US20140007992A1

US20140007992A1 - Method for Producing a Hot-Rolled Flat Steel Product

Info

Publication number: US20140007992A1
Application number: US13/978,969
Authority: US
Inventors: Evgeny Balichev; Jian Bian; Harald Hofmann
Original assignee: ThyssenKrupp Steel Europe AG
Current assignee: ThyssenKrupp Steel Europe AG
Priority date: 2011-01-11
Filing date: 2011-12-14
Publication date: 2014-01-09
Also published as: WO2012095232A1; ES2625281T3; JP2014505172A; CN103328120B; MX345374B; KR20130100215A; CN103328120A; DE102011000089A1; MX2013007874A; RU2013137456A; BR112013017753A2; CA2823095C; EP2663411A1; RU2554265C2; EP2663411B1; CA2823095A1

Abstract

A method for generating a flat steel product comprising the steps of: melting a steel melt, comprising, in addition to Fe and unavoidable impurities (in wt %) C: 0.5-1.3%, Mn: 18-26%, Al: 5.9-11.5%, Si: <1%, Cr: <8%, Ni: <3%, Mo: <2%, N: <0.1%, B: <0.1%, Cu: <5%, Nb: <1%, Ti: <1%, V: <1%, Ca: <0.05%, Zr: <0.1%, P: <0.04%, S: <0.04%; casting the steel melt into a cast strip; heating the cast strip to an initial hot-rolling temperature of 1100-1300° C. at a heating rate of at least 20 K/s; hot rolling the cast strip into a hot strip; cooling the hot strip within 10 seconds after the hot rolling at a cooling rate of at least 100 K/s to <400° C.; and winding the cooled hot strip into a coil at a coiling temperature of up to 400° C.

Description

The invention relates to a method for producing a hot-rolled flat steel product from a high-strength, highly ductile manganese steel, which in addition to a high Mn content has an Al content of 5.9-11.5 wt. %.
A steel of this kind and a method for the production thereof are known from, for example, DE-AS 1 262 613. According to the method described in that publication blocks with a low diameter are cast from molten steel with a suitable composition, which are then hot-rolled to form bar stock. Through heat treatment at 800-1250° C. the elongation and the notched impact strength of the material obtained in this way can be improved. From the stock obtained in this way, components for aircraft, floors, turbines, gears, valves and so on are to be made.
More recent developments have shown that the steels of the kind indicated above, because of a very good combination of characteristics of high strength, high deformability, a significantly reduced density and an associated minimised weight are particularly suited to flat products, thus to steel strips or sheets, in particular to the manufacture of components for motor vehicle manufacture, in particular the construction of car bodies or chassis parts.
The problem here, however, is that the steels concerned, because of their alloying state generated via conventional routes, as normally applied to steels with a high carbon content, are difficult to process. Thus the known steels have a high tendency towards core segregations of Mn and Al during casting and solidification. Furthermore with these there is an increased danger that surface cracks will result during continuous casting and the strand bending back while removing it from the casting mould. Furthermore, because of their low thermal conductivity as a rule long preheating times are necessary in order to bring the slabs cast from the steels in question up to the temperature necessary for hot rolling. The long oven dwell times of the slabs are associated with a pronounced tendency towards surface decarburisation. At the same time the low thermal conductivity brings with it the problem that during preheating, bloom and hot-rolling cracks form as a result of the recrystallisation inertia of the cool strip edges. Finally, the steels offer extremely high resistances, to heat and cold during hot- and cold-rolling which are considerably higher than with other high-alloy steels, such as for example RSH steels or conventional high-alloy Mn steels.
From U.S. Pat. No. 7,794,552 B2 a method is known for generating a flat steel product from such a conventionally composed, austenitic, high manganese content hot-rolled steel, which apart from iron and unavoidable impurities (in wt. %) contains 0.85%-1.05% C; 16%-19% Mn; up to 2% Si; up to 0.050% Al; up to 0.030% S; up to 0.050% P; up to 0.1% N, and, optionally, one or a plurality of elements chosen from “Cr, Mo, Ni, Cu, Nb, V”, provided that the Cr content is up to 1%; the Mo content is up to 1.5%; the Ni content is up to 1%; the Cu content is up to 5%; the Ti content is up to 0.50%; the Nb content is up to 0.50%; the V content is up to 0.50%. The recrystallised surface fraction of the steel strip or sheet being equal to 100% here, while the recrystallised surface fraction of precipitated carbides being equal to 0%. At the same time the average grain size of the steel will be ≦10 μm. The strength of the known steel created in this way is larger than 1200 MPa and the product of the strength and the elongation at break is larger than 65 000 MPa.
In order to achieve this, according to the known method a correspondingly composed steel melt is cast to form a semi-finished product, which can be a slab, thin slab or cast strip. The semi-finished product is heated to a temperature of 1100-1300° C. and at an end-of-rolling temperature of at least 900° C. rolled into a hot sheet. If necessary then a holding time is observed in order to achieve the desired complete recrystallisation of the strip surface. The hot strip obtained is then cooled at a cooling rate of at least 20° C./s to a maximum coiling temperature of 400° C. and wound into a coil. The hot strip obtained in this way can then, with intermediate annealing as necessary, be rolled into a cold strip.
The method known from U.S. Pat. No. 7,794,552 B2 is intended for steels where although during smelting Al can be used for deoxidation, the Al content is restricted to a maximum of 0.05 wt. %, in order to avoid the precipitation of AlN. The presence of AlN precipitations will accordingly pose a danger of the formation of cracks during deformation of the steel strip generated in the known manner.
Against the background of the prior art mentioned above, the object of the invention was to indicate an economical and reliably operating method for generating a flat steel product from a steel comprising a high Al content in addition to a high Mn content.
According to the invention this object is achieved by the method indicated in claim 1. Advantageous configurations of the method according to the invention are indicated in the dependent claims.
According to the invention for the production of a hot-rolled flat steel product initially a steel is melted comprising, in addition to iron and unavoidable impurities (in wt. %) C: 0.5-1.3%, Mn: 18-26%, Al: 5.9-11.5%, Si: less than 1%, Cr: less than 8%, Ni: less than 3%, Mo: less than 2%, N: less than 0.1%, B: less than 0.1%, Cu: less than 5%, Nb: less than 1%, Ti: less than 1%, V: less than 1%, Ca: less than 0.05%, Zr: less than 0.1%, P: less than 0.04%, and S: less than 0.04%.
In practical configurations of the invention here the contents of the alloying elements Si, Cr, Ni, Mo, N, B, Cu, Nb, Ti, V, Ca, Zr, P and S individually or in combination with one another are set according to the following (in wt. %): 0.1-0.4% Si, <3.0% Cr, <1.0% Ni, _<0.5% Mo, 0.005-0.04% N, <0.0050% B, <1% Cu, <0.2% Nb, <0.3% Ti, <0.3% V, <0.005% Ca, <0.005% Zr, 0.01-0.03% P or 0.005-0.02% S.
A steel melt with the abovementioned composition is then for example cast in a conventional two roller casting machine in a manner known per se to form a cast strip.
The advantage of casting the melt into a cast strip is known to be that with strip casting as a result of the rapid hardening less segregations occur. In high-alloy steels of the kind processed according to the invention this is particularly advantageous, because through a more even distribution of the alloying elements homogenous strip characteristics and optimum quality of the product obtained are achieved.
If for generating the cast strip a conventional two roller casting machine is used, in which the cast strip emerges in a vertical direction and by means of a strand guidance device is diverted in an arc into a horizontal direction of conveyance, then the cast strip cools on its way from the casting machine to the heating device typically at a cooling rate of 10-20 K/s to an intermediate temperature of as a rule not less than 700° C. According to the invention this temperature loss is kept as low as possible, so that the intrinsic casting heat of the cast strip upon leaving the casting machine is retained to the greatest possible extent as far as the heating device. In this way the amount of energy needed in the heating device for the increase in temperature to the initial hot rolling temperature carried out there can be minimised.
The heating of the cast strip to the respective initial hot rolling temperature in the range 1100-1300° C. takes place according to the invention at a heating rate of at least 20 K/s.
The cast strip which is in this way rapidly heated to the initial hot rolling temperature is then hot rolled in one or more passes to a hot strip.
Within 10 seconds of the end of hot rolling according to the invention cooling then commences, during which the hot strip obtained is cooled with a cooling rate of at least 100 K/s to <400° C. Through this rapid cooling the formation of components with an embrittling effect, such as carbides or intermetallic phases, is suppressed.
Finally, the cooled hot strip is wound at a coiling temperature of up to 400° C. to form a coil.
The individual work stages of the method according to the invention are performed in a continuous, uninterrupted sequence.
The invention is based on the knowledge that the production of a flat steel product free of edge or surface cracks from a steel having a high content of C, Mn and Al, is successful if from a melt with a corresponding composition a thin, maximum 5 mm, in particular 3-5 mm, thick, strip is cast. The thickness of the cast strip accordingly is already in the range of the thickness that the hot rolled flat product generated is ultimately to have.
The possibility used by the method according to the invention of casting a steel with a high content of C, Al and Mn, in strip casting and the associated rapid hardening of the steel after casting reduces the frequency of core segregations in the cast strip. Transversal cracks and crazing do not occur at all during casting of the cast strip and longitudinal cracks only to a very limited extent. When casting the strip in a two roller casting machine the occurrence of core segregations through variation in the casting roller force can be controlled.
The thin cast strip, according to the invention only a maximum of 5 mm, in particular 3-5 mm, thick, when leaving the roller gap already has a favourable cross-section with low bending stresses. Accordingly, the cast strip can be bent without problems from a vertical to a horizontal direction of conveyance, in which it passes through the further stations for its processing.
At the same time, through use of the strip casting the surface decarburisation is significantly reduced, since arduous slab heating is no longer necessary. The danger of crack formation during hot rolling is minimised due to the homogenised temperature distribution which is achieved during the rapid heating carried out according to the invention prior to hot rolling.
The cast strip according to the invention is characterised by a three-layer cast structure with dendritic marginal zones and a globular core.
The cast strip is heated using to the greatest possible extent the intrinsic casting heat upon leaving the casting machine to the required initial hot rolling temperature of 1100-1300° C. Here the heating takes place as quickly as possible, in particular at a heating rate of at least 20 K/s.
With the heating performed according to the invention the temperature increase achieved in the cast strip is typically up to 250° C., wherein the minimum increase in temperature is typically 50° C. Apart from avoiding the occurrence of undesired precipitations through the rapid heating of the strip performed according to the invention the temperature distribution across the width of the strip can be specifically set. Thus on the one hand it is possible, through the rapid heating to homogenise the temperature distribution. In order to achieve a certain deformation behaviour of the cast strip during the hot rolling process, on the other hand the heating can also be carried out in such a way that across the width of the cast strip a defined temperature profile occurs. In this way unevenness in the strip, deviations from directional stability and other geometric defects in the strip can be achieved, without the need for expensive additional measures.
For the accelerated heating to the initial hot rolling temperature an inductive heating device is in particular suitable, such as the one described in DE 10323796 B3. The advantage of using an induction furnace for rapid heating or soaking of the product to be rolled is that the rolled material after a short heating time can be heated to a precisely definable temperature.
The initial hot rolling temperature reached in the course of the rapid heating is selected in such a way that the rolling resistances, working against the cast strip during hot rolling, are minimised. This is in particular the case if the initial hot rolling temperature is at least 1050° C. The final hot rolling temperature of the hot rolling performed according to the invention is typically in the range 1000-1050° C. here. This stipulation is based on the knowledge that the steel to be processed according to the invention, because of its high aluminium content must be processed in a narrow temperature window.
The hot rolling of the cast band performed in-line with the strip casting reduces the process- and material-related core porosity of the cast strip, promotes homogeneity of the microstructure and thus improves the strip characteristics overall.
The hot rolling of the cast strip which is difficult to roll per se is also made easier by the fact that the cast strip prior to hot rolling already has a thickness close to the final dimension, so that in the course of the hot rolling only comparably low degrees of deformation need to be achieved. These are typically at least 10%, in particular 10-20%. Such low degrees of deformation can be achieved in a single pass, which further contributes towards optimising the economic efficiency of the method according to the invention.
The rapid cooling performed subsequent to the hot rolling with a cooling rate of at least 100 K/s ensures that in the hot strip obtained after leaving the final hot rolling mill no grain growth takes place. Furthermore, in this way at this point in the method according to the invention the precipitation of carbides, nitrides and carbonitrides is prevented. Typically the cooling rates achieved during the cooling following hot rolling are in the range 100-250 K/s.
In order to reliably prevent the start of grain growth, the cooling should as far as possible commence within the shortest possible timeframe from the end of hot rolling, but at the outside within 10 s.
In order to avoid oxidation of the melt and the cast strip on its way to the hot rolling device, in the method according to the invention the work steps performed prior to the hot rolling can be performed under a protective gas atmosphere. Inertisation carried out in the respective strip casting device of the meniscus region of the steel melt awaiting casting there reduces the formation of oxide coatings of the surfaces.
The hot strip obtained according to the invention has an austenitic-ferritic structure with a ferritic content of typically 5-50%.
Carbon can be present in a steel according to the invention in contents of 0.5-1.2 wt. %, wherein here in particular steels are considered whose C content is above 0.5 wt. %. The C content is important for the austenite formation and for the strength grade due to solid solution hardening, an increase in the stacking fault energy and the formation of carbides. Where the hot strip generated according to the invention is cold-rolled into a cold strip, in order to improve the yield strength of the cold strip by means of a specific over-ageing treatment following a final recrystallisation annealing on the cold strip an extremely fine carbide can be precipitated. At C contents of above 1.2 wt. % there is a danger of carbide occurring in quantities having an embrittling effect.
Manganese is present in a steel processed according to the invention in contents of 18-26 wt. %. Manganese is important for austenite formation and increases the stacking fault energy, which has a favourable effect on the processability and deformability.
A steel processed according to the invention has 5.9-11.5 wt. %, in particular >6-11.5 wt. % of Al. Aluminium reduces the density, has a solid solution hardening effect and increases the stacking fault energy. Aluminium also has a passivating effect and increases the resistance to corrosion. As a result of the very high stacking fault energy, the high contents of Al lead to the manifestation of the so-called “shear bend plasticity” as the dominating deformation mechanism with a particularly good combination of strengths and deformability. Excessively high aluminium contents, however, can bring about a highly embrittling DO₃order structure in the ferrite or excessive contents of Al-containing k-carbides ((Fe, MN)₃AlC) with an embrittling effect.
Si can be present in a steel processed according to the invention in contents of less than 1 wt. %, in particular 0.1-0.4 wt. %, in order to bring about solid solution hardening. Contents of Si in excess of 1 wt. % make welding and painting of the steel processed according to the invention more difficult however.
Cr, Ni and Mo likewise have a solid solution hardening effect and improve the oxidation and corrosion resistance of the steel processed according to the invention. At excessively high contents, however, Cr leads to the formation of special carbides, which can have a highly embrittling effect. Of optimum use are the positive effects of Cr, Ni and Mo, if, as specified by the invention, in a steel processed according to the invention the Cr content is restricted to less than 8 wt %, in particular less than wt. %, the Ni content to less than 3 wt. %, in particular less than 1 wt. %, and the No content to less than 2 wt. %, in particular to less than 0.5 wt. %.
Together with aluminium nitrogen forms nitrides and has a strength-increasing effect. Excessive contents of N, however, lead to coarse AlN, which can have a negative effect on the processability, the surface quality and the deformability of a steel processed according to the invention. Therefore the N content of a steel according to the invention is restricted to N<0.1 wt. %, in particular 0.005-0.04 wt. %.
The B content of a steel according to the invention is restricted to <0.1 wt. %, in particular less than 0.0050 wt. %. B has a strength-increasing effect and forms boron nitrides and carbides, which act as nucleation points for the occurrence of other carbides. Due to grain boundary precipitations, excessive B contents have an embrittling effect.
In steel processed according to the invention Cu has a solid solution hardening effect and increases the corrosion resistance. With excessively high Cu contents, however, there is a risk of hot cracking during hot rolling or hot joining. Therefore the Cu content of a steel processed according to the invention is restricted to less than 5 wt. %, in particular less than 1 wt. %.
The micro-alloying elements Nb, Ti and V lead to precipitations and grain refinement and thus contribute to the increase in strength. In addition, through the grain refinement effect, these elements reduce the tendency of the steel to develop weld cracks during hot joining.
Optimum use can be made of these effects if a steel processed according to the invention has Nb, Ti or V each in contents of less than 1.0 wt. %, and the Nb content is restricted in particular to <0.2 wt. %, the Ti content in particular to <0.3 wt. %, and the V content in particular to <0.3 wt. %.
Ca in contents of less than 0.05 wt. %, in particular <0.005 wt. %, spheroidises non-metallic materials such as Al₂O₃and FeS in steel processed according to the invention and improves the deformability. The formation of Ca aluminates converts alumina into slag and improves the purity.
In contents of less than 0.1 wt. %, in particular <0.005 wt. %, in steel processed according to the invention Zr has a solid solution hardening effect. Since, however, due to grain boundary segregations, Zr also has an embrittling effect, the content of this element in a steel processed according to the invention is restricted.
P and S segregate in a steel processed according to the invention at the grain boundaries and have an embrittling effect. As a result their content should be as low as possible, in particular lower than 0.04 wt. %. wherein the P content is advantageously 0.01-0.03 wt. % and the S content advantageously 0.005-0.02 wt. %.
In order to guarantee an optimum deformability of the hot strip obtained according to the invention, after winding and before further processing hot strip annealing is performed during which the hot strip obtained according to the invention is annealed at an annealing temperature of 1100-1200° C. If the hot strip annealing takes place in a continuous annealing furnace, annealing times of 60-300 s are required for this. Such hot strip annealing is expedient in particular if the Al content of the steel processed according to the invention is at least 10 wt %. In the case of such high Al contents it is also expedient, in order to avoid the formation of brittle phases, to have the cooling take place after hot rolling as quickly as possible, in particular at a cooling rate of at least 40 K/s.
The hot strip obtained according to the invention can optionally be pickled in the normal manner following coiling and used in the coated or uncoated state. It is similarly possible to coat the hot strip generated according to the invention following an optionally performed pickling in a manner known per se with a metallic protective coating, e.g. a corrosion-proofing coating. It is furthermore conceivable to provide the hot-rolled flat product generated according to the invention with coatings by which the deformation of the hot strip is simplified.
With the procedure according to the invention there is the possibility of cold-rolling the hot strips obtained according to the invention to form cold strip products, which can then undergo a recrystallisation annealing, over-ageing annealing (precipitation hardening by fine carbides) and various forms of surface refinement (Z, ZE, ZN, FAL). Here, for example, cold rolling and a subsequent recrystallisation annealing condenses and homogenises the microstructure in the core area.
If flat steel products with even lower thicknesses are required, then the hot strip generated according to the invention accordingly allows in a manner known per se a cold strip to be processed in one or more passes. This can if necessary in turn be surface coated if necessary to protect it against environmental influences.
Because the strip has already been cast at close to the final dimension and because of the only minor associated deformations that are necessary during hot and cold rolling, the intrinsically high resistance to hot rolling and cold rolling of the steel processed according to the invention has only an insignificant effect. This allows flat products of low thickness to be generated even from steels of the kind processed according to the invention which are problematical in terms of rolling treatment.
In the following the invention is explained in more detail using embodiments.
The Figure is a schematic representation of a production line 1 for production of a hot strip W.
The production line 1 which is set up for a continuous production sequence comprises a conventional two roller casting machine 1, in which a melt S is cast in the gap delimited by two rollers 2, 3 rotating in opposite directions into a cast strip G, the thickness of which is typically 3-5 mm. The cast strip G emerging in the vertical direction is, in a manner, likewise known per se, diverted via a strand guidance device in a horizontal direction of conveyance F, in which it is driven forward by means of a conveyor device 4 arranged at the end of the strand guidance.
The cast strip G aligned in this way and moving in the direction of conveyance F enters a heating device 5. On its way to the heating device 5 the cast strip G cools at a cooling rate of 10-20 K/s to an intermediate temperature.
In the heating device 5 the cast strip G entering there with the intermediate temperature is inductively heated by means of inductors 6 aligned transversally to the direction of conveyance F to an initial hot rolling temperature which is typically in the range 1100-1300° C., in particular at least 1150° C.
The increase in temperature of the cast strip G achieved by passing through the heating device as a result of the electromagnetic field generated by the inductors 6 is up to 300° C., typically 50-150° C. Here the inductors 6, as for example described in DE 103 23 796 B3, can be adjustable and controllable, such that on the one hand the cast strip G over its entire width heats evenly and on the other a defined temperature profile can be set in the cast strip G.
In order to avoid contact of the melt S and the cast strip G with the ambient atmosphere U, the two roller casting machine 1, the strand guidance device, the conveyor device 4 and the heating device 5 are kept under a protective gas atmosphere S.
After the heating device 5 the cast strip G enters a rolling mill 9, in which in a single pass it is hot rolled into a hot strip W with a thickness of typically 2.4-4.5 mm. The final hot rolling temperature, at which the hot strip W leaves the final rolling mill 9 in the direction of conveyance F, is generally in the range 1 000-1 050° C. here. The degrees of deformation achieved via the single feed rollers are generally in the range 10-30%.
Within 10 s of leaving the rolling mill 9 the hot strip W obtained is cooled in a cooling device 10 at a cooling rate of typically 100-200 K/s, to a coiling temperature in the range 300-400 at which the hot strip W is then wound by a coiling device 11 into a coil C.
Hot strip annealing in a heat treatment device not shown here can follow coiling.
In the production line 1 in the abovementioned manner four hot strips are generated from melts S1-S3, the compositions of which are given in Table 1.
The strips G cast from each of the melts S1-S3 are cooled on the way to the heating device 5 at a cooling rate in each case of approximately 15 K/s and in the heating device 5 heated by a temperature increase ΔT to the respective initial hot rolling temperature HAT and in the hot rolling mill 9 in three passes with a total degree of deformation φg and a final hot rolling temperature WET in each case hot rolled into a hot strip W with a thickness dWB. Immediately afterwards the hot strips W are in each case cooled at a cooling rate tk to the respective coiling temperature HAT, at which they are in each case coiled into a coil C. The parameters indicated in each case for the processing of the strips G cast from the steels S1-S3 ΔT, HAT, WET, φg, dW, tK and HAT are shown in Table 2.
The hot strip generated from the steel S3 following coiling also undergoes hot strip annealing at 1100° C. for 120 s in a continuous annealing furnace. In this way, even with the hot strip generated from this steel S3, despite its high C, Mn and Al content, surface defects can be reliably prevented.
Table 3 indicates the structure, together with the mechanical characteristics of hot strip thickness dWB, density ρWB, yield strength Rp0,2, tensile strength Rm, elongation A80, n-value and r-value of the hot strips generated from the steels S1-S3 by the procedure according to the invention explained here.
References
1 Production line
2, 3 Casting rollers
4 Conveyance device
5 Heating device
6 Inductors
9 Rolling mill
10 Cooling device
11 Coiling device
A Protective gas atmosphere
C Coil
F Direction of conveyance
G Cast strip
S Melt
U Ambient atmosphere
W Hot strip

TABLE 1

Steel	C	Mn	Al	Σ other

S1	0.55	18.0	6.0	≦0.14
S2	0.75	24.0	9.0	≦0.09
S3	1.25	26.0	11.5	≦0.15

Figures expressed as wt. %, remainder iron and unavoidable impurities

TABLE 2

		ΔT	WAT	WET	φg	tk	HAT
	Steel	[° C.]	[° C.]	[° C.]	[%]	[K/s]	[° C.]

	S1	150	1150	1020	17	200	300
	S2	120	1150	1030	15	200	300
	S3	150	1150	1025	14	200	300

TABLE 3

	dwB	ρWB	Rp0, 2	Rm	Ag

Steel	Structure	[mm]	[g/cm³]	[MPa]	[MPa]	[%]	n	r

S1	Austenitic-	3.1	7.2	519	761	33.1	0.26	0.90
	ferritic
S2	Austenitic-	3.0	7.0	680	900	32.5	0.23	0.69
	ferritic
S3	Austenitic-	3.0	6.8	680	920	35	0.26	0.76
	ferritic
	Fine
	precipitations
	of
	k-carbides

N.D. = Not determined

Claims

1. A method for producing a hot-rolled flat steel product comprising the following working steps:

melting a steel melt, comprising, in addition to iron and unavoidable impurities (in wt %)

C: 0.5-1.3%,

Mn: 18-26%,

Al: 5.9-11.5%,

Si: less than 1%,

Cr: less than 8%,

Ni: less than 3%,

Mo: less than 2%,

N: less than 0.1%,

B: less than 0.1%,

Cu: less than 5%,

Nb: less than 1%,

Ti: less than 1%,

V: less than 1%,

Ca: less than 0.05%,

Zr: less than 0.1%,

P: less than 0.04%,

S: less than 0.04%,

casting the steel melt into a cast strip,

heating the cast strip to an initial hot-rolling temperature of 1100-1300° C. at a heating rate of at least 20 K/s,

hot rolling the cast strip heated to the initial hot-rolling temperature into a hot strip,

cooling of the hot strip, the cooling starting within 10 seconds after the hot rolling at a cooling rate of at least 100 K/s to <400° C.,

winding the cooled hot strip into a coil at a coiling temperature of up to 400° C.

2. The method according to claim 1, wherein the steel melt contains (in wt. %) 0.1-0.4% Si, <3.0% Cr, <1.0% Ni, <0.5% Mo, 0.005-0.04% N, <0.0050% B, <1% Cu, <0.2% Nb, <0.3% Ti, <0.3% V, <0.005% Ca, <0.005% Zr, 0.01-0.03% P or 0.005-0.02% S.

3. The method according to claim 1, wherein the casting of the steel melt into a cast strip is performed in a two roller casting machine.

4. The method according to claim 1, wherein the thickness of the cast strip is a maximum of 5 mm.

5. The method according to claim 1, wherein the heating to the initial hot rolling temperature is performed by means of an inductively operating heating device.

6. The method according to claim 1, wherein the initial hot rolling temperature, to which the cast strip is heated, is at least 1150° C.

7. The method according to claim 1, wherein a degree of deformation achieved in the course of the hot rolling is at least 10%.

8. The method according to claim 1, wherein a final hot rolling temperature of the hot rolling is 1000-1050° C.

9. The method according to claim 1, wherein the hot rolling takes place in a single pass.

10. The method according to claim 1, wherein the cooling of the hot strip begins within 10 seconds of the end of hot rolling.

11. The method according to claim 1, wherein the working steps performed prior to hot rolling are carried out under a protective atmosphere.

12. The method according to claim 1, further comprising hot strip annealing the hot strip obtained at an annealing temperature of 900-1150° C.

13. The method according to claim 12, wherein the Al content of the cast strip is at least 10 wt. %.

14. The method according to claim 1, further comprising cold-rolling the hot strip into a cold strip.

15. The method according to claim 1, wherein a degree of deformation achieved in the course of the hot rolling is 10-20%.