Method for directly producing a trip steel strip in a cast-rolling combination plant

Abstract

The invention relates to a cast-rolling combination plant (10) which has a finish-rolling mill (55) and a cooling section (65), wherein: following the finish rolling of the finished strip (165), the finished strip (165) is fed to a first cooling group (166) in the cooling section (65), and a core of the finished strip (165) is force-cooled to a second exit temperature (TA2) in the first cooling group (166), the second exit temperature (TA2) being in the range of 620-700°C; when exiting the first cooling group (166), the core has a predominantly austenitic structure; the finished strip (165) is conveyed to a third cooling group (168) which is spaced from the first cooling group (166); while the finished strip (165) is being conveyed, the cooling rate of the core of the finished strip (165) changes to a second cooling rate; the second cooling rate of the core amounts to -25 K / s to 20 K / s, and, while the finished strip is being conveyed, a first portion of the austenitic structure in the finished strip (165) is converted into a ferritic structure; in the third cooling group (168), the core of the finished strip (165) is force-cooled to a third exit temperature (TA3) which is less than or equal to the bainite starting temperature (BS), so that a second portion of the austenite in the finished strip (165) is converted at least in part into a bainitic structure.

Description

[0001] Description

[0002] Process for the direct production of a TRIP steel strip in a combined casting and rolling plant

[0003] The invention relates to a method for producing a TRIP steel strip according to claim 1.

[0004] EP1045737 A1 discloses a method for producing a steel strip.

[0005] EP 4 101 552 A1 discloses a process for producing a micro-alloyed steel.

[0006] EP 0 952 235 A1 discloses a process for producing high-strength steels with high impact energy absorption properties.

[0007] It is an object of the invention to provide an improved method for producing a TRIP steel strip using a casting-rolling composite plant in continuous operation and a TRIP steel strip.

[0008] This object is achieved by a method according to claim 1. Advantageous embodiments are specified in the dependent claims.

[0009] It has been recognized that an improved method for the direct production of a TRIP steel strip in continuous operation can be provided in a combined casting and rolling plant having a finishing mill and a cooling section. A hot preliminary strip is fed to the finishing mill, which the finishing mill finish-rolls into a finished strip. Following the finish-rolling of the finished strip, the finished strip is fed to a first cooling group of the cooling section, and in the first cooling group, a core of the finished strip is forced-cooled to a second exit temperature such that the second exit temperature is in a range from 620°C to 700°C. Upon exiting the first cooling group, the core of the finished strip has a predominantly austenitic structure, preferably at least 90% phase content, in particular at least 95% phase content, in particular a completely austenitic structure.After leaving the first cooling group, the cooled finished strip is transported to a third cooling group of the cooling section, wherein during the transport of the finished strip between the first cooling group and the third cooling group, a second cooling rate of the core of the finished strip is established, wherein the second cooling rate of the core of the finished strip is from -25 K / s up to and including, in particular from 0 K / s up to and including 20 K / s. During transport in the second cooling group, a first part of the austenitic microstructure in the finished strip is converted into a ferritic microstructure. In the third cooling group, the core of the finished strip is force-cooled to a third exit temperature which is less than or equal to the bainite start temperature, so that a first part of the austenite of the finished strip is at least partially converted into bainite.The cooling can take place in the third cooling group in such a way that the first part of the austenite is ideally converted into a cementite-free bainite.

[0010] The advantage is that the TRIP strip is directly finished at the end of the process, regardless of its final thickness. In particular, an additional cold rolling step and / or an annealing step after the cooling line can be avoided. Due to the direct production of the TRIP strip, the process described above is particularly energy-efficient. In particular, a TRIP780 / HCT780T strip can be produced particularly cost-effectively using the process. Furthermore, it is ensured that the finished strip has already reached its final thickness during the hot rolling process and not during the cold rolling process.

[0011] The second cooling rate can also be negative due to the phase transformations that occur during transport and the resulting heat of transformation.

[0012] The low second cooling rate during transport, achieved, for example, by eliminating forced cooling, ensures a high degree of microstructure homogeneity in the TRIP strip. Furthermore, the low second cooling rate ensures a defined and reproducible phase proportion of metastable austenite and bainite with high process reliability.

[0013] The direct production of the finished rolled strip in continuous operation, i.e., when the finished rolled strip is mechanically connected to the roughing strip and the thin slab strand, has the further advantage of allowing extremely high reductions per roughing and / or finishing stand. This allows even high-strength grades to be hot-rolled directly to thin strip thicknesses, such as those required for lightweight automotive construction.

[0014] The entire time during which the second cooling rate prevails is also referred to as the holding time, during which only natural cooling can occur due to convention in the deactivated second cooling group, or the finished strip can heat up due to phase transformations during the holding time. The second, low cooling rate gives the microstructure sufficient time during transport between the first cooling group and the third cooling group to transform the desired phase fraction of at least 50% from austenite to ferrite at the set temperature.

[0015] Compared to batch hot rolling, the process described above, which is carried out exclusively in (quasi-)continuous operation, has the advantage of avoiding acceleration of the finished strip. This ensures that the process parameters required for the production of the TRIP strip, especially in the cooling section, can be reliably maintained over a long period of time. This ensures a high level of homogeneity across the entire produced TRIP strip.

[0016] Furthermore, the process described above ensures good castability and thus the casting speeds and mass flows required for the continuous process can be maintained. Furthermore, thanks to the process described above, the TRIP strip exhibits a low tendency toward surface defects, for example, caused by selective (high-temperature) oxidation of the austenite grain boundaries during continuous casting, as well as a low tendency toward internal oxidation and internal cracking.

[0017] Furthermore, the second exit temperature of 620°C to 700°C ensures rapid ferrite formation during transport between the first cooling group and the third cooling group, as well as the formation of globular ferrite. This has a positive effect on the mechanical properties of the TRIP steel's microstructure. Furthermore, the spatial distance between the first cooling group and the third cooling group can be shortened due to the rapid formation of globular ferrite.

[0018] In a further embodiment, the cooled finished strip is fed to a second cooling group of the cooling section after leaving the first cooling group, wherein forced cooling of the finished strip in the second cooling group is deactivated and the finished strip is transported in the second cooling group to a third cooling group of the cooling section. This configuration has the advantage that the spatial length of the first to third cooling groups can be adjusted depending on the conveying speed of the finished strip. The finished strip is wound up and cooled in the coil from the third exit temperature to an ambient temperature. When the wound finished strip is cooled to the ambient temperature, a remaining second portion of the austenite of the finished strip is enriched with carbon (which is essentially insoluble in the cementite-free bainite), so that a metastable residual austenite portion is formed.The metastable retained austenite transforms into martensite during cold forming, especially during rapid cold forming, such as in the event of a motor vehicle crash. The metastable retained austenite can also transform into martensite during cold forming of the finished rolled strip, making the component produced by forming, such as a car body part, particularly stiff and tough.

[0019] In a further embodiment, the finished strip is forced cooled in the first cooling group such that a first cooling rate of the core of the finished strip is established. In the third cooling group, the finished strip is forced cooled such that a third cooling rate of the core of the finished strip is established. The second cooling rate is lower than the first cooling rate and / or the third cooling rate. Preferably, the first cooling rate and / or the third cooling rate of the core of the finished strip is 20 K / s to 400 K / s, in particular 50 K / s to 200 K / s. This embodiment has the advantage that the first high cooling rate leads to rapid cooling into the (partially) ferritic range. This in turn promotes the rapid formation of homogeneous ferritite grains from the austenitic microstructure.The third cooling rate is necessary to prevent the remaining austenite from transforming into ferrite. Instead, thanks to the high second cooling rate, the remaining second portion of the austenite is largely transformed into bainite. The remaining austenite remains partially trapped between the bainite plates. Cementite precipitation is hindered by the alloying constituents silicon and / or aluminum, preventing the carbon from precipitating as iron carbide and thus enriching the remaining austenite between the bainite plates.

[0020] In a further embodiment, the core of the finished strip has a first exit temperature above the ferrite precipitation temperature (Ar3 temperature), in particular from 800 °C to 950 °C, in particular from 830 °C to 860 °C, when leaving the finishing rolling train.

[0021] In a further embodiment, the finished strip is transported from the first cooling group, preferably via the second (inactive) cooling group, to the third cooling group within a second time interval of 3 seconds to 8 seconds, in particular 4 seconds to 5 seconds. This embodiment ensures that a sufficiently long holding time is possible for ferrite formation in the finished strip, thus allowing a sufficiently large phase fraction of austenite to convert to ferrite.

[0022] In a further embodiment, the core of the finish-rolled finished strip leaves the second cooling group with a third exit temperature of 580°C up to and including 680°C, in particular of 620°C up to and including 660°C, and is transported to the third cooling group of the cooling section. Furthermore, upon exiting the finished strip from the third cooling group, the core of the finished strip has a fourth exit temperature, wherein upon exiting the finished strip 165 from the third cooling group 168, the core of the finished strip 165 has a fourth exit temperature TA4 of 180°C up to and including 450°C, in particular up to and including 330°C, in particular up to and including 360°C up to and including 420°C, in particular up to and including 330°C up to and including 390°C. Advantages of two paths

[0023] In another embodiment, the core of the finished strip is cooled from the fourth exit temperature to an ambient temperature within a fourth time interval of 24 hours to 72 hours. The ambient temperature is typically -20 °C to +50 °C. The slow (natural and convective) cooling of the finished strip ensures that there is sufficient time for the carbon, which is practically insoluble in ferrite, to diffuse into the retained austenite, thus forming a retained austenite that is metastable at ambient temperature.

[0024] In a further embodiment, the thickness of the pre-strip upon entry into the finishing rolling mill is 4 mm to 25 mm, in particular 6 mm to 18 mm. The finishing rolling mill reduces the thickness of the pre-strip to the finished strip to 0.6 mm to 6 mm, in particular to 0.8 mm to 2 mm.

[0025] It is particularly advantageous if the finished strip has a chemical composition in percent by weight of C inclusive of 0.15% up to and including 0.25%, in particular inclusive of 0.19% up to and including 0.21%, Mn 1.0% up to and including 2.0%, in particular 1.4% up to and including 1.6%, Si 1.0% up to and including 1.5%, in particular 1.1% up to and including 1.3%, Al inclusive of 0.3% up to and including 0.7%, in particular inclusive of 0.45% up to and including 0.55%, the remainder being Fe and unavoidable impurities.

[0026] It is particularly advantageous if the combined casting and rolling plant comprises a continuous casting machine with a mold and a single- or multi-stand roughing mill. A metallic melt is cast in the mold to form a partially solidified thin slab strand. The partially solidified thin slab strand is supported and deflected. The partially solidified thin slab strand is fed directly from the continuous casting machine to the roughing mill. The roughing mill rolls the thin slab strand into the preliminary strip, and the preliminary strip is fed uninterruptedly to the finishing mill. The uninterrupted feeding of the thin slab strand to the roughing mill and the uninterrupted feeding of the preliminary strip to the finishing mill allow the TRIP strip to be produced in continuous operation with particularly low energy consumption. Due to the uninterrupted production, the finished strip is mechanically connected to the pre-strip and mechanically to the thin slab strand.The TRIP tape is produced directly without the use of buffers or pre-tape separation, for example by shearing.

[0027] A high rolling temperature in the finishing rolling train and rolling to a particularly thin final thickness is ensured by arranging an intermediate heating system between the roughing rolling train and the finishing rolling train, the intermediate heating system heating a core of the preliminary strip by at least 100°C up to and including 300°C, in particular to 1100°C up to and including 1180°C, the heated preliminary strip being fed to the finishing rolling train.

[0028] The TRIP steel strip is manufactured using the process described above. The TRIP steel strip has a chemical composition in percent by weight of C inclusive of 0.15% up to and including 0.25%, in particular inclusive of 0.19% up to and including 0.21%, Mn 1.0% up to and including 2.0%, in particular 1.4% up to and including 1.6%, Si 1.0% up to and including 1.5%, in particular 1.1% up to and including 1.3%, Al inclusive of 0.3% up to and including 0.7%, in particular inclusive of 0.45% up to and including 0.55%, the remainder being Fe and unavoidable impurities. The finished strip has the following microstructure at room temperature, based on percent by volume: inclusive of 40% ferrite up to and including 60%, in particular inclusive of 45% up to and including 55% ferrite, inclusive of 8% up to and including 15% metastable retained austenite, the remainder being preferably cementite-free bainite. Preferably, the TRIP steel strip has a thickness of 0.6 mm up to and including 6 mm, in particular 0.8 mm up to and including 2 mm.

[0029] The invention is explained in more detail below with reference to the figures. These show:

[0030] FIG. 1 shows a schematic representation of a combined casting and rolling plant; FIG. 2 shows a section A of the combined casting and rolling plant marked in FIG. 1 in a symbolic representation;

[0031] FIG 4 is a diagram of the temperature of the steel between the metallic melt and the TRIP steel strip plotted against time during a pass through the combined casting and rolling plant; and

[0032] FIG 3 is a flow chart of a method for operating the combined casting and rolling plant shown in FIGS 1 and 2;

[0033] FIG 1 shows a schematic representation of a combined casting and rolling plant 10 for producing a TRIP steel strip 245.

[0034] The combined casting and rolling plant 10 comprises, for example, a continuous casting machine 15, a roughing train 20, preferably a first to third separating device 25, 30, 35, an intermediate heating device 45, preferably a descaler 50, a finishing train 55, a cooling section 65, at least one, preferably second, coiling device 70 and a control device 75.

[0035] The continuous casting machine 15 is embodied, for example, as a bow-type continuous casting machine. Another configuration of the continuous casting machine 15 is also conceivable. The continuous casting machine 15 has a ladle 95, a distributor 100, and a mold 105. During operation of the combined casting and rolling plant 10, the distributor 100 is filled with a metallic melt 110 by means of the ladle 95. The metallic melt 110 can be produced, for example, using a converter, for example in a Linz-Donawitz process. The metallic melt 110 can comprise steel, for example. The metallic melt 110 flows from the distributor 100 into the mold 105. In the mold 105, the metallic melt 110 is cast into a thin slab strand 115. The partially solidified thin slab strand 115 is pulled out of the mold 105 and, due to the design of the continuous casting machine 15 as a curved continuous casting machine, is deflected in an arc-shaped manner into a horizontal position, where it is supported and solidified.The thin slab strand 115 is conveyed away from the mold 105 in the conveying direction.

[0036] In a conveying direction of the thin slab strand 115, the roughing mill train 20 is arranged downstream of the continuous casting machine 15. In this embodiment, the roughing mill train 20 directly follows the continuous casting machine 15. The roughing mill train 20 can have one or more roughing stands 120, 121, 122. These are arranged one behind the other in the conveying direction of the thin slab strand 115. The number of roughing stands is essentially freely selectable and depends essentially on the format of the thin slab strand 115. The desired thickness of a transfer strip 125, which the roughing stands 120, 121, 122 roll, also plays a role. In this embodiment, three roughing stands 120, 121, 122 are provided for the roughing mill train 20 shown in FIG. 1. The roughing mill 20 is designed to roll the thin slab strand 115, which is hot when fed into the roughing mill 20, into the pre-strip 125.In a strand direction of the thin slab strand 115, a first roughing stand 120 is arranged upstream of a second roughing stand 121. In the strand direction of the thin slab strand 115, a third roughing stand 122 is arranged downstream of the second roughing stand 121.

[0037] In the exemplary embodiment, the first cutting device 25 and the second cutting device 30 are arranged downstream of the roughing train 20 with respect to the conveying direction of the preliminary strip 125. The second cutting device 30 is arranged at a distance from the roughing train 20 with respect to the conveying direction of the preliminary strip 125. A discharge device (not shown in FIG. 1) can be arranged between the first cutting device 25 and the second cutting device 30 in order to discharge a thin slab piece cut by the first cutting device 25 and the second cutting device 30. The second cutting device 30 can also be omitted. The first and second cutting devices 25, 30 can be designed, for example, as drum shears or pendulum shears.

[0038] In this embodiment, the intermediate heater 45 follows the second separating device 30, based on the conveying direction of the transfer strip 125. The intermediate heater 45 can, for example, be designed as an induction furnace. Another design of the intermediate heater 45 would also be possible. Based on the conveying direction of the transfer strip 125, the intermediate heater 45 is arranged upstream of the finishing rolling train 55 and the descaler 50. The descaler 50 is arranged directly upstream of the finishing rolling train 55 and downstream of the intermediate heater 45. The descaler 50 can also be omitted. The finishing rolling train 55 is arranged downstream of the descaler 50, based on the conveying direction of the transfer strip 125. In this embodiment, the finishing rolling train 55 has five finishing rolling stands 145, 146, 147, 148, 149. The finishing rolling stands 145, 146, 147, 148, 149 are arranged one behind the other with respect to the conveying direction of the preliminary strip 125.During operation of the integrated casting and rolling plant 10, the finishing stands 145, 146, 147, 148, 149 roll the preliminary strip 125 fed to the finishing rolling train 55 into a finished strip 165. With respect to the conveying direction of the supplied preliminary strip 125, a first finishing stand 145 is arranged upstream of the other finishing stands 146, 147, 148, 149. A second finishing stand 146 is arranged downstream of the first finishing stand 145 and upstream of a third to fifth finishing stand 147, 148, 149. The third finishing stand 147 is arranged downstream of the second finishing stand 146 and upstream of a fourth finishing stand 148. The fourth finishing stand 148 is arranged downstream of the third finishing stand 147 and upstream of the fifth finishing stand 149.

[0039] The control unit 75 has a control device 170, a data memory 175, and an interface 180. The data memory 175 is connected to the data storage device 170 via a first data connection 185. Likewise, the interface 180 is connected to the control device 170 via a second data connection 190.

[0040] A predefined first target temperature, a predefined second target temperature, and a predefined third target temperature are preferably stored in the data memory 175. Furthermore, a method for producing the TRIP steel strip 245 is stored in the data memory 175, on the basis of which method the control device 170 controls the components of the combined casting and rolling plant 10.

[0041] The interface 180 is further connected to the intermediate heating system 45 via a third data connection 195. A fourth data connection 200 connects the finishing rolling mill 55 to the interface 180. A fifth data connection 205 connects the cooling section 65 to the interface 180.

[0042] Furthermore, the integrated casting and rolling plant 10 can have a first temperature measuring device 80 and a second temperature measuring device 85. In addition, the integrated casting and rolling plant 10 can have a third temperature measuring device 172. The first temperature measuring device 80 and / or the second temperature measuring device 85 and / or the third temperature measuring device 172 can be designed, for example, as pyrometers. The first temperature measuring device 80 is arranged downstream of the intermediate heater 45 with respect to the conveying direction of the preliminary strip 125 and preferably upstream of the descaler 50. The second temperature measuring device 85 is arranged between the cooling section 65 and the finishing rolling train 55. The third temperature measuring device 172 can be arranged in the cooling section 65. The first temperature measuring device 80 is connected to the interface 180 via a sixth data connection 210.A seventh data connection 215 connects the second temperature measuring device 85 to the interface 180. The third temperature measuring device 172 is connected to the interface 180 by means of an eighth data connection 225.

[0043] FIG 2 shows a section A of the combined casting and rolling plant 10 marked in FIG 1 in a symbolic representation.

[0044] The cooling section 65 has a first cooling group 166, a second cooling group 167, at least one third cooling group 168, and preferably a roller conveyor 171. The first cooling group 166 is arranged upstream of the second cooling group 167 with respect to the conveying direction of the finished strip 165. The second cooling group 167 is, for example, directly adjacent to the first cooling group 166. With respect to the conveying direction of the finished strip 165, the third cooling group 168 is arranged directly downstream of the second cooling group 167. The third separating device 35 is arranged adjacent to the third cooling group 168. The first to third cooling groups 166, 167, 168 can each have a plurality of cooling beams 169, wherein the cooling beams 169 are arranged on the top and / or bottom of the finished strip 165.The roller conveyor 171 extends in the conveying direction along the first to third cooling groups 166, 167, 168 to transport the finished strip 165 in the cooling section 65 between the finishing rolling mill 55 and the third separating device 35. It should be noted that a respective length of the cooling groups 166, 167, 168 is exemplary. In particular, the length of the cooling groups 166, 167, 168 is determined by a transport speed of the finished strip 165. Depending on the transport speed, a length of the respective cooling groups 166, 167, 168 can be dynamically changed.

[0045] With respect to the conveying direction of the finished strip 165, the first cooling group 166 is, for example, shorter than the second cooling group 167 and the third cooling group 168. Furthermore, the third cooling group 168 is shorter than the second cooling group 167.

[0046] Between the first cooling group 166 and the first finishing rolling stand 145 arranged last in the conveying direction, the combined casting and rolling plant 10 can have a measuring section 60 which is arranged between the first cooling group 166 and the last first finishing rolling stand 145 with respect to the conveying device of the finished strip 165.

[0047] FIG. 3 shows a diagram of the temperature of the steel between the metallic melt 110 and the TRIP steel strip 245 plotted against time t during passage through the integrated casting and rolling plant 10. FIG. 4 shows a flow chart of a method for operating the integrated casting and rolling plant 10 shown in FIGS. 1 and 2. In FIG. 3, the integrated casting and rolling plant 10 already explained in FIG. 1 is shown schematically above the diagram in order to easily assign the individual temperatures graphically to the respective components of the integrated casting and rolling plant 10. The FIGS. are explained together below.

[0048] During operation of the combined casting and rolling plant 10, in a first process step 305, the mold 105 of the continuous casting machine 15 is closed with a dummy bar head (not shown in FIG. 1) and sealed with additional sealing means. The molten metal 110 is poured into the distributor 100 of the continuous casting machine 15 using the ladle 95. To begin the continuous casting, a plug is removed from a pouring tube of the continuous casting machine 15. The metallic melt 110 preferably has a chemical composition in weight percent of C inclusive of 0.15% up to and including 0.25%, in particular inclusive of 0.19% up to and including 0.21%, Mn 1.0% up to and including 2.0%, in particular 1.4% up to and including 1.6%, Si 1% up to and including 1.5%, in particular 1.1% up to and including 1.3%, Al inclusive of 0.3% up to and including 0.7%, in particular inclusive of 0.45% up to and including 0.55%, the remainder being Fe and unavoidable impurities. The metallic melt 110 can also have a different chemical composition.

[0049] The temperatures and process steps specified below refer to the chemical composition of the steel preferred in the embodiment in order to produce the TRIP steel strip 245 by means of the combined casting and rolling plant 10.

[0050] At the beginning of continuous casting, the metallic melt 110 in the mold 105 flows around the dummy strand head and solidifies at the dummy strand head. The dummy strand head is slowly drawn from the mold 105 of the continuous casting machine 15 toward the roughing train 20. Downstream of the dummy strand head in the conveying direction, the metallic melt 110 in the mold 105 cools at its contact surfaces with the mold 105 and forms a shell of the thin slab strand 115. The shell encloses a liquid core and holds the liquid core. At the mold exit, the thin slab strand 115 can, for example, have a thickness of 80 mm to 150 mm.

[0051] In the continuous casting machine 15, the thin slab strand 115 is deflected and further cooled on its way to the roughing mill train 20, so that the thin slab strand 115 solidifies from the outside to the inside. In the exemplary embodiment, the continuous casting machine 15, as explained above, is designed as a curved continuous casting machine, so that by deflecting the thin slab strand 115 by essentially 90° from the vertical, the thin slab strand 115 is fed to the roughing mill train 20 in a substantially horizontal manner. In a second method step 310, the thin slab strand 115 is rolled, as already explained above, in the roughing mill train 20 by the roughing mill stands 120, 121, 122 to form the preliminary strip 125.

[0052] A core temperature of the core of the thin slab strand 115 upon entering the roughing mill 20 with the above-mentioned chemical composition is approximately 1300 °C to 1450 °C. During each hot rolling step in the roughing mill 20, the core temperature of the core is reduced, so that the pre-strip 125 has a core temperature of approximately 980 °C to 1150 °C upon exiting the roughing mill 20.

[0053] It is particularly advantageous if a thickness reduction of the roughing strip 125 of 35 percent up to and including 60 percent occurs in the roughing pass at the first roughing stand 120 and / or the second roughing stand 121. A thickness reduction of the roughing strip 125 is preferably 35 percent up to and including 50 percent at the third roughing stand. The respective thickness reduction is based on a thickness of the roughing strip 125 upon exiting the respective roughing stand 120, 121, 122 to a thickness of the roughing strip 125 upon entering the corresponding roughing stand 120, 121, 122. This has the advantage that a thin roughing strip 125 leaves the roughing train 20 at the end of the roughing train 20.

[0054] In a third method step 315, the pre-strip 125 is guided through the first and second separating devices 25, 30, but the pre-strip 125 is not separated. Thus, the pre-strip 125 only passes through the first and second separating devices 25, 30. The pre-strip 125 continues to cool by convection, and a protective cover can be used to reduce the cooling during transport to the intermediate heater 45.

[0055] In a fourth method step 320, the control device 170 activates the intermediate heater 45 via the third data connection 195, so that the intermediate heater 45, which is configured, for example, as an induction furnace, heats the core temperature of the pre-strip 125 from 850°C to 950°C upon entering the intermediate heater 45 to approximately 1050°C to 1200°C. It is particularly advantageous if the pre-strip 125 leaves the intermediate heater 45 with a core temperature of 1100°C to 1180°C. This has the advantage of ensuring optimal hot-rolled strip surface quality.

[0056] In a fifth method step 325, the first temperature measuring device 80, which is designed, for example, as a first pyrometer, determines a first surface temperature T01 of the pre-strip 125 fed from the intermediate heater 45. The first temperature measuring device 80 provides first information about the first surface temperature TO1 of the pre-strip 125 between the intermediate heater 45 and the descaler 50 via the sixth data connection 210 of the interface 180, which provides the first information to the control device 170.

[0057] In a sixth method step 330, the control device 170 regulates the heating power of the intermediate heater 45 such that the determined first surface temperature TO1 of the pre-strip 125 between the intermediate heater 45 and the descaler 50 substantially corresponds to the first target temperature. In this case, the control device 170 can regularly repeat the fifth and sixth method steps 325, 330 in a loop at a predefined time interval.

[0058] In a seventh method step 335, the control device 170 activates the descaler 50 (if present). The descaler 50 descales the pre-strip 125. In doing so, the pre-strip 125 cools, for example, by 80°C to 100°C relative to the core of the pre-strip 125. In particular, it can be ensured that when the core temperature is between 1100°C and 1180°C at the outlet from the intermediate heater 45, optimal descaling performance of the descaler 50 is achieved.

[0059] With a first inlet temperature TE1 (relative to the core of the transfer strip 125), the transfer strip 125 is transported to the first finishing rolling stand 145 of the finishing rolling mill 55 in an eighth process step 340. The first inlet temperature TE1 is relative to the core of the transfer strip 125, with which the transfer strip 125 enters the first finishing rolling stand 145 in the conveying direction relative to the transfer strip 125 after the descaler 50. The first inlet temperature TE1 can be between 950°C and 1120°C, in particular between 950°C and 1050°C.

[0060] In a ninth method step 345, the preliminary strip 125 is finish-rolled to the finished strip 165, for example, using five finishing stands 145. It is particularly advantageous if, at the first finishing stand 145, a thickness reduction of the preliminary strip 125 to the finished rolled strip 165 of 35 percent up to and including 55 percent takes place in a first finishing roll pass. Preferably, at the second finishing stand 146, a thickness reduction of 30 percent up to and including 50 percent takes place in the second finishing roll pass. Furthermore, at the third finishing stand 147, a thickness reduction of 25 percent up to and including 40 percent takes place in the third finishing roll pass. At the fourth finishing stand 148, a thickness reduction of 20 percent up to and including 30 percent takes place, preferably in the fourth finishing roll pass.At the fifth finishing stand 149, a thickness reduction of 10 percent to 20 percent inclusive is preferably performed in the fifth finishing pass. The thickness reduction thus occurs primarily at the first to third finishing stands 145, 146, 147, resulting in repeated recrystallization of the microstructure in the finished rolled strip 165. This is made possible in particular by the high core temperature of 1100°C to 1180°C upon exiting the intermediate heating system. The high thickness reduction, especially at the first to third finishing stands 145, 146, 147, is only possible in continuous operation, since only a pull-through condition must be met, not a grip condition. Due to a preferably decreasing thickness reduction in the conveying direction of the finished strip 165 on the first to fifth finishing rolling stands 145, 146, 147, 148, 149, the finished rolled strip is particularly flat when leaving the finishing rolling train 55.

[0061] The finished strip 165 emerges with a thickness of 0.6 mm to 6 mm, including, and in particular, 0.8 mm to 2 mm. At each first finishing stand 145 of the finishing train 55, the preliminary strip 125 to be rolled into the finished strip 165 cools by approximately 50 °C, so that a stepped line forms in the temperature profile (see FIG. 4).

[0062] A first exit temperature TA1 of the finished strip 165 after passing through the finishing rolling mill 55 is approximately 800°C to 950°C, in particular from 830°C to 860°C inclusive. The first exit temperature TA1 is specified relative to the core of the finished strip 165. The first exit temperature TA1 can be specifically adjusted using the intermediate heater 45. Furthermore, the first exit temperature TA1 is preferably greater than a ferrite transformation temperature Ar3. Finish rolling via the first finishing rolling stands 145 preferably takes place in an austenitic microstructure. In particular, by ensuring the first exit temperature TA1 of 830 °C up to and including 860 °C with the core temperature of 1100 °C up to and including 1180 °C at the exit from the intermediate heating 45, a homogeneous, fine-grained austenite can be produced in the finished strip 165 at the exit from the finishing rolling train 55.Furthermore, the core temperature of 1100°C to 1180°C at the outlet from the intermediate heater 45 can prevent undesirable fayalite formation (in Si-alloyed steels) and the formation of scale scars on a surface of the finished rolled strip 165.

[0063] At the first exit temperature TA1, the finish-rolled finished strip 165 is transported further in the direction of the cooling section 65 in a tenth method step 350. The finished strip 165 is transported past the second temperature measuring device 85. The second temperature measuring device 85 can be designed as a pyrometer and measures a second surface temperature T02 of the finished strip 165 coming from the finishing rolling mill 55. The second temperature measuring device 85 provides information, which correlates with the first exit temperature TA1, to the control device 170 via the seventh data connection 215 and the interface 180. The control device 170 can take the second surface temperature T02 into account when controlling the intermediate heater 45. The second surface temperature T02 correlates, as already explained, with the first exit temperature TA1.However, the second surface temperature TO2 differs in its value from the first exit temperature TA1. This is due in particular to the fact that the second surface temperature TO2 refers to the surface of the finished strip 165, and the first exit temperature TA1 refers to the core of the finished strip 165. However, because the finished strip 165 is preferably only 0.6 mm to 6 mm thick, in particular 0.8 mm to 2 mm thick, the temperature difference between the first exit temperature TA1 and the second surface temperature TO2 is small (less than 10 °C).

[0064] In the exemplary embodiment, the control of the intermediate heater 45 by the control device 170 is carried out in such a way that the second surface temperature TO2 essentially corresponds to the second target temperature when controlling the intermediate heater 45. However, the second temperature measuring device 85 and / or the tenth method step 350 can also be omitted.

[0065] In an eleventh method step 355, the control device 170 activates the first cooling group 166 via the fifth data connection 205. The finished strip 165 is introduced into the first cooling group 166 at the first exit temperature TA1. Essentially, no phase transformation occurs in the finished strip 165 in the region between a final rolling pass of a final finishing stand and the entry into the first cooling group 166.

[0066] In the first cooling group 166, which preferably has one, in particular several, cooling beams 169, a cooling medium, for example water, optionally with an additive, is sprayed onto the hot, finish-rolled finished strip 165 by means of the cooling beams 169. This force-cools the finished strip 165 in the first cooling group 166. Upon exiting the first cooling group 166, the core of the finished strip 165 has a predominantly austenitic structure. The phase fraction of the austenitic structure is thus preferably more than 50%, particularly preferably 100%, upon exiting the first cooling group 166. The phase fraction refers to a volume percent.Preferably, a volume flow of the cooling medium is selected such that, within the first cooling group 166, the finished strip 165 is cooled from a second inlet temperature TE2, which essentially corresponds to the first outlet temperature TA1, to a second outlet temperature TA2, in particular from 550°C to 720°C, within a first time interval t1 at the first cooling rate. Cooling in the first cooling group 166 takes place, for example by controlling the volume flow of the cooling medium, such that the second outlet temperature TA2 is lower than Ae3. It is particularly advantageous if the second outlet temperature TA2 is from 620°C to 720°C inclusive.

[0067] It is particularly advantageous here if the flow rate of the cooling medium is selected such that a cooling capacity of the first cooling group 166 ensures a first cooling rate of the core of the finished strip 165 of at least 20 K / s up to and including 1000 K / s, in particular 20 K / s up to and including 500 K / s, in particular 20 K / s up to and including 400 K / s, in particular 50 K / s up to and including 200 K / s. Cooling in the core of the finished strip 165 in the first cooling group 166 preferably takes place continuously via the first cooling group 166.

[0068] The first cooling rate is ensured in the embodiment, for example, by the fact that, preferably with the arrangement of several cooling beams 169, a volume flow of about 100 m 3 / h up to 350 m 3 / h of the cooling medium is sprayed onto the finished strip 165 at a pressure of 2 bar to 4 bar. This ensures that within the short throughput time of the finished strip 165, for example with a transport speed of 4 m / s to 15 m / s through the first cooling group 166, the core of the finished strip 165 is cooled from the second inlet temperature TE2, for example from 800 °C to 950 °C inclusive, in particular 830 °C to 900 °C, to the second outlet temperature TA2. In order to ensure particularly precise control of the volume flow by the control device 170, a control valve can be provided for each cooling beam 169, which the control device 170 can control. As a result, the volume flow of the cooling medium can be continuously adjusted between 10% and 100% by the control device 170 for each cooling beam 169 of the first cooling group 166.

[0069] In a twelfth method step 360, the finished strip 165 is transported to the second cooling group 167 at the second exit temperature TA2. Transport within the cooling section 65 is effected by means of the roller conveyor 171. The control device 170 ensures that the second cooling group 167 is deactivated such that no cooling medium is conveyed onto the finished strip 165 within the second cooling group 167, thus preventing active forced cooling. Within the second cooling group 167, cooling of the finished strip 165 thus occurs only through radiation and convection cooling to the environment of the finished strip 165 within the second cooling group 167.

[0070] When the finished strip 165 enters the second cooling group 167 in the twelfth process step 360, a structure of the finished strip 165 is partially, in particular greater than 80 percent phase fraction, particularly preferably completely austenitic, since essentially no or only a slight phase transformation occurs in the finished strip 165 in the eleventh process step 355.

[0071] The third temperature measuring device 172 determines a third surface temperature TO3, which correlates with the second exit temperature TA2, after the finished strip 165 exits the first cooling group 166. The third temperature measuring device 172 provides third information about the third surface temperature TO3 via the eighth data connection 225 of the interface 180 and via the interface 180 of the control device 170.

[0072] When regulating the volume flow of the cooling medium in the first cooling group 166 in the eleventh method step 355, the control device 170 can also take into account the information about the third surface temperature TO3. In particular, the control device 170 can regulate the volume flow of the cooling medium that is guided, in particular sprayed, from the first cooling group 166 onto the finished strip 165 such that the third surface temperature TO3 substantially corresponds to the third target temperature. The third target temperature is selected such that the second outlet temperature TA2, which relates to the core, lies between the ferrite precipitation temperature Ae3 and the bainite start temperature BS.

[0073] Furthermore, when regulating the volume flow, the control device 170 can additionally take into account the second surface temperature TO2 in order to ensure a uniform first cooling rate in the first cooling group 166. In this case, the control device 170 can regularly repeat the eleventh and twelfth method steps 355, 360 in a loop at a predefined time interval.

[0074] In a thirteenth method step 365, the finished strip 165 is transported through the roller conveyor 171 in a warm, partially cooled state in the second cooling group 167 toward the third cooling group 168. As already mentioned above, the control device 170 keeps the second cooling group 167 deactivated, so that when the finished strip 165 passes through the second cooling group 167, no further cooling medium is applied to the finished strip 165 for further forced cooling of the finished strip 165.

[0075] In the twelfth and thirteenth process steps 360, 365, the finished strip 165 cools via the second cooling group 167 at a second cooling rate from the second exit temperature TA2 to a third exit temperature TA3. The second cooling rate is significantly lower than the first cooling rate. The second cooling rate is, for example, from -25 K / s up to and including 20 K / s, in particular from 0 K / s up to and including 20 K / s. The second cooling rate results primarily from a combined convective and radiative cooling of the finished strip 165 in the second cooling group 167 on the roller conveyor 171. Due to the forced cooling of the finished strip 165 in the eleventh process step 355 below the ferrite start temperature Ae3, a first part of the austenitic microstructure of the finished strip 165 transforms into ferrite during transport.

[0076] To pass through the second cooling group 167, the finished strip 165 requires a second time interval t2. The second time interval t2 is significantly longer than the first time interval t1. The second time interval t2 can last from 3 seconds up to and including 8 seconds, in particular from 4 seconds up to and including 5 seconds. In the second time interval t2, the finished strip 165 passes through the second cooling group 167 and is thus transported from the first cooling group 166 within the second time interval t2 via the second cooling group 167 into the third cooling group 168. The second time interval t2 serves as a holding time. In the second time interval t2, the mixed structure of austenite and ferrite continues to form in the finished strip 165. During transport in the second cooling group 167, the ferrite content in the structure of the finished strip 165 increases significantly.At the end of the second cooling group 167, the composition of the material of the finished strip 165 is as follows (based on volume percent): 40% to 80% ferrite, in particular 45% to 60% ferrite, the remainder essentially austenite. In particular, the homogeneous, fine-grained austenite (cf. ninth process step 345) promotes the rapid formation of ferrite during passage through the deactivated second cooling group 167. This allows the second cooling group 167 to be kept spatially short.

[0077] At the end of the second cooling group 167, the core of the finished strip 165 has the third exit temperature TA3, which is lower than the second exit temperature TA2. In particular, however, the third exit temperature TA3 is even higher than the austenite-ferrite transformation temperature Ar1. The third exit temperature TA3 can be between 580 °C and 710 °C, in particular between 650 °C and 690 °C.

[0078] The third exit temperature TA3 corresponds to a third inlet temperature TE3 with which the finished strip 165 enters the third cooling group 168 and is related to the core of the finished strip 165.

[0079] In a fourteenth method step 370, the control device 170 activates the third cooling group 168, if not already activated, via the fifth data connection 205. In the third cooling group 168, the cooling section 65 uses the cooling medium to cool the finished strip 165 from the third inlet temperature TE3 and thus from the third outlet temperature TA3 to a fourth outlet temperature TA4. In the third cooling group 168, the cooling medium is sprayed onto the warm finished strip 165, so that the finished strip 165 is forced-cooled in the third cooling group 168.

[0080] The fourth outlet temperature TA4 can, in particular, be from 300 °C to 450 °C inclusive and is thus lower than Ae1. Furthermore, the fourth outlet temperature TA4 is also significantly higher than an ambient temperature TU of approximately 20 °C to 40 °C of the combined casting and rolling plant 10.

[0081] The cooling of the finished strip 165 in the third cooling group 168 takes place in particular within a third time interval t3. In the third cooling group 168, the finished strip 165 is cooled at a third cooling rate which is significantly greater than the second cooling rate. The third cooling rate can be 20 K / s up to and including 1000 K / s, in particular 20 K / s up to and including 500 K / s, in particular 20 K / s up to and including 400 K / s, in particular 50 K / s up to and including 200 K / s. The cooling in the core of the finished strip 165 via the third cooling group 168 preferably takes place continuously. The third cooling rate can be different from the first cooling rate.

[0082] The third cooling rate is ensured in the embodiment in such a way that preferably a further volume flow of 100 m 3 / h up to 300 m 3 / h of the cooling medium is applied to the finished strip 165 at a pressure of 2 bar to 4 bar by means of the cooling beams 169 of the third cooling group 168. This ensures that within the short third time interval t3 of the finished strip 165, the core of the finished strip 165 is cooled by the second cooling group 168 from the third inlet temperature TE3 to the fourth outlet temperature TA4. Analogous to the first cooling group 166 in the third cooling group 168, each cooling beam 169 of the third cooling group 168 can also be designed such that a control valve controllable by the control device 170 is provided for it, in order to control them separately, preferably continuously and separately from the other cooling beam 169 of the third cooling group 168.As a result, a volume flow of the cooling medium within the third cooling group 168 can be continuously controlled between 0% and 100% by the control device 170 for each of the cooling beams 169 of the third cooling group 168.

[0083] Due to the rapid cooling of the finished strip 165 from the third exit temperature TA3 / third inlet temperature TE3 to the fourth exit temperature TA4, a second portion of the austenite is converted into cementite-free bainite. Cementite precipitation is inhibited in this embodiment by the alloying constituents Si and Al, which keeps the carbon C in solution. The thin-walled design of the finished strip 165 ensures a homogeneous conversion of the austenite into cementite-free bainite across the strip thickness.

[0084] In a fifteenth process step 375, the finished strip 165, cooled by the third cooling group 168 to the fourth exit temperature TA4, is guided through the third separating device 35 to the coiling device 70. In the coiling device 70, the finish-rolled finished strip 165, cooled to the fourth exit temperature TA4, is wound into a coil 250.

[0085] Since the coiling device 70 is arranged only a few meters away from one end of the cooling section 65, the fourth outlet temperature TA4 essentially corresponds to a fourth inlet temperature TE4 at which the finished strip 165 is guided into the coiling device 70. However, because the coiling device 70 is arranged at a distance from the cooling section 65 and the fourth outlet temperature TA4 is significantly greater than 180 °C, in particular in the range from 360 °C up to and including 420 °C, in particular in the range from 360 °C up to and including 390 °C, excess cooling medium can both drain off the finished strip 165 and dry off between the outlet of the finished strip 165 and the winding of the finished strip 165 in the coiling device 70 to form the coil 250, so that the finished strip 165 is preferably wound up dry.

[0086] After coil 250 has been wound, control device 170 can activate third separating device 35 and separate finished strip 165, so that the wound coil 250 can be removed from combined casting and rolling plant 10. This allows finished strip 165 to be continuously conveyed and wound into another coil 250. In particular, combined casting and rolling plant 10 can additionally comprise another separating device and additional coiling devices 70.

[0087] The finished coil 250 is essentially hollow-cylindrical and has an inner diameter and an outer diameter. It is advantageous if the ratio of the outer diameter to the inner diameter is between 2 and 2.8 inclusive.

[0088] In a sixteenth method step 380, the finished strip 165 is cooled in the wound state from the fourth exit temperature TA4 to the ambient temperature TU. During this process, the coil 250 is exposed to an environment. Immediately after winding, the coil cools at a fourth cooling rate of 20 K / h up to and including 30 K / h on an outer winding and / or an inner winding of the coil 250, preferably within the first hour after winding. In a center winding of the coil 250, which is arranged in the radial direction essentially centrally relative to the outer winding and the inner winding, a fifth cooling rate after winding is 1 K / h up to and including 20 K / h, preferably within the first hour after winding.

[0089] An average cooling rate at which the coil 250 cools from the fourth outlet temperature TA4 to the ambient temperature TU may preferably be 3 K / h up to and including 15 K / h, so that, for example, the coil 250 is cooled in a fourth time interval t4 of approximately 24 hours to 72 hours.

[0090] Cooling occurs by radiation and convection without additional active cooling. This natural cooling in the fourth time interval t4 enriches at least a third of the remaining retained austenite in the microstructure with carbon C, which is practically insoluble in ferrite, thus forming metastable retained austenite. The described cooling of the 250 coil without a heat shield has the advantage of reducing carbon supersaturation while simultaneously avoiding cementite precipitation in the retained austenite portion.

[0091] Alternatively, it is also conceivable that in the sixteenth process step 380, a heat hood is additionally placed on the coil 250, which thermally insulates the coil 250 from the environment in order to achieve a particularly slow and targeted cooling of the coil 250. It is particularly advantageous if the fourth exit temperature is 360°C to 390°C.

[0092] The heat hood allows the coil 250 (both on the inner and / or outer windings and / or the center winding) to cool at a sixth cooling rate of 1 K / h up to and including 10 K / h during the first hour after coiling under the heat hood. The average cooling rate, starting from the fourth exit temperature TA4, can preferably be 1 K / h up to and including 8 K / h under the heat hood.

[0093] After cooling the coil 250 at the center turn to a fifth exit temperature, which is approximately 10°C up to and including 100°C lower than the fourth exit temperature TA4, the heat shroud is removed in a seventeenth process step 385, and further cooling of the coil 250 takes place through the surroundings of the coil 250 to the ambient temperature TU. The average cooling rate after removal of the heat shroud can be 2 K / h up to and including 12 K / h.

[0094] The use of the heat hood has the advantage that the TRIP steel strip 245 has a particularly homogeneous structure and thus particularly homogeneous material properties.

[0095] After cooling to ambient temperature TU, the production of the finished strip 165 is complete, and the cooled finished strip 165 is now formed as TRIP steel strip 245. If the TRIP steel strip 245 is mechanically deformed in the subsequent production process, for example, due to a traffic accident or in a press, the metastable residual austenite forms martensite during deformation.

[0096] In particular, the TRIP steel strip 245 has the following chemical composition: from 0.15% up to and including 0.25%, in particular from 0.19% up to and including 0.21%, Mn 1.0% up to and including 2.0%, in particular from 1.4% up to 1.6%, Si 1.0% up to 1.5%, in particular from 1.1% up to 1.3%, Al up to and including 0.3% up to 0.7%, in particular from 0.45% up to 0.55%, balance Fe and unavoidable impurities.

[0097] Furthermore, the TRIP steel strip 245 has the following microstructure at ambient temperature TU (based on volume percent): from 40% ferrite up to and including 60%, in particular from 45% to 55% ferrite, from 8% to and including 15% metastable residual austenite, and preferably cementite-free bainite. Using the method described above and the combined casting and rolling plant 10 described above, the TRIP steel strip 245 can be produced with a particularly thin thickness, in particular 0.6 mm to 6 mm, in particular 0.8 mm to 2 mm, in a continuous casting and rolling process directly without cold rolling and annealing. This has the advantage that the energy required to produce the TRIP steel strip 245 is significantly lower, making it more environmentally friendly and cost-effective to produce the TRIP steel strip 245.

[0098] Even at high speeds, for example, at 10 m / s, the holding time, corresponding to the second time interval t2, between the exit of the finished strip 165 from the first cooling group 166 and its entry into the third cooling group 168 is ensured to be between 3 seconds and 8 seconds, in particular between 4 seconds and 5 seconds. This ensures that a sufficiently large proportion of ferrite is present in the finished strip 165 at the end of the second cooling group 167.

[0099] Furthermore, the above-described design of the combined casting-rolling plant 10 with the above-described method allows a high casting speed of 0.08 m / s to 0.1 m / s to be achieved with the specified thickness of the thin slab strand 115 of 100 mm to 150 mm.

[0100] Furthermore, it should be noted that the integrated casting and rolling plant 10 can also be designed differently than described in the figures. In particular, it would also be possible for the integrated casting and rolling plant 10 to have, for example, six finishing stands 145, wherein the last finishing stand 145 in the conveying direction, for example, is converted into a stand cooler in a preparatory step. For this purpose, in the preparatory step, work rolls can be removed from the finishing stand 145 by opening a changing device and replaced by one or more cooling beams. Furthermore, the cooling beam of the stand cooler can be aligned such that it points directly towards a passage through which the finished strip 165 is guided. When the changing device is closed, the cooling beams are fastened in the stand cooler.

[0101] By converting the last finishing stand 145 in the conveying direction, for example, into the stand cooler, the cooling section 65 is extended counter to the conveying direction of the finished strip, and the stand cooler forms a section of the first cooling group 166. Additionally, an intermediate cooler could also be arranged between the finishing stand 145 and the stand cooler. The TRIP steel strip 245 produced in continuous strand using the combined casting and rolling plant 10 and the process described in FIG. 3 is particularly suitable for the production of vehicle body panels and exhibits particularly good material properties. The increase in strength during forming is particularly suitable for highly stressed components, especially crash body components of motor vehicles.The transformation of metastable austenite into martensite occurs primarily during a crash, with the strength increase during deformation at high elongation at fracture allowing significant energy to be dissipated. TRIP steel strip 245 is also suitable for other vehicle components. TRIP steel strip 245 and the components manufactured with it are particularly tough and strong.

[0102] Furthermore, the combined casting and rolling plant 10 features particularly precise and stable process control thanks to its continuous operation and the direct production of the TRIP steel strip 245, ensuring high homogeneity of the hot strip. Since cold rolling and annealing are eliminated in the direct production of the TRIP steel strip 245 in continuous operation from the finished strip 165 material that has already cooled to ambient temperature, the energy required to produce the TRIP steel strip 245 is particularly low.

[0103] List of reference symbols

[0104] 10 Casting-rolling combined plant

[0105] 15 Continuous casting machine

[0106] 20 Roughing mill

[0107] 25 first separating device

[0108] 30 second separating device

[0109] 35 third separating device

[0110] 45 Intermediate heating

[0111] 50 descalers

[0112] 55 Finishing rolling mill

[0113] 60 measuring sections

[0114] 65 Cooling section

[0115] 70 reel device

[0116] 75 Control unit

[0117] 80 first temperature measuring device

[0118] 85 second temperature measuring device

[0119] 95 pan

[0120] 100 distributors

[0121] 105 mold

[0122] 110 metallic melt

[0123] 115 thin slab strand

[0124] 120 roughing stand

[0125] 125 Support act

[0126] 130 Discharge device

[0127] 135 first scaffolding group

[0128] 140 second scaffolding group

[0129] 145 finishing rolling stand

[0130] 156 upper work roll

[0131] 157 lower work roll

[0132] 165 finished strip

[0133] 166 first cooling group

[0134] 167 second cooling group

[0135] 168 third cooling group

[0136] 169 chilled beams

[0137] 170 Control device

[0138] 171 roller conveyor

[0139] 172 third temperature measuring device 175 data storage

[0140] 180 interface

[0141] 185 first data connection

[0142] 190 second data connection

[0143] 195 third data connection

[0144] 200 fourth data connection

[0145] 205 fifth data connection

[0146] 210 sixth data connection

[0147] 215 seventh data connection

[0148] 225 eighth data connection

[0149] 230 Sensor device

[0150] 245 TRIP steel band

[0151] 250 coils

[0152] 305 first procedural step

[0153] 310 second procedural step

[0154] 315 third procedural step

[0155] 320 fourth procedural step

[0156] 325 fifth procedural step

[0157] 330 sixth procedural step

[0158] 335 seventh procedural step

[0159] 340 eighth process step

[0160] 345 ninth procedural step

[0161] 350 tenth process step

[0162] 355 eleventh procedural step

[0163] 360 twelfth procedural step

[0164] 365 thirteenth procedural step

[0165] 370 fourteenth procedural step

[0166] 375 fifteenth procedural step

[0167] 380 sixteenth procedural step

[0168] 385 seventeenth procedural step

[0169] 400 first graph

[0170] 405 second graph

[0171] Ar1 austenite-ferrite transformation temperature

[0172] Ar3 Ferrite precipitation temperature t1 first time interval t2 second time interval t3 third time interval t4 fourth time interval

[0173] TA1 first outlet temperature

[0174] TA2 second outlet temperature

[0175] TA3 third outlet temperature

[0176] TA4 fourth outlet temperature

[0177] TE1 first inlet temperature

[0178] TE2 second inlet temperature

[0179] TE3 third inlet temperature

[0180] TE4 fourth inlet temperature

[0181] T01 first surface temperature

[0182] TO2 second surface temperature

[0183] TO3 third surface temperature

[0184] TU ambient temperature

Claims

Patent claims 1. Method for producing a TRIP steel strip (245) in a combined casting and rolling plant (10), - wherein the combined casting and rolling plant (10) comprises a finishing rolling mill (55) and a cooling section (65), - wherein a hot preliminary strip (125) is fed to the finishing rolling train (55), which the finishing rolling train (55) finish-rolls to a finished strip (165), - wherein, following the finish rolling of the finished strip (165), the finished strip (165) is fed to a first cooling group (166) of the cooling section (65) and in the first cooling group (166) a core of the finished strip (165) is forced to a second exit temperature (TA2) such that a first cooling rate of the core of the finished strip (165) is established, and that the second exit temperature (TA2) is in a range from 620°C to 700°C inclusive, - wherein the core of the finished strip (165) exits the first cooling group (166) has a predominantly, particularly preferably completely, austenitic structure, - wherein the finished strip (165) is cut within a second time interval (t2) of 3 seconds to 8 seconds, in particular from 4 seconds to 5 seconds, from the first cooling group (166) to a third cooling group (168) arranged at a distance from the first cooling group (166), - wherein the core of the finished rolled strip (165) comprises the second cooling group (167) with a third outlet temperature (TA3) of 580 °C up to and including 680 °C, in particular of 620 °C up to and including 660 °C, - where the third exit temperature (TA3) is greater than the austenite-ferrite transformation temperature (Ar1), - wherein a second cooling rate of the core of the finished strip (165) is established during the transport of the finished strip (165) between the first cooling group (166) and the third cooling group (168), - wherein the second cooling rate of the core of the finished strip (165) is -25 K / s up to and including 20 K / s, and during transport a first part of the austenitic structure in the finished strip (165) is transformed into a ferritic structure, - wherein in the third cooling group (168) the finished strip (165) is forced-cooled in such a way that a third cooling rate of the core of the finished strip (165) is established, and wherein in the third cooling group (168) the core of the finished strip (165) is forced-cooled to a third exit temperature (TA3) which is less than or equal to the bainite start temperature (BS), so that a second part of the austenite of the finished strip (165) is at least partially converted into a bainitic structure, - wherein the second cooling rate is lower than the first cooling rate and / or the third cooling rate, - wherein the first cooling rate and / or the third cooling rate of the core of the finished strip (165) is 20 K / s to 400 K / s, in particular 50 K / s to 200 K / s, - wherein, upon exit of the finished strip (165) from the third cooling group (168), the core of the finished strip (165) has a fourth exit temperature (TA4) of 180°C to 450°C inclusive, in particular 330°C to 420°C inclusive, in particular 360°C to 390°C inclusive, - wherein the finished strip (165) has a chemical composition in weight percent of C of 0.15% to 0.25%, in particular 0.19% to 0.21%, Mn 1.0% to 2.0%, in particular 1.4% to 1.6%, Si 1.0% to 1.5%, in particular 1.1% to 1.3%, Al 0.3% to 0.7%, in particular 0.45% to 0.55%, the remainder being Fe and unavoidable impurities, - wherein the TRIP steel strip (245) has the following microstructure at an ambient temperature (TU) (based on volume percent): from 40% ferrite up to and including 60%, in particular from 45% to 55% ferrite, from 8% to and including 15% metastable residual austenite and, preferably, cementite-free bainite.

2. Method according to claim 1, - wherein the cooled finished strip (165) is fed to a second cooling group (167) of the cooling section (65) after leaving the first cooling group (166), - wherein a forced cooling of the finished strip (165) in the second cooling group (167) is deactivated and the finished strip (165) in the second cooling group (167) is transported to a third cooling group (168) of the cooling section (65).

3. Method according to one of the preceding claims, - after passing through the cooling section (65), the finished strip (165) is wound up into a coil (250), - wherein the finished strip (165) is wound up in the coil (250) and cooled from the third outlet temperature (TA3) to the ambient temperature (TU), - wherein, upon cooling of the coiled finished strip (165) to ambient temperature (TU), a remaining third of the austenite of the finished strip (165) is enriched with carbon (C) in solution, so that a metastable austenite phase portion is formed.

4. Method according to one of the preceding claims, - wherein the core of the finished strip (165) has a first exit temperature (TA1) above a ferrite precipitation temperature (Ar3 temperature) upon exiting the finishing rolling train (55), in particular from 800 °C to 950 °C, in particular from 830 °C to 860 °C.

5. Method according to one of the preceding claims, - wherein the core of the finished strip (165) is cooled from the fourth exit temperature (TA4) to the ambient temperature (TU) within a fourth time interval (t4) of 24 hours to 72 hours.

6. Method according to one of the preceding claims, - wherein a thickness of the preliminary strip (125) upon entry into the finishing rolling train (55) is 4 mm to 25 mm, in particular 6 mm to 18 mm, - wherein the finishing rolling train (55) reduces the thickness of the preliminary strip (125) to the finished strip (165) to 0.6 mm to 6 mm, in particular to 0.8 mm to 2 mm.

7. Method according to one of the preceding claims, - wherein the combined casting and rolling plant (10) comprises a continuous casting machine (15) with a mold (105) and a single- or multi-stand roughing train (20), - wherein a metallic melt (110) is cast in the mold (105) to form a partially solidified thin slab strand (115), - wherein the partially solidified thin slab strand (115) is supported and deflected, - wherein the partially solidified thin slab strand (115) from the continuous casting machine is fed to the roughing train (20) without interruption, - wherein the roughing mill (20) rolls the thin slab strand (115) to the pre-strip (125), - wherein the preliminary strip (125) is fed to the finishing rolling train (55) without interruption.

8. Method according to claim 7, - wherein an intermediate heater (45) is arranged between the roughing train (20) and the finishing train (55), - wherein the intermediate heater (45) heats a core of the pre-strip (125) by at least 100°C up to and including 300°C, in particular to 1100°C up to and including 1180°C, - wherein the heated preliminary strip (125) is fed to the finishing rolling train (55).

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