Patent Application
20240327944
The present application claims priority to European Patent Application No. 23164462, filed Mar. 27, 2023, the contents of which are incorporated herein by reference.
The invention relates to a process for producing a TRIP steel strip as claimed and to a TRIP steel strip as claimed.
EP1045737 A1 describes a process for producing a steel strip.
It is an object of the invention to provide an improved process for producing a TRIP steel strip with an integrated casting-rolling plant in continuous operation and a TRIP steel strip.
This object is achieved with a process as claimed and with a TRIP steel strip as claimed. Advantageous embodiments are specified in the dependent claims.
It has been recognized that it is possible to provide an improved process for direct production of a TRIP steel strip in continuous operation in an integrated casting-rolling plant comprising a finishing mill train and a cooling sector. The finishing mill train is supplied with a hot precursor strip which is finish-milled into a finished strip by the finishing mill train After the finish-milling of the finished strip the finished strip is supplied to a first cooling group of the cooling sector and in the first cooling group a core of the finished strip is force-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, preferably to a phase proportion of at least 90%, in particular to a phase proportion of at least 95%, in particular completely, austenitic microstructure. After exiting the first cooling group the cooled finished strip is transported to a third cooling group of the cooling sector, 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 −25 K/s, in particular 0 K/s, to 20 K/s inclusive. During transport in the second cooling group a first portion of the austenitic microstructure is converted into a ferritic microstructure in the finished strip. In the third cooling group the core of the finished strip is force-cooled to a third exit temperature which is not more than the bainite starting temperature so that a first portion of the austenite of the finished strip is at least partially converted into bainite. The cooling may be carried out in the third cooling group in such a way that the first portion of the austenite is ideally converted into a cementite-free bainite.
This has the advantage that by means of the process the TRIP strip is produced directly in finished form at the end of the process irrespective of its final thickness. It is especially possible to eschew an additional cold-rolling step and/or an annealing step after the cooling sector. The direct production of the TRIP strip makes the above-described process particularly energy-efficient. The process especially makes it possible to produce a TRIP780/HCT780T strip in particularly cost-effective fashion. It is further ensured that the finished strip has already achieved its final thickness in the context of hot-rolling and not in the context of the cold-rolling step.
Due to the phase transitions taking place during transport and a resulting heat of transition the second cooling rate may also be negative.
The low second cooling rate during transport which is achieved for example by eschewing a forced cooling achieves a high homogeneity of the microstructure in the TRIP strip. A defined and reproducible phase proportion of metastable austenite and bainite is ensured with high process robustness by the low second cooling rate.
The direct production of the finish-milled strip in continuous operation, i.e. when the finish-milled strip is mechanically connected to the rough-milled strip and to the thin-slab strand, has the further advantage that extremely high reductions can be achieved per roughing mill and/or finishing mill stand. This means that even high-strength grades can be hot-rolled directly to low strip thicknesses as required for lightweight automotive construction for example.
The total time over which the second cooling rate prevails is also known as the hold time, in which only a natural cooling due to a convention can occur due to the deactivated second cooling group or the finished strip may undergo warming on account of phase transitions during the hold time. The second low cooling rate gives the microstructure sufficient time during transport between the first cooling group and the third cooling group to convert the desired phase proportion of at least 50% from austenite into ferrite at the established temperature.
Compared to batch hot-rolling, the process described above which is carried out exclusively in (quasi) continuous operation, has the advantage that acceleration of the finished strip can be avoided. This ensures that the process parameters necessary for production of the TRIP strip, especially in the region of the cooling sector, may be reliably maintained over long periods. This ensures high homogeneity over the entire produced TRIP strip.
The above-described process moreover ensures that good castability and thus the casting rates and mass flows necessary for the continuous process can be maintained. Furthermore, as a result of the above-described process the TRIP strip has a low tendency to surface defects, for example caused by selective (high temperature) oxidation of the austenite grain boundaries during continuous casting, and a low tendency to internal oxidation and internal cracking.
Furthermore, the second exit temperature of 620° C. to 700° C. inclusive ensures that rapid formation of ferrite is ensured during transport between the first cooling group and the third cooling group and that a globular ferrite is also formed. This has a positive effect on the mechanical properties of a microstructure of the TRIP steel. Furthermore, the spatial distance between the first cooling group and the third cooling group may be shortened by the rapid formation of the globular ferrite.
In a further embodiment after exiting the first cooling group the cooled finished strip is supplied to a second cooling group of the cooling sector, wherein a forced cooling of the finished strip in the second cooling group is deactivated and in the second cooling group the finished strip is transported to a third cooling group of the cooling sector. This design has the advantage that a spatial length of the first to third cooling group can be adapted according to a conveying rate of the finished strip.
The finished strip is coiled and cooled from the third exit temperature to an ambient temperature in the coil. Upon cooling of the coiled finished strip to ambient temperature a remaining second portion of the austenite of the finished strip is enriched with carbon (which is substantially insoluble in cementite-free bainite) to form a metastable residual austenite proportion. The metastable residual austenite is converted into martensite during cold especially deformation, during rapid cold deformation, for example in case of a crash of a motor vehicle. The metastable residual austenite can also be converted into martensite during cold deformation of the finish-milled finished strip, so that the component produced by forming techniques, for example a body component, is particularly stiff and tough.
In a further embodiment in the first cooling group the finished strip is force-cooled such that a first cooling rate of the core of the finished strip is established. In the third cooling group the finished strip is force-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. It is preferable when 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 inclusive, in particular 50 K/s to 200 K/s inclusive. This configuration has the advantage that the first high cooling rate leads to a rapid cooling into the (partially) ferritic range. This in turn promotes rapid formation of homogeneous ferrite grains from the austenitic structure. The third cooling rate is necessary to avoid conversion of the remaining austenite into ferrite. Instead, the high second cooling rate results in conversion of a large part of the remaining second portion of the austenite into bainite. The residual austenite remains present in part between bainite plates. Cementite precipitation is hindered by the alloying constituents silicon and/or aluminum, with the result that the carbon cannot precipitate as iron carbide and is thus enriched in the residual austenite remaining between the bainite plates.
In a further embodiment upon exiting the finishing mill train the core of the finished strip has a first exit temperature above the ferrite precipitation temperature (Ar3 temperature), in particular of 800° C. to 950° C., in particular of 830° C. to 860° C.
In a further embodiment the finished strip is transported from the first cooling group, preferably via the second (inactive) cooling group, into the third cooling group over a second time interval of 3 seconds to 8 seconds, in particular of 4 seconds to 5 seconds. This embodiment ensures that a sufficiently long hold time for ferrite formation in the finished strip is made possible and thus a sufficiently large phase proportion of austenite may be converted into ferrite.
In a further embodiment the core of the finish-milled finished strip exits the second cooling group at a third exit temperature of 580° C. to 680° C. inclusive, in particular of 620° C. to 660° C. inclusive and is transported into the third cooling group of the cooling sector. Furthermore, upon exiting of the finished strip from the third cooling group the core of the finished strip has a fourth exit temperature, wherein upon exiting 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. inclusive, in particular 360° C. to 420° C. inclusive, in particular 330° C. to 390° C. inclusive.
In a further embodiment the core of the finished strip is cooled from the fourth exit temperature to an ambient temperature over 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 that is practically insoluble in ferrite to defuse into the residual austenite, thus forming a residual austenite that is metastable at ambient temperature.
In a further embodiment a thickness of the precursor strip upon entry into the finishing mill train is 4 mm to 25 mm, in particular 6 mm to 18 mm. The finishing mill train reduces the thickness of the precursor strip to that of the finished strip of 0.6 mm to 6 mm inclusive, in particular to 0.8 mm to 2 mm inclusive.
It is particularly advantageous when the finished strip has a chemical composition in percent by weight of 0.15% to 0.25%, in particular 0.19% to 0.21%, inclusive of C, 1.0% to 2.0%, in particular 1.4% to 1.6%, of Mn, 1.0% to 1.5%, in particular 1.1% to 1.3%, of Si, 0.3% to 0.7%, in particular 0.45% to 0.55%, inclusive of Al, balance Fe and unavoidable impurities.
It is particularly advantageous when the integrated casting-rolling plant comprises a continuous casting machine having a mold and a single- or multi-stand roughing mill train, wherein a metallic melt is cast in the mold to afford a partially solidified thin-slab strand, wherein the partially solidified thin-slab strand is supported and deflected, wherein the roughing mill train is directly supplied with the partially solidified thin-slab strand from the continuous casting machine, wherein the roughing mill train rolls the thin-slab strand into the precursor strip, wherein the precursor strip is uninterruptedly supplied to the finishing mill train. As a result of the uninterrupted supplying of the thin-slab strand to the roughing mill train and the uninterrupted supplying of the precursor strip to the finishing mill train, the TRIP band is produced in continuous operation with particularly low energy input. Due to the uninterrupted production, the finished strip is mechanically connected to the precursor strip and mechanically connected to the thin-slab strand. The TRIP strip is manufactured directly without the use of intermediate storage means or precursor strip separation, for example via shears.
A high rolling temperature in the finishing mill train and a rolling to a particularly thin final thickness is ensured when an intermediate heating means is arranged between the roughing mill train and the finishing mill train, wherein the intermediate heating means heats a core of the precursor strip by at least 100° C. to 300° C. inclusive, in particular to 1100° C. to 1180° C., wherein the heated precursor strip is supplied to the finishing mill train.
The TRIP steel strip is manufactured using the above-described process. The TRIP steel strip has a chemical composition in percent by weight of 0.15% to 0.25%, in particular 0.19% to 0.21%, inclusive of C, 1.0% to 2.0%, in particular 1.4% to 1.6%, of Mn, 1.0% to 1.5%, in particular 1.1% to 1.3%, of Si, 0.3% to 0.7%, in particular 0.45% to 0.55%, inclusive of Al, balance Fe and unavoidable impurities. At room temperature the finished strip has the following microstructure based on percent by volume: from 40% to 60% inclusive ferrite, in particular from 45% to 55% inclusive ferrite, from 8% to 15% inclusive metastable residual austenite, balance preferably cementite-free bainite. The TRIP steel strip preferably has a thickness of 0.6 mm to 6 mm inclusive, in particular 0.8 mm to 2 mm inclusive.
The invention is more particularly elucidated below with reference to the figures. In the figures:
The integrated casting-rolling plant 10 comprises for example a continuous casting machine 15, a roughing mill train 20, preferably a first to third separating means 25, 30, 35, an intermediate heating means 45, preferably a descaler 50, a finishing mill train 55, a cooling sector 65, at least one, preferably two, coiling means 70 and a control unit 75.
The continuous casting machine 15 is designed for example as an arc continuous casting machine. A different configuration of the continuous casting machining 5 is also conceivable. The continuous casting machine 15 comprises a ladle 95, a distributor 100 and a mold 105. In operation of the integrated casting-rolling plant 10 the distributor 100 is filled with a metallic melt 110 using the ladle 95. The metallic melt 110 may be produced for example by means of a converter, for example in a Linz-Donawitz process. The metallic melt 110 may comprise steel for example. From distributor 100 the metallic melt 110 flows 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 drawn out of the mold 105 and arcuately deflected into a horizontal by the configuration of the continuous casting machine 15 as an arc continuous casting machine to be supported and solidified. The thin-slab strand 115 is conveyed away from the mold 105 in the conveying direction.
The roughing mill train 20 is arranged downstream of the continuous casting machine 15 in a conveying direction of the thin-slab strand 115. The roughing mill train 20 thus directly follows the continuous casting machine 15 in this embodiment.
The roughing mill train 20 may comprise one or more roughing mill stands 120, 121, 122. These are arranged in series in the conveying direction of the thin-slab strand 115. The number of roughing mill stands is essentially freely choosable and is essentially dependent on the dimensions of the thin-slab strand 115. A desired thickness of a precursor strip 125 milled by the roughing mill stands 120, 121, 122 also plays a role here. In this embodiment three roughing mill stands 120, 121, 122 are provided for the roughing mill train 20 shown in
In this embodiment the first separating means 25 and the second separating means 30 for example are arranged downstream of the roughing mill train 20 based on the conveying direction of the precursor strip 125. The second separating means 30 is arranged spaced apart from the roughing mill train 20 based on the conveying direction of the precursor strip 125. A discharging means (not shown in
In this embodiment the intermediate heating means 45 is arranged downstream of the second separating means 30 for example based on the conveying direction of the precursor strip 125. The intermediate heating means 45 may be designed for example as an induction furnace. A different configuration of the intermediate heating means 45 is also conceivable. The intermediate heating means 45 is arranged upstream of the finishing mill train 55 and the descaler 50 based on the conveying direction of the precursor strip 125. The descaler 50 is arranged directly upstream of the finishing mill train 55 and downstream of the intermediate heating means 45. The descaler 50 may also be eschewed. The finishing mill train 55 is arranged downstream of the descaler 50 based on the conveying direction of the precursor strip 125. In this embodiment the finishing mill train 55 has five finishing mill stands 145, 146, 147, 148, 149. The finishing mill stands 145, 146, 147, 148, 149 are arranged in series based on the conveying direction of the precursor strip 125. In operation of the integrated casting-rolling plant 10 the finishing mill stands 145, 146, 147, 148, 149 roll the precursor strip 125 supplied to the finishing mill train 55 into a finished strip 165. A first finishing mill stand 145 is arranged upstream of the other finishing mill stands 146, 147, 148, 149 based on the conveying direction of the supplied precursor strip 125. A second finishing mill stand 146 is arranged downstream of the first finishing mill stand 145 and upstream of a third to fifth finishing mill stand 147, 148, 149. The third finishing mill stand 147 is arranged downstream of the second finishing mill stand 146 and upstream of a fourth finishing mill stand 148. The fourth finishing mill stand 148 is arranged downstream of the third finishing mill stand 147 and upstream of the fifth finishing mill stand 149.
The control unit 75 comprises a control means 170, a data storage means 175 and an interface 180. The data storage means 175 is data-linked using a first data connection 185. Likewise, the interface 180 is data-linked with the control means 170 using a second data connection 190.
The data storage means 175 preferably stores a predefined first target temperature, a predefined second target temperature and a predefined third target temperature. The data storage means 175 also stores a process for producing the TRIP steel strip 245 on whose basis the control device 170 controls the components of the integrated casting-rolling plant 10.
The interface 180 is further data-linked with the second intermediate heating means 45 using a third data connection 195. A fourth data connection 200 data-links the finishing mill train 55 with the interface 180. A fifth data connection 205 data-links the cooling sector 65 with the interface 180.
The integrated casting-rolling system 10 may further comprise a first temperature measuring means 80 and a second temperature measuring means 85. The integrated casting-rolling system 10 may additionally comprise a third temperature measuring means 172. The first temperature measuring means 80 and/or the second temperature measuring means 85 and/or the third temperature measuring means 172 may be in the form of pyrometers for example. The first temperature measuring means 80 is arranged downstream of the intermediate heating means 45 and preferably arranged upstream of the descaler 50 based on the conveying direction of the precursor strip 125. The second temperature measuring means 85 is arranged between the cooling sector 65 and the finishing mill train 55. The third temperature measuring means 172 may be arranged in the cooling sector 65. The first temperature measuring means 80 is data-linked to the interface 180 using a sixth data connection 210. A seventh data connection 215 data-links the second temperature measuring means 85 with the interface 180. The third temperature measuring means 172 is data-linked to the interface 180 by means of an eighth data connection 225.
The cooling sector 65 comprises a first cooling group 166, a second cooling group 167, at least a third cooling group 168 and preferably a roller conveyor 171. The first cooling group 166 is arranged upstream of the second cooling group 167 based on the conveying direction of the finished strip 165. The second cooling group 167 is directly connected to the first cooling group 166 for example. The third cooling group 168 is arranged immediately downstream of the second cooling group 167 based on the conveying direction of the finished strip 165. The third separating means 35 is arranged downstream of the third cooling group 168. The first to third cooling groups 166, 167, 168 may each comprise a plurality of cooling beams 169, wherein the cooling beams 169 are arranged above and/or below the finished strip 165. The roller conveyor 171 extends in the conveying direction along the first to third cooling group 166, 167, 168 to transport the finished strip 165 in the cooling sector 65 between the finishing mill train 55 and the third separating means 35. It is noted that a respective length of the cooling group 166, 167, 168 is exemplary. The length of the cooling group 166, 167, 168 is in particular determined by a transport speed of the finished strip 165. A length of the respective cooling group 166, 167, 168 can be dynamically altered according to the transport speed.
Based on the conveying direction of the finished strip 165 the first cooling group 166 is made shorter than the second cooling group 167 and the third cooling group 168 for example. The third cooling group 168 is moreover made shorter than the second cooling group 167.
Between the first cooling group 166 and the first finishing mill stand 145 arranged last in the conveying direction, the integrated casting-rolling plant 10 may comprise a measuring sector 60 which is arranged between the first cooling group 166 and the last first finishing mill stand 145 based on the conveying direction of the finished strip 165.
Above the diagram,
In operation of the integrated casting-rolling plant 10 in a first process step 305 the mold 105 of the continuous casting machine 15 is closed by means of a dummy bar head (not shown in
The temperatures and process steps specified below refer to the chemical composition of the steel preferred in the embodiment to produce the TRIP steel strip 245 using the integrated casting-rolling plant 10.
At commencement of continuous casting the metallic melt 110 flows around the dummy bar head in the mold 105 and solidifies on the dummy bar head. The dummy bar head is slowly drawn out of the mold 105 of the continuous casting machine 15 in the direction of the roughing mill train 20. Downstream of the dummy bar 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 surrounds a liquid core and holds the liquid core. At the mold outlet the thin-slab strand 115 may have a thickness of 80 mm to 150 mm for example.
In the continuous casting machine 15 the thin-slab strand 115 is deflected and further cooled on the way to the roughing mill train 20 so that the thin-slab strand 115 solidifies from the outside in. In this embodiment the continuous casting machine 15 is for example configured as an arc continuous casting machine as is elucidated above so that through deflection of the thin-slab strand 115 by substantially 90° from the vertical the thin-slab strand 115 is supplied to the roughing mill train 20 in a substantially horizontal orientation.
In a second process step 310 the thin-slab strand 115 is rolled into the precursor strip 125 in the roughing mill train 20 by the roughing mill stands 120, 121, 122 as elucidated hereinabove.
A core temperature of the core of the thin-slab strand 115 upon entry into the roughing mill train 20 at the above mentioned chemical composition is about 1300° C. to 1450° C. Each hot-rolling step in the roughing mill train 20 reduces the core temperature of the core so that the precursor strip 125 has a core temperature of about 980° C. to 1150° C. upon exiting the roughing mill train 20.
It is particularly advantageous when the first roughing mill stand 120 and/or the second roughing mill stand 121 each bring about a thickness reduction of the precursor strip 125 of 35% to 60% inclusive in a rough-milling pass. A thickness reduction of the rough-milled strip 125 is preferably 35% to 50% inclusive at the third roughing mill stand. The respective thickness reduction is based on a thickness of the rough-milled strip 125 upon exiting the respective roughing mill stand 120, 121, 122 relative to a thickness of the rough-milled strip 125 upon entry into the corresponding roughing mill stand 120, 121, 122. This has the advantage that a thin rough-milled strip 125 exits the roughing mill train 20 at the end of the roughing mill train 20.
In a third process step 315 the precursor strip 125 is passed through the first and second separating means 25, 30, wherein a separating of the precursor strip 125 is not performed. The first and second separating means 25, 30 are thus only traversed. The precursor strip 125 undergoes further cooling by convection, wherein a protective cover can reduce the cooling during transport to the intermediate heating means 45.
In a fourth process step 320 the control means 170 activates the intermediate heating means 45 via the third data connection 195 so that the intermediate heating means 45, which is for example in the form of an induction furnace, increases the core temperature of the precursor strip 125 upon entry into the intermediate heating means 45 from 850° C. to 950° C. to about 1050° C. to 1200° C. It is particularly advantageous when the precursor strip 125 exits the intermediate heating means 45 at a core temperature of 1100° C. to 1180° C. This has the advantage of ensuring optimum surface quality of the hot-rolled strip.
In a fifth process step 325 the first temperature measuring means 80 which is for example in the form of a first pyrometer determines a first surface temperature TO1 of the precursor strip 125 discharged from the intermediate heating means 45. The first temperature measuring means 80 provides a first information about the first surface temperature TO1 of the precursor strip 125 between the intermediate heating means 45 and the descaler 50 via the sixth data connection 210 of the interface 180 which provides the first information of the control means 170.
In a sixth process step 330 the control means 170 controls a heating output of the intermediate heating means 45 such that the determined first surface temperature TO1 of the precursor strip 125 between the intermediate heating means 45 and the descaler 50 substantially corresponds to the first target temperature. The control means 170 can regularly repeat the fifth and sixth process step 325, 330 in a loop in a predefined time interval.
In a seventh process step 335 the control means 170 activates the descaler 50 (if present). The descaler 50 descales the precursor strip 125. This cools the precursor strip 125 by, for example, 80° C. to 100° C. based on the core of the precursor strip 125. It can especially be ensured that optimal descaling performance of the descaler 50 is effected when the core temperature at the exit of the intermediate heating means 45 is 1100° C. to 1180° C.
In an eighth process step 340 the precursor strip 125 is transported to the first finishing mill stand 145 of the finishing mill train 55 at a first entry temperature TE1 (based on the core of the precursor strip 125). The first entry temperature TE1 is based on the core of the precursor strip 125 and is the temperature at which the precursor strip 125 enters the first finishing mill stand 145 in the conveying direction based on the precursor strip 125 after the descaler 50. The first entry temperature TE1 may be between 950° C. and 1120° C., in particular between 950° C. and 1050° C.
In a ninth process step 345 the precursor strip 125 is finish-milled into the finished strip 165 using five finishing mill stands 145 for example. It is particularly advantageous when the first finishing mill stand 145 brings about a thickness reduction of the precursor strip 125 to the finished-milled strip 165 of 35% to 55% inclusive in a first finish-milling pass. It is preferable when the second finishing mill stand 146 brings about a thickness reduction of 30% to 50% inclusive in the second finish-milling pass. In addition, the third finishing mill stand 147 brings about a thickness reduction of 25% to 40% inclusive in the third finish-milling pass. The fourth finishing mill stand 148 preferably brings about a thickness reduction of 20% to 30% inclusive in the fourth finish-milling pass. The fifth finishing mill stand 149 preferably brings about a thickness reduction of 10% to 20% inclusive in the fifth finish-milling pass. The thickness reduction is thus carried out especially at the first to third finishing mill stands 145, 146, 147 so that repeated recrystallization of the microstructure in the finish-milled strip 165 is established. This is made possible especially by the high core temperature of 1100° C. to 1180° C. during discharging from the intermediate heating means. The high thickness reduction especially at the first to third finishing mill stands 145, 146, 147 is possible only in continuous operation since only a pull-through condition and not a gripping condition is to be satisfied. A preferably decreasing thickness reduction in the conveying direction of the finished strip 165 at the first to fifth finishing mill stand 145, 146, 147, 149 ensures is 148, that the finish-milled strip particularly flat upon exiting the finishing mill train 55.
The finished strip 165 exits with a thickness of 0.6 mm to 6 mm inclusive, in particular 0.8 mm to 2 mm inclusive. Each first finishing mill stand 145 of the finishing mill train 55 causes the precursor strip 125 that is to be milled into the finished strip 165 to cool by about 50° C., thus forming a stepped line in the temperature profile (cf.
A first exit temperature TA1 of the finished strip 165 after traversing the finishing mill train 55 is about 800° C. to 950° C., in particular from 830° C. to 860° C. inclusive. The first exit temperature TA1 is based on the core of the finished strip 165. The first exit temperature TA1 may be specifically adjusted using the intermediate heating means 45. Furthermore, the first exit temperature TA1 is preferably greater than a ferrite conversion temperature Ar3. The finish-milling via the first finishing mill stands 145 is preferably carried out in an austenitic microstructure. Ensuring the first exit temperature TA1 of 830° C. to 860° C. inclusive with the core temperature of 1100° C. to 1180° C. at the exit from the intermediate heating means 45 especially makes it possible to produce a homogeneous, fine-grained austenite in the finished strip 165 at the exit from the finishing mill train 55. Furthermore, the core temperature of 1100° C. to 1180° C. at the exit from the intermediate heating means 45 makes it possible to avoid unwanted fayalite formation (in the case of Si-alloyed steels) and the formation of scale scars on a surface of the finish-milled finished strip 165.
In a tenth process step 350 the finish-milled finished strip 165 is transported onwards in the direction of the cooling sector 65 at the first exit temperature TA1. The finished strip 165 is transported past the second temperature measuring means 85. The second temperature measuring means 85 may be in the form of a pyrometer and measures a second surface temperature TO2 of the finished strip 165 coming from the finishing mill train 55. The second temperature measuring means 85 provides an information which correlates with the first exit temperature TA1 via the seventh data connection 215 and the interface 180 of the control means 170. The control means 170 can take into account the second surface temperature TO2 in the control of the intermediate heating means 45. As elucidated hereinabove the second surface temperature TO2 correlates with the first exit temperature TA1. However the second surface temperature TO2 differs from the first exit temperature TA1 in terms of its value. This is especially because the second surface temperature TO2 relates to the surface of the finished strip 165 and the first exit temperature TA1 relates to the core of the finished strip 165. However, temperature difference between the first exit temperature TA1 and the second surface temperature TO2 is small (less than 10° C.) because the finished strip 165 has a thickness of preferably only 0.6 mm to 6 mm, in particular to 0.8 mm to 2 mm.
The controlling of the intermediate heating means 45 by the control means 170 is in this embodiment carried out for example such that the second surface temperature TO2 substantially corresponds to the second target temperature in the controlling of the intermediate heating means 45. However, the second temperature measuring means 85 and/or the tenth process step 350 may also be eschewed.
In an eleventh process step 355 the control means 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 transition occurs in the finished strip 165 in the region between a last milling pass of a last finishing mill stand and entry into the first cooling group 166.
In the first cooling group 166 which preferably comprises one, in particular two or more, cooling beams 169, the cooling beams 169 are used to spray a cooling medium, for example water, optionally with an additive, onto the hot finish-milled finished strip 165. This effects forced cooling of 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 microstructure. The phase proportion of the austenitic microstructure is thus preferably more than 50%, particularly preferably 100%, upon exiting the first cooling group 166. The phase proportion refers to percent by volume.
It is preferable when a volume flow of the cooling medium is chosen such that within the first cooling group 166 the finished strip 165 is cooled from a second entry temperature TE2, which substantially corresponds to the first exit temperature TA1, to a second exit temperature TA2 of in particular 550° C. to 720° C. over a first time interval t1 at the first cooling rate. The cooling in the first cooling group 166 is carried out in such a way, for example by controlling the volume flow of the cooling medium, that the second exit temperature TA2 is smaller than Ae3. It is particularly advantageous when the second exit temperature TA2 is 620° C. to 720° C. inclusive.
It is particularly advantageous when the conveying amount of the cooling medium is selected such that a cooling power 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 to 1000 K/s inclusive, in particular 20 K/s to 500 K/s inclusive, in particular 20 K/s to 400 K/s inclusive, in particular 50 K/s to 200 K/s inclusive. The cooling in the core of the finished strip 165 is preferably effected continuously via the first cooling group 166 in the first cooling group 166.
In this embodiment the first cooling rate is ensured for example by spraying a volume flow of about 100 m3/h to 350 m3/h of the cooling medium onto the finished strip 165 with a pressure of 2 bar to 4 bar preferably with the arrangement of two or more cooling beams 169. This ensures that over the short traversal time of the finished strip 165 for example with a transport velocity 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 entry temperature TE2 of for example 800° C. to 950° C. inclusive, in particular 830° C. to 900° C. inclusive, to the second exit temperature TA2. In order to ensure particularly precise control of the volume flow by the control means 170 each cooling beam 169 may have a control valve which may be actuated by the control means 170. This allows stepless control of the volume flow of the cooling medium for each cooling beam 169 of the first cooling group 166 between 10% and 100% via the control means 170.
In a twelfth process step 360 the finished strip 165 is transported into the second cooling group 167 at the second exit temperature TA2. Transport within the cooling sector 65 is carried out using the roller conveyor 171. The control means 170 ensures that the second cooling group 167 is deactivated in such a way that no cooling medium is conveyed onto the finished strip 165 and thus no active forced cooling is effected within the second cooling group 167. Within the second cooling group 167 cooling of the finished strip 165 therefore occurs only through cooling by radiation and convection to the environment of the finished strip 165 within the second cooling group 167.
Upon entry of the finished strip 165 into the second cooling group 167 in the twelfth process step 360 a microstructure of the finished strip 165 is partially, in particular to a phase proportion of more than 80%, particularly preferably completely, austenitic since in the eleventh process step 355 essentially only little phase transformation, if any, occurs in the finished strip 165.
The third temperature measuring means 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 means 172 provides a 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 means 170.
The control means 170 can also take into account the information about the third surface temperature TO3 in the controlling of the volume flow of the cooling medium in the first cooling group 166 in the eleventh process step 355. The control means 170 can especially control the volume flow of the cooling medium which is passed, in particular sprayed, from the first cooling group 166 onto the finished strip 165 in such a way that the third surface temperature TO3 substantially corresponds to the third target temperature. The third target temperature is selected such that the second exit temperature TA2 which refers to the core is between the ferrite precipitation temperature Ae3 and the bainite starting temperature BS.
When controlling the volume flow the control means 170 may additionally take into account the second surface temperature TO2 to ensure a uniform first cooling rate in the first cooling group 166. The control means 170 can regularly repeat the eleventh and twelfth process step 355, 360 in a loop in a predefined time interval.
In a thirteenth process step 365 the finished strip 165 is transported by the roller conveyor 171 in a warm, partially cooled state in the second cooling group 167 in the direction of the third cooling group 168. As elucidated above, the control means 170 keeps the second cooling group 167 in a deactivated state so that during traversal of the finished strip 165 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.
In the twelfth and thirteenth process steps 360, 365 the finished strip 165 cools from the second exit temperature TA2 to a third exit temperature TA3 at a second cooling rate via the second cooling group 167. The second cooling rate is markedly lower than the first cooling rate. The second cooling rate is for example −25 K/s to 20 K/s inclusive, in particular 0 K/s to 20 K/s inclusive. The second cooling rate results especially 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 to below the ferrite starting temperature Ae3 a first portion of the austenitic microstructure of the finished strip 165 is converted into ferrite during transport.
To traverse the second cooling group 167 the finished strip 165 requires a second time interval t2. The second time interval t2 is markedly longer than the first time interval t1. The second time interval t2 may have a duration of 3 seconds to 8 seconds inclusive, in particular from 4 seconds to 5 seconds inclusive. In the second time interval t2 the finished strip 165 passes through the second cooling group 167 and is thus transported over the second time interval t2 from the first cooling group 166 via the second cooling group 167 into the third cooling group 168. The second time interval t2 serves as a hold time. The mixed microstructure of austenite and ferrite is thus further formed in the finished strip 165 in the second time interval t2. During transport in the second cooling group 167 the ferrite proportion in the microstructure of the finished strip 165 increases markedly. At the end of the second cooling group 167 the composition of the material of the finished strip 165 is especially as follows (based on percent by volume): 40% to 80% ferrite, in particular 45% to 60% ferrite, balance essentially austenite. The homogeneous, fine-grained austenite (cf. ninth process step 345) especially favors the rapid formation of ferrite during traversal through the deactivated second cooling group 167. This allows the second cooling group 167 to be kept spatially short.
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. However, the third exit temperature TA3 is especially still greater than the austenite-ferrite transformation temperature Ar1. The third exit temperature TA3 may be 580° C. to 710° C., in particular 650° C. to 690° C.
The third exit temperature TA3 corresponds to a third entry temperature TE3 with which the finished strip 165 enters the third cooling group 168 and which refers to the core of the finished strip 165.
In a fourteenth process step 370 the control means 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 cools the finished strip 165 from the third entry temperature TE3 and thus from the third exit temperature TA3 to a fourth exit temperature TA4 using the cooling medium. In the third cooling group 168 the cooling medium is sprayed onto the warm finished strip 165 and the finished strip 165 is thus subjected to forced cooling in the third cooling group 168.
The fourth exit temperature TA4 may especially be from 300° C. to 450° C. inclusive and is thus lower than Ae1. The fourth exit temperature TA4 is moreover also markedly higher than an ambient temperature TU of about 20° C. to 40° C. of the integrated casting-rolling plant 10.
The cooling of the finished strip 165 in the third cooling group 168 is especially carried out 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 markedly greater than the second cooling rate. The third cooling rate may be 20 K/s to 1000 K/s inclusive, in particular 20 K/s to 500 K/s inclusive, in particular 20 K/s to 400 K/s inclusive, in particular 50 K/s to 200 K/s inclusive. The cooling in the core of the finished strip 165 is preferably effected continuously via the third cooling group 168. The third cooling rate may be different from the first cooling rate.
In this embodiment the third cooling rate is ensured in such a way that preferably a further volume flow of 100 m3/h to 300 m3/h of the cooling medium is applied to the finished strip 165 using the cooling beams 169 of the third cooling group 168 at a pressure of 2 bar to 4 bar. This ensures that within the short third time interval t3 of the finished strip 165 the core of the finished strip 165 is cooled from the third entry temperature TE3 to the fourth exit temperature TA4 by the second cooling group 168.
In addition, analogously to the first cooling group 166, in the third cooling group 168 each cooling beam 169 of the third cooling group 168 may be configured such that said beam is in each case provided with a control valve controllable by the control means 170 in order thus to independently actuate said beams preferably steplessly and independently of the respective other cooling beams 169 of the third cooling group 168. As a result, a volume flow of the cooling medium in the third cooling group 168 is steplessly controllable between 0% and 100% for each of the cooling beams 169 of the third cooling group 168 via the control means 170.
Due to the rapid cooling of the finished strip 165 from the third exit temperature TA3/third entry temperature TE3 to the fourth exit temperature TA4 a second proportion of the austenite is converted into cementite-free bainite. In this embodiment cementite precipitation is hindered by the alloying constituents Si and Al, as a result of which the carbon C remains in solution. The thin-walled configuration of the finished strip 165 ensures a homogeneous conversion of the austenite into cementite-free bainite over the strip thickness.
In a fifteenth process step 375 the finished strip 165 cooled to the fourth exit temperature TA4 by the third cooling group 168 is passed through the third separating means 35 toward the coiling means 70. In the coiling means 70 the finish-milled finished strip 165 cooled to the fourth exit temperature TA4 is coiled into a coil 250.
Since the coiling means 70 is arranged close to and only a few meters apart from an end of the cooling sector 65 the fourth exit temperature TA4 corresponds substantially to a fourth entry temperature TE4 with which the finished strip 165 is passed into the coiling means 70. However, since the coiling means 70 is arranged spaced apart from the cooling sector 65 and the fourth exit temperature TA4 is markedly above 180° C., in particular in the range from 360° C. to 420° C. inclusive, in particular in the range from 360° C. to 390° C. inclusive, excess cooling medium can both run off from the finished strip 165 and dry off therefrom between exiting of the finished strip 165 and coiling of the finished strip 165 into the coil 250 in the coiling means 70 and the finished strip 165 is therefore wound preferably in a dry state.
After the coiling of the coil 250 the control means 170 can activate the third separating means 35 and separate the finished strip 165 so that the wound coil 250 can be removed from the integrated casting-rolling plant 10. As a result, the finished strip 165 may continuously be conveyed further and wound into a further coil 250. The integrated casting-rolling plant 10 may especially comprise a further separating means and further coiling means 70.
The ready-wound coil 250 is substantially hollow-cylindrical and has an internal diameter and an external diameter. It is advantageous when a ratio of the external diameter to the internal diameter is 2 to 2.8 inclusive.
In a sixteenth process step 380 the finished strip 165 is cooled from the fourth exit temperature TA4 to the ambient temperature TU in the wound state. The coil 250 is exposed to an environment. Immediately after winding the coil cools at a fourth cooling rate of 20 K/h to 30 K/h inclusive at an outer winding and/or an inner winding of the coil 250, preferably within the first hour after winding. In a central winding of the coil 250 which in the radial direction is arranged substantially in a central position relative to the outer winding and the inner winding a fifth cooling rate after winding is 1 K/h to 20 K/h inclusive, preferably within the first hour after winding.
An average cooling rate with which the coil 250 cools from the fourth exit temperature TA4 to the ambient temperature TU may preferably be 3 K/h to 15 K/h inclusive so that the coil 250 cools in a fourth time interval t4 of about 24 hours to 72 hours for example.
The cooling occurs by radiation and convection without additional active cooling. As a result of this natural cooling of the fourth time interval t4 at least a remaining third portion of residual austenite in the microstructure is enriched with carbon C which is practically insoluble in ferrite, thus forming a metastable residual austenite. The described cooling of the coil 250 without a thermal hood has the advantage in terms of carbon supersaturation while simultaneously avoiding cementite precipitation in the residual austenite proportion.
It is alternatively also conceivable that in the sixteenth process step 380 a thermal hood is additionally placed on the coil 250 which thermally insulates the coil 250 relative to the environment to achieve a particularly slow and deliberate cooling of the coil 250. It is particularly advantageous when the fourth exit temperature is 360° C. to 390° C.
As a result of the thermal hood the coil 250 (both at the inner winding and/or the outer winding and/or the central winding) can cool under the thermal hood at a sixth cooling rate of 1 K/h to 10 K/h inclusive in the first hour after coiling. The average cooling rate starting from the fourth exit temperature TA4 may preferably be 1 K/h to 8 K/h under the thermal hood.
After cooling of the coil 250 at the central winding to a fifth exit temperature which is about 10° C. to 100° C. inclusive lower than the fourth exit temperature TA4 the thermal hood is removed in a seventeenth process step 385 and a further cooling of the coil 250 to the ambient temperature TU occurs via the environment of the coil 250. The average cooling rate after removal of the thermal hood may be 2 K/h to 12 K/h inclusive.
The use of the thermal hood has the advantage that the TRIP steel strip 245 has a particularly homogeneous microstructure and thus particularly homogeneous material properties.
After cooling to the ambient temperature TU the production of the finished strip 165 is complete and the cooled finished strip 165 is now in the form of a TRIP steel strip 245. If in a later production process the TRIP steel strip 245 is mechanically deformed, for example on account of a road traffic accident or in a press, the deformation causes the metastable residual austenite to be converted into martensite.
The TRIP steel strip 245 especially has the following chemical composition: 0.15% to 0.25%, in particular 0.19% to 0.21%, inclusive of C, 1.0% to 2.0%, in particular 1.4% to 1.6%, of Mn, 1.0% to 1.5%, in particular 1.1% to 1.3%, of Si, 0.3% to 0.7%, in particular 0.45% to 0.55%, inclusive of Al, balance Fe and unavoidable impurities.
At room temperature TU the TRIP steel strip 245 moreover has the following microstructure (based on percent by volume): from 40% to 60% inclusive, in particular from 45% to 55% inclusive, of ferrite, from 8% to 15% inclusive of metastable residual austenite and bainite, preferably cementite-free bainite.
The above-described process and the above-described integrated casting-rolling plant 10 thus allows direct production of the TRIP steel strip 245 in a particularly low thickness, in particular 0.6 mm to 6 mm, in particular 0.8 mm to 2 mm, in a continuous integrated casting-rolling plant without cold rolling and annealing. This has the advantage that the required energy to produce the TRIP steel strip 245 is markedly lower and the TRIP steel strip 245 is thus producible in a more environmentally friendly and cost-effective manner.
Even at high speeds, for example at 10 m/s, the hold time which corresponds to the second time interval t2, between the exit of the finished strip 165 from the first cooling group 166 and the entry into the third cooling group 168, of 3 seconds to 8 seconds, in particular of 4 seconds to 5 seconds, is ensured. 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.
The above-described configuration of the integrated casting-rolling plant 10 with the above-described process further makes it possible to achieve a high casting speed of 0.08 m/s to 0.1 m/s at the specified thickness of the thin-slab strand 115 of 100 mm to 150 mm.
It is further noted that the integrated casting-rolling plant 10 may also be configured in a manner distinct from that described in the figures. It would especially also be possible for the integrated casting-rolling plant 10 to have for example six finishing mill stands 145, wherein the last finishing mill stand 145 in the conveying direction for example is to this end reconfigured into a stand cooler in a preparation step. The preparation step may to this end comprise removing working rollers from the finishing mill stand 145 by opening a replacing means and replacing them with one or more cooling beams. The cooling beam of the stand cooler may further be oriented such that it is directly oriented in the direction of a feedthrough through which the finished strip 165 is passed. In the closed state of the replacing means the cooling beams are secured in the stand cooler.
Reconfiguring the last finishing mill stand 145 in the conveying direction for example into a stand cooler extends the cooling sector 65 counter to the conveying direction of the finished strip and the stand cooler forms a subsection of the first cooling group 166. An intermediate cooler may also be arranged between the finishing mill stand 145 and the stand cooler.
The TRIP steel strip 245 produced in a continuous strand using the integrated casting-rolling plant 10 and the process described in
The integrated casting-rolling plant 10 further exhibits a particularly exact and stable process mode through continuous operation and direct production of the TRIP steel strip 245, thus ensuring a high homogeneity of the hot-rolled strip. Since cold-rolling and annealing of material of the finished strip 165 that has already cooled to ambient temperature can be dispensed with in the direct production of the TRIP steel strip 245 in continuous operation the energy required to produce the TRIP steel strip 245 is particularly low.
| Number | Date | Country | Kind |
|---|---|---|---|
| 23164462.6 | Mar 2023 | EP | regional |
