Patent Application
20240271244
The present disclosure relates to a system for the thermomechanical rolling of long semi-finished steel products, a method for producing rod-shaped steel, preferably structural steel, from the long semi-finished steel products, along with a rod-shaped steel product that is obtainable by the method.
In the conventional production of rod-shaped steel products, the corresponding steel is initially hot-rolled using a plurality of rolling mill stands and then quenched after the rolling line. The large temperature drop causes a martensite ring to form around the circumference, giving the material the required strength. The resulting steel products are then separated into lengths of up to 12 m, cooled as evenly as possible from 650° ° C. to 100° ° C. on a cooling bed and then combined into bundles that are ready for transportation and further processing.
As an alternative to cooling on a cooling bed, such steel products can also be wound into compact coils. However, in contrast to the conventional method, the wound coil is subject to highly inhomogeneous cooling conditions, which cause an increased spread of the mechanical properties and which then have a negative effect during further processing, such as drawing over a die.
WO 2004/104237 A1, for example, discloses a method for winding metallic rods. According to the disclosed method, the finished-rolled rods are slowly cooled to a temperature in the range of 600 to 700° C. in a cooling and equalizing section, which is arranged downstream of the rolling unit in the transport direction and comprises a plurality of water tanks, and then fed to a coil winding device. This simply results in a microstructure with a core of ferrite and pearlite, which is surrounded by a ring-shaped structure of martensite and larger proportions of bainite.
The present disclosure provides a system for the thermomechanical rolling of long steel semi-finished products and a method for producing rod-shaped steel products, in particular structural steel, with which rod-shaped steel products, in particular structural steel, can be produced with a low spread of the mechanical material properties and/or with a constant quality with regard to their microstructure.
The system for thermomechanical rolling of long steel semi-finished products comprises a first rolling unit; a first thermomechanical sizing block arranged downstream of the first rolling unit in the transport direction; a first cooling device arranged between the first rolling unit and the first thermomechanical sizing block; a separating device arranged downstream of the first thermomechanical sizing block in the transport direction; a second cooling device arranged between the first thermomechanical sizing block and the separating device; and a coil winding device arranged downstream of the separating device in the transport direction.
In the same manner, the disclosure relates to a method for producing rod-shaped steel from long steel semi-finished products, in particular with a yield strength of at least 300 MPa, preferably with a yield strength of at least 400 MPa, even more preferably with a yield strength of at least 500 MPa, and most preferably with a yield strength of at least 600 MPa, wherein the long steel semi-finished product, which may have been heated to a temperature of at least 900° C., preferably to a temperature of at least 950° C., is initially pre-rolled in a first rolling unit and then cooled to a temperature of at least 850° C. in a subsequent first cooling device; is subsequently finish-rolled into the rod-shaped steel in a first thermomechanical sizing block arranged downstream of the first cooling device in the transport direction, which is cooled to a temperature in the range of 400° ° C. to 600° C. in a second cooling device downstream of the first thermomechanical sizing block; is subsequently cut in a separating device arranged downstream of the second cooling device in the transport direction; and then fed to a coil winding device arranged downstream of the separating device and wound into vertically and/or horizontally wound coils.
As part of the development of the present system, it has been shown that vertically and/or horizontally wound coils of a rod-shaped steel product, which on the one hand have a specific pearlitic-bainitic microstructure and on the other hand a reduced spread of the mechanical material values, can be obtained by a coordinated process control. Preferably, the rod-shaped steel products have a spread of the mechanical material values, in particular the yield strength Re of ≤25 MPa, more preferably a spread of the yield strength Re of ≤20 MPa, even more preferably a spread of the yield strength Re of ≤15 MPa, and most preferably a spread of the yield strength Re of ≤10 MPa, wherein the strength levels specified in the respective standards based on DIN 488 and other material values are improved in some cases.
A further advantage of thermomechanical rolling and the resulting grain refinement effects in the rod-shaped steel products is that the use of manganese or other strength-enhancing micro-alloying elements can be reduced or eliminated, which has a beneficial effect on production costs.
Furthermore, winding the rod-shaped steel products into vertically and/or horizontally wound coils has other advantages in terms of transportation and space requirements.
Further advantageous embodiments of the invention are indicated in the dependent formulated claims. The features listed individually in the dependent formulated claims can be combined with one another in a technologically useful manner and may define further embodiments of the invention. In addition, the features indicated in the claims are further specified and explained in the description, wherein further preferred embodiments of the invention are shown.
It should be noted that the temperatures stated here represent the average temperatures over the cross-section of the rolling stock and therefore cannot be equated with surface temperatures.
For the purposes of the present disclosure, the term “long semi-finished steel products” is understood to mean semi-finished steel products that are suitable for the production of the rod-shaped steel or steel products, in particular structural steel. Such long semi-finished steel products are also called billets and usually have a square or rectangular cross-section.
For the purposes of the present disclosure, the term “rod-shaped steel products” is understood to mean steel products or rod steel, in particular structural steel, which have a round cross-section with a diameter in the range of 6.0 to 50 mm, preferably 6.0 to 32 mm, and whose surface is ribbed and/or smooth. Such rod-shaped steel products are also known as “rebars.”
The first rolling unit, in which the long steel semi-finished product heated in advance to a temperature of at least 900° C., preferably to a temperature of at least 950° C., is pre-rolled, can be formed from a plurality of housingless rolling mill stands. Advantageously, the first rolling unit comprises at least ten, more preferably at least twelve, still more preferably at least fourteen, and most preferably sixteen of such housingless rolling mill stands.
In addition or alternatively, the first rolling unit can comprise hydraulically adjustable rolling mill stands instead of the housingless rolling mill stands.
In the transport direction behind the first rolling unit, the first cooling device is arranged in a first line section that extends between the first rolling unit and the first thermomechanical sizing block. The first cooling device can comprise one or, more preferably, two water tanks, which are then arranged at a distance from one another in the first line section. The cooling device is used to reduce the temperature of the rolling stock before the thermomechanical rolling step.
The cooling capacity of the water tanks of each cooling device can be set in a targeted manner on the basis of the volume flow of the cooling water, the number of active cooling tubes per water tank, the cooling tube diameter and/or the cooling water pressure and, if necessary, the cooling water temperature. The specifications can typically be predetermined by means of specific process models and adjusted by online control.
An exemplary water tank can have a water tank length of 6500 mm and comprise six cooling tubes, in each case 750 mm long. Such a water tank then typically has a maximum cooling water quantity of 230 m3/h and an adjustable cooling water pressure range of 1.5 to 6.0 bar.
The first line section is also preferably selected in such a manner that the rolling stock is also given sufficient time for adequate temperature equalization across the cross-section. The temperature in the rolling stock is equalized by conduction from the core to the surface. In order to achieve as uniform a temperature as possible over the entire cross-section of the rolling stock, it is particularly preferred that a temperature gradient of maximum 100° C., more preferably a temperature gradient of maximum 80° C., even more preferably a temperature gradient of maximum 60° C., and most preferably a temperature gradient of maximum 50° C. is set. The homogenization of the cross-section temperatures can be controlled indirectly between the respective stations by measuring the surface temperatures of the rolled long steel semi-finished product. Corresponding process models can also be used as a supplement.
The first line section can have a length of 80 to 120 m, more preferably a length of 90 to 100 m.
The rolled long semi-finished steel product cooled to a temperature of at least 850° C. in the first cooling device is then fed to the first thermomechanical sizing block, in which it is finish-rolled to the desired or specified, as the case may be, end diameter.
In a particularly advantageous embodiment, it is provided that the rolled long steel semi-finished product is fed to the first thermomechanical sizing block at a temperature of at least 700° C., preferably at a temperature of at least 730° C., more preferably at a temperature of at least 750° C. still more preferably at a temperature of at least 760° C., and most preferably at a temperature of at least 770° C. However, the temperature of the rolled long semi-finished steel products must not be too high, since otherwise the temperature gradient between the surface and the core temperatures, which is required for the metallurgical recrystallization processes and the associated grain refinement effects, and which is as low as possible, cannot be set. As such, the temperature at which the rolled long steel semi-finished product is fed to the first thermomechanical sizing block is limited to 850° C., preferably to 840° C., more preferably to 820° C., and most preferably to 800° C. Particularly preferably, it is provided that the rolled long semi-finished steel product is fed to the first thermomechanical sizing block at a temperature of 780° C.
In the thermomechanical sizing block, the highest forming or the highest reduction, as the case may be, takes place, which can preferably be 30 to 80%. The thermomechanical sizing block can be of single-stand, preferably two-stand, more preferably four-stand, even more preferably six-stand, and most preferably eight-stand design.
In a further advantageous embodiment, the system can comprise a second thermomechanical sizing block between the first thermomechanical sizing block and the second cooling device, which can also be of single-stand, preferably two-stand, more preferably four-stand, even more preferably six-stand, and most preferably eight-stand design. In this connection, it is particularly preferably provided that an intermediate cooling device is provided between the two thermomechanical sizing blocks, comprising one or two water tanks that are spaced apart from one another. For example, in a first advantageous embodiment, the first thermomechanical sizing block can be of four-stand design and the second thermomechanical sizing block can be of two-stand design. In a further advantageous embodiment, the first thermomechanical sizing block can, for example, be of four-stand design, and the second thermomechanical sizing block can also be of four-stand design. Any other combination is possible and conceivable with regard to the distribution of the stands between the two thermomechanical sizing blocks.
In addition, a thermomechanical sizing block formed in a basic design, for example a six-stand thermomechanical sizing block, could be divided into six single-stand thermomechanical sizing blocks, wherein an intermediate cooling device with at least one water tank is provided between each two of such six single-stand thermomechanical sizing blocks within the entire cascade of, for example, six single-stand thermomechanical sizing blocks.
The thermomechanical sizing blocks are generally known and marketed by the applicant under the brand name MEERdrive®.
The second cooling device is then arranged in a second line section downstream in the transport direction of the first, optionally second, thermomechanical sizing block, in which the long semi-finished steel products, which have been finish-rolled into rod-shaped steel, are cooled in order to stop further grain growth.
The second cooling device can comprise four to nine water tanks, more preferably five to eight water tanks. In a further embodiment, the second cooling device can comprise at least two, more preferably three, even more preferably four, and most preferably five water tanks, by means of which the rod-shaped steel is cooled in order, on the one hand, to equalize the temperature and, on the other hand, to prevent the formation of hardened structures in the form of martensite or bainite.
The second line section, which extends between the first or second thermomechanical sizing block, or the last stand of the sizing block, as the case may be, and the coil winding device, is also preferably selected in such a manner that the rolling stock is given sufficient time for adequate temperature equalization across the cross-section As such, preferably, a temperature gradient of at most 100° C., more preferably a temperature gradient of at most 80° C., still more preferably a temperature gradient of at most 60° C., and most preferably a temperature gradient of at most 50° C. is set in the long semi-finished steel product that has been finish-rolled into rod shape.
The second line section can advantageously have a transport length of 200 to 350 m, more preferably a transport length of 250 to 300 m. In this connection, it has been shown to be particularly preferable that cooling, which begins as soon as possible immediately after the last pass, i.e. after the first or second thermomechanical sizing block or the last stand of the sizing block, ensures control of the recrystallization processes and a high fine grain size, preferably with an average grain diameter of less than 12.0 μm, more preferably with an average grain diameter of less than 10.0 μm, even more preferably with an average grain diameter of less than 8.0 μm, and most preferably with an average grain diameter in the range of 5.0 to 6.5 μm.
Advantageously, it is therefore provided that the rod-shaped steel, which have a temperature in the range of 800° C. to 950° C. after the last pass, preferably a temperature in the range of 800° C. to 900° C., are fed to the second cooling device, in particular to the first water tank of the second cooling device, after a maximum of 100 ms, preferably after a maximum of 90 ms, even more preferably after a maximum of 80 ms, further preferably after a maximum of 70 ms, and most preferably after a maximum of 60 ms.
To prevent further grain growth, the rod-shaped steel is cooled to such an extent that an inlet temperature into the coil winding temperature in the range of 400° C. to 600° C., preferably an inlet temperature into the coil winding device of 450° C. to 550° C., is achieved.
In a further advantageous embodiment, the system can have a structure-sensor device arranged between the second cooling device and the separating device, via which any martensitic structure present, in particular a proportion of martensite in percent by area (A.- %), in the thermomechanically rolled rod-shaped steel product can be determined directly in the ongoing process. The structure-sensor device, which is arranged in the second line section, can advantageously be arranged in the transport direction directly in front of the coil winding device, directly in front of the separating device, and/or in the transport direction, possibly directly, behind the second cooling device, in particular behind the last water tank of the latter. An arrangement between two water tanks of the plurality of water tanks in the second cooling device is also possible.
In an advantageous embodiment, the system comprises a respective structure-sensor device downstream of each of the plurality of water tanks arranged within the second cooling device in the second line section. Thereby, each of the plurality of water tanks can be individually set and the formation of martensitic structures in the specific water tanks can be assigned.
The structure-sensor device can be used to identify the martensitic structure, in particular a proportion of martensite in A.- %, in the rod-shaped steel online in the ongoing process. In principle, all techniques known to the person skilled in the art at the time of application can be used as measuring methods. Advantageously, however, it is provided that the structure-sensor device comprises an ultrasonic measuring device, an X-ray measuring device, a radar measuring device and/or an electro-magnetic measuring device for identifying the undesirable martensite.
The structure-sensor device can advantageously be coupled with an open-loop or closed-loop control device via which, optionally with the aid of corresponding algorithms, active interventions can be made in the respective process steps in order to set the desired microstructure.
In a further aspect, the present disclosure further relates to a rod-shaped steel product, preferably produced by the process in accordance with the disclosure, in particular having a yield strength of at least 300 MPa, more preferably a yield strength of at least 400 MPa, even more preferably a yield strength of at least 500 MPa, and most preferably a yield strength of at least 600 MPa, having a proportion of martensite of at most 15.0 A.- %, preferably a proportion of martensite of at most 10.0 A.- %, more preferably a proportion of martensite of at most 8.0 A.- %, still more preferably a proportion of martensite of at most 6.0 A.- %, and most preferably a proportion of martensite of at most 5.0 A.- %.
Preferably, the rod-shaped steel, in particular structural steel, has the following composition in % by weight:
As further accompanying elements, the rod-shaped steel can comprise the following elements individually and/or in combination (in % by weight):
It is particularly preferred that the rod-shaped steel, in particular structural steel, has a carbon equivalent (Ceq) of ≤0.60, more preferably a carbon equivalent (Ceq) of ≤0.50.
The invention and the technical environment are explained in more detail below with reference to figures and examples. It should be noted that the invention is not intended to be limited by the exemplary embodiments shown. In particular, unless explicitly shown otherwise, it is also possible to extract partial aspects of the facts explained in the figures and combine them with other components and findings from the present description and/or figures. In particular, it should be noted that the figures and in particular the size relationships shown are only schematic. Identical reference signs designate identical objects, such that explanations from other figures can be used as a supplement if necessary.
To produce the corresponding rod-shaped steel 3, the long semi-finished steel products 2 are initially fed to a reheating furnace 4, in which the long semi-finished steel products 2 to be rolled are heated to a temperature of 900° C. to 1000° C.
The long semi-finished steel products 2 that are then heated are fed to a first rolling unit 5, in which they are pre-rolled in a cascade of sixteen housingless rolling mill stands (not shown). Thereby, a reduction in the range of 20 to 40% per pass is achieved in the respective rolling mill stand. The average temperature of the rolling stock in the first rolling unit 5 is 850° C. to 1000° C.
A first cooling device 6 is arranged behind the first rolling unit 5 in the transport direction in a first line section 7, which in the present case comprises two water tanks (not shown), in order to reduce the temperature of the 850° C. to 1000° C. hot rolling stock before the subsequent step of thermomechanical rolling. The first line section 7, which extends between the last housingless rolling mill stand of the first rolling unit 5 and a first thermomechanical sizing block 8, is also selected in such a manner that the rolling stock is given sufficient time for adequate temperature equalization in addition to the temperature reduction. The first line section can have a length of 90 m to 100 m.
The pre-rolled and cooled long semi-finished steel product 2, which by now has a round and/or oval cross-section, is then fed to a first thermomechanical sizing block 8 at a temperature in the range of 760° C. to 820° C. and finish-rolled to the desired or specified, as the case may be, end diameter, which can be 8 mm, 12 mm or 25 mm, for example. In one embodiment, the first thermomechanical sizing block 8 can be designed with six stands, wherein a reduction of 22 to 27% can be achieved per pass in the individual stands.
In a further embodiment, the first thermomechanical sizing block 8/8.1 can be supplemented by a second thermomechanical sizing block 8.2, which can also be of multi-stand design. In this embodiment, an intermediate cooling device 10 with two water tanks (not shown) is provided in an intermediate line section 9 formed between the two thermomechanical sizing blocks 8.1, 8.2. This intermediate line section 9 also has a specific length of 72 m, for example, in order to allow the rolling stock sufficient time for adequate temperature equalization between the two thermomechanical rolling steps.
The second cooling device 11 is then arranged behind the first or second thermomechanical sizing block 8, 8.1, 8.2 in the transport direction in a second line section 12, which extends between the first or second thermomechanical sizing block 8, 8.1, 8.2 and a coil winding device 13. In the second cooling device 11, the long semi-finished steel products 2, which are rolled into rod-shaped steel 3 and have a temperature of 800° C. to 900° C., are cooled by a cascade of four water tanks spaced one behind the other in order to prevent further grain growth and the formation of hardened structures in the form of martensite or bainite. For this purpose, cooling immediately after the last pass, which should be as short as possible, is necessary to control the recrystallization processes and to achieve a high fine grain size with an average grain diameter in the range of 6.0 to 10.0 μm. In order to allow the rolling stock sufficient time for sufficient temperature equalization on the way to the last station, the second line section 12 is also selected to be correspondingly long. This can have a length of 250 to 300 m, for example.
After cutting in a separating device 14, the rod-shaped steel 3 is then fed to the coil winding device 13, which is designed as a vertical coil winding device, at a coil winding temperature of 450° C. to 500° C.
Since the entire cooling process is unstable with respect to the respective target temperatures, and thus an abrupt formation of martensitic structures can occur in the course of process control, the system 1 can also comprise a structure-sensor device 15 arranged in the second line section 12.
Via the structure-sensor device 15, the formation of the martensitic structure, in particular a proportion of martensite in A.- %, in the produced rod-shaped steel 3 can be identified online in the ongoing process. To identify the undesirable martensite, the structure-sensing device 15 can comprise, for example, an ultrasonic sensing device, an X-ray sensing device, a radar sensing device and/or an electro-magnetic sensing device.
Via the dashed arrows. the possible positionings of the structure-sensor device 15 in the second line section 12 are shown. For example, this can be arranged upstream of the second cooling device 11, immediately upstream of the separating device 14 or immediately upstream of the coil winding device 13 in the transport direction. An arrangement between the water tanks of the plurality of water tanks in the second cooling device 11 or in the intermediate line section 9 is also possible.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2021 205 431.5 | May 2021 | DE | national |
This application is a national stage application, filed under 35 U.S.C. § 371, of International Patent Application PCT/EP2022/058216, filed on Mar. 29, 2022, which claims the benefit of German Patent Application DE 10 2021 205 431.5, filed on May 28, 2021.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/058216 | 3/29/2022 | WO |
