DE102005063058B3 - Producing cold rolled strip of ferritic stainless steel comprises controlled cooling before cold rolling - Google Patents

Abstract

Producing a cold rolled strip with a ferritic structure by casting a steel melt into a strip, optionally hot rolling the strip, coiling the hot rolled strip and subjecting it to one or more cold rolling operations comprises cooling the cast strip from an initial temperature (T1) of at least 1180[deg]C at a rate of at least 150[deg]C/second to an intermediate temperature (T2) of up to 1000[deg]C and then keeping it at a temperature (T3) of 900-1000[deg]C for at least 10 seconds.

Description

The The invention relates to a method for producing a cold strip with ferritic structure, in the case of cooling a ferritic structure forming molten steel is poured into a cast strip, in which the cast strip is hot rolled inline if necessary in which the hot rolled strip is unwound and in the off the hot rolled strip in one or more steps, the cold strip cold rolled.

Due to the high nickel price, substitution of austenitic stainless steel by ferritic stainless steel is increasingly taking place worldwide, which typically only contains Ni as a production-related accompanying element. A method of the type described, which is intended to make this possible, for example from the EP 0 881 305 B1 known. According to the known method, a stainless ferritic steel, the (in wt .-%) max. 0.12% C, max. 1% Mn, max. 1% Si, max. 0.04% P, max. 0.030% S, 16-18% Cr and the balance iron and unavoidable impurities, cast in a casting gap formed between the rolls of a two-roll caster to a cast strip. Subsequently, the cast strip is cooled, during which cooling it is avoided that the strip is kept in the austenite-ferrite transformation zone. After cooling, the strip is reeled at a temperature between 600 ° C and the temperature of the martensite transformation. Then the reeled tape is fed at a speed of max. 300 ° C / h cooled to a temperature between 200 ° C and the room temperature. Finally, a known per se bell annealing the coiled tape is performed.

at the usual for the Production of ferritic sheets taken over the route continuous casting of slabs is first the surface worked the slab, then reheated the slab, then the slab Slab in the hot strip mill hot rolled to hot strip and then reeled into a coil. That so hot strip obtained is then annealed, pickled and in several Strokes cold rolled. Finally The cold strip is usually bright annealed and trained.

Regardless of which the above-mentioned ways the hot strip is generated exists in ferritic stainless steel with a Cr content in the range of 17% cold-rolled strips the problem that in the course of a subsequent cold forming, specifically can occur during deep drawing, Zugrilligkeit or orange peel. With Ridgrability is strongly pronounced linear surface defects referred to, which aligned with ferritic chrome steels in the rolling direction are. The as "orange skin" (English orange peel) designated surface defects on the other hand appears undirected and is characterized by a scarred expression of surface characterized.

avoided can the emergence of treadness or orange peel thereby, that over one of the known production routes produced between hot strips The individual passes of cold rolling are lavishly annealed. This elaborate annealing steps to lead but to increased Cost of production, resulting in a higher market price of ferritic Stainless steel band across precipitate equivalent austenitic stainless steel.

In addition to the above-described prior art is from the DE 199 30 519 C1 a method for producing non-grain oriented electrical steel, wherein from a raw material, such as cast slabs, strips, slivers or thin slabs, made of a steel with (in% by weight) C: 0.001-0.05%, Si: ≤ 1 , 5%, Al: ≦ 0.4% with Si + 2Al ≦ 1.7%, Mn: 0.1-1.2%, optionally up to a total of 1.5% of alloying additives, such as P, Sn, Sb, Zr , V, Ti, N, Ni, Co, Nb and / or B, and the remainder being iron and customary accompanying elements, a hot strip is produced. For this purpose, according to the known method, the starting material is hot rolled directly from the casting heat or after a previous reheating to at least 1000 ° C and at most 1180 ° C amount reheating temperature in several forming passes and then coiled, during hot rolling at least the first forming pass in Austenitgebiet and at least another Umformstich is performed in the two-phase mixing area austenite / ferrite and wherein during the rolling in Zweiphasenmischgebiet a total change in shape ε _h of at least 35% is achieved. In this procedure, the magnetic properties of the resulting electric sheet are selectively influenced by a deformation during each durchformten in the course of hot rolling forming passes depending on the respective structural state. The decisive factor here is rolling in the two-phase mixed area, whereas the proportion of deformation in the ferrite area should be as low as possible. The known method is therefore particularly suitable for the processing of those Fe-Si alloys which have a pronounced two-phase mixing region between the austenite and ferrite regions.

Of the Invention was based on the object to call a method with the cold strips from ferritic stainless steels which cause the risk of the formation of orange peel or Zugrilligkeit is minimized in a cold forming.

These The object has been achieved according to the invention that in a method of the type specified in the cast Band between the pouring and the reel starting from a not lower than 1180 ° C lying Start temperature with a minimum of 150 ° C / s cooling rate amounts to one at most 1000 ° C amount Intermediate temperature cooled and subsequently for at least 10s at a 900-1000 ° C holding temperature is held.

practical Starting temperatures of intensive cooling are typically in the range of 1180-1270 ° C, in particular in the range of 1200-1250 ° C. When falling below the limit of 1180 ° C Austenite can already occur in quantities at the edge of the band, which is the success the method according to the invention impair would.

For the implementation of the inventive method are in particular those known per se steels, the to the class of 10-18% by weight Cr-containing, a ferritic microstructure forming stainless steels include and in the course of their cooling starting from the ferrite not completely in austenite and afterwards convert back to ferrite. Such steels contain besides iron and unavoidable impurities typically (in wt%) up to 0.08 C, 10-18 Wt% Cr, up to 1% Si, up to 1.5% Mn, up to 1% Ni, up to 0.04% P and up to 0.015% S. Where possible existing Ni content is not a metallurgical targeted addition, but the consequence of the manufacturing process and passes over the Ni-containing ones ladles, Converters or ovens in the melt. Typically, the Ni content is processed according to the invention steels in the range of 0.7-0.8 Wt .-%.

For the effect the method according to the invention The decisive factor is the combination of strip casting, rapid cooling of the cast tape and holding the tape over a sufficient, at least 10 seconds at a temperature in the range of 950 ± 50 ° C, in particular 950 ± 20 ° C, is. Surprised has been shown that in the cold forming of cold strips, the from heat treated in accordance with the invention cast tapes have been generated, neither Zugrilligkeit nor orange peel occurs, without requiring elaborate intermediate annealing must be performed between the stages of cold rolling.

Used according to the invention Steel alloys initially solidify ferritically in the course of strip casting. When cooling the solidified band then converts ferrite between 1200 ° C and 800 ° C partially in austenite. The thermodynamic cause lies in the low and with the temperature decreasing solubility of carbon in the Ferrite. Carbides, which can also absorb carbon, form only below 900 ° C. Austenite dissolves much more carbon.

at a usual one Cooling forms only at the grain boundaries austenite, since carbon in the ferrite again quickly diffused and migrate from the interior of the grain to the edges can. As a result, the ferrite grain boundaries are marked with austenite. As soon as carbides form at temperatures of less than 900 ° C, the Austenite part at the grain boundaries again. Because of the relatively slow but this is not complete, so that remains of austenite remain later in the temperature range of 200-300 ° C to martensite convert. The conventional method of working at the grain boundaries Remaining retained austenite thus fixes the coarse cast structure.

The The idea underlying the invention is now very fast to cool approximately the temperature with the maximum austenite content (950 ° C ± 50 ° C, in particular 950 ° C ± 20 ° C). To this This way, austenite formation at the kernal borders is minimized, because there are the diffusion path lengths for carbon and certainly for the substitutional elements (Cr, Ni, Mn, ...) in the short time the cooling are not sufficient.

simultaneously is in the range of a holding temperature of about 950 ° C, the Austenite formation driving force greatest and the temperature dependent diffusion coefficient so low that over Nucleation inside the grain form austenite particles. In consequence of the significantly reduced diffusion coefficient changes the distribution of in the respective alloy contained substitutional elements not or only little (Paraquilibrium). At the same time, the carbon supersaturation reduced.

One stops in according to the invention the cast strip material at this temperature above a Time of at least ten seconds, preferably twenty seconds, Thus austenite particles begin to separate out in the interior of the grain due to structural defects. It creates new grains in a ferritic matrix that breaks up the original cast structure. The particle density is greater, depending longer the holding time is. This excretion mechanism causes a Grain refining, the result of insensitivity inventively produced cold strips against Zugrilligkeit and orange skin formation leads.

The higher the cooling rate in the case of the intensive cooling of the cast strip according to the invention, the safer the suppression of formation of the invention which is aimed for occurs Austenite. Basically, therefore, the highest possible cooling rates are sought. However, practical experiments have confirmed in this connection that even at cooling rates of 150-250 ° C / s, the effects used in the invention safely occur.

outgoing from the above explained basic Methods according to the invention arise different variants of the invention, depending on the desired property combinations of each to be produced cold tapes, from the thermoforming behavior of the respective cast strip obtained, the available stationary plant engineering or the requirements of the operational logistics chosen can be. So you can, for example, first intensive cooling and then hot rolling or first rolls (above 1200 ° C in the ferrite) and then cool quickly. Further You can also the two-phase area between 1200 and 800 ° C very fast passed through. Then initially no austenite forms, but a more supersaturated in carbon Ferrite is frozen. Will then quickly from below 800 ° C on the Holding temperature warmed, In turn, austenite formation takes place inside the grain. A special fast reheating The holding temperature has a positive effect on the work result out. heated too slow, unwanted austenite forms at the grain boundaries, while the C supersaturation In the interior of the grain is degraded by carbon diffusion, thereby in the result the original one cast structure is fixed. Furthermore, one should not be too long in the temperature range from 800-900 ° C, because practical experiment prove that in this temperature range after about 100 seconds the carbon supersaturation is degraded by carbide formation.

You cool in the course of cooling off down to temperatures of less than 500 ° C, for example to room temperature, carbide can no longer form. The supersaturated ferrite is rather frozen and can later (offline) quickly back up to 950 ° C heated be used to form Austenitteilchen inside the grain.

According to the invention The cast strip is thus on in his intensive cooling invention cooled an intermediate temperature of 900-1000 ° C, so that for the invention quickly reaches critical temperature by a direct route becomes.

practical Experiments have confirmed that the inventive fast cooling down to the intermediate temperature and holding at the holding temperature between to water And reels also in such a process, the insensitivity of the obtained cold bands against Zugrilligkeit- and orange peel formation impact, between casting and reels on a hot rolling is dispensed with. In such cast bands in a conventional manner, for example via a two-roll caster be produced, the cast strip in terms of the homogeneity of his microstructure and its property distribution, however, usually between the strip casting and the reel hot rolled in at least one stitch. By the Hot forming will increase the density and rate of precipitation increased by austenite particles, since structural defects are introduced into the structure. The Exploit this mechanism for a grain refinement hangs crucial from the optimally chosen Forming conditions and the cooling and heating rates. A quick heating or cooling can in principle only on thin bands be achieved. Therefore, produced in the inventive manner by strip casting Band especially suitable for such a thermomechanical treatment.

Becomes in the method according to the invention performed a hot rolling, so it is preferred first a tape with a thickness of 1-5 mm, especially 2-3 mm, cast directly and the cast strip then inline with a Reduction of 5-60%, especially 10-40%, hot rolled. The mode of action of the method according to the invention makes it possible, the temperature control of the Bandes with regard to the required completed hot rolling so to choose that while the hot rolling on the deformation behavior of each processed Steel or the desired combination of properties of the obtained tape optimally tuned temperature conditions prevail.

Under Including a hot rolling thus resulting in the following variants of the method according to the invention:

Version 1:

• - pouring the cast tape;
• - Hot rolling of the strip having a hot-rolling temperature of not less than 1180 ° C, typically from 1180-1270 ° C (in the course hot rolling results in a first cooling of the cast strip);
• - in the Cooling commencing after hot rolling to 900-1000 ° C Temperature with a cooling rate of at least 150 ° C / s, In this case, intermediate and holding temperature equal to this Temperature are;
• - Hold of the band at the relevant, lying between 900-1000 ° C, in particular 950 ° C ± 20 ° C amount Temperature for at least 10 seconds.

in the The scope of this first variant of the method according to the invention is the cooling and Holding the tape performed only after hot rolling. there should in this case the cooling preferably use immediately after hot rolling, in practice so within less than three, especially within less than a second after leaving the last hot rolling mill. That way you can the pouring heat of the cast strip directly into the hot rolling step, so that not only high hot rolling temperatures are possible, but also the for the temperature control The band required energy consumption is reduced to a minimum is.

Variant 2:

• - pouring the cast tape;
• - cool down a 900-1000 ° C amount Intermediate temperature with a cooling rate of at least 150 ° C / s;
• - Hot rolling the band at the intermediate temperature;
• - Hold of the strip at a holding temperature which is also between 900-1000 ° C, in particular substantially equal to the intermediate temperature or 950 ° C ± 20 ° C, for at least 10 seconds.

at This variant of the invention is the cooling to the intermediate temperature before hot rolling and holding at the holding temperature after Hot rolling the cast strip completed. By hot rolling in the temperature range of 900-1000 ° C additional Dislocations in the structure of the hot rolled strip, which are used as nucleation sites for austenite formation subsequent holding at the holding temperature serve.

Variant 3:

• - pouring the cast tape;
• - cool down a 900-1000 ° C amount Intermediate temperature with a cooling rate of at least 150 ° C / s;
• - Hold of the strip at a holding temperature which is also between 900-1000 ° C, in particular substantially equal to the intermediate temperature or 950 ° C ± 20 ° C, for at least 10 seconds;
• - Hot rolling of the tape at the intermediate temperature.

According to this third variant is the cooling to the intermediate temperature and holding at the holding temperature completed before the hot rolling of the cast strip. The hot rolling of the structure with the previously completed hold at the hold temperature produced high density of austenite grains in the ferritic matrix leads to a high dislocation density, which in a subsequent recrystallization to a fine-grained structure leads. A such recrystallization usually becomes by a suitable recrystallization annealing treatment, such as in the manufacture of cold-rolled strips of the type in question is performed by default.

Variant 3:

• - pouring the cast tape;
• - Cool down with at least 150 ° C / s to an intermediate temperature below 900 ° C, in particular in the range of 800 ° C;
• - Hot rolling the band at the intermediate temperature;
• - fast Heat of the belt to a 950 ° C ± 50 ° C, in particular 950 ° C ± 20 ° C, amount holding temperature;
• - Hold of the band at the holding temperature.

The Hot rolling in the range of temperatures of less than 900 ° C, in particular in the range around 800 ° C, finds in this case in the pure ferrite area with one opposite one Rolls in the mixed area lower yield stress instead. This can Achieved a higher degree of deformation with reduced power consumption and low wear on the rollers if necessary and desired.

A according to the invention intensive Cooling in Temperature ranges below 900 ° C opens up the possibility of rolling the cast strip at temperatures well below 800 ° C or a further heat treatment at temperatures of less than 500 ° C, in particular less than 400 ° C, perform.

The inventive method settles in the variants described above particularly cost perform such strip casting, at those casting, if necessary Hot rolling and reeling as well as between casting and Reels performed inventive steps of cooling to the intermediate temperature and holding at the holding temperature in completed a continuously sequential step sequence become.

However, the effects used by the invention also allow a discontinuous processing of the individual steps of the method according to the invention. This can prove to be useful, for example, if a corresponding system technology is available or logistical reasons for a staggered implementation of the steps speak. From this results the following fourth variant of the invention.

Variant 4:

• - pouring the cast tape;
• - Cool down with at least 150 ° C / s to an intermediate temperature below 900 ° C, in particular below 800 ° C;
• - cool down less than 500 ° C, in particular to room temperature;
• - fast Heat to a hot rolling end temperature;
• - Hot rolling the strip at the hot rolling temperature;
• - fast Heat of the volume amounting to 950 ° C Holding temperature;
• - Hold of the band at the holding temperature.

According to this fourth variant of the invention, it is provided, the cast Band in the course of the cooling according to the invention one less than 900 ° C, in particular less than 800 ° C cooling intermediate temperature, with this cooling until can go to room temperature. Later the cast band will be then reheated to the holding temperature. Later, in this context, that between the cooling and holding further operations, such as hot rolling at a certain temperature, storing, dividing into slabs etc. performed can be.

Of Furthermore, it is possible Cool the cast strip to room temperature after pouring and only later Time initially on the for to warm up an optimal temperature and then warm up to bring the holding temperature and there over the required time to keep.

in the Case that when performing the method according to the invention Intermediate temperatures in the range of 800-900 ° C should be achieved for the reasons already mentioned when reheating to the holding temperature of this temperature range basically fast be passed. Therefore, an advantageous embodiment of the Invention that emanating from the respective intermediate temperature reheating to the holding temperature in 1-5 Seconds, especially in 2-3 Seconds, takes place.

in the Case that in the course of cooling reached intermediate temperatures well below 800 ° C, in particular in the range of room temperature or slightly above it, the tape must be for hot rolling for the reasons already mentioned reheated sufficiently quickly become. Therefore, the invention provides that the tape starting from the low intermediate temperature within 200s, in particular within 100s, is warmed to the respective hot rolling temperature, typically 700-800 ° C becomes. Is the heating up to 800 ° C too slow, can unwanted Excrete carbides. These lead to a premature reduction of supersaturation and thus to a significantly reduced density of Austenitteilchen with the result that the invention sought Grain refining is not achieved.

With The invention thus provides a method which, while avoiding it complex production steps allows both in terms of its properties and in terms of competitive price Produce product. The particular advantage of the method according to the invention consists in the fact that can be produced with him cold strips, the characterized by a homogeneous appearance and a zündernarbenfreie surface are. The latter is achieved in that already in the intensive cooling of the invention on the cast strip adhering scale is largely removed, so that if necessary, carried out hot rolling at best a minimized surface damage in Consequence of on the belt still existing tinder is caused.

When proceeding according to the invention can thus be dispensed with a complex, in the prior art regularly leading to an interruption of the continuous processing sequence descaling. Also can be saved by the inventive method, the effort that is still required in the prior art for the always necessary Haubenglühung the cold-rolled material. The possibility of cooling down to temperatures below 800 ° C. in the course of the process according to the invention also makes it possible, for example, to trim the cast or hot-rolled strip at temperatures, for example 600 ° C., where flatness problems can be largely avoided , Finally, due to the largely freely configurable temperature control, the method according to the invention allows an increase in the degree of hot forming in the strip casting line via, for example, a second rolling stand or smaller work roll diameter, whereby cold strips produced according to the invention produce a better depth / depth compared to cold strips produced according to the usual process route. Can have drawability. The rapid temperature changes required for carrying out the method according to the invention can be realized only by using the strip casting technique, since only the small strip thickness of the cast strip allows sufficiently rapid temperature changes over the entire strip cross section. Exemplary embodiments: The following In each case, as described above, to demonstrate the effect of the invention (experiments I-IV), a ferritic steel was used which contained (in% by weight) 0.043% C, 0.25% Si, 0.36% Mn, 0.021 % P, 0.002 S, 16.23% Cr, 0.49% Ni.

In a conventionally constructed casting mill is from a corresponding molten molten steel each produces a cast strip, the cast strip is hot rolled into a hot strip and the hot strip after all been reeled. The strip caster included one Two-roll casting machine, one in the conveying direction Following the cast strip inline to the casting machine arranged hot rolling mill and a in the conveying direction arranged behind the hot rolling mill Coiler. additionally are dependent from the respective experimental procedure an intensive water cooling device, inductively operating strip heating devices and electric holding ovens been used.

To the heat treatment according to the invention showed the obtained hot tapes a fine grained structure on, in contrast to conventionally described as "fine" structures one Variety of particles (martensite, retained austenite, carbide) in one relatively large, ferritic grain (= matrix) are located. The structure is accordingly finer overall, in itself, however, also inhomogeneous than conventional fine-grained trained Structure. Characteristic of the structure produced according to the invention bands is therefore the high number of particles per grain.

The Further processing in the experiments I-IV using the casting rolling mill produced according to the invention hot strips took place in a conventional manner with the inclusion of a box annealing, a pickling step, cold rolling without intermediate annealing, a bright glow and a dressage.

Out the cold strips thus obtained have been manufactured with a total degree of deformation of 70% sample parts. None of the samples showed orange peel or tearing.

Trial I:

In a first variant of the method according to the invention was the thickness of the cast strip 3 mm. After that from the casting gap of the two-roll casting machine leaking cast strip reached a strip temperature of 1180 ° C had, was carried out an intensive cooling with water. The cast band was within 2s on a Intermediate temperature of 950 ° C cooled.

The so cooled cast tape was subsequently added in a continuous, uninterrupted process sequence in an induction heating system for one Duration of 10s held at a holding temperature, in this case was equal to the intermediate temperature.

The such in accordance with the invention heat-treated cast strip was then hot rolled to a strip thickness of 2.5 mm.

On on the hot rolling stand cooled down the following exit roller table the strip to a reel temperature of about 550 ° C, before the coiler reached in which it has been wound into a coil.

The thus obtained hot strip had a columnar grain structure (about 100 microns wide and 500 μm long) with an equiaxed mid-band range (grain size 150 μm). The Grain boundaries were thin Hem made of martensite and carbides. In the interior of the grain were Recrystallized areas with a grain size of 20 microns. Next were fine in the microstructure distributed island-like particles consisting of carbides, martensite and Restaustenit passed. The particle density was typically 15-25 particles per grain.

Experiment II:

In a second attempt was made from the molten steel with the above Alloy first produced a cast, 2.8 mm thick tape. The cast band became in an induction heating system at a temperature of 1200 ° C held and then hot-rolled to a strip thickness of 2.1 mm at this temperature.

immediate After hot rolling, intensive cooling was carried out with water. It was the subsidized with about 1m / s Band cooled within 1s to an intermediate temperature of 950 ° C. The ribbon then came to an exit roller table, the first assigned to the hot rolling mill section via a length of 15m was provided with a cover, which ensured that Band in this first section about 15s maintained a substantially constant temperature. Then that cooled Tape still on the outlet roller table to a reel temperature of about 500 ° C off with it eventually has been coiled to a coil.

The microstructure of the hot strip obtained in the second test had the same columnar grain structure (about 100 μm wide and 500 μm long) with an equiaxed strip center region (grain size 150 μm) as the microstructure of the hot strip obtained in the first test. Also in this case, the grain boundaries showed a thin seam topped with martensite and carbides. Inside the grain were also recrystallized areas with an average grain size of 20 microns. Likewise, there were finely distributed island-like particles in the microstructure, which also existed as in the band of carbides, martensite and retained austenite contained after the first experiment. The particle density was typically 20-30 particles per grain.

Trial III:

In a third attempt was made first cast a 3 mm thick tape. After the cast strip has a temperature from 1180 ° C achieved intensive cooling with water, in which the strip was cooled to an intermediate temperature of 780 ° C within 3 seconds is. The cast and so cooled tape was then in a inductive heating system kept warm, heated to a hot rolling temperature of 800 ° C and subsequently hot rolled to a strip thickness of 2.5 mm at this hot rolling temperature. The tape cooled then on the outlet roller table to a reel temperature of about 550 ° C and has been reeled at this temperature.

From The resulting tape was partitioned at room temperature test panels. These were then inductively first within a period of 15s 800 ° C and then at 950 ° C heated. The time for heating between 800 ° C and 950 ° C was 2s.

through a holding furnace, the tape was then for 20s at a holding temperature of 950 ° C held. Subsequently a cooling took place in air.

The structure the so-heat-treated hot-rolled test panels also showed a stalking Grain structure (about 100 μm wide and 500 μm long) with an equiaxed mid-band range (grain size 150 μm). To the Grain boundaries were also thinner here Hem topped with martensite and carbides available. It was In the interior of the grain recrystallized areas with a grain size of 20 microns and in structure were finely distributed island-like particles consisting of carbides, Martensite and retained austenite passed. The particle density was typically 40-60 Particles per grain.

Trial IV:

It according to Experiment III, a 3 mm thick band was produced, which has been cooled intensively when reaching a strip temperature of 1180 ° C, until an intermediate temperature of 780 ° C was reached. Deviating from Trial III, however, was not just the step of stopping at the Holding temperature, but also the hot rolling step performed offline.

To This purpose was removed from the cast strip after it was at room temperature chilled was, panels divided and these panels from room temperature inside heated inductively from 30s to a hot rolling temperature of 800 ° C at the they were hot rolled to a strip thickness of 2.4 mm. After the renewed cooling down of the hot rolled sheets, they were brought to a holding temperature within 3 seconds of 950 ° C reheated.

through a holding furnace, the reheated tape was at the hold temperature for 20s held. Subsequently is the tape cooled in air Service.

Also in this embodiment showed the structure the hot-rolled sheets after holding at the holding temperature a stalked one Grain structure (about 100 μm wide and 500 μm long) with an equiaxed mid-band range (grain size 150 microns), wherein here too, the grain boundaries a thin hem with martensite and carbides and recrystallized inside the grain Areas with a grain size of 20 microns were. Next were fine as in the other experiments in the structure dispersed island-like particles consisting of carbides, martensite and retained austenite. The particle density was typically 40-60 particles per grain.

Claims (21)

A process for producing a ferritic cold-rolled strip in which molten steel forming a ferritic microstructure upon cooling is cast into a cast strip in which the cast strip is hot-rolled, if necessary, in-line, in which the hot-rolled strip is reeled and in the hot-rolled strip is cold rolled in one or more steps, the cold strip, characterized in that the cast strip between the casting and the reel, starting from a not lower than 1180 ° C lying start temperature with a minimum 150 ° C / s cooling rate to a maximum of 1000 ° C. is cooled intermediate temperature and then maintained for at least 10s at a 900-1000 ° C holding temperature. Method according to claim 1, characterized in that that the starting temperature of intensive cooling in the range of 1180-1270 ° C, in particular 1200-1250 ° C, is located. Method according to one of the preceding claims, characterized in that the molten steel is composed as follows (in% by weight): Cr: 10.5-18%, C: ≤ 0.08%, Si: ≦ 1% Mn: ≦ 1.5%, Ni: ≦ 1.00%, P: ≦ 0.04%, S: ≦ 0.015%, balance iron and unavoidable impurities. Method according to one of the preceding claims, characterized characterized in that the cooling rate 150-250 ° C / s. Method according to one of the preceding claims, characterized characterized in that the intermediate temperature is 900-1000 ° C. Method according to claim 5, characterized in that that the intermediate temperature is equal to the holding temperature. Method according to one of the preceding claims, characterized characterized in that the cast strip is hot rolled. Method according to claim 7, characterized in that that cooling and hold after hot rolling. Method according to claim 8, characterized in that that cooling of the hot rolled strip within less than three, in particular of less than a second after leaving the last hot rolling mill. Method according to claim 7, characterized in that that cooling to the intermediate temperature and holding at the holding temperature be completed before the hot rolling of the cast strip. Method according to claim 7, characterized in that that cooling to the intermediate temperature before hot rolling and holding at the holding temperature completed after hot rolling of the cast strip become. Method according to one of the preceding claims, characterized characterized in that the cast strip on cooling on one less than 900 ° C, in particular less than 800 ° C lying intermediate temperature and later cooled to the holding temperature reheated becomes. A method according to claim 7 and claim 12, characterized characterized in that the hot rolling amounts to less than 800 ° C Intermediate temperature performed becomes. Method according to one of claims 12 or 13, characterized that reheating within 1-5 Seconds. Method according to one of claims 12 to 14, characterized that casting, if necessary Hot rolling, reeling and the cooling performed between casting and reeling the intermediate temperature and holding at the holding temperature in one discontinuous procedure be completed. Method according to claim 15, characterized in that that the tape cools down to less than 500 ° C, in particular less than 400 ° C, temperature has cooled becomes. Method according to claim 16, characterized in that that the tape is cooled to room temperature after casting. Method according to one of claims 16 or 17, characterized that the band for hot rolling within 200s, especially within 100s, is heated to the respective hot rolling temperature. Method according to one of claims 1 to 14, characterized that casting, if necessary Hot rolling, reeling and the cooling performed between casting and reeling the intermediate temperature and holding at the holding temperature in one continuous procedure be completed. Method according to one of the preceding claims, characterized characterized in that the casting of the belt by means of a two-roller casting machine carried out becomes. Method according to one of the preceding claims, characterized characterized in that the thickness of the cast strip at most 6 mm.