EP2566989B1 - Process for hot rolling steel strips and hot rolling train - Google Patents

Description

FIELD OF THE INVENTION

The invention relates to a method for hot rolling of steel strips in a plurality of successive rolling stands, wherein the steel strips are rolled finished austenitic first and then, after an intensive interstand liquid cooling, in the ferritic state in one or more rolling stands to the final thickness, and a corresponding hot rolling mill.

Hot rolling is used when the rolling stock has a temperature above its recrystallization temperature during rolling. For steel, this is the range above about 780 ° C, usually is rolled at temperatures up to 1200 ° C warm.

During hot rolling of steel, the metal is usually in the austenitic state, where the iron atoms are arranged cubic face centered. Here, degrees of deformation, ie the ratio of initial thickness to input thickness, of up to 1: 200 are possible: for example, in a hot strip mill starting from a slab thickness of typically 240 mm, final thicknesses of 1.2 to 15 mm can be achieved. This is referred to as rolling in the austenitic state, when both the initial rolling and the final rolling temperature are in the austenitic region of the respective steel. The austenite area of a steel depends on the steel composition, but is usually above 800 ° C.

If, due to the defined material behavior, rolling takes place exclusively in the austenite region, then the final thickness can not be chosen arbitrarily small because the rolling stock cools during each rolling process and finally falls out of the austenite region. This can only be countered by the fact that the rolling stock already reaches the hot rolling stands with reduced thickness, ie was either reduced accordingly in a roughing mill or in a - decoupled - Dünnbandgießanlage is produced because the capacity of a Thin strip casting plant usually is not sufficient for a coupled operation. The reduction in the thickness of the rolling stock before hot rolling thus leads to significant capacity losses, the direct coupling of the hot rolling stands to a continuous casting or strip casting plant with high capacity for carrying out the so-called direct coupled or endless operation is usually not possible.

Instead of being austenitic, steel can also be hot rolled in the ferritic state. In the ferritic state, the iron atoms are cubic body centered, the steel is softer than in the austenitic state and can be easily deformed. Although the temperature of the steel in the ferritic state (ferrite region) is lower than in the austenite region, less rolling force is needed to reshape the steel. This is exploited to achieve even lower thicknesses and / or even greater widths in the finished steel strip. However, the low reshaping resistance of the ferrite is preferably limited to a relatively narrow temperature range of 100-150 ° C below the temperature where the complete equilibrium phase transition from austenite to ferrite occurs. This equilibrium austenite limit temperature of the steels of interest here, depending on the composition of the steel, is between 800 and 900 ° C and is known for most steel compositions. In the iron-carbon diagram, where the carbon content is plotted on the x-axis and the temperature on the y-axis, it can be seen as a line between the points G and P. Above the G-P line, both austenite (so-called gamma mixed crystals) and ferrite (so-called alpha mixed crystals) are present, below the line G-P, ie below the equilibrium austenite boundary temperature, only more ferrite (alpha mixed crystals) is present.

STATE OF THE ART

From the prior art, the combination of hot rolls in the austenitic state with subsequent hot rolling in the ferritic state is already known. The DE 196 00 990 A1 proposes for this purpose, the steel strip after the austenitic rolls to an intermediate thickness of 2-12 mm in one cool single cooling step and then in one or more steps, so in one or more rolling stands, to finish ferritically finished.

However, these measures alone can not ensure that controlled cooling of the steel strip occurs and, depending on the bandwidth of the steel strip, thickness, temperature before cooling, etc., actually reaches the ferritic state after cooling.

PRESENTATION OF THE INVENTION

It is therefore an object of the invention to provide a method that ensures for steel strips of different bandwidth, thickness and temperature before cooling that they are in the ferritic state after cooling, in which case at a fraction of> 90% ferritic microstructure Presence of the ferritic state can be spoken.

The object is achieved in that the final thickness of the steel strip is less than 3 mm, in particular less than 2.5 mm, preferably less than 1.49 mm, that the difference between the outlet temperature of the steel strip from the last stand before liquid cooling and the equilibrium Austenitgrenztemperatur by pre-control or regulation of this outlet temperature to not greater than 70 K, preferably not greater than 50 K, preferably less than 25 K is set, and that the liquid cooling between two stands in dependence on the length Lc of a cooling section is carried out by in the cooling section on both sides of the steel strip at least one liquid quantity Qu> 284 / (Lc ^1.42 ) liters per minute and per meter bandwidth, in particular Qu> 2 * 284 / (Lc ^1.42 ) liters per minute and per meter bandwidth, but not more than Qu = 7 * 284 / (Lc ^1.42 ) liters per minute and per meter of bandwidth, preferably Qu <4 * 284 / (Lc ^1.42 ) liters per minute and per meter of bandwidth is brought.

The inventive method is only well applicable if the final thickness is less than 3 mm, because only then in the cooling section sufficient cooling (the entire band cross-section) can take place.

That is for a bandwidth of the steel strip of 1 m and a length Lc of the cooling section of 1 m are on the cooling section on each side of the tape at least 284 liters per minute of liquid, usually water, apply, preferably 568 l / min and side of the band. If the steel strip is only 0.5 m wide and the length of the cooling section remains the same, apply at least 142 l / min to the upper and lower sides of the steel strip on the cooling line, preferably 284 l / min. If, at steel strip width 1 m, the cooling section is only 0.5 m long, significantly higher amounts of water are to be applied to the shorter cooling section, namely both the top and on the bottom 760 l / min, preferably even 1520 l / min.

The inventive method can be used both in regulation and in the precontrol of the outlet temperature of the steel strip from the last frame before the liquid cooling. Controlling the temperature requires that an actual value be measured, which is often not the case because the temperature is calculated using models. For pre-control, the outlet temperature is determined using other known data, such as process models.

But it can also be provided that the feedforward control or regulation of the outlet temperature of the steel strip from the last stand before the liquid cooling on quality and / or reduction-dependent tables or on simple grading and / or reduction-dependent mathematical relationships between width-specific mass flow rate (or the belt speed and the strip thickness) and inlet temperature in the first roll stand based.

An excerpt from such a table is given here by way of example for a five-stand hot rolling mill (finishing mill) for low throughputs. For different strip thicknesses (here only indicated for 8 mm in Table 1 and for 10 mm in Table 2) and for different inlet temperatures in the hot rolling line (here only indicated for 1070 ° C) are for different inlet velocities of the strip in the hot rolling line (V_band) the Temperature of the strip after the third rolling stand (T_3) and after the fourth rolling stand (T_4) indicated: Table 1 Strip thickness 8 mm Inlet temperature 1070 ° C according to scaffold 3 according to scaffolding 4 V_band [m / s] T_3 [° C] T_4 [° C] 0.8 870 809 0.85 891 836 0.9 917 866 0.95 942 897 1 965 920 1.05 986 942 1.1 1009 970 Strip thickness 10 mm Inlet temperature 1070 ° C according to scaffold 3 according to scaffolding 4 V_band [m / s] T_3 [° C] T_4 [° C] 0.7 895 840 0.75 934 881 0.8 962 915 0.85 987 945 0.9 1012 974

These tables are created based on experience and can then be used to pre-control the hot rolling mill. If the fourth rolling stand is the one before the liquid cooling, which is to leave the belt at a certain temperature, then the tables can see which run-in speeds of the belt in the hot rolling line (V_band), which strip thickness, what temperature of the strip after the third rolling stand (T_3), etc., correspond to this desired temperature and the temperature can be set by specifying the corresponding values - without control.

As a rule, water with an application temperature between 15 ° C and 60 ° C, preferably between 25 ° C and 40 ° C, is used as the cooling liquid.

By the amount of water according to the invention can be with appropriate control of the outlet temperature from the last stand before cooling, which is usually carried out by the so-called level 2 control of the hot rolling mill, achieve an intermediate cooling of the steel strip of more than 30 K to more than 100 K, so that full-ferrite structure of the steel strip is ensured before it enters the next mill, where then ferritic is rolled.

The process according to the invention can be used particularly well if the width-specific throughput through the rolling stands is less than 12 mm m / s, preferably less than 9.5 mm m / s. The throughput or volume flow is often specified in the rolling technique as a volume-specific volume flow, ie as a volume flow per unit width (1 m), and can be represented as the product of thickness of a band (usually in mm) and belt speed (usually in m / s). For carrying out the process according to the invention, this (broad specific) throughput should not be too high, that is to say be less than 15 mm / s, more preferably less than 12 mm / s, e.g. smaller 9.5 mm m / s.

The context of the invention was established by means of experiments. In order for the favorable properties of ferrite, its good ductility, to be exploited during hot rolling, the rolling stock, ie the steel strip, must be homogeneous and there must be no significant austenite portions in the steel strip which would significantly affect the amount of rolling force. The cooling must therefore ensure that in the steel strip after cooling and before entering the following rolling stands for ferritic rolling almost exclusively ferritic microstructure, ie at least 90% ferritic microstructure, preferably at least 95% ferritic microstructure, is present.

Depending on the strip thickness before the mill, before the cooling, the volume specific volume flow and the inlet temperature of the steel in the hot rolling line, which form the rolling stands for hot rolling, intensive cooling between at least two rolling stands is necessary to ensure the transformation into almost exclusively ferritic microstructure in which the average cooling rate T 'of the steel strip is to be at least 40 K / s, better still more than 60 K / s, preferably 90 K / s, within the rolling stand spacing (between the last rolling stand before and the first rolling stand after cooling).

The required cooling rate T '[K / s] is also dependent on the outlet temperature of the steel strip after the mill before cooling, on the exit speed from this mill and on the distance between the last mill before cooling and the first mill after cooling. Of course, the composition of the steel also plays a role.

The following considerations were made: The cooling rate T 'to be achieved within the cooling section length Lc can be calculated as follows: $$\mathrm{T\; \text{'}}=\mathrm{A}*\mathrm{B}*{\mathrm{v}}_{\mathrm{m}}/{\mathrm{L}}_{\mathrm{c}}$$

The factor A depends on the average outlet temperature Tm of the steel strip from the last stand before cooling and its distance to the equilibrium austenite limit temperature Ta and is preferably determined as follows: $$\mathrm{A}=\left[\mathrm{0}.\mathrm{5}...\mathrm{2}.\mathrm{0}\right]*\mathrm{40}+\left(\mathrm{tm}-\mathrm{Ta}\right)\mathrm{,}$$

Lc stands for the length of the cooling section within which the cooling rate T '[K / s] can be achieved. Alternatively, for Lc, the stand spacing between the last roll stand before and the first roll stand after cooling can also be used, then T 'becomes the average cooling rate between the two roll stands. v _m denotes the mean discharge speed from the last roll stand before cooling.

The dimensionless factor B reflects the iron content of the steel strip and is between 0.95 and approx. 1.95 when considering steels with> 98% Fe, where Fe denotes the iron content of the steel in% of the mass: $$\mathrm{B}=\mathrm{0}.\mathrm{95}+\mathrm{0}.\mathrm{5}*\left(\mathrm{100}-\mathrm{Fe}\right)$$

In order to ensure a sufficient cooling rate according to the invention, the temperature difference Tm-Ta (in the factor A) should not be greater than 70 K, more preferably less than 50 K, preferably less than 25 K. This can be ensured, for example, by the so-called Level 2 Automation, which controls the hot rolling mill.

The multiplication factor [0.5 ... 2.0] occurring at A is due to the significant scattering found in conversion and cooling tests at different cooling exposures to achieve 90 and 95% ferritization, respectively.

Based on this setting of the temperature difference (Tm-Ta), the known steel properties of the steel strips, which are rolled in the hot rolling mill, for a given length Lc of the cooling section results in an at least required cooling rate T ', from which the minimum required heat transfer coefficient can be determined and from which, in turn, the required amount of cooling liquid (cooling water) can be determined from mostly empirical or experimentally found relationships.

According to the invention, it can be provided that cooling only takes place between the penultimate and last roll stand of a hot rolling mill, that is, the steel strip is rolled in the austenitic state on the penultimate roll stand, then ferritization takes place by the cooling and in the last rolling stand the steel strip is rolled in the ferritic state ,

Or it can be provided that cooling takes place only between the third last and penultimate roll stand, so that the steel strip is rolled in the austenitic state on the third to last rolling stand, then takes place by the cooling ferritization and in the penultimate and last rolling mill, the steel strip is rolled in the ferritic state , This has the advantage that the penultimate rolling stand can be opened in case of insufficient cooling by the upstream cooling section, so this is not rolled. Insufficient cooling may occur, for example, if individual cooling devices, such as nozzles, fail or if the speed of the steel strip or mass flow rate is higher than expected.

Finally, it can also be provided that cooling takes place between the penultimate and last roll stands as well as between the third last and second to last roll stands. This has the advantage that the penultimate rolling mill can be opened in case of insufficient cooling by the upstream cooling section, so this is not rolled, and the steel strip can be cooled by the downstream second cooling in addition to the last stand, with the only the steel strip is rolled in the ferritic state. Of course, but also in case of sufficient cooling to the ferritic state before the penultimate stand with both the penultimate than Even with the last rolling mill, the steel strip will be rolled in the ferritic state.

In order to ensure rapid and intensive cooling of the already relatively thin strip (below 5 mm, in particular below 3.5 mm) after the austenitic rolling, it can be provided that the length Lc of the cooling section is between 5 and 30% of the distance between the preceding and the following Roll stand amounts. For example, the cooling section consists of at least two rows of spray nozzles per band side, wherein a row of nozzles results in a minimum length of the cooling section - in the sense of the present invention - of 350 mm.

Furthermore, it is the austenite ferrite conversion possible useful when the cooling section is located closer to the previous than the following rolling stand, in particular in connection with a relatively short cooling distance of 5 and 30% of the distance between the preceding and following mill stand. The cooling section should preferably be arranged at least 20% closer to the preceding than to the following rolling stand.

The method according to the invention is best used in installations where the distance between successive rolling stands between which the cooling takes place is between 3.5 and 7 m. At these intervals, it is certainly possible that after cooling of the steel strip still enough time for the most complete transformation of the structure into ferrite takes place.

The invention is also advantageous for steel strips with a width between 800 and 2200 mm applicable.

The strip thickness of the steel strip before cooling is generally 1.2 to 5 mm, in particular 1.5 to 3.5 mm, preferably 1.8 to 3.5 mm.

It is particularly advantageous in the method according to the invention that the steel strip can be finish rolled from continuously cast semi-finished products in directly successive work steps. Thus, a direct coupling of the hot rolling mill can be made to a continuous casting plant, so that steel strips with a strip thickness of less than 3 mm can be produced in a continuous process. For example, the steel strip could first be rough rolled in one to four steps, then heated again to at least 1100 ° C and then finish rolled in three to five steps. It need not be mentioned that between continuous casting plant and hot rolling plant of course in a known arrangement, other devices such as shears, ovens, cooling systems, Vorwalzanlagen, storage facilities, systems for descaling, etc. may be present. Of course, the hot rolling mill can also consist of more than five rolling stands, so that the steel strip can be finish rolled in more than five steps.

However, the invention can also be applied to a hot rolling plant arranged downstream of a strip casting plant.

A use according to the invention of a multi-stand hot rolling mill has between two successive rolling stands a cooling section for pressurizing the steel strip with liquid on both sides and an associated pilot control or regulating device, which is set so that at least in the cooling section on both sides of the steel strip as a function of the length Lc of the cooling section depending on a liquid quantity Qu> 284 / (Lc ^1.42 ) liters per minute and per meter of bandwidth, in particular Qu> 2 * 284 / (Lc ^1.42 ) liters per minute and per meter of bandwidth applied, and provides a pilot control or Regulation before, which sets the difference between the outlet temperature of the steel strip from the last stand before the cooling section and the equilibrium Austenitgrenztemperatur by controlling the outlet temperature to not greater than 70 K, preferably not greater than 50 K, preferably less than 20 K.

The cooling section is the route on which leaking liquid impinges on the steel strip.

As already described in the method according to the invention, the cooling section can be arranged between penultimate and last rolling stand and / or third last and second last rolling stand.

The length Lc of the cooling section may be between 5 and 30% of the distance between the preceding and following stand, in particular they consist of only one each over the width of arranged nozzle row per strip surface (ie one row of nozzles above as well as below).

The cooling section can be arranged closer to the preceding than to the following rolling stand, in particular by at least 20% closer to the preceding than to the following rolling stand.

The distance between successive rolling stands, between which a cooling section is arranged, should be best between 3.5 and 7 m.

The width of the hot rolling line and the cooling section will usually be designed for a bandwidth of the steel strip between 800 and 2200 mm.

The hot rolling mill may be connected to a continuous casting plant in such a way that the steel strip of continuously cast semi-finished products can be finish rolled in directly successive work steps. For example, this can result in a composite rolling mill with a hot rolling mill according to the invention, wherein the composite rolling mill has a roughing mill with one to four rolling stands, a heater for heating the steel strip from the roughing mill to about 1100 ° C and a hot rolling mill with three to five rolling stands for finish rolling. Of course, the hot rolling mill may include more than five scaffolding.

With the method according to the invention or the hot rolling mill, even at relatively small width-specific throughputs of steel strip (strip thickness times speed), for instance less than 0.438 m ² / min (that is 7.3 mm m / s) and moderate inlet temperatures of the steel strip into the hot rolling mill of less than 1050 ° C, especially less than 1020 ° C, can be achieved in only three rolling stands austenitic rolling strip thicknesses of significantly less than 3 mm. Due to the strong cooling according to the invention after austenitic rolling, for example after the third (or fourth) rolling stand, a final thickness of less than 1 mm can be achieved in one (or two) further rolling stand (s) by ferritic rolling, with the additional advantage that the last (or last two) rolling stand (s) require significantly lower rolling forces, resulting in energy savings for the hot rolling mill.

There are thus final thicknesses for steel strip of at least 1.2 mm reachable, where only austenitic rolling only considerably larger thicknesses than 1.2 mm, albeit well below 3 mm, can be achieved.

In any case, the entire hot rolling process is more stable than conventional methods because an undetermined partial ferritic rolling is excluded at the last or penultimate pass.

BRIEF DESCRIPTION OF THE FIGURES

The invention will be explained by way of example with reference to a schematic figure. The figure shows the side view of a hot rolling mill with cooling section.

WAYS FOR CARRYING OUT THE INVENTION

The steel strip 3 enters at the left edge of the figure in the hot rolling mill, consisting of the rolling stands F1 to F5, with an inlet temperature of less than 1050 ° C, preferably less than 1020 ° C, such as a roughing, which is connected to a continuous casting. The temperature of the steel strip 3 refers to the average value of the temperature averaged over the strip cross section. In the first three rolling stands F1 to F3, the steel strip 5 is rolled in the austenitic state, it leaves the rolling stand F3 with a typical strip thickness of less than 3 mm.

The cooling section 1 has here on both sides of the band to a plurality of spray nozzles 2, which are arranged in at least one row of nozzles per band side, and has a length Lc of at least 350 mm. The distance LF3 between the third roll stand F3 and the beginning of the cooling section 1 is here only a fraction of the distance LF4 between the fourth rolling stand F4 and the end of the cooling section 1.

The cooling section leads the steel strip 3 according to the invention both on the top and on the bottom of an inventive amount of water per minute and bandwidth in meters, whereby the steel strip 3 cools. Until the steel strip 3 enters the fourth rolling mill F4, an almost complete transformation of the microstructure into ferrite has taken place, so that the steel strip 3 is in the ferritic state is reduced in the fourth rolling mill. In the fifth rolling stand F5, the steel strip 3 is rolled in the ferritic state to its final thickness of less than 1.5 mm.

In general, the amount of water applied on both sides will be between two and four times the value of 284 / (Lc ^1.42 ) if the difference between the outlet temperature from the rolling stand F3 and the austenite boundary temperature is less than 50 K and the width-specific throughput is not too high is high, that is, for example, in the range of 5 to 12 mm m / s.

Conventionally, spray nozzles are used for applying the cooling water, which are arranged in rows in the width direction of the steel strip. If only one row of nozzles is used, this corresponds to a model cooling section 1 with a length of about 350 mm. This results e.g. with an oblique rotation / arrangement of the nozzles from the distance between the first and last point of impact of the water jet on the steel strip 3. From two rows of nozzles, so from a number of n nozzle rows, the length Lc of the cooling section is calculated from the distance between the first and last point of impact Water jet of a row of nozzles plus the (n-1) -fold mean distance between two rows of nozzles with each other.

ALTERNATE EMBODIMENT OF THE INVENTION

If the cooling section 1 were arranged between fourth F4 and fifth rolling stand F5 instead of between third F3 and fourth rolling stand F4, then the distance between third F3 and fourth rolling stand F4 could be smaller and the distance between fourth F4 and fifth rolling stand F5 greater. In any case, austenitic rolling would take place in the first four rolling stands F1 to F4, while ferritic rolling would only take place in the fifth rolling stand F5 after the most complete conversion of the structure of the steel strip 3 into ferrite.

LIST OF REFERENCE NUMBERS

1

cooling section

2

spray nozzles

3

steel strip

F1

first rolling stand

F2

second rolling stand

F3

third rolling stand

F4

fourth rolling stand

F5

fifth rolling stand

Lc

Length of the cooling section 1

lg

Distance between third and fourth rolling stand

LF3

Distance between third roll stand F3 and beginning of the cooling section 1

LC4

Distance between the fourth roll stand F4 and the end of the cooling section 1

Claims (24)

1. Process for hot rolling steel strips (3) in a plurality of successive roll stands (F1-F5), wherein the steel strips are finish-rolled to the end thickness in one or more roll stands firstly in the austenitic state and then, after intensive liquid cooling in an intermediate stand, in the ferritic state, characterized in that the end thickness of the steel strip (3) is less than 3 mm, in particular less than 2.5 mm, preferably less than 1.49 mm, in that the difference between the outlet temperature of the steel strip from the last roll stand (F3) before the liquid cooling and the equilibrium austenite limit temperature is set by the pilot control or regulation of said outlet temperature to no more than 70 K, preferably no more than 50 K, preferably less than 25 K, and in that the liquid cooling is effected between two roll stands depending on the length Lc of a cooling section (1) by applying to both sides of the steel strip (3), in the cooling section, at least in each case a quantity of liquid Qu > 284/(Lc^1.42) liters per minute and per meter of strip width, in particular Qu > 2*284/(Lc^1.42) liters per minute and per meter of strip width, but not more than Qu = 7*284/(Lc^1.42) liters per minute and per meter of strip width, preferably Qu < 4*284/(Lc^1.42) liters per minute and per meter of strip width.
2. Process according to Claim 1, characterized in that the width-specific throughput through the roll stands is less than 12 mm m/s, preferably less than 9.5 mm m/s.
3. Process according to Claim 1 or 2, characterized in that the pilot control or regulation of the outlet temperature of the steel strip from the last roll stand (F3) before the liquid cooling is based on tables dependent on quality and/or the degree of reduction or on simple mathematical relationships, dependent on quality and/or the degree of reduction, between width-specific mass throughput and inlet temperature into the first roll stand (F1).
4. Process according to one of Claims 1 to 3, characterized in that a mean cooling rate T'=A*B*v_m/Lc is set between the last roll stand (F3) before the liquid cooling with a cooling section (1) of the length L_c and the first roll stand (F4) after the liquid cooling,
   where A = [0.5...2]*40+(Tm-Ta) with the mean outlet temperature Tm of the steel strip (3) from the last roll stand (F3) before the liquid cooling and the equilibrium austenite limit temperature Ta,
   where B = 0.95+0.5*(100-Fe) with the iron content Fe of the steel in percent by mass,
   and where v_m denotes the strip velocity which is present between said two roll stands (F3, F4).
5. Process according to one of Claims 1 to 4, characterized in that the cooling liquid is water at an application temperature of between 15°C and 60°C, preferably between 25°C and 40°C.
6. Process according to one of Claims 1 to 5, characterized in that the cooling is effected between the penultimate (F4) and last (F5) roll stands and/or between the antepenultimate (F3) and penultimate (F4) roll stands.
7. Process according to Claim 6, characterized in that cooling is effected between the antepenultimate (F3) and penultimate (F4) roll stands and the penultimate roll stand (F4) is opened if the cooling is inadequate.
8. Process according to one of Claims 1 to 7, characterized in that the length Lc of the cooling section (1) amounts to between 5 and 30% of the distance between the preceding (F3) and following (F4) roll stands.
9. Process according to one of Claims 1 to 8, characterized in that the cooling section (1) is arranged closer to the preceding roll stand (F3) than to the following roll stand (F4), in particular is arranged closer to the preceding roll stand than to the following roll stand by at least 20%.
10. Process according to one of Claims 1 to 9, characterized in that the distance (Lg) between successive roll stands (F3, F4) between which the cooling takes place is between 3.5 and 7 m.
11. Process according to one of Claims 1 to 10, characterized in that the strip width of the steel strip (3) is between 800 and 2200 mm.
12. Process according to one of Claims 1 to 11, characterized in that the strip thickness of the steel strip (3) before the cooling section (1) is 1.2 to 5 mm, in particular 1.5 to 3.5 mm, preferably 1.8 to 3.5 mm.
13. Process according to one of Claims 1 to 12, characterized in that the steel strip (3) is finish-rolled from continuously cast semifinished product in directly successive operating steps.
14. Process according to Claim 13, characterized in that the steel strip (3) is firstly rough-rolled in one to four steps, is then heated to at least 1100°C again and is then finish-rolled in three to five steps.
15. Use of a multi-stand hot rolling train for carrying out the process according to one of Claims 1 to 12, characterized in that provision is made, between two successive roll stands (F3, F4), of a cooling section (1) for applying liquid to the steel strip (3) on both sides and an associated pilot control or regulating device, which is set in such a way that at least in each case a quantity of liquid Qu > 284/(Lc^1.42) liters per minute and per meter of strip width, in particular Qu > 2*284/(Lc^1.42) liters per minute and per meter of strip width, but not more than Qu = 7*284/(Lc^1.42) liters per minute and per meter of strip width, preferably Qu < 4*284/(Lc^1.42) liters per minute and per meter of strip width, is applied to both sides of the steel strip in the cooling section (1) depending on the length Lc of the cooling section, and in that provision is made of pilot control or regulation, which sets the difference between the outlet temperature of the steel strip (3) from the last roll stand (F3) before the cooling section (1) and the equilibrium austenite limit temperature by the regulation of the outlet temperature to no more than 70 K, preferably no more than 50 K, preferably less than 25 K.
16. Use of the hot rolling train according to Claim 15, characterized in that the pilot control or regulation of the outlet temperature of the steel strip (3) from the last roll stand (F3) before the cooling section (1) is based on tables dependent on quality and/or the degree of reduction or on simple mathematical relationships, dependent on quality and/or the degree of reduction, between width-specific mass throughput and inlet temperature into the first roll stand (F1).
17. Use of the hot rolling train according to Claim 15 or 16, characterized in that the cooling section (1) is arranged between the penultimate (F4) and last (F5) roll stands and/or between the antepenultimate (F3) and penultimate (F4) roll stands.
18. Use of the hot rolling train according to one of Claims 15 to 17, characterized in that the length Lc of the cooling section (1) amounts to between 5 and 30% of the distance between the preceding (F3) and following (F4) roll stands.
19. Use of the hot rolling train according to one of Claims 15 to 18, characterized in that the cooling section (1) is arranged closer to the preceding roll stand (F3) than to the following roll stand (F4), in particular is arranged closer to the preceding roll stand than to the following roll stand by at least 20%.
20. Use of the hot rolling train according to one of Claims 15 to 19, characterized in that the distance (Lg) between successive roll stands (F3, F4) between which a cooling section (1) is arranged is between 3.5 and 7 m.
21. Use of the hot rolling train according to one of Claims 15 to 20, characterized in that the width of the hot rolling train (F1-F5) and of the cooling section (1) is dimensioned for a strip width of the steel strip (3) of between 800 and 2200 mm.
22. Use of the hot rolling train according to one of Claims 15 to 21, characterized in that the cooling section (1) is formed in such a way that the cooling liquid used may be water at an application temperature of between 15°C and 60°C, preferably between 25°C and 40°C.
23. Use of the hot rolling train according to one of Claims 15 to 22, characterized in that it is connected to a continuous casting installation in such a way that the steel strip (3) can be finish-rolled from continuously cast semifinished product in directly successive operating steps.
24. Use of the combined rolling installation having a hot rolling train according to Claim 23, characterized in that the combined rolling installation has a rough rolling train with one to four roll stands, a heating device for heating the steel strip from the rough rolling train to above 1100°C and also a hot rolling train with three to five roll stands (F1-F5) for finish rolling.

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