EP1169486B1 - Method of producing a hot-rolled strip - Google Patents

Abstract

The invention relates to a method for producing hot strip which features good forming ability and increased strength. This is achieved in that a hot strip (W) which is produced in particular from continuous casting in the shape of reheated slabs or slabs obtained directly from the casting heat, from thin slabs or cast strip, based on a steel comprising (in mass %) C: 0.001-1.05%; Si: <=1.5%; Mn: 0.05-3.5%; Al: <=2.5%, if necessary further elements such as Cu, Ni, Mo, N, Ti, Nb, V, Zn, B, P, Cr, Ca and/or S, with the remainder being iron as well as the usual accompanying elements, is continuously finish rolled and subsequently continuously cooled, with cooling taking place in at least two subsequent cooling phases (tCK, tLK) of accelerated cooling, to a final temperature; with the first cooling phase (tCK) of accelerated cooling starting at the latest three seconds after the last pass of finish rolling; and with the hot strip (W) during the first cooling phase (tCK) of accelerated cooling being cooled at a cooling rate of at least 150° C./s.

Description

The invention relates to a method for generating a Steel hot strip, in which the hot strip after the Finishing rolling a carried out in several stages Cooling is subjected.

After cooling a hot strip after the in multiple stitches of finishing rolls comes into play a significant on the material properties of the tape Meaning too. By using an appropriate Among other things, the structure can be cooled as such and the proportions of the individual structure types influence this structure. So it is possible through cooling, for example, strength, toughness and To influence the hardness of a hot strip.

In the article "Hot rolled coils for special applications ", A. De Vito et al., BTF - special issue 1986, pages 137-141, are various studies described the influence of cooling in the Document hot strip production. Have these investigations shown that, for example, in the manufacture of a Dual-phase hot strip steel (DP hot strip steel) is useful is the cooling in after the finish rolling perform three stages. In the first and the last of these three stages, the belt passes through two conventionally trained, spaced from each other Laminar cooling sections, where cooling liquid is in the form a variety of in the conveying direction of the belt sequential veils on the ribbon is sprayed. The cooling rate achieved is in the first stage of cooling at around 70 ° C / s. The The belt is cooled in the third stage slower than in the first stage.

In the one between the laminar cooling sections Intermediate cooling takes place in the known Process in air, taking place at this stage cooling rate reached again much lower lies in the final stage of cooling.

It has been shown that the above explained known methods without the presence of Molybdenum in its composition DP hot-rolled steels Have them made with pronounced martensite and Ferrite shares are present. The concerned Hot strips have increased strength and toughness on.

At the same time, however, there must be a loss of ductility to be accepted. Beyond that it was found that with the known method The improvements achieved are not sufficient to achieve the especially with regard to the hardness of such manufactured hot strips to requirements fulfill.

In addition to the prior art explained above, JP 10-195588 A discloses a method for producing a hot strip from a slab, which (in% by weight) 0.02-0.2% C, 0.1-1, 5% Si, 0.5-3.0% Mn and one or more of the elements Cr and Mo each with 0.1-2.0%, the rest iron. The hot strip is hot-rolled according to the known method at a final temperature corresponding at least to the Ar ₃ temperature to thicknesses which are typically in the range from 0.8 to 1.4 mm. After leaving the hot rolling mill, the hot strips are continuously cooled in a first stage that begins 0.1 to 5 s after hot rolling with a cooling rate in the temperature range of 680 - 720 ° C that is more than 50 ° C. The first stage of cooling is followed by an intermediate cooling phase, during which the belt cools in air for up to 15 seconds. A third cooling phase then follows, in which cooling is carried out at a cooling rate of at least 30 ° C./s in a temperature range from 300 to 600 ° C. Finally the tape is coiled.

A comparable method is known from EP-A-0 719 868 known. According to this known method, a Slab with (in% by weight) 0.001 - 0.1% C, <1.5% Si, 0.5 - 3.0% Mn, 0.01-0.1% Al, and one or more of the Elements P (0.05 - 0.15%) and Cr (0.5 - 1.5%), the rest Iron, at a hot rolling end temperature of up to 850 ° C continuously rolled into hot strip. Within A first is carried out 0.5 s after the finish hot rolling Cooling down at temperatures between 650 - 750 ° C lie. The cooling rate is at least 30 ° C / s, typically 42 - 70 ° C / s. To the The first cooling stage is followed by an intermediate cooling phase at which the belt cools in air for 4 to 60 seconds. This is followed by a third cooling at cooling rates of at least 30 ° C / s to between 100 ° C and 500 ° C lying reel temperature.

The object of the invention is to provide a method create with which hot strips can be produced which have high formability and increased strength exhibit.

• Cu, Ni, Mo mit einem Anteil ≤ 0,8 %,
• N, Ti, Nb, V, Zn, B mit einem Anteil ≤ 0,5 %,
• P mit einem Anteil ≤ 0,09 %,
• Cr mit einem Anteil ≤ 1,5 % und / oder
• S mit einem Anteil ≤ 0,02 %,

und als Rest Eisen sowie übliche Begleitelemente enthält, wobei der Stahl ebenso wahlweise in der Flüssigphase mit Ca oder Ca-Trägerlegierungen behandelt sein kann, umfassend die folgenden Schritte:

• Kontinuierliches Fertigwalzen des Warmbandes (W),
• kontinuierliches Abkühlen des Warmbandes (W) in mindestens zwei aufeinander folgenden Kühlphasen (t_CK, t_LK) beschleunigter Kühlung auf eine Endtemperatur,
• wobei die erste Kühlphase (t_CK) beschleunigter Kühlung spätestens drei Sekunden nach dem letzten Walzstich des Fertigwalzens beginnt und
• wobei das Warmband (W) während der ersten Kühlphase (t_CK) beschleunigter Kühlung mit einer Abkühlgeschwindigkeit von mindestens 250 °C/s gekühlt wird.

This object is achieved according to the invention by a method for producing a hot strip, which is produced in particular from continuous casting in the form of reheated slabs or slabs used directly from the casting heat, from thin slabs or from cast strip based on a steel which (in mass%) C 0.001 - 1.05%, Si ≤ 1.5%, Mn 0.05 - 3.5%, al ≤ 2.5%, and optionally one or more of the elements

• Cu, Ni, Mo with a proportion ≤ 0.8%,
• N, Ti, Nb, V, Zn, B with a proportion ≤ 0.5%,
• P with a proportion ≤ 0.09%,
• Cr with a proportion ≤ 1.5% and / or
• S with a proportion ≤ 0.02%,

and the balance contains iron as well as usual accompanying elements, wherein the steel can also be optionally treated in the liquid phase with Ca or Ca carrier alloys, comprising the following steps:

• Continuous finishing of the hot strip (W),
• continuous cooling of the hot strip (W) in at least two successive cooling phases (t _CK , t _LK ) accelerated cooling to a final temperature,
• the first cooling phase (t _CK ) of accelerated cooling begins no later than three seconds after the last pass of the finish rolling and
• the hot strip (W) being cooled during the first cooling phase (t _CK ) of accelerated cooling at a cooling rate of at least 250 ° C./s.

According to the invention, the hot strip is cooled also in at least two consecutive through stages. The hot strip is in the the first cooling phase cooled much faster than the State of the art. This compact cooling during the first cooling phase has the consequence that the γ / α conversion of effective and targeted in the γ-region of hot-rolled strip is suppressed towards lower temperatures. In the then run through the second cooling phase The tape is then accelerated to the cooling brought desired final temperature. In this cooling phase the hardness-increasing second phases of the hot strip structure, like martensite, bainite and residual austenite, set. (At the end of the second cooling phase accelerated cooling can reach the final temperature of course, depending on the desired processing results required Act reel temperature.)

Around the γ / α conversion safely down to lower temperatures to suppress the phase of compact cooling according to the invention at the highest possible cooling rates and in as close as possible to the last stitch of the Finish rolling. Therefore the in cooling rate to be achieved in this phase at least 250 ° C / s.

Depending on the desired material properties can be used for the production of the hot strip Steel optionally contain additional elements. there in the event of their presence, the proportion (in mass%) of Cu, Ni, Mo not more than 0.8%, that of N, Ti, Nb, V, Zn, B not larger than 0.5%, that of P not larger than 0.09%, that of Cr not greater than 1.5% and that of S should not be greater than 0.02%.

Experiments have shown that, among other things in particular such steels of the aforementioned type for carrying out the method according to the invention are suitable, which contain 0.005 to 0.4 mass% silicon.

The method according to the invention is firstly for generating suitable for hot strips, which are based on steels are manufactured with low carbon contents. So is an advantageous variant of the invention Process characterized in that the steel (in Mass%) not more than 0.07% C, not more than 0.2% Si, not more than 0.6% Mn and not more than 0.08% Al contains the hot strip during finish rolling in Austenite area is rolled, the hot strip in the first Cooling phase of accelerated cooling starting from one Temperature above 850 ° C to a temperature of 680 is cooled to 750 ° C, the hot strip in the second Cooling phase accelerated cooling to a temperature of is cooled to less than 600 ° C and finally coiled becomes.

The method of manufacture according to the invention is likewise suitable for DP hot strip steels. A corresponding one Design of the method according to the invention characterized in that the steel (in mass%) 0.04 - 0.09% C, not more than 0.2% Si, 0.5 - 2.0% Mn, 0.02 - Contains 0.09% P and not more than 0.9% Cr, and that the Hot strip after finish rolling in the first cooling phase accelerated cooling based on a temperature above 800 ° C to a temperature of 650 to 730 ° C is cooled that the hot strip in the second cooling phase accelerated cooling cooled to less than 500 ° C and that the hot strip is then coiled.

Even with steels with a higher carbon content improvements in the procedure according to the invention of the material properties. According to one a further embodiment of the invention, a hot strip, which on a steel with (in mass%) 0.25 - 1.05% C, not more than 0.25% Si and not more than 0.6% Mn based, after finishing rolling in the first cooling phase accelerated cooling based on a temperature above 800 ° C to a temperature of 530 to 620 ° C cooled, in the second cooling phase accelerated cooling cooled to less than 500 ° C and then coiled. A hot strip produced in this way has also improved hardness and better Forming properties compared to conventionally produced Tapes.

In the case of an aluminum-containing TRIP hot strip, which (in mass%) contains 0.12-0.3% C, 1.2-3.5% Mn and 1.1-2.2% Al, and in the manner according to the invention after finish rolling in the first cooling phase, starting from a temperature which is between the Ar ₃ temperature and a temperature of Ar ₃ + 150 ° C., is cooled to a temperature which is up to 50 ° C. below the Ar ₃ temperature , is cooled to 350 to 550 ° C in the second cooling phase and is then coiled, improvements in strength can also be found with high formability.

Another advantageous variant of the invention The method is characterized in that the steel (in Mass%) 0.04 - 0.09% C, 0.5 - 1.5% Si, 0.5 - 2.0% Mn, 0.4 - 2.5% Al, no more than 0.09% P and no more than 0.9% Cr contains that the hot strip after Finishing rolls accelerated in the first cooling phase Cooling from a temperature above 800 ° C is cooled to a temperature of 650 to 730 ° C that the hot strip accelerated in the second cooling phase Cooling is cooled to less than 500 ° C and that Hot strip is then coiled. Such one Hot strip has DP and TRIP properties.

A structural steel with an increased ferrite content and from it The following particularly good formability can be thereby make the steel (in mass%) 0.07 - 0.22% C, 0.1 - 0.45% Si and 0.2 - 1.5% Mn contains that the Hot strip after finish rolling in the first cooling phase accelerated cooling based on a temperature above 800 ° C to a temperature of 650 to 730 ° C is cooled that the hot strip in the second cooling phase accelerated cooling cooled to less than 500 ° C and that the hot strip is then coiled. A hot strip can be made with the same steel composition with improved hardness, on the other hand, that the hot strip after finish rolling in the first Cooling phase of accelerated cooling starting from one Temperature above 800 ° C to a temperature of 580 is cooled to 650 ° C that the hot strip in the second Cooling phase of accelerated cooling to less than 500 ° C is cooled and that the hot strip is then coiled becomes. The hot strip cooled in this way has one reduced ferrite content, higher bainite and martensite contents on.

According to an appropriate embodiment of the Invention passes through the hot strip between the first Cooling phase and the second cooling phase accelerated Cooling an intermediate cooling phase during which the hot strip is exposed to air cooling. This intermediate cooling phase should take at least a second. In the course of the first phase more compact, i.e. strongly accelerated cooling subsequent intermediate phase, in the cooling in air takes place the austenite transformation in ferrite faster and reaches one larger scope than in the prior art, wherein at the same time a strong grain-refining effect watch is.

It has surprisingly been found that the procedure according to the invention can produce a hot strip, which compared to a conventional one Process in two laminar cooling stages with intermediate cooling in air-cooled hot strip same composition an increased hardness and a has a finer grain structure. At the same time the tape produced by the method according to the invention high strength and, unlike that after known methods produced tapes, a good one Formability on.

According to a preferred embodiment of the invention The first cooling phase therefore begins at the latest two Seconds after the last pass of finish rolling.

A further advantageous embodiment of the method, with which a hot strip of particularly good formability can be produced, is characterized in that at least one of the rolling passes is carried out during the finish rolling in the austenite region below a temperature of Ar ₃ + 80 ° C and that the total pass decrease during of finish rolling is more than 30%.

Depending on the nature and composition of the It is generation of the hot strip used steel expedient if the in particular as a thin slab primary material steel introduced into the respective rolling mill in the liquid phase with Ca or Ca carrier alloys is treated.

Depending on the desired work result, it can finally be advantageous if the hot strip in the second cooling phase with a cooling rate of is cooled at least 30 ° C / s.

Fig. 1

den eine Kühlstrecke umfassenden Endabschnitt einer Linie zum Herstellen von Warmbändern in seitlicher Ansicht;

Fig. 2

ein Diagramm, in welchem der Temperaturverlauf während des Abkühlens innerhalb der Kühlstrecke dargestellt ist;

Fig. 3

ein Diagramm, in welchem die umgewandelten Anteile eines zur Herstellung eines Warmbandes verwendeten Stahls über der Temperatur bei herkömmlicher und bei erfindungsgemäßer Verfahrensweise dargestellt sind.

The invention is explained in more detail below on the basis of a drawing illustrating an exemplary embodiment. In a schematic representation:

Fig. 1

the end section of a line for producing hot strips comprising a cooling section in a side view;

Fig. 2

a diagram in which the temperature profile during cooling is shown within the cooling section;

Fig. 3

a diagram in which the converted proportions of a steel used for the production of a hot strip against the temperature are shown in the conventional and in the inventive procedure.

Line 1 for producing a hot strip W comprises one Relay of several finishing stands, one of which is here only the last frame 2 is shown. In the Finishing rolling the hot strip W is on its The desired final thickness is rolled.

A short distance behind the last finished roll stand 2, a compact cooling device 3 is arranged. This Compact cooling device 3 includes not shown here Nozzles, via the cooling liquid, preferably water, under increased pressure on the top and bottom of the Hot strip W is brought. The volume flow of the Coolant is adjustable so that within the Compact cooling device 3 cooling speeds of 150 ° C / s to 1000 ° C / s can be achieved.

In the conveying direction F of the hot strip W at a distance from the Compact cooling device 3 is a second cooling device 4 arranged. The second cooling device 4 works Kind of a conventional laminar cooling, in which the Coolant through several in the conveying direction F arranged one behind the other, also not shown here Fan is brought onto the hot strip W in a fan-like manner. The Number of nozzles acted upon and / or Volume flow in the area of the laminar cooling device 4 applied coolant can be regulated in such a way that in the area of the laminar cooling device 4 Cooling speeds of 30 to 150 ° C / s reached become.

In the conveying direction F of the belt behind the laminar cooling device 4 is a reel device 5 arranged, in which the hot strip W to a coil is wrapped.

For example, one made from a multi-phase steel Hot strip W is used exclusively in the finishing mill in the austenite area with a total decrease of more rolled as 30%. If necessary, the hot strip W during the rolling of a thermomechanical treatment subjected.

After the hot strip W has left the last stand 2 of the finished rolling mill, it arrives in the compact cooling device 3 within a transfer phase t ₂ , which is shorter than two seconds. Upon entering the compact cooling device 3, the hot strip W becomes a first cooling phase t _CK continuously exposed to a compact cooling, during which the hot strip W is cooled from an inlet temperature ET _CK to an outlet temperature AT _CK . The cooling rates achieved are between 250 and 1000 ° C / s. The γ / α conversion of the hot strip steel is suppressed by the accelerated cooling of the hot strip W in the compact cooling device 3 within a short time t _z after the exit from the finished rolling stack.

The hot strip W then runs through a free stretch in which it is cooled in air for an intermediate _{cooling phase} t _PAUSE . The duration of the intermediate _{cooling phase} t _PAUSE is at least one second. During this time there is a partial conversion of the hot strip steel.

Finally, the hot strip W arrives in the laminar cooling device 4. In this, it is cooled from an inlet temperature ET _LK to an outlet temperature AT _LK within a second cooling phase t _LK . The set cooling rate is between 30 and 150 ° C / s. Depending on the respective chemical composition of the steel and the cooling rate chosen, second phases (bainite, martensite or residual austenite) are formed, through which the properties of the hot strip W are influenced. The excretion state of the hot strip W is also controlled in this way.

Finally, the hot strip W cooled in this way is Coiler device 5 coiled.

Table 1 compares the structure components and the hardness of hot strips produced from steels "Stahl" - "Stahl2", which were produced according to the method according to the invention explained above, with the structure components and the hardness of hot strips of the same composition, which are produced in a conventional manner have been cooled in air in two laminar cooling devices with intervening cooling.

The compositions of the steels "Stahl1" and "Stahl2" used to produce the hot strips are given in Table 2. C Mn P S Si Cu al N Cr Ni Ti Nb steel1 0.15 1.38 0.009 0,007 00:42 0.01 0.026 0.0041 0.02 0.02 0.02 0,018 Steel2 0.13 1.45 0,012 0,004 0.35 0.14 0.037 0.0064 0.04 0.16 0.034

In Fig. 3 is for the steel in a solid line the course CLK of that structural change, which occurs when a hot strip is first in the manner according to the invention for the time t _CK a compact cooling with a cooling rate of 250 ° C / s, then one Intermediate _{cooling phase} t _PAUSE and finally a laminar cooling _cycle for the time t _LK , contrasted with the LLK course of the structural transformation drawn in dashed lines, which occurs in a conventional combination of two laminar cooling systems with intermediate cooling in air.

It can be clearly seen that the upstream Compact cooling the proportion of hard phases, i.e. those that convert at low temperatures, increases. Thus, with the sequence of Compact / air / laminar cooling the converted portion UA of austenite at a temperature of 450 ° C only at about 60%. The conversion of the remaining shares of the Austenite then sets to a greater extent at temperatures below 400 ° C and is only at one Temperature of 320 ° C completed. In contrast, the converted portion UA in the case of the conventional Laminar / air / laminar cooling at 400 ° C already almost 90% achieved. The conversion of the then remaining austenite is already at 350 ° C completed.

Table 1 confirms the statement in FIG. 3. In each of the examined hot strips is when using the method according to the invention compared to conventional cooled bands a shift in the structural components in favor of the tougher martensite phases. With unchanged composition, this led to a significant increase in the hardness of each Hot strip.

At the same time, those manufactured according to the invention Sample a structure with a finer grain structure than that generated by the conventional method. This has to Consequence that the hot strips produced according to the invention a good one despite the increased shares of the hard phases Show formability. This fact has been confirmed also for a TRIP steel ((in mass%) C: 0.2%, Al: 1.8 %, Mn: 1.6%). Such steel pointed to conventional Production method an average ferrite grain diameter from 6 - 7 µm. With the procedure according to the invention this diameter is reduced to less than 3 µm.

REFERENCE NUMBERS

F

Conveying direction,

W

Hot strip,

1

Hot strip production line,

2

Finish rolling scaffold,

3

Compact cooling device,

4

Laminar cooling device,

5

Coiler,

t ₂

Transfer phase between leaving the Finishing mill stand 2 and the beginning of Compaktkühlung,

t _CK

first cooling phase, which the hot strip W needs to the length of the compact cooling device 3 to cover,

ET _CK

Inlet temperature of hot strip W when entering the compact cooling device 3,

AT _CK

Hot strip exit temperature W at exit from the compact cooling device 3,

t _PAUSE

Intermediate cooling phase during which the hot strip W is on Air is cooled

t _LK

second cooling phase, in which the hot strip W in the Laminar cooling device 4 is cooled,

ET _LK

Inlet temperature of hot strip W when entering the laminar cooling device 4,

AT _LK

Hot strip exit temperature W at exit from the laminar cooling device 4,

CLK

Course of the structural transformation that occurs if a hot strip is first compact cooling and then goes through laminar cooling,

LLK

Course LLK of structural change, which occurs in a Combination of two laminar cooling sets,

UA

respective converted proportion of austenite.

Claims (14)

1. A method for producing a hot strip (W) which strip is produced in particular from continuous casting in the shape of reheated slabs or slabs obtained directly from the casting heat, from thin slabs or cast strip, based on a steel comprising (in mass %) C 0.001 - 1.05 %; Si ≤ 1.5 %; Mn 0.05 - 3.5 %; Al ≤ 2.5 %; as well as optionally one or several of the following constituents

   Cu, Ni, Mo with an amount of ≤ 0.8 %;

   N, Ti, Nb, V, Zn, B with an amount of ≤ 0.5 %;

   P with an amount of ≤ 0.09 %;

   Cr with an amount of ≤ 1.5 %; and / or

   S with an amount of ≤ 0.02 %;

   and
   the remainder being iron as well as the usual accompanying elements,
   with the steel also optionally having been treated in the liquid phase with Ca or Ca carrier alloys,
   involving the following steps:

   continuous finish rolling of the hot strip (W);

   continuous cooling of the hot strip (W) in at least two subsequent cooling phases (t_CK, t_LK) of accelerated cooling, to a final temperature;

   with the first cooling phase (t_CK) of accelerated cooling starting at the latest three seconds after the last pass of finish rolling; and

   with the hot strip (W) during the first cooling phase (t_CK) of accelerated cooling being cooled at a cooling rate of at least 250 °C/s.
2. A method according to claim 1, characterised in that the steel comprises 0.005 to 0.4 mass % of silicon.
3. A method according to claim 1, characterised in that

   the steel comprises (in mass %) C ≤ 0.07 %; Si ≤ 0.2 %; Mn ≤ 0.6 %; Al ≤ 0.08 %;

   in that the hot strip (W) during finish rolling is rolled in the austenitic area;

   in that the hot strip (W) in the first cooling phase (t_CK) of accelerated cooling starting at a temperature above 850 °C is cooled to a temperature of 680 to 750 °C;

   in that the hot strip (W) in the second cooling phase (t_LK) of accelerated cooling is cooled to a temperature of less than 600 °C; and

   in that the hot strip (W) is subsequently coiled.
4. A method according to claim 1, characterised in that the steel (in mass %) comprises C 0.04 - 0.09 %; Si ≤ 0.2 %; Mn 0.5 - 2.0 %; P 0.02 - 0.09 %; Cr ≤ 0.9 %;

   in that the hot strip (W) after finish rolling in the first cooling phase (t_CK) of accelerated cooling starting from a temperature above 800 °C, is cooled to a temperature of 650 to 730 °C;

   in that in the second cooling phase (t_LK) of accelerated cooling, the hot strip (W) is cooled to less than 500 °C; and

   in that the hot strip (W) is subsequently coiled.
5. A method according to one of claims 1, characterised in that the steel comprises (in mass %) C 0.25 - 1.05 %; Si ≤ 0.25 %; Mn ≤ 0.6 %;

   in that the hot strip (W) after finish rolling in the first cooling phase (t_CK) of accelerated cooling starting from a temperature above 800 °C, is cooled to a temperature of between 530 and 620 °C;

   in that the hot strip (W) in the second cooling phase (t_LK) of accelerated cooling is cooled to a temperature of less than 500 °C; and

   in that the hot strip (W) is subsequently coiled.
6. A method according to claim 1, characterised in that the steel comprises (in mass %) C 0.12 - 0.3 %; Mn 1.2 - 3.5 %; Al 1.1 - 2.2 %;

   in that the hot strip (W) after finish rolling in the first cooling phase (t_CK) of accelerated cooling starting from a temperature between the Ar₃ temperature and a temperature of Ar₃ + 150 °C, is cooled to a temperature which is up to 50 °C below the Ar₃ temperature;

   in that the hot strip (W) in the second cooling phase (t_LK) of accelerated cooling is cooled to 350 to 550 °C; and

   in that the hot strip (W) is subsequently coiled.
7. A method according to claim 1, characterised in that the steel comprises (in mass %) C 0,04 - 0,09 %, Si 0,5 - 1,5 %, Mn 0,5 - 2,0 %, Al 0,4 - 2,5 %, P ≤ 0,09 %, Cr ≤ 0,9 %,

   in that the hot strip (W) after finish rolling in the first cooling phase (t_CK) of accelerated cooling starting from a temperature above 800 °C, is cooled to a temperature of 650 to 730 °C;

   in that the hot strip (W) in the second cooling phase (t_LK) of accelerated cooling is cooled to less than 500 °C; and

   in that the hot strip (W) is subsequently coiled.
8. A method according to claim 1, characterised in that the steel comprises (in mass %) C 0,07 - 0,22 %, Si 0,1 - 0,45 %, Mn 0,2 - 1,5 %,

   in that the hot strip (W) after finish rolling in the first cooling phase (t_CK) of accelerated cooling starting from a temperature above 800 °C, is cooled to a temperature of 650 to 730 °C;

   in that the hot strip (W) in the second cooling phase (t_LK) of accelerated cooling is cooled to less than 500 °C; and

   in that the hot strip (W) is subsequently coiled.
9. A method according to claim 1, characterised in that the steel comprises (in mass %) C 0,07 - 0,22 %, Si 0,1 - 0,45 %, Mn 0,2 - 1,5 %,

   in that the hot strip (W) after finish rolling in the first cooling phase (t_CK) of accelerated cooling starting from a temperature above 800 °C, is cooled to a temperature of 580 to 650 °C;

   in that the hot strip (W) in the second cooling phase (t_LK) of accelerated cooling is cooled to less than 500 °C; and

   in that the hot strip (W) is subsequently coiled.
10. A method according to one of the preceding claims, characterised in that between the first cooling phase (t_CK) of accelerated cooling and the second cooling phase (t_LK) of accelerated cooling the hot strip (W) passes through an intermediate cooling phase (t_PAUSE) during which the hot strip (W) is subjected to cooling by exposure to air.
11. A method according to claim 10, characterised in that the intermediate cooling phase (t_PAUSE) lasts for at least one second.
12. A method according to one of the preceding claims, characterised in that the first cooling phase (t_CK) of accelerated cooling starts at the latest two seconds after the last pass of finish rolling.
13. A method according to one of the preceding claims, characterised in that at least one of the passes during finish rolling is carried out in the austenitic range below a temperature of Ar₃ + 80 °C, and in that an overall pass reduction exceeding 30 % is achieved.
14. A method according to one of the preceding claims, characterised in that in the second cooling phase (t_LK) of accelerated cooling the hot strip (W) is cooled at a cooling rate of at least 30 °C/s.

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