Cr contents of 14%, 15% or 17%, Ni contents of 1.2%, 4%, 4.5% or 6.5%, Mn Contents of 10%, 8%, 3.5% or 4% and N contents of 0.13%, 0.05%, 0.12%, 0.10% or 0.05%. It is stated in the article that with increasing Cr or Mo content of the 8-ferrite content of slabs cast from the mentioned steels increases, whereas it decreases at elevated Ni or Cu contents. All references given in relation to the δ-ferrite content in the article refer to the cast state after solidification and before hot rolling.

Against the background of the prior art discussed above, the object of the invention was to provide a cold-rolled, inexpensive producible stainless steel flat product which, with good forming properties, has sufficient strength for a wide field of applications.

Likewise, a method for producing such a steel should be given.

With respect to the flat steel product, this object has been achieved according to the invention in that it has the features mentioned in claim 1. Advantageous embodiments of the flat steel product are in the on Claim 1 referred back claims.

The solution according to the invention of the above-mentioned object with regard to the method consists in that, during the production of a flat steel product, at least the working steps specified in claim 12 are run through. Advantageous embodiments of the method according to the invention are specified in the claims back to claim 12.

With the invention, a flat steel product produced from a stainless CrMnNiCu steel with higher Mn and Cu contents and low Ni content is available as a cost-effective alternative material to the 1.4301, which can advantageously be processed via strip casting to form a flat steel product.

The alloy constituents of the composite steel flat product according to the invention are selected so that its microstructure in the cold-rolled state has, in addition to austenite, a δ ferrite content ("delta ferrite content") of 8-13.5% by volume. This δ-ferrite content is dimensioned such that the flat steel product according to the invention, as a cold-rolled strip with good strength, has a corrosion resistance approaching the steel 1.4301. The mechanical properties of a cold-rolled flat steel product according to the invention, such as yield strength and tensile strength, are shifted towards higher values compared with steel 1.4301 and the elongation at break is shifted to low A80 values. The technological parameters for assessing the cold workability, such as the limit ratio and the dome height in the subsidence test, are in the lower Spread band range of the values determined for steel sheets produced from steel 1.4301.

Due to its special combination of properties, flat steel products according to the invention are therefore suitable as replacement for steel flat products produced from steel 1.4301 in the production of products falling into the field of "white goods" as well as for use in other applications in which steel sheets each have a distinct deep drawing and ironing shares are deformed to the respective product.

The steel from which a flat steel product according to the invention is made has (in% by weight): C: 0.05-0.14%, Si: 0.1-1.0%, Mn: 7,5 - 10,5%, Cr: > 17,5 - 22,0%, Ni: 1.0 - 4.0%, Cu: 1.0-3.0%, N: 0.03 - 0.2%, P: Max. 0.07%, S: Max. 0.01%, Not a word: Max. 0.5%, optionally one or more elements from the group "Ti, Nb, B, V, Al, Ca, As, Sn, Sb, Pb, Bi, H" with the following proviso Ti: Max. 0.02%, Nb: Max. 0.1%, B: Max. 0.004%, V: Max. 0.1%, al: 0.001-0.03%, Ca: 0.0005 - 0.003%, As: 0.003-0.015%, Sn: 0.003 - 0.01%, Pb: Max. 0.01%, H: Max. 0.0025%, and as the remainder Fe and unavoidable impurities.

Cr is contained primarily in order to improve the corrosion resistance in contents of more than 17.5% by weight to at most 22.0% by weight in the steel according to the invention. The requirement that in each case more than 17.5% by weight of Cr should be present in the steel according to the invention ensures that a corrosion resistance comparable to that of steel 1.4301 is achieved. This is achieved with particular certainty when the Cr content is at least 17.7% by weight, in particular at least 18.0% by weight. The results achieved by the invention occur in particular when the Cr content of the steel according to the invention is limited to 20 wt .-%.

C and N are strong Austenitbildner and also effectively increase the resistance to the formation of Umformmartensit in the processing of inventive steels. Therefore, the lower limit of the C content has been set to 0.05 wt%, and the lower limit of the N content has been set to 0.03 wt%.

By maintaining an upper limit of 0.14 wt .-% for the C content, the risk of chromium carbide formation in a Heat treatment, z. As in welding, and concomitant intercrystalline corrosion avoided.

N leads as an interstitial element to an increase in the yield strength and is therefore set to a maximum of 0.2 wt .-%. In order to ensure the best possible deformability, the N content is preferably limited to 0.12 wt .-%. Accordingly, the effect of nitrogen in a stainless steel according to the invention occurs, in particular, when its N content is at least 0.06% by weight, in particular from 0.06-0.10% by weight.

Si supports the formation of ferrite. Therefore, the Si content of a steel according to the invention is limited to max. 1 wt .-%, in particular 0.5 wt .-%, limited, wherein the undesirable effect of Si can be avoided in particular by the fact that the Si content of the steel according to the invention to max. 0.4 wt .-% is limited.

Mo alloy steel according to the invention is not selectively alloyed, since it supports on the one hand, the ferrite and on the other hand is expensive. Preferably, therefore, the Mo content is as low as possible. In particular, the Mo content can be lowered according to the invention to the extent that it is limited to ineffective quantities attributable to the production-related unavoidable impurities.

Ni is added to the steel according to the invention as austenite former, with a minimum content of 1% by weight being required in order to obtain the δ ferrite content ("delta ferrite content"). in a steel according to the invention to max. To ensure 25% in the hot strip and good forming properties, so that the desired, limited to a maximum of 15% delta ferrite cold strip invention is safely maintained. This effect is achieved particularly reliably if the Ni content is at least 1.5% by weight, in particular at least 2.0% by weight. By limiting the Ni content to at most 4 wt .-%, a significant reduction of the alloying agent costs compared to steel 1.4301 is achieved.

By adding the austenite formers Mn and Cu, the reduction of the Ni content is possible.

Copper has a austenite-stabilizing effect similar to that of nickel. Too high a copper content, however, may lead to the formation of copper-rich precipitates of reduced melting point, which could cause cracks, particularly during casting of the steel of the invention in a cast strip casting line or subsequent inline hot rolling. Therefore, the invention provides for an upper limit of 3% for copper. In order to ensure the effect of Cu in the steel according to the invention, a minimum Cu content of 1.5 wt .-%, in particular 2.0 wt .-% has proven to be favorable, which in practical experiments levels of 2.1 wt .-% and more have proven.

The austenite-forming action of Mn in a steel according to the invention occurs at Mn contents of at least 4% by weight. Out From an alloying, technical point of view, the Mn content is limited to max. 12 wt .-% limited, with an optimized effect of manganese is achieved in steel according to the invention, when the Mn content 7.5 - 10.5 wt .-%, is.

The contents of P and S are for P at max. 0.07 wt .-% and for S to max. 0.01 wt .-% limited in order to exclude the negative impact of these alloying elements on the deformability of a steel according to the invention as far as possible.

To set certain properties in a steel according to the invention, optionally contents of Ti, Nb, B, V, Al, Ca, As, Sn, Pb or H can be present.

Ti contents of up to 0.02 wt .-% are used both in the production of the flat steel product according to the invention on the continuous casting as well as on the so-called "strip casting" the prevention of cracks in the obtained band.

Nb contents of up to 0.1% by weight have a favorable effect on formability when produced both by continuous casting and by strip casting.

Boron may be added to steel in accordance with the invention in amounts of up to 0.004 wt% in the case of its strip-cast processing to counteract the risk of cracking. If the steel is cast in continuous casting, the presence of B contributes to the above Upper limit to avoid surface tears.

By adding Al at levels of 0.001-0.03% by weight, the degree of purity of the steel according to the invention can be improved. The same purpose is served by the presence of Ca at levels of from 0.0005 to 0.003% by weight.

By contents of As of 0.003 - 0.015 wt .-%, Sn of 0.003 - 0.01 wt .-%, Pb of up to 0.01 wt .-% and Bi of up to 0.01 wt .-% can at the processing of a steel according to the invention via strip casting, the risk of cracking can be minimized. In the case of continuous casting processing, these elements in the mentioned content ranges help to reduce the risk of occurrence of surface defects during hot rolling.

gilt
t kleiner als 1,3,
wobei mit %C der C-Gehalt, %N der N-Gehalt, %Si der Si-Gehalt, %Al der Al-Gehalt, %Mn der Mn-Gehalt, %Cr der Cr-Gehalt, %Ni der Ni-Gehalt, %Mo der Mo-Gehalt und %Cu der Cu-Gehalt der jeweiligen Stahlzusammensetzung bezeichnet sind. Dabei stellen sich die erfindungsgemäß angestrebten Eigenschaften dann besonders sicher ein, wenn t höchstens 1,2 beträgt.An optimum ratio of the austenite- and ferrite-forming alloy constituents with respect to the properties sought in particular in the cold-rolled state is then obtained, if for the factor $$\mathrm{t}=\frac{\%\mathrm{Cr}+\mathrm{2}\%\mathrm{Not\; a\; word}+\mathrm{1}.\mathrm{5}\%\mathrm{Si}+\mathrm{3}\%\mathrm{al}-\mathrm{5}}{\mathrm{0}.\mathrm{3}\%\mathrm{Mn}+\%\mathrm{Ni}+\mathrm{0}.\mathrm{5}\%\mathrm{Cu}+\mathrm{15}\left(\%\mathrm{C}+\%\mathrm{N}\right)+\mathrm{2}}$$

applies
t less than 1.3,
where with% C the C content,% N the N content,% Si the Si content,% Al the Al content,% Mn the Mn content,% Cr the Cr content,% Ni the Ni content ,% Mo is the Mo content and% Cu is the Cu content of the respective steel composition. Here are the inventively desired Properties are particularly safe if t is at most 1.2.

According to the invention, a cold-rolled steel product according to the invention, for example a cold-rolled steel strip or steel sheet, has an elongation A80 of at least 35%. In such a cold-rolled flat steel product according to the invention, the draw-off ratio during deep drawing of a rotationally symmetrical cup is 2.00.
"Boundary ratio" means the largest of the diameter of the blank, from which the cup is pulled, the drawing ratio formed in the first train to the diameter of the stamp used for deep-drawing of the cup, in which a cup can be deep-drawn without bottom tears or wrinkles with a certain hold-down force. The blank is clamped at its outer edge completely between a drawing ring and a hold-down. A punch with a diameter of 100 mm then penetrates the round blank and forms a dome in a deep-drawing process. This process continues until the sheet material breaks. The achieved under these circumstances, crack-free dome height is regularly 58 mm in a cold strip or sheet according to the invention. Accordingly, a flat steel product obtained according to the invention has a property combination which makes it optimally suitable for forming, for example by deep drawing or comparable operations.

The production of a cold-rolled steel flat product according to the invention generally comprises the Working sections "Melting, treatment and aftertreatment of steel in steelworks", "Production of cast strip by steel strip casting", "Hot rolling of cast strip or slabs", "Preparation (annealing and pickling / descaling) of hot strip for cold rolling" , "Cold rolling", "finish annealing of the cold strip" and "finishing (temper rolling, straightening, trimming) of the cold strip". Each of these working sections can include optional work steps, which are carried out, for example, depending on the available system technology and the requirements imposed by the user (customer).

For the production of a flat steel product according to the invention, therefore, a composite in erfindungemäßer steel is first melted. The melt thus composed is then cast into a cast strip in a two roll caster. The solidification of the steel according to the invention is carried out primarily ferritic and then austenitic due to the high Cr content and low Ni content. The high cooling rates on which the strip is cast favor the retention of distinct δ ferrite shares ("delta ferrite shares") in the hot strip.

Subsequently, the cast from the steel according to the invention strip is hot rolled inline following the strip casting in a continuous production process. In this way, a hot strip is produced with a typical thickness of 1 to 4 mm. On the way to the respective hot rolling stand, the cast strip can of course go through other workstations, such as a balance or reheat oven.

The processing of the steel according to the invention in a strip casting plant has the advantage that the molten steel to a strip with minimized, in particular to max. 4 mm, preferably max. 3.5 mm, limited thickness can be cast and then transformations with degrees of deformation of max. 50% is required to bring the cast strip to final thickness. In this way, it is possible to reliably produce a hot strip of steel according to the invention, despite its two-phase nature, which can subsequently be fed to cold strip for conventional further processing.

The procedure according to the invention has a particularly advantageous effect if the hot rolling takes place in a single hot rolling pass. The overall degree of deformation ε achieved in the course of hot rolling should be at most 50%, since otherwise an undesirable fine-grained microstructure forms.

The hot rolling temperatures with which the cast strip enters the first pass of the hot rolling are preferably in the range from 1050 to 1200 ° C.

The invention will be explained in more detail by means of exemplary embodiments.

Table 1 shows the chemical compositions of three alloys E1-E4 covered by the invention.

To produce melts composed according to these alloys E1-E4, alloyed and unalloyed scrap and ferroalloys have been melted together in an electric arc furnace in the steelworks.

Thereafter, the melt thus obtained from the electric arc furnace in an AOD (AOD = Argon Oxygen Decarburization) converter has been further treated. The main objective of this treatment was to reduce the carbon content to a target value by blowing in an oxygen-argon mixture.

After the AOD treatment, the melt has been poured into a pan. The high quality requirements for the properties of the molten steels then required aftertreatment. This was done in the secondary metallurgy, the pan or vacuum treatment of liquid crude steel. In addition to the homogenization of the melt and the observance of narrow temperature limits and / or exact temperatures, this step firstly pursued the goal of setting low contents of the elements carbon, nitrogen, hydrogen, phosphorus and some trace elements in the steel.

The correspondingly treated melt is then integrated in a conventional twin roll coater into a cast strip 2.5 to 3.5 mm thick and then directly integrated in a stitch into a hot strip of 1.5 gauge thickness - 2.5 mm hot rolled. The hot rolling end temperature was while 1100 ° C, for the hot rolling of hot strips of steels according to the invention always hot rolling end temperatures of 1050 - 1200 ° C at degrees of deformation of 25 - 50% come into question. The immediate succession of strip casting and hot rolling under the above conditions avoids the risk of cracking and surface defects resulting from conventional processing of the steel alloys of the present invention through a multi-stage hot rolling process due to the two-phase nature of the hot strips produced from them.

For comparison, two samples 4301.70, 4301.60 falling under the standard alloy of steel 1.4301 were melted, of which the sample 4301.70 in the twin roll caster with subsequent hot rolling in the manner described above for the samples E1 - E4 to hot strip with a thickness of 1.9 - 2.4 mm has been processed, while the sample 4301.60 has been conventionally cast into slabs and processed in several stages to 2.8 - 3.6 mm thick hot strip.

The hot strips produced in the manner described above have subsequently been prepared for cold rolling. They have been subjected to a heat treatment in the form of an annealing at a temperature which is typically in the range of 1000 to 1180 ° C. during the processing of hot strips according to the invention. In the embodiments described here, it was in each case 1050 ° C.

Subsequently, the hot strips were descaled in a known manner to free the hot strip surface from the oxide layer adhered thereto. Such descaling usually comprises a mechanical pre-descaling and pickling performed, for example, with the aid of a conventional scale breaker, in which the scale is largely completely removed from the metallic surface of the hot strip with a liquid pickling medium.

The thus annealed and cleanly stained so-called "white" hot strip was wound into coils and fed to the cold rolling stand.

The cold rolling of the hot strips to the required final thickness of 0.8 mm has been carried out without prior heating on a 20-roll cold rolling stand. This type of cold rolling mill is capable of applying the high forming forces required for the processing of stainless steels and at the same time ensures compliance with the tolerances required by the customer regarding surface quality and thickness. The degrees of deformation achieved in the course of cold rolling are typically in the range of 40-80% in the case of processing according to the invention.

The cold rolled solidified during cold rolling has been annealed recrystallizing at an annealing temperature of 1140 ° C to restore its forming properties required for further processing. For recrystallizing annealing suitable annealing temperatures are in the range of 1050-1180 ° C according to the invention flat steel products.

In the present embodiments, the recrystallizing annealing has been carried out on a conventional annealing and pickling line, in which the cold strip was first annealed in an open atmosphere and then freed again in the pickling section of the resulting scale. Alternatively, it is also possible with particularly high demands on the surface finish to perform the annealing under a protective gas atmosphere of a bright annealing line. The shiny metallic surface of the cold strip is retained, their gloss is enhanced by the final heat treatment in a protective gas atmosphere.

Finally, to finalize the customer's desired mechanical properties, flatness, surface texture, and gloss, the heat treated cold tapes were finally subjected to skin pass rolling. For this purpose, usually two- or four-roller skin pass mills are used with polished work rolls.

The δ-ferrite contents of the hot strips ("WB") produced from the steels E1-E4, 4301.70 and 4301.60 and their respective mechanical properties yield strength Rp, tensile strength Rm and elongation A80 are listed in Table 2. In the same way, in Table 2, 0.8 mm thick cold strips produced from steels E1 - E4, 4301.70 and 4301.60 in the manner described here are delta-ferrite content δ-ferrite, the granularity of their microstructure assessed according to ASTM and the yield strength Rp, tensile strength Rm and elongation A80.

For most of the cold tapes obtained from the inventive samples, the yield strength and tensile strength values are above the values of the cold tapes produced from Comparative Samples 4301.70 and 4301.60.

The elongation values A80 for the cold rolled strips produced from samples E1 - E4 are between 44.4% and 48.5% transverse to the rolling direction, while for the comparative samples 4301.70 and 4301.60 elongation values A80 of 53% and 57.6% could be determined.

The 8-ferrite content ("delta ferrite content") of the steel according to the invention in the cold-rolled strip is between 8.5% and 13% and thus significantly above the values determined for the two comparative samples. The significant proportions of δ-ferrite present in the samples according to the invention explain the lower elongation values. In addition, especially the cold strip produced from the samples E1-E4 with ASTM values of up to 10 is very fine-grained, which is considered as a possible cause of the high strength level. In addition, elements such as carbon and nitrogen or manganese increase the strength properties as interstitially or substitutionally dissolved atoms (in the form of a mixed crystal).

The technological characteristics of samples E1 and E4 suitable for evaluating formability and 4301.60 cold rolled tapes are listed in Table 3.

The dome height as a characteristic for the ironing ability of the cold strips produced from the samples E1 and E4 is within the range or slightly below the values that could be determined from the two comparative samples.

Also, the cut-off ratio of the cold tapes produced from samples E1 and E4 is in the range of the cut-off ratio of Sample 4301.60. The cold strips according to the invention thus have the same good deep drawing properties as the samples produced from the conventional steel 1.4301.

Accordingly, it is possible to produce components with a high thermoforming content and a large draw depth from inventive steel. When they are deformed, cold-rolled flat steel products produced in accordance with the invention exhibit a lower peak than cold strips which have been produced in a conventional manner by continuous casting from steel 1.4301. This proves an isotropic flow behavior of the steel according to the invention due to a lower rolling texture in the cold strip. Such behavior proves to be particularly favorable in many deep drawing processes. The r values in the transverse direction of cold rolling products produced according to the invention are in the range of the conventionally produced material.

If necessary, the cold-rolled strip obtained after temper rolling can be stretch-straightened and trimmed be subjected. As a rule, these production steps are carried out separately. If necessary, sanding lines can then provide the strips with different sanding patterns on the surface of the strip. For highest demands on the flatness of a stainless steel sheet, dressed or undressed cold strips are treated in strip stretching lines. Any residual stresses which may lead to the unevenness of a strip are thus compensated.

The invention thus provides a flat steel product which is made of a steel whose corrosion resistance is comparable to that of steel 1.4301. The δ-ferrite content ("delta-ferrite content") produced in accordance with the invention steel hot and cold strip is over the chem. In the course of the strip casting process selected as the processing method with subsequent hot rolling, possible rapid solidification is set so that ultimate elongation values well above 35%, in particular above 40%, are achieved and the technological forming properties lie within the range of dispersion of the material 1.4301. Table 1 sample C Si Mn Cr Not a word Ni N Cu E1 0.057 0.15 7.57 18.01 0.1 3.06 0.11 2.22 E2 0.06 0.11 7.6 18.0 0.1 3.08 0.09 2.31 E3 0.053 0.11 7.74 17.92 0.14 3.9 0.11 1.61 E4 0.09 0.09 8th 18.24 0.09 2.15 0.1 2.32 4301.70 0.04 0.2 1.24 18.14 0.25 8.52 0,049 0.26 4301.60 0.04 0.39 1.24 18.15 0.25 8.54 0,049 0.26 Data in% by weight hot strip cold strip robe δ-ferrite [%] Rp [MPa] Rm [MPa] Ag [%] δ-ferrite [%] Rp [MPa] Rm [MPa] Rp / Rm [%] A80 [%] Graininess according to ASTM E1 14 - - - 13 332 646 0.51 47.4 10 E2 14 336 651 43.2 11 341 637 0.54 44.4 10 E3 12 334 657 46 8.5 357 675 0.53 48.5 10 E4 13.9 360 685 41.6 11.7 390 705 0.55 41.2 10 4301.70 2.0 - 8.0 325 633 50 1-2 304 645 0.47 53 9 4301.60 1.0 - 2.0 - - - 0 285 624 0.46 57.6 8.5 "-" = Not determined sample Thickness [mm] Dome height [mm] Limiting drawing ratio Zipfelness [mm] E1 0.8 61.7 - 63.1 2.00 2.14 E4 0.8 63 - 65 2.06 2.44 4301.60 0.8 63 - 67 2,00 - 2,06 3.7 - 6.5

Claims (16)

1. Cold-rolled flat steel product, made from a stainless steel with a structure consisting of 8.5 - 13 % by volume δ-ferrite and for the remainder, austenite and with the following composition (in wt. -%): C: 0.05 - 0.14 %, Si: 0.1 - 1.0 %, Mn: 7.5 - 10.5 %, Cr: >17.5 - 22.0 %, Ni: 1.0 - 4.0 %, Cu: 1.0 - 3.0 %, N: 0.03 - 0.2 %, P: max. 0.07 %, S: max. 0.01 %, Mo: max. 0.5 %, optionally one or more elements from the group consisting of "Ti, Nb, B, V, Al, Ca, As, Sn, Sb, Pb, Bi, H" in accordance with Ti: max. 0.02 %, Nb: max. 0.1 %, B: max. 0.004 %, V: max. 0.1 % Al: 0.001 - 0.03 %, Ca: 0.0005 - 0.003 %, As: 0.003 - 0.015 %, Sn: 0.003 - 0.01 %, Pb: max. 0.01 %, Bi: max. 0.01 %, H: max. 0.0025 %, remainder Fe and unavoidable impurities, whereby for $$\mathrm{t}=\frac{\%\mathrm{Cr}+\mathrm{2}\%\mathrm{Mo}+\mathrm{1.5}\%\mathrm{Si}+\mathrm{3}\%\mathrm{Al}-\mathrm{5}}{\mathrm{0.3}\%\mathrm{Mn}+\%\mathrm{Ni}+\mathrm{0.5}\%\mathrm{Cu}+\mathrm{15}\left(\%\mathrm{C}+\%\mathrm{N}\right)+\mathrm{2}}$$

   

   t < 1.3 applies,
   whereby %C describes the C content, %N the N content, %Si the Si content, %Al the Al content, %Mn the Mn content, % Cr the Cr content, %Ni the Ni content, %Mo the Mo content and % Cu the Cu content of the particular steel composition of the flat product.
2. Cold-rolled flat steel product according to one of the above claims, characterised in that its Si content is 0.1 - 0.4 wt.-%.
3. Cold-rolled flat steel product according to one of the above claims, characterised in that its Mn content is 7.5 - 10.5 wt.-%.
4. Cold-rolled flat steel product according to one of the above claims, characterised in that its Cr content is max. 20 wt.-%.
5. Cold-rolled flat steel product according to one of the above claims, characterised in that its Cr content is at least 17.7 wt.-%, in particular at least 18.0 wt.-%.
6. Cold-rolled flat steel product according to one of the above claims, characterised in that its Ni content is at least 1.5 wt.-%.
7. Cold-rolled flat steel product according to one of the above claims, characterised in that it contains at least 1.5 wt.-% Cu.
8. Cold-rolled flat steel product according to Claim 7, characterised in that its Cu content is at least 2.0 wt.-%.
9. Cold-rolled flat steel product according to one of the above claims, characterised in that its N content is 0.03 - 0.10 wt.-%.
10. Cold-rolled flat steel product according to one of the above claims, characterised in that its elongation A80 is at least 35 %.
11. Cold-rolled flat steel product according to one of the above claims, characterised in that the limiting drawing ratio during deep drawing of a rotationally symmetrical cup is 2.00.
12. Method for producing a cold-rolled flat steel product, such as steel strip or steel sheet, whose characteristics are according to one of claims 1 to 11, comprising the following work steps:

    - Melting a steel that has the following composition (in wit.-%): C: 0.05 - 0.14 %, Si: 0.1 - 1.0 %, Mn: 7.5 - 10.5 %, Cr: > 17.5 - 22.0 %, Ni: 1.0 - 4.0 %, Cu: 1.0 - 3.0 %, N: 0.03 - 0.2 %, P: max. 0.07 %, S: max. 0.01 %, Mo: max. 0.5 %, optionally one or more elements from the group consisting of "Ti, Nb, B, V, Al, Ca, As, Sn, Sb, Pb, Bi, H" in accordance with Ti: max. 0.02 %, Nb: max. 0.1 %, B: max. 0.004 %, V: max. 0.1 %, Al: 0.001 - 0.03 %, Ca: 0.0005 - 0.003 %, As: 0.003 - 0.015 %, Sn: 0.003 - 0.01 %, Pb: max. 0.01 %, Bi: max. 0.01 %, H: max. 0.0025 %, remainder Fe and unavoidable impurities, whereby for $$\mathrm{t}=\frac{\%\mathrm{Cr}+\mathrm{2}\%\mathrm{Mo}+\mathrm{1.5}\%\mathrm{Si}+\mathrm{3}\%\mathrm{Al}-\mathrm{5}}{\mathrm{0.3}\%\mathrm{Mn}+\%\mathrm{Ni}+\mathrm{0.5}\%\mathrm{Cu}+\mathrm{15}\left(\%\mathrm{C}+\%\mathrm{N}\right)+\mathrm{2}}$$

    

    t < 1.3 applies,
    whereby %C describes the C content, %N the N content, %Si the Si content, %Al the Al content, %Mn the Mn content, % Cr the Cr content, %Ni the Ni content, %Mo the Mo content and % Cu the Cu content of the particular steel composition of the flat product,

    - Casting the molten steel in a twin-roll casting machine to form a cast strip;

    - Inline hot-rolling of the cast strip, following casting of the cast strip, to form a hot strip;

    - Cold-rolling of the hot strip to produce a cold strip, whereby the cold strip obtained has a structure that has 8.5 - 13 % by volume δ-ferrite and for the remainder, austenite.
13. Method according to claim 12, characterised in that the hot-rolling takes place in a single hot-rolling pass.
14. Method according to one of claims 12 or 13, characterised in that the degree of total deformation ε achieved during the hot-rolling is a maximum of 50 %.
15. Method according to one of claims 12 to 14, characterised in that the cast strip enters the first rolling pass at an initial hot-rolling temperature that lies in the range 1050 - 1200 °C.
16. Method according to one of claims 12 to 15, characterised in that the thickness of the cast strip is a maximum of 4 mm.